Sequential myofibrillar breakdown accompanies mitotic division of mammalian cardiomyocytes

Development, Proliferation, and Growth of the Mammalian Heart

During development, the embryonic heart grows by addition of cells from a highly proliferative progenitor pool and by subsequent precisely controlled waves of cardiomyocyte proliferation. In this period, the heart can compensate for cardiomyocyte loss by an increased proliferation rate of the remaining cardiomyocytes. This proliferative capacity is lost soon after birth, with heart growth continuing by an increase in cardiomyocyte volume. The failure of the injured adult heart to regenerate often leads to the development of heart failure, a major cause of death. With the recent observation of a small fraction of cardiomyocytes that appear to have retained the proliferative capacity within the adult heart, as well as the identification of developmental pathways such as the Hippo-signaling pathway that can invoke mature cardiomyocyte proliferation, more studies are taking a knowledge-based mechanistic approach to heart regeneration. A key question being asked is if this knowledge can be used therapeutically to reinitiate cardiomyocyte proliferation after injury such as myocardial infarction. In this respect, uncovering and understanding the mechanisms and conditions that give rise to a fully functional and adaptive heart in the developing embryo could provide us with the answers to many of the questions that are now being asked.

Cardiac regeneration: All work and no repair?

Structural changes in the developing heart may influence the limited regenerative capacity of the adult heart. We examine how the workload exerted on the adult mammalian heart may limit regenerative capability and discuss recent therapies that demonstrate beneficial effects through unloading the heart.

GSK-3β Inhibitor CHIR-99021 Promotes Proliferation Through Upregulating β-Catenin in Neonatal Atrial Human Cardiomyocytes

BACKGROUND

The renewal capacity of neonate human cardiomyocytes provides an opportunity to manipulate endogenous cardiogenic mechanisms for supplementing the loss of cardiomyocytes caused by myocardial infarction or other cardiac diseases. GSK-3β inhibitors have been recently shown to promote cardiomyocyte proliferation in rats and mice, thus may be ideal candidates for inducing human cardiomyocyte proliferation.

METHODS

Human cardiomyocytes were isolated from right atrial specimens obtained during routine surgery for ventricle septal defect and cultured with either GSK-3β inhibitor (CHIR-99021) or β-catenin inhibitor (IWR-1). Immunocytochemistry was performed to visualize 5-ethynyl-2'-deoxyuridine (EdU)-positive or Ki67-positive cardiomyocytes, indicative of proliferative cardiomyocytes.

RESULTS

GSK-3β inhibitor significantly increased β-catenin accumulation in cell nucleus, whereas β-catenin inhibitor significantly reduced β-catenin accumulation in cell plasma. In parallel, GSK-3β inhibitor increased EdU-positive and Ki67-positive cardiomyocytes, whereas β-catenin inhibitor decreased EdU-positive and Ki67-positive cardiomyocytes.

CONCLUSIONS

These results indicate that GSK-3β inhibitor can promote human atrial cardiomyocyte proliferation. Although it remains to be determined whether the observations in atrial myocytes could be directly applicable to ventricular myocytes, the current findings imply that Wnt/β-catenin pathway may be a valuable pathway for manipulating endogenous human heart regeneration.

Myocardial reparative functions of exosomes from mesenchymal stem cells are enhanced by hypoxia treatment of the cells via transferring microRNA-210 in an nSMase2-dependent way

Hypoxia treatment enhances paracrine effect of mesenchymal stem cells (MSCs). The aim of this study was to investigate whether exosomes from hypoxia-treated MSCs (Exo^H) are superior to those from normoxia-treated MSCs (Exo^N) for myocardial repair. Mouse bone marrow-derived MSCs were cultured under hypoxia or normoxia for 24 h, and exosomes from conditioned media were intramyocardially injected into infarcted heart of C57BL/6 mouse. Exo^H resulted in significantly higher survival, smaller scar size and better cardiac functions recovery. Exo^H conferred increased vascular density, lower cardiomyocytes (CMs) apoptosis, reduced fibrosis and increased recruitment of cardiac progenitor cells in the infarcted heart relative to Exo^N. MicroRNA analysis revealed significantly higher levels of microRNA-210 (miR-210) in Exo^H compared with Exo^N. Transfection of a miR-210 mimic into endothelial cells (ECs) and CMs conferred similar biological effects as Exo^H. Hypoxia treatment of MSCs increased the expression of neutral sphingomyelinase 2 (nSMase2) which is crucial for exosome secretion. Blocking the activity of nSMase2 resulted in reduced miR-210 secretion and abrogated the beneficial effects of Exo^H. In conclusion, hypoxic culture augments miR-210 and nSMase2 activities in MSCs and their secreted exosomes, and this is responsible at least in part for the enhanced cardioprotective actions of exosomes derived from hypoxia-treated cells.

The Light and Shadow of Senescence and Inflammation in Cardiovascular Pathology and Regenerative Medicine

Recent epidemiologic studies evidence a dramatic increase of cardiovascular diseases, especially associated with the aging of the world population. During aging, the progressive impairment of the cardiovascular functions results from the compromised tissue abilities to protect the heart against stress. At the molecular level, in fact, a gradual weakening of the cellular processes regulating cardiovascular homeostasis occurs in aging cells. Atherosclerosis and heart failure are particularly correlated with aging-related cardiovascular senescence, that is, the inability of cells to progress in the mitotic program until completion of cytokinesis. In this review, we explore the intrinsic and extrinsic causes of cellular senescence and their role in the onset of these cardiovascular pathologies. Additionally, we dissect the effects of aging on the cardiac endogenous and exogenous reservoirs of stem cells. Finally, we offer an overview on the strategies of regenerative medicine that have been advanced in the quest for heart rejuvenation.

Zebrafish heart regeneration: 15 years of discoveries

Cardiovascular disease is the leading cause of death worldwide. Compared to other organs such as the liver, the adult human heart lacks the capacity to regenerate on a macroscopic scale after injury. As a result, myocardial infarctions are responsible for approximately half of all cardiovascular related deaths. In contrast, the zebrafish heart regenerates efficiently upon injury through robust myocardial proliferation. Therefore, deciphering the mechanisms that underlie the zebrafish heart's endogenous regenerative capacity represents an exciting avenue to identify novel therapeutic strategies for inducing regeneration of the human heart. This review provides a historical overview of adult zebrafish heart regeneration. We summarize 15 years of research, with a special focus on recent developments from this fascinating field. We discuss experimental findings that address fundamental questions of regeneration research. What is the origin of regenerated muscle? How is regeneration controlled from a genetic and molecular perspective? How do different cell types interact to achieve organ regeneration? Understanding natural models of heart regeneration will bring us closer to answering the ultimate question: how can we stimulate myocardial regeneration in humans?

