Translating mosquito viromes into vector management strategies

Mosquitoes are best known for transmitting human and animal viruses. However, they also harbour mosquito-specific viruses (MSVs) as part of their microbiota. These are a group of viruses whose diversity and prevalence overshadow their medically relevant counterparts. Although metagenomics sequencing has remarkably accelerated the discovery of these viruses, what we know about them is often limited to sequence information, leaving much of their fundamental biology to be explored. Understanding the biology and ecology of MSVs can enlighten our knowledge of virus-virus interactions and lead to new innovations in the management of mosquito-borne viral diseases. We retrace the history of their discovery and discuss research milestones that would line the path from mosquito virome knowledge to vector management strategies.

Leveraging insect-specific viruses to elucidate mosquito population structure and dynamics

Several aspects of mosquito ecology that are important for vectored disease transmission and control have been difficult to measure at epidemiologically important scales in the field. In particular, the ability to describe mosquito population structure and movement rates has been hindered by difficulty in quantifying fine-scale genetic variation among populations. The mosquito virome represents a possible avenue for quantifying population structure and movement rates across multiple spatial scales. Mosquito viromes contain a diversity of viruses, including several insect-specific viruses (ISVs) and "core" viruses that have high prevalence across populations. To date, virome studies have focused on viral discovery and have only recently begun examining viral ecology. While nonpathogenic ISVs may be of little public health relevance themselves, they provide a possible route for quantifying mosquito population structure and dynamics. For example, vertically transmitted viruses could behave as a rapidly evolving extension of the host's genome. It should be possible to apply established analytical methods to appropriate viral phylogenies and incidence data to generate novel approaches for estimating mosquito population structure and dispersal over epidemiologically relevant timescales. By studying the virome through the lens of spatial and genomic epidemiology, it may be possible to investigate otherwise cryptic aspects of mosquito ecology. A better understanding of mosquito population structure and dynamics are key for understanding mosquito-borne disease ecology and methods based on ISVs could provide a powerful tool for informing mosquito control programs.

Wolbachia-Virus interactions and arbovirus control through population replacement in mosquitoes

Following transfer into the primary arbovirus vector Aedes aegypti, several strains of the intracellular bacterium Wolbachia have been shown to inhibit the transmission of dengue, Zika, and chikungunya viruses, important human pathogens that cause significant morbidity and mortality worldwide. In addition to pathogen inhibition, many Wolbachia strains manipulate host reproduction, resulting in an invasive capacity of the bacterium in insect populations. This has led to the deployment of Wolbachia as a dengue control tool, and trials have reported significant reductions in transmission in release areas. Here, we discuss the possible mechanisms of Wolbachia-virus inhibition and the implications for long-term success of dengue control. We also consider the evidence presented in several reports that Wolbachia may cause an enhancement of replication of certain viruses under particular conditions, and conclude that these should not cause any concerns with respect to the application of Wolbachia to arbovirus control.

Analysing inhibition of dengue virus in Wolbachia-infected mosquito cells following the removal of Wolbachia

Wolbachia pipientis is known to block replication of positive sense RNA viruses. Previously, we created an Aedes aegypti Aag2 cell line (Aag2.wAlbB) transinfected with the wAlbB strain of Wolbachia and a matching tetracycline-cured Aag2.tet cell line. While dengue virus (DENV) was blocked in Aag2.wAlbB cells, we found significant inhibition of DENV in Aag2.tet cells. RNA-Seq analysis of the cells confirmed removal of Wolbachia and lack of expression of Wolbachia genes that could have been due to lateral gene transfer in Aag2.tet cells. However, we noticed a substantial increase in the abundance of phasi charoen-like virus (PCLV) in Aag2.tet cells. When RNAi was used to reduce the PCLV levels, DENV replication was significantly increased. Further, we found significant changes in the expression of antiviral and proviral genes in Aag2.tet cells. Overall, the results reveal an antagonistic interaction between DENV and PCLV and how PCLV-induced changes could contribute to DENV inhibition.

