Studies on the binding site of the galactose-specific agglutinin PA-IL from Pseudomonas aeruginosa

Insights on Pseudomonas aeruginosa Carbohydrate Binding from Profiles of Cystic Fibrosis Isolates Using Multivalent Fluorescent Glycopolymers Bearing Pendant Monosaccharides

Pseudomonas aeruginosa contributes to frequent, persistent, and, often, polymicrobial respiratory tract infections for individuals with cystic fibrosis (CF). Chronic CF infections lead to bronchiectasis and a shortened lifespan. P. aeruginosa expresses numerous adhesins, including lectins known to bind the epithelial cell and mucin glycoconjugates. Blocking carbohydrate-mediated host-pathogen and intra-biofilm interactions critical to the initiation and perpetuation of colonization offer promise as anti-infective treatment strategies. To inform anti-adhesion therapies, we profiled the monosaccharide binding of P. aeruginosa from CF and non-CF sources, and assessed whether specific bacterial phenotypic characteristics affected carbohydrate-binding patterns. Focusing at the cellular level, microscopic and spectrofluorometric tools permitted the solution-phase analysis of P. aeruginosa binding to a panel of fluorescent glycopolymers possessing distinct pendant monosaccharides. All P. aeruginosa demonstrated significant binding to glycopolymers specific for α-D-galactose, β-D-N-acetylgalactosamine, and β-D-galactose-3-sulfate. In each culture, a small subpopulation accounted for the binding. The carbohydrate anomeric configuration and sulfate ester presence markedly influenced binding. While this opportunistic pathogen from CF hosts presented with various colony morphologies and physiological activities, no phenotypic, physiological, or structural feature predicted enhanced or diminished monosaccharide binding. Important to anti-adhesive therapeutic strategies, these findings suggest that, regardless of phenotype or clinical source, P. aeruginosa maintain a small subpopulation that may readily associate with specific configurations of specific monosaccharides. This report provides insights into whole-cell P. aeruginosa carbohydrate-binding profiles and into the context within which successful anti-adhesive and/or anti-virulence anti-infective agents for CF must contend.

Characterization of Cell Surface Glycan Profiles in Human and Mouse Corneas Using Lectin Microarrays

The advent of high-throughput sequencing technologies has facilitated the profiling of glycosylation genes at a single-cell level in complex biological systems, but the significance of these gene signatures to the composition of the glycocalyx remains ambiguous. Here, we used lectin microarrays to characterize the composition of cell surface glycans in human and mouse corneas and determine its relationship to single-cell transcriptomic data. Our results identify a series of cell surface glycan signatures that are unique to the different cell types of the human cornea and that correlate, to a certain extent, with the transcriptional expression of glycosylation genes. These include pathways involved in the biosynthesis of O-glycans in epithelial cells and core fucose on stromal and endothelial cell surfaces. Moreover, we show that human and mouse corneas display some structural differences in terms of cell surface glycan composition. These results could provide insights into the specialized function of individual cell types in the cornea and foster the identification of novel cornea-specific biomarkers.

Small Carbohydrate Derivatives as Potent Antibiofilm Agents

Biofilm formation by most pathogenic bacteria is considered as one of the key mechanisms associated with virulence and antibiotic resistance. Biofilm-forming bacteria adhere to the surfaces of biological or implant medical devices and create communities within their self-produced extracellular matrix that are difficult to treat by existing antibiotics. There is an urgent need to synthesize and screen structurally diverse molecules for their antibiofilm activity that can remove or minimize the bacterial biofilm. The development of carbohydrate-based small molecules as antibiofilm agents holds a great promise in addressing the problem of the eradication of biofilm-related infections. Owing to their structural diversity and specificity, the sugar scaffolds are valuable entities for developing antibiofilm agents. In this perspective, we discuss the literature pertaining to carbohydrate-based natural antibiofilm agents and provide an overview of the design, activity, and mode of action of potent synthetic carbohydrate-based molecules.

Chemical control over Asialo-GM1: A dual ligand for pili and Lectin A that activates swarming motility and facilitates adherence of Pseudomonas aeruginosa

Glycolipid, ganglio-N-tetraosylceramide (asialo-GM1), on the mammalian cells are known to be recognized by type IV pili of Pseudomonas aeruginosa. In this work, we show that asialo-GM1 can also be recognized by Lectin A (LecA), another adhesin protein of the P. aeruginosa, by a fluorescent polarization assay, a label-free bacterial motility enabled binding assay, and bacterial mutant studies. On hydrated semi-solid gel surfaces, asialo-GM1 enables swarming and twitching motilities, while on solid surfaces facilitates the bacterial adherence of P. aeruginosa. These results indicate that asialo-GM1 can modulate bioactivities, adherence, and motilities, that are controlled by opposite signaling pathways. We demonstrate that when a solution of pilin monomers or LecA proteins are spread on hydrated gel surfaces, the asialo-GM1 mediated swarming motility is inhibited. Treatment of artificial liposomes containing asialo-GM1 as a component of lipid bilayer with pilin monomers or LecA proteins caused transient leakage of encapsulated dye from liposomes. These results suggest that pili and LecA proteins not only bind to asialo-GM1 but can also cause asialo-GM1 mediated leakage. We also show that both pili and LecA mutants of P. aeruginosa adhere to asialo-GM1 coated solid surfaces, and that a class of synthetic ligands for pili and LecA inhibits both pili and LecA-mediated adherence of P. aeruginosa on asialo-GM1-coated surfaces.

