Adolescent feline heart contains a population of small, proliferative ventricular myocytes with immature physiological properties

Protein Kinase A Is a Master Regulator of Physiological and Pathological Cardiac Hypertrophy

BACKGROUND

The sympathoadrenergic system and its major effector PKA (protein kinase A) are activated to maintain cardiac output coping with physiological or pathological stressors. If and how PKA plays a role in physiological cardiac hypertrophy (PhCH) and pathological CH (PaCH) are not clear.

METHODS

Transgenic mouse models expressing the PKA inhibition domain (PKAi) of PKA inhibition peptide alpha (PKIalpha)-green fluorescence protein (GFP) fusion protein (PKAi-GFP) in a cardiac-specific and inducible manner (cPKAi) were used to determine the roles of PKA in physiological CH during postnatal growth or induced by swimming, and in PaCH induced by transaortic constriction (TAC) or augmented Ca2+ influx. Kinase profiling was used to determine cPKAi specificity. Echocardiography was used to determine cardiac morphology and function. Western blotting and immunostaining were used to measure protein abundance and phosphorylation. Protein synthesis was assessed by puromycin incorporation and protein degradation by measuring protein ubiquitination and proteasome activity. Neonatal rat cardiomyocytes (NRCMs) infected with AdGFP (GFP adenovirus) or AdPKAi-GFP (PKAi-GFP adenovirus) were used to determine the effects and mechanisms of cPKAi on myocyte hypertrophy. rAAV9.PKAi-GFP was used to treat TAC mice.

RESULTS

(1) cPKAi delayed postnatal cardiac growth and blunted exercise-induced PhCH; (2) PKA was activated in hearts after TAC due to activated sympathoadrenergic system, the loss of endogenous PKIα (PKA inhibition peptide α), and the stimulation by noncanonical PKA activators; (3) cPKAi ameliorated PaCH induced by TAC and increased Ca2+ influxes and blunted neonatal rat cardiomyocyte hypertrophy by isoproterenol and phenylephrine; (4) cPKAi prevented TAC-induced protein synthesis by inhibiting mTOR (mammalian target of rapamycin) signaling through reducing Akt (protein kinase B) activity, but enhancing inhibitory GSK-3α (glycogen synthase kinase-3α) and GSK-3β signals; (5) cPKAi reduced protein degradation by the ubiquitin-proteasome system via decreasing RPN6 phosphorylation; (6) cPKAi increased the expression of antihypertrophic atrial natriuretic peptide (ANP); (7) cPKAi ameliorated established PaCH and improved animal survival.

CONCLUSIONS

Cardiomyocyte PKA is a master regulator of PhCH and PaCH through regulating protein synthesis and degradation. cPKAi can be a novel approach to treat PaCH.

Distinct mononuclear diploid cardiac subpopulation with minimal cell-cell communications persists in embryonic and adult mammalian heart

A small proportion of mononuclear diploid cardiomyocytes (MNDCMs), with regeneration potential, could persist in adult mammalian heart. However, the heterogeneity of MNDCMs and changes during development remains to be illuminated. To this end, 12 645 cardiac cells were generated from embryonic day 17.5 and postnatal days 2 and 8 mice by single-cell RNA sequencing. Three cardiac developmental paths were identified: two switching to cardiomyocytes (CM) maturation with close CM-fibroblast (FB) communications and one maintaining MNDCM status with least CM-FB communications. Proliferative MNDCMs having interactions with macrophages and non-proliferative MNDCMs (non-pMNDCMs) with minimal cell-cell communications were identified in the third path. The non-pMNDCMs possessed distinct properties: the lowest mitochondrial metabolisms, the highest glycolysis, and high expression of Myl4 and Tnni1. Single-nucleus RNA sequencing and immunohistochemical staining further proved that the Myl4+Tnni1+ MNDCMs persisted in embryonic and adult hearts. These MNDCMs were mapped to the heart by integrating the spatial and single-cell transcriptomic data. In conclusion, a novel non-pMNDCM subpopulation with minimal cell-cell communications was unveiled, highlighting the importance of microenvironment contribution to CM fate during maturation. These findings could improve the understanding of MNDCM heterogeneity and cardiac development, thus providing new clues for approaches to effective cardiac regeneration.

RNA-Binding Protein LIN28a Regulates New Myocyte Formation in the Heart Through Long Noncoding RNA-H19

BACKGROUND

Developmental cardiac tissue holds remarkable capacity to regenerate after injury and consists of regenerative mononuclear diploid cardiomyocytes. On maturation, mononuclear diploid cardiomyocytes become binucleated or polyploid and exit the cell cycle. Cardiomyocyte metabolism undergoes a profound shift that coincides with cessation of regeneration in the postnatal heart. However, whether reprogramming metabolism promotes persistence of regenerative mononuclear diploid cardiomyocytes enhancing cardiac function and repair after injury is unknown. Here, we identify a novel role for RNA-binding protein LIN28a, a master regulator of cellular metabolism in cardiac repair after injury.

METHODS

LIN28a overexpression was tested using mouse transgenesis on postnatal cardiomyocyte numbers, cell cycle, and response to apical resection injury. With the use of neonatal and adult cell culture systems and adult and Mosaic Analysis with Double Markers myocardial injury models in mice, the effect of LIN28a overexpression on cardiomyocyte cell cycle and metabolism was tested. Last, isolated adult cardiomyocytes from LIN28a and wild-type mice 4 days after myocardial injury were used for RNA-immunoprecipitation sequencing.

RESULTS

LIN28a was found to be active primarily during cardiac development and rapidly decreases after birth. LIN28a reintroduction at postnatal day (P) 1, P3, P5, and P7 decreased maturation-associated polyploidization, nucleation, and cell size, enhancing cardiomyocyte cell cycle activity in LIN28a transgenic pups compared with wild-type littermates. Moreover, LIN28a overexpression extended cardiomyocyte cell cycle activity beyond P7 concurrent with increased cardiac function 30 days after apical resection. In the adult heart, LIN28a overexpression attenuated cardiomyocyte apoptosis, enhanced cell cycle activity, cardiac function, and survival in mice 12 weeks after myocardial infarction compared with wild-type littermate controls. Instead, LIN28a small molecule inhibitor attenuated the proreparative effects of LIN28a on the heart. Neonatal rat ventricular myocytes overexpressing LIN28a mechanistically showed increased glycolysis, ATP production, and levels of metabolic enzymes compared with control. LIN28a immunoprecipitation followed by RNA-immunoprecipitation sequencing in cardiomyocytes isolated from LIN28a-overexpressing hearts after injury identified long noncoding RNA-H19 as its most significantly altered target. Ablation of long noncoding RNA-H19 blunted LIN28a-induced enhancement on cardiomyocyte metabolism and cell cycle activity.

CONCLUSIONS

Collectively, LIN28a reprograms cardiomyocyte metabolism and promotes persistence of mononuclear diploid cardiomyocytes in the injured heart, enhancing proreparative processes, thereby linking cardiomyocyte metabolism to regulation of ploidy/nucleation and repair in the heart.

Mechanism-Based Cardiac Regeneration Strategies in Mammals

Heart failure in adults is a leading cause of morbidity and mortality worldwide. It can arise from a variety of diseases, with most resulting in a loss of cardiomyocytes that cannot be replaced due to their inability to replicate, as well as to a lack of resident cardiomyocyte progenitor cells in the adult heart. Identifying and exploiting mechanisms underlying loss of developmental cardiomyocyte replicative capacity has proved to be useful in developing therapeutics to effect adult cardiac regeneration. Of course, effective regeneration of myocardium after injury requires not just expansion of cardiomyocytes, but also neovascularization to allow appropriate perfusion and resolution of injury-induced inflammation and interstitial fibrosis, but also reversal of adverse left ventricular remodeling. In addition to overcoming these challenges, a regenerative therapy needs to be safe and easily translatable. Failure to address these critical issues will delay the translation of regenerative approaches. This review critically analyzes current regenerative approaches while also providing a framework for future experimental studies aimed at enhancing success in regenerating the injured heart.

