Construction and characterization of an expressed sequenced tag library for the mosquito vector Armigeres subalbatus

Extracellular vesicles secreted by Brugia malayi microfilariae modulate the melanization pathway in the mosquito host

Vector-borne, filarial nematode diseases cause significant disease burdens in humans and domestic animals worldwide. Although there is strong direct evidence of parasite-driven immunomodulation of mammalian host responses, there is less evidence of parasite immunomodulation of the vector host. We have previously reported that all life stages of Brugia malayi, a filarial nematode and causative agent of Lymphatic filariasis, secrete extracellular vesicles (EVs). Here we investigate the immunomodulatory effects of microfilariae-derived EVs on the vector host Aedes aegypti. RNA-seq analysis of an Ae. aegypti cell line treated with B. malayi microfilariae EVs showed differential expression of both mRNAs and miRNAs. AAEL002590, an Ae. aegypti gene encoding a serine protease, was shown to be downregulated when cells were treated with biologically relevant EV concentrations in vitro. Injection of adult female mosquitoes with biologically relevant concentrations of EVs validated these results in vivo, recapitulating the downregulation of AAEL002590 transcript. This gene was predicted to be involved in the mosquito phenoloxidase (PO) cascade leading to the canonical melanization response and correspondingly, both suppression of this gene using RNAi and parasite EV treatment reduced PO activity in vivo. Our data indicate that parasite-derived EVs interfere with critical immune responses in the vector host, including melanization.

An insight into the female and male Sabethes cyaneus mosquito salivary glands transcriptome

Mosquitoes are responsible for the death and debilitation of millions of people every year due to the pathogens they can transmit while blood feeding. While a handful of mosquitoes, namely those in the Aedes, Anopheles, and Culex genus, are the dominant vectors, many other species belonging to different genus are also involved in various pathogen cycles. Sabethes cyaneus is one of the many poorly understood mosquito species involved in the sylvatic cycle of Yellow Fever Virus. Here, we report the expression profile differences between male and female of Sa.cyaneus salivary glands (SGs). We find that female Sa.cyaneus SGs have 165 up-regulated and 18 down-regulated genes compared to male SGs. Most of the up-regulated genes have unknown functions, however, odorant binding proteins, such as those in the D7 protein family, and mucins were among the top 30 genes. We also performed various in vitro activity assays of female SGs. In the activity analysis we found that female SG extracts inhibit coagulation by blocking factor Xa and has endonuclease activity. Knowledge about mosquitoes and their physiology are important for understanding how different species differ in their ability to feed on and transmits pathogens to humans. These results provide us with an insight into the Sabethes SG activity and gene expression that expands our understanding of mosquito salivary glands.

Transcriptomic Insights into the Insect Immune Response to Nematode Infection

Insects in nature interact with a wide variety of microbial enemies including nematodes. These include entomopathogenic nematodes that contain mutualistic bacteria and together are able to infect a broad range of insects in order to complete their life cycle and multiply, filarial nematodes which are vectored by mosquitoes, and other parasitic nematodes. Entomopathogenic nematodes are commonly used in biological control practices and they form excellent research tools for understanding the genetic and functional bases of nematode pathogenicity and insect anti-nematode immunity. In addition, clarifying the mechanism of transmission of filarial nematodes by mosquitoes is critical for devising strategies to reduce disease transmission in humans. In all cases and in order to achieve these goals, it is vital to determine the number and type of insect host genes which are differentially regulated during infection and encode factors with anti-nematode properties. In this respect, the use of transcriptomic approaches has proven a key step for the identification of insect molecules with anti-nematode activity. Here, we review the progress in the field of transcriptomics that deals with the insect response to nematode infection. This information is important because it will expose conserved pathways of anti-nematode immunity in humans.

Cellular immunity in the insect Galleria mellonella against insect non-parasitic nematodes

Immunity to microbial infections is well understood; however, information regarding the immunity to parasitic multicellular organisms remains lacking. To understand innate host cellular immunity to nematodes, we compared the cellular response of the greater wax moth (Galleria mellonella) larvae against the non-parasitic, bacterial-feeding nematode Caenorhabditis elegans and pathogenic nematode Heterorhabditis bacteriophora. When intact first-instar or dauer larvae of C. elegans were injected into a G. mellonella larva, most of the nematodes were alive and not confined by the surrounding reaction by insect haemocytes (encapsulation), similarly as the pathogenic nematode, whereas most of the heat-killed nematodes of both species were severely encapsulated by 24 h after inoculation. Other non-parasitic nematodes were also not encapsulated. Surprisingly, C. elegans injected into the insect haemocoel grew and propagated in the live insect, resulting in death of the host insect. Our results suggest that C. elegans has some basic mechanisms to evade immunity of G. mellonenlla and grow in the haemocoel.

