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

Glycomimetics for the inhibition and modulation of lectins

Carbohydrates are essential mediators of many processes in health and disease. They regulate self-/non-self- discrimination, are key elements of cellular communication, cancer, infection and inflammation, and determine protein folding, function and life-times. Moreover, they are integral to the cellular envelope for microorganisms and participate in biofilm formation. These diverse functions of carbohydrates are mediated by carbohydrate-binding proteins, lectins, and the more the knowledge about the biology of these proteins is advancing, the more interfering with carbohydrate recognition becomes a viable option for the development of novel therapeutics. In this respect, small molecules mimicking this recognition process become more and more available either as tools for fostering our basic understanding of glycobiology or as therapeutics. In this review, we outline the general design principles of glycomimetic inhibitors (Section 2). This section is then followed by highlighting three approaches to interfere with lectin function, i.e. with carbohydrate-derived glycomimetics (Section 3.1), novel glycomimetic scaffolds (Section 3.2) and allosteric modulators (Section 3.3). We summarize recent advances in design and application of glycomimetics for various classes of lectins of mammalian, viral and bacterial origin. Besides highlighting design principles in general, we showcase defined cases in which glycomimetics have been advanced to clinical trials or marketed. Additionally, emerging applications of glycomimetics for targeted protein degradation and targeted delivery purposes are reviewed in Section 4.

Discovery of N-β-l-Fucosyl Amides as High-Affinity Ligands for the Pseudomonas aeruginosa Lectin LecB

The Gram-negative pathogen Pseudomonas aeruginosa causes severe infections mainly in immunocompromised or cystic fibrosis patients and is able to resist antimicrobial treatments. The extracellular lectin LecB plays a key role in bacterial adhesion to the host and biofilm formation. For the inhibition of LecB, we designed and synthesized a set of fucosyl amides, sulfonamides, and thiourea derivatives. Then, we analyzed their binding to LecB in competitive and direct binding assays. We identified β-fucosyl amides as unprecedented high-affinity ligands in the two-digit nanomolar range. X-ray crystallography of one α- and one β-anomer of N-fucosyl amides in complex with LecB revealed the interactions responsible for the high affinity of the β-anomer at atomic level. Further, the molecules showed good stability in murine and human blood plasma and hepatic metabolism, providing a basis for future development into antibacterial drugs.

Ball milled glyco-graphene oxide conjugates markedly disrupted Pseudomonas aeruginosa biofilms

The engineering of the surface of nanomaterials with bioactive molecules allows controlling their biological identity thus accessing functional materials with tuned physicochemical and biological profiles suited for specific applications. Then, the manufacturing process, by which the nanomaterial surface is grafted, has a significant impact on their development and innovation. In this regard, we report herein the grafting of sugar headgroups on a graphene oxide (GO) surface by exploiting a green manufacturing process that relies on the use of vibrational ball mills, a grinding apparatus in which the energy is transferred to the reacting species through collision with agate spheres inside a closed and vibrating vessel. The chemical composition and the morphology of the resulting glyco-graphene oxide conjugates (glyco-GO) are assessed by the combination of a series of complementary advanced techniques (i.e. UV-vis and Raman spectroscopy, transmission electron microscopy, and Magic Angle Spinning (MAS) solid-state NMR (ssNMR) providing in-depth insights into the chemical reactivity of GO in a mechanochemical route. The conjugation of monosaccharide residues on the GO surface significantly improves the antimicrobial activity of pristine GO against P. aeruginosa. Indeed, glyco-GO conjugates, according to the monosaccharide derivatives installed into the GO surface, affect the ability of sessile cells to adhere to a polystyrene surface in a colony forming assay. Scanning electron microscopy images clearly show that glyco-GO conjugates significantly disrupt an already established P. aeruginosa biofilm.

