Actions of sea anemone type 1 neurotoxins on voltage-gated sodium channel isoforms

As voltage-gated Na(+) channels are responsible for the conduction of electrical impulses in most excitable tissues in the majority of animals (except nematodes), they have become important targets for the toxins of venomous animals, from sea anemones to molluscs, scorpions, spiders and even fishes. During their evolution, different animals have developed a set of cysteine-rich peptides capable of binding different extracellular sites of this channel protein. A fundamental question concerning the mechanism of action of these toxins is whether they act at a common receptor site in Na(+) channels when exerting their different pharmacological effects, or at distinct receptor sites in different Na(v) channels subtypes whose particular properties lead to these pharmacological differences. The alpha-subunits of voltage-gated Na(+) channels (Na(v)1.x) have been divided into at least nine subtypes on the basis of amino acid sequences. Sea anemones have been extensively studied from the toxinological point of view for more than 40 years. There are about 40 sea anemone type 1 peptides known to be active on Na(v)1.x channels and all are 46-49 amino acid residues long, with three disulfide bonds and their molecular weights range between 3000 and 5000 Da. About 12 years ago a general model of Na(v)1.2-toxin interaction, developed for the alpha-scorpion toxins, was shown to fit also to action of sea anemone toxin such as ATX-II. According to this model these peptides are specifically acting on the type 3 site known to be between segments 3 and 4 in domain IV of the Na(+) channel protein. This region is indeed responsible for the normal Na(+) currents fast inactivation that is potently slowed by these toxins. This fundamental "gain-of-function" mechanism is responsible for the strong increase in the action potential duration. They constitute a class of tools by means of which physiologists and pharmacologists can study the structure/function relationships of channel proteins. As most of the structural and electrophysiological studies were performed on type 1 sea anemone sodium channel toxins, we will present a comprehensive and updated review on the current understanding of the physiological actions of these Na channel modifiers.