PROJECT SUMMARY/ABSTRACT MAYO CLINIC JACKSONVILLE Alzheimer's disease (AD) is the leading cause of dementia in the elderly with currently no disease-altering therapy. The accumulation, aggregation and deposition of amyloid-? (A?) peptides in the brain are central events in the pathogenesis of AD. Increasing evidence has shown that the ?4 allele of the apolipoprotein E (APOE) gene is the strongest genetic risk factor for AD among its three polymorphic alleles (?2, ?3 and ?4). ApoE regulates A? clearance, A? aggregation, and amyloid deposition in an isoform-dependent manner. ApoE4 also impairs cerebrovascular functions in the brain and has a reduced ability to regulate neuroinflammation. In addition to AD, apoE4 is associated with a greater risk for hypercholesterolemia and atherosclerosis. In periphery, apoE is highly expressed by the liver and plays critical roles in plasma lipoprotein metabolism. ApoE in the central nervous system (CNS) is synthesized de novo by astrocytes. It is known that blood-brain-barrier (BBB) restricts the transport of apoE into and out of the brain. However, because of the existence of apoE in both periphery and the brain, it has been difficult to address the specific contribution of apoE4 from these two domains to cerebrovascular dysfunction and AD-related pathologies. We hypothesize that peripheral apoE4 impairs CNS functions by modulating vascular integrity, inflammatory responses and amyloid pathogenesis, thus accelerating AD-related pathways. To address this, we have generated novel animal models to allow apoE isoform expression in either periphery or the brain, enabling us to address how peripheral apoE impacts the brain functions and AD-related pathways. In addition, we have established several innovative approaches, including in vivo two-photon microscopic imaging to examine BBB integrity, vasomotion and plaque formation, laser speckle contrast analysis to assess the cerebral blood flow, and in vivo microdialysis to measure brain A? and inflammatory cytokines in the brain interstitial fluid for this proposal. We will determine the effects of peripherally-expressed apoE3 or apoE4 on inflammatory responses, cerebrovascular functions, synapses and cognitive behaviors (Aim 1) and brain A? metabolism and amyloid pathology (Aim 2). Our studies will for the first time investigate how peripherally-expressed apoE isoforms affect CNS functions and AD-related pathways and pathologies. As apoE-directed therapy for AD will likely involve regulating apoE in both brain and periphery, our findings will provide mechanistic guidelines as to how apoE isoforms should be individually targeted for AD therapy.