Modulation of the Cardiomyocyte Cell Cycle in Genetically Altered Animals

Cardiomyocyte Proliferation from Fetal- to Adult- and from Normal- to Hypertrophy and Failing Hearts

Simple Summary

Death from injury to the heart from a variety of causes remains a major cause of mortality worldwide. The cardiomyocyte, the major contracting cell of the heart, is responsible for pumping blood to the rest of the body. During fetal development, these immature cardiomyocytes are small and rapidly divide to complete development of the heart by birth when they develop structural and functional characteristics of mature cells which prevent further division. All further growth of the heart after birth is due to an increase in the size of cardiomyocytes, hypertrophy. Following the loss of functional cardiomyocytes due to coronary artery occlusion or other causes, the heart is unable to replace the lost cells. One of the significant research goals has been to induce adult cardiomyocytes to reactivate the cell cycle and repair cardiac injury. This review explores the developmental, structural, and functional changes of the growing cardiomyocyte, and particularly the sarcomere, responsible for force generation, from the early fetal period of reproductive cell growth through the neonatal period and on to adulthood, as well as during pathological response to different forms of myocardial diseases or injury. Multiple issues relative to cardiomyocyte cell-cycle regulation in normal or diseased conditions are discussed.

Abstract

The cardiomyocyte undergoes dramatic changes in structure, metabolism, and function from the early fetal stage of hyperplastic cell growth, through birth and the conversion to hypertrophic cell growth, continuing to the adult stage and responding to various forms of stress on the myocardium, often leading to myocardial failure. The fetal cell with incompletely formed sarcomeres and other cellular and extracellular components is actively undergoing mitosis, organelle dispersion, and formation of daughter cells. In the first few days of neonatal life, the heart is able to repair fully from injury, but not after conversion to hypertrophic growth. Structural and metabolic changes occur following conversion to hypertrophic growth which forms a barrier to further cardiomyocyte division, though interstitial components continue dividing to keep pace with cardiac growth. Both intra- and extracellular structural changes occur in the stressed myocardium which together with hemodynamic alterations lead to metabolic and functional alterations of myocardial failure. This review probes some of the questions regarding conditions that regulate normal and pathologic growth of the heart.

Turning back the clock: A concise viewpoint of cardiomyocyte cell cycle activation for myocardial regeneration and repair

Patients with acute myocardial infarction (MI) could progress to end-stage congestive heart failure, which is one of the most significant problems in public health. From the molecular and cellular perspective, heart failure often results from the loss of cardiomyocytes-the fundamental contractile unit of the heart-and the damage caused by myocardial injury in adult mammals cannot be repaired, in part because mammalian cardiomyocytes undergo cell-cycle arrest during the early perinatal period. However, recent studies in the hearts of neonatal small and large mammals suggest that the onset of cardiomyocyte cell-cycle arrest can be reversed, which may lead to the development of entirely new strategies for the treatment of heart failure. In this Viewpoint, we summarize these and other provocative findings about the cellular and molecular mechanisms that regulate cardiomyocyte proliferation and how they may be targeted to turn back the clock of cardiomyocyte cell-cycle arrest and improve recovery from cardiac injury and disease.

CCND2 Overexpression Enhances the Regenerative Potency of Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes: Remuscularization of Injured Ventricle

RATIONALE

The effectiveness of transplanted, human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) for treatment of ischemic myocardial injury is limited by the exceptionally low engraftment rate.

OBJECTIVE

To determine whether overexpression of the cell cycle activator CCND2 (cyclin D2) in hiPSC-CMs can increase the graft size and improve myocardial recovery in a mouse model of myocardial infarction by increasing the proliferation of grafted cells.

