Aedes aegypti CCEae3A carboxylase expression confers carbamate, organophosphate and limited pyrethroid resistance in a model transgenic mosquito

Insecticide resistance is a serious threat to our ability to control mosquito vectors which transmit pathogens including malaria parasites and arboviruses. Understanding the underlying mechanisms is an essential first step in tackling the challenges presented by resistance. This study aimed to functionally characterise the carboxylesterase, CCEae3A, the elevated expression of which has been implicated in temephos resistance in Aedes aegypti and Aedes albopictus larvae. Using our GAL4/UAS expression system, already established in insecticide-sensitive Anopheles gambiae mosquitoes, we produced transgenic An. gambiae mosquitoes that express an Ae. aegypti CCEae3A ubiquitously. This new transgenic line permits examination of CCEae3A expression in a background in which there is not a clear orthologue in Vectorbase and allows comparison with existing An. gambiae GAL4-UAS lines. Insecticide resistance profiling of these transgenic An. gambiae larvae indicated significant increases in resistance ratio for three organophosphate insecticides, temephos (6), chloropyriphos (6.6) and fenthion (3.2) when compared to the parental strain. Cross resistance to adulticides from three major insecticide classes: organophosphates (malathion, fenitrothion and pirimiphos methyl), carbamates (bendiocarb and propoxur) and pyrethroid (alpha-cypermethrin) was also detected. Resistance to certain organophosphates and carbamates validates conclusions drawn from previous expression and phenotypic data. However, detection of resistance to pirimiphos methyl and alphacypermethrin has not previously been formally associated with CCEae3A, despite occurring in Ae. aegypti strains where this gene was upregulated. Our findings highlight the importance of characterising individual resistance mechanisms, thereby ensuring accurate information is used to guide future vector control strategies.

Actions of Camptothecin Derivatives on Larvae and Adults of the Arboviral Vector Aedes aegypti

Mosquito-borne viruses including dengue, Zika, and Chikungunya viruses, and parasites such as malaria and Onchocerca volvulus endanger health and economic security around the globe, and emerging mosquito-borne pathogens have pandemic potential. However, the rapid spread of insecticide resistance threatens our ability to control mosquito vectors. Larvae of Aedes aegypti were screened with the Medicines for Malaria Venture Pandemic Response Box, an open-source compound library, using INVAPP, an invertebrate automated phenotyping platform suited to high-throughput chemical screening of larval motility. We identified rubitecan (a synthetic derivative of camptothecin) as a hit compound that reduced A. aegypti larval motility. Both rubitecan and camptothecin displayed concentration dependent reduction in larval motility with estimated EC50 of 25.5 ± 5.0 µM and 22.3 ± 5.4 µM, respectively. We extended our investigation to adult mosquitoes and found that camptothecin increased lethality when delivered in a blood meal to A. aegypti adults at 100 µM and 10 µM, and completely blocked egg laying when fed at 100 µM. Camptothecin and its derivatives are inhibitors of topoisomerase I, have known activity against several agricultural pests, and are also approved for the treatment of several cancers. Crucially, they can inhibit Zika virus replication in human cells, so there is potential for dual targeting of both the vector and an important arbovirus that it carries.

Automated phenotyping of mosquito larvae enables high-throughput screening for novel larvicides and offers potential for smartphone-based detection of larval insecticide resistance

Pyrethroid-impregnated nets have contributed significantly to halving the burden of malaria but resistance threatens their future efficacy and the pipeline of new insecticides is short. Here we report that an invertebrate automated phenotyping platform (INVAPP), combined with the algorithm Paragon, provides a robust system for measuring larval motility in Anopheles gambiae (and An. coluzzi) as well as Aedes aegypti with the capacity for high-throughput screening for new larvicides. By this means, we reliably quantified both time- and concentration-dependent actions of chemical insecticides faster than using the WHO standard larval assay. We illustrate the effectiveness of the system using an established larvicide (temephos) and demonstrate its capacity for library-scale chemical screening using the Medicines for Malaria Venture (MMV) Pathogen Box library. As a proof-of-principle, this library screen identified a compound, subsequently confirmed to be tolfenpyrad, as an effective larvicide. We have also used the INVAPP / Paragon system to compare responses in larvae derived from WHO classified deltamethrin resistant and sensitive mosquitoes. We show how this approach to monitoring larval response to insecticides can be adapted for use with a smartphone camera application and therefore has potential for further development as a simple portable field-assay with associated real-time, geo-located information to identify hotspots.

