Structure-function analyses of Shiga toxin and the Shiga-like toxins

Engineered Bifunctional Proteins for Targeted Cancer Therapy: Prospects and Challenges

Bifunctional proteins (BFPs) are a class of therapeutic agents produced through genetic engineering and protein engineering, and are increasingly used to treat various human diseases, including cancer. These proteins usually have two or more biological functions-specifically recognizing different molecular targets to regulate the related signaling pathways, or mediating effector molecules/cells to kill tumor cells. Unlike conventional small-molecule or single-target drugs, BFPs possess stronger biological activity but lower systemic toxicity. Hence, BFPs are considered to offer many benefits for the treatment of heterogeneous tumors. In this review, the authors briefly describe the unique structural feature of BFP molecules and innovatively divide them into bispecific antibodies, cytokine-based BFPs (immunocytokines), and protein toxin-based BFPs (immunotoxins) according to their mode of action. In addition, the latest advances in the development of BFPs are discussed and the potential limitations or problems in clinical applications are outlined. Taken together, future studies need to be centered on understanding the characteristics of BFPs for optimizing and designing more effective such drugs.

Molecular Biology of Escherichia Coli Shiga Toxins' Effects on Mammalian Cells

Shiga toxins (Stxs), syn. Vero(cyto)toxins, are potent bacterial exotoxins and the principal virulence factor of enterohemorrhagic Escherichia coli (EHEC), a subset of Shiga toxin-producing E. coli (STEC). EHEC strains, e.g., strains of serovars O157:H7 and O104:H4, may cause individual cases as well as large outbreaks of life-threatening diseases in humans. Stxs primarily exert a ribotoxic activity in the eukaryotic target cells of the mammalian host resulting in rapid protein synthesis inhibition and cell death. Damage of endothelial cells in the kidneys and the central nervous system by Stxs is central in the pathogenesis of hemolytic uremic syndrome (HUS) in humans and edema disease in pigs. Probably even more important, the toxins also are capable of modulating a plethora of essential cellular functions, which eventually disturb intercellular communication. The review aims at providing a comprehensive overview of the current knowledge of the time course and the consecutive steps of Stx/cell interactions at the molecular level. Intervention measures deduced from an in-depth understanding of this molecular interplay may foster our basic understanding of cellular biology and microbial pathogenesis and pave the way to the creation of host-directed active compounds to mitigate the pathological conditions of STEC infections in the mammalian body.

Escherichia coli O157:H7-Clinical aspects and novel treatment approaches

Escherichia coli O157:H7 is a notorious pathogen often contracted by intake of contaminated water or food. Infection with this agent is associated with a broad spectrum of illness ranging from mild diarrhea and hemorrhagic colitis to the potentially fatal hemolytic uremic syndrome (HUS). Treating E. coli O157:H7 infection with antimicrobial agents is associated with an increased risk of severe sequelae such as HUS. The difficulty in treating this bacterium using conventional modalities of antimicrobial agent administration has sparked an interest in investigating new therapeutic approaches to this bacterium. These approaches have included the use of probiotic agents and natural products with variable success rates. In addition, novel modalities and regimen of antimicrobial agent administration have been assessed in an attempt at decreasing their association with aggravating infection outcomes.

The strain-specific dynamics of Escherichia coli O157:H7 faecal shedding in cattle post inoculation

This study reports analysis of faecal shedding dynamics in cattle for three Escherichia coli O157:H7 (ECO157) strains (S1, S2 and S3) of different genotype and ecological history, using experimental inoculation data. The three strains were compared for their shedding frequency and level of ECO157 in faeces. A multistate Markov chain model was used to compare shedding patterns of S1 and S2. Strains S1 and S2 were detected seven to eight times more often and at 10(4) larger levels than strain S3. Strains S1 and S2 had similar frequencies and levels of shedding. However, the total time spent in the shedding state during colonization was on average four times longer for S1 (15 days) compared to S2 (4 days). These results indicate that an ECO157 strain effect on the frequency, level, pattern and the duration of faecal shedding may need to be considered in control of ECO157 in the cattle reservoir.

