Fetal cardiomyocytes actively proliferate to form the primitive heart in utero in mammals, but they stop dividing shortly after birth. The identification of essential molecules maintaining this active cardiomyocyte proliferation is indispensable for potential adult heart regeneration. A recent study has shown that this proliferation depends on a low fetal oxygen condition before the onset of breathing at birth. We have established an isolation protocol for mouse fetal cardiomyocytes, performed under strict low oxygen conditions to mimic the intrauterine environment, that gives the highest proliferative activities thus far reported. Oxygen exposure during isolation/culture markedly inhibited cell division and repressed cell cycle-promoting genes, and subsequent genome-wide analysis identified Fam64a as a novel regulatory molecule. Fam64a was abundantly expressed in hypoxic fetal cardiomyocyte nuclei, but this expression was drastically repressed by oxygen exposure, and in postnatal cardiomyocytes following the onset of breathing and the resulting elevation of oxygen tension. Fam64a knockdown inhibited and its overexpression enhanced cardiomyocyte proliferation. Expression of a non-degradable Fam64a mutant suggested that optimum Fam64a expression and subsequent degradation by anaphase-promoting complex/cyclosome (APC/C) during the metaphase-to-anaphase transition are required for fetal cardiomyocyte division. We propose that Fam64a is a novel cell cycle promoter of hypoxic fetal cardiomyocytes in mice.

Loss of cardiomyocyte proliferative capacity after birth is a major obstacle for therapeutic heart regeneration in adult mammals. We and others have recently shown the importance of hypoxic in utero environments for active foetal cardiomyocyte proliferation. Here, we report the unexpected expression of novex-3, the short splice variant of the giant sarcomeric protein connectin (titin), in the cardiomyocyte nucleus specifically during the hypoxic foetal stage in mice. This nuclear localisation appeared to be regulated by the N-terminal region of novex-3, which contains the nuclear localisation signal. Importantly, the nuclear expression of novex-3 in hypoxic foetal cardiomyocytes was repressed at the postnatal stage following the onset of breathing and the resulting elevation of oxygen tension, whereas the sarcomeric expression remained unchanged. Novex-3 knockdown in foetal cardiomyocytes repressed cell cycle-promoting genes and proliferation, whereas novex-3 overexpression enhanced proliferation. Mechanical analysis by atomic force microscopy and microneedle-based tensile tests demonstrated that novex-3 expression in hypoxic foetal cardiomyocytes contributes to the elasticity/compliance of the nucleus at interphase and facilitates proliferation, by promoting phosphorylation-induced disassembly of multimer structures of nuclear lamins. We propose that novex-3 has a previously unrecognised role in promoting cardiomyocyte proliferation specifically at the hypoxic foetal stage.

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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.

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