Site-Directed φC31-Mediated Integration and Cassette Exchange in Anopheles Vectors of Malaria

Functional genomic analysis and related strategies for genetic control of malaria rely on validated and reproducible methods to accurately modify the genome of Anopheles mosquitoes. Amongst these methods, the φC31 system allows precise and stable site-directed integration of transgenes, or the substitution of integrated transgenic cassettes via recombinase-mediated cassette exchange (RMCE). This method relies on the action of the Streptomyces φC31 bacteriophage integrase to catalyze recombination between two specific attachment sites designated attP (derived from the phage) and attB (derived from the host bacterium). The system uses one or two attP sites that have been integrated previously into the mosquito genome and attB site(s) in the donor template DNA. Here we illustrate how to stably modify the genome of attP-bearing Anopheles docking lines using two plasmids: an attB-tagged donor carrying the integration or exchange template and a helper plasmid encoding the φC31 integrase. We report two representative results of φC31-mediated site-directed modification: the single integration of a transgenic cassette in An. stephensi and RMCE in An. gambiae mosquitoes. φC31-mediated genome manipulation offers the advantage of reproducible transgene expression from validated, fitness neutral genomic sites, allowing comparative qualitative and quantitative analyses of phenotypes. The site-directed nature of the integration also substantially simplifies the validation of the single insertion site and the mating scheme to obtain a stable transgenic line. These and other characteristics make the φC31 system an essential component of the genetic toolkit for the transgenic manipulation of malaria mosquitoes and other insect vectors.

New insecticide screening platforms indicate that Mitochondrial Complex I inhibitors are susceptible to cross-resistance by mosquito P450s that metabolise pyrethroids

Fenazaquin, pyridaben, tolfenpyrad and fenpyroximate are Complex I inhibitors offering a new mode of action for insecticidal malaria vector control. However, extended exposure to pyrethroid based products such as long-lasting insecticidal nets (LLINs) has created mosquito populations that are largely pyrethroid-resistant, often with elevated levels of P450s that can metabolise and neutralise diverse substrates. To assess cross-resistance liabilities of the Complex I inhibitors, we profiled their susceptibility to metabolism by P450s associated with pyrethroid resistance in Anopheles gambiae (CYPs 6M2, 6P3, 6P4, 6P5, 9J5, 9K1, 6Z2) and An. funestus (CYP6P9a). All compounds were highly susceptible. Transgenic An. gambiae overexpressing CYP6M2 or CYP6P3 showed reduced mortality when exposed to fenpyroximate and tolfenpyrad. Mortality from fenpyroximate was also reduced in pyrethroid-resistant strains of An. gambiae (VK7 2014 and Tiassalé 13) and An. funestus (FUMOZ-R). P450 inhibitor piperonyl butoxide (PBO) significantly enhanced the efficacy of fenpyroximate and tolfenpyrad, fully restoring mortality in fenpyroximate-exposed FUMOZ-R. Overall, results suggest that in vivo and in vitro assays are a useful guide in the development of new vector control products, and that the Complex I inhibitors tested are susceptible to metabolic cross-resistance and may lack efficacy in controlling pyrethroid resistant mosquitoes.

Laboratory transmission potential of British mosquitoes for equine arboviruses

BACKGROUND

There has been no evidence of transmission of mosquito-borne arboviruses of equine or human health concern to date in the UK. However, in recent years there have been a number of outbreaks of viral diseases spread by vectors in Europe. These events, in conjunction with increasing rates of globalisation and climate change, have led to concern over the future risk of mosquito-borne viral disease outbreaks in northern Europe and have highlighted the importance of being prepared for potential disease outbreaks. Here we assess several UK mosquito species for their potential to transmit arboviruses important for both equine and human health, as measured by the presence of viral RNA in saliva at different time points after taking an infective blood meal.

