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

Platelet-Derived Growth Factor-AB Promotes the Generation of Adult Bone Marrow–Derived Cardiac Myocytes

The directed generation of cardiac myocytes from endogenous stem cells offers the potential for novel therapies for cardiovascular disease. To facilitate the development of such approaches, we sought to identify and exploit the pathways directing the generation of cardiac myocytes from adult rodent bone marrow cells (BMCs). In vitro cultures supporting the spontaneous generation of functional cardiac myocytes from murine BMCs demonstrated induced expression of platelet-derived growth factor (PDGF)-A and -B isoforms with α- and β-myosin heavy chains as well as connexin43. Supplementation of PDGF-AB speeded the kinetics of myocyte development in culture by 2-fold. In a rat heart, myocardial infarction pretreatment model PDGF-AB also promoted the derivation of cardiac myocytes from BMCs, resulting in a significantly greater number of islands of cardiac myocyte bundles within the myocardial infarction scar compared with other treatment groups. However, gap junctions were detected only between the cardiac myocytes receiving BMCs alone, but not BMCs injected with PDGF-AB. Echocardiography and exercise testing revealed that the functional improvement of hearts treated with the combination of BMCs and PDGF-AB was no greater than with injections of BMCs or PDGF-AB alone. These studies demonstrated that PDGF-AB enhances the generation of BMC-derived cardiac myocytes in rodent hearts, but suggest that alterations in cellular patterning may limit the functional benefit from the combined injection of PDGF-AB and BMCs. Strategies based on the synergistic interactions of PDGF-AB and endogenous stem cells will need to maintain cellular patterning in order to promote the restoration of cardiac function after acute coronary occlusion. The full text of this article is available online at http://circres.ahajournals.org.

Effect of partial hepatectomy on in vivo engraftment after intravenous administration of human adipose tissue stromal cells in mouse

Their multidifferentiation potential makes human mesenchymal stem cells (hMSCs) candidates for cell-based therapeutic strategies for tissue injuries and for hematopoietic disorders by both local and systemic application. Despite their potential clinical utility in cellular and gene therapy, the fate of adipose stromal cells (ATSCs) after systemic administration is mostly unknown. In this study, we investigated the distribution of ATSCs injected intravenously and the effect of partial hepatectomy on their distribution. Adipose tissue stromal cells (hATSCs) were obtained from adipose tissues of adult human donors. Under appropriate culture conditions, hATSCs were induced to differentiate into osteocytes and adipocyte lineages. The hATSCs were marked by infection with the lacZ-adeno virus, and the distribution of injected cells was examined by X-Gal staining. Immunosuppression was achieved by the administration of cyclosporin into mice. The hATSCs were engrafted onto various tissues, including brain, thymus, heart, liver, and lung, after intravenous administration. Liver regeneration induced by partial hepatectomy enhanced the integration of hATCSCs into the liver. These results demonstrate that hATSCs have the ability to proliferate extensively in culture, and that they maintain their multilineage differentiation potential in vitro, establishing their progenitor cell nature. These cells are promising candidates for developing novel cell-based therapeutic approaches to postnatal tissue repair.

Seeing Within

Molecular imaging is a rapidly evolving discipline with the goal of developing tools to display and quantify molecular and cellular targets in vivo. The heart of this field is based on the rational design and screening of targeted and activatable imaging reporter agents to sense fundamental processes of biology. Parallel advances in small animal imaging systems and in agent synthesis have allowed molecular imaging applications to extend into the in vivo arena. These advances have permitted, for example, in vivo sensing of inflammation, apoptosis, cell trafficking, and gene expression. In this review, we first review core principles of molecular imaging with an emphasis on smart, activatable agent technology. We then discuss applications of state-of-the-art molecular probes to interrogate important aspects of cardiovascular biology, with a focus on atherosclerosis, thrombosis, and heart failure. In the ensuing years, we anticipate that fundamental aspects of cardiovascular biology will be detectable in vivo, and that promising molecular imaging agents will be translated into the clinical arena to guide diagnosis and therapy of human cardiovascular illness.

Fatal coronary artery disease after unrelated donor bone marrow transplantation

Several factors are responsible for the occurrence of cardiac complications after bone marrow transplantation (BMT). These factors include the cardiotoxic effects of radiation therapy, antineoplastic and immunosuppressive drugs, abnormal immunologic reactions associated with graft-vs-host disease, and infectious agents. We report the case of a 45-year-old woman with T-cell prolymphocytic leukemia and no prior risk factors for coronary artery disease in whom sudden cardiac death occurred 2 1/2 years after allogeneic BMT from an unrelated male donor. Autopsy revealed severe 3-vessel coronary disease with grade 4/4 stenosis. This process was primarily nonatherosclerotic, with intimal hyperplasia of undetermined etiology. Furthermore, fluorescence in situ hybridization to identify the donor Y chromosome with simultaneous immunofluorescence labeling of smooth muscle actin suggested the presence of donor cells that transformed into myocytes. Coronary artery disease is an important, albeit rare, complication of BMT. Donor hematopoietic cells may contribute to its pathogenesis.

Effect of curcumin on radiation-impaired healing of excisional wounds in mice

OBJECTIVE

To study the effect on wound contraction of pretreatment with various doses of curcumin (the most important active ingredient of the spice turmeric) in mice exposed to 6 Gy whole-body gamma radiation.

METHOD

A full-thickness skin wound was produced on the dorsum of Swiss albino mice treated with and without 25, 50, 100, 150 or 200 mg/kg body weight of curcumin before exposure to 6 Gy gamma radiation. Progression of wound contraction was monitored using video images of the wound at various days post-irradiation until full healing occurred. Mean wound healing times were also calculated.

