Direct evidence of the cyclooxygenase pathway of prostaglandin synthesis in arthropods: genetic and biochemical characterization of two crustacean cyclooxygenases

The Importance of Including Variable Exposure Concentrations When Assessing Toxicity of Sediment-Associated Pharmaceuticals to an Amphipod

Pharmaceuticals have been classified as an environmental concern due to their increasing consumption globally and potential environmental impact. We examined the toxicity of sediment-associated diclofenac and citalopram administered as both single compounds and in a mixture to the sediment-living amphipod Corophium volutator. This laboratory-based study addressed the following research questions: (1) What is the toxicity of sediment-associated diclofenac and citalopram to C. volutator? (2) Can the mixture effect be described with either of the two mixture models: concentration addition (CA) or independent action (IA)? (3) What is the importance of the choice of (i) exposure measure (start concentration, time-weighted average [TWA], full exposure profile) and (ii) effect model (concentration-response vs. the toxicokinetic-toxicodynamic model general unified threshold model for survival in its reduced form [GUTS-RED]) for the derived effect concentration values? Diclofenac was more toxic than citalopram to C. volutator as a single compound (10-day exposure). Diclofenac exposure to C. volutator provided median lethal concentrations (LC50s) within the same range (11 µg g-1 dry wt sediment) using concentration-response based on TWA and both GUTS-RED models. However, concentration-response based on measured start concentrations provided an approximately 90% higher LC50 (21.6 ± 2.0 µg g-1 dry wt sediment). For citalopram, concentration-response parameters were similar regardless of model or concentration used (LC50 85-97 µg g-1 dry wt sediment), however, GUTS-RED with the assumption of individual tolerance resulted in a lower LC50 (64.9 [55.3-74.8] µg g-1 dry wt sediment). The mixture of diclofenac and citalopram followed the CA quite closely, whereas the result was synergistic when using the IA prediction. In summary, concentration-response based on TWA and GUTS-RED provided similar and reasonably good fits compared to the data set. The implications are that GUTS-RED will provide a more flexible model, which, in principle, can extend beyond the experimental period and make predictions based on variable exposure profiles (toxicity at different time frames and at different variable exposure scenarios) compared to concentration-response, which provides contaminant toxicity at one point in time. Environ Toxicol Chem 2024;43:1767-1777. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

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.

BmPxt1 mediated immune response by regulating PGE2 in silkworm, Bombyx mori

Prostaglandins (PGs) mediates the immune response of insects to multiple stimuli. Mammalian cyclooxygenase (COXs) is a key enzyme in the synthesis of PGs, and peroxinectin (Pxt) may have similar functions in some sequenced insect genomes. As a representative of Lepidoptera, the silkworm also contains PGs, but its synthetic pathway is not clear. We cloned a full-length cDNA encoding a Pxt, designated as BmPxt1, from silkworm. Sequence alignment analysis showed that the protein encoded by BmPxt1 has a conserved domain similar to Pxts, and its catalytic site is shared with the Pxt of Manduca sexta, which also produces PGs. The expression of BmPxt1 gene was the highest in the hemocytes and was induced by Nuclear Polyhedrosis Virus (NPV) challenge in the detected tissues. Moreover, we found that dsPxt1 treatment deficiency down-regulated BmPxt1 transcript levels and efficiently inhibiting hemocyte-spreading and nodule formation in silkworm. Hemocyte-spreading, nodule formation, phenoloxidase (PO) and AMP genes (attacin, defencin and moricin) were also inhibited by aspirin, a COX inhibitor. Treatment by PGE2 but not arachidonic acid (AA) rescued the immunosuppression; PGs concentrations was also inhibited by aspirin. PGE2, but not AA, treatment rescued the PGs concentrations. The COX inhibitor, aspirin, impaired the innate immune response including nodulation, encapsulation, and melanization in silkworm, while PGE2, but not arachidonic acid (AA), partially reversed these effects of aspirin. Recombinant BmsPxt1 significantly induced PO activation in larvae hemolymph, PGs concentrations and encapsulation of agarose beads. Injection of recombinant BmsPxt1 into larvae resulted in increased transcript levels of AMP genes. Our results confirmed that BmPxt1 was involved in the synthesis of PGs in the innate immune response of silkworm larvae, and provided new information for the role of BmsPxt1 secreted by silkworm in activating PO and antimicrobial peptides.

Prostaglandin E2 mediates chorion formation of the Asian tiger mosquito, Aedes albopictus, at late oogenesis

Chorion-i.e., the eggshell-is formed during the late stage of oogenesis by follicular epithelium in the ovary. Although the endocrine signal(s) driving choriogenesis remain unclear in mosquitoes, this process in other insects has been suspected to involve the mediation of prostaglandins (PGs). This study tested the role of PG in the choriogenesis of the Asian tiger mosquito, Aedes albopictus, and its influence on controlling the expressions of genes associated with chorion formation by a transcriptome analysis. An immunofluorescence assay showed that PGE2 is localised in follicular epithelium. With the treatment of aspirin, an inhibitor of PG biosynthesis, at mid oogenesis, the PGE2 signal disappeared in the follicular epithelium led to significantly inhibited chorion formation along with a malformed eggshell. Ovary transcriptomes were assessed by RNASeq at the mid and late ovarian developmental stages. Differentially expressed genes (DEGs) exhibiting more than twofold changes in expression levels included 297 genes at mid stage and 500 genes at late stage. These DEGs at these two developmental stages commonly included genes associated with egg and chorion proteins of Ae. albopictus. Most chorion-associated genes were clustered in the 168 Mb region on a chromosome and exhibited significantly induced expressions at both ovarian developmental stages. The inhibition of PG biosynthesis significantly suppressed the expression of the chorion-associated genes while the addition of PGE2 rescued the gene expressions and led to recovery of choriogenesis. These results suggest that PGE2 mediates the choriogenesis of Ae. albopictus.

