Engineering RNA interference-based resistance to dengue virus type 2 in genetically modified Aedes aegypti

Global genomics of Aedes aegypti unveils widespread and novel infectious viruses capable of triggering a small RNA response

The mosquito Aedes aegypti is a prominent vector for arboviruses, but the breadth of mosquito viruses that infects this specie is not fully understood. In the broadest global survey to date of over 200 Ae. aegypti small RNA samples, we detected viral small interfering RNAs (siRNAs) and Piwi interacting RNAs (piRNAs) arising from mosquito viruses. We confirmed that most academic laboratory colonies of Ae. aegypti lack persisting viruses, yet two commercial strains were infected by a novel tombus-like virus. Ae. aegypti from North to South American locations were also teeming with multiple insect viruses, with Anphevirus and a bunyavirus displaying geographical boundaries from the viral small RNA patterns. Asian Ae. aegypti small RNA patterns indicate infections by similar mosquito viruses from the Americas and reveal the first wild example of dengue virus infection generating viral small RNAs. African Ae. aegypti also contained various viral small RNAs including novel viruses only found in these African substrains. Intriguingly, viral long RNA patterns can differ from small RNA patterns, indicative of viral transcripts evading the mosquitoes' RNA interference (RNAi) machinery. To determine whether the viruses we discovered via small RNA sequencing were replicating and transmissible, we infected C6/36 and Aag2 cells with Ae. aegypti homogenates. Through blind passaging, we generated cell lines stably infected by these mosquito viruses which then generated abundant viral siRNAs and piRNAs that resemble the native mosquito viral small RNA patterns. This mosquito small RNA genomics approach augments surveillance approaches for emerging infectious diseases.

Generating prophylactic immunity against arboviruses in vertebrates and invertebrates

The World Health Organization recently declared a global initiative to control arboviral diseases. These are mainly caused by pathogenic flaviviruses (such as dengue, yellow fever and Zika viruses) and alphaviruses (such as chikungunya and Venezuelan equine encephalitis viruses). Vaccines represent key interventions for these viruses, with licensed human and/or veterinary vaccines being available for several members of both genera. However, a hurdle for the licensing of new vaccines is the epidemic nature of many arboviruses, which presents logistical challenges for phase III efficacy trials. Furthermore, our ability to predict or measure the post-vaccination immune responses that are sufficient for subclinical outcomes post-infection is limited. Given that arboviruses are also subject to control by the immune system of their insect vectors, several approaches are now emerging that aim to augment antiviral immunity in mosquitoes, including Wolbachia infection, transgenic mosquitoes, insect-specific viruses and paratransgenesis. In this Review, we discuss recent advances, current challenges and future prospects in exploiting both vertebrate and invertebrate immune systems for the control of flaviviral and alphaviral diseases.

Limitations in harnessing oral RNA interference as an antiviral strategy in Aedes aegypti

Mosquitoes, particularly Aedes aegypti, are critical vectors for globally significant pathogenic viruses. This study examines the limitations of oral RNA interference (RNAi) as a strategy to disrupt viral transmission by Ae. aegypti. We hypothesized that double-stranded RNA (dsRNA) targeting the Zika virus (ZIKV) or chikungunya virus (CHIKV) genomes produced by engineered bacterial symbionts could trigger an antiviral response. Mosquitoes mono-colonized with Escherichia coli producing dsZIK or dsCHIK did not display reduced viral titers following exposure to virus-contaminated bloodmeals and failed to generate dsZIK- or dsCHIK-derived small interfering RNAs. To address potential limitations of bacterial dsRNA release, we explored dsRNA inoculation via feeding and injection. Although viral replication was impeded in mosquitoes injected with dsZIK or dsCHIK, no antiviral effect was observed in dsRNA-fed mosquitoes. These findings highlight complexities of implementing oral RNAi as an antiviral strategy in Ae. aegypti and warrant further exploration of local and systemic RNAi mechanisms.

Advances and challenges in synthetic biology for mosquito control

Mosquito-borne illnesses represent a significant global health peril, resulting in approximately one million fatalities annually. West Nile, dengue, Zika, and malaria are continuously expanding their global reach, driven by factors that escalate mosquito populations and pathogen transmission. Innovative control measures are imperative to combat these catastrophic ailments. Conventional approaches, such as eliminating breeding sites and using insecticides, have been helpful, but they face challenges such as insecticide resistance and environmental harm. Given the mounting severity of mosquito-borne diseases, there is promise in exploring innovative approaches using synthetic biology to bolster mosquitoes' resistance to pathogens, or even eliminate the mosquito vectors, as a means of control. This review outlines current strategies, future goals, and the importance of gene editing for global health defenses against mosquito-borne diseases.

Maxizyme-mediated suppression of chikungunya virus replication and transmission in transgenic Aedes aegypti mosquitoes

Chikungunya virus (CHIKV) is an emerging mosquito-borne pathogen of significant public health importance. There are currently no prophylactic vaccines or therapeutics available to control CHIKV. One approach to arbovirus control that has been proposed is the replacement of transmission-competent mosquitoes with those that are refractory to virus infection. Several transgene effectors are being examined as potentially useful for this population replacement approach. We previously demonstrated the successful use of hammerhead ribozymes (hRzs) as an antiviral effector transgene to control CHIKV infection of, and transmission by, Aedes mosquitoes. In this report we examine a maxizyme approach to enhance the catalytic activity and prevent virus mutants from escaping these ribozymes. We designed a maxizyme containing minimized (monomer) versions of two hRzs we previously demonstrated to be the most effective in CHIKV suppression. Three versions of CHIKV maxizyme were designed: Active (Mz), inactive (ΔMz), and a connected CHIKV maxizyme (cMz). The maxizymes with their expression units (Ae-tRNA val promoter and its termination signal) were incorporated into lentivirus vectors with selection and visualization markers. Following transformation, selection, and single-cell sorting of Vero cells, clonal cell populations were infected with CHIKV at 0.05 and 0.5 MOI, and virus suppression was assessed using TCID50-IFA, RT-qPCR, and caspase-3 assays. Five transgenic mosquito lines expressing cMz were generated and transgene insertion sites were confirmed by splinkerette PCR. Our results demonstrate that Vero cell clones expressing Mz exhibited complete inhibition of CHIKV replication compared to their respective inactive control version or the two parent hRzs. Upon oral challenge of transgenic mosquitoes with CHIKV, three out of the five lines were completely refractory to CHIKV infection, and all five lines tested negative for salivary transmission. Altogether, this study demonstrates that maxizymes can provide a higher catalytic activity and viral suppression than hRzs.

Manipulating the Destiny of Wild Populations Using CRISPR

Genetic biocontrol aims to suppress or modify populations of species to protect public health, agriculture, and biodiversity. Advancements in genome engineering technologies have fueled a surge in research in this field, with one gene editing technology, CRISPR, leading the charge. This review focuses on the current state of CRISPR technologies for genetic biocontrol of pests and highlights the progress and ongoing challenges of using these approaches.

Mosquito Transposon-Mediated Transgenesis

Transposon-mediated transgenesis of mosquito vectors of disease pathogens followed the early success of transgenesis in the vinegar fly, Drosophila melanogaster The P transposable element used in Drosophila does not function canonically in mosquitoes, and repeatable, routine transgenesis in mosquitoes was not accomplished until new transposable elements were discovered and validated. A number of distinct transposons were subsequently identified that mediate the introduction of exogenous DNA in a stable and heritable manner in mosquito species, including members of the genera Aedes, Anopheles, and Culex The most versatile element, piggyBac, is functional in all of these mosquito genera, as well as in many other insects in diverse orders, and has been used extensively outside the class. Transposon-mediated transgenesis of recessive and dominant marker genes and reporter systems has been used to define functional fragments of gene control sequences, introduce exogenous DNA encoding products beneficial to medical interests, and act as "enhancer traps" to identify endogenous genes with specific expression characteristics.

