Interactions between heavy metal chelators and botulinum neurotoxins at the mouse neuromuscular junction

High-Throughput Screening Uncovers Novel Botulinum Neurotoxin Inhibitor Chemotypes

Botulism is caused by potent and specific bacterial neurotoxins that infect host neurons and block neurotransmitter release. Treatment for botulism is limited to administration of an antitoxin within a short time window, before the toxin enters neurons. Alternatively, current botulism drug development targets the toxin light chain, which is a zinc-dependent metalloprotease that is delivered into neurons and mediates long-term pathology. Several groups have identified inhibitory small molecules, peptides, or aptamers, although no molecule has advanced to the clinic due to a lack of efficacy in advanced models. Here we used a homogeneous high-throughput enzyme assay to screen three libraries of drug-like small molecules for new chemotypes that modulate recombinant botulinum neurotoxin light chain activity. High-throughput screening of 97088 compounds identified numerous small molecules that activate or inhibit metalloprotease activity. We describe four major classes of inhibitory compounds identified, detail their structure-activity relationships, and assess their relative inhibitory potency. A previously unreported chemotype in any context of enzyme inhibition is described with potent submicromolar inhibition (Ki = 200-300 nM). Additional detailed kinetic analyses and cellular cytotoxicity assays indicate the best compound from this series is a competitive inhibitor with cytotoxicity values around 4-5 μM. Given the potency and drug-like character of these lead compounds, further studies, including cellular activity assays and DMPK analysis, are justified.

The neurosteroids, allopregnanolone and progesterone, induce autophagy in cultured astrocytes

Recent studies have suggested that neurosteroids such as pregnenolone, progesterone (PG) and their derivatives, have a role in activating autophagy in addition to diverse other functions. In our previous studies, we demonstrated that cellular free Zn(2+) is involved in oxidative stress-induced autophagy and autophagic cell death in astrocytes. In the present study, we examined the possibility that neurosteroids, allopregnanolone (Allo) and PG, also activate autophagy in cultured mouse astrocytes through modulation of intracellular Zn(2+). Exposure of astrocytes to 250 nM Allo or 500 nM PG caused cytosolic vacuoles to appear within a few hours of treatment onset. Live-cell confocal microscopy of astrocytes transfected with red fluorescent protein-conjugated LC3 (RFP-LC3), a marker for autophagic vacuoles (AVs), as well as transmission electron microscopy, revealed that these vacuoles were AVs. In addition, Western blots showed increases in LC3-II levels. Interestingly, mTOR and Akt were concurrently activated, and their blockade further increased LC3-II levels and caused some cell death. These results indicate that co-activation of mTOR and Akt may act to limit neurosteroid-induced autophagy and thus inhibit autophagic cell death. As in other cases of autophagy, cellular Zn(2+) levels increased after treatment with neurosteroids. The neurosteroid-induced increase in LC3-II levels was inhibited by addition of the Zn(2+) chelator TPEN. Both the increase in LC3-II levels and activation of Akt and mTOR by neurosteroids were all mediated by PG receptors, as the effects were blocked by the addition of RU-486, a PG receptor antagonist. Moreover, mutant huntingtin (mHtt) aggregates in GFP-mHttQ74-transfected astrocytes were substantially reduced by neurosteroid treatment, indicating that neurosteroid-induced autophagy may be functional. Present results demonstrate that Allo and PG activate autophagy in astrocytes. Notably, unlike several other autophagy inducers that, in excess, may cause autophagic cell death, Allo and PG are relatively non-toxic, possibly because of concurrent Akt and mTOR activation. Thus, as natural endogenous brain substances, Allo and PG may have a potential as therapeutic agents in neurodegenerative conditions in which abnormal protein aggregates are involved.

Role of zinc metallothionein-3 (ZnMt3) in epidermal growth factor (EGF)-induced c-Abl protein activation and actin polymerization in cultured astrocytes

