Fluorinated carbohydrates as lectin ligands: dissecting glycan-cyanovirin interactions by using 19F NMR spectroscopy

NMR spectroscopy and isothermal titration calorimetry (ITC) are powerful methods to investigate ligand-protein interactions. Here, we present a versatile and sensitive fluorine NMR spectroscopic approach that exploits the (19)F nucleus of (19)F-labeled carbohydrates as a sensor to study glycan binding to lectins. Our approach is illustrated with the 11 kDa Cyanovirin-N, a mannose binding anti-HIV lectin. Two fluoro-deoxy sugar derivatives, methyl 2-deoxy-2-fluoro-α-D-mannopyranosyl-(1→2)-α-D-mannopyranoside and methyl 2-deoxy-2-fluoro-α-D-mannopyranosyl-(1→2)-α-D-mannopyranosyl-(1→2)-α-D-mannopyranoside were utilized. Binding was studied by (19)F NMR spectroscopy of the ligand and (1)H-(15)N HSQC NMR spectroscopy of the protein. The NMR data agree well with those obtained from the equivalent reciprocal and direct ITC titrations. Our study shows that the strategic design of fluorinated ligands and fluorine NMR spectroscopy for ligand screening holds great promise for easy and fast identification of glycan binding, as well as for their use in reporting structural and/or electronic perturbations that ensue upon interaction with a cognate lectin.

Molecular recognition of complex-type biantennary N-glycans by protein receptors: a three-dimensional view on epitope selection by NMR

The current surge in defining glycobiomarkers by applying lectins rekindles interest in definition of the sugar-binding sites of lectins at high resolution. Natural complex-type N-glycans can present more than one potential binding motif, posing the question of the actual mode of interaction when interpreting, for example, lectin array data. By strategically combining N-glycan preparation with saturation-transfer difference NMR and modeling, we illustrate that epitope recognition depends on the structural context of both the sugar and the lectin (here, wheat germ agglutinin and a single hevein domain) and cannot always be predicted from simplified model systems studied in the solid state. We also monitor branch-end substitutions by this strategy and describe a three-dimensional structure that accounts for the accommodation of the α2,6-sialylated terminus of a biantennary N-glycan by viscumin. In addition, we provide a structural explanation for the role of terminal α2,6-sialylation in precluding the interaction of natural N-glycans with lectin from Maackia amurensis . The approach described is thus capable of pinpointing lectin-binding motifs in natural N-glycans and providing detailed structural explanations for lectin selectivity.

Lactose binding to human galectin-7 (p53-induced gene 1) induces long-range effects through the protein resulting in increased dimer stability and evidence for positive cooperativity

The product of p53-induced gene 1 is a member of the galectin family, i.e., galectin-7 (Gal-7). To move beyond structural data by X-ray diffraction, we initiated the study of the lectin by nuclear magnetic resonance (NMR) and circular dichroism spectroscopies, and molecular dynamics (MD) simulations. In concert, our results indicate that lactose binding to human Gal-7 induces long-range effects (minor conformational shifts and changes in structural dynamics) throughout the protein that result in stabilization of the dimer state, with evidence for positive cooperativity. Monte Carlo fits of (15)N-Gal-7 HSQC titrations with lactose using a two-site model yield K1 = 0.9 ± 0.6 × 10(3) M(-1) and K2 = 3.4 ± 0.8 × 10(3) M(-1). Ligand binding-induced stabilization of the Gal-7 dimer was supported by several lines of evidence: MD-based calculations of interaction energies between ligand-loaded and ligand-free states, gel filtration data and hetero-FRET spectroscopy that indicate a highly reduced tendency for dimer dissociation in the presence of lactose, CD-based thermal denaturation showing that the transition temperature of the lectin is significantly increased in the presence of lactose, and saturation transfer difference (STD) NMR using a molecular probe of the monomer state whose presence is diminished in the presence of lactose. MD simulations with the half-loaded ligand-bound state also provided insight into how allosteric signaling may occur. Overall, our results reveal long-range effects on Gal-7 structure and dynamics, which factor into entropic contributions to ligand binding and allow further comparisons with other members of the galectin family.