Chimeric Ligands of Pili and Lectin A Inhibit Tolerance, Persistence, and Virulence Factors of Pseudomonas aeruginosa over a Wide Range of Phenotypes

Bacteria readily form resilient phenotypes to counter environmental and antibiotic stresses. Here, we demonstrate a class of small molecules that inhibit a wide range of Pseudomonas aeruginosa phenotypes and enable antibiotics to kill previously tolerant bacteria, preventing the transition of tolerant bacteria into a persistent population. We identified two proteins, type IV pili and lectin LecA, as receptors for our molecules by methods including a new label-free assay based on bacterial motility sensing the chemicals in the environment, the chemical inhibition of bacteriophage adsorption on pili appendages of bacteria, and fluorescence polarization. Structure–activity relationship studies reveal a molecule that inhibits only pili appendage and a class of chimeric ligands that inhibit both LecA and pili. Important structural elements of the ligand are identified for each protein. This selective ligand binding identifies the phenotypes each protein receptor controls. Inhibiting LecA results in reducing biofilm formation, eliminating small colony variants, and is correlated with killing previously tolerant bacteria. Inhibiting pili appendages impedes swarming and twitching motilities and pyocyanin and elastase production. Because these phenotypes are controlled by a broad range of signaling pathways, this approach simultaneously controls the multiple signaling mechanisms preventing bacteria to elude antibiotic treatments.

Neutron crystallography reveals mechanisms used by Pseudomonas aeruginosa for host-cell binding

The opportunistic pathogen Pseudomonas aeruginosa, a major cause of nosocomial infections, uses carbohydrate-binding proteins (lectins) as part of its binding to host cells. The fucose-binding lectin, LecB, displays a unique carbohydrate-binding site that incorporates two closely located calcium ions bridging between the ligand and protein, providing specificity and unusually high affinity. Here, we investigate the mechanisms involved in binding based on neutron crystallography studies of a fully deuterated LecB/fucose/calcium complex. The neutron structure, which includes the positions of all the hydrogen atoms, reveals that the high affinity of binding may be related to the occurrence of a low-barrier hydrogen bond induced by the proximity of the two calcium ions, the presence of coordination rings between the sugar, calcium and LecB, and the dynamic behaviour of bridging water molecules at room temperature. These key structural details may assist in the design of anti-adhesive compounds to combat multi-resistance bacterial infections.

Pseudomonas aeruginosa employs lectins to bind to its host cells, and is known to be the major cause of lung infections. Lectin B (LecB) from Pseudomonas aeruginosa binds specifically to galactose and fucose and is important for pathogenicity, adhesion and biofilm formation. In this work, the neutron crystal structure (1.9 Å) of the deuterated LecB/Ca/fucose complex is reported. The structure, in combination with perdeuteration of the ligand and the receptor allowed the observation of hydrogen atoms, protonation states and hydrogen bonds involved in the interaction between pathogenic bacteria and host cells. Thus the study provides structural insights into the mechanism of high affinity binding of LecB to its targets.

Structural Diversities of Lectins Binding to the Glycosphingolipid Gb3

Glycolipids are present on the surfaces of all living cells and thereby represent targets for many protein receptors, such as lectins. Understanding the interactions between lectins and glycolipids is essential for investigating the functions of lectins and the dynamics of glycolipids in living membranes. This review focuses on lectins binding to the glycosphingolipid globotriaosylceramide (Gb3), an attractive host cell receptor, particularly for pathogens and pathogenic products. Shiga toxin (Stx), from Shigella dysenteriae or Escherichia coli, which is one of the most virulent bacterial toxins, binds and clusters Gb3, leading to local negative membrane curvature and the formation of tubular plasma membrane invaginations as the initial step for clathrin-independent endocytosis. After internalization, it is embracing the retrograde transport pathway. In comparison, the homotetrameric lectin LecA from Pseudomonas aeruginosa can also bind to Gb3, triggering the so-called lipid zipper mechanism, which results in membrane engulfment of the bacterium as an important step for its cellular uptake. Notably, both lectins bind to Gb3 but induce distinct plasma membrane domains and exploit mainly different transport pathways. Not only, several other Gb3-binding lectins have been described from bacterial origins, such as the adhesins SadP (from Streptococcus suis) and PapG (from E. coli), but also from animal, fungal, or plant origins. The variety of amino acid sequences and folds demonstrates the structural versatilities of Gb3-binding lectins and asks the question of the evolution of specificity and carbohydrate recognition in different kingdoms of life.

Synthesis of Glycodendrimers with Antiviral and Antibacterial Activity

Glycodendrimers are an important class of synthetic macromolecules that can be used to mimic many structural and functional features of cell-surface glycoconjugates. Their carbohydrate moieties perform key important functions in bacterial and viral infections, often regulated by carbohydrate-protein interactions. Several studies have shown that the molecular structure, valency and spatial organisation of carbohydrate epitopes in glycoconjugates are key factors in the specificity and avidity of carbohydrate-protein interactions. Choosing the right glycodendrimers almost always helps to interfere with such interactions and blocks bacterial or viral adhesion and entry into host cells as an effective strategy to inhibit bacterial or viral infections. Herein, the state of the art in the design and synthesis of glycodendrimers employed for the development of anti-adhesion therapy against bacterial and viral infections is described.

