Increased cardiac myocyte progenitors in failing human hearts

Regenerating cardiac cells: insights from the bench and the clinic

A major challenge in cardiovascular regenerative medicine is the development of novel therapeutic strategies to restore the function of cardiac muscle in the failing heart. The heart has historically been regarded as a terminally differentiated organ that does not have the potential to regenerate. This concept has been updated by the discovery of cardiac stem and progenitor cells that reside in the adult mammalian heart. Whereas diverse types of adult cardiac stem or progenitor cells have been described, we still do not know whether these cells share a common origin. A better understanding of the physiology of cardiac stem and progenitor cells should advance the successful use of regenerative medicine as a viable therapy for heart disease. In this review, we summarize current knowledge of the various adult cardiac stem and progenitor cell types that have been discovered. We also review clinical trials presently being undertaken with adult stem cells to repair the injured myocardium in patients with coronary artery disease.

Cardiac side population cells: moving toward the center stage in cardiac regeneration

Over the past decade, extensive work in animal models and humans has identified the presence of adult cardiac progenitor cells, capable of cardiomyogenic differentiation and likely contributors to cardiomyocyte turnover during normal development and disease. Among cardiac progenitor cells, there is a distinct subpopulation, termed "side population" (SP) progenitor cells, identified by their unique ability to efflux DNA binding dyes through an ATP-binding cassette transporter. This review highlights the literature on the isolation, characterization, and functional relevance of cardiac SP cells. We review the initial discovery of cardiac SP cells in adult myocardium as well as their capacity for functional cardiomyogenic differentiation and role in cardiac regeneration after myocardial injury. Finally, we discuss recent advances in understanding the molecular regulators of cardiac SP cell proliferation and differentiation, as well as likely future areas of investigation required to realize the goal of effective cardiac regeneration.

Bioenergetic and functional consequences of cellular therapy: activation of endogenous cardiovascular progenitor cells

RATIONALE

The mechanism by which endogenous progenitor cells contribute to functional and beneficial effects in stem cell therapy remains unknown.

OBJECTIVE

Utilizing a novel (31)P magnetic resonance spectroscopy-2-dimensional chemical shift imaging method, this study examined the heterogeneity and bioenergetic consequences of postinfarction left ventricular (LV) remodeling and the mechanisms of endogenous progenitor cell contribution to the cellular therapy.

METHODS AND RESULTS

Human embryonic stem cell-derived vascular cells (hESC-VCs) that stably express green fluorescent protein and firefly luciferase (GFP(+)/Luc(+)) were used for the transplantation. hESC-VCs may release various cytokines to promote angiogenesis, prosurvival, and antiapoptotic effects. Both in vitro and in vivo experiments demonstrated that hESC-VCs effectively inhibit myocyte apoptosis. In the mouse model, a fibrin patch-based cell delivery resulted in a significantly better cell engraftment rate that was accompanied by a better ejection fraction. In the swine model of ischemia-reperfusion, the patch-enhanced delivery of hESC-VCs resulted in alleviation of abnormalities including border zone myocardial perfusion, contractile dysfunction, and LV wall stress. These results were also accompanied by a pronounced recruitment of endogenous c-kit(+) cells to the injury site. These improvements were directly associated with a remarkable improvement in myocardial energetics, as measured by a novel in vivo (31)P magnetic resonance spectroscopy-2-dimensional chemical shift imaging technology.

CONCLUSIONS

The findings of this study demonstrate that a severely abnormal heterogeneity of myocardial bioenergetics in hearts with postinfarction LV remodeling can be alleviated by the hESC-VCs therapy. These findings suggest an important therapeutic target of peri-scar border zone and a promising therapeutic potential for using hESC-VCs together with the fibrin patch-based delivery system.

Towards regenerative therapy for cardiac disease

Development of regenerative therapeutic strategies to reverse the progression of advanced heart failure is one of the most urgent clinical needs of this century. Insights gained from clinical trials of adult stem cells, together with fundamental scientific advances in cardiac stem cell and regenerative biology, are beginning to yield potential new targets and strategies for regenerative therapies. Of particular importance are new scientific discoveries related to intrinsic cardiac regeneration, renewal factors that can trigger regeneration, and tissue-engineering technology, which are beginning to change the way investigators view the scientific and clinical position of cardiovascular regenerative therapy.

Pressure overload leads to an increase of cardiac resident stem cells

Recent studies suggest that the mammalian heart possesses some capacity for cardiac regeneration. This regenerative capacity is primarily documented postnatally and after myocardial infarction or pressure overload. Although the cell type that mediates endogenous regeneration is unclear, cardiac stem cells might be considered as potential candidates. To determine the number of c-kit + cardiac resident cells under conditions of pressure overload, we evaluated specimens derived from n = 8 patients with pressure overloaded single right ventricles in comparison to n = 4 explanted hearts from patients with dilated cardiomyopathy and n = 14 biopsies from children after heart transplantation. The age of the patients ranged from 16 days to 19 years. For quantification of cardiac stem cells, c-kit+/mast cell tryptase-/CD45- cells were counted and expressed as percent of the total nuclei. In specimens from patients with dilated cardiomyopathy, 0.13 ± 0.09% c-kit +/mast cell tryptase-/CD45- cells were detected. However, in specimens from patients with pressure overloaded single right ventricles, the numbers of c-kit+/mast cell tryptase-/CD45- cells were significantly higher (0.41 ±0.24%, p < 0.05). Under conditions of pressure overload, the right ventricle shows an approximately three-fold increase in c-kit+/mast cell tryptase-/CD45- cardiac resident cells. Despite the fact that this increased number of c-kit+ cells is not sufficient to prevent the failing heart from congestive heart failure, understanding the mechanism that leads to an increase of presumably cardiac resident stem cells under conditions of pressure overload might help to develop new strategies to enhance endogenous repair.

