Regenerating the heart

Regional and global protective effects of tissue kallikrein gene delivery to the peri-infarct myocardium

BACKGROUND

The kallikrein-kinin system participates in the maintenance of the cardiovascular phenotype. We previously demonstrated that human tissue kallikrein gene (hTK) transfer promotes the healing of ischemic limbs. The present investigation aimed to test the original hypothesis that hTK delivery to the peri-infarct myocardium would prevent post-ischemic heart failure.

METHODS AND RESULTS

Myocardial infarction (MI) was induced in anesthetized mice by permanently occluding the left coronary descendant. hTK was delivered to the peri-infarct myocardium via an adenoviral vector (Ad.hTK). Controls received Ad.Null or saline. Survival rate was similar among groups. Ad.hTK increased the number of circulating endothelial progenitor cells and promoted the growth of capillaries and arterioles in the peri-infarct myocardium. In addition, Ad.hTK increased the abundance of cardiac progenitor cells (CPCs) in the peri-infarct and suppressed the apoptotic death of peri-infarct cardiomyocytes in vivo and ex vivo. As a consequence of these beneficial effects, at 5 weeks from MI, hTK-transduced hearts were protected from post-MI ventricular dilatation and showed better systolic and diastolic functions.

CONCLUSIONS

Ad.hTK benefits the neovascularization and viability of peri-infarct myocardium and increases CPC abundance, thereby decreasing ventricular dysfunction. Our study significantly adds to the knowledge of the protective effects of TK gene transfer on ischemic diseases and opens new avenues for the treatment of post-MI cardiac failure.

Differentiation in vivo of cardiac committed human embryonic stem cells in postmyocardial infarcted rats

Human embryonic stem (HES) cells can give rise to cardiomyocytes in vitro. However, whether undifferentiated HES cells also feature a myocardial regenerative capacity after in vivo engraftment has not been established yet. We compared two HES cell lines (HUES-1 and I6) that were specified toward a cardiac lineage by exposure to bone morphogenetic protein-2 (BMP2) and SU5402, a fibroblast growth factor receptor inhibitor. Real-time polymerase chain reaction (PCR) revealed that the cardiogenic inductive factor turned on expression of mesodermal and cardiac genes (Tbx6, Isl1, FoxH1, Nkx2.5, Mef2c, and alpha-actin). Thirty immunosuppressed rats underwent coronary artery ligation and, 2 weeks later, were randomized and received in-scar injections of either culture medium (controls) or BMP2 (+/-SU5402)-treated HES cells. After 2 months, human cells were detected by anti-human lamin immunostaining, and their cardiomyocytic differentiation was evidenced by their expression of cardiac markers by reverse transcription-PCR and immunofluorescence using an anti-beta myosin antibody. No teratoma was observed in hearts or any other organ of the body. The ability of cardiac-specified HES cells to differentiate along the cardiomyogenic pathway following transplantation into infarcted myocardium raises the hope that these cells might become effective candidates for myocardial regeneration.

[Endothelial progenitor cells: new biomarkers and potential therapy in intensive care]

One of the most important breakthroughs in the field of vascular biology in the last decade was the discovery of endothelial progenitor cells (EPCs). These angiogenic cells dwell in bone marrow, and may be found in the general circulation spontaneously or in response to various stimuli such as ischemia, growth factor, pro-inflammatory cytokines, and drugs such as statins. There is growing evidence that EPCs can differentiate into mature endothelial cells and facilitate endothelial repair and angiogenesis in vivo. In recent years, consistent publications have shown that EPCs provide both diagnostic and prognostic information with respect to cardiovascular diseases, acute lung injury, and sepsis. Activation of EPCs from the bone marrow or injection of these cells may be used as a therapeutic option for the treatment of ischemic cardiovascular diseases.

Biphasic role for Wnt/beta-catenin signaling in cardiac specification in zebrafish and embryonic stem cells

Understanding pathways controlling cardiac development may offer insights that are useful for stem cell-based cardiac repair. Developmental studies indicate that the Wnt/beta-catenin pathway negatively regulates cardiac differentiation, whereas studies with pluripotent embryonal carcinoma cells suggest that this pathway promotes cardiogenesis. This apparent contradiction led us to hypothesize that Wnt/beta-catenin signaling acts biphasically, either promoting or inhibiting cardiogenesis depending on timing. We used inducible promoters to activate or repress Wnt/beta-catenin signaling in zebrafish embryos at different times of development. We found that Wnt/beta-catenin signaling before gastrulation promotes cardiac differentiation, whereas signaling during gastrulation inhibits heart formation. Early treatment of differentiating mouse embryonic stem (ES) cells with Wnt-3A stimulates mesoderm induction, activates a feedback loop that subsequently represses the Wnt pathway, and increases cardiac differentiation. Conversely, late activation of beta-catenin signaling reduces cardiac differentiation in ES cells. Finally, constitutive overexpression of the beta-catenin-independent ligand Wnt-11 increases cardiogenesis in differentiating mouse ES cells. Thus, Wnt/beta-catenin signaling promotes cardiac differentiation at early developmental stages and inhibits it later. Control of this pathway may promote derivation of cardiomyocytes for basic research and cell therapy applications.

Finding fluorescent needles in the cardiac haystack: tracking human mesenchymal stem cells labeled with quantum dots for quantitative in vivo three-dimensional fluorescence analysis

Stem cells show promise for repair of damaged cardiac tissue. Little is known with certainty, however, about the distribution of these cells once introduced in vivo. Previous attempts at tracking delivered stem cells have been hampered by the autofluorescence of host tissue and limitations of existing labeling techniques. We have developed a novel loading approach to stably label human mesenchymal stem cells with quantum dot (QD) nanoparticles. We report the optimization and validation of this long-term tracking technique and highlight several important biological applications by delivering labeled cells to the mammalian heart. The bright QD crystals illuminate exogenous stem cells in histologic sections for at least 8 weeks following delivery and permit, for the first time, the complete three-dimensional reconstruction of the locations of all stem cells following injection into the heart. Disclosure of potential conflicts of interest is found at the end of this article.

Thymosin beta-4 is essential for coronary vessel development and promotes neovascularization via adult epicardium

Ischemic heart disease leading to myocardial infarction causes irreversible cell loss and scarring and is a major cause of morbidity and mortality in humans. Significant effort in the field of cardiovascular medicine has been invested in the search for adult cardiac progenitor cells that may replace damaged muscle cells and/or contribute to new vessel formation (neovascularization) and in the identification of key factors, which may induce such progenitor cells to contribute to myocardial repair and collateral vessel growth. We recently demonstrated that the actin monomer-binding protein, thymosin beta-4 (Tbeta-4), when secreted from the myocardium provides a paracrine stimulus to the cells of the epicardium-derived cells (EPDCs) to promote their inward migration and differentiation into endothelial and smooth muscle cells to form the coronary vasculature. Translating this essential role for Tbeta-4 in coronary vessel development to the adult, we found that treatment of cultured adult explants with Tbeta-4 stimulated extensive outgrowth of epicardin-positive epicardial cells, which, as they migrated away from the explant, differentiated into procollagen type I, SMalphaA, and Flk1-positive cells indicative of fibroblasts, smooth muscle, and endothelial cells; thus releasing the adult epicardium from a quiescent state and restoring pluripotency. The ability of Tbeta-4 to promote coronary vessel development and potentially induce new vasculature in the adult is essential for cardiomyocyte survival and could contribute significantly toward the reported Tbeta4-induced cardioprotection and repair in the adult heart. Tbeta-4 is currently subject to multicenter phase 1 clinical trials for treatment of cardiovascular disease (http://www.regenerx.com), therefore, insight into the repair mechanism(s) induced by Tbeta-4 is an essential step toward harnessing therapeutic survival, migration, and repair properties of the peptide in the context of acute myocardial damage.

