CLIPB8 is part of the prophenoloxidase activation system in Anopheles gambiae mosquitoes

The expanded immunoregulatory protease network in mosquitoes is governed by gene coexpression

Serine protease cascades regulate key innate immune responses. Their activation cleaves a series of inactive protease zymogens, which then activate downstream effector enzymes, ensuring a rapid response. In mosquitoes, these cascades involve clip-domain serine proteases and their noncatalytic homologs (CLIPs), whose make-up and structural organization are not fully understood. Here, we systematically assessed the contribution of individual CLIPs to humoral immunity in Anopheles gambiae, by performing gene knockdown of 85 of the 110 CLIPs in adult females and assessing their individual contribution to humoral immunity regulation. By coupling RNAi with assays measuring antimicrobial activity and melanization, we identified 27 immunoregulatory CLIPs. Only one CLIP contributed to both immune responses, thus providing evidence for largely distinct protease subnetworks controlling melanization and antimicrobial activity. CLIPs regulating antimicrobial activity were found to contribute to antimicrobial peptide expression and to control infection resistance, as knockdowns reduced bacterial load and improved survival. Furthermore, our analysis of CLIP gene expression unveiled a immunoregulatory mechanism reliant on protease baseline coexpression rather than infection-induced upregulation. Together, results from this study improve our understanding of immune system regulation by proteolysis and identify a layer of regulation that could be manipulated for mosquito control purposes.

miR-11903a modulates CLIPB9-mediated pathogen defense and longevity in Aedes aegypti

Arthropod melanization is a crucial defense mechanism mediated by a complex cascade of CLIP domain serine proteases (CLIPs). In this study, it was confirmed that microRNA-11903a (miR-11903a) targets Aedes-CLIPB9 (AeCLIPB9) by bioinformatics prediction and dual-luciferase reporter assays. Following intrathoracic injection of miR-11903a agomir and antagomir, Real-time quantitative polymerase chain reaction confirmed that AeCLIPB9 is negatively regulated by miR-11903a. Spatiotemporal expression analysis revealed that miR-11903a is most abundant in 4th instar larvae, followed by pupae and adults, and highly expressed in the wings, head, and midgut of female adults. Following pathogen infection, AeCLIPB9 and miR-11903a exhibited opposite expression trends, indicating their potential roles in mosquito innate immunity. To further investigate the relationship between AeCLIPB9 and miR-11903a, double-strand CLIPB9 was synthesized and RNA interference was performed. Seven-d survival assays revealed that both AeCLIPB9 and miR-11903a were crucial immune factors in fighting pathogens. Finally, longevity assays demonstrated that miR-11903a influenced mosquito lifespan.

The expanded immunoregulatory protease network in mosquitoes is governed by gene co-expression

Serine protease cascades regulate key innate immune responses. In mosquitoes, these cascades involve clip-domain serine proteases and their non-catalytic homologs (CLIPs), forming a complex network whose make-up and structural organization is not fully understood. This study assessed the impact of 85 CLIPs on humoral immunity in Anopheles gambiae. By coupling RNAi with assays measuring antimicrobial activity and melanization, we identified 27 CLIPs as immunoregulators that together form two distinct subnetworks. CLIPs regulating antimicrobial activity were found to control infection resistance, as knockdowns reduced bacterial load and improved survival. Furthermore, our analysis of CLIP gene expression unveiled a novel immunoregulatory mechanism reliant on protease baseline co-expression rather than infection-induced upregulation. These findings underscore that despite its complexity mosquito immune regulation may be targeted for malaria interventions.

Comprehensive Proteomics Analysis of the Hemolymph Composition of Sugar-Fed Aedes aegypti Female and Male Mosquitoes

In arthropods, hemolymph carries immune cells and solubilizes and transports nutrients, hormones, and other molecules that are involved in diverse physiological processes including immunity, metabolism, and reproduction. However, despite such physiological importance, little is known about its composition. We applied mass spectrometry-based label-free quantification approaches to study the proteome of hemolymph perfused from sugar-fed female and male Aedes aegypti mosquitoes. A total of 1403 proteins were identified, out of which 447 of them were predicted to be extracellular. In both sexes, almost half of these extracellular proteins were predicted to be involved in defense/immune response, and their relative abundances (based on their intensity-based absolute quantification, iBAQ) were 37.9 and 33.2%, respectively. Interestingly, among them, 102 serine proteases/serine protease-homologues were identified, with almost half of them containing CLIP regulatory domains. Moreover, proteins belonging to families classically described as chemoreceptors, such as odorant-binding proteins (OBPs) and chemosensory proteins (CSPs), were also highly abundant in the hemolymph of both sexes. Our data provide a comprehensive catalogue of A. aegypti hemolymph basal protein content, revealing numerous unexplored targets for future research on mosquito physiology and disease transmission. It also provides a reference for future studies on the effect of blood meal and infection on hemolymph composition.

