Structural Profiling of Individual Glycosphingolipids in a Single Thin-Layer Chromatogram by Multiple Sequential Immunodetection Matched with Direct IR-MALDI-o-TOF Mass Spectrometry

Update on Glycosphingolipids Abundance in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the most frequent type of primary liver cancer. Low numbers of HCC patients being suitable for liver resection or transplantation and multidrug resistance development during pharmacotherapy leads to high death rates for HCC patients. Understanding the molecular mechanisms of HCC etiology may contribute to the development of novel therapeutic strategies for prevention and treatment of HCC. UDP-glucose ceramide glycosyltransferase (UGCG), a key enzyme in glycosphingolipid metabolism, generates glucosylceramide (GlcCer), which is the precursor for all glycosphingolipids (GSLs). Since UGCG gene expression is altered in 0.8% of HCC tumors, GSLs may play a role in cellular processes in liver cancer cells. Here, we discuss the current literature about GSLs and their abundance in normal liver cells, Gaucher disease and HCC. Furthermore, we review the involvement of UGCG/GlcCer in multidrug resistance development, globosides as a potential prognostic marker for HCC, gangliosides as a potential liver cancer stem cell marker, and the role of sulfatides in tumor metastasis. Only a limited number of molecular mechanisms executed by GSLs in HCC are known, which we summarize here briefly. Overall, the role GSLs play in HCC progression and their ability to serve as biomarkers or prognostic indicators for HCC, requires further investigation.

PapG subtype-specific binding characteristics of Escherichia coli towards globo-series glycosphingolipids of human kidney and bladder uroepithelial cells

Uropathogenic Escherichia coli (UPEC) are the primary cause of urinary tract infections (UTIs) in humans. P-fimbriae are key players for bacterial adherence to the uroepithelium through the Galα1-4Gal-binding PapG adhesin. The three identified classes I, II and III of PapG are supposed to adhere differently to host cell glycosphingolipids (GSLs) of the uroepithelial tract harboring a distal or internal Galα1-4Gal sequence. In this study, GSL binding characteristics were obtained in a nonradioactive adhesion assay using biotinylated E. coli UTI and urine isolates combined with enzyme-linked NeutrAvidin for detection. Initial experiments with reference globotriaosylceramide (Gb3Cer, Galα1-4Galβ1-4Glcβ1-1Cer), globotetraosylceramide (Gb4Cer, GalNAcβ1-3Galα1-4Galβ1-4Glcβ1-1Cer) and Forssman GSL (GalNAcα1-3GalNAcβ1-3Galα1-4Galβ1-4Glcβ1-1Cer) revealed balanced adhesion toward the three GSLs for PapG I-mediated attachment. In contrast, E. coli carrying PapG II or PapG III increasingly adhered to growing oligosaccharide chain lengths of Gb3Cer, Gb4Cer and Forssman GSL. Binding studies with GSLs from human A498 kidney and human T24 bladder epithelial cells, both being negative for the Forssman GSL, revealed the less abundant Gb4Cer vs. Gb3Cer as the prevalent receptor in A498 cells of E. coli expressing PapG II or PapG III. On the other hand, T24 cells exhibited a higher relative content of Gb4Cer vs. Gb3Cer alongside dominant binding of PapG II- or PapG III-harboring E. coli toward Gb4Cer and vastly lowered attachment to minor Gb3Cer. Further studies on PapG-mediated interaction with cell surface-exposed GSLs will improve our knowledge on the molecular mechanisms of P-fimbriae-mediated adhesion and may contribute to the development of antiadhesion therapeutics to combat UTIs.

Membrane assembly of Shiga toxin glycosphingolipid receptors and toxin refractiveness of MDCK II epithelial cells

Shiga toxins (Stxs) are the major virulence factors of Stx-producing Escherichia coli (STEC), which cause hemorrhagic colitis and severe extraintestinal complications due to injury of renal endothelial cells, resulting in kidney failure. Since kidney epithelial cells are suggested additional targets for Stxs, we analyzed Madin-Darby canine kidney (MDCK) II epithelial cells for presence of Stx-binding glycosphingolipids (GSLs), determined their distribution to detergent-resistant membranes (DRMs), and ascertained the lipid composition of DRM and non-DRM preparations. Globotriaosylceramide and globotetraosylceramide, known as receptors for Stx1a, Stx2a, and Stx2e, and Forssman GSL as a specific receptor for Stx2e, were found to cooccur with SM and cholesterol in DRMs of MDCK II cells, which was shown using TLC overlay assay detection combined with mass spectrometry. The various lipoforms of GSLs were found to mainly harbor ceramide moieties composed of sphingosine (d18:1) and C24:1/C24:0 or C16:0 FA. The cells were highly refractory toward Stx1a, Stx2a, and Stx2e, most likely due to the absence of Stx-binding GSLs in the apical plasma membrane determined by immunofluorescence confocal laser scanning microscopy. The results suggest that the cellular content of Stx receptor GSLs and their biochemical detection in DRM preparations alone are inadequate to predict cellular sensitivity toward Stxs.

