microRNAs (miRNAs) are non-coding RNAs that are now recognized as a major class of gene-regulating molecules widely distributed in metozoans and plants. miRNAs have been found to play important roles in apoptosis, cancer, development, differentiation, inflammation, longevity, and viral infection. There are a few reports describing miRNAs in the African malaria mosquito, Anopheles gambiae, on the basis of similarity to known miRNAs from other species. An. stephensi is the most important malaria vector in Asia and it is becoming a model Anopheline species for physiological and genetics studies.
We report the cloning and characterization of 27 distinct miRNAs from 17-day old An. stephensi female mosquitoes. Seventeen of the 27 miRNAs matched previously predicted An. gambiae miRNAs, offering the first experimental verification of miRNAs from mosquito species. Ten of the 27 are miRNAs previously unknown to mosquitoes, four of which did not match any known miRNAs in any organism. Twenty-five of the 27 Anopheles miRNAs had conserved sequences in the genome of a divergent relative, the yellow fever mosquito Aedes aegypti. Two clusters of miRNAs were found within introns of orthologous genes in An. gambiae, Ae. aegypti, and Drosophila melanogaster. Mature miRNAs were detected in An. stephensi for all of the nine selected miRNAs, including the four novel miRNAs (miR-x1- miR-x4), either by northern blot or by Ribonuclease Protection Assay. Expression profile analysis of eight of these miRNAs revealed distinct expression patterns from early embryo to adult stages in An. stephensi. In both An. stephensi and Ae. aegypti, the expression of miR-x2 was restricted to adult females and predominantly in the ovaries. A significant reduction of miR-x2 level was observed 72 hrs after a blood meal. Thus miR-x2 is likely involved in female reproduction and its function may be conserved among divergent mosquitoes. A mosquito homolog of miR-14, a regulator of longevity and apoptosis in D. melanogaster, represented 25% of all sequenced miRNA clones from 17-day old An. stephensi female mosquitoes. An. stephensi miR-14 displayed a relatively strong signal from late embryonic to adult stages. miR-14 expression is consistent during the adult lifespan regardless of age, sex, and blood feeding status. Thus miR-14 is likely important across all mosquito life stages.
This study provides experimental evidence for 23 conserved and four new microRNAs in An. stephensi mosquitoes. Comparisons between miRNA gene clusters in Anopheles and Aedes mosquitoes, and in D. melanogaster suggest the loss or significant change of two miRNA genes in Ae. aegypti. Expression profile analysis of eight miRNAs, including the four new miRNAs, revealed distinct patterns from early embryo to adult stages in An. stephensi. Further analysis showed that miR-x2 is likely involved in female reproduction and its function may be conserved among divergent mosquitoes. Consistent expression of miR-14 suggests that it is likely important across all mosquito life stages from embryos to aged adults. Understanding the functions of mosquito miRNAs will undoubtedly contribute to a better understanding of mosquito biology including longevity, reproduction, and mosquito-pathogen interactions, which are important to disease transmission.
The impact of global climate change on the transmission dynamics of infectious diseases is the subject of extensive debate. The transmission of mosquito-borne viral diseases is particularly complex, with climatic variables directly affecting many parameters associated with the prevalence of disease vectors. While evidence shows that warmer temperatures often decrease the extrinsic incubation period of an arthropod-borne virus (arbovirus), exposure to cooler temperatures often predisposes disease vector mosquitoes to higher infection rates. RNA interference (RNAi) pathways are essential to antiviral immunity in the mosquito; however, few experiments have explored the effects of temperature on the RNAi machinery.
We utilized transgenic “sensor” strains of Aedes aegypti to examine the role of temperature on RNA silencing. These “sensor” strains express EGFP only when RNAi is inhibited; for example, after knockdown of the effector proteins Dicer-2 (DCR-2) or Argonaute-2 (AGO-2). We observed an increase in EGFP expression in transgenic sensor mosquitoes reared at 18°C as compared with 28°C. Changes in expression were dependent on the presence of an inverted repeat with homology to a portion of the EGFP sequence, as transgenic strains lacking this sequence, the double stranded RNA (dsRNA) trigger for RNAi, showed no change in EGFP expression when reared at 18°C. Sequencing small RNAs in sensor mosquitoes reared at low temperature revealed normal processing of dsRNA substrates, suggesting the observed deficiency in RNAi occurs downstream of DCR-2. Rearing at cooler temperatures also predisposed mosquitoes to higher levels of infection with both chikungunya and yellow fever viruses.
This data suggest that microclimates, such as those present in mosquito breeding sites, as well as more general climactic variables may influence the dynamics of mosquito-borne viral diseases by affecting the antiviral immunity of disease vectors.
Although a link between the increased susceptibility of mosquitoes for arthropod-borne viruses and exposure to lower rearing temperatures has been known for many years, the molecular basis of this has remained unknown. We investigated this phenomenon using an engineered strain of mosquito where the expression of a reporter was dependant on the status of the RNA interference pathway (RNAi). Our studies indicate a correlation between the virus-susceptibility phenotype and temperature-dependent deficiencies in antiviral immunity. Specifically, we demonstrate that RNAi, a critical antiviral immune pathway in mosquito vectors of human disease, is impaired in insects reared at cooler temperatures. This suggests for the first time a molecular explanation for previously described observations, findings that may lead to a better understanding of how global climate change will affect the transmission of mosquito-borne viruses, and new criteria for evaluating genetic control strategies based on RNAi. Our studies also suggest a novel mechanism for arbovirus adaptation to otherwise incompetent vector species.
Paratransgenesis, the genetic manipulation of insect symbiotic microorganisms, is being considered as a potential method to control vector-borne diseases such as malaria. The feasibility of paratransgenic malaria control has been hampered by the lack of candidate symbiotic microorganisms for the major vector Anopheles gambiae. In other systems, densonucleosis viruses (DNVs) are attractive agents for viral paratransgenesis because they infect important vector insects, can be genetically manipulated and are transmitted to subsequent generations. However, An. gambiae has been shown to be refractory to DNV dissemination. We discovered, cloned and characterized the first known DNV (AgDNV) capable of infection and dissemination in An. gambiae. We developed a flexible AgDNV-based expression vector to express any gene of interest in An. gambiae using a two-plasmid helper-transducer system. To demonstrate proof-of-concept of the viral paratransgenesis strategy, we used this system to transduce expression of an exogenous gene (enhanced green fluorescent protein; EGFP) in An. gambiae mosquitoes. Wild-type and EGFP-transducing AgDNV virions were highly infectious to An. gambiae larvae, disseminated to and expressed EGFP in epidemiologically relevant adult tissues such as midgut, fat body and ovaries and were transmitted to subsequent mosquito generations. These proof-of-principle data suggest that AgDNV could be used as part of a paratransgenic malaria control strategy by transduction of anti-Plasmodium peptides or insect-specific toxins in Anopheles mosquitoes. AgDNV will also be extremely valuable as an effective and easy-to-use laboratory tool for transient gene expression or RNAi in An. gambiae.