Wolbachia infection-responsive immune genes suppress Plasmodium falciparum infection in Anopheles stephensi

Wolbachia, a maternally transmitted symbiotic bacterium of insects, can suppress a variety of human pathogens in mosquitoes, including malaria-causing Plasmodium in the Anopheles vector. However, the mechanistic basis of Wolbachia-mediated Plasmodium suppression in mosquitoes is not well understood. In this study, we compared the midgut and carcass transcriptomes of stably infected Anopheles stephensi with Wolbachia wAlbB to uninfected mosquitoes in order to discover Wolbachia infection-responsive immune genes that may play a role in Wolbachia-mediated anti-Plasmodium activity. We show that wAlbB infection upregulates 10 putative immune genes and downregulates 14 in midguts, while it upregulates 31 putative immune genes and downregulates 15 in carcasses at 24 h after blood-fed feeding, the time at which the Plasmodium ookinetes are traversing the midgut tissue. Only a few of these regulated immune genes were also significantly differentially expressed between Wolbachia-infected and non-infected midguts and carcasses of sugar-fed mosquitoes. Silencing of the Wolbachia infection-responsive immune genes TEP 4, TEP 15, lysozyme C2, CLIPB2, CLIPB4, PGRP-LD and two novel genes (a peritrophin-44-like gene and a macro domain-encoding gene) resulted in a significantly greater permissiveness to P. falciparum infection. These results indicate that Wolbachia infection modulates mosquito immunity and other processes that are likely to decrease Anopheles permissiveness to Plasmodium infection.

An obligate microsporidian parasite modulates defense against opportunistic bacterial infection in the yellow fever mosquito, Aedes aegypti

The ability of Aedes aegypti mosquitoes to transmit vertebrate pathogens depends on multiple factors, including the mosquitoes' life history traits, immune response, and microbiota (i.e., the microbes associated with the mosquito throughout its life). The microsporidium Edhazardia aedis is an obligate intracellular parasite that specifically infects Ae. aegypti mosquitoes and severely affects mosquito survival and other life history traits critical for pathogen transmission. In this work, we investigated how E. aedis impacts bacterial infection with Serratia marcescens in Ae. aegypti mosquitoes. We measured development, survival, and bacterial load in both larval and adult stages of mosquitoes. In larvae, E. aedis exposure was either horizontal or vertical and S. marcescens was introduced orally. Regardless of the route of transmission, E. aedis exposure resulted in significantly higher S. marcescens loads in larvae. E. aedis exposure also significantly reduced larval survival but subsequent exposure to S. marcescens had no effect. In adult females, E. aedis exposure was only horizontal and S. marcescens was introduced orally or via intrathoracic injection. In both cases, E. aedis infection significantly increased S. marcescens bacterial loads in adult female mosquitoes. In addition, females infected with E. aedis and subsequently injected with S. marcescens suffered 100% mortality which corresponded with a rapid increase in bacterial load. These findings suggest that exposure to E. aedis can influence the establishment and/or replication of other microbes in the mosquito. This has implications for understanding the ecology of mosquito immune defense and potentially disease transmission by mosquito vector species.

IMPORTANCE

The microsporidium Edhazardia aedis is a parasite of the yellow fever mosquito, Aedes aegypti. This mosquito transmits multiple viruses to humans in the United States and around the world, including dengue, yellow fever, and Zika viruses. Hundreds of millions of people worldwide will become infected with one of these viruses each year. E. aedis infection significantly reduces the lifespan of Ae. aegypti and is therefore a promising novel biocontrol agent. Here, we show that when the mosquito is infected with this parasite, it is also significantly more susceptible to infection by an opportunistic bacterial pathogen, Serratia marcescens. This novel discovery suggests the mosquito's ability to control infection by other microbes is impacted by the presence of the parasite.

