Background Cardiac stem cells (CSCs) delivered to the infarcted heart generate a large number of small fetal-neonatal cardiomyocytes which fail to acquire the differentiated phenotype. However, the interaction of CSCs with post-mitotic myocytes results in the formation of cells with adult characteristics. Methods and Results Based on in vitro and in vivo assays, we report that the commitment of human CSCs (hCSCs) to the myocyte lineage and the generation of mature working cardiomyocytes are influenced by microRNA-499 (miR-499) which is barely detectable in hCSCs, but is highly expressed in post-mitotic human cardiomyocytes. miR-499 traverses gap junction channels and translocates to structurally coupled hCSCs favoring their differentiation into functionally-competent cells. Expression of miR-499 in hCSCs represses the miR-499 target genes Sox6 and Rod1, enhancing cardiomyogenesis in vitro and after infarction in vivo. Although cardiac repair was detected in all cell-treated infarcted hearts, the aggregate volume of the regenerated myocyte mass and myocyte cell volume were greater in animals injected with hCSCs overexpressing miR-499. Treatment with hCSCs resulted in an improvement in ventricular function, consisting of a better preservation of developed pressure, and positive and negative dP/dt after infarction. An additional positive effect on cardiac performance occurred with miR-499, pointing to enhanced myocyte differentiation/hypertrophy as the mechanism by which miR-499 potentiated the restoration of myocardial mass and function in the infarcted heart. Conclusions The recognition that miR-499 promotes the differentiation of hCSCs into mechanically integrated cardiomyocytes has important clinical implications for the treatment of human heart failure.

An analysis of the clonality of cardiac progenitor cells (CPCs) and myocyte turnover in vivo requires genetic tagging of the undifferentiated cells so that the clonal marker of individual mother cells is traced in the specialized progeny. CPC niches in the atria and apex of the mouse heart were infected with a lentivirus carrying EGFP, and the destiny of the tagged cells was determined 1-5 months later. A common integration site was identified in isolated CPCs, cardiomyocytes, endothelial cells (ECs), and fibroblasts, documenting CPC self-renewal and multipotentiality and the clonal origin of the differentiated cell populations. Subsequently, the degree of EGFP-lentiviral infection of CPCs was evaluated 2-4 days after injection, and the number of myocytes expressing the reporter gene was measured 6 months later. A BrdU pulse-chasing protocol was also introduced as an additional assay for the analysis of myocyte turnover. Over a period of 6 months, each EGFP-positive CPC divided approximately eight times generating 230 cardiomyocytes; this value was consistent with the number of newly formed cells labeled by BrdU. To determine whether, human CPCs (hCPCs) are self-renewing and multipotent, these cells were transduced with the EGFP-lentivirus and injected after acute myocardial infarction in immunosuppressed rats. hCPCs, myocytes, ECs, and fibroblasts collected from the regenerated myocardium showed common viral integration sites in the human genome. Thus, our results indicate that the adult heart contains a pool of resident stem cells that regulate cardiac homeostasis and repair.F ate mapping protocols establish a lineage relationship between ancestors carrying the reporter gene and their descendents (1, 2), but do not provide information on the self-renewing property and clonogenicity of progenitor cells or clonal origin of daughter cells in vivo (3). Because of these limitations, viral gene-tagging remains the most accurate strategy for the analysis of stem cell growth (3-8). The semi-random insertion of retroviral and lentiviral vectors represents an effective tool for genetic marking, enabling the identification of the progeny generated by stem cell differentiation. Retroviruses and lentiviruses integrate permanently in the genome of the host cells; the insertion site of the viral genome is inherited by the population derived from the parental cell (6) and can be amplified by PCR. Thus, the detection of the sites of integration constitutes a unique approach for the documentation of self-renewal, clonogenicity, and multipotentiality of stem cells in vivo. So far, this methodology has been applied to the bone marrow (4-6) and the brain (3, 7, 8) and has not been used to characterize the mechanisms regulating cardiac homeostasis and pathology.The implementation of this technique in the adult heart is relevant for the incontrovertible demonstration of resident cardiac stem cells and the ability of the myocardium to undergo spontaneous regeneration. Moreover, the notion that cardiomyocytes have a long lifespan and ...

