Cytotoxicity of Clostridium difficile toxins A and B requires an active and functional SREBP-2 pathway

The antiarrhythmic drugs amiodarone and dronedarone inhibit intoxication of cells with pertussis toxin

Pertussis toxin (PT) is a virulent factor produced by Bordetella pertussis, the causative agent of whooping cough. PT exerts its pathogenic effects by ADP-ribosylating heterotrimeric G proteins, disrupting cellular signaling pathways. Here, we investigate the potential of two antiarrhythmic drugs, amiodarone and dronedarone, in mitigating PT-induced cellular intoxication. After binding to cells, PT is endocytosed, transported from the Golgi to the endoplasmic reticulum where the enzyme subunit PTS1 is released from the transport subunit of PT. PTS1 is translocated into the cytosol where it ADP-ribosylates inhibitory α-subunit of G-protein coupled receptors (Gαi). We showed that amiodarone and dronedarone protected CHO cells and human A549 cells from PT-intoxication by analyzing the ADP-ribosylation status of Gαi. Amiodarone had no effect on PT binding to cells or in vitro enzyme activity of PTS1 but reduced the signal of PTS1 in the cell suggesting that amiodarone interferes with intracellular transport of PTS1. Moreover, dronedarone mitigated the PT-mediated effect on cAMP signaling in a cell-based bioassay. Taken together, our findings underscore the inhibitory effects of amiodarone and dronedarone on PT-induced cellular intoxication, providing valuable insights into drug repurposing for infectious disease management.

Inhibition of Clostridioides difficile toxins TcdA and TcdB by the amiodarone derivative dronedarone

The dreaded nosocomial pathogen Clostridioides difficile causes diarrhea and severe inflammation of the colon, especially after the use of certain antibiotics. The bacterium releases two deleterious toxins, TcdA and TcdB, into the gut, which are mainly responsible for the symptoms of C. difficile-associated diseases (CDADs). Both toxins are capable of entering independently into various host cells, e.g., intestinal epithelial cells, where they mono-O-glucosylate and inactivate Rho and/or Ras GTPases, important molecular switches for various cellular functions. We have shown recently that the cellular uptake of the Clostridioides difficile toxins TcdA and TcdB (TcdA/B) is inhibited by the licensed class III antiarrhythmic drug amiodarone (Schumacher et al. in Gut Microbes 15(2):2256695, 2023). Mechanistically, amiodarone delays the cellular uptake of both toxins into target cells most likely by lowering membrane cholesterol levels and by interfering with membrane insertion and/or pore formation of TcdA/B. However, serious side effects, such as thyroid dysfunction and severe pulmonary fibrosis, limit the clinical use of amiodarone in patients with C. difficile infection (CDI). For that reason, we aimed to test whether dronedarone, an amiodarone derivative with a more favorable side effect profile, is also capable of inhibiting TcdA/B. To this end, we tested in vitro with various methods the impact of dronedarone on the intoxication of Vero and CaCo-2 cells with TcdA/B. Importantly, preincubation of both cell lines with dronedarone for 1 h at concentrations in the low micromolar range rendered the cells less sensitive toward TcdA/B-induced Rac1 glucosylation, collapse of the actin cytoskeleton, cell rounding, and cytopathic effects, respectively. Our study points toward the possibility of repurposing the already approved drug dronedarone as the preferable safer-to-use alternative to amiodarone for inhibiting TcdA/B in the (supportive) therapy of CDADs.

Trauma-toxicology: concepts, causes, complications

Trauma and toxic substances are connected in several aspects. On the one hand, toxic substances can be the reason for traumatic injuries in the context of accidental or violent and criminal circumstances. Examples for the first scenario is the release of toxic gases, chemicals, and particles during house fires, and for the second scenario, the use of chemical or biological weapons in the context of terroristic activities. Toxic substances can cause or enhance severe, life-threatening trauma, as described in this review for various chemical warfare, by inducing a tissue trauma accompanied by break down of important barriers in the body, such as the blood-air or the blood-gut barriers. This in turn initiates a "vicious circle" as the contribution of inflammatory responses to the traumatic damage enhances the macro- and micro-barrier breakdown and often results in fatal outcome. The development of sophisticated methods for detection and identification of toxic substances as well as the special treatment of the intoxicated trauma patient is summarized in this review. Moreover, some highly toxic substances, such as the protein toxins from the pathogenic bacterium Clostridioides (C.) difficile, cause severe post-traumatic complications which significantly worsens the outcome of hospitalized patients, in particular in multiply injured trauma patients. Therefore, novel pharmacological options for the treatment of such patients are necessarily needed and one promising strategy might be the neutralization of the toxins that cause the disease. This review summarizes recent findings on the molecular and cellular mechanisms of toxic chemicals and bacterial toxins that contribute to barrier breakdown in the human body as wells pharmacological options for treatment, in particular in the context of intoxicated trauma patients. "trauma-toxicology" comprises concepts regrading basic research, development of novel pharmacological/therapeutic options and clinical aspects in the complex interplay and "vicious circle" of severe tissue trauma, barrier breakdown, pathogen and toxin exposure, tissue damage, and subsequent clinical complications.

