The focus of P01 and Project 1 remains on the ‘neurovascular hypothesis’, which holds that changes in the cerebrovascular system contribute to cognitive decline, dementia and AD. We propose to test the hypothesis in APOE4 carriers (ε3/ε4; ε4/ε4) that are at high genetic risk for AD and develop with age accelerated cerebrovascular changes relative to APOE3 homozygotes (ε3/ε3) that are at lower risk for AD and develop slower cerebrovascular changes. Specifically, we hypothesize that cerebrovascular changes and breakdown in the blood-brain barrier (BBB) precede and predict preclinical cognitive decline and clinical progression from cognitively unimpaired (CU) to mild cognitive impairment (MCI), and MCI to dementia, in APOE4 carriers > APOE3 homozygotes independently of Aβ and tau AD biomarker changes, and prior to neurodegeneration. To test this, we will follow longitudinally 402 APOE4 and 465 APOE3 participants (ages 45-90), initially enrolled as CU (75%) and MCI (25%). We will use: 1) Advanced molecular biomarker assessment of BBB and NVU cell- specific biomarkers measured simultaneously in CSF and plasma with Aβ and tau biomarkers; 2) Advanced MRI assessment of regional BBB permeability in relation to CBF changes; 3) Preclinical AD staging using AT(N) biomarkers system; and 4) Cognitive decline by UDS 3.0 CDR and neuropsychological tests across different cognitive domains. Three aims will test our hypothesis in APOE4 carriers and APOE3 homozygotes to 1) Evaluate longitudinally biomarkers of BBB breakdown by serial CSF analysis in relation to serial Aβ and tau AD biomarker (CSF, PET) changes and cognitive decline (AIM 1); 2) BBB permeability by serial DCE-MRI measurements in relation to serial CSF biomarkers of BBB breakdown, Aβ and tau AD biomarker (CSF, PET) changes and cognitive decline (AIM 2); 3) BBB breakdown by serial DCE-MRI measurements in relation to serial CBF pCASL-MRI measurements, Aβ and tau AD biomarker (CSF, PET) changes and cognitive decline (AIM 3). We expect to show that BBB/vascular dysfunction plays a major role in preclinical and early cognitive decline in APOE4 carriers > APOE3 homozygotes, and that biomarkers of BBB breakdown in biofluids, and BBB and CBF MRI measures will serve as reliable new prognostic biomarkers for cognitive impairment, dementia and early AD. The results of this work may lead to new ways of thinking about pathogenesis, treatment and early prevention of cognitive impairment, dementia and AD for which we still do not have effective therapies.