Interaction of the Joining Region in Junctophilin-2 With the L-Type Ca2+ Channel Is Pivotal for Cardiac Dyad Assembly and Intracellular Ca2+ Dynamics

RATIONALE

Ca2+-induced Ca2+ release (CICR) in normal hearts requires close approximation of L-type calcium channels (LTCCs) within the transverse tubules (T-tubules) and RyR (ryanodine receptors) within the junctional sarcoplasmic reticulum. CICR is disrupted in cardiac hypertrophy and heart failure, which is associated with loss of T-tubules and disruption of cardiac dyads. In these conditions, LTCCs are redistributed from the T-tubules to disrupt CICR. The molecular mechanism responsible for LTCCs recruitment to and from the T-tubules is not well known. JPH (junctophilin) 2 enables close association between T-tubules and the junctional sarcoplasmic reticulum to ensure efficient CICR. JPH2 has a so-called joining region that is located near domains that interact with T-tubular plasma membrane, where LTCCs are housed. The idea that this joining region directly interacts with LTCCs and contributes to LTCC recruitment to T-tubules is unknown.

OBJECTIVE

To determine if the joining region in JPH2 recruits LTCCs to T-tubules through direct molecular interaction in cardiomyocytes to enable efficient CICR.

METHODS AND RESULTS

Modified abundance of JPH2 and redistribution of LTCC were studied in left ventricular hypertrophy in vivo and in cultured adult feline and rat ventricular myocytes. Protein-protein interaction studies showed that the joining region in JPH2 interacts with LTCC-α1C subunit and causes LTCCs distribution to the dyads, where they colocalize with RyRs. A JPH2 with induced mutations in the joining region (mutPG1JPH2) caused T-tubule remodeling and dyad loss, showing that an interaction between LTCC and JPH2 is crucial for T-tubule stabilization. mutPG1JPH2 caused asynchronous Ca2+-release with impaired excitation-contraction coupling after β-adrenergic stimulation. The disturbed Ca2+ regulation in mutPG1JPH2 overexpressing myocytes caused calcium/calmodulin-dependent kinase II activation and altered myocyte bioenergetics.

CONCLUSIONS

The interaction between LTCC and the joining region in JPH2 facilitates dyad assembly and maintains normal CICR in cardiomyocytes.

Changes in Cardiomyocyte Cell Cycle and Hypertrophic Growth During Fetal to Adult in Mammals

The failure of adult cardiomyocytes to reproduce themselves to repair an injury results in the development of severe cardiac disability leading to death in many cases. The quest for an understanding of the inability of cardiac myocytes to repair an injury has been ongoing for decades with the identification of various factors which have a temporary effect on cell‐cycle activity. Fetal cardiac myocytes are continuously replicating until the time that the developing fetus reaches a stage of maturity sufficient for postnatal life around the time of birth. Recent reports of the ability for early neonatal mice and pigs to completely repair after the severe injury has stimulated further study of the regulators of the cardiomyocyte cell cycle to promote replication for the remuscularization of injured heart. In all mammals just before or after birth, single‐nucleated hyperplastically growing cardiomyocytes, 1X2N, undergo ≥1 additional DNA replications not followed by cytokinesis, resulting in cells with ≥2 nuclei or as in primates, multiple DNA replications (polyploidy) of 1 nucleus, 2X2(+)N or 1X4(+)N. All further growth of the heart is attributable to hypertrophy of cardiomyocytes. Animal studies ranging from zebrafish with 100% 1X2N cells in the adult to some strains of mice with up to 98% 2X2N cells in the adult and other species with variable ratios of 1X2N and 2X2N cells are reviewed relative to the time of conversion. Various structural, physiologic, metabolic, genetic, hormonal, oxygenation, and other factors that play a key role in the inability of post‐neonatal and adult myocytes to undergo additional cytokinesis are also reviewed.

A low voltage activated Ca2+ current found in a subset of human ventricular myocytes

Low voltage activated (I_Ca-LVA) calcium currents including Cav1.3 and T-type calcium current (I_Ca-T) have not been reported in adult human left ventricular myocytes (HLVMs). We tried to examine their existence and possible correlation with etiology and patient characteristics in a big number of human LVMs isolated from explanted terminally failing (F) hearts, failing hearts with left ventricular assist device (F-LVAD) and nonfailing (NF) human hearts. LVA (I_Ca-LVA) was determined by subtracting L-type Ca²⁺ current (I_Ca-L) recorded with the holding potential of −50 mV from total Ca²⁺ current recorded with the holding potential of −90 mV or −70 mV. I_{Ca- LVA} was further tested with its sensitivity to 100 µM CdCl₂ and tetrodotoxin. Three HLVMs (3 of 137 FHLVMs) from 2 (N = 30 hearts) failing human hearts, of which one was idiopathic and the other was due to primary pulmonary hypertension, were found with I_Ca-LVA. I_Ca-LVA in one FHLVM was not sensitive to 100 µM CdCl₂ while I_Ca-LVA in another two FHLVMs was not sensitive to tetrodotoxin. It peaked at the voltage of −40~-20 mV and had a time-dependent decay faster than I_Ca-L but slower than sodium current (I_Na). I_Ca-LVA was not found in any HLVMs from NF (75 HLVMs from 17 hearts) or F-LVAD hearts (82 HLVMs from 18 hearts) but a statistically significant correlation could not be established. In conclusion, I_Ca-LVA was detected in some HLVMs of a small portion of human hearts that happened to be nonischemic failing hearts.

Telomeres and Telomerase in Heart Ontogenesis, Aging and Regeneration

The main purpose of the review article is to assess the contributions of telomere length and telomerase activity to the cardiac function at different stages of development and clarify their role in cardiac disorders. It has been shown that the telomerase complex and telomeres are of great importance in many periods of ontogenesis due to the regulation of the proliferative capacity of heart cells. The review article also discusses the problems of heart regeneration and the identification of possible causes of dysfunction of telomeres and telomerase.

Reviewing the Limitations of Adult Mammalian Cardiac Regeneration: Noncoding RNAs as Regulators of Cardiomyogenesis

The adult mammalian heart is incapable of regeneration following cardiac injury, leading to a decline in function and eventually heart failure. One of the most evident barriers limiting cardiac regeneration is the inability of cardiomyocytes to divide. It has recently become clear that the mammalian heart undergoes limited cardiomyocyte self-renewal throughout life and is even capable of modest regeneration early after birth. These exciting findings have awakened the goal to promote cardiomyogenesis of the human heart to repair cardiac injury or treat heart failure. We are still far from understanding why adult mammalian cardiomyocytes possess only a limited capacity to proliferate. Identifying the key regulators may help to progress towards such revolutionary therapy. Specific noncoding RNAs control cardiomyocyte division, including well explored microRNAs and more recently emerged long noncoding RNAs. Elucidating their function and molecular mechanisms during cardiomyogenesis is a prerequisite to advance towards therapeutic options for cardiac regeneration. In this review, we present an overview of the molecular basis of cardiac regeneration and describe current evidence implicating microRNAs and long noncoding RNAs in this process. Current limitations and future opportunities regarding how these regulatory mechanisms can be harnessed to study myocardial regeneration will be addressed.

Cardiomyocyte Polyploidy and Implications for Heart Regeneration

In mammals, most cardiomyocytes (CMs) become polyploid (they have more than two complete sets of chromosomes). The purpose of this review is to evaluate assumptions about CM ploidy that are commonly discussed, even if not experimentally demonstrated, and to highlight key issues that are still to be resolved. Topics discussed here include (a) technical and conceptual difficulties in defining a polyploid CM, (b) the candidate role of reactive oxygen as a proximal trigger for the onset of polyploidy, (c) the relationship between polyploidization and other aspects of CM maturation, (d) recent insights related to the regenerative role of the subpopulation of CMs that are not polyploid, and (e) speculations as to why CMs become polyploid at all. New approaches to experimentally manipulate CM ploidy may resolve some of these long-standing and fundamental questions.