Insight into the structural hierarchy of the protease cascade that regulates the mosquito melanization response

Serine protease cascades regulate important insect immune responses, including melanization and Toll pathway activation. In the context of melanization, central components of these cascades are clip domain serine proteases (CLIPs) including the catalytic, clip domain serine proteases (cSPs) and their non-catalytic homologs (cSPHs). Here, we define partially the structural hierarchy of An. gambiae cSPs of the CLIPB family, central players in melanization, and characterize their relative contributions to bacterial melanization and to mosquito susceptibility to bacterial infections. Using in vivo genetic analysis we show that the protease cascade branches downstream of the cSPs CLIPB4 and CLIPB17 into two branches one converging on CLIPB10 and the second on CLIPB8. We also show that the contribution of key cSPHs to melanization in vivo in response to diverse microbial challenges is more significant than any of the individual cSPs, possibly due to partial functional redundancy among the latter. Interestingly, we show that the key cSPH CLIPA8 which is essential for the efficient activation cleavage of CLIPBs in vivo is efficiently cleaved itself by several CLIPBs in vitro, suggesting that cSPs and cSPHs regulate signal amplification and propagation in melanization cascades by providing positive reinforcement upstream and downstream of each other.

Serine protease homolog pairs CLIPA4-A6, A4-A7Δ, and A4-A12 act as cofactors for proteolytic activation of prophenoloxidase-2 and -7 in Anopheles gambiae

Phenoloxidase (PO) catalyzed melanization and other insect immune responses are mediated by serine proteases (SPs) and their noncatalytic homologs (SPHs). Many of these SP-like proteins have a regulatory clip domain and are called CLIPs. In most insects studied so far, PO precursors are activated by a PAP (i.e., PPO activating protease) and its cofactor of clip-domain SPHs. Although melanotic encapsulation is a well-known refractory mechanism of mosquitoes against malaria parasites, it is unclear if a cofactor is required for PPO activation. In Anopheles gambiae, CLIPA4 is 1:1 orthologous to Manduca sexta SPH2; CLIPs A5-7, A12-14, A26, A31, A32, E6, and E7 are 11:4 orthologous to M. sexta SPH1a, 1b, 4, and 101, SPH2 partners in the cofactors. Here we produced proCLIPs A4, A6, A7Δ, A12, and activated them with CLIPB9 or M. sexta PAP3. A. gambiae PPO2 and PPO7 were expressed in Escherichia coli for use as PAP substrates. CLIPB9 was mutated to CLIPB9Xa by including a Factor Xa cleavage site. CLIPA7Δ was a deletion mutant with a low complexity region removed. After PAP3 or CLIPB9Xa processing, CLIPA4 formed a high Mr complex with CLIPA6, A7Δ or A12, which assisted PPO2 and PPO7 activation. High levels of specific PO activity (55-85 U/μg for PO2 and 1131-1630 U/μg for PO7) were detected in vitro, indicating that cofactor-assisted PPO activation also occurs in this species. The cleavage sites and mechanisms for complex formation and cofactor function are like those reported in M. sexta and Drosophila melanogaster. In conclusion, these data suggest that the three (and perhaps more) SPHI-II pairs may form cofactors for CLIPB9-mediated activation of PPOs for melanotic encapsulation in A. gambiae.

Insight into the structural hierarchy of the protease cascade that regulates the mosquito melanization response

Serine protease cascades regulate important insect immune responses, including melanization and Toll pathway activation. In the context of melanization, central components of these cascades are clip domain serine proteases (CLIPs) including the catalytic, clip domain serine proteases (cSPs) and their non-catalytic homologs (cSPHs). Here, we define partially the structural hierarchy of An. gambiae cSPs of the CLIPB family, central players in melanization, and characterize their relative contributions to bacterial melanization and to mosquito susceptibility to bacterial infections. Using in vivo genetic analysis we show that the protease cascade branches downstream of the cSPs CLIPB4 and CLIPB17 into two branches one converging on CLIPB10 and the second on CLIPB8. We also show that the contribution of key cSPHs to melanization in vivo in response to diverse microbial challenges is more significant than any of the individual cSPs, possibly due to partial functional redundancy among the latter. Interestingly, we show that the key cSPH CLIPA8 which is essential for the efficient activation cleavage of CLIPBs in vivo is efficiently cleaved itself by several CLIPBs in vitro, suggesting that cSPs and cSPHs regulate signal amplification and propagation in melanization cascades by providing positive reinforcement upstream and downstream of each other.

