Microwave-Assisted Synthesis of ß-N-Aryl Glycoamphiphiles with Diverse Supramolecular Assemblies and Lectin Accessibility.

Glycoamphiphiles have attracted considerable interest in a broad range of application fields owing to their solution and bulk-state self-assembly abilities. Despite their importance, the straightforward synthesis of glycoamphiphiles consisting of a hydrophilic carbohydrate linked to a hydrophobic aglycone remains one of the major challenges in glycosciences. Here, a rapid, simple, and efficient synthetic access to chemically stable glycoamphiphiles at physiological pH, namely, N-(ß-d-glycosyl)-2-alkylbenzamide, is reported. It leverages the nonreductive amination of unprotected carbohydrates with ortho-substituted aniline derivatives which could be readily obtained by reacting commercially available primary alkylamines with isatoic anhydride. This strategy avoids protection and deprotection of sugar hydroxyl groups and the use of reductive agents, which makes it advantageous in terms of atom and step economy. Moreover, in order to circumvent the cons of classical N-aryl glycosylation, we investigate the use of microwave as a heat source that provides fast, clean, and high-yield ß-N-arylation of unprotected carbohydrates. Their self-assembly into water led to multiple morphologies of dynamic supramolecular glycoamphiphiles that were characterized to assess their ability to bind to lectins from pathogenic bacteria. Biophysical interactions probed by isothermal titration microcalorimetry revealed micromolar affinities for most of the synthesized glycoamphiphiles.

Photoswitchable glycoligands targeting Pseudomonas aeruginosa LecA.

Biofilm formation is one of main causes of bacterial antimicrobial resistance infections. It is known that the soluble lectins LecA and LecB, produced by Pseudomonas aeruginosa, play a key role in biofilm formation and lung infection. Bacterial lectins are therefore attractive targets for the development of new antibiotic-sparing anti-infective drugs. Building synthetic glycoconjugates for the inhibition and modulation of bacterial lectins have shown promising results. Light-sensitive lectin ligands could allow the modulation of lectins activity with precise spatiotemporal control. Despite the potential of photoswitchable tools, few photochromic lectin ligands have been developed. We have designed and synthesized several O- and S-galactosyl azobenzenes as photoswitchable ligands of LecA and evaluated their binding affinity with isothermal titration calorimetry. We show that the synthesized monovalent glycoligands possess excellent photophysical properties and strong affinity for targeted LecA with K d values in the micromolar range. Analysis of the thermodynamic contribution indicates that the Z-azobenzene isomers have a systematically stronger favorable enthalpy contribution than the corresponding E-isomers, but due to stronger unfavorable entropy, they are in general of lower affinity. The validation of this proof-of-concept and the dissection of thermodynamics of binding will help for the further development of lectin ligands that can be controlled by light.

Elucidating the glycan-binding specificity and structure of Cucumis melo agglutinin, a new R-type lectin.

Plant lectins have garnered attention for their roles as laboratory probes and potential therapeutics. Here, we report the discovery and characterization of Cucumis melo agglutinin (CMA1), a new R-type lectin from melon. Our findings reveal CMA1's unique glycan-binding profile, mechanistically explained by its 3D structure, augmenting our understanding of R-type lectins. We expressed CMA1 recombinantly and assessed its binding specificity using multiple glycan arrays, covering 1,046 unique sequences. This resulted in a complex binding profile, strongly preferring C2-substituted, beta-linked galactose (both GalNAc and Fuca1-2Gal), which we contrasted with the established R-type lectin Ricinus communis agglutinin 1 (RCA1). We also report binding of specific glycosaminoglycan subtypes and a general enhancement of binding by sulfation. Further validation using agglutination, thermal shift assays, and surface plasmon resonance confirmed and quantified this binding specificity in solution. Finally, we solved the high-resolution structure of the CMA1 N-terminal domain using X-ray crystallography, supporting our functional findings at the molecular level. Our study provides a comprehensive understanding of CMA1, laying the groundwork for further exploration of its biological and therapeutic potential.

Cell-free expression and characterization of multivalent rhamnose-binding lectins using bio-layer interferometry.