Mimicking superinfection exclusion disrupts alphavirus infection and transmission in the yellow fever mosquito Aedes aegypti

Multiple viruses, including pathogenic viruses, bacteriophages, and even plant viruses, cause a phenomenon termed superinfection exclusion whereby a currently infected cell is resistant to secondary infection by the same or a closely related virus. In alphaviruses, this process is thought to be mediated, at least in part, by the viral protease (nsP2) which is responsible for processing the nonstructural polyproteins (P123 and P1234) into individual proteins (nsP1-nsP4), forming the viral replication complex. Taking a synthetic biology approach, we mimicked this naturally occurring phenomenon by generating a superinfection exclusion-like state in Aedes aegypti mosquitoes, rendering them refractory to alphavirus infection. By artificially expressing Sindbis virus (SINV) and chikungunya virus (CHIKV) nsP2 in mosquito cells and transgenic mosquitoes, we demonstrated a reduction in both SINV and CHIKV viral replication rates in cells following viral infection as well as reduced infection prevalence, viral titers, and transmission potential in mosquitoes.

Jamestown Canyon virus is transmissible by Aedes aegypti and is only moderately blocked by Wolbachia co-infection

Jamestown Canyon virus (JCV), a negative-sense arbovirus, is increasingly common in the upper Midwest of the USA. Transmitted by a range of mosquito genera, JCV's primary amplifying host is white-tailed deer. Aedes aegypti is responsible for transmitting various positive-sense viruses globally including dengue (DENV), Zika, chikungunya, and Yellow Fever. Ae. aegypti's distribution, once confined to the tropics, is expanding, in part due to climate change. Wolbachia, an insect endosymbiont, limits the replication of co-infecting viruses inside insects. The release and spread of the symbiont into Ae. aegypti populations have been effective in reducing transmission of DENV to humans, although the mechanism of Wolbachia-mediated viral blocking is still poorly understood. Here we explored JCV infection potential in Ae. aegypti, the nature of the vector's immune response, and interactions with Wolbachia infection. We show that Ae. aegypti is highly competent for JCV, which grows to high loads and rapidly reaches the saliva after an infectious blood meal. The mosquito immune system responds with strong induction of RNAi and JAK/STAT. Neither the direct effect of viral infection nor the energetic investment in immunity appears to affect mosquito longevity. Wolbachia infection blocked JCV only in the early stages of infection. Wolbachia-induced immunity was small compared to that of JCV, suggesting innate immune priming does not likely explain blocking. We propose two models to explain why Wolbachia's blocking of negative-sense viruses like JCV may be less than that of positive-sense viruses, relating to the slowdown of host protein synthesis and the triggering of interferon-like factors like Vago. In conclusion, we highlight the risk for increased human disease with the predicted future overlap of Ae. aegypti and JCV ranges. We suggest that with moderate Wolbachia-mediated blocking and distinct biology, negative-sense viruses represent a fruitful comparator model to other viruses for understanding blocking mechanisms in mosquitoes.

Large-scale reference-free analysis of flavivirus sequences in Aedes aegypti whole genome DNA sequencing data

Flaviviruses are a diverse group of RNA viruses, which include the etiological agents of Zika, dengue and yellow fever that are transmitted by mosquitoes. Flaviviruses do not encode reverse transcriptase and cannot reverse transcribe into DNA, yet DNA sequences of flaviviruses are found both integrated in the chromosomes of Aedes aegypti mosquitoes and as extrachromosomal sequences. We have previously examined the Ae. aegypti reference genome to identify flavivirus integrations and analyzed conservation of these sequences among whole-genome data of 464 Ae. aegypti collected across 10 countries globally. Here, we extended this analysis by identifying flavivirus sequences in these samples independently of the Ae. aegypti reference assembly. Our aim was to identify the complete set of viral sequences, including those absent in the reference genome, and their geographical distribution. We compared the identified sequences using BLASTn and applied machine learning methods to identify clusters of similar sequences. Apart from clusters of sequences that correspond to the four viral integration events that we had previously described, we identified 19 smaller clusters. The only cluster with a strong geographic association consisted of Cell-fusing agent virus-like sequences specific to Thailand. The remaining clusters did not have a geographic association and mostly consisted of near identical short sequences without strong similarity to any known flaviviral genomes. The short read sequencing data did not permit us to determine whether identified sequences were extrachromosomal or integrated into Ae. aegypti chromosomes. Our results suggest that Liverpool strain and field Ae. aegypti mosquitoes have a similar variety of conserved flaviviral DNA, whose functional role should be investigated in follow-up studies.

Transgene-induced cell death following dengue-2 virus infection in Aedes aegypti

Dengue viruses (DENVs) are mosquito-borne flaviviruses causing millions of human infections each year and pose a challenge for public health systems worldwide. Aedes aegypti is the principal vector species transmitting DENVs to humans. Controlling Ae. aegypti is difficult due to the abundance of breeding sites and increasing insecticide resistance in the vector populations. Developing new vector control strategies is critical for decreasing the disease burden. One potential approach is genetically replacing Ae. aegypti populations with vector populations highly resistant to DENV transmission. Here, we focus on an alternative strategy for generating dengue 2 virus (DENV-2) resistance in genetically-modified Ae. aegypti in which the mosquitoes express an inactive form of Michelob_x (Mx), an antagonist of the Inhibitor of Apoptosis (IAP), to induce apoptosis in those cells in which actively replicating DENV-2 is present. The inactive form of Mx was flanked by the RRRRSAG cleavage motif, which was recognized by the NS2B/NS3 protease of the infecting DENV-2 thereby releasing and activating Mx which then induced apoptosis. Our transgenic strain exhibited a significantly higher mortality rate than the non-transgenic control when infected with DENV-2. We also transfected a DNA construct containing inactive Mx fused to eGFP into C6/36 mosquito cells and indirectly observed Mx activation on days 3 and 6 post-DENV-2 infections. There were clear signs that the viral NS2B/NS3 protease cleaved the transgene, thereby releasing Mx protein into the cytoplasm, as was confirmed by the detection of eGFP expression in infected cells. The present study represents proof of the concept that virus infection can be used to induce apoptosis in infected mosquito cells.

A Brief History of the Discovery of RNA-Mediated Antiviral Immune Defenses in Vector Mosquitos

Arthropod-borne viruses (arboviruses) persist in a natural cycle that includes infections of humans or other vertebrates and transmission between vertebrates by infected arthropods, most commonly mosquitos. Arboviruses can cause serious, sometimes fatal diseases in humans and other vertebrates but cause little pathology in their mosquito vectors. Knowledge of the interactions between mosquito vectors and the arboviruses that they transmit is an important facet of developing schemes to control transmission. Mosquito innate immune responses to virus infection modulate virus replication in the vector, and understanding the components and mechanisms of the immune response could lead to improved methods for interrupting the transmission cycle. The most important aspect of mosquito antiviral defense is the exogenous small interfering RNA (exo-siRNA) pathway, one arm of the RNA interference (RNAi) silencing response. Our research as well as that of many other groups over the past 25 years to define this pathway are reviewed here. A more recently recognized but less well-understood RNA-mediated mosquito defense against arbovirus infections, the PIWI-interacting RNA (piRNA) pathway, is also described.

Factors Affecting Arbovirus Midgut Escape in Mosquitoes

Arboviral diseases spread by mosquitoes cause significant morbidity and mortality throughout much of the world. The treatment and prevention of these diseases through medication and vaccination is often limited, which makes controlling arboviruses at the level of the vector ideal. One way to prevent the spread of an arbovirus would be to stop its vector from developing a disseminated infection, which is required for the virus to make its way to the saliva of the mosquito to be potentially transmitted to a new host. The midgut of the mosquito provides one such opportunity to stop an arbovirus in its tracks. It has been known for many years that in certain arbovirus-vector combinations, or under certain circumstances, an arbovirus can infect and replicate in the midgut but is unable to escape from the tissue to cause disseminated infection. This situation is known as a midgut escape barrier. If we better understand why this barrier occurs, it might aid in the development of more informed control strategies. In this review, we discuss how the midgut escape barrier contributes to virus-vector specificity and possible mechanisms that may allow this barrier to be overcome in successful virus-vector combinations. We also discuss several of the known factors that either increase or decrease the likelihood of midgut escape.

