The structure of the tetanus toxin reveals pH-mediated domain dynamics

Structures of native SV2A reveal the binding mode for tetanus neurotoxin and anti-epileptic racetams

The synaptic vesicle glycoprotein 2A (SV2A) is a synaptic vesicle (SV) resident with homology to the major facilitator superfamily (MFS) and essential in vertebrate neurotransmission. Despite its unclear physiological role, SV2A is of high medical relevance as it is the target of the anti-epileptic drug Levetiracetam (LEV) and a receptor for clostridial neurotoxins (CNTs), among them presumably tetanus neurotoxin (TeNT). To obtain detailed insights about these molecular interactions we subjected native SV2A, purified from brain tissue, to cryo-EM. We discover that TeNT binds SV2A strikingly different from botulinum neurotoxin A and unveil the precise geometry of TeNT binding to dipartite SV2-ganglioside receptors. The structures deliver compelling support for SV2A as the protein receptor for TeNT in central neurons and reinforce the concepts of the dual receptor hypothesis for CNT entry into neurons. Further, our LEV-bound structure of SV2A reveals the drug-interacting residues, delineates a putative substrate pocket in SV2A and provides insights into the SV2-isoform-specificity of LEV. Our work has implications for CNT engineering from a hitherto unrecognized SV2 binding interface and for improved designs of anti-convulsant drugs in epilepsy treatment.

Cryo-EM structure of the botulinum neurotoxin A/SV2B complex and its implications for translocation

Botulinum neurotoxin A1 (BoNT/A1) belongs to the most potent toxins and is used as a major therapeutic agent. Neurotoxin conformation is crucial for its translocation to the neuronal cytosol, a key process for intoxication that is only poorly understood. To gain molecular insights into the steps preceding toxin translocation, we determine cryo-EM structures of BoNT/A1 alone and in complex with its receptor synaptic vesicle glycoprotein 2B (SV2B). In solution, BoNT/A1 adopts a unique, semi-closed conformation. The toxin changes its structure into an open state upon receptor binding with the translocation domain (HN) and the catalytic domain (LC) remote from the membrane, suggesting translocation incompatibility. Under acidic pH conditions, where translocation is initiated, receptor-bound BoNT/A1 switches back into a semi-closed conformation. This conformation brings the LC and HN close to the membrane, suggesting that a translocation-competent state of the toxin is required for successful LC transport into the neuronal cytosol.

Local Tetanus Begins with a Neuromuscular Junction Paralysis around the Site of Tetanus Neurotoxin Release due to Cleavage of the Vesicle-Associated Membrane Protein

Local tetanus develops when limited amounts of tetanus neurotoxin (TeNT) are released by Clostridium tetani generated from spores inside a necrotic wound. Within days, a spastic paralysis restricted to the muscles of the affected anatomical area develops. This paralysis follows the retrograde transport of TeNT inside the axons of motoneurons and its uptake by inhibitory interneurons with cleavage of a vesicle-associated membrane protein required for neurotransmitter release. Consequently, incontrollable excitation of motoneurons causes contractures of innervated muscles and leads to local spastic paralysis. Here, the initial events occurring close to the site of TeNT release were investigated in a mouse model of local tetanus. A peripheral flaccid paralysis was found to occur, before or concurrent to the spastic paralysis. At variance from the confined TeNT proteolytic activity taking place within motor neuron terminals, central protein cleavage was detected within inhibitory interneurons controlling motor neuron efferents innervating muscle groups distant from the site of TeNT release. These results indicate peripheral activity of TeNT in tetanus and explains why the spastic paralysis observed in local tetanus, although confined to single limbs, generally affects multiple muscles. The initial TeNT neuroparalytic activity can be detected by measuring the compound muscle action potential, providing a very early diagnosis and therapy, thus preventing the ensuing life-threatening generalized tetanus.

A DARPin promotes faster onset of botulinum neurotoxin A1 action

In this study, we characterize Designed Ankyrin Repeat Proteins (DARPins) as investigative tools to probe botulinum neurotoxin A1 (BoNT/A1) structure and function. We identify DARPin-F5 that completely blocks SNAP25 substrate cleavage by BoNT/A1 in vitro. X-ray crystallography reveals that DARPin-F5 inhibits BoNT/A1 activity by interacting with a substrate-binding region between the α- and β-exosite. This DARPin does not block substrate cleavage of BoNT/A3, indicating that DARPin-F5 is a subtype-specific inhibitor. BoNT/A1 Glu-171 plays a critical role in the interaction with DARPin-F5 and its mutation to Asp, the residue found in BoNT/A3, results in a loss of inhibition of substrate cleavage. In contrast to the in vitro results, DARPin-F5 promotes faster substrate cleavage of BoNT/A1 in primary neurons and muscle tissue by increasing toxin translocation. Our findings could have important implications for the application of BoNT/A1 in therapeutic areas requiring faster onset of toxin action combined with long persistence.

Ancient Clostridium DNA and variants of tetanus neurotoxins associated with human archaeological remains

The analysis of microbial genomes from human archaeological samples offers a historic snapshot of ancient pathogens and provides insights into the origins of modern infectious diseases. Here, we analyze metagenomic datasets from 38 human archaeological samples and identify bacterial genomic sequences related to modern-day Clostridium tetani, which produces the tetanus neurotoxin (TeNT) and causes the disease tetanus. These genomic assemblies had varying levels of completeness, and a subset of them displayed hallmarks of ancient DNA damage. Phylogenetic analyses revealed known C. tetani clades as well as potentially new Clostridium lineages closely related to C. tetani. The genomic assemblies encode 13 TeNT variants with unique substitution profiles, including a subgroup of TeNT variants found exclusively in ancient samples from South America. We experimentally tested a TeNT variant selected from an ancient Chilean mummy sample and found that it induced tetanus muscle paralysis in mice, with potency comparable to modern TeNT. Thus, our ancient DNA analysis identifies DNA from neurotoxigenic C. tetani in archaeological human samples, and a novel variant of TeNT that can cause disease in mammals.

The Tetrahymena bcd1 mutant implicates endosome trafficking in ciliate, cortical pattern formation

Ciliates, such as Tetrahymena thermophila, evolved complex mechanisms to determine both the location and dimensions of cortical organelles such as the oral apparatus (OA: involved in phagocytosis), cytoproct (Cyp: for eliminating wastes), and contractile vacuole pores (CVPs: involved in water expulsion). Mutations have been recovered in Tetrahymena that affect both the localization of such organelles along anterior-posterior and circumferential body axes and their dimensions. Here we describe BCD1, a ciliate pattern gene that encodes a conserved Beige-BEACH domain-containing protein a with possible protein kinase A (PKA)-anchoring activity. Similar proteins have been implicated in endosome trafficking and are linked to human Chediak-Higashi syndrome and autism. Mutations in the BCD1 gene broaden cortical organelle domains as they assemble during predivision development. The Bcd1 protein localizes to membrane pockets at the base of every cilium that are active in endocytosis. PKA activity has been shown to promote endocytosis in other organisms, so we blocked clathrin-mediated endocytosis (using "dynasore") and inhibited PKA (using H89). In both cases, treatment produced partial phenocopies of the bcd1 pattern mutant. This study supports a model in which the dimensions of diverse cortical organelle assembly-platforms may be determined by regulated balance between constitutive exocytic delivery and PKA-regulated endocytic retrieval of organelle materials and determinants.

