Botulinum neurotoxin X lacks potency in mice and in human neurons

Botulinum neurotoxins (BoNTs) are a class of toxins produced by Clostridium botulinum (C. botulinum) and other species of Clostridia. BoNT/X is a putative novel botulinum neurotoxin identified through genome sequencing and capable of SNARE cleavage, but its neurotoxic potential in humans and vertebrates remained unclear. The C. botulinum strain producing BoNT/X, Strain 111, encodes both a plasmid-borne bont/b2 as well as the chromosomal putative bont/x. This study utilized C. botulinum Strain 111 from Japan as well as recombinantly produced full-length BoNT/X to more fully analyze this putative pathogenic toxin. We confirmed production of full-length, catalytically active native BoNT/X by C. botulinum Strain 111, produced as a disulfide-bonded dichain polypeptide similar to other BoNTs. Both the purified native and the recombinant BoNT/X had high enzymatic activity in vitro but displayed very low potency in human-induced pluripotent stem cell-derived neuronal cells and in mice. Intraperitoneal injection of up to 50 µg of native BoNT/X in mice did not result in botulism; however, mild local paralysis was observed after injection of 2 μg into the gastrocnemius muscle. We further demonstrate that the lack of toxicity by BoNT/X is due to inefficient neuronal cell association and entry, which can be rescued by replacing the receptor binding domain of BoNT/X with that of BoNT/A. These data demonstrate that BoNT/X is not a potent vertebrate neurotoxin like the classical seven serotypes of BoNTs.

IMPORTANCE

The family of botulinum neurotoxins comprises the most potent toxins known to humankind. New members of this family of protein toxins as well as more distantly related homologs are being identified. The discovery of BoNT/X via bioinformatic screen in 2017 as a putative new BoNT serotype raised concern about its potential as a pathogenic agent with no available countermeasures. This study for the first time assessed both recombinantly produced and native purified BoNT/X for its vertebrate neurotoxicity.

Recent Developments in Vaccine Design: From Live Vaccines to Recombinant Toxin Vaccines

Vaccines are one of the most effective strategies to prevent pathogen-induced illness in humans. The earliest vaccines were based on live inoculations with low doses of live or related pathogens, which carried a relatively high risk of developing the disease they were meant to prevent. The introduction of attenuated and killed pathogens as vaccines dramatically reduced these risks; however, attenuated live vaccines still carry a risk of reversion to a pathogenic strain capable of causing disease. This risk is completely eliminated with recombinant protein or subunit vaccines, which are atoxic and non-infectious. However, these vaccines require adjuvants and often significant optimization to induce robust T-cell responses and long-lasting immune memory. Some pathogens produce protein toxins that cause or contribute to disease. To protect against the effects of such toxins, chemically inactivated toxoid vaccines have been found to be effective. Toxoid vaccines are successfully used today at a global scale to protect against tetanus and diphtheria. Recent developments for toxoid vaccines are investigating the possibilities of utilizing recombinant protein toxins mutated to eliminate biologic activity instead of chemically inactivated toxins. Finally, one of the most contemporary approaches toward vaccine design utilizes messenger RNA (mRNA) as a vaccine candidate. This approach was used globally to protect against coronavirus disease during the COVID-19 pandemic that began in 2019, due to its advantages of quick production and scale-up, and effectiveness in eliciting a neutralizing antibody response. Nonetheless, mRNA vaccines require specialized storage and transport conditions, posing challenges for low- and middle-income countries. Among multiple available technologies for vaccine design and formulation, which technology is most appropriate? This review focuses on the considerable developments that have been made in utilizing diverse vaccine technologies with a focus on vaccines targeting bacterial toxins. We describe how advancements in vaccine technology, combined with a deeper understanding of pathogen-host interactions, offer exciting and promising avenues for the development of new and improved vaccines.

Special Issue: Gram-Positive Bacterial Toxins

The Gram stain classifies most bacteria into one of two groups, Gram-negative or Gram-positive, based on the composition of their cell walls [...].

Botulinum Neurotoxin A4 Has a 1000-Fold Reduced Potency due to Three Single Amino Acid Alterations in the Protein Receptor Binding Domain

Botulinum neurotoxin subtype A4 (BoNT/A4) is ~1000-fold less potent than BoNT/A1. This study addresses the basis for low BoNT/A4 potency. Utilizing BoNT/A1-A4 and BoNT/A4-A1 Light Chain-Heavy Chain (LC-HC) chimeras, HC-A4 was responsible for low BoNT/A4 potency. Earlier studies showed BoNT/A1-receptor binding domain (Hcc) bound a β-strand peptide (556–564) and glycan-N559 within Luminal Domain 4 (LD4) of SV2C, the BoNT/A protein receptor. Relative to BoNT/A1, the Hcc of BoNT/A4 possesses two amino acid variants (D1141 and N1142) within the β-peptide binding interface and one amino acid variant (R1292) located near the SV2C glycan-N559. Introduction of BoNT/A4 β-strand peptide variant (D1141 and N1142) into BoNT/A1 reduced toxin potency 30-fold, and additional introduction of the BoNT/A4 glycan-N559 variant (D1141, N1142, and R1292) further reduced toxin potency to approach BoNT/A4. While introduction of BoNT/A1 glycan-N559 variant (G1292) into BoNT/A4 did not alter toxin potency, additional introduction of BoNT/A1 β-strand peptide variants (G1141, S1142, and G1292) resulted in potency approaching BoNT/A1 potency. Thus, outcomes from these functional and modeling studies indicate that in rodent models, disruption of Hcc -SV2C β-peptide and -glycan-N559 interactions mediate low BoNT/A4 potency, while in human motor neurons, disruption of Hcc-SV2C β-peptide alone mediates low BoNT/A4 potency, which link to a species-specific variation at SV2C563.

