Effect of alterations of basic amino acid residues of Escherichia coli heat-stable enterotoxin II on enterotoxicity

Enterotoxigenic Escherichia coli Heat-Stable Toxin and Ebola Virus Delta Peptide: Similarities and Differences

Enterotoxigenic Escherichia coli (ETEC) STb toxin exhibits striking structural similarity to Ebola virus (EBOV) delta peptide. Both ETEC and EBOV delta peptide are enterotoxins. Comparison of the structural and functional similarities and differences of these two toxins illuminates features that are important in induction of pathogenesis by a bacterial and viral pathogen.

Animal Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is the most common cause of E. coli diarrhea in farm animals. ETEC are characterized by the ability to produce two types of virulence factors: adhesins that promote binding to specific enterocyte receptors for intestinal colonization and enterotoxins responsible for fluid secretion. The best-characterized adhesins are expressed in the context of fimbriae, such as the F4 (also designated K88), F5 (K99), F6 (987P), F17, and F18 fimbriae. Once established in the animal small intestine, ETEC produce enterotoxin(s) that lead to diarrhea. The enterotoxins belong to two major classes: heat-labile toxins that consist of one active and five binding subunits (LT), and heat-stable toxins that are small polypeptides (STa, STb, and EAST1). This review describes the disease and pathogenesis of animal ETEC, the corresponding virulence genes and protein products of these bacteria, their regulation and targets in animal hosts, as well as mechanisms of action. Furthermore, vaccines, inhibitors, probiotics, and the identification of potential new targets by genomics are presented in the context of animal ETEC.

Biochemical and biological characterization of Escherichia coli STb His12 to Asn variant

We identified a variant of Escherichia coli STb toxin by PCR amplification of clinical isolates obtained from diseased pigs. The variant differed by only one amino acid at position 12 from His to Asn. This change was observed in 23 of the 100 randomly selected enterotoxigenic E. coli (ETEC) isolates tested. There was a positive correlation between the presence of the STa enterotoxin and the STb variant. As the variant represented a high percentage of the ETEC strains tested, we were interested in determining if the single amino acid change results in altered biological characteristics of the toxin. Circular dichroism analysis revealed that the secondary structure of the variant was similar to wildtype and that their thermal stabilities were similar. Surface plasmon resonance showed that the variant and the wildtype toxins possessed similar binding affinities for sulfatide but the variant exhibited a reduced binding capacity. A flow cytometry-based internalization assay showed that the variant toxin is more internalized into epithelial intestinal cells than the wildtype strain. However, this difference was minor. Overall, our results indicate that while wildtype STb and the variant share similar structural properties, modest differences exist in their internalization.

Studies on diarrheagenic Escherichia coli isolated from children with diarrhea in Myanmar

Escherichia coli isolates from 217 children in Myanmar with diarrhea were investigated for the presence of virulence genes related to diarrhea by colony hybridization and PCR. The genes examined were lt, stI, stII, stx1, stx2, eae, bfp, pCVD (which is the representative gene of plasmid of pCVD of EAEC), and ial (which is invasion-associated locus of the invasion plasmid found in EIEC). Isolates from 47 of 217 children (21.7%) possessed virulence genes characteristic of diarrheagenic E. coli. No instance was found of co-existence of different E. coli strains with different virulence genes in the same patient. Diarrheagenic E. coli are currently classified into five categories based on their virulence markers: ETEC, EHEC, EPEC, EAEC, and EIEC. Of the 47 isolates examined, 30 were EAEC, 12 were EPEC and 5 were ETEC. Susceptibility tests for antimicrobial agents showed that almost all diarrheagenic isolates were resistant to penicillin, tetracycline and streptomycin. However, the majority of strains were sensitive to cephalexin, nalidixic acid and norfloxacin. In particular, 42 of the 47 isolates were sensitive to norfloxacin, which is a fluoroquinolone. This study shows EAEC and EPEC are responsible for sporadic diarrhea in Myanmar and fluoroquinolones appear to be effective in the treatment of these patients.

