Regulation of cholesterol by y-secretase and ApoE: Implications for AD pathogenesis and synaptic function

阿尔茨海默病(AD)是美国最常见的神经退行性疾病，其影响比 超过500万美国人。突触是阿尔茨海默病发病过程中最早受影响的大脑组成部分， 提示阿尔茨海默病患者认知功能减退和神经元丢失始于突触功能障碍。尽管有很多 然而，目前还没有对AD发病机制的明确了解，也没有缓解或缓解AD的治疗方法 甚至阿尔茨海默病停止进展也是已知的。遗传学研究发现早老素和APP基因罕见突变 导致早发性家族性AD(FAD)，并描述了几个基因的常见变异，主要是载脂蛋白E 和TREM2基因，易患散发性AD，为AD发病机制提供潜在线索。早老化 突变会削弱γ-分泌酶的活性，这是一种膜内蛋白酶，可以裂解大量的 膜蛋白，包括APP。与FAD相关的早老素和APP突变均可提高产量 APP的卵裂产品Aβ。此外，所有AD患者都患有Aβ在大脑中的积聚，导致 A淀粉样蛋白Aβ假说认为阿尔茨海默病是由Aβ淀粉样蛋白在大脑中堆积诱发的。然而，治疗方法 阻止甚至逆转A-β在脑内积聚的方法在治疗AD方面并不有效。此外，ApoE 和TREM2与Aβ没有直接关系，但似乎影响小胶质细胞功能，炎症反应， 和/或脂类代谢。事实上，脂质含量的变化是阿尔茨海默病大脑的一个显著特征，这表明 β可能以一种与淀粉样蛋白形成无关的方式与AD发病有关。事实上，在初步阶段 实验中我们观察到γ-分泌酶活性的慢性下降，就像FAD- 早老素基因的相关突变导致突触传递显著减少和表达上调 胆固醇的合成。基于所有这些发现，我们在这里提出了一个跨学科的项目 它考察了γ分泌酶活性的变化在突触功能和脂质代谢中的作用 阿尔茨海默病的发病机制我们描述了将调查γ关系的四个具体目标- 分泌酶向突触传递，γ分泌酶活性通常被抑制的机制 胆固醇合成，以及慢性减少引起的胆固醇合成增加的可能性 γ分泌酶活性与观察到的突触损伤有关。此外，拟议的具体目标 我将探索ApoE4，即易患AD的ApoE变体，也通过改变脂代谢来发挥作用的可能性 在神经元中。这些实验将采用多学科方法，将在人类神经元中进行。 并将结合细胞生物学、转录学、CRISPR和电生理学技术 以探索潜在的机制。其中，这些实验将有助于我们理解 导致FAD和削弱γ分泌酶活性的早老素突变如何影响突触，并测试整体 FAD相关早老素突变和遗传ApoE变异易感于散发性AD的假说 通过调节神经元胆固醇水平的共同途径，进而影响突触功能。

Alzheimer's disease (AD) is the most common neurodegenerative disorder in the United States that affects more than 5 million Americans. Synapses are the earliest affected component of the brain during AD pathogenesis, suggesting that the cognitive decline and neuronal loss in AD initiates with synaptic dysfunction. Despite much effort, however, no definitive understanding of AD pathogenesis is available, and no therapies that alleviate or even stop progression of AD are known. Genetic studies identified rare mutations in presenilin and in APP genes that cause early-onset familial AD (FAD), and described common variants in several genes, chiefly the ApoE and TREM2 genes, that predispose to sporadic AD, providing potential clues to AD pathogenesis. Presenilin mutations impair the activity of γ-secretase, an intramembranous protease that cleaves a large number of membrane proteins, including APP. Presenilin and APP mutations associated with FAD both enhance production of Aβ, a cleavage product of APP. Moreover, all AD patients suffer from an accumulation of Aβ in brain, leading to the `amyloid Aβ hypothesis' whereby AD is induced by Aβ amyloid accumulation in brain. However, therapies that prevent or even reverse Aβ accumulation in brain have not been effective in treating AD. Furthermore, ApoE and TREM2 are not directly related to Aβ, but seem to influence microglial function, inflammatory responses, and/or lipid metabolism. Indeed, alterations in lipid content are a prominent feature of AD brains, suggesting that Aβ may be related to AD pathogenesis in a manner that is not related to amyloid formation. Indeed, in preliminary experiments we observed that a chronic decrease γ-secretase activity, as would be observed with FAD- associated mutations of presenilin genes, causes a major decrease in synaptic transmission and an upregulation of cholesterol synthesis. Based on the all of these findings together, we here propose an interdisciplinary project that examines the role of changes in γ-secretase activity in synaptic function and lipid metabolism as a potential pathogenetic mechanism in AD. We describe four specific aims that will investigate the relationship of γ- secretase to synaptic transmission, the mechanism by which γ-secretase activity normally suppresses cholesterol synthesis, and the possibility that increased cholesterol synthesis induced by a chronic decrease in γ-secretase activity is responsible for the observed synaptic impairments. Moreover, the proposed specific aims will explore the possibility that ApoE4, the ApoE variant predisposing to AD, also acts by altering lipid metabolism in neurons. These experiments will adopt a multidisciplinary approach that will be carried out in human neurons and in mouse brains, and will combine cell biology, transcriptomics, CRISPR, and electrophysiology techniques to explore the underlying mechanisms. Among others, these experiments will contribute to our understanding of how presenilin mutations that cause FAD and impair γ-secretase activity affect synapses, and test the overall hypothesis that FAD-associated presenilin mutations and genetic ApoE variants predisposing to sporadic AD act by a sucommon pathway regulating neuronal cholesterol levels, which in turn influences synaptic function.