Carbohydrate specificity of a galectin from chicken liver (CG-16)

Biophysical characterization of lectin–glycan interactions for therapeutics, vaccines and targeted drug-delivery

Lectin–glycan interactions play a role in biological processes, host–pathogen interactions and in disease. A more detailed understanding of these interactions is not only useful for the elucidation of their biological function but can also be applied in immunology, drug development and delivery and diagnostics. We review some commonly used biophysical techniques for studying lectin–glycan interactions; namely: frontal affinity chromatography, glycan/lectin microarray, surface plasmon resonance, electrochemical impedance spectroscopy, isothermal titration calorimetry, fluorescent assays, enzyme linked lectin sorbent assay and saturation transfer difference nuclear magnetic resonance spectroscopy. Each method is evaluated on efficiency, cost and throughput. We also consider the advantages and limitations of each technique and provide examples of their application in biology, drug discovery and delivery, immunology, glycoprofiling and biosensing.

Combining glycocluster synthesis with protein engineering: an approach to probe into the significance of linker length in a tandem-repeat-type lectin (galectin-4)

Complementarity in lectin-glycan interactions in situ is assumed to involve spatial features in both the lectin and the glycan, giving a functional meaning to structural aspects of the lectin beyond its carbohydrate-binding site. In combining protein engineering with glycocluster synthesis, it is shown that the natural linker length of a tandem-repeat-type human lectin (galectin-4) determines binding properties in two binding assays (using surface-presented glycoprotein and cell surface assays). The types of glycocluster tested included bivalent lactosides based on tertiary amides of terephthalic, isophthalic, 2,6-naphthalic and oxalic acids as well as bivalent H(type 2) trisaccharides grafted on secondary/tertiary terephthalamides and two triazole-linker-containing cores. The presented data reveal a marked change in susceptibility to the test compounds when turning the tandem-repeat-type to a proto-type-like display. The testing of glycoclusters is suggested as a general strategy to help to delineate the significance of distinct structural features of lectins beyond their contact sites to the glycan.

Carbohydrate specificity of chicken and human tandem-repeat-type galectins-8 in composition of cells

The network of adhesion/growth-regulatory galectins in chicken (chicken galectin, CG) has only one tandem-repeat-type protein, CG8. Using a cell-based assay and probing galectin reactivity with a panel of fluorescent neoglycoconjugates (glycoprobes), its glycan-binding profile was determined. For internal validation, human galectin-8 (HG8) was tested. In comparison to HG8, CG8 showed a rather similar specificity: both galectins displayed high affinity to blood group ABH antigens as well as to 3'-sialylated and 3'-sulfated lactosamine chains. The most remarkable difference was found to be an ability of HG8 (but not CG8) to bind the disaccharide Galβ1-3GlcNAc (Le(c)) as well as branched and linear oligolactosamines. The glycan-binding profile was shown to be influenced by glycocalix of the cell, where the galectin is anchored. Particularly, glycosidase treatment of galectin-loaded cells led to the change of the profile. Thus, we suppose the involvement of cis-glycans in the interaction of cell-anchored galectins with external glycoconjugates.

Galectins promote the interaction of influenza virus with its target cell

Influenza virus is known to bind sialoglycans located on the surface of the host cell. In addition, recent data suggest the involvement of other molecular targets in viral reception. Of note, a high density of terminal galactose residues is created on the surface of virions because of the influenza virus' own neuraminidase activity. Thus, we suggested the possibility for an interaction of the influenza virus with galactose-binding proteins--galectins. In the present work we studied the influence of several galectins on the adhesion and further internalization of virus into the cell; six virus strains and three cell lines were studied. Chicken galectins CG-1A and -2 as well as human galectins HGal-1 and -8 promote virus binding in dose dependent manner, but they do not influence the internalization stage. Also, galectins are able to restore the ability of influenza virus to infect desialylated cells up to the level of native cells. When CG-1A in physiological concentrations was loaded onto viruses, the adhesion level was higher than in the case of on-cell loading. The effect of adhesion increase depends on the glycan structure of target-cell as well as of virus. The aggregated data suggest a promotional effect of galectins during the stage of influenza virus binding with the surface of target-cell.

