Nicotinic acetylcholine receptors (nAChRs) containing the alpha7 gene product can influence a range of cellular events in neurons, depending on receptor location. On chick ciliary neurons, the receptors are concentrated on somatic spines, but little is known about mechanisms responsible for sequestering them there.
Rapsyn is a 43-kDa
protein essential for clustering
nicotinic receptors at the vertebrate neuromuscular junction. RT-PCR confirmed previous studies showing that the chick ciliary ganglion expresses
rapsyn transcripts, including several splice variants lacking part or all of exon 2. Heterologous expression of
rapsyn constructs, together with
nicotinic receptor constructs, shows that chicken full-length
rapsyn can induce clustering of both muscle and neuronal
nicotinic receptors. Splice variants lacking one or both leucine zipper motifs of exon 2 are unable to cluster the receptors, though, like full-length
rapsyn, they cluster themselves. Immunological analysis demonstrates the presence of full-length
rapsyn in chick muscle extracts but fails to detect either full-length or splice-variant versions of
rapsyn at significant levels in
ganglion extracts. The results suggest that
rapsyn does not cluster alpha7-nAChRs on ciliary neurons in any way similar to that of receptors at the neuromuscular junction where
rapsyn and the receptors are present in approximately equimolar amounts.