Existing cardiomyocytes generate cardiomyocytes at a low rate after birth in mice

Single cardiomyocyte nuclear transcriptomes reveal a lincRNA-regulated de-differentiation and cell cycle stress-response in vivo

Cardiac regeneration may revolutionize treatment for heart failure but endogenous progenitor-derived cardiomyocytes in the adult mammalian heart are few and pre-existing adult cardiomyocytes divide only at very low rates. Although candidate genes that control cardiomyocyte cell cycle re-entry have been implicated, expression heterogeneity in the cardiomyocyte stress-response has never been explored. Here, we show by single nuclear RNA-sequencing of cardiomyocytes from both mouse and human failing, and non-failing adult hearts that sub-populations of cardiomyocytes upregulate cell cycle activators and inhibitors consequent to the stress-response in vivo. We characterize these subgroups by weighted gene co-expression network analysis and discover long intergenic non-coding RNAs (lincRNA) as key nodal regulators. KD of nodal lincRNAs affects expression levels of genes related to dedifferentiation and cell cycle, within the same gene regulatory network. Our study reveals that sub-populations of adult cardiomyocytes may have a unique endogenous potential for cardiac regeneration in vivo.

Adult mammalian cardiomyocytes are predominantly binucleated and unable to divide. Using single nuclear RNA-sequencing of cardiomyocytes from mouse and human failing and non-failing adult hearts, See et al. show that some cardiomyocytes respond to stress by dedifferentiation and cell cycle re-entry regulated by lncRNAs.

Frequency of mononuclear diploid cardiomyocytes underlies natural variation in heart regeneration

Adult mammalian cardiomyocyte regeneration after injury is thought to be minimal. Mononuclear diploid cardiomyocytes (MNDCMs), a relatively small subpopulation in the adult heart, may account for the observed degree of regeneration, but this has not been tested. We surveyed 120 inbred mouse strains and found that the frequency of adult mononuclear cardiomyocytes was surprisingly variable (>7-fold). Cardiomyocyte proliferation and heart functional recovery after coronary artery ligation both correlated with pre-injury MNDCM content. Using genome-wide association, we identified Tnni3k as one gene that influences variation in this composition and demonstrated that Tnni3k knockout resulted in elevated MNDCM content and increased cardiomyocyte proliferation after injury. Reciprocally, overexpression of Tnni3k in zebrafish promoted cardiomyocyte polyploidization and compromised heart regeneration. Our results corroborate the relevance of MNDCMs in heart regeneration. Moreover, they imply that intrinsic heart regeneration is not limited nor uniform in all individuals, but rather is a variable trait influenced by multiple genes.

Fam64a is a novel cell cycle promoter of hypoxic fetal cardiomyocytes in mice

Fetal cardiomyocytes actively proliferate to form the primitive heart in utero in mammals, but they stop dividing shortly after birth. The identification of essential molecules maintaining this active cardiomyocyte proliferation is indispensable for potential adult heart regeneration. A recent study has shown that this proliferation depends on a low fetal oxygen condition before the onset of breathing at birth. We have established an isolation protocol for mouse fetal cardiomyocytes, performed under strict low oxygen conditions to mimic the intrauterine environment, that gives the highest proliferative activities thus far reported. Oxygen exposure during isolation/culture markedly inhibited cell division and repressed cell cycle-promoting genes, and subsequent genome-wide analysis identified Fam64a as a novel regulatory molecule. Fam64a was abundantly expressed in hypoxic fetal cardiomyocyte nuclei, but this expression was drastically repressed by oxygen exposure, and in postnatal cardiomyocytes following the onset of breathing and the resulting elevation of oxygen tension. Fam64a knockdown inhibited and its overexpression enhanced cardiomyocyte proliferation. Expression of a non-degradable Fam64a mutant suggested that optimum Fam64a expression and subsequent degradation by anaphase-promoting complex/cyclosome (APC/C) during the metaphase-to-anaphase transition are required for fetal cardiomyocyte division. We propose that Fam64a is a novel cell cycle promoter of hypoxic fetal cardiomyocytes in mice.

The use and abuse of Cre/Lox recombination to identify adult cardiomyocyte renewal rate and origin

The adult mammalian heart, including the human, is unable to regenerate segmental losses after myocardial infarction. This evidence has been widely and repeatedly used up-to-today to suggest that the myocardium, contrary to most adult tissues, lacks an endogenous stem cell population or more specifically a bona-fide cardiomyocyte-generating progenitor cell of biological significance. In the last 15 years, however, the field has slowly evolved from the dogma that no new cardiomyocytes were produced from shortly after birth to the present consensus that new cardiomyocytes are formed throughout lifespan. This endogenous regenerative potential increases after various forms of injury. Nevertheless, the degree/significance and more importantly the origin of adult new cardiomyocytes remains strongly disputed. Evidence from independent laboratories has shown that the adult myocardium harbours bona-fide tissue-specific cardiac stem cells (CSCs). Their transplantation and in situ activation have demonstrated the CSCs regenerative potential and have been used to develop regeneration protocols which in pre-clinical tests have shown to be effective in the prevention and treatment of heart failure. Recent reports purportedly tracking the c-kit⁺CSC's fate using Cre/lox recombination in the mouse have challenged the existence and regenerative potential of the CSCs and have raised scepticism about their role in myocardial homeostasis and regeneration. The validity of these reports, however, is controversial because they failed to show that the experimental approach used is capable to both identify and tract the fate of the CSCs. Despite these serious shortcomings, in contraposition to the CSCs, these publications have proposed the proliferation of existing adult fully-matured cardiomyocytes as the relevant mechanism to explain cardiomyocyte renewal in the adult. This review critically ponders the available evidence showing that the adult mammalian heart possesses a definable myocyte-generating progenitor cell of biological significance. This endogenous regenerative potential is expected to provide the bases for novel approaches of myocardial repair in the near future.

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.

