Analysis of glycosylphosphatidylinositol membrane anchors by electrospray ionization-mass spectrometry and collision induced dissociation

Overview of the role of kinetoplastid surface carbohydrates in infection and host cell invasion: prospects for therapeutic intervention

Kinetoplastid parasites are responsible for serious diseases in humans and livestock such as Chagas disease and sleeping sickness (caused by Trypanosoma cruzi and Trypanosoma brucei, respectively), and the different forms of cutaneous, mucocutaneous and visceral leishmaniasis (produced by Leishmania spp). The limited number of antiparasitic drugs available together with the emergence of resistance underscores the need for new therapeutic agents with novel mechanisms of action. The use of agents binding to surface glycans has been recently suggested as a new approach to antitrypanosomal design and a series of peptidic and non-peptidic carbohydrate-binding agents have been identified as antiparasitics showing efficacy in animal models of sleeping sickness. Here we provide an overview of the nature of surface glycans in three kinetoplastid parasites, T. cruzi, T. brucei and Leishmania. Their role in virulence and host cell invasion is highlighted with the aim of identifying specific glycan-lectin interactions and carbohydrate functions that may be the target of novel carbohydrate-binding agents with therapeutic applications.

Surface Glycans: A Therapeutic Opportunity for Kinetoplastid Diseases

Trypanosomal diseases are in need of innovative therapies that exploit novel mechanisms of action. The cell surface of trypanosomatid parasites is characterized by a dense coat of glycoconjugates with important functions in host cell recognition, immune evasion, infectivity, and cell function. The nature of parasite surface glycans is highly dynamic and changes during differentiation and in response to different stimuli through the action of glycosyltransferases and glycosidases. Here we propose a new approach to antiparasitic drug discovery that involves the use of carbohydrate-binding agents that bind specifically to cell-surface glycans, giving rise to cytotoxic events and parasite death. The potential and limitations of this strategy are addressed with a specific focus on the treatment of sleeping sickness.

Virus-like Particle Display of the α-Gal Epitope for the Diagnostic Assessment of Chagas Disease

The α-Gal antigen [Galα(1,3)Galβ(1,4)GlcNAcα] is an immunodominant epitope displayed by infective trypomastigote forms of Trypanosoma cruzi, the causative agent of Chagas disease. A virus-like particle displaying a high density of α-Gal was found to be a superior reagent for the ELISA-based serological diagnosis of Chagas disease and the assessment of treatment effectiveness. A panel of sera from patients chronically infected with T. cruzi, both untreated and benznidazole-treated, was compared with sera from patients with leishmaniasis and from healthy donors. The nanoparticle-α-Gal construct allowed for perfect discrimination between Chagas patients and the others, avoiding false negative and false positive results obtained with current state-of-the-art reagents. As previously reported with purified α-Gal-containing glycosylphosphatidylinositol-anchored mucins, the current study also showed concentrations of anti-α-Gal IgG to decrease substantially in patients receiving treatment with benznidazole, suggesting that the semiquantitative assessment of serum levels of this highly abundant type of antibody can report on disease status in individual patients.

GPIomics: global analysis of glycosylphosphatidylinositol-anchored molecules of Trypanosoma cruzi

Glycosylphosphatidylinositol (GPI) anchoring is a common, relevant posttranslational modification of eukaryotic surface proteins. Here, we developed a fast, simple, and highly sensitive (high attomole-low femtomole range) method that uses liquid chromatography-tandem mass spectrometry (LC-MSⁿ) for the first large-scale analysis of GPI-anchored molecules (i.e., the GPIome) of a eukaryote, Trypanosoma cruzi, the etiologic agent of Chagas disease. Our genome-wise prediction analysis revealed that approximately 12% of T. cruzi genes possibly encode GPI-anchored proteins. By analyzing the GPIome of T. cruzi insect-dwelling epimastigote stage using LC-MSⁿ, we identified 90 GPI species, of which 79 were novel. Moreover, we determined that mucins coded by the T. cruzi small mucin-like gene (TcSMUG S) family are the major GPI-anchored proteins expressed on the epimastigote cell surface. TcSMUG S mucin mature sequences are short (56–85 amino acids) and highly O-glycosylated, and contain few proteolytic sites, therefore, less likely susceptible to proteases of the midgut of the insect vector. We propose that our approach could be used for the high throughput GPIomic analysis of other lower and higher eukaryotes.

Glycoconjugate structures of parasitic protozoa

Glycoconjugates are abundant and ubiquitious on the surface of many protozoan parasites. Their tremendous diversity has implicated their critical importance in the life cycle of these organisms. This review highlights our current knowledge of the major glycoconjugates, with particular emphasis on their structures, of representative protozoan parasites, including Leishmania, Trypanosoma, Giardia, Plasmodia, and others.

