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

Transient expression of FRNK reveals stage-specific requirement for focal adhesion kinase activity in cardiac growth

Focal adhesion kinase (FAK) is strongly activated by integrins and growth factors and is essential for embryonic development. We previously showed that the C terminus of FAK is expressed as a separate protein termed FAK-related nonkinase (FRNK) in a smooth muscle cell-selective fashion and that FRNK functions to buffer FAK-dependent signals. We now show that FRNK is also transiently expressed in the neonatal myocardium, with peak levels occurring 5 to 7 days postnatal, just before cell cycle withdrawal. Using novel mouse models, we demonstrate that cardiac-selective expression of FRNK (leading to inhibition of FAK) starting at embryonic day 10.5 leads to a severe ventricular noncompaction defect associated with reduced cardiomyocyte proliferation. Remarkably, postnatal expression of nearly identical levels of FRNK is well tolerated and does not affect viability or anabolic cardiac growth. Nonetheless, FRNK expression in the adult heart does attenuate pathological cardiac hypertrophy following aortic banding, confirming and extending our previous data that this compensatory response is blunted in FAK null hearts. Our mechanistic studies in cultured neonatal cardiomyocytes reveal that FRNK expression induces p38/p27(kip)-dependent cell cycle withdrawal and attenuates extracellular signal-regulated kinase-dependent hypertrophic growth. These findings indicate that dynamic expression of FRNK in the neonatal heart may function to promote cardiomyocyte quiescence in an environment that is particularly rich in growth factors and growth promoting extracellular matrices.

Effect of traditional Chinese medicine Shu-mai-tang on angiogenesis, arteriogenesis and cardiac function in rats with myocardial ischemia

Ischemic heart disease is one of the leading causes of morbidity and mortality worldwide. Shu-mai-tang (SMT) is a traditional Chinese medicine for the treatment of ischemic heart disease. To better understand the underlying cardioprotection mechanisms of SMT on myocardial ischemia (MI), the effect of SMT on angiogenesis, arteriogenesis and cardiac function was investigated in a rat model of MI, as well as the potential mechanism. Rats with a ligated left anterior descending coronary artery (MI model) were randomized (24 rats/group) to receive SMT/LY294002 [phosphatidylinositol 3-kinase (PI3K) inhibitor], SMT or no treatment. A sham-operation group was included. It was demonstrated that 2 and 4 weeks after treatment the oral administration of SMT significantly increased capillaries and arterioles, suppressed myocardial fibrosis, as well as significantly increased cardiac phosphorylation of Akt, vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF-BB) levels and functional improvement. PI3K inhibitor down-regulated SMT-induced angiogenesis and arteriogenesis. These novel therapeutic properties of SMT to induce the reconstitution of stable blood supply networks, reverse LV remodeling may offer an alternative therapy for the treatment of ischemic heart disease. The potential mechanism may be that SMT promotes VEGF and PDGF-BB-mediated angiogenesis and arteriogenesis through the PI3K/Akt signaling pathway.

Stem cells and plasticity of skeletal muscle cell differentiation: potential application to cell therapy for degenerative muscular diseases

Degenerative muscular diseases, such as muscular dystrophies, have been the representative targets of regenerative cell therapy. Although satellite cells play central roles in skeletal muscle regeneration that intrinsically occurs after muscle injury, their application to cell therapy is confronted by difficulties. Other stem cells expected to be applicable to cell therapy include muscle-resident stem cells and nonmuscle-resident stem cells. Moreover, dedifferentiated cells of skeletal muscle might provide unique system for cell therapy. Terminally differentiated myotubes have plasticity of differentiation and dedifferentiate under certain experimental conditions, including the expression of SV40 large T antigen or the homeobox gene Msx1. The dedifferentiated cells exhibit multipotency to transdifferentiate into multiple mesenchymal origin cells. In addition, fibroblasts or undifferentiated myoblasts treated with a drug acquire multipotency. These cells may open new doors in cell therapy.

