Four phospholipase A2 genes encoded in the western flower thrips genome and their functional differentiation in mediating development and immunity

Eicosanoids are synthesized from phospholipids by the catalytic activity of phospholipase A2 (PLA2). Even though several PLA2s are encoded in the genome of different insect species, their physiological functions are not clearly discriminated. This study identified four PLA2 genes encoded in the western flower thrips, Frankliniella occidentalis. Two PLA2s (Fo-PLA2C and Fo-PLA2D) are predicted to be secretory while the other two PLA2s (Fo-PLA2A and Fo-PLA2B) are intracellular. All four PLA2 genes were expressed in all developmental stages, of which Fo-PLA2B and Fo-PLA2C were highly expressed in larvae while Fo-PLA2A and Fo-PLA2D were highly expressed in adults. Their expressions in different tissues were also detected by fluorescence in situ hybridization. All four PLA2s were detected in the larval and adult intestines and the ovary. Feeding double-stranded RNAs specific to the PLA2 genes specifically suppressed the target transcript levels. Individual RNA interference (RNAi) treatments led to significant developmental retardation, especially in the treatments specific to Fo-PLA2B and Fo-PLA2D. The RNAi treatments also showed that Fo-PLA2B and Fo-PLA2C expressions were required for the induction of immune-associated genes, while Fo-PLA2A and Fo-PLA2D expressions were required for ovary development. These results suggest that four PLA2s are associated with different physiological processes by their unique catalytic activities and expression patterns.

Immune responses of the Asian onion moth, Acrolepiopsis sapporensis, and their genetic factors from RNA-Seq analysis

A nonmodel insect, Acrolepiopsis sapporensis, has been analyzed in immune responses. The total hemocytes in the fifth instar larvae were 2.33 × 106  cells/mL. These hemocytes comprised at least five different types and different relative ratios: 47% granulocytes, 26% plasmatocytes, 11% oenocytoid, 8% prohemocytes, and 5% spherulocytes. Upon bacterial challenge, some of the hemocytes exhibited typical hemocyte-spreading behaviors, such as focal adhesion, and filopodial and lamellipodial cytoplasmic extensions. The hemocyte behaviors induced cellular immune responses demonstrated by nodule formation. In addition, the plasma collected from the immune-challenged larvae exhibited humoral immune responses by bacterial growth inhibition along with enhanced phenoloxidase enzyme activity. These cellular and humoral immune responses were further analyzed by determining the immune-associated genes from a transcriptome generated by RNA-Seq. A total of about 12 Gb sequences led to about 218,116 contigs, which were predicted to encode about 46,808 genes. Comparative expression analysis showed 8392 uniquely expressed genes in the immune-challenged larvae. Differentially expressed gene (DEG) analysis among the commonly expressed genes indicated that 782 genes were upregulated and 548 genes were downregulated in the expressions after bacterial challenge. These immune-associated genes included pattern recognition receptors, immune mediation/signaling genes, and various immune effectors. Specifically, the genetic components of the Toll, IMD, and JAK/STAT immune signaling pathways were included in the DEG database. These results demonstrate the immune responses of A. sapporensis larvae and suggest the genes associated with the immune responses in this nonmodel insect.

Dorsal switch protein 1 as a damage signal in insect gut immunity to activate dual oxidase via an eicosanoid, PGE2

