THE LOW-MOLECULAR-WEIGHT GTPASE RHO REGULATES MYOFIBRIL FORMATION ANDORGANIZATION IN NEONATAL RAT VENTRICULAR MYOCYTES - INVOLVEMENT OF RHO-KINASE

The assembly of contractile proteins into organized sarcomeric units i s one of the most distinctive features of cardiac myocyte hypertrophy. In a well characterized in vitro model system using cultured neonatal rat ventricular myocytes, a subset of G protein-coupled receptor agon ists has been shown to induce actin-myosin filament organization. Pret reatment of myocytes with C3 exoenzyme ADP-ribosylated Rho and inhibit ed the characteristic alpha(1)-adrenergic receptor agonist-induced myo fibrillar organization, suggesting involvement of the Rho GTPase in ca rdiac myofibrillogenesis. We used adenoviral mediated gene transfer to examine the effects of activated Rho and inhibitory mutants of one of its effecters, Rho kinase, in myocytes. Rho immunoreactivity was incr eased in the particulate fraction of myocytes infected with a recombin ant adenovirus expressing constitutively activated Rho. Rho-infected c ells demonstrated a striking increase in the assembly and organization of sarcomeric units and in the expression of the atrial natriuretic f actor protein. These Rho-induced responses were markedly inhibited by co-infection with adenoviruses expressing putative dominant negative f orms of Rho kinase. A parallel pathway involving Ras-induced myofibril lar organization and atrial natriuretic factor expression was only min imally affected. alpha(1)-Adrenergic receptor agonist-induced myofibri llogenesis was inhibited by some but not all of the Rho kinase mutants . Our data demonstrate that activated Rho has profound effects on myof ibrillar organization in cardiac myocytes and suggest that Rho kinase mediates Rho-induced hypertrophic responses.