[A reminder of the structure and function of the skeletal neuromuscular junction]

The skeletal neuromuscular junction has been considered as a model of chemical synapses due to its relatively simple organization. It is made up of three cellular partners including the motoneuron nerve terminals, the peri-synaptic Schwann cells and a specialized region of skeletal muscle fibers. It has been extensively studied revealing its ultrastructural complexity involving many molecular actors. The neuromuscular junction is a highly specialized structure, optimized for the rapid transmission of information from the presynaptic nerve terminal to the post-synaptic muscle fiber. This rapid transmission requires a very close apposition of plasmic membranes of pre- and post-synaptic partners, and a strict structural and molecular arrangement on both sides of the narrow synaptic cleft separating nerve terminal and muscle membranes. In this short review, we summarize the knowledge regarding pre- and post-synaptic ultrastructural specializations and give an overview of some functional aspects of neuromuscular transmission, including the quantal acetylcholine release process, which will help to better understand the pharmacological actions of botulinum toxins in esthetic and corrective dermatology.

Pore-forming epsilon toxin causes membrane permeabilization and rapid ATP depletion-mediated cell death in renal collecting duct cells

Clostridium perfringens epsilon toxin (ET) is a potent pore-forming cytotoxin causing fatal enterotoxemia in livestock. ET accumulates in brain and kidney, particularly in the renal distal-collecting ducts. ET binds and oligomerizes in detergent-resistant membranes (DRMs) microdomains and causes cell death. However, the causal linkage between membrane permeabilization and cell death is not clear. Here, we show that ET binds and forms 220-kDa insoluble complexes in plasma membrane DRMs of renal mpkCCD(cl4) collecting duct cells. Phosphatidylinositol-specific phospholipase C did not impair binding or the formation of ET complexes, suggesting that the receptor for ET is not GPI anchored. ET induced a dose-dependent fall in the transepithelial resistance and potential in confluent cells grown on filters, transiently stimulated Na+ absorption, and induced an inward ionic current and a sustained rise in [Ca2+]i. ET also induced rapid depletion of cellular ATP, and stimulated the AMP-activated protein kinase, a metabolic-sensing Ser/Thr kinase. ET also induced mitochondrial membrane permeabilization and mitochondrial-nuclear translocation of apoptosis-inducing factor, a potent caspase-independent cell death effector. Finally, ET induced cell necrosis characterized by a marked reduction in nucleus size without DNA fragmentation. DRM disruption by methyl-beta-cyclodextrin impaired ET oligomerization, and significantly reduced the influx of Na+ and [Ca2+]i, but did not impair ATP depletion and cell death caused by the toxin. These findings indicate that ET causes rapid necrosis of renal collecting duct cells and establish that ATP depletion-mediated cell death is not strictly correlated with the plasma membrane permeabilization and ion diffusion caused by the toxin.

[Mode of action of botulinum neurotoxin: pathological, cellular and molecular aspect]

Several bacteria of the Clostridium genus (C. botulinum) produce 150 kDa di-chainal protein toxins referred as botulinum neurotoxins or BoNTs. They associate with non-toxic companion proteins and form a complex termed botulinum toxin or BoTx. The latter is used in clinic for therapeutic purpose. BoNTs affect cholinergic nerve terminals in periphery where they block acetylcholine release, thereby causing dysautonomia and motorparalysis (i.e. botulism). The cellular action of BoNTs can be depicted according to a three steps model: binding, internalisation and intraneuronal action. The toxins heavy chain mediates binding to specific receptors followed by endocytotic internalisation of BoNT/receptor complex. BoNT receptors may comprise gangliosides and synaptic vesicle-associated proteins as synaptotagmins. Vesicle recycling induces BoNT internalisation. Upon acidification of vesicles, the light chain of the neurotoxin is translocated into the cytosol. Here, this zinc-endopeptidase cleaves one or two among three synaptic proteins (VAMP-synaptobrevin, SNAP25, and syntaxin). As the three protein targets of BoNT play major role in fusion of synaptic vesicles at the release sites, their cleavage is followed by blockage of neurotransmitter exocytosis. The duration of the paralytic effect of the BoNTs is determined by 1) the turnover of their protein target; 2) the time-life of the toxin light chain in the cytosol, and 3) the sprouting of new nerve-endings that are retracted when the poisoned nerve terminal had recovered its full functionality.

A role for phospholipase D1 in neurotransmitter release

Phosphatidic acid produced by phospholipase D (PLD) as a result of signaling activity is thought to play a role in membrane vesicle trafficking, either as an intracellular messenger or as a cone-shaped lipid that promotes membrane fusion. We recently described that, in neuroendocrine cells, plasma membrane-associated PLD1 operates at a stage of Ca(2+)-dependent exocytosis subsequent to cytoskeletal-mediated recruitment of secretory granules to exocytotic sites. We show here that PLD1 also plays a crucial role in neurotransmitter release. Using purified rat brain synaptosomes subjected to hypotonic lysis and centrifugation, we found that PLD1 is associated with the particulate fraction containing the plasma membrane. Immunostaining of rat cerebellar granule cells confirmed localization of PLD1 at the neuronal plasma membrane in zones specialized for neurotransmitter release (axonal neurites, varicosities, and growth cone-like structures). To determine the potential involvement of PLD1 in neurotransmitter release, we microinjected catalytically inactive PLD1(K898R) into Aplysia neurons and analyzed its effects on evoked acetylcholine (ACh) release. PLD1(K898R) produced a fast and potent dose-dependent inhibition of ACh release. By analyzing paired-pulse facilitation and postsynaptic responses evoked by high-frequency stimulations, we found that the exocytotic inhibition caused by PLD1(K898R) was not the result of an alteration in stimulus-secretion coupling or in vesicular trafficking. Analysis of the fluctuations in amplitude of the postsynaptic responses revealed that the PLD1(K898R) blocked ACh release by reducing the number of active presynaptic-releasing sites. Our results provide evidence that PLD1 plays a major role in neurotransmission, most likely by controlling the fusogenic status of presynaptic release sites.