Establishment of recombinant hybrid-IgG/IgA immunoglobulin specific for Shiga toxin

Shiga toxin 1 produced by enterohaemorrhagic Escherichia coli is an AB(5) toxin that is involved in the life-threatening haemolytic-uraemic syndrome. The B subunits (Stx1B) are cell-binding subunits. We previously established mouse hybridoma cell line producing IgA and IgG monoclonal antibodies (mAbs) against Stx1B. Here, we cloned cDNAs encoding each of the heavy, light and joining (J) chains from the hybridoma cell lines by means of the 5' rapid amplification of cDNA ends (RACE) PCR method. Upon assignment of the variable regions of the heavy and light chains to known germline sequences, we found substantial somatic hypermutation in the complementarity-determining regions in both the IgA and IgG mAbs. We also established a hybrid-IgG/IgA heavy chain having variable regions of the IgG mAb by means of recombinant PCR methods. Upon transient expression of the hybrid-IgG/IgA heavy, IgG-associated light and J chains in COS-1 cells, the translated dimeric hybrid-IgG/IgA bound to immobilized Stx1B, as revealed on ELISA. The production of dimeric hybrid-IgG/IgA was revealed on immunoblot analysis. The dimeric hybrid-IgG/IgA inhibited the binding of digoxigenin-conjugated Stx1B to natural ligands (CD77) displayed on Burkitt's lymphoma cell line Ramos. These results indicate that the replacement of variable regions resulted in the production of more useful recombinant dimeric IgA against Stx1B.

Decrease in Shiga toxin expression using a minimal inhibitory concentration of rifampicin followed by bactericidal gentamicin treatment enhances survival of Escherichia coli O157:H7-infected BALB/c mice

Background

Treatment of Escherichia coli O157:H7 infections with antimicrobial agents is controversial due to an association with potentially fatal sequelae. The production of Shiga toxins is believed to be central to the pathogenesis of this organism. Therefore, decreasing the expression of these toxins prior to bacterial eradication may provide a safer course of therapy.

Methods

The utility of decreasing Shiga toxin gene expression in E. coli O157:H7 with rifampicin prior to bacterial eradication with gentamicin was evaluated in vitro using real-time reverse-transcription polymerase chain reaction. Toxin release from treated bacterial cells was assayed for with reverse passive latex agglutination. The effect of this treatment on the survival of E. coli O157:H7-infected BALB/c mice was also monitored.

Results

Transcription of Shiga toxin-encoding genes was considerably decreased as an effect of treating E. coli O157:H7 in vitro with the minimum inhibitory concentration (MIC) of rifampicin followed by the minimum bactericidal concentration (MBC) of gentamicin (> 99% decrease) compared to treatment with gentamicin alone (50-75% decrease). The release of Shiga toxins from E. coli O157:H7 incubated with the MIC of rifampicin followed by addition of the MBC of gentamicin was decreased as well. On the other hand, the highest survival rate in BALB/c mice infected with E. coli O157:H7 was observed in those treated with the in vivo MIC equivalent dose of rifampicin followed by the in vivo MBC equivalent dose of gentamicin compared to mice treated with gentamicin or rifampicin alone.

Conclusions

The use of non-lethal expression-inhibitory doses of antimicrobial agents prior to bactericidal ones in treating E. coli O157:H7 infection is effective and may be potentially useful in human infections with this agent in addition to other Shiga toxin producing E. coli strains.

Recent insights into Pasteurella multocida toxin and other G-protein-modulating bacterial toxins

Over the past few decades, our understanding of the bacterial protein toxins that modulate G proteins has advanced tremendously through extensive biochemical and structural analyses. This article provides an updated survey of the various toxins that target G proteins, ending with a focus on recent mechanistic insights in our understanding of the deamidating toxin family. The dermonecrotic toxin from Pasteurella multocida (PMT) was recently added to the list of toxins that disrupt G-protein signal transduction through selective deamidation of their targets. The C3 deamidase domain of PMT has no sequence similarity to the deamidase domains of the dermonecrotic toxins from Escherichia coli (cytotoxic necrotizing factor [CNF]1-3), Yersinia (CNFY) and Bordetella (dermonecrotic toxin). The structure of PMT-C3 belongs to a family of transglutaminase-like proteins, with active site Cys-His-Asp catalytic triads distinct from E. coli CNF1.

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.

Large-scale synthesis of globotriose derivatives through recombinant E. coli

The carbohydrate chains decorating cell membranes and secreted proteins participate in a range of important biological processes. However, their ultimate significance and possible therapeutic potential have not been fully explored due to the lack of economical methods for their production. This study is an example of the use of a genetically engineered bacterial strain in the preparation of diverse oligosaccharides. Based on an ex vivo biosynthetic pathway, an artificial gene cluster was constructed by linking the genes of five associated enzymes on a plasmid vector. This plasmid was inserted into the E. coli NM522 strain to form globotriose-producing cells ('superbug' pLDR20-CKTUF). The specific strain was conveniently applied to the synthesis of globotriose trisaccharide and its derivatives, as potential neutralizers for Shiga toxin. This work demonstrates a novel and economical method for generating ligand diversity for carbohydrate drug development.

