Mass spectrometry of carbohydrate-containing biopolymers

The antifungal drug itraconazole inhibits vascular endothelial growth factor receptor 2 (VEGFR2) glycosylation, trafficking, and signaling in endothelial cells

Itraconazole is a safe and widely used antifungal drug that was recently found to possess potent antiangiogenic activity. Currently, there are four active clinical trials evaluating itraconazole as a cancer therapeutic. Tumor growth is dependent on angiogenesis, which is driven by the secretion of growth factors from the tumor itself. We report here that itraconazole significantly inhibited the binding of vascular endothelial growth factor (VEGF) to VEGF receptor 2 (VEGFR2) and that both VEGFR2 and an immediate downstream substrate, phospholipase C γ1, failed to become activated after VEGF stimulation. These effects were due to a defect in VEGFR2 trafficking, leading to a decrease in cell surface expression, and were associated with the accumulation of immature N-glycans on VEGFR2. Small molecule inducers of lysosomal cholesterol accumulation and mammalian target of rapamycin (mTOR) inhibition, two previously reported itraconazole activities, failed to recapitulate itraconazole's effects on VEGFR2 glycosylation and signaling. Likewise, glycosylation inhibitors did not alter cholesterol trafficking or inhibit mTOR. Repletion of cellular cholesterol levels, which was known to rescue the effects of itraconazole on mTOR and cholesterol trafficking, was also able to restore VEGFR2 glycosylation and signaling. This suggests that the new effects of itraconazole occur in parallel to those previously reported but are downstream of a common target. We also demonstrated that itraconazole globally reduced poly-N-acetyllactosamine and tetra-antennary complex N-glycans in endothelial cells and induced hypoglycosylation of the epidermal growth factor receptor in a renal cell carcinoma line, suggesting that itraconazole's effects extend beyond VEGFR2.

Structural differences in lipomannans from pathogenic and nonpathogenic mycobacteria that impact CD1b-restricted T cell responses

Mannosylated molecules on the Mycobacterium tuberculosis surface are important determinants in the immunopathogenesis of tuberculosis. To date, much attention has been paid to mannose-capped lipoarabinomannan, which mediates phagocytosis and intracellular trafficking of M. tuberculosis by engaging the macrophage mannose receptor and subsequently binds to intracellular CD1b molecules for presentation to T cells. Another important mannosylated lipoglycan on the M. tuberculosis surface is lipomannan (LM). Comparative structural detail of the LMs from virulent and avirulent strains is limited as is knowledge regarding their differential capacity to be recognized by the adaptive immune response. Here, we purified LM from the avirulent M. smegmatis and the virulent M. tuberculosis H(37)R(v), performed a comparative structural biochemical analysis, and addressed their ability to stimulate CD1b-restricted T cell clones. We found that M. tuberculosis H(37)R(v) produces a large neutral LM (TB-LM); in contrast, M. smegmatis produces a smaller linear acidic LM (SmegLM) with a high succinate content. Correspondingly, TB-LM was not as efficiently presented to CD1b-restricted T cells as SmegLM. Thus, here we correlate the structure-function relationships for LMs with CD1b-restricted T cell responses and provide evidence that the structural features of TB-LM contribute to its diminished T cell responsiveness.

Comparison of the baculovirus-insect cell and Pichia pastoris heterologous systems for the expression of the human tumor suppressor protein RNASET2

We report the expression of recombinant RNASET2, the only human member of the Rh/T2/S family of acid ribonucleases, in the yeast Pichia pastoris and the baculovirus-insect cell heterologous systems. In both models, the yield of recombinant protein was comparable and ranged between 5 mg/L (for a catalytically impaired mutant version of RNASET2) and 30 mg/L for the wild-type protein. Thus, the produced protein version rather than the expression system used appears to influence protein yield after optimization of culture conditions. The recombinant protein was found to undergo heterogeneous glycosylation in both systems, particularly in P. pastoris. Most importantly, the wild-type protein purified from both systems was found to be catalytically competent. The expression of recombinant RNASET2 in both systems will allow the implementation of functional assays in vivo and in vitro to better define the antioncogenic properties of this member of the Rh/T2/S ribonuclease family.

Glycomic analysis of human mast cells, eosinophils and basophils

In allergic diseases such as asthma, eosinophils, basophils and mast cells, through release of preformed and newly generated mediators, granule proteins and cytokines, are recognized as key effector cells. While their surface protein phenotypes, mediator release profiles, ontogeny, cell trafficking and genomes have been generally explored and compared, there has yet to be any thorough analysis and comparison of their glycomes. Such studies are critical to understand the contribution of carbohydrates to the induction and regulation of allergic inflammatory responses and are now possible using improved technologies for detecting and characterizing cell-derived glycans. We thus report here the application of high-sensitivity mass spectrometric-based glycomics methodologies to the analysis of N-linked glycans derived from isolated populations of human mast cells, eosinophils and basophils. The samples were subjected to matrix-assisted laser desorption ionization (MALDI) time-of-flight (TOF) screening analyses and MALDI-TOF/TOF sequencing studies. Results reveal substantive quantities of terminal N-acetylglucosamine containing structures in both the eosinophil and the basophil samples, whereas mast cells display greater relative quantities of sialylated terminal epitopes. For the first time, we characterize the cell surface glycan structures of principal allergic effector cells, which by interaction with glycan-binding proteins (e.g. lectins) have the possibility to dictate cellular functions, and might thus have important implications for the pathogenesis of inflammatory and allergic diseases.

MS-based glycomic strategies for probing the structural details of polylactosaminoglycan chain on N-glycans and glycoproteomic identification of its protein carriers

Most MS-based glycomic and glycoproteomic analyses focus on identifying changes in terminal glyco-epitopes represented by sialylation and fucosylation at specific positions of the terminal N-acetyllactosamine units. Much less attention was accorded to the underlying linear or branched poly-N-acetyllactosamine extension from the N-glycan trimannosyl core other than a simple inference of its presence due to mass data and hence glycosyl compositional assignment. Using the EA.hy926 cell line derived from human umbilical vein endothelial cells (HUVEC), we have systematically investigated the MALDI- and ESI-MS-based methodologies for probing the structural details of endothelial polylactosaminoglycans at both MS and MS(2) levels in conjunction with the use of endo-β-galactosidase to identify branching motifs and initiation sites. We showed that the polylactosaminoglycan chains on the N-glycans of EA.hy926 were less sialylated and fucosylated but more extended and branched than those of human umbilical vein endothelial cells, thus demonstrating a fundamental glycomic difference. For EA.hy926 that was investigated in more details, its polylactosaminoglycan chains were shown to be not restricted to extending from a specific antenna including the biologically important 6-arm position. Finally, experimental conditions for glycopeptide enrichment by tomato lectin were further optimized, which led to identification of over 40 candidate endothelial membrane protein carriers of polylactosaminoglycans by proteomic analysis.