Modeling cardiovascular disease in the zebrafish

The zebrafish possesses a host of advantages that have established it as an excellent model of vertebrate development. These include ease of genetic manipulation and transgenesis, optical clarity, and small size and cost. Biomedical researchers are increasingly exploiting these advantages to model human disease mechanisms. Here we review the use of the zebrafish for cardiovascular research. We summarize previous studies with the use of this organism to model such processes as thrombosis, collateral vessel development, inflammation, cardiomyopathy, and cardiac regeneration, evaluate its promise for novel drug discovery, and consider where the zebrafish fits into the framework of existing cardiovascular models.

MAPK signalling in cardiovascular health and disease: molecular mechanisms and therapeutic targets

Intracellular MAPK (mitogen-activated protein kinase) signalling cascades probably play an important role in the pathogenesis of cardiac and vascular disease. A substantial amount of basic science research has defined many of the details of MAPK pathway organization and activation, but the role of individual signalling proteins in the pathogenesis of various cardiovascular diseases is still being elucidated. In the present review, the role of the MAPKs ERK (extracellular signal-regulated kinase), JNK (c-Jun N-terminal kinase) and p38 MAPK in cardiac hypertrophy, cardiac remodelling after myocardial infarction, atherosclerosis and vascular restenosis will be examined, with attention paid to genetically modified murine model systems and to the use of pharmacological inhibitors of protein kinases. Despite the complexities of this field of research, attractive targets for pharmacological therapy are emerging.

Stemming heart failure with cardiac- or reprogrammed-stem cells

Despite extensive efforts to control myocyte growth by genetic targeting of the cell cycle machinery and small molecules for cardiac repair, adult myocytes themselves appeared to divide a limited number of times in response to a variety of cardiac muscle stresses. Rare tissue-resident stem cells are thought to exist in many adult organs that are capable of self-renewal and differentiation and possess a range of actions that are potentially therapeutic. Recent studies suggest that a population of cardiac stem cells (CSCs) is maintained after cardiac development in the adult heart in mammals including human beings; however, homeostatic cardiomyocyte replacement might be stem cell-dependent, and functional myocardial regeneration after cardiac muscle damage is not yet considered as sufficient to fully maintain or reconstitute the cardiovascular system and function. Although it is clear that adult CSCs have limitations in their capabilities to proliferate extensively and differentiate in response to injury in vivo for replenishing mature car-diomyocytes and potentially function as resident stem cells. Transplantation of CSCs expanded ex vivo seems to require an integrated strategy of cell growth-enhancing factor(s) and tissue engineering technologies to support the donor cell survival and subsequent proliferation and differentiation in the host microenvironment. There has been substantial interest regarding the evidence that mammalian fibroblasts can be genetically reprogrammed to induced pluripotent stem (iPS) cells, which closely resemble embryonic stem (ES) cell properties capable of differentiating into functional cardiomyocytes, and these cells may provide an alternative cell source for generating patient-specific CSCs for therapeutic applications.

Compartmentalised MAPK pathways

The mitogen-activated protein kinase (MAPK) pathway provides cells with the means to interpret external signal cues or conditions, and respond accordingly. This cascade regulates many cell functions such as differentiation, proliferation and migration. Through modulation of both the amplitude and duration of MAPK signalling, cells can control their responses to the multiple activators of the pathway. In addition, recent work has highlighted the importance of the cellular compartment from which the signalling occurs. Cells have developed intricate systems that enable them to localise MAPK components to specific subcellular domains in response to a particular stimulus. Consequently, different factors can activate the same kinase in separate locations. Crucial to this ability are molecular scaffolds, which act as signalling modules for MAPKs, confining them to the desired compartment. The participation of the MAPK network in fundamental physiological processes, such as cell proliferation and inflammation, and the derangement of the homeostasis that occurs in disease processes, renders MAPK a highly desirable target for therapeutic intervention. As we enhance our comprehension of scaffolds and other regulatory molecules, novel targets for drug design may be discovered that will afford selective and specific MAPK modulation.

