Physiol Rev 88: 451– 487, 2008;
doi:10.1152/physrev.00055.2006.
Properties of the Glomerular Barrier and Mechanisms
of Proteinuria
BÖRJE HARALDSSON, JENNY NYSTRÖM, AND WILLIAM M. DEEN
I. Introduction
II. Components of the Glomerular Barrier
A. Podocytes
B. Basement membrane
C. Endothelium
D. The endothelial cell surface layer
E. Mesangial cells
F. Plasma components
III. Methods to Study the Glomerular Barrier
A. Urinalysis in vivo
B. Micropuncture of single nephrons
C. The isolated perfused kidney
D. Tissue-uptake techniques
E. The isolated glomerulus
F. Isolated glomerular cells
G. Glomerular endothelial cells
H. Proteoglycan and GAG production
I. Isolated GBMs
J. Artificial membranes
IV. Solutes Used to Study Glomerular Permeability
A. Theoretically ideal solutes
B. Proteins
C. Dextran polymers
D. Ficoll polymers
E. Proteins versus synthetic polymers
V. Solute Properties That Affect Transglomerular Passage
A. Molecular size
B. Molecular charge
C. Configuration
D. Relative importance of size and charge selectivity
VI. Physical Principles and Theoretical Models
A. Hindered transport and membrane size selectivity
B. Sieving in homogeneous membranes
C. Effects of molecular charge, shape, and concentration on membrane selectivity
D. Sieving in multilayer membranes
VII. Mechanisms Behind Proteinuria and Nephrotic Syndromes
A. What is proteinuria?
B. Where do proteins in the urine come from?
C. The albumin retrieval hypothesis
D. Human diseases and animal models
VIII. Summary
A. What is the relevance of the individual components of the glomerular barrier?
B. What do we need to know more about?
C. Concluding remarks
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0031-9333/08 $18.00 Copyright © 2008 the American Physiological Society
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Department of Molecular and Clinical Medicine/Nephrology, Institute of Medicine, Göteborg University,
Sweden; and Department of Chemical Engineering, Massachusetts Institute of Technology,
Cambridge, Massachusetts
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HARALDSSON, NYSTRÖM, AND DEEN
Haraldsson B, Nyström J, Deen WM. Properties of the Glomerular Barrier and Mechanisms of Proteinuria.
Physiol Rev 88: 451– 487, 2008; doi:10.1152/physrev.00055.2006.—This review focuses on the intricate properties of
the glomerular barrier. Other reviews have focused on podocyte biology, mesangial cells, and the glomerular
basement membrane (GBM). However, since all components of the glomerular membrane are important for its
function, proteinuria will occur regardless of which layer is affected by disease. We review the properties of
endothelial cells and their surface layer, the GBM, and podocytes, discuss various methods of studying glomerular
permeability, and analyze data concerning the restriction of solutes by size, charge, and shape. We also review the
physical principles of transport across biological or artificial membranes and various theoretical models used to
predict the fluxes of solutes and water. The glomerular barrier is highly size and charge selective, in qualitative
agreement with the classical studies performed 30 years ago. The small amounts of albumin filtered will be
reabsorbed by the megalin-cubulin complex and degraded by the proximal tubular cells. At present, there is no
unequivocal evidence for reuptake of intact albumin from urine. The cellular components are the key players in
restricting solute transport, while the GBM is responsible for most of the resistance to water flow across the
glomerular barrier.
The intricate properties of the glomerular barrier
have fascinated researchers for decades (87). Today,
many of the molecular components of the barrier have
been revealed (234), but their functional interactions remain to be elucidated. The glomerular barrier is by far the
most complex biological membrane, with properties that
allow for high filtration rates of water, nonrestricted passage of small and middle-sized molecules, and almost
total restriction of serum albumin and larger proteins. In
humans, close to 180 liters of primary urine are produced
each day at capillary pressures far exceeding those in
other organs. Despite this tremendous work load, the
glomerulus remains intact year after year. Indeed, several
anticlogging mechanisms have been proposed, including
protein consumption by the mesangial cells (87), charge
repulsion in the glomerular basement membrane (GBM)
(146), a tangential-flow filtration mechanism (260), and a
gel permeation hypothesis (292, 346, 347).
This review focuses on the functional properties of
the glomerular barrier and the mechanisms of proteinuria
and nephrotic syndromes. Although recent discussions of
proteinuria have tended to focus on the role of the podocyte (319), a more complete understanding of the underlying mechanisms requires careful consideration of the
other glomerular components (323). To this end, we provide an overview of the role of glomerular endothelial
cells and the glycocalyx surrounding them and discuss the
GBM. For details on podocyte biology, readers are referred to Pavenstädt et al. (234). Also considered are
interactions between the barrier and plasma proteins,
such as orosomucoid and albumin, which further illustrate the multifaceted nature of the glomerular membrane. The principal components of the glomerular barrier are illustrated in Figure 1, together with the approximate glomerular filtration rate, plasma flow rate, and
albumin concentrations in humans.
Next we review the various methods used to study
glomerular permeability and discuss their virtues and limPhysiol Rev • VOL
FIG. 1. Schematic drawing of the glomerular barrier. Podo, podocytes; GBM, glomerular basement membrane; Endo, fenestrated endothelial cells; ESL, endothelial cell surface layer (often referred to as
the glycocalyx). Arrows indicate the filtration of plasma fluid across the
glomerular barrier, forming primary urine at a glomerular filtration rate
(GFR) of 125 ml/min in humans. The plasma flow rate (Qp) is close to
700 ml/min, giving a filtration fraction of 20%. Also shown are the
concentrations of albumin in serum (40 g/l) and estimated concentration
in primary urine 4 mg/l (i.e., 0.1% of that in plasma). The sieving coefficient of albumin across the glomerular barrier in humans is estimated
to be 10% of that in rodents.
itations. Misconceptions about the properties of the macromolecular tracers used to analyze permeability have led
to some confusion in the field. Therefore, we discuss the
“ideal solute” and compare its properties with those of the
solutes currently available for research. Subsequently, we
address the renal clearance of various solutes in humans
and intact animals and under different experimental conditions, discussing the results in terms of solute size,
shape, and charge. At the end of that section, the recent
controversy on glomerular charge selectivity is discussed
and analyzed.
Finally, we review physical principles and various
theoretical models that reflect our understanding of how
the glomerular barrier carries out its remarkable functions. “Simple” and multilayer barriers are discussed,
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I. INTRODUCTION
GLOMERULAR BARRIER AND PROTEINURIA
along with the implications of their behaviors for interpreting in vivo and in vitro data.
II. COMPONENTS OF THE
GLOMERULAR BARRIER
A. Podocytes
B. Basement Membrane
The basement membrane is composed of a fibrous
network consisting of type IV collagen (collagen ␣3, ␣4,
and ␣5 chains) (193, 271), laminin (mainly laminin 11;
Physiol Rev • VOL
␣5␤␥1) (205), and nidogen/entactin, together with proteoglycans such as agrin and perlecan, as well as glycoproteins. The basement membrane of the glomerular capillaries is much thicker (240 –370 nm) (163) than the
basement membranes in other vascular beds (40 – 80
nm) (288).
The collagen IV network may be described as the
backbone of the basement membrane. Mutations in the
collagen chains give rise to severe pathological conditions, the most well-known being Alport’s syndrome (hereditary glomerulonephritis) (14, 170, 195). Depending on
the location of the mutation, less severe conditions such
as thin glomerular membrane disease may occur (320).
The basement membrane contains large amounts of
proteoglycans, with mainly heparan sulfate chains attached. These chains may contribute to the selective
properties of the barrier (118, 145). However, the importance of the GBM in renal permselectivity has been debated. For a long time, it was considered to be the principal barrier (15, 43, 94, 303). Then in vitro studies of
isolated basement membranes suggested that additional
components were required to maintain glomerular permselectivity (20, 65). In another study, the inert probe Ficoll
and its negatively charged counterpart Ficoll sulfate were
used to investigate the charge effect of isolated GBMs
(28). There was no difference between the sieving curves
of negative and neutral Ficolls. This suggests that the bulk
of effective charge density within the glomerular barrier
lies in the endothelial or epithelial cell layers. However,
the GBM is still an important part of the glomerular
barrier, since it accounts for most of the restriction of the
fluid flux (65, 73). Moreover, recent data on laminin ␤2 in
patients (350) and knockout mice (132) seem to indicate
that the GBM may restrict solute flux as well. In the mice,
proteinuria preceded alterations in podocyte morphology,
suggesting the GBM to be an important part of the glomerular barrier (132). However, it could not be ruled out
that endothelial or epithelial functions were affected by
the loss of laminin and normal GBM structure (116).
C. Endothelium
The endothelial cells of the glomerulus have not been
well investigated, likely because they are located within
the glomerulus and because they are difficult to maintain
in culture without altering their phenotype. Glomerular
endothelial cells are unusually flattened, with a height
around the capillary loops of ⬃50 –150 nm (315). Capillaries with continuous endothelium are the most common
type and are typically found in skeletal muscle, cardiac
muscle, and skin (288). However, the glomerular capillaries have endothelial cells with a large fenestrated area
constituting 20 –50% of the entire endothelial surface (40).
The unusually high density of fenestrae is thought to
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The epithelial cells of the glomerulus have attracted
considerable interest in recent years. These specialized,
highly differentiated cells line the outside of the glomerular capillaries and thus face the Bowman’s capsule and
the primary urine. The cells have a large cell body and
long, extending cytoplasmic foot processes, which are
separated by a filtration slit that is ⬃25– 60 nm wide (150,
269, 342) and covered by a diaphragm. The molecular
components of the diaphragm have been extensively studied, and some proteins are vital for the maintenance of
normal glomerular permselectivity: the restriction of the
permeation of macromolecules across the glomerular barrier on the basis of molecular size, charge, and physical
configuration. One such molecule is nephrin, which when
mutated causes massive leakage of protein and severe
consequences for patients with congenital nephrotic syndrome of the Finnish type (153, 154).
The podocyte is a major component of the glomerular barrier, but its contribution to the restriction of fluid
and protein transport is unclear. The slit membrane has
porelike structures (342) whose dimensions are postulated to be 40 ⫻ 140 Å (254, 319), corresponding to a slit
half-width of 20 Å. With a Stokes-Einstein radius (the
most common measure of molecular size; see sect. VIA) of
36 Å, albumin cannot reach the urinary space across such
slits, except through “large pores” or “shunt pathways.”
Moreover, the slit membrane dimensions described above
also are too small to explain the sieving of other solutes.
The sieving coefficient (⌰) of a solute is a measure of its
transport rate in relation to that of water. Myoglobin with
a Stokes-Einstein radius of 18 Å has a glomerular sieving
coefficient close to 0.8 (348), but a slit half-width of 20 Å
implies total restriction (116), i.e., ⌰ close to zero. It is
therefore most likely that the slit dimensions were underestimated in Reference 254. In addition, several studies in
both mice and humans show that proteinuria can occur
without effacement of podocyte foot processes (296, 330).
Indeed, proteinuria may occur regardless of which layer
of the glomerular wall is damaged (116, 144).
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HARALDSSON, NYSTRÖM, AND DEEN
FIG. 3. A human glomerulus with podocytes stained green with
antibodies against synaptopodin and endothelial cells stained red with
Ulex europaeaus agglutinin I lectin.
D. The Endothelial Cell Surface Layer
On the luminal side, blood vessel walls are covered
with an endothelial cell surface layer (ESL) that is involved in blood coagulation (16, 174), modulation of angiogenesis (39, 96), rheology (62), and capillary barrier
function (129, 134, 135, 301). This layer has two components: the glycocalyx, which most commonly refers to the
plasma-membrane-bound part of the layer, and the endo-
FIG. 2. Scanning electron micrograph showing a mouse glomerulus (A) with several capillary loops, capillary lumen, and podocytes with their
foot processes. To the right (B) is a fenestrated glomerular capillary with its fenestrated endothelium surrounded by podocyte foot processes. Scale
bars: 10 ␮m (A) and 1 ␮m (B).
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allow high permeability to water and small solutes in the
glomerulus (73). A scanning electron micrograph of a
mouse glomerulus and a fenestrated capillary is shown in
Figure 2.
The fenestrae are ⬃60 nm in diameter (316), and
albumin, which is restricted by the glomerular wall, has a
diameter of only 3.6 nm. These measurements have been
used to suggest that endothelial cells do not contribute to
the permselectivity of the glomerular barrier. However,
the endothelial cell coat has charge-selective properties
and the barrier probably begins at the endothelial level.
For instance, the presence of a diaphragm across the
fenestrations in glomerular capillaries has been a matter
of debate. Rostgaard and Qvortrup (260) described a high
density of fibers in the fenestrae, probably consisting of
negatively charged proteoglycans. Other studies support a
permselective layer at the level of the endothelium (11, 52,
70, 134, 135, 212, 302). These studies include both morphological (267, 268) and functional data and provide
support for the notion that glomerular endothelial cells
and their cell coat should be considered in evaluating the
permselective properties of the glomerular barrier. A human glomerulus with the endothelial and podocyte cells
stained with different markers is shown in Figure 3.
Thus the endothelium seems to be an important component of the glomerular barrier, restricting macromolecules despite the presence of numerous large
fenestrae. Recently, there has been a renewed interest
in the cell surface coat that is produced by, and surrounds, the endothelial cells.
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GLOMERULAR BARRIER AND PROTEINURIA
contributes to the high permselectivity of the glomerular
wall.
1. The podocyte glycocalyx
Podocytes are covered by a glycocalyx containing
sulfated molecules such as GAG and sialyzated glycoconjugates (232). Most studies of the podocyte glycocalyx
were done using cationic dyes, which have, for example,
shown that heparan sulfate GAGs are present at the slit
membrane (261). The major known sialoprotein on podocytes is podocalyxin, a heavily glycosylated protein (151).
In rats treated with puromycin to mimic a nephrotic syndrome, the sialic acid content of podocalyxin was lowered, indicating that foot process effacement is linked to
a reduced negative charge on the podocyte surface (152).
Glomerular epithelial cells also produce proteoglycans
such as glypican-1 and syndecan-4 (238). The surface
anionic charge on podocytes is essential for maintaining
the foot process structure (25). It is also involved in
keeping a distance between parietal and visceral epithelial cells, thereby helping to maintain glomerular structure
and function (150).
E. Mesangial Cells
The main function of the glomerular mesangium is to
maintain the structure and function of the glomerular
barrier. The mesangium is composed of two entities: mesangial cells and the extracellular matrix. Like smooth
muscle cells, mesangial cells have contractile properties
(162, 277). Although this contractility might suggest that
mesangial cells influence filtration, their overall contribution to permselectivity is probably small. Rather, the contractility may regulate glomerular distensibility in response
to pressure.
Mesangial cells produce components of the mesangial matrix, which is thought to provide support for glomerular capillaries. The mesangial matrix consists of col-
FIG. 4. Electron microcraphs showing the glomerular barrier, with the capillary lumen above and the urinary space
below. The endothelial cell surface coat
(ESL) can be visualized with different
techniques (121). A: Cupromeronic Blue
was used to visualize the charged structures of the barrier. With this technique,
the GBM appears to have a homogeneous structure. B: staining with lanthanum results in higher contrast and
shows “bushlike” structures in the fenestrate that extend into the capillary lumen. Scale bars: 100 nm.
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thelial cell coat, a larger, more loosely associated part of
the layer. Originally described in the 1940s as a thin,
noncellular coat (44, 63), the glycocalyx was first visualized by staining with ruthenium red, a cationic dye that
binds to negatively charged molecules (178). The ESL is
composed of negatively charged glycoproteins, glycosaminoglycans (GAGs), and membrane-associated and secreted proteoglycans, and can be visualized by using different dyes (Fig. 4).
It has been challenging to visualize the glycocalyx
and the cell coat because of technical issues (e.g., dehydration and disruption during fixation and staining) and
because shear stress and plasma components are important aspects of the in vivo situation. Moreover, associated
plasma proteins such as orosomucoid and albumin affect
the composition (278, 279) and physical properties (114,
115, 117) of the endothelial cell coat. Earlier work using
different dyes to visualize the glycocalyx indicated a
thickness of 50 –100 nm (112, 289). However, intravital
microscopy studies with fluorescent tracers indicated a
thickness of 0.5 ␮m (339), while other measurements
suggested an even thicker layer, up to 1 ␮m (77). These
differences probably reflect the binding of the dyes to the
membrane-bound glycocalyx, which is present after fixation, whereas intravital microscopy reveals the thicker
ESL, which is present when the surrounding environment
is physiological. In two studies (121, 135), intralipid droplets were infused, and the distance from the droplets to
the vessel wall was calculated. The results indicated a
large ESL of ⬃200 – 400 nm. Digestion of proteoglycans
with hyaluronidase, heparinase, or chondroitinase reduced the thickness of the ESL (121, 135). Treatment with
chondroitinase also increased the fractional clearance of
albumin (135). Note that the enzymes used mainly affected the ESL and not the GBM or other structures of the
barrier, since the high molecular weights of the enzymes
made them remain mostly in the vascular compartment
during the experiment (135). Thus the ESL seems to be a
rather thick, negatively charged structure that most likely
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HARALDSSON, NYSTRÖM, AND DEEN
F. Plasma Components
The idea that components of plasma influence the
glomerular barrier is not entirely new (164), but it has
mainly concerned “permeability factors” that induce proteinuria (98). Several candidates have been suggested, but
causal associations have not been demonstrated (34). In
other capillary beds, low concentrations of albumin (⬍1%
of normal) increase permeability to water (63, 191). In the
kidney, however, the absence of albumin seems to increase glomerular permeability to macromolecules as
well (92). Orosomucoid (a plasma protein and acute
phase reactant normally present in concentrations between 0.1 and 1.0 g/l in human plasma) has a similar
permselective effect in several vascular beds (58, 114,
115), including the kidney (117, 139). The mechanism is
not clear, but it may involve fiber matrix interaction
(168), endothelial cell production of orosomucoid
(299), binding of orosomucoid to endothelial cells
(279), or an anti-inflammatory cAMP-mediated receptor
activation by orosomucoid (300).
In other vascular beds, plasma proteins increase the
thickness of the endothelial cell surface coat (often denoted as glycocalyx) (2). Any sufficiently concentrated
protein (such as albumin in normal plasma) would be
expected to increase the equilibrium partition coefficient
for itself and other macromolecules in glycocalyx and
possibly GBM, which in turn would influence glomerular
sieving (168) (see sect. VI). Thus there seem to be ample
possibilities for plasma proteins, such as orosomucoid, to
affect the glomerular barrier.
Figure 5 shows a schematic drawing of the glomerular barrier with focus on the endothelium. Podocytes
FIG. 5. Schematic drawing of the glomerular barrier with components of the glomerular endothelium [e.g., the integrins, Tie2, VEGF receptor
1 (VEGFR1), VEGFR2] and the endothelial cell surface coat (ESL). Several, or all, of the components of the ESL are produced by the endothelium
at certain rates and removed to blood or urine at similar rates during steady-state conditions. Membrane-bound proteoglycans (PG), such as
syndecan and glypican, which carry chondroitin sulfate (CS) side chains (syndecan) and/or heparan sulfate (HS) side chains (glypican and syndecan)
form the glycocalyx. The ESL is formed by secreted proteoglycans such as perlecan (mainly HS) and versican (mainly CS) together with secreted
glycosaminoglycans (GAG) (e.g., hyaluronan) and adsorbed plasma proteins (e.g., albumin and orosomucoid). Several other macromolecules not
shown in the figure are important components of the ESL and the glycocalyx. The podocytes produce substances such as ang1 and VEGF that affect
the endothelial cells. For a more detailed picture of the podocyte biology, please consult Reference 234.
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lagens (type I, III, IV, and V), laminin, fibronectin, and
proteoglycans with both heparan sulfate and chondroitin
sulfate chains (189, 215). Mesangial cells secrete several
growth factors, including interleukin (IL)-1, platelet-derived growth factor (PDGF), and insulin-like growth factor (IGF) (1, 8, 352). PDGF is a potent mitogen for mesangial cells. Transforming growth factor (TGF)-␤ has the
opposite effect and inhibits cell proliferation; it also stimulates proteoglycan synthesis in both mesangial cells and
podocytes in culture.
Mesangial cell proliferation and matrix expansion are
seen in several pathological conditions, such as IgA nephritis (143, 165) and diabetic nephropathy (102, 133,
276). As the mesangial matrix expands, it affects the
glomerular capillaries by reducing the area available for
filtration and may eventually even occlude the capillary
lumen. Matrix expansion due to cell proliferation also
leads to glomerulosclerosis.
GLOMERULAR BARRIER AND PROTEINURIA
influence endothelial properties by secreting hormones
such as vascular endothelial growth factor (VEGF) and
ANG I. The glycocalyx is composed of the membranebound proteoglycans such as syndecan and glypican. The
ESL is a much thicker structure, which also consists
of secreted proteoglycans (e.g., versican and perlecan),
hyaluronan, and adsorbed plasma proteins (e.g., orosomucoid and albumin).
III. METHODS TO STUDY THE
GLOMERULAR BARRIER
transport. For analysis of macromolecules such as albumin, however, certain technical problems emerge.
For example, if samples are obtained from the proximal
tubules and not from Bowman’s capsule, the urinary
concentration of albumin will be underestimated. Adsorption of protein to the surface of the glass pipette
may cause further underestimation. In addition, the
sampling procedure itself may damage the delicate barrier. In an attempt to circumvent some of these limitations, Tojo and Endou (317) plotted the albumin concentration against the tubular fluid to plasma concentration ratio (TF/P) of inulin (317). Retropolating to a
TF/P of 1.0 for inulin, they estimated the sieving coefficient for albumin to be 0.0006. Furthermore, their data
show that the albumin concentration is underestimated
by one order of magnitude when the TF/P is 2. In
contrast, the normal urinary excretion of albumin in
humans is ⬍20 mg/day, giving apparent sieving coefficients of ⬎10⫺5, reflecting the extensive tubular modification of the urinary content.
A. Urinalysis In Vivo
C. The Isolated Perfused Kidney
In vivo urinalysis is readily available for studies in
humans. The use of metabolically inert solutes such as
inulins (oligosaccharides derived from plants) is the gold
standard for estimating the glomerular filtration rate
(GFR) and for analyzing solute sieving.
It might seem more physiological to analyze endogenous proteins (308, 309), but such data are less useful
(203) due to the extensive reabsorption, possible degradation, or secretion by tubular cells. Attempts have been
made to correct for these limitations, but the accuracy of
the corrections is still uncertain (310, 311). Data concerning glomerular size selectivity in humans are mainly based
on studies using dextran (3, 106, 243, 244) or Ficoll (26).
Human glomerular charge selectivity has been studied
using sulfated dextran (105).
Patients with defects in the reabsorption machinery
of the proximal tubule (i.e., Fanconi syndrome) excrete
the proteins filtered across the glomerular wall. In a series
of elegant studies in such patients, modern proteomics
approaches such as matrix-assisted laser desorption/
ionization time of flight mass spectrometry (MALDI-TOF
MS) were used to analyze urine composition (59, 207,
208). The results showed that the megalin-cubulin complex in the proximal tubules was impaired (206), and
protein excretion was increased almost 100-fold (208).
This value is compatible with a highly selective glomerular barrier (45– 48, 68, 73, 116, 317).
B. Micropuncture of Single Nephrons
Micropuncture of single nephrons is an excellent
technique for studying pressures, flow, and small-solute
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This technique has been used for many years (343)
but with several modifications. For example, the kidney
can be perfused at normal temperatures (37°C) (30,
180, 257) or at reduced temperatures [4 – 8°C, cooled
isolated perfused kidney (cIPK)] (173, 240). As perfusate, one can use blood (33, 60, 248) or artificial solutions containing albumin (140), dextran (297), or other
colloids (38). Tubular modification of the primary urine
has been minimized by using chemical agents (222,
228), low temperatures (140, 213, 240, 302), and chemical fixation (51–53). Finally, the kidneys can be perfused with recirculating (33) or single-passage systems (140).
Perfusion with blood is theoretically more physiological but is technically difficult for several reasons, such as
the risk for hemolysis, complement activation, and thrombocyte degranulation. Therefore, most researchers interested in glomerular permeability have used artificial solutions based on albumin or other colloids. However, one
problem with a blood-free solution is that the maximum
amount of oxygen that can be dissolved in water does not
meet the metabolic demands of the kidney, as reflected by
morphological evidence of renal ischemia (173). Fluorocarbons can be used (91), but such agents might affect the
permeability per se. In the warm IPK, renal hypoxia leads
to albuminuria that gradually increases over time. The
kidneys may be better preserved if the perfusate contains
mannitol and amino acids (180, 333). Kidneys perfused at
reduced temperatures (8 –27°C) do not show ischemic
damage (173, 177); nevertheless, albumin clearance increased gradually over time (117).
