BackgroundMyzus persicae is a globally important aphid pest with a history of developing resistance to insecticides. Unusually, neonicotinoids have remained highly effective as control agents despite nearly two decades of steadily increasing use. In this study, a clone of M. persicae collected from southern France was found, for the first time, to exhibit sufficiently strong resistance to result in loss of the field effectiveness of neonicotinoids.ResultsBioassays, metabolism and gene expression studies implied the presence of two resistance mechanisms in the resistant clone, one based on enhanced detoxification by cytochrome P450 monooxygenases, and another unaffected by a synergist that inhibits detoxifying enzymes. Binding of radiolabeled imidacloprid (a neonicotinoid) to whole body membrane preparations showed that the high affinity [3H]-imidacloprid binding site present in susceptible M. persicae is lost in the resistant clone and the remaining lower affinity site is altered compared to susceptible clones. This confers a significant overall reduction in binding affinity to the neonicotinoid target: the nicotinic acetylcholine receptor (nAChR). Comparison of the nucleotide sequence of six nAChR subunit (Mpα1-5 and Mpβ1) genes from resistant and susceptible aphid clones revealed a single point mutation in the loop D region of the nAChR β1 subunit of the resistant clone, causing an arginine to threonine substitution (R81T).ConclusionPrevious studies have shown that the amino acid at this position within loop D is a key determinant of neonicotinoid binding to nAChRs and this amino acid change confers a vertebrate-like character to the insect nAChR receptor and results in reduced sensitivity to neonicotinoids. The discovery of the mutation at this position and its association with the reduced affinity of the nAChR for imidacloprid is the first example of field-evolved target-site resistance to neonicotinoid insecticides and also provides further validation of exisiting models of neonicotinoid binding and selectivity for insect nAChRs.

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Ca2+ in¯ux through NMDA receptors can initiate molecular changes in neurones which may underlie synaptic plasticity, neuronal development, survival and excitotoxicity. Signalling through the MAP kinase (Erk1/2) cascade may be central to these processes. We previously demonstrated that Ca 2+-permeable AMPA receptors activate Erk1/2 through a phosphatidylinositol 3-kinase (PI 3-kinase)-dependent mechanism. We now report that NMDA receptor activation of Erk1/2 was also blocked by inhibitors of PI 3-kinase (LY 294002, wortmannin). In addition, pre-treatment of neurones with pertussis toxin inhibited NMDA-induced Erk1/2 activation, indicating a role for heterotrimeric G i/o proteins. PI 3-kinase directs activation of the serine-threonine kinase Akt (PKB). Treatment of striatal neurones with glutamate induced a rapid Ca 2+ -dependent and PI 3-kinase-dependent phosphorylation of Akt (Ser473), which was not blocked by the Mek inhibitors PD98059 or U0126. Targets for Erk1/2 and Akt pathways include transcription factors. Glutamate-induced phosphorylation of cAMP response element binding protein (CREB; Ser133) was partially blocked with either PD98059, U0126, LY294002 or wortmannin but was very strongly inhibited on co-application of LY294002 and PD98059. We propose that NMDA receptor stimulation can activate Erk1/2 and Akt signalling pathways in a PI 3-kinase dependent manner which may target CREB in the nucleus.

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Primary cortical neurones exposed to an oxidative insult in the form of hydrogen peroxide (H 2 O 2 ) for 30 min showed a concentration-dependent increase in oxidative stress followed by a delayed NMDA receptor-dependent cell death measured 24 h later. Extracellular signal-regulated protein kinase (ERK1/2), c-jun N-terminal kinase (JNK) and the kinase Akt/PKB may regulate neuronal viability in response to oxidative insults. Using phospho-speci®c antibodies, a 15-min stimulation of neurones with H 2 O 2 (100 lM)1 mM) produced a concentration-dependent phosphorylation of ERK1/2 and Akt/PKB that was partly dependent on extracellular Ca 2+ and phosphatidylinositol 3-kinase (PI3-K). Higher concentrations of H 2 O 2 (1 mM) also stimulated a phosphorylation of JNK which was totally dependent on extracellular Ca 2+ but not PI3-K.H 2 O 2 -induced phosphorylation of ERK1/2, Akt/PKB or JNK were unaffected by the NMDA channel blocker MK801. Blocking ERK1/2 activation with the upstream inhibitor U0126 (10 lM) enhanced H 2 O 2 -induced (100±300 lM range) neurotoxicity and inhibited H 2 O 2 -mediated phosphorylation of the cyclic AMP regulatory binding protein (CREB), suggesting that ERK1/2 signals to survival under these conditions. At higher concentrations (mM), H 2 O 2 -stimulated a phosphorylation of c-jun. It is likely, therefore, that subjecting neurones to moderate oxidative-stress recruits pro-survival signals to CREB but during severe oxidative stress pro-death signals through JNK and c-jun are dominant.

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The nicotinic acetylcholine receptor pharmacological profile of sulfoxaflor in aphids is consistent with that of imidacloprid. Additionally, the insecticidal activity of sulfoxaflor and the current commercialised neonicotinoids is affected by the point mutation in FRC Myzus persicae. Therefore, it is suggested that sulfoxalfor be considered a neonicotinoid, and that this be taken into account when recommending insecticide rotation partnering for effective resistance management programmes.

The mechanisms governing the specific membrane targeting of adenylyl cyclase isoforms remain unknown. To address this issue, a series of chimeras were produced between the raft-targeted AC5 and the non-raft-targeted AC7, involving switching of their major domains. The AC5-AC7 chimeras were expressed in HEK 293 cells and lipid rafts were isolated from the bulk plasma membrane by either detergent-based or non-detergent-based fractionation methods. Additionally, confocal imaging was used to investigate the precise cellular targeting of the chimeras. Surprisingly, the two tandem six-transmembrane domains of AC5 were not required for localization to lipid rafts. Rather, AC5 localization depended on the complete cytoplasmic loops (C1 and C2); constructs with mixed domains were either retained in the endoplasmic reticulum or degraded. Similar conclusions are drawn for the lipid raft localization of the Ca 2؉ /calmodulinstimulable AC8; again, the C1 and C2 domains are critical. Thus, protein-protein interactions may be more important than protein-lipid interactions in targeting these calcium-sensitive enzymes to lipid rafts.