Tetanus toxin and botulinum neurotoxin-derived fusion molecules are effective bivalent vaccines

Tetanus toxin (TeNT) and botulinum neurotoxins (BoNTs) are neuroprotein toxins, with the latter being the most toxic known protein. They are structurally similar and contain three functional domains: an N-terminal catalytic domain (light chain), an internal heavy-chain translocation domain (HN domain), and a C-terminal heavy chain receptor binding domain (Hc domain or RBD). In this study, fusion functional domain molecules consisting of the TeNT RBD (THc) and the BoNT/A RBD (AHc) (i.e., THc-Linker-AHc and AHc-Linker-THc) were designed, prepared, and identified. The interaction of each Hc domain and the ganglioside receptor (GT1b) or the receptor synaptic vesicle glycoprotein 2 (SV2) was explored in vitro. Their immune response characteristics and protective efficacy were investigated in animal models. The recombinant THc-linker-AHc and AHc-linker-THc proteins with the binding activity had the correct size and structure, thus representing novel subunit vaccines. THc-linker-AHc and AHc-linker-THc induced high levels of specific neutralizing antibodies, and showed strong immune protective efficacy against both toxins. The high antibody titers against the two novel fusion domain molecules and against individual THc and AHc suggested that the THc and AHc domains, as antigens in the fusion functional domain molecules, do not interact with each other and retain their full key epitopes responsible for inducing neutralizing antibodies. Thus, the recombinant THc-linker-AHc and AHc-linker-THc molecules are strong and effective bivalent biotoxin vaccines, protecting against two biotoxins simultaneously. Our experimental design will be valuable to develop recombinant double-RBD fusion molecules as potent bivalent subunit vaccines against bio-toxins. KEY POINTS: • Double-RBD fusion molecules from two toxins had the correct structure and activity. • THc-linker-AHc and AHc-linker-THc efficiently protected against both biotoxins. • Such bivalent biotoxin vaccines based on the RBD are a valuable experimental design.

Biology activity and characterization of the functional L-HN fragment derivative of botulinum neurotoxin serotype E

OBJECTIVES

The mature botulinum neurotoxin (BoNT) is a long peptide chain consisting of a light chain (L) and a heavy chain (H) linked by a disulfide bond, where the heavy chain is divided into a translocation domain and an acceptor binding domain (Hc). In this study, we further explored the biology activity and characteristics of recombinant L-HN fragment (EL-HN) composed of the L and HN domains of BoNT/E in vivo and in vitro.

METHODS

Neurotoxicity of L-HN fragments from botulinum neurotoxins was assessed in mice. Cleavage of dichain EL-HN in vitro and in neuro-2a cells was assessed and compared with that of single chain EL-HN. Interaction of HN domain and the receptor synaptic vesicle glycoprotein 2C (SV2C) was explored in vitro and in neuro-2a cells only expressing SV2C.

RESULTS

We found that the 50% mouse lethal dose of the nicked dichain EL-HN fragment (EL-HN-DC) was 0.5 μg and its neurotoxicity was the highest among the L-HN's of the four serotypes of BoNT (A/B/E/F). The cleavage efficiency of EL-HN-DC toward synaptosome associated protein 25 (SNAP25) in vitro was 3-fold higher than that of the single chain at the cellular level, and showed 200-fold higher animal toxicity. The EL-HN-DC fragment might enter neuro-2a cells via binding to SV2C to efficiently cleave SNAP25.

CONCLUSIONS

The EL-HN fragment showed good biological activities in vivo and in vitro, and could be used as a drug screening model and to further explore the molecular mechanism of its transmembrane transport.

Tracing Foodborne Botulism Events Caused by Clostridium botulinum in Xinjiang Province, China, Using a Core Genome Sequence Typing Scheme

Foodborne botulism is a rare but life-threatening illness resulting from the action of a potent toxin mainly produced by Clostridium botulinum. It grows in an oxygen-deficient environment and is extremely viable in meat and soy products, making it one of the most virulent bacteria. How to track foodborne botulism events quickly and accurately has become a key issue. Here, we investigated two foodborne botulism events that occurred in Xinjiang in 2019 based on whole-genome sequencing and also successfully traced the relationship between clinical and food C. botulinum isolates using whole-genome core gene markers. All 59 isolates were classified as group I strains. Of the strains isolated in this study, 44 were found to be botulinum toxin A(B), and 15 isolates contained only the toxin B locus. Both the toxin A and B gene segments were located on the chromosome and organized in an ha cluster. Antibiotic resistance and virulence factors were also investigated. A set of 329 universal core gene markers were established using C. botulinum strains from a public database. These core gene markers were applied to the published C. botulinum genomes, and three outbreaks were identified. This work demonstrates that universal core gene markers can be used to trace foodborne botulism events, and we hope that our work will facilitate this effort in future. IMPORTANCE In this study, we analyzed 59 foodborne botulism (FB)-related strains isolated in Xinjiang Province, China. Our findings not only reveal the group classification, neurotoxin locus organization, antibiotic resistance and virulence factors of these strains but also establish a set of core gene markers for tracing foodborne botulism events, which was verified using published genomes. These findings indicate that these gene markers might be used as a potential tracing tool for FB events caused by C. botulinum group I strains, which have relatively stable genomic components.

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.

Engineering an Effective Human SNAP-23 Cleaving Botulinum Neurotoxin A Variant

Botulinum neurotoxin (BoNT) serotype A inhibits neurotransmitter release by cleaving SNAP-25 and represents an established pharmaceutical for treating medical conditions caused by hyperactivity of cholinergic nerves. Oversecretion from non-neuronal cells is often also the cause of diseases. Notably, excessive release of inflammatory messengers is thought to contribute to diseases such as chronic obstructive pulmonary disease, asthma, diabetes etc. The expansion of its application to these medical conditions is prevented because the major non-neuronal SNAP-25 isoform responsible for exocytosis, SNAP-23, is, in humans, virtually resistant to BoNT/A. Based on previous structural data and mutagenesis studies of SNAP-23 we optimized substrate binding pockets of the enzymatic domain for interaction with SNAP-23. Systematic mutagenesis and rational design yielded the mutations E148Y, K166F, S254A, and G305D, each of which individually increased the activity of LC/A against SNAP-23 between 3- to 23-fold. The assembled quadruple mutant showed approximately 2000-fold increased catalytic activity against human SNAP-23 in in vitro cleavage assays. A comparable increase in activity was recorded for the full-length BoNT/A quadruple mutant tested in cultivated primary neurons transduced with a fluorescently tagged-SNAP-23 encoding gene. Equipped with a suitable targeting domain this quadruple mutant promises to complete successfully tests in cells of the immune system.

Characterization of a membrane binding loop leads to engineering botulinum neurotoxin B with improved therapeutic efficacy

Botulinum neurotoxins (BoNTs) are a family of bacterial toxins with seven major serotypes (BoNT/A–G). The ability of these toxins to target and bind to motor nerve terminals is a key factor determining their potency and efficacy. Among these toxins, BoNT/B is one of the two types approved for medical and cosmetic uses. Besides binding to well-established receptors, an extended loop in the C-terminal receptor-binding domain (H_C) of BoNT/B (H_C/B) has been proposed to also contribute to toxin binding to neurons by interacting with lipid membranes (termed lipid-binding loop [LBL]). Analogous loops exist in the H_Cs of BoNT/C, D, G, and a chimeric toxin DC. However, it has been challenging to detect and characterize binding of LBLs to lipid membranes. Here, using the nanodisc system and biolayer interferometry assays, we find that H_C/DC, C, and G, but not H_C/B and H_C/D, are capable of binding to receptor-free lipids directly, with H_C/DC having the highest level of binding. Mutagenesis studies demonstrate the critical role of consecutive aromatic residues at the tip of the LBL for binding of H_C/DC to lipid membranes. Taking advantage of this insight, we then create a “gain-of-function” mutant H_C/B by replacing two nonaromatic residues at the tip of its LBL with tryptophan. Cocrystallization studies confirm that these two tryptophan residues do not alter the structure of H_C/B or the interactions with its receptors. Such a mutated H_C/B gains the ability to bind receptor-free lipid membranes and shows enhanced binding to cultured neurons. Finally, full-length BoNT/B containing two tryptophan mutations in its LBL, together with two additional mutations (E1191M/S1199Y) that increase binding to human receptors, is produced and evaluated in mice in vivo using Digit Abduction Score assays. This mutant toxin shows enhanced efficacy in paralyzing local muscles at the injection site and lower systemic diffusion, thus extending both safety range and duration of paralysis compared with the control BoNT/B. These findings establish a mechanistic understanding of LBL–lipid interactions and create a modified BoNT/B with improved therapeutic efficacy.

