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

Myocardial tissue engineering: creating a muscle patch for a wounded heart

Cardiac tissue engineering promises to revolutionize the treatment of patients with end-stage heart failure and provide new solutions to the serious problem of heart donor shortage. By its broad definition, tissue engineering involves the construction of tissue equivalents from donor cells seeded within three-dimensional polymeric scaffolds, then culturing and implanting of the cell-seeded scaffolds to induce and direct the growth of new, healthy tissue. Here, we present an up-to-date summary of research studies in cardiac tissue engineering, with an emphasis on the critical design principles.

Cardiac muscle plasticity in adult and embryo by heart-derived progenitor cells

The evidence of cardiomyocyte proliferation in damaged heart implied cardiac regeneration might occur by resident or extra cardiac stem cells. However, the specification and origin of these cells remain unknown. Here, we report using fluorescence-activated cell sorting that cardiac progenitor cells resided in adult heart and colocalized with small capillary vessels, within the stem cell antigen (Sca-1) population expressing high telomerase activity. Notably, hematopoietic stem cells capable of efflux Hoechst 33342, termed side population cells, also were identified within the heart-derived cells. The cardiac progenitor cells (CD45(-)/CD34(-)) express neither cardiac muscle nor endothelial cell markers at an undifferentiated stage. The exposure of 5-azacytidine induced cardiac differentiation, which depends, in part, on Bmpr1a, a type IA receptor for bone morphogenetic protein (BMP). The capability of adult Sca1(+) cells to adopt a cardiac muscle in embryogenesis was substantiated by blastocyst injection, using progenitors from the adult hearts of transgenic mice that harbor a bacterial artificial chromosome expressing GFP via the Nkx-2.5 locus. Intravenously injected progenitors, shortly after ischemic/reperfusion, homed and functionally differentiated 3.5% of total left ventricle in the host myocardium. Differentiation included both fusion-independent and fusion-associated components, proved by the Cre/loxP donor/recipient system. Our studies suggest that endogenous cardiac progenitors reside in the adult heart, regenerate cardiomyocytes functionally, and integrate into the existing heart circuitry.

Identification and isolation of hematopoietic stem cells

Hematopoietic stem cells (HSCs) are defined by their ability to repopulate all of the hematopoietic lineages in vivo and sustain the production of these cells for the life span of the individual. In the absence of reliable direct markers for HSCs, their identification and enumeration depends on functional long-term, multilineage, in vivo repopulation assays. The extremely low frequency of HSCs in any tissue and the absence of a specific HSC phenotype have made their purification and characterization a highly challenging goal. HSCs and primitive hematopoietic cells can be distinguished from mature blood cells by their lack of lineage-specific markers and presence of certain other cell-surface antigens, such as CD133 (for human cells) and c-kit and Sca-1 (for murine cells). Functional analyses of purified subpopulations of primitive hematopoietic cells have led to the development of several procedures for isolating cell populations that are highly enriched in cells with in vivo stem cell activity. Simplified methods for obtaining these cells at high yield have been important to the practical exploitation of such advances. This article reviews recent progress in identifying human and mouse HSCs and current techniques for their purification.

Stem cell plasticity: the growing potential of cellular therapy

The fundamental principle of stem cell biology is that cells with the potential for both self-renewal and terminal differentiation into one or more cell types may be found in a given tissue. The corollary of this principle is that the stem cells give rise to tissues in which they reside, the so-called expected tissues. Many exciting discoveries reported over the last several years challenge this paradigm by showing that there are not only tissue-specific stem cells that differentiate to the expected mature cells, but also that tissue stem cells can differentiate into unexpected cell lineages, suggesting an enormous plasticity of differentiation. Hematopoietic stem cells, which have drawn the most attention, mesenchymal stem cells, and neural stem cells have been the focus of many investigations. However, recent studies directed toward hematopoietic stem cells have disputed the concept of stem cell plasticity, suggesting that experimental artifact or somatic cell fusion may account for reported observations of plasticity. Although the data are mounting, stem cell plasticity, strictly defined, has yet to be rigorously proven. Animal models to meticulously define the biology and potential plasticity of stem cells and pilot clinical trials to begin to explore the biology and therapeutic potential of human stem cells will both be vital to advance the field over the coming years.

The biology of hematopoietic stem cells

Rarely has so much interest from the lay public, government, biotechnology industry, and special interest groups been focused on the biology and clinical applications of a single type of human cell as is today on stem cells, the founder cells that sustain many, if not all, tissues and organs in the body. Granting organizations have increasingly targeted stem cells as high priority for funding, and it appears clear that the evolving field of tissue engineering and regenerative medicine will require as its underpinning a thorough understanding of the molecular regulation of stem cell proliferation, differentiation, self-renewal, and aging. Despite evidence suggesting that embryonic stem (ES) cells might represent a more potent regenerative reservoir than stem cells collected from adult tissues, ethical considerations have redirected attention upon primitive cells residing in the bone marrow, blood, brain, liver, muscle, and skin, from where they can be harvested with relative sociological impunity. Among these, it is arguably the stem and progenitor cells of the mammalian hematopoietic system that we know most about today, and their intense study in rodents and humans over the past 50 years has culminated in the identification of phenotypic and molecular genetic markers of lineage commitment and the development of functional assays that facilitate their quantitation and prospective isolation. This review focuses exclusively on the biology of hematopoietic stem cells (HSCs) and their immediate progeny. Nevertheless, many of the concepts established from their study can be considered fundamental tenets of an evolving stem cell paradigm applicable to many regenerating cellular systems.

Gene and cell-based therapies for heart disease

Heart disease remains the prevalent cause of premature death and accounts for a significant proportion of all hospital admissions. Recent developments in understanding the molecular mechanisms of myocardial disease have led to the identification of new therapeutic targets, and the availability of vectors with enhanced myocardial tropism offers the opportunity for the design of gene therapies for both protection and rescue of the myocardium. Genetic therapies have been devised to treat complex diseases such as myocardial ischemia, heart failure, and inherited myopathies in various animal models. Some of these experimental therapies have made a successful transition to clinical trial and are being considered for use in human patients. The recent isolation of endothelial and cardiomyocyte precursor cells from adult bone marrow may permit the design of strategies for repair of the damaged heart. Cell-based therapies may have potential application in neovascularization and regeneration of ischemic and infarcted myocardium, in blood vessel reconstruction, and in bioengineering of artificial organs and prostheses. We expect that advances in the field will lead to the development of safer and more efficient vectors. The advent of genomic screening technology should allow the identification of novel therapeutic targets and facilitate the detection of disease-causing polymorphisms that may lead to the design of individualized gene and cell-based therapies.

Stem cells and repair of the heart

Stem-cell therapy provides the prospect of an exciting and powerful treatment to repair the heart. Although research has been undertaken in animals to analyse the safety and efficacy of this new approach, results have been inconclusive. The mechanism by which stem cells could improve cardiac function remains unclear. We describe the background to the concept of natural repair and the work that has been done to establish the role of stem cells in cardiac repair. Controversies have arisen in interpretation of experimental data. The important issues surrounding the application of stem-cell therapy to man are discussed critically. We discuss the future of this pioneering work in the setting of growing concerns about clinical studies in man without understanding the biological mechanisms involved, with the difficulties in funding this type of research.

