Overexpression of actin in AcMNPV-infected cells interferes with polyhedrin synthesis and polyhedra formation

The Mechanisms of Silkworm Resistance to the Baculovirus and Antiviral Breeding

Silkworm (Bombyx mori) is not only an economic insect but also a model organism for life science research. Bombyx mori nucleopolyhedrovirus (BmNPV) disease is a major infectious disease in the world's sericulture industry. The cocoon loss caused by this disease accounts for more than 60% of the total loss caused by all silkworm diseases. To date, there has been no effective solution for preventing and treating this disease. The most effective measure is to breed disease-resistant varieties. The quickest way to breed disease-resistant varieties is to apply genetic modification. However, this requires that we obtain disease resistance genes and know the mechanism of disease resistance. Since the discovery of disease-resistant resources in 1989, scholars in the sericulture industry around the world have been inspired to search for resistance genes. In the past two decades, with the help of multi-omics technologies, screening of resistance genes, gene localization, protein modification, virus-host interactions, etc., researchers have found some candidate genes that have been proposed to function at the cellular or individual level. Several disease-resistant varieties have been obtained and used in production through hybrid breeding, RNA interference, and genetic modification. This article summarizes and reviews the discovery of and research advances related to silkworm resistance to BmNPV. It is anticipated that the review will inspire scientific researchers to continue searching for disease resistance genes, clarify the molecular mechanism of silkworm disease resistance, and promote disease-resistant silkworm breeding.

Molecular mechanism responsible for the hyperexpression of baculovirus polyhedrin

One of the amazing phenomena in the baculovirus life cycle is the hyperexpression of the very late gene, polyhedrin (polh), causing the production of the occlusion bodies where progeny virions are embedded. However, to date, the molecular mechanism underlying its hyperexpression is not completely elucidated. Considering that, in this review, the mechanism responsible for its hyperexpression from the previous studies up to now was comprehensively summarized from three aspects, namely, the structure characteristics of the polh promoter and transcription regulation, the structure and translation regulation of the polh mRNA, and especially the regulators that influence the expression of polh gene. Moreover, this review will help us obtain a better understanding about the hyperexpression of polh, and also provide guidance for improving the expression efficiency of the foreign proteins by adopting the baculovirus expression vector system.

Discovery of anti-viral molecules and their vital functions in Bombyx mori

The silkworm Bombyx mori (B. mori), a lepidopteran model organism, has become an important model for molecular biology researches with its genome completely sequenced. Silkworms confront different types of virus diseases, mainly including those caused by Bombyx mori nucleopolyhedrovirus (BmNPV), Bombyx mori densovirus type 1 (BmDNV-1), Bombyx mori bidesovirus (BmBDV) which was termed as Bombyx mori densovirus type 2 (BmDNV-2) or Bombyx mori parvo-like virus (BmPLV) before in sericulture. B. mori offers excellent models to study the molecular mechanisms of insect innate immune responses to viruses. A variety of molecules and pathways have been identified to be involved in the immune responses in the silkworm to viruses, such as the antimicrobial peptides, prophenoloxidase-activating system, apoptosis, ROS, small RNA and related molecules. Here in this review, we summarize the current research advances in molecules involved in silkworm anti-virus pathways. Moreover, taking BmNPV as an example, we proposed a schematic model of molecules and pathways involved in silkworm immune responses against virus infection. We hope this review can facilitate further study of antiviral mechanisms in silkworm, and provide a reference for virus diseases in other organisms.

MacoNPV baculovirus midgut-specific gene expression during infection of the bertha armyworm, Mamestra configurata

Baculoviruses have two forms, occlusion derived virus (ODV) which is responsible for primary infection in host midgut tissue and budded virus (BV), which infects all other host tissues during secondary infection. This study examined the primary infection by ODV of midgut cells of bertha armyworm Mamestra configurata fourth instar larvae and measured the expression of viral genes over a time course of infection. Both digital PCR and RNA sequencing methods showed the profile of transcription to be different from those produced by AcMNPV BV infection of in vitro cell cultures. This included having unique collections of genes expressed early, as well as much greater late gene expression of p6.9 and much reduced expression of polh and p10. These differences likely reflect characteristics unique to the critical step of in vivo midgut cell infection, and provide insights into the processes that regulate viral gene expression in different host tissues.

