Chondroitin sulfate proteoglycan 4 functions as the cellular receptor for Clostridium difficile toxin B

The Conserved Cys-2232 in Clostridioides difficile Toxin B Modulates Receptor Binding

Clostridioides difficile toxins TcdA and TcdB are large clostridial glucosyltransferases which are the main pathogenicity factors in C. difficile-associated diseases. Four highly conserved cysteines are present in all large clostridial glucosyltransferases. In this study we focused on the conserved cysteine 2232 within the combined repetitive oligopeptide domain of TcdB from reference strain VPI10463 (clade I). Cysteine 2232 is not present in TcdB from hypervirulent strain R20291 (clade II), where a tyrosine is found instead. Replacement of cysteine 2232 by tyrosine in TcdB_{V PI10463} reduced binding to the soluble fragments of the two known TcdB receptors, frizzled-2 (FZD2) and poliovirus receptor-like protein-3/nectin-3 (PVRL3). In line with this, TcdB_R20291 showed weak binding to PVRL3 in pull-down assays which was increased when tyrosine 2232 was exchanged for cysteine. Surprisingly, we did not observe binding of TcdB_R20291 to FZD2, indicating that this receptor is less important for this toxinotype. Competition assay with the receptor binding fragments (aa 1101–1836) of TcdB_{V PI10463} and TcdB_R20291, as well as antibodies newly developed by antibody phage display, revealed different characteristics of the yet poorly described delivery domain of TcdB harboring the second receptor binding region. In summary, we found that conserved Cys-2232 in TcdB indirectly contributes to toxin–receptor interaction.

The chaperonin TRiC/CCT is essential for the action of bacterial glycosylating protein toxins like Clostridium difficile toxins A and B

Various bacterial protein toxins, including Clostridium difficile toxins A (TcdA) and B (TcdB), attack intracellular target proteins of host cells by glucosylation. After receptor binding and endocytosis, the toxins are translocated into the cytosol, where they modify target proteins (e.g., Rho proteins). Here we report that the activity of translocated glucosylating toxins depends on the chaperonin TRiC/CCT. The chaperonin subunits CCT4/5 directly interact with the toxins and enhance the refolding and restoration of the glucosyltransferase activities of toxins after heat treatment. Knockdown of CCT5 by siRNA and HSF1A, an inhibitor of TRiC/CCT, blocks the cytotoxic effects of TcdA and TcdB. In contrast, HSP90, which is involved in the translocation and uptake of ADP ribosylating toxins, is not involved in uptake of the glucosylating toxins. We show that the actions of numerous glycosylating toxins from various toxin types and different species depend on TRiC/CCT. Our data indicate that the TRiC/CCT chaperonin system is specifically involved in toxin uptake and essential for the action of various glucosylating protein toxins acting intracellularly on target proteins.

Marine glycosaminoglycan-like carbohydrates as potential drug candidates for infectious disease

Glycosaminoglycans (GAGs), present in the extracellular matrix, are exploited by numerous, distinct microbes for cellular attachment, adhesion, invasion and evasion of the host immune system. Glycosaminoglycans, including the widely used, clinical anticoagulant heparin and semi-synthetic analogues thereof, have been reported to inhibit and disrupt interactions between microbial proteins and carbohydrates present on the surface of host cells. However, the anticoagulant properties of unmodified, pharmaceutical heparin preparations preclude their capabilities as therapeutics for infectious disease states. Here, unique Glycosaminoglycan-like saccharides from various, distinct marine species are reported for their potential use as therapeutics against infectious diseases; many of which possess highly attenuated anticoagulant activities, while retaining significant antimicrobial properties.

Expression and (Lacking) Internalization of the Cell Surface Receptors of Clostridioides difficile Toxin B

Toxin-producing strains of Clostridioides difficile and Clostridium perfringens cause infections of the gastrointestinal tract in humans and ruminants, with the toxins being major virulence factors, essential for the infection, and responsible for the onset of severe symptoms. C. difficile toxin A (TcdA) and toxin B (TcdB), and the large cytotoxin (TpeL) from C. perfringens are single chain bacterial protein toxins with an AB-like toxin structure. The C-terminal delivery domain mediates cell entry of the N-terminal glycosyltransferase domain by receptor-mediated endocytosis. Several cell surface proteins have been proposed to serve as toxin receptors, including chondroitin-sulfate proteoglycan 4 (CSPG4), poliovirus receptor-like 3 (PVRL3), and frizzled-1/2/7 (FZD1/2/7) for TcdB and LDL-receptor-related protein-1 (LRP1) for TpeL. The expression of the TcdB receptors was investigated in human intestinal organoids (HIOs) and in cultured cell lines. HIOs from four human donors exhibited a comparable profile of receptor expression, with PVRL3, LRP1, and FZD7 being expressed and CSPG4 and FZD2 not being expressed. In human epithelial Caco-2 cells and HT29 cells as well as in immortalized murine fibroblasts, either receptor FZD2/7, CSPG4, PVRL3, and LRP1 was expressed. The question whether the toxins take advantage of the normal turnover of their receptors (i.e., constitutive endocytosis and recycling) from the cell surface or whether the toxins activity induce the internalization of their receptors has not yet been addressed. For the analysis of receptor internalization, temperature-induced uptake of biotinylated toxin receptors into immortalized mouse embryonic fibroblasts (MEFs) and Caco-2 cells was exploited. Solely LRP1 exhibited constitutive endocytosis from the plasma membrane to the endosome, which might be abused by TpeL (and possibly TcdB as well) for cell entry. Furthermore, internalization of CSPG4, PVRL3, FZD2, and FZD7 was observed neither in MEFs nor in Caco-2 cells. FZD2/7, CSPG4, and PVRL3 did thus exhibit no constitutive recycling. The presence of TcdB and the p38 activation induced by anisomycin were not able to induce or enhance CSPG4 or PVRL3 uptake in MEFs. In conclusion, FZD2/7, CSPG4, and PVRL3 seem to serve as cell surface binding receptors rather than internalizing receptors of TcdB.