How Botulinum Neurotoxin Light Chain A1 Maintains Stable Association with the Intracellular Neuronal Plasma Membrane

Botulinum neurotoxin serotype A (BoNT/A) is the most potent protein toxin for humans and is utilized as a therapy for numerous neurologic diseases. BoNT/A comprises a catalytic Light Chain (LC/A) and a Heavy Chain (HC/A) and includes eight subtypes (BoNT/A1-/A8). Previously we showed BoNT/A potency positively correlated with stable localization on the intracellular plasma membrane and identified a low homology domain (amino acids 268-357) responsible for LC/A1 stable co-localization with SNAP-25 on the plasma membrane, while LC/A3 was present in the cytosol of Neuro2A cells. In the present study, steady-state- and live-imaging of a cytosolic LC/A3 derivative (LC/A3V) engineered to contain individual structural elements of the A1 LDH showed that a 59 amino acid region (275-334) termed the MLD was sufficient to direct LC/A3V from the cytosol to the plasma membrane co-localized with SNAP-25. Informatics and experimental validation of the MLD-predicted R1 region (an α-helix, residues 275-300) and R2 region (a loop, α-helix, loop, residues 302-334) both contribute independent steps to the stable co-localization of LC/A1 with SNAP-25 on the plasma membrane of Neuro-2A cells. Understanding how these structural elements contribute to the overall association of LC/A1 on the plasma membrane may identify the molecular basis for the LC contribution of BoNT/A1 to high potency.

Genomic Diversity, Competition, and Toxin Production by Group I and II Clostridium botulinum Strains Used in Food Challenge Studies

Botulinum neurotoxins (BoNTs) produced by the bacteria Clostridium botulinum are the causative agent of human and animal botulism, a rare but serious and potentially deadly intoxication. Foodborne botulism is caused by the consumption of foods containing BoNTs, which results from contamination of foods with C. botulinum spores and toxin production by the bacteria during growth within the food. Validation of the safety of food products is essential in preventing foodborne botulism, however, limited guidance and standards exist for the selection of strains used in C. botulinum food challenge studies. Sequencing and genomics studies have revealed that C. botulinum is a large, diverse, and polyphyletic species, with physiologic and growth characteristics studied only in a few representatives. Little is known about potential growth competition or effects on toxin production between C. botulinum strains. In this study, we investigated an applied cocktail of ten C. botulinum strains, seven Group I and three Group II. Whole genome SNP alignments revealed that this strain cocktail encompasses the major clades of the Group I and II C. botulinum species. While growth competition appears to exist between several of the strains, the cocktail as a whole resulted in high levels of BoNT production.

Investigation of Salicylanilides as Botulinum Toxin Antagonists

Botulinum neurotoxin serotype A (BoNT/A) is recognized by the Centers for Disease Control and Prevention (CDC) as the most potent toxin and as a Tier 1 biowarfare agent. The severity and longevity of botulism stemming from BoNT/A is of significant therapeutic concern, and early administration of antitoxin-antibody therapy is the only approved pharmaceutical treatment for botulism. Small molecule therapeutic strategies have targeted both the heavy chain (HC) and the light chain (LC) catalytic active site and α-/β-exosites. The LC translocation mechanism has also been studied, but an effective, nontoxic inhibitor remains underexplored. In this work, we screened a library of salicylanilides as potential translocation inhibitors. Potential leads following a primary screen were further scrutinized to identify sal30, which has a cellular minimal concentration of a drug that is required for 50% inhibition (IC50) value of 141 nM. The inquiry of salicylanilide sal30's mechanism of action was explored through a self-quenched fluorogenic substrate conjugated to bovine serum albumin (DQ-BSA) fluorescence, confocal microscopy, and vacuolar H+-ATPase (V-ATPase) inhibition assays. The summation of these findings imply that endolysosomal proton translocation through the protonophore mechanism of sal30 causes endosome pH to increase, which in turn prevents LC translocation into cytosol, a process that requires an acidic pH. Thus, the inhibition of BoNT/A activity by salicylanilides likely occurs through disruption of pH-dependent endosomal LC translocation. We further probed BoNT inhibition by sal30 using additivity analysis studies with bafilomycin A1, a known BoNT/A LC translocation inhibitor, which indicated the absence of synergy between the two ionophores.

Preclinical Evidence for the Role of Botulinum Neurotoxin A (BoNT/A) in the Treatment of Peripheral Nerve Injury

Traumatic peripheral nerve injuries tend to be more common in younger, working age populations and can lead to long-lasting disability. Peripheral nerves have an impressive capacity to regenerate; however, successful recovery after injury depends on a number of factors including the mechanism and severity of the trauma, the distance from injury to the reinnervation target, connective tissue sheath integrity, and delay between injury and treatment. Even though modern surgical procedures have greatly improved the success rate, many peripheral nerve injuries still culminate in persistent neuropathic pain and incomplete functional recovery. Recent studies in animals suggest that botulinum neurotoxin A (BoNT/A) can accelerate nerve regeneration and improve functional recovery after injury to peripheral nerves. Possible mechanisms of BoNT/A action include activation or proliferation of support cells (Schwann cells, mast cells, and macrophages), increased angiogenesis, and improvement of blood flow to regenerating nerves.

Endogenous CRISPR-Cas Systems in Group I Clostridium botulinum and Clostridium sporogenes Do Not Directly Target the Botulinum Neurotoxin Gene Cluster

Most strains of proteolytic group I Clostridium botulinum (G1 C. botulinum) and some strains of Clostridium sporogenes possess genes encoding botulinum neurotoxin (BoNT), a potent neuroparalytic agent. Within G1 C. botulinum, conserved bont gene clusters of three major toxin serotypes (bont/A/B/F) can be found on conjugative plasmids and/or within chromosomal pathogenicity islands. CRISPR-Cas systems enable site-specific targeting of previously encountered mobile genetic elements (MGE) such as plasmids and bacteriophage through the creation of a spacer library complementary to protospacers within the MGEs. To examine whether endogenous CRISPR-Cas systems restrict the transfer of bont gene clusters across strains we conducted a bioinformatic analysis profiling endogenous CRISPR-Cas systems from 241 G1 C. botulinum and C. sporogenes strains. Approximately 6,200 CRISPR spacers were identified across the strains and Type I-B, III-A/B/D cas genes and CRISPR array features were identified in 83% of the strains. Mapping the predicted spacers against the masked strain and RefSeq plasmid dataset identified 56,000 spacer-protospacer matches. While spacers mapped heavily to targets within bont(+) plasmids, no protospacers were identified within the bont gene clusters. These results indicate the toxin is not a direct target of CRISPR-Cas but the plasmids predominantly responsible for its mobilization are. Finally, while the presence of a CRISPR-Cas system did not reliably indicate the presence or absence of a bont gene cluster, comparative genomics across strains indicates they often occupy the same hypervariable loci common to both species, potentially suggesting similar mechanisms are involved in the acquisition and curation of both genomic features.

