Carbohydrate structural units in glycoproteins and polysaccharides as important ligands for Gal and GalNAc reactive lectins

Whole-cell tumor vaccines desialylated to uncover tumor antigenic Gal/GalNAc epitopes elicit anti-tumor immunity

BACKGROUND

Aberrant sialoglycans on the surface of tumor cells shield potential tumor antigen epitopes, escape recognition, and suppress activation of immunocytes. α2,3/α2,6Gal- and α2,6GalNAc (Gal/GalNAc)-linked sialic acid residues of sialoglycans could affect macrophage galactose-type lectins (MGL) mediated-antigen uptake and presentation and promote sialic acid-binding immunoglobulin-like lectins (Siglecs) mediated-immunosuppression. Desialylating sialoglycans on tumor cells could present tumor antigens with Gal/GalNAc residues and overcome glyco-immune checkpoints. Thus, we explored whether vaccination with desialylated whole-cell tumor vaccines (DWCTVs) triggers anti-tumor immunity in ovarian cancer (OC).

METHODS

Sialic acid (Sia) and Gal/GalNAc residues on OC A2780, OVCAR3, and ID8 cells treated with α2-3 neuraminidase (α2-3NA) and α2-6NA, and Sigec-9 or Siglec-E and MGL on DCs pulsed with desialylated OC cells were identified using flow cytometry (FCM); RT-qPCR determined IFNG expression of T cells, TRBV was sequenced using Sanger sequencing and cytotoxicity of αβ T cells was measured with LDH assay; Anti-tumor immunity in vivo was validated via vaccination with desialylated whole-cell ID8 vaccine (ID8 DWCTVs).

RESULTS

Gal/GalNAc but not Sia residues were significantly increased in the desialylated OC cells. α2-3NA-modified DWCTV increased MGL but decreased Siglec-9 or Siglec E expression on DCs. MGL^bright/Siglec-9^dim DCs significantly up-regulated IFNG expression and CD4/CD8 ratio of T cells and diversified the TCR repertoire of αβ T-cells that showed enhanced cytotoxic activity. Vaccination with α2-3NA-modified ID8 DWCTVs increased MGL^bright/Siglec-E^dim DCs in draining lymph nodes, limited tumor growth, and extended survival in tumor-challenged mice.

CONCLUSION

Desialylated tumor cell vaccine could promote anti-tumor immunity and provide a strategy for OC immunotherapy in a clinical setting.

Glycoproteins Presenting Galactose and N-Acetylgalactosamine in Human Seminal Plasma as Potential Players Involved in Immune Modulation in the Fertilization Process

In light of recent research, there is increasing evidence showing that extracellular semen components have a significant impact on the immune reaction of the female partner, leading to the tolerogenic response enabling the embryo development and implantation as well as further progress of healthy pregnancy. Seminal plasma glycoproteins are rich in the unique immunomodulatory glycoepitopes that may serve as ligands for endogenous lectins that decorate the surface of immune cells. Such interaction may be involved in modulation of the maternal immune response. Among immunomodulatory glycans, Lewis type antigens have been of interest for at least two decades, while the importance of T/Tn antigens and related structures is still far from understanding. In the current work, we applied two plant lectins capable of distinguishing glycoepitopes with terminal GalNAc and Gal to identify glycoproteins that are their efficient carriers. By means of lectin blotting and lectin affinity chromatography followed by LC-MS, we identified lactotransferrin, prolactin inducible protein as well as fibronectin and semenogelins 1 and 2 as lectin-reactive. Net-O-glycosylation analysis results indicated that the latter three may actually carry T and/or Tn antigens, while in the case of prolactin inducible protein and lactotransferrin LacdiNAc and lactosamine glycoepitopes were more probable. STRING bioinformatics analysis linked the identified glycoproteins in the close network, indicating their involvement in immune (partially innate) processes. Overall, our research revealed potential seminal plasma ligands for endogenous Gal/GalNAc specific lectins with a possible role in modulation of maternal immune response during fertilization.

The Distribution Profile of Glycoconjugates in the Testis of Brown-Banded Bamboo Shark (Chiloscyllium punctatum) by Using Lectin Histochemistry

The testis of bamboo shark is characterized by diametric development leading to zonation architecture. Here, we investigated the staining pattern of 12 lectins in 6 groups of differential binding specificities within the germ, somatic, and interstitial cells of each zone. The neutral mucopolysaccharides appeared in the interstitial tissue in all the zones and became more significant in the spermatozoal–Sertoli cell junction. The cellular localization of the lectins varies in testicular zones and cell types. There was a gradual increase in glycosylation toward the degenerative zone. The increased intensity of most lectins in the interstitial cells indicates the association of glycoconjugates in their androgen-secreting activity. Statistical analyses showed a significant correlation between the groups of lectins and each lectin used, stronger response to lectins in the interstitial cells (ICs) than other cell types. Moreover, the response to glucosamine (GlcNAc), galactosamine (GalNAc), and fucose tended to be higher than glucose and galactose. Furthermore, the intensity of response was increased toward the degenerative zone. In addition, we can use peanut agglutinin (PNA) as an acrosomal marker in combination with other marker proteins for studying shark spermatogenesis. These findings refer to the crucial role of glycoconjugates in spermatogenesis in the bamboo shark testis.

