Wild-type Toxin Research Articles

Random Mutational Analysis Targeting Residue K155 within the Transmembrane β-Hairpin of the Mosquitocidal Mpp46Ab Toxin.

Mpp46Ab is a mosquito-larvicidal pore-forming toxin derived from Bacillus thuringiensis TK-E6. Pore formation is believed to be a central mode of Mpp46Ab action, and the cation selectivity of the channel pores, in particular, is closely related to its mosquito-larvicidal activity. In the present study, we constructed a mutant library in which residue K155 within the transmembrane β-hairpin was randomly replaced with other amino acid residues. Upon mutagenesis and following primary screening using Culex pipiens mosquito larvae, we obtained 15 mutants in addition to the wild-type toxin. Bioassays using purified proteins revealed that two mutants, K155E and K155I, exhibited toxicity significantly higher than that of the wild-type toxin. Although increased cation selectivity was previously reported for K155E channel pores, we demonstrated in the present study that the cation selectivity of K155I channel pores was also significantly increased. Considering the characteristics of the amino acids, the charge of residue 155 may not directly affect the cation selectivity of Mpp46Ab channel pores. Replacement of K155 with glutamic acid or isoleucine may induce a similar conformational change in the region associated with the ion selectivity of the Mpp46Ab channel pores. Mutagenesis targeting the transmembrane β-hairpin may be an effective strategy for enhancing the ion permeability of the channel pores and the resulting mosquito-larvicidal activity of Mpp46Ab.

Functional Characterization of a New Degradation Peptide BmTX4-P1 from Traditional Chinese Scorpion Medicinal Material.

Thermally processed Buthus martensii Karsch scorpion is an important traditional Chinese medical material that has been widely used to treat various diseases in China for over one thousand years. Our recent work showed that thermally processed Buthus martensii Karsch scorpions contain many degraded peptides; however, the pharmacological activities of these peptides remain to be studied. Here, a new degraded peptide, BmTX4-P1, was identified from processed Buthus martensii Karsch scorpions. Compared with the venom-derived wild-type toxin peptide BmTX4, BmTX4-P1 missed some amino acids at the N-terminal and C-terminal regions, while containing six conserved cysteine residues, which could be used to form disulfide bond-stabilized α-helical and β-sheet motifs. Two methods (chemical synthesis and recombinant expression) were used to obtain the BmTX4-P1 peptide, named sBmTX4-P1 and rBmTX4-P1. Electrophysiological experimental results showed that sBmTX4-P1 and rBmTX4-P1 exhibited similar activities to inhibit the currents of hKv1.2 and hKv1.3 channels. In addition, the experimental electrophysiological results of recombinant mutant peptides of BmTX4-P1 indicated that the two residues of BmTX4-P1 (Lys22 and Tyr31) were the key residues for its potassium channel inhibitory activity. In addition to identifying a new degraded peptide, BmTX4-P1, from traditional Chinese scorpion medicinal material with high inhibitory activities against the hKv1.2 and hKv1.3 channels, this study also provided a useful method to obtain the detailed degraded peptides from processed Buthus martensii Karsch scorpions. Thus, the study laid a solid foundation for further research on the medicinal function of these degraded peptides.

The role of the constriction in defining the molecular sensing trap of aerolysin-based pores.