Tumour suppressor p16(INK4a) - anoikis-favouring decrease in N/O-glycan/cell surface sialylation by down-regulation of enzymes in sialic acid biosynthesis in tandem in a pancreatic carcinoma model

Tumour suppressor p16(INK4a) is known to exert cell-cycle control via cyclin-dependent kinases. An emerging aspect of its functionality is the orchestrated modulation of N/O-glycosylation and galectin expression to induce anoikis in human Capan-1 pancreatic carcinoma cells. Using chemoselective N/O-glycan enrichment technology (glycoblotting) and product characterization, we first verified a substantial decrease in sialylation. Tests combining genetic (i.e. transfection with α2,6-sialyltransferase-specific cDNA) or metabolic (i.e. medium supplementation with N-acetylmannosamine to track down a bottleneck in sialic acid biosynthesis) engineering with cytofluorometric analysis of lectin binding indicated a role of limited substrate availability, especially for α2,6-sialylation, which switches off reactivity for anoikis-triggering homodimeric galectin-1. Quantitative MS analysis of protein level changes confirmed an enhanced galectin-1 presence along with an influence on glycosyltransferases (β1,4-galactosyltransferase-IV, α2,3-sialyltransferase-I) and detected p16(INK4a) -dependent down-regulation of two enzymes in the biosynthesis pathway for sialic acid [i.e. the bifunctional UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) and N-acetylneuraminic acid 9-phosphate synthase] (P < 0.001). By contrast, quantitative assessment for the presence of nuclear CMP-N-acetylneuraminic acid synthase (which is responsible for providing the donor for enzymatic sialylation that also acts as feedback inhibitor of the epimerase activity of GNE) revealed a trend for an increase. Partial restoration of sialylation in GNE-transfected cells supports the implied role of sialic acid availability for the glycophenotype. Fittingly, the extent of anoikis was reduced in double-transfected (p16(INK4a) /GNE) cells. Thus, a second means of modulating cell reactivity to the growth effector galectin-1 is established in addition to the common route of altering α2,6-sialyltransferase expression: regulating enzymes of the pathway for sialic acid biosynthesis.

Context-dependent multifunctionality of galectin-1: a challenge for defining the lectin as therapeutic target

INTRODUCTION

One route of translating the information encoded in the glycan chains of cellular glycoconjugates into physiological effects is via receptor (lectin) binding. A family of endogenous lectins, sharing folding, a distinct sequence signature and affinity for β-galactosides (thus termed galectins), does so effectively in a context-dependent manner.

AREAS COVERED

An overview is given on the multifunctional nature of galectins, with emphasis on galectin-1. The broad range of functions includes vital processes such as adhesion via glycan bridging, glycoconjugate transport or triggering signaling relevant, for example, for growth regulation. Besides distinct glycoconjugates, this lectin can also interact with certain proteins so that it can target counterreceptors at all sites of location, that is, in the cytoplasm and/or nucleus, at both sides of the membrane or extracellularly. Approaches to strategically exploit galectin activities with therapeutic intentions are outlined.

EXPERT OPINION

The wide versatility of sugar coding and the multifunctionality of galectin-1 explain why considering to turn the protein into a therapeutic target is an ambitious aim. Natural pathways shaped by physiologic master regulators (e.g., the tumor suppressor p16(INK4a)) are suggested to teach inspiring lessons as to how the lectin might be recruited to clinical service.

1H, 13C, and 15N backbone and side-chain chemical shift assignments for the 31 kDa human galectin-7 (p53-induced gene 1) homodimer, a pro-apoptotic lectin

Galectins are multifunctional proteins with carbohydrate/protein-binding properties and distinct expression profiles. Homodimeric galectin-7 (p53-induced gene 1) is a potent pro-apoptotic effector with clinical relevance. Here, we report (1)H, (13)C, and (15)N chemical shift assignments for human galectin-7 dimer as determined by using heteronuclear, triple resonance NMR spectroscopy.

Studies related to Norway spruce galactoglucomannans: chemical synthesis, conformation analysis, NMR spectroscopic characterization, and molecular recognition of model compounds

Galactoglucomannan (GGM) is a polysaccharide mainly consisting of mannose, glucose, and galactose. GGM is the most abundant hemicellulose in the Norway spruce (Picea abies), but is also found in the cell wall of flax seeds, tobacco plants, and kiwifruit. Although several applications for GGM polysaccharides have been developed in pulp and paper manufacturing and the food and medical industries, attempts to synthesize and study distinct fragments of this polysaccharide have not been reported previously. Herein, the synthesis of one of the core trisaccharide units of GGM together with a less-abundant tetrasaccharide fragment is described. In addition, detailed NMR spectroscopic characterization of the model compounds, comparison of the spectral data with natural GGM, investigation of the acetyl-group migration phenomena that takes place in the polysaccharide by using small model compounds, and a binding study between the tetrasaccharide model fragment and a galactose-binding protein (the toxin viscumin) are reported.

