Anopheles gambiae SRPN2 facilitates midgut invasion by the malaria parasite Plasmodium berghei

Mosquito genomics. Highly evolvable malaria vectors: the genomes of 16 Anopheles mosquitoes

Variation in vectorial capacity for human malaria among Anopheles mosquito species is determined by many factors, including behavior, immunity, and life history. To investigate the genomic basis of vectorial capacity and explore new avenues for vector control, we sequenced the genomes of 16 anopheline mosquito species from diverse locations spanning ~100 million years of evolution. Comparative analyses show faster rates of gene gain and loss, elevated gene shuffling on the X chromosome, and more intron losses, relative to Drosophila. Some determinants of vectorial capacity, such as chemosensory genes, do not show elevated turnover but instead diversify through protein-sequence changes. This dynamism of anopheline genes and genomes may contribute to their flexible capacity to take advantage of new ecological niches, including adapting to humans as primary hosts.

Functional characterization of Anopheles matrix metalloprotease 1 reveals its agonistic role during sporogonic development of malaria parasites

The ability to invade tissues is a unique characteristic of the malaria stages that develop/differentiate within the mosquitoes (ookinetes and sporozoites). On the other hand, tissue invasion by many pathogens has often been associated with increased matrix metalloprotease (MMP) activity in the invaded tissues. By employing cell biology and reverse genetics, we studied the expression and explored putative functions of one of the three MMPs encoded in the genome of the malaria vector Anopheles gambiae, namely, the Anopheles gambiae MMP1 (AgMMP1) gene, during the processes of blood digestion, midgut epithelium invasion by Plasmodium ookinetes, and oocyst development. We show that AgMMP1 exists in two alternative isoforms resulting from alternative splicing; one secreted (S-MMP1) and associated with hemocytes, and one membrane type (MT-MMP1) enriched in the cell attachment sites of the midgut epithelium. MT-MMP1 showed a remarkable response to ookinete midgut invasion manifested by increased expression, enhanced zymogen maturation, and subcellular redistribution, all indicative of an implication in the midgut epithelial healing that accompanies ookinete invasion. Importantly, RNA interference (RNAi)-mediated silencing of the AgMMP1 gene revealed a postinvasion protective function of AgMMP1 during oocyst development. The combined results link for the first time an MMP with vector competence and mosquito-Plasmodium interactions.

Mosquito defenses against Plasmodium parasites

Malaria, the human infectious disease caused by Plasmodium parasites, is transmitted by the bite of the mosquito Anopheles gambiae. Mosquitoes actively detect Plasmodium and mount efficient responses that eliminate the majority of invading parasites. Such responses include hemocyte-mediated defenses, activation of the complement-like system, melanization, and immune signaling cascades. This review aims to summarize our current knowledge of the mosquito immune responses to Plasmodium and to highlight the remaining gaps in our understanding of these events.

Morphological and proteomic characterization of midgut of the malaria vector Anopheles albimanus at early time after a blood feeding

The midgut of anopheline mosquito is the entry of Plasmodium, the causative agent of malaria.When the mosquito feeds on parasite infected host, Plasmodium parasites reach the midgut and must confront digestive enzymes, the innate immune response and go across the peritrophic matrix (PM), a thick extracellular sheath secreted by the mosquito midgut epithelial cells. Then, to continue its development, the parasite must reach the salivary glands to achieve transmission to a vertebrate host. We report here the morphological and biochemical descriptions of the midgut changes after a blood meal in Anopheles albimanus. Before blood feeding, midgut epithelial cells contained numerous electrondense vesicles distributed in the central to apical side. These vesicles were secreted to the luminal side of the midgut after a blood meal. At early times after blood ingest, the PM is formed near microvilli as a granulous amorphous material and after it consolidates forming a highly organized fibrillar structure, constituted by layers of electrondense and electronlucent regions. Proteomic comparative analysis of sugar and blood fed midguts showed several molecules that modify their abundance after blood intake; these include innate immunity, cytoskeletal, stress response, signaling, and digestive, detoxifying and metabolism enzymes. Biological significance In the midgut of mosquitoes during bloodfeeding, many simultaneous processes occur, including digestion, innate immune activities, cytoskeleton modifications, construction of a peritrophic matrix and hormone production, between others. Mechanical forces are very intense during bloodfeeding and epithelial and muscular cells must resist the stress, modifying the actin cytoskeleton and coordinating intracellular responses by signaling. Microorganisms present in midgut contents reproduce and interact with epithelial cells triggering innate immune response. When infectious agents are present in the blood meal they must traverse the peritrophic matrix, an envelope formed from secretion products of epithelial cells, and evade the immune system in order to reach the epithelium and continue their journey towards salivary glands, in preparation for the transmission to the new hosts. During all these processes, proteins of mosquitoes are modified in order to deal with mechanical and biological challenges, and the aim of this work is to study these changes.

Insect prophenoloxidase: the view beyond immunity

Insect prophenoloxidase (PPO) is an important innate immunity protein due to its involvement in cellular and humoral defense. It belongs to a group of type-3 copper-containing proteins that occurs in almost all organisms. Insect PPO has been studied for over a century, and the PPO activation cascade is becoming clearer. The insect PPO activation pathway incorporates several important proteins, including pattern-recognition receptors (PGRP, β GRP, and C-type lectins), serine proteases, and serine protease inhibitors (serpins). Due to their complexity, PPO activation mechanisms vary among insect species. Activated phenoloxidase (PO) oxidizes phenolic molecules to produce melanin around invading pathogens and wounds. The crystal structure of Manduca sexta PPO shows that a conserved amino acid, phenylalanine (F), can block the active site pocket. During activation, this blocker must be dislodged or even cleaved at the N-terminal sequence to expose the active site pockets and allow substrates to enter. Thanks to the crystal structure of M. sexta PPO, some domains and specific amino acids that affect PPO activities have been identified. Further studies of the relationship between PPO structure and enzyme activities will provide an opportunity to examine other type-3 copper proteins, and trace when and why their various physiological functions evolved. Recent researches show that insect PPO has a relationship with neuron activity, longevity, feces melanization (phytophagous insects) and development, which suggests that it is time for us to look back on insect PPO beyond the view of immunity in this review.

RNAi-mediated knockdown of serine protease inhibitor genes increases the mortality of Plutella xylostella challenged by destruxin A

Destruxin A is a mycotoxin that is secreted by entomopathogenic fungi which has a broad-spectrum insecticidal effect. Previous transcript and protein profiling analysis showed that destruxin A has significant effects on the expression of serine protease inhibitor genes (serpin-2, 4, 5) in the larvae of Plutella xylostella. In the current study, we aimed to understand the role of serpins under application of destruxin A. We obtained two full-length cDNA sequences of P. xylostella serpins, named serpin-4 and serpin-5, and cloned the serpin-2 gene whose full-length has already been published. Phylogenetic analysis indicated that these two serpin genes were highly clustered with other serpins associated with the immune response in other insects. The temporal and spatial expression of serpin-2, serpin-4 and serpin-5 were determined to be the highest in the fat body and hemolymph of 4^th larval stage using qRT-PCR and western blot detection techniques. RNA interference (RNAi) mediated knockdown of P. xylostella serpin genes was carried out by microinjection of double-stranded RNA (dsRNA). The expression levels of serpins decreased significantly after RNAi. Results showed that the depletion of serpins induced cecropins expression, increased phenoloxidase (PO) activity, body melanization and mortality in the larvae of P. xylostella under the same lethal concentration of destruxin A. The superimposed effects of serpins RNAi were similar with the destruxin A treatment upon mortality of P. xylostella larvae. We discovered for the first time that serpins play indispensable role in P. xylostella when challenged by destruxin A and deduced the possible function mechanism of destruxin A. Our findings are conducive to fully understanding the potential insecticidal mechanism of destruxin A and constitute a well-defined potential molecular target for novel insecticides.

Identification of immunity-related genes in Ostrinia furnacalis against entomopathogenic fungi by RNA-seq analysis

BACKGROUND

The Asian corn borer (Ostrinia furnacalis (Guenée)) is one of the most serious corn pests in Asia. Control of this pest with entomopathogenic fungus Beauveria bassiana has been proposed. However, the molecular mechanisms involved in the interactions between O. furnacalis and B. bassiana are unclear, especially under the conditions that the genomic information of O. furnacalis is currently unavailable. So we sequenced and characterized the transcriptome of O. furnacalis larvae infected by B. bassiana with special emphasis on immunity-related genes.