Postnatal Cardiac Development and Regenerative Potential in Large Mammals

The neonatal capacity for cardiac regeneration in mice is well studied and has been used to develop many potential strategies for adult cardiac regenerative repair following injury. However, translating these findings from rodents to designing regenerative therapeutics for adult human heart disease remains elusive. Large mammals including pigs, dogs, and sheep are widely used as animal models of humans in preclinical trials of new cardiac drugs and devices. However, very little is known about the fundamental cardiac cell biology and the timing of postnatal cardiac events that influence cardiomyocyte proliferation in these animals. There is emerging evidence that external physiological and environmental cues could be the key to understanding cardiomyocyte proliferative behavior. In this review, we survey available literature on postnatal development in various large mammal models to offer a perspective on the physiological and cellular characteristics that could be regulating cardiomyocyte proliferation. Similarities and differences between developmental milestones, cardiomyocyte maturational events, as well as environmental cues regulating cardiac development, are discussed for various large mammals, with a focus on postnatal cardiac regenerative potential and translatability to the human heart.

Fatty Acid Oxidation Promotes Cardiomyocyte Proliferation Rate but Does Not Change Cardiomyocyte Number in Infant Mice

Cardiomyocyte proliferation accounts for the increase of cardiac muscle during fetal mammalian heart development. Shortly after birth, cardiomyocyte transits from hyperplasia to hypertrophic growth. Here, we have investigated the role of fatty acid β-oxidation in cardiomyocyte proliferation and hypertrophic growth during early postnatal life in mice. A transient wave of increased cell cycle activity of cardiomyocyte was observed between postnatal day 3 and 5, that proceeded as cardiomyocyte hypertrophic growth and maturation. Assessment of cardiomyocyte metabolism in neonatal mouse heart revealed a myocardial metabolic shift from glycolysis to fatty acid β-oxidation that coincided with the burst of cardiomyocyte cell cycle reactivation and hypertrophic growth. Inhibition of fatty acid β-oxidation metabolism in infant mouse heart delayed cardiomyocyte cell cycle exit, hypertrophic growth and maturation. By contrast, pharmacologic and genetic activation of PPARα, a major regulator of cardiac fatty acid metabolism, induced fatty acid β-oxidation and initially promoted cardiomyocyte proliferation rate in infant mice. As the cell cycle proceeded, activation of PPARα-mediated fatty acid β-oxidation promoted cardiomyocytes hypertrophic growth and maturation, which led to cell cycle exit. As a consequence, activation of PPARα-mediated fatty acid β-oxidation did not alter the total number of cardiomyocytes in infant mice. These findings indicate a unique role of fatty acid β-oxidation in regulating cardiomyocyte proliferation and hypertrophic growth in infant mice.

Postnatal undernutrition alters adult female mouse cardiac structure and function leading to limited exercise capacity

KEY POINTS

Impaired growth during fetal life can reprogramme heart development and increase the risk for long-term cardiovascular dysfunction. It is uncertain if the developmental window during which the heart is vulnerable to reprogramming as a result of inadequate nutrition extends into the postnatal period. We found that adult female mice that had been undernourished only from birth to 3 weeks of age had disproportionately smaller hearts compared to males, with thinner ventricle walls and more mononucleated cardiomyocytes. In females, but not males, cardiac diastolic function, and heart rate responsiveness to adrenergic stimulation were limited and maximal exercise capacity was compromised. These data suggest that the developmental window during which the heart is vulnerable to reprogramming by inadequacies in nutrient intake may extend into postnatal life and such individuals could be at increased risk for a cardiac event as a result of strenuous exercise.

ABSTRACT

Adults who experienced undernutrition during critical windows of development are at increased risk for cardiovascular disease. The contribution of cardiac function to this increased disease risk is uncertain. We evaluated the effect of a short episode of postnatal undernutrition on cardiovascular function in mice at the whole animal, organ, and cellular levels. Pups born to control mouse dams were suckled from birth to postnatal day (PN) 21 on dams fed either a control (20% protein) or a low protein (8% protein) isocaloric diet. After PN21 offspring were fed the same control diet until adulthood. At PN70 V ̇ O 2 , max was measured by treadmill test. At PN80 cardiac function was evaluated by echocardiography and Doppler analysis at rest and following β-adrenergic stimulation. Isolated cardiomyocyte nucleation and Ca²⁺ transients (with and without β-adrenergic stimulation) were measured at PN90. Female mice that were undernourished and then refed (PUN), unlike male mice, had disproportionately smaller hearts and their exercise capacity, cardiac diastolic function, and heart rate responsiveness to adrenergic stimulation were limited. A reduced left ventricular end diastolic volume, impaired early filling, and decreased stored energy at the beginning of diastole contributed to these impairments. Female PUN mice had more mononucleated cardiomyocytes; under resting conditions binucleated cells had a functional profile suggestive of increased basal adrenergic activation. Thus, a brief episode of early postnatal undernutrition in the mouse can produce persistent changes to cardiac structure and function that limit exercise/functional capacity and thereby increase the risk for the development of a wide variety of cardiovascular morbidities.

The lifelong impact of fetal growth restriction on cardiac development

BACKGROUND

Maternal nutrient restriction (MNR) is a widespread cause of fetal growth restriction (FGR), an independent predictor of heart disease and cardiovascular mortality. Our objective was to examine the developmental and long-term impact of MNR-induced FGR on cardiac structure in a model that closely mimics human development.

METHODS

A reduction in total caloric intake spanning pregestation through to lactation in guinea pig sows was used to induce FGR. Proliferation, differentiation, and apoptosis of cardiomyocytes were assessed in late-gestation fetal, neonatal, and adult guinea pig hearts. Proteomic analysis and pathway enrichment were performed on fetal hearts.

RESULTS

Cardiomyocyte proliferation and the number of mononucleated cells were enhanced in the MNR-FGR fetal and neonatal heart, suggesting a delay in cardiomyocyte differentiation. In fetal hearts of MNR-FGR animals, apoptosis was markedly elevated and the total number of cardiomyocytes reduced, the latter remaining so throughout neonatal and into adult life. A reduction in total cardiomyocyte number in adult MNR-FGR hearts was accompanied by exaggerated hypertrophy and a disorganized architecture. Pathway analysis identified genes related to cell proliferation, differentiation, and survival.

CONCLUSIONS

FGR influences cardiomyocyte development during critical windows of development, leading to a permanent deficiency in cardiomyocyte number and compensatory hypertrophy in a rodent model that recapitulates human development.

New Myocyte Formation in the Adult Heart: Endogenous Sources and Therapeutic Implications

Death of adult cardiac myocytes and supportive tissues resulting from cardiovascular diseases such as myocardial infarction is the proximal driver of pathological ventricular remodeling that often culminates in heart failure. Unfortunately, no currently available therapeutic barring heart transplantation can directly replenish myocytes lost from the injured heart. For decades, the field has struggled to define the intrinsic capacity and cellular sources for endogenous myocyte turnover in pursuing more innovative therapeutic strategies aimed at regenerating the injured heart. Although controversy persists to this day as to the best therapeutic regenerative strategy to use, a growing consensus has been reached that the very limited capacity for new myocyte formation in the adult mammalian heart is because of proliferation of existing cardiac myocytes but not because of the activity of an endogenous progenitor cell source of some sort. Hence, future therapeutic approaches should take into consideration the fundamental biology of myocyte renewal in designing strategies to potentially replenish these cells in the injured heart.

Caveolae-localized L-type Ca2+ channels do not contribute to function or hypertrophic signalling in the mouse heart

Aims

L-type Ca2+ channels (LTCCs) in adult cardiomyocytes are localized to t-tubules where they initiate excitation-contraction coupling. Our recent work has shown that a subpopulation of LTCCs found at the surface sarcolemma in caveolae of adult feline cardiomyocytes can also generate a Ca2+ microdomain that activates nuclear factor of activated T-cells signaling and cardiac hypertrophy, although the relevance of this paradigm to hypertrophy regulation in vivo has not been examined.

Methods and results

Here we generated heart-specific transgenic mice with a putative caveolae-targeted LTCC activator protein that was ineffective in initiating or enhancing cardiac hypertrophy in vivo. We also generated transgenic mice with cardiac-specific overexpression of a putative caveolae-targeted inhibitor of LTCCs, and while this protein inhibited caveolae-localized LTCCs without effects on global Ca2+ handling, it similarly had no effect on cardiac hypertrophy in vivo. Cardiac hypertrophy was elicited by pressure overload for 2 or 12 weeks or with neurohumoral agonist infusion. Caveolae-specific LTCC activator or inhibitor transgenic mice showed no greater change in nuclear factor of activated T-cells activity after 2 weeks of pressure overload stimulation compared with control mice.