Drosophila pain sensitization and modulation unveiled by a novel pain model and analgesic drugs

In mammals, pain is regulated by the combination of an ascending stimulating and descending inhibitory pain pathway. It remains an intriguing question whether such pain pathways are of ancient origin and conserved in invertebrates. Here we report a new Drosophila pain model and use it to elucidate the pain pathways present in flies. The model employs transgenic flies expressing the human capsaicin receptor TRPV1 in sensory nociceptor neurons, which innervate the whole fly body, including the mouth. Upon capsaicin sipping, the flies abruptly displayed pain-related behaviors such as running away, scurrying around, rubbing vigorously, and pulling at their mouth parts, suggesting that capsaicin stimulated nociceptors in the mouth via activating TRPV1. When reared on capsaicin-containing food, the animals died of starvation, demonstrating the degree of pain experienced. This death rate was reduced by treatment both with NSAIDs and gabapentin, analgesics that inhibit the sensitized ascending pain pathway, and with antidepressants, GABAergic agonists, and morphine, analgesics that strengthen the descending inhibitory pathway. Our results suggest Drosophila to possess intricate pain sensitization and modulation mechanisms similar to mammals, and we propose that this simple, non-invasive feeding assay has utility for high-throughput evaluation and screening of analgesic compounds.

PGE2 mediation of egg development in Western flower thrip, Frankliniella occidentalis

Eicosanoids mediate various insect physiological processes, including reproduction. Especially, the eicosanoid prostaglandin E₂  (PGE₂ ) is known to mediate oocyte development in some insects. The explosive reproductive potential of the Western flower thrips, Frankliniella occidentalis, damages various agricultural crops. However, little is known about the underlying physiological processes of egg development in this pest. This study found that treatment with aspirin (ASP) (a specific cyclooxygenase (COX) inhibitor) used to inhibit PGE₂ biosynthesis during ovarian development significantly suppressed the reproduction of female F. occidentalis. However, the addition of PGE₂ to ASP-treated females significantly rescued the suppressed reproduction. PGE₂ was detected in growing ovarian follicles in an immunofluorescence assay. The hypothetical biosynthetic machinery of PGE₂ was predicted from the F. occidentalis genome and included phospholipase A₂  (PLA₂ ), COX-like peroxidase (POX), and PGE₂ synthase (PGES). Three specific PLA₂ s were highly expressed in female adults during active oogenesis. Specific POX and PGES genes also showed high expression during active oogenesis. The adverse effect of ASP treatment on oogenesis was observed in follicle formation in the germarium where the follicle numbers in an ovariole were decreased, which resulted in hypotrophied ovaries. This impairment was rescued by the addition of PGE₂ . ASP treatment also significantly inhibited chorion formation and suppressed gene expression associated with choriogenesis, which included chorion protein, mucin, and yellow while it did not inhibit vitellogenin gene expression. However, the addition of PGE₂ induced the expression of the target genes suppressed by ASP treatment and rescued chorion formation. These results suggest that PGE₂ mediated ovarian development by affecting follicle formation and choriogenesis in F. occidentalis.

Aspirin Inhibition of Prostaglandin Synthesis Impairs Mosquito Egg Development

Several endocrine signals mediate mosquito egg development, including 20-hydroxyecdysone (20E). This study reports on prostaglandin E₂ (PGE₂) as an additional, but core, mediator of oogenesis in a human disease-vectoring mosquito, Aedes albopictus. Injection of aspirin (an inhibitor of cyclooxygenase (COX)) after blood-feeding (BF) inhibited oogenesis by preventing nurse cell dumping into a growing oocyte. The inhibitory effect was rescued by PGE₂ addition. PGE₂ was found to be rich in nurse cells and follicular epithelium after BF. RNA interference (RNAi) treatments of PG biosynthetic genes, including PLA₂ and two COX-like peroxidases, prevented egg development. Interestingly, 20E treatment significantly increased the expressions of PG biosynthetic genes, while the RNAi of Shade (which is a 20E biosynthetic gene) expression prevented inducible expressions after BF. Furthermore, RNAi treatments of PGE₂ receptor genes suppressed egg production, even under PGE₂. These results suggest that a signaling pathway of BF-20E-PGE₂ is required for early vitellogenesis in the mosquito.

Identification and function analysis of BmPxtA in the immune response regulated by PGE2 of silkworm, Bombyx mori

Prostaglandins (PGs) can mediate the immune response of insects to infection. Mammalian cyclooxygenase (COXs) is a key enzyme in the synthesis of PGs, and Pxt may be its homologous gene in some sequenced insect genomes. As a representative of Lepidoptera, the silkworm also contains PGs, but the biosynthetic source of PGs is still unclear. In this study, Sequence analysis showed that peroxinectin (BmPxtA) gene of silkworm was closely related to human COX gene, and its homologous protein had conserved domains corresponding to human COX. The expression of BmPxtA gene was the highest in the hemocytes and was induced by Nuclear Polyhedrosis Virus (NPV) challenge in the detected tissues. The quantitative polymerase chain reaction (qPCR) results showed that silencing BmPxtA mediated by RNA interference (RNAi) inhibited the expression of immune-related pathway genes, and specifically suppressed hemocyte-spreading and nodule formation in silkworm; Hemocyte-spreading and nodule formation were also inhibited by aspirin, a COX inhibitor. Treatment by PGE₂ but not arachidonic acid (AA) rescued the immunosuppression; PGs concentrations was also inhibited by aspirin. PGE₂, but not AA, treatment rescued the PGs concentrations. These results suggest that BmPxtA gene is associated with PG biosynthesis in silkworm and the immune response of silkworm was affected by regulating the concentrations of PGs.