Role of curcumin, a naturally occurring phenolic compound of turmeric in accelerating the repair of excision wound, in mice whole-body exposed to various doses of γ-radiation

The healing of irradiated wounds has always been a central consideration in medical practice because radiation disrupts normal response to injury, leading to a protracted recovery period. The quest for clinically effective wound healing agents is important in the medical management of irradiated wounds. Therefore, the present study was conceptualized to investigate the effect of curcumin (natural yellow, diferuloylmethane), a major yellow pigment and an active component of turmeric on wound healing in mice exposed to whole-body gamma-radiation. A full-thickness wound was created on the dorsum of mice whole-body irradiated to 2, 4, 6, or 8 Gy. The progression of wound contraction was monitored periodically by capturing video images of the wound. The collagen, hexosamine, DNA, nitric oxide, and histological profiles were evaluated at various postirradiation days in mice treated and not treated with curcumin before exposure to 0 or 6 Gy. The whole-body exposure resulted in a dose-dependent delay in wound contraction and prolongation of wound healing time. Irradiation caused a significant reduction in collagen, hexosamine, DNA, and nitric oxide synthesis. Pretreatment with curcumin significantly enhanced the rate of wound contraction, decreased mean wound healing time, increased synthesis of collagen, hexosamine, DNA, and nitric oxide and improved fibroblast and vascular densities. This study demonstrates that curcumin pretreatment has a conducive effect on the irradiated wound and could be a substantial therapeutic strategy in initiating and supporting the cascade of tissue repair processes in irradiated wounds.

Heart infarct in NOD-SCID mice: therapeutic vasculogenesis by transplantation of human CD34+ cells and low dose CD34+KDR+ cells

Hematopoietic (Hem) and endothelial (End) lineages derive from a common progenitor cell, the hemangioblast: specifically, the human cord blood (CB) CD34+KDR+ cell fraction comprises primitive Hem and End cells, as well as hemangioblasts. In humans, the potential therapeutic role of Hem and End progenitors in ischemic heart disease is subject to intense investigation. Particularly, the contribution of these cells to angiogenesis and cardiomyogenesis in myocardial ischemia is not well established. In our studies, we induced myocardial infarct (MI) in the immunocompromised NOD-SCID mouse model, and monitored the effects of myocardial transplantation of human CB CD34+ cells on cardiac function. Specifically, we compared the therapeutic effect of unseparated CD34+ cells vs. PBS and mononuclear cells (MNCs); moreover, we compared the action of the CD34+KDR+ cell subfraction vs. the CD34+KDR- subset. CD34+ cells significantly improve cardiac function after MI, as compared with PBS/MNCs. Similar beneficial actions were obtained using a 2-log lower number of CD34+KDR+ cells, while the same number of CD34+KDR- cells did not have any effects. The beneficial effect of CD34+KDR+ cells may mostly be ascribed to their notable resistance to apoptosis and to their angiogenic action, since cardiomyogenesis was limited. Altogether, our results indicate that, within the CD34+ cell population, the CD34+KDR+ fraction is responsible for the improvement in cardiac hemodynamics and hence represents the candidate active CD34+ cell subset.

Myogenin and oxidative enzyme gene expression levels are elevated in rat soleus muscles after endurance training

The intent of this study was to determine whether endurance exercise training regulates increases in metabolic enzymes, which parallel modulations of myogenin and MyoD in skeletal muscle of rats. Adult Sprague-Dawley rats were endurance trained (TR) 5 days weekly for 8 wk on a motorized treadmill. They were killed 48 h after their last bout of exercise. Sedentary control (Con) rats were killed at the same time as TR animals. Myogenin, MyoD, citrate synthase (CS), cytochrome- c oxidase (COX) subunits II and VI, lactate dehydrogenase (LDH), and myosin light chain mRNA contents were determined in soleus muscles by using RT-PCR. Myogenin mRNA content was also estimated by using dot-blot hybridization. Protein expression levels of myogenin and MyoD were measured by Western blots. CS enzymatic activity was also measured. RT-PCR measurements showed that the mRNA contents of myogenin, CS, COX II, COX VI, and LDH were 25, 20, 17, 16, and 18% greater, respectively, in TR animals compared with Con animals ( P < 0.05). The ratio of myogenin to MyoD mRNA content estimated by RT-PCR in TR animals was 28% higher than that in Con animals ( P < 0.05). Myosin light chain expression was similar in Con and TR muscles. Results from dot-blot hybridization to a riboprobe further confirmed the increase in myogenin mRNA level in TR group. Western blot analysis indicated a 24% greater level of myogenin protein in TR animals compared with Con animals ( P < 0.01). The soleus muscles from TR animals had a 25% greater CS enzymatic activity than the Con animals ( P < 0.01). Moreover, myogenin mRNA and protein contents were positively correlated to CS activity and mRNA contents of CS, COX II, and COX VI ( P < 0.05). These data are consistent with the hypothesis that myogenin is in the pathway for exercise-induced changes in mitochondrial enzymes.

Mesenchymal stem cells: paradoxes of passaging

The field of stem cell biology continues to evolve with the ongoing characterization of multiple types of stem cells with their inherent potential for experimental and clinical application. Mesenchymal stem cells (MSC) are one of the most promising stem cell types due to their availability and the relatively simple requirements for in vitro expansion and genetic manipulation. Multiple populations described as "MSCs" have now been isolated from various tissues in humans and other species using a variety of culture techniques. Despite extensive in vitro characterization, relatively little has been demonstrated regarding their in vivo biology and therapeutic potential. Nevertheless, clinical trials utilizing MSCs are currently underway. The aim of this review is to critically analyze the field of MSC biology, particularly with respect to the current paradox between in vitro promise and in vivo efficacy. It is the authors' opinion that until this paradox is better understood, therapeutic applications will remain limited.

Vasculogenic potential of long term repopulating cord blood progenitors

In the adult, involvement of bone marrow-derived circulating endothelial progenitor cells (EPCs) in tissue revascularization (vasculogenesis) and the cooperation of hematopoietic cell subsets in supporting this process have been described in different experimental animal models. However, the effective contribution of such cells in restoring organ vascularization in a clinical setting needs to be clarified. In this study, a mouse transplantation model was engrafted by human cord blood hematopoietic stem and progenitor cells to follow the behavior of donor-derived endothelial and hematopoietic cells in the presence of a localized source of an angiogenic inducer. Human endothelial markers (CD31+/CD45-, VE-cadherin+) were always detectable in the bone marrow of transplanted mice, while they were only randomly detectable in peripheral neovascularization sites. To investigate the ability of human transplanted hematopoietic stem cells to support new vessel formation in response to altered homeostatic conditions, chimeric mice were further treated by systemic injection of human mononuclear cells (MNCs). Our data indicate that MNC administration in transplanted mice enhances vasculogenesis in the newly formed vessels. Taken together these results suggest that human-derived EPCs, long-term engrafting a xenotransplantation model, have hematopoietic and endothelial developmental potential, which can be modulated by altering the physiological conditions of host microenvironment.

Myelomonocytic cells are sufficient for therapeutic cell fusion in liver

Liver repopulation with bone marrow-derived hepatocytes (BMHs) can cure the genetic liver disease fumarylacetoacetate hydrolase (Fah) deficiency. BMHs emerge from fusion between donor bone marrow-derived cells and host hepatocytes. To use such in vivo cell fusion efficiently for therapy requires knowing the nature of the hematopoietic cells that fuse with hepatocytes. Here we show that the transplantation into Fah(-/-) mice of hematopoietic stem cells (HSCs) from lymphocyte-deficient Rag1(-/-) mice, lineage-committed granulocyte-macrophage progenitors (GMPs) or bone marrow-derived macrophages (BMMs) results in the robust production of BMHs. These results provide direct evidence that committed myelomonocytic cells such as macrophages can produce functional epithelial cells by in vivo fusion. Because stable bone marrow engraftment or HSCs are not required for this process, macrophages or their highly proliferative progenitors provide potential for targeted and well-tolerated cell therapy aimed at organ regeneration.

