Identification of a novel apoptosis suppressor gene from the baculovirus Lymantria dispar multicapsid nucleopolyhedrovirus

Apoptosis or Antiapoptosis? Interrupted Regulated Cell Death of Host Cells by Ascovirus Infection In Vitro

Ascoviruses are insect-specific viruses thought to utilize the cellular apoptotic processes of host larvae to produce numerous virion-containing vesicles. In this study, we first determined the biochemical characteristics of ascovirus-infected, in vitro-cultured insect cells and the possible antiapoptotic capacity of ascovirus-infected insect cells. The results indicated that the ascovirus infection in the first 24 h was different from the infection from 48 h to the later infection stages. In the early infection stage, the Spodoptera exigua host cells had high membrane permeability and cleaved gasdermin D (GSDMD) but uncleaved Casp-6 (SeCasp-6). In contrast, the later infection stage had no such increased membrane permeability and had cleaved SeCasp-6. Four different chemicals were used to induce apoptosis at different stages of ascovirus infection: hydrogen peroxide (H₂O₂) and actinomycin D (ActD) had similar effects on the ascovirus-infected cells, whereas cMYC inhibitors and tumor necrosis factor alpha (TNF-α) plus SM-164 apoptosis inducers (T/S) had similar effects on infected cells. The former two inducers inhibited viral DNA replication in most situations, while the latter two inducers inhibited viral DNA replication in the early stage of infection but promoted viral DNA replication in the later infection stage. Furthermore, immunoblotting assays verified that T/S treatment could increase the expression levels of viral major capsid protein (MCP) and the host inhibitor of apoptosis protein (SeIAP). Coimmunoprecipitation assays revealed interaction between SeIAP and SeCasps, but this interaction was disturbed in ascovirus-infected cells. This study details the in vitro infection process of ascovirus, indicating the utilization of pyroptosis for antiapoptosis cytopathology. IMPORTANCE Clarifying the relationship between different types of viral infections and host regulation of cell death (RCD) can provide insights into the interaction between viruses and host cells. Ascoviruses are insect-specific viruses with apoptosis-utilizing-like infection cytopathology. However, RCD does not only include apoptosis, and while in our previous transmission electron microscopic observations, ascovirus-infected cells did not show typical apoptotic characteristics (unpublished data), in this study, they did show increased membrane permeability. These results indicate that the cytopathology of ascovirus infection is a complex process in which the virus manipulates host RCD. The RCD of insect cells is quite different from that of mammals, and studies on the former are many fewer than those on the latter, especially in the case of RCD in lepidopteran insects. Our results will lay a foundation for understanding the RCD of lepidopteran insects and its function in the process of insect virus infection.

Bombyx mori C-Type Lectin (BmIML-2) Inhibits the Proliferation of B. mori Nucleopolyhedrovirus (BmNPV) through Involvement in Apoptosis

C-type lectins (CTLs) are widely distributed in mammals, insects, and plants, which act as pattern recognition receptors (PRRs) to recognize pathogens and initiate immune responses. In this study, we identified a C-type lectin gene called BmIML-2 from the silkworm Bombyx mori. Its open reading frame (ORF) encodes 314 amino acids, which contain dual tandem C-type lectin-like domain (CTLD). BmIML-2 is highly expressed in the fat body and is significantly induced at 24 h after BmNPV infection. Moreover, overexpression of BmIML-2 dramatically inhibited the proliferation of BmNPV, and knockdown assay via siRNA further validated the inhibition of BmIML-2 on viral proliferation. In addition, transcript level detection of apoptosis-related genes and observation of apoptosis bodies implied that overexpression of BmIML-2 promoted BmNPV-induced apoptosis. Immunofluorescence analysis indicated that BmIML-2 distributed throughout the cytoplasm and was slightly concentrated in the cell membrane. Taken together, our results suggest that BmIML-2 could inhibit in the proliferation of BmNPV by facilitating cell apoptosis.

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.

