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

Why and how do protective symbionts impact immune priming with pathogens in invertebrates?

Growing evidence demonstrates that invertebrates display adaptive-like immune abilities, commonly known as "immune priming". Immune priming is a process by which a host improves its immune defences following an initial pathogenic exposure, leading to better protection after a subsequent infection with the same - or different - pathogens. Nevertheless, beneficial symbionts can enhance similar immune priming processes in hosts, such as when they face repeated infections with pathogens. This "symbiotic immune priming" protects the host against pathogenic viruses, bacteria, fungi, or eukaryotic parasites. In this review, we explore the extent to which protective symbionts interfere and impact immune priming against pathogens from both a mechanical (proximal) and an evolutionary (ultimate) point of view. We highlight that the immune priming of invertebrates is the cornerstone of the tripartite interaction of hosts/symbionts/pathogens. The main shared mechanism of immune priming (induced by symbionts or pathogens) is the sustained immune response at the beginning of host-microbial interactions. However, the evolutionary outcome of immune priming leads to a specific discrimination, which provides enhanced tolerance or resistance depending on the type of microbe. Based on several studies testing immune priming against pathogens in the presence or absence of protective symbionts, we observed that both types of immune priming could overlap and affect each other inside the same hosts. As protective symbionts could be an evolutionary force that influences immune priming, they may help us to better understand the heterogeneity of pathogenic immune priming across invertebrate populations and species.

High Temperature Cycles Result in Maternal Transmission and Dengue Infection Differences Between Wolbachia Strains in Aedes aegypti

Environmental factors play a crucial role in the population dynamics of arthropod endosymbionts, and therefore in the deployment of Wolbachia symbionts for the control of dengue arboviruses. The potential of Wolbachia to invade, persist, and block virus transmission depends in part on its intracellular density. Several recent studies have highlighted the importance of larval rearing temperature in modulating Wolbachia densities in adults, suggesting that elevated temperatures can severely impact some strains, while having little effect on others. The effect of a replicated tropical heat cycle on Wolbachia density and levels of virus blocking was assessed using Aedes aegypti lines carrying strains wMel and wAlbB, two Wolbachia strains currently used for dengue control. Impacts on intracellular density, maternal transmission fidelity, and dengue inhibition capacity were observed for wMel. In contrast, wAlbB-carrying Ae. aegypti maintained a relatively constant intracellular density at high temperatures and conserved its capacity to inhibit dengue. Following larval heat treatment, wMel showed a degree of density recovery in aging adults, although this was compromised by elevated air temperatures. IMPORTANCE In the past decades, dengue incidence has dramatically increased all over the world. An emerging dengue control strategy utilizes Aedes aegypti mosquitoes artificially transinfected with the bacterial symbiont Wolbachia, with the ultimate aim of replacing wild mosquito populations. However, the rearing temperature of mosquito larvae is known to impact on some Wolbachia strains. In this study, we compared the effects of a temperature cycle mimicking natural breeding sites in tropical climates on two Wolbachia strains, currently used for open field trials. When choosing the Wolbachia strain to be used in a dengue control program it is important to consider the effects of environmental temperatures on invasiveness and virus inhibition. These results underline the significance of understanding the impact of environmental factors on released mosquitoes, in order to ensure the most efficient strategy for dengue control.

Molecular Rationale of Insect-Microbes Symbiosis-From Insect Behaviour to Mechanism

Insects nurture a panoply of microbial populations that are often obligatory and exist mutually with their hosts. Symbionts not only impact their host fitness but also shape the trajectory of their phenotype. This co-constructed niche successfully evolved long in the past to mark advanced ecological specialization. The resident microbes regulate insect nutrition by controlling their host plant specialization and immunity. It enhances the host fitness and performance by detoxifying toxins secreted by the predators and abstains them. The profound effect of a microbial population on insect physiology and behaviour is exploited to understand the host–microbial system in diverse taxa. Emergent research of insect-associated microbes has revealed their potential to modulate insect brain functions and, ultimately, control their behaviours, including social interactions. The revelation of the gut microbiota–brain axis has now unravelled insects as a cost-effective potential model to study neurodegenerative disorders and behavioural dysfunctions in humans. This article reviewed our knowledge about the insect–microbial system, an exquisite network of interactions operating between insects and microbes, its mechanistic insight that holds intricate multi-organismal systems in harmony, and its future perspectives. The demystification of molecular networks governing insect–microbial symbiosis will reveal the perplexing behaviours of insects that could be utilized in managing insect pests.

Novel Symbiotic Genome-Scale Model Reveals Wolbachia's Arboviral Pathogen Blocking Mechanism in Aedes aegypti

Wolbachia are endosymbiont bacteria known to infect arthropods causing different effects, such as cytoplasmic incompatibility and pathogen blocking in Aedes aegypti. Although several Wolbachia strains have been studied, there is little knowledge regarding the relationship between this bacterium and their hosts, particularly on their obligate endosymbiont nature and its pathogen blocking ability. Motivated by the potential applications on disease control, we developed a genome-scale model of two Wolbachia strains: wMel and the strongest Dengue blocking strain known to date: wMelPop. The obtained metabolic reconstructions exhibit an energy metabolism relying mainly on amino acids and lipid transport to support cell growth that is consistent with altered lipid and cholesterol metabolism in Wolbachia-infected mosquitoes. The obtained metabolic reconstruction was then coupled with a reconstructed mosquito model to retrieve a symbiotic genome-scale model accounting for 1,636 genes and 6,408 reactions of the Aedes aegypti-Wolbachia interaction system. Simulation of an arboviral infection in the obtained novel symbiotic model represents a metabolic scenario characterized by pathogen blocking in higher titer Wolbachia strains, showing that pathogen blocking by Wolbachia infection is consistent with competition for lipid and amino acid resources between arbovirus and this endosymbiotic bacteria.

Designing effective Wolbachia release programs for mosquito and arbovirus control

Mosquitoes carrying endosymbiotic bacteria called Wolbachia are being released in mosquito and arbovirus control programs around the world through two main approaches: population suppression and population replacement. Open field releases of Wolbachia-infected male mosquitoes have achieved over 95% population suppression by reducing the fertility of wild mosquito populations. The replacement of populations with Wolbachia-infected females is self-sustaining and can greatly reduce local dengue transmission by reducing the vector competence of mosquito populations. Despite many successful interventions, significant questions and challenges lie ahead. Wolbachia, viruses and their mosquito hosts can evolve, leading to uncertainty around the long-term effectiveness of a given Wolbachia strain, while few ecological impacts of Wolbachia releases have been explored. Wolbachia strains are diverse and the choice of strain to release should be made carefully, taking environmental conditions and the release objective into account. Mosquito quality control, thoughtful community awareness programs and long-term monitoring of populations are essential for all types of Wolbachia intervention. Releases of Wolbachia-infected mosquitoes show great promise, but existing control measures remain an important way to reduce the burden of mosquito-borne disease.

Blood Meals With Active and Heat-Inactivated Serum Modifies the Gene Expression and Microbiome of Aedes albopictus

The Asian "tiger mosquito" Aedes albopictus is currently the most widely distributed disease-transmitting mosquito in the world. Its geographical expansion has also allowed the expansion of multiple arboviruses like dengue, Zika, and chikungunya, to higher latitudes. Due to the enormous risk to global public health caused by mosquitoes species vectors of human disease, and the challenges in slowing their expansion, it is necessary to develop new and environmentally friendly vector control strategies. Among these, host-associated microbiome-based strategies have emerged as promising options. In this study, we performed an RNA-seq analysis on dissected abdomens of Ae. albopictus females from Manhattan, KS, United States fed with sugar and human blood containing either normal or heat-inactivated serum, to evaluate the effect of heat inactivation on gene expression, the bacteriome transcripts and the RNA virome of this mosquito species. Our results showed at least 600 genes with modified expression profile when mosquitoes were fed with normal vs. heat-inactivated-containing blood. These genes were mainly involved in immunity, oxidative stress, lipid metabolism, and oogenesis. Also, we observed bacteriome changes with an increase in transcripts of Actinobacteria, Rhodospirillaceae, and Anaplasmataceae at 6 h post-feeding. We also found that feeding with normal blood seems to particularly influence Wolbachia metabolism, demonstrated by a significant increase in transcripts of this bacteria in mosquitoes fed with blood containing normal serum. However, no differences were observed in the virome core of this mosquito population. These results suggest that heat and further inactivation of complement proteins in human serum may have profound effect on mosquito and microbiome metabolism, which could influence interpretation of the pathogen-host interaction findings when using this type of reagents specially when measuring the effect of Wolbachia in vector competence.

