Defense reactions by larvae of Aedes aegypti during infection by the aquatic fungus Lagenidium giganteum (Oomycete)

Differences in the proliferation trend of 'Microsporidium' sp. PL03 in Culex pipiens and C. torrentium larvae

Our study aimed to examine whether there are differences in the proliferation trend of microsporidia in mosquito larvae of the same genus (Culex spp.). DNA-barcoding and quantitative analyses were used to determine microsporidian rDNA copies in 'early' (L1 + L2) and 'late' (L3 + L4) Culex larvae in a natural population. In the study area, C. pipiens and C. torrentium larvae were infected by 'Microsporidium' sp. PL03 at similar levels. Infection by this microsporidian species probably elicits a notable immune response in C. pipiens, whereas in C. torrentium, it may evade or suppress the host immune response.

Serine pseudoproteases in physiology and disease

Serine proteases (SPs) constitute a very important family of enzymes, both physiologically and pathologically. The effects produced by these proteins have been explained by their proteolytic activity. However, the discovery of pharmacologically active SP molecules that show no enzymatic activity, as the so-called pseudo SPs or SP homologs (SPHs), has exposed a profoundly neglected possibility of nonenzymatic functions of these SP molecules. In this review, the most thoroughly described SPHs are presented. The main physiological domains in which SPHs operate appear to be in reproduction, embryonic development, immune response, host defense, and hemostasis. Hitherto unexplained actions of SPs should therefore be considered also as the result of the ligand-like attributes of SPs. The gain of a novel function by an SPH is a consequence of specific amino acid replacements that have resulted in a novel interaction interface or a 'catalytic trap'. Unraveling the SP/SPH interactome will provide a description of previously unknown physiological functions of SPs/SPHs, aiding the creation of innovative medical approaches.

Functional characterization of two clip domain serine proteases in innate immune responses of Aedes aegypti

Background

Clip domain serine proteases (CLIPs), a very diverse group of proteolytic enzymes, play a crucial role in the innate immunity of insects. Innate immune responses are the first line of defense in mosquitoes against the invasion of pathogenic microorganisms. The Toll pathway, immunodeficiency (IMD) pathway and melanization are the main processes of innate immunity in Aedes aegypti. CLIPS are classified into five subfamilies—CLIPA, CLIPB, CLIPC, CLIPD, and CLIPE—based on their sequence specificity and phylogenetic relationships. We report the functional characterization of the genes that code for two CLIPs in Ae. aegypti (Ae): Ae-CLIPB15 and Ae-CLIPB22.

Methods

Clustal Omega was used for multiple amino acid sequence alignment of Ae-CLIPB15 and Ae-CLIPB22 with different CLIP genes from other insect species. The spatiotemporal expression profiles of Ae-CLIPB15 and Ae-CLIPB22 were examined. We determined whether Ae-CLIPB15 and Ae-CLIPB22 respond to microbial challenge and tissue injury. RNA interference (RNAi) was used to explore the function of Ae-CLIPB15 and Ae-CLIPB22 in the defense of Ae. aegypti against bacterial and fungal infections. The expression levels of nuclear factor kappa B (NF-κB) transcription factors REL1 and REL2 in the Toll pathway and IMD pathway after bacterial infection were investigated. Finally, the change in phenoloxidase (PO) activity in Ae-CLIPB15 and Ae-CLIPB22 knockdown adults was investigated.

Results

We performed spatiotemporal gene expression profiling of Ae-CLIPB15 and Ae-CLIPB22 genes in Ae. aegypti using quantitative real-time polymerase chain reaction. These genes were expressed in different stages and tissues. The messenger RNA (mRNA) levels for both genes were also up-regulated by Gram-negative bacteria Escherichia coli, Gram-positive bacteria Staphylococcus aureus and fungal Beauveria bassiana infections, as well as in the tissue injury experiments. RNAi-mediated knockdown of Ae-CLIPB15 led to a significant decrease of PO activity in the hemolymph of Ae. aegypti, while other RNAi experiments revealed that both Ae-CLIPB15 and Ae-CLIPB22 were involved in immune defense against bacterial and fungal infections. The mRNA expression of NF-κB transcription factors REL1 and REL2 in the Toll pathway and IMD pathway differed between Ae-CLIPB15 and Ae-CLIPB22 knockdown mosquitoes infected with bacteria and wild type mosquitoes infected with bacteria.

