Putative null alleles of
unc-13 have severely impaired (almost paralyzed) locomotion, slow irregular pharyngeal pumping, and are resistant to acetylcholinesterase (AChE) inhibitors. They also accumulate abnormally high levels of acetylcholine (ACh). Motor and sensory neurons are misplaced in the mutants. Furthermore, abnormal connections between major interneurons through gap junctions as well as abnormal development of motor neurons have been found. The
unc-13 gene encodes a protein with the second messenger diacylglycerol, Ca2+ and phospholipid-binding domains. We are currently analyzing a role for
unc-13 in neurotransmission by making double and triple mutants between
unc-13 and other genes, including
ace-1,
ace-2,
ace-3,
dyn-1,
rab-3,
snt-1,
unc-18,
unc-17,
unc-64 and
unc-104, involved in neurotransmission. The
unc-13 ace double mutants had better locomotive activities than the
unc-13 single mutants. Hypersensitivity of the ace mutants against AChE inhibitors was also suppressed by
unc-13 mutations. These results suggest that the neurotransmitter ACh is deprived in neuro-neural and neuro-mascular junctions. However,
unc-13 ace double mutants accumulate ACh at markedly elevated levels, compared to
unc-13 or ace single mutants, suggesting that only regulated ACh release but not its constitutive release is impaired in
unc-13 mutants. Double mutants between
unc-13 and
rab-3 had behavior phenotypes resembling
unc-13 but the level of accumulated ACh was intermediate between the levels of
unc-13 and
rab-3. Mutants doubly defective in
unc-13 and
snt-1 had similar phenotypes to
unc-13 but ACh accumulation was the same as
snt-1. These results are consistent with evidence that RAB-3 and SNT-1 are involved in visicular transport and recycling, respectively. Based on the results, we propose at least two parallel pathways in Ca2+-regulated neurotransmitter release and UNC-13 is involved in the major pathway. Constitutive release of neurotransmitter seems to be unimpaired and, therefore, UNC-13 may function at the very last step of synaptic vesicle exocytosis as a major Ca2+ sensor. This proposed role for UNC-13 is consistent with its molecular structure of the protein described above.