| Summary: | Fragile X syndrome, resulting from the loss of function of the hFMR1 gene, is the most common heritable cause of intellectual disability. The human genome also encodes two closely related paralogs: hFXR1 and hFXR2. Drosophila that lack the dFMR1 gene recapitulate FXS-associated molecular, cellular and behavioral phenotypes. To test evolutionary conservation, we used tissue-targeted transgenic expression of all three human genes in the Drosophila disease model.
In neurons, dfmr1 null mutants exhibit elevated protein levels that alter the central brain and neuromuscular junction synaptic architecture, including an increase in synapse area, branching and bouton numbers. Importantly, hFMR1 can fully rescue both the molecular and cellular defects in neurons, whereas hFXR1 and hFXR2 provide absolutely no rescue. For non-neuronal requirements, we assayed male fecundity and testes function. dfmr1 null mutants are effectively sterile owing to disruption of the 9+2 microtubule organization in the sperm tail. All three human genes fully and equally rescue mutant fecundity and spermatogenesis defects. These results indicate that FMR1 function is evolutionarily conserved in neural mechanisms and cannot be compensated by either FXR1 or FXR2, but that all three proteins can substitute for each other non-neuronally.
It has been long hypothesized that the phosphorylation of serine 500 in human FMRP controls its function as an RNA-binding translational repressor. To test this hypothesis in vivo, we employed neuronally targeted expression of three human FMR1 transgenes, including wildtype (hFMR1), dephosphomimetic (S500A-hFMR1) and phosphomimetic (S500D-hFMR1). At the molecular and cellular levels, the phosphomimetic is able to rescue elevated protein levels and architecture overgrowth phenotypes, whereas the dephosphomimetic phenocopies the null condition. At the behavioral level, dfmr1 null mutants exhibit strongly impaired olfactory associative learning. The human phosphomimetic targeted only to the brain-learning center restores normal learning ability, whereas the dephosphomimetic provides absolutely no rescue. We conclude that human FMRP S500 phosphorylation is necessary for its in vivo function as a neuronal translational repressor and regulator of synaptic architecture, and for the manifestation of FMRP-dependent learning behavior.
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