The known unknowns of Powassan virus ecology

Powassan virus (POWV; Family: Flaviviridae, Genus: Flavivirus) is the sole North American member of the tick-borne encephalitis sero-complex. While associated with high rates of morbidity and mortality, POWV has historically been of little public health concern due to low incidence rates. However, over the last 20 yr, incidence rates have increased highlighting the growing epidemiological threat. Currently, there are no vaccines or therapeutics with tick habitat reduction, acaricide application, and public awareness programs being our primary means of intervention. The effectiveness of these control strategies is dependent on having a sound understanding of the virus's ecology. In this Forum, we review what is currently known about POWV ecology, identify gaps in our knowledge, and discuss prevailing and alternative hypotheses about transmission dynamics, reservoir hosts, and spatial focality.

RNAi turns 25:contributions and challenges in insect science

Since its discovery in 1998, RNA interference (RNAi), a Nobel prize-winning technology, made significant contributions to advances in biology because of its ability to mediate the knockdown of specific target genes. RNAi applications in medicine and agriculture have been explored with mixed success. The past 25 years of research on RNAi resulted in advances in our understanding of the mechanisms of its action, target specificity, and differential efficiency among animals and plants. RNAi played a major role in advances in insect biology. Did RNAi technology fully meet insect pest and disease vector management expectations? This review will discuss recent advances in the mechanisms of RNAi and its contributions to insect science. The remaining challenges, including delivery to the target site, differential efficiency, potential resistance development and possible solutions for the widespread use of this technology in insect management.

An 11-point time course midgut transcriptome across 72 h after bloodfeeding provides detailed temporal resolution of transcript expression in the arbovirus vector, Aedes aegypti

As the major vector for dengue, Zika, yellow fever, and chikungunya viruses, the mosquito Aedes aegypti is one of the most important insects in public health. These viruses are transmitted by bloodfeeding, which is also necessary for the reproduction of the mosquito. Thus, the midgut plays an essential role in mosquito physiology as the center for bloodmeal digestion and as an organ that serves as the first line of defense against viruses. Despite its importance, transcriptomic dynamics with fine temporal resolution across the entire digestion cycle have not yet been reported. To fill this gap, we conducted a transcriptomic analysis of A. aegypti female midguts across a 72-h bloodmeal digestion cycle for 11 time points, with a particular focus on the first 24 h. PCA analysis confirmed that 72 h is indeed a complete digestion cycle. Cluster and GO enrichment analysis showed the orchestrated modulation of thousands of genes to accomplish the midgut's role as the center for digestion, as well as nutrient transport with a clear progression with sequential emphasis on transcription, translation, energy production, nutrient metabolism, transport, and finally, autophagy by 24-36 h. We further determined that many serine proteases are robustly expressed as if to prepare for unexpected physiological challenges. This study provides a powerful resource for the analysis of genomic features that coordinate the rapid and complex transcriptional program induced by mosquito bloodfeeding.

Trypsin, the Major Proteolytic Enzyme for Blood Digestion in the Mosquito Midgut

When a female mosquito takes a blood meal, proteolytic activity surges in the midgut. Trypsin-like serine proteases are the major endoproteolytic enzyme induced by feeding in mosquitoes. The mosquito midgut lacks trypsin activity before the blood meal, but in most anautogenous mosquitoes, trypsin activity increases continuously up to 30 h after feeding and subsequently returns to baseline levels by 60 h. Trypsin activity in mosquitoes is restricted entirely to the posterior midgut lumen, where blood is stored and digested. Trypsin enzyme activity can be quantitatively measured using the artificial Nα-benzoyl-DL-arginine 4-nitroanilide hydrochloride substrate, a method described in our associated protocol.

Interactions between vector competence to chikungunya virus and resistance to deltamethrin in Aedes aegypti laboratory lines?

The urban mosquito species Aedes aegypti is the main vector of arboviruses worldwide. Mosquito control with insecticides is the most prevalent method for preventing transmission in the absence of effective vaccines and available treatments; however, the extensive use of insecticides has led to the development of resistance in mosquito populations throughout the world, and the number of epidemics caused by arboviruses has increased. Three mosquito lines with different resistance profiles to deltamethrin were isolated in French Guiana, including one with the I1016 knock-down resistant allele. Significant differences were observed in the cumulative proportion of mosquitoes with a disseminated chikungunya virus infection over time across these lines. In addition, some genes related to resistance (CYP6BB2, CYP6N12, GST2, trypsin) were variably overexpressed in the midgut at 7 days after an infectious bloodmeal in these three lines. Our work shows that vector competence for chikungunya virus varied between Ae. aegypti laboratory lines with different deltamethrin resistance profiles. More accurate verification of the functional association between insecticide resistance and vector competence remains to be demonstrated.

Genetic Adaptation by Dengue Virus Serotype 2 to Enhance Infection of Aedes aegypti Mosquito Midguts

Dengue viruses (DENVs), serotypes 1–4, are arthropod-borne viruses transmitted to humans by mosquitoes, primarily Aedes aegypti. The transmission cycle begins when Ae. aegypti ingest blood from a viremic human and the virus infects midgut epithelial cells. In studying viruses derived from the DENV2 infectious clone 30P-NBX, we found that when the virus was delivered to female Ae. aegypti in an infectious blood meal, the midgut infection rate (MIR) was very low. To determine if adaptive mutations in the DENV2 envelope (E) glycoprotein could be induced to increase the MIR, we serially passed 30P-NBX in Ae. aegypti midguts. After four passages, a single, non-conservative mutation in E protein domain II (DII) nucleotide position 1300 became dominant, resulting in replacement of positively-charged amino acid lysine (K) at position 122 with negatively-charged glutamic acid (E; K122E) and a significantly-enhanced MIR. Site directed mutagenesis experiments showed that reducing the positive charge of this surface-exposed region of the E protein DII correlated with improved Ae. aegypti midgut infection.

Cross-species microRNA transmission modulates flavivirus growth in mosquitoes

Mosquitoes can be infected with a variety of RNA viruses. Recently,Zhu et al. demonstrated that human microRNA hsa-miR-150-5p is acquired by mosquitoes during blood meals and protects the Dengue virus by downregulation of chymotrypsin AaCT-1 mRNA. This finding suggests the use of microRNA antagomirs as an antiviral approach in mosquitoes.

A human-blood-derived microRNA facilitates flavivirus infection in fed mosquitoes

Hematophagous arthropods, such as mosquitoes, naturally carry and transmit hundreds of arboviruses to humans. Blood meal is a predominant physical interface that shapes cross-species communications among humans, bloodsuckers, and arboviruses. Here, we identify a human-blood-derived microRNA, hsa-miR-150-5p, that interferes with a mosquito antiviral system to facilitate flavivirus infection and transmission. hsa-miR-150-5p is acquired with a blood meal into the mosquito hemocoel and persists for a prolonged time there. The agomir of hsa-miR-150-5p enhances, whereas the antagomir represses flaviviral infection in mosquitoes and transmission from mice to mosquitoes. Mechanistic studies indicate that hsa-miR-150-5p hijacks the mosquito Argonaute-1-mediated RNA interference system to suppress the expression of some chymotrypsins with potent virucidal activity. Mosquito chymotrypsins are essential for resisting systemic flavivirus infection in hemocoel tissues. Chymotrypsin homologs potentially targeted by miR-150-5p are also found in other hematophagous arthropods, demonstrating a conserved miR-150-5p-mediated cross-species RNAi mechanism that might determine flaviviral transmissibility in nature.

