Structure-based exploration of the ganglioside GM1 binding sites of Escherichia coli heat-labile enterotoxin and cholera toxin for the discovery of receptor antagonists

GM1a ganglioside-binding domain peptide inhibits host adhesion and inflammatory response of enterotoxigenic Escherichia coli heat-labile enterotoxin-B in HCT-8 cells

Enterotoxigenic Escherichia coli (ETEC) is a major cause of illness and death but has no effective therapy. The heat-labile enterotoxin LT is a significant virulence factor produced by ETEC. The heat-labile enterotoxin-B (LT-B) subunit may enter host cells by binding to monosialotetrahexosylganglioside-a (GM1a), a monosialoganglioside found on the plasma membrane surface of animal epithelial cells. This research was conducted to develop conformationally comparable peptides to the carbohydrate epitope of GM1a for the treatment of ETEC. We used the LT-B subunit to select LT-B-binding peptides that structurally resemble GM1a. The ganglioside microarray and docking simulations were used to identify three GM1a ganglioside-binding domain (GBD) peptides based on LT-B recognition. Peptides had an inhibiting effect on the binding of LT-B to GM1a. The binding capacity, functional inhibitory activity, and in vitro effects of the GBD peptides were evaluated using HCT-8 cells, a human intestinal epithelial cell line, to evaluate the feasibility of deploying GBD peptides to combat bacterial infections. KILSYTESMAGKREMVIIT was the most efficient peptide in inhibiting cellular absorption of LT-B in cells. Our findings offer compelling evidence that GM1a GBD-like peptides might act as new therapeutics to inhibit LT-B binding to epithelial cells and avoid the subsequent physiological consequences of LT.

Bacterial toxins and heart function: heat-labile Escherichia coli enterotoxin B promotes changes in cardiac function with possible relevance for sudden cardiac death

Bacterial toxins can cause cardiomyopathy, though it is not its most common cause. Some bacterial toxins can form pores in the membrane of cardiomyocytes, while others can bind to membrane receptors. Enterotoxigenic E. coli can secrete enterotoxins, including heat-resistant (ST) or labile (LT) enterotoxins. LT is an AB5-type toxin that can bind to specific cell receptors and disrupt essential host functions, causing several common conditions, such as certain diarrhea. The pentameric B subunit of LT, without A subunit (LTB), binds specifically to certain plasma membrane ganglioside receptors, found in lipid rafts of cardiomyocytes. Isolated guinea pig hearts and cardiomyocytes were exposed to different concentrations of purified LTB. In isolated hearts, mechanical and electrical alternans and an increment of heart rate variability, with an IC50 of ~0.2 μg/ml LTB, were observed. In isolated cardiomyocytes, LTB promoted significant decreases in the amplitude and the duration of action potentials. Na+ currents were inhibited whereas L-type Ca2+ currents were augmented at their peak and their fast inactivation was promoted. Delayed rectifier K+ currents decreased. Measurements of basal Ca2+ or Ca2+ release events in cells exposed to LTB suggest that LTB impairs Ca2+ homeostasis. Impaired calcium homeostasis is linked to sudden cardiac death. The results are consistent with the recent view that the B subunit is not merely a carrier of the A subunit, having a role explaining sudden cardiac death in children (SIDS) infected with enterotoxigenic E. coli, explaining several epidemiological findings that establish a strong relationship between SIDS and ETEC E. coli.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12551-023-01100-6.

Nanoparticle applications in food - a review

The use of nanotechnology in the food industry raises uncertainty in many respects. For years, achievements of nanotechnology have been applied mainly in biomedicine and computer science, but recently it has also been used in the food industry. Due to the extremely small (nano) scale, the properties and behavior of nanomaterials may differ from their macroscopic counterparts. They can be used as biosensors to detect reagents or microorganisms, monitor bacterial growth conditions, increase food durability e.g. when placed in food packaging, reducing the amount of certain ingredients without changing the consistency of the product (research on fat substitutes is underway), improve the taste of food, make some nutrients get better absorbed by the body, etc. There are companies on the market that are already introducing nanoparticles into the economy to improve their functionality, e.g. baby feeding bottles. This review focuses on the use of nanoparticles in the food industry, both organic (chitosan, cellulose, proteins) and inorganic (silver, iron, zinc oxide, titanium oxide, etc.). The use of nanomaterials in food production requires compliance with all legal requirements regarding the safety and quantity of nano-processed food products described in this review. In the future, new methods of testing nanoparticles should be developed that would ensure the effectiveness of compounds subjected to, for example, nano-encapsulation, i.e. whether the encapsulation process had a positive impact on the specific properties of these compounds. Nanotechnology has revolutionized our approach towards food engineering (from production to processing), food storage and the creation of new materials and products, and the search for new product applications.

Liposomes Prevent In Vitro Hemolysis Induced by Streptolysin O and Lysenin

The need for alternatives to antibiotics in the fight against infectious diseases has inspired scientists to focus on antivirulence factors instead of the microorganisms themselves. In this respect, prior work indicates that tiny, enclosed bilayer lipid membranes (liposomes) have the potential to compete with cellular targets for toxin binding, hence preventing their biological attack and aiding with their clearance. The effectiveness of liposomes as decoy targets depends on their availability in the host and how rapidly they are cleared from the circulation. Although liposome PEGylation may improve their circulation time, little is known about how such a modification influences their interactions with antivirulence factors. To fill this gap in knowledge, we investigated regular and long-circulating liposomes for their ability to prevent in vitro red blood cell hemolysis induced by two potent lytic toxins, lysenin and streptolysin O. Our explorations indicate that both regular and long-circulating liposomes are capable of similarly preventing lysis induced by streptolysin O. In contrast, PEGylation reduced the effectiveness against lysenin-induced hemolysis and altered binding dynamics. These results suggest that toxin removal by long-circulating liposomes is feasible, yet dependent on the particular virulence factor under scrutiny.