CLIPB4 Is a Central Node in the Protease Network that Regulates Humoral Immunity in Anopheles gambiae Mosquitoes

Insect humoral immune responses are regulated in part by protease cascades, whose components circulate as zymogens in the hemolymph. In mosquitoes, these cascades consist of clip-domain serine proteases (cSPs) and/or their non-catalytic homologs, which form a complex network, whose molecular make-up is not fully understood. Using a systems biology approach, based on a co-expression network of gene family members that function in melanization and co-immunoprecipitation using the serine protease inhibitor (SRPN)2, a key negative regulator of the melanization response in mosquitoes, we identify the cSP CLIPB4 from the African malaria mosquito Anopheles gambiae as a central node in this protease network. CLIPB4 is tightly co-expressed with SRPN2 and forms protein complexes with SRPN2 in the hemolymph of immune-challenged female mosquitoes. Genetic and biochemical approaches validate our network analysis and show that CLIPB4 is required for melanization and antibacterial immunity, acting as a prophenoloxidase (proPO)-activating protease, which is inhibited by SRPN2. In addition, we provide novel insight into the structural organization of the cSP network in An. gambiae, by demonstrating that CLIPB4 is able to activate proCLIPB8, a cSP upstream of the proPO-activating protease CLIPB9. These data provide the first evidence that, in mosquitoes, cSPs provide branching points in immune protease networks and deliver positive reinforcement in proPO activation cascades.

CLIPB4 is a central node in the protease network that regulates humoral immunity in Anopheles gambiae mosquitoes

Insect humoral immune responses are regulated in part by protease cascades, whose components circulate as zymogens in the hemolymph. In mosquitoes, these cascades consist of clip domain serine proteases (cSPs) and/or their non-catalytic homologs (cSPHs), which form a complex network, whose molecular make-up is not fully understood. Using a systems biology approach, based on a co-expression network of gene family members that function in melanization and co-immunoprecipitation using the serine protease inhibitor (SRPN)2, a key negative regulator of the melanization response in mosquitoes, we identify the cSP CLIPB4 from the African malaria mosquito Anopheles gambiae as a central node in this protease network. CLIPB4 is tightly co-expressed with SRPN2 and forms protein complexes with SRPN2 in the hemolymph of immune-challenged female mosquitoes. Genetic and biochemical approaches validate our network analysis and show that CLIPB4 is required for melanization and antibacterial immunity, acting as a prophenoloxidase (proPO)-activating protease, which is inhibited by SRPN2. In addition, we provide novel insight into the structural organization of the cSP network in An. gambiae, by demonstrating that CLIPB4 is able to activate proCLIPB8, a cSP upstream of the proPO-activating protease CLIPB9. These data provide the first evidence that, in mosquitoes, cSPs provide branching points in immune protease networks and deliver positive reinforcement in proPO activation cascades.

Immune gene expression in the mosquito vector Culex quinquefasciatus during an avian malaria infection

Plasmodium relictum is the most widespread avian malaria parasite in the world. It is listed as one of the 100 most dangerous invasive species, having been responsible for the extinction of several endemic bird species, and the near-demise of several others. Here we present the first transcriptomic study focused on the effect of P. relictum on the immune system of its vector (the mosquito Culex quinquefasciatus) at different times post-infection. We show that over 50% of immune genes identified as being part of the Toll pathway and 30%-40% of the immune genes identified within the Imd pathway are overexpressed during the critical period spanning the parasite's oocyst and sporozoite formation (8-12 days), revealing the crucial role played by both these pathways in this natural mosquito-Plasmodium combination. Comparison of infected mosquitoes with their uninfected counterparts also revealed some unexpected immune RNA expression patterns earlier and later in the infection: significant differences in expression of several immune effectors were observed as early as 30 min after ingestion of the infected blood meal. In addition, in the later stages of the infection (towards the end of the mosquito lifespan), we observed an unexpected increase in immune investment in uninfected, but not in infected, mosquitoes. In conclusion, our work extends the comparative transcriptomic analyses of malaria-infected mosquitoes beyond human and rodent parasites and provides insights into the degree of conservation of immune pathways and into the selective pressures exerted by Plasmodium parasites on their vectors.

A zone-of-inhibition assay to screen for humoral antimicrobial activity in mosquito hemolymph

