By Daniella Gimbosh
What is botulinum toxin – the mysterious chemical that has been termed the “miracle poison”? The bacterium Clostridium botulinum is responsible for the production of botulinum toxin, an extremely potent neurotoxin which may lead to poisoning known as botulism (Munchau & Bhatia, 2000). The effects of botulism on motor and parasympathetic functions are drastic due to acetylcholine vesicle exocytosis inhibition. However, this toxin is also useful in a wide variety of clinical applications, such as in the treatment of abnormal muscular activity and Botox. This “miracle poison” is therefore an extremely interesting area of exploration, from the mechanism of action of botulinum toxin and its effects on neurotransmitters, to its uses in medicine and potential future applications.
Usually, the propagation of action potentials leads to the release of acetylcholine in vesicles in a process known as exocytosis. Acetylcholine is an excitatory neurotransmitter which is released at synapses, specifically at neuromuscular junctions, which causes muscle contraction through the propagation of action potentials. How this process happens has been very well studied.
An action potential travels down a pre-synaptic neuron until it reaches an active synapse. The membrane of the pre-synaptic neuron is depolarised, leading to an influx of calcium ions through calcium-specific voltage-gated channels (Lu, 2015). These calcium ions stimulate the acetylcholine-containing vesicles to fuse with the membrane of the pre-synaptic neurons, resulting in neurotransmitter release through exocytosis as the acetylcholine quickly diffuses across the vast synaptic cleft. Acetylcholine receptors on the post-synaptic membrane bind the travelling neurotransmitter, allowing the action potential to be passed on and propagated again in the post-synaptic neuron. However, in the presence of botulinum toxin, this process looks very different: exocytosis of the acetylcholine vesicles is inhibited, and no further propagation of the action potential in the post-synaptic neuron takes place (Ritter et al., 2020).
The process of exocytosis across a synapse, in reality, involves a myriad of other proteins such as synaptosomal-associated proteins (SNAPs) and SNAP receptor proteins (SNAREs). SNAP-25 is a protein which allows for the formation of a SNARE complex (Purves et al., 2015) – this happens with the aid of many other helper proteins. When botulinum toxin is present, it binds to receptors on the SNAP-25 protein, leading to the cleavage of the SNARE complex. This means that vesicles containing acetylcholine cannot dock or fuse to the presynaptic membrane and, therefore, exocytosis of the neurotransmitter cannot take place. Since no vesicular exocytosis happens, no action potential can be propagated in the post-synaptic neuron, and no muscle contractions can occur. Botulinum toxin could, through this method of action, lead to paresis – partial muscle paralysis. This inhibition can, however, be overcome naturally as the SNARE complex is degraded and synthesised again through protein turnover, which removes the bound botulinum (Dressler & Adib Saberi, 2005).
Quite paradoxically, botulinum toxin has many therapeutic uses. It is often used clinically for treatment of hyperactive muscle disorders. Through the decrease of action potential propagation, a subsequent decrease in muscle contraction is achieved, which can be used to treat muscle diseases such as blepharospasm. Blepharospasm is a neurological disorder that may result in physical disability due to unilateral or bilateral eyelid twitching or even closure (Hellman & Torres-Russotto, 2014). It is the continual action potential propagation and muscle contraction which lead to the characteristic twitching of the disease. These chronic eyelid contractions can be treated with an extremely low dose of botulinum toxin Type A, which leads to a relatively long-term treatment for blepharospasm.
A more common application nowadays is the use of botulinum toxin Type A for cosmetic treatments under the name of “Botox” or “Dysport”. For this type of treatment, often used to minimise facial wrinkles, a dilute solution of botulinum toxin Type A is injected into a specific tissue, preventing muscle contraction in the desired muscle (Lim & Seet, 2007). A lack of muscle contraction leads to a smooth appearance on the skin surface, and thus, less wrinkles. Interestingly, a ‘side effect’ of this treatment is the loss of sweating in the treated area; SNARE complex cleavage by the toxin inhibits action potential propagation to the nerves in charge of stimulating the affected sweat gland. However, both Botox and Dysport treatments require secondary injections after some time as patients may develop toxin antibodies, leading to the recovery of the facial muscles.
There is no limit to the possibilities of botulinum toxin in future clinical applications. Research has implicated that the neurotoxin may be used in pain relief through the action of cholinergic synapses, as well as in the treatment of mental illnesses such as depression (Rasetti-Escargueil, Lemichez & Popoff, 2018). Scientists are delving deeper into the biochemical pathways of botulinum toxin, and as more is discovered about this miraculous poison, its potential therapeutic applications continue to widen.
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