The distribution of repeating [Gal beta 1,4GlcNAc beta 1,3] sequences in asparagine-linked oligosaccharides of the mouse lymphoma cell lines BW5147 and PHAR 2.1

Mass Spectrometry Analysis of Glycopeptides Enriched by Anion Exchange-Mediated Methods Reveals PolyLacNAc-Extended N-Glycans in Integrins and Tetraspanins in Melanoma Cells

Glycosylation is a key modulator of the functional state of proteins. Recent developments in large-scale analysis of intact glycopeptides have enabled the identification of numerous glycan structures that are relevant in pathophysiological processes. However, one motif found in N-glycans, poly-N-acetyllactosamine (polyLacNAc), still poses a substantial challenge to mass spectrometry-based glycoproteomic analysis due to its relatively low abundance and large size. In this work, we developed approaches for the systematic mapping of polyLacNAc-elongated N-glycans in melanoma cells. We first evaluated five anion exchange-based matrices for enriching intact glycopeptides and selected two materials that provided better overall enrichment efficiency. We then tested the robustness of the methodology by quantifying polyLacNAc-containing glycopeptides as well as changes in protein fucosylation and sialylation. Finally, we applied the optimal enrichment methods to discover glycopeptides containing polyLacNAc motifs in melanoma cells and found that integrins and tetraspanins are substantially modified with these structures. This study demonstrates the feasibility of glycoproteomic approaches for identification of glycoproteins with polyLacNAc motifs.

LeGenD: determining N-glycoprofiles using an explainable AI-leveraged model with lectin profiling

Glycosylation affects many vital functions of organisms. Therefore, its surveillance is critical from basic science to biotechnology, including biopharmaceutical development and clinical diagnostics. However, conventional glycan structure analysis faces challenges with throughput and cost. Lectins offer an alternative approach for analyzing glycans, but they only provide glycan epitopes and not full glycan structure information. To overcome these limitations, we developed LeGenD, a lectin and AI-based approach to predict N-glycan structures and determine their relative abundance in purified proteins based on lectin-binding patterns. We trained the LeGenD model using 309 glycoprofiles from 10 recombinant proteins, produced in 30 glycoengineered CHO cell lines. Our approach accurately reconstructed experimentally-measured N-glycoprofiles of bovine Fetuin B and IgG from human sera. Explanatory AI analysis with SHapley Additive exPlanations (SHAP) helped identify the critical lectins for glycoprofile predictions. Our LeGenD approach thus presents an alternative approach for N-glycan analysis.

N-linked glycans: an underappreciated key determinant of T cell development, activation, and function

N-linked glycosylation is a post-translational modification that results in the decoration of newly synthesized proteins with diverse types of oligosaccharides that originate from the amide group of the amino acid asparagine. The sequential and collective action of multiple glycosidases and glycosyltransferases are responsible for determining the overall size, composition, and location of N-linked glycans that become covalently linked to an asparagine during and after protein translation. A growing body of evidence supports the critical role of N-linked glycan synthesis in regulating many features of T cell biology, including thymocyte development and tolerance, as well as T cell activation and differentiation. Here, we provide an overview of how specific glycosidases and glycosyltransferases contribute to the generation of different types of N-linked glycans and how these post-translational modifications ultimately regulate multiple facets of T cell biology.

A Useful Guide to Lectin Binding: Machine-Learning Directed Annotation of 57 Unique Lectin Specificities

Glycans are critical to every facet of biology and medicine, from viral infections to embryogenesis. Tools to study glycans are rapidly evolving; however, the majority of our knowledge is deeply dependent on binding by glycan binding proteins (e.g., lectins). The specificities of lectins, which are often naturally isolated proteins, have not been well-defined, making it difficult to leverage their full potential for glycan analysis. Herein, we use a combination of machine learning algorithms and expert annotation to define lectin specificity for this important probe set. Our analysis uses comprehensive glycan microarray analysis of commercially available lectins we obtained using version 5.0 of the Consortium for Functional Glycomics glycan microarray (CFGv5). This data set was made public in 2011. We report the creation of this data set and its use in large-scale evaluation of lectin-glycan binding behaviors. Our motif analysis was performed by integrating 68 manually defined glycan features with systematic probing of computational rules for significant binding motifs using mono- and disaccharides and linkages. Combining machine learning with manual annotation, we create a detailed interpretation of glycan-binding specificity for 57 unique lectins, categorized by their major binding motifs: mannose, complex-type N-glycan, O-glycan, fucose, sialic acid and sulfate, GlcNAc and chitin, Gal and LacNAc, and GalNAc. Our work provides fresh insights into the complex binding features of commercially available lectins in current use, providing a critical guide to these important reagents.

A photo-cross-linking GlcNAc analog enables covalent capture of N-linked glycoprotein-binding partners on the cell surface

N-glycans are displayed on cell-surface proteins and can engage in direct binding interactions with membrane-bound and secreted glycan-binding proteins (GBPs). Biochemical identification and characterization of glycan-mediated interactions is often made difficult by low binding affinities. Here we describe the metabolic introduction of a diazirine photo-cross-linker onto N-acetylglucosamine (GlcNAc) residues of N-linked glycoproteins on cell surfaces. We characterize sites at which diazirine-modified GlcNAc is incorporated, as well as modest perturbations to glycan structure. We show that diazirine-modified GlcNAc can be used to covalently cross-link two extracellular GBPs, galectin-1 and cholera toxin subunit B, to cell-surface N-linked glycoproteins. The extent of cross-linking correlates with display of the preferred glycan ligands for the GBPs. In addition, covalently cross-linked complexes could be isolated, and protein components of cross-linked N-linked glycoproteins were identified by proteomics analysis. This method may be useful in the discovery and characterization of binding interactions that depend on N-glycans.

Tumor cell E-selectin ligands determine partialefficacy of bortezomib on spontaneous lung metastasis formation of solid human tumors in vivo

Extravasation of circulating tumor cells (CTCs) is critical for metastasis and is initiated by adhesive interactions between glycoligands on CTCs and E-selectin on endothelia. Here, we show that the clinically approved proteasome inhibitor bortezomib (BZM; Velcade) counteracts the cytokine-dependent induction of E-selectin in the lung mediated by the primary tumor, thereby impairing endothelial adhesion and thus spontaneous lung metastasis in vivo. However, the efficacy of BZM crucially depends on the tumor cells' E-selectin ligands, which determine distinct adhesion patterns. The canonical ligands sialyl-Lewis A (sLeA) and sLeX mediate particularly high-affinity E-selectin binding so that the incomplete E-selectin-reducing effect of BZM is not sufficient to disrupt adhesion or metastasis. In contrast, tumor cells lacking sLeA/X nevertheless bind E-selectin, but with low affinity, so that adhesion and lung metastasis are significantly diminished. Such low-affinity E-selectin ligands apparently consist of sialylated MGAT5 products on CD44. BZM no longer has anti-metastatic activity after CD44 knockdown in sLeA/X-negative tumor cells or E-selectin knockout in mice. sLeA/X can be determined by immunohistochemistry in cancer samples, which might aid patient stratification. These data suggest that BZM might act as a drug for inhibiting extravasation and thus distant metastasis formation in malignancies expressing low-affinity E-selectin ligands.