High sensitivity of mouse neuronal cells to tetanus toxin requires a GPI-anchored protein

Tetanus neurotoxin (TeNT) produced by Clostridium tetani specifically cleaves VAMP/synaptobrevin (VAMP) in central neurons, thereby causing inhibition of neurotransmitter release and ensuing spastic paralysis. Although polysialogangliosides act as components of the neurotoxin binding sites on neurons, evidence has accumulated indicating that a protein moiety is implicated as a receptor of TeNT. We have observed that treatment of cultured mouse neuronal cells with the phosphatidylinositol-specific phospholipase C (PIPLC) inhibited TeNT-induced cleavage of VAMP. Also, we have shown that the blocking effects of TeNT on neuroexocytosis can be prevented by incubation of Purkinje cell preparation with PIPLC. In addition, treatment of cultured mouse neuronal cells with cholesterol sequestrating agents such as nystatin and filipin, which disrupt clustering of GPI-anchored proteins in lipid rafts, prevented intraneuronal VAMP cleavage by TeNT. Our results demonstrate that high sensitivity of neurons to TeNT requires rafts and one or more GPI-anchored protein(s) which act(s) as a pivotal receptor for the neurotoxin.

Synapsin controls both reserve and releasable synaptic vesicle pools during neuronal activity and short-term plasticity in Aplysia

Neurotransmitter release is a highly efficient secretory process exhibiting resistance to fatigue and plasticity attributable to the existence of distinct pools of synaptic vesicles (SVs), namely a readily releasable pool and a reserve pool from which vesicles can be recruited after activity. Synaptic vesicles in the reserve pool are thought to be reversibly tethered to the actin-based cytoskeleton by the synapsins, a family of synaptic vesicle-associated phosphoproteins that have been shown to play a role in the formation, maintenance, and regulation of the reserve pool of synaptic vesicles and to operate during the post-docking step of the release process. In this paper, we have investigated the physiological effects of manipulating synapsin levels in identified cholinergic synapses of Aplysia californica. When endogenous synapsin was neutralized by the injection of specific anti-synapsin antibodies, the amount of neurotransmitter released per impulse was unaffected, but marked changes in the secretory response to high-frequency stimulation were observed, including the disappearance of post-tetanic potentiation (PTP) that was substituted by post-tetanic depression (PTD), and increased rate and extent of synaptic depression. Opposite changes on post-tetanic potentiation were observed when synapsin levels were increased by injecting exogenous synapsin I. Our data demonstrate that the presence of synapsin-dependent reserve vesicles allows the nerve terminal to release neurotransmitter at rates exceeding the synaptic vesicle recycling capacity and to dynamically change the efficiency of release in response to conditioning stimuli (e.g., post-tetanic potentiation). Moreover, synapsin-dependent regulation of the fusion competence of synaptic vesicles appears to be crucial for sustaining neurotransmitter release during short periods at rates faster than the replenishment kinetics and maintaining synchronization of quanta in evoked release.

[Analysis of synaptic neurotransmitter release mechanisms using bacterial toxins]

Several bacterial toxins are powerful and highly specific tools for studying basic mechanisms involved in cell biology. Whereas the clostridial neurotoxins are widely used by neurobiologists, many other toxins (i.e. toxins acting on small G-proteins or actin) are still overlooked. Botulinum neurotoxins (BoNT, serotypes A-G) and tetanus neurotoxin (TeNT), known under the generic term of clostridial neurotoxins, are characterized by their unique ability to selectively block neurotransmitter release. These proteins are formed of a light (Mr approximately 50) and a heavy (Mr approximately 100) chain which are disulfide linked. The cellular action of BoNT and TeNT involves several steps: heavy chain-mediated binding to the nerve ending membrane, endocytosis, and translocation of the light chain (their catalytic moiety) into the cytosol. The light chains each cleaves one of three, highly conserved, proteins (VAMP/synaptobrevin, syntaxin, and SNAP-25 also termed SNAREs) implicated in fusion of synaptic vesicles with plasma membrane at the release site. Hence, when these neurotoxins are applied extracellularly, they can be used as specific tools to inhibit evoked and spontaneous transmitter release from certain neurones whereas, when the membrane limiting steps are bypassed by the mean of intracellular applications, BoNTs orTeNT can be used to affect regulated secretion in various cell types. Several members of the Rho GTPase family have been involved in intracellular trafficking of synaptic vesicles and secretory organelles. As they are natural targets for several bacterial exoenzymes or cytotoxins, their role in neurotransmitter release can be probed by examining the action of these toxins on neurotransmission. Such toxins include: i) the non permeant C3 exoenzymes from C. botulinum or C. limosum which ADP-ribosylate and thereby inactivate Rho, ii) exoenzyme S from Pseudomonas aeruginosa which ADP-ribosylates different members of the Ras, Rab, Ral and Rap families, iii) toxin B from C. difficile which glucosylates Rho, Rac and CDC42, iv) lethal toxin from C. sordellii which glucosylates Rac, Ras and to a lesser extent, Rap and Ral, but not on Rho or CDC42, and v) CNF deamidases secreted by pathogenic strains of E. coli which activate Rho and, to a lesser extent, CDC42. Since these toxins or exoenzymes have no or little ability to enter into the neurones, they must be applied intraneuronally to bypass the membrane limiting steps. Injection of several of these toxins into Aplysia neurones allowed us to reveal a new role for Rac in the control of exocytosis. ADP-ribosylating enzymes, which specifically act on monomeric actin (C2 binary toxin from C. botulinum and iota toxin from C. perfringens), are potential tools to probe the role of actin filaments during secretion.

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.