Antibiotic stewardship for reptiles

This review discusses the general principles underlying responsible antibiotic usage in reptiles. Very little evidence underlies antibiotic usage in reptiles, and there are no published guidelines for responsible antibiotic usage. A literature search was performed to review the evidence for bacterial involvement in the pathology of selected common diseases of reptiles, allowing the development of recommendations for responsible antibiotic treatment of those diseases.

Irreversible inhibition of BoNT/A protease: proximity-driven reactivity contingent upon a bifunctional approach

Botulinum neurotoxin A (BoNT/A) is categorized as a Tier 1 bioterrorism agent and persists within muscle neurons for months, causing paralysis. A readily available treatment that abrogates BoNT/A's toxicity and longevity is a necessity in the event of a widespread BoNT/A attack and for clinical treatment of botulism, yet remains an unmet need. Herein, we describe a comprehensive warhead screening campaign of bifunctional hydroxamate-based inhibitors for the irreversible inhibition of the BoNT/A light chain (LC). Using the 2,4-dichlorocinnamic hydroxamic acid (DCHA) metal-binding pharmacophore modified with a pendent warhead, a total of 37 compounds, possessing 13 distinct warhead types, were synthesized and evaluated for time-dependent inhibition against the BoNT/A LC. Iodoacetamides, maleimides, and an epoxide were found to exhibit time-dependent inhibition and their k GSH measured as a description of reactivity. The epoxide exhibited superior time-dependent inhibition over the iodoacetamides, despite reacting with glutathione (GSH) 51-fold slower. The proximity-driven covalent bond achieved with the epoxide inhibitor was contingent upon the vital hydroxamate-Zn2+ anchor in placing the warhead in an optimal position for reaction with Cys165. Monofunctional control compounds exemplified the necessity of the bifunctional approach, and Cys165 modification was confirmed through high-resolution mass spectrometry (HRMS) and ablation of time-dependent inhibitory activity against a C165A variant. Compounds were also evaluated against BoNT/A-intoxicated motor neuron cells, and their cell toxicity, serum stability, and selectivity against matrix metalloproteinases (MMPs) were characterized. The bifunctional approach allows the use of less intrinsically reactive electrophiles to intercept Cys165, thus expanding the toolbox of potential warheads for selective irreversible BoNT/A LC inhibition. We envision that this dual-targeted strategy is amenable to other metalloproteases that also possess non-catalytic cysteines proximal to the active-site metal center.

COVALENT INHIBITION OF BOTULINUM NEUROTOXIN A - EXPLORATION OF WARHEAD REACTIVITY AND FUNCTION USING A BIFUNCTIONAL APPROACH

INTRODUCTION AND OBJECTIVES

Botulinum neurotoxin A (BoNT/A) is extremely toxic possessing an estimated intravenous LD₅₀ of 1-2 ng/kg and as such has been designated a category A bioterrorism agent.^{1, 2} BoNT/A also possesses an extremely long half-life and persists within muscle neurons for months to >1 year.³ Because of BoNT/A longevity, we have utilized covalent inhibition as a means to abrogate BoNT/A's toxicity. To this end, we describe an approach to designing inhibitors that possess both electrophilic warheads and metal-binding groups for the bifunctional inhibition of BoNT/A.

METHODS

Small molecule inhibitors that possessed electrophilic moieties were designed, using X-ray crystallography as guidance, to target both the zinc metal-binding region and Cys165 within the active site of BoNT/A. Synthesized compounds were evaluated for covalent inhibition using a continuous SNAPtide FRET assay⁴ and exhaustive dialysis. Compounds were also evaluated against a C165A variant. Compound reactivity, stability, MMP selectivity and cellular efficacy/toxicity was also evaluated.

RESULTS

Several electrophilic warhead types were confirmed to inhibit BoNT/A LC covalently with substantial differences in time-dependent inhibition between the WT and C165A variant. A trend in warhead reactivity was reflected in inhibitor stability and toxicity. Compounds exhibited moderate potency in a BoNT/A neuronal cellular assay but were not further explored due to undesirable therapeutic potential.

CONCLUSIONS

A fundamental framework for the bifunctional covalent inhibition of BoNT/A LC has been established. This approach has potential to be translated to other small molecule metal-binding inhibitors of BoNT/A LC with the vision that different pharmacophores, possessing improved physicochemical properties, will address BoNT/As toxicity and longevity within cells.

Identification of 3-hydroxy-1,2-dimethylpyridine-4(1H)-thione as a metal-binding motif for the inhibition of botulinum neurotoxin A

Botulinum neurotoxin serotype A (BoNT/A) is an important therapeutic target owing to its extremely potent nature, but also has potential use as a biowarfare agent. Currently, no therapeutic exists to reverse the long-lasting paralysis caused by BoNT/A. Herein, we describe the identification of 3-hydroxy-1,2-dimethylpyridine-4(1H)-thione (3,4-HOPTO) as a metal binding warhead for the inhibition of BoNT/A1. An initial screen of 96 metal binding fragments identified three derivatives containing the 3,4-HOPTO scaffold to inhibit the BoNT/A1 light chain (LC) at >95% at 1 mM. Additional screening of a 3,4-HOPTO sub-library identified structure-activity relationships (SARs) between N-substituted 3,4-HOPTO derivatives and the BoNT/A1 LC. Subsequent synthesis was conducted to improve on inhibitory potency - achieving low μM biochemical IC50 values. Representative compounds were evaluated in a cellular-based assay and showed promising μM activity.