Flavivirus Persistence in Wildlife Populations

A substantial number of humans are at risk for infection by vector-borne flaviviruses, resulting in considerable morbidity and mortality worldwide. These viruses also infect wildlife at a considerable rate, persistently cycling between ticks/mosquitoes and small mammals and reptiles and non-human primates and humans. Substantially increasing evidence of viral persistence in wildlife continues to be reported. In addition to in humans, viral persistence has been shown to establish in mammalian, reptile, arachnid, and mosquito systems, as well as insect cell lines. Although a considerable amount of research has centered on the potential roles of defective virus particles, autophagy and/or apoptosis-induced evasion of the immune response, and the precise mechanism of these features in flavivirus persistence have yet to be elucidated. In this review, we present findings that aid in understanding how vector-borne flavivirus persistence is established in wildlife. Research studies to be discussed include determining the critical roles universal flavivirus non-structural proteins played in flaviviral persistence, the advancement of animal models of viral persistence, and studying host factors that allow vector-borne flavivirus replication without destructive effects on infected cells. These findings underscore the viral–host relationships in wildlife animals and could be used to elucidate the underlying mechanisms responsible for the establishment of viral persistence in these animals.

Transcriptome comparison of dengue-susceptible and -resistant field derived strains of Colombian Aedes aegypti using RNA-sequencing

BACKGROUND

Forty percent of the world’s population live in areas where they are at risk from dengue fever, dengue hemorrhagic fever, and dengue shock syndrome. Dengue viruses are transmitted primarily by the mosquito Aedes aegypti. In Cali, Colombia, approximately 30% of field collected Ae. aegypti are naturally refractory to all four dengue serotypes.

OBJECTIVES

Use RNA-sequencing to identify those genes that determine refractoriness in feral mosquitoes to dengue. This information can be used in gene editing strategies to reduce dengue transmission.

METHODS

We employed a full factorial design, analyzing differential gene expression across time (24, 36 and 48 h post bloodmeal), feeding treatment (blood or blood + dengue-2) and strain (susceptible or refractory). Sequences were aligned to the reference Ae. aegypti genome for identification, assembled to visualize transcript structure, and analyzed for dynamic gene expression changes. A variety of clustering techniques was used to identify the differentially expressed genes.

FINDINGS

We identified a subset of genes that likely assist dengue entry and replication in susceptible mosquitoes and contribute to vector competence.

MAIN CONCLUSIONS

The differential expression of specific genes by refractory and susceptible mosquitoes could determine the phenotype, and may be used to in gene editing strategies to reduce dengue transmission.

Non-immune Traits Triggered by Blood Intake Impact Vectorial Competence

Blood-feeding arthropods are considered an enormous public health threat. They are vectors of a plethora of infectious agents that cause potentially fatal diseases like Malaria, Dengue fever, Leishmaniasis, and Lyme disease. These vectors shine due to their own physiological idiosyncrasies, but one biological aspect brings them all together: the requirement of blood intake for development and reproduction. It is through blood-feeding that they acquire pathogens and during blood digestion that they summon a collection of multisystemic events critical for vector competence. The literature is focused on how classical immune pathways (Toll, IMD, and JAK/Stat) are elicited throughout the course of vector infection. Still, they are not the sole determinants of host permissiveness. The dramatic changes that are the hallmark of the insect physiology after a blood meal intake are the landscape where a successful infection takes place. Dominant processes that occur in response to a blood meal are not canonical immunological traits yet are critical in establishing vector competence. These include hormonal circuitries and reproductive physiology, midgut permeability barriers, midgut homeostasis, energy metabolism, and proteolytic activity. On the other hand, the parasites themselves have a role in the outcome of these blood triggered physiological events, consistently using them in their favor. Here, to enlighten the knowledge on vector-pathogen interaction beyond the immune pathways, we will explore different aspects of the vector physiology, discussing how they give support to these long-dated host-parasite relationships.

Temperature modulates immune gene expression in mosquitoes during arbovirus infection

The principal vector of dengue, Zika and chikungunya viruses is the mosquito Aedes aegypti, with its ability to transmit pathogens influenced by ambient temperature. We use chikungunya virus (CHIKV) to understand how the mosquito transcriptome responds to arbovirus infection at different ambient temperatures. We exposed CHIKV-infected mosquitoes to 18, 28 and 32°C, and found that higher temperature correlated with higher virus levels, particularly at 3 days post infection, but lower temperature resulted in reduced virus levels. RNAseq analysis indicated significantly altered gene expression levels in CHIKV infection. The highest number of significantly differentially expressed genes was observed at 28°C, with a more muted effect at the other temperatures. At the higher temperature, the expression of many classical immune genes, including Dicer-2, was not substantially altered in response to CHIKV. The upregulation of Toll, IMD and JAK-STAT pathways was only observed at 28°C. Functional annotations suggested that genes in immune response and metabolic pathways related to energy supply and DNA replication were involved in temperature-dependent changes. Time post infection also led to substantially different gene expression profiles, and this varied with temperature. In conclusion, temperature significantly modulates mosquito gene expression in response to infection, potentially leading to impairment of immune defences at higher temperatures.

High-Throughput Method for Detection of Arbovirus Infection of Saliva in Mosquitoes Aedes aegypti and Ae. albopictus

Vector competence refers to the ability of a vector to acquire, maintain, and transmit a pathogen. Collecting mosquito saliva in medium-filled capillary tubes has become the standard for approximating arbovirus transmission. However, this method is time-consuming and labor-intensive. Here we compare the capillary tube method to an alternative high-throughput detection method the collection of saliva on paper cards saturated with honey, with (FTA card) and without (filter paper) reagents for the preservation of nucleic acid for Aedes aegypti and Aedes albopictus mosquitoes infected with two emerging genotypes of the chikungunya virus (CHIKV). Model results showed that the Asian genotype CHIKV dissemination in the harvested legs of both Ae. aegypti and Ae. albopictus increased the odds of females having a positive salivary infection and higher salivary viral titers, while for the IOL genotype the same effect was observed only for Ae. aegypti. Of the three tested detection methods, the FTA card was significantly more effective at detecting infected saliva of Ae. aegypti and Ae. albopictus females than the capillary tube and filter paper was as effective as the capillary tube for the Asian genotype. We did not find significant effects of the detection method in detecting higher viral titer for both Asian and IOL genotypes. Our results are discussed in light of the limitations of the different tested detection methods.

Transcriptome of the Aedes aegypti Mosquito in Response to Human Complement Proteins

Aedes aegypti is the primary mosquito vector of several human arboviruses, including the dengue virus (DENV). Vector control is the principal intervention to decrease the transmission of these viruses. The characterization of molecules involved in the mosquito physiological responses to blood-feeding may help identify novel targets useful in designing effective control strategies. In this study, we evaluated the in vivo effect of feeding adult female mosquitoes with human red blood cells reconstituted with either heat-inactivated (IB) or normal plasma (NB). The RNA-seq based transcript expression of IB and NB mosquitoes was compared against sugar-fed (SF) mosquitoes. In in vitro experiments, we treated Aag2 cells with a recombinant version of complement proteins (hC3 or hC5a) and compared transcript expression to untreated control cells after 24 h. The transcript expression analysis revealed that human complement proteins modulate approximately 2300 transcripts involved in multiple biological functions, including immunity. We also found 161 upregulated and 168 downregulated transcripts differentially expressed when human complement protein C3 (hC3) and human complement protein C5a (hC5a) treated cells were compared to the control untreated cells. We conclude that active human complement induces significant changes to the transcriptome of Ae. aegypti mosquitoes, which may influence the physiology of these arthropods.

A simplified method for blood feeding, oral infection, and saliva collection of the dengue vector mosquitoes

A simple device using folded Parafilm-M as an artificial blood feeder was designed for studying two important dengue vector mosquitoes, Aedes aegypti and Aedes albopictus. The efficiency of the artificial blood feeder was investigated by comparing the numbers of engorged mosquitoes that fed on the artificial blood feeder versus mice as a live blood source. Significantly more engorged females Aedes aegypti fed on the artificial blood feeder than on mice. In addition, the artificial feeder could serve as a useful apparatus for oral infection via artificial blood meals, and for saliva collection in mosquitoes. Our method enabled us to collect saliva from multiple mosquitoes at once, providing sufficient infected saliva for determination of the virus titer by plaque assay analysis. Our artificial feeder has the advantage that it is simple, inexpensive, and efficient.

