Glycosphingolipid structural analysis and glycosphingolipidomics

Sphingosines, or sphingoids, are a family of naturally occurring long-chain hydrocarbon derivatives sharing a common 1,3-dihydroxy-2-amino-backbone motif. The majority of sphingolipids, as their derivatives are collectively known, can be found in cell membranes in the form of amphiphilic conjugates, each composed of a polar head group attached to an N-acylated sphingoid, or ceramide. Glycosphingolipids (GSLs), which are the glycosides of either ceramide or myo-inositol-(1-O)-phosphoryl-(O-1)-ceramide, are a structurally and functionally diverse sphingolipid subclass; GSLs are ubiquitously distributed among all eukaryotic species and are found in some bacteria. Since GSLs are secondary metabolites, direct and comprehensive analysis (metabolomics) must be considered an essential complement to genomic and proteomic approaches for establishing the structural repertoire within an organism and deducing its possible functional roles. The glycosphingolipidome clearly comprises an important and extensive subset of both the glycome and the lipidome, but the complexities of GSL structure, biosynthesis, and function form the outlines of a considerable analytical problem, especially since their structural diversity confers by extension an enormous variability with respect to physicochemical properties. This chapter covers selected developments and applications of techniques in mass spectrometric (MS) that have contributed to GSL structural analysis and glycosphingolipidomics since 1990. Sections are included on basic characteristics of ionization and fragmentation of permethylated GSLs and of lithium-adducted nonderivatized GSLs under positive-ion electrospray ionization mass spectrometry (ESI-MS) and collision-induced mass spectrometry (CID-MS) conditions; on the analysis of sulfatides, mainly using negative-ion techniques; and on selected applications of ESI-MS and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) to emerging GSL structural, functional, and analytical issues. The latter section includes a particular focus on evolving techniques for analysis of gangliosides, GSLs containing sialic acid, as well as on characterizations of GSLs from selected nonmammalian eukaryotes, such as dipterans, nematodes, cestodes, and fungi. Additional sections focus on the issue of whether it is better to leave GSLs intact or remove the ceramide; on development and uses of thin-layer chromatography (TLC) blotting and TLC-MS techniques; and on emerging issues of high-throughput analysis, including the use of flow injection, liquid chromatography mass spectrometry (LC-MS), and capillary electrophoresis mass spectrometry (CE-MS).

Cerebroside analogues from marine-derived fungus Aspergillus flavipes

From the mycelium of the marine-derived fungus Aspergillus flavipes, isolated from the sea anemone Anthopleura xanthogrammica, two new cerebroside analogues, namely flavicerebrosides A (1): [(2S,2'R,3R,4E,8E)-N-2'-hydroxyoctadecanoyl-1-O-beta--galactopyranosyl-9-methyl-4,8-sphingadienine], and B (2): [(2S,2'R,3R,3'E,4E,8E)-N-2'-hydroxy-3'-octadecenoyl-1-O-beta--galactopyranosyl-9-methyl-4,8-sphingadienine], together with two known glycosphingolipids cerebrosides D (3) and C (4), were isolated. Their structures were identified by means of extensive spectroscopic analysis (IR, UV, 2D NMR, MS, CD) and chemical degradation. All four compounds showed cytotoxic activity against the KB cell line.

Structure and biological functions of fungal cerebrosides

Ceramide monohexosides (CMHs, cerebrosides) are glycosphingolipids composed of a hydrophobic ceramide linked to one sugar unit. In fungal cells, CMHs are very conserved molecules consisting of a ceramide moiety containing 9-methyl-4,8-sphingadienine in amidic linkage to 2-hydroxyoctadecanoic or 2-hydroxyhexadecanoic acids, and a carbohydrate portion consisting of one residue of glucose or galactose. 9-Methyl 4,8-sphingadienine-containing ceramides are usually glycosylated to form fungal cerebrosides, but the recent description of a ceramide dihexoside (CDH) presenting phytosphingosine in Magnaporthe grisea suggests the existence of alternative pathways of ceramide glycosylation in fungal cells. Along with their unique structural characteristics, fungal CMHs have a peculiar subcellular distribution and striking biological properties. In Pseudallescheria boydii, Candida albicans, Cryptococcus neoformans, Aspergillus nidulans, A. fumigatus, and Schizophyllum commune, CMHs are apparently involved in morphological transitions and fungal growth. The elucidation of structural and functional aspects of fungal cerebrosides may therefore contribute to the design of new antifungal agents inhibiting growth and differentiation of pathogenic species.

