Clostridium difficile toxin A-induced apoptosis is p53-independent but depends on glucosylation of Rho GTPases

The anti-inflammatory and anti-apoptotic effects of Achillea millefolium L. extracts on Clostridioides difficile ribotype 001 in human intestinal epithelial cells

BACKGROUND

Clostridioides difficile infection (CDI) is one of the most common health care-acquired infections. The dramatic increase in antimicrobial resistance of C. difficile isolates has led to growing demand to seek new alternative medicines against CDI. Achillea millefolium L. extracts exhibit strong biological activity to be considered as potential therapeutic agents. In this work, the inhibitory effects of A. millefolium, its decoction (DEC) and ethanol (ETOH) extracts, were investigated on the growth of C. difficile RT001 and its toxigenic cell-free supernatant (Tox-S) induced inflammation and apoptosis.

METHODS

Phytochemical analysis of extracts was performed by HPLC and GC analysis. The antimicrobial properties of extracts were evaluated against C. difficile RT001. Cell viability and cytotoxicity of Caco-2 and Vero cells treated with various concentrations of extracts and Tox-S were examined by MTT assay and microscopy, respectively. Anti-inflammatory and anti-apoptotic effects of extracts were assessed in Tox-S stimulated Caco-2 cells by RT-qPCR.

RESULTS

Analysis of the phytochemical profile of extracts revealed that the main component identified in both extracts was chlorogenic acid. Both extracts displayed significant antimicrobial activity against C. difficile RT001. Moreover, both extracts at concentration 50 µg/mL had no significant effect on cell viability compared to untreated cells. Pre-treatment of cells with extracts (50 µg/mL) significantly reduced the percentage of Vero cells rounding induced by Tox-S. Also, both pre-treatment and co-treatment of Tox-S stimulated Caco-2 cells with extracts significantly downregulated the gene expression level of IL-8, IL-1β, TNF-α, TGF-β, iNOS, Bax, caspase-9 and caspase-3 and upregulated the expression level of Bcl-2.

CONCLUSION

The results of the present study for the first time demonstrate the antimicrobial activity and protective effects of A. millefolium extracts on inflammatory response and apoptosis induced by Tox-S from C. difficile RT001 clinical strain in vitro. Further research is needed to evaluate the potential application of A. millefolium extracts as supplementary medicine for CDI prevention and treatment in clinical setting.

Role of the Alteration in Calcium Homeostasis in Cell Death Induced by Clostridioides difficile Toxin A and Toxin B

Clostridioides difficile (C. difficile), responsible for 15-25% of gastrointestinal infections, causes health problems mainly due to the toxic activity of toxins A and B (Tcds). These are responsible for its clinical manifestations, including diarrhea, pseudomembranous colitis, toxic megacolon and death, with a mortality of 5-30% in primary infection, that increase following relapses. Studies on Tcd-induced cell death have highlighted a key role of caspases, calpains, and cathepsins, with involvement of mitochondria and reactive oxygen species (ROS) in a complex signaling pathway network. The complex response in the execution of various types of cell death (apoptosis, necrosis, pyroptosis and pyknosis) depends on the amount of Tcd, cell types, and Tcd receptors involved, and could have as initial/precocious event the alterations in calcium homeostasis. The entities, peculiarities and cell types involved in these alterations will decide the signaling pathways activated and cell death type. Calcium homeostasis alterations can be caused by calcium influx through calcium channel activation, transient intracellular calcium oscillations, and leakage of calcium from intracellular stores. These increases in cytoplasmic calcium have important effects on all calcium-regulated molecules, which may play a direct role in several cell death types and/or activate other cell death effectors, such as caspases, calpains, ROS and proapoptotic Bcl-2 family members. Furthermore, some support for the possible role of the calcium homeostasis alteration in Tcd-induced cell death originates from the similarity with cytotoxic effects that cause pore-forming toxins, based mainly on calcium influx through plasma membrane pores.

Clostridioides difficile Toxin B Induced Senescence: A New Pathologic Player for Colorectal Cancer?

Clostridioides difficile (C. difficile) is responsible for a high percentage of gastrointestinal infections and its pathological activity is due to toxins A and B. C. difficile infection (CDI) is increasing worldwide due to the unstoppable spread of C. difficile in the anthropized environment and the progressive human colonization. The ability of C. difficile toxin B to induce senescent cells and the direct correlation between CDI, irritable bowel syndrome (IBS), and inflammatory bowel diseases (IBD) could cause an accumulation of senescent cells with important functional consequences. Furthermore, these senescent cells characterized by long survival could push pre-neoplastic cells originating in the colon towards the complete neoplastic transformation in colorectal cancer (CRC) by the senescence-associated secretory phenotype (SASP). Pre-neoplastic cells could appear as a result of various pro-carcinogenic events, among which, are infections with bacteria that produce genotoxins that generate cells with high genetic instability. Therefore, subjects who develop IBS and/or IBD after CDI should be monitored, especially if they then have further CDI relapses, waiting for the availability of senolytic and anti-SASP therapies to resolve the pro-carcinogenic risk due to accumulation of senescent cells after CDI followed by IBS and/or IBD.

Targeting the Inside of Cells with Biologicals: Toxin Routes in a Therapeutic Context

Numerous toxins translocate to the cytosol in order to fulfil their function. This demonstrates the existence of routes for proteins from the extracellular space to the cytosol. Understanding these routes is relevant to multiple aspects related to therapeutic applications. These include the development of anti-toxin treatments, the potential use of toxins as shuttles for delivering macromolecular cargo to the cytosol or the use of drugs based on toxins. Compared with other strategies for delivery, such as chemicals as carriers for macromolecular delivery or physical methods like electroporation, toxin routes present paths into the cell that potentially cause less damage and can be specifically targeted. The efficiency of delivery via toxin routes is limited. However, low-delivery efficiencies can be entirely sufficient, if delivered cargoes possess an amplification effect or if very few molecules are sufficient for inducing the desired effects. This is known for example from RNA-based vaccines that have been developed during the coronavirus disease 2019 pandemic as well as for other approved RNA-based drugs, which elicited the desired effect despite their typically low delivery efficiencies. The different mechanisms by which toxins enter cells may have implications for their technological utility. We review the mechanistic principles of the translocation pathway of toxins from the extracellular space to the cytosol, the delivery efficiencies, and therapeutic strategies or applications that exploit toxin routes for intracellular delivery.

Cytotoxic synergism of Clostridioides difficile toxin B with proinflammatory cytokines in subjects with inflammatory bowel diseases

Clostridioides difficile (C. difficile) is progressively colonizing humans and animals living with humans. During this process, hypervirulent strains and mutated toxin A and B of C. difficile (TcdA and TcdB) are originating and developing. While in healthy subjects colonization by C. difficile becomes a risk after the use of antibiotics that alter the microbiome, other categories of people are more susceptible to infection and at risk of relapse, such as those with inflammatory bowel disease (IBD). Recent in vitro studies suggest that this increased susceptibility could be due to the strong cytotoxic synergism between TcdB and proinflammatory cytokines the tumor necrosis factor-alpha and interferon-gamma (CKs). Therefore, in subjects with IBD the presence of an inflammatory state in the colon could be the driver that increases the susceptibility to C. difficile infection and its progression and relapses. TcdB is internalized in the cell via three receptors: chondroitin sulphate proteoglycan 4; poliovirus receptor-like 3; and Wnt receptor frizzled family. Chondroitin sulphate proteoglycan 4 and Wnt receptor frizzled family are involved in cell death by apoptosis or necrosis depending on the concentration of TcdB and cell types, while poliovirus receptor-like 3 induces only necrosis. It is possible that cytokines could also induce a greater expression of receptors for TcdB that are more involved in necrosis than in apoptosis. Therefore, in subjects with IBD there are the conditions: (1) For greater susceptibility to C. difficile infection, such as the inflammatory state, and abnormalities of the microbiome and of the immune system; (2) for the enhancement of the cytotoxic activity of TcdB +Cks; and (3) for a greater expression of TcdB receptors stimulated by cytokines that induce cell death by necrosis rather than apoptosis. The only therapeutic approach currently possible in IBD patients is monitoring of C. difficile colonization for interventions aimed at reducing tumor necrosis factor-alpha and interferon-gamma levels when the infection begins. The future perspective is to generate bacteriophages against C. difficile for targeted therapy.

