Regulation of lipid metabolism genes, lipid carrier protein lipophorin, and its receptor during immune challenge in the mosquito Aedes aegypti

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.

A hemocyte-derived Kunitz-BPTI-type chymotrypsin inhibitor, HlChI, from the ixodid tick Haemaphysalis longicornis, plays regulatory functions in tick blood-feeding processes

Inhibitors of proteases play key roles in the biological processes of vertebrate and invertebrate animals, including arthropod parasites. Here, we describe a cDNA that encodes a functionally active chymotrypsin inhibitor of the BPTI/Kunitz family of serine protease inhibitors from the hemocytes of the ixodid tick, Haemaphysalis longicornis, herein called HlChI. HlChI sequence is evolutionarily conserved and contains six cysteine residues and three disulfide bonds with a calculated molecular weight of 9.1 kDa. HlChI-specific mRNA was expressed in all developmental stages of ticks and the expression was up-regulated by host's blood-feeding processes. Endogenous HlChI was localized mainly in the hemocytes. HlChI potently inhibited bovine pancreatic α-chymotrypsin for hydrolyzing the fluorogenic substrate (IC(50) 8.32 nM, K(d) 5.35 ± 1.01 nM) and bovine casein digestion. However, HlChI weakly inhibited bovine pancreatic trypsin and could not affect the porcine elastase activity, suggesting its narrow specificity to chymotrypsin. HlChI was stable over the pH range 2-11 and heating up to 70 °C at pH 8. HlChI was highly stable to 8 M urea and 2% SDS at pH 8.0, when treated for 24 h at 37 °C. However, 0.2 M 2-mercaptoethanol caused complete but reversible inactivation of HlChI. Knockdown of HlChI gene by RNA interference (RNAi) caused death of the feeding ticks, failure of ticks to engorge and significantly reduced body weight gain. RNAi also resulted in significantly decreased egg conversion ratio and fecundity. These results suggest that HlChI is a chymotrypsin-specific inhibitor with high stability and may play regulatory functions in host's blood-feeding processes and tick reproduction.

Neutral lipid composition changes in the fat bodies of engorged females Rhipicephalus microplus ticks in response to fungal infections

The tick's fat body plays an essential role in energy storage and utilization. This study aimed to analyze the fat body neutral lipid composition in Rhipicephalus microplus engorged females. In the first study (physiological profile of untreated ticks), the lipid analysis took place over the course of 4 days; the engorged females were incubated at optimal conditions and their fat bodies were dissected daily. Fat body lipid analysis after fungal infection with Metarhizium anisopliae sensu lato (s.l.) or Beauveria bassiana s.l. was performed with four groups: one without any treatment, one that was inoculated with a solution of 0.1 % Tween 80 in water, and two groups that were inoculated with M. anisopliae or B. bassiana conidial suspensions. The fat bodies were dissected 24 and 48 h after infection. Lipid analysis was conducted by thin-layer chromatography on a silica plate. The results of the physiological profile showed that the amounts of triacylglycerol (TAG) and free cholesterol (CHO) decreased with time, whereas cholesterol ester (CHOE) increased on the second and fourth days. Following M. anisopliae or B. bassiana infection, there was an increase in the amount of CHO after 24 h, whereas the other lipid classes were not altered. M. anisopliae caused an increase in CHOE and TAG and a reduction in CHO at 48 h after infection; however, B. bassiana infection did not cause significant alterations in the concentrations of these lipids. M. anisopliae and B. bassiana infection changed the fat body metabolism of engorged female R. microplus ticks. This study provides the first report of changes in the neutral lipid composition of the R. microplus fat body.

Effect of Wolbachia on replication of West Nile virus in a mosquito cell line and adult mosquitoes

Wolbachia as an endosymbiont is widespread in insects and other arthropods and is best known for reproductive manipulations of the host. Recently, it has been shown that wMelpop and wMel strains of Wolbachia inhibit the replication of several RNA viruses, including dengue virus, and other vector-borne pathogens (e.g., Plasmodium and filarial nematodes) in mosquitoes, providing an alternative approach to limit the transmission of vector-borne pathogens. In this study, we tested the effect of Wolbachia on the replication of West Nile Virus (WNV). Surprisingly, accumulation of the genomic RNA of WNV for all three strains of WNV tested (New York 99, Kunjin, and New South Wales) was enhanced in Wolbachia-infected Aedes aegypti cells (Aag2). However, the amount of secreted virus was significantly reduced in the presence of Wolbachia. Intrathoracic injections showed that replication of WNV in A. aegypti mosquitoes infected with wMel strain of Wolbachia was not inhibited, whereas wMelPop strain of Wolbachia significantly reduced the replication of WNV in mosquitoes. Further, when wMelPop mosquitoes were orally fed with WNV, virus infection, transmission, and dissemination rates were very low in Wolbachia-free mosquitoes and were completely inhibited in the presence of Wolbachia. The results suggest that (i) despite the enhancement of viral genomic RNA replication in the Wolbachia-infected cell line the production of secreted virus was significantly inhibited, (ii) the antiviral effect in intrathoracically infected mosquitoes depends on the strain of Wolbachia, and (iii) replication of the virus in orally fed mosquitoes was completely inhibited in wMelPop strain of Wolbachia.

