Efficacy of a novel metalloprotease inhibitor on botulinum neurotoxin B activity

Fragment-Based Lead Discovery Strategies in Antimicrobial Drug Discovery

Fragment-based lead discovery (FBLD) is a powerful application for developing ligands as modulators of disease targets. This approach strategy involves identification of interactions between low-molecular weight compounds (100-300 Da) and their putative targets, often with low affinity (K_D ~0.1-1 mM) interactions. The focus of this screening methodology is to optimize and streamline identification of fragments with higher ligand efficiency (LE) than typical high-throughput screening. The focus of this review is on the last half decade of fragment-based drug discovery strategies that have been used for antimicrobial drug discovery.

Targeted 8-hydroxyquinoline fragment based small molecule drug discovery against neglected botulinum neurotoxin type F

OBJECTIVES

Botulinum neurotoxins are highly potent biological warfare agents. The unavailability of countermeasures against these neurotoxins has been a matter of extensive research. However, no clinical therapeutics has come to existence till date. The 8-hydroxyquinoline (8-HQ) scaffold is established privileged compound and its potential as drug candidate against BoNTs is recently being explored.

METHODS

In present work, three course studies were performed involving in silico, in vitro and in vivo cascade to screen 8-HQ small molecule inhibitors against BoNT/F intoxication. ~800 molecules obtained from open repositories were screened in silico and commercially obtained twenty-four 8-HQ derived small molecule inhibitors were evaluated against rBoNT/F light chain through fluorescence thermal shift (FTS) assay. Selected compounds were further evaluated through endopeptidase assay. Further binding affinity analysis was done through surface plasmon resonance (SPR) based Proteon™ XPR 36 system. Finally, the in vivo efficacy of these compounds was evaluated in mice model.

RESULTS

Three compounds NSC1011, NSC1014 and NSC84094 were found to be highly inhibitory after screening of 8-HQ compounds through FTS assay and endopeptidase assay. SPR based protein-small molecule interaction studies showed highest affinity binding of NSC1014 (K_D: 5.58E-06) with BoNT/F-LC. NSC1011, NSC1014, and NSC84094 displayed IC₅₀ of 30.47 ± 6.24, 14.91 ± 2.49 and 17.39 ± 2.74 μM, respectively, in endopeptidase assay. NSC1011 and NSC1014 displayed marked extension of survival time in mice model.

CONCLUSION

NSC1011 and NSC1014 have emerged as promising drug candidate against BoNT/F intoxication displaying higher potential than previously reported compounds.

Natural Compounds and Their Analogues as Potent Antidotes against the Most Poisonous Bacterial Toxin

Botulinum neurotoxins (BoNTs), the most poisonous proteins known to humankind, are a family of seven (serotype A to G) immunologically distinct proteins synthesized primarily by different strains of the anaerobic bacterium Clostridium botulinum Being the causative agents of botulism, the toxins block neurotransmitter release by specifically cleaving one of the three soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) proteins, thereby inducing flaccid paralysis. The development of countermeasures and therapeutics against BoNTs is a high-priority research area for public health because of their extreme toxicity and potential for use as biowarfare agents. Extensive research has focused on designing antagonists that block the catalytic activity of BoNTs. In this study, we screened 300 small natural compounds and their analogues extracted from Indian plants for their activity against BoNT serotype A (BoNT/A) as well as its light chain (LCA) using biochemical and cellular assays. One natural compound, a nitrophenyl psoralen (NPP), was identified to be a specific inhibitor of LCA with an in vitro 50% inhibitory concentration (IC₅₀) value of 4.74 ± 0.03 µM. NPP was able to rescue endogenous synaptosome-associated protein 25 (SNAP-25) from cleavage by BoNT/A in human neuroblastoma cells with an IC₅₀ of 12.2 ± 1.7 µM, as well as to prolong the time to the blocking of neutrally elicited twitch tensions in isolated mouse phrenic nerve-hemidiaphragm preparations.IMPORTANCE The long-lasting endopeptidase activity of BoNT is a critical biological activity inside the nerve cell, as it prompts proteolysis of the SNARE proteins, involved in the exocytosis of the neurotransmitter acetylcholine. Thus, the BoNT endopeptidase activity is an appropriate clinical target for designing new small-molecule antidotes against BoNT with the potential to reverse the paralysis syndrome of botulism. In principle, small-molecule inhibitors (SMIs) can gain entry into BoNT-intoxicated cells if they have a suitable octanol-water partition coefficient (log P) value and other favorable characteristics (P. Leeson, Nature 481:455-456, 2012, https://doi.org/10.1038/481455a). Several efforts have been made in the past to develop SMIs, but inhibitors effective under in vitro conditions have not in general been effective in vivo or in cellular models (L. M. Eubanks, M. S. Hixon, W. Jin, S. Hong, et al., Proc Natl Acad Sci U S A 104:2602-2607, 2007, https://doi.org/10.1073/pnas.0611213104). The difference between the in vitro and cellular efficacy presumably results from difficulties experienced by the compounds in crossing the cell membrane, in conjunction with poor bioavailability and high cytotoxicity. The screened nitrophenyl psoralen (NPP) effectively antagonized BoNT/A in both in vitro and ex vivo assays. Importantly, NPP inhibited the BoNT/A light chain but not other general zinc endopeptidases, such as thermolysin, suggesting high selectivity for its target. Small-molecule (nonpeptidic) inhibitors have better oral bioavailability, better stability, and better tissue and cell permeation than antitoxins or peptide inhibitors.