Curcumin and capsaicin regulate apoptosis and alleviate intestinal inflammation induced by Clostridioides difficile in vitro

Background

The dramatic upsurge of Clostridioides difficile infection (CDI) by hypervirulent isolates along with the paucity of effective conventional treatment call for the development of new alternative medicines against CDI. The inhibitory effects of curcumin (CCM) and capsaicin (CAP) were investigated on the activity of toxigenic cell-free supernatants (Tox-S) of C. difficile RT 001, RT 126 and RT 084, and culture-filtrate of C. difficile ATCC 700057.

Methods

Cell viability of HT-29 cells exposed to varying concentrations of CCM, CAP, C. difficile Tox-S and culture-filtrate was assessed by MTT assay. Anti-inflammatory and anti-apoptotic effects of CCM and CAP were examined by treatment of HT-29 cells with C. difficile Tox-S and culture-filtrate. Expression of BCL-2, SMAD3, NF-κB, TGF-β and TNF-α genes in stimulated HT-29 cells was measured using RT-qPCR.

Results

C. difficile Tox-S significantly (P < 0.05) reduced the cell viability of HT-29 cells in comparison with untreated cells. Both CAP and CCM significantly (P < 0.05) downregulated the gene expression level of BCL-2, SMAD3, NF-κB and TNF-α in Tox-S treated HT-29 cells. Moreover, the gene expression of TGF-β decreased in Tox-S stimulated HT-29 cells by both CAP and CCM, although these reductions were not significantly different (P > 0.05).

Conclusion

The results of the present study highlighted that CCM and CAP can modulate the inflammatory response and apoptotic effects induced by Tox-S from different clinical C. difficile strains in vitro. Further studies are required to accurately explore the anti-toxin activity of natural components, and their probable adverse risks in clinical practice.

Clostridioides difficile toxin B alone and with pro-inflammatory cytokines induces apoptosis in enteric glial cells by activating three different signalling pathways mediated by caspases, calpains and cathepsin B

Clostridioides difficile infection (CDI) causes nosocomial/antibiotic-associated gastrointestinal diseases with dramatically increasing global incidence and mortality rates. The main C. difficile virulence factors, toxins A and B (TcdA/TcdB), cause cytopathic/cytotoxic effects and inflammation. We demonstrated that TcdB induces caspase-dependent, mitochondria-independent enteric glial cell (EGC) apoptosis that is enhanced by the pro-inflammatory cytokines TNF-α and IFN-γ (CKs) by increasing caspase-3/7/9 and PARP activation. Because this cytotoxic synergism is important for CDI pathogenesis, we investigated the apoptotic pathways involved in TcdB- and TcdB + CK-induced apoptosis indepth. EGCs were pre-treated with the inhibitors BAF or Q-VD-OPh (pan-caspase), Z-DEVD-fmk (caspase-3/7), Z-IETD-fmk (caspase-8), PD150606 (calpains), and CA-074Me (cathepsin B) 1 h before TcdB exposure, while CKs were given 1.5 h after TcdB exposure, and assays were performed at 24 h. TcdB and TcdB + CKs induced apoptosis through three signalling pathways activated by calpains, caspases and cathepsins, which all are involved both in induction and execution apoptotic signalling under both conditions but to different degrees in TcdB and TcdB + CKs especially as regards to signal transduction mediated by these proteases towards downstream effects (apoptosis). Calpain activation by Ca²⁺ influx is the first pro-apoptotic event in TcdB- and TcdB + CK-induced EGC apoptosis and causes caspase-3, caspase-7 and PARP activation. PARP is also directly activated by calpains which are responsible of about 75% of apoptosis in TcdB and 62% in TcdB + CK which is both effector caspase-dependent and -independent. Initiator caspase-8 activation mediated by TcdB contributes to caspase-3/caspase-7 and PARP activation and is responsible of about 28% of apoptosis in both conditions. Caspase-3/caspase-7 activation is weakly responsible of apoptosis, indeed we found that it mediates 27% of apoptosis only in TcdB. Cathepsin B contributes to triggering pro-apoptotic signal and is responsible in both conditions of about 35% of apoptosis by a caspase-independent manner, and seems to regulate the caspase-3 and caspase-7 cleaved fragment levels, highlighting the complex interaction between these cysteine protease families activated during TcdB-induced apoptosis. Further a relevant difference between TcdB- and TcdB + CK-induced apoptosis is that TcdB-induced apoptosis increased slowly reaching at 72 h the value of 18.7%, while TcdB + CK-induced apoptosis increased strongly reaching at 72 h the value of 60.6%. Apoptotic signalling activation by TcdB + CKs is enriched by TNF-α-induced NF-κB signalling, inhibition of JNK activation and activation of AKT. In conclusion, the ability of C. difficile to activate three apoptotic pathways represents an important strategy to overcome resistance against its cytotoxic activity.

The Essential Role of Rac1 Glucosylation in Clostridioides difficile Toxin B-Induced Arrest of G1-S Transition

Clostridioides difficile infection (CDI) in humans causes pseudomembranous colitis (PMC), which is a severe pathology characterized by a loss of epithelial barrier function and massive colonic inflammation. PMC has been attributed to the action of two large protein toxins, Toxin A (TcdA) and Toxin B (TcdB). TcdA and TcdB mono-O-glucosylate and thereby inactivate a broad spectrum of Rho GTPases and (in the case of TcdA) also some Ras GTPases. Rho/Ras GTPases promote G1-S transition through the activation of components of the ERK, AKT, and WNT signaling pathways. With regard to CDI pathology, TcdB is regarded of being capable of inhibiting colonic stem cell proliferation and colonic regeneration, which is likely causative for PMC. In particular, it is still unclear, the glucosylation of which substrate Rho-GTPase is critical for TcdB-induced arrest of G1-S transition. Exploiting SV40-immortalized mouse embryonic fibroblasts (MEFs) with deleted Rho subtype GTPases, evidence is provided that Rac1 (not Cdc42) positively regulates Cyclin D1, an essential factor of G1-S transition. TcdB-catalyzed Rac1 glucosylation results in Cyclin D1 suppression and arrested G1-S transition in MEFs and in human colonic epithelial cells (HCEC), Remarkably, Rac1^−/− MEFs are insensitive to TcdB-induced arrest of G1-S transition, suggesting that TcdB arrests G1-S transition in a Rac1 glucosylation-dependent manner. Human intestinal organoids (HIOs) specifically expressed Cyclin D1 (neither Cyclin D2 nor Cyclin D3), which expression was suppressed upon TcdB treatment. In sum, Cyclin D1 expression in colonic cells seems to be regulated by Rho GTPases (most likely Rac1) and in turn seems to be susceptible to TcdB-induced suppression. With regard to PMC, toxin-catalyzed Rac1 glucosylation and subsequent G1-S arrest of colonic stem cells seems to be causative for decreased repair capacity of the colonic epithelium and delayed epithelial renewal.

