Upregulated NF-κB pathway proteins may underlie APOE44 associated astrocyte phenotypes in sporadic Alzheimer’s disease

Abstract

The Apolipoprotein-E4 allele (APOE) is the strongest genetic risk factor for sporadic Alzheimer’s disease but its role in disease pathogenesis is incompletely understood. The APOE gene encodes Apolipoprotein E (ApoE). Astrocytes are the main source of ApoE in the central nervous system (CNS) and are essential for homeostasis in health and disease. In response to CNS insult, a coordinated multicellular inflammatory response is triggered causing reactive astrogliosis with changes in astrocytic gene expression, cellular structure and function.

Human embryonic stem-cells with the ‘neutral’ APOE33 genotype were edited using CRISPR Cas-9 gene-editing to create isogenic APOE lines with an APOE44 genotype. Quiescent astrocytes were differentiated then stimulated with TNF-α, IL1α and C1q inducing an astrogliotic A1 phenotype. Several potentially pathological APOE44-related phenotypes were identified in both quiescent cells and reactive A1 astrocytes including significantly decreased phagocytosis, impaired glutamate and a defective immunomodulatory response.

In quiescent APOE44 astrocytes there was significantly decreased secretion of IL6, IL8 and several oxylipins. In A1 astrocytes there was a pro-inflammatory phenotype in APOE44 astrocytes with increases in GRO, ENA78, IL6 and IL8, a decrease in IL10 as well as significant differences in oxylipin expression. As TNF-α induced signaling in astrocytes is driven by Nuclear factor kappa B (NF-κB) proteins of this pathway were measured. Significantly higher levels of the p50, p65 and IκBα sub-units were found in both quiescent and A1 APOE44 astrocytes. This suggests that perturbation of NF-κB signaling may contribute to the damaging APOE44 cell phenotypes observed providing a new direction for targeted disease therapeutics.