Human genetic association studies point to immune response and lipid metabolism, in addition to amyloid-beta (Aβ) and tau, as major pathways in Alzheimer’s disease (AD) etiology. Accumulating evidence suggests that chronic neuroinflammation, mainly mediated by microglia and astrocytes, plays a causative role in neurodegeneration in AD. Our group and others have reported early and dramatic losses of brain sulfatide in AD cases and animal models that are mediated by ApoE in an isoform-dependent manner and accelerated by Aβ accumulation. To date, it remains unclear if changes in specific brain lipids are sufficient to drive AD-related pathology. To study the consequences of CNS sulfatide deficiency and gain insights into the underlying mechanisms, we developed a novel mouse model of adult-onset myelin sulfatide deficiency, i.e., tamoxifen-inducible myelinating glia-specific cerebroside sulfotransferase (CST) conditional knockout mice (CSTfl/fl/Plp1-CreERT), took advantage of constitutive CST knockout mice (CST−/−), and generated CST/ApoE double knockout mice (CST−/−/ApoE−/−), and assessed these mice using a broad range of methodologies including lipidomics, RNA profiling, behavioral testing, PLX3397-mediated microglia depletion, mass spectrometry (MS) imaging, immunofluorescence, electron microscopy, and Western blot. We found that mild central nervous system (CNS) sulfatide losses within myelinating cells are sufficient to activate disease-associated microglia and astrocytes, and to increase the expression of AD risk genes (e.g., Apoe, Trem2, Cd33, and Mmp12), as well as previously established causal regulators of the immune/microglia network in late-onset AD (e.g., Tyrobp, Dock, and Fcerg1), leading to chronic AD-like neuroinflammation and mild cognitive impairment. Notably, neuroinflammation and mild cognitive impairment showed gender differences, being more pronounced in females than males. Subsequent mechanistic studies demonstrated that although CNS sulfatide losses led to ApoE upregulation, genetically-induced myelin sulfatide deficiency led to neuroinflammation independently of ApoE. These results, together with our previous studies (sulfatide deficiency in the context of AD is mediated by ApoE and accelerated by Aβ accumulation) placed both Aβ and ApoE upstream of sulfatide deficiency-induced neuroinflammation, and suggested a positive feedback loop where sulfatide losses may be amplified by increased ApoE expression. We also demonstrated that CNS sulfatide deficiency-induced astrogliosis and ApoE upregulation are not secondary to microgliosis, and that astrogliosis and microgliosis seem to be driven by activation of STAT3 and PU.1/Spi1 transcription factors, respectively. Our results strongly suggest that sulfatide deficiency is an important contributor and driver of neuroinflammation and mild cognitive impairment in AD pathology.

中文翻译：

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成人发病的中枢神经系统髓鞘硫脂缺乏足以导致阿尔茨海默病样神经炎症和认知障碍

人类遗传关联研究指出，除了淀粉样蛋白-β (Aβ) 和 tau 外，免疫反应和脂质代谢也是阿尔茨海默病 (AD) 病因的主要途径。越来越多的证据表明，主要由小胶质细胞和星形胶质细胞介导的慢性神经炎症在 AD 的神经退行性变中起致病作用。我们的小组和其他人报告了在 AD 病例和动物模型中脑硫苷脂的早期和显着损失，这些损失由 ApoE 以异构体依赖性方式介导并由 Aβ 积累加速。迄今为止，尚不清楚特定脑脂质的变化是否足以驱动 AD 相关的病理学。为了研究 CNS 硫苷脂缺乏的后果并深入了解其潜在机制，我们开发了一种新的成年发病的髓鞘硫苷脂缺乏小鼠模型，即 他莫昔芬诱导的髓鞘形成神经胶质特异性脑苷脂磺基转移酶 (CST) 条件性敲除小鼠 (CSTfl/fl/Plp1-CreERT)，利用组成型 CST 敲除小鼠 (CST-/-)，并产生 CST/ApoE 双敲除小鼠 (CST- ///ApoE-/-)，并使用广泛的方法评估这些小鼠，包括脂质组学、RNA 分析、行为测试、PLX3397 介导的小胶质细胞耗竭、质谱 (MS) 成像、免疫荧光、电子显微镜和蛋白质印迹。我们发现髓鞘细胞内轻度中枢神经系统 (CNS) 硫苷脂损失足以激活与疾病相关的小胶质细胞和星形胶质细胞，并增加 AD 风险基因（例如 Apoe、Trem2、Cd33 和 Mmp12）的表达，以及正如先前确定的迟发性 AD 中免疫/小胶质细胞网络的因果调节因子（例如，Tyrobp、Dock 和 Fcerg1)，导致慢性 AD 样神经炎症和轻度认知障碍。值得注意的是，神经炎症和轻度认知障碍表现出性别差异，女性比男性更明显。随后的机制研究表明，尽管 CNS 硫苷脂丢失导致 ApoE 上调，但遗传诱导的髓磷脂硫苷脂缺乏导致独立于 ApoE 的神经炎症。这些结果，连同我们之前的研究（AD 中的硫苷脂缺乏由 ApoE 介导并由 Aβ 积累加速）将 Aβ 和 ApoE 置于硫苷脂缺乏诱导的神经炎症的上游，并提出了一个正反馈回路，其中硫苷脂损失可能是通过增加的 ApoE 表达放大。我们还证明，CNS 硫苷脂缺乏诱导的星形胶质细胞增生和 ApoE 上调并非继发于小胶质细胞增生，而且星形胶质细胞增生和小胶质细胞增生似乎分别由 STAT3 和 PU.1/Spi1 转录因子的激活驱动。我们的研究结果强烈表明，硫苷脂缺乏是 AD 病理学中神经炎症和轻度认知障碍的重要贡献者和驱动因素。