"Stuck on sugars - how carbohydrates regulate cell adhesion, recognition, and signaling"

We have explored the fundamental biological processes by which complex carbohydrates expressed on cellular glycoproteins and glycolipids and in secretions of cells promote cell adhesion and signaling. We have also explored processes by which animal pathogens, such as viruses, bacteria, and parasites adhere to glycans of animal cells and initiate disease. Glycans important in cell signaling and adhesion, such as key O-glycans, are essential for proper animal development and cellular differentiation, but they are also involved in many pathogenic processes, including inflammation, tumorigenesis and metastasis, and microbial and parasitic pathogenesis. The overall hypothesis guiding these studies is that glycoconjugates are recognized and bound by a growing class of proteins called glycan-binding proteins (GBPs or lectins) expressed by all types of cells. There is an incredible variety and diversity of GBPs in animal cells involved in binding N- and O-glycans, glycosphingolipids, and proteoglycan/glycosaminoglycans. We have specifically studied such molecular determinants recognized by selectins, galectins, and many other C-type lectins, involved in leukocyte recruitment to sites of inflammation in human tissues, lymphocyte trafficking, adhesion of human viruses to human cells, structure and immunogenicity of glycoproteins on the surfaces of human parasites. We have also explored the molecular basis of glycoconjugate biosynthesis by exploring the enzymes and molecular chaperones required for correct protein glycosylation. From these studies opportunities for translational biology have arisen, involving production of function-blocking antibodies, anti-glycan specific antibodies, and synthetic glycoconjugates, e.g. glycosulfopeptides, that specifically are recognized by GBPs. This invited short review is based in part on my presentation for the IGO Award 2019 given by the International Glycoconjugate Organization in Milan.

Altered N-glycosylation profiles as potential biomarkers and drug targets in diabetes

N-glycosylation is a ubiquitous protein modification, and N-glycosylation profiles are emerging as both biomarkers and functional effectors in various types of diabetes. Genome-wide association studies identified glycosyltransferase genes as candidate causal genes for type 1 and type 2 diabetes. Studies focused on N-glycosylation changes in type 2 diabetes demonstrated that patients can be distinguished from healthy controls based on N-glycome composition. In addition, individuals at an increased risk of future disease development could be identified based on N-glycome profiles. Moreover, accumulating evidence indicates that N-glycans have a major role in preventing the impairment of glucose-stimulated insulin secretion by maintaining the glucose transporter in proper orientation, indicating that interindividual variation in protein N-glycosylation might be a novel risk factor contributing to diabetes development. Defective N-glycosylation of T cells has been implicated in type 1 diabetes pathogenesis. Furthermore, studies of N-glycan alterations have successfully been used to identify individuals with rare types of diabetes (such as the HNF1A-MODY), and also to evaluate functional significance of novel diabetes-associated mutations. In conclusion, both N-glycans and glycosyltransferases emerge as potential therapeutic targets in diabetes.

St6gal1 knockdown alters HBV life cycle in HepAD38 cells

Complex glycans at the cell surface play important roles, and their alteration is known to modulate cellular activity. Previously, we found that HBV replication in HepAD38 altered cell-surface sialylated N-glycan through the upregulation of St6gal1, Mgat2, and Mgat4a expression. Here we studied the effects of knocking them down on HBV replication in HepAD38. Our results showed that St6gal1 knockdown (KD) reduced intracellular HBV rcDNA level by 90%, that Mgat2 KD did not change the intracellular HBV rcDNA level, and that Mgat4 KD increased the intracellular HBV rcDNA level by 19 times compared to Tet(-). The changes in intracellular rcDNA level were followed by the alteration of Pol and HBc expression. Our study suggests that St6gal1 KD contributes more to the HBV life cycle than Mgat2 or Mgat4a KD through the modification of intracellular L, Pol, and HBc expression.

Sugar-Binding Profiles of Chitin-Binding Lectins from the Hevein Family: A Comprehensive Study

Chitin-binding lectins form the hevein family in plants, which are defined by the presence of single or multiple structurally conserved GlcNAc (N-acetylglucosamine)-binding domains. Although they have been used as probes for chito-oligosaccharides, their detailed specificities remain to be investigated. In this study, we analyzed six chitin-binding lectins, DSA, LEL, PWM, STL, UDA, and WGA, by quantitative frontal affinity chromatography. Some novel features were evident: WGA showed almost comparable affinity for pyridylaminated chitotriose and chitotetraose, while LEL and UDA showed much weaker affinity, and DSA, PWM, and STL had no substantial affinity for the former. WGA showed selective affinity for hybrid-type N-glycans harboring a bisecting GlcNAc residue. UDA showed extensive binding to high-mannose type N-glycans, with affinity increasing with the number of Man residues. DSA showed the highest affinity for highly branched N-glycans consisting of type II LacNAc (N-acetyllactosamine). Further, multivalent features of these lectins were investigated by using glycoconjugate and lectin microarrays. The lectins showed substantial binding to immobilized LacNAc as well as chito-oligosaccharides, although the extents to which they bound varied among them. WGA showed strong binding to heavily sialylated glycoproteins. The above observations will help interpret lectin-glycoprotein interactions in histochemical studies and glyco-biomarker investigations.

Assembly, organization and regulation of cell-surface receptors by lectin–glycan complexes

Galectins are a family of β-galactoside-binding lectins carrying at least one consensus sequence in the carbohydrate-recognition domain. Properties of glycosylated ligands, such as N- and O-glycan branching, LacNAc (N-acetyl-lactosamine) content and the balance of α2,3- and α2,6-linked sialic acid dramatically influence galectin binding to a preferential set of counter-receptors. The presentation of specific glycans in galectin-binding partners is also critical, as proper orientation and clustering of oligosaccharide ligands on multiple carbohydrate side chains increase the binding avidity of galectins for particular glycosylated receptors. When galectins are released from the cells, they typically concentrate on the cell surface and the local matrix, raising their local concentration. Thus galectins can form their own multimers in the extracellular milieu, which in turn cross-link glycoconjugates on the cell surface generating galectin–glycan complexes that modulate intracellular signalling pathways, thus regulating cellular processes such as apoptosis, proliferation, migration and angiogenesis. Subtle changes in receptor expression, rates of protein synthesis, activities of Golgi enzymes, metabolite concentrations supporting glycan biosynthesis, density of glycans, strength of protein–protein interactions at the plasma membrane and stoichiometry may modify galectin–glycan complexes. Although galectins are key contributors to the formation of these extended glycan complexes leading to promotion of receptor segregation/clustering, and inhibition of receptor internalization by surface retention, when these complexes are disrupted, some galectins, particularly galectin-3 and -4, showed the ability to drive clathrin-independent mechanisms of endocytosis. In the present review, we summarize the data available on the assembly, hierarchical organization and regulation of conspicuous galectin–glycan complexes, and their implications in health and disease.