Wolbachia RNase HI contributes to virus blocking in the mosquito Aedes aegypti

The endosymbiotic bacterium Wolbachia pipientis blocks replication of several arboviruses in transinfected Aedes aegypti mosquitoes. However, the mechanism of virus blocking remains poorly understood. Here, we characterized an RNase HI gene from Wolbachia, which is rapidly induced in response to dengue virus (DENV) infection. Knocking down w RNase HI using antisense RNA in Wolbachia-transinfected mosquito cell lines and A. aegypti mosquitoes led to increased DENV replication. Furthermore, overexpression of wRNase HI, in the absence of Wolbachia, led to reduced replication of a positive sense RNA virus, but had no effect on a negative sense RNA virus, a familiar scenario in Wolbachia-infected cells. Altogether, our results provide compelling evidence for the missing link between early Wolbachia-mediated virus blocking and degradation of viral RNA. These findings and the successful pioneered knockdown of Wolbachia genes using antisense RNA in cell line and mosquitoes enable new ways to manipulate and study the complex endosymbiont-host interactions.

Virome and nrEVEome diversity of Aedes albopictus mosquitoes from La Reunion Island and China

Background

Aedes albopictus is a public health threat for its worldwide spread and ability to transmit arboviruses. Understanding mechanisms of mosquito immunity can provide new tools to control arbovirus spread. The genomes of Aedes mosquitoes contain hundreds of nonretroviral endogenous viral elements (nrEVEs), which are enriched in piRNA clusters and produce piRNAs, with the potential to target cognate viruses. Recently, one nrEVE was shown to limit cognate viral infection through nrEVE-derived piRNAs. These findings suggest that nrEVEs constitute an archive of past viral infection and that the landscape of viral integrations may be variable across populations depending on their viral exposure.

Methods

We used bioinformatics and molecular approaches to identify known and novel (i.e. absent in the reference genome) viral integrations in the genome of wild collected Aedes albopictus mosquitoes and characterize their virome.

Results

We showed that the landscape of viral integrations is dynamic with seven novel viral integrations being characterized, but does not correlate with the virome, which includes both viral species known and unknown to infect mosquitoes. However, the small RNA coverage profile of nrEVEs and the viral genomic contigs we identified confirmed an interaction among these elements and the piRNA and siRNA pathways in mosquitoes.

Conclusions

Mosquitoes nrEVEs have been recently described as a new form of heritable, sequence-specific mechanism of antiviral immunity. Our results contribute to understanding the dynamic distribution of nrEVEs in the genomes of wild Ae. albopictus and their interaction with mosquito viruses.

The Involvement of Atlastin in Dengue Virus and Wolbachia Infection in Aedes aegypti and Its Regulation by aae-miR-989

Endoplasmic reticulum (ER)-shaping atlastin proteins (ATLs) have been demonstrated to play a functional role during flavivirus replication in mammalian cells. For dengue virus (DENV), atlastin is required in the formation of the replication organelles and RNA replication, virion assembly, production of the infectious virus particles, and trafficking or directing the association of vesicle packets with furin. Here, we investigated the involvement of atlastin in DENV replication in the mosquito Aedes aegypti and explored the possibility of its manipulation by the endosymbiotic bacterium Wolbachia to interfere with DENV replication. Results showed the expression of Ae. aegypti atlastin gene (AaATL) was upregulated in DENV-infected Aag2 cells, and its silencing led to reduced DENV replication. Contrary to our assumption that AaATL could be downregulated by Wolbachia, we did not find evidence for that in Wolbachia-infected cell lines, but this was the case in mosquitoes. Further, silencing AaATL did not have any effect on Wolbachia density. Our results also suggest that aae-miR-989 miRNA negatively regulates AaATL. The oversupply of the miRNA mimic led to reduced DENV replication consistent with the positive role of AaATL in DENV replication. Overall, the results favor AaATL's involvement in DENV replication; however, there is no support that the protein is involved in Wolbachia-mediated DENV inhibition. In addition, the results contribute to discerning further possible overlapping functions of ATLs in mosquitoes and mammalian cells. IMPORTANCE Atlastin is a protein associated with the endoplasmic reticulum and has been shown to play a role in replication of flaviviruses in mammalian cells. This study aimed to investigate the role of mosquito Aedes aegypti atlastin (AaATL) in dengue virus replication and maintenance of Wolbachia, an endosymbiotic bacterium, in the mosquito. Our results suggest that AaATL facilitates dengue virus replication in mosquito cells, considering silencing the gene led to reductions in virus replication and virion production. Further, AaATL was found to be regulated by a mosquito microRNA, aae-miR-989. Despite an effect on dengue virus, AaATL silencing did not affect Wolbachia replication and maintenance in mosquito cells. The results shed light on the role of atlastins in mosquito-pathogen interactions and their overlapping roles in mosquito and mammalian cells.

