Toxicology and pharmacology of botulinum and tetanus neurotoxins: an update

Abstract

Tetanus and botulinum neurotoxins cause the neuroparalytic syndromes of tetanus and botulism, respectively, by delivering inside different types of neurons, metalloproteases specifically cleaving the SNARE proteins that are essential for the release of neurotransmitters. Research on their mechanism of action is intensively carried out in order to devise improved therapies based on antibodies and chemical drugs. Recently, major results have been obtained with human monoclonal antibodies and with single chain antibodies that have allowed one to neutralize the metalloprotease activity of botulinum neurotoxin type A1 inside neurons. In addition, a method has been devised to induce a rapid molecular evolution of the metalloprotease domain of botulinum neurotoxin followed by selection driven to re-target the metalloprotease activity versus novel targets with respect to the SNARE proteins. At the same time, an intense and wide spectrum clinical research on novel therapeutics based on botulinum neurotoxins is carried out, which are also reviewed here.

Keywords: Botulism; Cholinergic; Neurotoxins; Tetanus; Therapy; Toxicity.

Fig. 1

Internalization and trafficking of TeNT and BoNTsinside peripheral neurons. Bottom left panel: TeNT (light blue circle) and BoNTs (orange circle) bind to peripheral nerve terminals, most notably the motor axon terminals of the neuromuscular junction. Although TeNT binds also other types of nerve terminals, BoNTs bind selectively motorneurons and autonomic cholinergic nerves. Bottom right panel: TeNT binds via interaction with a polysialoganglioside (brown star) and nidogen molecule (green). BoNTs bind to a polysialoganglioside and to a protein receptor (SV2 or synaptotagmin, green). The TeNT-receptors complex enters the lumen of an endocytic vesicle that later on merges with a signalling endosome (SE) distinguished by the rab5 molecule (magenta). The replacement with rab7 (violet) determines the attachement of SE to the machinery (black tubules) that mediates their retroaxonal transport to the motorneuron perikaryon located within the spinal cord as shown in the top right panel. The BoNT-receptors complexes enter the lumen of a synaptic vesicle that leads to the translocation of the catalytic L chain in the cytosol of the presynaptic terminal (see Fig. 2 for more details), causing the local neuroparlaysis of botulism. Also the TeNT trimeric complex may enter the lumen of a synaptic vesicle and have a local effect, albeit to a much lesser extent than the BoNTs Similarly, BoNT/A is retraxonally transported as TeNT does, although the proportion and the specific trafficking route(s) are still unknown Top left panel: TeNT and BoNTs are discharged in the spinal cord fluids and bind inhibitory interneurons (Ia and IIa, glycinergic and gabaergic) or VoC cholinergic excitatory interneurons, respectively. As a consequence a spastic paralysis is caused by TeNT, whilst the action of BoNT contributes to the peripheral motorneurons’ paralysis

Fig. 2

The five steps of the molecular mechanism underlying the neuroparalytic action of tetanus neurotoxin (TeNT) and botulinum neurotoxins (BoNTs). Step 1: the neurotoxins bind to a polysialoganglioside molecule (brown star) and then to a protein receptor (detailed cartoons in the bottom right of the figure) via the C-terminal domain of the toxin (green). Step 2: this second binding drives the trimeric complex into the lumen of a synaptic vesicle. The vesicle is acidified (transition from light blue to red in the upper right panel) following the action of a proton pump (orange) to drive the accumulation of neurotransmitter (NT, pale orange) inside the synaptic vesicle. Step 3: low pH causes a structural transition of th neurotoxin that results in the membrane penetration of the HN domain (yellow) and translocation of the metalloprotease domain (red), which remains bound to HN via a disulfide bond (upper right panel). Translocation is assisted by the chaperone Hsp90 (light blue). Step 4: The disulfide bond is reduced by the NADH-Thioredoxin -Thioredoxin Reductase system (magenta and blue, respectively) (upper right panel). Step 5: the free metalloprotease domain reaches its specific SNARE protein target which is cleaved and inactivated by a single site cleavage, in such a way that neurotransmitter release from the synaptic vesicle is prevented. The left panel shows that TeNT cleaves only VAMP, whilst the various BoNT serotypes target VAMP or SNAP25 or Syntaxin (STX) at specific peptide bonds