Differential recognition of lipid domains by two Gb3-binding lectins

The two lectins LecA from Pseudomonas aeruginosa and the B-subunit of Shiga toxin from Shigella dysenteriae (StxB) share the glycosphingolipid globotriaosylceramide (Gb3) as receptor. Counterintuitively, we found that LecA and StxB segregated into different domains after recognizing Gb3 at the plasma membrane of cells. We hypothesized that the orientation of the carbohydrate head group of Gb3 embedded in the lipid bilayer differentially influences LecA and StxB binding. To test this hypothesis, we reconstituted lectin-Gb3 interaction using giant unilamellar vesicles and were indeed able to rebuild LecA and StxB segregation. Both, the Gb3 fatty acyl chain structure and the local membrane environment, modulated Gb3 recognition by LecA and StxB. Specifically, StxB preferred more ordered membranes compared to LecA. Based on our findings, we propose comparing staining patterns of LecA and StxB as an alternative method to assess membrane order in cells. To verify this approach, we re-established that the apical plasma membrane of epithelial cells is more ordered than the basolateral plasma membrane. Additionally, we found that StxB recognized Gb3 at the primary cilium and the periciliary membrane, whereas LecA only bound periciliary Gb3. This suggests that the ciliary membrane is of higher order than the surrounding periciliary membrane.

Recent advances in solution-processed photodetectors based on inorganic and hybrid photo-active materials

Due to their excellent and tailorable optoelectronic performance, low cost, facile fabrication, and compatibility with flexible substrates, solution-processed inorganic and hybrid photo-active materials have attracted extensive interest for next-generation photodetector applications. This review gives a comprehensive compilation of solution-processed photodetectors. The basic structures of the device and important parameters of photodetectors will be firstly summarized. Then the development of various solution processing technologies containing solution synthesis and liquid phase film-forming processes for the preparation of semiconductor films is described. From the materials science point of view, we give a comprehensive overview about the current status of solution processed semiconductor materials including inorganic and hybrid photo-active materials for the application of photodetectors. Moreover, challenges and future trends in the field of solution-processed photodetectors are proposed.

Conformational Change of Amyloid-β 40 in Association with Binding to GM1-Glycan Cluster

Aggregates of amyloid-β (Aβ) peptide are well known to be the causative substance of Alzheimer's disease (AD). Recent studies showed that monosialotetrahexosylganglioside (GM1) clusters induce the pathological aggregation of Aβ peptide responsible for the onset and development of AD. However, the effect of GM1-glycan cluster on Aβ conformations has yet to be clarified. Interactions between Aβ peptide and GM1-glycan cluster is important for the earliest stage of the toxic aggregation on GM1 cluster. Here, we performed all-atom molecular dynamics (MD) simulations of Aβ40 on a recently developed artificial GM1-glycan cluster. The artificial GM1-glycan cluster facilitates the characterization of interactions between Aβ40 and multiple GM1-glycans. We succeeded in observing the binding of Aβ40 to the GM1-glycan cluster in all of our MD simulations. Results obtained from these MD simulations indicate the importance of HHQ (13-15) segment of Aβ40 for the GM1-glycan cluster recognition. This result is consistent with previous experimental studies regarding the glycan recognition of Aβ peptide. The recognition mechanism of HHQ (13-15) segment is mainly explained by non-specific stacking interactions between side-chains of histidine and rings of sugar residues, in which the HHQ regime forms coil and bend structures. Moreover, we found that Aβ40 exhibits helix structures at C-terminal side on the GM1-glycan cluster. The helix formation is the initial stage of the pathological aggregation at ceramide moieties of GM1 cluster. The binding of Lys28 to Neu triggers the helix formation at C-terminus side because the formation of a salt bridge between Lys28 and Neu leads to change of intrachain interactions of Aβ40. Our findings suggest that the pathological helix formation of Aβ40 is initiated at GM1-glycan moieties rather than lipid ceramide moieties.

Glycopeptides and -Mimetics to Detect, Monitor and Inhibit Bacterial and Viral Infections: Recent Advances and Perspectives

The initial contact of pathogens with host cells is usually mediated by their adhesion to glycan structures present on the cell surface in order to enable infection. Furthermore, glycans play important roles in the modulation of the host immune responses to infection. Understanding the carbohydrate-pathogen interactions are of importance for the development of novel and efficient strategies to either prevent, or interfere with pathogenic infection. Synthetic glycopeptides and mimetics thereof are capable of imitating the multivalent display of carbohydrates at the cell surface, which have become an important objective of research over the last decade. Glycopeptide based constructs may function as vaccines or anti-adhesive agents that interfere with the ability of pathogens to adhere to the host cell glycans and thus possess the potential to improve or replace treatments that suffer from resistance. Additionally, synthetic glycopeptides are used as tools for epitope mapping of antibodies directed against structures present on various pathogens and have become important to improve serodiagnostic methods and to develop novel epitope-based vaccines. This review will provide an overview of the most recent advances in the synthesis and application of glycopeptides and glycopeptide mimetics exhibiting a peptide-like backbone in glycobiology.

Conformational properties of an artificial GM1 glycan cluster based on a metal-ligand complex

An artificial glycan cluster, in which 24 monosialotetrahexosylganglioside (GM1) glycans are transplanted to the interface of a metal-ligand complex, was recently proposed to investigate the interaction between GM1 glycan clusters and amyloidogenic proteins by NMR analysis. In this study, all-atom molecular dynamics simulations were performed to characterize the conformational properties of the artificial GM1 glycan cluster. We found that more than 65% of GM1 glycans are clustered by interchain hydrogen bonds. Interchain hydrogen bonds are mainly formed between Neu5Ac and Gal'. Pentamers were most frequently observed in the metal-ligand complex. GM1 glycans are tilted and hydrophobically interact with ligand moieties. The hydrophobic surface of the metal-ligand complex increases intrachain hydrogen bonds in each conformation of the GM1 glycans. The increase of intrachain hydrogen bonds stabilizes the local minimum conformations of the GM1 glycan in comparison with the monomeric one. Interchain hydrogen bonding between glycans and glycan-ligand hydrophobic interactions also contribute to this conformational stabilization. Our results provide the physicochemical properties of the new artificial GM1 glycan cluster under the thermal fluctuations for understanding its protein recognition and designing the drug material for amyloidogenic proteins.

