Saccharide Structures Research Articles

Computational Study of the Conformational Structures of Saccharides in Solution Based on J Couplings and the “Fast Sugar Structure Prediction Software”

This article reports on the implementation of J coupling calculations in our recently developed Fast Sugar Structure Prediction Software (FSPS). The FSPS combines a smart and exhaustive algorithm to search through conformational space with the calculation of different experimental nuclear magnetic resonance observables to establish the conformation of saccharides in solution. Using our algorithm in combination with NMR data, we investigate the solution structure of three simple disaccharides (methyl alpha-sophoroside, methyl alpha-laminarabioside, and methyl alpha-cellobioside) and one complex bacterial polysaccharide (Shigella flexneri 5a).

Potential Therapeutic Application of Chondroitin Sulfate/Dermatan Sulfate

Glycosaminoglycans (GAGs) are complex polysaccharides, which play important roles in cell growth, differentiation, morphogenesis, cell migration, and bacterial/viral infections. Major GAGs include heparin (Hep)/heparan sulfate, and chondroitin sulfate (CS)/dermatan sulfate (DS). Hep has been used for the treatment of thromboembolic disorders for more than 75 years, and has an established position in therapy today. CS/DS has attracted less attention and its clinical use is limited. However, CS/DS also have intriguing biological activities, which in turn should help in the development of CS/DS-based therapeutics. In this review, the following potential applications of CS/DS chains are discussed. (1) Sugar drugs for parasitic and viral infections. Particular CS variants appear to be involved in infections of various microbes, suggesting that CS/DS oligosaccharide sequences specifically interacting with microbes will lead to the development of inhibitory drugs for these infections. (2) Regenerative medicine. Biological activities of CS/DS chains possibly involve various growth factors, also known as Hep-binding growth factors. Specific CS/DS chains recruit growth/neurotrophic factors and/or potentiate their activities, suggesting that minute amounts of functional CS/DS chains can be utilized for tissue regeneration instead of signaling proteins. (3) Anti-tumor drugs. Specific saccharide structures in CS/DS chains appear to be involved in tumor cell proliferation and metastasis. The detection and identification of such CS/DS saccharide sequences would be an important contribution to cancer therapy.

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.

Engineering of PA-IIL lectin from Pseudomonas aeruginosa – Unravelling the role of the specificity loop for sugar preference

BackgroundLectins are proteins of non-immune origin capable of binding saccharide structures with high specificity and affinity. Considering the high encoding capacity of oligosaccharides, this makes lectins important for adhesion and recognition. The present study is devoted to the PA-IIL lectin from Pseudomonas aeruginosa, an opportunistic human pathogen capable of causing lethal complications in cystic fibrosis patients. The lectin may play an important role in the process of virulence, recognizing specific saccharide structures and subsequently allowing the bacteria to adhere to the host cells. It displays high values of affinity towards monosaccharides, especially fucose – a feature caused by unusual binding mode, where two calcium ions participate in the interaction with saccharide. Investigating and understanding the nature of lectin-saccharide interactions holds a great potential of use in the field of drug design, namely the targeting and delivery of active compounds to the proper site of action.ResultsIn vitro site-directed mutagenesis of the PA-IIL lectin yielded three single point mutants that were investigated both structurally (by X-ray crystallography) and functionally (by isothermal titration calorimetry). The mutated amino acids (22–23–24 triad) belong to the so-called specificity binding loop responsible for the monosaccharide specificity of the lectin. The mutation of the amino acids resulted in changes to the thermodynamic behaviour of the mutants and subsequently in their relative preference towards monosaccharides. Correlation of the measured data with X-ray structures provided the molecular basis for rationalizing the affinity changes. The mutations either prevent certain interactions to be formed or allow formation of new interactions – both of afore mentioned have strong effects on the saccharide preferences.ConclusionMutagenesis of amino acids forming the specificity binding loop allowed identification of one amino acid that is crucial for definition of the lectin sugar preference. Altering specificity loop amino acids causes changes in saccharide-binding preferences of lectins derived from PA-IIL, via creation or blocking possible binding interactions. This finding opens a gate towards protein engineering and subsequent protein design to refine the desired binding properties and preferences, an approach that could have strong potential for drug design.