High level expression and immunochemical characterization of botulinum neurotoxin type F light chain

Botulinum neurotoxins (BoNTs) are the most toxic biological substances known. Their potential use as biological warfare agent results in their classification as category A biowarfare agent by Centers for Disease Control and Prevention (CDC), USA. Presently, there are no approved detection system and pharmacological treatments for BoNT intoxication. Although a toxoid vaccine is available for immuno-prophylaxis, vaccines cannot reverse the effect of pre-translocated toxin. Direct handling of the live BoNTs for developing detection and therapeutics may pose fatal danger. This concern was addressed by purifying the recombinant catalytically active light chain of BoNT/F. BoNT/F-LC gene was amplified from the genomic DNA using specifically designed primers and expressed in Escherichia coli. Expression and purification profile were optimized under different conditions for biologically active light chain production. Specific polyclonal antibodies generated against type F illustrates in vivo neutralization in mice and rabbit. These antibodies play key role in conceiving the development of high throughput SPR based detection system which is a highly precise label free technique for protein interaction analysis. The presented work is first of its kind, signifying the production of highly stable and active rBoNT/F-LC and its immunochemical characterization. The study aids in paving the path towards developing a persistent detection system as well as in presenting comprehended scheme for in vitro small molecule therapeutics analysis.

Small molecule metalloprotease inhibitor with in vitro, ex vivo and in vivo efficacy against botulinum neurotoxin serotype A

Botulinum neurotoxins (BoNTs) are the most toxic substances known to mankind and are the causative agents of the neuroparalytic disease botulism. Their ease of production and extreme toxicity have caused these neurotoxins to be classified as Tier 1 bioterrorist threat agents and have led to a sustained effort to develop countermeasures to treat intoxication in case of a bioterrorist attack. While timely administration of an approved antitoxin is effective in reducing the severity of botulism, reversing intoxication requires different strategies. In the present study, we evaluated ABS 252 and other mercaptoacetamide small molecule active-site inhibitors of BoNT/A light chain using an integrated multi-assay approach. ABS 252 showed inhibitory activity in enzymatic, cell-based and muscle activity assays, and importantly, produced a marked delay in time-to-death in mice. The results suggest that a multi-assay approach is an effective strategy for discovery of potential BoNT therapeutic candidates.

Ex vivo inhibition of Clostridium botulinum neurotoxin types B, C, E, and F by small molecular weight inhibitors

Two small molecular weight inhibitors, compounds CB7969312 and CB7967495, that displayed inhibition of botulinum neurotoxin serotype A in a previous study, were evaluated for inhibition of botulinum neurotoxin serotypes B, C, E, and F. The small molecular weight inhibitors were assessed by molecular modeling, UPLC-based peptide cleavage assay; and an ex vivo assay, the mouse phrenic nerve - hemidiaphragm assay (MPNHDA). While both compounds were inhibitors of botulinum neurotoxin (BoNT) serotypes B, C, and F in the MPNHDA, compound CB7969312 was effective at lower molar concentrations than compound CB7967495. However, compound CB7967495 was significantly more effective at preventing BoNTE intoxication than compound CB7969312. In the UPLC-based peptide cleavage assay, CB7969312 was also more effective against LcC. Both compounds inhibited BoNTE, but not BoNTF, LcE, or LcF in the UPLC-based peptide cleavage assay. Molecular modeling studies predicted that both compounds would be effective inhibitors of BoNTs B, C, E, and F. But CB7967495 was predicted to be a more effective inhibitor of the four serotypes (B, C, E, and F) than CB7969312. This is the first report of a small molecular weight compound that inhibits serotypes B, C, E, and F in the ex vivo assay.

