Abstract 920: Mutual Potentiation Between Myofibril Assembly and Serum Response Factor in Cardiomyocyte Maturation

Abstract

Cardiomyocyte (CM) maturation is characterized by transcriptional, morphological and functional specializations that are essential for robust and sustained CM contractions throughout lifetime. The signal networks that govern CM maturation remain poorly defined, which obscures the role of CM maturation in inherited cardiomyopathies and myocardial regeneration and impairs efforts to engineer mature cardiac tissues in vitro . Our prior studies established the transcription factor serum response factor (SRF) as a key regulator of CM maturation: SRF regulates major CM maturation events including myofibril expansion, mitochondria biogenesis, transverse-tubule formation, and cellular hypertrophy. Myofibrillar genes were identified as direct SRF downstream targets that are required for other aspects of CM maturation. To further understand the role of myofibrils in CM maturation, here we report the generation and investigation of a floxed allele of Actn2 in mice, which encodes a central component of sarcomere Z-lines. We applied to these mice a low dose of adeno-associated virus that expressed Cre recombinase specifically in neonatal CMs to generate hearts with mosaic Actn2 mutation. This approach circumvented the confounding secondary effects of cardiac dysfunction in Actn2 mutants and revealed cell-autonomous gene functions. Strikingly, Actn2 ablation triggered dramatic transcriptional dysregulation in addition to the expected myofibrillar disassembly phenotypes in CMs, which strongly correlated with observations in SRF-depleted CMs. Actn2 mutation increased the amount of monomeric actin in CMs, which perturbed the nuclear localization of SRF cofactors MRTF-A/-B. Overexpression of a dominant-negative MRTF-A isoform was sufficient to recapitulate the transcriptional and morphological defects in Actn2 or Srf mutant CMs. Together, these data demonstrate mutual potentiation between myofibril assembly and MRTF-A/B-SRF signaling in CM maturation. This positive feedback loop underlies a novel mechanism by which mechanical forces regulate CM maturation, disruption of which likely contributes to cardiomyopathies caused by sarcomere gene mutations.