Disruption of synaptic development and ultrastructure by Drosophila NSF2 alleles

First identified as the cytosolic component that restored intra-Golgi vesicle trafficking following N-ethylmaleimide poisoning, N-ethylmaleimide-sensitive factor (NSF) was later shown to be an ATPase that participates in many vesicular trafficking events. Current models hold that NSF disassembles postfusion SNARE protein complexes, allowing them to participate in further rounds of vesicle cycling. To further understand the role of NSF in neural function, we have embarked on genetic studies of Drosophila NSF2. In one approach, we employed transgenic flies that carry a dominant-negative form of NSF2 (NSF(E/Q)). When expressed in neurons this construct suppresses synaptic transmission, increases activity-dependent fatigue of transmitter release, and reduces the functional size of the pool of vesicles available for release. Unexpectedly, it also induced pronounced overgrowth of the neuromuscular junction. The aim of the present study was twofold. First, we sought to determine if the neuromuscular junction (NMJ) overgrowth phenotype is present throughout development. Second, we examined NSF2(E/Q) larval synapses by serial section electron microscopy in order to determine if there are ultrastructural correlates to the observed physiological and morphological phenotypes. We indeed found that the NMJ overgrowth phenotype is present at the embryonic neuromuscular synapse. Likewise, at the ultrastructural level, we found considerable alterations in the number and distribution of synapses and active zones, whereas the number of vesicles present was not changed. From these data we conclude that a primary phenotype of the NSF2(E/Q) transgene is a developmental one and that alteration in the number and distribution of active zones contributes to the NSF2(E/Q) physiological phenotype.