Carbohydrates as antigenic determinants of glycoproteins

An Introduction to the Relationship Between Lewis x and Malignancy Mainly Related to Breast Cancer and Head Neck Squamous Cell Carcinoma (HNSCC)

Lewis x functions as an adhesion molecule in glycolipids and glycoproteins since it mediates homophilic and heterophilic attachment of normal and tumoral cells. During malignancy, altered glycosylation is a frequent event; accumulating data support the expression of Lewis x in tumors although controversial results have been described including its relationship with patient survival. This report has been developed as an introduction to the relationship between Lewis x expression and breast cancer and head and neck squamous cell carcinoma (HNSCC). Results obtained in our laboratory are presented in the context of the literature.

Polymorphisms in Fc Gamma Receptors and Susceptibility to Malaria in an Endemic Population

Repeated infections by Plasmodium falciparum result in a humoral response that could reduce disease symptoms and prevent the development of clinical malaria. The principal mechanism underlying this humoral response is that immunoglobulin G (IgG) binds directly to the parasites, thus causing their neutralization. However, the action of antibodies alone is not always sufficient to eliminate pathogens from an organism. One key element involved in the recognition of IgG that plays a crucial role in the destruction of the parasites responsible for spreading malaria is the family of Fc gamma receptors. These receptors are expressed on the surface of immune cells. Several polymorphisms have been detected in the genes encoding these receptors, associated with susceptibility or resistance to malaria in different populations. In this review, we describe identified polymorphisms within the family of Fc gamma receptors and the impact of these variations on the response of a host to infection as well as provide new perspectives for the design of an effective vaccine for malaria.

Targeting Glycoproteins as a therapeutic strategy for diabetes mellitus and its complications

OBJECTIVES

Glycoproteins are organic compounds formed from proteins and carbohydrates, which are found in many parts of the living systems including the cell membranes. Furthermore, impaired metabolism of glycoprotein components plays the main role in the pathogenesis of diabetes mellitus. The aim of this study is to investigate the influence of glycoprotein levels in the treatment of diabetes mellitus.

METHODS

All relevant papers in the English language were compiled by searching electronic databases, including Scopus, PubMed and Cochrane library. The keywords of glycoprotein, diabetes mellitus, glycan, glycosylation, and inhibitor were searched until January 2019.

RESULTS

Glycoproteins are pivotal elements in the regulation of cell proliferation, growth, maturation and signaling pathways. Moreover, they are involved in drug binding, drug transportation, efflux of chemicals and stability of therapeutic proteins. These functions, structure, composition, linkages, biosynthesis, significance and biological effects are discussed as related to their use as a therapeutic strategy for the treatment of diabetes mellitus and its complications.

CONCLUSIONS

The findings revealed several chemical and natural compounds have significant beneficial effects on glycoprotein metabolism. The comprehension of glycoprotein structure and functions are very essential and inevitable to enhance the knowledge of glycoengineering for glycoprotein-based therapeutics as may be required for the treatment of diabetes mellitus and its associated complications. Graphical abstract.

A validated collection of mouse monoclonal antibodies to human glycosyltransferases functioning in mucin-type O-glycosylation

Complex carbohydrates serve a wide range of biological functions in cells and tissues, and their biosynthesis involves more than 200 distinct glycosyltransferases (GTfs) in human cells. The kinetic properties, cellular expression patterns and subcellular topology of the GTfs direct the glycosylation capacity of a cell. Most GTfs are ER or Golgi resident enzymes, and their specific subcellular localization is believed to be distributed in the secretory pathway according to their sequential role in the glycosylation process, although detailed knowledge for individual enzymes is still highly fragmented. Progress in quantitative transcriptome and proteome analyses has greatly advanced our understanding of the cellular expression of this class of enzymes, but availability of appropriate antibodies for in situ monitoring of expression and subcellular topology have generally been limited. We have previously used catalytically active GTfs produced as recombinant truncated secreted proteins in insect cells for generation of mouse monoclonal antibodies (mAbs) to human enzymes primarily involved in mucin-type O-glycosylation. These mAbs can be used to probe subcellular topology of active GTfs in cells and tissues as well as their presence in body fluids. Here, we present several new mAbs to human GTfs and provide a summary of our entire collection of mAbs, available to the community. Moreover, we present validation of specificity for many of our mAbs using human cell lines with CRISPR/Cas9 or zinc finger nuclease (ZFN) knockout and knockin of relevant GTfs.

Diastereoselective synthesis of furanose and pyranose substituted glycine and alanine derivatives via proline-catalyzed asymmetric α-amination of aldehydes

A concise organocatalytic route toward the synthesis of furanose and pyranose substituted glycine and alanine derivatives is reported. These compounds are core structural units of some of the naturally available antibiotics and antifungal agents. Proline-catalyzed asymmetric α-amination of aldehydes derived from sugars is used as the key reaction to synthesize twelve sugar amino acid derivatives. The asymmetric transformations proceeded in good yields and with good to excellent diastereoselectivity. The application of the synthesized amino acids is demonstrated by synthesizing a tripeptide containing one of them.