LC-MS analysis of pig intestine sulfatides: interaction with Escherichia coli STb enterotoxin and characterization of molecular species present

STb, a 48-amino acid thermostable enterotoxin is produced by enterotoxigenic Escherichia coli strains and is responsible for diarrheal diseases in many animals, including man. Our laboratory recently identified a family of molecules, from a lipid extract of porcine intestinal epithelial cells, that could bind to STb. These molecules were identified as sulfatides as they reacted with a monoclonal antibody raised against this family of molecules. However, as the epitope recognized by this monoclonal antibody was the galactose 3-sulfate, a doubt could remain as to the exact nature of the identified receptors. The goal of this study was thus to confirm the chemical nature of the STb-binding molecule as sulfatides or as distinctive molecules comprising a sulfated galactosyl residue. Using a thin-layer chromatography-overlay method we confirmed using antibodies to STb that STb recognizes the commercial sulfatides and a band migrating at the same level from the intestinal tissue lipid extract obtained from an 8-week-old piglet. The compounds recovered from the silica gel plates were analyzed by mass spectrometry in electrospray negative-ionization mode. The most abundant ions observed had m/z values of 779, 795, 879 and 907. For commercial bovine brain sulfatides the ions 795, 879 and 907 have been attributed to hydroxylated sulfatides with a saturated fatty acid chain containing 16, 22 and 24 carbons, while the 779 ion contained a saturated fatty acid chain of 16 carbons. The general profile of the ions observed was similar to the already described commercial bovine brain sulfatides.

Region of heat-stable enterotoxin II of Escherichia coli involved in translocation across the outer membrane

Heat-stable enterotoxin II of Escherichia coli (STII) is synthesized as a precursor form consisting of pre- and mature regions. The pre-region is cleaved off from the mature region during translocation across the inner membrane, and the mature region emerges in the periplasm. The mature region, composed of 48 amino acid residues, is processed in the periplasm by DsbA to form an intramolecular disulfide bond between Cys-10 and Cys-48 and between Cys-21 and Cys-36. STII formed with these disulfide bonds is efficiently secreted out of the cell through the secretory system, including TolC. However, it remains unknown which regions of STII are involved in interaction with TolC. In this study, we mutated the STII gene and examined the secretion of these STIIs into the culture supernatant. A deletion of the part covering from amino acid residue 37 to the carboxy terminal end did not markedly reduce the efficiency of secretion of STII into the culture supernatant. On the other hand, the efficiency of secretion of the peptide covering from the amino terminal end to position 18 to the culture supernatant was significantly low. These observations indicated that the central region of STII from amino acid residue 19 to that at position 36 is involved in the secretion of STII into the milieu. The experiment using a dsbA-deficient strain of E. coli showed that the disulfide bond between Cys-21 and Cys-36 by DsbA is necessary for STII to adapt to the structure that can cross the outer membrane.

Binding to sulfatide and enterotoxicity of various Escherichia coli STb mutants

Binding of the 48 amino acid polypeptide of the mature heat-stable Escherichia coli enterotoxin b (STb) to the functional receptor sulfatide (SFT) constitutes the first step in inducing secretory diarrhoea in the intestinal lumen of animals. The NMR structure of this toxin dictated the choice of amino acids for site-directed mutagenesis to delineate the binding site of STb to SFT. Amino acids facing the solvent either in the loop or the hydrophobic alpha-helix were selected. Seventeen site-specific mutants of STb toxin were produced and purified by high-pressure liquid chromatography. Enterotoxicity of the 17 mutants was determined using a rat loop assay and binding was evaluated using a microtitre plate binding assay. Both hydrophobic and electrostatic interactions are important for STb attachment. When mutations (F37K, I41S and M42S) were introduced into the hydrophobic alpha-helix to lessen hydrophobicity, binding activity and enterotoxicity decreased by more than sixfold. The loop defined by C21 and C36 also made specific contributions. Mutants generated at basic residues (K22, K23 and R29) within this region exhibited both reduced binding activities and reduced toxic activities. For all STb mutants constructed and analysed, when binding to SFT was reduced, a reduction in toxicity equivalent or greater was noted, indicating that binding to SFT is a step that precedes the toxic effect observed for STb toxin. Significantly, when the negatively charged D30 was substituted for either alanine or valine, the binding to SFT was about twice that of native STb, whereas the enterotoxicity was reduced by half.