Shiga Toxin Glycosphingolipid Receptors in Human Caco-2 and HCT-8 Colon Epithelial Cell Lines

Shiga toxins (Stxs) released by enterohemorrhagic Escherichia coli (EHEC) into the human colon are the causative agents for fatal outcome of EHEC infections. Colon epithelial Caco-2 and HCT-8 cells are widely used for investigating Stx-mediated intestinal cytotoxicity. Only limited data are available regarding precise structures of their Stx receptor glycosphingolipids (GSLs) globotriaosylceramide (Gb3Cer) and globotetraosylceramide (Gb4Cer), and lipid raft association. In this study we identified Gb3Cer and Gb4Cer lipoforms of serum-free cultivated Caco-2 and HCT-8 cells, chiefly harboring ceramide moieties composed of sphingosine (d18:1) and C16:0, C22:0 or C24:0/C24:1 fatty acid. The most significant difference between the two cell lines was the prevalence of Gb3Cer with C16 fatty acid in HCT-8 and Gb4Cer with C22–C24 fatty acids in Caco-2 cells. Lipid compositional analysis of detergent-resistant membranes (DRMs), which were used as lipid raft-equivalents, indicated slightly higher relative content of Stx receptor Gb3Cer in DRMs of HCT-8 cells when compared to Caco-2 cells. Cytotoxicity assays revealed substantial sensitivity towards Stx2a for both cell lines, evidencing little higher susceptibility of Caco-2 cells versus HCT-8 cells. Collectively, Caco-2 and HCT-8 cells express a plethora of different receptor lipoforms and are susceptible towards Stx2a exhibiting somewhat lower sensitivity when compared to Vero cells.

Shiga toxin glycosphingolipid receptors and their lipid membrane ensemble in primary human blood-brain barrier endothelial cells

Shiga toxin (Stx)-mediated injury to microvascular endothelial cells in the brain significantly contributes to the pathogenesis of the hemolytic-uremic syndrome caused by enterohemorrhagic Escherichia coli (EHEC). Stxs are AB₅ toxins and the B-pentamers of the two major Stx subtypes Stx1a and Stx2a preferentially bind to the glycosphingolipid (GSL) globotriaosylceramide (Gb3Cer) expressed by human endothelial cells. Here we report on comprehensive structural analysis of the different lipoforms of Gb3Cer (Galα4Galβ4Glcβ1Cer) and globotetraosylceramide (Gb4Cer, GalNAcβ3Galα4Galβ4Glcβ1Cer, the less effective Stx receptor) of primary human brain microvascular endothelial cells and their association with lipid rafts. Detergent-resistant membranes (DRMs), obtained by sucrose density gradient ultracentrifugation, were used as lipid raft-analogous microdomains of the liquid-ordered phase and nonDRM fractions were employed as equivalents for the liquid-disordered phase of cell membranes. Structures of the prevalent lipoforms of Gb3Cer and Gb4Cer were those with Cer (d18:1, C16:0), Cer (d18:1, C22:0) and Cer (d18:1, C24:1/C24:0) determined by electrospray ionization mass spectrometry that was combined with thin-layer chromatography immunodetection using anti-Gb3Cer and anti-Gb4Cer antibodies as well as Stx1a and Stx2a subtypes. Association of Stx receptor GSLs was determined by co-localization with lipid raft-specific membrane protein flotillin-2 and canonical lipid raft marker sphingomyelin with Cer (d18:1, C16:0) and Cer (d18:1, C24:1/C24:0) in the liquid-ordered phase, whereas lyso-phosphatidylcholine was detectable exclusively in the liquid-disordered phase. Defining the precise microdomain structures of primary endothelial cells may help to unravel the initial mechanisms by which Stxs interact with their target cells and will help to develop novel preventive and therapeutic measures for EHEC-mediated diseases.

