Carbohydrates as antigenic determinants of glycoproteins

Synthesis of GDP-L-fucose by the human FX protein

FX is a homodimeric NADP(H)-binding protein of 68 kDa, first identified in human erythrocytes, from which it was purified to homogeneity. Its function has been unrecognized despite partial structural and genetic characterization. Recently, on the basis of partial amino acid sequence, it proved to be the human homolog of the murine protein P35B, a tumor rejection antigen. In order to address the biochemical role of FX, its primary structure was completed by cDNA sequencing. This sequence revealed a significant homology with many proteins from different organisms. Specifically, FX showed a remarkable similarity with a putative Escherichia coli protein, named Yefb, whose gene maps in a region of E. coli chromosome coding for enzymes involved in synthesis and utilization of GDP-D-mannose. Accordingly, a possible role of FX in this metabolism was investigated. The data obtained indicate FX as the enzyme responsible for the last step of the major metabolic pathway resulting in GDP-L-fucose synthesis from GDP-D-mannose in procaryotic and eucaryotic cells. Specifically, purified FX apparently catalyzes a combined epimerase and NADPH-dependent reductase reaction, converting GDP-4-keto-6-D-deoxymannose to GDP-L-fucose. This is the substrate of several fucosyltranferases involved in the correct expression of many glyconjugates, including blood groups and developmental antigens.

Simultaneous expression of keratan sulphate epitope (a sulphated poly-N-acetyllactosamine) and blood group ABH antigens in papillary carcinomas of the human thyroid gland

The monoclonal antibody 5-D-4 recognizes heavily sulphated forms of keratan sulphate epitope. It reacted strongly with the cell surfaces of most thyroid papillary carcinomas from all the individuals examined, independently of the blood group of the patients. Cells of follicular variants of papillary carcinomas were also labelled by 5-D-4. In contrast, no labelling with this antibody was observed in other types of thyroid neoplasms, or in normal tissues. The reactivity of 5-D-4 with papillary carcinomas was markedly reduced or abolished by prior digestion with endo-beta-galactosidase, keratanase II, or N-glycosidase F. Although keratanase digestion had no effect on 5-D-4 labelling, it revealed the binding sites of Griffonia simplicifolia agglutinin II (GSA-II), which recognizes terminal N-acetylglucosamine in a limited number of carcinoma cells from some individuals. Blood group ABH antigens, which are simultaneously expressed together with keratan sulphate epitope in cancer cells, were eliminated by digestion with endo-beta-galactosidase and N-glycosidase F, but were resistant to keratanase and keratanase II treatment. These results indicate that keratan sulphate oligosaccharides are cancer-associated and are probably oncofoetal antigens, as are the blood group antigens in human thyroid glands. The results suggests that poly-N-acetyllactosamine, which is ubiquitously and consistently produced in papillary carcinomas, is modified in two different ways: sulphation on the 6-position of at least some units of either galactose or N-acetylglucosamine or both, and decoration of non-reducing termini with the blood group antigens. Along with the endo-beta-galactosidase-GSA-II labelling procedure, labelling with 5-D-4 may be a useful diagnostic means for distinguishing papillary carcinoma from other types of thyroid neoplasms.

Influence of the carbohydrate moiety on the stability of glycoproteins

To study the role of oligosaccharides on the properties of glycoproteins, five glycoproteins (yeast external invertase, bovine serum fetuin, glucoamylase from Aspergillus niger, and chicken egg white ovotransferrin and avidin) of previously established glycan patterns were purified to homogeneity and deglycosylated with endo- and exo-glycosidases in native conditions. Thermal stability and conformational changes were measured by high-resolution differential scanning microcalorimetry and circular dicroism spectroscopy before and after they were deglycosylated. It was found that deglycosylation decreases protein thermal stability, as judged by the decrease in denaturation temperature and denaturation enthalpy, while it does not affect substantially the conformation as indicated by the CD spectra in the far UV range. The destabilization effect of deglycosylation seems to depend on the carbohydrate content, i.e., the maximum effect was observed for the most heavily glycosylated protein, irrespective of the types (N-linked or O-linked) or patterns (mono- or multi-branched) of the covalently attached carbohydrate chains. In addition, studies of the reversibility to heat denaturation revealed that deglycosylated proteins have a poorer thermal reversibility in calorimetric scans than their native counterparts and tend to aggregate during thermal inactivation at acidic pH. These results suggest that carbohydrate moieties, in addition to the apparent stabilizing effect, may prevent the unfolded or partially folded protein molecules from aggregation. Our results support the hypothesis that the general function of protein glycosylation is to aid in folding of the nascent polypeptide chain and in stabilization of the conformation of the mature glycoprotein.

Cross-reactivity between the major allergen from olive pollen and unrelated glycoproteins: evidence of an epitope in the glycan moiety of the allergen

Ole e 1, the major allergen from olive pollen, is a glycoprotein containing a single Asn-linked glycan moiety. Rabbit antiserum against this protein has been obtained; and its immunologic cross-reactivities in Western blotting with ascorbate oxidase, horseradish peroxidase, bromelain, ovalbumin, and honeybee venom phospholipase A2 have been studied. Ascorbate oxidase, peroxidase, and bromelain are recognized by the Ole e 1 antiserum. When these three proteins are deglycosylated by periodate treatment, such an immunologic reaction does not occur. The relative affinities of these proteins have been analyzed by direct and inhibition ELISA experiments. A commercially available antibody against horseradish peroxidase has also been considered in these studies. This antibody reacts with Ole e 1 but not with the periodate-deglycosylated allergen. Horseradish peroxidase, bromelain, and ascorbate oxidase are recognized by the IgE of sera from patients who are hypersensitive to olive tree pollen. This binding is also abolished by periodate treatment. The results are interpreted in terms of the presence of an epitope in the carbohydrate moiety of Ole e 1, which would contain a xylose involved in recognition by both IgE and IgG antibodies.

