"Rules of Engagement" of Protein-Glycoconjugate Interactions: A Molecular View Achievable by using NMR Spectroscopy and Molecular Modeling

Understanding the dynamics of protein-ligand interactions, which lie at the heart of host-pathogen recognition, represents a crucial step to clarify the molecular determinants implicated in binding events, as well as to optimize the design of new molecules with therapeutic aims. Over the last decade, advances in complementary biophysical and spectroscopic methods permitted us to deeply dissect the fine structural details of biologically relevant molecular recognition processes with high resolution. This Review focuses on the development and use of modern nuclear magnetic resonance (NMR) techniques to dissect binding events. These spectroscopic methods, complementing X-ray crystallography and molecular modeling methodologies, will be taken into account as indispensable tools to provide a complete picture of protein-glycoconjugate binding mechanisms related to biomedicine applications against infectious diseases.

The Hidden Conformation of Lewis x, a Human Histo-Blood Group Antigen, Is a Determinant for Recognition by Pathogen Lectins

Histo-blood group epitopes are fucosylated branched oligosaccharides with well-defined conformations in solution that are recognized by receptors, such as lectins from pathogens. We report here the results of a series of experimental and computational endeavors revealing the unusual distortion of histo-blood group antigens by bacterial and fungal lectins. The Lewis x trisaccharide adopts a rigid closed conformation in solution, while crystallography and molecular dynamics reveal several higher energy open conformations when bound to the Ralstonia solanacearum lectin, which is in agreement with thermodynamic and kinetic measurements. Extensive molecular dynamics simulations confirm rare transient Le(x) openings in solution, frequently assisted by distortion of the central N-acetyl-glucosamine ring. Additional directed molecular dynamic trajectories revealed the role of a conserved tryptophan residue in guiding the fucose into the binding site. Our findings show that conformational adaptation of oligosaccharides is of paramount importance in cell recognition and should be considered when designing anti-infective glyco-compounds.

Genomic Rearrangements and Functional Diversification of lecA and lecB Lectin-Coding Regions Impacting the Efficacy of Glycomimetics Directed against Pseudomonas aeruginosa

LecA and LecB tetrameric lectins take part in oligosaccharide-mediated adhesion-processes of Pseudomonas aeruginosa. Glycomimetics have been designed to block these interactions. The great versatility of P. aeruginosa suggests that the range of application of these glycomimetics could be restricted to genotypes with particular lectin types. The likelihood of having genomic and genetic changes impacting LecA and LecB interactions with glycomimetics such as galactosylated and fucosylated calix[4]arene was investigated over a collection of strains from the main clades of P. aeruginosa. Lectin types were defined, and their ligand specificities were inferred. These analyses showed a loss of lecA among the PA7 clade. Genomic changes impacting lec loci were thus assessed using strains of this clade, and by making comparisons with the PAO1 genome. The lecA regions were found challenged by phage attacks and PAGI-2 (genomic island) integrations. A prophage was linked to the loss of lecA. The lecB regions were found less impacted by such rearrangements but greater lecB than lecA genetic divergences were recorded. Sixteen combinations of LecA and LecB types were observed. Amino acid variations were mapped on PAO1 crystal structures. Most significant changes were observed on LecBPA7, and found close to the fucose binding site. Glycan array analyses were performed with purified LecBPA7. LecBPA7 was found less specific for fucosylated oligosaccharides than LecBPAO1, with a preference for H type 2 rather than type 1, and Lewis(a) rather than Lewis(x). Comparison of the crystal structures of LecBPA7 and LecBPAO1 in complex with Lewis(a) showed these changes in specificity to have resulted from a modification of the water network between the lectin, galactose and GlcNAc residues. Incidence of these modifications on the interactions with calix[4]arene glycomimetics at the cell level was investigated. An aggregation test was used to establish the efficacy of these ligands. Great variations in the responses were observed. Glycomimetics directed against LecB yielded the highest numbers of aggregates for strains from all clades. The use of a PAO1ΔlecB strain confirmed a role of LecB in this aggregation phenotype. Fucosylated calix[4]arene showed the greatest potential for a use in the prevention of P. aeruginosa infections.