Increased potency of cardiac stem cells compared with bone marrow mesenchymal stem cells in cardiac repair

Whereas cardiac-derived c-kit(+) stem cells (CSCs) and bone marrow-derived mesenchymal stem cells (MSCs) are undergoing clinical trials testing safety and efficacy as a cell-based therapy, the relative therapeutic and biologic efficacy of these two cell types is unknown. We hypothesized that human CSCs have greater ability than MSCs to engraft, differentiate, and improve cardiac function. We compared intramyocardial injection of human fetal CSCs (36,000) with two doses of adult MSCs (36,000 and 1,000,000) or control (phosphate buffered saline) in nonobese diabetic/severe combined immune deficiency mice after coronary artery ligation. The myocardial infarction-induced enlargement in left ventricular chamber dimensions was ameliorated by CSCs (p < .05 for diastolic and systolic volumes), as was the decline in ejection fraction (EF; p < .05). Whereas 1 × 10(6) MSCs partially ameliorated ventricular remodeling and improved EF to a similar degree as CSCs, 36,000 MSCs did not influence chamber architecture or function. All cell therapies improved myocardial contractility, but CSCs preferentially reduced scar size and reduced vascular afterload. Engraftment and trilineage differentiation was substantially greater with CSCs than with MSCs. Adult-cultured c-kit(+)CSCs were less effective than fetal, but were still more potent than high-dose MSCs. These data demonstrate enhanced CSC engraftment, differentiation, and improved cardiac remodeling and function in ischemic heart failure. MSCs required a 30-fold greater dose than CSCs to improve cardiac function and anatomy. Together, these findings demonstrate a greater potency of CSCs than bone marrow MSCs in cardiac repair.

Ischemia induces P-selectin-mediated selective progenitor cell engraftment in the isolated-perfused heart

Clinical trials infusing Bone Marrow Cells (BMCs) into injured hearts have produced measureable improvements in cardiac performance, but were insufficient to improve patient outcomes. Low engraftment rates are cited as probable contributor to limited improvements. To understand the mechanisms that control myocardial engraftment of BMCs following ischemia-reperfusion injury, in isolated-perfused mouse hearts, stop-flow ischemia was followed by variable-duration reperfusion (0-60 min) before addition of labeled syngenic BMCs to the perfusate. After a buffer-only wash, the heart was disaggregated. Retained BMCs (digest) and infused BMCs (aliquot) were compared by flow cytometry for c-kit and CD45 expression to determine the proportion of cell subtypes engrafted versus delivered (selectivity ratio). In these studies, a time-dependent selective retention of c-kit(+) cells was apparent starting at 30 min of reperfusion, at which time c-kit(+)/CD45(+) BMCs showed a selectivity ratio of 18 ± 2 (versus 2 ± 1 in sham-ischemic controls). To study the underlying mechanism for this selective retention, neutralizing antibodies for P-selectin or L-selectin were infused into the heart preparation and incubated with BMCs prior to BMC infusion. Blocking P-selectin in ischemic hearts ablated selectivity for c-kit(+)/CD45(+) BMCs at 30 min reperfusion (selectivity ratio of 3 ± 1) while selectivity persisted in the presence of L-selectin neutralization (selectivity ratio of 17 ± 2). To corroborate this finding, a parallel plate flow chamber was used to study capture and rolling dynamics of purified c-kit(+) versus c-kit- BMCs on various selectin molecules. C-kit(+) BMCs interacted weakly with L-selectin substrates (0.03 ± 0.01% adhered) but adhered strongly to P-selectin (0.28±0.04% adhered). C-kit- BMCs showed intermediate binding regardless of substrate (0.18 ± 0.04% adhered on L-selectin versus 0.17 ± 0.04% adhered on P-selectin). Myocardial ischemia-reperfusion stress induces selective engraftment of c-kit(+) bone marrow progenitor cells via P-selectin activation.

MicroRNA profiling predicts a variance in the proliferative potential of cardiac progenitor cells derived from neonatal and adult murine hearts

Cardiac progenitor cells (CPCs) are multipotent cells that may offer tremendous potentials for the regeneration of injured myocardium. To expand the limited number of CPCs for effective clinical regeneration of myocardium, it is important to understand their proliferative potentials. Single-cell based assays were utilized to purify c-kit(pos) CPCs from human and mouse hearts. MicroRNA profiling identified eight differentially expressed microRNAs in CPCs from neonatal and adult hearts. Notably, the predicted protein targets were predominantly involved in cellular proliferation-related pathways. To directly test this phenotypic prediction, the developmental variance in the proliferation of CPCs was tested. Ki67 protein expression and DNA kinetics were tested in human and mouse in vivo CPCs, and doubling times were tested in primary culture of mouse CPCs. The human embryonic and mouse neonatal CPCs showed a six-fold increase in Ki67 expressing cells, a two-fold increase in the number of cells in S/G2-M phases of cell cycle, and a seven-fold increase in the doubling time in culture when compared to the corresponding adult CPCs. The over-expression of miR-17-92 increased the proliferation in adult CPCs in vivo by two-fold. In addition, the level of retinoblastoma-like 2 (Rbl2/p130) protein was two-fold higher in adult compared to neonatal-mouse CPCs. In conclusion, we demonstrate a differentially regulated cohort of microRNAs that predicts differences in cellular proliferation in CPCs during postnatal development and target microRNAs that are involved in this transition. Our study provides new insights that may enhance the utilization of adult CPCs for regenerative therapy of the injured myocardium.

Evaluation of myocyte proliferation in alcoholic cardiomyopathy: telomerase enzyme activity (TERT) compared with Ki-67 expression

AIMS

Although the human heart was classically considered a terminal organ, recent studies have reported a myocyte proliferation response versus some aggressions. Excessive ethanol consumption induces development of cardiomyopathy (CMP) through myocyte apoptosis. We evaluated myocyte proliferation response in the heart of chronic alcoholic donors with telomerase activity (telomerase reverse transcriptase (TERT)) compared with Ki-67 nuclear expression.

METHODS

Heart samples were prospectively obtained from organ donors on life support. We included donors with (1) high lifetime alcohol consumption (n = 15), (2) longstanding hypertension (n = 14), (3) other causes of CMP (valve, coronary or idiopathic) (n = 8) and (4) previously healthy donors (n = 6). Groups 2 and 3 were subdivided according to the presence of CMP. Evaluation comprised parameters of ethanol consumption, left ventricular function by chest X-ray and 2D echocardiography, and histology and immunohistochemical studies. Myocyte proliferation was evaluated using an assay for Ki-67 expression and measuring telomerase gene activity by real-time PCR.

