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， 超过500万美国人。突触是AD发病过程中最早受影响的脑成分， 提示AD的认知能力下降和神经元丢失始于突触功能障碍。尽管有许多 然而，尽管作出了努力，但对AD的发病机制还没有明确的了解，也没有缓解或治疗AD的疗法。 甚至停止AD的进展是已知的。遗传学研究发现了早老素和APP基因的罕见突变 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ApoE 和TREM 2基因，易患散发性AD，提供了潜在的线索，AD的发病机制。Presenilin 突变削弱了γ-分泌酶的活性，γ-分泌酶是一种膜内蛋白酶， 与FAD相关的早老素和APP突变都能增强产生 此外，所有AD患者都患有A β在脑中的积累，导致Aβ在脑中的累积。 "淀粉样Aβ假说“，即AD是由Aβ淀粉样蛋白在脑中积累引起的。然而， 预防或甚至逆转Aβ在脑中积聚的药物在治疗AD中尚未有效。此外，ApoE 和TREM 2与Aβ不直接相关，但似乎影响小胶质细胞功能，炎症反应， 和/或脂质代谢。事实上，脂质含量的改变是AD大脑的一个突出特征，这表明， Aβ可能以与淀粉样蛋白形成无关的方式与AD发病机制相关。事实上，在初步 实验中，我们观察到慢性降低γ-分泌酶活性，如FAD- 早老素基因的相关突变，导致突触传递的主要减少和突触传递的上调。 胆固醇的合成。基于以上研究结果，我们提出了一个跨学科的研究项目 研究了γ-分泌酶活性变化在突触功能和脂质代谢中的作用， AD的发病机制我们描述了四个具体的目标，将调查γ- 分泌酶到突触传递，γ-分泌酶活性通常抑制的机制 胆固醇合成，以及由胆固醇的慢性减少引起的胆固醇合成增加的可能性。 γ-分泌酶活性是观察到的突触损伤的原因。此外，提出的具体目标 将探索ApoE 4（易患AD的ApoE变体）也通过改变脂质代谢发挥作用的可能性 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 并将联合收割机结合细胞生物学、转录组学、CRISPR和电生理学技术 探索潜在的机制。除其他外，这些实验将有助于我们了解 导致FAD和γ-分泌酶活性受损的早老素突变如何影响突触，并测试了 FAD相关早老素突变和遗传ApoE变异体诱发散发性AD作用的假设 通过调节神经元胆固醇水平的亚常见途径，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.