EED orchestration of heart maturation through interaction with HDACs is H3K27me3-independent

In proliferating cells, where most Polycomb repressive complex 2 (PRC2) studies have been performed, gene repression is associated with PRC2 trimethylation of H3K27 (H3K27me3). However, it is uncertain whether PRC2 writing of H3K27me3 is mechanistically required for gene silencing. Here, we studied PRC2 function in postnatal mouse cardiomyocytes, where the paucity of cell division obviates bulk H3K27me3 rewriting after each cell cycle. EED (embryonic ectoderm development) inactivation in the postnatal heart (Eed^CKO) caused lethal dilated cardiomyopathy. Surprisingly, gene upregulation in Eed^CKO was not coupled with loss of H3K27me3. Rather, the activating histone mark H3K27ac increased. EED interacted with histone deacetylases (HDACs) and enhanced their catalytic activity. HDAC overexpression normalized Eed^CKO heart function and expression of derepressed genes. Our results uncovered a non-canonical, H3K27me3-independent EED repressive mechanism that is essential for normal heart function. Our results further illustrate that organ dysfunction due to epigenetic dysregulation can be corrected by epigenetic rewiring.

DOI:http://dx.doi.org/10.7554/eLife.24570.001

Histone demethylases Kdm6ba and Kdm6bb redundantly promote cardiomyocyte proliferation during zebrafish heart ventricle maturation

Trimethylation of lysine 27 on histone 3 (H3K27me3) by the Polycomb repressive complex 2 (PRC2) contributes to localized and inherited transcriptional repression. Kdm6b (Jmjd3) is a H3K27me3 demethylase that can relieve repression-associated H3K27me3 marks, thereby supporting activation of previously silenced genes. Kdm6b is proposed to contribute to early developmental cell fate specification, cardiovascular differentiation, and/or later steps of organogenesis, including endochondral bone formation and lung development. We pursued loss-of-function studies in zebrafish to define the conserved developmental roles of Kdm6b. kdm6ba and kdm6bb homozygous deficient zebrafish are each viable and fertile. However, loss of both kdm6ba and kdm6bb shows Kdm6b proteins share redundant and pleiotropic roles in organogenesis without impacting initial cell fate specification. In the developing heart, co-expressed Kdm6b proteins promote cardiomyocyte proliferation coupled with the initial stages of cardiac trabeculation. While newly formed trabecular cardiomyocytes display a striking transient decrease in bulk cellular H3K27me3 levels, this demethylation is independent of collective Kdm6b. Our results indicate a restricted and likely locus-specific role for Kdm6b demethylases during heart ventricle maturation rather than initial cardiogenesis.

Assessing Cardiomyocyte Subtypes Following Transcription Factor-mediated Reprogramming of Mouse Embryonic Fibroblasts

Direct reprogramming of one cell type into another has recently emerged as a powerful paradigm for regenerative medicine, disease modeling, and lineage specification. In particular, the conversion of fibroblasts into induced cardiomyocyte-like myocytes (iCLMs) by Gata4, Hand2, Mef2c, and Tbx5 (GHMT) represents an important avenue for generating de novo cardiac myocytes in vitro and in vivo. Recent evidence suggests that GHMT generates a greater diversity of cardiac subtypes than previously appreciated, thus underscoring the need for a systematic approach to conducting additional studies. Before direct reprogramming can be used as a therapeutic strategy, however, the mechanistic underpinnings of lineage conversion must be understood in detail to generate specific cardiac subtypes. Here we present a detailed protocol for generating iCLMs by GHMT-mediated reprogramming of mouse embryonic fibroblasts (MEFs). We outline methods for MEF isolation, retroviral production, and MEF infection to accomplish efficient reprogramming. To determine the subtype identity of reprogrammed cells, we detail a step-by-step approach for performing immunocytochemistry on iCLMs using a defined set of compatible antibodies. Methods for confocal microscopy, identification, and quantification of iCLMs and individual atrial (iAM), ventricular (iVM), and pacemaker (iPM) subtypes are also presented. Finally, we discuss representative results of prototypical direct reprogramming experiments and highlight important technical aspects of our protocol to ensure efficient lineage conversion. Taken together, our optimized protocol should provide a stepwise approach for investigators to conduct meaningful cardiac reprogramming experiments that require identification of individual CM subtypes.

Concise Review: Mending a Broken Heart: The Evolution of Biological Therapeutics

Heart failure (HF), a common sequela of cardiovascular diseases, remains a staggering clinical problem, associated with high rates of morbidity and mortality worldwide. Advances in pharmacological, interventional, and operative management have improved patient care, but these interventions are insufficient to halt the progression of HF, particularly the end-stage irreversible loss of functional cardiomyocytes. Innovative therapies that could prevent HF progression and improve the function of the failing heart are urgently needed. Following successful preclinical studies, two main strategies have emerged as potential solutions: cardiac gene therapy and cardiac regeneration through stem and precursor cell transplantation. Many potential gene- and cell-based therapies have entered into clinical studies, intending to ameliorate cardiac dysfunction in patients with advanced HF. In this review, we focus on the recent advances in cell- and gene-based therapies in the context of cardiovascular disease, emphasizing the most advanced therapies. The principles and mechanisms of action of gene and cell therapies for HF are discussed along with the limitations of current approaches. Finally, we highlight the emerging technologies that hold promise to revolutionize the biological therapies for cardiovascular diseases. Stem Cells 2017;35:1131-1140.

Cardiac repair in a mouse model of acute myocardial infarction with trophoblast stem cells

Various stem cells have been explored for the purpose of cardiac repair. However, any individual stem cell population has not been considered as the ideal source. Recently, trophoblast stem cells (TSCs), a newly described stem cell type, have demonstrated extensive plasticity. The present study evaluated the therapeutic effect of TSCs transplantation for heart regeneration in a mouse model of myocardial infarction (MI) and made a direct comparison with the most commonly used mesenchymal stem cells (MSCs). Transplantation of TSCs and MSCs led to a remarkably improved cardiac function in contrast with the PBS control, but only the TSCs exhibited the potential of differentiation into cardiomyocytes in vivo. In addition, a significantly high proliferation level of both transplanted stem cells and resident cardiomyocytes was observed in the TSCs group. These findings primary revealed the therapeutic potential of TSCs in transplantation therapy for MI.

Redirecting cardiac growth mechanisms for therapeutic regeneration

Heart failure is a major source of morbidity and mortality. Replacing lost myocardium with new tissue is a major goal of regenerative medicine. Unlike adult mammals, zebrafish and neonatal mice are capable of heart regeneration following cardiac injury. In both contexts, the regenerative program echoes molecular and cellular events that occur during cardiac development and morphogenesis, notably muscle creation through division of cardiomyocytes. Based on studies over the past decade, it is now accepted that the adult mammalian heart undergoes a low grade of cardiomyocyte turnover. Recent data suggest that this cardiomyocyte turnover can be augmented in the adult mammalian heart by redeployment of developmental factors. These findings and others suggest that stimulating endogenous regenerative responses can emerge as a therapeutic strategy for human cardiovascular disease.