RNAi screens for genes involved in Golgi glycosylation

RNAi screening has gained popularity in recent years, due to its usefulness in systematic investigations of biological pathways. Combined with high-content screening and advances in imaging and analysis methods, it can enable detailed genetic characterization of cellular processes such as protein glycosylation, a major function of the Golgi apparatus. Glycosylation concerns about one third of all human proteins and regulates various cellular behaviors. Yet the methods available to study it are limited and not easily accessible. In this chapter, we detail a step-by-step method to systematically and quantitatively investigate glycosylation using fluorescent lectin staining, following high-throughput RNAi-based downregulation of gene activities. We also provide a workflow for downstream analysis of the data generated.

N-linked polylactosamine glycan synthesis is regulated by co-expression of β3GnT2 and GCNT2

Poly-N-acetyllactosamine (PLN) is a unique glycan composed of repeating units of the common disaccharide (Galβ1,4-GlcNAcβ1,3)n . The expression of PLN on glycoprotein core structures minimally requires enzyme activities for β1,4-galactosyltransferase (β4GalT) and β1,3-N-acetylglucosminyltransferase (β3GnT). Because β4GalTs are ubiquitous in most cells, PLN expression is generally ascribed to the tissue-specific transcription of eight known β3GnT genes in mice. In the olfactory epithelium (OE), β3GnT2 regulates expression of extended PLN chains that are essential for axon guidance and neuronal survival. N-glycan branching and core composition, however, can also modulate the extent of PLN modification. Here, we show for the first time that the β1,6-branching glycosyltransferase GCNT2 (formerly known as IGnT) is expressed at high levels specifically in the OE and other sensory ganglia. Postnatally, GCNT2 is maintained in mature olfactory neurons that co-express β3GnT2 and PLN. This highly specific co-expression suggests that GCNT2 and β3GnT2 function cooperatively in PLN synthesis. In support of this, β3GnT2 and GCNT2 co-transfection in HEK293T cells results in high levels of PLN expression on the cell surface and on adenylyl cyclase 3, a major carrier of PLN glycans in the OE. These data clearly suggest that GCNT2 functions in vivo together with β3GnT2 to determine PLN levels in olfactory neurons by regulating β1,6-branches that promote PLN extension.

Comprehensive list of lectins: origins, natures, and carbohydrate specificities

More than 100 years have passed since the first lectin ricin was discovered. Since then, a wide variety of lectins (lect means "select" in Latin) have been isolated from plants, animals, fungi, bacteria, as well as viruses, and their structures and properties have been characterized. At present, as many as 48 protein scaffolds have been identified as functional lectins from the viewpoint of three-dimensional structures as described in this chapter. In this chapter, representative 53 lectins are selected, and their major properties that include hemagglutinating activity, mitogen activity, blood group specificity, molecular weight, metal requirement, and sugar specificities are summarized as a comprehensive table. The list will provide a practically useful, comprehensive list for not only experienced lectin users but also many other non-expert researchers, who are not familiar to lectins and, therefore, have no access to advanced lectin biotechnologies described in other chapters.

Immobilized glycosylated Fmoc-amino acid for SPR: comparative studies of lectin-binding to linear or biantennary diLacNAc structures

A method to immobilize glycan-linked amino acids with protected α-amino groups, which are key intermediates to produce the desired neoglycoprotein, to a Biacore sensor chip was developed and its utility for interaction analyses was demonstrated. Two types of diN-acetyllactosamine (diLacNAc)-containing glycans, a core 2 hexasaccharide involving linear diLacNAc that is O-linked to N-(9-fluorenyl)methoxycarbonyl (Fmoc)-Thr and a biantennary diLacNAc that is N-linked to Fmoc-Asn, were used as ligands. For immobilization, the free carboxyl groups of the amino acid residues were activated with EDC/NHS, then reacted with the ethylenediamine-derivatized carboxymethyldextran sensor chip to obtain the desired ligand concentrations. Interactions of the ligands with five plant lectins were analyzed by surface plasmon resonance, and the bindings were compared. The resonance unit of each lectin was corrected by subtracting that of the reference cell on which the Fmoc-Thr-core 1 or Fmoc-Asn was immobilized as a ligand. The carbohydrate specificities of interactions were verified by preincubating lectins with their respective inhibitory sugar before injection. By steady state analysis, the Lycopersicon esculentum lectin showed a 27-fold higher affinity to linear diLacNAc than to biantennary diLacNAc, while Datura stramonium and Solanum tuberosum lectins both showed low Ka,apps of 10(6)M(-1) for these two ligands. In contrast, Ricinus communis agglutinin-120 showed a 3.2-fold higher Ka,app to biantennary LacNAc than to linear diLacNAc. A lectin purified from Pleurocybella porrigens mushroom interacted at the high affinity of 10(8)M(-1) with both linear and biantennary diLacNAcs, which identified it as a unique probe. This method provides a useful and sensitive system to analyze interactions by simulating the glycans on the cell surface.

Discovery of the natural anti-Gal antibody and its past and future relevance to medicine

This is a personal account of the discovery of the natural anti-Gal antibody, the most abundant natural antibody in humans, the reciprocal distribution of this antibody and its ligand the α-gal epitope in mammals and the immunological barrier this antibody has formed in porcine to human xenotransplantation. This barrier has been overcome in the recent decade with the generation of α1,3-galactosyltransferase gene-knockout pigs. However, anti-Gal continues to be relevant in medicine as it can be harnessed for various therapeutic effects. Anti-Gal converts tumor lesions injected with α-gal glycolipids into vaccines that elicit a protective anti-tumor immune response by in situ targeting of tumor cells for uptake by antigen-presenting cells. This antibody further accelerates wound and burn healing by interaction with α-gal nanoparticles applied to injured areas and induction of rapid recruitment and activation of macrophages. Anti-Gal/α-gal nanoparticle immune complexes may further induce rapid recruitment and activation of macrophages in ischemic myocardium and injured nerves, thereby inducing tissue regeneration and prevention of fibrosis.

Using lectins to harvest the plasma/serum glycoproteome

Aberrant protein glycosylation has been shown to be associated with disease processes and identification of disease-specific glycoproteins and glycosylation changes may serve as potential diagnostic and therapeutic biomarkers. However despite recent advances in proteomic-based biomarker discovery, this knowledge has not yet translated into an extensive mining of the glycoproteome for potential biomarkers. The major challenge for a comprehensive glycoproteomics analysis arises primarily from the enormous complexity and the large dynamic range in protein constituent in biological samples. Methods that specifically target glycoproteins are therefore necessary to facilitate their selective enrichment prior to their identification by MS-based analysis. The use of lectins, with selective affinities for specific carbohydrate epitopes, to enrich glycoprotein fractions coupled with modern MS, have greatly enhanced the identification of the glycoproteome. On account of their ability to specifically bind cell surface carbohydrates lectins have, during the recent past, found extensive applications in elucidation of the architecture and dynamics of cell surface carbohydrates, glycoconjugate purification, and structural characterization. Combined with complementary depletion and MS technologies, lectin affinity chromatography is becoming the most widely employed method of choice for biomarker discovery in cancer and other diseases.

