Synaptic components necessary for retrograde signaling triggered by calcium/calmodulin-dependent protein kinase II during synaptogenesis

The development and function of presynaptic terminals are tightly controlled by retrograde factors presented from postsynaptic cells. However, it remains elusive whether major constituents of synapses themselves are necessary for retrograde modulation during synaptogenesis. Here we show that the homophilic cell adhesion molecule Fasciclin II (FasII) as well as the scaffolding protein Discs large (DLG) is indispensable for retrograde signaling initiated by calcium/calmodulin-dependent protein kinase II (CaMKII) at developing Drosophila neuromuscular junctions. Postsynaptic activation of CaMKII increased the area of nerve terminals, the number of active zones, and the frequency of miniature excitatory synaptic currents in wild-type animals. However, all of these retrograde effects were abolished in the fasII or dlg mutant background. On the other hand, the retrograde effects remained in null mutants of the glutamate receptor subunit GluRIIA. Furthermore, we show that CaMKII-induced modulation was independent of the bone morphogenetic protein signaling that is important for retrograde control at mature larvae. These results highlight a novel function of FasII as well as DLG, and more broadly, illustrate that prime synaptic components are necessary for transferring target-derived signals to presynaptic cells at a certain developing synapse.