It has recently been shown that ionic current modulation by analytes visiting the pore formed by wild-type or mutant aerolysin (AeL) toxin is sensitive not only to molecular volume but also to molecular shape (1). One prerequisite of the remarkable sensitivity of this “whole molecule sensing” regime is the prolonged (1-100 ms) trapping of single molecules in the beta-barrel of the pore. At the same time, it is well known that macromolecular analytes are unable to enter this portion of the AeL-pore directly from the trans-side (2) but must enter from cis passinga constriction. We use high-bandwidth (0-100 kHz) recording of blockades induced by short polynucleotides (dA3,dA4,dA5) and oligoarginine peptides (R3,R5,R7) in conjunction with mutations of charged residues defining the trans-ward boundary of the constriction (D222C, R220S) to probe its role in the sensing mechanism. Both mutations induced rectification (D222C cis-ward; R220S trans-ward) and significantly affected the probability of transient deep blocks indicating analyte returns to the constriction after the first visit to the trap, with D222C potentiating and R220S reducing it for nuleotides. However, the initial deep block indicative of constriction passage remained in R220S. In the case of peptides, however, D222C left deep blocks largely unaffected while R220S nearly abolished returns and prolonged total blockage durations. These and other results indicate that the residues at the border between the constriction and the trap do not merely act by their charges but exert important steric influences and that further modifications of these residues may tailor the pore to various analytes. (1) Ouldali et al. 2020. Nat. Biotech. 38:176-181; Ensslen et al. 2022. JACS 144:16060-16068. (2) Baaken et al. 2015. ACS Nano. 9:6443-6449. (3) Iacovache et al. 2016. Nat Comms. 7:12062-8.

Trimming the disordered C-terminal extension of the protease inhibitor Hg1 produces a potent Kv1.3 blocker.

Animal venom contains a mixture of toxic polypeptides that act in synergy to enable the fast immobilization of prey or deter predators and competitors. This polypeptide arsenal is constantly replenished by recruiting protein scaffolds from various organs into the venom gland. The recruited proteins often retain their ancestral functions but acquire specific modifications associated with novel toxic functions. Here we report an in-depth analysis of Hg1, a Kunitz-fold protease inhibitor with moderate blocking activity on Kv1.3, isolated from the venom gland of the Hadrurus gertschi scorpion. Electrophysiological analyses, supported by MD simulations, suggested that in contrast to canonical K+ blocking peptides, Hg1 does not insert a lysine residue into the channel pore. The crystal structure of the toxin revealed an unusually long, disordered extension at the C-terminal region of the molecule. Deletion of the C-terminal extension increased the potency of the toxin by order of magnitude and restored the voltage sensitivity of the block, a hallmark of the canonical block mechanism. In silico analysis supported the notion that removal of the disordered C-terminal extension has lifted a geometric restraint that prevents the wild-type toxin from effectively blocking Kv1.3. We propose that Hg1 is an evolutionary intermediate between a potent protease inhibitor and a potent K+ channel blocker.

Pore formation-independent cell death induced by a β-barrel pore-forming toxin.

Vibrio cholerae cytolysin (VCC) is a β-barrel pore-forming toxin (β-PFT). It exhibits potent hemolytic activity against erythrocytes that appears to be a direct outcome of its pore-forming functionality. However, VCC-mediated cell-killing mechanism is more complicated in the case of nucleated mammalian cells. It induces apoptosis in the target nucleated cells, mechanistic details of which are still unclear. Furthermore, it has never been explored whether the ability of VCC to trigger programmed cell death is stringently dependent on its pore-forming activity. Here, we show that VCC can evoke hallmark features of the caspase-dependent apoptotic cell death even in the absence of the pore-forming ability. Our study demonstrates that VCC mutants with abortive pore-forming hemolytic activity can trigger apoptotic cell death responses and cytotoxicity, similar to those elicited by the wild-type toxin. VCC as well as its pore formation-deficient mutants display prominent propensity to translocate to the target cell mitochondria and cause mitochondrial membrane damage. Therefore, our results for the first time reveal that VCC, despite being an archetypical β-PFT, can kill target nucleated cells independent of its pore-forming functionality. These findings are intriguing for a β-PFT, whose destination is generally expected to remain limited on the target cell membranes, and whose mode of action is commonly attributed to the membrane-damaging pore-forming ability. Taken together, our study provides critical new insights regarding distinct implications of the two important virulence functionalities of VCC for the V. cholerae pathogenesis process: hemolytic activity for iron acquisition and cytotoxicity for tissue damage by the bacteria.