A cross-over inhibitor of the botulinum neurotoxin light chain B: a natural product implicating an exosite mechanism of action

Clostridium botulinum produces the most lethal toxins known to man, as such they are high risk terrorist threats, and alarmingly there is no approved therapeutic. We report the first cross-over small molecule inhibitor of these neurotoxins and propose a mechanism by which it may impart its inhibitory activity.

Small molecule inhibitors as countermeasures for botulinum neurotoxin intoxication

Botulinum neurotoxins (BoNTs) are the most potent of known toxins and are listed as category A biothreat agents by the U.S. CDC. The BoNT-mediated proteolysis of SNARE proteins inhibits the exocytosis of acetylcholine into neuromuscular junctions, leading to life-threatening flaccid paralysis. Currently, the only therapy for BoNT intoxication (which results in the disease state botulism) includes experimental preventative antibodies and long-term supportive care. Therefore, there is an urgent need to identify and develop inhibitors that will serve as both prophylactic agents and post-exposure ‘rescue’ therapeutics. This review focuses on recent progress to discover and develop small molecule inhibitors as therapeutic countermeasures for BoNT intoxication.

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.

Synthetic substrate for application in both high and low throughput assays for botulinum neurotoxin B protease inhibitors

A FRET peptide substrate was synthesized and evaluated for enzymatic cleavage by the BoNT/B light chain protease. The FRET substrate was found to be useful in both a high throughput assay to uncover initial 'hits' and a low throughput HPLC assay to determine kinetic parameters and modes of inhibition.

Investigations into small molecule non-peptidic inhibitors of the botulinum neurotoxins

Botulinum neurotoxins (BoNTs), proteins secreted by the bacteria genus Clostridium, represent a group of extremely lethal toxins and a potential bioterrorism threat. As the current therapeutic options are of a predominantly prophylactic nature and cannot be used en masse, new strategies and ultimately potential treatments are desperately needed to combat any widespread release of these neurotoxins. In these regards, our laboratory has been working on developing new alternatives to treat botulinum intoxication through the development of inhibitors of the light chain proteases, the etiological agent which causes BoNT intoxication. Such a strategy has required the construction of two high-throughput screens and small molecule non-peptidic libraries; excitingly, inhibitors of the BoNT/A protease have been uncovered and are being optimized via structure activity relationship studies.

The strange case of the botulinum neurotoxin: using chemistry and biology to modulate the most deadly poison

In the classic novella "The Strange Case of Dr. Jekyll and Mr. Hyde", Robert Louis Stevenson paints a stark picture of the duality of good and evil within a single man. Botulinum neurotoxin (BoNT), the most potent known toxin, possesses an analogous dichotomous nature: It shows a pronounced morbidity and mortality, but it is used with great effect in much lower doses in a wide range of clinical scenarios. Recently, tremendous strides have been made in the basic understanding of the structure and function of BoNT, which have translated into widespread efforts towards the discovery of biomacromolecules and small molecules that specifically modulate BoNT activity. Particular emphasis has been placed on the identification of inhibitors that can counteract BoNT exposure in the event of a bioterrorist attack. This Review summarizes the current advances in the development of therapeutics, including vaccines, peptides, and small-molecule inhibitors, for the prevention and treatment of botulism.

SNAP-25 substrate peptide (residues 180-183) binds to but bypasses cleavage by catalytically active Clostridium botulinum neurotoxin E

Clostridium botulinum neurotoxins are the most potent toxins to humans. The recognition and cleavage of SNAREs are prime evente in exhibiting their toxicity. We report here the crystal structure of the catalytically active full-length botulinum serotype E catalytic domain (BoNT E) in complex with SNAP-25 (a SNARE protein) substrate peptide Arg(180)-Ile(181)-Met(182)-Glu(183) (P1-P3'). It is remarkable that the peptide spanning the scissile bond binds to but bypasses cleavage by the enzyme and inhibits the catalysis fairly with K(i) approximately 69 microm. The inhibitory peptide occupies the active site of BoNT E and shows well defined electron density. The catalytic zinc and the conserved key residue Tyr(350) of the enzyme facilitate the docking of Arg(180) (P1) by interacting with its carbonyl oxygen that displaces the nucleophilic water. The general base Glu(212) side chain interacts with the main chain amino group of P1 and P1'. Conserved Arg(347) of BoNT E stabilizes the proper docking of the Ile(181) (P1') main chain, whereas the hydrophobic pockets stabilize the side chains of Ile(181) (P1') and Met(182) (P2'), and the 250 loop stabilizes Glu(183) (P3'). Structural and functional analysis revealed an important role for the P1' residue and S1' pocket in driving substrate recognition and docking at the active site. This study is the first of its kind and rationalizes the substrate cleavage strategy of BoNT E. Also, our complex structure opens up an excellent opportunity of structure-based drug design for this fast acting and extremely toxic high priority BoNT E.