Large-Scale Deployment and Establishment of Wolbachia Into the Aedes aegypti Population in Rio de Janeiro, Brazil

Traditional methods of vector control have proven insufficient to reduce the alarming incidence of dengue, Zika, and chikungunya in endemic countries. The bacterium symbiont Wolbachia has emerged as an efficient pathogen-blocking and self-dispersing agent that reduces the vectorial potential of Aedes aegypti populations and potentially impairs arboviral disease transmission. In this work, we report the results of a large-scale Wolbachia intervention in Ilha do Governador, Rio de Janeiro, Brazil. wMel-infected adults were released across residential areas between August 2017 and March 2020. Over 131 weeks, including release and post-release phases, we monitored the wMel prevalence in field specimens and analyzed introgression profiles of two assigned intervention areas, RJ1 and RJ2. Our results revealed that wMel successfully invaded both areas, reaching overall infection rates of 50-70% in RJ1 and 30-60% in RJ2 by the end of the monitoring period. At the neighborhood-level, wMel introgression was heterogeneous in both RJ1 and RJ2, with some profiles sustaining a consistent increase in infection rates and others failing to elicit the same. Correlation analysis revealed a weak overall association between RJ1 and RJ2 (r = 0.2849, p = 0.0236), and an association at a higher degree when comparing different deployment strategies, vehicle or backpack-assisted, within RJ1 (r = 0.4676, p < 0.0001) or RJ2 (r = 0.6263, p < 0.0001). The frequency knockdown resistance (kdr) alleles in wMel-infected specimens from both areas were consistently high over this study. Altogether, these findings corroborate that wMel can be successfully deployed at large-scale as part of vector control intervention strategies and provide the basis for imminent disease impact studies in Southeastern Brazil.

Wolbachia as translational science: controlling mosquito-borne pathogens

In this review we examine how exploiting the Wolbachia-mosquito relationship has become an increasingly popular strategy for controlling arbovirus transmission. Field deployments of Wolbachia-infected mosquitoes have led to significant decreases in dengue virus incidence via high levels of mosquito population suppression and replacement, emphasizing the success of Wolbachia approaches. Here, we examine how improved knowledge of Wolbachia-host interactions has provided key insight into the mechanisms of the essential phenotypes of pathogen blocking and cytoplasmic incompatibility. And we discuss recent studies demonstrating that extrinsic factors, such as ambient temperature, can modulate Wolbachia density and maternal transmission. Finally, we assess the prospects of using Wolbachia to control other vectors and agricultural pest species.

Combating mosquito-borne diseases using genetic control technologies

Mosquito-borne diseases, such as dengue and malaria, pose significant global health burdens. Unfortunately, current control methods based on insecticides and environmental maintenance have fallen short of eliminating the disease burden. Scalable, deployable, genetic-based solutions are sought to reduce the transmission risk of these diseases. Pathogen-blocking Wolbachia bacteria, or genome engineering-based mosquito control strategies including gene drives have been developed to address these problems, both requiring the release of modified mosquitoes into the environment. Here, we review the latest developments, notable similarities, and critical distinctions between these promising technologies and discuss their future applications for mosquito-borne disease control.

Intracellular Interactions Between Arboviruses and Wolbachia in Aedes aegypti

Aedes aegypti is inherently susceptible to arboviruses. The geographical expansion of this vector host species has led to the persistence of Dengue, Zika, and Chikungunya human infections. These viruses take advantage of the mosquito’s cell to create an environment conducive for their growth. Arboviral infection triggers transcriptomic and protein dysregulation in Ae. aegypti and in effect, host antiviral mechanisms are compromised. Currently, there are no existing vaccines able to protect human hosts from these infections and thus, vector control strategies such as Wolbachia mass release program is regarded as a viable option. Considerable evidence demonstrates how the presence of Wolbachia interferes with arboviruses by decreasing host cytoskeletal proteins and lipids essential for arboviral infection. Also, Wolbachia strengthens host immunity, cellular regeneration and causes the expression of microRNAs which could potentially be involved in virus inhibition. However, variation in the magnitude of Wolbachia’s pathogen blocking effect that is not due to the endosymbiont’s density has been recently reported. Furthermore, the cellular mechanisms involved in this phenotype differs depending on Wolbachia strain and host species. This prompts the need to explore the cellular interactions between Ae. aegypti-arboviruses-Wolbachia and how different Wolbachia strains overall affect the mosquito’s cell. Understanding what happens at the cellular and molecular level will provide evidence on the sustainability of Wolbachia vector control.

Fat and Happy: Profiling Mosquito Fat Body Lipid Storage and Composition Post-blood Meal

The fat body is considered the insect analog of vertebrate liver and fat tissue. In mosquitoes, a blood meal triggers a series of processes in the fat body that culminate in vitellogenesis, the process of yolk formation. Lipids are stored in the fat body in specialized organelles called lipid droplets that change in size depending on the nutritional and metabolic status of the insect. We surveyed lipid droplets in female Aedes aegypti fat body during a reproductive cycle using confocal microscopy and analyzed the dynamic changes in the fat body lipidome during this process using LC/MS. We found that lipid droplets underwent dynamic changes in volume after the mosquito took a blood meal. The lipid composition found in the fat body is quite complex with 117 distinct lipids that fall into 19 classes and sublcasses. Our results demonstrate that the lipid composition of the fat body is complex as most lipid classes underwent significant changes over the course of the vitellogenic cycle. This study lays the foundation for identifying unknown biochemical pathways active in the mosquito fat body, that are high-value targets for the development of novel mosquito control strategies.

Using Wolbachia to Eliminate Dengue: Will the Virus Fight Back?

Recent field trials have demonstrated that dengue incidence can be substantially reduced by introgressing strains of the endosymbiotic bacterium Wolbachia into Aedes aegypti mosquito populations. This strategy relies on Wolbachia reducing the susceptibility of Ae. aegypti to disseminated infection by positive-sense RNA viruses like dengue. However, RNA viruses are well known to adapt to antiviral pressures. Here, we review the viral infection stages where selection for Wolbachia-resistant virus variants could occur. We also consider the genetic constraints imposed on viruses that alternate between vertebrate and invertebrate hosts, and the likely selection pressures to which dengue virus might adapt in order to be effectively transmitted by Ae. aegypti that carry Wolbachia. While there are hurdles to dengue viruses developing resistance to Wolbachia, we suggest that long-term surveillance for resistant viruses should be an integral component of Wolbachia-introgression biocontrol programs.

Efficacy of Wolbachia-Infected Mosquito Deployments for the Control of Dengue

BACKGROUND

Aedes aegypti mosquitoes infected with the wMel strain of Wolbachia pipientis are less susceptible than wild-type A. aegypti to dengue virus infection.

METHODS

We conducted a cluster-randomized trial involving releases of wMel-infected A. aegypti mosquitoes for the control of dengue in Yogyakarta, Indonesia. We randomly assigned 12 geographic clusters to receive deployments of wMel-infected A. aegypti (intervention clusters) and 12 clusters to receive no deployments (control clusters). All clusters practiced local mosquito-control measures as usual. A test-negative design was used to assess the efficacy of the intervention. Patients with acute undifferentiated fever who presented to local primary care clinics and were 3 to 45 years of age were recruited. Laboratory testing was used to identify participants who had virologically confirmed dengue (VCD) and those who were test-negative controls. The primary end point was symptomatic VCD of any severity caused by any dengue virus serotype.

