AP-1/Fos-TGase2 axis mediates wounding-induced Plasmodium falciparum killing in Anopheles gambiae

Reduced Genetic Diversity of Key Fertility and Vector Competency Related Genes in Anopheles gambiae s.l. Across Sub-Saharan Africa

BACKGROUND

Insecticide resistance challenges the vector control efforts towards malaria elimination and proving the development of complementary tools. Targeting the genes that are involved in mosquito fertility and susceptibility to Plasmodium with small molecule inhibitors has been a promising alternative to curb the vector population and drive the transmission down. However, such an approach would require a comprehensive knowledge of the genetic diversity of the targeted genes to ensure the broad efficacy of new tools across the natural vector populations.

METHODS

Four fertility and parasite susceptibility genes were identified from a systematic review of the literature. The Single Nucleotide Polymorphisms (SNPs) found within the regions spanned by these four genes, genotyped across 2784 wild-caught Anopheles gambiae s.l. from 19 sub-Saharan African (SSA) countries, were extracted from the whole genome SNP data of the Ag1000G project (Ag3.0). The population genetic analysis on gene-specific data included the determination of the population structure, estimation of the differentiation level between the populations, evaluation of the linkage between the non-synonymous SNPs (nsSNPs), and a few statistical tests.

RESULTS

As potential targets for small molecule inhibitors to reduce malaria transmission, our set of four genes associated with Anopheles fertility and their susceptibility to Plasmodium comprises the mating-induced stimulator of oogenesis protein (MISO, AGAP002620), Vitellogenin (Vg, AGAP004203), Lipophorin (Lp, AGAP001826), and Haem-peroxidase 15 (HPX15, AGAP013327). The analyses performed on these potential targets of small inhibitor molecules revealed that the genes are conserved within SSA populations of An. gambiae s.l. The overall low Fst values and low clustering of principal component analysis between species indicated low genetic differentiation at all the genes (MISO, Vg, Lp and HPX15). The low nucleotide diversity (>0.10), negative Tajima's D values, and heterozygosity analysis provided ecological insights into the purifying selection that acts to remove deleterious mutations, maintaining genetic diversity at low levels within the populations. None of MISO nsSNPs were identified in linkage disequilibrium, whereas a few weakly linked nsSNPs with ambiguous haplotyping were detected at other genes.

CONCLUSIONS

This integrated finding on the genetic features of major malaria vectors' biological factors across natural populations offer new insights for developing sustainable malaria control tools. These loci were reasonably conserved, allowing for the design of effective targeting with small molecule inhibitors towards controlling vector populations and lowering global malaria transmission.

Effects of two transglutaminases on innate immune responses in the oriental armyworm, Mythimna separata

Transglutaminase (TGase) is a key enzyme that mediates hemolymph coagulation and is thought to contribute to the elimination of pathogenic microorganisms in invertebrates. The objective of this study was to elucidate the involvement of TGase in insect immune responses via functional analysis of this enzyme in the oriental armyworm, Mythimna separata, using recombinant proteins and RNA interference technique. We identified two TGase genes, mystgase1 and mystgase2, in Mythimna separata and found that both genes are expressed in all surveyed tissues in M. separata larvae. Significant changes were induced in hemocytes following Escherichia coli injection. Injection of Gram-positive bacteria (Micrococcus luteus) and Gram-negative bacteria (Escherichia coli and Serratia marcescens) into larvae triggered a time-specific induction of both mystgase1 and mystgase2 in hemocytes. Recombinant MysTGase1 and MysTGase2 proteins bound to both E. coli and M. luteus, localizing within bacterial clusters and resulting in agglutination in a Ca2+-dependent manner. The hemocytes of larvae injected with recombinant MysTGase1 or MysTGase2 exhibited enhanced phagocytic ability against E. coli, improved in vivo bacterial clearance, and increased resistance to S. marcescens, decreasing larval mortality rate. Conversely, RNA interference targeting mystgase1 or mystgase2 significantly reduced hemocyte phagocytic capability, decreased bacterial clearance, and increased susceptibility to S. marcescens infection, thereby increasing larval mortality rate. The findings of this study are anticipated to expand our understanding of the function of TGases within insect immune responses and may contribute to developing new pest control strategies.