RESULTS

Forty-three donors were included in the study, 35 having CMP. Nuclear Ki-67 activity was low in healthy controls and significantly increased in the other groups, mainly in those with CMP. Alcoholics with CMP had a non-significantly lower proliferation response than the other CMP groups. No proliferation activity was detected with TERT in any case.

CONCLUSION

Heart Ki-67 proliferation activity increases in organ donors with CMP, independently of its origin. Alcoholics presented non-significant lower myocyte proliferation capacity compared with the other groups of CMP. TERT activity was not a useful marker of proliferation in this model. Ki-67 is a better procedure to evaluate proliferation than TERT expression in alcohol-induced heart damage.

Myocardial AKT: the omnipresent nexus

One of the greatest examples of integrated signal transduction is revealed by examination of effects mediated by AKT kinase in myocardial biology. Positioned at the intersection of multiple afferent and efferent signals, AKT exemplifies a molecular sensing node that coordinates dynamic responses of the cell in literally every aspect of biological responses. The balanced and nuanced nature of homeostatic signaling is particularly essential within the myocardial context, where regulation of survival, energy production, contractility, and response to pathological stress all flow through the nexus of AKT activation or repression. Equally important, the loss of regulated AKT activity is primarily the cause or consequence of pathological conditions leading to remodeling of the heart and eventual decompensation. This review presents an overview compendium of the complex world of myocardial AKT biology gleaned from more than a decade of research. Summarization of the widespread influence that AKT exerts upon myocardial responses leaves no doubt that the participation of AKT in molecular signaling will need to be reckoned with as a seemingly omnipresent regulator of myocardial molecular biological responses.

Heart regeneration

Heart failure plagues industrialized nations, killing more people than any other disease. It usually results from a deficiency of specialized cardiac muscle cells known as cardiomyocytes, and a robust therapy to regenerate lost myocardium could help millions of patients every year. Heart regeneration is well documented in amphibia and fish and in developing mammals. After birth, however, human heart regeneration becomes limited to very slow cardiomyocyte replacement. Several experimental strategies to remuscularize the injured heart using adult stem cells and pluripotent stem cells, cellular reprogramming and tissue engineering are in progress. Although many challenges remain, these interventions may eventually lead to better approaches to treat or prevent heart failure.

Regeneration of skeletal muscle

Skeletal muscle has a robust capacity for regeneration following injury. However, few if any effective therapeutic options for volumetric muscle loss are available. Autologous muscle grafts or muscle transposition represent possible salvage procedures for the restoration of mass and function but these approaches have limited success and are plagued by associated donor site morbidity. Cell-based therapies are in their infancy and, to date, have largely focused on hereditary disorders such as Duchenne muscular dystrophy. An unequivocal need exists for regenerative medicine strategies that can enhance or induce de novo formation of functional skeletal muscle as a treatment for congenital absence or traumatic loss of tissue. In this review, the three stages of skeletal muscle regeneration and the potential pitfalls in the development of regenerative medicine strategies for the restoration of functional skeletal muscle in situ are discussed.

Current status of stem cell therapy in heart failure

Inspired by studies demonstrating the potential for new myocyte formation within adult mammalian hearts, an ongoing explosion of research is elucidating the biology of cardiac myogenesis and angiogenesis. Multiple lines of research suggest that disease-associated activation of endogenous cardiac repair processes are often insufficient to overcome the cell death resulting from myocardial infarction and chronic heart failure. In this context, this review highlights current evidence supporting endogenous cardiac repair mechanisms in human hearts, recent progress with clinical application of myocardial cell therapy, and complementary efforts to manipulate endogenous myocardial repair processes using a variety of tissue engineering strategies. The goal of this overview is to demonstrate that the insights and opportunities derived from each of these lines of inquiry are mutually complementary for ultimately achieving the goal of therapeutic cardiac regeneration.

Repair of the injured adult heart involves new myocytes potentially derived from resident cardiac stem cells

RATIONALE

The ability of the adult heart to generate new myocytes after injury is not established.

OBJECTIVE

Our purpose was to determine whether the adult heart has the capacity to generate new myocytes after injury, and to gain insight into their source.

METHODS AND RESULTS

Cardiac injury was induced in the adult feline heart by infusing isoproterenol (ISO) for 10 days via minipumps, and then animals were allowed to recover for 7 or 28 days. Cardiac function was measured with echocardiography, and proliferative cells were identified by nuclear incorporation of 5-bromodeoxyuridine (BrdU; 7-day minipump infusion). BrdU was infused for 7 days before euthanasia at days 10, 17, and 38 or during injury and animals euthanized at day 38. ISO caused reduction in cardiac function with evidence of myocyte loss from necrosis. During this injury phase there was a significant increase in the number of proliferative cells in the atria and ventricle, but there was no increase in BrdU+ myocytes. cKit+ cardiac progenitor cells were BrdU labeled during injury. During the first 7 days of recovery there was a significant reduction in cellular proliferation (BrdU incorporation) but a significant increase in BrdU+ myocytes. There was modest improvement in cardiac structure and function during recovery. At day 38, overall cell proliferation was not different than control, but increased numbers of BrdU+ myocytes were found when BrdU was infused during injury.

CONCLUSIONS

These studies suggest that ISO injury activates cardiac progenitor cells that can differentiate into new myocytes during cardiac repair.

A chemical biology approach to myocardial regeneration

Heart failure is one of the major causes of death in the Western world because cardiac muscle loss is largely irreversible and can lead to a relentless decline in cardiac function. Novel therapies are needed since the only therapy to effectively replace lost myocytes today is transplantation of the entire heart. The advent of embryonic and induced pluripotent stem cell (ESC/iPSC) technologies offers the unprecedented possibility of devising cell replacement therapies for numerous degenerative disorders. Not only are ESCs and iPSCs a plausible source of cardiomyocytes in vitro for transplantation, they are also useful tools to elucidate the biology of stem cells that reside in the adult heart and define signaling molecules that might enhance the limited regenerative capability of the adult human heart. Here, we review the extracellular factors that control stem cell cardiomyogenesis and describe new approaches that combine embryology with stem cell biology to discover drug-like small molecules that stimulate cardiogenesis and potentially contribute to the development of pharmaceutical strategies for heart muscle regeneration.