Changing concepts of bladder regeneration

During the last decade, there has been a dramatic increase in studies aimed at regeneration of the urinary bladder. Many studies employed animal-derived or synthetic materials as grafts for experimental bladder augmentation models, with or without additional measures to promote regeneration, such as autologous cell transplantation or growth factor loading. However, in spite of encouraging results in several reports, few methodologies have shown proven definitive clinical utility. One major problem in these studies is the lack of a clear distinction between native and regenerated bladder in total bladder function after augmentation. Another crucial problem is the absorption and shrinkage of larger grafts, which may result from insufficient vascular supply and smooth muscle regeneration. In contrast, researchers have recently attempted to establish alternative regenerative strategies for treating bladder diseases, and have employed far more diverse approaches according to the various pathological conditions to be treated. For total replacement of the bladder after cystectomy for invasive bladder cancer, urothelium-covered neobladder with non-urinary tract backbone remains a viable choice. In addition, functional bladder diseases such as urinary incontinence, weak detrusor, or non-compliant fibrotic bladder have also been major targets for many leading research groups in this field. These conditions are studied much more from different therapeutic standpoints, aiming at the prevention or reversal of pathological conditions in muscle remodeling or neural control. Such altered research direction would inevitably lead to less surgically based basic biological research, and also would include a far wider spectrum of adult and pediatric bladder diseases, from overactive bladder to dysfunctional voiding.

Subretinal transplantation of bone marrow mesenchymal stem cells delays retinal degeneration in the RCS rat model of retinal degeneration

Because there is no effective treatment for this retinal degeneration, potential application of cell-based therapy has attracted considerable attention. Several investigations support that bone marrow mesenchymal stem cells (MSCs) can be used for a broad spectrum of indications. Bone marrow MSCs exert their therapeutic effect in part by secreting trophic factors to promote cell survival. The current study investigates whether bone marrow MSCs secrete factor(s) to promote photoreceptor cell survival and whether subretinal transplantation of bone marrow MSCs promotes photoreceptor survival in a retinal degeneration model using Royal College of Surgeons (RCS) rats. In vitro, using mouse retinal cell culture, it was demonstrated that the conditioned medium of the MSCs delays photoreceptor cell apoptosis, suggesting that the secreted factor(s) from the MSCs promote photoreceptor cell survival. In vivo, the MSCs were injected into the subretinal space of the RCS rats and histological analysis, real-time RT-PCR and electrophysiological analysis demonstrated that the subretinal transplantation of MSCs delays retinal degeneration and preserves retinal function in the RCS rats. These results suggest that MSC is a useful cell source for cell-replacement therapy for some forms of retinal degeneration.

'Working' cardiomyocytes exhibiting plateau action potentials from human placenta-derived extraembryonic mesodermal cells

The clinical application of cell transplantation for severe heart failure is a promising strategy to improve impaired cardiac function. Recently, an array of cell types, including bone marrow cells, endothelial progenitors, mesenchymal stem cells, resident cardiac stem cells, and embryonic stem cells, have become important candidates for cell sources for cardiac repair. In the present study, we focused on the placenta as a cell source. Cells from the chorionic plate in the fetal portion of the human placenta were obtained after delivery by the primary culture method, and the cells generated in this study had the Y sex chromosome, indicating that the cells were derived from the fetus. The cells potentially expressed 'working' cardiomyocyte-specific genes such as cardiac myosin heavy chain 7beta, atrial myosin light chain, cardiac alpha-actin by gene chip analysis, and Csx/Nkx2.5, GATA4 by RT-PCR, cardiac troponin-I and connexin 43 by immunohistochemistry. These cells were able to differentiate into cardiomyocytes. Cardiac troponin-I and connexin 43 displayed a discontinuous pattern of localization at intercellular contact sites after cardiomyogenic differentiation, suggesting that the chorionic mesoderm contained a large number of cells with cardiomyogenic potential. The cells began spontaneously beating 3 days after co-cultivation with murine fetal cardiomyocytes and the frequency of beating cells reached a maximum on day 10. The contraction of the cardiomyocytes was rhythmical and synchronous, suggesting the presence of electrical communication between the cells. Placenta-derived human fetal cells may be useful for patients who cannot supply bone marrow cells but want to receive stem cell-based cardiac therapy.

Cellular therapy in Chagas' disease: potential applications in patients with chronic cardiomyopathy

Nearly a century after its discovery, Chagas' disease, caused by the protozoan Trypanosoma cruzi, remains a major health problem in Latin America. Although efforts in transmission control have contributed to a decrease in the number of new cases, approximately a third of chronic Chagasic individuals have or will develop the symptomatic forms of the disease, mainly cardiomyopathy. Chagas' disease is a progressively debilitating disease, which, at the final stages, there are no currently available treatments other than heart transplantation. In this scenario, cellular therapy is being tested as an alternative for millions of patients with heart dysfunction due to Chagas' disease. In this article, we review the studies of cellular therapy in animal models and in patients with Chagasic cardiomyopathy and the possible mechanisms by which cellular therapy may act in this disease.

The host immune response is essential for the beneficial effect of adult stem cells after myocardial ischemia

OBJECTIVE

Multipotent adult progenitor cells (MAPCs) are adult stem cells derived from bone marrow. We investigated the capacity of MAPCs to aid in tissue healing after myocardial ischemia in mice with different levels of immune competence.

METHODS

Adult murine C57BL/6 MAPCs were labeled with firefly luciferase and DsRed2 fluorescent protein and injected into the myocardium of immunocompetent C57BL/6 or T-, B- and natural killer-cell severe combined immunodeficient C57BL/6 Rag2/IL-2Rgammac(-/-) mice at the time of myocardial infarction (MI). Mice were sequentially analyzed using in vivo whole body bioluminescent imaging for MAPC persistence and high-resolution ultrasound biomicroscopy to assess cardiac function.

RESULTS

Luciferase signals emitted from donor MAPCs were significantly higher in Rag2/IL-2Rgammac(-/-) mice compared with C57BL/6 recipients of labeled MAPCs. At 100, 200, and 365 days after MI, left ventricular contractile function was significantly improved (and normalized) in C57BL/6 MAPC recipients. In contrast, despite a greater degree of MAPC persistence compared with C57BL/6 recipients, no cardiac improvement occurred in Rag2/IL-2Rgammac(-/-) recipients of MAPCs. The improved cardiac contractile performance in response to syngeneic MAPC infusion correlated with a prominent increase of vascular density in infarcted and peri-infarcted myocardium, which was dependent upon host immune competency.

CONCLUSION

These data indicate that immune competence of the recipient modulates the therapeutic impact of the adult nonhematopoietic stem cells infused after acute MI injury and that a more vigorous immune response is advantageous for therapeutic myocardial repair after MI.

