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

Lipid Metabolism in Insect Vectors of Diseases

According to the World Health Organization vector-borne diseases account for more than 17% of all infectious diseases, causing more than 700,000 deaths annually. Vectors are organisms that are able to transmit infectious pathogens between humans, or from animals to humans. Many of these vectors are hematophagous insects, which ingest the pathogen from an infected host during a blood meal, and later transmit it into a new host. Malaria, dengue, African trypanosomiasis, yellow fever, leishmaniasis, Chagas disease, and many others are examples of diseases transmitted by insects.Both the diet and the infection with pathogens trigger changes in many metabolic pathways, including lipid metabolism, compared to other insects. Blood contains mostly proteins and is very poor in lipids and carbohydrates. Thus, hematophagous insects attempt to efficiently digest and absorb diet lipids and also rely on a large de novo lipid biosynthesis based on utilization of proteins and carbohydrates as carbon source. Blood meal triggers essential physiological processes as molting, excretion, and oogenesis; therefore, lipid metabolism and utilization of lipid storage should be finely synchronized and regulated regarding that, in order to provide the necessary energy source for these events. Also, pathogens have evolved mechanisms to hijack essential lipids from the insect host by interfering in the biosynthesis, catabolism, and transport of lipids, which pose challenges to reproduction, survival, fitness, and other insect traits.In this chapter, we have tried to collect and highlight the current knowledge and recent discoveries on the metabolism of lipids in insect vectors of diseases related to the hematophagous diet and pathogen infection.

Exploring the effects of the acaricide cyflumetofen on the vital organs of the honey bee Apis mellifera (Hymenoptera: Apidae) workers

Bees are important for maintaining ecosystems, pollinating crops and producing marketable products. In recent years, a decline in bee populations has been reported, with multifactorial causes, including the intensification of pesticide use in agriculture. Among pesticides, cyflumetofen is an insecticide and acaricide used in apple, coffee and citrus crops, whose main pollinator is the honey bee Apis mellifera. Therefore, this bee is a potential target of cyflumetofen during foraging. This study evaluated the histopathological and cytological damage in the midgut, hypopharyngeal glands and fat body of A. mellifera workers exposed to LC50 of cyflumetofen. The midgut epithelium of exposed bees presented cytoplasmic vacuolization, release of vesicles and cell fragments, which indicate autophagy, increased production of digestive enzymes and cell death, respectively. The cytological analysis of the midgut revealed the dilation of the basal labyrinth and the presence of spherocrystals in the digestive cells. The hypopharyngeal glands produced greater amounts of secretion in treated bees, whereas no changes were observed in the fat body. The results indicate that acute exposure to cyflumetofen negatively affect A. mellifera, causing damage to the midgut and changes in the hypopharyngeal glands, which may compromise the survival and foraging of this pollinator.

Lipophorin: The Lipid Shuttle

Insects need to transport lipids through the aqueous medium of the hemolymph to the organs in demand, after they are absorbed by the intestine or mobilized from the lipid-producing organs. Lipophorin is a lipoprotein present in insect hemolymph, and is responsible for this function. A single gene encodes an apolipoprotein that is cleaved to generate apolipophorin I and II. These are the essential protein constituents of lipophorin. In some physiological conditions, a third apolipoprotein of different origin may be present. In most insects, lipophorin transports mainly diacylglycerol and hydrocarbons, in addition to phospholipids. The fat body synthesizes and secretes lipophorin into the hemolymph, and several signals, such as nutritional, endocrine, or external agents, can regulate this process. However, the main characteristic of lipophorin is the fact that it acts as a reusable shuttle, distributing lipids between organs without being endocytosed or degraded in this process. Lipophorin interacts with tissues through specific receptors of the LDL receptor superfamily, although more recent results have shown that other proteins may also be involved. In this chapter, we describe the lipophorin structure in terms of proteins and lipids, in addition to reviewing what is known about lipoprotein synthesis and regulation. In addition, we reviewed the results investigating lipophorin's function in the movement of lipids between organs and the function of lipophorin receptors in this process.

Temporal Transcriptomic Profiling Reveals Dynamic Changes in Gene Expression of Giant Freshwater Prawn upon Acute Saline-Alkaline Stresses

Bicarbonate and sulfate are among two primary ion constituents of saline-alkaline water, with excessive levels potentially causing metabolic disorders in crustaceans, affecting their molting and interrupting development. As an economically important crustacean species, the molecular adaptive mechanism of giant freshwater prawn Macrobrachium rosenbergii in response to the stress of bicarbonate and sulfate remains unexplored. To investigate the mechanism underlying NaHCO3, Na2SO4, and mixed NaHCO3, Na2SO4 stresses, M. rosenbergii larvae were exposed to the above three stress conditions, followed by total RNA extraction and high-throughput sequencing at eight distinct time points (0, 4, 8, 12, 24, 48, 72, and 96 h). Subsequent analysis revealed 13, 16, and 13 consistently identified differentially expressed genes (DEGs) across eight time points under three stress conditions. These consistently identified DEGs were significantly involved in the Gene Ontology (GO) terms of chitin-based cuticle development, protein-carbohydrate complex, structural constituent of cuticle, carnitine biosynthetic process, extracellular matrix, and polysaccharide catabolic process, indicating that alkaline stresses might potentially impact the energy metabolism, growth, and molting of M. rosenbergii larvae. Particularly, the transcriptome data revealed that DEGs associated with energy metabolism, immunity, and amino acid metabolism were enriched across multiple time points under three stress conditions. These DEGs are linked to Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, including glycolysis/glucogenesis, amino sugar and nucleotide sugar metabolism, and lysine degradation. Consistent enrichment findings across the three stress conditions support conclusions above. Together, these insights are instrumental in enhancing our understanding of the molecular mechanisms underlying the alkaline response in M. rosenbergii larvae. Additionally, they offer valuable perspectives on the regulatory mechanisms of freshwater crustaceans amid saline-alkaline water development.

Mapping the functional form of the trade-off between infection resistance and reproductive fitness under dysregulated immune signaling

Immune responses benefit organismal fitness by clearing parasites but also exact costs associated with immunopathology and energetic investment. Hosts manage these costs by tightly regulating the induction of immune signaling to curtail excessive responses and restore homeostasis. Despite the theoretical importance of turning off the immune response to mitigate these costs, experimentally connecting variation in the negative regulation of immune responses to organismal fitness remains a frontier in evolutionary immunology. In this study, we used a dose-response approach to manipulate the RNAi-mediated knockdown efficiency of cactus (IκBα), a central regulator of Toll pathway signal transduction in flour beetles (Tribolium castaneum). By titrating cactus activity across four distinct levels, we derived the shape of the relationship between immune response investment and traits associated with host fitness, including infection susceptibility, lifespan, fecundity, body mass, and gut homeostasis. Cactus knock-down increased the overall magnitude of inducible immune responses and delayed their resolution in a dsRNA dose-dependent manner, promoting survival and resistance following bacterial infection. However, these benefits were counterbalanced by dsRNA dose-dependent costs to lifespan, fecundity, body mass, and gut integrity. Our results allowed us to move beyond the qualitative identification of a trade-off between immune investment and fitness to actually derive its functional form. This approach paves the way to quantitatively compare the evolution and impact of distinct regulatory elements on life-history trade-offs and fitness, filling a crucial gap in our conceptual and theoretical models of immune signaling network evolution and the maintenance of natural variation in immune systems.