Multimodal Regulation of Cardiac Myocyte Proliferation

Efficient cardiac regeneration is closely associated with the ability of cardiac myocytes to proliferate. Fetal or neonatal mouse hearts containing proliferating cardiac myocytes regenerate even extensive injuries, whereas adult hearts containing mostly post-mitotic cardiac myocytes have lost this ability. The same correlation is seen in some homoiotherm species such as teleost fish and urodelian amphibians leading to the hypothesis that cardiac myocyte proliferation is a major driver of heart regeneration. Although cardiomyocyte proliferation might not be the only prerequisite to restore full organ function after cardiac damage, induction of cardiac myocyte proliferation is an attractive therapeutic option to cure the injured heart and prevent heart failure. To (re)initiate cardiac myocyte proliferation in adult mammalian hearts, a thorough understanding of the molecular circuitry governing cardiac myocyte cell cycle regulation is required. Here, we review the current knowledge in the field focusing on the withdrawal of cardiac myocytes from the cell cycle during the transition from neonatal to adult stages.

Heart regeneration and the cardiomyocyte cell cycle

Cardiovascular disease and in particular, heart failure are still main causes of death; therefore, novel therapeutic approaches are urgently needed. Loss of contractile substrate in the heart and limited regenerative capacity of cardiomyocytes are mainly responsible for the poor cardiovascular outcome. This is related to the postmitotic state of differentiated cardiomyocytes, which is partly due to their polyploid nature caused by cell cycle variants. As such, the cardiomyocyte cell cycle is a key player, and its manipulation could be a promising strategy for enhancing the plasticity of the heart by inducing cardiomyocyte proliferation. This review focuses on the cardiac cell cycle and its variants during postnatal growth, the different regenerative responses of the heart in dependance of the developmental stage and on manipulations of the cell cycle. Because a therapeutic goal is to induce authentic cell division in cardiomyocytes, recent experimental approaches following this strategy are also discussed.

Deletion of Gas2l3 in mice leads to specific defects in cardiomyocyte cytokinesis during development

GAS2L3 is a recently identified cytoskeleton-associated protein that interacts with actin filaments and tubulin. The in vivo function of GAS2L3 in mammals remains unknown. Here, we show that mice deficient in GAS2L3 die shortly after birth because of heart failure. Mammalian cardiomyocytes lose the ability to proliferate shortly after birth, and further increase in cardiac mass is achieved by hypertrophy. The proliferation arrest of cardiomyocytes is accompanied by binucleation through incomplete cytokinesis. We observed that GAS2L3 deficiency leads to inhibition of cardiomyocyte proliferation and to cardiomyocyte hypertrophy during embryonic development. Cardiomyocyte-specific deletion of GAS2L3 confirmed that the phenotype results from the loss of GAS2L3 in cardiomyocytes. Cardiomyocytes from Gas2l3-deficient mice exhibit increased expression of a p53-transcriptional program including the cell cycle inhibitor p21. Furthermore, loss of GAS2L3 results in premature binucleation of cardiomyocytes accompanied by unresolved midbody structures. Together these results suggest that GAS2L3 plays a specific role in cardiomyocyte cytokinesis and proliferation during heart development.

Cardiac Regeneration: Lessons From Development

Palliative surgery for congenital heart disease has allowed patients with previously lethal heart malformations to survive and, in most cases, to thrive. However, these procedures often place pressure and volume loads on the heart, and over time, these chronic loads can cause heart failure. Current therapeutic options for initial surgery and chronic heart failure that results from failed palliation are limited, in part, by the mammalian heart's low inherent capacity to form new cardiomyocytes. Surmounting the heart regeneration barrier would transform the treatment of congenital, as well as acquired, heart disease and likewise would enable development of personalized, in vitro cardiac disease models. Although these remain distant goals, studies of heart development are illuminating the path forward and suggest unique opportunities for heart regeneration, particularly in fetal and neonatal periods. Here, we review major lessons from heart development that inform current and future studies directed at enhancing cardiac regeneration.

Depletion of Tip60 from In Vivo Cardiomyocytes Increases Myocyte Density, Followed by Cardiac Dysfunction, Myocyte Fallout and Lethality

Tat-interactive protein 60 (Tip60), encoded by the Kat5 gene, is a member of the MYST family of acetyltransferases. Cancer biology studies have shown that Tip60 induces the DNA damage response, apoptosis, and cell-cycle inhibition. Although Tip60 is expressed in the myocardium, its role in cardiomyocytes (CMs) is unclear. Earlier studies here showed that application of cardiac stress to globally targeted Kat5^+/—haploinsufficient mice resulted in inhibition of apoptosis and activation of the CM cell-cycle, despite only modest reduction of Tip60 protein levels. It was therefore of interest to ascertain the effects of specifically and substantially depleting Tip60 from CMs using Kat5^{LoxP/-;Myh6-Cre} mice in the absence of stress. We report initial findings using this model, in which the effects of specifically depleting Tip60 protein from ventricular CMs, beginning at early neonatal stages, were assessed in 2–12 week-old mice. Although 5’-bromodeoxyuridine immunostaining indicated that CM proliferation was not altered at any of these stages, CM density was increased in 2 week-old ventricles, which persisted in 4 week-old hearts when TUNEL staining revealed inhibition of apoptosis. By week 4, levels of connexin-43 were depleted, and its patterning was dysmorphic, concomitant with an increase in cardiac hypertrophy marker expression and interstitial fibrosis. This was followed by systolic dysfunction at 8 weeks, after which extensive apoptosis and CM fallout occurred, followed by lethality as mice approached 12 weeks of age. In summary, chronic depletion of Tip60 from the ventricular myocardium beginning at early stages of neonatal heart development causes CM death after 8 weeks; hence, Tip60 protein has a crucial function in the heart.

