Current relevance of fungal and trypanosomatid glycolipids and sphingolipids: studies defining structures conspicuously absent in mammals

Recently, glycosphingolipids have been attracting attention due to their role on biological systems as second messengers or modulators of signal transduction, affecting several events, which range from apoptosis to regulation of the cell cycle. In pathogenic fungi, glycolipids are expressed in two classes: neutral monohexosylceramides (glucosyl-or galactosylceramide) and acidic glycosylinositol phosphorylceramides (the latter class carries longer glycan chains). It is worth to mention that monohexosylceramides exhibit significant structural differences in their lipid moieties compared to their mammalian counterparts, whereas the glycosylinositol phosphorylceramides exhibit remarkable structural differences in their carbohydrate moieties in comparison to mammal glycosphingolipids counterpart. We observed that glycosylinositol phosphorylceramides are capable of promoting immune response in infected humans. In addition, inhibiting fungal glycosphingolipid biosynthetic pathways leads to an inhibition of colony formation, spore germination, cell cycle, dimorphism and hyphal growth. Other pathogens, such as trypanosomatids, also present unique glycolipids, which may have an important role for the parasite development and/or disease establishment. Regarding host-pathogen interaction, cell membrane rafts, which are enriched in sphingolipids and sterols, participate in parasite/fungal infection. In this review, it is discussed the different biological roles of (glyco) (sphingo)lipids of pathogenic/opportunistic fungi and trypanosomatids.

Current research on the immune response to experimental sporotrichosis

Sporotrichosis is often manifested as a chronic granulomatous infection and the monocytes/macrophages play a central role in the host defense system. Surface components of Sporothrix schenckii have been characterized and suggestions have been made as to their possible role in pathogenicity. Ergosterol peroxide, cell-wall compounds (alkali-insoluble fraction-F1 and lipid extract-LEY), and exoantigen from the yeast form of the fungus have been characterized as virulence factors, activating both innate, by cytotoxins linked to the activation of reactive oxygen and nitrogen species (H2O2 and NO), and adaptive immune response to produce cytokines Th1 and Th2 profile. In this study, preliminary results have demonstrated that, in systemic sporotrichosis, TLR-4 triggers the innate immune response, activating an oxidative burst. These data represent the first report of the participation of TLR-4 in murine sporotrichosis, in the presence of lipids from the cell wall of S. schenckii. These results taken together may open new perspectives of study leading to an antifungal agent that could be used to benefit the entire population.

Yeast sphingolipids

Many advances in our understanding of fungal sphingolipids have been made in recent years. This review focuses on the types of sphingolipids that have been found in fungi and upon the genes in Saccharomyces cerevisiae, the common baker's yeast, that are necessary for sphingolipid metabolism. While only a small number of fungi have been examined, most contain sphingolipids composed of ceramide derivatized at carbon-1 with inositol phosphate. Further additions include mannose and then other carbohydrates. The second major class of fungal sphingolipids is the glycosylceramides, having either glucose or galactose attached to ceramide rather than inositol phosphate. The glycosylceramides sometimes contain additional carbohydrates. Knowledge of the genome sequence has expedited identification of S. cerevisiae genes necessary for sphingolipid metabolism. At least one gene is known for most steps in S. cerevisiae sphingolipid metabolism, but more are likely to be identified so that the 13 known genes are likely to grow in number. The AUR1 gene is necessary for addition of inositol phosphate to ceramide and has been identified as a target of several potent antifungal compounds. This essential step in yeast sphingolipid synthesis, which is not found in humans, appears to be an excellent target for the development of more effective antifungal compounds, both for human and for agricultural use.

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.

Structural determination of N-2'-hydroxyoctadecenoyl-1-O-beta-D-glucopyranosyl-9-methyl-4, 8-sphingadienine from species of Aspergillus

Ceramide monohexosides from Aspergillus fumigatus 2140 and 2109 strains and Aspergillus versicolor 550 strain, obtained by silica gel 60, and Iatrobeads chromatography were analysed using high-resolution 1D-, 2D-1H-NMR and 13C-NMR spectroscopy and fast atom bombardment mass spectrometry (FAB-MS). The ceramide monohexoside fraction (CMH) from A. fumigatus 2140 and A. versicolor 550 was identified as glucosylceramide, whereas glucose and galactose were present at a ratio of 1:1 in the CMH of A. fumigatus 2109. The major glycosphingolipid has a particular ceramide composition consisting of 9-methyl-4,8-sphingadienine linked to a 2-hydroxyoctadec-3-enoic acid. Although the structures presently described are similar to those of monohexosylceramides from other fungi, including edible ones, this is the first report on their occurrence in species pathogenic in humans.

Reactivity of chagasic sera with crude and highly purified glycosphingolipid fractions from Trypanosoma cruzi epimastigotes

The reactivities of sera from patients with Chagas disease or from T. cruzi-immunized rabbits with two different lipid preparations of T. cruzi were assessed using epimastigote antigens. Serum reactivities were determined using a quantitative enzyme-linked immunosorbent assay (ELISA). Antigen 1 represents the lower phase obtained from crude lipid extract after Folch partition (LCL). Antigen 2 is a highly purified glycosphingolipid fraction (GSL). The LCL antigen discriminated quite well the reactivities of Chagasic patients' sera and sera from healthy individuals, as well as between the serum from a T. cruzi-immunized rabbit (TIRS) and normal rabbit serum (NRS). A strong reactivity with GSL was obtained with TIRS. Reactivity with GSL was also obtained with human Chagasic sera. Compared to a group of normal individuals, the reactions of antibodies directed against lipid antigens were considerably increased in sera of patients with Chagas disease. Chagasic sera did not differentiate between glycolipids with terminal beta-glucosyl or beta-galactosyl non-reducing units. They discriminated, however, glucosylceramides with differences in the ceramide structure. To determine the specificity of Chagasic sera, antibodies isolated on LCL-immunosorbent (LCL-Ch Abs) as well as on laminin-immunosorbent (Lam-Ch Abs) were tested against laminin and LCL antigens. We found that Lam-Ch Abs reacted with murine laminin, whereas the reaction was negative with LCL. In contrast, the LCL-Ch Abs reacted either with LCL antigens or with laminin. The reactivity with laminin was strong in comparison with LCL.(ABSTRACT TRUNCATED AT 250 WORDS)