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

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

Transformation of a Metal Chelate into a "Catch and Anchor" Inhibitor of Botulinum A Protease.

Targeting the botulinum neurotoxin light chain (LC) metalloprotease using small-molecule metal chelate inhibitors is a promising approach to counter the effects of the lethal toxin. However, to overcome the pitfalls associated with simple reversible metal chelate inhibitors, it is crucial to investigate alternative scaffolds/strategies. In conjunction with Atomwise Inc., in silico and in vitro screenings were conducted, yielding a number of leads, including a novel 9-hydroxy-4H-pyrido [1,2-a]pyrimidin-4-one (PPO) scaffold. From this structure, an additional series of 43 derivatives were synthesized and tested, resulting in a lead candidate with a Ki of 150 nM in a BoNT/A LC enzyme assay and 17 µM in a motor neuron cell-based assay. These data combined with structure-activity relationship (SAR) analysis and docking led to a bifunctional design strategy, which we termed "catch and anchor" for the covalent inhibition of BoNT/A LC. Kinetic evaluation was conducted on structures prepared from this catch and anchor campaign, providing kinact/Ki values, and rationale for inhibition seen. Covalent modification was validated through additional assays, including an FRET endpoint assay, mass spectrometry, and exhaustive enzyme dialysis. The data presented support the PPO scaffold as a novel candidate for targeted covalent inhibition of BoNT/A LC.

Resolution of Two Steps in Botulinum Neurotoxin Serotype A1 Light Chain Localization to the Intracellular Plasma Membrane.

Botulinum neurotoxin serotype A (BoNT/A) is the most potent protein toxin to humans. BoNT/A light chain (LC/A) cleavage of the membrane-bound SNAP-25 has been well-characterized, but how LC/A traffics to the plasma membrane to target SNAP-25 is unknown. Of the eight BoNT/A subtypes (A1-A8), LC/A3 has a unique short duration of action and low potency that correlate to the intracellular steady state of LC/A, where LC/A1 is associated with the plasma membrane and LC/A3 is present in the cytosol. Steady-state and live imaging of LC/A3-A1 chimeras identified a two-step process where the LC/A N terminus bound intracellular vesicles, which facilitated an internal α-helical-rich domain to mediate LC/A plasma membrane association. The propensity of LC/A variants for membrane association correlated with enhanced BoNT/A potency. Understanding the basis for light chain intracellular localization provides insight to mechanisms underlying BoNT/A potency, which can be extended to applications as a human therapy.

Metal Ions Effectively Ablate the Action of Botulinum Neurotoxin A.

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

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

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

Multiple domains of tetanus toxin direct entry into primary neurons.

Tetanus toxin elicits spastic paralysis by cleaving VAMP-2 to inhibit neurotransmitter release in inhibitory neurons of the central nervous system. As the retrograde transport of tetanus neurotoxin (TeNT) from endosomes has been described, the initial steps that define how TeNT initiates trafficking to the retrograde system are undefined. This study examines TeNT entry into primary cultured cortical neurons by total internal reflection fluorescence (TIRF) microscopy. The initial association of TeNT with the plasma membrane was dependent upon ganglioside binding, but segregated from synaptophysin1 (Syp1), a synaptic vesicle (SV) protein. TeNT entry was unaffected by membrane depolarization and independent of SV cycling, whereas entry of the receptor-binding domain of TeNT (HCR/T) was stimulated by membrane depolarization and inhibited by blocking SV cycling. Measurement of the incidence of colocalization showed that TeNT segregated from Syp1, whereas HCR/T colocalized with Syp1. These studies show that while the HCR defines the initial association of TeNT with the cell membrane, regions outside the HCR define how TeNT enters neurons independent of SV cycling. This provides a basis for the unique entry of botulinum toxin and tetanus toxin into neurons.

Widespread sequence variations in VAMP1 across vertebrates suggest a potential selective pressure from botulinum neurotoxins.

