Highly focused transcriptional response of Anopheles coluzzii to O'nyong nyong arbovirus during the primary midgut infection

Transcriptomic and small RNA response to Mayaro virus infection in Anopheles stephensi mosquitoes

Mayaro virus (MAYV) is an arboviral pathogen in the genus Alphavirus that is circulating in South America with potential to spread to naïve regions. MAYV is also one of the few viruses with the ability to be transmitted by mosquitoes in the genus Anopheles, as well as the typical arboviral transmitting mosquitoes in the genus Aedes. Few studies have investigated the infection response of Anopheles mosquitoes. In this study we detail the transcriptomic and small RNA responses of An. stephensi to infection with MAYV via infectious bloodmeal at 2, 7, and 14 days post infection (dpi). 487 unique transcripts were significantly regulated, 78 putative novel miRNAs were identified, and an siRNA response is observed targeting the MAYV genome. Gene ontology analysis of transcripts regulated at each timepoint shows a number of proteases regulated at 2 and 7 dpi, potentially representative of Toll or melanization pathway activation, and repression of pathways related to autophagy and apoptosis at 14 dpi. These findings provide a basic understanding of the infection response of An. stephensi to MAYV and help to identify host factors which might be useful to target to inhibit viral replication in Anopheles mosquitoes.

Mayaro virus (MAYV) is a mosquito-borne Alphavirus responsible for outbreaks in South America and the Caribbean. In this study we infected Anopheles stephensi with MAYV and sequenced mRNA and small RNA to understand how MAYV infection impacts gene transcription and the expression of small RNAs in the mosquito vector. Genes involved with innate immunity and signaling pathways related to cell death are regulated in response to MAYV infection of An. stephensi, we also discovered novel miRNAs and describe the expression patterns of miRNAs, siRNAs, and piRNAs following bloodmeal ingestion. These results suggest that MAYV does induce a molecular response to infection in its mosquito vector species.

Role of Anopheles Mosquitoes in Cache Valley Virus Lineage Displacement, New York, USA

Cache Valley virus (CVV) is a mosquitoborne virus that infects livestock and humans. We report results of surveillance for CVV in New York, USA, during 2000–2016; full-genome analysis of selected CVV isolates from sheep, horse, humans, and mosquitoes from New York and Canada; and phenotypic characterization of selected strains. We calculated infection rates by using the maximum-likelihood estimation method by year, region, month, and mosquito species. The highest maximum-likelihood estimations were for Anopheles spp. mosquitoes. Our phylogenetic analysis identified 2 lineages and found evidence of segment reassortment. Furthermore, our data suggest displacement of CVV lineage 1 by lineage 2 in New York and Canada. Finally, we showed increased vector competence of An. quadrimaculatus mosquitoes for lineage 2 strains of CVV compared with lineage 1 strains.

Development of miRNA-Based Approaches to Explore the Interruption of Mosquito-Borne Disease Transmission

MicroRNA (miRNA or miR)-based approaches to interrupt the transmission of mosquito-borne diseases have been explored since 2005. A review of these studies and areas in which to proceed is needed. In this review, significant progress is reviewed at the level of individual miRNAs, and miRNA diversification and relevant confounders are described in detail. Current miRNA studies in mosquitoes include four steps, namely, identifying miRNAs, validating miRNA-pathogen interactions, exploring action mechanisms, and performing preapplication investigations. Notably, regarding the Plasmodium parasite, mosquito miRNAs generally bind to mosquito immunity- or development-related mRNAs, indirectly regulating Plasmodium infection; However, regarding arboviruses, mosquito miRNAs can bind to the viral genome, directly modifying viral replication. Thus, during explorations of miRNA-based approaches, researchers need select an ideal miRNA for investigation based on the mosquito species, tissue, and mosquito-borne pathogen of interest. Additionally, strategies for miRNA-based approaches differ for arboviruses and protozoan parasites.

