ALTERED EXPRESSION OF TROPOMODULIN IN CARDIOMYOCYTES DISRUPTS THE SARCOMERIC STRUCTURE OF MYOFIBRILS

Tropomodulin is a tropomyosin-binding protein that terminates ''pointe d-end'' actin filament polymerization. To test the hypothesis that reg ulation of tropomoduliniactin filament stoichiometry is critical for m aintenance of actin filament length, tropomodulin levels were altered in cells by infection with recombinant adenoviral expression vectors, which produce either sense or antisense tropomodulin mRNA. Neonatal ra t cardiomyocytes were infected, and sarcomeric actin filament organiza tion was examined. Confocal microscopy indicated that overexpression o f tropomodulin protein shortened actin filaments and caused myofibril degeneration. In contrast, decreased tropomodulin content resulted in the formation of abnormally long actin filament bundles. Despite chang es in myofibril structure caused by altered tropomodulin expression, t otal protein turnover of the cardiomyocytes was unaffected. Biochemica l analyses of infected cardiomyocytes indicated that changes in actin distribution, rather than altered actin content, accounted for myofibr il reorganization. Ultrastructural analysis showed thin-filament disar ray and revealed the presence of leptomeres after tropomodulin overexp ression. Tropomodulin-mediated effects constitute a novel mechanism to control actin filaments, and our findings demonstrate that regulated tropomodulin expression is necessary to maintain stabilized actin fila ment structures in cardiac muscle cells.