Hetero-Multivalency of Pseudomonas aeruginosa Lectin LecA Binding to Model Membranes

A single glycan-lectin interaction is often weak and semi-specific. Multiple binding domains in a single lectin can bind with multiple glycan molecules simultaneously, making it difficult for the classic “lock-and-key” model to explain these interactions. We demonstrated that hetero-multivalency, a homo-oligomeric protein simultaneously binding to at least two types of ligands, influences LecA (a Pseudomonas aeruginosa adhesin)-glycolipid recognition. We also observed enhanced binding between P. aeruginosa and mixed glycolipid liposomes. Interestingly, strong ligands could activate weaker binding ligands leading to higher LecA binding capacity. This hetero-multivalency is probably mediated via a simple mechanism, Reduction of Dimensionality (RD). To understand the influence of RD, we also modeled LecA’s two-step binding process with membranes using a kinetic Monte Carlo simulation. The simulation identified the frequency of low-affinity ligand encounters with bound LecA and the bound LecA’s retention of the low-affinity ligand as essential parameters for triggering hetero-multivalent binding, agreeing with experimental observations. The hetero-multivalency can alter lectin binding properties, including avidities, capacities, and kinetics, and therefore, it likely occurs in various multivalent binding systems. Using hetero-multivalency concept, we also offered a new strategy to design high-affinity drug carriers for targeted drug delivery.

Photorhabdus luminescens lectin A (PllA): A new probe for detecting α-galactoside-terminating glycoconjugates

Lectins play important roles in infections by pathogenic bacteria, for example, in host colonization, persistence, and biofilm formation. The Gram-negative entomopathogenic bacterium Photorhabdus luminescens symbiotically lives in insect-infecting Heterorhabditis nematodes and kills the insect host upon invasion by the nematode. The P. luminescens genome harbors the gene plu2096, coding for a novel lectin that we named PllA. We analyzed the binding properties of purified PllA with a glycan array and a binding assay in solution. Both assays revealed a strict specificity of PllA for α-galactoside-terminating glycoconjugates. The crystal structures of apo PllA and complexes with three different ligands revealed the molecular basis for the strict specificity of this lectin. Furthermore, we found that a 90° twist in subunit orientation leads to a peculiar quaternary structure compared with that of its ortholog LecA from Pseudomonas aeruginosa We also investigated the utility of PllA as a probe for detecting α-galactosides. The α-Gal epitope is present on wild-type pig cells and is the main reason for hyperacute organ rejection in pig to primate xenotransplantation. We noted that PllA specifically recognizes this epitope on the glycan array and demonstrated that PllA can be used as a fluorescent probe to detect this epitope on primary porcine cells in vitro In summary, our biochemical and structural analyses of the P. luminescens lectin PllA have disclosed the structural basis for PllA's high specificity for α-galactoside-containing ligands, and we show that PllA can be used to visualize the α-Gal epitope on porcine tissues.

Innovative Solutions to Sticky Situations: Antiadhesive Strategies for Treating Bacterial Infections

Bacterial adherence to host tissue is an essential process in pathogenesis, necessary for invasion and colonization and often required for the efficient delivery of toxins and other bacterial effectors. As existing treatment options for common bacterial infections dwindle, we find ourselves rapidly approaching a tipping point in our confrontation with antibiotic-resistant strains and in desperate need of new treatment options. Bacterial strains defective in adherence are typically avirulent and unable to cause infection in animal models. The importance of this initial binding event in the pathogenic cascade highlights its potential as a novel therapeutic target. This article seeks to highlight a variety of strategies being employed to treat and prevent infection by targeting the mechanisms of bacterial adhesion. Advancements in this area include the development of novel antivirulence therapies using small molecules, vaccines, and peptides to target a variety of bacterial infections. These therapies target bacterial adhesion through a number of mechanisms, including inhibition of pathogen receptor biogenesis, competition-based strategies with receptor and adhesin analogs, and the inhibition of binding through neutralizing antibodies. While this article is not an exhaustive description of every advancement in the field, we hope it will highlight several promising examples of the therapeutic potential of antiadhesive strategies.

The Pseudomonas aeruginosa lectin LecA triggers host cell signalling by glycosphingolipid-dependent phosphorylation of the adaptor protein CrkII

The human pathogen Pseudomonas aeruginosa induces phosphorylation of the adaptor protein CrkII by activating the non-receptor tyrosine kinase Abl to promote its uptake into host cells. So far, specific factors of P. aeruginosa, which induce Abl/CrkII signalling, are entirely unknown. In this research, we employed human lung epithelial cells H1299, Chinese hamster ovary cells and P. aeruginosa wild type strain PAO1 to study the invasion process of P. aeruginosa into host cells by using microbiological, biochemical and cell biological approaches such as Western Blot, immunofluorescence microscopy and flow cytometry. Here, we demonstrate that the host glycosphingolipid globotriaosylceramide, also termed Gb3, represents a signalling receptor for the P. aeruginosa lectin LecA to induce CrkII phosphorylation at tyrosine 221. Alterations in Gb3 expression and LecA function correlate with CrkII phosphorylation. Interestingly, phosphorylation of CrkII^Y221 occurs independently of Abl kinase. We further show that Src family kinases transduce the signal induced by LecA binding to Gb3, leading to Crk^Y221 phosphorylation. In summary, we identified LecA as a bacterial factor, which utilizes a so far unrecognized mechanism for phospho-CrkII^Y221 induction by binding to the host glycosphingolipid receptor Gb3. The LecA/Gb3 interaction highlights the potential of glycolipids to mediate signalling processes across the plasma membrane and should be further elucidated to gain deeper insights into this non-canonical mechanism of activating host cell processes.

