Functional analysis of two inhibitor of apoptosis (iap) orthologs from Helicoverpa armigera nucleopolyhedrovirus

The regulation of inhibitor of apoptosis proteins (IAPs) during the apoptosis of Cotesia chilonis

Inhibitor of apoptosis proteins (IAPs) are crucial components of apoptosis that perform vital roles in the regulation of caspase activity in organisms. In this study, two IAPs genes were identified from Cotesia chilonis, the dominant parasitic wasp of Chilo suppressalis. CcIAP1 gene is a typical IAP and contains two BIR domains and a RING domain, whereas CcIAP gene is an atypical IAP1 only containing two BIR domains. Phylogenetic analysis indicated that CcIAP1 and CcIAP were grouped with other Hymenopteran IAPs and IAP1 in C. suppressalis. Real-time quantitative PCR revealed that CcIAP1 and CcIAP genes were both highly induced at -6°C and 30°C, and expression was highest at the third instar stage. The expression of CcIAP1 and CcIAP genes were significantly induced during parasitism of C. suppressalis, and the 7-d time point resulted in the highest expression levels for both genes, in which was an advanced stage of larval development of C. chilonis. RNAi experiments showed that CcIAP1 gene was the key IAP in the regulation of apoptosis of C. chilonis and its host. In conclusion, CcIAP1 and CcIAP correlate with the development of C. chilonis and their responses to temperature stress.

Functional analyses of inhibitor of apoptosis protein 1 (IAP1) of Antheraea pernyi multinucleocapsid nucleopolyhedrovirus (AnpeNPV) in viral replication and occlusion body production

Inhibitor of apoptosis protein 1 (IAP1) of Antheraea pernyi multinucleocapsid nucleopolyhedrovirus (AnpeNPV) belongs to the baculovirus IAP1 type. The function of AnpeNPV-IAP1 in viral replication and occlusion body (OB) production remains unknown. In this study, we demonstrated that AnpeNPV-iap1 is a late gene. AnpeNPV-IAP1 mainly localizes to the nuclear ring zone and exhibits dynamic distribution in the cytoplasm and the virogenic stroma during AnpeNPV infection. AnpeNPV-IAP1 impacted the expression of a variety of viral genes at the very late phase of infection in Tn-Hi5 cells. The deletion of AnpeNPV-iap1 caused decreased expression levels of polyhedrin, morphological changes to OBs and reduced OB production in A. pernyi pupae, along with a lengthening of the lethal time of A. pernyi larvae. These results suggest that AnpeNPV-iap1 is involved in regulating viral gene expression, OB production and morphogenesis in A. pernyi.

Impact of Group II Baculovirus IAPs on Virus-Induced Apoptosis in Insect Cells

Apoptosis plays an important role in virus-host interactions and is a major element of the insect immune response. Exploring the regulatory mechanisms of virus-induced apoptosis through the expression of apoptotic genes holds important research and application value. Functional research on the reported inhibitor of apoptosis proteins (IAPs) mainly focuses on the group I baculovirus, while the functions of the group II baculovirus IAPs remains unclear. To explore its role in the regulation of the apoptosis of insect cells, we constructed the transient expression vector (pIE1 vectors) and the recombinant baculovirus expressing Bsiap genes (from the Buzura suppressaria nucleopolyhedrovirus) of the group II baculovirus. Apoptosis gene expression results and the virus-induced apoptosis rate show that the overexpression of BsIAP1 could promote apoptosis in insect cells. However, the overexpression of BsIAP2 and BsIAP3 decreases the expression of apoptotic genes, revealing an inhibitory effect. Results on the impact of baculovirus-induced apoptosis also confirm that BsIAP1 reduces viral nucleocapsid expression and the baculovirus titer, while BsIAP2 and BsIAP3 increase them significantly. Furthermore, compared with single expression, the co-expression of BsIAP2 and BsIAP3 significantly reduces the rate of virus-induced apoptosis and improves the expression of nucleocapsids and the titer of offspring virus, indicating the synergistic effect on BsIAP2 and BsIAP3. In addition, combined expression of all three BsIAPs significantly reduced levels of intracellular apoptosis-related genes (including apoptosis and anti-apoptosis genes), as well as apoptosis rate and progeny virus titer, indicating that life activities in insect cells are also inhibited. These findings reveal the relationship between apoptosis and group II baculovirus IAP, which provide an experimental and theoretical basis for further exploration of the molecular mechanism between group II baculoviruses and insect cells.