Identification of cardiac progenitors that survive in the ischemic human heart after ventricular myocyte death

Atypically-shaped cardiomyocytes (ACMs) are beating heart cells identified in the cultures of cardiomyocyte-removed fractions obtained from adult mouse hearts. Since ACMs spontaneously develop into beating cells in the absence of hormones or chemicals, these cells are likely to be a type of cardiac progenitors rather than stem cells. “Native ACMs” are found as small interstitial cells among ventricular myocytes that co-express cellular prion protein (PrP) and cardiac troponin T (cTnT) in mouse and human heart tissues. However, the endogenous behavior of human ACMs is unclear. In the present study, we demonstrate that PrP⁺ cTnT⁺ cells are present in the human heart tissue with myocardial infarction (MI). These cells were mainly found in the border of necrotic cardiomyocytes caused by infarcts and also in the hibernating myocardium subjected to the chronic ischemia. The ratio of PrP⁺ cTnT⁺ cells to the total cells observed in the normal heart tissue section of mouse and human was estimated to range from 0.3–0.8%. Notably, living human PrP⁺ cTnT⁺ cells were identified in the cultures obtained at pathological autopsy despite exposure to lethal ischemic conditions for hours after death. These findings suggest that ACMs could survive in the ischemic human heart and develop into a sub-population of cardiac myocytes.

Ascending Aortic Constriction Promotes Cardiomyocyte Proliferation in Neonatal Rats

Adult heart suffering from increased workload will undergo myocardial hypertrophy, subsequent cardiomyocyte (CM) death, and eventually heart failure. However, the effect of increasing afterload on the neonatal heart remains unknown. We performed ascending aortic constriction (AAC) in neonatal rats 8-12 hours after birth (P0, P indicates postpartum). Seven days after surgery, in vivo heart function was evaluated using cardiac ultrasonography. Haematoxylineosin and Masson staining were used to assess CM diameter and collagen deposition. Moreover, expression of both EdU and Ki67 were evaluated to determine DNA synthesis levels, and pH3 and aurora B as markers for mitosis in CMs. CM isolation was performed by heart perfusion at P0, P3, P5, and P7, respectively. CM number on P0 was 1.01 ± 0.29 × 10⁶. We found that CM cell cycle activation was significantly increased among constricted hearts, as demonstrated by increased Ki67, EdU, pH3, and aurora B positive cells/1000 CMs. At day 7 (P7), constriction group hearts manifested increased wall thickness (0.55 ± 0.05 mm versus 0.85 ± 0.10 mm, P < 0.01, n = 6), and improved hemodynamics as well as left ventricular ejection fraction (65.5 ± 3.7% versus 77.7 ± 4.8%, P < 0.01, n = 6). Of note, the population of CMs was also markedly increased in the constriction group (2.92 ± 0.27 × 10⁶ versus 3.41 ± 0.40 × 10⁶, P < 0.05, n = 6). In summary, we found that during the first week after birth significant numbers of neonatal CMs can reenter the cell cycle. Ascending aortic constriction promotes neonatal rat CM proliferation resulting in 16.7% more CMs in the heart.

Therapeutic Application of Adult Stem Cells in the Heart

Cell therapies have been explored as a potential treatment avenue to treat heart diseases, such as myocardial infarction, doxorubicin-induced cardiomyopathy, and heart failure. Embryonic and adult stem cells (ASCs) have been examined in animal and clinical settings. Unlike embryonic and induced pluripotent stem cells, ASCs do not pose a threat to form teratomas, nor do they have immune system concerns, making them ideal for therapeutic use in humans. In this review, we will investigate different characteristics and sources of adult stem cells and progenitor cells, as well as determine their efficacy in cell transplantation in experimental and clinical trials. In addition, we will propose other research avenues that may promote further understanding and use of ASCs in therapeutic designs.

Asxl2-/- Mice Exhibit De Novo Cardiomyocyte Production during Adulthood

Heart attacks affect more than seven million people worldwide each year. A heart attack, or myocardial infarction, may result in the death of a billion cardiomyocytes within hours. The adult mammalian heart does not have an effective mechanism to replace lost cardiomyocytes. Instead, lost muscle is replaced with scar tissue, which decreases blood pumping ability and leads to heart failure over time. Here, we report that the loss of the chromatin factor ASXL2 results in spontaneous proliferation and cardiogenic differentiation of a subset of interstitial non-cardiomyocytes. The adult Asxl2^-/- heart displays spontaneous overgrowth without cardiomyocyte hypertrophy. Thymidine analog labeling and Ki67 staining of 12-week-old hearts revealed 3- and 5-fold increases of proliferation rate for vimentin⁺ non-cardiomyocytes in Asxl2^-/- over age- and sex-matched wildtype controls, respectively. Approximately 10% of proliferating non-cardiomyocytes in the Asxl2^-/- heart express the cardiogenic marker NKX2-5, a frequency that is ~7-fold higher than that observed in the wildtype. EdU lineage tracing experiments showed that ~6% of pulsed-labeled non-cardiomyocytes in Asxl2^-/- hearts differentiate into mature cardiomyocytes after a four-week chase, a phenomenon not observed for similarly pulse-chased wildtype controls. Taken together, these data indicate de novo cardiomyocyte production in the Asxl2^-/- heart due to activation of a population of proliferative cardiogenic non-cardiomyocytes. Our study suggests the existence of an epigenetic barrier to cardiogenicity in the adult heart and raises the intriguing possibility of unlocking regenerative potential via transient modulation of epigenetic activity.

Decellularized zebrafish cardiac extracellular matrix induces mammalian heart regeneration

Heart attack is a global health problem that leads to significant morbidity, mortality, and health care burden. Adult human hearts have very limited regenerative capability after injury. However, evolutionarily primitive species generally have higher regenerative capacity than mammals. The extracellular matrix (ECM) may contribute to this difference. Mammalian cardiac ECM may not be optimally inductive for cardiac regeneration because of the fibrotic, instead of regenerative, responses in injured adult mammalian hearts. Given the high regenerative capacity of adult zebrafish hearts, we hypothesize that decellularized zebrafish cardiac ECM (zECM) made from normal or healing hearts can induce mammalian heart regeneration. Using zebrafish and mice as representative species of lower vertebrates and mammals, we show that a single administration of zECM, particularly the healing variety, enables cardiac functional recovery and regeneration of adult mouse heart tissues after acute myocardial infarction. zECM-treated groups exhibit proliferation of the remaining cardiomyocytes and multiple cardiac precursor cell populations and reactivation of ErbB2 expression in cardiomyocytes. Furthermore, zECM exhibits pro-proliferative and chemotactic effects on human cardiac precursor cell populations in vitro. These contribute to the structural preservation and correlate with significantly higher cardiac contractile function, notably less left ventricular dilatation, and substantially more elastic myocardium in zECM-treated hearts than control animals treated with saline or decellularized adult mouse cardiac ECM. Inhibition of ErbB2 activity abrogates beneficial effects of zECM administration, indicating the possible involvement of ErbB2 signaling in zECM-mediated regeneration. This study departs from conventional focuses on mammalian ECM and introduces a new approach for cardiac tissue regeneration.

