Hsp90 and Thioredoxin-Thioredoxin Reductase enable the catalytic activity of Clostridial neurotoxins inside nerve terminals

Overview of Bacterial Protein Toxins from Pathogenic Bacteria: Mode of Action and Insights into Evolution

Bacterial protein toxins are secreted by certain bacteria and are responsible for mild to severe diseases in humans and animals. They are among the most potent molecules known, which are active at very low concentrations. Bacterial protein toxins exhibit a wide diversity based on size, structure, and mode of action. Upon recognition of a cell surface receptor (protein, glycoprotein, and glycolipid), they are active either at the cell surface (signal transduction, membrane damage by pore formation, or hydrolysis of membrane compound(s)) or intracellularly. Various bacterial protein toxins have the ability to enter cells, most often using an endocytosis mechanism, and to deliver the effector domain into the cytosol, where it interacts with an intracellular target(s). According to the nature of the intracellular target(s) and type of modification, various cellular effects are induced (cell death, homeostasis modification, cytoskeleton alteration, blockade of exocytosis, etc.). The various modes of action of bacterial protein toxins are illustrated with representative examples. Insights in toxin evolution are discussed.

The Chaperonin TRiC/CCT Inhibitor HSF1A Protects Cells from Intoxication with Pertussis Toxin

Pertussis toxin (PT) is a bacterial AB5-toxin produced by Bordetella pertussis and a major molecular determinant of pertussis, also known as whooping cough, a highly contagious respiratory disease. In this study, we investigate the protective effects of the chaperonin TRiC/CCT inhibitor, HSF1A, against PT-induced cell intoxication. TRiC/CCT is a chaperonin complex that facilitates the correct folding of proteins, preventing misfolding and aggregation, and maintaining cellular protein homeostasis. Previous research has demonstrated the significance of TRiC/CCT in the functionality of the Clostridioides difficile TcdB AB-toxin. Our findings reveal that HSF1A effectively reduces the levels of ADP-ribosylated Gαi, the specific substrate of PT, in PT-treated cells, without interfering with enzyme activity in vitro or the cellular binding of PT. Additionally, our study uncovers a novel interaction between PTS1 and the chaperonin complex subunit CCT5, which correlates with reduced PTS1 signaling in cells upon HSF1A treatment. Importantly, HSF1A mitigates the adverse effects of PT on cAMP signaling in cellular systems. These results provide valuable insights into the mechanisms of PT uptake and suggest a promising starting point for the development of innovative therapeutic strategies to counteract pertussis toxin-mediated pathogenicity.

Requirement of Peptidyl-Prolyl Cis/Trans isomerases and chaperones for cellular uptake of bacterial AB-type toxins

Bacterial AB-type toxins are proteins released by the producing bacteria and are the causative agents for several severe diseases including cholera, whooping cough, diphtheria or enteric diseases. Their unique AB-type structure enables their uptake into mammalian cells via sophisticated mechanisms exploiting cellular uptake and transport pathways. The binding/translocation B-subunit facilitates binding of the toxin to a specific receptor on the cell surface. This is followed by receptor-mediated endocytosis. Then the enzymatically active A-subunit either escapes from endosomes in a pH-dependent manner or the toxin is further transported through the Golgi to the endoplasmic reticulum from where the A-subunit translocates into the cytosol. In the cytosol, the A-subunits enzymatically modify a specific substrate which leads to cellular reactions resulting in clinical symptoms that can be life-threatening. Both intracellular uptake routes require the A-subunit to unfold to either fit through a pore formed by the B-subunit into the endosomal membrane or to be recognized by the ER-associated degradation pathway. This led to the hypothesis that folding helper enzymes such as chaperones and peptidyl-prolyl cis/trans isomerases are required to assist the translocation of the A-subunit into the cytosol and/or facilitate their refolding into an enzymatically active conformation. This review article gives an overview about the role of heat shock proteins Hsp90 and Hsp70 as well as of peptidyl-prolyl cis/trans isomerases of the cyclophilin and FK506 binding protein families during uptake of bacterial AB-type toxins with a focus on clostridial binary toxins Clostridium botulinum C2 toxin, Clostridium perfringens iota toxin, Clostridioides difficile CDT toxin, as well as diphtheria toxin, pertussis toxin and cholera toxin.

