An aquaporin mediates cell shape change required for cellular immunity in the beet armyworm, Spodoptera exigua

Osmosis as nature's method for establishing optical alignment

For eyes to maintain optimal focus, precise coordination is required between lens optics and retina position, a mechanism that in vertebrates is governed by genetics, visual feedback, and possibly intraocular pressure (IOP).1 While the underlying processes have been intensely studied in vertebrates, they remain elusive in arthropods, though visual feedback may be unimportant.2 How do arthropod eyes remain functional while undergoing substantial growth? Here, we test whether a common physiological process, osmoregulation,3 could regulate growth in the sophisticated camera-type eyes of the predatory larvae of Thermonectus marmoratus diving beetles. Upon molting, their eye tubes elongate in less than an hour, and osmotic pressure measurements reveal that this growth is preceded by a transient increase in hemolymph osmotic pressure. Histological evaluation of support cells that determine the lens-to-retina spacing reveals swelling rather than the addition of new cells. In addition, as expected, treating larvae with hyperosmotic media post-molt leads to far-sighted (hyperopic) eyes due to a failure of proper lengthening of the eye tube and results in impaired hunting success. This study suggests that osmoregulation could be of ubiquitous importance for properly focused eyes.

Identification and characterization of cold-responsive aquaporins from the larvae of a crambid pest Agriphila aeneociliella (Eversmann) (Lepidoptera: Crambidae)

As small ectotherms, insects need to cope with the challenges of winter cold by regulating the water content through water transport. Aquaporins (AQPs) are key players to enhance the cold resistance by mediating essential homeostatic processes in many animals but remain poorly characterized in insects. Agriphila aeneociliella is a newly discovered winter wheat pest in China, and its early-stage larvae have strong tolerance to low temperature stress. Six AQP genes were identified, which belong to five AQP subfamilies (RPIP, Eglp, AQP12L, PRIP, DRIP). All of them contained six hydrophobic transmembrane helices (TMHs) and two relatively conservative Asparagine-Proline-Alanine motifs. The three-dimensional homology modeling showed that the six TMHs folded into an hourglass-like shape, and the imperceptible replace of four ar/R residues in contraction region had critical effects on changing the pore size of channels. Moreover, the transcript levels of AaAQP 1, 3, and 6 increased significantly with the treatment time below 0 °C. Combined with the results of pore radius variation, it is suggested that AaAQP1 and AaAQP3 may be considered to be the key anti-hypothermia proteins in A. aeneociliella by regulating rapid cell dehydration and allowing the influx of extracellular cold resistance molecules, thus avoiding death in winter.

Hemocyte Clusters Defined by scRNA-Seq in Bombyx mori: In Silico Analysis of Predicted Marker Genes and Implications for Potential Functional Roles

Within the hemolymph, insect hemocytes constitute a heterogeneous population of macrophage-like cells that play important roles in innate immunity, homeostasis and development. Classification of hemocytes in different subtypes by size, morphology and biochemical or immunological markers has been difficult and only in Drosophila extensive genetic analysis allowed the construction of a coherent picture of hemocyte differentiation from pro-hemocytes to granulocytes, crystal cells and plasmatocytes. However, the advent of high-throughput single cell technologies, such as single cell RNA sequencing (scRNA-seq), is bound to have a high impact on the study of hemocytes subtypes and their phenotypes in other insects for which a sophisticated genetic toolbox is not available. Instead of averaging gene expression across all cells as occurs in bulk-RNA-seq, scRNA-seq allows high-throughput and specific visualization of the differentiation status of individual cells. With scRNA-seq, interesting cell types can be identified in heterogeneous populations and direct analysis of rare cell types is possible. Next to its ability to profile the transcriptomes of individual cells in tissue samples, scRNA-seq can be used to propose marker genes that are characteristic of different hemocyte subtypes and predict their functions. In this perspective, the identities of the different marker genes that were identified by scRNA-seq analysis to define 13 distinct cell clusters of hemocytes in larvae of the silkworm, Bombyx mori, are discussed in detail. The analysis confirms the broad division of hemocytes in granulocytes, plasmatocytes, oenocytoids and perhaps spherulocytes but also reveals considerable complexity at the molecular level and highly specialized functions. In addition, predicted hemocyte marker genes in Bombyx generally show only limited convergence with the genes that are considered characteristic for hemocyte subtypes in Drosophila.