Isolation, Culture and Transduction of Adult Mouse Cardiomyocytes

Cultured cardiomyocytes can be used to study cardiomyocyte biology using techniques that are complementary to in vivo systems. For example, the purity and accessibility of in vitro culture enables fine control over biochemical analyses, live imaging, and electrophysiology. Long-term culture of cardiomyocytes offers access to additional experimental approaches that cannot be completed in short term cultures. For example, the in vitro investigation of dedifferentiation, cell cycle re-entry, and cell division has thus far largely been restricted to rat cardiomyocytes, which appear to be more robust in long-term culture. However, the availability of a rich toolset of transgenic mouse lines and well-developed disease models make mouse systems attractive for cardiac research. Although several reports exist of adult mouse cardiomyocyte isolation, few studies demonstrate their long-term culture. Presented here, is a step-by-step method for the isolation and long-term culture of adult mouse cardiomyocytes. First, retrograde Langendorff perfusion is used to efficiently digest the heart with proteases, followed by gravity sedimentation purification. After a period of dedifferentiation following isolation, the cells gradually attach to the culture and can be cultured for weeks. Adenovirus cell lysate is used to efficiently transduce the isolated cardiomyocytes. These methods provide a simple, yet powerful model system to study cardiac biology.

Ciona as a Simple Chordate Model for Heart Development and Regeneration

Cardiac cell specification and the genetic determinants that govern this process are highly conserved among Chordates. Recent studies have established the importance of evolutionarily-conserved mechanisms in the study of congenital heart defects and disease, as well as cardiac regeneration. As a basal Chordate, the Ciona model system presents a simple scaffold that recapitulates the basic blueprint of cardiac development in Chordates. Here we will focus on the development and cellular structure of the heart of the ascidian Ciona as compared to other Chordates, principally vertebrates. Comparison of the Ciona model system to heart development in other Chordates presents great potential for dissecting the genetic mechanisms that underlie congenital heart defects and disease at the cellular level and might provide additional insight into potential pathways for therapeutic cardiac regeneration.

Evolution, comparative biology and ontogeny of vertebrate heart regeneration

There are 64,000 living species of vertebrates on our planet and all of them have a heart. Comparative analyses devoted to understanding the regenerative potential of the myocardium have been performed in a dozen vertebrate species with the aim of developing regenerative therapies for human heart disease. Based on this relatively small selection of animal models, important insights into the evolutionary conservation of regenerative mechanisms have been gained. In this review, we survey cardiac regeneration studies in diverse species to provide an evolutionary context for the lack of regenerative capacity in the adult mammalian heart. Our analyses highlight the importance of cardiac adaptations that have occurred over hundreds of millions of years during the transition from aquatic to terrestrial life, as well as during the transition from the womb to an oxygen-rich environment at birth. We also discuss the evolution and ontogeny of cardiac morphological, physiological and metabolic adaptations in the context of heart regeneration. Taken together, our findings suggest that cardiac regenerative potential correlates with a low-metabolic state, the inability to regulate body temperature, low heart pressure, hypoxia, immature cardiomyocyte structure and an immature immune system. A more complete understanding of the evolutionary context and developmental mechanisms governing cardiac regenerative capacity would provide stronger scientific foundations for the translation of cardiac regeneration therapies into the clinic.

Endogenous Mechanisms of Cardiac Regeneration

Zebrafish possess a remarkable capacity for cardiac regeneration throughout their lifetime, providing a model for investigating endogenous cellular and molecular mechanisms regulating myocardial regeneration. By contrast, adult mammals have an extremely limited capacity for cardiac regeneration, contributing to mortality and morbidity from cardiac diseases such as myocardial infarction and heart failure. However, the viewpoint of the mammalian heart as a postmitotic organ was recently revised based on findings that the mammalian heart contains multiple undifferentiated cell types with cardiogenic potential as well as a robust regenerative capacity during a short period early in life. Although it occurs at an extremely low level, continuous cardiomyocyte turnover has been detected in adult mouse and human hearts, which could potentially be enhanced to restore lost myocardium in damaged human hearts. This review summarizes and discusses recent advances in the understanding of endogenous mechanisms of cardiac regeneration.

Matrix metalloproteinase-2 in oncostatin M-induced sarcomere degeneration in cardiomyocytes

Cardiomyocyte dedifferentiation may be an important source of proliferating cardiomyocytes facilitating cardiac repair. Cardiomyocyte dedifferentiation and proliferation induced by oncostatin-M (OSM) is characterized by sarcomere degeneration. However, the mechanism underlying sarcomere degeneration remains unclear. We hypothesized that this process may involve matrix metalloproteinase-2 (MMP-2), a key protease localized at the sarcomere in cardiomyocytes. We tested the hypothesis that MMP-2 is involved in the sarcomere degeneration that characterizes cardiomyocyte dedifferentiation. Confocal immunofluorescence and biochemical methods were used to explore the role of MMP-2 in OSM-induced dedifferentiation of neonatal rat ventricular myocytes (NRVM). OSM caused a concentration- and time-dependent loss of sarcomeric α-actinin and troponin-I in NRVM. Upon OSM-treatment, the mature sarcomere transformed to a phenotype resembling a less-developed sarcomere, i.e., loss of sarcomeric proteins and Z-disk transformed into disconnected Z bodies, characteristic of immature myofibrils. OSM dose dependently increased MMP-2 activity. Both the pan-MMP inhibitor GM6001 and the selective MMP-2 inhibitor ARP 100 prevented sarcomere degeneration induced by OSM treatment. OSM also induced NRVM cell cycling and increased methyl-thiazolyl-tetrazolium (MTT) staining, preventable by MMP inhibition. These results suggest that MMP-2 mediates sarcomere degeneration in OSM-induced cardiomyocyte dedifferentiation and thus potentially contributes to cardiomyocyte regeneration.

