APOE

Apolipoprotein E (APOE) is the single strongest genetic risk factor for late-onset Alzheimer’s disease, and its function is deeply intertwined with microglial states. While APOE is predominantly produced by astrocytes in the healthy brain, reactive microglia dramatically upregulate APOE expression as they transition into Disease-Associated Microglia (DAM) states.

In the context of Alzheimer's, APOE acts as a critical binding partner bridging amyloid plaques and microglial receptors—most notably TREM2. When microglia detect tissue damage, the massive upregulation of APOE serves as an immunometabolic bottleneck that shuts down homeostatic genes (like TGFBR1 and P2RY12) and pushes the cell toward active phagocytosis and containment of amyloid.

The APOE4 isoform—which structurally differs to APOE3 and APOE2—causes a toxic gain-of-function and loss-of-clearance. Microglia expressing APOE4 exhibit impaired lipid metabolism, stunted phagocytosis, and hyper-inflammatory responses. This disrupted axis causes microglia to fail at forming protective barriers around plaques, ultimately exacerbating tau spread and directly mediating neurodegeneration.