Bacterial AB toxins and host-microbe interactions

AB toxins are protein virulence factors secreted by many bacterial pathogens, contributing to the pathogenicity of the cognate bacteria. AB toxins consist of two functionally distinct components: the enzymatic "A" component for pathogenicity and the receptor-binding "B" component for toxin delivery. Consistently, unlike other virulence factors such as effectors, AB toxins do not require additional systems to deliver them to the target host cells. Target host cells are located in the infection site and/or located distantly from infected host cells. The first part of this review discusses the structural and functional features of single-peptide and multiprotein AB toxins in the context of host-microbe interactions, using several well-characterized examples. The second part of this review discusses toxin neutralization strategies, as well as applications of AB toxins relevant to developing intervention strategies against diseases.

Comparative Pharmacodynamics of Three Different Botulinum Toxin Type A Preparations following Repeated Intramuscular Administration in Mice

Botulinum neurotoxin type A (BoNT/A) causes muscle paralysis by blocking cholinergic signaling at neuromuscular junctions and is widely used to temporarily correct spasticity-related disorders and deformities. The paralytic effects of BoNT/A are time-limited and require repeated injections at regular intervals to achieve long-term therapeutic benefits. Differences in the level and duration of effectivity among various BoNT/A products can be attributed to their unique manufacturing processes, formulation, and noninterchangeable potency units. Herein, we compared the pharmacodynamics of three BoNT/A formulations, i.e., Botox^® (onabotulinumtoxinA), Xeomin^® (incobotulinumtoxinA), and Coretox^®, following repeated intramuscular (IM) injections in mice. Three IM injections of BoNT/A formulations (12 U/kg per dose), 12-weeks apart, were administered at the right gastrocnemius. Local paresis and chemodenervation efficacy were evaluated over 36 weeks using the digit abduction score (DAS) and compound muscle action potential (CMAP), respectively. One week after administration, all three BoNT/A formulations induced peak DAS and maximal reduction of CMAP amplitudes. Among the three BoNT/A formulations, only Coretox^® afforded a significant increase in paretic effects and chemodenervation with a prolonged duration of action after repeated injections. These findings suggest that Coretox^® may offer a better overall therapeutic performance in clinical settings.

Toxicology and pharmacology of botulinum and tetanus neurotoxins: an update

Tetanus and botulinum neurotoxins cause the neuroparalytic syndromes of tetanus and botulism, respectively, by delivering inside different types of neurons, metalloproteases specifically cleaving the SNARE proteins that are essential for the release of neurotransmitters. Research on their mechanism of action is intensively carried out in order to devise improved therapies based on antibodies and chemical drugs. Recently, major results have been obtained with human monoclonal antibodies and with single chain antibodies that have allowed one to neutralize the metalloprotease activity of botulinum neurotoxin type A1 inside neurons. In addition, a method has been devised to induce a rapid molecular evolution of the metalloprotease domain of botulinum neurotoxin followed by selection driven to re-target the metalloprotease activity versus novel targets with respect to the SNARE proteins. At the same time, an intense and wide spectrum clinical research on novel therapeutics based on botulinum neurotoxins is carried out, which are also reviewed here.

Novel Small Molecule Inhibitors That Prevent the Neuroparalysis of Tetanus Neurotoxin

Tetanus neurotoxin (TeNT) is a protein exotoxin produced by Clostridium tetani that causes the deadly spastic neuroparalysis of tetanus. It consists of a metalloprotease light chain and of a heavy chain linked via a disulphide bond. TeNT binds to the neuromuscular junction (NMJ) and it is retro-axonally transported into vesicular compartments to the spinal cord, where it is released and taken up by inhibitory interneuron. Therein, the catalytic subunit is translocated into the cytoplasm where it cleaves its target protein VAMP-1/2 with consequent blockage of the release of inhibitory neurotransmitters. Vaccination with formaldehyde inactivated TeNT prevents the disease, but tetanus is still present in countries where vaccination coverage is partial. Here, we show that small molecule inhibitors interfering with TeNT trafficking or with the reduction of the interchain disulphide bond block the activity of the toxin in neuronal cultures and attenuate tetanus symptoms in vivo. These findings are relevant for the development of therapeutics against tetanus based on the inhibition of toxin molecules that are being retro-transported to or are already within the spinal cord and are, thus, not accessible to anti-TeNT immunoglobulins.