Shiga toxin glycosphingolipid receptors of Vero-B4 kidney epithelial cells and their membrane microdomain lipid environment

Shiga toxins (Stxs) are produced by enterohemorrhagic Escherichia coli (EHEC), which cause human infections with an often fatal outcome. Vero cell lines, derived from African green monkey kidney, represent the gold standard for determining the cytotoxic effects of Stxs. Despite their global use, knowledge about the exact structures of the Stx receptor glycosphingolipids (GSLs) and their assembly in lipid rafts is poor. Here we present a comprehensive structural analysis of Stx receptor GSLs and their distribution to detergent-resistant membranes (DRMs), which were prepared from Vero-B4 cells and used as lipid raft equivalents. We identified globotriaosylceramide (Gb3Cer) and globotetraosylceramide (Gb4Cer) as the GSL receptors for Stx1a, Stx2a, and Stx2e subtypes using TLC overlay detection combined with MS. The uncommon Stx receptor, globopentaosylceramide (Gb5Cer, Galβ3GalNAcβ3Galα4Galβ4Glcβ1Cer), which was specifically recognized (in addition to Gb3Cer and Gb4Cer) by Stx2e, was fully structurally characterized. Lipoforms of Stx receptor GSLs were found to mainly harbor ceramide moieties composed of sphingosine (d18:1) and C24:0/C24:1 or C16:0 fatty acid. Moreover, co-occurrence with lipid raft markers, SM and cholesterol, in DRMs suggested GSL association with membrane microdomains. This study provides the basis for further exploring the functional impact of lipid raft-associated Stx receptors for toxin-mediated injury of Vero-B4 cells.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2009-2010

This review is the sixth update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2010. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, arrays and fragmentation are covered in the first part of the review and applications to various structural typed constitutes the remainder. The main groups of compound that are discussed in this section are oligo and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals. Many of these applications are presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions and applications to chemical synthesis.

Characterization of urinary tract infection-associated Shiga toxin-producing Escherichia coli

Enterohemorrhagic Escherichia coli (EHEC), a subgroup of Shiga toxin (Stx)-producing E. coli (STEC), is a leading cause of diarrhea and hemolytic-uremic syndrome (HUS) in humans. However, urinary tract infections (UTIs) caused by this microorganism but not associated with diarrhea have occasionally been reported. We geno- and phenotypically characterized three EHEC isolates obtained from the urine of hospitalized patients suffering from UTIs. These isolates carried typical EHEC virulence markers and belonged to HUS-associated E. coli (HUSEC) clones, but they lacked virulence markers typical of uropathogenic E. coli. One isolate exhibited a localized adherence (LA)-like pattern on T24 urinary bladder epithelial cells. Since the glycosphingolipids (GSLs) globotriaosylceramide (Gb3Cer) and globotetraosylceramide (Gb4Cer) are well-known receptors for Stx but also for P fimbriae, a major virulence factor of extraintestinal pathogenic E. coli (ExPEC), the expression of Gb3Cer and Gb4Cer by T24 cells and in murine urinary bladder tissue was examined by thin-layer chromatography and mass spectrometry. We provide data indicating that Stxs released by the EHEC isolates bind to Gb3Cer and Gb4Cer isolated from T24 cells, which were susceptible to Stx. All three EHEC isolates expressed stx genes upon growth in urine. Two strains were able to cause UTI in a murine infection model and could not be outcompeted in urine in vitro by typical uropathogenic E. coli isolates. Our results indicate that despite the lack of ExPEC virulence markers, EHEC variants may exhibit in certain suitable hosts, e.g., in hospital patients, a uropathogenic potential. The contribution of EHEC virulence factors to uropathogenesis remains to be further investigated.

Surface analysis of lipids by mass spectrometry: more than just imaging

Mass spectrometry is now an indispensable tool for lipid analysis and is arguably the driving force in the renaissance of lipid research. In its various forms, mass spectrometry is uniquely capable of resolving the extensive compositional and structural diversity of lipids in biological systems. Furthermore, it provides the ability to accurately quantify molecular-level changes in lipid populations associated with changes in metabolism and environment; bringing lipid science to the "omics" age. The recent explosion of mass spectrometry-based surface analysis techniques is fuelling further expansion of the lipidomics field. This is evidenced by the numerous papers published on the subject of mass spectrometric imaging of lipids in recent years. While imaging mass spectrometry provides new and exciting possibilities, it is but one of the many opportunities direct surface analysis offers the lipid researcher. In this review we describe the current state-of-the-art in the direct surface analysis of lipids with a focus on tissue sections, intact cells and thin-layer chromatography substrates. The suitability of these different approaches towards analysis of the major lipid classes along with their current and potential applications in the field of lipid analysis are evaluated.