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Several experimental approaches have been used to
unravel the complexities of glomerular permeability, including urinalysis in vivo, micropuncture of single
nephrons, isolated perfused kidneys, tissue-uptake techniques, isolated glomeruli, isolated glomerular cells, isolated basement membranes, and artificial membranes.
Each approach has merits and drawbacks.
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To study glomerular filtration, one must either inhibit
tubular cell activity or use inert tracers that are neither
secreted nor reabsorbed in the tubules (30). Tubular cell
toxins, such as lysine (150 mM), ammonium chloride (10
mM), chlorochine (0.1 mM), and cytochalasin B (0.02 mM)
(228), increase glomerular permeability (211) and cannot
be recommended. The cIPK model is most useful for
studies of macromolecular transport, since the equivalent
pore dimensions are similar in the cIPK model and in vivo
(123), a finding reported for warm IPKs as well (30) using
dextran as the inert tracer.
D. Tissue-Uptake Techniques
E. The Isolated Glomerulus
Isolated glomeruli have been used to estimate albumin permeability (274), based on osmotically induced
changes in glomerular volume. The underlying theory is
that solutes exert between 0 and 100% of the theoretical
osmotic pressure across membranes, the fraction of the
F. Isolated Glomerular Cells
Cells cannot be used to analyze the properties of the
complex glomerular barrier. However, studies in cells are
crucial for understanding how it is created and maintained. Glomerular endothelial (13, 23, 99, 233, 272, 273,
298, 324), epithelial (125, 199, 200), and mesangial cells
(156, 275) have been cultured and studied in isolation.
Human podocytes in culture are shown in Figure 6. Fewer
studies have focused on interactions between the different cell types (e.g., mesangial and endothelial cells) (156).
Glomerular cells produce molecules that may have endocrine and autocrine functions, such as VEGF (66, 103),
and their signaling effects are most likely important for
glomerular function (79, 85, 125, 142, 182, 304).
To study the importance and contribution of the
different cell types in the glomerulus, methods have been
developed to separate the various kinds of cells. Glomeruli are easy to harvest from kidney samples of humans
FIG. 6. Human podocytes in culture.
A: image obtained by light microscopy
shows small buds of foot processes beginning to form (white arrows). B: actin filaments stained by phalloidin.
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Glomerular function can also be studied by analyzing
the tissue uptake of radiolabeled tracers administered
in vivo. However, this approach assumes that the filtered
tracer is present in the urine, the tubular system, or the
tubular cells (19), at least shortly after administration
(312, 313). Radioactivity is counted in plasma, urine, and
the removed kidneys, allowing for estimates of clearance.
Comparisons between experimental groups of animals
(12) are hampered by the fact that there may be differences in hemodynamic conditions and in the number of
nephrons, and hence differences at the single-nephron
level affecting the filtration rate (179, 251). Such differences may affect the clearance of tracers even in the
absence of alterations at the membrane level, a phenomenon sometimes neglected, as will be discussed in
section VI.
theoretical osmotic pressure being equal to the osmotic
reflection coefficient (␴). Therefore, the authors interpret
changes in glomerular volume in terms of alterations in
␴alb. This is not an adequate term, however, since numerous factors (e.g., cellular contraction) can affect the glomerular volume apart from the permeability of albumin.
Moreover, reported ␴alb values of 0.5 or less in response
to damage (61, 188, 281, 285, 286) are unrealistically low.
Such values would imply urinary albumin losses for nephrotic patients in the range of several kilograms of albumin per day (180 l/d ⫻ 40 g/l ⫻ 0.5), far more than is
compatible with life. However, the model may still be
useful as a diagnostic tool in search for proteinuric factors in serum (42, 282–284, 318). The diffusional permeability of macromolecules has also been studied in isolated glomeruli (64, 65, 81, 82). In certain animals, it has
even been possible to microdissect and perfuse single
glomeruli for studies of glomerular transport (88).
459
GLOMERULAR BARRIER AND PROTEINURIA
G. Glomerular Endothelial Cells
Primary cultures of glomerular endothelial cells are
difficult to work with. The cellular phenotype is lost after
only a few passages, so reliable information can only be
gained from low-passage primary cultures. Most of the
earlier work on endothelial cells and their properties was
done on cells derived from large vessels (e.g., aortic and
umbilical vein endothelial cells). Although such studies
have yielded important information, there are differences
both between species and between different vascular
beds. A recent study comparing bovine aortic endothelial
cells to glomerular endothelial cells revealed significant
transcriptional differences between cells of macro- and
microvascular origin (280). To circumvent problems with
dedifferentiation, immortalized lines of glomerular endothelial cells were created.
Another important factor is shear stress. Endothelial
cells showed marked differences in expression and morphology in the presence and absence of shear stress (196).
Nevertheless, cultures of glomerular endothelial cells are
an important source of information for cell function. For
all studies on glomerular endothelium, it is important to
characterize the cells carefully using specific markers
(Fig. 7).
H. Proteoglycan and GAG Production
Proteoglycans are versatile molecules found in diverse areas of the body. The hallmark of a proteoglycan is
a protein core carrying at least one GAG chain. GAGs are
sulfated polysaccharides consisting of repeating disaccharide units of uronic acid (glucuronic acid or iduronic
acid) and amino sugar (galactosamine or glucosamine).
The GAG moieties of the proteoglycans are divided into
groups. Chondroitin sulfate and dermatan sulfate contain
galactosamine and are called galactosaminoglycans. Heparin and heparan sulfate contain glucosamine and are
therefore called glucosaminoglycans.
FIG. 7. Human glomerular endothelial
cells in primary culture. A: light microscopic image of normal cells. B: cells labeled with Dil-Ac-LDL, an endothelial cellspecific agent. C: cells labeled with an endothelial cell-specific lectin from Ulex
europaeus agglutinin I.
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and the most commonly used species such as rats and
mice. They are readily visualized by light microscopy and
may be microdissected from renal tissue. For a more
large-scale approach, glomeruli can be obtained by passing renal tissue through stainless steel sieves with specific
mesh sizes (13) or by using magnetic beads (305). To
single out the different cell types, the glomeruli must be
broken and the cells released (e.g., with collagenase or
other enzymes). For human kidney tissue, the age of the
donor is also of importance for the outcome (99), with the
cells from a young donor maintaining their phenotype
better. The cells may be cultured and then subcloned, or
specific cell sorting may be performed through FACS or
Percoll gradients.
Culturing the cells poses different challenges depending on the cell type. Mesangial cells were the first
cells of glomerular origin to be cultured. They rapidly
overgrow other cells in a coculture and are thus easier to
maintain in vitro than the other two cell types (89). It is
vital that the cultured cells are properly evaluated. Cells
in primary cultures may lose their phenotype and stop
expressing cell-specific markers. This has lately been addressed by creating immortalized lines of both human and
mouse glomerular endothelial cells (272, 331) and podocytes (200, 201, 270). Still, it is important to characterize
the cells for specific markers. Is an endothelial cell without specific markers such as platelet/endothelial cell adhesion molecule 1 still an endothelial cell? Is it relevant to
study a podocyte that does not express nephrin and
CD2AP?
460
HARALDSSON, NYSTRÖM, AND DEEN
proteoglycan expression in glomerular endothelial cells
and induced a nephrotic syndrome in rats (24).
I. Isolated GBMs
Isolated GBM has been used extensively to study the
transport of fluid and water-soluble solutes (65, 82, 93,
130). These studies have been of great importance for our
understanding of sieving across gels or fiber matrices. In
isolated GBM, charge selectivity has been studied using
dextrans (32) and Ficolls (28), and the effect of plasma
proteins on the selectivity of the GBM has been analyzed
(168). When extrapolating data from the isolated basement membranes to the GBM in vivo, one must consider
the possibility that certain components may have been
lost during the isolation procedure (28). Moreover, the
hydraulic conductivity and the sieving properties of isolated GBM can be modulated by elevating the hydrostatic
pressure on both sides of the membrane (253). There are,
however, strong arguments to suggest that the GBM has
similar properties in vivo and in vitro. The hydraulic
conductance of isolated GBM accounts for most of the
fluid restriction of the intact glomerular barrier (65, 73).
Therefore, it seems safe to conclude that the GBM normally restricts water transport but only marginally restricts the passage of solutes (28).
J. Artificial Membranes
Synthetic membranes of various types have been
used to examine the solute and membrane properties that
govern the transport of proteins and other macromolecular solutes. Results for pores or microchannels of precisely controlled dimensions have been reviewed (67, 69).
The desire to better understand and mimic materials such
as GBM has motivated several studies of transport
through hydrogels, including agarose and agarose-dextran
composites (138, 141, 158 –160, 167). Such studies are
gradually refining our understanding of the physics of
FIG. 8. Glomerular cells produce
various proteoglycans. These images
are from human podocytes in culture.
A: production of perlecan, labeled
green with specific antibodies. B: perlecan (green) and actin are labeled red
using phalloidin.
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Several enzymes regulate GAG chain synthesis and
sulfatation. The amount of attached sulfate groups determines how negatively charged the GAG will be. The galactosaminoglycans (chondroitin sulfate and dermatan sulfate) are modified by sulfotransferases (4-O or 6-O),
which add sulfate groups to the carbohydrate chain, and
2-O-sulfotransferase, which adds sulfate groups to the
uronic acid residue. The N-deacetylase N-sulfotransferase
enzyme family is involved in glucosaminoglycan (heparin
and heparin sulfate) sulfatation. There are also several
O-transferases that add sulfate groups to the glucuronic
acid moiety. In addition to the different GAG chains, there
are also several proteoglycan core proteins (Fig. 8).
In the glomerulus, the most described proteoglycans
are perlecan (131) and agrin (110, 239), both expressed in
the GBM together with another proteoglycan, collagen
VIII (113, 202). Perlecan carries heparan sulfate and sometimes chondroitin sulfate, but agrin carries only heparan
sulfate. These molecules contribute to the negative
charge of the GBM and may thus contribute to the permselectivity of the glomerular barrier. However, mice lacking perlecan heparan sulfate chains do not develop proteinuria or any other renal defects (258). This may be due
to compensatory mechanisms, but more likely it further
strengthens the findings that the GBM alone cannot be
responsible for the permselective properties of the glomerular barrier. Proteoglycans are present on the surface
of endothelial cells and podocytes and in the mesangial
cell matrix as well. Cultured immortalized human podocytes express both mRNA and protein for syndecan-1,
versican, and perlecan (24). The negative charges on
podocytes are thought to contribute to charge selectivity,
podocyte stability, and signaling. Bovine glomerular endothelial cells synthesize perlecan, a synthesis that may
be altered by treatment with TGF-␤ (148) or glucose
(109). Biglycan is expressed by glomerular endothelial
cells in tissue sections, and human glomerular endothelial
cells in culture express several proteoglycans (syndecan,
versican, glypican, and perlecan) along with their regulatory enzymes (sulfotransferases) (23). In addition, treatment with puromycin aminonucleoside downregulated
461
GLOMERULAR BARRIER AND PROTEINURIA
hindered transport through networks of cross-linked
polymers with fluid-filled interstices.
IV. SOLUTES USED TO STUDY
GLOMERULAR PERMEABILITY
A. Theoretically Ideal Solutes
B. Proteins
Endogenous proteins are highly relevant to study, but
they deviate from the ideal solute in many respects. By
definition, proteins are not inert, are never solid, seldom
have spherical shapes, are often compressible, and are
made of amino acids with a neutral, negative, or positive
charge, giving them a certain pH-sensitive net charge. To
complicate things further, charges may be expressed on
the surface or not, and the protein core may carry GAG
chains. The most common issue with respect to glomerular filtration is, however, the tubular handling of proteins. The most reliable estimates of the glomerular sieving of proteins come from studies in which reabsorption
is controlled.
Estimated sieving coefficients for albumin (⌰alb)
from studies controlled for tubular modification of the
TABLE
1.
Reported sieving coefficients for albumin in studies with “controlled” tubular modification
Species
Albumin Sieving Coefficient
Technique
Reference Nos.
Rat
Rat
Rat
Rat
Humans, Fanconi
Rat
Mouse
Rat
Rat
Rat
Rat
Rat
0.00033
0.00066
0.00060
0.00080
0.00008
0.0015
0.0023
0.0029
0.0080
0.0087
0.0450
0.0800
In vivo ⫹ lysine
Tissue uptake
Tissue uptake
Micropuncture
Urine proteomics
cIPK
cIPK
Tissue uptake
IPK no tubular cell inhibitor
Fixed kidney
IPK after tubular cell inhibitor
IPK after tubular cell inhibitor
310
179
18
317
208
123, 213, 214, 301, 302
134
19
228
53
225
228
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The ideal solute for studies of transport is a solid
inert sphere that does not affect the organism in any way
and is not metabolized during the observation period. For
studies of glomerular charge selectivity, solid spheres
should carry a known net charge without distorting molecular size or shape and should remain metabolically
inert. To assess the effect of shape, the solid molecules
should be made ellipsoid in a controlled and stable manner. Unfortunately, the ideal solute does not exist. Therefore, in interpreting the results of clearance studies with
“real world” solutes, their nonideal properties must be
taken into account. At present, Ficoll seems to be closest
to the “ideal” solute (see sect. VI).
urine are shown in Table 1. Micropuncture of rat proximal
tubules (317) gives values for ⌰alb (assuming a serum
albumin concentration of 30 g/l) that are 10-fold higher
than estimates from patients with Fanconi (Dents) syndrome (i.e., 8 ⫻ 10⫺5) (208). The latter value represents a
lower limit for the sieving coefficient, since there still may
be a residual reabsorption of albumin. Lower values (3 ⫻
10⫺4) were obtained in vivo in rats in which tubular
reabsorption was (partially? Ref. 314) inhibited with
lysine (310). The ⌰alb values from cIPK (8°C ) are two- to
threefold higher (122, 123, 176, 301, 302), but still close to
0.001. In contrast, estimates from IPK at 37°C are 10 times
higher (225, 228) and increase further, to 0.08, after treatment with a tubular cell inhibitor (228). The latter value is
not compatible with the notion of a highly selective glomerular barrier and gave rise to the so-called albumin
retrieval hypothesis (see sect. VII).
Glomerular sieving data for other proteins during
controlled tubular cell activity are scarce. In vivo, lysine
has been administered to produce proteinuria by inhibiting tubular cells (310). However, the albumin sieving coefficient calculated from such data is only half of that
obtained with micropuncture (317) or tissue-uptake techniques (19, 179) or from patients with Fanconi syndrome
(209) (Table 1). Indeed, lysine has variable and incomplete effects, and its mechanism of action is still unclear
(314). Therefore, the sieving data for ␤2-microglobin,
orosomucoid, IgG, and ␣2-macroglobin are likely to be
underestimated as well (310). To circumvent some of the
problems with administration of lysine, Lund et al. (179)
used a tissue-uptake technique to estimate glomerular
sieving of certain proteins. Although attractive, this technique is an indirect approach based on certain assumptions, as described in detail elsewhere (312). The clearance data from tissue-uptake studies (18, 19, 179) are
similar to those derived from measurements using neutral
Ficoll in vivo (26, 219). These values in turn are similar
to those from cIPK studies (122, 123, 176, 301, 302)
(Table 2).
462
TABLE
HARALDSSON, NYSTRÖM, AND DEEN
2.
Sieving coefficients for various proteins in selected studies
Type of Protein
Sieving Coefficient
Charge
References
35.5
35.5
28.4
32.0
32.0
19.6
17.5
54.0
46.0
0.0330
0.0260
0.1490
0.0700
0.1100
0.7700
0.7400
0.0023
0.0056
0
0
0
0
0
0
2
0
2
228
19
179
348
302
179
348
18
176
35.5
35.5
35.5
35.5
27.4
40.5
32.0
32.0
32.0
32.0
17.5
46.0
0.0021
0.0015
0.0006
0.0007
0.0770
0.0036
0.0690
0.0450
0.0170
0.0100
0.5100
0.0011
⫺23
⫺23
⫺23
⫺23
⫺13
⫺24
⫺6
⫺11
⫺11
⫺14
⫺6
⫺19
176
214
19
179
301
301
301
302
224
348
348
176
C. Dextran Polymers
D. Ficoll Polymers
Dextran has been extensively used for transport
studies in the kidney and other organs (250). It is an
␣-D-1,6-glucose-linked polymer with a few short side
branches that behave as a flexible hydrated random coil
rather than as a solid sphere. In the kidneys, dextran was
used 30 years ago in the classical studies of glomerular
size and charge selectivity (17, 36, 45– 48). However,
pore dimensions based on neutral dextran sieving data
were overestimated (219), owing in part to dextran’s
flexibility (218). Thus, compared with a rigid solute
such as Ficoll or a globular protein, the sieving coefficient of dextran may be an order of magnitude too
high (219).
Sulfated dextran raises other concerns. Notably, it
does not appear to be fully inert. Some batches of dextran
sulfate seem to bind to plasma proteins, and certain tubular cells may bind and incorporate the solute. As a
result of either effect, the concentration in urine will be
underestimated, and charge selectivity will be overestimated. Indeed, the glomerular charge density estimated
from studies of dextran sulfate is ⬃120 –170 meq/l (76)
but only 30 –50 meq/l in studies of proteins (176, 302, 346,
347) or Ficoll (123, 134, 301). Other elongated and/or
flexible molecules that behave like dextran have sieving
coefficients larger than expected from their StokesEinstein radii, such as hyaluronan (214) and bikunin
(175, 214). Thus there are good reasons to avoid dextran for studies of transport across biological membranes.
Ficolls are neutral, heavily cross-linked, sucrose-epichlorohydrin copolymers. The molecules behave as rigid
hydrated spheres and can be labeled with isotopes (26) or
fluorescent dyes (213). The normal molecular Stokes-Einstein radius for Ficoll 70 is 10 –70 Å (213). Ficolls are inert
and do not seem to be reabsorbed, secreted, or degraded
in kidneys in vivo. Indeed, neutral Ficoll is close to the
ideal solute for studies of transport across biological and
artificial membranes. Ficolls, including sulfated Ficoll, do
not bind to plasma proteins (28). Whereas sulfated Ficoll
seems to behave according to hydrodynamic laws (28),
there have been reports of anomalous behavior of carboxymethylated Ficoll (10, 107). The mechanism responsible for the high clearance for carboxymethylated Ficoll
remains to be elucidated (10, 107). The sieving coefficients for Ficoll from intact rats in vivo (219) and cIPK in
mice (134) are plotted against their molecular StokesEinstein radius in Figure 9.
Physiol Rev • VOL
E. Proteins Versus Synthetic Polymers
Some investigators suggest that proteins behave differently from Ficoll in their transport (264) across biological and artificial membranes (335). This view is based on
three arguments: 1) that Ficoll and protein glomerular
sieving data differ; 2) that Ficoll may not be uncharged;
and 3) that the Ficoll solute radius may change in response to changes in ionic strength.
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Neutral or slightly cationic proteins
nAlbumin
nAlbumin
Kappa-dimer
nHRP
nHRP
nMyoglobin
cMyoglobin
IgG
LDH-5
Anionic proteins
Albumin
Albumin
Albumin
Albumin
Ovalbumin
Orosomucoid
aHRP
aHRP
aHRP
aHRP
aMyoglobin
LDH-1
SE Radius
GLOMERULAR BARRIER AND PROTEINURIA
463
FIG. 9. Glomerular size selectivity is illustrated by a plot of sieving coefficients for neutral Ficoll versus molecular radius. Biological
data from two studies are presented together
with simulated curves. The experiments were
performed on intact rats (E) with a lognormal
pore distribution model (219) and data from
cIPK in mice (F) together with a fiber model
(134). Both models describe the biological
data with reasonable accuracy, as does a twopore model (212, 213, 301) not shown in the
graph.
during routine serum protein electrophoresis and behaves as a neutral solute even though FITC has a charge
of ⫺1 (cited in text in Ref. 301). Concerning the third
argument, HPLC gel filtration of sharp fractions of
Ficoll at normal, low, and high ionic strengths (as used
in Refs. 301, 302) showed that the Ficoll Stokes-Einstein radius was rather stable. It was not significantly
altered by reducing the ionic strength to 20 mM (⫺0.02 ⫾
0.18 Å, n ⫽ 10) or by increasing it to 520 mM (0.61 ⫾ 0.26
Å, n ⫽ 10, not significant) (B. Haraldsson and J. Nyström,
unpublished observation).
We conclude that Ficoll in many respects behaves as
an ideal solute. Its transport across artificial and biologi-
FIG. 10. Glomerular sieving coefficients for neutral Ficoll, neutral proteins,
and anionic proteins plotted against molecular radius. Two Ficoll sieving curves
are presented based on data from cIPK
studies in rats (301) and mice (134). The
neutral and anionic proteins can be found
in Table 2 together with references. The
anionic proteins differ considerably in net
charge, even for the same protein, depending on the experimental conditions (see,
e.g., anionic HRP in Table 2).
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Concerning the first argument, the glomerular sieving
data depend on the Peclet number (see sect. VI), and it
may be difficult to compare observations obtained under
different experimental conditions. It would be more relevant to determine if neutral spherical proteins differ from
Ficoll under controlled conditions, such as using artificial
membranes or isolated GBM. Indeed, such studies suggest
that proteins and Ficoll are rather similar (28, 71, 136,
141). Furthermore, neutral proteins seem to behave similarly to Ficoll in IPKs if studied under similar and standardized conditions (Fig. 10).
The second argument, that Ficoll may be charged
(335), is easily addressed. FITC-Ficoll moves slowly
464
HARALDSSON, NYSTRÖM, AND DEEN
cal membranes is quite similar to that of uncharged spherical proteins.
V. SOLUTE PROPERTIES THAT AFFECT
TRANSGLOMERULAR PASSAGE
A. Molecular Size
Physiol Rev • VOL
B. Molecular Charge
The first experimental evidence of capillary charge
selectivity was reported in 1969 by Areekul, who used
neutral and sulfated dextran (7). Several now-classical
studies suggested strong charge selectivity in the kidney,
with anionic macromolecules being more retarded than
their neutral counterparts of similar sizes (35, 36, 45– 48,
76). Similar effects were found using anionic, cationic,
and neutral HRP (246). This view prevailed for decades. In
1991, however, the use of dextran sulfate was called into
question (306). This led to a series of critical papers
questioning the very existence of glomerular charge selectivity.
1. Is there any glomerular charge selectivity?
Over the past decade, a large number of studies,
mainly from one laboratory, have suggested that glomer-
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Solutes are retarded in their passage across the glomerular barrier depending on their molecular size. This is
evident from a large number of human and experimental
studies using dextran (47), Ficoll (26, 213, 219), and proteins (179). Glomerular size selectivity can be expressed
in terms of several different theoretical models. According to the two-pore theory, the equivalent small pore
radius is close to 45–50 Å (26, 212, 213), the large pore
radius is close to 80 –100 Å (176, 212, 213, 310), and the
large pore pathway represents 0.003 or less of the total
hydraulic conductance (213). For the lognormal pore distribution model, the mean pore size seems to be close to
35– 45 Å, with a distribution parameter of 1.3–1.1; the
lower pore radius value is then combined with the
broader distribution parameter (26, 213, 219) (Fig. 9).
Finally, glomerular size selectivity has been described by
the charged-fiber model, according to which the fiber
volume is 6 –7% (52, 134) (Fig. 9). All these theoretical
models describe the sieving of solutes more or less to the
same degree. However, they evolved over time, and only
the latter model incorporates both size and charge effects.
As mentioned above, dextrans give falsely high values for the small pore radius, which typically is estimated
to be ⬃50 – 60 Å (37, 242, 263), almost 10 Å greater than
the “true” equivalent pore radius. However, pore size can
be markedly underestimated as well. Reports based on
protein analysis in vivo suggest a small pore radius of 30
Å or less (310). In the latter study, the work of Remuzzi
et al. (242) was cited erroneously as supporting such a
small pore size. The underestimation of the pore size
reflects the limitations of in vivo protein analysis, even in
the presence of tubular inhibition with lysine (310),which
is only partial (314). Indeed, using a tissue-uptake technique, authors from the same group estimated the pore
radius to be 38 Å (179). Naturally, the estimates of pore
radii depend on the biological conditions during the experiment, and occasionally low pore radii are found even
with Ficoll (217).
The studies above indicate that, for neutral solutes,
the sieving coefficient decreases as molecular size increases. This relationship can be described adequately
with at least four different transport theories: the twopore model, the lognormal pore distribution model (with
or without shunt), (neutral) fiber matrix theory, and the
negatively charged fiber model (136, 137). As mentioned
above, the negatively charged fiber model incorporates
both size and charge selectivity (52) and it fits well with
biological data (Fig. 9; see also Fig. 1 in Ref. 134). The
equations are well developed for the partition coefficient,
but the model is less mature in terms of estimations of the
reflection coefficients (see Ref. 351 and below for more
information). The size selectivity seen in Figure 9 for
Ficoll is from three different studies from rats in vivo
(219), rat cIPK (301), and mouse cIPK (134). Furthermore,
Figure 10 shows that neutral proteins behave as neutral
Ficoll (the two curves are from Refs. 134, 301).