The virulence factor LecB varies in clinical isolates: consequences for ligand binding and drug discovery

P. aeruginosa causes a substantial number of nosocomial infections and is the leading cause of death of cystic fibrosis patients. This Gram-negative bacterium is highly resistant against antibiotics and further protects itself by forming a biofilm. Moreover, a high genomic variability among clinical isolates complicates therapy. Its lectin LecB is a virulence factor and necessary for adhesion and biofilm formation. We analyzed the sequence of LecB variants in a library of clinical isolates and demonstrate that it can serve as a marker for strain family classification. LecB from the highly virulent model strain PA14 presents 13% sequence divergence with LecB from the well characterized PAO1 strain. These differences might result in differing ligand binding specificities and ultimately in reduced efficacy of drugs directed towards LecB. Despite several amino acid variations at the carbohydrate binding site, glycan array analysis showed a comparable binding pattern for both variants. A common high affinity ligand could be identified and after its chemoenzymatic synthesis verified in a competitive binding assay: an N-glycan presenting two blood group O epitopes (H-type 2 antigen). Molecular modeling of the complex suggests a bivalent interaction of the ligand with the LecB tetramer by bridging two separate binding sites. This binding rationalizes the strong avidity (35 nM) of LecB_PA14 to this human fucosylated N-glycan. Biochemical evaluation of a panel of glycan ligands revealed that LecB_PA14 demonstrated higher glycan affinity compared to LecB_PAO1 including the extraordinarily potent affinity of 70 nM towards the monovalent human antigen Lewis^a. The structural basis of this unusual high affinity ligand binding for lectins was rationalized by solving the protein crystal structures of LecB_PA14 with several glycans.

Mannose-centered aromatic galactoclusters inhibit the biofilm formation of Pseudomonas aeruginosa

Pseudomonas aeruginosa (PA) is a major public health care issue due to its ability to develop antibiotic resistance mainly through adhesion and biofilm formation. Therefore, targeting the bacterial molecular arsenal involved in its adhesion and the formation of its biofilm appears as a promising tool against this pathogen. The galactose-binding LecA (or PA-IL) has been described as one of the PA virulence factors involved in these processes. Herein, the affinity of three tetravalent mannose-centered galactoclusters toward LecA was evaluated with five different bioanalytical methods: HIA, ELLA, SPR, ITC and DNA-based glycoarray. Inhibitory potential towards biofilms was then assessed for the two glycoclusters with highest affinity towards LecA (Kd values of 157 and 194 nM from ITC measurements). An inhibition of biofilm formation of 40% was found for these galactoclusters at 10 μM concentration. Applications of these macromolecules in anti-bacterial therapy are therefore possible through an anti-adhesive strategy.

Carcinoma-associated fucosylated antigens are markers of the epithelial state and can contribute to cell adhesion through CLEC17A (Prolectin)

Terminal fucosylated motifs of glycoproteins and glycolipid chains are often altered in cancer cells. We investigated the link between fucosylation changes and critical steps in cancer progression: epithelial-to-mesenchymal transition (EMT) and lymph node metastasis.Using mammary cell lines, we demonstrate that during EMT, expression of some fucosylated antigens (e.g.: Lewis Y) is decreased as a result of repression of the fucosyltransferase genes FUT1 and FUT3. Moreover, we identify the fucose-binding bacterial lectin BC2L-C-Nt as a specific probe for the epithelial state.Prolectin (CLEC17A), a human lectin found on lymph node B cells, shares ligand specificities with BC2L-C-Nt. It binds preferentially to epithelial rather than to mesenchymal cells, and microfluidic experiments showed that prolectin behaves as a cell adhesion molecule for epithelial cells. Comparison of paired primary tumors/lymph node metastases revealed an increase of prolectin staining in metastasis and high FUT1 and FUT3 mRNA expression was associated with poor prognosis. Our data suggest that tumor cells invading the lymph nodes and expressing fucosylated motifs associated with the epithelial state could use prolectin as a colonization factor.