Helicobacter pylori β1,3-N-acetylglucosaminyltransferase for versatile synthesis of type 1 and type 2 poly-LacNAcs on N-linked, O-linked and I-antigen glycans

Poly-N-acetyllactosamine extensions on N- and O-linked glycans are increasingly recognized as biologically important structural features, but access to these structures has not been widely available. Here, we report a detailed substrate specificity and catalytic efficiency of the bacterial β3-N-acetylglucosaminyltransferase (β3GlcNAcT) from Helicobacter pylori that can be adapted to the synthesis of a rich diversity of glycans with poly-LacNAc extensions. This glycosyltransferase has surprisingly broad acceptor specificity toward type-1, -2, -3 and -4 galactoside motifs on both linear and branched glycans, found commonly on N-linked, O-linked and I-antigen glycans. This finding enables the production of complex ligands for glycan-binding studies. Although the enzyme shows preferential activity for type 2 (Galβ1-4GlcNAc) acceptors, it is capable of transferring N-acetylglucosamine (GlcNAc) in β1-3 linkage to type-1 (Galβ1-3GlcNAc) or type-3/4 (Galβ1-3GalNAcα/β) sequences. Thus, by alternating the use of the H. pylori β3GlcNAcT with galactosyltransferases that make the β1-4 or β1-3 linkages, various N-linked, O-linked and I-antigen acceptors could be elongated with type-2 and type-1 LacNAc repeats. Finally, one-pot incubation of di-LacNAc biantennary N-glycopeptide with the β3GlcNAcT and GalT-1 in the presence of uridine diphosphate (UDP)-GlcNAc and UDP-Gal, yielded products with 15 additional LacNAc units on the precursor, which was seen as a series of sequential ion peaks representing alternative additions of GlcNAc and Gal residues, on matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) analysis. Overall, our data demonstrate a broader substrate specificity for the H. pylori β3GlcNAcT than previously recognized and demonstrate its ability as a potent resource for preparative chemo-enzymatic synthesis of complex glycans.

Alteration of immune responses by N-acetylglucosaminyltransferase V during allergic airway inflammation

BACKGROUND

β-1,6-N-acetylglucosaminyltransferase V (Mgat5 or GlcNac-TV), which is involved in the glycosylation of proteins, is known to be important for down-regulation of TCR-mediated T-cell activation and negatively regulates induction of contact dermatitis and experimental autoimmune encephalomyelitis. However, the role of Mgat5 in the induction of allergic airway inflammation remains unclear.

METHODS

To elucidate the role of Mgat5 in the pathogenesis of allergic airway inflammation, ovalbumin (OVA)-induced airway inflammation was induced in Mgat5-deficient mice. The OVA-specific lymphocyte proliferation and cytokine production levels, OVA-specific IgG1, IgG2a and IgE levels in the serum, and the number of leukocytes and cytokine levels in the bronchoalveolar lavage (BAL) fluid were compared between wild-type and Mgat5-deficient mice.

RESULTS

OVA-specific lymphocyte proliferation and production of IFN-γ and IL-10, but not IL-4, were increased in Mgat5-deficient mice, suggesting that Th2-type immune responses are seemed to be suppressed by increased IFN-γ and IL-10 production in these mice. However, Th2-type responses such as OVA-specific IgG1, but not IgE, and IL-5 levels in BAL fluids were increased in Mgat5-deficient mice. Meanwhile, the number of eosinophils was normal, but the numbers of neutrophils, macrophages and lymphocytes were reduced, in these mutant mice during OVA-induced airway inflammation.

CONCLUSIONS

Mgat5-dependent glycosylation of proteins can modulate acquired immune responses, but it is not essential for the development of OVA-induced eosinophilic airway inflammation.

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.

Generation of antibodies recognizing an aberrant glycoform of human tissue inhibitor of metalloproteinase-1 (TIMP-1) using decoy immunization and phage display

Aberrant glycosylation of human tissue inhibitor of metalloproteinase-1 (TIMP-1) by N-acetylglucosaminyltransferase-V (GnT-V) was previously reported to be related to cancer progression. Here, we report on the antibodies recognizing the structural features initiated by an addition of N-linked β(1,6)-N-acetylglucosamine (GlcNAc) branch by GnT-V on TIMP-1. Two glycoforms of TIMP-1, TIMP1-L produced in Lec4 cells without GnT-V activity and TIMP1-B in GnT-V overexpressing transfectant cells, were purified from culture supernatant and used for generation of antibodies. TIMP1-L was injected in the left hind footpad of mice as decoy and TIMP1-B in the right hind footpad as immunogen. Phage-displayed scFv library was constructed from the B cells retrieved from the right popliteal lymph nodes and subjected to panning and screening. Phage ELISA of individual clones revealed the scFv clones with preferential binding activity to TIMP1-B, and they were converted into an scFv-Fc format for further characterization of binding specificity. Glycan binding assay of an antibody, 1-9F, revealed its differential specificity toward an extension of glycan structure initiated with β(1,6)-GlcNAc linkage and terminally decorated with a sialic acid. This study demonstrates feasibility of a new strategy combining decoy immunization with phage display for the efficient generation of antibodies tracking down structural features of different glycoforms.

Manipulating cell surface glycoproteins by targeting N-glycan-galectin interactions

The interaction of cell surface receptors and transporters with cognate ligands depends on their concentration, distribution, and organization at the cellular surface. The majority of cell surface receptors and transporters are co- and/or post-translationally modified with asparagine (N)-linked oligosaccharides (glycans). N-Glycan number and structure combine to control the concentration of glycoproteins at the cell surface through interactions with endogenous lectins such as galectins. ER/Golgi enzyme activity and hexosamine pathway supply of Golgi metabolites co-dependently regulate N-glycan biosynthesis and combine to provide adaptive control over cell growth and differentiation. Studies in mice and humans have revealed metabolic and genetic dysregulation of N-glycosylation in T-cell-mediated autoimmunity. In this chapter, we describe methods used to analyze N-glycan-galectin interactions in controlling the distribution and organization of cell surface receptors and transporters.

T cell receptor signaling co-regulates multiple Golgi genes to enhance N-glycan branching

T cell receptor (TCR) signaling enhances beta1,6GlcNAc-branching in N-glycans, a phenotype that promotes growth arrest and inhibits autoimmunity by increasing surface retention of cytotoxic T lymphocyte antigen-4 (CTLA-4) via interactions with galectins. N-Acetylglucosaminyltransferase V (MGAT5) mediates beta1,6GlcNAc-branching by transferring N-acetylglucosamine (GlcNAc) from UDP-GlcNAc to N-glycan substrates produced by the sequential action of Golgi alpha1,2-mannosidase I (MIa,b,c), MGAT1, alpha1,2-mannosidase II (MII, IIx), and MGAT2. Here we report that TCR signaling enhances mRNA levels of MIa,b,c and MII,IIx in parallel with MGAT5, whereas limiting levels of MGAT1 and MGAT2. Blocking the increase in MI or MII enzyme activity induced by TCR signaling with deoxymannojirimycin or swainsonine, respectively, limits beta1,6GlcNAc-branching, suggesting that enhanced MI and MII activity are both required for this phenotype. MGAT1 and MGAT2 have an approximately 250- and approximately 20-fold higher affinity for UDP-GlcNAc than MGAT5, respectively, and increasing MGAT1 expression paradoxically inhibits beta1,6GlcNAc branching by limiting UDP-GlcNAc supply to MGAT5, suggesting that restricted changes in MGAT1 and MGAT2 mRNA levels in TCR-stimulated cells serves to enhance availability of UDP-GlcNAc to MGAT5. Together, these data suggest that TCR signaling differentially regulates multiple N-glycan-processing enzymes at the mRNA level to cooperatively promote beta1,6GlcNAc branching, and by extension, CTLA-4 surface expression, T cell growth arrest, and self-tolerance.