Botulinum neurotoxins are a family of bacterial toxins, some of which are approved for medical and cosmetic uses. This study shows that introducing aromatic residues to a lipid binding loop improved therapeutic efficacy of botulinum neurotoxin B by enhancing its ability to bind to lipid membranes at motor nerve terminals.

Engineered botulinum neurotoxin B with improved binding to human receptors has enhanced efficacy in preclinical models

Although botulinum neurotoxin serotype A (BoNT/A) products are common treatments for various disorders, there is only one commercial BoNT/B product, whose low potency, likely stemming from low affinity toward its human receptor synaptotagmin 2 (hSyt2), has limited its therapeutic usefulness. We express and characterize two full-length recombinant BoNT/B1 proteins containing designed mutations E1191M/S1199Y (rBoNT/B1MY) and E1191Q/S1199W (rBoNT/B1QW) that enhance binding to hSyt2. In preclinical models including human-induced pluripotent stem cell neurons and a humanized transgenic mouse, this increased hSyt2 affinity results in high potency, comparable to that of BoNT/A. Last, we solve the cocrystal structure of rBoNT/B1MY in complex with peptides of hSyt2 and its homolog hSyt1. We demonstrate that neuronal surface receptor binding limits the clinical efficacy of unmodified BoNT/B and that modified BoNT/B proteins have promising clinical potential.

A mutated recombinant subunit vaccine protects mice and guinea pigs against botulinum type A intoxication

Botulinum neurotoxins (BoNTs) are the most potent toxins to mammals. A toxoid vaccine was previously used for prevention of botulinum intoxication; however, this vaccine is no longer available. Currently, no approved botulinum vaccines are available from the Food and Drug Administration (FDA). Recently, a recombinant host cell receptor-binding subunit created for use as a potential vaccine completed phase 2 clinical trials. The current study designed a vaccine candidate against BoNT type A (BoNT/A) using a structural design. Our vaccine candidate was the BoNT/A heavy chain C-terminal region (HCR) that contained the point mutation BA15 (R1269A) within the ganglioside-binding site. A Biacore affinity test showed that the affinity of BA15 for ganglioside GT1b was 100 times lower than that of the HCR. A SNAP25 cleavage assay revealed that immunized sera blocked SNAP25 cleavage of the BoNT/A toxin via BA15. In an in vivo experiment, mice and guinea pigs immunized with BA15 produced neutralizing antibodies that protected against 3,000 LD50 of BoNT/A. In conclusion, the results of both in vitro and in vivo assays showed that our BA15 vaccine candidate was similar to the recombinant host cell receptor-binding subunit vaccine. The inability of BA15to bind ganglioside shows that BA15 is a potential safe vaccine candidate.

Augmentation of VAMP-catalytic activity of botulinum neurotoxin serotype B does not result in increased potency in physiological systems

Botulinum neurotoxins (BoNTs) are used extensively as therapeutic agents. Serotypes A and B are available as marketed products. Higher doses of BoNT/B are required to reach an efficacy similar to that of products containing BoNT/A. Advances in our understanding of BoNT/B mechanism of action have afforded the opportunity to make rational modifications to the toxin aimed at increasing its activity. Recently, a mutation in the light chain of BoNT/B (S201P) was described that increases the catalytic activity of the isolated BoNT/B light chain in biochemical assays. In this study, we have produced two full-length recombinant BoNT/B toxins in E.coli-one wild type (rBoNT/B1) and one incorporating the S201P mutation (rBoNT/B1(S201P)). We have compared the activity of these two molecules along with a native BoNT/B1 in biochemical cell-free assays and in several biological systems. In the cell-free assay, which measured light-chain activity alone, rBoNT/B1(S201P) cleaved VAMP-2 and VAMP-1 substrate with an activity 3-4-fold higher than rBoNT/B1. However, despite the enhanced catalytic activity of rBoNT/B1(S201P), there was no significant difference in potency between the two molecules in any of the in vitro cell-based assays, using either rodent spinal cord neurons or cortical neurons. Similarly in ex vivo tissue preparations rBoNT/B1(S201P) was not significantly more potent than rBoNT/B1 at inhibiting either diaphragm or detrusor (bladder) muscle activity in C57BL/6N and CD1 mice. Finally, no differences between rBoNT/B1 and rBoNT/B1(S201P) were observed in an in vivo digit abduction score (DAS) assay in C57BL/6N mice, either in efficacy or safety parameters. The lack of translation from the enhanced BoNT/B1(S201P) catalytic activity to potency in complex biological systems suggests that the catalytic step is not the rate-limiting factor for BoNT/B to reach maximum efficacy. In order to augment the efficacy of BoNT/B in humans, strategies other than enhancing light chain activity may need to be considered.

Expression and purification of recombinant TAT-BoNT/A(1-448) under denaturing and native conditions

Botulinum toxin type A can temporarily inhibit muscle contraction. Currently, physicians administer this toxin as a bio-drug in treatment of some muscle contraction disorders. TAT-BoNT/A(1-448) is a functional recombinant protein derived from botulinum toxin light chain. Unlike the full length botulinum toxin, TAT-BoNT/A(1-448) is a self-permeable molecule which can pass through bio-surfaces so can be used as a topical therapeutic agent without injection. To maintain the functionality of TAT-BoNT/A(1-448), it is necessary to restore its normal folding upon expression and purification. In this study, we have investigated and optimized expression conditions for this novel recombinant protein. Under denaturing condition (1 mM IPTG, at 37°C), the chimeric protein was produced as inclusion body and required to be purified using denaturing agents (e.g. urea). Yet, lower incubation temperature (18°C) and less IPTG concentration (0.5 mM) induce a protein under native condition. In such condition, about 60% of the chimeric protein was expressed in soluble form.

A fully integrated microdevice for biobarcode assay based biological agent detection

An integrated microdevice, consisting of a micropump, a passive mixer, a magnetic separation chamber, and a microcapillary electrophoretic channel, was constructed for biobarcode assay based multiplex biological agent detection in a sample-to-answer-out manner within 30 min with high sensitivity.

Isolation and functional characterization of the novel Clostridium botulinum neurotoxin A8 subtype

Botulism is a severe neurological disease caused by the complex family of botulinum neurotoxins (BoNT). Based on the different serotypes known today, a classification of serotype variants termed subtypes has been proposed according to sequence diversity and immunological properties. However, the relevance of BoNT subtypes is currently not well understood. Here we describe the isolation of a novel Clostridium botulinum strain from a food-borne botulism outbreak near Chemnitz, Germany. Comparison of its botulinum neurotoxin gene sequence with published sequences identified it to be a novel subtype within the BoNT/A serotype designated BoNT/A8. The neurotoxin gene is located within an ha-orfX+ cluster and showed highest homology to BoNT/A1, A2, A5, and A6. Unexpectedly, we found an arginine insertion located in the HC domain of the heavy chain, which is unique compared to all other BoNT/A subtypes known so far. Functional characterization revealed that the binding characteristics to its main neuronal protein receptor SV2C seemed unaffected, whereas binding to membrane-incorporated gangliosides was reduced in comparison to BoNT/A1. Moreover, we found significantly lower enzymatic activity of the natural, full-length neurotoxin and the recombinant light chain of BoNT/A8 compared to BoNT/A1 in different endopeptidase assays. Both reduced ganglioside binding and enzymatic activity may contribute to the considerably lower biological activity of BoNT/A8 as measured in a mouse phrenic nerve hemidiaphragm assay. Despite its reduced activity the novel BoNT/A8 subtype caused severe botulism in a 63-year-old male. To our knowledge, this is the first description and a comprehensive characterization of a novel BoNT/A subtype which combines genetic information on the neurotoxin gene cluster with an in-depth functional analysis using different technical approaches. Our results show that subtyping of BoNT is highly relevant and that understanding of the detailed toxin function might pave the way for the development of novel therapeutics and tailor-made antitoxins.

