Nephrol Dial Transplant (2003) 18: 641–644
DOI: 10.1093/ndt/gfg028
Editorial Comments
Stem cells of the adult kidney: where are you from?
Takahito Ito
Department of Internal Medicine and Therapeutics, Division of Nephrology, Osaka University School of Medicine,
Suita, Japan
Keywords: bone marrow; kidney; side population; SP
cell; stem cell
Introduction
The structure of the kidney is the result of a
sophisticated interaction between several types of
immature cells. Research over the past few decades
has elucidated many of the processes underlying cell
proliferation, cell fate determination and differentiation. The discovery of genes related to nephrogenesis
has been of great help in understanding the sequence of
events in nephrogenesis. This information is available
at http:uugolgi.ana.ed.ac.ukukidhome.html [1]. Attempts
to identify further relevant genes are ongoing [2–4].
It has been a continuing challenge to identify the
stem cells involved in the genesis of this complicated
organ and to understand how they are regulated. The
difficulty begins with the definition of what is a stem
cell. Most definitions include as sine qua non requisites
the capacity of self-renewal and the potential to generate
several different types of differentiated progeny [5].
The criterion of self-renewal implies that when a stem
cell divides it can generate one new stem cell and one
descendant which further proliferates as a transitamplifying cell or a rapid-amplifying cell. This property is critical to maintain the pool of stem cells
during long periods or even during the life time.
In general, the differentiation potential of the
progeny of the most primitive stem cells is increasingly
restricted, while specific lineages emerge as the organ
develops. Although the kidney is a highly differentiated organ, a population of cells is present that
generates new cells either continuously or in response
to injury. Confusingly, these cells are often referred to
as stem cells because they are thought to constitute a
population of cells preserved from the early stage of
Correspondence and offprint requests to: Takahito Ito, MD,
Department of Internal Medicine and Therapeutics, Division of
Nephrology, Osaka University School of Medicine, Box A8, 2-2
Yamadaoka, Suita, Osaka 565-0871, Japan.
Email: [email protected]
#
nephrogenesis. If the repertoire of immature cells in the
adult kidney is restricted, so that only renal cells are
generated, they should be considered as progenitor
cells or committed stem cells rather than true stem
cells. In other words, cells that contribute to the repair
of the injured kidney are presumably distinct from the
stem cells which are involved in nephrogenesis.
Recently, a series of experiments has documented
that in different types of adult tissues, some cells can be
retrieved that are capable of generating an entire
spectrum of cells. For instance, in certain regions of the
adult brain, neurons are continuously replaced and
added by self-renewing neural stem cells. These cells
also generate non-neuronal cells including astrocytes,
oligodendrocytes, renal epithelial cells and skeletal
muscle cells [6–9]. Bone marrow-derived cells also
give rise to a wide range of non-haematopoietic cells
including renal cells in vivo and in vitro [10,11].
Leaving the self-renewal capacity for the moment,
these data suggest that immature cells in adult tissues
are truly multipotent.
Regulation of stem cells
When a stem cell encounters an environment which
provides direct interactions such as epithelial–
mesenchymal interactions and indirect communication
through secreting molecules such as cytokines, an
intrinsic differentiation programme of the cell is
triggered. To take the example of the kidney, this
organ is formed from the metanephrogenic mesenchyme. The ureteric bud arises from the progenitor
mesodermal field. As both the metanephrogenic
mesenchyme and the ureteric bud communicate with
each other in a dynamic reciprocal fashion, multipotent mesenchymal cells differentiate into various
types of renal epithelial or non-epithelial cells [12,13].
