Intestinal absorption of botulinum toxins of different molecular sizes in rats

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.

Translocation and dissemination of botulinum neurotoxin from the intestinal tract

Botulinum neurotoxins (BoNTs) are potent toxins which induce flaccid paralysis by inhibiting the release of acetylcholine at the neuromuscular junctions. They associate with non-toxic proteins (ANTPs or NAPs) to form complexes of various sizes which are resistant to acidic pH and protease degradation. BoNT trafficking from the digestive tract to the target neurons is still a matter of debate. BoNTs use different strategies to pass through the intestinal barrier including passage of BoNT complexes containing hemagglutinins (HAs) via M cells, HA-dependent perturbation of E-cadherin intercellular junctions between enterocytes and paracellular passage of BoNT complexes, and transcytosis of BoNT free of NAPs through certain intestinal epithelial cells. Then, BoNTs target neuronal cells, preferentially cholinergic neurons, in the intestinal mucosa and submucosa. The precise mode of BoNT dissemination until the final target neuro-muscular junctions is still elusive.

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.

Data describing inhibitory profiles of sugars against hemagglutination by the botulinum toxin complex of Clostridium botulinum serotypes C and D

Serotype C and D of Clostridium botulinum produce botulinum toxin complex (TC), which is comprised of botulinum neurotoxin, nontoxic nonhemagglutinin, and hemagglutinins (HAs). The TC is capable of aggregating equine erythrocytes via interaction between one of the HAs, namely HA-33, and sugar chains on the cell surface. This hemagglutination is inhibited by specific sugars. In this data article, we used four TCs from serotype C and D strains. The hemagglutination-inhibiting effects of 18 sugars and 8 glycoproteins were studied. The purified TC was mixed with the sugar to enable binding of the sugar to the TC; then, the erythrocytes were added to the mixture. Specific binding between the sugar and TC resulted in inhibition of cell aggregation. Here, data illustrating the inhibitory effects of various sugars and glycoproteins against hemagglutination induced by TC of C. botulinum serotypes C and D are presented.

Immunoprecipitation of native botulinum neurotoxin complexes from Clostridium botulinum subtype A strains

Botulinum neurotoxins (BoNTs) naturally exist as components of protein complexes containing nontoxic proteins. The nontoxic proteins impart stability of BoNTs in the gastrointestinal tract and during purification and handling. The two primary neurotoxin complexes (TCs) are (i) TC1, consisting of BoNT, nontoxin-nonhemagglutinin (NTNH), and hemagglutinins (HAs), and (ii) TC2, consisting of BoNT and NTNH (and possibly OrfX proteins). In this study, BoNT/A subtypes A1, A2, A3, and A5 were examined for the compositions of their TCs in culture extracts using immunoprecipitation (IP). IP analyses showed that BoNT/A1 and BoNT/A5 form TC1s, while BoNT/A2 and BoNT/A3 form TC2s. A Clostridium botulinum host strain expressing recombinant BoNT/A4 (normally present as a TC2) from an extrachromosomal plasmid formed a TC1 with complexing proteins from the host strain, indicating that the HAs and NTNH encoded on the chromosome associated with the plasmid-encoded BoNT/A4. Strain NCTC 2916 (A1/silent B1), which carries both an ha silent bont/b cluster and an orfX bont/a1 cluster, was also examined. IP analysis revealed that NCTC 2916 formed only a TC2 containing BoNT/A1 and its associated NTNH. No association between BoNT/A1 and the nontoxic proteins from the silent bont/b cluster was detected, although the HAs were expressed as determined by Western blotting analysis. Additionally, NTNH and HAs from the silent bont/b cluster did not form a complex in NCTC 2916. The stabilities of the two types of TC differed at various pHs and with addition of KCl and NaCl. TC1 complexes were more stable than TC2 complexes. Mouse serum stabilized TC2, while TC1 was unaffected.