Association of Shiga toxin glycosphingolipid receptors with membrane microdomains of toxin-sensitive lymphoid and myeloid cells

Glycosphingolipids (GSLs) of the globo-series constitute specific receptors for Shiga toxins (Stxs) released by certain types of pathogenic Escherichia coli strains. Stx-loaded leukocytes may act as transporter cells in the blood and transfer the toxin to endothelial target cells. Therefore, we performed a thorough investigation on the expression of globo-series GSLs in serum-free cultivated Raji and Jurkat cells, representing B- and T-lymphocyte descendants, respectively, as well as THP-1 and HL-60 cells of the monocyte and granulocyte lineage, respectively. The presence of Stx-receptors in GSL preparations of Raji and THP-1 cells and the absence in Jurkat and HL-60 cells revealed high compliance of solid-phase immunodetection assays with the expression profiles of receptor-related glycosyltransferases, performed by qRT-PCR analysis, and Stx2-caused cellular damage. Canonical microdomain association of Stx GSL receptors, sphingomyelin, and cholesterol in membranes of Raji and THP-1 cells was assessed by comparative analysis of detergent-resistant membrane (DRM) and nonDRM fractions obtained by density gradient centrifugation and showed high correlation based on nonparametric statistical analysis. Our comprehensive study on the expression of Stx-receptors and their subcellular distribution provides the basis for exploring the functional role of lipid raft-associated Stx-receptors in cells of leukocyte origin.

Expression of Shiga toxin 2e glycosphingolipid receptors of primary porcine brain endothelial cells and toxin-mediated breakdown of the blood-brain barrier

Shiga toxin (Stx) 2e, released by certain Stx-producing Escherichia coli, is presently the best characterized virulence factor responsible for pig edema disease, which is characterized by hemorrhagic lesions, neurological disorders and often fatal outcomes. Although Stx2e-mediated brain vascular injury is the key event in development of neurologic signs, the glycosphingolipid (GSL) receptors of Stx2e and toxin-mediated impairment of pig brain endothelial cells have not been investigated so far. Here, we report on the detailed structural characterization of Stx2e receptors globotriaosylceramide (Gb3Cer) and globotetraosylceramide (Gb4Cer), which make up the major neutral GSLs in primary porcine brain capillary endothelial cells (PBCECs). Various Gb3Cer and Gb4Cer lipoforms harboring sphingenine (d18:1) or sphinganine (d18:0) and mostly a long-chain fatty acid (C20-C24) were detected. A notable batch-to-batch heterogeneity of primary endothelial cells was observed regarding the extent of ceramide hydroxylation of Gb3Cer or Gb4Cer species. Gb3Cer, Gb4Cer and sphingomyelin preferentially distribute to detergent-resistant membrane fractions and can be considered lipid raft markers in PBCECs. Moreover, we employed an in vitro model of the blood-brain barrier (BBB), which exhibited strong cytotoxic effects of Stx2e on the endothelial monolayer and a rapid collapse of the BBB. These data strongly suggest the involvement of Stx2e in cerebral vascular damage with resultant neurological disturbance characteristic of edema disease.

Analysis of phospolipids and glycolipids by thin-layer chromatography-matrix-assisted laser desorption and ionization mass spectrometry

Thin-layer chromatography-matrix-assisted laser desorption and ionization mass spectrometry (TLC-MALDI-MS) of organic extracts from biological samples allows untargeted analysis and structural characterization of phospholipids and glycolipids ionized from the near-surface region of a sample separated on a TLC-plate. In particular, MALDI-MS enables the sensitive detection of many analytes directly from the solid surface of an ordinary TLC-plate even without previous staining. It will be shown that the detailed fatty acyl composition of phospholipids can be determined solely by TLC-MALDI-MS without previous derivatization, enzymatic cleavage and/or reversed phase separation. MALDI-time-of-flight (TOF) MS is thus a powerful method in this field due to its high sensitivity, low extent of induced fragmentation and simple, user-friendly performance. This review summarizes the so far available knowledge about combined TLC-MALDI-MS for phospholipid and glycolipid characterization together with the technical workflow and a survey of applications. Finally a perspective on the future of TLC-MALDI-MS is given.

