Re-engineering the target specificity of Clostridial neurotoxins - a route to novel therapeutics

The ability to chemically couple proteins to LH(N)-fragments of clostridial neurotoxins and create novel molecules with selectivity for cells other than the natural target cell of the native neurotoxin is well established. Such molecules are able to inhibit exocytosis in the target cell and have the potential to be therapeutically beneficial where secretion from a particular cell plays a causative role in a disease or medical condition. To date, these molecules have been produced by chemical coupling of the LH(N)-fragment and the targeting ligand. This is, however, not a suitable basis for producing pharmaceutical agents as the products are ill defined, difficult to control and heterogeneous. Also, the molecules described to date have targeted neuroendocrine cells that are susceptible to native neurotoxins, and therefore the benefit of creating a molecule with a novel targeting domain has been limited. In this paper, the production of a fully recombinant fusion protein from a recombinant gene encoding both the LH(N)-domain of a clostridial neurotoxin and a specific targeting domain is described, together with the ability of such recombinant fusion proteins to inhibit secretion from non-neuronal target cells. Specifically, a novel protein consisting of the LH(N)-domains of botulinum neurotoxin type C and epidermal growth factor (EGF) that is able to inhibit secretion of mucus from epithelial cells is reported. Such a molecule has the potential to prevent mucus hypersecretion in asthma and chronic obstructive pulmonary disease.

Botulinum neurotoxin structure, engineering, and novel cellular trafficking and targeting

Botulinum neurotoxins are multifaceted molecules, which are truly unique not only in their mode of action, but also their utility as a drug carrier either across the gut wall or to the nerve terminals. The molecule is divided in clear functional domains that can operate independently. This feature can be used to employ them as cargo carrier by linking other drugs or vaccines with the binding and translocation domains of BoNT. While the domain structures are largely independent of each other, the dynamic structure of these domains, especially that of the enzymatic domain (L chain), is quite different from the reported crystal structures for several BoNT serotypes and their enzymatic domain. This review discusses the comparative structures of BoNT in crystal and solution for their relevance to the molecular mechanism of BoNT action, especially in view of our recent discovery that the enzymatically active structure of the BoNT exists as a molten-globule and that of the endopeptidase domain as a novel PRIME conformation. Finally, a non-exhaustive discussion has been included to explain the long-lasting biological effects of certain serotypes of BoNT, based on the current knowledge of the structure-function of different serotypes of botulinum neurotoxins.

An Initial Assessment of the Systemic Pharmacokinetics of Botulinum Toxin

Botulinum toxin is an extraordinarily potent molecule that has an unusually long duration of action. Despite this, there is little information available on natural mechanisms for metabolism or elimination and virtually no information on pharmacologically induced mechanisms for metabolism and elimination. Therefore, a number of experiments were performed on laboratory animals that addressed two major issues: 1) the effect of blood on the structure, function, and biologic half-life of the toxin, and 2) the effect of neutralizing antibodies on half-life and elimination of circulating toxin. In the first series of studies, the metabolic transformation of toxin was assessed by incubating it in blood for varying lengths of time. At each time point, aliquots were examined to determine the amount of toxin, the structure of toxin, the catalytic activity of toxin, and the neuromuscular blocking activity of toxin. This work demonstrated that blood did not alter any characteristic of the toxin molecule. Experiments were also done in which toxin was administered to mice and rats at doses that produced clinical poisoning. The results demonstrated that the elimination half-life for native (nonmetabolized) toxin in blood and serum was 230 to 260 min. During the second series of studies, the rate of elimination of circulating toxin was studied in the presence of antibodies directed against the carboxyl-terminal half of the toxin molecule. This work demonstrated that neutralizing antibodies 1) enhanced clearance of toxin from the circulation and 2) enhanced tissue accumulation of toxin, particularly in liver and spleen.