T-cell growth, cell surface organization, and the galectin-glycoprotein lattice

Basal, activation, and arrest signaling in T cells determines survival, coordinates responses to pathogens, and, when dysregulated, leads to loss of self-tolerance and autoimmunity. At the T-cell surface, transmembrane glycoproteins interact with galectins via their N-glycans, forming a molecular lattice that regulates membrane localization, clustering, and endocytosis of surface receptors. Galectin-T-cell receptor (TCR) binding prevents ligand-independent TCR signaling via Lck by blocking spontaneous clustering and CD4-Lck recruitment to TCR, and in turn F-actin transfer of TCR/CD4-Lck complexes to membrane microdomains. Peptide-major histocompatibility complexes overcome galectin-TCR binding to promote TCR clustering and signaling by Lck at the immune synapse. Galectin also localizes the tyrosine phosphatase CD45 to microdomains and the immune synapse, suppressing basal and activation signaling by Lck. Following activation, membrane turnover increases and galectin binding to cytotoxic T-lymphocyte antigen-4 (CTLA-4) enhances surface expression by inhibiting endocytosis, thereby promoting growth arrest. Galectins bind surface glycoproteins in proportion to the branching and number of N-glycans per protein, the latter an encoded feature of protein sequence. N-glycan branching is conditional to the activity of Golgi N-acetylglucosaminyl transferases I, II, IV and V (Mgat1, 2, 4, and 5) and metabolic supply of their donor substrate UDP-GlcNAc. Genetic and metabolic control of N-glycan branching co-regulate homeostatic set-points for basal, activation, and arrest signaling in T cells and, when disturbed, result in T-cell hyperactivity and autoimmunity.

DSA affinity glycoproteome of human liver tissue

Due to the critical roles of glycoproteins in the activities of cells to tissues, mapping of liver glycoproteome may provide valuable basic information for finding disease marker proteins. In this study, Datura Stramonium Agglutinin (DSA) was chosen to enrich N-linked glycoproteins for its broader specificity with tri- or tetra-antennary complex type. DSA affinity glycoproteins' profiles of human liver tissue were generated by two-dimensional electrophoresis (2-DE) followed by glycoprotein staining based on multiplexed proteomics (MP) technology. 64+/-3 (n=3) protein spots were detected and 41 of glycoproteins were identified via peptide mass fingerprinting (PMF) using MALDI-TOF-MS/MS and annotated to IPI databases. Identified glycoproteins definitely take part in the regulation of cell cycle and metabolic processes. The detailed carbohydrate moiety of some glycoproteins might be concluded according to the literatures. The construction of DSA affinity glycoprotein database would contribute to the subsequent research.

Integrated analytical strategies for the study of phosphorylation and glycosylation in proteins

The post-translational modification (PTM) of proteins is a common biological mechanism for regulating protein localization, function, and turnover. The direct analysis of modifications is required because they are not coded by genes, and thus are not predictable. Different MS-based proteomic strategies are used for the analysis of PTMs, such as phosphorylation and glycosylation, and are composed of a structural simplification step of the protein followed by specific isolation step to extract the classes of modified peptides (also called "sub-proteomes") before mass spectrometry. This specific isolation step is necessary because PTMs occur at a sub-stoichiometric level and signal suppression of the modified fractions in the mass spectrometer occurs in the presence of the more-abundant non-modified counterpart. The request of innovative analytical strategies in PTM studies is the capability to localize the modification sites, give detailed structural information on the modification, and determine the isoform composition with increased selectivity, sensitivity, and throughput. This review focuses on the description of recent integrated analytical systems proposed for the analysis of PTMs in proteins, and their application to profile the glycoproteome and the phosphoproteome in biological samples. Comments on the difficulties and usefulness of the analytical strategies are given.

Analysis of glycopeptides using lectin affinity chromatography with MALDI-TOF mass spectrometry

Glycopeptides prepared from 1 nmol of a mixture of glycoproteins, transferrin, and ribonuclease B by lysylendopeptidase digestion were isolated by lectin and cellulose column chromatographies, and then they were analyzed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry and MALDI-quadrupole ion trap (QIT)-TOF mass spectrometry which enables the performance of MS ( n ) analysis. The lectin affinity preparation of glycopeptides with Sambucus nigra agglutinin and concanavalin A provides the glycan structure outlines for the sialyl linkage and the core structure of N-glycans. Such structural estimation was confirmed by MALDI-TOF MS and MALDI-QIT-TOF MS/MS. Amino acid sequences and location of glycosylation sites were determined by MALDI-QIT-TOF MS/MS/MS. Taken together, the combination of lectin column chromatography, MALDI-TOF MS, and MALDI-QIT-TOF MS ( n ) provides an easy way for the structural estimation of glycans and the rapid analysis of glycoproteomics.

The Galalpha1,3Galbeta1,4GlcNAc-R (alpha-Gal) epitope: a carbohydrate of unique evolution and clinical relevance

In 1985, we reported that a naturally occurring human antibody (anti-Gal), produced as the most abundant antibody (1% of immunoglobulins) throughout the life of all individuals, recognizes a carbohydrate epitope Galalpha1-3Galbeta1-4GlcNAc-R (the alpha-gal epitope). Since that time, an extensive literature has developed on discoveries related to the alpha-gal epitope and the anti-Gal antibody, including the barrier they form in xenotransplantation and their reciprocity in mammalian evolution. This review covers these topics and new avenues of clinical importance related to this unique antigen/antibody system (alpha-gal epitope/anti-Gal) in improving the efficacy of viral vaccines and in immunotherapy against cancer.

Polylactosamine on glycoproteins influences basal levels of lymphocyte and macrophage activation

beta1,3-N-acetylglucosaminyltransferase 2 (beta3GnT2) is a polylactosamine synthase that synthesizes a backbone structure of carbohydrate structures onto glycoproteins. Here we generated beta3GnT2-deficient (beta3GnT2(-/-)) mice and showed that polylactosamine on N-glycans was markedly reduced in their immunological tissues. In WT mice, polylactosamine was present on CD28 and CD19, both known immune costimulatory molecules. However, polylactosamine levels on these molecules were reduced in beta3GnT2(-/-) mice. beta3GnT2(-/-) T cells lacking polylactosamine were more sensitive to the induction of intracellular calcium flux on stimulation with anti-CD3epsilon/CD28 and proliferated more strongly than T cells from WT mice. beta3GnT2(-/-) B cells also showed hyperproliferation on BCR stimulation. Macrophages from beta3GnT2(-/-) mice had higher cell surface CD14 levels and enhanced responses to endotoxin. These results indicate that polylactosamine on N-glycans is a putative immune regulatory factor presumably suppressing excessive responses during immune reactions.