METHODS AND RESULTS

Human CCND2 was delivered to hiPSCs via lentiviral-mediated gene transfection. In cultured cells, markers for cell cycle activation and proliferation were ≈3- to 7-folds higher in CCND2-overexpressing hiPSC-CMs (hiPSC-CCND2^OECMs) than in hiPSC-CMs with normal levels of CCND2 (hiPSC-CCND2^WTCMs; P<0.01). The pluripotent genes (Oct 4, Sox2, and Nanog) decrease to minimal levels and undetectable levels at day 1 and 10 after differentiating to CMs. In the mouse myocardial infarction model, cardiac function, infarct size, and the number of engrafted cells were similar at week 1 after treatment with hiPSC-CCND2^OECMs or hiPSC-CCND2^WTCMs but was about tripled in hiPSC-CCND2^OECM-treated than in hiPSC-CCND2^WTCM-treated animals at week 4 (P<0.01). The cardiac function and infarct size were significantly better in both cell treatment groups' hearts than in control hearts, which was most prominent in hiPSC-CCND2^OECM-treated animals (P<0.05, each). No tumor formation was observed in any hearts.

CONCLUSIONS

CCND2 overexpression activates cell cycle progression in hiPSC-CMs that results in a significant enhanced potency for myocardial repair as evidenced by remuscularization of injured myocardium. This left ventricular muscle regeneration and increased angiogenesis in border zone are accompanied by a significant improvement of left ventricular chamber function.

Mitochondrial fission leads to Smac/DIABLO release quenched by ARC

Apoptosis plays a critical role for the development of a variety of cardiac diseases. Cardiomyocytes are enriched in mitochondria, while mitochondrial fission can regulate apoptosis. The molecular mechanism governing cardiomyocyte apoptosis remain to be fully elucidated. Our results showed that Smac/DIABLO is necessary for apoptosis in cardiomyocytes, and it is released from mitochondria into cytosol in response to apoptotic stimulation. Smac/DIABLO release is a consequence of mitochondrial fission mediated by dynamin-related protein-1 (Drp1). Upon release Smac/DIABLO binds to X-linked inhibitor of apoptosis protein (XIAP), resulting in the activation of caspase-9 and caspase-3. Their activation is a prerequisite for the initiation of apoptosis because the administration of z-LEHD-fmk and z-DQMD-fmk, two relatively specific inhibitors for caspase-9, and caspase-3, respectively, could significantly attenuate apoptosis. Smac/DIABLO release could not be blocked by these caspase inhibitors, indicating that it is an event upstream of caspase activation. ARC (apoptosis repressor with caspase recruitment domain), an abundantly expressed apoptotic repressor in cardiomyocytes, could inhibit mitochondrial fission and Smac/DIABLO release. Our data reveal that Smac/DIABLO is a target of ARC in counteracting apoptosis.

The human heart: a self-renewing organ

The dogma that the heart is a static organ which contains an irreplaceable population of cardiomyocytes prevailed in the cardiovascular field for the last several decades. However, the recent identification of progenitor cells that give rise to differentiated myocytes has prompted a re-interpretation of cardiac biology. The heart cannot be viewed any longer as a postmitotic organ characterized by a predetermined number of myocytes that is defined at birth and is preserved throughout life. The myocardium constitutes a dynamic entity in which new young parenchymal cells are formed to substitute old damaged dying myocytes. The regenerative ability of the heart was initially documented with a classic morphometric approach and more recently with the demonstration that DNA synthesis, mitosis, and cytokinesis take place in the newly formed myocytes of the normal and pathologic heart. Importantly, replicating myocytes correspond to the differentiated progeny of cardiac stem cells. These findings point to the possibility of novel therapeutic strategies for the diseased heart.

Potential and clinical utility of stem cells in cardiovascular disease

The recent identification of bone marrow-derived adult stem cells and other types of stem cells that could improve heart function after transplantation have raised high expectations. The basic mechanisms have been studied mostly in murine models. However, these experiments revealed controversial results on transdifferentiation vs transfusion of adult stem cells vs paracrine effects of these cells, which is still being debated. Moreover, the reproducibility of these results in precisely translated large animal models is still less well investigated. Despite these weaknesses results of several clinical trials including several hundreds of patients with ischemic heart disease have been published. However, there are no solid data showing that any of these approaches can regenerate human myocardium. Even the effectiveness of cell therapy in these approaches is doubtful. In future we need in this important field of regenerative medicine: i) more experimental data in large animals that are closer to the anatomy and physiology of humans, including data on dose effects, comparison of different cell types and different delivery routes; ii) a better understanding of the molecular mechanisms involved in the fate of transplanted cells; iii) more intensive research on genuine regenerative medicine, applying genetic regulation and cell engineering.

