Eicosanoids up-regulate production of reactive oxygen species by NADPH-dependent oxidase in Spodoptera exigua phagocytic hemocytes

Apolipoprotein D3 and LOX product play a role in immune-priming of a lepidopteran insect, Spodoptera exigua

Immune-priming occurs in insects after a prior pathogen exposure. However, its underlying mechanism in insects remains elusive. In the present work, immune-priming was detected in a lepidopteran insect, Spodoptera exigua. Specifically, a prior infection with a heat-killed pathogenic bacterium, Escherichia coli, led to increased survival upon the second infection of different pathogens. Plasma collected from larvae with the prior infection possessed the immune-priming factor(s) that significantly up-regulated cellular and humoral immune responses of naïve larvae. Our study also finds that variations in the timing of plasma collection for priming larvae resulted in distinct impacts on both cellular and humoral responses. However, when the active plasma exhibiting the immune-priming was heat-treated, it lost this priming activity, therefore suggesting that protein factor(s) play a role in this immune-priming. An immunofluorescence assay showed that the hemocytes collected from the immune-primed larvae highly reacted to a polyclonal antibody specific to a vertebrate lipocalin, apolipoprotein D (ApoD). Among 27 ApoD genes (Se-ApoD1 ∼ Se-ApoD27) of S. exigua, Se-ApoD3 was found to be highly induced during the immune-priming, in which it was shown to be expressed in hemocytes and fat body from a fluorescence in situ hybridization analysis. RNA interference of Se-ApoD3 expression significantly impaired the immune-priming of S. exigua larvae. Moreover, the inhibition of eicosanoid biosynthesis suppressed the immune-priming, in which treatment with a lipoxygenase (LOX) inhibitor-and not treatment with a cyclooxygenase inhibitor-suppressed immune-priming. Further, an addition of LOX product such as lipoxin A4 or lipoxin B4 significantly rescued the lost immune-priming activity. Taken together, these results suggest that a complex of ApoD3 and LOX product mediates the immune-priming activity of S. exigua.

A PLA2 deletion mutant using CRISPR/Cas9 coupled to RNASeq reveals insect immune genes associated with eicosanoid signaling

Eicosanoids mediate insect immune responses and synthesized by the catalytic activity of phospholipase A2 (PLA2). A uniquely encoded secretory PLA2 (sPLA2) is associated with immune responses of a lepidopteran insect, Spodoptera exigua. Its deletion mutant was generated using a CRISPR/Cas9 genome editing technology. Both wild and mutant lines were then immune-challenged, and the resulting transcripts were compared with their naïve transcripts by RNASeq using the Illumina-HiSeq platform. In total, 12,878 unigenes were further analyzed by differentially expressed gene tools. Over 69% of the expressed genes in S. exigua larvae are modulated in their expression levels by eicosanoids, recorded from CRISPR/Cas9 mutagenesis against an eicosanoid-synthetic gene, Se-sPLA2. Further, about 36% of the immune-associated genes are controlled by the eicosanoids in S. exigua. Indeed, the deletion mutant suffered significant immunosuppression in both cellular and humoral responses in response to bacterial challenge as well as severely reduced developmental and reproductive potentials.

Comprehensive review on glucose 6 phosphate dehydrogenase: A critical immunometabolic and redox switch in insects

Mounting an active immune response is energy intensive and demands the reallocation of nutrients to maintain the body's resistance and tolerance against infections. Central to this metabolic adaptation is Glucose-6-phosphate dehydrogenase (G6PDH), a housekeeping enzyme involve in pentose phosphate pathway (PPP). PPP play an essential role in generating ribose, which is critical for nicotinamide adenine dinucleotide phosphate (NADPH). It is vital for physiological and cellular processes such as generating nucleotides, fatty acids and reducing oxidative stress. The G6PDH is extremely conserved enzyme across species in PP shunt. The deficiency of enzymes leads to serious consequences on organism, particularly on adaptation and development. Acute deficiency can lead to impaired cell development, halted embryonic growth, reduce sensitivity to insulin, hypertension and increase inflammation. Historically, research focusing on G6PDH and PPP have primarily targeted diseases on mammalian. However, our review has investigated the unique functions of the G6PDH enzyme in insects and greatly improved mechanistic understanding of its operations. This review explore how G6PDH in insects plays a crucial role in managing the redox balance and immune related metabolism. This study aims to investigate the enzyme's role in different metabolic adaptations.