Isolation and characterization of Neurospora crassa mutants resistant to antifungal plant defensins

Twenty-five Neurospora crassa mutants obtained by chemical mutagenesis were screened for increased resistance to various antifungal plant defensins. Plant defensin-resistant N. crassa mutants were further tested for their cross-resistance towards other families of structurally different antimicrobial peptides. Two N. crassa mutants, termed MUT16 and MUT24, displaying resistance towards all plant defensins tested but not to structurally different antimicrobial peptides were selected for further characterization. MUT16 and MUT24 were more resistant towards plant defensin-induced membrane permeabilization as compared to the N. crassa wild-type. Based on the previously demonstrated key role of fungal sphingolipids in the mechanism of growth inhibition by plant defensins, membrane sphingolipids of MUT16 and MUT24 were analysed. Membranes of these mutants contained structurally different glucosylceramides, novel glycosylinositolphosphorylceramides, and an altered level of steryl glucosides. Evidence is provided to link these clear differences in sphingolipid profiles of N. crassa mutants with their resistance towards different plant defensins.

Unusual fatty acid composition of cerebrosides from the filamentous soil fungus Mortierella alpina

The cerebrosides produced by the soil filamentous fungus Mortierella alpina strain KG-1/95 account for about 13% of the total polar lipids extractable from lyophilised cells with chloroform/methanol mixtures. By means of 1H NMR and (13)C NMR spectroscopy, matrix-assisted laser-desorption ionisation mass spectrometry, and chemical degradation experiment, they have been shown to be 1-O-beta-D-glucopyranosyl-2-N-(2'-D-hydroxyalkanoyl)-9-methylsphinga-4(E),8(E)-dienines, the fatty acid composition of which is unusual and consists of 2-hydroxytridecanoic (4%), 2-hydroxytetradecanoic (60%), 2-hydroxypentadecanoic (20%), and 2-hydroxyhexadecanoic (16%) acids.

Characterization of glucosylceramides in Pseudallescheria boydii and their involvement in fungal differentiation

Pseudallescheria boydii is a fungal pathogen that causes disease in immunocompromised patients. Ceramide monohexosides (CMHs) were purified from lipidic extracts of this fungus, showing that, as described for several other species, P. boydii synthesizes glucosylceramides as major neutral glycosphingolipids. CMHs from P. boydii were analyzed by high-performance thin-layer chromatography, gas chromatography coupled to mass spectrometry, fast atom bombardment-mass spectrometry, and nuclear magnetic resonance. These combination of techniques allowed the identification of CMHs from P. boydii as molecules containing a glucose residue attached to 9-methyl-4,8-sphingadienine in amidic linkage to 2-hydroxyoctadecanoic or 2-hydroxyhexadecanoic acids. Antibodies from a rabbit infected with P. boydii recognized CMHs from this fungus. Antibodies to CMH were purified from serum and used in indirect immunofluorescence, which revealed that CMHs are detectable on the surface of mycelial and pseudohyphal but not conidial forms of P. boydii, suggesting a differential expression of glucosylceramides according with morphological phase. We also investigated the influence of antibodies to CMH on growth and germ tube formation in P. boydii. Cultures that were supplemented with these antibodies failed to form mycelium, but the latter was not affected once formed. Similar experiments were performed to evaluate whether antibodies to CMH would influence germ tube formation in Candida albicans, a fungal pathogen that synthesizes glucosylceramide and uses differentiation as a virulence factor. Addition of antiglucosylceramide antibodies to cultures of C. albicans clearly inhibited the generation of germ tubes. These results indicated that fungal CMHs might be involved in the differentiation and, consequently, play a role on the infectivity of fungal cells.