RESULTS

After successful introgression of wMel into the intervention clusters, 8144 participants were enrolled; 3721 lived in intervention clusters, and 4423 lived in control clusters. In the intention-to-treat analysis, VCD occurred in 67 of 2905 participants (2.3%) in the intervention clusters and in 318 of 3401 (9.4%) in the control clusters (aggregate odds ratio for VCD, 0.23; 95% confidence interval [CI], 0.15 to 0.35; P = 0.004). The protective efficacy of the intervention was 77.1% (95% CI, 65.3 to 84.9) and was similar against the four dengue virus serotypes. The incidence of hospitalization for VCD was lower among participants who lived in intervention clusters (13 of 2905 participants [0.4%]) than among those who lived in control clusters (102 of 3401 [3.0%]) (protective efficacy, 86.2%; 95% CI, 66.2 to 94.3).

CONCLUSIONS

Introgression of wMel into A. aegypti populations was effective in reducing the incidence of symptomatic dengue and resulted in fewer hospitalizations for dengue among the participants. (Funded by the Tahija Foundation and others; AWED ClinicalTrials.gov number, NCT03055585; Indonesia Registry number, INA-A7OB6TW.).

A potential third-order role of the host endoplasmic reticulum as a contact site in interkingdom microbial endosymbiosis and viral infection

The normal functioning of eukaryotic cells depends on the compartmentalization of metabolic processes within specific organelles. Interactions among organelles, such as those between the endoplasmic reticulum (ER) - considered the largest single structure in eukaryotic cells - and other organelles at membrane contact sites (MCSs) have also been suggested to trigger synergisms, including intracellular immune responses against pathogens. In addition to the ER-endogenous functions and ER-organelle MCSs, we present the perspective of a third-order role of the ER as a host contact site for endosymbiotic microbial non-pathogens and pathogens, from endosymbiont bacteria to parasitic protists and viruses. Although understudied, ER-endosymbiont interactions have been observed in a range of eukaryotic hosts, including protists, plants, algae, and metazoans. Host ER interactions with endosymbionts could be an ER function built from ancient, conserved mechanisms selected for communicating with mutualistic endosymbionts in specific life cycle stages, and they may be exploited by pathogens and parasites. The host ER-'guest' interactome and traits in endosymbiotic biology are briefly discussed. The acknowledgment and understanding of these possible mechanisms might reveal novel evolutionary perspectives, uncover the causes of unexplained cellular disorders and suggest new pharmacological targets.

Reduced competence to arboviruses following the sustainable invasion of Wolbachia into native Aedes aegypti from Southeastern Brazil

Field release of Wolbachia-infected Aedes aegypti has emerged as a promising solution to manage the transmission of dengue, Zika and chikungunya in endemic areas across the globe. Through an efficient self-dispersing mechanism, and the ability to induce virus-blocking properties, Wolbachia offers an unmatched potential to gradually modify wild Ae. aegypti populations turning them unsuitable disease vectors. Here we describe a proof-of-concept field trial carried out in a small community of Niterói, greater Rio de Janeiro, Brazil. Following the release of Wolbachia-infected eggs, we report here a successful invasion and long-term establishment of the bacterium across the territory, as denoted by stable high-infection indexes (> 80%). We have also demonstrated that refractoriness to dengue and Zika viruses, either thorough oral-feeding or intra-thoracic saliva challenging assays, was maintained over the adaptation to the natural environment of Southeastern Brazil. These findings further support Wolbachia's ability to invade local Ae. aegypti populations and impair disease transmission, and will pave the way for future epidemiological and economic impact assessments.

The Metabolic Response to Infection With Wolbachia Implicates the Insulin/Insulin-Like-Growth Factor and Hypoxia Signaling Pathways in Drosophila melanogaster

The endosymbiotic bacteria, Wolbachia, are best known for their ability to manipulate insect-host reproduction systems that enhance their vertical transmission within host populations. Increasingly, Wolbachia have been shown to depend on their hosts' metabolism for survival and in turn provision metabolites to their host. Wolbachia depends completely on the host for iron and as such iron has been speculated to be a fundamental aspect of Wolbachia-host interplay. However, the mechanisms by which dietary iron levels, Wolbachia, and its host interact remain to be elucidated. To understand the metabolic dependence of Wolbachia on its host, the possibility of metabolic provisioning and extraction, and the interplay with available dietary iron, we have used NMR-based metabolomics and compared metabolite profiles of Wolbachia-infected and uninfected Drosophila melanogaster flies raised on varying levels of dietary iron. We observed marked metabolite differences in the affected metabolite pathways between Wolbachia-infected and uninfected Drosophila, which were dependent on the dietary iron levels. Excess iron led to lipid accumulation, whereas iron deficiency led to changes in carbohydrate levels. This represents a major metabolic shift triggered by alterations in iron levels. Lipids, some amino acids, carboxylic acids, and nucleosides were the major metabolites altered by infection. The metabolic response to infection showed a reprogramming of the mitochondrial metabolism in the host. Based on these observations, we developed a physiological model which postulates that the host's insulin/insulin-like-growth factor pathway is depressed and the hypoxia signaling pathway is activated upon Wolbachia infection. This reprogramming leads to predominantly non-oxidative metabolism in the host, whereas Wolbachia maintains oxidative metabolism. Our data also support earlier predictions of the extraction of alanine from the host while provisioning riboflavin and ATP to the host.

Comprehensive Quantitative Proteome Analysis of Aedes aegypti Identifies Proteins and Pathways Involved in Wolbachia pipientis and Zika Virus Interference Phenomenon

Zika virus (ZIKV) is a global public health emergency due to its association with microcephaly, Guillain-Barré syndrome, neuropathy, and myelitis in children and adults. A total of 87 countries have had evidence of autochthonous mosquito-borne transmission of ZIKV, distributed across four continents, and no antivirus therapy or vaccines are available. Therefore, several strategies have been developed to target the main mosquito vector, Aedes aegypti, to reduce the burden of different arboviruses. Among such strategies, the use of the maternally-inherited endosymbiont Wolbachia pipientis has been applied successfully to reduce virus susceptibility and decrease transmission. However, the mechanisms by which Wolbachia orchestrate resistance to ZIKV infection remain to be elucidated. In this study, we apply isobaric labeling quantitative mass spectrometry (MS)-based proteomics to quantify proteins and identify pathways altered during ZIKV infection; Wolbachia infection; co-infection with Wolbachia/ZIKV in the A. aegypti heads and salivary glands. We show that Wolbachia regulates proteins involved in reactive oxygen species production, regulates humoral immune response, and antioxidant production. The reduction of ZIKV polyprotein in the presence of Wolbachia in mosquitoes was determined by MS and corroborates the idea that Wolbachia helps to block ZIKV infections in A. aegypti. The present study offers a rich resource of data that may help to elucidate mechanisms by which Wolbachia orchestrate resistance to ZIKV infection in A. aegypti, and represents a step further on the development of new targeted methods to detect and quantify ZIKV and Wolbachia directly in complex tissues.

Non-immune Traits Triggered by Blood Intake Impact Vectorial Competence

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

Wolbachia and Virus Alter the Host Transcriptome at the Interface of Nucleotide Metabolism Pathways

Wolbachia is a maternally transmitted bacterium that manipulates arthropod and nematode biology in myriad ways. The Wolbachia strain colonizing Drosophila melanogaster creates sperm-egg incompatibilities and protects its host against RNA viruses, making it a promising tool for vector control. Despite successful trials using Wolbachia-transfected mosquitoes for dengue control, knowledge of how Wolbachia and viruses jointly affect insect biology remains limited. Using the Drosophila melanogaster model, transcriptomics and gene expression network analyses revealed pathways with altered expression and splicing due to Wolbachia colonization and virus infection. Included are metabolic pathways previously unknown to be important for Wolbachia-host interactions. Additionally, Wolbachia-colonized flies exhibit a dampened transcriptomic response to virus infection, consistent with early blocking of virus replication. Finally, using Drosophila genetics, we show that Wolbachia and expression of nucleotide metabolism genes have interactive effects on virus replication. Understanding the mechanisms of pathogen blocking will contribute to the effective development of Wolbachia-mediated vector control programs.IMPORTANCE Recently developed arbovirus control strategies leverage the symbiotic bacterium Wolbachia, which spreads in insect populations and blocks viruses from replicating. While this strategy has been successful, details of how this "pathogen blocking" works are limited. Here, we use a combination of virus infections, fly genetics, and transcriptomics to show that Wolbachia and virus interact at host nucleotide metabolism pathways.