Synthetic Glycopeptides from the Mucin Family as Potential Tools in Cancer Immunotherapy

Compared to glycoproteins of healthy cells, glycoproteins of tumor cells are often aberrantly glycosylated. Thus, glycopeptide fragments of surface glycoproteins of tumor cells are of interest as tumor-associated antigens for the distinction between normal and tumor cells. Cancer immunotherapy directed at selectively targeting these tumor-associated glycoprotein structure alterations--deficient glycosylation and, thus, exposure of peptide epitopes which are masked in normal cells--is considered a promising approach for the treatment of cancer. For this purpose, glycoproteins from the mucin family are of particular interest. Mucins belong to a class of heavily O-glycosylated, high-molecular weight glycoproteins present on the surface of many epithelial cells. The mucin core protein consists of numerous tandem repeats rich in serine, threonine and proline. In their tumor-associated forms, epithelial mucins carry cryptic saccharide structures such as T(N)-, T-, sialyl-T(N)- and sialyl-T antigens and more complex oligosaccharides (e.g. Lewis(y)). In contrast to glycoproteins isolated from natural sources, synthetic glycopeptides can be obtained in high purity and with exactly defined structure. In this review, methodologies for the synthesis of mucin-type glycopeptides containing complex tumor-associated antigen structures are described. Due to the low immunogenicity often exhibited by synthetic tumor-associated glycopeptide antigens, their conjugation to carrier proteins or suitable T-cell epitopes is essential for the development of anti-tumor vaccines. The results of immunological evaluations of synthetic (glyco)peptides and oligosaccharides are described. Some of these synthetic vaccines show promising activities inducing proliferation of T-cells and cytotoxic T-cell responses.

Acceptor recognition of kojibiose phosphorylase from Thermoanaerobacter brockii: Syntheses of glycosyl glycerol and myo-inositol

The glucosyl transfer reaction of kojibiose phosphorylase (KP; EC 2.4.1.230) was examined using glycerol or myo-inositol as an acceptor. In the case of glycerol, KP produced two main transfer products: saccharides A and B. The structure of saccharide A was O-alpha-D-glucopyranosyl-(1-->1)-glycerol and that of saccharide B was O-alpha-D-glucopyranosyl-(1-->2)-O-alpha-D-glucopyranosyl-(1-->1)-glycerol. These results show that KP transferred a glucose residue to the hydroxyl group at position 1 of glycerol. On the other hand, when myo-inositol was used as an acceptor, KP produced four transfer products: saccharides 1-4. The structures of saccharides 1 and 2 were O-alpha-D-glucopyranosyl-(1-->1)- and O-alpha-D-glucopyranosyl-(1-->5)-myo-inositol, respectively; those of saccharides 3 and 4 were O-alpha-D-glucopyranosyl-(1-->2)-O-alpha-D-glucopyranosyl-(1-->1)- and O-alpha-D-glucopyranosyl-(1-->2)-O-alpha-D-glucopyranosyl-(1-->5)-myo-inositol, respectively. KP transferred a glucose residue to the hydroxyl group at position 1 or 5 of myo-inositol. On the basis of the structures of their glucosyl transfer products, glycerol and myo-inositol were found to have a common structure with three hydroxyl groups corresponding to the hydroxyl group of the glucose molecule at positions 2, 3 and 4. The conformation of these three hydroxyl groups in the structure is equatorial. This structure is the substrate recognition site of KP. It has been suggested that KP strictly recognizes the structures of glycerol and myo-inositol, and catalyzes the transfer reaction of a glucose residue to the hydroxyl group at position 1 in glycerol, and at position 1 or 5 in myo-inositol, corresponding to position 2 in glucose.

Structural Characterization of Complex Bacterial Glycolipids by Fourier Transform Mass Spectrometry

Bacterial glycolipids are complex amphiphilic molecules which are on the one hand of utmost importance for the organization and function of bacterial membranes, and which on the other hand play a major role in the activation of cells of the innate and adaptive immune system of the host. Already small alterations of their chemical structure may influence the biological activity tremendously. Due to their intrinsic biological heterogeneity [number and type of fatty acids, saccharide structures, and substitution with e.g. phosphate (P), 2-aminoethyl- (pyro)phosphate groups (P-Etn) or 4-amino-4-deoxyarabinose (Ara4N)], separation of the different components are a prerequisite for unequivocal chemical and NMR structural analyses. In this contribution the structural information which can be obtained from heterogeneous samples of glycolipids by Fourier transform (FT) ion cyclotron resonance mass spectrometric methods is described. By means of recently analysed complex biological samples the possibilities of high resolution electrospray ionization FT-MS are demonstrated. Capillary skimmer dissociation, as well as tandem mass spectrometry MS/MS analysis utilizing collision-induced dissociation and infrared multiphoton dissociation, are compared and their advantages to provide structural information of diagnostic importance are discussed.

