Abstract 14739: Induction of Native Cardiomyocyte Cell Cycle Re-Entry by Transplanted Cells as a Novel Mechanism of Therapeutic Regeneration in the Heart
Background: The extent of cardiomyocyte (CM) turnover in adult mammals remains unclear, while controversy surrounds the putative mechanisms of CM replenishment. Using a genetic fate mapping approach and objective methods of quantification, we investigated the ability of adult CMs to re-enter the cell cycle in the normal, infarcted and cell-treated heart.
Methods & Results: Tamoxifen pulsing of bitransgenic MerCreMer-ZEG mice (αMHC promoter driving expression of tamoxifen-inducible Cre), resulted in efficient (~80%) and specific labeling of CMs by GFP. Mice were then subjected to: a) sham surgery, b) myocardial infarction (MI), and c) MI followed by cardiosphere-derived cell (CDC) injection. Mice were pulsed with BrdU daily for up to 5 weeks. CMs, isolated by Langendorff enzymatic dissociation, underwent FACS-sorting for GFP (>98.9% GFP+, with minimal contamination by non-CMs) and subsequent flow cytometry for BrdU and Ki67. The normal adult mouse heart contains a small pool of cycling resident CMs (~0.08%/week, projecting to a CM turnover of ~1.3-4%/yr), which roughly doubles during the first 3 weeks after MI. Transplantation of CDCs further dramatically upregulates the percentage of cycling host CMs (to ~0.7%/week), while boosting heart function, reducing scar size and increasing viable myocardium. Immunocytochemistry of isolated CMs confirmed the flow cytometry results, while PCR microarray analysis showed upregulation of several genes associated with cell-cycle progression in CMs post MI, the expression of which was further amplified by CDC therapy. Cycling CMs are smaller, more mononucleated and primarily located in the infarct border zone. The observed phenomena could not be explained by progenitor cell proliferation, CM polyploidization, bi/multinucleation, cell fusion or DNA repair. Quantitative estimates (by comparing cycling in GFP+ and GFP- CM fractions) indicate that CM proliferation accounts for as much therapeutic regeneration as does recruitment of endogenous stem cells.
Conclusions: Bystander-induced proliferation of native cardiomyocytes represents a significant, previously-unrecognized mechanism of cardiac regeneration in response to cell therapy.
- © 2012 by American Heart Association, Inc.