Enteropathogenic Infections: Organoids Go Bacterial

Enteric infections represent a major health care challenge which is particularly prevalent in countries with restricted access to clean water and sanitation and lacking personal hygiene precautions, altogether facilitating fecal-oral transmission of a heterogeneous spectrum of enteropathogenic microorganisms. Among these, bacterial species are responsible for a considerable proportion of illnesses, hospitalizations, and fatal cases, all of which have been continuously contributing to ignite researchers' interest in further exploring their individual pathogenicity. Beyond the universally accepted animal models, intestinal organoids are increasingly valued for their ability to mimic key architectural and physiologic features of the native intestinal mucosa. As a consequence, they are regarded as the most versatile and naturalistic in vitro model of the gut, allowing monitoring of adherence, invasion, intracellular trafficking, and propagation as well as repurposing components of the host cell equipment. At the same time, infected intestinal organoids allow close characterization of the host epithelium's immune response to enteropathogens. In this review, (i) we provide a profound update on intestinal organoid-based tissue engineering, (ii) we report the latest pathophysiological findings defining the infected intestinal organoids, and (iii) we discuss the advantages and limitations of this in vitro model.

Targeting Multiple Binding Sites on Cholera Toxin B with Glycomimetic Polymers Promotes the Formation of Protein-Polymer Aggregates

The canonical binding site on the B subunit of cholera toxin (CTB) binds to GM1 gangliosides on host cells. However, the recently discovered noncanonical binding site on CTB with affinity for fucosylated molecules has raised the possibility that both sites can be involved in initiating intoxication. Previously, we showed that blocking CTB binding to human and murine small intestine epithelial cells can be increased by simultaneously targeting both binding sites with multivalent norbornene-based glycopolymers [ACS Infect. Dis. 2020, 6, 5, 1192-1203]. However, the mechanistic origin of the increased blocking efficacy was unclear. Herein, we observed that mixing CTB pentamers and glycopolymers that display fucose and galactose sugars results in the formation of large aggregates, which further inhibits binding of CTB to human granulocytes. Dynamic light scattering analysis, small-angle X-ray scattering analysis, transmission electron microscopy, and turbidimetric assays revealed that the facial directionality of CTB promotes interchain cross-linking, which in turn leads to self-assembly of protein-polymer networks. This cross-linking-induced self-assembly occurs only when the glycopolymer system contains both galactose and fucose. In an assay of the glycopolymer's ability to block CTB binding to human granulocytes, we observed a direct correlation between IC₅₀ and self-assembly size. The aggregation mechanism of inhibition proposed herein has potential utility for the development of low-cost macromolecular clinical therapeutics for cholera that do not have exotic architectures and do not require complex synthetic sequences.

A comprehensive review of therapeutic approaches available for the treatment of cholera

Objectives

The oral rehydration solution is the most efficient method to treat cholera; however, it does not interfere in the action mechanism of the main virulence factor produced by Vibrio cholerae, the cholera toxin (CT), and this disease still stands out as a problem for human health worldwide. This review aimed to describe therapeutic alternatives available in the literature, especially those related to the search for molecules acting upon the physiopathology of cholera.

Key findings

New molecules have offered a protection effect against diarrhoea induced by CT or even by infection from V. cholerae. The receptor regulator cystic fibrosis channel transmembrane (CFTR), monosialoganglioside (GM1), enkephalinase, AMP-activated protein kinase (AMPK), inhibitors of expression of virulence factors and activators of ADP-ribosylarginine hydrolase are the main therapeutic targets studied. Many of these molecules or extracts still present unclear action mechanisms.

Conclusions

Knowing therapeutic alternatives and their molecular mechanisms for the treatment of cholera could guide us to develop a new drug that could be used in combination with the rehydration solution.

Biopolymer Extracted from Anadenanthera colubrina (Red Angico Gum) Exerts Therapeutic Potential in Mice: Antidiarrheal Activity and Safety Assessment

Anadenanthera colubrina var. cebil (Griseb.) Altschul (Fabaceae family), commonly known as the red angico tree, is a medicinal plant found throughout Brazil’s semi-arid area. In this study, a chemical analysis was performed to investigate the antidiarrheal activity and safety profile of red angico gum (RAG), a biopolymer extracted from the trunk exudate of A. colubrina. Upon FT-IR spectroscopy, RAG showed bands in the regions of 1608 cm⁻¹, 1368 cm⁻¹, and 1029 cm⁻¹, which relate to the vibration of O–H water molecules, deformation vibration of C-O bands, and vibration of the polysaccharide C-O band, respectively, all of which are relevant to glycosidic bonds. The peak molar mass of RAG was 1.89 × 10⁵ g/mol, with the zeta potential indicating electronegativity. RAG demonstrated high yield and solubility with a low degree of impurity. Pre-treatment with RAG reduced the total diarrheal stool and enteropooling. RAG also enhanced Na+/K+-ATPase activity and reduced gastrointestinal transit, and thereby inhibited intestinal smooth muscle contractions. Enzyme-Linked Immunosorbent Assay (ELISA) demonstrated that RAG can interact with GM1 receptors and can also reduce E. coli-induced diarrhea in vivo. Moreover, RAG did not induce any signs of toxicity in mice. These results suggest that RAG is a possible candidate for the treatment of diarrheal diseases.

Inhibition of bacterial toxin recognition of membrane components as an anti-virulence strategy

Over recent years, the development of new antibiotics has not kept pace with the rate at which bacteria develop resistance to these drugs. For this reason, many research groups have begun to design and study alternative therapeutics, including molecules to specifically inhibit the virulence of pathogenic bacteria. Because many of these pathogenic bacteria release protein toxins, which cause or exacerbate disease, inhibition of the activity of bacterial toxins is a promising anti-virulence strategy. In this review, we describe several approaches to inhibit the initial interactions of bacterial toxins with host cell membrane components. The mechanisms by which toxins interact with the host cell membrane components have been well-studied over the years, leading to the identification of therapeutic targets, which have been exploited in the work described here. We review efforts to inhibit binding to protein receptors and essential membrane lipid components, complex assembly, and pore formation. Although none of these molecules have yet been demonstrated in clinical trials, the in vitro and in vivo results presented here demonstrate their promise as novel alternatives and/or complements to traditional antibiotics.