Structural Considerations for Building Synthetic Glycoconjugates as Inhibitors for Pseudomonas aeruginosa Lectins

Pseudomonas aeruginosa is a pathogenic bacterium, responsible for a large portion of nosocomial infections globally and designated as critical priority by the World Health Organisation. Its characteristic carbohydrate-binding proteins LecA and LecB, which play a role in biofilm-formation and lung-infection, can be targeted by glycoconjugates. Here we review the wide range of inhibitors for these proteins (136 references), highlighting structural features and which impact binding affinity and/or therapeutic effects, including carbohydrate selection; linker length and rigidity; and scaffold topology, particularly for multivalent candidates. We also discuss emerging therapeutic strategies, which build on targeting of LecA and LecB, such as anti-biofilm activity, anti-adhesion and drug-delivery, with promising prospects for medicinal chemistry.

Targeting undruggable carbohydrate recognition sites through focused fragment library design

Carbohydrate-protein interactions are key for cell-cell and host-pathogen recognition and thus, emerged as viable therapeutic targets. However, their hydrophilic nature poses major limitations to the conventional development of drug-like inhibitors. To address this shortcoming, four fragment libraries were screened to identify metal-binding pharmacophores (MBPs) as novel scaffolds for inhibition of Ca²⁺-dependent carbohydrate-protein interactions. Here, we show the effect of MBPs on the clinically relevant lectins DC-SIGN, Langerin, LecA and LecB. Detailed structural and biochemical investigations revealed the specificity of MBPs for different Ca²⁺-dependent lectins. Exploring the structure-activity relationships of several fragments uncovered the functional groups in the MBPs suitable for modification to further improve lectin binding and selectivity. Selected inhibitors bound efficiently to DC-SIGN-expressing cells. Altogether, the discovery of MBPs as a promising class of Ca²⁺-dependent lectin inhibitors creates a foundation for fragment-based ligand design for future drug discovery campaigns.

Recent Strategies to Combat Biofilms Using Antimicrobial Agents and Therapeutic Approaches

Biofilms are intricate bacterial assemblages that attach to diverse surfaces using an extracellular polymeric substance that protects them from the host immune system and conventional antibiotics. Biofilms cause chronic infections that result in millions of deaths around the world every year. Since the antibiotic tolerance mechanism in biofilm is different than that of the planktonic cells due to its multicellular structure, the currently available antibiotics are inadequate to treat biofilm-associated infections which have led to an immense need to find newer treatment options. Over the years, various novel antibiofilm compounds able to fight biofilms have been discovered. In this review, we have focused on the recent and intensively researched therapeutic techniques and antibiofilm agents used for biofilm treatment and grouped them according to their type and mode of action. We also discuss some therapeutic approaches that have the potential for future advancement.

Current Methods for Extraction and Concentration of Foodborne Bacteria with Glycan-Coated Magnetic Nanoparticles: A Review

Rapid and accurate food pathogen detection is an essential step to preventing foodborne illnesses. Before detection, removal of bacteria from the food matrix and concentration to detectable levels are often essential steps. Although many reviews discuss rapid concentration methods for foodborne pathogens, the use of glycan-coated magnetic nanoparticles (MNPs) is often omitted. This review seeks to analyze the potential of this technique as a rapid and cost-effective solution for concentration of bacteria directly from foods. The primary focus is the mechanism of glycan-coated MNP binding, as well as its current applications in concentration of foodborne pathogens. First, a background on the synthesis, properties, and applications of MNPs is provided. Second, synthesis of glycan-coated particles and their theorized mechanism for bacterial adhesion is described. Existing research into extraction of bacteria directly from food matrices is also analyzed. Finally, glycan-coated MNPs are compared to the magnetic separation technique of immunomagnetic separation (IMS) in terms of cost, time, and other factors. At its current state, glycan-coated MNPs require more research to fully identify the mechanism, potential for optimization, and extraction capabilities directly in food matrices. However, current research indicates glycan-coated MNPs are an incredibly cost-effective method for rapid food pathogen extraction and concentration.