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.

Blood Groups in Infection and Host Susceptibility

Blood group antigens represent polymorphic traits inherited among individuals and populations. At present, there are 34 recognized human blood groups and hundreds of individual blood group antigens and alleles. Differences in blood group antigen expression can increase or decrease host susceptibility to many infections. Blood groups can play a direct role in infection by serving as receptors and/or coreceptors for microorganisms, parasites, and viruses. In addition, many blood group antigens facilitate intracellular uptake, signal transduction, or adhesion through the organization of membrane microdomains. Several blood groups can modify the innate immune response to infection. Several distinct phenotypes associated with increased host resistance to malaria are overrepresented in populations living in areas where malaria is endemic, as a result of evolutionary pressures. Microorganisms can also stimulate antibodies against blood group antigens, including ABO, T, and Kell. Finally, there is a symbiotic relationship between blood group expression and maturation of the gastrointestinal microbiome.

Multifaceted glycodendrimers with programmable bioactivity through convergent, divergent, and accelerated approaches using polyfunctional cyclotriphosphazenes

The cyclotriphosphazene-based platform facilitates versatile synthesis of glycodendrimers active as inhibitors of two biomedically relevant lectins.

Aromatic thioglycoside inhibitors against the virulence factor LecA from Pseudomonas aeruginosa

Three small families of hydrolytically stable thioaryl glycosides were prepared as inhibitors of the LecA (PA-IL) virulence factor corresponding to the carbohydrate binding lectin from the bacterial pathogen Pseudomonas aeruginosa. The monosaccharidic arylthio β-d-galactopyranosides served as a common template for the major series that was also substituted at the O-3 position. Arylthio disaccharides from lactose and from melibiose constituted the other two series members. In spite of the fact that the natural ligand for LecA is a glycolipid of the globotriaosylceramide having an α-d-galactopyranoside epitope, this study illustrated that the β-d-galactopyranoside configuration having a hydrophobic aglycon could override the requirement toward the anomeric configuration of the natural sugar. The enzyme linked lectin assay together with isothermal titration microcalorimetry established that naphthyl 1-thio-β-d-galactopyranoside () gave the best inhibition with an IC50 twenty-three times better than that of the reference methyl α-d-galactopyranoside. In addition it showed a KD of 6.3 μM which was ten times better than that of the reference compound. The X-ray crystal structure of LecA with was also obtained.

Structure binding relationship of galactosylated Glycoclusters toward Pseudomonas aeruginosa lectin LecA using a DNA-based carbohydrate microarray

Pseudomonas aeruginosa (PA) is a major public health issue due to its impact on nosocomial infections as well as its impact on cystic fibrosis patient mortality. One of the main concerns is its ability to develop antibiotic resistance. Therefore, inhibition of PA virulence has been proposed as an alternative strategy to tackle PA based infections. LecA (or PA-IL), a galactose binding lectin from PA, is involved in its virulence. Herein, we aimed at designing high affinity synthetic ligands toward LecA for its inhibition and at understanding the key parameters governing the binding of multivalent galactosylated clusters. Twenty-five glycoclusters were synthesized and their bindings were studied on a carbohydrate microarray. Monosaccharide centered clusters and linear comb-like clusters were synthesized with different linkers separating the core and the galactosyl residues. Their length, flexibility, and aromaticity were varied. Our results showed that the binding profile of LecA to galactosylated clusters was dependent on both the core and the linker and also that the optimal linker was different for each core. Nevertheless, an aryl group in the linker structure drastically improved the binding to LecA. Our results also suggest that optimal distances are preferred between the core and the aromatic group and the core and the galactose.

Generating novel recombinant prokaryotic lectins with altered carbohydrate binding properties through mutagenesis of the PA-IL protein from Pseudomonas aeruginosa

BACKGROUND

Prokaryotic lectins offer significant advantages over eukaryotic lectins for the development of enhanced glycoselective tools. Amenability to recombinant expression in Escherichia coli simplifies their production and presents opportunities for further genetic manipulation to create novel recombinant prokaryotic lectins (RPLs) with altered or enhanced carbohydrate binding properties. This study explored the potential of the α-galactophilic PA-IL lectin from Pseudomonas aeruginosa for use as a scaffold structure for the generation of novel RPLs.

METHOD

Specific amino acid residues in the carbohydrate binding site of a recombinant PA-IL protein were randomly substituted by site-directed mutagenesis. The resulting expression clones were then functionally screened to identify clones expressing rPA-IL proteins with altered carbohydrate binding properties.

RESULTS

This study generated RPLs exhibiting diverse carbohydrate binding activities including specificity and high affinity for β-linked galactose and N-acetyl-lactosamine (LacNAc) displayed by N-linked glycans on glycoprotein targets. Key amino acid substitutions were identified and linked with specific carbohydrate binding activities. Ultimately, the utility of these novel RPLs for glycoprotein analysis and for selective fractionation and isolation of glycoproteins and their glycoforms was demonstrated.

