Glyceraldehyde-3-phosphate dehydrogenase is a mediator of hemocyte-spreading behavior and molecular target of immunosuppressive factor CrV1

Bracovirus Sneaks Into Apoptotic Bodies Transmitting Immunosuppressive Signaling Driven by Integration-Mediated eIF5A Hypusination

A typical characteristics of polydnavirus (PDV) infection is a persistent immunosuppression, governed by the viral integration and expression of virulence genes. Recently, activation of caspase-3 by Microplitis bicoloratus bracovirus (MbBV) to cleave Innexins, gap junction proteins, has been highlighted, further promoting apoptotic cell disassembly and apoptotic body (AB) formation. However, whether ABs play a role in immune suppression remains to be determined. Herein, we show that ABs transmitted immunosuppressive signaling, causing recipient cells to undergo apoptosis and dismigration. Furthermore, the insertion of viral–host integrated motif sites damaged the host genome, stimulating eIF5A nucleocytoplasmic transport and activating the eIF5A-hypusination translation pathway. This pathway specifically translates apoptosis-related host proteins, such as P53, CypA, CypD, and CypJ, to drive cellular apoptosis owing to broken dsDNA. Furthermore, translated viral proteins, such Vank86, 92, and 101, known to complex with transcription factor Dip3, positively regulated DHYS and DOHH transcription maintaining the activation of the eIF5A-hypusination. Mechanistically, MbBV-mediated extracellular vesicles contained inserted viral fragments that re-integrated into recipients, potentially via the homologous recombinant repair system. Meanwhile, this stimulation regulated activated caspase-3 levels via PI3K/AKT 308 and 473 dephosphorylation to promote apoptosis of granulocyte-like recipients Sf9 cell; maintaining PI3K/AKT 473 phosphorylation and 308 dephosphorylation inhibited caspase-3 activation leading to dismigration of plasmatocyte-like recipient High Five cells. Together, our results suggest that integration-mediated eIF5A hypusination drives extracellular vesicles for continuous immunosuppression.

Regulation of the Larval Transcriptome of Diatraea saccharalis (Lepidoptera: Crambidae) by Maternal and Other Factors of the Parasitoid Cotesia flavipes (Hymenoptera: Braconidae)

Koinobiont endoparasitoid wasps regulate the host's physiology to their own benefit during their growth and development, using maternal, immature and/or derived-tissue weaponry. The tools used to subdue the wasps' hosts interfere directly with host transcription activity. The broad range of host tissues and pathways affected impedes our overall understanding of the host-regulation process during parasitoid development. Next-generation sequencing and de novo transcriptomes are helpful approaches to broad questions, including in non-model organisms. In the present study, we used Illumina sequencing to assemble a de novo reference transcriptome of the sugarcane borer Diatraea saccharalis, to investigate the regulation of host gene expression by the larval endoparasitoid Cotesia flavipes. We obtained 174,809,358 reads and assembled 144,116 transcripts, of which 44,325 were putatively identified as lepidopteran genes and represented a substantial number of pathways that are well described in other lepidopteran species. Comparative transcriptome analyses of unparasitized versus parasitized larvae identified 1,432 transcripts of D. saccharalis that were up-regulated under parasitization by C. flavipes, while 1,027 transcripts were down-regulated. Comparison of the transcriptomes of unparasitized and pseudoparasitized D. saccharalis larvae led to the identification of 1,253 up-regulated transcripts and 972 down-regulated transcripts in the pseudoparasitized larvae. Analysis of the differentially expressed transcripts showed that C. flavipes regulated several pathways, including the Ca+2 transduction signaling pathway, glycolysis/gluconeogenesis, chitin metabolism, and hormone biosynthesis and degradation, as well as the immune system, allowing us to identify key target genes involved in the metabolism and development of D. saccharalis.

Facilitator roles of viruses in enhanced insect resistance to biotic stress

Virus-insect interactions are primarily parasitic, yet diverse mutualistic interactions, some of which are symbiogenic, also occur. These viruses can modify insect physiology and behavior so that hosts can gain resistance against various biotic challenges like pathogen and parasites. In the recent past, many insect mutualistic viruses have been reported. Viruses can show symbiogenic interactions with some insects, which have been explored at the molecular level. However, understanding about molecular mechanisms for many of the mutualistic viruses is still enigmatic. Exploration of these interactions and its mechanism can shed light on phenomenon of virus mediated biotic stress resistance in insects.

