Neuropil pattern formation and regulation of cell adhesion molecules in Drosophila optic lobe development depend on synaptobrevin

To investigate a possible involvement of synaptic machinery in Drosophila v isual system development, we studied the effects of a loss of function of n euronal synaptobrevin, a protein required for synaptic vesicle release. Exp ression of tetanus toxin light chain (which cleaves neuronal synaptobrevin) and genetic mosaics were used to analyze neuropil pattern formation and le vels of selected neural adhesion molecules in the optic lobe. We show that targeted toxin expression in the developing optic lobe results in disturban ces of the columnar organization of visual neuropils and of photoreceptor t erminal morphology. IrreC-rst immunoreactivity in neuropils is increased af ter widespread expression of toxin. In photoreceptors, targeted toxin expre ssion results in increased Fasciclin II and chaoptin but not IrreC-rst immu noreactivity. Axonal pathfinding and programmed cell death are not affected . In genetic mosaics, patches of photoreceptors that lack neuronal synaptob revin exhibit the same phenotypes observed after photoreceptor-specific tox in expression. Our results demonstrate the requirement of neuronal synaptob revin for regulation of cell adhesion molecules and development of the fine structure of the optic lobe. A possible causal link to fine-tuning process es that may include synaptic plasticity in the development of the Drosophil a CNS is discussed.