RESULTS

Irradiation caused significant delay in wound contraction and healing times. However, treatment with curcumin resulted in a dose-dependent increase in contraction when compared with a control. Greatest contraction was observed for 100 mg/kg curcumin, with statistically significant results at days three (p < 0.009), six (p < 0.05) and nine (p < 0.05) post-irradiation for this dose. Complete healing was achieved by day 23 post-irradiation in the curcumin-treated irradiation group.

CONCLUSION

Pretreatment with curcumin has a conductive effect on irradiated wounds. It could be a substantial therapeutic agent for ameliorating radiation-induced delay in wound repair in cases of combined injuries.

Cellular transplantation: hurdles remaining before widespread clinical use

PURPOSE OF REVIEW

The last few years have witnessed a growing interest in regenerative therapy of the failing heart by cell transplantation. Special emphasis has been put on skeletal myoblasts and bone marrow-derived stem cells, with a flurry of experimental studies generating overall positive but occasionally conflicting results. It is thus appropriate to review the most important of these studies in light of the major issues that still impede widespread clinical use of cell therapy.

RECENT FINDINGS

Recent laboratory data demonstrate the ability of autologous skeletal myoblasts to engraft into scarred myocardium and improve its function. Equally successful results have been reported with bone marrow-derived cells which, in contrast to myoblasts, are credited with a plasticity that might allow their transdifferentiation into cardiac or endothelial cells in response to organ-specific cues. However, some major questions remain unanswered; they include the choice of the optimal cell type in relation with the target patient population, the strategies for enhancing cell survival and functional integration, the clarification of the mechanisms of improvement, and the means of reducing invasiveness of cell delivery.

SUMMARY

Although laboratory research attempts to overcome these persisting hurdles, the accumulated body of evidence warrants implementation of clinical trials. The earliest ones have now documented the feasibility of cell therapy. It is now appropriate to conduct safety and efficacy studies which, if carefully done, should allow assessment of the extent to which this concept of regenerative therapy can be made a clinical reality.

Plasticity of Adult Stem Cells

Recent years have seen much excitement over the possibility that adult mammalian stem cells may be capable of differentiating across tissue lineage boundaries, and as such may represent novel, accessible, and very versatile effectors of therapeutic tissue regeneration. Yet studies proposing such "plasticity" of adult somatic stem cells remain controversial, and in general, existing evidence suggests that in vivo such unexpected transformations are exceedingly rare and in some cases can be accounted for by equally unexpected alternative explanations.

Human embryonic or adult stem cells: an overview on ethics and perspectives for tissue engineering

Over the past few years, research on animal and human stem cells has experienced tremendous advances which are almost daily loudly revealed to the public on the front-page of newspapers. The reason for such an enthusiasm over stem cells is that they could be used to cure patients suffering from spontaneous or injuries-related diseases that are due to particular types of cells functioning incorrectly, such as cardiomyopathy, diabetes mellitus, osteoporosis, cancers, Parkinson's disease, spinal cord injuries or genetic abnormalities. Currently, these diseases have slightly or non-efficient treatment options, and millions of people around the world are desperately waiting to be cured. Even if not any person with one of these diseases could potentially benefit from stem cell therapy, the new concept of "regenerative medicine" is unprecedented since it involves the regeneration of normal cells, tissues and organs which could allow to treat a patient whereby both, the immediate problem would be corrected and the normal physiological processes restored, without any need for subsequent drugs. However, conflicting ethical controversies surround this new medicine approach, inside and outside the medical community, especially when human embryonic stem cells (h-ESCs) are concerned. This ethical debate on clinical use of h-ESCs has recently encouraged the research on "adult" stem cells (ASCs) regarded as a less conflicting alternative for the future of regenerative medicine.

Epidermal cells accelerate the restoration of the blood flow in diabetic ischemic limbs

Epidermal progenitor cells (EpPCs) were long thought to be unipotent, giving rise only to other keratinocytes but recent studies question this assumption. Here, we investigated whether mouse EpPCs can adopt other antigenic and functional phenotypes. To test this, we injected freshly isolated and cultured EpPCs and transient amplifying cells into diabetic and non-diabetic mouse ischemic hindlimb and followed the cells' fate and the recovery of the ischemic limb blood flow over time. Both freshly isolated and cultured EpPCs and transient amplifying cells were incorporated into the vasculature of the ischemic limb 2 and 5 weeks post-injection, and some expressed endothelial cell but not keratinocyte antigens. Additionally, in the non-diabetic animals, first transient amplifying cells and then EpPCs accelerated the restoration of the blood flow. By contrast, in diabetic animals, only injected EpPCs or unsorted epidermal cells accelerated the restoration of the blood flow. These data indicate that epidermal cells can adopt non-skin phenotypes and functions, and that this apparent pluripotency is not lost by differentiation of EpPCs into transient amplifying cells. They also suggest that epidermal cell therapy might be of therapeutic value in the treatment of diabetic ischemia. Finally, because epidermal cells are readily accessible and expandable, they appear to be ideally suited for use as a non-viral gene delivery therapy.

Adult stem cell plasticity: fact or artifact?

There has been unprecedented recent interest in stem cells, mainly because of the hope they offer for cell therapy. Adult stem cells are an attractive source of cells for therapy, especially in view of the recent claims that they are remarkably plastic in their developmental potential when exposed to new environments. Some of these claims have been either difficult to reproduce or shown to be misinterpretations, leaving the phenomenon of adult stem cell plasticity under a cloud. There are, however, other examples of plasticity where differentiated cells or their precursors can be reprogrammed by extracellular cues to alter their character in ways that could have important implications for cell therapy and other forms of regenerative treatment.

