Historical Perspective of the Characterization of Conotoxins Targeting Voltage-Gated Sodium Channels

Abstract

Marine toxins have potent actions on diverse sodium ion channels regulated by transmembrane voltage (voltage-gated ion channels) or by neurotransmitters (nicotinic acetylcholine receptor channels). Studies of these toxins have focused on varied aspects of venom peptides ranging from evolutionary relationships of predator and prey, biological actions on excitable tissues, potential application as pharmacological intervention in disease therapy, and as part of multiple experimental approaches towards an understanding of the atomistic characterization of ion channel structure. This review examines the historical perspective of the study of conotoxin peptides active on sodium channels gated by transmembrane voltage, which has led to recent advances in ion channel research made possible with the exploitation of the diversity of these marine toxins.

Keywords: conotoxin; gating modifier; pore blocker; sodium channel; voltage-gated.

Figure 1

(A) Diagram of the sequence of the voltage-gated sodium channel, with voltage sensor S4 segments shaded in domains I–III (orange) and in domain IV (red) to denote the role of DIV S4 (domain IV S4 segment) in fast inactivation, with the inactivation particle identified as the IFM motif in the DIII–DIV linker. Receptor sites for marine toxins and extending to that for veratridine/batrachotoxin (site 2) are indicated, excluding insecticide and other receptor binding sites. Cryo-EM structures of toxins bound to sodium channels are shown below. (B) TTX bound within receptor site 1 in the pore domain of hNa_V1.4, with the toxin shaded (arrow). Binding site loci include acidic residues glutamate (orange), aspartate (blue), and aromatic residues (yellow): .pdb 6A95 [82]. (C) Spider toxin HwTX-IV bound to receptor site 4 in the extracellular S3–S4 linker of the voltage sensor domain (VSD) of domain II in hNa_V1.7, with DII pore domain (PD) included in this view: .pdb 7K48 [83]. (D) α-scorpoin toxin AaH2 bound to receptor site 3 in the extracellular S3–S4 linker of the voltage sensor domain of domain IV of hNa_V1.7: .pdb 6NT4 [84]. S4 positively charged residues are shown in green for (A,C,D) and illustrate the activated position of DIIS4 with HwTX-IV bound (C) and resting position of DIVS4 with AaH2 bound (D).

Figure 2

Example structures of functionally characterized conotoxins (µ-conotoxin GIIIA: .pdb 1TCK [92]; µO-conotoxin MfVIA: .pdb 2N7F [93]; δ-conotoxin EVIA: .pdb 1G1P [94]; ι-conotoxin RXIA: .pdb 2JRY [95]).

Figure 3

Voltage-dependent state transitions in sodium channels. Channels respond to weak depolarization by inactivating, or to strong depolarization by opening prior to inactivation. At right is shown the action of the site-1 toxin tetrodotoxin (TTX) to block sodium permeance in hNa_V1.4 channels (dotted line, blue) and is compared to the action of the site-3 toxin Anthopleurin-A (Ap-A) to slow the entry of these channels into fast inactivation (dotted line, red). Calibration; vertical 1 μA, horizontal 10 ms. Groome, unpublished.

Figure 4

Sequence alignment (A) of the pore helices for sodium channels hNa_V1.4 (B): .pdb 6AGF [82]) and rNa_V1.2 (C): .pdb 6J8E [152]). The selectivity filter (SF) is shown in red for the alignment, with acidic residues of the outer pore vestibule (OV) shown in sky blue. Residues within the pore helical structures in (B,C) are listed as acidic aspartate and glutamate, positive lysine, and neutral alanine. Sequence alignments were performed in Seaview 5 [153].

Figure 5

(A) Structure of KIIIA (.pdb 7SAV; [179]) (B) Cryo-EM structure of Na_V1.2 bound by KIIIA (.pdb 6J8E; [152]). (C) Enlargement of the P-loops of Na_V1.2 (tan helices) with KIIIA toxin residue K7 interacting with outer pore residues E945 and D1426, and R10 (shown in two possible orientations) interacting with D1426. Toxin residues W8, H12, and R14 are shown for which additional interactions within the P-loops, or with extracellular residues, were noted. DEKA selectivity filter residues are shown in red.