Structure of botulinum neurotoxin type D light chain at 1.65 A resolution: repercussions for VAMP-2 substrate specificity

The seven serotypes (A-G) of botulinum neurotoxins (BoNTs) function through their proteolytic cleavage of one of three proteins (SNAP-25, Syntaxin, and VAMP) that form the SNARE complex required for synaptic vesicle fusion. The different BoNTs have very specific protease recognition requirements, between 15 and 50 amino acids in length depending on the serotype. However, the structural details involved in substrate recognition remain largely unknown. Here is reported the 1.65 A resolution crystal structure of the catalytic domain of BoNT serotype D (BoNT/D-LC), providing insight into the protein-protein binding interaction and final proteolysis of VAMP-2. Structural analysis has identified a hydrophobic pocket potentially involved in substrate recognition of the P1' VAMP residue (Leu 60) and a second remote site for recognition of the V1 SNARE motif that is critical for activity. A structural comparison of BoNT/D-LC with BoNT/F-LC that also recognizes VAMP-2 one residue away from the BoNT/D-LC site provides additional molecular details about the unique serotype specific activities. In particular, BoNT/D prefers a hydrophobic interaction for the V1 motif of VAMP-2, while BoNT/F adopts a more hydrophilic strategy for recognition of the same V1 motif.

Unique Substrate Recognition by Botulinum Neurotoxins Serotypes A and E

Botulinum neurotoxins (BoNTs) are zinc proteases that cleave SNARE proteins to elicit flaccid paralysis by inhibiting the fusion of neurotransmitter-carrying vesicles to the plasma membrane of peripheral neurons. There are seven serotypes of BoNT, termed A-G. BoNT serotype A and serotype E cleave SNAP25 at residues 197-198 and 180-181, respectively. Unlike other zinc proteases, the BoNTs recognize extended regions of SNAP25 for cleavage. The basis for this extended substrate recognition and specificity is unclear. Saturation mutagenesis and deletion mapping identified residues 156-202 of SNAP25 as the optimal cleavage domain for BoNT/A, whereas the optimal cleavage domain for BoNT/E was shorter, comprising residues 167-186 of SNAP25. Two sub-sites were resolved within each optimal cleavage domain, which included a recognition or active site (AS) domain that contained the site of cleavage and a binding (B) domain, which contributed to substrate affinity. Within the AS domains, the P1', P3, and P5 sites of SNAP25 contributed to scissile bond cleavage by LC/A, whereas the P1' and P2 sites of SNAP25 contributed to scissile bond cleavage by LC/E. These studies provide insight into the development of strategies for small molecule inhibitors of the BoNTs.

Common binding site for disialyllactose and tri-peptide in C-fragment of tetanus neurotoxin

Clostridial neurotoxins are comprised of botulinum (BoNT) and tetanus (TeNT), which share significant structural and functional similarity. Crystal structures of the binding domain of TeNT complexed with disialyllactose (DiSia) and a tri-peptide Tyr-Glu-Trp (YEW) have been determined to 2.3 and 2.2 A, respectively. Both DiSia and YEW bind in a shallow cleft region on the surface of the molecule in the beta-trefoil domain, interacting with a set of common residues, Asp1147, Asp1214, Asn1216, and Arg1226. DiSia and YEW binding at the same site in tetanus toxin provides a putative site that could be occupied either by a ganglioside moiety or a peptide. Soaking experiments with a mixture of YEW and DiSia show that YEW competes with DiSia, suggesting that YEW can be used to block ganglioside binding. A comparison with the TeNT binding domain in complex with small molecules, BoNT/A and /B, provides insight into the different modes of ganglioside binding.

Recombinant C fragment of botulinum neurotoxin B serotype (rBoNTB (HC)) immune response and protection in the rhesus monkey

Botulinum neurotoxin B (BoNTB) is a distinct protein subtype of a family of neurotoxins with the potential for use in biological warfare or terrorist attacks. This study is one in a series evaluating the immunogenicity and protective effects of recombinant vaccines against the different subtypes of botulinum toxin. The recombinant subunit vaccines encoding the C fragment portion ( approximately 50 kDa) of the toxins are produced in the yeast, Pichia pastoris. In this study, groups of rhesus monkeys were vaccinated with three doses (1 and 5microg per dose) of rBoNTB(H(c)) vaccine. Total and neutralizing antibody titers were determined at various times during and postvaccination. Two groups of vaccinated monkeys plus non-vaccinated controls were actively challenged with B toxin by aerosol exposure. All monkeys receiving vaccine were protected from the toxin and no clinical signs of disease were observed, while controls displaying classic signs of botulism succumbed to the toxin challenge. Two additional groups of monkeys receiving the same vaccine regiment as the first two groups had significant levels of circulating neutralizing antibody titers up to 24 months postvaccination. This non-human primate study demonstrated the short- and long-term immunity afforded by the rBoNTB(H(c)) vaccine.