Potential costs of bacterial infection on storage protein gene expression and reproduction in queenless Apis mellifera worker bees on distinct dietary regimes

Insects are able to combat infection by initiating an efficient immune response that involves synthesizing antimicrobial peptides and a range of other defense molecules. These responses may be costly to the organism, resulting in it exploiting endogenous resources to maintain homeostasis or support defense to the detriment of other physiological needs. We used queenless worker bees on distinct dietary regimes that may alter hemolymph protein storage and ovary activation to investigate the physiological costs of infection with Serratia marcescens. The expression of the genes encoding the storage proteins vitellogenin and hexamerin 70a, the vitellogenin receptor, and vasa (which has a putative role in reproduction), was impaired in the infected bees. This impairment was mainly evident in the bees fed beebread, which caused significantly higher expression of these genes than did royal jelly or syrup, and this was confirmed at the vitellogenin and hexamerin 70a protein levels. Beebread was also the only diet that promoted ovary activation in the queenless bees, but this activation was significantly impaired by the infection. The expression of the genes encoding the storage proteins apolipophorins-I and -III and the lipophorin receptor was not altered by infection regardless the diet provided to the bees. Similarly, the storage of apolipophorin-I in the hemolymph was only slightly impaired by the infection, independently of the supplied diet. Taken together these results indicate that, infection demands a physiological cost from the transcription of specific protein storage-related genes and from the reproductive capacity.

Nutritional limitation and resistance to opportunistic Aspergillus parasites in honey bee larvae

Honey bees are threatened by land use changes which reduce the availability and diversity of pollen and nectar resources. There is concern that poor nutrition may be involved in recent population declines, either directly or due to indirect effects on immunocompetence. The larval stage is likely to be the most vulnerable to a poor diet, but the effects of larval nutrition on the disease susceptibility of bees are not well known. In this study we used laboratory-reared honey bee larvae to investigate the effects of diet quality on disease susceptibility to the opportunistic fungal parasites Aspergillus flavus, Aspergillus phoenicis and A. fumigatus. Larvae fed on a nutritionally poor diet were found to be significantly more susceptible to A. fumigatus. Larval resistance to A. fumigatus was enhanced by feeding with a diet supplemented with either dandelion or polyfloral pollens. This indicates that dandelion and polyfloral pollens contain elements that enhance resistance to this fungal disease, illustrating an interaction between nutrition and parasitism and emphasising the benefit of diverse floral resources in the environment to maintain honey bee health.

Time course proteomic profiling of cellular responses to immunological challenge in the sea urchin, Heliocidaris erythrogramma

Genome sequences and high diversity cDNA arrays have provided a detailed molecular understanding of immune responses in a number of invertebrates, including sea urchins. However, complementary analyses have not been undertaken at the level of proteins. Here, we use shotgun proteomics to describe changes in the abundance of proteins from coelomocytes of sea urchins after immunological challenge and wounding. The relative abundance of 345 reproducibly identified proteins were measured 6, 24 and 48 h after injection. Significant changes in the relative abundance of 188 proteins were detected. These included pathogen-binding proteins, such as the complement component C3 and scavenger receptor cysteine rich proteins, as well as proteins responsible for cytoskeletal remodeling, endocytosis and intracellular signaling. An initial systemic reaction to wounding was followed by a more specific response to immunological challenge involving proteins such as apolipophorin, dual oxidase, fibrocystin L, aminopeptidase N and α-2-macroglobulin.

Pre-feeding of a glycolipid binding protein LEC-8 from Caenorhabditis elegans revealed enhanced tolerance to Cry1Ac toxin in Helicoverpa armigera

Crystal toxins from Bacillus thuringiensis bind to glycolipids and glycoproteins using two different lectin domains in the toxin protein. Our previous observations suggested that the sequestration of crystal toxin depends on the functional interaction of a toxin lectin with glycolipids. Given the finding that competition of a galectin LEC-8 with Cry5B for binding to glycolipids resulting in reduced Bt toxicity in nematode, it is interesting to explore the role of LEC-8 in insects. Here, we reported that the LEC-8 can also be exploited by insect for their survival when they were fed with Bt toxin food. Bioassay with LEC-8 showed that pre-feeding of Helicoverpa armigera larvae reduced the Cry1Ac susceptibility. Both LEC-8 and Cry1Ac bind to the midgut glycolipid in a similar way. Further ELISA indicated that LEC-8 interacts with glycolipid from insect midgut, thus reduce Cry1Ac binding to glycolipid. This in turn enhances insect tolerance to Cry1Ac toxin. The sugar determinants of LEC-8 were studied by using haemagglutination (HA) and haemagglutination inhibition (HAI) assay. It was suggested that the terminal sugar of LEC-8 has multiple sugar binding property.

Insect tolerance to the crystal toxins Cry1Ac and Cry2Ab is mediated by the binding of monomeric toxin to lipophorin glycolipids causing oligomerization and sequestration reactions

Endotoxins from the soil bacterium Bacillus thuringiensis are used worldwide to control insect pests and vectors of diseases. Despite extensive use of the toxins as sprays and in transgenic crops, their mode of action is still not completely known. Here we show that two crystal toxins binding to different glycoprotein receptors have similar glycolipid binding properties. The glycolipid binding domain was identified in a recombinant peptide representing the domain II of the crystal toxin Cry1Ac (M-peptide). The recombinant M-peptide was isolated from bacterial lysates as a mixture of monomers and dimers and formed tetramers upon binding to glycolipid microvesicles from gut tissues and lipid particles from hemolymph plasma. Likewise, when mature toxins and M-peptides where mixed with plasma, these peptides bind to lipid particles and can be separated with lipophorin particles on low-density gradients. When mature toxin and M-peptides are added to lipid particles in increasing amounts, the peptide-particle complexes form higher aggregates that are similar to aggregates formed in low-density gradients in the presence of the toxin. This could indicate that glycolipids on lipid particles are possible targets for toxin monomers in the gut lumen, which upon binding to the glycolipids form tetramers and aggregate particles and thereby sequester the toxin inside the gut lumen before it can interact with receptors on the brush border membrane. The implication is that domain II interacting with glycolipids mediate tolerance to the toxin that is separate from interaction of the toxin with glycoprotein receptors causing toxicity.