Botulinum toxin complex increases paracellular permeability in intestinal epithelial cells via activation of p38 mitogen-activated protein kinase

Clostridium botulinum produces a large toxin complex (L-TC) that increases paracellular permeability in intestinal epithelial cells by a mechanism that remains unclear. Here, we show that mitogen-activated protein kinases (MAPKs) are involved in this permeability increase. Paracellular permeability was measured by FITC-dextran flux through a monolayer of rat intestinal epithelial IEC-6 cells, and MAPK activation was estimated from western blots. L-TC of C. botulinum serotype D strain 4947 increased paracellular dextran flux and activated extracellular signal-regulated kinase (ERK), p38, but not c-Jun N-terminal kinase (JNK) in IEC-6 cells. The permeability increase induced by L-TC was abrogated by the p38 inhibitor SB203580. These results indicate that L-TC increases paracellular permeability by activating p38, but not JNK and ERK.

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.

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.

HA-33 facilitates transport of the serotype D botulinum toxin across a rat intestinal epithelial cell monolayer

A large size botulinum toxin complex (L-TC) is composed of a single neurotoxin (BoNT), a single nontoxic nonhaemagglutinin (NTNHA) and a haemagglutinin (HA) complex. The HA complex is comprised of three HA-70 molecules and three arm structures of HA-33/HA-17 that consist of two HA-33 and a single HA-17. In addition to the mature L-TC, smaller TCs are present in cultures: M-TC (BoNT/NTNHA), M-TC/HA-70 and immature L-TCs with fewer HA-33/HA-17 arms than mature L-TC. Because L-TC displays higher oral toxicity than pure BoNT, it was presumed that nontoxic proteins are critical for food poisoning. In this study, the absorption of TCs across intestinal epithelial cells was assessed by examining the cell binding and monolayer transport of serotype D toxins in the rat intestinal epithelial cell line IEC-6. All TCs, including pure BoNT, displayed binding and transport, with mature L-TC showing the greatest potency. Inhibition experiments using antibodies revealed that BoNT, HA-70 and HA-33 could be responsible for the binding and transport. The findings here indicate that all TCs can transport across the cell layer via a sialic acid-dependent process. Nonetheless, binding and transport markedly increased with number of HA-33/HA-17 arms in the TC. We therefore conclude that the HA-33/HA-17 arm is not necessarily required for, but facilitates, transport of botulinum toxin complexes.

Endopeptidase activities of botulinum neurotoxin type B complex, holotoxin, and light chain

Botulinum neurotoxin (BoNT) serotype B (BoNT/B) is one of the serotypes of BoNT that causes deadly human botulism, though it is used clinically for treatment of many neuromuscular diseases. BoNT/B is produced by Clostridium botulinum, and it is secreted along with a group of neurotoxin-associated proteins (NAPs) in the form of a BoNT/B complex. The complex dissociates into a 150-kDa holotoxin and NAPs at alkaline pHs. The 150-kDa BoNT/B holotoxin can be nicked to produce a 50-kDa domain referred to as the light chain (LC) and a 100-kDa heavy chain, with the former possessing a unique endopeptidase activity. The two chains remain linked through a disulfide bond that can be reduced to separate the two chains. The endopeptidase activity is present in all three forms of the toxin (complex, purified BoNT/B holotoxin, and separated light chain), which are used by different researchers to develop detection methods and screen for inhibitors. In this research, the endopeptidase activities of the three forms, for the first time, were compared under the same conditions. The results show that enzyme activities of the three forms differ significantly and are largely dependent on nicking and disulfide reduction conditions. Under the conditions used, LC had the highest level of activity, and the complex had the lowest. The activity was enhanced by nicking of BoNT/B holotoxin and was enhanced even more by dithiothreitol (DTT) reduction after nicking. This information is useful for understanding the properties of BoNT endopeptidases and for comparing the efficacies of different inhibitors when they are tested with different forms of BoNT endopeptidase.

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.

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.