Glycobiology: progress, problems, and perspectives

This review highlights different aspects of glycobiology with analysis of recent progress in the study of biosynthesis, degradation, and biological role of glycoconjugates and of hereditary diseases related to the metabolism of these compounds. In addition, the review presents some analysis of the papers of other authors who have contributed to this special issue.

Generation of monoclonal antibodies to native active human glycosyltransferases

Complex carbohydrates serve a wide range of biological functions in cells and tissues. Their biosynthesis involves more than 200 distinct glycosyltransferases in human cells, and the expression, properties, and topology of these enzymes regulate the glycosylation patterns of proteins and lipids. Glycosyltransferases are ER-Golgi resident enzymes with slow turnover, which makes monitoring of protein expression a method more directly linked to enzyme function, than monitoring gene expression. In situ monitoring of expression and subcellular topology of glycosyltransferase proteins by immunological techniques using monoclonal antibodies therefore provides an excellent strategy to analyze the glycosylation process in cells. A major drawback has been difficulties in generating antibodies to glycosyltransferases and validating their specificities. Here we describe a simple strategy for generating and characterizing monoclonal antibodies to human glycosyltransferases. This strategy includes a process for recombinant production and purification of enzymes for immunization, a simple selection strategy for isolation of antibodies with optimal properties for in situ detection of enzyme expression, and a comprehensive strategy for characterizing the fine specificity of such antibodies.

Clinical usefulness of alterations in sialic acid, sialyl transferase and sialoproteins in breast cancer

Sialic acid, the end moieties of the carbohydrate chains are biologically important and essential for functions of glycoconjugates and are reported to be altered in cancer patients. Two hundred and twenty five breast cancer (BC) patients, 100 patients with benign breast disease (BBD) and 100 healthy females (controls) were enrolled for the study. Eight hundred and twenty four follow-up samples of 225 breast carcinoma patients were also evaluated. The association of sialic acid forms, sialyltransferase and α-2-6 sialoproteins levels with presence and extent as well as prognosis of breast carcinoma was studied. Serum sialic acid forms and sialyltransferase revealed significantly elevated levels among untreated breast cancer patients as compared to the controls, patients with BBD as well as cancer patients in remission. Non-responders showed comparable levels of the markers with those found in breast cancer patients at the time of diagnosis. Higher levels of sialic acid forms at diagnosis were associated with poor prognosis. A positive correlation between serum levels of different forms of sialic acids and extent of malignant disease was observed. The changes in serum proteins with terminal α-2-6 sialic acid correlated well with alterations in the levels of sialic acid forms and sialyltransferase. Malignant tissues showed elevated levels of sialic acid and sialyltransferase as compared to surrounding normal tissues.The results suggested potential utility of these markers in evaluation of clinical outcome.

Fc γ receptor IIIB (FcγRIIIB) polymorphisms are associated with clinical malaria in Ghanaian children

Plasmodium falciparum malaria kills nearly a million people annually. Over 90% of these deaths occur in children under five years of age in sub-Saharan Africa. A neutrophil mediated mechanism, the antibody dependent respiratory burst (ADRB), was recently shown to correlate with protection from clinical malaria. Human neutrophils constitutively express Fc gamma receptor-FcγRIIA and FcγRIIIB by which they interact with immunoglobulin (Ig) G (IgG)-subclass antibodies. Polymorphisms in exon 4 of FCGR2A and exon 3 of FCGR3B genes encoding FcγRIIA and FcγRIIIB respectively have been described to alter the affinities of both receptors for IgG. Here, associations between specific polymorphisms, encoding FcγRIIA p.H166R and FcγRIIIB-NA1/NA2/SH variants with clinical malaria were investigated in a longitudinal malaria cohort study. FcγRIIA-p.166H/R was genotyped by gene specific polymerase chain reaction followed by allele specific restriction enzyme digestion. FCGR3B-exon 3 was sequenced in 585 children, aged 1 to 12 years living in a malaria endemic region of Ghana. Multivariate logistic regression analysis found no association between FcγRIIA-166H/R polymorphism and clinical malaria. The A-allele of FCGR3B-c.233C>A (rs5030738) was significantly associated with protection from clinical malaria under two out of three genetic models (additive: p=0.0061; recessive: p=0.097; dominant: p=0.0076) of inheritance. The FcγRIIIB-SH allotype (CTGAAA) containing the 233A-allele (in bold) was associated with protection from malaria (p=0.049). The FcγRIIIB-NA2*03 allotype (CTGCGA), a variant of the classical FcγRIIIB-NA2 (CTGCAA) was associated with susceptibility to clinical malaria (p=0.0092). The present study is the first to report an association between a variant of FcγRIIIB-NA2 and susceptibility to clinical malaria and provides justification for further functional characterization of variants of the classical FcγRIIIB allotypes. This would be crucial to the improvement of neutrophil mediated functional assays such as the ADRB assay aimed at assessing the functionality of antibodies induced by candidate malaria vaccines.

