A painful tale about synthetic scorpion toxins

Within the voltage-gated Na⁺ (Na_v) channel gene family, the Na_v1.7 isoform (SCN9A) has been receiving a great deal of scientific and clinical attention after investigators uncovered its strategic role in various pain syndromes.¹ As a result, Na_v1.7 became somewhat of a Holy Grail for researchers in academia as well as the pharmaceutical industry who are interested in discovering novel, target-specific non-narcotic pain therapeutics. However, clinically-used Na_v channel drugs are prone to dose-limiting side effects because they typically target the conserved pore region and therefore do not discriminate between isoforms.² In contrast, Na_v channel voltage-sensing domains (VSDs) differ substantially between isoforms and regulate pore opening and closing (i.e. gating).³ As such, it should be possible to design effective drugs that target the gating process of a particular Na_v channel isoform without physically blocking the pore, a fascinating concept that has recently led to the discovery of Na_v1.7-specific small-molecule compounds.^4,5 To provide tools for catching prey or as a defense against predators, nature has crafted animal venoms to contain peptide toxins that selectively and subtly modulate Na_v channel gating by influencing VSD activity.³ These toxins have helped scientists to examine Na_v channel function in great detail and generate proof-of-principle data with respect to the role of particular isoforms in cellular excitability.³ However, few toxins have been described that specifically modulate Na_v1.7 function.

In issue 10(2) of Channels, Motin et al.⁶ report a detailed analysis of the effects of OD1, a toxin from the Odonthobuthus doriae scorpion, on Na_v1.7 gating. OD1 was previously classified as an α-scorpion toxin that inhibits fast inactivation by influencing VSD movement in domain IV (VSDIV) of Na_v channels.^3,7 Yet, detailed structure-function studies only became feasible after large-scale synthesis challenges were addressed.⁸ With access to synthetic OD1, Durek et al.⁸ were able to functionally screen OD1 analogs using a high-throughput fluorescence assay and identified two variants that selectively target Na_v1.7 with an increased potency of more than one order of magnitude.^6,8 Docking OD1 onto a model of human Na_v1.7 based on the bacterial Na_v channel Na_vAb (PDBID: 4EKW) reveals the intimate interaction of the mutated toxin region with the S3b-S4 loop in VSDIV (see Fig. 1), a structural motif known to interact with ligands that influence fast inactivation.³ After identifying potent analogs that slow the decay phase of macroscopic Na_v1.7 ionic currents, Motin et al. then proceeded to examine the working mechanism of OD1 on a single–channel level and summarize their results in a Markov model. Overall, they found that Na⁺current fast inactivation in the presence of toxin slows due to an increase in flickering behavior of single channels, where the channels close and open more readily, rather than to increase the channel mean open time. It remains to be determined if other minor, but significant effects on Na_v1.7 gating⁶ originate from the ability of OD1 to stabilize VSDIV in a particular state or influence VSD movements in other domains.

Figure 1.

Left panel, Interaction between OD1 (grey ribbons) and the S1–S4 voltage-sensing domain IV (VSDIV) of human Na_v1.7 (green ribbons) predicted using automated docking (Autodock Vina). The Na_v1.7 VSD structure was constructed by homology modelling using Na_vAB (PDBID: 4EKW) as the structural template. Key toxin residues are illustrated using red sticks. Right panel, Critical OD1 residues ⁹Asp, ¹⁰Asp and ¹¹Lys are shown in red/blue sticks. Corresponding putative interactions with residues within VSDIV of Na_v1.7 (E1545 and K1536) are indicated using spheres and arrows. Figure courtesy of DJ Adams and A Hung.

In a broader context, this study provides compelling evidence for the potential development of selective and potent ion channel modulators from naturally occurring animal toxins provided that synthesis hurdles can be overcome. These improved molecules could then find use as pharmacological tools to further examine the role of Na_v1.7 in normal and pathological tissues. Moreover, continuously improving synthetic methods such as peptide cyclization and minimization may overcome the obstacles that can occur between the early drug discovery stage and the clinical application of these toxins as analgesics.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.