Currently, it is unknown whether defects in stem cell growth and differentiation contribute to myocardial aging and chronic heart failure (CHF), and whether a compartment of functional human cardiac stem cells (hCSCs) persists in the decompensated heart. To determine whether aging and CHF are critical determinants of the loss in growth reserve of the heart, the properties of hCSCs were evaluated in 18 control and 23 explanted hearts. Age and CHF showed a progressive decrease in functionally competent hCSCs. Chronological age was a major predictor of five biomarkers of hCSC senescence: telomeric shortening, attenuated telomerase activity, telomere dysfunction-induced foci, and p21Cip1 and p16 INK4a expression. CHF had similar consequences for hCSCs, suggesting that defects in the balance between cardiomyocyte mass and the pool of nonsenescent hCSCs may condition the evolution of the decompensated myopathy. A correlation was found previously between telomere length in circulating bone marrow cells and cardiovascular diseases, but that analysis was restricted to average telomere length in a cell population, neglecting the fact that telomere attrition does not occur uniformly in all cells. The present study provides the first demonstration that dysfunctional telomeres in hCSCs are biomarkers of aging and heart failure. The biomarkers of cellular senescence identified here can be used to define the birth date of hCSCs and to sort young cells with potential therapeutic efficacy.

Background Little is known concerning the function of inositol 1,4,5-triphosphate receptors (IP3Rs) in the adult heart experimentally. Moreover, whether these Ca2+ release channels are present and play a critical role in human cardiomyocytes remains to be defined. IP3Rs may be activated following Gαq-protein-coupled receptors (GPCR) stimulation affecting Ca2+ cycling, enhancing myocyte performance and, potentially, favoring an increase in the incidence of arrhythmias. Methods and Results IP3R function was determined in human left ventricular (LV) myocytes and this analysis was integrated with assays in mouse myocytes to identify the mechanisms by which IP3Rs influence the electrical and mechanical properties of the myocardium. We report that IP3Rs are expressed and operative in human LV myocytes. Following GPCR activation, Ca2+ mobilized from the sarcoplasmic reticulum via IP3Rs contributes to the decrease in resting membrane potential, prolongation of the action-potential, and occurrence of early after-depolarizations. Ca2+ transient amplitude and cell shortening are enhanced, and extra-systolic and dysregulated Ca2+ elevations and contractions become apparent. These alterations in the electromechanical behavior of human cardiomyocytes are coupled with increased isometric twitch of the myocardium and arrhythmic events, suggesting that GPCR activation provide inotropic reserve, which is hampered by electrical instability and contractile abnormalities. Additionally, our findings support the notion that increases in Ca2+ load by IP3Rs promote Ca2+ extrusion by forward mode Na+/Ca2+ exchange, an important mechanism of arrhythmic events. Conclusions Thus, the GPCR/IP3R axis modulates the electromechanical properties of the human myocardium and its propensity to develop arrhythmias.

The aging myopathy manifests itself with diastolic dysfunction and preserved ejection fraction. We raised the possibility that, in a mouse model of physiological aging, defects in electromechanical properties of cardiomyocytes are important determinants of the diastolic characteristics of the myocardium, independently from changes in structural composition of the muscle and collagen framework. Here we show that an increase in the late Na+ current (INaL) in aging cardiomyocytes prolongs the action potential (AP) and influences temporal kinetics of Ca2+ cycling and contractility. These alterations increase force development and passive tension. Inhibition of INaL shortens the AP and corrects dynamics of Ca2+ transient, cell contraction and relaxation. Similarly, repolarization and diastolic tension of the senescent myocardium are partly restored. Thus, INaL offers inotropic support, but negatively interferes with cellular and ventricular compliance, providing a new perspective of the biology of myocardial aging and the aetiology of the defective cardiac performance in the elderly.

Rationale Hypoxia favors stem cell quiescence, while normoxia is required for their activation; but whether cardiac stem cell (CSC) function is regulated by the hypoxic/normoxic state of the cell is currently unknown. Objective A balance between hypoxic and normoxic CSCs may be present in the young heart, although this homeostatic control may be disrupted with aging. Defects in tissue oxygenation occur in the old myocardium, and this phenomenon may expand the pool of hypoxic CSCs, which are no longer involved in myocyte renewal. Methods and Results Here we show that the senescent heart is characterized by an increased number of quiescent CSCs with intact telomeres that cannot reenter the cell cycle and form a differentiated progeny. Conversely, myocyte replacement is controlled only by frequently dividing CSCs with shortened telomeres; these CSCs generate a myocyte population that is chronologically young but phenotypically old. Telomere dysfunction dictates their actual age and mechanical behavior. However, the residual subset of quiescent young CSCs can be stimulated in situ by stem cell factor reversing the aging myopathy. Conclusions Our findings support the notion that strategies targeting CSC activation and growth interfere with the manifestations of myocardial aging in an animal model. Although caution has to be exercised in the translation of animal studies to human beings, our data strongly suggests that a pool of functionally-competent CSCs persists in the senescent heart and this stem cell compartment can promote myocyte regeneration effectively, correcting partly the aging myopathy.