Homeodomain only protein x is down-regulated in human heart failure

Homeodomain only protein x (Hopx) is an unusual homeodomain protein that has diverse effects on cardiac growth. Manipulation of Hopx function in murine models is associated with cardiac hypertrophy, dilation and fibrosis. In the present study, we examined the expression profile of Hopx in various models of pathologic cardiac hypertrophy and failure. Hopx expression is significantly reduced in neonatal rat cardiac myocytes after α/β adrenergic receptor agonist treatment. Cardiac hypertrophy and failure induced by transaortic constriction in mice causes marked down-regulation of Hopx expression. Interestingly, HOPX expression was significantly reduced in hearts of humans with end-stage heart failure when compared to non-failing control hearts, and HOPX levels remain low after LVAD support. Our findings suggest that HOPX/Hopx expression is reduced in multiple examples of human and murine cardiac hypertrophy and failure.

Characterisation and cardiac directed differentiation of canine adult cardiac stem cells

This study describes the isolation and characterisation of adult canine cardiac stem cells, and explores their ability to differentiate into cardiac myocytes. Direct comparisons are also made with available human data. Atrial cardiac explants were taken from dogs post-mortem and cultured to isolate adult stem cells. Cells were able to survive successive passages in serum-free media, were able to form cardiospheres, and under controlled culture conditions were capable of clonal expansion, demonstrating their ability for self-renewal. Characterisation of these cells demonstrated the following marker profile: c-kit, GATA 4 and flk-1 positive; cardiac troponin T and NKx2.5 low. Cardiac lineage directed differentiation was performed based on the published literature. Gene expression studies demonstrated that cardiac directed differentiation was partially achieved, with up-regulation of cardiac troponin T and NKx2.5, and down-regulation of c-kit and endothelial lineage markers. However the cells did not express the ryanodine receptor or β(1)-adrenergic receptors and did not contract spontaneously.

The Role of Immunogenicity in Cardiovascular Disease

Recently, many of the complexities associated with cardiovascular diseases (CVD) have been unlocked. However, despite these breakthroughs, CVD and its related complications are the leading contributors of morbidity and mortality worldwide, which indicates the shortcomings of current treatment regimens and the need for continued research. Published data within the field clearly indicates that CVD are built on inflammation and autoimmune platforms, though a strong, fundamental understanding of the mechanisms remains elusive. Areas such as the mechanisms underlying increased immunogenicity of self-proteins in the cardiovascular system, the roles of immunogenic auto-antigens in eliciting inflammatory autoimmune responses, and the immunosuppressive mechanisms involved in controlling inflammatory and autoimmune cardiovascular diseases remain to be well-understood. We will delve into these topics and the advancements made within the field in this review. Specifically, we will concentrate on the innate and adaptive immune responses mediating immunogenicity; the mechanisms of inflammation and autoimmunity in atherogenesis; the mechanisms of inflammation and autoimmunity in diabetic atherosclerosis; immunogenicity and stem cell therapy; as well as immunogenicity and immunosuppression. In depth examination and comprehension of these topics will provide insight into the recent progress of the field and bring to the forefront potentially novel therapeutic avenues.

Myocyte turnover in the aging human heart

RATIONALE

The turnover of cardiomyocytes in the aging female and male heart is currently unknown, emphasizing the need to define human myocardial biology.

OBJECTIVE

The effects of age and gender on the magnitude of myocyte regeneration and the origin of newly formed cardiomyocytes were determined.

METHODS AND RESULTS

The interaction of myocyte replacement, cellular senescence, growth inhibition, and apoptosis was measured in normal female (n=32) and male (n=42) human hearts collected from patients 19 to 104 years of age who died from causes other than cardiovascular diseases. A progressive loss of telomeric DNA in human cardiac stem cells (hCSCs) occurs with aging and the newly formed cardiomyocytes inherit short telomeres and rapidly reach the senescent phenotype. Our data provide novel information on the superior ability of the female heart to sustain the multiple variables associated with the development of the senescent myopathy. At all ages, the female heart is equipped with a larger pool of functionally competent hCSCs and younger myocytes than the male myocardium. The replicative potential is higher and telomeres are longer in female hCSCs than in male hCSCs. In the female heart, myocyte turnover occurs at a rate of 10%, 14%, and 40% per year at 20, 60, and 100 years of age, respectively. Corresponding values in the male heart are 7%, 12%, and 32% per year, documenting that cardiomyogenesis involves a large and progressively increasing number of parenchymal cells with aging. From 20 to 100 years of age, the myocyte compartment is replaced 15 times in women and 11 times in men.

CONCLUSIONS

The human heart is a highly dynamic organ regulated by a pool of resident hCSCs that modulate cardiac homeostasis and condition organ aging.

Dedifferentiation and proliferation of mammalian cardiomyocytes

Background

It has long been thought that mammalian cardiomyocytes are terminally-differentiated and unable to proliferate. However, myocytes in more primitive animals such as zebrafish are able to dedifferentiate and proliferate to regenerate amputated cardiac muscle.

Methodology/Principal Findings

Here we test the hypothesis that mature mammalian cardiomyocytes retain substantial cellular plasticity, including the ability to dedifferentiate, proliferate, and acquire progenitor cell phenotypes. Two complementary methods were used: 1) cardiomyocyte purification from rat hearts, and 2) genetic fate mapping in cardiac explants from bi-transgenic mice. Cardiomyocytes isolated from rodent hearts were purified by multiple centrifugation and Percoll gradient separation steps, and the purity verified by immunostaining and RT-PCR. Within days in culture, purified cardiomyocytes lost their characteristic electrophysiological properties and striations, flattened and began to divide, as confirmed by proliferation markers and BrdU incorporation. Many dedifferentiated cardiomyocytes went on to express the stem cell antigen c-kit, and the early cardiac transcription factors GATA4 and Nkx2.5. Underlying these changes, inhibitory cell cycle molecules were suppressed in myocyte-derived cells (MDCs), while microRNAs known to orchestrate proliferation and pluripotency increased dramatically. Some, but not all, MDCs self-organized into spheres and re-differentiated into myocytes and endothelial cells in vitro. Cell fate tracking of cardiomyocytes from 4-OH-Tamoxifen-treated double-transgenic MerCreMer/ZEG mouse hearts revealed that green fluorescent protein (GFP) continues to be expressed in dedifferentiated cardiomyocytes, two-thirds of which were also c-kit⁺.