Structural basis of actin filament nucleation by tandem W domains

Spontaneous nucleation of actin is very inefficient in cells. To overcome this barrier, cells have evolved a set of actin filament nucleators to promote rapid nucleation and polymerization in response to specific stimuli. However, the molecular mechanism of actin nucleation remains poorly understood. This is hindered largely by the fact that actin nucleus, once formed, rapidly polymerizes into filament, thus making it impossible to capture stable multisubunit actin nucleus. Here, we report an effective double-mutant strategy to stabilize actin nucleus by preventing further polymerization. Employing this strategy, we solved the crystal structure of AMPPNP-actin in complex with the first two tandem W domains of Cordon-bleu (Cobl), a potent actin filament nucleator. Further sequence comparison and functional studies suggest that the nucleation mechanism of Cobl is probably shared by the p53 cofactor JMY, but not Spire. Moreover, the double-mutant strategy opens the way for atomic mechanistic study of actin nucleation and polymerization.

Suppression of AcMNPV replication by adf and thymosin protein up-regulation in a new testis cell line, Ha-shl-t

Host cytoskeletons facilitate the entry, replication, and egress of viruses because cytoskeletons are essential for viral survival. One mechanism of resisting viral infections involves regulating cytoskeletal polymerization/depolymerization. However, the molecular mechanisms of regulating these changes in cytoskeleton to suppress viral replication remain unclear. We established a cell line (named Ha-shl-t) from the pupal testis of Helicoverpa armigera (Lepidoptera: Noctuidae). The new testis cell line suppresses Autographa californica multiple nucleocapsid nucleopolyhedrovirus (AcMNPV) replication via disassembly of cytoskeleton. Up-regulation of thymosin (actin disassembling factor) and adf (actin depolymerizing factor) reduces F-actin. Silencing thymosin or adf or treating cells with the F-actin stabilizer phalloidin led to increased AcMNPV replication, while treating cells with an F-actin assembly inhibitor cytochalasin B decreased viral replication. We infer that Ha-shl-t cells utilize F-actin depolymerization to suppress AcMNPV replication by up-regulating thymosin and adf. We propose Ha-shl-t as a model system for investigating cytoskeletal regulation in antiviral action and testicular biology generally.

Direct redox regulation of F-actin assembly and disassembly by Mical

Different types of cell behavior, including growth, motility, and navigation, require actin proteins to assemble into filaments. Here, we describe a biochemical process that was able to disassemble actin filaments and limit their reassembly. Actin was a specific substrate of the multidomain oxidation-reduction enzyme, Mical, a poorly understood actin disassembly factor that directly responds to Semaphorin/Plexin extracellular repulsive cues. Actin filament subunits were directly modified by Mical on their conserved pointed-end, which is critical for filament assembly. Mical posttranslationally oxidized the methionine 44 residue within the D-loop of actin, simultaneously severing filaments and decreasing polymerization. This mechanism underlying actin cytoskeletal collapse may have broad physiological and pathological ramifications.

Changes in gene expression in the permissive larval host lightbrown apple moth (Epiphyas postvittana, Tortricidae) in response to EppoNPV (Baculoviridae) infection

Host cell and virus gene expression were measured five days after per os inoculation of 3rd instar lightbrown apple moth (LBAM) larvae with the Epiphyas postvittana nucleopolyhedrovirus (EppoNPV). Microarray analysis identified 84 insect genes that were up-regulated and 18 genes that were down-regulated in virus-infected larvae compared with uninfected larvae. From the 134 viral open reading frames represented on the microarray, 81 genes showed strong expression. Of the 38 functionally identifiable regulated insect genes, 23 coded for proteins that have roles in one of five processes; regulation of transcription and translation, induction of apoptosis, and maintenance of both juvenility and actin cytoskeletal integrity. Of the 34 functionally identifiable viral genes that were most strongly expressed, 12 had functions associated with these five processes, as did a further seven viral genes which were expressed at slightly lower levels. A survey of the LBAM-expressed sequence tag library identified further genes involved in these processes. In total, 135 insect genes and 38 viral genes were analysed by quantitative polymerase chain reaction. Twenty-one insect genes were strongly up-regulated and 31 genes strongly down-regulated. All 38 viral genes examined were highly expressed. These data suggest that induction of apoptosis and regulation of juvenility are the major 'battlegrounds' between virus and insect, with the majority of changes observed representing viral control of insect gene expression. Transcription and translational effects seem to be exerted largely through modulation of mRNA and protein degradation. Examples of attempts by the insect to repel the infection via changes in gene expression within these same processes were, however, also noted. The data also showed the extent to which viral transcription dominated in the infected insects at five days post inoculation.

