Regeneration of ischemic cardiac muscle and vascular endothelium by adult stem cells

Gastrointestinal stem cells

Turnover of the epithelial cell lineages within the gastrointestinal tract is a constant process, occurring every 2-7 days under normal homeostasis and increasing after damage. This process is regulated by multipotent stem cells, which give rise to all gastrointestinal epithelial cell lineages and can regenerate whole intestinal crypts and gastric glands. The stem cells of the gastrointestinal tract are as yet undefined, although it is generally agreed that they are located within a 'niche' in the intestinal crypts and gastric glands. Studies of allophenic tetraparental chimeric mice and targeted stem cell mutations suggest that a single stem cell undergoes asymmetrical division to produce an identical daughter cell, and thus replicate itself, and a committed progenitor cell which further differentiates into an adult epithelial cell type. The discovery of stem cell plasticity in many tissues, including the ability of transplanted bone marrow to transdifferentiate into intestinal subepithelial myofibroblasts, provides a potential use of bone marrow cells to deliver therapeutic genes to damaged tissues, for example, in treatment of mesenchymal diseases in the gastrointestinal tract, such as fibrosis and Crohn's disease. Studies are beginning to identify the molecular pathways that regulate stem cell proliferation and differentiation into adult gastrointestinal cell lineages, such as the Wnt and Notch/Delta signalling pathways, and the importance of mesenchymal-epithelial interactions in normal gastrointestinal epithelium and in development and disease.

A subpopulation of murine bone marrow cells fully differentiates along the myogenic pathway and participates in muscle repair in the mdx dystrophic mouse

Bone marrow (BM) transplantation in mice suggests the existence of pluripotent cells able to differentiate into skeletal muscle tissue, although sustained myofiber reconstitution has not yet been achieved. We investigated the myogenic potential of mouse BM cells and evaluated whether a BM fraction enriched for cells expressing skeletal muscle markers would ameliorate muscle repair, when compared to whole BM, into the dystrophic mdx mouse. We demonstrate that cells expressing striated-muscle-specific proteins are already present in the BM independently from experimentally forced myogenic conversion. We observed the presence of both markers of early myogenic program such as Pax3, Myf5, MyoD, desmin, and late myogenesis such as myosin heavy chain and alpha-sarcomeric actin. These myogenic cells are more represented in the early nonadherent BM fraction, which generates clones able to fully differentiate into myotubes. Transplantation in mdx mice by intravenous injection of whole BM and a tenfold BM myogenic enriched fraction resulted in BM reconstitution and limited dystrophin restoration. Taken together, these data show that a fraction of BM cells have a definite potential for differentiation along the skeletal muscle pathway and can be recruited by muscle repair mechanisms. They also indicate that factors limiting the degree of muscle recruitment and the host stem cell competition should be assessed in order to evaluate the usefulness of BM-derived myogenic cells into the context of cell-mediated gene therapy of inherited muscle diseases.

Vascular assembly in natural and engineered tissues

With the advent of molecular embryology and exploitation of genetic models systems, many genes necessary for normal blood vessel formation during early development have been identified. These genes include soluble effectors and their receptors, as well as components of cell-cell junctions and mediators of cell-matrix interactions. In vitro model systems (2-D and 3-D) to study paracrine and autocrine interactions of vascular cells and their progenitors have also been created. These systems are being combined to study the behavior of genetically altered cells to dissect and define the cellular role(s) of specific genes and gene families in directing the migration, proliferation, and differentiation needed for blood vessel assembly. It is clear that a complex spatial and temporal interplay of signals, including both genetic and environmental, modulates the assembly process. The development of real-time imaging and image analysis will enable us to gain further insights into this process. Collaborative efforts among vascular biologists, biomedical engineers, mathematicians, and physicists will allow us to bridge the gap between understanding vessel assembly in vivo and assembling vessels ex vivo.

Plasticity of marrow-derived stem cells

Many exciting discoveries reported over the past 3 years have caused us to expand the paradigm for understanding somatic stem cell plasticity. Within adult organs, there are not only specific stem cells that are capable of producing functional cells of one organ system, but also cells with the flexibility to differentiate into multiple other cell types. In the bone marrow, for example, in addition to hematopoietic stem cells and supportive stromal cells, there are cells with the potential to differentiate into mature cells of the heart, liver, kidney, lungs, GI tract, skin, bone, muscle, cartilage, fat, endothelium and brain. A subpopulation of cells in the brain can differentiate into all of the major cell types in the brain and also into hematopoietic and skeletal muscle cells. In this brief overview, several of these recent findings are summarized.