Molecular basis for disruption of E-cadherin adhesion by botulinum neurotoxin A complex

How botulinum neurotoxins (BoNTs) cross the host intestinal epithelial barrier in foodborne botulism is poorly understood. Here, we present the crystal structure of a clostridial hemagglutinin (HA) complex of serotype BoNT/A bound to the cell adhesion protein E-cadherin at 2.4 angstroms. The HA complex recognizes E-cadherin with high specificity involving extensive intermolecular interactions and also binds to carbohydrates on the cell surface. Binding of the HA complex sequesters E-cadherin in the monomeric state, compromising the E-cadherin-mediated intercellular barrier and facilitating paracellular absorption of BoNT/A. We reconstituted the complete 14-subunit BoNT/A complex using recombinantly produced components and demonstrated that abolishing either E-cadherin- or carbohydrate-binding of the HA complex drastically reduces oral toxicity of BoNT/A complex in vivo. Together, these studies establish the molecular mechanism of how HAs contribute to the oral toxicity of BoNT/A.

Identification of the synaptic vesicle glycoprotein 2 receptor binding site in botulinum neurotoxin A

Botulinum neurotoxins (BoNTs) inhibit neurotransmitter release by hydrolysing SNARE proteins. The most important serotype BoNT/A employs the synaptic vesicle glycoprotein 2 (SV2) isoforms A-C as neuronal receptors. Here, we identified their binding site by blocking SV2 interaction using monoclonal antibodies with characterised epitopes within the cell binding domain (HC). The site is located on the backside of the conserved ganglioside binding pocket at the interface of the HCC and HCN subdomains. The dimension of the binding pocket was characterised in detail by site directed mutagenesis allowing the development of potent inhibitors as well as modifying receptor binding properties.

Atoxic derivative of botulinum neurotoxin A as a prototype molecular vehicle for targeted delivery to the neuronal cytoplasm

We have previously described genetic constructs and expression systems that enable facile production of recombinant derivatives of botulinum neurotoxins (BoNTs) that retain the structural and trafficking properties of wt BoNTs. In this report we describe the properties of one such derivative, BoNT/A ad, which was rendered atoxic by introducing two amino acid mutations to the light chain (LC) of wt BoNT/A, and which is being developed as a molecular vehicle for delivering drugs to the neuronal cytoplasm. The neuronal binding, internalization, and intracellular trafficking of BoNT/A ad in primary hippocampal cultures was evaluated using three complimentary techniques: flow cytometry, immunohistochemistry, and Western blotting. Neuronal binding of BoNT ad was significantly increased when neurons were incubated in depolarizing medium. Flow cytometry demonstrated that BoNT/A ad internalized into neurons but not glia. After 24 hours, the majority of the neuron-bound BoNT/A ad became internalized, as determined by its resistance to pronase E-induced proteolytic degradation of proteins associated with the plasma membrane of intact cells. Significant amounts of the atoxic LC accumulated in a Triton X-100-extractable fraction of the neurons, and persisted as such for at least 11 days with no evidence of degradation. Immunocytochemical analysis demonstrated that the LC of BoNT/A ad was translocated to the neuronal cytoplasm after uptake and was specifically targeted to SNARE proteins. The atoxic LC consistently co-localized with synaptic markers SNAP-25 and VAMP-2, but was rarely co-localized with markers for early or late endosomes. These data demonstrate that BoNT/A ad mimics the trafficking properties of wt BoNT/A, confirming that our platform for designing and expressing BoNT derivatives provides an accessible system for elucidating the molecular details of BoNT trafficking, and can potentially be used to address multiple medical and biodefense needs.

Application of Real-Time PCR for Quantitative Detection ofClostridium botulinumType A Toxin Gene in Food

The TaqMan real-time PCR method for the quantitative detection of C. botulinum type A was developed based on sequence-specific hybridization probes. The validity of this assay was verified by using 10 genera of 20 strains, including reference strains of C. botulinum types A, B, C, D, E and F. The detection limit of this assay was evaluated on C. botulinum type A, using a 10-fold dilution series of DNA and spores . The DNA and spores were detected up to level of 0.1 ng/ml and 10(2)spores/ml, respectively. Spore spiked food sample preparation prior to the real-time PCR was performed by two methods, heat treatment and GuSCN. The detection limits after heat treatment showed 10(2) spores/ml for spiked sausage slurry, and 10(3) spores/ml for spiked canned corn slurry, while detection limits after GuSCN precipitation showed 10(2) spores/ml in both sausage and canned corn. Therefore the real-time PCR assay after GuSCN precipitation is useful for the quantification of C. botulinum type A because it showed identical CT values in both pure spore solutions and food slurries. We suggest that quantitative analysis of C. botulinum type A by TaqMan real-time PCR can be a rapid and accurate assessment method for botulinal risk in food samples.

Identification of fibroblast growth factor receptor 3 (FGFR3) as a protein receptor for botulinum neurotoxin serotype A (BoNT/A)

Botulinum neurotoxin serotype A (BoNT/A) causes transient muscle paralysis by entering motor nerve terminals (MNTs) where it cleaves the SNARE protein Synaptosomal-associated protein 25 (SNAP25₂₀₆) to yield SNAP25₁₉₇. Cleavage of SNAP25 results in blockage of synaptic vesicle fusion and inhibition of the release of acetylcholine. The specific uptake of BoNT/A into pre-synaptic nerve terminals is a tightly controlled multistep process, involving a combination of high and low affinity receptors. Interestingly, the C-terminal binding domain region of BoNT/A, H_C/A, is homologous to fibroblast growth factors (FGFs), making it a possible ligand for Fibroblast Growth Factor Receptors (FGFRs). Here we present data supporting the identification of Fibroblast Growth Factor Receptor 3 (FGFR3) as a high affinity receptor for BoNT/A in neuronal cells. H_C/A binds with high affinity to the two extra-cellular loops of FGFR3 and acts similar to an agonist ligand for FGFR3, resulting in phosphorylation of the receptor. Native ligands for FGFR3; FGF1, FGF2, and FGF9 compete for binding to FGFR3 and block BoNT/A cellular uptake. These findings show that FGFR3 plays a pivotal role in the specific uptake of BoNT/A across the cell membrane being part of a larger receptor complex involving ganglioside- and protein-protein interactions.

Botulinum neurotoxin serotype A (BoNT/A) is one of seven neurotoxins (BoNT/A-G), produced by the bacteria Clostridium botulinum that are both poisons and versatile therapeutics. These toxins enter motor neurons where they prevent the release of acetylcholine at the neuromuscular junction. The specific uptake of BoNT/A across the neuronal cell membrane is dependent on specific receptor interactions. Binding to high density ganglioside GT1b mediates the initial binding step and via a low affinity interaction concentrates BoNT/A on the cell surface. Once anchored in the membrane, lateral movements within the plasma membrane facilitate intermolecular interactions of BoNT/A with additional lower density but higher affinity protein receptors. Here we present data supporting the identification of Fibroblast Growth Factor Receptor 3 (FGFR3) as a high affinity receptor for BoNT/A. We show that BoNT/A binds to FGFR3 with high affinity and functions as an agonist ligand for FGFR3. The identification of this novel receptor for BoNT/A represents an important advance in the understanding of the mechanism of action of BoNT/A, especially on the initial steps of neuronal uptake, and can be the basis for the development of new specific countermeasures and new BoNT/A-based therapeutics.