Developmental and Tissue-Specific Structural Alterations of the Cell-Wall Polysaccharides of Arabidopsis thaliana Roots

The plant cell wall is a dynamic structure that plays important roles in growth and development and in the interactions of plants with their environment and other organisms. We have used monoclonal antibodies that recognize different carbohydrate epitopes present in plant cell-wall polysaccharides to locate these epitopes in roots of developing Arabidopsis thaliana seedlings. An epitope in the pectic polysaccharide rhamnogalacturonan I is observed in the walls of epidermal and cortical cells in mature parts of the root. This epitope is inserted into the walls in a developmentally regulated manner. Initially, the epitope is observed in atrichoblasts and later appears in trichoblasts and simultaneously in cortical cells. A terminal [alpha]-fucosyl-containing epitope is present in almost all of the cell walls in the root. An arabinosylated (1->6)-[beta]-galactan epitope is also found in all of the cell walls of the root with the exception of lateral root-cap cell walls. It is striking that these three polysaccharide epitopes are not uniformly distributed (or accessible) within the walls of a given cell, nor are these epitopes distributed equally across the two walls laid down by adjacent cells. Our results further suggest that the biosynthesis and differentiation of primary cell walls in plants are precisely regulated in a temporal, spatial, and developmental manner.

MAUB is a new mucin antigen associated with bladder cancer

The M344 tumor-associated antigen, expressed in 70% of superficial bladder tumors, is a sialylated carbohydrate present on a high molecular mass thiol-reducible secreted mucin, which we named MAUB for mucin antigen of the urinary bladder. Herein we studied the relationship between MAUB and other known mucins in the MGH-U3 bladder cancer line where MAUB expression is modulated by culture conditions. Northern blots, immunoradiometric assays, and Western blots showed that only MUC1 and MUC2 are expressed in this MAUB-positive cell line. MUC1 differs from MAUB by its molecular mass and its non-oligomeric nature, while MUC2 has similar molecular mass and response to culture conditions. However, in double determinant immunoradiometric assays, MAUB and MUC2 did not cross-react. Moreover, confocal microscopy showed different subcellular localization of the two antigens. Treatment of MGH-U3 cells with MUC2 antisense oligodeoxynucleotides resulted in decreased expression of MUC2 and increased expression of MAUB, ruling out the possibility that monoclonal antibody M344 recognizes a different glycosylated form of MUC2. In addition, we identified a tumor specimen expressing MAUB but no MUC2 antigen or mRNA. Together, these results suggest that there is expression of at least three mucins in MGH-U3 cells and that MAUB is a cancer-associated mucin distinct from those identified so far.

Specificity of C-glycoside complexation by mannose/glucose specific lectins

The binding of the mannose/glucose specific lectins from Canavalia ensiformis (concanavalin A) and Dioclea grandiflora to a series of C-glucosides were studied by titration microcalorimetry and fluorescence anisotropy titration. These closely related lectins share a specificity for the trimannoside methyl 3,6-di-O-(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside, and are a useful model system for addressing the feasibility of differentiating between lectins with overlapping carbohydrate specificities. The ligands were designed to address two issues: (1) how the recognition properties of non-hydrolyzable C-glycoside analogues compare with those of the corresponding O-glycosides and (2) the effect of presentation of more than one saccharide recognition epitope on both affinity and specificity. Both lectins bind the C-glycosides with affinities comparable to those of the O-glycoside analogues; however, the ability of both lectins to differentiate between gluco and manno diastereomers was diminished in the C-glycoside series. Bivalent norbornyl C-glycoside esters were bound by the lectin from Canavalia but only weakly by the lectin from Dioclea. In addition to binding the bivalent ligands, concanavalin A discriminated between C-2 epimers, with the manno configuration binding more tightly than the gluco. The stoichiometry of binding of the bivalent ligands to both di- and tetrameric lectin was two binding sites per ligand, rather than the expected 1:1 stoichiometry. Together, these results suggest that concanavalin A may possess more than one class of carbohydrate binding sites and that these additional sites show stereochemical discrimination similar to that of the previously identified monosaccharide binding site. The implications of these findings for possible in vivo roles of plant lectins and for the use of concanavalin A as a research tool are discussed.

Conformational studies on the selectin and natural killer cell receptor ligands sulfo- and sialyl-lacto-N-fucopentaoses (SuLNFPII and SLNFPII) using NMR spectroscopy and molecular dynamics simulations. Comparisons with the nonacidic parent molecule LNFPII

This investigation is focused on the conformational behavior of the blood group Lewisa (Le(a)-active pentasaccharide lacto-N-fucopentaose II (LNFPII) and its sulfated and sialylated analogs, SuLNFPII and SLNFPII. The latter two are more potent oligosaccharide ligands for the animal lectins, E- and L-selectin, and the natural killer cell receptor, NKR-P1, than are the shorter chain analogs based on the trisaccharide Le(a) domain. We report here that the three oligosaccharides based on the fucopentasaccharide have very similar average solution conformations as determined from NMR spectroscopical parameters, in particular 13C chemical shift differences. From restrained simulated annealing and restrained molecular dynamics (MD) simulations performed in order to determine the most probable conformational distributions around the glycosidic linkages we derive models for these oligosaccharides that are in good agreement with experimental parameters, such as rotating-frame Overhauser effects (ROE's) and long-range 1H,13C coupling constants across the glycosidic linkages. In these model structures the Le(a) domain at the non-reducing end of the longer chain oligosaccharides approximates the same rigid structure as in the shorter analogs. The Gal beta 1-4Glc linkage at the reducing end is also rather rigid, showing only little more flexibility than the Le(a) domain. However, the NeuAc alpha 2-3Gal linkage in SLNFPII, and the GlcNAc beta 1-3Gal linkage in all three oligosaccharides are flexible, in each case fluctuating mainly between two minimum energy structures: (phi = -81 degrees, psi = 8 degrees) and (phi = -160 degrees, psi = -20 degrees) for the NeuAc alpha 2-3Gal linkage, as reported previously for the isomeric sequence 3'-sialyl Le(x), and (phi = -25 degrees, psi = -26 degrees) and (phi = 20 degrees, psi = 24 degrees) for the GlcNAc beta 1-3Gal linkage. The flexibility of the latter linkage may allow the lactosyl domain at the reducing end to fit with little strain into extended carbohydrate binding sites on the recognition proteins, and, for the purposes of drug designs, it will be important to establish which conformational distribution is assumed for the GlcNAc beta 1-3Gal linkage in these longer chain oligosaccharides in the bound state.