Recent evidence indicates that zinc plays a major role in neurochemistry. Of the many zinc-binding proteins, metallothionein-3 (Mt3) is regarded as one of the major regulators of cellular zinc in the brain. However, biological functions of Mt3 are not yet well characterized. Recently, we found that lysosomal dysfunction in metallothionein-3 (Mt3)-null astrocytes involves down-regulation of c-Abl. In this study, we investigated the role of Mt3 in c-Abl activation and actin polymerization in cultured astrocytes following treatment with epidermal growth factor (EGF). Compared with wild-type (WT) astrocytes, Mt3-null cells exhibited a substantial reduction in the activation of c-Abl upon treatment with EGF. Consistent with previous studies, activation of c-Abl by EGF induced dissociation of c-Abl from F-actin. Mt3 added to astrocytic cell lysates bound F-actin, augmented F-actin polymerization, and promoted the dissociation of c-Abl from F-actin, suggesting a possible role for Mt3 in this process. Conversely, Mt3-deficient astrocytes showed significantly reduced dissociation of c-Abl from F-actin following EGF treatment. Experiments using various peptide fragments of Mt3 showed that a fragment containing the N-terminal TCPCP motif (peptide 1) is sufficient for this effect. Removal of zinc from Mt3 or pep1 with tetrakis(2-pyridylmethyl)ethylenediamine abrogated the effect of Mt3 on the association of c-Abl and F-actin, indicating that zinc binding is necessary for this action. These results suggest that ZnMt3 in cultured astrocytes may be a normal component of c-Abl activation in EGF receptor signaling. Hence, modulation of Mt3 levels or distribution may prove to be a useful strategy for controlling cytoskeletal mobilization following EGF stimulation in brain cells.

A nerve clamp electrode design for indirect stimulation of skeletal muscle

A nerve clamp electrode was developed to indirectly stimulate skeletal muscle innervated by α motor neurons as an alternative to conventional electrodes. The stimulating electrode device consists of a spring coil-activated nerve clamp mounted inside a 1-mL syringe barrel. Supramaximal pulses were generated by a Grass stimulator and delivered to the nerve segment via the nerve clamp electrode. The salient feature of the electrode is its ability to produce muscle contractions indirectly through stimulation of the attached nerve. Indirect muscle stimulation is critical for studying the paralytic actions of presynaptic-acting toxins such as botulinum neurotoxins (BoNT), a potent inhibitor of acetylcholine (ACh) release from α motor neurons. This device enables stimulation of muscle contraction indirectly as opposed to contraction from direct muscle stimulation. The electrode is able to stimulate indirect muscle contraction when tested on ex vivo preparations from rodent phrenic nerve-hemidiaphragm muscle in similar fashion to conventional electrodes. In addition, the electrode stimulated external intercostal nerve-muscle preparations. This was confirmed after applying BoNT serotype A, a potent inhibitor of ACh release, to induce muscle paralysis. Alternative methods, including suction and bipolar loop electrodes, were unsuccessful in stimulating indirect muscle contraction. Therefore, this novel electrode is useful for physiological assessment of nerve agents and presynaptic actions of toxins that cause muscle paralysis. This electrode is useful for stimulating nerve-muscle preparations for which the length of nerve is a concern.

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.

Comparison of extracellular and intracellular potency of botulinum neurotoxins

Levels of botulinum neurotoxin (BoNT) proteolytic activity were compared using a cell-free assay and living neurons to measure extracellular and intracellular enzymatic activity. Within the cell-free reaction model, BoNT serotypes A and E (BoNT/A and BoNT/E, respectively) were reversibly inhibited by chelating Zn2+ with N,N,N',N'-tetrakis (2-pyridylmethyl) ethylenediamine (TPEN). BoNT/E required relatively long incubation with TPEN to achieve total inhibition, whereas BoNT/A was inhibited immediately upon mixing. When naïve Zn2+-containing BoNTs were applied to cultured neurons, the cellular action of each BoNT was rapidly inhibited by subsequent addition of TPEN, which is membrane permeable. Excess Zn2+ added to the culture medium several hours after poisoning fully restored intracellular toxin activity. Unlike TPEN, EDTA irreversibly inhibited both BoNT/A and -E within the cell-free in vitro reaction. Excess Zn2+ did not reactivate the EDTA-treated toxins. However, application of EDTA-treated BoNT/A or -E to cultured neurons demonstrated normal toxin action in terms of both blocking neurotransmission and SNAP-25 proteolysis. Different concentrations of EDTA produced toxin preparations with incrementally reduced in vitro proteolytic activities, which, when applied to living neurons showed undiminished cellular potency. This suggests that EDTA renders the BoNT proteolytic domain conformationally inactive when tested with the cell-free reaction, but this change is corrected during entry into neurons. The effect of EDTA is unrelated to Zn2+ because TPEN could be applied to living cells before or after poisoning to produce rapid and reversible inhibition of both BoNTs. Therefore, bound Zn2+ is not required for toxin entry into neurons, and removal of Zn2+ from cytosolic BoNTs does not irreversibly alter toxin structure or function. We conclude that EDTA directly alters both BoNTs in a manner that is independent of Zn2+.