Pentavalent pillar[5]arene-based glycoclusters and their multivalent binding to pathogenic bacterial lectins

Anti-adhesive glycoclusters offer potential as therapeutic alternatives to classical antibiotics in treating infections. Pillar[5]arenes functionalised with either five galactose or five fucose residues were readily prepared using CuAAC reactions and evaluated for their binding to three therapeutically relevant bacterial lectins: LecA and Lec B from Pseudomonas aeuruginosa and BambL from Burkholderia ambifaria. Steric interactions were demonstrated to be a key factor in achieving good binding to LecA with more flexible galactose glycoclusters showing enhanced activity. In contrast binding to the fucose-selective lectins confirmed the importance of topology of the glycoclusters for activity with the pillar[5]arene ligand proving a selective ligand for BambL.

Pseudomonas aeruginosa lectin LecB inhibits tissue repair processes by triggering β-catenin degradation

Pseudomonas aeruginosa is an opportunistic pathogen that induces severe lung infections such as ventilator-associated pneumonia and acute lung injury. Under these conditions, the bacterium diminishes epithelial integrity and inhibits tissue repair mechanisms, leading to persistent infections. Understanding the involved bacterial virulence factors and their mode of action is essential for the development of new therapeutic approaches. In our study we discovered a so far unknown effect of the P. aeruginosa lectin LecB on host cell physiology. LecB alone was sufficient to attenuate migration and proliferation of human lung epithelial cells and to induce transcriptional activity of NF-κB. These effects are characteristic of impaired tissue repair. Moreover, we found a strong degradation of β-catenin, which was partially recovered by the proteasome inhibitor lactacystin. In addition, LecB induced loss of cell-cell contacts and reduced expression of the β-catenin targets c-myc and cyclin D1. Blocking of LecB binding to host cell plasma membrane receptors by soluble l-fucose prevented these changes in host cell behavior and signaling, and thereby provides a powerful strategy to suppress LecB function. Our findings suggest that P. aeruginosa employs LecB as a virulence factor to induce β-catenin degradation, which then represses processes that are directly linked to tissue recovery.

Pillar[5]arene-Based Glycoclusters: Synthesis and Multivalent Binding to Pathogenic Bacterial Lectins

The synthesis of pillar[5]arene-based glycoclusters has been readily achieved by CuAAC conjugations of azido- and alkyne-functionalized precursors. The lectin binding properties of the resulting glycosylated multivalent ligands have been studied by at least two complementary techniques to provide a good understanding. Three lectins were selected from bacterial pathogens based on their potential therapeutic applications as anti-adhesives, namely LecA and LecB from Pseudomonas aeruginosa and BambL from Burkholderia ambifaria. As a general trend, multivalency improved the binding to lectins and a higher affinity can be obtained by increasing to a certain limit the length of the spacer arm between the carbohydrate subunits and the central macrocyclic core.

Development of a competitive binding assay for the Burkholderia cenocepacia lectin BC2L-A and structure activity relationship of natural and synthetic inhibitors

Burkholderia cenocepacia is an opportunistic Gram-negative pathogen and especially hazardous for cystic fibrosis patients.

