The ACTC A295S Mutation Increases Heart and Skeletal Muscle Contractile Activity in Drosophila Models of Hypertrophic Cardiomyopathy

Abstract

Hypertrophic cardiomyopathy (HCM) is an autosomal dominant disease characterized by thickened heart walls, cardiac arrhythmias, myocyte disarray and interstitial fibrosis. Over 10 mutations in alpha-cardiac actin (ACTC) are associated with HCM. The ACTC Ala295Ser substitution causes a highly penetrant disease with diverse phenotypes. To investigate the in vivo consequences of the ACTC mutation we generated Drosophila lines with muscle-specific expression of A295S actin. Our unique models limit genetic diversity and pathological complexity to help resolve molecular mechanisms of diseases. For expression in the fly heart we used UAS-Act57BWT and UAS-Act57BA295S actin transgenes in conjunction with the HandGal4 cardiac-specific driver. Confocal microscopy confirmed heart-restricted expression of UAS-Act57BWTGFP and UAS-Act57BA295SGFP and co-polymerization of transgenic and endogenous actin. High-speed video microscopy and motion analysis revealed A295S actin expression significantly reduced diastolic volumes and increased systolic intervals consistent with elevated contractile properties at rest and during contraction. For indirect flight muscle (IFM)-specific actin expression, we used the IFM targeted Act88FWTand Act88FA295S transgenes, in an IFM actin null background. Muscle mechanics on heterozygous IFM fibers revealed that the A295S actin mutation increased power, the frequency of maximum power generation (fmax), and stiffness relative to control fibers. The higher fmax and stiffness are likely causes of an observed increase in wing beat frequency (WBF) of Act88FA295Sheterozygous flies. The WBF increase had a negative impact on flight ability which decreased further as mutant: wildtype actin ratios were increased. Overall, enhanced contractile activity of actin HCM cardiac myocytes and IFM fibers is consistent with the hypercontractile properties frequently associated with HCM mutations.