Integrated stress response associated with dark microglia contributes to neurodegeneration [snRNA-seq]

Microglia, the brain's primary resident immune cells, can assume various phenotypes with diverse functional outcomes on brain homeostasis. In Alzheimer's disease (AD), where microglia are a leading causal cell type, the identity of microglia subsets that drive neurodegeneration remains unresolved. Here, we identify a microglia phenotype characterized by a conserved stress signaling pathway, the integrated stress response (ISR). Using mouse models to activate or inhibit ISR in microglia, we show that ISR underlies the ultrastructurally distinct “dark” microglia subset linked to pathological synapse loss. Inducing microglial ISR in murine AD models exacerbates neurodegenerative pathologies, such as Tau pathology and synaptic terminal loss. Conversely, inhibiting microglial ISR in AD models ameliorates these pathologies. Mechanistically, we present evidence that ISR promotes the secretion of toxic long- chain lipids that impair neuron and oligodendrocyte homeostasis in vitro. Accordingly, inhibition of lipid synthesis in AD models ameliorates synaptic terminal loss. Our results demonstrate that activation of ISR within microglia represents a pathway contributing to neurodegeneration and suggest that this may be sustained, at least in part, by the secretion of long-chain lipids from ISR-activated microglia. Overall design: To determine the effect of ISR on microglial subsets, we performed single-nuclei sequencing on 5xFADTRAP, 5xFADiPKR, and 5xFADEif2ATRAP microglial nuclei.

小胶质细胞（Microglia）是大脑中主要的常驻免疫细胞，可呈现多种表型，对大脑内环境稳态产生多样的功能性影响。在阿尔茨海默病（AD）中，小胶质细胞是主要的致病细胞类型，但其驱动神经退行性变的具体亚群身份仍未明确。本研究鉴定出一种以保守应激信号通路——整合应激反应（integrated stress response, ISR）为特征的小胶质细胞表型。通过在小鼠模型中激活或抑制小胶质细胞内的ISR，我们证实ISR是与病理性突触丢失相关的、超微结构独特的“暗态”小胶质细胞亚群的分子基础。在小鼠AD模型中诱导小胶质细胞ISR会加重神经退行性病理改变，例如Tau病理与突触末梢丢失。反之，在AD模型中抑制小胶质细胞ISR则可改善上述病理状况。从机制上讲，本研究提供的证据表明ISR可促进毒性长链脂质的分泌，这类脂质在体外实验中会破坏神经元与少突胶质细胞的内环境稳态。据此，在AD模型中抑制脂质合成可改善突触末梢丢失现象。本研究结果证实，小胶质细胞内ISR的激活是一条促进神经退行性变的通路，且该通路至少部分可通过ISR激活的小胶质细胞分泌长链脂质得以维持。总体实验设计：为明确ISR对小胶质细胞亚群的影响，我们对5xFADTRAP、5xFADiPKR及5xFADEif2ATRAP小鼠的小胶质细胞核进行了单细胞核测序。