Comparison of Shiga-like toxin I B-subunit expression and localization in Escherichia coli and Vibrio cholerae by using trc or iron-regulated promoter systems

Screening through the PLICable promoter toolbox enhances protein production in Escherichia coli

Escherichia coli is a common host for recombinant protein production in which production titers are highly dependent on the employed expression system. Promoters are thereby a key element to control gene expression levels. In this study, a novel PLICable promoter toolbox was developed which enables in a single cloning step and after a screening experiment to identify out of ten IPTG-inducible promoters (T7, A3, lpp, tac, pac, Sp6, lac, npr, trc and syn) the most suitable one for high level protein production. The target gene is cloned under the control of different promoters in a single and efficient cloning step using the ligase-free cloning method PLICing (phosphorothioate-based ligase-independent gene cloning). The promoter toolbox was firstly validated using three well producible proteins (a cellulase from a metagenome library, a phytase from Yersinia mollaretii and an alcohol dehydrogenase from Pseudomonas putida) and then applied to two enzymes (3D1 DNA polymerase and glutamate dehydrogenase mutant) which are poorly produced in E. coli. By applying our PLICable pET-promoter toolbox, the authors were able to increase production by two-fold for 3D1 DNA polymerase (lac promoter) and 29-fold for glutamate dehydrogenase mutant H52Y (trc promoter).

Shiga toxin as a bacterial defense against a eukaryotic predator, Tetrahymena thermophila

Bacterially derived exotoxins kill eukaryotic cells by inactivating factors and/or pathways that are universally conserved among eukaryotic organisms. The genes that encode these exotoxins are commonly found in bacterial viruses (bacteriophages). In the context of mammals, these toxins cause diseases ranging from cholera to diphtheria to enterohemorrhagic diarrhea. Phage-carried exotoxin genes are widespread in the environment and are found with unexpectedly high frequency in regions lacking the presumed mammalian "targets," suggesting that mammals are not the primary targets of these exotoxins. We suggest that such exotoxins may have evolved for the purpose of bacterial antipredator defense. We show here that Tetrahymena thermophila, a bacterivorous predator, is killed when cocultured with bacteria bearing a Shiga toxin (Stx)-encoding temperate bacteriophage. In cocultures with Tetrahymena, the Stx-encoding bacteria display a growth advantage over those that do not produce Stx. Tetrahymena is also killed by purified Stx. Disruption of the gene encoding the StxB subunit or addition of an excess of the nontoxic StxB subunit substantially reduced Stx holotoxin toxicity, suggesting that this subunit mediates intake and/or trafficking of Stx by Tetrahymena. Bacterially mediated Tetrahymena killing was blocked by mutations that prevented the bacterial SOS response (recA mutations) or by enzymes that breakdown H(2)O(2) (catalase), suggesting that the production of H(2)O(2) by Tetrahymena signals its presence to the bacteria, leading to bacteriophage induction and production of Stx.

Macropinocytosis in Shiga toxin 1 uptake by human intestinal epithelial cells and transcellular transcytosis

Shiga toxin 1 and 2 production is a cardinal virulence trait of enterohemorrhagic Escherichia coli infection that causes a spectrum of intestinal and systemic pathology. However, intestinal sites of enterohemorrhagic E. coli colonization during the human infection and how the Shiga toxins are taken up and cross the globotriaosylceramide (Gb3) receptor-negative intestinal epithelial cells remain largely uncharacterized. We used samples of human intestinal tissue from patients with E. coli O157:H7 infection to detect the intestinal sites of bacterial colonization and characterize the distribution of Shiga toxins. We further used a model of largely Gb3-negative T84 intestinal epithelial monolayers treated with B-subunit of Shiga toxin 1 to determine the mechanisms of non-receptor-mediated toxin uptake. We now report that E. coli O157:H7 were found at the apical surface of epithelial cells only in the ileocecal valve area and that both toxins were present in large amounts inside surface and crypt epithelial cells in all tested intestinal samples. Our in vitro data suggest that macropinocytosis mediated through Src activation significantly increases toxin endocytosis by intestinal epithelial cells and also stimulates toxin transcellular transcytosis. We conclude that Shiga toxin is taken up by human intestinal epithelial cells during E. coli O157:H7 infection regardless of the presence of bacterial colonies. Macropinocytosis might be responsible for toxin uptake by Gb3-free intestinal epithelial cells and transcytosis. These observations provide new insights into the understanding of Shiga toxin contribution to enterohemorrhagic E. coli-related intestinal and systemic diseases.