The use of small molecules to investigate molecular mechanisms and therapeutic targets for treatment of botulinum neurotoxin A intoxication

Botulinum neurotoxins (BoNTs) are agents responsible for botulism, a disease characterized by peripheral neuromuscular blockade and subsequent flaccid paralysis. The potent paralytic ability of these toxins has resulted in their use as a therapeutic; however, BoNTs are also classified by the Centers for Disease Control and Prevention as one of the six highest-risk threat agents of bioterrorism. Consequently, a thorough understanding of the molecular mechanism of BoNT toxicity is crucial before effective inhibitors and, ultimately, an approved drug can be developed. In this article, we systematically detail BoNT intoxication by examining each of the discrete steps in this process. Additionally, rationally designed strategies for combating the toxicity of the most potent BoNT serotype are evaluated.

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.

Anomalous enhancement of botulinum toxin type A neurotoxicity in the presence of antitoxin

The neutralization of botulinum toxin serotype A with polyclonal equine antitoxin was studied in isolated mouse hemidiaphragms and compared to the same action in live mice. The biological activity of the toxin in the isolated muscle could be markedly reduced with excess antitoxin, estimated as 3:1 molar ratios of IgG Ab:toxin or better. Toxin neutralization in vivo required higher ratios of Ab:toxin, ranging from 30:1 at high toxin doses and increasing to 100:1 at 10xLD50 toxin. At equimolar Ab to toxin ratios in the isolated muscle, the biological activity of the toxin underwent a statistically significant increase. This paradoxical effect of the polyclonal antisera was serotype selective and independent of the presence or absence of hemagglutinin in the toxin. The enhancement of toxin activity was subsequently localized to occupancy of one of four epitopes on the toxin using monoclonal antibodies to mimic the effect of the antitoxin. The enhancement of toxin activity suggests that botulinum toxin may undergo a conformational change upon binding antibodies to certain domains. This phenomenon could contribute to the observed concentration dependent changes in neutralization efficacy with antitoxin in vivo.

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.

Pharmacologic characterization of botulinum toxin for basic science and medicine

The use of Botulinum neurotoxin (BoNT) is increasing in both clinical and basic science. Clinically, intramuscular injection of nanogram quantities of BoNT is fast becoming the treatment of choice for a spectrum of disorders including movement disorders such as torticollis, blepharospasm, Meige Disease, and hemifacial spasm (Borodic et al., 1991, 1994a; Jankovic and Brin, 1991; Clarke, 1992). Neuroscientists are using BoNTs as tools to develop a better understanding of the mechanisms underlying the neurotransmitter release process. Consequently, our ability to accurately and reliably quantify the biologic activity of botulinum toxin has become more important than ever. The accurate measurement of the pharmacologic activity of BoNTs has become somewhat problematic with the most significant problems occurring with the clinical use of the toxins. The biologic activity of BoNTs has been measured using a variety of techniques including assessment of whole animal responses to in vitro effects on neurotransmitter release. The purpose of this review is to examine the approaches employed to characterize, quantify and investigate the actions of the BoNTs and to provide a guide to aid investigators in determining which of these methods is most appropriate for their particular application or use.

Protection by the heavy metal chelator N,N,N',N'-tetrakis (2-pyridylmethyl)ethylenediamine (TPEN) against the lethal action of botulinum neurotoxin A and B

The ability of N,N,N',N'-tetrakis (2-pyridylmethyl)-ethyenediamine (TPEN) to protect against botulinum neurotoxin (BoNT) A and B was examined in vivo in mice. To determine the protective efficacy of TPEN, mice were injected i.p. with TPEN as a single bolus or as multiple injections 30 min before and 0, 2, 4 and 6 hr following i.v. challenges with BoNT-A or -B. TPEN treatment did not alter the 24 hr lethality of BoNT but did produce a significant delay in the time to death. For a moderate dose of serotype A (20 LD50), five divided doses of TPEN prolonged the time to death from 7.8 +/- 0.4 hr to 9.9 +/- 0.5 hr. For serotype B, examined under comparable conditions, the prolongation of the time to death was from 6.1 +/- 0.2 hr to 9.4 +/- 0.6 hr. The range of TPEN doses that could be examined in vivo was limited by its acute toxicity. Although low doses of TPEN (< or = 10 mg/kg) were well tolerated, higher doses (> or = 30 mg/kg) led to ataxia, loss of coordination, convulsions and death in 20.3 min or less. In clonal NG108-15 cells, TPEN was found to produce cytotoxicity as revealed by increases in the secretion of the marker enzyme lactate dehydrogenase (LDH), and enhanced reactivity with the vital dye trypan blue. From LDH concentration-response data determined 24 hr after addition of TPEN, the threshold concentration for observing cytotoxicity was 10 microM and the IC50 was 19.8 microM. At the highest TPEN concentration tested (100 microM), cytotoxicity was detected 8 hr after TPEN addition and increased in severity over a 3 day period. The cytotoxicity in NG108-15 cells appears to be distinct from the rapid-onset toxicity observed in whole animals. These results suggest that TPEN may be of potential benefit in delaying the lethal actions of BoNT-A and -B, but its use is limited by its initial and delayed toxicity. Since the therapeutic and toxic actions of TPEN are both related to zinc chelation, the use of TPEN would need to be restricted to low doses as part of a combination therapy.

