Voltage-gated sodium channels are essential for the initiation and propagation of the action potential in neurons and other excitable cells. Because of their critical roles in electrical signaling, sodium channels are targets of a variety of naturally occurring and synthetic neurotoxins, including several classes of insecticides. This review is intended to provide an update on the molecular biology of insect sodium channels and the molecular mechanism of pyrethroid resistance. Although mammalian and insect sodium channels share fundamental topological and functional properties, most insect species carry only one sodium channel gene, compared to multiple sodium channel genes found in each mammalian species. Recent studies showed that two posttranscriptional mechanisms, alternative splicing and RNA editing, are involved in generating functional diversity of sodium channels in insects. More than 50 sodium channel mutations have been identified to be responsible for or associated with knockdown resistance (kdr) to pyrethroids in various arthropod pests and disease vectors. Elucidation of molecular mechanism of kdr led to the identification of dual receptor sites of pyrethroids on insect sodium channels. Most of the kdr mutations appear to be located within or close to the two receptor sites. The accumulating knowledge of insect sodium channels and their interactions with insecticides provides a foundation for understanding the neurophysiology of sodium channels in vivo and the development of new and safer insecticides for effective control of arthropod pests and human disease vectors.

Pyrethroid insecticides are widely used as one of the most effective control measures in the global fight against agricultural arthropod pests and mosquito-borne diseases, including malaria and dengue. They exert toxic effects by altering the function of voltage-gated sodium channels, which are essential for proper electrical signaling in the nervous system. A major threat to the sustained use of pyrethroids for vector control is the emergence of mosquito resistance to pyrethroids worldwide. Here, we report the successful expression of a sodium channel, AaNa v 1-1, from Aedes aegypti in Xenopus oocytes, and the functional examination of nine sodium channel mutations that are associated with pyrethroid resistance in various Ae. aegypti and Anopheles gambiae populations around the world. Our analysis shows that five of the nine mutations reduce AaNa v 1-1 sensitivity to pyrethroids. Computer modeling and further mutational analysis revealed a surprising finding: Although two of the five confirmed mutations map to a previously proposed pyrethroid-receptor site in the house fly sodium channel, the other three mutations are mapped to a second receptor site. Discovery of this second putative receptor site provides a dual-receptor paradigm that could explain much of the molecular mechanisms of pyrethroid action and resistance as well as the high selectivity of pyrethroids on insect vs. mammalian sodium channels. Results from this study could impact future prediction and monitoring of pyrethroid resistance in mosquitoes and other arthropod pests and disease vectors.

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A large body of experimental data on Na+ channels is available, but the interpretation of these data in structural terms is difficult in the absence of a high-resolution structure. Essentially different electrophysiological and pharmacological properties of Na+ and K+ channels and poor identity of their sequences obstruct homology modeling of Na+ channels. In this work, we built the P-loops model of the Na+ channel, in which the pore helices are arranged exactly as in the MthK bacterial K+ channel. The conformation of the selectivity-filter region, which includes residues in positions -2 through +4 from the DEKA locus, was shaped around rigid molecules of saxitoxin and tetrodotoxin that are known to form multiple contacts with this region. Intensive Monte Carlo minimization that started from the MthK-like conformation produced practically identical saxitoxin- and tetrodotoxin-based models. The latter was tested to explain a wide range of experimental data that were not used at the model building stage. The docking of tetrodotoxin analogs unambiguously predicted their optimal orientation and the interaction energy that correlates with the experimental activity. The docking of mu-conotoxin produced a binding model consistent with experimentally known toxin-channel contacts. Monte Carlo-minimized energy profiles of tetramethylammonium pulled through the selectivity-filter region explain the paradoxical experimental data that this organic cation permeates via the DEAA but not the AAAA mutant of the DEKA locus. The model is also consistent with earlier proposed concepts on the Na+ channel selectivity as well as Ca2+ selectivity of the EEEE mutant of the DEKA locus. Thus, the model integrates available experimental data on the Na+ channel P-loops domain, and suggests that it is more similar to K+ channels than was believed before.

Pyrethroid insecticides are widely used to control insect pests and human disease vectors. Voltage-gated sodium channels are the primary targets of pyrethroid insecticides. Mutations in the sodium channel have been shown to be responsible for pyrethroid resistance, known as knockdown resistance (kdr), in various insects including mosquitoes. In Aedes aegypti mosquitoes, the principal urban vectors of dengue, zika, and yellow fever viruses, multiple single nucleotide polymorphisms in the sodium channel gene have been found in pyrethroid-resistant populations and some of them have been functionally confirmed to be responsible for kdr in an in vitro expression system, Xenopus oocytes. This mini-review aims to provide an update on the identification and functional characterization of pyrethroid resistance-associated sodium channel mutations from Aedes aegypti. The collection of kdr mutations not only helped us develop molecular markers for resistance monitoring, but also provided valuable information for computational molecular modeling of pyrethroid receptor sites on the sodium channel.

