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Blood Cells, Molecules, and Diseases 33 (2004) 211 – 215
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Mesenchymal stem cells: harnessing cell plasticity to tissue
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and organ repair
Dov Zipori*
Department of Molecular Cell Biology, Weizmann Institute of Science, Rehovot, 76100, Israel
Submitted 2 July 2004
Available online 23 September 2004
(Communicated by M. Litchman, M.D., 3 August 04)
Abstract
Plastic behavior of cells is a hallmark of embryonic development. The emergence of primary mesenchyme from within the inner cell mass
entails the first epithelial–mesenchymal transition step that is then followed by sequential transitions; the formation of new tissues and organs
requires transitions from mesenchyme into epithelium and vice versa. Although it is currently believed that in the adult such transitions do not
persist, the frequent occurrence of mesenchymal stem cells (MSCs) in various tissues of the adult organisms, and the reported plasticity of
such adult mesenchymal cells, raises the question as to whether the frequency of mesenchymal epithelial transitions in the adult have been
underestimated. Indeed, adult mesenchymal stem cells have been reported to differentiate in culture into a multitude of mature cell types
including epithelial cells. This opens the way to the use of these stem cells for the construction of new tissues and organs for therapeutic
purposes, but the question is still open as to whether mesenchymal stem cells transdifferentiate also in vivo. The molecular mechanism that
underlies the plasticity of mesenchymal stem cells and their capacity to transdifferentiate is unresolved. We found that these cells have a
promiscuous gene expression pattern; mesenchymal cells, whether primary or cloned cell lines, express T cell receptor (TCR) h and a genes,
along with other components of the TCR complex. These cells may therefore be in a standby state, in which many gene families are
expressed at a low level thereby making the cell readily capable of shifting fates.
D 2004 Elsevier Inc. All rights reserved.
Keywords: Mesenchymal stem cells; Cell plasticity; Tissue and organ repair
Introduction
The mesenchyme has long been recognized as being
much more than just bconnective tissueQ generating cell
population. Carcinomas, which are the most common tumor
cells, are epithelial derivatives, but obviously the mesenchymal stroma of the tumor is just as important for tumor
development. Yet studies on carcinogenesis and tumor
biology focus on the tumor cells while the properties of the
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This paper is based upon a presentation at a Focused Workshop on
Haploidentical Stem Cell Transplantation sponsored by the Leukemia and
Lymphoma Society held in Naples Italy from July 8–10, 2004.
* Fax: +972 8 9344125.
E-mail address: [email protected].
1079-9796/$ - see front matter D 2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.bcmd.2004.08.019
mesenchyme were set aside until very recently, when it
began to be realized that the tumor modifies its supportive
stroma. It appears that the reason for neglecting the stromal
component of tumors, and that of normal tissues, is not the
lack of understanding on the investigators’ side as to the
importance of mesenchyme. Rather, the properties of
epithelial cells are relatively simple to study since they
are fixed cells that express distinct markers, such as
cadherins, molecules that engage with the formation of
epithelial tissue and in the fixation of the cells to their
residence sites. By contrast, mesenchymal cells are motile,
occur in a multitude of variations, and do not express welldefined markers. In fact, even to date it is not a simple task
to define a mesenchymal cell and normally the definition is
based on discrimination from well-characterized cell types,
such as epithelial cells. Indeed, in the bone marrow,
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D. Zipori / Blood Cells, Molecules, and Diseases 33 (2004) 211–215
hemopoietic stem cells (HSC) have been recognized along
with mesenchymal components that make the stroma.
Despite the almost concomitant discovery, the characterization of HSC populations proceeded in a relatively fast
mode using a variety of cell markers such as Sca-1, ckit,
etc. To date, it is possible to delineate HSC populations
according to their differentiation stage using a series of
markers. On the other hand, the characterization of
mesenchymal stem cells and their descendents is deficient
up to this point. A variety of stem cell types such as
mesenchymal stem cells (MSCs), multipotential adult
progenitor cells (MAPC), etc., were described (see below).