Targeting the Central Pocket of the Pseudomonas aeruginosa Lectin LecA

Pseudomonas aeruginosa is an opportunistic ESKAPE pathogen that produces two lectins, LecA and LecB, as part of its large arsenal of virulence factors. Both carbohydrate-binding proteins are central to the initial and later persistent infection processes, i. e. bacterial adhesion and biofilm formation. The biofilm matrix is a major resistance determinant and protects the bacteria against external threats such as the host immune system or antibiotic treatment. Therefore, the development of drugs against the P. aeruginosa biofilm is of particular interest to restore efficacy of antimicrobials. Carbohydrate-based inhibitors for LecA and LecB were previously shown to efficiently reduce biofilm formations. Here, we report a new approach for inhibiting LecA with synthetic molecules bridging the established carbohydrate-binding site and a central cavity located between two LecA protomers of the lectin tetramer. Inspired by in silico design, we synthesized various galactosidic LecA inhibitors with aromatic moieties targeting this central pocket. These compounds reached low micromolar affinities, validated in different biophysical assays. Finally, X-ray diffraction analysis revealed the interactions of this compound class with LecA. This new mode of action paves the way to a novel route towards inhibition of P. aeruginosa biofilms.

Targeting extracellular lectins of Pseudomonas aeruginosa with glycomimetic liposomes

The antimicrobial resistance crisis requires novel approaches for the therapy of infections especially with Gram-negative pathogens. Pseudomonas aeruginosa is defined as priority 1 pathogen by the WHO and thus of particular interest. Its drug resistance is primarily associated with biofilm formation and essential constituents of its extracellular biofilm matrix are the two lectins, LecA and LecB. Here, we report microbial lectin-specific targeted nanovehicles based on liposomes. LecA- and LecB-targeted phospholipids were synthesized and used for the preparation of liposomes. These liposomes with varying surface ligand density were then analyzed for their competitive and direct lectin binding activity. We have further developed a microfluidic device that allowed the optical detection of the targeting process to the bacterial lectins. Our data showed that the targeted liposomes are specifically binding to their respective lectin and remain firmly attached to surfaces containing these lectins. This synthetic and biophysical study provides the basis for future application in targeted antibiotic delivery to overcome antimicrobial resistance.

Three-dimensional structures, dynamics and calcium-mediated interactions of the exopolysaccharide, Infernan, produced by the deep-sea hydrothermal bacterium Alteromonas infernus

The exopolysaccharide Infernan, from the bacterial strain GY785, has a complex repeating unit of nine monosaccharides established on a double-layer of sidechains. A cluster of uronic and sulfated monosaccharides confers to Infernan functional and biological activities. We characterized the 3-dimensional structures and dynamics along Molecular Dynamics trajectories and clustered the conformations in extended two-fold and five-fold helical structures. The electrostatic potential distribution over all the structures revealed negatively charged cavities explored for Ca²⁺ binding through quantum chemistry computation. The transposition of the model of Ca²⁺complexation indicates that the five-fold helices are the most favourable for interactions. The ribbon-like shape of two-fold helices brings neighbouring chains in proximity without steric clashes. The cavity chelating the Ca²⁺ of one chain is completed throughout the interaction of a sulfate group from the neighbouring chain. The resulting is a 'junction zone' based on unique chain-chain interactions governed by a heterotypic binding mode.

The assessment of Pseudomonas aeruginosa lectin LecA binding characteristics of divalent galactosides using multiple techniques

Pseudomonas aeruginosa is a widespread opportunistic pathogen that is capable of colonizing various human tissues and is resistant to many antibiotics. LecA is a galactose binding tetrameric lectin involved in adhesion, infection and biofilm formation. This study reports on the binding characteristics of mono- and divalent (chelating) ligands to LecA using different techniques. These techniques include Affinity Capillary Electrophoresis (ACE), Bio Layer Interferometry (BLI), Native Mass Spectrometry and a Thermal Shift Assay. Aspects of focus include: affinity, selectivity, binding kinetics and residence time. The affinity of a divalent ligand was determined to be in the low nanomolar range for all of the used techniques and with a ligand residence time of approximately 7 hours, while no strong binding was seen to related lectin tetramers. Each of the used techniques provides a unique and complementary insight into the chelation based binding mode of the divalent ligand to the LecA tetramer.