Multiple pocket recognition of SNAP25 by botulinum neurotoxin serotype 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. The molecular basis for SNAP25 recognition and cleavage by BoNT serotype E is currently unclear. Here we define the multiple pocket recognition of SNAP25 by LC/E. The initial recognition of SNAP25 is mediated by the binding of the B region of SNAP25 to the substrate-binding (B) region of LC/E comprising Leu166, Arg167, Asp127, Ala128, Ser129, and Ala130. The mutations at these residues affected substrate binding and catalysis. Three additional residues participate in scissile bond cleavage of SNAP25 by LC/E. The P3 site residues, Ile178, of SNAP25 interacted with the S3 pocket in LC/E through hydrophobic interactions. The S3 pocket included Ile47, Ile164, and Ile182 and appeared to align the P1' and P2 residues of SNAP25 with the S1' and S2 pockets of LC/E. The S1' pocket of LC/E included three residues, Phe191, Thr159, and Thr208, which contribute hydrophobic and steric interactions with the SNAP25 P1' residue Ile181. The S2 pocket residue of LC/E, Lys224, binds the P2 residue of SNAP25, Asp179, through ionic interactions. Deletion mapping indicates that main chain interaction(s) of residues 182-186 of SNAP25 contribute to substrate recognition by LC/E. Understanding the mechanism for substrate specificity provides insight for the development of inhibitors against the botulinum neurotoxins.

The evolving field of biodefence: therapeutic developments and diagnostics

Bioweapons are a clear threat to both military and civilian populations. Here, the latest advances in the pursuit of inhibitors against biothreat threat toxins, current therapeutic strategies for treating biodefence related pathogens, and strategies for improving detection and exposure survivability are covered.

There are numerous lead therapeutics that have emerged from drug discovery efforts. However, many of these are toxic and/or fail to possess conventional drug-like properties. One clear advantage of small (non-peptidic) molecules is that they possess scaffolds that are inherently more likely to evolve into real therapeutics.

One of the major obstacles impeding the translation of these lead therapeutics into viable drugs is the lack of involvement of the pharmaceutical industry, which has been discovering leads and translating them into drugs for decades. The expertise of the pharmaceutical industry therefore needs to be more effectively engaged in developing drugs against biothreat agents.

New methods for rapidly detecting and diagnosing biothreat agents are also in development. The detection and diagnosis of biothreats is inherently linked with treatment. The means for detecting the release of bioweapons are being deployed, and new technologies are shortening the timeframe between initial sample collection and conclusive agent determination. However, the organization of this process is imperfect.

At present, a unifying entity that orchestrates the biodefence response is clearly needed to reduce the time-to-drug process and redundancies in drug development efforts. Such a central entity could formulate and implement plans to coordinate all participants, including academic institutions, government agencies and the private sector. This could accelerate the development of countermeasures against high probability biothreat agents.

The threat of bioterrorism and the potential use of biological weapons against both military and civilian populations has become a major concern for governments around the world. For example, in 2001 anthrax-tainted letters resulted in several deaths, caused widespread public panic and exerted a heavy economic toll. If such a small-scale act of bioterrorism could have such a huge impact, then the effects of a large-scale attack would be catastrophic. This review covers recent progress in developing therapeutic countermeasures against, and diagnostics for, such agents.

Primary cell culture for evaluation of botulinum neurotoxin antagonists

The actions of botulinum neurotoxin (BoNT) were studied on evoked release of the neurotransmitter glycine in primary mouse spinal cord cells. 3[H]-glycine was taken up by cells in physiological solution and released by depolarization with 56 mM K+ in the presence of 2 mM Ca2+. Release of 3[H]-glycine was found to be inhibited by BoNT serotypes A, B and E with similar potency ratios to those observed in the acutely isolated mouse diaphragm muscle. When spinal cord cultures were exposed to BoNT/A for 24 h, inhibition of 3[H]-glycine release was detected at toxin concentrations as low as 10(-14) M, and complete inhibition was observed at concentration >or=10(-12) M. Preincubation of BoNT/A with polyclonal equine antiserum led to antagonism of toxin-induced inhibition of 3[H]-glycine release in spinal cord cells and to protection of mice from the lethal effects of BoNT/A. It is concluded that spinal cord neurons are a useful model for studying botulinum intoxication and for evaluating BoNT antagonists.