Catch and Anchor Approach To Combat Both Toxicity and Longevity of Botulinum Toxin A

Botulinum neurotoxins have remarkable persistence (∼weeks to months in cells), outlasting the small-molecule inhibitors designed to target them. To address this disconnect, inhibitors bearing two pharmacophores-a zinc binding group and a Cys-reactive warhead-were designed to leverage both affinity and reactivity. A series of first-generation bifunctional inhibitors was achieved through structure-based inhibitor design. Through X-ray crystallography, engagement of both the catalytic Zn2+ and Cys165 was confirmed. A second-generation series improved on affinity by incorporating known reversible inhibitor pharmacophores; the mechanism was confirmed by exhaustive dialysis, mass spectrometry, and in vitro evaluation against the C165S mutant. Finally, a third-generation inhibitor was shown to have good cellular activity and low toxicity. In addition to our findings, an alternative method of modeling time-dependent inhibition that simplifies assay setup and allows comparison of inhibition models is discussed.

Critical Analysis of Neuronal Cell and the Mouse Bioassay for Detection of Botulinum Neurotoxins

Botulinum Neurotoxins (BoNTs) are a large protein family that includes the most potent neurotoxins known to humankind. BoNTs delivered locally in humans at low doses are widely used pharmaceuticals. Reliable and quantitative detection of BoNTs is of paramount importance for the clinical diagnosis of botulism, basic research, drug development, potency determination, and detection in clinical, environmental, and food samples. Ideally, a definitive assay for BoNT should reflect the activity of each of the four steps in nerve intoxication. The in vivo mouse bioassay (MBA) is the ‘gold standard’ for the detection of BoNTs. The MBA is sensitive, robust, semi-quantitative, and reliable within its sensitivity limits. Potential drawbacks with the MBA include assay-to-assay potency variations, especially between laboratories, and false positives or negatives. These limitations can be largely avoided by careful planning and performance. Another detection method that has gained importance in recent years for research and potency determination of pharmaceutical BoNTs is cell-based assays, as these assays can be highly sensitive, quantitative, human-specific, and detect fully functional holotoxins at physiologically relevant concentrations. A myriad of other in vitro BoNT detection methods exist. This review focuses on critical factors and assay limitations of the mouse bioassay and cell-based assays for BoNT detection.

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.

Comparative functional analysis of mice after local injection with botulinum neurotoxin A1, A2, A6, and B1 by catwalk analysis

Botulinum neurotoxins (BoNTs) are potent neurotoxins and are the causative agent of botulism, as well as valuable pharmaceuticals. BoNTs are divided into seven serotypes that comprise over 40 reported subtypes. BoNT/A1 and BoNT/B1 are currently the only subtypes approved for pharmaceutical use in the USA. While several other BoNT subtypes including BoNT/A2 and/A6 have been proposed as promising pharmaceuticals, detailed characterization using in vivo assays are essential to determine their pharmaceutical characteristics compared to the currently used BoNT/A1 and/B1. Several methods for studying BoNTs in mice are being used, but no objective and quantitative assay for assessment of functional outcomes after injection has been described. Here we describe the use of CatWalk XT as a new analytical tool for the objective and quantitative analysis of the paralytic effect after local intramuscular injection of BoNT subtypes A1, A2, A6, and B1. Catwalk is a sophisticated gait and locomotion analysis system that quantitatively analyzes a rodent's paw print dimensions and footfall patterns while traversing a glass plate during unforced walk. Significant changes were observed in several gait parameters in mice after local intramuscular injection of all tested BoNT subtypes, however, no changes were observed in mice injected intraperitoneally with the same BoNTs. While a clear difference in time to peak paralysis was observed between BoNT/A1 and/B1, injection of all four toxins resulted in a deficit in the injected limb with the other limbs functionally compensating and with no qualitative differences between the four BoNT subtypes. The presented data demonstrate the utility of CatWalk as a tool for functional outcomes after local BoNT injection through its ability to collect large amounts of quantitative data and objectively analyze sensitive changes in static and dynamic gait parameters.

Substrate cleavage and duration of action of botulinum neurotoxin type FA ("H, HA")

Botulinum neurotoxin (BoNT) type FA is the only known naturally occurring chimeric BoNT of domains of BoNT/A and BoNT/F. BoNT/FA consists of an F5-like light chain (LC), a unique heavy chain (HC) translocation domain, and a HC receptor binding domain similar to BoNT/A1. Previous analyses of purified BoNT/FA have indicated a 5-10-fold greater potency in cultured human or rat neurons as compared to BoNT/A1 and a 400-500-fold greater potency compared to BoNT/B1. However, in vivo potency in mice was about 5-fold lower than BoNT/A1 or/B1. In this report, species specificity was examined by cell-based assays using primary neurons from mice and examining VAMP1 and 2 cleavage. The data indicated similar potency of BoNT/FA in primary mouse spinal cord neurons as previously observed in primary rat and human induced pluripotent stem cell (hiPSC) derived neuronal cell models, and equal enzymatic cleavage of mouse VAMP1 and 2 isoforms. Since the duration of action of BoNTs is due to continuous enzymatic activity of the LC in the neuronal cytosol, BoNT/FA was expected to have a short duration of action due to its F-type LC. In this report the duration of action of BoNT/FA was compared to that of BoNT/F1,/F5, and/B1 in both hiPSC derived neurons and in the in vivo mouse model. The data indicate a duration of action of BoNT/FA similar to BoNT/B1, while BoNT/F5 had a short duration of action similar to BoNT/F1.

Small molecule metalloprotease inhibitor with in vitro, ex vivo and in vivo efficacy against botulinum neurotoxin serotype A

Botulinum neurotoxins (BoNTs) are the most toxic substances known to mankind and are the causative agents of the neuroparalytic disease botulism. Their ease of production and extreme toxicity have caused these neurotoxins to be classified as Tier 1 bioterrorist threat agents and have led to a sustained effort to develop countermeasures to treat intoxication in case of a bioterrorist attack. While timely administration of an approved antitoxin is effective in reducing the severity of botulism, reversing intoxication requires different strategies. In the present study, we evaluated ABS 252 and other mercaptoacetamide small molecule active-site inhibitors of BoNT/A light chain using an integrated multi-assay approach. ABS 252 showed inhibitory activity in enzymatic, cell-based and muscle activity assays, and importantly, produced a marked delay in time-to-death in mice. The results suggest that a multi-assay approach is an effective strategy for discovery of potential BoNT therapeutic candidates.