Inhibition of the transcription of the Escherichia coli O157:H7 genes coding for shiga-like toxins and intimin, and its potential use in the treatment of human infection with the bacterium

Reverse-transcription PCR (RT-PCR) was used to assess the effects, in vitro, of rifampicin on the transcription of the eaeA, stx1 and stx2 genes (coding for intimin and shiga-like toxins I and II, respectively) of seven strains of Escherichia coli O157:H7 associated with human infection. Each strain was found to possess all three genes and, in the absence of rifampicin, all seven strains transcribed eaeA and stx2 (three strains did not transcribe their stx1 genes). Transcription of all three genes was inhibited (as witnessed by a negative result in the RT-PCR), however, when the strains were incubated, at 37 degrees C, with rifampicin at 4 microg/ml (found to be the minimum concentration of this antimicrobial agent that inhibited the multiplication of E. coli O157:H7). The results of an assay based on reversed passive latex agglutination (RPLA) revealed that exposure of the bacteria to 4 microg rifampicin/ml led to a 12-fold decrease in the release of shiga-like toxin I and a 16-fold decrease in the release of shiga-like toxin II. As rifampicin is capable of inhibiting the in-vitro transcription of the genes encoding the shiga-like toxins and intimin attachment protein of E. coli O157:H7, it may be useful in the treatment of human infections with strains of this bacterium. Studies are now underway to assess the in-vitro and in-vivo effects of rifampicin, at both transcription-inhibitory and bactericidal concentrations, on E. coli O157:H7. The effects of other agents on the inhibition of the expression or activity of the shiga-like toxins and intimin attachment protein will also be determined.

Efficient chemoenzymatic synthesis of globotriose and its derivatives with a recombinant alpha-(1-->4)-galactosyltransferase

A truncated alpha-(1-->4)-galactosyltransferase (LgtC) gene from Neisseria meningitidis was cloned. The recombinant glycosyltransferase was expressed in Escherichia coli BL21 (DE3) strain with high specific activity (5 units/mg protein). Its acceptor specificity was carefully characterized. Then the purified enzyme was utilized in highly efficient syntheses of globotriose and a variety of alpha-(1-->4)-galactosylated derivatives as potential antibacterial agents.

The use of Shiga-like toxin 1 in cancer therapy

The ribosome-inactivating protein, Shiga-like toxin-1 (SLT-1, SLT-I, Verotoxin 1, VT1) targets cells that express the glycolipid globotriaosylceramide (CD77) on their surface. The frequent occurrence of SLT-1 receptors on tumor cells derived from patients with hematological cancers (follicular lymphoma, multiple myeloma, chronic lymphocytic leukemia) and their absence on human CD34(+) hematopoietic stem cells suggest the ex vivo use of Shiga-like toxin-1 in purging CD77(+) tumor cells from autologous stem cell transplants. SLT-1 receptors are also commonly expressed on breast cancer, ovarian cancer and astrocytoma cells. In particular, the sensitivity of astrocytoma cell lines to this toxin provides an opportunity for using SLT-1 in vivo in the context of treating patients afflicted by this common form of brain tumor. Finally, the known structural features of SLT-1 allow one to contemplate altering its receptor specificity in an effort to target CD77(-) tumor cell populations.

Antiviral activity of shiga toxin 1: suppression of bovine leukemia virus-related spontaneous lymphocyte proliferation