In Table 2, the sieving coefficients for various neutral
proteins plotted in Figure 10 are given together with the
reference. There are two values for “neutral” albumin, one
obtained with a tissue-uptake technique (19) and the
other with IPKs (228). The latter value is likely to be an
underestimate, since tubular cell activities were not completely blocked. The charge-modified albumin may form
dimers, spontaneously regain its negative charge, and
bind to tissue structures, as described in detail elsewhere
(108). Indeed, values that are only 20% of those of Reference 228 have been reported for neutral albumin (179).
From the latter study, the value for ␬-dimer is obtained
together with data for neutral myoglobin. Similar data for
neutral horseradish peroxidase (HRP) have been reported
in some studies (246, 302, 348), and 30 –50% lower values
in others (179, 222). Finally, data have been reported for
IgG (18), lactate dehydrogenase-5 (176), and slightly cationic myoglobin (348). The protein sieving data of Lund
et al. (314) are systematically lower than expected. As
described above, the inhibition of tubular uptake by lysine
infusion seems to be incomplete and variable (314), leading to underestimations of the transglomerular passage of
proteins. We conclude that neutral Ficoll and neutral
proteins behave similarly in their passage across the
highly size-selective glomerular barrier.
GLOMERULAR BARRIER AND PROTEINURIA
Physiol Rev • VOL
10-fold for anionic albumin (228). The fact that the tubular
cell inhibitors raised the clearance for the neutral solutes
and for the anionic proteins suggests toxic effects on
glomerular size selectivity. Indeed, toxic effects on glomerular size and charge selectivity were found by Ohlson
et al. (213), which seriously questions the validity of the
original interpretations (222, 224, 228).
The third argument, that albumin is filtered in large
quantities and reabsorbed in intact form, is dealt with
below (see sect. VII). It seems safe to conclude that there
are ample experimental data to refute the hypothesis.
The fourth argument is that proteins such as albumin
are degraded (223, 227), causing protein excretion to be
underestimated (100). The validity of this argument has
been tested (209) and is discussed in detail in section VII.
The fifth argument is that anionic dextrans and Ficolls behave anomalously when filtered across glomerular
walls. The properties of Ficoll and dextran have been
discussed earlier in this review. There also have been
reports of anomalous behavior of dextran sulfate (41),
mainly suggested to be due to increased desulfation in
diabetic animals. For charge-modified anionic Ficoll, the
clearance is reported to be even more anomalous (107).
Thus carboxymethylated Ficoll and neutral Ficoll have
similar clearances for solutes with a Stokes-Einstein radius ⬍50 Å; however, for larger molecules, the sieving
coefficients differed by more than 100-fold (at 75 Å) (107).
Indeed, similar findings of facilitated transport of carboxymethylated Ficoll were reported over a broad molecular size range (15– 80 Å) (10). Both papers conclude that
anionic Ficoll is unsuitable for studies of solute transport
(10, 107). At least two important questions remain unresolved. First, is there active tubular secretion of carboxymethylated Ficoll? Second, is the anomalous behavior specific for carboxymethylated Ficoll or does it hold
also for the Ficoll sulfate used in studies of the GBM (28)?
Both these question are crucial, since Ficoll sulfate actually seems to behave quite like neutral Ficoll apart from
the net charge (28).
In the sixth argument, all data supporting the concept
of a negative charge barrier are dismissed as simply being
flawed (107, 264). Still, the fractional clearance for albumin at 37°C reported by Ohlson et al. (213) is close to the
value of 0.0075 reported by Comper et al. (228) during
control situations in the absence of tubular inhibitors.
However, the evidence for glomerular charge selectivity
does not rest solely on experiments with the cIPK, comparing neutral Ficoll and anionic proteins, as suggested by
some authors (10, 107, 264, 335). In Table 2 and Figure 10,
data from several experiments are presented for neutral
and anionic solutes together with sieving data for neutral
Ficoll. The charge-selective effect is obvious, even though
the observations were obtained under different conditions. We conclude that the evidence against a negative
glomerular charge barrier is weak at best. Let us there-
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ular charge selectivity does not exist at all (264). In arguing against a glomerular charge barrier, Comper and colleagues make several assertions: 1) that all studies using
dextran sulfate are invalid, due to problems with desulfation in the urine (57), binding to plasma proteins, glomerular cell uptake, and binding to GBM (306, 340, 341); 2)
that their own data on the glomerular clearance of neutral
and anionic HRP (aHRP) (222) and of neutral and chargemodified albumin (228) provide evidence against a charge
barrier (264); 3) that albumin is filtered in large quantities
across the glomerular wall and reabsorbed in intact form
(228); 4) that proteins are degraded in the urine and
excreted as small peptides not detected by most routine
techniques (223, 227); 5) that anionic dextrans and Ficolls
behave anomalously when filtered across glomerular
walls (10, 41, 107); and 6) that evidence of charge selectivity is artifactual, due to limitations of the investigative
techniques, mainly cIPK (264).
Are any of these arguments valid? Let us start with
the questions regarding dextran sulfate. Indeed, dextran
sulfate has been shown to bind to fibronectin (210) and to
plasma proteins, depending on the molecular weight of
the dextran molecules (105). In the latter study, after
measurement and correction of such effects, Guasch et al.
(105) still found evidence of a negative charge barrier.
The data on cellular uptake and binding of dextran sulfate
by glomerular cells and its subsequent desulfation were
reviewed recently in detail (73). In brief, the cellular
processing adduced by Comper and co-workers (266) is
rapidly saturable and therefore would not influence
steady-state clearance data, such as those reported by
Guasch et al. (105). We conclude that the use of dextran
sulfate overestimates the charge density of the glomerular
barrier, but the data are qualitatively correct.
Next, let us examine the second argument, that the
data on neutral HRP (nHRP), aHRP, and albumin provide
evidence against a negative charge barrier. The fractional
clearance of nHRP was 2.4 times higher than that of aHRP
under control conditions (224), a ratio identical to that
reported by Sörensson et al. (302) but less than the ratio
of 7 (348) or 9 (246) reported in earlier studies. Adding
150 mM lysine (n ⫽ 4) or 10 mM NH4Cl (n ⫽ 4) to the
perfusate elevated the fractional clearance of nHRP by
70% and of aHRP by 170%, giving a residual charge selectivity ratio of 1.6 (224). Also for albumin, the original data
of Osicka et al. (228) do suggest charge selectivity; in the
IPK perfused at 37°C, the fractional clearance was 0.0075
for native albumin and 0.0330 for neutral albumin (228).
The clearance of native albumin is 5–10 times higher than
values reported for cIPK or in vivo with controlled tubular
reabsorption (Table 1). Similar levels of clearance were
reported for neutral albumin by Bertolatus and Hunsicker
using a tissue-uptake technique (19), but their clearance
ratio was 43. After treatment with 150 mM lysine, fractional clearance increased 3-fold for neutral albumin and
465
466
HARALDSSON, NYSTRÖM, AND DEEN
fore review recent data regarding glomerular charge
selectivity.
2. Current understanding of glomerular
charge selectivity
Physiol Rev • VOL
C. Configuration
The effect of molecular shape on sieving was recognized early by Rennke and Venkatachalam (247), who
noted a sevenfold greater sieving coefficient for dextran
than for nHRP. It was later found that dextran is less
suitable for studies of permeability, since it behaves as a
flexible random coil (218) (see sect. IV). The effect of
solute shape is not, however, limited to dextran (72). The
plasma protein bikunin is an extremely elongated anionic
protein with a diffusion constant (and hence a StokesEinstein radius) close to that of albumin (290). However,
bikunin crosses the glomerular barrier 80 times faster
than albumin, despite similarities in molecular size and
charge (175). Furthermore, the sieving coefficient for the
GAG hyaluronan was three times that of bikunin and 240
times that of albumin despite similarities in size and
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Figure 10 illustrates molecular sieving as a function
of Stokes-Einstein radius for neutral Ficoll and for several
neutral and anionic proteins. The data were from several
studies using a variety of techniques, but with more or
less controlled tubular activity. It is evident that the neutral proteins are close to the Ficoll sieving curve, reflecting similar properties for neutral proteins and Ficoll. This
is in line with data obtained during highly controlled
conditions in artificial membranes or isolated GBM (28,
73). The sieving coefficients are considerably lower for all
anionic proteins than for neutral proteins, providing
strong support for glomerular charge selectivity. Thus,
irrespective of theoretical modeling, the net charge of a
molecule dramatically affects its transport. In Table 2,
sieving data for several neutral and anionic proteins are
shown.
Please note that, before correction, the raw data of
Comper and co-workers (222, 228) actually support the
notion of charge selectivity (Table 2). The authors
reached other conclusions based on the effect of the
tubular inhibitors, which reduced the clearance ratios
(neutral over anionic) for albumin and HRP (222, 228).
There are, however, reasons to believe that the agents
affect glomerular permeability per se (213). Indeed, the
reported dextran sieving values cover a rather narrow
range of molecular radii, and the sieving coefficients are
somewhat larger than reported for intact animals (47).
This is unexpected because perfused kidneys normally
have a lower area available for diffusion, which reduces
the sieving coefficient for small and intermediate-sized
solutes (123). On the other hand, inhibiting tubular cell
activity by reducing the temperature to 4 – 8°C lowers the
urine-to-plasma concentration ratio for Cr-EDTA from
20 –100 to 1.1–1.2 without significantly altering glomerular
charge and size selectivity (213).
The concept of glomerular charge selectivity has
been experimentally tested by altering the ionic strength
and pH of the perfusate. Sörensson et al. (302) found that
reducing the ionic strength from 152 to 34 mM lowered
the sieving coefficient (⌰) for aHRP by 40%; ⌰ for nHRP
was reduced by 15% (302). Similar effects were found for
other anionic proteins, including orosomucoid and albumin (301). Moreover, reducing the pH of the perfusate
increased the sieving coefficient significantly, in relation
to the diminished net negative charge of the protein, and
pretreatment with protamine had a similar effect (53),
consistent with previous observations (149, 334). Protamine has prominent effects on the intact glomerular barrier but not on isolated GBM (64), suggesting that glomer-
ular cells have a key role in charge selectivity (65). Indeed, our current understanding of the GBM indicates
that it has little charge selectivity (28).
Attempts have been made to alter the glomerular
charge barrier by using enzymes to digest GAGs (4, 101,
145, 256). Heparan sulfate proteoglycans are major
components of the GBM (255); in particular, perlecan
(197, 258) and agrin (161, 255) have been reported to
affect glomerular permeability. The endothelial cell surface coat (339) also contains GAGs. Heparanase, chondroitinase, and hyaluronidase may increase the sieving
coefficient for albumin by reducing the charge density
(134) and reducing the thickness of the endothelial cell
surface coat (135). In a recent report, mice with adriamycin-induced nephrosis expressed heparinase in their
glomeruli, which the control mice did not (161). We
conclude that the data obtained with various GAGdegrading enzymes support the notion of glomerular
charge selectivity.
Let us use one of the theoretical models to predict
the glomerular sieving coefficient for solutes of different
molecular size and charge. As mentioned below, all current models have limitations. In a recent paper, a heterogeneous charged fiber model was used to analyze the
beneficial effects of orosomucoid on rats with puromycininduced nephrotic syndrome (122). In Figure 11, the results
of such an analysis are presented as a plot of sieving
coefficient versus Stokes-Einstein radius. Curves are
shown for neutral solutes, slightly positively or negatively
charged solutes, and solutes with strong negative charge
densities similar to that of serum albumin. Naturally,
these curves represent an oversimplification of the glomerular barrier, due to the limitations of current transport
equations for a charged fibrous network. There is, however, a reasonable fit between the theoretical curves and
the biological data presented in Figure 9.
GLOMERULAR BARRIER AND PROTEINURIA
467
charge (214). In the latter study, the frictional ratios for
anionic solutes were related to their “apparent neutral
molecular radii” (214). The frictional ratio is 1.0 for a
sphere (like Ficoll) and increases with the degree of
elongation (1.3 for albumin, 1.8 for bikunin, and 2.3 for
hyaluronan). As calculated by Ohlson et al. (214), the apparent neutral molecular radii for the four solutes are,
respectively, 35.5, 55.1, 33.0, and 23.5 Å. We conclude
that molecular configuration substantially affects solute transport across the glomerular barrier if the solutes are elongated or random coils (71, 73, 214, 247).
However, smaller deviations from the ideal sphere
shape do not seem to affect transport to any significant
degree (28).
D. Relative Importance of Size
and Charge Selectivity
Size selectivity is most significant for neutral solutes in the steep portion of the sieving curve (i.e.,
Stokes-Einstein radius of 30 –50 Å; see solid curve in
Fig. 11). In that part of the curve, the sieving coefficient
falls by one order of magnitude for every 10-Å increase
in molecular radius. For neutral solutes up to the SEradius of 30 Å, the sieving coefficient only falls from 1.0 to
0.1. Also, increasing the molecular radius from 50 to 70 Å
reduces the sieving coefficient by a factor of five (Fig. 11).
Physiol Rev • VOL
The molecular radius interval between 30 and 40 Å is also
where charge selectivity is greatest. Thus a solute charge
density similar to that of albumin reduces the sieving coefficient for those solutes to 5–10% of that for a neutral solute.
The charge effect will depend on the charge density
of the solute and will occur over the entire molecular size
range. The molecular configuration may, however, overrule the effects of size and charge, and elongated solutes
of the size and charge of albumin may have sieving coefficients between 0.1 and 1.0 (214). Therefore, to predict
the transglomerular passage of a solute, one must know
its molecular size (diffusion constant, i.e., Stokes-Einstein
radius), its net charge (or charge density), and its configurations, as revealed, for example, by its frictional ratio
(214). For solutes that are not random coils or markedly
elongated, shape does not significantly affect their transport (28). The sieving coefficients presented in Figure 11
are therefore reasonably accurate for most solutes and
predict the effects of molecular size and charge.
VI. PHYSICAL PRINCIPLES
AND THEORETICAL MODELS
In reviewing the physical principles that govern the
interpretation of data on the glomerular permeability to
proteins and other macromolecules, our emphasis is on
factors that influence the sieving coefficient of a given
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FIG. 11. The fractional clearance or sieving coefficient of a solute plotted against its Stokes-Einstein radius. From the top, the curves represent
slightly positive solutes (with charge densities of 0.002 Coul/m2), neutral solutes, negatively charged solutes (with charge densities of ⫺0.006
Coul/m2), and highly negative solute charge similar to that of albumin (⫺0.022 Coul/m2). The clearance ratio of neutral over strongly negative solutes
was 20:1 or higher in the molecular radius range of 30 – 40 Å. The curves are based on a heterogeneous charged fiber model (122) based on the work
of Johnson and Deen (136), with a fiber radius of 4.5 Å, a relative fiber volume of 0.055, and a fiber charge density of ⫺0.2 Coul/m2. The charged
fiber gel consists of two parallel regions that differ in fiber density. One of the regions has only 10% of the fiber density of the rest of the gel. However,
this “nonselective” region constitutes only a minute fraction of the total gel and contributes to ⬃0.1% of the total hydraulic conductance of the
charged fiber gel. In other models, such high-permeable regions have been named “shunt pathways” or “large pores.” For more detail, please consult
http://kidney.med.gu.se/9.
468
HARALDSSON, NYSTRÖM, AND DEEN
Physiol Rev • VOL
A. Hindered Transport and Membrane
Size Selectivity
The defining feature of an ultrafiltration membrane is
that it can sieve macromolecules on the basis of molecular size. Solutes rejected in ultrafiltration have molecular
masses of ⬃1–1,000 kDa. However, the linear dimensions
of a molecule are more directly relevant to its ability to
pass through a small pore. The most common such measure of molecular size is the Stokes-Einstein radius (rs),
which is related to the diffusivity in bulk solution (D⬁) as
rs ⫽
kBT
6␲␮D⬁
(1)
where kB is Boltzmann’s constant, T is temperature, and ␮
is the viscosity of the solvent. This is the radius of a rigid
sphere that would have the given diffusivity. For water at
37°C, a molecule with rs ⫽ 1.0 nm has a diffusivity of
D⬁ ⫽ 3.3 ⫻ 10⫺10 m2/s. For the glomerulus, the range of
sizes of most interest for sieving is 2 ⱕ rs ⱕ 6 nm (as
discussed in sect. IV), with rs ⫽ 3.6 nm for serum albumin.
The local solute flux (N) in a porous or fibrous membrane can be written as
N⫽⫺KdD⬁
dC
⫹ KcvC
dx
(2)
where C is concentration, x is position, and v is fluid
velocity. This is simply Fick’s first law with a term added
for convection (bulk flow) and allowances made for hindrances to solute movement caused by the membrane.
The coefficients Kd and Kc are hindrance factors that
describe the effects of the membrane on solute diffusion
and convection, respectively. Thus the apparent diffusivity within the membrane is KdD⬁, and the solute velocity
due to bulk flow is Kcv. When Equation 2 is applied to
membranes with discrete pores, N, v, and C are interpreted as averages over the pore cross-section; for materials of more complex structure (e.g., arrays of fibers with
fluid-filled interstices), N and v are based on total surface
area and C is usually based on total volume (fluid plus
solid). If C is in molar units (mol/m3), then N has the units
mol䡠m⫺2 䡠s⫺1. Alternatively, mass units can be used for C
and N.
The basis for membrane size-selectivity is described
by theories that predict the values of Kd and Kc from rs
and the quantities that define the membrane nanostructure. A thorough discussion of such theories is beyond the
scope of this review, but an introduction to the main ideas
will make the subsequent material on sieving more understandable. In general, theories for hindered transport are
based on a combination of hydrodynamic and steric considerations. A macromolecule in solution behaves in part
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molecule. For ultrafiltration processes in general, the
sieving coefficient is the concentration in the filtrate divided by that in the retentate; for the glomerulus, it is
usually defined as the concentration in Bowman’s space
divided by that in arterial plasma. The sieving coefficient
of a solute is determined by multiple factors: the intrinsic
selectivity of the membrane based on solute size, charge,
and shape; the filtration rate of water; solute concentrations (if the retentate is not dilute); and, for a multilayer
barrier such as the glomerular capillary wall, the arrangement of the layers.
In general, one must distinguish between the concentration adjacent to the upstream surface of an ultrafiltration membrane and the average concentration in the retentate. In any sieving process, the concentration of rejected molecules tends to increase next to the upstream
membrane surface, a phenomenon called concentration
polarization. This increase drives the diffusion of rejected
solutes back into the bulk retentate. In industrial ultrafiltration, concentration polarization often leads to a
marked reduction in water filtration rates, due to the
osmotic effects of the retained solute and/or membrane
fouling (349). However, an analysis based on the conditions in glomerular capillaries indicates that the effects
there are negligible, because of the small capillary dimensions, moderate filtrate velocities, and mixing effects of
red blood cells (75). Accordingly, concentration polarization in the capillary lumen will not be considered here,
although an analogous phenomenon within a multilayer
membrane will be discussed.
Another key distinction involving retentate concentrations concerns their variations along the length of a
permeable tube, such as a glomerular capillary. As plasma
moves from the afferent to the efferent end, the concentration of a selectively retained solute must increase because of the proportionately greater removal of water; the
greater the filtration fraction of water and the smaller the
sieving coefficient, the larger the increase. Such axial
variations in concentration have been incorporated into
glomerular filtration models since the early 1970s (48, 72,
74, 78), and they have been important for understanding
the effects of plasma flow rate and other hemodynamic
variables on single-nephron GFR and sieving coefficients.
Thus a complete model of the glomerular filtration process must include mass balance equations that describe
concentration variations along a capillary. Nonetheless,
we have chosen not to discuss axial concentration
variations in detail and to focus instead on the determinants of the local sieving coefficient (i.e., the filtrateto-plasma concentration ratio at a given position along
a capillary). Those local factors are most crucial for
understanding barrier selectivity and are also most controversial.
GLOMERULAR BARRIER AND PROTEINURIA
other words, diffusion is severely hindered well before
the solute becomes a tight fit in the pore.
The pattern for convection is more complicated, in
that Kc first increases and then decreases as solute size
increases. The initial increase is due to the fact that the
finite size of the particle excludes its center from the layer
of fluid next to the pore wall, and therefore prevents it
from experiencing the most slowly moving fluid. Eventually, though, the hydrodynamic retardation due to the
pore wall dominates, and Kc declines. Note that convection tends to be less hindered than diffusion, such that
Kc/Kd ⱖ 1, with that ratio increasing with relative solute
size. A large value of Kc/Kd tends to aid sieving, as will be
seen. The functions H(␭), W(␭), and ⌽(␭), which are also
shown in Figure 12, will be discussed shortly.
Although the results in Figure 12 are for cylindrical
pores, the qualitative behavior of Kd and Kc with increasing molecular size is similar in slit pores (flat-wall channels) and even in fibrous media, where the transport paths
are more tortuous. Only one aspect of those results is
unique to cylindrical pores, the convective hindrance of a
solute that nearly fills the pore (␭ 3 1). Whereas Kc 3 1
in that limit for cylindrical pores, for slits and other pore
shapes Kc 3 0 for large solutes (67). The peculiar aspect
of a tightly fitting sphere in a cylindrical pore is that it
completely occludes the channel. With fluid unable to
bypass it, the sphere acts as a piston and moves at the
mean fluid velocity, rather than becoming immobile.
Hence, Kc 3 1 instead of 0.
There have been many applications of pore theory to
the glomerulus, as reviewed previously (181). Fiber-matrix theory is less completely developed. For hindered
transport through fibrous media, noteworthy recent studies include a discussion of diffusion in randomly oriented
fiber matrices (235) and a calculation of the osmotic
reflection coefficient for flow along the axes of parallel
fibers (351).
B. Sieving in Homogeneous Membranes
FIG. 12. Hindrance factors and partition coefficients for spherical
molecules in cylindrical pores plotted against relative solute size
(␭ ⫽rs/rp, i.e., solute over pore radius), illustrating membrane size
selectivity. The local hindrance factors of the membrane for diffusion
(Kd) and for convection (Kc) are plotted together with the partition
coefficient (⌽). Also shown are the overall hindrance factors for diffusion (H⫽ ⌽ 䡠 Kd) and for convection (W⫽ ⌽ 䡠 Kc). The expressions used
for Kd, Kc, H, W, and ⌽ are all given in Ref. 67.
Physiol Rev • VOL
The experimental measure of membrane selectivity
in ultrafiltration is the sieving coefficient (⌰). This section
begins by relating ⌰ to the hindrance factors just discussed, and then illustrates how sieving is influenced also
by the operating conditions of the ultrafiltration process.
This initial discussion is for a homogeneous or singlelayer membrane; multilayer barriers are considered later.
Suppose that a membrane consists of a single layer of
porous or fibrous material, extending from x ⫽ 0 to x ⫽ L.
The concentrations in the retentate and filtrate are denoted as C0 and CL, respectively. If the membrane thickness greatly exceeds the pore radius or interfiber spacing,
then near-equilibrium conditions will exist at the two
boundaries, such that
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as a hydrodynamic particle, and the presence of fixed
objects (e.g., pore walls or membrane fibers) tends to
increase the hydrodynamic drag on a particle and thereby
reduce its mobility (lowering Kd). Interactions between
the particle and the wall (or fiber) are mediated by viscous stresses and pressure variations in the fluid. Such
hydrodynamic interactions also cause the velocity of a
freely suspended particle to deviate from the mean fluid
velocity (affecting Kc). The finite size of the particle limits
the positions that its center can occupy, bringing steric
considerations into the calculation of the hindrance factors.
The theory of hindered transport is most completely
developed for the special case of spherical molecules in
long cylindrical or slit-shaped pores; diffusion and convection in fibrous media are less completely understood.
The theory for uniform pores, originating with the work of
Pappenheimer, Renkin, and co-workers more than 50
years ago (230, 245), has been reviewed elsewhere (67,
69). Included in those reviews are examples of results
from experiments using membrane pores or microfabricated channels of carefully controlled size, which generally confirm the theoretical predictions. The predictions
for cylindrical pores are shown in Figure 12, where the
hindrance factors are plotted as a function of relative
solute size (␭ ⫽ rs/rp, where rp is pore radius). Reflecting
the progressively greater hindrances to diffusion as relative solute size increases, Kd declines monotonically from
unity to zero. The intrapore diffusivity reaches half of the
free-solution value (i.e., Kd ⫽ 0.50) when ␭ ⫽ 0.24. In
469
470
HARALDSSON, NYSTRÖM, AND DEEN
C共0兲
C共L)
⫽⌽⫽
C0
CL
(3)
⌰⫽
⌽K c
W
⫺Pe ⫽
⫺Pe
1 ⫺ e ⫹ ⌽Kce
1 ⫺ e ⫹ We⫺Pe
(4)
⌽KcvL WvL
⫽
⌽KdD⬁ HD⬁
(5)
⫺Pe
Pe ⫽
where Pe, the Peclet number, is a dimensionless parameter that measures the importance of convection relative
to diffusion. From the first forms of Equations 4 and 5, it
is evident that the effects of membrane selectivity on
sieving are contained fully in the products ⌽Kc and ⌽Kd.