Assessing single-locus CRISPR/Cas9-based gene drive variants in the mosquito Aedes aegypti via single-generation crosses and modeling

The yellow fever mosquito Aedes aegypti is a major vector of arthropod-borne viruses, including dengue, chikungunya, and Zika viruses. A novel approach to mitigate arboviral infections is to generate mosquitoes refractory to infection by overexpressing antiviral effector molecules. Such an approach requires a mechanism to spread these antiviral effectors through a population, for example, by using CRISPR/Cas9-based gene drive systems. Critical to the design of a single-locus autonomous gene drive is that the selected genomic locus is amenable to both gene drive and appropriate expression of the antiviral effector. In our study, we used reverse engineering to target 2 intergenic genomic loci, which had previously shown to be highly permissive for antiviral effector gene expression, and we further investigated the use of 3 promoters (nanos, β2-tubulin, or zpg) for Cas9 expression. We then quantified the accrual of insertions or deletions (indels) after single-generation crossings, measured maternal effects, and assessed fitness costs associated with various transgenic lines to model the rate of gene drive fixation. Overall, MGDrivE modeling suggested that when an autonomous gene drive is placed into an intergenic locus, the gene drive system will eventually be blocked by the accrual of gene drive blocking resistance alleles and ultimately be lost in the population. Moreover, while genomic locus and promoter selection were critically important for the initial establishment of the autonomous gene drive, it was the fitness of the gene drive line that most strongly influenced the persistence of the gene drive in the simulated population. As such, we propose that when autonomous CRISPR/Cas9-based gene drive systems are anchored in an intergenic locus, they temporarily result in a strong population replacement effect, but as gene drive-blocking indels accrue, the gene drive becomes exhausted due to the fixation of CRISPR resistance alleles.

A perspective on the expansion of the genetic technologies to support the control of neglected vector-borne diseases and conservation

Genetic-based technologies are emerging as promising tools to support vector population control. Vectors of human malaria and dengue have been the main focus of these development efforts, but in recent years these technologies have become more flexible and adaptable and may therefore have more wide-ranging applications. Culex quinquefasciatus, for example, is the primary vector of avian malaria in Hawaii and other tropical islands. Avian malaria has led to the extinction of numerous native bird species and many native bird species continue to be threatened as climate change is expanding the range of this mosquito. Genetic-based technologies would be ideal to support avian malaria control as they would offer alternatives to interventions that are difficult to implement in natural areas, such as larval source reduction, and limit the need for chemical insecticides, which can harm beneficial species in these natural areas. This mosquito is also an important vector of human diseases, such as West Nile and Saint Louis encephalitis viruses, so genetic-based control efforts for this species could also have a direct impact on human health. This commentary will discuss the current state of development and future needs for genetic-based technologies in lesser studied, but important disease vectors, such as C. quinquefasciatus, and make comparisons to technologies available in more studied vectors. While most current genetic control focuses on human disease, we will address the impact that these technologies could have on both disease and conservation focused vector control efforts and what is needed to prepare these technologies for evaluation in the field. The versatility of genetic-based technologies may result in the development of many important tools to control a variety of vectors that impact human, animal, and ecosystem health.

The impact of transgenesis on mosquito fitness: A review

Transgenic mosquitoes developed by genetic manipulation, offer a promising strategy for the sustainable and effective control of mosquito-borne diseases. This strategy relies on the mass release of transgenic mosquitoes into the wild, where their transgene is expected to persist in the natural environment, either permanently or transiently, within the mosquito population. In such circumstances, the fitness of transgenic mosquitoes is an important factor in determining their survival in the wild. The impact of transgene expression, insertional mutagenesis, inbreeding depression related to laboratory adaptation, and the hitchhiking effect involved in developing homozygous mosquito lines can all have an effect on the fitness of transgenic mosquitoes. Therefore, real-time estimation of transgene-associated fitness cost is imperative for modeling and planning transgenic mosquito release programs. This can be achieved by directly comparing fitness parameters in individuals homozygous or hemizygous for the transgene and their wild-type counterparts, or by cage invasion experiments to monitor the frequency of the transgenic allele over multiple generations. Recent advancements such as site-specific integration systems and gene drives, provide platforms to address fitness issues in transgenic mosquitoes. More research on the fitness of transgenic individuals is required to develop transgenic mosquitoes with a low fitness cost.

Intron-derived small RNAs for silencing viral RNAs in mosquito cells

Aedes aegypti and Ae. albopictus are the main vectors of mosquito-borne viruses of medical and veterinary significance. Many of these viruses have RNA genomes. Exogenously provided, e.g. transgene encoded, small RNAs could be used to inhibit virus replication, breaking the transmission cycle. We tested, in Ae. aegypti and Ae. albopictus cell lines, reporter-based strategies for assessing the ability of two types of small RNAs to inhibit a chikungunya virus (CHIKV) derived target. Both types of small RNAs use a Drosophila melanogaster pre-miRNA-1 based hairpin for their expression, either with perfect base-pairing in the stem region (shRNA-like) or containing two mismatches (miRNA-like). The pre-miRNA-1 stem loop structure was encoded within an intron; this allows co-expression of one or more proteins, e.g. a fluorescent protein marker tracking the temporal and spatial expression of the small RNAs in vivo. Three reporter-based systems were used to assess the relative silencing efficiency of ten shRNA-like siRNAs and corresponding miRNA-like designs. Two systems used a luciferase reporter RNA with CHIKV RNA inserted either in the coding sequence or within the 3’ UTR. A third reporter used a CHIKV derived split replication system. All three reporters demonstrated that while silencing could be achieved with both miRNA-like and shRNA-like designs, the latter were substantially more effective. Dcr-2 was required for the shRNA-like siRNAs as demonstrated by loss of inhibition of the reporters in Dcr-2 deficient cell lines. These positive results in cell culture are encouraging for the potential use of this pre-miRNA-1-based system in transgenic mosquitoes.

Mosquitoes are important globally, spreading viral diseases worldwide. Chikungunya virus causes epidemics of disease in people. Here we have investigated using two types of small RNAs and pathways inherent in Aedes aegypti mosquitoes to target a piece of the chikungunya virus’s genome, potentially preventing viral replication. We express these small RNAs using a pre-miRNA-1 based system, inserted into the intron within a commonly used promoter. We have used reporter systems in cell lines which can give preliminary indications of how these systems might work in mosquitoes. Our results indicate that short-hairpin-like designs are more effective than micro-RNA-like designs at knocking down expression of their targets. This knock-down requires Dcr-2 indicating that the short-hairpin-like RNAs are likely using the endo-siRNA pathway to degrade mRNA which contains their complementary RNA.

Antiviral RNAi Mechanisms to Arboviruses in Mosquitoes: microRNA Profile of Aedes aegypti and Culex quinquefasciatus from Grenada, West Indies

Mosquito-borne arboviruses, such as dengue virus, West Nile virus, Zika virus and yellow fever virus, impose a tremendous cost on the health of populations around the world. As a result, much effort has gone into the study of the impact of these viruses on human infections. Comparatively less effort, however, has been made to study the way these viruses interact with mosquitoes themselves. As ingested arboviruses infect their midgut and subsequently other tissue, the mosquito mounts a multifaceted innate immune response. RNA interference, the central intracellular antiviral defense mechanism in mosquitoes and other invertebrates can be induced and modulated through outside triggers (small RNAs) and treatments (transgenesis or viral-vector delivery). Accordingly, modulation of this facet of the mosquito’s immune system would thereby suggest a practical strategy for vector control. However, this requires a detailed understanding of mosquitoes’ endogenous small RNAs and their effects on the mosquito and viral proliferation. This paper provides an up-to-date overview of the mosquito’s immune system along with novel data describing miRNA profiles for Aedes aegypti and Culex quinquefasiatus in Grenada, West Indies.