An Updated View on the Cellular Uptake and Mode-of-Action of Clostridioides difficile Toxins

Research on the human gut pathogen Clostridioides (C.) difficile and its toxins continues to attract much attention as a consequence of the threat to human health posed by hypervirulent strains. Toxin A (TcdA) and Toxin B (TcdB) are the two major virulence determinants of C. difficile. Both are single-chain proteins with a similar multidomain architecture. Certain hypervirulent C. difficile strains also produce a third toxin, namely binary toxin CDT (C. difficile transferase). C. difficile toxins are the causative agents of C. difficile-associated diseases (CDADs), such as antibiotics-associated diarrhea and pseudomembranous colitis. For that reason, considerable efforts have been expended to unravel their molecular mode-of-action and the cellular mechanisms responsible for their uptake. Many of these studies have been conducted in European laboratories. Here, we provide an update on our previous review (Papatheodorou et al. Adv Exp Med Biol, 2018) on important advances in C. difficile toxins research.

Oxysterols in Infectious Diseases

Oxysterols have emerged as important bioactive lipids in the immune response to infectious diseases. This chapter discusses our current knowledge of oxysterols and their receptors in bacterial and viral infections of the respiratory and gastrointestinal tracts. Oxysterols are produced in response to infections and have multiple roles including chemotaxis of immune cells to the site of infection and regulation of inflammation. Some oxysterols have been shown to possess antiviral or antibacterial activity.Lastly, we delve into the emerging mechanisms of action of oxysterols. Oxysterols can enhance host cell resistance via reduction of membrane accessible cholesterol, modulate membrane immune signalling, and impact inflammasome activation and efferocytosis.

Exploring the inhibitory potential of the antiarrhythmic drug amiodarone against Clostridioides difficile toxins TcdA and TcdB

The intestinal pathogen Clostridioides difficile is the leading cause of antibiotic-associated diarrhea and pseudomembranous colitis in humans. The symptoms of C. difficile-associated diseases (CDADs) are directly associated with the pathogen's toxins TcdA and TcdB, which enter host cells and inactivate Rho and/or Ras GTPases by glucosylation. Membrane cholesterol is crucial during the intoxication process of TcdA and TcdB, and likely involved during pore formation of both toxins in endosomal membranes, a key step after cellular uptake for the translocation of the glucosyltransferase domain of both toxins from endosomes into the host cell cytosol. The licensed drug amiodarone, a multichannel blocker commonly used in the treatment of cardiac dysrhythmias, is also capable of inhibiting endosomal acidification and, as shown recently, cholesterol biosynthesis. Thus, we were keen to investigate in vitro with cultured cells and human intestinal organoids, whether amiodarone preincubation protects from TcdA and/or TcdB intoxication. Amiodarone conferred protection against both toxins independently and in combination as well as against toxin variants from the clinically relevant, epidemic C. difficile strain NAP1/027. Further mechanistic studies suggested that amiodarone's mode-of-inhibition involves also interference with the translocation pore of both toxins. Our study opens the possibility of repurposing the licensed drug amiodarone as a novel pan-variant antitoxin therapeutic in the context of CDADs.

The Compound U18666A Inhibits the Intoxication of Cells by Clostridioides difficile Toxins TcdA and TcdB

The intestinal pathogen Clostridioides (C.) difficile is a major cause of diarrhea both in hospitals and outpatient in industrialized countries. This bacterium produces two large exotoxins, toxin A (TcdA) and toxin B (TcdB), which are directly responsible for the onset of clinical symptoms of C. difficile-associated diseases (CDADs), such as antibiotics-associated diarrhea and the severe, life-threatening pseudomembranous colitis. Both toxins are multidomain proteins and taken up into host eukaryotic cells via receptor-mediated endocytosis. Within the cell, TcdA and TcdB inactivate Rho and/or Ras protein family members by glucosylation, which eventually results in cell death. The cytotoxic mode of action of the toxins is the main reason for the disease. Thus, compounds capable of inhibiting the cellular uptake and/or mode-of-action of both toxins are of high therapeutic interest. Recently, we found that the sterol regulatory element-binding protein 2 (SREBP-2) pathway, which regulates cholesterol content in membranes, is crucial for the intoxication of cells by TcdA and TcdB. Furthermore, it has been shown that membrane cholesterol is required for TcdA- as well as TcdB-mediated pore formation in endosomal membranes, which is a key step during the translocation of the glucosyltransferase domain of both toxins from endocytic vesicles into the cytosol of host cells. In the current study, we demonstrate that intoxication by TcdA and TcdB is diminished in cultured cells preincubated with the compound U18666A, an established inhibitor of cholesterol biosynthesis and/or intracellular transport. U18666A-pretreated cells were also less sensitive against TcdA and TcdB variants from the epidemic NAP1/027 C. difficile strain. Our study corroborates the crucial role of membrane cholesterol for cell entry of TcdA and TcdB, thus providing a valuable basis for the development of novel antitoxin strategies in the context of CDADs.

