Upregulated NF-κB pathway proteins may underlieAPOE44associated astrocyte phenotypes in sporadic Alzheimer’s disease

Abstract

Sporadic Alzheimer's disease is the leading cause of dementia worldwide and the Apolipoprotein-E4 allele (APOE) is the strongest genetic risk factor but despite its importance, 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. Using a human embryonic stem-cell line with the neutral APOE33 genotype, we used CRISPR Cas-9 gene-editing technology to create isogenic APOE lines with an APOE44 genotype. We developed a modified protocol designed to produce quiescent astrocytes and then stimulated them to induce an astrogliotic A1 phenotype. Several potentially pathological APOE44-related phenotypes were identified in both quiescent cells and reactive A1 astrocytes including significantly decreased phagocytosis and impaired glutamate uptake in APOE44 astrocytes. There were also key differences in the inflammatory profiles of APOE33 and APOE44 astrocytes characterised by significantly decreased secretion of IL6, IL8 and several oxylipins in APOE44 quiescent astrocytes. 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 suggestive of a defect in their immunomodulatory function. As TNF-α induced signaling in astrocytes is driven by Nuclear factor kappa B (NF-κB) we investigated the proteins of this pathway and found significantly higher levels of the p65 and IκBα sub-units in both quiescent and A1 APOE44 astrocytes. This suggests that perturbation of NF-κB signaling may contribute to the damaging cell phenotypes that we observe and provides a new direction for targeted disease therapeutics. Given the large numbers of existing drugs that act on the NF-κB pathway, this could be realised in a relatively short timeframe.