ing. A mechanistic link between an inherited and an
acquired cardiac arrhythmia: HERG encodes the IKr potassium channel. Cell 81: 299–307, 1995.
11. Sanguinetti, M. C., and M. T. Keating. Role of delayed
rectifier potassium channels in cardiac repolarization
and arrhythmias. News Physiol. Sci. 12: 152–157, 1997.
12. Shibasaki, T. Conductance and kinetics of delayed rectifier potassium channels in nodal cells of the rabbit heart.
J. Physiol. (Lond.) 387: 227–250, 1987.
13. Spinelli, W., I. F. Moubarak, R. W. Parsons, and T. J.
Colatsky. Cellular electrophysiology of WAY-123,398,
a new class III antiarrhythmic agent: specificity of IK
block and lack of reverse use dependence in cat ventricular myocytes. Cardiovasc. Res. 27: 1580–1591,
1993.
14. Warmke, J. W., and B. Ganetzky. A family of potassium
channel genes related to eag in Drosophila and mammals. Proc. Natl. Acad. Sci. USA 91: 3438–3442, 1994.
15. Weinsberg, F., C. K. Bauer, and J. R. Schwarz. The class
III antiarrhythmic agent E-4031 selectively blocks the
inactivating inward-rectifying potassium current in rat
anterior pituitary tumor cells (GH3/B6 cells). Pflügers
Arch. 434: 1–10, 1997.
Probing Nanometer Structures with
Atomic Force Microscopy
Zhifeng Shao
Atomic force microscopy (AFM) can generate high-resolution images of the
surface of biological specimens and can also probe the interactions between
and within single macromolecules. Thus isolated heterogeneous biological
structures can be studied in submolecular detail with AFM.
L
“. . .AFM detects
deflections of a very
soft cantilever with a
sharp tip. . . .”
ight microscopy (LM) and electron
microscopy (EM) have become essential tools
in biology over the past decades. In fact, most of
our knowledge about the structure of cells and
organelles has been obtained with these wellestablished techniques. However, it is well
known that the resolution of LM is limited by diffraction and is therefore not appropriate for structural studies at the molecular level. EM, on the
other hand, can achieve atomic resolution, but
the intrinsic contrast of biological materials is so
low that either contrast-enhancing agents (i.e.,
negative stains or metal shadowing) or image
averaging (cryo-EM) is often required. Such fundamental limitations are not expected to be circumvented in the near future. However, a recent
and very exciting development, atomic force
Z. Shao is in the Department of Molecular Physiology and
Biological Physics of the University of Virginia, PO Box
10011, Charlottesville, VA 22906-0011, USA.
142
News Physiol. Sci. • Volume 14 • August 1999
microscopy (AFM), has demonstrated great
potential (1) as a complementary technique to
these traditional methods, which is not limited by
the same fundamental physical laws.
Different from any other microscopic technique, AFM detects deflections of a very soft cantilever with a sharp tip that is in physical contact
with the specimen while it raster scans across the
surface (Fig. 1). As such, its resolution will be
determined by the sharpness of the tip apex and
the deformation of the specimen under the tip
pressure (12). Neither of these factors is a fundamental limitation, and they will be continuously
improved over time. For hard, crystalline specimens, it is not difficult to obtain atomic scale features, even under ambient conditions (Fig. 1,
background). With biological macromolecules,
when reliably adsorbed to a flat substrate, a basic
requirement of this technique, the AFM has also
achieved considerable success either in aqueous
solution or under cryogenic temperatures (for
0886-1714/99 5.00 © 1999 Int. Union Physiol. Sci./Am.Physiol. Soc.
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4. Bauer, C. K., B. Engeland, I. Wulfsen, J. Ludwig, O.
Pongs, and J. R. Schwarz. RERG is a molecular correlate
of the inward-rectifying K current in clonal rat pituitary
cells. Receptors Channels 6: 19–29, 1998.
5. Bauer, C. K., W. Meyerhof, and J. R. Schwarz. An inwardrectifying K+ current in clonal rat pituitary cells and its
modulation by thyrotrophin-releasing hormone. J. Physiol. (Lond.) 429: 169–189, 1990.
6. Chiesa, N., B. Rosati, A. Arcangeli, M. Olivotto, and E.
Wanke. A novel role for HERG K+ channels: spike-frequency adaptation. J. Physiol. (Lond.) 501: 313–318,
1997.