Disassembly of myofibrils and potential imbalanced forces on Z-discs in cultured adult cardiomyocytes

Myofibrils are the main protein structures that generate force in the beating heart. Myofibril disassembly is related to many physiological and pathological processes. This study investigated, in a cultured rat adult cardiomyocyte model, the effect of force imbalance on myofibril disassembly. The imbalance of forces that were exerted on Z-discs was induced by the synergistic effect of broken intercalated discs and actin-myosin interaction. Cardiomyocytes with well-preserved intercalated discs were isolated from adult rat ventricles. The ultrastructure of cardiomyocyte was observed using a customized two-photon excitation fluorescence and second harmonic generation imaging system. The contraction of cardiomyocytes was recorded with a high-speed CCD camera, and the movement of cellular components was analyzed using a contractile imaging assay technique. The cardiomyocyte dynamic remodeling process was recorded using a time-lapse imaging system. The role of actin-myosin interaction in myofibril disassembly was investigated by incubating cardiomyocytes with blebbistatin (25 μM). Results demonstrated that the hierarchical disassembly process of myofibrils was initiated from cardiomyocyte free ends where intercalated discs had broken, during which the desmin network near the free cell ends was destroyed to release single myofibrils. Analysis of force (based on a schematic model of cardiomyocytes connected at intercalated discs) suggests that breaking of intercalated discs caused force imbalance on both sides of the Z-discs adjacent to the cell ends due to actin-myosin interaction. The damaged intercalated discs and actin-myosin interaction induced force imbalance on both sides of the Z-discs, which played an important role in the hierarchical disassembly of myofibrils. © 2016 Wiley Periodicals, Inc.

Comparative transcriptome profiling of the injured zebrafish and mouse hearts identifies miRNA-dependent repair pathways

Aims

The adult mammalian heart has poor regenerative capacity. In contrast, the zebrafish heart retains a robust capacity for regeneration into adulthood. These distinct responses are consequences of a differential utilization of evolutionary-conserved gene regulatory networks in the damaged heart. To systematically identify miRNA-dependent networks controlling cardiac repair following injury, we performed comparative gene and miRNA profiling of the cardiac transcriptome in adult mice and zebrafish.

Methods and results

Using an integrated approach, we show that 45 miRNA-dependent networks, involved in critical biological pathways, are differentially modulated in the injured zebrafish vs. mouse hearts. We study, more particularly, the miR-26a-dependent response. Therefore, miR-26a is down-regulated in the fish heart after injury, whereas its expression remains constant in the mouse heart. Targets of miR-26a involve activators of the cell cycle and Ezh2, a component of the polycomb repressive complex 2 (PRC2). Importantly, PRC2 exerts repressive functions on negative regulators of the cell cycle. In cultured neonatal cardiomyocytes, inhibition of miR-26a stimulates, therefore, cardiomyocyte proliferation. Accordingly, miR-26a knockdown prolongs the proliferative window of cardiomyocytes in the post-natal mouse heart.

Conclusions

This novel strategy identifies a series of miRNAs and associated pathways, in particular miR-26a, which represent attractive therapeutic targets for inducing repair in the injured heart.

Cardiac cytoarchitecture - why the "hardware" is important for heart function!

Cells that constitute fully differentiated tissues are characterised by an architecture that makes them perfectly suited for the job they have to do. This is especially obvious for cardiomyocytes, which have an extremely regular shape and display a paracrystalline arrangement of their cytoplasmic components. This article will focus on the two major cytoskeletal multiprotein complexes that are found in cardiomyocytes, the myofibrils, which are responsible for contraction and the intercalated disc, which mediates mechanical and electrochemical contact between individual cardiomyocytes. Recent studies have revealed that these two sites are also crucial in sensing excessive mechanical strain. Signalling processes will be triggered that## lead to changes in gene expression and eventually lead to an altered cardiac cytoarchitecture in the diseased heart, which results in a compromised function. Thus, understanding these changes and the signals that lead to them is crucial to design treatment strategies that can attenuate these processes. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

DAAM1 and DAAM2 are co-required for myocardial maturation and sarcomere assembly

Wnt ligands regulate heart morphogenesis but the underlying mechanisms remain unclear. Two Formin-related proteins, DAAM1 and 2, were previously found to bind the Wnt effector Disheveled. Here, since DAAM1 and 2 nucleate actin and mediate Wnt-induced cytoskeletal changes, a floxed-allele of Daam1 was used to disrupt its function specifically in the myocardium and investigate Wnt-associated pathways. Homozygous Daam1 conditional knockout (CKO) mice were viable but had misshapen hearts and poor cardiac function. The defects in Daam1 CKO mice were observed by mid-gestation and were associated with a loss of protrusions from cardiomyocytes invading the outflow tract. Further, these mice exhibited noncompaction cardiomyopathy (NCM) and deranged cardiomyocyte polarity. Interestingly, Daam1 CKO mice that were also homozygous for an insertion disrupting Daam2 (DKO) had stronger NCM, severely reduced cardiac function, disrupted sarcomere structure, and increased myocardial proliferation, suggesting that DAAM1 and DAAM2 have redundant functions. While RhoA was unaffected in the hearts of Daam1/2 DKO mice, AKT activity was lower than in controls, raising the issue of whether DAAM1/2 are only mediating Wnt signaling. Daam1-floxed mice were thus bred to Wnt5a null mice to identify genetic interactions. The hearts of Daam1 CKO mice that were also heterozygous for the null allele of Wnt5a had stronger NCM and more severe loss of cardiac function than Daam1 CKO mice, consistent with DAAM1 and Wnt5a acting in a common pathway. However, deleting Daam1 further disrupted Wnt5a homozygous-null hearts, suggesting that DAAM1 also has Wnt5a-independent roles in cardiac development.

Cardiomyocyte proliferation in cardiac development and regeneration: a guide to methodologies and interpretations

The newt and the zebrafish have the ability to regenerate many of their tissues and organs including the heart. Thus, a major goal in experimental medicine is to elucidate the molecular mechanisms underlying the regenerative capacity of these species. A wide variety of experiments have demonstrated that naturally occurring heart regeneration relies on cardiomyocyte proliferation. Thus, major efforts have been invested to induce proliferation of mammalian cardiomyocytes in order to improve cardiac function after injury or to protect the heart from further functional deterioration. In this review, we describe and analyze methods currently used to evaluate cardiomyocyte proliferation. In addition, we summarize the literature on naturally occurring heart regeneration. Our analysis highlights that newt and zebrafish heart regeneration relies on factors that are also utilized in cardiomyocyte proliferation during mammalian fetal development. Most of these factors have, however, failed to induce adult mammalian cardiomyocyte proliferation. Finally, our analysis of mammalian neonatal heart regeneration indicates experiments that could resolve conflicting results in the literature, such as binucleation assays and clonal analysis. Collectively, cardiac regeneration based on cardiomyocyte proliferation is a promising approach for improving adult human cardiac function after injury, but it is important to elucidate the mechanisms arresting mammalian cardiomyocyte proliferation after birth and to utilize better assays to determine formation of new muscle mass.