Modulation of the PGE2-Mediated Pathway in the Eclosion Blocking Effect of Flumethrin and Terpenoid Subfraction Isolated from Artemesia nilagirica in Rhipicephalus annulatus

Prostaglandins are a group of important cell-signaling molecules involved in the regulation of ovarian maturation, oocyte development, egg laying and associated behaviors in invertebrates. However, the presence of prostaglandin E₂ (PGE₂), the key enzymes for PGE₂ biosynthesis and its interference by drugs were not investigated previously in the ovary of ticks. The present study was undertaken to assess the modulation of the PGE₂-mediated pathway in the eclosion blocking effect of flumethrin and terpenoid subfraction isolated from Artemisia nilagirica in Rhipicephalus annulatus ticks. The acaricidal activities and chemical profiling of the terpenoid subfraction were performed. The localization of the cyclooxygenase1 (COX1) and prostaglandin E synthase (PGES) enzymes and the quantification of PGE₂ in the ovaries of the ticks treated with methanol (control), flumethrin and terpenoid subfraction were also undertaken. In addition, the vitellogenin concentration in hemolymph was also assayed. Both flumethrin and the terpenoid subfraction of A. nilagirica elicited a concentration-dependent inhibition of fecundity and blocking of hatching of the eggs. The COX1 could not be detected in the ovaries of treated and control ticks, while there was no significant difference observed in the concentration of vitellogenin (Vg) in them. The presence of PGES in the oocytes of control ticks was confirmed while the immunoreactivities against PGES were absent in the vitellogenic oocytes of ticks treated with flumethrin and terpenoid subfraction. The levels of PGE₂ were below the detection limit in the ovaries of the flumethrin-treated ticks, while it was significantly lower in the ovaries of the terpenoid subfraction-treated ticks. Hence, the prostaglandin E synthase and PGE₂ were identified as very important mediators for the signaling pathway for ovarian maturation and oviposition in ticks. In addition, the key enzyme for prostaglandin biosynthesis, PGES and the receptors for PGE₂ can be exploited as potential drug targets for tick control. The detection of PGES by immunohistochemistry and quantification of PGE₂ by LC-MSMS can be employed as valuable tools for screening newer compounds for their eclosion blocking acaricidal effects.

Biochemical characterization of the cyclooxygenase enzyme in penaeid shrimp

Cyclooxygenase (COX) is a two-step enzyme that converts arachidonic acid into prostaglandin H₂, a labile intermediate used in the production of prostaglandin E₂ (PGE₂) and prostaglandin F_2α (PGF_2α). In vertebrates and corals, COX must be N-glycosylated on at least two asparagine residues in the N-(X)-S/T motif to be catalytically active. Although COX glycosylation requirement is well-characterized in many species, whether crustacean COXs require N-glycosylation for their enzymatic function have not been investigated. In this study, a 1,842-base pair cox gene was obtained from ovarian cDNA of the black tiger shrimp Penaeus monodon. Sequence analysis revealed that essential catalytic residues and putative catalytic domains of P. monodon COX (PmCOX) were well-conserved in relation to other vertebrate and crustacean COXs. Expression of PmCOX in 293T cells increased levels of secreted PGE₂ and PGF_2α up to 60- and 77-fold, respectively, compared to control cells. Incubation of purified PmCOX with endoglycosidase H, which cleaves oligosaccharides from N-linked glycoproteins, reduced the molecular mass of PmCOX. Similarly, addition of tunicamycin, which inhibits N-linked glycosylation, in PmCOX-expressing cells resulted in PmCOX protein with lower molecular mass than those obtained from untreated cells, suggesting that PmCOX was N-glycosylated. Three potential glycosylation sites of PmCOX were identified at N79, N170 and N424. Mutational analysis revealed that although all three residues were glycosylated, only mutations at N170 and N424 completely abolished catalytic function. Inhibition of COX activity by ibuprofen treatment also decreased the levels of PGE₂ in shrimp haemolymph. This study not only establishes the presence of the COX enzyme in penaeid shrimp, but also reveals that N-glycosylation sites are highly conserved and required for COX function in crustaceans.

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.

Eicosanoid Signaling in Insect Immunology: New Genes and Unresolved Issues

This paper is focused on eicosanoid signaling in insect immunology. We begin with eicosanoid biosynthesis through the actions of phospholipase A₂, responsible for hydrolyzing the C18 polyunsaturated fatty acid, linoleic acid (18:2n-6), from cellular phospholipids, which is subsequently converted into arachidonic acid (AA; 20:4n-6) via elongases and desaturases. The synthesized AA is then oxygenated into one of three groups of eicosanoids, prostaglandins (PGs), epoxyeicosatrienoic acids (EETs) and lipoxygenase products. We mark the distinction between mammalian cyclooxygenases and insect peroxynectins, both of which convert AA into PGs. One PG, PGI₂ (also called prostacyclin), is newly discovered in insects, as a negative regulator of immune reactions and a positive signal in juvenile development. Two new elements of insect PG biology are a PG dehydrogenase and a PG reductase, both of which enact necessary PG catabolism. EETs, which are produced from AA via cytochrome P450s, also act in immune signaling, acting as pro-inflammatory signals. Eicosanoids signal a wide range of cellular immune reactions to infections, invasions and wounding, including nodulation, cell spreading, hemocyte migration and releasing prophenoloxidase from oenocytoids, a class of lepidopteran hemocytes. We briefly review the relatively scant knowledge on insect PG receptors and note PGs also act in gut immunity and in humoral immunity. Detailed new information on PG actions in mosquito immunity against the malarial agent, Plasmodium berghei, has recently emerged and we treat this exciting new work. The new findings on eicosanoid actions in insect immunity have emerged from a very broad range of research at the genetic, cellular and organismal levels, all taking place at the international level.

Characterization of prostanoid pathway and the control of its activity by the eyestalk optic ganglion in the female giant freshwater prawn, Macrobrachium rosenbergii

The giant freshwater prawn, Macrobrachium rosenbergii, is an economically valuable species that are distributed throughout the Asia-Pacific region. With the natural population declining due to overfishing, aquaculture of this species is deemed necessary. Hence, it is essential to understand the mechanisms regulating reproduction in order to increase their production. Prostaglandins (PGs) play an important role in reproduction in most vertebrates and several invertebrates. It has been proposed that crustaceans have PGs but the prostanoids pathway in the giant freshwater prawn is still unclear. In this study, we identified 25 prostanoid-related genes involved in the biosynthesis of active prostanoids in M. rosenbergii using in silico searches of transcriptome data. Comparative analysis of encoded proteins for the MroPGES2 gene with other species was performed to confirm their evolutionary conservation. Gene expression analysis revealed the correlation of MroPGES2 gene expression level with the progress of ovarian development. Eyestalk ablation increased the expression level of MroPGES2 gene compared to intact groups during the ovary maturation stages. Collectively, this study confirmed the existence of prostanoids in the giant freshwater prawn, as well as characterizing key gene MroPGES2 associated with the prostanoid pathway. We propose that MroPGES2 may play an important role in M. rosenbergii ovarian maturation and its expression is under the inhibitory control from the eyestalk optic ganglion hormones. Identification of genes in prostanoid pathway and their expressions enables future functional studies to be performed, which may lead to applications in the aquaculture of this species.