Beneficial effects of tyrosol on altered glycoprotein components in streptozotocin-induced diabetic rats

CONTEXT

Olive oil is the major source of tyrosol which is a natural phenolic antioxidant. Olive oil constitutes a major component of the Mediterranean diet that is linked to a reduced incidence of chronic diseases.

OBJECTIVE

This study evaluates the effects of tyrosol on altered glycoprotein components in streptozotocin-induced diabetic rats.

MATERIALS AND METHODS

Diabetes mellitus was induced in male Wistar rats by streptozotocin (40 mg/kg body weight). These rats were administered tyrosol (20 mg/kg body weight) and glibenclamide (600 μg/kg body weight) orally daily for 45 days. Plasma glucose, plasma insulin, glycoprotein components such as hexose, hexosamine, sialic acid and fucose in the plasma, liver and kidney, and histopathogy of tissues were analyzed.

RESULTS

Diabetic rats revealed significant (p < 0.05) increase in the levels of glucose, hexose, hexosamine, sialic acid and fucose (277.17, 152.45, 100.43, 79.69 and 49.29 mg/dL) in the plasma; decrease in the levels of palsma insulin (6.12 μU/mL) and sialic acid (4.36 and 5.03 mg/g) in the liver and kidney; significant (p < 0.05) increase in hexose (49.33 and 46.82 mg/g), hexosamine (22.68 and 33.20 mg/g) and fucose (31.63 and 32.44 mg/g) in the liver and kidney. Further, periodic acid-Schiff staining of tissues revealed positive-stain accumulation in diabetic rats. Tyrosol treatment showed significant (p < 0.05) effects on all the biochemical parameters and histopathology studied in streptozotocin- nduced diabetic rats. Also, the in vitro study revealed the antioxidant effect of tyrosol.

DISCUSSION AND CONCLUSIONS

Thus, tyrosol protects streptozotocin-induced diabetic rats from the altered glycoprotein components. Further, this study can be extrapolated to humans.

Postnatal morphological and lectin histochemical observation of the submucosal glands of rat nasopharynx

The development of submucosal glands of rat nasopharynx was studied with respect to their morphological maturation and glycoprotein alterations during the postnatal period. This study examined the histological morphology with hematoxylin-eosin and the binding pattern of lectins, soybean agglutinin (SBA), Dolichos biflorus agglutinin (DBA), Vicia villosa agglutinin (VVA), Ulex europaeus agglutinin-I (UEA-I), peanut agglutinin (PNA), wheat germ agglutinin (WGA), and succinylated WGA (sucWGA) on frozen sections from newborn into adulthood. At birth, nasopharyngeal glands consisted of rudimentary secretory units which by postnatal day 3 (PN3) showed the characteristic features of salivary glands comprised of mixed mucous and serous cells. With maturation, serous cells increased in number and were arranged in clusters. Lectin reactivity at birth was detected at the acinar cell basal membranes for DBA, SBA, VVA, UEA-1 and PNA. At PN3, lectins labeled the apical cytoplasm and basolateral membranes of mucous cells and progressively with maturation, extended from the apical to basal portions of the cytoplasm with variable reactivity of VVA, PNA and sucWGA. Serous cells were labeled by UEA-1 starting from PN10 and also by PNA in adults. Ducts showed variable lectin reaction on the luminal membrane with strong reactivity of DBA and UEA-1 at PN21. Taken together, lectin histochemistry indicated the transitional occurrence of glycoproteins depending on the stage of maturation of the glands. Moreover, these results emphasize the difference in the morphology and lectin histochemistry between the nasopharyngeal and palatine glands.

Effect of lectins from Diocleinae subtribe against oral Streptococci

Surface colonization is an essential step in biofilm development. The ability of oral pathogens to adhere to tooth surfaces is directly linked with the presence of specific molecules at the bacterial surface that can interact with enamel acquired pellicle ligands. In light of this, the aim of this study was to verify inhibitory and antibiofilm action of lectins from the Diocleinaesubtribe against Streptococcus mutans and Streptococcus oralis. The inhibitory action against planctonic cells was assessed using lectins from Canavaliaensi formis (ConA), Canavalia brasiliensis (ConBr), Canavalia maritima (ConM), Canavalia gladiata (CGL) and Canavalia boliviana (ConBol). ConBol, ConBr and ConM showed inhibitory activity on S. mutans growth. All lectins, except ConA, stimulated significantly the growth of S. oralis. To evaluate the effect on biofilm formation, clarified saliva was added to 96-well, flat-bottomed polystyrene plates, followed by the addition of solutions containing 100 or 200 µg/mL of the selected lectins. ConBol, ConM and ConA inhibited the S. mutans biofilms. No effects were found on S. oralis biofilms. Structure/function analysis were carried out using bioinformatics tools. The aperture and deepness of the CRD (Carbohydrate Recognition Domain) permit us to distinguish the two groups of Canavalia lectins in accordance to their actions against S. mutans and S. oralis. The results found provide a basis for encouraging the use of plant lectins as biotechnological tools in ecological control and prevention of caries disease.