Cardiovascular therapeutic aspects of cell therapy and stem cells

The recent advancements in stem cell biology, molecular and cell biology, and tissue engineering have paved the way to the development of a new biomedical discipline: regenerative medicine. The heart represents an attractive candidate for this emerging discipline since these emerging technologies could be used to potentially treat a variety of myocardial disorders. Here we describe our efforts in using stem cell and cell therapy strategies to restore the myocardial electromechanical properties. Specifically, our research has focused on the potential role of human embryonic stem cells (hESC) for myocardial regeneration (for the treatment of heart failure) and on using genetically engineered cell grafts to modify the myocardial electrophysiological properties (for the treatment of cardiac arrhythmias). The recently described hESC lines are unique pluripotent cell lines that can be propagated in the undifferentiated state in culture and coaxed to differentiate into cell derivatives of all three germ layers, including cardiomyocytes. The current article describes this unique cardiomyocyte differentiating system and details the molecular, ultrastructural, and functional properties of the generated hESC-derived cardiomyocytes (hESC-CMs). The ability of the hESC-CMs to integrate structurally and functionally with host cardiomyocytes in both in vitro and in vivo studies will be described as well as their ability to restore the myocardial electromechanical function in animal models of diseased hearts. We will next present detailed in vitro, in vivo, and computer simulation studies performed in our laboratory testing the hypothesis that cell grafts, engineered to express specific ion channels, can be used to modify the myocardial electrophysiological properties of cardiac tissue. The potential and drawbacks of this novel approach for the treatment of both tachyarrhythmias (using cell grafts expressing potassium channels) and bradyarrhythmias (using hESC coaxed to differentiate into pacemaking cells or conducting tissue) will be described.

Creation of myocardial tubes using cardiomyocyte sheets and an in vitro cell sheet-wrapping device

Regenerative medicine involving injection of isolated cells and transplantation of tissue-engineered myocardial patches, has received significant attention as an alternative method to repair damaged heart muscle. In the present study, as the next generation of myocardial tissue engineering we demonstrate the in vitro fabrication of pulsatile myocardial tubes using cell sheet engineering technologies. Three neonatal rat cardiomyocyte sheets, which were harvested from temperature-responsive culture dishes, were wrapped around fibrin tubes using a novel cell sheet-wrapping device. The tubular constructs demonstrated spontaneous, synchronized pulsation within 3h after cell sheet wrapping. Contractile force measurements showed that the contractile force increased in accordance with both increasing rest length (Starling mechanism) and increasing extracellular Ca(2+) concentration. Furthermore, the tissue-engineered myocardial tubes presented measurable inner pressure changes evoked by tube contraction (0.11+/-0.01mmHg, max 0.15mmHg, n=5). Histological analyses revealed both well-differentiated sarcomeres and diffuse gap junctions within the myocardial tissues that resembled native cardiac muscle. These data indicate that tissue-engineered myocardial tubes have native heart-like structure and function. These new myocardial tissue constructs should be useful for future applications in physiological studies and pharmacological screening, and present a possible core technology for the creation of engineered tissues capable of independent cardiac assistance.

Identification and selection of cardiomyocytes during human embryonic stem cell differentiation

Human embryonic stem cells (hESC) are pluripotent lines that can differentiate in vitro into cell derivatives of all three germ layers, including cardiomyocytes. Successful application of these unique cells in the areas of cardiovascular research and regenerative medicine has been hampered by difficulties in identifying and selecting specific cardiac progenitor cells from the mixed population of differentiating cells. We report the generation of stable transgenic hESC lines, using lentiviral vectors, and single-cell clones that express a reporter gene (eGFP) under the transcriptional control of a cardiac-specific promoter (the human myosin light chain-2V promoter). Our results demonstrate the appearance of eGFP-expressing cells during the differentiation of the hESC as embryoid bodies (EBs) that can be identified and sorted using FACS (purity>95%, viability>85%). The eGFP-expressing cells were stained positively for cardiac-specific proteins (>93%), expressed cardiac-specific genes, displayed cardiac-specific action-potentials, and could form stable myocardial cell grafts following in vivo cell transplantation. The generation of these transgenic hESC lines may be used to identify and study early cardiac precursors for developmental studies, to robustly quantify the extent of cardiomyocyte differentiation, to label the cells for in vivo grafting, and to allow derivation of purified cell populations of cardiomyocytes for future myocardial cell therapy strategies.

Absence of regeneration in the MRL/MpJ mouse heart following infarction or cryoinjury

BACKGROUND

Myocardial infarcts in mammals heal by scar formation rather than formation of new muscle tissue. The MRL/MpJ [Murphy Roths large (MRL) derived by the Murphy group of the Jackson Laboratory (MpJ)] mouse, however, has been reported to exhibit minimal scarring and subsequent cardiac regeneration after cryoinjury of the right ventricle. Other groups have reported that permanent and temporary ligation of the coronary artery resulted in scarring without regeneration.

METHODS

To clarify these contradictory results, we studied the temporal evolution of infarcts in MRL/MpJ and C57BL/6 control mice from 1 to 90 days post injury and the effects of intrathoracic cryoinjury to 28 days.

RESULTS

After infarction, the conversion from necrotic myocardium to granulation tissue and then to scar proceeded identically in the two groups. Infarct DNA synthesis, measured by incorporation of a 5-bromo-2-deoxyuridine pulse, peaked at 4 days in both strains and did not differ between strains at any time point. Endothelial cell and total vascular density in the both the infarcted and noninfarcted cardiac tissue did not differ between groups at any time. Histological analysis of directly cryoinjured right and left ventricular myocardium showed indistinguishable wound healing in both strains, and final scar size was identical in each group.

CONCLUSIONS

These studies demonstrate that both myocardial infarcts and cryoinjuries in MRL/MpJ mice heal by typical scar formation rather than muscle regeneration, in a manner very similar to C57BL/6 controls. We conclude that the MRL mouse is not a model for myocardial regeneration.

Progenitor cells and retinal angiogenesis

Nothing more dramatically captures the imagination of the visually impaired patient or the ophthalmologist treating them than the possibility of rebuilding a damaged retina or vasculature with "stem cells." Stem cells (SC) have been isolated from adult tissues and represent a pool of cells that may serve to facilitate rescue/repair of damaged tissue following injury or stress. We propose a new paradigm to "mature" otherwise immature neovasculature or, better yet, stabilize existing vasculature to hypoxic damage. This may be possible through the use of autologous bone marrow (BM) or cord blood derived hematopoietic SC that selectively target sites of neovascularization and gliosis where they provide vasculo- and neurotrophic effects. We have demonstrated that adult BM contains a population of endothelial and myeloid progenitor cells that can target activated astrocytes, a hallmark of many ocular diseases, and participate in normal developmental, or injury-induced, angiogenesis in the adult. Intravitreal injection of these cells from mice and humans can prevent retinal vascular degeneration ordinarily observed in mouse models of retinal degeneration; this vascular rescue correlates with functional neuronal rescue as well. The use of autologous adult BM derived SC grafts for the treatment of retinal vascular and degenerative diseases represents a novel conceptual approach that may make it possible to "mature" otherwise immature neovasculature, stabilize existing vasculature to hypoxic damage and/or rescue and protect retinal neurons from undergoing apoptosis. Such a therapeutic approach would obviate the need to employ destructive treatment modalities and would facilitate vascularization of ischemic and otherwise damaged retinal tissue.

Engineering the heart piece by piece: state of the art in cardiac tissue engineering

According to the National Transplant Society, more than 7000 Americans in need of organs die every year owing to a lack of lifesaving organs. Bioengineering 3D organs in vitro for subsequent implantation may provide a solution to this problem. The field of tissue engineering in its most rudimentary form is focused on the developed of transplantable organ substitutes in the laboratory. The objective of this article is to introduce important technological hurdles in the field of cardiac tissue engineering. This review starts with an overview of tissue engineering, followed by an introduction to the field of cardiovascular tissue engineering and finally summarizes some of the key advances in cardiac tissue engineering; specific topics discussed in this article include cell sourcing and biomaterials, in vitro models of cardiac muscle and bioreactors. The article concludes with thoughts on the utility of tissue-engineering models in basic research as well as critical technological hurdles that need to be addressed in the future.