Bacterial Species Associated With Human Inflammatory Bowel Disease and Their Pathogenic Mechanisms

Inflammatory bowel disease (IBD) is a chronic inflammatory condition of the gastrointestinal tract with unknown etiology. The pathogenesis of IBD results from immune responses to microbes in the gastrointestinal tract. Various bacterial species that are associated with human IBD have been identified. However, the microbes that trigger the development of human IBD are still not clear. Here we review bacterial species that are associated with human IBD and their pathogenic mechanisms to provide an updated broad understanding of this research field. IBD is an inflammatory syndrome rather than a single disease. We propose a three-stage pathogenesis model to illustrate the roles of different IBD-associated bacterial species and gut commensal bacteria in the development of human IBD. Finally, we recommend microbe-targeted therapeutic strategies based on the three-stage pathogenesis model.

Invisible steps for a global endemy: molecular strategies adopted by Clostridioides difficile

Clostridioides difficile infection (CDI) is on the rise worldwide and is associated with an increase in deaths and socio-health burden. C. difficile has become ubiquitous in anthropized environments because of the extreme resistance of its spores. Based on the epidemiological data and knowledge of molecular pathogenesis of C. difficile, it is possible to predict its progressive colonization of the human population for the following reasons: first, its global spread is unstoppable; second, the toxins (Tcds) produced by C. difficile, TcdA and TcdB, mainly cause cell death by apoptosis, but the surviving cells acquire a senescence state that favours persistence of C. difficile in the intestine; third, proinflammatory cytokines, tumour necrosis factor-α and interferon-γ, induced during CDI, enhance the cytotoxicity of Tcds and can increase the survival of senescent cells; fourth, Tcds block mobility and induce apoptosis in immune cells recruited at the infection site; and finally, after remission from primary infection or relapse, C. difficile causes functional abnormalities in the enteric glial cell (EGC) network that can result in irritable bowel syndrome, characterized by a latent inflammatory response that contributes to C. difficile survival and enhances the cytotoxic activity of low doses of TcdB, thus favouring further relapses. Since a 'global endemy' of C. difficile seems inevitable, it is necessary to develop an effective vaccine against Tcds for at-risk individuals, and to perform a prophylaxis/selective therapy with bacteriophages highly specific for C. difficile. We must be aware that CDI will become a global health problem in the forthcoming years, and we must be prepared to face this menace.

IL-16 and BCA-1 Serum Levels Are Associated with Disease Severity of C. difficile Infection

Clostridioides difficile infection (CDI) is associated with a high risk for complications and death, which requires identifying severe patients and treating them accordingly. We examined the serum level of six cytokines and chemokines (IL-16, IL-21, IL-23, IL-33, BCA-1, TRAIL) and investigated the association between them and patients’ disease severity. Concentrations of six cytokines and chemokines were measured using the MILLIPLEX^®MAP kit (Billerica, MA, USA) in serum samples attained from CDI patients within 24–48 h after laboratory confirmation of C. difficile presence. Demographic and clinical data were collected from medical records. The disease severity score was determined according to guidelines of the “Society for Healthcare Epidemiology of America and the Infectious Diseases Society of America” (SHEA-IDSA). Out of 54 patients, 20 (37%) had mild to moderate disease and 34 (63%) had severe disease. IL-16 (p = 0.005) and BCA-1 (p = 0.012) were associated with a more severe disease. In conclusion, IL-16 and BCA-1, along with other cytokines and chemokines, may serve as biomarkers for the early prediction of CDI severity in the future. An improved and more accessible assessment of CDI severity will contribute to the adjustment of the medical treatment, which will lead to a better patient outcome.

Clostridium difficile toxin A and toxin B inhibit YAP in the colonic epithelial cells

Toxin A (TcdA) and toxin B (TcdB), the two exotoxins of Clostridium difficile, are main causal agents for the colonic epithelium damage in Clostridium difficile infection (CDI). The Hippo pathway is crucial for the control of tissue homeostasis and regeneration of intestines. However, the dysregulation of Hippo pathway in CDI is unclear. Here we show that YAP and TAZ, the transcriptional coactivators downstream of the Hippo pathway, are sequestered in the cytoplasm, degraded, and inactivated by treatment with TcdA and TcdB in colonic epithelial cells. The overexpression of YAP restores the messenger RNA expressions of YAP target genes, attenuates the disruption of cytoskeleton and cell rounding, and rescues the cell proliferation of colonic epithelial cells under exposure of the two toxins. Our results demonstrate that inhibition of YAP and TAZ is involved in the pathogenesis of CDI, implicating that increasing YAP activity could be a potential therapeutic strategy for the CDI treatment.

Quantitative Phosphoproteome Analysis of Clostridioides difficile Toxin B Treated Human Epithelial Cells

The large clostridial glucosylating toxin B (TcdB) is a major virulence factor of the nosocomial pathogen Clostridioides difficile. TcdB inhibits small GTPases by glucosylation leading to impaired downstream signaling. TcdB also possesses a glucosyltransferase independent effect described as pyknosis. To elucidate the impact of TcdB and its glucosylation-inactive mutant TcdB_NXN on the kinome of human cells, SILAC labeled HEp-2 cells were treated with 2 nM TcdB for 8 h. Phosphopeptides were enriched using SCX chromatography, IMAC and TiO₂ followed shotgun mass spectrometry analysis. Overall 4,197 phosphopeptides were identified; more than 1,200 phosphosites responded to treatment with TcdB or TcdB_NXN. The data suggested that predominantly stress-activated MAPK-dependent signaling pathways were triggered by toxin B treatment.

Apoptosis of intestinal epithelial cells restricts Clostridium difficile infection in a model of pseudomembranous colitis

Clostridium difficile is the leading cause of pseudomembranous colitis in hospitalized patients. C. difficile enterotoxins TcdA and TcdB promote this inflammatory condition via a cytotoxic response on intestinal epithelial cells (IECs), but the underlying mechanisms are incompletely understood. Additionally, TcdA and TcdB engage the Pyrin inflammasome in macrophages, but whether Pyrin modulates CDI pathophysiology is unknown. Here we show that the Pyrin inflammasome is not functional in IECs and that Pyrin signaling is dispensable for CDI-associated IEC death and for in vivo pathogenesis. Instead, our studies establish that C. difficile enterotoxins induce activation of executioner caspases 3/7 via the intrinsic apoptosis pathway, and demonstrate that caspase-3/7-mediated IEC apoptosis is critical for in vivo host defense during early stages of CDI. In conclusion, our findings dismiss a critical role for inflammasomes in CDI pathogenesis, and identify IEC apoptosis as a host defense mechanism that restricts C. difficile infection in vivo.