Aberrant protein N-glycosylation impacts upon infection-related growth transitions of the haploid plant-pathogenic fungus Mycosphaerella graminicola

The ascomycete fungus Mycosphaerella graminicola is the causal agent of Septoria Tritici Blotch disease of wheat and can grow as yeast-like cells or as hyphae depending on environmental conditions. Hyphal growth is however essential for successful leaf infection. A T-DNA mutagenesis screen performed on haploid spores identified a mutant, which can undergo yeast-like growth but cannot switch to hyphal growth. For this reason the mutant was non-pathogenic towards wheat leaves. The gene affected, MgAlg2, encoded a homologue of Saccharomyces cerevisiae ScAlg2, an alpha-1,2-mannosyltransferase, which functions in the early stages of asparagine-linked protein (N-) glycosylation. Targeted gene deletion and complementation experiments confirmed that loss of MgAlg2 function prevented the developmental growth switch. MgAlg2 was able to functionally complement the S. cerevisiae ScAlg2-1 temperature sensitive growth phenotype. Spores of ΔMgAlg2 mutants were hypersensitive to the cell wall disrupting agent Calcofluor white and produced abnormally hypo-N-glycosylated proteins. Gene expression, proteome and glycoproteome analysis revealed that ΔMgAlg2 mutant spores show responses typically associated with the accumulation of mis-folded proteins. The data presented highlight key roles for protein N-glycosylation in regulating the switch to hyphal growth, possibly as a consequence of maintaining correct folding and localization of key proteins involved in this process.

High-performance liquid chromatography-mass spectrometry for mapping and sequencing glycosaminoglycan-derived oligosaccharides

Glycosaminoglycans (GAGs) have proven to be very difficult to analyze and characterize because of their high negative charge density, polydispersity and sequence heterogeneity. As the specificity of the interactions between GAGs and proteins results from the structure of these polysaccharides, an understanding of GAG structure is essential for developing a structure-activity relationship. Electrospray ionization (ESI) mass spectrometry (MS) is particularly promising for the analysis of oligosaccharides chemically or enzymatically generated by GAGs because of its relatively soft ionization capacity. Furthermore, on-line high-performance liquid chromatography (HPLC)-MS greatly enhances the characterization of complex mixtures of GAG-derived oligosaccharides, providing important structural information and affording their disaccharide composition. A detailed protocol for producing oligosaccharides from various GAGs, using controlled, specific enzymatic or chemical depolymerization, is presented, together with their HPLC separation, using volatile reversed-phase ion-pairing reagents and on-line ESI-MS structural identification. This analysis provides an oligosaccharide map together with sequence information from a reading frame beginning at the nonreducing end of the GAG chains. The preparation of oligosaccharides can be carried out in 10 h, with subsequent HPLC analysis in 1-2 h and HPLC-MS analysis taking another 2 h.

High-sensitivity O-glycomic analysis of mice deficient in core 2 {beta}1,6-N-acetylglucosaminyltransferases

Core 2 β1,6-N-acetylglucosaminyltransferase (C2GnT), which exists in three isoforms, C2GnT1, C2GnT2 and C2GnT3, is one of the key enzymes in the O-glycan biosynthetic pathway. These isoenzymes produce core 2 O-glycans and have been correlated with the biosynthesis of core 4 O-glycans and I-branches. Previously, we have reported mice with single and multiple deficiencies of C2GnT isoenzyme(s) and have evaluated the biological and structural consequences of the loss of core 2 function. We now present more comprehensive O-glycomic analyses of neutral and sialylated glycans expressed in the colon, small intestine, stomach, kidney, thyroid/trachea and thymus of wild-type, C2GnT2 and C2GnT3 single knockouts and the C2GnT1-3 triple knockout mice. Very high-quality data have emerged from our mass spectrometry techniques with the capability of detecting O-glycans up to at least 3500 Da. We were able to unambiguously elucidate the types of O-glycan core, branching location and residue linkages, which allowed us to exhaustively characterize structural changes in the knockout tissues. The C2GnT2 knockout mice suffered a major loss of core 2 O-glycans as well as glycans with I-branches on core 1 antennae especially in the stomach and the colon. In contrast, core 2 O-glycans still dominated the O-glycomic profile of most tissues in the C2GnT3 knockout mice. Analysis of the C2GnT triple knockout mice revealed a complete loss of both core 2 O-glycans and branched core 1 antennae, confirming that the three known isoenzymes are entirely responsible for producing these structures. Unexpectedly, O-linked mannosyl glycans are upregulated in the triple deficient stomach. In addition, our studies have revealed an interesting terminal structure detected on O-glycans of the colon tissues that is similar to the RM2 antigen from glycolipids.

Glycoproteomic analysis of embryonic stem cells: identification of potential glycobiomarkers using lectin affinity chromatography of glycopeptides

Numerous studies have recently focused on the identification of specific glycan biomarkers, given the important roles that protein linked glycans play, for example, during development and disease progression. The identification of protein glycobiomarkers, which are part of a very complex proteome, has involved the use of fractionation techniques such as lectin affinity chromatography. In this study, the glycoproteomic characterization of pluripotent murine embryonic stem cells (ES) and from ES cells that were differentiated into embroid bodies (EB) was performed using immobilized Concanavalin A (ConA). This procedure allowed the isolation of glycopeptides that express biantennary and hybrid N-linked structures (ConA2 fraction) as well as high mannose glycans (ConA3 fraction) that were abundant in both ES and EB stages. A total of 293 unique N-linked glycopeptide sequences (from 180 glycoproteins) were identified in the combined data sets from ES and EB cells. Of these glycopeptides, a total of 119 sequences were identified exclusively in only one of the lectin-bound fractions (24 in the ES-ConA2, 15 in the ES-ConA3, 16 in the EB-ConA2, and 64 in the EB-ConA3). Results from this study allowed the identification of individual N-glycosylation sites of proteins that express specific glycan types. The absence of some of these lectin-bound glycopeptides in a cell stage suggested that they were derived from proteins that were either expressed exclusively on a defined developmental stage or were expressed in both cell stages but carried the lectin-bound oligosaccharides in only one of them. Therefore, these lectin-bound glycopeptides can be considered as stage-specific glycobiomarkers.

Serum N-glycome biomarker for monitoring development of DENA-induced hepatocellular carcinoma in rat

BACKGROUND

There is a demand for serum markers for the routine assessment of the progression of liver cancer. We previously found that serum N-linked sugar chains are altered in hepatocellular carcinoma (HCC). Here, we studied glycomic alterations during development of HCC in a rat model.

RESULTS

Rat HCC was induced by the hepatocarcinogen, diethylnitrosamine (DENA). N-glycans were profiled using the DSA-FACE technique developed in our laboratory.In comparison with control rats, DENA rats showed a gradual but significant increase in two glycans (R5a and R5b) in serum total N-glycans during progression of liver cirrhosis and cancer, and a decrease in a biantennary glycan (P5). The log of the ratio of R5a to P1 (NGA2F) and R5b to P1 [log(R5a/P1) and log(R5b/P1)] were significantly (p < 0.0001) elevated in HCC rats, but not in rats with cirrhosis or fibrosis or in control rats. We thus propose a GlycoTest model using the above-mentioned serum glycan markers to monitor the progression of cirrhosis and HCC in the DENA-treated rat model. When DENA-treated rats were subsequently treated with farnesylthiosalicyclic acid, an anticancer drug, progression to HCC was prevented and GlycoTest markers (P5, R5a and R5b) reverted towards non-DENA levels, and the HCC-specific markers, log(R5a/P1) and log(R5b/P1), normalized completely.

CONCLUSIONS

We found an increase in core-alpha-1,6-fucosylated glycoproteins in serum and liver of rats with HCC, which demonstrates that fucosylation is altered during progression of HCC. Our GlycoTest model can be used to monitor progression of HCC and to follow up treatment of liver tumors in the DENA rat. This GlycoTest model is particularly important because a rapid non-invasive diagnostic procedure for tumour progression in this rat model would greatly facilitate the search for anticancer drugs.

