RETINOID X RECEPTOR GAMMA AS A POTENTIAL THERAPEUTIC TARGET FOR FRAGILE X SYNDROME

Fiscal Year 2014 Peer Reviewed Medical Research Program Topic Area: The proposed research project addresses the 'Fragile X Syndrome' topic area by seeking to understand the molecular mechanisms that underlie the disease and test a new therapeutic approach for its treatment. The proposed research project also addresses the gap and priority research area of 'epilepsy' by seeking to develop a novel pharmacological intervention for reducing seizures associated with fragile X syndrome and other forms of epilepsy.Idea to Be Studied: The proposed research project will test the hypothesis that reducing retinoid X receptor gamma subunit (RXR gamma) activity ameliorates the cellular, physiological, and behavioral impairments associated with fragile X syndrome.Objectives: The proposed research project seeks to achieve two main objectives as outlined in the two specific aims. In Aim 1, we will determine whether genetically removing RXR gamma ameliorates fragile X syndrome-related molecular, cellular, electrophysiological, and behavioral phenotypes in Fmr1 knockout mice. In Aim 2, we will assess the ability of pharmacological inhibitors of RXR gamma to reduce fragile X syndrome-related increases in neuronal excitability and seizure susceptibility in Fmr1 knockout mice.Innovative Aspect of the Research: The rationale for this proposal stems from our recent fortuitous observation that mice that carry a knockout mutation for Rxr gamma show impaired group 1 metabotropic glutamate receptor (mGluR) signaling. This novel, unpublished discovery was made in the course of an ongoing collaborative effort between the Principal Investigators to explore the molecular mechanisms that regulate synaptic transmission. Since hyperactivation of group 1 mGluR signaling is thought to contribute significantly to the development of the symptoms that characterize fragile X syndrome, our observation that loss of RXR gamma reduces group 1 mGluR signaling suggests that RXR gamma may be an effective therapeutic target for fragile X syndrome treatment or prevention. In the proposed research project, we will embark on a new series of experiments to explore this possibility.Study Design: To test the prediction that genetically removing RXR gamma ameliorates fragile X syndrome-related phenotypes, we will mate mice that carry a knockout mutation in the Rxr gamma gene with mice that carry a knockout mutation the Fmr1 gene, to generate animals that lack both the FMRP and the RXR gamma proteins, as well as animals that lack FMRP protein alone, the RXR gamma protein alone, and wild-type controls. We will then compare animals from each of these four groups to assess the effect of loss of RXR gamma on the presence and severity of fragile X syndrome-related phenotypes that have been described previously in animals that carry the Fmr1 mutation. These phenotypes will include increased dendritic spine density, enhanced synaptic depression, increased expression of FMRP target proteins, increased audiogenic seizure susceptibility, increased ambulatory activity, impaired motor performance, impaired novel object recognition, impaired social interaction and macroorchidism. To test the prediction that pharmacologically inhibiting RXR gamma reduces fragile X syndrome-related neuronal excitability and seizure susceptibility, we will test the effect of the RXR antagonist UVI3003 on group 1 mGluR-activated voltage-sensitive currents, bicuculline-induced epileptiform discharges, and audogenic seizure susceptibility in wild-type and Fmr1 knockout mice.Expected Results: If our hypotheses are correct, we expect to see reduced or absent fragile X syndrome-related phenotypes in animals that lack both RXR gamma and FMRP when compared to animals that lack FMRP alone. We also expect that RXR antagonist administration will block or reduce group 1 mGluR-activated voltage-sensitive currents and reduce the increased bicuculline-induced epileptiform discharges, and audogenic seizure susceptibility observed in Fmr1 knockout mice.Future Directions: If successful, the proposed research will form the basis for future preclinical testing to develop new pharmacological interventions for fragile X syndrome and fragile X syndrome-related seizures that act by targeting RXR gamma. This work will also form the basis for future efforts to better understand the molecular mechanisms that underlie epilepsy and autism spectrum disorders by examining the molecular basis for the interaction between RXR gamma and the group 1 mGluR signaling pathway and its effect on seizure susceptibility and autism-related impairments.