7. Corrette, B. J., C. K. Bauer, and J. R. Schwarz. Electrophysiology of anterior pituitary cells. In: The Electrophysiology of Neuroendocrine Cells, edited by H. Scherübl
and J. Hescheler. Boca Raton, FL: CRC, 1995, p.
101–143.
8. Jan, L. Y., and Y. N. Jan. Cloned potassium channels from
eukaryotes and prokaryotes. Annu. Rev. Neurosci. 20:
91–123, 1997.
9. Pongs, O. Molecular biology of voltage-dependent potassium channels. Physiol. Rev. 72: S69–S88, 1992.
10. Sanguinetti, M. C., C. Jiang, M. E. Curran, and M. T. Keat-
review, see Ref. 12). More recently, AFM has
been extended to probe inter- and intramolecular interactions, taking advantage of AFM as a
sensitive force sensor (3, 11). The purpose of this
article is to provide a brief review of the recent
achievements of biological AFM with several
examples to illustrate possible utilities of this
novel technique.
Molecular details have been resolved by
AFM under physiological solutions
AFM has been successfully applied to both soluble proteins and membrane proteins under
aqueous solutions (12). In some cases, the attainable resolution has been sufficient to resolve
some secondary structures on the surface of a
protein (9). One example is the cochaperonin
GroES, a heptameric 70-kDa protein from
Escherichia coli, as shown in Fig. 2A (9). The molecules are directly adsorbed to the surface of
mica. The seven spokes seen on top of each
GroES are formed by single β-turns, directly
resolved without image averaging. This perhaps
represents one of the highest-resolution structures
News Physiol. Sci. • Volume 14 • August 1999
“. . .the presence of
the lipid bilayer
provides an additional
stabilizing factor. . . .”
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FIGURE 1. In atomic force microscopy (AFM), a focused laser
beam is used to monitor the movement of the cantilever,
which can be controlled with a feedback circuit to control the
piezo scanner. The critical components are shown here: laser
diode (1), focusing lens (2), cantilever with tip (3), split diode
(4), specimen (5), and piezo scanner (6). When the bending
of the cantilever is maintained as a constant, the surface
topology is obtained (equiforce surface contour). With crystalline specimens, atomic resolution is not difficult to obtain,
as shown in the background, where a mica image is presented (lattice spacing: ~5Å). Because the cantilever is also a
very sensitive force sensor, it is also used to measure molecular interactions by monitoring the bending of the cantilever
when the tip is pulled away from the surface. To image biological materials, macromolecules must be adsorbed to a flat
substrate (mostly mica) with high stability. Otherwise, the
molecules can be easily swept away by the scanning tip.
so far resolved by AFM. All of the dimensions
measured in the AFM are in very good agreement
with the X-ray structure. One of the critical factors
found for attaining such high resolution is close
packing. Because even 1-nN probe force can produce a pressure of >1,000 atm within the tip-sample contact for an area of 1 nm2 (12), it is conceivable that when closely packed, not only can
the neighboring molecules stabilize the molecule
being imaged, but they can also help to reduce
the contact pressure to some degree. Chemical
crosslinking, in some instances, has also been
effective for achieving a higher resolution (9), suggesting that structural deformation caused by the
probe may still be greater than desired. Some
investigators have taken advantage of this greater
force to dissect macromolecules, such as the gap
junctions (6) and the chaperonin GroEL (9),
exposing their internal structures for imaging, an
aspect rather unique to the AFM.
Obviously, two-dimensional (2-D) crystals are
perhaps the most closely packed specimens, so
they might be expected to be ideal for AFM
imaging. Shown in Fig. 2B is an image of the
bacteriophage φ29 connector, through which the
virus genome is delivered into the host cell (10).
The connector itself has a tapered shape with 12
subunits resolved in the wide end, and 2 such
connectors form the unit cell in an antiparallel
arrangement. The dimensions, including the
height of the connector, the central channel, and
the diameter of both ends, have been determined
to nanometer precision. These results have been
used to refine the previous model of the connector and to help resolve the controversy over the
exact number of subunits.
For membrane proteins, the presence of the
lipid bilayer provides an additional stabilizing
factor, which has enabled several high-resolution
studies. One example is the cholera toxin B
oligomer (Fig. 2C) bound to its membrane receptor, the ganglioside Gm1 (14). The pentameric
structure of this 60-kDa toxin is well resolved. In
this case, because the binding affinity is so high
(pM range), the specimen was quite stable and a
substantial force (~1 nN) was required to remove
the toxin from its receptor. More recently, the
AFM has been successfully extended to the study
of several pore-forming bacterial toxins. An
example is shown in Fig. 2D, where an image of
α-hemolysin (α-toxin) from Staphylococcus
aureus, inserted into a pure lipid membrane, is
presented. In this supported bilayer, α-toxin readily formed these 2-D crystallites, and within each
oligomer, six subunits are clearly resolved (2).
