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 ...