Effects of Clostridium difficile toxin A on K562/A02 cell proliferation, apoptosis and multi-drug resistance

The aim of the present study was to investigate the cytotoxic effect and multi-drug resistance (MDR) of Clostridium difficile toxin A (TcdA) on K562/A02 cells, and understand its underlying molecular pathways. K562/A02 cells were treated with TcdA at different concentrations for 24, 48 and 72 h, and the inhibition effect and drug resistance of TcdA on K562/A02 cell proliferation was assessed by methyl thiazolyl tetrazolium colorimetric assay. Furthermore, cell cycle-apoptosis was analyzed by flow cytometry, P-glycoprotein (P-gp) expression was determined by western blot analysis and caspase-3 activity was measured using a caspase-3 activity kit. TcdA inhibited K562/A02 cell proliferation in a time- and dose-dependent manner. The inhibition rate of K562/A02 cells reached 8.76±0.76, 28.55±0.43, 47.89±0.27, 58.08±0.06 and 57.70±0.79% following treatment with 50, 100, 200, 400 and 800 ng/ml TcdA, respectively, for 24 h. K562/A02 cells in the G0/G1 phase increased and cells in the S phase decreased following treatment with TcdA (P<0.05), and the apoptotic rates in the 200 and 400 ng/ml concentration groups were 14.05 and 22.89%, respectively. In addition, TcdA (50 ng/ml) significantly inhibited the proliferation of K562/A02 cells and reduced the half maximal inhibitory concentration of these drugs in combination with chemotherapy drugs. The reversal folds were 3.09, 2.89 and 2.79, respectively. Furthermore, TcdA treatment was associated with the upregulation of P-gp in K562/A02 cells, and caspase-3 activity was observed to increase in K562/A02 cells following TcdA treatment, when compared with untreated controls (P<0.05). These findings suggested that TcdA may be able to inhibit K562/A02 cell growth, induce cell apoptosis by decreasing P-gp levels and caspase-3 activation, and partially reverse MDR. Further studies are required to evaluate the potential of TcdA as a candidate for the chemotherapy of hematologic malignancies.

Role of Single Procalcitonin Test on Admission as a Biomarker for Predicting the Severity of Clostridium difficile Infection

Objective: To evaluate whether serum Procalcitonin (PCT) at the early stage of infection can serve as a potential biomarker for determining Clostridium difficile infection (CDI) severity.

Methods: Fifty-four patients diagnosed with CDI were enrolled in the study. Serum samples were obtained within a median time of 24–48 h of the lab result for presence of C. difficile. PCT levels were measured by chemiluminescence immunoassay. Demographic, clinical, and prognostic data concerning the patients were retrospectively collected from medical records. The illness severity score was determined according to “Score indices for C. difficile infection severity.”

Results: We found that serum PCT levels were significantly higher in patients with moderate disease, compared to patients with mild disease (p = 0.0032). Additionally, PCT was correlated with mortality (p = 0.0002), white blood cell count (p = 0.019), and community-acquired disease (p = 0.0345).

Conclusion: Early measurement of PCT may serve as a biomarker for early prediction of CDI severity, which is of great importance due to the high risk of complications and death.

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.

Enteric glial cells are susceptible to Clostridium difficile toxin B

Clostridium difficile causes nosocomial/antibiotic-associated diarrhoea and pseudomembranous colitis. The major virulence factors are toxin A and toxin B (TcdB), which inactivate GTPases by monoglucosylation, leading to cytopathic (cytoskeleton alteration, cell rounding) and cytotoxic effects (cell-cycle arrest, apoptosis). C. difficile toxins breaching the intestinal epithelial barrier can act on underlying cells, enterocytes, colonocytes, and enteric neurons, as described in vitro and in vivo, but until now no data have been available on enteric glial cell (EGC) susceptibility. EGCs are crucial for regulating the enteric nervous system, gut homeostasis, the immune and inflammatory responses, and digestive and extradigestive diseases. Therefore, we evaluated the effects of C. difficile TcdB in EGCs. Rat-transformed EGCs were treated with TcdB at 0.1-10 ng/ml for 1.5-48 h, and several parameters were analysed. TcdB induces the following in EGCs: (1) early cell rounding with Rac1 glucosylation; (2) early G2/M cell-cycle arrest by cyclin B1/Cdc2 complex inactivation caused by p27 upregulation, the downregulation of cyclin B1 and Cdc2 phosphorylated at Thr161 and Tyr15; and (3) apoptosis by a caspase-dependent but mitochondria-independent pathway. Most importantly, the stimulation of EGCs with TNF-α plus IFN-γ before, concomitantly or after TcdB treatment strongly increased TcdB-induced apoptosis. Furthermore, EGCs that survived the cytotoxic effect of TcdB did not recover completely and showed not only persistent Rac1 glucosylation, cell-cycle arrest and low apoptosis but also increased production of glial cell-derived neurotrophic factor, suggesting self-rescuing mechanisms. In conclusion, the high susceptibility of EGCs to TcdB in vitro, the increased sensitivity to inflammatory cytokines related to apoptosis and the persistence of altered functions in surviving cells suggest an important in vivo role of EGCs in the pathogenesis of C. difficile infection.

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.

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.

Glucosylation Drives the Innate Inflammatory Response to Clostridium difficile Toxin A

Clostridium difficile is a major, life-threatening hospital-acquired pathogen that causes mild to severe colitis in infected individuals. The tissue destruction and inflammation which characterize C. difficile infection (CDI) are primarily due to the Rho-glucosylating toxins A and B. These toxins cause epithelial cell death and induce robust inflammatory signaling by activating the transcription factor NF-κB, leading to chemokine and cytokine secretion. The toxins also activate the inflammasome complex, which leads to secretion of the pyrogenic cytokine IL-1β. In this study, we utilized glucosylation-deficient toxin A to show that activation of the inflammasome by this toxin is dependent on Rho glucosylation, confirming similar findings reported for toxin B. We also demonstrated that tissue destruction and in vivo inflammatory cytokine production are critically dependent on the enzymatic activity of toxin A, suggesting that inhibiting toxin glucosyltransferase activity may be effective in combating this refractory disease.

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.

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.

The Role of Rho GTPases in Toxicity of Clostridium difficile Toxins

Clostridium difficile (C. difficile) is the main cause of antibiotic-associated diarrhea prevailing in hospital settings. In the past decade, the morbidity and mortality of C. difficile infection (CDI) has increased significantly due to the emergence of hypervirulent strains. Toxin A (TcdA) and toxin B (TcdB), the two exotoxins of C. difficile, are the major virulence factors of CDI. The common mode of action of TcdA and TcdB is elicited by specific glucosylation of Rho-GTPase proteins in the host cytosol using UDP-glucose as a co-substrate, resulting in the inactivation of Rho proteins. Rho proteins are the key members in many biological processes and signaling pathways, inactivation of which leads to cytopathic and cytotoxic effects and immune responses of the host cells. It is supposed that Rho GTPases play an important role in the toxicity of C. difficile toxins. This review focuses on recent progresses in the understanding of functional consequences of Rho GTPases glucosylation induced by C. difficile toxins and the role of Rho GTPases in the toxicity of TcdA and TcdB.

DXD motif-dependent and -independent effects of the chlamydia trachomatis cytotoxin CT166

The Gram-negative, intracellular bacterium Chlamydia trachomatis causes acute and chronic urogenital tract infection, potentially leading to infertility and ectopic pregnancy. The only partially characterized cytotoxin CT166 of serovar D exhibits a DXD motif, which is important for the enzymatic activity of many bacterial and mammalian type A glycosyltransferases, leading to the hypothesis that CT166 possess glycosyltransferase activity. CT166-expressing HeLa cells exhibit actin reorganization, including cell rounding, which has been attributed to the inhibition of the Rho-GTPases Rac/Cdc42. Exploiting the glycosylation-sensitive Ras(27H5) antibody, we here show that CT166 induces an epitope change in Ras, resulting in inhibited ERK and PI3K signaling and delayed cell cycle progression. Consistent with the hypothesis that these effects strictly depend on the DXD motif, CT166 with the mutated DXD motif causes neither Ras-ERK inhibition nor delayed cell cycle progression. In contrast, CT166 with the mutated DXD motif is still capable of inhibiting cell migration, suggesting that CT166 with the mutated DXD motif cannot be regarded as inactive in any case. Taken together, CT166 affects various fundamental cellular processes, strongly suggesting its importance for the intracellular survival of chlamydia.