Conclusions/Significance

Contradicting the prevailing view that they are terminally-differentiated, postnatal mammalian cardiomyocytes are instead capable of substantial plasticity. Dedifferentiation of myocytes facilitates proliferation and confers a degree of stemness, including the expression of c-kit and the capacity for multipotency.

Design and characterization of an injectable pericardial matrix gel: a potentially autologous scaffold for cardiac tissue engineering

Following ischemic injury in the heart, little to no repair occurs, causing a progressive degeneration of cardiac function that leads to congestive heart failure. Cardiac tissue engineering strategies have focused on designing a variety of injectable scaffolds that range in composition from single-component materials to complex extracellular matrix (ECM)-derived materials. In this study, the pericardial ECM, a commonly used biomaterial, was investigated for use as an injectable scaffold for cardiac repair. It was determined that a solubilized form of decellularized porcine pericardium could be injected and induced to gel in vivo, prompting investigation with human pericardium, which has the decided advantage of offering an autologous therapy. Characterization showed that the matrix gels retained components of the native pericardial ECM, with extant protein and glycosaminoglycan content identified. The results of an in vitro migration assay indicate that the porcine pericardial matrix is a stronger chemoattractant for relevant cell types, but in vivo results showed that the two materials caused statistically similar amounts of neovascularization, demonstrating feasibility as injectable treatments. Potential stem cell mobilization was supported by the presence of c-Kit+ cells within the matrix injection regions. With this work, the pericardium is identified as a novel source for an autologous scaffold for treating myocardial infarction.

Differentiation of human embryonic stem cells to cardiomyocytes for in vitro and in vivo applications

The ability of human embryonic stem cells to differentiate into spontaneously contracting cardiomyocyte-like cells has attracted substantial interest from the scientific community over the last decade. From having been difficult to control, human cardiomyogenesis in vitro is now becoming a process which, to a certain extent, can be effectively manipulated and directed. Although much research remains, new and improved protocols for guiding pluripotent stem cells to the cardiomyocyte lineage are accumulating in the scientific literature. However, the stem cell derived cardiomyocytes described to date, generally resemble immature embryonic/fetal cardiomyocytes, and they are in some functional and structural aspects different from adult cardiomyocytes. Thus, a future challenge will be to design strategies that eventually may allow the cells to reach a higher degree of maturation in vitro. Nevertheless, the cells which can be prepared using current protocols still have wide spread utility, and they have begun to find their way into the drug discovery platforms used in the pharmaceutical industry. In addition, stem cell derived cardiomyocytes and cardiac progenitors are anticipated to have a tremendous impact on how heart disease will be treated in the future. Here, we will discuss recent strategies for the generation of cardiomyocytes from human embryonic stem cells and recapitulate their features, as well as highlight some in vitro applications for the cells. Finally, opportunities in the area of cardiac regenerative medicine will be illustrated.

3-dimensional structures to enhance cell therapy and engineer contractile tissue

Experimental studies in animals and recent human clinical trials have revealed the current limitations of cellular transplantation, which include poor cell survival, lack of cell engraftment, and poor differentiation. Evidence in animals suggests that use of a 3-dimensional scaffold may enhance cell therapy and engineer myocardial tissue by improving initial cell retention, survival, differentiation, and integration. Several scaffolds of synthetic or natural origin are under development. Until now, contractility has been demonstrated in vitro only in biological scaffolds prepared from decellularized organs or tissue, or in collagenic porous scaffold obtained by crosslinking collagen fibers. While contractility of a cellularized collagen construct is poor, it can be greatly enhanced by tumor basement membrane extract. Recent advances in biochemistry have shown improved cell-matrix interactions by coupling adhesion molecules to achieve an efficient and safe bioartificial myocardium with no tumoral component. Fixation of adhesion molecules may also be a way to enhance cell homing and/or differentiation to increase local angiogenesis. Whatever the clinically successful combination ultimately proves to be, it is likely that cell therapy will require providing a supportive biochemical, physical, and spatial environment that will allow the cells to optimally differentiate and integrate within the target myocardial tissue.

{beta}1-Adrenergic receptor activation induces mouse cardiac myocyte death through both L-type calcium channel-dependent and -independent pathways

Cardiac diseases persistently increase the contractility demands of cardiac myocytes, which require activation of the sympathetic nervous system and subsequent increases in myocyte Ca(2+) transients. Persistent exposure to sympathetic and/or Ca(2+) stress is associated with myocyte death. This study examined the respective roles of persistent beta-adrenergic receptor (beta-AR) agonist exposure and high Ca(2+) concentration in myocyte death. Ventricular myocytes (VMs) were isolated from transgenic (TG) mice with cardiac-specific and inducible expression of the beta(2a)-subunit of the L-type Ca(2+) channel (LTCC). VMs were cultured, and the rate of myocyte death was measured in the presence of isoproterenol (ISO), other modulators of Ca(2+) handling and the beta-adrenergic system, and inhibitors of caspases and reactive oxygen species generation. The rate of myocyte death was greater in TG vs. wild-type myocytes and accelerated by ISO in both groups, although ISO did not increase LTCC current (I(Ca-L)) in TG-VMs. Nifedipine, an LTCC antagonist, only partially prevented myocyte death. These results suggest both LTCC-dependent and -independent mechanisms in ISO induced myocyte death. ISO increased the contractility of wild type and TG-VMs by enhancing sarcoplasmic reticulum function and inhibiting sarco(endo)plasmic reticulum Ca(2+)-ATPase, Na(+)/Ca(2+) exchanger, and CaMKII partially protected myocyte from death induced by both Ca(2+) and ISO. Caspase and reactive oxygen species inhibitors did not, but beta(2)-AR activation did, reduce myocyte death induced by enhanced I(Ca-L) and ISO stimulation. Our results suggest that catecholamines induce myocyte necrosis primarily through beta(1)-AR-mediated increases in I(Ca-L), but other mechanisms are also involved in rodents.

