Chloroquine derivatives block the translocation pores and inhibit cellular entry of Clostridium botulinum C2 toxin and Bacillus anthracis lethal toxin

The Clostridium botulinum C2 Toxin Subunit C2IIa Delivers Enzymes with Positively Charged N-Termini into the Cytosol of Target Cells

The binary Clostridium (C.) botulinum C2 toxin consists of two non-linked proteins. The proteolytically activated binding/transport subunit C2IIa forms barrel-shaped homoheptamers, which bind to cell surface receptors, mediate endocytosis, and translocate the enzyme subunit C2I into the cytosol of target cells. Here, we investigate whether C2IIa can be harnessed as a transporter for proteins/enzymes fused to polycationic tags, as earlier demonstrated for the related anthrax toxin transport subunit PA₆₃. To test C2IIa-mediated transport in cultured cells, reporter enzymes are generated by fusing different polycationic tags to the N- or C-terminus of other bacterial toxins' catalytic A subunits. C2IIa as well as PA₆₃ deliver N-terminally polyhistidine-tagged proteins more efficiently compared to C-terminally tagged ones. However, in contrast to PA₆₃, C2IIa does not efficiently deliver polylysine-tagged proteins into the cytosol of target cells. Moreover, untagged enzymes with a native cationic N-terminus are efficiently transported by both C2IIa and PA₆₃. In conclusion, the C2IIa-transporter serves as a transport system for enzymes that harbor positively charged amino acids at their N-terminus. The charge distribution at the N-terminus of cargo proteins and their ability to unfold in the endosome and subsequently refold in the cytosol determine transport feasibility and efficiency.

Chloroquine-analogues block anthrax protective antigen channels in steady-state and kinetic studies

The tripartite anthrax toxin from Bacillus anthracis represents the prototype of A-B type of toxins, where the effector A (an enzymatic subunit) is transported with the help of a binding component B into a target cell. Anthrax toxin consists of three different molecules, two effectors, lethal factor (LF) and edema factor (EF) and the binding component also known as protective antigen (PA). PA forms heptamers or octamers following binding to host cell's receptors and mediates the translocation of the effectors into the cytosol via the endosomal pathway. The cation-selective PA₆₃-channel is able to reconstitute in lipid membranes and can be blocked by chloroquine and other heterocyclic compounds. This suggests that the PA₆₃-channel contains a binding site for quinolines. In this study, we investigated the structure-function relationship of different quinolines for the block of the PA₆₃-channel. The affinity of the different chloroquine analogues to the PA₆₃-channel as provided by the equilibrium dissociation constant was measured using titrations. Some quinolines had a much higher affinity to the PA₆₃-channel than chloroquine itself. We also performed ligand-induced current noise measurements using fast Fourier transformation to get insight in the kinetics of the binding of some quinolines to the PA₆₃-channel. The on-rate constants of ligand binding were around 10⁸M^-1·s^-1 at 150mM KCl and were only little dependent on the individual quinoline. The off-rates varied between 4s^-1 and 160s^-1 and depended much more on the structure of the molecules than the on-rate constants. The possible use of the 4-aminoquinolines as a therapy is discussed.

The Pore-Forming Subunit C2IIa of the Binary Clostridium botulinum C2 Toxin Reduces the Chemotactic Translocation of Human Polymorphonuclear Leukocytes

