Characterization of cDNAs encoding p53 of Bombyx mori and Spodoptera frugiperda

The Bombyx mori G protein β subunit 1 (BmGNβ1) gene inhibits BmNPV infection

G protein β subunit 1 (GNβ1) has several functions, including cell growth regulation, the control of second messenger levels, and ion channel switching. Previous transcriptome analyses in our laboratory have shown that BmGNβ1 transcription is reduced following infection with Bombyx mori nucleopolyhedrovirus (BmNPV), but it is unknown what role this gene may have in the host response to BmNPV infection. In this study, the BmGNβ1 gene was cloned using the RACE method. After BmNPV infection, BmGNβ1 was downregulated in Baiyu strains in tissues such as the hemolymph and midgut. Indirect immunofluorescence showed that BmGNβ1 was localized to the cytoplasm. We further constructed a BmGNβ1-pIZ/V5-His-mCherry overexpression plasmid and designed siRNA to evaluate the role of BmGNβ1 in host response to infection. The results showed that BmGNβ1 overexpression inhibited BmNPV proliferation, while knockdown of BmGNβ1 was correlated with increased BmNPV proliferation. The siRNA-mediated reduction of BmGNβ1 was correlated with an increase in BmNPV infection of BmN cells, increased BmNPV vp39 transcription, and reduced survival time of BmNPV-infected B. mori. Overexpression of BmGNβ1 in BmN cells was also correlated with apoptosis and a modification in transcript levels of genes involved in host response to BmNPV infection (PI3K, AKT, Bmp53, BmFOXO, Caspase-1, Bmp21, BmPKN and BmCREB), suggesting that BmGNβ1 may influence the apoptotic host response of infected B. mori through the PI3K-AKT pathway. This study provides potential targets and theoretical support for breeding BmNPV-resistant silkworm varieties.

Determination of Key Components in the Bombyx mori p53 Apoptosis Regulation Network Using Y2H-Seq

The apoptosis pathway is highly conserved between invertebrates and mammals. Although genes encoding the classical apoptosis pathway can be found in the silkworm genome, the regulatory pathway and other apoptotic network genes have yet to be confirmed. Consequently, characterizing these genes and their underlying mechanisms could provide critical insights into the molecular basis of organ apoptosis and remodeling. A homolog of p53, a key apoptosis regulator in vertebrates, has been identified and cloned from Bombyx mori (Bmp53). This study confirmed via gene knockdown and overexpression that Bmp53 directly induces cell apoptosis and regulates the morphology and development of individuals during the metamorphosis stage. Furthermore, yeast two-hybrid sequencing (Y2H-Seq) identified several potential apoptotic regulatory interacting proteins, including the MDM2-like ubiquitination regulatory protein, which may represent an apoptosis factor unique to Bmp53 and which differs from that in other Lepidoptera. These results provide a theoretical basis for analyzing the various biological processes regulated by Bmp53 interaction groups and thus provide insight into the regulation of apoptosis in silkworms. The global interaction set identified in this study also provides a basic framework for future studies on apoptosis-dependent pupation in Lepidoptera.

Superoxide dismutase down-regulation and the oxidative stress is required to initiate pupation in Bombyx mori

Perhaps, oxidative stress progresses pupation in some Lepidopteran insects; however, the reasons for this remain obscure. In our previous study, we clarified Bombyx mori SOD1 (BmSOD1) and B. mori SOD2 (BmSOD2) proteins respond in common to ultraviolet irradiation (UV) oxidative stress and metamorphosis. This result strongly suggested pupation initiates by oxidative stress and might mediate by down-regulation of expression of BmSOD1 and BmSOD2 proteins. Thus, we examined about these relationships in B. mori in this study. In the microarray data reanalysis, we found the Notch signaling pathways as the common pathways in pupation and UV oxidative stress in B. mori. Also, we showed a molting hormone, 20-hydroxyecdysone, leads not only generation of superoxide but also downregulation of the expression of BmSOD proteins during pupation in B. mori. Our findings can contribute to a deeper understanding of how biological defense systems work against environmental oxidative stress.