A regulatory network of two galectins mediates the earliest steps of avian limb skeletal morphogenesis

BACKGROUND

The skeletal elements of vertebrate embryonic limbs are prefigured by rod- and spot-like condensations of precartilage mesenchymal cells. The formation of these condensations depends on cell-matrix and cell-cell interactions, but how they are initiated and patterned is as yet unresolved.

RESULTS

Here we provide evidence that galectins, β-galactoside-binding lectins with β-sandwich folding, play fundamental roles in these processes. We show that among the five chicken galectin (CG) genes, two, CG-1A, and CG-8, are markedly elevated in expression at prospective sites of condensation in vitro and in vivo, with their protein products appearing earlier in development than any previously described marker. The two molecules enhance one another's gene expression but have opposite effects on condensation formation and cartilage development in vivo and in vitro: CG-1A, a non-covalent homodimer, promotes this process, while the tandem-repeat-type CG-8 antagonizes it. Correspondingly, knockdown of CG-1A inhibits the formation of skeletal elements while knockdown of CG-8 enhances it. The apparent paradox of mutual activation at the gene expression level coupled with antagonistic roles in skeletogenesis is resolved by analysis of the direct effect of the proteins on precartilage cells. Specifically, CG-1A causes their aggregation, whereas CG-8, which has no adhesive function of its own, blocks this effect. The developmental appearance and regulation of the unknown cell surface moieties ("ligands") to which CG-1A and CG-8 bind were indicative of specific cognate- and cross-regulatory interactions.

CONCLUSION

Our findings indicate that CG-1A and CG-8 constitute a multiscale network that is a major mediator, earlier-acting than any previously described, of the formation and patterning of precartilage mesenchymal condensations in the developing limb. This network functions autonomously of limb bud signaling centers or other limb bud positional cues.

Human galectin-3 (Mac-2 antigen): defining molecular switches of affinity to natural glycoproteins, structural and dynamic aspects of glycan binding by flexible ligand docking and putative regulatory sequences in the proximal promoter region

BACKGROUND

Human galectin-3 (Mac-2 antigen) is a cell-type-specific multifunctional effector owing to selective binding of distinct cell-surface glycoconjugates harboring β-galactosides. The structural basis underlying the apparent preferences for distinct glycoproteins and for expression is so far unknown.

METHODS

We strategically combined solid-phase assays on 43 natural glycoproteins with a new statistical approach to fully flexible computational docking and also processed the proximal promoter region in silico.

RESULTS

The degree of branching in N-glycans and clustering of core 1 O-glycans are positive modulators for avidity. Sialylation of N-glycans in α2-6 linkage and of core 1 O-glycans in α2-3 linkage along with core 2 branching was an unfavorable factor, despite the presence of suited glycans in the vicinity. The lectin-ligand contact profile was scrutinized for six natural di- and tetrasaccharides enabling a statistical grading by analyzing flexible docking trajectories. The computational analysis of the proximal promoter region delineated putative sites for Lmo2/c-Ets-1 binding and new sites with potential for RUNX binding.

GENERAL SIGNIFICANCE

These results identify new features of glycan selectivity and ligand contact by combining solid-phase assays with in silico work as well as of reactivity potential of the promoter.

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.

Glycomic mapping of pseudomucinous human ovarian cyst glycoproteins: Identification of Lewis and sialyl Lewis glycotopes

Expression of sialyl Lewis x (sLe(x)) and sialyl Lewis a (sLe(a)) on cell-surface glycoproteins endows cells with the ability to adhere to E-, P-, and L-selectins present on endothelia, platelets, or leukocytes. Special arrangements of these glycotopes in cancers are thought to play a key role in metastasis. Previous studies have mostly described membrane-bound sLe(x) and sLe(a) activities. In this report, the major O-glycans of the secreted human ovarian cyst sialoglycoproteins from a Le(a+) nonsecretor individual (human ovarian cyst sample 350) were characterized by MS/MS analyses and immuno-/lectin-chemical assays. The results showed that HOC 350 carries a large number of epitopes for sLe(x), sLe(a), and Le(a) reactive antibodies. Advanced MS/MS sequencing coupled with mild periodate oxidation and exoglycosidase digestions further revealed that the O-glycans from HOC 350 are mostly of core 1 and 2 structures, extended and branched on the 3-arm with both type I and type II chains, complete with variable degrees of terminal sialylation and/or fucosylation to yield the sLe(x) or sLe(a) epitopes. Thus, the underlying core and peripheral backbone structures are similar to that of a previously proposed composite structural model for nonsialylated human ovarian cysts O-glycans, but with some notable distinguishing structural features in addition to sialylation.

