Abstract 3485: Cardiac Stem Cells Defined by Colony Forming Unit Assays Are From an Epicardial/Mesodermal but Not Neural-Crest, Cardiomyocyte or Bone-Marrow Origin

Support for the ambitious goal of therapeutic cardiac regeneration has come from the recent discovery of endogenous cardiac stem cells in the post-natal heart. Though the biological origin of these cells is presently poorly understood, the epicardium has been identified as an important source of cells for cardiac repair. We employed the colony forming unit fibroblast (CFU-F) assay to show that a subpopulation (cCFU-F) of Sca1+/CD31- cells from the adult murine heart are clonogenic, long term self renewing and multi-potent (properties of true stem cells). We hypothesised that cCFU-F are derived from a cardiac developmental and not bone marrow (BM) origin. To assess this, double transgenic (Cre/ZEG) mice were created to conduct Cre-Lox lineage tracing of cCFU-F. A possible epicardial, mesodermal, neural crest, committed cardiac or cardiomyocyte origin was investigated by placing Cre-recombinase under the control of GATA5, Mesp1, Wnt1, Nkx2–5 and MLC2v promoters. To investigate a possible BM origin, whole BM was transplanted from transgenic mice ubiquitously expressing green fluorescent protein (Β-actin-GFP) into lethally irradiated wild-type recipients with and without cardiac lead shielding. Percentage of cCFU-F colonies from respective lineage origins (n=3) were 79±6% epicardial, 81±7% mesodermal, 10±2% committed, 0±0% neural crest and 0±0% cardiomyocyte. Few Sca1+/CD31- cardiac cells were BM derived at 3 months (5.1±1%, n=5) and 6 months (0.5±0.4%, n=8) post-transplantation. Colony forming ability of cCFU-F in irradiated/non-shielded mice was lost and not rescued by BM transplantation either after normal aging or myocardial infarction (MI) to stimulate BM homing. Cardiac shielding preserved colony formation with all (100±0%) colonies derived from non-BM (GFP-cells) 8 weeks post transplantation after MI (n=3) or normal aging (n=4). In conclusion, cCFU-F endogenous cardiac stem cells are derived from a mesodermal and potentially epicardial origin but not from the neural crest, dedifferentiated cardiomyocytes or adult BM. Experiments with other epicardial specific inducible Cre lines are planned. Knowledge of cardiac stem cell origins will progress understanding of their biology possibly aiding translation to therapeutic strategies.