Expression and stability of the nontoxic component of the botulinum toxin complex

Clostridium botulinum produces botulinum neurotoxin (BoNT) as a large toxin complex associated with nontoxic-nonhemagglutinin (NTNHA) and/or hemagglutinin components. In the present study, high-level expression of full-length (1197 amino acids) rNTNHA from C. botulinum serotype D strain 4947 (D-4947) was achieved in an Escherichia coli system. Spontaneous nicking of the rNTNHA at a specific site was observed during long-term incubation in the presence of protease inhibitors; this was also observed in natural NTNHA. The rNTNHA assembled with isolated D-4947 BoNT with molar ratio 1:1 to form a toxin complex. The reconstituted toxin complex exhibited dramatic resistance to proteolysis by pepsin or trypsin at high concentrations, despite the fact that the isolated BoNT and rNTNHA proteins were both easily degraded. We provide definitive evidence that NTNHA plays a crucial role in protecting BoNT, which is an oral toxin, from digestion by proteases common in the stomach and intestine.

Clostridium botulinum serotype D neurotoxin and toxin complex bind to bovine aortic endothelial cells via sialic acid

Botulinum neurotoxin (BoNT) is produced as a large toxin complex (L-TC) associated with nontoxic nonhemagglutinin (NTNHA) and three hemagglutinin subcomponents (HA-70, -33 and -17). The binding properties of BoNT to neurons and L-TC to intestinal epithelial cells are well documented, while those to other tissues are largely unknown. Here, to obtain novel insights into the pathogenesis of foodborne botulism, we examine whether botulinum toxins bind to vascular endothelial cells. BoNT and 750 kDa L-TC (a complex of BoNT, NTNHA and HAs) of Clostridium botulinum serotype D were incubated with bovine aortic endothelial cells (BAECs), and binding to the cells was assessed using sodium dodecyl sulfate polyacrylamide gel electrophoresis and Western blot. Both BoNT and L-TC bound to BAECs, with L-TC showing stronger binding. Binding of BoNT and L-TC to BAECs was significantly inhibited by N-acetyl neuraminic acid in the cell culture medium or by treatment of the cells with neuraminidase. However, galactose, lactose or N-acetyl galactosamine did not significantly inhibit toxin binding to the cells. This is the first report demonstrating that BoNT and L-TC bind to BAECs via sialic acid, and this mechanism may be important in the trafficking pathway of BoNT in foodborne botulism.

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.

Mucosal vaccine development for botulinum intoxication

Botulism has classically been considered to be a food- and water-borne disease. However, it was recently classified by the US National Institute of Allergy and Infectious Diseases (National Institute of Health) and the US Centers for Disease Control and Prevention as a Category A agent. Thus, the botulinum exotoxin, a neurotoxin, could be easily disseminated by bioterrorists through the air-borne route with a high morbidity and mortality rate. In this regard, a high priority should be given to the development of a safe and effective mucosal vaccine to protect against botulinum neurotoxins (BoNTs) since it is well known that the mucosal immune system is the first line of defense against major pathogens. Further, mucosal immunization has been shown to induce both mucosal and systemic immunity to pathogens. By contrast, the current injection-type vaccine only provides protective immunity in the systemic compartment. Clearly, the development of a safe and effective mucosal vaccine against this toxin should be a high priority. In this regard, it has been shown that both nasal and oral immunization approaches have been taken in order to protect from BoNT intoxication. In this article, we will discuss the importance of the development of a mucosal vaccine against botulinum and introduce current aspects of BoNT mucosal vaccines, which show that they effectively prevent mucosal BoNT intoxication.

Detection of type A, B, E, and F Clostridium botulinum neurotoxins in foods by using an amplified enzyme-linked immunosorbent assay with digoxigenin-labeled antibodies

