Defining the roles of the baculovirus regulatory proteins IE0 and IE1 in genome replication and early gene transactivation

IE1 of Autographa californica Multiple Nucleopolyhedrovirus Activates Low Levels of Late Gene Expression in the Absence of Virus RNA Polymerase

Early and late gene expressions of baculoviruses have been known to rely on host RNA polymerase II and a virus-encoded RNA polymerase, separately. In this study, we found that Autographa californica multiple nucleopolyhedrovirus (AcMNPV) recombinant bacmids with the individual RNA polymerase subunit genes deleted could support low levels of expression of a reporter gene under the control of the promoter of a typical late gene, vp39, in transfected Sf9 cells. Through multistep subcloning of a genomic library of the virus and transient expression assay analysis, ie1 was identified to be the only viral gene that was responsible for activation of late gene expression in the absence of the viral RNA polymerase. Furthermore, IE1 was found to be capable of activating reporter gene expression from the promoters of additional late genes polh, p6.9, odv-e18, odv-e25, and gp41, independent of any additional viral factors. Deletion of ie1 from the virus genome eliminated late gene expression. The IE1-activated late gene expression was enhanced by the viral hr4b. It was shown to be insensitive to inhibition of α-amanitin and did not appear to have stable transcription start sites. It is proposed that IE1 may serve to recruit newly synthesized viral RNA polymerase to viral DNA by activating low levels of pretranscription of the late genes to create an appropriate DNA conformation. IMPORTANCE The late gene expression of baculovirus has been known to depend on the virus-encoded RNA polymerase, which consists of four subunits. The immediate-early gene ie1 was found to be required for viral early gene expression, late gene expression, and DNA replication. How it functions in late gene expression remains unclear. In this study, we found that AcMNPV IE1 could activate low levels of gene expression from late gene promoters independently of any additional viral factors, with nonspecific transcription start sites. This new finding will shed light on the role of IE1 in the regulation of late gene expression and the understanding of the mechanism of late gene transcription initiation.

Utility of Alternative Promoters for Foreign Gene Expression Using the Baculovirus Expression Vector System

The baculovirus expression vector system (BEVS) is a widely used platform for recombinant protein production for use in a wide variety of applications. Of particular interest is production of virus-like particles (VLPs), which consist of multiple viral proteins that self-assemble in strict stoichiometric ratios to mimic the structure of a virus but lacks its genetic material, while a significant amount of effort has been spent on optimizing expression ratios by co-infecting cells with multiple recombinant BEVs and modulating different process parameters, co-expressing multiple foreign genes from a single rBEV may offer more promise. However, there is currently a lack of promoters available with which to optimize co-expression of each foreign gene. To address this, previously published transcriptome data was used to identify promoters that have incrementally lower expression profiles and compared by expressing model cytoplasmic and secreted proteins. Bioinformatics was also used to identify sequence determinants that may be important for late gene transcription regulation, and translation initiation. The identified promoters and bioinformatics analyses may be useful for optimizing expression of foreign genes in the BEVS.

Unraveling the Genome-Wide Impact of Recombinant Baculovirus Infection in Mammalian Cells for Gene Delivery

Baculovirus expression systems have been widely used to produce recombinant mammalian proteins owing to the lack of viral replication in vertebrates. Although several lines of evidence have demonstrated impacts of baculovirus infection in mammalian hosts, genome-wide effects have not been fully elucidated. Here, we provide comparative transcriptome profiles of baculovirus and host-immune response genes in recombinant baculovirus-infected mammalian and insect cells. Specifically, to decipher the impacts of baculovirus infection in mammalian cells, we conducted total RNA-seq on human 293TT cells and insect Sf9 cells infected with recombinant baculovirus. We found that baculovirus genes were rarely expressed under the control of baculoviral promoters in 293TT cells. Although some baculovirus early genes, such as PE38 and IE-01, showed limited expression in 293TT cells, baculoviral late genes were mostly silent. We also found modest induction of a small number of mammalian immune response genes associated with Toll-like receptors, cytokine signaling, and complement in baculovirus-infected 293TT cells. These comprehensive transcriptome data will contribute to improving recombinant baculovirus as tools for gene delivery, gene therapy, and vaccine development.

