Primary Cilia-Mediated Regulation of Microglial Secretion in Alzheimer's Disease

Alzheimer's disease (AD) is a brain disorder manifested by a gradual decline in cognitive function due to the accumulation of extracellular amyloid plaques, disruptions in neuronal substance transport, and the degeneration of neurons. In affected neurons, incomplete clearance of toxic proteins by neighboring microglia leads to irreversible brain inflammation, for which cellular signaling is poorly understood. Through single-cell transcriptomic analysis, we discovered distinct regional differences in the ability of microglia to clear damaged neurites. Specifically, microglia in the septal region of wild type mice exhibited a transcriptomic signature resembling disease-associated microglia (DAM). These Lateral septum (LS)-enriched microglia (SEM) were associated with dense axonal bundles originating from the hippocampus. Further transcriptomic and proteomic approaches revealed that primary cilia, small hair-like structures found on cells, played a role in the regulation of microglial secretory function. Notably, primary cilia were transiently observed in less than 10% of microglia, and their presence was significantly reduced in microglia from AD mice. We observed significant changes in the expression and distribution of the secretome after inhibiting the primary cilia gene intraflagellar transport particle 88 (Ift88) in microglia. Intriguingly, inhibiting primary cilia in the SEM of AD mice resulted in the expansion of extracellular amyloid plaques and damage to adjacent neurites. These results indicate that DAM-like microglia are present in the LS, a critical target region for hippocampal nerve bundles, and that the primary ciliary signaling system regulates microglial secretion, affecting extracellular proteostasis. Age-related primary ciliopathy probably contributes to the selective sensitivity of microglia, thereby exacerbating AD. Targeting the primary ciliary signaling system could therefore be a viable strategy for modulating neuroimmune responses in AD treatments.阿尔茨海默病（Alzheimer's disease, AD）是一种脑部病症，其表现为因细胞外淀粉样斑块（extracellular amyloid plaques）沉积、神经元物质运输（neuronal substance transport）障碍以及神经元变性（degeneration of neurons）所引发的认知功能渐进性衰退。在受累神经元中，邻近小胶质细胞（neighboring microglia）无法彻底清除毒性蛋白，进而引发不可逆的脑部炎症，而该过程的细胞信号调控机制目前尚不明确。通过单细胞转录组分析（single-cell transcriptomic analysis），我们发现小胶质细胞清除受损神经突的能力存在显著的区域差异。具体而言，野生型小鼠隔区的小胶质细胞呈现出类似疾病相关小胶质细胞（disease-associated microglia, DAM）的转录组特征。这些富集于外侧隔核（Lateral septum, LS）的小胶质细胞（SEM）与源自海马体（hippocampus）的致密轴突束（dense axonal bundles）密切相关。进一步的转录组学与蛋白质组学方法（proteomic approaches）揭示，初级纤毛（primary cilia）——即细胞表面存在的毛发状小结构（small hair-like structures）——参与调控小胶质细胞分泌功能（microglial secretory function）。值得注意的是，仅不足10%的小胶质细胞会短暂表达初级纤毛，且在阿尔茨海默病模型小鼠的小胶质细胞中，初级纤毛的出现比例显著降低。我们观察到，在小胶质细胞中抑制初级纤毛相关基因鞭毛内运输蛋白88（intraflagellar transport particle 88, Ift88）的表达后，分泌组（secretome）的表达与分布均发生了显著变化。有趣的是，在阿尔茨海默病模型小鼠的SEM中抑制初级纤毛的功能，会导致细胞外淀粉样斑块的扩散以及邻近神经突的损伤。上述结果表明，类似疾病相关小胶质细胞的群体存在于外侧隔核这一海马神经束的关键靶区，且初级纤毛信号系统通过调控小胶质细胞的分泌功能，影响细胞外蛋白质稳态（extracellular proteostasis）。年龄相关性原发性纤毛病（Age-related primary ciliopathy）可能会加剧小胶质细胞的选择性易感性，进而恶化阿尔茨海默病的病情。因此，靶向初级纤毛信号系统或可成为调控阿尔茨海默病治疗中神经免疫应答（neuroimmune responses）的可行策略。