Latrunculin B facilitates Shiga toxin 1 transcellular transcytosis across T84 intestinal epithelial cells

Shiga toxins (Stx), released into the intestinal lumen by enterohemorrhagic Escherichia coli (EHEC), are major virulence factors responsible for gastrointestinal and systemic illnesses. These pathologies are believed to be due to the action of the toxins on endothelial cells, which express the Stx receptor, the glycosphingolipid Gb3. To reach the endothelial cells, Stx must translocate across the intestinal epithelial monolayer. This process is poorly understood. We investigated Stx1 movement across the intestinal epithelial T84 cell model and the role of actin turnover in this transcytosis. We showed that changes in the actin cytoskeleton due to latrunculin B, but not cytochalasin D or jasplakinolide, significantly facilitate toxin transcytosis across T84 monolayers. This trafficking is transcellular and completely inhibited by tannic acid, a cell impermeable plasma membrane fixative. This indicates that actin turnover could play an important role in Stx1 transcellular transcytosis across intestinal epithelium in vitro. Since EHEC attachment to epithelial cells causes an actin rearrangement, this finding may be highly relevant to Stx-induced disease.

Protection against Shiga toxin-producing Escherichia coli infection by transcutaneous immunization with Shiga toxin subunit B

Enterohemorrhagic Escherichia coli (EHEC) strains are important human food-borne pathogens. EHEC strains elaborate potent Shiga toxins (Stx1, and/or Stx2) implicated in the development of hemorrhagic colitis (HC) or hemolytic-uremic syndrome (HUS). In this report, we evaluated the immunogenicity and protective efficacy of Stx1 subunit B (StxB1) administered by transcutaneous immunization (TCI). Three groups of Dutch Belted rabbits received patches containing StxB1, StxB1 in combination with Escherichia coli heat-labile enterotoxin (LT), or LT alone. An additional group of naïve rabbits served as controls. The protective efficacy following TCI with StxB1 was assessed by challenging rabbits with a virulent Stx1-producing strain, RDEC-H19A, capable of inducing HC and HUS in rabbits. Antibodies specific to StxB1 from serum and bile samples were determined by enzyme-linked immunosorbent assay and toxin neutralization test. Rabbits immunized with StxB1 demonstrated improved weight gain and reduced Stx-induced histopathology. Rabbits receiving StxB or StxB1/LT showed a significant increase in serum immunoglobulin G titers specific to StxB1 as well as toxin neutralization titers. These data demonstrated that the StxB delivered by TCI could induce significant systemic immune responses. Thus, Stx subunit B vaccine delivered by a patch for a high-risk population may be a practical approach to prevent (and/or reduce) Stx-induced pathology.

Alternate routes for drug delivery to the cell interior: pathways to the Golgi apparatus and endoplasmic reticulum

The targeted delivery of drugs to the cell interior can be accomplished by taking advantage of the various receptor-mediated endocytic pathways operating in a particular cell. Among these pathways, the retrograde trafficking pathway from endosomes to the Golgi apparatus, and endoplasmic reticulum is of special importance since it provides a route to deliver drugs bypassing the acid pH, hydrolytic environment of the lysosome. The existence of pathways for drug or antigen delivery to the endoplasmic reticulum and Golgi apparatus has been to a large extent an outcome of research on the trafficking of A/B type-bacterial or plant toxins such as Shiga toxin within the cell. The targeting properties of these toxins reside in their B subunit. In this article we present an overview of the multiplicity of pathways to deliver drugs intracellularly. We highlight the retrograde trafficking pathway illustrated by Shiga toxin and Shiga-like toxin, and the potential role of the B subunit of these toxins as carriers of drugs, antigens and imaging agents.

A method for the purification of Shiga-like toxin 1 subunit B using a commercially available galabiose–agarose resin

We describe a procedure for the affinity purification of Shiga toxin 1 subunit B (SLTB) using a commercial galabiose-agarose resin. Recombinant SLTB was purified to 99% homogeneity in a single-step protocol, from the periplasmic extracts of Vibrio cholerae 0395 N1/pSBC54. SDS-PAGE of the affinity purified SLTB showed one band of 8 kDa MW. SLTB purified by this procedure retained its chemical and biological activity as demonstrated by re-binding to the galabiose-agarose resin, and receptor-mediated binding and uptake in Vero cells. The galabiose-agarose resin could isolate roughly 1mg of SLTB/mL of gel. The resin was stable over 3 years and 500 cycles/year of usage. Hence, this method is a straightforward approach to the large-scale preparation of SLTB at a reasonable cost.