Clostridial neurotoxins: structure-function led design of new therapeutics

The neurotoxins produced by various species of Clostridia are the causative agents of botulism and tetanus. The ability of the toxins, specifically those of the botulinum neurotoxin family, to disrupt neurotransmission has been exploited for use in several medical indications and now represents the therapeutic option of choice in a number of cases. Clostridial neurotoxins have been discovered to have a multi-domain structure that is shared between the various proteins of the family, and it has also been determined that each domain contributes a specific role to the holotoxin. The extensive use of recombinant expression approaches, along with solution of multiple crystallographic structures of individual domains, has enabled researchers to explore structurefunction relationships of the toxin domains more closely. These advances have facilitated a greater understanding of the potential use of individual domains for a wide variety of purposes, including the development of new therapeutics.

The use of Endopep-MS for the detection of botulinum toxins A, B, E, and F in serum and stool samples

Botulinum neurotoxin (BoNT) causes the disease botulism, which can be lethal if untreated. Previous work in our laboratory focused on developing Endopep-MS, a mass spectrometric-based endopeptidase method for the detection and differentiation of BoNT serotypes. We have expanded this effort to include an antibody capture method to partially purify and concentrate BoNT from serum and stool extract samples for the Endopep-MS assay. Because complex matrices such as serum and stool contain abundant endogenous proteases, this technique was needed to remove most proteases from the sample while concentrating BoNT from a sample size of 100 to 500 microl to 20 microl. When this antibody capture method is combined with the Endopep-MS reaction, limits of detection in 500mul of spiked human serum are 10 mouse LD50 (20 mouse LD50/ml) for BoNT A, 0.5 mouse LD50 (1 mouse LD50/ml) for BoNT B, 0.1 mouse LD50 (0.2 mouse LD50/ml) for BoNT E, and 0.5 mouse LD50 (1 mouse LD50/ml) for BoNT F. The limits of detection in spiked stool extracts are somewhat higher due to the high-protease environment of stool extract that also requires use of protease inhibitors. The entire method can be performed in as short a time as 4 h.

Hinge peptide combinatorial libraries for inhilbitors of botulinum neurotoxins and saxitoxin: Deconvolution strategy

Abstract Combinatorial library screening offers a rapid process for identifying potential therapies to toxins. Hinge peptide libraries, which rely on conformational diversity rather than traditional molecular diversity, reduce the need for huge numbers of syntheses and screening steps and greatly expedite the discovery process of active molecules. Hinge peptide libraries having the structures: Acetyl-X1-X2-hinge-X3-X4-NH2 (capped) and X1-hinge-X2-X3 (uncapped), where X1 through X4 are near-equimolar mixtures of twelve L-amino acids and hinge = 4-aminobutyric acid, were screened for inhibitory activity in bioassays for botulinum neurotoxins A and B (BoNT/A, BoNT/B) and saxitoxin. The zinc protease activity of the reduced light chains of BoNT/A and /B was assayed by measuring the cleavage of synthetic substrates. Saxitoxin activity was measured by the restoration of the viability of neuroblastoma cells treated with ouabain and veratridine. Deconvolution of libraries was accomplished by fixing one position at a time beginning with the C-terminus. Primary library subsets in which position 4 was fixed showed moderate levels of inhibition for BoNT/A. Secondary library subsets showed stronger inhibition in the bioassays. In each of the bioassays, inhibitory potency was stronger when the second position to be fixed was on the opposite side of the hinge, rather than on the same side with respect to the C-terminus, suggesting that the hinge facilitates the interaction of side chains. Inhibitors for all three of the toxins studied were discovered within library subsets, although not necessarily in primary subsets. These studies demonstrate that (1) the best strategy for deconvoluting hinge peptide libraries is by fixing residues alternately on each side of the hinge moiety, and (2) it is essential to investigate secondary subsets even when primary subsets are inactive. The present findings support the concept that the increased flexibility imposed by the inclusion of a central hinge residue in small peptides increases the opportunity for side chain interactions, providing a distinct advantage for hinge peptide libraries over conventional peptide libraries. Hinge peptide libraries are a rich source of novel ligands for modulation of biomechanisms. The library subsets uncovered in this study may possess peptides that will lead to effective therapies to neurotoxin poisoning.

