Elucidation of pyrethroid and DDT receptor sites in the voltage-gated sodium channel

Abstract

DDT and pyrethroid insecticides were among the earliest neurotoxins identified to act on voltage-gated sodium channels. In the 1960s, equipped with, at the time, new voltage-clamp techniques, Professor Narahashi and associates provided the initial evidence that DDT and allethrin (the first commercial pyrethroid insecticide) caused prolonged flow of sodium currents in lobster and squid giant axons. Over the next several decades, continued efforts by Prof. Narahashi's group as well as other laboratories led to a comprehensive understanding of the mechanism of action of DDT and pyrethroids on sodium channels. Fast forward to the 1990s, genetic, pharmacological and toxicological data all further confirmed voltage-gated sodium channels as the primary targets of DDT and pyrethroid insecticides. Modifications of the gating kinetics of sodium channels by these insecticides result in repetitive firing and/or membrane depolarization in the nervous system. This mini-review focuses on studies from Prof. Narahashi's pioneer work and more recent mutational and computational modeling analyses which collectively elucidated the elusive pyrethroid receptor sites as well as the molecular basis of differential sensitivities of insect and mammalian sodium channels to pyrethroids.

Keywords: DDT; Insecticides; Pyrethroids; Sodium channels.

Fig. 1

Structural formulae of representative pyrethroids and DDT.

Fig. 2

(A) Topology of sodium channel indicating residues within/around PyR1 and PyR2. The open circles and open triangles indicate, respectively, positions of mutations within/around PyR1 and PyR2, which affect action of pyrethroids. The filled circles and filled triangles indicate, respectively, positions of mutations within/around PyR1 and PyR2 that affect action of both pyrethroids and DDT, see (Du et al., 2015; Du et al., 2016) and references therein. Labels of respective positions are shown in boxes above transmembrane helices S5, S6 or below L45 linker-helices. To describe sequential positions of residues within the channel we use a residue-labeling scheme (Du et al., 2013; Zhorov and Tikhonov, 2004) where a label includes the domain number (1–4), segment type (k, the linker-helix L45; i, the inner helix S6; and o, the outer helix S5), and relative number of the residue in the segment. This provides the same labels to residues in the matching positions of the sequence alignment of sodium channels from different organisms and highlights symmetric location of residues in different channel domains. See Table 1 for more details. (B) The aligned sequences of Kv1.2 and AaNav1-1 channels. Residues predicted to contribute to PyR1 or control ligand access to PyR1 are highlighted. Residues predicted to contribute to PyR2 or control ligand access to PyR1 are underlined. Substitutions of these residues have been tested experimentally (Du et al., 2013, 2015, 2016; O’Reilly et al., 2006; Usherwood et al., 2007). Characters “k1”, “o1”, “i1”, etc. mark relative positions of residues, respectively, in the linker-helices L45, outer helices S5 and inner helices S6.

Fig. 3

Kv1.2-based model of the open AaNav1-1 channel pore module with two DMT molecules (A,B) and two DDT molecules (C,D) docked into PyR2 and revised PyR1 sites. Helices in domains I, II, III and IV are shown by pink, yellow, green and white ribbons, respectively. Arrows point to the kink regions between helices L45 and S5 of respective receptors. Known pyrethroid-sensing residues are shown as sticks and respective helices are labeled. Carbon, oxygen, nitrogen and hydrogen atoms of the ligands are, respectively, orange, red, blue, and gray. Bromine atoms in DMT are brown and chlorine atoms in DDT are green. Note that the bulkiest moieties of the insecticides (dimethylcyclopyl group of DMT and trichloromethyl group of DDT) bind similarly between L45, S5 and S6 helices, while aromatic substituents of the bulkiest groups extend between two domains. Figures A and B are Reproduced with permission from Molecular Pharmacology (Du Y, Nomura, Y, Zhorov BS, and Dong K. Rotational symmetry of two pyrethroid receptor sites in the mosquito sodium channel. 2015; 88: 273–280). Figures C and D are originally published in the Journal of Biological Chemistry (Du et al., 2016). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

Molecular basis of different pyrethroid sensitivities among insect and mammalian sodium channels. (A) Topology of the Nav1.4 protein indicating the residues that contribute to the resistance of mammalian sodium channels to pyrethroids. (B) Sequence alignments of mammalian and insect sodium channels in the regions that are critical for the binding and action of pyrethroids.