The diabetes autoantigen ICA69 and its Caenorhabditis elegans homologue, ric-19, are conserved regulators of neuroendocrine secretion

ICA69 is a diabetes autoantigen with no homologue of known function. Given that most diabetes autoantigens are associated with neuroendocrine secretor y vesicles, we sought to determine if this is also the case for ICA69 and w hether this protein participates in the process of neuroendocrine secretion . Western blot analysis of ICA69 tissue distribution in the mouse revealed a correlation between expression levels and secretory activity, with the hi ghest expression levels in brain, pancreas, and stomach mucosa. Subcellular fractionation of mouse brain revealed that although most of the ICA69 pool is cytosolic and soluble, a subpopulation is membrane-bound and coenriched with synaptic vesicles. We used immunostaining in the HIT insulin-secretin g beta-cell Line to show that ICA69 localizes in a punctate manner distinct from the insulin granules, suggesting an association with the synaptic-lik e microvesicles found in these cells. To pursue functional studies on ICA69 , we chose to use the model organism Caenorhabditis elegans, for which a ho mologue of ICA69 exists. We show that the promoter of the C. elegans ICA69 homologue is specifically expressed in all neurons and specialized secretor y cells. A deletion mutant was isolated and found to exhibit resistance to the drug aldicarb (an inhibitor of acetylcholinesterase), suggesting defect ive neurotransmitter secretion in the mutant. On the basis of the aldicarb resistance phenotype, we named the gene ric-19 (resistance to inhibitors of cholinesterase-19). The resistance to aldicarb was rescued by introducing a ric-19 transgene into the ric-19 mutant background. This is the first stu dy aimed at dissecting ICA69 function, and our results are consistent with the interpretation that ICA69/RIC-19 is an evolutionarily conserved cytosol ic protein participating in the process of neuroendocrine secretion via ass ociation with certain secretory vesicles.