Tumor suppressor p16INK4a--modulator of glycomic profile and galectin-1 expression to increase susceptibility to carbohydrate-dependent induction of anoikis in pancreatic carcinoma cells

Expression of the tumor suppressor p16(INK4a) after stable transfection can restore the susceptibility of epithelial tumor cells to anoikis. This property is linked to increases in the expression and cell-surface presence of the fibronectin receptor. Considering its glycan chains as pivotal signals, we assumed an effect of p16(INK4a) on glycosylation. To test this hypothesis for human Capan-1 pancreatic carcinoma cells, we combined microarray for selected glycosyltransferase genes with 2D chromatographic glycan profiling and plant lectin binding. Major differences between p16-positive and control cells were detected. They concerned expression of beta1,4-galactosyltransferases (down-regulation of beta1,4-galactosyltransferases-I/V and up-regulation of beta1,4-galactosyltransferase-IV) as well as decreased alpha2,3-sialylation of O-glycans and alpha2,6-sialylation of N-glycans. The changes are compatible with increased beta(1)-integrin maturation, subunit assembly and binding activity of the alpha(5)beta(1)-integrin. Of further functional relevance in line with our hypothesis, we revealed differential reactivity towards endogenous lectins, especially galectin-1. As a result of reduced sialylation, the cells' capacity to bind galectin-1 was enhanced. In parallel, the level of transcription of the galectin-1 gene increased conspicuously in p16(INK4a)-positive cells, and even figured prominently in a microarray on 1996 tumor-associated genes and in proteomic analysis. The cells therefore gain optimal responsiveness. The correlation between genetically modulated galectin-1 levels and anoikis rates in engineered transfectants inferred functional significance. To connect these findings to the fibronectin receptor, galectin-1 was shown to be co-immunoprecipitated. We conclude that p16(INK4a) orchestrates distinct aspects of glycosylation that are relevant for integrin maturation and reactivity to an endogenous effector as well as the effector's expression. This mechanism establishes a new aspect of p16(INK4a) functionality.

Dendritic cell-derived exosomes express a Streptococcus pneumoniae capsular polysaccharide type 14 cross-reactive antigen that induces protective immunoglobulin responses against pneumococcal infection in mice

Exosomes activate T cells in vivo, but whether exosomes are able to induce humoral immune responses is still unknown. We found that dendritic cells, but not other immune cells, constitutively release an exosome-associated glycoconjugate that is cross-reactive with the capsular polysaccharide of Streptococcus pneumoniae type 14 (Cps14-CRA). Cps14-CRA was localized to the cholesterol-enriched microdomains or rafts of the exosomes and was mapped to the beta1-->6 branched N-acetyl-lactosamine derivatives of the Cps14-CRA. Injection of CFA-primed naive mice with purified dendritic cell exosomes induced immunoglobulin (Ig) anti-Cps14 responses composed predominantly of IgM, IgG3, and IgG1. These responses were associated with protection against a lethal challenge with live S. pneumoniae type 14, but not with type 3 bacteria, and was correlated with the titer of elicited IgM and IgG3 anti-Cps14. These data show, for the first time, that exosomes can induce a humoral immune response to an associated unprocessed, autologous antigen. Although anti-Cps14 Ig responses are specifically demonstrated, these could reflect a broader mechanism that modulates both natural immunity and autoimmunity to other glycotopes.

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.