Transcriptional regulation of alpha1,3-galactosyltransferase in embryonal carcinoma cells by retinoic acid. Masking of Lewis X antigens by alpha-galactosylation

Treatment of mouse teratocarcinoma F9 cells with all-trans-retinoic acid (RA) causes a 9-fold increase in steady-state levels of mRNA for UDP-Gal:beta-D-Gal alpha1,3-galactosyltransferase (alpha1,3GT) beginning at 36 h. Enzyme activity rises in a similar fashion, which also parallels the induction of laminin and type IV collagen. Nuclear run-on assays indicate that this increase in alpha1,3GT in RA-treated F9 cells, like that of type IV collagen, is transcriptionally regulated. Differentiation also results in increased secretion of soluble alpha1,3GT activity into the growth media. The major alpha-galactosylated glycoprotein present in the media of RA-treated F9 cells, but not of untreated cells, was identified as laminin. Differentiation of F9 cells is accompanied by an increase in alpha-galactosylation of membrane glycoproteins and a decrease in expression of the stage-specific embryonic antigen, SSEA-1 (also known as the Lewis X antigen or LeX), which has the structure Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-R. However, flow cytometric analyses with specific antibodies and lectins, following treatment of cells with alpha-galactosidase, demonstrate that differentiated cells contain LeX antigens that are masked by alpha-galactosylation. Thus, RA induces alpha1,3GT at the transcriptional level, resulting in major alterations in the surface phenotype of the cells and masking of LeX antigens.

Fluorophore-assisted carbohydrate electrophoresis in the separation, analysis, and sequencing of carbohydrates

Carbohydrate analysis has traditionally been viewed as a specialty science, performed only in a few well-established laboratories using conventional carbohydrate analysis technology (e.g. NMR, gas chromatography-mass spectroscopy, high-performance liquid chromatography, capillary electrophoresis) combined with the specialized technical training that has been essential for accurate interpretation of the data. This tradition of specialized laboratories is changing, due primarily to an increase in the number of scientists performing routine carbohydrate analysis. As a result, many scientists who are not trained in traditional carbohydrate analytical techniques now need to be able to perform accurate carbohydrate analysis in their own laboratories. This has created a need for technically simple and inexpensive methods of carbohydrate analysis. In this review, we present application vignettes of a technically simple, yet analytically powerful method called fluorophore-assisted carbohydrate electrophoresis (FACE). FACE can be used for performing routine oligosaccharide profiling, monosaccharide analysis, and sequencing of a variety of carbohydrates.

Production of the chimerical plasminogen activator K2tu-PA in CHO cells

Development of a CHO cell-based production system for the hybrid plasminogen activator K2tu-PA is described. Using the major immediate-early promoter of mouse cytomegalovirus (MCMV) transient and stable expression levels were 3-10-fold higher than those obtained with several other strong promoters. Splicing and polyadenylation signals from the rabbit beta-globin gene were used downstream of the DNA segment coding for K2tu-PA. The strong enhancer moiety of the MCMV promoter also stimulated strongly the promoter of the dihydrofolate reductase (DHFR) gene, placed adjacently for selection/gene amplification purposes. One construct with opposing K2tu-PA and DHFR RNA transcripts yielded the highest expression level with a single copy of the plasmid, but K2tu-PA expression was consistently lost after amplification of such genes, possibly as a result of the formation of antisense RNA. With other constructs, K2tu-PA production leveled off at 6.5 micrograms per million cells per day despite a high gene copy number. This was due to a combination of inefficient mRNA translation and mRNA instability, caused by elements from the untranslated portions of tissue-type and urokinase-type plasminogen activator cDNA which were included in the expression vector. After elimination of these inhibitory DNA segments, 4-5-times higher expression levels were reached.

Human lymphocyte activation is associated with the early and high-level expression of the endogenous lectin CSL at the cell surface

Lymphocytes undergo activation in response to antigens, cytokines, lectins and antibodies interacting with specific cell-surface molecules or through substances influencing signal transduction pathways. This study shows that human T- and B-cells stimulated using phorbol esters or plant lectins express early (2 h using phorbol esters and 24 h using plant lectins) a high level of a polyvalent carbohydrate-binding protein, the cerebellar soluble lectin (CSL), which is in part externalized. The lectin, immunologically related to CDw70, interacts with specific glycoprotein ligands of the lymphocyte surface, including CD3 on T-cells and CD24 on B-cells. Major changes in phosphorylations associated with activation appear as largely CSL-dependent since they are specifically inhibited by anti-CSL Fab fragments. It is suggested that the lectin induces the clustering of specific cell-surface glycoproteins and plays the role of an endogenous amplifier of activation signals.

Characterization of a monoclonal antibody that recognizes an arabinosylated (1-->6)-beta-D-galactan epitope in plant complex carbohydrates

Monoclonal antibody CCRC-M7 is representative of a group of antibodies with similar binding specificity that were generated using the plant cell-wall pectic polysaccharide, rhamnogalacturonan I, as immunogen. The epitope recognized by CCRC-M7 is present in several plant polysaccharides and membrane glycoproteins. Selective enzymatic or chemical removal of arabinosyl residues from rhamnogalacturonan I reduced, but did not abolish, the ability of CCRC-M7 to bind to the polysaccharide. In contrast, enzymatic removal of both arabinosyl and galactosyl residues from rhamnogalacturonan I completely abolished binding of CCRC-M7 to the resulting polysaccharide. Competitive ELISAs using chemically defined oligosaccharides to compete for the CCRC-M7 binding site showed that oligosaccharides containing (1-->6)-linked beta-D-galactosyl residues were the best competitors among those tested, with the tri-, penta-, and hexa-saccharides being equally effective. The combined results from indirect and competitive ELISAs suggest that the minimal epitope recognized by CCRC-M7 encompasses a (1-->6)-linked beta-galactan containing at least three galactosyl residues with at least one arabinosyl residue attached.