The glycosylation of thymic microenvironments

The thymus is the principal organ for development of T-cells. Thymocyte precursors from bone marrow-derived progenitor cells enter the thymus where they differentiate involving several differentiation stages into mature T-cells that can leave the thymus to the periphery. Migration of thymocytes through the thymus and their development are tightly controlled by the interaction of thymocytes with components of the thymic microenvironments. Several studies have demonstrated the pivotal importance of glycosylation in cell-cell interactions or interactions of cells with extracellular matrix components (ECM) in various physiologic processes in the body. The knowledge on glycosylation of thymic microenvironments is however limited although the presence of C-type lectin receptors such as DC-SIGN, mannose receptor and DEC-205, which are specifically recognizing distinct carbohydrate moieties emphasize the importance of glycosylation in the thymus. In order to outline the distribution of glycoconjugates in microenvironments of the human thymus we studied the glycosylation of the human thymic microarchitecture by using plant lectins in situ. Eleven plant lectin-biotin conjugates with distinct specificity were used and analyzed by fluorescence microscopy. Mannose glycoconjugates, specifically detected by the lectins GNA and NPA, were abundant in the cortex but not in the medulla. Dendritic cells present in the thymic cortex were specifically co-stained with the galactose-specific lectins DSA and PNA. Several lectins bound to the thymic vasculature. The alpha2-fucose-specific lectin UEA stained thymic blood vessels in the interlobular space and medulla and capillaries in the cortex. In addition to UEA, thymic blood vessels and capillaries also reacted with the lectins DSA, PNA and the alpha-GalNac-specific lectin HPA. In contrast, lymph vessels present in the interlobular space do not interact with UEA, DSA and PNA, but only with HPA, revealing a disparate glycosylation pattern of lymph and blood vessels that may be important to determine the direction of thymocytes entering or leaving the thymus. In conclusion, the restricted expression patterns of carbohydrates defined microenvironments in the human thymus highlight the importance of glycosylation in various steps of T-cell development.

High-Throughput Carbohydrate Microarray Technology

One of our long-term interests is to explore the immunogenic sugar moieties that are important for “self-” and “nonself” discrimination and host immune responses. We have established a highthroughput platform of carbohydrate microarrays to facilitate these investigations. Using this technology, carbohydrate-containing macromolecules of distinct structural configurations, including polysaccharides, natural glycoconjugates, and mono- and oligosaccharides coupled to lipid, polyacrylamide, and protein carriers, have been tested for microarray construction without further chemical modification. Here, we discuss issues related to the establishment of this technology and areas that are highly promising for its application. We also provide an example to illustrate that the carbohydrate microarray is a discovery tool; it is particularly useful for identifying immunological sugar moieties, including differentially expressed complex carbohydrates of cancer cells and stem cells as well as sugar signatures of previously unrecognized microbial pathogens.

Carbohydrate Microarrays for Lectin Characterization and Glyco-Epitope Identification

Lectins can also be classified according to their binding specificity or selectivity with carbohydrates. This type of classification is helpful for selection of lectins as structural probes in biomedical applications. This chapter summarizes the concept and the updated information regarding the specificity-based lectin classification. Lectins are functionally classified based on their relative binding reactivities with the structural units of carbohydrate or glyco-epitopes. They are grouped according to their monosaccharide specificities and then further sub-grouped based on their reactivities with more complex structures. Carbohydrate specificities of biomedically important lectins are classified into six groups according to their specificities to monosaccharides. The chapter introduces a practical platform of carbohydrate microarrays that is useful for lectin characterization and classification. Finally, the chapter discusses a few examples to illustrate the application of this technology in lectin-related experimental investigations.

Global methods for protein glycosylation analysis by mass spectrometry

Mass spectrometry has been an analytical tool of choice for glycosylation analysis of individual proteins. Over the last 5 years several previously and newly developed mass spectrometry methods have been extended to global glycoprotein studies. In this review we discuss the importance of these global studies and the advances that have been made in enrichment analyses and fragmentation methods. We also briefly describe relevant sample preparation methods that have been used for the analysis of a single glycoprotein that could be extrapolated to global studies. Finally this review covers aspects of improvements and advances on the instrument front which are important to future global glycoproteomic studies.

Primary amine coding as a path to comparative proteomics

Various isotope coding strategies are being used today in the field of comparative proteomics. This article specifically reviews the strengths and limitations of various N-termini-directing strategies. N-termini-directed coding strategy allows for use of different chromatographic enrichment techniques. Since N-termini-directed coding strategies are global in nature, they can be utilized in studying PTMs as well as protein expression. Using different N-termini-directed coding strategies, both relative and absolute quantification of proteins can be achieved either in the MS mode or in the MS/MS mode. The review ends with the conclusion that significant improvements have been made in the last decade. Among various issues, a need still exists for a better understanding of the kinetic issues in proteomics, relative protein pool sizes for different proteins and the issue of stimulus-induced changes in protein aggregation. Another critical issue that needs to be addressed in great detail is the role of PTMs in regulation.

One-step biotinylation procedure for carbohydrates to study carbohydrate–protein interactions

Protein-carbohydrate interactions play crucial roles in numerous biological processes. To study these interactions, we developed a simple and fast procedure for the biotinylation of carbohydrates based on reductive amination. The method allows complete and stable biotinylation of small quantities of oligosaccharides and includes a rapid and simple procedure to remove excess labeling reagent. After biotinylation, the structural and biological integrity of the glycans was intact as determined by HPLC, mass spectrometry, and a plant lectin assay. By using the human C-type lectin DC-SIGN (dendritic cell-specific ICAM-3-grabbing nonintegrin), we demonstrate that the biotinylated glycans can be used in a glycan array to determine binding specificities of lectins. Moreover, we show that fluorescent beads coated with selected biotinylated glycans bind to DC-SIGN-expressing dendritic cells in vitro. Finally, by using biotinylated high-mannose N-glycans, we could visualize DC-SIGN-expressing cells in lymph node tissue. The availability of easy biotinylation methods for oligosaccharides such as those described here greatly facilitates the functional analysis of lectins. In addition, the biotinylated glycans will be great tools for investigating functional lectin receptors in situ.

Demonstration of the expression and the enzymatic activity of N-acetylglucosaminyltransferase IX in the mouse brain

We recently reported on a brain-specific beta1,6-N-acetylglucosaminyltransferase IX (GnT-IX, also referred to as GnT-VB), a GnT-V homologue, which acts on alpha-linked mannose of N-glycans and O-mannosyl glycans. To distinguish functions of GnT-IX with GnT-V, we examined the distribution of GnT-IX and GnT-V transcripts in mouse tissues by Northern blot analysis. The two enzymes were differentially expressed as has previously been observed in human tissues. GnT-IX transcripts were restricted to the cerebrum, cerebellum, thymus and testis, whereas GnT-V transcripts were expressed ubiquitously in mouse tissues. To investigate the localization of these enzymes in mouse tissues in more detail, a polyclonal antibody against GnT-IX was prepared. The antibody specifically recognized GnT-IX, but not GnT-V, in the Golgi apparatus, as confirmed by the use of GnT-IX and GnT-V transfectants. In agreement with the Northern blot analysis data, an immunohistochemical study showed substantial expression of GnT-IX in the brain, while no expression was observed in the liver. Moreover, to exclude GnT-V contamination, we performed an enzymatic assay for GnT-IX using a Mgat5 (GnT-V)-null mouse brain as an enzyme source and found the enzymatic activities do, in fact, exist in mouse brain. The reaction product was confirmed by high performance liquid chromatography and mass spectrometry. These results demonstrate, for the first time, that GnT-IX protein is actually expressed and may function as a glycosyltransferase in the brain.