Evidence for lectin activity of a plant receptor-like protein kinase by application of neoglycoproteins and bioinformatic algorithms

Detection of genes for putative receptor-like protein kinases, which contain an extracellular domain related to leguminous lectins, in plant genomes inspired the hypothesis that this part acts as sensor. Initial support for this concept came from proof for protein kinase activity. The next step, focusing on the protein of lombardy poplar (Populus nigra var. italica), is scrutiny for lectin activity. Consequently, we first pinpointed sets of high-scoring sequence pairs by extensive databank search. The calculations resulted in P-values in the range from 10(-14) to 10(-18) exclusively for leguminous lectins, the Pterocarpus angolensis agglutinin being front runner with P=3 x 10(-18) and thus most suitable template for modeling. The superimposition of the two folds gave notable similarity in the region responsible for binding carbohydrate and Ca(2+)/Mn(2+)-ions. Binding activity toward carbohydrates was detected by assaying a panel of (neo)glycoproteins as polyvalent probes, especially for alpha-l-rhamnose and glycans of asialofetuin. It was strictly dependent on Ca(2+)-ions, enhanced by Mn(2+)-ions and reached a K(D)-value of 34.3 nM for the neoglycoprotein with rhamnose as ligand. These results give further research direction to define physiological ligands, plant/bacterial rhamnose-containing saccharides and rhamnose-mimetic glycans or peptides being potential candidates.

Identification of peptide ligands for malignancy- and growth-regulating galectins using random phage-display and designed combinatorial peptide libraries

Members of the galectin family of endogenous lectins are involved in tumor growth regulation and in establishing characteristics of the malignant phenotype via protein-carbohydrate and protein-protein interactions. To identify peptide ligands with the potential to modulate these tumor-relevant interactions beneficially, complementary screening methods were employed, that is, both phage-display and a combinatorial pentapeptide library with the key YXY tripeptide core. Three representative prototype galectins were selected. The search for high-affinity ligands among phage-displayed random heptamers yielded enrichment after five selection cycles of the nonglycomimetic CQSPSARSC peptide in the case of the chicken homologue of galectin-1 but not the human protein, an indication for specificity. The most active glycomimetic from the combinatorial library of 5832 pentamers was WYKYW. Identification of peptide ligands for galectins with and without glycomimetic properties is thus possible. Our study documents the potential to combine the two library-based approaches for structural optimization of lead peptides.

Effects of polyvalency of glycotopes and natural modifications of human blood group ABH/Lewis sugars at the Galbeta1-terminated core saccharides on the binding of domain-I of recombinant tandem-repeat-type galectin-4 from rat gastrointestinal tract (G4-N)

In our recent publication, we defined core aspects of the carbohydrate specificity of domain-I of recombinant tandem-repeat-type galectin-4 from rat gastrointestinal tract (G4-N), especially its potent interaction with the linear tetrasaccharide Galbeta1-3GlcNAcbeta1-3Galbeta1-4Glc (Ibeta1-3L). The assumed role of galectin-4 as a microvillar raft stabilizer/organizer and as a malignancy-associated factor in hepatocellular and gastrointestinal carcinomas called for further refinement of its binding specificity. Thus, the effects of polyvalency of glycotopes and natural modifications of human blood group ABH/Lewis sugars at the terminal Galbeta1-core saccharides were thoroughly examined by the enzyme-linked lectinosorbent and lectin-glycan inhibition assays. The results indicate that (a) a high-density of polyvalent Galbeta1-3/4GlcNAc (I/II), Galbeta1-3GalNAc (T) and/or GalNAcalpha1-Ser/Thr (Tn) strongly favors G4-N/glycoform binding. These glycans were up to 2.3 x 10(6), 1.4 x 10(6), 8.8 x 10(5), and 1.4 x 10(5) more active than Gal, GalNAc, monomeric I/II and T, respectively; (b) while lFuc is a poor inhibitor, its presence as alpha1-2 linked to terminal Galbeta1-containing oligosaccharides, such as H active Ibeta1-3L, markedly enhances the reactivities of these ligands; (c) when blood group A (GalNAcalpha1-) or B (Galalpha1-) determinants are attached to terminal Galbeta1-3/4GlcNAc (or Glc) oligosaccharides, the reactivities are also increased; (d) with lFucalpha1-3/4 linked to sub-terminal GlcNAc, the reactivities of these haptens are reduced; and (e) short chain Le(a)/Le(x)/Le(y) and the short chains of sialyl Le(a)/Le(x) are poor inhibitors. These distinct binding features of G4-N establish the important concept of affinity enhancement by high density polyvalencies of glycotopes (vs. multi-antennary I/II) and by introduction of an ABH key sugar to Galbeta1-terminated core glycotopes. The polyvalent ligand binding properties of G4-N may help our understanding of its crucial role for cell membrane raft stability and provide salient information for the optimal design of blocking substances such as anti-tumoral glycodendrimers.