Early cell death induced by Clostridium difficile TcdB: Uptake and Rac1-glucosylation kinetics are decisive for cell fate

Toxin A and Toxin B (TcdA/TcdB) are large glucosyltransferases produced by Clostridium difficile. TcdB but not TcdA induces reactive oxygen species-mediated early cell death (ECD) when applied at high concentrations. We found that nonglucosylated Rac1 is essential for induction of ECD since inhibition of Rac1 impedes this effect. ECD only occurs when TcdB is rapidly endocytosed. This was shown by generation of chimeras using the trunk of TcdB from a hypervirulent strain. TcdB from hypervirulent strain has been described to translocate from endosomes at higher pH values and thus, meaning faster than reference type TcdB. Accordingly, intracellular delivery of the glucosyltransferase domain of reference TcdB by the trunk of TcdB from hypervirulent strain increased ECD. Furthermore, proton transporters such as sodium/proton exchanger (NHE) or the ClC-5 anion/proton exchanger, both of which contribute to endosomal acidification, also affected cytotoxic potency of TcdB: Specific inhibition of NHE reduced cytotoxicity, whereas transfection of cells with the endosomal anion/proton exchanger ClC-5 increased cytotoxicity of TcdB. Our data suggest that both the uptake rate of TcdB into the cytosol and the status of nonglucosylated Rac1 are key determinants that are decisive for whether ECD or delayed apoptosis is triggered.

Comparative pathogenesis of enteric clostridial infections in humans and animals

Several enteric clostridial diseases can affect humans and animals. Of these, the enteric infections caused by Clostridium perfringens and Clostridium difficile are amongst the most prevalent and they are reviewed here. C. perfringens type A strains encoding alpha toxin (CPA) are frequently associated with enteric disease of many animal mammalian species, but their role in these diseased mammals remains to be clarified. C. perfringens type B encoding CPA, beta (CPB) and epsilon (ETX) toxins causes necro-hemorrhagic enteritis, mostly in sheep, and these strains have been recently suggested to be involved in multiple sclerosis in humans, although evidence of this involvement is lacking. C. perfringens type C strains encode CPA and CPB and cause necrotizing enteritis in humans and animals, while CPA and ETX producing type D strains of C. perfringens produce enterotoxemia in sheep, goats and cattle, but are not known to cause spontaneous disease in humans. The role of C. perfringens type E in animal or human disease remains poorly defined. The newly revised toxinotype F encodes CPA and enterotoxin (CPE), the latter being responsible for food poisoning in humans, and the less prevalent antibiotic associated and sporadic diarrhea. The role of these strains in animal disease has not been fully described and remains controversial. Another newly created toxinotype, G, encodes CPA and necrotic enteritis toxin B-like (NetB), and is responsible for avian necrotic enteritis, but has not been associated with human disease. C. difficile produces colitis and/or enterocolitis in humans and multiple animal species. The main virulence factors of this microorganism are toxins A, B and an ADP-ribosyltransferase (CDT). Other clostridia causing enteric diseases in humans and/or animals are Clostridium spiroforme, Clostridium piliforme, Clostridium colinum, Clostridium sordellii, Clostridium chauvoei, Clostridium septicum, Clostridium botulinum, Clostridium butyricum and Clostridium neonatale. The zoonotic transmission of some, but not all these clostridsial species, has been demonstrated.

Structural basis for recognition of frizzled proteins by Clostridium difficile toxin B

Clostridium difficile infection is the most common cause of antibiotic-associated diarrhea in developed countries. The major virulence factor, C. difficile toxin B (TcdB), targets colonic epithelia by binding to the frizzled (FZD) family of Wnt receptors, but how TcdB recognizes FZDs is unclear. Here, we present the crystal structure of a TcdB fragment in complex with the cysteine-rich domain of human FZD2 at 2.5-angstrom resolution, which reveals an endogenous FZD-bound fatty acid acting as a co-receptor for TcdB binding. This lipid occupies the binding site for Wnt-adducted palmitoleic acid in FZDs. TcdB binding locks the lipid in place, preventing Wnt from engaging FZDs and signaling. Our findings establish a central role of fatty acids in FZD-mediated TcdB pathogenesis and suggest strategies to modulate Wnt signaling.

Cysteine Protease-Mediated Autocleavage of Clostridium difficile Toxins Regulates Their Proinflammatory Activity

BACKGROUND & AIMS

Clostridium difficile toxin A (TcdA) and C difficile toxin toxin B (TcdB), the major virulence factors of the bacterium, cause intestinal tissue damage and inflammation. Although the 2 toxins are homologous and share a similar domain structure, TcdA is generally more inflammatory whereas TcdB is more cytotoxic. The functional domain of the toxins that govern the proinflammatory activities of the 2 toxins is unknown.

METHODS

Here, we investigated toxin domain functions that regulate the proinflammatory activity of C difficile toxins. By using a mouse ilea loop model, human tissues, and immune cells, we examined the inflammatory responses to a series of chimeric toxins or toxin mutants deficient in specific domain functions.

RESULTS

Blocking autoprocessing of TcdB by mutagenesis or chemical inhibition, while reducing cytotoxicity of the toxin, significantly enhanced its proinflammatory activities in the animal model. Furthermore, a noncleavable mutant TcdB was significantly more potent than the wild-type toxin in the induction of proinflammatory cytokines in human colonic tissues and immune cells.