Local Delivery Strategies for Peptides and Proteins into the CNS: Status Quo, Challenges, and Future Perspectives

Over the past decades, peptides and proteins have been increasingly important in the treatment of various human diseases and conditions owing to their specificity, potency, and minimized off-target toxicity. However, the existence of the practically impermeable blood brain barrier (BBB) limits the entry of macromolecular therapeutics into the central nervous systems (CNS). Consequently, clinical translation of peptide/protein therapeutics for the treatment of CNS diseases has been limited. Over the past decades, developing effective delivery strategies for peptides and proteins has gained extensive attention, in particular with localized delivery strategies, due to the fact that they are capable of circumventing the physiological barrier to directly introduce macromolecular therapeutics into the CNS to improve therapeutic effects and reduce systemic side effects. Here, we discuss various local administration and formulation strategies that have shown successes in the treatment of CNS diseases using peptide/protein therapeutics. Lastly, we discuss challenges and future perspectives of these approaches.

High-Throughput IgG Epitope Mapping of Tetanus Neurotoxin: Implications for Immunotherapy and Vaccine Design

Tetanus is an acute, fatal disease caused by exotoxins released from Clostridium tetani during infections. A protective humoral immune response can be induced by vaccinations with pediatric and booster combinatorial vaccines that contain inactivated tetanus neurotoxin (TeNT) as a major antigen. Although some epitopes in TeNT have been described using various approaches, a comprehensive list of its antigenic determinants that are involved with immunity has not been elucidated. To this end, a high-resolution analysis of the linear B-cell epitopes in TeNT was performed using antibodies generated in vaccinated children. Two hundred sixty-four peptides that cover the entire coding sequence of the TeNT protein were prepared in situ on a cellulose membrane through SPOT synthesis and probed with sera from children vaccinated (ChVS) with a triple DTP-vaccine to map continuous B-cell epitopes, which were further characterized and validated using immunoassays. Forty-four IgG epitopes were identified. Four (TT-215-218) were chemically synthesized as multiple antigen peptides (MAPs) and used in peptide ELISAs to screen post-pandemic DTP vaccinations. The assay displayed a high performance with high sensitivity (99.99%) and specificity (100%). The complete map of linear IgG epitopes induced by vaccination with inactivated TeNT highlights three key epitopes involved in the efficacy of the vaccine. Antibodies against epitope TT-8/G can block enzymatic activity, and those against epitopes TT-41/G and TT-43/G can interfere with TeNT binding to neuronal cell receptors. We further show that four of the epitopes identified can be employed in peptide ELISAs to assess vaccine coverage. Overall, the data suggest a set of select epitopes to engineer new, directed vaccines.

Preparation and characterization of a neutralizing murine monoclonal antibody against tetanus toxin

After Clostridium tetani infects the human body, it propagates under anaerobic conditions and produces tetanus neurotoxin (TeNT). TeNT can affect the central nervous system, inhibit the release of neurotransmitters, and result in respiratory failure, which are the root causes of death in tetanus patients. Identifying monoclonal antibodies (mAbs) targeting TeNT with neutralizing activity is urgently needed for the prevention and treatment of tetanus infection. In this study, through immunizing BALB/c mice with tetanus toxoid (TT), we obtained six positive hybridoma cell lines (1A7, 2C7, 3A7, 3H4, 4C1, and 4E12). Antibody isotyping showed that the antibodies are all of the IgG1/κ subclass. Ascites fluid was prepared by allogeneic ascites induction and the antibodies were purified through protein G affinity chromatography columns. Purities of the produced murine mAbs were all greater than 95%. All six antibodies bound to linear epitopes, among which 3A7 bound to the TeNT/L domain and the other five antibodies bound to the TeNT/Hc domain. Moreover, the affinity constants of these six antibodies against the antigen were all in the nanomolar range, and the affinity of 4E12 antibody reached the picomolar range. Results from toxin-neutralization assays in mice showed that 2C7 antibody delayed animal death, while 1A7, 3A7, 3H4, and 4E12 antibodies conferred partial protection. Additionally, 4C1 antibody offered complete protection, as 200 μg of 4C1 antibody fully protected against toxin challenge with 10 LD₅₀ of TeNT and had a window period of 1 h. Antibody epitope grouping results revealed that the binding epitopes of 4C1 antibody were different from those of the other five antibodies. When 4C1 antibody was used in combination with another antibody, the neutralizing activities of antibodies were all evidently enhanced. Specifically, 4C1 combined with 3A7 antibody led to the greatest improvement in neutralizing activities, and 20 μg antibodies total (10 + 10 μg) fully protected against toxin challenge with 10 LD₅₀. When 4E12, 3A7, and 4C1 antibodies were used in combination, 18 μg antibodies total (6 + 6 + 6 μg) completely neutralized 10 LD₅₀ toxin. The present study derived murine mAbs with neutralizing activities and laid the foundation for follow-up therapeutic drug development for TeNT poisoning as well as establishment of TeNT detection methods.

In Silico Conformational Features of Botulinum Toxins A1 and E1 According to Intraluminal Acidification

Although botulinum neurotoxins (BoNTs) are among the most toxic compounds found in nature, their molecular mechanism of action is far from being elucidated. A key event is the conformational transition due to acidification of the interior of synaptic vesicles, leading to translocation of the BoNT catalytic domain into the neuronal cytosol. To investigate these conformational variations, homology modeling and atomistic simulations are combined to explore the internal dynamics of the sub-types BoNT/A1 (the most-used sub-type in medical applications) and BoNT/E1 (the most kinetically efficient sub-type). This first simulation study of di-chain BoNTs in closed and open states considers the effects of both neutral and acidic pH. The conformational mobility is driven by domain displacements of the ganglioside-binding site in the receptor binding domain, the translocation domain (HCNT) switch, and the belt α-helix, which present multiple conformations, depending on the primary sequence and the pH. Fluctuations of the belt α-helix are observed for closed conformations of the toxins and at acidic pH, while patches of more solvent-accessible residues appear under the same conditions in the core translocation domain HCNT. These findings suggest that, during translocation, the higher mobility of the belt could be transmitted to HCNT, leading to the favorable interaction of HCNT residues with the non-polar membrane environment.

Elucidation of critical pH-dependent structural changes in Botulinum Neurotoxin E

Botulinum Neurotoxins (BoNT) are the most potent toxins currently known. However, they also have therapeutic applications for an increasing number of motor related conditions due to their specificity, and low diffusion into the system. Although the start- and end- points for the BoNT mechanism of action are well-studied, a critical step remains poorly understood. It is theorised that BoNTs undergo a pH-triggered conformational shift, activating the neurotoxin by priming it to form a transmembrane (TM) channel. To test this hypothesis, we combined molecular dynamics (MD) simulations and small-angle x-ray scattering (SAXS), revealing a new conformation of serotype E (BoNT/E). This conformation was exclusively observed in simulations below pH 5.5, as determined by principal component analysis (PCA), and its theoretical SAXS profile matched an experimental SAXS profile obtained at pH 4. Additionally, a localised secondary structural change was observed in MD simulations below pH 5.5, in a region previously identified as instrumental for membrane insertion for serotype A (BoNT/A). These changes were found at a critical pH value for BoNTs in vivo, and may be relevant for their therapeutic use.

Rapid Detection of Clostridium tetani by Recombinase Polymerase Amplification Using an Exo Probe

Tetanus is a potentially fatal public health illness resulted from the neurotoxins generated by Clostridium tetani. C. tetani is not easily culturable and culturing the relevant bacteria from infected wounds has rarely been useful in diagnosis; PCR-based assays can only be conducted at highly sophisticated laboratories. Therefore, a real-time recombinase polymerase amplification assay (Exo-RPA) was constructed to identify the fragments of the neurotoxin gene of C. tetani. Primers and the exo probe targeting the conserved region were designed, and the resulting amplicons could be detected in less than 20 min, with a detection limit of 20 copies/reaction. The RPA assay displayed good selectivity, and there were no cross-reactions with other infectious bacteria common in penetrating wounds. Tests of target-spiked serum and pus extract revealed that RPA is robust to interfering factors and has great potential for further development for biological sample analysis. This method has been confirmed to be reliable for discriminating between toxic and nontoxic C. tetani strains. The RPA assay dramatically improves the diagnostic efficacy with simplified device architecture and is a promising alternative to real-time PCR for tetanus detection.