Paratransgenesis, the genetic manipulation of mosquito symbiotic microorganisms, is being considered as a potential strategy to control malaria. Microorganisms associated with Anopheles mosquitoes could be manipulated to alter the mosquito's ability to become infected with and transmit the malaria parasites, or reduce mosquito fecundity or lifespan. We identified the first potential microorganism (An. gambiae densovirus; AgDNV) for paratransgenesis of the major malaria vector Anopheles gambiae. AgDNV is highly infectious to An. gambiae larvae, disseminates to adult tissues and is transmitted vertically to subsequent generations. Recombinant AgDNV was able to transduce expression of an exogenous gene (EGFP) in An. gambiae cells and mosquitoes. EGFP-transducing virions infected mosquitoes, expressed EGFP in epidemiologically relevant tissues and were transmitted to offspring in a similar manner to wild-type virus. AgDNV could be used as part of a paratransgenic malaria control strategy by transduction of anti-Plasmodium genes or insect-specific toxins in Anopheles mosquitoes, as well as an easy-to-use system for transient gene expression and RNAi for basic laboratory research.
Immune responses mounted by the malaria vector Anopheles gambiae are largely regulated by the Toll and Imd (immune deficiency) pathways via the NF-kappaB transcription factors Rel1 and Rel2, which are controlled by the negative regulators Cactus and Caspar, respectively. Rel1- and Rel2-dependent transcription in A. gambiae has been shown to be particularly critical to the mosquito's ability to manage infection with the rodent malaria parasite Plasmodium berghei. Using RNA interference to deplete the negative regulators of these pathways, we found that Rel2 controls resistance of A. gambiae to the human malaria parasite Plasmodium falciparum, whereas Rel 1 activation reduced infection levels. The universal relevance of this defense system across Anopheles species was established by showing that caspar silencing also prevents the development of P. falciparum in the major malaria vectors of Asia and South America, A. stephensi and A. albimanus, respectively. Parallel studies suggest that while Imd pathway activation is most effective against P. falciparum, the Toll pathway is most efficient against P. berghei, highlighting a significant discrepancy between the human pathogen and its rodent model. High throughput gene expression analyses identified a plethora of genes regulated by the activation of the two Rel factors and revealed that the Toll pathway played a more diverse role in mosquito biology than the Imd pathway, which was more immunity-specific. Further analyses of key anti-Plasmodium factors suggest they may be responsible for the Imd pathway–mediated resistance phenotype. Additionally, we found that the fitness cost caused by Rel2 activation through caspar gene silencing was undetectable in sugar-fed, blood-fed, and P. falciparum-infected female A. gambiae, while activation of the Toll pathway's Rel1 had a major impact. This study describes for the first time a single gene that influences an immune mechanism that is able to abort development of P. falciparum in Anopheline species. Further, this study addresses aspects of the molecular, evolutionary, and physiological consequences of the observed phenotype. These findings have implications for malaria control since broad-spectrum immune activation in diverse anopheline species offers a viable and strategic approach to develop novel malaria control methods worldwide.
The relationship between malaria parasites and the mosquitoes that transmit them to humans comprises complex molecular interactions including mosquito immune responses. Anopheles can mount potent anti-Plasmodium immune responses; we show here that the gene caspar, which encodes a negative regulator of the immune signaling pathway Imd, controls mosquito resistance to the human malaria parasite. Silencing of this Imd pathway regulator results in complete resistance to human Plasmodium in three divergent Anopheline malaria vector species, yet does not cause complete resistance to a rodent Plasmodium species, indicating conservation of defense function among mosquito but not diverse parasite species. We also identify a panel of genes that are transcriptionally regulated by silencing of the caspar gene. Some of these genes contribute directly to parasite resistance. Finally, we show that the transient immune activation that renders mosquitoes resistant to the human malaria parasite has little to no effect on mosquito fitness as a measure of survival or fecundity under laboratory conditions. In sum, this study shows that the mosquito's immune pathway, Imd, can regulate resistance to Plasmodium through immune responses that entail several known anti-Plasmodium genes.
The RNA interference (RNAi) pathway acts as an innate antiviral immune response in Aedes aegypti, modulating arbovirus infection of mosquitoes. Sindbis virus (SINV; family: Togaviridae, genus: Alphavirus) is an arbovirus that infects Ae. aegypti in the laboratory. SINV strain TR339 encounters a midgut escape barrier (MEB) during infection of Ae. aegypti. The nature of this barrier is not well understood. To investigate the role of the midgut as the central organ determining vector competence for arboviruses, we generated transgenic mosquitoes in which the RNAi pathway was impaired in midgut tissue of bloodfed females. We used these mosquitoes to reveal effects of RNAi impairment in the midgut on SINV replication, midgut infection and dissemination efficiencies, and mosquito longevity.
As a novel tool for studying arbovirus-mosquito interactions, we engineered a transgenic mosquito line with an impaired RNAi pathway in the midgut of bloodfed females by silencing expression of the Aa-dcr2 gene. In midgut tissue of the transgenic Carb/dcr16 line, Aa-dcr2 expression was reduced ~50% between 1-7 days post-bloodmeal (pbm) when compared to the recipient mosquito strain. After infection with SINV-TR339EGFP, Aa-dcr2 expression levels were enhanced in both mosquito strains. In the RNAi pathway impaired mosquito strain SINV titers and midgut infection rates were significantly higher at 7 days pbm. There was also a strong tendency for increased virus dissemination rates among the transgenic mosquitoes. Between 7-14 days pbm, SINV was diminished in midgut tissue of the transgenic mosquitoes. Transgenic impairment of the RNAi pathway and/or SINV infection did not affect longevity of the mosquitoes.
We showed that RNAi impaired transgenic mosquitoes are a useful tool for studying arbovirus-mosquito interactions at the molecular level. Following ingestion by Ae. aegypti, the recombinant SINV-TR339EGFP was confronted with both MEB and a midgut infection barrier (MIB). Impairment of the RNAi pathway in the midgut strongly reduced both midgut barriers for the virus. This confirms that the endogenous RNAi pathway of Ae. aegypti modulates vector competence for SINV in the midgut. The RNAi pathway acts as a gatekeeper to the incoming virus by affecting infection rate of the midgut, intensity of infection, and dissemination from the midgut to secondary tissues.