Conclusions

Our findings suggest that Ae-CLIPB15 and Ae-CLIPB22 play a critical role in mosquito innate immunity, and that they are involved in immune responses to injury and infection. Their regulation of transcription factors and PO activity indicates that they also play a specific role in the regulation of innate immunity.

Graphical Abstract

CLIPB10 is a Terminal Protease in the Regulatory Network That Controls Melanization in the African Malaria Mosquito Anopheles gambiae

Humoral immune responses in animals are often tightly controlled by regulated proteolysis. This proteolysis is exerted by extracellular protease cascades, whose activation culminates in the proteolytic cleavage of key immune proteins and enzymes. A model for such immune system regulation is the melanization reaction in insects, where the activation of prophenoxidase (proPO) leads to the rapid formation of eumelanin on the surface of foreign entities such as parasites, bacteria and fungi. ProPO activation is tightly regulated by a network of so-called clip domain serine proteases, their proteolytically inactive homologs, and their serpin inhibitors. In Anopheles gambiae, the major malaria vector in sub-Saharan Africa, manipulation of this protease network affects resistance to a wide range of microorganisms, as well as host survival. However, thus far, our understanding of the molecular make-up and regulation of the protease network in mosquitoes is limited. Here, we report the function of the clip domain serine protease CLIPB10 in this network, using a combination of genetic and biochemical assays. CLIPB10 knockdown partially reversed melanotic tumor formation induced by Serpin 2 silencing in the absence of infection. CLIPB10 was also partially required for the melanization of ookinete stages of the rodent malaria parasite Plasmodium berghei in a refractory mosquito genetic background. Recombinant serpin 2 protein, a key inhibitor of the proPO activation cascade in An. gambiae, formed a SDS-stable protein complex with activated recombinant CLIPB10, and efficiently inhibited CLIPB10 activity in vitro at a stoichiometry of 1.89:1. Recombinant activated CLIPB10 increased PO activity in Manduca sexta hemolymph ex vivo, and directly activated purified M. sexta proPO in vitro. Taken together, these data identify CLIPB10 as the second protease with prophenoloxidase-activating function in An. gambiae, in addition to the previously described CLIPB9, suggesting functional redundancy in the protease network that controls melanization. In addition, our data suggest that tissue melanization and humoral melanization of parasites are at least partially mediated by the same proteases.

Antifungal immune responses in mosquitoes (Diptera: Culicidae): A review

Mosquitoes transmit many parasites and pathogens to humans that cause significant morbidity and mortality. As such, we are constantly looking for new methods to reduce mosquito populations, including the use of effective biological controls. Entomopathogenic fungi are excellent candidate biocontrol agents to control mosquitoes. Understanding the complex ecological, environmental, and molecular interactions between hosts and pathogens are essential to create novel, effective and safe biocontrol agents. Understanding how mosquitoes recognize and eliminate pathogens such as entomopathogenic fungi may allow us to create insect-order specific biocontrol agents to reduce pest populations. Here we summarize the current knowledge of fungal infection, colonization, development, and replication within mosquitoes and the innate immune responses of the mosquitoes towards the fungal pathogens, emphasizing those features required for an effective mosquito biocontrol agent.

Developmental and comparative perspectives on mosquito immunity

Diseases spread by mosquitoes have killed more people than those spread by any other group of arthropod vectors and remain an important factor in determining global health and economic stability. The mosquito innate immune system can act to either modulate infection with human pathogens or fight off entomopathogens and increase the fitness and longevity of infected mosquitoes. While work remains towards understanding the larval immune system and the development of the mosquito immune system, it has recently become clearer that environmental factors heavily shape the developing mosquito immune system and continue to influence the adult immune system as well. The adult immune system has been well-studied and is known to involve multiple tissues and diverse molecular mechanisms. This review summarizes and synthesizes what is currently understood about the development of the mosquito immune system and includes comparisons of immune components unique to mosquitoes among the blood-feeding arthropods as well as important distinguishing factors between the anopheline and culicine mosquitoes. An explanation is included for how mosquito immunity factors into vector competence and vectorial capacity is presented along with a model for the interrelationships between nutrition, microbiome, pathogen interactions and behavior as they relate to mosquito development, immune status, adult female fitness and ultimately, vectorial capacity. Novel discoveries in the fields of mosquito ecoimmunology, neuroimmunology, and intracellular antiviral responses are highlighted.