Development and validation of a targeted LC-MS/MS quantitation method to monitor cell culture expression of tetanus neurotoxin during vaccine production

The tetanus neurotoxin (TeNT) is one of the most toxic proteins known to man, which prior to the use of the vaccine against the TeNT producing bacteria Clostridium tetani, resulted in a 20% mortality rate upon infection. The clinical detrimental effects of tetanus have decreased immensely since the introduction of global vaccination programs, which depend on sustainable vaccine production. One of the major critical points in the manufacturing of these vaccines is the stable and reproducible production of high levels of toxin by the bacterial seed strains. In order to minimize time loss, the amount of TeNT is often monitored during and at the end of the bacterial culturing. The different methods that are currently available to assess the amount of TeNT in the bacterial medium suffer from variability, lack of sensitivity, and/or require specific antibodies. In accordance with the consistency approach and the three Rs (3Rs), both aiming to reduce the use of animals for testing, in-process monitoring of TeNT production could benefit from animal and antibody-free analytical tools. In this paper, we describe the development and validation of a new and reliable antibody free targeted LC-MS/MS method that is able to identify and quantify the amount of TeNT present in the bacterial medium during the different production time points up to the harvesting of the TeNT just prior to further upstream purification and detoxification. The quantitation method, validated according to ICH guidelines and by the application of the total error approach, was utilized to assess the amount of TeNT present in the cell culture medium of two TeNT production batches during different steps in the vaccine production process prior to the generation of the toxoid. The amount of TeNT generated under different physical stress conditions applied during bacterial culture was also monitored.

Structural Analysis of Botulinum Neurotoxins Type B and E by Cryo-EM

Botulinum neurotoxins (BoNTs) are the causative agents of a potentially lethal paralytic disease targeting cholinergic nerve terminals. Multiple BoNT serotypes exist, with types A, B and E being the main cause of human botulism. Their extreme toxicity has been exploited for cosmetic and therapeutic uses to treat a wide range of neuromuscular disorders. Although naturally occurring BoNT types share a common end effect, their activity varies significantly based on the neuronal cell-surface receptors and intracellular SNARE substrates they target. These properties are the result of structural variations that have traditionally been studied using biophysical methods such as X-ray crystallography. Here, we determined the first structures of botulinum neurotoxins using single-particle cryogenic electron microscopy. The maps obtained at 3.6 and 3.7 Å for BoNT/B and /E, respectively, highlight the subtle structural dynamism between domains, and of the binding domain in particular. This study demonstrates how the recent advances made in the field of single-particle electron microscopy can be applied to bacterial toxins of clinical relevance and the botulinum neurotoxin family in particular.

Exceptionally potent human monoclonal antibodies are effective for prophylaxis and treatment of tetanus in mice

We used human monoclonal antibodies (humAbs) to study the mechanism of neuron intoxication by tetanus neurotoxin and to evaluate these antibodies as a safe preventive and therapeutic substitute for hyperimmune sera to treat tetanus in mice. By screening memory B cells from immune donors, we selected 2 tetanus neurotoxin-specific mAbs with exceptionally high neutralizing activities and extensively characterized them both structurally and functionally. We found that these antibodies interfered with the binding and translocation of the neurotoxin into neurons by interacting with 2 epitopes, whose identification pinpoints crucial events in the cellular pathogenesis of tetanus. Our observations explain the neutralization ability of these antibodies, which we found to be exceptionally potent in preventing experimental tetanus when injected into mice long before the toxin. Moreover, their Fab derivatives neutralized tetanus neurotoxin in post-exposure experiments, suggesting their potential for therapeutic use via intrathecal injection. As such, we believe these humAbs, as well as their Fab derivatives, meet the requirements to be considered for prophylactic and therapeutic use in human tetanus and are ready for clinical trials.

Spectroscopic evidence of tetanus toxin translocation domain bilayer-induced refolding and insertion

Tetanus neurotoxin (TeNT) is an A-B toxin with three functional domains: endopeptidase, translocation (HCT), and receptor binding. Endosomal acidification triggers HCT to interact with and insert into the membrane, translocating the endopeptidase across the bilayer. Although the function of HCT is well defined, the mechanism by which it accomplishes this task is unknown. To gain insight into the HCT membrane interaction on both local and global scales, we utilized an isolated, beltless HCT variant (bHCT), which retained the ability to release potassium ions from vesicles. To examine which bHCT residues interact with the membrane, we widely sampled the surface of bHCT using 47 single-cysteine variants labeled with the environmentally sensitive fluorophore NBD. At neutral pH, no interaction was observed for any variant. In contrast, all NBD-labeled positions reported environmental change in the presence of acidic pH and membranes containing anionic lipids. We then examined the conformation of inserted bHCT using circular dichroism and intrinsic fluorescence. Upon entering the membrane, bHCT retained predominantly α-helical secondary structure, whereas the tertiary structure exhibited substantial refolding. The use of lipid-attached quenchers revealed that at least one of the three tryptophan residues penetrated deep into the hydrocarbon core of the membrane, suggesting formation of a bHCT transmembrane conformation. The possible conformational topology was further explored with the hydropathy analysis webtool MPEx, which identified a large, potential α-helical transmembrane region. Altogether, the spectroscopic evidence supports a model in which, upon acidification, the majority of TeNT bHCT entered the membrane with a concurrent change in tertiary structure.

A novel single-domain antibody multimer that potently neutralizes tetanus neurotoxin

Tetanus antitoxin, produced in animals, has been used for the prevention and treatment of tetanus for more than 100 years. The availability of antitoxins, ethical issues around production, and risks involved in the use of animal derived serum products are a concern. We therefore developed a llama derived single-domain antibody (VHH) multimer to potentially replace the conventional veterinary product. In total, 28 different tetanus neurotoxin (TeNT) binding VHHs were isolated, 14 of which were expressed in yeast for further characterization. Four VHH monomers (T2, T6, T15 and T16) binding TeNT with high affinity (KD < 1 nM), covering different antigenic domains as revealed by epitope binning, and including 3 monomers (T6, T15 and T16) that inhibited TeNT binding to neuron gangliosides, were chosen as building blocks to generate 11 VHH multimers. These multimers contained either 1 or 2 different TeNT binding VHHs fused to 1 VHH binding to either albumin (A12) or immunoglobulin (G13) to extend serum half-life in animals. Multimers consisting of 2 TeNT binding VHHs showed more than a 10-fold increase in affinity (KD of 4-23 pM) when compared to multimers containing only one TeNT binding VHH. The T6 and T16 VHHs showed synergistic in vivo TeNT neutralization and, when incorporated into a single VHH trimer (T6T16A12), they showed a very high TeNT neutralizing capacity (1,510 IU/mg).