RNAi can be achieved in insect herbivores by feeding them host plants stably transformed to express double stranded RNA (dsRNA) of selected midgut-expressed genes. However, the development of stably transformed plants is a slow and laborious process and here we developed a rapid, reliable and transient method. We used viral vectors to produce dsRNA in the host plant Nicotiana attenuata to transiently silence midgut genes of the plant's lepidopteran specialist herbivore, Manduca sexta. To compare the efficacy of longer, undiced dsRNA for insect gene silencing, we silenced N. attenuata's dicer genes (NaDCL1- 4) in all combinations in a plant stably transformed to express dsRNA targeting an insect gene.
Stable transgenic N. attenuata plants harboring a 312 bp fragment of MsCYP6B46 in an inverted repeat orientation (ir-CYP6B46) were generated to produce CYP6B46 dsRNA. After consuming these plants, transcripts of CYP6B46 were significantly reduced in M. sexta larval midguts. The same 312 bp cDNA was cloned in an antisense orientation into a TRV vector and Agro-infiltrated into N. attenuata plants. When larvae ingested these plants, similar reductions in CYP6B46 transcripts were observed without reducing transcripts of the most closely related MsCYP6B45. We used this transient method to rapidly silence the expression of two additional midgut-expressed MsCYPs. CYP6B46 transcripts were further reduced in midguts, when the larvae fed on ir-CYP6B46 plants transiently silenced for two combinations of NaDCLs (DCL1/3/4 and DCL2/3/4) and contained higher concentrations of longer, undiced CYP6B46 dsRNA.
Both stable and transient expression of CYP6B46 dsRNA in host plants provides a specific and robust means of silencing this gene in M. sexta larvae, but the transient system is better suited for high throughput analyses. Transiently silencing NaDCLs in ir-CYP6B46 plants increased the silencing of MsCYP6B46, suggested that insect's RNAi machinery is more efficient with longer lengths of ingested dsRNA.
A causative agent of human malaria, Plasmodium falciparum, is transmitted by Anopheles mosquitoes. The malaria parasite is under intensive attack from the mosquito's innate immune system during its sporogonic development. We have used genetic engineering to create immune-enhanced Anopheles stephensi mosquitoes through blood meal-inducible expression of a transgene encoding the IMD pathway-controlled NF-kB Rel2 transcription factor in the midgut and fat-body tissue. Transgenic mosquitoes showed greater resistance to Plasmodium and microbial infection as a result of timely concerted tissue-specific immune attacks involving multiple effectors. The relatively weak impact of this genetic modification on mosquito fitness under laboratory conditions encourages further investigation of this approach for malaria control.
Malaria is caused by the Plasmodium parasites which are transmitted by the Anopheles mosquitoes, and the mosquito's innate immune system plays an important role in blocking the parasite at several sporogonic stages. Our previous studies have shown that one of the major mosquito immune pathways, IMD, is important in the defense against the human malaria parasite in three major malaria vectors, Anopheles gambiae, A. stephensi, and A. albimanus. The transcription factor Rel2 is regulated by the IMD pathway and controls the expression of several potent anti-Plasmodium immune factors. In this study, we have used germ-line transformation technology to generate transgenic Anopheles stephensi mosquitoes with blood meal-inducible expression of Rel2 transcripts in both midgut and fat-body tissues. These transgenic mosquitoes were close to completely resistant to human malaria parasites as a result of the timely concerted tissue-specific expression of anti-Plasmodium effector genes. Under laboratory conditions, the fitness cost of this genetic modification was minimal, suggesting that this system could be utilized for the development of a malaria control strategy.
Transposon-mediated transformation was used to produce Anopheles
stephensi that express single-chain antibodies (scFvs) designed to
target the human malaria parasite, Plasmodium falciparum. The
scFvs, m1C3, m4B7, and m2A10, are derived from mouse monoclonal antibodies that
inhibit either ookinete invasion of the midgut or sporozoite invasion of
salivary glands. The scFvs that target the parasite surface, m4B7 and m2A10,
were fused to an Anopheles gambiae antimicrobial peptide,
Cecropin A. Previously-characterized Anopheles cis-acting DNA
regulatory elements were included in the transgenes to coordinate scFv
production with parasite development. Gene amplification and immunoblot analyses
showed promoter-specific increases in transgene expression in blood-fed females.
Transgenic mosquito lines expressing each of the scFv genes had significantly
lower infection levels than controls when challenged with P.
Malaria eradication will require vector-control strategies that are both
self-sustaining and not affected by migration of infected humans and mosquitoes.
Replacement of wild malaria-susceptible mosquito populations with transgenic
strains refractory to parasite development could interrupt the cycle of disease
transmission and support eradication efforts. Production of P.
falciparum-resistant mosquitoes is a necessary first step towards
investigating the population replacement strategy. Here we show that An.
stephensi engineered to produce P.
falciparum-targeting effector molecules are resistant to this important
human malaria parasite. Two of the three effector molecules represent a novel
combination of components derived from the immune systems of mosquitoes and
mice. An important feature of these molecules is that they are unlikely to
significantly harm the mosquito, as the mosquito component is an
Anopheles antimicrobial peptide with activity against
Plasmodium, while the other component is based on a murine
antibody selected for its ability to bind specifically to a parasite protein.
Transgenes with this design coupled with a gene-drive system could be used
alongside vaccines and drugs to provide sustainable local elimination of malaria
as part of a long-term strategy for eradication.
Tissue-specific promoters controlling the expression of transgenes in Anopheles mosquitoes represent a valuable tool both for studying the interaction between these malaria vectors and the Plasmodium parasites they transmit and for novel malaria control strategies based on developing Plasmodium-refractory mosquitoes by expressing anti-parasitic genes. With this aim we have studied the promoter regions of two genes from the most important malaria vector, Anopheles gambiae, whose expression is strongly induced upon blood feeding.
We analysed the A. gambiae Antryp1 and G12 genes, which we have shown to be midgut-specific and maximally expressed at 24 hours post-bloodmeal (PBM). Antryp1, required for bloodmeal digestion, encodes one member of a family of 7 trypsin genes. The G12 gene, of unknown function, was previously identified in our laboratory in a screen for genes induced in response to a bloodmeal. We fused 1.1 kb of the upstream regions containing the putative promoter of these genes to reporter genes and transformed these into the Indian malaria vector A. stephensi to see if we could recapitulate the expression pattern of the endogenous genes. Both the Antryp1 and G12 upstream regions were able to drive female-predominant, midgut-specific expression in transgenic mosquitoes. Expression of the Antryp1-driven reporter in transgenic A. stephensi lines was low, undetectable by northern blot analysis, and failed to fully match the induction kinetics of the endogenous Antryp1 gene in A. gambiae. This incomplete conservation of expression suggests either subtle differences in the transcriptional machinery between A. stephensi and A. gambiae or that the upstream region chosen lacked all the control elements. In contrast, the G12 upstream region was able to faithfully reproduce the expression profile of the endogenous A. gambiae gene, showing female midgut specificity in the adult mosquito and massive induction PBM, peaking at 24 hours.