Features of cardiomyocyte proliferation and its potential for cardiac regeneration

The human heart does not regenerate. Instead, following injury, human hearts scar. The loss of contractile tissue contributes significantly to morbidity and mortality. In contrast to humans, zebrafish and newts faithfully regenerate their hearts. Interestingly, regeneration is in both cases based on cardiomyocyte proliferation. In addition, mammalian cardiomyocytes proliferate during foetal development. Their proliferation reaches its maximum around chamber formation, stops shortly after birth, and subsequent heart growth is mostly achieved by an increase in cardiomyocyte size (hypertrophy). The underlying mechanisms that regulate cell cycle arrest and the switch from proliferation to hypertrophy are unclear. In this review, we highlight features of dividing cardiomyocytes, summarize the attempts to induce mammalian cardiomyocyte proliferation, critically discuss methods commonly used for its detection, and explore the potential and problems of inducing cardiomyocyte proliferation to improve function in diseased hearts.

Cardiomyocyte proliferation and protection against post-myocardial infarction heart failure by cyclin D1 and Skp2 ubiquitin ligase

AIMS

Cyclins and other cell-cycle regulators have been used in several studies to regenerate cardiomyocytes in ischaemic heart failure. However, proliferation of cardiomyocytes induced by nuclear-targeted cyclin D1 (D1NLS) stops after one or two rounds of cell cycles due in part to accumulation of p27Kip1, an inhibitor of cyclin-dependent kinase (CDK). Thus, expression of S-phase kinase-associated protein 2 (Skp2), a negative regulator of p27Kip1, significantly enhances the effect of D1NLS and CDK4 on cardiomyocyte proliferation in vitro. Here, we examined whether Skp2 can also improve cardiomyocyte regeneration and post-ischaemic cardiac performance in vivo.

METHODS AND RESULTS

Wistar rats underwent ischaemia/reperfusion injury by ligation of the coronary artery followed by injection of adenovirus vectors for D1NLS and CDK4 with or without Skp2. Enhanced proliferation of cardiomyocytes in the presence of Skp2 was demonstrated by increased expression of Ki67, a marker of proliferating cells (1.95% vs. 4.00%), and mitotic phosphorylated histone H3 (0.24% vs. 0.58%). Compared with rats that received only D1NLS and CDK4, expression of Skp2 improved left ventricular function as measured by the maximum and minimum rates of change in left ventricular pressure, the left ventricle end-diastolic pressure, left ventricle end-diastolic volume index, and the lung/body weight ratio.

CONCLUSION

Expression of Skp2 enhanced the effect of D1NLS and CDK4 on the proliferation of cardiomyocytes and further contributed to improved post-ischaemic cardiac function. Skp2 might be a versatile tool to improve the effect of cyclins on post-ischaemic regeneration of cardiomyocytes in vivo.

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.

Long-term but not short-term p38 mitogen-activated protein kinase inhibition improves cardiac function and reduces cardiac remodeling post-myocardial infarction

p38 mitogen-activated protein kinase (p38 MAPK) inhibition exerts beneficial effects on left ventricular (LV) remodeling and dysfunction. p38 MAPK activity is transiently increased soon after myocardial infarction (MI), suggesting brief inhibition may afford the same benefit as long-term inhibition. We examined chronic 12-week p38 MAPK inhibition compared with short-term (7-day) inhibition, and then we discontinued inhibition after MI. Post-MI rats at day 7 received either vehicle, 4-[4-(4-fluorophenyl)-1-(3-phenylpropyl)-5-(4-pyridinyl)-1H-imidazol-2-yl]-3-butyn-1-ol (RWJ67657; RWJ) for 12 weeks (long term; LT-RWJ), RWJ for 1 week and discontinued for 11 weeks (1-week RWJ), or continuous ramipril for 12 weeks. In separate groups of animals, 24 h after MI, vehicle or RWJ was administered for 7 days. Cardiac function was assessed by echocardiography and hemodynamic measurements. Percentage of fractional shortening improved after LT-RWJ and ramipril, but not after 1-week RWJ treatment. Likewise, LV contractility and maximal first derivative of left ventricular pressure (dP/dt(max)) was improved (12.5 and 14.4%) and LV end diastolic pressure (LVEDP) was reduced (49.4 and 54.6%) with both treatments. Functional outcomes were accompanied by regression of interstitial collagen I and alpha-smooth muscle actin expression in LV noninfarct, border, and infarct regions with LT-RWJ and ramipril treatment. Hypertrophy was reduced in noninfarct (18.3 and 12.2%) and border regions (16.3 and 12.0%) with both treatments, respectively. Animals receiving RWJ 24 h after MI for 7 days showed similar improvements in fractional shortening, dP/dt(max), LVEDP, including reduced fibrosis and hypertrophy. In vitro experiments confirmed a dose-dependent reduction in hypertrophy, with RWJ following tumor necrosis factor-alpha stimulation. Continuous but not short-term p38 MAPK blockade attenuates post-MI remodeling, which is associated with functional benefits on the myocardium.