Evolutionary and functional analyses of the interaction between the Bombyx mori inhibitor of apoptosis (IAP) and nucleopolyhedrovirus IAPs

As an important insect immune response, apoptosis plays a critical role in the interaction between baculoviruses and insect hosts. Previous reports have identified inhibitor of apoptosis (IAP) proteins in both insects and baculoviruses, but the relationship between these proteins is still not clearly understood. Here, we found that insect IAP proteins were clustered with baculovirus IAP3, suggesting that the baculovirus iap3 gene might be derived from the Lepidoptera or Diptera. We demonstrated that Bombyx mori inhibitor of apoptosis (Bmiap) gene had an inhibitory effect on apoptosis in silkworm cells. Further analysis of the effects of Bmiap genes on the proliferation of B. mori nucleopolyhedrovirus (BmNPV) showed that both the Bmiap and BmNPV iap genes increased BmNPV proliferation after BmNPV infected silkworm cells. Our results also indicated that BmNPV IAP1 and IAP2 directly interacted with BmIAP in silkworm cells, implying that the Bmiap gene might be hijacked by BmNPV iap genes during BmNPV infection. Taken together, our results provide important insights into the functional relationships of iap genes, and improve our knowledge of apoptosis in baculoviruses and insect hosts.

Genomic sequencing of Troides aeacus nucleopolyhedrovirus (TraeNPV) from golden birdwing larvae (Troides aeacus formosanus) to reveal defective Autographa californica NPV genomic features

Background

The golden birdwing butterfly (Troides aeacus formosanus) is a rarely observed species in Taiwan. Recently, a typical symptom of nuclear polyhedrosis was found in reared T. aeacus larvae. From the previous Kimura-2 parameter (K-2-P) analysis based on the nucleotide sequence of three genes in this isolate, polh, lef-8 and lef-9, the underlying virus did not belong to any known nucleopolyhedrovirus (NPV) species. Therefore, this NPV was provisionally named “TraeNPV”. To understand this NPV, the nucleotide sequence of the whole TraeNPV genome was determined using next-generation sequencing (NGS) technology.

Results

The genome of TraeNPV is 125,477 bp in length with 144 putative open reading frames (ORFs) and its GC content is 40.45%. A phylogenetic analysis based on the 37 baculoviral core genes suggested that TraeNPV is a Group I NPV that is closely related to Autographa californica nucleopolyhedrovirus (AcMNPV). A genome-wide analysis showed that TraeNPV has some different features in its genome compared with other NPVs. Two novel ORFs (Ta75 and Ta139), three truncated ORFs (pcna, he65 and bro) and one duplicated ORF (38.7 K) were found in the TraeNPV genome; moreover, there are fewer homologous regions (hrs) than there are in AcMNPV, which shares eight hrs within the TraeNPV genome. TraeNPV shares similar genomic features with AcMNPV, including the gene content, gene arrangement and gene/genome identity, but TraeNPV lacks 15 homologous ORFs from AcMNPV in its genome, such as ctx, host cell-specific factor 1 (hcf-1), PNK/PNL, vp15, and apsup, which are involved in the auxiliary functions of alphabaculoviruses.

Conclusions

Based on these data, TraeNPV would be clarified as a new NPV species with defective AcMNPV genomic features. The precise relationship between TraeNPV and other closely related NPV species were further investigated. This report could provide comprehensive information on TraeNPV for evolutionary insights into butterfly-infected NPV.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5713-2) contains supplementary material, which is available to authorized users.