In vitro and in vivo analyses of constitutive and in vivo-induced promoters in attenuated vaccine and vector strains of Vibrio cholerae

The optimal promoter for in vivo expression of heterologous antigens by live, attenuated vaccine vector strains of Vibrio cholerae is unclear; in vitro analyses of promoter activity may not accurately predict expression of antigens in vivo. We therefore introduced plasmids expressing the B subunit of cholera toxin (CtxB) under the control of a number of promoters into V. cholerae vaccine strain Peru2. We evaluated the tac promoter, which is constitutively expressed in V. cholerae, as well as the in vivo-induced V. cholerae heat shock htpG promoter and the in vivo-induced V. cholerae iron-regulated irgA promoter. The functionality of all promoters was confirmed in vitro. In vitro antigenic expression was highest in vaccine strains expressing CtxB under the control of the tac promoter (2 to 5 microgram/ml/unit of optical density at 600 nm [OD(600)]) and, under low-iron conditions, in strains containing the irgA promoter (5 microgram/ml/OD(600)). We orally inoculated mice with the various vaccine strains and used anti-CtxB immune responses as a marker for in vivo expression of CtxB. The vaccine strain expressing CtxB under the control of the tac promoter elicited the most prominent specific anti-CtxB responses in vivo (serum immunoglobulin G [IgG], P </= 0.05; serum IgA, P </= 0.05; stool IgA, P </= 0.05; bile IgA, P </= 0.05), despite the finding that the tac and irgA promoters expressed equivalent amounts of CtxB in vitro. Vibriocidal antibody titers were equivalent in all groups of animals. Our results indicate that in vitro assessment of antigen expression by vaccine and vector strains of V. cholerae may correlate poorly with immune responses in vivo and that of the promoters examined, the tac promoter may be best suited for expression from plasmids of at least certain heterologous antigens in such strains.

Syntheses and immunologic properties of Escherichia coli O157 O-specific polysaccharide and Shiga toxin 1 B subunit conjugates in mice

Escherichia coli O157 is the major cause of diarrhea-associated hemolytic uremic syndrome (HUS). Strains causing HUS contain either Shiga toxin 1 (Stx1) or Stx2, or both. In adult volunteers, conjugate vaccines of detoxified lipopolysaccharide (LPS) elicited bactericidal antibodies to E. coli O157. Here, the detoxified LPS was conjugated with improved schemes to the nontoxic B subunit of Stx1. Mice injected with these bivalent conjugates elicited both bactericidal antibodies to E. coli O157 and neutralization antibodies to Stx1.

In vivo transduction with shiga toxin 1-encoding phage

To facilitate the study of intestinal transmission of the Shiga toxin 1 (Stx1)-converting phage H-19B, Tn10d-bla mutagenesis of an Escherichia coli H-19B lysogen was undertaken. Two mutants containing insertions in the gene encoding the A subunit of Stx1 were isolated. The resultant ampicillin-resistant E. coli strains lysogenic for these phages produced infectious H-19B particles but not active toxin. These lysogens were capable of transducing an E. coli recipient strain in the murine gastrointestinal tract, thereby demonstrating that lysogens of Shiga toxin-converting phages give rise to infectious virions within the host gastrointestinal tract.

Coexpression of the B subunit of Shiga toxin 1 and EaeA from enterohemorrhagic Escherichia coli in Vibrio cholerae vaccine strains