Capillary gel electrophoresis separation of high-mannose type oligosaccharides derivatized by 1-aminopyrene-3,6,8-trisulfonic acid

In this paper we report the capillary gel electrophoresis separation of 1-aminopyrene-3,6,8-trisulfonic acid (APTS) labeled oligosaccharides, released enzymatically from bovine pancreatic ribonuclease B. The released and labeled high-mannose structures were identified by spiking the separated peaks with the appropriate commercially available individual oligosaccharides. Baseline separation of the three positional isomers of the mannose-7 and mannose-8 oligosaccharides was attained. Comparison of the electrophoretic mobilities of the high-mannose type branched carbohydrates to the linear molecules of maltooligosaccharides (glucose oligomers) have been shown using different gel concentrations in the running buffer system. We observed that increasing gel concentration in the running buffer causes an increase in the relative mobility values of the high-mannose type carbohydrate molecules compared to the linear glucose oligomers. Analysis of our data indicated that this increase in relative migration time was not due to sieving, but seemed to be related to the mannose content and hydrodynamic volume of the branched glycans as well as to the viscosity of the separation medium.

Expression of human chromosome 19p alpha(1,3)-fucosyltransferase genes in normal tissues. Alternative splicing, polyadenylation, and isoforms

The human alpha(1,3)-fucosyltransferase genes FUT3, FUT5, and FUT6 form a cluster on chromosome 19p13.3. Expression was studied using reverse transcriptase-polymerase chain reaction, rapid amplification of cDNA ends, and Northern analyses. FUT3 and FUT6 were expressed at high levels, while FUT5 expression was lower and restricted to fewer cell types. Alternatively spliced transcripts were identified for FUT3 and FUT6 in kidney, liver, and colon. A 2.37-kilobase pair (kb) FUT3 transcript, detected at high levels in kidney and colon, was absent in liver. FUT6 expression was characterized by a 3.5-kb transcript present in kidney and liver, and a 2.5-kb transcript in colon and liver. Two polyadenylation sites were shown for FUT5, but absence of consensus sequences suggests reduced efficiency for cleavage and polyadenylation. Two polyadenylation sites were also shown for FUT6, with the alternatively spliced downstream signal in tissues expressing high levels of FUT6. In these tissues, additional splicing results in isoforms with catalytic domain deletions. No detectable alpha(1,3)- or alpha(1,4)-fucosyltransferase activity was found in assays of cells transfected with FUT6 isoform cDNAs. Thus, tissue-specific post-transcriptional modifications are associated with expression patterns of FUT3, FUT5, and FUT6.

Conformational epitopes of pemphigus antigens (Dsg1 and Dsg3) are calcium dependent and glycosylation independent

The target molecule of pemphigus autoantibodies is a transmembrane desmosomal component, desmoglein 3 (Dsg3) in pemphigus vulgaris (PV) and Dsg1 in pemphigus foliaceus (PF). In this study, we examined the effects of calcium and glycosylation on the anti-genicity of the pemphigus antigens and on the generation of conformational epitopes. We used recombinant baculovirus proteins, PVIg and PFIg, which are considered to reflect accurately the native conformation of the extracellular domain of their respective proteins Dsg3 and Dsg1. These baculoproteins could immunoadsorb heterogeneous autoantibodies from the corresponding sera of PV and PF patients, completely blocking indirect immunofluorescence staining of normal human skin. Chelating calcium from the solution containing the baculoproteins using ethylenediaminetetraacetic acid (EDTA) or ethyleneglycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) abolished immunoadsorption by both PVIg and PFIg; however, immunoadsorption by the baculoproteins was restored after dialysis against 1 mM calcium. Nonglycosylated forms of both baculoproteins produced in the presence of tunicamycin retained their immunoadsorptive ability. Furthermore, immunoadsorption by the baculo-proteins was prevented irreversibly by treatment with low pH, high pH, and boiling, but not with the non-ionic detergent Nonidet P-40. These findings indicate that formation of the conformational epitopes on the pemphigus antigens is dependent on calcium but independent of glycosylation, and provide direct evidence that calcium plays an important role in determining the antigenic properties of the pemphigus antigens.

Expression and glycosylation of the filamentous brush border glycocalyx (FBBG) during rabbit enterocyte differentiation along the crypt-villus axis

The filamentous brush border glycocalyx forming the ‘enteric surface coat’ of the intestinal epithelium is composed in rabbits of a 400 kDa mucin-type glycoprotein, which was purified using the 3A4 monoclonal antibody. This monoclonal antibody recognizes a filamentous brush border glycocalyx-specific glycosidic structure containing an O-acetylated sialic acid, which is absent from all the other glycoproteins in the epithelium, with the exception of certain goblet cell mucins. Here we establish that only 50% of the rabbits tested synthesized this glycosidic structure. Upon immunolabeling surface epithelia and sections of jejunum from these rabbits, the carbohydrate epitope recognized by the 3A4 mAb was found to be present on the filamentous brush border glycocalyx of a variable number of enterocytes, which were patchily distributed over all the villi. This heterogeneous expression of 3A4 antigenicity, which was also observed in the crypts, suggests the existence of differences between the patterns of differentiation of enterocytes, which results in the expression of different pools of glycosyltransferases and/or acetyl transferases. In mature enterocytes, the 3A4 determinants were present only on the filamentous brush border glycocalyx, which is anchored solely to the membrane microdomain at the tip of brush border microvilli. However, expression of 3A4 antigenicity begins in the median third of crypts, in enterocytes with a short, thin brush border devoid of apical filamentous brush border glycocalyx. Here the 3A4 epitopes were present over the whole brush border membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluorophore-assisted carbohydrate electrophoresis technology and applications

Carbohydrates, in particular the complex carbohydrates conjugated to proteins and lipids, have important functions in a variety of biological systems. Their isolation and structural determination--prerequisites for elucidation of their biological functions--have been technical challenges for many decades. Almost all available chromatographic and electrophoretic methods as well as NMR and MS have been applied to carbohydrate analysis but none has proved satisfactory in terms of simplicity, sensitivity, reproducibility, cost and requirement for materials. Recently, a technique called fluorophore-assisted carbohydrate electrophoresis was developed which is very promising. It separates fluorescently-labeled carbohydrates on polyacrylamide gels and uses a charge-coupled device camera to detect and quantitate the products. This review describes the principles of the method and its applications to several aspects of research on carbohydrate-containing biological biomolecules.