The Aedes aegypti siRNA pathway mediates broad-spectrum defense against human pathogenic viruses and modulates antibacterial and antifungal defenses

The mosquito’s innate immune system defends against a variety of pathogens, and the conserved siRNA pathway plays a central role in the control of viral infections. Here, we show that transgenic overexpression of Dicer2 (Dcr2) or R2d2 resulted in an accumulation of 21-nucleotide viral sequences that was accompanied by a significant suppression of dengue virus (DENV), Zika virus (ZIKV), and chikungunya virus (CHIKV) replication, thus indicating the broad-spectrum antiviral response mediated by the siRNA pathway that can be applied for the development of novel arbovirus control strategies. Interestingly, overexpression of Dcr2 or R2d2 regulated the mRNA abundance of a variety of antimicrobial immune genes, pointing to additional functions of DCR2 and R2D2 as well as cross-talk between the siRNA pathway and other immune pathways. Accordingly, transgenic overexpression of Dcr2 or R2d2 resulted in a lesser proliferation of the midgut microbiota and increased resistance to bacterial and fungal infections.

This study shows that transgenic overexpression of siRNA pathway factors in mosquitoes mediates a broad-spectrum antiviral action against human pathogenic viruses such as dengue, Zika and Chikungunya virus, with implications for novel arbovirus control strategies; the siRNA pathway also regulates antimicrobial immune responses against bacterial and fungal infections.

Vector-Borne Viral Diseases as a Current Threat for Human and Animal Health—One Health Perspective

Over the last decades, an increase in the emergence or re-emergence of arthropod-borne viruses has been observed in many regions. Viruses such as dengue, yellow fever, or zika are a threat for millions of people on different continents. On the other hand, some arboviruses are still described as endemic, however, they could become more important in the near future. Additionally, there is a group of arboviruses that, although important for animal breeding, are not a direct threat for human health. Those include, e.g., Schmallenberg, bluetongue, or African swine fever viruses. This review focuses on arboviruses and their major vectors: mosquitoes, ticks, biting midges, and sandflies. We discuss the current knowledge on arbovirus transmission, ecology, and methods of prevention. As arboviruses are a challenge to both human and animal health, successful prevention and control are therefore only possible through a One Health perspective.

Generation of Transgenic Mosquitoes Harboring a Replication-Restricted Virus

Live microbe vaccines are designed to elicit strong cellular and antibody responses without developing the symptoms of the disease, and these are effective in preventing infectious diseases. A flying vaccinator (also known as a flying syringe) is a conceptual, genetically engineered hematophagous insect that is used to deliver vaccines such as an antigen from a parasite produced in mosquito saliva; bites from such insects may elicit antibody production by immunizing the host with an antigen through blood-feeding. In addition to a simple vaccine antigen, a flying vaccinator may potentially load a live attenuated microbe with an appropriate mechanism for sustaining its constitutive proliferation in the insect. In this study, a recombinant vesicular stomatitis virus (VSV) lacking the glycoprotein gene (VSV-G) was used to produce replication-restricted VSV (rrVSV) containing GFP. Transgenic Anopheles stephensi mosquitoes, in which the salivary glands expressed a VSV-G gene driven by an aapp salivary gland-specific promoter, were generated and injected intraperitoneally with rrVSV. The injected rrVSV entered the cells of the salivary gland and stimulated endogenous production of progeny rrVSV particles, as seen in rrVSV-infected Drosophila melanogaster expressing VSV-G. These data suggested the possibility of developing a valuable tool for delivering genetically attenuated virus vaccines via mosquito saliva, although efficient replication-restricted virus production is required.

A flavivirus-inducible gene expression system that modulates broad-spectrum antiviral activity against dengue and Zika viruses

Incidence of dengue virus (DENV) and Zika virus (ZIKV), two mosquito-borne flaviviruses, is increasing in large parts of the world. Vaccination and medication for these diseases are unsatisfactory. Here, we developed a novel antiviral approach, using a virus-inducible gene expression system, to block virus replication and transmission. Constructs containing the smallest replication units of dengue virus serotype 2 (DENV2) with negative-stranded DENV2 artificial genomes and genes of interest were established in an Aedes aegypti cell line, resulting in expression of target genes after DENV2 infection. Green fluorescent protein (GFP) assays confirmed the system was virus-inducible. When we used one of two apoptosis-related genes, A. aegypti michelob_x (AaMx) and inhibitor of apoptosis (IAP)-antagonist michelob_x-like protein (AaIMP) instead of GFP, the production of viral RNA and proteins were inhibited for all five viruses tested (DENV1-4 and ZIKV), and effector caspase activity was induced. The system thus inhibited the production of infectious virus particles in vitro, and in mosquitoes it did so after DENV2 infection. This is a novel broad-spectrum antiviral approach using a flavivirus-inducible gene-expression system, which could lead to new avenues for mosquito-borne disease control.

Lab-scale characterization and semi-field trials of Wolbachia Strain wAlbB in a Taiwan Wolbachia introgressed Ae. aegypti strain

Dengue fever is one of the most severe viral diseases transmitted by Aedes mosquitoes, with traditional approaches of disease control proving insufficient to prevent significant disease burden. Release of Wolbachia-transinfected mosquitoes offers a promising alternative control methodologies; Wolbachia-transinfected female Aedes aegypti demonstrate reduced dengue virus transmission, whilst Wolbachia-transinfected males cause zygotic lethality when crossed with uninfected females, providing a method for suppressing mosquito populations. Although highly promising, the delicate nature of population control strategies and differences between local species populations means that controlled releases of Wolbachia-transinfected mosquitoes cannot be performed without extensive testing on specific local Ae. aegypti populations. In order to investigate the potential for using Wolbachia to suppress local Ae. aegypti populations in Taiwan, we performed lab-based and semi-field fitness trials. We first transinfected the Wolbachia strain wAlbB into a local Ae. aegypti population (wAlbB-Tw) and found no significant changes in lifespan, fecundity and fertility when compared to controls. In the laboratory, we found that as the proportion of released male mosquitoes carrying Wolbachia was increased, population suppression could reach up to 100%. Equivalent experiments in semi-field experiments found suppression rates of up to 70%. The release of different ratios of wAlbB-Tw males in the semi-field system provided an estimate of the optimal size of male releases. Our results indicate that wAlbB-Tw has significant potential for use in vector control strategies aimed at Ae. aegypti population suppression in Taiwan. Open field release trials are now necessary to confirm that wAlbB-Tw mediated suppression is feasible in natural environments.

Transgenic refractory Aedes aegypti lines are resistant to multiple serotypes of dengue virus

The areas where dengue virus (DENV) is endemic have expanded rapidly, driven in part by the global spread of Aedes species, which act as disease vectors. DENV replicates in the mosquito midgut and is disseminated to the mosquito’s salivary glands for amplification. Thus, blocking virus infection or replication in the tissues of the mosquito may be a viable strategy for reducing the incidence of DENV transmission to humans. Here we used the mariner Mos1 transposase to create an Aedes aegypti line that expresses virus-specific miRNA hairpins capable of blocking DENV replication. These microRNA are driven by the blood-meal-inducible carboxypeptidase A promoter or by the polyubiquitin promoter. The transgenic mosquitoes exhibited significantly lower infection rates and viral titers for most DENV serotypes 7 days after receiving an infectious blood meal. The treatment was also effective at day 14 post infection after a second blood meal had been administered. In viral transmission assay, we found there was significantly reduced transmission in these lines. These transgenic mosquitoes were effective in silencing most of the DENV genome; such an approach may be employed to control a dengue fever epidemic.

Revisiting dengue virus-mosquito interactions: molecular insights into viral fitness

Dengue virus (DENV), like other viruses, closely interacts with the host cell machinery to complete its life cycle. Over the course of infection, DENV interacts with several host factors with pro-viral activities to support its infection. Meanwhile, it has to evade or counteract host factors with anti-viral activities which inhibit its infection. These molecular virus-host interactions play a crucial role in determining the success of DENV infection. Deciphering such interactions is thus paramount to understanding viral fitness in its natural hosts. While DENV-mammalian host interactions have been extensively studied, not much has been done to characterize DENV-mosquito host interactions despite its importance in controlling DENV transmission. Here, to provide a snapshot of our current understanding of DENV-mosquito interactions, we review the literature that identified host factors and cellular processes related to DENV infection in its mosquito vectors, Aedes aegypti and Aedes albopictus, with a particular focus on DENV-mosquito omics studies. This knowledge provides fundamental insights into the DENV life cycle, and could contribute to the development of novel antiviral strategies.