Analysis of nonhuman N-glycans as the minor constituents in recombinant monoclonal antibody pharmaceuticals

Minor N-linked glycans containing N-glycolylneuraminic acid residues and/or α-Gal epitopes (i.e., galactose-α1,3-galactose residues) have been reported to be present in recombinant monoclonal antibody (mAb) therapeutics. These contaminations are due to their production processes using nonhuman mammalian cell lines in culture media containing animal-derived materials. In case of the treatment of tumors, we inevitably use such mAbs by careful risk-benefit considerations to prolong patients' lives. However, expanding their clinical applications such as for rheumatism, asthma, and analgesia demands more careful evaluation of the product characteristics. The present work for detailed evaluations of N-glycans demonstrates the methods using capillary electrophoresis with laser-induced fluorescence detection (CE-LIF) and a combination of high-performance liquid chromatography and electrospray ionization time-of-flight mass spectrometry. The CE-LIF method provides excellent separation of both major and minor N-glycans from six commercial mAb pharmaceuticals within 30 min and clearly indicates that a possible trigger of immunogenicity in humans due to the presence of nonhuman N-glycans is present. We strongly believe that the proposed method will be a powerful tool for the analysis of N-glycans of recombinant mAb products in various development stages, such as clone selection, process control, and routine release testing to ensure safety and efficacy of the products.

Glycoprotein oligosaccharides as recognition structures

A series of observations--the pronounced changes in the expression and distribution of oligosaccharide antigens during embryonic development, cell differentiation and oncogenesis, the prominence of these changing structures (oncodevelopmental antigens) on the receptor for epidermal growth factor, and the stimulation of receptor autophosphorylation following their perturbation with antibodies--has suggested that the oligosaccharides of growth factor receptors and complementary lectins may be intimately involved in molecular recognition events in growth and differentiation processes. For elucidating oligosaccharide recognition by diverse cellular and secreted proteins and microbial adhesins, a new technique has been developed which involves the overlay of immobilized oligosaccharide probes (neoglycolipids) derived from glycoproteins and other sources. New insights have been gained into carbohydrate recognition by several mammalian lectins, and a novel receptor system has been discovered in Escherichia coli isolated from patients with urinary tract infections. This new technique seems ideal for elucidating oligosaccharide recognition in diverse biological settings, and for 'quality control' of the sugar chains of recombinant glycoproteins engineered for the purpose of administration to man.

Significance of carbohydrate components of cell surfaces

This article is focused on a family of carbohydrate structures which are (a) target antigens of autoantibodies and (b) onco-developmental antigens which change during embryonic development, cell differentiation, maturation and oncogenesis. Among the carrier molecules of these saccharide structures is the receptor for epidermal growth factor. Perturbation of these structures on the isolated receptor enhances autophosphorylation of the receptor glycoprotein. This suggests that the carbohydrate chains may be part of a growth regulatory network which may be 'tuned' or perturbed via interactions with endogenous lectins or by adhesins of infective agents. Certain sialylated forms of these oligosaccharide structures serve as receptors for a pathogen of man, Mycoplasma pneumoniae which, following infection, elicits anti-erythrocyte autoantibodies. These autoantibodies are directed against the backbone domain of the carbohydrate receptor and are therefore anti-receptor antibodies. These observations suggest that complex formation between the adhesins of infective agents and specific saccharides of host-cell membranes may be a 'new' mechanism for eliciting autoantibodies.

Fluorophore-assisted carbohydrate electrophoresis as detection method for carbohydrate-protein interactions

Fluorophore-assisted carbohydrate electrophoresis (FACE) is a straightforward, sensitive method for determining the presence and relative abundance of individual (oligo)saccharide in a(n) (oligo)saccharide mixture. The single terminal aldehydes of (oligo)saccharides were tagged with the charged fluorophore 8-aminonaphthalene-1,3,6-trisulfonate (ANTS), and separated with high resolution on the basis of size by polyacrylamide gel electrophoresis. ANTS fluorescence labeling is not biased by (oligo)saccharide length. Therefore, band fluorescence intensity is directly related to the relative abundance of individual (oligo)saccharide moieties in heterogeneous sample. In the same time, it also indicates that FACE can be used to investigate the interactions of carbohydrates and proteins.