Botulinum Neurotoxin Serotype F: Identification of Substrate Recognition Requirements and Development of Inhibitors with Low Nanomolar Affinity

Botulinum neurotoxins (BoNTs A-G) are zinc metalloendoproteases that exhibit extraordinary specificities for proteins involved in neurotransmitter release. In view of the extreme toxicities of these molecules, their applications in human medicine, and potential for misuse, it is of considerable importance to elucidate the mechanisms underlying substrate recognition and to develop inhibitors, with the ultimate goal of obtaining anti-botulinum drugs. We synthesized peptides based on vesicle-associated membrane protein (VAMP) to investigate the substrate requirements of BoNT F, which cleaves VAMP between residues Q58 and K59. The minimum substrate was a peptide containing VAMP residues 32-65, which includes only one of the two VAMP structural motifs thought to be required for botulinum substrate recognition. BoNT F exhibited a strict requirement for residues D57 (P(2)), K59 (P(1)'), and L60 (P(2)'), but peptides containing substitutions for R56 (P(3)), Q58 (P(1)), and S61 (P(3)') were cleaved. Therefore, the P(2), P(1)', and P(2)' residues of VAMP are of paramount importance for BoNT F substrate recognition near the scissile bond. K(i) values of uncleavable analogues were similar to K(m) values of the substrate, suggesting that substrate discrimination occurs at the cleavage step, not at the initial binding step. We then synthesized inhibitors of BoNT F that incorporated d-cysteine in place of glutamine 58, exhibited K(i) values of 1-2 nM, and required binding groups on the N-terminal but not the C-terminal side of the zinc ligand. The latter characteristic distinguishes BoNT F from other zinc metalloendoproteases, including BoNTs A and B.

Antitoxins: novel strategies to target agents of bioterrorism

Never before has there been such a strong possibility that biological agents might be used indiscriminately on civilian populations. This review focuses on the use of antitoxins - antibodies, receptor decoys, dominant-negative inhibitors of translocation, small-molecule inhibitors and substrate analogues - to counteract those biological weapons for which toxins are an important mechanism of disease pathogenesis.

Antagonism of botulinum toxin type A-induced cleavage of SNAP-25 in rat cerebral synaptosome by toosendanin

Toosendanin (TSN), a triterpenoid derivative extracted from Chinese traditional medicine, has been demonstrated to be an effective cure for experimental botulism. This study is designed to explore its antibotulismic mechanism by Western blotting. The results showed that TSN incubation did not change the electrophoresis pattern and the amounts of synaptosomal-associated protein of 25 kDa (SNAP-25), syntaxin and synaptobrevin/vesicle-associated membrane protein in rat cerebral synaptosomes, but made the synaptosomes completely resistant to botulinum neurotoxin A (BoNT/A)-mediated cleavage of SNAP-25. After binding of BoNT/A to synaptosomes, TSN still partially antagonized the toxin-mediated cleavage of SNAP-25. However, TSN-incubated synaptosomal membrane fraction did not resist the cleavage of SNAP-25 by the light chain of BoNT/A. It is suggested that the antibotulismic effect of TSN results from blocking the toxin's approach to its enzymatic substrate.

Thio-derived disulfides as potent inhibitors of botulinum neurotoxin type B: implications for zinc interaction

Botulinum neurotoxin type B causes the inhibition of acetylcholine release at the neuromuscular junction resulting in a flaccid paralysis designated botulism. This occurs through the cleavage of synaptobrevin, an intracellular critical component of neurotransmitter exocytosis, by the zinc-metallopeptidase activity of the smallest subunit of the toxin. Blocking the proteolytic activity may present an attractive approach to treat botulism as to date there is no efficient specific drug therapy available. We have therefore recently described a series of beta-amino-thiol derived pseudotripeptides able of inhibiting the toxin at low (10(-8) M) concentration. In this study, binding characteristics of the protein's active site are explored through various structural modifications of the thiol functionality which was supposed to be a key structural constituent for effective zinc-ion chelation. Surprisingly, sulfanyl-derivatives such as symmetric disulfides were shown to be better inhibitors than their thiol-counterparts, the most potent compound displaying a Ki value of 3.4 nM.

