More about Botulinum toxin

What is botulinum toxin?

Botulinum toxin is a protein produced by the common bacterium: Clostridium botulinum.

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

Different serotypes have distinct protein structures, modes of action and potencies.¹

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

Where does botulinum toxin come from?

Botulinum toxin is secreted under anaerobic conditions (without oxygen) by a bacterium commonly found in soil called Clostridium botulinum. In nature the toxin is found in association (complexed) with non-toxic proteins which include haemagglutinin proteins.¹

In a series of complex steps the toxin is extracted, purified and formulated under special manufacturing conditions to produce the vials of toxin used in medicine.

What does botulinum toxin do?

Botulinum toxin blocks transmission of messages from nerves to muscles 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 produced as a single polypeptide chain, weighing approximately 150 kDA, and are only weakly toxic. The chain is activated by a process of proteolytic cleavage, to form a 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

Acetylcholine is the primary neurotransmitter operating at the neuromuscular junction. Vesicles containing acetylcholine are stored in the presynaptic terminal of the nerve. Before releasing acetylcholine, the vesicles need to be transported to, and fuse with, the nerve terminal membrane. The transport of synaptic vesicles to the terminal 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 synaptic membrane, acetylcholine is released into the synaptic gap, diffuses across the gap, and binds to nicotinic cholinergic receptors on the postsynaptic membrane, triggering muscle contraction.

Binding and internalisation of botulinum toxin

When botulinum toxin is injected into the muscle it has significant affinity for peripheral cholinergic 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 and at this point it splits into a heavy chain and an active light chain.¹

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 exocytosis. 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

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.