Prostaglandin catabolism in Spodoptera exigua, a lepidopteran insect

Several prostaglandins (PGs) and PG-synthesizing enzymes have been identified from insects. PGs mediate cellular and humoral immune responses. However, uncontrolled and prolonged immune responses might have adverse effects on survival. PG catabolism in insects has not been reported. Here, using a transcriptomic analysis, we predicted the presence of two PG-degrading enzymes, PG dehydrogenase (SePGDH) and PG reductase (SePGR), in Spodoptera exigua, a lepidopteran insect. SePGDH and SePGR expression levels were upregulated after immune challenge. However, their expression peaks occurred after those of PG biosynthesis genes, such as those encoding PGE2 synthase or PGD2 synthase. SePGDH and SePGR expression levels were upregulated after injection with PGE2 or PGD2 In contrast, such upregulated expression was not detected after injection with leukotriene B4, an eicosanoid inflammatory mediator. RNA interference (RNAi) using double-stranded RNAs specific to SePGDH or SePGR suppressed their expression levels. The RNAi treatment resulted in an excessive and fatal melanization of larvae even after a non-pathogenic bacterial infection. Phenoloxidase (PO) activity mediating the melanization in larval plasma was induced by bacterial challenge or PGE2 injection. Although the induced PO activity decreased after 8 h in control larvae, those treated with dsRNAs specific to PG-degrading enzyme genes kept a high PO activity for a longer period. These results suggest that SePGDH and SePGR are responsible for PG degradation at a late phase of the immune response.

Prostaglandins regulate humoral immune responses in Aedes aegypti

Prostaglandins (PGs) are immuno-active lipids that mediate the immune response in invertebrates and vertebrates. In insects, PGs play a role on different physiological processes such as reproduction, ion transport and regulation of cellular immunity. However, it is unclear whether PGs play a role in invertebrate's humoral immunity, and, if so, which immune signaling pathways would be modulated by PGs. Here, we show that Aedes aegypti gut microbiota and Gram-negative bacteria challenge induces prostaglandin production sensitive to an irreversible inhibitor of the vertebrate cyclooxygenase, acetylsalicylic acid (ASA). ASA treatment reduced PG synthesis and is associated with decreased expression of components of the Toll and IMD immune pathways, thereby rendering mosquitoes more susceptible to both bacterial and viral infections. We also shown that a cytosolic phospholipase (PLAc), one of the upstream regulators of PG synthesis, is induced by the microbiota in the midgut after blood feeding. The knockdown of the PLAc decreased prostaglandin production and enhanced the replication of Dengue in the midgut. We conclude that in Ae. aegypti, PGs control the amplitude of the immune response to guarantee an efficient pathogen clearance.

Diet quality and NSAIDs promote changes in formation of prostaglandins by an aquatic invertebrate

We used the freshwater insect Hydropsyche sp. to investigate the impact of diets lacking arachidonic acid (ARA) and an environmentally relevant mixture of NSAIDs (Ibuprofen, Ketoprofen, Diclofenac and Naproxen at a nominal concentration of all compounds together 16.75 μg L^-1) on their metabolism of ARA and prostaglandins (PGs). The organisms were exposed for 16 days to four different treatments: a reference (FF), a diet lacking ARA (O), to NSAIDs in water (FFN) and to the combination of the two factors (ON). Mortality, biomass and bioconcentration of pharmaceuticals were investigated. The ARA and PGs levels in the organisms were monitored by utilising a targeted metabolomics approach. NSAIDs or dietary constraints did not produce significant differences in biomass or mortality of Hydropsyche sp. among treatments. In organisms exposed to NSAIDs, all pharmaceuticals were detected, except for Ketoprofen. Metabolomic approach determined the presence of PGH₂, PGE₁ and PGD₁. Levels of ARA diminished significantly in those organisms in treatment ON. The levels of PGs responded negatively to the absence of ARA in diet: PGH₂ diminished significantly with respect to the reference in treatment O while PGE₁ diminished significantly in treatment ON. Regarding the effects of NSAIDs on ARA metabolism, our results suggest that it was sensitive to NSAIDs, but effects were weak and did not imply a general decrease in the PGs. We confirmed that ARA was the main substrate for the synthesis of PGs in Hydropsyche sp, their absence or poor levels of ARA in diet, produced changes in the PG levels.

Variation in prostaglandin metabolism during growth of the diatom Thalassiosira rotula

Prostaglandins (PGs) are hormone-like mediators in many physiological and pathological processes that are present in all vertebrates, in some terrestrial and aquatic invertebrates, and have also been identified in some macroalgae. They have recently been reported also in marine microalgae but their role as chemical mediators is largely unknown. Here we studied the expression pattern of the PG biosynthetic pathway during different growth phases of the centric diatom Thalassiosira rotula and assessed the release of PGs in the surrounding environment for the first time. We show that enzymes responsible for PGs formation such as cyclooxygenase, prostaglandin E synthase 2-like and prostaglandin F synthase are mainly expressed at the end of the exponential phase and that PGs are released especially during the stationary and senescent phases, suggesting a possible signaling function for these compounds. Phylogenetic analysis of the limiting enzyme, COX, indicate the presence in diatoms of more than one enzyme related to the oxidative metabolism of fatty acids belonging to the peroxidase-cyclooxygenase superfamily. These findings suggest a more complex evolution and diversity of metabolic pathways leading to the synthesis of lipid mediators in diatoms.