Cell therapy of heart failure

Together with angiogenesis and gene therapy, cell transplantation is one of the newest treatments that have been proposed to improve the still grim outcome of patients with cardiac failure. The underlying rationale is that implantation of contractile cells into fibrous post-infarction scar can functionally 'regenerate' these areas. Primarily for practical reasons, autologous skeletal myoblasts have been the first to be tested in a clinical trial but other cell types can be considered, among which bone marrow stromal and hematopoietic stem cells are of particular interest because of their autologous origin and their purported transdifferentiation potential into cardiac and/or endothelial cells. However, several key issues still need to be addressed, including (i) the optimal type of donor cells, (ii) the mechanism by which cell engraftment improves cardiac function, actively (i.e., by increasing contractility) or passively (i.e. by limiting infarct expansion and remodelling), (iii) the optimisation of cell survival, and (iiii) the potential benefits of cell transplantation in non-ischaemic heart failure. Parallel to the numerous experimental studies designed to address these issues, initial clinical trials are underway or in preparation and, if properly designed and conducted, should allow to know whether the hopes raised by cellular therapy are met by clinically meaningful improvements in the outcomes of patients with heart failure.

Cardiomyocyte cell cycle regulation

Although rapid progress is being made in many areas of molecular cardiology, issues pertaining to the origins of heart-forming cells, the mechanisms responsible for cardiogenic induction, and the pathways that regulate cardiomyocyte proliferation during embryonic and adult life remain unanswered. In the present study, we review approaches and studies that have shed some light on cardiomyocyte cell cycle regulation. For reference, an initial description of cardiomyogenic induction and morphogenesis is provided, which is followed by a summary of published cell cycle analyses during these stages of cardiac ontology. A review of studies examining cardiomyocyte cell cycle analysis and de novo cardiomyogenic induction in the adult heart is then presented. Finally, studies in which cardiomyocyte cell cycle activity was experimentally manipulated in vitro and in vivo are reviewed. It is hoped that this compilation will serve to stimulate thought and experimentation in this intriguing area of cardiomyocyte cell biology.

Nestin-positive progenitor cells derived from adult human pancreatic islets of Langerhans contain side population (SP) cells defined by expression of the ABCG2 (BCRP1) ATP-binding cassette transporter

The disease diabetes mellitus arises as a consequence of a failure of the beta-cells in the islets of Langerhans of the pancreas to produce insulin in the amounts required to meet the needs of the body. Whole pancreas or islet transplants in patients with severe diabetes effectively restore insulin production. A lack of availability of donor pancreata requires the development of alternative sources of islets such as the ex vivo culture and differentiation of stem/progenitor cells. Earlier we discovered multipotential progenitor cells in islets isolated from adult human pancreata that express the neural stem cell marker nestin: nestin-positive islet-derived progenitor cells (NIPs). Recently it was shown that the exclusion of the Hoechst 33342 dye, which defines the pluripotential side population (SP) of hematopoietic stem cells, is mediated by the ATP-binding cassette transporter, ABCG2. Here we report that the human islet-derived NIPs contain a substantial subpopulation of SP cells that co-express ABCG2, MDR1, and nestin. Thus NIPs may be a potential source of adult pluripotential stem/progenitor cells useful for the production of islet tissue for transplantation into diabetic subjects.

Bone marrow as a source of endothelial cells and NeuN-expressing cells After stroke

BACKGROUND AND PURPOSE

After an ischemic event, bone marrow-derived cells may be involved in reparative processes. There is increasing evidence that bone marrow-derived stem cells may be a source of endothelial cells and organ-specific cells. Our objectives were to determine whether bone marrow-derived cells were a source of endothelial cells and neurons after cerebral ischemia.

METHODS

We transplanted bone marrow from male C57 BL/6-TgN (ACTbEGFP)1Osb mice, which express green fluorescent protein (GFP), into female C57 BL/6J mice. The recipient mice then underwent suture occlusion of the middle cerebral artery (MCA), and bone marrow- derived cells were tracked by GFP epifluorescence and Y chromosome probe.

RESULTS

Within 3 days and at 7 and 14 days after MCA occlusion, bone marrow-derived cells incorporated into the vasculature in the ischemic zone and expressed an endothelial cell phenotype. Few bone marrow-derived cells incorporated into the vasculature 24 hours after MCA occlusion. Some bone marrow-derived cells also expressed the neuronal marker NeuN at 7 and 14 days after ischemia.

CONCLUSIONS

Postnatal vasculogenesis occurs in the brain in the setting of a cerebral infarction. Bone marrow-derived cells are a source of endothelial cells and NeuN-expressing cells after cerebral infarction. This plasticity may be exploited in the future to enhance recovery after stroke.

Cell transplantation, myelin repair, and multiple sclerosis

A decade ago, therapeutic strategies to remyelinate the CNS in diseases such as multiple sclerosis had much experimental appeal, but translation of laboratory success into clinical treatments appeared to be a long way off. Within the past 12 months, however, the first patients with multiple sclerosis have received intracerebral implants of autologous myelinating cells. Here we review the clinical and biological problems presented by multiple sclerosis disease processes, and the background to the development of myelin-repair strategies. We attempt to highlight those areas where difficulties have yet to be resolved, and draw on various experimental findings to speculate on how remyelinating therapies are likely to develop in the foreseeable future.

