PROJECT SUMMARY The major goal of this proposal is to define critical cerebrovasculature pathways mediating the clearance of amyloid-β (Aβ), the aggregation of which into amyloid plaques represents a pathological hallmark of Alzheimer's disease (AD). In so doing, we will evaluate the specific role of LDL receptor-related protein 1 (LRP1), heparan sulfate proteoglycan (HSPG) and apolipoprotein E (apoE) isoforms in cerebrovascular clearance of Aβ and formation of cerebral amyloid angiopathy (CAA). The aggregation of Aβ in the brain is a direct result of its increased brain concentration due to an imbalance of its production and clearance. Although brain Aβ clearance is mediated by multiple pathways including intracellular degradation and extracellular degradation, much remains unknown about how the cerebrovasculature system clears Aβ through local cellular, blood-brain barrier (BBB), and perivascular drainage pathways. Given impaired clearance of Aβ drives late-onset AD (LOAD), we aim to improve understanding of the pathways regulating Aβ clearance, thereby establishing new targets for AD therapy and prevention that will benefit the vast majority of patients. During the previous funding cycle, we employed several conditional mouse models to demonstrate that deletion of the Aβ receptor LRP1 leads to slower Aβ clearance and exacerbated amyloid pathology, while deletion of another Aβ receptor HSPG in neurons produces the opposite effects. In addition, we and others have shown that apoE, a ligand for LRP1 and HSPG, modulates Aβ metabolism and pathology in an isoform- dependent manner with apoE4, whose gene allele represents the strongest genetic risk for AD, promoting amyloid deposition and the formation of CAA. Thus, the overall goal of this renewal application is to define the molecular mechanism underlying cerebrovascular clearance of Aβ. We hypothesize that the Aβ receptor LRP1 promotes, whereas HSPG inhibits, Aβ clearance along the cerebrovasculature in an apoE isoform-dependent manner impacting the formation of CAA and the distribution of Aβ pathology. In Aim 1, we will define the roles of LRP1 and HSPG in cerebrovascular function, clearance of Aβ, and formation of amyloid plaques and CAA using conditional mouse models inducing vasculature deletion of Aβ receptors at different ages and at different stages of plaque/CAA pathology. In Aim 2, we will analyze how apoE isoforms affect Aβ clearance and pathology in cerebrovasculature using cell type-specific and inducible mouse models. In Aim 3, we will define the molecular mechanisms through which LRP1, HSPG and apoE isoforms modulate Aβ metabolism, BBB integrity and vascular structure using reconstructed model systems from primary mouse cells or induced pluripotent stem cell (iPSC)-derived human cells to improve the likelihood of discoveries translatable to human AD. Finally, we plan to perform unbiased, single cell-type transcriptome analysis to uncover signaling pathways downstream of LRP1/HSPG/ap...