Various microbiota including beneficial symbionts reside in the insect gut. Infections of pathogens cause dysregulation of the microflora and threaten insect survival. Reactive oxygen species (ROS) have been used in the gut immune responses, in which its production is tightly regulated by controlling dual oxidase (Duox) activity via Ca2+ signal to protect beneficial microflora and gut epithelium due to its high cytotoxicity. However, it was not clear how the insects discriminate the pathogens from the various microbes in the gut lumen to trigger ROS production. An entomopathogenic nematode (Steinernema feltiae) infection elevated ROS level in the gut lumen of a lepidopteran insect, Spodoptera exigua. Dorsal switch protein 1 (DSP1) localized in the nucleus in the midgut epithelium was released into plasma upon the nematode infection and activated phospholipase A2 (PLA2). The activated PLA2 led to an increase of PGE2 level in the midgut epithelium, in which rising Ca2+ signal up-regulated ROS production. Inhibiting DSP1 release by its specific RNA interference (RNAi) or specific inhibitor, 3-ethoxy-4-methoxyphenol, treatment failed to increase the intracellular Ca2+ level and subsequently prevented ROS production upon the nematode infection. A specific PLA2 inhibitor treatment also prevented the up-regulation of Ca2+ and subsequent ROS production upon the nematode infection. However, the addition of PGE2 to the inhibitor treatment rescued the gut immunity. DSP1 release was not observed at infection with non-pathogenic pathogens but detected in plasma with pathogenic infections that would lead to damage to the gut epithelium. These results indicate that DSP1 acts as a damage-associated molecular pattern in gut immunity through DSP1/PLA2/Ca2+/Duox.

Immune Function of Endothelial Cells: Evolutionary Aspects, Molecular Biology and Role in Atherogenesis

Atherosclerosis is one of the key problems of modern medicine, which is due to the high prevalence of atherosclerotic cardiovascular diseases and their significant share in the structure of morbidity and mortality in many countries. Atherogenesis is a complex chain of events that proceeds over many years in the vascular wall with the participation of various cells. Endothelial cells are key participants in vascular function. They demonstrate involvement in the regulation of vascular hemodynamics, metabolism, and innate immunity, which act as leading links in the pathogenesis of atherosclerosis. These endothelial functions have close connections and deep evolutionary roots, a better understanding of which will improve the prospects of early diagnosis and effective treatment.

HMGB1-Like Dorsal Switch Protein 1 Triggers a Damage Signal in Mosquito Gut to Activate Dual Oxidase via Eicosanoids

Several mosquitoes transmit human pathogens by blood feeding, with the gut being the main entrance for the pathogens. Thus, the gut epithelium defends the pathogens by eliciting potent immune responses. However, it was unclear how the mosquito gut discriminates pathogens among various microflora in the lumen. This study proposed a hypothesis that a damage signal might be specifically induced by pathogens in the gut. The Asian tiger mosquito, Aedes albopictus, encodes dorsal switch protein 1 (Aa-DSP1) as a putative damage-associated molecular pattern (DAMP). Aa-DSP1 was localized in the nucleus of the midgut epithelium in naïve larvae. Upon infection by a pathogenic bacterium, Serratia marcescens, Aa-DSP1 was released to hemocoel and activated phospholipase A2 (PLA2). The activated PLA2 increased the level of prostaglandin E2 (PGE2) in the gut and subsequently increased Ca2+ signal to produce reactive oxygen species (ROS) via dual oxidase (Duox). Inhibition of Aa-DSP1 via RNA interference or specific inhibitor treatment failed to increase PGE2/Ca2+ signal upon the bacterial infection. Thus, the inhibitors specifically targeting eicosanoid biosynthesis significantly prevented the upregulation of ROS production in the gut and enhanced mosquito mortality after the bacterial infection. However, such inhibitory effects were rescued by adding PGE2. These suggest that Aa-DSP1 plays an important role in immune response of the mosquito gut as a DAMP during pathogen infection by triggering a signaling pathway, DSP1/PLA2/Ca2+/Duox.