A practical protocol for carbohydrate microarrays

We have established a high-throughput biochip platform for constructing carbohydrate microarrays. Using this technology, carbohydrate-containing macromolecules of diverse structures, including polysaccharides, natural glycoconjugates, and mono- and oligosaccharides coupled to carrier molecules, can be stably immobilized on a glass chip without chemical modification. Here, we describe a practical protocol for this technology. We hope that anyone who has access to a standard cDNA microarray facility will be able to explore this technology for his or her own research interest. We also provide an example to illustrate that the carbohydrate microarray is also a discovery tool; this is particularly useful for identifying immunologic sugar moieties, including complex carbohydrates of cancer cells and sugar signatures of previously unrecognized microbial pathogens.

The alpha-gal epitope and the anti-Gal antibody in xenotransplantation and in cancer immunotherapy

The alpha-gal epitope (Galalpha1-3Galbeta1-(3)4GlcNAc-R) is abundantly synthesized on glycolipids and glycoproteins of non-primate mammals and New World monkeys by the glycosylation enzyme alpha1,3galactosyltransferase (alpha1,3GT). In humans, apes and Old World monkeys, this epitope is absent because the alpha1,3GT gene was inactivated in ancestral Old World primates. Instead, humans, apes and Old World monkeys produce the anti-Gal antibody, which specifically interacts with alpha-gal epitopes and which constitutes approximately 1% of circulating immunoglobulins. Anti-Gal has functioned as an immunological barrier, preventing the transplantation of pig organs into humans, because anti-Gal binds to the alpha-gal epitopes expressed on pig cells. The recent generation of alpha1,3GT knockout pigs that lack alpha-gal epitopes has resulted in the elimination of this immunological barrier. Anti-Gal can be exploited for clinical use in cancer immunotherapy by targeting autologous tumour vaccines to APC, thereby increasing their immunogenicity. Autologous intact tumour cells from haematological malignancies, or autologous tumour cell membranes from solid tumours are processed to express alpha-gal epitopes by incubation with neuraminidase, recombinant alpha1,3GT and with uridine diphosphate galactose. Subsequent immunization with such autologous tumour vaccines results in in vivo opsonization by anti-Gal IgG binding to these alpha-gal epitopes. The interaction of the Fc portion of the vaccine-bound anti-Gal with Fcgamma receptors of APC induces effective uptake of the vaccinating tumour cell membranes by the APC, followed by effective transport of the vaccinating tumour membranes to the regional lymph nodes, and processing and presentation of the tumour-associated antigen (TAA) peptides. Activation of tumour-specific T cells within the lymph nodes by autologous TAA peptides may elicit an immune response that in some patients will be potent enough to eradicate the residual tumour cells that remain after completion of standard therapy. A similar expression of alpha-gal epitopes can be achieved by transduction of tumour cells with an adenovirus vector (or other vectors) containing the alpha1,3GT gene, thus enabling anti-Gal-mediated targeting of the vaccinating transduced cells to APC. Intratumoral delivery of the alpha1,3GT gene by various vectors results in the expression of alpha-gal epitopes. Such expression of the xenograft carbohydrate phenotype is likely to induce anti-Gal-mediated destruction of the tumour lesion, similar to rejection of xenografts by this antibody. Opsonization of the destroyed tumour cell membranes by anti-Gal IgG further targets them to APC, thus converting the tumour lesion, treated by the alpha1,3GT gene, into an in situ autologous tumour vaccine.

Measurement of the carbohydrate-binding specificity of lectins by a multiplexed bead-based flow cytometric assay

Carbohydrate binding underlies many cell recognition events. Here, we describe a multiplexed glyco-bead array method for determining the carbohydrate-binding specificities of plant lectins using a bead-based flow cytometric analysis. N-glycans including high mannose, hybrid, and complex types and O-glycans from glycoproteins were immobilized on multiplexed beads, and the specificities of 13 kinds of sugar chains were monitored within 2 h in a single reaction. This strategy is easy, rapid, reproducible, and suitable for small samples and allows the reliable and simultaneous elucidation of sugar-binding properties under identical conditions.

Use of Multidimensional Lectin Affinity Chromatography in Differential Glycoproteomics

This paper reports studies comparing the relative degree of sialylation among human serum glycoproteins carrying complex biantennary N-linked, hybrid, and high-mannose oligosaccharides. Comparisons were made by coupling lectin affinity selection with stable isotope coding of peptides from tryptic digests of serum. After proteolysis, samples were split and differentially acetylated with stable isotope coding agents according to either origin or the separation method by which they would be fractionated. A lectin column prepared from Sambucus nigra agglutinin (SNA) was used to select and compare the concentration of sialic acid containing glycopeptides. The relative standard deviation in quantification using this method was 4%. Using this method the concentration of sialic acid containing glycoproteins from a normal individual were compared to those in a pooled serum sample from a large number of normal individuals. It was found that sialylation varied less than 2-fold in all but four or five glycoproteins. Further studies were done on the degree of sialylation within glycoproteins. Samples labeled with the light isoform of the coding agent were applied to a set of serial lectin columns consisting of a concanavalin A (Con A) column coupled to an SNA column for selecting sialic acid appended to glycopeptides with complex biantennary N-linked, hybrid, and high-mannose glycans. In contrast, samples labeled with the heavy isoform of the coding agent were applied to a Con A lectin column alone to select glycopeptides containing complex biantennary N-linked, hybrid, and high-mannose glycans, without regard to sialylation. Glycopeptides thus selected were mixed, deglycosylated by PNGase F, and fractionated by reversed-phase chromatography (RPC). The RPC fractions were then analyzed by ESI-MS. The relative standard deviation of the method was 4%. All glycopeptides identified contained sialic acid except one. Peptides in which the relative abundance of isotopic isoforms was equal were considered to indicate that the protein parent was fully sialylated at that specific glycosylation site.

N-acetylglucosaminyltransferase V (Mgat5)-mediated N-glycosylation negatively regulates Th1 cytokine production by T cells

The differentiation of naive CD4(+) T cells into either proinflammatory Th1 or proallergic Th2 cells strongly influences autoimmunity, allergy, and tumor immune surveillance. We previously demonstrated that beta1,6GlcNAc-branched complex-type (N-acetylglucosaminyltransferase V (Mgat5)) N-glycans on TCR are bound to galectins, an interaction that reduces TCR signaling by opposing agonist-induced TCR clustering at the immune synapse. Mgat5(-/-) mice display late-onset spontaneous autoimmune disease and enhanced resistance to tumor progression and metastasis. In this study we examined the role of beta1,6GlcNAc N-glycan expression in Th1/Th2 cytokine production and differentiation. beta1,6GlcNAc N-glycan expression is enhanced by TCR stimulation independent of cell division and declines at the end of the stimulation cycle. Anti-CD3-activated splenocytes and naive T cells from Mgat5(-/-) mice produce more IFN-gamma and less IL-4 compared with wild-type cells, the latter resulting in the loss of IL-4-dependent down-regulation of IL-4Ralpha. Swainsonine, an inhibitor of Golgi alpha-mannosidase II, blocked beta1,6GlcNAc N-glycan expression and caused a similar increase in IFN-gamma production by T cells from humans and mice, but no additional enhancement in Mgat5(-/-) T cells. Mgat5 deficiency did not alter IFN-gamma/IL-4 production by polarized Th1 cells, but caused an approximately 10-fold increase in IFN-gamma production by polarized Th2 cells. These data indicate that negative regulation of TCR signaling by beta1,6GlcNAc N-glycans promotes development of Th2 over Th1 responses, enhances polarization of Th2 cells, and suggests a mechanism for the increased autoimmune disease susceptibility observed in Mgat5(-/-) mice.