Development of Potent Inhibitors of Botulinum Neurotoxin Type B

Botulinum neurotoxins are the most potent toxins known to date. They are zinc-metalloproteases able to cleave selectively an essential component of neurotransmitter exocytosis, causing the syndrome of botulism characterized by a flaccid paralysis. There is a great interest in designing antagonists of the action of these toxins. One way is to inhibit their catalytic activity. In this study, we report the design of such inhibitors directed toward BoNT/B. A study of the S(1) subsite specificity, using several beta-amino thiols, has shown that this subsite prefers a p-carboxybenzyl moiety. The specificity of the S(1)' and S(2)' subsites was studied using two libraries of pseudotripeptides containing the S(1) synthon derived from the best beta-amino thiol tested. Finally, a selection of various non natural amino acids for the recognition of the "prime" domain led to the most potent inhibitor of BoNT/B described to date with a K(i) value of 20 nM.

Fluorigenic substrates for the protease activities of botulinum neurotoxins, serotypes A, B, and F

The seven botulinum neurotoxins (BoNTs) are zinc metalloproteases that cleave neuronal proteins involved in neurotransmitter release and are among the most toxic natural products known. High-throughput BoNT assays are needed for use in antibotulinum drug discovery and to characterize BoNT protease activities. Compared to other proteases, BoNTs exhibit unusually stringent substrate requirements with respect to amino acid sequences and polypeptide lengths. Nonetheless, we have devised a strategy for development of fluorigenic BoNT protease assays, based on earlier structure-function studies, that has proven successful for three of the seven serotypes: A, B, and F. In synthetic peptide substrates, the P(1) and P(3)' residues were substituted with 2,4-dinitrophenyl-lysine and S-(N-[4-methyl-7-dimethylamino-coumarin-3-yl]-carboxamidomethyl)-cysteine, respectively. By monitoring the BoNT-catalyzed increase in fluorescence over time, initial hydrolysis rates could be obtained in 1 to 2 min when BoNT concentrations were 60 ng/ml (about 1 nM) or higher. Each BoNT cleaved its fluorigenic substrate at the same location as in the neuronal target protein, and kinetic constants indicated that the substrates were selective and efficient. The fluorigenic assay for BoNT B was used to characterize a new competitive inhibitor of BoNT B protease activity with a K(i) value of 4 micro M. In addition to real-time activity measurements, toxin concentration determinations, and kinetic studies, the BoNT substrates described herein may be directly incorporated into automated high-throughput assay systems to screen large numbers of compounds for potential antibotulinum drugs.

A high-affinity competitive inhibitor of type A botulinum neurotoxin protease activity

The peptide N-acetyl-CRATKML-amide is an effective inhibitor of type A botulinum neurotoxin (BoNT A) protease activity [Schmidt et al., FEBS Lett. 435 (1998) 61-64]. To improve inhibitor binding, the peptide was modified by replacing cysteine with other sulfhydryl-containing compounds. Ten peptides were synthesized. One peptide adapted the structure of captopril to the binding requirements of BoNT A, but it was a weak inhibitor, suggesting that angiotensin-converting enzyme is not a good model for BoNT A inhibitor development. However, replacing cysteine with 2-mercapto-3-phenylpropionyl yielded a peptide with K(i) of 330 nM, the best inhibitor of BoNT A protease activity reported to date. Additional modifications of the inhibitor revealed structural elements important for binding and supported our earlier findings that, with the exception of P1' arginine, subsites on BoNT A are not highly specific for particular amino acid side chains.

A novel mechanism for Clostridium botulinum neurotoxin inhibition

Clostridium botulinum neurotoxins are zinc endopeptidase proteins responsible for cleaving specific peptide bonds of proteins of neuroexocytosis apparatus. The ability of drugs to interfere with toxin's catalytic activity is being evaluated with zinc chelators and metalloprotease inhibitors. It is important to develop effective pharmacological treatment for the intact holotoxin before the catalytic domain separates and enters the cytosol. We present here evidence for a novel mechanism of an inhibitor binding to the holotoxin and for the chelation of zinc from our structural studies on Clostridium botulinum neurotoxin type B in complex with a potential metalloprotease inhibitor, bis(5-amidino-2-benzimidazolyl)methane, and provide snapshots of the reaction as it progresses. The binding and inhibition mechanism of this inhibitor to the neurotoxin seems to be unique for intact botulinum neurotoxins. The environment of the active site rearranges in the presence of the inhibitor, and the zinc ion is gradually removed from the active site and transported to a different site in the protein, probably causing loss of catalytic activity.