CD13 and ROR2 Permit Isolation of Highly Enriched Cardiac Mesoderm from Differentiating Human Embryonic Stem Cells

The generation of tissue-specific cell types from human embryonic stem cells (hESCs) is critical for the development of future stem cell-based regenerative therapies. Here, we identify CD13 and ROR2 as cell-surface markers capable of selecting early cardiac mesoderm emerging during hESC differentiation. We demonstrate that the CD13+/ROR2+ population encompasses pre-cardiac mesoderm, which efficiently differentiates to all major cardiovascular lineages. We determined the engraftment potential of CD13+/ROR2+ in small (murine) and large (porcine) animal models, and demonstrated that CD13+/ROR2+ progenitors have the capacity to differentiate toward cardiomyocytes, fibroblasts, smooth muscle, and endothelial cells in vivo. Collectively, our data show that CD13 and ROR2 identify a cardiac lineage precursor pool that is capable of successful engraftment into the porcine heart. These markers represent valuable tools for further dissection of early human cardiac differentiation, and will enable a detailed assessment of human pluripotent stem cell-derived cardiac lineage cells for potential clinical applications.

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CD13 and ROR2 separate hESC-derived MIXL1+ mesoderm from MIXL1+ endoderm

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CD13 and ROR2 select for a population of highly enriched pre-cardiac mesoderm

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CD13+/ROR2+ cells derived from hESCs engraft into porcine, but not murine hearts

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CD13+/ROR2+ cells differentiate to all major cardiac lineages in the pig heart

Innate heart regeneration: endogenous cellular sources and exogenous therapeutic amplification

INTRODUCTION

The -once viewed as heretical- concept of the adult mammalian heart as a dynamic organ capable of endogenous regeneration has recently gained traction. However, estimated rates of myocyte turnover vary wildly and the underlying mechanisms of cardiac plasticity remain controversial. It is still unclear whether the adult mammalian heart gives birth to new myocytes through proliferation of resident myocytes, through cardiomyogenic differentiation of endogenous progenitors or through both mechanisms.

AREAS COVERED

In this review, the authors discuss the cellular origins of postnatal mammalian cardiomyogenesis and touch upon therapeutic strategies that could potentially amplify innate cardiac regeneration.

EXPERT OPINION

The adult mammalian heart harbors a limited but detectable capacity for spontaneous endogenous regeneration. During normal aging, proliferation of pre-existing cardiomyocytes is the dominant mechanism for generation of new cardiomyocytes. Following myocardial injury, myocyte proliferation increases modestly, but differentiation of endogenous progenitor cells appears to also contribute to cardiomyogenesis (although agreement on the latter point is not universal). Since cardiomyocyte deficiency underlies almost all types of heart disease, development of therapeutic strategies that amplify endogenous regeneration to a clinically-meaningful degree is of utmost importance.

Building and re-building the heart by cardiomyocyte proliferation

The adult human heart does not regenerate significant amounts of lost tissue after injury. Rather than making new, functional muscle, human hearts are prone to scarring and hypertrophy, which can often lead to fatal arrhythmias and heart failure. The most-cited basis of this ineffective cardiac regeneration in mammals is the low proliferative capacity of adult cardiomyocytes. However, mammalian cardiomyocytes can avidly proliferate during fetal and neonatal development, and both adult zebrafish and neonatal mice can regenerate cardiac muscle after injury, suggesting that latent regenerative potential exists. Dissecting the cellular and molecular mechanisms that promote cardiomyocyte proliferation throughout life, deciphering why proliferative capacity normally dissipates in adult mammals, and deriving means to boost this capacity are primary goals in cardiovascular research. Here, we review our current understanding of how cardiomyocyte proliferation is regulated during heart development and regeneration.

Stimulatory Effects of Mesenchymal Stem Cells on cKit+ Cardiac Stem Cells Are Mediated by SDF1/CXCR4 and SCF/cKit Signaling Pathways

RATIONALE

Culture-expanded cells originating from cardiac tissue that express the cell surface receptor cKit are undergoing clinical testing as a cell source for heart failure and congenital heart disease. Although accumulating data support that mesenchymal stem cells (MSCs) enhance the efficacy of cardiac cKit(+) cells (CSCs), the underlying mechanism for this synergistic effect remains incompletely understood.

OBJECTIVE

To test the hypothesis that MSCs stimulate endogenous CSCs to proliferate, migrate, and differentiate via the SDF1/CXCR4 and stem cell factor/cKit pathways.

METHODS AND RESULTS

Using genetic lineage-tracing approaches, we show that in the postnatal murine heart, cKit(+) cells proliferate, migrate, and form cardiomyocytes, but not endothelial cells. CSCs exhibit marked chemotactic and proliferative responses when cocultured with MSCs but not with cardiac stromal cells. Antagonism of the CXCR4 pathway with AMD3100 (an SDF1/CXCR4 antagonist) inhibited MSC-induced CSC chemotaxis but stimulated CSC cardiomyogenesis (P<0.0001). Furthermore, MSCs enhanced CSC proliferation via the stem cell factor/cKit and SDF1/CXCR4 pathways (P<0.0001).

CONCLUSIONS

Together these findings show that MSCs exhibit profound, yet differential, effects on CSC migration, proliferation, and differentiation and suggest a mechanism underlying the improved cardiac regeneration associated with combination therapy using CSCs and MSCs. These findings have important therapeutic implications for cell-based therapy strategies that use mixtures of CSCs and MSCs.

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.