Small RNA Response to Infection of the Insect-Specific Lammi Virus and Hanko Virus in an Aedes albopictus Cell Line

RNA interference (RNAi)-mediated antiviral immunity is believed to be the primary defense against viral infection in mosquitoes. The production of virus-specific small RNA has been demonstrated in mosquitoes and mosquito-derived cell lines for viruses in all of the major arbovirus families. However, many if not all mosquitoes are infected with a group of viruses known as insect-specific viruses (ISVs), and little is known about the mosquito immune response to this group of viruses. Therefore, in this study, we sequenced small RNA from an Aedes albopictus-derived cell line infected with either Lammi virus (LamV) or Hanko virus (HakV). These viruses belong to two distinct phylogenetic groups of insect-specific flaviviruses (ISFVs). The results revealed that both viruses elicited a strong virus-derived small interfering RNA (vsiRNA) response that increased over time and that targeted the whole viral genome, with a few predominant hotspots observed. Furthermore, only the LamV-infected cells produced virus-derived Piwi-like RNAs (vpiRNAs); however, they were mainly derived from the antisense genome and did not show the typical ping-pong signatures. HakV, which is more distantly related to the dual-host flaviviruses than LamV, may lack certain unknown sequence elements or structures required for vpiRNA production. Our findings increase the understanding of mosquito innate immunity and ISFVs' effects on their host.

Pleiotropic Odorant-Binding Proteins Promote Aedes aegypti Reproduction and Flavivirus Transmission

Insect odorant-binding proteins (OBPs) are small soluble proteins that have been assigned roles in olfaction, but their other potential functions have not been extensively explored. Using CRISPR/Cas9-mediated disruption of Aedes aegyptiObp10 and Obp22, we demonstrate the pleiotropic contribution of these proteins to multiple processes that are essential for vectorial capacity. Mutant mosquitoes have impaired host-seeking and oviposition behavior, reproduction, and arbovirus transmission. Here, we show that Obp22 is linked to the male-determining sex locus (M) on chromosome 1 and is involved in male reproduction, likely by mediating the development of spermatozoa. Although OBP10 and OBP22 are not involved in flavivirus replication, abolition of these proteins significantly reduces transmission of dengue and Zika viruses through a mechanism affecting secretion of viral particles into the saliva. These results extend our current understanding of the role of insect OBPs in insect reproduction and transmission of human pathogens, making them essential determinants of vectorial capacity.

Susceptibility and interactions between Aedes mosquitoes and Zika viruses

Zika virus disease is caused by Zika virus infection, as transmitted by Aedes spp. mosquitoes. Many of the Zika virus strains isolated from patients display different pathogenicities toward humans. The vector mosquitoes for Zika virus are mainly of the Aedes genus, especially Aedes aegypti and Aedes albopictus. However, susceptibility and interactions between Aedes spp. mosquitoes and Zika viruses remain unclear. In this study, we chose two Zika virus strains (FSS13025 and PRVABC59) with different abilities to infect the primary vector mosquitoes Ae. aegypti and Ae. albopictus. The transcriptomes and small RNA profiles of infected and uninfected mosquitoes were comparatively analyzed, and differentially expressed genes were functionally examined using RNA interference. According to the results, the susceptibility of PRVABC59 was higher than that of FSS13025 in Aedes vector mosquitoes, and Ae. aegypti was more susceptible to Zika virus than was Ae. albopictus. For PRVABC59 infection, specific differential expression profiles correlated with Ae. aegypti and Ae. albopictus, and susceptibility was significantly affected when three targeted genes were successfully knocked down. Compared with PRVABC59, infection of Ae. albopictus with FSS13025 generated more 21-nt virus small interference RNA. It can be concluded that the susceptibility of vector Aedes spp. mosquitoes to Zika viruses varies and that the interactions between mosquitoes and Zika virus correlate with susceptibility.

Current Effector and Gene-Drive Developments to Engineer Arbovirus-Resistant Aedes aegypti (Diptera: Culicidae) for a Sustainable Population Replacement Strategy in the Field

Arthropod-borne viruses (arboviruses) such as dengue, Zika, and chikungunya viruses cause morbidity and mortality among human populations living in the tropical regions of the world. Conventional mosquito control efforts based on insecticide treatments and/or the use of bednets and window curtains are currently insufficient to reduce arbovirus prevalence in affected regions. Novel, genetic strategies that are being developed involve the genetic manipulation of mosquitoes for population reduction and population replacement purposes. Population replacement aims at replacing arbovirus-susceptible wild-type mosquitoes in a target region with those that carry a laboratory-engineered antiviral effector to interrupt arboviral transmission in the field. The strategy has been primarily developed for Aedes aegypti (L.), the most important urban arbovirus vector. Antiviral effectors based on long dsRNAs, miRNAs, or ribozymes destroy viral RNA genomes and need to be linked to a robust gene drive to ensure their fixation in the target population. Synthetic gene-drive concepts are based on toxin/antidote, genetic incompatibility, and selfish genetic element principles. The CRISPR/Cas9 gene editing system can be configurated as a homing endonuclease gene (HEG) and HEG-based drives became the preferred choice for mosquitoes. HEGs are highly allele and nucleotide sequence-specific and therefore sensitive to single-nucleotide polymorphisms/resistant allele formation. Current research efforts test new HEG-based gene-drive designs that promise to be less sensitive to resistant allele formation. Safety aspects in conjunction with gene drives are being addressed by developing procedures that would allow a recall or overwriting of gene-drive transgenes once they have been released.

Novel synthetic 3'-untranslated regions for controlling transgene expression in transgenic Aedes aegypti mosquitoes

Transgenic technology for mosquitoes is now more than two decades old, and a wide array of control sequences have been described for regulating gene expression in various life stages or specific tissues. Despite this, comparatively little attention has been paid to the development and validation of other transgene-regulating elements, especially 3'-untranslated regions (3'UTRs). As a consequence, the same regulatory sequences are often used multiple times in a single transgene array, potentially leading to instability of transgenic effector genes. To increase the repertoire of characterized 3'UTRs available for genetics-based mosquito control, we generated fifteen synthetic sequences based on the base composition of the widely used SV40 3'UTR sequence, and tested their ability to contribute to the expression of reporter genes EGFP or luciferase. Transient transfection in mosquito cells identified nine candidate 3'UTRs that conferred moderate to strong gene expression. Two of these were engineered into the mosquito genome through CRISPR/Cas9-mediated site-specific insertion and compared to the original SV40 3'UTR. Both synthetic 3'UTRs were shown to successfully promote transgene expression in all mosquito life stages (larva, pupa and adults), similar to the SV40 3'UTR, albeit with differences in intensity. Thus, the synthetic 3'UTR elements described here are suitable for regulating transgene expression in Ae. aegypti, and provide valuable alternatives in the design of multi-gene cassettes. Additionally, the synthetic-scramble approach we validate here could be used to generate additional functional 3'UTR elements in this or other organisms.

Prospects and Pitfalls: Next-Generation Tools to Control Mosquito-Transmitted Disease

Mosquito-transmitted diseases, including malaria and dengue, are a major threat to human health around the globe, affecting millions each year. A diverse array of next-generation tools has been designed to eliminate mosquito populations or to replace them with mosquitoes that are less capable of transmitting key pathogens. Many of these new approaches have been built on recent advances in CRISPR/Cas9-based genome editing. These initiatives have driven the development of pathogen-resistant lines, new genetics-based sexing methods, and new methods of driving desirable genetic traits into mosquito populations. Many other emerging tools involve microorganisms, including two strategies involving Wolbachia that are achieving great success in the field. At the same time, other mosquito-associated bacteria, fungi, and even viruses represent untapped sources of new mosquitocidal or antipathogen compounds. Although there are still hurdles to be overcome, the prospect that such approaches will reduce the impact of these diseases is highly encouraging.

