Botulinum neurotoxin C1 cleaves both syntaxin and SNAP-25 in intact and permeabilized chromaffin cells: correlation with its blockade of catecholamine release

Recent Advances in the Detection of Food Toxins Using Mass Spectrometry

Edibles are the only source of nutrients and energy for humans. However, ingredients of edibles have undergone many physicochemical changes during preparation and storage. Aging, hydrolysis, oxidation, and rancidity are some of the major changes that not only change the native flavor, texture, and taste of food but also destroy the nutritive value and jeopardize public health. The major reasons for the production of harmful metabolites, chemicals, and toxins are poor processing, inappropriate storage, and microbial spoilage, which are lethal to consumers. In addition, the emergence of new pollutants has intensified the need for advanced and rapid food analysis techniques to detect such toxins. The issue with the detection of toxins in food samples is the nonvolatile nature and absence of detectable chromophores; hence, normal conventional techniques need additional derivatization. Mass spectrometry (MS) offers high sensitivity, selectivity, and capability to handle complex mixtures, making it an ideal analytical technique for the identification and quantification of food toxins. Recent technological advancements, such as high-resolution MS and tandem mass spectrometry (MS/MS), have significantly improved sensitivity, enabling the detection of food toxins at ultralow levels. Moreover, the emergence of ambient ionization techniques has facilitated rapid in situ analysis of samples with lower time and resources. Despite numerous advantages, the widespread adoption of MS in routine food safety monitoring faces certain challenges such as instrument cost, complexity, data analysis, and standardization of methods. Nevertheless, the continuous advancements in MS-technology and its integration with complementary techniques hold promising prospects for revolutionizing food safety monitoring. This review discusses the application of MS in detecting various food toxins including mycotoxins, marine biotoxins, and plant-derived toxins. It also explores the implementation of untargeted approaches, such as metabolomics and proteomics, for the discovery of novel and emerging food toxins, enhancing our understanding of potential hazards in the food supply chain.

Crystal Structure of the Catalytic Domain of a Botulinum Neurotoxin Homologue from Enterococcus faecium: Potential Insights into Substrate Recognition

Clostridium botulinum neurotoxins (BoNTs) are the most potent toxins known, causing the deadly disease botulism. They function through Zn2+-dependent endopeptidase cleavage of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins, preventing vesicular fusion and subsequent neurotransmitter release from motor neurons. Several serotypes of BoNTs produced by Clostridium botulinum (BoNT/A-/G and/X) have been well-characterised over the years. However, a BoNT-like gene (homologue of BoNT) was recently identified in the non-clostridial species, Enterococcus faecium, which is the leading cause of hospital-acquired multi-drug resistant infections. Here, we report the crystal structure of the catalytic domain of a BoNT homologue from Enterococcus faecium (LC/En) at 2.0 Å resolution. Detailed structural analysis in comparison with the full-length BoNT/En AlphaFold2-predicted structure, LC/A (from BoNT/A), and LC/F (from BoNT/F) revealed putative subsites and exosites (including loops 1-5) involved in recognition of LC/En substrates. LC/En also appears to possess a conserved autoproteolytic cleavage site whose function is yet to be established.

Construction of a Yeast Cell-Based Assay System to Analyze SNAP25-Targeting Botulinum Neurotoxins

Herein, we describe a yeast cell-based assay system to analyze SNAP25-targeting botulinum neurotoxins (BoNTs). BoNTs are protein toxins, and, upon incorporation into neuronal cells, their light chains (BoNT-LCs) target specific synaptosomal N-ethylmaleimide-sensitive attachment protein receptor (SNARE) proteins, including synaptosomal-associated protein 25 (SNAP25). BoNT-LCs are metalloproteases, and each BoNT-LC recognizes and cleaves conserved domains in SNAREs termed the SNARE domain. In the budding yeast Saccharomyces cerevisiae, the SNAP25 ortholog Spo20 is required for production of the spore plasma membrane; thus, defects in Spo20 cause sporulation deficiencies. We found that chimeric SNAREs in which SNARE domains in Spo20 are replaced with those of SNAP25 are functional in yeast cells. The Spo20/SNAP25 chimeras, but not Spo20, are sensitive to digestion by BoNT-LCs. We demonstrate that spo20∆ yeasts harboring the chimeras exhibit sporulation defects when various SNAP25-targeting BoNT-LCs are expressed. Thus, the activities of BoNT-LCs can be assessed by colorimetric measurement of sporulation efficiencies. Although BoNTs are notorious toxins, they are also used as therapeutic and cosmetic agents. Our assay system will be useful for analyzing novel BoNTs and BoNT-like genes, as well as their manipulation.

Emerging Opportunities in Human Pluripotent Stem-Cells Based Assays to Explore the Diversity of Botulinum Neurotoxins as Future Therapeutics

Botulinum neurotoxins (BoNTs) are produced by Clostridium botulinum and are responsible for botulism, a fatal disorder of the nervous system mostly induced by food poisoning. Despite being one of the most potent families of poisonous substances, BoNTs are used for both aesthetic and therapeutic indications from cosmetic reduction of wrinkles to treatment of movement disorders. The increasing understanding of the biology of BoNTs and the availability of distinct toxin serotypes and subtypes offer the prospect of expanding the range of indications for these toxins. Engineering of BoNTs is considered to provide a new avenue for improving safety and clinical benefit from these neurotoxins. Robust, high-throughput, and cost-effective assays for BoNTs activity, yet highly relevant to the human physiology, have become indispensable for a successful translation of engineered BoNTs to the clinic. This review presents an emerging family of cell-based assays that take advantage of newly developed human pluripotent stem cells and neuronal function analyses technologies.

hiPSC-Derived Neurons Provide a Robust and Physiologically Relevant In Vitro Platform to Test Botulinum Neurotoxins

Botulinum neurotoxins (BoNTs) are zinc metalloproteases that block neurotransmitter release at the neuromuscular junction (NMJ). Their high affinity for motor neurons combined with a high potency have made them extremely effective drugs for the treatment of a variety of neurological diseases as well as for aesthetic applications. Current in vitro assays used for testing and developing BoNT therapeutics include primary rodent cells and immortalized cell lines. Both models have limitations concerning accuracy and physiological relevance. In order to improve the translational value of preclinical data there is a clear need to use more accurate models such as human induced Pluripotent Stem Cells (hiPSC)-derived neuronal models. In this study we have assessed the potential of four different human iPSC-derived neuronal models including Motor Neurons for BoNT testing. We have characterized these models in detail and found that all models express all proteins needed for BoNT intoxication and showed that all four hiPSC-derived neuronal models are sensitive to both serotype A and E BoNT with Motor Neurons being the most sensitive. We showed that hiPSC-derived Motor Neurons expressed authentic markers after only 7 days of culture, are functional and able to form active synapses. When cultivated with myotubes, we demonstrated that they can innervate myotubes and induce contraction, generating an in vitro model of NMJ showing dose-responsive sensitivity BoNT intoxication. Together, these data demonstrate the promise of hiPSC-derived neurons, especially Motor Neurons, for pharmaceutical BoNT testing and development.