Partial characterisation of carbohydrate-rich Echinococcus granulosus coproantigens

Coproantigen ELISA based tests for diagnosis of canine echinococcosis provide high specificity and sensitivity. However, the antigenic molecules present in faeces from infected dogs have not yet been characterised. While initial attempts to determine the molecular weights of Echinococcus granulosus coproantigens by SDS-PAGE and Western blotting with coproantigen reactive capture antibodies were equivocal, they suggested presence of a significant carbohydrate component. Periodate treatment of coproantigen positive faecal supernatants resulted in a significant reduction (53%) in ELISA activity, suggesting that carbohydrates are involved in the antigenic structure of E. granulosus coproantigens. Protease treatment of antigenic molecules resulted in an 11% reduction in absorbance in ELISA, indicating that protein components were also present which affected by enzyme activity. Lectin-binding ELISA assays indicated strong affinity of E. granulosus coproantigens to concanavalin agglutinin and Lens culinaris agglutinin, and moderate binding to wheat-germ agglutinin and peanut agglutinin. No binding was detectable to Ulex europaensis agglutinin-I, Bandeiraea simplicifolia or Dolichos biflorus agglutinin. These data indicate that E. granulosus coproantigens from infected dog faeces possibly contained alpha-D-mannose and/or alpha-D-glucose, beta-galactose and N-acetyl-beta-glucosamine residues. To verify the role of carbohydrate moieties in coproantigens, faecal samples were treated with exoglycosidase and tested in the coproantigen ELISA. Treatment with beta-galactosidase or N-acetyl-beta-glucosamine reduced ELISA activity by 44 and 30%, respectively. Incubation with a panel of other specific exoglycosidases including alpha-galactosidase as well as alpha-L-fucosidase, alpha-mannosidase, beta-mannosidase, alpha-glucosidase, beta-glucosidase, beta- fructosidase, or neuraminidase, did not alter coproantigen detection in ELISA. The results indicate that coproantigens present in faeces from E. granulosus naturally infected dogs were highly glycosylated and contain beta- galactose and N-acetyl-beta-glucosamine. The putative relationship of antigenic molecules with the tapeworm glycocalyx is discussed.

The role of surface carbohydrates on the interaction of microconidia of Trichophyton mentagrophytes with epithelial cells

The presence of carbohydrate-binding adhesins on the microconidia of Trichophyton mentagrophytes surface and their role on cellular interactions were investigated. Flow cytometry showed that this fungus recognizes the sugars mannose and galactose. The binding was inhibited by the addition of methyl alpha-D-mannopyranoside and methyl alpha-D-galactopyranoside, and showed higher fluorescence intensity at 37 degrees C than 28 degrees C. Trypsin treatment and heating of the cells reduced the binding, suggesting a (glyco) protein nature of the microconidia adhesin. The interaction of the fungus to Chinese hamster ovary epithelial cells and its glycosylation-deficient mutants demonstrated a higher adhesion index in Lec1 and Lec2 mutants, which express mannose and galactose, respectively, as the terminal carbohydrate on the cell surface. Endocytosed fungi were shown preferentially in Lec2 cells. Addition of the carbohydrates methyl alpha-D-mannopyranoside and methyl alpha-D-galactopyranoside to the interaction medium, pretreatment of Lec1 and Lec2 cells with lectins Concanavalina A and Arachis hypogaea and pretreatment with sodium periodate decreased the adhesion and the endocytic index. Examination of thin section by transmission electron microscopy showed that after fungal ingestion by Lec2 cells the fungi are enclosed in a 'loose'-type vacuole while the other cells are found within a 'tight'-type membrane-bound cytoplasmic vacuole. Our results suggest the occurrence of carbohydrate-specific adhesins on microconidia surface that recognize mannose and galactose. This may have a role in the adhesion process during the infectious process of dermatophytosis.

Efficient chemoenzymatic synthesis of O-linked sialyl oligosaccharides

The tumor associated Tn (GalNAcalpha(1-1)-Thr/Ser)- and T (Galbeta(1-3)-GalNAcalpha(1-1)Thr/Ser)-antigens and their sialylated derivatives are present on the surface of many cancer cells. Preparative synthesis of these sialylated T- and Tn-structures has been achieved mainly from a chemical synthetic approach due to the lack of the required glycosyltransferases. We demonstrate a flexible and efficient chemoenzymatic approach for using recombinant sialyltransferases including a chicken GalNAcalpha2,6-sialyltransferase (chST6GalNAc I) and a porcine Galbeta(1-3)GalNAcalpha-2,3-sialyltransferase (pST3Gal I). Using these enzymes, the common O-linked sialosides Neu5Acalpha(2-6)GalNAcalpha(1-1)Thr, Galbeta(1-3)[Neu5Acalpha(2-6)]GalNAcalpha(1-1)Thr, Neu5Acalpha(2-3)Galbeta(1-3)GalNAcalpha(1-1)Thr, and Neu5Acalpha(2-3)Galbeta(1-3)[Neu5Acalpha(2-6)]GalNAcalpha(1-1)Thr were readily prepared at preparative scale. The chST6GalNAc I was found to require at least one amino acid (Thr/Ser) for optimal activity, and is thus an ideal catalyst for synthesis of synthetic glycopeptides and glycoconjugates with O-linked glycans.