Deficiencies in acetyl-CoA carboxylase and fatty acid synthase 1 differentially affect eggshell formation and blood meal digestion in Aedes aegypti

To better understand the mechanism of de novo lipid biosynthesis in blood fed Aedes aegypti mosquitoes, we quantitated acetyl-CoA carboxylase (ACC) and fatty acid synthase 1 (FAS1) transcript levels in blood fed mosquitoes, and used RNAi methods to generate ACC and FAS1 deficient mosquitoes. Using the ketogenic amino acid (14)C-leucine as a metabolic precursor of (14)C-acetyl-CoA, we found that (14)C-triacylglycerol and (14)C-phospholipid levels were significantly reduced in both ACC and FAS1 deficient mosquitoes, confirming that ACC and FAS1 are required for de novo lipid biosynthesis after blood feeding. Surprisingly however, we also found that ACC deficient mosquitoes, but not FAS1 deficient mosquitoes, produced defective oocytes, which lacked an intact eggshell and gave rise to inviable eggs. This severe phenotype was restricted to the 1st gonotrophic cycle, suggesting that the eggshell defect was due to ACC deficiencies in the follicular epithelial cells, which are replaced after each gonotrophic cycle. Consistent with lower amounts of de novo lipid biosynthesis, both ACC and FAS1 deficient mosquitoes produced significantly fewer eggs than control mosquitoes in both the 1st and 2nd gonotrophic cycles. Lastly, FAS1 deficient mosquitoes, but not ACC deficient mosquitoes, showed delayed blood meal digestion, suggesting that a feedback control mechanism may coordinate rates of fat body lipid biosynthesis and midgut digestion during feeding. We propose that decreased ACC and FAS1 enzyme levels lead to reduced lipid biosynthesis and lower fecundity, whereas altered levels of the regulatory metabolites acetyl-CoA and malonyl-CoA account for the observed defects in eggshell formation and blood meal digestion, respectively.

West Nile virus encodes a microRNA-like small RNA in the 3' untranslated region which up-regulates GATA4 mRNA and facilitates virus replication in mosquito cells

West Nile virus (WNV) belongs to a group of medically important single-stranded, positive-sense RNA viruses causing deadly disease outbreaks around the world. The 3′ untranslated region (3′-UTR) of the flavivirus genome, in particular the terminal 3′ stem–loop (3′SL) fulfils multiple functions in virus replication and virus–host interactions. Using the Kunjin strain of WNV (WNV_KUN), we detected a virally encoded small RNA, named KUN-miR-1, derived from 3′SL. Transcription of WNV_KUN pre-miRNA (3′SL) in mosquito cells either from plasmid or Semliki Forest virus (SFV) RNA replicon resulted in the production of mature KUN-miR-1. Silencing of Dicer-1 but not Dicer-2 led to a reduction in the miRNA levels. Further, when a synthetic inhibitor of KUN-miR-1 was transfected into mosquito cells, replication of viral RNA was significantly reduced. Using cloning and bioinformatics approaches, we identified the cellular GATA4 mRNA as a target for KUN-miR-1. KUN-miR-1 produced in mosquito cells during virus infection or from plasmid DNA, SFV RNA replicon or mature miRNA duplex increased accumulation of GATA4 mRNA. Depletion of GATA4 mRNA by RNA silencing led to a significant reduction in virus RNA replication while a KUN-miR-1 RNA mimic enhanced replication of a mutant WNV_KUN virus producing reduced amounts of KUN-miR-1, suggesting that GATA4-induction via KUN-miR-1 plays an important role in virus replication.

Lipophorin acts as a shuttle of lipids to the milk gland during tsetse fly pregnancy

During pregnancy in the viviparous tsetse fly, lipid mobilization is essential for the production of milk to feed the developing intrauterine larva. Lipophorin (Lp) functions as the major lipid transport protein in insects and closely-related arthropods. In this study, we assessed the role of Lp and the lipophorin receptor (LpR) in the lipid mobilization process during tsetse reproduction. We identified single gene sequences for GmmLp and GmmLpR from the genome of Glossinamorsitansmorsitans, and measured spatial and temporal expression of gmmlp and gmmlpr during the female reproductive cycle. Our results show that expression of gmmlp is specific to the adult fat body and larvae. In the adult female, gmmlp expression is constitutive. However transcript levels increase in the larva as it matures within the mother's uterus, reaching peak expression just prior to parturition. GmmLp was detected in the hemolymph of pregnant females and larvae, but not in the uterine fluid or larval gut contents ruling out the possibility of direct transfer of GmmLp from mother to offspring. Transcripts for gmmlpr were detected in the head, ovaries, midgut, milk gland/fat body, ovaries and developing larva. Levels of gmmlpr remain stable throughout the first and second gonotrophic cycles with a slight dip observed during the first gonotrophic cycle. GmmLpR was detected in multiple tissues, including the midgut, fat body, milk gland, spermatheca and head. Knockdown of gmmlp by RNA interference resulted in reduced hemolymph lipid levels, delayed oocyte development and extended larval gestation. Similar suppresion of gmmlpr did not significantly reduce hemolymph lipid levels or oogenesis duration, but did extend the duration of larval development. Thus, GmmLp function as the primary shuttle for lipids originating from the midgut and fat body to the ovaries and milk gland to supply resources for developing oocytes and larval nourishment, respectively. Once in the milk gland however, lipids are apparently transferred into the developing larva not by lipophorin but by another carrier lipoprotein.