On the fate of skeletal myoblasts in a cardiac environment: down-regulation of voltage-gated ion channels

We have analysed the voltage-gated ion channels and fusion competence of skeletal muscle myoblasts labelled with green fluorescent protein (GFP) and the membrane dye PKH transplanted into the infarcted myocardium of syngenic rats. After cell transplantation the animals were killed and GFP(+)-PKH(+) myoblasts enzymatically isolated for subsequent studies of ionic currents through voltage-gated sodium, calcium and potassium channels. A down-regulation of all three types of ion channels after engraftment was observed. The fraction of cells with calcium (68%) and sodium channels (65%) declined to zero within 24 h and 1 week, respectively. Down-regulation of potassium currents (90% in control) occurred within 2 weeks to about 30%. Before injection myoblasts expressed predominantly transient outward potassium channels whereas after isolation from the myocardium exclusively rapid delayed rectifier channels. The currents recovered completely between 1 and 6 weeks under cell culture conditions. The down-regulation of ion channels and changes in potassium current kinetics suggest that the environment provided by infarcted myocardium affects expression of voltage-gated ion channels of skeletal myoblasts.

Stem cell therapy for ischemic heart disease

Recent experimental and clinical observations have suggested that cell transplantation could be of therapeutic value for the treatment of heart disease. This approach was based on the idea that transplanted donor cardiomyocytes would integrate with the host myocardium and thereby directly contribute to cardiac function. Surprisingly, the observation that non-cardiomyogenic cells could also improve cardiac function indicates that functional integration of donor cells might not be required to achieve a beneficial effect. More recently, several observations have suggested the presence of a greater than anticipated developmental repertoire in adult-derived stem cells, which, if further validated, would offer unprecedented opportunities for the restoration of cardiac function in diseased hearts. Here, we discuss current issues regarding the potential use of stem cell transplantation for the treatment of ischemic heart disease.

Haematopoietic stem cell transplantation for autoimmune disease: limits and future potential

Stem cell transplantation (SCT) for autoimmune disease is handicapped by a lack of definitive clinical trials able to demonstrate an overall benefit. This deficiency will become more problematic as the impetus grows to introduce and evaluate additional technologies intended to improve the safety and efficacy of the procedure. The development of effective surrogate analyses to predict outcome by measuring resurgent autoimmune clones or by genomic- and proteomic-based technologies to detect early disease recurrence may be of value in assessing the benefits of these modifications without the need for full-scale, long-term, randomized trials. The introduction of safer allogeneic transplantation techniques may increase the effectiveness of the procedure, while work on marrow stem cell plasticity and/or fusion suggests that SCT may serve not simply to halt the autoimmune process, but also to contribute cells capable of healing or regenerating diseased organs. Finally, the introduction of therapeutic transgenes into transplanted cells may further increase the effectiveness of SCT, although the regulatory complexities of gene therapy trials will probably delay this process. All these innovations will ensure that the next decade will see major changes in the practice and purpose of SCT for autoimmune disease.

Cell-based myocardial repair: How should we proceed?

Cell-based myocardial repair and regeneration heralds a new frontier in the treatment of cardiovascular disease. It provides an unprecedented opportunity to treat the underlying loss of cardiomyocytes that occurs after myocardial injury and that results in the cascade of events leading to heart failure. Yet, even as it progresses to the clinic, much remains to be understood about this technology. For example, controversies exist over the specific cells to be used, the cell dosages needed, how cells will impact the electrical activity of the myocardium, and even whether transplanted cells can actually improve myocardial function. We can perhaps answer these questions more quickly and more effectively - and thus benefit patients more rapidly - if we learn from the successes and failures of our gene therapy colleagues and take a prudent, step-wise approach from bench to bedside. To do so, we need only to promise what we can deliver, to do careful science, and then to deliver well on our promises. Although cellular cardiomyoplasty (cell transplantation for cardiac repair) shows great early clinical promise, its future as a new frontier in the treatment for cardiovascular disease will rest heavily on how we move forward in the next few years. Its success will heavily depend upon conducting carefully controlled, randomized double-blind clinical trials with appropriate endpoints, in the right patients. Choice of cell type, and mode of cell delivery, will also have to be considered, and may have to be matched to the patient. Irrespective of cell type, we can also be assured that cells offer both an opportunity for tissue repair and the potential for not yet understood outcomes. As with any frontier, there will be pitfalls and consequences to be considered that may surpass those of previous endeavors. But so too is the potential for previously unimagined success at treating the leading cause of death in the western world. In short, the promise for cardiovascular cell therapy is too great to be spoiled by ill-designed attempts that forget to account for both the natural propensities of cells and of the myocardium.

Phenotype correction of Fanconi anemia group A hematopoietic stem cells using lentiviral vector

Fanconi anemia (FA) is an autosomal recessive disease characterized by progressive bone marrow failure due to defective stem cell function. FA patients' cells are hypersensitive to DNA cross-linking agents such as mitomycin C (MMC), exposure to which results in cytogenetic aberrations and cell death. To date Moloney murine leukemia virus vectors have been used in clinical gene therapy. Recently, third-generation lentiviral vectors based on the HIV-1 genome have been developed for efficient gene transfer to hematopoietic stem cells. We generated a self-inactivating lentiviral vector expressing the FA group A cDNA driven by the murine stem cell virus U3 LTR promoter and used the vector to transduce side-population (SP) cells isolated from bone marrow of Fanconi anemia group A (Fanca) knockout mice. One thousand transduced SP cells reconstituted the bone marrow of sublethally irradiated Fanca recipient mice. Phenotype correction was demonstrated by stable hematopoiesis following MMC challenge. Using real-time PCR, one proviral vector DNA copy per cell was detected in all lineage-committed cells in the peripheral blood of both primary and secondary recipients. Our results suggest that the lentiviral vector transduces stem cells capable of self-renewal and long-term hematopoiesis in vivo and is potentially useful for clinical gene therapy of FA hematopoietic cells.

Cyclin A2 mediates cardiomyocyte mitosis in the postmitotic myocardium

Cell cycle withdrawal limits proliferation of adult mammalian cardiomyocytes. Therefore, the concept of stimulating myocyte mitotic divisions has dramatic implications for cardiomyocyte regeneration and hence, cardiovascular disease. Previous reports describing manipulation of cell cycle proteins have not shown induction of cardiomyocyte mitosis after birth. We now report that cyclin A2, normally silenced in the postnatal heart, induces cardiac enlargement because of cardiomyocyte hyperplasia when constitutively expressed from embryonic day 8 into adulthood. Cardiomyocyte hyperplasia during adulthood was coupled with an increase in cardiomyocyte mitosis, noted in transgenic hearts at all time points examined, particularly during postnatal development. Several stages of mitosis were observed within cardiomyocytes and correlated with the nuclear localization of cyclin A2. Magnetic resonance analysis confirmed cardiac enlargement. These results reveal a previously unrecognized critical role for cyclin A2 in mediating cardiomyocyte mitosis, a role that may significantly impact upon clinical treatment of damaged myocardium.

