Chronic and Evolving Inflammation after Traumatic Brain Injury: Microglial Priming and Neuropsychiatric Complications

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PROJECT SUMMARY/ABSTRACT: Traumatic brain injury (TBI) can lead to significant neuropsychiatric problems and neurodegenerative pathologies that develop with time after injury. These issues may be propagated by neuroinflammatory processes that continue well after the initial injury. Pt.3 We have reported that diffuse TBI in mice leads to “microglial priming” within cortical and hippocampal regions, in which the microglia remain in a sensitized state and are highly inflammatory following an immune challenge 30 days post injury (30 dpi). In this application, we show a distinct phase transition from acute (8-24 h) to sub-acute (7 d) and then to chronic (30 d) cortical-inflammation/microglia priming after TBI. Acutely, there was an inflammatory response after TBI that evolved into a subacute phase 7 dpi that was dominated by interferon (IFN) type I signaling. IFN responses are activated by cell distress and damage to promote an immune response that can prime innate immune cells, including microglia. We provide evidence of cortical neuronal damage 7 dpi with corresponding microglial activation. Pts.3&6 Single cell RNA seq (scRNAseq) of the cortex 7 dpi shows unique clusters of microglia, trauma-associated, that are influenced by IFNs. These microglia are involved in dendritic remodeling and suppression of neuronal homeostasis. At 30 dpi, there was cognitive impairment (associated with HPC & CTX), reduced network connectivity, and increased immune reactivity of primed microglia. Microglia are critical in these processes because microglial elimination (CSF1R antagonist) prevented TBI-induced neuroinflammation and IFN signaling, attenuated dendritic atrophy, and improved network connectivity. Thus, we hypothesize that increased interferon signaling is critical in promoting microglial priming and chronic neuroinflammation, dendritic remodeling, and cognitive decline. To address this, three aims are proposed using a midline fluid percussion injury in mice. In Aim-1, we will eliminate microglia to determine the influence of microglia on other CNS cells in the cortex and hippocampus acutely, sub- acutely, and chronically after TBI. ScRNAseq will be used to determine the transcriptome signature of microglia over time and in parallel with astrocytes, oligodendrocytes, and neurons at 3 these critical times after TBI. The focus will be determining which cells express IFNs and IFN receptors, and how they respond to increased IFN signaling with TBI. In Aim-2, we will determine if IFN signaling is critical in chronic neuroinflammation, pathology, cognitive decline, and microglial priming after TBI. Here, we will attenuate IFN signaling at the levels of IFN-a/b receptor activation (IFNαRKO, Mgl-IFNαRKO) and IFN production (STINGKO) to determine the extent to which these interventions ameliorate neuroinflammation, pathology, and microglial priming. In Aim-3, we will determine if TBI-induced microglial priming, chronic neuroinflammation, and cognitive decline 30 dpi are reversed by forced microglia turnover. We will remove microglia (CSF1R antagonist) when IFN responses are highest 7 dpi, and allow for microglial repopulation to 30 dpi. Completion of these aims will provide new insight on the IFN pathway that appears to be critical in the transition from acute to chronic inflammation mediated by microglia after TBI.