The identification of cardiac progenitor cells in mammals raises the possibility that the human heart contains a population of stem cells capable of generating cardiomyocytes and coronary vessels. The characterization of human cardiac stem cells (hCSCs) would have important clinical implications for the management of the failing heart. We have established the conditions for the isolation and expansion of c-kit-positive hCSCs from small samples of myocardium. Additionally, we have tested whether these cells have the ability to form functionally competent human myocardium after infarction in immunocompromised animals. Here, we report the identification in vitro of a class of human c-kit-positive cardiac cells that possess the fundamental properties of stem cells: they are self-renewing, clonogenic, and multipotent. hCSCs differentiate predominantly into cardiomyocytes and, to a lesser extent, into smooth muscle cells and endothelial cells. When locally injected in the infarcted myocardium of immunodeficient mice and immunosuppressed rats, hCSCs generate a chimeric heart, which contains human myocardium composed of myocytes, coronary resistance arterioles, and capillaries. The human myocardium is structurally and functionally integrated with the rodent myocardium and contributes to the performance of the infarcted heart. Differentiated human cardiac cells possess only one set of human sex chromosomes excluding cell fusion. The lack of cell fusion was confirmed by the Cre-lox strategy. Thus, hCSCs can be isolated and expanded in vitro for subsequent autologous regeneration of dead myocardium in patients affected by heart failure of ischemic and nonischemic origin.generation of human myocardium ͉ progenitor cells ͉ stem cell niches

Abstract-Heart failure is the leading cause of death in the elderly, but whether this is the result of a primary aging myopathy dictated by depletion of the cardiac progenitor cell (CPC) pool is unknown. Similarly, whether current lifespan reflects the ineluctable genetic clock or heart failure interferes with the genetically determined fate of the organ and organism is an important question. We have identified that chronological age leads to telomeric shortening in CPCs, which by necessity generate a differentiated progeny that rapidly acquires the senescent phenotype conditioning organ aging. CPC aging is mediated by attenuation of the insulin-like growth factor-1/insulin-like growth factor-1 receptor and hepatocyte growth factor/c-Met systems, which do not counteract any longer the CPC renin-angiotensin system, resulting in cellular senescence, growth arrest, and apoptosis. However, pulse-chase 5-bromodeoxyuridine-labeling assay revealed that the senescent heart contains functionally competent CPCs that have the properties of stem cells. This subset of telomerasecompetent CPCs have long telomeres and, following activation, migrate to the regions of damage, where they generate a population of young cardiomyocytes, reversing partly the aging myopathy. The senescent heart phenotype and heart failure are corrected to some extent, leading to prolongation of maximum lifespan. (Circ Res. 2008;102:597-606.)

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Coronary artery disease is the most common cause of cardiac failure in the Western world, and to date there is no alternative to bypass surgery for severe coronary atherosclerosis. We report that c-kitpositive cardiac progenitor cells (CPCs) activated with insulin-like growth factor 1 and hepatocyte growth factor before their injection in proximity of the site of occlusion of the left coronary artery in rats, engrafted within the host myocardium forming temporary niches. Subsequently, CPCs divided and differentiated into endothelial cells and smooth muscle cells and, to a lesser extent, into cardiomyocytes. The acquisition of vascular lineages appeared to be mediated by the up-regulation of hypoxia-inducible factor 1␣, which promoted the synthesis and secretion of stromal-derived factor 1 from hypoxic coronary vessels. Stromal-derived factor 1 was critical in the conversion of CPCs to the vascular fate. CPCs formed conductive and intermediate-sized coronary arteries together with resistance arterioles and capillaries. The new vessels were connected with the primary coronary circulation, and this increase in vascularization more than doubled myocardial blood flow in the infarcted myocardium. This beneficial effect, together with myocardial regeneration attenuated postinfarction dilated myopathy, reduced infarct size and improved function. In conclusion, locally delivered activated CPCs generate de novo coronary vasculature and may be implemented clinically for restoration of blood supply to the ischemic myocardium.coronary blood flow ͉ infarct size ͉ myocardial regeneration ͉ stem cells ͉ vasculogenesis

Targeted PET imaging with (68)Ga-pentixafor identifies the global and regional CXCR4 expression pattern in myocardium and systemic organs. CXCR4 upregulation after AMI coincides with inflammatory cell infiltration, but shows interindividual variability in patients. This may have implications for the response to CXCR4- or other inflammation-targeted therapy, and for subsequent ventricular remodeling.