Stem cells in cardiovascular disease: from cell biology to clinical therapy

Human life begins as a single fertilized cell. As adult human beings we are profoundly complex. This journey from single cell to complex being is attributable to the role of stem cells (i.e. cells that produce all the different types of cells and tissues that make up the human body). Recent interest has focused on the development of stem cells as a therapeutic option in the treatment of disease. Due to their ability both to replace and/or repair damaged tissue, stem cell therapy provides an ideal means to improve therapy for cardiac disorders associated with heart muscle injury. In particular, pre-clinical studies in animal models of acute myocardial infarction have shown great promise for both repairing damaged cardiac muscle and generating new blood vessel formation in the infarcted area. Stem-cell research therefore holds great therapeutic potential and is relevant, not only to basic science researchers, but also to clinicians (who may need to consider such cell-based therapy in the future) and to their patients.

Hematopoietic, vascular and cardiac fates of bone marrow-derived stem cells

Bone marrow contains many cell types, including stroma, vascular cells, adipocytes, osteoblasts and osteoclasts, as well as mesenchymal stem cells and hematopoietic stem cells. It was previously thought that cells within bone marrow solely functioned to regenerate cells within the marrow, as well as all circulating hematopoietic cells in peripheral blood. Recent reports, however, suggest that marrow-derived cells can also regenerate other cell types, including cardiac muscle, liver cell types, neuronal and non-neuronal cell types of the brain, as well as endothelial cells and osteoblasts. These multiple cell types could have originated from either of the stem cell populations within bone marrow or potentially other precursors. Therefore, it is not entirely clear whether each of these distinct cell lineages has a true progenitor within marrow or whether the marrow contains a multipotent population of cells that has been set aside during embryogenesis for postnatal repair and remodeling of a variety of tissues. It is clear, however, that directing the fate of bone marrow-derived progenitors (ie toward hematopoietic, vascular or cardiac cell fates) can only be accomplished if the phenotype of the stem cells is defined, and their homing and differentiation programs are elucidated. Much work is focused on these issues, wherein lie the key to harnessing the potential of adult stem cells for autologous cell and gene therapy.

Human CD34- hematopoietic stem cells: basic features and clinical relevance

CD34 has been widely used as a stem and progenitor cell marker, and clinical CD34+ stem cell transplantation (CD34+ SCT) has been performed for tumor purging and for prevention of graft-versus-host disease. Recently, CD34-negative hematopoietic stem cells (CD34 HSCs) were identified in mice and humans. Xenogeneic chimera engraftment assays with immunodeficient mice or preimmune fetal sheep resulted in identification of human CD34- HSCs in cord blood, bone marrow, and granulocyte colony-stimulating factor-mobilized peripheral blood, although no significant clonogenic activity was detected in vitro. These characteristics of CD34- HSCs make the assessment of clinical samples difficult. The generation of CD34+ HSCs from CD34 cells in vitro may be a surrogate assay for detecting CD34- HSC activity. This approach was used in recipients of CD34+ SCT and revealed the absence of a CD34 precursor population. The identification of a positive marker in CD34- HSCs and the development of a simpler and more efficient in vivo assay for CD34- HSCs may allow the diagnostic evaluation of human CD34- HSCs in various clinical procedures and diseases.

Efficient mobilization and recruitment of marrow-derived endothelial and hematopoietic stem cells by adenoviral vectors expressing angiogenic factors

Adult bone marrow (BM) is a rich reservoir for endothelial and hematopoietic stem and progenitor cells that contribute to revascularization of injured and tumor tissue. Physiological stress results in the release of specific chemo-cytokines that promote mobilization of stem cells to the circulation and direct their incorporation into the target tissues. In order to dissect the mechanism and identify the cellular mediators that regulate stem cell recruitment, we have developed an in vivo murine model, in which the plasma levels of chemokines are elevated by introducing adenoviral vectors (Advectors) expressing such chemokines. Among the known stem cell-active chemokines, the angiogenic factor VEGF through interaction with its receptors, VEGFR2 and VEGFR1 expressed on endothelial and hematopoietic stem cells, promotes mobilization and recruitment of these cells into the neo-angiogenic sites, thereby accelerating the revascularization process. Based on these studies, it has become apparent that mobilization of stem cells is a dynamic process and requires sequential release of chemocytokines, expression of adhesion molecules and activation of proteases that facilitate egress of cells from the BM to the circulation. Chemokine-activation of metalloproteinases is essential for the release of bio-active cytokines, thereby enhancing stem cell mobilization potential. Advectors are ideal for delivery of chemocytokines since they allow for long-term robust expression facilitating in vivo proliferation and mobilization of large numbers of an otherwise rare population of stem cells. VEGF-mobilized endothelial and hematopoietic stem cells provide for an enriched source of adult pluripotent cells that can be used for revascularization, tissue regeneration or gene therapy.