A promoterless gene for the Shiga toxin 1 B subunit (stxB1) has been placed under transcriptional control of the Vibrio cholerae heat shock gene htpG. A chromosomal enterohemorrhagic Escherichia coli fragment containing eaeA and 400 bp of upstream DNA was added to the construct, downstream of stxB1; no transcription terminators were located between the two genes. The plasmid construct was confirmed by DNA sequencing; in vitro transcription-translation studies demonstrated expression of EaeA from the plasmid. The htpGp-->stxB1, eaeA construct was inserted into lacZ on the chromosome of Peru2, an El Tor V. cholerae strain with both attRS1 sequences and the entire cholera toxin genetic element deleted, and into lacZ in JRB10, a Peru2 derivative that has a second copy of htpGp-->stxB1 also inserted in the V. cholerae virulence gene irgA. Two plasmid constructs, one containing stxB1 under the control of the tac promoter and another containing htpGp-->stxB1,eaeA, were transformed into Peru2. Expression of StxB1 by these constructs was quantified by enzyme-linked immunosorbent assay and was highest in the plasmid construct with stxB1 under the control of the tac promoter. Localization of EaeA to the outer membrane of the vector strains was demonstrated both by Western blotting and by immunofluorescence with an anti-EaeA antibody. A rabbit model for colonization by V. cholerae was used to compare the immune responses to the two heterologous antigens, StxB1 and EaeA, expressed by these strains. Rabbits immunized with Peru2 transformed with a plasmid carrying tac-->stxB1 developed neutralizing serum anti-StxB1 immunoglobulin G antibody responses. One of two rabbits immunized with a strain carrying a chromosomal copy of eaeA developed a marked immune response against EaeA. The plasmid construct containing htpGp-->stxB1,eaeA was unstable, producing low levels of StxB1 in vitro and not evoking anti-EaeA antibody responses in vivo following oral immunization. Chromosomal insertion of eaeA may be preferred for future expression of this antigen in V. cholerae vaccine constructs.

Protective immunity to Shiga-like toxin I following oral immunization with Shiga-like toxin I B-subunit-producing Vibrio cholerae CVD 103-HgR

This study addresses a mechanism for inducing systemic immunity to Shiga-like toxins by oral administration of a Shiga-like toxin I B-subunit-expressing Vibrio cholerae vaccine strain [CVD 103-HgR(pDA60)]. Two sets of three rabbits were given either CVD 103-HgR or CVD 103-HgR(pDA60) orally. All rabbits immunized with CVD 103-HgR(pDA60) developed neutralizing serum antibodies to Shiga-like toxin I. None of the controls developed such antibodies.

Maturational regulation of globotriaosylceramide, the Shiga-like toxin 1 receptor, in cultured human gut epithelial cells

Differentiated villus intestinal epithelial cells express globotriaosylceramide, the Shiga-like toxin 1 (SLT-1) receptor, and are sensitive to toxin-mediated cytotoxicity, whereas undifferentiated crypt cells neither express Gb3 nor respond to toxin. To investigate if SLT-1 receptors are maturationally regulated in human intestinal cells, we examined the effect of butyrate, a known transcriptional regulator of differentiation genes in many cell types, using cultured colonic cancer-derived epithelial cell lines. Exposure to butyrate increased villus cell marker enzymes such as alkaline phosphatase, sucrase, and lactase, expression of toxin receptors, and sensitivity to SLT-1 in villus-like CaCo-2A and HT-29 cells. These effects were reversibly inhibited by preincubation of CaCo-2A cells with actinomycin D or cycloheximide. Butyrate-treated CaCo-2A cells unable to bind fluoresceinated SLT-1 B subunit were undifferentiated as assessed by alkaline phosphatase activity. HT-29 cells induced to differentiate by another signal, glucose deprivation, upregulated receptor content and response to toxin. Crypt-like T-84 cells responded to butyrate with a modest increase in alkaline phosphatase and toxin binding, but no induction of sucrase or lactase, and no change in sensitivity to toxin. The results demonstrate that expression of SLT-1 toxin receptors and toxin sensitivity are coregulated with cellular differentiation in cultured intestinal cells.

