Metabolism of methoxychlor by the P450-monooxygenase CYP6G1 involved in insecticide resistance of Drosophila melanogaster after expression in cell cultures of Nicotiana tabacum

Up-regulation of CYP6G4 mediated by a CncC/maf binding-site-containing insertion confers resistance to multiple classes of insecticides in the house fly Musca domestica

Populations of many insect species have evolved a variety of resistance mechanisms in response to insecticide selection. Current knowledge about mutations responsible for insecticide resistance is largely achieved from studies on target-site resistance, while much less is known about metabolic resistance. Although it is well known that P450 monooxygenases are one of the major players involved in insecticide metabolism and resistance, understanding mutation(s) responsible for CYP-mediated resistance has been a big challenge. In this study, we used the house fly to pursue a better understanding of P450 mediated insecticide resistance at the molecular level. Metabolism studies illustrated that CYP6G4 had a broad-spectrum metabolic activity in metabolizing insecticides. Population genotyping revealed that the CYP6G4v1 allele harboring a DNA insertion (MdIS1) had been selected in many house fly populations on different continents. Dual luciferase reporter assays identified that the MdIS1 contained a CncC/Maf binding site, and electrophoretic mobility shift assay confirmed that transcription factor CncC was involved in the MdIS1-mediated regulation. This study highlights the common involvement of the CncC pathway in adaptive evolution, and provides an interesting case supportive of parallel evolution in P450-mediated insecticide resistance in insects.

Cytochrome P450s Are Essential for Insecticide Tolerance in the Endoparasitoid Wasp Meteorus pulchricornis (Hymenoptera: Braconidae)

With the widespread application of insecticides, parasitoid wasps may also be under risk when exposed to insecticides directly at their free-living stages. The endoparasitoid wasp Meteorus pulchricornis is the predominant natural enemy of many lepidopteran pests, such as Spodoptera litura and Helicoverpa armigera. The cytochrome P450 monooxygenases constitute a ubiquitous and complex superfamily of hydrophobic, haem-containing enzymes. P450s are involved in the detoxification of many xenobiotics. However, their exact roles in the tolerance mechanism in parasitoids toward insecticides has received less attention. Here, 28 P450 genes in M. pulchricornis were identified from a previously constructed transcriptome dataset. These P450 genes belonged to CYP2, -3, and -4, and mitochondrial clans. Subsequently, eight candidate MpulCYPs were selected from four CYP clans to validate their expression patterns under phoxim, cypermethrin, and chlorfenapyr exposure by qRT-PCR. The results showed that all three insecticides had significant effects on the expression of MpulCYPs. To further study the function of P450s, CYP369B3 was silenced, and its expression levels of CYP369B3 were significantly decreased. Survival analysis indicated that after dsRNA injection, the mortality rate of wasps was significantly increased when M. pulchricornis females were exposed to insecticides compared to control groups. Our findings provide a theoretical base for elucidating the mechanism of insecticide tolerance and promote functional research on P450 genes in parasitoid wasps.

The Foraging Gene, a New Environmental Adaptation Player Involved in Xenobiotic Detoxification

Foraging is vital for animals, especially for food. In Drosophila melanogaster, this behavior is controlled by the foraging gene (for) which encodes a cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG). In wild populations of Drosophila, rover individuals that exhibit long foraging trails and sitter individuals that exhibit short ones coexist and are characterized by high and low levels of PKG activity, respectively. We, therefore, postulated that rover flies are more exposed to environmental stresses, including xenobiotics contamination, than sitter flies. We then tested whether these flies differed in their ability to cope with xenobiotics by exposing them to insecticides from different chemical families. We performed toxicological tests and measured the activity and expression levels of different classes of detoxification enzymes. We have shown that a link exists between the for gene and certain cytochrome P450-dependent activities and that the expression of the insecticide-metabolizing cytochrome P450 Cyp6a2 is controlled by the for gene. An unsuspected regulatory pathway of P450s expression involving the for gene in Drosophila is revealed and we demonstrate its involvement in adaptation to chemicals in the environment. This work can serve as a basis for reconsidering adaptation to xenobiotics in light of the behavior of species, including humans.