The role of stem cells in skeletal and cardiac muscle repair

In postnatal muscle, skeletal muscle precursors (myoblasts) can be derived from satellite cells (reserve cells located on the surface of mature myofibers) or from cells lying beyond the myofiber, e.g., interstitial connective tissue or bone marrow. Both of these classes of cells may have stem cell properties. In addition, the heretical idea that post-mitotic myonuclei lying within mature myofibers might be able to re-form myoblasts or stem cells is examined and related to recent observations for similar post-mitotic cardiomyocytes. In adult hearts (which previously were not considered capable of repair), the role of replicating endogenous cardiomyocytes and the recruitment of other (stem) cells into cardiomyocytes for new cardiac muscle formation has recently attracted much attention. The relative contribution of these various sources of precursor cells in postnatal muscles and the factors that may enhance stem cell participation in the formation of new skeletal and cardiac muscle in vivo are the focus of this review. We concluded that, although many endogenous cell types can be converted to skeletal muscle, the contribution of non-myogenic cells to the formation of new postnatal skeletal muscle in vivo appears to be negligible. Whether the recruitment of such cells to the myogenic lineage can be significantly enhanced by specific inducers and the appropriate microenvironment is a current topic of intense interest. However, dermal fibroblasts appear promising as a realistic alternative source of exogenous myoblasts for transplantation purposes. For heart muscle, experiments showing the participation of bone marrow-derived stem cells and endothelial cells in the repair of damaged cardiac muscle are encouraging.

ABC transporters as phenotypic markers and functional regulators of stem cells

Characterization of molecules with tightly controlled expression patterns during differentiation represents an approach to understanding regulation of hematopoietic stem cell commitment. The multidrug resistance-1 (MDR1) gene product, P-glycoprotein, and the breast cancer resistance protein (BCRP) are expressed differentially during hematopoiesis, with the highest levels in primitive bone marrow stem cell populations that are CD34(low) and CD34(-), respectively. Roles for ATP-binding cassette (ABC) transporter superfamily members in conferring drug resistance have been extensively described. However, recent hematopoietic overexpression studies have begun to reveal previously unknown roles for ABC transporter function in normal and malignant hematopoiesis. Expression of MDR1 and BCRP transporters in the myeloid lineage has been reported in blasts from acute myeloid leukemia, but very low to undetectable in normal myelomonocytic cells. Retroviral-mediated dysregulated expression of the MDR1 transporter resulted in increased hematopoietic repopulating activity and myeloproliferative disease in mice. A distinct functional role for the BCRP transporter as a negative regulator of hematopoietic repopulating activity has recently been demonstrated using the same approach. Additionally, the presence of BCRP expression specifically on hematopoietic side-population stem cells and neural stem/progenitors, makes BCRP an attractive candidate marker for isolation of stem cells with the ability to respond to diverse environmental cues. Regulation of stem cell biology by ABC transporters has emerged as an important new field of investigation. In light of these findings, it will be critical to further characterize this family of proteins in hematopoietic lineage-restricted stem cells and in pluripotent stem cells capable of crossing lineage barriers.

Evidence for cardiomyocyte repopulation by extracardiac progenitors in transplanted human hearts

Human myocardium has long been considered to have essentially no intrinsic regenerative capacity. Recent studies in rodent models, however, have suggested the presence of an extracardiac stem cell population, perhaps in bone marrow, that is capable of some reconstitution of cardiomyocytes after injury. To determine whether similar mechanisms exist in the human heart, we evaluated human female allograft hearts transplanted into male patients. The presence of Y chromosomes in cardiomyocytes would indicate these cells arose from the recipient, rather than the donor heart. We identified 5 male patients who had retained a female heart at least 9 months before death and necropsy. Remarkably, in each case, the transplanted heart contained a minute but readily detectable fraction of Y chromosome-positive cardiomyocytes. The mean percentage of cardiomyocytes arising from the host was estimated to be 0.04% with a median of 0.016%. Most Y-positive cardiomyocytes were associated with regions of acute rejection, suggesting such chimerism involves an injury event. Furthermore, the sole patient whose immediate cause of death was allograft rejection showed a much higher percentage of host-derived cardiomyocytes, up to 29% in local, 1-mm(2) "hot spots." Thus, adult humans have extracardiac progenitor cells capable of migrating to and repopulating damaged myocardium, but this process occurs at very low levels.

The role of the stem cell leukemia (SCL) gene in hematopoietic and endothelial lineage specification

Anatomical observations made at the beginning of the twentieth century revealed an intimate association between the ontogeny of blood and endothelium and led to the hypothesis of a common cell of origin termed the hemangioblast. However, the precise nature of the cellular intermediates involved in the development of both lineages from uncommitted precursors to mature cell types is still the subject of ongoing studies, as are the molecular mechanisms driving this process. There is clear evidence that lineage-restricted transcription factors play a central role in the genesis of mature lineage committed cells from multipotent progenitors. Amongst these, the basic helix-loop-helix (bHLH) family is of key importance for cell fate determination in the development of the hematopoietic system and beyond. This article will review the current evidence for the common origin of blood and endothelium, focusing on the function of the bHLH protein encoded by the stem cell leukemia (SCL) gene, and its role as a pivotal regulator of hematopoiesis and vasculogenesis.