METHODOLOGY/PRINCIPAL FINDINGS

Illumina Hiseq2000 was used to sequence 4.64 and 4.72 Gb of the transcriptome from water-injected and B. bassiana-injected O. furnacalis larvae, respectively. De novo assembly generated 62,382 unigenes with mean length of 729 nt. All unigenes were searched against Nt, Nr, Swiss-Prot, COG, and KEGG databases for annotations using BLASTN or BLASTX algorithm with an E-value cut-off of 10(-5). A total of 35,700 (57.2%) unigenes were annotated to at least one database. Pairwise comparisons resulted in 13,890 differentially expressed genes, with 5,843 up-regulated and 8,047 down-regulated. Based on sequence similarity to homologs known to participate in immune responses, we totally identified 190 potential immunity-related unigenes. They encode 45 pattern recognition proteins, 33 modulation proteins involved in the prophenoloxidase activation cascade, 46 signal transduction molecules, and 66 immune responsive effectors, respectively. The obtained transcriptome contains putative orthologs for nearly all components of the Toll, Imd, and JAK/STAT pathways. We randomly selected 24 immunity-related unigenes and investigated their expression profiles using quantitative RT-PCR assay. The results revealed variant expression patterns in response to the infection of B. bassiana.

CONCLUSIONS/SIGNIFICANCE

This study provides the comprehensive sequence resource and expression profiles of the immunity-related genes of O. furnacalis. The obtained data gives an insight into better understanding the molecular mechanisms of innate immune processes in O. furnacalis larvae against B. bassiana.

Genome-block expression-assisted association studies discover malaria resistance genes in Anopheles gambiae

The malaria parasite-resistance island (PRI) of the African mosquito vector, Anopheles gambiae, was mapped to five genomic regions containing 80 genes, using coexpression patterns of genomic blocks. High-throughput sequencing identified 347 nonsynonymous single-nucleotide polymorphisms within these genes in mosquitoes from malaria-endemic areas in Kenya. Direct association studies between nonsynonymous single-nucleotide polymorphisms and Plasmodium falciparum infection identified three naturally occurring genetic variations in each of three genes (An. gambiae adenosine deaminase, fibrinogen-related protein 30, and fibrinogen-related protein 1) that were associated significantly with parasite infection. A role for these genes in the resistance phenotype was confirmed by RNA interference knockdown assays. Silencing fibrinogen-related protein 30 increased parasite infection significantly, whereas ablation of fibrinogen-related protein 1 transcripts resulted in mosquitoes nearly free of parasites. The discovered genes and single-nucleotide polymorphisms are anticipated to be useful in the development of tools for malaria control in endemic areas in Africa.

Towards the elements of successful insect RNAi

RNA interference (RNAi), the sequence-specific suppression of gene expression, offers great opportunities for insect science, especially to analyze gene function, manage pest populations, and reduce disease pathogens. The accumulating body of literature on insect RNAi has revealed that the efficiency of RNAi varies between different species, the mode of RNAi delivery, and the genes being targeted. There is also variation in the duration of transcript suppression. At present, we have a limited capacity to predict the ideal experimental strategy for RNAi of a particular gene/insect because of our incomplete understanding of whether and how the RNAi signal is amplified and spread among insect cells. Consequently, development of the optimal RNAi protocols is a highly empirical process. This limitation can be relieved by systematic analysis of the molecular physiological basis of RNAi mechanisms in insects. An enhanced conceptual understanding of RNAi function in insects will facilitate the application of RNAi for dissection of gene function, and to fast-track the application of RNAi to both control pests and develop effective methods to protect beneficial insects and non-insect arthropods, particularly the honey bee (Apis mellifera) and cultured Pacific white shrimp (Litopenaeus vannamei) from viral and parasitic diseases.

Immunisation against a serine protease inhibitor reduces intensity of Plasmodium berghei infection in mosquitoes

The mosquito innate immune response is able to clear the majority of Plasmodium parasites. This immune clearance is controlled by a number of regulatory molecules including serine protease inhibitors (serpins). To determine whether such molecules could represent a novel target for a malaria transmission-blocking vaccine, we vaccinated mice with Anopheles gambiae serpin-2. Antibodies against Anopheles gambiae serpin-2 significantly reduced the infection of a heterologous Anopheles species (Anopheles stephensi) by Plasmodium berghei, however this effect was not observed with Plasmodium falciparum. Therefore, this approach of targeting regulatory molecules of the mosquito immune system may represent a novel approach to transmission-blocking malaria vaccines.

Bacteria- and IMD pathway-independent immune defenses against Plasmodium falciparum in Anopheles gambiae

The mosquito Anopheles gambiae uses its innate immune system to control bacterial and Plasmodium infection of its midgut tissue. The activation of potent IMD pathway-mediated anti-Plasmodium falciparum defenses is dependent on the presence of the midgut microbiota, which activate this defense system upon parasite infection through a peptidoglycan recognition protein, PGRPLC. We employed transcriptomic and reverse genetic analyses to compare the P. falciparum infection-responsive transcriptomes of septic and aseptic mosquitoes and to determine whether bacteria-independent anti-Plasmodium defenses exist. Antibiotic treated aseptic mosquitoes mounted molecular immune responses representing a variety of immune functions upon P. falciparum infection. Among other immune factors, our analysis uncovered a serine protease inhibitor (SRPN7) and Clip-domain serine protease (CLIPC2) that were transcriptionally induced in the midgut upon P. falciparum infection, independent of bacteria. We also showed that SRPN7 negatively and CLIPC2 positively regulate the anti-Plasmodium defense, independently of the midgut-associated bacteria. Co-silencing assays suggested that these two genes may function together in a signaling cascade. Neither gene was regulated, nor modulated, by infection with the rodent malaria parasite Plasmodium berghei, suggesting that SRPN7 and CLIPC2 are components of a defense system with preferential activity towards P. falciparum. Further analysis using RNA interference determined that these genes do not regulate the anti-Plasmodium defense mediated by the IMD pathway, and both factors act as agonists of the endogenous midgut microbiota, further demonstrating the lack of functional relatedness between these genes and the bacteria-dependent activation of the IMD pathway. This is the first study confirming the existence of a bacteria-independent, anti-P. falciparum defense. Further exploration of this anti-Plasmodium defense will help clarify determinants of immune specificity in the mosquito, and expose potential gene and/or protein targets for malaria intervention strategies based on targeting the parasite in the mosquito vector.

The Anopheles innate immune system in the defense against malaria infection

The multifaceted innate immune system of insects is capable of fighting infection by a variety of pathogens including those causing human malaria. Malaria transmission by the Anopheles mosquito depends on the Plasmodium parasite's successful completion of its lifecycle in the insect vector, a process that involves interactions with several tissues and cell types as well as with the mosquito's innate immune system. This review will discuss our current understanding of the Anopheles mosquito's innate immune responses against the malaria parasite Plasmodium and the influence of the insect's intestinal microbiota on parasite infection.

Prophenoloxidase system and its role in shrimp immune responses against major pathogens

The global shrimp industry still faces various serious disease-related problems that are mainly caused by pathogenic bacteria and viruses. Understanding the host defense mechanisms is likely to be beneficial in designing and implementing effective strategies to solve the current and future pathogen-related problems. Melanization, which is performed by phenoloxidase (PO) and controlled by the prophenoloxidase (proPO) activation cascade, plays an important role in the invertebrate immune system in allowing a rapid response to pathogen infection. The activation of the proPO system, by the specific recognition of microorganisms by pattern-recognition proteins (PRPs), triggers a serine proteinase cascade, eventually leading to the cleavage of the inactive proPO to the active PO that functions to produce the melanin and toxic reactive intermediates against invading pathogens. This review highlights the recent discoveries of the critical roles of the proPO system in the shrimp immune responses against major pathogens, and emphasizes the functional characterizations of four major groups of genes and proteins in the proPO cascade in penaeid shrimp, that is the PRPs, serine proteinases, proPO and inhibitors.