Human infections with Shiga toxin (Stx)-producing Escherichia coli (STEC) cause hemorrhagic colitis. The Stxs belong to a large family of ribosome-inactivating proteins (RIPs) that are found in a variety of higher plants and some bacteria. Many RIPs have potent antiviral activity for the plants that synthesize them. STEC strains, both virulent and nonvirulent to humans, are frequently isolated from healthy cattle. Interestingly, despite intensive investigations, it is not known why cattle carry STEC. We tested the hypothesis that Stx has antiviral properties for bovine viruses by assessing the impact of Stx type 1 (Stx1) on bovine peripheral blood mononuclear cells (PBMC) from cows infected with bovine leukemia virus (BLV). PBMC from BLV-positive animals invariably displayed spontaneous lymphocyte proliferation (SLP) in vitro. Stx1 or the toxin A subunit (Stx1A) strongly inhibited SLP. Toxin only weakly reduced the pokeweed mitogen- or interleukin-2-induced proliferation of PBMC from normal (BLV-negative) cows and had no effect on concanavalin A-induced proliferation. The toxin activity in PBMC from BLV-positive cattle was selective for viral SLP and did not abrogate cell response to pokeweed mitogen- or interleukin-2-induced proliferation. Antibody to virus or Stx1A was most effective at inhibiting SLP if administered at the start of cell culture, indicating that both reagents likely interfere with BLV-dependent initiation of SLP. Stx1A inhibited expression of BLV p24 protein by PBMC. A well-defined mutant Stx1A (E167D) that has decreased catalytic activity was not effective at inhibiting SLP, suggesting the inhibition of protein synthesis is likely the mechanism of toxin antiviral activity. Our data suggest that Stx has potent antiviral activity and may serve an important role in BLV-infected cattle by inhibiting BLV replication and thus slowing the progression of infection to its malignant end stage.

Haemolytic-uraemic syndrome caused by vero toxin-producing Escherichia coli serotype Rough: K-: H49

The first case of haemolytic-uraemic syndrome (HUS) caused by Vero toxin-producing Escherichia coli (VTEC) which belonged to a novel serotype, Rough: K-: H49, is reported. The case was initially diagnosed as nephropathia epidemica caused by Puumala virus, but the subsequent diagnosis of HUS caused by VTEC was made after bacteriological investigation. The strain isolated fermented sorbitol produced VT2 toxin but not enterohaemolysin, nor did it carry the eaeA gene. In VTEC strains, the O antigen, the eaeA gene and enterohaemolysin production have been characterized as virulence-associated factors and believed to have an effect on pathogenesis of these strains to cause haemorrhagic colitis or HUS. The findings of this study demonstrate that there is a need for further studies to evaluate the pathogenetic mechanism of VTEC and need for easy diagnostic methods exploiting other properties than O157 antigen and non-fermentation of sorbitol to find all VTEC in human infections.

Disruption of an internal membrane-spanning region in Shiga toxin 1 reduces cytotoxicity

Shiga toxin type 1 (Stx1) belongs to the Shiga family of bipartite AB toxins that inactivate eukaryotic 60S ribosomes. The A subunit of Stxs are N-glycosidases that share structural and functional features in their catalytic center and in an internal hydrophobic region that shows strong transmembrane propensity. Both features are conserved in ricin and other ribosomal inactivating proteins. During eukaryotic cell intoxication, holotoxin likely moves retrograde from the Golgi apparatus to the endoplasmic reticulum. The hydrophobic region, spanning residues I224 through N241 in the Stx1 A subunit (Stx1A), was hypothesized to participate in toxin translocation across internal target cell membranes. The TMpred computer program was used to design a series of site-specific mutations in this hydrophobic region that disrupt transmembrane propensity to various degrees. Mutations were synthesized by PCR overlap extension and confirmed by DNA sequencing. Mutants StxAF226Y, A231D, G234E, and A231D-G234E and wild-type Stx1A were expressed in Escherichia coli SY327 and purified by dye-ligand affinity chromatography. All of the mutant toxins were similar to wild-type Stx1A in enzymatic activity, as determined by inhibition of cell-free protein synthesis, and in susceptibility to trypsin digestion. Purified mutant or wild-type Stx1A combined with Stx1B subunits in vitro to form a holotoxin, as determined by native polyacrylamide gel electrophoresis immunoblotting. StxA mutant A231D-G234E, predicted to abolish transmembrane propensity, was 225-fold less cytotoxic to cultured Vero cells than were the wild-type toxin and the other mutant toxins which retained some transmembrane potential. Furthermore, compared to wild-type Stx1A, A231D-G234E Stx1A was less able to interact with synthetic lipid vesicles, as determined by analysis of tryptophan fluorescence for each toxin in the presence of increasing concentrations of lipid membrane vesicles. These results provide evidence that this conserved internal hydrophobic motif contributes to Stx1 translocation in eukaryotic cells.