Conclusion

Our results indicate that LTCCs in the caveolae microdomain do not affect cardiac function and are not necessary for the regulation of hypertrophic signaling in the adult mouse heart.

Hypoxic Stress Decreases c-Myc Protein Stability in Cardiac Progenitor Cells Inducing Quiescence and Compromising Their Proliferative and Vasculogenic Potential

Cardiac progenitor cells (CPCs) have been shown to promote cardiac regeneration and improve heart function. However, evidence suggests that their regenerative capacity may be limited in conditions of severe hypoxia. Elucidating the mechanisms involved in CPC protection against hypoxic stress is essential to maximize their cardioprotective and therapeutic potential. We investigated the effects of hypoxic stress on CPCs and found significant reduction in proliferation and impairment of vasculogenesis, which were associated with induction of quiescence, as indicated by accumulation of cells in the G0-phase of the cell cycle and growth recovery when cells were returned to normoxia. Induction of quiescence was associated with a decrease in the expression of c-Myc through mechanisms involving protein degradation and upregulation of p21. Inhibition of c-Myc mimicked the effects of severe hypoxia on CPC proliferation, also triggering quiescence. Surprisingly, these effects did not involve changes in p21 expression, indicating that other hypoxia-activated factors may induce p21 in CPCs. Our results suggest that hypoxic stress compromises CPC function by inducing quiescence in part through downregulation of c-Myc. In addition, we found that c-Myc is required to preserve CPC growth, suggesting that modulation of pathways downstream of it may re-activate CPC regenerative potential under ischemic conditions.

Graphene Sheet-Induced Global Maturation of Cardiomyocytes Derived from Human Induced Pluripotent Stem Cells

Human induced pluripotent stem cells (hiPSCs) can proliferate infinitely. Their ability to differentiate into cardiomyocytes provides abundant sources for disease modeling, drug screening and regenerative medicine. However, hiPSC-derived cardiomyocytes (hiPSC-CMs) display a low degree of maturation and fetal-like properties. Current in vitro differentiation methods do not mimic the structural, mechanical, or physiological properties of the cardiogenesis niche. Recently, we present an efficient cardiac maturation platform that combines hiPSCs monolayer cardiac differentiation with graphene substrate, which is a biocompatible and superconductive material. The hiPSCs lines were successfully maintained on the graphene sheets and were able to differentiate into functional cardiomyocytes. This strategy markedly increased the myofibril ultrastructural organization, elevated the conduction velocity, and enhanced both the Ca²⁺ handling and electrophysiological properties in the absence of electrical stimulation. On the graphene substrate, the expression of connexin 43 increased along with the conduction velocity. Interestingly, the bone morphogenetic proteins signaling was also significantly activated during early cardiogenesis, confirmed by RNA sequencing analysis. Here, we reasoned that graphene substrate as a conductive biomimetic surface could facilitate the intrinsic electrical propagation, mimicking the microenvironment of the native heart, to further promote the global maturation of hiPSC-CMs. Our findings highlight the capability of electrically active substrates to influence cardiomyocyte development. We believe that application of graphene sheets will be useful for simple, fast, and scalable maturation of regenerated cardiomyocytes.

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.

Novel large-particle FACS purification of adult ventricular myocytes reveals accumulation of myosin and actin disproportionate to cell size and proteome in normal post-weaning development

RATIONALE

Quantifying cellular proteins in ventricular myocytes (MCs) is challenging due to tissue heterogeneity and the variety of cell sizes in the heart. In post-weaning cardiac ontogeny, rod-shaped MCs make up the majority of the cardiac mass while remaining a minority of cardiac cells in number. Current biochemical analyses of cardiac proteins do not correlate well the content of MC-specific proteins to cell type or size in normally developing tissue.

OBJECTIVE

To develop a new large-particle fluorescent-activated cell sorting (LP-FACS) strategy for the purification of adult rod-shaped MCs. This approach is developed to enable growth-scaled measurements per-cell of the MC proteome and sarcomeric proteins (i.e. myosin heavy chain (MyHC) and alpha-actin (α-actin)) content.

METHODS AND RESULTS

Individual cardiac cells were isolated from 21 to 94days old mice. An LP-FACS jet-in-air system with a 200-μm nozzle was defined for the first time to purify adult MCs. Cell-type specific immunophenotyping and sorting yielded ≥95% purity of adult MCs independently of cell morphology and size. This approach excluded other cell types and tissue contaminants from further analysis. MC proteome, MyHC and α-actin proteins were measured in linear biochemical assays normalized to cell numbers. Using the allometric coefficient α, we scaled the MC-specific rate of protein accumulation to growth post-weaning. MC-specific volumes (α=1.02) and global protein accumulation (α=0.94) were proportional (i.e. isometric) to body mass. In contrast, MyHC and α-actin accumulated at a much greater rate (i.e. hyperallometric) than body mass (α=1.79 and 2.19 respectively) and MC volumes (α=1.76 and 1.45 respectively).

CONCLUSION

Changes in MC proteome and cell volumes measured in LP-FACS purified MCs are proportional to body mass post-weaning. Oppositely, MyHC and α-actin are concentrated more rapidly than what would be expected from MC proteome accumulation, cell enlargement, or animal growth alone. LP-FACS provides a new standard for adult MC purification and an approach to scale the biochemical content of specific proteins or group of proteins per cell in enlarging MCs.

Dedifferentiation, Proliferation, and Redifferentiation of Adult Mammalian Cardiomyocytes After Ischemic Injury

BACKGROUND

Adult mammalian hearts have a limited ability to generate new cardiomyocytes. Proliferation of existing adult cardiomyocytes (ACMs) is a potential source of new cardiomyocytes. Understanding the fundamental biology of ACM proliferation could be of great clinical significance for treating myocardial infarction (MI). We aim to understand the process and regulation of ACM proliferation and its role in new cardiomyocyte formation of post-MI mouse hearts.

METHODS

β-Actin-green fluorescent protein transgenic mice and fate-mapping Myh6-MerCreMer-tdTomato/lacZ mice were used to trace the fate of ACMs. In a coculture system with neonatal rat ventricular myocytes, ACM proliferation was documented with clear evidence of cytokinesis observed with time-lapse imaging. Cardiomyocyte proliferation in the adult mouse post-MI heart was detected by cell cycle markers and 5-ethynyl-2-deoxyuridine incorporation analysis. Echocardiography was used to measure cardiac function, and histology was performed to determine infarction size.

RESULTS

In vitro, mononucleated and bi/multinucleated ACMs were able to proliferate at a similar rate (7.0%) in the coculture. Dedifferentiation proceeded ACM proliferation, which was followed by redifferentiation. Redifferentiation was essential to endow the daughter cells with cardiomyocyte contractile function. Intercellular propagation of Ca²⁺ from contracting neonatal rat ventricular myocytes into ACM daughter cells was required to activate the Ca²⁺-dependent calcineurin-nuclear factor of activated T-cell signaling pathway to induce ACM redifferentiation. The properties of neonatal rat ventricular myocyte Ca²⁺ transients influenced the rate of ACM redifferentiation. Hypoxia impaired the function of gap junctions by dephosphorylating its component protein connexin 43, the major mediator of intercellular Ca²⁺ propagation between cardiomyocytes, thereby impairing ACM redifferentiation. In vivo, ACM proliferation was found primarily in the MI border zone. An ischemia-resistant connexin 43 mutant enhanced the redifferentiation of ACM-derived new cardiomyocytes after MI and improved cardiac function.

CONCLUSIONS

Mature ACMs can reenter the cell cycle and form new cardiomyocytes through a 3-step process: dedifferentiation, proliferation, and redifferentiation. Intercellular Ca²⁺ signal from neighboring functioning cardiomyocytes through gap junctions induces the redifferentiation process. This novel mechanism contributes to new cardiomyocyte formation in post-MI hearts in mammals.

Tetrodotoxin-sensitive Ca2+ Currents, but No T-type Currents in Normal, Hypertrophied, and Failing Mouse Cardiomyocytes

AIMS

To obtain functional evidence that ICa,T is involved in the pathogenesis of cardiac hypertrophy and heart failure. We unexpectedly identified ICa(TTX) rather than ICa,T, therefore, we adjusted our aim to encompass these findings.