Proposed BoNT/A and /B Peptide Substrates Cannot Detect Multiple Subtypes in the Endopep-MS Assay

Botulinum neurotoxins (BoNTs) are a family of protein toxins consisting of seven known serotypes (BoNT/A-BoNT/G) and multiple subtypes within the serotypes, and all of which cause the disease botulism-a disease of great public health concern. Accurate detection of BoNTs in human clinical samples is therefore an important public health goal. To achieve this goal, our laboratory developed a mass spectrometry-based assay detecting the presence of BoNT via its enzymatic activity on a peptide substrate. Recently, publications reported the use of new peptide substrates to detect BoNT/A and /B with improved results over other peptide substrates. However, the authors did not provide results of their peptide substrate on multiple subtypes of BoNT. In this work, we describe the results of testing the new substrates with multiple BoNT/A and /B subtypes and find that the substrates cannot detect many subtypes of BoNT/A and /B.

Optimization of SNAP-25 and VAMP-2 Cleavage by Botulinum Neurotoxin Serotypes A-F Employing Taguchi Design-of-Experiments

The detection of catalytically active botulinum neurotoxins (BoNTs) can be achieved by monitoring the enzymatic cleavage of soluble NSF (N-ethylmaleimide-sensitive-factor) attachment protein receptor (SNARE) proteins by the toxins’ light chains (LC) in cleavage-based assays. Thus, for sensitive BoNT detection, optimal cleavage conditions for the clinically relevant A–F serotypes are required. Until now, a systematic evaluation of cleavage conditions for the different BoNT serotypes is still lacking. To address this issue, we optimized cleavage conditions for BoNT/A–F using the Taguchi design-of-experiments (DoE) method. To this aim, we analyzed the influence of buffer composition (pH, Zn²⁺, DTT (dithiothreitol), NaCl) as well as frequently used additives (BSA (bovine serum albumin), Tween 20, trimethylamine N-oxide (TMAO)) on BoNT substrate cleavage. We identified major critical factors (DTT, Zn²⁺, TMAO) and were able to increase the catalytic efficiency of BoNT/B, C, E, and F when compared to previously described buffers. Moreover, we designed a single consensus buffer for the optimal cleavage of all tested serotypes. Our optimized buffers are instrumental to increase the sensitivity of cleavage-based assays for BoNT detection. Furthermore, the application of the Taguchi DoE approach shows how the method helps to rationally improve enzymatic assays.

Construction of functional chimeras of syntaxin-1A and its yeast orthologue, and their application to the yeast cell-based assay for botulinum neurotoxin serotype C

BACKGROUND

Botulinum neurotoxins (BoNTs) prevent synaptic transmission because they hydrolyze synaptic N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). BoNT serotype C (BoNT/C) targets syntaxin-1A and SNAP-25, and is expected to be applied to cosmetic and therapeutic uses. SNAREs are evolutionally conserved proteins and in yeast a syntaxin-1A orthologue Sso1 is involved in exocytosis. The substrate specificity of BoNT/C is strict and it cannot cleave Sso1.

METHODS

Domain swapping and mutational screenings were performed to generate functional chimeras SNAREs of syntaxin-1A and Sso1. Such chimeras are expressed in yeast cells and assessed whether they are susceptible to BoNT/C digestion.

RESULTS

The Sso1 and syntaxin-1A chimera (Sso1/STX1A), in which the SNARE domain in Sso1 was replaced with that of syntaxin-1A, was not functional in yeast. The functional incompatibility of Sso1/STX1A was attributable to its accumulation in the ER. We found several mutations that could release Sso1/STX1A from the ER to make the chimera functional in yeast. Yeast cells harboring the mutant chimeras grew similarly to wild-type cells. However, unlike wild-type, yeast harboring the mutant chimeras exhibited a severe growth defect upon expression of BoNT/C. Results of further domain swapping analyses suggest that Sso1 is not digested by BoNT/C because it lacks a binding region to BoNT/C (α-exosite-binding region).

CONCLUSIONS

We obtained functional Sso1/STX1A chimeras, which can be applied to a yeast cell-based BoNT/C assay. BoNT/C can recognize these chimeras in a similar manner to syntaxin-1A.

GENERAL SIGNIFICANCE

The yeast cell-based BoNT/C assay would be useful to characterize and engineer BoNT/C.

A neurotoxin that specifically targets Anopheles mosquitoes

Clostridial neurotoxins, including tetanus and botulinum neurotoxins, generally target vertebrates. We show here that this family of toxins has a much broader host spectrum, by identifying PMP1, a clostridial-like neurotoxin that selectively targets anopheline mosquitoes. Isolation of PMP1 from Paraclostridium bifermentans strains collected in anopheline endemic areas on two continents indicates it is widely distributed. The toxin likely evolved from an ancestral form that targets the nervous system of similar organisms, using a common mechanism that disrupts SNARE-mediated exocytosis. It cleaves the mosquito syntaxin and employs a unique receptor recognition strategy. Our research has an important impact on the study of the evolution of clostridial neurotoxins and provides the basis for the use of P. bifermentans strains and PMP1 as innovative, environmentally friendly approaches to reduce malaria through anopheline control.

So far identified clostridial neurotoxins target vertebrates. Here, Contreras et al. isolate the clostridial-like neurotoxin PMP1 from Paraclostridium bifermentans strains and show that it selectively targets anopheline mosquitoes by targeting mosquito syntaxin.

Sensitive detection of type G botulinum neurotoxin through Endopep-MS peptide substrate optimization

Clostridium botulinum produces botulinum neurotoxins (BoNTs) that are one of the most poisonous substances. In order to respond to public health emergencies, there is a need to develop sensitive and specific methods for detecting botulinum toxin in various clinical matrices. Our laboratory has developed a mass spectrometry-based Endopep-MS assay that is able to rapidly detect and differentiate BoNT serotypes A–G by immunoaffinity capture of toxins and detection of unique cleavage products of peptide substrates. To improve the sensitivity of the Endopep-MS assay for the detection of BoNT serotype G, we report here the optimization of synthetic peptide substrates through systematic substitution, deletion, and incorporation of unnatural amino acids. Our data show that the resulting optimized peptides produced a significant improvement (two orders of magnitude) in assay sensitivity and allowed the detection of 0.01 mouseLD₅₀ toxin present in buffer solution.

Hsp90 Mediates Membrane Deformation and Exosome Release

Hsp90 is an essential chaperone that guards proteome integrity and amounts to 2% of cellular protein. We now find that Hsp90 also has the ability to directly interact with and deform membranes via an evolutionarily conserved amphipathic helix. Using a new cell-free system and in vivo measurements, we show this amphipathic helix allows exosome release by promoting the fusion of multivesicular bodies (MVBs) with the plasma membrane. We dissect the relationship between Hsp90 conformation and membrane-deforming function and show that mutations and drugs that stabilize the open Hsp90 dimer expose the helix and allow MVB fusion, while these effects are blocked by the closed state. Hence, we structurally separated the Hsp90 membrane-deforming function from its well-characterized chaperone activity, and we show that this previously unrecognized function is required for exosome release.

Light Chain Diversity among the Botulinum Neurotoxins

Botulinum neurotoxins (BoNT) are produced by several species of clostridium. There are seven immunologically unique BoNT serotypes (A⁻G). The Centers for Disease Control classifies BoNTs as 'Category A' select agents and are the most lethal protein toxins for humans. Recently, BoNT-like proteins have also been identified in several non-clostridia. BoNTs are di-chain proteins comprised of an N-terminal zinc metalloprotease Light Chain (LC) and a C-terminal Heavy Chain (HC) which includes the translocation and receptor binding domains. The two chains are held together by a disulfide bond. The LC cleaves Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). The cleavage of SNAREs inhibits the fusion of synaptic vesicles to the cell membrane and the subsequent release of acetylcholine, which results in flaccid paralysis. The LC controls the catalytic properties and the duration of BoNT action. This review discusses the mechanism for LC catalysis, LC translocation, and the basis for the duration of LC action. Understanding these properties of the LC may expand the applications of BoNT as human therapies.