N-Glycans on the link domain of human HARE/Stabilin-2 are needed for hyaluronan binding to purified ecto-domain, but not for cellular endocytosis of hyaluronan

The hyaluronic acid receptor for endocytosis (HARE)/Stabilin-2 is the primary systemic scavenger receptor for 13 ligands including hyaluronan (HA), heparin and chondroitin sulfates. Most ligand-binding sites are within the 190 kDa isoform, which contains approximately 25 kDa of N-glycans and is the C-terminal half of the full-length 315 kDa HARE. Glycoproteomic analyses of purified recombinant human 190-HARE ecto-domain identified a diverse population of glycans at 10 of 17 consensus sites. The most diversity (and the only sialylated structures) occurred at N(2280), within the HA-binding Link domain. To determine if these N-glycans are required for HA binding, we created human Flp-In 293 cell lines expressing membrane-bound or soluble ecto-domain variants of 190-HARE(N2280A). Membrane-bound HARE lacking Link domain N-glycans mediated rapid HA endocytosis, but purified 190-HARE(N2280A) ecto-domain showed little or no HA binding in ELISA-like, HA-HARE pull-down assays or by surface plasmon resonance analysis (which detected very high apparent affinity for 190-HARE ecto-domain binding to HA; K(d) = 5.2 nM). The results indicate that Link domain N-glycans stabilize interactions that facilitate HA binding to HARE.

Passive transfer of IgG anti-GM1 antibodies impairs peripheral nerve repair

Anti-GM1 antibodies are present in some patients with autoimmune neurological disorders. These antibodies are most frequently associated with acute immune neuropathy called Guillain-Barré syndrome (GBS). Some clinical studies associate the presence of these antibodies with poor recovery in GBS. The patients with incomplete recovery have failure of nerve repair, particularly axon regeneration. Our previous work indicates that monoclonal antibodies can inhibit axon regeneration by engaging cell surface gangliosides (Lehmann et al., 2007). We asked whether passive transfer of human anti-GM1 antibodies from patients with GBS modulate axon regeneration in an animal model. Human anti-GM1 antibodies were compared with other GM1 ligands, cholera toxin B subunit and a monoclonal anti-GM1 antibody. Our results show that patient derived anti-GM1 antibodies and cholera toxin beta subunit impair axon regeneration/repair after PNS injury in mice. Comparative studies indicated that the antibody/ligand-mediated inhibition of axon regeneration is dependent on antibody/ligand characteristics such as affinity-avidity and fine specificity. These data indicate that circulating immune effectors such as human autoantibodies, which are exogenous to the nervous system, can modulate axon regeneration/nerve repair in autoimmune neurological disorders such as GBS.

Structural characterisation of neutrophil glycans by ultra sensitive mass spectrometric glycomics methodology

Neutrophils are the most abundant white blood cells in humans and play a vital role in several aspects of the immune response. Numerous reports have implicated neutrophil glycosylation as an important factor in mediating these interactions. We report here the application of high sensitivity glycomics methodologies, including matrix assisted laser desorption ionisation (MALDI-TOF) and MALDI-TOF/TOF analyses, to the structural analysis of N- and O-linked carbohydrates released from two samples of neutrophils, prepared by two separate and geographically remote laboratories. The data produced demonstrates that the cells display a diverse range of sialylated and fucosylated complex glycans, with a high level of similarity between the two preparations.

Mass spectrometric analysis of mutant mice

Mass spectrometry (MS) has proven to be the preeminent tool for the rapid, high-sensitivity analysis of the primary structure of glycans derived from diverse biological sources including cells, fluids, secretions, tissues, and organs. These analyses are anchored by matrix-assisted laser desorption ionization time of flight (MALDI-TOF) analysis of permethylated derivatives of glycan pools released from the samples, to produce glycomic mass fingerprints. The application of complimentary techniques, such as chemical and enzymatic digestions, GC-MS linkage analysis, and tandem mass spectrometry (MS/MS) utilizing both electrospray (ES) and MALDI-TOF/TOF, together with bioinformatic tools allows the elucidation of incrementally more detailed structural information from the sample(s) of interest. The mouse as a model organism offers many advantages in the study of human biology, health, and disease; it is a mammal, shares 99% genetic homology with humans and its genome supports targeted mutagenesis in specific genes to produce knockouts efficiently and precisely. Glycomic analyses of tissues and organs from mice genetically deficient in one or more glycosylation gene and comparison with data collected from wild-type samples enables the facile identification of changes and perturbations within the glycome. The Consortium for Functional Glycomics (CFG) has been applying such MS-based glycomic analyses to a range of murine tissues from both wild-type and glycosylation-knockout mice in order to provide a repository of structural data for the glycobiology community. In this chapter, we describe in detail the methodologies used to prepare, derivatize, purify, and analyze glycan pools from mouse organs and tissues by MS. We also present a summary of data produced from the CFG systematic structural analysis of wild-type and knockout mouse tissues, together with a detailed example of a glycomic analysis of the Mgat4a knockout mouse.

Mass spectrometric analysis of sulfated N- and O-glycans

Sulfated N- and O-glycans carried on a myriad of cell-surface adhesion molecules and receptors are often not detected by current approaches in mass spectrometry (MS)-based glycomic mapping of cells and tissues. This is in part due to a natural lower abundance, compounded further by their negatively charged nature, which adversely disfavors their ionization and detection amid a sea of often much more abundant, nonsulfated, sialylated glycans. However, this particular limitation can actually be taken advantage of to effect highly selective enrichment and sensitive MS screening in negative ion mode, provided the ubiquitous sialic acids can first be neutralized. It has been demonstrated that permethylation not only fulfills this role adequately but further confers better MS/MS fragmentation characteristics for more efficient structural mapping and sequencing. Protocols and general practical considerations are described here which would enable one to readily prepare permethylated sulfated glycans, fractionate them away from the more abundant nonsulfated ones in simple steps for high-sensitivity MS analysis, and sensibly interpret the initial sulfoglycomic screening data thus obtained.

N-Glycosylation profiling of turtle egg yolk: expression of galabiose structure

To understand the roles of species-specific carbohydrates, systematic studies of interspecific glycan analyses are imperative. An extensive series of glycomics studies on approximately 180 kinds of bird eggs have demonstrated that 60-70% of the birds, which are closely related in phylogeny, express the alpha-Galp-(1-->4)-Galp structure on their egg glycoproteins. This prompted us to investigate the glycosylation profiles of eggs from an evolutionarily related organism, a sea turtle (reptilian). We performed N-glycosylation profiling of turtle egg yolk by using HPLC mapping in conjunction with mass spectrometric methods and thereby demonstrated that the alpha-Galp-(1-->4)-Galp groups are displayed on approximately 38% of total N-glycans. Our findings suggest that the ability to express the galabiose structure was acquired at an early stage of diversification in amniotes.

Glycomics profiling of Chinese hamster ovary cell glycosylation mutants reveals N-glycans of a novel size and complexity

Identifying biological roles for mammalian glycans and the pathways by which they are synthesized has been greatly facilitated by investigations of glycosylation mutants of cultured cell lines and model organisms. Chinese hamster ovary (CHO) glycosylation mutants isolated on the basis of their lectin resistance have been particularly useful for glycosylation engineering of recombinant glycoproteins. To further enhance the application of these mutants, and to obtain insights into the effects of altering one specific glycosyltransferase or glycosylation activity on the overall expression of cellular glycans, an analysis of the N-glycans and major O-glycans of a panel of CHO mutants was performed using glycomic analyses anchored by matrix-assisted laser desorption ionization-time of flight/time of flight mass spectrometry. We report here the complement of the major N-glycans and O-glycans present in nine distinct CHO glycosylation mutants. Parent CHO cells grown in monolayer versus suspension culture had similar profiles of N- and O-GalNAc glycans, although the profiles of glycosylation mutants Lec1, Lec2, Lec3.2.8.1, Lec4, LEC10, LEC11, LEC12, Lec13, and LEC30 were consistent with available genetic and biochemical data. However, the complexity of the range of N-glycans observed was unexpected. Several of the complex N-glycan profiles contained structures of m/z ∼13,000 representing complex N-glycans with a total of 26 N-acetyllactosamine (Galβ1–4GlcNAc)_n units. Importantly, the LEC11, LEC12, and LEC30 CHO mutants exhibited unique complements of fucosylated complex N-glycans terminating in Lewis^x and sialyl-Lewis^x determinants. This analysis reveals the larger-than-expected complexity of N-glycans in CHO cell mutants that may be used in a broad variety of functional glycomics studies and for making recombinant glycoproteins.

