Potential repair by cell grafting or mobilizing endogenous cells holds particular attraction in heart disease, where the meager capacity for cardiomyocyte proliferation likely contributes to the irreversibility of heart failure. Whether cardiac progenitors exist in adult myocardium itself is unanswered, as is the question whether undifferentiated cardiac precursor cells merely fuse with preexisting myocytes. Here we report the existence of adult heart-derived cardiac progenitor cells expressing stem cell antigen-1. Initially, the cells express neither cardiac structural genes nor Nkx2.5 but differentiate in vitro in response to 5 -azacytidine, in part depending on Bmpr1a, a receptor for bone morphogenetic proteins. Given intravenously after ischemia͞reperfusion, cardiac stem cell antigen 1 cells home to injured myocardium. By using a Cre͞Lox donor͞ recipient pair (␣MHC-Cre͞R26R), differentiation was shown to occur roughly equally, with and without fusion to host cells. C ardiomyocytes can be formed, at least ex vivo, from diverse adult pluripotent cells (1-5). Apart from therapeutic implications and obviating ethical concerns aroused by embryonic stem cell lines, adult cardiac progenitor cells might provide an explanation distinct from cell cycle reentry, for the reported rare occurrence of cycling ventricular muscle cells (6). However, recent publications suggest the failure of certain stem cells' specification into neurons, skeletal muscle, and myocardium in vivo (7,8) and recommend greater conservatism in evaluating claims of adult stem cell plasticity, for cogent reasons (9-11).The rarity of cardiogenic conversion by endogenous hematopoietic cells (2, 12), requirements for intracardiac injection (3), or mobilization by cytokines (13), uncertain proof for myocytes of host origin in transplanted human hearts (14), and the confounding possibility of cell fusion after grafting in vivo (15, 16) highlight unsettled issues surrounding stem cell plasticity in heart disease. For donor cell types already in clinical studies, the predominant in vivo effect of bone marrow or endothelial progenitor cells may be neoangiogenesis, not cardiac specification (17, 18), and skeletal myoblasts, despite integration and survival, are confounded by arrhythmias, perhaps reflecting lack of transdifferentiation (19). These obstacles underscore the need to seek cardiac progenitor cells beyond the few known sources. Materials and MethodsFlow Cytometry and Magnetic Enrichment. A ''total'' cardiac population was isolated from 6-to 12-wk-old C57BL͞6 mice by coronary perfusion with 0.025% collagenase, as for viable adult mouse cardiomyocytes (20). More typically, a ''myocytedepleted'' population was prepared, incubating minced myocardium in 0.1% collagenase (30 min, 37°C), lethal to most adult mouse cardiomyocytes (20). Cells were then filtered through 70-m mesh. Bone marrow cells (21) were compared, with or without collagenase and filtration. Cells were labeled with stem cell antigen 1 (Sca-1)-phycoerythrin (PE), Sca-1-FITC, c-kit-PE; CD4-...

Hypertrophic growth is a risk factor for mortality in heart diseases. Mechanisms are lacking for this global increase in RNA and protein per cell, which underlies hypertrophy. Hypertrophic signals cause phosphorylation of the RNA polymerase II C-terminal domain, required for transcript elongation. RNA polymerase II kinases include cyclin-dependent kinases-7 (Cdk7) and Cdk9, components of two basal transcription factors. We report activation of Cdk7 and -9 in hypertrophy triggered by signaling proteins (Galphaq, calcineurin) or chronic mechanical stress. Only Cdk9 was activated by acute load or, in culture, by endothelin. A preferential role for Cdk9 was shown in RNA polymerase II phosphorylation and growth induced by endothelin, using pharmacological and dominant-negative inhibitors. All four hypertrophic signals dissociated 7SK small nuclear RNA, an endogenous inhibitor, from cyclin T-Cdk9. Cdk9 was limiting for cardiac growth, shown by suppressing its inhibitor (7SK) in culture and preventing downregulation of its activator (cyclin T1) in mouse myocardium.Note: In the AOP version of this article, the numbering of the author affiliations was incorrect. This has now been fixed, and the affiliations appear correctly online and in print.

Cardiac muscle regeneration after injury is limited by ''irreversible'' cell cycle exit. Telomere shortening is one postulated basis for replicative senescence, via down-regulation of telomerase reverse transcriptase (TERT); telomere dysfunction also is associated with greater sensitivity to apoptosis. Forced expression of TERT in cardiac muscle in mice was sufficient to rescue telomerase activity and telomere length. Initially, the ventricle was hypercellular, with increased myocyte density and DNA synthesis. By 12 wk, cell cycling subsided; instead, cell enlargement (hypertrophy) was seen, without fibrosis or impaired function. Likewise, viral delivery of TERT was sufficient for hypertrophy in cultured cardiac myocytes. The TERT virus and transgene also conferred protection from apoptosis, in vitro and in vivo. Hyperplasia, hypertrophy, and survival all required active TERT and were not seen with a catalytically inactive mutation. Thus, TERT can delay cell cycle exit in cardiac muscle, induce hypertrophy in postmitotic cells, and promote cardiac myocyte survival.