In other words, sieving coefficients depend only on those
two products, and not on the three individual quantities
involved. Accordingly, when analyzing sieving, it is advantageous to switch from the local hindrance factors Kc and
Kd to overall hindrance factors defined as W ⫽ ⌽Kc and
H ⫽ ⌽Kd. That substitution yields the second forms of
Equations 4 and 5. An expression equivalent to Equation
4 was presented originally by Spiegler and Kedem (293)
some 40 years ago.
The behavior of W and H for spherical solutes in
cylindrical pores is shown in Figure 12. Because ⌽ ⬍ 1,
each overall hindrance factor is smaller than the corresponding local one; the differences are amplified as the
relative molecular size increases, because ⌽ becomes
smaller. When measured in terms of H, the rate of diffusion in a cylindrical pore is half that in free solution when
␭ ⫽ 0.14. In terms of W, convective transport is halved
when ␭ ⫽ 0.39. Thus solute transport in pores can be
hindered severely even when rs does not closely approach rp.
Equation 4 shows that the sieving coefficient depends on just two dimensionless parameters, W and Pe.
Physiol Rev • VOL
⌰⫽
W
W⫹Pe(1⫺W)
(Pe ⬍⬍ 1)
(6)
This result, which is accurate to within a few percent for
Pe ⬍ 0.1, indicates that ultrafiltration is effective even for
FIG. 13. Sieving coefficient (⌰) as a function of membrane Peclet
number (Pe) for four values of the convective hindrance factor (W).
These predictions for a homogeneous membrane are based on Equation 4.
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Thus ⌽, termed the partition coefficient, equals the equilibrium ratio of internal (membrane) to external concentration. If both external solutions are dilute and the solute
molecule is uncharged, the partition coefficient at both
sides is the same, as assumed in Equation 3. For spheres
in cylindrical pores, ⌽ ⫽ (1 ⫺ ␭)2, which is plotted in
Figure 12. The theoretical effects of solute charge, shape,
and concentration on partitioning are discussed in section VIC.
For ultrafiltration into a dead-end chamber (such as
Bowman’s space), the filtrate concentration is determined
by the ratio of the solute flux to the volume flux. That is,
CL ⫽ N/v. With the boundary conditions now established,
Equation 2 is integrated over the membrane thickness,
using the requirement from conservation of mass that N
and v be independent of x. The result for the sieving
coefficient (⌰ ⫽ CL/C0) is
Of course, the diffusional hindrance factor (H),filtrate
velocity (v), membrane thickness (L), and solute diffusivity in bulk solution (D⬁) all influence Pe, but none of those
quantities affects sieving independently. The dependence
of ⌰ on Pe for several values of W is illustrated in
Figure 13. In each case, ⌰ decreases from 1 to W as Pe
increases from 0 to ⬁; Pe ⫽ 5 is large enough to be
practically infinite. Recalling that Pe ⬀ vL, the dependence of ⌰ on Pe indicates that the selectivity of an
ultrafiltration membrane will be fully evident only if the
product of filtrate velocity and membrane thickness is
large enough to make convection dominant. Helping to
amplify the effects of convection is the fact that, typically,
W/H ⱖ 1 for porous or fibrous membranes, as discussed in
connection with Figure 12. When Pe is not large, the time
scale for diffusion is comparable to or less than the transit
time through the membrane, thereby allowing at least
partial equilibration of the filtrate with the retentate. If Pe
3 0, then ⌰ 3 1 and the intrinsic selectivity of the
membrane (as embodied in W) is completely masked.
The foregoing suggests that ultrafiltration will be an
ineffective separation process unless Pe for each macromolecule of interest exceeds some minimum value. As
indicated in Figure 13, the minimum Pe needed to achieve
a desired sieving coefficient depends on W. For example,
the value of Pe required to make ⌰ as small as 0.3 clearly
increases with increasing W. Expanding the exponentials
in Equation 4 for small Pe shows that
GLOMERULAR BARRIER AND PROTEINURIA
FIG. 14. Sieving in a homogeneous membrane for three hypothetical conditions. The baseline case corresponds to a pore radius (rp) of
5.0 nm. The other curves are those predicted for a selective increase
in pore radius to 6.0 nm or a selective decrease in the filtrate velocity to one-half of the baseline level. See text for other parameter
values.
Physiol Rev • VOL
monitored to avoid mistaking a flow-induced shift in a
sieving curve for one caused by a change in the membrane properties.
The foregoing discussion of sieving in homogeneous
membranes provides the background needed for a critical
examination of a model for albumin sieving proposed
recently by Smithies (292). The fundamental assumptions
in that model are as follows: 1) barrier selectivity resides
entirely in the GBM, 2) albumin transport across the GBM
is mainly by diffusion, and 3) diffusion there proceeds
independently of convection (water filtration). An implication of assumptions 2 and 3 is that variations in GFR
have almost no effect on the filtered load of albumin; if
GFR is increased or decreased, the albumin concentration
in Bowman’s space simply would be diluted to a greater
or lesser degree, with the concentration-GFR product
remaining nearly the same. If assumptions 1 and 2 are
correct, then the glomerular barrier behaves as a homogeneous membrane and Pe for albumin is small, making
Equation 6 applicable. For ⌰ to be small, as is true for
albumin (Table 1), Equation 6 requires that W ⬍⬍ Pe, in
which case it reduces to ⌰⬃W/Pe. Because Pe is proportional to v and therefore (if the number of nephrons
remains constant) proportional to GFR, this is consistent
with the idea that filtered load will not vary with GFR if
the barrier properties (i.e., W) are not affected. However,
the data of Rippe et al. (251) for Ficoll sieving in rats in
which GFR was manipulated show a dependence of ⌰ on
GFR that is far too weak to keep filtered loads constant.
Similar findings were reported for the sieving of albumin
and other proteins in rat kidneys (179). Thus the available
data do not support the idea that the filtered load of
albumin or similar-sized macromolecules is nearly independent of GFR.
Given the ample evidence that both endothelium and
epithelium participate in glomerular barrier selectivity
(see sects. VI and VII), assumption 1 in the Smithies model
is its most obvious oversimplification. However, assumption 3 is also incorrect in general, as may be seen by
returning to Equation 2. As noted earlier, steady-state
conservation of mass requires that N and v be independent of x. Because C decreases with increasing x, the
absolute value of the concentration gradient (⫺dC/dx)
must increase with x, leading to a C(x) profile that is
concave downward. The amount of curvature in C(x) is
determined by Pe. Thus flow influences diffusion by altering the shape of the concentration profile. Moreover, the
contributions of convection and diffusion to N each vary
with x so that, in general, it is meaningless to assert that
a specific fraction of the transmembrane flux is due to one
mechanism or the other. The exception is Pe ⫽ 0, where
there is no convection at all, but then there is also no
sieving!
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small Pe, as long as Pe greatly exceeds W. More precisely,
the requirement is that Pe ⬎⬎ W/(1 ⫺ W). In other words,
thin membranes or membranes subjected to low filtration
rates must be especially selective (i.e., have very small
values of W) to exhibit sieving.
The combined effects of pore radius and filtration
rate on sieving curves (plots of ⌰ vs. rs) are illustrated in
Figure 14. The baseline condition is a hypothetical situation in which rp ⫽ 5.0 nm and the product vL is such that
Pe ⫽ 1 for rs ⫽ 3.0 nm. The other predictions correspond
to a selective increase in pore radius (to 6.0 nm) or a
selective decrease in filtrate velocity (to one-half of the
baseline v). In each case, ⌰ declines from 1 to 0 as
molecular size increases. Because Pe is not uniformly
large, none of the curves in Figure 14 has quite the same
shape as the W curve in Figure 12. A selective increase in
pore radius shifts the entire sieving curve to the right, as
one might expect. The effects of a selective decrease in
filtration rate are more complicated, in that the sieving
curve of small molecules resembles that for an increased
pore radius, but the curve for large molecules is unchanged.
To understand these effects of filtration rate, one
must realize that each curve corresponds to a wide
range of Pe values, since Pe increases with molecular
size due both to decreases in D⬁ and increases in W/H.
If the initial value of Pe is moderate, a reduction in it
will increase the sieving coefficient, as shown in Figure
13. In Figure 14, the absence of any noticeable change
in ⌰ for the larger molecules, when v was reduced,
indicates that their Pe values remained large enough to
be in the plateau region of Figure 13. A practical conclusion from Figure 14 is that filtrate velocities must be
471
472
HARALDSSON, NYSTRÖM, AND DEEN
C. Effects of Molecular Charge, Shape, and
Concentration on Membrane Selectivity
2. Shape effects
1. Charge effects
The simplest way to describe the effects of charge on
⌽ is to model the solute of interest as a point charge with
a specified valence and to assume Donnan equilibria between the membrane and adjacent solutions. In such
calculations, the membrane is treated as a solution that
contains a certain concentration of fixed charges, in addition to mobile ions. The fixed charges may arise from
covalently attached acidic or basic groups or adsorbed
ions. Structural information such as pore radius, fiber
concentration, and fiber radius is neglected in Donnan
models. If an external solution contains nearly impermeant ions, such as large proteins, then they are viewed
as fixed charges in that solution. An excess of fixed negative charges in the membrane creates a negative intramembrane potential, which reduces ⌽ for anionic macromolecules and increases it for cationic ones. There have
been several applications of Donnan models to the glomerulus (53, 76, 301, 302, 313, 327, 348).
A more realistic approach is to recognize that membrane fixed charges will be localized on pore walls or on
the surface of membrane fibers, rather than distributed
uniformly over the membrane volume. A surface charge
creates a region of opposing charge in the adjacent solution called the diffuse double layer, and it is only within
that region that the electrical potential is perturbed by the
surface charge. The double-layer thickness is on the order
of the Debye length, which for physiological saline solutions is ⬃0.8 nm. Thus, within a porous or fibrous membrane that freely passes molecules up to 2 nm in radius,
and allows some passage of macromolecules as large as 6
nm, the electrical potential is not spatially uniform, as
assumed in the Donnan approach. Moreover, the charge
on a multivalent macromolecular solute will be distribPhysiol Rev • VOL
Equilibrium partition coefficients in uncharged systems have been computed for many combinations of solute shape and membrane structure. These include various
shapes of rigid particles in pores of uniform cross-section
(95, 155), spherical (211) or ellipsoidal (166) solutes in
random fiber matrices, and freely jointed chains in pores
(80) or random fiber matrices (344). The fiber matrices
considered include randomly positioned and oriented fibers of a single radius (211, 344) or mixtures with multiple
fiber radii (166). It is reasonable to approximate globular
proteins and Ficoll as rigid particles, whereas the freely
jointed chain may be a better model for dextran or other
linear polymers. Such molecules do not have a fixed
shape in solution and behave more as random coils.
Owing to the gel-like nature of the endothelial glycocalyx and GBM and the physiological importance of globular proteins, the partitioning of rigid particles in fiber
matrices is especially pertinent. In such systems, the effects of molecular shape seem to be of little importance,
unless the solutes are markedly elongated or they are
random coils (see sect. IV). For example, the hydrodynamic properties of albumin resemble those of a prolate
(elongated) ellipsoid with a major semiaxis of 7.0 nm and
a minor semiaxis of 2.1 nm (167). Using the excluded
volume theory (166), we can compare the partition
coefficient of such a molecule with that of a sphere of
the same Stokes-Einstein radius, 3.6 nm. In a fiber
matrix with a fiber radius of 1.0 nm, ⌽ for the ellipsoid
is predicted to be within 1% of that for the sphere of
equal diffusivity, for all fiber volume fractions from 0
and 0.2. This is consistent with partitioning (167) and
sieving studies (141) in agarose gels, in which the results for albumin and certain other globular proteins
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The effects of molecular charge and shape are incompletely understood, even for cylindrical or slit pores. A
major impediment to theoretical progress has been the
difficulty in solving the sets of differential equations that
describe the hydrodynamic forces acting on charged
and/or aspherical particles in small channels. Even more
formidable computational problems arise when the concentration of one or more macromolecules is large
enough to make solute-solute interactions important.
Given this incomplete state of knowledge, it has been
common to assume that Kc and Kd are affected mainly by
the Stokes-Einstein radius of the molecule and to focus on
how the other factors might affect the equilibrium partition coefficient ⌽. As already seen (Eq. 4), partitioning
influences sieving via the overall convective hindrance
factor, W ⫽ ⌽Kc; it does not affect Pe.
uted in some manner over its surface or volume, rather
than concentrated at a point. By computing positiondependent electrostatic potentials using the differential
equations that describe diffuse double layers, ⌽ has
been evaluated for solid or porous spheres in cylindrical pores (291) and for solid spheres in dilute fiber
matrices (136, 137).
An advantage of these more detailed calculations is
that they include the interplay between the effects of
molecular size and molecular charge. As the results show,
the effects of size and charge on ⌽ are not entirely separable. A limitation, in common with attempts to predict
size selectivity, is that the actual structure of a membrane
may be much more complex than the available idealizations (e.g., uniform cylindrical pores or randomly oriented
fibers). The fiber model has been suggested as an alternative to the Donnan model in describing glomerular
charge selectivity (52, 122, 134).
GLOMERULAR BARRIER AND PROTEINURIA
473
were nearly identical to those for Ficolls of equivalent
Stokes-Einstein radius.
been calculated for cylindrical pores (194), but not for
fiber matrices.
3. Concentration effects
D. Sieving in Multilayer Membranes
Physiol Rev • VOL
Because the glomerular filtration barrier consists of
three layers in series (fenestrated endothelium with its
glycocalyx, GBM, and epithelial filtration slits with their
slit diaphragms), it is important to understand how sieving in a multilayer membrane differs from that in a homogeneous one. We begin quite generally. Suppose that a
membrane consists of n layers arranged in series, numbered 1, 2, . . . n from the upstream to the downstream
side. The convective hindrance factor and Peclet number
for layer i are denoted as Wi and Pei, respectively. The
derivation of Equation 4 can be generalized to give an
expression for the sieving coefficient in layer i.Denoted as
⌰i, that sieving coefficient equals the downstream-to-upstream concentration ratio within that particular layer.
The result is (68)
⌰i ⫽
Wi
⌰i⫹1⌰i⫹2 . . . ⌰n共1 ⫺ e⫺Pei兲 ⫹ Wie⫺Pei
(7)
where ⌰n⫹1 ⫽ 1 by definition. For the most downstream
layer (i ⫽ n), Equation 7 reduces to Equation 4, indicating that ⌰n is a function only of Wn and Pen. In other
words, the sieving coefficient in the last layer is independent of the selectivity or flow conditions in the preceding
layers. However, as indicated by the product of sieving
coefficients in the denominator of Equation 7, all of the
other sieving coefficients are coupled to some extent,
each being affected by events downstream. This coupling
is most complete for the first layer (i ⫽ 1), in that ⌰1 is
influenced by all of the other sieving coefficients. The fact
that the sieving coefficient in a given layer influences all of
the upstream ones, but none of the downstream ones,
indicates that the ordering of the layers is important. The
effects of ordering in a two-layer membrane will be examined shortly.
The interdependence of the layer sieving coefficients
stems from two physical constraints. One is that the water
and solute fluxes across each layer must be the same.
Once a steady state has been reached, there can be no
time-dependent accumulation or depletion of water or
solute at any boundary between layers. The other constraint is that the concentration at the downstream side of
layer i is linked to that at the upstream side of layer i ⫹ 1.
The solute concentration profile in each layer must adjust
accordingly, and Equation 7 is a consequence of those
simultaneous adjustments.
For a multilayer barrier and dilute retentate, the overall sieving coefficient is calculated as
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In most applications of hindered transport theory to
ultrafiltration, whether in the glomerulus or other kinds of
barriers, each solute is assumed to cross the membrane
independently. However, repulsive forces among like
molecules, due to steric and electrostatic interactions,
cause ⌽ to increase with increasing concentration (5, 97).
For uncharged spheres, this effect is noticeable even
when the solute occupies only a small percentage of the
solution volume. Solute-solute interactions exist also
among unlike solutes. For example, increasing the concentration of an abundant solute is predicted to increase
both ⌽ of that solute and ⌽ of a second molecule present
at tracer concentrations (166). This prediction has been
confirmed quantitatively by partitioning measurements
for BSA (abundant solute) and Ficoll (tracer) in agarose
gels (167), and also helps to explain the effects of BSA on
Ficoll sieving in filters constructed from isolated GBM
(168). When ⌽ is concentration dependent, the partition
coefficient at the upstream side of an ultrafiltration membrane must be distinguished from that at the downstream
side (168); in Equation 4, the upstream value of ⌽ (or W)
must be used in the numerator and the downstream value
in the denominator.
Although rarely included in ultrafiltration models,
concentration effects may be physiologically significant.
For example, using a two-fiber model for GBM and assuming a solution protein concentration of 6 g/dl, the
partition coefficients of spherical tracers with 2.0 ⱕ rs ⱕ
5.0 nm were predicted to be 1.4 –3.9 times greater than in
the absence of protein, the percentage increases being
greater for the larger molecules (168). The effects of pure
BSA, or a 1:1 mixture of BSA and IgG, were predicted to
be nearly identical. Under these conditions, the partition
coefficient for BSA itself was predicted to increase 2.1fold, relative to a very dilute BSA solution. For the glomerulus in vivo, the effects of protein concentration on
plasma protein or exogenous tracer sieving will depend
on the extent to which large proteins such as albumin
penetrate the capillary wall. If high protein levels exist
only in the capillary lumen, the effect will be limited to the
partition coefficient between the lumen and endothelial
glycocalyx (168). This issue is discussed further after
Equation 8.
The aforementioned estimates of the concentration
dependence of ⌽ are for uncharged solutes and membranes. Electrostatic interactions are absent in agarose
gels (167) and minor in isolated GBM (28) but may be
quite important in the GAG-rich glycocalyx. The combined effects of solute charge and concentration have
474
HARALDSSON, NYSTRÖM, AND DEEN
⌰ ⫽ ⌰ 1⌰ 2 . . . ⌰ n
(8)
⌰1 ⫽
W1
⌰2共1 ⫺ e 兲 ⫹ W1e⫺Pe1
(9)
⫺Pe1
The influence of the downstream layer on sieving in the
upstream one is most striking for Pe1 ⬎⬎ 1, in which case
Equation 9 reduces to ⌰1 ⫽ W1/⌰2. If the upstream layer
is less selective than the downstream one, in the sense
that W1 ⬎ ⌰2, then ⌰1 ⬎ 1. This indicates that an internal
concentration polarization is created, such that the solute
concentration in layer 1 increases in the direction of
flow. Concentration polarization in the upstream layer is
avoided if W1 ⱕ ⌰2. However, for large Pe1, the overall
sieving coefficient for the two layers is always ⌰ ⫽ ⌰1⌰2 ⫽
W1. It is remarkable that, under these circumstances, ⌰ is
independent of the properties or flow conditions in the
downstream layer. For Pe1 ⬍⬍ 1, Equation 9 reduces to
⌰1 ⫽
W1
W1 ⫹ Pe1(⌰2 ⫺ W1)
(10)
which is the two-layer analog of Equation 6. In this case,
both ⌰1 and ⌰ depend on the properties of each membrane layer and are also affected (through Pe1) by the
filtration rate of water.
Suppose that layer 1 is the GBM and layer 2 is the
epithelium. For a molecule the size of albumin, it has been
estimated that Pe1 ⫽ 0.14 (73), based on the sieving and
diffusion properties of isolated rat GBM and the dimensions and flow rates for rats in vivo. With the use of Pe1 ⫽
0.14, together with the finding in isolated rat GBM that
W1 ⫽ 0.08 for a Ficoll molecule the size of albumin (73),
either Equation 7 or Equation 10 indicates that ⌰1 ⬃1 for
any small value of ⌰2. Thus it was concluded that if the
Physiol Rev • VOL
1. Case 1
If Pe1 ⬎⬎ 1, then it follows from Equations 7 and 8
that ⌰1 ⫽ W1/⌰2 and
⌰ ⫽ W1
(11)
As already discussed, the overall sieving coefficient is
independent of the filtration velocity in this case and is
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Of course, ⌰ measures how well the barrier functions as
a whole. If the retentate is not dilute, one or more additional, multiplicative factors will appear in Equation 8
(168), depending on the extent to which abundant solutes
penetrate the barrier. For example, if the major plasma
proteins are excluded from the endothelial glycocalyx,
such that high protein concentrations occur only in the
lumen, the additional factor equals the relative increase in
⌽ for the glycocalyx that is due to the plasma proteins
(168). In other words, if the luminal protein concentration
is sufficient to double ⌽ in the glycocalyx, Equation 8 will
contain an additional factor of 2. For simplicity, we will
proceed as if all solutions are dilute.
Examining the behavior of a two-layer membrane
provides insight into how pairs of layers of the glomerular
barrier may interact. For n ⫽ 2, Equation 7 applied to the
upstream layer becomes
slit diaphragm is a significant contributor to albumin sieving, the GBM is not (73).
The conclusion that the GBM does not influence
albumin sieving, just described, is at odds with the finding
that selective alterations in mouse GBM can lead to proteinuria (132). If in reality the GBM contributes significantly to albumin sieving in normal animals, W1 would
need to be orders of magnitude smaller than that measured in vivo. (For example, to yield ⌰1 ⫽ 0.1 at Pe1 ⫽
0.14, then W1 ⫽ 0.0014 for ⌰2 ⫽ 0.1 and W1 ⫽ 0.00014 for
⌰2 ⫽ 0.01.) It is possible that loss of macromolecular
constituents during GBM isolation led to a “looser” material with an elevated W. However, theories of flow through
fibrous media indicate that loss of, say, GAGs, also would
greatly elevate the hydraulic permeability (27). What is
most difficult to reconcile is that the hydraulic permeability in isolated GBM (measured simultaneously with Ficoll
sieving) was low enough to account for nearly all of the
known resistance to water filtration in vivo (73). That is,
a much “tighter” GBM in vivo would likely give unrealistically low values of single-nephron GFR.
In a multilayer barrier, physical interactions among
the layers also influence the dependence of the overall
sieving coefficient on the water filtration rate. As discussed already, ⌰ for a homogeneous membrane invariably declines as the filtration rate increases, until at high
Pe it reaches a minimum value equal to W. That is not
necessarily the case for a multilayer membrane, as will be
illustrated next.
For simplicity, we again consider a barrier with just
two layers. In general, the value of the Peclet number in a
given layer may be small, moderate, or large, suggesting
that there are nine combinations to consider when exploring how two-layer membranes behave. However, small
values of Pe1 or Pe2 are less interesting, because a layer
with a small Peclet number will tend to be “invisible”
(unless Wi ⬍ Pei). Of the four Peclet number combinations that remain, only three give different results for the
overall sieving coefficient, as discussed recently (68) and
explained in the three cases that follow. We will use
“Pe ⬃1” to indicate a moderate Pe and “Pe ⬎⬎ 1” to
indicate a large Pe. More precisely, Pe ⬃1 means that
neither exp(⫺Pe) nor 1 ⫺ exp(⫺Pe) is negligible compared with unity. If errors of ⬍20% are acceptable, that
will be true for Pe values ranging from ⬃0.2 to 2.
GLOMERULAR BARRIER AND PROTEINURIA
475
determined entirely by the selectivity of the upstream
layer. The downstream layer has no effect here, even if
Pe2 is not small. To obtain this result, Pe1 must be large
enough to make W1exp(⫺Pe1) negligible compared
with ⌰2.
2. Case 2
If Pe1 ⬃1 and Pe2 ⬎⬎ 1, then ⌰2 ⫽ W2 and
⌰⫽
W1W2
W2 ⫹ 共W1 ⫺ W2兲e⫺Pe1
(12)
3. Case 3
If Pe1 ⬃1 and Pe2 ⬃1, there is no algebraic simplification, and the resulting expression for ⌰ is too unwieldy
to offer much insight. However, if Pe1 ⫽ Pe2 ⫽ Pe, the
result is
⌰⫽
W1W2
W2 ⫹ 共W1 ⫺ W2兲e⫺Pe ⫺ W1共1 ⫺ W2兲e⫺2Pe
(13)
which is valid for any Pe. As in case 2, the sieving coefficient is flow dependent and influenced by the properties
of both layers. If Pe 3 0 in Equation 13, then ⌰ 3 1, as
seen before. If Pe ⬎⬎ 1, Equation 13 reduces to Equation
11. A comparison of Equations 12 and 13 reveals an
additional term in the denominator of the latter, which is
negative and which therefore tends to increase ⌰ at any
intermediate value of Pe. Thus the intrinsic selectivity of
the two layers is used less efficiently in the conditions
assumed for Equation 13 than in those for Equation 12.