In vivo and in vitro differentiation of myocytes from human bone marrow-derived multipotent progenitor cells

OBJECTIVE

Recent studies have shown that bone marrow (BM) contains cells capable of differentiating into myocytes in vivo. However, addition of demethylation drugs has been necessary to induce myocyte differentiation from BM cells in vitro, and precise mechanisms of BM cells' conversion to myocytes and the origin of those cells have not been established. We investigated the expression of myogenic markers during differentiation and maturation of myocytes from BM-derived multipotent adult progenitor cells (MAPC) under physiological culture condition.

MATERIALS AND METHODS

Frozen BM samples from 21 healthy donors were used as a source of MAPC. To induce myocyte differentiation MAPC was cultured in the presence of 5% FCS, VEGF, bFGF, and IGF-1, and the expressions of myocyte markers were examined at various time points. We also investigated engraftment and differentiation of MAPC-derived myocytes in vivo.

RESULTS

Frozen BM-derived MAPC, cultured under the physiological myogenic condition, demonstrated spatial expression patterns of several myocyte markers similar to that of authentic myocyte differentiation. When injected into murine muscles, MAPC treated with the myogenic condition engrafted and differentiated into myocyte marker-positive cells and myotubes in vivo.

CONCLUSION

For the first time, we were able to induce myocyte formation from BM cells under the physiological condition in vitro and demonstrated that treating cells with this condition prior to intramuscular injection increased efficiency of engraftment and differentiation in vivo.

Persistent expression of the ATP-binding cassette transporter, Abcg2, identifies cardiac SP cells in the developing and adult heart

Stem cells are important in the maintenance and repair of adult tissues. A population of cells, termed side population (SP) cells, has stem cell characteristics as they have been shown to contribute to diverse lineages. In this study, we confirm that Abcg2 is a determinant of the SP cell phenotype. Therefore, we examined Abcg2 expression during murine embryogenesis and observed robust expression in the blood islands of the E8.5 yolk sac and in developing tissues including the heart. During the latter stages of embryogenesis, Abcg2 identifies a rare cell population in the developing organs. We further establish that the adult heart contains an Abcg2 expressing SP cell population and these progenitor cells are capable of proliferation and differentiation. We define the molecular signature of cardiac SP cells and compare it to embryonic stem cells and adult cardiomyocytes using emerging technologies. We propose that the cardiac SP cell population functions as a progenitor cell population for the development, maintenance, and repair of the heart.

Bone Marrow Cells in the Repair and Modulation of Heart and Blood Vessels: Emerging Opportunities in Native and Engineered Tissue and Biomechanical Materials

Adult bone marrow-derived stem/progenitor cells have traditionally been considered to be tissue-specific cells with limited capacity for differentiation. However, recent discoveries have generated tremendous excitement regarding possible applications of stem cells, particularly bone marrow-derived stem cells, in the treatment of human diseases. The potential ability to regenerate cells of various different lineages has raised the therapeutic possibility of using these bone marrow-derived stem cells as a source of cells for tissue repair and regeneration. Tissue engineering is a rapidly expanding interdisciplinary field aimed at restoring function to tissues through the delivery of constructs which become integrated into the patient. The use of bone marrow-derived stem cells provides a less invasive source for cells applicable to tissue engineering, including cardiovascular tissues such as heart valves, blood vessels, and myocardium. Although these strategies are in the early stages of development, they are conceptually promising and offer important insights into the future treatment of various cardiovascular ailments.

Transplanted bone marrow cells repair heart tissue and reduce myocarditis in chronic chagasic mice

A progressive destruction of the myocardium occurs in approximately 30% of Trypanosoma cruzi-infected individuals, causing chronic chagasic cardiomyopathy, a disease so far without effective treatment. Syngeneic bone marrow cell transplantation has been shown to cause repair and improvement of heart function in a number of studies in patients and animal models of ischemic cardiopathy. The effects of bone marrow transplant in a mouse model of chronic chagasic cardiomyopathy, in the presence of the disease causal agent, ie, the T. cruzi, are described herein. Bone marrow cells injected intravenously into chronic chagasic mice migrated to the heart and caused a significant reduction in the inflammatory infiltrates and in the interstitial fibrosis characteristics of chronic chagasic cardiomyopathy. The beneficial effects were observed up to 6 months after bone marrow cell transplantation. A massive apoptosis of myocardial inflammatory cells was observed after the therapy with bone marrow cells. Transplanted bone marrow cells obtained from chagasic mice and from normal mice had similar effects in terms of mediating chagasic heart repair. These results show that bone marrow cell transplantation is effective for treatment of chronic chagasic myocarditis and indicate that autologous bone marrow transplant may be used as an efficient therapy for patients with chronic chagasic cardiomyopathy.

Stem cell-mediated muscle regeneration is enhanced by local isoform of insulin-like growth factor 1

We investigated the mechanism whereby expression of a transgene encoding a locally acting isoform of insulin-like growth factor 1 (mIGF-1) enhances repair of skeletal muscle damage. Increased recruitment of proliferating bone marrow cells to injured MLC/mIgf-1 transgenic muscles was accompanied by elevated bone marrow stem cell production in response to distal trauma. Regenerating MLC/mIgf-1 transgenic muscles contained increased cell populations expressing stem cell markers, exhibited accelerated myogenic differentiation, expressed markers of regeneration and readily converted cocultured bone marrow to muscle. These data implicate mIGF-1 as a powerful enhancer of the regeneration response, mediating the recruitment of bone marrow cells to sites of tissue damage and augmenting local repair mechanisms.

