Investigating CD47-SIRPa as novel protective signals during CNS synaptic pruning

 描述（由申请人提供）：小胶质细胞，大脑的常驻免疫细胞和吞噬细胞，正在成为健康大脑中发育突触回路的关键调节剂。我们实验室和其他人最近的研究表明，小胶质细胞吞噬发育中大脑的突触;然而，小胶质细胞如何知道靶向哪些突触仍然是一个主要的悬而未决的问题。 我们以前的工作表明，小胶质细胞介导的修剪是发育性突触细化的基础，这是形成成熟回路所需的一个重要过程，在成熟回路中，弱或不适当的突触被消除，剩余的连接被维持和加强。我们发现，小胶质细胞吞噬突触前输入是活动依赖性的，并由补体分子C1 q和C3，和小胶质细胞补体受体CR 3驱动。这些分子是先天免疫“吃我”信号，已知用于促进巨噬细胞吞噬凋亡细胞或碎片，缺乏这些信号的小鼠表现出突触输入的小胶质细胞吞噬减少和细化受损。这表明小胶质细胞介导的修剪可能类似于免疫系统中吞噬细胞对非自身物质的去除。然而，我们还不知道小胶质细胞如何精确地确定哪些输入要吞噬，哪些要避免，这是一个关于塑造精确，成熟连接所需的特异性的重要决定。 我们提出，保护性的“不要吃我”信号是必要的，以防止不适当的小胶质细胞吞噬必要的连接在突触细化，就像他们防止不适当的吞噬健康的自身细胞在免疫反应。我们的初步数据支持这一假设，因为“不要吃我”信号CD 47和SIRP β存在于发育中的大脑中，并且需要防止突触输入的过度小胶质细胞吞噬。我们将研究缺乏CD 47和SIRP β的小鼠的解剖学、功能和行为异常，以更好地了解过量小胶质细胞吞噬的后果。我们还将研究这些“不要吃我”信号是否以及如何受到活动的调节，以确定它们是否以活动依赖的方式指导小胶质细胞吞噬特定的突触。最后，由于已知“不要吃我”信号在神经退行性疾病患者的大脑中下调，我们将研究这些分子是否在亨廷顿病（HD）的小鼠模型中失调，从而可能导致异常小胶质细胞吞噬引起的突触丢失。 这项研究将是第一个证明突触保护是必要的，以防止不适当的小胶质细胞吞噬必要的连接在发展过程中。这项研究计划不仅将提供对小胶质细胞吞噬特定突触的调节机制的深入了解，还将提供对CNS神经退行性疾病中突触丢失的潜在机制的深入了解。

 DESCRIPTION (provided by applicant): Microglia, the brain's resident immune cells and phagocytes, are emerging as critical regulators of developing synaptic circuits in the healthy brain. Recent studies from our lab and others indicate that microglia engulf synapses in the developing brain; however, how microglia know which synapses to target remains a major open question. Our previous work demonstrates that microglia-mediated pruning underlies developmental synaptic refinement, an essential process required for the formation of mature circuits in which weak or inappropriate synapses are eliminated and remaining connections are maintained and strengthened. We found that microglial engulfment of presynaptic inputs is activity-dependent and driven by complement molecules C1q and C3, and microglial complement receptor CR3. These molecules are innate immune "eat me" signals known for promoting macrophage phagocytosis of apoptotic cells or debris, and mice lacking these signals exhibit reduced microglial engulfment of synaptic inputs and impaired refinement. This suggests that microglia-mediated pruning may be analogous to the removal of non-self material by phagocytes in the immune system. However, we do not yet know how microglia precisely determine which inputs to engulf and which to avoid, an important decision regarding the specificity needed to sculpt precise, mature connections. We propose that protective "don't eat me" signals are required to prevent inappropriate microglial engulfment of necessary connections during synaptic refinement, just as they prevent inappropriate engulfment of healthy self-cells by phagocytes during an immune response. Our preliminary data support this hypothesis, as "don't eat me" signals CD47 and SIRP are present in the developing brain and required to prevent excess microglial engulfment of synaptic inputs. We will investigate the anatomical, functional, and behavioral abnormalities in mice lacking CD47 and SIRP to better understand the consequences of excess microglial engulfment. We will also investigate whether and how these "don't eat me" signals are regulated by activity to determine if they direct microglia to engulf specific synapses in an activity-dependent manner. Finally, as "don't eat me" signals are known to be downregulated in the brains of patients with neurodegenerative diseases, we will examine whether these molecules are dysregulated in mouse models of Huntington's disease (HD) and could thereby underlie synapse loss caused by aberrant microglial engulfment. This study would be the first to demonstrate that synaptic protection is required to prevent inappropriate microglial engulfment of necessary connections during development. This research program will provide insight not only into the mechanisms regulating microglial engulfment of specific synapses, but also into possible mechanisms underlying synapse loss in CNS neurodegenerative diseases.