Cardiovascular developmental insights from embryos

We investigate cardiovascular (CV) developmental physiology and biomechanics in order to understand the dramatic acquisition of form and function during normal development and to identify the adaptive mechanisms that allow embryos to survive adverse genetic and epigenetic events. Cardiovascular patterning, morphogenesis, and growth occur via highly conserved genetic mechanisms. Structural and functional maturation of the embryonic heart is also conserved across a broad range of species with evidence for load dependence from onset of the heartbeat. The embryonic heart dynamically adapts to changes in biomechanical loading conditions and for reasons not yet clear, adapts better to increased than to decreased mechanical load. In mammals, maternal cardiovascular function dynamically impacts embryonic/fetal growth and hemodynamics and these interactions can now be studied longitudinally using high-resolution noninvasive techniques. Maternal exposure to hypoxia and to bioactive chemicals, such as caffeine, can rapidly impact embryonic/fetal cardiovascular function, growth, and outcome. Finally, tissue engineering approaches can be applied to investigate basic developmental aspects of the embryonic myocardium. We use isolated embryonic and fetal chick, mouse, or rat cardiac cells to generate 3D engineered early embryonic cardiac tissues (EEECT). EEECT retains the morphologic and proliferative features of embryonic myocardium, responds to increased mechanical load with myocyte hyperplasia, and may be an excellent future material for use in cardiac repair and regeneration. These insights into cardiovascular embryogenesis are relevant to identifying mechanisms for congenital cardiovascular malformations and for developing cell- and tissue-based strategies for myocardial repair.

Transplantation of undifferentiated murine embryonic stem cells in the heart: teratoma formation and immune response

Embryonic stem (ES) cells are promising for cardiac repair, but directing their differentiation toward cardiomyocytes remains challenging. We investigated whether the heart guides ES cells toward cardiomyocytes in vivo and whether allogeneic ES cells were immunologically tolerated. Undifferentiated mouse ES cells consistently formed cardiac teratomas in nude or immunocompetent syngeneic mice. Cardiac teratomas contained no more cardiomyocytes than hind-limb teratomas, suggesting lack of guided differentiation. ES cells also formed teratomas in infarcted hearts, indicating injury-related signals did not direct cardiac differentiation. Allogeneic ES cells also caused cardiac teratomas, but these were immunologically rejected after several weeks, in association with increased inflammation and up-regulation of class I and II histocompatibility antigens. Fusion between ES cells and cardiomyocytes occurred in vivo, but was rare. Infarct autofluorescence was identified as an artifact that might be mistaken for enhanced GFP expression and true regeneration. Hence, undifferentiated ES cells were not guided toward a cardiomyocyte fate in either normal or infarcted hearts, and there was no evidence for allogeneic immune tolerance of ES cell derivatives. Successful cardiac repair strategies involving ES cells will need to control cardiac differentiation, avoid introducing undifferentiated cells, and will likely require immune modulation to avoid rejection.

Cardiopoietic programming of embryonic stem cells for tumor-free heart repair

Embryonic stem cells have the distinct potential for tissue regeneration, including cardiac repair. Their propensity for multilineage differentiation carries, however, the liability of neoplastic growth, impeding therapeutic application. Here, the tumorigenic threat associated with embryonic stem cell transplantation was suppressed by cardiac-restricted transgenic expression of the reprogramming cytokine TNF-α, enhancing the cardiogenic competence of recipient heart. The in vivo aptitude of TNF-α to promote cardiac differentiation was recapitulated in embryoid bodies in vitro. The procardiogenic action required an intact endoderm and was mediated by secreted cardio-inductive signals. Resolved TNF-α–induced endoderm-derived factors, combined in a cocktail, secured guided differentiation of embryonic stem cells in monolayers produce cardiac progenitors termed cardiopoietic cells. Characterized by a down-regulation of oncogenic markers, up-regulation, and nuclear translocation of cardiac transcription factors, this predetermined population yielded functional cardiomyocyte progeny. Recruited cardiopoietic cells delivered in infarcted hearts generated cardiomyocytes that proliferated into scar tissue, integrating with host myocardium for tumor-free repair. Thus, cardiopoietic programming establishes a strategy to hone stem cell pluripotency, offering a tumor-resistant approach for regeneration.

The adult human heart as a source for stem cells: repair strategies with embryonic-like progenitor cells

Adequate cell-based repair of adult myocardium remains an elusive goal because most cells that are used cannot generate mature myocardium sufficient to promote large functional improvements. Embryonic stem cells can generate both mature cardiocytes and vasculature, but their use is hampered by associated teratoma formation and the need for an allogeneic source. The detection of sca-1(+), c-kit(+), or isl-1(+) cardiac precursors and the creation of cardiospheres from adult heart tissues suggest that a persistent population of immature progenitor cells is present in the mature myocardium. These cell populations probably represent stages along a continuum of cardiac stem cell development and differentiation. We report isolation from ventricle of uncommitted cardiac progenitor cells, which appear to resemble the more immature, common pool of embryonic lateral plate mesoderm progenitors that yield both myocardial and endocardial cells during normal cardiac development. Under controlled in vitro conditions and in vivo, these cells can differentiate into endothelial, smooth muscle, and cardiomyocyte lineages and can be isolated and expanded to clinically relevant numbers from adult rat myocardial tissue. In this article, we discuss the potential for autologous repair or even cardiac regeneration with cells that follow a developmental pathway similar to embryonic cardiac precursors but without the inherent limitations associated with undifferentiated embryonic stem cells.

Cell-based bone tissue engineering

The authors review the available data on bone tissue engineering and discuss possible new research areas that could help to make bone tissue engineering a clinical success.

Re-expression of proteins involved in cytokinesis during cardiac hypertrophy

Cardiomyocytes stop dividing after birth and postnatal heart growth is only achieved by increase in cell volume. In some species, cardiomyocytes undergo an additional incomplete mitosis in the first postnatal week, where karyokinesis takes place in the absence of cytokinesis, leading to binucleation. Proteins that regulate the formation of the actomyosin ring are known to be important for cytokinesis. Here we demonstrate for the first time that small GTPases like RhoA along with their downstream effectors like ROCK I, ROCK II and Citron Kinase show a developmental stage specific expression in heart, with high levels being expressed in cardiomyocytes only at stages when cytokinesis still occurs (i.e. embryonic and perinatal). This suggests that downregulation of many regulatory and cytoskeletal components involved in the formation of the actomyosin ring may be responsible for the uncoupling of cytokinesis from karyokinesis in rodent cardiomyocytes after birth. Interestingly, when the myocardium tries to adapt to the increased workload during pathological hypertrophy a re-expression of proteins involved in DNA synthesis and cytokinesis can be detected. Nevertheless, the adult cardiomyocytes do not appear to divide despite this upregulation of the cytokinetic machinery. The inability to undergo complete division could be due to the presence of stable, highly ordered and functional sarcomeres in the adult myocardium or could be because of the inefficiency of degradation pathways, which facilitate the division of differentiated embryonic cardiomyocytes by disintegrating myofibrils.