Lectins are important biological tools for binding glycans, but recombinant protein expression poses challenges for some lectin classes, limiting the pace of discovery and characterization. To discover and engineer lectins with new functions, workflows amenable to rapid expression and subsequent characterization are needed. Here, we present bacterial cell-free expression as a means for efficient, small-scale expression of multivalent, disulfide bond-rich, rhamnose-binding lectins. Furthermore, we demonstrate that the cell-free expressed lectins can be directly coupled with bio-layer interferometry analysis, either in solution or immobilized on the sensor, to measure interaction with carbohydrate ligands without purification. This workflow enables the determination of lectin substrate specificity and estimation of binding affinity. Overall, we believe that this method will enable high-throughput expression, screening, and characterization of new and engineered multivalent lectins for applications in synthetic glycobiology.

Increasing the Hydrophilicity of Cyclic Ketene Acetals Improves the Hydrolytic Degradation of Vinyl Copolymers and the Interaction of Glycopolymer Nanoparticles with Lectins.

Radical ring-opening polymerization (rROP) of cyclic ketene acetals (CKAs) with traditional vinyl monomers allows the synthesis of degradable vinyl copolymers. However, since the most commonly used CKAs are hydrophobic, most degradable vinyl copolymers reported so far degrade very slowly by hydrolysis under physiological conditions (phosphate-buffered saline, pH 7.4, 37 °C), which can be detrimental for biomedical applications. Herein, to design advanced vinyl copolymers by rROP with high CKA content and enhanced degradation profiles, we reported the copolymerization of 2-methylene-1,3,6-trioxocane (MTC) as a CKA with vinyl ether (VE) or maleimide (MI) derivatives. By performing a point-by-point comparison between the MTC/VE and MTC/MI copolymerization systems, and their counterparts based on 2-methylene-1,3-dioxepane (MDO) and 5,6-benzo-2-methylene-1,3-dioxepane (BMDO), we showed negligible impact on the macromolecular characteristics and similar reactivity ratios, suggesting successful substitution of MDO and BMDO by MTC. Interestingly, owing to the hydrophilicity of MTC, the obtained copolymers exhibited a faster hydrolytic degradation under both accelerated and physiological conditions. We then prepared MTC-based glycopolymers, which were formulated into surfactant-free nanoparticles, exhibiting excellent colloidal stability up to 4 months and complete degradation under enzymatic conditions. Importantly, MTC-based glyconanoparticles also showed a similar cytocompatibility toward two healthy cell lines and a much stronger lectin affinity than MDO-based glyconanoparticles.

Identification of New L-Fucosyl and L-Galactosyl Amides as Glycomimetic Ligands of TNF Lectin Domain of BC2L-C from Burkholderia cenocepacia.

The inhibition of carbohydrate-lectin interactions is being explored as an efficient approach to anti adhesion therapy and biofilm destabilization, two alternative antimicrobial strategies that are being explored against resistant pathogens. BC2L-C is a new type of lectin from Burkholderia cenocepacia that binds (mammalian) fucosides at the N-terminal domain and (bacterial) mannosides at the C-terminal domain. This double carbohydrate specificity allows the lectin to crosslink host cells and bacterial cells. We have recently reported the design and generation of the first glycomimetic antagonists of BC2L-C, ß-C- or ß-N-fucosides that target the fucose-specific N-terminal domain (BC2L-C-Nt). The low water solubility of the designed N-fucosides prevented a full examination of this promising series of ligands. In this work, we describe the synthesis and biophysical evaluation of new L-fucosyl and L-galactosyl amides, designed to be water soluble and to interact with BC2L-C-Nt. The protein-ligand interaction was investigated by Saturation Transfer Difference NMR, Isothermal Titration Calorimetry and crystallographic studies. STD-NMR experiments showed that both fucosyl and galactosyl amides compete with α-methyl fucoside for lectin binding. A new hit compound was identified with good water solubility and an affinity for BC2L-C-Nt of 159 µM (ITC), which represents a one order of magnitude gain over α-methyl fucoside. The x-ray structure of its complex with BC2L-C-Nt was solved at 1.55 Å resolution.

Neutralizing the Impact of the Virulence Factor LecA from Pseudomonas aeruginosa on Human Cells with New Glycomimetic Inhibitors.

