Definition
The conotoxins are paralytic poisons from Pacific cone snails that block the transmission of a nerve impulse from the nerve to the muscle at the neuromuscular junction.
Discovery
Kohn in 1976 studied cone snails, the venomous predators. Most Conus use their venoms for multiple purposes, including prey capture and defense. All 500 living species of cone snails have a highly sophisticated venom production apparatus and delivery system. They have specialized teeth, which in effect serve both as harpoon and disposable hypodermic needle for venom delivery 1,2,3.
Bulaj G et al., in 2003 demonstrated that the correct folding of a Conus peptide is facilitated by a posttranslationally modified amino acid, γ-carboxyglutamate. In addition, they showed that multiple isoforms of protein disulfide isomerase are major soluble proteins in Conus venom duct extracts. The results provided evidence for the type of adaptations required before cone snails could systematically explore the specialized biochemical world of ‘‘microproteins’’ that other organisms have not been able to systematically access 4.
Structural Characteristics
Most conotoxin protein genes initially produce a translation product that is 100 amino acids in length, a size sufficient to allow conventional folding by multiple intramolecular interactions. Toxin proteins found in venoms are generally smaller (50–100 aa), with additional stability provided by disulfide bonds. In effect, the cone snails have extended this tendency one step further, with some venom peptide superfamilies being the smallest highly structured but functionally diverse classes of gene products known (12–20 aa with two to three disulfide bonds). Conopeptide evolution has resulted in a large diversity of biological function being generated in each conotoxin superfamily. Thus, Conus peptide superfamilies are like other major classes of proteins produced through gene translation: structural and functional novelty can evolve, and thus, conotoxins are in many respects ‘‘microproteins’’ and differ from more conventional unstructured peptides. Each conotoxin superfamily has a characteristic arrangement of cysteine residues, which is assembled into a particular disulfide bonding configuration (the ‘‘disulfide framework’’). The latter is the primary determinant of polypeptide backbone structure. Despite hypermutation of the amino acids between Cys residues, the disulfide framework generally remains conserved within a superfamily, generating a characteristic scaffold. In principle, for peptides with six Cys residues (characteristic of at least four conotoxin superfamilies) there are 15 different of Conus peptides could have evolved: (i) specialized intramolecular interactions may stabilize conformation, and (ii) intermolecular interactions with extrinsic factors, perhaps within the endoplasmic reticulum (ER), may promote appropriate folding pathways within the ER 4. Conotoxins (or conopeptides) are named in a reasonably systemic manner: A Greek letter prefix denoting the structural class (α,μ,ω,δ,κ). Naturally occurring conotoxins include conotoxin GI, GIA and GII. In conotoxin GI, U is Glu, V4 is Asn, V5 is Pro, V6 is Ala, W is Gly, X is Arg, Y is His, Z is Tyr, V12 is Ser and R is --NH2.
Mode of Action
Many conotoxin have been found to be highly selective for a diverse range of mammalian ion channels and receptors associated with pain signaling pathways. All of these conotoxins act by preventing neuronal communication, but each targets a different aspect of the process to achieve this. The α-conotoxins target nicotinic ligand gated channels, the μ-conotoxins target the voltage-gated sodium channels and the ω-conotoxins target the voltage-gated calcium channels. α-conotoxin acts on nicotinic acetylcholine receptors. The effect is a paralysis similar to that seen with curare. δ-conontoxins acts on sodium channels. Unlike μ-conotoxins, they slow the inactivation of the sodium channel. μ-conotoxin acts on sodium channels. This is also the target for saxitoxin and tetrodotoxin and the effects are similar. κ-conotoxin acts on potassium channels. They are also known as shaker peptides because they block a potassium channel known as "Shaker" and as a result they induce tremors. ω-conotoxin acts on calcium channels associated with nerve impulse transmission at the neuromuscular junction. Conantonkins acts on NMDA glutamate receptors. This blocks nerve impulses that use glutamic acid rather than acetylcholine as the neurotransmitter.
Functions
Individual conotoxins vary greatly in lethality towards mammals. Some of the tremor inducing omega conotoxins is not lethal, whereas others of the same group are lethal at low levels.
The toxicity in rats and mice is usually reported for the toxins administered intracranially (into the brain). Some α-conotoxins have lethal doses as low as 25 μgrams/kg for mouse.
Synergistic interaction, the toxicity of the complex mixture of peptides that is cone snail venom may be much greater than the sum of its parts because of the synergistic interaction between toxins acting on different aspects of neural function 5.
At neuromuscular junction, conotoxins were found to act at the neuromuscular junction to inhibit the passage of an excitatory impulse across the junction, but which had no effect on muscle action potential.
Pharmacologic agents, their small size, structural stability, and target specificity make them attractive pharmacologic agents 6.
References
1. Kohn AJ, Nybakken JW, Mool V (1972). Radula tooth structure of the gastropod Conus imperialis. Science, 176:49-51.
2. Kohn AJ, Saunders PR, Wiener S (1960). Preliminary studies on the venom of the marine snail. Conus. Ann NY Acad Sci., 90:706-725.
3. Kohn AJ (1976). Chronological analysis of the species of Conus described during the 18 century. Zool J Linn Soc Lond., 58:39-59.
4. Bulaj G, Buczek O, Goodsell I, Jimenez EC, Kranski J, Nielsen JS, Garrett JE, Olivera BM (2003). Efficient oxidative folding of conotoxins and the radiation of venomous cone snails. PNAS., 100(2):14562–14568.
5. Terlau H, Olivera BM (2004). Conus venoms: a rich source of novel ion channels-targeted peptides. Physiol. Rev., 84:41-68.
6. Jones RM, Cartier GE. McIntosh JM, Bulaj G, Farrar VE, Olivera BM (2001). Composition and therapeutic utility of conotoxins from genus Conus. Expert Opinion on Therapeutic Patents, 11(4):603-623.