Abstract 18760: Resident c-kit-positive Cardiac Stem Cells Replenish Cardiomyocytes During Physiological Cardiac Growth

Abstract

How cell diversity is maintained within the adult heart through normal wear and tear and physio-pathologic remodeling remains an unanswered question. Intriguingly, exercise training produces physiological growth of the myocardium encompassing both myocyte hypertrophy and new myocyte formation. We assessed whether new myocyte formation during exercise training is the direct progeny of resident c-kit-positive (c-kit-pos) cardiac stem/progenitor cells (CSCs). Mice underwent a program of controlled swimming for 28 days. Untrained mice acted as sedentary controls. To distinguish new myocyte formation (detected by BrdU in vivo labeling) due to unexpected myocyte division or to stem cell differentiation, we used double-transgenic mice by crossbreeding the myocyte-specific Myh6-MerCreMer mice with p-lox-LacZ-Stop-lox/eYFP (ZYP) mice. To ascertain a direct relationship between c-kit-pos CSCs and new myocyte generation, we released into the myocardium of ZYP mice a lentiviral vector carrying Cre-Recombinase under the control of the c-kit promoter (Lenti-c-kit/Cre). This strategy permanently labels with YFP only resident c-kit-pos CSCs and allows to track their cell fate. Physical exercise produced rapid myogenic differentiation in c-kit-pos CSCs during the 28 days swimming program, which was followed by an increased number of new myocytes. The inducible myocyte-specific genetic fate-mapping strategy shows that new myocytes are not generated through the division of pre-existing myocytes but rather are replenished by stem/progenitor cells. Importantly, when chimeric GFP bone marrow-transplanted mice were enrolled in the training program, all new myocytes were GFP-negative, thus confirming they were not derived from cardiac homed bone marrow stem cells. Finally, through the Cre/YFP c-kit-pos CSC specific genetic recombination in vivo, we demonstrated that new myocyte formation during exercise training is the direct progeny of c-kit-pos CSC differentiation. These data based on genetic cell fate mapping provide the convincing direct evidence that cardiac workload triggers the in situ cardiac regenerative capacity of c-kit-pos CSCs. This property of the myocardium could be meaningfully exploited for cardiac regenerative therapies.