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

Cellular immunity in insects is accompanied by change in hemocyte shape. This study hypothesizes that cytoskeletal rearrangement is accompanied by transmembrane water transport to change cell volume, thus changing cell shape. A water-transporting pore (=aquaporin:AQP) has been identified in the beet armyworm, Spodoptera exigua. Its expression was detected in all developmental stages and tissues, although its transcription levels were different between biotic and abiotic conditions. Heterologous expression of Se-AQP in Sf9 cells showed that Se-AQP was localized on cell membrane. RNA interference (RNAi) using double-stranded RNA effectively suppressed its transcript levels. Under different ionic concentrations, hemocytes of RNAi-treated larvae did not change cell volume presumably due to malfunction in water transportation. Se-AQP might participate in glycerol transport because up-regulation of hemolymph glycerol titer after rapid cold-hardening was prevented by RNAi treatment against Se-AQP expression. The inhibitory effect of RNAi treatment on change of cell shape significantly impaired cellular immune responses such as phagocytosis and nodule formation upon bacterial challenge. RNAi treatment also significantly interfered with immature development of S. exigua. These results indicate that Se-AQP plays a crucial role in cell shape change that is required for cellular immunity and other physiological processes.

Parasitoid polydnaviruses and immune interaction with secondary hosts

Polydnaviruses (PDVs) are obligatory symbionts with parasitoid wasps. The PDV virions are produced solely in wasp (the primary host) calyx cells. They are injected into caterpillar hosts (the secondary host) during parasitoid oviposition, where they express irreplaceable actions to ensure survival and development of wasp larvae. Some of PDV gene products suppress host immune responses while others alter host growth, metabolism or endocrine system. Here, we treat new findings on PDV gene products and their action on immunity within secondary hosts.

RNA Interference in Moths: Mechanisms, Applications, and Progress

The vast majority of lepidopterans, about 90%, are moths. Some moths, particularly their caterpillars, are major agricultural and forestry pests in many parts of the world. However, some other members of moths, such as the silkworm Bombyx mori, are famous for their economic value. Fire et al. in 1998 initially found that exogenous double-stranded RNA (dsRNA) can silence the homolog endogenous mRNA in organisms, which is called RNA interference (RNAi). Soon after, the RNAi technique proved to be very promising not only in gene function determination but also in pest control. However, later studies demonstrate that performing RNAi in moths is not as straightforward as shown in other insect taxa. Nevertheless, since 2007, especially after 2010, an increasing number of reports have been published that describe successful RNAi experiments in different moth species either on gene function analysis or on pest management exploration. So far, more than 100 peer-reviewed papers have reported successful RNAi experiments in moths, covering 10 families and 25 species. By using classic and novel dsRNA delivery methods, these studies effectively silence the expression of various target genes and determine their function in larval development, reproduction, immunology, resistance against chemicals, and other biological processes. In addition, a number of laboratory and field trials have demonstrated that RNAi is also a potential strategy for moth pest management. In this review, therefore, we summarize and discuss the mechanisms and applications of the RNAi technique in moths by focusing on recent progresses.

A viral histone H4 suppresses insect insulin signal and delays host development

Parasitization by an endoparasitoid wasp, Cotesia plutellae, alters host development of Plutella xylostella by extending larval period and preventing metamorphosis. Insulin signal plays a crucial role in mediating insect development and controlling blood sugar level in insects. In this study, three insulin-like peptide genes (PxILP1-3) were predicted from the genome of P. xylostella. However, only PxILP1 was confirmed to be expressed in P. xylostella. Starvation suppressed the expression level of PxILP1 and up-regulated plasma trehalose level. RNA interference against PxILP1 mimicked starvation effect and extended the larval period of P. xylostella. Parasitized larvae exhibited significantly lower levels of PxILP1 expression compared to nonparasitized larvae. Injection of wasp-symbiotic polydnavirus C. plutellae bracovirus (CpBV) also suppressed PxILP1 expression and extended the larval period. Injection of a viral segment (CpBV-S30) containing a viral histone H4 (CpBV-H4) also suppressed PxILP1 expression. Co-injection of CpBV-S30 and double-stranded RNA (dsCpBV-H4) specific to CpBV-H4 rescued the suppression of PxILP1 expression. Injection of CpBV-S30 significantly extended larval development. Co-injection of CpBV-S30 with dsCpBV-H4 rescued the delay of larval development. Injection of a bovine insulin to parasitized larvae prevented parasitoid development. These results indicate that parasitism of C. plutellae can down-regulate host insulin signaling with the help of parasitic factor CpBV-H4.