Novel Heparan Sulfate Structures Revealed by Monoclonal Antibodies

The sulfated glycosaminoglycan heparan sulfate (HS) is found ubiquitously on cell surfaces, in the extracellular matrix, and intracellularly as HS proteoglycans. Because of the structural heterogeneity of HS, tissue-derived HS preparations represent a mixture of HS chains originating from different cell types and tissue loci. Monoclonal anti-HS antibodies have been employed to detect the localization of specific HS epitopes in tissues, but limited information has been available on the saccharide structures recognized by the antibodies. We have studied the saccharide epitope structures of four anti-HS antibodies, HepSS1, JM13, JM403, and 10E4, which all recognize distinct HS species as demonstrated by different patterns of immunoreactivity upon staining of embryonic rat and adult human tissues. The epitopes recognized by JM13 and HepSS1 were found almost exclusively in basement membrane HS, whereas JM403 and 10E4 reacted also with cell-associated HS species. The binding of HepSS1, JM403, and 10E4 to HS was dependent on the GlcN N-substitution of the polysaccharide rather than O-sulfation. HepSS1 thus interacted with N-sulfated HS domains, JM403 binding was critically dependent on N-unsubstituted GlcN residues, and 10E4 bound to "mixed" HS domains containing both N-acetylated and N-sulfated disaccharide units. By contrast, JM13 binding seemed to require the presence of 2-O-sulfated glucuronic acid residues.

Synthesis of novel types of divalent saccharide structures by a ketene acetal Claisen rearrangement

A simple allyl uronate was rearranged to give the branched product, which in turn could be transformed into a spiro-annellated bicyclic compound. More complex allyl uronates of d-and l-arabinose were rearranged to give novel methylene-bridged dimeric saccharide structures.

The Structure of the Polysaccharides Produced by Higher Basidiomyces Tremella mesenterica Ritz.: Fr. and Inonotus levis P. Karst.

Polysaccharides are the best known and most potent mushroom-derived substances, with various positive health eff ects including immunostimulation, lowering glucose level in blood, anticancer activity, and many others. Biologically active polysaccharides are widerspread among higher Basidiomycetes mushrooms, and most of them have unique structures in diff erent species. Th ere is still no clear correlation between biological activity and structural features of the polysaccharides. Th is situation results in part from poor knowledge of polysaccharide structures. Herein, we present the results from analyzing two polymers of Tremella mesenterica and Inonotus levis. Tremella mesenterica (Yellow Brain mushroom) possesses a wide spectrum of medicinal properties, including immunostimulating, protecting against radiation, antidiabetic, anti-infl ammatory, hypocholesterolemic, hepatoprotective, and antiallergic eff ects. Glucuronoxylomannan (GXM), an extracellular polysaccharide produced by single cell culture of T. mesenterica, has a number of benefi cial health eff ects. We developed a new strain of T. mesenterica, CBS 101939, which grows in submerged culture and off ers superior yields of one-cell biomass rich in exocellular heteropolysaccharide GXM. NMR and chemical analysis showed that it has a defi ned structure of repeating unit, which is O-acetylated at several points, stoichometrically at O-6 of two mannose residues, and partially at GlcA residue, which is present in non-acetylated or acetylated at positions 3 or 4 forms (see Fig. 1). Th ese results diff er from existing data on the structures of mushroom mannan-based poly sac charides, where mannan backbone was believed to be

