Abstract
<jats:p>The actin cytoskeleton of the cardiomyocyte is organized into two structurally and functionally distinct filament networks. The sarcomeric thin filaments, built primarily from cardiac α-actin (αCA), generate contractile force, whereas a separate subsarcolemmal cytoplasmic network, built from β-actin (Actb gene) and γ-actin (Actg1 gene), lies beneath the sarcolemma and is comparatively understudied in the mature heart. We hypothesized that this cytoplasmic actin network is required for sarcolemmal membrane integrity, signal transduction, and mechanosensing in the adult cardiomyocyte, and we generated cardiomyocyte-specific Actb and Actg1 double-gene deleted mice, using loxP (fl)-targeted Actbfl/fl and Actg1fl/fl alleles combined with an αMHC (Myh6) promoter-driven Cre-recombinase transgene, to test this directly. Deletion of cytoplasmic β-actin and γ-actin from cardiomyocytes (Actb/g1fl/fl-Myh6-Cre mice) drove compensatory upregulation of a γ-interferon like stress response with compensatory upregulation of skeletal α-actin (αSKA) and smooth muscle α-actin (αSMA) protein in adult cardiomyocytes, without altering baseline cardiac structure or function. To test if lost β-actin and γ-actin in the mouse heart impacts sarcolemmal stability, we crossed Actb/g1fl/fl-Myh6-Cre mice onto the dystrophin-deficient mdx background, which is characterized by a fragile sarcolemma. Unexpectedly, hearts from Actb/g1fl/fl-Myh6-Cre; mdx mice showed greater membrane stability than mdx hearts alone with intact Actb/Actg1. We also observed that Actb/g1fl/fl-Myh6-Cre mice subjected to chronic pressure overload by transverse aortic constriction (TAC) were protected and developed less cardiac hypertrophy, had better preserved systolic function, and improved survival. Together, these data indicate induction of the cytoplasmic β-actin/γ-actin network in the heart during disease stimulation is maladaptive and weakens the sarcolemma, and 2 downstream mechanisms are considered that could mediate this effect.</jats:p>