Bacterial adhesion, biofilm formation and host cell invasion of the ESKAPE pathogen Pseudomonas aeruginosa require the tetravalent lectins LecA and LecB, which are therefore drug targets to fight these infections. Recently, we have reported highly potent divalent galactosides as specific LecA inhibitors. However, they suffered from very low solubility and an intrinsic chemical instability due to two acylhydrazone motifs, which precluded further biological evaluation. Here, we isosterically substituted the acylhydrazones and systematically varied linker identity and length between the two galactosides necessary for LecA binding. The optimized divalent LecA ligands showed improved stability and were up to 1000-fold more soluble. Importantly, these properties now enabled their biological characterization. The lead compound L2 potently inhibited LecA binding to lung epithelial cells, restored wound closure in a scratch assay and reduced the invasiveness of P. aeruginosa into host cells.

Characterization of the interaction of multivalent glycosylated ligands with bacterial lectins by biolayer interferometry.

The study of multivalent carbohydrate-protein interactions remains highly complicated and sometimes rendered impossible due to aggregation problems. Biolayer interferometry is emerging as a tool to monitor such complex interactions. In this study, various glycoclusters and dendrimers were prepared and evaluated as ligands for lectins produced by pathogenic bacteria Pseudomonas aeruginosa (LecA and Lec B) and Burkholderia ambifaria (BambL). Reliable kinetic and thermodynamic parameters could be measured, and immobilization of either lectin or ligands resulted in high quality data. The methods gave results in full agreement with previous isothermal titration calorimetry experiments, and presented strong advantages because they require less quantity and purity for the biomolecules.

Degradable Glycopolyester-like Nanoparticles by Radical Ring-Opening Polymerization.

A small library of degradable polyester-like glycopolymers was successfully prepared by the combination of radical ring-opening copolymerization of 2-methylene-1,3-dioxepane as a cyclic ketene acetal (CKA) with vinyl ether (VE) derivatives and a Pd-catalyzed thioglycoconjugation. The resulting thioglycopolymers were formulated into self-stabilized thioglyconanoparticles, which were stable up to 4 months and were enzymatically degraded. Nanoparticles and their degradation products exhibited a good cytocompatibility on two healthy cell lines. Interactions between thioglyconanoparticles and lectins were investigated and highlighted the presence of both specific carbohydrate/lectin interactions and nonspecific hydrophobic interactions. Fluorescent thioglyconanoparticles were also prepared either by encapsulation of Nile red or by the functionalization of the polymer backbone with rhodamine B. Such nanoparticles were used to prove the cell internalization of the thioglyconanoparticles by lung adenocarcinoma (A549) cells, which underlined the great potential of P(CKA-co-VE) copolymers for biomedical applications.

Conformational transition in SPR experiments: impact of spacer length, immobilization mode and aptamer density on signal sign and amplitude.

Surface plasmon resonance (SPR) is an optical, real-time and label-free technique which represents a standard to study biomolecular interactions. While SPR signals are usually positive upon recognition, a few cases of negative signals have been reported because of significant conformational transition of the receptor upon the recognition of the target. In this study, we reported on the observation of negative or null SPR signals for an aptamer recognition with its low molecular weight target. The introduction of a spacer group for the aptamer immobilization led to a null SPR signal despite the device sensitivity and effective target recognition (a KD around 200 nM as demonstrated using a quartz crystal microbalance with dissipation monitoring and isothermal titration calorimetry). We demonstrated that this unconventional signal could be attributed to two opposite contributions: a positive one is afforded by the aptamer recognition and folding whereas a negative one results from the refractive index increment (RII) deviation upon the formation of the complex (ligand/analyte). We also demonstrated that the RII deviation is sensitive to the modification of the sequence flexibility and therefore depends on the anchoring procedure and the spacer length between the anchoring function and the site of recognition.

Hexavalent thiofucosides to probe the role of the Aspergillus fumigatus lectin FleA in fungal pathogenicity.

