Glia crosstalk in modulating synaptic dysfunctions in Alzheimer's disease (AD).

区域特异性兴奋性突触丢失与阿尔茨海默病(AD)认知功能下降密切相关(1，2)。在淀粉样变性的小鼠模型中，兴奋性突触外化磷脂酰丝氨酸，小胶质细胞通过特定的受体，主要是TREM2(3-7)，将其识别为Eat-Me信号。小胶质细胞吞噬的另一种机制需要与星形胶质细胞合作。星形胶质细胞是三体突触中突触信息的主要反应者，在损伤后与小胶质细胞发生密切的相互作用(8，9，6)。尽管星形胶质细胞在发育过程中吞噬突触，并在损伤过程中表达免疫清除相关基因，但有趣的是，在淀粉样变性模型中，我们没有看到星形胶质细胞吞噬兴奋性突触(Hong Lab未发表的数据)。相反，我们在突触受到影响的脑区看到星形胶质细胞分泌一种名为MFGE8的分子，该分子已被证明是吞噬细胞和外化磷脂酰丝氨酸的双功能连接物(Hong Lab未发表的数据，6)。使用CRISPR-Cas9技术阻断淀粉样变性小鼠模型中的星形胶质细胞MFGE8可防止小胶质细胞-突触吞噬，并改善这些小鼠的突触丢失。总之，这些数据表明，胶质细胞免疫突触串扰决定了突触消除在AD早期阶段的特异性，至少在兴奋性突触的背景下是这样。关于阿尔茨海默病抑制性突触丢失的研究很少。在小鼠淀粉样变性模型中，斑块前阶段主细胞上抑制性突触后电流的幅度/频率降低，并在后期(10，11，12，13)小白蛋白细胞中出现Nav1.1缺陷的神经元间丢失。Hong Lab显示，在淀粉样变性小鼠的海马区，突触后地理卟啉丢失，这表明中间神经元是早期变化的重要效应者(Hong Lab未发表的数据)。有趣的是，初步研究表明，淀粉样蛋白小鼠模型的小胶质细胞通过TREM2介导的突触消除可能只影响兴奋性突触，而不影响抑制性突触(Hong Lab未发表的数据，3，7)。这提出了一个有趣的问题：是什么导致抑制性突触丢失，以及星形胶质细胞和小胶质细胞在这一过程中所扮演的角色。总体而言，这可能会在神经胶质-突触串扰中引入一种新的范式，其中可能存在着如何在AD大脑中丢失抑制性突触与兴奋性突触的不同机制。首先，我将阐明各种‘Eat-me’信号在兴奋性突触和抑制性突触上的表达是否存在差异，例如，磷脂酰丝氨酸是否外化在兴奋性突触上，而不是抑制性突触上。在发育过程中，特定的GABA受体小胶质细胞选择性地吞噬CA1锥体细胞胞体周围的抑制性突触--在AD中测试这一点将是有趣的。另一种选择是来自不同中枢的小胶质细胞和突触非固有不同信号的贡献。星形胶质细胞对特定信号的表达可能决定了小胶质细胞的吞噬特异性。最后一条途径可能涉及星形胶质细胞的直接吞噬。星形胶质细胞可以通过分泌蛋白控制抑制突触的形成(15-17)。在淀粉样变性小鼠中，发现了一种新的球状星形细胞，其p62水平增加(洪的实验室未发表的数据)。P62表明具有自噬降解活性(18)。这种星形胶质细胞类型可能会释放影响选择性小胶质细胞吞噬的特定信号，也可能是吞噬的原因。除了上述建议，我还将对球状星形胶质细胞进行鉴定，以了解p62是否在发病机制、控制星形胶质细胞的形态发生和影响突触丢失方面发挥上游作用。我将评估原代培养的星形胶质细胞和神经元中p62缺失的影响，然后在体内研究突触变性与p62+星形胶质细胞出现的时间关系(19)。此外，为了了解胶质细胞串扰对突触吞噬特异性的影响，体内化学遗传学方法Split-Turboid tha

Region-specific excitatory synapse loss correlates strongly to Alzheimer's disease (AD) cognitive decline (1,2). In mouse models of amyloidosis, excitatory synapses externalize phosphatidylserine, which microglia recognize as an eat-me signal via specific receptors, mainly TREM2 (3-7). Another mechanism underlying microglia engulfment requires cooperation with astrocytes. Astrocytes are the main responders to synaptic information in tripartite synapses and develop an intimate interaction with microglia upon injury (8,9,6). Although astrocytes engulf synapses during development and express immune clearance-related genes during injury, interestingly, in models of amyloidosis, we do not see astrocytes engulfing excitatory synapses (Hong lab unpublished data). Instead, we see astrocytes in brain regions where synapses are affected secreting a molecule called MFGE8, which has been shown to act as a bifunctional linker of phagocytes and externalized phosphatidylserine (Hong lab unpublished data,6). Blocking astrocytic MFGE8 in mouse models of amyloidosis using CRISPR-Cas9 technology prevents microglia-synapse engulfment and ameliorates synapse loss in these mice. Altogether, these data suggest glia-immune-synapse crosstalk that determine the specificity of synapse elimination in early stages of AD, at least in context of excitatory synapses. Few studies have investigated the inhibitory synapse loss in AD. In mouse models of amyloidosis show decreased amplitude/frequency of inhibitory post-synaptic currents on principal cells at pre-plaque stages and interneurons loss with Nav1.1 defects in parvalbumin cells in later stages (10,11,12,13). Hong lab showed a loss of post-synaptic gephyrin in amyloidosis mice hippocampi, suggesting that interneurons are important effectors of early changes (Hong lab unpublished data). Interestingly, pilot studies suggest that TREM2-mediated synapse elimination by microglia in hippocampus of amyloid mouse models may affect only excitatory but not inhibitory synapses (Hong lab unpublished data,3,7). This raises an intriguing question of what mediates the loss of inhibitory synapse loss, and the roles of astrocytes and microglia in this process. Overall, this can introduce a new paradigm in glial-synapse crosstalk, where there may be distinct mechanisms to how inhibitory synapses are lost in the AD brain versus excitatory synapses. First, I will elucidate whether various 'eat-me' signals are expressed differentially on excitatory vs. inhibitory synapses, for e.g., if phosphatidylserine is externalized on excitatory synapses but not on inhibitory synapses. During development, specific GABA-receptive microglia selectively engulf inhibitory synapses around the CA1 pyramidal cells soma - it would be interesting to test this in AD (14). Another option is the contribution of microglia and synaptic non-intrinsic different signals from different hubs. Astrocytes' expression of specific signals may determine the microglia's engulfment specificity. A final route might involve astrocytes' direct engulfment. Astrocytes can control inhibitory synapse formation via secreted proteins (15-17). In amyloidosis mice, a new bulbous astrocyte with increased p62 level was found (Hong's lab unpublished data). p62 indicate autophagic degradative activity (18). This astrocyte type may release specific signals affecting the selective microglia's engulfment, or they may be responsible for engulfment.Besides the above suggestions, I will characterize the bulbous astrocytes to understand if p62 has an upstream role in pathogenesis, controlling astrocyte morphogenesis and affecting synapse loss. I will assess the effects of p62 loss in primary astrocyte and neuron cultures to then investigate in vivo the temporal relationship between synapse degeneration and p62+ astrocytes' appearance (19). Moreover, to understand the glia crosstalk effect on synapsis engulfment specificity, the in vivo chemico-genetic approach Split-TurboID tha