Oligosaccharide binding proteins from Bifidobacterium longum subsp. infantis reveal a preference for host glycans

Bifidobacterium longum subsp. infantis (B. infantis) is a common member of the infant intestinal microbiota, and it has been characterized by its foraging capacity for human milk oligosaccharides (HMO). Its genome sequence revealed an overabundance of the Family 1 of solute binding proteins (F1SBPs), part of ABC transporters and associated with the import of oligosaccharides. In this study we have used the Mammalian Glycan Array to determine the specific affinities of these proteins. This was correlated with binding protein expression induced by different prebiotics including HMO. Half of the F1SBPs in B. infantis were determined to bind mammalian oligosaccharides. Their affinities included different blood group structures and mucin oligosaccharides. Related to HMO, other proteins were specific for oligomers of lacto-N-biose (LNB) and polylactosamines with different degrees of fucosylation. Growth on HMO induced the expression of specific binding proteins that import HMO isomers, but also bind blood group and mucin oligosaccharides, suggesting coregulated transport mechanisms. The prebiotic inulin induced other family 1 binding proteins with affinity for intestinal glycans. Most of the host glycan F1SBPs in B. infantis do not have homologs in other bifidobacteria. Finally, some of these proteins were found to be adherent to intestinal epithelial cells in vitro. In conclusion, this study represents further evidence for the particular adaptations of B. infantis to the infant gut environment, and helps to understand the molecular mechanisms involved in this process.

Primary structure and specificity of a new member of galectin family from the Amethyst deceiver mushroom Laccaria amethystina

A new galectin was characterized in the Amethyst deceiver mushroom Laccaria amethystina. The complete amino acid (AA) sequence of the lectin, which exhibited β-galactoside specificity, was deduced from its peptide sequences. The AA sequence of L. amethystina galectin (LAG) showed high homology with those of the same genus, at 75.6% identity to Laccaria bicolor, and 35.5-65.0% to galectins of Agrocybe spp. and Coprinopsis cinerea. The AA sequence of LAG contained all but one conserved residue known to be involved in β-galactoside binding, with His at the position 57 residue replaced by Thr in LAG. Analysis of binding specificity by hemagglutination inhibition assay and enzyme-linked lectin-sorbent assay revealed high specificity of LAG towards O-glycoproteins.

Lectins as tools in glycoconjugate research

Lectins are ubiquitous proteins of nonimmune origin, present in plants, microorganisms, animals and humans which specifically bind defined monosugars or oligosaccharide structures. Great progress has been made in recent years in understanding crucial roles played by lectins in many biological processes. Elucidation of carbohydrate specificity of human and animal lectins is of great importance for better understanding of these processes. Long before the role of carbohydrate-protein interactions had been explored, many lectins, mostly of plant origin, were identified, characterized and applied as useful tools in studying glycoconjugates. This review focuses on the specificity-based lectin classification and the methods of measuring lectin-carbohydrate interactions, which are used for determination of lectin specificity or for identification and characterization of glycoconjugates with lectins of known specificity. The most frequently used quantitative methods are shortly reviewed and the methods elaborated and used in our laboratories, based on biotinylated lectins, are described. These include the microtiter plate enzyme-linked lectinosorbent assay, lectinoblotting and lectin-glycosphingolipid interaction on thin-layer plates. Some chemical modifications of lectin ligands on the microtiter plates and blots (desialylation, Smith degradation, beta-elimination), which extend the applicability of these methods, are also described.

Differential contributions of recognition factors of two plant lectins -Amaranthus caudatus lectin and Arachis hypogea agglutinin, reacting with Thomsen-Friedenreich disaccharide (Galbeta1-3GalNAcalpha1-Ser/Thr)

Previous reports on the carbohydrate specificities of Amaranthus caudatus lectin (ACL) and peanut agglutinin (PNA, Arachis hypogea) indicated that they share the same specificity for the Thomsen-Friedenreich (T(alpha), Galbeta1-3GalNAcalpha1-Ser/Thr) glycotope, but differ in monosaccharide binding--GalNAc>>Gal (inactive) for ACL; Gal>>GalNAc (weak) with respect to PNA. However, knowledge of the recognition factors of these lectins was based on studies with a small number monosaccharides and T-related oligosaccharides. In this study, a wider range of interacting factors of ACL and PNA toward known mammalian structural units, natural polyvalent glycotopes and glycans were examined by enzyme-linked lectinosorbent and inhibition assays. The results indicate that the main recognition factors of ACL, GalNAc was the only monosaccharide recognized by ACL as such, its polyvalent forms (poly GalNAcalpha1-Ser/Thr, Tn in asialo OSM) were not recognized much better. Human blood group precursor disaccharides Galbeta1-3/4GlcNAcbeta (I(beta)/II(beta)) were weak ligands, while their clusters (multiantennary II(beta)) and polyvalent forms were active. The major recognition factors of PNA were a combination of alpha or beta anomers of T disaccharide and their polyvalent complexes. Although I(beta)/II(beta) were weak haptens, their polyvalent forms played a significant role in binding. From the 50% molar inhibition profile, the shape of the ACL combining site appears to be a cavity type and most complementary to a disaccharide of Galbeta1-3GalNAc (T), while the PNA binding domain is proposed to be Galbeta1-3GalNAcalpha or beta1--as the major combining site with an adjoining subsite (partial cavity type) for a disaccharide, and most complementary to the linear tetrasaccharide, Galbeta1-3GalNAcbeta1-4Galbeta1-4Glc (T(beta)1-4L, asialo GM(1) sequence). These results should help us understand the differential contributions of polyvalent ligands, glycotopes and subtopes for the interaction with these lectins to binding, and make them useful tools to study glycosciences, glycomarkers and their biological functions.