Molecular and pharmacological properties of human embryonic stem cell-derived cardiomyocytes

Human embryonic stem cells (hESCs) can be coaxed to differentiate into specific cell types, including cardiomyocyte-like cells. These cells express cardiac-specific markers and display functional similarities to their adult counterparts. Based on these properties, hESC-derived cardiomyocytes have the potential to be extremely useful in various in vitro applications and to provide the opportunity for cardiac cell replacement therapies. However, before this can become a reality, the molecular and functional characteristics of these cells need to be investigated in more detail. In the present study we differentiate hESCs into cardiomyocyte-like cells via embryoid bodies (EBs). The fraction of spontaneously beating clusters obtained from the EBs averaged approximately 30% of the total number of EBs used. These cell clusters were isolated, dissociated into single-cell suspensions, and frozen for long-term storage. The cryopreserved cells could be successfully thawed and subcultured. Using electron microscopy, we observed Z discs and tight junctions in the hESC-derived cardiomyocytes, and by immunohistochemical analysis we detected expression of cardiac-specific markers (cTnI and cMHC). Notably, using BrdU labeling we also could demonstrate that some of the hESC-derived cardiomyocytes retain a proliferative capacity. Furthermore, pharmacological stimulation of the cells resulted in responses indicative of functional adrenergic and muscarinic receptor coupling systems. Taken together, these results lend support to the notion that hESCs can be used as a source for the procurement of cardiomyocytes for in vitro and in vivo applications.

Cardiac regeneration by resident stem and progenitor cells in the adult heart

Two main pieces of data have created a new field in cardiac research. First, the traditional view on the heart as a postmitotic organ has been challenged by the finding of small dividing cells in the heart expressing cardiac contractile proteins with stem cell properties and, second, cellular therapy of the diseased heart using a variety of different cells has shown encouraging effects on cardiac function. These findings immediately raise questions like "what is the identity and origin of the cardiac progenitor cells?","which molecular factors are involved in their mobilization and differentiation?", and "can these cells repair the damaged heart?" This review will address the state of current answers to these questions. Emerging evidence suggests that several subpopulations of cardiac stem or progenitor cells (CPCs) reside within the adult heart. CPCs with the ability to differentiate into all the constituent cells in the adult heart including cardiac myocytes, vascular smooth muscle and endothelial cells have been identified. Valuable knowledge has been obtained from the large number of animal studies and a number of small clinical trials that have utilized a variety of adult stem cells for regenerating infarcted hearts. However, contradictory reports on the regenerative potential of the CPCs exist, and the mechanisms behind the reported hemodynamic effects are intensely debated. Besides directly replenishing cardiac tissue, CPCs could also function by stimulating angiogenesis and improving survival of existing cells by secretion of paracrine factors. With this review we suggest that a better understanding of CPC biology will be pivotal for progressing therapeutic cardiac regeneration. This includes an extended knowledge of the molecular mechanisms behind their mobilization, differentiation, survival and integration in the myocardium.

Niche-to-niche migration of bone-marrow-derived cells

During ontogenesis, haematopoietic stem cells (HSCs) relocate between extra-embryonic and embryonic compartments. Similarly, site-specific homing of HSCs is ongoing during adulthood. With the expanding knowledge of HSC physiology, a new paradigm emerges in which HSCs and haematopoietic progenitor cells (HPCs) migrate to defined microenvironments within the bone marrow (BM) and to 'activated' or 'inducible' niches elsewhere. Here, we summarize current understanding of HSC niche characteristics, and the physiological and pathological mechanisms that guide HSC homing both within the BM and to distant niches in the periphery, promoting new vessel growth in tumours and ischaemia. Recent observations suggest that features of the HSC niche might also be recapitulated in pre-metastatic sites. Clusters of BM-derived HPCs promote invasion of disseminating cancer cells. Clear clinical benefits can be foreseen by modulating HSCs and their microenvironments, in promoting tissue regeneration, and inhibiting tumourigenesis and cancer metastasis.

Donor cell transplantation for myocardial disease: does it complement current pharmacological therapies?This paper is one of a selection of papers published in this Special Issue, entitled Young Investigators' Forum

Heart failure secondary to ischemic heart disease, hypertension, and myocardial infarction is a common cause of death in developed countries. Although pharmacological therapies are very effective, poor prognosis and shorter life expectancy of heart disease patients clearly indicate the need for alternative interventions to complement the present therapies. Since the progression of heart disease is associated with the loss of myocardial cells, the concept of donor cell transplantation into host myocardium is emerging as an attractive strategy to repopulate the damaged tissue. To this end, a number of donor cell types have been tested for their ability to increase the systolic function of diseased hearts in both experimental and clinical settings. Although initial clinical trials with bone marrow stem cells are encouraging, long-term consequences of such interventions are yet to be rigorously examined. While additional laboratory studies are required to address several issues in this field, there is also a clear need for further characterization of drug interactions with donor cells in these interventions. Here, we provide a brief summary of current pharmacological and cell-based therapies for heart disease. Further, we discuss the potential of various donor cell types in myocardial repair, mechanisms underlying functional improvement in cell-based therapies, as well as potential interactions between pharmacological and cell-based therapies.

Limited plasticity of mesenchymal stem cells cocultured with adult cardiomyocytes

In order to assess, in a controlled in vitro model, the differentiation potential of adult bone marrow derived stem cells we have developed a coculture procedure using adult rat cardiomyocytes and mesenchymal stem cells (MSCs) from transgenic GFP positive rats. We investigated in the cocultured MSCs the time course of cellular processes that are difficult to monitor in in vivo experiments. Adult rat cardiomyocytes and adult rat MSCs were cocultured for up to 7 days and analyzed by confocal microscopy. Several markers were studied by immunofluorescence technique. The fluorescent ST-BODIPY-Dihydropyridine was used to label calcium channels in living cells. Intracellular calcium was monitored with the fluorescent probe X-Rhod-1. Immunofluorescence experiments showed the presence of connexin-43 between cardiomyocytes and MSCs and between MSCs, while no sarcomeric structures were observed at any time of the coculture. We looked at the expression of calcium channels and development of voltage-dependent calcium signaling in cocultured MSCs. MSCs showed a time-dependent increase of labeling of ST-BODIPY-Dihydropyridine, reaching a relatively strong level after 72 h of coculture. The treatment with a non-fluorescent DHP, Nifedipine, completely abolished ST-BODIPY labeling. We investigated whether depolarization could modulate intracellular calcium. Depolarization-induced calcium transients increased in MSCs in relation to the coculture time. We conclude that MSCs cocultured with adult cardiomyocytes present preliminary evidence of voltage-dependent calcium modulation uncoupled with the development of nascent or adult myofibrils, thus showing a limited lineage specification and a low plasticity to differentiate in a full cardiomyocyte-like phenotype.

Photocrosslinkable hydrogel for myocyte cell culture and injection

Conventional treatment options for myocardial infarction are limited by the inability of mature myocardium to regenerate after injury. Although functional improvements after injection of cells and growth factors have been demonstrated, the clinical utility of this procedure has been hampered by poor cell localization, low survival, and rapid clearance of injected growth factors. The main objective of this study was to evaluate the applicability of a hydrogel, based on photocrosslinkable chitosan and acryloyl-poly(ethylene glycol)-RGDS (Az-chitosan/Acr-PEG-RGD) for myocyte cell culture and myocardial injection. Chitosan was modified with photoreactive azidobenzoic acid and Acr-PEG-RGD was synthesized by reacting YRGDS with an equimolar amount of acryloyl-PEG-N-hydroxysuccinimide. For injection and encapsulation each polymer was dissolved in Di-H(2)O (pH 6.4), the solutions were mixed and crosslinked by UV application (4 mW/cm(2)). C2C12 myoblasts proliferated and differentiated on hydrogels containing 5 mM RGD but not on the pure photocrosslinked chitosan. In vitro, the crosslinked hydrogels retained 80% of encapsulated VEGF for 24 days. Live/dead staining of neonatal rat cardiomyocytes encapsulated into Az-chitosan/Acr-PEG-RGD hydrogels indicated high cell viability upon UV crosslinking. Ex vivo, we localized the hydrogel on the surface and in the ventricle wall of an adult rat heart by brief (2 min) UV light application.