Activation of the immune deficiency pathway (IMD) reduces the mosquito heart rate via a nitric oxide-based mechanism

The immune deficiency pathway (IMD) is an important component of the antibacterial, antimalarial and antiviral response in mosquitoes. The IMD pathway also drives the infection induced migration of hemocytes to the heart. During an infection, periostial hemocytes kill pathogens in areas of high hemolymph flow and produce nitric oxide that reduces the heart rate. Here, we investigated the consequences of repressing the IMD pathway by silencing the transcription factor, rel2, or activating the pathway by silencing the negative regulator, caspar, in Anopheles gambiae. In uninfected mosquitoes, repression of the IMD pathway does not affect the circulatory system. However, activating the IMD pathway decreases the heart rate, and this correlates with increased transcription and activity of nitric oxide synthase (NOS), but not increased transcription of the lysozymes, LysC1 or LysC2. In infected mosquitoes, however, activation of the IMD pathway does not affect the heart rate but repression of the pathway decreases the heart rate. This latter phenotype correlates with increased transcription and activity of nitric oxide synthase, which is likely due to an increase in infection intensity. In conclusion, we demonstrate that a major immune signaling pathway that regulates periostial hemocyte aggregation, the IMD pathway, reduces the heart rate via a nitric oxide-based mechanism.

Activation of innate immunity selectively compromises mitochondrial complex I, proline oxidation, and flight activity in the major arbovirus vector Aedes aegypti

Aedes aegypti females are natural vectors of important arboviruses such as dengue, zika, and yellow fever. Mosquitoes activate innate immune response signaling pathways upon infection, as a resistance mechanism to fight pathogens and limit their propagation. Despite the beneficial effects of immune activation for insect vectors, phenotypic costs ultimately affect their fitness. However, the underlying mechanisms that mediate these fitness costs remain poorly understood. Given the high energy required to mount a proper immune response, we hypothesized that systemic activation of innate immunity would impair flight muscle mitochondrial function, compromising tissue energy demand and flight activity. Here, we investigated the dynamic effects of activation of innate immunity by intra-thoracic zymosan injection on A. aegypti flight muscle mitochondrial metabolism. Zymosan injection significantly increased defensin A expression in fat bodies in a time-dependent manner that compromised flight activity. Although oxidant levels in flight muscle were hardly altered, ATP-linked respiratory rates driven by mitochondrial pyruvate+proline oxidation were significantly reduced at 24 h upon zymosan injection. Oxidative phosphorylation coupling was preserved regardless of innate immune response activation along 24 h. Importantly, rotenone-sensitive respiration and complex I-III activity were specifically reduced 24 h upon zymosan injection. Also, loss of complex I activity compromised ATP-linked and maximal respiratory rates mediated by mitochondrial proline oxidation. Finally, the magnitude of innate immune response activation negatively correlated with respiratory rates, regardless of the metabolic states. Collectively, we demonstrate that activation of innate immunity is strongly associated with reduced flight muscle complex I activity with direct consequences to mitochondrial proline oxidation and flight activity. Remarkably, our results indicate a trade-off between dispersal and immunity exists in an insect vector, underscoring the potential consequences of disrupted flight muscle mitochondrial energy metabolism to arbovirus transmission.