The Antiviral Effects of the Symbiont Bacteria Wolbachia in Insects

Wolbachia is a maternally transmitted bacterium that lives inside arthropod cells. Historically, it was viewed primarily as a parasite that manipulates host reproduction, but more recently it was discovered that Wolbachia can also protect Drosophila species against infection by RNA viruses. Combined with Wolbachia's ability to invade insect populations due to reproductive manipulations, this provides a way to modify mosquito populations to prevent them transmitting viruses like dengue. In this review, we discuss the main advances in the field since Wolbachia's antiviral effect was discovered 12 years ago, identifying current research gaps and potential future developments. We discuss that the antiviral effect works against a broad range of RNA viruses and depends on the Wolbachia lineage. We describe what is known about the mechanisms behind viral protection, and that recent studies suggest two possible mechanisms: activation of host immunity or competition with virus for cellular resources. We also discuss how association with Wolbachia may influence the evolution of virus defense on the insect host genome. Finally, we investigate whether the antiviral effect occurs in wild insect populations and its ecological relevance as a major antiviral component in insects.

Identification of mesenchymal stromal cell survival responses to antimicrobial silver ion concentrations released from orthopaedic implants

Antimicrobial silver (Ag⁺) coatings on orthopaedic implants may reduce infection rates, but should not be to the detriment of regenerative cell populations, primarily mesenchymal stem/stromal cells (MSCs). We determined intramedullary silver release profiles in vivo, which were used to test relevant Ag⁺ concentrations on MSC function in vitro. We measured a rapid elution of Ag⁺ from intramedullary pins in a rat femoral implantation model, delivering a maximum potential concentration of 7.8 µM, which was below toxic levels determined for MSCs in vitro (EC₅₀, 33 µM). Additionally, we present in vitro data of the reduced colonisation of implants by Staphylococcus aureus. MSCs exposed to Ag⁺ prior to/during osteogenic differentiation were not statistically affected. Notably, at clonal density, the colony-forming capacity of MSCs was significantly reduced in the presence of 10 µM Ag⁺, suggesting that a subpopulation of clonal MSCs was sensitive to Ag⁺ exposure. At a molecular level, surviving colony-forming MSCs treated with Ag⁺ demonstrated a significant upregulation of components of the peroxiredoxin/thioredoxin pathway and processes involved in glutathione metabolism compared to untreated controls. Inhibition of glutathione synthesis using l-buthionine sulfoxamine eliminated MSC clonogenicity in the presence of Ag⁺, which was rescued by exogenous glutathione.

Wolbachia's Deleterious Impact on Aedes aegypti Egg Development: The Potential Role of Nutritional Parasitism

Simple Summary

Mosquito-borne viral diseases such as dengue, Zika and chikungunya cause a significant global health burden and are currently increasing in outbreak frequency and geographical reach. Wolbachia pipientis, an endosymbiotic bacterium, offers a solution to this. When Wolbachia is introduced into the main mosquito vector of these viruses, Aedes aegypti, it alters the mosquito’s reproductive biology, as well as reducing the ability of the mosquitoes to transmit viruses. These traits can be leveraged to reduce virus transmission within a community by mass releasing Wolbachia-infected mosquitoes. However, Wolbachia has some negative effects on Aedes aegypti fitness, particularly egg longevity, and the reason behind this remains ambiguous. Insect fitness is very important for the success for Wolbachia-biocontrol strategies as they rely on the released insects being competitive with the wild mosquito population. This review summarises the fitness effects of Wolbachia on Aedes aegypti and investigates the possible contribution of Wolbachia as a nutritional parasite in lowering host fitness. It proposes the next stages of research that can be conducted to address nutritional parasitism to aid in the expansion of Wolbachia-based disease management programs worldwide.

Abstract

The artificial introduction of the endosymbiotic bacterium, Wolbachia pipientis, into Aedes (Ae.) aegypti mosquitoes reduces the ability of mosquitoes to transmit human pathogenic viruses and is now being developed as a biocontrol tool. Successful introgression of Wolbachia-carrying Ae. aegypti into native mosquito populations at field sites in Australia, Indonesia and Malaysia has been associated with reduced disease prevalence in the treated community. In separate field programs, Wolbachia is also being used as a mosquito population suppression tool, where the release of male only Wolbachia-infected Ae. aegypti prevents the native mosquito population from producing viable eggs, subsequently suppressing the wild population. While these technologies show great promise, they require mass rearing of mosquitoes for implementation on a scale that has not previously been done. In addition, Wolbachia induces some negative fitness effects on Ae. aegypti. While these fitness effects differ depending on the Wolbachia strain present, one of the most consistent and significant impacts is the shortened longevity and viability of eggs. This review examines the body of evidence behind Wolbachia’s negative effect on eggs, assesses nutritional parasitism as a key cause and considers how these impacts could be overcome to achieve efficient large-scale rearing of these mosquitoes.

Tissue Tropisms and Transstadial Transmission of a Rickettsia Endosymbiont in the Highland Midge, Culicoides impunctatus (Diptera: Ceratopogonidae)

Microbial symbionts of disease vectors have garnered recent attention due to their ability to alter vectorial capacity. Their consideration as a means of arbovirus control depends on symbiont vertical transmission, which leads to spread of the bacteria through a population. Previous work has identified a Rickettsia symbiont present in several species of biting midges (Culicoides spp.), which transmit bluetongue and Schmallenberg arboviruses. However, symbiont transmission strategies and host effects remain underexplored. In this study, we describe the presence of Rickettsia in the ovarian suspensory ligament of Culicoides impunctatus. Infection of this organ suggests the connective tissue surrounding developing eggs is important for ensuring vertical transmission of the symbiont in midges and possibly other insects. Additionally, our results indicate Rickettsia localization in the fat body of Culicoides impunctatus. As the arboviruses spread by midges often replicate in the fat body, this location implies possible symbiont-virus interactions to be further investigated.

Rickettsia is a genus of intracellular bacteria which can manipulate host reproduction and alter sensitivity to natural enemy attack in a diverse range of arthropods. The maintenance of Rickettsia endosymbionts in insect populations can be achieved through both vertical and horizontal transmission routes. For example, the presence of the symbiont in the follicle cells and salivary glands of Bemisia whiteflies allows Belli group Rickettsia transmission via the germ line and plants, respectively. However, the transmission routes of other Rickettsia bacteria, such as those in the Torix group of the genus, remain underexplored. Through fluorescence in situ hybridization (FISH) and transmission electron microscopy (TEM) screening, this study describes the pattern of Torix Rickettsia tissue tropisms in the highland midge, Culicoides impunctatus (Diptera: Ceratopogonidae). Of note is the high intensity of infection of the ovarian suspensory ligament, suggestive of a novel germ line targeting strategy. Additionally, localization of the symbiont in tissues of several developmental stages suggests transstadial transmission is a major route for ensuring maintenance of Rickettsia within C. impunctatus populations. Aside from providing insights into transmission strategies, the presence of Rickettsia bacteria in the fat body of larvae indicates potential host fitness and vector capacity impacts to be investigated in the future.

IMPORTANCE Microbial symbionts of disease vectors have garnered recent attention due to their ability to alter vectorial capacity. Their consideration as a means of arbovirus control depends on symbiont vertical transmission, which leads to spread of the bacteria through a population. Previous work has identified a Rickettsia symbiont present in several species of biting midges (Culicoides spp.), which transmit bluetongue and Schmallenberg arboviruses. However, symbiont transmission strategies and host effects remain underexplored. In this study, we describe the presence of Rickettsia in the ovarian suspensory ligament of Culicoides impunctatus. Infection of this organ suggests the connective tissue surrounding developing eggs is important for ensuring vertical transmission of the symbiont in midges and possibly other insects. Additionally, our results indicate Rickettsia localization in the fat body of Culicoides impunctatus. As the arboviruses spread by midges often replicate in the fat body, this location implies possible symbiont-virus interactions to be further investigated.