Signaling between mammalian adiponectin and a mosquito adiponectin receptor reduces Plasmodium transmission

IMPORTANCE

When a female mosquito takes a blood meal from a mammalian host, components of the blood meal can affect mosquito fitness and indirectly influence pathogen infectivity. We identified a pathway involving an Anopheles gambiae adiponectin receptor, which, triggered by adiponectin from an incoming blood meal, decreases Plasmodium infection in the mosquito. Activation of this pathway negatively regulates lipophorin expression, an important lipid transporter that both enhances egg development and Plasmodium infection. This is an unrecognized cross-phyla interaction between a mosquito and its vertebrate host. These processes are critical to understanding the complex life cycle of mosquitoes and Plasmodium following a blood meal and may be applicable to other hematophagous arthropods and vector-borne infectious agents.

Lipid metabolism dynamic in Triatomine Rhodnius prolixus during acute Trypanosoma rangeli infection

During its life cycle, Trypanosoma rangeli invades the hemolymph of its invertebrate host and colonizes hemocytes and salivary glands. The parasite cannot synthesize some lipid classes, and during its cycle, it depends on the uptake of these molecules from its vertebrate and invertebrate hosts to meet growth and differentiation requirements. However, until now, knowledge on how the parasite affects the lipid physiology of individual insect organs has been largely unknown. Herein, the biochemical and molecular dynamics of triatomine R. prolixus lipid metabolism in response to acute T. rangeli infection were investigated. Biochemical and microscopic assays revealed the lipid droplet profile and the levels of the different identified lipid classes. In addition, a qRT‒PCR approach was used to determine the expression profile of 6 protein-coding genes involved in the R. prolixus lipid physiology. We observed that triacylglycerol (TAG), monoacylglycerol (MAG), phosphatidylethanolamine (PE) and phosphatidylcholine (PC) levels in the fat body decreased in infected insects. On the other hand, high levels of free fatty acids were observed in the hemolymph during infection. Analysis by confocal microscopy revealed a decrease in lipid droplets size from infected fat bodies, and investigations by scanning electron microscopy revealed a significant number of parasites adhered to the surface of the organ. T. rangeli infection upregulated the transcript levels of the protein-coding gene for the acetyl-CoA carboxylase, the first enzyme in the de novo fatty acid synthesis pathway, responsible for the production of malonyl-CoA. On the other hand, downregulation of lipophorin receptor was observed. In conclusion, this study reveals a new set of molecular events that occur within the vector in response to the challenge imposed by the parasite.

RNAi turns 25:contributions and challenges in insect science

Since its discovery in 1998, RNA interference (RNAi), a Nobel prize-winning technology, made significant contributions to advances in biology because of its ability to mediate the knockdown of specific target genes. RNAi applications in medicine and agriculture have been explored with mixed success. The past 25 years of research on RNAi resulted in advances in our understanding of the mechanisms of its action, target specificity, and differential efficiency among animals and plants. RNAi played a major role in advances in insect biology. Did RNAi technology fully meet insect pest and disease vector management expectations? This review will discuss recent advances in the mechanisms of RNAi and its contributions to insect science. The remaining challenges, including delivery to the target site, differential efficiency, potential resistance development and possible solutions for the widespread use of this technology in insect management.

Mapping the functional form of the trade-off between infection resistance and reproductive fitness under dysregulated immune signaling

Immune responses benefit organismal fitness by clearing parasites but also exact costs associated with immunopathology and energetic investment. Hosts manage these costs by tightly regulating the induction of immune signaling to curtail excessive responses and restore homeostasis. Despite the theoretical importance of turning off the immune response to mitigate these costs, experimentally connecting variation in the negative regulation of immune responses to organismal fitness remains a frontier in evolutionary immunology. In this study, we used a dose-response approach to manipulate the RNAi-mediated knockdown efficiency of cactus (IκBα), a central regulator of Toll pathway signal transduction in flour beetles (Tribolium castaneum). By titrating cactus activity along a continuous gradient, we derived the shape of the relationship between immune response investment and traits associated with host fitness, including infection susceptibility, lifespan, fecundity, body mass, and gut homeostasis. Cactus knock-down increased the overall magintude of inducible immune responses and delayed their resolution in a dsRNA dose-dependent manner, promoting survival and resistance following bacterial infection. However, these benefits were counterbalanced by dsRNA dose-dependent costs to lifespan, fecundity, body mass, and gut integrity. Our results allowed us to move beyond the qualitative identification of a trade-off between immune investment and fitness to actually derive its functional form. This approach paves the way to quantitatively compare the evolution and impact of distinct regulatory elements on life-history trade-offs and fitness, filling a crucial gap in our conceptual and theoretical models of immune signaling network evolution and the maintenance of natural variation in immune systems.

Alterations in energy metabolism of Rhodnius prolixus induced by Trypanosoma rangeli infection

Trypanosoma rangeli is a protozoan parasite that infects triatomines and mammals in the Americas, producing mixed infections with Trypanosoma cruzi, the etiological agent of Chagas disease. The former parasite is not pathogenic to humans, but has different levels of pathogenicity, as well as causing physiological and behavioral alterations, to its invertebrate hosts. In this study, we measured locomotory activity, and the glyceride accumulation profile in the hemolymph and fat body, as well as the expression of key genes related to triglyceride metabolism, of Rhodnius prolixus nymphs infected with T. rangeli. We found that the locomotory activity of the insects was correlated with the amount of triglycerides in the fat body. Infected nymphs had increased activity when starved, and also had an accumulation of glycerides in the fat body and hemolymph. These alterations were also associated with a higher expression of the diacylglycerol acyltransferase, lipophorin and lipophorin receptor genes in the fat body. We infer that T. rangeli is able to alter the energetic processes of its invertebrate host, in order to increase the availability of lipids to the parasite, which, in turn modifies the activity levels of the insect. These alterations are discussed with regard to their potential to increase the transmission rate of the parasite.

Lipids as a key element of insect defense systems

The relationship between insect pathogenic fungi and their insect hosts is a classic example of a co-evolutionary arms race between pathogen and target host: parasites evolve towards mechanisms that increase their advantage over the host, and the host increasingly strengthens its defenses. The present review summarizes the literature data describing the direct and indirect role of lipids as an important defense mechanism during fungal infection. Insect defense mechanisms comprise anatomical and physiological barriers, and cellular and humoral response mechanisms. The entomopathogenic fungi have the unique ability to digest the insect cuticle by producing hydrolytic enzymes with chitin-, lipo- and proteolytic activity; besides the oral tract, cuticle pays the way for fungal entry within the host. The key factor in insect resistance to fungal infection is the presence of certain types of lipids (free fatty acids, waxes or hydrocarbons) which can promote or inhibit fungal attachment to cuticle, and might also have antifungal activity. Lipids are considered as an important source of energy, and as triglycerides are stored in the fat body, a structure analogous to the liver and adipose tissue in vertebrates. In addition, the fat body plays a key role in innate humoral immunity by producing a range of bactericidal proteins and polypeptides, one of which is lysozyme. Energy derived from lipid metabolism is used by hemocytes to migrate to the site of fungal infection, and for phagocytosis, nodulation and encapsulation. One polyunsaturated fatty acid, arachidonic acid, is used in the synthesis of eicosanoids, which play several crucial roles in insect physiology and immunology. Apolipoprotein III is important compound with antifungal activity, which can modulate insect cellular response and is considered as important signal molecule.