Abstract-The recognition that the adult heart continuously renews its myocyte compartment raises the possibility that the age and lifespan of myocytes does not coincide with the age and lifespan of the organ and organism. If this were the case, myocyte turnover would result at any age in a myocardium composed by a heterogeneous population of parenchymal cells which are structurally integrated but may contribute differently to myocardial performance. To test this hypothesis, left ventricular myocytes were isolated from mice at 3 months of age and the contractile, electrical, and calcium cycling characteristics of these cells were determined together with the expression of the senescence-associated protein p16 INK4a and telomere length. The heart was characterized by the coexistence of young, aged, and senescent myocytes. Old nonreplicating, p16INK4a -positive, hypertrophied myocytes with severe telomeric shortening were present together with young, dividing, p16INK4a -negative, small myocytes with long telomeres. A class of myocytes with intermediate properties was also found. Physiologically, evidence was obtained in favor of the critical role that action potential (AP) duration and I CaL play in potentiating Ca 2ϩ cycling and the mechanical behavior of young myocytes or in decreasing Ca 2ϩ transients and the performance of senescent hypertrophied cells. The characteristics of the AP appeared to be modulated by the transient outward K ϩ current I to which was influenced by the different expression of the K ϩ channels subunits. Collectively, these observations at the physiological and structural cellular level document that by necessity the heart has to constantly repopulate its myocyte compartment to replace senescent poorly contracting myocytes with younger more efficient cells. Thus, cardiac homeostasis and myocyte turnover regulate cardiac function. Key Words: action potential profile Ⅲ excitation-contraction coupling Ⅲ myocyte volume Ⅲ telomere length Ⅲ senescence-associated proteins T he recent identification of a population of resident progenitor cells that controls cardiomyogenesis imposes a reinterpretation of the fundamental mechanisms of growth and senescence of the heart. 1 Traditionally, the heart was viewed as an organ characterized by a predetermined number of myocytes which is defined shortly after birth and is preserved throughout life. However, BrdU administration has documented that myocyte renewal occurs in the adult heart and that the rate of cell regeneration is slow, heterogeneous, and involves only a small percentage of cells at any given time. 2,3 In mammals, ventricular myocytes are replaced several times throughout life. 2,3 These findings were obtained by pulse-chase BrdU labeling assay 2 and by the rate of growth and commitment of cardiac progenitor cells. 3 Collectively, they challenge the notion that the age and lifespan of myocytes coincide with the age and lifespan of the organ and organism. According to the new paradigm, the continuous turnover of myocytes results in a heterogen...

Abstract-Recent studies suggest that rather than being terminally differentiated, the adult heart is a self-renewing organ with the capacity to generate new myocytes from cardiac stem/progenitor cells (CS/PCs). This study examined the hypotheses that new myocytes are generated during adolescent growth, to increase myocyte number, and these newly formed myocytes are initially small, mononucleated, proliferation competent, and have immature properties. Ventricular myocytes (VMs) and cKit ϩ (stem cell receptor) CS/PCs were isolated from 11-and 22-week feline hearts. Bromodeoxyuridine incorporation (in vivo) and p16INK4a immunostaining were measured to assess myocyte cell cycle activity and senescence, respectively. Telomerase activity, contractions, Ca 2ϩ transients, and electrophysiology were compared in small mononucleated (SMMs) and large binucleated (LBMs) myocytes. Heart mass increased by 101% during adolescent growth, but left ventricular myocyte volume only increased by 77%. Most VMs were binucleated (87% versus 12% mononucleated) and larger than mononucleated myocytes. A greater percentage of SMMs was bromodeoxyuridine positive (SMMs versus LBMs: 3.1% versus 0.8%; PϽ0.05), and p16INK4a negative and small myocytes had greater telomerase activity than large myocytes. Contractions and Ca 2ϩ transients were prolonged in SMMs versus LBMs and Ca 2ϩ release was disorganized in SMMs with reduced transient outward current and T-tubule density. The T-type Ca 2ϩ current, usually seen in fetal/neonatal VMs, was found exclusively in SMMs and in myocytes derived from CS/PC. Myocyte number increases during adolescent cardiac growth. These new myocytes are initially small and functionally immature, with patterns of ion channel expression normally found in the fetal/neonatal period (Circ Res. 2007;100:536-544.)

Nuclear factor B (NF-B) is a ubiquitous transcription factor activated by various stimuli implicated in ischemia-reperfusion injury. However, the role of NF-B in cardiac ischemia-reperfusion injury has not yet been well defined. Therefore, we investigated reperfusion damage in mice with targeted deletion of the NF-B subunit p50. Electrophoretic mobility shift assays validated NF-B activation in wild-type (WT) but not p50 knockout (KO) mice. KO and WT animals underwent 30 minutes of coronary artery ligation and 24 hours of reperfusion in vivo. Ischemia-reperfusion damage was significantly reduced in the p50 KO when compared with matching WT mice. Although adhesion molecules such as intercellular adhesion molecule were up-regulated in left ventricles of p50 KO animals, fewer neutrophils infiltrated the infarct area, suggesting leukocytes as a potential mediator of the protection observed in the p50 KO. This was confirmed in adoptive transfer experiments: whereas transplantation of KO bone marrow in KO animals sustained the protective effect on ischemiareperfusion injury, transplantation of WT bone marrow in KO animals abolished it. Thus, deletion of the NF-B subunit p50 reduces ischemia-reperfusion injury in vivo, associated with less neutrophil infiltration. Bone marrow transplantation experiments indicate that impaired NF-B activation in p50 KO leukocytes attenuates cardiac damage.