Metal Ions Effectively Ablate the Action of Botulinum Neurotoxin A

Botulinum neurotoxin serotype A (BoNT/A) causes a debilitating and potentially fatal illness known as botulism. The toxin is also a bioterrorism threat, yet no pharmacological antagonist to counteract its effects has reached clinical approval. Existing strategies to negate BoNT/A intoxication have looked to antibodies, peptides, or organic small molecules as potential therapeutics. In this work, a departure from the traditional drug discovery mindset was pursued, in which the enzyme's susceptibility to metal ions was exploited. A screen of a series of metal salts showed marked inhibitory activity of group 11 and 12 metals against the BoNT/A light chain (LC) protease. Enzyme kinetics revealed that copper (I) and (II) cations displayed noncompetitive inhibition of the LC (Ki ≈ 1 μM), while mercury (II) cations were 10-fold more potent. Crystallographic and mutagenesis studies elucidated a key binding interaction between Cys165 on BoNT/A LC and the inhibitory metals. As potential copper prodrugs, ligand-copper complexes were examined in a cell-based model and were found to prevent BoNT/A cleavage of the endogenous protein substrate, SNAP-25, even at low μM concentrations of complexes. Further investigation of the complexes suggested a bioreductive mechanism causing intracellular release of copper, which directly inhibited the BoNT/A protease. In vivo experiments demonstrated that copper (II) dithiocarbamate and bis(thiosemicarbazone) complexes could delay BoNT/A-mediated lethality in a rodent model, indicating their potential for treating the harmful effects of BoNT/A intoxication. Our studies illustrate that metals can be therapeutically viable enzyme inhibitors; moreover, enzymes that share homology with BoNT LCs may be similarly targeted with metals.

Assessment of ELISA as endpoint in neuronal cell-based assay for BoNT detection using hiPSC derived neurons

INTRODUCTION

Botulinum neurotoxins (BoNTs), the causative agents of botulism, are widely used as powerful bio-pharmaceuticals to treat neuro-muscular disorders. Due to the high potency and potential lethality of BoNTs, careful monitoring of the biologic activity of BoNT-based pharmaceuticals is required to ensure safe usage. For decades, the only approved method for potency determination of pharmaceutical BoNTs was the mouse bioassay (MBA), but in recent years improvements in cell-assay technologies have enabled MBA replacement by cell-based assays for specific product evaluations. This project details a method for quantitative and sensitive detection of biologic activity of BoNT/A1 in human induced pluripotent stem cell (hiPSC) derived neurons using an ELISA as a method to determine SNAP-25 cleavage by BoNT/A1 following toxin exposure.

METHODS

HiPSC derived neurons from two different sources were exposed to serial dilutions of BoNT/A1, and quantitative detection of toxin activity was evaluated and optimized in cell lysates using ELISA to detect cleaved SNAP-25.

RESULTS

The results from this study indicate that an ELISA using ultra TMB as a substrate quantitatively detects cleaved SNAP-25 in cell lysates of BoNT/A1 exposed hiPSC-derived neuronal cells with similar or greater sensitivity as Western blot (EC50~0.3U/well).

DISCUSSION

This study demonstrates a human specific and sensitive cell-based detection platform of BoNT/A1 activity using ELISA as an endpoint for quantitative detection of the SNAP-25 cleavage product. This assay is applicable to moderate to high-throughput formats and importantly employs non-cancerous human-specific neuronal cells for potency evaluation of a bio-pharmaceutical for human use.

Cellular Protection of SNAP-25 against Botulinum Neurotoxin/A: Inhibition of Thioredoxin Reductase through a Suicide Substrate Mechanism

Botulium neurotoxins (BoNTs) are among the most lethal toxins known to man. They are comprised of seven serotypes with BoNT/A being the most deadly; yet, there is no approved therapeutic for their intoxication or one that has even advanced to clinical trials. Botulinum neurotoxicity is ultimately governed through light chain (LC) protease SNARE protein cleavage leading to a loss of neurotransmitter release. Pharmacological attempts to ablate BoNT/A intoxication have sought to either nullify cellular toxin entry or critical biochemical junctions found within its intricate mechanism of action. In these regards, reports have surfaced of nonpeptidic small molecule inhibitors, but few have demonstrated efficacy in neutralizing cellular toxicity, a key prerequisite before rodent lethality studies can be initiated. On the basis of a lead discovered in our BoNT/A cellular assay campaign, we investigated a family of N-hydroxysuccinimide inhibitors grounded upon structure activity relationship (SAR) fundamentals. Molecules stemming from this SAR exercise were theorized to be protease inhibitors. However, this proposition was overturned on the basis of extensive kinetic analysis. Unexpectedly, inhibitor data pointed to thioredoxin reductase (TrxR), an essential component required for BoNT protease translocation. Also unforeseen was the inhibitors' mechanism of action against TrxR, which was found to be brokered through a suicide-mechanism utilizing quinone methide as the inactivating element. This new series of TrxR inhibitors provides an alternative means to negate the etiological agent responsible for BoNT intoxication, the LC protease.

Toward the discovery of dual inhibitors for botulinum neurotoxin A: concomitant targeting of endocytosis and light chain protease activity

Dyngo-4a™ has been found to be an endocytic inhibitor of BoNT/A neurotoxicity through dynamin inhibition. Herein, we demonstrate this molecule to have a previously unrecognized dual activity against BoNT/A, dynamin-protease inhibition. To establish the importance of this dual activity, detailed kinetic analysis of Dyngo-4a's inhibition of BoNT/A metalloprotease as well as cellular and animal toxicity studies have been described. The research presented is the first polypharmacological approach to counteract BoNT/A intoxication.