Differential affinities of Erythrina cristagalli lectin (ECL) toward monosaccharides and polyvalent mammalian structural units

Previous studies on the carbohydrate specificities of Erythrina cristagalli lectin (ECL) were mainly limited to analyzing the binding of oligo-antennary Galbeta1-->4GlcNAc (II). In this report, a wider range of recognition factors of ECL toward known mammalian ligands and glycans were examined by enzyme-linked lectinosorbent and inhibition assays, using natural polyvalent glycotopes, and a glycan array assay. From the results, it is shown that GalNAc was an active ligand, but its polyvalent structural units, in contrast to those of Gal, were poor inhibitors. Among soluble natural glycans tested for 50% molecular mass inhibition, Streptococcus pneumoniae type 14 capsular polysaccharide of polyvalent II was the most potent inhibitor; it was 2.1 x 10(4), 3.9 x 10(3) and 2.4 x 10(3) more active than Gal, tri-antennary II and monomeric II, respectively. Most type II-containing glycoproteins were also potent inhibitors, indicating that special polyvalent II and Galbeta1-related structures play critically important roles in lectin binding. Mapping all information available, it can be concluded that: [a] Galbeta1-->4GlcNAc (II) and some Galbeta1-related oligosaccharides, rather than GalNAc-related oligosaccharides, are the core structures for lectin binding; [b] their polyvalent II forms within macromolecules are a potent recognition force for ECL, while II monomer and oligo-antennary II forms play only a limited role in binding; [c] the shape of the lectin binding domains may correspond to a cavity type with Galbeta1-->4GlcNAc as the core binding site with additional one to four sugars subsites, and is most complementary to a linear trisaccharide, Galbeta1-->4GlcNAcbeta1-->6Gal. These analyses should facilitate the understanding of the binding function of ECL.

Differential binding to glycotopes among the layers of three mammalian retinal neurons by man-containing N-linked glycan, T(alpha) (Galbeta1-3GalNAcalpha1-), Tn (GalNAcalpha1-Ser/Thr) and I (beta)/II (beta) (Galbeta1-3/4GlcNAcbeta-) reactive lectins

Carbohydrate structures between retinal neurons and retinal pigment epithelium (RPE) play an important role in maintaining the integrity of retinal adhesion to underlying RPE, and in retinal detachment pathogenesis. Since relevant knowledge is still in the primary stage, glycotopes on the adult retina of mongrel canines (dog), micropigs and Sprague-Dawley rats were examined by lectino-histochemistry, using a panel of 16 different lectins. Paraffin sections of eyes were stained with biotinylated lectins, and visualized by streptavidin-peroxidase and diaminobenzidine staining. Mapping the affinity profiles, it is concluded that: (i) all sections of the retina reacted well with Morniga M, suggesting that N-linked glycans are present in all layers of the retina; (ii) no detectable human blood group ABH active glycotopes were found among retinal layers; (iii) outer and inner segments contained glycoconjugates rich in ligands reacting with T (alpha) (Galbeta1-3GalNAcalpha1-Ser/Thr) and Tn (GalNAcalpha1-Ser/Thr) specific lectins; (iv) cone cells of retina specifically bound peanut agglutinin (PNA), which recognizes T (alpha) residues and could be used as a specific marker for these photoreceptors; (v) the retinas of rat, dog and pig, had a similar binding profile but with different intensity; (vi) each retinal layer had its own binding characteristic. This information may provide useful background knowledge for normal retinal physiology and miscellaneous retinal diseases, including retinal detachment (RD) and age-related macular degeneration (ARMD).

Recognition profile of Morus nigra agglutinin (Morniga G) expressed by monomeric ligands, simple clusters and mammalian polyvalent glycotopes

The carbohydrate binding properties of a novel member of the subfamily of galactose-specific jacalin-related lectin isolated from the bark of black mulberry (Morus nigra) (Morniga G) was studied in detail by enzyme-linked lectinosorbent and inhibition assays using panels of monomeric saccharides, mammalian polyvalent glycotopes and polysaccharides. Among the natural glycans tested for lectin binding, Morniga G reacted best with glycoproteins (gps) presenting a high density of tumor-associated carbohydrate antigens Tn (GalNAcalpha1-Ser/Thr) and Talpha (Galbeta1-3GalNAcalpha1-). Their reactivities, on a nanogram basis, were up to 72.5, 3.9x10(3), 6.0x10(3), 8.8x10(3) and 2.9x10(4) times higher than that of Tn-containing glycopeptides (M.W.<3000 Da), monomeric T, Tn, GalNAc and Gal, respectively. It also reacted well with many multi-antennary N-glycans with II (Galbeta1-4GlcNAc) termini, ABH histo-blood group antigens and their precursors containing high densities of I/II and T/Tn glycotopes, and sialylated T/Tn. Among the mono-, di- and oligosaccharides tested, Thomsen-Friedenreich (T) disaccharide with aromatic aglycon [Galbeta1-3GalNAcalpha1-benzyl (Talpha1-benzyl)] and Tn glycopeptides were the best inhibitors. Molecular modeling and docking studies indicated the occurrence of a primary GalNAcalpha1- and Galbeta1-3GalNAc glycotope-binding site in Morniga G. Using a recently proposed system [Wu, A.M., 2003. Carbohydrate structural units in glycoproteins and polysaccharides as important ligands for Gal and GalNAc reactive lectins. J. Biomed. Sci. 10, 676-688], the binding properties of the combining sites of Morniga G can be defined as follows: (i) the monosaccharide specificity is GalNAc/Gal>>Man/Glc, GlcNAc and lFuc; (ii) the mammalian glycotope specificity is Talpha1-benzyl>T>Tn>GalNAcbeta1-3Gal (P), while B/E (Galalpha1-3/4Gal), I/II (Galbeta1-3/4GlcNAc), S (GalNAcbeta1-4Gal), F/A (GalNAcalpha1-3GalNAc/Gal) and L (Galbeta1-4Glc) are inactive; (iii) the most active ligand is T/Tn; (iv) simple clustered Tn or triantennary N-glycans with II termini (Tri-II) have limited impact; (v) high-density polyvalent glycotopes play a prominent role for enhancing Morniga G reactivity. These results provide evidence for the binding of this lectin to dense cell surface T/Tn glycoconjugates and facilitate future usage of this lectin in biotechnological and medical applications.