CONCLUSIONS

The carbohydrate binding properties of the PA-IL protein can be significantly altered using site-directed mutagenesis strategies to generate novel RPLs with diverse carbohydrate binding properties.

GENERAL SIGNIFICANCE

The novel RPLs reported would find a broad range of applications in glycobiology, diagnostics and in the analysis of biotherapeutics. The ability to readily produce these RPLs in gram quantities could enable them to find larger scale applications for glycoprotein or biotherapeutic purification.

Comprehensive list of lectins: origins, natures, and carbohydrate specificities

More than 100 years have passed since the first lectin ricin was discovered. Since then, a wide variety of lectins (lect means "select" in Latin) have been isolated from plants, animals, fungi, bacteria, as well as viruses, and their structures and properties have been characterized. At present, as many as 48 protein scaffolds have been identified as functional lectins from the viewpoint of three-dimensional structures as described in this chapter. In this chapter, representative 53 lectins are selected, and their major properties that include hemagglutinating activity, mitogen activity, blood group specificity, molecular weight, metal requirement, and sugar specificities are summarized as a comprehensive table. The list will provide a practically useful, comprehensive list for not only experienced lectin users but also many other non-expert researchers, who are not familiar to lectins and, therefore, have no access to advanced lectin biotechnologies described in other chapters.

RAFT-based tri-component fluorescent glycopolymers: synthesis, characterization and application in lectin-mediated bacterial binding study

A group of fluorescent statistical glycopolymers, prepared via reversible addition–fragmentation chain-transfer (RAFT)-based polymerizations, were successfully employed in lectin-mediated bacterial binding studies. The resultant glycopolymers contained three different monomers: N-(2-hydroxyethyl) acrylamide (HEAA), N-(2-aminoethyl) methacrylamide (AEMA) and N-(2-glyconamidoethyl)-methacrylamides possessing different pendant sugars. Low dispersities (≤1.32) and predictable degrees of polymerization were observed among the products. After the polymerization, the glycopolymers were further modified by different succinimidyl ester fluorophores targeting the primary amine groups on AEMA. With their binding specificities being confirmed by testing with lectin coated agarose beads, the glycopolymers were employed in bacterial binding studies, where polymers containing α-galactose or β-galactose as the pendant sugar were specifically bound by two clinically important pathogens Pseudomonas aeruginosa and Staphylococcus aureus, respectively. This is the first report of using RAFT-based glycopolymers in bacterial binding studies, and the ready access to tri-component statistical glycopolymers also warrants further exploration of their utility in other glycobiological applications.

Secondary sugar binding site identified for LecA lectin from Pseudomonas aeruginosa

The galactose-specific lectin LecA from Pseudomonas aeruginosa is a target for the development of new anti-infectious compounds. Sugar based molecules with anti-adhesive properties present great potential in the fight against bacterial infection and biofilm formation. LecA is specific for oligosaccharides with terminal α-galactoside residues and displays strong affinity for melibiose (αGal1-6Glc) with a Kd of 38.8 µM. The crystal structure of LecA/melibiose complex shows classical calcium-bridged binding of αGal in the primary binding site but also revealed a secondary sugar binding site with glucose bound. This sugar binding site is in close proximity to the galactose binding one, is independent of calcium and mainly involves interactions with a symmetry-related protein. This discovery would help to the design of new potent inhibitors targeting both binding sites.

CH-π "T-shape" interaction with histidine explains binding of aromatic galactosides to Pseudomonas aeruginosa lectin LecA

The galactose specific lectin LecA mediates biofilm formation in the opportunistic pathogen P. aeruginosa . The interaction between LecA and aromatic β-galactoside biofilm inhibitors involves an intermolecular CH-π T-shape interaction between C(ε1)-H of residue His50 in LecA and the aromatic ring of the galactoside aglycone. The generality of this interaction was tested in a diverse family of β-galactosides. LecA binding to aromatic β-galactosides (KD ∼ 8 μM) was consistently stronger than to aliphatic β-galactosides (KD ∼ 36 μM). The CH-π interaction was observed in the X-ray crystal structures of six different LecA complexes, with shorter than the van der Waals distances indicating productive binding. Related XH/cation/π-π interactions involving other residues were identified in complexes of aromatic glycosides with a variety of carbohydrate binding proteins such as concanavalin A. Exploiting such interactions might be generally useful in drug design against these targets.

Degradation of MSCRAMM target macromolecules in VLU slough by Lucilia sericata chymotrypsin 1 (ISP) persists in the presence of tissue gelatinase activity

Venous leg ulcer slough is unpleasant to the patient and difficult to manage clinically. It harbours infection, also preventing wound management materials and dressings from supporting the underlying viable tissues. In other words, slough has significant nuisance value in the tissue viability clinic. In this study, we have sought to increase our knowledge of slough by building upon a previous but limited analysis of this necrotic tissue. In particular, slough has been probed using Western blotting for the presence of proteins with the capacity to engage microbial surface components recognising adhesive matrix macromolecules. Although the samples were difficult to resolve, we detected fibrinogen, fibronectin, IgG, collagen, human serum albumin and matrix metalloproteinase-9. Furthermore, the effect of a maggot-derived debridement enzyme, chymotrypsin 1 on macromolecules in slough was confirmed across seven patient samples. The effect of chymotrypsin 1 on slough confirms our thesis that this potential debridement enzyme could be effective in removing slough along with its associated bacteria, given its observed resistance to intrinsic gelatinase activity. In summary, we believe that the data provide scientists and clinicians with further insights into the potential molecular interactions between bacteria, wound tissue and Lucilia sericata in a clinically problematic yet scientifically interesting wound ecosystem.

