The exocyst subunit Sec15 is critical for proper synaptic development and function at the Drosophila NMJ

The exocyst protein complex is important for targeted vesicle fusion in a variety of cell types, however, its function in neurons is still not entirely known. We found that presynaptic knockdown (KD) of the exocyst component sec15 by transgenic RNAi expression caused a number of unexpected morphological and physiological defects in the synapse. These include the development of active zones (AZ) devoid of essential presynaptic proteins, an increase in the branching of the presynaptic arbor, the appearance of satellite boutons, and a decrease in the amplitude of stimulated postsynaptic currents as well as a decrease in the frequency of spontaneous synaptic vesicle release. We also found the release of extracellular vesicles from the presynaptic neuron was greatly diminished in the Sec15 KDs. These effects were mimicked by presynaptic knockdown of Rab11, a protein known to interact with the exocyst. sec15 RNAi expression caused an increase in phosphorylated Mothers against decapentaplegic (pMad) in the presynaptic terminal, an indication of enhanced bone morphogenic protein (BMP) signaling. Some morphological phenotypes caused by Sec15 knockdown were reduced by attenuation of BMP signaling through knockdown of wishful thinking (Wit), while other phenotypes were unaffected. Individual knockdown of multiple proteins of the exocyst complex also displayed a morphological phenotype similar to Sec15 KD. We conclude that Sec15, functioning as part of the exocyst complex, is critically important for proper formation and function of neuronal synapses. We propose a model in which Sec15 is involved in the trafficking of vesicles from the recycling endosome to the cell membrane as well as possibly trafficking extracellular vesicles for presynaptic release and these processes are necessary for the correct structure and function of the synapse.