RNAi by Soaking Aedes aegypti Pupae in dsRNA

Simple Summary

Even after decades of control interventions, mosquito-borne diseases still pose a huge threat to humans. Understanding gene functions is important for discovering new targets for mosquito and mosquito-borne disease control. One of the well-established and widely used methods for understanding gene function is RNA-interference (RNAi). The use of RNAi is, however, restricted mostly to adult mosquitoes. A few studies have shown its applicability in mosquito larvae, and just one in pupae. The current double-stranded RNA (dsRNA) delivery methods for RNAi are labor-intensive and require microinjections into mosquitoes (adults, larvae, or pupae). In this study, we present a simple, fast, and less labor-intensive technique for RNAi in the pupal stage by soaking pupae in water containing dsRNA. This method will be useful in studying genes expressed in immature life stages of the mosquitoes and will hopefully open new avenues for identifying mosquito control targets in early life stages.

Abstract

RNA-interference (RNAi) is a standard technique for functional genomics in adult mosquitoes. However, RNAi in immature, aquatic mosquito stages has been challenging. Several studies have shown successful larval RNAi, usually in combination with a carrier molecule. Except for one study in malaria mosquito, Anopheles gambiae, none of the previous studies has explored RNAi in mosquito pupae. Even in the study that used RNAi in pupae, double stranded RNA (dsRNA) was introduced by microinjection. Here, we describe a successful method by soaking pupae in water containing dsRNA without any carrier or osmotic challenge. The knockdown persisted into adulthood. We expect that this simple procedure will be useful in the functional analysis of genes that highly express in pupae or newly emerged adults.

Infection of Aedes aegypti Mosquitoes with Midgut-Attenuated Sindbis Virus Reduces, but Does Not Eliminate, Disseminated Infection

Arboviruses are transmitted by specific vectors, and the reasons for this specificity are not fully understood. One contributing factor is the existence of tissue barriers within the vector such as the midgut escape barrier. We used microRNA (miRNA) targeting of Sindbis virus (SINV) to study how replication in midgut cells contributes to overcoming this barrier in the mosquito Aedes aegypti. SINV constructs were designed to be attenuated specifically in midgut cells by inserting binding sites for midgut-specific miRNAs into either the 3' untranslated region (MRE3'miRT) or the structural open reading frame (MRE-ORFmiRT) of the SINV genome. Both miRNA-targeted viruses replicated less efficiently than control viruses in the presence of these miRNAs. When mosquitoes were given infectious blood meals containing miRNA-targeted viruses, only around 20% (MRE3'miRT) or 40% (MRE-ORFmiRT) of mosquitoes developed disseminated infection. In contrast, dissemination occurred in almost all mosquitoes fed control viruses. Deep sequencing of virus populations from individual mosquitoes ruled out selection for mutations in the inserted target sequences as the cause for dissemination in these mosquitoes. In mosquitoes that became infected with miRNA-targeted viruses, titers were equivalent to those of mosquitoes infected with control virus in both the midgut and the carcass, and there was no evidence of a threshold titer necessary for dissemination. Instead, it appeared that if infection was successfully established in the midgut, replication and dissemination were largely normal. Our results support the hypothesis that replication is an important factor in allowing SINV to overcome the midgut escape barrier but hint that other factors are also likely involved. IMPORTANCE When a mosquito ingests an arbovirus during a blood meal, the arbovirus must escape from the midgut of the vector and infect the salivary glands in order to be transmitted to a new host. We used tissue-specific miRNA targeting to examine the requirement for Sindbis virus (SINV) to replicate in midgut epithelium in order to cause disseminated infection in the mosquito Aedes aegypti. Our results indicate that specifically reducing the ability of SINV to replicate in the mosquito midgut reduces its overall ability to establish infection in the mosquito, but if infection is established, replication and dissemination occur normally. These results are consistent with an importance for replication in the midgut epithelium in aiding arboviruses in crossing the midgut barrier.

A Thioester-Containing Protein Controls Dengue Virus Infection in Aedes aegypti Through Modulating Immune Response

Complement-like proteins in arthropods defend against invading pathogens in the early phases of infection. Thioester-containing proteins (TEPs), which exhibit high similarity to mammalian complement C3, are thought to play a key role in the innate immunity of arthropods. We identified and characterized anti-dengue virus (DENV) host factors, in particular complement-like proteins, in the mosquito Aedes aegypti. Our results indicate that TEP1 limits DENV infection in Ae. aegypti. We showed that TEP1 transcription is highly induced in mosquitoes following DENV infection. Silencing TEP1 resulted in the up-regulation of viral RNA and proteins. In addition, the production of infectious virus particles increased in the absence of TEP1. We generated a transgenic mosquito line with a TEP1 loss-of-function phenotype under a blood meal-inducible promoter. We showed that viral protein and titers increased in transgenic mosquitoes after an infectious blood meal. Interestingly, expression of transcription factor Rel2 and certain anti-microbial peptides (AMPs) were inhibited in transgenic mosquitoes. Overall, our results suggest that TEP1 regulates the immune response and consequently controls the replication of dengue virus in mosquitoes. This finding provides new insight into the molecular mechanisms of mosquito host factors in the regulation of DENV replication.

Transgenic Expression of Human C-Type Lectin Protein CLEC18A Reduces Dengue Virus Type 2 Infectivity in Aedes aegypti

The C-type lectins, one family of lectins featuring carbohydrate binding domains which participate in a variety of bioprocesses in both humans and mosquitoes, including immune response, are known to target DENV. A human C-type lectin protein CLEC18A in particular shows extensive glycan binding abilities and correlates with type-I interferon expression, making CLEC18A a potential player in innate immune responses to DENV infection; this potential may provide additional regulatory point in improving mosquito immunity. Here, we established for the first time a transgenic Aedes aegypti line that expresses human CLEC18A. This expression enhanced the Toll immune pathway responses to DENV infection. Furthermore, viral genome and virus titers were reduced by 70% in the midgut of transgenic mosquitoes. We found significant changes in the composition of the midgut microbiome in CLEC18A expressing mosquitoes, which may result from the Toll pathway enhancement and contribute to DENV inhibition. Transgenic mosquito lines offer a compelling option for studying DENV pathogenesis, and our analyses indicate that modifying the mosquito immune system via expression of a human immune gene can significantly reduce DENV infection.

Transcriptome Analysis of Responses to Dengue Virus 2 Infection in Aedes albopictus (Skuse) C6/36 Cells

Dengue virus (DENV), a member of the Flavivirus genus of the Flaviviridae family, can cause dengue fever (DF) and more serious diseases and thus imposes a heavy burden worldwide. As the main vector of DENV, mosquitoes are a serious hazard. After infection, they induce a complex host–pathogen interaction mechanism. Our goal is to further study the interaction mechanism of viruses in homologous, sensitive, and repeatable C6/36 cell vectors. Transcriptome sequencing (RNA-Seq) technology was applied to the host transcript profiles of C6/36 cells infected with DENV2. Then, bioinformatics analysis was used to identify significant differentially expressed genes and the associated biological processes. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) was performed to verify the sequencing data. A total of 1239 DEGs were found by transcriptional analysis of Aedes albopictus C6/36 cells that were infected and uninfected with dengue virus, among which 1133 were upregulated and 106 were downregulated. Further bioinformatics analysis showed that the upregulated DEGs were significantly enriched in signaling pathways such as the MAPK, Hippo, FoxO, Wnt, mTOR, and Notch; metabolic pathways and cellular physiological processes such as autophagy, endocytosis, and apoptosis. Downregulated DEGs were mainly enriched in DNA replication, pyrimidine metabolism, and repair pathways, including BER, NER, and MMR. The qRT-PCR results showed that the concordance between the RNA-Seq and RT-qPCR data was very high (92.3%). The results of this study provide more information about DENV2 infection of C6/36 cells at the transcriptome level, laying a foundation for further research on mosquito vector–virus interactions. These data provide candidate antiviral genes that can be used for further functional verification in the future.