Ectopic localizations of Golgi glycosyltransferases

Glycosyltransferases involved in N- and O-glycan chain elongation and termination are localized in the Golgi apparatus. Early evidence in support of this rule was based on fractionation techniques and was corroborated by numerous immunocytochemical studies. Usually these studies were confined to cultured cell lines exhibiting little differentiation features, such as HeLa cells. However, localization studies conducted in primary cell cultures (e.g., human umbilical vein endothelial cells), cells obtained ex vivo (e.g., sperm cells), and tissue sections (e.g., intestinal, renal, or hepatic tissue) often reveal ectopic localizations of glycosyltransferases usually at post-Golgi sites, including the plasma membrane. Hence, extracellular cues resulting from specific adhesion sites may influence post-Golgi trafficking routes, which may be reflected by ectopic localization of Golgi enzymes.

Genetic defects in N-glycosylation and cellular diversity in mammals

Glycoproteins in mammalian cells are modified with complex-type aspargine-linked glycans of variable chain lengths and composition. Observations of mice carrying mutations in glycosyltransferase genes imply that N-glycan structures regulate T-cell receptor clustering and hence sensitivity to agonists. We argue that the heterogeneity inherent in N-glycosylation contributes to cellular diversity and, thereby, to adaptability in the immune system.

Probing the catalytic mechanism of GDP-4-keto-6-deoxy-d-mannose Epimerase/Reductase by kinetic and crystallographic characterization of site-specific mutants

GDP-4-keto-6-deoxy-d-mannose epimerase/reductase is a bifunctional enzyme responsible for the last step in the biosynthesis of GDP-l-fucose, the substrate of fucosyl transferases. Several cell-surface antigens, including the leukocyte Lewis system and cell-surface antigens in pathogenic bacteria, depend on the availability of GDP-l-fucose for their expression. Therefore, the enzyme is a potential target for therapy in pathological states depending on selectin-mediated cell-to-cell interactions. Previous crystallographic investigations have shown that GDP-4-keto-6-deoxy-d-mannose epimerase/reductase belongs to the short-chain dehydrogenase/reductase protein homology family. The enzyme active-site region is at the interface of an N-terminal NADPH-binding domain and a C-terminal domain, held to bind the substrate. The design, expression and functional characterization of seven site-specific mutant forms of GDP-4-keto-6-deoxy-d-mannose epimerase/reductase are reported here. In parallel, the crystal structures of the native holoenzyme and of three mutants (Ser107Ala, Tyr136Glu and Lys140Arg) have been investigated and refined at 1. 45-1.60 A resolution, based on synchrotron data (R-factors range between 12.6 % and 13.9 %). The refined protein models show that besides the active-site residues Ser107, Tyr136 and Lys140, whose mutations impair the overall enzymatic activity and may affect the coenzyme binding mode, side-chains capable of proton exchange, located around the expected substrate (GDP-4-keto-6-deoxy-d-mannose) binding pocket, are selectively required during the epimerization and reduction steps. Among these, Cys109 and His179 may play a primary role in proton exchange between the enzyme and the epimerization catalytic intermediates. Finally, the additional role of mutated active-site residues involved in substrate recognition and in enzyme stability has been analyzed.

Structural characterization of oligosaccharides in recombinant soluble human interferon receptor 2 using fluorophore-assisted carbohydrate electrophoresis

The N-linked oligosaccharide profiles (banding patterns in gels) and structures of recombinant soluble human interferon receptor 2 (r-shIFNAR2) were determined using fluorophore-assisted carbohydrate electrophoresis (FACE, Glyko, Novato, CA). The method involves releasing N-linked oligosaccharide moieties from a glycoprotein by digestion with peptide-N glycanase (PNGase F), labeling the released oligosaccharides with the fluorescent dye 8-aminonaphthalene-1,3,6-trisulfonate (ANTS), and separating the labeled oligosaccharides by gel electrophoresis. The isolated oligosaccharides in the bands from the profiling gels can then be sequenced using exoglycosidases to reveal the oligosaccharide structures. The oligosaccharide profile of r-shIFNAR2 consists of at least nine oligosaccharide bands. The relative amount of oligosaccharide in each band can vary, depending on the culture conditions of the source cells. FACE structural analysis shows that r-shIFNAR2 contains only core-fucosylated N-linked oligosaccharides, most of which are fully sialylated (approximately 92%). The major types and relative amounts of the oligosaccharides from a representative sample are: disialylated, galactosylated, biantennary (15%); trisialylated, galactosylated, triantennary (19%), tetrasialylated, galactosylated, tetraantennary (30%), and N-acetyllactosamine-containing higher-order oligosaccharides including tri-, tetra-, and pentaantennary (28%). The remaining oligosaccharides are not fully sialylated and/or not fully galactosylated di-, tri-, and tetraantennary structures (approximately 5%) and unidentified structures (approximately 3%). A method for determining the types and structures of the N-acetyllactosamine containing oligosaccharides is also reported in this study.