Cell transplantation for the treatment of heart failure

Along with angiogenesis and gene therapy, cell transplantation is one of the newest treatment modalities proposed to improve the outcome of patients with cardiac failure. Both experimental and clinical data have now established that implantation of contractile cells into fibrous postinfarction scars can allow them to regain some functionality. Primarily for practical reasons, autologous skeletal myoblasts have been the first to be tested in a clinical trial, but other cell types can be considered among which bone marrow stromal and hematopoietic stem cells are of particular interest because of their presumed pluripotentiality and the possibility to use them as autografts. However, several key issues still need to be addressed including: (1) the advantages and disadvantages of these different donor cells; (2) the extent to which cell engraftment affects cardiac function actively (ie, by increasing contractility) or passively (ie, by limiting infarct expansion and remodeling); (3) the development of strategies targeted at enhancing cell survival; and (4) the identification of cardiac diseases for which cell engraftment may be most beneficial. In parallel to the numerous experimental studies designed to address these issues, initial clinical trials are underway or in preparation and it is mandatory to design and conduct them in a careful manner so as to ultimately know whether cellular transplantation holds its promise as a means of improving the outcomes of patients with heart failure.

Cells with hemopoietic potential residing in muscle are itinerant bone marrow-derived cells

OBJECTIVE

The nature of cells residing in muscle giving rise to hemopoietic colonies in vitro or hemopoietic reconstitution in vivo has been unclear. The goal of the present study was to characterize these cells and uncover their potential site of origin.

MATERIALS AND METHODS

Cells prepared from muscle were characterized for surface antigens (CD45, CD34, c-kit, Sca-1, CD31, VCAM-1), for their in vitro clonogenic capacity and in vivo repopulation potential either as unpurified cells or sorted subsets (CD45(+), CD45(-)). The presence of bone marrow (BM)-derived cells in muscle of mice reconstituted with marked BM cells before and after cytokine-induced mobilization was also examined.

RESULTS

Our data show: 1) The yield of CD45(+) cells is higher in muscle of neonates and young animals. Their composite phenotype does not favor contamination by blood. 2) The capacity of fresh muscle cell explants to give rise to colonies in vitro and hemopoietic reconstitution in vivo is associated with CD45(+) cells. 3) Irradiated recipients reconstituted with marked BM cells harbor marked BM-derived cells (CD45(+) or CD45(-)) in their muscle several months after transplant. 4) Cytokine-induced mobilization of transplanted animals modestly increases the yield of BM-derived cells recovered from muscle, unlike the yields from spleen, liver, or peripheral blood (PB).

CONCLUSIONS

Our data suggest a reinterpretation of previously published conclusions: hemopoietic colonies derived from fresh muscle explants do not originate from transdifferentiated muscle cells, but from BM-derived cells residing in muscle; the hemopoietic reconstituting potential of muscle cells is likewise attributed to these cells.

Bone marrow transplantation in mice leads to a minor population of hepatocytes that can be selectively amplified in vivo

Cell-based therapy may some day be a therapeutic alternative to liver transplantation. Recent observations indicating that hematopoietic stem cells can differentiate into hepatocytes have opened new therapeutic prospects. However, the clinical relevance of this phenomenon is unknown. We have previously developed a strategy based on the protective effect of Bcl-2 against Fas-mediated apoptosis to selectively amplify a small number of hepatocytes in vivo. We now show that this approach can be used to amplify a minor population of bone marrow-derived hepatocytes. Normal mice were transplanted with unfractionated bone marrow cells from transgenic animals expressing Bcl-2 under the control of a liver-specific promoter. Recipients were then submitted to weekly injections of the anti-Fas antibody, Jo2. Upon sacrifice, the liver of the recipients showed bone marrow-derived clusters of mature hepatocytes expressing Bcl-2, which showed that the hepatocyte progeny of a genetically modified bone marrow can be selectively expanded in vivo. In contrast, no Bcl-2 expression could be detected without the selective pressure of Jo2, suggesting that differentiation of bone marrow cells into mature hepatocytes is very inefficient under physiologic conditions. We conclude that a selection strategy will be required to achieve a therapeutic level of liver repopulation with bone marrow-derived hepatocytes.

Hematopoiesis and stem cells: plasticity versus developmental heterogeneity

Hematopoietic stem cells (HSCs) provide for blood formation throughout the life of the individual. Studies of HSCs form a conceptual framework for the analysis of stem cells of other organ systems. We review here the origin of HSCs during embryological development, the relationship between hematopoiesis and vascular development and the potential plasticity of HSCs and other tissue stem cells. Recent experiments in the mouse have been widely interpreted as evidence for unprecedented transdifferentiation of tissue stem cells. The use of enriched, but impure, cell populations allows for alternative interpretation. In considering these findings, we draw a distinction here between the plasticity of adult stem cells and the heterogeneity of stem cell types that pre-exist within tissues.

How cells change their phenotype

Recent attention has focused on the remarkable ability of adult stem cells to produce differentiated cells from embryologically unrelated tissues. This phenomenon is an example of metaplasia and shows that embryological commitments can be reversed or erased under certain circumstances. In some cases, even fully differentiated cells can change their phenotype (transdifferentiation). This review examines recently discovered cases of metaplasia, and speculates on the potential molecular and cellular mechanisms that underlie the switches, and their significance to developmental biology and medicine.

Stem cells: hype or hope?

Stem cells undergo self-renewal and differentiate into multiple lineages of mature cells. The identification of stem cells in diverse adult tissues and the findings that human embryonic stem cells can be proliferated and differentiated has kindled the imagination of both scientists and the public regarding future stem cell technology. These cells could constitute an unlimited supply of diverse cell types that can be used for cell transplantation or drug discovery. The new options raise several fundamental ethical issues. This review gives an overview of the scientific basis underlying the hope generated by stem cell research and discusses current ethical and funding regulations.

