The fragile X syndrome (FraX) mental retardation and autism spectrum disorder is caused by loss of FMR1 function. FMRP, the RNA-binding translation regulator encoded by FMR1, modulates synapse structure and function, but where and when its loss causes synaptic defects is unclear. Now, on ,Gatto and Broadie reveal FMRP's spatiotemporal roles in synaptogenesis by conditionally driving its expression in a FraX fly model. The constitutive presynaptic expression of dFMRP in these flies rescues their synaptic architectural defects but not normal neurotransmission. From these and other findings, they conclude that dFMRP has a crucial presynaptic role in regulating neuromuscular junction synaptic architecture, but acts post-synaptically in regulating neurotransmission strength. The authors also rescued synaptic structural defects in FraX flies by expressing dFMRP in early and late larval development; the rescue of defects in later development indicates that dFMRP can mediate late-stage plasticity to reverse some synaptic impairments. Thus, although FraX is primarily a developmental disease, late-stage therapeutic intervention might prove to be beneficial.
