Identification of β-chain of Fo F1 -ATPase in apoptotic cell population induced by Microplitis bicoloratus bracovirus and its role in the development of Spodoptera litura

Innexin hemichannel activation by Microplitis bicoloratus ecSOD monopolymer reduces ROS

The extracellular superoxide dismutases (ecSODs) secreted by Microplitis bicoloratus reduce the reactive oxygen species (ROS) stimulated by the Microplitis bicoloratus bracovirus. Here, we demonstrate that the bacterial transferase hexapeptide (hexapep) motif and bacterial-immunoglobulin-like (BIg-like) domain of ecSODs bind to the cell membrane and transiently open hemichannels, facilitating ROS reductions. RNAi-mediated ecSOD silencing in vivo elevated ROS in host hemocytes, impairing parasitoid larva development. In vitro, the ecSOD-monopolymer needed to be membrane bound to open hemichannels. Furthermore, the hexapep motif in the beta-sandwich of ecSOD49 and ecSOD58, and BIg-like domain in the signal peptides of ecSOD67 were required for cell membrane binding. Hexapep motif and BIg-like domain deletions induced ecSODs loss of adhesion and ROS reduction failure. The hexapep motif and BIg-like domain mediated ecSOD binding via upregulating innexins and stabilizing the opened hemichannels. Our findings reveal a mechanism through which ecSOD reduces ROS, which may aid in developing anti-redox therapy.

Microplitis bicoloratus bracovirus regulates cyclophilin A-apoptosis-inducing factor interaction to induce cell apoptosis in the insect immunosuppressive process

Microplitis bicoloratus bracovirus (MbBV) induces apoptosis in hemocytes of the host (Spodoptera litura) via the cyclophilin A (CypA)-mediated signaling pathway. However, the mechanisms underlying CypA-mediated signaling during apoptosis remain largely unknown. Therefore, in this study, we investigated how CypA and apoptosis-inducing factor (AIF) interact during MbBV-mediated apoptosis. Our findings showed that MbBV induces apoptosis through the CypA-AIF axis of insect immune suppression. In MbBV-infected Spli221 cells, both the expression of the cypa gene and the release of AIF from the mitochondria increased the number of apoptotic cells. CypA and AIF underwent concurrent cytoplasm-nuclear translocation. Conversely, blocking of AIF release from mitochondria not only inhibited the CypA-AIF interaction but also inhibited the cytoplasmic-nuclear translocation of AIF and CypA. Importantly, the survival of the apoptotic phenotype was significantly rescued in MbBV-infected Spli221 cells. In addition, we found that the cyclosporine A-mediated inhibition of CypA did not prevent the formation of the CypA and AIF complex; rather, this only suppressed genomic DNA fragmentation. In vitro experiments revealed direct molecular interactions between recombinant CypA and AIF. Taken together, our results demonstrate that the CypA-AIF interaction plays an important role in MbBV-induced innate immune suppression. This study will help to clarify aspects of insect immunological mechanisms and will be relevant to biological pest control.

Interactions of Vank proteins from Microplitis bicoloratus bracovirus with host Dip3 suppress eIF4E expression

Microplitis bicoloratus bracovirus (MbBV) inhibits the immune response of the host Spodoptera litura by disrupting nuclear factor (NF)-κB signaling and downstream gene expression. However, the underlying molecular mechanisms are not well understood. Herein, we report that viral ankyrin (Vank) proteins interacted with host dorsal-interacting protein 3 (Dip3) to selectively inhibit the transcription of eukaryotic translation initiation factor 4 E (eIF4E). Dip3 and Vank proteins were co-expressed and colocalized in the nucleus. Furthermore, ectopic expression of Dip3 rescued the transcription of some NF-κB-dependent genes suppressed by Vank proteins, including eIF4E. Co-immunoprecipitation and pull-down assays confirmed that Vank proteins interacted with and bound to full-length Dip3, which including MADF, DNA-binding protein, BESS, and protein-protein interaction motifs as well as non-motif sequences. In vivo, RNAi-mediated dip3 silencing decreased eIF4E levels and was accompanied by an immunosuppressive phenotype in S. litura. Our results provided novel insights into the regulation of host transcription during immune suppression by viral proteins that modulate nuclear NF-κB signaling.

Spodoptera litura cyclophilin A is required for Microplitis bicoloratus bracovirus-induced apoptosis during insect cellular immune response

Microplitis bicoloratus bracovirus (MbBV) is a polydnavirus found in the parasitic wasp M. bicoloratus. Although MbBV is a known inducer of apoptosis in host hemocytes, the mechanism by which this occurs remains elusive. In this study, we found that expression of cyclophilin A (CypA) was significantly upregulated in Spodoptera litura hemocytes at 6-day post-parasitization. Similar results were reported in High Five cells (Hi5 cells) infected by MbBV, suggesting that the upregulation of CypA is linked to MbBV infection in insect cells. cDNA encoding CypA was cloned from parasitized hemocytes of S. litura, and bioinformatic analyses showed that S. litura CypA belongs to the cyclophilin family of proteins. Overexpression of S. litura CypA in Hi5 cells revealed that the protein promotes MbBV-induced apoptosis in vitro. Conversely, suppression of the expression and activity of CypA protein significantly rescued the apoptotic phenotype observed in MbBV-infected Hi5 cells, suggesting that it plays a key role in this process. MbBV infection also promoted the cytoplasmic-nuclear translocation of CypA in Hi5 cells. Taken together, these results suggest that MbBV infection upregulates the expression of CypA, which is required for MbBV-mediated apoptosis. Our findings provide insight into the role that CypA plays in insect cellular immune response.

Inhibition of translation initiation factor eIF4A is required for apoptosis mediated by Microplitis bicoloratus bracovirus

Apoptotic hemocytes induced by Microplitis bicoloratus parasitism have been reported, and M. bicoloratus bracovirus (MbBV) is known to be the apoptosis inducer. However, the mechanism how MbBV regulates apoptosis remains unclear. eIF4A, one of translation initiation factors, was found from a Spodoptera litura transcriptome, the expression of which in the parasitized hemocytes of S. litura was inhibited in RT-qPCR analysis. The western blot also illustrated eIF4A at 6-day post-parasitization was inhibited in hemocytes. For testing interaction of MbBV-eIF4A-apoptosis, a cDNA clone encoding 1,266 bp of eIF4A was obtained from S. litura hemocytes and sequenced. Then, a 48 kDa V5-fusion protein of the eIF4A was detected by using the anti-V5 antibody at 72-h post-transfection in the High Five cells, which is located in the cell cytoplasm. In vitro, overexpression of eIF4A rescued the apoptotic High Five cells induced by MbBV. Conversely, in vivo, loss of eIF4A proteins by dsRNA feeding increased apoptosis of hemocytes. Furthermore, RNAi and parasitism significantly increased apoptosis of hemocytes in S. litura. These findings suggested that MbBV inhibited the expression of eIF4A, which was required for apoptosis mediated by MbBV. This study will contribute to biological pest control and enhance our understanding of molecular mechanisms underlying polydnavirus-parasitoid-host interaction.