An amplified enzyme-linked immunosorbent assay (ELISA) for the detection of Clostridium botulinum complex neurotoxins was evaluated for its ability to detect these toxins in food. The assay was found to be suitable for detecting type A, B, E, and F botulinum neurotoxins in a variety of food matrices representing liquids, solid, and semisolid food. Specific foods included broccoli, orange juice, bottled water, cola soft drinks, vanilla extract, oregano, potato salad, apple juice, meat products, and dairy foods. The detection sensitivity of the test for these botulinum complex serotypes was found to be 60 pg/ml (1.9 50% lethal dose [LD50]) for botulinum neurotoxin type A (BoNT/A), 176 pg/ml (1.58 LD50) for BoNT/B, 163 pg/ml for BoNT/E (4.5 LD50), and 117 pg/ml for BoNT/F (less than 1 LD50) in casein buffer. The test could also readily detect 2 ng/ml of neurotoxins type A, B, E, and F in a variety of food samples. For specificity studies, the assay was also used to test a large panel of type A C. botulinum, a smaller panel of proteolytic and nonproteolytic type B, E, and F neurotoxin-producing Clostridia, and nontoxigenic organisms using an overnight incubation of toxin production medium. The assay appears to be an effective tool for large-scale screening of the food supply in the event of a botulinum neurotoxin contamination event.

Characterization of botulinum progenitor toxins by mass spectrometry

Botulinum toxin analysis has renewed importance. This study included the use of nanochromatography-nanoelectrospray-mass spectrometry/mass spectrometry to characterize the protein composition of botulinum progenitor toxins and to assign botulinum progenitor toxins to their proper serotype and strain by using currently available sequence information. Clostridium botulinum progenitor toxins from strains Hall, Okra, Stockholm, MDPH, Alaska, and Langeland and 89 representing serotypes A through G, respectively, were reduced, alkylated, digested with trypsin, and identified by matching the processed product ion spectra of the tryptic peptides to proteins in accessible databases. All proteins known to be present in progenitor toxins from each serotype were identified. Additional proteins, including flagellins, ORF-X1, and neurotoxin binding protein, not previously reported to be associated with progenitor toxins, were present also in samples from several serotypes. Protein identification was used to assign toxins to a serotype and strain. Serotype assignments were accurate, and strain assignments were best when either sufficient nucleotide or amino acid sequence data were available. Minor difficulties were encountered using neurotoxin-associated protein identification for assigning serotype and strain. This study found that combined nanoscale chromatographic and mass spectrometric techniques can characterize C. botulinum progenitor toxin protein composition and that serotype/strain assignments based upon these proteins can provide accurate serotype and, in most instances, strain assignments using currently available information. Assignment accuracy will continue to improve as more nucleotide/amino acid sequence information becomes available for different botulinum strains.

Identification of the major steps in botulinum toxin action

Botulinum toxin is a uniquely potent substance synthesized by the organisms Clostridium botulinum, Clostridium baratii, and Clostridium butyricum. This toxin, which acts preferentially on peripheral cholinergic nerve endings to block acetylcholine release, is both an agent that causes disease (i.e., botulism) as well as an agent that can be used to treat disease (e.g., dystonia). The ability of botulinum toxin to produce its effects is largely dependent on its ability to penetrate cellular and intracellular membranes. Thus, toxin that is ingested or inhaled can bind to epithelial cells and be transported to the general circulation. Toxin that reaches peripheral nerve endings binds to the cell surface then penetrates the plasma membrane by receptor-mediated endocytosis and the endosome membrane by pH-induced translocation. Internalized toxin acts in the cytosol as a metalloendoprotease to cleave polypeptides that are essential for exocytosis. This review seeks to identify and characterize all major steps in toxin action, from initial absorption to eventual paralysis of cholinergic transmission.

Inhalational poisoning by botulinum toxin and inhalation vaccination with its heavy-chain component