Mass spectrometry based lipidomics: an overview of technological platforms

One decade after the genomic and the proteomic life science revolution, new ‘omics’ fields are emerging. The metabolome encompasses the entity of small molecules—Most often end products of a catalytic process regulated by genes and proteins—with the lipidome being its fat soluble subdivision. Within recent years, lipids are more and more regarded not only as energy storage compounds but also as interactive players in various cellular regulation cycles and thus attain rising interest in the bio-medical community. The field of lipidomics is, on one hand, fuelled by analytical technology advances, particularly mass spectrometry and chromatography, but on the other hand new biological questions also drive analytical technology developments. Compared to fairly standardized genomic or proteomic high-throughput protocols, the high degree of molecular heterogeneity adds a special analytical challenge to lipidomic analysis. In this review, we will take a closer look at various mass spectrometric platforms for lipidomic analysis. We will focus on the advantages and limitations of various experimental setups like ‘shotgun lipidomics’, liquid chromatography—Mass spectrometry (LC-MS) and matrix assisted laser desorption ionization-time of flight (MALDI-TOF) based approaches. We will also examine available software packages for data analysis, which nowadays is in fact the rate limiting step for most ‘omics’ workflows.

Lab-on-a-plate: extending the functionality of MALDI-MS and LDI-MS targets

We review the literature that describes how (matrix-assisted) laser desorption/ionization (MA)LDI target plates can be used not only as sample supports, but beyond that: as functional parts of analytical protocols that incorporate detection by MALDI-MS or matrix-free LDI-MS. Numerous steps of analytical procedures can be performed directly on the (MA)LDI target plates prior to the ionization of analytes in the ion source of a mass spectrometer. These include homogenization, preconcentration, amplification, purification, extraction, digestion, derivatization, synthesis, separation, detection with complementary techniques, data storage, or other steps. Therefore, we consider it helpful to define the "lab-on-a-plate" as a format for carrying out extensive sample treatment as well as bioassays directly on (MA)LDI target plates. This review introduces the lab-on-plate approach and illustrates it with the aid of relevant examples from the scientific and patent literature.

Lipid analysis by thin-layer chromatography--a review of the current state

High-performance thin-layer chromatography (HPTLC) is a widely used, fast and relatively inexpensive method of separating complex mixtures. It is particularly useful for smaller, apolar compounds and offers some advantages over HPLC. This review gives an overview about the special features as well as the problems that have to be considered upon the HPTLC analysis of lipids. The term "lipids" is used here in a broad sense and comprises fatty acids and their derivatives as well as substances related biosynthetically or functionally to these compounds. After a short introduction regarding the stationary phases and the methods how lipids can be visualized on an HPTLC plate, the individual lipid classes will be discussed and the most suitable solvent systems for their separation indicated. The focus will be on lipids that are most abundant in biological systems, i.e. cholesterol and its derivates, glycerides, sphingo- and glycolipids as well as phospholipids. Finally, a nowadays very important topic, the combination between HPTLC and mass spectrometric (MS) detection methods will be discussed. It will be shown that this is a very powerful method to investigate the identities of the HPTLC spots in more detail than by the use of common staining methods. Future aspects of HPTLC in the lipid field will be also discussed.

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

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.

An update of MALDI-TOF mass spectrometry in lipid research

Although matrix-assisted laser desorption and ionization (MALDI) mass spectrometry (MS)--often but not exclusively coupled with a time-of-flight (TOF) mass analyzer--is primarily established in the protein field, there is increasing evidence that MALDI MS is also very useful in lipid research: MALDI MS is fast, sensitive, tolerates sample impurities to a relatively high extent and provides very simple mass spectra without major fragmentation of the analyte. Additionally, MALDI MS devices originally purchased for "proteomics" can be used also for lipids without the need of major system alterations. After a short introduction into the method and the related ion-forming process, the MALDI mass spectrometric characteristics of the individual lipid (ranging from completely apolar hydrocarbons to complex glycolipids with the focus on glycerophospholipids) classes will be discussed and the progress achieved in the last years emphasized. Special attention will be paid to quantitative aspects of MALDI MS because this is normally considered to be the "weak" point of the method, particularly if complex lipid mixtures are to be analyzed. Although the detailed role of the matrix is not yet completely clear, it will be also explicitly shown that the careful choice of the matrix is crucial in order to be able to detect all compounds of interest. Two rather recent developments will be highlighted: "Imaging" MS is nowadays widely established and significant interest is paid in this context to the analysis of lipids because lipids ionize particularly well and are, thus, more sensitively detectable in tissue slices than other biomolecules such as proteins. It will also be shown that MALDI MS can be very easily combined with thin-layer chromatography (TLC) allowing the spatially-resolved screening of the entire TLC plate and the detection of lipids with a higher sensitivity than common staining protocols.