Overexpression of cardiac actin with baculovirus is promoter dependent

The influence of the promoter and an N-terminal hexahistidine tag on human cardiac actin (ACTC) expression and function was investigated using four baculovirus constructs. It was found that both non-tagged ACTC and hisACTC expression from the p10 promoter was higher than from the polh promoter. Characterization showed that an N-terminal hexahistidine tag has a negative effect on ACTC. Recombinant ACTC inhibits DNase-I and binds myosin S1, indicative of proper folding. Our data support the hypothesis that the actin protein down-regulates the polh promoter.

Development of an oligonucleotide-based DNA microarray for transcriptional analysis of Choristoneura fumiferana nucleopolyhedrovirus (CfMNPV) genes

A modified oligonucleotide-based two-channel DNA microarray was developed for characterization of temporal expression profiles of select Choristoneura fumiferana nucleopolyhedrovirus (CfMNPV) ORFs including its 7 unique ORFs. The microarray chip contained oligonucleotide probes for 23 CfMNPV ORFs and their complements as well as five host genes. Total RNA was isolated at different times post infection from Cf203 insect cells infected with CfMNPV. The cDNA was synthesized, fluorescent labelled with Cy3, and co-hybridized to the microarray chips along with Cy5-labelled viral genomic DNA, which served as equimolar reference standards for each probe. Transcription of the 7 CfMNPV unique ORFs was detected using DNA microarray analysis and their temporal expression profiles suggest that they are functional genes. The expression levels of three host genes varied throughout virus infection and therefore were unsuitable for normalization between microarrays. The DNA microarray results were compared to quantitative RT-PCR (qRT-PCR). Transcription of the non-coding (antisense) strands of some of the CfMNPV select genes including the polyhedrin gene, was also detected by array analysis and confirmed by qRT-PCR. The polyhedrin antisense transcript, based on long-range RT-PCR analysis, appeared to be a read-through product of an adjacent ORF in the same orientation as the antisense transcript.

Crystal structures of expressed non-polymerizable monomeric actin in the ADP and ATP states

Actin filament growth and disassembly, as well as affinity for actin-binding proteins, is mediated by the nucleotide-bound state of the component actin monomers. The structural differences between ATP-actin and ADP-actin, however, remain controversial. We expressed a cytoplasmic actin in Sf9 cells, which was rendered non-polymerizable by virtue of two point mutations in subdomain 4 (A204E/P243K). This homogeneous monomer, called AP-actin, was crystallized in the absence of toxins, binding proteins, or chemical modification, with ATP or ADP at the active site. The two surface mutations do not perturb the structure. Significant differences between the two states are confined to the active site region and sensor loop. The active site cleft remains closed in both states. Minor structural shifts propagate from the active site toward subdomain 2, but dissipate before reaching the DNase binding loop (D-loop), which remains disordered in both the ADP and ATP states. This result contrasts with previous structures of actin made monomeric by modification with tetramethylrhodamine, which show formation of an alpha-helix at the distal end of the D-loop in the ADP-bound but not the ATP-bound form (Otterbein, L. R., Graceffa, P., and Dominguez, R. (2001) Science 293, 708-711). Our reanalysis of the TMR-modified actin structures suggests that the nucleotide-dependent formation of the D-loop helix may result from signal propagation through crystal packing interactions. Whereas the observed nucleotide-dependent changes in the structure present significantly different surfaces on the exterior of the actin monomer, current models of the actin filament lack any actin-actin interactions that involve the region of these key structural changes.