Deletion mutant of PGE2 receptor using CRISPR-Cas9 exhibits larval immunosuppression and adult infertility in a lepidopteran insect, Spodoptera exigua

Prostaglandins (PGs) mediate various physiological processes in insects and other invertebrates, but there is very little information on PG receptors. This study identified a PGE₂ receptor (SePGE₂R) in the lepidopteran insect, Spodoptera exigua, and addressed its functional association with cellular immunity, development, and reproduction. SePGE₂R is expressed in most developmental stages and tissues. After SePGR₂R expression knock down by RNA interference (RNAi), larval nodule formation (clears bacterial infections from circulating hemolymph) was severely suppressed coupled with reduced F-actin growth in hemocytes. Treating female adults with RNAi prevented nurse cell dumping in follicles and interfered with oocyte development. SePGE₂R was heterologously expressed in Sf9 cells, in which the endogenous S. frugiperda PGE₂R was knocked down by small interfering RNA. This transiently expressed SePGE₂R responded to PGE₂, but not other PGs, with dose-dependent up-regulation of intracellular cAMP concentrations. Treating S. exigua larvae with PGE₂ led to activation of a trimeric Gαs subunit, protein kinase A (PKA), and Rho family small intracellular G proteins in hemocytes. A deletion mutant of SePGE₂R was generated using CRISPR/Cas9 which exhibited severely retarded larval development and adult reproduction. We infer that PGE₂R mediates insect immune and reproductive processes via a PKA signal pathway.

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.

Nitric oxide mediates antimicrobial peptide gene expression by activating eicosanoid signaling

Nitric oxide (NO) mediates both cellular and humoral immune responses in insects. Its mediation of cellular immune responses uses eicosanoids as a downstream signal. However, the cross-talk with two immune mediators was not known in humoral immune responses. This study focuses on cross-talk between two immune mediators in inducing gene expression of anti-microbial peptides (AMPs) of a lepidopteran insect, Spodoptera exigua. Up-regulation of eight AMPs was observed in S. exigua against bacterial challenge. However, the AMP induction was suppressed by injection of an NO synthase inhibitor, L-NAME, while little expressional change was observed on injecting its enantiomer, D-NAME. The functional association between NO biosynthesis and AMP gene expression was further supported by RNA interference (RNAi) against NO synthase (SeNOS), which suppressed AMP gene expression under the immune challenge. The AMP induction was also mimicked by NO alone because injecting an NO analog, SNAP, without bacterial challenge significantly induced the AMP gene expression. Interestingly, an eicosanoid biosynthesis inhibitor, dexamethasone (DEX), suppressed the NO induction of AMP expression. The inhibitory activity of DEX was reversed by the addition of arachidonic acid, a precursor of eicosanoid biosynthesis. AMP expression of S. exigua was also controlled by the Toll/IMD signal pathway. The RNAi of Toll receptors or Relish suppressed AMP gene expression by suppressing NO levels and subsequently reducing PLA2 enzyme activity. These results suggest that eicosanoids are a downstream signal of NO mediation of AMP expression against bacterial challenge.

A novel calcium-independent phospholipase A2 and its physiological roles in development and immunity of a lepidopteran insect, Spodoptera exigua

Phospholipase A₂ (PLA₂) catalyzes hydrolysis of ester linkage at sn-2 position of phospholipids. At least 15 groups (I-XV) of PLA₂ gene superfamily are associated with various physiological processes such as digestion, secretion, immunity, and maintenance of membrane integrity. This study suggests that various insects encode putative Group VI PLA₂s representing intracellular and calcium-independent PLA₂s (iPLA₂). These insect iPLA₂s are separated into at least two subgroups: iPLA₂A (Group VIA-like) and iPLA₂B (non-Group VIA). Most insects encode genes of iPLA₂B type, although their biological functions are currently unknown. This study predicted a novel iPLA₂ from Spodoptera exigua (a lepidopteran insect) (SeiPLA₂B) and analyzed its physiological functions by RNA interference (RNAi). SeiPLA₂B encodes 336 amino acid sequence with a predicted size of about 36.6 kDa and an isoelectric point at pH 8.61. It possesses a lipase catalytic site, but does not have ankyrin repeats in the amino terminal region. Phylogenetic analysis indicated that SeiPLA₂B was clustered with other Group VI iPLA₂s, in which SeiPLA₂B was closely associated with Group VIF gene while SeiPLA₂A was closely related to Group VIA gene. SeiPLA₂B was expressed in all developmental stages of S. exigua. In larval stage, SeiPLA₂B was expressed in fat body, hemocyte, and epidermis, but not in digestive tract. SeiPLA₂B RNAi significantly reduced PLA₂ enzyme activities and resulted in developmental retardation and immunosuppression. Though RNAi treatment did not significantly change fatty acid composition in fat body lipids, it significantly increased lipid peroxidation. Taken together, our results suggest that SeiPLA₂B plays important roles in the development and immunity of S. exigua.