Modulation of acyl-carnitines, the broad mechanism behind Wolbachia-mediated inhibition of medically important flaviviruses in Aedes aegypti

Wolbachia-infected mosquitoes are refractory to flavivirus infections, but the role of lipids in Wolbachia-mediated virus blocking remains to be elucidated. Here, we use liquid chromatography mass spectrometry to provide a comprehensive picture of the lipidome of Aedes aegypti (Aag2) cells infected with Wolbachia only, either dengue or Zika virus only, and Wolbachia-infected Aag2 cells superinfected with either dengue or Zika virus. This approach identifies a class of lipids, acyl-carnitines, as being down-regulated during Wolbachia infection. Furthermore, treatment with an acyl-carnitine inhibitor assigns a crucial role for acyl-carnitines in the replication of dengue and Zika viruses. In contrast, depletion of acyl-carnitines increases Wolbachia density while addition of commercially available acyl-carnitines impairs Wolbachia production. Finally, we show an increase in flavivirus infection of Wolbachia-infected cells with the addition of acyl-carnitines. This study uncovers a previously unknown role for acyl-carnitines in this tripartite interaction that suggests an important and broad mechanism that underpins Wolbachia-mediated pathogen blocking.

Dengue virus dominates lipid metabolism modulations in Wolbachia-coinfected Aedes aegypti

Competition between viruses and Wolbachia for host lipids is a proposed mechanism of Wolbachia-mediated virus blocking in insects. Yet, the metabolomic interaction between virus and symbiont within the mosquito has not been clearly defined. We compare the lipid profiles of Aedes aegypti mosquitoes bearing mono- or dual-infections of the Wolbachia wMel strain and dengue virus serotype 3 (DENV3). We found metabolic signatures of infection-induced intracellular events but little evidence to support direct competition between Wolbachia and virus for host lipids. Lipid profiles of dual-infected mosquitoes resemble those of DENV3 mono-infected mosquitoes, suggesting virus-driven modulation dominates over that of Wolbachia. Interestingly, knockdown of key metabolic enzymes suggests cardiolipins are host factors for DENV3 and Wolbachia replication. These findings define the Wolbachia-DENV3 metabolic interaction as indirectly antagonistic, rather than directly competitive, and reveal new research avenues with respect to mosquito × virus interactions at the molecular level.

Koh, Islam, Ye et al. describe lipid profiles of Aedes aegypti mosquitoes bearing mono- or dual-infections of Wolbachia (wMel) and dengue virus serotype 3 (DENV3), finding that virus modulation dominates the dual-infection lipid profile and that cardiolipins support DENV3 and Wolbachia replication. This study suggests that direct competition for lipids do not underlie Wolbachia-mediated virus blocking.

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.

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.

Artificial Selection Finds New Hypotheses for the Mechanism of Wolbachia-Mediated Dengue Blocking in Mosquitoes

Wolbachia is an intracellular bacterium that blocks virus replication in insects and has been introduced into the mosquito, Aedes aegypti for the biocontrol of arboviruses including dengue, Zika, and chikungunya. Despite ongoing research, the mechanism of Wolbachia-mediated virus blocking remains unclear. We recently used experimental evolution to reveal that Wolbachia-mediated dengue blocking could be selected upon in the A. aegypti host and showed evidence that strong levels of blocking could be maintained by natural selection. In this study, we investigate the genetic variation associated with blocking and use these analyses to generate testable hypotheses surrounding the mechanism of Wolbachia-mediated dengue blocking. From our results, we hypothesize that Wolbachia may block virus replication by increasing the regeneration rate of mosquito cells via the Notch signaling pathway. We also propose that Wolbachia modulates the host’s transcriptional pausing pathway either to prime the host’s anti-viral response or to directly inhibit viral replication.

Novel phenotype of Wolbachia strain wPip in Aedes aegypti challenges assumptions on mechanisms of Wolbachia-mediated dengue virus inhibition

The bacterial endosymbiont Wolbachia is a biocontrol tool that inhibits the ability of the Aedes aegypti mosquito to transmit positive-sense RNA viruses such as dengue and Zika. Growing evidence indicates that when Wolbachia strains wMel or wAlbB are introduced into local mosquito populations, human dengue incidence is reduced. Despite the success of this novel intervention, we still do not fully understand how Wolbachia protects mosquitoes from viral infection. Here, we demonstrate that the Wolbachia strain wPip does not inhibit virus infection in Ae. aegypti. We have leveraged this novel finding, and a panel of Ae. aegypti lines carrying virus-inhibitory (wMel and wAlbB) and non-inhibitory (wPip) strains in a common genetic background, to rigorously test a number of hypotheses about the mechanism of Wolbachia-mediated virus inhibition. We demonstrate that, contrary to previous suggestions, there is no association between a strain's ability to inhibit dengue infection in the mosquito and either its typical density in the midgut or salivary glands, or the degree to which it elevates innate immune response pathways in the mosquito. These findings, and the experimental platform provided by this panel of genetically comparable mosquito lines, clear the way for future investigations to define how Wolbachia prevents Ae. aegypti from transmitting viruses.

Viral RNA is a target for Wolbachia-mediated pathogen blocking

The ability of the endosymbiont Wolbachia pipientis to restrict RNA viruses is presently being leveraged to curb global transmission of arbovirus-induced diseases. Past studies have shown that virus replication is limited early in arthropod cells colonized by the bacterium, although it is unclear if this phenomenon is replicated in mosquito cells that first encounter viruses obtained through a vertebrate blood meal. Furthermore, these cellular events neither explain how Wolbachia limits dissemination of viruses between mosquito tissues, nor how it prevents transmission of infectious viruses from mosquitoes to vertebrate host. In this study, we try to address these issues using an array of mosquito cell culture models, with an additional goal being to identify a common viral target for pathogen blocking. Our results establish the viral RNA as a cellular target for Wolbachia-mediated inhibition, with the incoming viral RNA experiencing rapid turnover following internalization in cells. This early block in replication in mosquito cells initially infected by the virus thus consequently reduces the production of progeny viruses from these same cells. However, this is not the only contributor to pathogen blocking. We show that the presence of Wolbachia reduces the per-particle infectivity of progeny viruses on naïve mosquito and vertebrate cells, consequently limiting virus dissemination and transmission, respectively. Importantly, we demonstrate that this aspect of pathogen blocking is independent of any particular Wolbachia-host association and affects viruses belonging to Togaviridae and Flaviviridae families of RNA viruses. Finally, consistent with the idea of the viral RNA as a target, we find that the encapsidated virion RNA is less infectious for viruses produced from Wolbachia-colonized cells. Collectively, our findings present a common mechanism of pathogen blocking in mosquitoes that establish a link between virus inhibition in the cell to virus dissemination and transmission.

A Review: Wolbachia-Based Population Replacement for Mosquito Control Shares Common Points with Genetically Modified Control Approaches

The growing expansion of mosquito vectors has made mosquito-borne arboviral diseases a global threat to public health, and the lack of licensed vaccines and treatments highlight the urgent need for efficient mosquito vector control. Compared to genetically modified control strategies, the intracellular bacterium Wolbachia, endowing a pathogen-blocking phenotype, is considered an environmentally friendly strategy to replace the target population for controlling arboviral diseases. However, the incomplete knowledge regarding the pathogen-blocking mechanism weakens the reliability of a Wolbachia-based population replacement strategy. Wolbachia infections are also vulnerable to environmental factors, temperature, and host diet, affecting their densities in mosquitoes and thus the virus-blocking phenotype. Here, we review the properties of the Wolbachia strategy as an approach to control mosquito populations in comparison with genetically modified control methods. Both strategies tend to limit arbovirus infections but increase the risk of selecting arbovirus escape mutants, rendering these strategies less reliable.

