Identification and characterization of functional subunits of Clostridium botulinum type A progenitor toxin involved in binding to intestinal microvilli and erythrocytes

Pathogenicity and virulence of Clostridium botulinum

Clostridium botulinum, a polyphyletic Gram-positive taxon of bacteria, is classified purely by their ability to produce botulinum neurotoxin (BoNT). BoNT is the primary virulence factor and the causative agent of botulism. A potentially fatal disease, botulism is classically characterized by a symmetrical descending flaccid paralysis, which is left untreated can lead to respiratory failure and death. Botulism cases are classified into three main forms dependent on the nature of intoxication; foodborne, wound and infant. The BoNT, regarded as the most potent biological substance known, is a zinc metalloprotease that specifically cleaves SNARE proteins at neuromuscular junctions, preventing exocytosis of neurotransmitters, leading to muscle paralysis. The BoNT is now used to treat numerous medical conditions caused by overactive or spastic muscles and is extensively used in the cosmetic industry due to its high specificity and the exceedingly small doses needed to exert long-lasting pharmacological effects. Additionally, the ability to form endospores is critical to the pathogenicity of the bacteria. Disease transmission is often facilitated via the metabolically dormant spores that are highly resistant to environment stresses, allowing persistence in the environment in unfavourable conditions. Infant and wound botulism infections are initiated upon germination of the spores into neurotoxin producing vegetative cells, whereas foodborne botulism is attributed to ingestion of preformed BoNT. C. botulinum is a saprophytic bacterium, thought to have evolved its potent neurotoxin to establish a source of nutrients by killing its host.

Critical analysis in the advancement of cell-based assays for botulinum neurotoxin

The study on botulinum neurotoxins (BoNTs) has rapidly evolved for their structure and functions as opposed to them being poisons or cures. Since their discoveries, the scientific community has come a long way in understanding BoNTs' structure and biological activity. Given its current application as a tool for understanding neurocellular activity and as a drug against over 800 neurological disorders, relevant and sensitive assays have become critical for biochemical, physiological, and pharmacological studies. The natural entry of the toxin being ingestion, it has also become important to examine its mechanism while crossing the epithelial cell barrier. Several techniques and methodologies have been developed, for its entry, pharmacokinetics, and biological activity for identification, and drug efficacy both in vivo and in vitro conditions. However, each of them presents its own challenges. The cell-based assay is a platform that exceeds the sensitivity of mouse bioassay while encompassing all the steps of intoxication including cell binding, transcytosis, endocytosis, translocation and proteolytic activity. In this article we review in detail both the neuronal and nonneuronal based cellular interaction of BoNT involving its transportation, and interaction with the targeted cells, and intracellular activities.

Molecular engineering of a minimal E-cadherin inhibitor protein derived from Clostridium botulinum hemagglutinin

Hemagglutinin (HA), a nontoxic component of the botulinum neurotoxin (BoNT) complex, binds to E-cadherin and inhibits E-cadherin-mediated cell-cell adhesion. HA is a 470 kDa protein complex comprising six HA1, three HA2, and three HA3 subcomponents. Thus, to prepare recombinant full-length HA in vitro, it is necessary to reconstitute the macromolecular complex from purified HA subcomponents, which involves multiple purification steps. In this study, we developed NanoHA, a minimal E-cadherin inhibitor protein derived from Clostridium botulinum HA with a simple purification strategy needed for production. NanoHA, containing HA2 and a truncated mutant of HA3 (amino acids 380-626; termed as HA3^mini), is a 47 kDa single polypeptide (one-tenth the molecular weight of full-length HA, 470 kDa) engineered with three types of modifications: (i) a short linker sequence between the C terminus of HA2 and N terminus of HA3; (ii) a chimeric complex composed of HA2 derived from the serotype C BoNT complex and HA3^mini from the serotype B BoNT complex; and (iii) three amino acid substitutions from hydrophobic to hydrophilic residues on the protein surface. We demonstrated that NanoHA inhibits E-cadherin-mediated cell-cell adhesion of epithelial cells (e.g., Caco-2 and Madin-Darby canine kidney cells) and disrupts their epithelial barrier. Finally, unlike full-length HA, NanoHA can be transported from the basolateral side to adherens junctions via passive diffusion. Overall, these results indicate that the rational design of NanoHA provides a minimal E-cadherin inhibitor with a wide variety of applications as a lead molecule and for further molecular engineering.

Multi-dimensional nanoscale liquid chromatography and nano-electrospray ion-trap mass spectrometry for detection of Clostridium botulinum type C and the produced botulinum neurotoxin type C complex

Botulinum neurotoxin types C, D and their mosaic forms C/D and D/C produced mainly by Clostridium botulinum types C and D cause botulism in animals and belong to the most toxic substances for poultry and fish. In addition to intoxications, also toxoinfections with C. botulinum types C and D play a role that should not be underestimated, especially in veterinary medicine. Contrary to other botulinum neurotoxin complexes (BT x), the biosynthesis of these types is phage-encoded. Currently, the gold standard for neurotoxin detection in cases of clinical botulism is the mouse bioassay. In the last few years, alternatives for replacing this mouse bioassay have become increasingly interesting for the detection and characterisation of botulinum neurotoxins. Therefore, immunological techniques based mainly on antibodies, PCR or mass spectral methods have been developed. In this context, the most promising development is that of different endopeptidase assays. In our study, we were able to show that the 2D-nano-LC-MS/MS method presented by Klaubert et al. 2009 especially for detecting BT x A, B, E and F in complex culture media can also be used for detecting BT x C. The focus was therefore on transferring this method to detecting BT x C and pointing out necessary modifications of this current method. For method development, we used different culture preparations and sample conditions. To find out whether BT x C is just as stable against acetic peptic pretreatment as other BT x, we used sample preparations with and without peptic pretreatment. The decisive difference to previous publications is the detection of produced BT x C directly from culture supernatant of different strains of C. botulinum type C. In addition, we present a new approach of detecting protein fragments from C3 and C2 toxin and some specific host cell proteins of the bacterium Clostridium spp. in order to specify the carrier bacterium, therefore verifying the presence of an intact neurotoxin-encoding phage also without directly detecting BT x C and thus the possibility to produce neurotoxin. Herein, we describe a new method to examine environmental samples or suspected feed samples in cases of toxoinfections as well as finding out the causes of clinical botulism. This new approach is particularly interesting for veterinary medicine, especially for diseases like chronic botulism in cows or equine grass sickness.

Sialic Acids as Receptors for Pathogens

Carbohydrates have long been known to mediate intracellular interactions, whether within one organism or between different organisms. Sialic acids (Sias) are carbohydrates that usually occupy the terminal positions in longer carbohydrate chains, which makes them common recognition targets mediating these interactions. In this review, we summarize the knowledge about animal disease-causing agents such as viruses, bacteria and protozoa (including the malaria parasite Plasmodium falciparum) in which Sias play a role in infection biology. While Sias may promote binding of, e.g., influenza viruses and SV40, they act as decoys for betacoronaviruses. The presence of two common forms of Sias, Neu5Ac and Neu5Gc, is species-specific, and in humans, the enzyme converting Neu5Ac to Neu5Gc (CMAH, CMP-Neu5Ac hydroxylase) is lost, most likely due to adaptation to pathogen regimes; we discuss the research about the influence of malaria on this trait. In addition, we present data suggesting the CMAH gene was probably present in the ancestor of animals, shedding light on its glycobiology. We predict that a better understanding of the role of Sias in disease vectors would lead to more effective clinical interventions.