Hematopoietic myelomonocytic cells are the major source of hepatocyte fusion partners

Several recent reports have demonstrated that transplantation of bone marrow cells can result in the generation of functional hepatocytes. Cellular fusion between bone marrow-derived cells and host hepatocytes has been shown to be the mechanism of this phenomenon. However, the exact identity of the bone marrow cells that mediate cellular fusion has remained undetermined. Here we demonstrate that the hematopoietic progeny of a single hematopoietic stem cell (HSC) is sufficient to produce functional hepatic repopulation. Furthermore, transplantation of lymphocyte-deficient bone marrow cells and in vivo fate mapping of the myeloid lineage revealed that HSC-derived hepatocytes are primarily derived from mature myelomonocytic cells. In addition, using a Cre/lox-based strategy, we directly demonstrate that myeloid cells spontaneously fuse with host hepatocytes. Our findings raise the possibility that differentiated myeloid cells may be useful for future therapeutic applications of in vivo cellular fusion.

Gene transfer of stromal cell-derived factor-1alpha enhances ischemic vasculogenesis and angiogenesis via vascular endothelial growth factor/endothelial nitric oxide synthase-related pathway: next-generation chemokine therapy for therapeutic neovascularization

BACKGROUND

Stromal cell-derived factor-1alpha (SDF-1alpha) is implicated as a chemokine for endothelial progenitor cells (EPCs). We therefore hypothesized that SDF-1alpha gene transfer would induce therapeutic neovascularization in vivo by functioning as a chemokine of EPC.

METHODS AND RESULTS

To examine SDF-1alpha-induced mobilization of EPC, we used bone marrow-transplanted mice whose blood cells ubiquitously express beta-galactosidase (LacZ). We produced unilateral hindlimb ischemia in the mice and transfected them with plasmid DNA encoding SDF-1alpha or empty plasmids into the ischemic muscles. SDF-1alpha gene transfer mobilized EPCs into the peripheral blood, augmented recovery of blood perfusion to the ischemic limb, and increased capillary density associated with partial incorporation of LacZ-positive cells into the capillaries of the ischemic limb, suggesting that SDF-1alpha induced vasculogenesis and angiogenesis. SDF-1alpha gene transfer did not affect ischemia-induced expression of vascular endothelial growth factor (VEGF) but did enhance Akt and endothelial nitric oxide synthase (eNOS) activity. Blockade of VEGF or NOS prevented all such SDF-1alpha-induced effects.

CONCLUSIONS

SDF-1alpha gene transfer enhanced ischemia-induced vasculogenesis and angiogenesis in vivo through a VEGF/eNOS-related pathway. This strategy might become a novel chemokine therapy for next generation therapeutic neovascularization.

Myocardial aging and senescence: where have the stem cells gone?

Heart failure remains a leading cause of hospital admissions and mortality in the elderly, and current interventional approaches often fail to treat the underlying cause of pathogenesis. Preservation of structure and function in the aging myocardium is most likely to be successful via ongoing cellular repair and replacement, as well as survival of existing cardiomyocytes that generate contractile force. Research has led to a paradigm shift driven by application of stem cells to generate cardiovascular cell lineages. Early controversial findings of pluripotent precursors adopting cardiac phenotypes are now widely accepted, and current debate centers upon the efficiency of progenitor cell incorporation into the myocardium. Much work remains to be done in determining the relevant progenitor cell population and optimizing conditions for efficient differentiation and integration. Significant implications exist for treatment of pathologically damaged or aging myocardium since future interventional approaches will capitalize upon the use of cardiac stem cells as therapeutic reagents.

PECAM-1 is expressed on hematopoietic stem cells throughout ontogeny and identifies a population of erythroid progenitors

Platelet endothelial cell adhesion molecule-1 (PECAM-1) (CD31) is an adhesion molecule expressed on endothelial cells and subsets of leukocytes. Analysis of phenotypically defined hematopoietic stem cells (HSCs) from the yolk sac, fetal liver, and adult bone marrow demonstrates CD31 expression on these cells throughout development. CD31+ c-kit+ cells, but not CD31- c-kit+ cells, isolated from day-9.5 yolk sac give rise to multilineage hematopoiesis in vivo. Further evaluation of the CD31+ lineage marker-negative fraction of adult bone marrow reveals functionally distinct cell subsets. Transplantation of CD31+ Lin- c-kit- cells fails to protect lethally irradiated recipients, while CD31+ Lin- c-kit+ Sca-1- cells (CD31+ Sca-1-) provide radioprotection in the absence of long-term donor-derived hematopoiesis. Although donor-derived leukocytes were not detected in CD31+ Sca-1- recipients, donor-derived erythroid cells were transiently produced during the initial phases of bone marrow recovery. These results demonstrate CD31 expression on hematopoietic stem cells throughout ontogeny and identify a population of CD31+ short-term erythroid progenitors cells that confer protection from lethal doses of radiation.

Bone Marrow Cells and Myocardial Regeneration

Hematopoietic stem cell (HSC) plasticity and its clinical application have been studied profoundly in the past few years. Recent investigations indicate that HSC and other bone marrow stem cells can develop into other tissues. Because of the high morbidity and mortality of myocardial infarction and other heart disorders, myocardial regeneration is a good example of the clinical application of HSC plasticity in regenerative medicine. Preclinical studies in animals suggest that the use of this kind of treatment can reconstruct heart blood vessels, muscle, and function. Some clinical study results have been reported in the past 2 years. In 2003, reports of myocardial regeneration treatment increased significantly. Other studies include observations on the cell surface markers of transplanted cells and treatment efficacy. Some investigations, such as HSC testing, have focused on clinical applications using HSC plasticity and bone marrow transplantation to treat different types of disorders. In this review, we focus on the clinical application of bone marrow cells for myocardial regeneration.

Transplantation of Fetal Cardiomyocytes into Infarcted Rat Hearts Results in Long-Term Functional Improvement

Studies have demonstrated the feasibility of transplanting cardiomyocytes after myocardial infarction (MI). However, persistence and effects on left ventricular (LV) function have not been elucidated in long-term studies. Ventricular fetal cardiomyocytes from embryos of both sexes were injected into marginal regions of MI 4 weeks after suture occlusion of the left anterior descending artery in adult female rats. Two and 6 months after transplantation (Tx), engrafted cells were traced by immunohistochemical in situ hybridization for Y chromosomes or bromodeoxyuridine (BrdU) staining, LV dimensions and function were assessed by echocardiography, and LV pressure was assessed ex vivo in a Langendorff perfusion system. Immunohistochemistry for alpha-sarcomeric actin and Y chromosomes revealed the presence of transplanted cells in infarcted host myocardium at both 2 and 6 months. End-diastolic LV diameter markedly decreased after Tx and fractional shortening gradually increased after Tx (31.3 +/- 4.5% before Tx, 45.4 +/- 4.2% at 6 months; p<0.005). Wall area fraction and MI size were unaffected by Tx. In hearts with MI, but not in normal hearts, Tx led to the development of higher pressures (87 +/- 18 versus 38 +/- 8 mmHg, 6 months post-Tx versus nontreated). After catecholamine stimulation, both infarcted and normal hearts developed higher pressures after Tx (p<0.005), ultimately associated with reduced mortality after Tx versus nontreated. Transplanted cardiomyocyte-rich graft cells persist in host myocardium and mediate continuous improvement of LV function and survival in a rat model of MI even during long-term follow-up, possibly involving a catecholamine-sensitive mechanism.

Endothelial Progenitor Cells: More Than an Inflammatory Response?