Establishment of Wolbachia infection in Aedes aegypti from Pakistan via embryonic microinjection and semi-field evaluation of general fitness of resultant mosquito population

Background

Dengue is a mosquito-borne viral disease that is mainly spread by Aedes aegypti. It is prevalent on five continents, predominantly in tropical and sub-tropical zones across the world. Wolbachia bacteria have been extensively used in vector control strategies worldwide. The focus of the current study was to obtain a natural population of Ae. aegypti harbouring Wolbachia and to determine the impact of this bacteria on the new host in a semi-field environment.

Methods

Wolbachia-infected Aedes albopictus was collected from the city of Lahore, Punjab, Pakistan, and Wolbachia were successfully introduced into laboratory-reared Ae. aegypti via embryonic microinjection. The stable vertical transmission of wAlbB in the host population was observed for eight generations, and the impact of Wolbachia on the general fitness of the host was evaluated in semi-field conditions.

Results

In the laboratory and semi-field experiments, wAlbB Wolbachia presented a strong cytoplasmic incompatibility (CI) effect, evidenced as zero egg hatching, in crosses between Wolbachia-infected males and wild (uninfected) females of Ae. aegypti. Wolbachia infection had no noticeable impact on the general fitness (P > 0.05), fecundity, body size (females and males) and mating competitiveness of the new host, Ae. aegypti. However, there was a significant decrease in female fertility (egg hatch) (P < 0.001). In addition, under starvation conditions, there was a remarkable decrease (P < 0.0001) in the life span of Wolbachia-infected females compared to uninfected females (4 vs. > 5 days, respectively).

Conclusions

Wolbachia strain wAlbB has a great potential to control the dengue vector in Ae. aegypti populations by producing 100% CI with a limited burden on its host in natural field conditions. This strain can be used as a biological tool against vector-borne diseases.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05317-4.

Transcriptional response of Wolbachia-transinfected Aedes aegypti mosquito cells to dengue virus at early stages of infection

Mosquito-borne flaviviruses are responsible for viral infections and represent a considerable public health burden. Aedes aegypti is the principal vector of dengue virus (DENV), therefore understanding the intrinsic virus-host interactions is vital, particularly in the presence of the endosymbiont Wolbachia, which blocks virus replication in mosquitoes. Here, we examined the transcriptional response of Wolbachia-transinfected Ae. aegypti Aag2 cells to DENV infection. We identified differentially expressed immune genes that play a key role in the activation of anti-viral defence such as the Toll and immune deficiency pathways. Further, genes encoding cytosine and N⁶-adenosine methyltransferases and SUMOylation, involved in post-transcriptional modifications, an antioxidant enzyme, and heat-shock response were up-regulated at the early stages of DENV infection and are reported here for the first time. Additionally, several long non-coding RNAs were among the differentially regulated genes. Our results provide insight into Wolbachia-transinfected Ae. aegypti's initial virus recognition and transcriptional response to DENV infection.

Symbiotic Interactions Between Mosquitoes and Mosquito Viruses

Mosquitoes not only transmit human and veterinary pathogens called arboviruses (arthropod-borne viruses) but also harbor mosquito-associated insect-specific viruses (mosquito viruses) that cannot infect vertebrates. In the past, studies investigating mosquito viruses mainly focused on highly pathogenic interactions that were easier to detect than those without visible symptoms. However, the recent advances in viral metagenomics have highlighted the abundance and diversity of viruses which do not generate mass mortality in host populations. Over the last decade, this has facilitated the rapid growth of virus discovery in mosquitoes. The circumstances around the discovery of mosquito viruses greatly affected how they have been studied so far. While earlier research mainly focused on the pathogenesis caused by DNA and some double-stranded RNA viruses during larval stages, more recently discovered single-stranded RNA mosquito viruses were heavily studied for their putative interference with arboviruses in female adults. Thus, many aspects of mosquito virus interactions with their hosts and host-microbiota are still unknown. In this context, considering mosquito viruses as endosymbionts can help to identify novel research areas, in particular in relation to their long-term interactions with their hosts (e.g. relationships during all life stages, the stability of the associations at evolutionary scales, transmission routes and virulence evolution) and the possible context-dependent range of interactions (i.e. beneficial to antagonistic). Here, we review the symbiotic interactions of mosquito viruses considering different aspects of their ecology, such as transmission, host specificity, host immune system and interactions with other symbionts within the host cellular arena. Finally, we highlight related research gaps in mosquito virus research.

