Eicosanoids influence insect susceptibility to nucleopolyhedroviruses

Enhanced baculoviral virulence by suppressing the degradation of an insect immune resolvin, epoxyoctadecamonoenoic acid, in three lepidopteran insects

Epoxyoctadecamonoenoic acids (EpOMEs) are produced from linoleic acid by a cytochrome P450 monooxygenase (CYP) and play a crucial role in terminating excessive and unnecessary immune responses during the late infection stage in insects. This suggests that an increase in the EpOME level may enhance the virulence of insect pathogens against pests. This study tested this hypothesis using a specific inhibitor against soluble epoxide hydrolase (sEH) to degrade EpOMEs, which leads to elevated endogenous EpOME levels. A baculovirus, Autographa californica multiple nucleopolyhedrovirus (AcMNPV), was used to infect three different lepidopteran insects (Spodoptera exigua, Maruca vitrata, and Plutella xylostella) by oral feeding or hemocoelic injection treatments. Within one hour, the viral infection induced the expression of three different phospholipase A2 (PLA2) genes and, after 12 h, up-regulated the expressions of CYP and sEH genes in Spodopera exigua. As expected, AcMNPV virulence was suppressed by the addition of arachidonic acid (a catalytic product of PLA2) but was enhanced by the addition of either of the EpOME regioisomers. In addition, treatment with a specific sEH inhibitor (AUDA) increased AcMNPV virulence against three different lepidopteran insects, presumably by increasing endogenous EpOME levels. This enhanced effect of EpOMEs on virulence was further supported by specific RNA interference (RNAi), in which RNAi specific to CYP expression decreased AcMNPV virulence while a specific RNAi against sEH expression significantly enhanced virulence. In response to AcMNPV infection, TUNEL assay results showed that S. exigua larvae exhibited apoptosis in the midgut, fat body, and epidermis. Inhibition of apoptosis by a pan-caspase inhibitor, Z-VAD-FMK, significantly increased virulence. Similarly, the addition of AUDA to the viral treatment suppressed the gene expression of five inducible caspases and cytochrome C to suppress apoptosis, which led to a significant increase in the tissue viral titers. These results indicate that EpOMEs play a role in terminating excessive and unnecessary immune responses against viral infection during the late stage by down-regulating antiviral apoptosis in lepidopteran insects.

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.

Immunity of an insect herbivore to an entomovirus is affected by different host plants

BACKGROUND

Interactions between herbivorous insects and entomoviruses may depend on host plant, perhaps mediated through changes in herbivore innate immunity.

RESULTS

Caterpillars (Spodoptera exigua) fed Glycine max had high viral loads and low melanization rates together with low melanization enzyme [PO, DDC, TH] activities and gene expressions. Caterpillars fed Ipomoea aquatica had low viral loads and high melanization, gene activities and gene expressions while those fed Brassica oleracea or artificial diet had intermediate levels of each. Melanization rates were negatively correlated with viral loads and positively correlated with activity and expression of each of the three enzymes. Some diet effects on enzymes were constitutive because the same diets led to low (G. max) or high (I. aquatica) melanization related gene activities and expressions without infection.

CONCLUSION

Diet influences the interactions between insect herbivores and viruses by shaping the innate immune response both at the onset of infection and afterwards as viral loads accumulate over a period of days. In addition, diets that lead to low viral loads are associated with high activities and gene expressions of a variety of melanization related enzymes suggesting a common causative mechanism. © 2019 Society of Chemical Industry.

Can Herbivore-Induced Volatiles Protect Plants by Increasing the Herbivores' Susceptibility to Natural Pathogens?

