ApoE4 Alters ABCA1 membrane trafficking in astrocytes

Abstract

The APOE ϵ4 allele is the strongest genetic risk factor for late-onset Alzheimer's disease (AD). ApoE protein aggregation plays a central role in AD pathology, including the accumulation of β-amyloid (Aβ). Lipid-poor ApoE4 protein is prone to aggregate and lipidating ApoE4 protects it from aggregation. The mechanisms regulating ApoE4 aggregation in vivo are surprisingly not known. ApoE lipidation is controlled by the activity of the ATP binding cassette A1 (ABCA1). ABCA1 recycling and degradation is regulated by ADP-ribosylation factor 6 (ARF6). We found that ApoE4 promoted greater expression of ARF6 compared with ApoE3, trapping ABCA1 in late-endosomes and impairing its recycling to the cell membrane. This was associated with lower ABCA1-mediated cholesterol efflux activity, a greater percentage of lipid-free ApoE particles, and lower Aβdegradation capacity. Human CSFfromAPOE ϵ4/ϵ4 carriers showed a lower ability to induce ABCA1-mediated cholesterol efflux activity and greater percentage of aggregated ApoE protein compared with CSF fromAPOE ϵ3/ϵ3 carriers. Enhancing ABCA1 activity rescued impaired Aβ degradation in ApoE4-treated cells and reduced both ApoE and ABCA1 aggregation in the hippocampus of male ApoE4-targeted replacement mice. Together, our data demonstrate that aggregated and lipidϵpoor ApoE4 increases ABCA1 aggregation and decreases ABCA1 cell membrane recycling. Enhancing ABCA1 activityto reduce ApoE and ABCA1 aggregation is a potential therapeutic strategy for the prevention of ApoE4 aggregation-driven pathology.