Human Induced Pluripotent Stem Cell Derived Neuronal Cells Cultured on Chemically-Defined Hydrogels for Sensitive In Vitro Detection of Botulinum Neurotoxin

Botulinum neurotoxin (BoNT) detection provides a useful model for validating cell-based neurotoxicity screening approaches, as sensitivity is dependent on functionally competent neurons and clear quantitative endpoints are available for correlating results to approved animal testing protocols. Here, human induced pluripotent stem cell (iPSC)-derived neuronal cells were cultured on chemically-defined poly(ethylene glycol) (PEG) hydrogels formed by "thiol-ene" photopolymerization and tested as a cell-based neurotoxicity assay by determining sensitivity to active BoNT/A1. BoNT/A1 sensitivity was comparable to the approved in vivo mouse bioassay for human iPSC-derived neurons and neural stem cells (iPSC-NSCs) cultured on PEG hydrogels or treated tissue culture polystyrene (TCP) surfaces. However, maximum sensitivity for BoNT detection was achieved two weeks earlier for iPSC-NSCs that were differentiated and matured on PEG hydrogels compared to TCP. Therefore, chemically-defined synthetic hydrogels offer benefits over standard platforms when optimizing culture conditions for cell-based screening and achieve sensitivities comparable to an approved animal testing protocol.

Botulinum Neurotoxins Can Enter Cultured Neurons Independent of Synaptic Vesicle Recycling

Botulinum neurotoxins (BoNTs) are the causative agent of the severe and long-lasting disease botulism. At least seven different serotypes of BoNTs (denoted A-G) have been described. All BoNTs enter human or animal neuronal cells via receptor mediated endocytosis and cleave cytosolic SNARE proteins, resulting in a block of synaptic vesicle exocytosis, leading to the flaccid paralysis characteristic of botulism. Previous data have indicated that once a neuronal cell has been intoxicated by a BoNT, further entry of the same or other BoNTs is prevented due to disruption of synaptic vesicle recycling. However, it has also been shown that cultured neurons exposed to BoNT/A are still capable of taking up BoNT/E. In this report we show that in general BoNTs can enter cultured human or mouse neuronal cells that have previously been intoxicated with another BoNT serotype. Quantitative analysis of cell entry by assessing SNARE cleavage revealed none or only a minor difference in the efficiency of uptake of BoNTs into previously intoxicated neurons. Examination of the endocytic entry pathway by specific endocytosis inhibitors indicated that BoNTs are taken up by clathrin coated pits in both non pre-exposed and pre-exposed neurons. LDH release assays indicated that hiPSC derived neurons exposed consecutively to two different BoNT serotypes remained viable and healthy except in the case of BoNT/E or combinations of BoNT/E with BoNT/B, /D, or /F. Overall, our data indicate that previous intoxication of neuronal cells with BoNT does not inhibit further uptake of BoNTs.

In vivo onset and duration of action varies for botulinum neurotoxin A subtypes 1-5

To date, over 40 subtypes of botulinum neurotoxins (BoNTs) have been identified. BoNTs are classified into 7 serotypes distinguished primarily by their antigenic properties, but also characterized by their unique SNARE targets and cleavage sites, host specificity, and duration of action. Sequencing efforts in the last decade have identified several subtypes within the serotypes. Subtypes are currently defined as distinct based solely on amino acid sequence comparison, with a similarity cut-off of 2.5% difference. Ten subtypes have been identified for BoNT/A, which is the serotype associated with the most severe human botulism and also the most commonly used serotype for clinical purposes. Analyses of several of these subtypes have revealed distinct characteristics, ranging from differences in cell entry and enzyme kinetics to differences in potency in mice and cell-model specific potency. A long-term activity study in cultured primary neurons has indicated that BoNT/A1, 2, 4, and 5 have a similar duration of action, whereas BoNT/A3 has a significantly shorter duration of action. This report describes an in vivo mouse study, showing that after local injection BoNT/A2 resulted in faster onset of local paralysis than BoNT/A1, 3, 4, and 5, whereas BoNT/A3 resulted in significantly faster recovery of motor-neuron deficiency.

A Novel Botulinum Neurotoxin, Previously Reported as Serotype H, Has a Hybrid-Like Structure With Regions of Similarity to the Structures of Serotypes A and F and Is Neutralized With Serotype A Antitoxin

Botulism is a potentially fatal paralytic disease caused by the action of botulinum neurotoxin (BoNT) on nerve cells. There are 7 known serotypes (A-G) of BoNT and up to 40 genetic variants. Clostridium botulinum strain IBCA10-7060 was recently reported to produce BoNT serotype B (BoNT/B) and a novel BoNT, designated as BoNT/H. The BoNT gene (bont) sequence of BoNT/H was compared to known bont sequences. Genetic analysis suggested that BoNT/H has a hybrid-like structure containing regions of similarity to the structures of BoNT/A1 and BoNT/F5. This novel BoNT was serologically characterized by the mouse neutralization assay and a neuronal cell-based assay. The toxic effects of this hybrid-like BoNT were completely eliminated by existing serotype A antitoxins, including those contained in multivalent therapeutic antitoxin products that are the mainstay of human botulism treatment.

Activity of botulinum neurotoxin type D (strain 1873) in human neurons

Botulinum Neurotoxin type D (BoNT/D) causes periodic outbreaks of botulism in cattle and horses, but is rarely associated with human botulism. Previous studies have shown that humans responded poorly to peripheral injection of up to 10U of BoNT/D. Isolated human pyramidalis muscle preparations were resistant to BoNT/D, whereas isolated human intercostal muscle preparations responded to BoNT/D similarly as to other BoNT serotypes. In vitro data indicate that BoNT/D does not cleave human VAMP1 efficiently, and differential expression of the VAMP 1 and 2 isoforms may be responsible for the above observations. Here we examined sensitivity of cultured human neurons derived from human induced pluripotent stem cells to BoNT/D. Our data indicate that BoNT/D can enter and cleave VAMP 2 in human neurons, but at significantly lower efficiency than other BoNT serotypes. In addition, BoNT/D had a short duration of action in the cultured neurons, similar to that of BoNT/E. In vivo analyses indicated a slower time to death in mice, as well as a later onset and shorter duration of action than BoNT/A1. Finally, examination of BoNT/D activity in various rodent and human cell models resulted in dramatic differences in sensitivity, indicating a unique cell entry mechanism of BoNT/D.