Cardiomyogenic potential of C-kit(+)-expressing cells derived from neonatal and adult mouse hearts

BACKGROUND

C-kit is a receptor tyrosine kinase family member expressed in hematopoietic stem cells. C-kit is also transiently expressed in cardiomyocyte precursors during development and in a rare cell population in the normal adult heart. In the present study, the cardiomyogenic potential of c-kit(+) cells isolated from normal neonatal, normal adult, and infarcted adult mouse hearts was evaluated.

METHODS AND RESULTS

Magnetic activated cell sorting was used to prepare c-kit(+) cells from the hearts of ACT-EGFP/MHC-nLAC double transgenic mice. These animals exhibit widespread enhanced green fluorescent protein (EGFP) expression and cardiomyocyte-restricted nuclear beta-galactosidase activity, thus permitting simultaneous tracking of cell survival and differentiation. A subset of the c-kit(+) cells from double transgenic neonatal hearts acquired a cardiomyogenic phenotype when cocultured with fetal cardiomyocytes (2.4% of all EGFP(+) cells screened) but rarely when cultured alone or when cocultured with mouse fibroblasts (0.03% and 0.05% of the EGFP(+) cells screened, respectively). In contrast, c-kit(+) cells from normal adult double transgenic hearts failed to undergo cardiomyogenic differentiation when cocultured with nontransgenic fetal cardiomyocytes (>18 000 EGFP(+) cells screened) or when transplanted into normal or infarcted adult mouse hearts (14 EGFP(+) grafts examined). A single c-kit(+) cell from an infarcted double transgenic adult heart was observed to acquire a cardiomyogenic phenotype in coculture (>37 000 EGFP(+) cells screened).

CONCLUSIONS

These data suggest that the ability of cardiac-resident c-kit(+) cells to acquire a cardiomyogenic phenotype is subject to temporal limitations or, alternatively, that the cardiomyogenic population is lost. Elucidation of the underlying molecular basis may permit robust cardiomyogenic induction in adult-derived cardiac c-kit(+) cells.

Myocardial gene expression alterations in peripheral blood mononuclear cells of patients with idiopathic dilated cardiomyopathy

AIMS

To assess cardiac gene expression in peripheral blood cells of patients with idiopathic dilated cardiomyopathy (IDCM) and its relationship to echocardiographic left ventricular (LV) function.

METHODS AND RESULTS

A complete echocardiographic study and blood sampling were performed in 65 consecutive stable IDCM patients with LV ejection fraction (LVEF) 31.76 +/- 10.07% and chronic mild to moderate heart failure (NYHA functional class II to III) for > or =9 months. Blood samples from 19 healthy individuals were included for comparison. Transcript levels of myocardin, GATA4, alpha- and beta-myosin heavy chain (MHC), sarcoplasmic reticulum calcium ATPase 2 (SERCA2), and phospholamban were determined by quantitative real-time reverse transcription-polymerase chain reaction. Myocardin (24.88 +/- 4.93 vs. 3.98 +/- 1.12, P = 0.0048) and GATA4 (17.85 +/- 4.85 vs. 0.45 +/- 0.15, P = 0.0069 x 10(-5)) were upregulated in IDCM patients compared with controls, whereas SERCA2 (5.11 +/- 0.42 vs. 8.93 +/- 1.07, P = 0.001) was downregulated. In IDCM patients, myocardin (r = 0.279, P = 0.025), GATA4 (r = 0.314, P = 0.011), beta-MHC (r = 0.444, P=0.0002), and alpha-MHC (r = 0.272, P = 0.034) showed positive correlations, whereas SERCA2 (r = -0.264, P = 0.034) exhibited a negative correlation with LVEF. Patients with elevated LV filling pressures had lower myocardin (15.06 +/- 3.10 vs. 43.12 +/- 12.03, P = 0.048), GATA4 (8.96 +/- 2.17 vs. 34.38 +/- 12.60, P = 0.026), beta-MHC (10.59 +/- 4.05 vs. 16.43 +/- 4.91, P = 0.013), and alpha-MHC (0.27 +/- 0.08 vs. 0.79 +/- 0.20, P = 0.033) and higher SERCA2 (5.65 +/- 0.54 vs. 3.90 +/- 0.61, P = 0.037) levels. Patients with atrial fibrillation (AF) had higher SERCA2 levels compared with sinus rhythm patients (6.75 +/- 0.84 vs. 4.54 +/- 0.45, P = 0.017).

CONCLUSION

Our data indicate that cardiac gene expression alterations in peripheral blood cells of IDCM patients may reflect alterations in LV function, whereas the presence of AF may be associated with increased SERCA2 levels in these patients.

C-kit+ CD45- cells found in the adult human heart represent a population of endothelial progenitor cells

Although numerous reports support the existence of stem cells in the adult heart, few studies have been conducted using human cardiac tissue. Therefore, cells from human cardiac atrial biopsies were analyzed regarding progenitor properties. Expression of stem cell markers was analyzed using fluorescence-activated cell sorting. This identified a small population of C-kit+ cells, which could be further subdivided based on expression of CD45. The C-kit+ CD45+ population was determined to be of mast cell identity, while the C-kit+ CD45- population expressed mRNA of the endothelial lineage. Since the number of cells obtainable from biopsies was limited, a comparison between directly isolated and monolayer and explant cultured cells, respectively, was carried out. While both cultures retained a small population of mast cells, only monolayer culture produced a stable and relatively high percentage of C-kit+ CD45- cells. This population was found to co-express endothelial progenitor cell markers such as CD31, CD34, CXCR4, and FLK-1. The mRNA expression profile was similar to the one from directly isolated cells. When sorted cells were cultured in endothelial differentiation medium, the C-kit+ CD45- population retained its expression of endothelial markers to a large extent, but downregulated progenitor markers, indicating further differentiation into endothelial cells. We have confirmed that the human cardiac atrium contains a small C-kit+ CD45- population expressing markers commonly found on endothelial progenitor cells. The existence of an endothelial progenitor population within the heart might have future implications for developing methods of inducing neovascularization after myocardial infarction.