CONCLUSIONS

In this study, we identified a novel mechanism of regulating the biological activities of C difficile toxins in that cysteine protease-mediated autoprocessing regulates toxins' proinflammatory activities. Our findings provide new insight into the pathogenesis of C difficile infection and the design of therapeutics against the disease.

A neutralizing antibody that blocks delivery of the enzymatic cargo of Clostridium difficile toxin TcdB into host cells

Clostridium difficile infection is the leading cause of hospital-acquired diarrhea and is mediated by the actions of two toxins, TcdA and TcdB. The toxins perturb host cell function through a multistep process of receptor binding, endocytosis, low pH-induced pore formation, and the translocation and delivery of an N-terminal glucosyltransferase domain that inactivates host GTPases. Infection studies with isogenic strains having defined toxin deletions have established TcdB as an important target for therapeutic development. Monoclonal antibodies that neutralize TcdB function have been shown to protect against C. difficile infection in animal models and reduce recurrence in humans. Here, we report the mechanism of TcdB neutralization by PA41, a humanized monoclonal antibody capable of neutralizing TcdB from a diverse array of C. difficile strains. Through a combination of structural, biochemical, and cell functional studies, involving X-ray crystallography and EM, we show that PA41 recognizes a single, highly conserved epitope on the TcdB glucosyltransferase domain and blocks productive translocation and delivery of the enzymatic cargo into the host cell. Our study reveals a unique mechanism of C. difficile toxin neutralization by a monoclonal antibody, which involves targeting a process that is conserved across the large clostridial glucosylating toxins. The PA41 antibody described here provides a valuable tool for dissecting the mechanism of toxin pore formation and translocation across the endosomal membrane.

The role of toxins in Clostridium difficile infection

Clostridium difficile is a bacterial pathogen that is the leading cause of nosocomial antibiotic-associated diarrhea and pseudomembranous colitis worldwide. The incidence, severity, mortality and healthcare costs associated with C. difficile infection (CDI) are rising, making C. difficile a major threat to public health. Traditional treatments for CDI involve use of antibiotics such as metronidazole and vancomycin, but disease recurrence occurs in about 30% of patients, highlighting the need for new therapies. The pathogenesis of C. difficile is primarily mediated by the actions of two large clostridial glucosylating toxins, toxin A (TcdA) and toxin B (TcdB). Some strains produce a third toxin, the binary toxin C. difficile transferase, which can also contribute to C. difficile virulence and disease. These toxins act on the colonic epithelium and immune cells and induce a complex cascade of cellular events that result in fluid secretion, inflammation and tissue damage, which are the hallmark features of the disease. In this review, we summarize our current understanding of the structure and mechanism of action of the C. difficile toxins and their role in disease.

Heparan sulfate proteoglycans (HSPGs) and chondroitin sulfate proteoglycans (CSPGs) function as endocytic receptors for an internalizing anti-nucleic acid antibody

A subset of monoclonal anti-DNA autoantibodies enters a variety of living cells. Here, we aimed to identify the endocytic receptors recognized by an internalizing anti-nucleic acid autoantibody, the 3D8 single-chain variable fragment (scFv). We found that cell surface binding and internalization of 3D8 scFv were inhibited markedly in soluble heparan sulfate (HS)/chondroitin sulfate (CS)-deficient or -removed cells and in the presence of soluble HS and CS. 3D8 scFv colocalized intracellularly with either HS proteoglycans (HSPGs) or CSPGs in HeLa cells. 3D8 scFv was co-endocytosed and co-precipitated with representative individual HSPG and CSPG molecules: syndecan-2 (a transmembrane HSPG), glypican-3 (a glycosylphosphatidylinositol (GPI)-anchored HSPG); CD44 (a transmembrane CSPG); and brevican (a GPI-anchored CSPG). Collected data indicate that 3D8 scFv binds to the negatively charged sugar chains of both HSPGs and CSPGs and is then internalized along with these molecules, irrespective of how these proteoglycans are associated with the cell membrane. This is the first study to show that anti-DNA antibodies enter cells via both HSPGs and CSPGs simultaneously. The data may aid understanding of endocytic receptors that bind anti-DNA autoantibodies. The study also provides insight into potential cell membrane targets for macromolecular delivery.

Gleaning Insights from Fecal Microbiota Transplantation and Probiotic Studies for the Rational Design of Combination Microbial Therapies

Beneficial microorganisms hold promise for the treatment of numerous gastrointestinal diseases. The transfer of whole microbiota via fecal transplantation has already been shown to ameliorate the severity of diseases such as Clostridium difficile infection, inflammatory bowel disease, and others. However, the exact mechanisms of fecal microbiota transplant efficacy and the particular strains conferring this benefit are still unclear. Rationally designed combinations of microbial preparations may enable more efficient and effective treatment approaches tailored to particular diseases. Here we use an infectious disease, C. difficile infection, and an inflammatory disorder, the inflammatory bowel disease ulcerative colitis, as examples to facilitate the discussion of how microbial therapy might be rationally designed for specific gastrointestinal diseases. Fecal microbiota transplantation has already shown some efficacy in the treatment of both these disorders; detailed comparisons of studies evaluating commensal and probiotic organisms in the context of these disparate gastrointestinal diseases may shed light on potential protective mechanisms and elucidate how future microbial therapies can be tailored to particular diseases.

