Sphingosine Kinase 2 Potentiates Amyloid Deposition but Protects against Hippocampal Volume Loss and Demyelination in a Mouse Model of Alzheimer's Disease

Sphingosine 1-phosphate (S1P) is a potent vasculoprotective and neuroprotective signaling lipid, synthesized primarily by sphingosine kinase 2 (SK2) in the brain. We have reported pronounced loss of S1P and SK2 activity early in Alzheimer's disease (AD) pathogenesis, and an inverse correlation between hippocampal S1P levels and age in females, leading us to speculate that loss of S1P is a sensitizing influence for AD. Paradoxically, SK2 was reported to mediate amyloid beta (A beta) formation from amyloid precursor protein (APP) in vitro. To determine whether loss of S1P sensitizes to A beta-mediated neurodegeneration, we investigated whether SK2 deficiency worsens pathology and memory in male J20 (PDGFB-APP(SwInd)) mice. SK2 deficiency greatly reduced A beta content in J20 mice, associated with significant improvements in epileptiform activity and cross-frequency coupling measured by hippocampal electroencephalography. However, several key measures of APP(SwInd)-dependent neurodegeneration were enhanced on the SK2-null background, despite reduced A beta burden. These included hippocampal volume loss, oligodendrocyte attrition and myelin loss, and impaired performance in Y-maze and social novelty memory tests. Inhibition of the endosomal cholesterol exporter NPC1 greatly reduced sphingosine phosphorylation in glial cells, linking loss of SK2 activity and S1P in AD to perturbed endosomal lipid metabolism. Our findings establish SK2 as an important endogenous regulator of both APP processing to A beta, and oligodendrocyte survival, in vivo. These results urge greater consideration of the roles played by oligodendrocyte dysfunction and altered membrane lipid metabolic flux as drivers of neurodegeneration in AD.