Human neutrophils are activated by a peptide fragment of Clostridium difficile toxin B presumably via formyl peptide receptor

Clostridium difficile may induce antibiotic-associated diarrhoea and, in severe cases, pseudomembranous colitis characterized by tremendous neutrophil infiltration. All symptoms are caused by two exotoxins: TcdA and TcdB. We describe here the activation of isolated human blood neutrophils by TcdB and, moreover, by toxin fragments generated by limited proteolytical digestion. Kinetics and profiles of TcdB-induced rise in intracellular-free Ca(2+) and reactive oxygen species production were similar to that induced by fMLF, which activates the formyl peptide receptor (FPR) recognizing formylated bacterial peptide sequences. Transfection assays with the FPR-1 isoform hFPR26 in HEK293 cells, heterologous desensitization experiments and FPR inhibition via cyclosporine H strongly suggest activation of cells via FPR-1. Domain analyses revealed that the N-terminal glucosyltransferase domain of TcdB is a potent activator of FPR pointing towards an additional mechanism that might contribute to pathogenesis. This pro-inflammatory ligand effect can be triggered even by cleaved and, thus, non-cytotoxic toxin. In summary, we report (i) a ligand effect on neutrophils as completely new molecular mode of action, (ii) pathogenic potential of truncated or proteolytically cleaved 'non-cytotoxic' fragments and (iii) an interaction of the N-terminal glucosyltransferase domain instead of the C-terminal receptor binding domain of TcdB with target cells.

Critical roles of Clostridium difficile toxin B enzymatic activities in pathogenesis

TcdB is one of the key virulence factors of Clostridium difficile that is responsible for causing serious and potentially fatal colitis. The toxin contains at least two enzymatic domains: an effector glucosyltransferase domain for inactivating host Rho GTPases and a cysteine protease domain for the delivery of the effector domain into host cytosol. Here, we describe a novel intrabody approach to examine the role of these enzymes of TcdB in cellular intoxication. By screening a single-domain heavy chain (V(H)H) library raised against TcdB, we identified two V(H)H antibodies, 7F and E3, that specifically inhibit TcdB cysteine protease and glucosyltransferase activities, respectively. Cytoplasmic expression of 7F intrabody in Vero cells inhibited TcdB autoprocessing and delayed cellular intoxication, whereas E3 intrabody completely blocked the cytopathic effects of TcdB holotoxin. These data also demonstrate for the first time that toxin autoprocessing occurs after cysteine protease and glucosyltransferase domains translocate into the cytosol of target cells. We further determined the role of the enzymatic activities of TcdB in in vivo toxicity using a sensitive systemic challenge model in mice. Consistent with these in vitro results, a cysteine protease noncleavable mutant, TcdB-L543A, delayed toxicity in mice, whereas glycosyltransferase-deficient TcdB demonstrated no toxicity up to 500-fold of the 50% lethal dose (LD50) when it was injected systemically. Thus, glucosyltransferase but not cysteine protease activity is critical for TcdB-mediated cytopathic effects and TcdB systemic toxicity, highlighting the importance of targeting toxin glucosyltransferase activity for future therapy.

Recombinant Clostridium difficile toxin B induces endoplasmic reticulum stress in mouse colonal carcinoma cells

Clostridium difficile is the main cause of antibiotic-associated diarrhea and pseudomembranous colitis in humans and animals. Its pathogenicity is primarily linked to the secretion of two exotoxins (TcdA and TcdB). Although great progress in the toxic mechanism of TcdA and TcdB has been achieved, there are many conflicting reports about the apoptotic mechanism. More importantly, apoptotic endoplasmic reticulum (ER) stress has been reported in cells treated with Shiga toxins-another kind of cytotoxins that can cause diarrhea and colitis. Herein we checked whether TcdB can induce ER stress. The results showed that recombinant TcdB (rTcdB) activated molecular markers of unfolded protein response, suggesting that rTcdB induced ER stress in CT26 cells. However, rTcdB did not induce the up-regulation of C/EBP homologous protein (CHOP), a classic mediator of apoptotic ER stress, but it activated the precursor of cysteine aspartic acid-specific protease 12 (caspase-12), a controversial mediator of apoptotic ER stress. Besides, glucosyltransferase activity-deficient mutant recombinant TcdB induced ER stress, though it has no cytotoxic or cytopathic effect on CT26 cells. Altogether, these data demonstrated that ER stress induced by rTcdB is glucosyltransferase-independent, indicating that ER stress induced by rTcdB is non-apoptotic. This work also offers us a new insight into the molecular mechanism of CHOP protein expression regulation and the role of CHOP expression in ER stress.

Clostridium difficile toxin A attenuates Wnt/β-catenin signaling in intestinal epithelial cells

Clostridium difficile toxins A and B (TcdA and TcdB) are homologous glycosyltransferases that inhibit a group of small GTPases within host cells, but several mechanisms underlying their pathogenic activity remain unclear. In this study, we evaluated the effects of TcdA on the Wnt/β-catenin pathway, the major driving force behind the proliferation of epithelial cells in colonic crypts. IEC-6 and RKO cells stimulated with Wnt3a-conditioned medium were incubated with 10, 50, and 100 ng/ml of TcdA for 24 h, resulting in a dose-dependent inhibition of the Wnt signaling, as demonstrated by a T-cell factor (TCF) reporter assay. This was further confirmed by immunofluorescence staining for nuclear localization of β-catenin and Western blotting for β-catenin and c-Myc (encoded by a Wnt target gene). Moreover, our Western blot analysis showed a decrease in the β-catenin protein levels, which was reversed by z-VAD-fmk, a pan-caspase inhibitor. Nonetheless, TcdA was still able to inhibit the Wnt/β-catenin pathway even in the presence of z-VAD-fmk, lithium chloride (a GSK3β inhibitor), or constitutively active β-catenin, as determined by a TCF reporter assay. Furthermore, preincubation of RKO cells with TcdA for 12 h also attenuated Wnt3a-mediated activation of Wnt signaling, suggesting that inactivation of Rho GTPases plays a significant role in that inhibition. Taken together, these findings suggest that attenuation of the Wnt signaling by TcdA is important for TcdA antiproliferative effects.

Substrate specificity of clostridial glucosylating toxins and their function on colonocytes analyzed by proteomics techniques

Clostridium difficile is the major cause of intestinal infections in hospitals. The major virulence factors are toxin A (TcdA) and toxin B (TcdB), which belong to the group of clostridial glucosylating toxins (CGT) that inactivate small GTPases. After a 24 h incubation period with TcdA or a glucosyltransferase-deficient mutant TcdA (gdTcdA), quantitative changes in the proteome of colonic cells (Caco-2) were analyzed using high-resolution LC-MS/MS and the SILAC technique. The changes in abundance of more than 5100 proteins were quantified. Nearly 800 toxin-responsive proteins were identified that were involved in cell cycle, cell structure, and adhesion as well as metabolic processes. Several proteins localized to mitochondria or involved in lipid metabolism were consistently of higher abundance after TcdA treatment. All changes of protein abundance depended on the glucosyltransferase activity of TcdA. Glucosylation of the known targets of TcdA such as RhoA, RhoC, RhoG was detected by LC-MS/MS. In addition, an almost complete glucosylation of Rap1(A/B), Rap2(A/B/C) and a partial glucosylation of Ral(A/B) and (H/K/N)Ras were detected. The glucosylation pattern of TcdA was compared to that of other CGT like TcdB, the variant TcdB from C. difficile strain VPI 1470 (TcdBF), and lethal toxin from C. sordellii (TcsL).