Strong Inhibition of Cholera Toxin B Subunit by Affordable, Polymer-Based Multivalent Inhibitors

Cholera is a potentially fatal bacterial infection that affects a large number of people in developing countries. It is caused by the cholera toxin (CT), an AB₅ toxin secreted by Vibrio cholera. The toxin comprises a toxic A-subunit and a pentameric B-subunit that bind to the intestinal cell surface. Several monovalent and multivalent inhibitors of the toxin have been synthesized but are too complicated and expensive for practical use in developing countries. Meta-nitrophenyl α-galactoside (MNPG) is a known promising ligand for CT, and here mono- and multivalent compounds based on MNPG were synthesized. We present the synthesis of MNPG in greatly improved yields and its use while linked to a multivalent scaffold. We used economical polymers as multivalent scaffolds, namely, polyacrylamide, dextran, and hyperbranched polyglycerols (hPGs). Copper-catalyzed alkyne azide cycloaddition reaction (CuAAC) produced the inhibitors that were tested in an ELISA-type assay and an intestinal organoid swelling inhibition assay. The inhibitory properties varied widely depending on the type of polymer, and the most potent conjugates showed IC₅₀ values in the nanomolar range.

Antidiarrheal activity of a novel sulfated polysaccharide from the red seaweed Gracilaria cervicornis

ETHNOPHARMACOLOGICAL RELEVANCE

The use of marine seaweeds as a source of natural compounds with medicinal purposes is increasing in Western countries in the last decades, becoming an important alternative in the traditional medicine of many developing countries, where diarrhea still remains a severe public health problem, with high rates of mortality and morbidity. Sulfated polysaccharides (PLS) extracted from red seaweeds can exhibit therapeutic effects for the treatment of gastrointestinal disorders. Thus, the pharmacological properties of the PLS from Gracilaria cervicornis, an endemic seaweed found in the Brazilian northeast coast, was evaluated as an alternative natural medication for diarrhea.

AIM OF THE STUDY

This study aimed to evaluate the antidiarrheal activity of sulfated polysaccharides (PLS) extracted from the red seaweed G. cervicornis in Swiss mice pre-treated with castor oil or cholera toxin.

MATERIALS AND METHODS

The seaweed Gracilaria cervicornis was collected at Flecheiras beach (city of Trairí, State of Ceará, Brazil) and the PLS was obtained through enzymatic extraction and administered in mice (25-30 g) before diarrhea induction with castor oil or cholera toxin. For the evaluation of the total number of fecal output and diarrheal feces, the animals were placed in cages lined with adsorbent material. The evaluation of intestinal fluid accumulation (enteropooling) on castor oil-induced diarrhea in mice occurred by dissecting the small intestine and measuring its volume. The determination of Na⁺/K⁺-ATPase activity was measured in the small intestine supernatants by colorimetry, using commercial biochemistry kits. The gastrointestinal motility was evaluated utilizing an activated charcoal as a food tracer. The intestinal fluid secretion and chloride ion concentration were evaluated in intestinal closed loops in mice with cholera toxin-induced secretory diarrhea. The binding ability of PLS with GM1 and/or cholera toxin was evaluated by an Enzyme-Linked Immunosorbent Assay (ELISA).

RESULTS

The G. cervicornis PLS showed antidiarrheal effects in both acute and secretory diarrhea, reducing the total number of fecal output, diarrheic stools, intestinal fluid accumulation, and increasing small intestine Na⁺/K⁺-ATPase activity on castor oil-induced diarrhea. However, the PLS did not affect gastrointestinal motility, indicating that this compound has a different action mechanism than loperamide. In secretory diarrhea, the PLS decreased intestinal fluid secretion and small intestine chloride excretion, binding with GM1 and/or cholera toxin and blocking their attachment to the enterocyte cell surface.

CONCLUSIONS

In conclusion, PLS has a significant antidiarrheal effect in acute and secretory diarrhea. Further investigation is needed towards its use as a natural medicine to treat diarrhea.

Carbohydrate inhibitors of cholera toxin

Cholera is a diarrheal disease caused by a protein toxin released by Vibrio cholera in the host's intestine. The toxin enters intestinal epithelial cells after binding to specific carbohydrates on the cell surface. Over recent years, considerable effort has been invested in developing inhibitors of toxin adhesion that mimic the carbohydrate ligand, with particular emphasis on exploiting the multivalency of the toxin to enhance activity. In this review we introduce the structural features of the toxin that have guided the design of diverse inhibitors and summarise recent developments in the field.

GM1 ganglioside-independent intoxication by Cholera toxin

Cholera toxin (CT) enters and intoxicates host cells after binding cell surface receptors via its B subunit (CTB). We have recently shown that in addition to the previously described binding partner ganglioside GM1, CTB binds to fucosylated proteins. Using flow cytometric analysis of primary human jejunal epithelial cells and granulocytes, we now show that CTB binding correlates with expression of the fucosylated Lewis X (LeX) glycan. This binding is competitively blocked by fucosylated oligosaccharides and fucose-binding lectins. CTB binds the LeX glycan in vitro when this moiety is linked to proteins but not to ceramides, and this binding can be blocked by mAb to LeX. Inhibition of glycosphingolipid synthesis or sialylation in GM1-deficient C6 rat glioma cells results in sensitization to CT-mediated intoxication. Finally, CT gavage produces an intact diarrheal response in knockout mice lacking GM1 even after additional reduction of glycosphingolipids. Hence our results show that CT can induce toxicity in the absence of GM1 and support a role for host glycoproteins in CT intoxication. These findings open up new avenues for therapies to block CT action and for design of detoxified enterotoxin-based adjuvants.