Multivalent glycoconjugate syntheses and applications using aromatic scaffolds

Glycan-protein interactions are of utmost importance in several biological phenomena. Although the variety of carbohydrate residues in mammalian cells is limited to less than a dozen different sugars, their spatial topographical presentation in what is now associated as the "glycocodes" provides the fundamental keys for specific and high affinity "lock-in" recognition events associated with a wide range of pathologies. Toward deciphering our understanding of these glycocodes, chemists have developed new creative tools that included dendrimer chemistry in order to provide monodisperse multivalent glycoconjugates. This review provides a survey of the numerous aromatic architectures generated for the multivalent presentation of relevant carbohydrates using covalent attachment or supramolecular self-assemblies. The basic concepts toward their controlled syntheses will be described using modern synthetic procedures with a particular emphasis on powerful organometallic methodologies. The large variety of dendritic aromatic scaffolds, together with a brief survey of their unique biophysical and biological properties will be critically reviewed. The distinctiveness of the resulting multivalent glycoarchitectures, encompassing glycoclusters, glycodendrimers and molecularly defined self-assemblies, in forming well organized cross-linked lattices with multivalent carbohydrate binding proteins (lectins) together with their photophysical, medical, and imaging properties will also be briefly highlighted. The topic will be presented in increasing order of aromatic backbone complexities and will end with fullerenes together with self-assembled nanostructures, thus complementing the various scaffolds described in this special thematic issue dedicated to multivalent glycoscience.

Glycopeptide dendrimers as Pseudomonas aeruginosa biofilm inhibitors

Synthetic glycopeptide dendrimers composed of a branched oligopeptide tree structure appended with glycosidic groups at its multiple N-termini were investigated for binding to the Pseudomonas aeruginosa lectins LecB and LecA. These lectins are partly responsible for the formation of antibiotic resistant biofilms in the human pathogenic bacterium P. aeruginosa, which causes lethal airway infections in immune-compromised and cystic fibrosis patients. Glycopeptide dendrimers with high affinity to the lectins were identified by screening of combinatorial libraries. Several of these dendrimers, in particular the LecB specific glycopeptide dendrimers FD2 and D-FD2 and the LecA specific glycopeptide dendrimers GalAG2 and GalBG2, also efficiently block P. aeruginosa biofilm formation and induce biofilm dispersal in vitro. Structure-activity relationship and structural studies are reviewed, in particular the observation that multivalency is essential to the anti-biofilm effect in these dendrimers.

RapA2 is a calcium-binding lectin composed of two highly conserved cadherin-like domains that specifically recognize Rhizobium leguminosarum acidic exopolysaccharides

In silico analyses have revealed a conserved protein domain (CHDL) widely present in bacteria that has significant structural similarity to eukaryotic cadherins. A CHDL domain was shown to be present in RapA, a protein that is involved in autoaggregation of Rhizobium cells, biofilm formation, and adhesion to plant roots as shown by us and others. Structural similarity to cadherins suggested calcium-dependent oligomerization of CHDL domains as a mechanistic basis for RapA action. Here we show by circular dichroism spectroscopy, light scattering, isothermal titration calorimetry, and other methods that RapA2 from Rhizobium leguminosarum indeed exhibits a cadherin-like β-sheet conformation and that its proper folding and stability are dependent on the binding of one calcium ion per protein molecule. By further in silico analysis we also reveal that RapA2 consists of two CHDL domains and expand the range of CHDL-containing proteins in bacteria and archaea. However, light scattering assays at various concentrations of added calcium revealed that RapA2 formed neither homo-oligomers nor hetero-oligomers with RapB (a distinct CHDL protein), indicating that RapA2 does not mediate cellular interactions through a cadherin-like mechanism. Instead, we demonstrate that RapA2 interacts specifically with the acidic exopolysaccharides (EPSs) produced by R. leguminosarum in a calcium-dependent manner, sustaining a role of these proteins in the development of the biofilm matrix made of EPS. Because EPS binding by RapA2 can only be attributed to its two CHDL domains, we propose that RapA2 is a calcium-dependent lectin and that CHDL domains in various bacterial and archaeal proteins confer carbohydrate binding activity to these proteins.

Formation of GM1 ganglioside clusters on the lipid membrane containing sphingomyeline and cholesterol

GM1 gangliosides form a microdomain with sphingomyeline (SM) and cholesterol (Chol) and are deeply involved in the aggregation of amyloid beta (Aβ) peptides on neural membranes. We performed molecular dynamics simulations on two kinds of lipid bilayers containing GM1 ganglioside: GM1/SM/Chol and GM1/POPC. Both 10 and 100 ns simulations and another set of 10 ns simulations with different initial lipid arrangement essentially showed the same computational results. GM1 molecules in the GM1/SM/Chol membrane were condensed, whereas those in GM1/POPC membrane scattered. That is, the formation of GM1 cluster was observed only on the GM1/SM/Chol mixed membrane. There appeared numerous hydrogen bonds among glycan portions of the GM1 clusters due to the condensation. A comparison in distribution of lipid molecules between the two kinds of membranes suggested that cholesterol had important roles to prevent the membrane from interdigitation and to stabilize other lipids for interacting with each other. This property of cholesterol promotes the formation of GM1 clusters.