Genome mining and functional analysis of cytochrome P450 genes involved in insecticide resistance in Leucinodes orbonalis (Lepidoptera: Crambidae)

Genome-wide analysis of cytochrome P450 monooxygenase (CYP) genes from the advanced genome project of the Leucinodes orbonalis and the expression analysis provided significant information about the metabolism-mediated insecticide resistance. A total of 72 putative CYP genes were identified from the genome and transcriptome of L. orbonalis. The genes were classified under 30 families and 46 subfamilies based on the standard nomenclature. In the present study, a novel CYP gene, CYP324F1, was identified and it has not been reported from any other living system so far. Biochemical assays showed enhanced titers (5.81-18.5-fold) of O-demethylase of CYP in five field-collected populations. We selected 34 homologous CYP gene sequences, seemed to be involved in insecticide resistance for primer design and quantitative real-time PCR studies. Among the many overexpressed genes (>10 fold), the expression levels of CYP324F1 and CYP306A1 were prominent across all the field populations as compared with the susceptible iso-female line. Oral delivery of ds-CYP324F1 and ds-CYP306A1 directed against CYP324F1 and CYP306A1 to the larvae of one of the insecticide resistance populations caused reduced expression of these two transcripts in a dose-dependent manner (53.4%-85.0%). It appears that the increased titer of O-demethylase is the result of increased transcription level of CYP genes in resistant populations. The data provide insight for identifying the novel resistance management strategies against L. orbonalis.

Strategies for the construction of insect P450 fusion enzymes

Cytochrome P450 monooxygenases (P450s) are ubiquitous enzymes with a broad substrate spectrum. Insect P450s are known to catalyze reactions such as the detoxification of insecticides and the synthesis of hydrocarbons, which makes them useful for many industrial processes. Unfortunately, it is difficult to utilize P450s effectively because they must be paired with cytochrome P450 reductases (CPRs) to facilitate electron transfer from reduced nicotinamide adenine dinucleotide phosphate (NADPH). Furthermore, eukaryotic P450s and CPRs are membrane-anchored proteins, which means they are insoluble and therefore difficult to purify when expressed in their native state. Both challenges can be addressed by creating fusion proteins that combine the P450 and CPR functions while eliminating membrane anchors, allowing the production and purification of soluble multifunctional polypeptides suitable for industrial applications. Here we discuss several strategies for the construction of fusion enzymes combining insect P450 with CPRs.

Recombinant expression and characterization of Lucilia cuprina CYP6G3: Activity and binding properties toward multiple pesticides

The Australian sheep blowfly, Lucilia cuprina, is a primary cause of sheep flystrike and a major agricultural pest. Cytochrome P450 enzymes have been implicated in the resistance of L. cuprina to several classes of insecticides. In particular, CYP6G3 is a L. cuprina homologue of Drosophila melanogaster CYP6G1, a P450 known to confer multi-pesticide resistance. To investigate the basis of resistance, a bicistronic Escherichia coli expression system was developed to co-express active L. cuprina CYP6G3 and house fly (Musca domestica) P450 reductase. Recombinant CYP6G3 showed activity towards the high-throughput screening substrates, 7-ethoxycoumarin and p-nitroanisole, but not towards p-nitrophenol, coumarin, 7-benzyloxyresorufin, or seven different luciferin derivatives (P450-Glo™ substrates). The addition of house fly cytochrome b₅ enhanced the k_cat for p-nitroanisole dealkylation approximately two fold (17.8 ± 0.5 vs 9.6 ± 0.2 min^-1) with little effect on K_M (13 ± 1 vs 10 ± 1 μM). Inhibition studies and difference spectroscopy revealed that the organochlorine compounds, DDT and endosulfan, and the organophosphate pesticides, malathion and chlorfenvinphos, bind to the active site of CYP6G3. All four pesticides showed type I binding spectra with spectral dissociation constants in the micromolar range suggesting that they may be substrates of CYP6G3. While no significant inhibition was seen with the organophosphate, diazinon, or the neonicotinoid, imidacloprid, diazinon showed weak binding in spectral assays, with a Kd value of 23 ± 3 μM CYP6G3 metabolised diazinon to the diazoxon and hydroxydiazinon metabolites and imidacloprid to the 5-hydroxy and olefin metabolites, consistent with a proposed role of CYP6G enzymes in metabolism of phosphorothioate and neonicotinoid insecticides in other species.