Human bone marrow stem cells co-cultured with neonatal rat cardiomyocytes display limited cardiomyogenic plasticity

BACKGROUND AIMS

This study investigated whether neonatal rat cardiomyocytes (NRCM), when co-cultured, can induce transdifferentiation of either human mesenchymal stromal cells (MSC) or hematopoietic stem cells (HSC) into cardiomyocytes. Stem cells were obtained from patients with ischemic heart disease.

METHODS

Ex vivo-expanded MSC or freshly isolated HSC were used to set-up a co-culture system between NRCM and MSC or HSC. 5-azacytidin (5-aza) or dimethylsulfoxide (DMSO) was used as differentiation-inducing factor. Co-cultured stem cells were separated from NRCM by flow sorting, and cardiac gene expression was analyzed by reverse transcriptase-polymerase chain reaction. Cellular morphology was analyzed by immunofluorescence and transmission electron microscopy (TEM).

RESULTS

Co-culturing MSC induced expression of troponin T and GATA-4. However, no expression of alpha-actinin, myosin heavy chain or troponin I was detected. In the case of HSC, only expression of troponin T could be induced. Immunofluorescence and TEM confirmed the absence of sarcomeric organization in co-cultured MSC and HSC. Adding 5-aza or DMSO to the co-cultures did not influence differentiation.

CONCLUSIONS

This in vitro co-culture study obtained no convincing evidence of transdifferentiation of either MSC or HSC into functional cardiomyocytes. Nevertheless, induction of troponin T was observed in MSC and HSC, and GATA-4 in MSC. However, no morphologic changes could be detected by immunofluorescence or by TEM. These data could explain why only limited functional improvement was reported in clinical stem cell trials.

CD117-positive cells of the heart: progenitor cells or mast cells?

Human cardiac stem/progenitor cells and their potential for repair of heart injury are a current hot topic of research. CD117 has been used frequently as a marker for identification of stem/progenitor cells in the heart. However, cardiac mast cells, which are also CD117(+), have not been excluded by credible means when selecting putative cardiac progenitors by using CD117 as a marker. We evaluated the relationship between CD117(+) cells and mast cells in the left ventricle of human hearts (n=5 patients, ages 1 week-75 years) with the well-established mast cell markers tryptase, toluidine blue, and thionine. A large number (85-100%) of CD117(+) cells in the human heart were specifically identified as mast cells. In addition, mast cells showed weak or moderate CD45 immunostaining signals. These results indicate that the majority of CD117(+) cells in the heart are mast cells and that these cells are distinctly positive for CD45, although staining was weak or moderate. These results strongly suggest that the newly reported CD117(+)/CD45(dim/moderate) putative cardiac progenitor cells are mast cells. The significance of this observation in stem cell research of the heart is discussed.

Induced pluripotent stem cells: advances to applications

Induced pluripotent stem cell (iPS) technology has enriched the armamentarium of regenerative medicine by introducing autologous pluripotent progenitor pools bioengineered from ordinary somatic tissue. Through nuclear reprogramming, patient-specific iPS cells have been derived and validated. Optimizing iPS-based methodology will ensure robust applications across discovery science, offering opportunities for the development of personalized diagnostics and targeted therapeutics. Here, we highlight the process of nuclear reprogramming of somatic tissues that, when forced to ectopically express stemness factors, are converted into bona fide pluripotent stem cells. Bioengineered stem cells acquire the genuine ability to generate replacement tissues for a wide-spectrum of diseased conditions, and have so far demonstrated therapeutic benefit upon transplantation in model systems of sickle cell anemia, Parkinson’s disease, hemophilia A, and ischemic heart disease. The field of regenerative medicine is therefore primed to adopt and incorporate iPS cell-based advancements as a next generation stem cell platforms.

Enhancement of myocardial regeneration through genetic engineering of cardiac progenitor cells expressing Pim-1 kinase

BACKGROUND

Despite numerous studies demonstrating the efficacy of cellular adoptive transfer for therapeutic myocardial regeneration, problems remain for donated cells with regard to survival, persistence, engraftment, and long-term benefits. This study redresses these concerns by enhancing the regenerative potential of adoptively transferred cardiac progenitor cells (CPCs) via genetic engineering to overexpress Pim-1, a cardioprotective kinase that enhances cell survival and proliferation.

METHODS AND RESULTS

Intramyocardial injections of CPCs overexpressing Pim-1 were given to infarcted female mice. Animals were monitored over 4, 12, and 32 weeks to assess cardiac function and engraftment of Pim-1 CPCs with echocardiography, in vivo hemodynamics, and confocal imagery. CPCs overexpressing Pim-1 showed increased proliferation and expression of markers consistent with cardiogenic lineage commitment after dexamethasone exposure in vitro. Animals that received CPCs overexpressing Pim-1 also produced greater levels of cellular engraftment, persistence, and functional improvement relative to control CPCs up to 32 weeks after delivery. Salutary effects include reduction of infarct size, greater number of c-kit(+) cells, and increased vasculature in the damaged region.

CONCLUSIONS

Myocardial repair is significantly enhanced by genetic engineering of CPCs with Pim-1 kinase. Ex vivo gene delivery to enhance cellular survival, proliferation, and regeneration may overcome current limitations of stem cell-based therapeutic approaches.