These results differ from the heptameric X-ray
structure where the crystallization was achieved
with a detergent mixture. Even though this
143
“. . .one can directly
resolve the number of
subunits within
individual
oligomers. . . .”
144
important difference has not been reconciled,
this AFM study does demonstrate the relevance
of resolving molecular details under more natural conditions. It should be pointed out that the
α-toxin has long been considered a hexamer, but
the averaging techniques employed to arrive at
this conclusion were often under close scrutiny.
With the high contrast attainable in the AFM, this
is no longer a problem, as one can directly
resolve the number of subunits within individual
oligomers (Fig. 2D). Similar techniques have also
been applied to other bacterial toxins, including
the cholesterol-binding toxin perfringolysin, the
pore-forming aerolysin, and the vacuolating
News Physiol. Sci. • Volume 14 • August 1999
toxin vacA from Helicobacter pylori, implicated
as a major determinant of peptic ulcers.
At present, a major difficulty has been the
inability to reconstitute integral membrane proteins into supported phospholipid bilayers at very
high density, but 2-D crystalline sheets developed for EM should be equally suitable for the
AFM as demonstrated by Fig. 2B. With advances
of preparatory techniques, channels and transporters should be amenable to AFM, and the different conformations may be accessible to analysis at the nanometer scale.
In addition to imaging proteins, a noteworthy
application is the imaging of plasmid DNA
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FIGURE 2. Several representative examples of high-resolution AFM achieved under aqueous solution. A: a surface rendition of
the cochaperonin GroES (70 kDa) from E. coli. This structure is identical to that of the X-ray model, and a comparison indicates
that the 7 spokes at the center are composed of single β-turns, suggesting that secondary structural features might be resolvable
under most favorable conditions. Chemical crosslinking is shown to improve the resolution to some degree. Scale bar: 5 nm. B:
a surface rendition of the bacteriophage φ29 connector in a 2-dimensional (2-D) crystalline sheet. The connector is seen in an
antiparallel arrangement. Within each connector, individual subunits are resolved. Such 2-D crystals are also amenable to most
image processing routines to improve the fidelity of the results. Scale bar: 10 nm (x, y); 6 nm (z) (courtesy of Dr. D. Mueller, University of Basel). C: cholera toxin B oligomer (60 kDa) bound to its receptor Gm1 in a lipid bilayer. Such membrane-associated
proteins should be among the easiest for the AFM to study, and the technique has already been extended to study other proteins,
either naturally membrane-associating proteins or those through specific modifications. Scale bar: 5 nm (x, y); 2.5 nm (z). D: αhemolysin from S. aureus in a supported bilayer. This pore-forming toxin can insert into supported bilayers to form small 2-D
crystallites. However, both the subunit stoichiometry and the overall diameter revealed by AFM are different from the X-ray structure. The reconciliation of these differences may further our understanding of the oligomerization process of membrane proteins.
Scale bar: 10 nm (x, y); 1.5 nm (z).
adsorbed to a positively charged lipid bilayer (8).
Surprisingly, even with only partial charge neutralization, the supercoiled plasmids adopted a
highly condensed phase with quasi-parallel
packing on the bilayer surface. The stabilizing
effect of such close packing has enabled the
direct resolution of the major grooves of the
DNA double helix. This highly condensed phase
was later corroborated by X-ray studies of multilamellar aggregates, and such highly ordered
arrangements may be relevant to the efficiency of
gene delivery via cationic liposomes.
Because AFM is capable of imaging in aqueous solution, an exciting possibility is to capture
structural (conformational) changes in real time,
which would permit the making of “movies” of
macromolecules in action. Although this worthy
goal has not been fully achieved because of the
potential disturbance of the structure being studied by the tip and the relatively slow frame time
of the instrument (seconds), encouraging
progress has been made in the past few years,
indicating that at least under certain conditions,
such experiments can be successful. A simpler
but less biologically relevant example is shown
in Fig. 3a, where two successive frames depict
the progression of the ripple phase formation in
a gel-phase supported phosphatidylcholine
bilayer. In this case, the process takes many
hours to complete, perhaps because of the interaction with the substrate (mica), which has significantly slowed down the transition. A more
exciting example is the observation of the motion
and rearrangement of the actin cytoskeleton in
living glial cells (Fig. 3b). At the periphery of the
cell, the submembrane cytoskeleton can be
resolved as the AFM tip scans over the surface of
the cell, and the movement of the actin filaments
is clearly seen in these frames taken a few minutes apart (5). This and other similar studies show
that the cells remain viable under the probing tip
for many hours, and laser irradiation does not
seem to pose a serious problem for the cell.