Our studies on two putative blood-meal induced, midgut-specific promoters validate the use of G12 upstream regulatory regions to drive targeted transgene expression coinciding spatially and temporally with pre-sporogonic stages of Plasmodium parasites in the mosquito, offering the possibility of manipulating vector competence or performing functional studies on vector-parasite interactions.
Reverse genetics in the mosquito Anopheles gambiae by RNAi mediated gene silencing has led in recent years to an advanced understanding of the mosquito immune response against infections with bacteria and malaria parasites. We developed RNAi screens in An. gambiae hemocyte-like cells using a library of double-stranded RNAs targeting 109 genes expressed highly or specifically in mosquito hemocytes to identify novel regulators of the hemocyte immune response. Assays included phagocytosis of bacterial bioparticles, expression of the antimicrobial peptide CEC1, and basal and induced expression of the mosquito complement factor LRIM1. A cell viability screen was also carried out to assess dsRNA cytotoxicity and to identify genes involved in cell growth and survival. Our results identify 22 novel immune regulators, including proteins putatively involved in phagosome assembly and maturation (Ca2+ channel, v-ATPase and cyclin-dependent protein kinase), pattern recognition (fibrinogen-domain lectins and Nimrod), immune modulation (peptidase and serine protease homolog), immune signaling (Eiger and LPS-induced factor), cell adhesion and communication (Laminin B1 and Ninjurin) and immune homeostasis (Lipophorin receptor). The development of robust functional cell-based assays paves the way for genome-wide functional screens to study the mosquito immune response to infections with human pathogens.
The mosquito immune system relies on innate humoral and cellular reactions to fight infections, including those by malaria parasites that must pass through mosquitoes before they can infect humans. Therefore, a detailed molecular understanding of these reactions could assist the design of new ways to control the spread of malaria and other mosquito-borne diseases. Here we use a technique to silence in mosquito cultured cells genes that are highly and/or specifically expressed in mosquito hemocytes, the equivalent of human white blood cells, as a means to investigate their function in reactions of the mosquito immune system. Our study identifies several novel regulators of immune reactions including phagocytosis, the engulfment and subsequent destruction of bacteria and other pathogens by hemocytes, the production of antimicrobial peptides, which directly kill or inhibit the proliferation of microbes, and the basal and induced production of an important complement regulator. Complement is a robust reaction of mosquitoes against malaria parasites and bacteria through phagocytosis, lysis or melanization (the enclosure of pathogens in a melanin capsule). We also reveal intriguing molecular connections between these reactions such as phagocytosis and regulation of complement. Our study provides novel insights into mosquito immune system and its reactions against infections.
Phylogenetic and functional analysis was conducted on an Anopheles gambiae gene, ENSANGG00000017398. Based on phylogenetic analysis, this gene belongs to the same lineage as Heat shock protein cognate 70-4 (Hsc70-4) in Drosophila. Accordingly, we propose to name this gene Heat shock protein cognate 70B (HSC70B). We previously reported that expression of HSC70B and other genes including elongation factor-1α (EF-1α) and the agglutinin attachment subunit (agglutinin) were up-regulated in o'nyong-nyong virus (ONNV)-infected female An. gambiae. Double-stranded RNA interferences have been applied to further investigate HSC70B, EF-1α and the agglutinin functions in ONNV replication in An. gambiae.
Among these three RNAi silenced genes, only dsRNAs of HSC70B (dsHSC70B) promoted ONNV replication in adult An. gambiae compared to the control mosquitoes that were co-injected with ONNV and dsRNA of β-galactosidase (dsβ-gal). ONNV titers from mosquitoes co-injected with dsHSC70B were about 9-fold higher at 6 days post-injection (d.p.i.) as compared to the control mosquitoes. By using ONNV tagged with enhanced green fluorescent protein (ONNV-eGFP), co-injection of ONNV-eGFP with dsHSC70B also showed approximately 2 ~ 3-fold higher GFP expression rates than the controls in the head, thorax, and abdomen of the mosquito. Furthermore, co-injection of ONNV with dsHSC70B significantly reduced the lifespan of adult mosquitoes as compared with the control, co-injection of ONNV with dsβ-gal treated mosquitoes.
These results indicate that HSC70B plays important roles in homeostasis and suppression of ONNV replication in the vector, An. gambiae. Biological implications of these findings are that while mosquitoes allow ONNV to replicate in them, they also check viral titers so that ONNV infection will result in no harmful effect on mosquitoes. Therefore, mosquitoes can function as vectors of ONNV transmission to humans while ONNV infection in An. gambiae remains asymptomatic.
The endosymbiotic bacterium Wolbachia is being investigated as a potential control agent in several important vector insect species. Recent studies have shown that Wolbachia can protect the insect host against a wide variety of pathogens, resulting in reduced transmission of parasites and viruses. It has been proposed that compromised vector competence of Wolbachia-infected insects is due to up-regulation of the host innate immune system or metabolic competition. Anopheles mosquitoes, which transmit human malaria parasites, have never been found to harbor Wolbachia in nature. While transient somatic infections can be established in Anopheles, no stable artificially-transinfected Anopheles line has been developed despite numerous attempts. However, cultured Anopheles cells can be stably infected with multiple Wolbachia strains such as wAlbB from Aedes albopictus, wRi from Drosophila simulans and wMelPop from Drosophila melanogaster. Infected cell lines provide an amenable system to investigate Wolbachia-Anopheles interactions in the absence of an infected mosquito strain. We used Affymetrix GeneChip microarrays to investigate the effect of wAlbB and wRi infection on the transcriptome of cultured Anopheles Sua5B cells, and for a subset of genes used quantitative PCR to validate results in somatically-infected Anopheles mosquitoes. Wolbachia infection had a dramatic strain-specific effect on gene expression in this cell line, with almost 700 genes in total regulated representing a diverse array of functional classes. Very strikingly, infection resulted in a significant down-regulation of many immune, stress and detoxification-related transcripts. This is in stark contrast to the induction of immune genes observed in other insect hosts. We also identified genes that may be potentially involved in Wolbachia-induced reproductive and pathogenic phenotypes. Somatically-infected mosquitoes had similar responses to cultured cells. The data show that Wolbachia has a profound and unique effect on Anopheles gene expression in cultured cells, and has important implications for mechanistic understanding of Wolbachia-induced phenotypes and potential novel strategies to control malaria.