Large scale production of stem cells and their derivatives

Stem cells have been envisioned to become an unlimited cell source for regenerative medicine. Notably, the interest in stem cells lies beyond direct therapeutic applications. They might also provide a previously unavailable source of valuable human cell types for screening platforms, which might facilitate the development of more efficient and safer drugs. The heterogeneity of stem cell types as well as the numerous areas of application suggests that differential processes are mandatory for their in vitro culture. Many of the envisioned applications would require the production of a high number of stem cells and their derivatives in scalable, well-defined and potentially clinical compliant manner under current good manufacturing practice (cGMP). In this review we provide an overview on recent strategies to develop bioprocesses for the expansion, differentiation and enrichment of stem cells and their progenies, presenting examples for adult and embryonic stem cells alike.

A photopolymerizable hydrogel for 3-D culture of human embryonic stem cell-derived cardiomyocytes and rat neonatal cardiac cells

The purpose of this study was to assess the in vitro ability of two types of cardiomyocytes (cardiomyocytes derived from human embryonic stem cells (hESC-CM) and rat neonatal cardiomyocytes (rN-CM)) to survive and generate a functional cardiac syncytium in a three-dimensional in situ polymerizable hydrogel environment. Each cell type was cultured in a PEGylated fibrinogen (PF) hydrogel for up to two weeks while maturation and cardiac function were documented in terms of spontaneous contractile behavior and biomolecular organization. Quantitative contractile parameters including contraction amplitude and synchronization were measured by non-invasive image analysis. The rN-CM demonstrated the fastest maturation and the most significant spontaneous contraction. The hESC-CM maturation occurred between 10-14 days in culture, and exhibited less contraction amplitude and synchronization in comparison to the rN-CMs. The maturation of both cell types within the hydrogels was confirmed by cardiac-specific biomolecular markers, including alpha-sarcomeric actin, actinin, and connexin-43. Cellular responsiveness to isoproterenol, carbamylcholine and heptanol provided further evidence of the cardiac maturation in the 3-D PF hydrogel as well as identified a potential to use this system for in vitro drug screening. These findings indicate that the PF hydrogel biomaterial can be used as an in situ polymerizable biomaterial for stem cells and their cardiomyocyte derivatives.

Cardiomyocyte death and renewal in the normal and diseased heart

During post-natal maturation of the mammalian heart, proliferation of cardiomyocytes essentially ceases as cardiomyocytes withdraw from the cell cycle and develop blocks at the G0/G1 and G2/M transition phases of the cell cycle. As a result, the response of the myocardium to acute stress is limited to various forms of cardiomyocyte injury, which can be modified by preconditioning and reperfusion, whereas the response to chronic stress is dominated by cardiomyocyte hypertrophy and myocardial remodeling. Acute myocardial ischemia leads to injury and death of cardiomyocytes and nonmyocytic stromal cells by oncosis and apoptosis, and possibly by a hybrid form of cell death involving both pathways in the same ischemic cardiomyocytes. There is increasing evidence for a slow, ongoing turnover of cardiomyocytes in the normal heart involving death of cardiomyocytes and generation of new cardiomyocytes. This process appears to be accelerated and quantitatively increased as part of myocardial remodeling. Cardiomyocyte loss involves apoptosis, autophagy, and oncosis, which can occur simultaneously and involve different individual cardiomyocytes in the same heart undergoing remodeling. Mitotic figures in myocytic cells probably represent maturing progeny of stem cells in most cases. Mitosis of mature cardiomyocytes that have reentered the cell cycle appears to be a rare event. Thus, cardiomyocyte renewal likely is mediated primarily by endogenous cardiac stem cells and possibly by blood-born stem cells, but this biological phenomenon is limited in capacity. As a consequence, persistent stress leads to ongoing remodeling in which cardiomyocyte death exceeds cardiomyocyte renewal, resulting in progressive heart failure. Intense investigation currently is focused on cell-based therapies aimed at retarding cardiomyocyte death and promoting myocardial repair and possibly regeneration. Alteration of pathological remodeling holds promise for prevention and treatment of heart failure, which is currently a major cause of morbidity and mortality and a major public health problem. However, a deeper understanding of the fundamental biological processes is needed in order to make lasting advances in clinical therapeutics in the field.

