Ploidy Reversal As A Postnatal Developmental Program In Murine Hearts

Polyploidization is a normal cellular adaptation for a variety of cell types in mammals, including cardiomyocytes (CMs); however, the development of polyploidization understudied. Recent work suggests that genetics contribute to diverse displays of ploidy in the adult murine heart. Therefore, we hypothesized that the developmental progression to reach differing end-states may similarly be affected. Here, we assessed CM endowment, cell cycle, and ploidy temporally across two diverse inbred mouse strains, A/J and C57Bl/6J. Consistent with previous work, C57Bl/6J hearts displayed rapid cell cycle activation in the first postnatal week largely coinciding with the first round of endomitosis. Total CM numbers are relatively unchanged after P7, while final ploidy states are generally constant from P14 on. In contrast, A/J mice displayed depressed cell cycle activation in the first week of life. Polyploidy in A/Js reaches its peak at P21, whereby only ~3.5% of CMs remained mononuclear and diploid. Interestingly, from 3-6 weeks of age, we observed a dramatic expansion of total CM numbers and of the 1x2N subpopulation to ~8%, which could not be explained by proliferation of the residual diploid population. Instead, the expanded diploid population appears to come from a polyploid CM. This finding was confirmed by analysis identifying a population of CMs which had definitively completed cytokinesis by 6 weeks which was not present at 3 weeks. We believe this is the first report of ploidy reversal by a mammalian CM, a phenomenon first observed by the hepatocyte field. Ongoing single nuclear RNA seq is examining the transcriptomic differences between A/J and C57Bl/6J CMs at P21 to identify this unique “primed” population competent to undergo ploidy reversal. Preliminary results from this study suggest that AJ CM nuclei, regardless of their ploidy, are more likely to be in the cell cycle in comparison to C57. Understanding the developmental paths to end state CM endowment and ploidy states can help us understand the biology of organ size and reciprocally can be applied to stimulating regeneration.