Toll signal pathway activating eicosanoid biosynthesis shares its conserved upstream recognition components in a lepidopteran Spodoptera exigua upon infection by Metarhizium rileyi, an entomopathogenic fungus

Eicosanoids play crucial roles in mediating immune responses in insects. Upon a fungal infection, Toll signal pathway can mediate immune responses of Spodoptera exigua, a lepidopteran insect, by activating eicosanoid biosynthesis. However, upstream signal components of the Toll signal pathway activating eicosanoid biosynthesis remain unclear. This study predicted pattern recognition receptors (PRRs) and serine proteases (SPs) as upstream components of the Toll pathway with reference to known signal components of Manduca sexta, another lepidopteran insect. S. exigua infected with Metarhizium rileyi, an entomopathogenic fungus, activated phospholipase A₂ (PLA₂) and phenoloxidase (PO) enzymes along with marked increases of expression levels of genes encoding three specific antimicrobial peptides, cecropin, gallerimycin, and hemolin. Among ten Toll receptors encoded in the genome of S. exigua, seven Toll genes were associated with immune responses against fungal infection by M. rileyi through individual RNA interference (RNAi) screening. In addition, two Spätzles (ligands of Toll receptor) were required for Toll signaling against the fungal infection. All predicted upstream components of the Toll pathway were inducible by the fungal infection. Individual RNAi screening showed that three PRRs (βGRP-1, βGRP-2, and GNBP3) and five SPs (ModSP, HP21, HP5, HP6, and HP8) were required for immune responses of S. exigua mediated by Toll signal pathway against the fungal infection. However, two PO-activating proteases (PAP1 and PAP3) were not required for PLA₂ activation, although they were required for PO activation. These results suggest that PRRs and SPs conserved as upstream components in Toll signal pathway play crucial roles in triggering eicosanoid biosynthesis of S. exigua to mediate various immune responses against fungal infection.

PGE2 mediates hemocyte-spreading behavior by activating aquaporin via cAMP and rearranging actin cytoskeleton via Ca2

Spreading behavior of hemocytes (= insect blood cells) is essential for cellular immune responses against various microbial pathogens. It is activated by prostaglandin E₂ (PGE₂) via its membrane receptor associated with secondary messenger, cAMP, in insects. This study observed an increase of calcium ion (Ca²⁺) level after an acute increase of cAMP induced by PGE₂ treatment and clarified the intracellular signals underlying the hemocyte-spreading behavior. Inhibition of Ca²⁺ flux significantly impaired the hemocyte-spreading and subsequent cellular immune response, phagocytosis. The up-regulation of intracellular Ca²⁺ in response to PGE₂ was dependent on cAMP because RNA interference (RNAi) of PGE₂ receptor expression or inhibiting adenylate cyclase prevented Ca²⁺ mobilization. The up-regulation of Ca²⁺ was induced by inositol triphosphate (IP₃) via its specific IP₃ receptor. Furthermore, inhibition of ryanodine receptor impaired Ca²⁺ mobilization, suggesting Ca²⁺-induced Ca²⁺ release. However, the effective spreading behavior of hemocytes was dependent on both secondary messengers. Ca²⁺ signal stimulated by cAMP was required for activating small G proteins because RNAi treatments of small G proteins such as Rac1, RhoA, and Cdc42 failed to stimulate hemocyte-spreading. In contrast, aquaporin was activated by cAMP. Its activity was necessary for changing cell volume during hemocyte-spreading. These results indicate that PGE₂ mediates hemocyte-spreading via cAMP signal to activate aquaporin and via Ca²⁺ signal to activate actin cytoskeletal rearrangement.

Identification and physiological function of CsPrip, a new aquaporin in Chilo suppressalis

Aquaporin (AQP) transport solutes across cell membranes in both unicellular and multicellular organisms. In this study, the aquaporin CsPrip was identified in Chilo suppressalis, an important pest of rice. CsPrip was comprised of two variants, CsPrip_v1 and CsPrip_v2; the former variant was <103 bp was shorter than the latter, although both exhibited the same open reading frame (ORF). Transmembrane topology and protein structure analyses showed that CsPrip retained the conserved features of water-selective insect AQPs, including six transmembrane domains, two conserved hydrophobic asparagine-proline-alanine motifs and the aromatic/arginine constriction region. Expression in Xenopus oocytes revealed that CsPrip preferentially transported water and urea instead of trehalose and glycerol. The CsPrip transcript was expressed in multiple organs and tissues of C. suppressalis larvae and was most abundant in the hindgut and Malpighian tubules. CsPrip transcription was highest in male adults and was relatively stable throughout development. CsPrip expression in larvae was significantly altered by thermal stress, and relative humidity levels impacted CsPrip transcription in 3rd and 5th instar larvae. This study confirms that the aquaporin CsPrip performs multiple critical functions in maintaining water equilibrium in C. suppressalis.

Aquaporins in Immune Cells and Inflammation: New Targets for Drug Development

The mammalian immune system senses foreign antigens by mechanisms that involve the interplay of various kinds of immune cells, culminating in inflammation resolution and tissue clearance. The ability of the immune cells to communicate (via chemokines) and to shift shape for migration, phagocytosis or antigen uptake is mainly supported by critical proteins such as aquaporins (AQPs) that regulate water fluid homeostasis and volume changes. AQPs are protein channels that facilitate water and small uncharged molecules’ (such as glycerol or hydrogen peroxide) diffusion through membranes. A number of AQP isoforms were found upregulated in inflammatory conditions and are considered essential for the migration and survival of immune cells. The present review updates information on AQPs’ involvement in immunity and inflammatory processes, highlighting their role as crucial players and promising targets for drug discovery.

