Trends in Microbiology

Trends in Microbiology

Volume 11, Issue 9, September 2003, Pages 431-437
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The journey of tetanus and botulinum neurotoxins in neurons☆

https://doi.org/10.1016/S0966-842X(03)00210-5Get rights and content

Abstract

Anaerobic bacteria of the genus Clostridia are a major threat to human and animal health, being responsible for pathologies ranging from food poisoning to gas gangrene. In each of these, the production of sophisticated exotoxins is the main cause of disease. The most powerful clostridial toxins are tetanus and botulinum neurotoxins, the causative agents of tetanus and botulism. They are structurally organized into three domains endowed with distinct functions: high affinity binding to neurons, membrane translocation and specific cleavage of proteins controlling neuroexocytosis. Recent discoveries regarding the mechanism of membrane recruitment and sorting of these neurotoxins within neurons make them ideal tools to uncover essential aspects of neuronal physiology in health and disease.

Section snippets

The HC fragment and its binding to neurons

The absolute neurospecificity of clostridial neurotoxins is a major determinant of their outstanding toxicity [3]. The C-terminal portion of the heavy chain (HC) is responsible for neurospecific binding and, in the case of TeNT, retrograde transport [7] (Fig. 1a). Recombinant HC fragments bind to the functional receptors of TeNT and BoNTs, because they counteract the paralysis induced by parental neurotoxins in cultured neurons and isolated NMJ [12]. The crystal structures of the HC fragment of

TeNT and BoNTs receptors

TeNT and BoNTs bind to polysialogangliosides [7] and show a reduced activity in neurons in which ganglioside synthesis has been inhibited [25] or ablated [26]. However, their absolute neurospecificity and the overall lack of binding competition between TeNT and BoNTs [5] make it unlikely that polysialogangliosides represent the sole determinants for their binding to the neuronal surface. In agreement with the double lipid and protein receptor model [27], specific protein co-receptors for

Lipid rafts and clostridial neurotoxin binding

GPI-anchored proteins, together with cholesterol, gangliosides, and other sphingolipids, are enriched in microdomains of the plasma membrane termed lipid rafts, which act as functional platforms for signaling, ligand recognition and sorting 37, 38. The interaction of TeNT HC with two classes of raft-associated components, polysialogangliosides and GPI-anchored proteins, suggests that the binding of TeNT to neurons is mediated by lipid microdomains (Fig. 3). Accordingly, TeNT HC, as well as BoNT

Intracellular sorting and axonal retrograde transport

After binding to the neuronal plasma membrane, BoNTs and TeNT are targeted towards distinct regions of motor neurons (MNs) (Fig. 2). The action of BoNTs is mainly restricted to the NMJ, where they cause a long-lasting blockade of acetylcholine release 4, 7. The peripheral targeting of BoNTs is probably determined by the cellular properties of their protein receptors. BoNT-B binds to the intraluminal portion of synaptotagmin I and II [28], which is glycosylated, in a GT1b-dependent manner. This

Conclusions

Several studies on clostridial neurotoxins’ mechanism of action have advanced our understanding of the process of neuroexocytosis, promoting the functional characterization of SNARE proteins and their role in membrane fusion. However, novel applications based on the unique properties of BoNTs and TeNT still await full exploitation. One of the most attractive of these is the use of neurotoxins to dissect the neuronal basis of behaviour. The targeted expression of the L chain of TeNT in different

Supplementary material

See the supplementary video supplied with this article. Fluorescent tetanus neurotoxin (TeNT) is retrogradely transported in living rat motor neurons. Cells were incubated with 40 nM TeNT-Texas Red for 15 min at 37°C, washed and imaged by time-lapse, low-light microscopy. The cell body is located at the bottom of the picture. Frames were taken every 5 s. Shown here is a movie consisting of 45 frames played at 5 frames/s. The image is 7×37 μm.

Acknowledgements

We thank the many colleagues whose work could not be cited here owing to space limitations. We are indebted to O. Rossetto, J. Herreros and N. Fairweather for helpful comments during the preparation of this manuscript. Laboratory work by the authors is supported by Cancer Research UK.

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