The roles of serpins in mosquito immunology and physiology

In vector-borne diseases, the complex interplay between pathogen and its vector's immune system determines the outcome of infection and therefore disease transmission. Serpins have been shown in many animals to be key regulators of innate immune reactions. Their control over regulatory proteolytic cascades ultimately decides whether the recognition of a pathogen will lead to an appropriate immune response. In mosquitoes, serpins (SRPNs) regulate the activation of prophenoloxidase and thus melanization, contribute to malaria parasite lysis, and likely Toll pathway activation. Additionally, in culicine mosquitoes, SRPNs are able to regulate hemostasis in the vertebrate host, suggesting a crucial role during bloodfeeding. This review summarizes the annotation, transcriptional regulation, and current knowledge of SRPN function in the three mosquito species for which the complete genome sequence is available. Additionally, we give a brief overview of how SRPNs may be used to prevent transmission of vector-borne diseases.

The mosquito melanization response is implicated in defense against the entomopathogenic fungus Beauveria bassiana

Mosquito immunity studies have focused mainly on characterizing immune effector mechanisms elicited against parasites, bacteria and more recently, viruses. However, those elicited against entomopathogenic fungi remain poorly understood, despite the ubiquitous nature of these microorganisms and their unique invasion route that bypasses the midgut epithelium, an important immune tissue and physical barrier. Here, we used the malaria vector Anopheles gambiae as a model to investigate the role of melanization, a potent immune effector mechanism of arthropods, in mosquito defense against the entomopathogenic fungus Beauveria bassiana, using in vivo functional genetic analysis and confocal microscopy. The temporal monitoring of fungal growth in mosquitoes injected with B. bassiana conidia showed that melanin eventually formed on all stages, including conidia, germ tubes and hyphae, except the single cell hyphal bodies. Nevertheless, melanin rarely aborted the growth of any of these stages and the mycelium continued growing despite being melanized. Silencing TEP1 and CLIPA8, key positive regulators of Plasmodium and bacterial melanization in A. gambiae, abolished completely melanin formation on hyphae but not on germinating conidia or germ tubes. The detection of a layer of hemocytes surrounding germinating conidia but not hyphae suggested that melanization of early fungal stages is cell-mediated while that of late stages is a humoral response dependent on TEP1 and CLIPA8. Microscopic analysis revealed specific association of TEP1 with surfaces of hyphae and the requirement of both, TEP1 and CLIPA8, for recruiting phenoloxidase to these surfaces. Finally, fungal proliferation was more rapid in TEP1 and CLIPA8 knockdown mosquitoes which exhibited increased sensitivity to natural B. bassiana infections than controls. In sum, the mosquito melanization response retards significantly B. bassiana growth and dissemination, a finding that may be exploited to design transgenic fungi with more potent bio-control activities against mosquitoes.

Melanization is an important immune response and wound healing mechanism in arthropods that leads to melanin formation and deposition on microbial and wound surfaces, respectively. In the Anopheles gambiae mosquito that transmits the malaria parasite Plasmodium, melanization is dispensable for parasite killing. Further, we have shown that in Anopheles gambiae this immune process does not seem to play a role in defense against bacterial infections, which questions the role of melanization in mosquito immunity and the microbial pressure that drove its evolutionary path. Here, we infected mosquitoes with the entomopathogenic fungus Beauveria bassiana to study the role of melanization in anti-fungal defense. We show that mosquito blood cells, as well as specific immune proteins present in the mosquito blood participate in the melanization response to Beauveria bassiana. Our results also reveal that melanization does not abort the growth of the fungus but rather retards significantly its proliferation. The hyphal body stages, which are freely circulating single cells that can disseminate the infection appear earlier in mosquitoes exhibiting a compromised melanization response than in control mosquitoes. These findings provide novel insights into Beauveria bassiana-Anopheles gambiae interactions, which may be exploited to design transgenic fungi with enhanced bio-control potential against mosquitoes.

Biochemical characterization of Anopheles gambiae SRPN6, a malaria parasite invasion marker in mosquitoes

Serine proteinase inhibitors of the serpin family are well known as negative regulators of hemostasis, thrombolysis and innate immune responses. Additionally, non-inhibitory serpins serve functions as chaperones, hormone transporters, or anti-angiogenic factors. In the African malaria mosquito, Anopheles gambiae s.s., at least three serpins (SRPNs) are implicated in the innate immune response against malaria parasites. Based on reverse genetic and cell biological analyses, AgSRPN6 limits parasite numbers and transmission and has been postulated to control melanization and complement function in mosquitoes. This study aimed to characterize AgSRPN6 biophysically and determine its biochemical mode of action. The structure model of AgSRPN6, as predicted by I-Tasser showed the protein in the native serpin fold, with three central β-sheets, nine surrounding α-helices, and a protruding reactive center loop. This structure is in agreement with biophysical and functional data obtained from recombinant (r) AgSRPN6, produced in Escherichia coli. The physical properties of purified rAgSRPN6 were investigated by means of analytical ultracentrifugation, circular dichroism, and differential scanning calorimetry tools. The recombinant protein exists predominantly as a monomer in solution, is composed of a mixture of α-helices and β-sheets, and has a mid-point unfolding temperature of 56°C. Recombinant AgSRPN6 strongly inhibited porcine pancreatic kallikrein and to a lesser extent bovine pancreatic trypsin in vitro. Furthermore, rAgSRPN6 formed inhibitory, SDS-stable, higher molecular weight complexes with prophenoloxidase-activating proteinase (PAP)1, PAP3, and Hemolymph protein (HP)6, which are required for melanization in the lepidopteran model organism, Manduca sexta. Taken together, our results strongly suggest that AgSRPN6 takes on a native serpin fold and is an inhibitor of trypsin-like serine proteinases.

Regulation of anti-Plasmodium immunity by a LITAF-like transcription factor in the malaria vector Anopheles gambiae

The mosquito is the obligate vector for malaria transmission. To complete its development within the mosquito, the malaria parasite Plasmodium must overcome the protective action of the mosquito innate immune system. Here we report on the involvement of the Anopheles gambiae orthologue of a conserved component of the vertebrate immune system, LPS-induced TNFα transcription factor (LITAF), and its role in mosquito anti-Plasmodium immunity. An. gambiae LITAF-like 3 (LL3) expression is up-regulated in response to midgut invasion by both rodent and human malaria parasites. Silencing of LL3 expression greatly increases parasite survival, indicating that LL3 is part of an anti-Plasmodium defense mechanism. Electrophoretic mobility shift assays identified specific LL3 DNA-binding motifs within the promoter of SRPN6, a gene that also mediates mosquito defense against Plasmodium. Further experiments indicated that these motifs play a direct role in LL3 regulation of SRPN6 expression. We conclude that LL3 is a transcription factor capable of modulating SRPN6 expression as part of the mosquito anti-Plasmodium immune response.

The mosquito innate immune system serves as the primary defense response against invading pathogens, including that of the malaria parasite Plasmodium. The mosquito immune response is remarkably efficient in eliminating the parasite as indicated by the low prevalence of Plasmodium oocysts in wild caught mosquitoes. In an effort to understand the mechanisms of immune response, we report the first evidence of a LPS-induced TNF-α factor (LITAF)-like gene family in insects and describe the role of one member, LITAF-like 3 (LL3), in anti-Plasmodium immunity in the mosquito Anopheles gambiae. Silencing of LL3 greatly increases parasite survival. The gene appears to function as a transcription factor that binds to specific regions of the SRPN6 promoter, a known anti-Plasmodium gene, and modulates its transcript abundance. In summary, LL3 appears to be a novel component of the mosquito innate immune response.

CpG-containing oligodeoxynucleotides increases resistance of Anopheles mosquitoes to Plasmodium infection

Unmethylated CpG dinucleotide motifs in bacterial DNA or in synthetic oligodeoxynucleotides (ODN) are potent stimulators of the vertebrate innate immune system. However, the potential of these DNA species to modulate mosquito immunity have not been explored. In the present study, we investigated the effects of CpG-ODN on the outcome of Plasmodium infection in insects and on the modulation of mosquito immunity to Plasmodium. Anopheles stephensi and Anopheles gambiae mosquitoes inoculated with CpG-ODN showed significant reductions in the prevalence of Plasmodium infection, intensity of Plasmodium infection, and number of eggs produced. Microarrays were used to elucidate the transcriptional profiles of the fat bodies of CpG-ODN-treated mosquitoes. In total, 172 genes were differentially expressed, of which 136 were up-regulated and 36 were down-regulated. The major functional class of CpG-ODN-regulated genes encoded immune response-related proteins (31%). Within this group, genes associated with coagulation/wound healing were the most frequently represented (23%). Knockdown of a transglutaminase gene that was up-regulated by the CpG-ODN and chemical inhibition of the enzyme resulted in a significant increase in Plasmodium infection. Mosquitoes that were treated with CpG-ODNs were found to be less susceptible to Plasmodium infection. Transcriptional profiling of the fat body suggests that protection is associated with coagulation/wound healing. We show for the first time that transglutaminase activity plays a role in the control of Plasmodium infection.