Stem cell transplant into preimplantation embryo yields myocardial infarction-resistant adult phenotype

Stem cells are an emerging strategy for treatment of myocardial infarction, limited however to postinjury intervention. Preventive stem cell-based therapy to augment stress tolerance has yet to be considered for lifelong protection. Here, pluripotent stem cells were microsurgically introduced at the blastocyst stage of murine embryo development to ensure stochastic integration and sustained organ contribution. Engineered chimera displayed excess in body weight due to increased fat deposits, but were otherwise devoid of obesity-related morbidity. Remarkably, and in sharp contrast to susceptible nonchimeric offspring, chimera was resistant to myocardial infarction induced by permanent coronary occlusion. Infarcted nonchimeric adult hearts demonstrated progressive deterioration in ejection fraction, while age-matched 12-14-months-old chimera recovered from equivalent ischemic insult to regain within one-month preocclusion contractile performance. Electrical remodeling and ventricular enlargement with fibrosis, prominent in failing nonchimera, were averted in the chimeric cohort characterized by an increased stem cell load in adipose tissue and upregulated markers of biogenesis Ki67, c-Kit, and stem cell antigen-1 in the myocardium. Favorable outcome in infarcted chimera translated into an overall benefit in workload capacity and survival. Thus, prenatal stem cell transplant yields a cardioprotective phenotype in adulthood, expanding cell-based indications beyond traditional postinjury applications to include pre-emptive therapy.

Stem cell therapy for heart failure

Heart failure (HF) is a chronic disease and a significant global public health concern. Current medical treatment for HF can reduce symptoms but does little to decrease mortality and the need for cardiac transplantation. Novel therapies are needed to further decrease mortality and limit or eliminate the need for cardiac transplantation. Recently, several basic science and clinical trials have suggested that enhancing endogenous regeneration (repair) and exogenous cell therapy might be an approach to improve the function of the failing heart. This article reviews cell therapy clinical trials in patients with chronic HF. The three major subgroups of cells being studied in phase 1 and beginning phase 2 trials are skeletal myoblasts, bone marrow-derived mononuclear cells, and enriched subpopulations of bone marrow and cardiac stem cells. Techniques for stimulating upregulation of endogenous bone marrow progenitor cells in the circulating blood have raised serious safety issues and need to be carefully evaluated. Intracoronary infusion and both transepicardial and transendocardial direct injection of stem cells have been tested clinically and shown to be safe. Skeletal myoblast implantation has led to improved cardiac function, but studies show formation of skeletal muscle in the heart and a lack of electrical integration with surrounding myocardium, a cause for concern. Bone marrow-derived mononuclear cells and enriched subpopulations of cardiac and bone marrow stem cells have been studied extensively in animals and in recent clinical trials, with both controversy and success. There is still much room for improvement, but animal and human studies of enriched subpopulations of cardiac and bone marrow stem cells have shown that these cells are safe, have significant capability for cardiac repair, and offer the best chance for legitimate medical therapy for patients with chronic HF.

Natural and synthetic regulators of embryonic stem cell cardiogenesis

Debilitating cardiomyocyte loss underlies the progression to heart failure. Although there have been significant advances in treatment, current therapies are intended to improve or preserve heart function rather than regenerate lost myocardium. A major hurdle in implementing a cell-based regenerative therapy is the inefficient differentiation of cardiomyocytes from either endogenous or exogenous stem cell sources. Moreover, cardiomyocytes that develop in human embryonic stem cell (hESC) or human-induced pluripotent stem cell (hIPSC) cultures are comparatively immature, even after prolonged culture, and differences in their calcium handling, ion channel, and force generation properties relative to adult cardiomyocytes raise concerns of improper integration and function after transplantation. Thus, the discovery of natural and novel small molecule synthetic regulators of differentiation and maturation would accelerate the development of stem-cell-based myocardial therapies. Here, we document recent advances in defining natural signaling pathways that direct the multistep cardiomyogenic differentiation program and the development of small molecules that might be used to enhance differentiation as well as the potential characteristics of lead candidates for pharmaceutical stimulation of endogenous myocardial replacement.

Cytokines and myocardial regeneration: a novel treatment option for acute myocardial infarction

The regenerative capacity of the myocardium and its blood vessels has now been well demonstrated. The cytokines granulocyte colony-stimulating factor, erythropoietin, and stem cell factor may play a role in helping to stimulate cell regeneration under normal physiologic conditions and in patients with myocardial injury. After an ischemic insult, cytokines are released into the peripheral circulation and signal for the mobilization of stem cells. In experimental cardiac injury models, the addition of cytokines has been shown to improve myocardial function with and without the concurrent use of stem cell therapy. Preliminary studies in humans using cytokine therapy alone for treating myocardial infarction have been disappointing. Future studies in patients with myocardial injury need to examine the use of various combinations of cytokines, with and without the addition of intravascular stem cell infusions or direct stem cell injections.

c-Kit+ bone marrow stem cells differentiate into functional cardiac myocytes

The utility of bone marrow cells (BMCs) to regenerate cardiac myocytes is controversial. The present study examined the capacity of different types of BMCs to generate functional cardiac myocytes. Isolated c-kit(+) BMCs (BMSCs), c-kit(+) and crude BMCs from the adult feline femur were membrane stained with PKH26 dye or infected with a control enhanced green fluorescence protein transcript (EGFP)-adenovirus prior to co-culture upon neonatal rat ventricular myocytes (NRVM). Co-cultured cells were immuno-stained for c-kit, alpha-tropomyosin, alpha-actinin, connexin 43 (Cx43) and Ki67 and analyzed with confocal microscopy. Electrophysiology of BMSC derived myocytes were compared to NRVMs within the same culture dish. Gap junction function was analyzed by fluorescence recovery after photo-bleaching (FRAP). BMCs proliferated and differentiated into cardiac myocytes during the first 48 hours of co-culturing. These newly formed cardiac myocytes were able to contract spontaneously or synchronously with neighboring NRVMs. The myogenic rate of c-kit(+) BMSCs was significantly greater than c-kit(+) and crude BMCs (41.2 +/- 2.1, 6.1 +/- 1.2, and 17.1 +/- 1.5%, respectively). The newly formed cardiac myocytes exhibited an immature electrophysiological phenotype until they became electrically coupled to NRVMs through functional gap junctions. BMSCs did not become functional myocytes in the absence of NRVMs. In conclusion, c-kit(+) BMSCs have the ability to transdifferentiate into functional cardiac myocytes.