Abnormal Regulation of Mitochondrial Sphingolipids during Aging and Alzheimer's Disease

During pathogenesis of Alzheimer's disease (AD), mitochondria suffer alterations that lead to low energy production and reactive oxygen species formation. However, the mechanism of impaired mitochondria homeostasis in AD is not fully understood. We hypothesized that abnormal sphingolipid metabolism in mitochondria could be one of the contributing factors to mitochondrial dysfunction. Synaptic and non-synaptic mitochondria were isolated from 5xFAD and wild type (WT) mice at 3 and 7 months using Ficoll gradient ultracentrifugation, and their function was analyzed using Seahorse assay. Additionally, mitochondria were analyzed using mass spectrometry for proteomics and sphingolipidomics analyses. Sphingolipid levels were also determined in synaptic and non-synaptic mitochondria isolated from AD patients and healthy controls. We found that synaptic mitochondria isolated from 3-months old 5xFAD mice manifest diminished oxygen consumption as compared to WT. Consistently, proteomics analysis showed that proteins related to respiratory electron transport and oxidative phosphorylation were altered in 5xFAD mice. When quantifying the main sphingolipids in mitochondria, we found that Cer 18:0, Cer 22:0, and Cer 24:1 were increased already at 3 months in 5xFAD mice. No increase in ceramides was detected in mitochondria isolated from AD patients. However, increased levels of sphingosine were found in both 5xFAD mice and AD patients when compared to respective controls. We report that the regulation of sphingolipids in mitochondria is abnormal at 3 months of age in 5xFAD mice, as indicated by the accumulation of long-chain ceramides, which increases with age. Sphingosine levels are increased in both the mitochondria of 5xFAD mice and AD patients. Our data suggest that the sphingolipid composition is dysregulated in mitochondria early during AD pathogenesis.