Apolipoprotein E lipidation, rather than isoform identity, determines amyloid-β binding and astrocyte clearance

Inherited variations in the Apolipoprotein E (APOE) gene are the largest genetic determinant for late-onset Alzheimer's disease, with the APOEε4 allele conferring the highest risk. While APOE was shown to modulate amyloid beta (Aβ) pathology in a genotype-specific manner (APOEε4>APOEε3>APOEε2), it remains an open question whether these differences are due directly to isoform-specific interactions between ApoE and Aβ or indirect effects on Aβ clearance. To disentangle how single ApoE mutations confer vastly different effects on Aβ pathology, we investigated how both the ApoE isoform and lipidation modulate its binding to Aβ species, and its effect on Aβ uptake and cytotoxicity in human astrocytes. We found that ApoE lipidation, not isoform, has the biggest impact on its interaction with Aβ, on the uptake of Aβ by astrocytes, and on Aβ-induced cytotoxicity. Specifically, unlipidated ApoE preferentially interacts with Aβ oligomers and fibrils, which substantially inhibits their uptake by astrocytes. Conversely, lipidated ApoE showed no interaction with Aβ oligomers and had a reduced ability to inhibit Aβ uptake. Our observations suggest that previously reported ApoE isoform-specific differences in Aβ oligomer levels are not driven by intrinsic sequence-specific differences in the affinity between ApoE and Aβ, but potentially by isoform-specific differences in ApoE lipidation in the brain. We propose that the impaired lipidation of ApoE4 in the brain increases levels of unlipidated ApoE, which then bind toxic Aβ oligomers and reduce their clearance by astrocytes. Such a mechanism underscores the therapeutic potential of interventions aimed at increasing ApoE lipidation.