Cytosolic delivery and characterization of the TcdB glucosylating domain by using a heterologous protein fusion

Neutrophil inflammasomes sense the subcellular delivery route of translocated bacterial effectors and toxins

In neutrophils, caspase-11 cleaves gasdermin D (GSDMD), causing pyroptosis to clear cytosol-invasive bacteria. In contrast, caspase-1 also cleaves GSDMD but seems to not cause pyroptosis. Here, we show that this pyroptosis-resistant caspase-1 activation is specifically programmed by the site of translocation of the detected microbial virulence factors. We find that pyrin and NLRC4 agonists do not trigger pyroptosis in neutrophils when they access the cytosol from endosomal compartment. In contrast, when the same ligands penetrate through the plasma membrane, they cause pyroptosis. Consistently, pyrin detects extracellular Yersinia pseudotuberculosis ΔyopM in neutrophils, driving caspase-1-GSDMD pyroptosis. This pyroptotic response drives PAD4-dependent H3 citrullination and results in extrusion of neutrophil extracellular traps (NETs). Our data indicate that caspase-1, GSDMD, or PAD4 deficiency renders mice more susceptible to Y. pseudotuberculosis ΔyopM infection. Therefore, neutrophils induce pyroptosis in response to caspase-1-activating inflammasomes triggered by extracellular bacterial pathogens, but after they phagocytose pathogens, they are programmed to forego pyroptosis.

Anthrax toxin channel: What we know based on over 30 years of research

Protective antigen channel is the central component of the deadly anthrax exotoxin responsible for binding and delivery of the toxin's enzymatic lethal and edema factor components into the cytosol. The channel, which is more than three times longer than the lipid bilayer membrane thickness and has a 6-Å limiting diameter, is believed to provide a sophisticated unfoldase and translocase machinery for the foreign protein transport into the host cell cytosol. The tripartite toxin can be reengineered, one component at a time or collectively, to adapt it for the targeted cancer therapeutic treatments. In this review, we focus on the biophysical studies of the protective antigen channel-forming activity, small ion transport properties, enzymatic factor translocation, and blockage comparing it with the related clostridial binary toxin channels. We address issues linked to the anthrax toxin channel structural dynamics and lipid dependence, which are yet to become generally recognized as parts of the toxin translocation machinery.

The bacterial Ras/Rap1 site-specific endopeptidase RRSP cleaves Ras through an atypical mechanism to disrupt Ras-ERK signaling

The Ras-extracellular signal-regulated kinase pathway is critical for controlling cell proliferation, and its aberrant activation drives the growth of various cancers. Because many pathogens produce toxins that inhibit Ras activity, efforts to develop effective Ras inhibitors to treat cancer could be informed by studies of Ras inhibition by pathogens. Vibrio vulnificus causes fatal infections in a manner that depends on multifunctional autoprocessing repeats-in-toxin, a toxin that releases bacterial effector domains into host cells. One such domain is the Ras/Rap1-specific endopeptidase (RRSP), which site-specifically cleaves the Switch I domain of the small GTPases Ras and Rap1. We solved the crystal structure of RRSP and found that its backbone shares a structural fold with the EreA/ChaN-like superfamily of enzymes. Unlike other proteases in this family, RRSP is not a metalloprotease. Through nuclear magnetic resonance analysis and nucleotide exchange assays, we determined that the processing of KRAS by RRSP did not release any fragments or cause KRAS to dissociate from its bound nucleotide but instead only locally affected its structure. However, this structural alteration of KRAS was sufficient to disable guanine nucleotide exchange factor-mediated nucleotide exchange and prevent KRAS from binding to RAF. Thus, RRSP is a bacterial effector that represents a previously unrecognized class of protease that disconnects Ras from its signaling network while inducing limited structural disturbance in its target.