Cardiomyocyte cell cycle activation improves cardiac function after myocardial infarction

AIMS

Cardiomyocyte loss is a major contributor to the decreased cardiac function observed in diseased hearts. Previous studies have shown that cardiomyocyte-restricted cyclin D2 expression resulted in sustained cell cycle activity following myocardial injury in transgenic (MHC-cycD2) mice. Here, we investigated the effects of this cell cycle activation on cardiac function following myocardial infarction (MI).

METHODS AND RESULTS

MI was induced in transgenic and non-transgenic mice by left coronary artery occlusion. At 7, 60, and 180 days after MI, left ventricular pressure-volume measurements were recorded and histological analysis was performed. MI had a similar adverse effect on cardiac function in transgenic and non-transgenic mice at 7 days post-injury. No improvement in cardiac function was observed in non-transgenic mice at 60 and 180 days post-MI. In contrast, the transgenic animals exhibited a progressive and marked increase in cardiac function at subsequent time points. Improved cardiac function in the transgenic mice at 60 and 180 days post-MI correlated positively with the presence of newly formed myocardial tissue which was not apparent at 7 days post-MI. Intracellular calcium transient imaging indicated that cardiomyocytes present in the newly formed myocardium participated in a functional syncytium with the remote myocardium.

CONCLUSION

These findings indicate that cardiomyocyte cell cycle activation leads to improvement of cardiac function and morphology following MI and may represent an important clinical strategy to promote myocardial regeneration.

p53 initiates apoptosis by transcriptionally targeting the antiapoptotic protein ARC

p53 plays an important role in regulating apoptosis. However, the molecular mechanism by which it initiates the apoptotic program still remains to be fully understood. Here, we report that p53 can transcriptionally target the antiapoptotic protein, apoptosis repressor with caspase recruitment domain (ARC). Our results show that reactive oxygen species and anoxia lead to the up-regulation of p53 expression. Concomitantly, ARC is down-regulated at both the protein and mRNA levels. Knockdown of p53 expression can attenuate the decreases in ARC protein and mRNA levels, indicating that ARC down-regulation is a consequence of p53 activation. Strikingly, p53-induced ARC repression occurs in a transcription-dependent manner. We further demonstrate that the p53 up-regulated modulator of apoptosis (PUMA) and Bad are up-regulated in response to the stimulation with reactive oxygen species or anoxia, and p53 is responsible for their up-regulation. ARC can interact with PUMA or Bad via its N terminus. Such an interaction displaces the association of PUMA or Bad with Bcl-2. ARC repression by p53 leads to its failure to counteract the proapoptotic activity of PUMA and Bad. Thus, our data reveal a novel p53 apoptotic pathway in which it initiates apoptosis by transcriptionally repressing ARC.

The art of cobbling a running pump--will human embryonic stem cells mend broken hearts?

The heart is one of the least regenerative organs in the body, and highly vulnerable to the increasing incidence of cardiovascular diseases in an aging world population. Cell-based approaches aimed at cardiac repair have recently caused great public excitement. But clinical trials of patients' own skeletal myoblasts or bone marrow cells for transplantation have been disappointing. Human embryonic stem cells (hESCs) form bona fide cardiomyocytes in vitro which are readily generated in mass culture and are being tested in animal models of heart damage. The early results, while encouraging, underscore that much remains to be done. This review focuses on the many challenges that remain before hESCs-mediated repair of the human heart becomes a reality.