Functions of Armigeres subalbatus C-type lectins in innate immunity

C-type lectins (CTLs) are a superfamily of calcium-dependent carbohydrate binding proteins containing at least one carbohydrate-recognition domain (CRD) and they are present in almost all metazoans. Insect CTLs may function as pattern-recognition receptors and play important roles in innate immunity. In this study, we selected five AsCTLs from the mosquito Armigeres subalbatus, a natural vector of filarial nematodes, and performed both in vitro and in vivo studies to elucidate their functions in innate immunity. AsCTLMA15, AsCTLGA5 and AsCTL15 were mainly expressed in hemocytes, AsCTL16 was expressed in fat body, while AsCTLMA11 was expressed in both hemocytes and fat body, and only AsCTLMA11 and AsCTL16 were expressed at high levels in adult females. In vitro binding assays showed that all five recombinant AsCTLs could bind to different microbial cell wall components, including lipopolysaccharide (LPS), lipid A, peptidoglycan (PG), lipoteichoic acid (LTA), zymosan and laminarin (beta-1,3-glucan). Recombinant AsCTLs also bound to several Gram-negative and Gram-positive bacteria, and could agglutinate bacterial cells. Injection of double-stranded RNAs (dsRNAs) could significantly reduce expression of the five AsCTL mRNAs, and the survival of mosquitoes treated with dsRNA to AsCTLGA5 was significantly decreased after Escherichia coli infection, but did not change significantly after Micrococcus luteus infection compared to the control groups, suggesting that Ar. subalbatus AsCTLGA5 may participate in innate immunity against E. coli.

Cloning and characterization of the peptidoglycan recognition protein genes in the mosquito, Armigeres subalbatus (Diptera: Culicidae)

Peptidoglycan recognition proteins (PGRPs) are a group of proteins that are responsible for the recognition and, in some cases, binding of peptidoglycan (PGN), a unique cell wall component of bacteria, and initiation of immune responses to various types of pathogens. In the current study, full-length cDNA sequences of multiple PGRPs, identified via a database search, were cloned in the mosquito Armigeres subalbatus (Coquillett). During cloning, a novel transcript variant (isoform) of AsPGRP-LC (As: Ar. subalbatus) was also identified that shares a large 5' end fragment with AsPGRP-LC. All four AsPGRP genes (six transcripts) contain a conserved PGRP domain, an ortholog of the amidase-2 domain. Based on predicted functional domain, the six Ar. subalbatus PGRPs resemble both short (AsPGRP-S1) and long (AsPGRP-LBa, AsPGRP-LBb, AsPGRP-LCa, AsPGRP-LCb, and AsPGRP-LE) forms of PGRPs as in other insects. Sequence alignments showed that PGRPs are conserved across Dipterans. Phylogenetic analysis indicated that PGRPs represent an ancient gene family that has primarily diverged through speciation events among these Dipterans, with only a limited number of lineage specific gene duplications. Developmental profiling of the six AsPGRP transcripts using real-time polymerase chain reaction revealed that AsPGRP-LCa and AsPGRP-LCb are constitutively expressed at high levels in all developmental stages, while AsPGRP-S1, AsPGRP-LBa, AsPGRP-LBb, and AsPGRP-LE transcripts have low expression in most of the life stages and are increased only at certain times. Tissue profiling of the six AsPGRP transcripts showed that they are expressed in various patterns, even between the different isoforms of the same PGRP gene, indicating that these AsPGRPs may play different functions.

Insect immune responses to nematode parasites

Host innate immunity plays a central role in detecting and eliminating microbial pathogenic infections in both vertebrate and invertebrate animals. Entomopathogenic or insect pathogenic nematodes are of particular importance for the control of insect pests and vectors of pathogens, while insect-borne nematodes cause serious diseases in humans. Recent work has begun to use the power of insect models to investigate host-nematode interactions and uncover host antiparasitic immune reactions. This review describes recent findings on innate immune evasion strategies of parasitic nematodes and host cellular and humoral responses to the infection. Such information can be used to model diseases caused by human parasitic nematodes and provide clues indicating directions for research into the interplay between vector insects and their invading tropical parasites.