Extracellular vesicles restrict dengue virus fusion in Aedes aegypti cells

Aedes aegypti is the primary vector of dengue virus (DENV), and acquires this virus from a vertebrate host during blood feeding. Previous literature has shown that vertebrate blood factors such as complement protein C5a and low-density lipoprotein (LDL) influence DENV acquisition in the mosquito. Here, we show that extracellular vesicles in cell culture medium inhibit DENV infection in mosquito cells. Specifically, extracellular vesicles enter into mosquito cells and inhibit an early stage of infection. Extracellular vesicles had no effect on virus cell attachment or entry. Instead, extracellular vesicles restricted virus membrane fusion. Extracellular vesicles only inhibited DENV infection in mosquito cells and not vertebrate cells. These data highlight a novel virus-vector-host interaction that limits virus infection in mosquito cells by restricting virus membrane fusion.

Antiviral and virucidal activities of Lucilia cuprina maggots' excretion/secretion (Diptera: Calliphoridae): first work

Maggots of Lucilia sericata and L. cuprina are a backbone of the maggot debridement therapy. Further, the excretion/secretion (E/S) of these maggots has antibacterial and antifungal activities, nevertheless the antiviral activity of E/S for these maggots still out the focus. This study aimed to evaluate the E/S of L. cuprina maggots against the Rift Valley Fever (RVF) and Coxsackie B4 (CB4) viruses for first time. After collection of the E/S, its cytotoxicity on Vero cells was evaluated and the safe concentration was determined which used to investigate the antiviral and virucidal effect of E/S on the selected viruses. The E/S decreased the titers of the tested viruses compared with that of untreated viruses. The outcome data refer to that the E/S of L. cuprina consider as a promising antiviral and virucidal agent.

Biological Adaptations Associated with Dehydration in Mosquitoes

Diseases that are transmitted by mosquitoes are a tremendous health and socioeconomic burden with hundreds of millions of people being impacted by mosquito-borne illnesses annually. Many factors have been implicated and extensively studied in disease transmission dynamics, but knowledge regarding how dehydration impacts mosquito physiology, behavior, and resulting mosquito-borne disease transmission remain underdeveloped. The lapse in understanding on how mosquitoes respond to dehydration stress likely obscures our ability to effectively study mosquito physiology, behavior, and vectorial capabilities. The goal of this review is to develop a profile of factors underlying mosquito biology that are altered by dehydration and the implications that are related to disease transmission.

A salivary protein of Aedes aegypti promotes dengue-2 virus replication and transmission

Although dengue is the most prevalent arthropod-borne viral disease in humans, no effective medication or vaccine is presently available. Previous studies suggested that mosquito salivary proteins influence infection by the dengue virus (DENV) in the mammalian host. However, the effects of salivary proteins on DENV replication within the Aedes aegypti mosquito remain largely unknown. In this study, we investigated the effect of a specific salivary protein (named AaSG34) on DENV serotype 2 (DENV2) replication and transmission. We showed that transcripts of AaSG34 were upregulated in the salivary glands of Aedes aegypti mosquitoes after a meal of blood infected with DENV2. Transcripts of the dengue viral genome and envelop protein in the salivary glands were significantly diminished after an infectious blood meal when AaSG34 was silenced. The effect of AaSG34 on DENV2 transmission was investigated in Stat1-deficient mice. The intradermal inoculation of infectious mosquito saliva induced hemorrhaging in the Stat1-deficient mice; however, saliva from the AaSG34-silenced mosquitoes did not induce hemorrhaging, suggesting that AaSG34 enhances DENV2 transmission. This is the first report to demonstrate that the protein AaSG34 promotes DENV2 replication in mosquito salivary glands and enhances the transmission of the virus to the mammalian host.

Mosquito adaptations to hematophagia impact pathogen transmission

Mosquito-borne diseases such as Dengue fever, Chikungunya, and Malaria are critical threats to public health in many parts of the world. Female mosquitoes have evolved multiple adaptive mechanisms to hematophagy, including the ability to efficiently draw and digest blood, as well as the ability to eliminate excess fluids and toxic by-products of blood digestion. Pathogenic agents enter the mosquito digestive tract with the blood meal and need to travel through the midgut and into the hemocele in order to reach the salivary glands and infect a new host. Pathogens need to adjust to these hostile gut, hemocele, and salivary gland environments, and when possible influence the physiology and behavior of their hosts to enhance transmission.

Optimization of double-stranded RNAi intrathoracic injection method in Aedes aegypti

RNA interference is widely used to analyze gene functions via phenotypic knockdown of target transcripts in mosquitoes, which transmit numerous mosquito-borne diseases. Functional analysis of mosquito genes is indispensable to understand and reduce transmission of mosquito-borne diseases in mosquitoes. Intrathoracic injection of double-stranded RNA (dsRNA) remains the simplest and most customizable method in mosquitoes for functional analysis of the genes of interest. However, achieving consistent and effective knockdown by dsRNAi is often elusive and may require extensive optimization. We tested the effectiveness of gene silencing by intrathoracic injection of four different quantities of dsRNA targeting two Ae. aegypti genes, cysteine desulfurylase (Nfs1) and short-chain dehydrogenase (SDH). We found that Nfs1 gene has a lower expression level upon silencing than SDH gene. In the case of the gene that is easier to silence, Nfs1 gene expression was significantly silenced by all four tested quantities of dsRNA up to 21 d.p.i., but silencing of SDH, the gene that is difficult to silence, was less effective, with knockdown lasting up to 9 d.p.i. only when 1,000 ng of dsRNA was used. Based on our observation, intrathoracic injection of 500 ng of dsRNAs per mosquito is recommended to achieve effective knockdown for well-silenced transcripts such as Nfs1 for up to 3 weeks. This includes most in vivo bioassays involving arboviral infections in Ae. aegypti. The estimated quantities of dsRNA described in this study should be applicable to most Ae. aegypti dsRNAi studies and thus provide a guideline to develop efficient dsRNAi in other experimental investigations.

A transcriptomic survey of the impact of environmental stress on response to dengue virus in the mosquito, Aedes aegypti

Populations of Aedes aegypti naturally exhibit variable susceptibility to dengue viruses. This natural variation can be impacted by nutritional stress resulting from larval-stage crowding, indicating the influence of environment components on the adult mosquito immune response. In particular, larval crowding was previously shown to reduce the susceptibility of adult females of a Trinidad field isolate of A. aegypti to the dengue serotype 2 (JAM1409) virus. Here, we present the first whole transcriptome study to address the impact of environmental stress on A. aegypti response to dengue virus. We examined expression profiles of adult females resulting from crowded and optimum reared larvae from the same Trinidad isolate at two critical early time points-3 and 18 hours post dengue virus infected blood meal. We exposed specimens to either a dengue or naïve blood meal, and then characterized the response in ten gene co-expression modules based on their transcriptional associations with environmental stress and time. We further analyzed the top 30 hub or master regulatory genes in each of the modules, and validated our results via qRT-PCR. These hub genes reveal which functions are critical to the mechanisms that confer dengue virus refractoriness or susceptibility to stress conditioned A. aegypti, as well as the time points at which they are most important.