METHODS AND RESULTS

We investigated (1) Cav3.1 (α1G) transgenic (Tg) mice compared with nontransgenic (tTA-Ntg); (2) Cav3.1-deficient mice (Cav3.1) compared with wild type (Wt) after chemically and surgically induced cardiac remodeling; and (3) spontaneous hypertensive rats and thoracic aortic constriction (TAC) rats. Whole-cell patch-clamp technique was used to measure ICa in ventricular myocytes. Cav3.1-Tg expressed ICa,T (-18.35 ± 1.02 pA/pF at -40 mV) without signs of compromised cardiac function. While we failed to detect ICa,T after hypertrophic stimuli, instead we demonstrated that both Wt and Cav3.1 mouse exhibit ICa(TTX). Using TAC rats, only 2 of 24 VMs showed ICa,T under our experimental conditions. Without TTX, ICa(TTX) occurred in VMs from Wt, spontaneous hypertensive rats, and TAC rats also.

CONCLUSIONS

These findings demonstrate for the first time that mouse VMs express ICa(TTX). We suggest that future studies should take into consideration the measuring conditions when interpreting ICa,T reappearance in ventricular myocytes in response to hypertrophic stress. Contamination with ICa(TTX) could possibly confuse the relevance of the data.

Bioenergetics of the aging heart and skeletal muscles: Modern concepts and controversies

Age-related alterations in the bioenergetics of the heart and oxidative skeletal muscle tissues are of crucial influence on their performance. Until now the prevailing concept of aging was the mitochondrial theory, the increased production of reactive oxygen species, mediated by deficiency in the activity of respiratory chain complexes. However, studies with mitochondria in situ have presented results which, to some extent, disagree with previous ones, indicating that the mitochondrial theory of aging may be overestimated. The studies reporting age-related decline in mitochondrial function were performed using mainly isolated mitochondria. Measurements on this level are not able to take into account the system level properties. The relevant information can be obtained only from appropriate studies using cells or tissue fibers. The functional interactions between the components of Intracellular Energetic Unit (ICEU) regulate the energy production and consumption in oxidative muscle cells. The alterations of these interactions in ICEU should be studied in order to find a more effective protocol to decelerate the age-related changes taking place in the energy metabolism. In this article, an overview is given of the present theories and controversies of causes of age-related alterations in bioenergetics. Also, branches of study, which need more emphasis, are indicated.

Antiarrhythmic effect of growth factor-supplemented cardiac progenitor cells in chronic infarcted heart

c-Kit(pos) cardiac progenitor cells (CPCs) represent a successful approach in healing the infarcted heart and rescuing its mechanical function, but electrophysiological consequences are uncertain. CPC mobilization promoted by hepatocyte growth factor (HGF) and IGF-1 improved electrogenesis in myocardial infarction (MI). We hypothesized that locally delivered CPCs supplemented with HGF + IGF-1 (GFs) can concur in ameliorating electrical stability of the regenerated heart. Adult male Wistar rats (139 rats) with 4-wk-old MI or sham conditions were randomized to receive intramyocardial injection of GFs, CPCs, CPCs + GFs, or vehicle (V). Enhanced green fluorescent protein-tagged CPCs were used for cell tracking. Vulnerability to stress-induced arrhythmia was assessed by telemetry-ECG. Basic cardiac electrophysiological properties were examined by epicardial multiple-lead recording. Hemodynamic function was measured invasively. Hearts were subjected to anatomical, morphometric, immunohistochemical, and molecular biology analyses. Compared with V and at variance with individual CPCs, CPCs + GFs approximately halved arrhythmias in all animals, restoring cardiac anisotropy toward sham values. GFs alone reduced arrhythmias by less than CPCs + GFs, prolonging ventricular refractoriness without affecting conduction velocity. Concomitantly, CPCs + GFs reactivated the expression levels of Connexin-43 and Connexin-40 as well as channel proteins of key depolarizing and repolarizing ion currents differently than sole GFs. Mechanical function and anatomical remodeling were equally improved by all regenerative treatments, thus exhibiting a divergent behavior relative to electrical aspects. Conclusively, we provided evidence of distinctive antiarrhythmic action of locally injected GF-supplemented CPCs, likely attributable to retrieval of Connexin-43, Connexin-40, and Cav1.2 expression, favoring intercellular coupling and spread of excitation in mended heart.

Persistent increases in Ca(2+) influx through Cav1.2 shortens action potential and causes Ca(2+) overload-induced afterdepolarizations and arrhythmias

Persistent elevation of Ca(2+) influx due to prolongation of the action potential (AP), chronic activation of the β-adrenergic system and molecular remodeling occurs in stressed and diseased hearts. Increases in Ca(2+) influx are usually linked to prolonged myocyte action potentials and arrhythmias. However, the contribution of chronic enhancement of Cav1.2 activity on cardiac electrical remodeling and arrhythmogenicity has not been completely defined and is the subject of this study. Chronically increased Cav1.2 activity was produced with a cardiac specific, inducible double transgenic (DTG) mouse system overexpressing the β2a subunit of Cav (Cavβ2a). DTG myocytes had increased L-type Ca(2+) current (ICa-L), myocyte shortening, and Ca(2+) transients. DTG mice had enhanced cardiac performance, but died suddenly and prematurely. Telemetric electrocardiograms revealed shortened QT intervals in DTG mice. The action potential duration (APD) was shortened in DTG myocytes due to significant increases of potassium currents and channel abundance. However, shortened AP in DTG myocytes did not fully limit excess Ca(2+) influx and increased the peak and tail ICa-L. Enhanced ICa promoted sarcoplasmic reticulum (SR) Ca(2+) overload, diastolic Ca(2+) sparks and waves, and increased NCX activity, causing increased occurrence of early and delayed afterdepolarizations (EADs and DADs) that may contribute to premature ventricular beats and ventricular tachycardia. AV blocks that could be related to fibrosis of the AV node were also observed. Our study suggests that increasing ICa-L does not necessarily result in AP prolongation but causes SR Ca(2+) overload and fibrosis of AV node and myocardium to induce cellular arrhythmogenicity, arrhythmias, and conduction abnormalities.

Revascularization of chronic hibernating myocardium stimulates myocyte proliferation and partially reverses chronic adaptations to ischemia

BACKGROUND

The time course and extent of recovery after revascularization of viable dysfunctional myocardium are variable. Although fibrosis is a major determinant, myocyte structural and molecular remodeling may also play important roles.

OBJECTIVES

This study sought to determine whether persistent myocyte loss and/or irreversibility of protein changes that develop in hibernating myocardium have an impact on functional recovery in the absence of infarction.

METHODS

Swine implanted with a chronic left anterior descending artery (LAD) stenosis to produce hibernating myocardium underwent percutaneous revascularization, with serial functional recovery evaluated for 1 month (n = 12). Myocardial tissue was evaluated to assess myocyte size, nuclear density, and proliferation indexes in comparison with those of normal animals and nonrevascularized controls. Proteomic analysis by 2-dimensional differential in-gel electrophoresis was used to determine the reversibility of molecular adaptations of hibernating myocytes.

RESULTS

At 3 months, physiological features of hibernating myocardium were confirmed, with depressed LAD wall thickening and no significant infarction. Revascularization normalized LAD flow reserve, with no immediate change in LAD wall thickening. Regional LAD wall thickening slowly improved but remained depressed 1 month post-percutaneous coronary intervention. Surprisingly, revascularization was associated with histological evidence of myocytes re-entering the growth phase of the cell cycle and increases in the number of c-Kit(+) cells. Myocyte nuclear density returned to normal, whereas regional myocyte hypertrophy regressed. Proteomic analysis demonstrated heterogeneous effects of revascularization. Up-regulated stress and cytoskeletal proteins normalized, whereas reduced contractile and metabolic proteins persisted.

CONCLUSIONS

Delayed recovery of hibernating myocardium in the absence of scar may reflect persistent reductions in the amounts of contractile and metabolic proteins. Although revascularization appeared to stimulate myocyte proliferation, the persistence of small immature myocytes may have contributed to delayed functional recovery.