Characterization and functional assay of apsup (Lyxy105) from Lymantria xylina multiple nucleopolyhedrovirus (LyxyMNPV)

The baculoviral anti-apoptotic genes, p35 and iap (inhibitor of apoptosis), play important roles in the initiation of viral infection. Recently, a new anti-apoptotic gene (apoptosis suppressor, apsup) was identified in Lymantria dispar multiple nucleopolyhedrovirus (LdMNPV). An apsup homolog gene, Lyxy105 (ly-apsup), was also predicted in the Lymantria xylina multiple nucleopolyhedrovirus (LyxyMNPV) genome. In this study, we attempt to perform a gene expression analysis and a functional assay of ly-apsup to demonstrate its anti-apoptotic activity and identify the functional domain of this protein. The transcription of the ly-apsup gene region was detected from 12 h post-infection (hpi) and increased significantly after 24-72 hpi. Comparison of the putative amino acid sequences to those of 18 baculoviral homolog proteins showed high amino acid identity to the LdMNPV sequences. Moreover, five conserved protein domains (named as domains I-V) were found. Therefore, protein functional assays were conducted on full-length proteins and different truncation clones. The overexpression of each clone was confirmed by western blot analysis, and the data revealed that a cleavage of ~ 5 kDa at the N-terminal region of the full-length, domains I-IV (1-241) and I-III (1-178), proteins occurred. The results of the functional analysis showed that full-length Ly-apsup and Ly-apsup with domain I (1-70) could inhibit Drosophila-RPR protein (D-RPR)-induced and actinomycin D (ActD)-induced apoptoses. In addition, the domains I and I-II (1-126) regions showed higher anti-apoptotic activity than the other domains in both D-RPR-induced and ActD-induced cell apoptoses. In conclusion, domain I of Ly-apsup may play an important role in the anti-apoptotic activity of this protein; cleavage of the Ly-apsup N-terminus may lead to decreased anti-apoptotic activity.

Function analysis and application of IAP1/2 of Autographa californica multiple nucleopolyhedrovirus

We confirmed the function of Ac-IAPs and developed a useful AcMNPV-iap2-egfp, which provided theoretical foundation for using and modifying AcMNPV.

Transcriptome dynamics in the asexual cycle of the chordate Botryllus schlosseri

Background

We performed an analysis of the transcriptome during the blastogenesis of the chordate Botryllus schlosseri, focusing in particular on genes involved in cell death by apoptosis. The tunicate B. schlosseri is an ascidian forming colonies characterized by the coexistence of three blastogenetic generations: filter-feeding adults, buds on adults, and budlets on buds. Cyclically, adult tissues undergo apoptosis and are progressively resorbed and replaced by their buds originated by asexual reproduction. This is a feature of colonial tunicates, the only known chordates that can reproduce asexually.

Results

Thanks to a newly developed web-based platform (http://botryllus.cribi.unipd.it), we compared the transcriptomes of the mid-cycle, the pre-take-over, and the take-over phases of the colonial blastogenetic cycle. The platform is equipped with programs for comparative analysis and allows to select the statistical stringency. We enriched the genome annotation with 11,337 new genes; 581 transcripts were resolved as complete open reading frames, translated in silico into amino acid sequences and then aligned onto the non-redundant sequence database. Significant differentially expressed genes were classified within the gene ontology categories. Among them, we recognized genes involved in apoptosis activation, de-activation, and regulation.

Conclusions

With the current work, we contributed to the improvement of the first released B. schlosseri genome assembly and offer an overview of the transcriptome changes during the blastogenetic cycle, showing up- and down-regulated genes. These results are important for the comprehension of the events underlying colony growth and regression, cell proliferation, colony homeostasis, and competition among different generations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2598-1) contains supplementary material, which is available to authorized users.

