Abstract 121: Cellular Origins of Postnatal Cardiomyogenesis in the Normal, Infarcted and Cell-Treated Heart

Abstract

Background: Controversy surrounds the putative mechanisms of cardiomyocyte (CM) replenishment in adult mammals. Using genetic fate mapping and bone marrow allotransplantation, we investigated the postnatal origins of CMs in the normal, infarcted and cell-treated heart. Methods & Results: Tamoxifen pulsing of non-infarcted bitransgenic MerCreMer-ZEG mice (αMHC promoter driving inducible Cre) resulted in specific labeling of CMs by GFP. Comparison of BrdU incorporation (after 5 weeks of BrdU administration) in GFP+ and GFP- CMs (by flow cytometry, immunocytochemistry and histology) indicated that in the normal adult mouse heart, CM turnover occurs predominantly through proliferation of resident small mononucleated CMs at an annual rate of ~1.3%, independent from CM polyploidization, multinucleation, cell fusion or DNA repair. After myocardial infarction (MI), new CM formation in the border zone arises from both progenitor cells (PCs) as well as pre-existing CMs. Transplantation of cardiosphere-derived cells (CDCs) upregulates both host CM cycling and recruitment of PCs. To investigate the identity of endogenous PCs, bitransgenic mice underwent MI, followed by tamoxifen pulsing. One week later, FACS of myocyte-depleted hearts resulted in isolation of small non-myocyte GFP+ cells, which exhibit activated αMHC post-MI (~1.5% of cells in risk area vs <0.2% in sham controls). These novel PCs (Sca1+ [53%]/ nestin+ [11%]/ ckit-/ ABCG2-) express cardiac transcription factors (Nkx 2.5, GATA4, MEF2C, FOXC2), cardiogenic molecules (BMP2) and cardiac structural proteins (at both transcript & protein level), beat spontaneously in vitro and form CMs when injected into recipient hearts. Bone marrow transplantation demonstrated that recruited PCs do not arise from hematogenous seeding. Cell therapy with CDCs stimulates recruitment of PCs (~4.2% of cells in risk area), while improving heart function and increasing viable myocardium. Conclusions: The normal adult mouse heart replenishes lost CMs through proliferation of pre-existing CMs. After MI, proliferation of resident CMs increases, but new CMs also arise from recruited PCs. CDCs boost both CM proliferation as well as recruitment of endogenous PCs, resulting in myocardial regeneration.