The binary C2 toxin of Clostridium (C.) botulinum consists of two non-linked proteins, the enzyme subunit C2I and the separate binding/transport subunit C2II. To exhibit toxic effects on mammalian cells, proteolytically activated C2II (C2IIa) forms barrel-shaped heptamers that bind to carbohydrate receptors which are present on all mammalian cell types. C2I binds to C2IIa and the toxin complexes are internalized via receptor-mediated endocytosis. In acidified endosomal vesicles, C2IIa heptamers change their conformation and insert as pores into endosomal membranes. These pores serve as translocation-channels for the subsequent transport of C2I from the endosomal lumen into the cytosol. There, C2I mono-ADP-ribosylates G-actin, which results in depolymerization of F-actin and cell rounding. Noteworthy, so far morphological changes in cells were only observed after incubation with the complete C2 toxin, i.e., C2IIa plus C2I, but not with the single subunits. Unexpectedly, we observed that the non-catalytic transport subunit C2IIa (but not C2II) alone induced morphological changes and actin alterations in primary human polymorphonuclear leukocytes (PMNs, alias neutrophils) from healthy donors ex vivo, but not macrophages, epithelial and endothelial cells, as detected by phase contrast microscopy and fluorescent microscopy of the actin cytoskeleton. This suggests a PMN selective mode of action for C2IIa. The cytotoxicity of C2IIa on PMNs was prevented by C2IIa pore blockers and treatment with C2IIa (but not C2II) rapidly induced Ca²⁺ influx in PMNs, suggesting that pore-formation by C2IIa in cell membranes of PMNs is crucial for this effect. In addition, incubation of primary human PMNs with C2IIa decreased their chemotaxis ex vivo through porous culture inserts and in co-culture with human endothelial cells which is closer to the physiological extravasation process. In conclusion, the results suggest that C2IIa is a PMN-selective inhibitor of chemotaxis. This provides new knowledge for a pathophysiological role of C2 toxin as a modulator of innate immune cells and makes C2IIa an attractive candidate for the development of novel pharmacological strategies to selectively down-modulate the excessive and detrimental PMN recruitment into organs after traumatic injuries.

Anthrax toxin channel: What we know based on over 30 years of research

Protective antigen channel is the central component of the deadly anthrax exotoxin responsible for binding and delivery of the toxin's enzymatic lethal and edema factor components into the cytosol. The channel, which is more than three times longer than the lipid bilayer membrane thickness and has a 6-Å limiting diameter, is believed to provide a sophisticated unfoldase and translocase machinery for the foreign protein transport into the host cell cytosol. The tripartite toxin can be reengineered, one component at a time or collectively, to adapt it for the targeted cancer therapeutic treatments. In this review, we focus on the biophysical studies of the protective antigen channel-forming activity, small ion transport properties, enzymatic factor translocation, and blockage comparing it with the related clostridial binary toxin channels. We address issues linked to the anthrax toxin channel structural dynamics and lipid dependence, which are yet to become generally recognized as parts of the toxin translocation machinery.

Characterization and Pharmacological Inhibition of the Pore-Forming Clostridioides difficile CDTb Toxin

The clinically highly relevant Clostridioides (C.) difficile releases several AB-type toxins that cause diseases such as diarrhea and pseudomembranous colitis. In addition to the main virulence factors Rho/Ras-glycosylating toxins TcdA and TcdB, hypervirulent strains produce the binary AB-type toxin CDT. CDT consists of two separate proteins. The binding/translocation B-component CDTb facilitates uptake and translocation of the enzyme A-component CDTa to the cytosol of cells. Here, CDTa ADP-ribosylates G-actin, resulting in depolymerization of the actin cytoskeleton. We previously showed that CDTb exhibits cytotoxicity in the absence of CDTa, which is most likely due to pore formation in the cytoplasmic membrane. Here, we further investigated this cytotoxic effect and showed that CDTb impairs CaCo-2 cell viability and leads to redistribution of F-actin without affecting tubulin structures. CDTb was detected at the cytoplasmic membrane in addition to its endosomal localization if CDTb was applied alone. Chloroquine and several of its derivatives, which were previously identified as toxin pore blockers, inhibited intoxication of Vero, HCT116, and CaCo-2 cells by CDTb and CDTb pores in vitro. These results further strengthen pore formation by CDTb in the cytoplasmic membrane as the underlying cytotoxic mechanism and identify pharmacological pore blockers as potent inhibitors of cytotoxicity induced by CDTb and CDTa plus CDTb.