The formation of new capillaries (angiogenesis) may be of clinical importance in facilitating reperfusion and regeneration of hibernating cardiac tissue after myocardial infarction and in microvascular ischemia. Evidence is accumulating that as part of the response to hypoxia, bone marrow-derived circulating endothelial progenitor cells (CEPs) are mobilized and subsequently differentiate into proper endothelial cells. There are also indications that such CEPs can facilitate endothelial repair and angiogenesis in vivo. It is not clear yet, however, whether these CEPs are essential for these adaptive processes or what the relative contribution of CEP is compared with that of other mononuclear inflammatory cells that are mobilized to areas of ischemia. Moreover, there are still many uncertainties about how cardiovascular risk factors alter CEP function. Particularly when therapeutically mobilizing CEPs, a further understanding of this issue is essential to assess the risk of potentially harmful side effects of altered CEP function.

Mechanical, cellular, and molecular factors interact to modulate circulating endothelial cell progenitors

It appears that there are two classes of human circulating endothelial cell (EC) progenitors, CD34+and CD34–CD14+cells. Attention has focused on CD34+cells, yet CD34–CD14+monocytic cells are far more abundant and may represent the most common class of circulating EC progenitor. Little is known about molecular or physiological factors that regulate putative CD34–CD14+EC progenitor function, although factors secreted by other blood and cardiovascular cells to which they are exposed probably affect their behavior. Hypoxia and stretch are two important physiological stimuli known to trigger growth factors in cardiovascular cells and accordingly may modulate EC progenitors. To investigate the impact of these environmental parameters on EC progenitors, EC production in CD34–CD14+cultures was evaluated. Our data indicate that neither stretch nor hypoxia alters EC production by EC progenitors directly but do so indirectly through their effects on cardiovascular cells. Conditioned media (CM) from coronary artery smooth muscle cells inhibit EC production in culture, and this inhibition is stronger if the coronary smooth muscle cells have been subjected to cyclic stretch. In contrast, cardiomyocyte CM increases EC cell number, an effect that is potentiated if the myocytes have been subjected to hypoxia. Significantly, EC progenitor responses to CM are altered by the presence of CD34–CD14–peripheral blood mononuclear cells (PBMCs). Moreover, CD34–CD14–PBMCs attenuate EC progenitor responsiveness to the angiogenic factors basic fibroblast growth factor (FGF-2), vascular endothelial cell growth factor-A165, and erythropoietin while inducing EC progenitor death in the presence of transforming growth factor-β1in vitro

Stem cell plasticity, cell fusion, and transdifferentiation

One of the most contentious issues in biology today concerns the existence of stem cell plasticity. The term "plasticity" refers to the capacity of tissue-derived stem cells to exhibit a phenotypic potential that extends beyond the differentiated cell phenotypes of their resident tissue. Although evidence of stem cell plasticity has been reported by multiple laboratories, other scientists have not found the data persuasive and have remained skeptical about these new findings. This review will provide an overview of the stem cell plasticity controversy. We will examine many of the major objections that have been made to challenge the stem cell plasticity data. This controversy will be placed in the context of the traditional view of stem cell potential and cell phenotypic diversification. What the implications of cell plasticity are, and how its existence may modulate our present understanding of stem cell biology, will be explored. In addition, we will examine a topic that is usually not included within a discussion of stem cell biology--the direct conversion of one differentiated cell type into another. We believe that these observations on the transdifferentiation of differentiated cells have direct bearing on the issue of stem cell plasticity, and may provide insights into how cell phenotypic diversification is realized in the adult and into the origin of cell phenotypes during evolution.

Bone marrow–derived hematopoietic cells generate cardiomyocytes at a low frequency through cell fusion, but not transdifferentiation

Recent studies have suggested that bone marrow cells might possess a much broader differentiation potential than previously appreciated. In most cases, the reported efficiency of such plasticity has been rather low and, at least in some instances, is a consequence of cell fusion. After myocardial infarction, however, bone marrow cells have been suggested to extensively regenerate cardiomyocytes through transdifferentiation. Although bone marrow-derived cells are already being used in clinical trials, the exact identity, longevity and fate of these cells in infarcted myocardium have yet to be investigated in detail. Here we use various approaches to induce acute myocardial injury and deliver transgenically marked bone marrow cells to the injured myocardium. We show that unfractionated bone marrow cells and a purified population of hematopoietic stem and progenitor cells efficiently engraft within the infarcted myocardium. Engraftment was transient, however, and hematopoietic in nature. In contrast, bone marrow-derived cardiomyocytes were observed outside the infarcted myocardium at a low frequency and were derived exclusively through cell fusion.

Regenerative capacity of the myocardium: implications for treatment of heart failure

Research into myocardial regeneration has an exciting future, shown by the results of experimental and clinical work challenging the dogma that the heart is a postmitotic non-regenerating organ. Such studies have initiated a lively debate about the feasibility of novel treatment approaches leading to the recovery of damaged myocardial tissue. The possibility of reconstituting dead myocardium by endogenous cardiomyocyte replication, transplantation, or activation of stem cells--or even cloning of an artificial heart--is being advanced, and will be a major subject of future research. Although health expenditure for heart failure in the industrial world is high, we are still a long way from being able to treat the cause of reduced myocardial contractility. Despite the hopes of some people, conventional treatment for heart failure does not achieve myocardial regeneration. We present a virtual case report of a patient with acute myocardial infarction; we discuss treatment options, including strategies aimed at organ regeneration.

Cell therapy for bone disease: a review of current status

Bone marrow is a reservoir of pluripotent stem/progenitor cells for mesenchymal tissues. Upon in vitro expansion, in vivo bone-forming efficiency of bone marrow stromal cells (BMSCs) is dramatically lower in comparison with fresh bone marrow, and their in vitro multidifferentiation potentials are gradually lost. Nevertheless, when BMSCs are isolated and expanded in the presence of fibroblast growth factor 2, the percentage of cells able to differentiate into the osteogenic, chondrogenic, and adipogenic lineages is greater. Osteogenic progenitors are not exclusive to skeletal tissues. We could also think of cells in different adult tissues as potentially capable of following an osteochondrogenic differentiation pathway, but, under normal physiological conditions, they are inhibited in this process by the environment and/or the adjacent cell populations. When, for some reason such as pathology, the environment changes dramatically and the inhibiting condition is removed, these cells could become osteoblasts. Bone is repaired via local delivery of cells within a scaffold. Bone formation was first assessed in small animal models. Large animal models were successively developed to prove the feasibility of the tissue engineering approach in a model closer to a real clinical situation. Eventually, pilot clinical studies were performed. Extremely appealing is the possibility of using mesenchymal progenitors in the therapy of genetic bone diseases via systemic infusion. There is experimental evidence to suggest that mesenchymal progenitors delivered by this route engraft with a very low efficiency and do not produce relevant and durable clinical effects. Under some conditions, where the local microenvironment is either altered (i.e., injury) or under important remodeling processes (i.e., fetal growth), engraftment of stem and progenitor cells seems to be enhanced. A better understanding of their engraftment mechanisms will, hopefully, extend the field of therapeutic applications of mesenchymal progenitors.