Insight into the regulation of the Nrf2 pathway in response to ascovirus infection in Spodoptera exigua

BACKGROUND

Ascoviruses are a type of entomopathogenic microorganism with high biological pest control potential and are expected to contribute to the natural control of lepidopteran pests. However, knowledge of the molecular mechanism underlying the biocidal activity of ascovirus on its host insects remains limited.

RESULTS

In this study, the relative enzyme activity of superoxide dismutase and peroxidase, as well as the expression level of Spodoptera exigua peroxidase (SePOD), were found to be significantly increased at 6 h post infection with Heliothis virescens ascovirus 3h (HvAV-3h). H₂ O₂ accumulation and enhanced expression of NADPH Oxidase (SeNOX) were also observed. In addition, Nuclear Factor erythroid 2-Related Factor 2 (SeNrf2) and muscle aponeurosis fibromatosis (SeMaf) were overexpressed following infection with HvAV-3h. Silencing of SeNrf2 decreased the expression of SePOD, whereas the mortality of SeNrf2-silenced larvae and viral genome copy number also increased. Further RNA interference of SeNOX significantly decreased expression of SeNrf2 and SePOD and therefore increased the mortality and viral genome copy number of the ascovirus-infected host.

CONCLUSION

The HvAV-3h activated Nrf2/ARE pathway of S. exigua and reactive oxygen species were found to respond to ascovirus infection by regulating alterations in antioxidant enzyme genes mediated by the host Nrf2/ARE pathway. These findings enhance our knowledge of ascovirus-host interactions and lay the foundation for the application of ascoviruses in biological pest control. © 2022 Society of Chemical Industry.

Cannabidiol (CBD) Supports the Honeybee Worker Organism by Activating the Antioxidant System

In the experiment, we tested the effect of 30% CBD oil on the activity of the antioxidant system (superoxide dismutase, catalase, glutathione peroxidase, glutathione), the level of total antioxidant capacity, and the concentrations of ions (calcium, magnesium, and phosphorus) in honeybee workers in the hive test. For this purpose, we prepared hives containing all stages of the development of honey bees and started the experiment by adding 200 marked, one-day old bees to each colony (intended for hemolymph collection). In the test, we created three groups (two colonies per group): (1) Experimental with CBD oil mixed with sugar syrup (CSy); (2) experimental with CBD oil on textile strips (CSt); and (3) control with pure sugar syrup only (C). Every week, we collected hemolymph from the marked bees. In the experiment, all antioxidant enzyme activities were higher for the experimental groups CSy and CSt compared to group C. The highest concentrations/levels were obtained for the CSy group. Concentrations of calcium, magnesium, and phosphorus ions were also higher for the experimental groups compared to the C group (the highest concentration for the CSy group). We conclude that CBD oil positively contributes to stimulating the antioxidant system of honeybees.

Repat33 Acts as a Downstream Component of Eicosanoid Signaling Pathway Mediating Immune Responses of Spodoptera exigua, a Lepidopteran Insect

Repat (=response to pathogen) is proposed for an immune-associated gene family from Spodoptera exigua, a lepidopteran insect. In this gene family, 46 members (Repat1-Repat46) have been identified. They show marked variations in their inducible expression patterns in response to infections by different microbial pathogens. However, their physiological functions in specific immune responses and their interactions with other immune signaling pathways remain unclear. Repat33 is a gene highly inducible by bacterial infections. The objective of this study was to analyze the physiological functions of Repat33 in mediating cellular and humoral immune responses. Results showed that Repat33 was expressed in all developmental stages and induced in immune-associated tissues such as hemocytes and the fat body. RNA interference (RNAi) of Repat33 expression inhibited the hemocyte-spreading behavior which impaired nodule formation of hemocytes against bacterial infections. Such RNAi treatment also down-regulated expression levels of some antimicrobial genes. Interestingly, Repat33 expression was controlled by eicosanoids. Inhibition of eicosanoid biosynthesis by RNAi against a phospholipase A₂ (PLA₂) gene suppressed Repat33 expression while an addition of arachidonic acid (a catalytic product of PLA₂) to RNAi treatment recovered such suppression of Repat33 expression. These results suggest that Repat33 is a downstream component of eicosanoids in mediating immune responses of S. exigua.