Botulinum Hemagglutinin: Critical Protein for Adhesion and Absorption of Neurotoxin Complex in Host Intestine

Botulinum hemagglutinin (HA) is one of the auxiliary protein components of the botulinum neurotoxin (BoNT) complex, the most lethal toxin known. HA promotes the intestinal absorption of BoNT by at least two mechanisms, resulting in high oral toxicity. One of the mechanisms is the attachment of large progenitor toxin complexes (L-PTCs) to the cell surface of the intestinal epithelium by the carbohydrate-binding activity of HA. The other is epithelial barrier disruption by the E-cadherin-binding activity of HA. The carbohydrate-binding activity of HA also promotes attachment to the basolateral cell surface, which increases the frequency of contact between HA and E-cadherin. Together, the carbohydrate-binding activity of HA is critical for the intestinal absorption of BoNTs. The trimeric triskelion-shaped structure of HA confers the multivalent binding to its ligands and increases the pathogenic biological activities of HA.

[Mechanism of intestinal absorption of botulinum neurotoxin complex]

Botulinum neurotoxins (BoNTs) produced by the anaerobic bacterium Clostridium botulinum and related species cause botulism, a neuroparalytic disease associated with a high mortality. BoNTs are always produced as large protein complexes (progenitor toxin complexes, PTCs) through association with non-toxic components (NAPs) including hemagglutinin (HA) and non-toxic non-hemagglutinin (NTNHA). Food-borne botulism is caused by the ingestion of PTCs. PTCs in the gastrointestinal tract cross the intestinal epithelial barrier, enter the blood stream, and reach the nerve endings, where BoNTs cleave the SNAREs required for vesicle fusion. Consequently, BoNTs inhibit neurotransmitter release and cause paralysis. To cause food-borne botulism, BoNTs must traverse the intestinal epithelial barrier. However, the mechanism used to cross this barrier remains unclear. Using an in vitro epithelial barrier system, we previously showed that the interaction of HA with E-cadherin results in disruption of tight junctions. Furthermore, we previously reported that microfold (M) cells in the follicle-associated epithelium (FAE) of mouse Peyer's patches (PPs) are major sites where type A1 BoNT breaches the intestinal epithelial barrier. Here, I would like to demonstrate an ingenious invasion mechanism of the BoNT complex.

Role of critical elements in botulinum neurotoxin complex in toxin routing across intestinal and bronchial barriers

The highly potent botulinum neurotoxin serotype A (BoNT/A) inhibits neurotransmitter release at neuromuscular junctions resulting in flaccid muscle paralysis, respiratory arrest and death. In order to reach their neuronal cell targets, BoNT/A must cross epithelial cell barriers lining the intestines and airways. The toxin is produced as a large protein complex comprised of the neurotoxin and non-toxic neurotoxin-associated proteins (NAPs). Although NAPs are known to protect the toxin from harsh environments, their role in the movement of BoNT/A across epithelial barriers has not been fully characterized. In the current study, movement of the toxin across epithelial cells was examined macroscopically using a sensitive near infrared fluorescence transcytosis assay and microscopically using fluorescently labeled toxin and confocal microscopy. The studies show that the BoNT/A complex internalizes more rapidly than the pure toxin. The studies also show that one NAP protein, hemaglutinin 33 (Hn33), enhanced both the binding and movement of a deactivated recombinant botulinum neurotoxin A (DrBoNT) across epithelial cell monolayers and that the toxin associates with Hn33 on the cell surface. Collectively, the data demonstrate that, in addition to their protective role, NAPs and Hn33 play an important role in BoNT/A intoxication.

Characterization of Hemagglutinin Negative Botulinum Progenitor Toxins

Botulism is a disease involving intoxication with botulinum neurotoxins (BoNTs), toxic proteins produced by Clostridium botulinum and other clostridia. The 150 kDa neurotoxin is produced in conjunction with other proteins to form the botulinum progenitor toxin complex (PTC), alternating in size from 300 kDa to 500 kDa. These progenitor complexes can be classified into hemagglutinin positive or hemagglutinin negative, depending on the ability of some of the neurotoxin-associated proteins (NAPs) to cause hemagglutination. The hemagglutinin positive progenitor toxin complex consists of BoNT, nontoxic non-hemagglutinin (NTNH), and three hemagglutinin proteins; HA-70, HA-33, and HA-17. Hemagglutinin negative progenitor toxin complexes contain BoNT and NTNH as the minimally functional PTC (M-PTC), but not the three hemagglutinin proteins. Interestingly, the genome of hemagglutinin negative progenitor toxin complexes comprises open reading frames (orfs) which encode for three proteins, but the existence of these proteins has not yet been extensively demonstrated. In this work, we demonstrate that these three proteins exist and form part of the PTC for hemagglutinin negative complexes. Several hemagglutinin negative strains producing BoNT/A, /E, and /F were found to contain the three open reading frame proteins. Additionally, several BoNT/A-containing bivalent strains were examined, and NAPs from both genes, including the open reading frame proteins, were associated with BoNT/A. The open reading frame encoded proteins are more easily removed from the botulinum complex than the hemagglutinin proteins, but are present in several BoNT/A and /F toxin preparations. These are not easily removed from the BoNT/E complex, however, and are present even in commercially-available purified BoNT/E complex.

Botulinum Neurotoxins: Biology, Pharmacology, and Toxicology

The study of botulinum neurotoxins (BoNT) is rapidly progressing in many aspects. Novel BoNTs are being discovered owing to next generation sequencing, but their biologic and pharmacological properties remain largely unknown. The molecular structure of the large protein complexes that the toxin forms with accessory proteins, which are included in some BoNT type A1 and B1 pharmacological preparations, have been determined. By far the largest effort has been dedicated to the testing and validation of BoNTs as therapeutic agents in an ever increasing number of applications, including pain therapy. BoNT type A1 has been also exploited in a variety of cosmetic treatments, alone or in combination with other agents, and this specific market has reached the size of the one dedicated to the treatment of medical syndromes. The pharmacological properties and mode of action of BoNTs have shed light on general principles of neuronal transport and protein-protein interactions and are stimulating basic science studies. Moreover, the wide array of BoNTs discovered and to be discovered and the production of recombinant BoNTs endowed with specific properties suggest novel uses in therapeutics with increasing disease/symptom specifity. These recent developments are reviewed here to provide an updated picture of the biologic mechanism of action of BoNTs, of their increasing use in pharmacology and in cosmetics, and of their toxicology.

