Semaphorins have been intensively studied for their role in dendritic and axonal pathfinding, but less is known about their potential role in synapse formation. In the adult giant fiber (GF) system of fruit flies (Drosophila melanogaster), we show that transmembrane Semaphorin 1a (Sema-1a) is involved in synapse formation in addition to its role in guidance during pathfinding. Cell-autonomous rescue experiments showed that Sema-1a is involved in assembly of a central synapse and that it is required both pre- and postsynaptically. We also found that pre- but not postsynaptic gain-of-function Sema-1a was able to disrupt the GF-motor neuron synapse and that the phenotype depended on a proline-rich intracellular domain that contains a putative Enabled binding site. We suggest that transmembrane Sema-1a is part of a bi-directional signaling system that leads to the formation of the GF synapse and possibly acts as both a ligand and a receptor.

In order to determine the timing of events during the assembly of a neural circuit in Drosophila we targeted expression of the temperature-sensitive shibire gene to the giant fiber system and then disrupted endocytosis at various times during development. The giant fiber retracted its axon or incipient synapses when endocytosis was blocked at critical times, and we perceived four phases to giant fiber development: an early pathfinding phase, an intermediate phase of synaptogenesis, a late stabilization process and, finally, a mature synapse. By co-expressing shibire ts and semaphorin 1a we provided evidence that Semaphorin 1a was one of the proteins being regulated by endocytosis and its removal was a necessary part of the program for synaptogenesis. Temporal control of targeted expression of the semaphorin 1a gene showed that acute excess Semaphorin 1a had a permanent disruptive effect on synapse formation.