Mosquito Midgut Prostaglandin Release Establishes Systemic Immune Priming

Anopheles gambiae mosquitoes that have been infected with Plasmodium mount a more effective immune response to a subsequent infection. Priming is established when Plasmodium invasion of the mosquito midgut allows contact of the gut microbiota with epithelial cells. This event is followed by a systemic release of a hemocyte differentiation factor (HDF) consisting of Lipoxin A4 bound to Evokin, a lipocalin carrier, which increases the proportion of circulating hemocytes. We show that mosquito midgut cells produce and release prostaglandin E2 (PGE2), which attracts hemocytes to the midgut surface and enhances their patrolling activity. Systemic injection of prostaglandins (PGs) recapitulates the priming response and enhances antiplasmodial immunity by triggering HDF production. Although insects lack cyclooxygenases, two heme peroxidases, HPX7 and HPX8, catalyze essential steps in PG biosynthesis in mosquitoes. Mosquito midgut PGE2 release attracts hemocytes and establishes a long-lasting enhanced systemic cellular immune response to Plasmodium infection.

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Plasmodium invasion or bacterial exposure triggers midgut prostaglandin synthesis

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Prostaglandins attract mosquito hemocytes and increase their patrolling activity

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Two midgut peroxidases, HPX7 and HPX8, catalyze midgut prostaglandin synthesis

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Systemic release of midgut prostaglandins is essential to establish immune priming

Inhibition of prostaglandin biosynthesis leads to suppressed ovarian development in Spodoptera exigua

Prostaglandins (PGs) are a group of eicosanoids that are C20 oxygenated polyunsaturated fatty acids. PGs can mediate various physiological processes such as immunity, salivary secretion, excretion, and reproduction in insects. The objective of this study was to determine the effect of PG on oocyte development in Spodoptera exigua, a lepidopteran insect known to biosynthesize PGs. Polytrophic ovarioles of S. exigua females exhibited follicle development in germarium, in which oocytes were distinct from nurse cells. During vitellogenesis, nurse cells degenerated by losing cytoplasm called "nurse cell dumping" while oocytes showed increase in cell volume. When PG biosynthesis inhibitors such as ibuprofen or aspirin were applied, nurse cell dumping was not complete and no chorion was formed, thus preventing egg formation. However, addition of PGE₂ significantly rescued such inhibition and resumed oocyte development and choriogenesis. To support the observation with genetic factor, RNA interference (RNAi) specific to peroxynectins (Pxts: Se-Pxt1 and Se-Pxt2) known to act as insect cyclooxygenase was performed to suppress PG biosynthesis. Both Se-Pxt1 and Se-Pxt2 were highly expressed in the ovary of control female. RNAi treatment against Se-Pxt1 or Se-Pxt2 specifically suppressed target genes and inhibited oocyte development. Addition of PGE₂ to adults treated with RNAi rescued the suppressed development of oocytes. Results of this study suggest that PGs can stimulate oocyte development as autocrine/paracrine mediators of vitellogenesis and choriogenesis in insects.

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.

Eicosanoid Diversity of Stony Corals

Oxylipins are well-established lipid mediators in plants and animals. In mammals, arachidonic acid (AA)-derived eicosanoids control inflammation, fever, blood coagulation, pain perception and labor, and, accordingly, are used as drugs, while lipoxygenases (LOX), as well as cyclooxygenases (COX) serve as therapeutic targets for drug development. In soft corals, eicosanoids are synthesized on demand from AA by LOX, COX, and catalase-related allene oxide synthase-lipoxygenase (cAOS-LOX) and hydroperoxide lyase-lipoxygenase (cHPL-LOX) fusion proteins. Reef-building stony corals are used as model organisms for the stress-related genomic studies of corals. Yet, the eicosanoid synthesis capability and AA-derived lipid mediator profiles of stony corals have not been determined. In the current study, the genomic and transcriptomic data about stony coral LOXs, AOS-LOXs, and COXs were analyzed and the eicosanoid profiles and AA metabolites of three stony corals, Acropora millepora, A. cervicornis, and Galaxea fascicularis, were determined by reverse-phase high-performance liquid chromatography (RP-HPLC) coupled with MS-MS and a radiometric detector. Our results confirm that the active LOX and AOS-LOX pathways are present in Acropora sp., which correspond to the genomic/sequence data reported earlier. In addition, LOX, AOS-LOX, and COX products were detected in the closely related species G. fascicularis. In conclusion, the functional 8R-LOX and/or AOS-LOX pathways are abundant among corals, while COXs are restricted to certain soft and stony coral lineages.

Animal-like prostaglandins in marine microalgae

Diatoms are among the most successful primary producers in ocean and freshwater environments. Deriving from a secondary endosymbiotic event, diatoms have a mixed genome containing bacterial, animal and plant genes encoding for metabolic pathways that may account for their evolutionary success. Studying the transcriptomes of two strains of the diatom Skeletonema marinoi, we report, for the first time in microalgae, an active animal-like prostaglandin pathway that is differentially expressed in the two strains. Prostaglandins are hormone-like mediators in many physiological and pathological processes in mammals, playing a pivotal role in inflammatory responses. They are also present in macroalgae and invertebrates, where they act as defense and communication mediators. The occurrence of animal-like prostaglandins in unicellular photosynthetic eukaryotes opens up new intriguing perspectives on the evolution and role of these molecules in the marine environment as possible mediators in cell-to-cell signaling, eventually influencing population dynamics in the plankton.

Biologically Active Oxylipins from Enzymatic and Nonenzymatic Routes in Macroalgae

Marine algae are rich and heterogeneous sources of great chemical diversity, among which oxylipins are a well-recognized class of natural products. Algal oxylipins comprise an assortment of oxygenated, halogenated, and unsaturated functional groups and also several carbocycles, varying in ring size and position in lipid chain. Besides the discovery of structurally diverse oxylipins in macroalgae, research has recently deciphered the role of some of these metabolites in the defense and innate immunity of photosynthetic marine organisms. This review is an attempt to comprehensively cover the available literature on the chemistry, biosynthesis, ecology, and potential bioactivity of oxylipins from marine macroalgae. For a better understanding, enzymatic and nonenzymatic routes were separated; however, both processes often occur concomitantly and may influence each other, even producing structurally related molecules.