Characterization of the neurotoxin produced by isolates associated with avian botulism

Several varieties of birds are affected by type C botulism. We conducted neutralization tests of culture supernatants of isolates from cases of avian botulism. Whereas the toxin produced by isolates derived from mammalian botulism was neutralized only with type C antitoxin, the toxins of all isolates related to avian botulism were neutralized with both type C and D antitoxins. An analysis of nucleotide sequences with several strains revealed that the neurotoxin gene in the isolates from avian botulism comprises two thirds of the type C neurotoxin gene and one third of the type D neurotoxin gene. This indicates that the neurotoxin of avian isolates is a mosaic of type C and D neurotoxins. We prepared three sets of primers to differentiate the gene for the mosaic form from the conserved genes of type C and D neurotoxins. The results of polymerase chain reaction with these primers indicated that all avian botulism-related isolates and specimens possess the gene for the mosaic form of the neurotoxin. The toxins purified from avian and mammalian isolates exhibited the same degree of lethality in mice, but the former showed greater toxicity to chickens than the latter. These results indicate that the mosaic neurotoxin is probably a pathogenic agent causing some forms of avian botulism.

Botulinum neurotoxin detection and differentiation by mass spectrometry

A new rapid, mass spectrometry-based method to detect and differentiate botulinal neurotoxins is described.

Botulinum neurotoxins (BoNTs) are proteases that cleave specific cellular proteins essential for neurotransmitter release. Seven BoNT serotypes (A–G) exist; 4 usually cause human botulism (A, B, E, and F). We developed a rapid, mass spectrometry–based method (Endopep-MS) to detect and differentiate active BoNTs A, B, E, and F. This method uses the highly specific protease activity of the toxins with target peptides specific for each toxin serotype. The product peptides derived from the endopeptidase activities of BoNTs are detected by matrix-assisted laser-desorption ionization time-of-flight mass spectrometry. In buffer, this method can detect toxin equivalents of as little as 0.01 mouse lethal dose (MLD)₅₀ and concentrations as low as 0.62 MLD₅₀/mL. A high-performance liquid chromatography–tandem mass spectrometry method for quantifying active toxin, where the amount of toxin can be correlated to the amount of product peptides, is also described.

Where has all the botox gone? (How) will we ever learn… Using monoclonal antibodies to track botulinum toxin

Botulinum toxin (BTX) is an important therapeutic tool in the treatment of overactive skeletal and smooth muscles, as well as hypersecretory and painful disorders. Despite advances in our understanding of how BTX works, much remains to be elucidated, such as how BTX ameliorates pain, how it produces weakness remote from the site of injection and the fate of the heavy and light chain components of the BTX molecule following endocytosis into the presynaptic membrane. BTX, conjugated to radionuclides, allows investigators to track the molecule both in vitro and in vivo. However, altering the BTX molecule may cause structural changes or pharmacokinetic and pharmacodynamic alterations, and disrupt its normal action. We propose instead to bind the biomarkers (appropriate dyes, radionuclides or MRI contrast agents) to monoclonal antibodies directed against either heavy or light chain components of BTX, thus allowing administration of native (i.e. unaltered) BTX.

Botulinum toxin injections to reduce adiposity: Possibility, or fat chance?

Obese individuals often suffer from negative self-image. Many, even those with a normal body mass index, resort to pharmacotherapy (lipase inhibitors or appetite suppressants), mesotherapy and surgery (gastric volume reduction, liposuction or apronectomy) in a bid to remove excess adipose tissue. These treatments are associated with inherent morbidity and even mortality, and hence should not be undertaken lightly. The observation that denervation of adipose tissue results in lipoatrophy leads us to postulate that chemodenervation using botulinum toxin may achieve the same result, i.e. fat loss, and we explore the methods by which selective fat loss may be achieved. We concede that removal of subcutaneous fat does not, however, reduce the risks associated with the metabolic syndrome, as visceral (intra-abdominal) fat is not reduced by the removal of subcutaneous fat.

Botulism: the challenge of diagnosis and treatment

Botulism is a rare paralytic disease caused by a neurotoxin produced from the spore-forming bacterium Clostridium botulinum and in rare cases Clostridium butyricum and Clostridium baratii. Botulism has 4 naturally occurring syndromes: foodborne, wound, infant botulism, and adult intestinal toxemia. Inhalational botulism could result from aerosolization of botulinum toxin, and iatrogenic botulism can result from injection of toxin. All of these produce the same clinical syndrome of symmetrical cranial nerve palsies followed by descending, symmetric flaccid paralysis of voluntary muscles, which may progress to respiratory compromise and death. Treatment includes meticulous intensive care that includes mechanical ventilation, when necessary, and administration of antitoxin.

