Transgenic overexpression of gamma-cytoplasmic actin protects against eccentric contraction-induced force loss in mdx mice

Background: gamma-cytoplasmic (gamma-(cyto)) actin levels are elevated in dystrophin-deficient mdx mouse skeletal muscle. The purpose of this study was to determine whether further elevation of gamma-(cyto) actin levels improve or exacerbate the dystrophic phenotype of mdx mice. Methods: We transgenically overexpressed gamma-(cyto) actin, specifically in skeletal muscle of mdx mice (mdx-TG), and compared skeletal muscle pathology and force-generating capacity between mdx and mdx-TG mice at different ages. We investigated the mechanism by which gamma-(cyto) actin provides protection from force loss by studying the role of calcium channels and stretch-activated channels in isolated skeletal muscles and muscle fibers. Analysis of variance or independent t-tests were used to detect statistical differences between groups. Results: Levels of gamma-(cyto) actin in mdx-TG skeletal muscle were elevated 200-fold compared to mdx skeletal muscle and incorporated into thin filaments. Overexpression of gamma-(cyto) actin had little effect on most parameters of mdx muscle pathology. However, gamma-(cyto) actin provided statistically significant protection against force loss during eccentric contractions. Store-operated calcium entry across the sarcolemma did not differ between mdx fibers compared to wild-type fibers. Additionally, the omission of extracellular calcium or the addition of streptomycin to block stretch-activated channels did not improve the force-generating capacity of isolated extensor digitorum longus muscles from mdx mice during eccentric contractions. Conclusions: The data presented in this study indicate that upregulation of gamma-(cyto) actin in dystrophic skeletal muscle can attenuate force loss during eccentric contractions and that the mechanism is independent of activation of stretch-activated channels and the accumulation of extracellular calcium.