Targeted gene expression in the transgenic Aedes aegypti using the binary Gal4-UAS system

In this study, we report the establishment of the binary Gal4/UAS system for the yellow fever mosquito Aedes aegypti. We utilized the 1.8-kb 5' upstream region of the vitellogenin gene (Vg) to genetically engineer mosquito lines with the Vg-Gal4 activator and established UAS-EGFP responder transgenic mosquito lines to evaluate the binary Gal4/UAS system. The results show that the Vg-Gal4 driver leads to a high level of tissue-, stage- and sex-specific expression of the EGFP reporter in the fat body of Vg-Gal4/UAS-EGFP hybrids after blood-meal activation. In addition, the applicability of this system to study hormonal regulation of gene expression was demonstrated in in vitro organ culture experiments in which the EGFP reporter was highly activated in isolated fat bodies of previtellogenic Vg-Gal4/UAS-EGFP females incubated in the presence of 20-hydroxyecdysone (20E). Hence, this study has opened the door for further refinement of genetic tools in mosquitoes.

Effects of Plasmodium gallinaceum on hemolymph physiology of Aedes aegypti during parasite development

Insect disease vectors show diminished fecundity when infected with Plasmodium. This phenomenon has already been demonstrated in laboratory models such as Aedes aegypti, Anopheles gambiae and Anopheles stephensi. This study demonstrates several changes in physiological processes of A. aegypti occurring upon infection with Plasmodium gallinaceum, such as reduced ecdysteroid levels in hemolymph as well as altered expression patterns for genes involved in vitellogenesis, lipid transport and immune response. Furthermore, we could show that P. gallinaceum infected A. aegypti presented a reduction in reproductive fitness, accompanied by an activated innate immune response and increase in lipophorin expression, with the latter possibly representing a nutritional resource for Plasmodium sporozoites.

Up-regulation of lipophorin (Lp) and lipophorin receptor (LpR) gene in the mosquito, Culex quinquefasciatus (Diptera: Culicidae), infected with the filarial parasite, Wuchereria bancrofti (Spirurida: Onchocercidae)

In mosquitoes, including Culex quinquefasciatus, immune molecules are known to be upregulated or produced de novo upon exposure to parasites or pathogens. These molecules are regulatory in nature acting against parasite or pathogen infection and development. Similarly, there are molecules that are upregulated to facilitate parasite development in the vector mosquitoes. Lipophorin, a major lipid transporting lipoprotein in the hemolymph of insects, is implicated as a helper molecule in the clotting mechanism and facilitator of parasite and pathogen development in mosquitoes. In the present study, upregulation of a 240 kDa protein was detected in C. quinquefasciatus infected with the human lymphatic filarial parasite, Wuchereria bancrofti. It was identified as a lipophorin through nano-Lc-MS/MS analysis. Transcription of the lipophorin receptor gene also was identified through RACE-PCR. C. quinquefasciatus is the vector of W. bancrofti, and it allows successful development of the parasite. The role of upregulated lipophorin and transcription of its receptor gene in this mosquito could be implicated as a facilitator for the parasite development.

Quantitative analysis of replication and tropisms of Dengue virus type 2 in Aedes albopictus

Dengue virus serotype 2 (DENV-2) RNA replication profiles and tropisms were studied by using quantitative RT-PCR (q-RTPCR) in intrathoracically infected Aedes albopictus. The virus RNA replication profiles were diverse in mosquito organs. In fat body, brain, salivary gland, and malpighian tubes, it peaked at 8, 23, 23, and 27 days post-infection, respectively, and then, all declined. In midgut, it increased all the time and had no trend of decline. In ovary, it had no apparent increase. Subsequent Western blotting of DENV-2 E protein had similar results. Using ribosomal protein 7 (rpS7) as an internal control, we found that, in salivary gland, brain, fat body, and midgut, the average DENV-2 RNA levels (DENV-2 RNA/rpS7 mRNA) were 1,028, 464, 5.6, and 6.2, respectively; in malpighian tubes, it was 1, and in ovary, it was far less than 1. These results suggest that infection profiles and tropism of DENV-2 RNA in Ae. albopictus organs are significantly different.