The type of blood used to feed Aedes aegypti females affects their cuticular and internal free fatty acid (FFA) profiles

Aedes aegypti, the primary vector of various arthropod-borne viral (arboviral) diseases such as dengue and Zika, is a popular laboratory model in vector biology. However, its maintenance in laboratory conditions is difficult, mostly because the females require blood meals to complete oogenesis, which is often provided as sheep blood. The outermost layer of the mosquito cuticle is consists of lipids which protects against numerous entomopathogens, prevents desiccation and plays an essential role in signalling processes. The aim of this work was to determine how the replacement of human blood with sheep blood affects the cuticular and internal FFA profiles of mosquitoes reared in laboratory culture. The individual FFAs present in cuticular and internal extracts from mosquito were identified and quantified by GC-MS method. The normality of their distribution was checked using the Kolmogorov-Smirnov test and the Student's t-test was used to compare them. GC-MS analysis revealed similar numbers of internal and cuticular FFAs in the female mosquitoes fed sheep blood by membrane (MFSB) and naturally fed human blood (NFHB), however MFSB group demonstrated 3.1 times greater FFA concentrations in the cuticular fraction and 1.4 times the internal fraction than the NFHB group. In the MFSB group, FFA concentration was 1.6 times higher in the cuticular than the internal fraction, while for NFHB, FFA concentration was 1.3 times lower in the cuticular than the internal fraction. The concentration of C18:3 acid was 223 times higher in the internal fraction than the cuticle in the MHSB group but was absent in the NFHB group. MFSB mosquito demonstrate different FFA profiles to wild mosquitoes, which might influence their fertility and the results of vital processes studied under laboratory conditions. The membrane method of feeding mosquitoes is popular, but our research indicates significant differences in the FFA profiles of MFSB and NFHB. Such changes in FFA profile might influence female fertility, as well as other vital processes studied in laboratory conditions, such as the response to pesticides. Our work indicates that sheep blood has potential shortcomings as a substitute feed for human blood, as its use in laboratory studies may yield different results to those demonstrated by free-living mosquitoes.

Autophagy: a necessary defense against extreme cadmium intoxication in a multigenerational 2D experiment

Autophagy is a natural process that aims to eliminate malfunctioning cell parts, organelles or molecules under physiological conditions. It is also induced in response to infection, starvation or oxidative stress to provide energy in case of an energy deficit. The aim of this 2-dimensional study was to test if, and if so, how, this process depends on the concentration of cadmium in food (with Cd concentrations from 0 to 352 μg of Cd per g of food (dry weight)-D1 dimension) and the history of selection pressure (160 vs 20 generations of exposure to Cd-D2 dimension). For the study, the 5th instar larvae of a unique strain of the moth Spodoptera exigua that was selected for cadmium tolerance for 160 generations (44 μg of Cd per g of food (dry weight)), as well as 20-generation (11, 22 and 44 μg of Cd per g of food (dry weight)) and control strains, were used. Autophagy intensity was measured by means of flow cytometry and compared with life history parameters: survivability and duration of the 3rd larval stage. The highest values of autophagy markers were found in the groups exposed to the highest Cd concentration and corresponded (with a significant correlation coefficient) to an increased development duration or decreased survivorship in the respective groups. In conclusion, autophagy is probably initiated only if any other defense mechanisms, e.g., antioxidative mechanisms, are not efficient. Moreover, in individuals from pre-exposed populations, the intensity of autophagy is lower.