Botulinum toxin is the etiologic agent responsible for the disease botulism, which is characterized by peripheral neuromuscular blockade. Botulism is ordinarily encountered as a form of oral poisoning. The toxin is absorbed from the lumen of the gut to reach the general circulation and is then distributed to peripheral cholinergic nerve endings. However, there is a widespread presumption that botulinum toxin can also act as an inhalation poison, which would require that it be absorbed from the airway. Experiments have been done to show that both pure toxin and progenitor toxin (a complex with auxiliary proteins) are inhalation poisons. Interestingly, the data indicate that auxiliary proteins are not necessary to protect the toxin or to facilitate its absorption. When studied on rat primary alveolar epithelial cells or on immortalized human pulmonary adenocarcinoma (Calu-3) cells, botulinum toxin displayed both specific binding and transcytosis. The rate of transport was greater in the apical-to-basolateral direction than in the basolateral-to-apical direction. Transcytosis was energy dependent, and it was blocked by serotype-specific antibody. The results demonstrated that the holotoxin was not essential for the process of binding and transcytosis. Both in vivo and in vitro experiments showed that the heavy-chain component of the toxin was transported across epithelial monolayers, which indicates that the structural determinants governing binding and transcytosis are found in this fragment. The heavy chain was not toxic, and therefore it was tested for utility as an inhalation vaccine against the parent molecule. This fragment was shown to evoke complete protection against toxin doses of at least 10(4) times the 50% lethal dose.

Mucosal immunisation with Clostridium botulinum type C 16 S toxoid and its non-toxic component

Clostridium botulinum types C and D produce a 16 S (500 kDa) toxin that is formed by conjugation of neurotoxin with a non-toxic component (nonTox). The amino acid sequences of type C and D nonTox components are almost identical. In a previous report it was proposed that nonTox is necessary for the effective absorption of the toxin from the small intestine. This suggested the hypothesis that mucosal immunity against nonTox in the small intestine might prevent the absorption of both C- and D-16 S toxins. The nonTox was purified from a mutant strain, (C)-N71, that does not produce neurotoxin. This nonTox or detoxified C-16 S toxin were mixed with adjuvant (a mutant form of heat-labile toxin of Escherichia coli), and inoculated into mice via the nasal or oral route, or both. The mice inoculated nasally four times with nonTox or toxoid produced high levels of antibodies (including IgA) against the immunogens, both in intestinal fluids and sera. When these nonTox-immunised mice were challenged orally with 2 and 20 oral minimum lethal doses (MLD) of C- or D-16 S toxins, the same results were obtained with both C and D; the mice survived after challenge with 2 MLD of either C or D but were killed by 20 MLD of either toxin although the time to death was significantly longer than in the control non-immunised mice. These results indicate that the local anti-nonTox antibodies reduce absorption of both C- and D-16 S toxins from the small intestine. The C-16 S toxoid-immunised mice showed similar behaviour with type D toxin challenge, probably due to the same mechanism, but were protected against 20 MLD of C-16 S toxin.

In vitro reconstitution of the Clostridium botulinum type D progenitor toxin

Clostridium botulinum type D strain 4947 produces two different sizes of progenitor toxins (M and L) as intact forms without proteolytic processing. The M toxin is composed of neurotoxin (NT) and nontoxic-nonhemagglutinin (NTNHA), whereas the L toxin is composed of the M toxin and hemagglutinin (HA) subcomponents (HA-70, HA-17, and HA-33). The HA-70 subcomponent and the HA-33/17 complex were isolated from the L toxin to near homogeneity by chromatography in the presence of denaturing agents. We were able to demonstrate, for the first time, in vitro reconstitution of the L toxin formed by mixing purified M toxin, HA-70, and HA-33/17. The properties of reconstituted and native L toxins are indistinguishable with respect to their gel filtration profiles, native-PAGE profiles, hemagglutination activity, binding activity to erythrocytes, and oral toxicity to mice. M toxin, which contained nicked NTNHA prepared by treatment with trypsin, could no longer be reconstituted to the L toxin with HA subcomponents, whereas the L toxin treated with proteases was not degraded into M toxin and HA subcomponents. We conclude that the M toxin forms first by assembly of NT with NTNHA and is subsequently converted to the L toxin by assembly with HA-70 and HA-33/17.

