Neuroinflammation, myelin and behavior: Temporal patterns following mild traumatic brain injury in mice

Traumatic brain injury (TBI) results in white matter injury (WMI) that is associated with neurological deficits. Neuroinflammation originating from microglial activation may participate in WMI and associated disorders. To date, there is little information on the time courses of these events after mild TBI. Therefore we investigated (i) neuroinflammation, (ii) WMI and (iii) behavioral disorders between 6 hours and 3 months after mild TBI. For that purpose, we used experimental mild TBI in mice induced by a controlled cortical impact. (i) For neuroinflammation, IL-1b protein as well as microglial phenotypes, by gene expression for 12 microglial activation markers on isolated CD11b+ cells from brains, were studied after TBI. IL-1b protein was increased at 6 hours and 1 day. TBI induced a mixed population of microglial phenotypes with both pro-inflammatory, anti-inflammatory and immunomodulatory markers from 6 hours to 3 days post-injury. At 7 days, microglial activation was completely resolved. (ii) Three myelin proteins were assessed after TBI on ipsi- and contralateral corpus callosum, as this structure is enriched in white matter. TBI led to an increase in 2',3'-cyclic-nucleotide 3'-phosphodiesterase, a marker of immature and mature oligodendrocyte, at 2 days post-injury; a bilateral demyelination, evaluated by myelin basic protein, from 7 days to 3 months post-injury; and an increase in myelin oligodendrocyte glycoprotein at 6 hours and 3 days post-injury. Transmission electron microscopy study revealed various myelin sheath abnormalities within the corpus callosum at 3 months post-TBI. (iii) TBI led to sensorimotor deficits at 3 days post-TBI, and late cognitive flexibility disorder evidenced by the reversal learning task of the Barnes maze 3 months after injury. These data give an overall invaluable overview of time course of neuroinflammation that could be involved in demyelination and late cognitive disorder over a time-scale of 3 months in a model of mild TBI. This model could help to validate a pharmacological strategy to prevent post-traumatic WMI and behavioral disorders following mild TBI.

创伤性脑损伤（Traumatic Brain Injury, TBI）可引发白质损伤（White Matter Injury, WMI），后者与神经功能缺损密切相关。源自小胶质细胞活化的神经炎症，可能参与白质损伤及其相关病症的病理进程。目前针对轻度创伤性脑损伤后上述事件的时间进程相关研究仍较为匮乏。为此，本研究围绕轻度创伤性脑损伤后6小时至3个月的三个核心维度展开探究：（i）神经炎症状态；（ii）白质损伤情况；（iii）行为学异常。
本研究采用可控皮层撞击法构建小鼠轻度创伤性脑损伤模型。（i）针对神经炎症检测：我们分离小鼠脑组织中的CD11b+细胞，通过检测12种小胶质细胞活化标志物的基因表达以分析小胶质细胞表型，并同步检测白细胞介素1β（IL-1β）蛋白水平。结果显示：IL-1β蛋白在损伤后6小时及1天即显著升高；创伤后6小时至3天，小胶质细胞呈现促炎、抗炎与免疫调节混合表型；至损伤后7天，小胶质细胞活化完全消退。
（ii）针对白质损伤检测：我们对损伤侧及对侧胼胝体中的三种髓鞘蛋白进行了定量检测（该脑区富含白质纤维）。结果表明：创伤后2天，未成熟与成熟少突胶质细胞标志物2',3'-环核苷酸3'-磷酸二酯酶（2',3'-cyclic-nucleotide 3'-phosphodiesterase）水平升高；创伤后7天至3个月，通过髓鞘碱性蛋白（myelin basic protein, MBP）检测证实双侧胼胝体出现脱髓鞘病变；创伤后6小时及3天，髓鞘少突胶质细胞糖蛋白（myelin oligodendrocyte glycoprotein, MOG）水平升高。透射电子显微镜观察显示，创伤后3个月，小鼠胼胝体内部可见多种髓鞘结构异常。
（iii）针对行为学异常评估：创伤后3天，小鼠出现感觉运动功能缺损；创伤后3个月，通过巴恩斯迷宫（Barnes maze）反转学习实验证实其存在晚期认知灵活性障碍。
本研究系统阐明了轻度创伤性脑损伤后3个月内，可能参与脱髓鞘病变及晚期认知障碍发生的神经炎症时间进程，为相关领域提供了极具价值的整体研究图景。该小鼠模型可用于验证针对轻度创伤性脑损伤后创伤性白质损伤及行为学异常的药理学干预策略。