Muscle-derived hematopoietic stem cells are hematopoietic in origin

It has recently been shown that mononuclear cells from murine skeletal muscle contain the potential to repopulate all major peripheral blood lineages in lethally irradiated mice, but the origin of this activity is unknown. We have fractionated muscle cells on the basis of hematopoietic markers to show that the active population exclusively expresses the hematopoietic stem cell antigens Sca-1 and CD45. Muscle cells obtained from 6- to 8-week-old C57BL/6-CD45.1 mice and enriched for cells expressing Sca-1 and CD45 were able to generate hematopoietic but not myogenic colonies in vitro and repopulated multiple hematopoietic lineages of lethally irradiated C57BL/6-CD45.2 mice. These data show that muscle-derived hematopoietic stem cells are likely derived from the hematopoietic system and are a result not of transdifferentiation of myogenic stem cells but instead of the presence of substantial numbers of hematopoietic stem cells in the muscle. Although CD45-negative cells were highly myogenic in vitro and in vivo, CD45-positive muscle-derived cells displayed only very limited myogenic activity and only in vivo.

Contribution of marrow-derived progenitors to vascular and cardiac regeneration

Adult bone marrow is a rich reservoir of tissue-specific pluripotent stem and progenitor cells. Accumulating evidence suggest that these cells have the potential of contributing to tissue revascularization and cardiac regeneration. Physiological stress results in the release of specific chemokines and cytokines that promote mobilization of stem cells to the peripheral circulation. Incorporation of these mobilized cells contributes to formation of functional vasculature and sets up stage for tissue regeneration. Vascular Endothelial Growth Factor (VEGF) through interaction with its receptors VEGFR2 and VEGFR1 expressed on endothelial and hematopoietic stem cells promote recruitment of these cells into the sites of tissue injury accelerating vascular healing. Similarly, subset of CD34 + marrow derived cells are mobilized and recruited to the ischemic myocardium, differentiating into cardiac and vascular cells, restoring cardiac function. Identification of cellular mediators and tissue specific chemocytokines that facilitate selective recruitment of marrow-derived stem and progenitor cells to specific organs, will open up new avenues to accelerate cardiovascular regeneration and tissue revascularization.

Skeletal muscle stem cells do not transdifferentiate into cardiomyocytes after cardiac grafting

Skeletal muscle cell-derived grafts in the heart may benefit myocardial performance after infarction. Several studies have suggested that skeletal muscle stem cells (satellite cells) from adult muscle undergo transdifferentiation into cardiomyocytes after grafting into the heart, but expression of cardiac markers in graft cells has not been rigorously confirmed. To determine the fate of satellite cell-derived grafts in the heart, adult rat satellite cells were tagged in vitro with bromodeoxyuridine (BrdU) and grafted into normal hearts of syngeneic rats. At 4 and 12 weeks the graft cells formed multinucleated, cross-striated myofibers that expressed fast skeletal myosin heavy chain (MHC), thus indicating a mature skeletal muscle phenotype. Double staining for the BrdU tag and cardiac-specific markers was employed to identify transdifferentiation. Aside from four questionable cells, none of the 11 grafts examined expressed alpha-MHC, cardiac troponin I, or atrial natriuretic peptide. At 4 weeks, grafts expressed beta -MHC, a hallmark of slow twitch myofibers. By 12 weeks, however, the myofibers had atrophied and downregulated beta-MHC. Grafts never expressed the intercalated disk proteins N-cadherin or connexin43, hence electromechanical coupling did not occur. In conclusion, satellite cells differentiate into mature skeletal muscle and do not express cardiac-specific genes after grafting into the heart. Thus, transdifferentiation into cardiomyocytes did not occur.

Stem cells from the Mammalian blastocyst

Early differentiation of the mammalian embryo leads to the development of two distinct lineages-the inner cell mass (ICM) and the trophectoderm. Cells of the ICM are pluripotent and give rise to all tissues of the fetus, while trophectoderm cells are restricted in their potential to the trophoblast cell layers of the placenta. In the mouse, apparently immortal stem cell lines can be obtained from both cell types. These cell lines, embryonic stem (ES) cells and trophoblast stem (TS) cells, are morphologically and molecularly distinct and depend on different signaling pathways for their maintenance. They also show different cell fates when introduced into early embryos to generate chimeras. However, a change in the levels of expression of a key regulator of pluripotency, Oct4, can push ES cells towards the TS phenotype, when grown in TS cell conditions. Stem cell potential in the early embryo thus appears to depend on a combination of the levels of expression of key intrinsic regulators and the appropriate extrinsic environmental factors. Manipulation of both intrinsic and extrinsic regulators may be needed to reveal the full potential of stem cells from other stages of development and the adult.

Myocyte renewal and ventricular remodelling

Remaining young at heart is a desirable but elusive goal. Unbeknown to us, however, myocyte regeneration may accomplish just that. Continuous cell renewal in the adult myocardium was thought to be impossible, but multipotent cardiac stem cells may be able to renew the myocardium and, under certain circumstances, can be coaxed to repair the broken heart after infarction.