RESULTS

The following wild-caught British mosquitoes were evaluated for their potential as vectors of zoonotic equine arboviruses: Ochlerotatus detritus for Venezuelan equine encephalitis virus (VEEV) and Ross River virus (RRV), and Culiseta annulata and Culex pipiens for Japanese encephalitis virus (JEV). Production of RNA in saliva was demonstrated at varying efficiencies for all mosquito-virus pairs. Ochlerotatus detritus was more permissive for production of RRV RNA in saliva than VEEV RNA. For RRV, 27.3% of mosquitoes expectorated viral RNA at 7 days post-infection when incubated at 21 °C and 50% at 24 °C. Strikingly, 72% of Cx. pipiens produced JEV RNA in saliva after 21 days at 18 °C. For some mosquito-virus pairs, infection and salivary RNA titres reduced over time, suggesting unstable infection dynamics.

CONCLUSIONS

This study adds to the number of Palaearctic mosquito species that demonstrate expectoration of viral RNA, for arboviruses of importance to human and equine health. This work adds to evidence that native mosquito species should be investigated further for their potential to vector zoonotic mosquito-borne arboviral disease of equines in northern Europe. The evidence that Cx. pipiens is potentially an efficient laboratory vector of JEV at temperatures as low as 18 °C warrants further investigation, as this mosquito is abundant in cooler regions of Europe and is considered an important vector for West Nile Virus, which has a comparable transmission ecology.

Isolation and transcriptomic analysis of Anopheles gambiae oenocytes enables the delineation of hydrocarbon biosynthesis

The surface of insects is coated in cuticular hydrocarbons (CHCs); variations in the composition of this layer affect a range of traits including adaptation to arid environments and defence against pathogens and toxins. In the African malaria vector, Anopheles gambiae quantitative and qualitative variance in CHC composition have been associated with speciation, ecological habitat and insecticide resistance. Understanding how these modifications arise will inform us of how mosquitoes are responding to climate change and vector control interventions. CHCs are synthesised in sub-epidermal cells called oenocytes that are very difficult to isolate from surrounding tissues. Here we utilise a transgenic line with fluorescent oenocytes to purify these cells for the first time. Comparative transcriptomics revealed the enrichment of biological processes related to long chain fatty acyl-CoA biosynthesis and elongation of mono-, poly-unsaturated and saturated fatty acids and enabled us to delineate, and partially validate, the hydrocarbon biosynthetic pathway in An. gambiae.

Improved reference genome of Aedes aegypti informs arbovirus vector control

Female Aedes aegypti mosquitoes infect more than 400 million people each year with dangerous viral pathogens including dengue, yellow fever, Zika and chikungunya. Progress in understanding the biology of mosquitoes and developing the tools to fight them has been slowed by the lack of a high-quality genome assembly. Here we combine diverse technologies to produce the markedly improved, fully re-annotated AaegL5 genome assembly, and demonstrate how it accelerates mosquito science. We anchored physical and cytogenetic maps, doubled the number of known chemosensory ionotropic receptors that guide mosquitoes to human hosts and egg-laying sites, provided further insight into the size and composition of the sex-determining M locus, and revealed copy-number variation among glutathione S-transferase genes that are important for insecticide resistance. Using high-resolution quantitative trait locus and population genomic analyses, we mapped new candidates for dengue vector competence and insecticide resistance. AaegL5 will catalyse new biological insights and intervention strategies to fight this deadly disease vector.

An improved, fully re-annotated Aedes aegypti genome assembly (AaegL5) provides insights into the sex-determining M locus, chemosensory systems that help mosquitoes to hunt humans and loci involved in insecticide resistance and will help to generate intervention strategies to fight this deadly disease vector.