Roles of Symbiotic Microorganisms in Arboviral Infection of Arthropod Vectors

Arthropod vectors serve as native reservoirs and transmitters of hundreds of arboviruses. In arthropod vectors, symbiotic microorganisms residing in the gut lumen and/or hemocoelic tissues maintain complicated relationships with their host and influence multiple aspects of vector physiology. Recently, accumulating evidence has established an important role for symbiotic microorganisms in vector-virus interactions which could potentially be used to control viral transmission. Herein, we review recent progress on symbiotic microbe-arbovirus interactions and summarize the molecular mechanisms by which commensal microbes act on hosts and arboviruses. Understanding the sophisticated interactions among arthropod vectors, microbiota, and arboviruses may offer new strategies for the prevention of arboviral diseases in the future.

A Role for the Insulin Receptor in the Suppression of Dengue Virus and Zika Virus in Wolbachia-Infected Mosquito Cells

Wolbachia-infected mosquitoes are refractory to super-infection with arthropod-borne pathogens, but the role of host cell signaling proteins in pathogen-blocking mechanisms remains to be elucidated. Here, we use an antibody microarray approach to provide a comprehensive picture of the signaling response of Aedes aegypti-derived cells to Wolbachia. This approach identifies the host cell insulin receptor as being downregulated by the bacterium. Furthermore, siRNA-mediated knockdown and treatment with a small-molecule inhibitor of the insulin receptor kinase concur to assign a crucial role for this enzyme in the replication of dengue and Zika viruses in cultured mosquito cells. Finally, we show that the production of Zika virus in Wolbachia-free live mosquitoes is impaired by treatment with the selective inhibitor mimicking Wolbachia infection. This study identifies Wolbachia-mediated downregulation of insulin receptor kinase activity as a mechanism contributing to the blocking of super-infection by arboviruses.

Extracellular vesicles restrict dengue virus fusion in Aedes aegypti cells

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

Titer regulation in arthropod-Wolbachia symbioses

Symbiosis between intracellular bacteria (endosymbionts) and animals are widespread. The alphaproteobacterium Wolbachia pipientis is known to maintain a variety of symbiotic associations, ranging from mutualism to parasitism, with a wide range of invertebrates. Wolbachia infection might deeply affect host fitness (e.g. reproductive manipulation and antiviral protection), which is thought to explain its high prevalence in nature. Bacterial loads significantly influence both the infection dynamics and the extent of bacteria-induced host phenotypes. Hence, fine regulation of bacterial titers is considered as a milestone in host-endosymbiont interplay. Here, we review both environmental and biological factors modulating Wolbachia titers in arthropods.

Wolbachia transinfections in Culex quinquefasciatus generate cytoplasmic incompatibility

Culex quinquefasciatus is an important mosquito vector of a number of viral and protozoan pathogens of humans and animals, and naturally carries the endosymbiont Wolbachia pipientis, strain wPip. Wolbachia are used in two distinct vector control strategies: firstly, population suppression caused by mating incompatibilities between mass-released transinfected males and wild females; and secondly, the spread of pathogen transmission-blocking strains through populations. Using embryonic microinjection, two novel Wolbachia transinfections were generated in C. quinquefasciatus using strains native to the mosquito Aedes albopictus: a wAlbB single infection, and a wPip plus wAlbA superinfection. The wAlbB infection showed full bidirectional cytoplasmic incompatibility (CI) with wild-type C. quinquefasciatus in reciprocal crosses. The wPipwAlbA superinfection showed complete unidirectional CI, and therefore population invasion potential. Whereas the wAlbB strain showed comparatively low overall densities, similar to the native wPip, the wPipwAlbA superinfection reached over 400-fold higher densities in the salivary glands compared to the native wPip, suggesting it may be a candidate for pathogen transmission blocking.

Highly efficient photocontrolled targeted delivery of siRNA by a cyclodextrin-based supramolecular nanoassembly

A binary supramolecular nanoassembly that can efficiently load siRNA into A549 cancer cells and inhibited cell growth by photo-irradiation was fabricated using α-CD-modified hyaluronic acid and an azobenzene-modified diphenylalanine derivative.

FlySilico: Flux balance modeling of Drosophila larval growth and resource allocation

Organisms depend on a highly connected and regulated network of biochemical reactions fueling life sustaining and growth promoting functions. While details of this metabolic network are well established, knowledge of the superordinate regulatory design principles is limited. Here, we investigated by iterative wet lab and modeling experiments the resource allocation process during the larval development of Drosophila melanogaster. We chose this system, as survival of the animals depends on the successful allocation of their available resources to the conflicting processes of growth and storage metabolite deposition. First, we generated “FlySilico”, a curated metabolic network of Drosophila, and performed time-resolved growth and metabolite measurements with larvae raised on a holidic diet. Subsequently, we performed flux balance analysis simulations and tested the predictive power of our model by simulating the impact of diet alterations on growth and metabolism. Our predictions correctly identified the essential amino acids as growth limiting factor, and metabolic flux differences in agreement with our experimental data. Thus, we present a framework to study important questions of resource allocation in a multicellular organism including process priorization and optimality principles.

Virus evolution in Wolbachia-infected Drosophila

Wolbachia, a common vertically transmitted symbiont, can protect insects against viral infection and prevent mosquitoes from transmitting viral pathogens. For this reason, Wolbachia-infected mosquitoes are being released to prevent the transmission of dengue and other arboviruses. An important question for the long-term success of these programmes is whether viruses can evolve to escape the antiviral effects of Wolbachia. We have found that Wolbachia altered the outcome of competition between strains of the DCV virus in Drosophila. However, Wolbachia still effectively blocked the virus genotypes that were favoured in the presence of the symbiont. We conclude that Wolbachia did cause an evolutionary response in viruses, but this has little or no impact on the effectiveness of virus blocking.

Microbial Control of Intestinal Homeostasis via Enteroendocrine Cell Innate Immune Signaling

A community of commensal microbes, known as the intestinal microbiota, resides within the gastrointestinal tract of animals and plays a role in maintenance of host metabolic homeostasis and resistance to pathogen invasion. Enteroendocrine cells, which are relatively rare in the intestinal epithelium, have evolved to sense and respond to these commensal microbes. Specifically, they express G-protein-coupled receptors and functional innate immune signaling pathways that recognize products of microbial metabolism and microbe-associated molecular patterns, respectively. Here we review recent evidence from Drosophila melanogaster that microbial cues recruit antimicrobial, mechanical, and metabolic branches of the enteroendocrine innate immune system and argue that this response may play a role not only in maintaining host metabolic homeostasis but also in intestinal resistance to invasion by bacterial, viral, and parasitic pathogens.

TRIM69 Inhibits Vesicular Stomatitis Indiana Virus

Vesicular stomatitis Indiana virus (VSIV), formerly known as vesicular stomatitis virus (VSV) Indiana (VSVIND), is a model virus that is exceptionally sensitive to the inhibitory action of interferons (IFNs). Interferons induce an antiviral state by stimulating the expression of hundreds of interferon-stimulated genes (ISGs). These ISGs can constrain viral replication, limit tissue tropism, reduce pathogenicity, and inhibit viral transmission. Since VSIV is used as a backbone for multiple oncolytic and vaccine strategies, understanding how ISGs restrict VSIV not only helps in understanding VSIV-induced pathogenesis but also helps us evaluate and understand the safety and efficacy of VSIV-based therapies. Thus, there is a need to identify and characterize the ISGs that possess anti-VSIV activity. Using arrayed ISG expression screening, we identified TRIM69 as an ISG that potently inhibits VSIV. This inhibition was highly specific as multiple viruses, including influenza A virus, HIV-1, Rift Valley fever virus, and dengue virus, were unaffected by TRIM69. Indeed, just one amino acid substitution in VSIV can govern sensitivity/resistance to TRIM69. Furthermore, TRIM69 is highly divergent in human populations and exhibits signatures of positive selection that are consistent with this gene playing a key role in antiviral immunity. We propose that TRIM69 is an IFN-induced inhibitor of VSIV and speculate that TRIM69 could be important in limiting VSIV pathogenesis and might influence the specificity and/or efficacy of vesiculovirus-based therapies.IMPORTANCE Vesicular stomatitis Indiana virus (VSIV) is a veterinary pathogen that is also used as a backbone for many oncolytic and vaccine strategies. In natural and therapeutic settings, viral infections like VSIV are sensed by the host, and as a result the host cells make proteins that can protect them from viruses. In the case of VSIV, these antiviral proteins constrain viral replication and protect most healthy tissues from virus infection. In order to understand how VSIV causes disease and how healthy tissues are protected from VSIV-based therapies, it is crucial that we identify the proteins that inhibit VSIV. Here, we show that TRIM69 is an antiviral defense that can potently and specifically block VSIV infection.