Stability of selected reference genes in Sf9 cells treated with extrinsic apoptotic agents

As a tightly controlled cell death process, apoptosis eliminates unwanted cells and plays a vital role in multicellular organisms. Previous study have demonstrated that apoptosis occurred in Spodoptera frugiperda cultured Sf9 cells, which triggered by diverse apoptotic stimuli, including azadirachtin, camptothecin and ultraviolet. Due to its simplicity, high sensitivity and reliable specificity, RT-qPCR has been used widespread for analyzing expression levels of target genes. However, the selection of reference genes influences the accuracy of results profoundly. In this study, eight genes were selected for analyses of their suitability as references for normalizing RT-PCR data in Sf9 cells treated with apoptotic agents. Five algorithms, including NormFinder, BestKeeper, Delta Ct method, geNorm, and RefFinder, were used for stability ranking. Based on comprehensively analysis, the expression stability of selected genes varied in cells with different apoptotic stimuli. The best choices for cells under different apoptosis conditions were listed: EF2 and EF1α for cells treated with azadirachtin; RPL13 and RPL3 for cells treated with camptothecin; EF1α and β-1-TUB for cells irradiated under ultraviolet; and EF1α and EF2 for combinational analyses of samples. Our results not only facilitate a more accurate normalization for RT-qPCR data, but also provide the reliable assurance for further studies of apoptotic mechanisms under different stimulus in Sf9 cells.

The functions of caspase in whitefly Bemisia tabaci apoptosis in response to ultraviolet irradiation

Whiteflies (Bemisia tabaci) are phloem feeders, and some invasive species are composed of cryptic species complexes that cause extensive crop damage, particularly via the direct transmission of plant viruses. Apoptosis is a type of programmed cell death essential for organismal development and tissue homeostasis. The caspases belong to a family of cysteine proteases that play a central role in the initiation of apoptosis in many organisms. Here, we employed a comprehensive genomics approach to identity caspases in B. tabaci Middle East Asia Minor 1 (MEAM1), an invasive whitefly that carries a cryptic species complex that is devastating to crops. Four caspase genes were identified, and their motif compositions were predicted. Structures were relatively conserved in both putative effector and initiator caspases. Expression patterns of caspase genes differed across insect developmental stages. Three caspase genes were induced immediately after ultraviolet (UV) treatment. Expression levels of Bt-caspase-1 and Bt-caspase-3b increased in the midgut and salivary glands during apoptosis induced by UV treatments, whereas silencing of both genes reduced UV-triggered apoptosis. Our study demonstrates that Bt-caspase-1 and Bt-caspase-3b, respectively, act as putative initiator and effector apoptotic caspases in the MEAM1 whitefly.

Antiviral immune responses of Bombyx mori cells during abortive infection with Autographa californica multiple nucleopolyhedrovirus

Lepidopteran cells rely on multiple antiviral responses to defend against baculovirus infections, including apoptosis, global protein synthesis shutdown, and rRNA degradation. Here, we characterized apoptosis and rRNA degradation in Autographa californica multiple nucleopolyhedrovirus (AcMNPV)-infected Bombyx mori cells, a system resulting in abortive infection, in relation to viral DNA replication and viral late gene expression. RNAi-mediated silencing of viral DNA replication-related genes prevented apoptosis, but not rRNA degradation, in B. mori cells infected with p35-deficient AcMNPV. Additionally, AcMNPV, but not B. mori nucleopolyhedrovirus (BmNPV), drastically reduced B. mori cellular iap1 transcript levels and p35-deficient AcMNPV induced more prominent apoptosis than did p35-deficient BmNPV. These results, together with previous results that global protein synthesis shutdown follows viral DNA replication, demonstrate that rRNA degradation is the primary antiviral response that abolishes productive AcMNPV infection of B. mori cells. Our results also demonstrate that B. mori cells induce apoptosis to a different extent depending on NPV species.

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.