Background Two opposite views of cardiac growth are currently held: one views the heart as a static organ, characterized by a large number of cardiomyocytes, which are present at birth and live as long as the organism; and the other views the heart a highly plastic organ in which the myocyte compartment is restored several times during the course of life. Methods and Results The average age of cardiomyocytes, vascular endothelial cells (ECs), and fibroblasts and their turnover rates were measured by retrospective 14C birth dating of cells in 19 normal hearts, 2 to 78 years of age, and in 17 explanted failing hearts, 22 to 70 years of age. We report that the human heart is characterized by a significant turnover of ventricular myocytes, ECs, and fibroblasts, physiologically and pathologically. Myocyte, EC, and fibroblast renewal is very high shortly after birth, decreases during postnatal maturation, remains relatively constant in the adult organ, and increases dramatically with age. From 20 to 78 years of age, the adult human heart replaces entirely its myocyte, EC, and fibroblast compartment ~8, ~6, and ~8 times, respectively. Myocyte, EC, and fibroblast regeneration is further enhanced with chronic heart failure (CHF). Conclusions The human heart is a highly dynamic organ that retains a remarkable degree of plasticity throughout life and in the presence of CHF. However, the ability to regenerate cardiomyocytes, vascular ECs, and fibroblasts cannot prevent the manifestations of myocardial aging or oppose the negative effects of ischemic and idiopathic dilated cardiomyopathy.

Rationale Embryonic and fetal myocardial growth is characterized by a dramatic increase in myocyte number, but whether the expansion of the myocyte compartment is dictated by activation and commitment of resident cardiac stem cells (CSCs), division of immature myocytes or both is currently unknown. Objectives In this study, we tested whether prenatal cardiac development is controlled by activation and differentiation of CSCs and whether division of c-kit-positive CSCs in the mouse heart is triggered by spontaneous Ca2+ oscillations. Results We report that embryonic-fetal c-kit-positive CSCs are self-renewing, clonogenic and multipotent in vitro and in vivo. The growth and commitment of c-kit-positive CSCs is responsible for the generation of the myocyte progeny of the developing heart. The close correspondence between values computed by mathematical modeling and direct measurements of myocyte number at E9, E14, E19 and one day after birth strongly suggests that the organogenesis of the embryonic heart is dependent on a hierarchical model of cell differentiation regulated by resident CSCs. The growth promoting effects of c-kit-positive CSCs are triggered by spontaneous oscillations in intracellular Ca2+, mediated by IP3 receptor activation, which condition asymmetric stem cell division and myocyte lineage specification. Conclusions Myocyte formation derived from CSC differentiation is the major determinant of cardiac growth during development. Division of c-kit-positive CSCs in the mouse is promoted by spontaneous Ca2+ spikes, which dictate the pattern of stem cell replication and the generation of a myocyte progeny at all phases of prenatal life and up to one day after birth.

Rationale Understanding the mechanisms that regulate trafficking of human cardiac stem cells (hCSCs) may lead to development of new therapeutic approaches for the failing heart. Objectives We tested whether the motility of hCSCs in immunosuppressed infarcted animals is controlled by the guidance system that involves the interaction of Eph receptors with ephrin ligands. Methods and Results Within the cardiac niches, cardiomyocytes expressed preferentially the ephrin A1 ligand, while hCSCs possessed the EphA2 receptor. Treatment of hCSCs with ephrin A1 resulted in the rapid internalization of the ephrin A1-EphA2 complex, post-translational modifications of Src kinases, and morphological changes consistent with the acquisition of a motile cell phenotype. Ephrin A1 enhanced the motility of hCSCs in vitro, and their migration in vivo following acute myocardial infarction. At two weeks after infarction, the volume of the regenerated myocardium was two-fold larger in animals injected with ephrin A1-activated hCSCs than in animals receiving control hCSCs; this difference was dictated by a greater number of newly formed cardiomyocytes and coronary vessels. The increased recovery in myocardial mass with ephrin A1-treated hCSCs was characterized by further restoration of cardiac function and by a reduction in arrhythmic events. Conclusions Ephrin A1 promotes the motility of EphA2-positive hCSCs, facilitates their migration to the area of damage, and enhances cardiac repair. Thus, in situ stimulation of resident hCSCs with ephrin A1 or their ex vivo activation prior to myocardial delivery improves cell targeting to sites of injury, possibly providing a novel strategy for the management of the diseased heart.

BackgroundDiabetes is associated with prolongation of the QT interval of the electrocardiogram and enhanced dispersion of ventricular repolarization, factors that, together with atherosclerosis and myocardial ischemia, may promote the occurrence of electrical disorders. Thus, we tested the possibility that alterations in transmembrane ionic currents reduce the repolarization reserve of myocytes, leading to action potential (AP) prolongation and enhanced beat‐to‐beat variability of repolarization.Methods and ResultsDiabetes was induced in mice with streptozotocin (STZ), and effects of hyperglycemia on electrical properties of whole heart and myocytes were studied with respect to an untreated control group (Ctrl) using electrocardiographic recordings in vivo, ex vivo perfused hearts, and single‐cell patch‐clamp analysis. Additionally, a newly developed algorithm was introduced to obtain detailed information of the impact of high glucose on AP profile. Compared to Ctrl, hyperglycemia in STZ‐treated animals was coupled with prolongation of the QT interval, enhanced temporal dispersion of electrical recovery, and susceptibility to ventricular arrhythmias, defects observed, in part, in the Akita mutant mouse model of type I diabetes. AP was prolonged and beat‐to‐beat variability of repolarization was enhanced in diabetic myocytes, with respect to Ctrl cells. Density of Kv K+ and L‐type Ca2+ currents were decreased in STZ myocytes, in comparison to cells from normoglycemic mice. Pharmacological reduction of Kv currents in Ctrl cells lengthened AP duration and increased temporal dispersion of repolarization, reiterating features identified in diabetic myocytes.ConclusionsReductions in the repolarizing K+ currents may contribute to electrical disturbances of the diabetic heart.