Pathogenesis and diagnosis of Shiga toxin-producing Escherichia coli infections

Since their initial recognition 20 years ago, Shiga toxin-producing Escherichia coli (STEC) strains have emerged as an important cause of serious human gastrointestinal disease, which may result in life-threatening complications such as hemolytic-uremic syndrome. Food-borne outbreaks of STEC disease appear to be increasing and, when mass-produced and mass-distributed foods are concerned, can involve large numbers of people. Development of therapeutic and preventative strategies to combat STEC disease requires a thorough understanding of the mechanisms by which STEC organisms colonize the human intestinal tract and cause local and systemic pathological changes. While our knowledge remains incomplete, recent studies have improved our understanding of these processes, particularly the complex interaction between Shiga toxins and host cells, which is central to the pathogenesis of STEC disease. In addition, several putative accessory virulence factors have been identified and partly characterized. The capacity to limit the scale and severity of STEC disease is also dependent upon rapid and sensitive diagnostic procedures for analysis of human samples and suspect vehicles. The increased application of advanced molecular technologies in clinical laboratories has significantly improved our capacity to diagnose STEC infection early in the course of disease and to detect low levels of environmental contamination. This, in turn, has created a potential window of opportunity for future therapeutic intervention.

Inactivation of two haemolytic toxin genes in Aeromonas hydrophila attenuates virulence in a suckling mouse model

The contribution of two unrelated Aeromonas hydrophila beta-haemolytic toxins to virulence was assessed in a suckling mouse model. The first haemolysin gene, isolated from an A. hydrophila A6 cosmid bank, encoded a potential gene product of 621 amino acids and a predicted molecular size of 69.0 kDa. The inferred amino acid sequence showed 89% identity to the AHH1 haemolysin of A. hydrophila ATCC 7966, and 51% identity to the HlyA haemolysin of Vibrio cholerae EI Tor strain O17. The second haemolysin gene (designated aerA), which encodes aerolysin, a pore-forming toxin, was partially cloned by PCR for the purpose of mutant construction. This PCR product was a 1040 bp fragment from the C-terminal region of aerA. It is proposed that the 69.0 kDa V. cholerae-HlyA-like haemolysin gene be termed hlyA to contrast with the aerA terminology for the aerolysin. A suicide vector was used to inactivate both the hlyA and aerA genes in A. hydrophila A6. When assessed in the suckling mouse model, only the hlyA aerA double mutant showed a statistically significant reduction in virulence--a 20-fold change in LD50 (Scheffe test, P < 0.05). Cytotoxicity to buffalo green monkey kidney cell monolayers and haemolysis on horse blood agar were eliminated only in the hlyA aerA double mutants. This is the first report of cloning and mutagenesis of two unrelated haemolytic toxin genes in the same strain of a mesophilic aeromonad. For A. hydrophila, a two-toxin model provides a more complete explanation of virulence.

Investigation of ribosome binding by the Shiga toxin A1 subunit, using competition and site-directed mutagenesis

The enzymatic subunit of Shiga toxin (StxA1) is a member of the ribosome-inactivating protein (RIP) family, which includes the ricin A chain as well as other examples of plant toxins. StxA1 catalytically depurinates a well-conserved GAGA tetra-loop of 28S rRNA which lies in the acceptor site of eukaryotic ribosomes. The specific activities of native StxA1, as well as mutated forms of the enzyme with substitutions in catalytic site residues, were measured by an in vitro translation assay. Electroporation was developed as an alternative method for the delivery of purified A1 polypeptides into Vero cells. Site-directed mutagenesis coupled with N-bromosuccinimide modification indicated that the sole tryptophan residue of StxA1 is required for binding it to the 28S rRNA backbone. Northern analysis established that the catalytic site substitutions reduced enzymatic activity by specifically interfering with the capacity of StxA1 to depurinate 28S rRNA. Ribosomes were protected from StxA1 by molar excesses of tRNA and free adenine, indicating that RIPs have the capacity to enter the acceptor site groove prior to binding and depurinating the GAGA tetra-loop.

Intervention of carbohydrate recognition by proteins and nucleic acids

Carbohydrates in biological systems are often associated with specific recognition and signaling processes leading to important biological functions and diseases. Considerable efforts have been directed toward understanding and mimicking the recognition processes and developing effective agents to control the processes. The pace of discovery research in glycobiology and development of carbohydrate-based therapeutics, however, has been relatively slow due to the lack of appropriate strategies and methods available for carbohydrate-related research. This review summarizes some of the most recent developments in the field, with particular emphasis on work from our laboratories regarding the use of chemoenzymatic strategies to tackle the carbohydrate recognition problem. Highlights include the study of selectin-carbohydrate and aminoglycoside-RNA interactions and development of agents for the intervention of these recognition processes.