It is informative to synthesize a two-layer membrane
mathematically, starting with one layer and then adding a
second with the same or different properties. Keeping in
mind that the sieving coefficients for large molecules
(e.g., albumin) should be as small as possible, we can
inquire whether the overall barrier function is improved
or worsened by adding a second layer. The results of such
calculations are given in Figure 15, which shows the
overall sieving coefficient as a function of the Peclet
number in layer 1. The curve for the single membrane was
generated using W ⫽ 0.01 in Equation 4 and is identical to
the bottom curve in Figure 13. If we now add a second
layer that has the same properties as the first, creating a
“double membrane,” there tends to be a downward shift
in ⌰ for any given value of Pe1. For the arbitrary parameter values that we chose for Figure 15, the maximum
Physiol Rev • VOL
FIG. 15. Sieving in single-layer and two-layer membranes. The
overall sieving coefficient (⌰) is plotted as a function of the Peclet
number in layer 1 (Pe1). Curves are shown for a single-layer membrane
(W1 ⫽ 0.01); a membrane in which a second, identical layer has been
added upstream or downstream from the first (W1 ⫽ W2 ⫽ 0.01); and a
membrane with a second, less selective layer added upstream from the
first (W1 ⫽ 0.1, W2 ⫽ 0.01). In the two-layer membranes, it was assumed
that Pe1 ⫽ Pe2.
effect (a 45% reduction in ⌰) occurs at a Pe value of ⬃0.1.
Because the two layers have the same intrinsic selectivity,
the double membrane is the same as a homogeneous one
with twice the original thickness. Recalling that the Peclet
number is proportional to membrane thickness, it is apparent that for the double membrane the Peclet number
to be used in Equation 4 must increase from Pe1 to 2Pe1.
The reduction in ⌰ occurs only at small-to-moderate
Pe1, because when Pe1 is large, there is no benefit in
doubling it. That is, for large Pe1 the sieving coefficient
is already at its minimum value, ⌰ ⫽ W. The assertion
that adding an identical second layer merely doubles
the apparent value of Pe can be verified by setting W1 ⫽
W2 ⫽ W in Equation 13.
If the additional layer is an order of magnitude less
selective, the results are very sensitive to its position. If it
is placed downstream, such that W1 ⫽ 0.01, W2 ⫽ 0.1, and
Pe2 ⫽ Pe1, barrier function is improved. However, the
values of ⌰ obtained from Equation 13 (not shown) are at
most 7% lower than those for the single-layer membrane,
changes that are too small to be evident on the log scale
in Figure 15. If the additional layer is placed upstream,
such that W1 ⫽ 0.1, W2 ⫽ 0.01, and Pe2 ⫽ Pe1, barrier
function is worsened. In this case, Equation 13 predicts
marked elevations in ⌰ at both moderate and large values
of Pe1, relative to the single-layer membrane. For Pe1 ⬎ 5
(somewhat beyond the range of Fig. 15), the overall sieving coefficient closely approaches that of the upstream
layer, consistent with Equation 11.
The functional degradation seen when the less selective layer is placed upstream reflects internal concentration polarization within the first membrane layer. In gen-
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Here the sieving coefficient is flow dependent and is
influenced by the selectivity of both layers. Since Pe1 is
proportional to the filtration rate of water, ⌰ will increase
with the filtration rate if W1 ⬎ W2 and decrease with
filtration rate if W1 ⬍ W2.
476
HARALDSSON, NYSTRÖM, AND DEEN
VII. MECHANISMS BEHIND PROTEINURIA AND
NEPHROTIC SYNDROMES
A. What is Proteinuria?
Proteinuria is said to be present when the urine
contains more than 300 mg protein per day (or 200 mg/l).
In the nephrotic syndrome, ⬎3.5 g/day of proteins are
excreted in the urine. Urine with daily protein excretion
of 30 –300 mg (20 –200 mg/l) reflects microalbuminuria.
Individual laboratories may give slightly different definitions based on the technique used and patient characteristics (e.g., age and sex). Many clinicians prefer to use
albumin excretion in relation to that for creatinine to
correct for differences in urine dilution. Normally, the
albumin-creatinine concentration ratio is ⬍30 ␮g/mg (128,
183). Today, protein analysis of the urine is used more as
a prognostic index for cardiovascular disease than as a
means of detecting patients with renal disorders (31, 111,
119, 336).
B. Where do Proteins in the Urine Come From?
Proteins in the final urine have different origins. First,
proteins cross the glomerular barrier, and tubular cell
reabsorption modifies their final concentration. Second,
there is tubular secretion of proteins from the blood.
Third, proteins may be synthesized by the cells themselves and released into the urine. Fourth, the proteins
may be added to the urine at a later stage (e.g., by excretion from the prostate gland in men). Let us examine
these alternatives in more detail.
Patients with glomerulonephritis have proteinuria,
hematuria, elevated blood pressure, and morphological
Physiol Rev • VOL
alterations in glomerular structures. It has therefore been
natural to assume that the proteinuria is caused by defects in the glomerular barrier. Recently, this view has
been questioned and an alternative hypothesis has been
proposed, the so-called albumin retrieval hypothesis (84).
The hypothesis has attracted certain interest, and it is
therefore discussed below, together with the overwhelming evidence against it.
C. The Albumin Retrieval Hypothesis
According to the albumin-retrieval hypothesis, the
proteinuria that occurs with diabetic nephropathy (226,
229) or other renal disorders (265) reflects defects in the
tubular system with intact glomerular permeability (265,
266). The morphological changes in the glomerulus are
therefore considered coincidental and of no pathophysiological significance. The hypothesis rests on four assumptions: 1) that albumin is retrieved from the tubule
fluid in intact form (84); 2) that the glomerular sieving
coefficient for albumin is close to 0.08 (228), which is two
to three orders of magnitude higher than other estimates
(see Fig. 10); 3) that the uptake of albumin is mediated by
the megalin-cubulin complex (264); and 4) that albumin is
markedly degraded and fragmented in the urine (84).
None of these assumptions is valid.
First, the question of whether there is any tubular
uptake of intact albumin is not a new one. In 1966,
Maunsbach studied the tubular uptake of autologous rat
albumin (185) and concluded that such uptake must be
extremely small or nonexistent (184). Others have confirmed and extended these findings. Measurements of the
kinetics of albumin uptake in proximal tubules (231) led
to the conclusion that albumin is taken up by the tubular
cells at the luminal side, and its amino acids are released
on the other side to blood (231). The molecular mechanism is the megalin-cubulin complex (21, 22).
Second, a glomerular sieving coefficient for albumin
close to 0.08 is not compatible with life and must be due
to flaws in the experimental technique. A simple example
illustrates the magnitude of the problem. Thus, for a
person with a glomerular filtration rate of 180 liters/day,
a serum concentration of albumin close to 40 g/l, and a
sieving coefficient for albumin of 0.08, the daily losses of
albumin would be more than half a kilogram (180 ⫻ 40 ⫻
0.08 ⫽ 576 g). With the assumption that a reuptake of
intact albumin exists (despite compelling evidence
against it, see above), such amounts are higher than theoretically achievable using detailed models of tubular
uptake (169).
Third, it has been suggested that the retrieval of
intact albumin is mediated by the megalin-cubulin complex (264). Indeed, megalin and cubulin are strongly
linked to the uptake of albumin and several other impor-
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eral, the hallmark of internal polarization is a sieving
coefficient in one or more membrane layers that exceeds
unity. For the example under consideration (W1 ⫽ 0.1,
W2 ⫽ 0.01, Pe2 ⫽ Pe1), ⌰1 ⫽ 2.1 at Pe1 ⫽ 1.0. Thus, in
moving across this two-layer membrane, the solute concentration first increases and then decreases. At low values of Pe1, the initial increase is negligible, and the twolayer membrane functions nearly as well as the singlelayer membrane (Fig. 15).
Overall, the results in Figure 15 suggest that ⌰ varies
inversely with filtration rate only if sieving in the most
downstream layer is not dominant. Accordingly, the tendency for ⌰ to vary inversely with single-nephron GFR (or
with GFR at constant nephron number), which has been
observed for dextran (47), Ficoll (251), and proteins
(179), supports the view that the endothelium and/or
GBM contribute significantly to sieving. This does not
exclude a role for the epithelium in sieving; it suggests
only that it is not the single dominant structure.
GLOMERULAR BARRIER AND PROTEINURIA
D. Human Diseases and Animal Models
Proteinuria is a hallmark of renal disease, and patients may suffer from glomerulonephritis, diabetic nephropathy, and numerous other conditions. Among the
conditions that cause nephrotic syndrome (with heavy
proteinuria, edema, low serum albumin concentrations,
and hyperlipidemia), minimal change nephrosis is the
most common disorder in children, while membranous
glomerulonephritis is predominant in elderly patients. Diabetes mellitus affects 170 million people worldwide, and
the number is expected to double over the next 25 years
(102). This makes diabetic nephropathy the most common cause of proteinuria, and occasionally the amounts
excreted reach nephrotic levels. Some patients suffer
Physiol Rev • VOL
from focal segmental glomerulosclerosis, and others have
primary or secondary forms of membranoproliferative
glomerulonephritis. In all these cases, the diagnosis is
obtained from morphological analysis of renal biopsies. In
the future, quantitative PCR will be used to assess the
expression profiles of different proteins (325).
Today we know the molecular defect underlying
many genetic disorders (321). Patients with nephrotic
syndrome of the Finnish type have a defect in the nephrin
gene, NPHS1(153). Children with steroid-resistant nephrotic syndrome are likely to have a problem with the
podocin gene, NPHS2(29). LMX1B, a gene that is mutated
in patients with nail-patella syndrome, is required for
podocyte differentiation (192). Indeed, several proteins
are known to be important for an intact glomerular barrier (see Refs. 83, 90, 103, 104, 142, 190, 237, 249, 320, 321,
326, 345). However, many renal disorders do not seem to
be explained by genetic factors, and the underlying mechanisms remain unknown.
Because of our rudimentary understanding of renal
pathophysiology, most treatments are nonspecific, cytotoxic, and anti-inflammatory, with potentially harmful
side effects. As a result, many patients with glomerulonephritis are given conservative treatments such as diuretics
and antihypertensive medicine alone. Preeclampsia is of
particular interest because the disease involves endothelial dysfunction (147, 186). Women with preeclampsia
have glomerular endotheliosis with swelling and vacuolization of the cells (187). Podocytes are not affected and
the GBM is intact, suggesting that the proteinuria is due
solely to endothelial damage. Moreover, VEGF is involved in the process (86, 186). Proteinuria also occurs
in Fanconi syndrome due to defective tubular reabsorption of filtered protein (207–209) (see above).
Proteinuria also occurs after ischemia-reperfusion,
which is evident from studies on transplanted kidneys (9,
294). A longer period of ischemia (60 min) causes nonselective damage to the glomerular barrier (252). If, however, the ischemia is shorter (15–20 min), the damage
following reperfusion mainly affects glomerular charge
selectivity (6, 252). These changes occur with no apparent
damage to the podocytes, the GBM, or the endothelial
cells, suggesting more subtle effects of the endothelial
cell surface layer (6). In other organs, similar short ischemic periods have been shown to selectively damage the
endothelial glycocalyx (198, 236, 262). Also, perfused kidneys have been reported to lose sulfated proteoglycans
and GAGs after ischemia (333), supporting the idea that
ischemia reperfusion causes proteinuria through damage
of the endothelial surface layer.
Several animal models have been used to mimic kidney disease in humans. Most studies of nephrotic syndrome have been done using nephrosis-inducing agents
such as puromycin aminonucleoside (122, 123, 216, 220,
225) and adriamycin (18, 19, 161). More recently, studies
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tant solutes in many species, including humans (49, 337).
It is also known that the turnover of the megalin-cubulin
complex is controlled by a chloride channel, ClC-5 (50,
124), which opens new therapeutic avenues (157). However, the megalin-cubulin complex cannot be responsible
for the reuptake of intact albumin, since these complexes
are internalized into lyzosomes, and the albumin is degraded (49).
Fourth, one research group concluded that albumin
fragments are present in urine and require special techniques to be detected (54, 56, 84, 100, 120, 221, 265, 295).
That conclusion is based on the assumption that their
method, which uses both the Biuret reagent and an HPLC
technique, is superior to the methods used by others. The
Biuret reagent is assumed to be sensitive and specific for
protein detection. Norden et al. (209) tested this assumption using state-of-the-art proteomics matrix-assisted laser desorption/ionization time of flight mass spectrometry
and Nano liquid chromatography electrospray tandem
mass spectrometry, to select 14 nonpeptide components
from urine, mix them, and test the samples using the
Biuret reagent.
Although there were no proteins present in these
samples, the Biuret reagent gave a protein value of 0.4 g/l,
and similar errors were found with the HPLC technique
(209). Moreover, a proteomics analysis of the fate of
albumin in patients with Fanconi syndrome and in normal
controls showed no evidence of significant fragmentation
of albumin in the urine (209). As mentioned in section IV,
patients with Fanconi syndrome excrete large quantities
of amino acids and proteins due to a defect in their
tubular reabsorption process. These findings provide additional support for our current understanding of the
megalin-cubulin complex, in which albumin is internalized by lyzosomes, degraded, and released to the blood in
the form of amino acids (49, 207). Considering the paucity
of experimental support, it is surprising that the albumin
retrieval hypothesis has survived so many years.
477
478
HARALDSSON, NYSTRÖM, AND DEEN
VIII. SUMMARY
In life, we search for simple answers to complex
questions. With respect to the glomerular barrier, scientists have presented data suggesting that the GBM, the
podocyte, or the endothelium is the one and only important component. In this review, we argue for an integrative view of the glomerular barrier and of the mechanisms
behind proteinuria. From the theoretical analysis presented above, it is evident that defects in any of the three
glomerular layers may result in proteinuria. Indeed, a
quest is under way for a more global analysis of glomerular proteins affected by disease (305).
A. What is the Relevance of the Individual
Components of the Glomerular Barrier?
The slit membrane of the podocytes is the most
complex structure in the glomerular barrier and definitely
of great importance for glomerular selectivity. As discussed already, several diseases in humans and animals
are caused by gene polymorphisms that affect proteins
involved in the slit membrane. Whether the podocyte
layer contributes to both charge and size selectivity is a
matter of debate. There simply are no data to support
opinions on this matter. The GBM, on the other hand,
shows some size selectivity but little charge selectivity, at
least when studied in isolated form using current protocols (28). Finally, the capillary endothelium is gaining
more and more attention (323). The high frequency of
large fenestrae has led many investigators to assume that
the endothelium does not contribute to the barrier, apart
from keeping blood cells from the GBM. However, data
from other capillary beds with fenestrated endothelia
(307) suggest these structures are as selective as continPhysiol Rev • VOL
uous capillaries, such as those in skeletal muscle (250).
Indeed, the sieving coefficient for albumin in peripheral
capillaries is ⬍0.1, due to the properties of the endothelial
surface coat or to the fiber matrix nature of underlying
basement membrane-like structures (55, 259). In Figure 5,
we have illustrated the endothelial cell surface layer with
some key components. The endothelial surface coat has
been studied quite extensively in other organs (204, 328,
329, 332, 338), but the visualization is mainly indirect, and
disease models are lacking (323). Nevertheless, it seems
safe to conclude that glomerular capillary endothelium is
at least as selective as that in intestine and in salivary and
pancreatic glands. If so, the concentration of albumin in
the fluid entering the GBM is likely to be 10% or less of
that in plasma.
The highly differentiated and specialized podocyte is
of utmost importance for an intact barrier. Several genetic
defects in the podocyte affect its structure (321, 322) and
cause diseases with proteinuria. In addition, through the
production of growth factors such as VEGF, the podocyte
affects the GBM and the endothelium indirectly (66).
Regardless of where the defect is, the result is proteinuria.
Thus the glomerular barrier with its specialized components truly acts as an integrative and delicate structure.
Over the next decade, a deeper understanding of the
barrier will lead to insight into the pathogenic mechanisms of kidney diseases and provide specific therapies.
The glomerular barrier is highly size and charge selective
in a manner that can be predicted using modern transport
theories (see Fig. 11). In particular, it markedly restricts
the passage of large anionic proteins such as albumin.
If these capillaries were as permeable as peripheral capillaries in skeletal muscle, a normal man would lose close
to a kilogram of albumin per day in the urine (116).
Fortunately, we lose ⬍1% of that amount into the primary
urine and ⬍30 mg/day of albumin in the final urine. The
components of the glomerulus that create such a magnificent barrier are the podocytes, the GBM, and the fenestrated endothelial cells with their endothelial surface
layer of GAGs.
Most known pathologies have been found in podocytes, in particular in proteins of the podocyte slit membrane (319, 323). The GBM constitutes most of the glomerular hydraulic resistance (65), while being highly permeable to solutes. Thus the GBM does not seem to be
particularly charge or size selective (28). Still, some GBM
lesions, such as mutations of the laminin ␤2-chain (350),
produce proteinuria even in the absence of detectable
changes in the cellular components (132). This finding is
not readily compatible with functional studies of the GBM
(68), unless laminin is present in the endothelial cell
surface layer as well. The endothelium has hitherto been
neglected (68, 70, 116). However, recent data on preeclampsia (147, 171, 172, 187) show that this endothelial
disease may affect glomerular capillaries and cause pro-
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in mouse knockout models have provided more information on the specific actions of individual proteins such as
laminin (350) and lamb2 (132), suggesting a major role for
the GBM in proteinuric conditions. Furthermore, induction of B7-1 in podocytes gives a nephrotic syndrome in
mice (241). Mice lacking the podocyte slit membrane
CD2-associated protein develop congenital nephrotic syndrome (287). There are mice that develop a disease resembling focal segmental glomerulosclerosis (125, 126)
and many other models (127). Most investigators try to
develop tissue-specific inducible genetic models to suppress or overexpress a certain protein. More sophisticated techniques can be used to study glomerular size and
charge selectivity in these mice (133–135), rather than
simply determining if proteinuria is present. The variety
of animal models and the continuing discovery of genes
mutated in patients with kidney disease give us a firm
platform for elucidating the mechanisms of proteinuria.
GLOMERULAR BARRIER AND PROTEINURIA
teinuria through the actions of VEGF (147). Indeed, glomerular lesions similar to those in human preeclampsia
were found in a mouse model with altered expression of
VEGF-A in the podocytes (85). Studies on kidneys subjected to mild ischemia support the notion that proteinuria may be caused by defects in the endothelial surface
layer (6).
B. What Do We Need to Know More About?
C. Concluding Remarks
We have presented an integrative view of the glomerular barrier, according to which each component of the
barrier must maintain its integrity to preserve normal
function. The GBM appears to be less selective than the
cellular components. The statement that podocyte damage leads to proteinuria is still accurate; it is, however,
equally true that damage to the endothelium causes proteinuria. In the final section of this review, we have highlighted important questions that need to be addressed for
a better understanding of one of nature’s great wonders,
the glomerular barrier.
Physiol Rev • VOL
ACKNOWLEDGMENTS
Address for reprint requests and other correspondence: B.
Haraldsson, Dept. of Molecular and Clinical Medicine/Nephrology, The Sahlgrenska Academy at Göteborg University, SU/
Sahlgrenska, SE-413 45 Gothenburg, Sweden (e-mail: borje.
[email protected]).
GRANTS
We are grateful for financial support of this work from the
Swedish Research Council 9898, the Ingabritt and Arne Lundberg Research Foundation, ALF/LUA funds of the VG region, the
Swedish Diabetes Association, and the Swedish National Kidney
Foundation.
REFERENCES
1. Abboud HE, Poptic E, DiCorleto P. Production of platelet-derived growth factorlike protein by rat mesangial cells in culture.
J Clin Invest 80: 675– 683, 1987.
2. Adamson RH, Clough G. Plasma proteins modify the endothelial
cell glycocalyx of frog mesenteric microvessels. J Physiol 445:
473– 486, 1992.
3. Ala-Houhala I, Pasternack A. Fractional dextran and protein
clearances in glomerulonephritis and in diabetic nephropathy. Clin
Sci 72: 289 –296, 1987.
4. Amon H, Gayer J. Electron microscope studies on the influence of
hyaluronidase on the basal membrane of the glomerular capillaries
with special reference to the problem of permeability. Klinische
Wochenschrift 41: 163–172, 1963.
5. Anderson JL, Brannon JH. Concentration dependence of the
distribution coefficient for macromolecules in porous media. J
Polymer Sci 19: 405– 421, 1981.
6. Andersson M, Nilsson UA, Hjalmarsson C, Haraldsson B, Nystrom Sorensson J. Mild renal ischemia-reperfusion reduces
charge and size selectivity of the glomerular barrier. Am J Physiol
Renal Physiol 292: F1802–F1809, 2007.
7. Areekul S. Reflection coefficients of neutral and sulphate-substituted dextran molecules in the isolated perfused rabbit ear. Acta
Soc Med Ups 74: 129 –138, 1969.
8. Aron DC, Rosenzweig JL, Abboud HE. Synthesis and binding of
insulin-like growth factor I by human glomerular mesangial cells.
J Clin Endocrinol Metab 68: 585–591, 1989.
9. Artz MA, Dooper PM, Meuleman EJ, van der Vliet JA, Wetzels
JF. Time course of proteinuria after living-donor kidney transplantation. Transplantation 76: 421– 423, 2003.
10. Asgeirsson D, Rippe B, Venturoli D, Rippe C. Increased glomerular permeability to negatively charged Ficoll relative to neutral
Ficoll in rats. Am J Physiol Renal Physiol 291: F1083–F1089, 2006.
11. Avasthi PS, Koshy V. The anionic matrix at the rat glomerular
endothelial surface. Anat Rec 220: 258 –266, 1988.
12. Bakoush O, Tencer J, Torffvit O, Tenstad O, Skogvall I, Rippe
B. Increased glomerular albumin permeability in old spontaneously
hypertensive rats. Nephrol Dial Transplant 19: 1724 –1731, 2004.
13. Ballermann BJ. Regulation of bovine glomerular endothelial cell
growth in vitro. Am J Physiol Cell Physiol 256: C182–C189, 1989.
14. Barker DF, Hostikka SL, Zhou J, Chow LT, Oliphant AR,
Gerken SC, Gregory MC, Skolnick MH, Atkin CL, Tryggvason
K. Identification of mutations in the COL4A5 collagen gene in
Alport syndrome. Science 248: 1224 –1227, 1990.
15. Batsford SR, Rohrbach R, Vogt A. Size restriction in the glomerular capillary wall: importance of lamina densa. Kidney Int 31:
710 –717, 1987.
16. Benedict CR, Pakala R, Willerson JT. Endothelial-dependent
procoagulant and anticoagulant mechanisms. Recent advances in
understanding. Texas Heart Inst J 21: 86 –90, 1994.
17. Bennett CM, Glassock RJ, Chang RL, Deen WM, Robertson
CR, Brenner BM, Troy JL, ueki IR, Rasmussen B. Permselectivity of the glomerular capillary wall. Studies of experimental
88 • APRIL 2008 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.2 on July 4, 2017
Despite recent advances, the mechanisms underlying
acquired glomerular disorders are poorly understood. As
a result, the therapeutic arsenal is limited and blunt.
Regarding the properties of the glomerular barrier, more
biological data are needed on the transglomerular passage of proteins that differ in size, charge, and shape.
Different technical approaches should be adopted, since
each method has advantages and drawbacks. In particular, the impact of molecular configuration must be studied
in detail. Only limited amounts of biological data have
been obtained under conditions of controlled tubular cell
activity. We need to learn more about the individual components of the barrier. The podocyte is the most complex
and fascinating part of the glomerular membrane, and
despite a decade of intense research, much remains to be
learned about this structure. For example, it will be important to understand in greater detail the communication between podocytes and the endothelium. It will also
be important to elucidate the endothelial and epithelial
production of proteoglycans and GAGs. What substances
are produced by these cells and at what rates? What is the
composition of the endothelial cell surface layer as shown
in Figure 5, and what is the turnover rate of this layer?
What is the role of the mesangial cell? We need to know
how various agents affect glomerular permeability during
health and disease. Finally, we must use the information
at hand to develop better therapies for the many patients
with kidney disease.
479
480
18.
19.
20.
21.
22.
23.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
glomerulonephritis in the rat using dextran sulfate. J Clin Invest
57: 1287–1294, 1976.
Bertolatus JA, Abuyousef M, Hunsicker LG. Glomerular sieving of high molecular weight proteins in proteinuric rats. Kidney
Int 31: 1257–1266, 1987.
Bertolatus JA, Hunsicker LG. Glomerular sieving of anionic and
neutral bovine albumins in proteinuric rats. Kidney Int 28: 467–
476, 1985.
Bertolatus JA, Klinzman D. Macromolecular sieving by glomerular basement membrane in vitro: effect of polycation or biochemical modifications. Microvasc Res 41: 311–327, 1991.
Birn H, Christensen EI. Renal albumin absorption in physiology
and pathology. Kidney Int 69: 440 – 449, 2006.