Novel neutralizing human monoclonal antibodies against tetanus neurotoxin

Tetanus is a fatal disease caused by tetanus neurotoxin (TeNT). TeNT is composed of a light chain (Lc) and a heavy chain, the latter of which is classified into two domains, N-terminus Hn and C-terminus Hc. Several TeNT-neutralizing antibodies have been reported, but it remains unclear which TeNT domains are involved in neutralization. To further understand the mechanism of these antibodies, we isolated TeNT-reactive human antibody clones from peripheral blood mononuclear cells. We then analyzed the reactivity of the isolated antibody clones to each protein domain and their inhibition of Hc-ganglioside GT1b binding, which is critical for TeNT toxicity. We also investigated the TeNT-neutralizing ability of isolated antibody clones and showed that an Hn-reactive clone protected strongly against TeNT toxicity in mice. Furthermore, combination treatment of Hn-reactive antibody clones with both Hc-reactive and TeNT mix (the mixture of Hc, Hn, and Lc proteins)–reactive antibody clones enhanced the neutralizing effect. These results indicated that antibody clones targeting Hn effectively neutralized TeNT. In addition, the use of a cocktail composed of Hc-, Hn-, and TeNT mix–reactive antibodies provided enhanced protection compared to the use of each antibody alone.

Insights into Clostridium tetani: From genome to bioreactors

Tetanus vaccination is of major importance for public health in most countries in the world. The World Health Organization indicated that 15,000 tetanus cases were reported in 2018 (Organization, World Health, 2019). Currently, vaccine manufacturers use tetanus toxin produced by Clostridium tetani fermentation in complex media. The complex components, commonly derived from animal sources, introduce potential variability in cultures. To achieve replicable fermentation and to avoid toxic or allergic reactions from animal-source compounds, several studies have tried to switch from complex to chemically defined media without affecting toxin titers. The present review introduces the current knowledge on i) C. tetani strain diversity, whole-genome sequences and metabolic networks; ii) toxin regulation and synthesis; and iii) culture media, cultivation processes and growth requirements. We critically reviewed the reported data on metabolism in C. tetani and completed comparative genomic and proteomic analyses with other Clostridia species. We integrated genomic data based on whole-genome sequence annotation, supplemented with cofactor specificities determined by protein sequence identity, in a new map of C. tetani central metabolism. This is the first data review that integrates insights from omics experiments on C. tetani. The overview of C. tetani physiology described here could provide support for the design of new chemically defined media devoid of complex sources for toxin production.

Structural basis of tetanus toxin neutralization by native human monoclonal antibodies

Four potent native human monoclonal antibodies (mAbs) targeting distinct epitopes on tetanus toxin (TeNT) are isolated with neutralization potency ranging from approximately 17 mg to 6 mg each that are equivalent to 250 IU of human anti-TeNT immunoglobulin. TT0170 binds fragment B, and TT0069 and TT0155 bind fragment AB. mAb TT0067 binds fragment C and blocks the binding of TeNT to gangliosides. The co-crystal structure of TT0067 with fragment C of TeNT at a 2.0-Å resolution demonstrates that mAb TT0067 directly occupies the W pocket of one of the receptor binding sites on TeNT, resulting in blocking the binding of TeNT to ganglioside on the surface of host cells. This study reveals at the atomic level the mechanism of action by the TeNT neutralizing antibody. The key neutralization epitope on the fragment C of TeNT identified in our work provides the critical information for the development of fragment C of TeNT as a better and safer tetanus vaccine.

Tetanus and tetanus neurotoxin: From peripheral uptake to central nervous tissue targets

Tetanus is a deadly but preventable disease caused by a protein neurotoxin produced by Clostridium tetani. Spores of C. tetani may contaminate a necrotic wound and germinate into a vegetative bacterium that releases a toxin, termed tetanus neurotoxin (TeNT). TeNT enters the general circulation, binds to peripheral motor neurons and sensory neurons, and is transported retroaxonally to the spinal cord. It then enters inhibitory interneurons and blocks the release of glycine or GABA causing a spastic paralysis. This review attempts to correlate the metalloprotease activity of TeNT and its trafficking and localization into the vertebrate body to the nature and sequence of appearance of the symptoms of tetanus.

Tetanus vaccine-induced human neutralizing antibodies provide full protection against neurotoxin challenge in mice

Clostridium tetani causes life-threatening disease by producing tetanus neurotoxin (TeNT), one of the most toxic protein substances. Toxicosis can be prevented and cured by administration of anti-TeNT neutralizing antibodies. Here, we identified a series of monoclonal antibodies (mAbs) derived from memory B cells of a healthy adult immunized with the C-terminal domain of TeNT (TeNT-Hc). Thirteen mAbs bound to both tetanus toxoid (TT) and TeNT-Hc, while two mAbs recognized only TT. VH3-23 was the most frequently used germline gene in these TT-binding mAbs, and the pairwise identity values of the VH gene sequences ranged from 27% to 69%. Three of these mAbs-T3, T7, and T9-6-completely protected mice from challenge with 2× LD50 of TeNT, and two (T2 and T18) significantly prolonged the survival time. The five neutralizing mAbs recognized distinct epitopes on TT, with binding affinities ranging from 0.123 to 11.9 nM. Our study provides promising therapeutic candidates for tetanus.

Structural flexibility of the tetanus neurotoxin revealed by crystallographic and solution scattering analyses

Although the tetanus neurotoxin (TeNT) delivers its protease domain (LC) across the synaptic vesicle lumen into the cytosol via its receptor binding domain (H_C) and translocation domain (H_N), the molecular mechanism coordinating this membrane translocation remains unresolved. Here, we report the high-resolution crystal structures of full-length reduced TeNT (rTeNT, 2.3 Å), TeNT isolated H_N (TeNT/iH_N, 2.3 Å), TeNT isolated H_C (TeNT/iH_C, 1.5 Å), together with the solution structures of TeNT/iH_N and beltless TeNT/iH_N (TeNT/blH_N). TeNT undergoes significant domains rotation of the H_N and LC were demonstrated by structural comparison of rTeNT and non-reduced-TeNT (nrTeNT). A linker loop connects the H_N and H_C is essential for the self-domain rotation of TeNT. The TeNT-specific C869-C1093 disulfide bond is sensitive to the redox environment and its disruption provides linker loop flexibility, which enables domain arrangement of rTeNT distinct from that of nrTeNT. Furthermore, the mobility of C869 in the linker loop and the sensitivity to redox condition of C1093 were confirmed by crystal structure analysis of TeNT/iH_C. On the other hand, the structural flexibility of H_N was investigated by crystallographic and solution scattering analyses. It was found that the region (residues 698-769), which follows the translocation region had remarkable change in TeNT/iH_N. Besides, the so-called belt region has a high propensity to swing around the upper half of TeNT/iH_N at acidic pH. It provides the first overview of the dynamics of the Belt in solution. These newly obtained structural information that shed light on the transmembrane mechanism of TeNT.

