Neuronal entry and high neurotoxicity of botulinum neurotoxin A require its N-terminal binding sub-domain

Structure and activity of botulinum neurotoxin X

Botulinum neurotoxins (BoNTs) are the most potent toxins known and are used to treat an increasing number of medical disorders. All BoNTs are naturally co-expressed with a protective partner protein (NTNH) with which they form a 300 kDa complex, to resist acidic and proteolytic attack from the digestive tract. We have previously identified a new botulinum neurotoxin serotype, BoNT/X, that has unique and therapeutically attractive properties. We present the cryo-EM structure of the BoNT/X-NTNH/X complex at 3.1 Å resolution. Unexpectedly, the BoNT/X complex is stable and protease resistant at both neutral and acidic pH and disassembles only in alkaline conditions. Using the stabilizing effect of NTNH, we isolated BoNT/X and showed that it has very low potency both in vitro and in vivo . Given the high catalytic activity and translocation efficacy of BoNT/X, low activity of the full toxin is likely due to the receptor-binding domain, which presents weak ganglioside binding and exposed hydrophobic surfaces.

Understanding the Pivotal Role of the Vagus Nerve in Health from Pandemics

The COVID-19 pandemic seems endless with the regular emergence of new variants. Is the SARS-CoV-2 virus particularly evasive to the immune system, or is it merely disrupting communication between the body and the brain, thus pre-empting homeostasis? Retrospective analysis of the COVID-19 and AIDS pandemics, as well as prion disease, emphasizes the pivotal but little-known role of the 10th cranial nerve in health. Considering neuroimmunometabolism from the point of view of the vagus nerve, non-invasive bioengineering solutions aiming at monitoring and stimulating the vagal tone are subsequently discussed as the next optimal and global preventive treatments, far beyond pandemics.

Botulinum Neurotoxin Chimeras Suppress Stimulation by Capsaicin of Rat Trigeminal Sensory Neurons In Vivo and In Vitro

Chimeras of botulinum neurotoxin (BoNT) serotype A (/A) combined with /E protease might possess improved analgesic properties relative to either parent, due to inheriting the sensory neurotropism of the former with more extensive disabling of SNAP-25 from the latter. Hence, fusions of /E protease light chain (LC) to whole BoNT/A (LC/E-BoNT/A), and of the LC plus translocation domain (HN) of /E with the neuronal acceptor binding moiety (HC) of /A (BoNT/EA), created previously by gene recombination and expression in E. coli., were used. LC/E-BoNT/A (75 units/kg) injected into the whisker pad of rats seemed devoid of systemic toxicity, as reflected by an absence of weight loss, but inhibited the nocifensive behavior (grooming, freezing, and reduced mobility) induced by activating TRPV1 with capsaicin, injected at various days thereafter. No sex-related differences were observed. c-Fos expression was increased five-fold in the trigeminal nucleus caudalis ipsi-lateral to capsaicin injection, relative to the contra-lateral side and vehicle-treated controls, and this increase was virtually prevented by LC/E-BoNT/A. In vitro, LC/E-BoNT/A or /EA diminished CGRP exocytosis from rat neonate trigeminal ganglionic neurons stimulated with up to 1 µM capsaicin, whereas BoNT/A only substantially reduced the release in response to 0.1 µM or less of the stimulant, in accordance with the /E protease being known to prevent fusion of exocytotic vesicles.

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.

Why Are Botulinum Neurotoxin-Producing Bacteria So Diverse and Botulinum Neurotoxins So Toxic?

Botulinum neurotoxins (BoNTs) are the most lethal toxins among all bacterial, animal, plant and chemical poisonous compounds. Although a great effort has been made to understand their mode of action, some questions are still open. Why, and for what benefit, have environmental bacteria that accidentally interact with their host engineered so diverse and so specific toxins targeting one of the most specialized physiological processes, the neuroexocytosis of higher organisms? The extreme potency of BoNT does not result from only one hyperactive step, but in contrast to other potent lethal toxins, from multi-step activity. The cumulative effects of the different steps, each having a limited effect, make BoNTs the most potent lethal toxins. This is a unique mode of evolution of a toxic compound, the high potency of which results from multiple steps driven by unknown selection pressure, targeting one of the most critical physiological process of higher organisms.

A SNAP-25 cleaving chimera of botulinum neurotoxin /A and /E prevents TNFα-induced elevation of the activities of native TRP channels on early postnatal rat dorsal root ganglion neurons

Transient receptor potential (TRP) vallinoid 1 (TRPV1) and ankyrin 1 (TRPA1) are two transducing channels expressed on peripheral sensory nerves involved in pain sensation. Upregulation of their expression, stimulated by inflammatory cytokines and growth factors in animal pain models, correlate with the induction of nociceptive hyper-sensitivity. Herein, we firstly demonstrate by immuno-cytochemical labelling that TNFα augments the surface content of these channels on rat cultured dorsal root ganglion (DRG) neurons which, in turn, enhances the electrophysiological and functional responses of the latter to their specific agonists. A molecular basis underlying this TNFα-dependent enhancement was unveiled by pre-treating DRGs with a recently-published chimeric protein, consisting of the protease light chain (LC) of botulinum neurotoxin (BoNT) serotype E fused to full-length BoNT/A (LC/E-BoNT/A). This cleaves synaptosomal-associated protein of Mr 25k (SNAP-25) and reported previously to exhibit anti-nociceptive activity in a rat model of neuropathic pain. Low pM concentrations of this chimera were found to prevent the TNFα-stimulated delivery of TRPV1/A1 to the neuronal plasmalemma and, accordingly, decreased their incremental functional activities relative to those of control cells, an effect accompanied by SNAP-25 cleavage. Advantageously, LC/E-BoNT/A did not reduce the basal surface contents of the two channels or their pharmacological responses. Thus, use of multiple complementary methodologies provides evidence that LC/E-BoNT/A abolishes the TNFα-dependent augmented, but not resting, surface trafficking of TRPV1/A1. As TNFα is known to induce nociceptive hyper-sensitivity in vivo, our observed inhibition by LC/E-BoNT/A of its action in vitro could contribute to its potential alleviation of pain.

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

The tetanus neurotoxin (TeNT) is a highly potent toxin produced by Clostridium tetani that inhibits neurotransmission of inhibitory interneurons, causing spastic paralysis in the tetanus disease. TeNT differs from the other clostridial neurotoxins by its unique ability to target the central nervous system by retrograde axonal transport. The crystal structure of the tetanus toxin reveals a "closed" domain arrangement stabilised by two disulphide bridges, and the molecular details of the toxin's interaction with its polysaccharide receptor. An integrative analysis combining X-ray crystallography, solution scattering and single particle electron cryo-microscopy reveals pH-mediated domain rearrangements that may give TeNT the ability to adapt to the multiple environments encountered during intoxication, and facilitate binding to distinct receptors.