Aspergillus fumigatus is a pathogenic fungus infecting the respiratory system and responsible for a variety of life-threatening lung diseases. A fucose-binding lectin named FleA which has a controversial role in A. fumigatus pathogenesis was recently identified. New chemical probes with high affinity and enzymatic stability are needed to explore the role of FleA in the infection process. In this study, we developed potent FleA antagonists based on optimized and non-hydrolysable thiofucoside ligands. We first synthesized a set of monovalent sugars showing micromolar affinity for FleA by isothermal titration calorimetry. The most potent derivative was co-crystallized with FleA to gain insights into the binding mode in operation. Its chemical multimerization on a cyclodextrin scaffold led to an hexavalent compound with a significantly enhanced binding affinity (Kd = 223 ± 21 nM) thanks to a chelate binding mode. The compound could probe the role of bronchial epithelial cells in a FleA-mediated response to tissue invasion.

Stereoselective Synthesis of Fluorinated Galactopyranosides as Potential Molecular Probes for Galactophilic Proteins: Assessment of Monofluorogalactoside-LecA Interactions.

The replacement of hydroxyl groups by fluorine atoms on hexopyranoside scaffolds may allow access to invaluable tools for studying various biochemical processes. As part of ongoing activities toward the preparation of fluorinated carbohydrates, a systematic investigation involving the synthesis and biological evaluation of a series of mono- and polyfluorinated galactopyranosides is described. Various monofluorogalactopyranosides, a trifluorinated, and a tetrafluorinated galactopyranoside have been prepared using a Chiron approach. Given the scarcity of these compounds in the literature, in addition to their synthesis, their biological profiles were evaluated. Firstly, the fluorinated compounds were investigated as antiproliferative agents using normal human and mouse cells in comparison with cancerous cells. Most of the fluorinated compounds showed no antiproliferative activity. Secondly, these carbohydrate probes were used as potential inhibitors of galactophilic lectins. The first transverse relaxation-optimized spectroscopy (TROSY) NMR experiments were performed on these interactions, examining chemical shift perturbations of the backbone resonances of LecA, a virulence factor from Pseudomonas aeruginosa. Moreover, taking advantage of the fluorine atom, the 19 F NMR resonances of the monofluorogalactopyranosides were directly monitored in the presence and absence of LecA to assess ligand binding. Lastly, these results were corroborated with the binding potencies of the monofluorinated galactopyranoside derivatives by isothermal titration calorimetry experiments. Analogues with fluorine atoms at C-3 and C-4 showed weaker affinities with LecA as compared to those with the fluorine atom at C-2 or C-6. This research has focused on the chemical synthesis of "drug-like" low-molecular-weight inhibitors that circumvent drawbacks typically associated with natural oligosaccharides.

Multivalent Fucosides with Nanomolar Affinity for the Aspergillus fumigatus Lectin FleA Prevent Spore Adhesion to Pneumocytes.

FleA (or AFL), a fucose lectin, was recently identified in the opportunistic mold Aspergillus fumigatus, which causes fatal lung infections in immunocompromised patients. We designed di-, hexa- and octavalent fucosides with various spacer arm lengths to block the hexameric FleA through chelation. Microcalorimetry measurements showed that the ethylene glycol (EG) spacer arm length has a strong influence on the binding affinity of the divalent fucosides. The relationship between the EG length and chelate binding efficiency to FleA was explored according to polymer theory. Hexa- and octavalent compounds based on cyclodextrin and octameric silsesquioxane scaffolds were nanomolar FleA inhibitors, surpassing their monovalent fucose analogue by more than three orders of magnitude. Importantly, some of the fucosides were highly efficient in preventing fungal spore adhesion to bronchoepithelial cells, with half maximal inhibitory concentration values in the micromolar range. We propose that the synergistic antiadhesive effect observed can be ascribed to chelate binding to FleA and to the formation of conidium aggregates, as observed by optical microscopy. These fucosides are promising tools that can be used to better understand the role of FleA in conidia pathogenicity and host defenses against invasive aspergillosis.

Tetraphenylethylene-based glycoclusters with aggregation-induced emission (AIE) properties as high-affinity ligands of bacterial lectins.

