A novel role for astrocyte-secreted synaptogenic factor Hevin/SPARCL1 in microglia-mediated synaptic pruning in response to visual experience

摘要 突触回路是高等动物行为和认知的基础。适当建立和维持 这些回路需要在生命早期形成和去除的连接数量上保持平衡。增加 有证据表明，突触形成和消除受损会导致突触病理 存在于许多神经系统疾病中。我们实验室和其他实验室进行了严格的体外和体内研究， 证明了星形胶质细胞和小胶质细胞在介导突触形成和消除中的关键作用， 分别该领域的进一步工作已经开始描述这两种神经胶质细胞类型如何交流 来平衡突触的形成和消除然而，对信号的详细分子理解， 介导这种通信，这种通信在电路发展中的作用还没有得到充分的研究。 处理。在我的初步研究中，我发现了星形胶质细胞衍生的突触生成因子， Hevin/Sparcl 1，作为小胶质细胞的直接信号。Hevin在眼睛睁开后被蛋白水解， 与丘脑皮质突触密度的净减少期相吻合。另外，海文的C-末端 结合并激活TLR 4，导致小胶质细胞TLR 2表达增加。最近使用A 开发的TLR 2敲入报告细胞系，我进一步表明TLR 2的表达仅限于小胶质细胞， 发育中的皮质，并且可用于鉴定两个小胶质细胞群体，TLR 2-低对TLR 2-高。 有趣的是，我发现TLR 2高的小胶质细胞有一个高的溶酶体区室，这表明它们代表了一个小胶质细胞。 吞噬细胞数量更多的小胶质细胞根据这些发现，本提案将检验这一假设 星形胶质细胞分泌的Hevin以感觉经验依赖的方式被切割，产生C- 突触发育所需的末端片段。我进一步提出，海文信号本地 通过表达TLR 4的小胶质细胞来引发周围小胶质细胞的亚群， 吞噬细胞标记它们增加TLR 2的表达。这一假设将在两个 目标。目的1阐明Hevin蛋白水解裂解在突触发育和可塑性中的作用， 将这一事件与感官体验联系起来目标2将定义一种分子机制， 星形胶质细胞衍生的突触发生因子与增强的小胶质细胞介导的突触消除。这些发现 这项研究将首次确定从星形胶质细胞到小胶质细胞的分子信号通路， 协调星形胶质细胞和小胶质细胞的功能，以响应感官体验。

ABSTRACT Synaptic circuits underlie behavior and cognition in higher animals. Proper establishment and maintenance of these circuits requires a balance in the number of connections formed and removed early in life. Increasing evidence suggests that impaired synapse formation and elimination contribute to the synaptic pathologies found in many neurological disorders. Rigorous in vitro and in vivo work from our lab and others have demonstrated the critical role for astrocytes and microglia in mediating synapse formation and elimination, respectively. Further work in the field has begun to describe how these two glial cell types may communicate to balance synapse formation and elimination. However, a detailed molecular understanding of the signals that mediate this communication and the role of such communication in circuit development have not been fully addressed. In my preliminary studies, I have identified the astrocyte-derived synaptogenic factor, Hevin/Sparcl1, as a direct signal to microglia. Hevin is proteolytically cleaved just after eye opening which coincides with a period of net decrease in thalamocortical synapse density. Additionally, Hevin’s C-terminal binds and activates TLR4 leading to an increase in the expression of microglia TLR2. Using a recently developed TLR2 knock in reporter line, I further show that TLR2 expression is restricted to microglia in the developing cortex and can be used to identify two populations of microglia, TLR2-low vs. TLR2-high. Intriguingly, I found that TLR2-high microglia have a high lysosomal compartment suggesting they represent a more phagocytic population of microglia. Based on these findings, this proposal will test the hypothesis that astrocyte secreted Hevin is cleaved in a sensory experience dependent manner to produce a C- terminal fragment that is required for synapse development. I further propose that Hevin signals locally through microglia expressed TLR4 to prime a subpopulation of the surrounding microglia to be more phagocytic marking them with an increased expression of TLR2. This hypothesis will be tested in two aims. Aim 1 will elucidate the role of Hevin proteolytic cleavage in synapse development and plasticity and link this event to sensory experience. Aim 2 will define a molecular mechanism that links processing of an astrocyte-derived synaptogenic factor with heightened microglia mediated synapse elimination. The findings from this study will for the first time identify a molecular signaling pathway from astrocytes to microglia that coordinates astrocyte and microglia function in response to sensory experience.