Sulfatide from the pig jejunum brush border epithelial cell surface is involved in binding of Escherichia coli enterotoxin b

Using a quantitative dot blot overlay assay of polyvinylidene difluoride membranes, we investigated the ability of Escherichia coli heat-stable enterotoxin b (STb) to bind to various glycolipids of defined structure. STb bound strongly to acidic glycosphingolipids, including sulfatide (or 3'-sulfogalactosylceramide) and several gangliosides, but not significantly to their derivatives, galactosylceramide and asialogangliosides, respectively. STb exhibited the highest binding affinity for sulfatide. STb bound to pure sulfatide in a dose-dependent and saturable manner, with a detection level of a few nanograms. The binding was not inhibited by tetramethylurea, which is a strong disrupter of hydrophobic interactions, or by the anionic sulfated polymer of glucose, dextran sulfate, indicating that the binding is not due solely to either hydrophobic or ionic interactions via the sulfate group of the sulfatide. The specificity of the binding was confirmed by the finding that a 500-fold molar excess of sulfatide inhibited STb binding by approximately 45%, whereas no competition was obtained with galactosylceramide under the same conditions. Taken together, our data indicated that a galactose residue linked to a sulfate group is required for the binding specificity of STb. Then, total lipids extracted either from the mucous layer or from the epithelial cells of the pig jejunum brush border, the natural target of STb, were analyzed by thin-layer chromatography (TLC). Both extracts contained a lipidic molecule with a relative mobility on a TLC plate similar to that of the sulfatide standard. The migrated lipid extracted directly from a preparative TLC plate was confirmed to be sulfatide, as it was recognized by laminin, a sulfated glycolipid binding protein, and by a monoclonal antibody directed against sulfatide. In an overlay assay on PVDF membranes, STb bound to the sulfatide prepared from porcine jejunum as well as to the sulfatide standard. Thus, these findings suggest that the terminal oligosaccharide sequence Gal(3SO4)beta1- on sulfatide could mediate binding of STb to its target cells and, in support of a recent report (E. Rousset, J. Harel, and J. D. Dubreuil, Microb. Pathog. 24:277-288, 1998), probably terminal sialic acid residue on another glycosphingolipid. Moreover, pretreatment in the ligated intestinal loop assay with laminin or sulfatase altered the biological activity of STb. In summary, we present data indicating that sulfatide represents a functional receptor for the STb toxin.

Binding characteristics of Escherichia coli enterotoxin b (STb) to the pig jejunum and partial characterization of the molecule involved

Escherichia coli heat-stable enterotoxin b (STb) causes severe diarrhoea in weaning piglets. STb most probably has to bind to intestinal epithelial cells in order to achieve its effect. Using biotinylated biologically active STb, we developed a semi-quantitative binding assay using indirect fluorescence microscopy. We demonstrated the attachment of the biotinylated toxin to microvilli of the pig jejunum. However, binding was abolished when biotinylated STb was either boiled or treated with 2-mercaptoethanol, treatments known to abolish biological activity. Different characteristics of STb attachment to the pig small intestine were determined. The reaction was rapid and reached maximum intensity after approximately 10 min. The binding was pH dependent showing an optimum at pH 5.8. Incubation at either 4 degrees C, 25 degrees C or 37 degrees C did not affect the binding. No competition was observed with non-biotinylated STb. However, preincubation of biotinylated STb with streptavidin conjugated to horseradish peroxidase completely abolished the binding. Pig tissues other than jejunum demonstrated binding towards STb including duodenum, ileum, caecum, colon, liver, lung, spleen and kidney. The molecule involved was then partially characterized. Metaperiodate treatment of the jejunum sections abrogated binding but protease treatment had no effect. Enzymatic treatments of jejunal sections demonstrated that N- and O-glycosidases, and several exoglycosidases did not affect binding, whereas reduced binding was observed with ceramide glycanase and alpha-glucosidase, and was completely abolished following neuraminidase treatment. Overall, our results suggest that in vitro STb binding was rapid, pH dependent, temperature independent, not restricted to jejunum and involves a molecule that seems to be composed of a ceramide moiety, terminal neuraminic acid and/or alpha-linked terminal glucose residue(s).

Occurrence, distribution, and associations of O and H serogroups, colonization factor antigens, and toxins of enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is a leading cause of infectious diarrhea worldwide. Four categories of antigens have been commonly studied: O serogroup, H serogroup, colonization factor antigens (CFA), and toxins. A database has been complied from published reports of nearly 1,000 ETEC isolates from 18 locations and analyzed to determine the occurrence, distribution, and associations of O serogroup, H serogroup, CFA, and toxin type. Tables listing the associations of antigens are presented. This analysis documents the widespread nature and variety of ETEC. Even the most common combination of antigens, O6:H16 CFA/II LTST, accounted for only 11% of the ETEC isolates in the database. It was isolated from 12 locations. Many phenotypes occurred only once. CFA detection based on enzyme-linked antibodies with polyclonal sera is suggested as the preferred assay. A combination of CFA and toxin-based antigens is suggested as the most practical vaccine.