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.

Identification of a Botulinum Neurotoxin-like Toxin in a Commensal Strain of Enterococcus faecium

Botulinum neurotoxins (BoNTs), produced by various Clostridium strains, are a family of potent bacterial toxins and potential bioterrorism agents. Here we report that an Enterococcus faecium strain isolated from cow feces carries a BoNT-like toxin, designated BoNT/En. It cleaves both VAMP2 and SNAP-25, proteins that mediate synaptic vesicle exocytosis in neurons, at sites distinct from known BoNT cleavage sites on these two proteins. Comparative genomic analysis determines that the E. faecium strain carrying BoNT/En is a commensal type and that the BoNT/En gene is located within a typical BoNT gene cluster on a 206 kb putatively conjugative plasmid. Although the host species targeted by BoNT/En remains to be determined, these findings establish an extended member of BoNTs and demonstrate the capability of E. faecium, a commensal organism ubiquitous in humans and animals and a leading cause of hospital-acquired multi-drug-resistant (MDR) infections, to horizontally acquire, and possibly disseminate, a unique BoNT gene cluster.

Novel Distinctive Roles of Docking Proteins in Short-term Synaptic Plasticity of Frog Neuromuscular Transmission Revealed by Botulinum Neurotoxins

Short-term synaptic plasticity (SSP) is a basic mechanism for temporal processing of neural information in synaptic transmission. Facilitation, the fastest component of SSP, has been extensively investigated with regard to Ca²⁺ signaling and other relevant substances. However, systematic analyses on the slower components of SSP, originated by Magleby and Zengel, have remained stagnant for decades, as few chemicals directly modifying these slower components have been identified. In combination with refined experimental protocols designed to study the stimulation frequency-dependence of SSP and botulinum neurotoxins A and C (BoNT-A and BoNT-C), we investigated SSP of frog neuromuscular transmission to clarify the roles of synaptosomal-associated protein of 25 kDa (SNAP-25) and syntaxin, SNARE proteins exclusively participating in vesicular events including docking, priming and exocytosis. We found that BoNT-A treatment eliminated slow potentiation, and BoNT-C poisoning abolished intermediate augmentation, two components of SSP. Fast facilitation was maintained after double poisoning with BoNT-A and -C, but the postsynaptic response became biphasic. A novel depression, termed repression, emerged by double poisoning. Repression was different from depletion because it developed even at a low-frequency stimulation of 1 Hz. We conclude that SNAP-25 and syntaxin not only play roles as cooperative exocytotic machinery, but also have roles in SSP.

K+-Cl- co-transporter 2 (KCC2) - a membrane trafficking perspective

K⁺-Cl^- co-transporter 2 (KCC2/SLC12A5) is a neuronal specific cation chloride co-transporter which is active under isotonic conditions, and thus a key regulator of intracellular Cl^- levels. It also has an ion transporter-independent structural role in modulating the maturation and regulation of excitatory glutamatergic synapses. KCC2 levels are developmentally regulated, and a postnatal upregulation of KCC2 generates a low intracellular chloride concentration that allows the neurotransmitters γ-aminobutyric acid (GABA) and glycine to exert inhibitory neurotransmission through its Cl^- permeating channel. Functional expression of KCC2 at the neuronal cell surface is necessary for its activity, and impairment in KCC2 cell surface transport and/or internalization may underlie a range of neuropathological conditions. Although recent advances have shed light on a range of cellular mechanisms regulating KCC2 activity, little is known about its membrane trafficking itinerary and regulatory proteins. In this review, known membrane trafficking signals, pathways and mechanisms pertaining to KCC2's functional surface expression are discussed.

Botulinum neurotoxin C mutants reveal different effects of syntaxin or SNAP-25 proteolysis on neuromuscular transmission

Botulinum neurotoxin serotype C (BoNT/C) is a neuroparalytic toxin associated with outbreaks of animal botulism, particularly in birds, and is the only BoNT known to cleave two different SNARE proteins, SNAP-25 and syntaxin. BoNT/C was shown to be a good substitute for BoNT/A1 in human dystonia therapy because of its long lasting effects and absence of neuromuscular damage. Two triple mutants of BoNT/C, namely BoNT/C S51T/R52N/N53P (BoNT/C α-51) and BoNT/C L200W/M221W/I226W (BoNT/C α-3W), were recently reported to selectively cleave syntaxin and have been used here to evaluate the individual contribution of SNAP-25 and syntaxin cleavage to the effect of BoNT/C in vivo. Although BoNT/C α-51 and BoNT/C α-3W toxins cleave syntaxin with similar efficiency, we unexpectedly found also cleavage of SNAP-25, although to a lesser extent than wild type BoNT/C. Interestingly, the BoNT/C mutants exhibit reduced lethality compared to wild type toxin, a result that correlated with their residual activity against SNAP-25. In spite of this, a local injection of BoNT/C α-51 persistently impairs neuromuscular junction activity. This is due to an initial phase in which SNAP-25 cleavage causes a complete blockade of neurotransmission, and to a second phase of incomplete impairment ascribable to syntaxin cleavage. Together, these results indicate that neuroparalysis of BoNT/C at the neuromuscular junction is due to SNAP-25 cleavage, while the proteolysis of syntaxin provides a substantial, but incomplete, neuromuscular impairment. In light of this evidence, we discuss a possible clinical use of BoNT/C α-51 as a botulinum neurotoxin endowed with a wide safety margin and a long lasting effect.

The seven established Botulinum Neurotoxins serotypes (BoNT/A to G) and the many BoNT subtypes, the causative agents of botulism, are the most poisonous substances known (lethal doses in the low ng/kg range). Due to their toxicological properties, BoNTs are Janus-faced toxins: potent pathogenic factors and potential bioterrorism agents as well as safe and efficacious therapeutics. BoNTs exert their neuroparalytic action by cleaving SNARE proteins, either SNAP-25 or synaptobrevin/VAMP, which mediate neurotransmitter release at the neuromuscular junction; BoNT/C is the only serotype shown to cleave SNAP-25 and syntaxin-1 in vitro. Our study shows for the first time that this parallel cleavage also occurs in vivo. By using mutated toxins reported to be syntaxin-selective, we found that SNAP-25 proteolysis at the neuromuscular junction is the key determinant of BoNT/C lethality as it completely blocks nerve-muscle transmission. Conversely, syntaxin-1 cleavage only attenuates nerve terminal activity without inactivating the synapse, leading to only a partial decrease of neuromuscular functionality. As a result, the BoNT/C mutants have dramatically reduced lethality, but still modulate neuromuscular junction activity upon intramuscular injection. This aspect is particularly relevant considering the possible use of syntaxin-specific BoNT/C derivatives to improve the present clinical utilization of BoNTs.