The cells in the metanephrogenic mesenchyme, however, are special in that the ureteric bud can branch
only when the bud enters a cross-talk with the
metanephrogenic mesenchyme. In contrast, non-renal
tissues, such as the neural tube, enable the metanephrogenic mesenchyme to form renal tubules [14]. It
2003 European Renal Association–European Dialysis and Transplant Association
642
is therefore conceivable that the metanephrogenic
mesenchymal cells are totally committed to form
renal tissue and are not true stem cells. Intriguingly,
neural cells exist along the border of the mesenchyme
in the early stages of nephrogenesis [15]. They may
be implicated in nephrogenesis since—as discussed
above—the neural tube induces formation of renal
tubules in metanephrogenic mesenchyme.
If we assume that true stem cells exist in the adult
kidney, it is necessary that the microenvironment
actively inhibits them from taking on unwanted fates.
It is assumed that neighbouring cells, so-called ‘niche
cells’, induce a state of dormancy in stem cells [16]. It
remains to be elucidated, however, whether such a
control system truly operates in the adult kidney.
Bone marrow as a reservoir
It is possible that fresh stem cells from other tissues are
recruited into the kidney. In 1998, Ferrari et al. [17]
demonstrated that following injury a proportion of
skeletal muscle fibres was actually formed by bone
marrow-derived cells. This was documented in chimeric animals following transplantation of genetically
marked bone marrow cells. Currently, it is widely
accepted that bone marrow-derived cells can give rise
to a wide range of differentiated cells such as hepatocytes, cardiomyocytes and smooth muscle cells. For
the adult kidney, Imasawa et al. [18] and Ito et al. [19]
independently reported that bone marrow-derived cells
have the potential to differentiate into glomerular
mesangial cells in mice or rats. Repopulating mesangial cells seem to enter the glomerulus through the
juxtaglomerular zone [19,20]. Bone marrow-derived
cells also produce renal epithelial cells in vivo and this
has also been found in humans [21–23]. The fact that a
single bone marrow-derived cell (or a fraction of
purified haematopoietic cells) can give rise to multiple
types of epithelial cells, strongly suggests that multipotent stem cells are present in the adult bone marrow
[21,24].
The bone marrow is a heterogeneous tissue comprising haematopoietic cells, macrophages, sinusoidal
endothelial cells, stromal cells and fat cells. Furthermore, there is a relevant type of stem cell in bone
marrow: mesenchymal stem cells, presumably related
to stromal fibroblastic cells, that provide biological
and physical support to haematopoietic stem cells in
the marrow. In the 1970s, Friedenstein et al. [25]
established a culture of fibroblastic adherent cells from
bone marrow. Later, Prockop [26] and Pittenger et al.
[27] found that the fibroblastic adherent cells include
mesenchymal stem cells which are capable of producing mesenchymal tissues such as bone, cartilage, fat
and smooth muscle. Such fibroblastic cells are often
identified with the marrow stromal cells. When bone
marrow is cultured using a different protocol, conditions can be found which support haematopoiesis
in vitro with high efficiency [28]. Iwatani et al. [29]
Nephrol Dial Transplant (2003) 18: Editorial Comments
successfully converted rat bone marrow-derived adherent cells into mesangial-like cells in vitro by using
platelet-derived growth factor-BB and collagen type
IV. In the context of the previous report on glomerular
remodelling [19], this would suggest that mesenchymal
stem cells in the adult bone marrow are capable of
producing mesangial cells in vivo, at least under
pathological conditions. It should be noted, however,
that a controversy continues as to whether marrow
stromal cells are transplantable or whether marrow
stromal cells circulate in blood just like mobilized
haematopoietic stem cells [30]. It is possible that there
is no strict borderline between haematopoietic stem
cells and mesenchymal stem cells, but that gradual
transitions exist. Recently, Reyes and Verfaille [31]
and Jiang et al. [32,33] established multipotent
adherent stem cells from human and rodent bone
marrow stromal cells. These cells have been named
multipotent adult progenitor cells (MAPCs). They
proliferate without showing signs of senescence in vitro.
They are able to differentiate not only into blood
cells, but also into mesodermal, neuroectodermal and
endodermal cells.