Nucleopolyhedrovirus infection and/or parasitism by Microplitis pallidipes Szepligeti affect hemocyte apoptosis of Spodoptera exigua (Hübner) larvae

We determined the effects of parasitism by the endoparasitoid Microplitis pallidipes Szepligeti and/or nucleopolyhedrovirus (NPV) infection on hemocyte apoptosis of Spodoptera exigua (Hübner) larvae. Compared to healthy (control) larvae, larvae that were parasitized, virus-infected, or both all showed a significant increase in hemocyte apoptosis during 48-h observation period. The peaks of hemocyte apoptosis in parasitized, virus-infected and parasitized+infected larvae were at 12, 24 and 48 h after treatment, and were 86.7±1.9, 87.4±3.6 and 76.5±1.6%, respectively. Meanwhile, compared to parasitized larvae, hemocyte apoptosis in jointly parasitized and infected larvae increased by 12.9%, 18.7% and 2.8% at 8, 36 and 48 h respectively, and decreased by 39.0% and 9.1% at 12 and 24h. Compared to virus-infected larvae, hemocyte apoptosis in jointly parasitized and infected larvae increased by 13.4%, 2.4% and 15.3% at 8, 36 and 48 h, respectively, and decreased by 4.0% and 29.9% at 12 and 24h. Our study found that joint and separate parasitism and SeNPV infection induced hemocyte apoptosis of S. exigua larvae. It also revealed that NPV infection promoted host hemocyte apoptosis induced by parasitism at early egg and larval stages of M. pallidipes in host larvae, but inhibited the same effect at late egg stage of M. pallidipes in host larvae, and that parasitism promoted host hemocyte apoptosis induced by NPV infection at early egg and larval stages of M. pallidipes in host larvae, but inhibited the same effect at late egg stage of M. pallidipes in host larvae.

The Baculovirus Antiapoptotic p35 Protein Functions as an Inhibitor of the Host RNA Interference Antiviral Response

RNA interference (RNAi) is considered an ancient antiviral defense in diverse organisms, including insects. Virus infections generate double-strand RNAs (dsRNAs) that trigger the RNAi machinery to process dsRNAs into virus-derived short interfering RNAs (vsiRNAs), which target virus genomes, mRNAs, or replication intermediates. Viruses, in turn, have evolved viral suppressors of RNAi (VSRs) to counter host antiviral RNAi. Following recent discoveries that insects mount an RNAi response against DNA viruses, in this study, we found that Autographa californica multiple nucleopolyhedrovirus (AcMNPV) infection similarly induces an RNAi response in Spodoptera frugiperda cells by generating a large number of vsiRNAs postinfection. Interestingly, we found that AcMNPV expresses a potent VSR to counter RNAi. The viral p35 gene, which is well known as an inhibitor of apoptosis, was found to be responsible for the suppression of RNAi in diverse insect and mammalian cells. The VSR activity of p35 was further confirmed by a p35-null AcMNPV that did not suppress the response. In addition, our results showed that the VSR activity is not due to inhibition of dsRNA cleavage by Dicer-2 but acts downstream in the RNAi pathway. Furthermore, we found that the VSR activity is not linked to the antiapoptotic activity of the protein. Overall, our results provide evidence for the existence of VSR activity in a double-stranded DNA virus and identify the responsible gene, which is involved in the inhibition of RNAi as well as apoptosis.

IMPORTANCE

Our findings demonstrate the occurrence of an insect RNAi response against a baculovirus (AcMNPV) that is highly utilized in microbial control, biological and biomedical research, and protein expression. Moreover, our investigations led to the identification of a viral suppressor of RNAi activity and the gene responsible for the activity. Notably, this gene is also a potent inhibitor of apoptosis. The outcomes signify the dual role of a virus-encoded protein in nullifying two key antiviral responses, apoptosis and RNAi.

