Temporal requirements of the fragile X mental retardation protein in the regulation of synaptic structure

Abstract

Fragile X syndrome (FraX), caused by the loss-of-function of one gene (FMR1), is the most common inherited form of both mental retardation and autism spectrum disorders. The FMR1 product (FMRP) is an mRNA-binding translation regulator that mediates activity-dependent control of synaptic structure and function. To develop any FraX intervention strategy, it is essential to define when and where FMRP loss causes the manifestation of synaptic defects, and whether the reintroduction of FMRP can restore normal synapse properties. In the Drosophila FraX model, dFMRP loss causes neuromuscular junction (NMJ) synapse overelaboration (overgrowth, overbranching, excess synaptic boutons), accumulation of development-arrested satellite boutons, and altered neurotransmission. We used the Gene-Switch method to conditionally drive dFMRP expression to define the spatiotemporal requirements in synaptic mechanisms. Constitutive induction of targeted neuronal dFMRP at wild-type levels rescues all synaptic architectural defects in Drosophila Fmr1 (dfmr1)-null mutants, demonstrating a presynaptic requirement for synapse structuring. By contrast, presynaptic dFMRP expression does not ameliorate functional neurotransmission defects, indicating a postsynaptic dFMRP requirement. Strikingly, targeted early induction of dFMRP effects nearly complete rescue of synaptic structure defects, showing a primarily early-development role. In addition, acute dFMRP expression at maturity partially alleviates dfmr1-null defects, although rescue is not as complete as either early or constitutive dFMRP expression, showing a modest capacity for late-stage structural plasticity. We conclude that dFMRP predominantly acts early in synaptogenesis to modulate architecture, but that late dFMRP introduction at maturity can weakly compensate for early absence of dFMRP function.

Interference of the complex between NCS-1 and Ric8a with phenothiazines regulates synaptic function and is an approach for fragile X syndrome

“…5 A and B). The data confirmed that neuron volume is increased in the mutant (13,36). Furthermore, we provide direct counting of synapses, which are also increased in the mutant (Fig.…”

“…5 A and C). As previously reported, expression of NCS-1 is reduced and the number of synapses is increased in the fly fmr1 mutant (13,20). However, simultaneous overexpression of NCS-1 and Ric8a suppresses the synaptic phenotypes of each genetic alteration and yields a normal number of synapses (19).…”

“…Similar spine effects are reported for ASDs (11). Benefiting from the conservation of the fmr1 gene, work in Drosophila has shown that loss of function mutations increase synapse number, whereas the excess of function condition yields the opposite effect, synapse loss (12,13). As for human subjects, fmr1 mutants in Drosophila are deficient in associative learning and memory (14,15).…”

“…Fragile X is a convenient case for this assay, because the synaptic phenotype is well-characterized and the genetic origin is conserved (1). Mutants in fmr1 show essentially the same synaptic phenotype in humans, mice, and flies (5,(11)(12)(13). Importantly, the fly homolog of NCS-1, Frq2, is among the mRNAs deficiently translated in fmr1 mutants (20).…”

Therapeutic Strategies in Fragile X Syndrome: Dysregulated mGluR Signaling and Beyond

“…These reports have provided considerable insight into the mechanism that might underlie abnormal synapse development and dendritic spine morphology in FXS. In Drosophila, FMRP is essential for synapse formation and experience-dependent axonal pruning during development (Gatto and Broadie, 2008;Tessier and Broadie, 2008). Studies in Fmr1 KO mice have further revealed that FMRP regulates protein synthesis-dependent axon pruning, dendritic spine elimination, and actin-dependent stabilization of spines.…”