Fungal secondary metabolites as modulators of interactions with insects and other arthropods

Fungi share a diverse co-evolutionary history with animals, especially arthropods. In this review, we focus on the role of secondary metabolism in driving antagonistic arthropod-fungus interactions, i.e., where fungi serve as a food source to fungal grazers, compete with saprophagous insects, and attack insects as hosts for growth and reproduction. Although a wealth of studies on animal-fungus interactions point to a crucial role of secondary metabolites in deterring animal feeding and resisting immune defense strategies, causal evidence often remains to be provided. Moreover, it still remains an unresolved puzzle as to what extent the tight regulatory control of secondary metabolite formation in some model fungi represents an evolved chemical defense system favored by selective pressure through animal antagonists. Given these gaps in knowledge, we highlight some co-evolutionary aspects of secondary metabolism, such as induced response, volatile signaling, and experimental evolution, which may help in deciphering the ecological importance and evolutionary history of secondary metabolite production in fungi.

Insecticide control of vector-borne diseases: when is insecticide resistance a problem?

Many of the most dangerous human diseases are transmitted by insect vectors. After decades of repeated insecticide use, all of these vector species have demonstrated the capacity to evolve resistance to insecticides. Insecticide resistance is generally considered to undermine control of vector-transmitted diseases because it increases the number of vectors that survive the insecticide treatment. Disease control failure, however, need not follow from vector control failure. Here, we review evidence that insecticide resistance may have an impact on the quality of vectors and, specifically, on three key determinants of parasite transmission: vector longevity, competence, and behaviour. We argue that, in some instances, insecticide resistance is likely to result in a decrease in vector longevity, a decrease in infectiousness, or in a change in behaviour, all of which will reduce the vectorial capacity of the insect. If this effect is sufficiently large, the impact of insecticide resistance on disease management may not be as detrimental as previously thought. In other instances, however, insecticide resistance may have the opposite effect, increasing the insect's vectorial capacity, which may lead to a dramatic increase in the transmission of the disease and even to a higher prevalence than in the absence of insecticides. Either way-and there may be no simple generality-the consequence of the evolution of insecticide resistance for disease ecology deserves additional attention.

The major yolk protein vitellogenin interferes with the anti-plasmodium response in the malaria mosquito Anopheles gambiae

Functional gene analysis in malaria mosquitoes reveals molecules underpinning the trade-off between efficient reproduction and the antiparasitic response.

When taking a blood meal on a person infected with malaria, female Anopheles gambiae mosquitoes, the major vector of human malaria, acquire nutrients that will activate egg development (oogenesis) in their ovaries. Simultaneously, they infect themselves with the malaria parasite. On traversing the mosquito midgut epithelium, invading Plasmodium ookinetes are met with a potent innate immune response predominantly controlled by mosquito blood cells. Whether the concomitant processes of mosquito reproduction and immunity affect each other remains controversial. Here, we show that proteins that deliver nutrients to maturing mosquito oocytes interfere with the antiparasitic response. Lipophorin (Lp) and vitellogenin (Vg), two nutrient transport proteins, reduce the parasite-killing efficiency of the antiparasitic factor TEP1. In the absence of either nutrient transport protein, TEP1 binding to the ookinete surface becomes more efficient. We also show that Lp is required for the normal expression of Vg, and for later Plasmodium development at the oocyst stage. Furthermore, our results uncover an inhibitory role of the Cactus/REL1/REL2 signaling cassette in the expression of Vg, but not of Lp. We reveal molecular links that connect reproduction and immunity at several levels and provide a molecular basis for a long-suspected trade-off between these two processes.

Malaria annually claims the lives of almost 1 million infants and imposes a major socio-economic burden on Africa and other tropical regions. Meanwhile, the detailed biological interactions between the malaria parasite and its Anopheles mosquito vector remain largely enigmatic. What we do know is that the majority of malaria parasites are normally eliminated by the mosquito's immune response. Mosquitoes accidentally acquire an infection by sucking parasite-laden blood, but this belies the primary function of the blood in the provisioning of nutrients for egg development in the insect's ovaries. We have found that the molecular processes involved in delivering blood-acquired nutrients to maturing eggs diminish the efficiency of parasite killing by the mosquito immune system. Conversely, molecular pathways that set the immune system on its maximal capacity for parasite killing preclude the efficient development of the mosquito's eggs. Our results reveal some of the molecules that underpin this example of the trade-offs between reproduction and immunity, a concept that has long intrigued biologists.

Glycerophospholipid acquisition in Plasmodium - a puzzling assembly of biosynthetic pathways

Throughout the Plasmodium life cycle, malaria parasites repeatedly undergo rapid cellular growth and prolific divisions, necessitating intense membrane neogenesis and, in particular, the acquisition of high amounts of phospholipids. At the intraerythrocytic stage, glycerophospholipids are the main parasite membrane constituents, which mostly originate from the Plasmodium-encoded enzymatic machinery. Several proteins and entire pathways have been characterized and their features reported, thereby generating a global view of glycerophospholipid synthesis across Plasmodium spp. The malaria parasite displays a panoply of pathways that are seldom found together in a single organism. The major glycerophospholipids are synthesized via ancestral prokaryotic CDP-diacylglycerol-dependent pathways and eukaryotic-type de novo pathways. The parasite exhibits additional reactions that bridge some of these routes and are otherwise restricted to some organisms, such as plants, while base-exchange mechanisms are largely unexplored in Plasmodium. Marked differences between Plasmodium spp. have also been reported in phosphatidylcholine and phosphatidylethanolamine synthesis. Little is currently known about glycerophospholipid acquisition at non-erythrocytic stages, but recent data reveal that intrahepatocytic parasites, oocysts and sporozoites import various host lipids, and that de novo fatty acid synthesis is only crucial at the late liver stage. More studies on the different Plasmodium developmental stages are needed, to further assemble the different pieces of this glycerophospholipid synthesis puzzle, which contains highly promising therapeutic targets.