Genomic advances in the study of the mosquito vector during avian malaria infection

Invertebrate host–parasite associations are one of the keystones in order to understand vector-borne diseases. The study of these specific interactions provides information not only about how the vector is affected by the parasite at the gene-expression level, but might also reveal mosquito strategies for blocking the transmission of the parasites. A very well-known vector for human malaria is Anopheles gambiae. This mosquito species has been the main focus for genomics studies determining essential key genes and pathways over the course of a malaria infection. However, to-date there is an important knowledge gap concerning other non-mammophilic mosquito species, for example some species from the Culex genera which may transmit avian malaria but also zoonotic pathogens such as West Nile virus. From an evolutionary perspective, these 2 mosquito genera diverged 170 million years ago, hence allowing studies in both species determining evolutionary conserved genes essential during malaria infections, which in turn might help to find key genes for blocking malaria cycle inside the mosquito. Here, we extensively review the current knowledge on key genes and pathways expressed in Anopheles over the course of malaria infections and highlight the importance of conducting genomic investigations for detecting pathways in Culex mosquitoes linked to infection of avian malaria. By pooling this information, we underline the need to increase genomic studies in mosquito–parasite associations, such as the one in Culex–Plasmodium, that can provide a better understanding of the infection dynamics in wildlife and reduce the negative impact on ecosystems.

The immune deficiency and c-Jun N-terminal kinase pathways drive the functional integration of the immune and circulatory systems of mosquitoes

The immune and circulatory systems of animals are functionally integrated. In mammals, the spleen and lymph nodes filter and destroy microbes circulating in the blood and lymph, respectively. In insects, immune cells that surround the heart valves (ostia), called periostial haemocytes, destroy pathogens in the areas of the body that experience the swiftest haemolymph (blood) flow. An infection recruits additional periostial haemocytes, amplifying heart-associated immune responses. Although the structural mechanics of periostial haemocyte aggregation have been defined, the genetic factors that regulate this process remain less understood. Here, we conducted RNA sequencing in the African malaria mosquito, Anopheles gambiae, and discovered that an infection upregulates multiple components of the immune deficiency (IMD) and c-Jun N-terminal kinase (JNK) pathways in the heart with periostial haemocytes. This upregulation is greater in the heart with periostial haemocytes than in the circulating haemocytes or the entire abdomen. RNA interference-based knockdown then showed that the IMD and JNK pathways drive periostial haemocyte aggregation and alter phagocytosis and melanization on the heart, thereby demonstrating that these pathways regulate the functional integration between the immune and circulatory systems. Understanding how insects fight infection lays the foundation for novel strategies that could protect beneficial insects and harm detrimental ones.

Silencing Transglutaminase Genes TGase2 and TGase3 Has Infection-Dependent Effects on the Heart Rate of the Mosquito Anopheles gambiae

Transglutaminases are pleiotropic enzymes that in mosquitoes participate in the formation of the mating plug and the wound-induced antimalarial response. Moreover, one transglutaminase, TGase3, negatively regulates the infection-induced aggregation of hemocytes on the heart. Given that TGase3 is an inhibitor of periostial hemocyte aggregation, we used RNAi-based gene silencing followed by intravital video imaging to scrutinize whether any of the three transglutaminases encoded in the genome of the mosquito, Anopheles gambiae, play a role in modulating the heart rate of uninfected and infected mosquitoes. Initially, we confirmed that an infection decreases the heart rate. Then, we uncovered that silencing TGase1 does not impact heart physiology, but silencing TGase2 results in a constant heart rate regardless of infection status, eliminating the infection-induced decrease in the heart rate. Finally, silencing TGase3 decreases the heart rate in uninfected mosquitoes but increases the heart rate in infected mosquitoes. We conclude that TGase2 and TGase3 modulate heart physiology and demonstrate that factors not classically associated with insect circulatory physiology are involved in the functional integration of the immune and circulatory systems of mosquitoes.

Rhodnius prolixus Hemolymph Immuno-Physiology: Deciphering the Systemic Immune Response Triggered by Trypanosoma cruzi Establishment in the Vector Using Quantitative Proteomics