Western array analysis of cell cycle protein changes during the hyperplastic to hypertrophic transition in heart development

Cardiac myocytes proliferate most rapidly during the hyperplastic phase of heart development; however, the level of cell cycle activity is drastically down regulated after birth. Further growth of the heart is achieved by hypertrophic growth of cardiac myocytes. The mechanism that controls the switch from hyperplastic proliferation to hypertrophic growth in cardiac myocytes is unknown. Understanding this fundamental mechanism of cardiac myocyte biology would be most beneficial for studies directed towards myocardial regeneration. In this study, we identified changes in the expression of proteins involved in cell cycle regulation during the hyperplastic to hypertrophic transition of cardiac myocytes. Using a high-throughput immunoblotting technique, we examined 200+ proteins in primary cultures of cardiac myocytes at different developmental time points to determine the important regulators of this transition. In addition, we also analyzed samples from an immortalized cardiac myocyte cell line to compare expression levels of cell cycle regulatory proteins to our primary cultures. Our findings by this uncovered proteomic screen identified several potential key regulatory proteins and provide insight into the important components of cardiac myocyte cell cycle regulation.

Cell-based approaches for cardiac repair

Many forms of cardiovascular disease are associated with cardiomyocyte loss via necrosis and/or apoptosis. The cumulative loss of contractile cells ultimately results in diminished cardiac function. Numerous approaches have been employed to reduce the rate of cardiomyocyte loss, or alternatively, to repopulate the heart with new cardiomyocytes. Strategies aimed at repopulating the heart include cardiomyocyte cell therapy, myogenic stem cell therapy, and cell cycle activation therapy. All three approaches are based on the assumption that the de novo cardiomyocytes will participate in a functional syncytium with the surviving myocardium. This review will discuss the current status of interventions aimed at repopulating the heart with functional cardiomyocytes.

Plasticity and cardiovascular applications of multipotent adult progenitor cells

Cardiovascular disease is the leading cause of death worldwide, which has encouraged the search for new therapies that enable the treatment of patients in palliative and curative ways. In the past decade, the potential benefit of transplantation of cells that are able to substitute for the injured tissue has been studied with several cell populations, such as stem cells. Some of these cell populations, such as myoblasts and bone marrow cells, are already being used in clinical trials. The laboratory of CM Verfaillie has studied primitive progenitors, termed multipotent adult progenitor cells, which can be isolated from adult bone marrow. These cells can differentiate in vitro at the single-cell level into functional cells that belong to the three germ layers and contribute to most, if not all, somatic cell types after blastocyst injection. This remarkably broad differentiation potential makes this particular cell population a candidate for transplantation in tissues in need of regeneration. Here, we focus on the regenerative capacity of multipotent adult progenitor cells in several ischemic mouse models, such as acute and chronic myocardial infarction and limb ischemia.

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.

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.

Overexpression of bone morphogenetic protein 10 in myocardium disrupts cardiac postnatal hypertrophic growth

Postnatal cardiac hypertrophies have traditionally been classified into physiological or pathological hypertrophies. Both of them are induced by hemodynamic load. Cardiac postnatal hypertrophic growth is regarded as a part of the cardiac maturation process that is independent of the cardiac working load. However, the functional significance of this biological event has not been determined, mainly because of the difficulty in creating an experimental condition for testing the growth potential of functioning heart in the absence of hemodynamic load. Recently, we generated a novel transgenic mouse model (alphaMHC-BMP10) in which the cardiac-specific growth factor bone morphogenetic protein 10 (BMP10) is overexpressed in postnatal myocardium. These alphaMHC-BMP10 mice appear to have normal cardiogenesis throughout embryogenesis, but develop to smaller hearts within 6 weeks after birth. alphaMHC-BMP10 hearts are about half the normal size with 100% penetrance. Detailed morphometric analysis of cardiomyocytes clearly indicated that the compromised cardiac growth in alphaMHC-BMP10 mice was solely because of defect in cardiomyocyte postnatal hypertrophic growth. Physiological analysis further demonstrated that the responses of these hearts to both physiological (e.g. exercise-induced hypertrophy) and pathological hypertrophic stimuli remain normal. In addition, the alphaMHC-BMP10 mice develop subaortic narrowing and concentric myocardial thickening without obstruction by four weeks of age. Systematic analysis of potential intracellular pathways further suggested a novel genetic pathway regulating this previously undefined cardiac postnatal hypertrophic growth event. This is the first demonstration that cardiac postnatal hypertrophic growth can be specifically modified genetically and dissected out from physiological and pathological hypertrophies.

