Neurotoxins may be synthetic or endogenous compounds derived from species as diverse as bacteria, fungi, spiders, marine life, and man. Seven botulinum neurotoxin (BoNT) serotypes (A, B, C, D, E, F, and G), which are produced by Clostridium botulinum and other Clostridium species, inhibit neurotransmitter release from nerve terminals. These serotypes of BoNT are antigenically dissimilar, utilize distinct but related mechanisms of action, and are not interchangeable. Only BoNT serotypes A and B are approved by the Food and Drug Administration for treatment of neurological disorders and for cosmetic indications.
Neuroexocytosis, a multistage process leading to the fusion of synaptic vesicles with the plasma membrane, involves proteins collectively called SNAREs (soluble N-ethylmaleimide–sensitive factor [NSF] attachment protein receptors). Following calcium entry into the nerve terminal, 3 SNAREs form a highly stable SNARE complex which is required for fusion of synaptic vesicles with the inner surface of the plasmalemma. Membrane fusion allows the subsequent release of acetylcholine (ACh) from synaptic vesicles into the neuromuscular synaptic cleft, resulting in an action potential in the muscle that causes it to contract. After BoNT binds to its receptor and is internalized into the nerve terminal, the BoNT proteolytically cleaves its SNARE substrate, thereby blocking neuroexocytosis. Although it is generally assumed that the effects of BoNTs are restricted to the peripheral nervous system, studies suggest that BoNTs, especially at high doses, may affect higher structures in the brain. BoNT may also alter the excitability of central neural circuits, both at spinal and cortical levels, by modulating peripheral sensory inputs.
The engineering of BoNTs is a crucial step in the evolution of neurotoxins, both as research tools and for clinical therapy. Modifying the pharmacological properties of neurotoxins through protein engineering may expand and improve the efficacy of future of neurotoxin-based therapies. Through the use of recombinant technology, combining advantageous therapeutic features of each serotype has led to the development of a chimeric recombinant toxin that effectively blocks the release of pain peptides. For example, targeting a chimera of BoNT-E and BoNT-A to nociceptive neurons is a potential new therapy for pain.
The effects of BoNT occur 2 to 5 days after injection and can last 3 months or longer, but they wear off gradually as a result of pharmacokinetic and intracellular events. To achieve the best possible outcome, treatment with BoNT should be tailored to the individual needs of the patient. Understanding the mechanism of action of BoNT-A has led to the worldwide treatment of more than 100 human conditions due to hyperactivity of nerves supplying various muscles or glands.