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.

Role of adhesion in arthropod immune recognition

The recognition and inactivation of toxins and pathogens are mediated by a combination of cell-free and cellular mechanisms. A number of soluble and membrane-bound pattern recognition molecules interact with elicitors to become involved in both cell-free inactivation as well as cellular uptake reactions. Here we describe the possible recognition and effector function of key arthropod immune proteins, such as peroxinectin, hemolin, and hemomucin, as an outcome of changes in adhesiveness, which drive self-assembly reactions leading to cell-free coagulation and cellular uptake reactions. The fact that some of these proteins are essential for immune and developmental functions in some species, but are not found in closely related species, may point to the existence of multiprotein assemblies, which are conserved at the mechanistic level and can function with more than one combination of protein constituents.

Insect fat body: energy, metabolism, and regulation

The fat body plays major roles in the life of insects. It is a dynamic tissue involved in multiple metabolic functions. One of these functions is to store and release energy in response to the energy demands of the insect. Insects store energy reserves in the form of glycogen and triglycerides in the adipocytes, the main fat body cell. Insect adipocytes can store a great amount of lipid reserves as cytoplasmic lipid droplets. Lipid metabolism is essential for growth and reproduction and provides energy needed during extended nonfeeding periods. This review focuses on energy storage and release and summarizes current understanding of the mechanisms underlying these processes in insects.

The Kunitz-like modulatory protein haemangin is vital for hard tick blood-feeding success

Ticks are serious haematophagus arthropod pests and are only second to mosquitoes as vectors of diseases of humans and animals. The salivary glands of the slower feeding hard ticks such as Haemaphysalis longicornis are a rich source of bioactive molecules and are critical to their biologic success, yet distinct molecules that help prolong parasitism on robust mammalian hosts and achieve blood-meals remain unidentified. Here, we report on the molecular and biochemical features and precise functions of a novel Kunitz inhibitor from H. longicornis salivary glands, termed Haemangin, in the modulation of angiogenesis and in persistent blood-feeding. Haemangin was shown to disrupt angiogenesis and wound healing via inhibition of vascular endothelial cell proliferation and induction of apoptosis. Further, this compound potently inactivated trypsin, chymotrypsin, and plasmin, indicating its antiproteolytic potential on angiogenic cascades. Analysis of Haemangin-specific gene expression kinetics at different blood-feeding stages of adult ticks revealed a dramatic up-regulation prior to complete feeding, which appears to be functionally linked to the acquisition of blood-meals. Notably, disruption of Haemangin-specific mRNA by a reverse genetic tool significantly diminished engorgement of adult H. longicornis, while the knock-down ticks failed to impair angiogenesis in vivo. To our knowledge, we have provided the first insights into transcriptional responses of human microvascular endothelial cells to Haemangin. DNA microarray data revealed that Haemangin altered the expression of 3,267 genes, including those of angiogenic significance, further substantiating the antiangiogenic function of Haemangin. We establish the vital roles of Haemangin in the hard tick blood-feeding process. Moreover, our results provide novel insights into the blood-feeding strategies that enable hard ticks to persistently feed and ensure full blood-meals through the modulation of angiogenesis and wound healing processes.

Genome-wide transcriptomic profiling of Anopheles gambiae hemocytes reveals pathogen-specific signatures upon bacterial challenge and Plasmodium berghei infection

Background

The mosquito Anopheles gambiae is a major vector of human malaria. Increasing evidence indicates that blood cells (hemocytes) comprise an essential arm of the mosquito innate immune response against both bacteria and malaria parasites. To further characterize the role of hemocytes in mosquito immunity, we undertook the first genome-wide transcriptomic analyses of adult female An. gambiae hemocytes following infection by two species of bacteria and a malaria parasite.

Results

We identified 4047 genes expressed in hemocytes, using An. gambiae genome-wide microarrays. While 279 transcripts were significantly enriched in hemocytes relative to whole adult female mosquitoes, 959 transcripts exhibited immune challenge-related regulation. The global transcriptomic responses of hemocytes to challenge with different species of bacteria and/or different stages of malaria parasite infection revealed discrete, minimally overlapping, pathogen-specific signatures of infection-responsive gene expression; 105 of these represented putative immunity-related genes including anti-Plasmodium factors. Of particular interest was the specific co-regulation of various members of the Imd and JNK immune signaling pathways during malaria parasite invasion of the mosquito midgut epithelium.

Conclusion

Our genome-wide transcriptomic analysis of adult mosquito hemocytes reveals pathogen-specific signatures of gene regulation and identifies several novel candidate genes for future functional studies.

Trade-off between immune stimulation and expression of storage protein genes

Proteins stored in insect hemolymph may serve as a source of amino acids and energy for metabolism and development. The expression of the main storage proteins was assessed in bacterial-challenged honey bees using real-time (RT)-PCR and Western blot. After ensuring that the immune system had been activated by measuring the ensuing expression of the innate immune response genes, defensin-1 (def-1) and prophenoloxidase (proPO), we verified the expression of four genes encoding storage proteins. The levels of vitellogenin (vg) mRNA and of the respective protein were significantly lowered in bees injected with bacteria or water only (injury). An equivalent response was observed in orally-infected bees. The levels of apolipophorin II/I (apoLp-II/I) and hexamerin (hex 70a) mRNAs did not significantly change, but levels of Hex 70a protein subunit showed a substantial decay after bacterial challenge or injury. Infection also caused a strong reduction in the levels of apoLp-III transcripts. Our findings are consistent with a down-regulation of the expression and accumulation of storage proteins as a consequence of activation of the immune system, suggesting that this phenomenon represents a strategy to redirect resources to combat injury or infection.