Expression of Autographa californica multiple nucleopolyhedrovirus genes in mammalian cells and upregulation of the host beta-actin gene

The gene expression of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) was examined in two types of mammalian cells, human HeLa14 and hamster BHK cells. DNA microarray analysis followed by reverse transcription-PCR identified at least 12 viral genes transcribed in both HeLa14 cells and BHK cells inoculated with AcMNPV. 5' rapid amplification of cDNA ends was carried out to examine the transcriptional fidelity of these genes in HeLa14 cells. The transcription of ie-1, ie-0 and gp64 was initiated at a baculovirus early gene motif, CAGT, accompanied by a TATA motif. In addition, the same splicing observed for ie-0 mRNA in Sf9 cells occurred in HeLa14 cells. While the transcription initiation sites for pe38 and p6.9 were not located in the CAGT motif, most of them were in a typical eukaryotic RNA polymerase II promoter structure (a conventional TATA motif and/or an initiator). Interestingly, the expression of beta-actin was upregulated in the mammalian cells inoculated with AcMNPV. Subsequent experiments using UV-inactivated virus confirmed the upregulation, suggesting that de novo synthesis of viral products is not required for the event. These results indicated that the AcMNPV genome acts as a template for transcription in mammalian cells through the usual infection pathway, though there is no evidence for the functional expression of viral genes at present.

Structural and ultrastructural changes during the infection of UFL-AG-286 cells with the baculovirus AgMNPV

During infection of the permissive insect cell line UFL-AG-286 by the baculovirus Anticarsia gemmatalis nucleopolyhedrovirus (AgMNPV-2D) several morphological changes occur. By 12 h postinfection (h p.i.), the infected cells became round and exhibited a decrease in the number of cytoplasmic projections. By 24 h p.i., it was possible to detect a virogenic stroma inside the cell nucleus, and after 48 h p.i., polyhedral inclusion bodies were observed. Some of these morphological modifications are probably due to changes in the cytoskeleton of the cell and this possibility was substantiated by the observation that the distribution of actin and microtubules was dramatically modified upon infection. Several viral-induced proteins were also produced during infection and a sharp decrease in overall protein synthesis was observed. These results are very similar to those obtained with other cell lines infected with different baculoviruses, indicating a similar mechanism of infection. Copyright 1998 Academic Press.

Nucleopolyhedrovirus interactions with their insect hosts

It is clear from this brief review that our understanding of the molecular cross-talk between insects and their baculovirus pathogens is still very limited. Studies in cell culture have taught us a great deal about the basic baculovirus molecular machinery and how it is regulated, and in many cases this information has been predictive of what occurs in infected insects. Frequently, however, studies in cell culture do not adequately predict the infection process in insect hosts, as demonstrated by viral mutants (some of which were discussed in this review) that behave identically to wild-type virus in cell culture but differ markedly in larvae. More baculovirus studies, therefore, need to be conducted in vivo if we are to improve our understanding of the complex interactions between baculoviruses and their hosts. Conducting baculovirus studies in insects (or at least in primary cell culture) also offers the opportunity to address questions that reach beyond the baculovirus community in significance. For example, almost all of our knowledge of viral fusion mechanisms comes from infection of cells in culture where the pH is neutral or acidic and the temperature is constant at 27 degrees or 37 degrees C. An answer to the question of how the ODV envelope fuses with the microvillar membrane of columnar epithelial cells in the highly alkaline midgut environment at low temperatures will not only be important for an improved understanding of baculovirus infection in the natural world, but will also constitute a new chapter on viral entry mechanisms. Similarly, the answer to the question of how baculovirus nucleocapsids move basally within microvilli promises to involve factors and/or a mechanism not yet described by cell biologists, and so will constitute a valuable contribution to both baculovirology and cell biology. There are many more such examples of biological mechanisms that can be uniquely explored within the context of baculoviruses and their insect hosts, some of which have been highlighted in this review. As more and more young investigators realize the importance of combining a knowledge of virology, molecular technology, and insect biology, however, many of the outstanding mysteries will be solved.