Clostridium botulinum strain Af84 contains three neurotoxin gene clusters: bont/A2, bont/F4 and bont/F5

Sanger and shotgun sequencing of Clostridium botulinum strain Af84 type Af and its botulinum neurotoxin gene (bont) clusters identified the presence of three bont gene clusters rather than the expected two. The three toxin gene clusters consisted of bont subtypes A2, F4 and F5. The bont/A2 and bont/F4 gene clusters were located within the chromosome (the latter in a novel location), while the bont/F5 toxin gene cluster was located within a large 246 kb plasmid. These findings are the first identification of a C. botulinum strain that contains three botulinum neurotoxin gene clusters.

Two-component signal transduction system CBO0787/CBO0786 represses transcription from botulinum neurotoxin promoters in Clostridium botulinum ATCC 3502

Blocking neurotransmission, botulinum neurotoxin is the most poisonous biological substance known to mankind. Despite its infamy as the scourge of the food industry, the neurotoxin is increasingly used as a pharmaceutical to treat an expanding range of muscle disorders. Whilst neurotoxin expression by the spore-forming bacterium Clostridium botulinum appears tightly regulated, to date only positive regulatory elements, such as the alternative sigma factor BotR, have been implicated in this control. The identification of negative regulators has proven to be elusive. Here, we show that the two-component signal transduction system CBO0787/CBO0786 negatively regulates botulinum neurotoxin expression. Single insertional inactivation of cbo0787 encoding a sensor histidine kinase, or of cbo0786 encoding a response regulator, resulted in significantly elevated neurotoxin gene expression levels and increased neurotoxin production. Recombinant CBO0786 regulator was shown to bind to the conserved −10 site of the core promoters of the ha and ntnh-botA operons, which encode the toxin structural and accessory proteins. Increasing concentration of CBO0786 inhibited BotR-directed transcription from the ha and ntnh-botA promoters, demonstrating direct transcriptional repression of the ha and ntnh-botA operons by CBO0786. Thus, we propose that CBO0786 represses neurotoxin gene expression by blocking BotR-directed transcription from the neurotoxin promoters. This is the first evidence of a negative regulator controlling botulinum neurotoxin production. Understanding the neurotoxin regulatory mechanisms is a major target of the food and pharmaceutical industries alike.

Botulinum neurotoxin produced by the spore-forming bacterium Clostridium botulinum is the most poisonous biological substance known to mankind. By blocking neurotransmission, the neurotoxin causes a flaccid paralysis called botulism which may to lead to death upon respiratory muscle collapse. Despite its infamy as the scourge of the food industry, the neurotoxin is attracting increasing interest as a pharmaceutical to treat an expanding range of muscle disorders. Whilst neurotoxin production by C. botulinum appears tightly regulated, to date only positive regulatory elements, thus enhancing the neurotoxin production, have been implicated in this control. The identification of negative regulators, responsible for down-tuning the neurotoxin synthesis, has proven to be elusive, but would offer novel approaches both for the production of safe foods and for the development of therapeutic neurotoxins. Here, we report a two-component signal transduction system that negatively regulates botulinum neurotoxin production. Understanding the neurotoxin regulatory mechanisms is a major target of the food and pharmaceutical industries alike.

Botulinum neurotoxin devoid of receptor binding domain translocates active protease

Clostridium botulinum neurotoxin (BoNT) causes flaccid paralysis by disabling synaptic exocytosis. Intoxication requires the tri-modular protein to undergo conformational changes in response to pH and redox gradients across endosomes, leading to the formation of a protein-conducting channel. The approximately 50 kDa light chain (LC) protease is translocated into the cytosol by the approximately 100 kDa heavy chain (HC), which consists of two modules: the N-terminal translocation domain (TD) and the C-terminal Receptor Binding Domain (RBD). Here we exploited the BoNT modular design to identify the minimal requirements for channel activity and LC translocation in neurons. Using the combined detection of substrate proteolysis and single-channel currents, we showed that a di-modular protein consisting only of LC and TD was sufficient to translocate active protease into the cytosol of target cells. The RBD is dispensable for cell entry, channel activity, or LC translocation; however, it determined a pH threshold for channel formation. These findings indicate that, in addition to its individual functions, each module acts as a chaperone for the others, working in concert to achieve productive intoxication.

Enhanced immune responses using plasmid DNA replicon vaccine encoding the Hc domain of Clostridium botulinum neurotoxin serotype A

In current study, the immunogenicity of a plasmid DNA replicon vaccine (pSCARSHc) encoding the Hc domain of Clostridium botulinum neurotoxin serotype A (AHc) was investigated and compared with a conventional plasmid DNA vaccine (pcDNASHc) encoding the same antigen. In vitro, pSCARSHc incorporating Semliki Forest virus (SFV) replicon could express AHc protein and induce apoptosis of transfected cells. Comparison with the conventional plasmid DNA vaccine (pcDNASHc) yielded several interesting results. First, our self-designed pSCARSHc could induce relatively higher AHc-specific antibodies and lymphocyte proliferative responses in immunized Balb/c mice, especially at low doses. Second, while both pSCARSHc and pcDNASHc induced Th2-type immune responses, the ratio of IgG1 to IgG2a was lower in pSCARSHc groups and the Th2- and Th1-type humoral immune responses induced by pSCARSHc were also stronger than that of the pcDNASHc vaccine. Third, it was shown that the sera from pSCARSHc-vaccinated mice conferred more efficient protection than those from pcDNASHc-vaccinated mice by BoNT/A neutralization assay. Finally, mice immunized with pSCARSHc could also elicit more efficient protection against BoNT/A than pcDNASHc. These results indicate that our plasmid DNA replicon vaccine can provide strong immunogenicity and should be a potential alternative strategy to conventional DNA vaccines in developing an efficacious vaccine against C. botulinum neurotoxin serotype A.

Variations in expression and release of botulinum neurotoxin in Clostridium botulinum type A strains

In this study, we characterized the production and release of botulinum neurotoxin during growth of two Clostridium botulinum strains, Hall A and NCTC2916, of distinct gene organizations. The intra- and extracellular fractions of the bacterial cells harvested at various stages of growth were analyzed for the neurotoxin. Both strains exhibited a temporal neurotoxin gene expression; however, these two strains differ in their patterns of growth, toxin production, toxin release, and post-translational nicking. Comparing to the NCTC2916 strain, the Hall A strain showed an extended stationary phase, delayed autolysis, and earlier expression and release of neurotoxin. Understanding the differences between these two toxin-producing strains may provide insights into the toxinogenesis of C. botulinum.

Interaction of syntaxin 1A with the N-terminus of Kv4.2 modulates channel surface expression and gating

Kv4.2 channels are responsible in the heart for the Ca2+-independent transient outward currents and are important in regulating myocardial excitability and Ca2+ homeostasis. We have identified previously the expression of syntaxin 1A (STX1A) on the cardiac ventricular myocyte plasma membranes, and its modulation of cardiac ATP-sensitive K+ channels. We speculated that STX1A interacts with other cardiac ion channels, thus we examined the interaction of STX1A with Kv4.2 channels. Co-immunoprecipitation and GST pulldown assays demonstrated a direct interaction of STX1A with the Kv4.2 N-terminus. We next investigated the functional alterations of Kv4.2 gating by STX1A in Xenopus oocytes. Coexpression of Kv4.2 with STX1A (1) resulted in a reduction of Kv4.2 current amplitude; (2) caused a depolarizing shift of the steady-state inactivation curve; (3) enhanced the rate of current decay; and (4) accelerated the rate of recovery from inactivation. Additional coexpression of botulinum neurotoxin C, which cleaves STX1A, reversed the effects of STX1A on Kv4.2. STX1A inhibited partially the gating changes by KChIP2, suggesting a competitive interaction of these proteins for an overlapping binding region on the N-terminus of Kv4.2. Indeed, the N-terminal truncation mutants of Kv4.2 (Kv4.2Delta2-40 and Kv4.2Delta7-11), which form part of the KChIP2 binding site, displayed reduced sensitivity to STX1A modulation. Our study suggests that STX1A directly modulates Kv4.2 current amplitude and gating through its interaction with an overlapping region of the KChIP binding motif domain on the Kv4.2 N-terminus.