PGE2 upregulates gene expression of dual oxidase in a lepidopteran insect midgut via cAMP signalling pathway

In insect midgut, prostaglandins (PGs) play a crucial role in defending bacterial and malarial pathogens. However, little is known about the PG signalling pathway in the midgut. A dual oxidase (Se-Duox) with presumed function of catalysing reactive oxygen species (ROS) production in the midgut was identified in beet armyworm, Spodoptera exigua. Se-Duox was expressed in all developmental stages, exhibiting relatively high expression levels in the midgut of late larval instars. Se-Duox expression was upregulated upon bacterial challenge. RNA interference (RNAi) of Se-Duox expression significantly suppressed ROS levels in the midgut lumen. The suppression of ROS levels increased insecticidal activity of Serratia marcescens after oral infection. Interestingly, treatment with a PLA₂ inhibitor prevented the induction of Se-Duox expression in response to bacterial challenge. On the other hand, addition of its catalytic product rescued the induction of Se-Duox expression. Especially, PG synthesis inhibitor significantly suppressed Se-Duox expression, while the addition of PGE₂ or PGD₂ rescued the inhibition. Subsequent PG signals involved cAMP and downstream components because specific inhibitors of cAMP signal components such as adenylate cyclase (AC) and protein kinase A (PKA) significantly inhibited Se-Duox expression. Indeed, addition of a cAMP analogue stimulated Se-Duox expression in the midgut. Furthermore, individual RNAi specific to PGE₂ receptor (a trimeric G-protein subunit), AC, PKA or cAMP-responsive element-binding protein resulted in suppression of Se-Duox expression. These results suggest that PGs can activate midgut immunity via cAMP signalling pathway by inducing Se-Duox expression along with increased ROS levels.

Prostaglandins influence protein phosphorylation in established insect cell line

Prostaglandins (PGs) are oxygenated metabolites of arachidonic acid and two other C20 polyunsaturated fatty acids. Among other actions in invertebrates, PGs act in ovarian development, renal functions, immunity, hemocyte migration, and gene/protein expression. Reversible phosphorylation is a major mechanism of regulating protein functions in eukaryotic cells and for some mammalian proteins it is influenced by PGs. We posed the hypothesis that PGs influence protein phosphorylation within insect cells, which we tested with the established insect cell line, BCIRL-HzAM1. After 20, 30, or 40 min incubations in the presence of one of three PGs (at 15 μM), PGA₂ , PGE₁ , or PGF_2α , separate sets of cells were processed for analysis by two-dimensional electrophoresis followed by tandem mass spectrometry. We recorded significant phosphorylation changes in 31 proteins, decreases in 15, and increases in 15, and one protein with increased or decreased phosphorylation, depending on PG treatment. Increasing PG exposure times led to changes in fewer proteins, 20 min incubations led to changes in 16 proteins, 30 min to changes in 13, and 40 min to changes in 2 proteins. The proteins were identified by bioinformatic analyses, including transcript description, calculated molecular weights and isoelectric points, MOlecular Weight SEarch score, total ion score, numbers of peptides, percent protein coverage, E-value, and highest peptide score. The data presented in this paper firmly support our hypothesis that PGs influence protein phosphorylation within insect cells and adds a novel PG-signaled function to insect biology.

Biosynthesis and immunity of epoxyeicosatrienoic acids in a lepidopteran insect, Spodoptera exigua

Eicosanoids mediate both cellular and humoral immune responses in insects. Epoxyeicosatrienoic acids (EETs) are a group of eicosanoids containing epoxide formed by epoxygenase (EPX) activity of cytochrome P450 (CYP). Although EETs have been considered to mediate immune responses in some insects, their synthetic machinery was little understood in insects. This study monitored EETs in a lepidopteran insect, Spodoptera exigua, immunized with bacteria and found all four EETs (5,6-EET, 8,9-EET, 11,12-EET, and 14,15-EET) from larval fat body at 247-1,736 pg/g levels. Then to predict EPXs, 140 CYPs were collected from S. exigua transcriptomes and compared with human EPXs. Four CYPs (SeEPX1-SeEPX4) sharing homologies with human EPXs were chosen and assessed in subsequent expression and functional analyses. All four EPXs were expressed in all development stages. In larval stage, all four EPXs were expressed in immune-associated tissues such as fat body and hemocytes. Furthermore, their expression levels were highly enhanced by bacterial challenge in different tissues. RNA interference (RNAi) using gene-specific double stranded RNA injection suppressed their expression levels by more than 55%. RNAi treatments interfered with hemocyte-spreading behavior and nodule formation upon bacterial challenge except RNAi treatment against SeEPX2. All four EETs stimulated cellular immune response measured by nodule formation in S. exigua. The suppressed immune responses by the RNAi treatments against three SeEPXs were rescued by the addition of 8,9-EET. However, other three EETs gave their specific rescue effect depending on SeEPX types under RNAi. In humoral immune response, all four RNAi treatments suppressed expression of antimicrobial peptide genes. This study reports the presence of all four EETs in larval fat body of S. exigua and suggests that four SeEPXs are associated with immune responses mediated by EETs.