The Binary Toxin CDT of Clostridium difficile as a Tool for Intracellular Delivery of Bacterial Glucosyltransferase Domains

Binary toxins are produced by several pathogenic bacteria. Examples are the C2 toxin from Clostridium botulinum, the iota toxin from Clostridium perfringens, and the CDT from Clostridium difficile. All these binary toxins have ADP-ribosyltransferases (ADPRT) as their enzymatically active component that modify monomeric actin in their target cells. The binary C2 toxin was intensively described as a tool for intracellular delivery of allogenic ADPRTs. Here, we firstly describe the binary toxin CDT from C. difficile as an effective tool for heterologous intracellular delivery. Even 60 kDa glucosyltransferase domains of large clostridial glucosyltransferases can be delivered into cells. The glucosyltransferase domains of five tested large clostridial glucosyltransferases were successfully introduced into cells as chimeric fusions to the CDTa adapter domain (CDTaN). Cell uptake was demonstrated by the analysis of cell morphology, cytoskeleton staining, and intracellular substrate glucosylation. The fusion toxins were functional only when the adapter domain of CDTa was N-terminally located, according to its native orientation. Thus, like other binary toxins, the CDTaN/b system can be used for standardized delivery systems not only for bacterial ADPRTs but also for a variety of bacterial glucosyltransferase domains.

Induced autoprocessing of the cytopathic Makes caterpillars floppy-like effector domain of the Vibrio vulnificus MARTX toxin

The multifunctional-autoprocessing repeats-in-toxin (MARTX(Vv)) toxin that harbours a varied repertoire of effector domains is the primary virulence factor of Vibrio vulnificus. Although ubiquitously present among Biotype I toxin variants, the 'Makes caterpillars floppy-like' effector domain (MCF(Vv)) is previously unstudied. Using transient expression and protein delivery, MCF(Vv) and MCF(Ah) from the Aeromonas hydrophila MARTX(Ah)) toxin are shown for the first time to induce cell rounding. Alanine mutagenesis across the C-terminal subdomain of MCF(Vv) identified an Arg-Cys-Asp (RCD) tripeptide motif shown to comprise a cysteine protease catalytic site essential for autoprocessing of MCF(Vv). The autoprocessing could be recapitulated in vitro by the addition of host cell lysate to recombinant MCF(Vv), indicating induced autoprocessing by cellular factors. The RCD motif is also essential for cytopathicity, suggesting autoprocessing is essential first to activate the toxin and then to process a cellular target protein resulting in cell rounding. Sequence homology places MCF(Vv) within the C58 cysteine protease family that includes the type III secretion effectors YopT from Yersinia spp. and AvrPphB from Pseudomonas syringae. However, the catalytic site RCD motif is unique compared with other C58 peptidases and is here proposed to represent a new subgroup of autopeptidase found within a number of putative large bacterial toxins.

Channel-forming bacterial toxins in biosensing and macromolecule delivery

To intoxicate cells, pore-forming bacterial toxins are evolved to allow for the transmembrane traffic of different substrates, ranging from small inorganic ions to cell-specific polypeptides. Recent developments in single-channel electrical recordings, X-ray crystallography, protein engineering, and computational methods have generated a large body of knowledge about the basic principles of channel-mediated molecular transport. These discoveries provide a robust framework for expansion of the described principles and methods toward use of biological nanopores in the growing field of nanobiotechnology. This article, written for a special volume on "Intracellular Traffic and Transport of Bacterial Protein Toxins", reviews the current state of applications of pore-forming bacterial toxins in small- and macromolecule-sensing, targeted cancer therapy, and drug delivery. We discuss the electrophysiological studies that explore molecular details of channel-facilitated protein and polymer transport across cellular membranes using both natural and foreign substrates. The review focuses on the structurally and functionally different bacterial toxins: gramicidin A of Bacillus brevis, α-hemolysin of Staphylococcus aureus, and binary toxin of Bacillus anthracis, which have found their "second life" in a variety of developing medical and technological applications.

Cytotoxicity of the Vibrio vulnificus MARTX toxin effector DUF5 is linked to the C2A subdomain