Wolbachia are bacteria that infect many insect species, but do not infect Anopheles mosquitoes. These mosquitoes transmit Plasmodium parasites, which cause malaria in humans. Wolbachia infection in Aedes aegypti mosquitoes reduces their ability to transmit diverse pathogens including viruses, nematodes and bird malaria parasites. Wolbachia-induced stimulation of the mosquito's innate immune system has been suggested as a mechanism conferring this pathogen interference. Since no Wolbachia-infected Anopheles mosquito strain exists, we used infected cultured Anopheles cells to examine the effect of infection on Anopheles gene expression. Wolbachia had a profound influence on Anopheles gene expression. Many of the genes regulated by Wolbachia have been seen in other studies to influence Plasmodium levels in mosquitoes, but interestingly and in contrast to other mosquitoes, many of the host genes were suppressed rather than induced.
Transgenic RNAi holds promise as a simple, low-cost, and fast method for reverse genetics in mammals. It may be particularly useful for producing animal models for hypomorphic gene function. Inducible RNAi that permits spatially and temporally controllable gene silencing in vivo will enhance the power of transgenic RNAi approach. Furthermore, because microRNA (miRNA) targeting specific genes can be expressed simultaneously with protein coding genes, incorporation of fluorescent marker proteins can simplify the screening and analysis of transgenic RNAi animals.
We sought to optimally express a miRNA simultaneously with a fluorescent marker. We compared two construct designs. One expressed a red fluorescent protein (RFP) and a miRNA placed in its 3' untranslated region (UTR). The other expressed the same RFP and miRNA, but the precursor miRNA (pre-miRNA) coding sequence was placed in an intron that was inserted into the 3'-UTR. We found that the two constructs expressed comparable levels of miRNA. However, the intron-containing construct expressed a significantly higher level of RFP than the intron-less construct. Further experiments indicate that the 3'-UTR intron enhances RFP expression by its intrinsic gene-expression-enhancing activity and by eliminating the inhibitory effect of the pre-miRNA on the expression of RFP. Based on these findings, we incorporated the intron-embedded pre-miRNA design into a conditional expression construct that employed the Cre-loxP system. This construct initially expressed EGFP gene, which was flanked by loxP sites. After exposure to Cre recombinase, the transgene stopped EGFP expression and began expression of RFP and a miRNA, which silenced the expression of specific cellular genes.
We have designed and tested a conditional miRNA-expression construct and showed that this construct expresses both the marker genes strongly and can silence the target gene efficiently upon Cre-mediated induction of the miRNA expression. This construct can be used to increase the efficiency of making cell lines or transgenic animals that stably express miRNA targeting specific genes.
The RNA interference pathway functions as an antiviral defense in invertebrates. In order to generate a phenotypic marker which “senses” the status of the RNAi pathway in Aedes aegypti, transgenic strains were developed to express EGFP and DsRED marker genes in the eye, as well as double-stranded RNA homologous to a portion of the EGFP gene. Transgenic “sensor” mosquitoes exhibited robust eye-specific DsRED expression with little EGFP, indicating RNAi-based silencing. Cloning and high-throughput sequencing of small RNAs confirmed that the inverted-repeat transgene was successfully processed into short-interfering RNAs by the mosquito RNAi pathway. When the Ae. aegypti homologues of the genes DCR-2 or AGO-2 were knocked-down, a clear increase in EGFP fluorescence was observed in the mosquito eyes. Knockdown of DCR-2 was also associated with an increase in EGFP mRNA levels, as determined by Northern blot and real-time PCR. Knockdown of AGO-3, a gene involved in the germline-specific piRNA pathway, did not restore EGFP expression at either the mRNA or protein level. This transgenic sensor strain can now be used to identify other components of the mosquito RNAi pathway and has the potential to be used in the identification of arboviral suppressors of RNAi.
Aedes aegypti; RNAi; transgenic mosquito; dicer; argonaute
Functional screens based on dsRNA-mediated gene silencing identified several Anopheles gambiae genes that limit Plasmodium berghei infection. However, some of the genes identified in these screens have no effect on the human malaria parasite Plasmodium falciparum; raising the question of whether different mosquito effector genes mediate anti-parasitic responses to different Plasmodium species.
Four new An. gambiae (G3) genes were identified that, when silenced, have a different effect on P. berghei (Anka 2.34) and P. falciparum (3D7) infections. Orthologs of these genes, as well as LRIM1 and CTL4, were also silenced in An. stephensi (Nijmegen Sda500) females infected with P. yoelii (17XNL). For five of the six genes tested, silencing had the same effect on infection in the P. falciparum-An. gambiae and P. yoelii-An. stephensi parasite-vector combinations. Although silencing LRIM1 or CTL4 has no effect in An. stephensi females infected with P. yoelii, when An. gambiae is infected with the same parasite, silencing these genes has a dramatic effect. In An. gambiae (G3), TEP1, LRIM1 or LRIM2 silencing reverts lysis and melanization of P. yoelii, while CTL4 silencing enhances melanization.
There is a broad spectrum of compatibility, the extent to which the mosquito immune system limits infection, between different Plasmodium strains and particular mosquito strains that is mediated by TEP1/LRIM1 activation. The interactions between highly compatible animal models of malaria, such as P. yoelii (17XNL)-An. stephensi (Nijmegen Sda500), is more similar to that of P. falciparum (3D7)-An. gambiae (G3).