Capillary and slab gel electrophoresis profiling of oligosaccharides

Capillary electrophoresis with laser-induced fluorescent detection, a one-dimensional version of the well-established planar analytical method of polyacrylamide gel electrophoresis, has been proven to be a powerful new microanalytical method for profiling complex carbohydrates. In this paper a comparison is presented between the planar high concentration polyacrylamide gel electrophoresis method and capillary electrophoresis of different carbohydrates with respect to performance and efficiency. N-Linked oligosaccharides were released from several glycoproteins, including fetuin, human immunodeficiency virus (HIV) envelope recombinant glycoprotein (GP-120), alpha 1-acid glycoprotein and ribonuclease B, using recombinant peptide-N-glycosidase F (PNGase F). Both separation methods involve labeling of the released carbohydrates at the reducing end with the fluorescent dye, disodium 8-amino-1,3,6-naphthalene trisulfonate (ANTS). Fluorophore labeling was followed by separation of the labeled oligosaccharides either by high concentration polyacrylamide gel electrophoresis or capillary electrophoresis.

Hydrophobic glycosides of N-acetylglucosamine can act as primers for polylactosamine synthesis and can affect glycolipid synthesis in vivo

Several hydrophobic glycosides of N-acetylglucosamine (GlcNAc) served as primers for polylactosamine synthesis when added to Chinese hamster ovary (CHO) cells. The modified glycosides, containing one to six lactosamine repeats in linear array, were sialylated and secreted into the culture medium. The relative efficiencies of the glycosides to serve as primers were dependent on the nature of the aglycone and on the anomeric configuration of the GlcNAc residue. The same compounds were tested for their effects on glycolipid synthesis in CHO cells. All of the beta-glycosides significantly inhibited the synthesis of the lactoseries glycolipid GM3 whereas the alpha-glycoside was inactive. The compound GlcNAc alpha 1-O-benzyl- was the most efficient primer of polylactosamine synthesis and had no effect on glycolipid synthesis. This compound may have potential for the assay of the polylactosamine synthetic capacity of living cells.

Acceptor specificity of different length constructs of human recombinant alpha 1,3/4-fucosyltransferases. Replacement of the stem region and the transmembrane domain of fucosyltransferase V by protein A results in an enzyme with GDP-fucose hydrolyzing activity

The acceptor specificity of recombinant full-length, membrane-bound fucosyltransferases, expressed in COS-7 cells, and soluble, protein-A chimeric forms of alpha 1,3-fucosyltransferase (Fuc-T) III, Fuc-TIV, and Fuc-TV was analyzed toward a broad panel of oligosaccharide, glycolipid, and glycoprotein substrates. Our results on the full-length enzymes confirm and extend previous studies. However, chimeric Fuc-Ts showed increased activity toward glycoproteins, whereas chimeric Fuc-TIII and Fuc-TV had a decreased activity with glycosphingolipids, compared to the full-length enzymes. Unexpectedly, chimeric Fuc-TV exhibited a GDP-fucose hydrolyzing activity. In substrates with multiple acceptor sites, the preferred site of fucosylation was identified. Fuc-TIII and Fuc-TV catalyzed fucose transfer exclusively to OH-3 of glucose in lacto-N-neotetraose and lacto-N-tetraose, respectively, as was demonstrated by 1H NMR spectroscopy. Thin layer chromatography immunostaining revealed that FucT-IV preferred the distal GlcNAc residue in nLc6Cer, whereas Fuc-TV preferred the proximal Gl-cNAc residue. Incubation of Fuc-TIV or Fuc-TV with VI3NeuAcnLc6Cer resulted in products with the sialyl-LewisX epitope as well as the VIM-2 structure. To identify polar groups on acceptors that function in enzyme binding, deoxygenated substrate analogs were tested as acceptors. All three Fuc-Ts had an absolute requirement for a hydroxyl at C-6 of galactose in addition to the accepting hydroxyl at C-3 or C-4 of GlcNAc.

Effect of protein chemical hydrophobization on antiglucose oxidase immunoglobulin production in mouse

One problem resulting from the therapeutic use of enzymes is the adverse immunological reactions. In order to study the immunoglobulin production elicited into mice by different derivatives of an enzyme, glucose oxidase was chosen as a model. The immunoglobulin productions induced by apoglucose oxidase, prepared by removing flavine adenine dinucleotide from the native enzyme through an acidic treatment and devoid of enzymatic activity, by metaperiodate-oxidized glucose oxidase that lost about 50% of its carbohydrate moiety, and by propyl aliphatic chains-coupled glucose oxidase were as intense as that induced by native glucose oxidase. On the other hand, coupling hexyl aliphatic chains to the enzyme did change its ability to stimulate antibody production. This hydrophobized preparation induced a low titer of antibody after repeated intravenous or subcutaneous injections. This result suggests a simple strategy for reducing the immunogenicity of foreign proteins and for decreasing the risk of immunological complications in enzyme therapy.

Major organ-specific glycoproteins in isolated brain and kidney membranes identified as Na,K-ATPase subunits by combined glycan-, lectin-, and immunoblotting

In the present work combined glycan-, lectin-, and immunoblotting of isolated brain and kidney membranes shows that the alpha and beta subunits of Na,K-ATPase are the most abundant glycoproteins. Further, Datura stramonium and Galanthus nivalis agglutinins recognize the Na,K-ATPase subunits in a mutually exclusive manner in membranes from human, rabbit and rat brain or human, rabbit, rat, pig and dog kidney indicating the presence of species-independent organ-typical glycoforms. The glycosylation status is not related to the ouabain-sensitivity. Taken together, the data reveals organ-specific glycoforms of Na,K-ATPase which might have roles for organ identification and recognition.