Bi- to tetravalent glycoclusters: synthesis, structure-activity profiles as lectin inhibitors and impact of combining both valency and headgroup tailoring on selectivity

The emerging functional versatility of cellular glycans makes research on the design of synthetic inhibitors a timely topic. In detail, the combination of ligand (or headgroup or contact site) structure with spatial parameters that depend on topological and geometrical factors underlies the physiological selectivity of glycan-protein (lectin) recognition. We herein tested a panel of bi-, tri- and tetravalent compounds against two plant agglutinins and adhesion/growth-regulatory lectins (galectins). In addition, we examined the impact of headgroup tailoring (converting lactose to 2'-fucosyllactose) in combination with valency increase in two assay types of increasing biorelevance (from solid-phase binding to cell binding). Compounds were prepared using copper-catalysed azide alkyne cycloaddition from peracetylated lactosyl or 2'-fucosyllactosyl azides. Significant inhibition was achieved for the plant toxin with a tetravalent compound. Different levels of sensitivity were noted for the three groups of the galectin family. The headgroup extension to 2'-fucosyllactose led to a selectivity gain, especially for the chimera-type galectin-3. Valency increase established discrimination against the homodimeric proteins, whereas the combination of valency with the headgroup extension led to discrimination against the tandem-repeat-type galectin-8 for chicken galectins but not human galectins-3 and -4. Thus, detailed structure-activity profiling of glycoclusters combined with suitably modifying the contact site for the targeted lectin will help minimize cross-reactivity among this class of closely related proteins.

Loss of adhesion/growth-regulatory galectin-9 from squamous cell epithelium in head and neck carcinomas

Galectins are potent effectors of cell adhesion and growth regulation. Their expression as comples network necessitates systematic study of each member of this family. Toward this aim, we here focus on the tandem-repeat-type galectin-9. Its presence is monitored in normal squamous epithelium of the head and neck, the surgical margin, and four types of squamous cell carcinoma. Lectin presence was detected in cells of the basal layer of the epithelium. All galectin-9-negative epithelia showed aberrant positivity for keratins 14 and 19. The surgical margin presented either a normal pattern of galectin-9 and keratin presence or a mosaic-like presence/absence of galectin-9 and aberrant expression of both keratins 14 and 19. All studied specimens of squamous cell carcinoma were negative for galectin-9. When biotinylated galectin-9, or its N-terminal domain, was tested, no significant tissue reactivity for both probes was observed. Neuraminidase treatment generated reactivity to the N-domain. In conclusion, galectin-9 is expressed in the majority of samples of normal epithelium, along with regular presence of keratins 14 or 19. This lectin can represent a potential marker of normality in the cases of the studied squamous cell epithelia.

Autoantibodies against galectin-2 peptides as biomarkers for the antiphospholipid syndrome

Autoantibodies against opsonins of dying and dead cells mediate Fcγ receptor-dependent phagocytosis of autologous apoptotic and necrotic cells and hereby tend to elicit inflammation instead of silent clearance. We analysed sera of patients with chronic autoimmune diseases for the occurrence of IgG autoantibodies recognizing galectins. These pluripotent effectors can also bind to apoptotic or necrotic cells. Patients with antiphospholipid syndrome (APS; n = 104) and systemic lupus erythematosus (SLE; n = 62) were examined, healthy donors ( n = 31) served as controls. Selected peptides of galectin (Gal)-2 were employed for peptide-based ELISAs. Levels of anti-Gal-2PEP-IgG were significantly increased in SLE and APS when compared with controls. In addition, patients with APS showed significantly higher levels of anti-Gal-2PEP-IgG compared with patients with SLE. Anti-Gal-2PEP-IgG may, therefore, be considered novel biomarkers for APS.

Down-regulation of the epidermal growth factor receptor by altering N-glycosylation: emerging role of β1,4-galactosyltransferases

BACKGROUND/AIM

Blocking N-glycosylation of the epidermal growth factor receptor (EGFR) by tunicamycin inhibits its cellular accumulation. Due to the toxic potential of this drug, finding less drastic routes to reduce EGFR expression is desirable.

MATERIALS AND METHODS

Four glycosylation mutants of Chinese hamster ovary (CHO) cells with defects in N-glycan processing and branch-end maturation were tested for EGFR gene expression, production, functionality and routing after transfection with a vector encoding for human EGFR.

RESULTS

Lack of conversion of paucimannosidic to hybrid/complex-type N-glycans and drastic reductions in sialylation/galactosylation did not lead to major effects. In contrast, EGFR expression in a mutant with reduced presence of β1,4-galactosyltransferases-I-VI was markedly reduced. Misrouting or defects in transfection/transcription were excluded.

CONCLUSION

β1,4-Galactosyl-transferases warrant for further attention as effector(s) in order to attenuate EGFR-dependent signaling.