Wolbachia-induced inhibition of O'nyong nyong virus in Anopheles mosquitoes is mediated by Toll signaling and modulated by cholesterol

Enhanced host immunity and competition for metabolic resources are two main competing hypotheses for the mechanism of Wolbachia-mediated pathogen inhibition in arthropods. Using an Anopheles mosquito - somatic Wolbachia infection - O'nyong nyong virus (ONNV) model, we demonstrate that the mechanism underpinning Wolbachia-mediated virus inhibition is up-regulation of the Toll innate immune pathway. However, the viral inhibitory properties of Wolbachia were abolished by cholesterol supplementation. This result was due to Wolbachia-dependent cholesterol-mediated suppression of Toll signaling rather than competition for cholesterol between Wolbachia and virus. The inhibitory effect of cholesterol was specific to Wolbachia-infected Anopheles mosquitoes and cells. These data indicate that both Wolbachia and cholesterol influence Toll immune signaling in Anopheles mosquitoes in a complex manner and provide a functional link between the host immunity and metabolic competition hypotheses for explaining Wolbachia-mediated pathogen interference in mosquitoes. In addition, these results provide a mechanistic understanding of the mode of action of Wolbachia-induced pathogen blocking in Anophelines, which is critical to evaluate the long-term efficacy of control strategies for malaria and Anopheles-transmitted arboviruses.

Chronic depletion of vertebrate lipids in Aedes aegypti cells dysregulates lipid metabolism and inhibits innate immunity without altering dengue infectivity

Aedes aegypti is the primary vector of dengue virus (DENV) and other arboviruses. Previous literature suggests that vertebrate and invertebrate lipids and the nutritional status of mosquitoes modify virus infection. Here, we developed a vertebrate lipid-depleted Ae. aegypti cell line to investigate if chronic depletion of vertebrate lipids normally present in a blood meal and insect cell culture medium would impact cell growth and virus infection. Chronic depletion of vertebrate lipids reduced cell size and proliferation, although cells retained equivalent total intracellular lipids per cell by reducing lipolysis and modifying gene expression related to sugar and lipid metabolism. Downregulation of innate immunity genes was also observed. We hypothesized that chronic depletion of vertebrate lipids would impact virus infection; however, the same amount of DENV was produced per cell. This study reveals how Ae. aegypti cells adapt in the absence of vertebrate lipids, and how DENV can replicate equally well in cells that contain predominately vertebrate or invertebrate lipids.

Aedes aegypti is a major threat to public health. Ae. aegypti is the primary vector of dengue virus types 1–4 (DENV 1–4), zika virus (ZIKV), chikungunya virus (CHIKV), and yellow fever virus (YFV). Ae. aegypti acquires arboviruses from a vertebrate host during blood feeding. Blood feeding introduces vertebrate-specific factors into the mosquito that may be important for both mosquito and virus. This study reveals that Ae. aegypti adapts to depletion of vertebrate lipids by inhibiting lipolysis and promoting de novo synthesis of invertebrate lipids, and that DENV can replicate equally well without high concentrations of cholesterol and other vertebrate lipid species. Understanding how disease vectors adapt to nutritional changes will identify novel strategies for vector control and disease mitigation.

Trypanosoma cruzi modulates lipid metabolism and highjacks phospholipids from the midgut of Rhodnius prolixus

Chagas disease is potentially life-threatening and caused by the protozoan parasite Trypanosoma cruzi. The parasite cannot synthesize some lipids and depends on the uptake of these lipids from its vertebrate and invertebrate hosts. To achieve this, T. cruzi may need to modify the physiology of the insect host for its own benefit. In this study, we investigated the interaction of T. cruzi (Y strain) with its insect vector Rhodnius prolixus and how it manipulates the vector lipid metabolism. We observed a physiological change in lipid flux in of infected insects. In the fat body of infected insects, triacylglycerol levels decreased by 80.6% and lipid storage droplet-1(LSD-1) mRNA levels were lower, when compared to controls. Lipid sequestration by infected midguts led to increased levels of 5' AMP-activated protein kinase (AMPK) phosphorylation and activation in the fat body, inhibiting the synthesis of fatty acids and stimulating their oxidation. This led to reduced lipid levels in the fat body of infected insets, despite the fact that T. cruzi does not colonize this tissue. There was a 3-fold increase, in lipid uptake and synthesis in the midgut of infected insects. Finally, our results suggest that the parasite modifies the lipid flux and metabolism of its vector R. prolixus through the increase in lipid delivery from the fat body to midgut that are then scavenge by T cruzi.

Physiological and transcriptional immune responses of a non-model arthropod to infection with different entomopathogenic groups

Insect immune responses to multiple pathogen groups including viruses, bacteria, fungi, and entomopathogenic nematodes have traditionally been documented in model insects such as Drosophila melanogaster, or medically important insects such as Aedes aegypti. Despite their potential importance in understanding the efficacy of pathogens as biological control agents, these responses are infrequently studied in agriculturally important pests. Additionally, studies that investigate responses of a host species to different pathogen groups are uncommon, and typically focus on only a single time point during infection. As such, a robust understanding of immune system responses over the time of infection is often lacking in many pest species. This study was conducted to understand how 3rd instar larvae of the major insect pest Helicoverpa zea responded through the course of an infection by four different pathogenic groups: viruses, bacteria, fungi, and entomopathogenic nematodes; by sampling at three different times post-inoculation. Physiological immune responses were assessed at 4-, 24-, and 48-hours post-infection by measuring hemolymph phenoloxidase concentrations, hemolymph prophenoloxidase concentrations, hemocyte counts, and encapsulation ability. Transcriptional immune responses were measured at 24-, 48-, and 72-hours post-infection by quantifying the expression of PPO2, Argonaute-2, JNK, Dorsal, and Relish. This gene set covers the major known immune pathways: phenoloxidase cascade, siRNA, JNK pathway, Toll pathway, and IMD pathway. Our results indicate H. zea has an extreme immune response to Bacillus thuringiensis bacteria, a mild response to Helicoverpa armigera nucleopolyhedrovirus, and little-to-no detectable response to either the fungus Beauveria bassiana or Steinernema carpocapsae nematodes.

The metabolism and role of free fatty acids in key physiological processes in insects of medical, veterinary and forensic importance

Insects are the most widespread group of organisms and more than one million species have been described. These animals have significant ecological functions, for example they are pollinators of many types of plants. However, they also have direct influence on human life in different manners. They have high medical and veterinary significance, stemming from their role as vectors of disease and infection of wounds and necrotic tissue; they are also plant pests, parasitoids and predators whose activities can influence agriculture. In addition, their use in medical treatments, such as maggot therapy of gangrene and wounds, has grown considerably. They also have many uses in forensic science to determine the minimum post-mortem interval and provide valuable information about the movement of the body, cause of the death, drug use, or poisoning. It has also been proposed that they may be used as model organisms to replace mammal systems in research. The present review describes the role of free fatty acids (FFAs) in key physiological processes in insects. By focusing on insects of medical, veterinary significance, we have limited our description of the physiological processes to those most important from the point of view of insect control; the study examines their effects on insect reproduction and resistance to the adverse effects of abiotic (low temperature) and biotic (pathogens) factors.

