Darrin H. Brager - US grants

? DESCRIPTION (provided by applicant): We propose to investigate the physiological consequences of two recently identified dendritic channelopathies in hippocampal pyramidal neurons from the fmr1-/y mouse model of Fragile X syndrome (FXS). Despite their critical importance in the regulation of neuronal function, there have been surprisingly few physiological investigations of ion channel function in FXS. This is particularly noteworthy, because FMRP, the protein that is missing in FXS, binds to more than twenty mRNAs encoding a number of ion channel proteins, including the putative A-type K+ channel subunit KV4.2 and the h-channel subunit HCN2, and modulates ion channel function via protein-protein interactions. A-type K+ channels and h-channels have a strong influence over the integrative properties of hippocampal dendrites in part because of their very high dendritic expression and specific biophysical properties. We propose to use whole-cell and cell-attached electrophysiological recording in combination with single cell calcium imaging to investigate how changes in Ih and IKA alter the integrative properties of the distal dendrites of CA1 pyramidal neurons in the fmr1-/y mouse. We will also use electrophysiology in combination with immunohistochemistry and western blotting to investigate whether these two channelopathies persist across the dorsal-ventral axis of the hippocampus. Lastly, we will investigate whether regional restoration of FMRP expression in adult mice can rescue the cellular and behavioral abnormalities that occur in FXS. This project will provide the first physiological investigation of the impact of channelopathies on dendritic function in Fragile X syndrome.

Project Summary/Abstract. Fragile X syndrome (FX) is a widespread type of inherited intellectual disability. Effective treatments that target mechanisms underlying FX are currently lacking. FX is the foremost monogenic cause of autism spectrum disorders, and thus many individuals with FX exhibit abnormal social behaviors. Individuals with FX also often engage in aberrant spatial behaviors such as ?elopement?, wandering off and getting lost. The hippocampus is a brain structure that is particularly vulnerable to FX. Much evidence suggests that hippocampal areas CA2 and CA1 are important for social behaviors and spatial memory, respectively. Yet, few studies have investigated whether disturbances in neurophysiological mechanisms of social and spatial memory functions in CA2 and CA1 underlie social behavioral and spatial memory impairments in FX. This project?s goal is to address this gap in knowledge by investigating the extent to which subcellular, cellular, circuit, and network mechanisms of social and spatial memory operations in the hippocampus are impaired in rodent models of FX. The studies will employ state-of-the-art in vivo and in vitro electrophysiological techniques. In vivo approaches will be used to assess whether aberrant cellular and network mechanisms are related to deficits in social exploration and spatial memory. In vitro experiments will uncover cellular mechanisms underlying altered intrinsic properties and plasticity in CA2 and aberrant inhibition in CA1. Models of FX in two species, specifically Fmr1 knockout (KO) rats and mice, will be used, allowing comparison of FX pathophysiology across species. Specific Aim 1 will assess whether the strength of inputs to CA2 neurons during exploration of social stimuli is weaker in Fmr1 KO rats than wildtype rats. This Aim will also use sophisticated behavioral tracking software to determine whether Fmr1 KO rats show aberrant behavioral patterns during social exploration. Specific Aim 2 will employ whole cell and patch clamp recordings, including recordings directly from dendrites, in hippocampal slices to test whether CA2 neurons in Fmr1 KO rats and mice show impaired synaptic plasticity and responses to the social neuropeptide, oxytocin. Specific Aim 3 will test whether coordination of spike sequences from ensembles of CA1 neurons, believed to be an important network mechanism of spatial memory processing, is disrupted in Fmr1 KO rats performing spatial memory tasks. Coordination of spiking across ensembles of hippocampal neurons requires properly timed activation of specific CA1 interneurons. Thus, disrupted coordination of CA1 spike sequences in FX may reflect disturbances in CA1 interneurons. Specific Aim 4 will employ whole cell recordings of specific classes of CA1 interneurons and inhibitory inputs to CA1 pyramidal cells to test the hypothesis that inhibitory circuits are disrupted in FX. Successful completion of these Aims will provide novel insights about specific mechanisms underlying aberrant social and spatial behaviors in FX. Gaining a deeper understanding of FX mechanisms is expected to suggest novel targets for intervention in FX.