Crosslinking of mammalian lectin (galectin-1) by complex biantennary saccharides

Galectins are beta-galactoside-binding proteins that occur intra- and extracellularly in many animal tissues. They have been proposed to form networks of glycoconjugates on the cell surface, where they may modulate various cell response pathways such as growth, activation and adhesion. The high resolution X-ray crystallographic analyses of three crystal forms of bovine galectin-1 in complex with biantennary saccharides of N-acetyllactosamine type reveal infinite chains of lectin dimers cross-linked through N-acetyllactosamine units located at the end of the oligosaccharide antenna. The oligosaccharide adopts a different low energy conformation in each of the three crystal forms.

Oligosaccharide ligands for NKR-P1 protein activate NK cells and cytotoxicity

A diversity of high-affinity oligosaccharide ligands are identified for NKR-P1, a membrane protein on natural killer (NK) cells which contains an extracellular Ca(2+)-dependent lectin domain. Interactions of such oligosaccharides on the target cell surface with NKR-P1 on the killer cell surface are crucial both for target cell recognition and for delivery of stimulatory or inhibitory signals linked to the NK cytolytic machinery. NK-resistant tumour cells are rendered susceptible by preincubation with liposomes expressing NKR-P1 ligands, suggesting that purging of tumour or virally infected cells in vivo may be a therapeutic possibility.

Immunogenicity of the N-glycans of peanut peroxidase

The three tryptic glycopeptides of cationic peanut peroxidase (C. PRX) and the sole one of anionic peanut peroxidase (A. PRX) were individually coupled to bovine serum albumin to raise antisera. The three categories of antibodies directed towards three N-glycans of C. PRX (anti-GLa, anti-GLb and anti-GLc) were isolated from antisera with glycan-conjugated ECH Sepharose 4B affinity columns and the distribution of epitopes on the N-glycans was investigated. The reactivity of anti-GLa, anti-GLb and anti-GLc is inhibited 25-40% by 1 M fucose, compared with a slight inhibition by N-acetylglycosamine and xylose. Mannose and galactose showed no inhibition to anti-GLa and only a slight inhibition to anti-GLb and anti-GLc. All of anti-GLa, anti-GLb and anti-GLc recognize A. PRX and horseradish peroxidase but do not recognize fetuin. Also, their reactivity is inhibited by bromelain by more than 70%. The three categories of antibodies present high homogeneity and appear to be directed mainly towards the core structure [Xyl] (Man)3 [Fuc] (GlcNAc)2. An effective and simple method to screen antibodies with carbohydrate specificities is described herein.

Synthesis and structural studies of asparagine-modified 2-deoxy-alpha-N-glycopeptides associated with the renin-angiotensin system

Following addition of N-iodosuccinimide to glycals, reductive hydrogenolysis and ring opening gave 2-deoxy-alpha-N-glycopeptides carrying a deaminated asparagine unit. This reaction could be performed employing glucal, galactal, L-rhamnal, L-fucal and lactal to give the corresponding glycoconjugate building blocks 11, 12, 17, 22, 27 and 32. Further NIS-mediated glycosylation of the rhamno derivative 21 led to simple trisaccharide peptide adducts 45. Peptide synthesis of the gluco building unit with different preassembled oligopeptides afforded glycoconjugates 36, 39, 41 and 42 assumed to be of interest as potential inhibitors of the renin-angiotensin system.

Oligosaccharides and discordant xenotransplantation

The initiating factor in the hyperacute rejection of pig organs by human or non-human primates is believed to be related to the presence of preformed "natural" antibodies in the host. In 1991, we demonstrated that human anti-pig antibodies were IgG, IgM and IgA and bound most strongly to oligosaccharides with an alpha galactose (alpha Gal) terminal residue. These included (i) alpha Gal-R (alpha galactose), (ii) alpha Gall-3 beta Gal-R (B disaccharide), (iii) alpha Gall-3 beta Gall-4 beta GlcNAc-R (linear B type 2 trisaccharide) and (iv) alpha Gall-3 beta Gall-4 beta Glc-R (linear B type 6 trisaccharide) where R is (CH2) 8COOCH3. In vitro studies using both the chromium release assay and a live/dead staining technique demonstrated that the cytotoxicity of human sera towards pig cells can be significantly reduced or abolished by immunoadsorption of the serum with immunoaffinity columns of an alpha Gal structure, particularly those with an alpha 1-3 linkage, and not by a large selection of other carbohydrates. Similarly, human anti-pig antibodies can be largely inhibited or "neutralized" by the addition of an alpha 1-3Gal di- or trisaccharide to the serum. Staining of pig vascular endothelium utilizing a panel of carbohydrate-specific lectins and immunoaffinity antibodies demonstrated the presence of three different carbohydrate epitopes, namely (i) alpha Gall-3 beta Gall-4 beta GlcNAc-R (linear B type 2 trisaccharide (ii) alpha NeuAc2-3 beta Gall-4 beta GlcNAc-R (sialyl-N-acetyllactosamine), and (iii) beta Gall-4 beta GlcNAc-R (N-acetyllactosamine). We have investigated organs from several breeds of pig and have concluded that the alpha Gal epitope is either monomorphic or at least has a high incidence in porcine species, since we have not found any pig negative for this antigen. Human vascular endothelial cells have at their surface the same lactosamine-ended precursor and sialylated chains as pigs, but instead of terminal alpha Gal they express the fucosylated polymorphic ABH histo-blood group epitopes. As we have found no evidence that human or baboon plasma contain antibodies directed against sialic acid or lactosamine, and as human tissues contain both of these carbohydrates, it seems unlikely that either of these epitopes plays a role in the vascular rejection that takes place when pig organs are transplanted into primates. Unfortunately, the alpha Gal disaccharide and trisaccharides were not available to us in the large quantities required for extracorporeal immunoadsorption or continuous intravenous infusion in adult baboons.(ABSTRACT TRUNCATED AT 400 WORDS)

