Individual C. elegans body wall muscles express two pharmacologically distinct nicotinic receptors: a levamisole-sensitive receptor and a levamisole-insensitive receptor. Recently we identified the alpha-7 like nicotinic receptor subunit ACR-16 as an essential component of the levamisole-insensitive receptor (Touroutine et.al. 2005). Several proteins regulating the trafficking and synaptic localization of the levamisole-sensitive receptor have been identified including UNC-50, UNC-74 and LEV-10. However, with the exception of RIC-3, which regulates the function of both body wall muscle nicotinic receptors, the molecular mechanisms specifically controlling ACR-16 receptor assembly, trafficking and localization are unknown.
To identify molecules required for ACR-16 function, we have generated transgenic worms expressing an integrated ACR-16::GFP construct under the control of the muscle specific promoter of
myo-3. This construct completely rescues the behavioral phenotype of
unc-63(
x37);
acr-16(
ok789) - worms in which both nicotinic receptor responses are absent, to that of the levamisole-resistant
unc-63(
x37) single mutant. Furthermore, the amplitude of the cholinergic current attributed to the ACR-16 receptor is fully restored in muscle patch clamp recordings, suggesting that the GFP tag does not significantly affect ACR-16 receptor function. Immunofluorescent staining with antibodies to the presynaptic cholinergic vesicular transporter protein UNC-17, confirmed that ACR-16::GFP is localized to post-synaptic sites. To validate our reagent as a reliable tool for a genetic screen, we explored the localization of ACR-16::GFP in
ric-3 mutants: in these mutants ACR-16::GFP puncta are completely absent.
We are now conducting a forward semi-clonal screen to identify mutants in which ACR-16::GFP is absent or mislocalized. In a pilot screen of 800 haploid genomes, we have identified several candidates in which ACR-16::GFP is mislocalized. We are currently SNP mapping these mutants.