Functional defects in Clostridium difficile TcdB toxin uptake identify CSPG4 receptor-binding determinants

Clostridium difficile is a major nosocomial pathogen that produces two exotoxins, TcdA and TcdB, with TcdB thought to be the primary determinant in human disease. TcdA and TcdB are large, multidomain proteins, each harboring a cytotoxic glucosyltransferase domain that is delivered into the cytosol from endosomes via a translocation domain after receptor-mediated endocytosis of toxins from the cell surface. Although there are currently no known host cell receptors for TcdA, three cell-surface receptors for TcdB have been identified: CSPG4, NECTIN3, and FZD1/2/7. The sites on TcdB that mediate binding to each receptor are not defined. Furthermore, it is not known whether the combined repetitive oligopeptide (CROP) domain is involved in or required for receptor binding. Here, in a screen designed to identify sites in TcdB that are essential for target cell intoxication, we identified a region at the junction of the translocation and the CROP domains that is implicated in CSPG4 binding. Using a series of C-terminal truncations, we show that the CSPG4-binding site on TcdB extends into the CROP domain, requiring three short repeats for binding and for full toxicity on CSPG4-expressing cells. Consistent with the location of the CSPG4-binding site on TcdB, we show that the anti-TcdB antibody bezlotoxumab, which binds partially within the first three short repeats, prevents CSPG4 binding to TcdB. In addition to establishing the binding region for CSPG4, this work ascribes for the first time a role in TcdB CROPs in receptor binding and further clarifies the relative roles of host receptors in TcdB pathogenesis.

Amino Acid Differences in the 1753-to-1851 Region of TcdB Influence Variations in TcdB1 and TcdB2 Cell Entry

Clostridium difficile TcdB2 enters cells with a higher efficiency than TcdB1 and exhibits an overall higher level of toxicity. However, the TcdB2-specific sequences that account for more efficient cell entry have not been reported. In this study, we examined the contribution of carboxy-terminal sequence differences to TcdB activity by comparing the binding, uptake, and endosomal localization of TcdB1 and TcdB2 or selected recombinant fragments of these proteins. Our findings suggest that sequence differences in the amino acid 1753 to 1851 region proximal to the combined repetitive oligopeptide domain (CROP) support enhanced uptake of TcdB2 and localization of toxin in acidified endosomes. In the absence of this region, the CROP domains of both forms of the toxin exhibited similar levels of cell interaction, while the addition of amino acids 1753 to 1851 greatly increased toxin binding by only TcdB2. Moreover, the amino acid 1753 to 2366 fragment of TcdB2, but not TcdB1, accumulated to detectable levels in acidified endosomes. Unexpectedly, we discovered an unusual relationship between endocytosis and the efficiency of cell binding for TcdB1 and TcdB2 wherein inhibition of endocytosis by a chemical inhibitor or incubation at a low temperature resulted in a dramatic reduction in cell binding. These findings provide information on sequence variations that may contribute to differences in TcdB1 and TcdB2 toxicity and reveal a heretofore unknown connection between endocytosis and cell binding for this toxin. IMPORTANCE TcdB is a major virulence factor produced by Clostridium difficile, a leading cause of antibiotic-associated diarrhea. Hypervirulent strains of C. difficile encode a variant of TcdB (TcdB2) that is more toxic than toxin derived from historical strains (TcdB1). Though TcdB1 and TcdB2 exhibit 92% overall identity, a 99-amino-acid region previously associated with cell entry and spanning amino acids 1753 to 1851 has only 77% sequence identity. Results from the present study indicate that the substantial sequence variation in this region could contribute to the differences in cell entry between TcdB1 and TcdB2 and possibly explain TcdB2's heightened toxicity. Finally, during the course of these studies, an unusual aspect of TcdB cell entry was discovered wherein cell binding appeared to depend on endocytosis. These findings provide insight into TcdB's variant forms and their mechanisms of cell entry.

Clostridium difficile Toxin Biology

Clostridium difficile is the cause of antibiotics-associated diarrhea and pseudomembranous colitis. The pathogen produces three protein toxins: C. difficile toxins A (TcdA) and B (TcdB), and C. difficile transferase toxin (CDT). The single-chain toxins TcdA and TcdB are the main virulence factors. They bind to cell membrane receptors and are internalized. The N-terminal glucosyltransferase and autoprotease domains of the toxins translocate from low-pH endosomes into the cytosol. After activation by inositol hexakisphosphate (InsP6), the autoprotease cleaves and releases the glucosyltransferase domain into the cytosol, where GTP-binding proteins of the Rho/Ras family are mono-O-glucosylated and, thereby, inactivated. Inactivation of Rho proteins disturbs the organization of the cytoskeleton and affects multiple Rho-dependent cellular processes, including loss of epithelial barrier functions, induction of apoptosis, and inflammation. CDT, the third C. difficile toxin, is a binary actin-ADP-ribosylating toxin that causes depolymerization of actin, thereby inducing formation of the microtubule-based protrusions. Recent progress in understanding of the toxins' actions include insights into the toxin structures, their interaction with host cells, and functional consequences of their actions.

Intrinsic Toxin-Derived Peptides Destabilize and Inactivate Clostridium difficile TcdB

Clostridium difficile infection (CDI) is a major cause of hospital-associated, antibiotic-induced diarrhea, which is largely mediated by the production of two large multidomain clostridial toxins, TcdA and TcdB. Both toxins coordinate the action of specific domains to bind receptors, enter cells, and deliver a catalytic fragment into the cytosol. This results in GTPase inactivation, actin disassembly, and cytotoxicity. TcdB in particular has been shown to encode a region covering amino acids 1753 to 1851 that affects epitope exposure and cytotoxicity. Surprisingly, studies here show that several peptides derived from this region, which share the consensus sequence 1769NVFKGNTISDK1779, protect cells from the action of TcdB. One peptide, PepB2, forms multiple interactions with the carboxy-terminal region of TcdB, destabilizes TcdB structure, and disrupts cell binding. We further show that these effects require PepB2 to form a higher-order polymeric complex, a process that requires the central GN amino acid pair. These data suggest that TcdB1769-1779 interacts with repeat sequences in the proximal carboxy-terminal domain of TcdB (i.e., the CROP domain) to alter the conformation of TcdB. Furthermore, these studies provide insights into TcdB structure and functions that can be exploited to inactivate this critical virulence factor and ameliorate the course of CDI.IMPORTANCEClostridium difficile is a leading cause of hospital-associated illness that is often associated with antibiotic treatment. To cause disease, C. difficile secretes toxins, including TcdB, which is a multidomain intracellular bacterial toxin that undergoes conformational changes during cellular intoxication. This study describes the development of peptide-based inhibitors that target a region of TcdB thought to be critical for structural integrity of the toxin. The results show that peptides derived from a structurally important region of TcdB can be used to destabilize the toxin and prevent cellular intoxication. Importantly, this work provides a novel means of toxin inhibition that could in the future develop into a C. difficile treatment.