Uptake of Clostridial Neurotoxins into Cells and Dissemination

Clostridial neurotoxins, botulinum neurotoxins (BoNT) and tetanus neurotoxin (TeNT), are potent toxins, which are responsible for severe neurological diseases in man and animals. BoNTs induce a flaccid paralysis (botulism) by inhibiting acetylcholine release at the neuromuscular junctions, whereas TeNT causes a spastic paralysis (tetanus) by blocking the neurotransmitter release (glycine, GABA) in inhibitory interneurons within the central nervous system. Clostridial neurotoxins recognize specific receptor(s) on the target neuronal cells and enter via a receptor-mediated endocytosis. They transit through an acidic compartment which allows the translocation of the catalytic chain into the cytosol, a prerequisite step for the intracellular activity of the neurotoxins. TeNT migrates to the central nervous system by using a motor neuron as transport cell. TeNT enters a neutral pH compartment and undergoes a retrograde axonal transport to the spinal cord or brain, where the whole undissociated toxin is delivered and interacts with target neurons. Botulism most often results from ingestion of food contaminated with BoNT. Thus, BoNT passes through the intestinal epithelial barrier mainly via a transcytotic mechanism and then diffuses or is transported to the neuromuscular junctions by the lymph or blood circulation. Indeed, clostridial neurotoxins are specific neurotoxins which transit through a transport cell to gain access to the target neuron, and use distinct trafficking pathways in both cell types.

In Silico Study to Develop a Lectin-Like Protein from Mushroom Agaricus bisporus for Pharmaceutical Application

A lectin-like protein of unknown function designated as LSMT was recently discovered in the edible mushroom Agaricus bisporus. The protein shares high structural similarity to HA-33 from Clostridium botulinum (HA33) and Ricin-B-like lectin from the mushroom Clitocybe nebularis (CNL), which have been developed as drug carrier and anti-cancer, respectively. These homologous proteins display the ability to penetrate the intestinal epithelial cell monolayer, and are beneficial for oral administration. As the characteristics of LSMT are unknown, a structural study in silico was performed to assess its potential pharmaceutical application. The study suggested potential binding to target ligands such as HA-33 and CNL although the nature, specificity, capacity, mode, and strength may differ. Further molecular docking experiments suggest that interactions between the LSMT and tested ligands may take place. This finding indicates the possible use of the LSMT protein, initiating new research on its use for pharmaceutical purposes.

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.

The long journey of botulinum neurotoxins into the synapse

Botulinum neurotoxins (BoNT) cause the disease botulism, a flaccid paralysis of the muscle. They are also very effective, widely used medicines applied locally in sub-nanogram quantities. BoNTs are released together with several non-toxic, associated proteins as progenitor toxin complexes (PCT) by Clostridium botulinum to become highly potent oral poisons ingested via contaminated food. They block the neurotransmission in susceptible animals and humans already in nanogram quantities due to their specific ability to enter motoneurons and to cleave only selected neuronal proteins involved in neuroexocytosis. BoNTs have developed a sophisticated strategy to passage the gastrointestinal tract and to be absorbed in the intestine of the host to finally attack neurons. A non-toxic non-hemagglutinin (NTNHA) forms a binary complex with BoNT to protect it from gastrointestinal degradation. This binary M-PTC is one component of the bi-modular 14-subunit ∼760 kDa large progenitor toxin complex. The other component is the structurally and functionally independent dodecameric hemagglutinin (HA) complex which facilitates the absorption on the intestinal epithelium by glycan binding. Subsequent to its transcytosis the HA complex disrupts the tight junction of the intestinal barrier from the basolateral side by binding to E-cadherin. Now, the L-PTC can also enter the circulation by paracellular routes in much larger quantities. From here, the dissociated BoNTs reach the neuromuscular junction and accumulate via interaction with polysialo gangliosides, complex glycolipids, on motoneurons at the neuromuscular junction. Subsequently, additional specific binding to luminal segments of synaptic vesicles proteins like SV2 and synaptotagmin leads to their uptake. Finally, the neurotoxins shut down the synaptic vesicle cycle, which they had exploited before to enter their target cells, via specific cleavage of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, which constitute the core components of the cellular membrane fusion machinery.

Clostridium botulinum type C hemagglutinin affects the morphology and viability of cultured mammalian cells via binding to the ganglioside GM3

Botulinum neurotoxin is conventionally divided into seven serotypes, designated A-G, and is produced as large protein complexes through associations with non-toxic components, such as hemagglutinin (HA) and non-toxic non-HA. These non-toxic proteins dramatically enhance the oral toxicity of the toxin complex. HA is considered to have a role in toxin transport through the intestinal epithelium by carbohydrate binding and epithelial barrier-disrupting activity. Type A and B HAs disrupt E-cadherin-mediated cell adhesion, and, in turn, the intercellular epithelial barrier. Type C HA (HA/C) disrupts the barrier function by affecting cell morphology and viability, the mechanism of which remains unknown. In this study, we identified GM3 as the target molecule of HA/C. We found that sialic acid binding of HA is essential for the activity. It was abolished when cells were pre-treated with an inhibitor of ganglioside synthesis. Consistent with this, HA/C bound to a-series gangliosides in a glycan array. In parallel, we isolated clones resistant to HA/C activity from a susceptible mouse fibroblast strain. These cells lacked expression of ST-I, the enzyme that transfers sialic acid to lactosylceramide to yield GM3. These clones became sensitive to HA/C activity when GM3 was expressed by transfection with the ST-I gene. The sensitivity of fibroblasts to HA/C was reduced by expressing ganglioside synthesis genes whose products utilize GM3 as a substrate and consequently generate other a-series gangliosides, suggesting a GM3-specific mechanism. Our results demonstrate that HA/C affects cells in a GM3-dependent manner.

Botulinum toxin A complex exploits intestinal M cells to enter the host and exert neurotoxicity

To cause food-borne botulism, botulinum neurotoxin (BoNT) in the gastrointestinal lumen must traverse the intestinal epithelial barrier. However, the mechanism by which BoNT crosses the intestinal epithelial barrier remains unclear. BoNTs are produced along with one or more non-toxic components, with which they form progenitor toxin complexes (PTCs). Here we show that serotype A1 L-PTC, which has high oral toxicity and makes the predominant contribution to causing illness, breaches the intestinal epithelial barrier from microfold (M) cells via an interaction between haemagglutinin (HA), one of the non-toxic components, and glycoprotein 2 (GP2). HA strongly binds to GP2 expressed on M cells, which do not have thick mucus layers. Susceptibility to orally administered L-PTC is dramatically reduced in M-cell-depleted mice and GP2-deficient (Gp2^−/−) mice. Our finding provides the basis for the development of novel antitoxin therapeutics and delivery systems for oral biologics.

It is unclear how ingested botulinum neurotoxin invades the host to cause illness. Here, the authors show that the toxin complex containing neurotoxin, hemagglutinin (HA), and NTNHA proteins traverses the epithelial barrier via HA-glycoprotein 2 interaction and endocytosis by Peyer’s patch microfold cells.