Systematic identification of plausible pathways to potential harm via problem formulation for investigational releases of a population suppression gene drive to control the human malaria vector Anopheles gambiae in West Africa

BACKGROUND

Population suppression gene drive has been proposed as a strategy for malaria vector control. A CRISPR-Cas9-based transgene homing at the doublesex locus (dsxFCRISPRh) has recently been shown to increase rapidly in frequency in, and suppress, caged laboratory populations of the malaria mosquito vector Anopheles gambiae. Here, problem formulation, an initial step in environmental risk assessment (ERA), was performed for simulated field releases of the dsxFCRISPRh transgene in West Africa.

METHODS

Building on consultative workshops in Africa that previously identified relevant environmental and health protection goals for ERA of gene drive in malaria vector control, 8 potentially harmful effects from these simulated releases were identified. These were stratified into 46 plausible pathways describing the causal chain of events that would be required for potential harms to occur. Risk hypotheses to interrogate critical steps in each pathway, and an analysis plan involving experiments, modelling and literature review to test each of those risk hypotheses, were developed.

RESULTS

Most potential harms involved increased human (n = 13) or animal (n = 13) disease transmission, emphasizing the importance to subsequent stages of ERA of data on vectorial capacity comparing transgenics to non-transgenics. Although some of the pathways (n = 14) were based on known anatomical alterations in dsxFCRISPRh homozygotes, many could also be applicable to field releases of a range of other transgenic strains of mosquito (n = 18). In addition to population suppression of target organisms being an accepted outcome for existing vector control programmes, these investigations also revealed that the efficacy of population suppression caused by the dsxFCRISPRh transgene should itself directly affect most pathways (n = 35).

CONCLUSIONS

Modelling will play an essential role in subsequent stages of ERA by clarifying the dynamics of this relationship between population suppression and reduction in exposure to specific potential harms. This analysis represents a comprehensive identification of plausible pathways to potential harm using problem formulation for a specific gene drive transgene and organism, and a transparent communication tool that could inform future regulatory studies, guide subsequent stages of ERA, and stimulate further, broader engagement on the use of population suppression gene drive to control malaria vectors in West Africa.

Filtering the Junk: Assigning Function to the Mosquito Non-Coding Genome

The portion of the mosquito genome that does not code for proteins contains regulatory elements that likely underlie variation for important phenotypes including resistance and susceptibility to infection with arboviruses and Apicomplexan parasites. Filtering the non-coding genome to uncover these functional elements is an expanding area of research, though identification of non-coding regulatory elements is challenging due to the lack of an amino acid-like code for the non-coding genome and a lack of sequence conservation across species. This review focuses on three types of non-coding regulatory elements: (1) microRNAs (miRNAs), (2) long non-coding RNAs (lncRNAs), and (3) enhancers, and summarizes current advances in technical and analytical approaches for measurement of each of these elements on a genome-wide scale. The review also summarizes and highlights novel findings following application of these techniques in mosquito-borne disease research. Looking beyond the protein-coding genome is essential for understanding the complexities that underlie differential gene expression in response to arboviral or parasite infection in mosquito disease vectors. A comprehensive understanding of the regulation of gene and protein expression will inform transgenic and other vector control methods rooted in naturally segregating genetic variation.

Transmission potential of Mayaro virus by Aedes albopictus, and Anopheles quadrimaculatus from the USA

BACKGROUND

Mayaro virus (MAYV; Alphavirus, Togaviridae) is an emerging pathogen endemic in South American countries. The increase in intercontinental travel and tourism-based forest excursions has resulted in an increase in MAYV spread, with imported cases observed in Europe and North America. Intriguingly, no local transmission of MAYV has been reported outside South America, despite the presence of potential vectors.

METHODS

We assessed the vector competence of Aedes albopictus from New York and Anopheles quadrimaculatus for MAYV.

RESULTS

The results show that Ae. albopictus from New York and An. quadrimaculatus are competent vectors for MAYV. However, Ae. albopictus was more susceptible to infection. Transmission rates increased with time for both species, with rates of 37.16 and 64.44% for Ae. albopictus, and of 25.15 and 48.44% for An. quadrimaculatus, respectively, at 7 and 14 days post-infection.

CONCLUSIONS

Our results suggest there is a risk of further MAYV spread throughout the Americas and autochthonous transmission in the USA. Preventive measures, such as mosquito surveillance of MAYV, will be essential for early detection.