Multi-tissue GAL4-mediated gene expression in all Anopheles gambiae life stages using an endogenous polyubiquitin promoter

The ability to manipulate the Anopheles gambiae genome and alter gene expression effectively and reproducibly is a prerequisite for functional genetic analysis and for the development of novel control strategies in this important disease vector. However, in vivo transgenic analysis in mosquitoes is limited by the lack of promoters active ubiquitously. To address this, we used the GAL4/UAS system to investigate the promoter of the An. gambiae Polyubiquitin-c (PUBc) gene and demonstrated its ability to drive expression in mosquito cell culture before incorporation into An. gambiae transgenic driver lines. To generate such lines, piggyBac-mediated insertion was used to identify genomic regions able to sustain widespread expression and to create φC31 docking lines at these permissive sites. Patterns of expression induced by PUBc-GAL4 drivers carrying single intergenic insertions were assessed by crossing with a novel responder UAS-mCD8:mCherry line that was created by φC31-mediated integration. Amongst the drivers created at single, unique chromosomal integration loci, two were isolated that induced differential expression levels in a similar multiple-tissue spatial pattern throughout the mosquito life cycle. This work expands the tools available for An. gambiae functional analysis by providing a novel promoter for investigating phenotypes resulting from widespread multi-tissue expression, as well as identifying and tagging genomic sites that sustain broad transcriptional activity.

A maleness gene in the malaria mosquito Anopheles gambiae

The molecular pathways controlling gender are highly variable and have been identified in only a few nonmammalian model species. In many insects, maleness is conferred by a Y chromosome-linked M factor of unknown nature. We have isolated and characterized a gene, Yob, for the M factor in the malaria mosquito Anopheles gambiae Yob, activated at the beginning of zygotic transcription and expressed throughout a male's life, controls male-specific splicing of the doublesex gene. Silencing embryonic Yob expression is male-lethal, whereas ectopic embryonic delivery of Yob transcripts yields male-only broods. This female-killing property may be an invaluable tool for creation of conditional male-only transgenic Anopheles strains for malaria control programs.

Efficient ΦC31 integrase-mediated site-specific germline transformation of Anopheles gambiae

Current transgenic methodology developed for mosquitoes has not been applied widely to the major malaria vector Anopheles gambiae, which has proved more difficult to genetically manipulate than other mosquito species and dipteran insects. In this protocol, we describe ΦC31-mediated site-specific integration of transgenes into the genome of A. gambiae. The ΦC31 system has many advantages over 'classical' transposon-mediated germline transformation systems, because it allows integration of large transgenes at specific, characterized genomic locations. Starting from a general protocol, we have optimized steps from embryo collection to co-injection of transgene-containing plasmid and in vitro-produced ΦC31 integrase mRNA. We also provide tips for screening transgenic larvae. The outlined procedure provides robust transformation in A. gambiae, resulting in homozygous transgenic lines in ∼2-3 months.

Demasculinization of the Anopheles gambiae X chromosome

Background

In a number of organisms sex-biased genes are non-randomly distributed between autosomes and the shared sex chromosome X (or Z). Studies on Anopheles gambiae have produced conflicting results regarding the underrepresentation of male-biased genes on the X chromosome and it is unclear to what extent sexual antagonism, dosage compensation or X-inactivation in the male germline, the evolutionary forces that have been suggested to affect the chromosomal distribution of sex-biased genes, are operational in Anopheles.

Results

We performed a meta-analysis of sex-biased gene expression in Anopheles gambiae which provides evidence for a general underrepresentation of male-biased genes on the X-chromosome that increased in significance with the observed degree of sex-bias. A phylogenomic comparison between Drosophila melanogaster, Aedes aegypti and Culex quinquefasciatus also indicates that the Anopheles X chromosome strongly disfavours the evolutionary conservation of male-biased expression and that novel male-biased genes are more likely to arise on autosomes. Finally, we demonstrate experimentally that transgenes situated on the Anopheles gambiae X chromosome are transcriptionally silenced in the male germline.