Clostridioides difficile Toxin A Remodels Membranes and Mediates DNA Entry Into Cells to Activate Toll-Like Receptor 9 Signaling

BACKGROUND & AIMS

Clostridioides difficile toxin A (TcdA) activates the innate immune response. TcdA co-purifies with DNA. Toll-like receptor 9 (TLR9) recognizes bacterial DNA to initiate inflammation. We investigated whether DNA bound to TcdA activates an inflammatory response in murine models of C difficile infection via activation of TLR9.

METHODS

We performed studies with human colonocytes and monocytes and macrophages from wild-type and TLR9 knockout mice incubated with TcdA or its antagonist (ODN TTAGGG) or transduced with vectors encoding TLR9 or small-interfering RNAs. Cytokine production was measured with enzyme-linked immunosorbent assay. We studied a transduction domain of TcdA (TcdA_57-80), which was predicted by machine learning to have cell-penetrating activity and confirmed by synchrotron small-angle X-ray scattering. Intestines of CD1 mice, C57BL6J mice, and mice that express a form of TLR9 that is not activated by CpG DNA were injected with TcdA, TLR9 antagonist, or both. Enterotoxicity was estimated based on loop weight to length ratios. A TLR9 antagonist was tested in mice infected with C difficile. We incubated human colon explants with an antagonist of TLR9 and measured TcdA-induced production of cytokines.

RESULTS

The TcdA_57-80 protein transduction domain had membrane remodeling activity that allowed TcdA to enter endosomes. TcdA-bound DNA entered human colonocytes. TLR9 was required for production of cytokines by cultured cells and in human colon explants incubated with TcdA. TLR9 was required in TcdA-induced mice intestinal secretions and in the survival of mice infected by C difficile. Even in a protease-rich environment, in which only fragments of TcdA exist, the TcdA_57-80 domain organized DNA into a geometrically ordered structure that activated TLR9.

CONCLUSIONS

TcdA from C difficile can bind and organize bacterial DNA to activate TLR9. TcdA and TcdA fragments remodel membranes, which allows them to access endosomes and present bacterial DNA to and activate TLR9. Rather than inactivating the ability of DNA to bind TLR9, TcdA appears to chaperone and organize DNA into an inflammatory, spatially periodic structure.

Low Density Lipoprotein Receptor-Related Protein-1 (LRP1) Is Involved in the Uptake of Clostridioides difficile Toxin A and Serves as an Internalizing Receptor

Toxin producing Clostridioides difficile strains cause gastrointestinal infections with the large glucosylating protein toxins A (TcdA) and B (TcdB) being major virulence factors responsible for the onset of symptoms. TcdA and TcdB enter their target cells via receptor-mediated endocytosis. Inside the cell, the toxins glucosylate and thereby inactivate small GTPases of the Rho-/Ras subfamilies resulting in actin reorganization and cell death. The receptors of TcdA are still elusive, glycoprotein 96 (gp96), the low density lipoprotein receptor family (LDLR) and sulfated glycosaminoglycans (sGAGs) have most recently been suggested as receptors for TcdA. In this study, we provide evidence on rapid endocytosis of Low density lipoprotein Receptor-related Protein-1 (LRP1) into fibroblasts and Caco-2 cells by exploiting biotinylation of cell surface proteins. In contrast, gp96 was not endocytosed either in the presence or absence of TcdA. The kinetics of internalization of TfR and LRP1 were comparable in the presence and the absence of TcdA, excluding that TcdA facilitates its internalization by triggering internalization of its receptors. Exploiting fibroblasts with a genetic deletion of LRP1, TcdA was about one order of magnitude less potent in LRP1-deficient cells as compared to the corresponding control cells. In contrast, TcdB exhibited a comparable potency in LRP1-proficient and -deficient fibroblasts. These findings suggested a role of LRP1 in the cellular uptake of TcdA but not of TcdB. Correspondingly, binding of TcdA to the cell surface of LRP1-deficient fibroblasts was reduced as compared with LRP1-proficient fibroblasts. Finally, TcdA bound to LRP1 ligand binding type repeat cluster II (amino acid 786–1,165) and cluster IV (amino acid 3332-3779). In conclusion, LRP1 appears to serve as an endocytic receptor and gp96 as a non-endocytic receptor for TcdA.