Mosquito-fungus interactions and antifungal immunity

The mosquito immune system has evolved in the presence of continuous encounters with fungi that range from food to foes. Herein, we review the field of mosquito-fungal interactions, providing an overview of current knowledge and topics of interest. Mosquitoes encounter fungi in their aquatic and terrestrial habitats. Mosquito larvae are exposed to fungi on plant detritus, within the water column, and at the water surface. Adult mosquitoes are exposed to fungi during indoor and outdoor resting, blood and sugar feeding, mating, and oviposition. Fungi enter the mosquito body through different routes, including ingestion and through active or passive breaches in the cuticle. Oral uptake of fungi can be beneficial to mosquitoes, as yeasts hold nutritional value and support larval development. However, ingestion of or surface contact with fungal entomopathogens leads to colonization of the mosquito with often lethal consequences to the host. The mosquito immune system recognizes fungi and mounts cellular and humoral immune responses in the hemocoel, and possibly epithelial immune responses in the gut. These responses are regulated transcriptionally through multiple signal transduction pathways. Proteolytic protease cascades provide additional regulation of antifungal immunity. Together, these immune responses provide an efficient barrier to fungal infections, which need to be overcome by entomopathogens. Therefore, fungi constitute an excellent tool to examine the molecular underpinnings of mosquito immunity and to identify novel antifungal peptides. In addition, recent advances in mycobiome analyses can now be used to examine the contribution of fungi to various mosquito traits, including vector competence.

Mosquito Immunobiology: The Intersection of Vector Health and Vector Competence

As holometabolous insects that occupy distinct aquatic and terrestrial environments in larval and adult stages and utilize hematophagy for nutrient acquisition, mosquitoes are subjected to a wide variety of symbiotic interactions. Indeed, mosquitoes play host to endosymbiotic, entomopathogenic, and mosquito-borne organisms, including protozoa, viruses, bacteria, fungi, fungal-like organisms, and metazoans, all of which trigger and shape innate infection-response capacity. Depending on the infection or interaction, the mosquito may employ, for example, cellular and humoral immune effectors for septic infections in the hemocoel, humoral infection responses in the midgut lumen, and RNA interference and programmed cell death for intracellular pathogens. These responses often function in concert, regardless of the infection type, and provide a robust front to combat infection. Mosquito-borne pathogens and entomopathogens overcome these immune responses, employing avoidance or suppression strategies. Burgeoning methodologies are capitalizing on this concerted deployment of immune responses to control mosquito-borne disease.

Transstadial transmission of larval hemocoelic infection negatively affects development and adult female longevity in the mosquito Anopheles gambiae

During all life stages, mosquitoes are exposed to pathogens, and employ an immune system to resist or limit infection. Although much attention has been paid to how adult mosquitoes fight infection, little is known about how an infection during the larval stage affects the biology of the resultant adult. In this study, we investigated whether a bacterial infection in the hemocoel of the African malaria mosquito, Anopheles gambiae, is transstadially transmitted from larvae to adults (both females and males), and whether immune stimulation in the hemocoel as a larva alters development or biological traits of the adult. Specifically, larvae were injected in the hemocoel with either fluorescent microspheres or Escherichia coli, and the following traits were examined: transstadial transmission, larval development to adulthood, adult survival, and adult body size. Our results show that transstadial transmission of hemocoel contents occurs from larvae to pupae and from pupae to adults, but that bacterial prevalence and intensity varies with age. Injury, immune stimulation or infection decreases the proportion of larvae that undergo pupation and eclosion, infection decreases the longevity of adult females, and treatment has complex effects on the body size of the resultant adults. The present study adds larval hemocoelic infection to the known non-genetic factors that reduce overall fitness by negatively affecting development and adult biological traits that influence mosquito vector competence.

Anopheles gambiae larvae mount stronger immune responses against bacterial infection than adults: evidence of adaptive decoupling in mosquitoes

BACKGROUND

The immune system of adult mosquitoes has received significant attention because of the ability of females to vector disease-causing pathogens while ingesting blood meals. However, few studies have focused on the immune system of larvae, which, we hypothesize, is highly robust due to the high density and diversity of microorganisms that larvae encounter in their aquatic environments and the strong selection pressures at work in the larval stage to ensure survival to reproductive maturity. Here, we surveyed a broad range of cellular and humoral immune parameters in larvae of the malaria mosquito, Anopheles gambiae, and compared their potency to that of newly-emerged adults and older adults.