Human peptide α-defensin-1 interferes with Clostridioides difficile toxins TcdA, TcdB, and CDT

The human pathogenic bacterium Clostridioides difficile produces two exotoxins TcdA and TcdB, which inactivate Rho GTPases thereby causing C. difficile-associated diseases (CDAD) including life-threatening pseudomembranous colitis. Hypervirulent strains produce additionally the binary actin ADP-ribosylating toxin CDT. These strains are hallmarked by more severe forms of CDAD and increased frequency and severity. Once in the cytosol, the toxins act as enzymes resulting in the typical clinical symptoms. Therefore, targeting and inactivation of the released toxins are of peculiar interest. Prompted by earlier findings that human α-defensin-1 neutralizes TcdB, we investigated the effects of the defensin on all three C. difficile toxins. Inhibition of TcdA, TcdB, and CDT was demonstrated by analyzing toxin-induced changes in cell morphology, substrate modification, and decrease in transepithelial electrical resistance. Application of α-defensin-1 protected cells and human intestinal organoids from the cytotoxic effects of TcdA, TcdB, CDT, and their combination which is attributed to a direct interaction between the toxins and α-defensin-1. In mice, the application of α-defensin-1 reduced the TcdA-induced damage of intestinal loops in vivo. In conclusion, human α-defensin-1 is a specific and potent inhibitor of the C. difficile toxins and a promising agent to develop novel therapeutic options against C. difficile infections.

Rescue of tight junctional localization of a claudin-16 mutant D97S by antimalarial medicine primaquine in Madin-Darby canine kidney cells

Magnesium ion (Mg²⁺) is paracellularly reabsorbed through claudin-16 (CLDN16) in the thick ascending limb (TAL) of Henle's loop in the kidney. Genetic disorders of CLDN16 cause mislocalization of CLDN16, resulting in hypomagnesemia. There is no effective treatment for hypomagnesemia except for magnesium administration. Here, we searched for a novel drug to restore tight junctional localization of a CLDN16 mutant. A D97S mutant, which has a mutation in the first extracellular loop (ECL) of CLDN16, was mainly colocalized with endosome marker, whereas wild-type (WT) CLDN16 was colocalized with ZO-1, an adaptor protein of tight junctions. The protein stability of the D97S mutant was lower than that of WT. The expression level of the D97S mutant was increased by lactacystin, a proteasomal inhibitor. Endocytosis inhibitors increased the tight junctional localization of the D97S mutant. We found that primaquine, an antimalarial agent, increased the protein stability and cell surface localization of the D97S mutant, but the localization of other mutants, which have mutations in the cytosolic domain or second ECL, was not affected. Transepithelial Mg²⁺ flux was increased by primaquine in D97S mutant-expressing cells. The expression of chaperon proteins, proteasome activity, and lactate dehydrogenase release were decreased by primaquine, and the proportion of viable cells increased. In contrast, these effects were not observed in WT CLDN16-expressing cells. These results suggested that primaquine increases the tight junctional localization of the D97S mutant, resulting in a reduction in ER stress and cellular injury. Primaquine may become an effective treatment drug for selected patients with mutant CLDN16.

Potential Therapeutic Effects of Mepacrine against Clostridium perfringens Enterotoxin in a Mouse Model of Enterotoxemia

Clostridium perfringens enterotoxin (CPE) is a pore-forming toxin that causes the symptoms of common bacterial food poisoning and several non-foodborne human gastrointestinal diseases, including antibiotic-associated diarrhea and sporadic diarrhea. In some cases, CPE-mediated disease can be very severe or fatal due to the involvement of enterotoxemia. Therefore, the development of potential therapeutics against CPE action during enterotoxemia is warranted. Mepacrine, an acridine derivative drug with broad-spectrum effects on pores and channels in mammalian membranes, has been used to treat protozoal intestinal infections in human patients. A previous study showed that the presence of mepacrine inhibits CPE-induced pore formation and activity in enterocyte-like Caco-2 cells, reducing the cytotoxicity caused by this toxin in vitro Whether mepacrine is similarly protective against CPE action in vivo has not been tested. When the current study evaluated whether mepacrine protects against CPE-induced death and intestinal damage using a murine ligated intestinal loop model, mepacrine protected mice from the enterotoxemic lethality caused by CPE. This protection was accompanied by a reduction in the severity of intestinal lesions induced by the toxin. Mepacrine did not reduce CPE pore formation in the intestine but inhibited absorption of the toxin into the blood of some mice. Protection from enterotoxemic death correlated with the ability of this drug to reduce CPE-induced hyperpotassemia. These in vivo findings, coupled with previous in vitro studies, support mepacrine as a potential therapeutic against CPE-mediated enterotoxemic disease.

