Jay R. Gibson - US grants

DESCRIPTION (provided by applicant): Fragile X Syndrome (FXS) is the most common inherited form of mental retardation and is caused by loss of function mutations in the Fragile X Mental Retardation gene (FMR1). It results in debilitating behavioral and cognitive impairments, and to date, there are no effective treatments. Because up to 30% of FXS patients are also autistic, understanding the etiology of FXS may also help us understand Autism Spectrum Disorder (ASD). Patients with FXS, as well as other forms of mental retardation, have abnormal dendritic spines, suggesting that abnormal synaptic function contributes to the cognitive deficits of this disease. In the FXS mouse model, the Fmr1 knockout mouse (Fmr1 KO), there is altered synaptic plasticity suggesting a possible cellular mechanism for the cognitive deficits. However, the final alterations in baseline synaptic function, network connectivity, and network function have remained allusive. Because neocortex is widely hypothesized to be critical to cognition, it is reasonable to hypothesize that synaptic and network function changes in this structure may directly underlie the cognitive deficits in FXS and autism. Accordingly, we have found clear, profound changes in the Fmr1 KO mouse at 3 levels of neurobiological function in the neocortex - synaptic, cellular, and network. Most saliently, excitatory drive is decreased 2-fold onto one class of inhibitory neuron and the excitability of excitatory neurons is increased. Together these changes suggest that neocortical circuitry is hyperexcitable. This is confirmed by changes in network function observed in vitro. These changes are consistent with the hypothesis that the balance of excitation and inhibition is altered in mental retardation and autism. Utilizing electrophysiological methods in acute brain slice preparations, we propose the following specific aims: 1) determine where and when in the neocortex functional changes exist, 2) determine the mechanisms underlying synaptic and cellular changes, and 3) determine the cellular underpinnings and mechanisms of network function change. Our results will help us understand the neurobiological mechanisms underlying FXS and autism and could lead to the development of treatments to reverse these syndromes in humans. PUBLIC HEALTH RELEVANCE: We have found alterations in neocortical circuitry in the mouse model of Fragile X Syndrome - the Fmr1 KO mouse. These alterations include synaptic and network function, and they indicate that neocortical networks are hyperexcitable. Accordingly, these changes could underlie the increased sensitivity to sensory stimuli or even cognitive disabilities in Fragile X Syndrome patients. Our proposed line of study investigating these alterations will provide potential cellular targets for genetic and drug treatments for both Fragile X Syndrome and autism since both disorders often occur together.

A typical neocortical pyramidal neuron integrates information from thousands of excitatory inputs from both ?local? circuits, within a cortical region, and from many ?long-range? circuits from other cortical and non-cortical regions, both ipsi- and contralaterally. These long-range connections develop postnatally, are highly specified, and regulated by sensory experience. Remarkably, >50% of excitatory synapses a typical neocortical pyramidal neuron receives are from long-range connections, but virtually nothing is known of how functional long-range connections develop, are regulated by experience or if and how long-range and local inputs are balanced. In the last grant cycle, we discovered that the activity-dependent transcription factor, Myocyte-Enhancer Factor 2C (MEF2C) functions postnatally and cell autonomously to regulate the balance of local and long-range excitatory connectivity onto layer (L) 2/3 neocortical neurons in primary somatosensory cortex. Specifically, MEF2C promotes connectivity from multiple local excitatory circuits onto L2/3 neurons, while weakening callosal inputs from contralateral somatosensory cortex. Importantly, sensory experience is necessary for MEF2C regulation of both local and callosal circuits suggesting that MEF2C as a key player in experience-dependent, input-specific development of cortical circuits. Our findings have important implications for neurodevelopmental disorders. Brain mapping studies in humans with autism spectrum disorder (ASD) and schizophrenia (SCZ) reveal imbalances in local vs. long-range functional cortical connectivity. Furthermore, loss of function mutations in Mef2c are linked with intellectual disability (ID), ASD, epilepsy and SCZ in humans and mice. We hypothesize that sensory experience-driven neural activity regulates MEF2C-dependent transcriptional control of target genes to mediate input-specific development and plasticity of cortical circuits. Thus, a corollary of this hypothesis predicts that loss of function mutations in Mef2c or its effectors would result in imbalances of local and long-range connectivity, abnormal sensory-related behaviors and neuropsychiatric disease. To test these hypotheses: In Aim 1, we will use state-of-the-art optogenetic and photostimulation circuit mapping methods to determine if Mef2c deletion generally strengthens long-range inputs onto L2/3 neurons, bidirectionally regulates inputs based on their dendritic location and maintains the input-specificity of mature circuits. Aim 2: MEF2C functions both to repress transcription of target genes and stimulate their transcription in response to neural activity. Using MEF2C mutants, we will determine which of these functions mediates experience-dependent, input-specific regulation of cortical circuits. Aim 3: FMRP, mGluR5 and Arc are required for MEF2 regulation of synapses in culture neurons and associated with ASD, ID and SCZ. Here, we will test their role in MEF2C-mediated input-specific development of cortical circuits. Aim 4: To identify candidate genes that regulate cortical circuit input specificity, we will identify the postnatal MEF2C- and experience-dependent transcriptomes in L2/3 neocortical neurons using fluorescent-activated cell sorting (FACS) and RNA sequencing.