Dual Oxidase-Derived Reactive Oxygen Species Against Bacillus thuringiensis and Its Suppression by Eicosanoid Biosynthesis Inhibitors

Two entomopathogenic bacteria, Xenorhabdus and Photorhabdus, are known to be able to synthesize and secrete eicosanoid biosynthesis inhibitors (EIBs) that can enhance pathogenicity of Bacillus thuringiensis (Bt) against different target insects. Such enhancements can be explained by the suppression of immune responses in the hemocoel by EIBs. However, little is known about the role of EIBs in the defense against Bt pathogenicity in the gut. This study was focused on the role of insect gut immunity in the defense against Bt pathogenicity, in which the cooperative effect of bacterial metabolites was assessed. Screening 14 different bacterial strains, bacterial culture broth of Photorhabdus temperata subsp. temperata ANU101 (Ptt) gave the highest cooperative effect on Bt virulence along with significant inhibitory activity against phospholipase A₂ (PLA₂) of Plutella xylostella. In gut lumen, Ptt culture broth suppressed the generation of reactive oxygen species (ROS) induced by Bt treatment and facilitated bacterial growth, similar to vitamin E, an antioxidant. To analyze the ROS source, dual oxidase (Px-Duox) and NADPH-dependent oxidase (Px-Nox) genes were predicted from P. xylostella genome and their expressions were confirmed in larval gut. RNA interference (RNAi) of Px-Duox expression reduced ROS levels in both gut epithelium and lumen while RNAi of Px-Nox expression reduced ROS levels only in gut epithelium. Ptt extract significantly suppressed gene expression levels of Px-Duox and Px-Nox, leading to lower ROS concentrations in the gut lumen. Three commercial PLA₂ inhibitors significantly increased the insecticidal activity of Bt by suppressing ROS levels in the gut lumen. These results indicate that Ptt extract containing EBIs can prevent up-regulation of ROS level in the midgut in response to Bt infection and enhance the virulence of Bt against P. xylostella.

Inhibitors of Eicosanoid Biosynthesis Reveal that Multiple Lipid Signaling Pathways Influence Malaria Parasite Survival in Anopheles gambiae

Eicosanoids are bioactive signaling lipids derived from the oxidation of fatty acids that act as important regulators of immune homeostasis and inflammation. As a result, effective anti-inflammatory drugs have been widely used to reduce pain and inflammation which target key eicosanoid biosynthesis enzymes. Conserved from vertebrates to insects, the use of these eicosanoid pathway inhibitors offer opportunities to evaluate the roles of eicosanoids in less-characterized insect systems. In this study, we examine the potential roles of eicosanoids on malaria parasite survival in the mosquito Anopheles gambiae. Using Plasmodium oocyst numbers to evaluate parasite infection, general or specific inhibitors of eicosanoid biosynthesis pathways were evaluated. Following the administration of dexamethasone and indomethacin, respective inhibitors of phospholipid A2 (PLA2) and cyclooxygenase (COX), oocyst numbers were unaffected. However, inhibition of lipoxygenase (LOX) activity through the use of esculetin significantly increased oocyst survival. In contrast, 12-[[(tricyclo[3.3.1.13,7]dec-1-ylamino)carbonyl]amino]-dodecanoic acid (AUDA), an inhibitor of epoxide hydroxylase (EH), decreased oocyst numbers. These experiments were further validated through RNAi experiments to silence candidate genes homologous to EH in An. gambiae to confirm their contributions to Plasmodium development. Similar to the results of AUDA treatment, the silencing of EH significantly reduced oocyst numbers. These results imply that specific eicosanoids in An. gambiae can have either agonist or antagonistic roles on malaria parasite survival in the mosquito host.