Early Golgi Abnormalities and Neurodegeneration upon Loss of Presynaptic Proteins Munc18-1, Syntaxin-1, or SNAP-25

The loss of presynaptic proteins Munc18-1, syntaxin-1, or SNAP-25 is known to produce cell death, but the underlying features have not been compared experimentally. Here, we investigated these features in cultured mouse CNS and DRG neurons. Side-by-side comparisons confirmed massive cell death, before synaptogenesis, within 1-4 DIV upon loss of t-SNAREs (syntaxin-1, SNAP-25) or Munc18-1, but not v-SNAREs (synaptobrevins/VAMP1/2/3 using tetanus neurotoxin (TeNT), also in TI-VAMP/VAMP7 knock-out (KO) neurons). A condensed cis-Golgi was the first abnormality observed upon Munc18-1 or SNAP-25 loss within 3 DIV. This phenotype was distinct from the Golgi fragmentation observed in apoptosis. Cell death was too rapid after syntaxin-1 loss to study Golgi abnormalities. Syntaxin-1 and Munc18-1 depend on each other for normal cellular levels. We observed that endogenous syntaxin-1 accumulates at the Golgi of Munc18-1 KO neurons. However, expression of a non-neuronal Munc18 isoform that does not bind syntaxin-1, Munc18-3, in Munc18-1 KO neurons prevented cell death and restored normal cis-Golgi morphology, but not synaptic transmission or syntaxin-1 targeting. Finally, we observed that DRG neurons are the only Munc18-1 KO neurons that do not degenerate in vivo or in vitro In these neurons, cis-Golgi abnormalities were less severe, with no changes in Golgi shape. Together, these data demonstrate that cell death upon Munc18-1, syntaxin-1, or SNAP-25 loss occurs via a degenerative pathway unrelated to the known synapse function of these proteins and involving early cis-Golgi abnormalities, distinct from apoptosis.SIGNIFICANCE STATEMENT This study provides new insights in a neurodegeneration pathway triggered by the absence of specific proteins involved in synaptic transmission (syntaxin-1, Munc18-1, SNAP-25), whereas other proteins involved in the same molecular process (synaptobrevins, Munc13-1/2) do not cause degeneration. Massive cell death occurs in cultured neurons upon depleting syntaxin-1, Munc18-1, and/or SNAP-25, well before synapse formation. This study characterizes several relevant cellular phenotypes, especially early cis-Golgi abnormalities, distinct from abnormalities observed during apoptosis, and rules out several other phenotypes as causal (defects in syntaxin-1 targeting and synaptic transmission). As proteins, such as syntaxin-1, Munc18-1, or SNAP-25, modulate α-synuclein neuropathy and/or are dysregulated in Alzheimer's disease, understanding this type of neurodegeneration may provide new links between synaptic defects and neurodegeneration in humans.

Bacterial Signaling to the Nervous System through Toxins and Metabolites

Mammalian hosts interface intimately with commensal and pathogenic bacteria. It is increasingly clear that molecular interactions between the nervous system and microbes contribute to health and disease. Both commensal and pathogenic bacteria are capable of producing molecules that act on neurons and affect essential aspects of host physiology. Here we highlight several classes of physiologically important molecular interactions that occur between bacteria and the nervous system. First, clostridial neurotoxins block neurotransmission to or from neurons by targeting the SNARE complex, causing the characteristic paralyses of botulism and tetanus during bacterial infection. Second, peripheral sensory neurons-olfactory chemosensory neurons and nociceptor sensory neurons-detect bacterial toxins, formyl peptides, and lipopolysaccharides through distinct molecular mechanisms to elicit smell and pain. Bacteria also damage the central nervous system through toxins that target the brain during infection. Finally, the gut microbiota produces molecules that act on enteric neurons to influence gastrointestinal motility, and metabolites that stimulate the "gut-brain axis" to alter neural circuits, autonomic function, and higher-order brain function and behavior. Furthering the mechanistic and molecular understanding of how bacteria affect the nervous system may uncover potential strategies for modulating neural function and treating neurological diseases.

Regulation of Ca2+ channels by SNAP-25 via recruitment of syntaxin-1 from plasma membrane clusters

SNAP-25 regulates Ca²⁺ channels in an unknown manner. Endogenous and exogenous SNAP-25 inhibit Ca²⁺ currents indirectly by recruiting syntaxin-1 from clusters on the plasma membrane, thereby making it available for Ca²⁺ current inhibition. Thus the cell can regulate Ca²⁺ influx by expanding or contracting syntaxin-1 clusters.

SNAP-25 regulates Ca²⁺ channels, with potentially important consequences for diseases involving an aberrant SNAP-25 expression level. How this regulation is executed mechanistically remains unknown. We investigated this question in mouse adrenal chromaffin cells and found that SNAP-25 inhibits Ca²⁺ currents, with the B-isoform being more potent than the A-isoform, but not when syntaxin-1 is cleaved by botulinum neurotoxin C. In contrast, syntaxin-1 inhibits Ca²⁺ currents independently of SNAP-25. Further experiments using immunostaining showed that endogenous or exogenous SNAP-25 expression recruits syntaxin-1 from clusters on the plasma membrane, thereby increasing the immunoavailability of syntaxin-1 and leading indirectly to Ca²⁺ current inhibition. Expression of Munc18-1, which recruits syntaxin-1 within the exocytotic pathway, does not modulate Ca²⁺ channels, whereas overexpression of the syntaxin-binding protein Doc2B or ubMunc13-2 increases syntaxin-1 immunoavailability and concomitantly down-regulates Ca²⁺ currents. Similar findings were obtained upon chemical cholesterol depletion, leading directly to syntaxin-1 cluster dispersal and Ca²⁺ current inhibition. We conclude that clustering of syntaxin-1 allows the cell to maintain a high syntaxin-1 expression level without compromising Ca²⁺ influx, and recruitment of syntaxin-1 from clusters by SNAP-25 expression makes it available for regulating Ca²⁺ channels. This mechanism potentially allows the cell to regulate Ca²⁺ influx by expanding or contracting syntaxin-1 clusters.

Altered mechanisms underlying the abnormal glutamate release in amyotrophic lateral sclerosis at a pre-symptomatic stage of the disease

Abnormal Glu release occurs in the spinal cord of SOD1(G93A) mice, a transgenic animal model for human ALS. Here we studied the mechanisms underlying Glu release in spinal cord nerve terminals of SOD1(G93A) mice at a pre-symptomatic disease stage (30days) and found that the basal release of Glu was more elevated in SOD1(G93A) with respect to SOD1 mice, and that the surplus of release relies on synaptic vesicle exocytosis. Exposure to high KCl or ionomycin provoked Ca(2+)-dependent Glu release that was likewise augmented in SOD1(G93A) mice. Equally, the Ca(2+)-independent hypertonic sucrose-induced Glu release was abnormally elevated in SOD1(G93A) mice. Also in this case, the surplus of Glu release was exocytotic in nature. We could determine elevated cytosolic Ca(2+) levels, increased phosphorylation of Synapsin-I, which was causally related to the abnormal Glu release measured in spinal cord synaptosomes of pre-symptomatic SOD1(G93A) mice, and increased phosphorylation of glycogen synthase kinase-3 at the inhibitory sites, an event that favours SNARE protein assembly. Western blot experiments revealed an increased number of SNARE protein complexes at the nerve terminal membrane, with no changes of the three SNARE proteins and increased expression of synaptotagmin-1 and β-Actin, but not of an array of other release-related presynaptic proteins. These results indicate that the abnormal exocytotic Glu release in spinal cord of pre-symptomatic SOD1(G93A) mice is mainly based on the increased size of the readily releasable pool of vesicles and release facilitation, supported by plastic changes of specific presynaptic mechanisms.

