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.