The Route of Infection Influences the Contribution of Key Immunity Genes to Antibacterial Defense in Anopheles gambiae

Insect systemic immune responses to bacterial infections have been mainly studied using microinjections, whereby the microbe is directly injected into the hemocoel. While this methodology has been instrumental in defining immune signaling pathways and enzymatic cascades in the hemolymph, it remains unclear whether and to what extent the contribution of systemic immune defenses to host microbial resistance varies if bacteria invade the hemolymph after crossing the midgut epithelium subsequent to an oral infection. Here, we address this question using the pathogenic Serratia marcescens (Sm) DB11 strain to establish systemic infections of the malaria vector Anopheles gambiae, either by septic Sm injections or by midgut crossing after feeding on Sm. Using functional genetic studies by RNAi, we report that the two humoral immune factors, thioester-containing protein 1 and C-type lectin 4, which play key roles in defense against Gram-negative bacterial infections, are essential for defense against systemic Sm infections established through injection, but they become dispensable when Sm infects the hemolymph following oral infection. Similar results were observed for the mosquito Rel2 pathway. Surprisingly, blocking phagocytosis by cytochalasin D treatment did not affect mosquito susceptibility to Sm infections established through either route. Transcriptomic analysis of mosquito midguts and abdomens by RNA-seq revealed that the transcriptional response in these tissues is more pronounced in response to feeding on Sm. Functional classification of differentially expressed transcripts identified metabolic genes as the most represented class in response to both routes of infection, while immune genes were poorly regulated in both routes. We also report that Sm oral infections are associated with significant downregulation of several immune genes belonging to different families, specifically the clip-domain serine protease family. In sum, our findings reveal that the route of infection not only alters the contribution of key immunity genes to host antimicrobial defense but is also associated with different transcriptional responses in midguts and abdomens, possibly reflecting different adaptive strategies of the host.

Diverse Defenses: A Perspective Comparing Dipteran Piwi-piRNA Pathways

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

De novo profiling of RNA viruses in Anopheles malaria vector mosquitoes from forest ecological zones in Senegal and Cambodia

BACKGROUND

Mosquitoes are colonized by a large but mostly uncharacterized natural virome of RNA viruses, and the composition and distribution of the natural RNA virome may influence the biology and immunity of Anopheles malaria vector populations.

RESULTS

Anopheles mosquitoes were sampled in malaria endemic forest village sites in Senegal and Cambodia, including Anopheles funestus, Anopheles gambiae group sp., and Anopheles coustani in Senegal, and Anopheles hyrcanus group sp., Anopheles maculatus group sp., and Anopheles dirus in Cambodia. The most frequent mosquito species sampled at both study sites are human malaria vectors. Small and long RNA sequences were depleted of mosquito host sequences, de novo assembled and clustered to yield non-redundant contigs longer than 500 nucleotides. Analysis of the assemblies by sequence similarity to known virus families yielded 115 novel virus sequences, and evidence supports a functional status for at least 86 of the novel viral contigs. Important monophyletic virus clades in the Bunyavirales and Mononegavirales orders were found in these Anopheles from Africa and Asia. The remaining non-host RNA assemblies that were unclassified by sequence similarity to known viruses were clustered by small RNA profiles, and 39 high-quality independent contigs strongly matched a pattern of classic RNAi processing of viral replication intermediates, suggesting they are entirely undescribed viruses. One thousand five hundred sixty-six additional high-quality unclassified contigs matched a pattern consistent with Piwi-interacting RNAs (piRNAs), suggesting that strand-biased piRNAs are generated from the natural virome in Anopheles. To functionally query piRNA effect, we analyzed piRNA expression in Anopheles coluzzii after infection with O'nyong nyong virus (family Togaviridae), and identified two piRNAs that appear to display specifically altered abundance upon arbovirus infection.

CONCLUSIONS

Anopheles vectors of human malaria in Africa and Asia are ubiquitously colonized by RNA viruses, some of which are monophyletic but clearly diverged from other arthropod viruses. The interplay between small RNA pathways, immunity, and the virome may represent part of the homeostatic mechanism maintaining virome members in a commensal or nonpathogenic state, and could potentially influence vector competence.