Understanding the development of Trypanosoma cruzi within the triatomine vector at the molecular level should provide novel targets for interrupting parasitic life cycle and affect vectorial competence. The aim of the current study is to provide new insights into triatomines immunology through the characterization of the hemolymph proteome of Rhodnius prolixus, a major Chagas disease vector, in order to gain an overview of its immune physiology. Surprisingly, proteomics investigation of the immunomodulation of T. cruzi-infected blood reveals that the parasite triggers an early systemic response in the hemolymph. The analysis of the expression profiles of hemolymph proteins from 6 h to 24 h allowed the identification of a broad range of immune proteins expressed already in the early hours post-blood-feeding regardless of the presence of the parasite, ready to mount a rapid response exemplified by the significant phenol oxidase activation. Nevertheless, we have also observed a remarkable induction of the immune response triggered by an rpPGRP-LC and the overexpression of defensins 6 h post-T. cruzi infection. Moreover, we have identified novel proteins with immune properties such as the putative c1q-like protein and the immunoglobulin I-set domain-containing protein, which have never been described in triatomines and could play a role in T. cruzi recognition. Twelve proteins with unknown function are modulated by the presence of T. cruzi in the hemolymph. Determining the function of these parasite-induced proteins represents an exciting challenge for increasing our knowledge about the diversity of the immune response from the universal one studied in holometabolous insects. This will provide us with clear answers for misunderstood mechanisms in host-parasite interaction, leading to the development of new generation strategies to control vector populations and pathogen transmission.

Transglutaminase 3 negatively regulates immune responses on the heart of the mosquito, Anopheles gambiae

The immune and circulatory systems of insects are functionally integrated. Following infection, immune cells called hemocytes aggregate around the ostia (valves) of the heart. An earlier RNA sequencing project in the African malaria mosquito, Anopheles gambiae, revealed that the heart-associated hemocytes, called periostial hemocytes, express transglutaminases more highly than hemocytes elsewhere in the body. Here, we further queried the expression of these transglutaminase genes and examined whether they play a role in heart-associated immune responses. We found that, in the whole body, injury upregulates the expression of TGase2, whereas infection upregulates TGase1, TGase2 and TGase3. RNAi-based knockdown of TGase1 and TGase2 did not alter periostial hemocyte aggregation, but knockdown of TGase3 increased the number of periostial hemocytes during the early stages of infection and the sequestration of melanin by periostial hemocytes during the later stages of infection. In uninfected mosquitoes, knockdown of TGase3 also slightly reduced the number of sessile hemocytes outside of the periostial regions. Taken altogether, these data show that TGase3 negatively regulates periostial hemocyte aggregation, and we hypothesize that this occurs by negatively regulating the immune deficiency pathway and by altering hemocyte adhesion. In conclusion, TGase3 is involved in the functional integration between the immune and circulatory systems of mosquitoes.

Ecological plasticity to ions concentration determines genetic response and dominance of Anopheles coluzzii larvae in urban coastal habitats of Central Africa

In Central Africa, the malaria vector Anopheles coluzzii is predominant in urban and coastal habitats. However, little is known about the environmental factors that may be involved in this process. Here, we performed an analysis of 28 physicochemical characteristics of 59 breeding sites across 5 urban and rural sites in coastal areas of Central Africa. We then modelled the relative frequency of An. coluzzii larvae to these physicochemical parameters in order to investigate environmental patterns. Then, we assessed the expression variation of 10 candidate genes in An. coluzzii, previously incriminated with insecticide resistance and osmoregulation in urban settings. Our results confirmed the ecological plasticity of An. coluzzii larvae to breed in a large range of aquatic conditions and its predominance in breeding sites rich in ions. Gene expression patterns were comparable between urban and rural habitats, suggesting a broad response to ions concentrations of whatever origin. Altogether, An. coluzzii exhibits a plastic response to occupy both coastal and urban habitats. This entails important consequences for malaria control in the context of the rapid urban expansion in Africa in the coming years.