Murine Models Demonstrate Distinct Vasculogenic and Cardiomyogenic cKit+ Lineages in the Heart

After 2 recent genetic studies in mice addressing the developmental origins and regenerative activity of cardiac cKit+ cells, 2 additional reports by Sultana et al and Liu et al provide further information on the expression of cKit in the embryonic and adult hearts. Here, we synthesize the findings from the 4 distinct cKit models to gain insights into the biology of this important cell type.

Multipotency and cardiomyogenic potential of human adipose-derived stem cells from epicardium, pericardium, and omentum

Background

Acute myocardial infarction (MI) leads to an irreversible loss of proper cardiac function. Application of stem cell therapy is an attractive option for MI treatment. Adipose tissue has proven to serve as a rich source of stem cells (ADSCs). Taking into account the different morphogenesis, anatomy, and physiology of adipose tissue, we hypothesized that ADSCs from different adipose tissue depots may exert a diverse multipotency and cardiogenic potential.

Methods

The omental, pericardial, and epicardial adipose tissue samples were obtained from organ donors and patients undergoing heart transplantation at our institution. Human foreskin fibroblasts were used as the control group. Isolated ADSCs were analyzed for adipogenic and osteogenic differentiation capacity and proliferation potential. The immunophenotype and constitutive gene expression of alkaline phosphatase (ALP), GATA4, Nanog, and OCT4 were analyzed. DNA methylation inhibitor 5-azacytidine was exposed to the cells to stimulate the cardiogenesis. Finally, reprogramming towards cardiomyocytes was initiated with exogenous overexpression of seven transcription factors (ESRRG, GATA4, MEF2C, MESP1, MYOCD, TBX5, ZFPM2) previously applied successfully for fibroblast transdifferentiation toward cardiomyocytes. Expression of cardiac troponin T (cTNT) and alpha-actinin (Actn2) was analyzed 3 weeks after initiation of the cardiac differentiation.

Results

The multipotent properties of isolated plastic adherent cells were confirmed with expression of CD29, CD44, CD90, and CD105, as well as successful differentiation toward adipocytes and osteocytes; with the highest osteogenic and adipogenic potential for the epicardial and omental ADSCs, respectively. Epicardial ADSCs demonstrated a lower doubling time as compared with the pericardium and omentum-derived cells. Furthermore, epicardial ADSCs revealed higher constitutive expression of ALP and GATA4. Increased Actn2 and cTNT expression was observed after the transduction of seven reprogramming factors, with the highest expression in the epicardial ADSCs, as compared with the other ADSC subtypes and fibroblasts.

Conclusions

Human epicardial ADSCs revealed a higher cardiomyogenic potential as compared with the pericardial and omental ADSC subtypes as well as the fibroblast counterparts. Epicardial ADSCs may thus serve as the valuable subject for further studies on more effective methods of adult stem cell differentiation toward cardiomyocytes.

Telocytes in exercise-induced cardiac growth

Exercise can induce physiological cardiac growth, which is featured by enlarged cardiomyocyte cell size and formation of new cardiomyocytes. Telocytes (TCs) are a recently identified distinct interstitial cell type, existing in many tissues and organs including heart. TCs have been shown to form a tandem with cardiac stem/progenitor cells in cardiac stem cell niches, participating in cardiac regeneration and repair. Although exercise‐induced cardiac growth has been confirmed as an important way to promote cardiac regeneration and repair, the response of cardiac TCs to exercise is still unclear. In this study, 4 weeks of swimming training was used to induce robust healthy cardiac growth. Exercise can induce an increase in cardiomyocyte cell size and formation of new cardiomyocytes as determined by Wheat Germ Lectin and EdU staining respectively. TCs were identified by three immunofluorescence stainings including double labelling for CD34/vimentin, CD34/platelet‐derived growth factor (PDGF) receptor‐α and CD34/PDGF receptor‐β. We found that cardiac TCs were significantly increased in exercised heart, suggesting that TCs might help control the activity of cardiac stem/progenitor cells, cardiomyocytes or endothelial cells. Adding cardiac TCs might help promote cardiac regeneration and renewal.

Towards regenerating the mammalian heart: challenges in evaluating experimentally induced adult mammalian cardiomyocyte proliferation

In recent years, there has been a dramatic increase in research aimed at regenerating the mammalian heart by promoting endogenous cardiomyocyte proliferation. Despite many encouraging successes, it remains unclear if we are any closer to achieving levels of mammalian cardiomyocyte proliferation for regeneration as seen during zebrafish regeneration. Furthermore, current cardiac regenerative approaches do not clarify whether the induced cardiomyocyte proliferation is an epiphenomena or responsible for the observed improvement in cardiac function. Moreover, due to the lack of standardized protocols to determine cardiomyocyte proliferation in vivo, it remains unclear if one mammalian regenerative factor is more effective than another. Here, we discuss current methods to identify and evaluate factors for the induction of cardiomyocyte proliferation and challenges therein. Addressing challenges in evaluating adult cardiomyocyte proliferation will assist in determining 1) which regenerative factors should be pursued in large animal studies; 2) if a particular level of cell cycle regulation presents a better therapeutic target than another (e.g., mitogenic receptors vs. cyclins); and 3) which combinatorial approaches offer the greatest likelihood of success. As more and more regenerative studies come to pass, progress will require a system that not only can evaluate efficacy in an objective manner but can also consolidate observations in a meaningful way.

No Evidence for Cardiomyocyte Number Expansion in Preadolescent Mice

The magnitude of cardiomyocyte generation in the adult heart has been heavily debated. A recent report suggests that during mouse preadolescence, cardiomyocyte proliferation leads to a 40% increase in the number of cardiomyocytes. Such an expansion would change our understanding of heart growth and have far-reaching implications for cardiac regeneration. Here, using design-based stereology, we found that cardiomyocyte proliferation accounted for 30% of postnatal DNA synthesis; however, we were unable to detect any changes in cardiomyocyte number after postnatal day 11. (15)N-thymidine and BrdU analyses provided no evidence for a proliferative peak in preadolescent mice. By contrast, cardiomyocyte multinucleation comprises 57% of postnatal DNA synthesis, followed by cardiomyocyte nuclear polyploidisation, contributing with 13% to DNA synthesis within the second and third postnatal weeks. We conclude that the majority of cardiomyocytes is set within the first postnatal week and that this event is followed by two waves of non-replicative DNA synthesis. This Matters Arising paper is in response to Naqvi et al. (2014), published in Cell. See also the associated Correspondence by Soonpaa et al. (2015), and the response by Naqvi et al. (2015), published in this issue.