Detection of the HA-33 protein in botulinum neurotoxin type G complex by mass spectrometry

Background

The disease botulism is caused by intoxication with botulinum neurotoxins (BoNTs), extremely toxic proteins which cause paralysis. This neurotoxin is produced by some members of the Clostridium botulinum and closely related species, and is produced as a protein complex consisting of the neurotoxin and neurotoxin-associated proteins (NAPs). There are seven known serotypes of BoNT, A-G, and the composition of the NAPs can differ between these serotypes. It was previously published that the BoNT/G complex consisted of BoNT/G, nontoxic-nonhemagglutinin (NTNH), Hemagglutinin 70 (HA-70), and HA-17, but that HA-33, a component of the protein complex of other serotypes of BoNT, was not found.

Methods

Components of the BoNT/G complex were first separated by SDS-PAGE, and bands corresponding to components of the complex were digested and analyzed by LC-MS/MS.

Results

Gel bands were identified with sequence coverages of 91 % for BoNT/G, 91 % for NTNH, 89 % for HA-70, and 88 % for HA-17. Notably, one gel band was also clearly identified as HA-33 with 93 % sequence coverage.

Conclusions

The BoNT/G complex consists of BoNT/G, NTNH, HA-70, HA-17, and HA-33. These proteins form the progenitor form of BoNT/G, similar to all other HA positive progenitor toxin complexes.

Solubility of the catalytic domains of Botulinum neurotoxin serotype E subtypes

The Clostridium botulinum neurotoxins (BoNTs) are the most potent protein toxins known to humans. There are seven serotypes of the BoNTs (A-G), among which serotypes A, B, E and F are known to cause natural human intoxication. To date, eleven subtypes of LC/E, termed E1∼E11, have been identified. The LCs of BoNT/E were insoluble, prohibiting studies towards understanding the mechanisms of toxin action and substrate recognition. In this work, the molecular basis of insolubility of the recombinant LCs of two representative subtypes of BoNT/E, E1(Beluga) and E3 (Alaska), was determined. Hydrophobicity profile and structural modeling predicted a C-terminal candidate region responsible for the insolubility of LC/Es. Deletion of C-terminal 19 residues of LC/E(1-400) resulted in enhanced solubility, from 2 to ∼50% for LC/EAlaska and from 16 to ∼95% for LC/EBeluga. In addition, resides 230-236 were found to contribute to a different solubility level of LC/EAlaska when compared to LC/EBeluga. Substituting residues (230)TCI(232) in LC/EAlaska to the corresponding residues of (230)KYT(232) in LC/EBeluga enhanced the solubility of LC/EAlaska to a level approaching that of LC/EBeluga. Among these LC/Es and their derivatives, LC/EBeluga 1-400 was the most soluble and stable protein. Each LC/E derivative possessed similar catalytic activity, suggesting that the C-terminal region of LC/Es contributed to protein solubility, but not catalytic activity. In conclusion, this study generated a soluble and stable recombinant LC/E and provided insight into the structural components that govern the solubility and stability of the LCs of other BoNT serotypes and Tetanus toxin.

Botulinum neurotoxin: Progress in negating its neurotoxicity; and in extending its therapeutic utility via molecular engineering. MiniReview

While the poisonous effects of botulinum neurotoxin (BoNT) have been recognized since antiquity, the overall actions and mechanisms of effects of BoNT have been elucidated primarily over the past several decades. The general utility of BoNT is described in the paper, but the focus is mainly on the approaches towards negating the toxic effects of BoNT, and on the projection of an engineered BoNT molecule serving as a Trojan Horse to deliver a therapeutic load for treatment of a host of medical disorders. The BoNT molecule is configured with a binding domain, a zinc-dependent protease with specificity primarily for vesicular proteins, and a translocation domain for delivery of the metalloprotease into the cytoplasm. The anti-toxin approaches for BoNT include the use of vaccines, antibodies, block of BoNT binding or translocation, inhibition of metalloprotease activity, impeded translocation of the protease/catalytic domain, and inhibition of the downstream Src signaling pathway. Projections of BoNT as a therapeutic include its targeting to non-cholinergic nerves, also targeting to non-neuronal cells for treatment of hypersecretory disorders (e.g., cystic fibrosis), and treatment of hormonal disorders (e.g., acromegaly). Still in the exploratory phase, there is the expectation of major advances in BoNT neuroprotective strategies and burgeoning utility of engineered BoNTs as therapeutics.

Mass Spectrometric Detection of Bacterial Protein Toxins and Their Enzymatic Activity

Mass spectrometry has recently become a powerful technique for bacterial identification. Mass spectrometry approaches generally rely upon introduction of the bacteria into a matrix-assisted laser-desorption time-of-flight (MALDI-TOF) mass spectrometer with mass spectrometric recognition of proteins specific to that organism that form a reliable fingerprint. With some bacteria, such as Bacillus anthracis and Clostridium botulinum, the health threat posed by these organisms is not the organism itself, but rather the protein toxins produced by the organisms. One such example is botulinum neurotoxin (BoNT), a potent neurotoxin produced by C. botulinum. There are seven known serotypes of BoNT, A–G, and many of the serotypes can be further differentiated into toxin variants, which are up to 99.9% identical in some cases. Mass spectrometric proteomic techniques have been established to differentiate the serotype or toxin variant of BoNT produced by varied strains of C. botulinum. Detection of potent biological toxins requires high analytical sensitivity and mass spectrometry based methods have been developed to determine the enzymatic activity of BoNT and the anthrax lethal toxins produced by B. anthracis. This enzymatic activity, unique for each toxin, is assessed with detection of the toxin-induced cleavage of strategically designed peptide substrates by MALDI-TOF mass spectrometry offering unparalleled specificity. Furthermore, activity assays allow for the assessment of the biological activity of a toxin and its potential health risk. Such methods have become important diagnostics for botulism and anthrax. Here, we review mass spectrometry based methods for the enzymatic activity of BoNT and the anthrax lethal factor toxin.

CRISPR/Cas9-Mediated Genomic Deletion of the Beta-1, 4 N-acetylgalactosaminyltransferase 1 Gene in Murine P19 Embryonal Carcinoma Cells Results in Low Sensitivity to Botulinum Neurotoxin Type C

Botulinum neurotoxins produced by Clostridium botulinum cause flaccid paralysis by inhibiting neurotransmitter release at peripheral nerve terminals. Previously, we found that neurons derived from the murine P19 embryonal carcinoma cell line exhibited high sensitivity to botulinum neurotoxin type C. In order to prove the utility of P19 cells for the study of the intracellular mechanism of botulinum neurotoxins, ganglioside-knockout neurons were generated by deletion of the gene encoding beta-1,4 N-acetylgalactosaminyltransferase 1 in P19 cells using the clustered regularly interspaced short palindromic repeats combined with Cas9 (CRISPR/Cas9) system. By using this system, knockout cells could be generated more easily than with previous methods. The sensitivity of the generated beta-1,4 N-acetylgalactosaminyltransferase 1-depleted P19 neurons to botulinum neurotoxin type C was decreased considerably, and the exogenous addition of the gangliosides GD1a, GD1b, and GT1b restored the susceptibility of P19 cells to botulinum neurotoxin type C. In particular, addition of a mixture of these three ganglioside more effectively recovered the sensitivity of knockout cells compared to independent addition of GD1a, GD1b, or GT1b. Consequently, the genome-edited P19 cells generated by the CRISPR/Cas9 system were useful for identifying and defining the intracellular molecules involved in the toxic action of botulinum neurotoxins.