A basic introduction to single particles cryo-electron microscopy

<abstract> <p>In the last years, cryogenic-electron microscopy (cryo-EM) underwent the most impressive improvement compared to other techniques used in structural biology, such as X-ray crystallography and NMR. Electron microscopy was invented nearly one century ago but, up to the beginning of the last decades, the 3D maps produced through this technique were poorly detailed, justifying the term “blobbology” to appeal to cryo-EM. Recently, thanks to a new generation of microscopes and detectors, more efficient algorithms, and easier access to computational power, single particles cryo-EM can routinely produce 3D structures at resolutions comparable to those obtained with X-ray crystallography. However, unlike X-ray crystallography, which needs crystallized proteins, cryo-EM exploits purified samples in solution, allowing the study of proteins and protein complexes that are hard or even impossible to crystallize. For these reasons, single-particle cryo-EM is often the first choice of structural biologists today. Nevertheless, before starting a cryo-EM experiment, many drawbacks and limitations must be considered. Moreover, in practice, the process between the purified sample and the final structure could be trickier than initially expected. Based on these observations, this review aims to offer an overview of the principal technical aspects and setups to be considered while planning and performing a cryo-EM experiment.</p> </abstract>

The 25 kDa HCN Domain of Clostridial Neurotoxins Is Indispensable for Their Neurotoxicity

The extraordinarily potent clostridial neurotoxins (CNTs) comprise tetanus neurotoxin (TeNT) and the seven established botulinum neurotoxin serotypes (BoNT/A-G). They are composed of four structurally independent domains: the roles of the catalytically active light chain, the translocation domain H_N, and the C-terminal receptor binding domain H_CC are largely resolved, but that of the H_CN domain sandwiched between H_N and H_CC has remained unclear. Here, mutants of BoNT/A, BoNT/B, and TeNT were generated by deleting their H_CN domains or swapping H_CN domains between each other. Both deletion and replacement of TeNT H_CN domain by H_CNA and H_CNB reduced the biological activity similarly, by ~95%, whereas BoNT/A and B deletion mutants displayed >500-fold reduced activity in the mouse phrenic nerve hemidiaphragm assay. Swapping H_CN domains between BoNT/A and B hardly impaired their biological activity, but substitution with H_CNT did. Binding assays revealed that in the absence of H_CN, not all receptor binding sites are equally well accessible. In conclusion, the presence of H_CN is vital for CNTs to exert their neurotoxicity. Although structurally similar, the H_CN domain of TeNT cannot equally substitute those of BoNT and vice versa, leaving the possibility that H_CNT plays a different role in the intoxication mechanism of TeNT.

Tetanus in Animals — Summary of Knowledge

Tetanus is a neurologic non-transmissible disease (often fatal) of humans and other animals with a worldwide occurrence. Clostridium tetani is the spore producing bacillus which causes the bacterial disease. In deep penetrating wounds the spores germinate and produce a toxin called tetanospasmin. The main characteristic sign of tetanus is a spastic paralysis. A diagnosis is usually based on the clinical signs because the detection in the wound and the cultivation of C. tetani is very difficult. Between animal species there is considerable variability in the susceptibility to the bacillus. The most sensitive animal species to the neurotoxin are horses. Sheep and cattle are less sensitive and tetanus in these animal species are less common. Tetanus in cats and dogs are rare and dogs are less sensitive than cats. Clinically two forms of tetanus have been recognized, i. e. localized and generalized. The available treatment is not specific because the toxin in neuronal cells cannot be accessed by antitoxin antibodies. The aim of the therapy is to: neutralise the unbound neurotoxin, inhibit C. tetani growth in the wound, and provide supportive care to mitigate the effects of the neurotoxin. The treatment is difficult with an unclear prognosis.

Inactivated tetanus as an immunological smokescreen: A major step towards harnessing tetanus-based therapeutics

BACKGROUND AND PURPOSE

Tetanus neurotoxin has many potential therapeutic applications, due to its ability to increase localised muscle tone when injected directly into a muscle. It is a closely related molecule to botulinum neurotoxin (most commonly known as Botox), which has been widely used to release muscle tension for therapeutic and cosmetic applications. However, tetanus toxin has been relegated to the "maybe pile" for protein therapeutics - as most of the population is vaccinated, leading to highly effective antibody-mediated protection against the toxin. The potential for tetanus-based therapeutics remains substantial if the problem of pre-existing immunity can be resolved.

EXPERIMENTAL APPROACH

A well-established murine model of localised muscular contraction was utilised. We administered functional tetanus toxin combined with an immunogenic, but functionally inactive, decoy molecule.

KEY RESULTS

Incorporation of the decoy molecule greatly reduces the dose of active toxin required to induce a localised increase in muscle tone in mice vaccinated with the human toxoid vaccine.

CONCLUSION AND IMPLICATIONS

Our results clearly demonstrate that the barriers to developing a tetanus toxin therapeutic are not insurmountable and the technology presented here is the first major step towards realising the therapeutic potential of this powerful neurotoxin. Opening the therapeutic potential of tetanus toxin will have huge implications for the wide range of diseases caused by low-tone muscle.

Evaluation of a recombinant tetanus toxin subunit vaccine

Tetanus is an acute, fatal disease caused by exotoxin produced by Clostridium tetani. The current vaccine against tetanus is based on inactivated tetanus toxin (TeNT). To develop a recombinant TeNT vaccine suitable for replacement of full-length tetanus toxoid (TT) vaccine for use in humans, a recombinant non-tagged isoform of the Hc domain of the tetanus toxin (THc) was expressed in Escherichia coli and purified by sequential chromatography steps. The immunogenicity and protective effect of the THc antigen were explored and compared with those of TT in Balb/c mice. The THc-based subunit vaccine provided complete protection against TeNT challenge following a high dosage as a toxoid vaccine. While the anti-THc and neutralising antibody titres were higher for the THc-based vaccine than the TT vaccine because protective epitopes are located on the THc domain. Frequency- and dose-dependent immunoprotection were also observed in THc-immunised mice. Mice immunised with one injection of 1 μg or 4 μg THc antigen were completely protected against 10² or 10³ 50% mouse lethal dose (LD₅₀) of TeNT, respectively. Furthermore, the THc protein was found to recognise and bind to ganglioside GT1b in a dose-dependent manner, and anti-THc sera antibodies also inhibited binding between THc and GT1b. Antigen on the form of recombinant non-tagged THc domain expressed in E. coli achieved strong immunoprotective potency, suggesting that it could be developed into a candidate subunit vaccine against tetanus as an alternative to the current TT vaccine.

A Novel High-Potency Tetanus Vaccine

Chemical inactivation is a clinically effective mechanism to detoxify protein toxins to produce vaccines against microbial infections and to serve as a platform for production of conjugate polysaccharide vaccines. This method is widely used for the production of protein toxin vaccines, including tetanus toxoid. However, chemical modification alters the protein structure with unknown effects on antigenicity. Here, a recombinant full-length tetanus toxin (TT) is engineered with 8 mutations (8MTT) that inactivate three toxin functions: catalysis, translocation, and receptor binding. 8MTT is nontoxic and elicits a potent immune response in outbred mice. 8MTT also represents a malleable platform for the production of conjugate vaccines, which can facilitate a rapid vaccine response against emerging microbial pathogens.

Chemically inactivated tetanus toxoid (CITT) is clinically effective and widely used. However, CITT is a crude nonmalleable vaccine that contains hundreds of Clostridium tetani proteins, and the active component is present in variable and sometimes minor percentages of vaccine mass. Recombinant production of a genetically inactivated tetanus vaccine offers an opportunity to replace and improve the current tetanus vaccine. Previous studies showed the feasibility of engineering full-length tetanus toxin (TT) in Escherichia coli. In the present study, full-length TT was engineered with eight individual amino acid mutations (8MTT) to inactivate catalysis, translocation, and host receptor-binding functions, retaining 99.4% amino acid identity to native tetanus toxin. 8MTT purified as a 150-kDa single-chain protein, which trypsin nicked to a 100-kDa heavy chain and 50-kDa light chain. The 8MTT was not toxic for outbred mice and was >50 million-fold less toxic than native TT. Relative to CITT, 8MTT vaccination elicited a strong immune response and showed good vaccine potency against TT challenge. The strength of the immune response to both vaccines varied among individual outbred mice. These data support 8MTT as a candidate vaccine against tetanus and a malleable candidate conjugate vaccine platform to enhance the immune response to polysaccharides and other macromolecular molecules to facilitate a rapid response to emerging microbial pathogens.