Characterization and reconstitution of functional hemagglutinin of the Clostridium botulinum type C progenitor toxin

The purified progenitor toxin of Clostridium botulinum type C strain 6814 (C-6814) forms a large complex composed of 150-kDa neurotoxin (NT), 130-kDa nontoxic-nonhemagglutinin (NTNHA), and hemagglutinin (HA) components. The HA component consisted of a mixture of several subcomponents with molecular masses of 70, 55, 33, 26-21 and 17 kDa. We isolated the HA subcomponents from the progenitor toxin by chromatography in the presence of denaturants. The isolated HA subcomponents, designated as i-HA-33, i-HA-55, i-HA-70 and i-HA-33/17, were nearly homogeneous on SDS/PAGE, but the HA-17 and HA-26-21 components were not purified. Some HA subcomponents, designated as f-HA-33 and f-HA-33/17 complex, existed free of the progenitor toxin in the culture medium and they were separately purified. Every HA subcomponent so far isolated shows binding activity to erythrocytes. The hemagglutination activities of each HA subcomponent had a titer of 25 for the f-HA-33/17 complex, and below 23 for the other f- and i-HA subcomponents, while the parent progenitor L toxin was 28. The reconstitution of various combinations of f- and i-HA subcomponents was attempted via mixing and tested for hemagglutination activity. When the i-HA-33/17 complex and i-HA-55 were mixed, the hemagglutination activity was recovered to a titer of 29, which was slightly higher than that of the parent toxin. These data imply that a combination of at least HA-33, -17 and -55 subcomponents is required for full hemagglutination activity of the botulinum progenitor toxin, but each single HA subcomponent shows weak or no aggregation of erythrocytes.

Pure botulinum neurotoxin is absorbed from the stomach and small intestine and produces peripheral neuromuscular blockade

Clostridium botulinum serotype A produces a neurotoxin composed of a 100-kDa heavy chain and a 50-kDa light chain linked by a disulfide bond. This neurotoxin is part of a ca. 900-kDa complex, formed by noncovalent association with a single nontoxin, nonhemagglutinin subunit and a family of hemagglutinating proteins. Previous work has suggested, although never conclusively demonstrated, that neurotoxin alone cannot survive passage through the stomach and/or cannot be absorbed from the gut without the involvement of auxiliary proteins in the complex. Therefore, this study compared the relative absorption and toxicity of three preparations of neurotoxin in an in vivo mouse model. Equimolar amounts of serotype A complex with hemagglutinins, complex without hemagglutinins, and purified neurotoxin were surgically introduced into the stomach or into the small intestine. In some experiments, movement of neurotoxin from the site of administration was restricted by ligation of the pylorus. Comparison of relative toxicities demonstrated that at adequate doses, complex with hemagglutinins, complex without hemagglutinins, and pure neurotoxin can be absorbed from the stomach. The potency of neurotoxin in complex was greater than that of pure neurotoxin, but the magnitude of this difference diminished as the dosage of neurotoxin increased. Qualitatively similar results were obtained when complex with hemagglutinins, complex without hemagglutinins, and pure neurotoxin were placed directly into the intestine. This work establishes that pure botulinum neurotoxin serotype A is toxic when administered orally. This means that pure neurotoxin does not require hemagglutinins or other auxiliary proteins for absorption from the gastrointestinal system into the general circulation.

Binding and transcytosis of botulinum neurotoxin by polarized human colon carcinoma cells

T-84 and Caco-2 human colon carcinoma cells and Madin-Darby canine kidney (MDCK) cells were used to study binding and transcytosis of iodinated Clostridium botulinum neurotoxin serotypes A, B, and C, as well as tetanus toxin. Specific binding and transcytosis were demonstrated for serotypes A and B in intestinal cells. Using serotype A as an example, the rate of transcytosis by T-84 cells was determined in both apical to basolateral (11.34 fmol/h/cm2) as well as basolateral to apical (8.98 fmol/h/cm2) directions, and by Caco-2 cells in the apical to basolateral (8.42 fmol/h/cm2) direction. Serotype A retained intact di-chain structure during transit through T-84 or Caco-2 cells, and when released on the basolateral side was toxic in vivo to mice and in vitro on mouse phrenic nerve-hemidiaphragm preparations. Serotype C and tetanus toxin did not bind effectively to T-84 cells, nor were they efficiently transcytosed (8-10% of serotype A). MDCK cells did not bind or efficiently transcytose (0.32 fmol/h/cm2) botulinum toxin. Further characterization demonstrated that the rate of transcytosis for serotype A in T-84 cells was increased 66% when vesicle sorting was disrupted by 5 microM brefeldin A, decreased 42% when microtubules were disrupted by 10 microM nocodazole, and decreased 74% at 18 degreesC. Drugs that antagonize toxin action at the nerve terminal, such as bafilomycin A1 (which prevents acidification of endosomes) and methylamine HCl (which neutralizes acidification of endosomes), produced only a modest inhibitory effect on the rate of transcytosis (17-22%). These results may provide an explanation for the mechanism by which botulinum toxin escapes the human gastrointestinal tract, and they may also explain why specific serotypes cause human disease and others do not.