Increased survivorship following bacterial infection by the mosquito Aedes aegypti as compared to Anopheles gambiae correlates with increased transcriptional induction of antimicrobial peptides

Mosquitoes defend themselves from pathogens by mounting cellular and humoral innate immune responses. Bioinformatic analyses have revealed considerable divergence in immune gene repertoires between mosquito species, but interspecies empirical comparisons of immune responses are lacking. Here, we present a comparative analysis of the antimicrobial responses of two distantly related disease vectors: Aedes aegypti and Anopheles gambiae. Survival studies showed that Ae. aegypti are more proficient in surviving a bacterial infection than An. gambiae, and this correlates with Ae. aegypti's superior ability to kill bacteria in their hemocoels. Hemocytes from both species swiftly phagocytose bacteria, but phagocytosis does not explain Ae. aegypti's increased robustness: An. gambiae contain more circulating hemocytes and display a higher phagocytic index, but the phagocytic capacity of individual hemocytes is greater in Ae. aegypti. Then, profiling of 19 immunity genes revealed that transcriptional induction following infection is significantly elevated in Ae. aegypti when compared to An. gambiae, with the largest change seen in the transcription of cecropin and defensin. These data show that Ae. aegypti is better equipped to survive a bacterial infection than An. gambiae, and this correlates with Ae. aegypti's increased transcriptional induction of antimicrobial peptides and other humoral immune factors in response to infection.

Complex effects of temperature on mosquito immune function

Over the last 20 years, ecological immunology has provided much insight into how environmental factors shape host immunity and host–parasite interactions. Currently, the application of this thinking to the study of mosquito immunology has been limited. Mechanistic investigations are nearly always conducted under one set of conditions, yet vectors and parasites associate in a variable world. We highlight how environmental temperature shapes cellular and humoral immune responses (melanization, phagocytosis and transcription of immune genes) in the malaria vector, Anopheles stephensi. Nitric oxide synthase expression peaked at 30°C, cecropin expression showed no main effect of temperature and humoral melanization, and phagocytosis and defensin expression peaked around 18°C. Further, immune responses did not simply scale with temperature, but showed complex interactions between temperature, time and nature of immune challenge. Thus, immune patterns observed under one set of conditions provide little basis for predicting patterns under even marginally different conditions. These quantitative and qualitative effects of temperature have largely been overlooked in vector biology but have significant implications for extrapolating natural/transgenic resistance mechanisms from laboratory to field and for the efficacy of various vector control tools.

Intracellular immune responses of dipteran insects

Vector-borne diseases, transmitted by bloodsucking arthropods, pose worldwide socio-medical and economical problems. Some of the major human infectious diseases, such as malaria, Dengue fever, and yellow fever, are transmitted by mosquitoes. While the majority of pathogens enjoy extracellular life styles in insects, viruses and some endosymbionts are strictly intracellular. Here, we summarize our knowledge on defense reactions against intracellular microorganisms in dipteran insects and discuss the potential of insects as models to study human pathogens.

Crystal structure of native Anopheles gambiae serpin-2, a negative regulator of melanization in mosquitoes

Serpins are the dominant group of protease inhibitors in metazoans that control a wide variety of biological processes including major innate immune reactions. One of these inhibitors, SRPN2, controls melanization in mosquitoes – a powerful, arthropod-specific innate immune response. SRPN2 depletion from the hemolymph of adult female mosquitoes significantly reduces longevity and therefore this serpin is a potential target for novel insecticides. We report here the crystal structure of SRPN2 in its native conformation from the African malaria mosquito, Anopheles gambiae to 1.75 Å resolution. SRPN2 adopts a similar fold as observed for other serpins with a core of three β-sheets surrounded by nine α-helices with an exposed reactive center loop (RCL) that extends from the protein body. Similar to other native serpin structures, several residues within the reactive center loop were disordered and could not be modeled. Intriguingly, the N-terminal hinge of the RCL in SRPN2 was found to be inserted into β-sheet A, suggesting a potential activation mechanism analogous to heparin-mediated activation of Antithrombin III.

Characterization of a regulatory unit that controls melanization and affects longevity of mosquitoes

Melanization is an innate immune response in arthropods that encapsulates and kills invading pathogens. One of its rate-limiting steps is the activation of prophenoloxidase (PPO), which is controlled by an extracellular proteinase cascade and serpin inhibitors. The molecular composition of this system is largely unknown in mosquitoes with the exception of serpin-2 (SRPN2), which was previously identified as a key negative regulator of melanization. Using reverse genetic and biochemical techniques, we identified the Anopheles gambiae clip-serine proteinase CLIPB9 as a PPO-activating proteinase, which is inhibited by SRPN2. Double knockdown of SRPN2 and CLIPB9 reversed the pleiotrophic phenotype induced by SRPN2 silencing. This study identifies the first inhibitory serpin-serine proteinase pair in mosquitoes and defines a regulatory unit of melanization. Additionally, the interaction of CLIPB9 and SRPN2 affects the life span of adult female mosquitoes and therefore constitutes a well-defined potential molecular target for novel late-life acting insecticides.

Mosquito immunity

Throughout their lifetime, mosquitoes are exposed to pathogens during feeding, through breaks in their cuticle and following pathogen-driven cuticular degradation. To resist infection, mosquitoes mount innate cellular and humoral immune responses that are elicited within minutes of exposure and can lead to pathogen death via three broadly defined mechanisms: lysis, melanization and hemocyte-mediated phagocytosis. This chapter reviews our current understanding of the mosquito immune system, with an emphasis on the physical barriers that prevent pathogens from entering the body, the organs and tissues that regulate immune responses and the mechanistic and molecular bases of immunity.

Evolutionary genomics of Glossina morsitans immune-related CLIP domain serine proteases and serine protease inhibitors

Several species of haematophagous tsetse flies (genus Glossina) are vectors for trypanosomes, the parasitic protozoans that cause Human African Trypanosomiasis (HAT). Although there was a reduced incidence of HAT in the mid 1960s, decreased disease surveillance has led to a resurgence of HAT in sub-Saharan Africa. Despite being efficient vectors for HAT transmission, the prevalence of G. morsitans infection by trypanosomes in the wild is surprisingly minimal. The precise mechanisms by which G. morsitans remain refractory to trypanosome infection are largely unknown although it has been demonstrated that G. morsitans mounts a strong immune response to invading pathogens. This study identifies G. morsitans immune-related CLIP domain serine proteases and their inhibitors, serine protease inhibitors (serpin) genes. It further establishes their evolutionary relationships with counterparts in Drosophila melanogaster, Anopheles gambiae, Bombyx mori, Manduca sexta and Culex quinquefasciatus. Multiple sequence alignments show conservation of most secondary structure elements for both CLIPs and serpins. Amino acid composition of the serpin reactive site loop (RSL) indicates that the G. morsitans serpins act through an inhibitory mechanism to the target serine protease. Similar to D. melanogaster and unlike A. gambiae, the transcriptome data suggest that G. morsitans does not contain gene expansions in their CLIP-domain serine protease and serpin families. The presence of alternatively spliced variants in the G. morsitans serpins transcriptome data mirrors that of the D. melanogaster transcriptome.