Lipid Interactions Between Flaviviruses and Mosquito Vectors

Mosquito-borne flaviviruses, such as dengue (DENV), Zika (ZIKV), yellow fever (YFV), West Nile (WNV), and Japanese encephalitis (JEV) viruses, threaten a large part of the human populations. In absence of therapeutics and effective vaccines against each flaviviruses, targeting viral metabolic requirements in mosquitoes may hold the key to new intervention strategies. Development of metabolomics in the last decade opened a new field of research: mosquito metabolomics. It is now clear that flaviviruses rely on mosquito lipids, especially phospholipids, for their cellular cycle and propagation. Here, we review the biosyntheses of, biochemical properties of and flaviviral interactions with mosquito phospholipids. Phospholipids are structural lipids with a polar headgroup and apolar acyl chains, enabling the formation of lipid bilayer that form plasma- and endomembranes. Phospholipids are mostly synthesized through the de novo pathway and remodeling cycle. Variations in headgroup and acyl chains influence phospholipid physicochemical properties and consequently the membrane behavior. Flaviviruses interact with cellular membranes at every step of their cellular cycle. Recent evidence demonstrates that flaviviruses reconfigure the phospholipidome in mosquitoes by regulating phospholipid syntheses to increase virus multiplication. Identifying the phospholipids involved and understanding how flaviviruses regulate these in mosquitoes is required to design new interventions.

Trypanosoma cruzi Affects Rhodnius prolixus Lipid Metabolism During Acute Infection

The interaction between Rhodnius prolixus and Trypanosoma cruzi has huge medical importance because it responds to the transmission of Chagas disease, a neglected tropical disease that affects about eight million people worldwide. It is known that trypanosomatid pathogens depend on active lipid endocytosis from the insect host to meet growth and differentiation requirements. However, until now, knowledge on how the parasite affects the lipid physiology of individual insect organs was largely unknown. Herein, the biochemical and molecular dynamics of the triatomine R. prolixus lipid metabolism in response to T. cruzi acute infection were investigated. A qRT-PCR approach was used to determine the expression profile of 12 protein-coding genes involved in R. prolixus lipid physiology. In addition, microscopic and biochemical assays revealed the lipid droplet profile and the levels of the different identified lipid classes. Finally, spectrometry analyses were used to determine fatty acid and sterol composition and their modulation towards the infection. T. cruzi infection downregulated the transcript levels of protein-coding genes for lipid biosynthetic and degrading pathways in individual triatomine organs. On the other hand, upregulation of lipid receptor transcripts indicates an attempt to capture more lipids from hemolymphatic lipoproteins. Consequently, several lipid classes (such as monoacylglycerol, diacylglycerol, triacylglycerol, cholesteryl ester, phosphatidylcholine, and phosphatidylethanolamine) were involved in the response to the parasite challenge, although modulating only the insect fat body. T. cruzi never leaves the insect gut and yet it modulates non-infected tissues, suggesting that the association between the parasite and the vector organs is reached by cell signaling molecules. This hypothesis raises several intriguing issues to inspire future studies in the parasite-vector interaction field.

Lipidomic Profiling Reveals Distinct Differences in Sphingolipids Metabolic Pathway between Healthy Apis cerana cerana larvae and Chinese Sacbrood Disease

Chinese sacbrood disease (CSD), which is caused by Chinese sacbrood virus (CSBV), is a major viral disease in Apis cerana cerana larvae. Analysis of lipid composition is critical to the study of CSBV replication. The host lipidome profiling during CSBV infection has not been conducted. This paper identified the lipidome of the CSBV-larvae interaction through high-resolution mass spectrometry. A total of 2164 lipids were detected and divided into 20 categories. Comparison of lipidome between healthy and CSBV infected-larvae showed that 266 lipid species were altered by CSBV infection. Furthermore, qRT-PCR showed that various sphingolipid enzymes and the contents of sphingolipids in the larvae were increased, indicating that sphingolipids may be important for CSBV infection. Importantly, Cer (d14:1 + hO/21:0 + O), DG (41:0e), PE (18:0e/18:3), SM (d20:0/19:1), SM (d37:1), TG (16:0/18:1/18:3), TG (18:1/20:4/21:0) and TG (43:7) were significantly altered in both CSBV_24 h vs. CK_24 h and CSBV_48 h vs. CK_48 h. Moreover, TG (39:6), which was increased by more than 10-fold, could be used as a biomarker for the early detection of CSD. This study provides evidence that global lipidome homeostasis in A. c. cerana larvae is remodeled after CSBV infection. Detailed studies in the future may improve the understanding of the relationship between the sphingolipid pathway and CSBV replication.

Dietary and Plasmodium challenge effects on the cuticular hydrocarbon profile of Anopheles albimanus

The cuticular hydrocarbon (CHC) profile reflects the insects' physiological states. These include age, sex, reproductive stage, and gravidity. Environmental factors such as diet, relative humidity or exposure to insecticides also affect the CHC composition in mosquitoes. In this work, the CHC profile was analyzed in two Anopheles albimanus phenotypes with different degrees of susceptibility to Plasmodium, the susceptible-White and resistant-Brown phenotypes, in response to the two dietary regimes of mosquitoes: a carbon-rich diet (sugar) and a protein-rich diet (blood) alone or containing Plasmodium ookinetes. The CHCs were analyzed by gas chromatography coupled to mass spectrometry or flame ionization detection, identifying 19 CHCs with chain lengths ranging from 20 to 37 carbons. Qualitative and quantitative changes in CHCs composition were dependent on diet, a parasite challenge, and, to a lesser extent, the phenotype. Blood-feeding caused up to a 40% reduction in the total CHC content compared to sugar-feeding. If blood contained ookinetes, further changes in the CHC profile were observed depending on the Plasmodium susceptibility of the phenotypes. Higher infection prevalence caused greater changes in the CHC profile. These dietary and infection-associated modifications in the CHCs could have multiple effects on mosquito fitness, impacts on disease transmission, and tolerance to insecticides.

Comprehensive Quantitative Proteome Analysis of Aedes aegypti Identifies Proteins and Pathways Involved in Wolbachia pipientis and Zika Virus Interference Phenomenon

Zika virus (ZIKV) is a global public health emergency due to its association with microcephaly, Guillain-Barré syndrome, neuropathy, and myelitis in children and adults. A total of 87 countries have had evidence of autochthonous mosquito-borne transmission of ZIKV, distributed across four continents, and no antivirus therapy or vaccines are available. Therefore, several strategies have been developed to target the main mosquito vector, Aedes aegypti, to reduce the burden of different arboviruses. Among such strategies, the use of the maternally-inherited endosymbiont Wolbachia pipientis has been applied successfully to reduce virus susceptibility and decrease transmission. However, the mechanisms by which Wolbachia orchestrate resistance to ZIKV infection remain to be elucidated. In this study, we apply isobaric labeling quantitative mass spectrometry (MS)-based proteomics to quantify proteins and identify pathways altered during ZIKV infection; Wolbachia infection; co-infection with Wolbachia/ZIKV in the A. aegypti heads and salivary glands. We show that Wolbachia regulates proteins involved in reactive oxygen species production, regulates humoral immune response, and antioxidant production. The reduction of ZIKV polyprotein in the presence of Wolbachia in mosquitoes was determined by MS and corroborates the idea that Wolbachia helps to block ZIKV infections in A. aegypti. The present study offers a rich resource of data that may help to elucidate mechanisms by which Wolbachia orchestrate resistance to ZIKV infection in A. aegypti, and represents a step further on the development of new targeted methods to detect and quantify ZIKV and Wolbachia directly in complex tissues.

