A flow cytometry–based in vitro assay reveals that formation of apolipoprotein E (ApoE)–amyloid beta complexes depends on ApoE isoform and cell type

Abstract

Apolipoprotein E (ApoE) is a secreted apolipoprotein with three isoforms, E2, E3, and E4, that binds to lipids and facilitates their transport in the extracellular environment of the brain and the periphery. The E4 allele is a major genetic risk factor for the sporadic form of Alzheimer’s disease (AD), and studies of human brain and mouse models have revealed that E4 significantly exacerbates the deposition of amyloid beta (A). It has been suggested that this deposition could be attributed to the formation of soluble ApoE isoform–specific ApoE–A complexes. However, previous studies have reported conflicting results regarding the directionality and strength of those interactions. In this study, using a series of flow cytometry assays that maintain the physiological integrity of ApoE–A complexes, we systematically assessed the association of A with ApoE2, E3, or E4. We used ApoE secreted from HEK cells or astrocytes overexpressing ApoE fused with a GFP tag. As a source of soluble A peptide, we used synthetic A40 or A42 or physiological A secreted from CHO cell lines overexpressing WT or V717F variant amyloid precursor protein (APP). We observed significant interactions between the different ApoE isoforms and A, with E4 interacting with A more strongly than the E2 and E3 isoforms. We also found subtle differences depending on the A type and the ApoE-producing cell type. In conclusion, these results indicate that the strength of the ApoE–A association depends on the source of A or ApoE.