Collectively, the adult bone marrow seems to
harbour stem cells which contribute to the generation
of non-haematopoietic somite cells. Nevertheless, it
remains unknown whether the bone marrow-derived
stem cells have to be recruited de novo into the adult
kidney or whether they are present in the kidney in a
dormant state.
Side population (SP) cells
An intriguing method to purify haematopoietic stem
cells from the murine bone marrow was reported in
1996 [34]. This technique does not rely on surface
markers that are generally used to isolate haematopoietic stem cells, but on the differential efflux of a
vital DNA binding dye, Hoechst 33342. Because of its
unique fluorescence pattern during flow cytometry
analysis, the cells are referred to as side population
(SP) cells or as Hoechstlow cells. The SP cells, which are
mostly CD34-negative, are small and round in shape
and can be obtained from non-haematopoietic tissues
[35–37]. One of the multidrug-resistance genes, ATPbinding cassette superfamily G (white) member 2
(ABCG2), is mainly responsible for the dye efflux
[38]. Because SP cells can be obtained from almost all
species and adult tissues tested so far, and because they
are multipotent, it is assumed that the efflux of
Hoechst 33342 dye is a common trait of multipotent
tissue stem cells [36,37,39,40].
Iwatani et al. [41] purified SP cells from adult rat
kidney and tested their differentiation potentials.
When kidney-derived SP cells were administered by
intravenous injection into irradiated rats, the engrafted
cells contributed to hepatocytes, skeletal muscle cells
and haematopoietic lineages, but not to renal cells
including mesangial cells and renal epithelial cells. This
Nephrol Dial Transplant (2003) 18: Editorial Comments
was true even after inducing renal injury in the Thy1
glomerulonephritis model, where, following injury, no
transplanted SP cells lodged in the kidney. These
results imply that the SP cells in the adult rat kidney
are not engaged via renal tissue-specific interactions.
Obviously they are not committed to the kidney.
Additionally, it is unlikely that bone marrow-derived
mesangial cells are the progeny of SP cells in the
kidney. According to their data, at least some SP cells
in the kidney are derived from bone marrow.
SP cells represent a heterogeneous cell population.
Skeletal muscle contains two types of SP cells: CD45q
and CD45 . Expression of CD45 is an index of haematopoietic lineage. At present, it is known that CD45q
SP cells contribute to haematopoietic lineages, while
CD45 SP cells can differentiate into non-haematopoietic
lineages including skeletal muscle [37,40]. Regardless
of the expression of CD45, it remains unknown whether
SP cells in non-haematopoietic tissues are totally
derived from bone marrow, and whether they are
constitutively replaced.
Perspective
In 2002, Terada et al. [42] and Ying et al. [43] reported
that immature cells are capable of fusion with other
types of cells in vitro. After fusion, the immature cells
adopt the gene expression corresponding to a tetraploid (4n) cell [42,43]. If such fusion events occur in vivo,
the interpretation of experiments using transplantation of immature stem cells will become problematic.
For example, expression of a transgene in the skeletal
muscle might simply reflect cell fusion, and could not
be taken as evidence of differentiation from a stem
cell. In the case of skeletal muscle cells, fusion of nuclei is
not necessary to incorporate the transgene because
the skeletal muscle fibre is a multinucleated cell.
Despite recent progress in stem cell biology,
an important question remains unresolved, namely
whether each of the different cell lineages in the adult
kidney has its own progenitor cells or whether there
are universal stem cells residing within the kidney or
elsewhere capable of generating different renal cell
types. It is still unknown whether immature stem cells
have been set aside or not during nephrogenesis
for postnatal repair and remodelling of the kidney.
Nevertheless, for the clinician the exciting perspective
is provided that bone marrow-derived cells can be put
to therapeutic use to repair various types of injured
tissues, even though it remains unclear at present
whether they are physiological stem cells or not.
Conflict of interest statement. None declared.
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2003 European Renal Association–European Dialysis and Transplant Association