Early plasma ceramide and sphingomyelin levels reflect APOE genotype but not familial Alzheimer's disease gene mutations in female 5xFAD mice, with brain-region specific sphingolipid alterations

Pathophysiological changes associated with Alzheimer's disease (AD) begin decades before dementia onset, with age and APOE ε4 genotype as major risk factors [1-4]. Primary risk factors for developing AD include aging and number of copies of the apolipoprotein E (APOE) ε4 allele. Altered sphingolipid metabolism is increasingly implicated in early AD. However, the relationship between early plasma and brain sphingolipid changes-particularly in the context of APOE genotype-remains poorly defined. In this study, we analyzed plasma and brain sphingolipid profiles in transgenic AD mice carrying human APOE3 or APOE4 variants, with or without familial AD mutations (E3FAD and E4FAD). Using liquid chromatography-tandem mass spectrometry (LC-MS/MS), we assessed 110 sphingolipid species across four major classes (ceramides (Cers), hexosylceramides (HexCers), lactosylceramides (LacCers), and sphingomyelins (SMs)) at 2, 4, and 6 months in plasma and at 6 months in brain tissue in the cortex, hippocampus, striatum, and cerebellum. Our results demonstrate that early plasma sphingolipid alterations are largely driven by APOE genotype rather than AD pathology. Specifically, APOE4 carriers showed significant increases in SM species and reductions in Cer species compared to APOE3 carriers, independent of age or AD genotype. Brain lipid profiles showed minimal changes across genotypes after region correction. However, combined p-value analyses revealed APOE- and EFAD-dependent differences in the composition of primarily cortical sphingolipids. ROC analyses demonstrated high discriminative power of plasma sphingolipids for APOE, but not for AD genotype. These findings suggest that early plasma lipid profiles in female 5xFAD mice are more strongly influenced by APOE genotype than by overt AD pathology, potentially reflecting systemic pathways linked to APOE4-associated AD risk, while early disease-associated changes in the brain appear to be subtle and region-specific. These results underscore the importance of accounting for APOE genotype in early-stage AD lipidomic studies and in the interpretation of peripheral lipid biomarkers.