Expression of carbohydrate xenoantigens on porcine peripheral nerve

The use of thin easily revascularized cutaneous nerve autografts, which has been the gold standard, or the alternative use of nerve allografts or artificial grafts for nerve reconstructing have all their pros and cons. Nerve xenotransplantation may offer a potential alternative. In a potential pig to human nerve xenograft transplantation set-up several porcine antigen barriers have to be considered such as carbohydrate antigens system like the blood group A/O, the Galalpha1-3Gal (alphaGal) and the Hanganutziu-Deicher (HD) antigens. The swine leukocyte protein antigens system may also have to bee considered. The knowledge of the antigen expression on pig peripheral nerves is today limited. The present study describes the distribution of glycolipid based carbohydrate xenoantigens in ischiadicus nerve from blood group A and O pigs. Glycolipid fractions were separated on thin layer chromatography plates and immunostained with human AB sera, biotinylated Griffonia simplicifolia isolectin B4, monoclonal antibodies reacting with the HD antigen and with blood group A antigens based on different core saccharide structures. In addition, the subcellular distribution of alphaGal and HD antigens were studied by light- and electron-microscopical immunohistochemistry. The total amount of neutral glycolipids was 15 mg/g tissue for both blood group A and O nerves with mono-glycosylceramides as the dominating component. The total amount of acidic glycolipids (gangliosides and sulpholipids) was 9 mg/g tissue for both the blood group O and A nerves with sulphatides as the dominating components. Analyses of the glycolipid fractions showed strong expression of both the alphaGal and the HD antigens in nerves from both blood group A and O pigs. In addition, small amounts of blood group A antigens were expressed in nerves from blood group A pigs. Staining of neutral glycolipids from blood group A pigs using monoclonal antibodies reacting with A antigen having different core structures suggested that the A epitope expressed on pig ischiadicus nerves is based on the type 1 core chain structure. Light and electron microscopical studies on the alphaGal and HD-antigen distribution revealed that the neural cells were alphaGal antigen negative. Endothelial cells of blood vessels, and lymphatic and perineural cells expressed alphaGal antigen. Both endothelial cells and myelinized axons revealed positively labelled for the HD antigen.

Structural characterization of water-insoluble nonstarchy polysaccharides of oats and barley

Water-insoluble dietary fibre of dehulled oat and barley grain was isolated with enzymatic hydrolysis of starch and protein and further extracted sequentially. The extractants used were saturated barium hydroxide, water, 1 M potassium hydroxide and 4 M sodium hydroxide. The fractions were dialysed against pure water, freeze-dried and weighed. Repeatable recoveries of extracts were obtained. The ground grains and extracted fractions were characterized for their fat and protein content. The fractions were hydrolysed and characterized by measuring their neutral sugar composition using gas chromatography. Comparison of the amounts of cellulosic and noncellulosic polysaccharides was made. Further characterization of fractions was performed with solid-state 13C CP/MAS NMR spectroscopy and IR spectroscopy. The differences in composition of insoluble material between oats and barley were small. The distribution of masses and monosaccharides of the five fractions obtained were similar and the measured spectra show the same characteristic signals as saccharide structures.

Mannan-binding proteins from boar seminal plasma

The interaction of boar seminal plasma proteins and sperm with yeast mannan was investigated by the enzyme-linked binding assay (ELBA) and specific detection of proteins after SDS electrophoresis and blotting using biotinylated derivative of the polysaccharide. Heparin-binding proteins (especially AQN 1 and DQH proteins) and their aggregated forms showed affinity to yeast mannan. Besides that, these proteins were shown to bind to oviductal epithelium. The mannan-binding activity of boar proteins and sperm was inhibited most efficiently by ovomucoid, ovalbumin and N-glycans released from ovalbumin, but not with d-glucose, d-mannose and their phosphates. On the other hand, yeast mannan inhibited both the interaction of boar seminal plasma and sperm with heparin and the binding of these proteins to porcine oviductal epithelium.Yeast mannan immobilized to divinyl sulfone-activated Sepharose was used for the isolation of mannan-binding proteins. Proteins adsorbed to the immobilized polysaccharide were analyzed by RP-HPLC, SDS electrophoresis and N-terminal amino acid sequencing. AQN and AWN spermadhesins and DQH protein (names are derived from the N-terminal amino acid sequence) were identified as components of the isolated fraction.The results suggest an involvement of mannan-binding proteins in the formation of the sperm oviductal reservoir in pig. The ability of these proteins to interact both the complex d-mannose-containing saccharide structures and the heparin may also play an important role in sperm release from the oviductal reservoir or the capacitation process.

Transglycosylation of glycosyl residues to cyclic tetrasaccharide by Bacillus stearothermophilus cyclomaltodextrin glucanotransferase using cyclomaltodextrin as the glycosyl donor.

