Gp. Feng et al., RAPSYN CLUSTERS NEURONAL ACETYLCHOLINE-RECEPTORS BUT IS INESSENTIAL FOR FORMATION OF AN INTERNEURONAL CHOLINERGIC SYNAPSE, The Journal of neuroscience, 18(11), 1998, pp. 4166-4176
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.