Knowledge of the nature of cardiac progenitor cells is important for an understanding of the development of heart diseases, and as a potential route to regenerative therapy using stem cells. Two groups now report previously unknown cardiac myocyte lineages isolated from the proepicardium of the mouse. Cai et al. identify a population of progenitor cells that express the transcription factor Tbx18, and Zhou et al. identify cells marked by the transcription factor Wt1. Both types of progenitor are pluripotent, capable of producing several different classes of heart cell, making them promising candidates for use in cardiac repair. In this paper, a new type of cardiac progenitor, marked by the transcription factor Wt1, is shown to reside in the proepicardium and epicardium. It is shown that during normal heart development, a subset of Wt1+ precursors differentiated into fully functional cardiomyocytes, and they arise from Wt1 progenitors that express Nkx2-5 and Isl1, suggesting that they share a developmental origin with multipotent Nkx2 5+/Isl1+ progenitors. The heart is formed from cardiogenic progenitors expressing the transcription factors Nkx2-5 and Isl1 (refs 1 and 2). These multipotent progenitors give rise to cardiomyocyte, smooth muscle and endothelial cells, the major lineages of the mature heart3,4. Here we identify a novel cardiogenic precursor marked by expression of the transcription factor Wt1 and located within the epicardium—an epithelial sheet overlying the heart. During normal murine heart development, a subset of these Wt1+ precursors differentiated into fully functional cardiomyocytes. Wt1+ proepicardial cells arose from progenitors that express Nkx2-5 and Isl1, suggesting that they share a developmental origin with multipotent Nkx2-5+ and Isl1+ progenitors. These results identify Wt1+ epicardial cells as previously unrecognized cardiomyocyte progenitors, and lay the foundation for future efforts to harness the cardiogenic potential of these progenitors for cardiac regeneration and repair.

Heart growth is tightly controlled so that the heart reaches a predetermined size. Fetal heart growth occurs through cardiomyocyte proliferation, whereas postnatal heart growth involves primarily physiological cardiomyocyte hypertrophy. The Hippo kinase cascade is an important regulator of organ growth. A major target of this kinase cascade is YAP1, a transcriptional coactivator that is inactivated by Hippo kinase activity. Here, we used both genetic gain and loss of Yap1 function to investigate its role in regulating proliferative and physiologic hypertrophic heart growth. Fetal Yap1 inactivation caused marked, lethal myocardial hypoplasia and decreased cardiomyocyte proliferation, whereas fetal activation of YAP1 stimulated cardiomyocyte proliferation. Enhanced proliferation was particularly dramatic in trabecular cardiomyocytes that normally exit from the cell cycle. Remarkably, YAP1 activation was sufficient to stimulate proliferation of postnatal cardiomyocytes, both in culture and in the intact heart. A dominant negative peptide that blocked YAP1 binding to TEAD transcription factors inhibited YAP1 proliferative activity, indicating that this activity requires YAP1–TEAD interaction. Although Yap1 was a critical regulator of cardiomyocyte proliferation, it did not influence physiological hypertrophic growth of cardiomyocytes, because postnatal Yap1 gain or loss of function did not significantly alter cardiomyocyte size. These studies demonstrate that Yap1 is a crucial regulator of cardiomyocyte proliferation, cardiac morphogenesis, and myocardial trabeculation. Activation of Yap1 in postnatal cardiomyocytes may be a useful strategy to stimulate cardiomyocyte expansion in therapeutic myocardial regeneration.

The prospect of cell-based therapy in cardiovascular regenerative medicine comes a step closer with the demonstration that a peptide can stimulate a progenitor cell population in the adult heart to act as a source of new cardiomyocytes. The stem or progenitor cells, thought to be derived from the epicardium, are activated by the small actin monomer binding protein thymosin β4, which has previously been shown to restore vascular potential to adult epicardium-derived progenitor cells after injury. The discovery of a resident source of myocardial progenitors will stimulate a search for small molecules and other factors that promote optimal progenitor activation and replacement of destroyed myocardium. A significant bottleneck in cardiovascular regenerative medicine is the identification of a viable source of stem/progenitor cells that could contribute new muscle after ischaemic heart disease and acute myocardial infarction1. A therapeutic ideal—relative to cell transplantation—would be to stimulate a resident source, thus avoiding the caveats of limited graft survival, restricted homing to the site of injury and host immune rejection. Here we demonstrate in mice that the adult heart contains a resident stem or progenitor cell population, which has the potential to contribute bona fide terminally differentiated cardiomyocytes after myocardial infarction. We reveal a novel genetic label of the activated adult progenitors via re-expression of a key embryonic epicardial gene, Wilm’s tumour 1 (Wt1), through priming by thymosin β4, a peptide previously shown to restore vascular potential to adult epicardium-derived progenitor cells2 with injury. Cumulative evidence indicates an epicardial origin of the progenitor population, and embryonic reprogramming results in the mobilization of this population and concomitant differentiation to give rise to de novo cardiomyocytes. Cell transplantation confirmed a progenitor source and chromosome painting of labelled donor cells revealed transdifferentiation to a myocyte fate in the absence of cell fusion. Derived cardiomyocytes are shown here to structurally and functionally integrate with resident muscle; as such, stimulation of this adult progenitor pool represents a significant step towards resident-cell-based therapy in human ischaemic heart disease.

The epicardium makes essential cellular and paracrine contributions to the growth of the fetal myocardium and the formation of the coronary vasculature. However, whether the epicardium has similar roles postnatally in the normal and injured heart remains enigmatic. Here, we have investigated this question using genetic fate-mapping approaches in mice. In uninjured postnatal heart, epicardial cells were quiescent. Myocardial infarction increased epicardial cell proliferation and stimulated formation of epicardium-derived cells (EPDCs), which remained in a thickened layer on the surface of the heart. EPDCs did not adopt cardiomyocyte or coronary EC fates, but rather differentiated into mesenchymal cells expressing fibroblast and smooth muscle cell markers. In vitro and in vivo assays demonstrated that EPDCs secreted paracrine factors that strongly promoted angiogenesis. In a myocardial infarction model, EPDC-conditioned medium reduced infarct size and improved heart function. Our findings indicate that epicardium modulates the cardiac injury response by conditioning the subepicardial environment, potentially offering a new therapeutic strategy for cardiac protection.