Evaluation of an enzyme-linked immunosorbent assay (ELISA) kit for the detection of botulinum neurotoxins A, B, E, and F in selected food matrices

The mouse bioassay (MBA) is the only accepted standard method for detection of botulinum neurotoxins (BoNTs) in foods. The ELISA method has several advantages over the MBA and is therefore widely used for in vitro detection of BoNTs. The US Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) conducted a precollaborative study to evaluate the applicability of Botulinum Toxin ELISA kits for the detection of BoNT serotypes A, B, E, and F in a variety of food matrices. In this study, food samples (e.g., broccoli, salami, smoked salmon, green beans, orange juice, tomato juice, low-fat plain yogurt, whole milk, liquid infant formula milk, and liquid eggs) were spiked with high, medium, and low concentration BoNT serotypes A, B, E, and F. Samples (unspiked and spiked) were tested at both laboratories by the ELISA kits. All food samples were positive for BoNTs by ELISA in both laboratories at medium and high spiking levels; a positive ELISA result in low spiked samples was both serotype and laboratory dependent. Overall, the ELISA method appears to be an effective preliminary screening method for BoNT detection in food matrices.

Functional dissection of the Clostridium botulinum type B hemagglutinin complex: identification of the carbohydrate and E-cadherin binding sites

Botulinum neurotoxin (BoNT) inhibits neurotransmitter release in motor nerve endings, causing botulism, a condition often resulting from ingestion of the toxin or toxin-producing bacteria. BoNTs are always produced as large protein complexes by associating with a non-toxic protein, non-toxic non-hemagglutinin (NTNH), and some toxin complexes contain another non-toxic protein, hemagglutinin (HA), in addition to NTNH. These accessory proteins are known to increase the oral toxicity of the toxin dramatically. NTNH has a protective role against the harsh conditions in the digestive tract, while HA is considered to facilitate intestinal absorption of the toxin by intestinal binding and disruption of the epithelial barrier. Two specific activities of HA, carbohydrate and E-cadherin binding, appear to be involved in these processes; however, the exact roles of these activities in the pathogenesis of botulism remain unclear. The toxin is conventionally divided into seven serotypes, designated A through G. In this study, we identified the amino acid residues critical for carbohydrate and E-cadherin binding in serotype B HA. We constructed mutants defective in each of these two activities and examined the relationship of these activities using an in vitro intestinal cell culture model. Our results show that the carbohydrate and E-cadherin binding activities are functionally and structurally independent. Carbohydrate binding potentiates the epithelial barrier-disrupting activity by enhancing cell surface binding, while E-cadherin binding is essential for the barrier disruption.

Type A botulinum neurotoxin complex proteins differentially modulate host response of neuronal cells

Type A Botulinum neurotoxin (BoNT/A), the most potent poison known to mankind, is produced by Clostridium botulinum type A as a complex with neurotoxin-associated proteins (NAPs). Currently BoNT/A in purified and complex forms are both available in therapeutic and cosmetic applications to treat neuromuscular disorders. Whereas Xeomin(®) (incobotulinumtoxin A, Merz Pharmaceuticals, Germany) is free from complexing proteins, Botox(®) (onabotulinumtoxin A, Allergan, USA) contains NAPs, which by themselves have no known role in the intracellular biochemical process involved in the blockade of neurotransmitter release. Since the fate and possible interactions of NAPs with patient tissues after intramuscular injection are not known, it was the aim of this study to evaluate the binding of BoNT/A and/or the respective NAPs to cells derived from neuronal and non-neuronal human tissues, and to further explore neuronal cell responses to different components of BoNT/A. BoNT/A alone, the complete BoNT/A complex, and the NAPs alone, all bind to neuronal SH-SY5Y cells. The BoNT/A complex and NAPs additionally bind to RMS13 skeletal muscle cells, TIB-152 lymphoblasts, Detroit 551 fibroblasts besides the SH-SY5Y cells. However, no binding to these non-neuronal cells was observed with pure BoNT/A. Although BoNT/A, both in its purified and complex forms, bind to SH-SY5Y, the intracellular responses of the SH-SY5Y cells to these BoNT/A components are not clearly understood. Examination of inflammatory cytokine released from SH-SY5Y cells revealed that BoNT/A did not increase the release of inflammatory cytokines, whereas exposure to NAPs significantly increased release of IL-6, and MCP-1, and exposure to BoNT/A complex significantly increased release of IL-6, MCP-1, IL-8, TNF-α, and RANTES vs. control, suggesting that different components of BoNT/A complex induce significantly differential host response in human neuronal cells. Results suggest that host response to different compositions of BoNT/A based therapeutics may play important role in local and systemic symptoms in patients.

Crystallization and preliminary X-ray analysis of a novel haemagglutinin component of the toxin complex of serotype C Clostridium botulinum

The botulinum toxin complex, the causative agent of botulism, passes through the intestinal wall via sugar-chain-dependent cell binding of a haemagglutinin of 33 kDa molecular weight (HA-33). The amino-acid sequence of the C-terminal half of HA-33 of the serotype C strain Yoichi (C-Yoichi) shares only 46% identity with those of the major serotype C strains. Additionally, C-Yoichi HA-33 exhibits a unique sugar-binding specificity. In the present work, C-Yoichi HA-33 was expressed in Escherichia coli and crystallized. Diffraction data were collected at a resolution of 2.2 Å. The crystals belonged to space group R3. The complete detailed protein structure will yield insight into how the unique HA-33 protein recognizes sugar moieties.

Crystal structure of Clostridium botulinum whole hemagglutinin reveals a huge triskelion-shaped molecular complex

Clostridium botulinum HA is a component of the large botulinum neurotoxin complex and is critical for its oral toxicity. HA plays multiple roles in toxin penetration in the gastrointestinal tract, including protection from the digestive environment, binding to the intestinal mucosal surface, and disruption of the epithelial barrier. At least two properties of HA contribute to these roles: the sugar-binding activity and the barrier-disrupting activity that depends on E-cadherin binding of HA. HA consists of three different proteins, HA1, HA2, and HA3, whose structures have been partially solved and are made up mainly of β-strands. Here, we demonstrate structural and functional reconstitution of whole HA and present the complete structure of HA of serotype B determined by x-ray crystallography at 3.5 Å resolution. This structure reveals whole HA to be a huge triskelion-shaped molecule. Our results suggest that whole HA is functionally and structurally separable into two parts: HA1, involved in recognition of cell-surface carbohydrates, and HA2-HA3, involved in paracellular barrier disruption by E-cadherin binding.

Structure of a bimodular botulinum neurotoxin complex provides insights into its oral toxicity

Botulinum neurotoxins (BoNTs) are produced by Clostridium botulinum and cause the fatal disease botulism, a flaccid paralysis of the muscle. BoNTs are released together with several auxiliary proteins as progenitor toxin complexes (PTCs) to become highly potent oral poisons. Here, we report the structure of a ∼760 kDa 14-subunit large PTC of serotype A (L-PTC/A) and reveal insight into its absorption mechanism. Using a combination of X-ray crystallography, electron microscopy, and functional studies, we found that L-PTC/A consists of two structurally and functionally independent sub-complexes. A hetero-dimeric 290 kDa complex protects BoNT, while a hetero-dodecameric 470 kDa complex facilitates its absorption in the harsh environment of the gastrointestinal tract. BoNT absorption is mediated by nine glycan-binding sites on the dodecameric sub-complex that forms multivalent interactions with carbohydrate receptors on intestinal epithelial cells. We identified monosaccharides that blocked oral BoNT intoxication in mice, which suggests a new strategy for the development of preventive countermeasures for BoNTs based on carbohydrate receptor mimicry.

