Rapsyn variants in ciliary ganglia and their possible effects on clustering of nicotinic receptors

Abstract

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.