Overexpression of the Endosomal Anion/Proton Exchanger ClC-5 Increases Cell Susceptibility toward Clostridium difficile Toxins TcdA and TcdB

Virulent C. difficile toxins TcdA and TcdB invade host intestinal epithelia by endocytosis and use the acidic environment of intracellular vesicles for further processing and activation. We investigated the role of ClC-5, a chloride/proton exchanger expressed in the endosomes of gastrointestinal epithelial cells, in the activation and processing of C. difficile toxins. Enhanced intoxication by TcdA and TcdB was observed in cells expressing ClC-5 but not ClC-4, another chloride/proton exchanger with similar function but different localization. In accordance with the established physiological function of ClC-5, its expression lowered the endosomal pH in HEK293T cells by approximately 0.6 units and enhanced approximately 5-fold the internalization of TcdA. In colon HT29 cells, 34% of internalized TcdA localized to ClC-5-containing vesicles defined by colocalization with Rab5, Rab4a, and Rab7 as early and early-to-late of endosomes but not as Rab11-containing recycling endosomes. Impairing the cellular uptake of TcdA by deleting the toxin CROPs domain did not abolish the effects of ClC-5. In addition, the transport-incompetent mutant ClC-5 E268Q similarly enhanced both endosomal acidification and intoxication by TcdA but facilitated the internalization of the toxin to a lower extent. These data suggest that ClC-5 enhances the cytotoxic action of C. difficile toxins by accelerating the acidification and maturation of vesicles of the early and early-to-late endosomal system. The dispensable role of electrogenic ion transport suggests that the voltage-dependent nonlinear capacitances of mammalian CLC transporters serve important physiological functions. Our data shed light on the intersection between the endocytotic cascade of host epithelial cells and the internalization pathway of the large virulence C. difficile toxins. Identifying ClC-5 as a potential specific host ion transporter hijacked by toxins produced by pathogenic bacteria widens the horizon of possibilities for novel therapies of life-threatening gastrointestinal infections.

Epitopes and Mechanism of Action of the Clostridium difficile Toxin A-Neutralizing Antibody Actoxumab

The exotoxins toxin A (TcdA) and toxin B (TcdB) are produced by the bacterial pathogen Clostridium difficile and are responsible for the pathology associated with C. difficile infection (CDI). The antitoxin antibodies actoxumab and bezlotoxumab bind to and neutralize TcdA and TcdB, respectively. Bezlotoxumab was recently approved by the FDA for reducing the recurrence of CDI. We have previously shown that a single molecule of bezlotoxumab binds to two distinct epitopes within the TcdB combined repetitive oligopeptide (CROP) domain, preventing toxin binding to host cells. In this study, we characterize the binding of actoxumab to TcdA and examine its mechanism of toxin neutralization. Using a combination of approaches including a number of biophysical techniques, we show that there are two distinct actoxumab binding sites within the CROP domain of TcdA centered on identical amino acid sequences at residues 2162-2189 and 2410-2437. Actoxumab binding caused the aggregation of TcdA especially at higher antibody:toxin concentration ratios. Actoxumab prevented the association of TcdA with target cells demonstrating that actoxumab neutralizes toxin activity by inhibiting the first step of the intoxication cascade. This mechanism of neutralization is similar to that observed with bezlotoxumab and TcdB. Comparisons of the putative TcdA epitope sequences across several C. difficile ribotypes and homologous repeat sequences within TcdA suggest a structural basis for observed differences in actoxumab binding and/or neutralization potency. These data provide a mechanistic basis for the protective effects of the antibody in vitro and in vivo, including in various preclinical models of CDI.

Clostridium difficile Toxin A Undergoes Clathrin-Independent, PACSIN2-Dependent Endocytosis

Clostridium difficile infection affects a significant number of hospitalized patients in the United States. Two homologous exotoxins, TcdA and TcdB, are the major virulence factors in C. difficile pathogenesis. The toxins are glucosyltransferases that inactivate Rho family-GTPases to disrupt host cellular function and cause fluid secretion, inflammation, and cell death. Toxicity depends on receptor binding and subsequent endocytosis. TcdB has been shown to enter cells by clathrin-dependent endocytosis, but the mechanism of TcdA uptake is still unclear. Here, we utilize a combination of RNAi-based knockdown, pharmacological inhibition, and cell imaging approaches to investigate the endocytic mechanism(s) that contribute to TcdA uptake and subsequent cytopathic and cytotoxic effects. We show that TcdA uptake and cellular intoxication is dynamin-dependent but does not involve clathrin- or caveolae-mediated endocytosis. Confocal microscopy using fluorescently labeled TcdA shows significant colocalization of the toxin with PACSIN2-positive structures in cells during entry. Disruption of PACSIN2 function by RNAi-based knockdown approaches inhibits TcdA uptake and toxin-induced downstream effects in cells indicating that TcdA entry is PACSIN2-dependent. We conclude that TcdA and TcdB utilize distinct endocytic mechanisms to intoxicate host cells.