Biophysical characterization of the stability of the 150-kilodalton botulinum toxin, the nontoxic component, and the 900-kilodalton botulinum toxin complex species

Botulinum neurotoxin serotype A is initially released from the bacterium Clostridium botulinum as a stable 900-kDa complex. The serotype A 900-kDa complex is one of the forms of the toxin being used as a therapeutic agent for the treatment of various neuromuscular disorders. Previous experiments have demonstrated that the 900-kDa complex form of the toxin protects the toxin from the harsh conditions of the gastrointestinal tract. To provide molecular level details of the stability and equilibrium of the 900-kDa complex, the nontoxic component, and the toxic (botulinum neurotoxin) component, the three species have been investigated with a series of biophysical techniques at the molecular level (dynamic light scattering, proteolysis, circular dichroism, pH incubations, and agglutination assays). These experiments were conducted under harsh conditions which mimic those found along the gastrointestinal tract. Separately, exposure to denaturing and proteolytic conditions degrades both the botulinum neurotoxin and the nontoxic component. In the 900-kDa complex, the botulinum neurotoxin is protected during exposure to the gastrointestinal environment and the nontoxic component is slightly modified. Surprisingly, the toxin protects the ability of the nontoxic component to agglutinate erythrocytes. Contrary to previous reports, the purified 900-kDa complex did not have agglutination ability until after exposure to the proteolytic conditions. These experiments provide new evidence and detail for the theory that the nontoxic component and the toxic component protect one another during exposure to harsh conditions, and a molecular model is presented for the passage of the toxin through the gastrointestinal tract.

The haemagglutinin of Clostridium botulinum type C progenitor toxin plays an essential role in binding of toxin to the epithelial cells of guinea pig small intestine, leading to the efficient absorption of the toxin

Binding of the purified type C 7S (neurotoxin), 12S and 16S botulinum toxins to epithelial cells of ligated small intestine or colon of the guinea pig (in vivo test) and to pre-fixed gastrointestinal tissue sections (in vitro test) was analysed. The 16S toxin bound intensely to the microvilli of epithelial cells of the small intestine in both in vivo and in vitro tests, but did not bind to cells of the stomach or colon. The neurotoxin and 12S toxin did not bind to epithelial cells of the small intestine or to cells of the stomach or colon. Absorption of the toxins was assessed by determining the toxin titre in the sera of guinea pigs 6-8 h after the intra-intestinal administration of the toxins. When the 16S toxin [1 x 10(5) minimum lethal dose (MLD)] was injected, 200-660 MLD ml-1 was detected in the sera, whereas when the 12S toxin (2 x 10(5) MLD) or 7S toxin (2 x 10(5) MLD) was injected, little toxin activity was detected in the sera. Therefore, the haemagglutinin of type C 16S toxin is apparently very important in the binding and absorption of botulinum toxin in the small intestine.