Analysis of Shiga toxin subunit association by using hybrid A polypeptides and site-specific mutagenesis

Shiga toxin (STX), a bacterial toxin produced by Shigella dysenteriae type 1, is a hexamer composed of five receptor-binding B subunits which encircle an alpha-helix at the carboxyl terminus of the enzymatic A polypeptide. Hybrid toxins constructed by fusing the A polypeptide sequences of STX and Shiga-like toxin type II were used to confirm that the carboxyl terminus of the A subunits governs association with the B pentamers. The alpha-helix of the 293-amino-acid STX A subunit contains nine residues (serine 279 to methionine 287) which penetrate the nonpolar pore of the B-subunit pentamer. Site-directed mutagenesis was used to establish the involvement of two residues bordering this alpha-helix, aspartic acid 278 and arginine 288, in coupling the C terminus of StxA to the B pentamer. Amino acid substitutions at StxB residues arginine 33 and tryptophan 34, which are on the membrane-contacting surface of the pentamer, reduced cytotoxicity without affecting holotoxin formation. Although these B-subunit mutations did not involve receptor-binding residues, they may have induced an electrostatic repulsion between the holotoxin and the mammalian cell membrane or disrupted cytoplasmic translocation.

Evidence that the A2 fragment of Shiga-like toxin type I is required for holotoxin integrity

Escherichia coli Shiga-like toxin type I (SLT-I) is a potent cytotoxin consisting of an enzymatically active A subunit and a pentameric B subunit that mediates toxin binding to susceptible eukaryotic cells. Evidence that the carboxy-terminal 38 amino acids of the A subunit are involved in holotoxin 1A:5B association is presented. We compared the ability of purified recombinant SLT-I B subunit (Slt-IB) to combine in vitro with purified recombinant SLT-I A subunit (Slt-IA; full-length subunit A includes amino acids 1 to 293) and its ability to combine with purified recombinant SLT-I A1 subunit (Slt-IA1; truncated subunit A includes amino acids 1 to 255). Each mixture was analyzed for biological and physical evidence of toxin assembly. Although Slt-IA successfully combined with Slt-IB to form a molecular species similar to holotoxin that was detectable by nondenaturing polyacrylamide gel electrophoresis and immunoblotting and yielded a molecule which was cytotoxic to cultured Vero cells, Slt-IA1 did not have this ability. Slt-IA1 was 36-fold more active than Slt-IA in an in vitro protein synthesis inhibition assay. These findings suggest that the Slt-IA2 fragment is crucial for formation of SLT holotoxin and stabilizes the interaction between the A and B subunits.

Cytotoxicity of a shiga toxin A subunit-CD4 fusion protein to human immunodeficiency virus-infected cells

Shiga toxin (STX) is a ribosome-inactivating cytotoxin produced by Shigella dysenteriae serotype 1. The enzymatic domain of the STX A polypeptide has been defined by introducing amino- and carboxy-terminal deletions in the polypeptide and assessing activity in a cell-free translation system. Three recombinant forms of StxA which possess enzymatic activity were genetically fused to a 165-amino-acid polypeptide derived from CD4, the cellular receptor for human immunodeficiency virus type 1 (HIV-1). This strategy eliminated the STX receptor-binding subunit and directed the hybrid toxins to cells expressing the HIV-1 surface glycoprotein gp120. A bacterial lysate containing these toxin chimeras killed the HIV-1-infected T-cell line 8E5 but was not cytotoxic toward the uninfected parental cell line A3.01. This cytotoxic activity was specifically inhibited by monoclonal antibodies which block the interaction between CD4 and gp120. These StxA-CD4 hybrids add to the repertoire of recombinant fusion proteins which possess the capacity to selectively kill HIV-1-infected T cells.

Southwestern Internal Medicine Conference: Shiga-like toxins in hemolytic-uremic syndrome and thrombotic thrombocytopenic purpura

The majority of cases of hemolytic-uremic syndrome and a smaller proportion of cases of thrombotic thrombocytopenic purpura have recently been shown to result from a toxin produced by enteric bacteria, referred to as verotoxin, or Shiga-like toxin. The predominant toxin-producing bacterial strain in North America is E. coli O157:H7, which causes hemorrhagic colitis in humans after ingestion of contaminated meat. The toxin is believed to gain entry to the circulation from the bowel wall; it then binds to specific glycolipid receptors abundant on renal vascular endothelial cells. The toxin inactivates ribosomes inside the cells, thereby killing them and producing the clinical manifestations of hemolytic-uremic syndrome. Recognition of the etiology of hemolytic-uremic syndrome may lead to better prospects for prevention and treatment.