The multifunctional-autoprocessing repeats-in-toxin (MARTX) toxins are bacterial protein toxins that serve as delivery platforms for cytotoxic effector domains. The domain of unknown function in position 5 (DUF5) effector domain is present in at least six different species' MARTX toxins and as a hypothetical protein in Photorhabdus spp. Its presence increases the potency of the Vibrio vulnificus MARTX toxin in mouse virulence studies, indicating DUF5 directly contributes to pathogenesis. In this work, DUF5 is shown to be cytotoxic when transiently expressed in HeLa cells. DUF5 localized to the plasma membrane dependent upon its C1 domain and the cells become rounded dependent upon its C2 domain. Both full-length DUF5 and the C2 domain caused growth inhibition when expressed in Saccharomyces cerevisiae. A structural model of DUF5 was generated based on the structure of Pasteurella multocida toxin facilitating localization of the cytotoxic activity to a 186 amino acid subdomain termed C2A. Within this subdomain, an alanine scanning mutagenesis revealed aspartate-3721 and arginine-3841 as residues critical for cytotoxicity. These residues were also essential for HeLa cell intoxication when purified DUF5 fused to anthrax toxin lethal factor was delivered cytosolically. Thermal shift experiments indicated that these conserved residues are important to maintain protein structure, rather than for catalysis. The Aeromonas hydrophila MARTX toxin DUF5(Ah) domain was also cytotoxic, while the weakly conserved C1-C2 domains from P. multocida toxin were not. Overall, this study is the first demonstration that DUF5 as found in MARTX toxins has cytotoxic activity that depends on conserved residues in the C2A subdomain.

Vibrio vulnificus biotype 3 multifunctional autoprocessing RTX toxin is an adenylate cyclase toxin essential for virulence in mice

Vibrio vulnificus is an environmental organism that causes both food-borne and wound infections with high morbidity and mortality in humans. The annual incidence and global distribution of infections associated with this pathogen are increasing with climate change. In the late 1990s, an outbreak of tilapia-associated wound infections in Israel was linked to a previously unrecognized variant of V. vulnificus designated biotype 3. The sudden emergence and clonality of the outbreak suggest that this strain may be a true newly emergent pathogen with novel virulence properties compared to those of other V. vulnificus strains. In a subcutaneous infection model to mimic wound infection, the multifunctional autoprocessing RTX (MARTX) toxin of biotype 3 strains was shown to be an essential virulence factor contributing to highly inflammatory skin wounds with severe damage affecting every tissue layer. We conducted a sequencing-based analysis of the MARTX toxin and found that biotype 3 MARTX toxin has an effector domain structure distinct from that of either biotype 1 or biotype 2. Of the two new domains identified, a domain similar to Pseudomonas aeruginosa ExoY was shown to confer adenylate cyclase activity on the MARTX toxin. This is the first demonstration that the biotype 3 MARTX toxin is essential for virulence and that the ExoY-like MARTX effector domain is a catalytically active adenylate cyclase.

Identification of a His-Asp-Cys catalytic triad essential for function of the Rho inactivation domain (RID) of Vibrio cholerae MARTX toxin

Vibrio cholerae is the causative agent of the severe diarrheal disease cholera. For V. cholerae to colonize the intestinal epithelium, accessory toxins such as the multifunctional autoprocessing repeats-in-toxin (MARTX(Vc)) toxin are required. MARTX toxins are composite toxins comprised of arrayed effector domains that carry out distinct functions inside the host cell. Among the three effector domains of MARTX(Vc) is the Rho inactivation domain (RID(Vc)) known to cause cell rounding through inactivation of small RhoGTPases. Using alanine scanning mutagenesis in the activity subdomain of RID(Vc), four residues, His-2782, Leu-2851, Asp-2854, and Cys-3022, were identified as impacting RID(Vc) function in depolymerization of the actin cytoskeleton and inactivation of RhoA. Tyr-2807 and Tyr-3015 were identified as important potentially for forming the active structure for substrate contact but are not involved in catalysis or post translational modifications. Finally, V. cholerae strains modified to carry a catalytically inactive RID(Vc) show that the rate and efficiency of MARTX(Vc) actin cross-linking activity does not depend on a functional RID(Vc), demonstrating that these domains function independently in actin depolymerization. Overall, our results indicate a His-Asp-Cys catalytic triad is essential for function of the RID effector domain family shared by MARTX toxins produced by many Gram-negative bacteria.