Minor contribution of bone marrow-derived endothelial progenitors to the vascularization of murine gliomas

Until recently, it was generally accepted that the vascularization of solid tumors occurred exclusively through the sprouting and co-option from pre-existing blood vessels. Growing evidence now suggests that bone marrow-derived endothelial progenitor cells (EP) circulate in the blood and may play an important role in the formation of new blood vessels in certain tumors. Whether endothelial progenitors participate in the vascularization of brain tumors has not yet been evaluated. In this study, we examined the contribution of EP to tumor angiogenesis in a murine glioma tumor model. Donor bone marrow cells obtained from transgenic mice constitutively expressing beta-galactosidase or GFP either ubiquitously or transcriptionally regulated by an endothelial specific promotor Tie-2 were injected into lethally irradiated adult mice. After bone marrow reconstitution by donor cells, mice were implanted with syngeneic GL261 murine glioma cells. Morphological and confocal 3-dimensional analysis showed that the majority of the engrafted donor marrow cells were expressing hematopoietic and/or microglia markers, but did not appreciably contribute to the tumor vasculature. Implantation of glioma cells genetically engineered to overexpress VEGF produced highly vascularized tumors. However, the number of endothelial progenitors incorporated in the tumor vasculature did not increase. These data strongly suggest that neovascularization in the brain might fundamentally be regulated by the sprouting of pre-existing vessels and implicate that circulating endothelial progenitors do not play a significant role in this process.

Genetic programme of cardiogenesis: implications for therapeutic application

It has become accepted that new cardiomyocytes can be derived from stem cells. Although the potential for therapeutic application is evident, the reported efficiency of differentiation varies greatly from 0.02 to 54%. To obtain clinically relevant numbers of newly differentiated cardiac cells, stem cell differentiation should be as efficient as possible. A plausible way to increase the efficiency of differentiation of stem cells into cardiomyocytes is through the introduction of cardiac specific regulatory genes in the stem cells. This review summarises the role of several key transcription factors in cardiogenesis.

Stem Cell Plasticity in the Hematopoietic System

Bone marrow (BM) contains hematopoietic stem cells, which differentiate into all mature blood cells, and marrow stromal cells that provide the microenvironment for hematopoietic stem/progenitor cells along with the capability to differentiate into mature cells of multiple mesenchymal tissues including fat, bone, and cartilage. Recent studies indicate that adult BM also contains cells that can differentiate into nonhematopoietic cells of ectodermal, mesodermal, and endodermal tissues other than hematopoietic tissues, including liver, pancreas, kidney, lung, skin, gastrointestinal tract, heart, skeletal muscles, and neural tissues. Studies reporting the multipotentiality of BM cells have become a focus of interest because they suggest that clinical applications could be at hand using easily obtainable cells in the treatment of tissue damage or degenerative diseases. Presently, however, definitive evidence explaining the mechanism of this multipotentiality of BM stem cells is lacking. In this review, we summarize recent progress and controversies in investigation of the multipotentiality of adult BM-derived stem cells to differentiate into nonhematopoietic tissues.

Proliferation and Differentiation Processes in the Heart Muscle Elements in Different Phylogenetic Groups

This article reviews, discusses, and summarizes data about the generative behavior of muscle tissue cells, the mechanisms of its regulation, and the organization of the endocrine function of the heart in the main phylogenetic groups. With respect to the ratio of processes of proliferation and differentiation, cell organization, and growth mechanism, muscle tissues of propulsive organs can be divided into three types, each revealed in one of three main groups of animals, lophotrochozoans, ecdysozoans, and chordates. Ecdysterone is likely to play the key role in the regulation of proliferation and differentiation processes in the heart muscle of crustaceans, and, most probably, also of molluscs. In each of the three main phylogenetic groups the endocrine function of the heart consisting of secretion of natriuretic peptides has a peculiar organization. Vertebrate cardiomyocytes are known to combine contractile and endocrine differentiation. Such functional dualism is absent in heart muscle elements of Lophotrochozoa and Ecdysozoa; in the heart of lopfotrochozoans, secretion of natriuretic peptides is performed by endothelial cells and their derivatives. Homology of the heart muscle in the animal kingdom as well as possible mechanisms of genomic and epigenomic regulation of different types of cardiomyogenesis are discussed.

Stem cells and the formation of the myocardium in the vertebrate embryo

A major goal in cardiovascular biology is to repair diseased or damaged hearts with newly generated myocardial tissue. Stem cells offer a potential source of replacement myocytes for restoring cardiac function. Yet little is known about the nature of the cells that are able to generate myocardium and the conditions they require to form heart tissue. A source of information that may be pertinent to addressing these issues is the study of how the myocardium arises from progenitor cells in the early vertebrate embryo. Accordingly, this review will examine the initial events of cardiac developmental biology for insights into the identity and characteristics of the stem cells that can be used to generate myocardial tissue for therapeutic purposes.

GENERATION OF HYBRID CELL LINES WITH ENDOTHELIAL POTENTIAL FROM SPONTANEOUS FUSION OF ADULT BONE MARROW CELLS WITH EMBRYONIC FIBROBLAST FEEDER

We have previously isolated mouse embryonic cell lines with endothelial potential using a simple empirical approach. In an attempt to isolate similar cell lines from adult mouse bone marrow (BM), BM cells were cultured on mitotically inactive mouse embryonic fibroblast (MEF) feeder cells. Several cell lines with putative endothelial potential were generated. They expressed endothelial-specific genes and formed vascular-like structures when plated on matrigel. When transplanted into appropriate mouse models, they incorporated into the endothelium of the vasculature. Similar cell lines were also obtained using human or porcine BM. None of these lines induced tumor formation when transplanted into immunodeficient Rag1-/- mice. However, all the lines were aneuploid with genetic markers from BM samples and the MEF feeder, suggesting that they resulted from a non-species-specific fusion of a BM cell and mitotically inactive MEF. Together, these lines demonstrated for the first time that BM cells can also undergo fusion with commonly used mitotically inactive feeder cells. Although these fusion cell lines were culture artifacts, their derivation would be useful in understanding fusion of BM cells with other cell types, and their endothelial potential will also be useful in characterizing endothelial differentiation.