An entomopathogenic bacterium, Xenorhabdus hominickii ANU101, produces oxindole and suppresses host insect immune response by inhibiting eicosanoid biosynthesis

An entomopathogenic bacterium, Xenorhabdus hominickii ANU101, was isolated from an entomopathogenic nematode, Steinernema monticolum. X. hominickii exhibited significant insecticidal activities at ≥6.6×10² colony-forming units per larva against a lepidopteran insect, Spodoptera exigua with hemocoelic injection. The insecticidal activity of X. hominickii was reduced by an addition of arachidonic acid (AA, a catalytic product of PLA₂), but enhanced by an addition by dexamethasone (DEX, a specific inhibitor of PLA₂). S. exigua could defend the bacterial infection by forming hemocyte nodules. However, live X. hominickii significantly reduced the hemocytic nodulation compared to similar treatment with heat-killed X. hominickii. An addition of AA to live X. hominickii significantly rescued the immunosuppression. X. hominickii also inhibited phenoloxidase activity in hemolymph of S. exigua larvae. Furthermore, the bacteria suppressed gene expressions of antimicrobial peptides, such as attacin-1, attacin-2, defensin, gallerimycin and transferrin-1 of S. exigua. An organic extract of X. hominickii-cultured broth with ethyl acetate possessed oxindole and significantly suppressed hemocyte nodulation. Again, an addition of AA diminished the inhibitory activity of the organic extract against hemocyte nodulation. Oxindole alone inhibited hemocyte nodulation and PLA₂ enzyme activity. These results suggest that the entomopathogenicity of X. hominickii comes from its inhibitory activity against eicosanoid biosynthesis of target insects.

Prostaglandin mediates down-regulation of phenoloxidase activation of Spodoptera exigua via plasmatocyte-spreading peptide-binding protein

Insect immunity is innate and highly efficient to defend against various pathogens. However, uncontrolled excessive immune responses would be highly detrimental and energy-consuming processes. An insect cytokine, plasmatocyte-spreading peptide (SePSP), induces hemocyte-spreading behavior as well as activates phenoloxidase (PO) in the beet armyworm, Spodoptera exigua. A hemocyte transcriptome of S. exigua contains a partial sequence of a putative PSP-binding protein (SePSP-BP1). SePSP-BP1 was expressed in most larval stages except in the last instar. However, a bacterial challenge induced SePSP-BP1 expression in the last instar especially in hemocytes and fat body. Injecting a double-stranded RNA specific to SePSP-BP1 (dsPSP-BP1) suppressed the induction of SePSP-BP1 expression in response to bacterial challenge. The larvae treated with dsPSP-BP1 suffered high mortality to infection of nonpathogenic bacteria due to uncontrolled high PO activity. SePSP significantly induced PO activity. The eicosanoid synthesis inhibitor, dexamethasone (DEX), inhibited SePSP-mediated PO activation. However, treatment with prostaglandin E2 (PGE2) induced a transient increase of PO activity under DEX treatment. Treatment of dsPSP decreased the duration of PO activation induced by PGE2, while treatment of dsPSP-BP1 increased the induced period. These results suggest that prostaglandin mediates PSP signals in both upregulation of PO activity and its subsequent downregulation via SePSP-BP1.