Aluminum hydroxide adjuvant diverts the uptake and trafficking of genetically detoxified pertussis toxin to lysosomes in macrophages.

Aluminum salts have been successfully utilized as adjuvants to enhance the immunogenicity of vaccine antigens since the 1930s. However, the cellular mechanisms behind the immune adjuvanticity effect of these materials in antigen‐presenting cells are poorly understood. In this study, we investigated the uptake and trafficking of aluminum oxy‐hydroxide (AlOOH), in RAW 264.7 murine and U‐937 human macrophages‐like cells. Furthermore, we determined the impact that the adsorption to AlOOH particulates has on the trafficking of a Bordetella pertussis vaccine candidate, the genetically detoxified pertussis toxin (gdPT). Our results indicate that macrophages internalize AlOOH by constitutive macropinocytosis assisted by the filopodial protrusions that capture the adjuvant particles. Moreover, we show that AlOOH has the capacity to nonspecifically adsorb IgG, engaging opsonic phagocytosis, which is a feature that may allow for more effective capture and uptake of adjuvant particles by antigen‐presenting cells (APCs) at the site of vaccine administration. We found that AlOOH traffics to endolysosomal compartments that hold degradative properties. Importantly, while we show that gdPT escapes degradative endolysosomes and traffics toward the retrograde pathway, as reported for the wild‐type pertussis toxin, the adsorption to AlOOH diverts gdPT to traffic to the adjuvant’s lysosome‐type compartments, which may be key for MHC‐II‐driven antigen presentation and activation of CD4+ T cell. Thus, our findings establish a direct link between antigen adsorption to AlOOH and the intracellular trafficking of antigens within antigen‐presenting cells and bring to light a new potential mechanism for aluminum adjuvancy. Moreover, the in‐vitro single‐cell approach described herein provides a general framework and tools for understanding critical attributes of other vaccine formulations.

Discrimination of the Activity of Low-Affinity Wild-Type and High-Affinity Mutant Recombinant BoNT/B by a SIMA Cell-Based Reporter Release Assay.

Botulinum neurotoxin (BoNT) is used for the treatment of a number of ailments. The activity of the toxin that is isolated from bacterial cultures is frequently tested in the mouse lethality assay. Apart from the ethical concerns inherent to this assay, species-specific differences in the affinity for different BoNT serotypes give rise to activity results that differ from the activity in humans. Thus, BoNT/B is more active in mice than in humans. The current study shows that the stimulus-dependent release of a luciferase from a differentiated human neuroblastoma–based reporter cell line (SIMA-hPOMC1-26-Gluc) was inhibited by clostridial and recombinant BoNT/A to the same extent, whereas both clostridial and recombinant BoNT/B inhibited the release to a lesser extent and only at much higher concentrations, reflecting the low activity of BoNT/B in humans. By contrast, the genetically modified BoNT/B-MY, which has increased affinity for human synaptotagmin, and the BoNT/B protein receptor inhibited luciferase release effectively and with an EC50 comparable to recombinant BoNT/A. This was due to an enhanced uptake into the reporter cells of BoNT/B-MY in comparison to the recombinant wild-type toxin. Thus, the SIMA-hPOMC1-26-Gluc cell assay is a versatile tool to determine the activity of different BoNT serotypes providing human-relevant dose-response data.