Cytotoxicity of Clostridium difficile toxin B does not require cysteine protease-mediated autocleavage and release of the glucosyltransferase domain into the host cell cytosol

Clostridium difficile virulence requires secretion of two exotoxins: TcdA and TcdB. The precise mechanism of toxin uptake and delivery is undefined, but current models predict that the cysteine protease domain (CPD)-mediated autocleavage and release of glucosyltransferase domain (GTD) are crucial for intoxication. To determine the importance of CPD-mediated cleavage to TcdB cytotoxicity, we generated two mutant toxins--TcdB-C698S and TcdB-H653A--and assayed their abilities to intoxicate cells. The CPD mutants include an intact GTD but lack the cysteine protease activity. The mutants had reduced potency in that their effect on cells was delayed and required higher concentrations than wild-type TcdB. They did eventually cause cell rounding, glucosylation of Rho GTPases, and apoptosis that was indistinguishable from that caused by TcdB. Although the mutant toxins caused a complete cell rounding, they failed to release their GTD into cytosol, whereas wild-type TcdB displayed significant autocleavage and release of GTD. We conclude that the cysteine protease-mediated autocleavage and release of GTD is not a prerequisite for the cytotoxic activity of TcdB, but rather limits the potency and speed of Rho GTPase glucosylation. Our findings revise and refine the current model for the mode of the action and cellular trafficking of TcdB.

Clostridium difficile Toxin B causes epithelial cell necrosis through an autoprocessing-independent mechanism

Clostridium difficile is the most common cause of antibiotic-associated nosocomial infection in the United States. C. difficile secretes two homologous toxins, TcdA and TcdB, which are responsible for the symptoms of C. difficile associated disease. The mechanism of toxin action includes an autoprocessing event where a cysteine protease domain (CPD) releases a glucosyltransferase domain (GTD) into the cytosol. The GTD acts to modify and inactivate Rho-family GTPases. The presumed importance of autoprocessing in toxicity, and the apparent specificity of the CPD active site make it, potentially, an attractive target for small molecule drug discovery. In the course of exploring this potential, we have discovered that both wild-type TcdB and TcdB mutants with impaired autoprocessing or glucosyltransferase activities are able to induce rapid, necrotic cell death in HeLa and Caco-2 epithelial cell lines. The concentrations required to induce this phenotype correlate with pathology in a porcine colonic explant model of epithelial damage. We conclude that autoprocessing and GTD release is not required for epithelial cell necrosis and that targeting the autoprocessing activity of TcdB for the development of novel therapeutics will not prevent the colonic tissue damage that occurs in C. difficile – associated disease.

Clostridium difficile is an anaerobic spore-forming bacterium that infects the human colon and causes diarrhea, pseudomembranous colitis, and toxic megacolon. Most people that develop disease symptoms have undergone antibiotic treatment, which alters the normal gut flora and allows C. difficile to flourish. C. difficile secretes two toxins, TcdA and TcdB, that are responsible for the fluid secretion, inflammation, and colonic tissue damage associated with disease. The emergence of hypervirulent strains of C. difficile that are linked to increased morbidity and mortality highlights the need for new therapeutic strategies. One strategy is to inhibit the function of the toxins, thereby decreasing damage to the colon while the patient clears the infection with antibiotics. Toxin function is thought to depend on an autoprocessing event that releases a catalytic ‘effector’ portion of the toxin into the host cell. In the course of trying to identify small molecules that would inhibit such a function, we found that TcdB induces a rapid necrosis in epithelial cells that is not dependent on autoprocessing. The physiological relevance of this observation is confirmed in colonic explants and suggests that inhibiting TcdB autoprocessing will not prevent the colonic tissue damage observed in C. difficile associated diseases.

Toward a structural understanding of Clostridium difficile toxins A and B

Clostridium difficile is a toxin-producing bacterium that is a frequent cause of hospital-acquired and antibiotic-associated diarrhea. The incidence, severity, and costs associated with C. difficile associated disease are substantial and increasing, making C. difficile a significant public health concern. The two primary toxins, TcdA and TcdB, disrupt host cell function by inactivating small GTPases that regulate the actin cytoskeleton. This review will discuss the role of these two toxins in pathogenesis and the structural and molecular mechanisms by which they intoxicate cells. A focus will be placed on recent publications highlighting mechanistic similarities and differences between TcdA, TcdB, and different TcdB variants.

Clostridium difficile toxins: mediators of inflammation

Clostridium difficile is a significant problem in hospital settings as the most common cause of nosocomial diarrhea worldwide. C. difficile infections (CDIs) are characterized by an acute intestinal inflammatory response with neutrophil infiltration. These symptoms are primarily caused by the glucosylating toxins, TcdA and TcdB. In the past decade, the frequency and severity of CDIs have increased markedly due to the emergence of so-called hypervirulent strains that overproduce cytotoxic glucosylating toxins relative to historical strains. In addition, these strains produce a third toxin, binary toxin or C. difficile transferase (CDT), that may contribute to hypervirulence. Both the glucosylating toxins and CDT covalently modify target cell proteins to cause disassembly of the actin cytoskeleton and induce severe inflammation. This review summarizes our current knowledge of the mechanisms by which glucosylating toxins and CDT disrupt target cell function, alter host physiology and stimulate immune responses.

Systems analysis of the transcriptional response of human ileocecal epithelial cells to Clostridium difficile toxins and effects on cell cycle control

Background

Toxins A and B (TcdA and TcdB) are Clostridium difficile's principal virulence factors, yet the pathways by which they lead to inflammation and severe diarrhea remain unclear. Also, the relative role of either toxin during infection and the differences in their effects across cell lines is still poorly understood. To better understand their effects in a susceptible cell line, we analyzed the transciptome-wide gene expression response of human ileocecal epithelial cells (HCT-8) after 2, 6, and 24 hr of toxin exposure.

Results

We show that toxins elicit very similar changes in the gene expression of HCT-8 cells, with the TcdB response occurring sooner. The high similarity suggests differences between toxins are due to events beyond transcription of a single cell-type and that their relative potencies during infection may depend on differential effects across cell types within the intestine. We next performed an enrichment analysis to determine biological functions associated with changes in transcription. Differentially expressed genes were associated with response to external stimuli and apoptotic mechanisms and, at 24 hr, were predominately associated with cell-cycle control and DNA replication. To validate our systems approach, we subsequently verified a novel G₁/S and known G₂/M cell-cycle block and increased apoptosis as predicted from our enrichment analysis.

Conclusions

This study shows a successful example of a workflow deriving novel biological insight from transcriptome-wide gene expression. Importantly, we do not find any significant difference between TcdA and TcdB besides potency or kinetics. The role of each toxin in the inhibition of cell growth and proliferation, an important function of cells in the intestinal epithelium, is characterized.

Differential binding and internalization of Clostridium difficile toxin A by human peripheral blood monocytes, neutrophils and lymphocytes

Colitis due to Clostridium difficile infection is mediated by secreted toxins A and B and is characterized by infiltration by cells from the systemic circulation. The aim of our study was to investigate interactions between fluorescently labelled toxin A and peripheral blood monocytes, neutrophils and lymphocytes. Purified toxin A was labelled with Alexa Fluor® 488 (toxin A(488)) and incubated with isolated human peripheral blood mononuclear cells or washed whole blood cells for varying time intervals at either 37 or 4 °C/ice. The ability of trypan blue to quench cell surface-associated (but not cytoplasmic) fluorescence was also investigated. At 37 °C, toxin A(488) -associated fluorescence in monocytes peaked at 1 h (majority internalized), with subsequent loss associated with cell death. In contrast to monocytes, binding of toxin A(488) in neutrophils was greater on ice than at 37 °C. Studies using trypan blue suggested that over 3 h at 37 °C, most of the toxin A(488)-associated fluorescence in neutrophils remained at the cell surface. Over 48 h (37 °C and ice/4 °C), there was minimal toxin A(488)-associated fluorescence in lymphocytes. These studies suggest major differences in interactions between toxin A and circulating cells that infiltrate the mucosa during colonic inflammation in C. difficile infection.