Resolving the Molecular Steps in Clostridial Neurotoxin Light Chain Translocation

The clostridial neurotoxins (CNTs), botulinum toxin and tetanus toxin, are the most toxic proteins for humans. Neurotoxicity is based upon the specificity of the CNTs for neural host receptors and substrates. CNTs are organized into three domains, a Light Chain (LC) that is a metalloprotease and a Heavy Chain (HC) that has two domains, an N-terminal LC translocation domain (HCN) and a C-terminal receptor binding domain (HCC). While catalysis and receptor binding functions of the CNTs have been developed, our understanding of LC translocation is limited. This is due to the intrinsic complexity of the translocation process and limited tools to assess the step-by-step events in LC translocation. Recently, we developed a novel, cell-based TT-reporter to measure LC translocation as the translocation of a β-lactamase reporter across a vesicle membrane in neurons. Using this approach, we identified a role for a cis-Loop, located within the HCN, in LC translocation. In this commentary, we describe our current understanding of how CNTs mediate LC translocation and place the role of the cis-Loop in the LC translocation process relative to other independent functions that have been implicated in LC translocation. Understanding the basis for LC translocation will enhance the use of CNTs in vaccine development and as human therapies.

Fully Human Monoclonal Antibodies Effectively Neutralizing Botulinum Neurotoxin Serotype B

Botulinum neurotoxin (BoNT) is the most potent natural toxin known. Of the seven BoNT serotypes (A to G), types A, B, E, and F cause human botulism. Treatment of human botulism requires the development of effective toxin-neutralizing antibodies without side effects such as serum sickness and anaphylaxis. In this study, we generated fully human monoclonal antibodies (HuMAbs) against serotype B BoNT (BoNT/B1) using a murine–human chimera fusion partner cell line named SPYMEG. Of these HuMAbs, M2, which specifically binds to the light chain of BoNT/B1, showed neutralization activity in a mouse bioassay (approximately 10 i.p. LD₅₀/100 µg of antibody), and M4, which binds to the C-terminal of heavy chain, showed partial protection. The combination of two HuMAbs, M2 (1.25 µg) and M4 (1.25 µg), was able to completely neutralize BoNT/B1 (80 i.p. LD₅₀) with a potency greater than 80 i.p. LD₅₀/2.5 µg of antibodies, and was effective both prophylactically and therapeutically in the mouse model of botulism. Moreover, this combination showed broad neutralization activity against three type B subtypes, namely BoNT/B1, BoNT/B2, and BoNT/B6. These data demonstrate that the combination of M2 and M4 is promising in terms of a foundation for new human therapeutics for BoNT/B intoxication.

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.

Development of An Innovative and Quick Method for the Isolation of Clostridium botulinum Strains Involved in Avian Botulism Outbreaks

Avian botulism is a serious neuroparalytic disease mainly caused by a type C/D botulinum neurotoxin produced by Clostridium botulinum group III, one of the entwined bacterial species from the Clostridiumnovyisensulato genospecies. Its isolation is very challenging due to the absence of selective media and the instability of the phage carrying the gene encoding for the neurotoxin. The present study describes the development of an original method for isolating C. botulinum group III strains. Briefly, this method consists of streaking the InstaGene matrix extraction pellet on Egg Yolk Agar plates and then collecting the colonies with lipase and lecithinase activities. Using this approach, it was possible to isolate 21 C. novyi sensu lato strains from 22 enrichment broths of avian livers, including 14 toxic strains. This method was successfully used to re-isolate type C, D, C/D, and D/C strains from liver samples spiked with five spores per gram. This method is cheap, user-friendly, and reliable. It can be used to quickly isolate toxic strains involved in avian botulism with a 64% success rate and C. novyi sensu lato with a 95% rate. This opens up new perspectives for C. botulinum genomic research, which will shed light on the epidemiology of avian botulism.

New Engineered-Botulinum Toxins Inhibit the Release of Pain-Related Mediators

Targeted delivery of potent inhibitor of cytokine/pain-mediator into inflammatory or pain-sensing cells is a promising avenue for treating chronic pain, a world-wide major healthcare burden. An unmet need exists for a specific and effective delivery strategy. Herein, we describe a new approach using sortase to site-specifically ligate a non-toxic botulinum neurotoxin D (BoNT/D) core-therapeutic (synaptobrevin-cleaving protease and translocation domains) to cell-specific targeting ligands. An engineered core-therapeutic was efficiently ligated to IL-1β ligand within minutes. The resultant conjugate specifically entered into cultured murine primary macrophages, cleaved synaptobrevin 3 and inhibited LPS/IFN-γ evoked IL-6 release. Likewise, a CGRP receptor antagonist ligand delivered BoNT/D protease into sensory neurons and inhibited K⁺-evoked substance P release. As cytokines and neuropeptides are major regulators of inflammation and pain, blocking their release by novel engineered inhibitors highlights their therapeutic potential. Our report describes a new and widely-applicable strategy for the production of targeted bio-therapeutics for numerous chronic diseases.