Recent development in the design of small 'drug-like' and nanoscale glycomimetics against Escherichia coli infections

Glycoconjugates are involved in several pathological processes. Glycomimetics that can favorably emulate complex carbohydrate structures, while competing with natural ligands as inhibitors, are gaining considerable attention owing to their improved hydrolytic stability, binding affinity, and pharmacokinetic (PK) properties. Of particular interest are the families of α-d-mannopyranoside analogs, which can be used as inhibitors against adherent invasive Escherichia coli infections. Bacterial resistance to modern antibiotics triggers the search for new alternative antibacterial strategies that are less susceptible to acquiring resistance. In this review, we highlight recent progress in the chemical syntheses of this family of compounds, one of which having reached clinical trials against Crohn's disease (CD).

Novel approaches to glycomimetic design: development of small molecular weight lectin antagonists

Introduction: The direct binding of carbohydrates or those presented on glycoproteins or glycolipids to proteins is the primary effector of many biological responses. One class of carbohydrate-binding proteins, lectins are important in all forms of life. Their functions in animals include regulating cell adhesion, glycoprotein synthesis, metabolism, and mediating immune system response while in bacteria and viruses a lectin-mediated carbohydrate-protein interaction between host cells and the pathogen initiates pathogenesis of the infection.Areas covered: In this review, the authors outline the structural and functional pathogenesis of lectins from bacteria, amoeba, and humans. Mimics of a carbohydrate are referred to as glycomimetics, which are much smaller in molecular weight and are devised to mimic the key binding interactions of the carbohydrate while also allowing additional contacts with the lectin. This article emphasizes the various approaches used over the past 10-15 years in the rational design of glycomimetic ligands.Expert opinion: Medicinal chemistry efforts enabled by X-ray structural biology have identified small-molecule glycomimetic lectin antagonists that have entered or are nearing clinical trials. A common theme in these strategies is the use of biaryl ring systems to emulate the carbohydrate interactions with the lectin.

Directing Drugs to Bugs: Antibiotic-Carbohydrate Conjugates Targeting Biofilm-Associated Lectins of Pseudomonas aeruginosa

Chronic infections by Pseudomonas aeruginosa are characterized by biofilm formation, which effectively enhances resistance toward antibiotics. Biofilm-specific antibiotic delivery could locally increase drug concentration to break antimicrobial resistance and reduce the drug's peripheral side effects. Two extracellular P. aeruginosa lectins, LecA and LecB, are essential structural components for biofilm formation and thus render a possible anchor for biofilm-targeted drug delivery. The standard-of-care drug ciprofloxacin suffers from severe systemic side effects and was therefore chosen for this approach. We synthesized several ciprofloxacin-carbohydrate conjugates and established a structure-activity relationship. Conjugation of ciprofloxacin to lectin probes enabled biofilm accumulation in vitro, reduced the antibiotic's cytotoxicity, but also reduced its antibiotic activity against planktonic cells due to a reduced cell permeability and on target activity. This work defines the starting point for new biofilm/lectin-targeted drugs to modulate antibiotic properties and ultimately break antimicrobial resistance.

The Pseudomonas aeruginosa Lectin LecB Causes Integrin Internalization and Inhibits Epithelial Wound Healing

Pseudomonas aeruginosa is a ubiquitous environmental bacterium that is one of the leading causes of nosocomial infections. P. aeruginosa is able to switch between planktonic, intracellular, and biofilm-based lifestyles, which allows it to evade the immune system as well as antibiotic treatment. Hence, alternatives to antibiotic treatment are urgently required to combat P. aeruginosa infections. Lectins, like the fucose-specific LecB, are promising targets, because removal of LecB resulted in decreased virulence in mouse models. Currently, several research groups are developing LecB inhibitors. However, the role of LecB in host-pathogen interactions is not well understood. The significance of our research is in identifying cellular mechanisms of how LecB facilitates P. aeruginosa infection. We introduce LecB as a new member of the list of bacterial molecules that bind integrins and show that P. aeruginosa can move forward underneath attached epithelial cells by loosening cell-basement membrane attachment in a LecB-dependent manner.