Aedes aegypti HPX8C modulates immune responses against viral infection

Mosquitoes act as vectors of numerous pathogens that cause human diseases. Dengue virus (DENV) transmitted by mosquito, Aedes aegypti, is responsible for dengue fever epidemics worldwide with a serious impact on human health. Currently, disease control mainly relies on vector targeted intervention strategies. Therefore, it is imperative to understand the molecular mechanisms underlying the innate immune response of mosquitoes against pathogens. In the present study, the expression profiles of immunity-related genes in the midgut responding to DENV infection by feeding were analyzed by transcriptome and quantitative real-time PCR. The level of Antimicrobial peptides (AMPs) increased seven days post-infection (d.p.i.), which could be induced by the Toll immune pathway. The expression of reactive oxygen species (ROS) genes, including antioxidant genes, such as HPX7, HPX8A, HPX8B, HPX8C were induced at one d.p.i. and peaked again at ten d.p.i. in the midgut. Interestingly, down-regulation of the antioxidant gene HPX8C by RNA interference led to reduction in the virus titer in the mosquito, probably due to the elevated levels of ROS. Application of a ROS inhibitor and scavenger molecules further established the role of oxygen free radicals in the modulation of the immune response to DENV infection. Overall, our comparative transcriptome analyses provide valuable information about the regulation of immunity related genes in the transmission vector in response to DENV infection. It further allows us to identify novel molecular mechanisms underlying the host-virus interaction, which might aid in the development of novel strategies to control mosquito-borne diseases.

HPX8C is a heme-containing peroxidase, which can move reactive oxygen species (ROS) damage to the organism by reducing H₂O₂ to H₂O. Previously, the peroxidase gene has been shown to modulate midgut immunity and regulate anti-malarial response in mosquitoes. In this study, the classical immune signaling pathways, Toll and IMD genes might be late responses against the viruses. HPX8C was demonstrated here to play a role in antiviral immunity against DENV infection in Ae. Aegypti mosquitoes. HPX8C expression was induced by DENV infection and continued to increase with an elevated virus titer. In HPX8C-depleted mosquitoes, the ROS level was found to be increased with a corresponding decrease in the DENV and ZIKV virus titer. Therefore, it was speculated that HPX8C mediated immune responses against the DENV in the mosquito in the late stage of viral infection, which could be controlled by Toll pathway.

Prostaglandins and Other Eicosanoids in Insects: Biosynthesis and Biological Actions

This essay reviews the discoveries, synthesis, and biological significance of prostaglandins (PGs) and other eicosanoids in insect biology. It presents the most current - and growing - understanding of the insect mechanism of PG biosynthesis, provides an updated treatment of known insect phospholipase A2 (PLA2), and details contemporary findings on the biological roles of PGs and other eicosanoids in insect physiology, including reproduction, fluid secretion, hormone actions in fat body, immunity and eicosanoid signaling and cross-talk in immunity. It completes the essay with a prospectus meant to illuminate research opportunities for interested readers. In more detail, cellular and secretory types of PLA2, similar to those known on the biomedical background, have been identified in insects and their roles in eicosanoid biosynthesis documented. It highlights recent findings showing that eicosanoid biosynthetic pathway in insects is not identical to the solidly established biomedical picture. The relatively low concentrations of arachidonic acid (AA) present in insect phospholipids (PLs) (< 0.1% in some species) indicate that PLA2 may hydrolyze linoleic acid (LA) as a precursor of eicosanoid biosynthesis. The free LA is desaturated and elongated into AA. Unlike vertebrates, AA is not oxidized by cyclooxygenase, but by a specific peroxidase called peroxinectin to produce PGH2, which is then isomerized into cell-specific PGs. In particular, PGE2 synthase recently identified converts PGH2 into PGE2. In the cross-talks with other immune mediators, eicosanoids act as downstream signals because any inhibition of eicosanoid signaling leads to significant immunosuppression. Because host immunosuppression favors pathogens and parasitoids, some entomopathogens evolved a PLA2 inhibitory strategy activity to express their virulence.