Purification and Characterization of Liver Lectins from a Lizard,Sceloporus spinosus

This study discusses the purification of soluble beta-galactose lectins obtained from the lizard liver of Sceloporus spinosus. The first lectin named lizard hepatic lectin-1 (LHL-1) presented a molecular weight of 31,750, with an isoelectric point of 4.25. The highest specific hemagglutinating activity was achieved using human blood type A1: N-acetylgalactosamine (GalNAc)-galactose (Gal)-fucose (Fuc). Carbohydrate inhibition assays indicated a higher lectin specificity for GalNAc. For LHL-2 the molecular weight obtained was 23,850 with an isoelectric point of 3.25. The highest carbohydrate specificity was observed for Gal. These lizard hepatic lectins are similar to the mammal hepatic lectins previously reported. However, it is different from the alligator hepatic lectin (AHL). The homology analyses of LHL-1 resulted in 100% identity with the Steroidogenic acute regulatory protein (StAR), while LHL-2 was similar to adenylate kinase (75% identity). We suggest that these liver lectins are related to the inherent functions of liver previously reported.

Human Galectin-2: Novel Inducer of T Cell Apoptosis with Distinct Profile of Caspase Activation

Galectin-2 is structurally closely related to galectin-1, but has a distinct expression profile primarily confined to the gastrointestinal tract. Prominent differences in the proximal promoter regions between galectins-2 and -1 concern Sp1-, hepatocyte NF-3, and T cell-specific factor-1 binding sites. Of note, these sequence elements are positioned equally in the respective regions for human and rat galectins-2. Labeled galectin-2 binds to T cells in a beta-galactoside-specific manner. In contrast to galectin-1, the glycoproteins CD3 and CD7 are not ligands, while the shared affinity to beta1 integrin (or a closely associated glycoprotein) accounts for a substantial extent of cell surface binding. The carbohydrate-dependent binding of galectin-2 induces apoptosis in activated T cells. Fluorogenic substrate and inhibitor assays reveal involvement of caspases-3 and -9, in accordance with cleavage of the DNA fragmentation factor. Enhanced cytochrome c release, disruption of the mitochondrial membrane potential, and an increase of the Bax/Bcl-2 ratio by opposite regulation of expression of both proteins add to the evidence that the intrinsic apoptotic pathway is triggered. Cell cycle distribution and expression of regulatory proteins remained unaffected. Notably, galectins-1 and -7 reduce cyclin B1 expression, defining functional differences between the structurally closely related galectins. Cytokine secretion of activated T cells was significantly shifted to the Th2 profile. Our study thus classifies galectin-2 as proapoptotic effector for activated T cells, raising a therapeutic perspective. Of importance for understanding the complex galectin network, it teaches the lesson that selection of cell surface ligands, route of signaling, and effects on regulators of cell cycle progression are markedly different between structurally closely related galectins.

Saccharides as efficacious solubilisers for highly lipophilic compounds in aqueous media

The bioavailability of lipophilic substrates is critical for biotransformations with isolated enzymes as well as with whole cells. With the example of a series of lipophilic ketones the suitability of saccharides as potent solubilisers for highly lipophilic substrates was demonstrated. Best results were obtained for d-glucose, which increased substrate solubility up to 50 times. In whole-cell biocatalysis the sugar acts both as solubiliser and as carbon source for which reason this procedure does not impair cell physiology and is unique in being environmentally benign. The capability of saccharides to solubilise lipophilic compounds in aqueous media sources from their ability to form hydrophilic and lipophilic domains at hydrophobic interfaces, thus forming cyclodextrin-like structures around the lipophilic substrate.