Cardiovascular disease: potential impact of stem cell therapy

Atherosclerotic vascular disease becomes a clinical problem when there is sufficient atherosclerotic plaque burden and/or endothelial dysfunction to cause a limitation of nutrient blood flow to tissues. However, once myocardial infarction has occurred, there is little, if any, way to stimulate the growth of new blood vessels or cardiac muscle to replace that which has been lost. The potential use of hematopoietic stem cells (HSCs) to treat cardiovascular disease has recently been suggested from preclinical and clinical studies. HSCs are precursors of all the blood cells, but they may also give rise to cells of the vascular system, endothelial cells in the form of endothelial progenitor cells (EPCs). Clinical trials have been conducted in patients with either acute myocardial infarction or limb ischemia to determine the initial effectiveness and safety of this treatment approach. These studies demonstrated the potential clinical effectiveness of this stem cell approach to the treatment of patients with acute myocardial ischemia and limb ischemia. Today, more preclinical studies are planned to elucidate the mechanism by which transplanted stem cells can home and differentiate into these endothelial cells and cardiac muscle cells. At the same time, new clinical trials are planned to evaluate both chronic, stable as well as acute myocardial ischemia and limb ischemia with CD34+ and CD133+ stem cells, as well as with further selected EPCs and mesenchymal stem cells.

Scalable production of embryonic stem cell-derived cardiomyocytes

Cardiomyocyte transplantation could offer a new approach to replace scarred, nonfunctional myocardium in a diseased heart. Clinical application of this approach would require the ability to generate large numbers of donor cells. The purpose of this study was to develop a scalable, robust, and reproducible process to derive purified cardiomyocytes from genetically engineered embryonic stem (ES) cells. ES cells transfected with a fusion gene consisting of the alpha-cardiac myosin heavy chain (MHC) promoter driving the aminoglycoside phosphotransferase (neomycin resistance) gene were used for cardiomyocyte enrichment. The transfected cells were aggregated into embyroid bodies (EBs), inoculated into stirred suspension cultures, and differentiated for 9 days before selection of cardiomyocytes by the addition of G418 with or without retinoic acid (RA). Throughout the culture period, EB and viable cell numbers were measured. In addition, flow cytometric analysis was performed to monitor sarcomeric myosin (a marker for cardiomyocytes) and Oct-4 (a marker for undifferentiated ES cells) expression. Enrichment of cardiomyocytes was achieved in cultures treated with either G418 and retinoic acid (RA) or with G418 alone. Eighteen days after differentiation, G418-selected flasks treated with RA contained approximately twice as many cells as the nontreated flasks, as well as undetectable levels of Oct-4 expression, suggesting that RA may promote cardiac differentiation and/or survival. Immunohistological and electron microscopic analysis showed that the harvested cardiomyocytes displayed many features characteristic of native cardiomyocytes. Our results demonstrate the feasibility of large-scale production of viable, ES cell-derived cardiomyocytes for tissue engineering and/or implantation, an approach that should be transferable to other ES cell derived lineages, as well as to adult stem cells with in vitro cardiomyogenic activity.

Evidence for Fusion Between Cardiac and Skeletal Muscle Cells

Cardiomyoplasty with skeletal myoblasts may benefit cardiac function after infarction. Recent reports indicate that adult stem cells can fuse with other cell types. Because myoblasts are “fusigenic” cells by nature, we hypothesized they might be particularly likely to fuse with cardiomyocytes. To test this, neonatal rat cardiomyocytes labeled with LacZ and green fluorescent protein (GFP) were cocultured with unlabeled C2C12 myoblasts. After 3 days, we observed a small population of skeletal myotubes that expressed LacZ and GFP, indicating cell fusion. To test whether such fusion occurred in vivo, LacZ-expressing C2C12 myoblasts were grafted into normal nude mouse hearts. At 2 weeks after grafting, cells at the graft-host interface expressed both LacZ and cardiac-specific myosin light chain 2v (MLC2v). To test more definitively whether fusion between skeletal and cardiac muscle could occur, we used a Cre/lox reporter system that activated LacZ only upon cell fusion. When neonatal cardiomyocytes from α-myosin heavy chain promoter (α-MHC)-Cre mice were cocultured with myoblasts from floxed-lacZ reporter mice, LacZ was activated in a subset of cells, indicating cell fusion occurred in vitro. Finally, we grafted the floxed-lacZ myoblasts into normal hearts of α-MHC-Cre + and α-MHC-Cre − mice (n=5 each). Hearts analyzed at 4 days and 1 week after transplantation demonstrated activation of LacZ when the skeletal muscle cells were implanted into hearts of α-MHC-Cre + mice, but not after implantation into α-MHC-Cre − mice. These data indicate that skeletal muscle cell grafting gives rise to a subpopulation of skeletal-cardiac hybrid cells with a currently unknown phenotype. The full text of this article is available online at http://circres.ahajournals.org .

Low angiogenic potency induced by the implantation of ex vivo expanded CD117+stem cells

Ex vivo expansion of stem cells might be a feasible method of resolving the problem of limited cell supply in cell-based therapy. The implantation of expanded CD34+endothelial progenitor cells has the capacity to induce angiogenesis. In this study, we tried to induce angiogenesis by implanting expanded CD117+stem cells derived from mouse bone marrow. After 2 wk of culture with the addition of several growth factors, the CD117+stem cells expanded ∼20-fold and had an endothelial phenotype with high expression of CD34 and vascular endothelial-cadherin. However, >70% of these ex vivo expanded cells had a senescent phenotype by β-galactosidase staining, and their survival and incorporation were poor after implantation into the ischemic limbs of mice. Compared with the PBS injection only, the microvessel density and the percentage of limb blood flow were significantly higher after the implantation of 2 × 105freshly collected CD117+cells ( P < 0.01) but not after the implantation of 2 × 105expanded CD117+cells ( P > 0.05). These data indicate that ex vivo expansion of CD117+stem cells has low potency for inducing therapeutic angiogenesis, which might be related to the cellular senescence during ex vivo expansion.

Embryonic Cell Lines with Endothelial Potential: An In Vitro System for Studying Endothelial Differentiation

Objective— Endothelial differentiation is a fundamental process in angiogenesis and vasculogenesis with implications in development, normal physiology, and pathology. To better understand this process, an in vitro cellular system that recapitulates endothelial differentiation and is amenable to experimental manipulations is required.

Methods and Results— Embryonic cell lines that differentiate exclusively into endothelial cells were derived from early mouse embryos using empirical but reproducible culture techniques without viral or chemical transformation. The cells were not pluripotent and expressed reduced levels of Oct 4 and Rex-1 . They were non-tumorigenic with a population doubling time of ≈15 hours. When plated on matrigel, they readily differentiated to form patent tubular structures with diameters of 30 to 150 μm. The differentiated cells endocytosed acetylated low-density lipoprotein (LDL) and began to express endothelial-specific markers such as CD34, CD31, Flk-1, TIE2, P-selectin, Sca-1, and thy-1. They also expressed genes essential for differentiation and maintenance of endothelial lineages, eg, Flk-1, vascular endothelial growth factor (VEGF), and angiopoietin-1. When transplanted into animal models, these cells incorporated into host vasculature.

Conclusions— These cell lines can undergo in vitro and in vivo endothelial differentiation that recapitulated known endothelial differentiation pathways. Therefore, they are ideal for establishing an in vitro cellular system to study endothelial differentiation.

