The complex genetics of heart regeneration

Medical University Of South Carolina, Charleston SC

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Abstract

Project Summary Mammalian heart development does not end with birth; major changes occur in the early postnatal heart that lay the foundation for subsequent physiology and pathology. A critical transition occurs shortly after birth in the outcome of cardiomyocyte (CM) cell cycle: in the embryo, CMs complete cell cycle and divide (proliferate), whereas postnatally, most CMs interrupt cell cycle after S-phase DNA replication but before cytokinesis and become polyploid. The same events in the normal postnatal heart are reiterated in the injured adult heart: CMs are induced by injury to enter cell cycle and then mostly interrupt rather than complete it, resulting in failure to replace lost CMs. A critical gap in knowledge relates to the mechanisms that cause postnatal CMs to interrupt or complete cell cycle; for a process that is so fundamental to both embryonic heart growth and postnatal heart regeneration, it is surprising how little is known of the cellular and molecular features that underlie these events. In this project renewal, we propose to explore two novel pathways that influence mammalian CM cell cycle completion, ploidy and regeneration. Both Aims of this project relate to oxygen or reactive oxygen, which rise in the postnatal period. Both Aims invoke signaling mechanisms (rather than nonspecific damage or toxicity) that transmit oxygenation into the physiological outcome of cell cycle interruption and CM polyploidy. Aim 1 builds on our prior discovery of Tnni3k, which is a CM-specific kinase. Tnni3k activity promotes CM polyploidy, whereas absence of Tnni3k activity improves CM cell cycle completion. We discovered that Tnni3k is hyperactivated by peroxiredoxin mediated oxidation. The studies proposed use a range of cell and animal models to expand on this insight in normal heart biology and in heart regeneration. Aim 2 involves FIH1, which we newly identified as a regulator of CM polyploidy. FIH1 is an oxygen-dependent asparagine hydroxylase, and through a genetic strategy described in the proposal, we also identified Pik3r4 as a candidate substrate of FIH1 action. Pik3r4 is a component of the midbody machinery that initiates abscission, the terminal step of cytokinesis and the step at which most mouse CMs fail when becoming polyploid. The proposed analyses define this interaction and its consequences in normal heart biology and in heart regeneration. Our earlier insights established a relationship between CM ploidy, proliferation and regeneration. The studies in this application seek to extend this understanding to its underlying mechanistic components.