Progress in deciphering the information content of the 'glycome'--a crescendo in the closing years of the millennium

The closing years of the second millennium have been uplifting for carbohydrate biology. Optimism that oligosaccharide sequences are bearers of crucial biological information has been borne out by the constellation of efforts of carbohydrate chemists, biochemists, immunochemists, and cell- and molecular biologists. The direct involvement of specific oligosaccharide sequences in protein targeting and folding, and in mechanisms of infection, inflammation and immunity is now unquestioned. With the emergence of families of proteins with carbohydrate-binding activities, assignments of information content for defined oligosaccharide sequences will become more common, but the pinpointing and elucidation of the bioactive domains on oligosaccharides will continue to pose challenges even to the most experienced carbohydrate biologists. The neoglycolipid technology incorporates some of the key requirements for this challenge: namely the resolution of complex glycan mixtures, and ligand binding coupled with sequence determination by mass spectrometry.

High-affinity binding to the GM-CSF receptor requires intact N-glycosylation sites in the extracellular domain of the β subunit

The human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor consists of 2 glycoprotein subunits, GMR and GMRβ. GMR in isolation binds to GM-CSF with low affinity. GMRβ does not bind GM-CSF by itself, but forms a high-affinity receptor in association with GMR. Previously, it was found that N-glycosylation of GMR is essential for ligand binding. The present study investigated the role of N-glycosylation of the β subunit on GM-CSF receptor function. GMRβ has 3 potential N-glycosylation sites in the extracellular domain at Asn58, Asn191, and Asn346. Single mutants and triple mutants were constructed, converting asparagine in the target sites to aspartic acid or alanine. A single mutation at any of the 3 consensus N-glycosylation sites abolished high-affinity GM-CSF binding in transfected COS cells. Immunofluorescence and subcellular fractionation studies demonstrated that all of the GMRβ mutants were faithfully expressed on the cell surface. Reduction of apparent molecular weight of the triple mutant proteins was consistent with loss of N-glycosylation. Intact N-glycosylation sites of GMRβ in the extracellular domain are not required for cell surface targeting but are essential for high-affinity GM-CSF binding.

High-affinity binding to the GM-CSF receptor requires intact N-glycosylation sites in the extracellular domain of the β subunit

The human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor consists of 2 glycoprotein subunits, GMR and GMRβ. GMR in isolation binds to GM-CSF with low affinity. GMRβ does not bind GM-CSF by itself, but forms a high-affinity receptor in association with GMR. Previously, it was found that N-glycosylation of GMR is essential for ligand binding. The present study investigated the role of N-glycosylation of the β subunit on GM-CSF receptor function. GMRβ has 3 potential N-glycosylation sites in the extracellular domain at Asn58, Asn191, and Asn346. Single mutants and triple mutants were constructed, converting asparagine in the target sites to aspartic acid or alanine. A single mutation at any of the 3 consensus N-glycosylation sites abolished high-affinity GM-CSF binding in transfected COS cells. Immunofluorescence and subcellular fractionation studies demonstrated that all of the GMRβ mutants were faithfully expressed on the cell surface. Reduction of apparent molecular weight of the triple mutant proteins was consistent with loss of N-glycosylation. Intact N-glycosylation sites of GMRβ in the extracellular domain are not required for cell surface targeting but are essential for high-affinity GM-CSF binding.

H (0) blood group determinant is present on soluble human L-selectin expressed in BHK-cells

In the present study we show that the H (0) blood group determinant Fuc alpha1-2Gal beta1-4GlcNAc beta1-R is present on N-linked glycans of soluble human L-selectin recombinantly expressed in baby hamster kidney (BHK) cells. The glycans were isolated using complementary HPLC techniques and characterized by a combination of exoglycosidase digestion and mass spectrometry. The linkage of the fucose residues was determined by incubation of the glycans with specific fucosidases. The H blood determinant Fuc alpha1-2Gal beta1-4GlcNAc beta1 was detected for bi-, 2,4 branched tri- and tetraantennary structures. To our knowledge, the proposed oligosaccharide structures represent a new glycosylation motif for recombinant glycoproteins expressed on BHK cells.