Engineered heart tissue for regeneration of diseased hearts

Cardiac tissue engineering aims at providing contractile heart muscle constructs for replacement therapy in vivo. At present, most cardiac tissue engineering attempts utilize heart cells from embryonic chicken and neonatal rats and scaffold materials. Over the past years our group has developed a novel technique to engineer collagen/matrigel-based cardiac muscle constructs, which we termed engineered heart tissue (EHT). EHT display functional and morphological properties of differentiated heart muscle and can be constructed in different shape and size from collagen type I, extracellular matrix proteins (Matrigel((R))), and heart cells from neonatal rats and embryonic chicken. First implantation studies in syngeneic Fischer 344 rats provided evidence of EHT survival and integration in vivo. This review will focus on our experience in tissue engineering of cardiac muscle. Mainly, EHT construction, matrix requirements, potential applications of different cell types including stem cells, and our first implantation experiences will be discussed. Despite many critical and unresolved questions, we believe that cardiac tissue engineering in general has an interesting perspective for the replacement of malfunctioning myocardium and reconstruction of congenital malformations.

Hematopoietic origin of microglial and perivascular cells in brain

BACKGROUND

Bone marrow (BM)-derived cells differentiate into a wide variety of cell types. BM contains a heterogeneous population of stem and progenitor cells including hematopoietic stem cells, marrow stromal cells, and perhaps other progenitor cells. To establish unequivocally the transdifferentiation capability of a hematopoietic cell to a nonhematopoietic cell (endothelial cells, neurons, and glial cells), it is imperative to demonstrate that a single cell or clone of that single cell (clonal analysis) differentiates into cells comprising vessels or other cells in the brain.

METHODS

We generated mice that exhibited a high level of hematopoietic reconstitution from a single enhanced green fluorescent protein (EGFP) stem cell. To achieve this, we combined FACS sorting and cell culture to generate a population of cells derived from a single hematopoietic stem cell (Lin-, CD34-, c-kit+, and Sca-1+). Clonal populations of cells were then transplanted into lethally irradiated recipient mice. After 3-4 months of engraftment, some mice underwent middle cerebral artery (MCA) suture occlusion. EGFP immunocytochemistry and dual labeling was performed with cell-specific markers on tissue from various time points.

RESULTS

In all transplanted mice, EGFP+ highly ramified cells were seen in the brain parenchyma. These cells stained with RCA120 lectin and had the characteristics of parenchymal microglial cells. In brains without infarction and in uninfarcted brain regions of mice that underwent MCA occlusion, there were many EGFP+ cells in a perivascular distribution, associated with both small and larger blood vessels. The cells were tightly apposed to the vessel wall and some had long processes that enveloped the endothelial cells. After MCA occlusion, there was an influx of EGFP expressing cells in the ischemic tissue that colocalized with the "neovascularization." These EGFP+ cells were wrapped around endothelial cells in an albuminal location and did not coexpress von Willebrand Factor or CD31. We detected rare dual-labeled EGFP and NeuN-expressing cells. We detected two staining patterns. The more frequent pattern was phagocytosis of NeuN cells by EGFP expressing cells. However, we also detected rarer cells where the EGFP and NeuN appeared to be colocalized by confocal microscopy.

CONCLUSIONS

HSC differentiate into parenchymal microglial cells and perivascular cells in the brain. The numbers of these cells increase after cerebral ischemia. The HSC is therefore one source of parenchymal microglial cells and a source for perivascular cells. After a cerebral infarction, there are rare HSC-derived cells that stain with the neuronal marker, NeuN. However, the more common pattern appears to represent phagocytosis of damaged neurons by EGFP+ microglial cells.

Molecular phenotype of airway side population cells

Lung epithelial-specific stem cells have been localized to discrete microenvironments throughout the adult conducting airway. Properties of these cells include pollutant resistance, multipotent differentiation, and infrequent proliferation. Goals of the present study were to use Hoechst 33342 efflux, a property of stem cells in other tissues, to purify and further characterize airway stem cells. Hoechst 33342 effluxing lung cells were identified as a verapamil-sensitive side population by flow cytometry. Lung side population cells were further subdivided on the basis of hematopoietic (CD45 positive) or nonhematopoietic (CD45 negative) origin. Nonhematopoietic side population cells were enriched for stem cell antigen-1 reactivity and expressed molecular markers specific to both airway and mesenchymal lineages. Analysis of the molecular phenotype of airway-derived side population cells indicates that they are similar to neuroepithelial body-associated variant Clara cells. Taken together, these data suggest that the nonhematopoietic side population isolated from lung is enriched for previously identified airway stem cells.

Multiparameter analysis of murine bone marrow side population cells

We describe the multiparameter flow cytometric analysis of the relationship between side population (SP) formation and well-characterized, antigen-defined stem cell subsets. We also compared the competitive repopulation ability of subsets defined by the SP profile to those identified by antigenic markers. The vast majority of SP cells possessed a primitive cell phenotype (c-kit+, SCA-1+, Thy-1+, CD31+, CD135neg, lineageneg), but only a minority of antigen-defined subsets were SP cells. Hence, although SP cells are identified independently of antigenic markers, they are not distinct from established stem cell phenotypes, but are a small subset of them. Approximately half of SP cells expressed CD34 at readily detectable levels, and one-third of SP cells possessed the primitive c-kit+, SCA-1+, lineageneg, CD34neg cell phenotype. Since most SP cells are a subset of c-kit+, Thy-1+, lineageneg, SCA-1+ cells (KTLS), we determined whether the SP cell subset represents a further enrichment in long-term repopulating cell content. The SP+ subset of KTLS+ cells was more enriched for competitive repopulation units than the SP- fraction of KTLS+ cells. Hence, the SP cell fraction highlights a subset of the long-term repopulating cells found within the already highly purified KTLS fraction.

Human pancreatic islet-derived progenitor cell engraftment in immunocompetent mice

The potential for the use of stem/progenitor cells for the restoration of injured or diseased tissues has garnered much interest recently, establishing a new field of research called regenerative medicine. Attention has been focused on embryonic stem cells derived from human fetal tissues. However, the use of human fetal tissue for research and transplantation is controversial. An alternative is the isolation and utilization of multipotent stem/progenitor cells derived from adult donor tissues. We have previously reported on the isolation, propagation, and partial characterization of a population of stem/progenitor cells isolated from the pancreatic islets of Langerhans of adult human donor pancreata. Here we show that these human adult tissue-derived cells, nestin-positive islet-derived stem/progenitor cells, prepared from human adult pancreata survive engraftment and produce tissue chimerism when transplanted into immunocompetent mice either under the kidney capsule or by systemic injection. These xenografts seem to induce immune tolerance by establishing a mixed chimerism in the mice. We propose that a population of stem/progenitor cells isolated from the islets of the pancreas can cross xenogeneic transplantation immune barriers, induce tissue tolerance, and grow.

Short-term human prostate primary xenografts: an in vivo model of human prostate cancer vasculature and angiogenesis

Transgenic spontaneously occurring and transplantable xenograft models of adenocarcinoma of the prostate (CaP) are established tools for the study of CaP progression and metastasis. However, no animal model of CaP has been characterized that recapitulates the response of the human prostate vascular compartment to the evolving tumor microenvironment during CaP progression. We report that primary xenografts of human CaP and of noninvolved areas of the human prostate peripheral zone transplanted to athymic nude mice provide a unique model of human angiogenesis occurring in an intact human prostate tissue microenvironment. Angiogenesis in human kidney primary xenografts established from human renal cell carcinoma and noninvolved kidney tissue, a highly vascular organ and cancer, was compared with angiogenesis in xenografts from the relatively less vascularized prostate. Immunohistochemical identification of the human versus mouse host origin of the endothelial cells and of human endothelial cell proliferation in the human prostate and human kidney xenografts demonstrated that: (a) the majority of the vessels in primary xenografts of benign and malignant tissue of both organs were lined with human endothelial cells through the 30-day study period; (b) the mean vessel density was increased in both the CaP and benign prostate xenografts relative to the initial tissue, whereas there was no significant difference in mean vessel density in the renal cell carcinoma and benign kidney xenografts compared with the initial tissue; and (c) the number of vessels with proliferating endothelial cells in primary xenografts of CaP and benign prostate increased compared with their respective initial tissue specimens, whereas the number of vessels with proliferating endothelial cells decreased in the benign kidney xenografts. Short-term primary human prostate xenografts, therefore, represent a valuable in vivo model for the study of human angiogenesis within a human tissue microenvironment and for comparison of angiogenesis in CaP versus benign prostate.