RESULTS

We found that larvae kill bacteria in their hemocoel with equal or greater efficiency compared to newly-emerged adults, and that antibacterial ability declines further with adult age, indicative of senescence. This phenotype correlates with more circulating hemocytes and a differing spatial arrangement of sessile hemocytes in larvae relative to adults, as well as with the individual hemocytes of adults carrying a greater phagocytic burden. The hemolymph of larvae also possesses markedly stronger antibacterial lytic and melanization activity than the hemolymph of adults. Finally, infection induces a stronger transcriptional upregulation of immunity genes in larvae than in adults, including differences in the immunity genes that are regulated.

CONCLUSIONS

These results demonstrate that immunity is strongest in larvae and declines after metamorphosis and with adult age, and suggest that adaptive decoupling, or the independent evolution of larval and adult traits made possible by metamorphosis, has occurred in the mosquito lineage.

Biological Control Strategies for Mosquito Vectors of Arboviruses

Historically, biological control utilizes predatory species and pathogenic microorganisms to reduce the population of mosquitoes as disease vectors. This is particularly important for the control of mosquito-borne arboviruses, which normally do not have specific antiviral therapies available. Although development of resistance is likely, the advantages of biological control are that the resources used are typically biodegradable and ecologically friendly. Over the past decade, the advancement of molecular biology has enabled optimization by the manipulation of genetic materials associated with biological control agents. Two significant advancements are the discovery of cytoplasmic incompatibility induced by Wolbachia bacteria, which has enhanced replacement programs, and the introduction of dominant lethal genes into local mosquito populations through the release of genetically modified mosquitoes. As various arboviruses continue to be significant public health threats, biological control strategies have evolved to be more diverse and become critical tools to reduce the disease burden of arboviruses.

Drosophila melanogaster clip-domain serine proteases: Structure, function and regulation

Mammalian chymotrypsin-like serine proteases (SPs) are one of the best-studied family of enzymes with roles in a wide range of physiological processes, including digestion, blood coagulation, fibrinolysis and humoral immunity. Extracellular SPs can form cascades, in which one protease activates the zymogen of the next protease in the chain, to amplify physiological or pathological signals. These extracellular SPs are generally multi-domain proteins, with pro-domains that are involved in protein-protein interactions critical for the sequential organization of the cascades, the control of their intensity and their proper localization. Far less is known about invertebrate SPs than their mammalian counterparts. In insect genomes, SPs and their proteolytically inactive homologs (SPHs) constitute large protein families. In addition to the chymotrypsin fold, many of these proteins contain additional structural domains, often with conserved mammalian orthologues. However, the largest group of arthropod SP regulatory modules is the clip domains family, which has only been identified in arthropods. The clip-domain SPs are extracellular and have roles in the immune response and embryonic development. The powerful reverse-genetics tools in Drosophila melanogaster have been essential to identify the functions of clip-SPs and their organization in sequential cascades. This review focuses on the current knowledge of Drosophila clip-SPs and presents, when necessary, data obtained in other insect models. We will first cover the biochemical and structural features of clip domain SPs and SPHs. Clip-SPs are implicated in three main biological processes: the control of the dorso-ventral patterning during embryonic development; the activation of the Toll-mediated response to microbial infections and the prophenoloxydase cascade, which triggers melanization. Finally, we review the regulation of SPs and SPHs, from specificity of activation to inhibition by endogenous or pathogen-encoded inhibitors.

The role of mosquito olfaction in oviposition site location and in the avoidance of unsuitable hosts

Developments in the exploitation of mosquito olfaction are traced, in collaborative studies with various groups, from the first identification of a mosquito pheromone through to a discussion of non-host avoidance. The characterization of the oviposition pheromone for mosquitoes in the genus Culex, e.g. Culex quinquefasciatus, as a novel chiral lactone ester provided the impetus for a number of sophisticated asymmetric syntheses and economical large-scale routes to racemic products. The latter have provided material for successful field trials in three continents. During the course of this field work, we obtained evidence that semiochemicals originating directly from the oviposition site are essential for activity of the oviposition pheromone. Recent studies are elucidating the nature of these agents and their geographical variability. Initially, we used synthetic oviposition pheromone to attract mosquitoes to sites treated with a biorational larvicide. However, recyclable biological control agents offer better prospects for resource-poor regions. A biotechnological approach to pheromone production has been devised involving the generation of inexpensive starting materials by the cultivation of a higher plant. New studies on dipterous pests feeding on farm animals indicate a semiochemically based mechanism by which unsuitable individuals within the host species are avoided. There appears to be an analogous process in which mosquitoes avoid certain potential human hosts, thereby raising prospects for the development of novel, rationally identified repellents once the semiochemical/olfactory interactions have been fully elucidated.