His180 in the pore-lining α4 of the Bacillus thuringiensis Cry4Aa δ-endotoxin is crucial for structural arrangements of the α4-α5 transmembrane hairpin and hence biotoxicity

One proposed toxic mechanism of Bacillus thuringiensis Cry δ-endotoxins involves pore formation in target membranes by the α4-α5 transmembrane hairpin constituting their pore-forming domain. Here, nine selected charged and uncharged polar residues in the pore-lining α4 of the Cry4Aa mosquito-active toxin were substituted with Ala. All mutant toxins, i.e., D169A, R171A, Q173A, H178A, Y179A, H180A, Q182A, N183A and E187A, were over-expressed in Escherichia coli as 130-kDa protoxin inclusions at levels comparable to the wild-type toxin. Bioassays against Aedes aegypti larvae revealed that only H178A and H180A mutants displayed a drastic reduction in biotoxicity, albeit almost complete insolubility observed for H178A, but not for H180A inclusions. Further mutagenic analysis showed that replacements of His180 with charged (Arg, Lys, Asp, Glu), small uncharged polar (Ser, Cys) or small non-polar (Gly, Val) residues severely impaired the biotoxicity, unlike substitutions with relatively large uncharged (Asn, Gln, Leu) or aromatic (Phe, Tyr, Trp) residues. Similar to the trypsin-activated wild-type toxin, both bio-active and -inactive H180 mutants were still capable of releasing entrapped calcein from lipid vesicles and producing cation-selective channels with ~130-pS maximum conductance. Analysis of the Cry4Aa structure revealed the existence of a hydrophobic cavity near the critical His180 side-chain. Analysis of simulated structures revealed that His180-to-smaller residue conversions create a gap disrupting such cavity's hydrophobicity and hence structural arrangements of the α4-α5 hairpin. Altogether, our data disclose a critical involvement in Cry4Aa-biotoxicity of His180 exclusively present in the lumen-facing α4 for providing proper environment for the α4-α5 hairpin prior to membrane-inserted pore formation.

Structural and dynamical effects of targeted mutations on μO-Conotoxin MfVIA: Molecular simulation studies

Conotoxins are a group of cysteine-rich, neurotoxic peptides isolated from the venom of marine cone snails. MfVIA is a member of the μO-conotoxin family, and acts as an inhibitor of subtype 1.8 voltage-gated sodium ion channels (NaV1.8). The unique selectivity of MfVIA as an inhibitor of NaV1.8 makes it an ideal peptide for elucidation of the physiological functions of this voltage-gated ion channel. Previous experimental studies of point mutations of MfVIA showed that the double mutant [E5K,E8K] exhibited greater activity at NaV1.8 relative to the wild-type toxin. The present study employs molecular dynamics (MD) simulations to examine the effects of various mutations at these key residues (E5 and E8) on the structure and dynamics of MfVIA. Five double mutants were studied, in which the positions 5 and 8 residues were mutated to amino acids with a range of different physicochemical properties, namely [E5A,E8A], [E5D,E8D], [E5F,E8F], [E5K,E8K], and [E5R,E8R]. Except for [E5D,E8D], all of the mutants tend to show decreased contacts at the N-terminus owing to the loss of the R1-E5 salt bridge relative to that of the wild-type, which subsequently lead to greater exposure and flexibility of the N-terminus for most of the mutant peptides studied, potentially rendering them more able to interact with other species, including NaV1.8. Molecular docking studies of the peptides to NaV1.8 via different binding mechanisms suggest that the [E5R, E8R] mutant may be especially worthy of further investigation owing to its predicted binding mode, which differs markedly from those of the other peptides in this study.

Preclinical in vitro and in vivo profile of a highly-attenuated, broadly efficacious pneumolysin genetic toxoid

