Resident c-kit + cells in the heart are not cardiac stem cells

Identifying a bona fide population of cardiac stem cells (CSCs) is a critical step for developing cell-based therapies for heart failure patients. Previously, cardiac c-kit + cells were reported to be CSCs with a potential to become myocardial, endothelial and smooth muscle cells in vitro and after cardiac injury. Here we provide further insights into the nature of cardiac c-kit + cells. By targeting the c-kit locus with multiple reporter genes in mice, we find that c-kit expression rarely co-localizes with the expression of the cardiac progenitor and myogenic marker Nkx2.5, or that of the myocardial marker, cardiac troponin T (cTnT). Instead, c-kit predominantly labels a cardiac endothelial cell population in developing and adult hearts. After acute cardiac injury, c-kit + cells retain their endothelial identity and do not become myogenic progenitors or cardiomyocytes. Thus, our work strongly suggests that c-kit + cells in the murine heart are endothelial cells and not CSCs.