The Receptor Site and Mechanism of Action of Sodium Channel Blocker Insecticides

Yongqiang Zhang,Yuzhe Du,Dingxin Jiang,Caitlyn Behnke,Yoshiko Nomura,Boris S Zhorov,Ke Dong

doi:10.1074/jbc.m116.742056

Abstract

Sodium channels are excellent targets of both natural and synthetic insecticides with high insect selectivity. Indoxacarb, its active metabolite DCJW, and metaflumizone (MFZ) belong to a relatively new class of sodium channel blocker insecticides (SCBIs) with a mode of action distinct from all other sodium channel-targeting insecticides, including pyrethroids. Electroneutral SCBIs preferably bind to and trap sodium channels in the inactivated state, a mechanism similar to that of cationic local anesthetics. Previous studies identified several SCBI-sensing residues that face the inner pore of sodium channels. However, the receptor site of SCBIs, their atomic mechanisms, and the cause of selective toxicity of MFZ remain elusive. Here, we have built a homology model of the open-state cockroach sodium channel BgNav1-1a. Our computations predicted that SCBIs bind in the inner pore, interact with a sodium ion at the focus of P1 helices, and extend their aromatic moiety into the III/IV domain interface (fenestration). Using model-driven mutagenesis and electrophysiology, we identified five new SCBI-sensing residues, including insect-specific residues. Our study proposes the first three-dimensional models of channel-bound SCBIs, sheds light on the molecular basis of MFZ selective toxicity, and suggests that a sodium ion located in the inner pore contributes to the receptor site for electroneutral SCBIs.

Highlights

Voltage-gated sodium channels are transmembrane proteins whose activation triggers fast inflow of sodium ions into the cell, causing the rising phase of the action potential
The selectivity filter is composed of Asp, Glu, Lys, and Ala residues from the ascending parts of the four P-loops and divides the ion-conducting pathway into the following two parts: the outer pore, which is exposed to the extracellular space, and the inner pore, which in the open channel is exposed to the cytoplasm
None of the currently available high resolution structures of P-loop channels can be directly used to build a homology model of the open sodium channel, which would be suitable for the computational search for binding sites of large sodium channel blocker insecticides (SCBIs) molecules

Summary

Introduction

Voltage-gated sodium channels are transmembrane proteins whose activation triggers fast inflow of sodium ions into the cell, causing the rising phase of the action potential. Intensive use of pyrethroids over the last decades has led to selection of numerous mutations in sodium channels in various arthropod populations, which confer resistance to pyrethroids [4]. Identification of these sodium channel mutations has facilitated the development of molecular markers for early detection of pyrethroid resistance in various populations around the world [5]. These mutations help define the receptor sites for pyrethroids on sodium channels and reveal the molecular mechanisms of the differential sensitivities of insect and mammalian sodium channels to pyrethroids [6]. Identification of more residues that are critical for the binding and action of SCBIs would provide further molecular markers for more accurate and earlier detection of resistance genotypes and might delay the development of resistance in natural populations

Methods

Results

Conclusion