Cardiac myosin binding protein C regulates postnatal myocyte cytokinesis

Homozygous cardiac myosin binding protein C-deficient (Mybpc(t/t)) mice develop dramatic cardiac dilation shortly after birth; heart size increases almost twofold. We have investigated the mechanism of cardiac enlargement in these hearts. Throughout embryogenesis myocytes undergo cell division while maintaining the capacity to pump blood by rapidly disassembling and reforming myofibrillar components of the sarcomere throughout cell cycle progression. Shortly after birth, myocyte cell division ceases. Cardiac MYBPC is a thick filament protein that regulates sarcomere organization and rigidity. We demonstrate that many Mybpc(t/t) myocytes undergo an additional round of cell division within 10 d postbirth compared with their wild-type counterparts, leading to increased numbers of mononuclear myocytes. Short-hairpin RNA knockdown of Mybpc3 mRNA in wild-type mice similarly extended the postnatal window of myocyte proliferation. However, adult Mybpc(t/t) myocytes are unable to fully regenerate the myocardium after injury. MYBPC has unexpected inhibitory functions during postnatal myocyte cytokinesis and cell cycle progression. We suggest that human patients with homozygous MYBPC3-null mutations develop dilated cardiomyopathy, coupled with myocyte hyperplasia (increased cell number), as observed in Mybpc(t/t) mice. Human patients, with heterozygous truncating MYBPC3 mutations, like mice with similar mutations, have hypertrophic cardiomyopathy. However, the mechanism leading to hypertrophic cardiomyopathy in heterozygous MYBPC3(+/-) individuals is myocyte hypertrophy (increased cell size), whereas the mechanism leading to cardiac dilation in homozygous Mybpc3(-/-) mice is primarily myocyte hyperplasia.

Dynamic Alterations to α-Actinin Accompanying Sarcomere Disassembly and Reassembly during Cardiomyocyte Mitosis

Although mammals are thought to lose their capacity to regenerate heart muscle shortly after birth, embryonic and neonatal cardiomyocytes in mammals are hyperplastic. During proliferation these cells need to selectively disassemble their myofibrils for successful cytokinesis. The mechanism of sarcomere disassembly is, however, not understood. To study this, we performed a series of immunofluorescence studies of multiple sarcomeric proteins in proliferating neonatal rat ventricular myocytes and correlated these observations with biochemical changes at different cell cycle stages. During myocyte mitosis, α-actinin and titin were disassembled as early as prometaphase. α-actinin (representing the sarcomeric Z-disk) disassembly precedes that of titin (M-line), suggesting that titin disassembly occurs secondary to the collapse of the Z-disk. Sarcomere disassembly was concurrent with the dissolution of the nuclear envelope. Inhibitors of several intracellular proteases could not block the disassembly of α-actinin or titin. There was a dramatic increase in both cytosolic (soluble) and sarcomeric α-actinin during mitosis, and cytosolic α-actinin exhibited decreased phosphorylation compared to sarcomeric α-actinin. Inhibition of cyclin-dependent kinase 1 (CDK1) induced the quick reassembly of the sarcomere. Sarcomere dis- and re-assembly in cardiomyocyte mitosis is CDK1-dependent and features dynamic differential post-translational modifications of sarcomeric and cytosolic α-actinin.

Actin cytoskeletal remodeling with protrusion formation is essential for heart regeneration in Hippo-deficient mice

The mammalian heart regenerates poorly, and damage commonly leads to heart failure. Hippo signaling is an evolutionarily conserved kinase cascade that regulates organ size during development and prevents adult mammalian cardiomyocyte regeneration by inhibiting the transcriptional coactivator Yap, which also responds to mechanical signaling in cultured cells to promote cell proliferation. To identify Yap target genes that are activated during cardiomyocyte renewal and regeneration, we performed Yap chromatin immunoprecipitation sequencing (ChIP-Seq) and mRNA expression profiling in Hippo signaling-deficient mouse hearts. We found that Yap directly regulated genes encoding cell cycle progression proteins, as well as genes encoding proteins that promote F-actin polymerization and that link the actin cytoskeleton to the extracellular matrix. Included in the latter group were components of the dystrophin glycoprotein complex, a large molecular complex that, when defective, results in muscular dystrophy in humans. Cardiomyocytes near the scar tissue of injured Hippo signaling-deficient mouse hearts showed cellular protrusions suggestive of cytoskeletal remodeling. The hearts of mdx mutant mice, which lack functional dystrophin and are a model for muscular dystrophy, showed impaired regeneration and cytoskeleton remodeling, but normal cardiomyocyte proliferation, after injury. Our data showed that, in addition to genes encoding cell cycle progression proteins, Yap regulated genes that enhance cytoskeletal remodeling. Thus, blocking the Hippo pathway input to Yap may tip the balance so that Yap responds to mechanical changes associated with heart injury to promote repair.

Dedifferentiation, Transdifferentiation, and Proliferation: Mechanisms Underlying Cardiac Muscle Regeneration in Zebrafish

The adult mammalian heart is increasingly recognized as a regenerative organ with a measurable capacity to replenish cardiomyocytes throughout its lifetime, illuminating the possibility of stimulating endogenous regenerative capacity to treat heart diseases. Unlike mammals, certain vertebrates possess robust capacity for regenerating a damaged heart, providing a model to understand how regeneration could be augmented in injured human hearts. Facilitated by its rich history in the study of heart development, the teleost zebrafish Danio rerio has been established as a robust model to investigate the underlying mechanism of cardiac regeneration. This review discusses the current understanding of the endogenous mechanisms behind cardiac regeneration in zebrafish, with a particular focus on cardiomyocyte dedifferentiation, transdifferentiation, and proliferation.

Advances in understanding the mechanism of zebrafish heart regeneration

The adult mammalian heart was once believed to be a post-mitotic organ without any capacity for regeneration, but recent findings have challenged this dogma. A modified view assigns the mammalian heart a measurable capacity for regeneration throughout its lifetime, with the implication that endogenous regenerative capacity can be therapeutically stimulated in the injury setting. Although extremely limited in adult mammals, the natural capacity for organ regeneration is a conserved trait in certain vertebrates. Urodele amphibians and teleosts are well-known examples of such animals that can efficiently regenerate various organs including the heart as adults. By understanding how these animals regenerate a damaged heart, one might obtain valuable insights into how regeneration can be augmented in injured human hearts. Among the regenerative vertebrate models, the teleost zebrafish, Danio rerio, is arguably the best characterized with respect to cardiac regenerative responses. Knowledge is still limited, but a decade of research in this model has led to results that may help to understand how cardiac regeneration is naturally stimulated and maintained. This review surveys recent advances in the field and discusses current understanding of the endogenous mechanisms of cardiac regeneration in zebrafish.

