Depolarization-induced PKC activities suppress myogenin-driven AChR γ gene promoter activity

Abstract

NICOTINIC acetylcholine receptor ( AChR) is a pentameric complex (a2Py/e8), which is composed of four different subunits; its expression is regulated by neurally evoked electrical activity. Before innervation in skeletal muscle during embryonic stage, AChR is expressed on the fiber surface; after synaptic formation, the expression of AChR is restricted to the motor endplate. Denervation of the neonate or blockage of the electrical activity by sodium channel inhibitor enhances the AChR levels over the entire plasma membrane markedly. Conversely, reinnervation or electrostimulation of denervated "supersensitive" muscle leads to down-regulation of the AChR gene expression in extrajunction. T o understand the coupling mechanism of membrane depolarization and transcriptional activation of AChR genes is becoming more and more interesting. Klarsfeld et a l . ['I demonstrated that calcium and PKC are involved in the inhibition of AChR a subunit biosynthesis induced by electrical activity. Calcium influx through 1, type calcium channel on depolarized membrane rapidly inactivates the AChR geneL2]. Membrane depolarization activates nuclear PKC, which almost instantly leads to the inhibition of AChR genes[31 . I t appears as if a factor required for the expression of these genes is inactivated by phosphorylation. The kinetics of receptor gene expression following denervation and electrostimulation suggests that this factor should have autocatalytic properties[41. So myogenin, a member of myogenic regulatory factors, is a plausible candidate, and this hypothesis is supported by ~ f t i m i e ' ~ ' and other researchers. We have studied the effects of calcium and PKC on the coupling mechanism of excitationtranscription of AChR subunit gene using 50 mmol/L potassium instead of electrostimulation of CzClz cells, and have successfully mimicked the roles of myogenin in this signaling' pathway.