Shared larval rearing environment, sex, female size and genetic diversity shape Ae. albopictus bacterial microbiota

The Asian tiger mosquito Aedes albopictus became of public health concern as it can replicate and transmit viral and filarial pathogens with a strong invasive success over the world. Various strategies have been proposed to reduce mosquito population's vectorial capacity. Among them, symbiotic control of mosquito borne disease offers promising perspectives. Such method is likely to be affected by the dynamics of mosquito-associated symbiotic communities, which might in turn be affected by host genotype and environment. Our previous study suggested a correlation between mosquitoes’ origin, genetic diversity and midgut bacterial diversity. To distinguish the impact of those factors, we have been studying the midgut bacterial microbiota of two Ae. albopictus populations from tropical (La Réunion) and temperate (Montpellier) origins under controlled laboratory conditions. the two populations experienced random mating or genetic bottleneck. Microbiota composition did not highlight any variation of the α and β-diversities in bacterial communities related to host’s populations. However, sizes of the mosquitoes were negatively correlated with the bacterial α-diversity of females. Variations in mosquito sex were associated with a shift in the composition of bacterial microbiota. The females’ mosquitoes also exhibited changes in the microbiota composition according to their size and after experiencing a reduction of their genetic diversity. These results provide a framework to investigate the impact of population dynamics on the symbiotic communities associated with the tiger mosquito.

Natural Variation in Resistance to Virus Infection in Dipteran Insects

The power and ease of Drosophila genetics and the medical relevance of mosquito-transmitted viruses have made dipterans important model organisms in antiviral immunology. Studies of virus-host interactions at the molecular and population levels have illuminated determinants of resistance to virus infection. Here, we review the sources and nature of variation in antiviral immunity and virus susceptibility in model dipteran insects, specifically the fruit fly Drosophila melanogaster and vector mosquitoes of the genera Aedes and Culex. We first discuss antiviral immune mechanisms and describe the virus-specificity of these responses. In the following sections, we review genetic and microbiota-dependent variation in antiviral immunity. In the final sections, we explore less well-studied sources of variation, including abiotic factors, sexual dimorphism, infection history, and endogenous viral elements. We borrow from work on other pathogen types and non-dipteran species when it parallels or complements studies in dipterans. Understanding natural variation in virus-host interactions may lead to the identification of novel restriction factors and immune mechanisms and shed light on the molecular determinants of vector competence.

An Aedes aegypti-associated fungus increases susceptibility to dengue virus by modulating gut trypsin activity

Transmission of dengue virus (DENV) requires successful completion of the infection cycle in the Aedes aegypti vector, which is initiated in the midgut tissue after ingestion of an infectious blood meal. While certain Ae. aegypti midgut-associated bacteria influence virus infection, little is known about the midgut-associated fungi (mycobiota), and how its members might influence susceptibility to DENV infection. We show that a Talaromyces (Tsp_PR) fungus, isolated from field-caught Ae. aegypti, render the mosquito more permissive to DENV infection. This modulation is attributed to a profound down-regulation of digestive enzyme genes and trypsin activity, upon exposure to Tsp_PR-secreted factors. In conclusion, we show for the first time that a natural mosquito gut-associated fungus can alter Ae. aegypti physiology in a way that facilitates pathogen infection.

Perturbed cholesterol and vesicular trafficking associated with dengue blocking in Wolbachia-infected Aedes aegypti cells

Wolbachia are intracellular maternally inherited bacteria that can spread through insect populations and block virus transmission by mosquitoes, providing an important approach to dengue control. To better understand the mechanisms of virus inhibition, we here perform proteomic quantification of the effects of Wolbachia in Aedes aegypti mosquito cells and midgut. Perturbations are observed in vesicular trafficking, lipid metabolism and in the endoplasmic reticulum that could impact viral entry and replication. Wolbachia-infected cells display a differential cholesterol profile, including elevated levels of esterified cholesterol, that is consistent with perturbed intracellular cholesterol trafficking. Cyclodextrins have been shown to reverse lipid accumulation defects in cells with disrupted cholesterol homeostasis. Treatment of Wolbachia-infected Ae. aegypti cells with 2-hydroxypropyl-β-cyclodextrin restores dengue replication in Wolbachia-carrying cells, suggesting dengue is inhibited in Wolbachia-infected cells by localised cholesterol accumulation. These results demonstrate parallels between the cellular Wolbachia viral inhibition phenotype and lipid storage genetic disorders.

Wolbachia infection of mosquitoes can block dengue virus infection and is tested in field trials, but the mechanism of action is unclear. Using proteomics, Geoghegan et al. here identify effects of Wolbachia on cholesterol homeostasis and dengue virus replication in Aedes aegypti.

The midgut transcriptome of Aedes aegypti fed with saline or protein meals containing chikungunya virus reveals genes potentially involved in viral midgut escape

BACKGROUND

The mosquito Aedes aegypti is the primary vector for medically important arthropod-borne viruses, including chikungunya virus (CHIKV). Following oral acquisition, an arbovirus has to persistently infect several organs in the mosquito before becoming transmissible to another vertebrate host. A major obstacle an arbovirus has to overcome during its infection cycle inside the mosquito is the midgut escape barrier, representing the exit mechanism arboviruses utilize when disseminating from the midgut. To understand the transcriptomic basis of midgut escape and to reveal genes involved in the process, we conducted a comparative transcriptomic analysis of midgut samples from mosquitoes which had received a saline meal (SM) or a protein meal (PM) (not) containing CHIKV.

RESULTS

CHIKV which was orally acquired by a mosquito along with a SM or PM productively infected the midgut epithelium and disseminated to secondary tissues. A total of 27 RNA-Seq libraries from midguts of mosquitoes that had received PM or SM (not) containing CHIKV at 1 and 2 days post-feeding were generated and sequenced. Fewer than 80 genes responded differentially to the presence of CHIKV in midguts of mosquitoes that had acquired the virus along with SM or PM. SM feeding induced differential expression (DE) of 479 genes at day 1 and 314 genes at day 2 when compared to midguts of sugarfed mosquitoes. By comparison, PM feeding induced 6029 DE genes at day 1 and 7368 genes at day 2. Twenty-three DE genes encoding trypsins, metalloproteinases, and serine-type endopeptidases were significantly upregulated in midguts of mosquitoes at day 1 following SM or PM ingestion. Two of these genes were Ae. aegypti late trypsin (AeLT) and serine collagenase 1 precursor (AeSP1). In vitro, recombinant AeLT showed strong matrix metalloproteinase activity whereas recombinant AeSP1 did not.

CONCLUSIONS

By substituting a bloodmeal for SM, we identified midgut-expressed genes not involved in blood or protein digestion. These included genes coding for trypsins, metalloproteinases, and serine-type endopeptidases, which could be involved in facilitating midgut escape for arboviruses in Ae. aegypti. The presence of CHIKV in any of the ingested meals had relatively minor effects on the overall gene expression profiles in midguts.

Transcriptome Analysis of the Midgut of the Chinese Oak Silkworm Antheraea pernyi Infected with Antheraea pernyi Nucleopolyhedrovirus

The Antheraea pernyi nucleopolyhedrovirus (ApNPV) is an exclusive pathogen of A. pernyi. The intense interactions between ApNPV and A. pernyi cause a series of physiological and pathological changes to A. pernyi. However, no detailed report exists regarding the molecular mechanisms underlying the interactions between ApNPV and A. pernyi. In this study, four cDNA libraries of the A. pernyi midgut, including two ApNPV-infected groups and two control groups, were constructed for transcriptomic analysis to provide new clues regarding the molecular mechanisms that underlie these interactions. The transcriptome of the A. pernyi midgut was de novo assembled using the Trinity platform because of the lack of a genome resource for A. pernyi. Compared with the controls, a total of 5,172 differentially expressed genes (DEGs) were identified, including 2,183 up-regulated and 2,989 down-regulated candidates, of which 2,965 and 911 DEGs were classified into different GO categories and KEGG pathways, respectively. The DEGs involved in A. pernyi innate immunity were classified into several categories, including heat-shock proteins, apoptosis-related proteins, serpins, serine proteases and cytochrome P450s. Our results suggested that these genes were related to the immune response of the A. pernyi midgut to ApNPV infection via their essential roles in regulating a variety of physiological processes. Our results may serve as a basis for future research not only on the molecular mechanisms of ApNPV invasion but also on the anti-ApNPV mechanism of A. pernyi.