A neonatal blueprint for cardiac regeneration

Adult mammals undergo minimal regeneration following cardiac injury, which severely compromises cardiac function and contributes to the ongoing burden of heart failure. In contrast, the mammalian heart retains a transient capacity for cardiac regeneration during fetal and early neonatal life. Recent studies have established the importance of several evolutionarily conserved mechanisms for heart regeneration in lower vertebrates and neonatal mammals including induction of cardiomyocyte proliferation, epicardial cell activation, angiogenesis, extracellular matrix deposition and immune cell infiltration. In this review, we provide an up-to-date account of the molecular and cellular basis for cardiac regeneration in lower vertebrates and neonatal mammals. The historical context for these recent findings and their ramifications for the future development of cardiac regenerative therapies are also discussed.

A cardiomyocyte-specific Wdr1 knockout demonstrates essential functional roles for actin disassembly during myocardial growth and maintenance in mice

Actin dynamics are critical for muscle development and function, and mutations leading to deregulation of actin dynamics cause various forms of heritable muscle diseases. AIP1 is a major cofactor of the actin depolymerizing factor/cofilin in eukaryotes, promoting actin depolymerizing factor/cofilin-mediated actin disassembly. Its function in vertebrate muscle has been unknown. To investigate functional roles of AIP1 in myocardium, we generated conditional knockout (cKO) mice with cardiomyocyte-specific deletion of Wdr1, the mammalian homolog of yeast AIP1. Wdr1 cKO mice began to die at postnatal day 13 (P13), and none survived past P24. At P12, cKO mice exhibited cardiac hypertrophy and impaired contraction of the left ventricle. Electrocardiography revealed reduced heart rate, abnormal P wave, and abnormal T wave at P10 and prolonged QT interval at P12. Actin filament (F-actin) accumulations began at P10 and became prominent at P12 in the myocardium of cKO mice. Within regions of F-actin accumulation in myofibrils, the sarcomeric components α-actinin and tropomodulin-1 exhibited disrupted patterns, indicating that F-actin accumulations caused by Wdr1 deletion result in disruption of sarcomeric structure. Ectopic cofilin colocalized with F-actin aggregates. In adult mice, Wdr1 deletion resulted in similar but much milder phenotypes of heart hypertrophy, F-actin accumulations within myofibrils, and lethality. Taken together, these results demonstrate that AIP1-regulated actin dynamics play essential roles in heart function in mice.

[Proliferation of adult mammalian ventricular cardiomyocytes: a sporadic but feasible phenomenon]

Proliferation of adult mammalian ventricular cardiomyocytes has been ruled out by some researchers, who have argued that these cells are terminally differentiated; however, this dogma has been rejected because other researchers have reported that these cells can present the processes necessary to proliferate, that is, DNA synthesis, mitosis and cytokinesis when the heart is damaged experimentally through pharmacological and surgical strategies or due to pathological conditions concerning the cardiovascular system. This review integrates some of the available works in the literature evaluating the DNA synthesis, mitosis and cytokinesis in these myocytes, when the myocardium is damaged, with the purpose of knowing if their proliferation can be considered as a feasible phenomenon. The review is concluded with a reflection about the perspectives of the knowledge generated in this area.

Regulation of cardiomyocyte proliferation during development and regeneration

The regulation of cardiomyocyte proliferation is important for heart development and regeneration. The proliferation patterns of cardiomyocytes are closely related to heart morphogenesis, size, and functions. The proliferation levels are high during early embryogenesis; however, mammalian cardiomyocytes exit the cell cycle irreversibly soon after birth. The cell cycle exit inhibits cardiac regeneration in mammals. On the other hand, cardiomyocytes of adult zebrafish and probably newts can proliferate after cardiac injury, and the hearts can be regenerated. Therefore, the ability to reproliferate determines regenerative ability. As in other cells, the relationship between proliferation and differentiation is very interesting, and is closely related to cardiac development, regeneration and homeostasis. In this review, these topics are discussed.

Extra- and intracellular factors regulating cardiomyocyte proliferation in postnatal life

One of the striking differences that distinguish the adult from the embryonic heart in mammals and set it apart from the heart in urodeles and teleosts is the incapacity of cardiomyocytes to respond to damage by proliferation. While the molecular reasons underlying these characteristics still await elucidation, mounting evidence collected over the last several years indicates that cardiomyocyte proliferation can be modulated by different extracellular molecules. The exogenous administration of selected growth factors is capable of inducing neonatal and, in some instances, also adult cardiomyocyte proliferation. Other diffusible factors can regulate the proliferation and cardiac commitment of endogenous or implanted stem cells. While the individual role of these factors in the paracrine control of normal heart homeostasis still needs to be defined, this information is relevant for the development of novel therapeutic strategies for cardiac regeneration. In addition, recent evidence indicates that postnatal cardiomyocyte proliferation is controlled by genetically defined pathways, such as the Hippo pathway, and can be modulated by perturbing the endogenous cardiomyocyte microRNA network; the identification of the cytokines that activate these molecular circuits holds great potential for clinical translation.

Cell electrospinning cardiac patches for tissue engineering the heart

The investigative studies performed and presented in this communication demonstrate the ability for cell electrospinning to directly handle living primary cardiac myocytes from which living cardiac fibers and scaffolds are generated. This platform technology investigated in these studies holds great promise for cardiac medicine and surgery to diagnostics and bio-analysis of cardiac tissues at all states.

Binucleation of cardiomyocytes: the transition from a proliferative to a terminally differentiated state

Cardiomyocytes possess a unique ability to transition from mononucleate to the mature binucleate phenotype in late fetal development and around birth. Mononucleate cells are proliferative, whereas binucleate cells exit the cell cycle and no longer proliferate. This crucial period of terminal differentiation dictates cardiomyocyte endowment for life. Adverse early life events can influence development of the heart, affecting cardiomyocyte number and contributing to heart disease late in life. Although much is still unknown about the mechanisms underlying the binucleation process, many studies are focused on molecules involved in cell cycle regulation and cytokinesis as well as epigenetic modifications that can occur during this transition. Better understanding of these mechanisms could provide a basis for recovering the proliferative capacity of cardiomyocytes.