Transplanted adult hematopoietic stems cells differentiate into functional endothelial cells

During early embryogenesis, blood vessels and hematopoietic cells arise from a common precursor cell, the hemangioblast. Recent studies have identified endothelial progenitor cells in the peripheral blood, and there is accumulating evidence that a subset of these cells is derived from precursors in the bone marrow. Here we show that adult bone marrow-derived, phenotypically defined hematopoietic stem cells (c-kit+, Sca-1+, lineage-) give rise to functional endothelial cells. With the exception of the brain, donor-derived cells are rapidly integrated into blood vessels. Durably engrafted endothelial cells express CD31, produce von Willebrand factor, and take up low-density lipoprotein. Analysis of DNA content indicates that donor-derived endothelial cells are not the products of cell fusion. Self-renewal of stem cells with hematopoietic and endothelial cell potential was revealed by serial transplantation studies. The clonal origin of both hematopoietic and endothelial cell outcomes was established by the transfer of a single cell. These results suggest that adult bone marrow-derived hematopoietic stem cells may serve as a reservoir for endothelial cell progenitors.

Adult Bone Marrow-Derived Hemangioblasts, Endothelial Cell Progenitors, and EPCs

Long before their existence was proven, work with blood islands pointed to the existence of hemangioblasts in the embryo, and it was widely accepted that such cells existed. In contrast, though evidence for adult hemangioblasts appeared at least as early as 1932, until quite recently, it was commonly assumed that there were no adult hemangioblasts. Over the past decade, these views have changed, and it is now generally accepted that a subset of bone marrow cells or their progeny can and do function as adult hemangioblasts. This chapter will examine the basic biology of bone marrow-derived hemangioblasts and endothelial cell progenitors (angioblasts) and the relationship of these adult cells to their embryonic counterparts. Efforts to define the endothelial cell progenitor phenotype will also be discussed, though to date, there is no consensus on the definitive adult phenotype, probably because there are multiple phenotypes and because the cells are plastic. Also examined are the putative roles of bone marrow-derived cells in vascular homeostasis and repair, including both their ability to differentiate and contribute directly to vascular repair, as well as to promote vascular growth by secreting pro-angiogenic factors. Finally, the use of bone marrow cells as therapeutic tools will be addressed.

Cell-based Therapy to Regenerate Myocardium: from Bench to Bedside

The field of cell-based therapy to regenerate myocardium has been expanding rapidly, with significant advances being made in both the laboratory and the clinical area. In this article we review this field, including our experiences and discuss remaining issues and possibilities for future clinical applications.

Primitive adult hematopoietic stem cells can function as osteoblast precursors

Osteoblasts are continually recruited from stem cell pools to maintain bone. Although their immediate precursor is a plastic-adherent mesenchymal stem cell able to generate tissues other than bone, increasing evidence suggests the existence of a more primitive cell that can differentiate to both hematopoietic and mesenchymal cells. We show here that the "side population" (SP) of marrow stem cells, defined by their ability to rapidly expel a DNA-binding dye and to regenerate the hematopoietic compartment, can differentiate to osteoblasts through a mesenchymal intermediate. When transplanted into lethally irradiated mice, single gene-marked murine SP cells reconstituted depleted osteoprogenitor pools, such that a large proportion of the osteogenic cells in the epiphysis of long bone carried the donor SP cell marker. These findings suggest that the developmental capacity of SP cells is not restricted to the hematopoietic lineages but extends to osteogenic differentiation. This property not only elucidates a previously unrecognized step in osteoblast development, but also has intriguing implications for the use of SP cells in clinical orthopedics and stem cell-based disorders of bone.

Spallanzani's mouse: a model of restoration and regeneration

The ability to regenerate is thought to be a lost phenotype in mammals, though there are certainly sporadic examples of mammalian regeneration. Our laboratory has identified a strain of mouse, the MRL mouse, which has a unique capacity to heal complex tissue in an epimorphic fashion, i.e., to restore a damaged limb or organ to its normal structure and function. Initial studies using through-and-through ear punches showed rapid full closure of the ear holes with cartilage growth, new hair follicles, and normal tissue architecture reminiscent of regeneration seen in amphibians as opposed to the scarring usually seen in mammals. Since the ear hole closure phenotype is a quantitative trait, this has been used to show-through extensive breeding and backcrossing--that the trait is heritable. Such analysis reveals that there is a complex genetic basis for this trait with multiple loci. One of the major phenotypes of the MRL mouse is a potent remodeling response with the absence or a reduced level of scarring. MRL healing is associated with the upregulation of the metalloproteinases MMP-2 and MMP-9 and the downregulation of their inhibitors TIMP-2 and TIMP-3, both present in inflammatory cells such as neutrophils and macrophages. This model has more recently been extended to the heart. In this case, a cryoinjury to the right ventricle leads to near complete scarless healing in the MRL mouse whereas scarring is seen in the control mouse. In the MRL heart, bromodeoxyuridine uptake by cardiomyocytes filling the wound site can be seen 60 days after injury. This does not occur in the control mouse. Function in the MRL heart, as measured by echocardiography, returns to normal.

