Microglial remodeling of the extracellular matrix in memory circuits

项目摘要 突触和回路的结构重塑对于经验依赖性可塑性是必不可少的，如在 将学到的经验巩固为长期记忆。免疫功能障碍与 许多认知障碍，包括精神分裂症和神经退行性疾病， 对脑内巨噬细胞（称为小胶质细胞）的功能感兴趣，这是主要的免疫细胞 在脑实质中。我们先前发表了IL-1家族细胞因子白细胞介素-33，其由 星形胶质细胞在发育过程中，向其在小胶质细胞上表达的专性受体IL 1 RL 1发出信号， 小胶质细胞活化和吞噬功能。在构成本提案基础的初步数据中，我们确定了 一个新的群体的IL-33表达神经元在两个成年人的大脑区域：海马和额叶皮质。在 在海马的详细结构分析中，我们发现神经元特异性的IL-33或小胶质细胞的缺失， IL-33 R的特异性缺失导致树突棘减少，棘可塑性的标记物（称为棘）减少， 头部丝状伪足和受损的神经发生。此外，这种信号传导途径的缺失导致了 背景恐惧条件反射：小鼠能够正常地学习识别恐惧背景，但具有渐进的恐惧条件反射。 在15-30天后，他们区分恐惧背景和中性背景的能力下降 训练从机制上讲，我们发现细胞外基质蛋白（硫酸软骨素蛋白聚糖短链）， 和聚集蛋白聚糖）在IL-33缺陷动物的海马中积累。我们发现小胶质细胞吞噬聚集蛋白聚糖， 而IL-33信号的缺失减少了这种吞噬。我们还开发了一种神经元功能增益 构建体组成性分泌IL-33。我们发现这足以增加海马棘的数量， 小胶质细胞ECM吞噬，并清除树突棘周围的ECM。根据这些初步数据， 该提案将测试神经元衍生的IL-33驱动ECM的小胶质细胞重塑的中心假设， 促进回路可塑性，支持记忆巩固。目的一是探讨细胞凋亡的分子调控， IL-33从神经元的释放及其活性依赖性，并测试两种候选蛋白酶介导的小胶质细胞 对IL-33的反应。目的二将测试这种神经元-小胶质细胞信号传导对ECM的影响 额叶皮层的组成，重点是神经元周围网络，并确定其对连接的影响。 皮层微电路Aim 3将在情境恐惧条件反射任务中使用钙成像，以测试神经元 活动模式在从最近记忆到遥远记忆的过渡过程中发生变化，解决了以下关键问题： 巩固记忆的结构变化总之，这些研究将系统地剖析 一种新的细胞回路调节健康大脑的可塑性。这项工作的成果 理解与小胶质细胞和免疫相关的许多疾病中的认知功能障碍的意义 系统，包括精神分裂症和神经退行性疾病，如阿尔茨海默病。

PROJECT SUMMARY Structural remodeling of synapses and circuits is essential to experience-dependent plasticity, as occurs during the consolidation of learned experiences into long-term memory. Immune dysfunction has been implicated in numerous cognitive disorders, including schizophrenia and neurodegenerative diseases, leading to increasing interest in the function of brain-resident macrophages known as microglia, which are the dominant immune cell in the brain parenchyma. We previously published that the IL-1 family cytokine Interleukin-33, which is made by astrocytes during development, signals to its obligate receptor IL1RL1 expressed on microglia to promote microglial activation and phagocytic function. In preliminary data that forms the basis for this proposal, we identify a novel population of IL-33 expressing neurons in two adult brain regions: hippocampus and frontal cortex. In detailed structural analyses in the hippocampus, we find that neuron-specific deletion of IL-33 or microglial- specific deletion of IL-33 R leads to fewer dendritic spines, diminished markers of spine plasticity known as spine head filopodia, and impaired neurogenesis. Furthermore, loss of this signaling pathway leads to deficits in contextual fear conditioning: mice are able to normally learn to recognize a fear context but have a progressive decrease in their ability to discriminate the fear context from a neutral context emerging at 15-30 days post training. Mechanistically, we find that extracellular matrix proteins (the chondroitin sulfate proteoglycans brevican and aggrecan) accumulate in the hippocampus of IL-33 deficient animals. We find that microglia engulf aggrecan, and that loss of IL-33 signaling diminishes this engulfment. We also developed a neuronal gain of function construct that constitutively secretes IL-33. We find this is sufficient to increase hippocampal spine numbers, microglial ECM engulfment, and to clear ECM around dendritic spines. Based on these preliminary data, this proposal will test the central hypothesis that neuron-derived IL-33 drives microglial remodeling of ECM to promote circuit plasticity in support of memory consolidation. Aim One will explore the molecular regulation of IL-33 release from neurons and its activity dependence, and test two candidate proteases mediating microglial remodeling in response to IL-33. Aim Two will test the impact of this neuron-microglia signaling on the ECM composition of the frontal cortex with a focus on perineuronal nets and determine its impact on connectivity of the cortical microcircuit. Aim 3 will use calcium imaging in a contextual fear conditioning task to test how neuronal activity patterns shift during the transition from recent to remote memories, addressing key questions regarding the structural changes that underlie memory consolidation. Together, these studies will systematically dissect the role of a novel cellular circuit regulating plasticity in the healthy brain. The outcome of this work has implications for understanding cognitive dysfunction in numerous diseases linked to microglia and the immune system , including schizophrenia and neurodegenerative conditions such as Alzheimer's Disease.