We present evidence that synapse retraction occurs during normal synaptic growth at the Drosophila neuromuscular junction (NMJ). An RNAi-based screen to identify the molecular mechanisms that regulate synapse retraction identified Arp-1/centractin, a subunit of the dynactin complex. Arp-1 dsRNA enhances synapse retraction, and this effect is phenocopied by a mutation in P150/Glued, also a dynactin component. The Glued protein is enriched within the presynaptic nerve terminal, and presynaptic expression of a dominant-negative Glued transgene enhances retraction. Retraction is associated with a local disruption of the synaptic microtubule cytoskeleton. Electrophysiological, ultrastructural, and immunohistochemical data support a model in which presynaptic retraction precedes disassembly of the postsynaptic apparatus. Our data suggests that dynactin functions locally within the presynaptic arbor to promote synapse stability.