Proteomic analysis of Aedes aegypti midgut during post-embryonic development and of the female mosquitoes fed different diets

In this work we analyzed protein expression in the Aedes aegypti midgut during the larval (fourth instar, L4), pupal, and adult stages [including newly emerged (NE), sugar-fed (SF) and blood-fed (BF) females]. Two-dimensional electrophoresis showed 13 spots in the midgut of larvae, 95 in the midgut of pupae, 90 in the midgut of NE, and 76 in the midgut of SF or BF females. In the larval midguts, high serpin expression was noted, while in the pupae, protein abundance was lower than in the NE, SF, and BF females. The spots related to proteins linked to energy production, protein metabolism, signaling, and transport were highly expressed in the NE stage, while spots related proteins involved in translation were abundant in SF and BF females. The differential abundance of proteins in the midgut of A. aegypti at different developmental stages supports the necessity for midgut development during immature stage followed by the necessity of proteins related to digestion in adults.

Per os infectivity of white spot syndrome virus (WSSV) in white-legged shrimp (Litopenaeus vannamei) and role of peritrophic membrane

As earlier observations on peroral infectivity of WSSV in white-legged shrimp are conflicting, here, a standardized peroral intubation technique was used to examine (i) the role of the physical composition of the viral inoculum and (ii) the barrier function of the PM. In a first experiment, the infectivity of a WSSV stock was compared by determining the SID₅₀ by intramuscular injection, peroral inoculation or via feeding. The following titers were obtained: 10^8.77 SID₅₀/g by intramuscular injection, 10^1.23 SID₅₀/g by peroral inoculation and 10^0.73 SID₅₀/g by feeding. These results demonstrated that 10^7.54–10^8.03 infectious virus is needed to infect shrimp by peroral inoculation and via feeding. Next, it was examined if damage of the PM may increase the susceptibility for WSSV by peroral route. The infectivity of a virus stock was tested upon peroral inoculation of shrimp with and without removal of the PM and compared with the infectivity upon intramuscular inoculation. The virus titers obtained upon intramuscular injection and peroral inoculation of shrimp with and without PM were 10^8.63, 10^1.13 and 10^1.53 SID₅₀/mL, respectively. This experiment confirmed the need of 10^7.1–10^7.5 infectious virus to infect shrimp via peroral route and showed that the removal of the PM slightly but not significantly (p > 0.05) facilitated the infection of shrimp. This study indicated that WSSV contaminated feed is poorly infectious via peroral route, whereas it is highly infectious when injected into shrimp. The PM plays a minor role as internal barrier of shrimp against WSSV infection.

Deltamethrin Resistance Mechanisms in Aedes aegypti Populations from Three French Overseas Territories Worldwide

Background

Aedes aegypti is a cosmopolite mosquito, vector of arboviruses. The worldwide studies of its insecticide resistance have demonstrated a strong loss of susceptibility to pyrethroids, the major class of insecticide used for vector control. French overseas territories such as French Guiana (South America), Guadeloupe islands (Lesser Antilles) as well as New Caledonia (Pacific Ocean), have encountered such resistance.

Methodology/Principal Findings

We initiated a research program on the pyrethroid resistance in French Guiana, Guadeloupe and New Caledonia. Aedes aegypti populations were tested for their deltamethrin resistance level then screened by an improved microarray developed to specifically study metabolic resistance mechanisms. Cytochrome P450 genes were implicated in conferring resistance. CYP6BB2, CYP6M11, CYP6N12, CYP9J9, CYP9J10 and CCE3 genes were upregulated in the resistant populations and were common to other populations at a regional scale. The implication of these genes in resistance phenomenon is therefore strongly suggested. Other genes from detoxification pathways were also differentially regulated. Screening for target site mutations on the voltage-gated sodium channel gene demonstrated the presence of I1016 and C1534.

Conclusion /significance

This study highlighted the presence of a common set of differentially up-regulated detoxifying genes, mainly cytochrome P450 genes in all three populations. GUA and GUY populations shared a higher number of those genes compared to CAL. Two kdr mutations well known to be associated to pyrethroid resistance were also detected in those two populations but not in CAL. Different selective pressures and genetic backgrounds can explain such differences. These results are also compared with those obtained from other parts of the world and are discussed in the context of integrative research on vector competence.

Aedes aegypti is vector of Dengue, Chikungunya and Zika viruses, all causing emerging or re-emerging diseases worldwide. Fighting these diseases relies on the control of the vector. Therefore, insecticides have been extensively used worldwide, resulting in the development of insecticide resistance. In the French overseas territories, resistance to pyrethroids has been monitored for many years with high levels in the South American French territories. We then investigated the mechanisms underlying this resistance in populations from French Guiana, Guadeloupe and New Caledonia. Transcription levels of detoxification genes were measured and alongside screening for target site mutations. Upregulation of cytochrome P450 genes and carboxylesterases were observed in all three populations. Mutations related to pyrethroid resistance in position 1016 and 1534 of the voltage-gated sodium channel gene were also observed. French Guiana and Guadeloupe populations presented a closer profile of resistance mechanisms whereas the New Caledonia population had a more restricted profile. Such differences can be explained by different vector control practices, regional insecticide uses and genetic backgrounds. These results are also compared with others obtained from other parts of the world and are discussed with the perspective of integrative research on vector competence.

Dengue Virus Infection of Aedes aegypti Requires a Putative Cysteine Rich Venom Protein

Dengue virus (DENV) is a mosquito-borne flavivirus that causes serious human disease and mortality worldwide. There is no specific antiviral therapy or vaccine for DENV infection. Alterations in gene expression during DENV infection of the mosquito and the impact of these changes on virus infection are important events to investigate in hopes of creating new treatments and vaccines. We previously identified 203 genes that were ≥5-fold differentially upregulated during flavivirus infection of the mosquito. Here, we examined the impact of silencing 100 of the most highly upregulated gene targets on DENV infection in its mosquito vector. We identified 20 genes that reduced DENV infection by at least 60% when silenced. We focused on one gene, a putative cysteine rich venom protein (SeqID AAEL000379; CRVP379), whose silencing significantly reduced DENV infection in Aedes aegypti cells. Here, we examine the requirement for CRVP379 during DENV infection of the mosquito and investigate the mechanisms surrounding this phenomenon. We also show that blocking CRVP379 protein with either RNAi or specific antisera inhibits DENV infection in Aedes aegypti. This work identifies a novel mosquito gene target for controlling DENV infection in mosquitoes that may also be used to develop broad preventative and therapeutic measures for multiple flaviviruses.

Flavivirus sfRNA suppresses antiviral RNA interference in cultured cells and mosquitoes and directly interacts with the RNAi machinery

Productive arbovirus infections require mechanisms to suppress or circumvent the cellular RNA interference (RNAi) pathway, a major antiviral response in mosquitoes. In this study, we demonstrate that two flaviviruses, Dengue virus and Kunjin virus, significantly repress siRNA-mediated RNAi in infected human cells as well as during infection of the mosquito vector Culex quinquefasciatus. Arthropod-borne flaviviruses generate a small structured non-coding RNA from the viral 3' UTR referred to as sfRNA. Analysis of infections with a mutant Kunjin virus that is unable to generate appreciable amounts of the major sfRNA species indicated that RNAi suppression was associated with the generation of the non-coding sfRNA. Co-immunoprecipitation of sfRNA with RNAi mediators Dicer and Ago2 suggest a model for RNAi suppression. Collectively, these data help to establish a clear role for sfRNA in RNAi suppression and adds to the emerging impact of viral long non-coding RNAs in modulating aspects of anti-viral immune processes.

In-depth characterization of trypsin-like serine peptidases in the midgut of the sugar fed Culex quinquefasciatus

BACKGROUND

Culex quinquefasciatus is a hematophagous insect from the Culicidae family that feeds on the blood of humans, dogs, birds and livestock. This species transmits a wide variety of pathogens between humans and animals. The midgut environment is the first location of pathogen-vector interactions for blood-feeding mosquitoes and the expression of specific peptidases in the early stages of feeding could influence the outcome of the infection. Trypsin-like serine peptidases belong to a multi-gene family that can be expressed in different isoforms under distinct physiological conditions. However, the confident assignment of the trypsin genes that are expressed under each condition is still a challenge due to the large number of trypsin-coding genes in the Culicidae family and most likely because they are low abundance proteins.