Myocardial tissue engineering and regeneration as a therapeutic alternative to transplantation

Ischemic cardiomyopathy leading to congestive heart failure remains the leading source of morbidity and mortality in Western society and medical management of this condition offers only palliative treatment. While allogeneic heart transplantation can both extend and improve the quality of life for patients with end-stage heart failure, this therapeutic option is limited by donor organ shortage. Even after successful transplantation, chronic cardiac rejection in the form of cardiac allograft vasculopathy can severely limit the lifespan of the transplanted organ. Current experimental efforts focus on cellular cardiomyoplasty, myocardial tissue engineering, and myocardial regeneration as alternative approaches to whole organ transplantation. Such strategies may offer novel forms of therapy to patients with end-stage heart failure within the near future.

Spontaneous cardiomyocyte differentiation from adipose tissue stroma cells

Cardiomyocyte regeneration is limited in adult life. Thus, the identification of a putative source of cardiomyocyte progenitors is of great interest to provide a usable model in vitro and new perspective in regenerative therapy. As adipose tissues were recently demonstrated to contain pluripotent stem cells, the emergence of cardiomyocyte phenotype from adipose-derived cells was investigated. We demonstrated that rare beating cells with cardiomyocyte features could be identified after culture of adipose stroma cells without addition of 5-azacytidine. The cardiomyocyte phenotype was first identified by morphological observation, confirmed with expression of specific cardiac markers, immunocytochemistry staining, and ultrastructural analysis, revealing the presence of ventricle- and atrial-like cells. Electrophysiological studies performed on early culture revealed a pacemaker activity of the cells. Finally, functional studies showed that adrenergic agonist stimulated the beating rate whereas cholinergic agonist decreased it. Taken together, this study demonstrated that functional cardiomyocyte-like cells could be directly obtained from adipose tissue. According to the large amount of this tissue in adult mammal, it could represent a useful source of cardiomyocyte progenitors.

Sustained Expansion and Transgene Expression of Coagulation Factor VIII–Transduced Cord Blood–Derived Endothelial Progenitor Cells

Objective— Although hemophilia A seems particularly suitable for gene therapy because even low amounts of plasma coagulation factor VIII (FVIII) provide a significant clinical benefit to the patients, the ideal target cell for recombinant FVIII expression and gene therapy approaches remains to be identified. In this study, we tested the capacity of cord blood–derived endothelial progenitor cells (CBECs) for FVIII expression on stable lentiviral transduction.

Methods and Results— CD34 + endothelial progenitor cells (EPCs) from cord blood were differentiated into CBECs. Endothelial phenotype was characterized, and lentiviral transduction of early-passage CBECs with a vector encoding FVIII and EGFP did not alter their functional properties and proliferative potential. CBEC could be expanded by 5 to 9 orders of magnitude, thus allowing the expansion of up to 10 15 FVIII-secreting CBECs, starting from as little as 10 6 CD34 + cells. CBECs proved to be highly suitable for FVIII secretion, with 0.35 to 0.39 IU FVIII:C/5×10 4 cells per 48 hours (7.0 to 7.8 IU FVIII:C/10 6 cells per 48 hours), which remained stable over the expansion period.

Conclusions— Our data indicate that CBECs are attractive target cells for inherited coagulation disorders such as hemophilia A, which on lentiviral transduction can be readily expanded to large numbers of transplantable gene-modified cells in vitro.

Exploitation of stem cell plasticity

For many years, adult haemopoietic stem cells (HSCs) have been considered 'plastic' in their proliferative and differentiation capacities. Recently, evidence that supports newer concepts of adult stem cell plasticity has been reported. In particular, stem cells from haemopoietic tissues seem to have 'extraordinary' abilities to generate or switch between haemopoietic and nonhaemopoietic lineages, exhibiting an unexpected degree of developmental or differentiation potential. The mechanisms by which cell fate reprogramming occurs are still poorly understood. Nevertheless, an increasing number of studies is challenging one of the main dogmas in biology, namely that mammalian cell differentiation follows established programmes in a hierarchical fashion, and once committed to a particular somatic cell lineage, cells do not change into another somatic lineage. The 'nonhierarchical', 'reversible' phenotype of stem cells in haemopoietic tissues, if it exists, would be an advantage that could be exploited in regenerative medicine. Here, we review the recent advances in HSC biology and discuss the general concepts of adult stem cell plasticity with respect to these cells and how these might be exploited clinically.

[Current concepts in stem cell research]

In the last few years, advances in stem cell research have opened up new horizons in the treatment of human diseases and in regenerative medicine. It is not unusual to find news on stem cell research in newspapers and other media. This review describes some basic concepts in research needed to understand the medical literature on stem cells and to provide the information and bibliography necessary to be up to date in one of the subjects that has generated the greatest number of publications in the last few years.

Current status and future prospects for acute myocardial infarction therapy

Considerable new evidence has accumulated in randomized trials of myocardial reperfusion. The trials of catheter-based reperfusion compared with fibrinolytics have shown an advantage for angioplasty and stenting over pharmacologic therapy, even accounting for delays in transporting patients from facilities with intervention capabilities. On the basis of the recent trials, it is recommended that a regional center for infarct intervention be set up akin to regional trauma centers in the United States. Now that we have entered the third decade of myocardial reperfusion therapy, we can expect iterative improvement in all aspects, and ultimately a fused approach of pharmacology and mechanical therapies-to achieve the optimal outcomes and continue to lower the toll of fatality and morbidity of acute myocardial infarction (MI).