The major insect lipoprotein is a lipid source to mosquito stages of malaria parasite

Once mosquito midgut barrier was crossed malaria parasite faces a extensive metabolic developmental program in order to ensure its transmission. In the hemolymph of the mosquito the dynamics of lipid metabolism is conducted by a major lipoprotein, lipophorin (Lp). It was recently shown that Lp is engaged in the mosquito immune response to parasite infection. However, it is not clear if Lp is uptaken by the parasite. Here, we show that oocysts are able to uptake mosquito Lp. The uptake of FITC-labeled Lp was demonstrated in midgut-associated oocysts. Alternatively, to confirm Lp incorporation by oocysts we have conducted another set of experiments with iodinated Lp ((125)I-Lp). Oocysts were able to incorporate (125)I-Lp and the process is both time and temperature dependent. This set of results indicated that no matter oocysts are attached to mosquito midgut wall they bear a lipid sequestering machinery from its surroundings. Phospholipid transfer to sporozoites was also demonstrated. In conclusion, these results demonstrate for the first time that malaria parasite undergoes lipid uptake while in the invertebrate host.

Legumains from the hard tick Haemaphysalis longicornis play modulatory roles in blood feeding and gut cellular remodelling and impact on embryogenesis

The biology and vectorial capacity of haematophagous ticks are directly related to effective blood feeding and digestion. The midgut-associated proteases in ticks are involved in the blood (Hb) digestion cascade, the molecular mechanisms of which are yet poorly understood. Our previous studies indicated that Haemaphysalis longicornis midgut-specific asparaginyl endopeptidases/legumains, HlLgm and HlLgm2, act in the Hb digestion cascade. Here, we investigated the potential of these enzymes in blood feeding and digestion, midgut remodelling and reproduction of ticks by employing RNA interference (RNAi) techniques. Injection of HlLgm- and HlLgm2 gene-specific double-stranded RNAs into unfed adult female H. longicornis caused gene-specific transcriptional and translational disruptions. RNAi impacted on tick blood feeding leading to death of the feeding ticks, failure of ticks to reach repletion and significant reductions in engorged tick body weight. Histological examination revealed that deletion of legumains resulted in damage to the midgut tissues and disruption of normal cellular remodelling during feeding. Gene knock-down also caused significantly delayed onset of oviposition, reduced number of eggs and, most strikingly, structurally deformed eggs that failed to hatch suggesting imperfect embryogenesis. Synergistic impacts of RNAi were reflected on all parameters evaluated when HlLgm and HlLgm2 were silenced together. These findings suggest that legumains may play modulatory roles in blood feeding and digestion, midgut cellular remodelling and embryogenesis in H. longicornis. Deletion of legumains in H. longicornis would help in controlling the tick population and thereby transmission of diseases to their hosts.

Beta-secretase: structure, function, and evolution

The most popular current hypothesis is that Alzheimer's disease (AD) is caused by aggregates of the amyloid peptide (Abeta), which is generated by cleavage of the Abeta protein precursor (APP) by beta-secretase (BACE-1) followed by gamma-secretase. BACE-1 cleavage is limiting for the production of Abeta, making it a particularly good drug target for the generation of inhibitors that lower Abeta. A landmark discovery in AD was the identification of BACE-1 (a.k.a. Memapsin-2) as a novel class of type I transmembrane aspartic protease. Although BACE-2, a homologue of BACE-1, was quickly identified, follow up studies using knockout mice demonstrated that BACE-1 was necessary and sufficient for most neuronal Abeta generation. Despite the importance of BACE-1 as a drug target, development has been slow due to the incomplete understanding of its function and regulation and the difficulties in developing a brain penetrant drug that can specifically block its large catalytic pocket. This review summarizes the biological properties of BACE-1 and attempts to use phylogenetic perspectives to understand its function. The article also addresses the challenges in discovering a selective drug-like molecule targeting novel mechanisms of BACE-1 regulation.

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.

Resource depletion in Aedes aegypti mosquitoes infected by the microsporidia Vavraia culicis

Parasitic infection is often associated with changes in host life-history traits, such as host development. Many of these life-history changes are ultimately thought to be the result of a depletion or reallocation of the host's resources driven either by the host (to minimize the effects of infection) or by the parasite (to maximize its growth rate). In this paper we investigate the energetic budget of Aedes aegypti mosquito larvae infected by Vavraia culicis, a microsporidian parasite that transmits horizontally between larvae, and which has been previously shown to reduce the probability of pupation of its host. Our results show that infected larvae have significantly less lipids, sugars and glycogen than uninfected larvae. These differences in resources were not due to differences in larval energy intake (feeding rate) or expenditure (metabolic rate). We conclude that the lower energetic resources of infected mosquitoes are the result of the high metabolic demands that microsporidian parasites impose on their hosts. Given the fitness advantages for the parasite of maintaining the host in a larval stage, we discuss whether resource depletion may also be a parasite mechanism to prevent the pupation of the larvae and thus maximize its own transmission.