Polymorphisms in Anopheles gambiae immune genes associated with natural resistance to Plasmodium falciparum

Many genes involved in the immune response of Anopheles gambiae, the main malaria vector in Africa, have been identified, but whether naturally occurring polymorphisms in these genes underlie variation in resistance to the human malaria parasite, Plasmodium falciparum, is currently unknown. Here we carried out a candidate gene association study to identify single nucleotide polymorphisms (SNPs) associated with natural resistance to P. falciparum. A. gambiae M form mosquitoes from Cameroon were experimentally challenged with three local wild P. falciparum isolates. Statistical associations were assessed between 157 SNPs selected from a set of 67 A. gambiae immune-related genes and the level of infection. Isolate-specific associations were accounted for by including the effect of the isolate in the analysis. Five SNPs were significantly associated to the infection phenotype, located within or upstream of AgMDL1, CEC1, Sp PPO activate, Sp SNAKElike, and TOLL6. Low overall and local linkage disequilibrium indicated high specificity in the loci found. Association between infection phenotype and two SNPs was isolate-specific, providing the first evidence of vector genotype by parasite isolate interactions at the molecular level. Four SNPs were associated to either oocyst presence or load, indicating that the genetic basis of infection prevalence and intensity may differ. The validity of the approach was verified by confirming the functional role of Sp SNAKElike in gene silencing assays. These results strongly support the role of genetic variation within or near these five A. gambiae immune genes, in concert with other genes, in natural resistance to P. falciparum. They emphasize the need to distinguish between infection prevalence and intensity and to account for the genetic specificity of vector-parasite interactions in dissecting the genetic basis of Anopheles resistance to human malaria.

Anopheles gambiae is the main malaria vector in Africa, transmitting the parasite when it blood feeds on human hosts. The parasite undergoes several developmental stages in the mosquito to complete its life cycle, during which time it is confronted by the mosquito's immune system. The resistance of mosquitoes to malaria infection is highly variable in wild populations and is known to be under strong genetic control, but to date the specific genes responsible for this variation remain to be identified. The present study uncovers variations in A. gambiae immune genes that are associated with natural resistance to Plasmodium falciparum, the deadliest human malaria parasite. The association of some mosquito genetic loci with the level of infection depended on the P. falciparum isolate, suggesting that resistance is determined by interactions between the genome of the mosquito and that of the parasite. This finding highlights the need to account for the natural genetic diversity of malaria parasites in future research on vector-parasite interactions. The loci uncovered in this study are potential targets for developing novel malaria control strategies based on natural mosquito resistance mechanisms.

Manduca sexta serpin-5 regulates prophenoloxidase activation and the Toll signaling pathway by inhibiting hemolymph proteinase HP6

Insect immune responses include prophenoloxidase (proPO) activation and Toll pathway initiation, which are mediated by serine proteinase cascades and regulated by serpins. Manduca sexta hemolymph proteinase-6 (HP6) is a component of both pathways. It cleaves and activates proPO activating proteinase 1 (PAP1) and hemolymph proteinase-8 (HP8), which activates proSpätzle. Inhibitors of HP6 could have the capability of regulating both of these innate immune proteinase cascade pathways. Covalent complexes of HP6 with serpin-4 and serpin-5 were previously isolated from M. sexta plasma using immunoaffinity chromatography with serpin antibodies. We investigated the inhibition of purified, recombinant HP6 by serpin-4 and serpin-5. Both serpin-4 and serpin-5 formed SDS-stable complexes with HP6 in vitro, and they inhibited the activation of proHP8 and proPAP1. Serpin-5 inhibited HP6 more efficiently than did serpin-4. Injection of serpin-5 into larvae resulted in decreased bacteria-induced antimicrobial activity in hemolymph and reduced the bacteria-induced expression of attacin, cecropin and hemolin genes in fat body. Injection of serpin-4 had a weaker effect on antimicrobial peptide expression. These results indicate that serpin-5 may regulate the activity of HP6 to modulate proPO activation and antimicrobial peptide production during immune responses of M. sexta.

Invasion of mosquito salivary glands by malaria parasites: prerequisites and defense strategies

The interplay between vector and pathogen is essential for vector-borne disease transmission. Dissecting the molecular basis of refractoriness of some vectors may pave the way to novel disease control mechanisms. A pathogen often needs to overcome several physical barriers, such as the peritrophic matrix, midgut epithelium and salivary glands. Additionally, the arthropod vector elicites immune responses that can severely limit transmission success. One important step in the transmission of most vector-borne diseases is the entry of the disease agent into the salivary glands of its arthropod vector. The salivary glands of blood-feeding arthropods produce a complex mixture of molecules that facilitate blood feeding by inhibition of the host haemostasis, inflammation and immune reactions. Pathogen entry into salivary glands is a receptor-mediated process, which requires molecules on the surface of the pathogen and salivary gland. In most cases, the nature of these molecules remains unknown. Recent advances in our understanding of malaria parasite entry into mosquito salivary glands strongly suggests that specific carbohydrate molecules on the salivary gland surface function as docking receptors for malaria parasites.

Differential transcript expression between the microfilariae of the filarial nematodes, Brugia malayi and B. pahangi

Background

Brugia malayi and B. pahangi are two closely related nematodes that cause filariasis in humans and animals. However, B. pahangi microfilariae are able to develop in and be transmitted by the mosquito, Armigeres subalbatus, whereas most B. malayi are rapidly melanized and destroyed within the mosquito hemocoel. A cross-species microarray analysis employing the B. malayi V2 array was carried out to determine the transcriptional differences between B. malayi and B. pahangi microfilariae with similar age distribution.

Results

Following microarray data analysis, a list of preferentially expressed genes in both microfilariae species was generated with a false discovery rate estimate of 5% and a signal intensity ratio of 2 or higher in either species. A total of 308 probes were preferentially expressed in both species with 149 probes, representing 123 genes, in B. pahangi microfilariae and 159 probes, representing 107 genes, in B. malayi microfilariae. In B. pahangi, there were 76 (62%) up-regulated transcripts that coded for known proteins that mapped into the KEGG pathway compared to 61 (57%) transcripts in B. malayi microfilariae. The remaining 47 (38%) transcripts in B. pahangi and 46 (43%) transcripts in B. malayi microfilariae were comprised almost entirely of hypothetical genes of unknown function. Twenty-seven of the transcripts in B. pahangi microfilariae coded for proteins that associate with the secretory pathway compared to thirty-nine in B. malayi microfilariae. The data obtained from real-time PCR analysis of ten genes selected from the microarray list of preferentially expressed genes showed good concordance with the microarray data, indicating that the microarray data were reproducible.

Conclusion

In this study, we identified gene transcripts that were preferentially expressed in the microfilariae of B. pahangi and B. malayi, some of which coded for known immunomodulatory proteins. These comparative transcriptome data will be of interest to researchers keen on understanding the inherent differences, at the molecular level, between B. malayi and B. pahangi microfilariae especially because these microfilariae are capable of surviving in the same vertebrate host but elicit different immune response outcomes in the mosquito, Ar. subalbatus.

Distinct melanization pathways in the mosquito Aedes aegypti

Serine protease cascades are involved in blood coagulation and immunity. In arthropods, they regulate melanization, which plays an important role in immune defense and wound healing. However, the mechanisms underlying melanization pathways are not completely characterized. We found that in the mosquito Aedes aegypti, there are two distinct melanization activation pathways carried out by different modules of serine proteases and their specific inhibitors serpins. Immune melanization proteases (IMP-1 and IMP-2) and Serpin-1 mediate hemolymph prophenoloxidase cleavage and immune response against the malaria parasite. Tissue melanization, exemplified by the formation of melanotic tumors, is controlled by tissue melanization protease (CLIPB8), IMP-1, and Serpin-2. In addition, serine proteases CLIPB5 and CLIPB29 are involved in activation of Toll pathway by fungal infection or by infection-independent manner, respectively. Serpin-2 is implicated in the latter activation of Toll pathway. This study revealed the complexity underlying melanization and Toll pathway in mosquitoes.

