RAPSYN CLUSTERS NEURONAL ACETYLCHOLINE-RECEPTORS BUT IS INESSENTIAL FOR FORMATION OF AN INTERNEURONAL CHOLINERGIC SYNAPSE

Nicotinic acetylcholine receptors (AChRs) are clustered at high densit y in the postsynaptic membranes of skeletal neuromuscular junctions an d cholinergic interneuronal synapses. A cytoplasmic protein, rapsyn, i s essential for AChR clustering in muscle. Here, we asked whether raps yn mediates neuronal AChR clustering at cholinergic synapses in a mamm alian sympathetic ganglion, the superior cervical ganglion (SCG). Seve ral observations supported this possibility: (1) AChR clusters contain ing the alpha 3-5 and beta 2 subunits, homologs of the muscle AChR sub units, are present at SCG synapses; (2) rapsyn RNA is readily detectab le in the SCG; and (3) expression of recombinant rapsyn in heterologou s cells induces aggregation of coexpressed neuronal AChR subunits. How ever, rapsyn protein was undetectable at ganglionic synaptic sites. Mo reover, aggregates of neuronal AChRs induced in heterologous cells by full-length rapsyn remained intracellular, whereas rapsyn-induced clus ters of muscle AChRs reached the cell surface. Additional studies reve aled a second rapsyn RNA species in SCG generated by alternative splic ing and competent to encode a novel short rapsyn isoform. However, thi s isoform clustered neither neuronal nor muscle AChRs in heterologous cells. Most telling, the number, size, and density of AChR clusters in SCG did not differ significantly between neonatal mice bearing a targ eted mutation of the rapsyn gene and littermate controls. Thus, rapsyn is dispensable for clustering of ganglionic neuronal nicotinic AChRs.