Four molecules of the 33 kDa haemagglutinin component of the Clostridium botulinum serotype C and D toxin complexes are required to aggregate erythrocytes

Normally, large-sized botulinum toxin complexes (L-TC) of serotype C and D are composed of a single neurotoxin, a single non-toxic non-haemagglutinin, two HA-70 molecules, four HA-33 molecules and four HA-17 molecules that assemble to form a 650 kDa L-TC. The 540 and 610 kDa TC species (designated here as L-TC2and L-TC3, respectively) were purified in addition to the 650 kDa L-TC from the culture supernatants of serotype D strains (D-4947 and D-CB16) and serotype C strains (C-6814 and C-Yoichi). The 650 kDa L-TC from D-4947, D-CB16 and C-6814 showed haemagglutination and erythrocyte-binding activity, but their L-TC2and L-TC3species had only binding activity. In contrast, every TC species from C-Yoichi having the C-terminally truncated variant of HA-33 exhibited neither haemagglutination activity nor erythrocyte-binding activity. Four strain-specific HA-33/HA-17 complexes were isolated from the 650 kDa L-TC of each strain. The 650 kDa HA-hybrid L-TCs were reconstituted by various combinations of isolated HA-33/HA-17 complexes and haemagglutination-negative L-TC2or L-TC3from each strain. HA-hybrid 650 kDa L-TC, including at least one HA-33/HA-17 complex derived from C-Yoichi, lost haemagglutination activity, leading to the conclusion that the binding of four HA-33 molecules is required for haemagglutination activity of botulinum L-TC. The results of the modelling approach indicated that the structure of a variant C-Yoichi HA-33 molecule reveals clear deformation of theβ-trefoil domain responsible for the carbohydrate recognition site.

Structural analysis of botulinum neurotoxin serotype F light chain: implications on substrate binding and inhibitor design

The seven serologically distinct Clostridium botulinum neurotoxins (BoNTs A-G) are zinc endopeptidases which block the neurotransmitter release by cleaving one of the three proteins of the soluble N-ethylmaleimide-sensitive-factor attachment protein receptor complex (SNARE complex) essential for the fusion of vesicles containing neurotransmitters with target membranes. These metallopeptidases exhibit unique specificity for the substrates and peptide bonds they cleave. Development of countermeasures and therapeutics for BoNTs is a priority because of their extreme toxicity and potential misuse as biowarfare agents. Though they share sequence homology and structural similarity, the structural information on each one of them is required to understand the mechanism of action of all of them because of their specificity. Unraveling the mechanism will help in the ultimate goal of developing inhibitors as antibotulinum drugs for the toxins. Here, we report the high-resolution structure of active BoNT/F catalytic domain in two crystal forms. The structure was exploited for modeling the substrate binding and identifying the S1' subsite and the putative exosites which are different from BoNT/A or BoNT/B. The orientation of docking of the substrate at the active site is consistent with the experimental BoNT/A-LC:SNAP-25 peptide model and our proposed model for BoNT/E-LC:SNAP-25.

Expression, purification, and characterization of Clostridium botulinum type B light chain

A full-length synthetic gene encoding the light chain of botulinum neurotoxin serotype B, approximately 50 kDa (BoNT/B LC), has been cloned into a bacterial expression vector pET24a+. BoNT/B LC was expressed in Escherichia coli BL21.DE3.pLysS and isolated from the soluble fraction. The resultant protein was purified to homogeneity by cation chromatography and was determined to be >98% pure as assessed by SDS-polyacrylamide gel stained with SilverXpress and analyzed by densitometry. Mass spectroscopic analysis indicated the protein to be 50.8 kDa, which equaled the theoretically expected mass. N-terminal sequencing of the purified protein showed the sequence corresponded to the known reported sequence. The recombinant BoNT/B light chain was found to be highly stable, catalytically active, and has been used to prepare antisera that neutralizes against BoNT/B challenge. Characterization of the protein including pH, temperature, and the stability of the protein in the presence or absence of zinc is described within. The influence of pH differences, buffer, and added zinc on secondary and tertiary structure of BoNT/B light chain was analyzed by circular dichroism and tryptophan fluorescence measurements. Optimal conditions for obtaining maximum metalloprotease activity and stabilizing the protein for long term storage were determined. We further analyzed the thermal denaturation of BoNT/B LC as a function of temperature to probe the pH and added zinc effects on light chain stability. The synthetic BoNT/B LC has been found to be highly active on its substrate (vesicle associated membrane protein-2) and, therefore, can serve as a useful reagent for BoNT/B research.