Multi-scale molecular dynamics study of cholera pentamer binding to a GM1-phospholipid membrane

The AB5 type toxin produced by the Vibrio cholerae bacterium is the causative agent of the cholera disease. The cholera toxin (CT) has been shown to bind specifically to GM1 glycolipids on the membrane surface. This binding of CT to the membrane is the initial step in its endocytosis and has been postulated to cause significant disruption to the membrane structure. In this work, we have carried out a combination of coarse-grain and atomistic simulations to study the binding of CT to a membrane modelled as an asymmetrical GM1-DPPC bilayer. Simulation results indicate that the toxin binds to the membrane through only three of its five B subunits, in effect resulting in a tilted bound configuration. Additionally, the binding of the CT can increase the area per lipid of GM1 leaflet, which in turn can cause the membrane regions interacting with the bound subunits to experience significant bilayer thinning and lipid tail disorder across both the leaflets.

Polymer antidotes for toxin sequestration

Toxins delivered by envenomation, secreted by microorganisms, or unintentionally ingested can pose an immediate threat to life. Rapid intervention coupled with the appropriate antidote is required to mitigate the threat. Many antidotes are biological products and their cost, methods of production, potential for eliciting immunogenic responses, the time needed to generate them, and stability issues contribute to their limited availability and effectiveness. These factors exacerbate a world-wide challenge for providing treatment. In this review we evaluate a number of polymer constructs that may serve as alternative antidotes. The range of toxins investigated includes those from sources such as plants, animals and bacteria. The development of polymeric heavy metal sequestrants for use as antidotes to heavy metal poisoning faces similar challenges, thus recent findings in this area have also been included. Two general strategies have emerged for the development of polymeric antidotes. In one, the polymer acts as a scaffold for the presentation of ligands with a known affinity for the toxin. A second strategy is to generate polymers with an intrinsic affinity, and in some cases selectivity, to a range of toxins. Importantly, in vivo efficacy has been demonstrated for each of these strategies, which suggests that these approaches hold promise as an alternative to biological or small molecule based treatments.

Cholera: pathophysiology and emerging therapeutic targets

Cholera is a diarrheal disease that remains an important global health problem with several hundreds of thousands of reported cases each year. This disease is caused by intestinal infection with Vibrio cholerae, which is a highly motile gram-negative bacterium with a single-sheathed flagellum. In the course of cholera pathogenesis, V. cholerae expresses a transcriptional activator ToxT, which subsequently transactivates expressions of two crucial virulence factors: toxin-coregulated pilus and cholera toxin (CT). These factors are responsible for intestinal colonization of V. cholerae and induction of fluid secretion, respectively. In intestinal epithelial cells, CT binds to GM1 ganglioside receptors on the apical membrane and undergoes retrograde vesicular trafficking to endoplasmic reticulum, where it exploits endoplasmic reticulum-associated protein degradation systems to release a catalytic A1 subunit of CT (CT A1) into cytoplasm. CT A1, in turn, catalyzes ADP ribosylation of α subunits of stimulatory G proteins, leading to a persistent activation of adenylate cyclase and an elevation of intracellular cAMP. Increased intracellular cAMP in human intestinal epithelial cells accounts for pathogenesis of profuse diarrhea and severe fluid loss in cholera. This review provides an overview of the pathophysiology of cholera diarrhea and discusses emerging drug targets for cholera, which include V. cholerae virulence factors, V. cholerae motility, CT binding to GM1 receptor, CT internalization and intoxication, as well as cAMP metabolism and transport proteins involved in cAMP-activated Cl(-) secretion. Future directions and perspectives of research on drug discovery and development for cholera are discussed.

Picomolar inhibition of cholera toxin by a pentavalent ganglioside GM1os-calix[5]arene

Cholera toxin (CT), the causative agent of cholera, displays a pentavalent binding domain that targets the oligosaccharide of ganglioside GM1 (GM1os) on the periphery of human abdominal epithelial cells. Here, we report the first GM1os-based CT inhibitor that matches the valency of the CT binding domain (CTB). This pentavalent inhibitor contains five GM1os moieties linked to a calix[5]arene scaffold. When evaluated by an inhibition assay, it achieved a picomolar inhibition potency (IC50 = 450 pM) for CTB. This represents a significant multivalency effect, with a relative inhibitory potency of 100,000 compared to a monovalent GM1os derivative, making GM1os-calix[5]arene one of the most potent known CTB inhibitors.

Targeting bacterial toxins

Protein toxins constitute the main virulence factors of several species of bacteria and have proven to be attractive targets for drug development. Lead candidates that target bacterial toxins range from small molecules to polymeric binders, and act at each of the multiple steps in the process of toxin-mediated pathogenicity. Despite recent and significant advances in the field, a rationally designed drug that targets toxins has yet to reach the market. This Review presents the state of the art in bacterial toxin targeted drug development with a critical consideration of achieved breakthroughs and withstanding challenges. The discussion focuses on A-B-type protein toxins secreted by four species of bacteria, namely Clostridium difficile (toxins A and B), Vibrio cholerae (cholera toxin), enterohemorrhagic Escherichia coli (Shiga toxin), and Bacillus anthracis (anthrax toxin), which are the causative agents of diseases for which treatments need to be improved.

Synthesis of 1,2,3-triazole linked galactopyranosides and evaluation of cholera toxin inhibition

We report the synthesis of a series of bivalent 1,2,3-triazole linked galactopyranosides as potential inhibitors of cholera toxin (CT). The inhibitory activity of the bivalent series was examined (ELISA) and the series showed low inhibitory activity (millimolar IC(50)s). Conversely, the monomeric galactotriazole analogues were strong inhibitors of cholera toxin (IC(50) = 71-75 μM).

Screening a fragment cocktail library using ultrafiltration

Ultrafiltration provides a generic method to discover ligands for protein drug targets with millimolar to micromolar K(d), the typical range of fragment-based drug discovery. This method was tailored to a 96-well format, and cocktails of fragment-sized molecules, with molecular masses between 150 and 300 Da, were screened against medical structural genomics target proteins. The validity of the method was confirmed through competitive binding assays in the presence of ligands known to bind the target proteins.

