Regulation of microglial plasticity by TLR4 and microRNAs

 描述(申请人提供)：成人中枢神经系统可以适应和补偿中枢神经系统损伤或神经退行性疾病后的神经功能缺陷，这种影响通常导致的临床症状不像神经元损伤的程度那么严重。小胶质细胞是中枢神经系统固有的免疫细胞，在大脑健康和疾病中发挥着重要作用，是重要的神经元伙伴，但对其活动如何参与中枢神经系统修复和适应慢性神经炎性损伤的内源性机制知之甚少。在这里，我们重点研究在一种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表型向抗炎/神经支持性表型转变的机制。我们将使用慢性间歇性缺氧(CIH)，这是睡眠呼吸暂停的一个特征，会在人类和动物模型中导致显著的认知和学习障碍，超过一半的其他神经疾病和损伤患者都会经历这种情况。我们已经确定了脑出血神经病理学的两个主要阶段，一个是与海马神经元丢失和海马可塑性受损(LTP)重合的急性促炎阶段，另一个是小胶质细胞产生神经营养因子和神经认知行为部分恢复的晚期阶段。我们的数据表明，Toll样受体4(TLR4)的信号活性可能有助于小胶质细胞在脑出血病理晚期的有益作用。TLR4是一种先天免疫受体，最常与促炎性小胶质细胞反应有关。我们发现，在CIH过程中，TLR4依赖的抗炎microRNA上调，其功能是抑制典型的MyD88依赖的TLR4信号通路的促炎活性。TLR4信号还促进了脑源性神经营养因子的产生，这是海马LTP和神经元存活所必需的，我们发现小胶质细胞BDNF的表达在时间上反映了早期海马LTP的丢失和脑出血后学习能力的恢复。因此，我们的主要假设是，早期的促炎TLR4信号后来被依赖于TLR4的miRNAs抑制，最终促进小胶质细胞向神经营养表型的转变，帮助限制神经元损伤，并支持在持续的脑出血病理过程中海马神经可塑性的恢复。利用转基因小鼠的遗传/生化研究、多色流式细胞术和海马片电生理学的共同力量，将实现四个具体目标。AIMS将测试TLR4及其信号通路在协调小胶质细胞的有益效应中的作用，以及在脑出血后期对小胶质细胞BDNF限制神经元丢失和恢复海马可塑性的需求。这些研究的结果将确定小胶质细胞适应耐受神经炎性疾病的机制，以限制损伤并帮助促进神经元功能的恢复。对于间歇性低氧和/或小胶质细胞在其中起作用的许多疾病，这里确定的机制可能被用于治疗，以增强正在进行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.