(1)H NMR-based metabolomics of Daphnia magna responses after sub-lethal exposure to triclosan, carbamazepine and ibuprofen

Pharmaceuticals and personal care products are a class of emerging contaminants that are present in wastewater effluents, surface water, and groundwater around the world. There is a need to determine rapid and reliable bioindicators of exposure and the toxic mode of action of these contaminants to aquatic organisms. (1)H nuclear magnetic resonance (NMR)-based metabolomics in combination with multivariate statistical analysis was used to determine the metabolic profile of Daphnia magna after exposure to a range of sub-lethal concentrations of triclosan (6.25-100μg/L), carbamazepine (1.75-14mg/L) and ibuprofen (1.75-14mg/L) for 48h. Sub-lethal triclosan exposure suggested a general oxidative stress condition and the branched-chain amino acids, glutamine, glutamate, and methionine emerged as potential bioindicators. The aromatic amino acids, serine, glycine and alanine are potential bioindicators for sub-lethal carbamazepine exposure that may have altered energy metabolism. The potential bioindicators for sub-lethal ibuprofen exposure are serine, methionine, lysine, arginine and leucine, which showed a concentration-dependent response. The differences in the metabolic changes were related to the dissimilar modes of toxicity of triclosan, carbamazepine and ibuprofen. (1)H NMR-based metabolomics gave an improved understanding of how these emerging contaminants impact the keystone species D. magna.

Variation of prostaglandin E2 concentrations in ovaries and its effects on ovarian maturation and oocyte proliferation in the giant fresh water prawn, Macrobrachium rosenbergii

Prostaglandins (PGs) are important bioactive mediators for many physiological functions. In some decapod crustaceans, prostaglandin E2 (PGE2) has been detected in reproductive organs, and may play a role in the control of ovarian maturation. However, in the freshwater prawn, Macrobrachium rosenbergii, the presences of PGE2 and key enzymes for PGE2 biosynthesis, as well as its effects on ovarian maturation have not yet been investigated. In this study we reported the presence of PGE2, cyclooxygenase1 (COX1) and prostaglandin E synthase (PGES) in the ovarian tissues of M. rosenbergii, using immunohistochemistry. Intense immunoreactivities of PGE2 (PGE2-ir), COX1 (Cox1-ir) and PGES (PGES-ir) were detected in previtellogenic oocytes (Oc1 and Oc2), while the immunoreactivities were absent in the late vitellogenic oocytes (Oc4). This finding supports the hypothesis that the PGE2 biosynthesis occurs in the ovary of this prawn. To ascertain this finding we used LC-MS/MS to quantitate PGE2 concentrations during ovarian developmental cycle. The levels of PGE2 were significantly higher in the early ovarian stages (St I and II) than in the late stages (St III and IV). Moreover, we found that administration of PGE2 stimulated the ovarian maturation in this species by shortening the length of the ovarian cycle, increasing ovarian-somatic index, oocyte proliferation, and vitellogenin (Vg) level in the hemolymph.

Reconstruction of cyclooxygenase evolution in animals suggests variable, lineage-specific duplications, and homologs with low sequence identity

Cyclooxygenase (COX) enzymatically converts arachidonic acid into prostaglandin G/H in animals and has importance during pregnancy, digestion, and other physiological functions in mammals. COX genes have mainly been described from vertebrates, where gene duplications are common, but few studies have examined COX in invertebrates. Given the increasing ease in generating genomic data, as well as recent, although incomplete descriptions of potential COX sequences in Mollusca, Crustacea, and Insecta, assessing COX evolution across Metazoa is now possible. Here, we recover 40 putative COX orthologs by searching publicly available genomic resources as well as ~250 novel invertebrate transcriptomic datasets. Results suggest the common ancestor of Cnidaria and Bilateria possessed a COX homolog similar to those of vertebrates, although such homologs were not found in poriferan and ctenophore genomes. COX was found in most crustaceans and the majority of molluscs examined, but only specific taxa/lineages within Cnidaria and Annelida. For example, all octocorallians appear to have COX, while no COX homologs were found in hexacorallian datasets. Most species examined had a single homolog, although species-specific COX duplications were found in members of Annelida, Mollusca, and Cnidaria. Additionally, COX genes were not found in Hemichordata, Echinodermata, or Platyhelminthes, and the few previously described COX genes in Insecta lacked appreciable sequence homology (although structural analyses suggest these may still be functional COX enzymes). This analysis provides a benchmark for identifying COX homologs in future genomic and transcriptomic datasets, and identifies lineages for future studies of COX.

A cyclooxygenase-2-dependent prostaglandin E2 biosynthetic system in the Golgi apparatus

Cyclooxygenases (COXs) catalyze the committed step in prostaglandin (PG) biosynthesis. COX-1 is constitutively expressed and stable, whereas COX-2 is inducible and short lived. COX-2 is degraded via endoplasmic reticulum (ER)-associated degradation (ERAD) following post-translational glycosylation of Asn-594. COX-1 and COX-2 are found in abundance on the luminal surfaces of the ER and inner membrane of the nuclear envelope. Using confocal immunocytofluorescence, we detected both COX-2 and microsomal PGE synthase-1 (mPGES-1) but not COX-1 in the Golgi apparatus. Inhibition of trafficking between the ER and Golgi retarded COX-2 ERAD. COX-2 has a C-terminal STEL sequence, which is an inefficient ER retention signal. Substituting this sequence with KDEL, a robust ER retention signal, concentrated COX-2 in the ER where it was stable and slowly glycosylated on Asn-594. Native COX-2 and a recombinant COX-2 having a Golgi targeting signal but not native COX-1 exhibited efficient catalytic coupling to mPGES-1. We conclude that N-glycosylation of Asn-594 of COX-2 occurs in the ER, leading to anterograde movement of COX-2 to the Golgi where the Asn-594-linked glycan is trimmed prior to retrograde COX-2 transport to the ER for ERAD. Having an inefficient ER retention signal leads to sluggish Golgi to ER transit of COX-2. This permits significant Golgi residence time during which COX-2 can function catalytically. Cytosolic phospholipase A2α, which mobilizes arachidonic acid for PG synthesis, preferentially translocates to the Golgi in response to physiologic Ca(2+) mobilization. We propose that cytosolic phospholipase A2α, COX-2, and mPGES-1 in the Golgi comprise a dedicated system for COX-2-dependent PGE2 biosynthesis.