Birn H, Willnow TE, Nielsen R, Norden AG, Bonsch C,
Moestrup SK, Nexo E, Christensen EI. Megalin is essential for
renal proximal tubule reabsorption and accumulation of transcobalamin-B(12). Am J Physiol Renal Physiol 282: F408 –F416, 2002.
Björnson A, Moses J, Ingemansson A, Haraldsson B, Sörensson J. Primary human glomerular endothelial cells produce proteoglycans, puromycin affects their posttranslational modification.
Am J Physiol Renal Physiol 288: F748 –F756, 2005.
Björnson Granqvist A, Ebefors K, Saleem MA, Mathieson PW,
Haraldsson B, Sörensson Nyström J. Podocyte proteoglycan
synthesis is involved in the development of nephrotic syndrome.
Am J Physiol Renal Physiol 292: F722–F730, 2006.
Blau EB, Haas JE. Glomerular sialic acid and proteinuria in
human renal disease. Lab Invest 28: 477– 481, 1973.
Blouch K, Deen WM, Fauvel JP, Bialek J, Derby G, Myers BD.
Molecular configuration and glomerular size selectivity in healthy
and nephrotic humans. Am J Physiol Renal Physiol 273: F430 –
F437, 1997.
Bolton GR, Deen WM. Limitations in the application of fibermatrix models to glomerular basement membrane. In: Membrane
Transport and Renal Physiology, edited by Layton HE, Weinstein
AM. New York: Springer-Verlag, 2002, p. 141–156.
Bolton GR, Deen WM, Daniels BS. Assessment of the charge
selectivity of glomerular basement membrane using Ficoll sulfate.
Am J Physiol Renal Physiol 274: F889 –F896, 1998.
Boute N, Gribouval O, Roselli S, Benessy F, Lee H, Fuchshuber A, Dahan K, Gubler MC, Niaudet P, Antignac C. NPHS2,
encoding the glomerular protein podocin, is mutated in autosomal
recessive steroid-resistant nephrotic syndrome. Nature Genet 24:
349 –354, 2000.
Boyce NW, Holdsworth SR. Glomerular permselectivity in the
isolated perfused rat kidney. Am J Physiol Renal Fluid Electrolyte
Physiol 249: F780 –F784, 1985.
Brantsma AH, Bakker SJ, Hillege HL, de Zeeuw D, de Jong
PE, Gansevoort RT. Urinary albumin excretion and its relation
with C-reactive protein and the metabolic syndrome in the prediction of type 2 diabetes. Diabetes Care 28: 2525–2530, 2005.
Bray J, Robinson GB. Influence of charge on filtration across
renal basement membrane films in vitro. Kidney Int 25: 527–533,
1984.
Breimer ME, Svalander CT, Haraldsson B, Björck S. Physiological and histological characterisation of a pig kidney in vitro
perfusion model for xenotransplantation studies. Scand J Urol
Nephrol 30: 213–221, 1996.
Brenchley PEC. Vascular permeability factors in steroid-sensitive
nephrotic syndrome and focal segmental glomerulosclerosis. Nephrol Dial Transplant 18 Suppl 6: 21–25, 2003.
Brenner BM, Baylis C, Deen WM. Transport of molecules across
renal glomerular capillaries. Physiol Rev 56: 502–534, 1976.
Brenner BM, Bohrer MP, Baylis C, Deen WM. Determinants of
glomerular permselectivity: insights derived from observations in
vivo. Kidney Int 12: 229 –237, 1977.
Bridges CR Jr, Rennke HG, Deen WM, Troy JL, Brenner BM.
Reversible hexadimethrine-induced alterations in glomerular structure and permeability. J Am Soc Nephrol 1: 1095–1108, 1991.
Brink HM, Moons WM, Slegers JF. Glomerular filtration in the
isolated perfused kidney. I. Sieving of macromolecules. Pflügers
Arch 397: 42– 47, 1983.
Brown MD, Egginton S, Hudlicka O, Zhou AL. Appearance of
the capillary endothelial glycocalyx in chronically stimulated rat
Physiol Rev • VOL
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
58.
59.
skeletal muscles in relation to angiogenesis. Exp Physiol 81: 1043–
1046, 1996.
Bulger RE, Eknoyan G, Purcell DJ 2nd, Dobyan DC. Endothelial characteristics of glomerular capillaries in normal, mercuric
chloride-induced, gentamicin-induced acute renal failure in the rat.
J Clin Invest 72: 128 –141, 1983.
Burne MJ, Adal Y, Cohen N, Panagiotopoulos S, Jerums G,
Comper WD. Anomalous decrease in dextran sulfate clearance in
the diabetic rat kidney. Am J Physiol Renal Physiol 274: F700 –
F708, 1998.
Cattran D, Neogi T, Sharma R, McCarthy ET, Savin VJ. Serial
estimates of serum permeability activity and clinical correlates in
patients with native kidney focal segmental glomerulosclerosis.
J Am Soc Nephrol 14: 448 – 453, 2003.
Caulfield JP, Farquhar MG. The permeability of glomerular capillaries to graded dextrans. Identification of the basement membrane as the primary filtration barrier. J Cell Biol 63: 883–903, 1974.
Chambers R, Zweifach BW. Intercellular cement and capillary
permeability. Physiol Rev 27: 436 – 463, 1947.
Chang RLS, Deen WM, Robertson CR, Bennett CM, Glassock
RJ, Brenner BM, Troy JL, Ueki IF, Rasmussen B. Permselectivity of the glomerular capillary wall. Studies of experimental
glomerulonephritis in the rat using neutral dextran. J Clin Invest
57: 1272–1286, 1976.
Chang RLS, Deen WM, Robertson CR, Brenner BM. Permselectivity of the glomerular capillary wall. III. Restricted transport of
polyanions. Kidney Int 8: 212–218, 1975.
Chang RLS, Ueki IF, Troy JL, Deen WM, Robertson CR, Brenner BM. Permselectivity of the glomerular capillary wall to macromolecules. II. Experimental studies in rats using neutral dextran.
Biophys J 15: 887–906, 1975.
Chang RLS, Robertson CR, Deen WM, Brenner BM. Permselectivity of the glomerular capillary wall to macromolecules. I. Theoretical considerations. Biophys J 15: 861– 886, 1975.
Christensen EI, Birn H. Megalin and cubilin: synergistic endocytic receptors in renal proximal tubule. Am J Physiol Renal
Physiol 280: F562–F573, 2001.
Christensen EI, Devuyst O, Dom G, Nielsen R, Van der Smissen P, Verroust P, Leruth M, Guggino WB, Courtoy PJ. Loss of
chloride channel ClC-5 impairs endocytosis by defective trafficking
of megalin and cubilin in kidney proximal tubules. Proc Natl Acad
Sci USA 100: 8472– 8477, 2003.
Ciarimboli G, Bokenkamp A, Schurek HJ, Fels LM, Kilian I,
Maess B, Stolte H. The “fixed” charge of glomerular capillary wall
as determinant of permselectivity. Renal Failure 23: 365–376, 2001.
Ciarimboli G, Hjalmarsson C, Bokenkamp A, Schurek HJ,
Haraldsson B. Dynamic alterations of glomerular charge density
in fixed rat kidneys suggest involvement of endothelial cell coat.
Am J Physiol Renal Physiol 285: F722–F730, 2003.
Ciarimboli G, Schurek HJ, Zeh M, Flohr H, Bokenkamp A,
Fels LM, Kilian I, Stolte H. Role of albumin and glomerular
capillary wall charge distribution on glomerular permselectivity:
studies on the perfused-fixed rat kidney model. Pflügers Arch 438:
883– 891, 1999.
Clavant SP, Sastra SA, Osicka TM, Comper WD. The analysis
and characterisation of immuno-unreactive urinary albumin in
healthy volunteers. Clin Biochem 39: 143–151, 2006.
Clough G. Relationship between microvascular permeability and
ultrastructure. Prog Biophys Mol Biol 55: 47– 69, 1991.
Comper WD, Osicka TM. Detection of urinary albumin. Adv
Chronic Kidney Dis 12: 170 –176, 2005.
Comper WD, Tay M, Wells X, Dawes J. Desulphation of dextran
sulphate during kidney ultrafiltration. Biochem J 297: 31–34, 1994.
Curry FE, Rutledge JC, Lenz JF. Modulation of microvessel wall
charge by plasma glycoprotein orosomucoid. Am J Physiol Heart
Circ Physiol 257: H1354 –H1359, 1989.
Cutillas PR, Chalkley RJ, Hansen KC, Cramer R, Norden AG,
Waterfield MD, Burlingame AL, Unwin RJ. The urinary proteome in Fanconi syndrome implies specificity in the reabsorption
of proteins by renal proximal tubule cells. Am J Physiol Renal
Physiol 287: F353–F364, 2004.
88 • APRIL 2008 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.2 on July 4, 2017
24.
HARALDSSON, NYSTRÖM, AND DEEN
GLOMERULAR BARRIER AND PROTEINURIA
Physiol Rev • VOL
85.
86.
87.
88.
89.
90.
91.
92.
93.
94.
95.
96.
97.
98.
99.
100.
101.
102.
103.
104.
105.
106.
albumin to the rat renal vein—the albumin retrieval pathway. Renal
Failure 23: 347–363, 2001.
Eremina V, Quaggin SE. The role of VEGF-A in glomerular development and function. Curr Opin Nephrol Hypertens 13: 9 –15,
2004.
Eremina V, Sood M, Haigh J, Nagy A, Lajoie G, Ferrara N,
Gerber HP, Kikkawa Y, Miner JH, Quaggin SE. Glomerularspecific alterations of VEGF-A expression lead to distinct congenital and acquired renal diseases. J Clin Invest 111: 707–716, 2003.
Farquhar MG. Editorial: the primary glomerular filtration barrier— basement membrane or epithelial slits? Kidney Int 8: 197–
211, 1975.
Fels LM, Sanz-Altamira PM, Decker B, Elger B, Stolte H.
Filtration characteristics of the single isolated perfused glomerulus
of Myxine glutinosa. Renal Physiol Biochem 16: 276 –284, 1993.
Foidart JB, Dechenne CA, Mahieu P, Creutz CE, de Mey J.
Tissue culture of normal rat glomeruli. Isolation and morphological
characterization of two homogeneous cell lines. Invest Cell Pathol
2: 15–26, 1979.
Franceschini N, North KE, Kopp JB, McKenzie L, Winkler C.
NPHS2 gene, nephrotic syndrome and focal segmental glomerulosclerosis: a HuGE review. Genet Med 8: 63–75, 2006.
Franke H, Malyusz M, Runge D. Improved sodium and PAH
transport in the isolated fluorocarbon-perfused rat kidney.
Nephron 22: 423– 431, 1978.
Fujihara CK, Arcos-Fajardo M, Brandao De Almeida Prado E,
Jose Brandao De Almeida Prado M, Sesso A, Zatz R. Enhanced
glomerular permeability to macromolecules in the Nagase analbuminemic rat. Am J Physiol Renal Physiol 282: F45–F50, 2002.
Gekle D, von Bruchhausen F, Fuchs G. On the size of the pore
equivalent in isolated basement membrane of the rat kidney.
Pflügers Arch 289: 180 –190, 1966.
Ghitescu L, Desjardins M, Bendayan M. Immunocytochemical
study of glomerular permeability to anionic, neutral and cationic
albumins. Kidney Int 42: 25–32, 1992.
Giddings JC, Kucera E, Russel CP, Myers MN. Statistical theory
for the equilibrium distribution of rigid molecules in inert porous
networks. Exclusion chromatography. J Phys Chem 72: 4397– 4408,
1968.
Gitay-Goren H, Soker S, Vlodavsky I, Neufeld G. The binding
of vascular endothelial growth factor to its receptors is dependent
on cell surface-associated heparin-like molecules. J Biol Chem 267:
6093– 6098, 1992.
Glandt ED. Distribution equilibrium between a bulk phase and
small pores. AlChE J 27: 51–59, 1981.
Glassock RJ. Circulating permeability factors in the nephrotic
syndrome: a fresh look at an old problem. J Am Soc Nephrol 14:
541–543, 2003.
Green DF, Hwang KH, Ryan US, Bourgoignie JJ. Culture of
endothelial cells from baboon and human glomeruli. Kidney Int 41:
1506 –1516, 1992.
Greive KA, Osicka TM, Russo LM, Comper WD. Fragmentation
of filtered proteins and implications for glomerular protein sieving
in Fanconi syndrome. Kidney Int 61: 1549 –1550, 2002.
Groggel GC, Stevenson J, Hovingh P, Linker A, Border WA.
Changes in heparan sulfate correlate with increased glomerular
permeability. Kidney Int 33: 517–523, 1988.
Groop PH, Forsblom C, Thomas MC. Mechanisms of disease:
pathway-selective insulin resistance and microvascular complications of diabetes. Nat Clin Pract Endocrinol Metab 1: 100 –110,
2005.
Guan F, Villegas G, Teichman J, Mundel P, Tufro A. Autocrine
VEGF-A system in podocytes regulates podocin and its interaction
with CD2AP. Am J Physiol Renal Physiol 291: F422–F428, 2006.
Guan N, Ding J, Zhang J, Yang J. Expression of nephrin, podocin,
alpha-actinin, WT1 in children with nephrotic syndrome. Pediatr
Nephrol 18: 1122–1127, 2003.
Guasch A, Deen WM, Myers BD. Charge selectivity of the glomerular filtration barrier in healthy and nephrotic humans. J Clin
Invest 92: 2274 –2282, 1993.
Guasch A, Hashimoto H, Sibley RK, Deen WM, Myers BD.
Glomerular dysfunction in nephrotic humans with minimal
88 • APRIL 2008 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.2 on July 4, 2017
60. Cuypers Y, Vandenreyt I, Bipat R, Toelsie J, Van Damme B,
Steels P. The functional state of the isolated rabbit kidney perfused with autologous blood. Pflügers Arch 440: 634 – 642, 2000.
61. Dahly-Vernon AJ, Sharma M, McCarthy ET, Savin VJ, Ledbetter SR, Roman RJ. Transforming growth factor-beta, 20-HETE
interaction, glomerular injury in Dahl salt-sensitive rats. Hypertension 45: 643– 648, 2005.
62. Damiano ER. The effect of the endothelial-cell glycocalyx on the
motion of red blood cells through capillaries. Microvasc Res 55:
77–91, 1998.
63. Danielli JF. Capillary permeability and oedema in the perfused
frog. J Physiol 98: 109 –129, 1940.
64. Daniels BS. Increased albumin permeability in vitro following
alterations of glomerular charge is mediated by the cells of the
filtration barrier. J Lab Clin Med 124: 224 –230, 1994.
65. Daniels BS, Hauser EB, Deen WM, Hostetter TH. Glomerular
basement membrane: in vitro studies of water and protein permeability. Am J Physiol Renal Fluid Electrolyte Physiol 262: F919 –
F926, 1992.
66. Datta K, Li J, Karumanchi SA, Wang E, Rondeau E, Mukhopadhyay D. Regulation of vascular permeability factor/vascular
endothelial growth factor (VPF/VEGF-A) expression in podocytes.
Kidney Int 66: 1471–1478, 2004.
67. Dechadilok P, Deen WM. Hindrance factors for diffusion and
convection in pores. Ind Engin Chem Res 45: 6953– 6959, 2006.
68. Deen WM. Cellular contributions to glomerular size-selectivity.
Kidney Int 69: 1295–1297, 2006.
69. Deen WM. Hindered transport of large molecules in liquid-filled
pores. AIChE J 33: 1409 –1425, 1987.
70. Deen WM. What determines glomerular capillary permeability?
J Clin Invest 114: 1412–1414, 2004.
71. Deen WM, Bohrer MP, Epstein NB. Effects of molecular size and
configuration on diffusion in microporous membranes. AIChE J 27:
952–959, 1981.
72. Deen WM, Bridges CR, Brenner BM, Myers BD. Heteroporous
model of glomerular size selectivity: application to normal and
nephrotic humans. Am J Physiol Renal Fluid Electrolyte Physiol
249: F374 –F389, 1985.
73. Deen WM, Lazzara MJ, Myers BD. Structural determinants of
glomerular permeability. Am J Physiol Renal Physiol 281: F579 –
F596, 2001.
74. Deen WM, Robertson CR, Brenner BM. A model of glomerular
ultrafiltration in the rat. Am J Physiol 223: 1178 –1183, 1972.
75. Deen WM, Robertson CR, Brenner BM. Concentration polarization in an ultrafiltering capillary. Biophys J 14: 412– 431, 1974.
76. Deen WM, Satvat B, Jamieson JM. Theoretical model for glomerular filtration of charged solutes. Am J Physiol Renal Fluid
Electrolyte Physiol 238: F126 –F139, 1980.
77. Desjardins C, Duling BR. Microvessel hematocrit: measurement
and implications for capillary oxygen transport. American
J Physiol Heart Circ Physiol 252: H494 –H503, 1987.
78. Du Bois R, Decoodt P, Gassée JP, Verniory A, Lambert PP.
Determination of glomerular intracapillary and transcapillary pressure gradients from sieving data. Pflügers Arch 356: 299 –316, 1974.
79. Durvasula RV, Shankland SJ. Podocyte injury and targeting therapy: an update. Curr Opin Nephrol Hypertens 15: 1–7, 2006.
80. Casassa EF. Equilibrium distribution of flexible polymer chains
between a macroscopic solution phase and small voids. J Polym
Sci Polym Lett 5: 773–778, 1967.
81. Edwards A, Daniels BS, Deen WM. Hindered transport of macromolecules in isolated glomeruli. II. Convection and pressure
effects in basement membrane. Biophys J 72: 214 –222, 1997.
82. Edwards A, Deen WM, Daniels BS. Hindered transport of macromolecules in isolated glomeruli. I. Diffusion across intact and
cell-free capillaries. Biophys J 72: 204 –213, 1997.
83. El-Aouni C, Herbach N, Blattner SM, Henger A, Rastaldi MP,
Jarad G, Miner JH, Moeller MJ, St-Arnaud R, Dedhar S,
Holzman LB, Wanke R, Kretzler M. Podocyte-specific deletion
of integrin-linked kinase results in severe glomerular basement
membrane alterations and progressive glomerulosclerosis. J Am
Soc Nephrol 17: 1334 –1344, 2006.
84. Eppel GA, Osicka TM, Pratt LM, Jablonski P, Howden B,
Glasgow EF, Comper WD. The return of glomerular filtered
481
482
107.
108.
109.
110.
111.
113.
114.
115.
116.
117.
118.
119.
120.
121.
122.
123.
124.
125.
126.
changes or focal glomerulosclerosis. Am J Physiol Renal Fluid
Electrolyte Physiol 260: F728 –F737, 1991.
Guimaraes MA, Nikolovski J, Pratt LM, Greive K, Comper
WD. Anomalous fractional clearance of negatively charged Ficoll
relative to uncharged Ficoll. Am J Physiol Renal Physiol 285:
F1118 –F1124, 2003.
Gyenge CC, Tenstad O, Wiig H. In vivo determination of steric
and electrostatic exclusion of albumin in rat skin and skeletal
muscle. J Physiol 552: 907–916, 2003.
Ha TS, Duraisamy S, Faulkner JL, Kasinath BS. Regulation of
glomerular endothelial cell proteoglycans by glucose. J Korean
Med Sci 19: 245–252, 2004.
Hagen SG, Michael AF, Butkowski RJ. Immunochemical and
biochemical evidence for distinct basement membrane heparan
sulfate proteoglycans. J Biol Chem 268: 7261–7269, 1993.
Halbesma N, Kuiken DS, Brantsma AH, Bakker SJ, Wetzels
JF, De Zeeuw D, De Jong PE, Gansevoort RT. Macroalbuminuria is a better risk marker than low estimated GFR to identify
individuals at risk for accelerated GFR loss in population screening. J Am Soc Nephrol 17: 2582–2590, 2006.
Haldenby KA, Chappell DC, Winlove CP, Parker KH, Firth JA.
Focal and regional variations in the composition of the glycocalyx
of large vessel endothelium. J Vasc Res 31: 2–9, 1994.
Halfter W, Dong S, Schurer B, Cole GJ. Collagen XVIII is a
basement membrane heparan sulfate proteoglycan. J Biol Chem
273: 25404 –25412, 1998.
Haraldsson B, Rippe B. Orosomucoid as one of the serum components contributing to normal capillary permselectivity in rat
skeletal muscle. Acta Physiol Scand 129: 127–135, 1987.
Haraldsson B, Rippe B. Serum factors other than albumin are
needed for the maintenance of normal capillary permeability in rat
hindlimb muscle. Acta Physiol Scand 123: 427– 436, 1985.
Haraldsson B, Sörensson J. Why do we not all have proteinuria?
An update of our current understanding of the glomerular barrier.
News Physiol Sci 19: 7–10, 2004.
Haraldsson BS, Johnsson EK, Rippe B. Glomerular permselectivity is dependent on adequate serum concentrations of orosomucoid. Kidney Int 41: 310 –316, 1992.
Hassell JR, Robey PG, Barrach HJ, Wilczek J, Rennard SI,
Martin GR. Isolation of a heparan sulfate-containing proteoglycan
from basement membrane. Proc Natl Acad Sci USA 77: 4494 – 4498,
1980.
Hillege HL, Janssen WM, Bak AA, Diercks GF, Grobbee DE,
Crijns HJ, Van Gilst WH, De Zeeuw D, De Jong PE. Microalbuminuria is common, also in a nondiabetic, nonhypertensive population, an independent indicator of cardiovascular risk factors and
cardiovascular morbidity. J Internal Med 249: 519 –526, 2001.
Hilliard LM, Osicka TM, Clavant SP, Robinson PJ, NikolicPaterson DJ, Comper WD. Characterization of the urinary albumin degradation pathway in the isolated perfused rat kidney. J Lab
Clin Med 147: 36 – 44, 2006.
Hjalmarsson C, Johansson BR, Haraldsson B. Electron microscopic evaluation of the endothelial surface layer of glomerular
capillaries. Microvasc Res 67: 9 –17, 2004.
Hjalmarsson C, Lidell ME, Haraldsson B. Beneficial effects of
orosomucoid on the glomerular barrier in puromycin aminonucleoside-induced nephrosis. Nephrol Dial Transplant 21: 1223–1230,
2006.
Hjalmarsson C, Ohlson M, Haraldsson B. Puromycin aminonucleoside damages the glomerular size barrier with minimal effects on charge density. Am J Physiol Renal Physiol 281: F503–
F512, 2001.
Hryciw DH, Ekberg J, Pollock CA, Poronnik P. ClC-5: a chloride channel with multiple roles in renal tubular albumin uptake.
Int J Biochem Cell Biol 38: 1036 –1042, 2006.
Huber TB, Benzing T. The slit diaphragm: a signaling platform to
regulate podocyte function. Curr Opin Nephrol Hypertens 14:
211–216, 2005.
Huber TB, Kwoh C, Wu H, Asanuma K, Godel M, Hartleben B,
Blumer KJ, Miner JH, Mundel P, Shaw AS. Bigenic mouse
models of focal segmental glomerulosclerosis involving pairwise
interaction of CD2AP, Fyn, synaptopodin. J Clin Invest 116: 1337–
1345, 2006.
Physiol Rev • VOL
127. Huber TB, Reinhardt HC, Exner M, Burger JA, Kerjaschki D,
Saleem MA, Pavenstädt H. Expression of functional CCR and
CXCR chemokine receptors in podocytes. J Immunol 168: 6244 –
6252, 2002.
128. Hutchison AS, O’Reilly DS, MacCuish AC. Albumin excretion
rate, albumin concentration, albumin/creatinine ratio compared for
screening diabetics for slight albuminuria. Clin Chem 34: 2019 –
2021, 1988.
129. Huxley VH, Williams DA. Role of a glycocalyx on coronary
arteriole permeability to proteins: evidence from enzyme treatments. Am J Physiol Heart Circ Physiol 278: H1177–H1185, 2000.
130. Igarashi S, Nagase M, Oda T, Honda N. Molecular sieving by
glomerular basement membrane isolated from normal and nephrotic rabbits. Clin Chim Acta 68: 255–258, 1976.
131. Iozzo RV, Cohen IR, Grassel S, Murdoch AD. The biology of
perlecan: the multifaceted heparan sulphate proteoglycan of basement membranes and pericellular matrices. Biochem J 302: 625–
639, 1994.
132. Jarad G, Cunningham J, Shaw AS, Miner JH. Proteinuria precedes podocyte abnormalities inLamb2⫺/⫺ mice, implicating the
glomerular basement membrane as an albumin barrier. J Clin
Invest 116: 2272–2279, 2006.
133. Jeansson M, Granqvist AB, Nyström JS, Haraldsson B. Functional and molecular alterations of the glomerular barrier in longterm diabetes in mice. Diabetologia 49: 2200 –2209, 2006.
134. Jeansson M, Haraldsson B. Glomerular size and charge selectivity in the mouse after exposure to glucosaminoglycan-degrading
enzymes. J Am Soc Nephrol 14: 1756 –1765, 2003.