Pneumolysin is a highly conserved, cholesterol-dependent cytolysin that is an important Streptococcus pneumoniae virulence factor and an attractive target for vaccine development. To attenuate pneumolysin toxicity, a genetic toxoid was constructed with two amino acid changes, G293S and L460D, termed PLY-D, that reduced cytolytic activity > 125,000-fold. In mice, PLY-D elicited high anti-PLY IgG antibody titers that neutralized the cytolytic activity of the wild-type toxin in vitro. To evaluate the protective efficacy of PLY-D, mice were immunized intramuscularly and then challenged intranasally with a lethal dose of 28 clinical isolates of S. pneumoniae originating from different geographical locations, disease states (i.e. bacteremia, pneumonia), or body sites (i.e. sputum, blood). PLY-D immunization conferred significant protection from challenge with 17 of 20 serotypes (85%) and 22 of 28 strains (79%). Further, we demonstrated that immunization with PLY-D provided statistically significant improvement in survival against challenge with serotype 4 and 18C strains compared to mice immunized with a pneumococcal conjugate vaccine Prevnar 13® (PCV13). Co-administration of PLY–D and PCV13 conferred greater protection against challenge with a serotype 6B strain than immunization with either vaccine alone. These data indicate that PLY-D is a broadly protective antigen with the potential to serve as a serotype-independent vaccine against invasive pneumococcal disease either alone or in combination with PCVs.

Disulfide bond of Mycoplasma pneumoniae community-acquired respiratory distress syndrome toxin is essential to maintain the ADP-ribosylating and vacuolating activities.

Mycoplasma pneumoniae is the leading cause of bacterial community-acquired pneumonia among hospitalised children in United States and worldwide. Community-acquired respiratory distress syndrome (CARDS) toxin is a key virulence determinant of M. pneumoniae. The N-terminus of CARDS toxin exhibits ADP-ribosyltransferase (ADPRT) activity, and the C-terminus possesses binding and vacuolating activities. Thiol-trapping experiments of wild-type (WT) and cysteine-to-serine-mutated CARDS toxins with alkylating agents identified disulfide bond formation at the amino terminal cysteine residues C230 and C247. Compared with WT and other mutant toxins, C247S was unstable and unusable for comparative studies. Although there were no significant variations in binding, entry, and retrograde trafficking patterns of WT and mutated toxins, C230S did not elicit vacuole formation in intoxicated cells. In addition, the ADPRT domain of C230S was more sensitive to all tested proteases when compared with WT toxin. Despite its in vitro ADPRT activity, the reduction of C230S CARDS toxin-mediated ADPRT activity-associated IL-1β production in U937 cells and the recovery of vacuolating activity in the protease-released carboxy region of C230S indicated that the disulfide bond was essential not only to maintain the conformational stability of CARDS toxin but also to properly execute its cytopathic effects.

Specific targeting of a pore-forming toxin (listeriolysin O) to LHRH-positive cancer cells using LHRH targeting peptide

Conventional drug delivery systems have many limitations including cytotoxicity and affecting non-specific cells. Cell-targeting peptides (CTPs) as a potential class of targeting moiety have some advantages over previous targeting moieties such as monoclonal antibodies, offer additional benefits to design systems using CTPs. Here we have engineered listeriolysin O (LLO) pore-forming toxin by adding a luteinizing hormone-releasing hormone (LHRH) targeting peptide to its N-terminus. Two versions of the toxin, with and without targeting peptide, were sub-cloned into a bacterial expression plasmid. BL21 DE3 cells were used for induction of expression and recombinant proteins were purified using nickel-immobilized metal affinity chromatography column. In order to treat MDA-MB-231 and SKOV3 cell lines as LHRH receptor positive and negative cells, two mentioned LLO toxins were used to evaluate their cytotoxicity and specificity. Our results reveal that the IC50 of LLO toxin on MDA-MB-231 and SKOV3 cells was 0.32 and 0.41 μg/ml respectively. Furthermore, IC50 of fusion LHRH-LLO toxin on the cells was 0.88 and 19.55 μg/ml. Cytotoxicity of engineered LHRH-LLO toxin on negative cells was significantly 48-fold lower than wild-type LLO toxin. But this difference has been lowered to only 2.7-fold less cytotoxicity in positive cells. To the best of our knowledge, the current work as the first study regarding engineered toxin revealed that CDC family members could be used to target the specific cell-type.

Targeting MCF-7 Cell Line by Listeriolysin O Pore Forming Toxin Fusion with AHNP Targeted Peptide.