Clostridium difficile is a bacterial pathogen that causes nearly half a million infections each year in the United States. It infects the human colon and causes diarrhea, colitis and, in some cases, death. C. difficile infection is mediated by the action of two large homologous toxins, TcdA and TcdB. Disruption of host cell function by these toxins requires entry into cells. There are multiple ways for pathogens and virulence factors such as viruses and toxins to enter host cells. The entry mechanism is often directed by a cell surface receptor and can impact the trafficking and virulence properties of the pathogenic factor. Investigating the internalization strategy can provide critical insight into the mechanism of action for specific pathogens and virulence factors. In our current study, we sought to determine the strategy utilized by TcdA to enter host cells. We show that TcdA uptake occurs by a clathrin- and caveolae-independent endocytic mechanism that is mediated by PACSIN2 and dynamin. We also show that TcdA and TcdB can utilize different routes of entry, which may have implications regarding their cytotoxic mechanisms. In summary, our results provide new insights into the mechanism of cellular intoxication by TcdA and the role of PACSIN2 in endocytosis.

Evidence for dual receptor-binding sites in Clostridium difficile toxin A

TcdA (308 kDa) and TcdB (270 kDa) disrupt the integrity of the intestinal epithelial barrier and provide an environment favorable for Clostridium difficile colonization. Recent evidence suggests that entry of TcdA into cells is mediated by at least two domains. Here, we report the characterization of a second receptor-binding domain (RBD2) for TcdA. While both the isolated combined repetitive oligopeptides (CROPs) and RBD2 fragments are rapidly internalized into cells under physiologic conditions, only the CROPs domain appreciably accumulates at the cell surface. Once internalized, CROPs and RBD2 are trafficked to late endosomal compartments. An internal deletion of RBD2 from TcdA holotoxin ablated toxicity in HT29 cells. These data are consistent with the recently proposed dual receptor model of cellular entry.

Simultaneous generation of multi-gene knockouts in human cells

Genome‐editing techniques enable the generation of gene knockouts in various mammalian cell lines. However, it remains technically challenging to completely disrupt a targeted gene using a canonical method in a timely manner. To improve the efficiency of producing reliable genomic modifications, we designed a method using a linear donor fragment containing a reporter system. Combined with a homologous recombination‐independent knock‐in strategy, we successfully enriched those cell clones that specifically carry the target gene mutations. We observed a much improved success rate when generating single‐ and multiple‐gene knockouts in a one‐step procedure using this special protocol coupled with the CRISPR/Cas9 system. This new approach further empowers the molecular biological study of genes and their functions.

Frizzled proteins are colonic epithelial receptors for C. difficile toxin B

Clostridium difficile toxin B (TcdB) is a critical virulence factor that causes diseases associated with C. difficile infection. Here we carried out CRISPR-Cas9-mediated genome-wide screens and identified the members of the Wnt receptor frizzled family (FZDs) as TcdB receptors. TcdB binds to the conserved Wnt-binding site known as the cysteine-rich domain (CRD), with the highest affinity towards FZD1, 2 and 7. TcdB competes with Wnt for binding to FZDs, and its binding blocks Wnt signalling. FZD1/2/7 triple-knockout cells are highly resistant to TcdB, and recombinant FZD2-CRD prevented TcdB binding to the colonic epithelium. Colonic organoids cultured from FZD7-knockout mice, combined with knockdown of FZD1 and 2, showed increased resistance to TcdB. The colonic epithelium in FZD7-knockout mice was less susceptible to TcdB-induced tissue damage in vivo. These findings establish FZDs as physiologically relevant receptors for TcdB in the colonic epithelium.

Clostridium difficile Toxins TcdA and TcdB Cause Colonic Tissue Damage by Distinct Mechanisms

As the major cause of antibiotic-associated diarrhea, Clostridium difficile is a serious problem in health care facilities worldwide. C. difficile produces two large toxins, TcdA and TcdB, which are the primary virulence factors in disease. The respective functions of these toxins have been difficult to discern, in part because the cytotoxicity profiles for these toxins differ with concentration and cell type. The goal of this study was to develop a cell culture model that would allow a side-by-side mechanistic comparison of the toxins. Conditionally immortalized, young adult mouse colonic (YAMC) epithelial cells demonstrate an exquisite sensitivity to both toxins with phenotypes that agree with observations in tissue explants. TcdA intoxication results in an apoptotic cell death that is dependent on the glucosyltransferase activity of the toxin. In contrast, TcdB has a bimodal mechanism; it induces apoptosis in a glucosyltransferase-dependent manner at lower concentrations and glucosyltransferase-independent necrotic death at higher concentrations. The direct comparison of the responses to TcdA and TcdB in cells and colonic explants provides the opportunity to unify a large body of observations made by many independent investigators.

Recombinant Mucin-Type Fusion Proteins with a Galα1,3Gal Substitution as Clostridium difficile Toxin A Inhibitors

The capability of a recombinant mucin-like fusion protein, P-selectin glycoprotein ligand-1/mouse IgG2b (PSGL-1/mIgG2b), carrying Galα1,3Galβ1,4GlcNAc determinants to bind and inhibit Clostridium difficile toxin A (TcdA) was investigated. The fusion protein, produced by a glyco-engineered stable CHO-K1 cell line and designated C-PGC2, was purified by affinity and gel filtration chromatography from large-scale cultures. Liquid chromatography-mass spectrometry was used to characterize O-glycans released by reductive β-elimination, and new diagnostic ions to distinguish Galα1,3Gal- from Galα1,4Gal-terminated O-glycans were identified. The C-PGC2 cell line, which was 20-fold more sensitive to TcdA than the wild-type CHO-K1, is proposed as a novel cell-based model for TcdA cytotoxicity and neutralization assays. The C-PGC2-produced fusion protein could competitively inhibit TcdA binding to rabbit erythrocytes, making it a high-efficiency inhibitor of the hemagglutination property of TcdA. The fusion protein also exhibited a moderate capability for neutralization of TcdA cytotoxicity in both C-PGC2 and CHO-K1 cells, the former with and the latter without cell surface Galα1,3Galβ1,4GlcNAc sequences. Future studies in animal models of C. difficile infection will reveal its TcdA-inhibitory effect and therapeutic potential in C. difficile-associated diseases.