Improved cardiac function in infarcted mice after treatment with pluripotent embryonic stem cells

Because pluripotent embryonic stem cells (ESCs) are able to differentiate into any tissue, they are attractive agents for tissue regeneration. Although improvement of cardiac function has been observed after transplantation of pluripotent ESCs, the extent to which these effects reflect ESC-mediated remuscularization, revascularization, or paracrine mechanisms is unknown. Moreover, because ESCs may generate teratomas, the ability to predict the outcome of cellular differentiation, especially when transplanting pluripotent ESCs, is essential; conversely, a requirement to use predifferentiated ESCs would limit their application to highly characterized subsets that are available in limited numbers. In the experiments reported here, we transplanted low numbers of two murine ESC lines, respectively engineered to express a beta-galactosidase gene from either a constitutive (elongation factor) or a cardiac-specific (alpha-myosin heavy chain) promoter, into infarcted mouse myocardium. Although ESC-derived tumors formed within the pericardial space in 21% of injected hearts, lacZ histochemistry revealed that engraftment of ESC was restricted to the ischemic myocardium. Echocardiographic monitoring of ESC-injected hearts that did not form tumors revealed functional improvements by 4 weeks postinfarction, including significant increases in ejection fraction, circumferential fiber shortening velocity, and peak mitral blood flow velocity. These experiments indicate that the infarcted myocardial environment can support engraftment and cardiomyogenic differentiation of pluripotent ESCs, concomitant with partial functional recovery.

Gene expression analysis of zebrafish heart regeneration

Mammalian hearts cannot regenerate. In contrast, zebrafish hearts regenerate even when up to 20% of the ventricle is amputated. The mechanism of zebrafish heart regeneration is not understood. To systematically characterize this process at the molecular level, we generated transcriptional profiles of zebrafish cardiac regeneration by microarray analyses. Distinct gene clusters were identified based on temporal expression patterns. Genes coding for wound response/inflammatory factors, secreted molecules, and matrix metalloproteinases are expressed in regenerating heart in sequential patterns. Comparisons of gene expression profiles between heart and fin regeneration revealed a set of regeneration core molecules as well as tissue-specific factors. The expression patterns of several secreted molecules around the wound suggest that they play important roles in heart regeneration. We found that both platelet-derived growth factor-a and -b (pdgf-a and pdgf-b) are upregulated in regenerating zebrafish hearts. PDGF-B homodimers induce DNA synthesis in adult zebrafish cardiomyocytes. In addition, we demonstrate that a chemical inhibitor of PDGF receptor decreases DNA synthesis of cardiomyocytes both in vitro and in vivo during regeneration. Our data indicate that zebrafish heart regeneration is associated with sequentially upregulated wound healing genes and growth factors and suggest that PDGF signaling is required.

Comparison of gene expression in regenerating heart and fin points to common as well as tissue-specific mechanisms. PDGFs expressed in the heart induce DNA synthesis in cardiac myocytes.

Heart valve tissue engineering: concepts, approaches, progress, and challenges

Potential applications of tissue engineering in regenerative medicine range from structural tissues to organs with complex function. This review focuses on the engineering of heart valve tissue, a goal which involves a unique combination of biological, engineering, and technological hurdles. We emphasize basic concepts, approaches and methods, progress made, and remaining challenges. To provide a framework for understanding the enabling scientific principles, we first examine the elements and features of normal heart valve functional structure, biomechanics, development, maturation, remodeling, and response to injury. Following a discussion of the fundamental principles of tissue engineering applicable to heart valves, we examine three approaches to achieving the goal of an engineered tissue heart valve: (1) cell seeding of biodegradable synthetic scaffolds, (2) cell seeding of processed tissue scaffolds, and (3) in-vivo repopulation by circulating endogenous cells of implanted substrates without prior in-vitro cell seeding. Lastly, we analyze challenges to the field and suggest future directions for both preclinical and translational (clinical) studies that will be needed to address key regulatory issues for safety and efficacy of the application of tissue engineering and regenerative approaches to heart valves. Although modest progress has been made toward the goal of a clinically useful tissue engineered heart valve, further success and ultimate human benefit will be dependent upon advances in biodegradable polymers and other scaffolds, cellular manipulation, strategies for rebuilding the extracellular matrix, and techniques to characterize and potentially non-invasively assess the speed and quality of tissue healing and remodeling.

Advanced Tools for Tissue Engineering: Scaffolds, Bioreactors, and Signaling

This article contains the collective views expressed at the second session of the workshop "Tissue Engineering--The Next Generation,'' which was devoted to the tools of tissue engineering: scaffolds, bioreactors, and molecular and physical signaling. Lisa E. Freed and Farshid Guilak discussed the integrated use of scaffolds and bioreactors as tools to accelerate and control tissue regeneration, in the context of engineering mechanically functional cartilage and cardiac muscle. Edward Guo focused on the opportunities that tissue engineering generates for studies of mechanobiology and on the need for tissue engineers to learn about mechanical forces during tissue and organ genesis. Martha L. Gray focused on the potential of biomedical imaging for noninvasive monitoring of engineered tissues and on the opportunities biomedical imaging can generate for the development of new markers. Robert Tranquillo reviewed the approach to tissue engineering of a spectrum of avascular habitually loaded tissues- blood vessels, heart valves, ligaments, tendons, cartilage, and skin. Jeffrey W. Holmes offered the perspective of a "reverse paradigm''--the use of tissue constructs in quantitative studies of cell-matrix interactions, cell mechanics, matrix mechanics, and mechanobiology. Milica Radisic discussed biomimetic design of tissue-engineering systems, on the example of synchronously contractile cardiac muscle. Michael V. Sefton proposed a new, simple approach to the vascularization of engineered tissues. This session stressed the need for advanced scaffolds, bioreactors, and imaging technologies and offered many enlightening examples on how these advanced tools can be utilized for functional tissue engineering and basic research in medicine and biology.

Concise review: stem cells, myocardial regeneration, and methodological artifacts

This review discusses the current controversy about the role that endogenous and exogenous progenitor cells have in cardiac homeostasis and myocardial regeneration following injury. Although great enthusiasm was created by the possibility of reconstituting the damaged heart, the opponents of this new concept of cardiac biology have interpreted most of the findings supporting this possibility as the product of technical artifacts. This article challenges this established, static view of cardiac growth and favors the notion that the mammalian heart has the inherent ability to replace its cardiomyocytes through the activation of a pool of resident primitive cells or the administration of hematopoietic stem cells.

Multipotent embryonic isl1+ progenitor cells lead to cardiac, smooth muscle, and endothelial cell diversification

Cardiogenesis requires the generation of endothelial, cardiac, and smooth muscle cells, thought to arise from distinct embryonic precursors. We use genetic fate-mapping studies to document that isl1(+) precursors from the second heart field can generate each of these diverse cardiovascular cell types in vivo. Utilizing embryonic stem (ES) cells, we clonally amplified a cellular hierarchy of isl1(+) cardiovascular progenitors, which resemble the developmental precursors in the embryonic heart. The transcriptional signature of isl1(+)/Nkx2.5(+)/flk1(+) defines a multipotent cardiovascular progenitor, which can give rise to cells of all three lineages. These studies document a developmental paradigm for cardiogenesis, where muscle and endothelial lineage diversification arises from a single cell-level decision of a multipotent isl1(+) cardiovascular progenitor cell (MICP). The discovery of ES cell-derived MICPs suggests a strategy for cardiovascular tissue regeneration via their isolation, renewal, and directed differentiation into specific mature cardiac, pacemaker, smooth muscle, and endothelial cell types.