Analysis of gene expression in induced pluripotent stem cell-derived human neurons exposed to botulinum neurotoxin A subtype 1 and a type A atoxic derivative

Botulinum neurotoxin type A1 (BoNT/A1) is a potent protein toxin responsible for the potentially fatal human illness botulism. Notwithstanding, the long-lasting flaccid muscle paralysis caused by BoNT/A has led to its utility as a powerful and versatile bio-pharmaceutical. The flaccid paralysis is due to specific cleavage of neuronal SNAREs by BoNTs. However, actions of BoNTs on intoxicated neurons besides the cleavage of SNAREs have not been studied in detail. In this study we investigated by microarray analysis the effects of BoNT/A and a catalytically inactive derivative (BoNT/A ad) on the transcriptome of human induced pluripotent stem cell (hiPSC)-derived neurons at 2 days and 2 weeks after exposure. While there were only minor changes in expression levels at 2 days post exposure, at 2 weeks post exposure 492 genes were differentially expressed more than 2-fold in BoNT/A1-exposed cells when compared to non-exposed populations, and 682 genes were differentially expressed in BoNT/A ad-exposed cells. The vast majority of genes were similarly regulated in BoNT/A1 and BoNT/A ad-exposed neurons, and the few genes differentially regulated between BoNT/A1 and BoNT/A ad-exposed neurons were differentially expressed less than 3.5 fold. These data indicate a similar response of neurons to BoNT/A1 and BoNT/A ad exposure. The most highly regulated genes in cells exposed to either BoNT/A1 or BoNT/A ad are involved in neurite outgrowth and calcium channel sensitization.

Persistence of botulinum neurotoxin a subtypes 1-5 in primary rat spinal cord cells

Botulinum neurotoxins (BoNTs) are the most poisonous substances known and cause the severe disease botulism. BoNTs have also been remarkably effective as therapeutics in treating many neuronal and neuromuscular disorders. One of the hallmarks of BoNTs, particularly serotype A, is its long persistence of 2-6 months in patients at concentrations as low as fM or pM. The mechanisms for this persistence are currently unclear. In this study we determined the persistence of the BoNT/A subtypes 1 through 5 in primary rat spinal neurons. Remarkably, the duration of intracellular enzymatic activity of BoNT/A1, /A2, /A4 and /A5 was shown to be at least 10 months. Conversely, the effects of BoNT/A3 were observed for up to ∼5 months. An intermittent dosing with BoNT/E showed intracellular activity of the shorter acting BoNT/E for 2–3 weeks, followed by reoccurrence and persistence of BoNT/A-induced SNAP-25 cleavage products.

Synthesis/biological evaluation of hydroxamic acids and their prodrugs as inhibitors for Botulinum neurotoxin A light chain

Botulinum neurotoxin A (BoNT/A) is the most potent toxin known. Unfortunately, it is also a potential bioweapon in terrorism, which is without an approved therapeutic treatment once cellular intoxication takes place. Previously, we reported how hydroxamic acid prodrug carbamates increased cellular uptake, which translated to successful inhibition of this neurotoxin. Building upon this research, we detail BoNT/A protease molecular modeling studies accompanied by the construction of small library of hydroxamic acids based on 2,4-dichlorocinnamic hydroxamic acid scaffold and their carbamate prodrug derivatization along with the evaluation of these molecules in both enzymatic and cellular models.

Targeting botulinum A cellular toxicity: a prodrug approach

The botulinum neurotoxin light chain (LC) protease has become an important therapeutic target for postexposure treatment of botulism. Hydroxamic acid based small molecules have proven to be potent inhibitors of LC/A with nanomolar Ki values, yet they lack cellular activity conceivably due to low membrane permeability. To overcome this potential liability, we investigated two prodrug strategies, 1,4,2-dioxazole and carbamate, based on our 1-adamantylacetohydroxamic acid scaffold. The 1,4,2-dioxazole prodrug did not demonstrate cellular activity, however, carbamates exhibited cellular potency with the most active compound displaying an EC50 value of 20 μM. Cellular trafficking studies were conducted using a "fluorescently silent" prodrug that remained in this state until cellular uptake was complete, which allowed for visualization of the drug's release inside neuronal cells. In sum, this research sets the stage for future studies leveraging the specific targeting and delivery of these prodrugs, as well as other antibotulinum agents, into neuronal cells.

Progress in cell based assays for botulinum neurotoxin detection

Botulinum neurotoxins (BoNTs) are the most potent human toxins known and the causative agent of botulism, and are widely used as valuable pharmaceuticals. The BoNTs are modular proteins consisting of a heavy chain and a light chain linked by a disulfide bond. Intoxication of neuronal cells by BoNTs is a multi-step process including specific cell binding, endocytosis, conformational change in the endosome, translocation of the enzymatic light chain into the cells cytosol, and SNARE target cleavage. The quantitative and reliable potency determination of fully functional BoNTs produced as active pharmaceutical ingredient (API) requires an assay that considers all steps in the intoxication pathway. The in vivo mouse bioassay has for years been the 'gold standard' assay used for this purpose, but it requires the use of large numbers of mice and thus causes associated costs and ethical concerns. Cell-based assays are currently the only in vitro alternative that detect fully functional BoNTs in a single assay and have been utilized for years for research purposes. Within the last 5 years, several cell-based BoNT detection assays have been developed that are able to quantitatively determine BoNT potency with similar or greater sensitivity than the mouse bioassay. These assays now offer an alternative method for BoNT potency determination. Such quantitative and reliable BoNT potency determination is a crucial step in basic research, in the development of pharmaceutical BoNTs, and in the quantitative detection of neutralizing antibodies.

Evaluation of adamantane hydroxamates as botulinum neurotoxin inhibitors: synthesis, crystallography, modeling, kinetic and cellular based studies

Botulinum neurotoxins (BoNTs) are the most lethal biotoxins known to mankind and are responsible for the neuroparalytic disease botulism. Current treatments for botulinum poisoning are all protein based and thus have a limited window of treatment opportunity. Inhibition of the BoNT light chain protease (LC) has emerged as a therapeutic strategy for the treatment of botulism as it may provide an effective post exposure remedy. Using a combination of crystallographic and modeling studies a series of hydroxamates derived from 1-adamantylacetohydroxamic acid (3a) were prepared. From this group of compounds, an improved potency of about 17-fold was observed for two derivatives. Detailed mechanistic studies on these structures revealed a competitive inhibition model, with a K(i)=27 nM, which makes these compounds some of the most potent small molecule, non-peptidic BoNT/A LC inhibitors reported to date.