Multifunctional multivalency: a focused library of polymeric cholera toxin antagonists

Structural pre-organization of the multivalent ligands is important for successful interaction with multimeric proteins. Polymer-based heterobifunctional ligands that contain pendant groups prearranged into heterodimers can be used to probe the active site and surrounding area of the receptor. Here we describe the synthesis and activities of a series of galactose conjugates on polyacrylamide and dextran. Conjugation of a second fragment resulted in nanomolar inhibitors of cholera toxin, while the galactose-only progenitors showed no detectable activity.

The assembly state between magnetic nanosensors and their targets orchestrates their magnetic relaxation response

The target-induced clustering of magnetic nanoparticles is typically used for the identification of clinically relevant targets and events. A decrease in the water proton transverse NMR relaxation time, or T(2), is observed upon clustering, allowing the sensitive and accurate detection of target molecules. We have discovered a new mechanistically unique nanoparticle-target interaction resulting in a T(2) increase and demonstrate herein that this increase, and its associated r(2) relaxivity decrease, are also observed upon the interaction of the nanoparticles with ligands or molecular entities. Small molecules, proteins, and a 15-bp nucleic acid sequence were chemically conjugated to polyacrylic-acid-coated iron oxide nanoparticles, and all decreased the original nanoparticle r(2) value. Further experiments established that the r(2) decrease was inversely proportional to the number of ligands bound to the nanoparticle and the molecular weight of the bound ligand. Additional experiments revealed that the T(2)-increasing mechanism was kinetically faster than the conventional clustering mechanism. Most importantly, under conditions that result in T(2) increases, as little as 5.3 fmol of Bacillus anthracis plasmid DNA (pX01 and pX02), 8 pmol of the cholera toxin B subunit (Ctb), and even a few cancer cells in blood were detected. Transition from the binding to the clustering mechanism was observed in the carbohydrate-, Ctb-, and DNA-sensing systems, simply by increasing the target concentration significantly above the nanoparticle concentration, or using Ctb in its pentameric form as opposed to its monomer. Collectively, these results demonstrate that the molecular architectures resulting from the interaction between magnetic nanosensors and their targets directly govern water proton NMR relaxation. We attribute the observed T(2) increases to the bound target molecules partially obstructing the diffusion of solvent water molecules through the superparamagnetic iron oxide nanoparticles' outer relaxation spheres. Finally, we anticipate that this novel interaction can be incorporated into new clinical and field detection applications, due to its faster kinetics relative to the conventional nanoparticle-clustering assays.

Highly sensitive detection of protein toxins by surface plasmon resonance with biotinylation-based inline atom transfer radical polymerization amplification

Ultrasensitive detection of proteins is of great importance to proteomics studies. We report here a method to enhance detection sensitivity in surface plasmon resonance (SPR) spectroscopy by coupling a polymerization initiator to a biospecific interaction and inducing inline atom transfer radical polymerization (ATRP) for amplifying SPR response. Bacterial cholera toxin (CT) is chosen as the model protein that has been covalently immobilized on the surface for demonstrating the principle. The specific recognition is achieved by use of biotinylated anti-CT, which allows initiators with a biotin tag to be fixed at the protein binding site through a neutravidin bridge and triggers the localized growth of polymer brushes of poly(hydroxyl-ethyl methacrylate) (PHEMA) via an ATRP mechanism. To further enhance the signal, a second ATRP reaction is conducted that takes advantage of the hydroxyl groups of PHEMA brushes from the first step to form hyperbranched polymers onto the sensing surface. The two consecutive ATRP steps significantly improve SPR detection, allowing low amounts of CT that yield no direct measurement to be quantified with large signals. The resulting polymer film has been characterized by optical and atomic force microscopy. Ascorbic acid (AA) is employed as deoxygen reagent in the catalyst mixture that effectively suppresses oxygen interference, shortening the reaction time and making it possible for applying this ATRP approach to flow injection based SPR detection. A calibration curve of PHEMA amplification for CT detection based on surface coverage has been obtained that displays a correlation in a range from 8.23 x 10(-15) to 3.61 x 10(-12) mol/cm(2) with a limit of detection of 6.27 x 10(-15) mol/cm(2). The versatile biotin-neutravidin interaction used here should allow adaptation of ATRP enhancement to many other systems that include DNA, RNA, peptides, and carbohydrates, opening new avenues for ultrasensitive analysis of biomolecules with flow-injection assay and SPR spectroscopy.

Heat-labile enterotoxin: beyond G(m1) binding

Enterotoxigenic Escherichia coli (ETEC) is a significant source of morbidity and mortality worldwide. One major virulence factor released by ETEC is the heat-labile enterotoxin LT, which is structurally and functionally similar to cholera toxin. LT consists of five B subunits carrying a single catalytically active A subunit. LTB binds the monosialoganglioside G_M1, the toxin’s host receptor, but interactions with A-type blood sugars and E. coli lipopolysaccharide have also been identified within the past decade. Here, we review the regulation, assembly, and binding properties of the LT B-subunit pentamer and discuss the possible roles of its numerous molecular interactions.

Inhibition of binding of the AB5-type enterotoxins LT-I and cholera toxin to ganglioside GM1 by galactose-rich dietary components

Cholera, travelers' diarrhea, or colibacillosis in pigs can possibly be prevented or attenuated by dietary provision of competitive inhibitors that react with the GM1-binding sites of the enterotoxins cholera toxin (CT), human Escherichia coli heat-labile enterotoxin of serogroup I (LTh-I), and porcine LT-I (LTp-I). The interfering efficiency of natural substances with binding of the toxins to the gangliosid receptor GM1 was tested using a specially adapted GM1-coated-microtiter-well enzyme-linked immunosorbent assay. The substances tested for their GM1 displacing capacity were galactose-containing or -related saccharides from bovine milk, skim milk powder, galactan from gum arabic, food stabilizers as well as ground fenugreek seed and soy bean constituents that contain galactomannans, the galactopolysaccharides agar and agarose, and larch wood and other plant materials that contain arabinogalactans. Skim milk powder, compared with the pure milk saccharides tested, interfered to a higher extent with LTh-I (65-66% inhibition at 5 mg test substance/mL) and CT binding (63-67% inhibition at 5 mg test substance/mL) when supplied before or simultaneously with the toxins in the GM1-enzyme-linked immunosorbent assay. Ground fenugreek seed counteracted GM1 binding of 5 ng LTh-I/mL as well as 5 ng and 1 microg LTp-I/mL (43-65% inhibition at 5 mg test substance/mL), and 4 ng CT/mL (61-92% inhibition at 5 mg test substance/mL) very efficiently when supplied before the toxin-GM1 complex had formed. With 50 mg/mL fenugreek seed, inhibition percentages of even 92-99% were reached for LTh-I and CT binding. Efforts to resolve already bound toxin from GM1 with the test substances were less effective than preincubations and concurrent incubations.

