Targeting voltage sensors in sodium channels with spider toxins

Abstract

Voltage-activated sodium (Nav) channels are essential in generating and propagating nerve impulses, placing them amongst the most widely targeted ion channels by toxins from venomous organisms. An increasing number of spider toxins have been shown to interfere with the voltage-driven activation process of mammalian Nav channels, possibly by interacting with one or more of their voltage sensors. This review focuses on our existing knowledge of the mechanism by which spider toxins affect Nav channel gating and the possible applications of these toxins in the drug discovery process.

Figure 1. Spider toxins that target voltage-activated ion channels

a. Cartoon representing a top view of a Nav channel (left) and a Kv channel (right). The central Na⁺- or K⁺-selective pore is surrounded by the four voltage sensors of the four domains (DI-DIV). In the Nav channel, the paddles are not identical and are therefore colored differently. In the Kv channel, the paddles are identical and therefore have the same color. b. Cartoon of a side view of a Nav channel imbedded in a lipid membrane. Each domain (DI-DIV) consists of six transmembrane segments (S1-S6) of which S1-S4 form the voltage sensor and the S5-S6 segments of each domain come together to form the Na⁺-selective pore of the channel. c. Sequence alignment of spider toxins with an ICK motif and three disulfide bridges that target Nav, Kv, and/or Cav channels (indicated by grey circles). Residues that have been shown to be a part of the functionally important surfaces of SGTx1, ProTx-II, and Magi5 are indicated in green. %C = % conserved residues.

Figure 2. Interactions between spider toxins and Nav channels

a. Effects of 100nM ProTx-I, 100nM SGTx1, and 1μM Magi5 on rNav1.2a channels expressed in Xenopus laevis oocytes and recorded with the two-electrode voltage-clamp technique. Left, sodium currents elicited by a depolarization to a suitable membrane voltage before (black) and after toxin addition (red), are shown. Right, corresponding conductance-voltage relationships are shown (n=3; error bars are s.e.m.). b. NMR solution structures of SGTx1 and Magi5. Residue coloring is as follows: blue, basic; red, acidic; green, hydrophobic; white, histidine; pink, serine/threonine/asparagine. Backbone fold is shown on top in dark grey. Images were created using DSViewer Pro and Protein Data Bank accession IDs 1LA4 for SGTx1 and 2GX1 for Magi5.

Figure 3. Sensitivity of rNav1.2a paddle chimeras to ProTx-I and ProTx-II

Effects of 100nM ProTx-I and ProTx-II on Kv2.1 and rNav1.2 paddle chimeras are shown where paddle motifs were transferred from each voltage-sensing domain from rNav1.2a into Kv2.1. For each toxin and construct, potassium currents were elicited by depolarizations near the foot of the voltage-activation curve (top). Currents are shown before (black) and in the presence of toxin (colored). Normalized tail current voltage-activation relationships are also shown (bottom), where tail current amplitude (I/I_max) is plotted against test voltage before (black) and in the presence of toxins (colored). n=3-5; error bars are s.e.m.

Box figure

Ribbon representation of the X-ray structure of a paddle chimera between the Kv2.1 and Kv1.2 channel viewed from the external side of the membrane (top view) and from within the membrane (side view). The S3b-S4 paddle motif is colored blue, the pore domain (S5-S6) is colored yellow and possible lipid molecules are colored grey. Basic residues in S4 are shown as stick representations (Protein Data Bank accession ID is 2R9R). The side view of the chimeric channel shows the S1–S4 voltage-sensing domain and its interface with the pore domain together with the possible location of lipid molecules.