Ionizing radiation and heart risks

Cardiovascular disease and cancer are the two leading causes of morbidity and mortality worldwide. As advancements in radiation therapy (RT) have significantly increased the number of cancer survivors, the risk of radiation-induced cardiovascular disease (RICD) in this group is a growing concern. Recent epidemiological data suggest that accidental or occupational exposure to low dose radiation, in addition to therapeutic ionizing radiation, can result in cardiovascular complications. The progression of radiation-induced cardiotoxicity often takes years to manifest but is also multifaceted, as the heart may be affected by a variety of pathologies. The risk of cardiovascular disease development in RT cancer survivors has been known for 40 years and several risk factors have been identified in the last two decades. However, most of the early work focused on clinical symptoms and manifestations, rather than understanding cellular processes regulating homeostatic processes of the cardiovascular system in response to radiation. Recent studies have suggested that a different approach may be needed to refute the risk of cardiovascular disease following radiation exposure. In this review, we will focus on how different radiation types and doses may induce cardiovascular complications, highlighting clinical manifestations and the mechanisms involved in the pathophysiology of radiation-induced cardiotoxicity. We will finally discuss how current and future research on heart development and homeostasis can help reduce the incidence of RICD.

Effect of Remote Ischaemic Conditioning in Oncology Patients Undergoing Chemotherapy: Rationale and Design of the ERIC-ONC Study--A Single-Center, Blinded, Randomized Controlled Trial

Cancer survival continues to improve, and thus cardiovascular consequences of chemotherapy are increasingly important determinants of long‐term morbidity and mortality. Conventional strategies to protect the heart from chemotherapy have important hemodynamic or myelosuppressive side effects. Remote ischemic conditioning (RIC) using intermittent limb ischemia‐reperfusion reduces myocardial injury in the setting of percutaneous coronary intervention. Anthracycline cardiotoxicity and ischemia‐reperfusion injury share common biochemical pathways in cardiomyocytes. The potential for RIC as a novel treatment to reduce subclinical myocyte injury in chemotherapy has never been explored and will be investigated in the Effect of Remote Ischaemic Conditioning in Oncology (ERIC‐ONC) trial (clinicaltrials.gov NCT 02471885). The ERIC‐ONC trial is a single‐center, blinded, randomized, sham‐controlled study. We aim to recruit 128 adult oncology patients undergoing anthracycline‐based chemotherapy treatment, randomized in a 1:1 ratio into 2 groups: (1) sham procedure or (2) RIC, comprising 4, 5‐minute cycles of upper arm blood pressure cuff inflations and deflations, immediately before each cycle of chemotherapy. The primary outcome measure, defining cardiac injury, will be high‐sensitivity troponin‐T over 6 cycles of chemotherapy and 12 months follow‐up. Secondary outcome measures will include clinical, electrical, structural, and biochemical endpoints comprising major adverse cardiovascular clinical events, incidence of cardiac arrhythmia over 14 days at cycle 5/6, echocardiographic ventricular function, N‐terminal pro‐brain natriuretic peptide levels at 3 months follow‐up, and changes in mitochondrial DNA, micro‐RNA, and proteomics after chemotherapy. The ERIC‐ONC trial will determine the efficacy of RIC as a novel, noninvasive, nonpharmacological, low‐cost cardioprotectant in cancer patients undergoing anthracycline‐based chemotherapy.

Pharmacological Therapy in the Heart as an Alternative to Cellular Therapy: A Place for the Brain Natriuretic Peptide?

The discovery that stem cells isolated from different organs have the ability to differentiate into mature beating cardiomyocytes has fostered considerable interest in developing cellular regenerative therapies to treat cardiac diseases associated with the loss of viable myocardium. Clinical studies evaluating the potential of stem cells (from heart, blood, bone marrow, skeletal muscle, and fat) to regenerate the myocardium and improve its functional status indicated that although the method appeared generally safe, its overall efficacy has remained modest. Several issues raised by these studies were notably related to the nature and number of injected cells, as well as the route and timing of their administration, to cite only a few. Besides the direct administration of cardiac precursor cells, a distinct approach to cardiac regeneration could be based upon the stimulation of the heart's natural ability to regenerate, using pharmacological approaches. Indeed, differentiation and/or proliferation of cardiac precursor cells is controlled by various endogenous mediators, such as growth factors and cytokines, which could thus be used as pharmacological agents to promote regeneration. To illustrate such approach, we present recent results showing that the exogenous administration of the natriuretic peptide BNP triggers “endogenous” cardiac regeneration, following experimental myocardial infarction.

Heart regeneration

Regenerating an injured heart holds great promise for millions of patients suffering from heart diseases. Since the human heart has very limited regenerative capacity, this is a challenging task. Numerous strategies aiming to improve heart function have been developed. In this review we focus on approaches intending to replace damaged heart muscle by new cardiomyocytes. Different strategies for the production of cardiomyocytes from human embryonic stem cells or human induced pluripotent stem cells, by direct reprogramming and induction of cardiomyocyte proliferation are discussed regarding their therapeutic potential and respective advantages and disadvantages. Furthermore, different methods for the transplantation of pluripotent stem cell-derived cardiomyocytes are described and their clinical perspectives are discussed. 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.

Personalizing cardiac regenerative therapy: At the heart of Pim1 kinase

During cardiac aging, DNA damage and environmental stressors contribute to telomeric shortening and human cardiac progenitor cells acquire a senescent phenotype that leads to decreased stem cell function. Reversion of this phenotype through genetic modification is essential to advance regenerative therapy. Studies in the cardiac specific overexpression and subcellular targeting of Pim1 kinase demonstrate its influence on regeneration, proliferation, survival, metabolism and senescence. The cardioprotective effects of Pim1 modification can be picked apart and enhanced by targeting the kinase to distinct subcellular compartments, allowing for selection of specific phenotypic traits after molecular modification. In this perspective, we examine the therapeutic implications of Pim1 to encourage the personalization of cardiac regenerative therapy.

Cardiac Regeneration and Stem Cells

After decades of believing the heart loses the ability to regenerate soon after birth, numerous studies are now reporting that the adult heart may indeed be capable of regeneration, although the magnitude of new cardiac myocyte formation varies greatly. While this debate has energized the field of cardiac regeneration and led to a dramatic increase in our understanding of cardiac growth and repair, it has left much confusion in the field as to the prospects of regenerating the heart. Studies applying modern techniques of genetic lineage tracing and carbon-14 dating have begun to establish limits on the amount of endogenous regeneration after cardiac injury, but the underlying cellular mechanisms of this regeneration remained unclear. These same studies have also revealed an astonishing capacity for cardiac repair early in life that is largely lost with adult differentiation and maturation. Regardless, this renewed focus on cardiac regeneration as a therapeutic goal holds great promise as a novel strategy to address the leading cause of death in the developed world.