In response to insect herbivory, plants mobilize various defenses. Defense responses include the release of herbivore-induced plant volatiles (HIPVs) that can serve as signals to alert undamaged tissues and to attract natural enemies of the herbivores. Some HIPVs can have a direct negative impact on herbivore survival, but it is not well understood by what mechanisms. Here, we tested the hypothesis that exposure to HIPVs renders insects more susceptible to natural pathogens. Exposure of the caterpillars of the noctuid Spodoptera exigua to indole and linalool, but not exposure to (Z)-3-hexenyl acetate, increased the susceptibility to Spodoptera exiguamultiple nucleopolyhedrovirus (SeMNPV). We also found that exposure to indole, but not exposure to linalool or (Z)-3-hexenyl acetate, increased the pathogenicity of Bacillus thuringiensis Additional experiments revealed significant changes in microbiota composition after forty-eight hours of larval exposure to indole. Overall, these results provide evidence that certain HIPVs can strongly enhance the susceptibility of caterpillars to pathogens, possibly through effects on the insect gut microbiota. These findings suggest a novel mechanism by which HIPVs can protect plants from herbivorous insects.IMPORTANCE Multitrophic interactions involving insect pests, their natural enemies, microorganisms, and plant hosts are increasingly being recognized as relevant factors in pest management. In response to herbivory attacks, plants activate a wide range of defenses that aim to mitigate the damage. Attacked plants release herbivore-induced plant volatiles (HIPVs), which can act as priming signals for other plants and attract natural enemies of herbivores, and which may have a direct negative impact on herbivore survival. In the present work, we show that exposure of the insects to the induced volatiles could increase the insects' susceptibility to the entomopathogens naturally occurring in the plant environment. These findings suggest a novel role for plant volatiles by influencing insect interactions with natural pathogens, probably mediated by alterations in the insect microbiota composition. In addition, this work provides evidence for selectable plant traits (production of secondary metabolites) that can have an influence on the ecology of the pests and could be relevant in the improvement of pest management strategies using natural entomopathogens.

A non-venomous sPLA2 of a lepidopteran insect: Its physiological functions in development and immunity

Eicosanoids are oxygenated C20 polyunsaturated fatty acids that mediate various physiological processes in insects. Eicosanoid biosynthesis begins with a C20 precursor, arachidonic acid (5,8,11,14-eicosatetraenoic acid: AA). AA is usually released from phospholipids at sn-2 position by catalytic activity of phospholipase A₂ (PLA₂). Although various PLA₂s classified into 16 gene families (= Groups) are known in various biological systems, few PLA₂s are known in insects. Only two PLA₂s involved in intracellular calcium independent PLA₂ (iPLA₂) group have been identified in lepidopteran insects with well known eicosanoid physiology. This study reports the first secretory PLA₂ (sPLA₂) in lepidopteran insects. A partial open reading frame (ORF) of PLA₂ was obtained by interrogating Spodoptera exigua transcriptome. Subsequent 3'-RACE resulted in a full ORF (Se-sPLA2A) encoding 194 amino acid sequence containing signal peptide, calcium-binding domain, and catalytic site. Phylogenetic analysis indicated that Se-sPLA₂A was clustered with other Group III sPLA₂s. Se-sPLA₂A was expressed in most larval instars except late last instar. Its expression was inducible by immune challenge and juvenile hormone analog injection. RNA interference of Se-sPLA₂A significantly suppressed cellular immunity and impaired larval development. These results suggest that non-venomous sPLA₂ plays a crucial role in immune and developmental processes in S. exigua, a lepidopteran insect.

Prostaglandin A2 influences gene expression in an established insect cell line (BCIRL-HzAM1) cells

Prostaglandins (PGs) and other eicosanoids are oxygenated metabolites of arachidonic acid and two other C(20) polyunsaturated fatty acids. While most well studied in mammals, PGs exert important actions in insects and virtually all other invertebrates. We have been researching the mechanisms of PG actions in established insect cell lines and reported earlier that two PGs, PGA(1) and PGE(1), influence gene and protein expression in HzAM1 cells. Here we report on further experiments with three 2-series PGs, PGA(2), PGE(2) and PGF(2α). In separate experiments we treated cells with each of the three PGs for 12 and 24h and then analyzed cell lysates by 2-D electrophoresis. Analysis of the gels by Delta2D software showed that PGA(2) influenced expression of 60 proteins while PGE(2) and PGF(2α) treatments led to expression changes for only a few proteins. All spots representing changes in protein expression were processed for analysis by MALDI TOF/TOF mass spectrometry. Bioinformatic analysis of the resulting sequences yielded in silico identifications of all proteins. The apparent changes in some proteins were confirmed by quantitative PCR, which demonstrated that changes in protein expression were parallel to changes in mRNA expression. We assorted the proteins into functional categories, including 1/cell structure and function; 2/cell protection and immunity; 3/energetics and metabolism; 4/nucleotide processing; 5/protein action and processing and 6/signal transduction. These findings substantially extend our idea that one mechanism of PG actions in insect cells is the modulation of gene and protein expression.

