PLA2 mediates the innate immune response in Asian corn borer, Ostrinia furnacalis

A secreted phospholipase A2 (BmsPLA2 ) regulates melanization of immunity through BmDDC in the silkworm Bombyx mori

Insect immune-associated phospholipase A2 (PLA2 ) is an important target of pathogen invasion. Melanization, an effective defense response, has significant correlations with other immune responses to coordinate immune attack against invaders. However, the effect of PLA2 on melanization has not yet been reported in insects or other arthropods. In this work, we cloned a PLA2 gene (BmsPLA2 ), and its protein had characteristic features of secreted PLA2 (sPLA2 ). After injection of bacteria, BmsPLA2 expression and sPLA2 activity in hemolymph significantly increased. BmsPLA2 fluorescence was transferred from the cytoplasm to the cell membranes of circulating hemocytes. These results indicated that BmsPLA2 was related to hemolymph immunity in silkworms. Interestingly, reducing BmsPLA2 by RNA interference decreased melanosis (melanistic hemocytes) levels in vivo and in vitro, while BmsPLA2 overexpression had the opposite effect. The larval survival and melanization rate in the hemocoel both slowed depending on the PLA2 inhibitor dosage. These results demonstrated that BmsPLA2 plays a role in melanization during the immune process of silkworms. Surprisingly, the level of BmDDC matched the degree of melanization in various observations. BmDDC expression showed a significant increase, with the peak occurring later than that of BmsPLA2 after injection of bacteria, implying that BmsPLA2 was activated prior to BmDDC. Moreover, the alteration of BmsPLA2 by RNA interference or overexpression led to altered BmDDC levels. These results suggested that BmsPLA2 regulates the melanization response in silkworms through BmDDC. Our study proposes a new regulatory mechanism of the melanization response and new directions for understanding the complex immune networks of insects.

An Efficient and Simple Method for Collecting Haemolymph of Cerambycidae (Insecta: Coleoptera) Adults

Cerambycid beetles (Cerambycidae) are major forest pests, posing a serious threat to the security of forest resources worldwide. Extensive research has focused on the control of cerambycid beetles from physiological and biochemical perspectives. Despite the important roles of insect haemolymph in physiological processes, efficient collection methods for Cerambycidae are lacking. For the efficient and easy collection of large amounts of pure haemolymph from adult cerambycid beetles, a new method, named net centrifugation, was developed. Three species of cerambycid beetles with large differences in size, Anoplophora chinensis, Monochamus saltuarius and Saperda populnea, were selected for the study. Haemolymph was collected by the newly developed net centrifugation method-in which an inner nylon net is used during centrifugation under optimised conditions, and a relatively small wound is generated on the insect-as well as the traditional tearing method and double centrifugation method. Among the three methods evaluated, the net centrifugation method caused the least damage to cerambycid beetles during the whole operation. This method resulted in the most haemolymph from a single beetle, with the lowest turbidity, mostly pure haemocytes in the precipitate, the clearest haemolymph smears by microscopy and the highest quality of RNA extracted from haemocytes. The net centrifugation method has a high collection efficiency, providing important technical support for haemolymph extraction and entomological research.

A secretory phospholipase A2 of a fungal pathogen contributes to lipid droplet homeostasis, assimilation of insect-derived lipids, and repression of host immune responses