Conclusion

The data presented here support the hypothesis that the observed demasculinization of the Anopheles X chromosome is driven by X-chromosome inactivation in the male germline and by sexual antagonism. The demasculinization appears to be the consequence of a loss of male-biased expression, rather than a failure in the establishment or the extinction of male-biased genes.

The Anopheles gambiae alpha-tubulin-1b promoter directs neuronal, testes and developing imaginal tissue specific expression and is a sensitive enhancer detector

A knowledge gap in mosquito functional genetic analysis is the dearth of characterized regulatory regions that can target tissue specific transgene expression. To broaden the tools available, a promoter region of the Anopheles gambiaeα-tubulin1b gene has been assayed following fusion to the green fluorescent protein (GFP) reporter gene and stable transformation of An. gambiae. In eight transgenic lines, the Angtub α1b regulatory region directed a core profile of tissue specific expression in the head, chordotonal organs, ventral nerve cord and testes. This profile overlaps those seen for α2-tubulin expression in Drosophila melanogaster and Bombyx mori. In addition, widespread position dependant expression was observed in other specific tissues that were unique to each line. For example, in different lines, expression was observed in larval and adult muscles, fatbody, cuticle and midgut secretory cells. The majority of genomic transgene insertions were mapped to within 10 kb of a gene, suggesting that the Angtub α1b basal promoter is particularly sensitive to enhancers and may be suitable to form the basis of a sensitive enhancer trapping construct, in combination with a binary expression system such as Gal4-UAS.

Optimization of the Gal4-UAS system in an Anopheles gambiae cell line

The development of the bipartite Gal4-UAS system in Anopheles gambiae would improve the functional characterization of genes in this important malaria vector. Towards this aim, we used Gal4 driver plasmids to successfully activate expression of the reporter gene, luciferase, from UAS responder plasmids when cotransfected into an An. gambiae cell line. To optimize Gal4-regulated gene expression in mosquitoes, we compared the efficiency of a series of alternative Gal4 transactivators to drive reporter gene expression from responder plasmids incorporating different numbers of tandemly arrayed Gal4 binding sites or upstream activation sequences (UAS). The results indicated that the native Gal4 is only weakly active in these cells. Modified forms of Gal4, including those carrying minimal VP16 activation domains, as well as a deleted form of Gal4, give up to 20-fold greater activity than the native protein, when used in conjunction with a responder plasmid having 14 UAS repeats. The identification of Gal4-UAS vectors that are efficiently expressed in a mosquito cell line should facilitate the transfer of this versatile expression system to An. gambiae, and potentially to other insects of medical importance.

The RNA-Seq approach to studying the expression of mosquito mitochondrial genes

In this study, we used extensive expressed sequence tag evidence obtained through 454 and Solexa next-generation sequencing to explore mtDNA transcription in male and female first instar larvae of Aedes aegypti and adults of Aedes aegypti, Anopheles gambiae, and Anopheles quadrimaculatus. Relative abundances of individual transcripts differed considerably within each sample, consistent with the differential stability of messenger RNA species. Large differences were also observed between species and between larval and adult stages; however, the male and female larval samples were remarkably similar. Quantitative PCR analysis of selected genes, cox1, l-rRNA and nd5, in larvae and adults of Ae. aegypti and in An. gambiae adults was consistent with the RNA-Seq-based quantification of expression. Finally, the absence of a conserved mtDNA region involved in transcriptional control in other dipterans suggests that mosquitoes have evolved a distinct mechanism of regulation of gene expression in the mitochondrion.