Epitope mapping of monoclonal antibodies directed against lipophosphoglycan of Leishmania major promastigotes

Monoclonal antibodies (MAbs) were generated against Leishmania major promastigote lipophosphoglycan (LPG) to use as tools in defining functional epitopes of this major cell surface glycoconjugate. Epitope mapping of four MAbs, designated 4A2-A2, 2G11-A3, 5E6-D10 and 5E10-F2, revealed that the phosphorylated oligosaccharide repeat unit PO4-6[Gal(beta 1-3)]Gal(beta 1-4)Man alpha 1-, P3, is a highly immunogenic epitope which has previously been demonstrated, by chemical analyses, to be a repeat unit specific to L. major. Two antibodies, 4A2-A2 and 5E10-F2, also recognised the repeat unit PO4-6[Ara(beta 1-2)Gal(beta 1-3)]Gal(beta 1-4)Man alpha 1-, 4Pa, with less affinity than P3, while 2G11-A3 recognised P4a with greater affinity than for P3. The L. major metacyclic-specific antibody 3F12 only recognised repeat units terminating with arabinose residues. In particular, 3F12 recognised P4a, which is upregulated in metacyclic LPG compared to the procyclic form of the molecule. The oligosaccharides P3, P4a and P5a are specific to L. major LPG. The epitopes of 4A2-A2, 2G11-A3, 5E6-D10 and 5E10-F2 were found on the cell surface and in the flagellar pocket of both procyclic and metacyclic V121 promastigotes, but were only detected at very low levels on amastigotes. The repeat unit P3 is able to inhibit attachment of procyclic promastigotes to the midgut of the sandfly vector, but neither Fab fragments of the four antibodies nor purified P3 could inhibit attachment of metacyclic promastigotes to the macrophage cell line J774. It was also shown that human sera from patients with cutaneous leishmaniasis recognised purified P3. The data suggests that while P3 is an immunogen in the natural course of infection of the human host, P3 plays no role in attachment and internalisation of promastigotes into the macrophages of the mammalian host.

Identification of organ-specific glycosylation of a membrane protein in two tissues using lectins

Since glycosylation of proteins is performed by the host cell, and variable sugar groupings can confer heterogeneity on the same polypeptide, we wished to see whether membrane proteins, in particular the ubiquitous transmembrane Na,K-ATPase, could be glycosylated differently in different organs. Using a highly sensitive enzyme-linked antibody detection system of bound digoxigenin-labelled lectins on nitrocellulose sheets containing electroblotted alpha and beta subunits of kidney and brain Na,K-ATPase, isolated from various rat strains, in combination with isoform-specific immunoblots, we discovered that brain Na,K-ATPase was highly mannosylated in contrast to renal Na,K-ATPase. Thus, we describe the existence of organ-related glycoforms of an integral ubiquitous membrane protein, i.e. diversification of the same polypeptide by organ-typical sugars. At the same time, the presence of the same glycosylation pattern can make distinct protein isoforms occurring in a same organ more homogeneous. Such organ-related glycoforms may serve for tissue identification and as tissue-specific receptors.

The use of polyacrylamide gel electrophoresis for the analysis of acidic glycans labeled with the fluorophore 2-aminoacridone

A method is described for the electrophoretic analysis of fluorophore-labeled acidic glycans. Various glycoproteins were treated with the enzyme, peptide N-glycosidase F, to release mixtures of asparagine-linked glycans that were then labeled with the fluorophore 2-aminoacridone, and the resulting derivatives electrophoresed in high-density polyacrylamide gels. The acidic glycans were separated with high resolution and distinct, well-resolved fluorescent band patterns were produced. Neutral saccharides did not electrophorese in the system. Information on the structure of the glycans was revealed by the changes in the band patterns observed after the mixtures of the glycan fluorophore derivatives were treated with either of the enzymes neuraminidase or beta-galactosidase. Quantities of glycoprotein as little as 10 micrograms were analysed without difficulty. The electrofluorograms were viewed using a digital imaging system based on a cooled charge-coupled device. The method was also demonstrated using purified acidic glycans of known structure. The method was easy to use, enabled the sensitive analysis of multiple samples in parallel and should be a useful addition to the range of glycan analytical techniques.

The adhesive specificity of the soluble human lectin, IgE-binding protein, toward lipid-linked oligosaccharides. Presence of the blood group A, B, B-like, and H monosaccharides confers a binding activity to tetrasaccharide (lacto-N-tetraose and lacto-N-neotetraose) backbones

The immunoglobulin E-binding protein, epsilon BP (also known as CBP35, Mac-2, L-34, and L-29), is a beta-galactoside-binding protein of approximately 30 kDa and a member of the animal lectin family termed S-type or S-Lac. Multiple biological activities have been attributed to this lectin such as mediation of IgE binding to the surface of Langerhans cells and activation of mast cells through binding to the high affinity IgE receptor. In order to better understand the cell-binding activity and the proposed role for epsilon BP as a biological response modifier, we have studied the specificity of binding of the radioiodinated epsilon BP to a series of lipid-linked, structurally defined oligosaccharide sequences of the lacto/neolacto family. The results show that the minimum lipid-linked oligosaccharides that can support epsilon BP binding are pentasaccharides of the lacto/neolacto series and that the lectin binds more strongly to oligosaccharides of this family that bear the blood group A, B, or B-like determinants than to those bearing blood group H. This preferential binding of epsilon BP is also manifest with whole cells, as erythrocytes of blood groups A and B are more strongly bound by epsilon BP than those of blood group O. Blood group Le(a) and Le(x) sequences are not bound by the lectin.(ABSTRACT TRUNCATED AT 250 WORDS)

H and B human blood-group antigen expression in cochlear hair cells is modulated by thyroxine

The presence of human blood-group antigens in developing and adult hypothyroid rat cochleas was analyzed using antibodies directed against antigens H and B. During postnatal development, hypothyroid rat cochleas exhibited a highly selective expression of both B and H antigens, mainly at the hair cell level. Labeling for antigen B was found throughout the hair cells, whereas the antibody directed against antigen H selectively labeled the apical part of these cells. These immunostaining patterns were similar to those found in normal (euthyroid) rat cochleas, but antigenic expression periods were clearly prolonged. Thus, whereas in normal rat cochleas, the B and H antigenic expression disappears from postnatal day (PD) 9 on, in cochleas of hypothyroid rats the reactivity was intense until PD15; it decreased from this developmental stage, and was negative or only faintly positive at PD30. Therefore, in congenital hypothyroidism, hair cell immunoreactivity is present at developmental stages that are negative in normal rat cochleas. These results suggest that human blood-group antigen expression on the developing cochlear hair cells of rats is modulated by thyroxine and that thyroxine is necessary for the temporal expression pattern and secretion of normal glycoproteins.