Antibody mapping to domains of botulinum neurotoxin serotype A in the complexed and uncomplexed forms

The domain organization of the botulinum neurotoxin serotype A was studied by using antibody mapping of 44 monoclonal single-chain variable fragments. The analysis was carried out on (i) the individual domains of botulinum neurotoxin holotoxin (binding, translocation, and catalytic), (ii) botulinum neurotoxin holotoxin, (iii) the botulinum neurotoxin holotoxin in complex with the nontoxic portion, and (iv) botulinum neurotoxin holotoxin and nontoxic portion of the complex recombined in vitro. All 44 antibodies mapped to individual domains of botulinum neurotoxin. Forty of the 44 single-chain variable fragments bound the botulinum neurotoxin holotoxin relative to the isolated domains, suggesting that 4 epitopes are covered when the individual domains are in the holotoxin form. Only 20 of the antibodies showed a positive reaction to the toxin while in complex with the nontoxic portion. All of the covered epitopes were mapped to the binding domain of botulinum neurotoxin, which suggested that the binding domain is in direct contact with the nontoxic portion in the complex. Based on the antibody mapping to the different domains of the botulinum neurotoxin holotoxin and the entire complex, a model of the botulinum neurotoxin complex is proposed.

Clostridium botulinum toxins: nature and preparation for clinical use

C. botulinum neurotoxins are acutely toxic materials and act by inhibiting release of the neurotransmitter acetylcholine. The specific nature of this inhibition is discussed and the preparation and purification of Type A toxin specifically for clinical use is described.

Oral toxicities of Clostridium botulinum type A and B toxins from different strains

The production and the oral toxicity for mice of Clostridium botulinum type A and B toxins of different strains were studied. All five type B strains produced both 16S (large or L) and 12S (medium or M) toxins, although the relative amounts varied with the strains. The culture supernatant of type B Okra strain was the most potent in oral toxicity. The L toxin of this culture was about 700 times more toxic in feeding tests with mice than the L toxin from type B strain NH-2, whereas the M toxins of the two strains had the same oral toxicity. These results indicate that the oral toxicity of type B toxin varies with the culture strain. Oral toxicities of L toxin produced by type A strains 62A and 97 were comparable but were 10 times higher than those of their M toxins. Hybrids of toxic and nontoxic components separated from L toxins of type B strains Okra and NH-2 revealed that the high oral toxicity of the B-L toxin of strain Okra is attributable not to the toxic but to the nontoxic component of the toxin. The present study suggests that the 16S molecular-sized toxin elaborated by a certain strain of C. botulinum type B is implicated in the high fatality rate in type B human botulism.

A report on the pathology of type A botulism

An autopsy case of type A botulism, a sacrifice during the first outbreak in Japan, was presented. The patient exhibited typical neuromuscular symptoms and died on the 8th day from the onset of illness. Type A Clostridium botulinum was demonstrated from the stools and the toxin from the blood serum during the course of disease. The infection source of botulinus bacilli, however, was not clarified. The autopsy diagnoses were made as follows; 1) type A botulism, 2) bronchopneumonia, 3) respirator brain, 4) congestion, 5) hemorrhage in vagal nerve, myocardium and endometrium, 6) gastric erosion, 7) cloudy swelling of kidneys, 8) enterocolitis, 9) focal necrosis of liver and adrenals, 10) demyelination of cranial nerves, and 11) focal hyaline degeneration of striated and smooth muscles and myocardium. The botulinus toxin was confirmed from the samples of cardiac blood or autopsy.

Intraintestinal toxin in infant mice challenged intragastrically with Clostridium botulinum spores

Conventionally raised suckling mice were injected intragastrically with 10(5) spores of a Clostridium botulinum type A culture. Botulism was not observed, but 80% or more of mice challenged when 8 to 11 days old had botulinum toxin in the large intestine 3 days later. Mice younger than 7 days or older than 15 days were resistant to the challenge. When in vivo toxin production was started by spores given to 9-day-old mice, toxin was present in the intestine at 1 through 7 days postchallenge but with greatest consistency between 1 and 4 days. Total toxin in an intestine ranged up to 1,920 50% lethal doses as titrated intraperitoneally in adult mice. The dose infecting 50% of a group of 9-day-old mice was 700 (95% confidence limits of 170 to 3,000) spores per animal. Toxin was formed in the lumen of the large intestine; it was not associated with the ileum. Injection of 10(5) spores intraperitoneally into 9-day-old mice resulted in toxin production in the large intestines of 30% of the test animals.