LacdiNAc-Glycans Constitute a Parasite Pattern for Galectin-3-Mediated Immune Recognition

Although Gal beta 1-4GlcNAc (LacNAc) moieties are the most common constituents of N-linked glycans on vertebrate proteins, GalNAc beta 1-4GlcNAc (LacdiNAc, LDN)-containing glycans are widespread in invertebrates, such as helminths. We postulated that LDN might be a molecular pattern for recognition of helminth parasites by the immune system. Using LDN-based affinity chromatography and mass spectrometry, we have identified galectin-3 as the major LDN-binding protein in macrophages. By contrast, LDN binding was not observed with galectin-1. Surface plasmon resonance (SPR) analysis and a solid phase binding assay demonstrated that galectin-3 binds directly to neoglycoconjugates carrying LDN glycans. In addition, galectin-3 bound to Schistosoma mansoni soluble egg Ags and a mAb against the LDN glycan inhibited this binding, suggesting that LDN glycans within S. mansoni soluble egg Ags contribute to galectin-3 binding. Immunocytochemistry demonstrated high levels of galectin-3 in liver granulomas of S. mansoni-infected hamsters, and a colocalization of galectin-3 and LDN glycans was observed on the parasite eggshells. Finally, we demonstrate that galectin-3 can mediate recognition and phagocytosis of LDN-coated particles by macrophages. These findings provide evidence that LDN-glycans constitute a parasite pattern for galectin-3-mediated immune recognition.

Glycan arrays lead to the discovery of autoimmunogenic activity of SARS-CoV

Using carbohydrate microarrays, we characterized the carbohydrate binding activity of SARS-CoV neutralizing antibodies elicited by an inactivated SARS-CoV vaccine. In these antibodies, we detected undesired autoantibody reactivity specific for the carbohydrate moieties of an abundant human serum glycoprotein asialo-orosomucoid (ASOR). This observation provides important clues for the selection of specific immunologic probes to examine whether SARS-CoV expresses antigenic structures that mimic the host glycan. We found that lectin PHA-L (Phaseolus vulgaris L.), which is specific for a defined complex carbohydrate of ASOR, stained the SARS-CoV-infected cells specifically and intensively. Taken together, we present immunologic evidence that a carbohydrate structure of SARS-CoV shares antigenic similarity with host glycan complex carbohydrates. The experimental approaches we applied in this study are likely applicable for the identification of immunologic targets of other viral pathogens.

Molecular cloning and characterization of human GnT-IX, a novel beta1,6-N-acetylglucosaminyltransferase that is specifically expressed in the brain

A novel beta1,6-N-acetylglucosaminyltransferase (beta1, 6GnT) cDNA was identified by a BLAST search using the amino acid sequence of human GnT-V as a query. The full-length sequence was determined by a combination of 5'-rapid amplification of cDNA end analysis and a further data base search. The open reading frame encodes a 792 amino acid protein with a type II membrane protein structure typical of glycosyltransferases. The entire sequence identity to human GnT-V is 42%. When pyridylaminated (PA) agalacto biantennary N-linked oligosaccharide was used as an acceptor substrate, the recombinant enzyme generated a novel product other than the expected GnT-V product, (GlcNAcbeta1,2-Manalpha1,3-)[GlcNAcbeta1,2-(GlcNAcbeta1,6-)Manalpha1,6-]Manbeta1,4-GlcNAcbeta1,4-GlcNAc-PA. This new product was identified as [GlcNAcbeta1,2-(GlcNAcbeta1,6-)Manalpha1,3-][Glc-NAcbeta1,2-(GlcNAcbeta1,6-)Manalpha1,6-]Manbeta1,4-GlcNAcbeta1,4-GlcNAc-PA by mass spectrometry and 1H NMR. Namely, the new GnT (designated as GnT-IX) has beta1,6GnT activity not only to the alpha1,6-linked mannose arm but also to the alpha1,3-linked mannose arm of N-glycan, forming a unique structure that has not been reported to date. Northern blot analysis showed that the GnT-IX gene is exclusively expressed in the brain, whereas the GnT-V gene is expressed ubiquitously. These results suggest that GnT-IX is responsible for the synthesis of a unique oligosaccharide structure in the brain.

Ah, sweet mystery of death! Galectins and control of cell fate

Control of cell death is critical in eukaryotic development, immune system homeostasis, and control of tumorigenesis. The galectin family of lectins is implicated in all of these processes. Other families of molecules function as death receptors or death effectors, but galectins are uniquely capable of acting both extracellularly and intracellularly to control cell death. Extracellularly, galectins cross-link glycan ligands to transduce signals that lead directly to death or that influence other signals regulating cell fate. Intracellular expression of galectins can modulate other signals controlling cell viability. Individual galectins can act on multiple cell types, and multiple galectins can act on the same cell. Understanding how galectins regulate cell viability and function will broaden our knowledge of the roles of galectins in basic biological processes and facilitate development of therapeutic applications for galectins in autoimmunity, transplant-related disease, and cancer.

UDP-N-acetylglucosamine:alpha-6-D-mannoside beta1,6 N-acetylglucosaminyltransferase V (Mgat5) deficient mice

Targeted gene mutations in mice that cause deficiencies in protein glycosylation have revealed functions for specific glycans structures in embryogenesis, immune cell regulation, fertility and cancer progression. UDP-N-acetylglucosamine:alpha-6-D-mannoside beta1,6 N-acetylglucosaminyltransferase V (GlcNAc-TV or Mgat5) produces N-glycan intermediates that are elongated with poly N-acetyllactosamine to create ligands for the galectin family of mammalian lectins. We generated Mgat5-deficient mice by gene targeting methods in embryonic stem cells, and observed a complex phenotype in adult mice including susceptibility to autoimmune disease, reduced cancer progression and a behavioral defect. We found that Mgat5-modified N-glycans on the T cell receptor (TCR) complex bind to galectin-3, sequestering TCR within a multivalent galectin-glycoprotein lattice that impedes antigen-dependent receptor clustering and signal transduction. Integrin receptor clustering and cell motility are also sensitive to changes in Mgat5-dependent N-glycosylation. These studies demonstrate that low affinity but high avidity interactions between N-glycans and galectins can regulate the distribution of cell surface receptors and their responsiveness to agonists.