Bombyx mori homolog of tumor suppressor p53 is involved in apoptosis-mediated antiviral immunity of B. mori cells infected with nucleopolyhedrovirus

Apoptosis is important in antiviral immunity and affects viral multiplication and pathogenesis. Here, we showed that Bombyx mori cells transiently expressing B. mori homolog of the tumor suppressor p53 (Bm-p53) protein underwent apoptosis accompanied by elevated caspase-3-like protease activity and processing of B. mori Dronc (Bm-Dronc). RNAi-mediated silencing of bm-p53 expression, which significantly diminished accumulation of bm-p53 transcript and Bm-p53 protein, prevented apoptosis of B. mori cells infected with a recombinant B. mori nucleopolyhedrovirus defective in the anti-apoptotic p35 gene (vBmΔp35) and abolished the activation of caspase-3-like protease and processing of Bm-Dronc. Apoptosis in vBmΔp35-infected B. mori cells is associated with viral DNA replication, suggesting involvement of the DNA damage response. The Bm-p53 pro-apoptotic function is also found in Spodoptera frugiperda and Lymantria dispar cells. These results indicate that apoptosis induction in vBmΔp35-infected B. mori cells is a Bm-p53-mediated process promoted by the commencement of viral DNA replication.

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.

The interaction between baculoviruses and their insect hosts

Baculoviruses are double-stranded circular DNA viruses that infect arthropods via the midgut. Because of their superiority as eukaryotic expression systems and their importance as biopesticides, extensive research on the functions of baculovirus genes as well as on the host response to baculovirus infection has been carried out, including transcriptomic and proteomic analyses of the midgut. The morphological and cellular changes caused by baculovirus infection are also important to better understand the infection pathway. Thanks to these previous studies, we now have a clearer picture of the mechanisms of action of the virus and of host immunity. In this paper, we systematically reviewed studies on the interaction between baculoviruses and their insect hosts. By better understanding these interactions, baculoviruses can be developed for use as more efficient biopesticides to improve agricultural development in the future.

Functional analysis of inhibitor of apoptosis 1 of the silkworm Bombyx mori

Recent advances in genome-wide surveys have revealed a number of lepidopteran insect homologs of mammalian and Drosophila genes that are responsible for apoptosis regulation. However, the underlying molecular mechanisms for apoptosis regulation in lepidopteran insect cells remain poorly understood. In the present study, we demonstrated that the transfection of Bombyx mori BM-N cells with dsRNA against the B. mori cellular iap1 gene (cbm-iap1) induces severe apoptosis that is accompanied by an increase of caspase-3-like protease activity. In these apoptotic cells, the cleaved form of the endogenous initiator caspase Dronc (Bm-Dronc) was detected, indicating that cBm-IAP1 protein depletion by RNAi silencing resulted in the activation of Bm-Dronc. In transient expression assays in BM-N cells, cBm-IAP1 suppressed the apoptosis triggered by Bm-Dronc overexpression and depressed the elevation of caspase-3-like protease activity, but also increased the cleaved form of Bm-Dronc protein. cBm-IAP1 also suppressed the caspase-3-like protease activity stimulated by Bm-caspase-1 overexpression. Co-immunoprecipitation experiments demonstrated that cBm-IAP1 strongly interacts with Bm-Dronc, but only has weak affinity for Bm-caspase-1. Transient expression analyses showed that truncated cBm-IAP1 proteins defective in the BIR1, BIR2 or RING domain were unable to suppress Bm-Dronc-induced apoptosis. In addition, BM-N cells expressing truncated cBm-IAP1 proteins underwent apoptosis, suggesting that intact cBm-IAP1, which has anti-apoptotic activity, was replaced or displaced by the overexpressed truncated cBm-IAP1 proteins, which are incapable of interfering with the apoptotic caspase cascade. Taken together, the present results demonstrate that cBm-IAP1 is a vital negative regulator of apoptosis in BM-N cells and functions by preventing the activation and/or activity of Bm-Dronc and Bm-caspase-1.

Using the Baculovirus/Insect Cell System to Study Apoptosis

Apoptosis is a physiological program of cell suicide conserved in invertebrates and vertebrates. Apoptosis is crucial to the normal development of organisms and in tissue homeostasis by promoting elimination of unwanted cells, including damaged or virus-infected cells. Due to the importance of programmed cell death for the survival of the organism, a tight regulation is exerted at various activation levels of the cell-death machinery. The utilization of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) to identify genes that inhibit the apoptotic process will be described using a transfection-based approach, illustrated by identification of the p49 gene.