Recommended mass spectrometry-based strategies to identify botulinum neurotoxin-containing samples

Botulinum neurotoxins (BoNTs) cause the disease called botulism, which can be lethal. BoNTs are proteins secreted by some species of clostridia and are known to cause paralysis by interfering with nerve impulse transmission. Although the human lethal dose of BoNT is not accurately known, it is estimated to be between 0.1 μg to 70 μg, so it is important to enable detection of small amounts of these toxins. Our laboratory previously reported on the development of Endopep-MS, a mass-spectrometric‑based endopeptidase method to detect, differentiate, and quantify BoNT immunoaffinity purified from complex matrices. In this work, we describe the application of Endopep-MS for the analysis of thirteen blinded samples supplied as part of the EQuATox proficiency test. This method successfully identified the presence or absence of BoNT in all thirteen samples and was able to successfully differentiate the serotype of BoNT present in the samples, which included matrices such as buffer, milk, meat extract, and serum. Furthermore, the method yielded quantitative results which had z-scores in the range of -3 to +3 for quantification of BoNT/A containing samples. These results indicate that Endopep-MS is an excellent technique for detection, differentiation, and quantification of BoNT in complex matrices.

Botulinum neurotoxin serotypes detected by electrochemical impedance spectroscopy

Botulinum neurotoxin is one of the deadliest biological toxins known to mankind and is able to cause the debilitating disease botulism. The rapid detection of the different serotypes of botulinum neurotoxin is essential for both diagnosis of botulism and identifying the presence of toxin in potential cases of terrorism and food contamination. The modes of action of botulinum neurotoxins are well-established in literature and differ for each serotype. The toxins are known to specifically cleave portions of the SNARE proteins SNAP-25 or VAMP; an interaction that can be monitored by electrochemical impedance spectroscopy. This study presents a SNAP-25 and a VAMP biosensors for detecting the activity of five botulinum neurotoxin serotypes (A-E) using electrochemical impedance spectroscopy. The biosensors are able to detect concentrations of toxins as low as 25 fg/mL, in a short time-frame compared with the current standard methods of detection. Both biosensors show greater specificity for their compatible serotypes compared with incompatible serotypes and denatured toxins.

Enhanced detection of type C botulinum neurotoxin by the Endopep-MS assay through optimization of peptide substrates

It is essential to have a simple, quick and sensitive method for the detection and quantification of botulinum neurotoxins, the most toxic substances and the causative agents of botulism. Type C botulinum neurotoxin (BoNT/C) represents one of the seven members of distinctive BoNT serotypes (A to G) that cause botulism in animals and avians. Here we report the development of optimized peptide substrates for improving the detection of BoNT/C and /CD mosaic toxins using an Endopep-MS assay, a mass spectrometry-based method that is able to rapidly and sensitively detect and differentiate all types of BoNTs by extracting the toxin with specific antibodies and detecting the unique cleavage products of peptide substrates. Based on the sequence of a short SNAP-25 peptide, we conducted optimization through a comprehensive process including length determination, terminal modification, single and multiple amino acid residue substitution, and incorporation of unnatural amino acid residues. Our data demonstrate that an optimal peptide provides a more than 200-fold improvement over the substrate currently used in the Endopep-MS assay for the detection of BoNT/C1 and /CD mosaic. Using the new substrate in a four-hour cleavage reaction, the limit of detection for the BoNT/C1 complex spiked in buffer, serum and milk samples was determined to be 0.5, 0.5 and 1mouseLD50/mL, respectively, representing a similar or higher sensitivity than that obtained by traditional mouse bioassay.

Novel therapeutic uses and formulations of botulinum neurotoxins: a patent review (2012 - 2014)

INTRODUCTION

Botulinum neurotoxins (BoNTs) are among the most toxic of known biological molecules and function as acetylcholine release inhibitors and neuromuscular blocking agents. Paradoxically, these properties also make them valuable therapeutic agents for the treatment of movement disorders, urological conditions and hypersecretory disorders. Greater understanding of their molecular mechanism of action and advances in protein engineering has led to significant efforts to improve and expand their function with a view towards broadening their therapeutic potential.

AREAS COVERED

Searches of Espacenet and Google Patent have revealed a number of patents related to BoNTs. This review will focus on novel therapeutic uses and formulations disclosed during 2012 - 2014. The seven patents discussed will include nanoformulations of FDA-approved BoNTs, additional BoNT subtypes and novel BoNT variants and chimeras created through protein engineering. Supporting patents and related publications are also briefly discussed.

EXPERT OPINION

The clinical and commercial success of BoNTs has prompted investigation into novel BoNTs or BoNT-mediated chimeras with promising in vitro results. Distinct strategies including the use of nanoformulations and targeted delivery have been implemented to identify new indication and improved functionality. Greater understanding of their systemic exposure, efficacy and safety profiles will be required for further development.

Functional characterization of botulinum neurotoxin serotype H as a hybrid of known serotypes F and A (BoNT F/A)

A unique strain of Clostridium botulinum (IBCA10-7060) was recently discovered which produces two toxins: botulinum neurotoxin (BoNT) serotype B and a novel BoNT reported as serotype H. Previous molecular assessment showed that the light chain (LC) of the novel BoNT most resembled the bont of the light chain of known subtype F5, while the C-terminus of the heavy chain (HC) most resembled the binding domain of serotype A. We evaluated the functionality of both toxins produced in culture by first incorporating an immunoaffinity step using monoclonal antibodies to purify BoNT from culture supernatants and tested each immune-captured neurotoxin with full-length substrates vesicle-associated membrane protein 2 (VAMP-2), synaptosomal-associated protein 25 (SNAP-25), syntaxin, and shortened peptides representing the substrates. The BoNT/B produced by this strain behaved as a typical BoNT/B, having immunoaffinity for anti-B monoclonal antibodies and cleaving both full length VAMP-2 and a peptide based on the sequence of VAMP-2 in the expected location. As expected, there was no activity toward SNAP-25 or syntaxin. The novel BoNT demonstrated immunoaffinity for anti-A monoclonal antibodies but did not cleave SNAP-25 as expected for BoNT/A. Instead, the novel BoNT cleaved VAMP-2 and VAMP-2-based peptides in the same location as BoNT/F5. This is the first discovery of a single botulinum neurotoxin with BoNT/A antigenicity and BoNT/F light chain function. This work suggests that the newly reported serotype H may actually be a hybrid of previously known BoNT serotype A and serotype F, specifically subtype F5.

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

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

Detection of Botulinum Toxins A, B, E, and F in Foods by Endopep-MS

Botulism is caused by exposure to botulinum neurotoxins (BoNTs). BoNTs are proteins secreted by some species of clostridia; these neurotoxins are known to interfere with nerve impulse transmission, thus causing paralysis. Botulism may be contracted through consumption of food either naturally or intentionally contaminated with BoNT. The human lethal dose of BoNT is not known but is estimated to be between 0.1 and 70 μg; thus, it is important to be able to detect small amounts of this toxin in foods to ensure food safety and to identify the source of an outbreak. Our laboratory previously reported on the development of Endopep-MS, a mass-spectrometric-based endopeptidase method for the detection and differentiation of BoNT. This method can detect BoNT at levels below the historic standard mouse bioassay in clinical samples such as serum, stool, and culture supernatants. We have now expanded this assay to detect BoNT in over 50 foods including representative products that were involved in actual botulism investigations. The foods tested by the Endopep-MS included those with various acidities, viscosities, and fat levels. Dairy and culturally diverse products were also included. This work demonstrates that the Endopep-MS method can be used to detect BoNT/A, /B, /E, and /F in foods at levels spiked below that of the limit of detection of the mouse bioassay. Furthermore, we successfully applied this method to investigate several foods associated with botulism outbreaks.