Anopheles aquasalis Infected by Plasmodium vivax displays unique gene expression profiles when compared to other malaria vectors and plasmodia

Malaria affects 300 million people worldwide every year and is endemic in 22 countries in the Americas where transmission occurs mainly in the Amazon Region. Most malaria cases in the Americas are caused by Plasmodium vivax, a parasite that is almost impossible to cultivate in vitro, and Anopheles aquasalis is an important malaria vector. Understanding the interactions between this vector and its parasite will provide important information for development of disease control strategies. To this end, we performed mRNA subtraction experiments using A. aquasalis 2 and 24 hours after feeding on blood and blood from malaria patients infected with P. vivax to identify changes in the mosquito vector gene induction that could be important during the initial steps of infection. A total of 2,138 clones of differentially expressed genes were sequenced and 496 high quality unique sequences were obtained. Annotation revealed 36% of sequences unrelated to genes in any database, suggesting that they were specific to A. aquasalis. A high number of sequences (59%) with no matches in any databases were found 24 h after infection. Genes related to embryogenesis were down-regulated in insects infected by P. vivax. Only a handful of genes related to immune responses were detected in our subtraction experiment. This apparent weak immune response of A. aquasalis to P. vivax infection could be related to the susceptibility of this vector to this important human malaria parasite. Analysis of some genes by real time PCR corroborated and expanded the subtraction results. Taken together, these data provide important new information about this poorly studied American malaria vector by revealing differences between the responses of A. aquasalis to P. vivax infection, in relation to better studied mosquito-Plasmodium pairs. These differences may be important for the development of malaria transmission-blocking strategies in the Americas.

Discovery of Plasmodium modulators by genome-wide analysis of circulating hemocytes in Anopheles gambiae

Insect hemocytes mediate important cellular immune responses including phagocytosis and encapsulation and also secrete immune factors such as opsonins, melanization factors, and antimicrobial peptides. However, the molecular composition of these important immune cells has not been elucidated in depth, because of their scarcity in the circulating hemolymph, their adhesion to multiple tissues and the lack of primary culture methods to produce sufficient material for a genome-wide analysis. In this study, we report a genome-wide molecular characterization of circulating hemocytes collected from the hemolymph of adult female Anopheles gambiae mosquitoes--the major mosquito vector of human malaria in subSaharan Africa. Their molecular profile identified 1,485 transcripts with enriched expression in these cells, and many of these genes belong to innate immune gene families. This hemocyte-specific transcriptome is compared to those of Drosophila melanogaster and two other mosquitoes, Aedes aegypti and Armigeres subalbatus. We report the identification of two genes as ubiquitous hemocyte markers and several others as hemocyte subpopulation markers. We assess, via an RNAi screen, the roles in development of Plasmodium berghei of 63 genes expressed in hemocytes and provide a molecular comparison of the transcriptome of these cells during malaria infection.

Anopheles gambiae innate immunity

The successful development of Plasmodium in Anopheles mosquitoes is governed by complex molecular and cellular interactions that we are just beginning to understand. Anopheles immune system has received particular attention as genetic evidence points clearly to its critical role in eliminating the majority of parasites invading the midgut epithelium. Several factors regulating Plasmodium development have been identified and tentatively assigned to the individual steps leading to mosquito immune reactions; non-self-recognition, signal modulation, signal transduction and effector mechanisms. Detailed knowledge of these steps and their underlying molecular mechanisms may offer novel perspectives to abort Plasmodium development in the vector. Here, we summarize our current knowledge of mosquito innate immunity highlighting both, recent advances and areas where additional research is required.

Generation, annotation, and analysis of ESTs from midgut tissue of adult female Anopheles stephensi mosquitoes

Background

Malaria is a tropical disease caused by protozoan parasite, Plasmodium, which is transmitted to humans by various species of female anopheline mosquitoes. Anopheles stephensi is one such major malaria vector in urban parts of the Indian subcontinent. Unlike Anopheles gambiae, an African malaria vector, transcriptome of A. stephensi midgut tissue is less explored. We have therefore carried out generation, annotation, and analysis of expressed sequence tags from sugar-fed and Plasmodium yoelii infected blood-fed (post 24 h) adult female A. stephensi midgut tissue.

Results

We obtained 7061 and 8306 ESTs from the sugar-fed and P. yoelii infected mosquito midgut tissue libraries, respectively. ESTs from the combined dataset formed 1319 contigs and 2627 singlets, totaling to 3946 unique transcripts. Putative functions were assigned to 1615 (40.9%) transcripts using BLASTX against UniProtKB database. Amongst unannotated transcripts, we identified 1513 putative novel transcripts and 818 potential untranslated regions (UTRs). Statistical comparison of annotated and unannotated ESTs from the two libraries identified 119 differentially regulated genes. Out of 3946 unique transcripts, only 1387 transcripts were mapped on the A. gambiae genome. These also included 189 novel transcripts, which were mapped to the unannotated regions of the genome. The EST data is available as ESTDB at .

Conclusion

3946 unique transcripts were successfully identified from the adult female A. stephensi midgut tissue. These data can be used for microarray development for better understanding of vector-parasite relationship and to study differences or similarities with other malaria vectors. Mapping of putative novel transcripts from A. stephensi on the A. gambiae genome proved fruitful in identification and annotation of several genes. Failure of some novel transcripts to map on the A. gambiae genome indicates existence of substantial genomic dissimilarities between these two potent malaria vectors.

Inferring selection in the Anopheles gambiae species complex: an example from immune-related serine protease inhibitors

Background

Mosquitoes of the Anopheles gambiae species complex are the primary vectors of human malaria in sub-Saharan Africa. Many host genes have been shown to affect Plasmodium development in the mosquito, and so are expected to engage in an evolutionary arms race with the pathogen. However, there is little conclusive evidence that any of these mosquito genes evolve rapidly, or show other signatures of adaptive evolution.

Methods

Three serine protease inhibitors have previously been identified as candidate immune system genes mediating mosquito-Plasmodium interaction, and serine protease inhibitors have been identified as hot-spots of adaptive evolution in other taxa. Population-genetic tests for selection, including a recent multi-gene extension of the McDonald-Kreitman test, were applied to 16 serine protease inhibitors and 16 other genes sampled from the An. gambiae species complex in both East and West Africa.

Results

Serine protease inhibitors were found to show a marginally significant trend towards higher levels of amino acid diversity than other genes, and display extensive genetic structuring associated with the 2La chromosomal inversion. However, although serpins are candidate targets for strong parasite-mediated selection, no evidence was found for rapid adaptive evolution in these genes.

Conclusion

It is well known that phylogenetic and population history in the An. gambiae complex can present special problems for the application of standard population-genetic tests for selection, and this may explain the failure of this study to detect selection acting on serine protease inhibitors. The pitfalls of uncritically applying these tests in this species complex are highlighted, and the future prospects for detecting selection acting on the An. gambiae genome are discussed.

The serpin gene family in Anopheles gambiae

Serpins (serine protease inhibitors) regulate some innate immune responses of insects by inhibiting endogenous proteases. In this study, we characterized the serpin (SRPN) gene family in the mosquito Anopheles gambiae, the major malaria vector in Sub-Saharan Africa. We identified 18 A. gambiae SRPN genes, all on chromosomes 2 and 3, through searches of genomic DNA and EST databases. In addition to SRPN10, previously documented to exhibit alternative splicing, we found three splicing isoforms of SRPN4. We completed sequencing of cDNAs for the A. gambiae serpins to obtain complete coding sequence information and to verify or improve gene predictions. The predicted SRPN9 and 15 in the initial genome annotation were determined to be a single gene (SRPN9). Sixteen of the serpins contained putative secretion signal sequences. Multiple sequence alignments showing conserved residues important in structural conformation, including the consensus pattern within the hinge region, indicated that most of the A. gambiae serpins may be inhibitory. Phylogenetic analyses confirmed that SRPN1, 2, 3, 8, 9 and 10 formed phylogenetic clusters with known inhibitory serpins from Drosophila melanogaster and Manduca sexta. Many of the A. gambiae serpins were expressed during all life stages. However, SRPN7, 8, 12, and 19 were expressed at very low levels in the adult stage. SRPN13 was expressed mostly in eggs and young larvae, whereas SRPN5 and 14 were expressed mostly in adults. Such differences in expression pattern suggest that the serpins are involved in multiple physiological processes. Determining the biological functions of the mosquito serpins will require future work to identify the proteases they inhibit in vivo.