Background:Tumor-targeting peptides are attracting subjects in cancer therapy. These peptides, which are widely studied, deliver therapeutic agents to the specific sites of tumors. In this study, we produced a new form of recombinant listeriolysin O (LLO) with genetically fused Anti-HER2/neu peptide (AHNP) sequence adding to its C-terminal end. The aim of the study was to engineer this pore-forming toxin to make it much more specific to tumor cells.Materials and Method and Results:Two forms of the toxin (with and without peptide) were subcloned into a bacterial expression plasmid. Subcloning was performed using a polymerase chain reaction (PCR) product as a megaprimer in a quick-change PCR to introduce the whole insert gene into the expression plasmid. After expression of two recombinant forms of LLO in BL21 DE3 cells, purification was performed using Ni-NTA affinity column. MDA-MB-231 and MCF-7 cell lines (as negative and positive controls, respectively) were treated with both LLO toxins to evaluate their cytotoxicity and specificity. The IC50 of LLO on MDA-MB-231 and MCF-7 cells was 21 and 5 ng/ml, respectively. In addition, IC50 for the fusion AHNP-LLO toxin was 140 and 60 ng/ml, respectively. It was found that the cytotoxicity of the new engineered AHNP-LLO toxin has decreased by about 9x compared to the wild-type toxin and the specificity of the AHNP-LLO toxin has been also reduced.Conclusions:Results show that the C-terminal of the LLO should not be modified and it seems that N-terminal of the toxin should be preferred for engineering and adding peptide modules.

Enhancement of insect susceptibility and larvicidal efficacy of Cry4Ba toxin by calcofluor

BackgroundMosquitoes transmit many vector-borne infectious diseases including malaria, dengue, chikungunya, yellow fever, filariasis, and Japanese encephalitis. The insecticidal δ-endotoxins Cry4, Cry11, and Cyt produced from Bacillus thuringiensis have been used for bio-control of mosquito larvae. Cry δ-endotoxins are synthesised as inactive protoxins in the form of crystalline inclusions in which they are processed to active toxins in larval midgut lumen. Previously, we demonstrated that the activated Cry4Ba toxin has to alter the permeability of the peritrophic membrane (PM), allowing toxin passage across PM to reach specific receptors on microvilli of larval midgut epithelial cells, where the toxin undergoes conformational changes, followed by membrane insertion and pore formation, resulting in larval death. A peritrophic membrane (PM)-binding calcofluor has been proposed to inhibit chitin formation and enhance baculovirus infection of lepidopteran Trichoplusia ni.MethodsIn this study, Aedes aegypti larvae were fed with the calcofluor and Cry4Ba toxin to investigate the effect of this agent on the toxicity of the Cry4Ba toxin.ResultsCalcofluor displayed an enhancing effect when co-fed with the Cry4Ba wild-type toxin. The agent could restore the killing activity of the partially active Cry4Ba mutant E417A/Y455A toward Ae. aegypti larvae. PM destruction was observed after larval challenge with calcofluor together with the toxin. Interestingly, calcofluor increased Cry4Ba toxin susceptibility toward semi-susceptible Culex quinquefasciatus larvae. However, calcofluor alone or in combination with the toxin showed no mortality effect on non-susceptible fresh-water fleas, Moina macrocopa.ConclusionsOur results suggest that PM may contribute to the resistance of the mosquito larvae to Cry4Ba toxin. The PM-permeability alternating calcofluor might be a promising candidate for enhancing insect susceptibility, which will consequently improve Cry4Ba efficacy in field settings in the future.

Contribution of toxic shock syndrome toxin-1 to systemic inflammation investigated by a mouse model of cervicovaginal infection with Staphylococcus aureus.

