Examining the role of specific NMDA receptor subunits in cortical circuit dysfunction in Fragile X Syndrome

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PROJECT SUMMARY Sensory hypersensitivity is a common symptom in autism and Fragile X Syndrome (FXS) and is thought to be a result of cortical circuit dysregulation. EEG studies in humans with FXS and the FXS mouse model, the Fmr1 KO, reveal cortical circuit hyperexcitability and synchrony deficits such as enhanced resting state power in the gamma band and reduced sensory-driven synchrony. In acute slices, this hyperexcitability can be observed as prolonged persistent activity states, called UP states, and increased gamma band power during UP states. I hypothesize that circuit mechanisms that mediate hyperexcitability and prolonged UP states in the neocortex may contribute to EEG phenotypes in FXS. Using positive and negative allosteric modulators (PAMs/NAMs) specific for GluN2C/D subunits of NMDA receptors, I have revealed an upregulation of GluN2C/D function in the Fmr1 KO cortex that contributes to circuit hyperexcitability. Specifically, GluN2C/D PAMs increase UP state duration and gamma power during the UP states, while NAMs rescue UP state duration. Remarkably, these interventions only affected the Fmr1 KO, not their wildtype (WT) littermates suggesting that GluN2C/D function is upregulated in the Fmr1 KO and leads to cortical circuit dysfunction. Typically, GluN2C/D subunits are expressed in cortical inhibitory neurons and astrocytes. Since my results are not consistent with effects on inhibitory neurons, GluN2C/D subunits may be misexpressed in excitatory neurons or upregulated in astrocytes in the Fmr1 KO. I hypothesize that GluN2C/D expression and/or function is increased in excitatory neurons and/or astrocytes in Fmr1 KO mice and this contributes to hyperexcitability and altered synchrony of cortical circuits. The goal of the proposed project is to test this hypothesis and determine expressional and functional changes in Glun2C/D that may contribute to cortical hyperexcitability and synchrony following three Specific Aims. Aim 1. To determine change in cortical protein and RNA expression levels as well as cell specific expression of GluN2C/D subunits in Fmr1 KO cortex. Aim 2. To determine cell specific functional contribution of GluN2C/D subunits to NMDA-mediated currents in WT and Fmr1 KO cortex. Aim 3. To determine the contribution of GluN2C/D NMDARs to in vivo sensory driven circuit excitability and altered synchrony using multi-electrode array EEG and measurement of audiogenic seizures in the Fmr1 KO mouse in collaboration with Dr. Devin Binder at UC Riverside. These experiments will not only provide insights into the molecular and cellular basis of GluN2C/D contribution to cortical circuit dysfunction, but also examine GluN2C/D subunits as a potential target for therapeutic development using translational biomarkers.