External Ca Dependency of Synaptic Transmission in Drosophila synaptotagmin I Mutants

Okamoto, Tomonori, Takuya Tamura, Kazuhiro Suzuki, and Yoshiaki Kidokoro. External Ca dependency of synaptic transmission in Drosophila synaptotagmin I mutants. J Neurophysiol 94: 1574–1586, 2005; doi:10.1152/jn.00205.2005. To resolve some of differences in reports on the function of Synaptotagmin I (Syt I), we re-examined synaptic transmission at the neuromuscular junction of Drosophila embryos that have mutations in the Syt I gene (syt I). Two major questions addressed were which Ca binding domain, C2A or C2B, sense Ca and is Syt I a negative regulator of spontaneous vesicle fusion. Synaptic currents were induced by nerve stimulation or by high K treatment in external solutions containing various Ca concentrations. In a null allele, syt I, synchronous synaptic currents were rarely observed but not abolished. The quantal content was about 1/60 of control but increased linearly with [Ca ]e with a slope of 0.95 (N) in the double logarithmic plot, in contrast to 3.01 in control. The slope of 1.06 in an allele, syt I, which lacks the second Ca binding domain, C2B, was not different from in syt I. In another allele, syt I, in which one amino acid in C2B is mutated, synchronous synaptic transmission was also impaired and N was 1.54, which is significantly smaller than in control. In high K saline, the [Ca ]e dependency of vesicle release in syt I AD4 was lower than in controls, whereas that in syt I was even lower than in syt I, suggesting that syt I is inhibiting vesicle fusion. These findings led us to conclude that C2B, not C2A, senses Ca , and Syt I is a negative regulator of vesicle fusion.