A two-stage multiplex method for quantitative analysis of botulinum neurotoxins type A, B, E, and F by MALDI-TOF mass spectrometry

In this publication, we report on the development of a quantitative enzymatic method for the detection of four botulinum neurotoxin (BoNT) serotypes responsible for human botulism by MALDI-TOF mass spectrometry. Factors that might affect the linearity and dynamic range for detection of BoNT cleavage products were initially examined, including the amount of peptide substrate and internal standard, the timing of cleavage reaction, and the components in the reaction solution. It was found that a long incubation time produced sensitive results, but was not capable of determining higher toxin concentrations, whereas a short incubation time was less sensitive so that lower toxin concentrations were not detected. In order to overcome these limitations, a two-stage analysis strategy was applied. The first stage analysis involved a short incubation period (e.g., 30 min). If no toxin was detected at this stage, the cleavage reaction was allowed to continue and the samples were analyzed at a second time point (4 h), so that toxin levels lower than 1 mouse LD50 or 55 attomoles per milliliter (55 amol/mL) could be quantified. By combining the results from two-stage quantification, 4 or 5 orders of magnitude in dynamic range were achieved for the detection of the serotypes of BoNT/A, BoNT/B, BoNT/E, or BoNT/F. The effect of multiplexing the assay by mixing substrates for different BoNT serotypes into a single reaction was also investigated in order to reduce the numbers of the cleavage reactions and save valuable clinical samples.

Botulinum toxin A, brain and pain

Botulinum neurotoxin type A (BoNT/A) is one of the most potent toxins known and a potential biological threat. At the same time, it is among the most widely used therapeutic proteins used yearly by millions of people, especially for cosmetic purposes. Currently, its clinical use in certain types of pain is increasing, and its long-term duration of effects represents a special clinical value. Efficacy of BoNT/A in different types of pain has been found in numerous clinical trials and case reports, as well as in animal pain models. However, sites and mechanisms of BoNT/A actions involved in nociception are a matter of controversy. In analogy with well known neuroparalytic effects in peripheral cholinergic synapses, presently dominant opinion is that BoNT/A exerts pain reduction by inhibiting peripheral neurotransmitter/inflammatory mediator release from sensory nerves. On the other hand, growing number of behavioral and immunohistochemical studies demonstrated the requirement of axonal transport for BoNT/A's antinociceptive action. In addition, toxin's enzymatic activity in central sensory regions was clearly identified after its peripheral application. Apart from general pharmacology, this review summarizes the clinical and experimental evidence for BoNT/A antinociceptive activity and compares the data in favor of peripheral vs. central site and mechanism of action. Based on literature review and published results from our laboratory we propose that the hypothesis of peripheral site of BoNT/A action is not sufficient to explain the experimental data collected up to now.

Differential interaction of tomosyn with syntaxin and SNAP25 depends on domains in the WD40 β-propeller core and determines its inhibitory activity

Neuronal exocytosis depends on efficient formation of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes and is regulated by tomosyn, a SNARE-binding protein. To gain new information about tomosyn's activity, we characterized its mobility and organization on the plasma membrane (PM) in relation to other SNARE proteins and inhibition of exocytosis. By using direct stochastic optical reconstruction microscopy (dSTORM), we found tomosyn to be organized in small clusters adjacent to syntaxin clusters. In addition, we show that tomosyn is present in both syntaxin-tomosyn complexes and syntaxin-SNAP25-tomosyn complexes. Tomosyn mutants that lack residues 537-578 or 897-917 from its β-propeller core diffused faster on the PM and exhibited reduced binding to SNAP25, suggesting that these mutants shift the equilibrium between tomosyn-syntaxin-SNAP25 complexes on the PM to tomosyn-syntaxin complexes. As these deletion mutants impose less inhibition on exocytosis, we suggest that tomosyn inhibition is mediated via tomosyn-syntaxin-SNAP25 complexes and not tomosyn-syntaxin complexes. These findings characterize, for the first time, tomosyn's dynamics at the PM and its relation to its inhibition of exocytosis.

Three enzymatically active neurotoxins of Clostridium botulinum strain Af84: BoNT/A2, /F4, and /F5

Botulinum neurotoxins (BoNTs) are produced by various species of clostridia and are potent neurotoxins which cause the disease botulism, by cleaving proteins needed for successful nerve transmission. There are currently seven confirmed serotypes of BoNTs, labeled A-G, and toxin-producing clostridia typically only produce one serotype of BoNT. There are a few strains (bivalent strains) which are known to produce more than one serotype of BoNT, producing either both BoNT/A and /B, BoNT/A and /F, or BoNT/B and /F, designated as Ab, Ba, Af, or Bf. Recently, it was reported that Clostridium botulinum strain Af84 has three neurotoxin gene clusters: bont/A2, bont/F4, and bont/F5. This was the first report of a clostridial organism containing more than two neurotoxin gene clusters. Using a mass spectrometry based proteomics approach, we report here that all three neurotoxins, BoNT/A2, /F4, and /F5, are produced by C. botulinum Af84. Label free MS(E) quantification of the three toxins indicated that toxin composition is 88% BoNT/A2, 1% BoNT/F4, and 11% BoNT/F5. The enzymatic activity of all three neurotoxins was assessed by examining the enzymatic activity of the neurotoxins upon peptide substrates, which mimic the toxins' natural targets, and monitoring cleavage of the substrates by mass spectrometry. We determined that all three neurotoxins are enzymatically active. This is the first report of three enzymatically active neurotoxins produced in a single strain of Clostridium botulinum.

Comparison of the catalytic properties of the botulinum neurotoxin subtypes A1 and A5

Clostridium botulinum neurotoxins (BoNTs) cause the life-threatening disease botulism through the inhibition of neurotransmitter release by cleaving essential SNARE proteins. There are seven serologically distinctive types of BoNTs and many subtypes within a serotype have been identified. BoNT/A5 is a recently discovered subtype of type A botulinum neurotoxin which possesses a very high degree of sequence similarity and identity to the well-studied A1 subtype. In the present study, we examined the endopeptidase activity of these two BoNT/A subtypes and our results revealed significant differences in substrate binding and cleavage efficiency between subtype A5 and A1. Distinctive hydrolysis efficiency was observed between the two toxins during cleavage of the native substrate SNAP-25 versus a shortened peptide mimic. N-terminal truncation studies demonstrated that a key region of the SNAP-25, including the amino acid residues at 151 through 154 located in the remote binding region of the substrate, contributed to the differential catalytic properties between A1 and A5. Elevated binding affinity of the peptide substrate resulted from including these important residues and enhanced BoNT/A5's hydrolysis efficiency. In addition, mutations of these amino acid residues affect the proteolytic performance of the two toxins in different ways. This study provides a better understanding of the biological activity of these toxins, their performance characteristics in the Endopep-MS assay to detect BoNT in clinical samples and foods, and is useful for the development of peptide substrates.

