RNA-binding FMRP and Staufen sequentially regulate the coracle scaffold to control synaptic glutamate receptor and bouton development

Both mRNA-binding Fragile X Mental Retardation Protein (FMRP) and mRNA-binding Staufen regulate synaptic bouton formation and glutamate receptor (GluR) levels at the Drosophila neuromuscular junction (NMJ) glutamatergic synapse. Here, we test whether these RNA-binding proteins (RBPs) act jointly in a common mechanism. We find both dfmr1 and staufen mutants, and trans-heterozygous double mutants, display increased synaptic bouton formation and GluRIIA accumulation. With cell-targeted RNAi, we show a downstream Staufen role within postsynaptic muscle. With immunoprecipitation, we show that FMRP binds staufen mRNA to stabilize postsynaptic transcripts. Staufen is known to target actin-binding, GluRIIA anchor Coracle, and we confirm that Staufen binds to coracle mRNA. We find that FMRP and Staufen act sequentially to co-regulate postsynaptic Coracle expression, and we show Coracle, in turn, controls GluRIIA levels and synaptic bouton development. Consistently, we find dfmr1, staufen and coracle mutants elevate neurotransmission strength. We find FMRP, Staufen and Coracle all suppress pMad activation, providing a trans-synaptic signaling linkage between postsynaptic GluRIIA levels and presynaptic bouton development. This work supports an FMRP—Staufen—Coracle—GluRIIA—pMad pathway regulating structural and functional synapse development.