Effects of Clostridium difficile Toxin A on the proteome of colonocytes studied by differential 2D electrophoresis

Clostridium difficile is a spore-forming anaerobic pathogen, commonly associated with severe diarrhea or life-threatening pseudomembraneous colitis. Its main virulence factors are the single-chain, multi-domain toxin A (TcdA) and B (TcdB). Their glucosyltransferase domain selectively inactivates Rho proteins leading to a reorganization of the cytoskeleton. To study exclusively glucosyltransferase-dependent molecular effects of TcdA, human colonic cells (Caco-2) were treated with recombinant wild type TcdA and the glucosyltransferase deficient variant of the toxin, TcdA(gd) for 24h. Changes in the protein pattern of the colonic cells were investigated by 2-D DIGE and LCMS/MS methodology combined with detailed proteome mapping. gdTcdA did not induce any detectable significant changes in the protein pattern. Comparing TcdA-treated cells with a control group revealed seven spots of higher and two of lower intensity (p<0.05). Three proteins are involved in the assembly of the cytoskeleton (β-actin, ezrin, and DPYL2) and four are involved in metabolism and/or oxidative stress response (ubiquitin, DHE3, MCCB, FABPL) and two in regulatory processes (FUBP1, AL1A1). These findings correlate well to known effects of TcdA like the reorganization of the cytoskeleton and stress the importance of Rho protein glucosylation for the pathogenic effects of TcdA.

Impact of clostridial glucosylating toxins on the proteome of colonic cells determined by isotope-coded protein labeling and LC-MALDI

Background

The anaerobe Clostridium difficile produces two major virulence factors toxin A and B that inactivate Rho proteins by glucosylation of a pivotal threonine residue. Purified toxins induce reorganization of the cytoskeleton and cell death in colonic cells. Whether all toxin effects on target cells depend on catalytic glucosyltransferase activity is unclear at present. Thus, we conducted a proteome approach to compare the protein profile of target cells treated either with wild type toxin A (rTcdA wt) or with a catalytically inactive mutant toxin A (mutant rTcdA). Relative protein quantification was feasible using isotope-coded protein labeling techniques (ICPL) and mass spectrometry (LC-MALDI).

Results

Altogether we found a significant differential expression of thirty proteins after treatment with rTcdA wt or mutant rTcdA. Mutant rTcdA caused up-regulation of seven proteins and sixteen proteins were responsive to rTcdA wt after 5 h. Long-term effect of rTcdA wt on protein expression was the down-regulation of eleven proteins. Up- or down-regulation of several proteins was verified by western blot analysis confirming the MS results.

Conclusion

Our results indicate incubation time-dependent effects of the clostridial glucosylating toxin A on colonic cells. The rTcdA wt impact more cellular functions than actin cytoskeleton reorganization and apoptosis. Furthermore, these data give insight into glucosyltransferase independent effects of clostridial glucosylating toxins on target cells after short incubation time. Additionally, our data reveal pro-inflammatory and proliferative effects of mutant rTcdA after short-term incubation.

Rho/Ras-GTPase-dependent and -independent activity of clostridial glucosylating toxins

Clostridial glucosylating toxins are the main virulence factors of clostridia responsible for gangrene and/or colitis. These toxins have been well characterized to inactivate Rho/Ras-GTPases through glucosylation. However, the signalling pathways downstream of Rho/Ras-GTPases leading to the intracellular effects of these toxins are only partially known. Rac-dependent modification of focal adhesion complexes and phosphoinositide metabolism seem to be key processes involved in actin filament depolymerization and disorganization of intercellular junctions. In addition, clostridial glucosylating toxins induce Rho/Ras-independent intracellular effects such as activation of mitogen-activated protein kinase pathways, which are used by some of these toxins to trigger an inflammatory response.

Down-regulation of interleukin-16 in human mast cells HMC-1 by Clostridium difficile toxins A and B

Toxin A (TcdA) and toxin B (TcdB) are the major virulence factors of Clostridium difficile and are the causative agents for clinical symptoms, such as secretory diarrhoea and pseudomembranous colitis. Mast cells are essentially involved in the toxin-induced colonic inflammatory processes. To study the direct effects of these toxins on the expression of inflammatory genes, a DNA microarray containing evaluated probes of 90 selected inflammatory genes was applied to the immature mast cell line HMC-1. TcdA and TcdB induced up-regulation of only a limited number of genes within the early phase of cell treatment. Interleukin-8 (IL-8), transcription factor c-jun and heme oxygenase-1 messenger RNA (mRNA) increased more than 2-fold. In contrast, IL-16, known as a CD4(+) T-cell chemoattractant factor and the chemokine receptor cKit were down-regulated. Stimulation of HMC-1 cells with IL-8 had no effect on IL-16 mRNA level, indicating that both cytokines were independently affected by the toxins. Regulation of both cytokines, however, depended on glucosylation of Rho GTPases as tested by application of enzyme-deficient TcdA or TcdB. Down-regulation of total and secreted IL-16 protein was checked by enzyme-linked immunosorbent assay. The data implicate that TcdA and TcdB affect lymphocyte migration by modulating release of the chemoattractant factor IL-16 from mast cells. In addition, this is the first report showing that Rho GTPases are involved in the regulation of IL-16 expression.

Difference in the biological effects of Clostridium difficile toxin B in proliferating and non-proliferating cells

Toxin A (TcdA) and toxin B (TcdB) from Clostridium difficile are the causative agents of the C. difficile-associated diarrhea (CDAD) and its severe form, the pseudomembranous colitis. TcdA and TcdB both glucosylate and thereby inactivate low molecular weight GTP-binding proteins of the Rho, Rac, and Cdc42 subfamilies. In cultured cell lines, TcdB induces actin re-organization and bi-nucleation ("cytopathic effects") and cell death ("cytotoxic effects"). In this study, the role of cell cycle progression in the cytopathic and the cytotoxic effects of TcdB is evaluated by a differential analysis of these effects in proliferating and non-proliferating cells. Density-synchronized murine fibroblasts and confluent HT29 colonocytes are exploited as cell culture models for non-proliferating cells. Cell death is analyzed in terms of a loss of cell viability, phosphatidylserine exposure, and DNA fragmentation. In proliferating cells, TcdB blocks cell proliferation and induces apoptotic cell death. In contrast, TcdB induces non-apoptotic cell death in non-proliferating cells. TcdB-induced cell rounding turns out to be independent of cell cycle progression. Cell cycle progression is an important determinant in the biological effects of TcdB. With respect to the pathology of CDAD, this study leads to the new hypothesis that necrotic cell death of terminally differentiated colonocytes and inhibition of epithelial renewal of the colon contribute to the pathogenesis of CDAD.

Autoproteolytic cleavage mediates cytotoxicity of Clostridium difficile toxin A

Toxin A and toxin B from Clostridium difficile are the causative agents of the antibiotic-associated pseudomembranous colitis. They are of an A/B structure type and possess inositol hexakisphosphate-inducible autoproteolytic activity to release their glucosyltransferase domain to the cytoplasm of target cells. In this study, we investigated the effect of extracellular and intracellular autoproteolytic cleavage on the function of TcdA. Extracellular cleavage led to functional inactivation albeit TcdA was less susceptible to inositol hexakisphosphate-induced autoproteolysis than TcdB. A non-cleavable TcdA mutant (TcdA A541 G542 A543) was generated to investigate whether autoproteolysis is a prerequisite for intracellular function of TcdA. Although the EC(50) regarding cell rounding was about 75-fold reduced in short-term assay, non-cleavable TcdA was able to induce complete cell rounding and apoptosis after 36 h comparable to wildtype TcdA when continuously present. Studies with limited uptake of toxins revealed progressive Rac1 glucosylation and complete cell rounding for TcdA, whereas the effect induced by non-cleavable TcdA was reversible. These findings argue for cytosolic accumulation of the released glucosyltransferase domain of wild-type TcdA and rapid degradation of the non-cleavable TcdA. In summary, extracellular cleavage functionally inactivates TcdA (and TcdB), whereas intracellular autoproteolytic cleavage is not essential for function of TcdA but defines its potency.