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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: Physiological hypertrophy in the developing heart has been considered the product of an increase in volume of preexisting fetal cardiomyocytes in the absence of myocyte formation. Objective: In this study, we tested whether the mouse heart at birth has a pool of cardiac stem cells (CSCs) that differentiate into myocytes contributing to the postnatal expansion of the parenchymal cell compartment. Methods and Results: We have found that the newborn heart contains a population of c-kit-positive CSCs that are lineage negative, self-renewing, and multipotent. CSCs express the Notch1 receptor and show the nuclear localization of its active fragment, N1ICD. In 60% of cases, N1ICD was coupled with the presence of Nkx2.5, indicating that the commitment of CSCs to the myocyte lineage is regulated by Notch1. Importantly, overexpression of N1ICD in neonatal CSCs significantly expanded the proportion of transit-amplifying myocytes.To establish whether these in vitro findings had a functional counterpart in vivo, the Notch pathway was blocked in newborn mice by administration of a ␥-secretase inhibitor. This intervention resulted in the development of a dilated myopathy and high mortality rates. Ventricular decompensation was characterized by a 62% reduction in amplifying myocytes, which resulted in a 54% decrease in myocyte number. A t birth, the myocardium must accommodate the abrupt changes in the patterns of blood flow and circulatory resistance and the increasing demands of the rapidly growing organism. Physiological hypertrophy in the developing heart has been considered the result of an increase in volume of preexisting fetal cardiomyocytes, whereas the contribution of new myocytes to the postnatal expansion in ventricular mass was viewed as negligible at best. 1 However, the documentation of high levels of DNA synthesis in myocytes together with apoptotic cell death 2 indicate that the postnatal maturation of the heart is a complex process in which cardiomyogenesis may play a critical role in the acquisition of the adult heart phenotype.By viral tagging and clonal marking, we have shown that cardiomyogenesis in the adult mouse heart depends on the activation and differentiation of a pool of cardiac stem cells (CSCs) that express the c-kit antigen. 3 On the basis of these findings, the question can be raised whether primitive cells with similar properties are present in the neonatal heart and the generation of new myocytes is dictated by lineage commitment of these cells. According to the traditional model of growth in self-renewing organs, stem cells give rise to progenitorprecursor cells, which then become highly dividing amplifying cells that eventually reach terminal differentiation and growth arrest. 4 This hierarchically structured mechanism of parenchymal cell generation may account for the expansion in myocyte number and volume of the neonatal heart. CSC-derived amplifying myocytes replicate and concurrently differentiate, mimicking cellular hyperplasia and hypertrophy. 5 The possibility exists t...

Inflammation driven by immune cells and pro-inflammatory cytokines is implicated in pancreaticInsulin insufficiency, resulting from injury to the insulinproducing ␤ cells in pancreatic islets, plays a pivotal role in the development of diabetes mellitus. Accumulating evidence suggests that inflammatory processes driven by T cells and macrophages are involved in ␤-cell injury.

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In stem cell-regulated organs, a subset of niches is characterized by low oxygen tension. This metabolic adaptation offers a selective advantage to stem cells favoring the preservation of their quiescent undifferentiated phenotype. The objective of this work was to determine whether in the mouse heart cardiac niches constitute a heterogeneous compartment composed of hypoxic and normoxic niches, and whether differences in O 2 concentration affect the function of c-kit-positive cardiac stem cells (CSCs).To test this possibility, we studied first the in vivo uptake of the hypoxic marker pimonidazole (PIMO), which identifies intracellular O 2 concentration <10 mmHg. Mice were sacrificed 2 hours after intraperitoneal administration of PIMO, and PIMO-labeling was analyzed. By immunolabeling, 15% of cardiac niches were characterized by a hypoxic microenvironment and more than 20% of isolated CSCs were PIMO-positive, as measured by flow-cytometry. The cell cycle protein Ki67 was restricted to the PIMO-negative CSC class, which contained early committed cells expressing c-kit together with the myocyte specific transcription factors GATA4 and Nkx2.5. Mice were then administered tirapazamine, a compound that kills selectively hypoxic cells. One day later, the fraction of PIMO-positive CSCs was markedly decreased but, at 5 days, this compartment was partly reconstituted. This compensatory response was coupled with increased proliferation of PIMO-negative CSCs, suggesting that normoxic CSCs have the ability to replenish hypoxic niches following injury. Subsequently, the effects of hypoxia were studied in human CSCs (hCSCs) exposed in vitro to 1% O 2 . With respect to cells cultured in normoxia, 1% O 2 led to upregulation of HIF1α in hCSCs which also showed lower levels of BrdU incorporation. These cellular responses were accompanied by an increase in transcripts for the stemness genes c-kit, Oct4, Nanog and Sox2, and a decrease in mRNA for myocyte and vascular genes. Apoptosis, measured by TdT labeling, did not differ in normoxic and hypoxic hCSCs. In conclusion, our data indicate that hypoxic and normoxic niches coexist in the myocardium, and that intracellular hypoxia regulates the quiescent primitive CSC phenotype.