A Rho-related GTPase is involved in Ca(2+)-dependent neurotransmitter exocytosis

Rho, Rac, and Cdc42 monomeric GTPases are well known regulators of the actin cytoskeleton and phosphoinositide metabolism and have been implicated in hormone secretion in endocrine cells. Here, we examine their possible implication in Ca(2+)-dependent exocytosis of neurotransmitters. Using subcellular fractionation procedures, we found that RhoA, RhoB, Rac1, and Cdc42 are present in rat brain synaptosomes; however, only Rac1 was associated with highly purified synaptic vesicles. To determine the synaptic function of these GTPases, toxins that impair Rho-related proteins were microinjected into Aplysia neurons. We used lethal toxin from Clostridium sordellii, which inactivates Rac; toxin B from Clostridium difficile, which inactivates Rho, Rac, and Cdc42; and C3 exoenzyme from Clostridium botulinum and cytotoxic necrotizing factor 1 from Escherichia coli, which mainly affect Rho. Analysis of the toxin effects on evoked acetylcholine release revealed that a member of the Rho family, most likely Rac1, was implicated in the control of neurotransmitter release. Strikingly, blockage of acetylcholine release by lethal toxin and toxin B could be completely removed in <1 s by high frequency stimulation of nerve terminals. Further characterization of the inhibitory action produced by lethal toxin suggests that Rac1 protein regulates a late step in Ca(2+)-dependent neuroexocytosis.

Calcium-dependent regulation of rab3 in short-term plasticity

The Rab3 proteins are monomeric GTP-binding proteins associated with secretory vesicles. In their active GTP-bound state, Rab3 proteins are involved in the regulation of hormone secretion and neurotransmitter release. This action is thought to involve specific effectors, including two Ca2+-binding proteins, Rabphilin and Rim. Rab3 acts late in the exocytotic process, in a cell domain in which the intracellular Ca2+ concentration is susceptible to rapid changes. Therefore, we examined the possible Ca2+-dependency of the regulatory action of GTP-bound Rab3 and wild-type Rab3 on neuroexocytosis at identified cholinergic synapses in Aplysia californica. The effects of recombinant GTPase-deficient Aplysia-Rab3 (apRab3-Q80L) or wild-type apRab3 were studied on evoked acetylcholine release. Intraneuronal application of apRab3-Q80L in identified neurons of the buccal ganglion of Aplysia led to inhibition of neurotransmission; wild-type apRab3 was less effective. Intracellular chelation of Ca2+ ions by EGTA greatly potentiated the inhibitory action of apRab3-Q80L. Train and paired-pulse facilitation, two Ca2+-dependent forms of short-term plasticity induced by a rise in intraterminal Ca2+ concentration, were increased after injection of apRab3-Q80L. This result suggests that the inhibition exerted by GTP-bound Rab3 on neuroexocytosis is reduced during transient augmentations of intracellular Ca2+ concentration. Therefore, a Ca2+-dependent modulation of GTP-bound Rab3 function may contribute to short-term plasticity.

[Action mechanisms of botulinum neurotoxins and tetanus neurotoxins]

Tetanus (TeNT) neurotoxin and botulinum (BoNT, serotypes A-G) neurotoxins are di-chain bacterial proteins of MW-150 kDa which are also termed as clostridial neurotoxins. They are the only causative agents of two severe neuroparalytic diseases, namely tetanus and botulism. The peripheral muscle spasms which characterise tetanus are due to a blockade of inhibitory (GABAergic and glycinergic) synapses in the central nervous system leading to a motor neurones desinhibition. In contrast, botulism symptoms are only peripheral. They are consequent to a near irreversible and highly selective inhibition of acetyl-choline release at the motor nerve endings innervating skeletal muscles. During the past decade, the cellular and molecular modes of action of clostridial neurotoxins has been near completely elucidated. After a binding step of the neurotoxins to specific membrane acceptors located only on nerve terminals, BoNTs and TeNT are internalized into neurons. Inside their target neurones, the intracellularly active moiety (their light chain) is translocated from the endosomal compartment to the cytosol. The neurotoxins' light chains are zinc-dependent (endopeptidases which are specific for one among three synaptic proteins (VAMP/synaptobrevin, syntaxin or SNAP-25) implicated in neurotransmitter exocytosis. The presence of distinct targets for BoNTs and TeNT correlates well with the observed quantal alterations of neurotransmitter release which characterize certain toxin serotypes. In addition, evidence for a second, non-proteolytic, inhibitory mechanism of action has been provided recently. Most likely, this additional blocking action involves the activation of neurone transglutaminases. Due to their specific action on key proteins of the exocytosis apparatus, clostridial neurotoxins are now widely used as molecular tools to study exocytosis.

Evidence for a functional link between Rab3 and the SNARE complex

Rab3 is a monomeric GTP-binding protein associated with secretory vesicles which has been implicated in the control of regulated exocytosis. We have exploited Rab3 mutant proteins to investigate the function of Rab3 in the process of neurotransmitter release from Aplysia neurons. A GTPase-deficient Rab3 mutant protein was found to inhibit acetylcholine release suggesting that GTP hydrolysis by Rab3 is rate-limiting in the exocytosis process. This effect was abolished by a mutation in the effector domain, and required the association of Rab3 with membranes. In order to determine the step at which Rab3 interferes with the secretory process, tetanus and botulinum type A neurotoxins were applied to Aplysia neurons pre-injected with the GTPase-deficient Rab3 mutant protein. These neurotoxins are Zn(2+)-dependent proteases that cleave VAMP/synaptobrevin and SNAP-25, two proteins which can form a ternary complex (termed the SNARE complex) with syntaxin and have been implicated in the docking of synaptic vesicles at the plasma membrane. The onset of toxin-induced inhibition of neurotransmitter release was strongly delayed in these cells, indicating that the mutant Rab3 protein led to the accumulation of a toxin-insensitive component of release. Since tetanus and botulinum type A neurotoxins cannot attack their targets, VAMP/synaptobrevin and SNAP-25, when the latter are engaged in the SNARE complex, we propose that Rab3 modulates the activity of the fusion machinery by controlling the formation or the stability of the SNARE complex.