Ubiquitous and Tissue-specific RNA Targeting in Drosophila Melanogaster using CRISPR/CasRx

CasRx, a member of the RNA-targeting Cas13 family, is a promising new addition of the CRISPR/Cas technologies in efficient gene transcript reduction with an attractive off-target profile at both cellular and organismal levels. It is recently reported that the CRISPR/CasRx system can be used to achieve ubiquitous and tissue-specific gene transcript reduction in Drosophila melanogaster. This paper details the methods from the recent work, consisting of three parts: 1) ubiquitous in vivo endogenous RNA targeting using a two-component CasRx system; 2) ubiquitous in vivo exogenous RNA targeting using a three-component CasRx system; and 3) tissue-specific in vivo RNA targeting using a three-component CasRx system. The effects of RNA targeting observed include targeted gene specific phenotypic changes, targeted RNA transcript reduction, and occasional lethality phenotypes associated with high expression of CasRx protein and collateral activity. Overall, these results showed that the CasRx system is capable of target RNA transcript reduction at the organismal level in a programmable and efficient manner, demonstrating that in vivo transcriptome targeting, and engineering is feasible and lays the foundation for future in vivo CRISPR-based RNA targeting technologies.

Genetic transformation of mosquitoes by microparticle bombardment

Mosquitoes constitute the major living beings causing human deaths in the world. They are vectors of malaria, yellow fever, dengue, zika, filariases, chikungunya, among other diseases. New strategies to control/eradicate mosquito populations are based on newly developed genetic manipulation techniques. However, genetic transformation of mosquitoes is a major technical bottleneck due to low efficiency, the need of sophisticated equipment, and highly trained personnel. The present report shows the transgenerational genetic transformation of Aedes aegypti, using the particle inflow gun (PIG), by integrating the ecfp gene in the AAEL000582 mosquito gene with the CRISPR-Cas9 technique, achieving a mean efficiency of 44.5% of bombarded individuals (G0) that showed ECFP expression in their tissues, and a mean of 28.5% transformation efficiency measured on G1 individuals. The same transformation technique was used to integrate the egfp/scorpine genes cloned in the Minos transposon pMinHygeGFP into the Anopheles albimanus genome, achieving a mean efficiency of 43.25% of bombarded individuals (G0) that showed EGFP expression in their tissues. Once the technique was standardized, transformation of Ae. aegypti neonate larvae and An. albimanus eggs was achieved when exposed to gold microparticle bombardment. Integration of genes and heterologous protein expression were confirmed by PCR, sequencing, fluorescent microscopy, mass spectrometry, Western blot and dot blot analyses. Transgenerational inheritance of the transgenes was observed only on Ae. aegypti, as all transformed An. albimanus individuals died at the pupal stage of the G0 generation.

Antiviral Compounds for Blocking Arboviral Transmission in Mosquitoes

Mosquito-borne arthropod-borne viruses (arboviruses) such as the dengue virus (DENV), Zika virus (ZIKV), and chikungunya virus (CHIKV) are important human pathogens that are responsible for significant global morbidity and mortality. The recent emergence and re-emergence of mosquito-borne viral diseases (MBVDs) highlight the urgent need for safe and effective vaccines, therapeutics, and vector-control approaches to prevent MBVD outbreaks. In nature, arboviruses circulate between vertebrate hosts and arthropod vectors; therefore, disrupting the virus lifecycle in mosquitoes is a major approach for combating MBVDs. Several strategies were proposed to render mosquitoes that are refractory to arboviral infection, for example, those involving the generation of genetically modified mosquitoes or infection with the symbiotic bacterium Wolbachia. Due to the recent development of high-throughput screening methods, an increasing number of drugs with inhibitory effects on mosquito-borne arboviruses in mammalian cells were identified. These antivirals are useful resources that can impede the circulation of arboviruses between arthropods and humans by either rendering viruses more vulnerable in humans or suppressing viral infection by reducing the expression of host factors in mosquitoes. In this review, we summarize recent advances in small-molecule antiarboviral drugs in mammalian and mosquito cells, and discuss how to use these antivirals to block the transmission of MBVDs.

Next-generation tools to control biting midge populations and reduce pathogen transmission

Biting midges of the genus Culicoides transmit disease-causing agents resulting in a significant economic impact on livestock industries in many parts of the world. Localized control efforts, such as removal of larval habitat or pesticide application, can be logistically difficult, expensive and ineffective if not instituted and maintained properly. With these limitations, a population-level approach to the management of Culicoides midges should be investigated as a means to replace or supplement existing control strategies. Next-generation control methods such as Wolbachia- and genetic-based population suppression and replacement are being investigated in several vector species. Here we assess the feasibility and applicability of these approaches for use against biting midges. We also discuss the technical and logistical hurdles needing to be addressed for each method to be successful, as well as emphasize the importance of addressing community engagement and involving stakeholders in the investigation and development of these approaches.

Cryoprotectant toxicity and hypothermic sensitivity among Anopheles larvae

Laboratory rearing of mosquitoes is commonly practiced by researchers studying mosquito-borne infectious diseases and vector control methods. In the absence of cryopreservation methods to stabilize unique or genetically modified strains, mosquito lines must be continuously maintained, a laborious process that risks selection effects, contamination, and genetic drift. Towards the development of a cryopreservation protocol, several commonly used cryoprotectants were systematically characterized here both individually and as cocktails. Among first instar, feeding-stage An. gambiae and An. stephensi larvae, cryoprotectant toxicity followed the order of dimethyl sulfoxide > ethylene glycol > methanol. The resulting LD₅₀ values were used as the basis for the development of cryoprotectant cocktail solutions, where formulation optimization was streamlined using Taguchi methods of experimental design. Sensitivity to hypothermia was further evaluated to determine the feasibility of cryoprotectant loading at reduced temperatures and slow cooling approaches to cryopreservation. The information described here contributes to the knowledge base necessary to inform the development of a cryopreservation protocol for Anopheles larvae.

Split drive killer-rescue provides a novel threshold-dependent gene drive

A wide range of gene drive mechanisms have been proposed that are predicted to increase in frequency within a population even when they are deleterious to individuals carrying them. This also allows associated desirable genetic material ("cargo genes") to increase in frequency. Gene drives have garnered much attention for their potential use against a range of globally important problems including vector borne disease, crop pests and invasive species. Here we propose a novel gene drive mechanism that could be engineered using a combination of toxin-antidote and CRISPR components, each of which are already being developed for other purposes. Population genetics mathematical models are developed here to demonstrate the threshold-dependent nature of the proposed system and its robustness to imperfect homing, incomplete penetrance of toxins and transgene fitness costs, each of which are of practical significance given that real-world components inevitably have such imperfections. We show that although end-joining repair mechanisms may cause the system to break down, under certain conditions, it should persist over time scales relevant for genetic control programs. The potential of such a system to provide localised population suppression via sex ratio distortion or female-specific lethality is also explored. Additionally, we investigate the effect on introduction thresholds of adding an extra CRISPR base element, showing that this may either increase or decrease dependent on parameter context.

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.

Adequacy and sufficiency evaluation of existing EFSA guidelines for the molecular characterisation, environmental risk assessment and post-market environmental monitoring of genetically modified insects containing engineered gene drives

Advances in molecular and synthetic biology are enabling the engineering of gene drives in insects for disease vector/pest control. Engineered gene drives (that bias their own inheritance) can be designed either to suppress interbreeding target populations or modify them with a new genotype. Depending on the engineered gene drive system, theoretically, a genetic modification of interest could spread through target populations and persist indefinitely, or be restricted in its spread or persistence. While research on engineered gene drives and their applications in insects is advancing at a fast pace, it will take several years for technological developments to move to practical applications for deliberate release into the environment. Some gene drive modified insects (GDMIs) have been tested experimentally in the laboratory, but none has been assessed in small-scale confined field trials or in open release trials as yet. There is concern that the deliberate release of GDMIs in the environment may have possible irreversible and unintended consequences. As a proactive measure, the European Food Safety Authority (EFSA) has been requested by the European Commission to review whether its previously published guidelines for the risk assessment of genetically modified animals (EFSA, 2012 and 2013), including insects (GMIs), are adequate and sufficient for GDMIs, primarily disease vectors, agricultural pests and invasive species, for deliberate release into the environment. Under this mandate, EFSA was not requested to develop risk assessment guidelines for GDMIs. In this Scientific Opinion, the Panel on Genetically Modified Organisms (GMO) concludes that EFSA's guidelines are adequate, but insufficient for the molecular characterisation (MC), environmental risk assessment (ERA) and post-market environmental monitoring (PMEM) of GDMIs. While the MC,ERA and PMEM of GDMIs can build on the existing risk assessment framework for GMIs that do not contain engineered gene drives, there are specific areas where further guidance is needed for GDMIs.

