Agatoxins: ion channel specific toxins from the american funnel web spider, Agelenopsis aperta

Abstract

Agatoxins from Agelenopsis aperta venom target three classes of ion channels, including transmitter-activated cation channels, voltage-activated sodium channels, and voltage-activated calcium channels. The α-agatoxins are non-competitive, use-dependent antagonists of glutamate receptor channels, and produce rapid but reversible paralysis in insect prey. Their actions are facilitated by the μ-agatoxins, which shift voltage-dependent activation of neuronal sodium channels to more negative potentials, causing spontaneous transmitter release and repetitive action potentials. The ω-agatoxins target neuronal calcium channels, modifying their properties in distinct ways, either through gating modification (ω-Aga-IVA) or by reduction of unitary current (ω-Aga-IIIA). The α-agatoxins and ω-agatoxins modify both insect and vertebrate ion channels, while the μ-agatoxins are selective for insect channels. Agatoxins have been used as selective pharmacological probes for characterization of ion channels in the brain and heart, and have been evaluated as candidate biopesticides.

Introduction

The vast majority of spiders, including the American funnel web spider, Agelenopsis aperta subdue prey by envenomation. It is remarkable that, despite the mostly negative reputation spiders have among the general populace, most employ venoms that are innocuous to mammals but very active against insects. The total number of extant spider species can only be speculated upon, but it is surely in the tens of thousands, and perhaps over 1,00,000 (Coddington and Levi, 1991). Of these, the number of venoms that are medically important is likely to be far less than 1% of this total. The impressive biological activity of spider venoms against insects was one of our original motivations for investigating them as possible sources of leads for novel physiological processes that could be targeted by new insecticides.

The discovery of three classes of agatoxins from A. aperta venom, each specific for a different ion channel class, has provided insights at several levels. The venom is a highly concentrated and diverse cocktail of toxins that together produce rapid and long-lasting block of neuromuscular transmission. Some of the toxins are very selective for insect ion channels, while others affect both insect and mammalian ion channels and receptors, in some instances with surprising affinity and selectivity for the latter. While mammalian activity was unexpected at the time, the conservation of ion channels and receptors in diverse animal groups revealed by genome studies now makes it more understandable. It is ironic that, despite our original focus on the discovery of insect-selective toxins, a great deal of effort has been devoted to characterizing toxins that modify ion channels in the mammalian brain.

Regarding phylogenetic specificity of venom toxins, it is noteworthy that some of the most impressive insect-specific spider toxins have come from spiders that are lethal to humans. Although the first atracotoxins isolated and characterized from Atrax and Hadronyche species were mammalian toxic (see the review by Nicholson et al. in this volume), King and his colleagues have demonstrated even the most medically important spiders make toxins that are extremely insect-selective (see the review by Tedford et al. in this volume). It is clear spider venoms, along with those of Conus and scorpions, contain a diverse pharmacopoeia of toxins that recognize surface architectures of receptors and channels from a variety of phylogenetic groups, in many instances with remarkable yet unpredictable specificity.

A. aperta is common to the Southwestern United States, where it spins a flat silk web on grass or shrubs that converges into a funnel-shaped lair (Fig. 1). Here the spider hides in wait for prey to become ensnared in its silk, whereupon it emerges and attacks with one or more bites. A member of the family Agelenidae, Agelenopsis is now grouped among the Entelegynae and Amaurobioidea based on cladistic analysis utilizing a multitude of characters (Griswold et al., 1999). Properties of the agatoxins identified from its venom are summarized here, both from the perspectives of predator-prey interaction and pharmacological profiles.