Single hematopoietic stem cells generate skeletal muscle through myeloid intermediates

Recent studies have shown that cells from the bone marrow can give rise to differentiated skeletal muscle fibers. However, the mechanisms and identities of the cell types involved have remained unknown, and the validity of the observation has been questioned. Here, we use transplantation of single CD45+ hematopoietic stem cells (HSCs) to demonstrate that the entire circulating myogenic activity in bone marrow is derived from HSCs and their hematopoietic progeny. We also show that ongoing muscle regeneration and inflammatory cell infiltration are required for HSC-derived contribution, which does not occur through a myogenic stem cell intermediate. Using a lineage tracing strategy, we show that myofibers are derived from mature myeloid cells in response to injury. Our results indicate that circulating myeloid cells, in response to inflammatory cues, migrate to regenerating skeletal muscle and stochastically incorporate into mature myofibers.

Plasticity of marrow-derived stem cells

Bone marrow (BM) contains hematopoietic stem cells (HSCs), which differentiate into every type of mature blood cell; endothelial cell progenitors; and marrow stromal cells, also called mesenchymal stem cells (MSCs), which can differentiate into mature cells of multiple mesenchymal tissues including fat, bone, and cartilage. Recent findings indicate that adult BM also contains cells that can differentiate into additional mature, nonhematopoietic cells of multiple tissues including epithelial cells of the liver, kidney, lung, skin, gastrointestinal (GI) tract, and myocytes of heart and skeletal muscle. Experimental results obtained in vitro and in vivo are the subject of this review. The emphasis is on how these experiments were performed and under what conditions differentiation from bone marrow to epithelial and neural cells occurs. Questions arise regarding whether tissue injury is necessary for this differentiation and the mechanisms by which it occurs. We also consider which bone marrow subpopulations are capable of this differentiation. Only after we have a better understanding of the mechanisms involved and of the cells required for this differentiation will we be able to fully harness adult stem cell plasticity for clinical purposes.

Islet Regeneration During the Reversal of Autoimmune Diabetes in NOD Mice

Nonobese diabetic (NOD) mice are a model for type 1 diabetes in humans. Treatment of NOD mice with end-stage disease by injection of donor splenocytes and complete Freund's adjuvant eliminates autoimmunity and permanently restores normoglycemia. The return of endogenous insulin secretion is accompanied by the reappearance of pancreatic beta cells. We now show that live donor male or labeled splenocytes administered to diabetic NOD females contain cells that rapidly differentiate into islet and ductal epithelial cells within the pancreas. Treatment with irradiated splenocytes is also followed by islet regeneration, but at a slower rate. The islets generated in both instances are persistent, functional, and apparent in all NOD hosts with permanent disease reversal.

Stem cell plasticity revisited: CXCR4-positive cells expressing mRNA for early muscle, liver and neural cells ‘hide out’ in the bone marrow

It has been suggested that bone marrow (BM)-derived hematopoietic stem cells transdifferentiate into tissue-specific stem cells (the so-called phenomenon of stem cell plasticity), but the possibility of committed tissue-specific stem cells pre-existing in BM has not been given sufficient consideration. We hypothesized that (i) tissue-committed stem cells circulate at a low level in the peripheral blood (PB) under normal steady-state conditions, maintaining a pool of stem cells in peripheral tissues, and their levels increase in PB during stress/tissue injury, and (ii) they could be chemoattracted to the BM where they find a supportive environment and that the SDF-1-CXCR4 axis plays a prominent role in the homing/retention of these cells to BM niches. We performed all experiments using freshly isolated cells to exclude the potential for 'transdifferentiation' of hematopoietic stem or mesenchymal cells associated with in vitro culture systems. We detected mRNA for various early markers for muscle (Myf-5, Myo-D), neural (GFAP, nestin) and liver (CK19, fetoprotein) cells in circulating (adherent cell-depleted) PB mononuclear cells (MNC) and increased levels of expression of these markers in PB after mobilization by G-CSF (as measured using real-time RT-PCR). Furthermore, SDF-1 chemotaxis combined with real-time RT-PCR analysis revealed that (i) these early tissue-specific cells reside in normal murine BM, (ii) express CXCR4 on their surface and (iii) can be enriched (up to 60 x) after chemotaxis to an SDF-1 gradient. These cells were also highly enriched within purified populations of murine Sca-1(+) BM MNC as well as of human CD34(+)-, AC133(+)- and CXCR4-positive cells. We also found that the expression of mRNA for SDF-1 is upregulated in damaged heart, kidney and liver. Hence our data provide a new perspective on BM not only as a home for hematopoietic stem cells but also a 'hideout' for already differentiated CXCR4-positive tissue-committed stem/progenitor cells that follow an SDF-1 gradient, could be mobilized into PB, and subsequently take part in organ/tissue regeneration.

Transdifferentiation of human peripheral blood CD34+-enriched cell population into cardiomyocytes, endothelial cells, and smooth muscle cells in vivo

BACKGROUND

Adult human peripheral blood cells have been shown to differentiate into mature cells of nonhematopoietic tissues, such as hepatocytes and epithelial cells of the skin and gastrointestinal track. We investigated whether these cells could also transdifferentiate into human cardiomyocytes, mature endothelial cells, and smooth muscle cells in vivo.

METHODS AND RESULTS

Myocardial infarction was created in SCID mice by occluding the left anterior descending coronary artery, after which adult peripheral blood CD34+ cells were injected into the tail vein. Hearts were harvested 2 months after injection and stained for human leukocyte antigen (HLA) and markers for cardiomyocytes, endothelial cells, and smooth muscle cells. Cardiomyocytes, endothelial cells, and smooth muscle cells that bear HLA were identified in the infarct and peri-infarct regions of the mouse hearts. In a separate experiment, CD34+ cells were injected intraventricularly into mice without experimental myocardial infarction. HLA-positive myocytes and smooth muscle cells could only be identified in 1 of these mice killed at different time points.