In 2006, we reported a mariner (Mos1)-transformed Aedes aegypti line, Carb77, which was highly resistant to dengue-2 virus (DENV2). Carb77 mosquitoes expressed a DENV2-specific inverted-repeat (IR) RNA in midgut epithelial cells after ingesting an infectious bloodmeal. The IR-RNA formed double-stranded DENV2-derived RNA, initiating an intracellular antiviral RNA interference (RNAi) response. However, Carb77 mosquitoes stopped expressing the IR-RNA after 17 generations in culture and lost their DENV2-refractory phenotype. In the current study, we generated new transgenic lines having the identical transgene as Carb77. One of these lines, Carb109M, has been genetically stable and refractory to DENV2 for >33 generations. Southern blot analysis identified two transgene integration sites in Carb109M. Northern blot analysis detected abundant, transient expression of the IR-RNA 24 h after a bloodmeal. Carb109M mosquitoes were refractory to different DENV2 genotypes but not to other DENV serotypes. To further test fitness and stability, we introgressed the Carb109M transgene into a genetically diverse laboratory strain (GDLS) by backcrossing for five generations and selecting individuals expressing the transgene's EGFP marker in each generation. Comparison of transgene stability in replicate backcross 5 (BC5) lines versus BC1 control lines demonstrated that backcrossing dramatically increased transgene stability. We subjected six BC5 lines to five generations of selection based on EGFP marker expression to increase the frequency of the transgene prior to final family selection. Comparison of the observed transgene frequencies in the six replicate lines relative to expectations from Fisher's selection model demonstrated lingering fitness costs associated with either the transgene or linked deleterious genes. Although minimal fitness loss (relative to GDLS) was manifest in the final family selection stage, we were able to select homozygotes for the transgene in one family, Carb109M/GDLS.BC5.HZ. This family has been genetically stable and DENV2 refractory for multiple generations. Carb109M/GDLS.BC5.HZ represents an important line for testing proof-of-principle vector population replacement.
Expression of a DENV2 sequence-derived IR RNA in the mosquito midgut initiates an antiviral intracellular RNAi response that efficiently blocks DENV2 infection and profoundly impairs vector competence for that virus in Aedes aegypti. DENV2-specific IR RNA expression in the Carb109M strain has maintained the RNAi-based, refractory phenotype for 33 generations in laboratory culture. The two transgene integration sites were stable after multiple generations and following introgression into a genetically-diverse (GDLS) Ae. aegypti population. Introgression of the transgene into the GDLS genetic background changed GDLS from a DENV2 susceptible phenotype to a DENV2 refractory phenotype. The DENV2 refractory homozygous line, Carb109M/GDLS.BC5.HZ, exhibits (relative to GDLS) minimal fitness loss associated with the transgene. This line could be a potential candidate for proof-of-principle field studies.
A number of studies have shown that both innate and adaptive immune defense mechanisms greatly influence the course of human dengue virus (DENV) infections, but little is known about the innate immune response of the mosquito vector Aedes aegypti to arbovirus infection. We present evidence here that a major component of the mosquito innate immune response, RNA interference (RNAi), is an important modulator of mosquito infections. The RNAi response is triggered by double-stranded RNA (dsRNA), which occurs in the cytoplasm as a result of positive-sense RNA virus infection, leading to production of small interfering RNAs (siRNAs). These siRNAs are instrumental in degradation of viral mRNA with sequence homology to the dsRNA trigger and thereby inhibition of virus replication. We show that although dengue virus type 2 (DENV2) infection of Ae. aegypti cultured cells and oral infection of adult mosquitoes generated dsRNA and production of DENV2-specific siRNAs, virus replication and release of infectious virus persisted, suggesting viral circumvention of RNAi. We also show that DENV2 does not completely evade RNAi, since impairing the pathway by silencing expression of dcr2, r2d2, or ago2, genes encoding important sensor and effector proteins in the RNAi pathway, increased virus replication in the vector and decreased the extrinsic incubation period required for virus transmission. Our findings indicate a major role for RNAi as a determinant of DENV transmission by Ae. aegypti.
Dengue viruses, globally the most prevalent arboviruses, are transmitted to humans by persistently infected Aedes aegypti mosquitoes. Understanding the mechanisms mosquitoes use to modulate infections by these agents of serious human diseases should give us critical insights into virus–vector interactions leading to transmission. RNA interference (RNAi) is an innate defense mechanism used by invertebrates to inhibit RNA virus infections; however, little is known about the antiviral role of RNAi in mosquitoes. RNAi is triggered by double-stranded RNA, leading to degradation of RNA with sequence homology to the dsRNA trigger. We show that dengue virus type 2 (DENV2) infection of Ae. aegypti by the natural route generates dsRNA and DENV2-specific small interfering RNAs, hallmarks of the RNAi response; nevertheless, persistent infection of mosquitoes occurs, suggesting that DENV2 circumvents RNAi. We also show that DENV2 infection is modulated by RNAi, since impairment by silencing expression of genes encoding important sensor and effector proteins in the RNAi pathway increases virus replication in the vector and decreases the incubation period before virus transmission. Our findings indicate a significant role for RNAi in determining the mosquito vector's potential for transmitting human diseases.
In plants, RNA- based gene silencing mediated by small RNAs functions at the transcriptional or post-transcriptional level to negatively regulate target genes, repetitive sequences, viral RNAs and/or transposon elements. Post-transcriptional gene silencing (PTGS) or the RNA interference (RNAi) approach has been achieved in a wide range of plant species for inhibiting the expression of target genes by generating double-stranded RNA (dsRNA). However, to our knowledge, successful RNAi-application to knock-down endogenous genes has not been reported in the important staple food crop banana.
Using embryogenic cell suspension (ECS) transformed with ß-glucuronidase (GUS) as a model system, we assessed silencing of gusAINT using three intron-spliced hairpin RNA (ihpRNA) constructs containing gusAINT sequences of 299-nt, 26-nt and 19-nt, respectively. Their silencing potential was analysed in 2 different experimental set-ups. In the first, Agrobacterium-mediated co-transformation of banana ECS with a gusAINT containing vector and an ihpRNA construct resulted in a significantly reduced GUS enzyme activity 6–8 days after co-cultivation with either the 299-nt and 19-nt ihpRNA vectors. In the second approach, these ihpRNA constructs were transferred to stable GUS-expressing ECS and their silencing potential was evaluated in the regenerated in vitro plants. In comparison to control plants, transgenic plants transformed with the 299-nt gusAINT targeting sequence showed a 4.5 fold down-regulated gusA mRNA expression level, while GUS enzyme activity was reduced by 9 fold. Histochemical staining of plant tissues confirmed these findings. Northern blotting used to detect the expression of siRNA in the 299-nt ihpRNA vector transgenic in vitro plants revealed a negative relationship between siRNA expression and GUS enzyme activity. In contrast, no reduction in GUS activity or GUS mRNA expression occurred in the regenerated lines transformed with either of the two gusAINT oligo target sequences (26-nt and 19-nt).
RNAi-induced silencing was achieved in banana, both at transient and stable level, resulting in significant reduction of gene expression and enzyme activity. The success of silencing was dependent on the targeted region of the target gene. The successful generation of transgenic ECS for second transformation with (an)other construct(s) can be of value for functional genomics research in banana.
Electronic supplementary material
The online version of this article (doi:10.1186/1756-0500-7-655) contains supplementary material, which is available to authorized users.