The X-ray structure of the bacterial sodium channel NavAb provides a new template for the study of sodium and calcium channels. Unlike potassium channels, NavAb contains P2 helices in the outer-pore region. Because the sequence similarity between eukaryotic and prokaryotic sodium channels in this region is poor, the structural similarity is unclear. We analyzed it by using experimental data on tetrodotoxin block of sodium channels. Key tetrodotoxin-binding residues are outer carboxylates in repeats I, II, and IV, three positions downstream from the selectivity-filter residues. In a NavAb-based model of Nav1 channels derived from the sequence alignment without insertions/deletions, the outer carboxylates did not face the pore and therefore did not interact with tetrodotoxin. The hypothesis that the evolutionary appearance of Nav1 channels involved point deletions in an ancestral channel between the selectivity filter and the outer carboxylates allowed building of a NavAbbased model with tetrodotoxin-channel contacts similar to those proposed previously. This hypothesis also allowed us to reproduce in Nav1 the folding-stabilizing contacts between long-side chain residues in P1 and P2, which are seen in NavAb. The NavAb-based inner-pore model of Nav1 preserved major features of our previous KcsA-based models, including the access pathway for ligands through the repeat III/IV interface and their interactions with specific residues. Thus, structural properties of eukaryotic voltage-gated sodium channels that are suggested by functional data were reproduced in the NavAbbased models built by using the unaltered template structure but with adjusted sequence alignment. Sequences of eukaryotic calcium channels aligned with NavAb without insertions/ deletions, which suggests that NavAb is a promising basis for the modeling of calcium channels.

In the last decade, the idea of common organization of certain ion channel families exhibiting diverse physiological and pharmacological properties has received strong experimental support. Transmembrane topologies and patterns of the porefacing residues are conserved in P-loop channels that include high-selective cation channels and certain ligand-gated channels. X-ray structures of bacterial K + channels, KcsA, Ion channels are transmembrane proteins that mediate passive transport of ions. The channels greatly diverge in their structural, physiological and pharmacological properties. According to the preferable permeant ion, they are generally categorized as K + , Na + , Ca 2+ and Cl -channels. Regarding the mechanism of activation, the channels are divided into superfamilies of voltage-gated and ligand-gated channels (Hille 2001). In the 90s, it has been established that the pore region of the voltage-gated Na + , K + and Ca 2+ channels includes a common structural motif, namely, two transmembrane helices separated by a membrane re-entrant loop (P-loop) incorporating the selectivity filter (Fig. 1). Surprisingly, glutamate-gated cation selective channels also share certain characteristics with P-loop channels. The fact that many channels of essentially different functional properties have a common fold concords with the general concept that the number of proteins with specific structural and functional properties is much larger than the number of folding patterns of these proteins. Appreciation of structural similarity between functionally distinct proteins is important. The crystallographic study of a bacterial K + channel from Streptomyces lividans, KcsA (Doyle et al. 1998), revolutionized our understanding of P-loop channels. The KcsA channel was crystallized in the closed state. In view of this structure, numerous experimental data on other P-loop channels were explained, including structure-activity relationship, chemical cross-linking, site-directed mutagenesis and ligand-receptor interactions. The next fundamental step was the X-ray study of a bacterial K + channel from Methanobacterium thermoautotrophicum ( Abbreviations used: AMPA, a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid; AMPAR, AMPA-binding type of glutamate receptor;

Voltage-gated sodium channels are critical for the generation and propagation of action potentials. They are the primary target of several classes of insecticides, including DDT, pyrethroids and sodium channel blocker insecticides (SCBIs). DDT and pyrethroids preferably bind to open sodium channels and stabilize the open state, causing prolonged currents. In contrast, SCBIs block sodium channels by binding to the inactivated state. Many sodium channel mutations are associated with knockdown resistance (kdr) to DDT and pyrethroids in diverse arthropod pests. Functional characterization of kdr mutations together with computational modelling predicts dual pyrethroid receptor sites on sodium channels. In contrast, the molecular determinants of the SCBI receptor site remain largely unknown. In this review, we summarize current knowledge about the molecular mechanisms of action of pyrethroids and SCBIs, and highlight the differences in the molecular interaction of these insecticides with insect versus mammalian sodium channels.