Overcoming antibiotic resistance in Pseudomonas aeruginosa biofilms using glycopeptide dendrimers

Antibiotic resistance in the opportunistic pathogen Pseudomonas aeruginosa is partly caused by biofilms forming a physical barrier to antibiotic penetration. Here we focused on modifying tetravalent glycopeptide dendrimer ligands of P. aeruginosa lectins LecB or LecA to increase their biofilm inhibition activity. First heteroglycoclusters were investigated displaying one pair each of LecB specific fucosyl groups and LecA specific galactosyl groups and binding simultaneously to both lectins, one of which gave the first fully resolved crystal structure of a peptide dendrimer as LecB complex providing a structural model for dendrimer-lectin interactions (PDB ; 5D2A). Biofilm inhibition was increased by introducing additional cationic residues in these dendrimers but resulted in bactericidal effects similar to those of non-glycosylated polycationic antimicrobial peptide dendrimers. In a second approach dendrimers displaying four copies of the natural LecB ligand Lewisa were prepared leading to slightly stronger LecB binding and biofilm inhibition. Finally synergistic application of a LecB specific non-bactericidal antibiofilm dendrimer with the antibiotic tobramycin at sub-inhibitory concentrations of both compounds allowed effective biofilm inhibition and dispersal.

Cinnamide Derivatives of d-Mannose as Inhibitors of the Bacterial Virulence Factor LecB from Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic Gram-negative pathogen with high antibiotic resistance. Its lectin LecB was identified as a virulence factor and is relevant in bacterial adhesion and biofilm formation. Inhibition of LecB with carbohydrate-based ligands results in a decrease in toxicity and biofilm formation. We recently discovered two classes of potent drug-like glycomimetic inhibitors, that is, sulfonamides and cinnamides of d-mannose. Here, we describe the chemical synthesis and biochemical evaluation of more than 20 derivatives with increased potency compared to the unsubstituted cinnamide. The structure-activity relationship (SAR) obtained and the extended biophysical characterization allowed the experimental determination of the binding mode of these cinnamides with LecB. The established surface binding mode now allows future rational structure-based drug design. Importantly, all glycomimetics tested showed extended receptor residence times with half-lives in the 5-20 min range, a prerequisite for therapeutic application. Thus, the glycomimetics described here provide an excellent basis for future development of anti-infectives against this multidrug-resistant pathogen.

Structural Insight into Multivalent Galactoside Binding to Pseudomonas aeruginosa Lectin LecA

Multivalent galactosides inhibiting Pseudomonas aeruginosa biofilms may help control this problematic pathogen. To understand the binding mode of tetravalent glycopeptide dendrimer GalAG2 [(Gal-β-OC6H4CO-Lys-Pro-Leu)4(Lys-Phe-Lys-Ile)2Lys-His-Ile-NH2] to its target lectin LecA, crystal structures of LecA complexes with divalent analog GalAG1 [(Gal-β-OC6H4CO-Lys-Pro-Leu)2Lys-Phe-Lys-Ile-NH2] and related glucose-triazole linked bis-galactosides 3u3 [Gal-β-O(CH2)n-(C2HN3)-4-Glc-β-(C2HN3)-[β-Glc-4-(N3HC2)]2-(CH2)n-O-β-Gal (n = 1)] and 5u3 (n = 3) were obtained, revealing a chelate bound 3u3, cross-linked 5u3, and monovalently bound GalAG1. Nevertheless, a chelate bound model better explaining their strong LecA binding and the absence of lectin aggregation was obtained by modeling for all three ligands. A model of the chelate bound GalAG2·LecA complex was also obtained rationalizing its unusually tight LecA binding (KD = 2.5 nM) and aggregation by lectin cross-linking. The very weak biofilm inhibition with divalent LecA inhibitors suggests that lectin aggregation is necessary for biofilm inhibition by GalAG2, pointing to multivalent glycoclusters as a unique opportunity to control P. aeruginosa biofilms.

Biologically Active Heteroglycoclusters Constructed on a Pillar[5]arene-Containing [2]Rotaxane Scaffold

A synthetic approach combining recent concepts for the preparation of multifunctional nanomolecules (click chemistry on multifunctional scaffolds) with supramolecular chemistry (self‐assembly to prepare rotaxanes) gave easy access to a large variety of sophisticated [2]rotaxane heteroglycoclusters. Specifically, compounds combining galactose and fucose have been prepared to target the two bacterial lectins (LecA and LecB) from the opportunistic pathogen Pseudomonas aeruginosa.

