Clostridium difficile infection (CDI) causes antibiotic-associated diarrhea and pseudomembranous colitis. Hypervirulent strains of the pathogen, which are responsible for increased morbidity and mortality of CDI, produce the binary actin-ADP ribosylating toxin Clostridium difficile transferase (CDT) in addition to the Rho-glucosylating toxins A and B. CDT depolymerizes the actin cytoskeleton, increases adherence and colonization of Clostridia by induction of microtubule-based cell protrusions and, eventually, causes death of target cells. Using a haploid genetic screen, we identified the lipolysis-stimulated lipoprotein receptor as the membrane receptor for CDT uptake by target cells. Moreover, we show that Clostridium perfringens iota toxin, which is a related binary actin-ADP ribosylating toxin, enters target cells via the lipolysis-stimulated lipoprotein receptor. Identification of the toxin receptors is essential for understanding of the toxin uptake and provides a most valuable basis for antitoxin strategies.bacterial ADP-ribosyltransferases | receptor binding

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Clostridium difficile toxin A (TcdA) and toxin B (TcdB), C. sordellii lethal toxin (TcsL) and C. novyi α-toxin (TcnA) are important pathogenicity factors, which represent the family of the clostridial glucosylating toxins (CGTs). Toxin A and B are associated with antibiotic-associated diarrhea and pseudomembraneous colitis. Lethal toxin is involved in toxic shock syndrome after abortion and α-toxin in gas gangrene development. CGTs enter cells via receptor-mediated endocytosis and require an acidified endosome for translocation of the catalytic domain into the cytosol. Here we studied the endocytic processes that mediate cell internalization of the CGTs. Intoxication of cells was monitored by analyzing cell morphology, status of Rac glucosylation in cell lysates and transepithelial resistance of cell monolayers. We found that the intoxication of cultured cells by CGTs was strongly delayed when cells were preincubated with dynasore, a cell-permeable inhibitor of dynamin, or chlorpromazine, an inhibitor of the clathrin-dependent endocytic pathway. Additional evidence about the role of clathrin in the uptake of the prototypical CGT family member toxin B was achieved by expression of a dominant-negative inhibitor of the clathrin-mediated endocytosis (Eps15 DN) or by siRNA against the clathrin heavy chain. Accordingly, cells that expressed dominant-negative caveolin-1 were not protected from toxin B-induced cell rounding. In addition, lipid rafts impairment by exogenous depletion of sphingomyelin did not decelerate intoxication of HeLa cells by CGTs. Taken together, our data indicate that the endocytic uptake of the CGTs involves a dynamin-dependent process that is mainly governed by clathrin.

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Background: TpeL is a member of the family of clostridial glucosylating toxins, produced by Clostridium perfringens. Results: TpeL enters target cells by self-mediated entry and mono-glycosylates Ras proteins at Thr-35. Conclusion: TpeL inhibits Ras signaling and induces apoptosis in target cells. Significance: TpeL is a new glucosylating toxin produced by C. perfringens.

Clostridium spiroforme produces the binary actin-ADP-ribosylating toxin CST (C. spiroforme toxin), which has been proposed to be responsible for diarrhea, enterocolitis, and eventually death, especially in rabbits. Here we report on the recombinant production of the enzyme component (CSTa) and the binding component (CSTb) of C. spiroforme toxin in Bacillus megaterium. By using the recombinant toxin components, we show that CST enters target cells via the lipolysis-stimulated lipoprotein receptor (LSR), which has been recently identified as the host cell receptor of the binary toxins Clostridium difficile transferase (CDT) and Clostridium perfringens iota toxin. Microscopic studies revealed that CST, but not the related Clostridium botulinum C2 toxin, colocalized with LSR during toxin uptake and traffic to endosomal compartments. Our findings indicate that CST shares LSR with C. difficile CDT and C. perfringens iota toxin as a host cell surface receptor.

Clostridium sordellii lethal toxin and Clostridium novyi ␣-toxin, which are virulence factors involved in the toxic shock and gas gangrene syndromes, are members of the family of clostridial glucosylating toxins. The toxins inactivate Rho/Ras proteins by glucosylation or attachment of GlcNAc (␣-toxin). Here, we studied the activation of the autoproteolytic processing of the toxins by inositol hexakisphosphate (InsP 6 ) and compared it with the processing of Clostridium difficile toxin B. In the presence of low concentrations of InsP 6 (<1 M), toxin fragments consisting of the N-terminal glucosyltransferase (or GlcNActransferase) domains and the cysteine protease domains (CPDs) of C. sordellii lethal toxin, C. novyi ␣-toxin, and C. difficile toxin B were autocatalytically processed. The cleavage sites of lethal toxin (Leu-543) and ␣-toxin (Leu-548) and the catalytic cysteine residues (Cys-698 of lethal toxin and Cys-707 of ␣-toxin) were identified. Affinity of the CPDs for binding InsP 6 was determined by isothermal titration calorimetry. In contrast to fulllength toxin B and ␣-toxin, autocatalytic cleavage and InsP 6 binding of full-length lethal toxin depended on low pH (pH 5) conditions. The data indicate that C. sordellii lethal toxin and C. novyi ␣-toxin are InsP 6 -dependently processed. However, full-length lethal toxin, but not its short toxin fragments consisting of the glucosyltransferase domain and the CPD, requires a pH-sensitive conformational change to allow binding of InsP 6 and subsequent processing of the toxin.

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