Manipulation of electrostatic and saccharide linker interactions in the design of efficient glycopolypeptide-based cholera toxin inhibitors

Multivalent, glycopolymer inhibitors designed for the treatment of disease and pathogen infection have shown improvements in binding correlated with general changes in glycopolymer architecture and composition. We have previously demonstrated that control of glycopolypeptide backbone extension and ligand spacing significantly impacts the inhibition of the cholera toxin B subunit pentamer (CT B(5)) by these polymers. In the studies reported here, we elucidate the role of backbone charge and linker length in modulating the inhibition event. Peptides of the sequence AXPXG (where X is a positive, neutral or negative amino acid), equipped with the alkyne functionality of propargyl glycine, were designed and synthesized via solid-phase peptide synthetic methods and glycosylated via Cu(I)-catalyzed alkyne-azide cycloaddition reactions. The capacity of the glycopeptides to inhibit the binding of the B(5) subunit of cholera toxin was evaluated. These studies indicated that glycopeptides with a negatively charged backbone show improved inhibition of the binding event relative to the other glycopeptides. In addition, variations in the length of the linker between the peptide and the saccharide ligand also affected the inhibition of CT by the glycopeptides. Our findings suggest that, apart from appropriate saccharide spacing and polypeptide chain extension, saccharide linker conformation and the systematic placement of charges on the polypeptide backbone are also significant variables that can be tuned to improve the inhibitory potencies of glycopolypeptide-based multivalent inhibitors.

Design and creativity in synthesis of multivalent neoglycoconjugates

From the authors' opinion, this chapter constitutes a modest extension of the seminal and inspiring contribution of Stowell and Lee on neoglycoconjugates published in this series [C. P. Stowell and Y. C. Lee, Adv. Carbohydr. Chem. Biochem., 37 (1980) 225-281]. The outstanding progresses achieved since then in the field of the "glycoside cluster effect" has witnessed considerable creativity in the design and synthetic strategies toward a vast array of novel carbohydrate structures and reflects the dynamic activity in the field even since the recent chapter by the Nicotra group in this series [F. Nicotra, L. Cipolla, F. Peri, B. La Ferla, and C. Radaelli, Adv. Carbohydr. Chem. Biochem., 61 (2007) 353-398]. Beyond the more classical neoglycoproteins and glycopolymers (not covered in this work) a wide range of unprecedented and often artistically beautiful multivalent and monodisperse nanostructures, termed glycodendrimers for the first time in 1993, has been created. This chapter briefly surveys the concept of multivalency involved in carbohydrate-protein interactions. The topic is also discussed in regard to recent steps undertaken in glycobiology toward identification of lead candidates using microarrays and modern analytical tools. A systematic description of glycocluster and glycodendrimer synthesis follows, starting from the simplest architectures and ending in the most complex ones. Presentation of multivalent glycostructures of intermediate size and comprising, calix[n]arene, porphyrin, cyclodextrin, peptide, and carbohydrate scaffolds, has also been intercalated to better appreciate the growing synthetic complexity involved. A subsection describing novel all-carbon-based glycoconjugates such as fullerenes and carbon nanotubes is inserted, followed by a promising strategy involving dendrons self-assembling around metal chelates. The chapter then ends with those glycodendrimers that have been prepared using commercially available dendrimers possessing varied functionalities, or systematically synthesized using either divergent or convergent strategies.

Carbohydrate-based anti-adhesive inhibition of Vibrio cholerae toxin binding to GM1-OS immobilized into artificial planar lipid membranes

We have studied 'food grade' sialyloligosaccharides (SOS) as anti-adhesive drugs or receptor analogues, since the terminal sialic acid residue has already been shown to contribute significantly to the adhesion and pathogenesis of the Vibrio cholerae toxin (Ctx). GM1-oligosaccharide (GM1-OS) was immobilized into a supporting POPC lipid bilayer onto a surface plasmon resonance (SPR) chip, and the interaction between uninhibited Ctx and GM1-OS-POPC was measured. SOS inhibited 94.7% of the Ctx binding to GM1-OS-POPC at 10mg/mL. The SOS EC(50) value of 5.521mg/mL is high compared with 0.2811microg/mL (182.5rhoM or 1.825x10(-10)M) for GM1-OS. The commercially available sialyloligosaccharide (SOS) mixture Sunsial E((R)) is impure, containing one monosialylated and two disialylated oligosaccharides in the ratio 9.6%, 6.5% and 17.5%, respectively, and 66.4% protein. However, these inexpensive food-grade molecules are derived from egg yolk and could be used to fortify conventional food additives, by way of emulsifiers, sweeteners and/or preservatives. The work further supports our hypothesis that SOS could be a promising natural anti-adhesive glycomimetic against Ctx and prevent subsequent onset of disease.