Modular self-assembling biomaterials for directing cellular responses

Self-assembling biomaterials are promising as cell-interactive matrices because they can be constructed in a modular fashion, which enables the independent and simultaneous tuning of several of their physicochemical and biological properties. Such modularity facilitates the optimization of multi-component matrices for use in complex biological environments such as 3-D cell culture or scaffolds for regenerative medicine. This Highlight will discuss recent strategies for producing modular self-assembling biomaterials, with a particular focus on how ligand presentation and matrix mechanics can be controlled in modular ways. In addition, it will discuss key hurdles that remain for employing these materials as cell-interactive scaffolds in biomedical applications, particularly those that relate to how they may interface with the immune system.

Stem cell factor/c-kit signaling mediated cardiac stem cell migration via activation of p38 MAPK

OBJECTIVE

It was reported that there are cardiac stem cells (CSCs) in the rat heart, and they could reconstitute well-differentiated myocardium that are formed by blood-carrying new vessels and myocytes. However, how do the CSCs migrate into the peri-infarcted areas after myocardial infarction (MI)? It remains entirely unknown about the signal transduction involved in the migration of CSCs.

METHODS AND RESULTS

Rat heart MI was induced by left coronary artery ligation. Both immunohistochemical staining and Western blotting analysis was performed to detect the expression of SCF protein, and RT-PCR was conducted for the expression of SCF mRNA. Cardiac stem cells were isolated from rat hearts, and a cardiac stem cell migration assay was performed using a 48-well chemotaxis chamber system. On day 5 after MI in rats, the expression of stem cell factor (SCF) mRNA and protein was significantly increased in the peri-infarcted area, which was matched with more accumulation of CSCs in the region and improvement of cardiac function, which was blocked by p38 MAPK selective inhibitor SB203580. In in vitro experiments, SCF induced CSC migration in a concentration-dependent manner, and the antibody against SCF receptor (c-kit) blocked the SCF-induced CSC migration. Western blot analysis showed that the phosphorylated p38 MAPK (Phospho-p38 MAPK) was highly increased in the SCF-treated CSCs, and the inhibition of p38 MAPK activity significantly attenuated SCF-induced the migration of CSCs.

CONCLUSION

It demonstrated that SCF/c-kit signaling may mediate the migration of CSCs via activation of p38 MAPK.

Zebrafish Heart Regeneration as a Model for Cardiac Tissue Repair

Heart disease remains the leading cause of mortality throughout the world. Mammals have an extremely limited capacity to repair lost or damaged heart tissue, thus encouraging biologists to seek out models for heart regeneration. Zebrafish exhibit a robust regenerative capacity in a variety of tissues including the fin, spinal cord, retina, and heart, making it the sole regenerative vertebrate organism currently amenable to genetic manipulation. Future studies will utilize functional approaches to tease apart zebrafish heart regeneration in hopes of unlocking our own regenerative potential.