Efficacy of certain quinolines as pharmacological antagonists in botulinum neurotoxin poisoning

Various 4- and 8-aminoquinolines, which are effective antimalarial agents, were examined as potential pretreatment compounds for prolongation of the time to 50% block of nerve-elicited muscle twitches in isolated mouse diaphragms exposed to botulinum type A neurotoxin. The 4-aminoquinolines (chloroquine, amodiaquine) and quinacrine, an acridine derivative similar to chloroquine, prolonged the time required for botulinum type A neurotoxin to block neuromuscular transmission by more than 3-fold; 8-aminoquinolines (primaquine and WR242511) had no antibotulinum type A neurotoxin activity. Pyrimethamine, an antimalarial drug lacking the quinoline ring structure, was also ineffective. Rank order potencies based on equimolar effective concentrations for the test compounds were quinacrine > amodiaquine > chloroquine > quinine or quinidine. Maximum protection from botulinum type A neurotoxin-induced neuromuscular block was achieved when muscles were exposed to drug prior to or simultaneously with the toxin. A delay of more than 20 min abolished the protective ability of the antimalarial agents, presumably owing to the release of the toxin from endosomes in quantities sufficient to initiate neuromuscular block. All of the test compounds except quinine and quinidine depressed muscle contractions when concentrations exceeded 20 microM. In addition, amodiaquine at 50 microM induced muscle contracture. A combination of agents at low concentrations that act at different steps of botulinum type A neurotoxin poisoning potentiated the prolongation of time to 50% block in an approximately additive fashion. Thus N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (2 microM) and quinacrine (5 microM), when administered in combination, produced up to a 4-fold increase in time to 50% block. A similar level of protection with quinacrine alone required a 4-fold increase in the aminoquinoline concentration. Although the mechanism of protection by these antimalarial agents is probably through the raising of endosomal pH, the possibility that some of these drugs could also act by inhibiting toxin-induced channel formation cannot be ruled out.

Effect of 3,4-diaminopyridine on rat extensor digitorum longus muscle paralyzed by local injection of botulinum neurotoxin

The actions of the K+ channel blocker, 3,4-diaminopyridine (3,4-DAP), were studied in the rat extensor digitorum longus (EDL) muscle following local inhibition of neuromuscular transmission by botulinum neurotoxin (BoNT). Local paralysis of the EDL muscle was induced by s.c. injections of BoNT serotypes A, B, E or F over the anterior tibialis muscle. One to 14 days later, the rats were anesthetized with urethane, and isometric twitch tensions following stimulation of the peroneal nerve were measured in situ. Muscles were paralyzed within 24 hr of administration of 5 mouse LD50 units (U) of BoNT/A and remained inhibited for the entire 14-day period of observation. Similar levels of inhibition, but of shorter duration, were observed after local injection of 20 U of BoNT/E, 10(4) U of BoNT/B or 20 U of BoNT/F. 3,4-DAP (4 mg/kg, i.v.) potentiated twitch tensions markedly in BoNT/A intoxicated muscle. The increase in tension developed rapidly (halftime = 5.81 +/- 0.6 min), persisted for approximately 1 hr, then decayed slowly with a halftime of 25.2 +/- 4.6 min. Subsequent administration of 3,4-DAP restored tensions to the original maxima, and this procedure could be repeated up to eight times with no decrement. The action of 3,4-DAP was comparable when given 1, 2, 3 or 7 days after BoNT/A and enhanced when administered 14 days after toxin injection. 3,4-DAP was less effective in reversing BoNT/E-induced muscle paralysis and nearly ineffective in antagonizing the paralytic actions of BoNT/B or BoNT/F. The results indicate that 3,4-DAP is of benefit in BoNT/A and BoNT/E intoxication, but is of marginal value after exposure to serotypes B and F.