However, these studies also show that the cell
membrane is much too compliant for the AFM,
and severe deformation often occurs during
imaging. As a result, the resolution achieved has
been relatively low, and membrane components,
such as individual receptors or transporters, have
not been resolved. Whether this can be significantly improved remains to be seen.
At the molecular level, a significant development is the observation of RNA polymerase transcribing a piece of DNA (7). This is an extremely
“. . .the submembrane
cytoskeleton can be
resolved. . . .”
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Structural dynamics may be accessible to
AFM observations
difficult experiment, because to retain the activity of the enzyme, the adhesion to the substrate
must be weak. As a result, the molecules could
be easily scraped away by the scanning tip. To
overcome this problem, a novel imaging mode,
the tapping mode, in which the tip oscillates at a
relatively high frequency and only makes transient contact with the sample in each cycle, must
be used. As shown in Fig. 3c, the movement of
the RNA polymerase is clearly captured in these
successive frames. This is certainly a major technical advance, although at this stage the resolution is relatively low and the enzymatic activity
much reduced. Thus major improvements are
still required before this type of application can
yield useful new information.
Flexible structures are well resolved in the
cryo-AFM at high resolution
Despite the recent success, a major limitation
remains: the deformation of flexible structures
under the AFM tip. As many studies attest, large
molecular complexes, from immunoglobulins to
the membrane of cells, are too soft for high-resolution imaging at room temperature. To overcome these problems, several groups attempted
to build AFMs that could image at cryogenic
temperatures. However, because of technical
complications and especially the problem of protecting the specimen from contamination
(because of condensation of water and other
contaminants onto the specimen), this goal was
only recently achieved with a system operated in
liquid nitrogen vapor under ambient pressure (4).
At a temperature close to that of liquid nitrogen,
the structural rigidity of most macromolecules
has been shown to be much greater, and the
adhesion to the substrate is also improved. Most
importantly, the trapping effect of the liquid
nitrogen pool in the system has effectively eliminated surface contamination. Combined with a
stable temperature control and high mechanical
stability, the cryo-AFM has achieved the same
resolution as that at room temperature, and
imaging of biological structures has been highly
reproducible (4).
Several interesting applications of the cryoAFM are shown in Fig. 4. Figure 4a is a stereograph of purified immunoglobulin M (IgM),
adsorbed to mica. The specimen was dehydrated
before freezing, but no other treatment was
applied. Although the general dimensions are
consistent with those observed by EM with negatively stained specimens, which has served as
the basis for the current model of IgM, the threedimensional (3-D) structure of IgM as seen in the
cryo-AFM clearly indicates that IgM has a nonNews Physiol. Sci. • Volume 14 • August 1999
145
planar conformation with the Fc domains (center) protruding out from the plane of the Fab
domains, in contrast to the previous planar
model. How to accommodate a nonplanar IgM
into the current molecular model of complement activation (classical pathway) remains to
be worked out. Figure 4b shows an image of
smooth muscle myosin (15). Of particular interest is the ability to clearly resolve the regulatory
and motor domains within each individual
myosin head, where the dimensions agree with
146
News Physiol. Sci. • Volume 14 • August 1999
the crystal structure of S1 solved by Rayment
and colleagues. This result raises the possibility
that when intact myosin interacts with filamentous actin, which has also been imaged at high
resolution revealing the actin monomers, conformational changes within the myosin head
under various conditions may be directly
observable, given the 4- to10-nm movement of
the cross-bridge stroke suggested by most models. Without the need for averaging over an
ensemble of molecules, heterogeneous speci-
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FIGURE 3. Examples of real-time observation of dynamic processes with the AFM. a: With a supported gel-phase phosphatidylcholine bilayer, the so-called ripple phase can be induced with external agents or temperature. In these images taken
some minutes apart, the progression of the ripples is clearly depicted. Such observations may provide a more complete description of this interesting but not fully understood phenomenon. b: With living glial cells, the submembrane skeletal structures have
been observed. Toward the edge of the cell where the thickness is much smaller than that near the nucleus, the
movement/rearrangement of the actin filaments can be seen in these images taken a few minutes apart (courtesy of Dr. E. Henderson, Iowa State University). c: With a carefully controlled adhesion to the substrate, the transcription process of E. coli RNA
polymerase was also observed in real time (transcription rate was much lower when enzyme was adsorbed to substrate). Note
the change in length of the DNA on either side of the enzyme in these images taken ~4 min apart (courtesy of Drs. P. K. Hansma,
