Synaptic, microcircuit and network mechanisms of cortical dysfunction in mouse models of Fragile X Syndrome

Project 1 (P1) investigators have identified EEG phenotypes in humans with Fragile X Syndrome (FXS) that correlate with clinical outcomes, such as enhanced resting state gamma power and deficits in sensory- driven cortical coherence (See Overall). These phenotypes are remarkably conserved in a mouse model of FXS, the Fmr1 knockout (KO), as shown by Project 2 (P2) team, suggesting similar circuit dysfunction in mice and humans with loss of function of Fmr1. We have identified mechanisms of circuit alterations in the Fmr1 KO mouse that we hypothesize contribute to these EEG phenotypes. Specifically, we observe hyperexcitability of local circuits ex vivo (or in acute slices) and in vivo by two key phenotypes: Prolonged Circuit Activation (PCA) and enhanced gamma power. PCA is observed ex vivo in primary somatosensory (S1) cortex during spontaneously occurring activity or in response to thalamic stimulation. PCA is also observed in the cortex in vivo in response to sensory (sound or touch) stimulation or during hippocampal sharp-wave/ripples (SWRs). We observe enhanced gamma power and interlayer gamma synchrony between the L2/3 →L5 cortical layers ex vivo as well as enhanced gamma power across multiple brain regions in vivo in Fmr1 KO mice. We hypothesize that the mechanisms of PCA and increased gamma power in neocortical microcircuits contribute to enhanced resting state gamma power and/or deficits in sensory-evoked cortical synchronization observed in Fmr1 KO mice and humans with FXS and preliminary data support this hypothesis. We have identified a novel circuit mechanism of cortical PCA that suggests a therapeutic target for FXS. Antagonists of GluN2C/D-subunit-containing NMDA receptors (NMDARs) correct cortical circuit hyperexcitability and auditory hypersensitivity in Fmr1 KO mice. A cellular basis for this effect may be the functional misexpression of GluN2C/D in Fmr1 KO cortical Layer (L) 5 excitatory neurons. We hypothesize that GluN2C/D functional misexpression in excitatory neurons in the Fmr1 KO mice contributes to cortical circuit hyperexcitability and deficits in sensory-driven synchrony observed in the EEG. We will test this hypothesis in Aim 1, in collaboration with P2, using mice with cell-type specific deletion of GluN2C or GluN2D. Our results also suggest cortical layer specific hyperexcitability and altered interlayer synchrony may be mediated by GluN2C/D. We will test this hypothesis in Aim 2 using multi-electrode array recordings across cortex in response to thalamic stimulation. To link cellular and synaptic mechanisms with in vivo brain network function, in Aim 3 we will perform longitudinal in vivo electrophysiological recordings simultaneously across multiple brain areas in behaving mice and compare results with surface EEG in P2 and P1. Motivated by results from P1 and P2, Aim 4 will examine variability of micro- and macro-circuit phenotypes as a function of FMRP expression levels and sex and test mechanistic hypotheses for effects of estrogen on circuit activity.