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Abstract
The number, position, and configuration of double bonds in lipids affect membrane fluidity and the recruitment of signaling proteins. Studies on mammalian sphingolipids have focused on those with a saturated sphinganine or mono-unsaturated sphingosine long chain base. Using high-resolution liquid chromatography-tandem mass spectrometry (LC-MS/MS), we observed a marked accumulation of lipids containing a di-unsaturated sphingadiene base in the hippocampus of mice lacking the metabolic enzyme sphingosine kinase 2 (SphK2). The double bonds were localized to positions C4–C5 and C14–C15 of sphingadiene using ultraviolet photodissociation-tandem mass spectrometry (UVPD-MS/MS). Phosphorylation of sphingoid bases by sphingosine kinase 1 (SphK1) or SphK2 forms the penultimate step in the lysosomal catabolism of all sphingolipids. Both SphK1 and SphK2 phosphorylated sphinga-4,14-diene as efficiently as sphingosine, however deuterated tracer experiments in an oligodendrocyte cell line demonstrated that ceramides with a sphingosine base are more rapidly metabolized than those with a sphingadiene base. Since SphK2 is the dominant sphingosine kinase in brain, we propose that the accumulation of sphingadiene-based lipids in SphK2-deficient brains results from the slower catabolism of these lipids, combined with a bottleneck in the catabolic pathway created by the absence of SphK2. We have therefore uncovered a previously unappreciated role for SphK2 in lipid quality control.
Highlights
Sphingolipids are a large and diverse family of lipids, heavily enriched in the mammalian brain.They are important regulators of cell signaling and physiology in eukaryotes [1,2]
Employing the emerging technique of UVPD-MS/MS, the double bonds were unambiguously assigned to positions C4–C5 and C14–C15 in the sphingoid base backbone of these lipids. We show that these sphingadiene-containing lipids are catabolized more slowly than the more abundant sphingosine-based sphingolipids, and, propose that sphingadiene lipids accumulate in the brains of sphingosine kinase 2 (SphK2)−/− mice due to poor degradation via the sphingosine kinase-sphingosine 1-phosphate (S1P) lyase pathway
Lipids identified using LipidSearch software were manually verified for peak shape and correct product ions, resulting in a total of 357 confirmed lipids in SphK2−/− and 363 in sphingosine kinase 1 (SphK1)−/− mice
Summary
Sphingolipids are a large and diverse family of lipids, heavily enriched in the mammalian brain. They are important regulators of cell signaling and physiology in eukaryotes [1,2]. The common constituent of sphingolipids is their sphingoid base, which in mammals is predominantly 18-carbon sphingosine that contains a double bond at the C4–5 position (d18:1) (Figure 1a). Lipidomic studies often quantify sphingolipids with the less abundant saturated dihydrosphingosine (d18:0) backbone [2]. Metabolites 2020, 10, x FOR PEER REVIEW often quantify sphingolipids with the less abundant saturated dihydrosphingosine (d18:0). Metabolites 2020, 10, 236 backbone [2].
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