Stimulation of protein synthesis in cultured skeletal muscle by a trophic protein from sciatic nerves

Abstract

A trophic protein purified from chicken sciatic nerves, sciatin, was added to postmitotic myotube cultures of chicken embryonic skeletal muscle to determine sciatin's effect on protein turnover. At a concentration of 20 to 25 μg/culture dish, sciatin selectively stimulated the rate of incorporation of [ 14C]leucine into muscle protein by as much as 36% without affecting the rate of protein degradation. The increased rate of [ 14C]leucine incorporation was first order to 8 h after labeling with isotope but became second order during the next 16 h. The stimulatory effect of sciatin on the rate of [ 14C]leucine incorporation was not due to a differential increase in the intracellular pool of free [ 14C]leucine because sciatin failed to significantly alter the amount of trichloroacetate-soluble [ 14C]leucine in treated cultures. Evidence obtained from experiments using the protein synthesis inhibitors actinomycin D and cycloheximide or from labeling experiments with [ 3H]uridine indicated that sciatin evoked a nonspecific increase in the rate of protein synthesis which involved synthesis of RNA. The presence of tetrodotoxin (0.1 μg/dish), an agent which abolishes spontaneous muscle contractions, had no effect on sciatin-stimulated protein synthesis, suggesting that this action of sciatin did not require muscle activity. Stimulation of protein synthesis by sciatin did not appear to be mediated by the concentration of cyclic nucleotides because neither theophylline (an inhibitor of phosphodiesterase) nor imidazole (an inhibitor of adenyl cyclase) affected its action. These data suggest that the trophic influence mediated by sciatin may be involved in the homeostatic maintenance of protein turnover in skeletal muscle. Futhermore, the model used in the present report provides a means to delineate the respective role of neurally derived trophic influences and muscle activity in the regulation of specific muscle proteins.