Non-immune Traits Triggered by Blood Intake Impact Vectorial Competence

Blood-feeding arthropods are considered an enormous public health threat. They are vectors of a plethora of infectious agents that cause potentially fatal diseases like Malaria, Dengue fever, Leishmaniasis, and Lyme disease. These vectors shine due to their own physiological idiosyncrasies, but one biological aspect brings them all together: the requirement of blood intake for development and reproduction. It is through blood-feeding that they acquire pathogens and during blood digestion that they summon a collection of multisystemic events critical for vector competence. The literature is focused on how classical immune pathways (Toll, IMD, and JAK/Stat) are elicited throughout the course of vector infection. Still, they are not the sole determinants of host permissiveness. The dramatic changes that are the hallmark of the insect physiology after a blood meal intake are the landscape where a successful infection takes place. Dominant processes that occur in response to a blood meal are not canonical immunological traits yet are critical in establishing vector competence. These include hormonal circuitries and reproductive physiology, midgut permeability barriers, midgut homeostasis, energy metabolism, and proteolytic activity. On the other hand, the parasites themselves have a role in the outcome of these blood triggered physiological events, consistently using them in their favor. Here, to enlighten the knowledge on vector-pathogen interaction beyond the immune pathways, we will explore different aspects of the vector physiology, discussing how they give support to these long-dated host-parasite relationships.

Different amplitudes of temperature fluctuation induce distinct transcriptomic and metabolomic responses in the dung beetle Phanaeus vindex

Most studies exploring molecular and physiological responses to temperature have focused on constant temperature treatments. To gain a better understanding of the impact of fluctuating temperatures, we investigated the effects of increased temperature variation on Phanaeus vindex dung beetles across levels of biological organization. Specifically, we hypothesized that increased temperature variation is energetically demanding. We predicted that thermal sensitivity of metabolic rate and energetic reserves would be reduced with increasing fluctuation. To test this, we examined the responses of dung beetles to constant (20°C), low fluctuation (20±5°C), or high fluctuation (20±12°C) temperature treatments using respirometry, assessment of energetic reserves and HPLC-MS-based metabolomics. We found no significant differences in metabolic rate or energetic reserves, suggesting increased fluctuations were not energetically demanding. To understand why there was no effect of increased amplitude of temperature fluctuation on energetics, we assembled and annotated a de novo transcriptome, finding non-overlapping transcriptomic and metabolomic responses of beetles exposed to different fluctuations. We found that 58 metabolites increased in abundance in both fluctuation treatments, but 15 only did so in response to high-amplitude fluctuations. We found that 120 transcripts were significantly upregulated following acclimation to any fluctuation, but 174 were upregulated only in beetles from the high-amplitude fluctuation treatment. Several differentially expressed transcripts were associated with post-translational modifications to histones that support a more open chromatin structure. Our results demonstrate that acclimation to different temperature fluctuations is distinct and may be supported by increasing transcriptional plasticity. Our results indicate for the first time that histone modifications may underlie rapid acclimation to temperature variation.

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.

Dengue virus dominates lipid metabolism modulations in Wolbachia-coinfected Aedes aegypti

Competition between viruses and Wolbachia for host lipids is a proposed mechanism of Wolbachia-mediated virus blocking in insects. Yet, the metabolomic interaction between virus and symbiont within the mosquito has not been clearly defined. We compare the lipid profiles of Aedes aegypti mosquitoes bearing mono- or dual-infections of the Wolbachia wMel strain and dengue virus serotype 3 (DENV3). We found metabolic signatures of infection-induced intracellular events but little evidence to support direct competition between Wolbachia and virus for host lipids. Lipid profiles of dual-infected mosquitoes resemble those of DENV3 mono-infected mosquitoes, suggesting virus-driven modulation dominates over that of Wolbachia. Interestingly, knockdown of key metabolic enzymes suggests cardiolipins are host factors for DENV3 and Wolbachia replication. These findings define the Wolbachia-DENV3 metabolic interaction as indirectly antagonistic, rather than directly competitive, and reveal new research avenues with respect to mosquito × virus interactions at the molecular level.

Koh, Islam, Ye et al. describe lipid profiles of Aedes aegypti mosquitoes bearing mono- or dual-infections of Wolbachia (wMel) and dengue virus serotype 3 (DENV3), finding that virus modulation dominates the dual-infection lipid profile and that cardiolipins support DENV3 and Wolbachia replication. This study suggests that direct competition for lipids do not underlie Wolbachia-mediated virus blocking.

The Role of Peptide Hormones in Insect Lipid Metabolism

Lipids are the primary storage molecules and an essential source of energy in insects during reproduction, prolonged periods of flight, starvation, and diapause. The coordination center for insect lipid metabolism is the fat body, which is analogous to the vertebrate adipose tissue and liver. The fat body is primarily composed of adipocytes, which accumulate triacylglycerols in intracellular lipid droplets. Genomics and proteomics, together with functional analyses, such as RNA interference and CRISPR/Cas9-targeted genome editing, identified various genes involved in lipid metabolism and elucidated their functions. However, the endocrine control of insect lipid metabolism, in particular the roles of peptide hormones in lipogenesis and lipolysis are relatively less-known topics. In the current review, the neuropeptides that directly or indirectly affect insect lipid metabolism are introduced. The primary lipolytic and lipogenic peptide hormones are adipokinetic hormone and the brain insulin-like peptides (ILP2, ILP3, ILP5). Other neuropeptides, such as insulin-growth factor ILP6, neuropeptide F, allatostatin-A, corazonin, leucokinin, tachykinins and limostatin, might stimulate lipolysis, while diapause hormone-pheromone biosynthesis activating neuropeptide, short neuropeptide F, CCHamide-2, and the cytokines Unpaired 1 and Unpaired 2 might induce lipogenesis. Most of these peptides interact with one another, but mostly with insulin signaling, and therefore affect lipid metabolism indirectly. Peptide hormones are also involved in lipid metabolism during reproduction, flight, diapause, starvation, infections and immunity; these are also highlighted. The review concludes with a discussion of the potential of lipid metabolism-related peptide hormones in pest management.

Extracellular vesicles restrict dengue virus fusion in Aedes aegypti cells

Aedes aegypti is the primary vector of dengue virus (DENV), and acquires this virus from a vertebrate host during blood feeding. Previous literature has shown that vertebrate blood factors such as complement protein C5a and low-density lipoprotein (LDL) influence DENV acquisition in the mosquito. Here, we show that extracellular vesicles in cell culture medium inhibit DENV infection in mosquito cells. Specifically, extracellular vesicles enter into mosquito cells and inhibit an early stage of infection. Extracellular vesicles had no effect on virus cell attachment or entry. Instead, extracellular vesicles restricted virus membrane fusion. Extracellular vesicles only inhibited DENV infection in mosquito cells and not vertebrate cells. These data highlight a novel virus-vector-host interaction that limits virus infection in mosquito cells by restricting virus membrane fusion.

Developmental and comparative perspectives on mosquito immunity

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

Effects of the entomopathogenic fungus Metarhizium flavoviride on the fat body lipid composition of Zophobas morio larvae (Coleoptera: Tenebrionidae)

Insects employ different defense strategies against fungal infections and chemicals. We aimed to identify the lipid compositions of the fat body of Zophobas morio larvae before and after fungal infection with the entomopathogenic fungus Metarhizium flavoviride. We used gas chromatography-mass spectrometry to analyze lipid extracts of the fat body isolated of Z. morio 2, 5, and 7 days after fungal infection (treatment group) and compared it with the lipid extracts in a control group injected with physiological isotonic saline. In all the samples, fatty acids were the most abundant compound found in the fat body extracts, with hexadecanoic acid/C16:0 being the most abundant lipid. However, the types and concentrations of lipids changed after fungal infection, likely as an immune response. The most considerable changes occurred in the concentrations of long-chain fatty acids, i.e., hexadecanoic acid/C16:0, octadecenoic acid/C18:1, and octadecanoic acid/C18:0. Contents of methyl ester increased significantly after infection, but that of other esters, especially octanoic acid decyl ester/OADE, decreased on the 5th day after infection. To the best of our knowledge, this is the first detailed analysis of the changes in the lipid composition of the fat body of Z. morio larvae as a result of fungal infection. Our results suggest that entomopathogenic fungal infection affects the internal lipid composition of insects, potentially as a way of adjusting to such infection. These results can help understand infection processes and defense strategies of insects against fungal infection. Ultimately, they can contribute to the creation of more effective chemicals against pest insects.