Tetanus Toxin cis-Loop Contributes to Light-Chain Translocation

How protein toxins translocate their catalytic domain across a cell membrane is the least understood step in toxin action. This study utilized a reporter, β-lactamase, that was genetically fused to full-length, nontoxic tetanus toxin (βlac-TT) in discovery-based live-cell assays to study LC translocation. Directed mutagenesis identified a role for K⁷⁶⁸ in LC translocation. K⁷⁶⁸ was located between α15 and α16 (termed the cis-loop). Cellular assays showed that K⁷⁶⁸ did not interfere with other toxin functions, including cell binding, intracellular trafficking, and pore formation. The equivalent K⁷⁶⁸ is conserved among the clostridial neurotoxin family of proteins as a conserved structural motif. The cis-loop appears to contribute to LC translocation.

The clostridial neurotoxins (CNTs) comprise tetanus toxin (TT) and botulinum neurotoxin (BoNT [BT]) serotypes (A to G and X) and several recently identified CNT-like proteins, including BT/En and the mosquito BoNT-like toxin Pmp1. CNTs are produced as single proteins cleaved to a light chain (LC) and a heavy chain (HC) connected by an interchain disulfide bond. LC is a zinc metalloprotease (cleaving soluble N-ethylmaleimide-sensitive factor attachment protein receptors [SNAREs]), while HC contains an N-terminal translocation domain (HCN) and a C-terminal receptor binding domain (HCC). HCN-mediated LC translocation is the least understood function of CNT action. Here, β-lactamase (βlac) was used as a reporter in discovery-based live-cell assays to characterize TT-mediated LC translocation. Directed mutagenesis identified a role for a charged loop (⁷⁶⁷DKE⁷⁶⁹) connecting α15 and α16 (cis-loop) within HCN in LC translocation; aliphatic substitution inhibited LC translocation but not other toxin functions such as cell binding, intracellular trafficking, or HCN-mediated pore formation. K⁷⁶⁸ was conserved among the CNTs. In molecular simulations of the HCN with a membrane, the cis-loop did not bind with the cell membrane. Taken together, the results of these studies implicate the cis-loop in LC translocation, independently of pore formation.

IMPORTANCE How protein toxins translocate their catalytic domain across a cell membrane is the least understood step in toxin action. This study utilized a reporter, β-lactamase, that was genetically fused to full-length, nontoxic tetanus toxin (βlac-TT) in discovery-based live-cell assays to study LC translocation. Directed mutagenesis identified a role for K⁷⁶⁸ in LC translocation. K⁷⁶⁸ was located between α15 and α16 (termed the cis-loop). Cellular assays showed that K⁷⁶⁸ did not interfere with other toxin functions, including cell binding, intracellular trafficking, and pore formation. The equivalent K⁷⁶⁸ is conserved among the clostridial neurotoxin family of proteins as a conserved structural motif. The cis-loop appears to contribute to LC translocation.

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.

Tetanus in animals

Tetanus is a neurologic disease of humans and animals characterized by spastic paralysis. Tetanus is caused by tetanus toxin (TeNT) produced by Clostridium tetani, an environmental soilborne, gram-positive, sporulating bacterium. The disease most often results from wound contamination by soil containing C. tetani spores. Horses, sheep, and humans are highly sensitive to TeNT, whereas cattle, dogs, and cats are more resistant. The diagnosis of tetanus is mainly based on the characteristic clinical signs. Identification of C. tetani at the wound site is often difficult.

Designs of Antigen Structure and Composition for Improved Protein-Based Vaccine Efficacy

Today, vaccinologists have come to understand that the hallmark of any protective immune response is the antigen. However, it is not the whole antigen that dictates the immune response, but rather the various parts comprising the whole that are capable of influencing immunogenicity. Protein-based antigens hold particular importance within this structural approach to understanding immunity because, though different molecules can serve as antigens, only proteins are capable of inducing both cellular and humoral immunity. This fact, coupled with the versatility and customizability of proteins when considering vaccine design applications, makes protein-based vaccines (PBVs) one of today's most promising technologies for artificially inducing immunity. In this review, we follow the development of PBV technologies through time and discuss the antigen-specific receptors that are most critical to any immune response: pattern recognition receptors, B cell receptors, and T cell receptors. Knowledge of these receptors and their ligands has become exceptionally valuable in the field of vaccinology, where today it is possible to make drastic modifications to PBV structure, from primary to quaternary, in order to promote recognition of target epitopes, potentiate vaccine immunogenicity, and prevent antigen-associated complications. Additionally, these modifications have made it possible to control immune responses by modulating stability and targeting PBV to key immune cells. Consequently, careful consideration should be given to protein structure when designing PBVs in the future in order to potentiate PBV efficacy.

Neuronal selectivity of botulinum neurotoxins

Botulinum neurotoxins (BoNTs) are highly potent toxins responsible for a severe disease, called botulism. They are also efficient therapeutic tools with an increasing number of indications ranging from neuromuscular dysfunction to hypersecretion syndrome, pain release, depression as well as cosmetic application. BoNTs are known to mainly target the motor-neurons terminals and to induce flaccid paralysis. BoNTs recognize a specific double receptor on neuronal cells consisting of gangliosides and synaptic vesicle protein, SV2 or synaptotagmin. Using cultured neuronal cells, BoNTs have been established blocking the release of a wide variety of neurotransmitters. However, BoNTs are more potent in motor-neurons than in the other neuronal cell types. In in vivo models, BoNT/A impairs the cholinergic neuronal transmission at the motor-neurons but also at neurons controlling secretions and smooth muscle neurons, and blocks several neuronal pathways including excitatory, inhibitory, and sensitive neurons. However, only a few reports investigated the neuronal selectivity of BoNTs in vivo. In the intestinal wall, BoNT/A and BoNT/B target mainly the cholinergic neurons and to a lower extent the other non-cholinergic neurons including serotonergic, glutamatergic, GABAergic, and VIP-neurons. The in vivo effects induced by BoNTs on the non-cholinergic neurons remain to be precisely investigated. We report here a literature review of the neuronal selectivity of BoNTs.

Helix N-Cap Residues Drive the Acid Unfolding That Is Essential in the Action of the Toxin Colicin A

Numerous bacterial toxins and other virulence factors use low pH as a trigger to convert from water-soluble to membrane-inserted states. In the case of colicins, the pore-forming domain of colicin A (ColA-P) has been shown both to undergo a clear acidic unfolding transition and to require acidic lipids in the cytoplasmic membrane, whereas its close homologue colicin N shows neither behavior. Compared to that of ColN-P, the ColA-P primary structure reveals the replacement of several uncharged residues with aspartyl residues, which upon replacement with alanine induce an unfolded state at neutral pH. Here we investigate ColA-P's structural requirement for these critical aspartyl residues that are largely situated at the N-termini of α helices. As previously shown in model peptides, the charged carboxylate side chain can act as a stabilizing helix N-Cap group by interacting with free amide hydrogen bond donors. Because this could explain ColA-P destabilization when the aspartyl residues are protonated or replaced with alanyl residues, we test the hypothesis by inserting asparagine, glutamine, and glutamate residues at these sites. We combine urea (fluorescence and circular dichroism) and thermal (circular dichroism and differential scanning calorimetry) denaturation experiments with 1H-15N heteronuclear single-quantum coherence nuclear magnetic resonance spectroscopy of ColA-P at different pH values to provide a comprehensive description of the unfolding process and confirm the N-Cap hypothesis. Furthermore, we reveal that, in urea, the single domain ColA-P unfolds in two steps; low pH destabilizes the first step and stabilizes the second.