Protein glycosylation: implications for in vivo functions and therapeutic applications

The glycosylation machinery in eukaryotic cells is available to all proteins that enter the secretory pathway. There is a growing interest in diseases caused by defective glycosylation, and in therapeutic glycoproteins produced through recombinant DNA technology route. The choice of a bioprocess for commercial production of recombinant glycoprotein is determined by a variety of factors, such as intrinsic biological properties of the protein being expressed and the purpose for which it is intended, and also the economic target. This review summarizes recent development and understanding related to synthesis of glycans, their functions, diseases, and various expression systems and characterization of glycans. The second section covers processing of N- and O-glycans and the factors that regulate protein glycosylation. The third section deals with in vivo functions of protein glycosylation, which includes protein folding and stability, receptor functioning, cell adhesion and signal transduction. Malfunctioning of glycosylation machinery and the resultant diseases are the subject of the fourth section. The next section covers the various expression systems exploited for the glycoproteins: it includes yeasts, mammalian cells, insect cells, plants and an amoeboid organism. Biopharmaceutical properties of therapeutic proteins are discussed in the sixth section. In vitro protein glycosylation and the characterization of glycan structures are the subject matters for the last two sections, respectively.

Bioaffinity based immobilization of enzymes

Procedures that utilize the affinities of biomolecules and ligands for the immobilization of enzymes are gaining increasing acceptance in the construction of sensitive enzyme-based analytical devices as well as for other applications. The strong affinity of polyclonal/monoclonal antibodies for specific enzymes and those of lectins for glycoenzymes bearing appropriate oligosaccharides have been generally employed for the purpose. Potential of affinity pairs like cellulose-cellulose binding domain bearing enzymes and immobilized metal ionsurface histidine bearing enzymes has also been recognised. The bioaffinity based immobilization procedures usually yield preparations exhibiting high catalytic activity and improved stability against denaturation. Bioaffinity based immobilizations are usually reversible facilitating the reuse of support matrix, orient the enzymes favourably and offer the possibility of enzyme immobilization directly from partially pure enzyme preparations or even cell lysates. Enzyme lacking innate ability to bind to various affinity supports can be made to bind to them by chemically or genetically linking the enzymes with appropriate polypeptides/domains like the cellulose binding domain, protein A, histidine-rich peptides, single chain antibodies, etc.

IgE-binding and histamine-release capabilities of the main carbohydrate component isolated from the major allergen of olive tree pollen, Ole e 1

BACKGROUND

Pollen from olive trees (Olea europaea ) is a cause of pollinosis and an aggravating of asthma in Mediterranean regions. Recently, Ole e 1, the major allergen from olive tree pollen, has been isolated and its amino acid sequence has been elucidated. It is a glycoprotein whose carbohydrate moiety is involved in an IgE-binding epitope responsible for cross-reactivity among plant glycoproteins. However, the allergenicity of the free carbohydrate side chains remains to be clarified.

OBJECTIVE

The purpose of this study was to isolate the main carbohydrate component of Ole e 1 allergen and analyze its IgE-binding and histamine-release capabilities.

METHODS

Deglycosylation treatment of Ole e 1 with PNGase F and gel exclusion chromatography were used to isolate the main sugar component of the allergen. Sera of patients who are allergic to olive pollen and sera sensitive to Ole e 1 have been used in dot blotting assays of IgE binding to the isolated carbohydrate. Heparinized whole blood obtained from patients sensitive to Ole e 1 were stimulated by the free carbohydrate; the resulting histamine release was measured.

RESULTS

The main sugar component of Ole e 1 has been isolated. Free carbohydrate was able to bind IgE from sera of patients allergic to olive pollen; the sera of 65% of these patients contained anticarbohydrate reacting IgE, and 100% of those patients were sensitive to Ole e 1. The free carbohydrate promoted in vitro histamine release from basophils of sensitized patients.

CONCLUSION

The carbohydrate moieties of allergenic glycoproteins can constitute significant determinants on the binding to IgE of the sera from patients who are hypersensitive and can be responsible for inducing histamine release from blood cells.