Expanding the canon: Non-classical mosquito genes at the interface of arboviral infection

Mosquito transmitted viruses cause significant morbidity and mortality in human populations. Despite the use of insecticides and other measures of vector control, arboviral diseases are on the rise. One potential solution for limiting disease transmission to humans is to render mosquitoes refractory to viral infection through genetic modification. Substantial research effort in Drosophila, Aedes and Anopheles has helped to define the major innate immune pathways, including Toll, IMD, Jak/Stat and RNAi, however we still have an incomplete picture of the mosquito antiviral response. Transcriptional profiles of virus-infected insects reveal a much wider range of pathways activated by the process of infection. Within these lists of genes are unexplored mosquito candidates of viral defense. Wolbachia species are endosymbiotic bacteria that naturally limit arboviral infection in mosquitoes. Our understanding of the Wolbachia-mediated viral blocking mechanism is poor, but it does not appear to operate via the classical immune pathways. Herein, we reviewed the transcriptomic response of mosquitoes to multiple viral species and put forth consensus gene types/families outside the immune canon whose expression responds to infection, including cytoskeleton and cellular trafficking, the heat shock response, cytochromes P450, cell proliferation, chitin and small RNAs. We then examine emerging evidence for their functional role in viral resistance in diverse insect and mammalian hosts and their potential role in Wolbachia-mediated viral blocking. These candidate gene families offer novel avenues for research into the nature of insect viral defense.

Interaction of RNA viruses of the natural virome with the African malaria vector, Anopheles coluzzii

Mosquitoes are colonized by a little-studied natural virome. Like the bacterial microbiome, the virome also probably influences the biology and immunity of mosquito vector populations, but tractable experimental models are lacking. We recently discovered two novel viruses in the virome of wild Anopheles and in colonies of the malaria vector Anopheles coluzzii: Anopheles C virus and Anopheles cypovirus. Here, we describe biological interactions between these two viruses and An. coluzzii mosquitoes. Viral abundance varies reproducibly during mosquito development. DNA forms of these viruses were not detected, and thus viral persistence is likely based on vertical transmission of RNA genomes. At least Anopheles C virus is vertically transmitted by an intraembryonic route. Relative abundance of the two viruses is inversely correlated in individual mosquitoes. One possible mechanism for this could be interactions with host immunity, and functional genomic analysis indicated differential influence of at least the Toll and JAK/STAT immune signaling pathways upon the viruses. The nonrandom distributions and interactions with host immunity suggest that these and other members of the natural virome may constitute a source of unrecognized heterogeneity in mosquito vector populations.

MicroRNAs from saliva of anopheline mosquitoes mimic human endogenous miRNAs and may contribute to vector-host-pathogen interactions

During blood feeding haematophagous arthropods inject into their hosts a cocktail of salivary proteins whose main role is to counteract host haemostasis, inflammation and immunity. However, animal body fluids are known to also carry miRNAs. To get insights into saliva and salivary gland miRNA repertoires of the African malaria vector Anopheles coluzzii we used small RNA-Seq and identified 214 miRNAs, including tissue-enriched, sex-biased and putative novel anopheline miRNAs. Noteworthy, miRNAs were asymmetrically distributed between saliva and salivary glands, suggesting that selected miRNAs may be preferentially directed toward mosquito saliva. The evolutionary conservation of a subset of saliva miRNAs in Anopheles and Aedes mosquitoes, and in the tick Ixodes ricinus, supports the idea of a non-random occurrence pointing to their possible physiological role in blood feeding by arthropods. Strikingly, eleven of the most abundant An. coluzzi saliva miRNAs mimicked human miRNAs. Prediction analysis and search for experimentally validated targets indicated that miRNAs from An. coluzzii saliva may act on host mRNAs involved in immune and inflammatory responses. Overall, this study raises the intriguing hypothesis that miRNAs injected into vertebrates with vector saliva may contribute to host manipulation with possible implication for vector-host interaction and pathogen transmission.