The consequences of Brugia malayi infection on the flight and energy resources of Aedes aegypti mosquitoes

Evidence from experimental infection studies has shown that infected mosquitoes exhibit altered host-seeking behaviours, with suppression and activation of behaviours dependent on the parasite’s development stage. The mechanisms are poorly characterised; however, infections can impact mosquito energy reserves, thereby influencing key life-history traits and behaviours. In addition, filarial infection is likely detrimental to flight due to damage caused by developing worms. This study aimed to evaluate the impacts of Brugia malayi infection on Aedes aegypti flight parameters: distance, average speed, maximum speed and number of flight bursts, using a tethered flight mill. In addition, we explored whether differences in flight capacity may be due to the effect of infection on glycogen and lipid reserves. Infection with filarial worms significantly reduced flight distance but increased the number of flight bursts. Exposure to microfilaermic blood led to a significant decrease in average and maximum flight speeds even in the absence of an established infection. Mosquitoes fed on microfilaraemic blood showed reduced levels of glycogen (−37.9%) and lipids (−49.7%) compared to controls at nine days post-exposure. However, a one-hour period of flight activity caused an increase in lipid content for both infected and control mosquitoes. Consequential flight incapacitation may serve in explaining the heterogeneous distribution of lymphatic filariasis.

Exposure of Larvae of the Solitary Bee Osmia bicornis to the Honey Bee Pathogen Nosema ceranae Affects Life History

Wild bees are important pollinators of wild plants and agricultural crops and they are threatened by several environmental stressors including emerging pathogens. Honey bees have been suggested as a potential source of pathogen spillover. One prevalent pathogen that has recently emerged as a honey bee disease is the microsporidian Nosema ceranae. While the impacts of N. ceranae in honey bees are well documented, virtually nothing is known about its effects in solitary wild bees. The solitary mason bee Osmia bicornis is a common pollinator in orchards and amenable to commercial management. Here, we experimentally exposed larvae of O. bicornis to food contaminated with N. ceranae and document spore presence during larval development. We measured mortality, growth parameters, and timing of pupation in a semi-field experiment. Hatched individuals were assessed for physiological state including fat body mass, wing muscle mass, and body size. We recorded higher mortality in the viable-spore-exposed group but could only detect a low number of spores among the individuals of this treatment. Viable-spore-treated individuals with higher head capsule width had a delayed pupation start. No impact on the physiological status could be detected in hatched imagines. Although we did not find overt evidence of O. bicornis infection, our findings indicate that exposure of larvae to viable N. ceranae spores could affect bee development.

Pyriproxyfen, a juvenile hormone analog, damages midgut cells and interferes with behaviors of Aedes aegypti larvae

Juvenile hormone analogs (JHA) are known to interfere with growth and biosynthesis of insects with potential for insecticide action. However, there has been comparatively few data on morphological effects of JHA on insect organs. To determine pyriproxyfen effects on Aedes aegypti larvae, we conducted toxicity, behavioral bioassays and assessed ultrastructural effects of pyriproxyfen on midgut cells. A. aegypti larvae were exposed in aqueous solution of pyriproxyfen LC₅₀ concentrations and evaluated for 24 h. This study fulfilled the toxic prevalence of pyriproxyfen to A. aegypti larvae (LC₅₀ = 8.2 mg L^-1). Behavioral observations confirmed that pyriproxyfen treatment significantly changes swimming behavior of larvae, limiting its displacement and speed. The pyriproxyfen causes remarkable histopathological and cytotoxic alterations in the midgut of larvae. Histopathological study reveals presence of cytoplasmic vacuolization and damage to brush border of the digestive cells. The main salient lesions of cytotoxic effects are occurrence of cell debris released into the midgut lumen, cytoplasm rich in lipid droplets, autophagosomes, disorganized microvilli and deformed mitochondria. Data suggest that pyriproxyfen can be used to help to control and eradicate this insect vector.

Lipid metabolism in insect disease vectors

More than a third of the world population is at constant risk of contracting some insect-transmitted disease, such as Dengue fever, Zika virus disease, malaria, Chagas' disease, African trypanosomiasis, and others. Independent of the life cycle of the pathogen causing the disease, the insect vector hematophagous habit is a common and crucial trait for the transmission of all these diseases. This lifestyle is unique, as hematophagous insects feed on blood, a diet that is rich in protein but relatively poor in lipids and carbohydrates, in huge amounts and low frequency. Another unique feature of these insects is that blood meal triggers essential metabolic processes, as molting and oogenesis and, in this way, regulates the expression of various genes that are involved in these events. In this paper, we review current knowledge of the physiology and biochemistry of lipid metabolism in insect disease vectors, comparing with classical models whenever possible. We address lipid digestion and absorption, hemolymphatic transport, and lipid storage by the fat body and ovary. In this context, both de novo fatty acid and triacylglycerol synthesis are discussed, including the related fatty acid activation process and the intracellular lipid binding proteins. As lipids are stored in order to be mobilized later on, e.g. for flight activity or survivorship, lipolysis and β-oxidation are also considered. All these events need to be finely regulated, and the role of hormones in this control is summarized. Finally, we also review information about infection, when vector insect physiology is affected, and there is a crosstalk between its immune system and lipid metabolism. There is not abundant information about lipid metabolism in vector insects, and significant current gaps in the field are indicated, as well as questions to be answered in the future.

Dynamic remodeling of lipids coincides with dengue virus replication in the midgut of Aedes aegypti mosquitoes

We describe the first comprehensive analysis of the midgut metabolome of Aedes aegypti, the primary mosquito vector for arboviruses such as dengue, Zika, chikungunya and yellow fever viruses. Transmission of these viruses depends on their ability to infect, replicate and disseminate from several tissues in the mosquito vector. The metabolic environments within these tissues play crucial roles in these processes. Since these viruses are enveloped, viral replication, assembly and release occur on cellular membranes primed through the manipulation of host metabolism. Interference with this virus infection-induced metabolic environment is detrimental to viral replication in human and mosquito cell culture models. Here we present the first insight into the metabolic environment induced during arbovirus replication in Aedes aegypti. Using high-resolution mass spectrometry, we have analyzed the temporal metabolic perturbations that occur following dengue virus infection of the midgut tissue. This is the primary site of infection and replication, preceding systemic viral dissemination and transmission. We identified metabolites that exhibited a dynamic-profile across early-, mid- and late-infection time points. We observed a marked increase in the lipid content. An increase in glycerophospholipids, sphingolipids and fatty acyls was coincident with the kinetics of viral replication. Elevation of glycerolipid levels suggested a diversion of resources during infection from energy storage to synthetic pathways. Elevated levels of acyl-carnitines were observed, signaling disruptions in mitochondrial function and possible diversion of energy production. A central hub in the sphingolipid pathway that influenced dihydroceramide to ceramide ratios was identified as critical for the virus life cycle. This study also resulted in the first reconstruction of the sphingolipid pathway in Aedes aegypti. Given conservation in the replication mechanisms of several flaviviruses transmitted by this vector, our results highlight biochemical choke points that could be targeted to disrupt transmission of multiple pathogens by these mosquitoes.

