Screening for synaptic defects revealed a locus involved in presynaptic and postsynaptic functions in Drosophila embryos.

To identify genes involved in synaptic functions, we screened lethal enhancer trap lines by monitoring synaptic activities at the neuromuscular junction in Drosophila embryos. It was found that MY7919, thus isolated, has moderate defects in both pre- and postsynaptic functions. The mean amplitudes of spontaneous as well as evoked synaptic currents were smaller than those in wild-type. The failure rate was higher than normal at any given concentration of external Ca(2+), indicating that presynaptic functions were impaired. In addition, the mean amplitude of miniature synaptic currents was smaller, and the unitary current amplitudes of junctional glutamate receptor channels were slightly but significantly smaller. Thus, postsynaptic functions were also altered. The gene was cloned and found to be identical to the previously reported apontic (=tracheae defective) locus, which is believed to be a transcription factor expressed in the central nervous system (CNS) as well as in the head, tracheae, and heart. Immunohistochemical analysis using an antiapontic antibody revealed that the protein is localized to nuclei. Null alleles of the apontic locus were obtained by imprecise excision of the enhancer trap vector. Synaptic activities in null mutants were not different from those of the original allele, even though null homozygotes had uncontracted ventral nerve cords and more severe behavioral phenotypes. The morphology of the neuromuscular junction of the null mutant was qualitatively similar to that of wild-type, with the presence of typical pre- and postsynaptic specializations, but with some suggestions of quantitative differences. This strategy for screening mutants with synaptic defects will reveal more genes directly or indirectly affecting synaptic transmission.