The RING for gypsy moth control: Topical application of fragment of its nuclear polyhedrosis virus anti-apoptosis gene as insecticide

Numerous studies suggest a cellular origin for the Lymantria dispar multicapsid nuclear polyhedrosis virus (LdMNPV) anti-apoptosis genes IAPs, thus opening a possibility to use the fragments of these genes for modulation of host metabolism. We report here the strong insecticidal and metabolic effect of single-stranded antisense DNA fragment from RING (really interesting new gene) domain of gypsy moth LdMNPV IAP-3 gene: specifically, on reduction of biomass (by 35%) and survival of L. dispar caterpillars. The treatment with this DNA fragment leads to a significantly higher mortality rates of female insects (1.7 fold) accompanied with the signs of apoptosis. Additionally, we show increased expression of host IAP-1, caspase-4 and gelsolin genes in eggs laid by survived females treated with RING DNA fragment accompanied with calcium and magnesium imbalance, indicating that the strong stress reactions and metabolic effects are not confined to treated insects but likely led to apoptosis in eggs too. The proposed new approach for insect pest management, which can be considered as advancement of "microbial pesticides", is based on the application of the specific virus DNA, exploiting the knowledge about virus-pest interactions and putting it to the benefit of mankind.

Novel apoptosis suppressor Apsup from the baculovirus Lymantria dispar multiple nucleopolyhedrovirus precludes apoptosis by preventing proteolytic processing of initiator caspase Dronc

We previously identified a novel baculovirus-encoded apoptosis suppressor, Apsup, from the baculovirus Lymantria dispar multiple nucleopolyhedrovirus (LdMNPV). Apsup inhibits the apoptosis of L. dispar Ld652Y cells triggered by infection with p35-defective Autographa californica MNPV (vAcΔp35) and exposure to actinomycin D or UV light. Here, we examined the functional role of Apsup in apoptosis regulation in insect cells. Apsup prevented apoptosis and the proteolytic processing of L. dispar initiator caspase Dronc (Ld-Dronc) in Ld652Y cells triggered by overexpression of Ld-Dronc, LdMNPV inhibitor-of-apoptosis 3 (IAP3), or Hyphantria cunea MNPV IAP1. In vAcΔp35-infected apoptotic Ld652Y cells, Apsup restricted apoptosis induction and prevented processing of endogenous Ld-Dronc. Conversely, upon RNA interference (RNAi)-mediated silencing of apsup, LdMNPV-infected Ld652Y cells, which typically support high-titer virus replication, underwent apoptosis, accompanied by the processing of endogenous Ld-Dronc. Furthermore, endogenous Ld-Dronc coimmunoprecipitated with transiently expressed Apsup, indicating that Apsup physically interacts with Ld-Dronc. Apsup prevented the apoptosis of Sf9 cells triggered by vAcΔp35 infection but did not inhibit apoptosis or activation of caspase-3-like protease in vAcΔp35-infected Drosophila melanogaster S2 cells. Apsup also inhibited the proteolytic processing of L. dispar effector caspase Ld-caspase-1 in the transient expression assay but did not physically interact with Ld-caspase-1. These results demonstrate that Apsup inhibits apoptosis in Ld652Y cells by preventing the proteolytic processing of Ld-Dronc. Together with our previous findings showing that Apsup prevents the processing of both overexpressed Ld-Dronc and Bombyx mori Dronc, these results also demonstrate that Apsup functions as an effective apoptotic suppressor in various lepidopteran, but not dipteran, insect cells.

Induction of apoptosis by the Amsacta moorei entomopoxvirus

CF-70-B2 cells derived from the spruce budworm (Choristoneura fumiferana) undergo apoptosis when infected with Amsacta moorei entomopoxvirus (AMEV), as characterized by membrane blebbing, formation of apoptotic bodies, TdT-mediated dUTP nick-end labelling (TUNEL) staining, condensed chromatin and induction of caspase-3/7 activity. The apoptotic response was reduced when cells were infected with UV-inactivated AMEV, but not when infected in the presence of the DNA synthesis inhibitor, cytosine β-d-arabinofuranoside. Hence, only pre-DNA replication events were involved in inducing the antiviral response in CF-70-B2 cells. The virus eventually overcame the host’s antiviral response and replicated to high progeny virus titres accompanied by high levels of caspase-3/7 activity. The CF-70-B2 cells were less productive of progeny virus in comparison to LD-652, a Lymantria dispar cell line routinely used for propagation of AMEV. At late stages of infection, LD-652 cells also showed characteristics of apoptosis such as oligosomal DNA fragmentation, TUNEL staining, condensed chromatin and increased caspase-3/7 activity. Induction of apoptosis in LD-652 cells was dependent on viral DNA replication and/or late gene expression. A significantly reduced rate of infection was observed in the presence of general caspase inhibitors Q-VD-OPH and Z-VAD-FMK, indicating caspases may be involved in productive virus infection.