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

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

JNK signaling regulates oviposition in the malaria vector Anopheles gambiae

The reproductive fitness of the Anopheles gambiae mosquito represents a promising target to prevent malaria transmission. The ecdysteroid hormone 20-hydroxyecdysone (20E), transferred from male to female during copulation, is key to An. gambiae reproductive success as it licenses females to oviposit eggs developed after blood feeding. Here we show that 20E-triggered oviposition in these mosquitoes is regulated by the stress- and immune-responsive c-Jun N-terminal kinase (JNK). The heads of mated females exhibit a transcriptional signature reminiscent of a JNK-dependent wounding response, while mating—or injection of virgins with exogenous 20E—selectively activates JNK in the same tissue. RNAi-mediated depletion of JNK pathway components inhibits oviposition in mated females, whereas JNK activation by silencing the JNK phosphatase puckered induces egg laying in virgins. Together, these data identify JNK as a potential conduit linking stress responses and reproductive success in the most important vector of malaria.

Metabolic balancing by miR-276 shapes the mosquito reproductive cycle and Plasmodium falciparum development

The blood-feeding behavior of Anopheles females delivers essential nutrients for egg development and drives parasite transmission between humans. Plasmodium growth is adapted to the vector reproductive cycle, but how changes in the reproductive cycle impact parasite development remains unclear. Here, we show that the bloodmeal-induced miR-276-5p fine-tunes the expression of branched-chain amino acid transferase to terminate the reproductive cycle. Silencing of miR-276 prolongs high rates of amino acid (AA) catabolism and increases female fertility, suggesting that timely termination of AA catabolism restricts mosquito investment into reproduction. Prolongation of AA catabolism in P. falciparum-infected females also compromises the development of the transmissible sporozoite forms. Our results suggest that Plasmodium sporogony exploits the surplus mosquito resources available after reproductive investment and demonstrate the crucial role of the mosquito AA metabolism in within-vector parasite proliferation and malaria transmission.

Plasmodium growth is adapted to the reproductive cycle of mosquitoes, but underlying mechanisms are unclear. Here, Lampe et al. show that the blood-meal induced miR-276 balances the termination of the mosquito amino acid catabolism and egg development, providing nutrients for Plasmodium sporozoite development.

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

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

Microbial Pre-exposure and Vectorial Competence of Anopheles Mosquitoes

Anopheles female mosquitoes can transmit Plasmodium, the malaria parasite. During their aquatic life, wild Anopheles mosquito larvae are exposed to a huge diversity of microbes present in their breeding sites. Later, adult females often take successive blood meals that might also carry different micro-organisms, including parasites, bacteria, and viruses. Therefore, prior to Plasmodium ingestion, the mosquito biology could be modulated at different life stages by a suite of microbes present in larval breeding sites, as well as in the adult environment. In this article, we highlight several naturally relevant scenarios of Anopheles microbial pre-exposure that we assume might impact mosquito vectorial competence for the malaria parasite: (i) larval microbial exposures; (ii) protist co-infections; (iii) virus co-infections; and (iv) pathogenic bacteria co-infections. In addition, significant behavioral changes in African Anopheles vectors have been associated with increasing insecticide resistance. We discuss how these ethological modifications may also increase the repertoire of microbes to which mosquitoes could be exposed, and that might also influence their vectorial competence. Studying Plasmodium–Anopheles interactions in natural microbial environments would efficiently contribute to refining the transmission risks.

Purification and biochemical characterization of a novel transglutaminase from Mythimna separata larvae (Noctuidae, Lepidoptera)

A novel transglutaminase (MsTGase) from Mythimna separata larvae was separated and purified; its biochemical property and enzymatic catalytic activities were investigated. MsTGase was obtained chromatographically by the precipitation of Sephadex G-100 gel and DEAE-Cellulose-52 ion-exchange column with 48-fold purification and a reproducible yield of approximately 12%. Molecular weight of the MsTGase was 63.5 KDa and its N-terminal amino acid sequence was GKIEEG-LVI. Michaelis constant of the MsTGase for the substrate N-CBZ-Gln-Gly was 12.83mM with a V_max of 7.99U/mL. Optimum conditions for MsTGase activity were at 42°C and pH7.5. The enzyme didn't possess metal ion at its catalytic active site; its activity could be significantly inhibited by Mg²⁺, but activated by Ca²⁺. Chlorpyrifos and spinosad showed a strong potential to increase MsTGase activity, supporting the view that MsTGase was a novel target. Moreover, the formation of intermolecular cross-links of casein and bovine serum albumin polymerized by MsTGase in the presence of DTT was observed. These findings pave the way for future studies on the physiological role of MsTGase and the potential impact of its regulation on MsTGase-associated pest management.

