Sequence variations at I260 and A1731 contribute to persistent currents in Drosophila sodium channels

Double sodium channel mutation, I265T/L1014F, is possibly related to pyrethroid-resistant in Thrips palmi

Thrips palmi Karny (Thysanoptera: Thripidae) can harm a variety of agricultural crops and transmit plant viruses, causing heavy economic losses. In the Hainan province of China, pyrethroids were sprayed widely to control T. palmi, which leaded to resistance to pyrethroids in T. palmi. The bioassay has shown that the resistance ratio of T. palmi to pyrethroids increases annually. Resistance ratio to λ-cyhalothrin has increased from 10.711 to 23.321 and to cypermethrin has increased from 5.507 to 23.051 for 3 years, 2020-2022. The double mutation (I265T/L1014F) was identified from the field strain for the first time, which were located in the domains I and II of the voltage-gated sodium channel of T. palmi, respectively. The double mutation is probably the reason for the higher resistance of T. palmi in Hainan. The frequencies of the double mutation were 53.33% in HN2020, 70.00% in HN2021, and 96.67% in HN2022. Results indicated that T. palmi had developed different degrees of resistance to pyrethroids in Hainan. This study provides theoretical guidance for the use of insecticides in the field control of thrips.

Charge substitutions at the voltage-sensing module of domain III enhance actions of site-3 and site-4 toxins on an insect sodium channel

Scorpion α-toxins bind at the pharmacologically-defined site-3 on the sodium channel and inhibit channel inactivation by preventing the outward movement of the voltage sensor in domain IV (IVS4), whereas scorpion β-toxins bind at site-4 on the sodium channel and enhance channel activation by trapping the voltage sensor of domain II (IIS4) in its outward position. However, limited information is available on the role of the voltage-sensing modules (VSM, comprising S1-S4) of domains I and III in toxin actions. We have previously shown that charge reversing substitutions of the innermost positively-charged residues in IIIS4 (R4E, R5E) increase the activity of an insect-selective site-4 scorpion toxin, Lqh-dprIT3-c, on BgNav1-1a, a cockroach sodium channel. Here we show that substitutions R4E and R5E in IIIS4 also increase the activity of two site-3 toxins, LqhαIT from Leiurusquinquestriatus hebraeus and insect-selective Av3 from Anemonia viridis. Furthermore, charge reversal of either of two conserved negatively-charged residues, D1K and E2K, in IIIS2 also increase the action of the site-3 and site-4 toxins. Homology modeling suggests that S2-D1 and S2-E2 interact with S4-R4 and S4-R5 in the VSM of domain III (III-VSM), respectively, in the activated state of the channel. However, charge swapping between S2-D1 and S4-R4 had no compensatory effects on gating or toxin actions, suggesting that charged residue interactions are complex. Collectively, our results highlight the involvement of III-VSM in the actions of both site 3 and site 4 toxins, suggesting that charge reversing substitutions in III-VSM allosterically facilitate IIS4 or IVS4 voltage sensor trapping by these toxins.

Distinct modulating effects of TipE-homologs 2-4 on Drosophila sodium channel splice variants

The Drosophila melanogaster TipE protein is thought to be an insect sodium channel auxiliary subunit functionally analogous to the β subunits of mammalian sodium channels. Besides TipE, four TipE-homologous proteins (TEH1-4) have been identified. It has been reported that TipE and TEH1 have both common and distinct effects on the gating properties of splice variants of the Drosophila sodium channel, DmNav. However, limited information is available on the effects of TEH2, TEH3 and TEH4 on the function of DmNav channel variants. In this study, we found that TEH2 increased the amplitude of peak current, but did not alter the gating properties of three examined DmNav splice variants expressed in Xenopus oocytes. In contrast, TEH4 had no effect on peak current, yet altered the gating properties of all three channel variants. Furthermore, TEH4 enhanced persistent current and slowed sodium current decay. The effects of TEH3 on DmNav variants are similar to those of TEH4, but the data were collected from a small portion of oocytes because co-expression of TEH3 with DmNav variants generated a large leak current in the majority of oocytes examined. In addition, TEH3 and TEH4 enhanced the expression of endogenous currents in oocytes. Taken together, our results reveal distinct roles of TEH proteins in modulating the function of sodium channels and suggest that TEH proteins might provide an important layer of regulation of membrane excitability in vivo. Our results also raise an intriguing possibility of TEH3/TEH4 as auxiliary subunits of other voltage-gated ion channels besides sodium channels.