Cardioprotective Actions of TGFβRI Inhibition Through Stimulating Autocrine/Paracrine of Survivin and Inhibiting Wnt in Cardiac Progenitors

Heart failure due to myocardial infarction (MI) is a major cause of morbidity and mortality in the world. We found previously that A83-01, a TGFβRI inhibitor, could facilitate cardiac repair in post-MI mice and induce the expansion of a Nkx2.5 + cardiomyoblast population. This study aimed to investigate the key autocrine/paracrine factors regulated by A83-01 in the injured heart and the mechanism of cardioprotection by this molecule. Using a previously described transgenic Nkx2.5 enhancer-green fluorescent protein (GFP) reporter mice, we isolated cardiac progenitor cells (CPC) including Nkx2.5-GFP + (Nkx2.5+), sca1+, and Nkx2.5+/sca1 + cells. A83-01 was found to induce proliferation of these three subpopulations mainly through increasing Birc5 expression in the MEK/ERK-dependent pathway. Survivin, encoded by Birc5, could also directly proliferate Nkx2.5 + cells and enhance cultured cardiomyocytes viability. A83-01 could also reverse the downregulation of Birc5 in postinjured mice hearts (n = 6) to expand CPCs. Moreover, the increased Wnt3a in postinjured hearts could decrease CPCs, which could be reversed by A83-01 via inhibiting Fzd6 and Wnt1-induced signaling protein 1 expressions in CPCs. Next, we used inducible αMHC-cre/mTmG mice to label cardiomyocytes with GFP and nonmyocytes with RFP. We found A83-01 preserved more GFP + myocytes (68.6% ± 3.1% vs. 80.9% ± 3.0%; p < .05, n = 6) and fewer renewed RFP + myocytes (0.026% ± 0.005% vs. 0.062% ± 0.008%; p < .05, n = 6) in parallel with less cardiac fibrosis in isoprenaline-injected mice treated with A83-01. TGFβRI inhibition in an injured adult heart could both stimulate the autocrine/paracrine activity of survivin and inhibit Wnt in CPCs to mediate cardioprotection and improve cardiac function.

Acute inflammation stimulates a regenerative response in the neonatal mouse heart

Cardiac injury in neonatal 1-day-old mice stimulates a regenerative response characterized by reactive cardiomyocyte proliferation, which is distinguished from the fibrotic repair process in adults. Acute inflammation occurs immediately after heart injury and has generally been believed to exert a negative effect on heart regeneration by promoting scar formation in adults; however, little is known about the role of acute inflammation in the cardiac regenerative response in neonatal mice. Here, we show that acute inflammation induced cardiomyocyte proliferation after apical intramyocardial microinjection of immunogenic zymosan A particles into the neonatal mouse heart. We also found that cardiac injury-induced regenerative response was suspended after immunosuppression in neonatal mice, and that cardiomyocytes could not be reactivated to proliferate after neonatal heart injury in the absence of interleukin-6 (IL-6). Furthermore, cardiomyocyte-specific deletion of signal transducer and activator of transcription 3 (STAT3), the major downstream effector of IL-6 signaling, decreased reactive cardiomyocyte proliferation after apical resection. Our results indicate that acute inflammation stimulates the regenerative response in neonatal mouse heart, and suggest that modulation of inflammatory signals might have important implications in cardiac regenerative medicine.

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.

A Murine Myh6MerCreMer Knock-In Allele Specifically Mediates Temporal Genetic Deletion in Cardiomyocytes after Tamoxifen Induction

A mouse model that mediates temporal, specific, and efficient myocardial deletion with Cre-LoxP technology will be a valuable tool to determine the function of genes during heart formation. Mhy6 encodes a cardiac muscle specific protein: alpha-myosin heavy chain. Here, we generated a new Myh6-MerCreMer (Myh6(MerCreMer/+)) inducible Cre knock-in mouse by inserting a MerCreMer cassette into the Myh6 start codon. By crossing knock-in mice with Rosa26 reporter lines, we found the Myh6(MerCreMer/+) mice mediate complete Cre-LoxP recombination in cardiomyocytes after tamoxifen induction. X-gal staining and immunohistochemistry analysis revealed that Myh6-driven Cre recombinase was specifically activated in cardiomyocytes at embryonic and adult stages. Furthermore, echocardiography showed that Myh6(MerCreMer/+) mice maintained normal cardiac structure and function before and after tamoxifen administration. These results suggest that the new Myh6(MerCreMer/+) mouse can serve as a robust tool to dissect the roles of genes in heart development and function. Additionally, myocardial progeny during heart development and after cardiac injury can be traced using this mouse line.

Wnt/β-catenin signaling in heart regeneration

The ability to repair damaged or lost tissues varies significantly among vertebrates. The regenerative ability of the heart is clinically very relevant, because adult teleost fish and amphibians can regenerate heart tissue, but we mammals cannot. Interestingly, heart regeneration is possible in neonatal mice, but this ability is lost within 7 days after birth. In zebrafish and neonatal mice, lost cardiomyocytes are regenerated via proliferation of spared, differentiated cardiomyocytes. While some cardiomyocyte turnover occurs in adult mammals, the cardiomyocyte production rate is too low in response to injury to regenerate the heart. Instead, mammalian hearts respond to injury by remodeling of spared tissue, which includes cardiomyocyte hypertrophy. Wnt/β-catenin signaling plays important roles during vertebrate heart development, and it is re-activated in response to cardiac injury. In this review, we discuss the known functions of this signaling pathway in injured hearts, its involvement in cardiac fibrosis and hypertrophy, and potential therapeutic approaches that might promote cardiac repair after injury by modifying Wnt/β-catenin signaling. Regulation of cardiac remodeling by this signaling pathway appears to vary depending on the injury model and the exact stages that have been studied. Thus, conflicting data have been published regarding a potential role of Wnt/β-catenin pathway in promotion of fibrosis and cardiomyocyte hypertrophy. In addition, the Wnt inhibitory secreted Frizzled-related proteins (sFrps) appear to have Wnt-dependent and Wnt-independent roles in the injured heart. Thus, while the exact functions of Wnt/β-catenin pathway activity in response to injury still need to be elucidated in the non-regenerating mammalian heart, but also in regenerating lower vertebrates, manipulation of the pathway is essential for creation of therapeutically useful cardiomyocytes from stem cells in culture. Hopefully, a detailed understanding of the in vivo role of Wnt/β-catenin signaling in injured mammalian and non-mammalian hearts will also contribute to the success of current efforts towards developing regenerative therapies.