Plasmodium berghei P47 is essential for ookinete protection from the Anopheles gambiae complement-like response

Malaria is a mosquito-borne disease affecting millions of people every year. The rodent parasite Plasmodium berghei has served as a model for human malaria transmission studies and played a pivotal role in dissecting the mosquito immune response against infection. The 6-cysteine protein P47, known to be important for P. berghei female gamete fertility, is shown to serve a different function in Plasmodium falciparum, protecting ookinetes from the mosquito immune response. Here, we investigate the function of P. berghei P47 in Anopheles gambiae mosquito infections. We show that P47 is expressed on the surface of both female gametocytes and ookinetes where it serves distinct functions in promoting gametocyte-to-ookinete development and protecting ookinetes from the mosquito complement-like response, respectively. The latter function is essential, as ookinetes lacking P47 are targeted for killing while traversing the mosquito midgut cells and eliminated upon exposure to hemolymph proteins of the complement-like system. Silencing key factors of the complement-like system restores oocyst development and disease transmission to rodent hosts. Our data establish a dual role of P. berghei P47 in vivo and reinforce the use of this parasite to study the impact of the mosquito immune response on human malaria transmission.

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.

Variation in susceptibility of African Plasmodium falciparum malaria parasites to TEP1 mediated killing in Anopheles gambiae mosquitoes

Anopheles gambiae s.s. mosquitoes are efficient vectors for Plasmodium falciparum, although variation exists in their susceptibility to infection. This variation depends partly on the thioester-containing protein 1 (TEP1) and TEP depletion results in significantly elevated numbers of oocysts in susceptible and resistant mosquitoes. Polymorphism in the Plasmodium gene coding for the surface protein Pfs47 modulates resistance of some parasite laboratory strains to TEP1-mediated killing. Here, we examined resistance of P. falciparum isolates of African origin (NF54, NF165 and NF166) to TEP1-mediated killing in a susceptible Ngousso and a refractory L3–5 strain of A. gambiae. All parasite clones successfully developed in susceptible mosquitoes with limited evidence for an impact of TEP1 on transmission efficiency. In contrast, NF166 and NF165 oocyst densities were strongly reduced in refractory mosquitoes and TEP1 silencing significantly increased oocyst densities. Our results reveal differences between African P. falciparum strains in their capacity to evade TEP1-mediated killing in resistant mosquitoes. There was no significant correlation between Pfs47 genotype and resistance of a given P. falciparum isolate for TEP1 killing. These data suggest that polymorphisms in this locus are not the sole mediators of immune evasion of African malaria parasites.

The dual roles of Armigeres subalbatus prophenoloxidase V in parasite melanization and egg chorion melanization in the mosquito Ar. subalbatus

Phenoloxidases (POs) play key roles in various physiological functions in insects, e.g., cuticular sclerotization, wound healing, egg tanning, cuticle formation and melanotic encapsulaction of pathogens. Previously, we identified five POs, designated As-pro-PO I-V, from the mosquito Armigeres subalbatus and demonstrated that the functions of As-pro-PO I, II and III, were associated with filarial parasite melanization, blood feeding and cuticle formation, respectively. In the present study, we delineate the dual functions of As-pro-PO V. We found that the level of As-pro-PO V mRNA in mosquitoes was significantly increased after microfilaria challenge or blood feeding, and decreased to normal level after oviposition. Knockdown of As-pro-PO V by dsRNA resulted in significant decreases in the degree of microfilaria melanization, egg chronic melanization rates and egg hatching rates in Ar. subalbatus. Further transfection and electrophoretic mobility-shift assays verified the As-pro-PO V gene might regulated by both AP-1, a putative immune-related regulatory element and CdxA, a developmental regulatory element. The binding of AP-1 and CdxA motif with mosquito nuclear extracts was significantly enhanced after microfilaria challenge and blood-feeding in Ar. subalbatus, respectively. These results indicate that As-pro-PO V is a critical enzyme that is required for both an effective melanization immune response and egg chorion melanization in this mosquito.