Interaction of Escherichia coli heat-stable enterotoxin B with cultured human intestinal epithelial cells

Binding of Escherichia coli heat-stable enterotoxin B (STb) to the human intestinal epithelial cell lines T84 and HT29 and to polarized T84 cells was studied to define the initial interaction of this peptide toxin with target cells. Equilibrium and competitive binding isotherms showed that 125I-STb bound specifically to T84 and HT29 cells; however, the toxin-epithelial cell interactions could be characterized by low-affinity binding (< or = 10(5) M(-1)) to a high number of binding sites (> or = 10(6) per cell). STb binding to T84 and HT29 cells as a function of 125I-STb concentration did not approach saturation at levels well above the effective biological concentration of STb for fluid secretion. Treatment of the 125I-STb-bound T84 and HT29 cells with an acidic saline solution to remove surface-bound toxin revealed that only approximately 55% +/- 10% of 125I-STb could be removed by this treatment at 4 degrees C, suggesting that approximately half of the bound STb was stably associated with the plasma membrane and/or internalized into the cytoplasm. Similar results were obtained when binding and internalization experiments were conducted at 22 and 37 degrees C. Immunofluorescence studies demonstrated that the strongest signal for STb appeared in the plasma membrane even after acid treatment. Toxin-treated cells also displayed diffuse cytoplasmic staining, indicating that once cell bound, STb did not appear to preferentially associate with membrane vesicles or cellular organelles. Binding and subsequent internalization of 125I-STb were not affected by treatment of the cells with trypsin, endoglycosidase F/peptide N-glycosidase F, Vibrio cholerae neuraminidase, tunicamycin, or 5 mM sodium chlorate, which blocks sulfation of surface proteoglycans. In addition, the internalization process was not altered by preincubation of the cells with the cytoskeleton inhibitors cytochalasin D and colchicine or cellular perturbants (i.e., 0.45 M sucrose and 5 mM sodium azide), indicating that cell surface proteins or carbohydrates did not function as STb receptors. The binding of 125I-STb to polarized T84 cells was also examined, and the total and nonspecific binding isotherms were found to overlap, indicating that the apical surface of polarized T84 cells did not contain a specific receptor for STb. In comparison to undifferentiated cells, twice the amount of bound STb (approximately 80% +/- 10%) was removable from polarized T84 cells after treatment with acidic solution. The percentage of surface-bound STb to polarized T84 cells did not vary significantly with the transepithelial electrical resistance of the cells or when STb was applied basolaterally. Together, our results indicate that STb binds with relatively low affinity to the plasma membrane of cultured intestinal epithelial cells and polarized T84 cells, probably to membrane lipids, and becomes stably associated with the lipid bilayer. The fact that a significant portion of the bound STb becomes free in the cytoplasm, even at a low temperature, suggests that the bound toxin may directly traverse the membrane bilayer.

Structure of neurotoxin B-IV from the marine worm Cerebratulus lacteus: a helical hairpin cross-linked by disulphide bonding

B-IV is a 55-residue, crustacean-selective, neurotoxin secreted by Cerebratulus lacteus, a large marine worm found along the northeastern coast of North America. The 3D structure of this molecule in aqueous solution has been determined by 1H NMR spectroscopy at 600 MHz. The molecule has a well-defined helical hairpin structure, with the branches of the hairpin linked by four disulphide bonds. The disulphide connectivities were established from the NMR data to be 1-8/2-7/3-6/4-5, which differed from those determined previously by chemical means, where 1-7 and 2-8 connectivities were found. Each branch of the hairpin is largely alpha-helical, with the helices in the N and C-terminal branches encompassing residues 11 to 23 and 34 to 49, respectively. The loop connecting the branches of the hairpin contains two inverse gamma-turns centred on residues 24 and 25, a type I beta-turn at residues 28 to 31 and a type II beta-turn at residues 30 to 33. Arg17, -25 and -34, which are important for activity, are all on the same face of the molecule, while Trp30, which is also important for activity, is on the opposite face. Structure comparisons show that the B-IV structure is quite similar to those of Rop (ColE1 repressor of primer) and the heat-stable enterotoxin B from Escherichia coli. These structural similarities are discussed in relation to possible mechanisms of action of B-IV.