CRISPR/Cas9-mediated disruption of the immediate early-0 and 2 as a therapeutic approach to Bombyx mori nucleopolyhedrovirus in transgenic silkworm

The CRISPR/Cas9 system is a powerful tool for the treatment of infectious diseases. In our previous study, we knocked out the Bombyx mori nucleopolyhedrovirus (BmNPV) key genes and BmNPV-dependent host factor to generate transgenic antiviral strains. To further expand the range of target genes for BmNPV and more effectively prevent and control pathogenic infections, we performed gene editing and antiviral analysis by constructing a target-directed baculovirus early transcriptional activator immediate early-0 (ie-0) and 2 (ie-2) transgenic silkworm line. We hybridized it with Cas9 transgenic line to produce a double-positive transgenic Cas9(+)/sgIE0-sgIE2(+) line that could activate the CRISPR gene editing system. We first demonstrated that the system is capable of efficiently editing target genes and resulting in fragment deletions in the BmNPV genome. Survival rate of the transgenic Cas9(+)/sgIE0-sgIE2(+) line reached 65% after inoculation with 1 × 10⁶ occlusion bodies/larva. Molecular analysis showed that BmNPV DNA replication and viral gene expression level in the transgenic Cas9(+)/sgIE0-sgIE2(+) line were significantly inhibited compared with the control Cas9(-)/sgIE0-sgIE2(-) line. These results indicated that IE-0 and IE-2, as baculovirus early transcriptional activators, can be used as target sites for gene therapy and that multigene editing could expand the range of target sites for research to create silkworm resistance breeds.

Real-Time Expression Analysis of Selected Anticarsia gemmatalis multiple nucleopolyhedrovirus Gene Promoters during Infection of Permissive, Semipermissive and Nonpermissive Cell Lines

Baculovirus infection follows a transcriptionally controlled sequence of gene expression that occurs by activation of different viral gene promoter sequences during infection. This sequence of promoter activation may be disrupted by cellular defenses against viral infection, which might interfere with viral progeny formation. In this work, the activity of the ie1, gp64, lef-1, vp39, p6.9 and polh promoters of the Anticarsia gemmatalis multiple nucleopolyhedrovirus was assessed during infection of permissive, semipermissive and nonpermissive cell lines by a novel methodology that detects reporter protein luminescence in real-time. This technique allowed us to characterize in rich detail the AgMNPV promoters in permissive cell lines and revealed differential profiles of expression in cells with limited permissivity that correlate well with limitations in viral DNA replication. Semipermissive and nonpermissive cell lines presented delays and restrictions in late and very late promoter expression. Cells undergoing apoptosis did not inhibit late gene expression; however, viral progeny formation is severely affected. This work demonstrates the application of the real-time luminescence detection methodology and how the promoter expression profile may be used to diagnose cellular permissivity to baculovirus infection.

Autographa californica Multiple Nucleopolyhedrovirus AC83 is a Per Os Infectivity Factor (PIF) Protein Required for Occlusion-Derived Virus (ODV) and Budded Virus Nucleocapsid Assembly as well as Assembly of the PIF Complex in ODV Envelopes

Baculovirus occlusion-derived virus (ODV) initiates infection of lepidopteran larval hosts by binding to the midgut epithelia, which is mediated by per os infectivity factors (PIFs). Autographa californica multiple nucleopolyhedrovirus (AcMNPV) encodes seven PIF proteins, of which PIF1 to PIF4 form a core complex in ODV envelopes to which PIF0 and PIF6 loosely associate. Deletion of any pif gene results in ODV being unable to bind or enter midgut cells. AC83 also associates with the PIF complex, and this study further analyzed its role in oral infectivity to determine if it is a PIF protein. It had been proposed that AC83 possesses a chitin binding domain that enables transit through the peritrophic matrix; however, no chitin binding activity has ever been demonstrated. AC83 has been reported to be found only in the ODV envelopes, but in contrast, the Orgyia pseudotsugata MNPV AC83 homolog is associated with both ODV nucleocapsids and envelopes. In addition, unlike known pif genes, deletion of ac83 eliminates nucleocapsid formation. We propose a new model for AC83 function and show AC83 is associated with both ODV nucleocapsids and envelopes. We also further define the domain required for nucleocapsid assembly. The cysteine-rich region of AC83 is also shown not to be a chitin binding domain but a zinc finger domain required for the recruitment or assembly of the PIF complex to ODV envelopes. As such, AC83 has all the properties of a PIF protein and should be considered PIF8. In addition, pif7 (ac110) is reported as the 38th baculovirus core gene.IMPORTANCE ODV is essential for the per os infectivity of the baculovirus AcMNPV. To initiate infection, ODV binds to microvilli of lepidopteran midgut cells, a process which requires a group of seven virion envelope proteins called PIFs. In this study, we reexamined the function of AC83, a protein that copurifies with the ODV PIFs, to determine its role in the oral infection process. A zinc finger domain was identified and a new model for AC83 function was proposed. In contrast to previous studies, AC83 was found to be physically located in both the envelope and nucleocapsid of ODV. By deletion analysis, the AC83 domain required for nucleocapsid assembly was more finely delineated. We show that AC83 is required for PIF complex formation and conclude that it is a true per os infectivity factor and should be called PIF8.