Review
Botulinum and tetanus neurotoxins: structure, function and therapeutic utility

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Abstract

The toxic products of the anaerobic bacteria Clostridium botulinum, Clostridium butyricum, Clostridium barati and Clostridium tetani are the causative agents of botulism and tetanus. The ability of botulinum neurotoxins to disrupt neurotransmission, often for prolonged periods, has been exploited for use in several medical applications and the toxins, as licensed pharmaceutical products, now represent the therapeutics of choice for the treatment for several neuromuscular conditions. Research into the structures and activities of botulinum and tetanus toxins has revealed features of these proteins that might be useful in the design of improved vaccines, effective inhibitors and novel biopharmaceuticals. Here, we discuss the relationships between structure, mechanism of action and therapeutic use.

Section snippets

CNT structure

To understand the components of BoNT responsible for its mechanism of action, it is necessary to introduce the basic CNT structure. CNTs are synthesized as single-chain polypeptides of ∼150 kDa and are subsequently cleaved to form di-chain molecules (Fig. 1), in which the light (LC) and heavy chains (HC) are linked by a single disulfide bond. The 50-kDa LC acts as a zinc-dependent endopeptidase. The heavy chain contains two functional domains, each of ∼50 kDa. The N-terminal half (HN) is the

Mechanism of action

BoNT intoxication (summarized in Fig. 3) occurs through a multi-step process involving each of the toxin functional domains, and can be described as the outcome of three discrete stages.

Binding and internalization

Data obtained for the structure of the HC domains of BoNT/A [4], BoNT/B [5] and TeNT [6] reveals that the binding domain is composed of two sub-domains. The N-terminal half adopts a jelly-roll, lectin-like fold and the C-terminal half has a β-trefoil fold. The binding domain binds to gangliosides on the surface of motorneurons. As described previously, BoNTs bind to gangliosides of the GT1b and GD1b classes with low affinity [24]. Gangliosides consist of sialic acid-containing oligosaccharide

Medical applications of BoNTs

Despite being extremely poisonous to humans, BoNTs are highly effective therapeutic agents and accomplished research tools. It was observed at the beginning of the 20th century that, by injection of botulinum toxin, localized paralysis of individual muscle groups could be achieved without causing botulism in the recipient (reviewed in Ref. [50]). Preparations of BoNT/A have been approved by the US Food and Drug Administration as a biopharmaceutical for use in treating strabismus, blepharospasm

Future directions

Although some progress has been made in recent years, identification and characterization of the protein receptors for the BoNTs and determination of the mechanism of specificity of CNT binding domains for their receptors is an outstanding problem. Furthermore, understanding the mechanism of LC translocation and activation within the motorneuron, including the effects of pH on the tertiary structures of BoNTs, will be crucial for rational design of engineered BoNT therapeutics. Further

Acknowledgements

We apologize for the omission of many relevant citations imposed by space limitations. We wish to acknowledge the helpful comments of Bernard Poulain and Neil Isaacs during preparation of this manuscript. Our work on BoNTs is supported by a joint post-graduate studentship to K.T. between the University of Bath and CAMR, UK. K.R.A. wishes to acknowledge the Royal Society and the Leverhulme Trust, UK for a Senior Research Fellowship.

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      In fact, an accurate structure of TeNT, derived from x-ray crystallography, was only solved in 2017 (Masuyer et al., 2017). Tetanus toxin has a three sub-domain structure, split into two chains joined by a disulphide bond in the active form, very similar in structure and function to the botulinum toxins (Beise et al., 1994; Eisel et al., 1986; Helting and Zwisler, 1977; Masuyer et al., 2017, 2011; Turton et al., 2002). The main difference between tetanus and botulinum is that tetanus is transported to the CNS, where it disrupts the synaptic vesicle fusion complex in the inhibitory interneuron, thereby increasing the neurological signal to the corresponding muscle (Bercsenyi et al., 2012; Caleo and Schiavo, 2009; Lalli et al., 2003; Ovespian et al., 2015; Restani et al., 2012).

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