A drosophila model of cellular and molecular mechanisms of fragile x syndrome.

Abstract

Fragile X Syndrome (FXS) is the most common form of inherited mental retardation. In this thesis, I describe my work using a Drosophila model to study cellular, molecular and genetic mechanisms that underlie the cognitive dysfunction in FXS. First, I used the MARCM technique to generate clones of Drosophila fragile X mutant retardation 1 (dfmr1) mutant neurons within an otherwise wild-type brain. I focused on the Mushroom Body, a well-characterized brain region of learning and memory. The dfmr1 null mutant neurons display overgrowth and overbranching in cell bodies, dendrites and axons. Consistently, dfmr1 over-expression neurons results in simpler cellular architecture. These results indicate that dfmr1 is a negative regulator of neuronal architectural complexity. Second, using immunocytochemistry and confocal imaging fluorescence intensity quantification, I investigated the regulatory function of the dfmr1 protein (dFMRP) on ionotropic glutamate receptors (iGluR) at the Drosophila NMJ synapses, and the relationship between dFMRP function and Drosophila metabotropic glutamate receptor (DmGluRA) synaptic signaling. I found that dFMRP regulates two iGluR classes in opposite directions. In contrast, DmGluRA negatively regulates both iGluR classes in common. Double null mutants of dfmr1 and dmGluRA always display an additive effect of the two single mutant phenotypes, which suggests independent, convergent pathways between dFMRP and DmGluRA regulation. Thirdly, I examined mechanistic relationships between dFMRP and DmGluRA by assaying protein expression, behavior and neuron structure in both the brain and NMJ synapse; in single mutants, double mutants and with an mGluR antagonist. These results show that DmGluRA and dFMRP convergently regulate presynaptic properties. Taken together, my work has clarified the cellular function of dFMRP on neuronal and synaptic architecture, uncovered new molecular mechanisms showing that dFMRP regulates class-specific iGluR levels in synaptic terminals, and elucidated the mechanistic relationship between dFMRP function and DmGluRA signaling.