The mode by which these populations are related and
whether they represent steps of increasing differentiation
within a descending cascade is completely unclear. One
well-studied function of the bone marrow mesenchyme is
the capacity to promote long-term hemopoiesis and stem
cell renewal. Hemopoietic cell growth and differentiation
within the microenvironment is dependent upon interactions
of the HSC with the bone marrow stroma, which is a
complex tissue, composed of a number of vascular and
connective tissue cell types. The stroma was found to
elaborate cell surface and secreted signaling molecules that
account, at least partially, for the ability of the stroma to
regulate hemopoiesis [1–3]. This in vivo dependence of
hemopoietic cells, and HSC in particular, on stromal cell
signaling is well simulated by long-term bone marrow
cultures which are based on the use of stromal cells, as a
stem cell supportive adherent layer, providing conditions
for long-term progenitor cell proliferation associated with
long-term myelopoiesis and erythropoiesis [4,5]. Yet, what
are the specific properties of mesenchymal cells that support
hematopoiesis versus those that do not? Are stromal cells
supportive of hemopoiesis an end stage of differentiation
starting from a mesenchymal stem cell? This is not at all
clear. It is therefore the poor characterization of the
mesenchymal phenotype and particularly the mesenchymal
stem cell phenotype that causes the retardation of research.
The following discussion relates to some of the major
aspects of contemporary research relating to the mesenchyme: Firstly, I describe herein some mesenchymal
populations. Secondly, the properties of mesenchymal stem
cells that make them an appropriate tool for tissue and
organ repair are discussed. Thirdly, a possible molecular
basis of mesenchymal cellular plasticity is presented and
finally, the mode by which better characterization of these
cells may be achieved is suggested.
Mesenchymal cell populations
Friedenstien et al. [6] first described bone marrowderived fibroblastoid cell populations that could transfer the
hemopoietic environment of their tissue of origin into
ectopic sites. Thus, fibroblasts derived from the bone
marrow induced the formation of bone and bone marrow
at the site of implantation under the kidney capsule. This
pioneering study led to the identification of cultured stromal
cells from the bone marrow as a scaffold for the formation
of long-term hemopoietic activity and hemopoietic stem cell
renewal in culture. Furthermore, these studied first
delineated the osteogenic properties of these marrowderived mesenchymal cells. It was later understood that
these cells are in fact highly plastic and portray a variety of
phenotypes. In the early 1980s, we derived a series of cell
lines from mouse bone marrow. These cell lines were
characterized as adipocytes, endothelial-like cells, fibroblastoid cells, as well as others, with mix fibroendothelial
features [7,8]. Later studies confirmed this heterogeneity in
the phenotypes that may be derived from the bone marrow
mesenchyme. For many years it was known that MSCs,
which reside in the bone marrow, give rise to the mesodermderived components and differentiate in vitro to osteoblasts,
adipocytes, chondrocytes, and myocytes (Fig. 1). The
methods for isolation and culture of MSC vary. Many
investigators use plastic adherence as a major mode for
separation between the stromal component of the bone
marrow and the hemopoietic cells. The use of minimal
medium, which is not supplemented by cytokines, facilitated the growth of colony forming units-fibroblasts (CFUF) that proliferate to form macroscopic fibroblastoid
colonies in vitro. These can be further propagated by serial
passage to become cell strains [9,10] and eventually cell
lines [7,8]. The MSCs derived in this manner, whether
primary cultures of cell lines, often retain the capacity to
differentiate into several mesodermal directions, including
osteogenesis, adipogenesis, and may serve effectively in
supporting long-term hemopoiesis initiated by HSC [11]. A
second common approach is based on the isolation of
precursor populations by means of positive or negative
selections using cell surface markers [12–15]. One example
is a recent study indicating that human bone marrow MSC
can be directly selected by virtue of expression of CD49a,
the a1-integrin subunit of the very late antigen (VLA)-1,
which is a receptor for collagen and laminin [16]. This
population is CD49a+ CD45med/low and differentiates into
several mesodermal directions. All CFU-F found in human
bone marrow are included within the CD49a+ CD45med/low
fraction. In contrast to the above MSCs that are apparently
restricted to mesodermal lineages, Jiang et al. [17] described
the isolation and characterization of cells copurifying with
MSCs, termed MAPCs, that are by far more plastic than
previously ascribed to mesenchymal cells and differentiate
in vitro not only into mesodermal derivatives but also into
cells of visceral mesoderm, neuroectoderm, and endoderm
[18] (Fig. 1). When injected into an early blastocyst, single
MAPCs contribute to most, if not all, somatic cell type (Fig.