Food-borne botulinum neurotoxin (BoNT) poisoning results in fatal muscle paralysis. But how can BoNT–a large protein released by the bacteria clostridia–survive the hostile gastrointestinal (GI) tract to gain access to neurons that control muscle contraction? Here, we report the complete structure of a bimodular ∼760 kDa BoNT/A large progenitor toxin complex (L-PTC), which is composed of BoNT and four non-toxic bacterial proteins. The architecture of this bacterial machinery mimics an Apollo lunar module, whereby the “ascent stage” (a ∼290 kDa module) protects BoNT from destruction in the GI tract and the 3-arm “descent stage” (a ∼470 kDa module) mediates absorption of BoNT by binding to host carbohydrate receptors in the small intestine. This new finding has helped us identify the carbohydrate-binding sites and the monosaccharide IPTG as a prototypical oral inhibitor, which extends survival following lethal BoNT/A intoxication of mice. Hence, pre-treatment with small molecule inhibitors based on carbohydrate receptor mimicry can provide temporary protection against BoNT entry into the circulation.

Molecular assembly of botulinum neurotoxin progenitor complexes

Botulinum neurotoxin (BoNT) is produced by Clostridium botulinum and associates with nontoxic neurotoxin-associated proteins to form high-molecular weight progenitor complexes (PCs). The PCs are required for the oral toxicity of BoNT in the context of food-borne botulism and are thought to protect BoNT from destruction in the gastrointestinal tract and aid in absorption from the gut lumen. The PC can differ in size and protein content depending on the C. botulinum strain. The oral toxicity of the BoNT PC increases as the size of the PC increases, but the molecular architecture of these large complexes and how they contribute to BoNT toxicity have not been elucidated. We have generated 2D images of PCs from strains producing BoNT serotypes A1, B, and E using negative stain electron microscopy and single-particle averaging. The BoNT/A1 and BoNT/B PCs were observed as ovoid-shaped bodies with three appendages, whereas the BoNT/E PC was observed as an ovoid body. Both the BoNT/A1 and BoNT/B PCs showed significant flexibility, and the BoNT/B PC was documented as a heterogeneous population of assembly/disassembly intermediates. We have also determined 3D structures for each serotype using the random conical tilt approach. Crystal structures of the individual proteins were placed into the BoNT/A1 and BoNT/B PC electron density maps to generate unique detailed models of the BoNT PCs. The structures highlight an effective platform that can be engineered for the development of mucosal vaccines and the intestinal absorption of oral biologics.

Analysis of the mechanisms that underlie absorption of botulinum toxin by the inhalation route

Botulinum toxin is a highly potent oral and inhalation poison, which means that the toxin must have an efficient mechanism for penetration of epithelial barriers. To date, three models for toxin passage across epithelial barriers have been proposed: (i) the toxin itself undergoes binding and transcytosis; (ii) an auxiliary protein, HA35, transports toxin from the apical to the basal side of epithelial cells; and (iii) an auxiliary protein, HA35, acts on the basal side of epithelial cells to disrupt tight junctions, and this permits paracellular flux of toxin. These models were evaluated by studying toxin absorption following inhalation exposure in mice. Three types of experiments were conducted. In the first, the potency of pure neurotoxin was compared with that of progenitor toxin complex, which contains HA35. The results showed that the rate and extent of toxin absorption, as well as the potency of absorbed toxin, did not depend upon, nor were they enhanced by, the presence of HA35. In the second type of experiment, the potencies of pure neurotoxin and progenitor toxin complex were compared in the absence or presence of antibodies on the apical side of epithelial cells. Antibodies directed against the neurotoxin protected against challenge, but antibodies against HA35 did not. In the final type of experiment, the potency of pure neurotoxin and toxin complex was compared in animals pretreated to deliver antibodies to the basal side of epithelial cells. Once again, antibodies directed against the neurotoxin provided resistance to challenge, but antibodies directed against HA35 did not. Taken collectively, the data indicate that the toxin by itself is capable of crossing epithelial barriers. The data do not support any hypothesis in which HA35 is essential for toxin penetration of epithelial barriers.

Two-component systems are involved in the regulation of botulinum neurotoxin synthesis in Clostridium botulinum type A strain Hall

Clostridium botulinum synthesizes a potent neurotoxin (BoNT) which associates with non-toxic proteins (ANTPs) to form complexes of various sizes. The bont and antp genes are clustered in two operons. In C. botulinum type A, bont/A and antp genes are expressed during the end of the exponential growth phase and the beginning of the stationary phase under the control of an alternative sigma factor encoded by botR/A, which is located between the two operons. In the genome of C. botulinum type A strain Hall, 30 gene pairs predicted to encode two-component systems (TCSs) and 9 orphan regulatory genes have been identified. Therefore, 34 Hall isogenic antisense strains on predicted regulatory genes (29 TCSs and 5 orphan regulatory genes) have been obtained by a mRNA antisense procedure. Two TCS isogenic antisense strains showed more rapid growth kinetics and reduced BoNT/A production than the control strain, as well as increased bacterial lysis and impairment of the bacterial cell wall structure. Three other TCS isogenic antisense strains induced a low level of BoNT/A and ANTP production. Interestingly, reduced expression of bont/A and antp genes was shown to be independent of botR/A. These results indicate that BoNT/A synthesis is under the control of a complex network of regulation including directly at least three TCSs.

Preferential entry of botulinum neurotoxin A Hc domain through intestinal crypt cells and targeting to cholinergic neurons of the mouse intestine

Botulism, characterized by flaccid paralysis, commonly results from botulinum neurotoxin (BoNT) absorption across the epithelial barrier from the digestive tract and then dissemination through the blood circulation to target autonomic and motor nerve terminals. The trafficking pathway of BoNT/A passage through the intestinal barrier is not yet fully understood. We report that intralumenal administration of purified BoNT/A into mouse ileum segment impaired spontaneous muscle contractions and abolished the smooth muscle contractions evoked by electric field stimulation. Entry of BoNT/A into the mouse upper small intestine was monitored with fluorescent HcA (half C-terminal domain of heavy chain) which interacts with cell surface receptor(s). We show that HcA preferentially recognizes a subset of neuroendocrine intestinal crypt cells, which probably represent the entry site of the toxin through the intestinal barrier, then targets specific neurons in the submucosa and later (90–120 min) in the musculosa. HcA mainly binds to certain cholinergic neurons of both submucosal and myenteric plexuses, but also recognizes, although to a lower extent, other neuronal cells including glutamatergic and serotoninergic neurons in the submucosa. Intestinal cholinergic neuron targeting by HcA could account for the inhibition of intestinal peristaltism and secretion observed in botulism, but the consequences of the targeting to non-cholinergic neurons remains to be determined.