Differences in the multiple step process of inhibition of neurotransmitter release induced by tetanus toxin and botulinum neurotoxins type A and B at Aplysia synapses

In order to gain insights into the steps (binding, uptake, intracellular effect) which differ in the inhibitory actions of tetanus toxin and botulinum neurotoxins types A or B, their temperature dependencies were investigated at identified cholinergic and non-cholinergic synapses in Aplysia. Upon lowering the temperature from 22 degrees C to 10 degrees C, extracellularly applied botulinum neurotoxin type A and B appeared unable to inhibit transmitter release whilst tetanus toxin exhibited a residual activity. Binding of each toxin to the neuronal membrane appeared virtually unaltered following this temperature change. By contrast, the intracellular effects of botulinum neurotoxin type B and tetanus toxin were strongly attenuated by temperature reduction whereas the inhibitory action of botulinum neurotoxin type A was only moderately reduced. Importantly, this discrepancy relates to the known proteolytic cleavage of different synaptic proteins by these two toxin groups. Since both the binding and intracellular activity of botulinum neurotoxin type A are minimally affected at 10 degrees C, its inability to inhibit neurotransmission at this low temperature when applied extracellularly indicated attenuation of its uptake. Due to the strict temperature dependence of the intracellular action of tetanus toxin and botulinum neurotoxin type B, but not A, an examination of the effects of changes in temperature on the internalization step was facilitated by the use of heterologous mixtures of the toxins' heavy and light chains. At 10 degrees C, heavy chain from tetanus toxin but not from botulinum neurotoxin type B mediated uptake of botulinum neurotoxin type A light chain. Collectively, these results provide evidence that, at least in Aplysia, the uptake mechanism for botulinum neurotoxin types A and B differs from that of tetanus toxin.

Tetanus toxin inhibits neuroexocytosis even when its Zn(2+)-dependent protease activity is removed

Tetanus toxin (TeTX) is a dichain protein that blocks neuroexocytosis, an action attributed previously to Zn(2+)-dependent proteolysis of synaptobrevin (Sbr) by its light chain (LC). Herein, its cleavage of Sbr in rat cerebrocortical synaptosomes was shown to be minimized by captopril, an inhibitor of certain metalloendoproteases, whereas this agent only marginally antagonized the inhibition of noradrenaline release, implicating a second action of the toxin. This hypothesis was proven by preparing three mutants (H233A, E234A, H237A) of the LC lacking the ability to cleave Sbr and reconstituting them with native heavy chain. The resultant dichains were found to block synaptosomal transmitter release, albeit with lower potency than that made from wild type LC; as expected, captopril attenuated only the inhibition caused by the protease-active wild type toxin. Moreover, these protease-inactive toxins or their LCs blocked evoked quantal release of transmitter when micro-injected inside Aplysia neurons. TeTX was known to stimulate in vitro a Ca(2+)-dependent transglutaminase (TGase) (Facchiano, F., and Luini, A. (1992) J. Biol. Chem. 267, 13267-13271), an affect found here to be reduced by an inhibitor of this enzyme, monodansylcadaverine. Accordingly, treatment of synaptosomes with the latter antagonized the inhibition of noradrenaline release by TeTX while not affecting Sbr cleavage. This drug also attenuated the inhibitory action of all the mutants. Hence, it is concluded that TeTX inhibits neurotransmitter release by proteolysis of Sbr and a protease-independent activation of a neuronal TGase.

Inhibition of neurotransmitter release by synthetic proline-rich peptides shows that the N-terminal domain of vesicle-associated membrane protein/synaptobrevin is critical for neuro-exocytosis

Tetanus toxin and clostridial neurotoxins type B, D, F, and G inhibit intracellular Ca(2+)-dependent neurotransmitter release via the specific proteolytic cleavage of vesicle-associated membrane protein (VAMP)/synaptobrevin, a highly conserved 19-kDa integral protein of the small synaptic vesicle membrane. This results in the release of the larger part of the cytosolic domain of this synaptic protein into the cytoplasm. Microinjection of synthetic peptides corresponding to this fragment into identified presynaptic neurons of Aplysia californica led to a potent, long lasting, and dose-dependent inhibition (approximately 50% at 10 MicroM) of acetylcholine release, probably by hindering endogenous VAMP/synaptobrevin from interacting with synaptic proteins involved in exocytosis. Structure activity studies showed that this effect is confined to the N-terminal domain of VAMP/synaptobrevin isoform II and is related to the presence of a proline-rich motif (PGGPXGX3PP or PAAPXGX3PP). At higher concentrations, the inhibitory effect was lower and only transient, suggesting that the N-terminal proline-rich domain of VAMP/synaptobrevin plays opposing roles in neurotransmitter release very likely by interacting with different synaptic proteins. This probably occurs by disruption of the recently reported in vitro VAMP-synaptophysin interaction that involves the N-terminal domain of VAMP II and was proposed to hinder synatophysin-related formation of a fusion pore. The observed recovery of neurotransmitter release following injection of high concentration of N-terminal fragments of VAMP II brings a strong in vivo support to this hypothesis. The minimum active peptide GPGGPQGGMQPPREQS could be used for rationally designing potent synthetic blockers of neurotransmission.

