NEURAL INFLUENCE ON PROTEIN-KINASE-C ISOFORM EXPRESSION IN SKELETAL-MUSCLE

Protein kinase C (PKC) is a family of enzymes involved in synapse form ation and signal transduction at the neuromuscular junction. Two PKC i soforms, classical PKC alpha and novel PKC theta, have been shown to b e enriched in skeletal muscle or localized to the endplate. We examine d the role of nerve in regulating the expression of these PKC isoforms in rat skeletal muscle by denervating diaphragm muscle and measuring PKC protein expression at various postoperative times. nPKC theta prot ein levels decreased 65% after denervation, whereas cPKC alpha levels increased 80% compared with control hemidiaphragms. These results sugg est that innervation regulates PKC theta and alpha isoform expression in skeletal muscle. To explore further how nerve regulates PKC express ion, we characterized PKC isoform expression in rat myotubes deprived of neural input. Myoblast expression of nPKC theta was low, and the in crease in nPKC theta expression that occurred during differentiation i nto myotubes resulted in levels of nPKC theta significantly below adul t skeletal muscle. cPKC alpha expression in myoblasts increased during differentiation to levels that exceeded expression in adult skeletal muscle. Coculturing myotubes with a neuroblastoma X glioma hybrid clon al cell line (NG108-15) increased nPKC theta expression, but not cPKC alpha, suggesting that nPKC theta in skeletal muscle and myotubes is r egulated by nerve contact or by a factor(s) provided by nerve. Treatin g myotubes with tetrodotoxin did not affect either basal- or NG108-15 cell-stimulated nPKC theta expression. Together these results suggest that expression of nPKC theta in skeletal muscle is regulated by a tra nsynaptic interaction with nerve that specifically influences nPKC the ta expression.