Regulation of microglial plasticity by TLR4 and microRNAs

 描述（由申请方提供）：成年CNS可以适应和补偿CNS损伤或神经退行性疾病后的神经功能缺陷，这种效应导致的临床症状通常不如神经元损伤严重。小胶质细胞，CNS常驻免疫细胞，在脑健康和疾病中起着重要作用，是必不可少的神经元伙伴，但很少有人了解它们的活动如何有助于CNS修复和适应慢性神经炎性损伤的内源性机制。在这里，我们专注于在影响数百万美国人的慢性疾病的过程中控制小胶质细胞从促炎向抗炎/神经支持表型转变的机制。我们将使用暴露于慢性间歇性缺氧（CIH），这是睡眠呼吸暂停的一个标志，在人类和动物模型中导致显著的认知和学习障碍，并且在超过一半的其他神经系统疾病和损伤患者中经历过。我们已经确定了CIH神经病理学的两个主要阶段，一个急性促炎阶段，与海马神经元损失和海马可塑性受损（长时程增强; LTP）相一致，和一个晚期阶段，其中小胶质细胞产生神经营养因子和神经认知行为部分恢复。我们的数据表明，Toll样受体4（TLR 4）（一种最常与促炎性小胶质细胞应答相关的先天性免疫受体）的信号传导活性可能有助于小胶质细胞在CIH病理学后期的有益作用。我们发现CIH过程中TLR 4依赖性的抗炎microRNA上调，其功能是抑制经典MyD 88依赖性TLR 4信号通路的促炎活性。TLR 4信号还促进海马LTP和神经元存活所需的BDNF产生，我们发现小胶质细胞BDNF表达在时间上反映了CIH期间早期海马LTP损失和后期学习恢复。因此，我们的总体假设是，早期促炎性TLR 4信号传导后来被TLR 4依赖性miRNA抑制，最终促进小胶质细胞向神经营养表型的转变，有助于限制神经元损伤并支持持续CIH病理过程中海马神经可塑性的恢复。四个具体的目标将追求使用转基因小鼠的遗传/生化研究，多色流式细胞术，和海马切片电生理学的共享权力。这些目标将测试TLR 4及其信号通路在协调小胶质细胞的有益作用中的作用，以及在CIH后期限制神经元损失和恢复海马可塑性中对小胶质细胞BDNF的需求。这些研究的结果将确定小胶质细胞适应持久神经炎症疾病的机制，以限制损伤并帮助促进神经元功能的恢复。这里确定的机制可以用于治疗益处，以增强间歇性缺氧和/或小胶质细胞发挥作用的许多疾病的持续的内源性神经修复机制。

 DESCRIPTION (provided by applicant): The adult CNS can adapt to and compensate for deficits in neurologic function after CNS injury or neurodegenerative disease, an effect that contributes to typically less severe clinical symptoms than the magnitude of neuronal damage. Microglia, CNS resident immune cells, play fundamental roles in brain health and disease and are essential neuron partners, but little is understood about how their activities contribute to endogenous mechanisms of CNS repair and adaptation to a chronic neuroinflammatory insult. Here we focus on the mechanisms that control microglial transition from the pro-inflammatory to the anti- inflammatory/neurosupportive phenotype over the course of a chronic disease that affects millions of Americans. We will use exposure to chronic intermittent hypoxia (CIH), a hallmark of sleep apnea that causes significant cognitive and learning impairments in humans and animal models, and that is experienced in more than half of patients with other neurological diseases and injuries. We have identified two primary phases of CIH neuropathology, an acute pro-inflammatory phase that coincides with hippocampal neuron loss and impaired hippocampal plasticity (long-term potentiation; LTP), and a late phase in which microglia produce neurotrophic factors and neurocognitive behaviors partially recover. Our data suggest that the signaling activities of the Toll-like receptor 4 (TLR4), an innate immune receptor that is most often associated with pro- inflammatory microglial responses, may contribute to the beneficial effects of microglia late in CIH pathology. We find TLR4-dependent upregulation of an anti-inflammatory microRNA during CIH that functions to suppress the pro-inflammatory activities of the canonical MyD88-dependent TLR4 signaling pathway. TLR4 signaling also promotes BDNF production which is required for hippocampal LTP and neuron survival, and we find that microglial BDNF expression temporally mirrors early hippocampal LTP loss and later recovery of learning during CIH. Thus, our overarching hypothesis is that early pro-inflammatory TLR4 signaling is later repressed by TLR4-dependent miRNAs that ultimately promote microglial transition to the neurotrophic phenotype, helping to limit neuronal damage and support recovery of hippocampal neuroplasticity during continued CIH pathology. Four specific aims will be pursued using the shared power of genetic/biochemical studies in transgenic mice, multi-color flow cytometry, and hippocampal slice electrophysiology. The aims will test the role of TLR4 and its signaling pathways in orchestrating the beneficial effects of microglia, as well as the requirement for microglial BDNF in limiting neuronal loss and recovering hippocampal plasticity in the late phase of CIH. Results from these studies will identify mechanisms by which microglia adapt to enduring neuroinflammatory disease to limit injury and help promote recovery of neuronal function. Mechanisms identified here may be harnessed for therapeutic benefit to enhance ongoing, endogenous neural repair mechanisms for many disorders in which intermittent hypoxia and/or microglia play a role.