Cyclomaltodextrin glucanotransferase (EC 2.4.1.19, abbreviated as CGTase) derived from Bacillus stearothermophilus produced a series of transfer products from a mixture of cyclomaltohexaose and cyclic tetrasaccharide (cyclo[-->6)-alpha-D-Glcp-(1-->3)-alpha-D-Glcp-(1-->6)-alpha-D-Glcp-(1-->3)-alpha-D-Glcp-(1-->], CTS). Of the transfer products, only two components, saccharides A and D, remained and accumulated after digestion with glucoamylase. The total combined yield of the saccharides reached 63.4% of total sugars, and enzymatic and instrumental analyses revealed the structures of both saccharides. Saccharide A was identified as 4-mono-O-alpha-glucosyl-CTS, [-->6)-[alpha-D-Glcp-(1-->4)]-alpha-D-Glcp-(1-->3)-alpha-D-Glcp-(1-->6)-alpha-D-Glcp-(1-->3)-alpha-D-Glcp-(1-->], and sachharide D was 4,4'-di-O-alpha-glucosyl-CTS, [-->6)-[alpha-D-Glcp-(1-->4)]-alpha-D-Glcp-(1-->3)-alpha-D-Glcp-(1-->6)-[alpha-D-Glcp-(1-->4)]-alpha-D-Glcp-(1-->3)-alpha-D-Glcp-(1-->]. These structures led us to conclude that the glycosyltransfer catalyzed by CGTase was specific to the C4-OH of the 6-linked glucopyranosyl residues in CTS.

Synthetic Glycopeptides for the Development of Antitumour Vaccines

Glycoproteins of tumour cells often are aberrantly glycosylated. In its tumour-associated form, the epithelial mucin MUC1 carries short saccharide structures such as TN, T, sialyl-TN, and sialyl-T antigens. Due to the incomplete saccharide components, peptide epitopes of the backbone become accessible to the immune system. For the construction of synthetic antitumour vaccines, glycopeptides have been synthesized which contain tumour-associated saccharide antigens and peptide sequences from the tandem repeat portion of MUC1. In the synthesis of these glycopeptides, preformed glycosyl–amino acid building blocks are applied in solution- or solid-phase strategies. Examples are given for the use of N-Fmoc-protected TN, T, sialyl-TN, and sialyl-T antigen–serine and –threonine conjugates in syntheses of glycopeptides from the tandem repeat region from MUC1 and the N-terminal portion of leukosialin (CD43). In solid-phase syntheses allylic linkers (cleavable under neutral conditions), acid-sensitive linkers, and the novel 2-phenyl-2-trimethylsilyl-ethyl ester linker (detachable with fluoride under neutral conditions) have successfully been applied. The synthetic tumour-associated glycopeptide antigens, identical to nature, often exhibit low immunogenicity. Therefore, their conjugation to carrier proteins is demanded. To this aim, glycopeptide antigens have been equipped with biotin labels or conjugated with T-cell peptide epitopes. Some of these synthetic glycopeptide antigens induce proliferation of T-cells and a cytotoxic T-cell response.

Facilitated transport of saccharides through a supported liquid membrane containing a neutral lipophilic resorcinarene carrier

The facilitated transport of several types of saccharidess has been studied across a supported liquid membrane containing a resorcinarene carrier. The rate-determining step is believed to be the migration of a carrier-saccharide-water ternary complex in the organic solvent. The transport kinetics obey a saturation law, that allows the calculation of the stability constants of the various complexes. The stability constants are related to the saccharide structures: important factors are chain length, configuration and substitution of key hydroxyl groups.

Heparin/Heparan Sulfate Domains in Binding and Signaling of Fibroblast Growth Factor 8b

The role of heparin and heparan sulfate in the binding and signaling of fibroblast growth factors (FGFs) has been subject to intense investigation, but the studies have largely been confined to two species (FGF1 and FGF2) of the family with approximately 20 members. We have investigated the structural requirements for heparin/heparan sulfate in binding and activation of FGF8 (splice variant b). We present evidence that the minimal FGF8b-binding saccharide domain encompasses 5-7 monosaccharide units. The N-, 2-O-, and 6-O-sulfate substituents of heparin/heparan sulfate (HS) are all involved in the interaction, preferentially in the form of trisulfated IdoUA(2-OSO(3))-GlcNSO(3)(6-OSO(3)) disaccharide constituents. These structural characteristics resemble those described earlier for FGF1. By contrast, the saccharide structures required for the biological activity of FGF8b differed significantly from those characteristic for FGF1 and FGF2. Experiments with cells lacking active HS indicated that extended >/=14-mer heparin domains were needed to enhance cell proliferation and Erk phosphorylation by FGF8b, whereas in cells stimulated with FGF1 or FGF2 the corresponding responses were achieved by much shorter, 6-8-mer, oligosaccharides. Furthermore, still longer domains were needed to activate FGF8b in cells with "non-optimal" FGF receptor expression. Collectively, our data suggest that the heparin/HS structures enhancing the biological activity of FGFs were influenced by the FGF species involved as well as by the cellular composition of FGF receptors.

