Analyses of aphid sodium channels and prediction of pesticide interactions

For the foreseeable future the worldwide production of food crops is likely to be dependant on the use of agrochemicals to control pests and diseases. It is estimated that 25-75% (varies between crops) of current agricultural output would be lost without pest control and despite efforts to develop non-chemical methods the control of insect pests is still largely achieved by the use of insecticides. However, the use of many groups of insecticides is severely threatened by the ability of insects to evolve resistance mechanisms that render the chemicals ineffective. Such resistance poses a serious threat to agriculture both in the UK and worldwide. Resistance may result from either an increase in the ability of the insect to detoxify the insecticide or by changes in the proteins with which the insecticide interacts. Two types of insecticide, DDT and the synthetic pyrethroids (the latter currently account for around 17% of the world insecticide market), act on sodium channel proteins found in insect nerve cell membranes. The correct functioning of these channels is essential for normal transmission of nerve impulses and this process is rapidly disrupted by binding of the insecticides, leading to paralysis and eventual death. Some insect pest populations have evolved modifications of the sodium channel protein which inhibit the binding of the insecticide and result in the insect developing resistance. Previous work done in our laboratories (at Rothamsted Research and elsewhere) has led to the identification of specific sites on the sodium channel that are changed in resistant insects providing clues on the location of the DDT and pyrethroid binding sites. With the advent of the full genome sequence for an aphid, it will be possible to study the sodium channel in a group of important agricultural pests, where resistance is widespread. Thus the key aim of the current project is to define insecticide binding sites on the aphid channel using computer-generated models. It is anticipated that this will eventually enable the design of novel molecules with high insecticidal activity on hitherto resistant insects.

DDT and Pyrethroid insecticides, often referred to as 'knockdown insecticides' because they paralyse flying insects, show high potency and selectivity for insect sodium channels, although the specific site (or sites) of action of these compounds on the channel protein is not well characterised. Sodium channels in insects with knock down resistance (kdr) and super-kdr have lower sensitivities to DDT and pyrethroids resulting from point mutations within the gene encoding the sodium channel. These mutations are highly conserved across a range of resistant pest species and the identification of these (and other related mutations) together with knowledge of the structure-activity of DDT and different pyrethroids are now providing clues on the residues in the channel protein that might be involved in the binding of these important insecticides. We have generated a preliminary homology model for the binding sites of DDT and pyrethroids to the sodium channel protein. The current project will exploit the newly available aphid genome sequences and use further computer modelling techniques to study the interactions between these insecticides and other ligands with the channel of an important agricultural pest. The main goal is to produce a detailed and accurate picture of the binding sites of DDT and pyrethroids and other ligands on the channel that in the longer term can be used to design new insecticides especially those potent on resistant channels.