Protein structure controls the processing of the N-linked oligosaccharides and glycosylphosphatidylinositol glycans of variant surface glycoproteins expressed in bloodstream form Trypanosoma brucei

The variant surface glycoproteins (VSGs) of Trypanosoma brucei are a family of homodimeric glycoproteins that adopt similar shapes. An individual trypanosome expresses one VSG at a time in the form of a dense protective mono-layer on the plasma membrane. VSG genes are expressed from one of several polycistronic transcription units (expression sites) that contain several expression site associated genes. We used a transformed trypanosome clone expressing two different VSGs (VSG121 and VSG221) from the same expression site (that of VSG221) to establish whether the genotype of the trypanosome clone or the VSG structure itself controls VSG N-linked oligosaccharide and GPI anchor glycan processing. In-gel release and fluorescent labeling of N-linked oligosaccharides and on-blot fluorescent labeling and release of GPI anchor glycans were employed to compare the carbohydrate structures of VSG121 and VSG221 when expressed individually in wild-type trypanosome clones and when expressed together in the transformed trypanosome clone. The data indicate that the genotype of the trypanosome clone has no effect on the N-linked oligosaccharide structures present on a given VSG variant and only a minor effect on the GPI anchor glycans. The latter is most likely an effect of changes in inter-VSG packing when two VGSs are expressed simultaneously. Thus, N-linked oligosaccharide and GPI anchor processing enzymes appear to be constitutively expressed in bloodstream form African trypanosomes and the tertiary and quaternary structures of the VSG homodimers appear to dictate the processing and glycoform microheterogeneity of surface-expressed VSGs.

Structural analysis of O-linked oligosaccharide-alditols by electrospray-tandem mass spectrometry after mild periodate oxidation and derivatization with 2-aminopyridine

O-linked oligosaccharide-alditols were analyzed by a combination of high-performance liquid chromatography (HPLC) and electrospray-tandem mass spectrometry (ESI-MS/MS). First, oligosaccharide-alditols were treated with sodium meta-periodate under conditions where core N-acetylgalactosaminitol is specifically degraded. The resulting fragments were labeled with 2-aminopyridine and purified on a reversed-phase column. Pyridylamino oligosaccharides yielded protonated molecular ions in positive-ion ES-MS and gave Y-series sequence ions, arising from glycosidic cleavages, by ESI-tandem mass spectrometry. Information on sugar sequence and branching of oligosaccharides linked at C6 and C3 to the N-acetylgalactosaminitol can be obtained. A systematic study of various oligosaccharide-alditols demonstrated that this approach constitutes a powerful tool for the structural characterization of O-glycans available only in limited quantities.

Structure of the glycosylphosphatidylinositol membrane anchor glycan of a class-2 variant surface glycoprotein from Trypanosoma brucei

The neutral glycan fraction of the glycosylphosphatidylinositol (GPI) membrane anchor of a class-2 variant surface glycoprotein (VSG) from Trypanosoma brucei was isolated following aqueous hydrogen fluoride dephosphorylation and nitrous acid deamination of the purified glycoprotein. The neutral glycans were fractionated by high-pH anion exchange chromatography and gel-filtration and six major glycan structures were solved by a combination of one and two-dimensional NMR, composition analysis, methylation linkage analysis and electrospray-mass spectrometry. The glycans were similar to those previously described for class-1 VSGs, in that they contained the linear trimannosyl sequence Manalpha1-2Manalpha1-6Man and a complex alpha-galactose branch of up to Galalpha1-2Galalpha1-6(Galalpha1-2)Gal, but most also contained an additional galactose residue attached alpha1-2 to the non-reducing terminal mannose residue and about one-third contained an additional galactose residue attached beta1-3 to the middle mannose residue. The additional complexity of the class-2 VSG GPI glycans is discussed in terms of a biosynthetic model that explains the full range of mature GPI structures that can be expressed on different VSG classes by the same trypanosome clone.

Analysis of the structural heterogeneity of laminarin by electrospray-ionisation-mass spectrometry

Electrospray-ionisation-mass spectrometry (ESIMS) was used in conjunction with chemical derivatisation and degradation procedures to analyse the size heterogeneity and branching structure of laminarin from the brown alga, Laminaria digitata. Laminarin is a beta-(1-->3)-linked D-glucan with occasional beta-(1-->6)-linked branches. Electrospray-ionisation-mass spectrometry of permethylated laminarin distinguished two homologous series of molecules, a minor G-series containing 22-28 glucosyl residues, and a more abundant M-series containing 20-30 glucosyl residues linked to a mannitol residue. The relative abundance of all these molecular species could be determined simultaneously from a single mass spectrum, with a mean mass error of 0.6 atomic mass units and a mean mass accuracy of 0.011%. Both series had a mean degree of polymerisation of 25 glucosyl residues, and an approximately 3:1 molar ratio of M-series to G-series molecules was maintained across the range of molecular sizes. Treatment of laminarin with periodate, followed by reduction with borohydride, degraded terminal glucosyl residues on both the main chain and the branches, and allowed the detection of isomers differing solely in their degree of branching. M-series molecules were thus shown to contain 0, 1, 2, 3 or 4 branches, with an average of 1.3 branches per molecule; branched G-series molecules were also detected. Subsequent treatment with acid (Smith degradation) showed that 75% of the branches were single glucosyl residues. This study thus shows how the speed, resolution and mass accuracy of electrospray-ionisation-mass spectrometry can be used in the detailed structural analysis of a polydisperse polysaccharide.