The Role of RUNX1 in CM Cell Cycle Activity and its Impact on Cardiac Regeneration

Kaelin A. Akins, Samantha K. Swift, Mary E. Kolell¹, Michael Flinn, Samantha Paddock, Caitlin O'Meara, Michaela Patterson

Department of Cell Biology, Neurobiology, and Anatomy, and the Cardiovascular Center, Medical College of Wisconsin

Factors responsible for cardiomyocyte proliferation could serve as a potential therapeutic to stimulate endogenous myocardial regeneration following insult, such as ischemic injury. A previously published forward genetics approach assessing the frequency of a rare and presumed proliferative-competent subpopulation of cardiomyocytes, mononuclear diploid cardiomyocytes (MNDCMs), led us to the transcription factor, RUNX1. Here, we examine the effect of RUNX1 on cardiomyocyte cell cycle during postnatal development and cardiac regeneration using cardiomyocyte-specific gain- and loss-of-function mouse models. We hypothesize that RUNX1 overexpression (OE) increases cardiomyocyte cell cycle activity with expansion of the MNDCMs population extending the neonatal regenerative window and positively impacting cardiac regeneration.

RUNX1 expression in cardiomyocytes peaks in the postnatal window and becomes negligible in adulthood. RUNX1 KO decreased postnatal cardiomyocyte cell activity, while RUNX1 OE extended the period of cell cycle activation. This expanded cell cycle activation observed in RUNX1 OE mice is complete with cytokinesis resulting in an expansion of the MNDCM population and total cardiomyocyte endowment. After injury, RUNX1 expression modestly increased in cardiomyocytes. Therefore, we wondered if RUNX1 could similarly regulate cardiomyocyte cell cycle in neonatal regeneration. RUNX1 KO neonates with a P1 resection showed no difference in cell cycle activity, cardiac function, and fibrosis suggestingRUNX1 is not required for neonatal heart regeneration. RUNX1 OE neonatal animals with a P6 MI displayed no difference in cardiomyocyte cell cycle activity at 7days post injury, however, we observed an increase in cell cycle activity 28 days post injury accompanied by exacerbated cardiac function. Examination of the ploidy dynamics revealed that RUNX1 OE neonates displayed a marked increase in the 4N population with decreased 8N and 16N populations, perhaps congruent to ploidy reversal. Taken together, RUNX1 indeed influences cardiomyocyte cell cycle activity in development, a phenomenon which does not perfectly translate in the context of injury.