Model for studying Clostridium botulinum neurotoxin using differentiated motor neuron-like NG108-15 cells

Cancerous cell lines have traditionally shown low sensitivity to laboratory or pharmaceutical preparations of botulinum neurotoxin. The work presented here demonstrates that the mouse neuroblastoma/rat glioma hybrid cell line NG108-15 is capable of more sensitively detecting BoNT/A1 than any cell line previously described. This cell line has previously been described to have motor neuron like characteristics, therefore making it a good model to study BoNTs. Differentiation of NG108-15 cells in serum-free medium containing retinoic acid and purmorphamine dramatically increased sensitivity of the neurons to BoNT/A (EC(50) = ~16 LD(50) U). Additional pre-treatment with triasialoganglioside GT1B prior to toxin exposure reduced the EC(50) further to ~11 LD(50) U. Co-culture of the neurons with C2C12 myotubes also significantly increased BoNT/A sensitivity of NG108-15 cells (EC(50) = 26 U) in the absence of differentiation factors.

Learning from the past: historical aspects of bacterial toxins as pharmaceuticals

Botulinum neurotoxins are the most poisonous substances known to humankind, but also are the bacterial toxins most frequently used as pharmaceuticals to benefit humans. The discovery of botulinum toxins and development into a useful drug is unique and fascinating, dating back to the early 19th century, when Justinus Kerner first recognized that botulism was caused by a biological toxin and suggested its use for medicinal purposes. This was translated into reality in 1980, when Alan Scott for the first time used the toxins to successfully treat strabismus. Now a subset of botulinum toxins are widely used for cosmetic applications, treatment of various movement disorders, pain and many other syndromes, and further developments using other botulinum toxins or recombinant molecules engineered from subdomains are promising.

Novel application of human neurons derived from induced pluripotent stem cells for highly sensitive botulinum neurotoxin detection

Human induced pluripotent stem cells (hiPSC) hold great promise for providing various differentiated cell models for in vitro toxigenicity testing. For Clostridium botulinum neurotoxin (BoNT) detection and mechanistic studies, several cell models currently exist, but none examine toxin function with species-specific relevance while exhibiting high sensitivity. The most sensitive cell models to date are mouse or rat primary cells and neurons derived from mouse embryonic stem cells, both of which require significant technical expertise for culture preparation. This study describes for the first time the use of hiPSC-derived neurons for BoNT detection. The neurons used in this study were differentiated and cryopreserved by Cellular Dynamics International (Madison, WI) and consist of an almost pure pan-neuronal population of predominantly gamma aminoisobutyric acidergic and glutamatergic neurons. Western blot and quantitative PCR data show that these neurons express all the necessary receptors and substrates for BoNT intoxication. BoNT/A intoxication studies demonstrate that the hiPSC-derived neurons reproducibly and quantitatively detect biologically active BoNT/A with high sensitivity (EC(50) ∼0.3 U). Additionally, the quantitative detection of BoNT serotypes B, C, E, and BoNT/A complex was demonstrated, and BoNT/A specificity was confirmed through antibody protection studies. A direct comparison of BoNT detection using primary rat spinal cord cells and hiPSC-derived neurons showed equal or increased sensitivity, a steeper dose-response curve and a more complete SNARE protein target cleavage for hiPSC-derived neurons. In summary, these data suggest that neurons derived from hiPSCs provide an ideal and highly sensitive platform for BoNT potency determination, neutralizing antibody detection and for mechanistic studies.

Neuronal targeting, internalization, and biological activity of a recombinant atoxic derivative of botulinum neurotoxin A

Non-toxic derivatives of botulinum neurotoxin A (BoNT/A) have potential use as neuron-targeting delivery vehicles, and as reagents to study intracellular trafficking. We have designed and expressed an atoxic derivative of BoNT/A (BoNT/A ad) as a full-length 150 kDa molecule consisting of a 50 kDa light chain (LC) and a 100 kDa heavy chain (HC) joined by a disulfide bond and rendered atoxic through the introduction of metalloprotease-inactivating point mutations in the light chain. Studies in neuronal cultures demonstrated that BoNT/A ad cannot cleave synaptosomal-associated protein 25 (SNAP25), the substrate of wt BoNT/A, and that it effectively competes with wt BoNT/A for binding to endogenous neuronal receptors. In vitro and in vivo studies indicate accumulation of BoNT/A ad at the neuromuscular junction of the mouse diaphragm. Immunoprecipitation studies indicate that the LC of BoNT/A ad forms a complex with SNAP25 present in the neuronal cytosolic fraction, demonstrating that the atoxic LC retains the SNAP25 binding capability of the wt toxin. Toxicity of BoNT/A ad was found to be reduced approximately 100,000-fold relative to wt BoNT/A.

Sensitive and quantitative detection of botulinum neurotoxin in neurons derived from mouse embryonic stem cells

Botulinum neurotoxins (BoNTs), the most poisonous protein toxins known, represent a serious bioterrorism threat but are also used as a unique and important bio-pharmaceutical to treat an increasing myriad of neurological disorders. The only currently accepted detection method by the United States Food and Drug Administration for biological activity of BoNTs and for potency determination of pharmaceutical preparations is the mouse bioassay (MBA). Recent advances have indicated that cell-based assays using primary neuronal cells can provide an equally sensitive and robust detection platform as the MBA to reliably and quantitatively detect biologically active BoNTs. This study reports for the first time a BoNT detection assay using mouse embryonic stem cells to produce a neuronal cell culture. The data presented indicate that this assay can reliably detect BoNT/A with a similar sensitivity as the MBA.

Botulinum neurotoxin subtype A2 enters neuronal cells faster than subtype A1

Botulinum neurotoxins (BoNTs), the causative agent of human botulism, are the most potent naturally occurring toxins known. BoNT/A1, the most studied BoNT, is also used as an important biopharmaceutical. In this study, the biological activity of BoNT/A1 is compared to that of BoNT/A2 using neuronal cell models. The data obtained indicate faster and increased intoxication of neuronal cells by BoNT/A2 than BoNT/A1, and that the mechanism underlying this increased toxicity is faster and more efficient cell entry that is independent of ganglioside binding. These results have important implications for the development of new BoNT based therapeutics and BoNT countermeasures.