Chimerism of the transplanted heart

BACKGROUND

Cases in which a male patient receives a heart from a female donor provide an unusual opportunity to test whether primitive cells translocate from the recipient to the graft and whether cells with the phenotypic characteristics of those of the recipient ultimately reside in the donor heart. The Y chromosome can be used to detect migrated undifferentiated cells expressing stem-cell antigens and to discriminate between primitive cells derived from the recipient and those derived from the donor.

METHODS

We examined samples from the atria of the recipient and the atria and ventricles of the graft by fluorescence in situ hybridization to determine whether Y chromosomes were present in eight hearts from female donors implanted into male patients. Primitive cells bearing Y chromosomes that expressed c-kit, MDR1, and Sca-1 were also investigated.

RESULTS

Myocytes, coronary arterioles, and capillaries that had a Y chromosome made up 7 to 10 percent of those in the donor hearts and were highly proliferative. As compared with the ventricles of control hearts, the ventricles of the transplanted hearts had markedly increased numbers of cells that were positive for c-kit, MDR1, or Sca-1. The number of primitive cells was higher in the atria of the hosts and the atria of the donor hearts than in the ventricles of the donor hearts, and 12 to 16 percent of these cells contained a Y chromosome. Undifferentiated cells were negative for markers of bone marrow origin. Progenitor cells expressing MEF2, GATA-4, and nestin (which identify the cells as myocytes) and Flk1 (which identifies the cells as endothelial cells) were identified.

CONCLUSIONS

Our results show a high level of cardiac chimerism caused by the migration of primitive cells from the recipient to the grafted heart. Putative stem cells and progenitor cells were identified in control myocardium and in increased numbers in transplanted hearts.

Stem cells: hype and reality

This update discusses what is known regarding embryonic and adult tissue-derived pluripotent stem cells, including the mechanisms underlying self-renewal without senescence, differentiation in multiple cell types both in vitro and in vivo, and future potential clinical uses of such stem cells. In Section I, Dr. Lansdorp reviews the structure and function of telomerase, the enzyme that restores telomeric ends of chromosomes upon cell division, highly present in embryonic stem cells but not adult stem cells. He discusses the structure and function of telomerase and signaling pathways activated by the enzyme, with special emphasis on normal and leukemic hematopoietic stem cells. In Section II, Dr. Pera reviews the present understanding of mammalian pluripotent embryonic stem cells. He discusses the concept of pluripotentiality in its embryonic context, derivation of stem cells from embryonic or fetal tissue, the basic properties of the stem cells, and methods to produce specific types of differentiated cell from stem cells. He examines the potential applications of stem cells in research and medicine and some of the barriers that must be crossed to achieve these goals. In Section III, Dr. Verfaillie reviews the present understanding of pluripotency of adult stem cells. She discusses the concept of stem cell plasticity, a term used to describe the greater potency described by several investigators of adult tissue-derived stem cells, critically reviews the published studies demonstrating stem cell plasticity, and possible mechanisms underlying such plasticity, and examines the possible role of pluripotent adult stem cells in research and medicine.

Transplantation of embryonic stem cells improves cardiac function in postinfarcted rats

Massive loss of cardiac myocytes after myocardial infarction (MI) is a common cause of heart failure. The present study was designed to investigate the improvement of cardiac function in MI rats after embryonic stem (ES) cell transplantation. MI in rats was induced by ligation of the left anterior descending coronary artery. Cultured ES cells used for cell transplantation were transfected with the marker green fluorescent protein (GFP). Animals in the treated group received intramyocardial injection of ES cells in injured myocardium. Compared with the MI control group injected with an equivalent volume of the cell-free medium, cardiac function in ES cell-implanted MI animals was significantly improved 6 wk after cell transplantation. The characteristic phenotype of engrafted ES cells was identified in implanted myocardium by strong positive staining to sarcomeric alpha-actin, cardiac alpha-myosin heavy chain, and troponin I. GFP-positive cells in myocardium sectioned from MI hearts confirmed the survival and differentiation of engrafted cells. In addition, single cells isolated from cell-transplanted MI hearts showed rod-shaped GFP-positive myocytes with typical striations. The present data demonstrate that ES cell transplantation is a feasible and novel approach to improve ventricular function in infarcted failing hearts.

Thrombocytosis: too much of a good thing?

Thrombocytosis is due to (a) a variety of disorders that cause reactive stimulation of platelet production, (b) familial mutations, or (c) essential thrombocythemia (ET) and other myeloproliferative disorders (MPDs). The MPDs are clonal abnormalities of the pluripotent hematopoietic stem cell. Dysregulation of megakaryocytopoiesis in ET involves defective binding of thrombopoietin by platelets and megakaryocytes resulting in increased levels of plasma free thrombopoietin, and increased sensitivity of megakaryocytes to thrombopoietin leading to their increased proliferation. Bleeding and thrombosis are the major causes of morbidity and mortality in ET and the other MPDs. The elevated platelet count and qualitative platelet defects have been implicated in the pathophysiology of these hemostatic problems. However, these platelet abnormalities do not correlate well with clinical complications. It is proposed that bleeding and thrombosis could be due to vascular abnormalities that result from dysfunctional hematopoietic stem cell-derived endothelial cells.