The opportunistic bacterium Pseudomonas aeruginosa produces the fucose-specific lectin LecB, which has been identified as a virulence factor. LecB has a tetrameric structure with four opposing binding sites and has been shown to act as a cross-linker. Here, we demonstrate that LecB strongly binds to the glycosylated moieties of β1-integrins on the basolateral plasma membrane of epithelial cells and causes rapid integrin endocytosis. Whereas internalized integrins were degraded via a lysosomal pathway, washout of LecB restored integrin cell surface localization, thus indicating a specific and direct action of LecB on integrins to bring about their endocytosis. Interestingly, LecB was able to trigger uptake of active and inactive β1-integrins and also of complete α3β1-integrin–laminin complexes. We provide a mechanistic explanation for this unique endocytic process by showing that LecB has the additional ability to recognize fucose-bearing glycosphingolipids and causes the formation of membrane invaginations on giant unilamellar vesicles. In cells, LecB recruited integrins to these invaginations by cross-linking integrins and glycosphingolipids. In epithelial wound healing assays, LecB specifically cleared integrins from the surface of cells located at the wound edge and blocked cell migration and wound healing in a dose-dependent manner. Moreover, the wild-type P. aeruginosa strain PAO1 was able to loosen cell-substrate adhesion in order to crawl underneath exposed cells, whereas knockout of LecB significantly reduced crawling events. Based on these results, we suggest that LecB has a role in disseminating bacteria along the cell-basement membrane interface.

Anti-biofilm Agents against Pseudomonas aeruginosa: A Structure-Activity Relationship Study of C-Glycosidic LecB Inhibitors

Biofilm formation is a key mechanism of antimicrobial resistance. We have recently reported two classes of orally bioavailable C-glycosidic inhibitors of the Pseudomonas aeruginosa lectin LecB with antibiofilm activity. They proved efficient in target binding, were metabolically stable, nontoxic, selective, and potent in inhibiting formation of bacterial biofilm. Here, we designed and synthesized six new carboxamides and 24 new sulfonamides for a detailed structure-activity relationship for two clinically representative LecB variants. Sulfonamides generally showed higher inhibition compared to carboxamides, which was rationalized based on crystal structure analyses. Substitutions at the thiophenesulfonamide increased binding through extensive contacts with a lipophilic protein patch. These metabolically stable compounds showed a further increase in potency toward the target and in biofilm inhibition assays. In general, we established the structure-activity relationship for these promising antibiofilm agents and showed that modification of the sulfonamide residue bears future optimization potential.

Glycans in drug discovery

Glycans are key players in many biological processes. They are essential for protein folding and stability and act as recognition elements in cell-cell and cell-matrix interactions. Thus, being at the heart of medically relevant biological processes, glycans have come onto the scene and are considered hot spots for biomedical intervention. The progress in biophysical techniques allowing access to an increasing molecular and structural understanding of these processes has led to the development of effective therapeutics. Indeed, strategies aimed at designing glycomimetics able to block specific lectin-carbohydrate interactions, carbohydrate-based vaccines mimicking self- and non-self-antigens as well as the exploitation of the therapeutic potential of glycosylated antibodies are being pursued. In this mini-review the most prominent contributions concerning recurrent diseases are highlighted, including bacterial and viral infections, cancer or immune-related pathologies, which certainly show the great promise of carbohydrates in drug discovery.

Phenylpropanoids and its derivatives: biological activities and its role in food, pharmaceutical and cosmetic industries

Phenylpropanoids and their derivatives are plant secondary metabolites widely present in fruits, vegetables, cereal grains, beverages, spices and herbs. They are known to have multifaceted effects which include antimicrobial, antioxidant, anti-inflammatory, antidiabetic, anticancer activities and as well as exhibits renoprotective, neuroprotective, cardioprotective and hepatoprotective effects. Owing to their antioxidant, antimicrobial and photoprotective properties, these compounds have wide application in the food (preservation, packaging films and edible coating), pharmaceutical, cosmetic and other industries such as textile (colorant), biofuel (antioxidant additive) and sensors (sensing biologically relevant molecules). Phenylpropanoids are present in commercially available dietary supplements and skin care products. In this review, we have presented the current knowledge on the biosynthesis, occurrence, biological activities of phenylpropanoids and their derivatives, along with the mechanism of action and their potential applications in various industries.