Galactooligosaccharides (GOS) inhibit Vibrio cholerae toxin binding to its GM1 receptor

It is widely reported that cholera toxin (Ctx) remains a significant cause of gastrointestinal disease globally, particularly in developing countries where access to clean drinking water is at a premium. Vaccines are prohibitively expensive and have shown only short-term protection. Consequently, there is scope for continued development of novel treatment strategies. One example is the use of galactooligosaccharides (GOS) as functional mimics for the cell-surface toxin receptor (GM1). In this study, GOS fractions were fractionated using cation exchange chromatography followed by structural characterization using a combination of hydrophilic interaction liquid chromatography (HILIC) and electrospray ionization mass spectrometry (ESI-MS) such that their molecular weight profiles were known. Each profile was correlated against biological activity measured using a competitive inhibitory GM1-linked ELISA. GOS fractions containing >5% hexasaccharides (DP(6)) exhibited >90% binding, with EC(50) values between 29.27 and 56.04 mg/mL. Inhibition by GOS DP(6) was dose dependent, with an EC(50) value of 5.10 mg/mL (5.15 microM MW of 990 Da). In removing low molecular weight carbohydrates that do possess prebiotic, nutraceutical, and/or biological properties and concentrating GOS DP(5) and/or DP(6), Ctx antiadhesive activity per unit of (dry) weight was improved. This could be advantageous in the manufacture of pharmaceutical or nutraceutical formulations for the treatment or prevention of an acute or chronic disease associated with or caused by the adhesion and/or uptake of a Ctx or HLT.

Cholera toxin inhibitors studied with high-performance liquid affinity chromatography: a robust method to evaluate receptor-ligand interactions

Anti-adhesion drugs may be an alternative to antibiotics to control infection of micro-organisms. The well-characterized interaction between cholera toxin and the cellular glycolipid GM1 makes it an attractive model for inhibition studies in general. In this report, we demonstrate a high-performance liquid affinity chromatography approach called weak affinity chromatography to evaluate cholera toxin inhibitors. The cholera toxin B-subunit was covalently coupled to porous silica and a (weak) affinity column was produced. The K(D) values of galactose and meta-nitrophenyl alpha-D-galactoside were determined with weak affinity chromatography to be 52 and 1 mM, respectively, which agree well with IC(50) values previously reported. To increase inhibition potency multivalent inhibitors have been developed and the interaction with multivalent glycopolypeptides was also evaluated. The affinity of these compounds was found to correlate with the galactoside content but K(D) values were not obtained because of the inhomogeneous response and slow off-rate from multivalent interactions. Despite the limitations in obtaining direct K(D) values of the multivalent galactopolypeptides, weak affinity chromatography represents an additional and valuable tool in the evaluation of monovalent as well as multivalent cholera toxin inhibitors. It offers multiple advantages, such as a low sample consumption, high reproducibility and short analysis time, which are often not observed in other methods of analysis.

Identification of Escherichia coli enterotoxin inhibitors from traditional medicinal herbs by in silico, in vitro, and in vivo analyses

ETHNOPHARMACOLOGICAL RELEVANCE

Glycyrrhiza uralensis has been used for the treatment of gastrointestinal disorders, such as diarrhea, in several ancient cultures. Glycyrrhizin is the principal component of liquorice and lots of pharmacological effects have been demonstrated.

AIM OF THE STUDY

Heat-labile enterotoxin (LT), the virulence factor of enterotoxigenic Escherichia coli, induces diarrhea by initially binding to the GM1 on the surfaces of intestinal epithelial cells and consequently leading to the massive loss of fluid and ions from cells. Therefore, we evaluated the inhibitory effects of traditional medicinal herbs (TMH) on the B subunit of LT (LTB) and GM1 interaction.

MATERIALS AND METHODS

The inhibitory effects of TMH on LTB-GM1 interaction were evaluated by GM1-enzyme-linked immunosorbent assay (ELISA). The likely active phytochemicals of these TMH were then predicted by in silico model (docking) and analyzed by in vitro (GM1-ELISA) and in vivo (patent mouse gut assay) models.

RESULTS

We found that various TMH, which have been ethnomedically used for the treatment of diarrhea, inhibited the LTB-GM1 interaction. Docking data showed that triterpenoids were the most active phytochemicals and the oleanane-type triterpenoids presented better LTB-binding abilities than other types of triterpenoids. Moreover, by in vitro and in vivo models, we demonstrated that glycyrrhizin was the most effective oleanane-type triterpenoid that significantly suppressed both the LTB-binding ability (IC50=3.26+/-0.17 mM) and the LT-induced fluid accumulation in mice.

CONCLUSIONS

We found an LT inhibitor, glycyrrhizin, from TMH by in silico, in vitro, and in vivo analyses.

Concentrations of ganglioside type M1 and immunoglobulin G in colostrum are inversely related to bacterial infection at early lactation in cows

The levels of IgG and ganglioside type M1 in the colostrum of cows and heifers were analyzed to examine their utility in predicting acquisition of intramammary infection (IMI) during the first weeks postpartum. In general, high levels of IgG and ganglioside type M1 in cows were associated with lower new incidence of IMI, and linear discriminate analysis based on these 2 variables yielded 69.4% successful classification into cows that did or did not acquire new IMI. This analysis was less successful in heifers because a high proportion of them joined the herd when already infected with bacteria in their udders. It is suggested that application of a wider range of measures that reflect the immune status would enable the identification of most cows prone to new IMI.

Sialyloligosaccharides inhibit cholera toxin binding to the GM1 receptor

It is recognised that cholera toxin (Ctx) is a significant cause of gastrointestinal disease globally, particularly in developing countries where access to uncontaminated drinking water is at a premium. Ctx vaccines are prohibitively expensive and only give short-term protection. Consequently, there is scope for the development of alternative control strategies or prophylactics. This may include the use of oligosaccharides as functional mimics for the cell-surface toxin receptor (GM1). Furthermore, the sialic acid component of epithelial receptors has already been shown to contribute significantly to the adhesion and pathogenesis of Ctx. Here, we demonstrate the total inhibition of Ctx using GM1-competitive ELISA with 25mgmL(-1) of a commercial preparation of sialyloligosaccharides (SOS). The IC(50) value was calculated as 5.21mgmL(-1). One-hundred percent inhibition was also observed at all concentrations of Ctx-HRP tested with 500ngmL(-1) GM1-OS. Whilst SOS has much lower affinity for Ctx than GM1-OS, the commercial preparation is impure containing only 33.6% carbohydrate; however, the biantennary nature of SOS appears to give a significant increase in potency over constituent monosaccahride residues. It is proposed that SOS could be used as a conventional food additive, such as in emulsifiers, stabilisers or sweeteners, and are classified as nondigestible oligosaccharides that pass into the small intestine, which is the site of Ctx pathogenesis.