135. Jeansson M, Haraldsson B. Morphological and functional evidence for an important role of the endothelial cell glycocalyx in the
glomerular barrier. Am J Physiol Renal Physiol 290: F111–F116,
2006.
136. Johnson EM, Deen WM. Electrostatic effects on the equilibrium
partitioning of spherical colloids in random fibrous media.. J Colloid Interface Sci 178: 749 –756, 1996.
137. Johnson EM, Deen WM. Errata: electrostatic effects on the equilibrium partitioning of spherical colloids in random fibrous media.
J Colloid Interface Sci 195: 268 –268, 1997.
138. Johnson EM, Deen WM. Hydraulic permeability of agarose gels.
AIChE J 42: 1220 –1224, 1996.
139. Johnsson E, Haraldsson B. Addition of purified orosomucoid
preserves the glomerular permeability for albumin in isolated perfused rat kidneys. Acta Physiol Scand 147: 1– 8, 1993.
140. Johnsson E, Haraldsson B. An isolated perfused rat kidney preparation designed for assessment of glomerular permeability characteristics. Acta Physiol Scand 144: 65–73, 1992.
141. Johnston ST, Deen WM. Hindered convection of Ficoll and proteins in agarose gels. Ind Eng Chem Res 41: 340 –346, 2002.
142. Johnstone DB, Holzman LB. Clinical impact of research on the
podocyte slit diaphragm. Nature Clin Pract 2: 271–282, 2006.
143. Julian BA, Novak J. IgA nephropathy: an update. Curr Opin
Nephrol Hypertens 13: 171–179, 2004.
144. Kalluri R. Proteinuria with and without renal glomerular podocyte
effacement. J Am Soc Nephrol 17: 2383–2389, 2006.
145. Kanwar YS, Linker A, Farquhar MG. Increased permeability of
the glomerular basement membrane to ferritin after removal of
glycosaminoglycans (heparan sulfate) by enzyme digestion. J Cell
Biol 86: 688 – 693, 1980.
146. Kanwar YS, Rosenzweig LJ. Clogging of the glomerular basement membrane. J Cell Biol 93: 489 – 494, 1982.
147. Karumanchi SA, Maynard SE, Stillman IE, Epstein FH,
Sukhatme VP. Preeclampsia: a renal perspective. Kidney Int 67:
2101–2113, 2005.
148. Kasinath BS. Glomerular endothelial cell proteoglycans—regulation by TGF-beta 1. Arch Biochem Biophys 305: 370 –377, 1993.
149. Kelley VE, Cavallo T. Glomerular permeability: transfer of native
ferritin in glomeruli with decreased anionic sites. Lab Invest 39:
547–553, 1978.
150. Kerjaschki D. Dysfunctions of cell biological mechanisms of visceral epithelial cell (podocytes) in glomerular diseases. Kidney Int
45: 300 –313, 1994.
88 • APRIL 2008 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.2 on July 4, 2017
112.
HARALDSSON, NYSTRÖM, AND DEEN
GLOMERULAR BARRIER AND PROTEINURIA
Physiol Rev • VOL
172.
173.
174.
175.
176.
177.
178.
179.
180.
181.
182.
183.
184.
185.
186.
187.
188.
189.
190.
191.
192.
SA. Soluble endoglin and other circulating antiangiogenic factors
in preeclampsia. N Engl J Med 355: 992–1005, 2006.
Levine RJ, Maynard SE, Qian C, Lim KH, England LJ, Yu KF,
Schisterman EF, Thadhani R, Sachs BP, Epstein FH, Sibai
BM, Sukhatme VP, Karumanchi SA. Circulating angiogenic factors and the risk of preeclampsia. N Engl J Med 350: 672– 683, 2004.
Lieberthal W, Rennke HG, Sandock KM, Valeri CR, Levinsky
NG. Ischemia in the isolated erythrocyte-perfused rat kidney. Protective effect of hypothermia. Renal Physiol Biochem 11: 60 – 69,
1988.
Lijnen HR, Collen D. Endothelium in hemostasis and thrombosis.
Prog Cardiovasc Dis 39: 343–350, 1997.
Lindström KE, Blom A, Johnsson E, Haraldsson B, Fries E.
High glomerular permeability of bikunin despite similarity in
charge and hydrodynamic size to serum albumin. Kidney Int 51:
1053–1058, 1997.
Lindström KE, Johnsson E, Haraldsson B. Glomerular charge
selectivity for proteins larger than serum albumin as revealed by
lactate dehydrogenase isoforms. Acta Physiol Scand 162: 481– 488,
1998.
Lindström KE, Rönnstedt L, Jaremko G, Haraldsson B. Physiological and morphological effects of perfusing isolated rat kidneys with hyperosmolal mannitol solutions. Acta Physiol Scand
166: 231–238, 1999.
Luft JH. Fine structures of capillary and endocapillary layer as
revealed by ruthenium red. Federation Proc 25: 1773–1783, 1966.
Lund U, Rippe A, Venturoli D, Tenstad O, Grubb A, Rippe B.
Glomerular filtration rate dependence of sieving of albumin and
some neutral proteins in rat kidneys. Am J Physiol Renal Physiol
284: F1226 –F1234, 2003.
Maack T. Physiological evaluation of the isolated perfused rat
kidney. Am J Physiol Renal Fluid Electrolyte Physiol 238: F71–
F78, 1980.
Maddox DA, Deen WM, Brenner BM. Glomerular filtration. In:
Handbook of Physiology. Renal Physiology.Bethesda, MD: Am.
Physiol. Soc., 1992, sect. 8, vol. I, chapt. 13, p. 545– 638.
Marrero MB, Banes-Berceli AK, Stern DM, Eaton DC. Role of
the JAK/STAT signaling pathway in diabetic nephropathy. Am J
Physiol Renal Physiol 290: F762–F768, 2006.
Mattix HJ, Hsu CY, Shaykevich S, Curhan G. Use of the albumin/creatinine ratio to detect microalbuminuria: implications of
sex and race. J Am Soc Nephrol 13: 1034 –1039, 2002.
Maunsbach AB. Absorption of 125I-labeled homologous albumin
by rat kidney proximal tubule cells. A study of microperfused
single proximal tubules by electron microscopic autoradiography
and histochemistry 1966. J Am Soc Nephrol 8: 323–351, 1997.
Maunsbach AB. Absorption of I-125-labeled homologous albumin
by rat kidney proximal tubule cells. A study of microperfused
single proximal tubules by electron microscopic autoradiography
and histochemistry. J Ultrastruct Res 15: 197–241, 1966.
Maynard SE, Min JY, Merchan J, Lim KH, Li J, Mondal S,
Libermann TA, Morgan JP, Sellke FW, Stillman IE, Epstein
FH, Sukhatme VP, Karumanchi SA. Excess placental soluble
fms-like tyrosine kinase 1 (sFlt1) may contribute to endothelial
dysfunction, hypertension, proteinuria in preeclampsia. J Clin
Invest 111: 649 – 658, 2003.
Maynard SE, Venkatesha S, Thadhani R, Karumanchi SA.
Soluble Fms-like tyrosine kinase 1 and endothelial dysfunction in
the pathogenesis of preeclampsia. Pediatr Res 57: 1R–7R, 2005.
McCarthy ET, Sharma R, Sharma M, Li JZ, Ge XL, Dileezpan
KN, Savin VJ. TNF-alpha increases albumin permeability of isolated rat glomeruli through the generation of superoxide. J Am Soc
Nephrol 9: 433– 438, 1998.
Mene P, Simonson MS, Dunn MJ. Physiology of the mesangial
cell. Physiol Rev 69: 1347–1424, 1989.
Meyrier A. Mechanisms of disease: focal segmental glomerulosclerosis. Nature Clin Pract 1: 44 –54, 2005.
Michel CC, Phillips ME. The effect of Ficoll 70 and bovine serum
albumin on the permeability properties of individually perfused
frog mesenteric capillaries. J Physiol 291: 39P, 1979.
Miner JH, Morello R, Andrews KL, Li C, Antignac C, Shaw AS,
Lee B. Transcriptional induction of slit diaphragm genes by Lmx1b
88 • APRIL 2008 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.2 on July 4, 2017
151. Kerjaschki D, Sharkey DJ, Farquhar MG. Identification and
characterization of podocalyxin—the major sialoprotein of the renal glomerular epithelial cell. J Cell Biol 98: 1591–1596, 1984.
152. Kerjaschki D, Vernillo AT, Farquhar MG. Reduced sialylation
of podocalyxin–the major sialoprotein of the rat kidney glomerulus–in aminonucleoside nephrosis. Am J Pathol 118: 343–349, 1985.
153. Kestila M, Lenkkeri U, Mannikko M, Lamerdin J, McCready
P, Putaala H, Ruotsalainen V, Morita T, Nissinen M, Herva R,
Kashtan CE, Peltonen L, Holmberg C, Olsen A, Tryggvason K.
Positionally cloned gene for a novel glomerular protein–nephrin–is
mutated in congenital nephrotic syndrome. Mol Cell 1: 575–582,
1998.
154. Kestila M, Mannikko M, Holmberg C, Gyapay G, Weissenbach
J, Savolainen ER, Peltonen L, Tryggvason K. Congenital nephrotic syndrome of the Finnish type maps to the long arm of
chromosome 19. Am J Hum Genet 54: 757–764, 1994.
155. Limbach KW, Nitsche JM, Wei J. Partitioning of nonspherical
molecules between bulk solution and porous solids. AIChE J 35:
42–52, 1989.
156. Kitahara T, Hiromura K, Ikeuchi H, Yamashita S, Kobayashi
S, Kuroiwa T, Kaneko Y, Ueki K, Nojima Y. Mesangial cells
stimulate differentiation of endothelial cells to form capillary-like
networks in a three-dimensional culture system. Nephrol Dial
Transplant 20: 42– 49, 2005.
157. Klassen RB, Crenshaw K, Kozyraki R, Verroust PJ, Tio L,
Atrian S, Allen PL, Hammond TG. Megalin mediates renal uptake of heavy metal metallothionein complexes. Am J Physiol
Renal Physiol 287: F393–F403, 2004.
158. Kosto KB, Deen WM. Diffusivities of macromolecules in composite hydrogels. AIChE J 50: 2648 –2658, 2004.
159. Kosto KB, Deen WM. Hindered convection of macromolecules in
hydrogels. Biophys J 88: 277–286, 2005.
160. Kosto KB, Panuganti S, Deen WM. Equilibrium partitioning of
Ficoll in composite hydrogels. J Colloid Interface Sci 277: 404 – 409,
2004.
161. Kramer A, van den Hoven M, Rops A, Wijnhoven T, van den
Heuvel L, Lensen J, van Kuppevelt T, van Goor H, van der
Vlag J, Navis G, Berden JH. Induction of glomerular heparanase
expression in rats with adriamycin nephropathy is regulated by
reactive oxygen species and the renin-angiotensin system. J Am
Soc Nephrol 17: 2513–2520, 2006.
162. Kreisberg JI, Venkatachalam M, Troyer D. Contractile properties of cultured glomerular mesangial cells. Am J Physiol Renal
Physiol 249: F457–F463, 1985.
163. Kriz W, Kaissling B. Structural organization of the mammalian
kidney. In: The Kidney Physiology and Pathophysiology, edited by
Seldin GW, Giebisch G. New York: Raven, 1992, p. 707–777.
164. Lagrue G, Xheneumont S, Branellec A, Hirbec G, Weil B. A
vascular permeability factor elaborated from lymphocytes. I. Demonstration in patients with nephrotic syndrome. Biomedicine 23:
37– 40, 1975.
165. Lai AS, Lai KN. Molecular basis of IgA nephropathy. Curr Mol
Med 5: 475– 487, 2005.
166. Lazzara MJ, Blankschtein D, Deen WM. Effects of multisolute
steric interactions on membrane partition coefficients. J Colloid
Interface Sci 226: 112–122, 2000.
167. Lazzara MJ, Deen WM. Effects of concentration on the partitioning of macromolecule mixtures in agarose gels. J Colloid Interface
Sci 272: 288 –297, 2004.
168. Lazzara MJ, Deen WM. Effects of plasma proteins on sieving of
tracer macromolecules in glomerular basement membrane. Am J
Physiol Renal Physiol 281: F860 –F868, 2001.
169. Lazzara MJ, Deen WM. Model of albumin reabsorption in the
proximal tubule. Am J Physiol Renal Physiol 292: F430 –F439,
2007.
170. Lemmink HH, Mochizuki T, van den Heuvel LP, Schroder CH,
Barrientos A, Monnens LA, van Oost BA, Brunner HG, Reeders ST, Smeets HJ. Mutations in the type IV collagen alpha 3
(COL4A3) gene in autosomal recessive Alport syndrome. Hum Mol
Genet 3: 1269 –1273, 1994.
171. Levine RJ, Lam C, Qian C, Yu KF, Maynard SE, Sachs BP,
Sibai BM, Epstein FH, Romero R, Thadhani R, Karumanchi
483
484
193.
194.
195.
196.
198.
199.
200.
201.
202.
203.
204.
205.
206.
207.
208.
209.
210.
is required in podocyte differentiation. J Clin Invest 109: 1065–
1072, 2002.
Miner JH, Sanes JR. Collagen IV alpha 3, alpha 4, alpha 5 chains
in rodent basal laminae: sequence, distribution, association with
laminins, developmental switches. J Cell Biol 127: 879 – 891, 1994.
Mitchell BD, Deen WM. Theoretical effects of macromolecule
concentration and charge on membrane rejection coefficients. J
Membr Sci 19: 75–100, 1984.
Mochizuki T, Lemmink HH, Mariyama M, Antignac C, Gubler
MC, Pirson Y, Verellen-Dumoulin C, Chan B, Schroder CH,
Smeets HJ. Identification of mutations in the alpha 3(IV) and
alpha 4(IV) collagen genes in autosomal recessive Alport syndrome. Nature Genet 8: 77– 81, 1994.
Morigi M, Zoja C, Figliuzzi M, Remuzzi G, Remuzzi A. Supernatant of endothelial cells exposed to laminar flow inhibits mesangial cell proliferation. Am J Physiol Cell Physiol 264: C1080 –C1083,
1993.
Morita H, Yoshimura A, Inui K, Ideura T, Watanabe H, Wang
L, Soininen R, Tryggvason K. Heparan sulfate of perlecan is
involved in glomerular filtration. J Am Soc Nephrol 16: 1703–1710,
2005.
Mulivor AW, Lipowsky HH. Inflammation- and ischemia-induced
shedding of venular glycocalyx. Am J Physiol Heart Circ Physiol
286: H1672–H1680, 2004.
Mundel P, Kriz W. Cell culture of podocytes. Exp Nephrol 4:
263–266, 1996.
Mundel P, Reiser J, Kriz W. Induction of differentiation in cultured rat and human podocytes. J Am Soc Nephrol 8: 697–705, 1997.
Mundel P, Reiser J, Zuniga Mejia Borja A, Pavenstädt H,
Davidson GR, Kriz W, Zeller R. Rearrangements of the cytoskeleton and cell contacts induce process formation during differentiation of conditionally immortalized mouse podocyte cell lines. Exp
Cell Res 236: 248 –258, 1997.
Muragaki Y, Timmons S, Griffith CM, Oh SP, Fadel B, Quertermous T, Olsen BR. Mouse Col18a1 is expressed in a tissuespecific manner as three alternative variants and is localized in
basement membrane zones. Proc Natl Acad Sci USA 92: 8763– 8767,
1995.
Myers BD, Guasch A. Selectivity of the glomerular filtration barrier in healthy and nephrotic humans. Am J Nephrol 13: 311–317,
1993.
Nieuwdorp M, van Haeften TW, Gouverneur MC, Mooij HL,
van Lieshout MH, Levi M, Meijers JC, Holleman F, Hoekstra
JB, Vink H, Kastelein JJ, Stroes ES. Loss of endothelial glycocalyx during acute hyperglycemia coincides with endothelial dysfunction and coagulation activation in vivo. Diabetes 55: 480 – 486,
2006.
Noakes PG, Miner JH, Gautam M, Cunningham JM, Sanes JR,
Merlie JP. The renal glomerulus of mice lacking s-laminin/laminin
beta 2: nephrosis despite molecular compensation by laminin beta
1. Nature Genet 10: 400 – 406, 1995.
Norden AG, Lapsley M, Igarashi T, Kelleher CL, Lee PJ,
Matsuyama T, Scheinman SJ, Shiraga H, Sundin DP, Thakker
RV, Unwin RJ, Verroust P, Moestrup SK. Urinary megalin deficiency implicates abnormal tubular endocytic function in Fanconi
syndrome. J Am Soc Nephrol 13: 125–133, 2002.
Norden AG, Lapsley M, Lee PJ, Pusey CD, Scheinman SJ, Tam
FW, Thakker RV, Unwin RJ, Wrong O. Fragmentation of filtered
proteins and implications for glomerular protein sieving in Fanconi
syndrome. Kidney Int 62: 349, 2002.
Norden AG, Lapsley M, Lee PJ, Pusey CD, Scheinman SJ, Tam
FW, Thakker RV, Unwin RJ, Wrong O. Glomerular protein sieving and implications for renal failure in Fanconi syndrome. Kidney
Int 60: 1885–1892, 2001.
Norden AG, Sharratt P, Cutillas PR, Cramer R, Gardner SC,
Unwin RJ. Quantitative amino acid and proteomic analysis: very
low excretion of polypeptides ⬎750 Da in normal urine. Kidney Int
66: 1994 –2003, 2004.
Ogamo A, Nagai A, Nagasawa K. Binding of heparin fractions and
other polysulfated polysaccharides to plasma fibronectin: effects of
molecular size and degree of sulfation of polysaccharides. Biochim
Biophys Acta 841: 30 – 41, 1985.
Physiol Rev • VOL
211. Ogston AG. The spaces in a uniform random suspension of fibres.
Trans Faraday Soc 54: 1754 –1757, 1958.
212. Ohlson M, Sörensson J, Haraldsson B. A gel-membrane model
of glomerular charge and size selectivity in series. Am J Physiol
Renal Physiol 280: F396 –F405, 2001.
213. Ohlson M, Sörensson J, Haraldsson B. Glomerular size and
charge selectivity in the rat as revealed by FITC-ficoll and albumin.
Am J Physiol Renal Physiol 279: F84 –F91, 2000.
214. Ohlson M, Sörensson J, Lindström K, Blom AM, Fries E,
Haraldsson B. Effects of filtration rate on the glomerular barrier
and clearance of four differently shaped molecules. Am J Physiol
Renal Physiol 281: F103–F113, 2001.
215. Ohyama K, Seyer JM, Raghow R, Kang AH. Extracellular matrix
phenotype of rat mesangial cells in culture. Biosynthesis of collagen types I, III, IV, V and a low molecular weight collagenous
component and their regulation by dexamethasone. J Lab Clin Med
116: 219 –227, 1990.
216. Oken DE, Flamenbaum W. Micropuncture studies of proximal
tubule albumin concentrations in normal and nephrotic rats. J Clin
Invest 50: 1498 –1505, 1971.
217. Oliver JD 3rd, Anderson S, Troy JL, Brenner BM, Deen WM.
Determination of glomerular size-selectivity in the normal rat with
Ficoll. J Am Soc Nephrol 3: 214 –228, 1992.
218. Oliver JD 3rd, Deen WM. Random-coil model for glomerular
sieving of dextran. Bull Math Biol 56: 369 –389, 1994.
219. Oliver JD 3rd, Simons JL, Troy JL, Provoost AP, Brenner BM,
Deen WM. Proteinuria and impaired glomerular permselectivity in
uninephrectomized fawn-hooded rats. Am J Physiol Renal Fluid
Electrolyte Physiol 267: F917–F925, 1994.
220. Olson JL, Rennke HG, Venkatachalam MA. Alterations in the
charge and size selectivity barrier of the glomerular filter in aminonucleoside nephrosis in rats. Lab Invest 44: 271–279, 1981.
221. Osicka TM, Comper WD. Characterization of immunochemically
nonreactive urinary albumin. Clin Chem 50: 2286 –2291, 2004.
222. Osicka TM, Comper WD. Glomerular charge selectivity for anionic and neutral horseradish peroxidase. Kidney Int 47: 1630 –
1637, 1995.
223. Osicka TM, Comper WD. Protein degradation during renal passage in normal kidneys is inhibited in experimental albuminuria.
Clin Sci 93: 65–72, 1997.
224. Osicka TM, Comper WD. Tubular inhibition destroys charge selectivity for anionic and neutral horseradish peroxidase. Biochim
Biophys Acta 1381: 170 –178, 1998.
225. Osicka TM, Hankin AR, Comper WD. Puromycin aminonucleoside nephrosis results in a marked increase in fractional clearance
of albumin. Am J Physiol Renal Physiol 277: F139 –F145, 1999.
226. Osicka TM, Houlihan CA, Chan JG, Jerums G, Comper WD.
Albuminuria in patients with type 1 diabetes is directly linked to
changes in the lysosome-mediated degradation of albumin during
renal passage. Diabetes 49: 1579 –1584, 2000.
227. Osicka TM, Panagiotopoulos S, Jerums G, Comper WD. Fractional clearance of albumin is influenced by its degradation during
renal passage. Clin Sci 93: 557–564, 1997.
228. Osicka TM, Pratt LM, Comper WD. Glomerular capillary wall
permeability to albumin and horseradish peroxidase. Nephrology
199 –212, 1996.
229. Osicka TM, Yu Y, Panagiotopoulos S, Clavant SP, Kiriazis Z,
Pike RN, Pratt LM, Russo LM, Kemp BE, Comper WD, Jerums
G. Prevention of albuminuria by aminoguanidine or ramipril in
streptozotocin-induced diabetic rats is associated with the normalization of glomerular protein kinase C. Diabetes 49: 87–93, 2000.
230. Pappenheimer JR, Renkin EM, Borrero LM. Filtration,diffusion
and molecular sieving through peripheral capillary membranes. A
contribution to the pore theory of capillary permeability. Am J
Physiol 167: 13– 46, 1951.
231. Park CH, Maack T. Albumin absorption and catabolism by isolated perfused proximal convoluted tubules of the rabbit. J Clin
Invest 73: 767–777, 1984.
232. Pavenstadt H. The charge for going by foot: modifying the surface
of podocytes. Exp Nephrol 6: 98 –103, 1998.
233. Pavenstädt H, Henger A, Briner V, Greger R, Schollmeyer P.
Extracellular ATP regulates glomerular endothelial cell function. J
Auton Pharmacol 16: 389 –391, 1996.
88 • APRIL 2008 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.2 on July 4, 2017
197.
HARALDSSON, NYSTRÖM, AND DEEN
GLOMERULAR BARRIER AND PROTEINURIA
Physiol Rev • VOL
256. Rosenzweig LJ, Kanwar YS. Removal of sulfated (heparan sulfate) or nonsulfated (hyaluronic acid) glycosaminoglycans results
in increased permeability of the glomerular basement membrane to
125
I-bovine serum albumin. Lab Invest 47: 177–184, 1982.
257. Ross BD. The isolated perfused rat kidney. Clin Sci Mol Med Suppl
55: 513–521, 1978.
258. Rossi M, Morita H, Sormunen R, Airenne S, Kreivi M, Wang L,
Fukai N, Olsen BR, Tryggvason K, Soininen R. Heparan sulfate
chains of perlecan are indispensable in the lens capsule but not in
the kidney. EMBO J 22: 236 –245, 2003.
259. Rostgaard J, Qvortrup K. Electron microscopic demonstrations
of filamentous molecular sieve plugs in capillary fenestrae. Microvasc Res 53: 1–13, 1997.
260. Rostgaard J, Qvortrup K. Sieve plugs in fenestrae of glomerular
capillaries–site of the filtration barrier? Cells Tissues Organs 170:
132–138, 2002.
261. Roth J, Brown D, Orci L. Regional distribution of N-acetyl-Dgalactosamine residues in the glycocalyx of glomerular podocytes.
J Cell Biol 96: 1189 –1196, 1983.
262. Rubio-Gayosso I, Platts SH, Duling BR. Reactive oxygen species mediate modification of glycocalyx during ischemia-reperfusion injury. Am J Physiol Heart Circ Physiol 290: H2247–H2256,
2006.
263. Ruggenenti P, Mosconi L, Sangalli F, Casiraghi F, Gambara V,
Remuzzi G, Remuzzi A. Glomerular size-selective dysfunction in
NIDDM is not ameliorated by ACE inhibition or by calcium channel
blockade. Kidney Int 55: 984 –994, 1999.
264. Russo LM, Bakris GL, Comper WD. Renal handling of albumin:
a critical review of basic concepts and perspective. Am J Kidney
Dis 39: 899 –919, 2002.
265. Russo LM, Comper WD, Osicka TM. Mechanism of albuminuria
associated with cardiovascular disease and kidney disease. Kidney
Int Suppl S67–S68, 2004.