Serum response factor regulates immediate early host gene expression in Toxoplasma gondii-infected host cells

Toxoplasma gondii is a wide spread pathogen that can cause severe and even fatal disease in fetuses and immune-compromised hosts. As an obligate intracellular parasite, Toxoplasma must alter the environment of its host cell in order to establish its replicative niche. This is accomplished, in part, by secretion of factors into the host cell that act to modulate processes such as transcription. Previous studies demonstrated that genes encoding transcription factors such as c-jun, junB, EGR1, and EGR2 were amongst the host genes that were the most rapidly upregulated following infection. In cells stimulated with growth factors, these genes are regulated by a transcription factor named Serum Response Factor. Serum Response Factor is a ubiquitously expressed DNA binding protein that regulates growth and actin cytoskeleton genes via MAP kinase or actin cytoskeletal signaling, respectively. Here, we report that Toxoplasma infection leads to the rapid activation of Serum Response Factor. Serum Response Factor activation is a Toxoplasma-specific event since the transcription factor is not activated by the closely related protozoan parasite, Neospora caninum. We further demonstrate that Serum Response Factor activation requires a parasite-derived secreted factor that signals via host MAP kinases but independently of the host actin cytoskeleton. Together, these data define Serum Response Factor as a host cell transcription factor that regulates immediate early gene expression in Toxoplasma-infected cells.

Variations in TcdB activity and the hypervirulence of emerging strains of Clostridium difficile

Hypervirulent strains of Clostridium difficile have emerged over the past decade, increasing the morbidity and mortality of patients infected by this opportunistic pathogen. Recent work suggested the major C. difficile virulence factor, TcdB, from hypervirulent strains (TcdB_HV) was more cytotoxic in vitro than TcdB from historical strains (TcdB_HIST). The current study investigated the in vivo impact of altered TcdB tropism, and the underlying mechanism responsible for the differences in activity between the two forms of this toxin. A combination of protein sequence analyses, in vivo studies using a Danio rerio model system, and cell entry combined with fluorescence assays were used to define the critical differences between TcdB_HV and TcdB_HIST. Sequence analysis found that TcdB was the most variable protein expressed from the pathogenicity locus of C. difficile. In line with these sequence differences, the in vivo effects of TcdB_HV were found to be substantially broader and more pronounced than those caused by TcdB_HIST. The increased toxicity of TcdB_HV was related to the toxin's ability to enter cells more rapidly and at an earlier stage in endocytosis than TcdB_HIST. The underlying biochemical mechanism for more rapid cell entry was identified in experiments demonstrating that TcdB_HV undergoes acid-induced conformational changes at a pH much higher than that of TcdB_HIST. Such pH-related conformational changes are known to be the inciting step in membrane insertion and translocation for TcdB. These data provide insight into a critical change in TcdB activity that contributes to the emerging hypervirulence of C. difficile.

Clostridium difficile is a spore-forming bacterium that contaminates hospitals and infects patients undergoing antibiotic therapy. C. difficile is now the leading cause of hospital-acquired diarrhea in developed countries. Most concerning has been the recent increase in mortality of C. difficile patients due to the emergence of a hypervirulent strain of this pathogen. Results from the current study suggest this change in disease severity may be due to new strains producing a variant form of C. difficile's major virulence factor, TcdB. The findings indicate TcdB from hypervirulent strains targets a much broader range of cells in vivo and is able to translocate into target cells more quickly than TcdB from historical strains of C. difficile. The more rapid cell entry by TcdB from hypervirulent C. difficile appears to be due to the toxin's capacity to undergo conformational changes necessary for membrane translocation at a higher pH than TcdB from historical strains. To date, very little has been learned about the underlying reasons for the increased virulence of emerging C. difficile strains. These findings provide insight into this problem and suggest variations in TcdB activity could be an important contributing factor to the hypervirulence of emerging strains of C. difficile.

Identification of a conserved membrane localization domain within numerous large bacterial protein toxins

Vibrio cholerae is the causative agent of the diarrheal disease cholera. Many virulence factors contribute to intestinal colonization and disease including the Multifunctional Autoprocessing RTX toxin (MARTX(Vc)). The Rho-inactivation domain (RID) of MARTX(Vc) is responsible for inactivating the Rho-family of small GTPases, which leads to depolymerization of the actin cytoskeleton. Based on a deletion analysis of RID to determine the minimal functional domain, we have identified a subdomain at the N terminus of RID that is homologous to the membrane targeting C1 domain of Pasteurella multocida toxin. A GFP fusion to this subdomain from RID colocalized with a plasma membrane marker when transiently expressed within HeLa cells and can be found in the membrane fraction following subcellular fractionation. This C1-like subdomain is present in multiple families of bacterial toxins, including all of the clostridial glucosyltransferase toxins and various MARTX toxins. GFP-fusions to these homologous domains are also membrane associated, indicating that this is a conserved membrane localization domain (MLD). We have identified three residues (Y23, S68, R70) as necessary for proper localization of one but not all MLDs. In addition, we found that substitution of the RID MLD with the MLDs from two different effector domains from the Vibrio vulnificus MARTX toxin restored RID activity, indicating that there is functional overlap between these MLDs. This study describes the initial recognition of a family of conserved plasma membrane-targeting domains found in multiple large bacterial toxins.

