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APOE2, E3 AND E4 DIFFERENTIALLY MODULATE HOMEOSTATIC MECHANISMS IN ISOGENIC IPSC-DERIVED ASTROCYTES
Abstract
Aims
The APOE4 allele is the major genetic risk factor for AD while APOE3 is defined as average risk and APOE2 is protective. Despite recent advances, the fundamental role of different APOE alleles in brain homeostasis is still poorly understood. Here, we aim to uncover the functional role of APOE2, E3 and E4 in human astrocytes.
Methods
We differentiated human APOE-isogenic iPSCs (E4, E3, E2 and APOE-knockout (KO)) to functional astrocytes (hereafter “iAstrocytes”). iAstrocytes at baseline and after activation with Interleukin-1β were analyzed for proteomic profiles using unlabelled mass spectrometry. iAstrocyte functions and properties, such as uptake of glutamate and beta-amyloid, release of cytokine and lipid/cholesterol metabolism were characterized by various assays.
Results
APOE4 iAstrocytes showed lowest capacity for uptake of beta-amyloid and glutamate, while uptake was highest in APOE2 and APOE-KO cells. We observed a genotype-dependent reduction of cholesterol and lipid metabolic and biosynthetic proteomic pathways, and an increase in immunoregulatory pathways at baseline (E4>E3>E2). Cholesterol efflux and biosynthesis were reduced in E4 iAstrocytes, and subcellular localization of cholesterol in lysosomes was increased. APOE4 iAstrocytes showed an increase in inflammatory proteomic pathway, accompanied by highest release of cytokines at baseline, while APOE2 and APOE-KO iAstrocytes show lowest cytokine release (APOE4>E3>E2>KO).
Conclusions
We show that APOE plays a major role in several physiological and metabolic processes in human astrocytes with APOE4 pushing iAstrocytes to a disease-relevant phenotype, causing dysregulated cholesterol/lipid homeostasis, increased inflammatory signalling and reduced β-amyloid uptake while APOE2 iAstrocytes show opposing (protective) effects.