The role of the single interchains disulfide bond in tetanus and botulinum neurotoxins and the development of antitetanus and antibotulism drugs

A large number of bacterial toxins consist of active and cell binding protomers linked by an interchain disulfide bridge. The largest family of such disulfide-bridged exotoxins is that of the clostridial neurotoxins that consist of two chains and comprise the tetanus neurotoxins causing tetanus and the botulinum neurotoxins causing botulism. Reduction of the interchain disulfide abolishes toxicity, and we discuss the experiments that revealed the role of this structural element in neuronal intoxication. The redox couple thioredoxin reductase-thioredoxin (TrxR-Trx) was identified as the responsible for reduction of this disulfide occurring on the cytosolic surface of synaptic vesicles. We then discuss the very relevant finding that drugs that inhibit TrxR-Trx also prevent botulism. On this basis, we propose that ebselen and PX-12, two TrxR-Trx specific drugs previously used in clinical trials in humans, satisfy all the requirements for clinical tests aiming at evaluating their capacity to effectively counteract human and animal botulism arising from intestinal toxaemias such as infant botulism.

Identification of Formaldehyde-Induced Modifications in Diphtheria Toxin

Diphtheria toxoid is produced by detoxification of diphtheria toxin with formaldehyde. This study was performed to elucidate the chemical nature and location of formaldehyde-induced modifications in diphtheria toxoid. Diphtheria toxin was chemically modified using 4 different reactions with the following reagents: (1) formaldehyde and NaCNBH₃, (2) formaldehyde, (3) formaldehyde and NaCNBH₃ followed by formaldehyde and glycine, and (4) formaldehyde and glycine. The modifications were studied by SDS-PAGE, primary amino group determination, and liquid chromatography-electrospray mass spectrometry of chymotryptic digests. Reaction 1 resulted in quantitative dimethylation of all lysine residues. Reaction 2 caused intramolecular cross-links, including the NAD⁺-binding cavity and the receptor-binding site. Moreover, A fragments and B fragments were cross-linked by formaldehyde on part of the diphtheria toxoid molecules. Reaction 3 resulted in formaldehyde-glycine attachments, including in shielded areas of the protein. The detoxification reaction typically used for vaccine preparation (reaction 4) resulted in a combination of intramolecular cross-links and formaldehyde-glycine attachments. Both the NAD⁺-binding cavity and the receptor-binding site of diphtheria toxin were chemically modified. Although CD4⁺ T-cell epitopes were affected to some extent, one universal CD4⁺ T-cell epitope remained almost completely unaltered by the treatment with formaldehyde and glycine.

Tetanus antitoxin potency assessment by surface plasmon resonance and ToBI test

Potency testing of tetanus antitoxin must be performed in vivo, in a very painful, stressful and prone to high variability assay. It is, therefore, mandatory to find alternatives to this kind of potency assessment. Immunochemical tests as ELISA or ToBI test are already available but usually results in a poor correlation to the in vivo protection. Considering research and development of mono and oligoclonal antibodies against tetanus and the improvement of equine polyclonal antitoxin production and control, we developed an alternative instrumental test for tetanus antitoxin by using surface plasmon resonance. Tetanus antitoxin from hyperimmune equine sera (16 batches) were tested and the results indicated excellent concordance and correlation to the in vivo test (Lin's ρ = 0.9). This innovative approach should now be improved in order to extend it to oligoclonal and monoclonal human antibodies aiming to replace mice for the potency assessment of tetanus antitoxin especially during research and development steps.

The population structure of Clostridium tetani deduced from its pan-genome

Clostridium tetani produces a potent neurotoxin, the tetanus neurotoxin (TeNT) that is responsible for the worldwide neurological disease tetanus, but which can be efficiently prevented by vaccination with tetanus toxoid. Until now only one type of TeNT has been characterized and very little information exists about the heterogeneity among C. tetani strains. We report here the genome sequences of 26 C. tetani strains, isolated between 1949 and 2017 and obtained from different locations. Genome analyses revealed that the C. tetani population is distributed in two phylogenetic clades, a major and a minor one, with no evidence for clade separation based on geographical origin or time of isolation. The chromosome of C. tetani is highly conserved; in contrast, the TeNT-encoding plasmid shows substantial heterogeneity. TeNT itself is highly conserved among all strains; the most relevant difference is an insertion of four amino acids in the C-terminal receptor-binding domain in four strains that might impact on receptor-binding properties. Other putative virulence factors, including tetanolysin and collagenase, are encoded in all genomes. This study highlights the population structure of C. tetani and suggests that tetanus-causing strains did not undergo extensive evolutionary diversification, as judged from the high conservation of its main virulence factors.

Structural Biology and Molecular Modeling to Analyze the Entry of Bacterial Toxins and Virulence Factors into Host Cells

Understanding the functions and mechanisms of biological systems is an outstanding challenge. One way to overcome it is to combine together several approaches such as molecular modeling and experimental structural biology techniques. Indeed, the interplay between structural and dynamical properties of the system is crucial to unravel the function of molecular machinery’s. In this review, we focus on how molecular simulations along with structural information can aid in interpreting biological data. Here, we examine two different cases: (i) the endosomal translocation toxins (diphtheria, tetanus, botulinum toxins) and (ii) the activation of adenylyl cyclase inside the cytoplasm (edema factor, CyA, ExoY).

Tetanus

Tetanus is a vaccine-preventable disease that still commonly occurs in many low-income and middle-income countries, although it is rare in high-income countries. The disease is caused by the toxin of the bacterium Clostridium tetani and is characterised by muscle spasms and autonomic nervous system dysfunction. Global vaccination initiatives have had considerable success but they continue to face many challenges. Treatment for tetanus aims to control spasms and reduce cardiovascular instability, and consists of wound debridement, antitoxin, antibiotics, and supportive care. Recent research has focused on intravenous magnesium sulphate and intrathecal antitoxin administration as methods of spasm control that can avoid the need for ventilatory support. Nevertheless, without access to mechanical ventilation, mortality from tetanus remains high. Even with such care, patients require several weeks of hospitalisation and are vulnerable to secondary problems, such as hospital-acquired infections.

An oligoclonal combination of human monoclonal antibodies able to neutralize tetanus toxin in vivo

The use of antibody-based therapy to treat a variety of diseases and conditions is well documented. The use of antibodies as an antidote to treat tetanus infections was one of the first examples of immunotherapy and remains the standard of care for cases involving potential infections. Plasma-derived immunoglobulins obtained from human or horse pose risks of infection from undetectable emergent viruses or may cause anaphylaxis. Further, there is a lack of consistency between lots. In the search for new formulations, we obtained a series of clonally related human monoclonal antibodies (mAbs) derived from B cells sorted from donors that presented anti-tetanus neutralizing titers. Donors were revaccinated prior to blood collection. Different strategies were used for single-cell sorting, since it was challenging to identify cells at a very low frequency: memory B cell sorting using fluorescent-labeled tetanus toxoid and toxin as baits, and plasmablast sorting done shortly after revaccination. Screening of the recombinant mAbs with the whole tetanus toxin allowed us to select candidates with therapeutic potential, since mAbs to different domains can contribute additively to the neutralizing effect. Because of selective binding to different domains, we tested mAbs individually, or in mixtures of two or three, in the neutralizing in vivo assay specified by Pharmacopeia for the determination of polyclonal hyperimmune sera potency. An oligoclonal mixture of three human mAbs completely neutralized the toxin injected in the animals, signaling an important step for clinical mAb development.

