Apolipoprotein E (apoE) colocalizes with amyloid-β (Aβ) in Alzheimer disease (AD) plaques and in synapses, and evidence suggests that direct interactions between apoE and Aβ are important for apoE's effects in AD. The present work examines the hypothesis that apoE receptors mediate uptake of apoE/Aβ complex into synaptic terminals. Western blot analysis shows multiple SDS-stable assemblies in synaptosomes from human AD cortex; apoE/Aβ complex was markedly increased in AD compared with aged control samples. Complex formation between apoE and Aβ 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 Aβ concentration into LDLR and LRP1-positive synaptosomes, along with free and esterified cholesterol. Synaptic Aβ was increased by apoE4 in control and AD samples. These results are consistent with uptake of apoE/Aβ complex and associated lipids into synaptic terminals, with subsequent Aβ clearance in control synapses and accumulation in AD synapses.
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