Purification and characterization of a small cationic protein from the tobacco hornworm Manduca sexta

The prophenoloxidase (proPO) activation system is an important defense mechanism in arthropods, and activation of proPO to active phenoloxidase (PO) involves a serine proteinase cascade. Here, we report the purification and characterization of a small cationic protein CP8 from the tobacco hornworm, Manduca sexta, which can stimulate proPO activation. BLAST search showed that Manduca CP8 is similar to a fungal proteinase inhibitor-1 (AmFPI-1), an inducible serine proteinase inhibitor-1 (ISPI-1), and other small cationic proteins with unknown functions. However, we showed that Manduca CP8 did not inhibit proteinase activity, but stimulated proPO activation in plasma. When small amount (0.1 microg) of purified native CP8 or BSA was added to cell-free plasma samples and incubated for 20 min, low PO activity was observed in both groups. But significantly higher PO activity was observed in the CP8-group than in the BSA-group when more proteins (0.5 microg) were added and incubated for 20 min. However, when the plasma samples were incubated with proteins for 30 min, high PO activity was observed in both the CP8 and BSA groups regardless of the amount of proteins added. Moreover, when PO in the plasma was pre-activated with Micrococcus luteus, addition of CP8 did not have an effect on PO activity, and CP8/bacteria mixture did not stimulate PO activity to a higher level than did BSA/bacteria. These results suggest that CP8 helps activate proPO more rapidly at the initial stage. CP8 mRNA was specifically expressed in fat body and its mRNA level decreased when larvae were injected with saline or bacteria. However, CP8 protein concentration in hemolymph did not change significantly in larvae injected with saline or microorganisms.

Challenges and approaches for mosquito targeted malaria control

Malaria is one of today's most serious diseases with an enormous socioeconomic impact. While anti-malarial drugs have existed for some time and vaccines development may be underway, the most successful malaria eradication programs have thus far relied on attacking the mosquito vector that spreads the disease causing agent Plasmodium. Here we will review past, current and future perspectives of malaria vector control strategies and how these approaches have taken a promising turn thanks recent advances in functional genomics and molecular biology.

A comparative analysis of serpin genes in the silkworm genome

Serine protease inhibitors (serpins) are a superfamily of proteins, most of which control protease-mediated processes by inhibiting their cognate enzymes. Sequencing of the silkworm genome provides an opportunity to investigate serpin structure, function, and evolution at the genome level. There are thirty-four serpin genes in Bombyx mori. Six are highly similar to their Manduca sexta orthologs that regulate innate immunity. Three alternative exons in serpin1 gene and four in serpin28 encode a variable region including the reactive site loop. Splicing of serpin2 pre-mRNA yields variations in serpin2A, 2A' and 2B. Sequence similarity and intron positions reveal the evolutionary pathway of seven serpin genes in group C. RT-PCR indicates an increase in the mRNA levels of serpin1, 3, 5, 6, 9, 12, 13, 25, 27, 32 and 34 in fat body and hemocytes of larvae injected with bacteria. These results suggest that the silkworm serpins play regulatory roles in defense responses.

The serpin saga; development of a new class of virus derived anti-inflammatory protein immunotherapeutics

Serine proteinase inhibitors, also called serpins, are an ancient grouping of proteins found in primitive organisms from bacteria, protozoa and horseshoe crabs and thus likely present at the time of the dinosaurs, up to all mammals living today. The innate or inflammatory immune system is also an ancient metazoan regulatory system, providing the first line of defense against infection or injury. The innate inflammatory defense response evolved long before acquired, antibody dependent immunity. Viruses have developed highly effective stratagems that undermine and block a wide variety of host inflammatory and immune responses. Some of the most potent of these immune modifying strategies utilize serpins that have also been developed over millions of years, including the hijacking by some viruses for defense against host immune attacks. Serpins represent up to 2-10 percent of circulating plasma proteins, regulating actions as wide ranging as thrombosis, inflammation, blood pressure control and even hormone transport. Targeting serpin-regulated immune or inflammatory pathways makes evolutionary sense for viral defense and many of these virus-derived inhibitory proteins have proven to be highly effective, working at very low concentrations--even down to the femptomolar to picomolar range. We are studying these viral anti-inflammatory proteins as a new class of immunomodulatory therapeutic agents derived from their native viral source. One such viral serpin, Serp-1 is now in clinical trial (conducted by VIRON Therapeutics, Inc.) for acute unstable coronary syndromes (unstable angina and small heart attacks), representing a 'first in class' therapeutic study. Several other viral serpins are also currently under investigation as anti-inflammatory or anti-immune therapeutics. This chapter describes these original studies and the ongoing analysis of viral serpins as a new class of virus-derived immunotherapeutic.

Mosquito RUNX4 in the immune regulation of PPO gene expression and its effect on avian malaria parasite infection

Prophenoloxidases (PPOs) are key enzymes of the melanization reaction, which is a prominent defense mechanism of arthropods. The mosquito Aedes aegypti has ten PPO genes in the genome, four of which (PPO1, PPO3, PPO5, and PPO8) were expressed in response to microbial infection. Cactus depletion resulted in transcriptional activation of these four genes, suggesting this up-regulation to be under the control of the Toll pathway. The silencing of Cactus also led to developmental arrest and death of the avian malaria parasite, Plasmodium gallinaceum. We discovered that RUNT-related transcription factor 4 (RUNX4), the orthologue of Drosophila Lozenge, bound to the RUNT binding motif in the promoter of mosquito PPO genes and stimulated the expression of Drosophila PPO-A1 and PPO3 in S2 cell line. The immune effects caused by Cactus depletion were eliminated by double knockdown of Cactus/RUNX4. These findings suggest that RUNX4 regulates PPO gene expression under the control of the Toll pathway and plays a critical role in restricting parasite development.

Anopheles gambiae APL1 is a family of variable LRR proteins required for Rel1-mediated protection from the malaria parasite, Plasmodium berghei

Background

We previously identified by genetic mapping an Anopheles gambiae chromosome region with strong influence over the outcome of malaria parasite infection in nature. Candidate gene studies in the genetic interval, including functional tests using the rodent malaria parasite Plasmodium berghei, identified a novel leucine-rich repeat gene, APL1, with functional activity against P. berghei.

Principal Findings

Manual reannotation now reveals APL1 to be a family of at least 3 independently transcribed genes, APL1A, APL1B, and APL1C. Functional dissection indicates that among the three known APL1 family members, APL1C alone is responsible for host defense against P. berghei. APL1C functions within the Rel1-Cactus immune signaling pathway, which regulates APL1C transcript and protein abundance. Gene silencing of APL1C completely abolishes Rel1-mediated host protection against P. berghei, and thus the presence of APL1C is required for this protection. Further highlighting the influence of this chromosome region, allelic haplotypes at the APL1 locus are genetically associated with and have high explanatory power for the success or failure of P. berghei parasite infection.

Conclusions

APL1C functions as a required transducer of Rel1-dependent immune signal(s) to efficiently protect mosquitoes from P. berghei infection, and allelic genetic haplotypes of the APL1 locus display distinct levels of susceptibility and resistance to P. berghei.

Conserved mosquito/parasite interactions affect development of Plasmodium falciparum in Africa

In much of sub-Saharan Africa, the mosquito Anopheles gambiae is the main vector of the major human malaria parasite, Plasmodium falciparum. Convenient laboratory studies have identified mosquito genes that affect positively or negatively the developmental cycle of the model rodent parasite, P. berghei. Here, we use transcription profiling and reverse genetics to explore whether five disparate mosquito gene regulators of P. berghei development are also pertinent to A. gambiae/P. falciparum interactions in semi-natural conditions, using field isolates of this parasite and geographically related mosquitoes. We detected broadly similar albeit not identical transcriptional responses of these genes to the two parasite species. Gene silencing established that two genes affect similarly both parasites: infections are hindered by the intracellular local activator of actin cytoskeleton dynamics, WASP, but promoted by the hemolymph lipid transporter, ApoII/I. Since P. berghei is not a natural parasite of A. gambiae, these data suggest that the effects of these genes have not been drastically altered by constant interaction and co-evolution of A. gambiae and P. falciparum; this conclusion allowed us to investigate further the mode of action of these two genes in the laboratory model system using a suite of genetic tools and infection assays. We showed that both genes act at the level of midgut invasion during the parasite's developmental transition from ookinete to oocyst. ApoII/I also affects the early stages of oocyst development. These are the first mosquito genes whose significant effects on P. falciparum field isolates have been established by direct experimentation. Importantly, they validate for semi-field human malaria transmission the concept of parasite antagonists and agonists.