Identification of the Shiga toxin A-subunit residues required for holotoxin assembly

Recent X-ray crystallographic analyses have demonstrated that the receptor-binding (B) subunits of Shiga toxin (STX) are arranged as a doughnut-shaped pentamer. The C terminus of the enzymatic (A) subunit presumably penetrates the nonpolar pore of the STX B pentamer, and the holotoxin is stabilized by noncovalent interactions between the polypeptides. We identified a stretch of nine nonpolar amino acids near the C terminus of StxA which were required for subunit association by using site-directed mutagenesis to introduce progressive C-terminal deletions in the polypeptide and assessing holotoxin formation by a receptor analog enzyme-linked immunosorbent assay, immunoprecipitation, and a cytotoxicity assay. Tryptophan and aspartic acid residues which form the N-terminal boundary, as well as two arginine residues which form the C-terminal boundary of the nine-amino-acid sequence, were implicated as the stabilizers of subunit association. Our model proposes that residues 279 to 287 of the 293-amino-acid STX A subunit penetrate the pore while the tryptophan, aspartic acid, and 2 arginine residues interact with other charged or aromatic amino acids outside the pore on the planar surfaces of the STX B pentamer.

Minimum domain of the Shiga toxin A subunit required for enzymatic activity

The minimum sequence of the enzymatic (A) subunit of Shiga toxin (STX) required for activity was investigated by introducing N-terminal and C-terminal deletions in the molecule. Enzymatic activity was assessed by using an in vitro translation system. A 253-amino-acid STX A polypeptide, which is recognized as the enzymatically active portion of the 293-amino-acid A subunit, expressed less than wild-type levels of activity. In addition, alteration of the proposed nicking site between Ala-253 and Ser-254 by site-directed mutagenesis apparently prevented proteolytic processing but had no effect on the enzymatic activity of the molecule. Therefore, deletion analysis was used to identify amino acid residue 271 as the C terminus of the enzymatically active portion of the STX A subunit. STX A polypeptides with N-terminal and C-terminal deletions were released into the periplasmic space of Escherichia coli by fusion to the signal peptide and the first 22 amino acids of Shiga-like toxin type II, a member of the STX family. Although these fusion proteins expressed less than wild-type levels of enzymatic activity, they confirmed the previous finding that Tyr-77 is an active-site residue. Therefore, the minimum domain of the A polypeptide which was required for the expression of enzymatic activity was defined as StxA residues 75 to 268.

Antibodies to shiga holotoxin and to two synthetic peptides of the B subunit in sera of patients with Shigella dysenteriae 1 dysentery

Acute- and convalescent-phase sera from 18 Thai patients and convalescent-phase sera from two Israeli patients and one Bangladeshi patient with Shigella dysenteriae 1 (Shiga) dysentery were tested by enzyme-linked immunosorbent assay to detect antibodies that bind S. dysenteriae lipopolysaccharide (LPS), Shiga holotoxin, or two synthetic peptides representing epitopes from the B subunit of Shiga toxin. Paired sera from 24 Maryland adults with Shigella flexneri 2a or Shigella sonnei diarrhea served as negative controls. Of the 16 paired Thai serum samples tested for immunoglobulin G LPS antibody, 10 had greater than or equal to 4-fold rises (the two subjects with the highest convalescent-phase titers exhibited toxin-neutralizing activity); acute-phase specimens from four of the remaining six individuals already had elevated Shiga LPS titers in their acute specimens ranging from 1:800 to 1:12,800. Similarly, convalescent-phase sera from the two Israeli patients and the Bangladeshi patient revealed LPS titers of 1:800 to 1:3,200. In contrast, none of the Maryland volunteers with S. flexneri or S. sonnei diarrhea manifested rises in Shiga anti-LPS (P less than 0.00001 versus 10 of 16 Thai patients). Only 4 of the 18 Thai patients had significant rise in antibody to purified Shiga toxin, while one of the two Israeli patients and the one Bangladeshi patient had elevated convalescent-phase titers. None of the sera that reacted with Shiga holotoxin had antibody that bound to the peptides. This report, which describes a search for serum antibodies that bind Shiga toxin in patients with Shiga dysentery, demonstrates such antibodies in only a minority of patients with bacteriologically confirmed disease. During Shiga dysentery, Shiga toxin may be elaborated in such small quantities in vivo that it fails to elicit an immune response in most patients even though it may exert biological effects. In this behavior Shiga toxin resembles tetanus toxin, another potent exotoxin that fails to elicit antitoxic responses in people who recover from clinical tetanus.