X-ray Crystal Structures of Short Antimicrobial Peptides as Pseudomonas aeruginosa Lectin B Complexes

Herein, we report X-ray crystal structures of 11-13 residue antimicrobial peptides (AMPs) active against Pseudomonas aeruginosa as complexes of fucosylated d-enantiomeric sequences with the P. aeruginosa lectin LecB. These represent the first crystal structures of short AMPs. In 24 individual structures of eight different peptides, we found mostly α-helices assembled as two-helix or four-helix bundles with a hydrophobic core and cationic residues pointing outside. Two of the analogs formed an extended structure engaging in multiple contacts with the lectin. Molecular dynamics (MD) simulations showed that α-helices are stabilized by bundle formation and suggested that the N-terminal acyl group present in the linker to the fucosyl group can extend the helix by one additional H-bond and increase α-helix amphiphilicity. Investigating N-terminal acylation led to AMPs with equivalent and partly stronger antibacterial effects compared to the free peptide.

Carbohydrate Ligands on Magnetic Nanoparticles for Centrifuge-Free Extraction of Pathogenic Contaminants in Pasteurized Milk

Rapid detection of bacterial contamination in the food supply chain is critically important for food safety monitoring. Reliable extraction and concentration of bacteria from complex matrices is required to achieve high detection sensitivity, especially in situations of low contamination and infective dose. Carbohydrate ligands that attach to microbial cell-surface epitopes are promising economical and biocompatible substitutes for cell-targeting ligands and antibodies. Two different carbohydrate ligands immobilized onto magnetic nanoparticles (MNPs) were easily suspended in liquid food (milk) and allowed expedient extraction of microbes within minutes, without the need for centrifugation or loss in capture capacity. In this pilot study, 25-mL samples of undiluted milk were spiked with 5 mg of MNPs and artificially contaminated with bacteria at 3 to 5 log CFU/mL. MNPs and bacteria formed MNP-cell complexes, which were rapidly separated from the milk matrix with a simple magnet to allow supernatant removal. MNP-cell complexes were then concentrated by resuspension in 1 mL of fresh milk and plated per Bacteriological Analytical Manual procedures. Capture was carried out in vitamin D, 2% reduced fat, and fat-free milk spiked with Salmonella Enteritidis, Escherichia coli O157:H7, and Bacillus cereus for a combined total of 18 experiments (three replicates each). An additional eight experiments were conducted to investigate the effect of competitive bacteria on capture. All experiments were carried out over several months to account for environmental variations. Capture efficiency, on a log basis, for all combinations of milk and bacteria was 73 to 90%. Long-term exposure of the MNPs to milk did not markedly affect capture efficiency. These carbohydrate-functionalized MNPs have potential as nonspecific receptors for rapid extraction of bacteria from complex liquids, opening the door to discovery of biocompatible ligands that can reliably target pathogens in our food.

Emerging nano-biosensing with suspended MNP microbial extraction and EANP labeling

Emerging nano-biosensing with suspended MNP microbial extraction and EANP labeling may ensure a secure microbe-free food supply, as rapid response detection of microbial contamination is of utmost importance. Many biosensor designs have been proposed over the past two decades, covering a broad range of binding ligands, signal amplification, and detection mechanisms. These designs may consist of self-contained test strips developed from the base up with complicated nanoparticle chemistry and intricate ligand immobilization. Other methods use multiple step-wise additions, many based upon ELISA 96-well plate technology with fluorescent detection. In addition, many biosensors use expensive antibody receptors or DNA ligands. But many of these proposed designs are impracticable for most applications or users, since they don't FIRST address the broad goals of any biosensor: Field operability, Inexpensive, with Real-time detection that is both Sensitive and Specific to target, while being as Trouble-free as possible. Described in this review are applications that utilize versatile magnetic nanoparticles (MNP) extraction, electrically active nanoparticles (EANP) labeling, and carbohydrate-based ligand chemistry. MNP provide rapid pathogen extraction from liquid samples. EANP labeling improves signal amplification and expands signaling options to include optical and electrical detection. Carbohydrate ligands are inexpensive, robust structures that are increasingly synthesized for higher selectivity. Used in conjunction with optical or electrical detection of gold nanoparticles (AuNP), carbohydrate-functionalized MNP-cell-AuNP nano-biosensing advances the goal of being the FIRST biosensor of choice in detecting microbial pathogens throughout our food supply chain.

