Transient induction of cell cycle promoter Fam64a improves cardiac function through regulating Klf15-dependent cardiomyocyte differentiation in mice

The introduction of fetal or neonatal signatures such as cell cycle promoting genes into damaged adult hearts has been vigorously pursued as a promising strategy for stimulating proliferation and regeneration of adult cardiomyocytes, which normally cannot divide. However, cell division of cardiomyocytes requires preceding dedifferentiation with sarcomere disassembly and calcium dysregulation, which, in principle, compromises contractile function. To overcome this intrinsic dilemma, we explored the feasibility of optimizing the induction protocol of the cell cycle promoter in mice. As a model of this approach, we used Fam64a, a fetal-specific cardiomyocyte cell cycle promoter that we have recently identified. We first analyzed transgenic mice maintaining long-term cardiomyocyte-specific expression of Fam64a after birth, when endogenous expression was abolished. Despite having an enhanced proliferation of postnatal cardiomyocytes, these mice showed age-related cardiac dysfunction characterized by sustained cardiomyocyte dedifferentiation, which was reminiscent of the dilemma. Mechanistically, Fam64a inhibited glucocorticoid receptor-mediated transcriptional activation of Klf15, a key regulator that drives cardiomyocyte differentiation, thereby directing cardiomyocytes toward immature undifferentiated states. In contrast, transient induction of Fam64a in cryoinjured wildtype adult mice hearts improved functional recovery with augmented cell cycle activation of cardiomyocytes. These data indicate that optimizing the intensity and duration of the stimulant to avoid excessive cardiomyocyte dedifferentiation could pave the way toward developing efficient strategy for successful heart regeneration.