Synapse formation at the mammalian neuromuscular junction requires at least two postsynaptic proteins, the receptor tyrosine kinase MuSK and the acetylcholine-receptor associated protein rapsyn. These proteins interact with presynaptically expressed agrin. C. elegans has genes encoding proteins similar to all three of these proteins. The MuSK homologue, the product of the
kin-8 gene (a.k.a
cam-1), is expressed in neurons and muscle. Animals that expressed a translational fusion in which the kinase domain of KIN-8 was replaced by the green fluorescent protein (GFP) or that have been injected with RNAi were uncoordinated (Unc) and egg-laying defective(Egl). Furthermore, the level of
kin-8 mRNA was reduced in
unc-4 and
unc-37 mutant animals (these latter genes encode transcription factors that control the choice of presynaptic partners for the VA motor neurons of the ventral cord). In addition, the number of ventral cord motor neurons expressing a
kin-8::lacZ fusion was reduced in an
unc-4 background. These results suggest that
kin-8 may regulate neuronal activity, and possibly synapse formation, in C. elegans. Similar findings were found with the C. elegans rapsyn homologue, the product of the
rap-1 gene. RAP-1 was expressed in many cells that also express KIN-8 and although RNAi with
rap-1 did not produce a pronounced phenotype in wild-type animals, it yielded a phenotype identical to that of
kin-8 RNAi in a hypomorphic
kin-8 mutant (gift from M. Koga and Y. Oshima). In contrast, RNAi for
agr-1, the gene for the C. elegans agrin homologue, did not produce the same phenotype. We also examined the synapses formed by the interneurons (AVA, AVD, AVE) and the A-type motorneurons in the ventral cord using a vamp::gfp construct (provided by M. Nonet) driven by a
sek-1 promoter (the
sek-1 gene is expressed in AVA,AVD,AVE and some other tissues, M. Tanaka et al. WBG 15(4):30). The
sek-1::vamp::gfp is expressed in a similar pattern as reported by Tanaka et al. and punctated staining can be seen along the ventral cord. In the
unc-4(
e120) and
unc-37(
e262) backgrounds, the ventral cord staining become weak and diffuse, indicating loss of synapse formation. Similar results were obtained when
kin-8 RNAi was injected into wild-type animals carrying the
sek-1::vamp::gfp construct.