Heterologous antigen expression in Vibrio cholerae vector strains

Live attenuated vector strains of Vibrio cholerae were derived from Peru-2, a Peruvian El Tor Inaba strain deleted for the cholera toxin genetic element and attRS1 sequences, which was developed as a live, oral vaccine strain. A promoterless gene encoding the Shiga-like toxin I B subunit (slt-IB) was inserted in the V. cholerae virulence gene irgA by in vivo marker exchange, such that slt-IB was under transcriptional control of the iron-regulated irgA promoter. slt-IB was also placed under transcriptional control of the V. cholerae heat shock promoter, htpGp, and introduced into either the irgA or lacZ locus, or both loci, on the chromosome of Peru-2, generating JRB10, JRB11, or JRB12, respectively. A new technique was used to perform allelic exchange with lacZ. This method uses plasmid p6891MCS, a pBR327 derivative containing cloned V. cholerae lacZ, to insert markers of interest into the V. cholerae chromosome. Recombinants can be detected by simple color screening and antibiotic selection. In vitro measurements of Slt-IB produced by the vector strains suggested that expression of Slt-IB from the irgA and htpG promoters was synergistic and that two copies of the gene for Slt-IB increased expression over a single copy. The V. cholerae vectors colonized the gastrointestinal mucosa of rabbits after oral immunization, as demonstrated by very high serum antibody responses to V. cholerae antigens. Comparison of the serologic responses to the B subunit of cholera toxin (CtxB) following orogastric inoculation either with the wild-type C6709 or with Peru-10, a strain containing ctxB regulated by htpGp, suggested that both the cholera toxin and heat shock promoters were active in vivo, provoking comparable immunologic responses. Orogastric inoculation of rabbits with vector strains evoked serum immunoglobulin G (IgG) responses to Slt-IB in two of the four strains tested; all four strains produced biliary IgA responses. No correlation was observed between the type of promoter expressing slt-IB and the level of serum IgG or biliary IgA response, but the vector strain containing two copies of the gene for slt-IB evoked greater serum IgG responses than strains containing a single copy, consistent with the increased expression of Slt-IB from this strain observed in vitro. A comparison of the serum and biliary antibody responses to Slt-IB expressed from htpGp versus CtxB expressed from the same promoter suggested that CtxB is a more effective orally delivered immunogen.

Expression and purification of Shiga-like toxin II B subunits

Shiga-like toxins (SLTs), which are produced by certain strains of Escherichia coli, are composed of enzymatically active A and B subunit multimers responsible for the toxin's binding. We have previously purified large amounts of the SLT-I B subunit by using a hyperexpression vector in Vibrio cholerae under the control of the trc promoter. In this study we examined various expression vectors to maximize yields of the SLT-II B subunit. The SLT-II B subunit has been expressed by using both the T7 promoter and the tac promoter in E. coli. When expressed from a plasmid containing the structural gene for SLT-II B deleted of the leader sequence, SLT-II B was able to form multimers when cross-linked, although SLT-II B production from this plasmid was unreproducible. SLT-II B expressed in all three systems appeared to form unstable multimers, which did not readily bind to a monoclonal antibody which preferentially recognizes B subunit multimers. SLT-II B expression was not increased by moving any of the plasmids into V. cholerae. Polyclonal antibodies raised to SLT-II B in rabbits recognized B subunit in SLT-II holotoxin yet were poorly neutralizing. SLT-II B was also expressed as a fusion protein with maltose-binding protein and could be cleaved from maltose-binding protein with factor Xa. Although the expression vectors were able to make large amounts of SLT-II B, as determined by Western blotting (immunoblotting), the levels of purified SLT-II B subunit were low compared with those obtained previously for SLT-I B subunit, probably because of instability of the multimeric SLT-II B subunit.

Use of the Vibrio cholerae irgA gene as a locus for insertion and expression of heterologous antigens in cholera vaccine strains

Vibrio cholerae may be a particularly effective organism for use in delivering heterologous antigens to stimulate a common mucosal immune response. A live attenuated vaccine strain of V. cholerae was constructed from the ctxA deletion mutant 0395-N1, containing the B subunit of Shiga-like toxin I under the transcriptional control of the iron-regulated irgA promoter. The B subunit of Shiga-like toxin I is identical to the B subunit of Shiga toxin (StxB). irgA encodes the major iron-regulated outer membrane protein of V. cholerae, which is a known virulence factor for this organism. Clones of the structural gene irgA from the classical V. cholerae strain 0395, with the gene for the Shiga-like toxin I B subunit inserted under the control of the irgA promoter, were used to introduce an internal deletion of irgA into the chromosome of 0395-N1 by in vivo marker exchange, using the suicide vector plasmid pCVD442. This plasmid contains the sacB gene from Bacillus subtilis, which allowed positive selection for loss of plasmid sequences on exposure to sucrose. The construction of vaccine strains was confirmed by Southern hybridization studies and outer membrane protein analysis. The expression of StxB in the vaccine strain VAC2 following growth in high- or low-iron conditions was shown to be tightly iron-regulated by Western blot analysis and by quantification of StxB using a sandwich enzyme-linked immunosorbent assay. The production of StxB by VAC2 under low-iron conditions was greater than that of the reference strain Shigella dysenteriae 60R.(ABSTRACT TRUNCATED AT 250 WORDS)