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.

sPLA2 behaves like a prophylactic agent and mediates cellular and humoral immune responses in Plutella xylostella

Most immune effectors are inducible to microbial pathogen infection while some are already present to act as prophylactic immunity against as yet unseen infection. This study identified secretory phospholipase A₂ (sPLA₂ ) as a prophylactic factor in diamondback moth, Plutella xylostella. Western blotting using a polyclonal antibody raised against other lepidopteran sPLA₂ reacted specifically with ∼25 kDa protein, which was present at approximately 0.4 mM in the plasma of naïve larvae. Interrogation of P. xylostella transcriptomes revealed an open-reading frame for sPLA₂ (Px-sPLA₂ ), exhibiting high homology with other Group III sPLA₂ s. Px-sPLA₂ was expressed in all developmental stages. In the larval stage, bacterial challenge induced its expression in hemocytes and fat body but not in gut or epidermis. RNA interference (RNAi) suppressed Px-sPLA₂ messenger RNA level and sPLA₂ activity in plasma. An inhibition zone assay showed that Px-sPLA₂ exhibited antibacterial activities against different species, because specific RNAi knockdown impaired the activity. The RNAi treatment also suppressed the cellular immune response assessed by hemocyte nodule formation and humoral immune response assessed by antimicrobial peptide gene expression. Finally, benzylideneacetone (BZA, a specific sPLA₂ inhibitor) treatment inhibited plasma sPLA₂ activity of naive larvae in a dose-dependent manner. An addition of BZA significantly increased the bacterial virulence of an entomopathogen, Bacillus thuringiensis. These results suggest that Px-sPLA₂ is an immune-associated factor of P. xylostella and its relatively high level of concentration in the plasma of naive larvae strongly suggests its role as a prophylactic factor in defending against pathogens at early infection stages.

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.

Prostaglandin D2 synthase and its functional association with immune and reproductive processes in a lepidopteran insect, Spodoptera exigua

Several prostaglandins (PGs) have been identified in different insect species. However, their biosynthesis and physiological roles in insects remain unclear. PGD₂ is synthesized by isomerization from PGH₂ in mammals. This study identified a PGD₂ synthase (SePGDS) in a lepidopteran insect, Spodoptera exigua. It showed sequence homology (32.8%) with human PGDS. Based on its conserved active site residues, its N-terminal tyrosine (Y8) was predicted to mediate electron relay from glutathione to PGH₂ substrate, which was distinct from the catalysis of PGE₂ (=PGD₂ isomer) synthase (SePGES). SePGDS was highly expressed in larval and adult stages. RNA interference (RNAi) of SePGDS expression resulted in immunosuppression of cellular immune responses by suppressing the expression of actin polymerization-associated genes. It also suppressed the expression of some antimicrobial genes. Such immunosuppression induced by RNAi treatment was specifically rescued by the addition of PGD₂, but not its precursor, arachidonic acid. Such RNAi treatment in adults prevented egg development in females by inhibiting choriogenesis. RNAi treatment also suppressed nurse cell dumping to growing oocytes. However, the addition of PGD₂ rescued egg development of RNAi-treated females. These results suggest that SePGDS is responsible for the production of PGD₂ which mediates immune and reproductive processes of S. exigua.

PGE2 mediates cytoskeletal rearrangement of hemocytes via Cdc42, a small G protein, to activate actin-remodeling factors in Spodoptera exigua (Lepidoptera: Noctuidae)

Prostaglandin E₂ (PGE₂ ) mediates cellular immune responses in insects by stimulating hemocyte-spreading behavior that is driven by actin remodeling to form filopodial or lamellipodial cytoplasmic extensions. In Spodoptera exigua (Lepidoptera: Noctuidae), Cdc42, a small G protein, played a crucial role in mediating PGE₂ signal on hemocyte-spreading behavior. Hemocyte-spreading behavior requires actin cytoskeletal rearrangement. A plethora of actin-related proteins have been predicted to have functional links with Cdc42. Here, we selected four actin-associated genes (Actin-related protein 2 [Arp2], Profilin, Cofilin, and Fascin) and evaluated their influences on cytoskeletal rearrangement in S. exigua. Bioinformatic analysis confirmed their gene identities. Transcript analysis using reverse-transcription polymerase chain reaction indicated that all four actin-associated genes were expressed in most developmental stages, showing high expression levels in larval hemocytes. RNA interference (RNAi) against these genes was performed by injecting double-stranded RNA (dsRNA) to hemocoel. Under RNAi condition, the hemocyte-spreading behavior was significantly impaired except for dsRNA treatment against Cofilin, an actin-depolymerizing factor. Alteration of cytoskeletal rearrangement appeared to vary after different RNAi treatments. RNAi against Arp2 markedly suppressed lamellipodial extension while RNAi against Profilin or Fascin adversely influenced filopodial extension. RNAi of these actin-associated factors prevented cellular immune responses measured by nodule formation against bacterial challenge. Under RNAi conditions, addition of PGE₂ did not well induce hemocyte-spreading behavior, suggesting that these actin-associated factors might act downstream of the hormone signaling pathway. These results suggest that PGE₂ can mediate hemocyte-spreading behavior via Cdc42 to activate downstream actin polymerization/branching/bundling factors, thus inducing actin cytoskeletal rearrangement.