Actin Crosslinking Toxins of Gram-Negative Bacteria

Actin crosslinking toxins produced by Gram-negative bacteria represent a small but unique class of bacterial protein toxins. For each of these toxins, a discrete actin crosslinking domain (ACD) that is a distant member of the ATP-dependent glutamine synthetase family of protein ligases is translocated to the eukaryotic cell cytosol. This domain then incorporates a glutamate-lysine crosslink between actin monomers, resulting in destruction of the actin cytoskeleton. Recent studies argue that the function of these toxins during infection is not destruction of epithelial layers, but rather may specifically target phagocytic cells to promote survival of bacteria after the onset of innate immune defenses. This review will summarize key experiments performed over the past 10 years to reveal the function of these toxins.

Critical intermediate steps in Clostridium sordellii lethal toxin-induced apoptosis

Clostridium sordellii lethal toxin (TcsL) inactivates small GTPases via glucosylation and induces apoptosis in mammalian cells; however, signaling events that link substrate modification with modulation of the mitochondria in these cells has not been determined. Experiments in the current study examined TcsL modulation of the Akt signaling pathway and related downstream targets. Early in TcsL intoxication, cells demonstrated a dramatic decrease in phosphorylated Akt, and this event required toxin enzymatic activity. The decrease in phosphorylated Akt was followed by caspase-dependent processing of Bcl-x(L) and Bid, revealing the connection between GTPase inactivation and mitochondrial-mediated apoptosis observed in TcsL-intoxicated cells. Levels of glycogen synthase kinase-3beta declined during later times of TcsL intoxication, suggesting a second intermediate step in apoptosis. Collectively, these data provide insight into the cascade of signaling events that lead to apoptotic death of TcsL-intoxicated cells.

Inactivation of small Rho GTPases by the multifunctional RTX toxin from Vibrio cholerae

Many bacterial toxins target small Rho GTPases in order to manipulate the actin cytoskeleton. The depolymerization of the actin cytoskeleton by the Vibrio cholerae RTX toxin was previously identified to be due to the unique mechanism of covalent actin cross-linking. However, identification and subsequent deletion of the actin cross-linking domain within the RTX toxin revealed that this toxin has an additional cell rounding activity. In this study, we identified that the multifunctional RTX toxin also disrupts the actin cytoskeleton by causing the inactivation of small Rho GTPases, Rho, Rac and Cdc42. Inactivation of Rho by RTX was reversible in the presence of cycloheximide and by treatment of cells with CNF1 to constitutively activate Rho. These data suggest that RTX targets Rho GTPase regulation rather than affecting Rho GTPase directly. A novel 548-amino-acid region of RTX was identified to be responsible for the toxin-induced inactivation of the Rho GTPases. This domain did not carry GAP or phosphatase activities. Overall, these data show that the RTX toxin reversibly inactivates Rho GTPases by a mechanism distinct from other Rho-modifying bacterial toxins.

Identification of Clostridium difficile toxin B cardiotoxicity using a zebrafish embryo model of intoxication

Clostridium difficile toxin B (TcdB) has been studied extensively by using cell-free systems and tissue culture, but, like many bacterial toxins, the in vivo targets of TcdB are unknown and have been difficult to elucidate with traditional animal models. In the current study, the transparent Danio rerio (zebrafish) embryo was used as a model for imaging of in vivo TcdB localization and organ-specific damage in real time. At 24 h after treatment, TcdB was found to localize at the pericardial region, and zebrafish exhibited the first signs of cardiovascular damage, including a 90% reduction in systemic blood flow and a 20% reduction in heart rate. Within 72 h of exposure to TcdB, the ventricle chamber of the heart became deformed and was unable to contract or pump blood, and the fish exhibited extensive pericardial edema. In line with the observed defects in ventricle contraction, TcdB was found to directly disrupt coordinated contractility and rhythmicity in primary cardiomyocytes. Furthermore, using a caspase-3 inhibitor, we were able to block TcdB-related cardiovascular damage and prevent zebrafish death. These findings present an insight into the in vivo targets of TcdB, as well as demonstrate the strength of the zebrafish embryo as a tractable model for identification of in vivo targets of bacterial toxins and evaluation of novel candidate therapeutics.