In insects, antibacterial immunity largely depends on the activation of downstream signaling and effector responses, leading to the synthesis and secretion of soluble effector molecules, such as antimicrobial peptides (AMPs). AMPs are acute infection response peptides secreted into the hemolymph upon bacterial stimulation. The transcription of innate immunity genes encoding for AMPs is highly dependent on several signaling cascade pathways, such as the Toll pathway. In the African malaria mosquito, Anopheles gambiae, AMPs hold a special interest as their upregulation have been shown to limit the growth of malaria parasites, bacteria, and fungi. Most of the current knowledge on the regulation of insect AMPs in microbial infection have been obtained from Drosophila. However, largely due to the lack of convenient assays, the regulation of antimicrobial activity in mosquito hemolymph is still not completely understood. In this study, we report a zone of inhibition assay to identify the contribution of AMPs and components of the Toll pathway to the antimicrobial activity of A. gambiae hemolymph. As a proof of principle, we demonstrate that Micrococcus luteus challenge induces antimicrobial activity in the adult female mosquito hemolymph, which is largely dependent on defensin 1. Moreover, by using RNAi to silence Cactus, REL1, and MyD88, we showed that Cactus kd induces antimicrobial activity in the mosquito hemolymph, whereas the antimicrobial activity in REL1 kd and MyD88 kd is reduced after challenge. Finally, while injection itself is not sufficient to induce antimicrobial activity, our results show that it primes the response to bacterial challenge. Our study provides information that increases our knowledge of the regulation of antimicrobial activity in response to microbial infections in mosquitoes. Furthermore, this assay represents an ex vivo medium throughput assay that can be used to determine the upstream regulatory elements of antimicrobial activity in A. gambiae hemolymph.

Functional constraints on insect immune system components govern their evolutionary trajectories

Roles of constraints in shaping evolutionary outcomes are often considered in the contexts of developmental biology and population genetics, in terms of capacities to generate new variants and how selection limits or promotes consequent phenotypic changes. Comparative genomics also recognizes the role of constraints, in terms of shaping evolution of gene and genome architectures, sequence evolutionary rates, and gene gains or losses, as well as on molecular phenotypes. Characterizing patterns of genomic change where putative functions and interactions of system components are relatively well described offers opportunities to explore whether genes with similar roles exhibit similar evolutionary trajectories. Using insect immunity as our test case system, we hypothesize that characterizing gene evolutionary histories can define distinct dynamics associated with different functional roles. We develop metrics that quantify gene evolutionary histories, employ these to characterize evolutionary features of immune gene repertoires, and explore relationships between gene family evolutionary profiles and their roles in immunity to understand how different constraints may relate to distinct dynamics. We identified three main axes of evolutionary trajectories characterized by gene duplication and synteny, maintenance/stability and sequence conservation, and loss and sequence divergence, highlighting similar and contrasting patterns across these axes amongst subsets of immune genes. Our results suggest that where and how genes participate in immune responses limit the range of possible evolutionary scenarios they exhibit. The test case study system of insect immunity highlights the potential of applying comparative genomics approaches to characterize how functional constraints on different components of biological systems govern their evolutionary trajectories.

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.

The CLIP-domain serine protease CLIPC9 regulates melanization downstream of SPCLIP1, CLIPA8, and CLIPA28 in the malaria vector Anopheles gambiae

The arthropod melanization immune response is activated by extracellular protease cascades predominantly comprised of CLIP-domain serine proteases (CLIP-SPs) and serine protease homologs (CLIP-SPHs). In the malaria vector, Anopheles gambiae, the CLIP-SPHs SPCLIP1, CLIPA8, and CLIPA28 form the core of a hierarchical cascade downstream of mosquito complement that is required for microbial melanization. However, our understanding of the regulatory relationship of the CLIP-SPH cascade with the catalytic CLIP-SPs driving melanization is incomplete. Here, we report on the development of a novel screen to identify melanization pathway components based on the quantitation of melanotic mosquito excreta, eliminating the need for microdissections or hemolymph enzymatic assays. Using this screen, we identified CLIPC9 and subsequent functional analyses established that this protease is essential for the melanization of both Escherichia coli and the rodent malaria parasite Plasmodium berghei. Mechanistically, septic infection with E. coli promotes CLIPC9 cleavage and both full-length and cleaved CLIPC9 localize to this bacterium in a CLIPA8-dependent manner. The steady state level of CLIPC9 in the hemolymph is regulated by thioester-containing protein 1 (TEP1), suggesting it functions downstream of mosquito complement. In support, CLIPC9 cleavage is inhibited following SPCLIP1, CLIPA8, and CLIPA28 knockdown positioning it downstream of the CLIP-SPH cascade. Moreover, like CLIPA8 and CLIPA28, CLIPC9 processing is negatively regulated by serine protease inhibitor 2 (SRPN2). This report demonstrates how our novel excretion-based approach can be utilized to dissect the complex protease networks regulating mosquito melanization. Collectively, our findings establish that CLIPC9 is required for microbial melanization in An. gambiae and shed light on how the CLIP-SPH cascade regulates this potent immune response.