Malaria is a parasitic infectious disease transmitted by mosquitoes. It impacts half the population of the world and kills 1 to 3 million people every year, the vast majority of whom are children aged below 5 in sub-Saharan Africa. There, the deadliest parasite is Plasmodium falciparum and its most important vector is the mosquito Anopheles gambiae. This study identifies for the first time specific A. gambiae genes that demonstrably regulate the density of mosquito infection by P. falciparum parasites circulating in malaria patients in Africa. These genes function in mosquito lipid transport and intracellular actin cytoskeleton dynamics, and act as an agonist and an antagonist, respectively, of the parasite ookinete-to-oocyst developmental transition. Importantly, our study validates for P. falciparum the concept of mosquito genes that support or hinder parasite development, a concept that we defined previously using a laboratory model system. Thus, the work constitutes a major contribution to understanding meaningful mosquito/parasite interactions in natural transmission conditions.

A cysteine protease is critical for Babesia spp. transmission in Haemaphysalis ticks

Vector ticks possess a unique system that enables them to digest large amounts of host blood and to transmit various animal and human pathogens, suggesting the existence of evolutionally acquired proteolytic mechanisms. We report here the molecular and reverse genetic characterization of a multifunctional cysteine protease, longipain, from the babesial parasite vector tick Haemaphysalis longicornis. Longipain shares structural similarity with papain-family cysteine proteases obtained from invertebrates and vertebrates. Endogenous longipain was mainly expressed in the midgut epithelium and was specifically localized at lysosomal vacuoles and possibly released into the lumen. Its expression was up-regulated by host blood feeding. Enzymatic functional assays using in vitro and in vivo substrates revealed that longipain hydrolysis occurs over a broad range of pH and temperature. Haemoparasiticidal assays showed that longipain dose-dependently killed tick-borne Babesia parasites, and its babesiacidal effect occurred via specific adherence to the parasite membranes. Disruption of endogenous longipain by RNA interference revealed that longipain is involved in the digestion of the host blood meal. In addition, the knockdown ticks contained an increased number of parasites, suggesting that longipain exerts a killing effect against the midgut-stage Babesia parasites in ticks. Our results suggest that longipain is essential for tick survival, and may have a role in controlling the transmission of tick-transmittable Babesia parasites.

Ticks are important ectoparasites among the blood-feeding arthropods and serve as vectors of many deadly diseases of humans and animals. Of tick-transmitted pathogens, Babesia, an intracellular haemoprotozoan parasite causing a malaria-like disease, called babesiosis, gain increasing interest due to its zoonotic significance. When vector ticks acquire the protozoa via blood-meals, they invade midgut and undergo several developmental stages prior to exit through salivary glands. It has long been conceived that midguts of these ticks evolve diverse innate immune mechanisms and perform blood digestion critical for tick survival. A cysteine proteinase, longipain, was identified from the three-host tick Haemaphysalis longicornis, which shows potent parasiticidal activity. Longipain is localized in midgut epithelium and its expression is induced by blood feeding. This protein is passively secreted into midgut lumen where it exerts enzymatic degradation of blood-meals. A series of experiments unveil that longipain-knockdown ticks when fed on Babesia-infected dog, exhibited a significantly increased numbers of parasites compared with controls. Longipain has shown to interact on the surface of Babesia parasites in vitro and in vivo, and is thought to mediate direct killing of the parasites, suggesting that longipain may be a potential chemotherapeutic target against babesiosis and ticks themselves.

The proPO-system: pros and cons for its role in invertebrate immunity

Melanisation is an important immune response in many invertebrates. Recent evidence also strongly implies that the melanisation (prophenoloxidase activating) cascade is intimately associated with the appearance of factors stimulating cellular defence by aiding phagocytosis and encapsulation reactions. However, some controversy exists in the field, and at least in flies and mosquitoes, the successful combat of some pathogens does not seem to be dependent on phenoloxidase activity. This may be because of redundancy among separate immune mechanisms, inappropriate testing, species differences or a combination thereof. Recently, by using RNA interference against phenoloxidase or in specific host-pathogen interactions where the pathogen prevents melanin production by the host, convincing data have confirmed the importance of this cascade in invertebrate innate immunity.

The molecular evolution of four anti-malarial immune genes in the Anopheles gambiae species complex

Background

If the insect innate immune system is to be used as a potential blocking step in transmission of malaria, then it will require targeting one or a few genes with highest relevance and ease of manipulation. The problem is to identify and manipulate those of most importance to malaria infection without the risk of decreasing the mosquito's ability to stave off infections by microbes in general. Molecular evolution methodologies and concepts can help identify such genes. Within the setting of a comparative molecular population genetic and phylogenetic framework, involving six species of the Anopheles gambiae complex, we investigated whether a set of four pre-selected immunity genes (gambicin, NOS, Rel2 and FBN9) might have evolved under selection pressure imposed by the malaria parasite.

Results

We document varying levels of polymorphism within and divergence between the species, in all four genes. Introgression and the sharing of ancestral polymorphisms, two processes that have been documented in the past, were verified in this study in all four studied genes. These processes appear to affect each gene in different ways and to different degrees. However, there is no evidence of positive selection acting on these genes.

Conclusion

Considering the results presented here in concert with previous studies, genes that interact directly with the Plasmodium parasite, and play little or no role in defense against other microbes, are probably the most likely candidates for a specific adaptive response against P. falciparum. Furthermore, since it is hard to establish direct evidence linking the adaptation of any candidate gene to P. falciparum infection, a comparative framework allowing at least an indirect link should be provided. Such a framework could be achieved, if a similar approach like the one involved here, was applied to all other anopheline complexes that transmit P. falciparum malaria.

The parasite invasion marker SRPN6 reduces sporozoite numbers in salivary glands of Anopheles gambiae

For malaria transmission to occur, Plasmodium sporozoites must infect the salivary glands of their mosquito vectors. This study reports that Anopheles gambiae SRPN6 participates in a local salivary gland epithelial response against the rodent malaria parasite, Plasmodium berghei. We showed previously that SRPN6, an immune inducible midgut invasion marker, influences ookinete development. Here we report that SRPN6 is also specifically induced in salivary glands with the onset of sporozoite invasion. The protein is located in the basal region of epithelial cells in proximity to invading sporozoites. Knockdown of SRPN6 during the late phase of sporogony by RNAi has no effect on oocyst rupture but significantly increases the number of sporozoites present in salivary glands. Despite several differences between the passage of Plasmodium through the midgut and the salivary glands, this study identifies a striking overlap in the molecular responses of these two epithelia to parasite invasion.

Phenoloxidase Is an Important Component of the Defense against Aeromonas hydrophila Infection in a Crustacean, Pacifastacus leniusculus

The melanization cascade, in which phenoloxidase is the terminal enzyme, appears to play a key role in recognition of and defense against microbial infections in invertebrates. Here, we show that phenoloxidase activity and melanization are important for the immune defense toward a highly pathogenic bacterium, Aeromonas hydrophila, in the freshwater crayfish, Pacifastacus leniusculus. RNA interference-mediated depletion of crayfish prophenoloxidase leads to increased bacterial growth, lower phagocytosis, lower phenoloxidase activity, lower nodule formation, and higher mortality when infected with this bacterium. In contrast, if RNA interference of pacifastin, an inhibitor of the crayfish prophenoloxidase activation cascade, is performed, it results in lower bacterial growth, increased phagocytosis, increased nodule formation, higher phenoloxidase activity, and delayed mortality. Our data therefore suggest that phenoloxidase is required in crayfish defense against an infection by A. hydrophila, a highly virulent and pathogenic bacterium to crayfish.

Population genetics of Plasmodium resistance genes in Anopheles gambiae: no evidence for strong selection

Anopheles mosquitoes are the primary vectors for malaria in Africa, transmitting the disease to more than 100 million people annually. Recent functional studies have revealed mosquito genes that are crucial for Plasmodium development, but there is presently little understanding of which genes mediate vector competence in the wild, or evolve in response to parasite-mediated selection. Here, we use population genetic approaches to study the strength and mode of natural selection on a suite of mosquito immune system genes, CTL4, CTLMA2, LRIM1, and APL2 (LRRD7), which have been shown to affect Plasmodium development in functional studies. We sampled these genes from two African populations of An. gambiae s.s., along with several closely related species, and conclude that there is no evidence for either strong directional or balancing selection on these genes. We highlight a number of challenges that need to be met in order to apply population genetic tests for selection in Anopheles mosquitoes; in particular the dearth of suitable outgroup species and the potential difficulties that arise when working within a closely-related species complex.