The Actin cross-linking domain of the Vibrio cholerae RTX toxin directly catalyzes the covalent cross-linking of actin

Vibrio cholerae is a Gram-negative bacterial pathogen that exports enterotoxins to alter host cells and to elicit diarrheal disease. Among the secreted toxins is the multifunctional RTX toxin, which causes cell rounding and actin depolymerization by covalently cross-linking actin monomers into dimers, trimers, and higher multimers. The region of the toxin responsible for cross-linking activity is the actin cross-linking domain (ACD). In this study, we further investigated the role of the ACD in the actin cross-linking reaction. We show that the RTX toxin cross-links actin independently of tissue transglutaminase, thus eliminating an indirect model of ACD activity. We demonstrate that a fusion protein of the ACD and the N-terminal portion of lethal factor from Bacillus anthracis (LF(N)ACD) has cross-linking activity in vivo and in crude cell extracts. Furthermore, we determined that LF(N)ACD directly catalyzes the formation of covalent linkages between actin molecules in vitro and that Mg(2+) and ATP are essential cofactors for the cross-linking reaction. In addition, G-actin is proposed as a cytoskeletal substrate of the RTX toxin in vivo. Future studies of the in vitro cross-linking reaction will facilitate characterization of the enzymatic properties of the ACD and contribute to our knowledge of the novel mechanism of covalent actin cross-linking.

Clostridium difficile toxins: mechanism of action and role in disease

As the leading cause of hospital-acquired diarrhea, Clostridium difficile colonizes the large bowel of patients undergoing antibiotic therapy and produces two toxins, which cause notable disease pathologies. These two toxins, TcdA and TcdB, are encoded on a pathogenicity locus along with negative and positive regulators of their expression. Following expression and release from the bacterium, TcdA and TcdB translocate to the cytosol of target cells and inactivate small GTP-binding proteins, which include Rho, Rac, and Cdc42. Inactivation of these substrates occurs through monoglucosylation of a single reactive threonine, which lies within the effector-binding loop and coordinates a divalent cation critical to binding GTP. By glucosylating small GTPases, TcdA and TcdB cause actin condensation and cell rounding, which is followed by death of the cell. TcdA elicits effects primarily within the intestinal epithelium, while TcdB has a broader cell tropism. Important advances in the study of these toxins have been made in the past 15 years, and these are detailed in this review. The domains, subdomains, and residues of these toxins important for receptor binding and enzymatic activity have been elegantly studied and are highlighted herein. Furthermore, there have been major advances in defining the role of these toxins in modulating the inflammatory events involving the disruption of cell junctions, neuronal activation, cytokine production, and infiltration by polymorphonuclear cells. Collectively, the present review provides a comprehensive update on TcdA and TcdB's mechanism of action as well as the role of these toxins in disease.

Comparison of wild type with recombinant Clostridium difficile toxin A

Toxins A and B from Clostridium difficile are single-chain proteins of 308,000 and 270,000 Da, respectively. They possess transferase activity to monoglucosylate proteins of the Rho GTPase family whereby Rho, Rac, and Cdc42 are the canonical substrates. For application of these toxins as specific Rho GTPase inhibitors the highest possible purity is of crucial interest. We, therefore, expressed recombinant His-tagged toxin A using the Bacillus megaterium expression system. Specific antisera raised against the native toxin A from C. difficile and the recombinant toxin, respectively, showed identical sensitivity and specificity in Western blot and ELISA analyses towards both toxins. By comparison of both toxins in functional studies we showed that the recombinant toxin was about two times more cytotoxic than the native toxin, and the glucosyltransferase-activity of the recombinant toxin was even 10-fold increased. However, recombinant toxin A showed one essential difference to the classically purified one. The reported transferase-independent effect of toxin A to release cytochrome c from isolated mitochondria was not exhibited by the recombinant toxin A. This putative mitochondrial effect decreased with increased purity of toxin A, and was absent with recombinant toxin, strongly suggesting an clostridial contamination responsible. In summary, we tested the recombinant toxin A to be at least an adequate substitute for the native toxin, bearing the advantage of a rapid single-step purification and the absence of biological active contaminations.

