The impact of brain lipoprotein structure and composition on amyloid beta metabolism and Alzheimer's disease - PROJECT SUMMARY/ABSTRACT Alzheimer's disease (AD) is a debilitating brain disorder characterized by memory loss and cognitive decline, affecting millions worldwide. The intricate interplay between lipid metabolism and AD pathology is increasingly recognized, and brain lipoproteins (BLps) are thought to be key players given they serve as the primary lipid transport vehicles in the central nervous system (CNS). BLps reside in the cerebrospinal fluid (CSF) that bathes the brain but unfortunately, due to their low abundance and limited accessibility, a comprehensive understanding of BLps has remained elusive. We developed a fluorescent lipoprotein profiling (FLP) technology that hurdles this abundance problem to deeply characterize BLps in small volumes of CSF. Using our FLP and mass spectrometry (LC-MS/MS), we revealed that BLps consist of a spectrum of discretely sized subpopulations that exhibit remarkable proteomic diversity. We posit that the dominant organizing scaffold on BLps, apolipoprotein (APO)E, acts as a master regulator of BLp composition by adopting distinct structural conformations to facilitate docking with complementary partners that impart unique functions. APOE has three isoforms—E2, E3, and E4 differentiated by cysteine-arginine mutations at residues 112 and 158 and carriers of APOE4 have increased amyloid plaque burden, and substantially increased lifetime risk for developing AD. We hypothesize that conformational differences between APOE isoforms alter BLp compositional signatures and their ability to bind and traffic amyloid beta (Aβ), the building block of amyloid plaques. Our objectives are two-fold: First, to assess the impact of APOE isoforms on BLp distribution, composition, and ability to bind Aβ. To do so, we will perform a comparative analysis of BLp patterns in individuals with different APOE genotypes and levels of cognitive health using our FLP and LC-MS/MS, shedding light on the AD-relevant BLp subspecies. Second, we aim to unravel the structural details of lipid-bound APOE isoforms that govern their interactions with Aβ. Using cryo-EM and novel structural proteomics approaches, we will derive high-resolution structural maps of Aβ complexed with different APOE isoforms on reconstituted HDL and determine how “accessory” proteins found on BLps modulate their interaction with Aβ. These studies will provide a detailed molecular map of BLp metabolism in the CNS which can be leveraged for the development of targeted therapeutic approaches to treat and prevent AD.