Translational aspects of cardiac cell therapy

Cell therapy has been intensely studied for over a decade as a potential treatment for ischaemic heart disease. While initial trials using skeletal myoblasts, bone marrow cells and peripheral blood stem cells showed promise in improving cardiac function, benefits were found to be short-lived likely related to limited survival and engraftment of the delivered cells. The discovery of putative cardiac ‘progenitor’ cells as well as the creation of induced pluripotent stem cells has led to the delivery of cells potentially capable of electromechanical integration into existing tissue. An alternative strategy involving either direct reprogramming of endogenous cardiac fibroblasts or stimulation of resident cardiomyocytes to regenerate new myocytes can potentially overcome the limitations of exogenous cell delivery. Complimentary approaches utilizing combination cell therapy and bioengineering techniques may be necessary to provide the proper milieu for clinically significant regeneration. Clinical trials employing bone marrow cells, mesenchymal stem cells and cardiac progenitor cells have demonstrated safety of catheter based cell delivery, with suggestion of limited improvement in ventricular function and reduction in infarct size. Ongoing trials are investigating potential benefits to outcome such as morbidity and mortality. These and future trials will clarify the optimal cell types and delivery conditions for therapeutic effect.

Hypoxia fate mapping identifies cycling cardiomyocytes in the adult heart

Although the adult mammalian heart is incapable of meaningful functional recovery following substantial cardiomyocyte loss, it is now clear that modest cardiomyocyte turnover occurs in adult mouse and human hearts, mediated primarily by proliferation of pre-existing cardiomyocytes. However, fate mapping of these cycling cardiomyocytes has not been possible thus far owing to the lack of identifiable genetic markers. In several organs, stem or progenitor cells reside in relatively hypoxic microenvironments where the stabilization of the hypoxia-inducible factor 1 alpha (Hif-1α) subunit is critical for their maintenance and function. Here we report fate mapping of hypoxic cells and their progenies by generating a transgenic mouse expressing a chimaeric protein in which the oxygen-dependent degradation (ODD) domain of Hif-1α is fused to the tamoxifen-inducible CreERT2 recombinase. In mice bearing the creERT2-ODD transgene driven by either the ubiquitous CAG promoter or the cardiomyocyte-specific α myosin heavy chain promoter, we identify a rare population of hypoxic cardiomyocytes that display characteristics of proliferative neonatal cardiomyocytes, such as smaller size, mononucleation and lower oxidative DNA damage. Notably, these hypoxic cardiomyocytes contributed widely to new cardiomyocyte formation in the adult heart. These results indicate that hypoxia signalling is an important hallmark of cycling cardiomyocytes, and suggest that hypoxia fate mapping can be a powerful tool for identifying cycling cells in adult mammals.

Generation of cardiac progenitor cells through epicardial to mesenchymal transition

The epithelial to mesenchymal transition (EMT) is a biological process that drives the formation of cells involved both in tissue repair and in pathological conditions, including tissue fibrosis and tumor metastasis by providing cancer cells with stem cell properties. Recent findings suggest that EMT is reactivated in the heart following ischemic injury. Specifically, epicardial EMT might be involved in the formation of cardiac progenitor cells (CPCs) that can differentiate into endothelial cells, smooth muscle cells, and, possibly, cardiomyocytes. The identification of mechanisms and signaling pathways governing EMT-derived CPC generation and differentiation may contribute to the development of a more efficient regenerative approach for adult heart repair. Here, we summarize key literature in the field.

Arrhythmia in stem cell transplantation

Stem cell regenerative therapies hold promise for treating diseases across the spectrum of medicine. While significant progress has been made in the preclinical stages, the clinical application of cardiac cell therapy is limited by technical challenges. Certain methods of cell delivery, such as intramyocardial injection, carry a higher rate of arrhythmias. Other potential contributors to the arrhythmogenicity of cell transplantation include reentrant pathways caused by heterogeneity in conduction velocities between graft and host as well as graft automaticity. In this article, the arrhythmogenic potential of cell delivery to the heart is discussed.

An emerging consensus on cardiac regeneration

Cardiac regeneration is a rapidly evolving and controversial field of research. The identification some 12 years ago of progenitor cells that reside within the heart spurred enthusiasm for cell-based regenerative therapies. However, recent evidence has called into question both the presence of a biologically important stem cell population in the heart and the ability of exogenously derived cells to promote regeneration through direct formation of new cardiomyocytes. Here, we discuss recent developments that suggest an emerging consensus on the ability of different cell types to regenerate the adult mammalian heart.

Using exercise to measure and modify cardiac function

Exercise is the archetype of physiologic demands placed on the cardiovascular system. Acute responses provide an informative assessment of cardiovascular function and fitness, while repeated exercise promotes cardiovascular health and evokes important molecular, structural, and functional changes contributing to its effects in primary and secondary prevention. Here we examine the use of exercise in murine models, both as a phenotypic assay and as a provocative intervention. We first review the advantages and limitations of exercise testing for assessing cardiac function, then highlight the cardiac structural and cellular changes elicited by chronic exercise and key molecular pathways that mediate these effects.

How to make a cardiomyocyte

During development, cardiogenesis is orchestrated by a family of heart progenitors that build distinct regions of the heart. Each region contains diverse cell types that assemble to form the complex structures of the individual cardiac compartments. Cardiomyocytes are the main cell type found in the heart and ensure contraction of the chambers and efficient blood flow throughout the body. Injury to the cardiac muscle often leads to heart failure due to the loss of a large number of cardiomyocytes and its limited intrinsic capacity to regenerate the damaged tissue, making it one of the leading causes of morbidity and mortality worldwide. In this Primer we discuss how insights into the molecular and cellular framework underlying cardiac development can be used to guide the in vitro specification of cardiomyocytes, whether by directed differentiation of pluripotent stem cells or via direct lineage conversion. Additional strategies to generate cardiomyocytes in situ, such as reactivation of endogenous cardiac progenitors and induction of cardiomyocyte proliferation, will also be discussed.