Large clostridial cytotoxins

The large clostridial cytotoxins are a family of structurally and functionally related exotoxins from Clostridium difficile (toxins A and B), C. sordellii (lethal and hemorrhagic toxin) and C. novyi (alpha-toxin). The exotoxins are major pathogenicity factors which in addition to their in vivo effects are cytotoxic to cultured cell lines causing reorganization of the cytoskeleton accompanied by morphological changes. The exotoxins are single-chain protein toxins, which are constructed of three domains: receptor-binding, translocation and catalytic domain. These domains reflect the self-mediated cell entry via receptor-mediated endocytosis, translocation into the cytoplasm, and execution of their cytotoxic activity by an inherent enzyme activity. Enzymatically, the toxins catalyze the transfer of a glucosyl moiety from UDP-glucose to the intracellular target proteins which are the Rho and Ras GTPases. The covalent attachment of the glucose moiety to a conserved threonine within the effector region of the GTPases renders the Rho-GTPases functionally inactive. Whereas the molecular mode of cytotoxic effects is fully understood, the mechanisms leading to inflammatory processes in the context of disease (e.g., antibiotic-associated pseudomembranous colitis caused by Clostridium difficile) are less clear.

Clostridium sordellii lethal toxin is maintained in a multimeric protein complex

Clostridium sordellii lethal toxin (TcsL) is distinct among large clostridial toxins (LCTs), as it is markedly reduced in its rate of intoxication at pH 8.0 yet is cytotoxic at pH 4.0. Results from the present study suggest that TcsL's slow rate of intoxication at pH 8.0 is linked to formation of a high-molecular-weight complex containing dissociable pH 4.0-sensitive polypeptides. The cytosolic delivery of TcsL's enzymatic domain by using a surrogate cell entry system resulted in cytopathic effect rates similar to those of other LCTs at pH 8.0, further indicating that rate-limiting steps occurred at the point of cell entry. Since these rate-limiting steps could be overcome at pH 4.0, TcsL was examined across a range of pH values and was found to dissociate into distinct 45- to 55-kDa polypeptides between pH 4.0 and pH 5.0. The polypeptides reassociated when shifted back to pH 8.0. At pH 8.0, this complex was resistant to sodium dodecyl sulfate (SDS) and multiple proteases; however, following dissociation, the polypeptides became protease sensitive. Dissociation of TcsL, and cytotoxicity, could be blocked by preincubation with ethylene glycol bis(sulfosuccinimidylsuccinate), resulting in cross-linking of the polypeptides. TcsL was also examined at pH 8.0 by using SDS-agarose gel electrophoresis and transmission electron microscopy and was found to exist in a higher-molecular-weight complex which resolved at a size exceeding 750 kDa and also dissociated at pH 4.0. However, this complex did not reassemble following a shift back to pH 8.0. Collectively, these data suggest that TcsL is maintained in a protease-resistant, high-molecular-weight complex, which dissociates at pH 4.0, leading to cytotoxicity.

Cellular uptake of Clostridium difficile toxin B. Translocation of the N-terminal catalytic domain into the cytosol of eukaryotic cells