Recognition factors of Ricinus communis agglutinin 1 (RCA1)

Ricinus communis agglutinin (RCA1) is one of the most important applied lectins that has been widely used as a tool to study cell surfaces and to purify glycans. Although the carbohydrate specificity of RCA1 has been described, the information obtained was mainly focused on inhibition of simple Galbeta1-related oligosaccharides and simple clusters. Here, all possible recognition factors of RCA1 of glycan binding were examined by enzyme-linked lectinosorbent (ELLSA) and inhibition assays, using known mammalian Gal/GalNAc carbohydrate structural units and natural polyvalent glycans. Among the glycoproteins (gps) tested and expressed as 50% nanogram inhibition, the high-density polyvalent Galbeta1-4GlcNAc (II) glycotopes occurring in natural gps, such as Pneumococcus type 14 capsular polysaccharide which is composed of repeating poly II residues, resulted in 9.0 x 10(4), 1.5 x 10(5), 2.3 x 10(4) and 2.1 x 10(4)-fold higher affinities to RCA1 than the monomeric Gal, linear I/II and Tri-antennary-II (Tri-II). Of the ligands tested and expressed as nanomoles of 50% inhibition, Tri-II was the best, being about 2, 4, 25.6 and 33.3 times better inhibitor than Di-II, II, I (Galbeta1-3GlcNAc) and Gal, respectively. From the results of this study, it is concluded that: (a) Galbeta1-4GlcNAc and other Galbeta1-related oligosaccharides are essential for lectin binding and their polyvalent form in macromolecules should be the most important recognition factor for RCA1; (b) the combining site of RCA1 may be a groove type, recognizing Galbeta1-4GlcNAc (II) as the major binding site; (c) its combining size may be large enough to accommodate a tetrasaccharide of beta-anomeric Gal at the non-reducing end and most complementary to human blood group I Ma active trisaccharide (Galbeta1-4GlcNAcbeta1-6Gal) and lacto-N-neotetraose (Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc); (d) RCA1 has a preference for the beta-anomer of Gal oligosaccharides with a Galbeta1-4 linkage > Galbeta1-6 > or = Galbeta1-3; (e) configuration of carbon-2, -3 -4 and -6 in Gal are essential for binding; (f) hydrophobic interaction in the vicinity of the binding site useful for sugar accommodation increases affinity. These results should be helpful for understanding the functional role of RCA1 and for characterizing glycotopes of mammalian complex carbohydrates.

Carbohydrate specificity of an insecticidal lectin isolated from the leaves of Glechoma hederacea (ground ivy) towards mammalian glycoconjugates

Preliminary studies indicated that the potent insecticidal lectin, Gleheda, from the leaves of Glechoma hederacea (ground ivy) preferentially agglutinates human erythrocytes carrying the Tn (GalNAcalpha1-Ser/Thr) antigen. However, no details have been reported yet with respect to the fine specificity of the lectin. To corroborate the molecular basis of the insecticidal activity and physiological function of Gleheda, it is necessary to identify the recognition factors that are involved in the Gleheda-glycotope interaction. In the present study, the requirement of high-density multivalent carbohydrate structural units for Gleheda binding and a fine-affinity profile were evaluated using ELLSA (enzyme-linked lectinosorbent assay) with our extended glycan/ligand collections, a glycan array and molecular modelling. From the results, we concluded that a high-density of exposed multivalent Tn-containing glycoproteins (natural armadillo and asialo ovine salivary glycoproteins) were the most potent factors for Gleheda binding. They were, on a nanogram basis, 6.5x10(5), 1.5x10(4) and 3.1x10(3) times more active than univalent Gal (galactose), GalNAc (N-acetylgalactosamine) and Tn respectively. Among mono- and oligo-saccharides examined, simple clustered Tn (molecular mass <3000 Da) from ovine salivary glycoprotein was the best, being 37.5 and 1.7x10(3) times better than GalNAc and Gal respectively. GalNAc glycosides were significantly more active than Gal glycosides, indicating that the N-acetamido group at C-2 plays an important role in Gleheda binding. The results of glycan array support the conclusions drawn with respect to the specificity of Gleheda based on the ELLSA assays. These findings combined with the results of the molecular modelling and docking indicate the occurrence of a primary GalNAcalpha1-binding site in the Gleheda monomer. However, the extraordinary binding feature of Gleheda for glycoproteins demonstrates the importance of affinity enhancement by high-density multivalent glycotopes in the ligand-lectin interactions in biological processes.