Myeloid progenitors differentiate into microglia and promote vascular repair in a model of ischemic retinopathy

Vision loss associated with ischemic diseases such as retinopathy of prematurity and diabetic retinopathy are often due to retinal neovascularization. While significant progress has been made in the development of compounds useful for the treatment of abnormal vascular permeability and proliferation, such therapies do not address the underlying hypoxia that stimulates the observed vascular growth. Using a model of oxygen-induced retinopathy, we demonstrate that a population of adult BM-derived myeloid progenitor cells migrated to avascular regions of the retina, differentiated into microglia, and facilitated normalization of the vasculature. Myeloid-specific hypoxia-inducible factor 1alpha (HIF-1alpha) expression was required for this function, and we also demonstrate that endogenous microglia participated in retinal vascularization. These findings suggest what we believe to be a novel therapeutic approach for the treatment of ischemic retinopathies that promotes vascular repair rather than destruction.

Myometrial cells induce angiogenesis and salvage damaged myocardium

Characteristically, uterine myometrial cells (MCs) are proliferative, inducing angiogenesis within the female reproductive organ. We evaluated whether MCs implanted into myocardium could also induce angiogenesis and restore heart function after injury. MCs were isolated from the adult rat uterus and cultured for three studies: 1) Intracellular VEGF levels were measured in MCs cultured with progesterone (10−11, 10−9, and 10−7M) ( n = 6 tests per group). 2) Blood vessel density was evaluated 8 days after MCs (3 × 106or 6 × 106), smooth muscle cells (SMCs), or endothelial cells ( n = 6 rats per group) were injected with matrigel into the subcutaneous tissue of adult rats. 3) MCs, SMCs (5 × 106/rat), or media were injected into a transmural scar 3 wk after cryoinjury in rat hearts ( n = 12 rats per group), and heart function, blood vessel density, and myocardial scar size and thickness were evaluated 5 wk later. In study 1, cultured MCs expressed VEGF, with levels significantly ( P < 0.05) upregulated by progesterone at an optimal dose of 10−11M. In study 2, MCs injected into the subcutaneous tissue with matrigel induced significantly more blood vessels, especially large-diameter vessels, than did SMCs or endothelial cells ( P < 0.01 for all groups). This angiogenic effect was greatest ( P < 0.01) at higher doses of MCs and was enhanced by progesterone (10−11M). In study 3, MCs implanted into the injured myocardium increased blood vessel density at the implant area, reduced scar size, and improved cardiac function relative to SMCs and media. Overall, MCs induced angiogenesis in vitro and in vivo, prevented cardiac remodeling, and improved heart functional recovery after cardiac injury.

Therapeutic angiogenesis: Translating experimental concepts to medically relevant goals

Angiogenesis is central to many physiological and pathological phenomena. In physiological angiogenesis, new vessels are well shaped and their growth is finely tuned to match the metabolic needs of tributary tissues. Accordingly, neovascularization is activated by physical exercise and destabilized by non-use. In contrast, pathological blood vessels that are observed in retinal neovascularization, cancer or in ischemic tissues are leaky, irregularly shaped, and tend to form arterial-venous fistulae. A great deal of attention is focused on new approaches for medical manipulation of vascular growth. These methods are aimed at facilitating the reperfusion of ischemic tissues or eradicating pathological vasculature. In this position paper, we challenge the rationale of therapeutic angiogenesis for the cure of myocardial and peripheral ischemia. Therapeutic angiogenesis aims at combating the insufficiency of, or insensitivity to angiogenic factors in the setting of atherosclerotic-induced arterial occlusion. However, clinical evidence indicates that such a defect is not common among patients with ischemic disease, as a whole. Genetic and environmental factors could account for the great heterogeneity in the expression of the master angiogenic factors. Future improvements in the strategy would require the introduction of in vitro assays and in vivo imaging systems for assessing human angiogenesis. Finally, the promise is to find individualized angiogenesis-based therapies for a genuine cure of ischemia and prevention of organ failure.

Multimodality noninvasive imaging demonstrates in vivo cardiac regeneration after mesenchymal stem cell therapy

OBJECTIVES

The purpose of this study was to test the hypothesis, with noninvasive multimodality imaging, that allogeneic mesenchymal stem cells (MSCs) produce and/or stimulate active cardiac regeneration in vivo after myocardial infarction (MI).

BACKGROUND

Although intramyocardial injection of allogeneic MSCs improves global cardiac function after MI, the mechanism(s) underlying this phenomenon are incompletely understood.

METHODS

We employed magnetic resonance imaging (MRI) and multi-detector computed tomography (MDCT) imaging in MSC-treated pigs (n = 10) and control subjects (n = 12) serially for a 2-month period after anterior MI. A sub-endocardial rim of tissue, demonstrated with MDCT, was assessed for regional contraction with MRI tagging. Rim thickness was also measured on gross pathological specimens, to confirm the findings of the MDCT imaging, and the size of cardiomyocytes was measured in the sub-endocardial rim and the non-infarct zone.

RESULTS

Multi-detector computed tomography demonstrated increasing thickness of sub-endocardial viable myocardium in the infarct zone in MSC-treated animals (1.0 +/- 0.2 mm to 2.0 +/- 0.3 mm, 1 and 8 weeks after MI, respectively, p = 0.028, n = 4) and a corresponding reduction in infarct scar (5.1 +/- 0.5 mm to 3.6 +/- 0.2 mm, p = 0.044). No changes occurred in control subjects (n = 4). Tagging MRI demonstrated time-dependent recovery of active contractility paralleling new tissue appearance. This rim was composed of morphologically normal cardiomyocytes, which were smaller in MSC-treated versus control subjects (11.6 +/- 0.2 mum vs. 12.6 +/- 0.2 mum, p < 0.05).

CONCLUSIONS

With serially obtained MRI and MDCT, we demonstrate in vivo reappearance of myocardial tissue in the MI zone accompanied by time-dependent restoration of contractile function. These data are consistent with a regenerative process, highlight the value of noninvasive multimodality imaging to assess the structural and functional basis for myocardial regenerative strategies, and have potential clinical applications.

Systemically transferred hematopoietic stem cells home to the subretinal space and express RPE-65 in a mouse model of retinal pigment epithelium damage

Stem cell regeneration of damaged tissue has recently been reported in many different organs. Since the loss of retinal pigment epithelium (RPE) in the eye is associated with a major cause of visual loss - specifically, age-related macular degeneration - we investigated whether hematopoietic stem cells (HSC) given systemically can home to the damaged subretinal space and express markers of RPE lineage. Green fluorescent protein (GFP) cells of bone marrow origin were used in a sodium iodate (NaIO(3)) model of RPE damage in the mouse. The optimal time for adoptive transfer of bone marrow-derived stem cells relative to the time of injury and the optimal cell type [whole bone marrow, mobilized peripheral blood, HSC, facilitating cells (FC)] were determined by counting the number of GFP(+) cells in whole eye flat mounts. Immunocytochemistry was performed to identify the bone marrow origin of the cells in the RPE using antibodies for CD45, Sca-1, and c-kit, as well as the expression of the RPE-specific marker, RPE-65. The time at which bone marrow-derived cells were adoptively transferred relative to the time of NaIO(3) injection did not significantly influence the number of cells that homed to the subretinal space. At both one and two weeks after intravenous (i.v.) injection, GFP(+) cells of bone marrow origin were observed in the damaged subretinal space, at sites of RPE loss, but not in the normal subretinal space. The combined transplantation of HSC+FC cells appeared to favor the survival of the homed stem cells at two weeks, and RPE-65 was expressed by adoptively transferred HSC by four weeks. We have shown that systemically injected HSC homed to the subretinal space in the presence of RPE damage and that FC promoted survival of these cells. Furthermore, the RPE-specific marker RPE-65 was expressed on adoptively transferred HSC in the denuded areas.