Development of a delivery vehicle for intracellular transport of botulinum neurotoxin antagonists

A targeted delivery vehicle (DV) was developed for intracellular transport of emerging botulinum neurotoxin (BoNT) antagonists. The DV consisted of the isolated heavy chain (HC) of BoNT/A coupled to a 10-kDa amino dextran via the heterobifunctional linker 3-(2-pyridylthio)-propionyl hydrazide. The HC served to target BoNT-sensitive cells and promote internalization of the complex, while the dextran served as a platform to deliver model therapeutic molecules to the targeted cells. To determine the ability of this chimeric glycoprotein to enter neurons, dextran and HC were labeled independently with the fluorescent dyes Oregon green 488 and Cy3, respectively. Internalization of DV was monitored in primary cortical cells using laser confocal microscopy. Incubation of cells for 24 h with DV resulted in discrete punctate labeling of both soma and processes. The Cy3 and Oregon green 488 signals were generally co-localized, suggesting that the complex remained in the same intracellular compartment during the initial 24 h. The DV-associated fluorescence was reduced progressively by co-application of increasing concentrations of unlabeled BoNT/A holotoxin. The results suggest that the BoNT/A HC is able to mediate internalization of a coupled dextran, even though the latter bears no resemblance to the BoNT/A light chain (LC). The HC of BoNT/A thus offers promise as a selective carrier to deliver BoNT antagonists to the nerve terminal cytoplasm for inhibiting the proteolytic activity of internalized BoNT/A LC.

High-throughput assays for botulinum neurotoxin proteolytic activity: serotypes A, B, D, and F

Botulinum neurotoxins (BoNT) are zinc metalloproteases that cleave and inactivate cellular proteins essential for neurotransmitter release. Because the paralytic effect of BoNT is a consequence of its enzymatic activity, selective inhibitors may be useful as drugs or as tools for further research. To expedite inhibitor discovery, we developed high-throughput, solid-phase protease activity assays for four of the seven BoNT serotypes: A, B, D, and F. Each assay consisted of a cleavable oligopeptide, based on the natural substrate sequence, labeled with fluorescein and covalently attached to maleimide-activated multiwell plates. Solutions of holotoxin or nontoxic catalytic domain of BoNT were incubated in substrate-coated wells, with or without test compounds, followed by transfer and assay of solubilized product in a multiwell fluorometer. Routine toxin concentrations ranged from 10 to 100 ng/ml, but concentrations as low as 2 ng/ml gave reproducible signals. The fluorescence assays were selective, gave very low background readings, and were stable upon prolonged storage. Using the nontoxic catalytic domain of BoNT A, we determined the relative inhibitory potencies of a family of structurally related pseudotripeptide compounds. Unlike previous methods, our assays did not employ antibodies or reverse-phase extraction steps, only well-to-well transfers, and were easily adapted to a high-throughput automated environment.

High-throughput fluorogenic assay for determination of botulinum type B neurotoxin protease activity

Botulinum neurotoxins are responsible for botulism, a flaccid muscular paralysis caused by inhibition of acetylcholine release at the neuromuscular junction. This occurs by cleavage of conserved proteins involved in exocytosis such as synaptobrevin by the zinc metallopeptidase activity of the light chain of some botulinum neurotoxins. Botulism, for which there is presently no therapy available, is a relatively widespread disease that may result in death. Consequently, the development of drugs able to inhibit the hydrolytic activity of these neurotoxins is of great interest. Design and screening of such inhibitors could be largely facilitated by using high-throughput assays. With this aim, a novel in vitro test for quantifying the proteolytic activity of botulinum type B neurotoxin was developed. The substrate is the 60--94 fragment of human synaptobrevin-1 which was modified by introduction of the fluorescent amino acid l-pyrenylalanine in position 74 and a p-nitrophenylalanyl residue as quenching group in position 77. The cleavage of Syb 60-94 [Pya(74), Nop(77)] by the toxin active chain occurs selectively between residues 76 and 77 as in the case of the unmodified synaptobrevin and is directly quantified by measuring the strong fluorescence of the formed metabolite Syb 60-76 [Pya(74)]. This is the easiest, quickest, and cheapest assay described to date for measuring the proteolytic activity of botulinum type B neurotoxin. It can be easily automated for high-throughput screening. Moreover, amounts of about 3.5 pg/ml of botulinum type B neurotoxin could be detected by this method.