Affinity analysis of lectin interaction with immobilized C- and O- gylcosides studied by surface plasmon resonance assay

A biosensor based on the surface plasmon resonance (SPR) principle was used for kinetic analysis of lectin interactions with different immobilized saccharide structures. A novel affinity ligands β- d-glycopyranosylmethylamines derived from common d-aldohexoses linked to the carboxymethyl dextran layer of the SPR sensor surface served for interactions with a wide range of lectins. The method of preparation and use of the β- d-mannopyranosyl glycosylated sensor surface was described. The results of affinity analysis of lectin–ligand interactions were evaluated and compared with data obtained from measurements using commercially available p-aminophenyl α- d-glycopyranosides. Possible applications and advantages of C- and O-glycosylated SPR biosensors are discussed.

Binding of Heparin/Heparan Sulfate to Fibroblast Growth Factor Receptor 4

Fibroblast growth factors (FGFs) are heparin-binding polypeptides that affect the growth, differentiation, and migration of many cell types. FGFs signal by binding and activating cell surface FGF receptors (FGFRs) with intracellular tyrosine kinase domains. The signaling involves ligand-induced receptor dimerization and autophosphorylation, followed by downstream transfer of the signal. The sulfated glycosaminoglycans heparin and heparan sulfate bind both FGFs and FGFRs and enhance FGF signaling by mediating complex formation between the growth factor and receptor components. Whereas the heparin/heparan sulfate structures involved in FGF binding have been studied in some detail, little information has been available on saccharide structures mediating binding to FGFRs. We have performed structural characterization of heparin/heparan sulfate oligosaccharides with affinity toward FGFR4. The binding of heparin oligosaccharides to FGFR4 increased with increasing fragment length, the minimal binding domains being contained within eight monosaccharide units. The FGFR4-binding saccharide domains contained both 2-O-sulfated iduronic acid and 6-O-sulfated N-sulfoglucosamine residues, as shown by experiments with selectively desulfated heparin, compositional disaccharide analysis, and a novel exoenzyme-based sequence analysis of heparan sulfate oligosaccharides. Structurally distinct heparan sulfate octasaccharides differed in binding to FGFR4. Sequence analysis suggested that the affinity of the interaction depended on the number of 6-O-sulfate groups but not on their precise location.

N-Acetyllactosamine and the CT Carbohydrate Antigen Mediate Agrin-Dependent Activation of MuSK and Acetylcholine Receptor Clustering in Skeletal Muscle

Galβ1,3GalNAc and Galβ1,4GlcNAc are the subterminal saccharide structures present on the CT carbohydrate antigen GalNAcβ1,4[NeuAcα2,3]-Galβ1-(3GalNAc or 4GlcNAc)-R, which is localized at the mammalian neuromuscular junction. Here we show that Galβ1,3GalNAc, Galβ1,4GlcNAc, and the CT carbohydrate antigen affect postsynaptic assembly in cultured muscle cells. Treatment of C2C12 myotubes with benzyl-O-α-GalNAc or neuraminidase increased peanut agglutinin (PNA) expression and AChR clustering. Induction of AChR clustering was blocked by PNA and by muscle agrin. Addition of Galβ1,4GlcNAc or Galβ1,3GalNAc increased AChR clustering in myotubes and muscle-specific kinase (MuSK) autophosphorylation in vitro, while NeuAcα2,3Galβ1,4GlcNAc and Galβ1,4Glc did not. Neural agrin activated MuSK in vitro if the lactosamine-containing mucin domain was present, and this activation was blocked in large part by Galβ1,3GalNAc and Galβ1,4GlcNAc. Agrin fragments and MuSK bound to these disaccharides with differing specificities. Overexpression of the CT carbohydrate antigen also increased AChR clustering and MuSK autophosphorylation in the presence of neural agrin. These data suggest a model in which different portions of the CT carbohydrate structure contribute to agrin-dependent signal transduction.