Eicosanoids: Exploiting Insect Immunity to Improve Biological Control Programs

Insects, like all invertebrates, express robust innate, but not adaptive, immune reactions to infection and invasion. Insect immunity is usually resolved into three major components. The integument serves as a physical barrier to infections. Within the hemocoel, the circulating hemocytes are the temporal first line of defense, responsible for clearing the majority of infecting bacterial cells from circulation. Specific cellular defenses include phagocytosis, microaggregation of hemocytes with adhering bacteria, nodulation and encapsulation. Infections also stimulate the humoral component of immunity, which involves the induced expression of genes encoding antimicrobial peptides and activation of prophenoloxidase. These peptides appear in the hemolymph of challenged insects 6-12 hours after the challenge. Prostaglandins and other eicosanoids are crucial mediators of innate immune responses. Eicosanoid biosynthesis is stimulated by infection in insects. Inhibition of eicosanoid biosynthesis lethally renders experimental insects unable to clear bacterial infection from hemolymph. Eicosanoids mediate specific cell actions, including phagocytosis, microaggregation, nodulation, hemocyte migration, hemocyte spreading and the release of prophenoloxidase from oenocytoids. Some invaders have evolved mechanisms to suppress insect immunity; a few of them suppress immunity by targeting the first step in the eicosanoid biosynthesis pathways, the enzyme phospholipase A₂. We proposed research designed to cripple insect immunity as a technology to improve biological control of insects. We used dsRNA to silence insect genes encoding phospholipase A₂, and thereby inhibited the nodulation reaction to infection. The purpose of this article is to place our view of applying dsRNA technologies into the context of eicosanoid actions in insect immunity. The long-term significance of research in this area lies in developing new pest management technologies to contribute to food security in a world with a rapidly growing human population.

Benzylideneacetone, an eicosanoid biosynthesis inhibitor enhances baculovirus pathogenicity in the diamondback moth, Plutella xylostella

Benzylideneacetone (BZA) is a monoterpenoid compound produced by an entomopathogenic bacterium, Xenorhabdus nematophila. BZA inhibits phospholipase A(2) to suppress biosynthesis of eicosanoids that mediate immune responses in insects. In response to per os infection of Autographa californica multiple nucleopolyhedrosis virus (AcMNPV), the diamondback moth, Plutella xylostella, developed red spots on the midgut epithelium. The midgut exhibiting red spot formation suffered abnormal cell integrity, such as genomic DNA fragmentation and condensed spots in the nucleoplasm. The number of red spots increased with viral dose and incubation time after the viral treatment. BZA inhibited the formation of the midgut red spots in a dose-dependent manner. However, the inhibitory effect of BZA on the red spot formation was reversed by addition of arachidonic acid, suggesting that the red spot response may be mediated by eicosanoids. BZA treatment resulted in significant enhancement of AcMNPV occlusion body (OB) pathogenicity to P. xylostella.

Contributions of immune responses to developmental resistance in Lymantria dispar challenged with baculovirus

How the innate immune system functions to defend insects from viruses is an emerging field of study. We examined the impact of melanized encapsulation, a component of innate immunity that integrates both cellular and humoral immune responses, on the success of the baculovirus Lymantria dispar multiple nucleocapsid nucleopolyhedrovirus (LdMNPV) in its host L. dispar. L. dispar exhibits midgut-based and systemic, age-dependent resistance to LdMNPV within the fourth instar; the LD(50) in newly molted larvae is approximately 18-fold lower than in mid-instar larvae (48-72h post-molt). We examined the role of the immune system in systemic resistance by measuring differences in hemocyte immunoresponsiveness to foreign targets, hemolymph phenoloxidase (PO) and FAD-glucose dehydrogenase (GLD) activities, and melanization of infected tissue culture cells. Mid-instar larvae showed a higher degree of hemocyte immunoresponsiveness, greater potential PO activity (pro-PO) at the time the virus is escaping the midgut to enter the hemocoel (72h post-inoculation), greater GLD activity, and more targeted melanization of infected tissue, which correlate with reduced viral success in the host. These findings support the hypothesis that innate immune responses can play an important role in anti-viral defenses against baculoviruses and that the success of these defenses can be age-dependent.