Enabling techniques and strategic workflow for sulfoglycomics based on mass spectrometry mapping and sequencing of permethylated sulfated glycans

Sulfate modifications on terminal epitopes of N- and O-glycans have increasingly been implicated as critical determinants mediating a diverse range of biological recognition functions. To address these low abundance but important sulfated glycans, and the sulfoglycome in general, further development of enrichment strategies and enabling mass spectrometry (MS)-based mapping techniques are needed. In this report, we demonstrate that the sulfated glycans, with and without additional sialylation, can be successfully permethylated by the sodium hydroxide slurry method and be distinguished from phosphorylated glycans by virtue of this derivatization. In conjunction with simple microscale postderivatization fractionation steps, permethyl derivatives fully retaining the negatively charged sulfate moiety and separated from the nonsulfated ones, can be efficiently detected and sequenced de novo by advanced MS/MS in the positive-ion mode. In particular, we show that the highly sequence and linkage informative high energy collision induced dissociation (CID) MS/MS afforded by MALDI-TOF/TOF can be extended to sulfoglycomic applications. The sulfated parent ion selected for CID MS/MS was found to mostly retain the sulfate moiety and therefore allow efficient fragmentation via the usual array of glycosidic, cross ring, and concerted double cleavages. Collectively, the optimized strategy enables a high sensitivity detection and critical mapping of the sulfoglycome such as the one derived from lymph node tissues or cell lines in both negative and positive-ion modes. Novel sulfated epitopes were identified from a crude mouse lymph node preparation, which fully attested to the practical utility of the methodology developed.

Targeted glycoproteomic identification of cancer cell glycosylation

GalMBP is a fragment of serum mannose-binding protein that has been modified to create a probe for galactose-containing ligands. Glycan array screening demonstrated that the carbohydrate-recognition domain of GalMBP selectively binds common groups of tumor-associated glycans, including Lewis-type structures and T antigen, suggesting that engineered glycan-binding proteins such as GalMBP represent novel tools for the characterization of glycoproteins bearing tumor-associated glycans. Blotting of cell extracts and membranes from MCF7 breast cancer cells with radiolabeled GalMBP was used to demonstrate that it binds to a selected set of high molecular weight glycoproteins that could be purified from MCF7 cells on an affinity column constructed with GalMBP. Proteomic and glycomic analysis of these glycoproteins by mass spectrometry showed that they are forms of CD98hc that bear glycans displaying heavily fucosylated termini, including Lewis(x) and Lewis(y) structures. The pool of ligands was found to include the target ligands for anti-CD15 antibodies, which are commonly used to detect Lewis(x) antigen on tumors, and for the endothelial scavenger receptor C-type lectin, which may be involved in tumor metastasis through interactions with this antigen. A survey of additional breast cancer cell lines reveals that there is wide variation in the types of glycosylation that lead to binding of GalMBP. Higher levels of binding are associated either with the presence of outer-arm fucosylated structures carried on a variety of different cell surface glycoproteins or with the presence of high levels of the mucin MUC1 bearing T antigen.

An Aeromonas caviae genomic island is required for both O-antigen lipopolysaccharide biosynthesis and flagellin glycosylation

Aeromonas caviae Sch3N possesses a small genomic island that is involved in both flagellin glycosylation and lipopolysaccharide (LPS) O-antigen biosynthesis. This island appears to have been laterally acquired as it is flanked by insertion element-like sequences and has a much lower G+C content than the average aeromonad G+C content. Most of the gene products encoded by the island are orthologues of proteins that have been shown to be involved in pseudaminic acid biosynthesis and flagellin glycosylation in both Campylobacter jejuni and Helicobacter pylori. Two of the genes, lst and lsg, are LPS specific as mutation of them results in the loss of only a band for the LPS O-antigen. Lsg encodes a putative Wzx flippase, and mutation of Lsg affects only LPS; this finding supports the notion that flagellin glycosylation occurs within the cell before the flagellins are exported and assembled and not at the surface once the sugar has been exported. The proteins encoded by flmA, flmB, neuA, flmD, and neuB are thought to make up a pseudaminic acid biosynthetic pathway, and mutation of any of these genes resulted in the loss of motility, flagellar expression, and a band for the LPS O-antigen. Furthermore, pseudaminic acid was shown to be present on both flagellin subunits that make up the polar flagellum filament, to be present in the LPS O-antigen of the A. caviae wild-type strain, and to be absent from the A. caviae flmD mutant strain.

Enhanced expression of beta 3-galactosyltransferase 5 activity is sufficient to induce in vivo synthesis of extended type 1 chains on lactosylceramides of selected human colonic carcinoma cell lines

In general, an elevated expression of beta 3-galactosyltransferase (beta 3GalT) activity contributed by beta 3GalT5 correlates well with increased biosynthesis and expression of type 1 chain (Gal beta 1-3GlcNAc beta 1-) derivatives such as Lewis A and sialyl Lewis A, which are mostly recognized as terminal epitopes and not further extended. Most known beta 3-N-acetylglucosaminyltransferases show a higher activity toward extending type 2 chain (Gal beta 1-4GlcNAc beta 1-), and an over-expression of beta 3GalT5 could suppress the formation of the type 2 chain poly-N-acetyllactosaminoglycans. The potential of extending instead the predominant type 1 chain termini synthesized under such circumstances was, however, not investigated, partly due to technical difficulty in unambiguous identification of extended type 1 chains. Using an advanced mass spectrometry-based glycomic mapping and glycan sequencing approach, we show here that type 1 chains carried on the lacto-series glycosphingolipids of colonic carcinoma cells can be extended when the endogenous beta 3GalT activity relative to competing beta 4GalT activity, as defined against a common GlcNAc beta 1-3Gal beta 1-4Glc acceptor, is sufficiently high, as found in Colo205 and SW1116, but not in DLD-1 cells. In support of this positive correlation, the lacto-series glycosphingolipids isolated from stably transfected DLD-1 clones over-expressing beta 3GalT5 were shown to comprise fucosylated dimeric type 1 chains, whereas a mock transfectant and the DLD-1 parent carried only fucosylated dimeric type 2 chains on their lactosylceramides. It suggests that while the natural expression of extended type 1 chain is likely to be determined by many contributing factors including the relative amounts of competing glycosyltransferases and the UDP-Gal level, the enhanced expression of beta 3GalT5 is sufficient to promote in vivo extension of type 1 chains by furnishing a significantly higher amount of type 1 chain precursors relative to competing type 2 chains.