Botulism is a severe and often fatal disease in man and animals characterized by flaccid paralysis. Clostridium botulinum produces a potent neurotoxin (botulinum neurotoxin) responsible for all the symptoms of botulism. Botulism is most often acquired by ingesting preformed botulinum neurotoxin in contaminated food or after intestinal colonization by C. botulinum under certain circumstances, such as in infant botulism, and toxin production in the intestine. The first step of the disease consists in the passage of the botulinum neurotoxin through the intestinal barrier, which is still poorly understood. We investigated the trafficking of the botulinum neurotoxin in a mouse intestinal loop model, using fluorescent HcA (half C-terminal domain of the heavy chain). We observed that HcA preferentially recognizes neuroendocrine intestinal crypt cells, which likely represent the entry site of the toxin through the intestinal barrier, then targets specific neurons, mainly cholinergic neurons, in the submucosa, and later (90–120 min) in the musculosa leading to local paralytic effects such as inhibition of intestinal peristaltism. These results represent an important advance in the understanding of the initial steps of botulism intoxication and can be the basis for the development of new specific countermeasures against botulism.

Molecular diversity of the two sugar-binding sites of the β-trefoil lectin HA33/C (HA1) from Clostridium botulinum type C neurotoxin

A critical role in internalizing the Clostridium botulinum neurotoxin into gastrointestinal cells is played by nontoxic components complexed with the toxin. One of the components, a β-trefoil lectin has been known as HA33 or HA1. The HA33 from C. botulinum type A (HA33/A) has been predicted to have a single sugar-binding site, while type C HA33 (HA33/C) has two sites. Here we constructed HA33/C mutants and evaluated the binding capacities of the individual sites through mucin-assay and isothermal titration calorimetry. The mutant W176A (site I knockout) had a K(d) value of 31.5mM for galactose (Gal) and 61.3mM for N-acetylgalactosamine (GalNAc), while the K(d) value for N-acetylneuraminic acid (Neu5Ac) was too high to be determined. In contrast, the double mutant N278A/Q279A (site II knockout) had a K(d) value of 11.8mM for Neu5Ac. We also determined the crystal structures of wild-type and the F179I mutant in complex with GalNAc at site II. The results suggest that site I of HA33/C is quite unique in that it mainly recognizes Neu5Ac, and site II seems less important for the lectin specificity. The architectures and the properties of the sugar-binding sites of HA33/C and HA33/A were shown to be drastically different.

Microarray analysis of differentially regulated genes in human neuronal and epithelial cell lines upon exposure to type A botulinum neurotoxin

Among the seven serotypes (A-G), type A botulinum neurotoxin (BoNT/A) is the most prevalent etiologic agent and the most potent serotype to cause foodborne botulism, characterized by flaccid muscle paralysis. Upon ingestion, BoNT/A crosses epithelial cell barriers to reach lymphatic and circulatory systems and blocks acetylcholine release at the pre-synaptic cholinergic nerve terminals of neuromuscular junctions (NMJs) resulting in paralysis. One of the unique features of BoNT/A intoxication is its neuroparalytic longevity due to its persistent catalytic activity. The persistent presence of the toxin inside the cell can induce host cell responses. To understand the pathophysiology and host response at the cellular level, gene expression changes upon exposure of human HT-29 colon carcinoma (epithelial) and SH-SY5Y neuroblastoma cell lines to BoNT/A complex were investigated using microarray analysis. In HT-29 cells, 167 genes were up-regulated while 60 genes were down-regulated, whereas in SH-SY5Y cells about 223 genes were up-regulated and 18 genes were down-regulated. Modulation of genes and pathways involved in neuroinflammatory, ubiquitin-proteasome degradation, phosphatidylinositol, calcium signaling in SH-SY5Y cells, and genes relevant to focal adhesion, cell adhesion molecules, adherens and gap junction related pathways in HT-29 cells suggest a massive host response to BoNT/A. A clear differential response in epithelial and neuronal cells indicates that the genes affected may play a distinct role in BoNTs cellular mode of action, involving these two types of host cells.

Towards new uses of botulinum toxin as a novel therapeutic tool

The uses of botulinum toxin in the fields of neurology, ophthalmology, urology, rehabilitation medicine and aesthetic applications have been revolutionary for the treatment of patients. This non-invasive therapeutic has continually been developed since first discovered in the 1970s as a new approach to what were previously surgical treatments. As these applications develop, so also the molecules are developing into tools with new therapeutic properties in specific clinical areas. This review examines how the botulinum toxin molecule is being adapted to new therapeutic uses and also how new areas of use for the existing molecules are being identified. Prospects for future developments are also considered.

The botulinum toxin complex meets E-cadherin on the way to its destination

Botulinum neurotoxin (BoNT) causes the disease botulism, which is characterized by flaccid paralysis, in humans and animals. The metalloprotease activity of BoNT inhibits neurotransmitter release at neuro-muscular junctions. In most cases, poisoning occurs when BoNT is ingested. Therefore, BoNT must pass through the epithelial barrier of the gastrointestinal tract to enter the systemic circulation and reach the target site. BoNT forms large protein complexes by associating with non-toxic components referred to as non-toxic non-hemagglutinin (NTNH) and hemagglutinin (HA). These proteins protect BoNT from the low pH and proteases in the digestive tract. We recently determined that HA has an unexpected function of disrupting the intercellular epithelial barrier by directly binding to E-cadherin. HA binds to E-cadherin and disrupts its function in a species-specific manner, and this interaction is essential to disrupt tight junctions. This activity is thought to facilitate the absorption of BoNT through the paracellular route of the intestinal epithelium in susceptible species.

Laetiporus sulphureus lectin and aerolysin protein family

The parasitic mushroom Laetiporus sulphureus produces a family of lectins (LSL's) sharing 80-90% sequence identity that possesses a low but significant sequence similarity to the bacterial pore-forming toxins mosquitocidal toxin Mtx-2 from Bacillus sphaericus and a toxin from Clostridium septicum. The crystal structure of one member of the L. sulphureus lectins family (LSLa) reveals unexpected structural similarities to the 1-pore-forming toxins from the aerolysin family, namely, aerolysin from the Gram-negative bacterium Aeromonas hydrophila, epsilon-toxin from Clostridium perfringens and parasporin from B. thuringiensis. This similarity presumably indicates that the hemolytic activity of LSLa proceeds through a molecular mechanism that involves the formation of oligomeric transmembrane beta-barrels. Comparison of the crystal structures of the above mentioned proteins reveals common pore-forming modules, which are then distributed both in bacteria and fungi. Currently, it can be stated that the above three dimensional structures have been key in revealing structural similarities that were elusive at the sequence level. A potential corollary from this is that structural studies aimed at determining high resolution structures of aerolysin-like pore-forming toxins, whose biological activity involves large conformational changes, are mandatory to define protein domains or structural motifs with membrane-binding properties.

Interaction of botulinum toxin with the epithelial barrier

Botulinum neurotoxin (BoNT) is a protein toxin (approximately 150 kDa), which possesses a metalloprotease activity. Food-borne botulism is manifested when BoNT is absorbed from the digestive tract to the blood stream and enters the peripheral nerves, where the toxin cleaves core proteins of the neuroexocytosis apparatus and elicits the inhibition of neurotransmitter release. The initial obstacle to orally ingested BoNT entering the body is the epithelial barrier of the digestive tract. Recent cell biology and molecular biology studies are beginning to elucidate the mechanism by which this large protein toxin crosses the epithelial barrier. In this review, we provide an overview of the structural features of botulinum toxins (BoNT and BoNT complex) and the interaction of these toxins with the epithelial barrier.