Toxic shock syndrome toxin-1 (TSST-1), a superantigen produced by Staphylococcus aureus is a causative agent of toxic shock syndrome (TSS) that is frequently associated with tampon use. It has long been suggested that TSS is induced when TSST-1 circulates through the body. However, the systemic distribution of TSST-1 from vagina or uterus has never been demonstrated. In this study, a mouse cervicovaginal infection model was established. Transcervical inoculation with a virulence strain of S. aureus and its derivative TSST-1-deficient mutant demonstrated that TSST-1 distributed to the bloodstream and spleen, and promoted systemic inflammation without bacteremia. Transcervical administration with the wild-type toxin and a superantigen-deficient mutant of TSST-1 (mTSST-1) demonstrated that the superantigenic activity of TSST-1 was essential to stimulate the systemic inflammation. Furthermore, this activity was not promoted by co-transcervical inoculation with lipopolysaccharides. The circulating TSST-1 and systemic inflammation rapidly reduced at 48h after administration, suggesting that persistence of S. aureus in the uterus may be involved in long-term complications of TSS. Transcervical inoculation with mTSST-1-producing S. aureus showed that this toxin promoted bacterial number, uterine tissue damage, and localization of bacterial cells around uterine cavity. The results suggest that TSST-1 enhances S. aureus burden in uterine cavity, the secreted TSST-1 distributes into circulation system, and then systemic inflammation is induced.

Determinants of Raft Partitioning of the Helicobacter pylori Pore-Forming Toxin VacA.

Helicobacter pylori, a Gram-negative bacterium, is a well-known risk factor for gastric cancer. H. pylori vacuolating cytotoxin A (VacA) is a secreted pore-forming toxin that induces a wide range of cellular responses. Like many other bacterial toxins, VacA has been hypothesized to utilize lipid rafts to gain entry into host cells. Here, we used giant plasma membrane vesicles (GPMVs) as a model system to understand the preferential partitioning of VacA into lipid rafts. We show that a wild-type (WT) toxin predominantly associates with the raft phase. Acid activation of VacA enhances binding of the toxin to GPMVs but is not required for raft partitioning. VacA mutant proteins with alterations at the amino terminus (resulting in impaired membrane channel formation) and a nonoligomerizing VacA mutant protein retain the ability to preferentially associate with lipid rafts. Consistent with these results, the isolated VacA p55 domain was capable of binding to lipid rafts. We conclude that the affinity of VacA for rafts is independent of its capacity to oligomerize or form membrane channels.

Targeting oncogenic Ras by the Clostridium perfringens toxin TpeL.

Clostridium perfringens toxin TpeL belongs to the family of large clostridial glycosylating toxins. The toxin causes N-acetylglucosaminylation of Ras proteins at threonine35 thereby inactivating the small GTPases. Here, we show that all main types of oncogenic Ras proteins (H-Ras, K-Ras and N-Ras) are modified by the toxin in vitro and in vivo. Toxin-catalyzed modification of Ras was accompanied by inhibition of the MAP kinase pathway. Importantly, TpeL inhibited the paradoxical activation of the MAP kinase pathway induced by the BRAF inhibitor Vemurafenib in the human melanoma cell line SBCL2. The toxin also blocked Ras signaling in a zebrafish embryo model expressing oncogenic H-RasG12V, resulting in a reduction of melanocyte number. By using the binding and translocation component of anthrax toxin (protective antigen), the glucosyltransferase domain of TpeL was effectively introduced into target cells that were not sensitive to native TpeL toxin. To reach a higher specificity towards cancer cells, a chimeric TpeL toxin was engineered that possessed the knob region of adenovirus serotype 35 fiber, which interacts with CD46 of target cells frequently overexpressed in cancer cells. The chimeric TpeL fusion toxin efficiently inhibited Ras and MAP kinases in human pancreatic cancer Capan-2 cells, which were insensitive to the wild-type toxin. The data reveal that TpeL and TpeL-related immunotoxins provide a new toolset as Ras-inactivating agents.