Structural glycobiology: a game of snakes and ladders

Oligo- and polysaccharides are infamous for being extremely flexible molecules, populating a series of well-defined rotational isomeric states under physiological conditions. Characterization of this heterogeneous conformational ensemble has been a major obstacle impeding high-resolution structure determination of carbohydrates and acting as a bottleneck in the effort to understand the relationship between the carbohydrate structure and function. This challenge has compelled the field to develop and apply theoretical and experimental methods that can explore conformational ensembles by both capturing and deconvoluting the structural and dynamic properties of carbohydrates. This review focuses on computational approaches that have been successfully used in combination with experiment to detail the three-dimensional structure of carbohydrates in a solution and in a complex with proteins. In addition, emerging experimental techniques for three-dimensional structural characterization of carbohydrate-protein complexes and future challenges in the field of structural glycobiology are discussed. The review is divided into five sections: (1) The complexity and plasticity of carbohydrates, (2) Predicting carbohydrate-protein interactions, (3) Calculating relative and absolute binding free energies for carbohydrate-protein complexes, (4) Emerging and evolving techniques for experimental characterization of carbohydrate-protein structures, and (5) Current challenges in structural glycoscience.

Multivalent drug design and inhibition of cholera toxin by specific and transient protein-ligand interactions

Multivalent inhibitors of the cholera toxin B pentamer are potential therapeutic drugs for treating cholera and serve as models for demonstrating multivalent ligand effects through a structure-based approach. A crucial yet often overlooked aspect of multivalent drug design is the length, rigidity and chemical composition of the linker used to connect multiple binding moieties. To specifically study the role of chemical linkers in multivalent ligand design, we have synthesized a series of compounds with one and two binding motifs connected by several different linkers. These compounds have affinity for and potency against the cholera toxin B pentamer despite the fact that none can simultaneously bind two toxin receptor sites. Results from saturation transfer difference NMR reveal transient, non-specific interactions between the cholera toxin and linker groups contribute significantly to overall binding affinity of monovalent compounds. However, the same random protein-ligand interactions do not appear to affect binding of bivalent molecules. Moreover, the binding affinities and potencies of these 'non-spanning' bivalent ligands appear to be wholly independent of linker length. Our detailed analysis identifies multiple effects that account for the improved inhibitory potencies of bivalent ligands and suggest approaches to further improve the activity of this class of compounds.

Architecture Effects on the Binding of Cholera Toxin by Helical Glycopolypeptides

A variety of binding events in biological systems are mediated by multivalent interactions between oligosaccharides and saccharide receptors present on pathogens and cell surfaces. In particular, given the important role of multivalent interaction between proteins and carbohydrates in the initial step of pathogen recognition, many glycosylated molecules and polymers have been synthesized in order to mimic the carbohydrate ligands and to inhibit the binding of the pathogen to its target. In this work, we extend our evaluation of the impact of the architecture of well-defined glycopolypeptides on the inhibition of binding of the cholera toxin B pentamer (CT B(5)) subunit. Here we report the production of two families of α-helical glycopolypeptides which were synthesized via a combination of protein engineering and chemical methods. The presentation of pendant saccharides on the polypeptide backbones, as well as their valencies, can be well controlled via these methods. Control of the backbone conformation, introduced in this report, is also possible via these strategies. The polypeptides and glycopolypeptides were characterized via SDS-PAGE analysis, (1)H NMR, and MALDI-TOF mass spectrometry. Their conformation and hydrodynamic volume were characterized by circular dichroic (CD) spectroscopy and gel permeation chromatography (GPC), respectively. The binding of CT B(5) by these glycopolypeptides was evaluated via direct enzyme-linked immunosorbent assay (DELA). The effects of spacing and conformation were elucidated by comparison of the binding exhibited by helical glycopolypeptides with that of random-coil glycopolypeptides.

Solid-phase capture of pathogenic bacteria by using gangliosides and detection with real-time PCR

We developed a method for concentrating pathogens from samples without enrichment. Immobilized gangliosides concentrated bacteria for detection with real-time PCR. A sensitivity of approximately 4 CFU/ml (3 h) in samples without competing microflora was achieved. Samples with competing microflora had a sensitivity of 40,000 CFU/ml. The variance was less than one cycle.

Bile and unsaturated fatty acids inhibit the binding of cholera toxin and Escherichia coli heat-labile enterotoxin to GM1 receptor

Cholera toxin (CT) is an archetypal bacterial toxin that binds with a high affinity to the receptor ganglioside GM1 on the intestinal epithelial surface and that causes the severe watery diarrhea characteristic of the disease cholera. Blockage of the interaction of CT with the GM1 receptor is an attractive approach for therapeutic intervention. We report here that crude bile prevents the interaction of CT with GM1 and reduces CT-mediated fluid accumulation in the rabbit intestine. The unsaturated fatty acids detected in crude bile, arachidonic, linoleic, and oleic acids, were found to be the most effective. Crude bile and the unsaturated fatty acids interacted with CT but not GM1 to prevent CT-GM1 binding. Neither crude bile nor the unsaturated fatty acids had any effect on the subunit structure of CT. The binding of CT to unsaturated fatty acids resulted in a shift of the apparent pI of CT from 6.8 to 8.2 and a marked decrease in intrinsic fluorescence. The Kd was calculated from fluorescence quenching assays. It was demonstrated by the rabbit ileal loop model that practically no fluid accumulated in the intestinal loops when CT was administered together with inhibitory concentrations of linoleic acid. The bile present in the intestine was sufficient to inhibit the activity of up to 300 ng CT. Bile and unsaturated fatty acids also inhibited the binding of Escherichia coli heat-labile enterotoxin (LT) to GM1, and no fluid accumulation was observed in rabbit ileal loops when LT was administered together with linoleic acid.