Screening of a Drug Library Identifies Inhibitors of Cell Intoxication by CNF1

Cytotoxic necrotizing factor 1 (CNF1) is a toxin produced by pathogenic strains of Escherichia coli responsible for extra-intestinal infections. CNF1 deamidates Rac1, thereby triggering its permanent activation and worsening inflammatory reactions. Activated Rac1 is prone to proteasomal degradation. There is no targeted therapy against CNF1, despite its clinical relevance. In this work we developed a fluorescent cell-based immunoassay to screen for inhibitors of CNF1-induced Rac1 degradation among 1120 mostly approved drugs. Eleven compounds were found to prevent CNF1-induced Rac1 degradation, and five also showed a protective effect against CNF1-induced multinucleation. Finally, lasalocid, monensin, bepridil, and amodiaquine protected cells from both diphtheria toxin and CNF1 challenges. These data highlight the potential for drug repurposing to fight several bacterial infections and Rac1-based diseases.

The Potential Therapeutic Agent Mepacrine Protects Caco-2 Cells against Clostridium perfringens Enterotoxin Action

Clostridium perfringens enterotoxin (CPE) causes the gastrointestinal (GI) symptoms of a common bacterial food poisoning and several nonfoodborne human GI diseases. A previous study showed that, via an undetermined mechanism, the presence of mepacrine blocks CPE-induced electrophysiologic activity in artificial membranes. The current study now demonstrates that mepacrine also inhibits CPE-induced cytotoxicity in human enterocyte-like Caco-2 cells and that mepacrine does not directly inactivate CPE. Instead, this drug reduces both CPE pore formation and CPE pore activity in Caco-2 cells. These results suggest mepacrine as a therapeutic candidate for treating CPE-mediated GI diseases.

Clostridium perfringens enterotoxin (CPE) causes the diarrhea associated with a common bacterial food poisoning and many antibiotic-associated diarrhea cases. The severity of some CPE-mediated disease cases warrants the development of potential therapeutics. A previous study showed that the presence of mepacrine inhibited CPE-induced electrophysiology effects in artificial lipid bilayers lacking CPE receptors. However, that study did not assess whether mepacrine inactivates CPE or, instead, inhibits a step in CPE action. Furthermore, CPE action in host cells is complex, involving the toxin binding to receptors, receptor-bound CPE oligomerizing into a prepore on the membrane surface, and β-hairpins in the CPE prepore inserting into the membrane to form a pore that induces cell death. Therefore, the current study evaluated the ability of mepacrine to protect cells from CPE. This drug was found to reduce CPE-induced cytotoxicity in Caco-2 cells. This protection did not involve mepacrine inactivation of CPE, indicating that mepacrine affects one or more steps in CPE action. Western blotting then demonstrated that mepacrine decreases CPE pore levels in Caco-2 cells. This mepacrine-induced reduction in CPE pore levels did not involve CPE binding inhibition but rather an increase in CPE monomer dissociation due to mepacrine interactions with Caco-2 membranes. In addition, mepacrine was also shown to inhibit CPE pores when already present in Caco-2 cells. These in vitro studies, which identified two mepacrine-sensitive steps in CPE-induced cytotoxicity, add support to further testing of the therapeutic potential of mepacrine against CPE-mediated disease.

IMPORTANCEClostridium perfringens enterotoxin (CPE) causes the gastrointestinal (GI) symptoms of a common bacterial food poisoning and several nonfoodborne human GI diseases. A previous study showed that, via an undetermined mechanism, the presence of mepacrine blocks CPE-induced electrophysiologic activity in artificial membranes. The current study now demonstrates that mepacrine also inhibits CPE-induced cytotoxicity in human enterocyte-like Caco-2 cells and that mepacrine does not directly inactivate CPE. Instead, this drug reduces both CPE pore formation and CPE pore activity in Caco-2 cells. These results suggest mepacrine as a therapeutic candidate for treating CPE-mediated GI diseases.