In vitro and in vivo roles of glucocorticoid and vitamin D receptors in the control of cardiomyocyte proliferative potential

Abstract

Cardiomyocyte (CM) proliferative potential varies considerably across species. While lower vertebrates and neonatal mammals retain robust capacities for CM proliferation, adult mammalian CMs lose proliferative potential due to cell-cycle withdrawal and polyploidization, failing to mount a proliferative response to regenerate lost CMs after cardiac injury. The decline of murine CM proliferative potential occurs in the neonatal period when the endocrine system undergoes drastic changes for adaptation to extrauterine life. We recently demonstrated that thyroid hormone (TH) signaling functions as a primary factor driving CM proliferative potential loss in vertebrates. Whether other hormonal pathways govern this process remains largely unexplored. Here we showed that agonists of glucocorticoid receptor (GR) and vitamin D receptor (VDR) suppressed neonatal CM proliferation in vitro. We next examined CM nucleation and proliferation in mutant mice lacking GR or VDR specifically in CMs, but we observed no difference between mutant and control littermates. Additionally, we generated compound mutant mice that lack GR or VDR and express dominant-negative TH receptor alpha in their CMs, and similarly observed no increase in CM proliferative potential compared to dominant-negative TH receptor alpha mice alone. Thus, although GR and VDR activation in cultured CMs is sufficient to inhibit CM proliferation, they seem to be dispensable for CM cell-cycle exit and binucleation in vivo. In addition, given the recent report that VDR activation in zebrafish promotes CM proliferation and tissue regeneration, our results suggest distinct roles of VDR in zebrafish and rodent CM cell-cycle regulation. BACKGROUNDCardiovascular disease is the leading killer in the United States, mostly due to heart failure after myocardial infarction (MI) 1,2 . After ischemic injury like MI induces CM death in the hearts of adult mammals, lost CMs are not replenished and fibrotic tissue permanently replaces previously functional cardiac muscle. However, lower vertebrates like zebrafish (Danio rerio), newts (Notophthalmus viridescens), and axolotls (Ambystoma mexicanum) display robust CM proliferation and myocardial regeneration after cardiac injury 2-4 . Intriguingly, neonatal mammals also transiently possess considerable CM proliferative potential. Ischemic heart injury induced in newborn mice at P0 or P1 resolves as fibrosis-free, fully-regenerated cardiac muscle by 21 days post-injury, mediated by existing CMs that proliferate to reconstitute the lost myocardium 5,6 . This transient CM proliferative potential is lost in mice after the first week as neonatal CMs binucleate and permanently exit the cell cycle 7 . Because the regeneration-competent hearts of lower vertebrates and neonatal mammals consist predominantly of mononuclear diploid CMs while the non-regenerative hearts of adult mammals consist primarily of polyploid CMs, this developmental polyploidization -documented in both mice and humans -is implicated as a critical inhibitor of CM proliferative...