METHODS

We used zymography for the biochemical characterization of the peptidase profile of the midgut from C. quinquefasciatus females fed on sugar. Protein samples were also submitted to SDS-PAGE followed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis for peptidase identification. The peptidases sequences were analyzed with bioinformatics tools to assess their distinct features.

RESULTS

Zymography revealed that trypsin-like serine peptidases were responsible for the proteolytic activity in the midgut of females fed on sugar diet. After denaturation in SDS-PAGE, eight trypsin-like serine peptidases were identified by LC-MS/MS. These peptidases have structural features typical of invertebrate digestive trypsin peptidases but exhibited singularities at the protein sequence level such as: the presence of different amino acids at the autocatalytic motif and substrate binding regions as well as different number of disulfide bounds. Data mining revealed a group of trypsin-like serine peptidases that are specific to C. quinquefasciatus when compared to the culicids genomes sequenced so far.

CONCLUSION

We demonstrated that proteomics approaches combined with bioinformatics tools and zymographic analysis can lead to the functional annotation of trypsin-like serine peptidases coding genes and aid in the understanding of the complexity of peptidase expression in mosquitoes.

Exome and transcriptome sequencing of Aedes aegypti identifies a locus that confers resistance to Brugia malayi and alters the immune response

Many mosquito species are naturally polymorphic for their abilities to transmit parasites, a feature which is of great interest for controlling vector-borne disease. Aedes aegypti, the primary vector of dengue and yellow fever and a laboratory model for studying lymphatic filariasis, is genetically variable for its capacity to harbor the filarial nematode Brugia malayi. The genome of Ae. aegypti is large and repetitive, making genome resequencing difficult and expensive. We designed exome captures to target protein-coding regions of the genome, and used association mapping in a wild Kenyan population to identify a single, dominant, sex-linked locus underlying resistance. This falls in a region of the genome where a resistance locus was previously mapped in a line established in 1936, suggesting that this polymorphism has been maintained in the wild for the at least 80 years. We then crossed resistant and susceptible mosquitoes to place both alleles of the gene into a common genetic background, and used RNA-seq to measure the effect of this locus on gene expression. We found evidence for Toll, IMD, and JAK-STAT pathway activity in response to early stages of B. malayi infection when the parasites are beginning to die in the resistant genotype. We also found that resistant mosquitoes express anti-microbial peptides at the time of parasite-killing, and that this expression is suppressed in susceptible mosquitoes. Together, we have found that a single resistance locus leads to a higher immune response in resistant mosquitoes, and we identify genes in this region that may be responsible for this trait.

The role of RNA interference (RNAi) in arbovirus-vector interactions

RNA interference (RNAi) was shown over 18 years ago to be a mechanism by which arbovirus replication and transmission could be controlled in arthropod vectors. During the intervening period, research on RNAi has defined many of the components and mechanisms of this antiviral pathway in arthropods, yet a number of unexplored questions remain. RNAi refers to RNA-mediated regulation of gene expression. Originally, the term described silencing of endogenous genes by introduction of exogenous double-stranded (ds)RNA with the same sequence as the gene to be silenced. Further research has shown that RNAi comprises three gene regulation pathways that are mediated by small RNAs: the small interfering (si)RNA, micro (mi)RNA, and Piwi-interacting (pi)RNA pathways. The exogenous (exo-)siRNA pathway is now recognized as a major antiviral innate immune response of arthropods. More recent studies suggest that the piRNA and miRNA pathways might also have important roles in arbovirus-vector interactions. This review will focus on current knowledge of the role of the exo-siRNA pathway as an arthropod vector antiviral response and on emerging research into vector piRNA and miRNA pathway modulation of arbovirus-vector interactions. Although it is assumed that arboviruses must evade the vector’s antiviral RNAi response in order to maintain their natural transmission cycles, the strategies by which this is accomplished are not well defined. RNAi is also an important tool for arthropod gene knock-down in functional genomics studies and in development of arbovirus-resistant mosquito populations. Possible arbovirus strategies for evasion of RNAi and applications of RNAi in functional genomics analysis and arbovirus transmission control will also be reviewed.

Role of the Vector in Arbovirus Transmission

Many arboviral diseases are uncontrolled, and the viruses that cause them are globally emerging or reemerging pathogens that produce significant disease throughout the world. The increased spread and prevalence of disease are occurring during a period of substantial scientific growth in the vector-borne disease research community. This growth has been supported by advances in genomics and proteomics, and by the ability to genetically alter disease vectors. For the first time, researchers are elucidating the molecular details of vector host-seeking behavior, the susceptibility of disease vectors to arboviruses, the immunological control of infection in disease vectors, and the determinants that facilitate transmission of arboviruses from a vector to a host. These discoveries are facilitating the development of novel strategies to combat arboviral disease, including the release of transgenic mosquitoes harboring dominant lethal genes, the introduction of arbovirus-blocking microbes into mosquito populations, and the development of acquisition- and transmission-blocking therapeutics. Understanding the role of the vector in arbovirus transmission has provided critical practical and theoretical tools to control arboviral disease.

The role of viral persistence in flavivirus biology

In nature, vector borne flaviviruses are persistently cycled between either the tick or mosquito vector and small mammals such as rodents, skunks, and swine. These viruses account for considerable human morbidity and mortality worldwide. Increasing and substantial evidence of viral persistence in humans, which includes the isolation of RNA by RT PCR and infectious virus by culture, continues to be reported. Viral persistence can also be established in vitro in various human, animal, arachnid, and insect cell lines in culture. Although some research has focused on the potential roles of defective virus particles, evasion of the immune response through the manipulation of autophagy and/or apoptosis, the precise mechanism of flavivirus persistence is still not well understood. We propose additional research for further understanding of how viral persistence is established in different systems. Avenues for additional studies include determining whether the multifunctional flavivirus protein NS5 has a role in viral persistence, the development of relevant animal models of viral persistence, and investigating the host responses that allow vector borne flavivirus replication without detrimental effects on infected cells. Such studies might shed more light on the viral–host relationships and could be used to unravel the mechanisms for establishment of persistence.

Persistent infections by vector borne flaviviruses are an important, but inadequately studied topic.

Influence of mosquito genotype on transcriptional response to dengue virus infection

The mosquito Aedes aegypti is the principal vector that transmits dengue virus (DENV) to humans. The primary factors that trigger a susceptible or refractory interaction of A. aegypti with DENV are not well understood. In this study, our aim is to characterize the influence of vector genotype on differential gene expression of susceptible vs. refractory A. aegypti strains to DENV infection. To accomplish that, we identified differential expression of a set of complementary DNAs (cDNAs; n = 9,504) of the D2S3 (susceptible) and Moyo-D (refractory) strains of A. aegypti to DENV serotype 2 (JAM1409) and compared these results to the differential expression of cDNAs in a different susceptible vector genotype (Moyo-S) relative to the same refractory genotype (Moyo-D) identified from our previous study. We observed that, although the number of differentially expressed transcripts (DETs) was similar in both the studies, about ~95% of the DETs were distinct between Moyo-D/D2S3 vs. Moyo-D/Moyo-S. This suggested that A. aegypti response, to infection of a given genotype of dengue, is largely dependent upon the vector genotype. However, we observed a set of common DETs among the vector strains that were associated with predicted functions such as endocytosis, regulation of autophagy, peroxisome, and lipid metabolism that may be relatively universal in conferring mosquito response to DENV infection.