Characterization of a new α-galactosyl-binding lectin from the mushroom Clavaria purpurea

A galactose specific lectin (CpL) was purified from the Clavaria purpurea mushroom by affinity chromatography. CpL agglutinated only trypsin-treated rabbit erythrocytes. On enzyme linked lectin sorbent assay (ELLSA), the lectin bound with thyroglobulin and asialo bovine submaxillary mucin (BSM). The fine sugar binding specificity of CpL was elucidated using inhibition of hemagglutination and sugar immobilized gold nano-particles (SGNP). The results indicated a preference of CpL towards α-galactosyl sugar chains. Among several monosaccharides and disaccharides assayed for dissociation effect on the SGNP-CpL complex, Galα1-3Gal and raffinose were the best inhibitors. The partial amino acid sequence showed two QXW motifs in CpL and similarity towards members of the ricin B superfamily.

Inhibition of the bacterial lectins of Pseudomonas aeruginosa with monosaccharides and peptides

Pseudomonas aeruginosa (PA) can cause infections in compromised hosts by interacting with the glycocalyx of host epithelial cells. It binds to glycostructures on mucosal surfaces via two lectins, which are carbohydrate-binding proteins, named PA-IL and PA-IIL, and blocking this interaction is, thus, an attractive anti-adhesive strategy. The aim of this study was to determine by ciliary beat frequency (CBF) analysis whether monosaccharides or peptides mimicking glycostructures represent blockers of PA lectin binding to human airway cilia. The treatment with monosaccharides and peptides alone did not change the CBF compared to controls and the tested compounds did not influence the cell morphology or survival, with the exception of peptide pOM3. PA-IL caused a decrease of the CBF within 24 h. D-galactose as well as the peptides mimicking HNK-1, polysialic acid and fucose compensated the CBF-modulating effect of PA-IL with different affinities. PA-IIL also bound to the human airway cilia in cell culture and resulted in a decrease of the CBF within 24 h. L(-)-fucose and pHNK-1 blocked the CBF-decreasing effect of PA-IIL. The HNK-1-specific glycomimetic peptide had a high affinity for binding to both PA-IL and PA-IIL, and inhibited the ciliotoxic effect of both lectins, thus, making it a strong candidate for a therapeutic anti-adhesive drug.

Selenoglycosides in silico: ab initio-derived reparameterization of MM4, conformational analysis using histo-blood group ABH antigens and lectin docking as indication for potential of bioactivity

The identification of glycan epitopes such as the histo-blood group ABH determinants as docking sites for bacterial/viral infections and signals in growth regulation fuels the interest to develop non-hydrolysable mimetics for therapeutic applications. Inevitably, the required substitution of the linkage oxygen atom will alter the derivative's topology. Our study addresses the question of the impact of substitution of oxygen by selenium. In order to characterize spatial parameters and flexibility of selenoglycosides, we first performed ab initio calculations on model compounds to refine the MM4 force field. The following application of the resulting MM4R version appears to reduce the difference to ab initio data when compared to using the MM4 estimator. Systematic conformational searches on the derivatives of histo-blood group ABH antigens revealed increased flexibility with acquisition of additional low-energy conformer(s), akin to the behavior of S-glycosides. Docking analysis using the Glide program for eight test cases indicated potential for bioactivity, giving further experimental investigation a clear direction to testing Se-glycosides as lectin ligands.

Blocking of Pseudomonas aeruginosa and Ralstonia solanacearum Lectins by plant and microbial branched polysaccharides used as food additives

Pseudomonas aeruginosa antibiotic resistance prompted the search for glycodecoys that would block its lectin-dependent adhesion to human cells. We have used the lectins of this pathogen, PA-IL (galactophilic LecA) and PA-IIL (fucophilic LecB), and two additional pathogenic bacterial lectins, CV-IIL (fucophilic, of Chromobacterium violaceum) and RS-IIL (mannophilic, of Ralstonia solanacearum), for assaying the pathogenic lectin-blocking abilities of some plant and microbial polysaccharidic food additives, adding the mannophilic plant lectin Con A as a reference. Locust-bean and guar galactomannans and acacia gum very strongly inhibited PA-IL. The other lectins, excluding CV-IIL, were very strongly inhibited by yeast mannan. Xanthan and inulin were weak inhibitors. The differential blocking of these lectins by galactosylated branches of plant polysaccharides and by mannan matched their inhibition by avian egg whites, human milk, and royal jelly (protecting animal embryos and neonates from infections). The nondigestability and nontoxicity of the food additives are advantageous for curing gastrointestinal and external infections.

Role of LecA and LecB lectins in Pseudomonas aeruginosa-induced lung injury and effect of carbohydrate ligands

Pseudomonas aeruginosa is a frequently encountered pathogen that is involved in acute and chronic lung infections. Lectin-mediated bacterium-cell recognition and adhesion are critical steps in initiating P. aeruginosa pathogenesis. This study was designed to evaluate the contributions of LecA and LecB to the pathogenesis of P. aeruginosa-mediated acute lung injury. Using an in vitro model with A549 cells and an experimental in vivo murine model of acute lung injury, we compared the parental strain to lecA and lecB mutants. The effects of both LecA- and Lec B-specific lectin-inhibiting carbohydrates (alpha-methyl-galactoside and alpha-methyl-fucoside, respectively) were evaluated. In vitro, the parental strain was associated with increased cytotoxicity and adhesion on A549 cells compared to the lecA and lecB mutants. In vivo, the P. aeruginosa-induced increase in alveolar barrier permeability was reduced with both mutants. The bacterial burden and dissemination were decreased for both mutants compared with the parental strain. Coadministration of specific lectin inhibitors markedly reduced lung injury and mortality. Our results demonstrate that there is a relationship between lectins and the pathogenicity of P. aeruginosa. Inhibition of the lectins by specific carbohydrates may provide new therapeutic perspectives.