The metallo-proteinase activity of tetanus and botulism neurotoxins

Tetanus and botulinum neurotoxins are produced by several Clostridia and cause the paralytic syndromes of tetanus and botulism by blocking neurotransmitter release at central and peripheral synapses, respectively. They consist of two disulfide-linked polypeptides: H (100 kDa) is responsible for neurospecific binding and cell penetration of L (50 kDa), a zinc-endopeptidase specific for three protein subunits of the neuroexocytosis apparatus. Tetanus neurotoxin and botulinum neurotoxin serotypes B, D, F and G cleave at single sites, which differ for each neurotoxin, VAMP/synaptobrevin, a membrane protein of the synaptic vesicles. Botulinum A and E neurotoxins cleave SNAP-25, a protein of the presynaptic membrane, at two different carboxyl-terminal peptide bonds. Serotype C cleaves specifically syntaxin, another protein of the nerve plasmalemma. The target specificity of these metallo-proteinases relies on a double recognition of their substrates based on interactions with the cleavage site and with a non-contiguous segment that contains a structural motif common to VAMP, SNAP-25 and syntaxin.

Tetanus and botulinum neurotoxins are zinc proteases specific for components of the neuroexocytosis apparatus

Tetanus and botulinum neurotoxins bind to nerve cells, penetrate the cytosol, and block neurotransmitter release. Comparison of their amino-acid sequences shows the presence of the highly conserved His-Glu-x-x-His zinc-binding motif of zinc-endopeptidases (HExxH). Atomic absorption measurements of clostridial neurotoxins show the presence of one atom of zinc/toxin molecule bound to the light chain. The toxin-bound zinc ion is essential for the neurotoxins inhibition of neurotransmitter release in Aplysia neurons injected with the toxins. Phosphoramidon, a very specific inhibitor of zinc-endopeptidases, blocks the intracellular activity of the clostridial neurotoxins. Highly purified preparations of the light chain of tetanus and botulinum B and F neurotoxins cleaved specifically VAMP/synaptobrevin, an integral membrane protein of small synaptic vesicles, both in vivo and in vitro. From these studies, it can be concluded that the clostridial neurotoxins responsible for tetanus and botulism block neuroexocytosis via the proteolytic cleavage of specific components of the neuroexocytotic machinery.

[Molecular mechanism of action of tetanus toxin and botulinum neurotoxins]

Tetanus toxin and botulinum neurotoxins are di-chain proteins of 150 kD molecular weight. They are produced by bacteria of the Clostridium genus. These toxins act on the nervous system by inhibiting neurotransmitter release (glycine and GABA in the case of tetanus toxin; acetylcholine in the case of botulinum neurotoxins) thus inducing the spastic or flaccid paralysis that characterizes tetanus and botulism, respectively. Their cellular mechanism of action involves three main steps, namely binding to the neurone membrane, internalization and intracellular blockade of the release mechanism for neurotransmitters. Membrane acceptors for these toxins are not yet fully identified; they would consist of membrane gangliosides and proteins. The internalization step would be achieved by endocytosis. Recent findings show that both binding and internalization are mediated only by the heavy chain of the toxins whereas the intracellular blockade of neurotransmitter release involves their light chain alone. The light chain has been identified as a zinc metalloprotease and its substrates would be proteins involved in the neurotransmitter release mechanism. The target of tetanus toxin and of botulinum neurotoxin type B is VAMP/synaptobrevin, a membrane protein of the synaptic vesicles of nerve cell terminals.

Antagonism of the intracellular action of botulinum neurotoxin type A with monoclonal antibodies that map to light-chain epitopes

mAbs were produced in mice against highly purified, renatured light chain (LC) of botulinum neurotoxin A (BoNT A) that was immobilised on nitrocellulose to avoid the undesirable use of toxoids. Subcutaneous implants of relatively high amounts (up to 10 micrograms each) of LC allowed its slow release into the systemic circulation and, thus, yielded much higher antibody titres against the underivatized antigen than had hitherto been obtained by conventional immunization. Seven stable hybridoma cell lines were established which secrete mAb of IgG1 and IgG2b subclasses reactive specifically with BoNT A and LC, in native and denatured states, without showing any cross-reactivity with types B, E, F or tetanus toxin. The pronounced reactivities of three mAbs towards refolded LC or intact toxin, observed in immunobinding and precipitation assays, relative to that seen in Western blots imply a preference for conformational epitopes. Though mAbs 4, 5 and 7 failed to neutralize the lethality of BoNT in vivo, administration intraneurally of mAb7 prevented the inhibition of transmitter release normally induced by subsequent extracellular administration of BoNT A. Notably, the latter mAb reacted with a synthetic peptide corresponding to amino acids 28-53 in the N-terminus of the LC, a highly conserved region in Clostridial neurotoxins reported to be essential for maintaining the tertiary structure of the chain. Most importantly, when mAbs 4 or 7 were microinjected inside ganglionic neurons of Aplysia, each reversed, though transiently, the blockade of acetylcholine release by the toxin; this novel finding is discussed in relation to the nature of the zinc-dependent protease activity of the toxin.

A role for the interchain disulfide or its participating thiols in the internalization of botulinum neurotoxin A revealed by a toxin derivative that binds to ecto-acceptors and inhibits transmitter release intracellularly

Botulinum neurotoxin type A consists of a disulfide-linked light and heavy chain, with an intradisulfide present within the C-terminal half of the latter. The functional consequences of reducing these bonds and alkylating the thiols were investigated. Modification of free cysteine residues had no effect on the toxicity in mouse bioassays or on acetylcholine release in the mouse nerve-diaphragm and the buccal ganglion of Aplysia californica. However, reduction of the toxin prior to alkylation drastically decreased neuroparalytic potency; yet, this derivative inhibited transmitter release if injected directly into a presynaptic neuron in the Aplysia ganglion or added to bovine permeabilized adrenal chromaffin cells. Its antagonism of the action of botulinum neurotoxin A at mammalian motor nerve endings and Aplysia neurons indicates retention of the ability to bind to the toxin's productive ecto-acceptors. Thus, the abolition of the toxicity of extracellularly applied botulinum neurotoxin A by the cleavage of both disulfides, and the alkylation of the half-cystines involved, results from ineffective uptake. Modified forms of the isolated chains of botulinum neurotoxin A were utilized to determine which of the disulfides were necessary for internalization. Alkylation of the cysteines in the light and heavy chains, including those involved in the interchain bond but excluding those of the intact disulfide in the heavy chain, revealed that the intermolecular bond must be present, or the thiols concerned unmodified, for botulinum neurotoxin A to undergo membrane translocation into Aplysia neurons.