Collaborative contribution of six cytochrome P450 monooxygenase genes to fenpropathrin resistance in Tetranychus cinnabarinus (Boisduval)

Cytochrome P450 monooxygenases (P450s), as an important family of detoxification enzymes, participate in the metabolism of agrochemicals in almost all agricultural pests and play important roles in the development of insecticide resistance. Two P450 genes (CYP389B1 and CYP392A26) were identified and their expression patterns were investigated in our previous study. In this study, four more P450 gene sequences (CYP391A1, CYP384A1, CYP392D11 and CYP392A28) from the Clan 2, Clan 3 and Clan 4 families were identified and characterized. Quantitative PCR analysis showed that these four P450 genes were highly expressed in a fenpropathrin-resistant (FeR) strain of Tetranychus cinnabarinus. In addition, their expressions were much more sensitive to fenpropathrin induction in the FeR strain than the susceptible strain. Gene-silencing experiments via double-stranded RNA feeding were carried out. The results showed that mRNA levels of these six P450 genes were reduced in the FeR strain and the activities of P450s were decreased. Consequently mite susceptibilities to fenpropathrin were increased. Interestingly, silencing all six P450 genes simultaneously had an even greater effect on resistance than silencing them individually. This study increases our understanding of the molecular mechanisms of insecticide detoxification, suggesting that the overexpression of these six P450 genes might play important roles in fenpropathrin resistance in T. cinnabarinus collaboratively.

Cytochrome P450 CYP4DE1 and CYP6CW3v2 contribute to ethiprole resistance in Laodelphax striatellus (Fallén)

Laodelphax striatellus Fallén (Hemiptera: Delphacidae), a destructive pest of rice, has developed high resistance to multiple insecticides, threatening the success of pest management programmes. The present study investigated ethiprole resistance mechanisms in a field population that is highly resistant to ethiprole. That population was used to establish a laboratory population that was subjected to further selection to produce a resistant strain. Target genes were cloned and compared between the resistant and the susceptible strains, the role of detoxification enzymes was examined, and the relative expression levels of 71 detoxification enzyme genes were tested using quantitative real time (RT)-PCR. The laboratory selection enhanced the resistance from 107-fold to 180-fold. The Rdl-type target site mutation seldom occurred in the resistant strain and is unlikely to represent the major mechanism underlying the observed resistance. Of the three important detoxification enzymes, only P450 monooxygenase was found to be associated with ethiprole resistance. Moreover, two genes, CYP4DE1 and CYP6CW3v2, were found to be overexpressed in the resistant strain. Furthermore, gene-silencing via a double-stranded RNA feeding test was carried out, and the results showed that the mRNA levels of CYP4DE1 and CYP6CW3v2 were reduced in the resistant strain, whereas ethiprole susceptibility was increased. These results suggest that CYP4DE1 and CYP6CW3v2 play an important role in ethiprole resistance in L. striatellus.

Identification of cytochrome P450 monooxygenase genes and their expression profiles in cyhalothrin-treated Colorado potato beetle, Leptinotarsa decemlineata

Based on a Leptinotarsa decemlineata transcriptome dataset and the GenBank sequences, a total of 74 cytochrome P450 monooxygenase genes (Cyps) were identified. These genes fell into CYP2 clan, mitochondrial clan, CYP3 clan and CYP4 clan, and were classified into 19 families and 35 subfamilies according to standard nomenclature. Two new families were discovered in CYP4 clan, and were named CYP412 and CYP413 respectively. Four new families that were recently discovered in Tribolium castaneum, including mitochondrial family CYP353, CYP3 clan families CYP345 and CYP347, and CYP4 clan family CYP350, were also found in L. decemlineata. The phylogenetic trees of CYPs from L. decemlineata and other representative insect species were constructed, and these trees provided evolutionary insight for the genetic distance. Our results facilitate further researches to understand the functions and evolution of L. decemlineata Cyp genes. In order to find cyhalothrin-inducible Cyp genes, the expression levels of Cyps belonging to CYP12, CYP6, CYP9 and CYP4 families were determined by quantitative reverse transcriptase-PCR in cyhalothrin-treated and control fourth-instar larvae. Nine Cyp genes, i.e., Cyp12H2, Cyp6BH2, Cyp6BJ1, Cyp6BQ17, Cyp6EG1, Cyp6EH1, Cyp6EJ1 Cyp4BN13v1 and Cyp4BN15, were highly expressed in cyhalothrin-treated larvae. These CYPs are the candidates that are involved in cyhalothrin detoxification.

