Serotonin (5-HT) stimulates egg-laying in nematodes and multiple 5-HT receptors appear to be involved in the process. In the free-living nematode, Caenorhabditis elegans, the 5-HT stimulation of egg-laying is abolished in
ser-1 null worms that lack a 5-HT2 like receptor and the phenotype is rescued by
ser-1 expression in vulval muscle. A type I PDZ motif (ETFL) at the C-terminus of both the C.elegans (SER-1) and Ascaris suum (AS-1) 5-HT2-like receptors is not essential for rescue, but appears to optimize signaling. The C-termini of both SER-1 and AS-1 bind specifically to PDZ domain 10 of the multi-PDZ domain containing protein MPZ-1 (C52A11.4), based on yeast 2 hybrid screening, GST pulldown and co-immunoprecipitation. Most importantly, the overexpression of MPZ-1 PDZ domain 10 in vulval muscle, predicted to disrupt the SER-1:MPZ-1 interaction, also specifically inhibits 5-HT stimulated egg-laying. The expression of
mpz-1 is complex and differentially driven by multiple promoters. Expression is confined to about 60 neurons and body wall and vulval muscles. However, only 3 pairs of these neurons also appear express
ser-1 (RMH, RMF, RMD). Indeed, MPZ-1 also appears to interact with other GPCRs with acidic amino acids in the -3 position of their PDZ motifs. The expression of GFP-tagged MPZ-1 PDZ domains 6, 9 or 10 driven by specific
mpz-1 promoters is uniform throughout neurons or muscles. In contrast, the expression of GFP-tagged MPZ-1 PDZ domain 8 is punctate. In neurons, PDZ domain 8 colocalizes with synaptobrevin, suggesting a synaptic localization and in body wall muscle is associated with the dense bodies, suggesting a role in tight junctions. These studies suggest that the SER-1:MPZ-1 interaction is essential for optimal signaling and that MPZ-1 may be involved in the assembly, trafficking or localization of multiple GPCRs.