Carbohydrate recognition factors of the lectin domains present in the Ricinus communis toxic protein (ricin)

Ricin (RCA60) is a potent cytotoxic protein with lectin domains, contained in the seeds of the castor bean Ricinus communis. It is a potential biohazard. To corroborate the biological properties of ricin, it is essential to understand the recognition factors involved in the ricin-glycotope interaction. In previous reports, knowledge of the binding properties of ricin was limited to oligosugars and glycopeptides with different specificities. Here, recognition factors of the lectin domains in ricin were examined by enzyme-linked lectinosorbent (ELLSA) and inhibition assays, using mammalian Gal/GalNAc structural units and corresponding polyvalent forms. Except for blood group GalNAcalpha1-3Gal (A) active and Forssman (GalNAcalpha1-3GalNAc, F) disaccharides, ricin has a broad range of affinity for mammalian disaccharide structural units-Galbeta1-4Glcbeta1-(Lbeta), Galbeta1-4GlcNAc (II), Galbeta1-3GlcNAc (I), Galbeta1-3GalNAcalpha1-(Talpha), Galbeta1-3GalNAcbeta1-(Tbeta), Galalpha1-3Gal (B), Galalpha1-4Gal (E), GalNAcbeta1-3Gal (P), GalNAcalpha1-Ser/Thr (Tn) and GalNAcbeta1-4Gal (S). Among the polyvalent glycotopes tested, ricin reacted best with type II-containing glycoproteins (gps). It also reacted well with several T (Thomsen-Friedenreich), tumor-associated Tn and blood group Sd. (a+)-containing gps. Except for bird nest and Tamm-Horsfall gps (THGP), this lectin reacted weakly or not at all with ABH-blood type and sialylated gps. From the present and previous results, it can be concluded that: (i) the combining sites of these lectin domains should be a shallow-groove type, recognizing Galbeta1-4Glcbeta1- and Galbeta1-3(4)GlcNAcbeta- as the major binding site; (ii) its size may be as large as a tetrasaccharide and most complementary to lacto-N-tetraose (Galbeta1-3GlcNAc beta1-3Galbeta1-4Glc) and lacto-N-neotetraose (Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc); (iii) the polyvalency of glycotopes, in general, enhances binding; (iv) a hydrophobic interaction in the vicinity of the binding site for sugar accommodation, increases the affinity for Galbeta-. This study should assist in understanding the glyco-recognition factors involved in carbohydrate-toxin interactions in biological processes. The effect of the polyvalent P/S glycotopes on ricin binding should be examined. However, this is hampered by the lack of availability of suitable reagents.

Polyvalent GalNAcalpha1-->Ser/Thr (Tn) and Galbeta1-->3GalNAcalpha1-->Ser/Thr (T alpha) as the most potent recognition factors involved in Maclura pomifera agglutinin-glycan interactions

The agglutinin isolated from the seeds of Maclura pomifera (MPA) recognizes a mucin-type disaccharide sequence, Galbeta1-->3GalNAc (T) on a human erythrocyte membrane. We have utilized the enzyme-linked lectinosorbent assay (ELLSA) and inhibition assay to more systematically analyze the carbohydrate specificity of MPA with glyco-recognition factors and mammalian Gal/GalNAc structural units in lectin-glycoform interactions. From the results, it is concluded that the high densities of polyvalent GalNAcalpha1-->Ser/Thr (Tn) and Galbeta1-->3GalNAcalpha1-->Ser/Thr (T(alpha)) glycotopes in macromolecules are the most critical factors for MPA binding, being on a nanogram basis 2.0 x 10(5), 4.6 x 10(4) and 3.9 x 10(4) more active than monovalent Gal, monomeric T and Tn glycotope, respectively. Other carbohydrate structural units in mammalian glycoconjugates, such as human blood group Sd (a+) related disaccharide (GalNAcbeta1-->4Gal) and Pk/P1 active disaccharide (Galalpha1-->4Gal) were inactive. These results demonstrate that the configurations of carbon-4 and carbon-2 are essential for MPA binding and establish the importance of affinity enhancement by high-density polyvalencies of Tn/T glycotopes in MPA-glycan interactions. The overall binding profile of MPA can be defined in decreasing order as high density of polyvalent Tn/T(alpha) (M.W. > 4.0 x 10(4)) >> Tn-containing glycopeptides (M.W. < 3.0 x 10(3)) > monomeric T/Tn and P (GalNAcbeta1-->3Gal) > GalNAc > Gal >> Man, L: ARA: , D: Fuc and Glc (inactive). Our findings should aid in the selection of this lectin for elucidating functions of carbohydrate chains in life processes and for applications in the biomedical sciences.