The Antiviral Small-Interfering RNA Pathway Induces Zika Virus Resistance in Transgenic Aedes aegypti

The resurgence of arbovirus outbreaks across the globe, including the recent Zika virus (ZIKV) epidemic in 2015–2016, emphasizes the need for innovative vector control methods. In this study, we investigated ZIKV susceptibility to transgenic Aedes aegypti engineered to target the virus by means of the antiviral small-interfering RNA (siRNA) pathway. The robustness of antiviral effector expression in transgenic mosquitoes is strongly influenced by the genomic insertion locus and transgene copy number; we therefore used CRISPR/Cas9 to re-target a previously characterized locus (Chr2:321382225) and engineered mosquitoes expressing an inverted repeat (IR) dsRNA against the NS3/4A region of the ZIKV genome. Small RNA analysis revealed that the IR effector triggered the mosquito’s siRNA antiviral pathway in bloodfed females. Nearly complete (90%) inhibition of ZIKV replication was found in vivo in both midguts and carcasses at 7 or 14 days post-infection (dpi). Furthermore, significantly fewer transgenic mosquitoes contained ZIKV in their salivary glands (p = 0.001), which led to a reduction in the number of ZIKV-containing saliva samples as measured by transmission assay. Our work shows that Ae. aegypti innate immunity can be co-opted to engineer mosquitoes resistant to ZIKV.

The Lethal(2)-Essential-for-Life [L(2)EFL] Gene Family Modulates Dengue Virus Infection in Aedes aegypti

Efforts to determine the mosquito genes that affect dengue virus replication have identified a number of candidates that positively or negatively modify amplification in the invertebrate host. We used deep sequencing to compare the differential transcript abundances in Aedes aegypti 14 days post dengue infection to those of uninfected A. aegypti. The gene lethal(2)-essential-for-life [l(2)efl], which encodes a member of the heat shock 20 protein (HSP20) family, was upregulated following dengue virus type 2 (DENV-2) infection in vivo. The transcripts of this gene did not exhibit differential accumulation in mosquitoes exposed to insecticides or pollutants. The induction and overexpression of l(2)efl gene products using poly(I:C) resulted in decreased DENV-2 replication in the cell line. In contrast, the RNAi-mediated suppression of l(2)efl gene products resulted in enhanced DENV-2 replication, but this enhancement occurred only if multiple l(2)efl genes were suppressed. l(2)efl homologs induce the phosphorylation of eukaryotic initiation factor 2α (eIF2α) in the fruit fly Drosophila melanogaster, and we confirmed this finding in the cell line. However, the mechanism by which l(2)efl phosphorylates eIF2α remains unclear. We conclude that l(2)efl encodes a potential anti-dengue protein in the vector mosquito.

Diverse Defenses: A Perspective Comparing Dipteran Piwi-piRNA Pathways

Animals face the dual threat of virus infections hijacking cellular function and transposons proliferating in germline genomes. For insects, the deeply conserved RNA interference (RNAi) pathways and other chromatin regulators provide an important line of defense against both viruses and transposons. For example, this innate immune system displays adaptiveness to new invasions by generating cognate small RNAs for targeting gene silencing measures against the viral and genomic intruders. However, within the Dipteran clade of insects, Drosophilid fruit flies and Culicids mosquitoes have evolved several unique mechanistic aspects of their RNAi defenses to combat invading transposons and viruses, with the Piwi-piRNA arm of the RNAi pathways showing the greatest degree of novel evolution. Whereas central features of Piwi-piRNA pathways are conserved between Drosophilids and Culicids, multiple lineage-specific innovations have arisen that may reflect distinct genome composition differences and specific ecological and physiological features dividing these two branches of Dipterans. This perspective review focuses on the most recent findings illuminating the Piwi/piRNA pathway distinctions between fruit flies and mosquitoes, and raises open questions that need to be addressed in order to ameliorate human diseases caused by pathogenic viruses that mosquitoes transmit as vectors.

The RNAi Pathway Is Important to Control Mayaro Virus Infection in Aedes aegypti but not for Wolbachia-Mediated Protection

Mayaro virus (MAYV), a sylvatic arbovirus belonging to the Togaviridae family and Alphavirus genus, is responsible for an increasing number of outbreaks in several countries of Central and South America. Despite Haemagogus janthinomys being identified as the main vector of MAYV, laboratory studies have already demonstrated the competence of Aedes aegypti to transmit MAYV. It has also been demonstrated that the WolbachiawMel strain is able to impair the replication and transmission of MAYV in Ae. aegypti. In Ae. aegypti, the small interfering RNA (siRNA) pathway is an important antiviral mechanism; however, it remains unclear whether siRNA pathway acts against MAYV infection in Ae. aegypti. The main objective of this study was to determine the contribution of the siRNA pathway in the control of MAYV infection. Thus, we silenced the expression of AGO2, an essential component of the siRNA pathway, by injecting dsRNA-targeting AGO2 (dsAGO2). Our results showed that AGO2 is required to control MAYV replication upon oral infection in Wolbachia-free Ae. aegypti. On the other hand, we found that Wolbachia-induced resistance to MAYV in Ae. aegypti is independent of the siRNA pathway. Our study brought new information regarding the mechanism of viral protection, as well as on Wolbachia mediated interference.

Cas13b-dependent and Cas13b-independent RNA knockdown of viral sequences in mosquito cells following guide RNA expression

Aedes aegypti and Aedes albopictus mosquitoes are vectors of the RNA viruses chikungunya (CHIKV) and dengue that currently have no specific therapeutic treatments. The development of new methods to generate virus-refractory mosquitoes would be beneficial. Cas13b is an enzyme that uses RNA guides to target and cleave RNA molecules and has been reported to suppress RNA viruses in mammalian and plant cells. We investigated the potential use of the Prevotella sp. P5-125 Cas13b system to provide viral refractoriness in mosquito cells, using a virus-derived reporter and a CHIKV split replication system. Cas13b in combination with suitable guide RNAs could induce strong suppression of virus-derived reporter RNAs in insect cells. Surprisingly, the RNA guides alone (without Cas13b) also gave substantial suppression. Our study provides support for the potential use of Cas13b in mosquitoes, but also caution in interpreting CRISPR/Cas data as we show that guide RNAs can have Cas-independent effects.

Ross River Virus Provokes Differentially Expressed MicroRNA and RNA Interference Responses in Aedes aegypti Mosquitoes

Alphaviruses are globally distributed and predominately transmitted by mosquitoes. Aedes species are common vectors for the clinically important alphaviruses-Chikungunya, Sindbis, and Ross River (RRV) viruses-with Aedes aegypti also being a vector for the flaviviruses dengue, Yellow Fever, and Zika viruses. Ae. aegypti was putatively implicated in the large 1979-1980 South Pacific Islands outbreak of RRV-the leading cause of arboviral disease in Australia today. The RNA interference (RNAi) defense response in mosquitoes involves a number of small RNAs, with their kinetics induced by alphaviruses being poorly understood, particularly at the tissue level. We compared the small RNA profiles between RRV-infected and noninfected Ae. aegypti midgut and fat body tissues at 2, 6, and 12 days post-inoculation (dpi). RRV induced an incremental RNAi response, yielding short interfering and P-element-induced-wimpy-testis (PIWI)-interacting RNAs. Fourteen host microRNAs were differentially expressed due to RRV with the majority in the fat body at 2 dpi. The largely congruent pattern of microRNA regulation with previous reports for alphaviruses and divergence from those for flaviviruses suggests a degree of conservation, whereas patterns of microRNA expression unique to this study provide novel insights into the tissuespecific hostvirus attributes of Ae. aegypti responses to this previously unexplored oldworld alphavirus.