MicroRNA induced cardiac reprogramming in vivo: evidence for mature cardiac myocytes and improved cardiac function

RATIONALE

A major goal for the treatment of heart tissue damaged by cardiac injury is to develop strategies for restoring healthy heart muscle through the regeneration and repair of damaged myocardium. We recently demonstrated that administration of a specific combination of microRNAs (miR combo) into the infarcted myocardium leads to direct in vivo reprogramming of noncardiac myocytes to cardiac myocytes. However, the biological and functional consequences of such reprogramming are not yet known.

OBJECTIVE

The aim of this study was to determine whether noncardiac myocytes directly reprogrammed using miRNAs in vivo develop into mature functional cardiac myocytes in situ, and whether reprogramming leads to improvement of cardiac function.

METHODS AND RESULTS

We subjected fibroblast-specific protein 1-Cre mice/tandem dimer Tomato (tdTomato) mice to cardiac injury by permanent ligation of the left anterior descending coronary artery and injected lentiviruses encoding miR combo or a control nontargeting miRNA. miR combo significantly increased the number of reprogramming events in vivo. Five to 6 weeks after injury, morphological and physiological properties of tdTomato(-) and tdTomato(+) cardiac myocyte-like cells were analyzed ex vivo. tdTomato(+) cells expressed cardiac myocyte markers, sarcomeric organization, excitation-contraction coupling, and action potentials characteristic of mature ventricular cardiac myocytes (tdTomato(-) cells). Reprogramming was associated with improvement of cardiac function, as analyzed by serial echocardiography. There was a time delayed and progressive improvement in fractional shortening and other measures of ventricular function, indicating that miR combo promotes functional recovery of damaged myocardium.

CONCLUSIONS

The findings from this study further validate the potential use of miRNA-mediated reprogramming as a therapeutic approach to promote cardiac regeneration after myocardial injury.

Imatinib activates pathological hypertrophy by altering myocyte calcium regulation

BACKGROUND

Imatinib mesylate is a selective tyrosine-kinase inhibitor used in the treatment of multiple cancers, most notably chronic myelogenous leukemia. There is evidence that imatinib can induce cardiotoxicity in cancer patients. Our hypothesis is that imatinib alters calcium regulatory mechanisms and can contribute to development of pathological cardiac hypertrophy.

METHODS AND RESULTS

Neonatal rat ventricular myocytes (NRVMs) were treated with clinical doses (low: 2 μM; high: 5 μM) of imatinib and assessed for molecular changes. Imatinib increased peak systolic Ca(2+) and Ca(2+) transient decay rates and Western analysis revealed significant increases in phosphorylation of phospholamban (Thr-17) and the ryanodine receptor (Ser-2814), signifying activation of calcium/calmodulin-dependent kinase II (CaMKII). Imatinib significantly increased NRVM volume as assessed by Coulter counter, myocyte surface area, and atrial natriuretic peptide abundance seen by Western. Imatinib induced cell death, but did not activate the classical apoptotic program as assessed by caspase-3 cleavage, indicating a necrotic mechanism of death in myocytes. We expressed AdNFATc3-green fluorescent protein in NRVMs and showed imatinib treatment significantly increased nuclear factor of activated T cells translocation that was inhibited by the calcineurin inhibitor FK506 or CaMKII inhibitors.

CONCLUSION

These data show that imatinib can activate pathological hypertrophic signaling pathways by altering intracellular Ca(2+) dynamics. This is likely a contributing mechanism for the adverse cardiac effects of imatinib.

Proliferative activity of cadriomyocytes in chronic hypercholesterolemia

We studied proliferative activity of cardiomyocytes (using proliferation marker Ki-67) and compared it with their total number in the heart under conditions of experimental chronic hypercholesterolemia and its combination with hypothyroidism. It was found that Ki-67-positive cells are primarily located in the subepicardial layer near the heart base in both intact and experimental animals. Replicative cardiomyocyte pool in intact rats constituted 1.67 ± 0.33‰ of the total cardiomyocyte population; after 68-day atherogenic diet with exogenous cholesterol alone, the replicative cardiomyocyte pool decreased by 16 % (to 1.40 ± 0.24‰). Treatment with mercazolil against the background of exogenous cholesterol increased this parameter by 40 % (to 2.33 ± 0.88‰). Changes in replicative activity of cardiomyocytes correlated with their total number in the heart and organ weight. We conclude that replicative cardiomyocyte pool primarily includes non-terminally differentiated cardiomyocytes (small mononuclear cardiomyocytes) and their proliferation maintains the total number of cardiomyocytes in the heart under conditions of cytopathic influences and provides the basis for physiological and reparative regeneration of the myocardium.

GABAergic contributions to alcohol responsivity during adolescence: insights from preclinical and clinical studies

There is a considerable body of literature demonstrating that adolescence is a unique age period, which includes rapid and dramatic maturation of behavioral, cognitive, hormonal and neurobiological systems. Most notably, adolescence is also a period of unique responsiveness to alcohol effects, with both hyposensitivity and hypersensitivity observed to the various effects of alcohol. Multiple neurotransmitter systems are undergoing fine-tuning during this critical period of brain development, including those that contribute to the rewarding effects of drugs of abuse. The role of developmental maturation of the γ-amino-butyric acid (GABA) system, however, has received less attention in contributing to age-specific alcohol sensitivities. This review integrates GABA findings from human magnetic resonance spectroscopy studies as they may translate to understanding adolescent-specific responsiveness to alcohol effects. Better understanding of the vulnerability of the GABA system both during adolescent development, and in psychiatric conditions that include alcohol dependence, could point to a putative mechanism, boosting brain GABA, that may have increased effectiveness for treating alcohol use disorders.

Mouse strain determines cardiac growth potential

Rationale

The extent of heart disease varies from person to person, suggesting that genetic background is important in pathology. Genetic background is also important when selecting appropriate mouse models to study heart disease. This study examines heart growth as a function of strain, specifically C57BL/6 and DBA/2 mouse strains.

Objective

In this study, we test the hypothesis that two strains of mice, C57BL/6 and DBA/2, will produce varying degrees of heart growth in both physiological and pathological settings.

Methods and Results

Differences in heart dimensions are detectable by echocardiography at 8 weeks of age. Percentages of cardiac progenitor cells (c-kit+ cells) and mononucleated cells were found to be in a higher percentage in DBA/2 mice, and more tri- and quad-nucleated cells were in C57BL/6 mice. Cardiomyocyte turnover shows no significant changes in mitotic activity, however, there is more apoptotic activity in DBA/2 mice. Cardiomyocyte cell size increased with age, but increased more in DBA/2 mice, although percentages of nucleated cells remained the same in both strains. Two-week isoproterenol stimulation showed an increase in heart growth in DBA/2 mice, both at cardiomyocyte and whole heart level. In isoproterenol-treated DBA/2 mice, there was also a greater expression level of the hypertrophy marker, ANF, compared to C57BL/6 mice.

Conclusion

We conclude that the DBA/2 mouse strain has a more immature cardiac phenotype, which correlates to a cardiac protective response to hypertrophy in both physiological and pathological stimulations.

Bone-derived stem cells repair the heart after myocardial infarction through transdifferentiation and paracrine signaling mechanisms

RATIONALE

Autologous bone marrow-derived or cardiac-derived stem cell therapy for heart disease has demonstrated safety and efficacy in clinical trials, but functional improvements have been limited. Finding the optimal stem cell type best suited for cardiac regeneration is the key toward improving clinical outcomes.

OBJECTIVE

To determine the mechanism by which novel bone-derived stem cells support the injured heart.

METHODS AND RESULTS

Cortical bone-derived stem cells (CBSCs) and cardiac-derived stem cells were isolated from enhanced green fluorescent protein (EGFP+) transgenic mice and were shown to express c-kit and Sca-1 as well as 8 paracrine factors involved in cardioprotection, angiogenesis, and stem cell function. Wild-type C57BL/6 mice underwent sham operation (n=21) or myocardial infarction with injection of CBSCs (n=67), cardiac-derived stem cells (n=36), or saline (n=60). Cardiac function was monitored using echocardiography. Only 2/8 paracrine factors were detected in EGFP+ CBSCs in vivo (basic fibroblast growth factor and vascular endothelial growth factor), and this expression was associated with increased neovascularization of the infarct border zone. CBSC therapy improved survival, cardiac function, regional strain, attenuated remodeling, and decreased infarct size relative to cardiac-derived stem cells- or saline-treated myocardial infarction controls. By 6 weeks, EGFP+ cardiomyocytes, vascular smooth muscle, and endothelial cells could be identified in CBSC-treated, but not in cardiac-derived stem cells-treated, animals. EGFP+ CBSC-derived isolated myocytes were smaller and more frequently mononucleated, but were functionally indistinguishable from EGFP- myocytes.