Clostridium difficile colitis: pathogenesis and host defence

Clostridium difficile is a major cause of intestinal infection and diarrhoea in individuals following antibiotic treatment. Recent studies have begun to elucidate the mechanisms that induce spore formation and germination and have determined the roles of C. difficile toxins in disease pathogenesis. Exciting progress has also been made in defining the role of the microbiome, specific commensal bacterial species and host immunity in defence against infection with C. difficile. This Review will summarize the recent discoveries and developments in our understanding of C. difficile infection and pathogenesis.

Identification of an Essential Region for Translocation of Clostridium difficile Toxin B

Clostridium difficile toxin A (TcdA) and toxin B (TcdB) are the major virulence factors involved in C. difficile-associated diarrhea and pseudomembranous colitis. TcdA and TcdB both contain at least four distinct domains: the glucosyltransferase domain, cysteine protease domain, receptor binding domain, and translocation domain. Few studies have investigated the translocation domain and its mechanism of action. Recently, it was demonstrated that a segment of 97 amino acids (AA 1756-1852, designated D97) within the translocation domain of TcdB is essential for the in vitro and in vivo toxicity of TcdB. However, the mechanism by which D97 regulates the action of TcdB in host cells and the important amino acids within this region are unknown. In this study, we discovered that a smaller fragment, amino acids 1756-1780, located in the N-terminus of the D97 fragment, is essential for translocation of the effector glucosyltransferase domain into the host cytosol. A sequence of 25AA within D97 is predicted to form an alpha helical structure and is the critical part of D97. The deletion mutant TcdB∆1756-1780 showed similar glucosyltransferase and cysteine protease activity, cellular binding, and pore formation to wild type TcdB, but it failed to induce the glucosylation of Rho GTPase Rac1 of host cells. Moreover, we found that TcdB∆1756-1780 was rapidly degraded in the endosome of target cells, and therefore its intact glucosyltransferase domain was unable to translocate efficiently into host cytosol. Our finding provides an insight into the molecular mechanisms of action of TcdB in the intoxication of host cells.

Clostridium difficile Toxins A and B: Insights into Pathogenic Properties and Extraintestinal Effects

Clostridium difficile infection (CDI) has significant clinical impact especially on the elderly and/or immunocompromised patients. The pathogenicity of Clostridium difficile is mainly mediated by two exotoxins: toxin A (TcdA) and toxin B (TcdB). These toxins primarily disrupt the cytoskeletal structure and the tight junctions of target cells causing cell rounding and ultimately cell death. Detectable C. difficile toxemia is strongly associated with fulminant disease. However, besides the well-known intestinal damage, recent animal and in vitro studies have suggested a more far-reaching role for these toxins activity including cardiac, renal, and neurologic impairment. The creation of C. difficile strains with mutations in the genes encoding toxin A and B indicate that toxin B plays a major role in overall CDI pathogenesis. Novel insights, such as the role of a regulator protein (TcdE) on toxin production and binding interactions between albumin and C. difficile toxins, have recently been discovered and will be described. Our review focuses on the toxin-mediated pathogenic processes of CDI with an emphasis on recent studies.

Clostridium difficile infection

Infection of the colon with the Gram-positive bacterium Clostridium difficile is potentially life threatening, especially in elderly people and in patients who have dysbiosis of the gut microbiota following antimicrobial drug exposure. C. difficile is the leading cause of health-care-associated infective diarrhoea. The life cycle of C. difficile is influenced by antimicrobial agents, the host immune system, and the host microbiota and its associated metabolites. The primary mediators of inflammation in C. difficile infection (CDI) are large clostridial toxins, toxin A (TcdA) and toxin B (TcdB), and, in some bacterial strains, the binary toxin CDT. The toxins trigger a complex cascade of host cellular responses to cause diarrhoea, inflammation and tissue necrosis - the major symptoms of CDI. The factors responsible for the epidemic of some C. difficile strains are poorly understood. Recurrent infections are common and can be debilitating. Toxin detection for diagnosis is important for accurate epidemiological study, and for optimal management and prevention strategies. Infections are commonly treated with specific antimicrobial agents, but faecal microbiota transplants have shown promise for recurrent infections. Future biotherapies for C. difficile infections are likely to involve defined combinations of key gut microbiota.