The Structure and Classification of Botulinum Toxins

Botulinum neurotoxins (BoNTs) are a family of bacterial protein toxins produced by various Clostridium species. They are traditionally classified into seven major serotypes (BoNT/A-G). Recent progress in sequencing microbial genomes has led to an ever-growing number of subtypes, chimeric toxins, BoNT-like toxins, and remotely related BoNT homologs, constituting an expanding BoNT superfamily. Recent structural studies of BoNTs, BoNT progenitor toxin complexes, tetanus neurotoxin (TeNT), toxin-receptor complexes, and toxin-substrate complexes have provided mechanistic understandings of toxin functions and the molecular basis for their variations. The growing BoNT superfamily of toxins present a natural repertoire that can be explored to develop novel therapeutic toxins, and the structural understanding of their variations provides a knowledge basis for engineering toxins to improve therapeutic efficacy and expand their clinical applications.

A viral-fusion-peptide-like molecular switch drives membrane insertion of botulinum neurotoxin A1

Botulinum neurotoxin (BoNT) delivers its protease domain across the vesicle membrane to enter the neuronal cytosol upon vesicle acidification. This process is mediated by its translocation domain (H_N), but the molecular mechanism underlying membrane insertion of H_N remains poorly understood. Here, we report two crystal structures of BoNT/A1 H_N that reveal a novel molecular switch (termed BoNT-switch) in H_N, where buried α-helices transform into surface-exposed hydrophobic β-hairpins triggered by acidic pH. Locking the BoNT-switch by disulfide trapping inhibited the association of H_N with anionic liposomes, blocked channel formation by H_N, and reduced the neurotoxicity of BoNT/A1 by up to ~180-fold. Single particle counting studies showed that an acidic environment tends to promote BoNT/A1 self-association on liposomes, which is partly regulated by the BoNT-switch. These findings suggest that the BoNT-switch flips out upon exposure to the acidic endosomal pH, which enables membrane insertion of H_N that subsequently leads to LC delivery.

The translocation domain (H_N) of Botulinum neurotoxins (BoNTs) mediates the delivery of the BoNT light chain (LC) into neuronal cytosol. Here the authors provide insights into H_N membrane insertion by determining the crystal structure of BoNT/A1 H_N at acidic pH, which reveals a molecular switch in H_N, where buried α-helices are transformed into surface-exposed hydrophobic β-hairpins.

Botulinum and Tetanus Neurotoxins

Botulinum neurotoxins (BoNTs) and tetanus neurotoxin (TeNT) are the most potent toxins known and cause botulism and tetanus, respectively. BoNTs are also widely utilized as therapeutic toxins. They contain three functional domains responsible for receptor-binding, membrane translocation, and proteolytic cleavage of host proteins required for synaptic vesicle exocytosis. These toxins also have distinct features: BoNTs exist within a progenitor toxin complex (PTC), which protects the toxin and facilitates its absorption in the gastrointestinal tract, whereas TeNT is uniquely transported retrogradely within motor neurons. Our increasing knowledge of these toxins has allowed the development of engineered toxins for medical uses. The discovery of new BoNTs and BoNT-like proteins provides additional tools to understand the evolution of the toxins and to engineer toxin-based therapeutics. This review summarizes the progress on our understanding of BoNTs and TeNT, focusing on the PTC, receptor recognition, new BoNT-like toxins, and therapeutic toxin engineering.

Detection of Clostridium tetani Neurotoxins Inhibited In Vivo by Botulinum Antitoxin B: Potential for Misleading Mouse Test Results in Food Controls

The presence of botulinum neurotoxin-producing Clostridia (BPC) in food sources is a public health concern. In favorable environmental conditions, BPC can produce botulinum neurotoxins (BoNTs) outside or inside the vertebrate host, leading to intoxications or toxico-infectious forms of botulism, respectively. BPC in food are almost invariably detected either by PCR protocols targeted at the known neurotoxin-encoding genes, or by the mouse test to assay for the presence of BoNTs in the supernatants of enrichment broths inoculated with the tested food sample. The sample is considered positive for BPC when the supernatant contains toxic substances that are lethal to mice, heat-labile and neutralized in vivo by appropriate polyclonal antibodies raised against purified BoNTs of different serotypes. Here, we report the detection in a food sample of a Clostridium tetani strain that produces tetanus neurotoxins (TeNTs) with the above-mentioned characteristics: lethal for mice, heat-labile and neutralized by botulinum antitoxin type B. Notably, neutralization occurred with two different commercially available type B antitoxins, but not with type A, C, D, E and F antitoxins. Although TeNT and BoNT fold very similarly, evidence that antitoxin B antiserum can neutralize the neurotoxic effect of TeNT in vivo has not been documented before. The presence of C. tetani strains in food can produce misleading results in BPC detection using the mouse test.

The Chaperonin GroEL: A Versatile Tool for Applied Biotechnology Platforms

The nucleotide-free chaperonin GroEL is capable of capturing transient unfolded or partially unfolded states that flicker in and out of existence due to large-scale protein dynamic vibrational modes. In this work, three short vignettes are presented to highlight our continuing advances in the application of GroEL biosensor biolayer interferometry (BLI) technologies and includes expanded uses of GroEL as a molecular scaffold for electron microscopy determination. The first example presents an extension of the ability to detect dynamic pre-aggregate transients in therapeutic protein solutions where the assessment of the kinetic stability of any folded protein or, as shown herein, quantitative detection of mutant-type protein when mixed with wild-type native counterparts. Secondly, using a BLI denaturation pulse assay with GroEL, the comparison of kinetically controlled denaturation isotherms of various von Willebrand factor (vWF) triple A domain mutant-types is shown. These mutant-types are single point mutations that locally disorder the A1 platelet binding domain resulting in one gain of function and one loss of function phenotype. Clear, separate, and reproducible kinetic deviations in the mutant-type isotherms exist when compared with the wild-type curve. Finally, expanding on previous electron microscopy (EM) advances using GroEL as both a protein scaffold surface and a release platform, examples are presented where GroEL-protein complexes can be imaged using electron microscopy tilt series and the low-resolution structures of aggregation-prone proteins that have interacted with GroEL. The ability of GroEL to bind hydrophobic regions and transient partially folded states allows one to employ this unique molecular chaperone both as a versatile structural scaffold and as a sensor of a protein's folded states.

Protein Toxins That Utilize Gangliosides as Host Receptors

Subsets of protein toxins utilize gangliosides as host receptors. Gangliosides are preferred receptors due to their extracellular localization on the eukaryotic cell and due to their essential nature in host physiology. Glycosphingolipids, including gangliosides, are mediators of signal transduction within and between eukaryotic cells. Protein toxins possess AB structure-function organization, where the A domain encodes a catalytic function for the posttranslational modification of a host macromolecule, including proteins and nucleic acids, and a B domain, which encodes host receptor recognition, including proteins and glycosphingolipids, alone or in combination. Protein toxins use similar strategies to bind glycans by pockets and loops, generally employing hydrogen bonding and aromatic stacking to stabilize interactions with sugars. In some cases, glycan binding facilitates uptake, while in other cases, cross-linking or a second receptor is necessary to stimulate entry. The affinity that protein toxins have for host glycans is necessary for tissue targeting, but not always sufficient to cause disease. In addition to affinity for binding the glycan, the lipid moiety also plays an important role in productive uptake and tissue tropism. Upon endocytosis, the protein toxin must escape to another intracellular compartment or into cytosol to modify a host substrate, modulating host signaling, often resulting in cytotoxic or apoptotic events in the cell, and a unique morbidity for the organism. The study of protein toxins that utilize gangliosides as host receptors has illuminated numerous eukaryotic cellular processes, identified the basis for developing interventions to prevent disease through vaccines and control bacterial diseases through therapies. In addition, subsets of these protein toxins have been utilized as therapeutic agents to treat numerous human inflictions.