We conclude that Nrg has a function in synapse formation by organizing microtubules in the synaptic terminal. This novel synaptic function is conserved in human L1-CAM but is not common to all L1-type proteins. Finally, our findings suggest that some aspects of L1-CAM-related neurological disorders in humans may result from a disruption in synapse formation rather than in axon pathfinding.

The signaling mechanisms that allow the conversion of a growth cone into a mature and stable synapse are yet to be completely understood. Ubiquitination plays key regulatory roles in synaptic development and may be involved in this process. Previous studies identified the Drosophila ubiquitin conjugase bendless (ben) to be important for central synapse formation however the precise role it plays has not been elucidated. Our studies indicate that Ben plays a pivotal role in synaptic growth and maturation. We have determined that an incipient synapse is present with a high penetrance in ben mutants suggesting that Ben is required for a developmental step after target recognition. We used cell autonomous rescue experiments to show that Ben has a presynaptic role in synapse growth. We then harnessed the TARGET system to transiently express UAS-ben in a ben mutant background and identified a well-defined critical period for Ben function in establishing a full-grown, mature synaptic terminal. We demonstrate that the protein must be present at a time point before but not during the actual growth process. We also provide phenotypic evidence demonstrating that Ben is not a part of the signal transduction pathway involving the well-characterized ubiquitin ligase highwire (hiw). We conclude that Bendless functions as a novel developmental switch that permits the transition from axonal growth and incipient synapse formation to synaptic growth and maturation in the central nervous system.

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