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      Microglial Activation in Traumatic Brain Injury

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          Abstract

          Microglia have a variety of functions in the brain, including synaptic pruning, CNS repair and mediating the immune response against peripheral infection. Microglia rapidly become activated in response to CNS damage. Depending on the nature of the stimulus, microglia can take a number of activation states, which correspond to altered microglia morphology, gene expression and function. It has been reported that early microglia activation following traumatic brain injury (TBI) may contribute to the restoration of homeostasis in the brain. On the other hand, if they remain chronically activated, such cells display a classically activated phenotype, releasing pro-inflammatory molecules, resulting in further tissue damage and contributing potentially to neurodegeneration. However, new evidence suggests that this classification is over-simplistic and the balance of activation states can vary at different points. In this article, we review the role of microglia in TBI, analyzing their distribution, morphology and functional phenotype over time in animal models and in humans. Animal studies have allowed genetic and pharmacological manipulations of microglia activation, in order to define their role. In addition, we describe investigations on the in vivo imaging of microglia using translocator protein (TSPO) PET and autoradiography, showing that microglial activation can occur in regions far remote from sites of focal injuries, in humans and animal models of TBI. Finally, we outline some novel potential therapeutic approaches that prime microglia/macrophages toward the beneficial restorative microglial phenotype after TBI.

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          Most cited references 126

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          The Microglial Sensome Revealed by Direct RNA Sequencing

          Microglia, the principal neuroimmune sentinels of the brain, continuously sense changes in their environment and respond to invading pathogens, toxins and cellular debris. Microglia exhibit plasticity and can assume neurotoxic or neuroprotective priming states that determine their responses to danger. We used direct RNA sequencing, without amplification or cDNA synthesis, to determine the quantitative transcriptomes of microglia of healthy adult and aged mice. We validated our findings by fluorescent dual in-situ hybridization, unbiased proteomic analysis and quantitative PCR. We report here that microglia have a distinct transcriptomic signature and express a unique cluster of transcripts encoding proteins for sensing endogenous ligands and microbes that we term the “sensome”. With aging, sensome transcripts for endogenous ligand recognition are downregulated, whereas those involved in microbe recognition and host defense are upregulated. In addition, aging is associated with an overall increase in expression of microglial genes involved in neuroprotection.
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            The epidemiology and impact of traumatic brain injury: a brief overview.

            Traumatic brain injury (TBI) is an important public health problem in the United States and worldwide. The estimated 5.3 million Americans living with TBI-related disability face numerous challenges in their efforts to return to a full and productive life. This article presents an overview of the epidemiology and impact of TBI.
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              Microglial and macrophage polarization—new prospects for brain repair.

              The traditional view of the adult brain as a static organ has changed in the past three decades, with the emergence of evidence that it remains plastic and has some regenerative capacity after injury. In the injured brain, microglia and macrophages clear cellular debris and orchestrate neuronal restorative processes. However, activation of these cells can also hinder CNS repair and expand tissue damage. Polarization of macrophage populations toward different phenotypes at different stages of injury might account for this dual role. This Perspectives article highlights the specific roles of polarized microglial and macrophage populations in CNS repair after acute injury, and argues that therapeutic approaches targeting cerebral inflammation should shift from broad suppression of microglia and macrophages towards subtle adjustment of the balance between their phenotypes. Breakthroughs in the identification of regulatory molecules that control these phenotypic shifts could ultimately accelerate research towards curing brain disorders.
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                Author and article information

                Contributors
                Journal
                Front Aging Neurosci
                Front Aging Neurosci
                Front. Aging Neurosci.
                Frontiers in Aging Neuroscience
                Frontiers Media S.A.
                1663-4365
                28 June 2017
                2017
                : 9
                Affiliations
                Division of Brain Sciences, Department of Medicine, Imperial College London London, United Kingdom
                Author notes

                Edited by: Isidre Ferrer, University of Barcelona, Spain

                Reviewed by: Kerry O'Banion, University of Rochester, United States; Kathleen A. Maguire-Zeiss, Georgetown University, United States

                *Correspondence: Magdalena Sastre m.sastre@ 123456imperial.ac.uk
                Article
                10.3389/fnagi.2017.00208
                5487478
                Copyright © 2017 Donat, Scott, Gentleman and Sastre.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

                Page count
                Figures: 3, Tables: 2, Equations: 0, References: 171, Pages: 20, Words: 14567
                Funding
                Funded by: Wellcome Trust 10.13039/100004440
                Award ID: 105603/Z/14/Z
                Categories
                Neuroscience
                Review

                Neurosciences

                traumatic brain injury, cci, microglia, neuroinflammation, tspo, polarization states

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