SYNAPSE-ASSOCIATED EXPRESSION OF AN ACETYLCHOLINE RECEPTOR-INDUCING PROTEIN, ARIA HEREGULIN, AND ITS PUTATIVE RECEPTORS, ERBB2 AND ERBB3, IN DEVELOPING MAMMALIAN MUSCLE/

Developing motor axons induce synaptic specializations in muscle fiber s, including preferential transcription of acetylcholine receptor (ACh R) subunit genes by subsynaptic nuclei. One candidate nerve-derived si gnaling molecule is AChR-inducing activity (ARIA)/heregulin, a ligand of the erbB family of receptor tyrosine kinases. Here, we asked whethe r ARIA and erbB kinases are expressed in patterns compatible with thei r proposed signaling roles. In developing muscle, ARIA was present not only at synaptic sites, but also in extrasynaptic regions of the musc le fiber, ARIA was synthesized, rather than merely taken up, by muscle cells, as indicated by the presence of ARIA mRNA in muscle and of ARI A protein in a clonal muscle tell line. ARIA-responsive myotubes expre ssed both erbB2 and erbB3, but little EGFR/erbB1 or erbB4. In adults, erbB2 and erbB3 were localized to the postsynaptic membrane. ErbB3 was restricted to the postsynaptic membrane perinatally, at a time when A RIA was still broadly distributed. Thus, our data are consistent with a model in which ARIA interacts with erbB kinases on the muscle cell s urface to provide a local signal that induces synaptic expression of A ChR genes. However, much of the ARIA is produced by muscle, not nerve, and the spatially restricted response may result from the localizatio n of erbB kinases as well as of ARIA. Finally, we show that erbB3 is n ot concentrated at synaptic sites in mutant mice that lack rapsyn, a c ytoskeletal protein required for AChR clustering, suggesting that path ways for synaptic AChR expression acid clustering interact. (C) 1995 A cademic Press, Inc.