B. Smith, N. Thomson, and S. Kasas, University of California, Santa Barbara).
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FIGURE 4. Several applications of cryo-AFM. a: A stereo view of purified human immunoglobulin M (IgM) adsorbed to mica.
Note the central protrusion, suggesting that the Fc domains and the J chain are not in the same plane as the Fab domains. In
other words, each immunoglobulin G (IgG)-like subunit is bent. The implications of this structure on the molecular model of
complement activation (classical pathway) have not been examined. b: Smooth muscle myosin purified from turkey gizzard.
Note the excellent contrast of the image and the well-defined molecular contours. Within each myosin head, the regulatory
domains can be recognized from the motor domain (right; scale bar: 20 nm). This raises the possibility of directly observing the
structural changes during contraction. c: Chromatin fibers purified from chicken erythrocytes. Each nucleosome (~12-15 nm) is
well resolved, along with the linker DNA between the nucleosomes. Given the resolution, other components, if present, such
as a transcribing RNA polymerase or transcription factor complexes, should be resolvable.
mens, characteristic of the weakly bound states,
although challenging, should remain amenable
to analysis. Such studies have the potential to
provide additional insight into the working
mechanism of muscle contraction. Another
interesting example is purified chromatin fragNews Physiol. Sci. • Volume 14 • August 1999
147
“. . .using modified
nanotubes as the tip
to sense the chemical
interaction. . . .”
Probing ligand receptor interaction and
protein folding
Because the AFM is also a very sensitive force
sensor when soft cantilevers (k ,
, 0.01 nN/nm)
are used, it has been used to probe molecular
interactions between single molecules. Although
the sensitivity of the optical detection system has
the ability to detect sub-piconewton forces, the
thermal fluctuation of the cantilever has limited
this measurement to the 10 pN range. The possibility of reducing the limiting thermal noise is currently under investigation in several laboratories.
Force measurements can be conducted with
relative ease with simple modifications to the tip.
For example, with biotin attached to a tip and
streptavidin immobilized on a substrate, the
force required to pull the two apart has been
measured to be on the order of several hundred
piconewtons (3). This is a relatively large force,
but given the extremely high affinity of biotin to
streptavidin, this magnitude of force is not surprising. However, it should be pointed out that it
148
News Physiol. Sci. • Volume 14 • August 1999
has been difficult to directly link rupture force to
the binding free energy, because during the rupture process the AFM has been probing nonequilibrium states. Several solutions to this difficulty
have been suggested but are beyond the scope of
this article.
A further extension of this technique is to
observe the unfolding of multi-domain fibrous
proteins, such as the giant muscle protein titin
(11). With one end of the protein attached to the
tip and the other to the substrate, upon retraction of the tip, the unfolding of each Ig-like
domain was observed in real time. It was shown
that before unfolding, each domain behaves
elastically, but at the threshold, it unfolds to an
extended state of 25 nm in length. The threshold
force was shown to depend on the pulling
speed, clearly indicating the nonequilibrium
nature of these measurements. So far, the opposite, but more interesting, process of folding of a
protein has not been directly observed because
of technical limitations. An improved force sensitivity and precise positional control would be
required for achieving such a goal. Even for a
partial construction of the energy landscape, a
detailed description of the folding pathway must
be obtained.
The recent demonstration of using modified
nanotubes as the tip to sense the chemical interaction between the tip and the sample should
have great potential to provide a high spatial resolution map of the specimen surface (13).
Because in this case, the dimension of the tip is
well controlled and only the end of the carbon
nanotube is functionalized, the reproducibility of
such experiments should improve and the lifetime of the tip should extend. The latter is
required if it is to be useful for imaging purposes.