Identification of two clip domain serine proteases involved in the pea aphid's defense against bacterial and fungal infection

Phenoloxidases (POs) are required for the pea aphid's defense against bacterial and fungal infection. Prophenoloxidases (PPOs) are proteolytically converted to its active form PO through a clip domain serine protease cascade. In this study, we identified five clip domain serine proteases in the pea aphids. The messenger RNA levels of two of them, Ap_SPLP and Ap_VP, were upregulated by Gram-positive bacterium Staphylococcus aureus and fungus Beauveria bassiana infections. Double-stranded RNA-based expression knockdown of these two genes resulted in reduced PO activity of the aphid hemolymph, higher loads of S. aureus and B. bassiana in the aphids, and lower survival rates of the aphids after infections. Our data suggest that Ap_SPLP and Ap_VP are involved in PPO activation pathway in the pea aphid.

Identification of a Conserved Prophenoloxidase Activation Pathway in Cotton Bollworm Helicoverpa armigera

Melanization is a prominent insect humoral response for encapsulation of and killing invading pathogens. It is mediated by a protease cascade composed of a modular serine protease (SP), and clip domain SPs (cSPs), which converts prophenoloxidase (PPO) into active phenoloxidase (PO). To date, melanization pathway in cotton bollworm Helicoverpa armigera, an important agricultural pest, remains largely unclear. To biochemically reconstitute the pathway in vitro, the putative proteases along with modified proteases containing the factor Xa cleavage site were expressed by Drosophila S2 cell expression system. Purified recombinant proteins were used to examine their role in activating PPO. It is revealed that cascade is initiated by a modular SP-SP41, followed by cSP1 and cSP6. The three-step SP41/cSP1/cSP6 cascade could further activate PPO, and the PO activity was significantly enhanced in the presence of two cSP homologs (cSPHs), cSPH11 and cSPH50, suggesting the latter are cofactors for PPO activation. Moreover, baculovirus infection was efficiently blocked by the reconstituted PPO activation cascade, and the effect was boosted by cSPH11 and cSPH50. Taken together, we unraveled a conserved PPO activation cascade in H. armigera, which is similar to that exists in lepidopteran biochemical model Manduca sexta and highlighted its role in antagonizing viral infection.

Insect Hemolymph Immune Complexes

Insects possess powerful immune systems that have evolved to defend against wounding and environmental pathogens such as bacteria, fungi, protozoans, and parasitoids. This surprising sophistication is accomplished through the activation of multiple immune pathways comprised of a large array of components, many of which have been identified and studied in detail using both genetic manipulations and traditional biochemical techniques. Recent advances indicate that certain pathways activate arrays of proteins that interact to form large functional complexes. Here we discuss three examples from multiple insects that exemplify such processes, including pathogen recognition, melanization, and coagulation. The functionality of each depends on integrating recognition with the recruitment of immune effectors capable of healing wounds and destroying pathogens. In both melanization and coagulation, protein interactions also appear to be essential for enzymatic activities tied to the formation of melanin and for the recruitment of hemocytes. The importance of these immune complexes is highlighted by the evolution of mechanisms in pathogens to disrupt their formation, an example of which is provided. While technically difficult to study, and not always readily amenable to dissection through genetics, modern mass spectrometry has become an indispensable tool in the study of these higher-order protein interactions. The formation of immune complexes should be viewed as an essential and emerging frontier in the study of insect immunity.

The mosquito melanization response requires hierarchical activation of non-catalytic clip domain serine protease homologs

Serine protease cascades regulate important insect immune responses namely melanization and Toll pathway activation. An important component of these cascades are clip-domain serine protease homologs (cSPHs), which are non-catalytic, but essential for activating the enzyme prophenoloxidase (PPO) in the melanization response during septic infections. The activation of cSPHs requires their proteolytic cleavage, yet factors that control their activation and the complexity of their interactions within these cascades remain unclear. Here, we report the identification of CLIPA28 as a novel immune-related cSPH in the malaria vector Anopheles gambiae. Functional genetic analysis using RNA interference (RNAi) revealed that CLIPA28 is essential for the melanization of Plasmodium berghei parasites in refractory mosquitoes, and for mosquito resistance to fungal infections. We further show, using combined biochemical and genetic approaches, that CLIPA28 is member of a network of at least four cSPHs, whereby members are activated in a hierarchical manner following septic infections. Depletion of the complement-like protein TEP1 abolished the activation of this network after septic infections, whereas, depletion of the serine protease inhibitor 2 (SRPN2) triggered enhanced network activation, even in naïve mosquitoes, culminating in a dramatic reduction in cSPHs hemolymph levels, which paralleled that of PPO. Our data suggest that cSPHs are engaged in complex and multilayered interactions within serine protease cascades that regulate melanization, and identify TEP1 and SRPN2 as two master regulators of the cSPH network.