Botulinum neurotoxins (BoNT/A-G), the most potent toxins known, act by cleaving three SNARE proteins required for synaptic vesicle exocytosis. Previous studies on BoNTs have generally utilized the major SNARE homologues expressed in brain (VAMP2, syntaxin 1, and SNAP-25). However, BoNTs target peripheral motor neurons and cause death by paralyzing respiratory muscles such as the diaphragm. Here we report that VAMP1, but not VAMP2, is the SNARE homologue predominantly expressed in adult rodent diaphragm motor nerve terminals and in differentiated human motor neurons. In contrast to the highly conserved VAMP2, BoNT-resistant variations in VAMP1 are widespread across vertebrates. In particular, we identified a polymorphism at position 48 of VAMP1 in rats, which renders VAMP1 either resistant (I48) or sensitive (M48) to BoNT/D. Taking advantage of this finding, we showed that rat diaphragms with I48 in VAMP1 are insensitive to BoNT/D compared to rat diaphragms with M48 in VAMP1. This unique intra-species comparison establishes VAMP1 as a physiological toxin target in diaphragm motor nerve terminals, and demonstrates that the resistance of VAMP1 to BoNTs can underlie the insensitivity of a species to members of BoNTs. Consistently, human VAMP1 contains I48, which may explain why humans are insensitive to BoNT/D. Finally, we report that residue 48 of VAMP1 varies frequently between M and I across seventeen closely related primate species, suggesting a potential selective pressure from members of BoNTs for resistance in vertebrates.

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.

Entry of a recombinant, full-length, atoxic tetanus neurotoxin into Neuro-2a cells.

Tetanus neurotoxin (TeNT) and botulinum neurotoxin (BoNT) are clostridial neurotoxins (CNTs) responsible for the paralytic diseases tetanus and botulism, respectively. CNTs are AB toxins with an N-terminal zinc-metalloprotease light chain that is linked by a disulfide bond to a C-terminal heavy chain that includes a translocation domain and a receptor-binding domain (HCR). Current models predict that the HCR defines how CNTs enter and traffic in neurons. Recent studies implicate that domains outside the HCR contribute to CNT trafficking in neurons. In the current study, a recombinant, full-length TeNT derivative, TeNT(RY), was engineered to analyze TeNT cell entry. TeNT(RY) was atoxic in a mouse challenge model. Using Neuro-2a cells, a mouse neuroblastoma cell line, TeNT HCR (HCR/T) and TeNT(RY) were found to bind gangliosides with similar affinities and specificities, consistent with the HCR domain containing receptor binding function. Temporal studies showed that HCR/T and TeNT(RY) entered Neuro-2a cells slower than the HCR of BoNT/A (HCR/A), transferrin, and cholera toxin B. Intracellular localization showed that neither HCR/T nor TeNT(RY) localized with HCR/A or synaptic vesicle protein 2, the protein receptor for HCR/A. HCR/T and TeNT(RY) exhibited only partial intracellular colocalization, indicating that regions outside the HCR contribute to the intracellular TeNT trafficking. TeNT may require this complex functional entry organization to target neurons in the central nervous system.

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

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

Botulinum neurotoxin D-C uses synaptotagmin I and II as receptors, and human synaptotagmin II is not an effective receptor for type B, D-C and G toxins.

Botulinum neurotoxins (BoNTs) are classified into seven types (A-G), but multiple subtype and mosaic toxins exist. These subtype and mosaic toxins share a high sequence identity, and presumably the same receptors and substrates with their parental toxins. Here, we report that a mosaic toxin, type D-C (BoNT/D-C), uses different receptors from its parental toxin BoNT/C. BoNT/D-C, but not BoNT/C, binds directly to the luminal domains of synaptic vesicle proteins synaptotagmin (Syt) I and II, and requires expression of SytI/II to enter neurons. The SytII luminal fragment containing the toxin-binding site can block the entry of BoNT/D-C into neurons and reduce its toxicity in vivo in mice. We also found that gangliosides increase binding of BoNT/D-C to SytI/II and enhance the ability of the SytII luminal fragment to block BoNT/D-C entry into neurons. These data establish SytI/II, in conjunction with gangliosides, as the receptors for BoNT/D-C, and indicate that BoNT/D-C is functionally distinct from BoNT/C. We further found that BoNT/D-C recognizes the same binding site on SytI/II where BoNT/B and G also bind, but utilizes a receptor-binding interface that is distinct from BoNT/B and G. Finally, we also report that human and chimpanzee SytII has diminished binding and function as the receptor for BoNT/B, D-C and G owing to a single residue change from rodent SytII within the toxin binding site, potentially reducing the potency of these BoNTs in humans and chimpanzees.

