Overexpression of nPKC θ is inhibitory for agrin‐induced nicotinic acetylcholine receptor clustering in C 2 C 12 myotubes

Protein kinase C (PKC) activity has been implicated in nicotinic acetylcholine receptor (nAChR) cluster disruption but the specific PKC isoforms involved have not been identified. We first tested whether phorbol esters, which activate PKCs, regulate agrin‐induced nAChR clustering in C 2 C 12 cells. We found that extended phorbol ester treatment (6 hr) increased nAChR clustering by two‐fold. This increase correlated in time with downregulation of PKCs, as indicated by the disappearance of cPKC α, suggesting that the presence of PKCs is inhibitory for maximal nAChR clustering. To address the question whether nPKC θ, a specific PKC isoform restricted in expression to skeletal muscle and localized to neuromuscular junctions, regulates agrin‐induced nAChR cluster formation we overexpressed an nPKC θ ‐green fluorescent protein (GFP) fusion protein in C 2 C 12 myotubes. The number of nAChR clusters was significantly reduced in nPKC θ‐GFP compared to GFP overexpressing myotubes at less‐than‐maximal clustering concentrations of agrin. These data indicate that nPKC θ activity inhibits nAChR cluster formation. To examine whether nPKC θ activation by phorbol esters regulates agrin‐induced nAChR clustering, we treated overexpressing myotubes overnight with maximal agrin concentrations followed by phorbol esters for 1 hr. Phorbol ester treatment reduced preexisting nAChR cluster numbers in nPKC θ‐GFP compared to GFP‐overexpressing myotubes, suggesting that stimulating nPKC θ activity disrupts nAChR clusters in the presence of maximal clustering concentrations of agrin. Together these findings, that nPKC θ activity inhibits agrin‐induced nAChR cluster formation and disrupts preexisting agrin‐induced nAChR clusters, suggest that nPKC θ activity is inhibitory for agrin function. © 2002 Wiley‐Liss, Inc.