The formin Fhod3 is required for sarcomere reassembly after mitosis in postnatal cardiomyocytes

Fhod3, a member of the formin family of proteins that regulate actin polymerization, is essential for the organization of cardiac sarcomeres during embryonic cardiogenesis. We previously demonstrated that the perinatal deletion of Fhod3 in mice results in lethality with disorganized sarcomeres, whereas the deletion in the adult period is non-lethal and does not affect sarcomere morphology. To clarify the underlying mechanism of this developmental stage-dependent outcome, we examined the effects of Fhod3 depletion in primary cultured cardiomyocytes derived from Fhod3^flox neonates. Long-range time-lapse imaging throughout the entire culture period revealed that Fhod3 depletion around birth using muscle creatine kinase-Cre caused sarcomere disruption only in cells that had undergone mitosis and not in cells that had not undergone mitosis, suggesting that sarcomere breakdown by Fhod3 deletion occurs after mitosis. To verify this, we varied the timing of Fhod3 deletion using a tamoxifen-inducible αMHC-MerCreMer line. Cells treated with tamoxifen from day 1 in vitro (DIV1) exhibited a disruption of sarcomeres, similar to that observed in the Fhod3-depleted cells by MCK-Cre around birth. In contrast, cells treated from DIV9, when postnatal mitosis was almost complete, showed organized sarcomere despite Fhod3 depletion. Thus, in the presence of Fhod3, sarcomere reassembly after mitosis was successful, whereas it failed in the absence of Fhod3, indicating the indispensable role of Fhod3 in sarcomere reorganization after mitosis.