Cardiac regeneration: repopulating the heart

Many forms of pediatric and adult heart disease result from a deficiency in cardiomyocyte number. Through repopulation of the heart with new cardiomyocytes (that is, induction of regenerative cardiac growth), cardiac disease potentially can be reversed, provided that the newly formed myocytes structurally and functionally integrate in the preexisting myocardium. A number of approaches have been utilized to effect regenerative growth of the myocardium in experimental animals. These include interventions aimed at enhancing the ability of cardiomyocytes to proliferate in response to cardiac injury, as well as transplantation of cardiomyocytes or myogenic stem cells into diseased hearts. Here we review efforts to induce myocardial regeneration. We also provide a critical review of techniques currently used to assess cardiac regeneration and functional integration of de novo cardiomyocytes.

Cellular plasticity in vertebrate regeneration

Within the animal kingdom, there are several examples of organisms with remarkable regenerative abilities. Among vertebrates, newts appear to be the most adept at replacing lost structures and injured organs and can regenerate their limbs, tails, spinal cords, jaws, retinas, lenses, optic nerves, intestines, and heart ventricles. This regenerative ability is dependent on the induction of an unusual degree of cellular plasticity near the site of injury. Mature cells lose their differentiated characteristics and revert to proliferating progenitor cells that will later redifferentiate to replace the lost or injured tissues. This degree of cellular plasticity appears to be restricted to those vertebrates with the most remarkable regenerative abilities and is not observed in mammals. However, in the last several years, there have been a few studies suggesting that certain factors present in newt tissues can induce a dedifferentiation response in mammalian cells. These results suggest that the knowledge gained from studying the molecular basis of cellular plasticity in newts and other regeneration-competent model organisms might one day be used to enhance the regenerative potential in mammals.

The cardiomyocyte cell cycle

Many forms of cardiac disease are characterized by cardiomyocyte death due to necrosis, apoptosis and/or oncosis. Recently, the notion of promoting cardiac regeneration as a means to replace damaged heart tissue has engendered considerable interest. One approach to accomplish heart muscle regeneration entails promoting cardiomyocyte cell cycle activity in the surviving myocardium. Genetically modified mice have provided useful model systems to test the efficacy of specific pathways to promote cardiomyocyte proliferation in normal and diseased hearts. For example, expression of a heart-restricted dominant interfering version of p193 (an E3 ubiquitin ligase also known as Cul7) resulted in an induction of cardiomyocyte cell cycle activity at the infarct border zone and ventricular septum 4 weeks after permanent coronary artery occlusion. A concomitant reduction in hypertrophic cardiomyocyte growth was also observed in this model, suggesting that cell cycle activation partially counteracted the adverse ventricular remodelling that occurs post-infarction. In other studies, targeted expression of cyclin D2 promoted cardiomyocyte cell cycle activity in adult hearts. The level of cardiomyocyte cell cycle activity increased after myocardial infarction, ultimately resulting in a marked increase in cardiomyocyte number and a concomitant regression of infarct size. Collectively, these data suggest that modulation of cardiomyocyte cell cycle activity can be exploited to promote regenerative growth in injured hearts.

Regenerating the heart

Cell-based cardiac repair offers the promise of rebuilding the injured heart from its component parts. Work began with committed cells such as skeletal myoblasts, but recently the field has expanded to explore an array of cell types, including bone marrow cells, endothelial progenitors, mesenchymal stem cells, resident cardiac stem cells, and both mouse and human embryonic stem cells. A related strategy for cardiac repair involves cell mobilization with factors such as cytokines. Translation of cell-based approaches to the clinic has progressed rapidly, and clinical trials using autologous skeletal myoblasts and bone marrow cells are under way. Many challenges remain before the vision of healing an infarct by muscle regeneration can be realized. Future research is likely to focus on improving our ability to guide the differentiation of stem cells, control their survival and proliferation, identify factors that mediate their homing and modulate the heart's innate inflammatory and fibrotic responses.