Cattle infection response network and its functional modules

BACKGROUND

Weighted Gene Co-expression Network analysis, a powerful technique used to extract co-expressed gene pattern from mRNA expression data, was constructed to infer common immune strategies used by cattle in response to five different bacterial species (Escherichia coli, Mycobacterium avium, Mycobacterium bovis, Salmonella and Staphylococcus aureus) and a protozoa (Trypanosoma Congolense) using 604 publicly available gene expression microarrays from 12 cattle infection experiments.

RESULTS

A total of 14,999 transcripts that were differentially expressed (DE) in at least three different infection experiments were consolidated into 15 modules that contained between 43 and 4441 transcripts. The high number of shared DE transcripts between the different types of infections indicated that there were potentially common immune strategies used in response to these infections. The number of transcripts in the identified modules varied in response to different infections. Fourteen modules showed a strong functional enrichment for specific GO/pathway terms related to "immune system process" (71%), "metabolic process" (71%), "growth and developmental process" (64%) and "signaling pathways" (50%), which demonstrated the close interconnection between these biological pathways in response to different infections. The largest module in the network had several over-represented GO/pathway terms related to different aspects of lipid metabolism and genes in this module were down-regulated for the most part during various infections. Significant negative correlations between this module's eigengene values, three immune related modules in the network, and close interconnection between their hub genes, might indicate the potential co-regulation of these modules during different infections in bovine. In addition, the potential function of 93 genes with no functional annotation was inferred based on neighbor analysis and functional uniformity among associated genes. Several hypothetical genes were differentially expressed during experimental infections, which might indicate their important role in cattle response to different infections.

CONCLUSIONS

We identified several biological pathways involved in immune response to different infections in cattle. These findings provide rich information for experimental biologists to design experiments, interpret experimental results, and develop novel hypothesis on immune response to different infections in cattle.

Systemic Israeli acute paralysis virus (IAPV) infection in bumblebees (Bombus terrestris) through feeding and injection

Israeli acute paralysis virus (IAPV) can cause a systemic infection, resulting in mortality in both Apis and Bombus spp. bees. However, little is known about the virus infection dynamics within bee tissues. Here, we established systemic IAPV infections in reared bumblebee Bombus terrestris workers through feeding and injection and investigated the mortality, tissue tropism and viral localization. Injection of approximately 500 IAPV (IAPV^inj stock) particles resulted in acute infection, viral loads within tissues that were relatively stable from bee to bee, and a distinctive tissue tropism, making this method suitable for studying systemic IAPV infection in bumblebees. Feeding with approximately 1 × 10⁶ particles of the same virus stock did not result in systemic infection. A high-concentration stock of IAPV (IAPV^fed stock) allowed us to feed bumblebees with approximately 1 × 10⁹ viral particles, which induced both chronic and acute infection. We also observed a higher variability in viral titers within tissues and less clear tissue tropism during systemic infection, making feeding with IAPV^fed stock less optimal for studying IAPV systemic infection. Strikingly, both infection methods and stocks with different viral loads gave a similar viral localization pattern in the brain and midgut of bumblebees with an acute infection. The implications of these findings in the study of the local immunity in bees and barriers to oral transmission are discussed. Our data provide useful information on the establishment of a systemic viral infection in bees.

RNA interference in mosquito: understanding immune responses, double-stranded RNA delivery systems and potential applications in vector control

RNA interference (RNAi) refers to the process of post-transcriptional silencing of cellular mRNA by the application of double-stranded RNA (dsRNA). RNAi strategies have been widely employed to regulate gene expression in plants and animals including insects. With the availability of the full genome sequences of major vector mosquitoes, RNAi has been increasingly used to conduct genetic studies of human pathogens in mosquito vectors and to study the evolution of insecticide resistance in mosquitoes. This review summarizes the recent progress in our understanding of mosquito-pathogen interactions using RNAi and various methods of dsRNA delivery in mosquitoes at different stages. We also discuss potential applications of this technology to develop novel tools for vector control.

Hormone and receptor interplay in the regulation of mosquito lipid metabolism

Mosquitoes transmit devastating human diseases because they need vertebrate blood for egg development. Metabolism in female mosquitoes is tightly coupled with blood meal-mediated reproduction, which requires an extremely high level of energy consumption. Functional analysis has shown that major genes encoding for enzymes involved in lipid metabolism (LM) in the mosquito fat bodies are down-regulated at the end of the juvenile hormone (JH)-controlled posteclosion (PE) phase but exhibit significant elevation in their transcript levels during the post-blood meal phase (PBM), which is regulated mainly by 20-hydroxyecdysone (20E). Reductions in the transcript levels of genes encoding triacylglycerol (TAG) catabolism and β-oxidation enzymes were observed to correlate with a dramatic accumulation of lipids in the PE phase; in contrast, these transcripts were elevated significantly and lipid stores were diminished during the PBM phase. The RNAi depletion of Methoprene-tolerant (Met) and ecdysone receptor (EcR), receptors for JH and 20E, respectively, reversed the LM gene expression and the levels of lipid stores and metabolites, demonstrating the critical roles of these hormones in LM regulation. Hepatocyte nuclear factor 4 (HNF4) RNAi-silenced mosquitoes exhibited down-regulation of the gene transcripts encoding TAG catabolism and β-oxidation enzymes and an inability to use lipids effectively, as manifested by TAG accumulation. The luciferase reporter assay showed direct regulation of LM-related genes by HNF4. Moreover, HNF4 gene expression was down-regulated by Met and activated by EcR and Target of rapamycin, providing a link between nutritional and hormonal regulation of LM in female mosquitoes.

Stress responses sculpt the insect immune system, optimizing defense in an ever-changing world

A whole organism, network approach can help explain the adaptive purpose of stress-induced changes in immune function. In insects, mediators of the stress response (e.g. stress hormones) divert molecular resources away from immune function and towards tissues necessary for fight-or-flight behaviours. For example, molecules such as lipid transport proteins are involved in both the stress and immune responses, leading to a reduction in disease resistance when these proteins are shifted towards being part of the stress response system. Stress responses also alter immune system strategies (i.e. reconfiguration) to compensate for resource losses that occur during fight-or flight events. In addition, stress responses optimize immune function for different physiological conditions. In insects, the stress response induces a pro-inflammatory state that probably enhances early immune responses.

Plasmodium Oocysts: Overlooked Targets of Mosquito Immunity

Although the ability of mosquitoes to limit Plasmodium infection is well documented, many questions remain as to how malaria parasites are recognized and killed by the mosquito host. Recent evidence suggests that anti-Plasmodium immunity is multimodal, with different immune mechanisms regulating ookinete and oocyst survival. However, most experiments determine the number of mature oocysts, without considering that different immune mechanisms may target different developmental stages of the parasite. Complement-like proteins have emerged as important determinants of early immunity targeting the ookinete stage, yet the mechanisms by which the mosquito late-phase immune response limits oocyst survival are less understood. Here, we describe the known components of the mosquito immune system that limit oocyst development, and provide insight into their possible mechanisms of action.

Serpins in arthropod biology

Serpins are the largest known family of serine proteinase inhibitors and perform a variety of physiological functions in arthropods. Herein, we review the field of serpins in arthropod biology, providing an overview of current knowledge and topics of interest. Serpins regulate insect innate immunity via inhibition of serine proteinase cascades that initiate immune responses such as melanization and antimicrobial peptide production. In addition, several serpins with anti-pathogen activity are expressed as acute-phase serpins in insects upon infection. Parasitoid wasps can downregulate host serpin expression to modulate the host immune system. In addition, examples of serpin activity in development and reproduction in Drosophila have also been discovered. Serpins also function in host-pathogen interactions beyond immunity as constituents of venom in parasitoid wasps and saliva of blood-feeding ticks and mosquitoes. These serpins have distinct effects on immunosuppression and anticoagulation and are of interest for vaccine development. Lastly, the known structures of arthropod serpins are discussed, which represent the serpin inhibitory mechanism and provide a detailed overview of the process.