Antibodies against rat brain vesicle-associated membrane protein (synaptobrevin) prevent inhibition of acetylcholine release by tetanus toxin or botulinum neurotoxin type B

Tetanus and botulinum B neurotoxins are zinc endopeptidases that cleave vesicle-associated membrane protein (VAMP or synaptobrevin) at a single peptide bond. To test the possibility that in vivo also the toxin-induced blockade of neurotransmission is due to cleavage of VAMP, rat brain VAMP-specific antibodies were raised in rabbits. IgGs purified from one antiserum, which bind specifically to rat brain VAMP, also specifically recognize proteins from Aplysia californica in immunoblotting. When injected into neurons in the buccal ganglion of Aplysia, these IgGs did not affect the release of acetylcholine but effectively prevented the inhibitory action of both toxins on neurotransmitter release, thus indicating that the block of neurotransmission by these neurotoxins is consequent to the cleavage of VAMP or specific interaction with VAMP.

Novel targets and catalytic activities of bacterial protein toxins

Among bacterial protein toxins with intracellular targets, tetanus and botulinum toxins form a group with unique properties. They are absolutely neurospecific and act in the cytosol of neurons. Recent evidence indicates that they are zinc proteases specific for proteins of the neuroexocytosis apparatus.

Tetanus and botulinum-B neurotoxins block neurotransmitter release by proteolytic cleavage of synaptobrevin

Clostridial neurotoxins, including tetanus toxin and the seven serotypes of botulinum toxin (A-G), are produced as single chains and cleaved to generate toxins with two chains joined by a single disulphide bond (Fig. 1). The heavy chain (M(r) 100,000 (100K)) is responsible for specific binding to neuronal cells and cell penetration of the light chain (50K), which blocks neurotransmitter release. Several lines of evidence have recently suggested that clostridial neurotoxins could be zinc endopeptidases. Here we show that tetanus and botulinum toxins serotype B are zinc endopeptidases, the activation of which requires reduction of the interchain disulphide bond. The protease activity is localized on the light chain and is specific for synaptobrevin, an integral membrane protein of small synaptic vesicles. The rat synaptobrevin-2 isoform is cleaved by both neurotoxins at the same single site, the peptide bond Gln 76-Phe 77, but the isoform synaptobrevin-1, which has a valine at the corresponding position, is not cleaved. The blocking of neurotransmitter release of Aplysia neurons injected with tetanus toxin or botulinum toxins serotype B is substantially delayed by peptides containing the synaptobrevin-2 cleavage site. These results indicate that tetanus and botulinum B neurotoxins block neurotransmitter release by cleaving synaptobrevin-2, a protein that, on the basis of our results, seems to play a key part in neurotransmitter release.

Tetanus toxin is a zinc protein and its inhibition of neurotransmitter release and protease activity depend on zinc

Tetanus and botulinum neurotoxins are the most potent toxins known. They bind to nerve cells, penetrate the cytosol and block neurotransmitter release. Comparison of their predicted amino acid sequences reveals a highly conserved segment that contains the HexxH zinc binding motif of metalloendopeptidases. The metal content of tetanus toxin was then measured and it was found that one atom of zinc is bound to the light chain of tetanus toxin. Zinc could be reversibly removed by incubation with heavy metal chelators. Zn2+ is coordinated by two histidines with no involvement in cysteines, suggesting that it plays a catalytic rather than a structural role. Bound Zn2+ was found to be essential for the tetanus toxin inhibition of neurotransmitter release in Aplysia neurons injected with the light chain. The intracellular activity of the toxin was blocked by phosphoramidon, a very specific inhibitor of zinc endopeptidases. Purified preparations of light chain showed a highly specific proteolytic activity against synaptobrevin, an integral membrane protein of small synaptic vesicles. The present findings indicate that tetanus toxin, and possibly also the botulinum neurotoxins, are metalloproteases and that they block neurotransmitter release via this protease activity.

Minimal essential domains specifying toxicity of the light chains of tetanus toxin and botulinum neurotoxin type A

To define conserved domains within the light (L) chains of clostridial neurotoxins, we determined the sequence of botulinum neurotoxin type B (BoNT/B) and aligned it with those of tetanus toxin (TeTx) and BoNT/A, BoNT/C1, BoNT/D, and BoNT/E. The L chains of BoNT/B and TeTx share 51.6% identical amino acid residues whereas the degree of identity to other clostridial neurotoxins does not exceed 36.5%. Each of the L chains contains a conserved motif, HExxHxxH, characteristic for metalloproteases. We then generated specific 5'- and 3'-deletion mutants of the L chain genes of TeTx and BoNT/A and tested the biological properties of the gene products by microinjection of the corresponding mRNAs into identified presynaptic cholinergic neurons of the buccal ganglia of Aplysia californica. Toxicity was determined by measurement of neurotransmitter release, as detected by depression of postsynaptic responses to presynaptic stimuli (Mochida, S., Poulain, B., Eisel, U., Binz, T., Kurazono, H., Niemann, H., and Tauc, L. (1990) Proc. Natl. Acad. Sci. U. S. A. 87, 7844-7848). Our studies allow the following conclusions. 1) Residues Cys439 of TeTx and Cys430 of BoNT/A, both of which participate in the interchain disulfide bond, play no role in the toxification reaction. 2) Derivatives of TeTx that lacked either 8 amino- or 65 carboxyl-terminal residues are still toxic, whereas those lacking 10 amino- or 68 carboxyl-terminal residues are nontoxic. 3) For BoNT/A, toxicity could be demonstrated only in the presence of added nontoxic heavy (H) chain. A deletion of 8 amino-terminal or 32 carboxyl-terminal residues from the L chain had no effect on toxicity, whereas a removal of 10 amino-terminal or 57 carboxyl-terminal amino acids abolished toxicity. 4) The synergistic effect mediated by the H chain is linked to the carboxyl-terminal portion of the H chain, as demonstrated by injection of HC-specific mRNA into neurons containing the L chain. This finding suggests that the HC domain of the H chain becomes exposed to the cytosol during or after the putative translocation step of the L chain.