Structural and RNAi characterization of the German cockroach lipophorin receptor, and the evolutionary relationships of lipoprotein receptors

BACKGROUND

Lipophorin receptors (LpRs) have been described in a number of insects, but functional studies have been reported only in locusts and mosquitoes. The aim of the present work was to characterize the LpR of the cockroach Blattella germanica, not only molecularly but also functionally using RNAi techniques, and to place LpRs in a phylogenetical context among lipoprotein receptors.

RESULTS

We cloned a putative LpR from B. germanica (BgLpR) using RT-PCR methods. Two isoforms of BgLpR that differ from each other by an insertion/deletion of 24 amino acids were obtained from the fat body and the ovary. A phylogenetical analysis of lipoprotein receptors showed that BgLpR grouped with other sequences annotated as LpR in a cluster placed as a sister group of vertebrate low density lipoprotein receptors (LDLR) + lipoprotein receptor-related proteins 8 (LPR8) + vitellogenin receptors (VgR) + very low density lipoprotein receptors (VLDLR). The two BgLpR isoforms are expressed in different adult female tissues (fat body, ovary, brain, midgut, muscle) and in embryos. In ovaries and fat body, the two isoforms are similarly expressed during the first gonadotrophic cycle. mRNA levels in the fat body increase in parallel to vitellogenesis, whereas they decrease in the ovaries. BgLpR protein levels increase in parallel to vitellogenesis in both organs. Treatment with juvenile hormone increases BgLpR protein. RNAi experiments show that females with lower BgLpR expression have less lipophorin in the growing oocytes with respect to controls.

CONCLUSION

The two isoforms of BgLpR are structurally similar to other LpRs. Phylogenetical analyses show that LpRs and the group formed by vertebrate LDLR + LPR8 + VgR + VLDLR, diverged from a common ancestor and diversified in parallel. The different expression patterns in the fat body and the ovary, comparing mRNA and protein, indicate that the corresponding mechanisms regulating BgLpR expression are different. Experiments with JH III suggest that such a hormone regulates the expression of BgLpR posttranscriptionally. RNAi experiments indicate that BgLpR is a functional lipophorin receptor.

Apolipocrustacein, formerly vitellogenin, is the major egg yolk precursor protein in decapod crustaceans and is homologous to insect apolipophorin II/I and vertebrate apolipoprotein B

BACKGROUND

In animals, the biogenesis of some lipoprotein classes requires members of the ancient large lipid transfer protein (LLTP) superfamily, including the cytosolic large subunit of microsomal triglyceride transfer protein (MTP), vertebrate apolipoprotein B (apoB), vitellogenin (Vtg), and insect apolipophorin II/I precursor (apoLp-II/I). In most oviparous species, Vtg, a large glycolipoprotein, is the main egg yolk precursor protein.

RESULTS

This report clarifies the phylogenetic relationships of LLTP superfamily members and classifies them into three families and their related subfamilies. This means that the generic term Vtg is no longer a functional term, but is rather based on phylogenetic/structural criteria. In addition, we determined that the main egg yolk precursor protein of decapod crustaceans show an overall greater sequence similarity with apoLp-II/I than other LLTP, including Vtgs. This close association is supported by the phylogenetic analysis, i.e. neighbor-joining, maximum likelihood and Bayesian inference methods, of conserved sequence motifs and the presence of three common conserved domains: an N-terminal large lipid transfer module marker for LLTP, a DUF1081 domain of unknown function in their central region exclusively shared with apoLp-II/I and apoB, and a von Willebrand-factor type D domain at their C-terminal end. Additionally, they share a conserved functional subtilisin-like endoprotease cleavage site with apoLp-II/I, in a similar location.

CONCLUSION

The structural and phylogenetic data presented indicate that the major egg yolk precursor protein of decapod crustaceans is surprisingly closely related to insect apoLp-II/I and vertebrate apoB and should be known as apolipocrustacein (apoCr) rather than Vtg. These LLTP may arise from an ancient duplication event leading to paralogs of Vtg sequences. The presence of LLTP homologs in one genome may facilitate redundancy, e.g. involvement in lipid metabolism and as egg yolk precursor protein, and neofunctionalization and subfunctionalization, e.g. involvement in clotting cascade and immune response, of extracellular LLTP members. These protein-coding nuclear genes may be used to resolve phylogenetic relationships among the major arthropod groups, especially the Pancrustacea-major splits.

GATA transcription factor required for immunity to bacterial and fungal pathogens

In the past decade, Caenorhabditis elegans has been used to dissect several genetic pathways involved in immunity; however, little is known about transcription factors that regulate the expression of immune effectors. C. elegans does not appear to have a functional homolog of the key immune transcription factor NF-κB. Here we show that that the intestinal GATA transcription factor ELT-2 is required for both immunity to Salmonella enterica and expression of a C-type lectin gene, clec-67, which is expressed in the intestinal cells and is a good marker of S. enterica infection. We also found that ELT-2 is required for immunity to Pseudomonas aeruginosa, Enterococcus faecalis, and Cryptococcus neoformans. Lack of immune inhibition by DAF-2, which negatively regulates the FOXO transcription factor DAF-16, rescues the hypersusceptibility to pathogens phenotype of elt-2(RNAi) animals. Our results indicate that ELT-2 is part of a multi-pathogen defense pathway that regulates innate immunity independently of the DAF-2/DAF-16 signaling pathway.