CONCLUSIONS

CBSCs improve survival, cardiac function, and attenuate remodeling through the following 2 mechanisms: (1) secretion of proangiogenic factors that stimulate endogenous neovascularization, and (2) differentiation into functional adult myocytes and vascular cells.

T-type Ca²⁺ channels regulate the exit of cardiac myocytes from the cell cycle after birth

T-type Ca(2+) channels (TTCCs) are expressed in the fetal heart and then disappear from ventricular myocytes after birth. The hypothesis examined in this study was the α1G TTCCs' influence in myocyte maturation and their rapid withdrawal from the cell cycle after birth.

METHODS

Cardiac myocytes were isolated from neonatal and adult wild type (WT), α1G-/- and α1G over expressing (α1GDT) mice. Bromodeoxyuridine (BrdU) uptake, myocyte nucleation, cell cycle analysis, and T-type Ca(2+) currents were measured.

RESULTS

All myocytes were mono-nucleated at birth and 35% of WT myocytes expressed functional TTCCs. Very few neonatal myocytes had functional TTCCs in α1G-/- hearts. By the end of the first week after birth no WT or α1G-/- had functional TTCCs. During the first week after birth about 25% of WT myocytes were BrdU+ and became bi-nucleated. Significantly fewer α1G-/- myocytes became bi-nucleated and fewer of these myocytes were BrdU+. Neonatal α1G-/- myocytes were also smaller than WT. Adult WT and α1G-/- hearts were similar in size, but α1G-/- myocytes were smaller and a greater % were mono-nucleated. α1G over expressing hearts were smaller than WT but their myocytes were larger.

CONCLUSIONS

The studies performed show that loss of functional TTCCs is associated with bi-nucleation and myocyte withdrawal from the cell cycle. Loss of α1G TTCCs slowed the transition from mono- to bi-nucleation and resulted in an adult heart with a greater number of small cardiac myocytes. These results suggest that TTCCs are involved in the regulation of myocyte size and the exit of myocytes from the cell cycle during the first week after birth.

Cardiomyocyte proliferation and progenitor cell recruitment underlie therapeutic regeneration after myocardial infarction in the adult mouse heart

Cardiosphere-derived cells (CDCs) have been shown to regenerate infarcted myocardium in patients after myocardial infarction (MI). However, whether the cells of the newly formed myocardium originate from the proliferation of adult cardiomyocytes or from the differentiation of endogenous stem cells remains unknown. Using genetic fate mapping to mark resident myocytes in combination with long-term BrdU pulsing, we investigated the origins of postnatal cardiomyogenesis in the normal, infarcted and cell-treated adult mammalian heart. In the normal mouse heart, cardiomyocyte turnover occurs predominantly through proliferation of resident cardiomyocytes at a rate of ∼1.3–4%/year. After MI, new cardiomyocytes arise from both progenitors as well as pre-existing cardiomyocytes. Transplantation of CDCs upregulates host cardiomyocyte cycling and recruitment of endogenous progenitors, while boosting heart function and increasing viable myocardium. The observed phenomena cannot be explained by cardiomyocyte polyploidization, bi/multinucleation, cell fusion or DNA repair. Thus, CDCs induce myocardial regeneration by differentially upregulating two mechanisms of endogenous cell proliferation.

Cardiomyocyte proliferation contributes to heart growth in young humans

The human heart is believed to grow by enlargement but not proliferation of cardiomyocytes (heart muscle cells) during postnatal development. However, recent studies have shown that cardiomyocyte proliferation is a mechanism of cardiac growth and regeneration in animals. Combined with evidence for cardiomyocyte turnover in adult humans, this suggests that cardiomyocyte proliferation may play an unrecognized role during the period of developmental heart growth between birth and adolescence. We tested this hypothesis by examining the cellular growth mechanisms of the left ventricle on a set of healthy hearts from humans aged 0-59 y (n = 36). The percentages of cardiomyocytes in mitosis and cytokinesis were highest in infants, decreasing to low levels by 20 y. Although cardiomyocyte mitosis was detectable throughout life, cardiomyocyte cytokinesis was not evident after 20 y. Between the first year and 20 y of life, the number of cardiomyocytes in the left ventricle increased 3.4-fold, which was consistent with our predictions based on measured cardiomyocyte cell cycle activity. Our findings show that cardiomyocyte proliferation contributes to developmental heart growth in young humans. This suggests that children and adolescents may be able to regenerate myocardium, that abnormal cardiomyocyte proliferation may be involved in myocardial diseases that affect this population, and that these diseases might be treatable through stimulation of cardiomyocyte proliferation.

Pretreatment with xenon protected immature rabbit heart from ischaemia/reperfusion injury by opening of the mitoKATP channel

BACKGROUND

The noble gas anaesthetic, xenon has previously been shown to protect the adult myocardium from ischaemia/reperfusion (I/R) injury, however its effect on immature myocardium is unclear. The aim of this study was to investigate the effect of xenon on the isolated immature heart.

METHODS

Isolated, immature (2-3weeks old) New Zealand rabbit hearts were perfused with Krebs-Henseleit buffer via Langendorff-mode. After 20min of baseline equilibration, hearts were pretreated with 75% xenon, 75% xenon+100μM diazoxide, or 75% xenon+100μM 5-hydroxydecanoate, and then subjected to 1h of global ischaemia and 3h of reperfusion.

RESULTS

Pretreatment with 75% xenon significantly improved cardiac function (P<0.01 vs. the I/R group, respectively), limited myocardial infarct size (20.83±2.16%, P<0.01 vs. 35.82±2.14% of the I/R group), reduced cardiac enzyme release (CK-MB, 1.00±0.19IU/L, P<0.01 vs. 0.44±0.14IU/L of the I/R group; LDH, 6.15±1.06IU/L P<0.01 vs. 3.49±0.37IU/L of the I/R group) and decreased apoptosis (6.17±0.56%, P<0.01 vs. 11.31±0.93% of the I/R group). In addition, the mitochondrial structure changes caused by I/R injury were largely prevented by 75% xenon pretreatment (1.37±0.16, P<0.01 vs. 2.32±0.13 of the I/R group). The mitochondrial adenosine triphosphate-sensitive potassium (mitoKATP) channel opener diazoxide did not influence the effect of xenon, but the specific mitoKATP channel blocker 5-hydroxydecanoate completely abolished this effect.

CONCLUSIONS

Our study demonstrated that pretreatment with 75% xenon protected immature heart from I/R injury, and this protection was probably mediated by preservation of myocardial mitochondria and opening of mitoKATP channel.

The need for standardized protocols for future clinical trials of cell therapy

Multiple clinical trials have been conducted to determine the outcome of stem cell transplantation on cardiac function. However, marked variability in design across these trials has generated ambiguity in interpretation of their results. This review systematically evaluates the currently available protocols to illustrate the need for a standardized protocol for future trials.

Cellular therapy promotes endogenous stem cell repair

Cellular transplantation for cardiac repair has emerged as an exciting treatment option for patients with myocardial infarction (MI) and heart failure. Animal models of post-infarction left ventricular remodeling have demonstrated an improvement in left ventricular (LV) function, decrease in scar size, and amelioration of adverse cardiac remodeling after stem cell transplantation. These beneficial effects occur despite minimal engraftment and negligible differentiation of transplanted cells. Evidence of the heart capability to self-renew continues to mount; however, the extent to which this occurs is still unclear. Although there is a specific population of cardiac stem cells capable of differentiating into cardiomyocytes, they alone are not capable of fully regenerating tissue damaged by MI. Therefore, paracrine mechanisms may be responsible for activating endogenous stem cells to promote regeneration and prevent apoptosis. These structural beneficial effects may reduce regional wall stresses, consequently leading to long-term host myocardium gene/protein expression changes, which may subsequently result in improvement in LV function.

