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Small Molecule Inhibitor Screen Reveals Calcium Channel Signaling as a Mechanistic Mediator of Clostridium difficile TcdB-Induced Necrosis.

ABSTRACT: Clostridioides difficile is the leading cause of nosocomial diarrhea in the United States. The primary virulence factors are two homologous glucosyltransferase toxins, TcdA and TcdB, that inactivate host Rho-family GTPases. The glucosyltransferase activity has been linked to a "cytopathic" disruption of the actin cytoskeleton and contributes to the disruption of tight junctions and the production of pro-inflammatory cytokines. TcdB is also a potent cytotoxin that causes epithelium necrotic damage through an NADPH oxidase (NOX)-dependent mechanism. We conducted a small molecule screen to identify compounds that confer protection against TcdB-induced necrosis. We identified an enrichment of "hit compounds" with a dihydropyridine (DHP) core which led to the discovery of a key early stage calcium signal that serves as a mechanistic link between TcdB-induced NOX activation and reactive oxygen species (ROS) production. Disruption of TcdB-induced calcium signaling (with both DHP and non-DHP molecules) is sufficient to ablate ROS production and prevent subsequent necrosis in cells and in a mouse model of intoxication.

SUBMITTER: Farrow MA

PROVIDER: S-EPMC7230009 | biostudies-literature |

REPOSITORIES: biostudies-literature

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CdtR Regulates TcdA and TcdB Production in Clostridium difficile.

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

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Project description:Clostridium difficile, a highly drug-resistant Gram-positive, spore-forming bacterium, remains a leading cause of hospital-acquired diarrhoea and antibiotic-associated colitis. Clinically, only a handful of antibiotics are used for treating C. difficile infection (CDI), suggesting a necessity for the development of new treatment options. Here we performed a high-throughput screen of 2000 drug-like compounds for inhibition of C. difficile. From this screen, one compound, 5-nitro-1,10-phenanthroline (5-NP), showed potent bactericidal effects in vitro. In addition, this compound displayed high potency towards other Clostridium spp. as well as Mycobacterium bovis but not towards other tested Gram-positive and Gram-negative bacteria. Furthermore, we show that this inhibition may proceed through a metal chelation-dependent mechanism. More importantly, preliminary evidence suggests moderate efficacy for this compound in treating CDI in a murine infection model. These results present a possible basis for the further development of this compound as an antibiotic treatment for CDI.

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Project description:Clostridium difficile is a leading cause of nosocomial infections. The major virulence factors of this pathogen are the multi-domain toxins TcdA and TcdB. These toxins contain a cysteine protease domain (CPD) that autoproteolytically releases a cytotoxic effector domain upon binding intracellular inositol hexakisphosphate. Currently, there are no known inhibitors of this protease. Here, we describe the rational design of covalent small molecule inhibitors of TcdB CPD. We identified compounds that inactivate TcdB holotoxin function in cells and solved the structure of inhibitor-bound protease to 2.0 Å. This structure reveals the molecular basis of CPD substrate recognition and informed the synthesis of activity-based probes for this enzyme. The inhibitors presented will guide the development of therapeutics targeting C. difficile, and the probes will serve as tools for studying the unique activation mechanism of bacterial toxin CPDs.

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