Biologically based myocardial regeneration: is there a role for the surgeon?

PURPOSE OF REVIEW

Despite advances in medical, percutaneous, and surgical treatment, there is an increasing burden of ischemic cardiovascular disease and heart failure. Over the past decade, a large number of preclinical and clinical studies have evaluated various biologic agents to treat these diseases.

RECENT FINDINGS

Although the safety and feasibility of growth factor therapy, using vascular endothelial growth factors and fibroblast growth factors, for myocardial angiogenesis has been well established in a number of clinical trials, their ability to induce clinically significant improvements in symptoms remains uncertain. Numerous candidates have been proposed for cell-based therapies to improve myocardial perfusion and function and have demonstrated efficacy in preclinical studies. These cell types include skeletal myoblasts, bone-marrow derived cells, endothelial progenitors, and mesenchymal stem cells. Early clinical trials have demonstrated feasibility of cell harvest and implantation.

SUMMARY

Biologic myocardial regeneration is a new and rapidly evolving area for the treatment of cardiovascular disease. Translation of these biologic entities into clinically useful therapeutic agents will require a better mechanistic understanding of their effects on the myocardium and the coronary circulation, optimization of delivery techniques, and systematic evaluation in large, randomized, placebo-controlled studies.

Hybrid cardiomyocytes derived by cell fusion in heterotopic cardiac xenografts

Cardiomyocytes expressing host markers, such as the Y chromosome in sex-mismatched transplants, have been described in human allografts, suggesting that circulating cells can contribute to cardiac regeneration. It has not been established, however, whether host-derived cardiomyocytes result from transdifferentiation of stem cells or cell fusion. To address this issue, we used heterotopic heart xenografts and looked for markers of donor and recipient cells. Golden Syrian hamsters or transgenic mice expressing nuclear beta-galactosidase under the control of the cardiac troponin I promoter served as organ donors, while GFP+ transgenic rats were used as recipients. GFP+ cells, including abundant CD-45+ inflammatory cells and rare undifferentiated cells expressing early cardiac markers (GATA-4 or MEF2C), were found in xenografts harvested two weeks after surgery. In addition, rare GFP+ mature cardiomyocytes were found in 7 of 8 hamster xenografts and 6 of 6 mouse xenografts. The proportion of these cells was very low (0.0001% to 0.0344% in hamster xenografts) but similar to the one observed in control rat heart allografts. Without exception, all GFP+ cardiomyocytes also expressed donor markers, i.e., hamster membrane antigens or lacZ, so they must derive from cell fusion, not transdifferentiation.

Differential gene expression associated with migration of mesenchymal stem cells to conditioned medium from tumor cells or bone marrow cells

Distinct signals that guide migration of mesenchymal stem cells (MSCs) to specific in vivo targets remain unknown. We have used rat MSCs to investigate the molecular mechanisms involved in such migration. Rat MSCs were shown to migrate to tumor microenvironment in vivo, and an in vitro migration assay was used under defined conditions to permit further mechanistic investigations. We hypothesized that distinct molecular signals are involved in the homing of MSCs to tumor sites and bone marrow. To test this hypothesis, gene expression profiles of MSCs exposed in vitro to conditioned medium (CM) from either tumor cells or bone marrow were compared. Analysis of the microarray gene expression data revealed that 104 transcripts were upregulated in rat MSCs exposed to CM from C85 human colorectal cancer cells for 24 hours versus control medium. A subset of 12 transcripts were found to be upregulated in rat MSCs that were exposed to tumor cell CM but downregulated when MSCs were exposed to bone marrow CM and included CXCL-12 (stromal cell-derived factor-1 [SDF-1]), CXCL-2, CINC-2, endothelial cell specific molecule-1, fibroblast growth factor-7, nuclear factor-kappaB p105, and thrombomodulin. Exposure to tumor cell CM enhanced migration of MSCs and correlated with increased SDF-1 protein production. Moreover, knockdown of SDF-1 expression in MSCs inhibited migration of these cells to CM from tumor cells, but not bone marrow cells, confirming the importance of SDF-1 expression by MSCs in this differential migration. These results suggest that increased SDF-1 production by MSCs acts in an autocrine manner and is required for migratory responses to tumor cells.

FGF1/p38 MAP kinase inhibitor therapy induces cardiomyocyte mitosis, reduces scarring, and rescues function after myocardial infarction

Mammalian cardiomyocytes have limited proliferation potential, and acutely injured mammalian hearts do not regenerate adequately. Instead, injured myocardium develops fibrosis and scarring. Here we show that FGF1/p38 MAP kinase inhibitor treatment after acute myocardial injury in 8- to 10-week-old rats increases cardiomyocyte mitosis. At 3 months after injury, 4 weeks of FGF1/p38 MAP kinase inhibitor therapy results in reduced scarring and wall thinning, with markedly improved cardiac function. In contrast, p38 MAP kinase inhibition alone fails to rescue heart function despite increased cardiomyocyte mitosis. FGF1 improves angiogenesis, possibly contributing to the survival of newly generated cardiomyocytes. Our data indicate that FGF1 and p38 MAP kinase, proteins involved in cardiomyocyte proliferation and angiogenesis during development, may be delivered therapeutically to enhance cardiac regeneration.

Engraftment of engineered ES cell-derived cardiomyocytes but not BM cells restores contractile function to the infarcted myocardium

Cellular cardiomyoplasty is an attractive option for the treatment of severe heart failure. It is, however, still unclear and controversial which is the most promising cell source. Therefore, we investigated and examined the fate and functional impact of bone marrow (BM) cells and embryonic stem cell (ES cell)-derived cardiomyocytes after transplantation into the infarcted mouse heart. This proved particularly challenging for the ES cells, as their enrichment into cardiomyocytes and their long-term engraftment and tumorigenicity are still poorly understood. We generated transgenic ES cells expressing puromycin resistance and enhanced green fluorescent protein cassettes under control of a cardiac-specific promoter. Puromycin selection resulted in a highly purified (>99%) cardiomyocyte population, and the yield of cardiomyocytes increased 6-10-fold because of induction of proliferation on purification. Long-term engraftment (4-5 months) was observed when co-transplanting selected ES cell-derived cardiomyocytes and fibroblasts into the injured heart of syngeneic mice, and no teratoma formation was found (n = 60). Although transplantation of ES cell-derived cardiomyocytes improved heart function, BM cells had no positive effects. Furthermore, no contribution of BM cells to cardiac, endothelial, or smooth muscle neogenesis was detected. Hence, our results demonstrate that ES-based cell therapy is a promising approach for the treatment of impaired myocardial function and provides better results than BM-derived cells.

Heart repair and stem cells

Of the medical conditions currently being discussed in the context of possible treatments based on cell transplantation therapy, few have received more attention than the heart. Much focus has been on the potential application of bone marrow-derived cell preparations, which have already been introduced into double-blind, placebo-controlled clinical trials. The consensus is that bone marrow may have therapeutic benefit but that this is not based on the ability of bone marrow cells to transdifferentiate into cardiac myocytes. Are there potential stem cell sources of cardiac myocytes that may be useful in replacing those lost or dysfunctional after myocardial infarction? Here, this question is addressed with a review of the recent literature.