Immunomodulatory activity of oenothein B isolated from Epilobium angustifolium

Epilobium angustifolium has been traditionally used to treat of a number of diseases; however, not much is known regarding its effect on innate immune cells. In this study, we report that extracts of E. angustifolium activated functional responses in neutrophils and monocyte/macrophages. Activity-guided fractionation, followed by mass spectroscopy and NMR analysis, resulted in the identification of oenothein B as the primary component responsible for phagocyte activation. Oenothein B, a dimeric hydrolysable tannin, dose-dependently induced a number of phagocyte functions in vitro, including intracellular Ca(2+) flux, production of reactive oxygen species, chemotaxis, NF-kappaB activation, and proinflammatory cytokine production. Furthermore, oenothein B was active in vivo, inducing keratinocyte chemoattractant production and neutrophil recruitment to the peritoneum after intraperitoneal administration. Biological activity required the full oenothein B structure, as substructures of oenothein B (pyrocatechol, gallic acid, pyrogallol, 3,4-dihydroxybenzoic acid) were all inactive. The ability of oenothein B to modulate phagocyte functions in vitro and in vivo suggests that this compound is responsible for at least part of the therapeutic properties of E. angustifolium extracts.

Sialic acid-dependent binding and transcytosis of serotype D botulinum neurotoxin and toxin complex in rat intestinal epithelial cells

A large toxin complex (L-TC) produced by Clostridium botulinum is composed of neurotoxin (BoNT), non-toxic non-hemagglutinin (NTNHA) and hemagglutinin subcomponents (HA-70, -33 and -17). In animal botulism, BoNT or L-TC is internalized by intestinal epithelial cells. Previous studies showed that L-TC binds to intestinal cells via sugar chains on the cell surface, but the role of toxin binding to sugar chains in the toxin absorption from intestine is unclear. To clarify whether the toxin binding to sugar chains on intestinal cell surface leads to its transcytosis across the cells, we examined binding and permeation of BoNT and L-TC of C. botulinum serotype D strain 4947 to the rat intestinal epithelial cell line IEC-6 in semi-permeable filters in Transwell systems. Both BoNT and L-TC bound to and permeated the cell monolayers, with L-TC showing greater binding and permeation. In addition, both binding and permeation of toxins were potently inhibited by N-acetyl neuraminic acid in the cell culture medium or by treatment of the cells with neuraminidase. However, neither galactose, lactose nor N-acetyl galactosamine inhibited binding or permeation of toxins. These results support the idea that permeation of both BoNT and L-TC through the intestinal cell layer depends on prior binding to sialic acid on the cell surface. This is the first report demonstrating that the binding of botulinum toxins to cell surface sialic acid leads to their transcytosis through intestinal epithelial cells.

Disruption of the epithelial barrier by botulinum haemagglutinin (HA) proteins – differences in cell tropism and the mechanism of action between HA proteins of types A or B, and HA proteins of type C

Orally ingested botulinum neurotoxin (BoNT) causes food-borne botulism, but BoNT must pass through the gut lining and enter the bloodstream. We have previously found that type B haemagglutinin (HA) proteins in the toxin complex play an important role in the intestinal absorption of BoNT by disrupting the paracellular barrier of the intestinal epithelium, and therefore facilitating the transepithelial delivery of BoNT. Here, we show that type A HA proteins in the toxin complex have a similar disruptive activity and a greater potency than type B HA proteins in the human intestinal epithelial cell lines Caco-2 and T84 and in the canine kidney epithelial cell line MDCK I. In contrast, type C HA proteins in the toxin complex (up to 300 nM) have no detectable effect on the paracellular barrier in these human cell lines, but do show a barrier-disrupting activity and potent cytotoxicity in MDCK I. These findings may indicate that type A and B HA proteins contribute to the development of food-borne botulism, at least in humans, by facilitating the intestinal transepithelial delivery of BoNTs, and that the relative inability of type C HA proteins to disrupt the paracellular barrier of the human intestinal epithelium is one of the reasons for the relative absence of food-borne human botulism caused by type C BoNT.

Molecular properties of each subcomponent in Clostridium botulinum type B haemagglutinin complex

The role of each subcomponent of Clostridium botulinum serotype B haemagglutinin (HA), which is one component of 16S toxin, and consists of four subcomponents (HA1, 2, 3a, and 3b), was investigated. In order to identify the subcomponent contributing to the stability of a neurotoxin in the gastro-intestinal tract, each recombinant HA (rHA) subcomponent was incubated with gastro-intestinal proteases. Although rHA1 and rHA3 were stable to these proteases except for specific cleavage, rHA2 was not. Anti-free whole HA serum reacted with neither rHA2 nor HA2 in 16S toxin on both Western blot and ELISA, while anti-rHA2 serum reacted with both rHA2 and HA2 in 16S toxin on Western blots, although it did not react with 16S toxin in ELISA. Binding or haemagglutination activity against erythrocytes was found in rHA1 and rHA3, but not in rHA2. In addition, only HA1 bound to the intestinal section. These results indicate that the HA (and 16S toxin) complex is assembled in the way that HA1 and HA3 (HA3a plus HA3b) encase HA2, followed by modification with trypsin-like bacterial protease, leading to the conclusion that HA1 and HA3 act as protective factors for the neurotoxin and as attachment factors to host cells.

Development and partial characterization of high-affinity monoclonal antibodies for botulinum toxin type A and their use in analysis of milk by sandwich ELISA

Botulinum neurotoxins (BoNT), produced by the anaerobic bacterium Clostridium botulinum, cause severe neuroparalytic disease and are considered the most toxic biological agents known. While botulism is rare in the U.S. it often is fatal if not treated quickly, and recovery is long, requiring intensive treatment. BoNT is synthesized as a 150 kDa precursor protein (holotoxin), which is then enzymatically cleaved to form two subunit chains linked by a single disulfide bond. The 'gold standard' for BoNT detection relies on a mouse bioassay. This is a time consuming (up to 4 days) assay and it lacks specificity, however, it gives a sensitivity (mouse LD(50)) of approximately 10 pg mL(-1). Most BoNT immunoassays are much less sensitive. In this study we describe the development of four high-affinity (dissociation constants (Kd's) in the low pM range) monoclonal antibodies (mAbs) that specifically bind BoNT serotype A (BoNT/A). These antibodies, designated F1-2, F1-5, F1-40, and F2-43 are IgG1 subclass mAbs with kappa light chains and they specifically bind BoNT serotype A. Western blot analyses following SDS-PAGE demonstrate that mAbs F1-2 and F1-5 bind the 100 kDa heavy chain subunit and that mAb F1-40 binds the 50 kDa light chain. The fourth antibody demonstrated strong binding to the 150 kDa holotoxin in the ELISA and on Western blots following electrophoresis on native gels. However binding in Western blot studies was not observed for mAb F2-43 following SDS-PAGE. A highly sensitive sandwich ELISA, capable of detecting as little as 2 pg/mL BoNT/A was developed using mAbs F1-2 and F1-40. Such an assay represents a realistic, high sensitivity alternative to the mouse bioassay.

