No REST for 5-HT Neurons Following Traumatic Brain Injury - PROJECT SUMMARY/ABSTRACT Traumatic brain injury (TBI), including closed-head injury (CHI), is a leading cause of disability in the United States. Chronic neuropsychiatric comorbidities are associated with TBI, including depression, anxiety, and social withdrawal, however our understanding of the molecular mechanisms driving these sequalae remains extremely limited. Exacerbating this issue is a lack of any FDA approved pharmacotherapies for TBI. Selective serotonin reuptake inhibitors (SSRIs) are commonly prescribed to treat various neuropsychiatric disorders; however, these drugs typically fail to achieve desired efficacy in individuals with TBI. Serotonin (5-HT) is a monoaminergic neurotransmitter linked to the etiology of various neuropsychiatric disorders including depression, anxiety and altered social function, however the effects of various forms of TBI on 5-HT neurotransmission are not well understood. Using a preclinical model for TBI/CHI, we have discovered significant alterations in 5-HT levels and signaling that originates from 5-HT neurons located with the raphe nucleus (RN). RNA sequencing analysis has revealed significant repression in the expression of transcripts related to the identity and function of specific 5- HT neuron subpopulations within the RN following TBI. Using informatics-based analyses combined with immunohistology, we have identified a transcription factor (TF), neuron restrictive silencer factor (NRSF/REST), novel in the context of mammalian 5-HT neurons, that we hypothesize drives alterations in function and/or identity of specific subpopulations of 5-HT neurons. This is a remarkable as aberrant 5-HT neuron activity canonically drives the generation of behaviours related to neuropsychiatric disorders, and 5-HT neuron subpopulations are critically dependent on the maintenance of their basal transcriptional profiles to maintain their identities and function. Specifically, we hypothesize that NRSF activity within 5-HT neurons following TBI, alters the functional landscape of these neurons, driving aberrant behavior states associated with the generation of neuropsychiatric disorders. Our Objectives are therefore to determine the cell specific transcriptional alterations occurring within defined 5-HT neuron subpopulations following TBI; delineate the roles of NRSF within 5-HT neuron populations; and determine whether increases in NRSF expression and activity underly altered 5-HT signaling and behaviors following TBI. To pursue these objectives, we have three Specific Aims: Aim 1. Identify and characterize TBI- induced alterations in the transcriptional identity of specific RN 5-HT neuron subpopulations; Aim 2: Delineate the role of TBI-induced NRSF expression within 5-HT neurons on the maintenance and identity of 5-HT neuron subpopulations; and Aim 3: Determine whether the blockade of NRSF signaling within 5-HT neurons ameliorates TBI-elicited alterations in 5-HT signaling and behaviors reminiscent of neuropsychiatric sequalae. Cumulatively, work proposed herein will, for the first time, delineate the specific actions of TBI on 5-HT neuron subpopulations of the RN, experimentally delineate the role of NRSF within 5-HT neurons within the brain and determine whether NRSF represents a novel candidate for the attenuation of TBI-induced neuropsychiatric complications.
