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-...

The transforming-growth-factor-beta-activated kinase TAK1 is a member of the mitogen-activated protein kinase kinase kinase family, which couples extracellular stimuli to gene transcription. The in vivo function of TAK1 is not understood. Here, we investigated the potential involvement of TAK1 in cardiac hypertrophy. In adult mouse myocardium, TAK1 kinase activity was upregulated 7 days after aortic banding, a mechanical load that induces hypertrophy and expression of transforming growth factor beta. An activating mutation of TAK1 expressed in myocardium of transgenic mice was sufficient to produce p38 mitogen-activated protein kinase phosphorylation in vivo, cardiac hypertrophy, interstitial fibrosis, severe myocardial dysfunction, 'fetal' gene induction, apoptosis and early lethality. Thus, TAK1 activity is induced as a delayed response to mechanical stress, and can suffice to elicit myocardial hypertrophy and fulminant heart failure.

Receptors for bone morphogenetic proteins (BMPs), members of the transforming growth factor-␤ (TGF␤) superfamily, are persistently expressed during cardiac development, yet mice lacking type II or type IA BMP receptors die at gastrulation and cannot be used to assess potential later roles in creation of the heart. Here, we used a Cre͞lox system for cardiac myocyte-specific deletion of the type IA BMP receptor, ALK3. ALK3 was specifically required at mid-gestation for normal development of the trabeculae, compact myocardium, interventricular septum, and endocardial cushion. Cardiac muscle lacking ALK3 was specifically deficient in expressing TGF␤2, an established paracrine mediator of cushion morphogenesis. Hence, ALK3 is essential, beyond just the egg cylinder stage, for myocyte-dependent functions and signals in cardiac organogenesis. Bone morphogenetic proteins (BMPs), named for their firstdiscovered role in bone differentiation, mediate a diverse spectrum of developmental events, such as dorsal-ventral patterning, mesoderm specification, and the spacing of embryo implantation (1, 2). BMPs are postulated to mediate cardiac development in mammals, by extrapolation from the role of decapentaplegic, a related factor in Drosophila (3), their function in cardiac looping in fish (4) or frogs (5), and, especially, studies of cardiac myogenesis in avians (6, 7). Avian explant studies also implicate BMPs as an essential paracrine signal from myocardium adjacent to the endocardial cushion during the epithelialmesenchymal transformation and later events that ultimately give rise to the atrio-ventricular (AV) valves (8). This process is known to involve TGF␤2 and TGF␤3 as well (9-12), but the molecular connection between BMPs and TGF␤ is uncertain.Despite inferences from the distribution of BMPs during heart formation (13), and BMP effects on cardiac differentiation in teratocarcinoma cells (14), evidence analogous to that for other species is lacking in mammals themselves. One barrier to using conventional germline deletions to resolve these issues in mice is the greater potential for redundancy among BMP family members than in flies. In the mid-gestation heart, BMP2 and -4 are found, respectively, in myocardial layers of the AV canal and in the AV cushion itself, BMP5, -6, and -7 are initially homogeneous throughout the myocardium, and BMP10, which is heart-specific, is found exclusively in trabeculae (14,15). This diversity, with overlapping and nonoverlapping programs of expression, suggests the utility of addressing the question via BMP receptors, instead, which are smaller in number.BMPs bind two serine͞threonine kinase receptors, type II (BMPRII) and type I (ALK3͞BMPR-IA and ALK6͞BMPR-IB), which form a heteromeric signaling complex acting in series, as for other TGF␤ family receptors (16). In the presence of ligand, the type II receptor phosphorylates the type I receptors, which activate signaling by intracellular effectors including Smad transcription factors (16). ALK3 is ubiquitous throughout development, wherea...