Mapping of functional regions of Clostridium perfringens type A enterotoxin

Studies were conducted to allow construction of an initial map of the structure-versus-function relationship of the Clostridium perfringens type A enterotoxin (CPE). Removal of the N-terminal 25 amino acids of CPE increased the primary cytotoxic effect of CPE but did not affect binding. CPE sequences required for at least four epitopes were also identified.

The pathogenic mechanisms of Shiga toxin and the Shiga-like toxins

It is now well documented that some enteric bacteria which cause diarrhoeal and/or dysenteric disease produce, at high levels, one or more of a family of protein toxins referred to as Shiga toxin and Shiga-like toxins (SLTs; alternatively called verocytotoxins or VTs). Within the past few years, there have been considerable advancements made in our understanding of the biochemistry and molecular biology of Shiga toxin and SLTs. However, the precise role of the toxins in mediating colonic disease, as well as their contribution to the development of extra-intestinal sequelae (e.g. the haemolytic uraemic syndrome and neurological disorders), remain less clear. In this MicroReview, we will briefly summarize recent progress in Shiga toxin- and SLT-related research and present evidence supporting the concept that these toxins contribute to pathogenesis by directly damaging vascular endothelial cells, thereby disrupting the homeostatic properties of these cells. We will also discuss data which suggest that toxin-mediated damage in the kidney may not be limited to glomerular endothelial cells but may include tubular epithelial cells. Thus, the role of the toxins in renal disease may not be limited to the glomeruli, as was initially hypothesized when the association of infection with toxin-producing strains and the development of acute renal failure was established.

Importance of arginine at position 170 of the A subunit of Vero toxin 1 produced by enterohemorrhagic Escherichia coli for toxin activity

Comparison of the primary structures of the A subunits of Vero toxin 1 (VT1), Vero toxin 2 (VT2), and two variants of VT2 (VT2vp and VT2vh) and the ricin A chain revealed three conserved regions (amino acid residues 51-55, 167-171 and 202-207 from the N-terminus of VT1). All three regions of the ricin A chain corresponded in position to the active site of ricin proposed by X-ray crystal diffraction analysis. To determine the relative importance of the conserved amino acid residues for toxin activity of VT1, we prepared VT1 mutants with single amino-acid substitutions by oligonucleotide-directed site-specific mutagenesis. A total of 22 mutants were prepared to examine 14 conserved residues, and their cytotoxicities to Vero cells and inhibitory activities on protein synthesis in a rabbit reticulocyte lysate were compared with those of wild-type VT1. Replacement of glutamic acid at position 167 by glutamine and of arginine at position 170 by leucine reduced both activities drastically. These results suggest that, in addition to the glutamic acid at position 167 reported previously, arginine at position 170 also plays an important role in the toxin activity of VT1. A possible chemical mechanism of the enzymatic (N-glycosidase) activity of VT1 is proposed based on the relative activities of various mutants.

Genetic basis of virulence in Shigella species

Shigella species and enteroinvasive strains of Escherichia coli cause disease by invasion of the colonic epithelium, and this invasive phenotype is mediated by genes carried on 180- to 240-kb plasmids. In addition, at least eight loci on the Shigella chromosome are necessary for full expression of virulence. The products of these genes can be classified as (i) virulence determinants that directly affect the ability of shigellae to survive in the intestinal tissues, e.g., the aerobactin siderophore (iucABCD and iutA), superoxide dismutase (sodB), and somatic antigen expression (rfa and rfb); (ii) cytotoxins that contribute to the severity of disease, e.g., the Shiga toxin (stx) and a putative analog of this toxin (flu); and (iii) regulatory loci that affect the expression of plasmid genes, e.g., ompR-envZ, which mediates response to changes in osmolarity, virR (osmZ), which mediates response to changes in temperature, and kcpA, which affects the translation of the plasmid virG (icsA) gene which is associated with intracellular bacterial mobility and intracellular bacterial spread. A single plasmid regulatory gene (virF) controls a virulence-associated plasmid regulon including virG (icsA) and two invasion-related loci, i.e., (i) ipaABCD, encoding invasion plasmid antigens that may be structural components of the Shigella invasion determinant; and (ii) invAKJH (mxi), which is necessary for insertion of invasion plasmid antigens into the outer membrane.