Melanization is a spectacular immune response of insects that culminates in the deposition of melanin on microbial surfaces leading to their death. Despite more than half a century of investigation of this immune reaction in several insect models, important knowledge gaps, specifically concerning the regulation of this response, remain, especially that its output is tightly regulated by complex genetic and biochemical interactions between members of structurally diverse immune gene families. Here, we identify a new hemolymph protein that is involved in the melanization of malaria parasites and show that it forms, together with other proteins of the same gene family, a network, whereby members are activated in an orderly manner during mosquito septic infections. Furthemore, we show that this network is controlled by two structurally distinct immune proteins which act as master regulators of its activation. Dissecting the complexity of the regulatory mechanisms mediating insect melanization responses, may facilitate fine manipulation of mosquito immunity to reduce vectorial capacity.

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.

Stimulation of a protease targeting the LRIM1/APL1C complex reveals specificity in complement-like pathway activation in Anopheles gambiae

The complement-like pathway of the African malaria mosquito Anopheles gambiae provides protection against infection by diverse pathogens. A functional requirement for a core set of proteins during infections by rodent and human malaria parasites, bacteria, and fungi suggests a similar mechanism operates against different pathogens. However, the extent to which the molecular mechanisms are conserved is unknown. In this study we probed the biochemical responses of complement-like pathway to challenge by the Gram-positive bacterium Staphyloccocus aureus. Western blot analysis of the hemolymph revealed that S. aureus challenge activates a TEP1 convertase-like activity and promotes the depletion of the protein SPCLIP1. S. aureus challenge did not lead to an apparent change in the abundance of the LRIM1/APL1C complex compared to challenge by the Gram-negative bacterium, Escherichia coli. Following up on this observation using a panel of LRIM1 and APL1C antibodies, we found that E. coli challenge, but not S. aureus, specifically activates a protease that cleaves the C-terminus of APL1C. Inhibitor studies in vivo and in vitro protease assays suggest that a serine protease is responsible for APL1C cleavage. This study reveals that despite different challenges converging on activation of a TEP1 convertase-like activity, the mosquito complement-like pathway also includes pathogen-specific reactions.

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

Background

Anopheles mosquitoes are efficient vectors of human malaria, but it is unknown why they do not transmit viruses as well as Aedes and Culex mosquitoes. The only arbovirus known to be consistently transmitted by Anopheles mosquitoes is O’nyong nyong virus (ONNV, genus Alphavirus, family Togaviridae). The interaction of Anopheles mosquitoes with RNA viruses has been relatively unexamined.

Results

We transcriptionally profiled the African malaria vector, Anopheles coluzzii, infected with ONNV. Mosquitoes were fed on an infectious bloodmeal and were analyzed by Illumina RNAseq at 3 days post-bloodmeal during the primary virus infection of the midgut epithelium, before systemic dissemination. Virus infection triggers transcriptional regulation of just 30 host candidate genes. Most of the regulated candidate genes are novel, without known function. Of the known genes, a significant cluster includes candidates with predicted involvement in carbohydrate metabolism. Two candidate genes encoding leucine-rich repeat immune (LRIM) factors point to possible involvement of immune protein complexes in the mosquito antiviral response. The primary ONNV infection by bloodmeal shares little transcriptional response in common with ONNV infection by intrathoracic injection, nor with midgut infection by the malaria parasites, Plasmodium falciparum or P. berghei. Profiling of A. coluzzii microRNA (miRNA) identified 118 known miRNAs and 182 potential novel miRNA candidates, with just one miRNA regulated by ONNV infection. This miRNA was not regulated by other previously reported treatments, and may be virus specific. Coexpression analysis of miRNA abundance and messenger RNA expression revealed discrete clusters of genes regulated by Imd and JAK/STAT, immune signaling pathways that are protective against ONNV in the primary infection.

Conclusions

ONNV infection of the A. coluzzii midgut triggers a remarkably limited gene regulation program of mostly novel candidate genes, which likely includes host genes deployed for antiviral defense, as well as genes manipulated by the virus to facilitate infection. Functional dissection of the ONNV-response candidate genes is expected to generate novel insight into the mechanisms of virus-vector interaction.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4918-0) contains supplementary material, which is available to authorized users.