MKK6-p38 MAPK signaling pathway enhances survival but not bone-resorbing activity of osteoclasts

Phosphorylated p38 mitogen-activating kinase (MAPK) is observed in osteoclasts under in vivo inflammatory situations. However, the role of p38 MAPK in osteoclast function has not been elucidated, because all external stimuli tested hitherto failed to induce the phosphorylation of p38 MAPK in osteoclasts in culture. In this study, a constitutively active form of MKK6 (MKK6CA) was expressed in osteoclasts using adenoviral gene transfer in vitro. MKK6CA expressed in osteoclasts phosphorylated p38 MAPK and enhanced the survival of osteoclasts. Dentine-resorbing activity of osteoclasts was not enhanced by the MKK6CA expression. These results suggest that p38 MAPK signaling plays a critical role in the survival of osteoclasts in inflammatory diseases.

Potential of p38-MAPK inhibitors in the treatment of ischaemic heart disease

Chronic heart failure is debilitating, often fatal, expensive to treat and common. In most patients it is a late consequence of myocardial infarction (MI). The intracellular signals following infarction that lead to diminished contractility, apoptosis, fibrosis and ultimately heart failure are not fully understood but probably involve p38-mitogen activated protein kinases (p38), a family of serine/threonine kinases which, when activated, cause cardiomyocyte contractile dysfunction and death. Pharmacological inhibitors of p38 suppress inflammation and are undergoing clinical trials in rheumatoid arthritis, Chrohn's disease, psoriasis and surgery-induced tissue injury. In this review, we discuss the mechanisms, circumstances and consequences of p38 activation in the heart. The purpose is to evaluate p38 inhibition as a potential therapy for ischaemic heart disease.

Mitogen-activated protein kinases in heart development and diseases

Mitogen-activated protein (MAP) kinases belong to a highly conserved family of Ser-Thr protein kinases in the human kinome and have diverse roles in broad physiological functions. The 4 best-characterized MAP kinase pathways, ERK1/2, JNK, p38, and ERK5, have been implicated in different aspects of cardiac regulation, from development to pathological remodeling. Recent advancements in the development of kinase-specific inhibitors and genetically engineered animal models have revealed significant new insights about MAP kinase pathways in the heart. However, this explosive body of new information also has yielded many controversies about the functional role of specific MAP kinases as either detrimental promoters or critical protectors of the heart during cardiac pathological processes. These uncertainties have raised questions on whether/how MAP kinases can be targeted to develop effective therapies against heart diseases. In this review, recent studies examining the role of MAP kinase subfamilies in cardiac development, hypertrophy, and survival are summarized.

p38alpha MAPK can positively or negatively regulate Rac-1 activity depending on the presence of serum

The small GTP-ase Rac-1 can trigger p38 MAPK activation and, in turn, p38alpha can regulate signalling pathways that potentially impinge on Rac-1 activity. We have investigated the cross-talk between p38alpha and Rac-1 and found that p38alpha regulates the association between Rac-1 and caveolin-1 in serum-deprived cardiomyocytes. This interaction depends on cell attachment and correlates with higher levels of active Rac-1. Actin organization might regulate the formation of Rac-1-caveolin-1 complexes. In contrast, the Rac-1-caveolin-1 interaction is almost undetectable in the presence of serum, where Rac-1 activity is negatively regulated by p38alpha. Our results indicate that p38alpha can differentially contribute to Rac-1 activation depending on the presence of serum.

Periostin induces proliferation of differentiated cardiomyocytes and promotes cardiac repair

Adult mammalian hearts respond to injury with scar formation and not with cardiomyocyte proliferation, the cellular basis of regeneration. Although cardiogenic progenitor cells may maintain myocardial turnover, they do not give rise to a robust regenerative response. Here we show that extracellular periostin induced reentry of differentiated mammalian cardiomyocytes into the cell cycle. Periostin stimulated mononucleated cardiomyocytes to go through the full mitotic cell cycle. Periostin activated alphaV, beta1, beta3 and beta5 integrins located in the cardiomyocyte cell membrane. Activation of phosphatidylinositol-3-OH kinase was required for periostin-induced reentry of cardiomyocytes into the cell cycle and was sufficient for cell-cycle reentry in the absence of periostin. After myocardial infarction, periostin-induced cardiomyocyte cell-cycle reentry and mitosis were associated with improved ventricular remodeling and myocardial function, reduced fibrosis and infarct size, and increased angiogenesis. Thus, periostin and the pathway that it regulates may provide a target for innovative strategies to treat heart failure.