Plasmid encoded neurotoxin genes in Clostridium botulinum serotype A subtypes

Clostridium botulinum, an important pathogen of humans and animals, produces botulinum neurotoxin (BoNT), the most poisonous toxin known. We have determined by pulsed-field gel electrophoresis (PFGE) and Southern hybridizations that the genes encoding BoNTs in strains Loch Maree (subtype A3) and 657Ba (type B and subtype A4) are located on large (approximately 280 kb) plasmids. This is the first demonstration of plasmid-borne neurotoxin genes in Clostridium botulinum serotypes A and B. The finding of BoNT type A and B genes on extrachromosomal elements has important implications for the evolution of neurotoxigenicity in clostridia including the origin, expression, and lateral transfer of botulinum neurotoxin genes.

Trivalent vaccine against botulinum toxin serotypes A, B, and E that can be administered by the mucosal route

Most reports dealing with vaccines against botulinum toxin have focused on the injection route of administration. This is unfortunate, because a mucosal vaccine is likely to be more efficacious for patients and pose fewer risks to health care workers and to the environment. Therefore, efforts were made to generate a mucosal vaccine that provides protection against the botulinum serotypes that typically cause human illness (serotypes A, B, and E). This work demonstrated that carboxy-terminal peptides derived from each of the three serotypes were able to bind to and penetrate human epithelial barriers in vitro, and there was no cross inhibition of membrane binding and transcytosis. The three polypeptides were then tested in vivo as a trivalent vaccine that could be administered to mice by the intranasal route. The results indicated that the mucosal vaccine evoked high secretory titers of immunoglobulin A (IgA), as well as high circulating titers of IgG and IgA, and it also evoked a high level of resistance to challenge with toxin. The immunoglobulin responses and the levels of resistance to challenge were increased by coadministration of adjuvants, such as chitosan and vitamin E. At least three mechanisms were identified to account for the antibody-induced resistance: (i) blockade of toxin absorption across epithelial cells, (ii) enhanced clearance of toxin from the circulation, and (iii) blockade of toxin action at the neuromuscular junction. These results are a compelling demonstration that a mucosal vaccine against multiple serotypes of botulinum toxin has been identified.

Mucosal vaccine targeting improves onset of mucosal and systemic immunity to botulinum neurotoxin A

Absence of suitable mucosal adjuvants for humans prompted us to consider alternative vaccine designs for mucosal immunization. Because adenovirus is adept in binding to the respiratory epithelium, we tested the adenovirus 2 fiber protein (Ad2F) as a potential vaccine-targeting molecule to mediate vaccine uptake. The vaccine component (the host cell-binding domain to botulinum toxin (BoNT) serotype A) was genetically fused to Ad2F to enable epithelial binding. The binding domain for BoNT was selected because it lies within the immunodominant H chain as a beta-trefoil (Hcbetatre) structure; we hypothesize that induced neutralizing Abs should be protective. Mice were nasally immunized with the Hcbetatre or Hcbetatre-Ad2F, with or without cholera toxin (CT). Without CT, mice immunized with Hcbetatre produced weak secretory IgA (sIgA) and plasma IgG Ab response. Hcbetatre-Ad2F-immunized mice produced a sIgA response equivalent to mice coimmunized with CT. With CT, Hcbetatre-Ad2F-immunized mice showed a more rapid onset of sIgA and plasma IgG Ab responses that were supported by a mixed Th1/Th2 cells, as opposed to mostly Th2 cells by Hcbetatre-dosed mice. Mice immunized with adjuvanted Hcbetatre-Ad2F or Hcbetatre were protected against lethal BoNT serotype A challenge. Using a mouse neutralization assay, fecal Abs from Hcbetatre-Ad2F or Hcbetatre plus CT-dosed mice could confer protection. Parenteral immunization showed that the inclusion of Ad2F enhances anti-Hcbetatre Ab titers even in the absence of adjuvant. This study shows that the Hcbetatre structure can confer protective immunity and that use of Hcbetatre-Ad2F gives more rapid and sustained mucosal and plasma Ab responses.

Quantitative Duplex PCR of Clostridium botulinum Types A and B Neurotoxin Genes

A duplex quantitative polymerase chain reaction (PCR) assay for Clostridium botulinum types A and B was developed. The sensitivity and specificity of the assay were verified by using 6 strains of type A, 7 strains of type B, and 14 genera of 42 non-C. botulinum types A and B strains, including C. botulinum types C, D, E, F, and G. In pure culture, the detection limit was 10(2) CFU/ mL for type A and 10(3) CFU/mL for type B. In mushroom broth, increases in the amounts of C. botulinum types A and B could be monitored separately (the quantifiable range was 10(2) to 10(6) for type A and 10(2) to 10(7) for type B) from each sample that contained a large number of background bacteria, and toxin could be detected much earlier than with mouse assay. These results suggest that duplex quantitative PCR methods are useful to detect and quantify C. botulinum types A and/ or B toxin genes.

Determination of neurotoxin gene expression in Clostridium botulinum type A by quantitative RT-PCR

Real time reverse transcription (RT)-PCR was used to quantify the expression of the botulinum neurotoxin type A (BoNT/A) gene (cntA) by normalization with the expression of 16S rRNA. The method were confirmed by monitoring the mRNA levels of cntA during growth in five type A strains. In all but one of the strains the expression of cntA mRNA was maximal in the late exponential phase, and approximately 35-fold greater than in the early exponential phase. The concentration of the extracellular BoNT/A complex detected by ELISA was highest in stationary phase. Sodium nitrite and sorbic acid completely inhibited growth at 20 ppm and 4 mg ml-1, respectively. CntA expression became lower in proportion to the concentration of sorbic acid, and this reduction was confirmed by mouse bioassay. Our results show that real time RT-PCR can be used to quantify levels of C. botulinum type A neurotoxin transcripts and to assess the effects of food additives on botulinal risk.

Expression, Purification and Transduction of PEP-1-Botulinum Neurotoxin Type A (PEP-1-BoNT/A) into Skin

Botulinum neurotoxin A (BoNT/A) has been used therapeutically to treat muscular hypercontractions and sudomotor hyperactivity and it has been reported that BoNT/A might have analgesic properties in headache. PEP-1 peptide is a known carrier peptide that delivers full-length native proteins in vitro and in vivo. In this study, a BoNT/A gene were fused with PEP-1 peptide in a bacterial expression vector to produce a genetic in-frame PEP-1-BoNT/A fusion protein. The expressed and purified PEP-1-BoNT/A fusion proteins were efficiently transduced into cells in a time- and dose-dependent manner when added exogenously in a culture medium. In addition, immunohistochemical analysis revealed that PEP-1-BoNT/A fusion protein efficiently penetrated into the epidermis as well as the dermis of the subcutaneous layer, when sprayed on mice skin. These results suggest that PEP-1-BoNT/A fusion protein provide an efficient strategy for therapeutic delivery in various human diseases related to this protein.

A novel type A2 neurotoxin gene cluster in Clostridium botulinum strain Mascarpone

The partial nucleotide sequence ( approximately 10 kb) of the cluster of genes encoding the botulinum neurotoxin complex in Clostridium botulinum type A strain Mascarpone was determined. The analysis revealed six ORFs (orfs), which were organized as in the type A2 and type A3 botulinum neurotoxin gene clusters of strains Kyoto-F and NCTC 2916, respectively. While the orfs at the proximal and distal ends of the sequence (orfX2 and bont/A genes) shared a high level of similarity with the corresponding sequences of strain Kyoto-F, the segment encompassing the orfX1 and botR/A genes within the sequence exhibited a higher degree of homology to the related region in strain NCTC 2916. The mosaic structure of the Mascarpone neurotoxin gene cluster suggests recombinational exchanges.