Banana; Embryogenic cell suspension; GUS expression; ihpRNA vector; Transgene silencing
Transmission of malaria is dependent on the successful completion of the Plasmodium lifecycle in the Anopheles vector. Major obstacles are encountered in the midgut tissue, where most parasites are killed by the mosquito's immune system. In the present study, DNA microarray analyses have been used to compare Anopheles gambiae responses to invasion of the midgut epithelium by the ookinete stage of the human pathogen Plasmodium falciparum and the rodent experimental model pathogen P. berghei. Invasion by P. berghei had a more profound impact on the mosquito transcriptome, including a variety of functional gene classes, while P. falciparum elicited a broader immune response at the gene transcript level. Ingestion of human malaria-infected blood lacking invasive ookinetes also induced a variety of immune genes, including several anti-Plasmodium factors. Twelve selected genes were assessed for effect on infection with both parasite species and bacteria using RNAi gene silencing assays, and seven of these genes were found to influence mosquito resistance to both parasite species. An MD2-like receptor, AgMDL1, and an immunolectin, FBN39, showed specificity in regulating only resistance to P. falciparum, while the antimicrobial peptide gambicin and a novel putative short secreted peptide, IRSP5, were more specific for defense against the rodent parasite P. berghei. While all the genes that affected Plasmodium development also influenced mosquito resistance to bacterial infection, four of the antimicrobial genes had no effect on Plasmodium development. Our study shows that the impact of P. falciparum and P. berghei infection on A. gambiae biology at the gene transcript level is quite diverse, and the defense against the two Plasmodium species is mediated by antimicrobial factors with both universal and Plasmodium-species specific activities. Furthermore, our data indicate that the mosquito is capable of sensing infected blood constituents in the absence of invading ookinetes, thereby inducing anti-Plasmodium immune responses.
The malarial parasite Plasmodium has to traverse the gut wall of the Anopheles mosquito in order to complete its lifecycle and to be transmitted between hosts. At the midgut stage of infection, the mosquito activates immune responses to eliminate most invading parasites. The features of these immune responses are not very well understood and have mainly been examined using the rodent parasite model P. berghei. Here the authors investigated the relationship between the Anopheles gambiae responses against the human pathogen P. falciparum, the rodent parasite P. berghei, and bacterial infections, at both the gene expression and functional levels. The mosquito responses against these pathogens were quite diverse, and the defense against the two malaria parasite species involved both common and species-specific components. Malaria-infected blood was sufficient to activate anti-Plasmodium immune responses, even in the absence of midgut invasion. Through this mechanism, the mosquito can initiate its defense against Plasmodium prior to invasion of the gut. Mosquito genes that could negatively influence Plasmodium development were also capable of regulating the resistance to bacterial infection, but several of the antibacterial genes had no effect on Plasmodium; thus, the mosquito apparently utilizes its antibacterial defense systems against the malaria parasite.
Malaria is one of the most important infectious diseases in the world, and it has many economic and social impacts on populations, especially in poor countries. Transmission-blocking vaccines (TBVs) are valuable tools for malaria eradication. A study on Anopheles gambiae revealed that polyclonal antibodies to carboxypeptidase B1 of A. gambiae can block sexual parasite development in the mosquito midgut. Hence, it was introduced as a TBV target in regions where A. gambiae is the main malaria vector. However, in Iran and neighboring countries as far as China, the main malaria vector is Anopheles stephensi. Also, the genome of this organism has not been sequenced yet. Therefore, in this study, carboxypeptidase B1 of A. stephensi was characterized by genomic and proteomic approaches. Furthermore, its expression pattern after ingestion of Plasmodium falciparum gametocytes and the effect of anti-CPBAs1 antibodies on sexual parasite development were evaluated. Our results revealed that the cpbAs1 expression level was increased after ingestion of the mature gametocytes of P. falciparum and that anti-CPBAs1 directed antibodies could significantly reduce the mosquito infection rate in the test group compared with the control group. Therefore, according to our findings and with respect to the high similarity of carboxypeptidase enzymes between the two main malaria vectors in Africa (A. gambiae) and Asia (A. stephensi) and the presence of other sympatric vectors, CPBAs1 could be introduced as a TBV candidate in regions where A. stephensi is the main malaria vector, and this will broaden the scope for the potential wider application of CPBAs1 antigen homologs/orthologs.
Mosquitoes are intermediate hosts for numerous disease causing organisms. Vector control is one of the most investigated strategy for the suppression of mosquito-borne diseases. Anopheles stephensi is one of the vectors of malaria parasite Plasmodium vivax. The parasite undergoes major developmental and maturation steps within the mosquito midgut and little is known about Anopheles-associated midgut microbiota. Identification and characterization of the mosquito midgut flora is likely to contribute towards better understanding of mosquito biology including longevity, reproduction and mosquito-pathogen interactions that are important to evolve strategies for vector control mechanisms.
Lab-reared and field-collected A. stephensi male, female and larvae were screened by "culture-dependent and culture-independent" methods. Five 16S rRNA gene library were constructed form lab and field-caught A. stephensi mosquitoes and a total of 115 culturable isolates from both samples were analyzed further. Altogether, 68 genera were identified from midgut of adult and larval A. stephensi, 53 from field-caught and 15 from lab-reared mosquitoes. A total of 171 and 44 distinct phylotypes having 85 to 99% similarity with the closest database matches were detected among field and lab-reared A. stephensi midgut, respectively. These OTUs had a Shannon diversity index value of 1.74–2.14 for lab-reared and in the range of 2.75–3.49 for field-caught A. stephensi mosquitoes. The high species evenness values of 0.93 to 0.99 in field-collected adult and larvae midgut flora indicated the vastness of microbial diversity retrieved by these approaches. The dominant bacteria in field-caught adult male A. stephensi were uncultured Paenibacillaceae while in female and in larvae it was Serratia marcescens, on the other hand in lab-reared mosquitoes, Serratia marcescens and Cryseobacterium meninqosepticum bacteria were found to be abundant.
More than fifty percent of the phylotypes were related to uncultured class of bacteria. Interestingly, several of the bacteria identified are related to the known symbionts in other insects. Few of the isolates identified in our study are found to be novel species within the gammaproteobacteria which could not be phylogenetically placed within known classes. To the best of our knowledge, this is the first attempt to study the midgut microbiota of A. stephensi from lab-reared and field-collected adult and larvae using "culture-dependent and independent methods".
RNA interference (RNAi) is a valuable tool in the investigation of gene function. The purpose of this study was to examine the availability, target cell types and efficiency of RNAi in the mouse seminiferous epithelium.