Complement-related proteins control the flavivirus infection of Aedes aegypti by inducing antimicrobial peptides

The complement system functions during the early phase of infection and directly mediates pathogen elimination. The recent identification of complement-like factors in arthropods indicates that this system shares common ancestry in vertebrates and invertebrates as an immune defense mechanism. Thioester (TE)-containing proteins (TEPs), which show high similarity to mammalian complement C3, are thought to play a key role in innate immunity in arthropods. Herein, we report that a viral recognition cascade composed of two complement-related proteins limits the flaviviral infection of Aedes aegypti. An A. aegypti macroglobulin complement-related factor (AaMCR), belonging to the insect TEP family, is a crucial effector in opposing the flaviviral infection of A. aegypti. However, AaMCR does not directly interact with DENV, and its antiviral effect requires an A. aegypti homologue of scavenger receptor-C (AaSR-C), which interacts with DENV and AaMCR simultaneously in vitro and in vivo. Furthermore, recognition of DENV by the AaSR-C/AaMCR axis regulates the expression of antimicrobial peptides (AMPs), which exerts potent anti-DENV activity. Our results both demonstrate the existence of a viral recognition pathway that controls the flaviviral infection by inducing AMPs and offer insights into a previously unappreciated antiviral function of the complement-like system in arthropods.

Hosts are equipped with sophisticated machineries for detecting and eliminating invading viruses before they cause significant physiological damage. Unlike mammals which have both innate and adaptive immune systems, insects rely solely on the innate immune system to limit viral infection. Mosquitoes are natural vectors for many human pathogenic viruses, such as dengue virus (DENV) and yellow fever virus. Despite lacking an immunoglobulin-based humoral response, mosquitoes arm themselves with a functional complement-like system to ward off invading pathogens. Herein, we show that a system composed of complement-related factors recognizes and limits flaviviruses by inducing antimicrobial peptides expression. Understanding antiviral mechanisms in arthropods may provide novel strategies for limiting arboviral transmission in nature.

Effect of diet delivered various concentrations of double-stranded RNA in silencing a midgut and a non-midgut gene of Helicoverpa armigera

Ribonucleic acid interference (RNAi) is a sequence-specific gene silencing mechanism induced by double-stranded RNA (dsRNA). Recently, RNAi has gained popularity as a reverse genetics tool owing to its tremendous potential in insect pest management, which includes Helicoverpa armigera. However, its efficiency is mainly governed by dsRNA concentration, frequency of application, target gene, etc. Therefore, to obtain a robust RNAi response in H. armigera, we evaluated various concentrations of dsRNA and its frequency of applications delivered through diet in silencing a midgut gene, chymotrypsin and a non-midgut gene, juvenile hormone acid methyl transferase (jhamt) of H. armigera. The extent of target gene silencing was determined by employing reverse transcription quantitative real-time polymerase chain reaction (RT-qPCR). Our study revealed four significant findings: (i) single application of dsRNA elicited a delayed and transient silencing, while multiple applications resulted in early and persistent silencing of the above genes; (ii) silencing of the non-midgut gene (jhamt) through diet delivered dsRNA revealed prevalence of systemic silencing probably due to communication of silencing signals in this pest; (iii) the extent of silencing of chymotrypsin was positively correlated with dsRNA concentration and was negatively correlated with jhamt; (iv) interestingly, over-expression (15–18 folds) of an upstream gene, farnesyl diphosphate synthase (fpps), in juvenile hormone (JH) biosynthetic pathway at higher concentrations of jhamt dsRNA was the plausible reason for lesser silencing of jhamt. This study provides an insight into RNAi response of target genes, which is essential for RNAi design and implementation as a pest management strategy.

West Nile Virus: biology, transmission, and human infection

West Nile Virus was introduced into the Western Hemisphere during the late summer of 1999 and has been causing significant and sometimes severe human diseases since that time. This article briefly touches upon the biology of the virus and provides a comprehensive review regarding recent discoveries about virus transmission, virus acquisition, and human infection and disease.

Nature, nurture and evolution of intra-species variation in mosquito arbovirus transmission competence

Mosquitoes vary in their competence or ability to transmit arthropod-borne viruses (arboviruses). Many arboviruses cause disease in humans and animals. Identifying the environmental and genetic causes of variation in mosquito competence for arboviruses is one of the great challenges in public health. Progress identifying genetic (nature) and environmental (nurture) factors influencing mosquito competence for arboviruses is reviewed. There is great complexity in the various traits that comprise mosquito competence. The complex interactions between environmental and genetic factors controlling these traits and the factors shaping variation in Nature are largely unknown. The norms of reaction of specific genes influencing competence, their distributions in natural populations and the effects of genetic polymorphism on phenotypic variation need to be determined. Mechanisms influencing competence are not likely due to natural selection because of the direct effects of the arbovirus on mosquito fitness. More likely the traits for mosquito competence for arboviruses are the effects of adaptations for other functions of these competence mechanisms. Determining these other functions is essential to understand the evolution and distributions of competence for arboviruses. This information is needed to assess risk from mosquito-borne disease, predict new mosquito-arbovirus systems, and provide novel strategies to mitigate mosquito-borne arbovirus transmission.

Complex modulation of the Aedes aegypti transcriptome in response to dengue virus infection

Dengue fever is the most important arboviral disease world-wide, with Aedes aegypti being the major vector. Interactions between the mosquito host and dengue viruses (DENV) are complex and vector competence varies among geographically-distinct Ae. aegypti populations. Additionally, dengue is caused by four antigenically-distinct viral serotypes (DENV1–4), each with multiple genotypes. Each virus genotype interacts differently with vertebrate and invertebrate hosts. Analyses of alterations in mosquito transcriptional profiles during DENV infection are expected to provide the basis for identifying networks of genes involved in responses to viruses and contribute to the molecular-genetic understanding of vector competence. In addition, this knowledge is anticipated to support the development of novel disease-control strategies. RNA-seq technology was used to assess genome-wide changes in transcript abundance at 1, 4 and 14 days following DENV2 infection in carcasses, midguts and salivary glands of the Ae. aegypti Chetumal strain. DENV2 affected the expression of 397 Ae. aegypti genes, most of which were down-regulated by viral infection. Differential accumulation of transcripts was mainly tissue- and time-specific. Comparisons of our data with other published reports reveal conservation of functional classes, but limited concordance of specific mosquito genes responsive to DENV2 infection. These results indicate the necessity of additional studies of mosquito-DENV interactions, specifically those focused on recently-derived mosquito strains with multiple dengue virus serotypes and genotypes.

Effect of Moringa oleifera flower extract on larval trypsin and acetylcholinesterase activities in Aedes aegypti

Aedes aegypti control is crucial to reducing dengue fever. Aedes aegypti larvae have developed resistance to organophosporous insecticides and the use of natural larvicides may help manage larval resistance by increasing elements in insecticide rotation programs. Here, we report on larvicidal activity of Moringa oleifera flower extract against A. aegypti L(1), L(2), L(3), and L(4) as well as the effect of flower extract on gut trypsin and whole-larval acetylcholinesterase from L(4.) In addition, the heated flower extract was investigated for larvicidal activity against L(4) and effect on larval gut trypsin. Moringa oleifera flower extract contains a proteinaceous trypsin inhibitor (M. oleifera flower trypsin inhibitor, MoFTI), triterpene (β-amyrin), sterol (β-sitosterol) as well as flavonoids (kaempferol and quercetin). Larvicidal activity was detected against L(2), L(3), and L(4) (LC(50) of 1.72%, 1.67%, and 0.92%, respectively). Flower extract inhibited L(4) gut trypsin (MoFTI K(i) = 0.6 nM) and did not affect acetylcholinesterase activity. In vivo assay showed that gut trypsin activity from L(4) treated with M. oleifera flower extract decreased over time (0-1,440 min) and was strongly inhibited (98.6%) after 310 min incubation; acetylcholinesterase activity was not affected. Thermal treatment resulted in a loss of trypsin inhibitor and larvicidal activities, supporting the hypothesis that flower extract contains a proteinaceous trypsin inhibitor that may be responsible for the deleterious effects on larval mortality.