The Tip of the Four N-Terminal α-Helices of Clostridium sordellii Lethal Toxin Contains the Interaction Site with Membrane Phosphatidylserine Facilitating Small GTPases Glucosylation

Clostridium sordellii lethal toxin (TcsL) is a powerful virulence factor responsible for severe toxic shock in man and animals. TcsL belongs to the large clostridial glucosylating toxin (LCGT) family which inactivates small GTPases by glucosylation with uridine-diphosphate (UDP)-glucose as a cofactor. Notably, TcsL modifies Rac and Ras GTPases, leading to drastic alteration of the actin cytoskeleton and cell viability. TcsL enters cells via receptor-mediated endocytosis and delivers the N-terminal glucosylating domain (TcsL-cat) into the cytosol. TcsL-cat was found to preferentially bind to phosphatidylserine (PS)-containing membranes and to increase the glucosylation of Rac anchored to the lipid membrane. We have previously reported that the N-terminal four helical bundle structure (1–93 domain) recognizes a broad range of lipids, but that TcsL-cat specifically binds to PS and phosphatidic acid. Here, we show using mutagenesis that the PS binding site is localized on the tip of the four-helix bundle which is rich in positively-charged amino acids. Residues Y14, V15, F17, and R18 on loop 1, between helices 1 and 2, in coordination with R68 from loop 3, between helices 3 and 4, form a pocket which accommodates L-serine. The functional PS-binding site is required for TcsL-cat binding to the plasma membrane and subsequent cytotoxicity. TcsL-cat binding to PS facilitates a high enzymatic activity towards membrane-anchored Ras by about three orders of magnitude as compared to Ras in solution. The PS-binding site is conserved in LCGTs, which likely retain a common mechanism of binding to the membrane for their full activity towards membrane-bound GTPases.

Reactive Oxygen Species as Additional Determinants for Cytotoxicity of Clostridium difficile Toxins A and B

Clostridium difficile infections can induce mild to severe diarrhoea and the often associated characteristic pseudomembranous colitis. Two protein toxins, the large glucosyltransferases TcdA and TcdB, are the main pathogenicity factors that can induce all clinical symptoms in animal models. The classical molecular mode of action of these homologous toxins is the inhibition of Rho GTPases by mono-glucosylation. Rho-inhibition leads to breakdown of the actin cytoskeleton, induces stress-activated and pro-inflammatory signaling and eventually results in apoptosis of the affected cells. An increasing number of reports, however, have documented further qualities of TcdA and TcdB, including the production of reactive oxygen species (ROS) by target cells. This review summarizes observations dealing with the production of ROS induced by TcdA and TcdB, dissects pathways that contribute to this phenomenon and speculates about ROS in mediating pathogenesis. In conclusion, ROS have to be considered as a discrete, glucosyltransferase-independent quality of at least TcdB, triggered by different mechanisms.

Receptors and Binding Structures for Clostridium difficile Toxins A and B

Two characteristics of toxins A and B from C. difficile (TcdA, TcdB) are important for the understanding of the pathogenic effect of these homologous toxins. First, these toxins are huge single-chain but multidomain proteins that display their action intracellularly within the cytosol of host cells. And second, albeit various cell types highly differ in their sensitivity toward these toxins, no toxin-resistant cell type has been described yet. Investigation of receptor-mediated uptake of these toxins is very ambitious. It demands discrimination between cell surface binding, interaction with more than one functional receptor responsible for uptake as well as other functional receptors that recognize bacterial pathogens and are not necessarily related with endocytosis. The current understanding of a complex uptake process is that TcdB interacts with at least two facultative receptors that mediate entry into host cells by redundant endocytotic pathways. Although both homologous toxins do obviously not share the same receptors, this principle of redundant binding domains found for TcdB does also account for TcdA.

Binding and entry of Clostridium difficile toxin B is mediated by multiple domains

Clostridium difficile is responsible for a number of serious gastrointestinal diseases caused primarily by two exotoxins, TcdA and TcdB. These toxins enter host cells by binding unique receptors, at least partially via their combined repetitive oligopeptides (CROPs) domains. Our study investigated structural determinants necessary for binding and entry of TcdB. Deletion analyses identified TcdB residues 1372-1493 as essential for cytotoxicity in three cell lines. Consistent with this observation, overlapping TcdB fragments (residues 1372-1848, 1372-1493 and 1493-1848) were able to independently bind cells. Our data provide new evidence supporting a more complex model of clostridial glucosylating toxin uptake than previously suggested.

Identification of an epithelial cell receptor responsible for Clostridium difficile TcdB-induced cytotoxicity

Clostridium difficile is the leading cause of hospital-acquired diarrhea in the United States. The two main virulence factors of C. difficile are the large toxins, TcdA and TcdB, which enter colonic epithelial cells and cause fluid secretion, inflammation, and cell death. Using a gene-trap insertional mutagenesis screen, we identified poliovirus receptor-like 3 (PVRL3) as a cellular factor necessary for TcdB-mediated cytotoxicity. Disruption of PVRL3 expression by gene-trap mutagenesis, shRNA, or CRISPR/Cas9 mutagenesis resulted in resistance of cells to TcdB. Complementation of the gene-trap or CRISPR mutants with PVRL3 resulted in restoration of TcdB-mediated cell death. Purified PVRL3 ectodomain bound to TcdB by pull-down. Pretreatment of cells with a monoclonal antibody against PVRL3 or prebinding TcdB to PVRL3 ectodomain also inhibited cytotoxicity in cell culture. The receptor is highly expressed on the surface epithelium of the human colon and was observed to colocalize with TcdB in both an explant model and in tissue from a patient with pseudomembranous colitis. These data suggest PVRL3 is a physiologically relevant binding partner that can serve as a target for the prevention of TcdB-induced cytotoxicity in C. difficile infection.

Proteoglycan form and function: A comprehensive nomenclature of proteoglycans

We provide a comprehensive classification of the proteoglycan gene families and respective protein cores. This updated nomenclature is based on three criteria: Cellular and subcellular location, overall gene/protein homology, and the utilization of specific protein modules within their respective protein cores. These three signatures were utilized to design four major classes of proteoglycans with distinct forms and functions: the intracellular, cell-surface, pericellular and extracellular proteoglycans. The proposed nomenclature encompasses forty-three distinct proteoglycan-encoding genes and many alternatively-spliced variants. The biological functions of these four proteoglycan families are critically assessed in development, cancer and angiogenesis, and in various acquired and genetic diseases where their expression is aberrant.