Comparative study of the glycan specificities of cell-bound human tandem-repeat-type galectin-4, -8 and -9

Adhesion/growth-regulatory galectins (gals) exert their functionality by the cis/trans-cross-linking of distinct glycans after initial one-point binding. In order to define the specificity of ensuing association events leading to cross-linking, we recently established a cell-based assay using fluorescent glycoconjugates as flow cytometry probes and tested it on two human gals (gal-1 and -3). Here we present a systematic study of tandem-repeat-type gal-4, -8 and -9 loaded on Raji cells resulting in the following key insights: (i) all three gals bound to oligolactosamines; (ii) binding to ligands with Galβ1-3GlcNAc or Galβ1-3GalNAc as basic motifs was commonly better than that to canonical Galβ1-4GlcNAc; (iii) all three gals bound to 3'-O-sulfated and 3'-sialylated disaccharides mentioned above better than that to parental neutral forms and (iv) histo-blood group ABH antigens were the highest affinity ligands in both the cell and the solid-phase assay. Fine specificity differences were revealed as follows: (i) gal-8 and -9, but not gal-4, bound to disaccharide Galβ1-3GlcNAc; (ii) increase in binding due to negatively charged substituents was marked only in the case of gal-4 and (iii) gal-4 and -8 bound preferably to histo-blood group A glycans, whereas gal-9 targeted B-type glycans. Experiments with single carbohydrate recognition domains (CRDs) of gal-4 showed that the C-CRD preferably bound to ABH glycans, whereas the N-CRD associated with oligolactosamines. In summary, the comparative analysis disclosed the characteristic profiles of glycan reactivity for the accessible CRD of cell-bound gals. These results indicate the distinct sets of functionality for these three members of the same subgroup of human gals.

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.

Evaluation of peracetic acid sanitizers efficiency against spores isolated from spoiled cans in suspension and on stainless steel surfaces

The aim of this study was to determine the inactivation effect of industrial formulations of peracetic acid biocides on bacterial spores adhering to stainless steel surfaces. A standardized protocol was used to validate biocide activity against spores in suspension. To validate sporicidal activity under practical conditions, we developed an additional protocol to simulate industrial sanitization of stainless steel surfaces with a foam sanitizer. Spores of three spore-forming bacteria, Clostridium sporogenes PA3679, Geobacillus stearothermophilus, and Moorella thermoacetica/thermoautotrophica, were sprayed onto stainless steel as bioaerosols. Sporicidal activity was high against the C. sporogenes spore suspension, with more than 5 log CFU ml(-1) destroyed at all liquid biocide contact times. Sporicidal activity also was high against G. stearothermophilus and M. thermoacetica/thermoautotrophica spores after 30 min of contact, but we found no population reduction at the 5-min contact time for the highest sporicide concentration tested. The foam biocide effectively inactivated C. sporogenes spores adhered to stainless steel but had a reduced decontamination effect on other species. For G. stearothermophilus spores, sanitization with the foam sporicide was more efficient on horizontal steel than on vertical steel, but foam sanitization was ineffective against M. thermoacetica/thermoautotrophica whatever the position. These results highlight that decontamination efficiency may differ depending on whether spores are suspended in an aqueous solution or adhered to a stainless steel surface. Biocide efficiency must be validated using relevant protocols and bacteria representative of the microbiological challenges and issues affecting each food industry.

New aspects of galectin functionality in nuclei of cultured bone marrow stromal and epidermal cells: biotinylated galectins as tool to detect specific binding sites

Nuclear presence of galectins suggests a role of these endogenous lectins in the regulation of transcription, pre-mRNA splicing and transport processes. Therefore, detection and localization of nuclear binding sites for galectins by a new methodological step, has potential to further functional analysis. In the first step of our model study we monitored the nuclear expression of galectins-1 and -3 in cultured stromal cells of human bone marrow and human/porcine keratinocytes. To enable detection and localization of galectin-specific binding sites, we used purified galectins biotinylated without loss of activity as cytochemical tool. The degree of labeling of the probes was determined by adapting two-dimensional gel electrophoresis and calculating pI changes in response to stepwise chemical modification of basic and acidic side chains by the biotinylation reagents. Binding studies revealed positivity for galectin-1, whereas galectins-3, -5, and -7 were not reactive with nuclear sites under identical conditions in bone marrow stromal cells and keratinocytes prepared from hair follicle enriched for stem cells. Inhibition by lactose indicated an involvement of the carbohydrate recognition domain in nuclear binding of galectin-1. Colocalization of the galectin-1-dependent signal with the SC35 splicing factor and sensitivity toward RNase treatment argued in favor of galectin binding in nuclear speckles, albeit only for a small fraction of the cells. Epidermal cells positive for galectin-1-binding sites expressed DeltaNp63 known as a potential marker of stem cells. Based on cytokeratin expression cells with nuclear binding of labeled galectin-1 were basal and not suprabasal cells. Regarding proliferation, no relationship to the expression of a proliferation marker, Ki-67, was observed. The nucleolar signal colocalized with fibrillarin and nucleophosmin/B23 as representatives of nucleolar proteins in both types of studied cells. In conclusion, the application of labeled galectins to localize accessible binding sites adds a new aspect to the functional analysis of these lectins in the nucleus.