Holotoxin Activity of Botulinum Neurotoxin Subtype A4 Originating from a Nontoxigenic Clostridium botulinum Expression System.

Clostridium botulinum subtype A4 neurotoxin (BoNT/A4) is naturally expressed in the dual-toxin-producing C. botulinum strain 657Ba at 100× lower titers than BoNT/B. In this study, we describe purification of recombinant BoNT/A4 (rBoNT/A4) expressed in a nonsporulating and nontoxigenic C. botulinum expression host strain. The rBoNT/A4 copurified with nontoxic toxin complex components provided in trans by the expression host and was proteolytically cleaved to the active dichain form. Activity of the recombinant BoNT/A4 in mice and in human neuronal cells was about 1,000-fold lower than that of BoNT/A1, and the recombinant BoNT/A4 was effectively neutralized by botulism heptavalent antitoxin. A previous report using recombinant truncated BoNT/A4 light chain (LC) expressed in Escherichia coli has indicated reduced stability and activity of BoNT/A4 LC compared to BoNT/A1 LC, which was surmounted by introduction of a single-amino-acid substitution, I264R. In order to determine whether this mutation would also affect the holotoxin activity of BoNT/A4, a recombinant full-length BoNT/A4 carrying this mutation as well as a second mutation predicted to increase solubility (L260F) was produced in the clostridial expression system. Comparative analyses of the in vitro, cellular, and in vivo activities of rBoNT/A4 and rBoNT/A4-L260F I264R showed 1,000-fold-lower activity than BoNT/A1 in both the mutated and nonmutated BoNT/A4. This indicates that these mutations do not alter the activity of BoNT/A4 holotoxin. In summary, a recombinant BoNT from a dual-toxin-producing strain was expressed and purified in an endogenous clostridial expression system, allowing analysis of this toxin.

Botulinum neurotoxin X lacks potency in mice and in human neurons.

Botulinum neurotoxins (BoNTs) are a class of toxins produced by Clostridium botulinum (C. botulinum) and other species of Clostridia. BoNT/X is a putative novel botulinum neurotoxin identified through genome sequencing and capable of SNARE cleavage, but its neurotoxic potential in humans and vertebrates remained unclear. The C. botulinum strain producing BoNT/X, Strain 111, encodes both a plasmid-borne bont/b2 as well as the chromosomal putative bont/x. This study utilized C. botulinum Strain 111 from Japan as well as recombinantly produced full-length BoNT/X to more fully analyze this putative pathogenic toxin. We confirmed production of full-length, catalytically active native BoNT/X by C. botulinum Strain 111, produced as a disulfide-bonded dichain polypeptide similar to other BoNTs. Both the purified native and the recombinant BoNT/X had high enzymatic activity in vitro but displayed very low potency in human-induced pluripotent stem cell-derived neuronal cells and in mice. Intraperitoneal injection of up to 50 µg of native BoNT/X in mice did not result in botulism; however, mild local paralysis was observed after injection of 2 µg into the gastrocnemius muscle. We further demonstrate that the lack of toxicity by BoNT/X is due to inefficient neuronal cell association and entry, which can be rescued by replacing the receptor binding domain of BoNT/X with that of BoNT/A. These data demonstrate that BoNT/X is not a potent vertebrate neurotoxin like the classical seven serotypes of BoNTs. IMPORTANCE: The family of botulinum neurotoxins comprises the most potent toxins known to humankind. New members of this family of protein toxins as well as more distantly related homologs are being identified. The discovery of BoNT/X via bioinformatic screen in 2017 as a putative new BoNT serotype raised concern about its potential as a pathogenic agent with no available countermeasures. This study for the first time assessed both recombinantly produced and native purified BoNT/X for its vertebrate neurotoxicity.

Enhancing the protective immune response against botulism.