The metabolism of 6-deoxyhexoses in bacterial and animal cells

L-fucose and L-rhamnose are two 6-deoxyhexoses naturally occurring in several complex carbohydrates. In prokaryotes both of them are found in polysaccharides of the cell wall, while in animals only L-fucose has been described, which mainly participates to the structure of glycoconjugates, either in the cell membrane or secreted in biological fluids, such as ABH blood groups and Lewis system antigens. L-fucose and L-rhamnose are synthesized by two de novo biosynthetic pathways starting from GDP-D-mannose and dTDP-D-glucose, respectively, which share several common features. The first step for both pathways is a dehydration reaction catalyzed by specific nucleotide-sugar dehydratases. This leads to the formation of unstable 4-keto-6-deoxy intermediates, which undergo a subsequent epimerization reaction responsible for the change from D- to L-conformation, and then a NADPH-dependent reduction of the 4-keto group, with the consequent formation of either GDP-L-fucose or dTDP-L-rhamnose. These compounds are then the substrates of specific glycosyltransferases which are responsible for insertion of either L-fucose or L-rhamnose in the corresponding glycoconjugates. The enzyme involved in the first step of GDP-L-fucose biosynthesis in E. coli, i.e., GDP-D-mannose 4,6 dehydratase, has been recently expressed as recombinant protein and characterized in our laboratory. We have also cloned and fully characterized a human protein, formerly named FX, and an E. coli protein, WcaG, which display both the epimerase and the reductase activities, thus indicating that only two enzymes are required for GDP-L-fucose production. Fucosylated complex glycoconjugates at the cell surface can then be recognized by specific counter-receptors in interacting cells, these mechanisms initiating important processes including inflammation and metastasis. The second pathway starting from dTDP-D-glucose leads to the synthesis of antibiotic glycosides or, alternatively, to the production of dTDP-L-rhamnose. While several sets of data are available on the first enzyme of the pathway, i.e., dTDP-D-glucose dehydratase, the enzymes involved in the following steps still need to be identified and characterized.

High affinity cross-reacting mAb generated by minimal mimicry: implications for the pathogenesis of anti-nuclear autoantibodies and immunosuppression

The antigen recognition of a profoundly immunosuppressive mAb, mAb 2E1, in vivo was investigated. In addition to the 62-kDa effector cell protease receptor 1, mAb 2E1 bound the 32-kDa T cell adhesion receptor E2 (CD99) and the 86-kDa p80 subunit of the nuclear antigen complex Ku. These molecules share no overall sequence similarity. Peptide mapping experiments identified the mAb 2E1 cross-reacting epitopes as the sequences 66GSFSDADLAD75 in E2 and 571GGAHFSVSSLAEG583 in p80 of Ku, sharing a minimal homology motif FSXXXLA, in which X is a nonconserved amino acid. Each of these peptides separately inhibited the binding of mAb 2E1 to E2, effector cell protease receptor 1, and p80 of Ku in a dose-dependent manner. Scatchard plot analysis of 125I-labeled mAb 2E1 binding to peripheral blood mononuclear cells revealed a high-affinity interaction with a dissociation constant of 7 x 10(-10) M. An anti-E2 mAb bound the same epitope 66GSFSDADLAD75 recognized by mAb 2E1 but failed to react with p80 of Ku and was not immunosuppressive. These findings demonstrate that high-affinity cross-reacting mAbs can be generated by mimicry of a minimal surface on unrelated molecules. This model of minimal mimicry may determine the nuclear reactivity of certain autoantibodies to Ku and contribute to aberrant immunosuppression in vivo.

Expression, purification and characterization of GDP-D-mannose 4,6-dehydratase from Escherichia coli

GDP-D-mannose dehydratase (GMD) catalyzes the first step of the pathway that converts GDP-D-mannose to GDP-L-fucose in bacteria, plants and mammals. Recently, the gene coding for GMD has been identified and sequenced in E. coli. Based on this sequence, we have expressed and purified GMD in E. coli as a glutathione transferase (GST) fusion protein. The fused GST-GMD protein and the thrombin-cleaved GMD were then characterized. The catalytically active form of both enzyme species seems to be a hexamer of 410 and 250 kDa, respectively. The GST-GMD fusion protein has a Km of 0.22 +/- 0.04 mM and a specific activity of 2.3 +/- 0.2 micromol/h/mg. Ca2+ and Mg2+ activate GMD, while GDP-L-beta-fucose, the end-product of the pathway, inhibits it specifically. The GST-GMD fusion protein contains one mole of tightly bound NADP+ per mole of hexamer. Apparently, this NADP+ is involved in the catalytic mechanism of GMD.

Modular organization of carbohydrate recognition domains in animal lectins

In spite of the great diversity of animal lectins, a common characteristic is their ability to bind sugars by means of discrete, modular carbohydrate recognition domains, CRDs. Three different groups of animal lectins-galectins, P-type and C-type lectins- have different types of CRDs which they arrange in a number of combinations, in three dimensions, in order to increase the affinity for oligosaccharides associated with glycoconjugates. The necessity of combining multiple CRDs in a native lectin molecule in order to increase the affinity for multiple ligands is of great importance physiologically, since many of the carbohydrate structures associated with proteins exist in a variety of different conformations. Recent work has clarified the structural basis for carbohydrate recognition by some of these lectins.