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  • Review Article
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Botulinum neurotoxins: genetic, structural and mechanistic insights

Nature Reviews Microbiology volume 12, pages 535–549 (2014)Cite this article

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Key Points

  • Botulinum neurotoxins (BoNTs) are produced by neurotoxigenic clostridia and are a diverse group that consists of approximately 40 different BoNT types (and various subtypes), all of which cause persistent paralysis of peripheral nerve terminals — a condition known as botulism.

  • Recent studies have solved various structures of BoNT complexes, which has provided insights into their modes of entry into the general circulation as well as the ability of these toxins to survive for long periods of time in the ex vivo environment.

  • The molecular basis of the specificity of BoNT binding to nerve terminals is explored, as well as the ensuing cellular events, including toxin endocytosis and the targeting and cleavage of SNARE proteins.

  • A molecular model for the essential process of membrane translocation of the metalloprotease domain of BoNTs into the neuronal cytosol is presented.

  • Open questions and future areas of research are outlined with respect to the development of novel therapeutic agents that are based on BoNTs.

Abstract

Botulinum neurotoxins (BoNTs) are produced by anaerobic bacteria of the genus Clostridium and cause a persistent paralysis of peripheral nerve terminals, which is known as botulism. Neurotoxigenic clostridia belong to six phylogenetically distinct groups and produce more than 40 different BoNT types, which inactivate neurotransmitter release owing to their metalloprotease activity. In this Review, we discuss recent studies that have improved our understanding of the genetics and structure of BoNT complexes. We also describe recent insights into the mechanisms of BoNT entry into the general circulation, neuronal binding, membrane translocation and neuroparalysis.

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Figure 1: Animal and human botulism.
Figure 2: Structure of isolated BoNT molecules and BoNT complexes.
Figure 3: Binding and trafficking of botulinum neurotoxins inside nerve terminals.
Figure 4: Model for the molecular events that occur during L-chain translocation across the synaptic vesicle membrane.

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Acknowledgements

The authors thank T. Binz, G. Franciosa, R. Kammerer, F. Lista, M. Montal, S. Pellet and G. Schiavo for their comments. The authors apologize to colleagues whose work could not be cited owing to space limitations. Research in the authors' laboratory is supported by University of Padova, Italy, Fondazione CARIPARO, the Axonomics Poject of the Provincia Autonoma di Trento and the Italian Ministry of Defence.

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Author notes

  1. Ornella Rossetto and Marco Pirazzini: These authors contributed equally to this work.

Authors and Affiliations

  1. Department of Biomedical Sciences, University of Padova, Via Ugo Bassi 58/B, Padova, 35131, Italy

    Ornella Rossetto, Marco Pirazzini & Cesare Montecucco

  2. National Research Council Institute of Neuroscience, University of Padova, Via Ugo Bassi 58/B, Padova, 35131, Italy

    Ornella Rossetto, Marco Pirazzini & Cesare Montecucco

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  1. Ornella Rossetto
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  2. Marco Pirazzini
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Correspondence to Cesare Montecucco.

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DATABASES

3BTA

1EPW

3FFZ

3VOC

PowerPoint slides

Glossary

Neurotransmitter

An endogenous chemical that transmits signals across a synapse from a neuron to a postsynaptic cell.

Metalloprotease

A proteolytic enzyme that is defined by the presence of an essential active-site metal ion, which is most often zinc.

Mouse lethal dose

Corresponds to the toxin dose that is required to kill 50% of exposed mice; it is usually expressed as the median lethal dose (LD50). The mouse LD50 of BoNTs is 0.1–1 ng per kg for subtype 1 of the seven serotypes.

Cholinergic nerve terminals

Axonal terminals that use acetylcholine for neurotransmission.

M cells

Specialized epithelial cells of the follicle-associated epithelium of the gastrointestinal tract that are involved in the rapid uptake and presentation of particular antigens and microorganisms to immune cells of the lymphoid follicle, thereby inducing an effective immune response.

Neuroendocrine crypt cells

Cells that are distributed throughout the intestinal epithelium and that secrete peptide hormones in an endocrine or paracrine manner from dense core or neurosecretory granules.

Synaptic vesicles

Neuronal vesicles that store and release neurotransmitters or neuropeptides at the synapse.

Presynaptic receptor

A receptor that is localized on the surface of the presynaptic membrane; it is either protein or lipid in nature.

Synaptotagmin

(Syt). A protein that spans the membrane of synaptic vesicles and binds to Ca2+ to trigger the fusion of synaptic vesicles with the plasma membrane of the neuron.

SV2

A protein that spans the membrane of synaptic vesicles and has an unknown function. Following fusion of the synaptic vesicle to the plasma membrane, the luminal domain of SV2 becomes exposed to the extracellular medium and functions as a receptor for botulinum neurotoxins.

Phrenic nerve hemidiaphragm

An ex vivo preparation that includes a portion of the diaphragm, as well as the axon and nerve terminal of the phrenic nerve. This nerve contains motor, as well as sensory and sympathetic, fibres and controls the contraction of the diaphragm muscle via the release of acetylcholine. Its inhibition by botulinum neurotoxins blocks respiration, which causes death.

Patch clamp technique

An electrophysiological technique that is based on microelectrodes that are sealed on the plasma membrane of a cell, which enables the measurement of electrical activity and the properties of ion channels.

pH sensor

In the context of this Review; amino acid residues that change protonation state according to variations in pH. A change in protein structure may consequently occur, owing to altered hydrogen bonding and electrostatic interactions.

Neuroexocytosis nanomachine

A molecular machine of nanometre dimensions that is used for the release of neurotransmitters.

Serum sickness

An illness of humans that is caused by a hypersensitive reaction to proteins in antiserum derived from a non-human source; it usually occurs 4–10 days after exposure.

Camelid-like antibodies

Single-domain antibodies that are derived from the heavy-chain antibodies of camelids; they are a new generation of therapeutic agents and immunoreagents.

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Rossetto, O., Pirazzini, M. & Montecucco, C. Botulinum neurotoxins: genetic, structural and mechanistic insights. Nat Rev Microbiol 12, 535–549 (2014). https://doi.org/10.1038/nrmicro3295

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