Eicosanoid-mediated immunity in insects

Eicosanoid is a collective term for oxygenated metabolites of C20 polyunsaturated fatty acids. As seen in mammals, eicosanoids play crucial roles in mediating various physiological processes, including immune responses, in insects. Upon microbial pathogen infection, non-self recognition signals are propagated to nearly immune effectors such as hemocytes and fat body using various immune mediators, in which eicosanoid signals act as the ultimate downstream mediator. The chemical diversity of eicosanoids may operate to mediate various immune responses. Some entomopathogenic bacteria suppress eicosanoid biosynthesis, which inhibits host insect immunity and promotes their pathogenicity. This review introduces immune responses mediated by various eicosanoids. Then it explains the cross-talks of eicosanoids with other immune mediators including cytokines, biogenic monoamines, and nitric oxide to clarify the complexity of insect immune mediation. Finally, we highlight the biological significance of eicosanoids by demonstrating bacterial pathogenicity inhibiting a key enzyme - phospholipase A₂ - in eicosanoid biosynthesis using their secondary metabolites to defend host insect immune attack.

Prostaglandin-mediated recovery from bacteremia delays larval development in fall armyworm, Spodoptera frugiperda

Insect immunity includes a surveillance system that detects and signals infections, coupled with hemocytic and humoral immune functions. These functions are signaled and coordinated by several biochemicals, including biogenic amines, insect cytokines, peptides, and prostaglandins (PGs). The actions of these mediators are coordinated within cells by various forms of cross-talk among the signaling systems and they result in effective reactions to infection. While this is well understood, we lack information on how immune-mediated recovery influences subsequent juvenile development in surviving insects. We investigated this point by posing the hypothesis that PG signaling is necessary for larval recovery, although the recovery imposes biological costs, registered in developmental delays and failures in surviving individuals. Here, we report that nodulation responses to infections by the bacterium, Serratia marcescens, increased over time up to 5 h postinfection, with no further nodulation; it increased in a linear manner with increasing bacterial dosages. Larval survivorship decreased with increasing bacterial doses. Treating larvae with the PG-biosynthesis inhibitor, indomethacin, led to sharply decreased nodulation reactions to infection, which were rescued in larvae cotreated with indomethacin and the PG-precursor, arachidonic acid. Although nodulation was fully rescued, all bacterial challenged larvae suffered reduced survivorship compared to controls. Bacterial infection led to reduced developmental rates in larvae, but not pupae. Adult emergence from pupae that developed from experimental larvae was also decreased. Taken together, our data potently bolster our hypothesis.

Emamectin benzoate induces ROS-mediated DNA damage and apoptosis in Trichoplusia Tn5B1-4 cells

Emamectin benzoate (EMB), a novel macrocyclic lactone insecticide, possesses high efficacy and beneficial selective toxicity in agriculture, but so far the EMB-induced cytotoxic action in arthropod insect remains unclear. The present studies were carried out to characterize the property of EMB on the induction of reactive oxygen species (ROS)-mediated DNA damage and apoptosis in Trichoplusia Tn5B1-4 cell model. Following the exposure to EMB at 2.5, 5, 10 or 15 μM, the cells changed to be round, suspended and aggregated, and the decline of cell proliferating ability and cell viability was positively related with the exposure time. Median inhibitory concentration (IC₅₀) of EMB on cell viability was 3.72 μM during 72 h exposure. Apoptosis was induced in 29.8% (24 h) and 39.5% (48 h) of the cells by EMB at 15 μM, showing chromatin condensation in nuclei. The content of ROS in the cells increased rapidly as the concentration of EMB increased, and the pre-incubation of the cells with vitamin E significantly reduced the ROS accumulation. In the treatment of 15 μM EMB, the migrated cell nucleus with DNA strand breaks appeared a teardrop, pear-shaped, or large fan-like tail, and 63.1% of γH2AX-positive cells contained more than four foci, accompanying with high expression level of caspase-3 in time-dependent manner, which consequently led to cell apoptotic death. These evidences in ROS-mediated DNA damage and cell apoptosis induced by EMB may be helpful for deep understanding the cytotoxic action of EMB based on cell model.