On the basis of these latest developments, it is
almost certain that in the near future, one should
be able to attach any molecule to the tip as the
sensor to “fish” out its partner in the cell membrane, thus addressing such fundamental issues
as domain organization, receptor localization,
and functional redistribution. Another recent
report suggests that surface charges on a biological specimen may be directly mapped at severalnanometer resolution with the tapping-mode
AFM under aqueous solutions at low ionic
strength, raising the possibility of directly resolving an active ion channel in a membrane.
It is not an exaggeration to consider the development of the AFM, as well as other related probe
techniques, as the most exciting advance in the
field of biological microscopy in recent years. Its
limitations notwithstanding, AFM does provide a
most direct approach for structural studies of biological surfaces and for probing into the details of
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ments, as shown in Fig. 4c. In this image, each
individual nucleosome is well resolved and
appears to have a rather random orientation
with a lateral dimension similar to those determined by EM and X-ray crystallography. Furthermore, almost all linker DNA segments can be
recognized, providing a direct measure of their
length, a critical parameter in the construction
of a packing model for the chromatin, the
higher-order structure of which remains unresolved. With a closer examination of the images,
it is suggestive that the linker histones might
have been resolved. If so, its effect on the structure of chromatin could be directly examined.
A major developmental effort is the combination of the cryo-AFM with the well-established
technique of deep etch and freeze fracture,
because at present specimens are dehydrated
before imaging. Rapid freezing should protect
biological structures from degradation or prevent
dissociation of oligomeric proteins during cooling, as well as trapping intermediate states of a
complex. With better-preserved specimens, it is
also possible to further improve the resolution of
the cryo-AFM. Furthermore, fracture will allow
the elucidation of the integral membrane segments of a protein, providing a new technique
for determining its membrane topology. An
extension of the fracture technique is to combine
etch and sectioned removal of the specimen surface. If well controlled, this will allow the 3-D
imaging of large structures at nanometer resolution, which could not be achieved with any
available technology.
macromolecular interactions. With the rapid
technical advance of this novel technology, there
is no doubt that many exciting discoveries will be
uncovered in the years to come, and the AFM will
soon establish itself as one of the widely applied
techniques in biomedical research. We have not
yet reached the limit of its potential.
References
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2. Czajkowsky, D., S. Sheng, and Z. Shao. Staphylococcal
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3. Florin, E., V. Moy, and H. Gaub. Adhesion forces
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5. Henderson, E., P. Hayelon, and D. Sakaguchi. Actin filament dynamics in living glial cells imaged by atomic
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I thank Prof. A. V. Somlyo and D. Czajkowsky for a critical
reading of the manuscript. I also thank Drs. D. Mueller, E.
Henderson, P. K. Hansma, B. Smith, N. Thomson, and S.
Kasas for the use of their images and S. Sheng for the preparation of the figures.
The work from this laboratory was supported by the
National Institutes of Health, the National Science Foundation, and the American Heart Association.
I apologize for not being able to include many other
excellent references because of strict space limitations.
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Hepatic Regeneration—Revisiting the
Myth of Prometheus
Victor Ankoma-Sey
Myriad signals such as growth factors, cytokines, growth inhibitors, hormones,
ions, extracellular matrix, and the resident hepatic cells are involved in the
regulation of hepatic regeneration. These regulatory factors ultimately mediate
changes in gene expression, a critical step in this well-orchestrated
restorative process.
T
he liver is a remarkable organ, given its inherent capacity to fully restore itself after significant hepatic tissue loss either from partial hepatectomy (PHx) or acute liver injury. The tremendous regenerative potential of the liver has been
recognized since ancient times. In classical
Greek mythology, Prometheus, after stealing the
V. Ankoma-Sey is in the Division of Gastroenterology, Hepatology, and Nutrition at the University of Texas-Houston
Medical School, 6431 Fannin, Houston, TX 77030, USA.
0886-1714/99 5.00 © 1999 Int. Union Physiol. Sci./Am.Physiol. Soc.
secret of fire and introducing it to earthlings, was
punished by having an eagle of Zeus feast daily
on his liver. His punishment was the ultimate torture, as his liver regenerated eternally while the
eagle continued his perpetual daily feeding sessions from a constantly replenished source.
The classical model of hepatic regeneration is
that of partial hepatectomy in which ~70% of the
liver is resected. The remaining lobes enlarge and
reconstitute the original size of the liver. Hepatic
regeneration after PHx in the rat takes 5-7 days.
News Physiol. Sci. • Volume 14 • August 1999
“. . .having an eagle
of Zeus feast daily on
his liver.”
149