Microglia-dependent mechanisms governing neural circuit plasticity

这项提案的目标是确定小胶质细胞和感觉经验如何整合， 将突触重塑成精确的功能性大脑地图。数以万亿计的突触形成高度精确的 大脑中代表身体各个部位的地形图。这些地图的形成和维护是由 感官体验（视觉、触觉等），包括消除较不活跃的突触和形成， 维持其他突触。尽管经过50多年的研究， 经验表明，去除或维持特定的突触仍然是一个悬而未决的问题。我们做出了 最初令人兴奋和意想不到的观察是，小胶质细胞，即驻留的CNS巨噬细胞，吞噬并 消除了发育中的视网膜神经系统中一部分不太活跃的突触。此外，减少 小胶质细胞介导的突触吞噬50%（补体受体3 KO）导致持续的 视网膜神经节数目增加。这项工作建立了一种新的思维方式突触 重塑和启发了几个重要的新问题：小胶质细胞介导的突触重塑 实现功能电路所必需的？小胶质细胞重塑成人大脑中的突触吗？如何 神经活动调节小胶质细胞介导的突触重塑？视网膜膝状体系统是有限的 来解决这些问题。我们需要一个强大的系统来研究突触重塑， 突触在整个生命过程中的可塑性，用于测量功能的易处理的分析，以及地形图 具有高的空间和时间分辨率。老鼠的桶状皮层符合所有这些标准， 经验调节小胶质细胞将发育和成熟的突触塑造成功能性突触的假说， 大脑回路我们新的初步数据首次表明，小胶质细胞吞噬兴奋性丘脑皮质 (TC)突触的发展桶皮质和以下的感觉剥夺（胡须去除）， 新生儿此外，缺乏小胶质细胞（集落刺激因子1受体KO; CSF 1 R KO）的小鼠具有缺陷 在适当的桶结构和TC输入消除后，晶须剥夺的发展， 在缺乏小胶质细胞特异性趋化因子受体（Fractalkine受体KO， CX3CR1 KO）。我们现在将使用高分辨率静态和功能成像以及分子成像的组合， 生物学在小鼠桶皮层：1）确定是否小胶质细胞雕刻发展皮层电路， 脑功能图（Aim 1）。2)确定小胶质细胞是否调节神经元的经验依赖性可塑性， 新生儿和成人的皮质地图（目标2）。3)确定小胶质细胞如何重塑突触以响应 神经活动的变化（目标3）。答案将揭示调节感官体验的新机制 调节大脑结构和功能地图的发展，将确定实现可塑性的新方法， 在成人大脑中，并将提供新的机制洞察如何突触重塑在多种情况下， （发育、学习和记忆、疾病等）。

The goal of this proposal is to determine how microglia and sensory experience integrate to remodel synapses into precise, functional brain maps. Trillions of synapses form highly precise topographic maps in the brain representing each part of the body. These maps are shaped and maintained by sensory experience (vision, touch, etc.), including elimination of less active synapses and formation and maintenance of other synapses. Despite over 50 years of research, the underlying mechanisms by which experience dictates removal or maintenance of specific synapses still remains an open question. We made the initial exciting and unexpected observation that microglia, the resident CNS macrophages, engulfed and eliminated a subset of less active synapses in the developing retinogeniculate system. Further, reducing microglia-mediated engulfment of synapses by 50% (complement receptor 3 KO) resulted in sustained increases in retinogeniculate synapse number. This work established a new way of thinking about synaptic remodeling and inspired several important new questions: Is microglia-mediated synaptic remodeling necessary for achieving functional circuits? Do microglia remodel synapses in the adult brain? How does neural activity regulate microglia-mediated synaptic remodeling? The retinogeniculate system was limiting for addressing these questions. We required a robust system for studying synaptic remodeling that involved plasticity of synapses throughout life, tractable assays for measuring function, and a topographic arrangment with high spatial and temporal resolution. The mouse barrel cortex fit all these critera and will enable us to test the hypothesis that experience regulates microglia to shape developing and mature syanpses into functional brain circuits. Our new preliminary data show for the first time that microglia engulf excitatory thalamocortical (TC) synapses in the developing barrel cortex and following sensory deprivation (whisker removal) in the neonate. Further, mice deficient in microglia (colony stimulating factor 1 receptor KO; CSF1R KO) have defects in the development of approriate barrel architecture and TC input elimination following whisker deprivation is completely blocked in mice deficient in a microglia-specific chemokine receptor (fractalkine receptor KO, CX3CR1 KO). We will now use a combination of high resolution static and functional imaging and molecular biology in the mouse barrel cortex to: 1) Determine whether microglia sculpt developing cortical circuits into functional brain maps (Aim 1). 2) Determine whether microglia regulate experience-dependent plasticity of cortical maps in the neonate and adult (Aim 2). 3) Identify how microglia remodel synapses in response to changes in neural activity (Aim 3). Answers will uncover new mechanisms regulating how sensory experience regulates the development of structural and functional brain maps, will identify new ways to achieve plasticity in the adult brain, and will provide new mechanistic insight into how synapses remodel in multiple contexts (development, learning and memory, disease, etc.).