Evidence for a radiation-responsive 'p53 gateway' contributing significantly to the radioresistance of lepidopteran insect cells

Recently, we have demonstrated that microRNA-31 (miR-31) overexpression is inherent to radiation-induced cell death in the highly radioresistant Sf9 insect cells, and regulates pro-apoptotic Bax translocation to mitochondria. In the present study, we report that at sub-lethal radiation doses for Sf9 cells, miR-31 is significantly downregulated and is tightly regulated by an unusual mechanism involving p53. While ectopic overexpression of a well-conserved Sfp53 caused typical apoptosis, radiation-induced p53 accumulation observed selectively at sub-lethal doses failed to induce cell death. Further investigation of this paradoxical response revealed an intriguing phenomenon that sub-lethal radiation doses result in accumulation of a ‘hyper-phosphorylated’ Sfp53, which in turn binds to miR-31 genomic location and suppresses its expression to prevent cell death. Interestingly, priming cells with sub-lethal doses even prevented the apoptosis induced by lethal radiation or ectopic Sfp53 overexpression. On the other hand, silencing p53 increased radiation-induced cell death by inhibiting miR-31 downregulation. This study thus shows the existence of a unique radiation-responsive ‘p53 gateway’ preventing miR-31-mediated apoptosis in Sf9 cells. Since Sfp53 has a good functional homology with human p53, this study may have significant implications for effectively modulating the mammalian cell radioresistance.

Transcriptome analysis of Spodoptera frugiperda Sf9 cells reveals putative apoptosis-related genes and a preliminary apoptosis mechanism induced by azadirachtin

As an important botanical pesticide, azadirachtin demonstrates broad insecticidal activity against many agricultural pests. The results of a previous study indicated the toxicity and apoptosis induction of azadirachtin in Spodoptera frugiperda Sf9 cells. However, the lack of genomic data has hindered a deeper investigation of apoptosis in Sf9 cells at a molecular level. In the present study, the complete transcriptome data for Sf9 cell line was accomplished using Illumina sequencing technology, and 97 putative apoptosis-related genes were identified through BLAST and KEGG orthologue annotations. Fragments of potential candidate apoptosis-related genes were cloned, and the mRNA expression patterns of ten identified genes regulated by azadirachtin were examined using qRT-PCR. Furthermore, Western blot analysis showed that six putative apoptosis-related proteins were upregulated after being treated with azadirachtin while the protein Bcl-2 were downregulated. These data suggested that both intrinsic and extrinsic apoptotic signal pathways comprising the identified potential apoptosis-related genes were potentially active in S. frugiperda. In addition, the preliminary results revealed that caspase-dependent or caspase-independent apoptotic pathways could function in azadirachtin-induced apoptosis in Sf9 cells.

The p53 gene with emphasis on its paralogues in mosquitoes

The p53 gene is highly important in human cancers, as it serves as a tumor-suppressor gene. Subsequently, two p53 homologues, i.e., p73 and p63, with high identity of amino acids were identified, leading to construction of the p53 family. The p53 gene is highly important in human cancer because it usually transcribes genes that function by causing apoptosis in mammalian cells. In contrast, p63 and p73 tend to be more important in modulating development than inducing cell death, even though they share similar protein structures. Relatively recently, p53 was also identified in mosquitoes and many other insect species. Uniquely, its structure lacks the sterile alpha motif domain which is a putative protein-protein interaction domain and exclusively exists at the C-terminal region in p73 and p63 in mammals. A phylogenetic analysis revealed that the p53 gene derived from mosquitoes is composed of two paralogues, p53-1 and p53-2. Of these, only p53-2 is responsively upregulated by dengue 2 virus (DENV2) in C6/36 cells which usually survive the infection. This indicates that the p53 gene is closely related to DENV infection in mosquito cells. The specific significance of p53-2's involvement in cell survival from virus-induced stress is described and briefly discussed in this report.

