APOE4 alters the lipid droplet proteome and modulates droplet dynamics

Excess lipid droplet (LD) accumulation is associated with several pathological states, including Alzheimer's disease (AD). However, the mechanism(s) by which changes in LD composition and dynamics contribute to pathophysiology of these disorders remains unclear. Apolipoprotein E (ApoE) is a droplet associated protein with a common risk variant (E4) that confers the largest increase in genetic risk for late-onset AD. E4 is associated with both increased neuroinflammation and excess LD accumulation. In the current study, we sought to quantitatively profile the lipid and protein composition of LDs between the 'neutral' E3 and risk variant E4, to gain insight into potential LD-driven contributions to AD pathogenesis. Targeted replacement mice expressing human E3 or E4 were injected with saline or lipopolysaccharide (LPS), and after 24 hours, hepatic lipid droplets were isolated for proteomic and lipidomic analyses. Lipidomics revealed a shift in the distribution of glycerophospholipids in E4 LDs with a concomitant increase in phosphatidylcholine species, and overall, the baseline profile of E4 LDs resembled that of the LPS-treated groups. Quantitative proteomics showed that LDs from E4 mice are enriched for proteins involved in protein/vesicle transport but have decreased levels of proteins involved in fatty acid β-oxidation. Interestingly, proteins associated with LDs showed substantial overlap with previously published lists of AD postmortem tissue and microglia 'omics studies, suggesting a potential role for LDs in modulating AD risk or progression. Given this, we exposed primary microglia from the same E3 or E4 mice to exogenous lipid, inflammatory stimulation, necroptotic N2A cells (nN2A), or a combination of treatments to evaluate LD formation and its impact on the cells' immune state. Microglia from E4 mice accumulated more LDs in every condition tested - at baseline and following addition of fatty acids, LPS stimulation, or nN2As. E4 microglia also secreted significantly more cytokines (TNF, IL-1β, IL-10) than E3 microglia in the control, oleic acid, and nN2A treatment conditions, yet showed a blunted response to LPS. In sum, these results suggest that E4 microglia accumulate more LDs compared to E3 microglia and that E4 is associated with a basal LD composition that resembles a pro-inflammatory cell. Together with the high overlap of the LD proteome with established AD-associated datasets, these data further support the idea that alterations in LD dynamics, particularly within microglia, may contribute to the increased risk for AD associated with APOE4 .