Translational Control of Host Gene Expression by a Cys-Motif Protein Encoded in a Bracovirus

Translational control is a strategy that various viruses use to manipulate their hosts to suppress acute antiviral response. Polydnaviruses, a group of insect double-stranded DNA viruses symbiotic to some endoparasitoid wasps, are divided into two genera: ichnovirus (IV) and bracovirus (BV). In IV, some Cys-motif genes are known as host translation-inhibitory factors (HTIF). The genome of endoparasitoid wasp Cotesia plutellae contains a Cys-motif gene (Cp-TSP13) homologous to an HTIF known as teratocyte-secretory protein 14 (TSP14) of Microplitis croceipes. Cp-TSP13 consists of 129 amino acid residues with a predicted molecular weight of 13.987 kDa and pI value of 7.928. Genomic DNA region encoding its open reading frame has three introns. Cp-TSP13 possesses six conserved cysteine residues as other Cys-motif genes functioning as HTIF. Cp-TSP13 was expressed in Plutella xylostella larvae parasitized by C. plutellae. C. plutellae bracovirus (CpBV) was purified and injected into non-parasitized P. xylostella that expressed Cp-TSP13. Cp-TSP13 was cloned into a eukaryotic expression vector and used to infect Sf9 cells to transiently express Cp-TSP13. The synthesized Cp-TSP13 protein was detected in culture broth. An overlaying experiment showed that the purified Cp-TSP13 entered hemocytes. It was localized in the cytosol. Recombinant Cp-TSP13 significantly inhibited protein synthesis of secretory proteins when it was added to in vitro cultured fat body. In addition, the recombinant Cp-TSP13 directly inhibited the translation of fat body mRNAs in in vitro translation assay using rabbit reticulocyte lysate. Moreover, the recombinant Cp-TSP13 significantly suppressed cellular immune responses by inhibiting hemocyte-spreading behavior. It also exhibited significant insecticidal activities by both injection and feeding routes. These results indicate that Cp-TSP13 is a viral HTIF.

Altered immune function of Octodonta nipae (Maulik) to its pupal endoparasitoid, Tetrastichus brontispae Ferrière

Most studies on the contribution of the altered immune response by endoparasitoid have been restricted to the interactions between Ichneumonoidea and their hosts, while effects of parasitism by Chalcidoidea on the hosts have rarely been characterized except some wasps such as Pteromalidae. Endoparasitoid Tetrastichus brontispae Ferrière, belonging to Eulophidae (Hymenoptera), has a great potential to control some Coleopteran beetles such as Octodonta nipae, one invasive species in southern China. However, the physiological mechanism underlying the escape from the melanotic encapsulation in O. nipae pupae has not been demonstrated. In the present study, effects of parasitism on the immune function of its pupal host O. nipae were investigated. The combining results that granulocytes and plasmatocytes could phagocytize bacteria from 2 to 48h and granulocytes, plasmatocytes and oenocytoids were prophenoloxidase/phenoloxidase positive hemocytes indicated that granulocytes, plasmatocytes and oenocytoids were the main immunocompetent hemocytes in O. nipae pupae. Parasitism by T. brontispae resulted in a significant increase in the percentage of hemocytes viability and spreading at 96h, growing percentage of granulocytes at 24h but no effects on the total hemocyte counts, and an enhanced phenoloxidase activity only at 12 and 72h while a significantly longer melanization time of the hemolymph at 96h following parasitism. These results indicate that mixtures of systemic active and local active regulation are used for T. brontispae to escape host encapsulation in O. nipae pupae. The present study contributes to the understanding of the diversity of virulence strategies used by parasitoids.