Biosecurity Threat Posed by Botulinum Toxin

The deliberate release of biological agents with terrorist or criminal intent continues to pose concerns in the current geopolitical situation. Therefore, attention is still needed to ensure preparedness against the potential use of pathogens as unconventional weapons. Botulinum neurotoxin (BoNT) is one such biological threat, characterized by an extremely low lethal dose, high morbidity and mortality when appropriately disseminated, and the capacity to cause panic and social disruption. This paper addresses the risks of a potential release of the botulinum neurotoxin and summarizes the relevant aspects of the threat.

Role of AIP56 disulphide bond and its reduction by cytosolic redox systems for efficient intoxication

Apoptosis-inducing protein of 56 kDa (AIP56) is a major virulence factor of Photobacterium damselae subsp. piscicida, a gram-negative pathogen that infects warm water fish species worldwide and causes serious economic losses in aquacultures. AIP56 is a single-chain AB toxin composed by two domains connected by an unstructured linker peptide flanked by two cysteine residues that form a disulphide bond. The A domain comprises a zinc-metalloprotease moiety that cleaves the NF-kB p65, and the B domain is involved in binding and internalisation of the toxin into susceptible cells. Previous experiments suggested that disruption of AIP56 disulphide bond partially compromised toxicity, but conclusive evidences supporting the importance of that bond in intoxication were lacking. Here, we show that although the disulphide bond of AIP56 is dispensable for receptor recognition, endocytosis, and membrane interaction, it needs to be intact for efficient translocation of the toxin into the cytosol. We also show that the host cell thioredoxin reductase-thioredoxin system is involved in AIP56 intoxication by reducing the disulphide bond of the toxin at the cytosol. The present study contributes to a better understanding of the molecular mechanisms operating during AIP56 intoxication and reveals common features shared with other AB toxins.

Botulinum neurotoxins A, B, C, E, and F preferentially enter cultured human motor neurons compared to other cultured human neuronal populations

Human-induced pluripotent stem cell (hiPSC)-derived neurons can be exquisitely sensitive to botulinum neurotoxins (BoNTs), exceeding sensitivity of the traditionally used mouse bioassay. In this report, four defined hiPSC-derived neuronal populations including primarily GABAergic, glutamatergic, dopaminergic, and motor neurons were examined for BoNT/A, B, C, D, E, and F sensitivity. The data indicate that sensitivity varies markedly for the BoNTs tested. Motor neurons are significantly more sensitive than other neuron types for all BoNTs except BoNT/D. Examination of SNARE protein levels and BoNT-specific cell surface protein receptors reveals few differences between the cell types except greater expression levels of the receptor protein SV2C and synapsin-IIa in motor neurons. This indicates that differential toxicity of BoNTs for motor neurons compared to other neuronal cell types involves multiple mechanisms.

Efficacy of Early Endoscopic Intervention for Restoring Normal Swallowing Function in Patients with Lateral Medullary Infarction

Dysphagia is considered to be a significant barrier for recovery after lateral medullary infarction (LMI). However, there is still no gold standard treatment for dysphagia. The aim of this study was to explore an effect of an early treatment options for swallowing dysfunction after acute LMI. Medical records of acute LMI patients who had been admitted to the department of rehabilitation medicine from January 2014 to December 2017 were reviewed retrospectively. We compared the clinical efficacy of conventional dysphagia rehabilitation to early endoscopic intervention using either botulinum toxin injection into cricopharyngeal muscle or endoscopic balloon dilatation of the muscle. Outcomes, such as duration of parental feeding, albumin level at diet transition to enteral feeding, and complications, were analyzed. A total of 18 patients with LMI were included. While eight patients (8/9, 88.89%) in the endoscopic group were capable of orally ingesting their diet after intervention, the conversion from tube feeding to an oral diet was possible in only five patients (5/9, 55.56%) of the conventional group during hospitalization. However, the difference between the two groups was not significant (p-value ≤ 0.147, chi-square test). Only the final dietary level at the time of discharge was higher level in endoscopic group. The conversion interval from tube feeding to oral diet was also comparable between groups. There was no re-conversion from the oral diet to tube feeding in patients of either group during the median follow-up period of 20 months. Early endoscopic intervention may be a better option for dysphagia with LMI, compared to conventional dysphagia rehabilitation. However, a larger and prospective trial may be needed to confirm our observations.