Transcriptomic and proteomic responses of sweetpotato whitefly, Bemisia tabaci, to thiamethoxam

BACKGROUND

The sweetpotato whitefly, Bemisia tabaci (Hemiptera: Aleyrodidae), is one of the most widely distributed agricultural pests. Although it has developed resistance to many registered insecticides including the neonicotinoid insecticide thiamethoxam, the mechanisms that regulate the resistance are poorly understood. To understand the molecular basis of thiamethoxam resistance, "omics" analyses were carried out to examine differences between resistant and susceptible B. tabaci at both transcriptional and translational levels.

RESULTS

A total of 1,338 mRNAs and 52 proteins were differentially expressed between resistant and susceptible B. tabaci. Among them, 11 transcripts had concurrent transcription and translation profiles. KEGG analysis mapped 318 and 35 differentially expressed genes and proteins, respectively, to 160 and 59 pathways (p<0.05). Thiamethoxam treatment activated metabolic pathways (e.g., drug metabolism), in which 118 transcripts were putatively linked to insecticide resistance, including up-regulated glutathione-S-transferase, UDP glucuronosyltransferase, glucosyl/glucuronosyl transferase, and cytochrome P450. Gene Ontology analysis placed these genes and proteins into protein complex, metabolic process, cellular process, signaling, and response to stimulus categories. Quantitative real-time PCR analysis validated "omics" response, and suggested a highly overexpressed P450, CYP6CX1, as a candidate molecular basis for the mechanistic study of thiamethoxam resistance in whiteflies. Finally, enzymatic activity assays showed elevated detoxification activities in the resistant B. tabaci.

CONCLUSIONS

This study demonstrates the applicability of high-throughput omics tools for identifying molecular candidates related to thiamethoxam resistance in an agricultural important insect pest. In addition, transcriptomic and proteomic analyses provide a solid foundation for future functional investigations into the complex molecular mechanisms governing the neonicotinoid resistance in whiteflies.

Pymetrozine is hydroxylated by CYP6CM1, a cytochrome P450 conferring neonicotinoid resistance in Bemisia tabaci

BACKGROUND

Resistance to neonicotinoid insecticides such as imidacloprid in the cotton whitefly, Bemisia tabaci, is linked to its hydroxylation by constitutively overexpressed CYP6CM1, a cytochrome P450 enzyme. Here, an investigation was conducted to establish whether CYP6CM1 functionally expressed in Sf9 cells also detoxifies pymetrozine, a selective homopteran feeding blocker known to be cross-resistant to neonicotinoids in whiteflies.

RESULTS

Incubation of pymetrozine with functionally expressed Bemisia CYP6CM1 and subsequent LC-MS/MS analysis revealed a rapid formation of two pymetrozine metabolites by hydroxylation of its heterocyclic 1,2,4-triazine ring system. Enzyme kinetics revealed a Km value of 5.9 ± 0.3 µM and a time-dependent depletion of pymetrozine.

CONCLUSION

The known cross-resistance between imidacloprid, other neonicotinoid insecticides and pymetrozine in B. tabaci is most likely conferred by the very same detoxification mechanism, i.e. a monooxygenase-based hydroxylation mechanism linked to the overexpression of CYP6CM1. These insecticide chemistries should not be alternated in whitefly resistance management strategies.

Identification and validation of a gene causing cross-resistance between insecticide classes in Anopheles gambiae from Ghana

In the last decade there have been marked reductions in malaria incidence in sub-Saharan Africa. Sustaining these reductions will rely upon insecticides to control the mosquito malaria vectors. We report that in the primary African malaria vector, Anopheles gambiae sensu stricto, a single enzyme, CYP6M2, confers resistance to two classes of insecticide. This is unique evidence in a disease vector of cross-resistance associated with a single metabolic gene that simultaneously reduces the efficacy of two of the four classes of insecticide routinely used for malaria control. The gene-expression profile of a highly DDT-resistant population of A. gambiae s.s. from Ghana was characterized using a unique whole-genome microarray. A number of genes were significantly overexpressed compared with two susceptible West African colonies, including genes from metabolic families previously linked to insecticide resistance. One of the most significantly overexpressed probe groups (false-discovery rate-adjusted P < 0.0001) belonged to the cytochrome P450 gene CYP6M2. This gene is associated with pyrethroid resistance in wild A. gambiae s.s. populations) and can metabolize both type I and type II pyrethroids in recombinant protein assays. Using in vitro assays we show that recombinant CYP6M2 is also capable of metabolizing the organochlorine insecticide DDT in the presence of solubilizing factor sodium cholate.