Molecular cloning, characterization, and expression analysis of p53 from the oriental river prawn, Macrobrachium nipponense, in response to hypoxia

The tumor suppressor gene p53 plays a critical role in safeguarding the integrity of the genome in mammalian cells. It acts as a sequence-specific transcription factor. Once p53 is activated by a variety of cellular stresses, it transactivates downstream target genes and regulates the cell cycle and apoptosis. However, little is known about the functions of the p53 pathway in prawns in response to hypoxia. In this study, the cDNA of p53 from the oriental river prawn, Macrobrachium nipponense, (Mnp53) was cloned using a combination of homology cloning and rapid amplification of cDNA ends. The full-length cDNA of Mnp53 has 2130 bp, including an open reading frame of 1125 bp that encodes a polypeptide of 374 amino acids with a predicted molecular weight of 41.9 kDa and a theoretical isoelectric point of 6.9. Quantitative real-time (qRT)-PCR assays revealed that Mnp53 was ubiquitously expressed in all examined tissues, but at high levels in the hepatopancreas. In addition, we studied respiratory bursts and reactive oxygen species (ROS) production in the hepatopancreas of M. nipponense. Our results suggest that oxidative stress occurred in prawns in response to hypoxia and that apoptosis was associated with an increase in caspase-3 mRNA expression. qRT-PCR and western blot results confirmed that hypoxic stress induced the upregulation of Mnp53 at mRNA and protein levels. Furthermore, immunohistochemistry showed remarkable changes in immunopositive staining after the same hypoxic treatment. These results suggest that hypoxia-induced oxidative stress may cause apoptosis and cooperatively stimulate the expression of Mnp53.

p53 is involved in shrimp survival via its regulation roles on MnSOD and GPx in response to acute environmental stresses

The tumor suppressor gene p53 plays a critical role in safeguarding the integrity of genome in mammalian cells. It acts as a sequence-specific transcription factor. Once activated by a variety of cellular stresses, p53 transactivates downstream target genes, through which it regulates cell cycle and apoptosis. However, little is known about p53 as well as its downstream target genes in invertebrates. A full length cDNA that encodes a 453-amino-acid p53 protein (Lvp53) was characterized in the Pacific white shrimp (Litopenaeus vannamei) to explore the potential relationships between p53 and two antioxidant enzyme genes: Mn-superoxide dismutase (MnSOD) and glutathione peroxidase (GPx) in eliminating cell stresses in L. vannamei. Sequence analysis revealed a close phylogenetic relationship between Lvp53 and that of Marsupenaeus japonicus, and a high degree of conservation in critical amino acids residues is involved in DNA and zinc binding among species. Quantitative real-time PCR showed that Lvp53 was expressed with varied levels in all the 11 tissues under investigation. In response to acute pH challenge, the relative expression of Lvp53 was induced in a pH- and time-dependent manner, with the peak observed at pH 6.1 and after 24 h of treatment, in which condition, both the relative mRNA expressions and the enzymatic activities of LvMnSOD and LvGPx were increased correspondingly. In response to acute cadmium (Cd) exposure, the relative expression of Lvp53 was upregulated in a time- and concentration-dependent manner, with the maximum detected at Cd 6.6 μM and after 48 h of exposure, in which case, both the transcripts and the enzymatic activities of LvMnSOD and LvGPx were also induced. After Lvp53 transcripts were declined by double-strand RNA injection, the relative mRNA expressions of LvMnSOD and LvGPx were decreased correspondingly. Meanwhile, pH 6.1 or 6.6 μM Cd could not induce the transcripts or the enzymatic activities of LvMnSOD or LvGPx any more in Lvp53-silenced shrimp, but increased shrimp mortalities. These results indicated the involvement of Lvp53, LvMnSOD and LvGPx in mediating cell stress caused by suboptimal pH and elevated levels of Cd in L. vannamei, and that the expressions of LvMnSOD and LvGPx were positively regulated by Lvp53, which is a potential mechanism for shrimp to survive the oxidative stress that occurs during short-term exposure to Cd or challenge with acidic pH. This finding will contribute to better understanding of p53 signaling pathways and redox regulation in invertebrate organisms.