Contribution of individual disulfide bonds to biological action of Escherichia coli heat-stable enterotoxin B

Heat-stable enterotoxins (STs) of Escherichia coli are peptides which alter normal gut physiology by stimulating the loss of water and electrolytes. The action of heat-stable toxin B (STb) is associated with an increase in levels of lumenal 5-hydroxytryptamine and prostaglandin E2, known mediators of intestinal secretion. In addition, the toxin is responsible for elevation of cytosolic calcium ion levels in cultured cells. STb is a 48-amino-acid basic peptide containing four cysteine residues and two disulfide bonds. Previous work indicates that disulfide bonds are required for intestinal secretory activity, and yet the relative contribution of the two bonds to toxin stability and action is presently unclear. Site-directed mutagenesis was used to alter the cysteine residues of STb to assess the role of the individual disulfide bonds in toxin activity. Our results indicate that loss of a single disulfide bond was sufficient to abolish the intestinal secretory and G protein-coupled calcium ion influx activities associated with STb toxicity. Loss of toxin action was not a function of increased sensitivity of STb mutants to proteolysis, since mutant toxins displayed proteolytic decay rates equivalent to that of wild-type STb. Circular dichroism spectroscopy of mutant STb toxins indicated that single-disulfide-bond elimination did not apparently affect the toxin secondary structure of one mutant, STbC33S,C71S. In contrast, the alpha-helical content of the other disulfide bond mutant, STbC44S,C59G, was significantly altered, as was that of reduced and alkylated authentic STb. Since both Cys-Cys mutant STbs were completely nontoxic, the absence of biological activity cannot be explained by dramatic secondary structural changes alone; keys to the conformational requirements for STb toxicity undoubtedly reside in the three-dimensional structure of this peptide.

The structure of Escherichia coli heat-stable enterotoxin b by nuclear magnetic resonance and circular dichroism

The heat-stable enterotoxin b (STb) is secreted by enterotoxigenic Escherichia coli that cause secretory diarrhea in animals and humans. It is a 48-amino acid peptide containing two disulfide bridges, between residues 10 and 48 and 21 and 36, which are crucial for its biological activity. Here, we report the solution structure of STb determined by two- and three-dimensional NMR methods. Approximate interproton distances derived from NOE data were used to construct structures of STb using distance-geometry and simulated annealing procedures. The NMR-derived structure shows that STb is helical between residues 10 and 22 and residues 38 and 44. The helical structure in the region 10-22 is amphipathic and exposes several polar residues to the solvent, some of which have been shown to be important in determining the toxicity of STb. The hydrophobic residues on the opposite face of this helix make contacts with the hydrophobic residues of the C-terminal helix. The loop region between residues 21 and 36 has another cluster of hydrophobic residues and exposes Arg 29 and Asp 30, which have been shown to be important for intestinal secretory activity. CD studies show that reduction of disulfide bridges results in a dramatic loss of structure, which correlates with loss of function. Reduced STb adopts a predominantly random-coil conformation. Chromatographic measurements of concentrations of native, fully reduced, and single-disulfide species in equilibrium mixtures of STb in redox buffers indicate that the formation of the two disulfide bonds in STb is only moderately cooperative. Similar measurements in the presence of 8 M urea suggest that the native secondary structure significantly stabilizes the disulfide bonds.

Disulfide bond formation and secretion of Escherichia coli heat-stable enterotoxin II

The Escherichia coli heat-stable enterotoxin II (STII) is a typical extracellular toxin consisting of 48 amino acid residues, of which 4 are cysteine. There are two disulfide bonds, one between Cys-10 and Cys-48 and one between Cys-21 and Cys-36. We examined the involvement of DsbA in the formation of the disulfide bonds of STII and the role of each in the secretion of STII. A dsbA mutant was transformed with a plasmid harboring the STII gene, and STII was not detected either in the cells or in the culture supernatant. Reducing the level of STII brought about the dsbA mutation restored by introducing the wild-type dsbA gene into the mutant strain. These results showed that DsbA is involved in forming the disulfide bonds of STII and that STII without these disulfide bonds is degraded during secretion. We substituted these four cysteine residues in vivo by oligonucleotide-directed site-specific mutagenesis. The amino acid sequence of the purified STII (C48S) and pulse-chase studies revealed that two intermolecular disulfide bonds must be formed to be efficiently secreted and that cleavage between amino acid residues 14 and 15 is probably the first step in the proteolytic degradation of STII.