Mutations in presenilin-1 (PS1) or Presenilin-2 (PS-2), the catalytic subunit of γ-secretase, lead to the early- onset form of Alzheimer’s disease (EOAD). However, the causes of late-onset AD (LOAD) are under active investigation. The pathological features and functional connectivity of both forms of AD are similar, suggesting that γ-secretase may play a causative role in the late-onset form of AD. Moreover, the pathological role of γ- secretase in EOAD has not been fully elucidated. Only a small fraction of the γ-secretase complex is catalytically active and the function and activation of the inactive complex is unknown. The objectives of this proposal are to elucidate the mechanism of action of γ-secretase modulatory proteins (GSMPs) in the regulation of γ-secretase and examine their function in AD. Here we propose to investigate how γ-secretase activity is activated by cerebrovascular disease mediated hypoxia and γ-secretase activating protein (GSAP). We will elucidate the molecular basis of GSAP in modulation of γ-secretase for the processing of APP and Notch1. We will determine the underlying mechanism by which Hif1α activates γ-secretase. Lastly, we will investigate the mechanisms by which GSAP- and Hif1α- regulate γ-secretase activity in AD mouse models. The long-term goals of this proposal are to elucidate the mechanism of γ-secretase modulation and identify novel γ-secretase regulatory proteins as well as assess their relevance to AD. Our studies focus on the molecular basis of γ-secretase modulation by modulatory proteins and have potentially significant impacts on understanding the disease mechanism and developing therapeutics.