The role of stem cells for treatment of cardiovascular disease

Cardiovascular disease is a global cause of mortality and morbidity. Current treatments fail to address the underlying scarring and cell loss, which are the causes of ischaemic heart failure. Cellular transplantation can overcome these problems and new impetus has been injected into this field following the isolation of human embryonic and adult stem cells. These cells have shown remarkable ability to produce cardiomyocytes and vascular cells in vitro and in vivo. Initial transplantation studies have demonstrated functional benefits and it is hoped further randomised clinical trials will concur with initial findings. Much basic science remains to be unearthed, such as the signals for homing, differentiation and engraftment of transplanted cells. Further matters of concern are the role of cell fusion and the mechanisms by which transplanted cells improve cardiac function. In spite of initial progress made in stem cell therapy there is still much to be done and we are some way off from achieving the goal of effective cellular regeneration.

Stem cell plasticity: from transdifferentiation to macrophage fusion

The past 5 years have witnessed an explosion of interest in using adult-derived stem cells for cell and gene therapy. This has been driven by a number of findings, in particular, the possibility that some adult stem cells can differentiate into non-autologous cell types, and also the discovery of multipotential stem cells in adult bone marrow. These discoveries suggested a quasi-alchemical nature of cells derived from adult organs, thus raising new and exciting therapeutic possibilities. Recent data, however, argue against the whole idea of stem cell 'plasticity', and bring into question the therapeutic strategies based upon this concept. Here, we will review the current state of knowledge in the field and discuss some of the clinical implications.

Profoundly reduced neovascularization capacity of bone marrow mononuclear cells derived from patients with chronic ischemic heart disease

BACKGROUND

Cell therapy with bone marrow-derived stem/progenitor cells is a novel option for improving neovascularization and cardiac function in ischemic heart disease. Circulating endothelial progenitor cells in patients with coronary heart disease are impaired with respect to number and functional activity. However, whether this impairment also extends to bone marrow-derived mononuclear cells (BM-MNCs) in patients with chronic ischemic cardiomyopathy (ICMP) is unclear.

METHODS AND RESULTS

BM-MNCs were isolated from bone marrow aspirates in 18 patients with ICMP (ejection fraction, 38+/-11%) and 8 healthy control subjects (controls). The number of hematopoietic stem/progenitor cells (CD34+/CD133+), CD49d(+) (VLA-4) cells, and CXCR4+ cells did not differ between the 2 groups. However, the colony-forming capacity of BM-MNCs from patients with ICMP was significantly lower compared with BM-MNCs from healthy controls (37.3+/-25.0 versus 113.8+/-70.4 granulocyte-macrophage colony-forming units; P=0.009). Likewise, the migratory response to stromal cell-derived factor 1 (SDF-1) and vascular endothelial growth factor (VEGF) was significantly reduced in BM-MNCs derived from patients with ICMP compared with BM-MNCs from healthy controls (SDF-1, 46.3+/-26.2 versus 108.6+/-40.4 cells/microscopic field, P<0.001; VEGF, 34+/-24.2 versus 54.8+/-29.3 cells/microscopic field, P=0.027). To assess the in vivo relevance of these findings, we tested the functional activity of BM-MNCs to improve neovascularization in a hindlimb animal model using nude mice. Two weeks after ligation of the femoral artery and intravenous injection of 5x10(5) BM-MNCs, laser Doppler-derived relative limb blood flow in mice treated with BM-MNCs from patients with ICMP was significantly lower compared with mice treated with BM-MNCs from healthy controls (0.45+/-0.14 versus 0.68+/-0.15; P<0.001). The in vivo neovascularization capacity of BM-MNCs closely correlated with the in vitro assessment of SDF-1-induced migration (r=0.78; P<0.001) and colony-forming capacity (r=0.74; P<0.001).

CONCLUSIONS

BM-MNCs isolated from patients with ICMP have a significantly reduced migratory and colony-forming activity in vitro and a reduced neovascularization capacity in vivo despite similar content of hematopoietic stem cells. This functional impairment of BM-MNCs from patients with ICMP may limit their therapeutic potential for clinical cell therapy.

Haematopoietic stem cells adopt mature haematopoietic fates in ischaemic myocardium

Under conditions of tissue injury, myocardial replication and regeneration have been reported. A growing number of investigators have implicated adult bone marrow (BM) in this process, suggesting that marrow serves as a reservoir for cardiac precursor cells. It remains unclear which BM cell(s) can contribute to myocardium, and whether they do so by transdifferentiation or cell fusion. Here, we studied the ability of c-kit-enriched BM cells, Lin- c-kit+ BM cells and c-kit+ Thy1.1(lo) Lin- Sca-1+ long-term reconstituting haematopoietic stem cells to regenerate myocardium in an infarct model. Cells were isolated from transgenic mice expressing green fluorescent protein (GFP) and injected directly into ischaemic myocardium of wild-type mice. Abundant GFP+ cells were detected in the myocardium after 10 days, but by 30 days, few cells were detectable. These GFP+ cells did not express cardiac tissue-specific markers, but rather, most of them expressed the haematopoietic marker CD45 and myeloid marker Gr-1. We also studied the role of circulating cells in the repair of ischaemic myocardium using GFP+-GFP- parabiotic mice. Again, we found no evidence of myocardial regeneration from blood-borne partner-derived cells. Our data suggest that even in the microenvironment of the injured heart, c-kit-enriched BM cells, Lin- c-kit+ BM cells and c-kit+ Thy1.1(lo) Lin- Sca-1+ long-term reconstituting haematopoietic stem cells adopt only traditional haematopoietic fates.

Haematopoietic stem cells do not transdifferentiate into cardiac myocytes in myocardial infarcts

The mammalian heart has a very limited regenerative capacity and, hence, heals by scar formation. Recent reports suggest that haematopoietic stem cells can transdifferentiate into unexpected phenotypes such as skeletal muscle, hepatocytes, epithelial cells, neurons, endothelial cells and cardiomyocytes, in response to tissue injury or placement in a new environment. Furthermore, transplanted human hearts contain myocytes derived from extra-cardiac progenitor cells, which may have originated from bone marrow. Although most studies suggest that transdifferentiation is extremely rare under physiological conditions, extensive regeneration of myocardial infarcts was reported recently after direct stem cell injection, prompting several clinical trials. Here, we used both cardiomyocyte-restricted and ubiquitously expressed reporter transgenes to track the fate of haematopoietic stem cells after 145 transplants into normal and injured adult mouse hearts. No transdifferentiation into cardiomyocytes was detectable when using these genetic techniques to follow cell fate, and stem-cell-engrafted hearts showed no overt increase in cardiomyocytes compared to sham-engrafted hearts. These results indicate that haematopoietic stem cells do not readily acquire a cardiac phenotype, and raise a cautionary note for clinical studies of infarct repair.