SYNAPSES FXS

Characterization of the molecular components of synapses in Fragile-X mental retardation syndrome: new insights into the FMRP regulatory mechanisms

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 Obiettivo del progetto (Objective)

'Fragile-X mental retardation syndrome (FXS) is caused by mutations in the Fmr1 gene that codes for the FMRP protein. FMRP is an RNA binding protein involved in mRNA metabolism. The translational dysregulation of a subset of FMRP target mRNAs is probably the major contribution to FXS. A hallmark of the FXS phenotype is the elevated spine density with immature morphology. We aim to understand the causes of this spine dysmorphogenesis by unraveling the molecular composition of synapses in a mouse model for FXS and also get insights into the fine-tune regulatory mechanisms of FMRP. This general aim will be tackled by the following objectives: (i) Characterization of the postsynaptic core proteome of hippocampal and cortical synapses in Fmr1 knock-out (KO) mouse model under synaptic plasticity activation at two developmental stages; (ii) Quantitative analysis of the synaptic mRNAs in Fmr1 KO; (iii) Study the function of synaptic microRNAs on mRNA expression and the role that FMRP plays in this miRNA-regulation, and (iv) identification of the regulatory dendritic translational machinery by isolation of the native Arc and PSD-95 mRNAs, two FMRP targets. These objectives will be approached by a novel combination of high sensitive techniques such as Tandem Affinity Purification (TAP) method, microarrays and RNA pull down in Fmr1 KO and TAP knock-in mouse models. The wide expertise of the host lab in mRNA metabolism and in FXS, the use of well characterized mouse models and the fine design of the experimental work-flow make this project with high rate of success. The main outcomes will be to list the dysregulated mRNAs and proteins in synapses of FXS, if this dysregulation is brain region specific and change during development and what mechanisms behind mRNA translation affect this dysregulation. All this will be very relevant to understand the ethiology of the disease and settle the bases for future drug design to ameliorate the grade of mental retardation in FXS patients.'

Introduzione (Teaser)

Discovering the molecular determinants of Fragile X syndrome (FXS) could help understand how the disease manifests during development. Most importantly, it could unravel putative targets for clinical intervention.

Descrizione progetto (Article)

FXS is an inherited mental retardation condition that affects boys. It is caused by a mutation in the fragile X mental retardation gene (FMR1), which affects neuronal dendritic spines and the formation and function of synapses. The fragile X mental retardation protein (FMRP) is an RNA binding protein that regulates the transport, stability and translation of neuronal mRNAs. Deregulation of this process probably underlies the molecular aetiology of FXS.

To elucidate the molecular mechanisms of FXS, scientists on the EU-funded SYNAPSES FXS project focused on neuronal synapses and the identification of affected molecules. To achieve this, they used different transgenic mouse models, including the mouse equivalent of FXS that carries a mutation in the FMR1 gene.

They isolated the proteome of the postsynaptic membranes from the mouse hippocampus and cortex and are in the process of comparing it with normal synapses. This will unveil the molecular signalling networks that get dysregulated in FXS. So far, genome-wide association studies have disclosed a number of FMRP mRNA targets associated with autism, FXS and other neurological diseases.

Further analyses of FXS synapses identified an altered profile of microRNA expression, clearly underscoring the importance of gene regulation in synapses lacking FMRP. From a regulatory perspective, the consortium addressed the mechanism of FMRP-mediated regulation of the PSD-95 and Arc targets. They discovered that another protein, FXRP2, interacts with FMRP to mediate repression or activation of these targets.

Taken together, the findings of the SYNAPSES FXS study provide fundamental insight into the molecular events that take place in FXS. The identified molecules constitute targets for potential therapeutic exploitation, thereby facilitating amelioration of disease progression.