Highly fucosylated N-glycan ligands for mannan-binding protein expressed specifically on CD26 (DPPVI) isolated from a human colorectal carcinoma cell line, SW1116

The serum mannan-binding protein (MBP) is a host defense C-type lectin specific for mannose, N-acetylglucosamine, and fucose residues, and exhibits growth inhibitory activity toward human colorectal carcinoma cells. The MBP-ligand oligosaccharides (MLO) isolated from a human colorectal carcinoma cell line, SW1116, are large, multiantennary N-glycans with highly fucosylated polylactosamine-type structures having Le(b)-Le(a) or tandem repeats of the Le(a) structure at their nonreducing ends. In this study, we isolated the major MBP-ligand glycoproteins from SW1116 cell lysates with an MBP column and identified them as CD26/dipeptidyl peptidase IV (DPPIV) (110 kDa) and CD98 heavy chain (CD98hc)/4F2hc (82 kDa). Glycosidase digestion revealed that CD26 contained such complex-type N-glycans that appear to mediate the MBP binding. MALDI-MS of the N-glycans released from CD26 by PNGase F demonstrated conclusively that CD26 is the major MLO-carrying protein. More interestingly, a comparison of the N-glycans released from the MBP-binding and non-MBP-binding glycopeptides suggested that complex-type N-glycans carrying a minimum of 4 Le(a)/Le(b) epitopes arranged either as multimeric tandem repeats or terminal epitopes on multiantennary structures are critically important for the high affinity binding to MBP. Analysis of the N-glycan attachment sites demonstrated that the high affinity MLO was expressed preferentially at some N-glycosylation sites, but this site preference was not so stringent. Finally, hypothetical 3D models of tandem repeats of the Le(a) epitope and the MBP-Lewis oligosaccharide complex were presented.

Lipoarabinomannan of Mycobacterium: mannose capping by a multifunctional terminal mannosyltransferase

Biosynthesis of phosphatidylinositol (PI)-containing lipoarabinomannan (LAM) and lipomannan (LM) of Mycobacterium spp. follows a conserved pathway involving multiple membrane-associated, substrate-specific mannosyltransferases (ManTs) responsible for the sequential addition of alpha-mannopyranosyl (Manp) units donated by decaprenyl-P-Manp on the periplasmic side of the plasma membrane. Because of their receptor-binding and immunomodulatory properties, the alpha(1-->2)-linked di- and tri-Manp motifs that functionalize the nonreducing arabinan termini of LAM (ManLAM) in Mycobacterium tuberculosis are of crucial importance. We now show that the M. tuberculosis ManT, Rv2181, is required for the addition of these alpha(1-->2)-linked Manp residues but also at other locations of the LAM molecule. Structural analyses of the LM and LAM variants produced by a M. tuberculosis Rv2181 knockout mutant revealed the presence of but a single Manp residue on the nonreducing arabinan termini of LAM and also a complete absence of alpha(1-->2)-linked Man branching on the mannan backbones of LM and LAM. A recombinant strain was constructed in ManLAM-deficient Mycobacterium smegmatis that coexpressed Rv2181 and Rv1635c-the ManT responsible for the addition of the first Manp capping residue of ManLAM. Analysis revealed LAM termini fully capped with di- and tri-Manp motifs in addition to alpha(1-->2)Man branching on the mannan backbones of LM and LAM, confirming the involvement of the alpha(1-->2)ManT Rv2181 in the dual role of Man capping and mannan-core branching, and in the process generated a rapidly growing, ManLAM-containing strain, a tool for the study of the role of ManLAM in the pathogenesis of tuberculosis.

Glycomics and proteomics analyses of mouse uterine luminal fluid revealed a predominance of Lewis Y and X epitopes on specific protein carriers

Sperm motility and maturation are known to be affected by a host of factors encountered en route in both male and female genital tracts prior to fertilization. Using a concerted proteomics and glycomics approach with advanced mass spectrometry-based glycan sequencing capability, we show in this work that 24p3, an abundant mouse uterine luminal fluid (ULF) glycoprotein also called lipocalin 2 (Lcn2), is highly fucosylated in the context of carrying multiple Lewis X and Y epitopes on complex type N-glycans at its single glycosylation site. The predominance of Lewis X/Y along with Neu5Acalpha2-6 sialylation was found to be a salient feature of the ULF glycome, and several other protein carriers were additionally identified including the highly abundant lactotransferrin, which is N-glycosylated at two sites, both with a similar range of highly fucosylated N-glycans. A comparative glycomics analysis of the male genital tract fluids revealed that there is a gradient of glycomic complexity from the cauda to caput regions of the epididymis, varying from high mannose to sialylated complex type N-glycans but mostly devoid of fucosylation. The seminal vesicle fluid glycome, on the other hand, carries equally abundant multimeric Lewis X structures but is distinctively lacking in additional fucosylation of the terminal galactose to give the Lewis Y epitope typifying the glycome of female ULF. One-dimensional shotgun proteomics analysis identified over 40 proteins in the latter, many of which are reported for the first time, and a majority are notably involved in immune defense and antigen processing. Further sperm binding and motility assays suggest that the Lewis X/Y epitopes do contribute to the sperm motility-enhancing activity of 24p3, whereas lactotransferrin is largely inactive in this context despite being similarly glycosylated. These findings underline the importance of glycoproteomics in delineating both the specific glycan structures and their carriers in assigning glycobiological functions.

Inhibition of the sodium/potassium ATPase impairs N-glycan expression and function

Aberrant N-linked glycans promote the malignant potential of cells by enhancing the epithelial-to-mesenchymal transition and the invasive phenotype. To identify small molecule inhibitors of N-glycan biosynthesis, we developed a chemical screen based on the ability of the tetravalent plant lectin L-phytohemagglutinin (L-PHA) to bind and crosslink surface glycoproteins with beta1,6GlcNAc-branched complex type N-glycans and thereby induce agglutination and cell death. In this screen, Jurkat cells were treated with a library of off-patent chemicals (n = 1,280) to identify molecules that blocked L-PHA-induced death. The most potent hit from this screen was the cardiac glycoside (CG) dihydroouabain. In secondary assays, a panel of CGs was tested for their effects on L-PHA-induced agglutination and cell death. All of the CGs tested inhibited L-PHA-induced death in Jurkat cells, and the most potent CG tested was digoxin with an EC(50) of 60 +/- 20 nmol/L. Digoxin also increased the fraction of some concanavalin A-binding N-glycans. Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, digoxin specifically increased GlcNAc(1)Man(3)GlcNAc(2)Fuc(1) and GlcNAc(2)Man(3)GlcNAc(2)Fuc(1) oligosaccharides demonstrating an impairment of the N-glycan pathway. Consistent with this effect on the N-glycan pathway, digoxin inhibited N-glycosylation-mediated processes of tumor cell migration and invasion. Furthermore, digoxin prevented distant tumor formation in two mouse models of metastatic prostate cancer. Thus, taken together, our high throughput screen identified CGs as modifiers of the N-glycan pathway. These molecules can be used as tools to better understand the role of N-glycans in normal and malignant cells. Moreover, these results may partly explain the anticancer effect of CGs in cardiovascular patients.

New insights into the functions and N-glycan structures of factor X activator from Russell's viper venom

The coagulation factor X activator from Russell's viper venom (RVV-X) is a heterotrimeric glycoprotein. In this study, its three subunits were cloned and sequenced from the venom gland cDNAs of Daboia siamensis. The deduced heavy chain sequence contained a C-terminal extension with four additional residues to that published previously. Both light chains showed 77-81% identity to those of a homologous factor X activator from Vipera lebetina venom. Far-western analyses revealed that RVV-X could strongly bind protein S, in addition to factors X and IX. This might inactivate protein S and potentiate the disseminated intravascular coagulation syndrome elicited by Russell's viper envenomation. The N-glycans released from each subunit were profiled and sequenced by MALDI-MS and MS/MS analyses of the permethyl derivatives. All the glycans, one on each light chain and four on the heavy chain, showed a heterogeneous pattern, with a combination of variable terminal fucosylation and sialylation on multiantennary complex-type sugars. Amongst the notable features were the presence of terminal Lewis and sialyl-Lewis epitopes, as confirmed by western blotting analyses. As these glyco-epitopes have specific receptors in the vascular system, they possibly contribute to the rapid homing of RVV-X to the vascular system, as supported by the observation that slower and fewer fibrinogen degradation products are released by desialylated RVV-X than by native RVV-X.