Multivalency effects on Pseudomonas aeruginosa biofilm inhibition and dispersal by glycopeptide dendrimers targeting lectin LecA

The galactose specific lectin LecA partly mediates the formation of antibiotic resistant biofilms by Pseudomonas aeruginosa, an opportunistic pathogen causing lethal airways infections in immunocompromised and cystic fibrosis patients, suggesting that preventing LecA binding to natural saccharides might provide new opportunities for treatment. Here 8-fold (G3) and 16-fold (G4) galactosylated analogs of GalAG2, a tetravalent G2 glycopeptide dendrimer LecA ligand and P. aeruginosa biofilm inhibitor, were obtained by convergent chloroacetyl thioether (ClAc) ligation between 4-fold or 8-fold chloroacetylated dendrimer cores and digalactosylated dendritic arms. Hemagglutination inhibition, isothermal titration calorimetry and biofilm inhibition assays showed that G3 dendrimers bind LecA slightly better than their parent G2 dendrimers and induce complete biofilm inhibition and dispersal of P. aeruginosa biofilms, while G4 dendrimers show reduced binding and no biofilm inhibition. A binding model accounting for the observed saturation of glycopeptide dendrimer galactosyl groups and LecA binding sites is proposed based on the crystal structure of a G3 dendrimer LecA complex.

The interplay of autophagy and β-Catenin signaling regulates differentiation in acute myeloid leukemia

The major feature of leukemic cells is an arrest of differentiation accompanied by highly active proliferation. In many subtypes of acute myeloid leukemia, these features are mediated by the aberrant Wnt/β-Catenin pathway. In our study, we established the lectin LecB as inducer of the differentiation of the acute myeloid leukemia cell line THP-1 and used it for the investigation of the involved processes. During differentiation, functional autophagy and low β-Catenin levels were essential. Corresponding to this, a high β-Catenin level stabilized proliferation and inhibited autophagy, resulting in low differentiation ability. Initiated by LecB, β-Catenin was degraded, autophagy became active and differentiation took place within hours. Remarkably, the reduction of β-Catenin sensitized THP-1 cells to the autophagy-stimulating mTOR inhibitors. As downmodulation of E-Cadherin was sufficient to significantly reduce LecB-mediated differentiation, we propose E-Cadherin as a crucial interaction partner in this signaling pathway. Upon LecB treatment, E-Cadherin colocalized with β-Catenin and thereby prevented the induction of β-Catenin target protein expression and proliferation. That way, our study provides for the first time a link between E-Cadherin, the aberrant Wnt/β-Catenin signaling, autophagy and differentiation in acute myeloid leukemia. Importantly, LecB was a valuable tool to elucidate the underlying molecular mechanisms of acute myeloid leukemia pathogenesis and may help to identify novel therapy approaches.