Larval food quantity affects the capacity of adult mosquitoes to transmit human malaria

Adult traits of holometabolous insects are shaped by conditions experienced during larval development, which might impact interactions between adult insect hosts and parasites. However, the ecology of larval insects that vector disease remains poorly understood. Here, we used Anopheles stephensi mosquitoes and the human malaria parasite Plasmodium falciparum, to investigate whether larval conditions affect the capacity of adult mosquitoes to transmit malaria. We reared larvae in two groups; one group received a standard laboratory rearing diet, whereas the other received a reduced diet. Emerging adult females were then provided an infectious blood meal. We assessed mosquito longevity, parasite development rate and prevalence of infectious mosquitoes over time. Reduced larval food led to increased adult mortality and caused a delay in parasite development and a slowing in the rate at which parasites invaded the mosquito salivary glands, extending the time it took for mosquitoes to become infectious. Together, these effects increased transmission potential of mosquitoes in the high food regime by 260-330%. Such effects have not, to our knowledge, been shown previously for human malaria and highlight the importance of improving knowledge of larval ecology to better understand vector-borne disease transmission dynamics.

A Critical Role for CLSP2 in the Modulation of Antifungal Immune Response in Mosquitoes

Entomopathogenic fungi represent a promising class of bio-insecticides for mosquito control. Thus, detailed knowledge of the molecular mechanisms governing anti-fungal immune response in mosquitoes is essential. In this study, we show that CLSP2 is a modulator of immune responses during anti-fungal infection in the mosquito Aedes aegypti. With a fungal infection, the expression of the CLSP2 gene is elevated. CLSP2 is cleaved upon challenge with Beauveria bassiana conidia, and the liberated CLSP2 CTL-type domain binds to fungal cell components and B. bassiana conidia. Furthermore, CLPS2 RNA interference silencing significantly increases the resistance to the fungal challenge. RNA-sequencing transcriptome analysis showed that the majority of immune genes were highly upregulated in the CLSP2-depleted mosquitoes infected with the fungus. The up-regulated immune gene cohorts belong to melanization and Toll pathways, but not to the IMD or JAK-STAT. A thioester-containing protein (TEP22), a member of α2-macroglobulin family, has been implicated in the CLSP2-modulated mosquito antifungal defense. Our study has contributed to a greater understanding of immune-modulating mechanisms in mosquitoes.

Analogs of LDL Receptor Ligand Motifs in Dengue Envelope and Capsid Proteins as Potential Codes for Cell Entry

It is established that cell entry of low density lipoprotein particles (LLPs) containing Apo B100 and Apo E is mediated by receptors and GAGs. Receptor ligand motifs, XBBBXXBX, XBBXBX, and ΨBΨXB, and mono- and bipartite NLS sequences are abundant in Apo E and Apo B100 as well as in envelope and capsid proteins of Dengue viruses 1-4 (DENV1-4). Synthetic, fluorescence-labeled peptides of sequences in DENV2 envelope protein, and DENV3 capsid that include these motifs were used to conduct a qualitative assessment of cell binding and entry capacity using HeLa cells. DENV2 envelope peptide, Dsp2EP, ⁰⁵⁶⁴Gly-Gly⁰⁵⁹⁵, was shown to bind and remain at the cell surface. In contrast, DENV3 capsid protein peptide, Dsp3CP, ⁰⁰⁰²Asn-Gln⁰⁰²⁸, readily enters HeLa cells and accumulates at discrete loci in the nucleus. FITC-labeled dengue synthetic peptides colocalize with Low Density Lipoprotein-CM-DiI and Apo E-CM-DiI to a degree that suggests that Dengue viruses may utilize cell entry pathways used by LLPs.

MicroRNAs as mediators of insect host-pathogen interactions and immunity

Insects are the most successful group of animals on earth, owing this partly to their very effective immune responses to microbial invasion. These responses mainly include cellular and humoral responses as well as RNA interference (RNAi). Small non-coding RNAs (snRNAs) produced through RNAi are important molecules in the regulation of gene expression in almost all living organisms; contributing to important processes such as development, differentiation, immunity as well as host-microorganism interactions. The main snRNAs produced by the RNAi response include short interfering RNAs, microRNAs and piwi-interacting RNAs. In addition to the host snRNAs, some microorganisms encode snRNAs that affect the dynamics of host-pathogen interactions. In this review, we will discuss the latest developments in regards to the role of microRNA in insect host-pathogen interactions and provide some insights into this rapidly developing area of research.

Role of microRNAs in arbovirus/vector interactions

The role of microRNAs (miRNAs) as small non-coding RNAs in regulation of gene expression has been recognized. They appear to be involved in regulation of a wide range of cellular pathways that affect several biological processes such as development, the immune system, survival, metabolism and host-pathogen interactions. Arthropod-borne viruses impose great economic and health risks around the world. Recent advances in miRNA biology have shed some light on the role of these small RNAs in vector-virus interactions. In this review, I will reflect on our current knowledge on the role of miRNAs in arbovirus-vector interactions and the potential avenues for their utilization in limiting virus replication and/or transmission.

Effects of infection by Trypanosoma cruzi and Trypanosoma rangeli on the reproductive performance of the vector Rhodnius prolixus

The insect Rhodnius prolixus is responsible for the transmission of Trypanosoma cruzi, which is the etiological agent of Chagas disease in areas of Central and South America. Besides this, it can be infected by other trypanosomes such as Trypanosoma rangeli. The effects of these parasites on vectors are poorly understood and are often controversial so here we focussed on possible negative effects of these parasites on the reproductive performance of R. prolixus, specifically comparing infected and uninfected couples. While T. cruzi infection did not delay pre-oviposition time of infected couples at either temperature tested (25 and 30°C) it did, at 25°C, increase the e-value in the second reproductive cycle, as well as hatching rates. Meanwhile, at 30°C, T. cruzi infection decreased the e-value of insects during the first cycle and also the fertility of older insects. When couples were instead infected with T. rangeli, pre-oviposition time was delayed, while reductions in the e-value and hatching rate were observed in the second and third cycles. We conclude that both T. cruzi and T. rangeli can impair reproductive performance of R. prolixus, although for T. cruzi, this is dependent on rearing temperature and insect age. We discuss these reproductive costs in terms of potential consequences on triatomine behavior and survival.

Lipophorins can adhere to dsRNA, bacteria and fungi present in the hemolymph of the desert locust: a role as general scavenger for pathogens in the open body cavity

Desert locusts are characterized by a highly sensitive and effective RNA interference (RNAi) response. Moreover, delivery of dsRNA into the open body cavity will elicit potent silencing effects throughout the body. On the other hand, many other insect species, such as Bombyx mori and Drosophila melanogaster, lack the ability to efficiently spread the RNAi-signal. In this study, we demonstrated that, in the serum of the desert locust, lipophorins adhere to dsRNA-fragments. Lipophorins can be subdivided into high density and low density lipophorins (HDLp and LDLp), according to their buoyant density, and we showed that both types of lipophorins can interact with dsRNA fragments. Furthermore, in the presence of (gram-positive) bacteria or fungi, LDLp induce the formation of pathogen aggregates, while no clear aggregation effects were detected in the presence of HDLp.