The need for a vaccine against botulism has increased since the discontinuation of the pentavalent (ABCDE) botulinum toxoid vaccine by the Centers for Disease Control and Prevention. The botulinum toxins (BoNTs) are the primary virulence factors and vaccine components against botulism. BoNTs comprise three domains which are involved in catalysis (LC), translocation (HCT), and host receptor binding (HCR). Recombinant HCR subunits have been used to develop the next generation of BoNT vaccines. Using structural studies and the known entry properties of BoNT/A, an HCR subunit vaccine against BoNT/A that contained the point mutation W1266A within the ganglioside binding pocket was designed. HCR/A(W1266A) did not enter primary neurons, and the crystal structure of HCR/A(W1266A) was virtually identical to that of wild-type HCR/A. Using a mouse model, experiments were performed using a high-dose vaccine and a low-dose vaccine. At a high vaccine dose, HCR/A and HCR/A(W1266A) elicited a protective immune response to BoNT/A challenge. At the low-dose vaccination, HCR/A(W1266A) was a more protective vaccine than HCR/A. α-HCR IgG titers correlated with protection from BoNT challenge, although titers to block HCR/A entry were greater in serum in HCR/A-vaccinated mice than in HCR/A(W1266A)-vaccinated mice. This study shows that removal of receptor binding capacity enhances potency of the subunit HCR vaccine. Vaccines that lack receptor binding capacity have the added property of limited off-target toxicity.

Characterization of botulinum neurotoxin A subtypes 1 through 5 by investigation of activities in mice, in neuronal cell cultures, and in vitro.

Botulinum neurotoxins (BoNTs) are synthesized by Clostridium botulinum and exist as seven immunologically distinct serotypes designated A through G. For most serotypes, several subtypes have now been described based on nominal differences in the amino acid sequences. BoNT/A1 is the most well-characterized subtype of the BoNT/A serotype, and many of its properties, including its potency, its prevalence as a food poison, and its utility as a pharmaceutical, have been thoroughly studied. In contrast, much remains unknown of the other BoNT/A subtypes. In this study, BoNT/A subtype 1 (BoNT/A1) to BoNT/A5 were characterized utilizing a mouse bioassay, an in vitro cleavage assay, and several neuronal cell-based assays. The data indicate that BoNT/A1 to -5 have distinct in vitro and in vivo toxicological properties and that, unlike those for BoNT/A1, the neuronal and mouse results for BoNT/A2 to -5 do not correlate with their enzymatic activity. These results indicate that BoNT/A1 to -5 have distinct characteristics, which are of importance for a greater understanding of botulism and for pharmaceutical applications.

Botulinum neurotoxin D uses synaptic vesicle protein SV2 and gangliosides as receptors.

Botulinum neurotoxins (BoNTs) include seven bacterial toxins (BoNT/A-G) that target presynaptic terminals and act as proteases cleaving proteins required for synaptic vesicle exocytosis. Here we identified synaptic vesicle protein SV2 as the protein receptor for BoNT/D. BoNT/D enters cultured hippocampal neurons via synaptic vesicle recycling and can bind SV2 in brain detergent extracts. BoNT/D failed to bind and enter neurons lacking SV2, which can be rescued by expressing one of the three SV2 isoforms (SV2A/B/C). Localization of SV2 on plasma membranes mediated BoNT/D binding in both neurons and HEK293 cells. Furthermore, chimeric receptors containing the binding sites for BoNT/A and E, two other BoNTs that use SV2 as receptors, failed to mediate the entry of BoNT/D suggesting that BoNT/D binds SV2 via a mechanism distinct from BoNT/A and E. Finally, we demonstrated that gangliosides are essential for the binding and entry of BoNT/D into neurons and for its toxicity in vivo, supporting a double-receptor model for this toxin.

Association of toxin-producing Clostridium botulinum with the macroalga Cladophora in the Great Lakes.