266. Russo LM, Sandoval RM, McKee M, Osicka TM, Collins AB,
Brown D, Molitoris BA, Comper WD. The normal kidney filters
nephrotic levels of albumin retrieved by proximal tubule cells:
retrieval is disrupted in nephrotic states. Kidney Int 71: 504 –513,
2007.
267. Ryan GB, Hein SJ, Karnovsky MJ. Glomerular permeability to
proteins. Effects of hemodynamic factors on the distribution of
endogenous immunoglobulin G and exogenous catalase in the rat
glomerulus. Lab Invest 34: 415– 427, 1976.
268. Ryan GB, Karnovsky MJ. Distribution of endogenous albumin in
the rat glomerulus: role of hemodynamic factors in glomerular
barrier function. Kidney Int 9: 36 – 45, 1976.
269. Salant DJ. The structural biology of glomerular epithelial cells in
proteinuric diseases. Curr Opin Nephrol Hypertens 3: 569 –574,
1994.
270. Saleem MA, O’Hare MJ, Reiser J, Coward RJ, Inward CD,
Farren T, Xing CY, Ni L, Mathieson PW, Mundel P. A conditionally immortalized human podocyte cell line demonstrating
nephrin and podocin expression. J Am Soc Nephrol 13: 630 – 638,
2002.
271. Sanes JR, Engvall E, Butkowski R, Hunter DD. Molecular
heterogeneity of basal laminae: isoforms of laminin and collagen IV
at the neuromuscular junction and elsewhere. J Cell Biol 111:
1685–1699, 1990.
272. Satchell SC, Tasman CH, Singh A, Ni L, Geelen J, von Ruhland CJ, O’Hare MJ, Saleem MA, van den Heuvel LP, Mathieson PW. Conditionally immortalized human glomerular endothelial
cells expressing fenestrations in response to VEGF. Kidney Int 69:
1633–1640, 2006.
273. Savage CO. The biology of the glomerulus: endothelial cells. Kidney Int 45: 314 –319, 1994.
274. Savin VJ, Sharma R, Lovell HB, Welling DJ. Measurement of
albumin reflection coefficient with isolated rat glomeruli. J Am Soc
Nephrol 3: 1260 –1269, 1992.
275. Scheinman JI, Fish AJ, Brown DM, Michael AJ. Human glomerular smooth muscle (mesangial) cells in culture. Lab Invest 34:
150 –158, 1976.
276. Schena FP, Gesualdo L. Pathogenetic mechanisms of diabetic
nephropathy. J Am Soc Nephrol 16 Suppl 1: S30 –S33, 2005.
88 • APRIL 2008 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.2 on July 4, 2017
234. Pavenstädt H, Kriz W, Kretzler M. Cell biology of the glomerular
podocyte. Physiol Rev 83: 253–307, 2003.
235. Phillips RJ. A hydrodynamic model for hindered diffusion of
proteins and micelles in hydrogels. Biophys J 79: 3350 –3353, 2000.
236. Platts SH, Linden J, Duling BR. Rapid modification of the glycocalyx caused by ischemia-reperfusion is inhibited by adenosine
A2A receptor activation. Am J Physiol Heart Circ Physiol 284:
H2360 –H2367, 2003.
237. Pollak MR. The genetic basis of FSGS and steroid-resistant nephrosis. Semin Nephrol 23: 141–146, 2003.
238. Pyke C, Kristensen P, Ostergaard PB, Oturai PS, Romer J.
Proteoglycan expression in the normal rat kidney. Nephron 77:
461– 470, 1997.
239. Raats CJ, van den Born J, Bakker MA, Oppers-Walgreen B,
Pisa BJ, Dijkman HB, Assmann KJ, Berden JH. Expression of
agrin, dystroglycan, utrophin in normal renal tissue and in experimental glomerulopathies. Am J Pathol 156: 1749 –1765, 2000.
240. Rabkin R, Kitabchi AE. Factors influencing the handling of insulin by the isolated rat kidney. J Clin Invest 62: 169 –175, 1978.
241. Reiser J, von Gersdorff G, Loos M, Oh J, Asanuma K, Giardino L, Rastaldi MP, Calvaresi N, Watanabe H, Schwarz K,
Faul C, Kretzler M, Davidson A, Sugimoto H, Kalluri R,
Sharpe AH, Kreidberg JA, Mundel P. Induction of B7-1 in podocytes is associated with nephrotic syndrome. J Clin Invest 113:
1390 –1397, 2004.
242. Remuzzi A, Perico N, Sangalli F, Vendramin G, Moriggi M,
Ruggenenti P, Remuzzi G. ACE inhibition and ANG II receptor
blockade improve glomerular size-selectivity in IgA nephropathy.
Am J Physiol Renal Physiol 276: F457–F466, 1999.
243. Remuzzi A, Puntorieri S, Battaglia C, Bertani T, Remuzzi G.
Angiotensin converting enzyme inhibition ameliorates glomerular
filtration of macromolecules and water and lessens glomerular
injury in the rat. J Clin Invest 85: 541–549, 1990.
244. Remuzzi A, Viberti G, Ruggenenti P, Battaglia C, Pagni R,
Remuzzi G. Glomerular response to hyperglycemia in human diabetic nephropathy. Am J Physiol Renal Fluid Electrolyte Physiol
259: F545–F552, 1990.
245. Renkin EM, Robinson RR. Glomerular filtration. N Engl J Med
290: 785–792, 1974.
246. Rennke HG, Patel Y, Venkatachalam MA. Glomerular filtration
of proteins: clearance of anionic, neutral, cationic horseradish
peroxidase in the rat. Kidney Int 13: 278 –288, 1978.
247. Rennke HG, Venkatachalam MA. Glomerular permeability of
macromolecules. Effect of molecular configuration on the fractional clearance of uncharged dextran and neutral horseradish
peroxidase in the rat. J Clin Invest 63: 713–717, 1979.
248. Rijkmans BG, Buurman WA, Kootstra G. Six-day canine kidney
preservation. Hypothermic perfusion combined with isolated blood
perfusion. Transplantation 37: 130 –134, 1984.
249. Rinta-Valkama J, Palmen T, Lassila M, Holthofer H. Podocyteassociated proteins FAT, alpha-actinin-4 and filtrin are expressed in
Langerhans islets of the pancreas. Mol Cell Biochem 294: 117–125,
2007.
250. Rippe B, Haraldsson B. Transport of macromolecules across
microvascular walls: the two-pore theory. Physiol Rev 74: 163–219,
1994.
251. Rippe C, Asgeirsson D, Venturoli D, Rippe A, Rippe B. Effects
of glomerular filtration rate on Ficoll sieving coefficients (theta) in
rats. Kidney Int 69: 1326 –1332, 2006.
252. Rippe C, Rippe A, Larsson A, Asgeirsson D, Rippe B. Nature of
glomerular capillary permeability changes following acute renal
ischemia/reperfusion (I/R) injury in rats. Am J Physiol Renal
Physiol 292: F1362–F1368, 2006.
253. Robinson GB, Walton HA. Glomerular basement membrane as a
compressible ultrafilter. Microvasc Res 38: 36 – 48, 1989.
254. Rodewald R, Karnovsky MJ. Porous substructure of the glomerular slit diaphragm in the rat and mouse. J Cell Biol 60: 423– 433,
1974.
255. Rops AL, van der Vlag J, Lensen JF, Wijnhoven TJ, van den
Heuvel LP, van Kuppevelt TH, Berden JH. Heparan sulfate
proteoglycans in glomerular inflammation. Kidney Int 65: 768 –785,
2004.
485
486
HARALDSSON, NYSTRÖM, AND DEEN
Physiol Rev • VOL
298. Sörensson J, Björnson A, Ohlson M, Ballermann BJ, Haraldsson B. Synthesis of sulfated proteoglycans by bovine glomerular
endothelial cells in culture. Am J Physiol Renal Physiol 284: F373–
F380, 2003.
299. Sörensson J, Matejka GL, Ohlson M, Haraldsson B. Human
endothelial cells produce orosomucoid, an important component
of the capillary barrier. Am J Physiol Heart Circ Physiol 276:
H530 –H534, 1999.
300. Sörensson J, Ohlson M, Björnson A, Haraldsson B. Orosomucoid has a cAMP-dependent effect on human endothelial cells and
inhibits the action of histamine. Am J Physiol Heart Circ Physiol
278: H1725–H1731, 2000.
301. Sörensson J, Ohlson M, Haraldsson B. A quantitative analysis of
the glomerular charge barrier in the rat. Am J Physiol Renal
Physiol 280: F646 –F656, 2001.
302. Sörensson J, Ohlson M, Lindström K, Haraldsson B. Glomerular charge selectivity for horseradish peroxidase and albumin at
low and normal ionic strengths. Acta Physiol Scand 163: 83–91,
1998.
303. Takami H, Naramoto A, Shigematsu H, Ohno S. Ultrastructure
of glomerular basement membrane by quick-freeze and deep-etch
methods. Kidney Int 39: 659 – 664, 1991.
304. Takeda T. Podocyte cytoskeleton is connected to the integral
membrane protein podocalyxin through Na⫹/H⫹-exchanger regulatory factor 2 and ezrin. Clin Exp Nephrol 7: 260 –269, 2003.
305. Takemoto M, Asker N, Gerhardt H, Lundkvist A, Johansson
BR, Saito Y, Betsholtz C. A new method for large scale isolation
of kidney glomeruli from mice. Am J Pathol 161: 799 – 805, 2002.
306. Tay M, Comper WD, Singh AK. Charge selectivity in kidney
ultrafiltration is associated with glomerular uptake of transport
probes. Am J Physiol Renal Fluid Electrolyte Physiol 260: F549 –
F554, 1991.
307. Taylor AE, Granger DN. Exchange of macromolecules across the
microcirculation. In: Handbook of Physiology. The Cardiovascular
System. Microcirculation.Bethesda, MD: Am. Physiol. Soc., 1984,
sect. 2, vol. IV, pt. 2, chapt. 11, p. 467–520.
308. Taylor GM, Neuhaus TJ, Shah V, Dillon S, Barratt TM. Charge
and size selectivity of proteinuria in children with idiopathic nephrotic syndrome. Pediatr Nephrol 11: 404 – 410, 1997.
309. Ten Dam MA, ten Kate RW, Werter CJ, van Kamp GJ, Meuwissen SG, Donker AJ. Measurement of the glomerular sieving
coefficient of endogenous proteins in man. Contr Nephrol 83: 47–
52, 1990.
310. Tencer J, Frick IM, Öquist BW, Alm P, Rippe B. Size-selectivity
of the glomerular barrier to high molecular weight proteins: upper
size limitations of shunt pathways. Kidney Int 53: 709 –715, 1998.
311. Tencer J, Torffvit O, Thysell H, Rippe B, Grubb A. Urine
IgG2/IgG4-ratio indicates the significance of the charge selective
properties of the glomerular capillary wall for the macromolecular
transport in glomerular diseases. Nephrol Dial Transplant 14:
1425–1429, 1999.
312. Tenstad O, Roald AB, Grubb A, Aukland K. Renal handling of
radiolabelled human cystatin C in the rat. Scand J Clin Lab Invest
56: 409 – 414, 1996.
313. Tenstad O, Williamson HE, Clausen G, Oien AH, Aukland K.
Glomerular filtration and tubular absorption of the basic polypeptide aprotinin. Acta Physiol Scand 152: 33–50, 1994.
314. Thelle K, Christensen EI, Vorum H, Orskov H, Birn H. Characterization of proteinuria and tubular protein uptake in a new
model of oral L-lysine administration in rats. Kidney Int 69: 1333–
1340, 2006.
315. Tisher CC, Brenner BM. Structure and Function of the Glomerulus. Philadelphia, PA: Lippincott, 1994.
316. Tisher CC, Madsen KM. Anatomy of the Kidney. In: The Kidney,
edited by Brenner B, Rector F. Philadelphia, PA: Saunders, 1991,
p. 3–75.
317. Tojo A, Endou H. Intrarenal handling of proteins in rats using
fractional micropuncture technique. Am J Physiol Renal Fluid
Electrolyte Physiol 263: F601–F606, 1992.
318. Trachtman H, Greenbaum LA, McCarthy ET, Sharma M, Gauthier BG, Frank R, Warady B, Savin VJ. Glomerular permeability activity: prevalence and prognostic value in pediatric patients
88 • APRIL 2008 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.2 on July 4, 2017
277. Schlondorff D. The glomerular mesangial cell: an expanding role
for a specialized pericyte. FASEB J 1: 272–281, 1987.
278. Schnitzer JE, Carley WW, Palade GE. Specific albumin binding
to microvascular endothelium in culture. Am J Physiol Heart Circ
Physiol 254: H425–H437, 1988.
279. Schnitzer JE, Pinney E. Quantitation of specific binding of orosomucoid to cultured microvascular endothelium: role in capillary
permeability. Am J Physiol Heart Circ Physiol 263: H48 –H55,
1992.
280. Sengoelge G, Luo W, Fine D, Perschl AM, Fierlbeck W,
Haririan A, Sorensson J, Rehman TU, Hauser P, Trevick JS,
Kulak SC, Wegner B, Ballermann BJ. A SAGE-based comparison between glomerular and aortic endothelial cells. Am J Physiol
Renal Physiol 288: F1290 –F1300, 2005.
281. Sharma M, Sharma R, Ge XL, Fish BL, McCarthy ET, Savin VJ,
Cohen EP, Moulder JE. Early detection of radiation-induced
glomerular injury by albumin permeability assay. Radiat Res 155:
474 – 480, 2001.
282. Sharma M, Sharma R, McCarthy ET, Savin VJ. The focal segmental glomerulosclerosis permeability factor: biochemical characteristics and biological effects. Exp Biol Med 229: 85–98, 2004.
283. Sharma M, Sharma R, McCarthy ET, Savin VJ. “The FSGS
factor”: enrichment and in vivo effect of activity from focal segmental glomerulosclerosis plasma. J Am Soc Nephrol 10: 552–561,
1999.
284. Sharma M, Sharma R, Reddy SR, McCarthy ET, Savin VJ.
Proteinuria after injection of human focal segmental glomerulosclerosis factor. Transplantation 73: 366 –372, 2002.
285. Sharma R, Khanna A, Sharma M, Savin VJ. Transforming
growth factor-beta1 increases albumin permeability of isolated rat
glomeruli via hydroxyl radicals. Kidney Int 58: 131–136, 2000.
286. Sharma R, Suzuki K, Nagase H, Savin VJ. Matrix metalloproteinase (stromelysin-1) increases the albumin permeability of isolated rat glomeruli. J Lab Clin Med 128: 297–303, 1996.
287. Shih NY, Li J, Karpitskii V, Nguyen A, Dustin ML, Kanagawa
O, Miner JH, Shaw AS. Congenital nephrotic syndrome in mice
lacking CD2-associated protein. Science 286: 312–315, 1999.
288. Simionescu M, Simionescu N. Ultrastructure of the microvascular wall: functional correlations. In: Handbook of Physiology. The
Cardiovascular System. Microcirculation.Bethesda, MD: Am.
Physiol. Soc., 1984, sect. 2, vol. IV, pt. 2, chapt. 3, p. 41–101.
289. Sims DE, Westfall JA, Kiorpes AL, Horne MM. Preservation of
tracheal mucus by nonaqueous fixative. Biotech Histochem 66:
173–180, 1991.
290. Sjöberg EM, Blom A, Larsson BS, Alston-Smith J, Sjöquist M,
Fries E. Plasma clearance of rat bikunin: evidence for receptormediated uptake. Biochemical J 308: 881– 887, 1995.
291. Smith FG III, Deen WM. Electrostatic effects on the partitioning
of spherical colloids between dilute bulk solution and cylindrical
pores. J Colloid Interface Sci 91: 571–590, 1983.
292. Smithies O. Why the kidney glomerulus does not clog: a gel
permeation/diffusion hypothesis of renal function. Proc Natl Acad
Sci USA 100: 4108 – 4113, 2003.
293. Spiegler KS, Kedem O. Thermodynamics of hyperfiltration
(reverse osmosis): criteria for efficient membranes. Desalination
1: 311–326, 1966.
294. Stefanidis I, Heintz B, Stocker G, Mrowka C, Sieberth HG,
Haubeck HD. Association between heparan sulfate proteoglycan
excretion and proteinuria after renal transplantation. J Am Soc
Nephrol 7: 2670 –2676, 1996.
295. Strong KJ, Osicka TM, Comper WD. Urinary-peptide excretion
by patients with and volunteers without diabetes. J Lab Clin Med
145: 239 –246, 2005.
296. Sugimoto H, Hamano Y, Charytan D, Cosgrove D, Kieran M,
Sudhakar A, Kalluri R. Neutralization of circulating vascular
endothelial growth factor (VEGF) by anti-VEGF antibodies and
soluble VEGF receptor 1 (sFlt-1) induces proteinuria. J Biol Chem
278: 12605–12608, 2003.
297. Sumpio BE, Chaudry IH, Baue AE. Reduction of the drug-induced nephrotoxicity by ATP-MgCl2. 1. Effects on the cis-diamminedichloroplatinum-treated isolated perfused kidneys. J Surg
Res 38: 429 – 437, 1985.
GLOMERULAR BARRIER AND PROTEINURIA
319.
320.
321.
322.
323.
324.
326.
327.
328.
329.
330.
331.
332.
333.
334.
335.
336.
Physiol Rev • VOL
337.
338.
339.
340.
341.
342.
343.
344.
345.
346.
347.
348.
349.
350.
351.
352.
with urinary albumin excretion in men and women. J Am Soc
Nephrol 14: 1330 –1335, 2003.
Verroust PJ. Pathophysiology of cubilin: of rats, dogs and men.
Nephrol Dial Transplant 17 Suppl 9: 55–56, 2002.
Vink H, Duling BR. Capillary endothelial surface layer selectively
reduces plasma solute distribution volume. Am J Physiol Heart
Circ Physiol 278: H285–H289, 2000.
Vink H, Duling BR. Identification of distinct luminal domains for
macromolecules, erythrocytes, leukocytes within mammalian capillaries. Circ Res 79: 581–589, 1996.
Vyas SV, Comper WD. Dextran sulfate binding to isolated rat
glomeruli and glomerular basement membrane. Biochim Biophys
Acta 1201: 367–372, 1994.
Vyas SV, Parker JA, Comper WD. Uptake of dextran sulphate by
glomerular intracellular vesicles during kidney ultrafiltration. Kidney Int 47: 945–950, 1995.
Wartiovaara J, Ofverstedt LG, Khoshnoodi J, Zhang J,
Makela E, Sandin S, Ruotsalainen V, Cheng RH, Jalanko H,
Skoglund U, Tryggvason K. Nephrin strands contribute to a
porous slit diaphragm scaffold as revealed by electron tomography.
J Clin Invest 114: 1475–1483, 2004.
Weiss C, Passow H, Rothstein A. Autoregulation of flow in
isolated rat kidney in absence of red cells. Am J Physiol 196:
1115–1118, 1959.
White JA, Deen WM. Equilibrium partitioning of flexible macromolecules in fibrous membranes and gels. Macromolecules 33:
8504 – 8511, 2000.
Wolf G, Stahl RA. CD2-associated protein and glomerular disease.
Lancet 362: 1746 –1748, 2003.
Wolgast M, Källskog Ö, Wahlström H. Characteristics of the
glomerular capillary membrane of the rat kidney as a hydrated gel.
I. Hypothetical structure. Acta Physiol Scand 158: 213–224, 1996.
Wolgast M, Källskog Ö, Wahlström H. Characteristics of the
glomerular capillary membrane of the rat kidney as a hydrated gel.
II. On the validity of the model. Acta Physiol Scand 158: 225–232,
1996.
Wolgast M, Öjteg G. Electrophysiology of renal capillary membranes: gel concept applied and Starling model challenged. Am J
Physiol Renal Fluid Electrolyte Physiol 254: F364 –F373, 1988.
Zeman LJ, Zydney AL. Microfiltration and Ultrafiltration. New
York: Dekker, 1996, p. 350 –352.
Zenker M, Aigner T, Wendler O, Tralau T, Muntefering H,
Fenski R, Pitz S, Schumacher V, Royer-Pokora B, Wuhl E,
Cochat P, Bouvier R, Kraus C, Mark K, Madlon H, Dotsch J,
Rascher W, Maruniak-Chudek I, Lennert T, Neumann LM,
Reis A. Human laminin beta2 deficiency causes congenital nephrosis with mesangial sclerosis and distinct eye abnormalities. Hum
Mol Genet 13: 2625–2632, 2004.
Zhang X, Curry FR, Weinbaum S. Mechanism of osmotic flow in
a periodic fiber array. Am J Physiol Heart Circ Physiol 290:
H844 –H852, 2006.
Zoja C, Wang JM, Bettoni S, Sironi M, Renzi D, Chiaffarino F,
Abboud HE, Van Damme J, Mantovani A, Remuzzi G. Interleukin-1 beta and tumor necrosis factor-alpha induce gene expression
and production of leukocyte chemotactic factors, colony-stimulating factors, and interleukin-6 in human mesangial cells. Am J
Pathol 138: 991–1003, 1991.
88 • APRIL 2008 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.2 on July 4, 2017
325.
with idiopathic nephrotic syndrome. Am J Kidney Dis 44: 604 – 610,
2004.
Tryggvason K. Unraveling the mechanisms of glomerular ultrafiltration: nephrin, a key component of the slit diaphragm. J Am Soc
Nephrol 10: 2440 –2445, 1999.
Tryggvason K, Patrakka J. Thin basement membrane nephropathy. J Am Soc Nephrol 17: 813– 822, 2006.
Tryggvason K, Patrakka J, Wartiovaara J. Hereditary proteinuria syndromes and mechanisms of proteinuria. N Engl J Med 354:
1387–1401, 2006.
Tryggvason K, Pikkarainen T, Patrakka J. Nck links nephrin to
actin in kidney podocytes. Cell 125: 221–224, 2006.
Tryggvason K, Wartiovaara J. How does the kidney filter
plasma? Physiology 20: 96 –101, 2005.
Uchida K, Uchida S, Nitta K, Yumura W, Marumo F, Nihei H.
Glomerular endothelial cells in culture express and secrete vascular endothelial growth factor. Am J Physiol Renal Fluid Electrolyte
Physiol 266: F81–F88, 1994.
Waga I, Yamamoto J, Sasai H, Munger WE, Hogan SL, Preston
GA, Sun HW, Jennette JC, Falk RJ, Alcorta DA. Altered mRNA
expression in renal biopsy tissue from patients with IgA nephropathy. Kidney Int 64: 1253–1264, 2003.
Walz G. Slit or pore? A mutation of the ion channel TRPC6 causes
FSGS. Nephrol Dial Transplant 20: 1777–1779, 2005.
Van Damme M, Prevost M. Transport of charged macromolecules
across a biological charged membrane. Comput Prog Biomed 19:
107–117, 1985.
Van den Berg B, Vink H. Glycocalyx perturbation: cause or consequence of damage to the vasculature? Am J Physiol Heart Circ
Physiol 290: H2174 –H2175, 2006.
Van den Berg BM, Vink H, Spaan JA. The endothelial glycocalyx
protects against myocardial edema. Circ Res 92: 592–594, 2003.
Van den Berg JG, van den Bergh Weerman MA, Assmann KJ,
Weening JJ, Florquin S. Podocyte foot process effacement is not
correlated with the level of proteinuria in human glomerulopathies.
Kidney Int 66: 1901–1906, 2004.
Van Setten PA, van Hinsbergh VW, van der Velden TJ, van de
Kar NC, Vermeer M, Mahan JD, Assmann KJ, van den Heuvel
LP, Monnens LA. Effects of TNF alpha on verocytotoxin cytotoxicity in purified human glomerular microvascular endothelial cells.
Kidney Int 51: 1245–1256, 1997.
VanTeeffelen JW, Dekker S, Fokkema DS, Siebes M, Vink H,
Spaan JA. Hyaluronidase treatment of coronary glycocalyx increases reactive hyperemia but not adenosine hyperemia in dog
hearts. Am J Physiol Heart Circ Physiol 289: H2508 –H2513, 2005.
Vassiliou P, Tay M, Comper WD. Partial ischemia and proteinuria during isolated kidney perfusion is accompanied by the release
of vascular [35S]heparan sulfate. Biochem Int 19: 1241–1251, 1989.
Vehaskari VM, Chang CT, Stevens JK, Robson AM. The effects
of polycations on vascular permeability in the rat. A proposed role
for charge sites. J Clin Invest 73: 1053–1061, 1984.
Venturoli D, Rippe B. Ficoll and dextran vs. globular proteins as
probes for testing glomerular permselectivity: effects of molecular
size, shape, charge, deformability. Am J Physiol Renal Physiol 288:
F605–F613, 2005.
Verhave JC, Hillege HL, Burgerhof JG, Navis G, de Zeeuw D,
de Jong PE. Cardiovascular risk factors are differently associated
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