The
unc-31 gene encodes the C. elegans homolog of CAPS (calcium activated protein for secretion) 1 . CAPS has been shown to be required for the calcium dependent exocytosis of large dense core vesicles which contain peptides, biogenic amines, and hormones. However, it is not known whether CAPS plays a role in the exocytosis of small clear synaptic vesicles which contain classical neurotransmitters such as acetylcholine, GABA, and glutamate. We have undertaken a genetic and an electrophysiological approach to better define the function of CAPS in C. elegans neurotransmission.
unc-31 mutants have a number of behavioral defects. They are uncoordinated and are defective in both egg laying and pharyngeal pumping. The behavioral phenotypes observed in
unc-31 mutants are not due to developmental defects. We demonstrated that axon morphology and synapse distribution are normal in
unc-31 mutants. In addition, the uncoordinated phenotype of
unc-31 animals was rescued by expressing UNC-31 in adults using a heat shock construct, suggesting that the UNC-31 protein is required for neuronal function rather than for neuronal development.
unc-31 mutants exhibited a decrease in neurotransmission at the neuromuscular junction.
unc-31 mutants exhibited a 50% reduction in evoked release of neurotransmitter and a 50% reduction in the frequency of miniature postsynaptic events. These data suggest that the uncoordinated phenotype observed in
unc-31 mutants is due to a decrease in small clear synaptic vesicle release at the neuromuscular junction. If the requirement for CAPS at motor neurons acts via peptide regulation of motorneurons then we can make two predictions: (1) that if it is a polysynaptic effect, the uncoordinated phenotype of
unc-31 should be rescued by CAPS expression in cells other than in motorneurons, and (2) the Unc-31 phenotype should be rescued by mutations in G protein signaling in motorneurons. We are currently expressing UNC-31 in different tissues and in different subsets of neurons in
unc-31 mutants and looking for rescue of the mutant phenotype. To determine if
unc-31 function is acting via a peptide signaling mechanism we conducted a suppressor screen. In this screen a single allele of
egl-30 , which encodes Gq a , was identified. This allele is a gain-of-function activated form of Gq a . In addition, we identified multiple loss-of-function alleles of
goa-1 , which encodes Go a .
egl-30 is required for a signaling cascade that leads to localization of the UNC-13 protein to nerve terminal membranes. This localization of UNC-13 leads to an increase in acetylcholine released at the neuromuscular junction. GOA-1 antagonizes Gq a and hence the amount of UNC-13 localized at the plasma membrane. These data suggest that CAPS is acting via 7-pass transmembrane receptors and G proteins to alter the basal levels of motorneuron function. Thus, it is likely that CAPS facilitates the release of large dense core vesicles that modulate release of classical neurotransmitters rather than directly facilitating the release of small clear core vesicles. 1 Livingstone, D. 1991, Thesis.