Growth and Maintenance of Wolbachia in Insect Cell Lines

Simple Summary

Wolbachia is an intracellular bacterium that occurs in arthropods and in filarial worms. First described nearly a century ago in the reproductive tissues of Culex pipiens mosquitoes, Wolbachia is now known to occur in roughly 50% of insect species, and has been considered the most abundant intracellular bacterium on earth. In insect hosts, Wolbachia modifies reproduction in ways that facilitate spread of the microbe within the host population, but otherwise is relatively benign. In this “gene drive” capacity, Wolbachia provides a tool for manipulating mosquito populations. In mosquitoes, Wolbachia causes cytoplasmic incompatibility, in which the fusion of egg and sperm nuclei is disrupted, and eggs fail to hatch, depending on the presence/absence of Wolbachia in the parent insects. Recent findings demonstrate that Wolbachia from infected insects can be transferred into mosquito species that do not host a natural infection. When transinfected into Aedes aegypti, an important vector of dengue and Zika viruses, Wolbachia causes cytoplasmic incompatibility and, in addition, decreases the mosquito’s ability to transmit viruses to humans. This review addresses the maintenance of Wolbachia in insect cell lines, which provide a tool for high-level production of infectious bacteria. In vitro technologies will improve use of Wolbachia for pest control, and provide the microbiological framework for genetic engineering of this promising biocontrol agent.

Abstract

The obligate intracellular microbe, Wolbachia pipientis (Rickettsiales; Anaplasmataceae), is a Gram-negative member of the alpha proteobacteria that infects arthropods and filarial worms. Although closely related to the genera Anaplasma and Ehrlichia, which include pathogens of humans, Wolbachia is uniquely associated with invertebrate hosts in the clade Ecdysozoa. Originally described in Culex pipiens mosquitoes, Wolbachia is currently represented by 17 supergroups and is believed to occur in half of all insect species. In mosquitoes, Wolbachia acts as a gene drive agent, with the potential to modify vector populations; in filarial worms, Wolbachia functions as a symbiont, and is a target for drug therapy. A small number of Wolbachia strains from supergroups A, B, and F have been maintained in insect cell lines, which are thought to provide a more permissive environment than the natural host. When transferred back to an insect host, Wolbachia produced in cultured cells are infectious and retain reproductive phenotypes. Here, I review applications of insect cell lines in Wolbachia research and describe conditions that facilitate Wolbachia infection and replication in naive host cells. Progress in manipulation of Wolbachia in vitro will enable genetic and biochemical advances that will facilitate eventual genetic engineering of this important biological control agent.

Transcriptional Response of Wolbachia to Dengue Virus Infection in Cells of the Mosquito Aedes aegypti

Aedes aegypti transmits one of the most significant mosquito-borne viruses, dengue virus (DENV). The absence of effective vaccines and clinical treatments and the emergence of insecticide resistance in A. aegypti necessitate novel vector control strategies. A new approach uses the endosymbiotic bacterium Wolbachia pipientis to reduce the spread of arboviruses. However, the Wolbachia-mediated antiviral mechanism is not well understood. To shed light on this mechanism, we investigated an unexplored aspect of Wolbachia-virus-mosquito interaction. We used RNA sequencing to examine the transcriptional response of Wolbachia to DENV infection in A. aegypti Aag2 cells transinfected with the wAlbB strain of Wolbachia. Our results suggest that genes encoding an endoribonuclease (RNase HI), a regulator of sigma 70-dependent gene transcription (6S RNA), essential cellular, transmembrane, and stress response functions and primary type I and IV secretion systems were upregulated, while a number of transport and binding proteins of Wolbachia, ribosome structure, and elongation factor-associated genes were downregulated due to DENV infection. Furthermore, bacterial retrotransposon, transposable, and phage-related elements were found among the up- and downregulated genes. We show that Wolbachia elicits a transcriptional response to virus infection and identify differentially expressed Wolbachia genes mostly at the early stages of virus infection. These findings highlight Wolbachia's ability to alter its gene expression in response to DENV infection of the host cell. IMPORTANCE Aedes aegypti is a vector of several pathogenic viruses, including dengue, Zika, chikungunya, and yellow fever viruses, which are of importance to human health. Wolbachia is an endosymbiotic bacterium currently used in transinfected mosquitoes to suppress replication and transmission of dengue viruses. However, the mechanism of Wolbachia-mediated virus inhibition is not fully understood. While several studies have shown mosquitoes' transcriptional responses to dengue virus infection, none have investigated these responses in Wolbachia, which may provide clues to the inhibition mechanism. Our results suggest changes in the expression of a number of functionally important Wolbachia genes upon dengue virus infection, including those involved in stress responses, providing insights into the endosymbiont's reaction to virus infection.