Lectinochemical studies on the glyco-recognition factors of a Tn (GalNAcα1→Ser/Thr) specific lectin isolated from the seeds of Salvia sclarea

The lectin extracted from the seeds of Salvia sclarea (SSL) recognizes the Tn antigen (GalNAc alpha1-->Ser/Thr) expressed in certain human carcinomas. In previous studies, knowledge of the binding properties of SSL was restricted to GalNAcalpha1--> related oligosaccharides and glycopeptides. Thus, the requirements of functional groups in monosaccharide and high-density polyvalent carbohydrate structural units for SSL binding and an updated affinity profile were further evaluated by enzyme-linked lectinosorbent (ELLSA) and inhibition assays. Among the glycoproteins (gps) tested for interaction, a high density of exposed Tn-containing glycoproteins such as in the armadillo salivary Tn glycoprotein and asialo ovine salivary glycoprotein reacted best with SSL. When the gps were tested for inhibition of SSL binding, which was expressed as 50% nanogram inhibition, the high density polyvalent Tn present in macromolecules was the most potent inhibitor. Among the monosaccharide and carbohydrate structural units studied, which were expressed as nanomole inhibition, GalNAc alpha1-->3GalNAc beta1-->3Gal alpha1-->4Gal beta1-->4Glc (Fp), GalNAc alpha1-->3Gal beta1-->4Glc (A(L)), GalNAc alpha1-->3GalNAc beta1-->Me (F beta), GalNAc alpha1-->3GalNAc alpha1-->Me (F alpha) and GalNAc alpha1--> Ser/Thr (Tn) were the most active ligands, being 2.5-5.0 x 10(3) and 1.25-2.5 times more active than Gal and GalNAc, respectively. From the results, it is suggested that the combining site of SSL is a shallow groove type, recognizing the monosaccharide of GalNAc as the major binding site or Tn up to the Forssman pentasaccharide (Fp). It can be concluded that the three critical factors for SSL binding are the -NH CH(3)CO at carbon-2 in Gal, the configuration of carbon-3 in GalNAc, and the polyvalent Tn (GalNAc alpha1-->Ser/Thr) present in macromolecules. These results should assist in understanding the glyco-recognition factors involved in carbohydrate-lectin interactions in biological processes. The effect of the polyvalent F alpha, F beta and GalNAc beta1-->3Gal alpha1--> (P alpha) glycotopes on binding should be examined. However, this is hampered by the lack of availability of suitable reagents.

Further characterization of the binding properties of two monoclonal antibodies recognizing human Tn red blood cells

The terminal alpha anomeric Ga1NAc residue is an essential sugar for the Tn glycotope, human blood group A determinant, and Forssman antigen. In a previous study [King M.J., Parson S.F., Wu A,M., Jones N., Transfusion 31: 142-149, 1991] we defined two monoclonal antibodies (MoAbs, BRIC66 and BRIC111) reacting with human Tn red blood cells. However, more advanced studies of these two MoAbs were hampered by the lack of availability of Gal/GalNAc related glycotopes. In order to use these antibodies as powerful probes to elucidate structural changes during life processes, we have characterized in detail the combining sites of these two MoAbs using enzyme-linked immunosorbent (ELISA) and inhibition assays with an extended glycan/ligand collection. From the results, it has been established that BRIC66 demonstrated multiple specificities and its reactivity towards glycotopes was defined as: Ga1NAc alpha1-->Ser/Thr (Tn) > or = Ga1NAc alpha1-->3(LFuc alpha1-->2)Gal (Ah) > Ga1NAcalpha1-->3Galbeta1-->4Glc (AL) > Ga1NAalpha1-->3Gal (A) GalNAc alpha1-->3GalNAc >> Gal or Glc. Another MoAb, BRIC111, mainly bound Tn-glycophorin. The best ligand for this MoAb was Tn-containing glycopeptides (M.W. < 3.0 x 10(3) Da) from asialo ovine salivary mucin (OSM), which was approximately 70 and 58 times more active than Ga1NAc and monomeric Ga1NAc alpha1-->Ser/Thr (Tn), respectively, suggesting that the active glycotopes present in glycophorin for BRIC111 binding also exist in OSM. The N-acetyl group at carbon-2 and configuration at carbon-2 and carbon-4 of the alpha anomeric Ga1NAc are required for the binding of either MoAb. Identification of these binding properties should aid in the selection of these MoAbs and the conditions required for biological studies and clinical applications.

Determination of structural and functional overlap/divergence of five proto-type galectins by analysis of the growth-regulatory interaction with ganglioside GM1in silicoandin vitroon human neuroblastoma cells

The growth-regulatory interplay between ganglioside GM1 on human SK-N-MC neuroblastoma cells and an endogenous lectin provides a telling example for glycan (polysaccharide) functionality. Galectin-1 is the essential link between the sugar signal and the intracellular response. The emerging intrafamily complexity of galectins raises the question on defining extent of their structural and functional overlap/divergence. We address this problem for proto-type galectins in this system: ganglioside GM1 as ligand, neuroblastoma cells as target. Using the way human galectin-1 interacts with this complex natural ligand as template, we first defined equivalent positioning for distinct substitutions in the other tested proto-type galectins, e.g., Lys63 vs. Leu60/Gln72 in galectins-2 and -5. As predicted from our in silico work, the tested proto-type galectins have affinity for the pentasaccharide of ganglioside GM1. In contrast to solid-phase assays, cell surface presentation of the ganglioside did not support binding of galectin-5, revealing the first level of regulation. Next, a monomeric proto-type galectin (CG-14) can impair galectin-1-dependent negative growth control by competitively blocking access to the shared ligand without acting as effector. Thus, the quaternary structure of proto-type galectins is an efficient means to give rise to functional divergence. The identification of this second level of regulation is relevant for diagnostic monitoring. It might be exploited therapeutically by producing galectin variants tailored to interfere with galectin activities associated with the malignant phenotype. Moreover, the given strategy for comparative computational analysis of extended binding sites has implications for the rational design of galectin-type-specific ligands.