Anopheles gambiae P450 reductase is highly expressed in oenocytes and in vivo knockdown increases permethrin susceptibility

We describe an in vivo model for investigation of detoxification mechanisms of the mosquito Anopheles gambiae, important for the development of malaria control programmes. Cytochrome P450s are involved in metabolic insecticide resistance and require NADPH cytochrome P450 reductase (CPR) to function. Here we demonstrate that the major sites of adult mosquito CPR expression are oenocytes, mid-gut epithelia and head appendages. High CPR expression was also evident in Drosophila oenocytes indicating a general functional role in these insect cells. RNAi mediated knockdown drastically reduced CPR expression in oenocytes, and to a lesser extent in mid-gut epithelia; the head was unaffected. These flies showed enhanced sensitivity to permethrin, demonstrating a key role for abdominal/mid-gut P450s in pyrethroid metabolism, aiding the development of insecticides.

Conditional expression in the malaria mosquito Anopheles stephensi with Tet-On and Tet-Off systems

We report successful conditional gene expression in the malaria vector, Anopheles stephensi, on the basis of binary systems consisting of gene driver and responder transgenic lines generated by Minos-mediated germline transformation. An A. gambiae tissue-specific enhancer derived from a serpin (SRPN10) gene was utilized to control the temporal and spatial expression of doxycycline (dox)-sensitive transcriptional regulators in the driver lines. The "Tet-Off" driver utilized the tetracycline-controlled transcriptional activator (tTA) that is unable to bind and activate transcription from tetracycline operators (TetO) in the presence of dox; the "Tet-on" driver utilized the reverse tTA (rtTA) that, conversely, binds and activates TetO operators in the presence of dox. The responder lines carried insertions encompassing a LacZ reporter gene, cis-regulated by a TetO-P-element hybrid promoter. The progeny of crosses between driver and responder lines expressed beta-galactosidase under dual, tissue-specific and dox-mediated regulation. In adult rtTA/TetOPlacZ progeny, dox treatment rapidly induced beta-galactosidase activity throughout the midgut epithelium and especially in malaria parasite-invaded epithelial cells. Transactivator-dependent, dox-mediated regulation was observed in hemocytes and pericardial cells using both systems. Conditional tissue-specific regulation is a powerful tool for analyzing gene function in mosquitoes and potentially for development of strategies to control disease transmission.

Genetic manipulation of insect vectors as a strategy for the control of vector-borne disease

A variety of very effective methods have been employed for suppressing insect vector populations, including the application of biological control agents and the elimination of breeding sites, with a continuing and heavy reliance on the use of chemical insecticides. However, the development of insecticide resistance by vector insects, the cost of developing and registering new insecticidal compounds, and the increase in legislation to combat the detrimental effects of insecticidal residues on the environment, have emphasized the need to assess alternative strategies for vector control. What is required is a completely novel approach to either suppress vector populations, or to alter their ability to transmit disease-causing organisms in such a way as to have a profound and long-lasting effect on disease transmission. Genetic manipulation of insect vectors may provide just such an approach. The major requirements for genome manipulation in insects and the progress which has been made to create transgenic vector insects are reviewed. The potential applications of this methodology are then explored in the context of its future use for the control of vector-borne diseases.

Stable transformation of mosquito cell lines using a hsp70::neo fusion gene

Mosquito cell culture transfection will allow the advancement of genetic studies of these important disease-transmitting insects. Towards this end, we report the generation of stably transformed Aedes aegypti Mos20 cells using a plasmid construct containing the Tn5 neo gene, the Drosophila melanogaster hsp70 promoter, an SV40 intron and poly adenylation sequence, and a pBR 322 backbone. The apparent frequency of transfection, as measured by transient resistance of cell colonies to Geneticin (G418), ranged between 1 x 10(-4) and 1 x 10(-5), whereas the mean frequency of transformation, as assessed by establishment of cloned lines, was 3.3 x 10(-6). The stable cell lines display typical characteristics common to mammalian cell lines transformed with plasmids, including stable resistance to G418 after removal of selection, and co-transformation with unlinked plasmids. However, in contrast to the report of transformation of Ae. albopictus cells [Monroe et al., Proc. Natl. Acad. Sci. USA 89 (1992) 5725-5729], the plasmids within transformed Ae. aegypti cells have a wide range of copy number (3 to 5000), are extensively rearranged, and are only found integrated into the chromosome.