Secretory phospholipase A2s (sPLA2s) are found in a wide range of organisms from bacteria to higher plants and animals and are involved in varied and cellular processes. However, roles of these enzymes in microbial pathogens remain unclear. Here, an sPLA2 (BbPLA2) was characterized in the filamentous insect pathogenic fungus, Beauveria bassiana. BbPLA2 was exclusively expressed in insect hemolymph-derived cells (hyphal bodies), and its expression was induced by insect-derived nutrients and lipids, and nutrient starvation. High levels of secretion of BbPLA2 were observed as well as its distribution in hyphal body lipid drops (LDs). Overexpression of BbPLA2 increased the ability of B. bassiana to utilize insect-derived nutrients and lipids, and promoted LD accumulation, indicating functions for BbPLA2 in mediating LD homeostasis and assimilation of insect-derived lipids. Strains overexpressing BbPLA2 showed moderately increased virulence, including more efficient penetration of the insect cuticle and evasion of host immune responses as compared to the wild type strain. In addition, B. bassiana-activated host immune genes were downregulated in the BbPLA2 overexpression strain, but upregulated by infections with a ΔBbPLA2 strain. These data demonstrate that BbPLA2 contributes to LD homeostasis, assimilation of insect-derived lipids, and repression of host immune responses.

Nitric Oxide-Induced Calcineurin A Mediates Antimicrobial Peptide Production Through the IMD Pathway

Nitric oxide (NO) at a high concentration is an effector to kill pathogens during insect immune responses, it also functions as a second messenger at a low concentration to regulate antimicrobial peptide (AMP) production in insects. Drosophila calcineurin subunit CanA1 is a ubiquitous serine/threonine protein phosphatase involved in NO-induced AMP production. However, it is unclear how NO regulates AMP expression. In this study, we used a lepidopteran pest Ostrinia furnacalis and Drosophila S2 cells to investigate how NO signaling affects the AMP production. Bacterial infections upregulated the transcription of nitric oxide synthase 1/2 (NOS1/2), CanA and AMP genes and increased NO concentration in larval hemolymph. Inhibition of NOS or CanA activity reduced the survival of bacteria-infected O. furnacalis. NO donor increased NO level in plasma and upregulated the production of CanA and certain AMPs. In S2 cells, killed Escherichia coli induced NOS transcription and boosted NO production, whereas knockdown of NOS blocked the NO level increase caused by E. coli. As in O. furnacalis larvae, supplementation of the NO donor increased NO level in the culture medium and AMP expression in S2 cells. Suppression of the key pathway genes showed that the IMD (but not Toll) pathway was involved in the upregulation of CecropinA1, Defensin, Diptericin, and Drosomycin by killed E. coli. Knockdown of NOS also reduced the expression of CanA1 and AMPs induced by E. coli, indicative of a role of NO in the AMP expression. Furthermore, CanA1 RNA interference and inhibition of its phosphatase activity significantly reduced NO-induced AMP expression, and knockdown of IMD suppressed NO-induced AMP expression. Together, these results suggest that NO-induced AMP production is mediated by CanA1 via the IMD pathway.

Molecular Characterization of Two Genes Encoding Novel Ca2+-Independent Phospholipase A2s from the Silkworm, Bombyx mori

Eicosanoids are crucial downstream signals in the insect immune responses. Phospholipase A2 (PLA2) catalyzes phospholipids, the initial step in eicosanoid biosynthesis. In mammals, the biological roles of Ca²⁺-independent Phospholipase A2 (iPLA2) have been extensively studied; however, only a few studies have attempted to explore iPLA2 functions in insects. In this study, we identified two iPLA2 genes (designated as BmiPLA2A and BmiPLA2B) in the silkworm, Bombyx mori. BmiPLA2A had a 2427 base pair (bp) open reading frame (ORF) that coded for a protein with 808 amino acids. In contrast, BmiPLA2B had a 1731 bp ORF that coded for a protein with 576 amino acids. Domain analysis revealed that BmiPLA2A had six ankyrin repeat domains, but BmiPLA2B lacks these domains. BmiPLA2A and BmiPLA2B were transcribed widely in various tissues and developmental stages with different expression patterns. The administration of 20-hydroxyecdysone increased their expression levels in the epidermis and hemocytes. Furthermore, challenged with virus, fungus, Gram-negative bacteria, and Gram-positive bacteria induced the expression of BmiPLA2A and BmiPLA2B with variable degrees along with different time points. Our findings imply that BmiPLA2A and BmiPLA2B may have important biological roles in the development and innate immunity of B. mori.

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.