Global cross-talk of genes of the mosquito Aedes aegypti in response to dengue virus infection

Background

The mosquito Aedes aegypti is the primary vector of dengue virus (DENV) infection in humans, and DENV is the most important arbovirus across most of the subtropics and tropics worldwide. The early time periods after infection with DENV define critical cellular processes that determine ultimate success or failure of the virus to establish infection in the mosquito.

Methods and Results

To identify genes involved in these processes, we performed genome-wide transcriptome profiling between susceptible and refractory A. aegypti strains at two critical early periods after challenging them with DENV. Genes that responded coordinately to DENV infection in the susceptible strain were largely clustered in one specific expression module, whereas in the refractory strain they were distributed in four distinct modules. The susceptible response module in the global transcriptional network showed significant biased representation with genes related to energy metabolism and DNA replication, whereas the refractory response modules showed biased representation across different metabolism pathway genes including cytochrome P450 and DDT [1,1,1-Trichloro-2,2-bis(4-chlorophenyl) ethane] degradation genes, and genes associated with cell growth and death. A common core set of coordinately expressed genes was observed in both the susceptible and refractory mosquitoes and included genes related to the Wnt (Wnt: wingless [wg] and integration 1 [int1] pathway), MAPK (Mitogen-activated protein kinase), mTOR (mammalian target of rapamycin) and JAK-STAT (Janus Kinase - Signal Transducer and Activator of Transcription) pathways.

Conclusions

Our data revealed extensive transcriptional networks of mosquito genes that are expressed in modular manners in response to DENV infection, and indicated that successfully defending against viral infection requires more elaborate gene networks than hosting the virus. These likely play important roles in the global-cross talk among the mosquito host factors during the critical early DENV infection periods that trigger the appropriate host action in susceptible vs. refractory mosquitoes.

Dengue virus is primarily transmitted by Aedes aegypti mosquitoes. Control of the vector mosquito is the major practice to prevent dengue. However, it is not well known how the virus can infect some mosquito strains but fail to do so with other refractory strains. To address that question, we conducted whole genome microarray based gene expression studies between susceptible and refractory strains of A. aegypti to identify gene expression patterns following challenge with dengue virus. Our analysis of the early infection periods reveals that a large number of genes are involved in a highly coordinated manner either to host or defend against the virus. Genes responding to dengue infection were clustered in seven expression modules. Genes associated with susceptibility to infection were largely clustered in one expression module, while those associated with refractoriness were distributed in four distinct modules. A common core set of genes expressed in both susceptible and refractory individuals were clustered in two expression modules. We identified genes and specific pathways that potentially regulate compatible or non-compatible interactions between dengue virus and the mosquito, most notably energy metabolism and DNA replication in the susceptible response in contrast to cell growth and death in the refractory response.

Alterations in the Aedes aegypti transcriptome during infection with West Nile, dengue and yellow fever viruses

West Nile (WNV), dengue (DENV) and yellow fever (YFV) viruses are (re)emerging, mosquito-borne flaviviruses that cause human disease and mortality worldwide. Alterations in mosquito gene expression common and unique to individual flaviviral infections are poorly understood. Here, we present a microarray analysis of the Aedes aegypti transcriptome over time during infection with DENV, WNV or YFV. We identified 203 mosquito genes that were ≥5-fold differentially up-regulated (DUR) and 202 genes that were ≥10-fold differentially down-regulated (DDR) during infection with one of the three flaviviruses. Comparative analysis revealed that the expression profile of 20 DUR genes and 15 DDR genes was quite similar between the three flaviviruses on D1 of infection, indicating a potentially conserved transcriptomic signature of flaviviral infection. Bioinformatics analysis revealed changes in expression of genes from diverse cellular processes, including ion binding, transport, metabolic processes and peptidase activity. We also demonstrate that virally-regulated gene expression is tissue-specific. The overexpression of several virally down-regulated genes decreased WNV infection in mosquito cells and Aedes aegypti mosquitoes. Among these, a pupal cuticle protein was shown to bind WNV envelope protein, leading to inhibition of infection in vitro and the prevention of lethal WNV encephalitis in mice. This work provides an extensive list of targets for controlling flaviviral infection in mosquitoes that may also be used to develop broad preventative and therapeutic measures for multiple flaviviruses.

Dengue (DENV), West Nile (WNV) and Yellow Fever (YFV) viruses are responsible for severe human disease and mortality worldwide. There is no vaccine available for dengue or West Nile virus and no specific antiviral is available for any of these viral infections. These viruses are transmitted to humans through the bite of a mosquito vector. Understanding the effects of viral infection on gene expression in the mosquito is crucial to the development of effective antiviral treatments for mosquitoes and may enable researchers to interrupt the human-insect infection cycle. Here we investigate the alterations in gene expression across the entire Aedes aegypti genome during infection with DENV, YFV and WNV over time. We describe several genes that share a similar expression profile during infection with all three viruses. We also use a WNV mosquito cell, mosquito and mouse model to show that virally downregulated genes are inhibitory to infection when overexpressed and that viral regulation of mosquito genes is tissue-specific. Our results provide an extensive amount of data highlighting viral gene targets in the mosquito during infection. This data may also be used to develop broad-spectrum anti-flaviviral treatments in mosquitoes.

In vitro activation and enzyme kinetic analysis of recombinant midgut serine proteases from the Dengue vector mosquito Aedes aegypti

Background

The major Dengue virus vector Aedes aegypti requires nutrients obtained from blood meal proteins to complete the gonotrophic cycle. Although bioinformatic analyses of Ae. aegypti midgut serine proteases have provided evolutionary insights, very little is known about the biochemical activity of these digestive enzymes.

Results

We used peptide specific antibodies to show that midgut serine proteases are expressed as zymogen precursors, which are cleaved to the mature form after blood feeding. Since midgut protein levels are insufficient to purify active proteases directly from blood fed mosquitoes, we engineered recombinant proteins encoding a heterologous enterokinase cleavage site to permit generation of the bona fide mature form of four midgut serine proteases (AaET, AaLT, AaSPVI, AaSPVII) for enzyme kinetic analysis. Cleavage of the chromogenic trypsin substrate BApNA showed that AaET has a catalytic efficiency (k_cat/K_M) that is ~30 times higher than bovine trypsin, and ~2-3 times higher than AaSPVI and AaSPVII, however, AaLT does not cleave BApNA. To measure the enzyme activities of the mosquito midgut proteases using natural substrates, we developed a quantitative cleavage assay based on cleavage of albumin and hemoglobin proteins. These studies revealed that the recombinant AaLT enzyme was indeed catalytically active, and cleaved albumin and hemoglobin with equivalent efficiency to that of AaET, AaSPVI, and AaSPVII. Structural modeling of the AaLT and AaSPVI mature forms indicated that AaLT is most similar to serine collagenases, whereas AaSPVI appears to be a classic trypsin.

Conclusions

These data show that in vitro activation of recombinant serine proteases containing a heterologous enterokinase cleavage site can be used to investigate enzyme kinetics and substrate cleavage properties of biologically important mosquito proteases.

An in vivo transfection approach elucidates a role for Aedes aegypti thioester-containing proteins in flaviviral infection

Mosquitoes transmit pathogens that cause infectious diseases of global importance. Techniques to easily introduce genes into mosquitoes, however, limit investigations of the interaction between microbes and their arthropod vectors. We now show that a cationic liposome significantly enhances delivery and expression of plasmid DNA in Aedes aegypti and Anopheles gambiae mosquitoes. We then introduced the genes for Ae. aegypti thioester-containing proteins (AeTEPs), which are involved in the control of flaviviral infection, into mosquitoes using this technique. In vivo transfection of AeTEP-1 into Ae. aegypti significantly reduced dengue virus infection, suggesting that the approach can further our understanding of pathogen-mosquito interactions.