The genome of a baculovirus isolated from Hemileuca sp. encodes a serpin ortholog

The genome sequence of a baculovirus from Hemileuca sp. was determined. The genome is 140,633 kb, has a G+C content of 38.1 %, and encodes 137 putative open-reading frames over 50 amino acids. 126 of these ORFs showed similarity to other baculovirus genes in the database including all 37 core genes. Of the remaining 11 predicted genes, one is related to a lepidopteran serpin gene. This is the first report of a baculovirus encoding a member of this family of serine protease inhibitors, and to our knowledge the first report of a viral serpin outside the Poxviridae. The genome also contained three homologous repeat sequences. Phylogenetic analysis indicated that the virus is a group II Alphabaculovirus and belongs to a lineage that includes Orgyia leucostigma, Ectropis obliqua, Apocheima cinerarium, and Euproctis pseudoconspersa nucleopolyhedroviruses.

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.

The Lymantria dispar IPLB-Ld652Y cell line transcriptome comprises diverse virus-associated transcripts

The enhanced viral susceptibility of the gypsy moth (Lymantria dispar)-derived IPLB-Ld652Y cell line has made it a popular in vitro system for studying virus-related phenomena in the Lepidoptera. Using both single-pass EST sequencing and 454-based pyrosequencing, a transcriptomic library of 14,368 putatively unique transcripts (PUTs) was produced comprising 8,476,050 high-quality, informative bases. The gene content of the IPLB-Ld652Y transcriptome was broadly assessed via comparison with the NCBI non-redundant protein database, and more detailed functional annotation was inferred by comparison to the Swiss-Prot subset of UniProtKB. In addition to L. dispar cellular transcripts, a diverse array of both RNA and DNA virus-associated transcripts was identified within the dataset, suggestive of a high level of viral expression and activity in IPLB-Ld652Y cells. These sequence resources will provide a sound basis for developing testable experimental hypotheses by insect virologists, and suggest a number of avenues for potential research.

Cloning and characterization of a dronc homologue in the silkworm, Bombyx mori

We cloned and characterized a novel Bombyx mori homologue (bm-dronc) of Drosophila melanogaster dronc (dm-dronc), which could encode a polypeptide of 438 amino acid residues. Bm-Dronc shares relatively low amino acid sequence identities of 25% and 26% with Dm-Dronc and Aedes aegypti Dronc (Aa-Dronc), respectively. Bm-Dronc has the sequence QACRG surrounding the catalytic site (C), which is consistent with the QAC(R/Q/G)(G/E) consensus sequence in most caspases but distinct from the sequences PFCRG and SICRG of Dm-Dronc and Aa-Dronc, respectively. Bm-Dronc possesses a long N-terminal prodomain containing a caspase recruitment domain (CARD), a p20 domain and a p10 domain, exhibiting cleavage activities on synthetic substrates Ac-VDVAD-AMC, Ac-IETD-AMC and Ac-LEHD-AMC, which are preferred by human initiator caspases-2, -8 and -9, respectively. Bm-Dronc transiently expressed in insect cells and Escherichia coli cells underwent spontaneous cleavage and caused apoptosis and stimulation of caspase-3-like protease activity in various lepidopteran cell lines, but not in the dipteran cell line D. melanogaster S2. The apoptosis and the stimulation of caspase-3-like protease activity induced by Bm-Dronc overexpression were abrogated upon transfection with either a double-stranded RNA against bm-dronc or a plasmid expressing functional anti-apoptotic protein Hycu-IAP3 encoded by the baculovirus Hyphantria cunea multiple nucleopolyhedrovirus (MNPV). Apoptosis induction in BM-N cells by infection with a p35-defective Autographa californica MNPV or exposure to actinomycin D and UV promoted the cleavage of Bm-Dronc. These results indicate that Bm-Dronc serves as the initiator caspase responsible for the induction of caspase-dependent apoptosis.