Enhanced longevity by ibuprofen, conserved in multiple species, occurs in yeast through inhibition of tryptophan import

The common non-steroidal anti-inflammatory drug ibuprofen has been associated with a reduced risk of some age-related pathologies. However, a general pro-longevity role for ibuprofen and its mechanistic basis remains unclear. Here we show that ibuprofen increased the lifespan of Saccharomyces cerevisiae, Caenorhabditis elegans and Drosophila melanogaster, indicative of conserved eukaryotic longevity effects. Studies in yeast indicate that ibuprofen destabilizes the Tat2p permease and inhibits tryptophan uptake. Loss of Tat2p increased replicative lifespan (RLS), but ibuprofen did not increase RLS when Tat2p was stabilized or in an already long-lived strain background impaired for aromatic amino acid uptake. Concomitant with lifespan extension, ibuprofen moderately reduced cell size at birth, leading to a delay in the G1 phase of the cell cycle. Similar changes in cell cycle progression were evident in a large dataset of replicatively long-lived yeast deletion strains. These results point to fundamental cell cycle signatures linked with longevity, implicate aromatic amino acid import in aging and identify a largely safe drug that extends lifespan across different kingdoms of life.

Aging is the greatest risk factor for many diseases, which together account for the majority of global deaths and healthcare costs. Here we show that the common drug ibuprofen increases the lifespan of yeast, worms and flies, indicative of conserved longevity effects. In budding yeast, an excellent model of cellular longevity mechanisms, ibuprofen's pro-longevity action is independent of its known anti-inflammatory role. We show that the critical function of ibuprofen in longevity is to inhibit the uptake of aromatic amino acids, by destabilizing the high-affinity tryptophan permease. We further show that ibuprofen alters cell cycle progression. Mirroring the effects of ibuprofen, we found that most yeast long-lived mutants were also similarly affected in cell cycle progression. These findings identify a safe drug that extends the lifespan of divergent organisms and reveal fundamental cellular properties associated with longevity.

Bacterial and algal orthologs of prostaglandin H₂synthase: novel insights into the evolution of an integral membrane protein

Prostaglandin H₂synthase (PGHS; EC 1.14.99.1), a bi-functional heme enzyme that contains cyclooxygenase and peroxidase activities, plays a central role in the inflammatory response, pain, and blood clotting in higher eukaryotes. In this review, we discuss the progenitors of the mammalian enzyme by using modern bioinformatics and homology modeling to draw comparisons between this well-studied system and its orthologs from algae and bacterial sources. A clade of bacterial and algal orthologs is described that have salient structural features distinct from eukaryotic counterparts, including the lack of a dimerization and EGF-like domains, the absence of gene duplicates, and minimal membrane-binding domains. The functional implications of shared and variant features are discussed.

Roles of peroxinectin in PGE2-mediated cellular immunity in Spodoptera exigua

BACKGROUND

Prostaglandins (PGs) mediate insect immune responses to infections and invasions. Although the presence of PGs has been confirmed in several insect species, their biosynthesis in insects remains a conundrum because orthologs of the mammalian cyclooxygenases (COXs) have not been found in the known insect genomes. PG-mediated immune reactions have been documented in the beet armyworm, Spodoptera exigua. The purpose of this research is to identify the source of PGs in S. exigua.

PRINCIPAL FINDINGS

Peroxidases (POXs) are a sister group of COX genes. Ten putative POXs (SePOX-A ∼ SePOX-J) were expressed in S. exigua. Expressions of SePOX-F and -H were induced by bacterial challenge and expressed in the hemocytes and the fat body. RNAi of each POX was performed by hemocoelic injection of their specific double-stranded RNAs. dsPOX-F or, separately, dsPOX-H, but not the other eight dsRNA constructs, specifically suppressed hemocyte-spreading behavior and nodule formation; these two reactions were also inhibited by aspirin, a COX inhibitor. PGE2, but not arachidonic acid, treatment rescued the immunosuppression. Sequence analysis indicated that both POX genes were clustered with peroxinectin (Pxt) and their cognate proteins shared some conserved domains corresponding to the Pxt of Drosophila melanogaster.

CONCLUSIONS

SePOX-F and -H are Pxt-like genes associated with PG biosynthesis in S. exigua.

N-glycosylation site occupancy in human prostaglandin H synthases expressed in Pichia pastoris

Prostaglandin H synthases (PGHSs) are N-glycosylated membrane proteins that catalyse the committed step in prostaglandin synthesis. Unlike PGHS-2, the production of recombinant PGHS-1 in non-mammalian expression systems is complicated. The majority of the heterologous enzyme is inactive due to misfolding. Correct N-glycosylation is proposed to be obligatory for proper folding of mammalian PGHSs. In this study, human PGHS-1 and -2 (hPGHS-1 and -2) were expressed in the yeast Pichia pastoris. Recombinant hPGHS-2 was catalytically active, whereas hPGHS-1 was inactive. Accumulation of non-glycosylated hPGHSs was not observed in the crude lysate of the yeast cells. The N-glycosylation patterns of the purified recombinant proteins were characterised using nano-LC/MS/MS. The isoforms exhibited similar N-glycosylation site occupancy. The results indicate that there are more complex grounds for the inactivity of the recombinant hPGHS-1 produced in yeast.