Hemocyte differentiation mediates the mosquito late-phase immune response against Plasmodium in Anopheles gambiae

Plasmodium parasites must complete development in the mosquito vector for transmission to occur. The mosquito innate immune response is remarkably efficient in limiting parasite numbers. Previous work has identified a LPS-induced TNFα transcription factor (LITAF)-like transcription factor, LITAF-like 3 (LL3), which significantly influences parasite numbers. Here, we demonstrate that LL3 does not influence invasion of the mosquito midgut epithelium or ookinete-to-oocyst differentiation but mediates a late-phase immune response that decreases oocyst survival. LL3 expression in the midgut and hemocytes is activated by ookinete midgut invasion and is independent of the mosquito microbiota, suggesting that LL3 may be a component of a wound-healing response. LL3 silencing abrogates the ability of mosquito hemocytes to differentiate and respond to parasite infection, implicating hemocytes as critical modulators of the late-phase immune response.

Malaria immunity in man and mosquito: insights into unsolved mysteries of a deadly infectious disease

Malaria is a mosquito-borne disease caused by parasites of the obligate intracellular Apicomplexa phylum the most deadly of which, Plasmodium falciparum, prevails in Africa. Malaria imposes a huge health burden on the world's most vulnerable populations, claiming the lives of nearly one million children and pregnant women each year. Although there is keen interest in eradicating malaria, we do not yet have the necessary tools to meet this challenge, including an effective malaria vaccine and adequate vector control strategies. Here we review what is known about the mechanisms at play in immune resistance to malaria in both the human and mosquito hosts at each step in the parasite's complex life cycle with a view toward developing the tools that will contribute to the prevention of disease and death and, ultimately, to the goal of malaria eradication. In so doing, we hope to inspire immunologists to participate in defeating this devastating disease.

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.

Dihydroisoxazole inhibitors of Anopheles gambiae seminal transglutaminase AgTG3

BACKGROUND

Current vector-based malaria control strategies are threatened by the rise of biochemical and behavioural resistance in mosquitoes. Researching mosquito traits of immunity and fertility is required to find potential targets for new vector control strategies. The seminal transglutaminase AgTG3 coagulates male Anopheles gambiae seminal fluids, forming a 'mating plug' that is required for male reproductive success. Inhibitors of AgTG3 can be useful both as chemical probes of A. gambiae reproductive biology and may further the development of new chemosterilants for mosquito population control.

METHODS

A targeted library of 3-bromo-4,5-dihydroxoisoxazole inhibitors were synthesized and screened for inhibition of AgTG3 in a fluorescent, plate-based assay. Positive hits were tested for in vitro activity using cross-linking and mass spectrometry, and in vivo efficacy in laboratory mating assays.

RESULTS

A targeted chemical library was screened for inhibition of AgTG3 in a fluorescent plate-based assay using its native substrate, plugin. Several inhibitors were identified with IC50 < 10 μM. Preliminary structure-activity relationships within the library support the stereo-specificity and preference for aromatic substituents in the chemical scaffold. Both inhibition of plugin cross-linking and covalent modification of the active site cysteine of AgTG3 were verified. Administration of an AgTG3 inhibitor to A. gambiae males by intrathoracic injection led to a 15% reduction in mating plug transfer in laboratory mating assays.

CONCLUSIONS

A targeted screen has identified chemical inhibitors of A. gambiae transglutaminase 3 (AgTG3). The most potent inhibitors are known inhibitors of human transglutaminase 2, suggesting a common binding pose may exist within the active site of both enzymes. Future efforts to develop additional inhibitors will provide chemical tools to address important biological questions regarding the role of the A. gambiae mating plug. A second use for transglutaminase inhibitors exists for the study of haemolymph coagulation and immune responses to wound healing in insects.