The baculovirus sulfhydryl oxidase Ac92 (P33) interacts with the Spodoptera frugiperda P53 protein and oxidizes it in vitro

The Autographa californica M nucleopolyhedrovirus (AcMNPV) sulfhydryl oxidase Ac92 is essential for production of infectious virions. Ac92 also interacts with human p53 and enhances human p53-induced apoptosis in insect cells, but it is not known whether any relationship exists between Ac92 and native p53 homologs from insect hosts of AcMNPV. We found that Ac92 interacted with SfP53 from Spodoptera frugiperda in infected cells and oxidized SfP53 in vitro. However, Ac92 did not interact with or oxidize a mutant of SfP53 predicted to lack DNA binding. Silencing Sfp53 expression did not rescue the ability of an ac92-knockout virus to produce infectious virus. Similarly, ac92 expression did not affect SfP53-stimulated caspase activity or the localization of SfP53. Thus, although Ac92 binds to SfP53 during AcMNPV replication and oxidizes SfP53 in vitro, we could not detect any effects of this interaction on AcMNPV replication in cultured cells.

SfDronc, an initiator caspase involved in apoptosis in the fall armyworm Spodoptera frugiperda

Initiator caspases are the first caspases that are activated following an apoptotic stimulus, and are responsible for cleaving and activating downstream effector caspases, which directly cause apoptosis. We have cloned a cDNA encoding an ortholog of the initiator caspase Dronc in the lepidopteran insect Spodoptera frugiperda. The SfDronc cDNA encodes a predicted protein of 447 amino acids with a molecular weight of 51 kDa. Overexpression of SfDronc induced apoptosis in Sf9 cells, while partial silencing of SfDronc expression in Sf9 cells reduced apoptosis induced by baculovirus infection or by treatment with UV or actinomycin D. Recombinant SfDronc exhibited several expected biochemical characteristics of an apoptotic initiator caspase: 1) SfDronc efficiently cleaved synthetic initiator caspase substrates, but had very little activity against effector caspase substrates; 2) mutation of a predicted cleavage site at position D340 blocked autoprocessing of recombinant SfDronc and reduced enzyme activity by approximately 10-fold; 3) SfDronc cleaved the effector caspase Sf-caspase-1 at the expected cleavage site, resulting in Sf-caspase-1 activation; and 4) SfDronc was strongly inhibited by the baculovirus caspase inhibitor SpliP49, but not by the related protein AcP35. These results indicate that SfDronc is an initiator caspase involved in caspase-dependent apoptosis in S. frugiperda, and as such is likely to be responsible for the initiator caspase activity in S. frugiperda cells known as Sf-caspase-X.

Polydnaviruses of Parasitic Wasps: Domestication of Viruses To Act as Gene Delivery Vectors

Symbiosis is a common phenomenon in which associated organisms can cooperate in ways that increase their ability to survive, reproduce, or utilize hostile environments. Here, we discuss polydnavirus symbionts of parasitic wasps. These viruses are novel in two ways: (1) they have become non-autonomous domesticated entities that cannot replicate outside of wasps; and (2) they function as a delivery vector of genes that ensure successful parasitism of host insects that wasps parasitize. In this review we discuss how these novelties may have arisen, which genes are potentially involved, and what the consequences have been for genome evolution.

Baculovirus infection induces a DNA damage response that is required for efficient viral replication

Several mammalian viruses have been shown to induce a cellular DNA damage response during replication, and in some cases, this response is required for optimal virus replication. However, nothing is known about whether a DNA damage response is stimulated by DNA viruses in invertebrates. Cell cycle arrest and apoptosis are two of the downstream effects of the DNA damage response, and both are stimulated by baculovirus infection, suggesting a possible relationship between baculoviruses and the DNA damage response. In the study described in this report, we found that replication of the baculovirus Autographa californica M nucleopolyhedrovirus (AcMNPV) in the cell line Sf9, derived from the lepidopteran insect Spodoptera frugiperda, stimulated a DNA damage response, as indicated by an increased abundance of the S. frugiperda P53 protein (SfP53) and phosphorylation of the histone variant protein H2AX. Stimulation of the DNA damage response was dependent on viral DNA replication. Inhibition of the DNA damage response prevented both the increase in SfP53 accumulation and H2AX phosphorylation and also caused a 10- to 100-fold reduction in virus production, along with decreased viral DNA replication and late gene expression. However, silencing of Sfp53 expression by RNA interference did not significantly affect AcMNPV replication or induction of apoptosis by a mutant of AcMNPV lacking the antiapoptotic gene p35, indicating that these processes are not dependent on SfP53 in Sf9 cells.