Avian botulism, a paralytic disease of birds, often occurs on a yearly cycle and is increasingly becoming more common in the Great Lakes. Outbreaks are caused by bird ingestion of neurotoxins produced by Clostridium botulinum, a spore-forming, gram-positive, anaerobe. The nuisance, macrophytic, green alga Cladophora (Chlorophyta; mostly Cladophora glomerata L.) is a potential habitat for the growth of C. botulinum. A high incidence of botulism in shoreline birds at Sleeping Bear Dunes National Lakeshore (SLBE) in Lake Michigan coincides with increasingly massive accumulations of Cladophora in nearshore waters. In this study, free-floating algal mats were collected from SLBE and other shorelines of the Great Lakes between June and October 2011. The abundance of C. botulinum in algal mats was quantified and the type of botulism neurotoxin (bont) genes associated with this organism were determined by using most-probable-number PCR (MPN-PCR) and five distinct bont gene-specific primers (A, B, C, E, and F). The MPN-PCR results showed that 16 of 22 (73%) algal mats from the SLBE and 23 of 31(74%) algal mats from other shorelines of the Great Lakes contained the bont type E (bont/E) gene. C. botulinum was present up to 15000 MPN per gram dried algae based on gene copies of bont/E. In addition, genes for bont/A and bont/B, which are commonly associated with human diseases, were detected in a few algal samples. Moreover, C. botulinum was present as vegetative cells rather than as dormant spores in Cladophora mats. Mouse toxin assays done using supernatants from enrichment of Cladophora containing high densities of C. botulinum (>1000 MPN/g dried algae) showed that Cladophora-borne C. botulinum were toxin-producing species (BoNT/E). Our results indicate that Cladophora provides a habitat for C. botulinum, warranting additional studies to better understand the relationship between this bacterium and the alga, and how this interaction potentially contributes to botulism outbreaks in birds.

Botulinum neurotoxins B and E translocate at different rates and exhibit divergent responses to GT1b and low pH.

Botulinum neurotoxins (BoNTs, serotypes A-G) are the most deadly substances known. Here, we investigated how BoNT/E, a serotype that causes human botulism, translocates into the cytosol of neurons. Analogous to BoNT/B, BoNT/E required binding of the coreceptor, GT1b, to undergo significant secondary structural changes and transform into a hydrophobic protein at low pH. These data indicate that both serotypes act as coincidence detectors for both GT1b and low pH, to undergo translocation. However, BoNT/E translocated much more rapidly than BoNT/B. Also, BoNT/E required only GT1b, and not low pH, to oligomerize, whereas BoNT/B required both. In further contrast to the case of BoNT/B, low pH alone altered the secondary structure of BoNT/E to some degree and resulted in its premature inactivation. Hence, comparison of two BoNT serotypes revealed that these agents exhibit both convergent and divergent responses to receptor interactions, and pH, in the translocation pathway.

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

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

SV2 mediates entry of tetanus neurotoxin into central neurons.

Tetanus neurotoxin causes the disease tetanus, which is characterized by rigid paralysis. The toxin acts by inhibiting the release of neurotransmitters from inhibitory neurons in the spinal cord that innervate motor neurons and is unique among the clostridial neurotoxins due to its ability to shuttle from the periphery to the central nervous system. Tetanus neurotoxin is thought to interact with a high affinity receptor complex that is composed of lipid and protein components; however, the identity of the protein receptor remains elusive. In the current study, we demonstrate that toxin binding, to dissociated hippocampal and spinal cord neurons, is greatly enhanced by driving synaptic vesicle exocytosis. Moreover, tetanus neurotoxin entry and subsequent cleavage of synaptobrevin II, the substrate for this toxin, was also dependent on synaptic vesicle recycling. Next, we identified the potential synaptic vesicle binding protein for the toxin and found that it corresponded to SV2; tetanus neurotoxin was unable to cleave synaptobrevin II in SV2 knockout neurons. Toxin entry into knockout neurons was rescued by infecting with viruses that express SV2A or SV2B. Tetanus toxin elicited the hyper excitability in dissociated spinal cord neurons - due to preferential loss of inhibitory transmission - that is characteristic of the disease. Surprisingly, in dissociated cortical cultures, low concentrations of the toxin preferentially acted on excitatory neurons. Further examination of the distribution of SV2A and SV2B in both spinal cord and cortical neurons revealed that SV2B is to a large extent localized to excitatory terminals, while SV2A is localized to inhibitory terminals. Therefore, the distinct effects of tetanus toxin on cortical and spinal cord neurons are not due to differential expression of SV2 isoforms. In summary, the findings reported here indicate that SV2A and SV2B mediate binding and entry of tetanus neurotoxin into central neurons.