Presynaptic neurotoxins with enzymatic activities

Toxins that alter neurotransmitter release from nerve terminals are of considerable scientific and clinical importance. Many advances were recently made in the understanding of their molecular mechanisms of action and use in human therapy. Here, we focus on presynaptic neurotoxins, which are very potent inhibitors of the neurotransmitter release because they are endowed with specific enzymatic activities: (1) clostridial neurotoxins with a metallo-proteolytic activity and (2) snake presynaptic neurotoxins with a phospholipase A2 activity.

Sugar-binding sites of the HA1 subcomponent of Clostridium botulinum type C progenitor toxin

Clostridium botulinum type C 16S progenitor toxin contains a hemagglutinin (HA) subcomponent, designated HA1, which appears to play an important role in the effective internalization of the toxin in gastrointestinal epithelial cells and in creating a broad specificity for the oligosaccharide structure that corresponds to various targets. In this study, using the recombinant protein fused to glutathione S-transferase, we investigated the binding specificity of the HA1 subcomponent to sugars and estimated the binding sites of HA1 based on X-ray crystallography and soaking experiments using various sugars. N-Acetylneuraminic acid, N-acetylgalactosamine, and galactose effectively inhibited the binding that occurs between glutathione S-transferase-HA1 and mucins, whereas N-acetylglucosamine and glucose did not inhibit it. The crystal structures of HA1 complex with N-acetylneuraminic acid, N-acetylgalactosamine, and galactose were also determined. There are two sugar-binding sites, sites I and II. Site I corresponds to the electron densities noted for all sugars and is located at the C-terminal beta-trefoil domain, while site II corresponds to the electron densities noted only for galactose. An aromatic amino acid residue, Trp176, at site I has a stacking interaction with the hexose ring of the sugars. On the other hand, there is no aromatic residue at site II; thus, the interaction with galactose seems to be poor. The double mutant W176A at site I and D271F at site II has no avidity for N-acetylneuraminic acid but has avidity for galactose. In this report, the binding specificity of botulinum C16S toxin HA1 to various sugars is demonstrated based on its structural features.

Receptor-mediated transcytosis of botulinum neurotoxin A through intestinal cell monolayers

Botulism is mainly acquired by the oral route, and botulinum neurotoxin (BoNT) escapes the gastrointestinal tract by crossing the digestive epithelial barrier prior to gaining access to the nerve endings. Here, we show that biologically active BoNT/A crosses intestinal cell monolayers via a receptor-mediated transcytosis, including a transport inhibition at 4 degrees C and a passage at 37 degrees C in a saturable manner within 30-60 min. BoNT/A passage rate was about 10-fold more efficient through the intestinal crypt cell line m-IC(cl2), than through the carcinoma Caco-2 or T84 cells, and was not increased when BoNT/A was associated with the non-toxic proteins (botulinum complex). Like for neuronal cells, BoNT/A binding to intestinal cells was mediated by the half C-terminal domain as tested by fluorescence-activated cytometry and by transcytosis competition assay. A 'double receptor model' has been proposed in which BoNT/A interacts with gangliosides of GD(1b) and GT(1b) series as well as SV2 protein. Gangliosides of GD(1b) and GT(1b) series and recombinant intravesicular SV2-C domain partially impaired BoNT/A transcytosis, suggesting a putative role of gangliosides and SV2 or a related protein in BoNT/A transcytosis through Caco-2 and m-IC(cl2) cells.

Role of nontoxic components of serotype D botulinum toxin complex in permeation through a Caco-2 cell monolayer, a model for intestinal epithelium

Botulinum neurotoxin (BoNT) is produced as a large toxin complex (TC) associated with nontoxic nonhemagglutinin (NTNHA) and three hemagglutinin subcomponents (HA-70, -33 and -17). To assess the role of nontoxic components in the oral intoxication of botulinum TCs, we investigated the permeability of serotype D strain 4947 BoNT and its various TC species through cultured Caco-2 cell monolayers. The L-TC species (complexes composed of BoNT, NTNHA, HA-70, HA-33 and HA-17) showed potent permeability through the cell layer, whereas free BoNT, M-TC (BoNT and NTNHA complexes) and M-TC/HA-70 showed little or no permeability. Cell binding tests demonstrated that HA-33/HA-17 complexes bound to cells, whereas other components did not. These findings suggest that BoNT in the 650-kDa L-TC permeates into the cell mainly in an HA-33/HA-17-mediated manner, although free BoNT can permeate into the cell. As free BoNT and M-TC were susceptible to digestion with gastrointestinal juice, it is likely that L-TC species containing HA-33 caused higher oral toxicity in mice than others. We conclude that the HA-33 subcomponent plays a critical role in the permeation of TCs into intestinal epithelium, and that other HA subcomponents protect BoNT against gastrointestinal digestion.

Efficacy and safety of botulinum neurotoxin type A free of complexing proteins (NT 201) in cervical dystonia and blepharospasm

Botulinum neurotoxin type A (BTX-A) weakens voluntary muscle power and is an effective therapy for focal dystonia, including cervical dystonia (CD) and benign essential blepharospasm (BEB). Antibody formation against BTX-A may lead to failure of treatment. NT 201 (XEOMIN®) is a formulation of pure BTX-A that is free of complexing proteins and therefore may have a reduced immunogenic potential in comparison with other BTX-A preparations. In clinical trials, NT 201 was found to be effective, well-tolerated and at least noninferior to a BTX-A preparation containing complexing proteins in the treatment of CD and BEB. There were no differences between the two therapies in terms of onset of action, duration and waning of effect. Future research is aimed at evaluating the benefits of NT 201.

A Novel Subunit Structure of Clostridium botulinum Serotype D Toxin Complex with Three Extended Arms

The botulinum neurotoxins (BoNTs) are the most potent toxins known in nature, causing the lethal disease known as botulism in humans and animals. The BoNTs act by inhibiting neurotransmitter release from cholinergic synapses. Clostridium botulinum strains produce large BoNTs toxin complexes, which include auxiliary non-toxic proteins that appear not only to protect BoNTs from the hostile environment of the digestive tract but also to assist BoNT translocation across the intestinal mucosal layer. In this study, we visualize for the first time a series of botulinum serotype D toxin complexes using negative stain transmission electron microscopy (TEM). The complexes consist of the 150-kDa BoNT, 130-kDa non-toxic non-hemagglutinin (NTNHA), and three kinds of hemagglutinin (HA) subcomponents: 70-kDa HA-70, 33-kDa HA-33, and 17-kDa HA-17. These components assemble sequentially to form the complex. A novel TEM image of the mature L-TC revealed an ellipsoidal-shaped structure with "three arms" attached. The "body" section was comprised of a single BoNT, a single NTNHA and three HA-70 molecules. The arm section consisted of a complex of HA-33 and HA-17 molecules. We determined the x-ray crystal structure of the complex formed by two HA-33 plus one HA-17. On the basis of the TEM image and biochemical results, we propose a novel 14-mer subunit model for the botulinum toxin complex. This unique model suggests how non-toxic components make up a "delivery vehicle" for BoNT.