A supramolecular approach to protein labeling. A novel fluorescent bioassay for concanavalin a activity

[reaction: see text] A new method for the bioassay of concanavalin A is based on the interaction of saccharide-substituted oligopyrrolic macrocycles with lectins. A general sensing mechanism involves the interaction of aggregated and primarily nonfluorescent labels with the target protein, label deaggregation, site-specific binding, and fluorescence signaling. Addition of saccharides to the fluorescent lectin-macrocycle complex leads to the release of the label and partial quenching of fluorescence due to reaggregation. Specificity of the protein-probe binding is discussed.

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.

Analysis of selected blood and immune cell responses to carbohydrate-dependent surface binding of proto- and chimera-type galectins

Cell surface glycans present docking sites to endogenous lectins. With growing insight into the diversity of lectin families it becomes important to answer the question on the activity profiles of individual family members. Focusing on galectins (beta-galactoside-binding proteins without Ca(2+)-requirement sharing the jelly-roll-like folding pattern), this study was performed to assess the potency of proto-type galectins (galectins-1 and -7 and CG-16) and the chimera-type galectin-3 to elicit selected cell responses by carbohydrate-dependent surface binding and compare the results. The galectins, except for galectin-1, were found to enhance detergent (SDS)-induced hemolysis of human erythrocytes to different degrees. Their ability to confer increased membrane osmofragility thus differs. Aggregation of neutrophils, thymocytes and platelets was induced by the proto-type galectin-1 but not -7, by CG-16 and also galectin-3. Cell-type-specific quantitative differences and the importance of the fine-specificity of the galectin were clearly apparent. In order to detect cellular responses based on galectin binding and bridging of cells the formation of haptenic-sugar-resistant (HSR) intercellular contacts (an indicator of post-binding signaling) was monitored. It was elicited by CG-16 and galectin-1 but not galectin-3, revealing another level at which activities of individual galectins can differ. Acting as potent elicitor of neutrophil aggregation, CG-16-dependent post-binding effects were further analyzed. Carbohydrate-dependent binding to the neutrophils' surface led to a sustained increase of cytoplasmic Ca2+ concentration in a dose-dependent manner. The ability of CG-16 to activate H2O2 generation by human peripheral blood neutrophils was primed by the Ca(2+)-ionophor ionomycin and by cytochalasin B. In a general context, these results emphasize that--besides plant lectins as laboratory tools--animal lectins can trigger cell reaction cascades, implying potential in vivo relevance for the measured activities. Within the family of galectins, the activity profiles depend on the target cell type and the individual galectin. Notably, proto-type galectins do not necessarily share a uniform capacity as elicitor.

Effect of polyvalencies of glycotopes on the binding of a lectin from the edible mushroom, Agaricus bisporus

Agaricus bisporus agglutinin (ABA) isolated from edible mushroom has a potent anti-proliferative effect on malignant colon cells with considerable therapeutic potential as an anti-neoplastic agent. Since previous studies on the structural requirement for binding were limited to molecular or submolecular levels of Galbeta1-3GalNAc (T; Thomsen-Friedenreich disaccharide glycotope; where Gal represents D-galactopyranose and GalNAc represents 2-acetamido-2-deoxy-D-galactopyranose) and its derivatives, the binding properties of ABA were further investigated using our collection of glycans by enzyme-linked lectinosorbent assay and lectin-glycan inhibition assay. The results indicate that polyvalent Galbeta1-related glycotopes, GalNAcalpha1-Ser/Thr (Tn), and their cryptoforms, are the most potent factor for ABA binding. They were up to 5.5x10(5) and 4.7x10(6) times more active than monomeric T and GalNAc respectively. The affinity of ABA for ligands can be ranked as: multivalent T (alpha) (Galbeta1-3GalNAcalpha1-), Tn and I / II (Galbeta1-3GlcNac/Galbeta1-4GlcNAc, where GlcNAc represents 2-acetamido-2-deoxy-D-glucopyranose)>>>>monomeric T (alpha) and Tn > I >>GalNAc>>> II, L (Galbeta1-4Glc, where Glc represents D-glucopyranose) and Gal (inactive). These specific binding features of ABA establish the importance of affinity enhancement by high-density polyvalent (versus multiantennary I / II) glycotopes and facilitate our understanding of the lectin receptor recognition events relevant to its biological activities.