Structural requirements of anti-GD1a antibodies determine their target specificity

The acute motor axonal neuropathy (AMAN) variant of Guillain-Barré syndrome (GBS) is associated with anti-GD1a and anti-GM1 IgG antibodies. The basis of preferential motor nerve injury in this disease is not clear, however, because biochemical studies demonstrate that sensory and motor nerves express similar quantities of GD1a and GM1 gangliosides. To elucidate the pathophysiology of AMAN, we have developed several monoclonal antibodies (mAbs) with GD1a reactivity and reported that one mAb, GD1a-1, preferentially stained motor axons in human and rodent nerves. To understand the basis of this preferential motor axon staining, several derivatives of GD1a were generated by various chemical modifications of N-acetylneuraminic (sialic) acid residues (GD1a NeuAc 1-amide, GD1a NeuAc ethyl ester, GD1a NeuAc 1-alcohol, GD1a NeuAc 1-methyl ester, GD1a NeuAc 7-alcohol, GD1a NeuAc 7-aldehyde) on this ganglioside. Binding of anti-GD1a mAbs and AMAN sera with anti-GD1a Abs to these derivatives was examined. Our results indicate that mAbs with selective motor axon staining had a distinct pattern of reactivity with GD1a-derivatives compared to mAbs that stain both motor and sensory axons. The fine specificity of the anti-GD1a antibodies determines their motor selectivity, which was validated by cloning a new mAb (GD1a-E6) with a chemical and immunocytochemical binding pattern similar to that of GD1a-1 but with two orders of magnitude higher affinity. Control studies indicate that selective binding of mAbs to motor nerves is not due to differences in antibody affinity or ceramide structural specificity. Since GD1a-reactive mAb with preferential motor axon staining showed similar binding to sensory- and motor nerve-derived GD1a in a solid phase assay, we generated computer models of GD1a based on binding patterns of different GD1a-reactive mAbs to different GD1a-derivatives. These modelling studies suggest that critical GD1a epitopes recognized by mAbs are differentially expressed in motor and sensory nerves. The GD1a-derivative binding patterns of AMAN sera resembled those with motor-specific mAbs. On the basis of these findings we postulate that both the fine specificity and ganglioside orientation/exposure in the tissues contribute to target recognition by anti-ganglioside antibodies and this observation provides one explanation for preferential motor axon injury in AMAN.

GlycoWorkbench: a tool for the computer-assisted annotation of mass spectra of glycans

Mass spectrometry is the main analytical technique currently used to address the challenges of glycomics as it offers unrivalled levels of sensitivity and the ability to handle complex mixtures of different glycan variations. Determination of glycan structures from analysis of MS data is a major bottleneck in high-throughput glycomics projects, and robust solutions to this problem are of critical importance. However, all the approaches currently available have inherent restrictions to the type of glycans they can identify, and none of them have proved to be a definitive tool for glycomics. GlycoWorkbench is a software tool developed by the EUROCarbDB initiative to assist the manual interpretation of MS data. The main task of GlycoWorkbench is to evaluate a set of structures proposed by the user by matching the corresponding theoretical list of fragment masses against the list of peaks derived from the spectrum. The tool provides an easy to use graphical interface, a comprehensive and increasing set of structural constituents, an exhaustive collection of fragmentation types, and a broad list of annotation options. The aim of GlycoWorkbench is to offer complete support for the routine interpretation of MS data. The software is available for download from: http://www.eurocarbdb.org/applications/ms-tools.

Quantitation by isobaric labeling: applications to glycomics

The study of glycosylation patterns (glycomics) in biological samples is an emerging field that can provide key insights into cell development and pathology. A current challenge in the field of glycomics is to determine how to quantify changes in glycan expression between different cells, tissues, or biological fluids. Here we describe a novel strategy, quantitation by isobaric labeling (QUIBL), to facilitate comparative glycomics. Permethylation of a glycan with (13)CH 3I or (12)CH 2DI generates a pair of isobaric derivatives, which have the same nominal mass. However, each methylation site introduces a mass difference of 0.002922 Da. As glycans have multiple methylation sites, the total mass difference for the isobaric pair allows separation and quantitation at a resolution of approximately 30000 m/Delta m. N-Linked oligosaccharides from a standard glycoprotein and human serum were used to demonstrate that QUIBL facilitates relative quantitation over a linear dynamic range of 2 orders of magnitude and permits the relative quantitation of isomeric glycans. We applied QUIBL to quantitate glycomic changes associated with the differentiation of murine embryonic stem cells to embryoid bodies.

Glycomic studies of Drosophila melanogaster embryos

With the complete genome sequence of Drosophila melanogaster defined a systematic approach towards understanding the function of glycosylation has become possible. Structural assignment of the entire Drosophila glycome during specific developmental stages could provide information that would shed further light on the specific roles of different glycans during development and pinpoint the activity of certain glycosyltransferases and other glycan biosynthetic genes that otherwise might be missed through genetic analyses. In this paper the major glycoprotein N- and O-glycans of Drosophila embryos are described as part of our initial undertaking to characterize the glycome of Drosophila melanogaster. The N-glycans are dominated by high mannose and paucimannose structures. Minor amounts of mono-, bi- and tri-antennary complex glycans were observed with GlcNAc and Galbeta1-4GlcNAc non-reducing end termini. O-glycans were restricted to the mucin-type core 1 Galbeta1-3GalNAc sequence.

Distinctive characteristics of MALDI-Q/TOF and TOF/TOF tandem mass spectrometry for sequencing of permethylated complex type N-glycans

Concerted MALDI-MS profiling and CID MS/MS sequencing of permethylated glycans is one of the most effective approaches for high throughput glycomics applications. In essence, the identification of larger complex type N-glycans necessitates an unambiguous definition of any modification on the trimannosyl core and the complement of non-reducing terminal sequences which constitute the respective antennary structures. Permethylation not only affords analyses of both neutral and sialylated glycans at comparable ease and sensitivity but also yields more sequence-informative fragmentation pattern. Facile glycosidic cleavages directed mostly at N-acetylglucosamine under low energy CID, as implemented on a quadrupole/time-of-flight (Q/TOF) instrument, often afford multiple losses of the attached antenna resulting in characteristic ions related to the number of antennary branches on the trimannosyl core. Non-reducing terminal epitopes can be easily deduced but information on the linkage specific substituent on the terminal units is often missing. The high energy CID MS/MS afforded by TOF/TOF instrument can fill in the gap by giving an array of additional cross-ring and satellite ions. Glycosidic cleavages occurring specifically in concert with loss of 2-linked or 3-linked substituents provide an effective way to identify the branch-specific antennary extension. These characteristics are shown here to be effective in deriving the sequences of additionally galactosylated, sialylated and fucosylated terminal N-acetyllactosamine units and their antennary location. Together, a highly reproducible fragmentation pattern can be formulated to simplify spectral assignment. This work also provides first real examples of sequencing multiply sialylated complex type N-glycans by high energy CID on a TOF/TOF instrument.