Lectinochemical studies on the affinity of Anguilla anguilla agglutinin for mammalian glycotopes

Anguilla anguilla agglutinin (AAA) is a fucose-specific lectin found in the serum of the fresh water eel. It is suggested to be associated with innate immunity by recognizing disease-associated cell surface glycans, and has been widely used as a reagent in hematology and glycobiology. In order to gain a better understanding of AAA for further applications, it is necessary to elucidate its binding profile with mammalian glycotopes. We, therefore, analyzed the detailed carbohydrate specificity of AAA by enzyme-linked lectinosorbent assay (ELLSA) with our extended glycan/ligand collection and lectin-glycan inhibition assay. Among the glycans tested, AAA reacted well with nearly all human blood group Ah (GalNAcalpha1-->3[LFucalpha1-->2]Gal), Bh (Galalpha1-->3[LFucalpha1-->2]Gal), H LFucalpha1-->2Gal) and Leb (Fucalpha1-->2Galbeta1-->3[Fucalpha1-->4]GlcNAc) active glycoproteins (gps), but not with blood group Lea (Galbeta1-->3[Fucalpha1-->4]GlcNAc) substances, suggesting that residues and optimal density of alpha1-2 linked LFuc to Gal at the non-reducing end of glycoprotein ligands are essential for lectin-carbohydrate interactions. Blood group precursors, Galbeta1-3GalNAc (T), GalNAcalpha1-Ser/Thr (Tn) containing glycoproteins and N-linked plasma gps, gave only negligible affinity. Among the mammalian glycotopes tested, Ah, Bh and H determinants were the best, being about 5 to 6.7 times more active than LFuc, but were weaker than p-nitrophenylalphaFuc indicating that hydrophobic environment surrounding the LFuc moiety enhance the reactivity. The hierarchy of potency of oligo- and monosaccharides can be ranked as follows: p-nitrophenyl-alphaFuc > Ah, Bh and H > LFuc > LFucalpha1-->2Galbeta1-->4Glc (2'-FL) and Galbeta1-->4[LFucalpha1-->3]Glc (3'-FL), while LNDFH I (Leb hexa-), Lea, Lex (Galbeta1-->4[Fucalpha1-->3]GlcNAc), and LDFT (gluco-analogue of Ley) were inactive. From the present observations, it can be concluded that the combining site of AAA should be a small cavity-type capable of recognizing mainly H/crypto H and of binding to specific polyvalent ABH and Leb glycotopes.

Recognition profile of Bauhinia purpurea agglutinin (BPA)

Bauhinia purpurea agglutinin (BPA) is a Galbeta1-3GalNAc (T) specific leguminous lectin that has been widely used in multifarious cytochemical and immunological studies of cells and tissues under pathological or malignant conditions. Despite these diverse applications, knowledge of its carbohydrate specificity was mainly limited to molecular or submolecular T disaccharides. Thus, the requirement of high density polyvalent or multi-antennary carbohydrate structural units for BPA binding and an updated affinity profile were further evaluated by enzyme-linked lectinosorbent (ELLSA) and inhibition assays. Among the glycoproteins (gps) tested and expressed as 50% nanogram inhibition, the high density polyvalent GalNAcalpha1-Ser/Thr (Tn) and Galbeta1-3/4GlcNAc (I/II) glycotopes present on macromolecules generated a great enhancement of binding affinity for BPA as compared to their monomers. The most potent inhibitors were a Tn-containing gp (asialo OSM) and a I/II containing gp (human blood group precursor gp), which were up to 1.7 x 10(4) and 2.3 x 10(3) times more potent than monovalent Gal and GalNAc, respectively. However, multi-antennary glycopeptides, such as tri-antennary Galbeta1-4GlcNAc, which was slightly more active than II or Gal, gave only a minor contribution. Regarding the carbohydrate structural units studied by the inhibition assay, blood group GalNAcbeta1-3/4Gal (P/S) active glycotopes were active ligands. The overall binding profile of BPA was: high density polyvalent T/Tn and II clusters >>> Tn-glycopeptides (M.W. <3.0 x 10(3))/Talpha monomer > monovalent P/S > Tn monomer and GalNAc > tri-antennary II > Gal >> Man and Glc (inactive). These findings give evidence for the binding of this lectin to dense cell surface T, Tn and I/II glycoconjugates and should facilitate future usage of this lectin in biotechnological and medical applications.

Polyvalency of Tn (GalNAcα1→Ser/Thr) glycotope as a critical factor forVicia villosaB4and glycoprotein interactions

Vicia villosa B(4) (VVL-B(4)) is an important lectin for detecting exposed Tn (GalNAcalpha1-Ser/Thr) determinants on cancer cells. In order to elucidate the binding factors involved in VVL-B(4) and glycotope interaction, the binding properties of this lectin were analyzed by enzyme-linked lectinosorbent and inhibition assays. From the results, it is concluded that the most critical factor affecting VVL-B(4) binding is polyvalency at the alpha anomer of Gal with -NH CH(3)CO at carbon-2 (Tn epitope), which enhances the reactivity by 3.3x10(5) times over monovalent Gal. The reactivities of glycotopes can be ranked as follows: high density Tn cluster >>Tn glycopeptides (MW<3.0x10(3) >> monomeric Tn to tri- Tn glycopeptides >>> other GalNAcalpha/beta-related structural units>Gal and Galalpha- or beta-linked ligands, demonstrating the essential role of the polyvalency of Tn glycotopes in the enhancement of the binding.