Altered gene expression profile of Wolbachia pipientis wAlbB strain following transinfection from its native host Aedes albopictus to Aedes aegypti cells

Wolbachia is an obligate intracellular bacterial symbiont prevalent among arthropods and nematodes. To survive and reproduce, Wolbachia interacts with and modifies host subcellular structures, while sensing and responding to changes within the cellular environment. In mutualistic associations, Wolbachia may provision the host with metabolites, or help to maintain the chemical homeostasis of the host cell. Some strains can rapidly invade insect populations by manipulating host reproductive biology, while also preventing viral replication, allowing their use in vector control of arthropod-borne viruses. The Aedes albopictus-derived strain wAlbB is promising in this regard. When transinfected into the Yellow fever mosquito, Aedes aegypti, wAlbB reaches high frequencies within wild populations, and strongly inhibits viral transmission. Despite its obvious potential, much is still unknown about the molecular interactions between Wolbachia and host that enable its use in vector control. Furthermore, most Wolbachia transinfection research to date has focused on host effects. In the current study, we used a cell line model to explore the effect of transinfection of wAlbB from Ae. albopictus to Ae. aegypti. Using RNA sequencing, we show that several genes associated with host-symbiont interactions were downregulated by transinfection, with the greatest downregulation exhibited by prophage-associated genes.

Wolbachia-Mitochondrial DNA Associations in Transitional Populations of Rhagoletis cerasi

The endosymbiont Wolbachia can manipulate arthropod host reproduction by inducing cytoplasmic incompatibility (CI), which results in embryonic mortality when infected males mate with uninfected females. A CI-driven invasion of Wolbachia can result in a selective sweep of associated mitochondrial haplotype. The co-inheritance of Wolbachia and host mitochondrial DNA can therefore provide significant information on the dynamics of an ongoing Wolbachia invasion. Therefore, transition zones (i.e., regions where a Wolbachia strain is currently spreading from infected to uninfected populations) represent an ideal area to investigate the relationship between Wolbachia and host mitochondrial haplotype. Here, we studied Wolbachia-mitochondrial haplotype associations in the European cherry fruit fly, Rhagoletis cerasi, in two transition zones in the Czech Republic and Hungary, where the CI-inducing strain wCer2 is currently spreading. The wCer2-infection status of 881 individuals was compared with the two known R. cerasi mitochondrial haplotypes, HT1 and HT2. In accordance with previous studies, wCer2-uninfected individuals were associated with HT1, and wCer2-infected individuals were mainly associated with HT2. We found misassociations only within the transition zones, where HT2 flies were wCer2-uninfected, suggesting the occurrence of imperfect maternal transmission. We did not find any HT1 flies that were wCer2-infected, suggesting that Wolbachia was not acquired horizontally. Our study provides new insights into the dynamics of the early phase of a Wolbachia invasion.

Wolbachia modulates prevalence and viral load of Culex pipiens densoviruses in natural populations

The inadequacy of standard mosquito control strategies calls for ecologically safe novel approaches, for example the use of biological agents such as the endosymbiotic α-proteobacteria Wolbachia or insect-specific viruses (ISVs). Understanding the ecological interactions between these "biocontrol endosymbionts" is thus a fundamental step. Wolbachia are transmitted vertically from mother to offspring and modify their hosts' phenotypes, including reproduction (e.g., cytoplasmic incompatibility) and survival (e.g., viral interference). In nature, Culex pipiens (sensu lato) mosquitoes are always found infected with genetically diverse Wolbachia called wPip that belong to five phylogenetic groups. In recent years, ISVs have also been discovered in these mosquito species, although their interactions with Wolbachia in nature are unknown. Here, we studied the interactions between a widely prevalent ISV, the Culex pipiens densovirus (CpDV, Densovirinae), and Wolbachia in northern Tunisian C. pipiens populations. We showed an influence of different Wolbachia groups on CpDV prevalence and a general positive correlation between Wolbachia and CpDV loads. By investigating the putative relationship between CpDV diversification and wPip groups in the different sites, we detected a signal linked to wPip groups in CpDV phylogeny in sites where all larvae were infected by the same wPip group. However, no such signal was detected where the wPip groups coexisted, suggesting CpDV horizontal transfer between hosts. Overall, our results provide good evidence for an ecological influence of Wolbachia on an ISV, CpDV, in natural populations and highlight the importance of integrating Wolbachia in our understanding of ISV ecology in nature.