Glycomic mapping of O- and N-linked glycans from major rat sublingual mucin

Carbohydrate moieties of salivary mucins play various roles in life processes, especially as a microbial trapping agent. While structural details of the salivary O-glycans from several mammalian sources are well studied, very little information is currently available for the corresponding N-glycans. The existence of N-glycans alongside O-glycans on mucin isolated from rat sublingual gland has previously been implicated by total glycosyl compositional analysis but the respective structural data are both lacking. The advent of facile glycomic mapping and sequencing methods by mass spectrometry (MS) has enabled a structural reinvestigation into many previously unsolved issues. For the first time, high energy collision induced dissociation (CID) MALDI-MS/MS as implemented on a TOF/TOF instrument was applied to permethyl derivatives of mucin type O-glycans and N-glycans, from which the linkage specific fragmentation pattern could be established. The predominant O-glycans carried on the rat sublingual mucin were defined as sialylated core 3 and 4 types whereas the N-glycans were determined to be non-bisected hybrid types similarly carrying a sialylated type II chain. The masking effect of terminal sialylation on the tight binding of rat sublingual mucin to Galbeta1-->4GlcNAc specific lectins and three oligomannose specific lectins were clearly demonstrated in this study.

Glycomic mapping of pseudomucinous human ovarian cyst glycoproteins: Identification of Lewis and sialyl Lewis glycotopes

Expression of sialyl Lewis x (sLe(x)) and sialyl Lewis a (sLe(a)) on cell-surface glycoproteins endows cells with the ability to adhere to E-, P-, and L-selectins present on endothelia, platelets, or leukocytes. Special arrangements of these glycotopes in cancers are thought to play a key role in metastasis. Previous studies have mostly described membrane-bound sLe(x) and sLe(a) activities. In this report, the major O-glycans of the secreted human ovarian cyst sialoglycoproteins from a Le(a+) nonsecretor individual (human ovarian cyst sample 350) were characterized by MS/MS analyses and immuno-/lectin-chemical assays. The results showed that HOC 350 carries a large number of epitopes for sLe(x), sLe(a), and Le(a) reactive antibodies. Advanced MS/MS sequencing coupled with mild periodate oxidation and exoglycosidase digestions further revealed that the O-glycans from HOC 350 are mostly of core 1 and 2 structures, extended and branched on the 3-arm with both type I and type II chains, complete with variable degrees of terminal sialylation and/or fucosylation to yield the sLe(x) or sLe(a) epitopes. Thus, the underlying core and peripheral backbone structures are similar to that of a previously proposed composite structural model for nonsialylated human ovarian cysts O-glycans, but with some notable distinguishing structural features in addition to sialylation.

Glycomics Analysis of Schistosoma mansoni Egg and Cercarial Secretions

The parasitic helminth Schistosoma mansoni is a major public health concern in many developing countries. Glycoconjugates, and in particular the carbohydrate component of these products, represent the main immunogenic challenge to the host and could therefore represent one of the crucial determinants for successful parasite establishment. Here we report a comparative glycomics analysis of the N- and O-glycans derived from glycoproteins present in S. mansoni egg (egg-secreted protein) and cercarial (0-3-h released protein) secretions by a combination of mass spectrometric techniques. Our results show that S. mansoni secrete glycoproteins with glycosylation patterns that are complex and stage-specific. Cercarial stage secretions were dominated by N-glycans that were core-xylosylated, whereas N-glycans from egg secretions were predominantly core-difucosylated. O-Glycan core structures from cercarial secretions primarily consisted of the core sequence Galbeta1-->3(Galbeta1-->6)GalNAc, whereas egg-secreted O-glycans carried the mucin-type core 1 (Galbeta1-->3GalNAc) and 2 (Galbeta1-->3(GlcNAcbeta1-->6)GalNAc) structures. Additionally we identified a novel O-glycan core in both secretions in which a Gal residue is linked to the protein. Terminal structures of N- and O-glycans contained high levels of fucose and include stage-specific structures. These glycan structures identified in S. mansoni secretions are potentially antigenic motifs and ligands for carbohydrate-binding proteins of the host immune system.

A rapid mass spectrometric strategy for the characterization of N- and O-glycan chains in the diagnosis of defects in glycan biosynthesis

Glycosylation of proteins is a very complex process which involves numerous factors such as enzymes or transporters. A defect in one of these factors in glycan biosynthetic pathways leads to dramatic disorders named congenital disorders of glycosylation (CDG). CDG can affect the biosynthesis of not only protein N-glycans but also O-glycans. The structural analysis of glycans on serum glycoproteins is essential to solving the defect. For this reason, we propose in this paper a strategy for the simultaneous characterization of both N- and O-glycan chains isolated from the serum glycoproteins. The serum (20 microL) is used for the characterization of N-glycans which are released by enzymatic digestion with PNGase F. O-glycans are chemically released by reductive elimination from whole serum glycoproteins using 10 microL of the serum. Using strategies based on mass spectrometric analysis, the structures of N- and O-glycan chains are defined. These strategies were applied on the sera from one patient with CDG type IIa, and one patient with a mild form of congenital disorder of glycosylation type II (CDG-II) that is caused by a deficiency in the Cog1 subunit of the complex.

The sperm agglutination antigen-1 (SAGA-1) glycoforms of CD52 are O-glycosylated

CD52 is composed of a 12 amino acid peptide with N-linked glycans bound to the single potential glycosylation site at position 3, and a glycosylphosphatidylinositol-anchor attached at the C-terminus. Some glycoforms of this molecule expressed in the male reproductive tract are recognized by complement-dependent sperm-immobilizing antibodies in infertile patients making this antigen an important target for immunocontraception and fertility studies. Although the amount of posttranslational modification is already remarkable for such a small polypeptide, O-glycosylation of CD52 has additionally been implicated by several studies, but never rigorously characterized. In this report, we show clear evidence for the presence of O-glycans in CD52 preparations immunopurified using the murine S19 monoclonal antibody generated against sperm agglutination antigen-1 (SAGA-1), a male reproductive tract specific form of CD52. The O-glycans have been characterized by MALDI-TOF and tandem mass spectrometry after reductive elimination and permethylation. The data indicate that the major SAGA-1 O-glycans are core 1 and 2 mucin-type structures, with and without sialic acid (NeuAc(0-2)Hex(1-3)HexNAc(1-2)HexNAcitol). Minor fucosy- lated O-glycans are also present including some struc- tures with putative Le(y) epitopes (NeuAc(0-1)Fuc(1-3)Hex(1-2) HexNAc(0-1)HexNAcitol). Analysis of O-glycopeptides by tandem mass spectrometry provided an additional level of support for the O-glycosylation of SAGA-1. Elucidation of the O-glycosylation of SAGA-1 adds to the complexity of this molecule and may help to explain its biological activity.

New insights into the biosynthesis of mycobacterial lipomannan arising from deletion of a conserved gene

Genetic construction of a mutant strain (designated MSMEG4245) of Mycobacterium smegmatis, defective in a broadly conserved gene for a putative glycosyltransferase of the glycosyltransferase-C superfamily, results in a phenotype marked by the virtual absence of the phosphatidylinositol-containing lipomannan and lipoarabinomannan, replaced instead by a novel truncated form of lipomannan. The normal spectrum of phosphatidylinositol mannosides, long presumed precursors of these lipoglycans, was retained. Matrix-assisted laser desorption/ionization-time of flight/mass spectrometry of the mutated form of lipomannan shows a family of phosphatidylinositol-anchored lipomannans with from only 5 to 20 Manp residues as compared with lipomannan from the wild type strain consisting of 21-34 Manp residues but with few changes in the branching pattern. Thus, MSMEG4245 is apparently a key mannosyltransferase, required for the proper elongation of lipomannan to its normal state and subsequent synthesis of lipoarabinomannan. The corresponding ortholog in Mycobacterium tuberculosis H37Rv has been identified as Rv2174. This previously unrecognized feature of the biosynthesis of lipomannan/lipoarabinomannan allows a significant revision of structural and biosynthetic schemata and provides a molecular basis of selectivity in biosynthesis, as conferred by the MSMEG4245 gene.