Fucose and fucose-containing sugar epitopes enhance hippocampal long-term potentiation in the freely moving rat

Male Wistar rats were intrahippocampally injected with L-fucose and the sugar epitope 2'-fucosyl-lactose prior to induction of long-term potentiation (LTP). Both substances had only a minimal and short-lasting depressive effect on the monosynaptically evoked field potential recorded in the dorsal blade of the dentate gyrus of freely moving rats upon stimulation of the perforant pathway. However, LTP induced by fractionated tetanization of the perforant pathway, which declined within 24 h in control animals injected with Lactose, remained at the initial level even 48 h after tetanization (difference to the control group significant with P < 0.01). The results support earlier findings which have indicated a participation of fucosylated macromolecules in the maintenance of LTP. Different molecular mechanisms concerning the effect of both substances and the significance of the data in elucidation of the relationship between LTP and memory formation are discussed.

Synthesis and solution structure of an S-glycosylated cyclic hexapeptide. Evidence for conformational change induced by glycosylation

Synthesis and conformational analysis of the S-glycosylated cyclic hexapeptide cyclo(-D-Pro1-Phe2-Cys3(tetra-O-acetyl-beta-D-galactopyranosyl)-++ +Trp4-Lys(Z)5- Phe6-) I was carried out to examine the influence of a saccharide residue in position i of a standard beta-turn on the formation of reverse turns and on the biological activity. Synthesis was carried out in the liquid phase employing a galactosylated cysteine building block. The cyclization reagents DPPA/NaHCO3 avoided high dilution conditions. Spectroscopic data were extracted from homo- and heteronuclear 2D-NMR techniques (TOCSY, NOESY, HMQC, HMQC-TOCSY, HMBCS-270). For structural refinement restrained molecular dynamics (MD) simulations in vacuo and with explicit DMSO as solvent were performed. Finally, simulations in DMSO without experimental restraints provided insight in stability and dynamics of the structural model. A comparison of the S-glycosylated Cys3 peptide with the analogous Thr3 peptide exhibits a similar overall conformation of the hexapeptide [beta II' D-Pro-Phe and another beta-turn about Trp4-Lys5(Z)]. However, the latter shows a distinct dynamic flip beta I, beta II in the glycopeptide, whereas the Thr-analogue only populates beta I. This influence is attributed to a beta I stabilizing effect of a hydrogen bridge of Thr-O gamma in position i to the NH of the amino acid in position i + 2, which is lacking in the glycosylated compound.

Structure of S-lectin, a developmentally regulated vertebrate beta-galactoside-binding protein

The crystal structure of a 14-kDa bovine spleen S-lectin complexed with the disaccharide N-acetyllactosamine at 1.9-A resolution reveals a surprising structural relationship to legume lectins, despite the lack of sequence homology. Two monomers associate to form an extended beta-sandwich, each with the same jelly roll topology typical of legume lectins but with dramatically trimmed loops and with different dimer association. Each monomer binds one N-acetyllactosamine molecule in a topologically and spatially different site than that of legume lectins. The carbohydrate-binding site provides an unprecedented paradigm for carbohydrate binding, with a unique network of salt bridges. The specificity for beta-galactose arises from intricate interactions that constrain the position of the O4 atom.

Glycosylation site of the major allergen from olive tree pollen. Allergenic implications of the carbohydrate moiety

The electrophoretic analysis of purified Ole e I, the major allergen from Olea europaea pollen, reveals the presence of two main variants, glycosylated (20.0 kDa) and non-glycosylated (18.5 kDa) components. The glycosylated variant has been identified as a concanavalin A-binding glycoprotein. Its carbohydrate moiety has a molecular mass of about 1.3 kDa (5% weight of the glycosylated allergen), based on mass spectrometry analysis. Enzymatic treatment of native Ole e I with the specific glycosidase PNGase F accounts for an oligosaccharide N-linked to the polypeptide chain. This treatment does not sensibly modify the secondary structure of the protein but diminishes the affinity of the allergen for specific IgE antibodies. Tryptic digestion of Ole e I reveals the presence of a single carbohydrate-containing peptide. This peptide was recognized by the sera of hypersensitive individuals. The amino acid sequence of this peptide is Phe-Lys-Leu-Asn-Thr-Val-Asn-Gly-Thr-Thr-Arg, asparagine at the seventh being the carbohydrate attaching site. The obtained data are discussed in terms of the potential role of the sugar moiety in the allergenic activity of Ole e I.

Protein glycosylation. Structural and functional aspects

During the last decade, there have been enormous advances in our knowledge of glycoproteins and the stage has been set for the biotechnological production of many of them for therapeutic use. These advances are reviewed, with special emphasis on the structure and function of the glycoproteins (excluding the proteoglycans). Current methods for structural analysis of glycoproteins are surveyed, as are novel carbohydrate-peptide linking groups, and mono- and oligo-saccharide constituents found in these macromolecules. The possible roles of the carbohydrate units in modulating the physicochemical and biological properties of the parent proteins are discussed, and evidence is presented on their roles as recognition determinants between molecules and cells, or cell and cells. Finally, examples are given of changes that occur in the carbohydrates of soluble and cell-surface glycoproteins during differentiation, growth and malignancy, which further highlight the important role of these substances in health and disease.