Human mesenchymal stem cells differentiate to a cardiomyocyte phenotype in the adult murine heart

BACKGROUND

Cellular cardiomyoplasty has been proposed as an alternative strategy for augmenting the function of diseased myocardium. We investigated the potential of human mesenchymal stem cells (hMSCs) from adult bone marrow to undergo myogenic differentiation once transplanted into the adult murine myocardium.

METHODS AND RESULTS

A small bone marrow aspirate was taken from the iliac crest of healthy human volunteers, and hMSCs were isolated as previously described. The stem cells, labeled with lacZ, were injected into the left ventricle of CB17 SCID/beige adult mice. At 4 days after injection, none of the engrafted hMSCs expressed myogenic markers. A limited number of cells survived past 1 week and over time morphologically resembled the surrounding host cardiomyocytes. Immunohistochemistry revealed de novo expression of desmin, beta-myosin heavy chain, alpha-actinin, cardiac troponin T, and phospholamban at levels comparable to those of the host cardiomyocytes; sarcomeric organization of the contractile proteins was observed. In comparison, neither cardiac troponin T nor phospholamban was detected in the myotubes formed in vitro by MyoD-transduced hMSCs.

CONCLUSIONS

The purified hMSCs from adult bone marrow engrafted in the myocardium appeared to differentiate into cardiomyocytes. The persistence of the engrafted hMSCs and their in situ differentiation in the heart may represent the basis for using these adult stem cells for cellular cardiomyoplasty.

Stem cells: is there a future in plastics?

The concept that ostensibly tissue-specific stem cells can give rise to cells of heterologous lineages has gained support from studies using purified hematopoietic stem cells and sensitive donor-cell tracking methods. The ability to exploit these findings in clinical settings will probably depend on new insights into the mechanisms by which such stem cells or their progeny migrate to sites of organ damage and differentiate to cell types competent to participate in tissue regeneration.

A complex linkage in the developmental pathway of endothelial and hematopoietic cells

During normal vertebrate development, hematopoietic and endothelial cells form closely situated and interacting populations. Although the close proximity of cells to each other does not necessarily mean that they are relatives, accumulating evidence indicates that hematopoietic and endothelial cells are indeed close kin; they share common progenitors and each is able to become the other under certain circumstances. This article summarizes recent advances in the developmental relationship between hematopoietic and endothelial cells.

The last frontier in cardiovascular health: a landmark lecture for the XVII World Congress of the International Society for Heart Research

Following World War II, Vannevar Bush described science as an "endless frontier" that should be made accessible to all Americans. Since then, cardiovascular health has improved markedly, largely because substantial investments in biomedical research led to numerous therapies and prevention strategies for cardiovascular disease. Despite these advances, however, science remains an endless frontier and we continue to face an infinite array of opportunities for improving cardiovascular health. A standard definition for "frontier" is the "farthermost limit of knowledge or achievement". The limits of our knowledge are expanding at an ever accelerated pace. Unfortunately, we do not always apply what we know, and therefore fail to achieve all we could. For example, we have known for two decades that heart attack patients benefit from beta-blockers, but even today, the drugs are not always prescribed. And, health disparities continue to exist among races and communities. Therefore, the "last frontier of cardiovascular health" is the translation and application of our knowledge to improve the cardiovascular health of all people. We will not reach the farthermost limit of achievement without new knowledge. But, in our zeal to expand our knowledge of cardiovascular diseases, we must remember to ensure that what we learn is rapidly applied to improve cardiovascular health.

Thrombocytosis and thrombocythemia

Thrombocytosis is caused by three major pathophysiological mechanisms: (1) reactive or secondary thrombocytosis; (2) familial thrombocytosis; and (3) clonal thrombocytosis, including essential thrombocythemia and related myeloproliferative disorders. Recent work has begun to elucidate the abnormal megakaryocytopoiesis of essential thrombocythemia, which is associated with paradoxically elevated plasma levels of thrombopoietin. The clonal nature of all cases of essential thrombocythemia has been challenged. Thrombotic complications are the major causes of morbidity and mortality in this disease. Indications for platelet cytoreduction and antiplatelet therapy, as well as complications of treatment, are being clarified.

Have the bloody cells gone to our heads?

Recent studies have shown that cells expressing neuronal antigens can be derived from a bone marrow transplant. A new report lends support to and extends these previous results by presenting compelling morphological evidence for the generation and integration of highly differentiated bone marrow-derived neurons.

Stem cells in tissue engineering

The concept of producing 'spare parts' of the body for replacement of damaged or lost organs lies at the core of the varied biotechnological practices referred to generally as tissue engineering. Use of postnatal stem cells has the potential to significantly alter the perspective of tissue engineering. Successful long-term restoration of continuously self-renewing tissues such as skin, for example, depends on the use of extensively self-renewing stem cells. The identification and isolation of stem cells from a number of tissues provides appropriate targets for prospective gene therapies.