1, shown by an embryo contour). On transplantation into a
nonirradiated host, MAPCs engraft and differentiate into the
hematopoietic lineage, as well as into epithelium of liver,
lung, and gut. MAPCs proliferate extensively for more than
100 population doublings without obvious senescence or
loss of differentiation potential. Furthermore, MAPCs have
D. Zipori / Blood Cells, Molecules, and Diseases 33 (2004) 211–215
213
Fig. 1. The heterogeneity of mesenchymal cell populations: The bone marrow, as well as other organs, has been reported to be an ample source of mesenchymal
stem cells. These cells have been given different designations, mainly due the different modes used for their derivation and also due to differences in the
spectrum of differentiation directions they exhibit (arrows).
Fig. 2. Mesenchymal cells express TCR components: One possible interpretation of this finding is that this is part of a general phenomenon of promiscuous
gene expression pattern of MSCs that enables numerous potential differentiation directions. MSCs are thus in a standby state, ready to assume several pathways
of differentiation upon appropriate induction. The stem cell in panel (a) is shown to express many genes at low level (many small colored dotes). By contrast,
the mature cell in panel (b) expresses fewer genes but some are highly expressed (larger dotes). An additional possibility is that TCR is in fact functional in
mesenchymal cells and is involved in cell growth control (c) or may also be involved in cell-to-cell interactions (d).
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D. Zipori / Blood Cells, Molecules, and Diseases 33 (2004) 211–215
been shown to be isolated not only from different species
like human, mouse, and rat but also from different organs
like muscle and brain.
Apparently, the different modes of derivation of mesenchymal cells in vitro yield cells with divergent phenotypes
and different functional capacities. At the moment, it is
unclear whether these various MSC populations represent
separate entities that exist in vivo, or whether they emerge
upon in vitro culture.
Thus, mesenchymal cells have a highly plastic phenotype. Yet, do they form a differentiation cascade, similar to
the hemopoietic tree? Can one arrange the mesenchymal
population in a sequential order of increasing differentiation
stages? This author does not support this view. Conversely,
it is suggested that mesenchymal cells are in an unstable
state that enables them to shift from one position to the other
and to assume a dramatically different phenotype within a
relatively short time. The mode by which this is achieved is
discussed below, but before going into this issue let us first
consider how can the incredible plasticity of mesenchymal
stem cells be harnessed to clinical uses.
Advantages of adult mesenchymal stem cells as a tool for
cell and gene therapy
Several major obstacles stand in the way of use of
embryonic stem cells in human therapy. Although these
cells show a pluripotent differentiation capacity and have
been derived from humans and thus are available for human
experimentation, they pose several disadvantages: Primarily,
in the mouse, these cells have been found to cause the
formation of teratomas and teratocarcinomas, even when
induced to differentiate, due to few residual cells that remain
as stem cells. Secondly, but not of less importance, is the
fact that the embryonic stem cell lines are derived from
individual embryos and carry the genetic characteristics of
the donor. They are recognized as foreign by the recipient
and may be rejected. Alternatively, they may give rise to
lymphoid progeny that will recognize the recipient as
foreign and induce damage. Finally, these cells are rare.