Neuritogenic cerebrosides from an edible Chinese mushroom. Part 2: Structures of two additional termitomycesphins and activity enhancement of an inactive cerebroside by hydroxylation

Termitomycesphins E and F, novel cerebrosides that are hydroxylated around the middle of the long-chain base (LCB), have been isolated from the edible Chinese mushroom Termitomyces albuminosus (Berk.) Heim. ('Jizong' in Chinese) together with termitomycesphins A-D, and shown to induce neuronal differentiation in rat PC12 cells. Their stereostructures have been determined based on their chemical derivatization and spectroscopic analysis. The major cerebroside obtained from the same mushroom was not hydroxylated around the middle of the LCB and was inactive against PC12 cells, suggesting the importance of the extra hydroxyl group on LCB. The Di- and tetrahydroxylation of this inactive cerebroside resulted in the enhancement of its neuritogenic activity.

Characterization of cerebrosides from the thermally dimorphic mycopathogen Histoplasma capsulatum: expression of 2-hydroxy fatty N-acyl (E)-Delta(3)-unsaturation correlates with the yeast-mycelium phase transition

Cerebroside (monohexosylceramide) components were identified in neutral lipids extracted from both the yeast and mycelial forms of the thermally dimorphic mycopathogen Histoplasma capsulatum. The components were purified from both forms and their structures elucidated by 1- and 2-dimensional nuclear magnetic resonance (NMR) spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and low energy tandem collision-induced dissociation mass spectrometry (ESI-MS/CID-MS). Both components were characterized as beta-glucopyranosylceramides (GlcCers) containing (4E,8E)-9-methyl-4,8-sphingadienine as the long-chain base, attached to 18-carbon 2-hydroxy fatty N-acyl components. However, while the fatty acid of the yeast form GlcCer was virtually all N-2'-hydroxyoctadecanoate, the mycelium form GlcCer was characterized by almost exclusive expression of N-2'-hydroxy-(E)-delta(3)-octadecenoate. These results suggest that the yeast-mycelium transition is accompanied by up-regulation of an as yet uncharacterized ceramide or cerebroside 2-hydroxy fatty N-acyl (E)-delta(3)-desaturase activity. They also constitute further evidence for the existence of two distinct pathways for ceramide biosynthesis in fungi, since glycosylinositol phosphorylceramides (GIPCs), the other major class of fungal glycosphingolipids, are found with ceramides consisting of 4-hydroxysphinganine (phytosphingosine) and longer chain 2-hydroxy fatty acids. In addition to identification of the major glucocerebroside components, minor components (< 5%) detectable by molecular weight differences in the ESI-MS profiles were also characterized by tandem ESI-MS/CID-MS analysis. These minor components were identified as variants differing in fatty acyl chain length, or the absence of the sphingoid 9-methyl group or (E)-delta(8)-unsaturation, and are hypothesized to be either biosynthetic intermediates or the result of imperfect chemical transformation by the enzymes responsible for these features. Possible implications of these findings with respect to chemotaxonomy, compartmentalization of fungal glycosphingolipid biosynthetic pathways, and regulation of morphological transitions in H.capsulatum and other dimorphic fungi are discussed.

General and efficient syntheses of C(18)-4,8-sphingadienines via S(N)2'-type homoallylic coupling reactions mediated by thioether-stabilized copper reagents

The stereoselective syntheses of C(18)-4,8-sphingadienines 3 and 4 as analogues of sphingosine 1 are described. The key step in these syntheses involved a novel S(N)2'-type homoallylic coupling reaction between the corresponding thioether-stabilized allylic copper reagents and the allylic mesylate 7. The thioether-stabilized allylic copper reagents were easily prepared and retained the configuration of their double bond during the coupling reactions, thus overcoming the problem of isomerization which was normally associated with the use of allylic organometallic reagents in such applications.