Binding profile of Artocarpus integrifolia agglutinin (Jacalin)

Artocarpus integrifolia agglutinin (Jacalin) from the seeds of jack fruits has attracted considerable attention for its diverse biological activities and has been recognized as a Galbeta1-->3GalNAc (T) specific lectin. In previous studies, the information of its binding was limited to the inhibition results of monosaccharides and several T related disaccharides, but its interaction with other carbohydrate structural units occurring in natural glycans has not been characterized. For this reason, the binding profile of this lectin was studied by enzyme linked lectinosorbent assay (ELLSA) with our glycan/ligand collection. Among glycoproteins (gps) tested for binding, high density of multi-Galbeta1-->3GalNAcalpha1--> (mT(alpha)) and GalNAcalpha1-->Ser/Thr (mTn) containing gps reacted most avidly with Jacalin. As inhibitors expressed as nanograms yielding 50% inhibition, these mT(alpha) and mTn containing glycans were about 7.1 x 10(3), 4.0 x 10(5), and 7.8 x 10(5) times more potent than monomeric T(alpha), GalNAc, and Gal. Of the sugars tested and expressed as nanomoles for 50% inhibition, Tn containing peptides, T(alpha), and the human P blood group active disaccharide (P(alpha), GalNAcbeta1-->3Galalpha1-->) were the best and about 283 times more active than Gal. We conclude that the most potent ligands for this lectin are mTn, mT, and possibly P(alpha) glycotopes, while GalNAcbeta1-->4Galbeta1-->, GalNAcalpha1-->3Gal, GalNAcalpha1-->3GalNAc, and Galalpha1-->3Gal determinants were poor inhibitors. Thus, the overall binding profile of Jacalin can be defined in decreasing order as high density of mTn, and mT(alpha) >>> simple Tn cluster > monomeric T(alpha) > monomeric P(alpha) > monomeric Tn > monomeric T > GalNAc > Gal > Methylalpha1-->Man z.Gt; Man and Glc (inactive). Our finding should aid in the selection of this lectin for biological applications.

Fine specificity of domain-I of recombinant tandem-repeat-type galectin-4 from rat gastrointestinal tract (G4-N)

Galectins, a family of beta-galactoside-specific endogenous lectins, are involved in regulating diverse activities such as proliferation/apoptosis, cell-cell (matrix) interaction and cell migration. It is presently unclear to what extent the carbohydrate fine specificities of the combining sites of mammalian galectins overlap. To address this issue, we performed an analysis of the carbohydrate-recognition domain (CRD-I) near the N-terminus of recombinant rat galectin-4 (G4-N) by the biotin/avidin-mediated microtitre plate lectin-binding assay with natural glycoproteins (gps)/polysaccharide and by the inhibition of galectin-glycan interactions with a panel of glycosubstances. Among the 35 glycans tested for lectin binding, G4-N reacted best with human blood group ABH precursor gps, and asialo porcine salivary gps, which contain high densities of the blood group Ii determinants Galbeta1-3GalNAc (the mucin-type sugar sequence on the human erythrocyte membrane) and/or GalNAcalpha1-Ser/Thr ( Tn ), whereas this lectin domain reacted weakly or not at all with most sialylated gps. Among the oligosaccharides tested by the inhibition assay, Galbeta1-3GlcNAcbeta1-3Galbeta1-4Glc was the best. It was 666.7 and 33.3 times more potent than Gal and Galbeta1-3GlcNAc, respectively. G4-N has a preference for the beta-anomer of Gal at the non-reducing ends of oligosaccharides with a Galbeta1-3 linkage, over Galbeta1-4 and Galbeta1-6. The fraction of Tn glycopeptide from asialo ovine submandibular glycoprotein was 8.3 times more active than Galbeta1-3GlcNAc. The overall carbohydrate specificity of G4-N can be defined as Galbeta1-3GlcNAcbeta1-3Galbeta1-4Glc (lacto- N -tetraose)>Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc (lacto- N -neo-tetraose) and Tn clusters>Galbeta1-4Glc and GalNAcbeta1-3Gal>Galbeta1-3GalNAc>Galbeta1-3GlcNAc>Galbeta1-4GlcNAc>GalNAc>Gal. The definition of this binding profile provides the basis to detect differential binding properties relative to the other galectins with ensuing implications for functional analysis.