Mutations of two acidic residues at the cytoplasmic end of segment IIIS6 of an insect sodium channel have distinct effects on pyrethroid resistance

Mutations in sodium channels are known to confer knockdown resistance to pyrethroid insecticides, such as permethrin and cypermethrin, in various agricultural pests and disease vectors. Double mutations, D³ⁱ²⁸V and E³ⁱ³²G, were detected in cypermethrin-resistant Helicoverpa armigera and Heliothis virescens populations. However, the role of the two mutations in pyrethroid resistance remains unclear. In this study, we introduced the mutations into the cockroach sodium channel, BgNa_v1-1a, and examined their effects on channel gating and pyrethroid sensitivity in Xenopus oocytes. D³ⁱ²⁸V alone and the double mutation, D³ⁱ²⁸V/E³ⁱ³²G, shifted the voltage dependence of activation in the depolarizing direction by 15 mV and 20 mV, respectively, whereas E³ⁱ³²G had no significant effect. D³ⁱ²⁸V reduced the amplitude of tail currents induced by permethrin and NRDC 157 (Type I pyrethroids) and deltamethrin and cypermethrin (Type II pyrethroids), whereas E³ⁱ³²G alone had no effect. Intriguingly, the amplitude of Type II pyrethroid-induced tail current from D³ⁱ²⁸V/E³ⁱ³²G channels was similar to that of BgNa_v1-1a channels, but the decay of the tail currents was accelerated. Such effects were not observed for Type I pyrethroid-induced tail currents. Computational analysis based on the model of dual pyrethroid receptors on insect sodium channels predicted D³ⁱ²⁸V and E³ⁱ³²G exert their effects on channel gating and pyrethroid action via allosteric mechanisms. Our results not only illustrate the distinct effect of the D³ⁱ²⁸V/E³ⁱ³²G double mutations on Type I vs. Type II pyrethroids, but also reinforce the concept that accelerated decay of tail currents can be an effective mechanism of pyrethroid resistance to Type II pyrethroids.