Isolation and culture of neonatal mouse cardiomyocytes

Cultured neonatal cardiomyocytes have long been used to study myofibrillogenesis and myofibrillar functions. Cultured cardiomyocytes allow for easy investigation and manipulation of biochemical pathways, and their effect on the biomechanical properties of spontaneously beating cardiomyocytes. The following 2-day protocol describes the isolation and culture of neonatal mouse cardiomyocytes. We show how to easily dissect hearts from neonates, dissociate the cardiac tissue and enrich cardiomyocytes from the cardiac cell-population. We discuss the usage of different enzyme mixes for cell-dissociation, and their effects on cell-viability. The isolated cardiomyocytes can be subsequently used for a variety of morphological, electrophysiological, biochemical, cell-biological or biomechanical assays. We optimized the protocol for robustness and reproducibility, by using only commercially available solutions and enzyme mixes that show little lot-to-lot variability. We also address common problems associated with the isolation and culture of cardiomyocytes, and offer a variety of options for the optimization of isolation and culture conditions.

Human embryonic stem cell-derived cardiomyocytes migrate in response to gradients of fibronectin and Wnt5a

An improved understanding of the factors that regulate the migration of human embryonic stem cell-derived cardiomyocytes (hESC-CMs) would provide new insights into human heart development and suggest novel strategies to improve their electromechanical integration after intracardiac transplantation. Since nothing has been reported as to the factors controlling hESC-CM migration, we hypothesized that hESC-CMs would migrate in response to the extracellular matrix and soluble signaling molecules previously implicated in heart morphogenesis. To test this, we screened candidate factors by transwell assay for effects on hESC-CM motility, followed by validation via live-cell imaging and/or gap-closure assays. Fibronectin (FN) elicited a haptotactic response from hESC-CMs, with cells seeded on a steep FN gradient showing nearly a fivefold greater migratory activity than cells on uniform FN. Studies with neutralizing antibodies indicated that adhesion and migration on FN are mediated by integrins α-5 and α-V. Next, we screened 10 soluble candidate factors by transwell assay and found that the noncanonical Wnt, Wnt5a, elicited an approximately twofold increase in migration over controls. This effect was confirmed using the gap-closure assay, in which Wnt5a-treated hESC-CMs showed approximately twofold greater closure than untreated cells. Studies with microfluidic-generated Wnt5a gradients showed that this factor was chemoattractive as well as chemokinetic, and Wnt5a-mediated responses were inhibited by the Frizzled-1/2 receptor antagonist, UM206. In summary, hESC-CMs show robust promigratory responses to FN and Wnt5a, findings that have implications on both cardiac development and cell-based therapies.

Direct visualization of cell division using high-resolution imaging of M-phase of the cell cycle

Current approaches to monitor and quantify cell division in live cells, and reliably distinguish between acytokinesis and endoreduplication, are limited and complicate determination of stem cell pool identities. Here we overcome these limitations by generating an in vivo reporter system using the scaffolding protein anillin fused to enhanced green fluorescent protein, to provide high spatiotemporal resolution of mitotic phase. This approach visualizes cytokinesis and midbody formation as hallmarks of expansion of stem and somatic cells, and enables distinction from cell cycle variations. High-resolution microscopy in embryonic heart and brain tissues of enhanced green fluorescent protein–anillin transgenic mice allows live monitoring of cell division and quantitation of cell cycle kinetics. Analysis of cell division in hearts post injury shows that border zone cardiomyocytes in the infarct respond with increasing ploidy, but not cell division. Thus, the enhanced green fluorescent protein–anillin system enables monitoring and measurement of cell division in vivo and markedly simplifies in vitro analysis in fixed cells.

Current methods for detecting proliferation in live cells cannot distinguish between dividing cells and cells that are progressing through the cell cycle. Here, a method is described that detects anillin in the contractile ring and in the midbody of cells during M-phase, providing a more accurate detection of dividing cells.

Functional screening identifies miRNAs inducing cardiac regeneration

In mammals, enlargement of the heart during embryonic development is primarily dependent on the increase in cardiomyocyte numbers. Shortly after birth, however, cardiomyocytes stop proliferating and further growth of the myocardium occurs through hypertrophic enlargement of the existing myocytes. As a consequence of the minimal renewal of cardiomyocytes during adult life, repair of cardiac damage through myocardial regeneration is very limited. Here we show that the exogenous administration of selected microRNAs (miRNAs) markedly stimulates cardiomyocyte proliferation and promotes cardiac repair. We performed a high-content microscopy, high-throughput functional screening for human miRNAs that promoted neonatal cardiomyocyte proliferation using a whole-genome miRNA library. Forty miRNAs strongly increased both DNA synthesis and cytokinesis in neonatal mouse and rat cardiomyocytes. Two of these miRNAs (hsa-miR-590 and hsa-miR-199a) were further selected for testing and were shown to promote cell cycle re-entry of adult cardiomyocytes ex vivo and to promote cardiomyocyte proliferation in both neonatal and adult animals. After myocardial infarction in mice, these miRNAs stimulated marked cardiac regeneration and almost complete recovery of cardiac functional parameters. The miRNAs identified hold great promise for the treatment of cardiac pathologies consequent to cardiomyocyte loss.

The Notch pathway controls fibrotic and regenerative repair in the adult heart

Aims

In the adult heart, Notch signalling regulates the response to injury. Notch inhibition leads to increased cardiomyocyte apoptosis, and exacerbates the development of cardiac hypertrophy and fibrosis. The role of Notch in the mesenchymal stromal cell fraction, which contains cardiac fibroblasts and cardiac precursor cells, is, however, largely unknown. In the present study, we evaluate, therefore, whether forced activation of the Notch pathway in mesenchymal stromal cells regulates pathological cardiac remodelling.