Towards understanding the functional role of the glycosyltransferases involved in the biosynthesis of Moraxella catarrhalis lipooligosaccharide

The glycosyltransferase enzymes (Lgts) responsible for the biosynthesis of the lipooligosaccharide-derived oligosaccharide structures from Moraxella catarrhalis have been investigated. This upper respiratory tract pathogen is responsible for a spectrum of illnesses, including otitis media (middle ear infection) in children, and contributes to exacerbations of chronic obstructive pulmonary disease in elderly patients. To investigate the function of the glycosyltransferase enzymes involved in the biosynthesis of lipooligosaccharide of M. catarrhalis and to gain some insight into the mechanism of serotype specificity for this microorganism, mutant strains of M. catarrhalis were produced. Examination by NMR and MS of the oligosaccharide structures produced by double-mutant strains (2951lgt1/4Delta and 2951lgt5/4Delta) and a single-mutant strain (2951lgt2Delta) of the bacterium has allowed us to propose a model for the serotype-specific expression of lipooligosaccharide in M. catarrhalis. According to this model, the presence/absence of Lgt4 and the Lgt2 allele determines the lipooligosaccharide structure produced by a strain. Furthermore, it is concluded that Lgt4 functions as an N-acetylglucosylamine transferase responsible for the addition of an alpha-D-GlcNAc (1-->2) glycosidic linkage to the (1-->4) branch, and also that there is competition between the glycosyltransferases Lgt1 and Lgt4. That is, in the presence of an active Lgt4, GlcNAc is preferentially added to the (1-->4) chain of the growing oligosaccharide, instead of Glc. In serotype B strains, which lack Lgt4, Lgt1 adds a Glc at this position. This implies that active Lgt4 has a much higher affinity/specificity for the beta-(1-->4)-linked Glc on the (1-->4) branch than does Lgt1.

Tools for glycomics: relative quantitation of glycans by isotopic permethylation using 13CH3I

Analysis of oligosaccharides by mass spectrometry (MS) has enabled the investigation of the glycan repertoire of organisms with high resolution and sensitivity. It is difficult, however, to correlate the expression of glycosyltransferases with the glycan structures present in a particular cell type or tissue because the use of MS for quantitative purposes has significant limitations. For this reason, in order to develop a technique that would allow relative glycan quantification by MS analysis between two samples, a procedure was developed for the isotopic labeling of oligosaccharides with (13)C-labeled methyl iodide using standard permethylation conditions. Separate aliquots of oligosaccharides from human milk were labeled with (12)C or (13)C methyl iodide; the labeled and non-labeled glycans were mixed in known proportions, and the mixtures analyzed by MS. Results indicated that the isotopic labeling described here was capable of providing relative quantitative data with a dynamic range of at least two orders of magnitude, adequate linearity, and reproducibility with a coefficient of variation that was 13% on average. This procedure was used to analyze N-linked glycans released from various mixtures of glycoproteins, such as alpha-1 acid glycoprotein, human transferrin, and bovine fetuin, using MS techniques that included matrix assisted laser desorption ionization-time of flight MS and electrospray ionization with ion cyclotron resonance-Fourier transformation MS. The measured (12)C:(13)C ratios from mixtures of glycans permethylated with either (12)CH(3)I or (13)CH(3)I were consistent with the theoretical proportions. This technique is an effective procedure for relative quantitative glycan analysis by MS.

A strategy for identification of oligosaccharide structures using observational multistage mass spectral library

Glycosylation is the most widespread posttranslational modification in eukaryotes; however, the role of oligosaccharides attached to proteins has been little studied because of the lack of a sensitive and easy analytical method for oligosaccharide structures. Recently, tandem mass spectrometric techniques have been revealing that oligosaccharides might have characteristic signal intensity profiles. We describe here a strategy for the rapid and accurate identification of the oligosaccharide structures on glycoproteins using only mass spectrometry. It is based on a comparison of the signal intensity profiles of multistage tandem mass (MSn) spectra between the analyte and a library of observational mass spectra acquired from structurally defined oligosaccharides prepared using glycosyltransferases. To smartly identify the oligosaccharides released from biological materials, a computer suggests which ion among the fragment ions in the MS/MS spectrum should yield the most informative MS3 spectrum to distinguish similar oligosaccharides. Using this strategy, we were able to identify the structure of N-linked oligosaccharides in immunoglobulin G as an example.

Sequencing of oligoarabinosyl units released from mycobacterial arabinogalactan by endogenous arabinanase: identification of distinctive and novel structural motifs

The mycobacterial D-arabinofuran is a common constituent of both cell wall mycolyl-arabinogalactan (AG) and the associated lipoarabinomannan (LAM), and is thus accorded critical structural and immunological roles. Despite a well-recognized importance, progress in understanding its full structural characteristics beyond the nonreducing terminal motifs has hitherto been limited by available analytical tools. An endogenous arabinanase activity recently isolated from Mycobacterium smegmatis was previously shown to be capable of releasing large oligoarabinosyl units from AG. Advanced tandem mass spectrometry utilizing both low and high energy collision induced dissociation now afforded a facile way to map and directly sequence the digestion products which were dominated by distinctive Ara18 and Ara19 structural units, together with Ara7 and lesser amount of Ara11 and Ara12. Significantly, evidence was obtained for the first time which validated the linkages and branching pattern of the previously inferred Ara22 structural motif of AG, on which the preferred cleavage sites of the novel arabinanase could be localized. The established linkage-specific MS/MS fragmentation characteristics further led to identification of a galactosamine substituent on the C2 position of a portion of the internal 3,5-branched Ara residue of the AG of Mycobacterium tuberculosis, but not that of the nonpathogenic, fast growing M. smegmatis.

Resistance to Bacillus thuringiensis toxin in Caenorhabditis elegans from loss of fucose

A mutation in the Caenorhabditis elegans bre-1 gene was isolated in a screen for Bacillus thuringiensis toxin-resistant (bre) mutants to the Cry5B crystal toxin made by B. thuringiensis. bre-1 mutant animals are different from the four other cloned bre mutants in that their level of resistance is noticeably lower. bre-1 animals also display a significantly reduced brood size at 25 degrees C. Here we cloned the bre-1 gene and characterized the bre-1 mutant phenotype. bre-1 encodes a protein with significant homology to a GDP-mannose 4,6-dehydratase, which catalyzes the first step in the biosynthesis of GDP-fucose from GDP-mannose. Injection of GDP-fucose but not fucose into C. elegans intestinal cells rescues bre-1 mutant phenotypes. Thus, C. elegans lacks a functional fucose salvage pathway. Furthermore, we demonstrate that bre-1 mutant animals are defective in production of fucosylated glycolipids and that bre-1 mutant animals make quantitatively reduced levels of glycolipid receptors for Cry5B. We finally show that bre-1 mutant animals, although viable, show a lack of fucosylated N- and O-glycans, based on mass spectrometric evidence. Thus, C. elegans can survive with little fucose and can develop resistance to crystal toxin by loss of a monosaccharide biosynthetic pathway.