Botulinum toxin is a uniquely potent substance synthesized by the organisms Clostridium botulinum, Clostridium baratii, and Clostridium butyricum. This toxin, which acts preferentially on peripheral cholinergic nerve endings to block acetylcholine release, is both an agent that causes disease (i.e., botulism) as well as an agent that can be used to treat disease (e.g., dystonia). The ability of botulinum toxin to produce its effects is largely dependent on its ability to penetrate cellular and intracellular membranes. Thus, toxin that is ingested or inhaled can bind to epithelial cells and be transported to the general circulation. Toxin that reaches peripheral nerve endings binds to the cell surface then penetrates the plasma membrane by receptor-mediated endocytosis and the endosome membrane by pH-induced translocation. Internalized toxin acts in the cytosol as a metalloendoprotease to cleave polypeptides that are essential for exocytosis. This review seeks to identify and characterize all major steps in toxin action, from initial absorption to eventual paralysis of cholinergic transmission.

The heavy chains of both botulinum neurotoxin type B and tetanus toxin form channels in planar bilayer membranes. These channels have pH-dependent and voltagedependent properties that are remarkably similar to those previously described for diphtheria toxin. Selectivity experiments with anions and cations show that the channels formed by the heavy chains of all three toxins are large; thus, these channels could serve as "tunnel proteins" for translocation of active peptide fragments. These findings support the hypothesis that the active fragments of botulinum neurotoxin and tetanus toxin, like that of diphtheria toxin, are translocated across the membranes of acidic vesicles.Diphtheria toxin (1), botulinum neurotoxin (2), and tetanus toxin (3) are proteins that are similar in origin and macrostructure. All three toxins are synthesized by bacteria (Corynebacterium diphtheriae, Clostridium botulinum, and Clostridium tetani) as single polypeptide chains (diphtheria toxin, "'60 kDa; clostridial neurotoxins, ==150 kDa). When exposed to trypsin or trypsin-like enzymes, they are cleaved to yield two-chain molecules in which a heavy-chain polypeptide is linked by a disulfide bond to a light-chain polypeptide. The two-chain structure is the active form of the three toxins.Various techniques have been used to generate polypeptide fragments from the toxins. The most straightforward of these is disulfide-bond reduction, which releases the heavy chain from the light chain. Alternatively, the clostridial neurotoxins have been exposed to limited proteolysis (e.g., with papain) to generate a fragment B and fragment C. Finally, traditional techniques have been used to select mutant organisms that synthesize incomplete toxins, such as the CRM45 fragment of diphtheria toxin, from which the B45 fragment can be formed. The various toxins and their fragments are illustrated in Fig. 1 Previous studies have shown that the amino terminus of the heavy chain from diphtheria toxin (18) and whole diphtheria toxin (19) form channels in lipid bilayers, and it has been proposed (18) that these channels provide the pathway for the light chain to cross membranes. Here we report that the heavy chains of both botulinum neurotoxin type B and tetanus toxin also form channels in lipid bilayers. Furthermore, for all three toxins, channel formation is maximal when the protein-containing (cis) side of the artificial membrane is at low pH (-4.0) and the opposite (trans) side is at pH -7.0, a pH gradient comparable to that across the membranes of acidic vesicles in cells. The channels for all three toxins are very large, as determined by selectivity experiments with large anions and cations, and this finding is compatible with the idea that the channels function as "tunnel proteins" for translocation of fully extended active fragments. In addition, tetanus toxin channels display a voltage dependence similar to that of diphtheria toxin channels, opening when positive voltages are applied to the cis side of artificial membranes and closing when nega...

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alpha-Latrotoxin stimulates neurotransmitter release probably by binding to two receptors, CIRL/latrophilin 1 (CL1) and neurexin Ialpha. We have now produced recombinant alpha-latrotoxin (LtxWT) that is as active as native alpha-latrotoxin in triggering synaptic release of glutamate, GABA and norepinephrine. We have also generated three alpha-latrotoxin mutants with substitutions in conserved cysteine residues, and a fourth mutant with a four-residue insertion. All four alpha-latrotoxin mutants were found to be unable to trigger release. Interestingly, the insertion mutant LtxN4C exhibited receptor-binding affinities identical to wild-type LtxWT, bound to CL1 and neurexin Ialpha as well as LtxWT, and similarly stimulated synaptic hydrolysis of phosphatidylinositolphosphates. Therefore, receptor binding by alpha-latrotoxin and stimulation of phospholipase C are insufficient to trigger exocytosis. This conclusion was confirmed in experiments with La3+ and Cd2+. La3+ blocked release triggered by LtxWT, whereas Cd2+ enhanced it. Both cations, however, had no effect on the stimulation by LtxWT of phosphatidylinositolphosphate hydrolysis. Our data show that receptor binding by alpha-latrotoxin and activation of phospholipase C do not by themselves trigger exocytosis. Thus receptors recruit alpha-latrotoxin to its point of action without activating exocytosis. Exocytosis probably requires an additional receptor-independent activity of alpha-latrotoxin that is selectively inhibited by the LtxN4C mutation and by La3+.