Apolipoprotein E/Amyloid-β Complex Accumulates in Alzheimer Disease Cortical Synapses via Apolipoprotein E Receptors and Is Enhanced by AP0E4

Apolipoprotein E (apoE) colocalizes with amyloid-beta (A beta) in Alzheimer disease (AD) plaques and in synapses, and evidence suggests that direct interactions between apoE and A beta are important for apoE's effects in AD. The present work examines the hypothesis that apoE receptors mediate uptake of apoE/A beta complex into synaptic terminals. Western blot analysis shows multiple SDS-stable assemblies in synaptosomes from human AD cortex; apoE/A beta complex was markedly increased in AD compared with aged control samples. Complex formation between apoE and A beta was confirmed by coimmunoprecipitation experiments. The apoE receptors low-density lipoprotein receptor (LDLR) and LDLR-related protein 1 (LRP1) were quantified in synaptosomes using flow cytometry, revealing up-regulation of LRP1 in early-and late-stage AD. Dual-labeling flow cytometry analysis of LRP1- and LDLR positives indicate most (approximately 65%) of LDLR and LRP1 is associated with postsynaptic density-95 (PSD-95)-positive synaptosomes, indicating that remaining LRP1 and LDLR receptors are exclusively presynaptic. Flow cytometry analysis of Nile red labeling revealed a reduction in cholesterol esters in AD synaptosomes. Dual-labeling experiments showed apoE and A beta concentration into LDLR and LRP1-positive synaptosomes, along with free and esterified cholesterol. Synaptic A beta was increased by apoE4 in control and AD samples. These results are consistent with uptake of apoE/A beta complex and associated lipids into synaptic terminals, with subsequent A beta clearance in control synapses and accumulation in AD synapses.