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.

Characterization of the antibody response to the receptor binding domain of botulinum neurotoxin serotypes A and E

Clostridium botulinum neurotoxins (BoNTs) are the most toxic proteins for humans. The current clostridial-derived vaccines against BoNT intoxication have limitations including production and accessibility. Conditions were established to express the soluble receptor binding domain (heavy-chain receptor [HCR]) of BoNT serotypes A and E in Escherichia coli. Sera isolated from mice and rabbits immunized with recombinant HCR/A1 (rHCR/A1) from the classical type A-Hall strain (ATCC 3502) (BoNT/A1) and rHCR/E from BoNT serotype E Beluga (BoNT/E(B)) neutralized the homologous serotype of BoNT but displayed differences in cross-recognition and cross-protection. Enzyme-linked immunosorbent assay and Western blotting showed that alpha-rHCR/A1 recognized epitopes within the C terminus of the HCR/A and HCR/E, while alpha-rHCR/E recognized epitopes within the N terminus or interface between the N and C termini of the HCR proteins. alpha-rHCR/E(B) sera possessed detectable neutralizing capacity for BoNT/A1, while alpha-rHCR/A1 did not neutralize BoNT/E. rHCR/A was an effective immunogen against BoNT/A1 and the Kyoto F infant strain (BoNT/A2), but not BoNT serotype E Alaska (BoNT/E(A)), while rHCR/E(B) neutralized BoNT/E(A), and under hyperimmunization conditions protected against BoNT/A1 and BoNT/A2. The protection elicited by rHCR/A1 to BoNT/A1 and BoNT/A2 and by rHCR/E(B) to BoNT/E(A) indicate that immunization with receptor binding domains elicit protection within sub-serotypes of BoNT. The protection elicited by hyperimmunization with rHCR/E against BoNT/A suggests the presence of common neutralizing epitopes between the serotypes E and A. These results show that a receptor binding domain subunit vaccine protects against serotype variants of BoNTs.

A new wrinkle on pain relief: re-engineering clostridial neurotoxins for analgesics

Botulinum neurotoxins are used to treat of a range of chronic neuromuscular conditions and, increasingly, conditions involving non-neuromuscular transmission, both cholinergic and non-cholinergic, including chronic pain. However, their clinical use is limited by the potential for adverse effects related to the neuromuscular activity, which results from the selectivity of the toxin for the neuromuscular junction. The elucidation of the structure of the botulinum toxin molecule and its relationship to neurotoxin function has enabled the design of novel molecules incorporating selected aspects of toxin function. This review considers the suitability of engineered neurotoxins as the basis for novel therapeutic proteins and the opportunity of developing analgesics based on these neurotoxins.

Crystal structure of the C3bot-RalA complex reveals a novel type of action of a bacterial exoenzyme

C3 exoenzymes from bacterial pathogens ADP-ribosylate and inactivate low-molecular-mass GTPases of the Rho subfamily. Ral, a Ras subfamily GTPase, binds the C3 exoenzymes from Clostridium botulinum and C. limosum with high affinity without being a substrate for ADP ribosylation. In the complex, the ADP-ribosyltransferase activity of C3 is blocked, while binding of NAD and NAD-glycohydrolase activity remain. Here we report the crystal structure of C3 from C. botulinum in a complex with GDP-bound RalA at 1.8 A resolution. C3 binds RalA with a helix-loop-helix motif that is adjacent to the active site. A quaternary complex with NAD suggests a mode for ADP-ribosyltransferase inhibition. Interaction of C3 with RalA occurs at a unique interface formed by the switch-II region, helix alpha3 and the P loop of the GTPase. C3-binding stabilizes the GDP-bound conformation of RalA and blocks nucleotide release. Our data indicate that C. botulinum exoenzyme C3 is a single-domain toxin with bifunctional properties targeting Rho GTPases by ADP ribosylation and Ral by a guanine nucleotide dissociation inhibitor-like effect, which blocks nucleotide exchange.