Differences in the temperature dependencies of uptake of botulinum and tetanus toxins in Aplysia neurons

The respective neuroselective actions of botulinum type A (BoNT) and tetanus (TeTx) neurotoxins on cholinergic and non-cholinergic synapses of Aplysia are mainly due to differences in their extracellular neuronal targetting. Further information was gained on this neuroselectivity by examining the temperature dependencies of binding, internalization and intracellular action of both toxins. After reduction of temperature from 22 degrees C to 10 degrees C, the binding of neither BoNT nor TeTx was significantly altered whereas the neuronal uptake of BoNT, but not of TeTx, was prevented. Although TeTx internalization could be detected at the low temperature, its intracellular activity was greatly attenuated compared to that of BoNT. It is inferred that the uptake mechanisms are different for these two related but distinct toxins.

Clostridial neurotoxins: from toxins to therapeutic tools?

Tetanus toxin and botulinum toxins are powerful neurotoxins which block neurotransmitter release through an unknown mechanism my means of their light chains. The heavy chains provide the machinery for neuroselective binding, internalization, retrograde intraaxonal transport, and translocation of the L-chains into the cytosole. We have cloned and sequenced the structural genes of tetanus toxin and of five serologically distinct botulinum toxins to identify structurally and functionally conserved subdomains. The minimum essential domains of the L-chains of tetanus and botulinum toxin type A were identified by combined in vitro transcription and microinjection of L-chain specific mRNA into identified presynaptic neurons of Aplysia californica. In addition, a nontoxic mutant of tetanus was generated by replacing histidine(237) by a proline residue. The development of nontoxic neuroselective transporter molecules carrying various marker enzymes is discussed.

Heterologous combinations of heavy and light chains from botulinum neurotoxin A and tetanus toxin inhibit neurotransmitter release in Aplysia

The neuroparalytic activities of botulinum neurotoxin type A (BoNT A), tetanus toxin (TeTx), or homologous and heterologous combinations of their constituent polypeptides were examined at cholinergic and non-cholinergic synapses of Aplysia californica. When applied extracellularly, BoNT A or a mixture of its heavy (HC) and light (LC) chains were far more potent in blocking transmitter release at cholinergic than non-cholinergic synapses. The reverse was true for TeTx or a mixture its constituent chains. Such selectivity was assigned to differences in neuronal targetting and uptake of the neurotoxins since both exhibited similar potencies when injected directly into the cell body of either cell type. When bath-applied, heterologous combinations of the toxins' HC and LC appeared as effective as the parent neurotoxins from whence each HC was derived. Moreover, targetting/internalization was attributable to the analogous N-terminal moieties, H2 and beta 2, of the HC from BoNT A and TeTx. Thus, it may be postulated that the latter regions possess two functional domains, one being distinct and responsible for the divergent neuronal specificity, whereas the other serves a common role in translocating the LC of either toxin. Also, it was shown that the C-terminal portion of the HC of TeTx is unable to play the intracellular role of its counterpart in BoNT A.

Exogenous mRNA encoding tetanus or botulinum neurotoxins expressed in Aplysia neurons

Injection of exogenous mRNA purified from various tissue preparations into cellular translation systems such as Xenopus oocytes has allowed expression of complex proteins (e.g., receptors for neurotransmitters). No evidence for expression of injected exogenous mRNA, however, has been reported in terminally differentiated neurons. If achieved, it would allow the study of long-lasting changes of properties of nerve cells in their functional context. To obtain evidence of such expression, we chose two proteins that produce a detectable effect even at very low intracellular concentrations. Tetanus toxin and botulinum neurotoxin fulfill this criterion, being the most potent neurotoxins known. Both toxins block neurotransmitter release at nanomolar intracellular concentrations. These di-chain proteins, consisting of a light chain and a heavy chain, have recently been sequenced. Their active sites are located (or partly located) on the light chain. mRNAs encoding the light chain of either toxin were transcribed in vitro from the cloned and specifically truncated genes of Clostridium tetani and Clostridium botulinum, respectively, and injected into presynaptic cholinergic neurons of the buccal ganglia of Aplysia californica. Depression of neurotransmitter release appeared in less than 1 hr, demonstrating successful expression of foreign mRNA injected into a neuron in situ.

Receptor-mediated presynaptic facilitation of quantal release of acetylcholine induced by pralidoxime in Aplysia

1. Possible interactions of contrathion (pralidoxime sulfomethylate), a reactivator of phosphorylated acetylcholinesterase (AChE), with the regulation of cholinergic transmission were investigated on an identified synapse in the buccal ganglion of Aplysia californica. 2. Transmitter release was evoked either by a presynaptic action potential or, under voltage clamp, by a long depolarization of the presynaptic cell. At concentrations higher than 10(-5) M, bath-applied contrathion decreased the amplitude of miniature postsynaptic currents and increased their decay time. At the same time, the quantal release of ACh was transiently facilitated. The facilitatory effect of contrathion was prevented by tubocurarine but not by atropine. Because in this preparation, these drugs block, respectively, the presynaptic nicotinic-like and muscarinic-like receptors involved in positive and negative feedback of ACh release, we proposed that contrathion activates presynaptic nicotinic-like receptors. 3. Differential desensitization of the presynaptic receptors is proposed to explain the transience of the facilitatory action of contrathion on ACh release. 4. The complexity of the synaptic action of contrathion raises the possibility that its therapeutic effects in AChE poisonings are not limited to AChE reactivation.