Activation of Aedes aegypti prophenoloxidase-3 and its role in the immune response against entomopathogenic fungi

Serine protease cascade-mediated melanization is an important innate immune response in insects and crustaceans, which involves the proteolytic activation of prophenoloxidase (PPO). In this study, we investigated the role of Aedes aegypti PPO3 in antifungal immune defence. We expressed and purified recombinant PPO3 (rPPO3) in Escherichia coli and demonstrated that rPPO3 was activated by ethanol and, to a lesser extent, by cetylpyridinium chloride. In the presence of Cu²⁺ , rPPO3 exhibited enzyme activity. Immunoblot results revealed that the rPPO3 was cleaved by the haemolymph from immune-challenged mosquitoes or purified Ostrinia furnacalis serine protease 105 in vitro. The cleaved rPPO3 converted dopamine to toxic intermediates that killed fungal conidia of Beauveria bassiana in vitro. In mosquitoes challenged with Be. bassiana, cleavage of rPPO3 produced a 50 kDa phenoloxidase (PO) fragment. Further analysis revealed that the survival rate of mosquitoes with fungal infection increased significantly following injection of rPPO3 into the haemocoel. Taken together, our results suggest that proteolytic cleavage of the mosquito PPO3 plays an important role in the antifungal immune response. This has led to a better understanding of the mechanism of PPO activation in the mosquito and the role of melanization in the antifungal immune response.

Serine protease-related proteins in the malaria mosquito, Anopheles gambiae

Insect serine proteases (SPs) and serine protease homologs (SPHs) participate in digestion, defense, development, and other physiological processes. In mosquitoes, some clip-domain SPs and SPHs (i.e. CLIPs) have been investigated for possible roles in antiparasitic responses. In a recent test aimed at improving quality of gene models in the Anopheles gambiae genome using RNA-seq data, we observed various discrepancies between gene models in AgamP4.5 and corresponding sequences selected from those modeled by Cufflinks, Trinity and Bridger. Here we report a comparative analysis of the 337 SP-related proteins in A. gambiae by examining their domain structures, sequence diversity, chromosomal locations, and expression patterns. One hundred and ten CLIPs contain 1 to 5 clip domains in addition to their protease domains (PDs) or non-catalytic, protease-like domains (PLDs). They are divided into five subgroups: CLIPAs (22) are clip_1-5-PLD; CLIPBs (29), CLIPCs (12) and CLIPDs (14) are mainly clip-PD; most CLIPEs (33) have a domain structure of PD/PLD-PLD-clip-PLD_0-1. While expression of the CLIP genes in group-1 is generally low and detected in various tissue- and stage-specific RNA-seq libraries, some putative GPs/GPHs (i.e. single domain gut SPs/SPHs) in group-2 are highly expressed in midgut, whole larva or whole adult libraries. In comparison, 46 SPs, 26 SPHs, and 37 multi-domain SPs/SPHs (i.e. PD/PLD-PLD_≥1) in group-3 do not seem to be specifically expressed in digestive tract. There are 16 SPs and 2 SPH containing other types of putative regulatory domains (e.g. LDLa, CUB, Gd). Of the 337 SP and SPH genes, 159 were sorted into 46 groups (2-8 members/group) based on similar phylogenetic tree position, chromosomal location, and expression profile. This information and analysis, including improved gene models and protein sequences, constitute a solid foundation for functional analysis of the SP-related proteins in A. gambiae.

Arthropod Innate Immune Systems and Vector-Borne Diseases

Arthropods, especially ticks and mosquitoes, are the vectors for a number of parasitic and viral human diseases, including malaria, sleeping sickness, Dengue, and Zika, yet arthropods show tremendous individual variation in their capacity to transmit disease. A key factor in this capacity is the group of genetically encoded immune factors that counteract infection by the pathogen. Arthropod-specific pattern recognition receptors and protease cascades detect and respond to infection. Proteins such as antimicrobial peptides, thioester-containing proteins, and transglutaminases effect responses such as lysis, phagocytosis, melanization, and agglutination. Effector responses are initiated by damage signals such as reactive oxygen species signaling from epithelial cells and recognized by cell surface receptors on hemocytes. Antiviral immunity is primarily mediated by siRNA pathways but coupled with interferon-like signaling, antimicrobial peptides, and thioester-containing proteins. Molecular mechanisms of immunity are closely linked to related traits of longevity and fertility, and arthropods have the capacity for innate immunological memory. Advances in understanding vector immunity can be leveraged to develop novel control strategies for reducing the rate of transmission of both ancient and emerging threats to global health.