Cysteine Protease-Mediated Autocleavage of Clostridium difficile Toxins Regulates Their Proinflammatory Activity.

Background & AimsClostridium difficile toxin A (TcdA) and C difficile toxin toxin B (TcdB), the major virulence factors of the bacterium, cause intestinal tissue damage and inflammation. Although the 2 toxins are homologous and share a similar domain structure, TcdA is generally more inflammatory whereas TcdB is more cytotoxic. The functional domain of the toxins that govern the proinflammatory activities of the 2 toxins is unknown.MethodsHere, we investigated toxin domain functions that regulate the proinflammatory activity of C difficile toxins. By using a mouse ilea loop model, human tissues, and immune cells, we examined the inflammatory responses to a series of chimeric toxins or toxin mutants deficient in specific domain functions.ResultsBlocking autoprocessing of TcdB by mutagenesis or chemical inhibition, while reducing cytotoxicity of the toxin, significantly enhanced its proinflammatory activities in the animal model. Furthermore, a noncleavable mutant TcdB was significantly more potent than the wild-type toxin in the induction of proinflammatory cytokines in human colonic tissues and immune cells.ConclusionsIn this study, we identified a novel mechanism of regulating the biological activities of C difficile toxins in that cysteine protease-mediated autoprocessing regulates toxins’ proinflammatory activities. Our findings provide new insight into the pathogenesis of C difficile infection and the design of therapeutics against the disease.

In vivo tropism of Salmonella Typhi toxin to cells expressing a multiantennal glycan receptor.

Typhoid fever is a life-threatening disease, but little is known about the molecular bases for its unique clinical presentation. Typhoid toxin, a unique virulence factor of Salmonella Typhi (the cause of typhoid fever), recapitulates in an animal model many symptoms of typhoid fever. Typhoid toxin binding to its glycan receptor Neu5Ac is central, but, due to the ubiquity of Neu5Ac, how typhoid toxin causes specific symptoms remains elusive. Here we show that typhoid toxin displays in vivo tropism to cells expressing multiantennal glycoprotein receptors, particularly on endothelial cells of arterioles in the brain and immune cells, which is in line with typhoid symptoms. Neu5Ac displayed by multiantennal N-glycans, rather than a single Neu5Ac, appears to serve as the high-affinity receptor, as typhoid toxin possesses five identical binding pockets per toxin. Human counterparts also express the multiantennal Neu5Ac receptor. Here we also show that mice immunized with inactive typhoid toxins and challenged with wild-type typhoid toxin presented neither the characteristic in vivo tropism nor symptoms. These mice were protected against a lethal-dose toxin challenge, but Ty21a-vaccinated mice were not. Cumulatively, these results reveal remarkable features describing how a bacterial exotoxin induces virulence exclusively in specific cells at the organismal level.

Rational design of botulinum neurotoxin A1 mutants with improved oxidative stability

Botulinum neurotoxins (BoNTs) are the most potent toxic proteins to mankind known but applied in low doses trigger a localized muscle paralysis that is beneficial for the therapy of several neurological disorders and aesthetic treatment. The paralytic effect is generated by the enzymatic activity of the light chain (LC) that cleaves specifically one of the SNARE proteins responsible for neurotransmitter exocytosis. The activity of the LC in a BoNT-containing therapeutic can be compromised by denaturing agents present during manufacturing and/or in the cell. Stabilization of the LC by reducing vulnerability towards denaturants would thus be advantageous for the development of BoNT-based therapeutics. In this work, we focused on increasing the stability of LC of BoNT/A1 (LC/A1) towards oxidative stress. We tackled this task by rational design of mutations at cysteine and methionine LC/A1 sites. Designed mutants showed improved oxidative stability in vitro and equipotency to wildtype toxin in vivo. Our results suggest that suitable modification of the catalytic domain can lead to more stable BoNTs without impairing their therapeutic efficacy.