Characterization of the zinc binding activity of the rubella virus nonstructural protease

The rubella virus (RUB) nonstructural (NS) protein (NSP) ORF encodes a protease that cleaves the NSP precursor (240 kDa) at a single site to produce two products. A cleavage site mutation was introduced into a RUB infectious cDNA clone and found to be lethal, demonstrating that cleavage of the NSP precursor is necessary for RUB replication. Based on computer alignments, the RUB NS protease was predicted to be a papain-like cysteine protease (PCP) with the residues Cys1152 and His1273 as the catalytic dyad; however, the RUB NS protease was recently found to require divalent cations such as Zn, Co, and Cd for activity (X. Liu, S. L. Ropp, R. J. Jackson, and T. K. Frey, J. Virol. 72:4463-4466, 1998). To analyze the function of metal cation binding in protease activity, Zn binding studies were performed using the minimal NS protease domain within the NSP ORF. When expressed as a maltose binding protein (MBP) fusion protein by bacteria, the NS protease exhibited activity both in the bacteria and in vitro following purification when denatured and refolded in the presence of Zn. Atomic absorption analysis detected 1.6 mol of Zn bound per mol of protein refolded in this manner. Expression of individual domains within the protease as MBP fusions and analysis by a Zn(65) binding assay revealed two Zn binding domains: one located at a predicted metal binding motif beginning at Cys1175 and the other one close to the cleavage site. Mutagenesis studies showed that Cys1175 and Cys1178 in the first domain and Cys1227 and His1273, the His in the predicted catalytic site, in the second domain are essential for zinc binding. All of these residues are also necessary for the protease activity, as were several other Cys residues not involved in Zn binding. Far-UV circular dichroism (CD) analysis of the MBP-NS protease fusion protein showed that the protease domain contained a large amount of alpha-helical structure, which is consistent with the results of secondary-structural prediction. Both far-UV-CD and fluorescence studies suggested that Zn did not exert a major effect on the overall structure of the fusion protein. Finally, protease inhibitor assays found that the protease activity can be blocked by both metal ion chelators and the metalloprotease inhibitor captopril. In conjunction with the finding that the previously predicted catalytic site, His1273, is essential for zinc binding, this suggests that the RUB NS protease is actually a novel virus metalloprotease rather than a PCP.

How botulinum and tetanus neurotoxins block neurotransmitter release

Botulinum neurotoxins (BoNT, serotypes A-G) and tetanus neurotoxin (TeNT) are bacterial proteins that comprise a light chain (M(r) approximately 50) disulfide linked to a heavy chain (M(r) approximately 100). By inhibiting neurotransmitter release at distinct synapses, these toxins cause two severe neuroparalytic diseases, tetanus and botulism. The cellular and molecular modes of action of these toxins have almost been deciphered. After binding to specific membrane acceptors, BoNTs and TeNT are internalized via endocytosis into nerve terminals. Subsequently, their light chain (a zinc-dependent endopeptidase) is translocated into the cytosolic compartment where it cleaves one of three essential proteins involved in the exocytotic machinery: vesicle associated membrane protein (also termed synaptobrevin), syntaxin, and synaptosomal associated protein of 25 kDa. The aim of this review is to explain how the proteolytic attack at specific sites of the targets for BoNTs and TeNT induces perturbations of the fusogenic SNARE complex dynamics and how these alterations can account for the inhibition of spontaneous and evoked quantal neurotransmitter release by the neurotoxins.

Neurotoxins affecting neuroexocytosis

Nerve terminals are specific sites of action of a very large number of toxins produced by many different organisms. The mechanism of action of three groups of presynaptic neurotoxins that interfere directly with the process of neurotransmitter release is reviewed, whereas presynaptic neurotoxins acting on ion channels are not dealt with here. These neurotoxins can be grouped in three large families: 1) the clostridial neurotoxins that act inside nerves and block neurotransmitter release via their metalloproteolytic activity directed specifically on SNARE proteins; 2) the snake presynaptic neurotoxins with phospholipase A(2) activity, whose site of action is still undefined and which induce the release of acethylcholine followed by impairment of synaptic functions; and 3) the excitatory latrotoxin-like neurotoxins that induce a massive release of neurotransmitter at peripheral and central synapses. Their modes of binding, sites of action, and biochemical activities are discussed in relation to the symptoms of the diseases they cause. The use of these toxins in cell biology and neuroscience is considered as well as the therapeutic utilization of the botulinum neurotoxins in human diseases characterized by hyperfunction of cholinergic terminals.