Mosquito genomics: progress and challenges

The whole-genome sequencing of mosquitoes has facilitated our understanding of fundamental biological processes at their basic molecular levels and holds potential for application to mosquito control and prevention of mosquito-borne disease transmission. Draft genome sequences are available for Anopheles gambiae, Aedes aegypti, and Culex quinquefasciatus. Collectively, these represent the major vectors of African malaria, dengue fever and yellow fever viruses, and lymphatic filariasis, respectively. Rapid advances in genome technologies have revealed detailed information on genome architecture as well as phenotype-specific transcriptomics and proteomics. These resources allow for detailed comparative analyses within and across populations as well as species. Next-generation sequencing technologies will likely promote a proliferation of genome sequences for additional mosquito species as well as for individual insects. Here we review the current status of genome research in mosquitoes and identify potential areas for further investigations.

Mosquito transcriptome profiles and filarial worm susceptibility in Armigeres subalbatus

BACKGROUND

Armigeres subalbatus is a natural vector of the filarial worm Brugia pahangi, but it kills Brugia malayi microfilariae by melanotic encapsulation. Because B. malayi and B. pahangi are morphologically and biologically similar, comparing Ar. subalbatus-B. pahangi susceptibility and Ar. subalbatus-B. malayi refractoriness could provide significant insight into recognition mechanisms required to mount an effective anti-filarial worm immune response in the mosquito, as well as provide considerable detail into the molecular components involved in vector competence. Previously, we assessed the transcriptional response of Ar. subalbatus to B. malayi, and now we report transcriptome profiling studies of Ar. subalbatus in relation to filarial worm infection to provide information on the molecular components involved in B. pahangi susceptibility.

METHODOLOGY/PRINCIPAL FINDINGS

Utilizing microarrays, comparisons were made between mosquitoes exposed to B. pahangi, B. malayi, and uninfected bloodmeals. The time course chosen facilitated an examination of key events in the development of the parasite, beginning with the very start of filarial worm infection and spanning to well after parasites had developed to the infective stage in the mosquito. At 1, 3, 6, 12, 24 h post infection and 2-3, 5-6, 8-9, and 13-14 days post challenge there were 31, 75, 113, 76, 54, 5, 3, 13, and 2 detectable transcripts, respectively, with significant differences in transcript abundance (increase or decrease) as a result of parasite development.

CONCLUSIONS/SIGNIFICANCE

Herein, we demonstrate that filarial worm susceptibility in a laboratory strain of the natural vector Ar. subalbatus involves many factors of both known and unknown function that most likely are associated with filarial worm penetration through the midgut, invasion into thoracic muscle cells, and maintenance of homeostasis in the hemolymph environment. The data show that there are distinct and separate transcriptional patterns associated with filarial worm susceptibility as compared to refractoriness, and that an infection response in Ar. subalbatus can differ significantly from that observed in Ae. aegypti, a common laboratory model.

Mosquito transcriptome changes and filarial worm resistance in Armigeres subalbatus

Background

Armigeres subalbatus is a natural vector of the filarial worm Brugia pahangi, but it rapidly and proficiently kills Brugia malayi microfilariae by melanotic encapsulation. Because B. malayi and B. pahangi are morphologically and biologically similar, the Armigeres-Brugia system serves as a valuable model for studying the resistance mechanisms in mosquito vectors. We have initiated transcriptome profiling studies in Ar. subalbatus to identify molecular components involved in B. malayi refractoriness.

Results

These initial studies assessed the transcriptional response of Ar. subalbatus to B. malayi at 1, 3, 6, 12, 24, 48, and 72 hrs after an infective blood feed. In this investigation, we initiated the first holistic study conducted on the anti-filarial worm immune response in order to effectively explore the functional roles of immune-response genes following a natural exposure to the parasite. Studies assessing the transcriptional response revealed the involvement of unknown and conserved unknowns, cytoskeletal and structural components, and stress and immune responsive factors. The data show that the anti-filarial worm immune response by Ar. subalbatus to be a highly complex, tissue-specific process involving varied effector responses working in concert with blood cell-mediated melanization.

Conclusion

This initial study provides a foundation and direction for future studies, which will more fully dissect the nature of the anti-filarial worm immune response in this mosquito-parasite system. The study also argues for continued studies with RNA generated from both hemocytes and whole bodies to fully expound the nature of the anti-filarial worm immune response.