CONCLUSIONS

Adult peripheral blood CD34+ cells can transdifferentiate into cardiomyocytes, mature endothelial cells, and smooth muscle cells in vivo. However, transdifferentiation is augmented significantly by local tissue injury. The use of peripheral blood CD34+ cells for cell-based therapy should greatly simplify the procurement of cells for the regeneration of damaged myocardium.

Protective effects of transplanted and mobilized bone marrow stem cells on mice with severe acute pancreatitis

AIM: To evaluate the protective effects of transplanted and mobilized bone marrow stem cells (BMSCs) on mice with severe acute pancreatitis (SAP) and to probe into their possible mechanisms.

METHODS: A mouse model of SAP induced by intraperitoneal injections of L-arginine was employed in the present study. Two hundred female Balb/c mice weighing 18-22 g were randomly assigned into 4 groups. Group A was the stem cell mobilized group treated by injection of granulocyte-colony stimulating factor (G-CSF) into mice for 4 d at a dose of 40 μg·kg^-1·d^-1 before induction of SAP. Group B was the group of BMSCs transplantation, in which the mice were given the isolated BMSCs via the tail vein 4 d prior to induction of SAP. Group C served as the model control and only SAP was induced. The mice without induction of SAP in group D acted as the normal control. At the time of animal sacrifice at 24, 48 and 72 h after induction of SAP, blood samples were obtained and prepared to detect serum amylase, while the abdominal viscera were examined both grossly and microscopically for the observation of pathological changes.

RESULTS: The mortality of mice in the model control, groups A and B was 34%, 8% and 10% respectively within 72 h after induction of SAP. The serum level of amylase in the model control was significantly increased at all time points after induction of SAP as compared with that of the normal control (P < 0.05-0.01). When the mice were pretreated with BMSCs’ transplantation or G-CSF injection, their serum level of amylase was significantly reduced at 48 h and 72 h after induction of SAP in comparison with that of the model control (P < 0.05-0.01). In accordance with these observations, both gross and microscopic examinations revealed that the pathological changes of SAP in mice pretreated with BMSCs transplantation or G-CSF injection were considerably attenuated as compared with those in the model control at all observed time points.

CONCLUSION: Both transplanted allogenic and mobilized autologous BMSCs can protect mouse pancreas from severe damage in the process of SAP.

Cardiac progenitor cells from adult myocardium: homing, differentiation, and fusion after infarction

Potential repair by cell grafting or mobilizing endogenous cells holds particular attraction in heart disease, where the meager capacity for cardiomyocyte proliferation likely contributes to the irreversibility of heart failure. Whether cardiac progenitors exist in adult myocardium itself is unanswered, as is the question whether undifferentiated cardiac precursor cells merely fuse with preexisting myocytes. Here we report the existence of adult heart-derived cardiac progenitor cells expressing stem cell antigen-1. Initially, the cells express neither cardiac structural genes nor Nkx2.5 but differentiate in vitro in response to 5'-azacytidine, in part depending on Bmpr1a, a receptor for bone morphogenetic proteins. Given intravenously after ischemia/reperfusion, cardiac stem cell antigen 1 cells home to injured myocardium. By using a Cre/Lox donor/recipient pair (alphaMHC-Cre/R26R), differentiation was shown to occur roughly equally, with and without fusion to host cells.

Phenotype and hematopoietic potential of side population cells throughout embryonic development

Adult murine bone marrow hematopoietic stem cells (HSCs) can be purified by sorting Hoechst 33342-extruding side population (SP) cells. Herein we investigated whether SP cells reside within embryonic tissues and exhibit hematopoietic progenitor activity. We isolated yolk sac (YS) and embryonic tissues 7.5 to 11.5 days after coitus (dpc), resolved an SP in each, and demonstrated that these SP cells exhibit distinct phenotypic and functional characteristics throughout development. YS and embryonic SP isolated 8.0 dpc expressed vascular endothelial-cadherin (VE-cadherin) and vascular endothelial receptor 2 (Flk-1), markers not expressed by bone marrow SP but expressed by endothelial cells and progenitors. SP at this stage did not express CD45 or produce hematopoietic colonies in vitro. In contrast, SP isolated 9.5 to 11.5 dpc contained a significantly higher proportion of cells expressing cKit and CD45, markers highly expressed by bone marrow SP. Furthermore, YS SP isolated 9.5 to 11.5 dpc demonstrated 40- to 90-fold enrichment for hematopoietic progenitor activity over unfractionated tissue. Our data indicate that YS and embryonic SP cells detected prior to the onset of circulation express the highest levels of endothelial markers and do not generate blood cells in vitro; however, as development progresses, they acquire hematopoietic potential and phenotypic characteristics similar to those of bone marrow SP.

Clinical applications of tissue engineered constructs

The reconstruction of soft tissue defects poses a challenge for plastic surgeons and tissue engineers. The construction of a biologically, functionally, and cosmetically successful replacement part will involve the combination of a composite that contains endoderm, mesoderm, and ectoderm. It will be active in immune surveillance and function. It must be durable to withstand the stress and strain encountered by the skin. Such a composite will involve the use of bone, cartilage, muscle, blood vessels, nerves, connective tissue, dermis, and epidermis. Fortunately, many of these tissues are among the best studied by tissue engineers. The future of this field will likely involve to some degree the co-mingling of current reconstructive modalities, including the techniques of prefabrication and pre-lamination, with more aggressive and successful tissue engineering technology and the rapidly developing science of stem cell biology. Tissues synthesized in vitro with better structure, color, and texture can be pre-laminated to a site that has already been prefabricated. Prefabrication of a bio-absorbable matrix can create a well perfused scaffold onto which larger subunits can be prelaminated. The future of this field of endeavor is exciting, and, with further research, experience, and interdisciplinary collaboration, bioengineered tissue constructs will become a reality.