A recombinant fungal lectin for labeling truncated glycans on human cancer cells

Cell surface glycoconjugates present alterations of their structures in chronic diseases and distinct oligosaccharide epitopes have been associated with cancer. Among them, truncated glycans present terminal non-reducing β-N-acetylglucosamine (GlcNAc) residues that are rare on healthy tissues. Lectins from unconventional sources such as fungi or algi provide novel markers that bind specifically to such epitopes, but their availability may be challenging. A GlcNAc-binding lectin from the fruiting body of the fungus Psathyrella velutina (PVL) has been produced in good yield in bacterial culture. A strong specificity for terminal GlcNAc residues was evidenced by glycan array. Affinity values obtained by microcalorimetry and surface plasmon resonance demonstrated a micromolar affinity for GlcNAcβ1-3Gal epitopes and for biantennary N-glycans with GlcNAcβ1-2Man capped branches. Crystal structure of PVL complexed with GlcNAcβ1-3Gal established the structural basis of the specificity. Labeling of several types of cancer cells and use of inhibitors of glycan metabolism indicated that rPVL binds to terminal GlcNAc but also to sialic acid (Neu5Ac). Analysis of glycosyltransferase expression confirmed the higher amount of GlcNAc present on cancer cells. rPVL binding is specific to cancer tissue and weak or no labeling is observed for healthy ones, except for stomach glands that present unique αGlcNAc-presenting mucins. In lung, breast and colon carcinomas, a clear delineation could be observed between cancer regions and surrounding healthy tissues. PVL is therefore a useful tool for labeling agalacto-glycans in cancer or other diseases.

Structural insights into Aspergillus fumigatus lectin specificity: AFL binding sites are functionally non-equivalent

The Aspergillus fumigatus lectin AFL was recently described as a new member of the AAL lectin family. As a lectin from an opportunistic pathogen, it might play an important role in the interaction of the pathogen with the human host. A detailed study of structures of AFL complexed with several monosaccharides and oligosaccharides, including blood-group epitopes, was combined with affinity data from SPR and discussed in the context of previous findings. Its six binding sites are non-equivalent, and owing to minor differences in amino-acid composition they exhibit a marked difference in specific ligand recognition. AFL displays a high affinity in the micromolar range towards oligosaccharides which were detected in plants and also those bound on the human epithelia. All of these results indicate AFL to be a complex member of the lectin family and a challenging target for future medical research and, owing to its binding properties, a potentially useful tool in specific biotechnological applications.

Algal lectin binding to core (α1-6) fucosylated N-glycans: structural basis for specificity and production of recombinant protein

We determined the specificity of BTL, a lectin from the red marine alga Bryothamnion triquetrum, toward fucosylated oligosaccharides. BTL showed a strict specificity for the core α1,6-fucosylation, which is an important marker for cancerogenesis and quality control of therapeutical antibodies. The double fucosylation α1,6 and α1,3 was also recognized, but the binding was totally abolished in the sole presence of the α1,3-fucosylation. A more detailed analysis of the specificity of BTL showed a preference for bi- and tri-antennary nonbisected N-glycans. Sialylation or fucosylation at the nonreducing end of N-glycans did not affect the recognition by the lectin. BTL displayed a strong affinity for a core α1,6-fucosylated octasaccharide with a Kd of 12 μM by titration microcalorimetry. The structural characterization of the interaction between BTL and the octasaccharide was obtained by STD-NMR. It demonstrated an extended epitope for recognition that includes the fucose residue, the distal GlcNAc and one mannose residue. Recombinant rBTL was obtained in Escherichia coli and characterized. Its binding properties for carbohydrates were studied using hemagglutination tests and glycan array analysis. rBTL was able to agglutinate rabbit erythrocytes with strong hemagglutination activity only after treatment with papain and trypsin, indicating that its ligands were not directly accessible at the cell surface. The hemagglutinating properties of rBTL confirm the correct folding and functional state of the protein. The results show BTL as a potent candidate for cancer diagnosis and as a reagent for the preparation and quality control of antibodies lacking core α1,6-fucosylated N-glycans.

Multivalent thioglycopeptoids via photoclick chemistry: potent affinities towards LecA and BC2L-A lectins

The straightforward access to peptoid-based multivalent thioglycoclusters displaying 1-thio-β-d-galactose or 1-thio-α/β-d-mannose and their evaluation towards two bacterial lectins are described.

Fucofullerenes as tight ligands of RSL and LecB, two bacterial lectins

A series of water-soluble glycofullerenes containing up to 24 fucose residues were tested against the two bacterial lectins LecB and RSL, and C60(E)12 bearing 24 fucose residues appeared to be the best known inhibitor of both lectins to date.