Ancestral genetic complexity of arachidonic acid metabolism in Metazoa

Eicosanoids play an important role in inducing complex and crucial physiological processes in animals. Eicosanoid biosynthesis in animals is widely reported; however, eicosanoid production in invertebrate tissue is remarkably different to vertebrates and in certain respects remains elusive. We, for the first time, compared the orthologs involved in arachidonic acid (AA) metabolism in 14 species of invertebrates and 3 species of vertebrates. Based on parsimony, a complex AA-metabolic system may have existed in the common ancestor of the Metazoa, and then expanded and diversified through invertebrate lineages. A primary vertebrate-like AA-metabolic system via cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P450 (CYP) pathways was further identified in the basal chordate, amphioxus. The expression profiling of AA-metabolic enzymes and lipidomic analysis of eicosanoid production in the tissues of amphioxus supported our supposition. Thus, we proposed that the ancestral complexity of AA-metabolic network diversified with the different lineages of invertebrates, adapting with the diversity of body plans and ecological opportunity, and arriving at the vertebrate-like pattern in the basal chordate, amphioxus.

Why do a wide variety of animals retain multiple isoforms of cyclooxygenase?

Cyclooxygenase (COX) has been cloned from the phyla Cnidaria, Mollusca, Arthropoda, and Chordata of the animal kingdom. Many organisms have multiple COX isoforms that have arisen from gene duplication. It is not well understood why there are multiple COX isoforms in the same organism, or when duplication of the COX gene occurred. Here, we summarize the current knowledge of the evolutionary history of COX in the animal kingdom and discuss the reasons why the multiple COX system has been retained so widely. The phylogenetic analysis suggests that all COX genes in animals may descend from a common ancestor and that the duplication of an ancestral COX gene might occur within each lineage after the divergence of the animal. In most instances, the expressions of multiple COX isoforms are separately regulated and these isoforms play different and important pathophysiological roles in each organism. This may be the reason why multiple COX isoforms are widely retained.

Insights into the prostanoid pathway in the ovary development of the penaeid shrimp Penaeus monodon

The prostanoid pathway converts polyunsaturated fatty acids (PUFAs) into bioactive lipid mediators, including prostaglandins, thromboxanes and prostacyclins, all of which play vital roles in the immune and reproductive systems in most animal phyla. In crustaceans, PUFAs and prostaglandins have been detected and often associated with female reproductive maturation. However, the presence of prostanoid biosynthesis genes remained in question in these species. In this study, we outlined the prostanoid pathway in the black tiger shrimp Penaeus monodon based on the amplification of nine prostanoid biosynthesis genes: cytosolic phospholipase A2, hematopoietic prostaglandin D synthase, glutathione-dependent prostaglandin D synthase, prostaglandin E synthase 1, prostaglandin E synthase 2, prostaglandin E synthase 3, prostaglandin F synthase, thromboxane A synthase and cyclooxygenase. TBLASTX analysis confirmed the identities of these genes with 51-99% sequence identities to their closest homologs. In addition, prostaglandin F2α (PGF2α), which is a product of the prostaglandin F synthase enzyme, was detected for the first time in P. monodon ovaries along with the previously identified PUFAs and prostaglandin E2 (PGE2) using RP-HPLC and mass-spectrometry. The prostaglandin synthase activity was also observed in shrimp ovary homogenates using in vitro activity assay. When prostaglandin biosynthesis was examined in different stages of shrimp ovaries, we found that the amounts of prostaglandin F synthase gene transcripts and PGF2α decreased as the ovaries matured. These findings not only indicate the presence of a functional prostanoid pathway in penaeid shrimp, but also suggest a possible role of the PGF2α biosynthesis in shrimp ovarian development.

Thermal preferences of wintering snails Planorbarius corneus (L.) exposed to lipopolysaccharide and zymosan

Fever is regarded as a physiological response to infection both in endothermic and ectothermic animals. In ectotherms, fevers are achieved only behaviorally, and has been described in many vertebrates' and few invertebrates' groups. In snails only symptoms of reverse fever as a response to trematode invasion were found. Present work reports on the effects of two different pyrogens - lipopolysaccharide extracted from Escherichia coli (LPS), and zymosan - from Saccharomyces cerevisiae on the thermal behavior of wintering (studied during a winter season) specimens of the Planorbarius corneus (L.). Using the thermal gradient protocol we demonstrate that the individuals of this snail species responded with behavioral fevers to dosages of pyrogens. LPS injection to the surface of the snail's foot at a dose of 10 μg/g resulted in a significant increase in preferred temperature at 5h after injection. Similarly zymosan at a dose of 0.5 and 1.0 μg/g - caused fever at 8h and 9h respectively. Average temperature chosen by feverish animals after latency period reached 28.7±0.41 °C (LPS), 28.1±0.43 °C (zymosan 1.0 μg/g) or 25.5±0.33 °C (zymosan 0.5 μg/g). We conclude, therefore, that snails are capable of reacting with fever to selected pathogen associated factors, and P. corneus can be used as a model to study a behavioral fever phenomenon in invertebrate animals.

Plasmatocyte-spreading peptide influences hemocyte behavior via eicosanoids

Hemocyte-spreading behavior is required for expressing a cellular immune response, nodulation, which clears the vast majority of invading microbes from circulation. The nodulation response is completed by a layer of plasmatocytes, which spread over the nodule and initiate a malanization process leading to darkened nodules. Plasmatocyte-spreading peptide (PSP), the first reported insect cytokine, is responsible for mediating the spreading and attachment of some subclasses of plasmatocytes to nodules. Prostaglandins (PGs), one group of eicosanoids formed from arachidonic acid (AA), also mediate plasmatocyte spreading (PS), although the potential interactions between the PSP and PG signal transduction pathways have not been investigated. We tested our hypothesis that PSP acts via biosynthesis of eicosanoids, specifically PGs, in the beet armyworm, Spodoptera exigua. In this study, we report that (1) PSP and PGE(2) independently stimulated Ca(++)-dependent PS, (2) inhibitors of PG biosynthesis reversibly blocked PS, (3) dsRNA silencing the gene encoding proPSP blocked PS, which was rescued by PSP and by AA, (4) PSP-stimulated PS was reversibly impaired by inhibitors of PG biosynthesis, and (5) the inhibitor-impaired spreading was rescued by AA. Taken together, these points strongly support our model showing that PSP acts via a plasmatocyte-surface receptor, which stimulates biosynthesis of the PGs responsible for mediating plasmatocytes spreading.