Clostridium difficile toxin B (269 kDa) is one of the causative agents of antibiotic-associated diarrhea and pseudomembranous colitis. Toxin B acts in the cytosol of eukaryotic target cells where it inactivates Rho GTPases by monoglucosylation. The catalytic domain of toxin B is located at the N terminus (amino acid residues 1-546). The C-terminal and the middle region of the toxin seem to be involved in receptor binding and translocation. Here we studied whether the full-length toxin or only a part of the holotoxin is translocated into the cytosol. Vero cells were treated with recombinant glutathione S-transferase-toxin B, and thereafter, toxin B fragments were isolated by affinity precipitation of the glutathione S-transferase-tagged protein from the cytosolic fraction of intoxicated cells. The toxin fragment (approximately 65 kDa) was recognized by an antibody against the N terminus of toxin B and was identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry analysis as the catalytic domain of toxin B. The toxin fragment located in the cytosol possessed glucosyltransferase activity that could modify RhoA in vitro, but it was not able to intoxicate intact cells. After treatment of Vero cells with a radiolabeled fragment of toxin B (amino acid residues 547-2366), radioactivity was identified in the membrane fraction of Vero cells but not in the cytosolic fraction of Vero cells. Furthermore, analysis of cells by fluorescence microscopy revealed that the C terminus of toxin B was located in endosomes, whereas the N terminus was detected in the cytosol. Protease inhibitors, which were added to the cell medium, delayed intoxication of cells by toxin B and pH-dependent translocation of the toxin from the cell surface across the cell membrane. The data indicate that toxin B is proteolytically processed during its cellular uptake process.

Mutational analysis of the enzymatic domain of Clostridium difficile toxin B reveals novel inhibitors of the wild-type toxin

Toxin B (TcdB), a major Clostridium difficile virulence factor, glucosylates and inactivates the small GTP-binding proteins Rho, Rac, and Cdc42. In the present study we provide evidence that enzymatically inactive fragments of the TcdB enzymatic domain are effective intracellular inhibitors of native TcdB. Site-directed and deletion mutants of the TcdB enzymatic region (residues 1 to 556), lacking receptor binding and cell entry domains, were analyzed for attenuation of glucosyltransferase and glucosylhydrolase activity. Five of six derivatives from TcdB(1-556) were found to be devoid of enzymatic activity. In order to facilitate cell entry, mutants were genetically fused to lfn, which encodes the protective antigen binding region of anthrax toxin lethal factor and mediates the cell entry of heterologous proteins. In line with reduced enzymatic activity, the mutants also lacked cytotoxicity. Remarkably, pretreatment or cotreatment of cells with four of the mutants provided protection against the cytotoxic effects of native TcdB. Furthermore, a CHO cell line expressing enzymatically active TcdB(1-556) was also protected by the mutant-derived inhibitors, suggesting that inhibition occurred at an intracellular location. Protection also was afforded by the inhibitor to cells treated with Clostridium sordellii lethal toxin (TcsL), which uses the same cosubstrate as TcdB but shares Rac only as a common substrate target. Finally, the inhibitor did not provide protection against Clostridium novyi alpha-toxin (Tcnalpha), which shares similar substrates with TcdB yet uses a different cosubstrate. This is the first report to demonstrate that the potential exists to inhibit toxins at their intracellular site of action by using inactive mutants.

Clostridium difficile toxin B activates dual caspase-dependent and caspase-independent apoptosis in intoxicated cells

Clostridium difficile toxin B (TcdB) inactivates the small GTPases Rho, Rac and Cdc42 during intoxication of mammalian cells. In the current work, we show that TcdB has the potential to stimulate caspase-dependent and caspase-independent apoptosis. The apoptotic pathways became evident when caspase-3-processed-vimentin was detected in TcdB-treated HeLa cells. Caspase-3 activation was subsequently confirmed in TcdB-intoxicated HeLa cells. Interestingly, caspase inhibitor delayed TcdB-induced cell death, but did not alter the time-course of cytopathic effects. A similar effect was also observed in MCF-7 cells, which are deficient in caspase-3 activity. The time-course to cell death was almost identical between cells treated with TcdB plus caspase inhibitor and cells intoxicated with the TcdB enzymatic domain (TcdB1-556). Unlike TcdB treated cells, intoxication with TcdB1-556 or expression of TcdB1-556 in a transfected cell line, did not stimulate caspase-3 activation yet cells exhibited cytopathic effects and cell death. Although TcdB1-556 treated cells did not demonstrate caspase-3 activation these cells were apoptotic as determined by differential annexin-V/propidium iodide staining and nucleosomal DNA fragmentation. These data indicate TcdB triggers caspase-independent apoptosis as a result of substrate inactivation and may evoke caspase-dependent apoptosis due to a second, yet undefined, activity of TcdB. This is the first example of a bacterial virulence factor with the potential to stimulate multiple apoptotic pathways in host cells.