APOE 4/4 promotes dysfunctional and inflammatory phenotypes concomitant with impaired maturation of hiPSC-derived astrocytes

Alzheimer's disease (AD) stands as the main cause of dementia among the elderly, with the ε4 allele of apolipoprotein E (APOE) being the primary genetic risk factor. The impact of APOE alleles on astrocyte maturation and function, however, has not been thoroughly understood. To address this, researchers produced human induced pluripotent stem cell (hiPSC)-derived astrocytes from AD patients carrying ε3/ε3 and ε4/ε4 alleles, as well as from healthy controls (HC).

They also utilized isogenic gene-edited hiPSC lines with each APOE allele and an APOE knock-out line to determine allele-specific phenotypes and differentiate between gain- and loss-of-function mechanisms.

Upon examination, APOE 4/4 astrocyte cultures demonstrated diminished percentages of GFAP- and S100β-positive cells in comparison to APOE 2/2 and APOE 3/3. There were no alterations in GLT-1- and AQP4-positive cells. Regardless of the genotype, astrocytes of all types responded to IL-1β + TNF by elevating proinflammatory cytokine expression and release, and to both IL-1β + TNF and Aβ1-42 by altering morphology.

APOE 4/4 astrocytes exhibited increased IL-6 mRNA and morphological branching upon IL-1β + TNF stimulation. Remarkably, under normal conditions, APOE 4/4 astrocytes revealed significant reductions in glutamate uptake capacity and cell size, accompanied by heightened IL-6 release and CXCL3 mRNA expression. In Aβ1-42 uptake assays, a greater proportion of Aβ+ astrocytes were present in APOE 4/4 cultures compared to APOE KO cultures.

The majority of these phenotypes were absent in APOE KO astrocytes, suggesting that the effects of APOE 4/4 were primarily mediated through gain-of-function mechanisms.

The researchers concluded that APOE 4/4 alters astrocyte morphological and molecular maturation while promoting inflammation, disrupting glutamate and Aβ handling under normal circumstances. The findings indicate that the ε4/ε4 genotype disrupts astrocyte development and fundamental processes of cellular homeostasis early in the etiopathology of Alzheimer's disease.