Botulinum Toxin in Restless Legs Syndrome-A Randomized Double-Blind Placebo-Controlled Crossover Study

Background: Restless Legs Syndrome (RLS) is a common movement disorder with an estimated prevalence of up to 12%. Previous small studies with onabotulinumtoxin A (OnaA) for RLS have shown inconsistent results. Methods: Twenty-four patients with an International RLS score (IRLS) of >11 (moderate-severe) were enrolled in this blinded, placebo-controlled crossover study. Twenty-one patients completed the evaluations at 4, 6, and 8 weeks after each injection. One-hundred units of Incobotulinumtoxin A (IncoA) or normal saline were injected into tibialis anterior, gastrocnemius, and biceps femoris muscles each side. Results: Improvement from a severe (IRLS >21) to a mild/moderate (IRLS ≤20) score was significant at four weeks (p = 0.0036) and six weeks (p = 0.0325) following IncoA administration compared to placebo. Additionally, there was significant improvement in pain score at six weeks as measured by Visual Analogue Scale (p = 0.04) and the Johns Hopkins Quality of Life Questionnaire (p = 0.01) in the IncoA group. Definite or marked improvement on Patient Global Impression of Change was seen in 7 out of 21 patients in the IncoA group vs. 1 out of 21 patients in the placebo group at 4 weeks (p = 0.012). Conclusion: IncoA injection lead to a reduction in severity of RLS symptoms, pain score, and quality of life, without any adverse effects.

Preparation of Cerebellum Granule Neurons from Mouse or Rat Pups and Evaluation of Clostridial Neurotoxin Activity and Their Inhibitors by Western Blot and Immunohistochemistry

Cerebellar Granule Neurons (CGN) from post-natal rodents have been widely used as a model to study neuronal development, physiology and pathology. CGN cultured in vitro maintain the same features displayed in vivo by mature cerebellar granule cells, including the development of a dense neuritic network, neuronal activity, neurotransmitter release and the expression of neuronal protein markers. Moreover, CGN represent a convenient model for the study of Clostridial Neurotoxins (CNT), most notably known as Tetanus and Botulinum neurotoxins, as they abundantly express both CNT receptors and intraneuronal substrates, i.e., Soluble N-ethylmaleimide-sensitive factor activating protein receptors (SNARE proteins). Here, we describe a protocol for obtaining a highly pure culture of CGN from postnatal rats/mice and an easy procedure for their intoxication with CNT. We also illustrate handy methods to evaluate CNT activity and their inhibition.

Engineering of Botulinum Neurotoxins for Biomedical Applications

Botulinum neurotoxins (BoNTs) have been used as therapeutic agents in the clinical treatment of a wide array of neuromuscular and autonomic neuronal transmission disorders. These toxins contain three functional domains that mediate highly specific neuronal cell binding, internalization and cytosolic delivery of proteolytic enzymes that cleave proteins integral to the exocytosis of neurotransmitters. The exceptional cellular specificity, potency and persistence within the neuron that make BoNTs such effective toxins, also make them attractive models for derivatives that have modified properties that could potentially expand their therapeutic repertoire. Advances in molecular biology techniques and rapid DNA synthesis have allowed a wide variety of novel BoNTs with alternative functions to be assessed as potential new classes of therapeutic drugs. This review examines how the BoNTs have been engineered in an effort to produce new classes of therapeutic molecules to address a wide array of disorders.

Novel Botulinum Neurotoxins: Exploring Underneath the Iceberg Tip

Botulinum neurotoxins (BoNTs), the etiological agents of botulism, are the deadliest toxins known to humans. Yet, thanks to their biological and toxicological features, BoNTs have become sophisticated tools to study neuronal physiology and valuable therapeutics for an increasing number of human disorders. BoNTs are produced by multiple bacteria of the genus Clostridium and, on the basis of their different immunological properties, were classified as seven distinct types of toxin. BoNT classification remained stagnant for the last 50 years until, via bioinformatics and high-throughput sequencing techniques, dozens of BoNT variants, novel serotypes as well as BoNT-like toxins within non-clostridial species have been discovered. Here, we discuss how the now “booming field” of botulinum neurotoxin may shed light on their evolutionary origin and open exciting avenues for future therapeutic applications.