Methods and results

We generated transgenic mice overexpressing the Notch ligand Jagged1 on the surface of cardiomyocytes to activate Notch signalling in adjacent myocyte and non-myocyte cells. In neonatal transgenic mice, activated Notch sustained cardiac precursor and myocyte proliferation after birth, and led to increased numbers of cardiac myocytes in adult mice. In the adult heart under pressure overload, Notch inhibited the development of cardiomyocyte hypertrophy and transforming growth factor-β/connective tissue growth factor-mediated cardiac fibrosis. Most importantly, Notch activation in the stressed adult heart reduced the proliferation of myofibroblasts and stimulated the expansion of stem cell antigen-1-positive cells, and in particular of Nkx2.5-positive cardiac precursor cells.

Conclusions

We conclude that Notch is pivotal in the healing process of the injured heart. Specifically, Notch regulates key cellular mechanisms in the mesenchymal stromal cell population, and thereby controls the balance between fibrotic and regenerative repair in the adult heart. Altogether, these findings indicate that Notch represents a unique therapeutic target for inducing regeneration in the adult heart via mobilization of cardiac precursor cells.

The role of neuregulin/ErbB2/ErbB4 signaling in the heart with special focus on effects on cardiomyocyte proliferation

The signaling complex consisting of the growth factor neuregulin-1 (NRG1) and its tyrosine kinase receptors ErbB2 and ErbB4 has a critical role in cardiac development and homeostasis of the structure and function of the adult heart. Recent research results suggest that targeting this signaling complex may provide a viable strategy for treating heart failure. Clinical trials are currently evaluating the effectiveness and safety of intravenous administration of recombinant NRG1 formulations in heart failure patients. Endogenous as well as administered NRG1 has multiple possible activities in the adult heart, but how these are related is unknown. It has recently been demonstrated that NRG1 administration can stimulate proliferation of cardiomyocytes, which may contribute to repair failing hearts. This review summarizes the current knowledge of how NRG1 and its receptors control cardiac physiology and biology, with special emphasis on its role in cardiomyocyte proliferation during myocardial growth and regeneration.

Multiple influences of blood flow on cardiomyocyte hypertrophy in the embryonic zebrafish heart

Cardiomyocyte hypertrophy is a complex cellular behavior involving coordination of cell size expansion and myofibril content increase. Here, we investigate the contribution of cardiomyocyte hypertrophy to cardiac chamber emergence, the process during which the primitive heart tube transforms into morphologically distinct chambers and increases its contractile strength. Focusing on the emergence of the zebrafish ventricle, we observed trends toward increased cell surface area and myofibril content. To examine the extent to which these trends reflect coordinated hypertrophy of individual ventricular cardiomyocytes, we developed a method for tracking cell surface area changes and myofibril dynamics in live embryos. Our data reveal a previously unappreciated heterogeneity of ventricular cardiomyocyte behavior during chamber emergence: although cardiomyocyte hypertrophy was prevalent, many cells did not increase their surface area or myofibril content during the observed timeframe. Despite the heterogeneity of cell behavior, we often found hypertrophic cells neighboring each other. Next, we examined the impact of blood flow on the regulation of cardiomyocyte behavior during this phase of development. When blood flow through the ventricle was reduced, cell surface area expansion and myofibril content increase were both dampened, and the behavior of neighboring cells did not seem coordinated. Together, our studies suggest a model in which hemodynamic forces have multiple influences on cardiac chamber emergence: promoting both cardiomyocyte enlargement and myofibril maturation, enhancing the extent of cardiomyocyte hypertrophy, and facilitating the coordination of neighboring cell behaviors.

Knockdown of embryonic myosin heavy chain reveals an essential role in the morphology and function of the developing heart

The expression and function of embryonic myosin heavy chain (eMYH) has not been investigated within the early developing heart. This is despite the knowledge that other structural proteins, such as alpha and beta myosin heavy chains and cardiac alpha actin, play crucial roles in atrial septal development and cardiac function. Most cases of atrial septal defects and cardiomyopathy are not associated with a known causative gene, suggesting that further analysis into candidate genes is required. Expression studies localised eMYH in the developing chick heart. eMYH knockdown was achieved using morpholinos in a temporal manner and functional studies were carried out using electrical and calcium signalling methodologies. Knockdown in the early embryo led to abnormal atrial septal development and heart enlargement. Intriguingly, action potentials of the eMYH knockdown hearts were abnormal in comparison with the alpha and beta myosin heavy chain knockdowns and controls. Although myofibrillogenesis appeared normal, in knockdown hearts the tissue integrity was affected owing to apparent focal points of myocyte loss and an increase in cell death. An expression profile of human skeletal myosin heavy chain genes suggests that human myosin heavy chain 3 is the functional homologue of the chick eMYH gene. These data provide compelling evidence that eMYH plays a crucial role in important processes in the early developing heart and, hence, is a candidate causative gene for atrial septal defects and cardiomyopathy.

MiR-15 family regulates postnatal mitotic arrest of cardiomyocytes

RATIONALE

Mammalian cardiomyocytes withdraw from the cell cycle during early postnatal development, which significantly limits the capacity of the adult mammalian heart to regenerate after injury. The regulatory mechanisms that govern cardiomyocyte cell cycle withdrawal and binucleation are poorly understood.

OBJECTIVE

Given the potential of microRNAs (miRNAs) to influence large gene networks and modify complex developmental and disease phenotypes, we searched for miRNAs that were regulated during the postnatal switch to terminal differentiation.

METHODS AND RESULTS

Microarray analysis revealed subsets of miRNAs that were upregulated or downregulated in cardiac ventricles from mice at 1 and 10 days of age (P1 and P10). Interestingly, miR-195 (a member of the miR-15 family) was the most highly upregulated miRNA during this period, with expression levels almost 6-fold higher in P10 ventricles relative to P1. Precocious overexpression of miR-195 in the embryonic heart was associated with ventricular hypoplasia and ventricular septal defects in β-myosin heavy chain-miR-195 transgenic mice. Using global gene profiling and argonaute-2 immunoprecipitation approaches, we showed that miR-195 regulates the expression of a number of cell cycle genes, including checkpoint kinase 1 (Chek1), which we identified as a highly conserved direct target of miR-195. Finally, we demonstrated that knockdown of the miR-15 family in neonatal mice with locked nucleic acid-modified anti-miRNAs was associated with an increased number of mitotic cardiomyocytes and derepression of Chek1.

CONCLUSIONS

These findings suggest that upregulation of the miR-15 family during the neonatal period may be an important regulatory mechanism governing cardiomyocyte cell cycle withdrawal and binucleation.