Advances in signaling in vertebrate regeneration as a prelude to regenerative medicine

While all animals have evolved strategies to respond to injury and disease, their ability to functionally recover from loss of or damage to organs or appendages varies widely damage to skeletal muscle, but, unlike amphibians and fish, they fail to regenerate heart, lens, retina, or appendages. The relatively young field of regenerative medicine strives to develop therapies aimed at improving regenerative processes in humans and is predicated on >40 years of success with bone marrow transplants. Further progress will be accelerated by implementing knowledge about the molecular mechanisms that regulate regenerative processes in model organisms that naturally possess the ability to regenerate organs and/or appendages. In this review we summarize the current knowledge about the signaling pathways that regulate regeneration of amphibian and fish appendages, fish heart, and mammalian liver and skeletal muscle. While the cellular mechanisms and the cell types involved in regeneration of these systems vary widely, it is evident that shared signals are involved in tissue regeneration. Signals provided by the immune system appear to act as triggers of many regenerative processes. Subsequently, pathways that are best known for their importance in regulating embryonic development, in particular fibroblast growth factor (FGF) and Wnt/beta-catenin signaling (as well as others), are required for progenitor cell formation or activation and for cell proliferation and specification leading to tissue regrowth. Experimental activation of these pathways or interference with signals that inhibit regenerative processes can augment or even trigger regeneration in certain contexts.

Cardiac myocyte cell cycle control in development, disease, and regeneration

Cardiac myocytes rapidly proliferate during fetal life but exit the cell cycle soon after birth in mammals. Although the extent to which adult cardiac myocytes are capable of cell cycle reentry is controversial and species-specific differences may exist, it appears that for the vast majority of adult cardiac myocytes the predominant form of growth postnatally is an increase in cell size (hypertrophy) not number. Unfortunately, this limits the ability of the heart to restore function after any significant injury. Interest in novel regenerative therapies has led to the accumulation of much information on the mechanisms that regulate the rapid proliferation of cardiac myocytes in utero, their cell cycle exit in the perinatal period, and the permanent arrest (terminal differentiation) in adult myocytes. The recent identification of cardiac progenitor cells capable of giving rise to cardiac myocyte-like cells has challenged the dogma that the heart is a terminally differentiated organ and opened new prospects for cardiac regeneration. In this review, we summarize the current understanding of cardiomyocyte cell cycle control in normal development and disease. In addition, we also discuss the potential usefulness of cardiomyocyte self-renewal as well as feasibility of therapeutic manipulation of the cardiac myocyte cell cycle for cardiac regeneration.

Getting to the heart of regeneration in zebrafish

A scientific and clinical prerogative of the 21st century is to stimulate the regenerative ability of the human heart. While the mammalian heart shows little or no natural regeneration in response to injury, certain non-mammalian vertebrates possess an elevated capacity for cardiac regeneration. Adult zebrafish restore ventricular muscle removed by surgical resection, events that involve little or no scarring. Recent studies have begun to reveal cellular and molecular mechanisms of this regenerative process that have exciting implications for human cardiac biology and disease.

Can the cardiomyocyte cell cycle be reprogrammed?

Cardiac repair following myocardial injury is restricted due to the limited proliferative potential of adult cardiomyocytes. The ability of mammalian cardiomyocytes to proliferate is lost shortly after birth as cardiomyocytes withdraw from the cell cycle and differentiate. We do not fully understand the molecular and cellular mechanisms that regulate this cell cycle withdrawal, although if we could it might lead to the discovery of novel therapeutic targets for improving cardiac repair following myocardial injury. For the last decade, researchers have investigated cardiomyocyte cell cycle control, commonly using transgenic mouse models or recombinant adenoviruses to manipulate cell cycle regulators in vivo or in vitro. This review discusses cardiomyocyte cell cycle regulation and summarises recent data from studies manipulating the expressions and activities of cell cycle regulators in cardiomyocytes. The validity of therapeutic strategies that aim to reinstate the proliferative potential of cardiomyocytes to improve myocardial repair following injury will be discussed.