Multiple domains of botulinum neurotoxin contribute to its inhibition of transmitter release in Aplysia neurons

The binding, internalization, and inhibition of transmitter release by botulinum neurotoxin (BoNT) was investigated using the intact toxin, its heavy (HC) or light (LC) chains, and a proteolytic fragment thereof. In Aplysia neurons, blockade of acetylcholine release upon external application of BoNT types A or E was prevented by reducing the temperature to 10 degrees C, due to arresting intoxication at the membrane binding step. At this low temperature, type A HC, H2 (comprised of the N-terminal of HC), or H2L (H2 disulfide-linked to LC) antagonized the neuroparalytic action of BoNT A or E, indicating that the latter bind saturably to common ecto-acceptor via the H2 region. In contrast, H2L was unable to counteract BoNT-induced paralysis at the murine neuromuscular junction. In accordance with this species difference, unlike native BoNT, saturable binding of 125I-labeled H2L could not be detected in mammalian peripheral or central nerve terminals. Possibly, more stringent structural requirements form the basis of the toxin's greater effectiveness in inhibiting neurotransmission at mouse nerve muscle synapses than Aplysia nerve terminals. In further identification of functional domains in the toxin, an unprocessed single-chain form of BoNT type E was found to be ineffective when applied extra- or intracellularly to Aplysia neurons. Notably, bath application of the latter to a neuron preinjected with HC, but not H2L or LC, resulted in a blockade of release. This shows that the single-chain species can become internalized and requires, not only LC, but also processed HC for its inhibitory action; consistently, the proteolyzed form of BoNT E was active.

Inhibition of transmitter release by botulinum neurotoxin A. Contribution of various fragments to the intoxication process

1. The contribution of a proteolytic fragment (H2L) of botulinum neurotoxin type A (comprised of the aminoterminal region of the heavy-chain disulphide-linked to the light chain) to inhibition of neurotransmitter release was investigated, using central cholinergic synapses of Aplysia, rodent nerve-diaphragm preparations and cerebrocortical synaptosomes. 2. No reduction in neurotransmitter release was observed following external application to these preparations of highly purified H2L or after intracellular injection into Aplysia neurons. 3. The lack of activity was not the result of alteration in the light chain of H2L during preparation of the latter because (a) renaturation of this light chain with intact heavy chain produced a toxic di-chain form and (b) simultaneous application of heavy chain and light chain from H2L inhibited transmitter release in Aplysia. 4. Bath application of H2L and heavy chain together inhibited release of transmitter; however, at the neuromuscular junction the potency of this mixture was much lower than that of native toxin. A similar blockade resulted when heavy chain was applied intracellularly and H2L added to the bath, demonstrating that H2L is taken up into cholinergic neurons of Aplysia. This uptake is shown to be mediated by the amino-terminal moiety of heavy chain (H2), because bath application of light chain plus H2 led to a decrease in acetylcholine release from a neuron that had been injected with heavy chain. 5. A role within the neuron is implicated for a carboxy-terminal portion of heavy chain (H1) since intracellular injection of light chain and H2 did not affect transmitter release. Although the situation is unclear in mammalian nerves, these collective findings indicate that blockade of transmitter release in Aplysia neurons requires the intracellular presence of light chain and H1 (by inference), whilst H2 contributes to the internalization step.

Light chain of tetanus toxin intracellularly inhibits acetylcholine release at neuro-neuronal synapses, and its internalization is mediated by heavy chain

The ability of the two-chain form of tetanus toxin (TeTx), its constituent light (LC) or heavy (HC) chains, and papain fragment to block evoked acetylcholine (ACh) release in the buccal ganglia of Aplysia californica was studied electrophysiologically. Extracellularly applied, TeTx or its B fragment (consisting of LC and beta 2, the amino-terminal portion of HC) blocked ACh release, whereas LC, HC, or the beta 2 fragment did not affect it. Toxicity was restored when LC was bath applied together with HC or the beta 2 fragment. When injected into the presynaptic neuron, TeTx, the B fragment or LC, but not HC, induced inhibition of ACh release. These results indicate that the blockade of ACh release by TeTx is mimicked by intracellular action of LC, the internalization of which is mediated by the HC via its amino-terminal moiety.

Involvement of the constituent chains of botulinum neurotoxins A and B in the blockade of neurotransmitter release

1. The abilities of botulinum neurotoxins, types A and B (single and two-chain forms) to inactivate an intraneuronal component required for transmitter release were quantified in a phrenic-nerve-diaphragm preparation, cerebrocortical synaptosomes or the buccal ganglion of Aplysia californica and compared with the mouse toxicity assay. 2. Homogeneous preparations of the individually renatured polypeptide chains of both toxin types showed low residual toxicity in the whole animal and had no effect on neurotransmission in all three systems, when tested singly. 3. Mixtures of individually renatured heavy chain, from type A or B, and either light chain proved very effective in blocking the evoked release of acetylcholine when bath-applied to the buccal ganglion of Aplysia whilst they were relatively inactive on mammalian nerve terminals, indicating a less efficient uptake of the polypeptides in the latter. 4. When renatured together, the homologous, but not the heterologous, chains of each toxin type yielded toxic, disulphide-linked two-chain species. 5. A role for the heavy chain alone in acceptor recognition and membrane translocation was implicated by the blockade of acetylcholine release produced when light chain was applied to a ganglion of Aplysia previously bathed in heavy chain and washed extensively. No blockade was observed when the order of application of the two chains was reversed. 6. These findings are discussed in the context of the intracellular requirement for both the constituent toxin chains for toxicity, and in the apparent need for these chains to be linked via a disulphide bond for uptake in rodents but not in Aplysia.