APOE4 reshapes the lipid droplet proteome and modulates microglial inflammatory responses

Excess lipid droplet (LD) accumulation is implicated in various diseases, including Alzheimer's disease (AD), yet the mechanisms underlying this accumulation remain unclear. Apolipoprotein E (ApoE) is a droplet-associated protein, and its E4 variant confers the greatest genetic risk for late-onset AD while also being linked to increased neuroinflammation and LD accumulation. In this study, we compared the lipid and protein composition of hepatic LDs in targeted replacement mice expressing human E3 (neutral) or E4 (risk variant), under both baseline conditions and following lipopolysaccharide (LPS) administration. Lipidomic analysis revealed that E4 LDs exhibit a shift in glycerophospholipid distribution, with an increase in phosphatidylcholine species, such that their baseline profile resembles that of LPS-treated LDs. Quantitative proteomics indicated that E4 LDs are enriched in proteins related to vesicle transport but show decreased levels of proteins involved in fatty acid β-oxidation. Notably, many LD-associated proteins overlapped with those identified in AD postmortem and microglial 'omics studies, suggesting a role for LDs in AD pathogenesis. To further explore these findings, primary microglia from E3 and E4 mice were exposed to exogenous lipids, LPS, and necroptotic N2A cells. Under most conditions, E4 microglia accumulated more LDs and secreted higher levels of proinflammatory cytokines (TNF, IL-1β, IL-10) compared to E3 microglia, although their LPS response was blunted. These data suggest that altered LD dynamics in E4 microglia may contribute to the increased AD risk associated with APOE4.