Diastereomer-specific quantification of bioactive hexosylceramides from bacteria and mammals

Mammals synthesize, cell-type specifically, the diastereomeric hexosylceramides, beta-galactosylceramide (GalCer) and beta-glucosylceramide (GlcCer), which are involved in several diseases, such as sphingolipidosis, diabetes, chronic kidney diseases, or cancer. In contrast, Bacteroides fragilis, a member of the human gut microbiome, and the marine sponge, Agelas mauritianus, produce alpha-GalCer, one of the most potent stimulators for invariant natural killer T cells. To dissect the contribution of these individual stereoisomers to pathologies, we established a novel hydrophilic interaction chromatography-based LC-MS2 method and separated (R > 1.5) corresponding diastereomers from each other, independent of their lipid anchors. Testing various bacterial and mammalian samples, we could separate, identify (including the lipid anchor composition), and quantify endogenous beta-GlcCer, beta-GalCer, and alpha-GalCer isomers without additional derivatization steps. Thereby, we show a selective decrease of beta-GlcCers versus beta-GalCers in cell-specific models of GlcCer synthase-deficiency and an increase of specific beta-GlcCers due to loss of beta-glucoceramidase 2 activity. Vice versa, beta-GalCer increased specifically when cerebroside sulfotransferase (Gal3st1) was deleted. We further confirm beta-GalCer as substrate of globotriaosylceramide synthase for galabiaosylceramide synthesis and identify additional members of the human gut microbiome to contain immunogenic alpha-GalCers. Finally, this method is shown to separate corresponding hexosylsphingosine standards, promoting its applicability in further investigations.-