Light Chain of Botulinum Neurotoxin Serotype A: Structural Resolution of a Catalytic Intermediate,

Botulinum neurotoxin serotype A (BoNT/A, 1296 residues) is a zinc metalloprotease that cleaves SNAP25 to inhibit the fusion of neurotransmitter-carrying vesicles to the plasma membrane of peripheral neurons. BoNT/A is a disulfide-linked di-chain protein composed of an N-terminal, thermolysin-like metalloprotease light chain domain (LC/A, 448 residues) and a C-terminal heavy chain domain (848 residues) that can be divided into two subdomains, a translocation subdomain and a receptor binding subdomain. LC/A cleaves SNAP25 between residues Gln197-Arg198 and, unlike thermolysin, recognizes an extended region of SNAP25 for cleavage. The structure of a recombinant LC/A (1-425) treated with EDTA (No-Zn LC/A) was determined. The overall structure of No-Zn LC/A is similar to that reported for the holotoxin, except that it lacks the Zn ion, indicating that the role of Zn is catalytic not structural. In addition, structures of a noncatalytic mutant LC/A (Arg362Ala/Tyr365Phe) complexed with and without an inhibitor, ArgHX, were determined. The overall structure and the active site conformation for the mutant are the same as wild type. When the inhibitor binds to the active site, the carbonyl and N-hydroxyl groups form a bidentate ligand to the Zn ion and the arginine moiety binds to Asp369, suggesting that the inhibitor-bound structure mimics a catalytic intermediate with the Arg moiety binding at the P1' site. Consistent with this model, mutation of Asp369 to Ala decreases the catalytic activity of LC/A by approximately 600-fold, and the residual activity is not inhibited by ArgHX. These results provide new information on the reaction mechanism and insight into the development of strategies for small molecule inhibitors of BoNTs.

Expression of botulinum neurotoxins A and E, and associated non-toxin genes, during the transition phase and stability at high temperature: analysis by quantitative reverse transcription-PCR

Production of botulinum neurotoxin A (BoNT/A) and associated non-toxic proteins (ANTPs), which include a non-toxic non-haemagglutinin (NTNH/A) as well as haemagglutinins (HAs), was found previously to be dependent upon an RNA polymerase alternative sigma factor (BotR/A). Expression of the botR/A, bont/A and antp genes, monitored by reverse transcription and real-time PCR analysis, occurred concomitantly at the transition between the exponential and stationary growth phases of Clostridium botulinum A. The botR/A expression level was about 100-fold less than those of the bont/A and antp genes. Therefore, BotR/A is an alternative sigma factor controlling the botulinum A locus genes during the transition phase. The highest toxin concentration was released into the culture supernatant 12 h after maximum expression of the botR/A, bont/A and antp genes, without any apparent bacterial lysis. Toxin levels were then stable over 5 days in cultures at 37 degrees C, whereas a dramatic decrease in lethal activity was observed between 24 and 48 h in cultures at 44 degrees C. High temperature did inhibit transcription, since expression levels of the botR/A, bont/A and antp genes were similar in cultures at 37 and 44 degrees C. However, incubation at 44 degrees C triggered a calcium-dependent protease that degraded BoNT/A and NTNH/A, but not HAs. In C. botulinum E, which contains no gene related to botR, the bont/E and p47 genes were also expressed during the transition phase, and no protease activation at 44 degrees C was evident.

Expression of an anti-botulinum toxin A neutralizing single-chain Fv recombinant antibody in transgenic tobacco

Botulinum neurotoxins (BoNTs) are the most poisonous substances known and are thus classified as high-risk threats for use as bioterror agents. To examine the potential of transgenic plants as bioreactors for the production of BoNT antidotes, we transformed tobacco with an optimized, synthetic gene encoding a botulinum neurotoxin A (BoNT/A) neutralizing single-chain Fv (scFv) recombinant antibody fragment. In vitro mouse muscle twitch assays demonstrated the functional utility of this scFv extracted from tobacco for neutralizing the paralytic effects of BoNT/A at neuromuscular junctions. Based on the efficiency of the scFv capture process and the dose required to antidote a human being, 1-2 ha of this tobacco could yield up to 4 kg of scFv, which would be enough to contribute to the manufacture of 1,000,000 therapeutic doses of a monoclonal antibody (mAb) cocktail capable of neutralizing the effects of BoNT poisoning. Transgenic plants could provide an inexpensive production platform for expression of multiple mAbs toward the creation of polyclonal therapies (i.e. pooled mAbs) as the next improvement in recombinant antibody therapy.

Cloning, expression, and purification of C-terminal quarter of the heavy chain of botulinum neurotoxin type A

Botulinum neurotoxins (BoNTs) are highly potent toxins that inhibit neurotransmitter release from peripheral cholinergic synapses. BoNTs consist of a toxifying light chain (LC; 50 kDa) and a binding-translocating heavy chain (HC; 100 kDa) linked through a disulfide bond. The complete sequence of BoNT/A consists of 1296 amino acid residues. The beta-trefoil domain for BoNT/A to which gangliosides bind starts at Ser 1092 and this fragment represents the C-half of the C-terminus of the heavy chain (C-quarter HC or HCQ). The recombinant HCQ DNA was successfully cloned into an expression vector (pET15b), which was used to transform Escherichia coli strain BL21-Star (DE3) for expression. Expression of HCQ was obtained by an extended post-induction time of 15 h at 30 degrees C. The recombinant histidine tagged HCQ protein was isolated and purified by nickel affinity gel column chromatography and its molecular weight was verified by gel electrophoresis. The HCQ was positively identified by antibodies raised against BoNT/A employing immunological dot-blot and Western blot assays. HCQ was shown to bind with synaptotagmin (a known BoNT/A receptor) and gangliosides, indicating that the expressed and purified HCQ protein retains a functionally active conformation.

Sequence variation within botulinum neurotoxin serotypes impacts antibody binding and neutralization

The botulinum neurotoxins (BoNTs) are category A biothreat agents which have been the focus of intensive efforts to develop vaccines and antibody-based prophylaxis and treatment. Such approaches must take into account the extensive BoNT sequence variability; the seven BoNT serotypes differ by up to 70% at the amino acid level. Here, we have analyzed 49 complete published sequences of BoNTs and show that all toxins also exhibit variability within serotypes ranging between 2.6 and 31.6%. To determine the impact of such sequence differences on immune recognition, we studied the binding and neutralization capacity of six BoNT serotype A (BoNT/A) monoclonal antibodies (MAbs) to BoNT/A1 and BoNT/A2, which differ by 10% at the amino acid level. While all six MAbs bound BoNT/A1 with high affinity, three of the six MAbs showed a marked reduction in binding affinity of 500- to more than 1,000-fold to BoNT/A2 toxin. Binding results predicted in vivo toxin neutralization; MAbs or MAb combinations that potently neutralized A1 toxin but did not bind A2 toxin had minimal neutralizing capacity for A2 toxin. This was most striking for a combination of three binding domain MAbs which together neutralized >40,000 mouse 50% lethal doses (LD(50)s) of A1 toxin but less than 500 LD(50)s of A2 toxin. Combining three MAbs which bound both A1 and A2 toxins potently neutralized both toxins. We conclude that sequence variability exists within all toxin serotypes, and this impacts monoclonal antibody binding and neutralization. Such subtype sequence variability must be accounted for when generating and evaluating diagnostic and therapeutic antibodies.

Characterization of the antibody response to the receptor binding domain of botulinum neurotoxin serotypes A and E

Clostridium botulinum neurotoxins (BoNTs) are the most toxic proteins for humans. The current clostridial-derived vaccines against BoNT intoxication have limitations including production and accessibility. Conditions were established to express the soluble receptor binding domain (heavy-chain receptor [HCR]) of BoNT serotypes A and E in Escherichia coli. Sera isolated from mice and rabbits immunized with recombinant HCR/A1 (rHCR/A1) from the classical type A-Hall strain (ATCC 3502) (BoNT/A1) and rHCR/E from BoNT serotype E Beluga (BoNT/E(B)) neutralized the homologous serotype of BoNT but displayed differences in cross-recognition and cross-protection. Enzyme-linked immunosorbent assay and Western blotting showed that alpha-rHCR/A1 recognized epitopes within the C terminus of the HCR/A and HCR/E, while alpha-rHCR/E recognized epitopes within the N terminus or interface between the N and C termini of the HCR proteins. alpha-rHCR/E(B) sera possessed detectable neutralizing capacity for BoNT/A1, while alpha-rHCR/A1 did not neutralize BoNT/E. rHCR/A was an effective immunogen against BoNT/A1 and the Kyoto F infant strain (BoNT/A2), but not BoNT serotype E Alaska (BoNT/E(A)), while rHCR/E(B) neutralized BoNT/E(A), and under hyperimmunization conditions protected against BoNT/A1 and BoNT/A2. The protection elicited by rHCR/A1 to BoNT/A1 and BoNT/A2 and by rHCR/E(B) to BoNT/E(A) indicate that immunization with receptor binding domains elicit protection within sub-serotypes of BoNT. The protection elicited by hyperimmunization with rHCR/E against BoNT/A suggests the presence of common neutralizing epitopes between the serotypes E and A. These results show that a receptor binding domain subunit vaccine protects against serotype variants of BoNTs.