The experimental model was based on transgenic mice expressing EGFP (enhanced green fluorescent protein). RNAi was induced by in vivo transfection of plasmid vectors encoding for short hairpin RNAs (shRNAs) targeting EGFP. shRNAs were transfected in vivo by microinjection into the seminiferous tubules via the rete testis followed by square wave electroporation. As a transfection reporter, expression of red fluorescent protein (HcRed 1) was used. Cell types, the efficiency of both transfections and RNAi were all evaluated.
Sertoli cells were the main transfected cells. A reduction of about 40% in the level of EGFP protein was detected in cells successfully transfected both in vivo and in vitro. However, the efficiency of in vivo transfection was low.
In adult seminiferous epithelial cells, in vivo post-transcriptional gene silencing mediated by RNAi via shRNA is efficient in Sertoli cells. Similar levels of RNAi were detected both in vivo and in vitro. This also indicates that Sertoli cells have the necessary silencing machinery to repress the expression of endogenous genes via RNAi.
RNA interference (RNAi) can potently reduce target gene expression in mammalian cells and is in wide use for loss-of-function studies. Several recent reports have demonstrated that short double-stranded RNAs (dsRNAs), used to mediate RNAi, can also induce an interferon-based response resulting in changes in the expression of many interferon-responsive genes. Off-target gene silencing has also been described, bringing into question the validity of certain RNAi-based approaches for studying gene function. We have targeted the plasminogen activator inhibitor-2 (PAI-2 or SERPINB2) mRNA using lentiviral vectors for delivery of U6 promoter-driven PAI-2-targeted short hairpin RNA (shRNA) expression. PAI-2 is reported to have anti-apoptotic activity, thus reduction of endogenous expression may be expected to make cells more sensitive to programmed cell death.
As expected, we encountered a cytotoxic phenotype when targeting the PAI-2 mRNA with vector-derived shRNA. However, this predicted phenotype was a potent non-specific effect of shRNA expression, as functional overexpression of the target protein failed to rescue the phenotype. By decreasing the shRNA length or modifying its sequence we maintained PAI-2 silencing and reduced, but did not eliminate, cytotoxicity. ShRNA of 21 complementary nucleotides (21 mers) or more increased expression of the oligoadenylate synthase-1 (OAS1) interferon-responsive gene. 19 mer shRNA had no effect on OAS1 expression but long-term selective pressure on cell growth was observed. By lowering lentiviral vector titre we were able to reduce both expression of shRNA and induction of OAS1, without a major impact on the efficacy of gene silencing.
Our data demonstrate a rapid cytotoxic effect of shRNAs expressed in human tumor cell lines. There appears to be a cut-off of 21 complementary nucleotides below which there is no interferon response while target gene silencing is maintained. Cytotoxicity or OAS1 induction could be reduced by changing shRNA sequence or vector titre, but stable gene silencing could not be maintained in extended cell culture despite persistent marker gene expression from the RNAi-inducing transgene cassette. These results underscore the necessity of careful controls for immediate and long-term RNAi use in mammalian cell systems.
The genetic basis of a host's resistance to parasites has important epidemiological and evolutionary consequences. Understanding this genetic basis can be complicated by non-genetic factors, such as environmental quality, which may influence the expression of genetic resistance and profoundly alter patterns of disease and the host's response to selection. In particular, understanding the environmental influence on the genetic resistance of mosquitoes to malaria gives valuable knowledge concerning the use of malaria-resistant transgenic mosquitoes as a measure of malaria control. We made a step towards this understanding by challenging eight isofemale lines of the malaria vector Anopheles stephensi with the rodent malaria parasite Plasmodium yoelii yoelii and by feeding the mosquitoes with different concentrations of glucose. The isofemale lines differed in infection loads (the numbers of oocysts), corroborating earlier studies showing a genetic basis of resistance. In contrast, the proportion of infected mosquitoes did not differ among lines, suggesting that the genetic component underlying infection load differs from the genetic component underlying infection rate. In addition, the mean infection load and, in particular, its heritable variation in mosquitoes depended on the concentration of glucose, which suggests that the environment affects the expression and the evolution of the mosquitoes' resistance in nature. We found no evidence of genotype-by-environment interactions, i.e. the lines responded similarly to environmental variation. Overall, these results indicate that environmental variation can significantly reduce the importance of genes in determining the resistance of mosquitoes to malaria infection.
Anopheles stephensi; Plasmodium yoelii yoelii; resistance; environment; malaria
Malaria affects 300 million people worldwide every year and 450,000 in Brazil. In coastal areas of Brazil, the main malaria vector is Anopheles aquasalis, and Plasmodium vivax is responsible for the majority of malaria cases in the Americas. Insects possess a powerful immune system to combat infections. Three pathways control the insect immune response: Toll, IMD, and JAK-STAT. Here we analyze the immune role of the A. aquasalis JAK-STAT pathway after P. vivax infection. Three genes, the transcription factor Signal Transducers and Activators of Transcription (STAT), the regulatory Protein Inhibitors of Activated STAT (PIAS) and the Nitric Oxide Synthase enzyme (NOS) were characterized. Expression of STAT and PIAS was higher in males than females and in eggs and first instar larvae when compared to larvae and pupae. RNA levels for STAT and PIAS increased 24 and 36 hours (h) after P. vivax challenge. NOS transcription increased 36 h post infection (hpi) while this protein was already detected in some midgut epithelial cells 24 hpi. Imunocytochemistry experiments using specific antibodies showed that in non-infected insects STAT and PIAS were found mostly in the fat body, while in infected mosquitoes the proteins were found in other body tissues. The knockdown of STAT by RNAi increased the number of oocysts in the midgut of A. aquasalis. This is the first clear evidence for the involvement of a specific immune pathway in the interaction of the Brazilian malaria vector A. aquasalis with P. vivax, delineating a potential target for the future development of disease controlling strategies.
Malaria is endemic in 22 countries in the Americas where the Anopheles aquasalis mosquito is an important malaria vector and the Plasmodium vivax parasite is responsible for most malaria cases. This natural vector-parasite pair is difficult to study due to the lack of cultivating system for P. vivax, and to the lack of genome data for A. aquasalis. Moreover, almost all previous studies are based on African and Asian anopheline species. Understanding the interaction mechanisms between mosquito vectors and plasmodia is important for the development of malaria control strategies. Our results showed that the JAK-STAT immune pathway is activated in A. aquasalis after P. vivax challenge and is important to maintain the low levels of P. vivax load observed in this vector. Our results add to the understanding of the A. aquasalis interaction with P. vivax and lead to possible explanations for this vector competence in P. vivax transmission. All information generated here may be used to direct the development of new or specific strategies to block malaria transmission by A. aquasalis in some parts of the Americas.