All these three reservations do not exist in the context of
adult mesenchymal stem cells. Indeed, there are no reports,
thus far, on the formation of tumors resulting from adult
MSCs. These cells are not very immunogenic and furthermore possess a capacity to suppress immune responses. It
was reported recently that haploidentical mesenchymal stem
cells abrogate severe acute graft-versus-host disease in a
human patient [19]. We have found previously, that
mesenchymal cells suppress the generation of B lineage
cells by elaborating activin A, a transforming growth factor
h family member [20,21]. This may be part of the
mechanism through which the mesenchyme down regulates
the immune response. Since these cells are found in the
adult and may be derived from a variety of tissue and organ
sources, they are rather abundant [22,23]. An additional
property of adult MSCs is their ability to suppress tumor
growth and to serve as a gene therapy vehicle [24].
Obviously, these properties make adult stem cells favorable
candidates for use in human therapy.
The molecular basis for mesenchymal stem cell plasticity
As already implied above, one major question regarding
the mesenchymal phenotype is how do the cells manage to
change their phenotype so effectively. Several investigators
have shown that mesenchymal cells apparently express a
variety of gene products prior to their differentiation. We
have shown that mesenchymal cells express the T cell
receptor (TCR) gene [25]. This unexpected finding goes
along with the possibility that stem cells are expressing a
variety of gene families that characterize differentiated
progeny. In this respect, they are in a standby state, being
prepared to make a dramatic leap into a required direction, if
necessary. Thus, the transcription machinery is operating at
the low level but is not silenced. It is therefore enough that
the environment will signal one direction so that the cell
will, within a short while, be able to respond by commencing differentiation (Fig. 2a & b). One other possibility,
which is not contradictory to the above, is that the expressed
genes also encode, at a low level, the expression of
corresponding proteins. What could TCR proteins do in
mesenchymal cells though? We have found the that the
structure of TCR in mesenchymal cells is different from the
classical molecule found in T cells in that it lacks the V
region components due to the lack of recombinases in
mesenchymal cells. The mRNA molecule that is transcribed
is therefore a truncated form. Our experiments have shown
that mesenchymal cells may possess a TCR-like antigen
encoded by this transcript. The expression of this TCR
transcript was correlated to the growth properties of the cells
and to their potential to form tumors. It is possible therefore
that the expression of this form of TCR in the mesenchyme
is not only a part of the plasticity mechanism but in fact
TCR, as well as other expressed genes, may have additional
functions (Fig. 2c & d). This possibility is supported by the
findings that similar forms of TCR have been found in
neuronal cells [26,27] and because TCRg was found to
encode a protein, through an alternate reading frame, which
is specifically expressed in prostate cells [28].
Further trends in mesenchymal cell research and the
clinical applications
In view of the above, it is apparently essential to better
study the gene expression profile of mesenchymal cell
populations. This is not simple to do since, as explained
above, these cells are in constant transition and the methods
to be used with this cell type should be modified
accordingly. Once a clear mesenchymal fingerprint is
established, it would be possible to better understand the
molecular basis for cellular plasticity. This is clearly
important, from the theoretical point of view, but it is
D. Zipori / Blood Cells, Molecules, and Diseases 33 (2004) 211–215
equally important for practical purposes. Indeed, we are
limited to date in our capacity to control the fate of
mesenchymal cells. Better understanding of the plastic
nature of these cells will enable their direction to the
desired pathway and will create new tools for tissue and
organ replacement in human disease.
Acknowledgments
Supported by the Gabrielle Rich Center for Transplantation Biology and by research grants from the
Minerva Foundation, Germany, Mr. Michael Krasny,
Daniel and Rhonda Shapiro, Jerrald and Helene Wulff,
Dr. and Mrs. Murry Goldberg, and Isabelle and Leonard
Goldenson Assoc. DZ is an incumbent of the Joe and Celia
Weinstein Professorial Chair at the Weizmann Institute of
Science.
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