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More about Botulinum toxin1

What is botulinum toxin and where does it come from?

3D protein structure of botulinum toxin type A

Botulinum toxin is a protein produced by the bacterium Clostridium
Botulinum, which can be commonly found in soil. Under anaerobic conditions Clostridium Botulinum secretes botulinum toxin.

Seven different serotypes of toxin are produced by the bacteria: A, B, C, D, E, F and G.

Each toxin serotype has a distinct protein structure, mode of action and
potency.1

The most potent serotype is type A which this is the most commonly used toxin in
medicine. Serotype B is also available as a medicine.

In nature the toxin is found in association (complexed) with non-toxic proteins which include haemagglutinin proteins.1

What does botulinum toxin do?

Clostridium botulinum bacteria

Botulinum toxin blocks transmission of messages from nerves to muscles (neuromuscular transmission) and therefore weakens muscles temporarily.

Accidental ingestion of large quantities of botulinum toxin, e.g. from improperly canned foods, can lead to an acute paralytic illness called botulism.1

Used in small therapeutic quantities botulinum toxin can selectively weaken overactive muscles temporarily.

How long does botulinum toxin last?

Clinically detectable effects of botulinum toxin last for 2 - 4 months in most situations, although in some cases the effects can last longer.2

Structure of botulinum toxin

Botulinum toxins are proteins that have similar molecular structures and weights of around 140-170 kDA. The toxins are initially produced as single polypeptide chains, and are only weakly toxic. The chain is then activated by a process of proteolytic cleavage, to form the highly toxic di-chain molecule, comprised of a heavy chain and a light chain, linked by a disulphide bond.1

Mode of action of botulinum toxin

Normal neuromuscular transmission

In normal neuromuscular transmission, acetylcholine is the primary neurotransmitter which, when released from the nerve terminal into a neuromuscular junction, promotes muscle contraction.

Acetylcholine is a substance which is stored in vesicles in nerve endings (presynaptic terminals). Before releasing acetylcholine into the neuromuscular junction, the vesicles need to be transported to, and fuse with, the nerve terminal membrane (presynaptic membrane). The transport of vesicles to the presynaptic membrane, and the eventual release of acetylcholine, is controlled by a group of proteins called the SNARE complex - (the Soluble N-ethylmaleimide sensitive factor attachment protein receptor complex). The SNARE complex consists of a number of proteins, but the three main ones are:1

  • SNAP-25 (synaptosomal-associated protein of 25kDa)
  • Synaptobrevin (also known as vesicle-associated membrane protein or VAMP)
  • Syntaxin

Following fusion of the vesicle with the presynaptic membrane, acetylcholine is released into the synaptic gap, diffuses across the gap, and binds to nicotinic cholinergic receptors on the postsynaptic membrane (on the surface of the muscle).

Normal neuromuscular transmission


Binding and internalisation of botulinum toxin

When botulinum toxin is injected into the muscle it has significant affinity for peripheral cholinergic (involving acetylcholine) nerve endings, where it binds rapidly, specifically and irreversibly to receptors on the presynaptic membrane - the heavy chain facilitates this binding and internalisation of the toxin. The bound toxin is taken up into the nerve terminal by endocytosis. The resulting endosome containing toxin migrates into the cytosol of the nerve terminal and at this point it splits into a heavy chain and an active light chain.1

Binding and internalisation of botulinum toxin


Inhibition of acetylcholine release

The active light chain of botulinum toxin has a specific affinity to cleave certain proteins involved in the mechanism of acetylcholine release. Botulinum toxin type A cleaves SNAP-25 so that:1

  • Acetylcholine vesicles cannot fuse with the presynaptic membrane
  • Acetylcholine is not released
  • Neuromuscular transmission is blocked
  • Muscle weakness and paralysis occurs

Inhibition of acetylcholine release


Recovery of function

The blocked neuromuscular junction triggers the growth of new nerve sprouts by budding. Eventually these allow resumption of neuromuscular transmission, leading to a recovery of muscle function. Thus, the clinical effect of botulinum toxin is temporary. In time, the original neuromuscular junction may recover from botulinum toxin and the axonal sprouts regress.1

References

1. Rossetto O, Montecucco C. Chapter 2. How botulinum toxins work. In: Moore P, Naumann M, editors. Handbook of Botulinum Toxin Treatment. 2nd Ed. Blackwell Science 2003.

2. Moore P, Naumann M. Chapter 3. General and clinical aspects of treatment with botulinum toxin. In: Moore P, Naumann M, editors. Handbook of Botulinum Toxin Treatment. 2nd Ed. Blackwell Science 2003.