Pathoblockers or antivirulence drugs as a new option for the treatment of bacterial infections

The rapid development of antimicrobial resistance is threatening mankind to such an extent that the World Health Organization expects more deaths from infections than from cancer in 2050 if current trends continue. To avoid this scenario, new classes of anti-infectives must urgently be developed. Antibiotics with new modes of action are needed, but other concepts are also currently being pursued. Targeting bacterial virulence as a means of blocking pathogenicity is a promising new strategy for disarming pathogens. Furthermore, it is believed that this new approach is less susceptible towards resistance development. In this review, recent examples of anti-infective compounds acting on several types of bacterial targets, e.g., adhesins, toxins and bacterial communication, are described.

Development and optimization of a competitive binding assay for the galactophilic low affinity lectin LecA from Pseudomonas aeruginosa

Infections with the Gram-negative bacterium Pseudomonas aeruginosa result in a high mortality among immunocompromised patients and those with cystic fibrosis. The pathogen can switch from planktonic life to biofilms, and thereby shields itself against antibiotic treatment and host immune defense to establish chronic infections. The bacterial protein LecA, a C-type lectin, is a virulence factor and an integral component for biofilm formation. Inhibition of LecA with its carbohydrate ligands results in reduced biofilm mass, a potential Achilles heel for treatment. Here, we report the development and optimization of a fluorescence polarization-based competitive binding assay with LecA for application in screening of potential inhibitors. As a consequence of the low affinity of d-galactose for LecA, the fluorescent ligand was optimized to reduce protein consumption in the assay. The assay was validated using a set of known inhibitors of LecA and IC50 values in good agreement with the known Kd values were obtained. Finally, we employed the optimized assay to screen sets of synthetic thio-galactosides and natural blood group antigens and report their structure-activity relationship. In addition, we evaluated a multivalent fluorescent assay probe for LecA and report its applicability in an inhibition assay.

Glycomimetic, Orally Bioavailable LecB Inhibitors Block Biofilm Formation of Pseudomonas aeruginosa

The opportunistic Gram-negative bacterium Pseudomonas aeruginosa is a leading pathogen for infections of immuno-compromised patients and those suffering from cystic fibrosis. Its ability to switch from planktonic life to aggregates, forming the so-called biofilms, is a front-line mechanism of antimicrobial resistance. The bacterial carbohydrate-binding protein LecB is an integral component and necessary for biofilm formation. Here, we report a new class of drug-like low molecular weight inhibitors of the lectin LecB with nanomolar affinities and excellent receptor binding kinetics and thermodynamics. This class of glycomimetic inhibitors efficiently blocked biofilm formation of P. aeruginosa in vitro while the natural monovalent carbohydrate ligands failed. Furthermore, excellent selectivity and pharmacokinetic properties were achieved. Notably, two compounds showed good oral bioavailability, and high compound concentrations in plasma and urine were achieved in vivo.

Covalent Lectin Inhibition and Application in Bacterial Biofilm Imaging

Biofilm formation by pathogenic bacteria is a hallmark of chronic infections. In many cases, lectins play key roles in establishing biofilms. The pathogen Pseudomonas aeruginosa often exhibiting various drug resistances employs its lectins LecA and LecB as virulence factors and biofilm building blocks. Therefore, inhibition of the function of these proteins is thought to have potential in developing "pathoblockers" preventing biofilm formation and virulence. A covalent lectin inhibitor specific to a carbohydrate binding site is described for the first time. Its application in the LecA-specific in vitro imaging of biofilms formed by P. aeruginosa is also reported.

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.

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.

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.