The BIR and BIR-like domains of Bombyx mori nucleopolyhedrovirus IAP2 protein are required for efficient viral propagation

The baculovirus Bombyx mori nucleopolyhedrovirus (BmNPV) possesses two genes, iap1 and iap2, which encode inhibitor of apoptosis (IAP) proteins. We previously showed that although both genes are dispensable for viral propagation, iap2 is required for efficient viral propagation in cultured cells. BmNPV IAP2 contains three putative functional domains: a baculovirus IAP repeat (BIR), a BIR-like (BIRL) domain, and a RING finger domain. To identify the domain affecting viral growth, we generated a series of BmNPV bacmids expressing iap2 derivatives lacking one or two domains, or possessing a single amino acid substitution to abolish IAP2 ubiquitin ligase activity. We examined their properties in both cultured cells and B. mori larvae. We found that either the BIR or BIRL domain of IAP2 plays an important role in BmNPV infection, and that the RING finger domain, which is required for ubiquitin ligase activity, does not greatly contribute to BmNPV propagation. This is the first study to identify functional domains of the baculovirus IAP2 protein.

Genome scale analysis of differential mRNA expression of Helicoverpa zea insect cells infected with a H. armigera baculovirus

Knowledge of baculovirus-insect host interactions at a genome-scale level is important for developing a number of baculovirus-based applications, but the gathering of such knowledge is hindered by the lack of genomic sequences in most insect hosts. In this study, expression kinetics of 24,206 Helicoverpa zea insect transcripts and 134 Helicoverpa armigera nucleopolyhedrovirus (HearNPV) genes at 0, 12, 24 and 48 h post-infection (hpi) were simultaneously analyzed using microarrays, which were developed from sequences obtained by deep transcriptome sequencing. Host genes in pathways important for infection such as those for energy generation, anti-viral peptides, apoptosis, detoxification, DNA polymerase activities, RNA polymerase activities, translation initiation, protein processing and cell cycle arrest were identified. Differential expression was linked to changes in the number of intracellular and extracellular viral genomes and occlusion bodies. The first comprehensive elucidation of HearNPV-H. zea expression kinetics was obtained.

Baculovirus Lymantria dispar multiple nucleopolyhedrovirus IAP2 and IAP3 do not suppress apoptosis, but trigger apoptosis of insect cells in a transient expression assay

Ld652Y cells derived from the gypsy moth, Lymantria dispar, are permissive for productive infection with L. dispar multiple nucleopolyhedrovirus (LdMNPV), but undergo apoptosis upon infection with various other NPVs, including those isolated from Bombyx mori, Hyphantria cunea, Spodoptera exigua, Orgyia pseudotsugata, and Spodoptera litura. In this study, we examined whether LdMNPV-encoded inhibitor of apoptosis 2 (Ld-IAP2) and 3 (Ld-IAP3) are involved in apoptosis suppression in LdMNPV-infected Ld652Y cells. We found that neither Ld-IAP2 nor Ld-IAP3 was able to suppress the apoptosis of Ld652Y cells induced by p35-defective Autographa californica MNPV (vAcΔp35). However, both Ld-IAP2 and Ld-IAP3 induced apoptosis in Ld652Y cells in a transient expression assay. The apoptosis induced by Ld-IAP3 was accompanied by the stimulation of caspase-3-like protease activity and cleavage of the B. mori homolog of the initiator caspase Dronc, and was precluded by the LdMNPV-encoded apoptosis suppressor protein Apsup and H. cunea MNPV IAP3. Inconsistent with the results obtained previously in SpIm, Ld652Y and High Five cells infected with NPVs from H. cunea, O. pseudotsugata, and A. californica, respectively, considerable stimulation of caspase-3-like protease activity was not observed in LdMNPV-infected Ld652Y cells, likely due to the strong apoptosis suppression activity of Apsup. These results, together with the previous finding that RNAi-mediated silencing of apsup induces apoptosis of LdMNPV-infected Ld652Y cells, indicate that Apsup, but not Ld-IAP2 or Ld-IAP3, is primarily responsible for the suppression of apoptosis in LdMNPV-infected Ld652Y cells. However, it remains inconclusive whether Ld-IAP2 and Ld-IAP3 function as pro-apoptotic proteins in LdMNPV-infected Ld652Y cells.