Mutations in presenilin-1 (PS1) or Presenilin-2 (PS-2), the catalytic subunit of ?-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 ?-secretase may play a causative role in the late-onset form of AD. Moreover, the pathological role of ?- secretase in EOAD has not been fully elucidated. Only a small fraction of the ?-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 ?-secretase modulatory proteins (GSMPs) in the regulation of ?-secretase and examine their function in AD. Here we propose to investigate how ?-secretase activity is activated by cerebrovascular disease mediated hypoxia and ?-secretase activating protein (GSAP). We will elucidate the molecular basis of GSAP in modulation of ?-secretase for the processing of APP and Notch1. We will determine the underlying mechanism by which Hif1? activates ?-secretase. Lastly, we will investigate the mechanisms by which GSAP- and Hif1?- regulate ?-secretase activity in AD mouse models. The long-term goals of this proposal are to elucidate the mechanism of ?-secretase modulation and identify novel ?-secretase regulatory proteins as well as assess their relevance to AD. Our studies focus on the molecular basis of ?-secretase modulation by modulatory proteins and have potentially significant impacts on understanding the disease mechanism and developing therapeutics.