Neural influence on protein kinase C isoform expression in skeletal muscle

Abstract

Protein kinase C (PKC) is a family of enzymes involved in synapse formation and signal transduction at the neuromuscular junction. Two PKC isoforms, classical PKC α and novel PKC θ, have been shown to be enriched in skeletal muscle or localized to the endplate. We examined 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 θ protein levels decreased 65% after denervation, whereas cPKC α levels increased 80% compared with control hemidiaphragms. These results suggest that innervation regulates PKC θ and α isoform expression in skeletal muscle. To explore further how nerve regulates PKG expression, we characterized PKC isoform expression in rat myotubes deprived of neural input. Myoblast expression of nPKC θ was low, and the increase in nPKC θ expression that occurred during differentiation into myotubes resulted in levels of nPKG θ significantly below adult skeletal muscle. cPKC α expression in myoblasts increased during differentiation to levels that exceeded expression in adult skeletal muscle. Coculturing myotubes with a neuroblastoma x glioma hybrid clonal cell line (NG108-15) increased nPKC θ expression, but not cPKC α, suggesting that nPKC θ in skeletal muscle and myotubes is regulated by nerve contact or by a factor(s) provided by nerve. Treating myotubes with tetrodotoxin did not affect either basal- or NG108-15 cell-stimulated nPKC θ expression. Together these results suggest that expression of nPKC θ in skeletal muscle is regulated by a transynaptic interaction with nerve that specifically influences nPKC θ expression.