Inhibition of doxorubicin-induced senescence by PPARδ activation agonists in cardiac muscle cells: cooperation between PPARδ and Bcl6

Senescence and apoptosis are two distinct cellular programs that are activated in response to a variety of stresses. Low or high doses of the same stressor, i.e., the anticancer drug doxorubicin, may either induce apoptosis or senescence, respectively, in cardiac muscle cells. We have demonstrated that PPARδ, a ligand-activated transcriptional factor that controls lipid metabolism, insulin sensitivity and inflammation, is also involved in the doxorubicin-induced senescence program. This occurs through its interference with the transcriptional repressor protein B cell lymphoma-6 (Bcl6). Low doses of doxorubicin increase the expression of PPARδ that sequesters Bcl6, thus preventing it from exerting its anti-senescent effects. We also found that L-165041, a specific PPARδ activator, is highly effective in protecting cardiomyocytes from doxorubicin-induced senescence through a Bcl6 related mechanism. In fact, L-165041 increases Bcl6 expression via p38, JNK and Akt activation, and at the same time it induces the release of Bcl6 from PPARδ, thereby enabling Bcl6 to bind to its target genes. L-165041 also prevented apoptosis induced by higher doses of doxorubicin. However, while experiments performed with siRNA analysis techniques very clearly showed the weight of Bcl6 in the cellular senescence program, no role was found for Bcl6 in the anti-apoptotic effects of L-165041, thus confirming that senescence and apoptosis are two very distinct stress response cellular programs. This study increases our understanding of the molecular mechanism of anthracycline cardiotoxicity and suggests a potential role for PPARδ agonists as cardioprotective agents.

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

RATIONALE

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

OBJECTIVE

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

METHODS AND RESULTS

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

CONCLUSIONS

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

Down-regulation of replication factor C-40 (RFC40) causes chromosomal missegregation in neonatal and hypertrophic adult rat cardiac myocytes

Background

Adult mammalian cardiac myocytes are generally assumed to be terminally differentiated; nonetheless, a small fraction of cardiac myocytes have been shown to replicate during ventricular remodeling. However, the expression of Replication Factor C (RFC; RFC140/40/38/37/36) and DNA polymerase δ (Pol δ) proteins, which are required for DNA synthesis and cell proliferation, in the adult normal and hypertrophied hearts has been rarely studied.

Methods

We performed qRT-PCR and Western blot analysis to determine the levels of RFC and Pol δ message and proteins in the adult normal cardiac myocytes and cardiac fibroblasts, as well as in adult normal and pulmonary arterial hypertension induced right ventricular hypertrophied hearts. Immunohistochemical analyses were performed to determine the localization of the re-expressed DNA replication and cell cycle proteins in adult normal (control) and hypertrophied right ventricle. We determined right ventricular cardiac myocyte polyploidy and chromosomal missegregation/aneuploidy using Fluorescent in situ hybridization (FISH) for rat chromosome 12.

Results

RFC40-mRNA and protein was undetectable, whereas Pol δ message was detectable in the cardiac myocytes isolated from control adult hearts. Although RFC40 and Pol δ message and protein significantly increased in hypertrophied hearts as compared to the control hearts; however, this increase was marginal as compared to the fetal hearts. Immunohistochemical analyses revealed that in addition to RFC40, proliferative and mitotic markers such as cyclin A, phospho-Aurora A/B/C kinase and phospho-histone 3 were also re-expressed/up-regulated simultaneously in the cardiac myocytes. Interestingly, FISH analyses demonstrated cardiac myocytes polyploidy and chromosomal missegregation/aneuploidy in these hearts. Knock-down of endogenous RFC40 caused chromosomal missegregation/aneuploidy and decrease in the rat neonatal cardiac myocyte numbers.

Conclusion

Our novel findings suggest that transcription of RFC40 is suppressed in the normal adult cardiac myocytes and its insufficient re-expression may be responsible for causing chromosomal missegregation/aneuploidy and in cardiac myocytes during right ventricular hypertrophy.

Carbon nanotubes promote growth and spontaneous electrical activity in cultured cardiac myocytes

Nanoscale manipulations of the extracellular microenvironment are increasingly attracting attention in tissue engineering. Here, combining microscopy, biological, and single-cell electrophysiological methodologies, we demonstrate that neonatal rat ventricular myocytes cultured on substrates of multiwall carbon nanotubes interact with carbon nanotubes by forming tight contacts and show increased viability and proliferation. Furthermore, we observed changes in the electrophysiological properties of cardiomyocytes, suggesting that carbon nanotubes are able to promote cardiomyocyte maturation.

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.

Heme oxygenase-1 induction protects the heart and modulates cellular and extracellular remodelling after myocardial infarction in rats

Heme oxygenase-1 (HO-1) is a cytoprotective enzyme, which regulates cell proliferation and has potential antifibrogenic properties. In the present study, we investigated the effects of pre-emptive HO-1 induction by cobalt protoporphyrin IX on the healing of myocardial infarction in rats. The proliferation and repair of cardiac cells was assessed by immunostaining of Ki67 and proliferating cell nuclear antigen, and apoptosis of cardiomyocytes by terminal deoxynucleotidyl transferase dUTP nick end labelling. Compared with control hearts, HO-1 induction reduced apoptosis and increased proliferation and repair of cardiomyocytes in the infarct border area during the first few days after infarction. Concomitantly, HO-1 decreased accumulation and proliferation of fibroblasts, and down-regulated procollagen type I expression in the infarct area. Furthermore, HO-1 increased expression of the anti-inflammatory cytokine, transforming growth factor-β1, suggesting that the cardioprotective effect of HO-1 in the early phase of infarct healing may result partly from the suppression of the inflammatory response. In the remote myocardium, HO-1 inhibited both proliferation and apoptosis of cardiomyocytes, attenuated heart failure-induced increase in the repair of cardiomyocytes and decreased perivascular fibrosis, thereby potentially alleviating adverse ventricular remodelling. The cardioprotective effects of HO-1 in the late phase of infarct healing may be mediated partly by down-regulation of the profibrotic connective tissue growth factor (CTGF), as HO-1 decreased CTGF expression at week 4. In conclusion, our findings suggest an important role for HO-1 in maintaining cellular homeostasis in the postinfarction heart. Modulation of the HO-1 pathway may provide a new therapeutic approach to enhance the recovery of myocardial infarction and protect against pathological myocardial changes.

β-myosin heavy chain is induced by pressure overload in a minor subpopulation of smaller mouse cardiac myocytes

RATIONALE

Induction of the fetal hypertrophic marker gene β-myosin heavy chain (β-MyHC) is a signature feature of pressure overload hypertrophy in rodents. β-MyHC is assumed present in all or most enlarged myocytes.

OBJECTIVE

To quantify the number and size of myocytes expressing endogenous β-MyHC by a flow cytometry approach.

METHODS AND RESULTS

Myocytes were isolated from the left ventricle of male C57BL/6J mice after transverse aortic constriction (TAC), and the fraction of cells expressing endogenous β-MyHC was quantified by flow cytometry on 10,000 to 20,000 myocytes with use of a validated β-MyHC antibody. Side scatter by flow cytometry in the same cells was validated as an index of myocyte size. β-MyHC-positive myocytes constituted 3 ± 1% of myocytes in control hearts (n=12), increasing to 25 ± 10% at 3 days to 6 weeks after TAC (n=24, P<0.01). β-MyHC-positive myocytes did not enlarge with TAC and were smaller at all times than myocytes without β-MyHC (≈70% as large, P<0.001). β-MyHC-positive myocytes arose by addition of β-MyHC to α-MyHC and had more total MyHC after TAC than did the hypertrophied myocytes that had α-MyHC only. Myocytes positive for β-MyHC were found in discrete regions of the left ventricle in 3 patterns: perivascular, in areas with fibrosis, and in apparently normal myocardium.

CONCLUSIONS

β-MyHC protein is induced by pressure overload in a minor subpopulation of smaller cardiac myocytes. The hypertrophied myocytes after TAC have α-MyHC only. These data challenge the current paradigm of the fetal hypertrophic gene program and identify a new subpopulation of smaller working ventricular myocytes with more myosin.