Decreased Lipidated ApoE-receptor Interactions Confer Protection Against Pathogenicity of ApoE and Its Lipid Cargoes in Lysosomes

While apolipoprotein E (APOE) is the strongest genetic modifier for late-onset Alzheimer's disease (LOAD), the molecular mechanisms underlying isoform-dependent risk and the relevance of ApoE-associated lipids remain elusive. Here, we report that impaired low-density lipoprotein (LDL) receptor (LDLR) binding of lipidated ApoE2 (lipApoE2) avoids LDLR recycling defects observed with lipApoE3/E4 and decreases the uptake of cholesteryl esters (CEs), which are lipids linked to neurodegeneration. In human neurons, the addition of ApoE carrying polyunsaturated fatty acids (PUFAs)-CE revealed an allelic series (ApoE4 > ApoE3 > ApoE2) associated with lipofuscinosis, an age-related lysosomal pathology resulting from lipid peroxidation. Lipofuscin increased lysosomal accumulation of tau fibrils and was elevated in the APOE4 mouse brain with exacerbation by tau pathology. Intrahippocampal injection of PUFA-CE-lipApoE4 was sufficient to induce lipofuscinosis in wild-type mice. Finally, the protective Christchurch mutation also reduced LDLR binding and phenocopied ApoE2. Collectively, our data strongly suggest decreased lipApoE-LDLR interactions minimize LOAD risk by reducing the deleterious effects of endolysosomal targeting of ApoE and associated pathogenic lipids.