On the structural diversity of Shiga toxin glycosphingolipid receptors in lymphoid and myeloid cells determined by nanoelectrospray ionization tandem mass spectrometry

Abstract

Shiga toxin (Stx, synonymous to verotoxin, VT) binds with high and low affinity to the globo-series neutral glycosphingolipids (GSLs), globotriaosylceramide (Gb3Cer or Galalpha4Galbeta4Glcbeta1Cer, also known as CD77) and globotetraosylceramide (Gb4Cer or GalNAcbeta3Galalpha4Galbeta4Glcbeta1Cer), respectively, which represent the targets of Stxs on many different cell types. B-cell-derived Raji cells and THP-1 cells of monocytic origin are widely used for the investigation of Stx-mediated cellular response, because Stx is known to cause cell death in both cell lines. Despite their functional importance, the Stx receptors of Raji and THP-1 cells have so far not been investigated. This prompted us to explore the structures of their GSL receptors in detail by means of nanoelectrospray ionization quadrupole time-of-flight mass spectrometry (nanoESI-QTOF-MS) with collision-induced dissociation (CID) in conjunction with Stx1 as well as anti-Gb3Cer and anti-Gb4Cer antibodies. Using the combination of a thin-layer chromatography (TLC) overlay assay and MS(1) and MS(2) analysis we identified Gb3Cer (d18:1, C24:1/C24:0) as the prevalent Stx1-receptor accompanied by less abundant Gb3Cer (d18:1, C16:0) in the neutral GSL fraction of Raji cells. The same Gb3Cer species but with almost equal proportions of the C24:1/C24:0 and C16:0 variants were found in THP-1 cells. In addition, unusual hydroxylated Gb3Cer (d18:1, C24:1/C24:0) and Gb3Cer (d18:1, C26:1) could be identified in trace quantities in both cell lines. As the most obvious difference between Raji and THP-1 cells we observed the expression of Gb4Cer in THP-1 cells, whereas Raji cells failed to express this elongation product of Gb3Cer. Both short- and long-chain fatty acid carrying Gb4Cer (d18:1, C16:0) and Gb4Cer (d18:1, C24:1/C24:0), respectively, were the prevalent Gb4Cer variants. This first report on the differential expression of Gb3Cer and Gb4Cer and their structural diversity in lymphoid and myeloid cell lines supports the hypothesis that such heterogeneities might play a functional role in the molecular assembly of GSLs in membrane organization and cellular signaling of Stx-susceptible cells.