Membrane Cholesterol Is Crucial for Clostridium difficile Surface Layer Protein Binding and Triggering Inflammasome Activation

Clostridium difficile, an obligate anaerobic gram-positive bacillus, generates spores and is commonly found colonizing the human gut. Patients with C. difficile infection (CDI) often exhibit clinical manifestations of pseudomembranous colitis or antibiotic-associated diarrhea. Surface layer proteins (SLPs) are the most abundant proteins in the C. difficile cell wall, suggesting that they might involve in immune recognition. Our previous results demonstrated that C. difficile triggers inflammasome activation. Here, we found SLPs as well as C. difficile induced inflammasome activation, and in a dose-dependent manner. In addition, the cholesterol-rich microdomains on the cell membrane (also referred to as lipid rafts) are thought to be crucial for bacterial adhesion and signal transduction. We demonstrated that lipid rafts participated in C. difficile SLPs binding to the cell membrane. Fluorescence microscopy showed that membrane cholesterol depletion by methyl-β-cyclodextrin (MβCD) reduced the association of SLPs with the cell surface. The coalescence of SLPs in the cholesterol-rich microdomains was confirmed in C. difficile-infected cells. Furthermore, the inflammasome activations induced by SLPs or C. difficile were abrogated by MβCD. Our results demonstrate that SLPs recruit the lipid rafts, which may be a key step for C. difficile colonization and inducing inflammasome activation.

Toxin B Variants from Clostridium difficile Strains VPI 10463 and NAP1/027 Share Similar Substrate Profile and Cellular Intoxication Kinetics but Use Different Host Cell Entry Factors

Clostridium difficile induces antibiotic-associated diarrhea due to the release of toxin A (TcdA) and toxin B (TcdB), the latter being its main virulence factor. The epidemic strain NAP1/027 has an increased virulence attributed to different factors. We compared cellular intoxication by TcdB_NAP1 with that by the reference strain VPI 10463 (TcdB_VPI). In a mouse ligated intestinal loop model, TcdB_NAP1 induced higher neutrophil recruitment, cytokine release, and epithelial damage than TcdB_VPI. Both toxins modified the same panel of small GTPases and exhibited similar in vitro autoprocessing kinetics. On the basis of sequence variations in the frizzled-binding domain (FBD), we reasoned that TcdB_VPI and TcdB_NAP1 might have different receptor specificities. To test this possibility, we used a TcdB from a NAP1 variant strain (TcdB_NAP1v) unable to glucosylate RhoA but with the same receptor-binding domains as TcdB_NAP1. Cells were preincubated with TcdB_NAP1v to block cellular receptors, prior to intoxication with either TcdB_VPI or TcdB_NAP1. Preincubation with TcdB_NAP1v blocked RhoA glucosylation by TcdB_NAP1 but not by TcdB_VPI, indicating that the toxins use different host factors for cell entry. This crucial difference might explain the increased biological activity of TcdB_NAP1 in the intestine, representing a contributing factor for the increased virulence of the NAP1/027 strain.

Thrombin generation is increased in patients with Clostridium difficile colitis - a pilot study

Background: There are only limited data in the literature on the thrombotic risk of patients with Clostridium difficile (CD) colitis, although this disease is widespread throughout the world. Objective: The aim of this study was to explore thrombin generation in these patients - the best way to evaluate their coagulation. Methods: A prospective observational study was conducted during 15 months on hospitalized patients with CD colitis. Thrombin generation was performed in platelet-poor plasma using a Ceveron^® alpha analyzer and was compared with a group of volunteer control subjects. Results: Thirty-three patients and 51 control subjects were enrolled in the study. Two biomarkers - mean velocity index and peak thrombin - were significantly higher in patient group, compared to the control subjects (p = 0.010, respectively, p = 0.0395). This pattern of thrombin generation suggests that patients with CD colitis without septic shock have a potential thrombotic risk. The mean velocity index significantly correlated with the estimated related risk of death according to the Charlson age-comorbidity index. Conclusions: The higher values of thrombin generation suggest that CD colitis increases the thromboembolic risk. The pattern of thrombin generation could identify patients with particularly higher thromboembolic risk. They are potential candidates for thromboprophylaxis strategies and monitorization.