Molecular cloning of two prophenoloxidase genes from the mosquito Aedes aegypti

The biosynthesis of melanotic materials is an important process in the life of a mosquito. Melanin production is critical for many diverse processes such as egg chorion tanning, cuticular sclerotization, and melanotic encapsulation of metazoan parasites. Prophenoloxidase plays a critical role in this biochemical cascade. Two cDNAs, one full length and one partial clone, and two genomic clones encoding prophenoloxidase (pro-PO) were isolated from the yellow fever mosquito, Aedes aegypti. The full-length cDNA, pAaProPO1, is 2286 bp long with a 2055 bp open reading frame encoding a 685 amino acid protein that shares 89% identity with Armigeres subalbatus pro-PO. It contains two putative copper binding domains (amino acids 197-243 and 346-423) that are homologous to other insect pro-POs. AaProPO1 messenger RNA (mRNA) was detected by reverse transcription polymerase chain reaction (RT-PCR) only from third-stage larvae and not in adult mosquitoes after blood feeding, during the melanotic encapsulation of Dirofilaria immitis microfilariae or following exposure to bacteria. A 750 bp fragment of the second cDNA (pAaProPO2) was cloned using RT-PCR from mRNA obtained from 14-day postovipostional eggs. AaProPO2 mRNA was not found in any other life stages, and may be in low abundance or transiently expressed. AaProPO2 and AaProPO1 each contain three introns that are 60, 68 and 58 bp and 61, 69 and 59 bp long, respectively, and the intron sequences of these two genes are not similar.

Host-parasite relationships of Ascogregarina chagasi (Eugregarinorida, Aseptatorina, Lecudinidae) in Lutzomyia longipalpis (Diptera: Psychodidae)

The life cycle of Ascogregarina chagasi in larvae and in adult female Lutzomyia longipalpis was studied by light and electron microscopy. Sporozoites and young gamonts were attached to the epithelial lining of the larval midgut via an osmiophilic contact zone. The mucron of young gamonts was bordered by an invaginated pellicular fold, and an electron-opaque vesicular structure was observed adjacent to it. Sporozoites possessed an apical complex and were bound by a double membrane with underlying subpellicular microtubules. The gamont pellicle was uniformly corrugated and consisted of two cortical membranes and a plasma membrane. Mature gamonts and gametocysts were found in the posterior ectoperitrophic space of third and fourth instar larval midguts and in the haemocoel of adult flies. Gametocysts in adult females adhered to the genital accessory glands, where they were encased in electron-dense capsules secreted by the fly through haemocyte-mediated humoral immune reactions. Oocytes were spindle-shaped and bound by a double-layered wall with a discernible polar plug at each end. Sporulation was endogenous, occurring within gametocysts in the midguts of larvae or the accessory glands of adult females. FITC-phalloidine staining of all stages of A. chagasi except mature gametocysts produced bright fluorescence, indicating the presence of a diffuse, actin-like protein in the cytoplasm.

Studies on prophenoloxidase activation in the mosquito Aedes aegypti L

This study, the first of its kind in a mosquito vector species, demonstrates the feasibility of studying prophenoloxidase activation in an insect containing not more than a few microliters of hemolymph. Mosquito phenoloxidase was found to be in an inactive proenzyme form, prophenoloxidase. Mosquito prophenoloxidase required bivalent cation for its activation; Ca2+ was found to be the most efficient for activation. Concomitant amidase activity was also observed prior to phenoloxidase activity. Through Western blotting, using a cross-reactive silkworm antiprophenoloxidase antibody, our results strongly suggest that mosquito prophenoloxidase activation resulted from limited proteolysis. Protease inhibitor studies reinforced this contention showing the involvement of (a) serine protease(s) with trypsin-like activity in the activation of mosquito prophenoloxidase.

An immune response-dependent mechanism for the vertical transmission of an entomopathogen

An exceptionally efficient mechanism for the vertical transmission of a parasitic gregarine is dependent on the insect host's immune response. Gametocysts of Ascogregarina chagasi on the genital accessory glands of adult female sand flies (Lutzomyia longipalpis) become encapsulated through hemocyte-mediated immune reactions. Oocysts of A. chagasi, ejected into the lumen of the glands owing to pressure exerted by this capsule, become glued to eggshells and are subsequently ingested by larvae. In L. longipalpis with an experimentally suppressed encapsulation reaction, fewer accessory glands contained oocysts of A. chagasi.