Structural organization of the functional domains of Clostridium difficile toxins A and B

Clostridium difficile toxins A and B are members of an important class of virulence factors known as large clostridial toxins (LCTs). Toxin action involves four major steps: receptor-mediated endocytosis, translocation of a catalytic glucosyltransferase domain across the membrane, release of the enzymatic moiety by autoproteolytic processing, and a glucosyltransferase-dependent inactivation of Rho family proteins. We have imaged toxin A (TcdA) and toxin B (TcdB) holotoxins by negative stain electron microscopy to show that these molecules are similar in structure. We then determined a 3D structure for TcdA and mapped the organization of its functional domains. The molecule has a "pincher-like" head corresponding to the delivery domain and two tails, long and short, corresponding to the receptor-binding and glucosyltransferase domains, respectively. A second structure, obtained at the acidic pH of an endosome, reveals a significant structural change in the delivery and glucosyltransferase domains, and thus provides a framework for understanding the molecular mechanism of LCT cellular intoxication.

Killing of rat basophilic leukemia cells by lethal toxin from Clostridium sordellii: critical role of phosphatidylinositide 3'-OH kinase/Akt signaling

Clostridium sordellii lethal toxin (TcsL) belongs to the family of clostridial glucosylating toxins. TcsL exhibits glucosyltransferase activity to inactivate Rho and Ras proteins. On cultured cells, TcsL causes actin reorganization ("cytopathic effect") and apoptotic cell death ("cytotoxic effect"). This study is based on the concept that the cytotoxic effects of TcsL depend on the glucosylation of critical substrate proteins rather than on the glucosyltransferase activity per se. The cytotoxic effects of TcsL depend on the glucosyltransferase activity of TcsL, as neither chemically inactivated TcsL nor a glucosyltransferase-deficient mutant version of TcsL caused it. The TcsL homologous toxin B from Clostridium difficile serotype F strain 1470 (TcdBF) also failed to cause cytotoxic effects. Correlation of the toxins' respective protein substrate specificities highlighted (H/K/N)Ras as critical substrate proteins for the cytotoxic effects. (H/K/N)Ras are critical upstream regulators of phosphatidylinositide 3'-OH kinase (PI3K)/Akt survival signaling. Tauroursodeoxycholic acid (TUDCA) classified to activate PI3K/Akt signaling downstream of apoptosis-inducing stimuli prevented the cytotoxic effects of TcsL. In conclusion, (H/K/N)Ras glucosylation and subsequent inhibition of PI3K/Akt signaling are critical for the cytotoxic effects of TcsL.

c-Abl mediates endothelial apoptosis induced by inhibition of integrins alphavbeta3 and alphavbeta5 and by disruption of actin

Inhibition of integrins alphavbeta3 and alphavbeta5 in human brain microvascular endothelial cells (HBMECs) by the function-blocking peptide RGDfV induces loss of spreading on vitronectin, cell detachment, and apoptosis. We demonstrate that cell detachment is not required for apoptosis because plating on bovine serum albumin-blocked poly-L-lysine (allows attachment, but not integrin ligation and cell spreading) also induced apoptosis. Latrunculin B (LatB), which inhibits F-actin polymerization, induced transient loss of HBMEC spreading on vitronectin, but not their detachment, and induced apoptosis despite recovery of cell spreading. However, LatB did not cause apoptosis in 5 tumor cell lines. In HBMECs, both LatB and RGDfV induced transient Y412 and Y245 phosphorylation of endogenous c-Abl, a nonreceptor tyrosine kinase that reciprocally regulates F-actin. LatB also induced nuclear translocation of c-Abl in HBMECs. STI-571 (imatinib), a targeted therapy for BCR-ABL1(+) leukemias and inhibitor of c-Abl, platelet-derived growth factor receptor, and c-Kit, decreased endothelial apoptosis. LatB-induced HBMEC apoptosis, and its inhibition by STI-571 also occurred in a 3-dimensional collagen model, supporting physiologic relevance. Last, siRNA to c-Abl (but not nonspecific siRNA) also inhibited RGDfV- and LatB-induced apoptosis. Thus, endogenous c-Abl mediates endothelial apoptosis induced by inhibition of integrins alphavbeta3/alphavbeta5 or by LatB-induced disruption of F-actin.

Clostridial toxins

Clostridia produce the highest number of toxins of any type of bacteria and are involved in severe diseases in humans and other animals. Most of the clostridial toxins are pore-forming toxins responsible for gangrenes and gastrointestinal diseases. Among them, perfringolysin has been extensively studied and it is the paradigm of the cholesterol-dependent cytolysins, whereas Clostridium perfringens epsilon-toxin and Clostridium septicum alpha-toxin, which are related to aerolysin, are the prototypes of clostridial toxins that form small pores. Other toxins active on the cell surface possess an enzymatic activity, such as phospholipase C and collagenase, and are involved in the degradation of specific cell-membrane or extracellular-matrix components. Three groups of clostridial toxins have the ability to enter cells: large clostridial glucosylating toxins, binary toxins and neurotoxins. The binary and large clostridial glucosylating toxins alter the actin cytoskeleton by enzymatically modifying the actin monomers and the regulatory proteins from the Rho family, respectively. Clostridial neurotoxins proteolyse key components of neuroexocytosis. Botulinum neurotoxins inhibit neurotransmission at neuromuscular junctions, whereas tetanus toxin targets the inhibitory interneurons of the CNS. The high potency of clostridial toxins results from their specific targets, which have an essential cellular function, and from the type of modification that they induce. In addition, clostridial toxins are useful pharmacological and biological tools.

Essential role of the glucosyltransferase activity in Clostridium difficile toxin-induced secretion of TNF-alpha by macrophages

Clostridium difficile causes serious and potentially fatal inflammatory diseases of the colon. Two large protein toxins, TcdA and TcdB, have been clearly implicated in pathogenesis. The goal of this study was to determine whether the glucosyltransferase activity of the toxins is critical for the induction of tumor necrosis factor-alpha (TNF-alpha), an important cytokine mediating both local and systematic inflammatory response. A dose-dependent TNF-alpha secretion was demonstrated in murine macrophage cell line RAW 264.7 after exposure to TcdA or TcdB. TNF-alpha production was blocked by anti-toxin antibodies, indicating that the cytokine-driven response is mediated by the toxins. Both toxins disrupted the cytoskeleton of host cells, while cytoskeleton disruptions using Cytochalasin-D and latrunculin B did not affect TNF-alpha production. The TNF-alpha synthesis was inhibited by reagents that target clathrin-dependent endocytosis or prevent endosomal acidification, suggesting that the endocytosis pathway is necessary for the induction of TNF-alpha. Furthermore, knockout of the enzymatic activity by mutating two key amino acids in the catalytic domain of TcdA abolished its cytokine-inducing activity. Our studies demonstrated a crucial role of the glucosyltransferase activity of C. difficile toxins in the induction of TNF-alpha in macrophages.