Cytotoxicity of botulinum neurotoxins reveals a direct role of syntaxin 1 and SNAP-25 in neuron survival

Botulinum neurotoxins (BoNT/A-G) are well-known to act by blocking synaptic vesicle exocytosis. Whether BoNTs disrupt additional neuronal functions has not been addressed. Here we report that cleavage of syntaxin 1 (Syx 1) by BoNT/C and cleavage of SNAP-25 by BoNT/E both induce degeneration of cultured rodent and human neurons. Furthermore, although SNAP-25 cleaved by BoNT/A can still support neuron survival, it has reduced capacity to tolerate additional mutations and also fails to pair with syntaxin isoforms other than Syx 1. Syx 1 and SNAP-25 are well-known for mediating synaptic vesicle exocytosis, but we found that neuronal death is due to blockage of plasma membrane recycling processes that share Syx 1/SNAP-25 for exocytosis, independent of blockage of synaptic vesicle exocytosis. These findings reveal neuronal cytotoxicity for a subset of BoNTs and directly link Syx 1/SNAP-25 to neuron survival as the prevalent SNARE proteins mediating multiple fusion events on neuronal plasma membranes.

Assembly and function of the botulinum neurotoxin progenitor complex

Botulinum neurotoxins (BoNTs) are among the most poisonous substances known to man, but paradoxically, BoNT-containing medicines and cosmetics have been used with great success in the clinic. Accidental BoNT poisoning mainly occurs through oral ingestion of food contaminated with Clostridium botulinum. BoNTs are naturally produced in the form of progenitor toxin complexes (PTCs), which are high molecular weight (up to ~900 kDa) multiprotein complexes composed of BoNT and several non-toxic neurotoxin-associated proteins (NAPs). NAPs protect the inherently fragile BoNTs against the hostile environment of the gastrointestinal (GI) tract and help BoNTs pass through the intestinal epithelial barrier before they are released into the general circulation. These events are essential for ingested BoNTs to gain access to motoneurons, where they inhibit neurotransmitter release and cause muscle paralysis. In this review, we discuss the structural basis for assembly of NAPs and BoNT into the PTC that protects BoNT and facilitate its delivery into the bloodstream.

Improved detection of botulinum neurotoxin serotype A by Endopep-MS through peptide substrate modification

Botulinum neurotoxins (BoNTs) are a family of seven toxin serotypes that are the most toxic substances known to humans. Intoxication with BoNT causes flaccid paralysis and can lead to death if untreated with serotype-specific antibodies. Supportive care, including ventilation, may be necessary. Rapid and sensitive detection of BoNT is necessary for timely clinical confirmation of clinical botulism. Previously, our laboratory developed a fast and sensitive mass spectrometry (MS) method termed the Endopep-MS assay. The BoNT serotypes are rapidly detected and differentiated by extracting the toxin with serotype-specific antibodies and detecting the unique and serotype-specific cleavage products of peptide substrates that mimic the sequence of the BoNT native targets. To further improve the sensitivity of the Endopep-MS assay, we report here the optimization of the substrate peptide for the detection of BoNT/A. Modifications on the terminal groups of the original peptide substrate with acetylation and amidation significantly improved the detection of BoNT/A cleavage products. The replacement of some internal amino acid residues with single or multiple substitutions led to further improvement. An optimized peptide increased assay sensitivity 5-fold with toxin spiked into buffer solution or different biological matrices.

Tetanus toxin and botulinum toxin a utilize unique mechanisms to enter neurons of the central nervous system

Botulinum neurotoxins (BoNTs) and tetanus neurotoxin (TeNT) are the most toxic proteins for humans. While BoNTs cause flaccid paralysis, TeNT causes spastic paralysis. Characterized BoNT serotypes enter neurons upon binding dual receptors, a ganglioside and a neuron-specific protein, either synaptic vesicle protein 2 (SV2) or synaptotagmin, while TeNT enters upon binding gangliosides as dual receptors. Recently, TeNT was reported to enter central nervous system (CNS) neurons upon synaptic vesicle cycling that was mediated by the direct binding to SV2, implying that TeNT and BoNT utilize common mechanisms to enter CNS neurons. This prompted an assessment of TeNT entry into CNS neurons, using the prototypic BoNT serotype A as a reference for SV2-mediated entry into synaptic vesicles, analyzing the heavy-chain receptor binding domain (HCR) of each toxin. Synaptic vesicle cycling stimulated the entry of HCR/A into neurons, while HCR/T entered neurons with similar levels of efficiency in depolarized and nondepolarized neurons. ImageJ analysis identified two populations of cell-associated HCR/T in synaptic vesicle cycling neurons, a major population which segregated from HCR/A and a minor population which colocalized with HCR/A. HCR/T did not inhibit HCR/A entry into neurons in competition experiments and did not bind SV2, the protein receptor for BoNT/A. Intoxication experiments showed that TeNT efficiently cleaved VAMP2 in depolarized neurons and neurons blocked for synaptic vesicle cycling. These experiments demonstrate that TeNT enters neurons by two pathways, one independent of stimulated synaptic vesicle cycling and one by synaptic vesicles independent of SV2, showing that TeNT and BoNT/A enter neurons by unique mechanisms.

Purification and characterization of a novel subtype a3 botulinum neurotoxin

Botulinum neurotoxins (BoNTs) produced by Clostridium botulinum are of considerable importance due to their being the cause of human and animal botulism, their potential as bioterrorism agents, and their utility as important pharmaceuticals. Type A is prominent due to its high toxicity and long duration of action. Five subtypes of type A BoNT are currently recognized; BoNT/A1, -/A2, and -/A5 have been purified, and their properties have been studied. BoNT/A3 is intriguing because it is not effectively neutralized by polyclonal anti-BoNT/A1 antibodies, and thus, it may potentially replace BoNT/A1 for patients who have become refractive to treatment with BoNT/A1 due to antibody formation or other modes of resistance. Purification of BoNT/A3 has been challenging because of its low levels of production in culture and the need for innovative purification procedures. In this study, modified Mueller-Miller medium was used in place of traditional toxin production medium (TPM) to culture C. botulinum A3 (CDC strain) and boost toxin production. BoNT/A3 titers were at least 10-fold higher than those produced in TPM. A purification method was developed to obtain greater than 95% pure BoNT/A3. The specific toxicity of BoNT/A3 as determined by mouse bioassay was 5.8 × 10(7) 50% lethal doses (LD(50))/mg. Neutralization of BoNT/A3 toxicity by a polyclonal anti-BoNT/A1 antibody was approximately 10-fold less than the neutralization of BoNT/A1 toxicity. In addition, differences in symptoms were observed between mice that were injected with BoNT/A3 and those that were injected with BoNT/A1. These results indicate that BoNT/A3 has novel biochemical and pharmacological properties compared to those of other subtype A toxins.

Discovery of a novel enzymatic cleavage site for botulinum neurotoxin F5

Botulinum neurotoxins (BoNTs) cause botulism by cleaving proteins necessary for nerve transmission. There are seven serotypes of BoNT, A-G, characterized by their response to antisera. Many serotypes are further distinguished into differing subtypes based on amino acid sequence, some of which result in functional differences. Our laboratory previously reported that all tested subtypes within each serotype have the same site of enzymatic activity. Recently, three new subtypes of BoNT/F; /F3, /F4, and /F5, were reported. Here, we report that BoNT/F5 cleaves substrate synaptobrevin-2 in a different location than the other BoNT/F subtypes, between (54)L and (55)E. This is the first report of cleavage of synaptobrevin-2 in this location.