Evolutionary origin and function of NOX4-art, an arthropod specific NADPH oxidase

BACKGROUND

NADPH oxidases (NOX) are ROS producing enzymes that perform essential roles in cell physiology, including cell signaling and antimicrobial defense. This gene family is present in most eukaryotes, suggesting a common ancestor. To date, only a limited number of phylogenetic studies of metazoan NOXes have been performed, with few arthropod genes. In arthropods, only NOX5 and DUOX genes have been found and a gene called NOXm was found in mosquitoes but its origin and function has not been examined. In this study, we analyzed the evolution of this gene family in arthropods. A thorough search of genomes and transcriptomes was performed enabling us to browse most branches of arthropod phylogeny.

RESULTS

We have found that the subfamilies NOX5 and DUOX are present in all arthropod groups. We also show that a NOX gene, closely related to NOX4 and previously found only in mosquitoes (NOXm), can also be found in other taxonomic groups, leading us to rename it as NOX4-art. Although the accessory protein p22-phox, essential for NOX1-4 activation, was not found in any of the arthropods studied, NOX4-art of Aedes aegypti encodes an active protein that produces H₂O₂. Although NOX4-art has been lost in a number of arthropod lineages, it has all the domains and many signature residues and motifs necessary for ROS production and, when silenced, H₂O₂ production is considerably diminished in A. aegypti cells.

CONCLUSIONS

Combining all bioinformatic analyses and laboratory work we have reached interesting conclusions regarding arthropod NOX gene family evolution. NOX5 and DUOX are present in all arthropod lineages but it seems that a NOX2-like gene was lost in the ancestral lineage leading to Ecdysozoa. The NOX4-art gene originated from a NOX4-like ancestor and is functional. Although no p22-phox was observed in arthropods, there was no evidence of neo-functionalization and this gene probably produces H₂O₂ as in other metazoan NOX4 genes. Although functional and present in the genomes of many species, NOX4-art was lost in a number of arthropod lineages.

Rhodnius prolixus: from physiology by Wigglesworth to recent studies of immune system modulation by Trypanosoma cruzi and Trypanosoma rangeli

This review is dedicated to the memory of Professor Sir Vincent B. Wigglesworth (VW) in recognition of his many pioneering contributions to insect physiology which, even today, form the basis of modern-day research in this field. Insects not only make vital contributions to our everyday lives by their roles in pollination, balancing eco-systems and provision of honey and silk products, but they are also outstanding models for studying the pathogenicity of microorganisms and the functioning of innate immunity in humans. In this overview, the immune system of the triatomine bug, Rhodnius prolixus, is considered which is most appropriate to this dedication as this insect species was the favourite subject of VW's research. Herein are described recent developments in knowledge of the functioning of the R. prolixus immune system. Thus, the roles of the cellular defences, such as phagocytosis and nodule formation, as well as the role of eicosanoids, ecdysone, antimicrobial peptides, reactive oxygen and nitrogen radicals, and the gut microbiota in the immune response of R. prolixus are described. The details of many of these were unknown to VW although his work gives indications of his awareness of the importance to R. prolixus of cellular immunity, antibacterial activity, prophenoloxidase and the gut microbiota. This description of R. prolixus immunity forms a backdrop to studies on the interaction of the parasitic flagellates, Trypanosoma cruzi and Trypanosoma rangeli, with the host defences of this important insect vector. These parasites remarkably utilize different strategies to avoid/modulate the triatomine immune response in order to survive in the extremely hostile host environments present in the vector gut and haemocoel. Much recent information has also been gleaned on the remarkable diversity of the immune system in the R. prolixus gut and its interaction with trypanosome parasites. This new data is reviewed and gaps in our knowledge of R. prolixus immunity are identified as subjects for future endeavours. Finally, the publication of the T. cruzi, T. rangeli and R. prolixus genomes, together with the use of modern molecular techniques, should lead to the enhanced identification of the determinants of infection derived from both the vector and the parasites which, in turn, could form targets for new molecular-based control strategies.