The C-terminus of botulinum neurotoxin type A light chain contributes to solubility, catalysis, and stability

Botulinum neurotoxin type A (BoNT/A) is the etiological agent responsible for botulism, a disease characterized by peripheral neuromuscular blockade. BoNT/A is produced by Clostridium botulinum as a single chain protein that is activated by proteolytic cleavage to form a 50 kDa light chain (LC, 448 amino acids) and a disulfide bond-linked 100 kDa heavy chain (HC, 847 amino acids). Whilst HC comprises the receptor binding and translocation domains, LC is a Zn2+-endopeptidase that cleaves at a single glutaminyl-arginine bond corresponding to residues 197 and 198 at the C-terminus of SNAP25. Cleavage of SNAP25 uncouples the neural exocytosis docking/fusion machinery. LC/A (LC 1-448) and several C-terminal deletion proteins of LC/A were engineered and expressed as His-tagged fusion proteins in Escherichia coli. LC 1-448 was purified, but precipitated upon storage. Approximately 40% of LC 1-448 was a covalent dimer due to the formation of inter-chain disulfide bond formation at Cys430. Conversion of Cys430 to Ser abolished dimer formation of LC 1-448, but did not improve solubility. Three C-terminal deletion peptides were engineered; LC 1-425 and LC 1-418 were expressed and could be purified as soluble and stable proteins, whilst LC 1-398 was soluble, but not stable to storage. Kinetic studies showed that LC 1-448 and LC 1-425 efficiently cleaved GST-SNAP25 and the fluorescent substrate SNAPtide, while LC 1-418 catalyzed the cleavage of both the SNAP25 and the fluorescent substrate SNAPtide with a similar Km, but at a 10-fold slower kcat. Thus, regions within the C-terminus of LC/A contribute to solubility, stability, and catalysis.

Development and application of Real-Time PCR assays to detect fragments of the Clostridium botulinum types A, B, and E neurotoxin genes for investigation of human foodborne and infant botulism

Real-time PCR assays for detection of Clostridium botulinum neurotoxin (BoNT) gene fragments specific to BoNTA, B, and E were developed as alternatives to the mouse bioassay. The expected specificities of the PCR assays were demonstrated by in silico analysis as well as empirical testing of target DNA extracted from 83 pure cultures of C. botulinum, and 44 bacteria from other species. The sensitivities of the assays were found to be equivalent to 16, 10, and 141 genomes for BoNT A, B, and E, respectively. The assays were shown to be applicable to both purified DNA, as well as crude DNA extracted from cultures and enrichment broths. The assays were evaluated using DNA extracted directly from clinical and food specimens as well as from inoculated broths using material collected from seven confirmed and one suspected case of botulism. The appropriate BoNT genes were detected in material from seven of the eight cases of botulism and provided a supportive diagnosis faster than the conventional bioassay. These assays have already proven useful for pubic health microbiological investigation of suspected cases of human botulism by substantially improving the diagnostic process.

Preparation of specifically activatable endopeptidase derivatives of Clostridium botulinum toxins type A, B, and C and their applications

Clostridium botulinum neurotoxins are potently toxic proteins of 150 kDa with specific endopeptidase activity for SNARE proteins involved in vesicle docking and release. Following treatment with trypsin, a fragment of botulinum neurotoxin serotype A that lacks the C-terminal domain responsible for neuronal cell binding, but retains full catalytic activity, can be obtained. Known as the LH(N) fragment, we report the development of a recombinant expression and purification scheme for the isolation of comparable fragments of neurotoxin serotypes B and C. Expressed as maltose-binding protein fusions, both have specific proteolytic sites present between the fusion tag and the light chain to facilitate removal of the fusion, and between the light chain endopeptidase and the H(N) translocation domains to facilitate activation of the single polypeptide. We have also used this approach to prepare a new variant of LH(N)/A with a specific activation site that avoids the need to use trypsin. All three LH(N)s are enzymatically active and are of low toxicity. The production of specifically activatable LH(N)/A, LH(N)/B, and LH(N)/C extends the opportunities for exploitation of neurotoxin fragments. The potential utility of these fragments is discussed.

Cloth-based hybridization array system for the detection of Clostridium botulinum type A, B, E, and F neurotoxin genes

A simple cloth-based hybridization array system was developed for the characterization of Clostridium botulinum isolates based on the botulinum neurotoxin serotype. Bacterial isolates were subjected to a multiplex PCR incorporating digoxigenin-dUTP and primers targeting the four botulinum neurotoxin gene serotypes (A, B, E, and F) predominantly involved in human illness, followed by hybridization of the amplicons with an array of toxin gene-specific oligonucleotide probes immobilized on polyester cloth and subsequent immunoenzymatic assay of the bound digoxigenin label. This system provided sensitive and specific detection of the different botulinum neurotoxin gene markers in a variety of C. botulinum strains, exhibiting the expected patterns of reactivity with a panel of target and nontarget organisms.

A molecular basis underlying differences in the toxicity of botulinum serotypes A and E

Botulinum neurotoxins (BoNTs) block neurotransmitter release through their specific proteolysis of the proteins responsible for vesicle exocytosis. Paradoxically, two serotypes of BoNTs, A and E, cleave the same molecule, synaptosome-associated protein with relative molecular mass 25K (SNAP-25), and yet they cause synaptic blockade with very different properties. Here we compared the action of BoNTs A and E on the plasma membrane fusion machinery composed of syntaxin and SNAP-25. We now show that the BoNT/A-cleaved SNAP-25 maintains its association with two syntaxin isoforms in vitro, which is mirrored by retention of SNAP-25 on the plasma membrane in vivo. In contrast, BoNT/E severely compromises the ability of SNAP-25 to bind the plasma membrane syntaxin isoforms, leading to dissociation of SNAP-25. The distinct properties of botulinum intoxication, therefore, can result from the ability of shortened SNAP-25 to maintain its association with syntaxins-in the case of BoNT/A poisoning resulting in unproductive syntaxin/SNAP-25 complexes that impede vesicle exocytosis.

The Role of Exoproteases in Governing Intraneuronal Metabolism of Botulinum Toxin

Botulinum toxin type A has a long duration of action, and thus it can block transmitter release for several weeks to several months. However, little is known about the precise mechanism that accounts for termination of toxin action. Therefore, experiments were done to gauge the effects of aminopeptidases and carboxypeptidases on the structure and function of the toxin. Exoproteases were added to the holotoxin, the native light chain, and a recombinant light chain. Treated toxin and light chain were examined for their effects on neuromuscular transmission and on isolated substrate. The data showed that aminopeptidase attack did not alter the N-terminus of the toxin/light chain, nor did it produce losses in biological activity. Carboxypeptidase attack did alter the C-terminus of the light chain, but not sufficiently to alter biological activity. The data suggest that the tertiary structure of the light chain confers upon the molecule substantial resistance to exoproteases.