Comparative analysis of ceramide structural modification found in fungal cerebrosides by electrospray tandem mass spectrometry with low energy collision-induced dissociation of Li+ adduct ions

Fungal cerebrosides (monohexosylceramides, or CMHs) exhibit a number of ceramide structural modifications not found in mammalian glycosphingolipids, which present additional challenges for their complete characterization. The use of Li+ cationization, in conjunction with electrospray ionization mass spectrometry and low energy collision-induced dissociation tandem mass spectrometry (ESI-MS/CID-MS), was found to be particularly effective for detailed structural analysis of complex fungal CMHs, especially minor components present in mixtures at extremely low abundance. A substantial increase in both sensitivity and fragmentation was observed on collision-induced dissociation of [M + Li]+ versus [M + Na]+ of the same CMH components analyzed under similar conditions. The effects of particular modifications on fragmentation were first systematically evaluated by analysis of a wide variety of standard CMHs expressing progressively more functionalized ceramides. These included bovine brain galactocerebrosides with non-hydroxy and 2-hydroxy fatty N-acylation; a plant glucocerebroside having (E/Z)-delta8 in addition to (E)-delta4 unsaturation of the sphingoid base; and a pair of fungal cerebrosides known to be further modified by a branching 9-methyl group on the sphingoid moiety, and to have a 2-hydroxy fatty N-acyl moiety either fully saturated or (E)-delta3 unsaturated. The method was then applied to characterization of both major and minor components in CMH fractions from a non-pathogenic mycelial fungus, Aspergillus niger; and from pathogenic strains of Candida albicans (yeast form); three Cryptococcus spp. (all yeast forms); and Paracoccidioides brasiliensis (both yeast and mycelium forms). The major components of all species examined differed primarily (and widely) in the level of 2-hydroxy fatty N-acyl delta3 unsaturation, but among the minor components a significant degree of additional structural diversity was observed, based on differences in sphingoid or N-acyl chain length, as well as on the presence or absence of the sphingoid delta8 unsaturation or 9-methyl group. Some variants were isobaric, and were not uniformly present in all species, affirming the need for MS/CID-MS analysis for full characterization of all components in a fungal CMH fraction. The diversity in ceramide distribution observed may reflect significant species-specific differences among fungi with respect to cerebroside biosynthesis and function.

Structural analyses of a precursory substance of bitterness: new polyisoprenepolyols isolated from an edible mushroom (Hypsizigusmarmoreus) by fast atom bombardment mass spectrometry

New polyisoprenepolyols (hypsiziprenol AA and BA) were isolated from an edible mushroom (Hypsizigus marmoreus). These polyols occur as a mixture of homologous polyisoprene derivatives with 40-70 carbon atoms. Analyses by FAB/MS in the positive and negative ion modes are complementary with each other in that the former provides information on the number of hydroxy groups present while the latter specifies the isoprenoid sequence, and thus become a powerful tool for analyzing the structures of polyisoprenepolyols. No polyisoprenepolyols obtained here were found to have antitumor activity on NCI-H292 and EL-4 cell lines.

Structural characterization of neutral glycosphingolipids from Fusarium species

Glycosphingolipids were extracted from hyphae of Fusarium solani and from an unnamed Fusarium species, and were purified by silica and Iatrobead column chromatography. Their structures were determined by compositional analysis, nuclear magnetic resonance spectroscopy, gas chromatography/mass spectrometry and by fast atom bombardment mass spectrometry of the native and peracetylated materials, which defined their sugar, long-chain base and fatty acid compositions. The locations of the double bonds in the bases were established by 2D NMR spectroscopy and by novel mass spectrometric approaches, including collisional activation of the protonated and lithium-cationized glycosphingolipids, and of the sphingadienene-derived fragment ion at m/z 276. From these results we propose that the structures of the glycosphingolipids from F. solani and Fusarium sp. are N-2'-hydroxyoctadecanoyl-1-O-beta-D-glucopyranosyl-9-methyl-4, 8-sphingadienine and N-2'-hydroxyoctadecenoyl-1-O-beta-D-glucopyranosyl-9-methyl-4, 8-sphingadienine, respectively.