U18666A inhibits classical swine fever virus replication through interference with intracellular cholesterol trafficking

The level of cholesterol in host cells has been demonstrated to affect viral infection. Our previous studies showed that cholesterol-rich membrane rafts mediated the entry of classical swine fever virus (CSFV) into PK-15 or 3D4/21 cells, but the role of cholesterol post entry was still not clear. In this study, we found that CSFV replication before fusion was affected when the cholesterol trafficking in infected cells was disrupted using a cholesterol transport inhibitor, U18666A. Our data showed that U18666A affected both the fusion and replication steps in the life cycle of the virus, but not its binding and entry steps. The subsequent experiments confirmed that niemann-pick C1 (NPC1), a lysosomal membrane protein that helps cholesterol to leave the lysosome, was affected by U18666A, which led to the accumulation of cholesterol in lysosomes and inhibition of CSFV replication. Imipramine, a cationic hydrophobic amine similar to U18666A, also inhibited CSFV replication via similar mechanism. Surprisingly, the antiviral effect of U18666A was restored by the histone deacetylase inhibitor (HDACi), Vorinostat, which suggested that HDACi reverted the dysfunction of NPC1, and intra-cellular cholesterol accumulation disappeared and CSFV replicability resumed. Together, these data indicated that CSFV transformed from early endosome and late endosome into lysosome after endocytosis for further replication and that U18666A was a potential drug candidate for anti-pestivirus treatment.

Quantitative methods for assessing local and bodywide contributions to Wolbachia titer in maternal germline cells of Drosophila

BACKGROUND

Little is known about how bacterial endosymbionts colonize host tissues. Because many insect endosymbionts are maternally transmitted, egg colonization is critical for endosymbiont success. Wolbachia bacteria, carried by approximately half of all insect species, provide an excellent model for characterizing endosymbiont infection dynamics. To date, technical limitations have precluded stepwise analysis of germline colonization by Wolbachia. It is not clear to what extent titer-altering effects are primarily mediated by growth rates of Wolbachia within cell lineages or migration of Wolbachia between cells.

RESULTS

The objective of this work is to inform mechanisms of germline colonization through use of optimized methodology. The approaches are framed in terms of nutritional impacts on Wolbachia. Yeast-rich diets in particular have been shown to suppress Wolbachia titer in the Drosophila melanogaster germline. To determine the extent of Wolbachia sensitivity to diet, we optimized 3-dimensional, multi-stage quantification of Wolbachia titer in maternal germline cells. Technical and statistical validation confirmed the identity of Wolbachia in vivo, the reproducibility of Wolbachia quantification and the statistical power to detect these effects. The data from adult feeding experiments demonstrated that germline Wolbachia titer is distinctly sensitive to yeast-rich host diets in late oogenesis. To investigate the physiological basis for these nutritional impacts, we optimized methodology for absolute Wolbachia quantification by real-time qPCR. We found that yeast-rich diets exerted no significant effect on bodywide Wolbachia titer, although ovarian titers were significantly reduced. This suggests that host diets affects Wolbachia distribution between the soma and late stage germline cells. Notably, relative qPCR methods distorted apparent wsp abundance, due to altered host DNA copy number in yeast-rich conditions. This highlights the importance of absolute quantification data for testing mechanistic hypotheses.

CONCLUSIONS

We demonstrate that absolute quantification of Wolbachia, using well-controlled cytological and qPCR-based methods, creates new opportunities to determine how bacterial abundance within the germline relates to bacterial distribution within the body. This methodology can be applied to further test germline infection dynamics in response to chemical treatments, genetic conditions, new host/endosymbiont combinations, or potentially adapted to analyze other cell and tissue types.

Curious entanglements: interactions between mosquitoes, their microbiota, and arboviruses

Mosquitoes naturally harbor a diverse community of microorganisms that play a crucial role in their biology. Mosquito-microbiota interactions are abundant and complex. They can dramatically alter the mosquito immune response, and impede or enhance a mosquito's ability to transmit medically important arboviral pathogens. Yet critically, given the massive public health impact of arboviral disease, few such interactions have been well characterized. In this review, we describe the current state of knowledge of the role of microorganisms in mosquito biology, how microbial-induced changes to mosquito immunity moderate infection with arboviruses, cases of mosquito-microbial-virus interactions with a defined mechanism, and the molecular interactions that underlie the endosymbiotic bacterium Wolbachia's ability to block virus infection in mosquitoes.

How do Wolbachia modify the Drosophila ovary? New evidences support the "titration-restitution" model for the mechanisms of Wolbachia-induced CI

Background

Cytoplasmic incompatibility (CI) is the most common phenotype induced by endosymbiont Wolbachia and results in embryonic lethality when Wolbachia-modified sperm fertilize eggs without Wolbachia. However, eggs carrying the same strain of Wolbachia can rescue this embryonic death, thus producing viable Wolbachia-infected offspring. Hence Wolbachia can be transmitted mainly by hosts’ eggs. One of the models explaining CI is “titration-restitution”, which hypothesized that Wolbachia titrated-out some factors from the sperm and the Wolbachia in the egg would restitute the factors after fertilization. However, how infected eggs rescue CI and how hosts’ eggs ensure the proliferation and transmission of Wolbachia are not well understood.

Results

By RNA-seq analyses, we first compared the transcription profiles of Drosophila melanogaster adult ovaries with and without the wMel Wolbachia and identified 149 differentially expressed genes (DEGs), of which 116 genes were upregulated and 33 were downregulated by Wolbachia infection. To confirm the results obtained from RNA-seq and to screen genes potentially associated with reproduction, 15 DEGs were selected for quantitative RT-PCR (qRT-PCR). Thirteen genes showed the same changing trend as RNA-seq analyses. To test whether these genes are associated with CI, we also detected their expression levels in testes. Nine of them exhibited different changing trends in testes from those in ovaries. To investigate how these DEGs were regulated, sRNA sequencing was performed and identified seven microRNAs (miRNAs) that were all upregulated in fly ovaries by Wolbachia infection. Matching of miRNA and mRNA data showed that these seven miRNAs regulated 15 DEGs. Wolbachia-responsive genes in fly ovaries were involved in biological processes including metabolism, transportation, oxidation-reduction, immunity, and development.

Conclusions

Comparisons of mRNA and miRNA data from fly ovaries revealed 149 mRNAs and seven miRNAs that exhibit significant changes in expression due to Wolbachia infection. Notably, most of the DEGs showed variation in opposite directions in ovaries versus testes in the presence of Wolbachia, which generally supports the “titration-restitution” model for CI. Furthermore, genes related to metabolism were upregulated, which may benefit maximum proliferation and transmission of Wolbachia. This provides new insights into the molecular mechanisms of Wolbachia-induced CI and Wolbachia dependence on host ovaries.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5977-6) contains supplementary material, which is available to authorized users.