Modulation of botulinum neurotoxin A catalytic domain stability by tyrosine phosphorylation

Botulinum neurotoxin A (BoNT A) is a substrate of the Src family of tyrosine kinases. Here, we report that the BoNT A light chain (LC) is phosphorylated in the tyrosine-71 located at N-terminus. Covalent modification of this residue notably increases the thermal stability of the endopeptidase activity, without affecting its catalytic efficacy. Similarly, mutation of this residue specifically affected the protein stability but not its endopeptidase function. Fusion of the Tat-translocating domain to the N-terminus of the enzyme produced a cell permeable, functional enzyme, as evidenced by immunocytochemistry and by the cleavage of cytosolic SNAP25 in intact PC12 cells. Noteworthy, truncation of cellular SNAP25 was reduced in cells when the Src kinase activity was inhibited with a specific antagonist, implying that tyrosine phosphorylation of BoNT A LC modulates the in vivo proteolytic activity of the neurotoxin. Taken together, these findings substantiate the tenet that tyrosine phosphorylation of BoNT A LC could be an important modulatory strategy of the neurotoxin stability and suggest that the phosphorylated neurotoxin may be a relevant molecule in vivo.

Botulinum neurotoxin type D enables cytosolic delivery of enzymatically active cargo proteins to neurones via unfolded translocation intermediates

Multi-domain bacterial protein toxins are being explored as potential carriers for targeted delivery of biomolecules. Previous approaches employing isolated receptor binding subunits disallow entry into the cytosol. Strategies in which catalytic domains are replaced with cargo molecules are presumably inefficient due to co-operation of domains during the endosomal translocation step. Here, we characterize a novel transport vehicle in which cargo proteins are attached to the amino terminus of the full-length botulinum neurotoxin type D (BoNT/D). The intrinsic enzymatic activity of the neurotoxin allowed quantification of the efficacy of cargo delivery to the cytosol. Dihydrofolate reductase and BoNT type A (BoNT/A) light chain (LC) were efficiently conveyed into the cytosol, whereas attachment of firefly luciferase or green fluorescent protein drastically reduced the toxicity. Luciferase and BoNT/A LC retained their catalytic activity as evidenced by luciferin conversion or SNAP-25 hydrolysis in the cytosol of synaptosomes, respectively. Conformationally stabilized dihydrofolate reductase as cargo considerably decreased the toxicity indicative for the requirement of partial unfolding of cargo protein and catalytic domain as prerequisite for efficient translocation across the endosomal membrane. Thus, enzymatically inactive clostridial neurotoxins may serve as effective, safe carriers for delivering proteins in functionally active form to the cytosol of neurones.

Substrate recognition strategy for botulinum neurotoxin serotype A

Clostridal neurotoxins (CNTs) are the causative agents of the neuroparalytic diseases botulism and tetanus. CNTs impair neuronal exocytosis through specific proteolysis of essential proteins called SNAREs. SNARE assembly into a low-energy ternary complex is believed to catalyse membrane fusion, precipitating neurotransmitter release; this process is attenuated in response to SNARE proteolysis. Site-specific SNARE hydrolysis is catalysed by the CNT light chains, a unique group of zinc-dependent endopeptidases. The means by which a CNT properly identifies and cleaves its target SNARE has been a subject of much speculation; it is thought to use one or more regions of enzyme-substrate interaction remote from the active site (exosites). Here we report the first structure of a CNT endopeptidase in complex with its target SNARE at a resolution of 2.1 A: botulinum neurotoxin serotype A (BoNT/A) protease bound to human SNAP-25. The structure, together with enzyme kinetic data, reveals an array of exosites that determine substrate specificity. Substrate orientation is similar to that of the general zinc-dependent metalloprotease thermolysin. We observe significant structural changes near the toxin's catalytic pocket upon substrate binding, probably serving to render the protease competent for catalysis. The novel structures of the substrate-recognition exosites could be used for designing inhibitors specific to BoNT/A.

Differentiation of the gene clusters encoding botulinum neurotoxin type A complexes in Clostridium botulinum type A, Ab, and A(B) strains

We describe a strategy to identify the clusters of genes encoding components of the botulinum toxin type A (boNT/A) complexes in 57 strains of Clostridium botulinum types A, Ab, and A(B) isolated in Italy and in the United States from different sources. Specifically, we combined the results of PCR for detecting the ha33 and/or p47 genes with those of boNT/A PCR-restriction fragment length polymorphism analysis. Three different type A toxin gene clusters were revealed; type A1 was predominant among the strains from the United States, whereas type A2 predominated among the Italian strains, suggesting a geographic distinction between strains. By contrast, no relationship between the toxin gene clusters and the clinical or food source of strains was evident. In two C. botulinum type A isolates from the United States, we recognized a third type A toxin gene cluster (designated type A3) which was similar to that previously described only for C. botulinum type A(B) and Ab strains. Total genomic DNA from the strains was subjected to pulsed-filed gel electrophoresis and randomly amplified polymorphic DNA analyses, and the results were consistent with the boNT/A gene clusters obtained.

Nucleotide sequence and transcriptional analysis of the type A2 neurotoxin gene cluster in Clostridium botulinum

The nucleotide sequences of the upstream regions of the botulinum neurotoxin type A1 (BoNT/A1) cluster of Clostridium botulinum strain NCTC 2916 and the BoNT/A2 cluster of strain Kyoto-F were determined. A novel gene, designated orfx3, was identified following the orfx2 gene in both clusters. ORF-X2 and ORF-X3 exhibit similarity to the BoNT cluster associated P-47 protein. The BoNT/A1 and BoNT/A2 clusters share a similar gene arrangement, but exhibit differences in the spacing between certain genes. Sequences with similarity to transposases were identified in these intergenic regions, suggesting that these differences arose from an ancestral insertion event. Transcriptional analysis of the BoNT/A2 cluster revealed that the genes of the cluster are primarily synthesized as three polycistronic transcripts. Two divergent polycistronic transcripts, one encoding the orfx1, orfx2, and orfx3 genes, the second encoding the p47, ntnh, and bont/a2 genes, are transcribed from conserved BoNT cluster promoters. The third polycistronic transcript, expressed at low levels, encodes the positive regulatory botR gene and the orfx genes. This is the first complete analysis of a botulinum toxin A2 cluster.

Identification of type A, B, E, and F botulinum neurotoxin genes and of botulinum neurotoxigenic clostridia by denaturing high-performance liquid chromatography

Denaturing high-performance liquid chromatography (DHPLC) is a recently developed technique for rapid screening of nucleotide polymorphisms in PCR products. We used this technique for the identification of type A, B, E, and F botulinum neurotoxin genes. PCR products amplified from a conserved region of the type A, B, E, and F botulinum toxin genes from Clostridium botulinum, neurotoxigenic C. butyricum type E, and C. baratii type F strains were subjected to both DHPLC analysis and sequencing. Unique DHPLC peak profiles were obtained with each different type of botulinum toxin gene fragment, consistent with nucleotide differences observed in the related sequences. We then evaluated the ability of this technique to identify botulinal neurotoxigenic organisms at the genus and species level. A specific short region of the 16S rRNA gene which contains genus-specific and in some cases species-specific heterogeneity was amplified from botulinum neurotoxigenic clostridia and from different food-borne pathogens and subjected to DHPLC analysis. Different peak profiles were obtained for each genus and species, demonstrating that the technique could be a reliable alternative to sequencing for the rapid identification of food-borne pathogens, specifically of botulinal neurotoxigenic clostridia most frequently implicated in human botulism.

[Modification and high level expression of protective antigen fragment of botulinum neurotoxin serotype A]

Designed a pair of primers through modifying N-terminal bases (5bps) of gene after ATG but not changing amino acid, and amplified a smaller mutated gene sequnce (468bp) containing two protective antigenic determinants from pBlue-BoNTaHc, N-terminal codon of mutated gene fragment is changed from low to high frenqence in E. coli. Mutated gene was ligated into pGEM-T vector and sequenced, then, cloned into a expression plasmid pBV220. As a result, cloned gene was expressed in insoluble form by temperature inducing (from 30 degrees C to 42 degrees C) in E. coli. Expression product is 40% of total proteins and is of specific binding activity to antibody in ELISA. The successful modification and high level expression of protective fragment of botulinum neurotoxin serotype A(BoNTaHc468) gene is conducive to further study on antitoxin and vaccine.