Tetraphenylethylene (TPE) is fluorescent through aggregation induced emission (AIE) in water. Herein, TPE was used as the core of glycoclusters that target the bacterial lectins LecA and LecB of Pseudomonas aeruginosa. Synthesis of these TPE-based glycoclusters was accomplished by using azide-alkyne "click" chemistry. The AIE properties of the resulting glycoclusters could be readily verified, but imaging could not be pursued due to the overlap of the fluorescence signals from cells and bacteria. Nonetheless, the glycoclusters displayed nanomolar affinities toward LecA and LecB. Further evaluation in a cell-based anti-adhesive assay highlighted a limited decrease in adhesion (20%) for the fucosylated glycocluster. This confirmed that these TPE-based glycoclusters are indeed LecA and LecB high-affinity ligands. Nevertheless, the hypotheses involving their application in imaging or anti-adhesive therapy could not be verified.

Virtual Screening Against Carbohydrate-Binding Proteins: Evaluation and Application to Bacterial Burkholderia ambifaria Lectin.

Bacterial adhesion to human epithelia via lectins constitutes a therapeutic opportunity to prevent infection. Specifically, BambL (the lectin from Burkholderia ambifaria) is implicated in cystic fibrosis, where lectin-mediated bacterial adhesion to fucosylated lung epithelia is suspected to play an important role. We employed structure-based virtual screening to identify inhibitors of BambL-saccharide interaction with potential therapeutic value. To enable such discovery, a virtual screening protocol was iteratively developed via 194 retrospective screening protocols against 4 bacterial lectins (BambL, BC2L-A, FimH, and LecA) with known ligands. Specific attention was given to the rigorous evaluation of retrospective screening, including calculation of analytical errors for enrichment metrics. The developed virtual screening workflow used crystallographic constraints, pharmacophore filters, and a final manual selection step. The protocol was applied to BambL, predicting 15 active compounds from virtual libraries of approximately 7 million compounds. Experimental validation using fluorescence polarization confirmed micromolar inhibitory activity for two compounds, which were further characterized by isothermal titration calorimetry and surface plasmon resonance. Subsequent testing against LecB from Pseudomonas aeruginosa demonstrated binding specificity of one of the hit compounds. This report demonstrates the utility of virtual screening protocols, integrating ligand-based pharmacophore filtering and structure-based constraints, in the search for bacterial lectin inhibitors.

Perylenediimide-based glycoclusters as high affinity ligands of bacterial lectins: synthesis, binding studies and anti-adhesive properties.

The synthesis of eight perylenediimide-based glycoclusters was readily performed from hexa- and tetra-propargylated cores through azide-alkyne "click" conjugation. Variations in the carbohydrate epitope (Glc, Gal, Man, Fuc) and the linker arm provided molecular diversity. Interactions with LecA and LecB, two proteins involved in the adhesion of Pseudomonas aeruginosa to host tissues, were evaluated by microcalorimetry (ITC). In both cases high affinities were obtained with Kd values in the nanomolar range. Further evaluation of their anti-adhesive properties using cultured epithelial cells demonstrated their potent anti-adhesive activities against Pseudomonas aeruginosa with only 30-40% residual adhesion observed. The fluorescence properties of the PDI core were then investigated by confocal microscopy on cell-bacteria cultures. However, the red fluorescence signal of the PDI-based glycocluster was too weak to provide significant data. The present study provides another type of anti-adhesive glycocluster against bacterial infection with a large aromatic PDI core.

Dynamic Cooperative Glycan Assembly Blocks the Binding of Bacterial Lectins to Epithelial Cells.

Pathogens frequently rely on lectins for adhesion and cellular entry into the host. Since these interactions typically result from multimeric binding of lectins to cell-surface glycans, novel therapeutic strategies are being developed with the use of glycomimetics as competitors of such interactions. Herein we study the benefit of nucleic acid based oligomeric assemblies with PNA-fucose conjugates. We demonstrate that the interactions of a lectin with epithelial cells can be inhibited with conjugates that do not form stable assemblies in solution but benefit from cooperativity between ligand-protein interactions and PNA hybridization to achieve high affinity. A dynamic dimeric assembly fully blocked the binding of the fucose-binding lectin BambL of Burkholderia ambifaria, a pathogenic bacterium, to epithelial cells with an efficiency of more than 700-fold compared to l-fucose.

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.

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.

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.