<i>Drosophila</i>Amphiphysin Functions during Synaptic Fasciclin II Membrane Cycling

Recent studies have revealed that endocytosis and exocytosis of postsynaptic receptors play a major role in the regulation of synaptic function, particularly during long-term potentiation and long-term depression. Interestingly, many of the proteins implicated in exocytosis and endocytosis of synaptic vesicles are also involved in postsynaptic protein cycling. In vertebrates, Amphiphysin is postulated to function during endocytosis in nerve terminals; however, several recent reports using a Drosophila amphiphysin (damph) null mutant have failed to substantiate such a role at fly synapses. In addition, Damph is surprisingly enriched at the postsynapse. Here we used the glutamatergic larval neuromuscular junction to study the synaptic role of Damph. By selectively labeling internal and external pools of the cell adhesion molecule Fasciclin II (FasII), and by using a novel in vivo surface FasII immunocapture protocol, we show that the level of external FasII is decreased in damph mutants although the total level of FasII remains constant. In vivo FasII internalization assays indicate that the reincorporation of FasII molecules into the cell surface is severely inhibited in damph mutants. Moreover, we show that blocking soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) function in postsynaptic muscle cells interferes with FasII exocytosis. These experiments suggest that in Drosophila, Damph functions during SNARE-dependent postsynaptic FasII membrane cycling. This study challenges the notion that synaptic Amphiphysin is involved exclusively in endocytosis and suggests a novel role for this protein in postsynaptic exocytosis.