Infection of Aedes albopictus Mosquito C6/36 Cells with the wMelpop Strain of Wolbachia Modulates Dengue Virus-Induced Host Cellular Transcripts and Induces Critical Sequence Alterations in the Dengue Viral Genome

Dengue virus (DENV) causes frequent epidemics infecting ∼390 million people annually in over 100 countries. There are no approved vaccines or antiviral drugs for treatment of infected patients. However, there is a novel approach to control DENV transmission by the mosquito vectors, Aedes aegypti and Aedes albopictus, using the Wolbachia symbiont. The wMelPop strain of Wolbachia suppresses DENV transmission and shortens the mosquito life span. However, the underlying mechanism is poorly understood. To clarify this mechanism, either naive A. albopictus (C6/36) or wMelPop-C6/36 cells were infected with DENV serotype 2 (DENV2). Analysis of host transcript profiles by transcriptome sequencing (RNAseq) revealed that the presence of wMelPop dramatically altered the mosquito host cell transcription in response to DENV2 infection. The viral RNA evolved from wMelPop-C6/36 cells contained low-frequency mutations (∼25%) within the coding region of transmembrane domain 1 (TMD1) of E protein. Mutations with >97% frequencies were distributed within other regions of E, the NS5 RNA-dependent RNA polymerase (NS5POL) domain, and the TMDs of NS2A, NS2B, and NS4B. Moreover, while DENV2-infected naive C6/36 cells showed syncytium formation, DENV2-infected wMelPop-C6/36 cells did not. The Wolbachia-induced mutant DENV2 can readily infect and replicate in naive C6/36 cells, whereas in mutant DENV2-infected BHK-21 or Vero cells, virus replication was delayed. In LLC-MK2 cells, the mutant failed to produce plaques. Additionally, in BHK-21 cells, many mutations in the viral genome reverted to the wild type (WT) and compensatory mutations in NS3 gene appeared. Our results indicate that wMelPop impacts significantly the interactions of DENV2 with mosquito and mammalian host cells.IMPORTANCE Mosquito-borne diseases are of global significance causing considerable morbidity and mortality throughout the world. Dengue virus (DENV; serotypes 1 to 4), a member of the Flavivirus genus of the Flaviviridae family, causes millions of infections annually. Development of a safe vaccine is hampered due to absence of cross-protection and increased risk in secondary infections due to antibody-mediated immune enhancement. Infection of vector mosquitoes with Wolbachia bacteria offers a novel countermeasure to suppress DENV transmission, but the mechanisms are poorly understood. In this study, the host transcription profiles and viral RNA sequences were analyzed in naive A. albopictus (C6/36) and wMelPop-C6/36 cells by RNAseq. Our results showed that the wMelPop symbiont caused profound changes in host transcription profiles and morphology of DENV2-infected C6/36 cells. Accumulation of several mutations throughout DENV2 RNA resulted in loss of infectivity of progeny virions. Our findings offer new insights into the mechanism of Wolbachia-mediated suppression of DENV transmission.

Two-By-One model of cytoplasmic incompatibility: Synthetic recapitulation by transgenic expression of cifA and cifB in Drosophila

Wolbachia are maternally inherited bacteria that infect arthropod species worldwide and are deployed in vector control to curb arboviral spread using cytoplasmic incompatibility (CI). CI kills embryos when an infected male mates with an uninfected female, but the lethality is rescued if the female and her embryos are likewise infected. Two phage WO genes, cifA_wMel and cifB_wMel from the wMel Wolbachia deployed in vector control, transgenically recapitulate variably penetrant CI, and one of the same genes, cifA_wMel, rescues wild type CI. The proposed Two-by-One genetic model predicts that CI and rescue can be recapitulated by transgenic expression alone and that dual cifA_wMeland cifB_wMel expression can recapitulate strong CI. Here, we use hatch rate and gene expression analyses in transgenic Drosophila melanogaster to demonstrate that CI and rescue can be synthetically recapitulated in full, and strong, transgenic CI comparable to wild type CI is achievable. These data explicitly validate the Two-by-One model in wMel-infected D. melanogaster, establish a robust system for transgenic studies of CI in a model system, and represent the first case of completely engineering male and female animal reproduction to depend upon bacteriophage gene products.

Releases of Wolbachia-infected mosquitos are underway worldwide because Wolbachia block replication of Zika and Dengue viruses and spread themselves maternally through arthropod populations via cytoplasmic incompatibility (CI). The CI drive system depends on a Wolbachia-induced sperm modification that results in embryonic lethality when an infected male mates with an uninfected female, but this lethality is rescued when the female and her embryos are likewise infected. We recently reported that the phage WO genes, cifA and cifB, cause the sperm modification and cifA rescues the embryonic lethality caused by the wMel Wolbachia strain deployed in vector control. These reports motivated proposal of the Two-by-One model of CI whereby two genes cause lethality and one gene rescues it. Here we provide unequivocal support for the model in the Wolbachia strain used in vector control via synthetic methods that recapitulate CI and rescue in the absence of a Wolbachia infections. Our results reveal the set of phage WO genes responsible for this powerful genetic drive system, act as a proof-of-concept that these genes alone can induce gene drive like crossing patterns, and establish methodologies and hypotheses for future studies of CI in Drosophila. We discuss the implications of the Two-by-One model towards functional mechanisms of CI, the emergence of incompatibility between Wolbachia strains, vector control applications, and CI gene nomenclature.

Lipid Metabolism as a Source of Druggable Targets for Antiviral Discovery against Zika and Other Flaviviruses

The Zika virus (ZIKV) is a mosquito-borne flavivirus that can lead to birth defects (microcephaly), ocular lesions and neurological disorders (Guillain-Barré syndrome). There is no licensed vaccine or antiviral treatment against ZIKV infection. The effort to understand the complex interactions of ZIKV with cellular networks contributes to the identification of novel host-directed antiviral (HDA) candidates. Among the cellular pathways involved in infection, lipid metabolism gains attention. In ZIKV-infected cells lipid metabolism attributed to intracellular membrane remodeling, virion morphogenesis, autophagy modulation, innate immunity and inflammation. The key roles played by the cellular structures associated with lipid metabolism, such as peroxisomes and lipid droplets, are starting to be deciphered. Consequently, there is a wide variety of lipid-related antiviral strategies that are currently under consideration, which include an inhibition of sterol regulatory element-binding proteins (SREBP), the activation of adenosine-monophosphate activated kinase (AMPK), an inhibition of acetyl-Coenzyme A carboxylase (ACC), interference with sphingolipid metabolism, blockage of intracellular cholesterol trafficking, or a treatment with cholesterol derivatives. Remarkably, most of the HDAs identified in these studies are also effective against flaviviruses other than ZIKV (West Nile virus and dengue virus), supporting their broad-spectrum effect. Considering that lipid metabolism is one of the main cellular pathways suitable for pharmacological intervention, the idea of repositioning drugs targeting lipid metabolism as antiviral candidates is gaining force.

The effect of Wolbachia on gene expression in Drosophila paulistorum and its implications for symbiont-induced host speciation

Background

The Neotropical fruit fly Drosophila paulistorum (Diptera: Drosophilidae) is a species complex in statu nascendi comprising six reproductively isolated semispecies, each harboring mutualistic Wolbachia strains. Although wild type flies of each semispecies are isolated from the others by both pre- and postmating incompatibilities, mating between semispecies and successful offspring development can be achieved once flies are treated with antibiotics to reduce Wolbachia titer. Here we use RNA-seq to study the impact of Wolbachia on D. paulistorum and investigate the hypothesis that the symbiont may play a role in host speciation. For that goal, we analyze samples of heads and abdomens of both sexes of the Amazonian, Centro American and Orinocan semispecies of D. paulistorum.

Results

We identify between 175 and 1192 differentially expressed genes associated with a variety of biological processes that respond either globally or according to tissue, sex or condition in the three semispecies. Some of the functions associated with differentially expressed genes are known to be affected by Wolbachia in other species, such as metabolism and immunity, whereas others represent putative novel phenotypes involving muscular functions, pheromone signaling, and visual perception.

Conclusions

Our results show that Wolbachia affect a large number of biological functions in D. paulistorum, particularly when present in high titer. We suggest that the significant metabolic impact of the infection on the host may cause several of the other putative and observed phenotypes. We also speculate that the observed differential expression of genes associated with chemical communication and reproduction may be associated with the emergence of pre- and postmating barriers between semispecies, which supports a role for Wolbachia in the speciation of D. paulistorum.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5816-9) contains supplementary material, which is available to authorized users.