Linearization of baculovirus DNA enhances the recovery of recombinant virus expression vectors

A Time and Cost-Effective Pipeline for Expression Screening and Protein Production in Insect Cells Based on the HR-Bac Toolbox to Generate Recombinant Baculoviruses

The baculovirus expression vector system (BEVS) is recognized as a powerful platform for producing challenging proteins and multiprotein complexes both in academia and industry. Since a baculovirus was first used to produce heterologous human IFN-β protein in insect cells, the BEVS has continuously been developed and its applications expanded. We have recently established a multigene expression toolbox (HR-bac) composed of a set of engineered bacmids expressing a fluorescent marker to monitor virus propagation and a library of transfer vectors. Unlike platforms that rely on Tn7-medidated transposition for the construction of baculoviruses, HR-bac relies on homologous recombination, which allows to evaluate expression constructs in 2 weeks and is thus perfectly adapted to parallel expression screening. In this chapter, we detail our standard operating procedures for the preparation of the reagents, the construction and evaluation of baculoviruses, and the optimization of protein production for both intracellularly expressed and secreted proteins.

End-to-End Semi-automated Mid-scale Protein Screening Platform for Drug Discovery Research

The drug discovery landscape is ever-evolving and constantly demands revolutionary technology advancements in protein expression and production laboratories. We have built a higher-throughput mid-scale semi-automated protein expression and screening platform to accelerate drug discovery research. The workflow described here enables comprehensive expression and purification screening assessment of challenging or difficult-to-express recombinant proteins in a fast and efficient manner by delivering small but sufficient amounts of high-quality proteins. The platform has been implemented for a wide range of applications that include identification of optimal constructs and chaperones for poorly expressing proteins, assessment of co-expression partners for expressing stable multiprotein complexes, and suitable buffer/additive screening for insoluble or aggregation-prone proteins. The approach allows parallel expression, purification, and characterization of 24 different samples using co-infection or a polycistronic approach in insect cells and enables parallel testing of multiple parameters to improve protein yields. The strategy has been successfully adopted for screening intracellular and secreted proteins in Escherichia coli, mammalian transient expression, and baculovirus expression vector systems. Proteins purified from this platform are used for several structural and functional screens, such as negative staining, biochemical activity assays, mass spectrometry, surface plasmon resonance, and DNA-encoded chemical library screens. In this article, for simplicity, we have focused on detailed expression and purification screening of intracellular protein complexes from insect cells. © 2023 Wiley Periodicals LLC. Basic Protocol 1: Baculovirus generation via homologous recombination Support Protocol 1: Anti-glycoprotein 64 antibody assay Basic Protocol 2: Generation of insect cell biomass expressing target protein(s) Basic Protocol 3: Mid-scale affinity purification Support Protocol 2: Automated method for affinity purification on Hamilton STAR Basic Protocol 4: Size exclusion chromatography Support Protocol 3: Chromeleon 7 operation on Vanquish Duo.

An easy method to generate recombinant pseudorabies virus expressing the capsid protein of Porcine circovirus type 2d

Introduction

Homologous recombination is an effective way to generate recombinant viruses for vaccine research such as pseudorabies virus (PRV) and adenovirus. Its efficiency can be affected by the integrity of viral genome and the linearization sites.

Methods

In the study, we described a simple approach to isolate the viral DNA with high genomic integrity for large DNA viruses and a time-saving method to generate recombinant PRVs. Several cleavage sites in the PRV genome were investigated by using the EGFP as a reporter gene for identification of PRV recombination.

Results

Our study showed that cleavage sites of XbaI and AvrII are ideal for PRV recombination which showed higher recombinant efficiency than others. The recombinant PRV-EGFP virus can be easily plaque purified in 1-2 weeks after the transfection. By using PRV-EGFP virus as the template and XbaI as the linearizing enzyme, we successfully constructed the PRV-PCV2d_ORF2 recombiant virus within a short period by simply transfecting the linearized PRV-EGFP genome and PCV2d_ORF2 donor vector into BHK-21 cells. This easy and efficient method for producing recombinant PRV might be adapted in other DNA viruses for the generation of recombinant viruses.

Biosynthesis of a VLP-type nanocarrier specific to cancer cells using the BEVS expression system for targeted drug delivery

OBJECTIVE

Canine parvovirus (CPV) is a small virus without an envelope that consists of three viral proteins including VP1, VP2, and VP3. Exclusively, the VP2 can form a typically CPV-sized virus-like particle (CPV-VLP) that can be used as a biological nanocarrier for diagnostic and therapeutic purposes since these VLPs can target cancer cells specially through the transferrin surface receptors (TFRs). Consequently, we aimed to produce these nanocarriers to be used for specific targeting of cancer cells.

METHODS

Sf9 insect cells were transfected with constructed recombinant bacmid shuttle vector encoding an enhanced green fluorescent protein (EGFP) and CPV-VP2 by the cationic lipids of Cellfectin II. Subsequently, two recombinant baculoviruses expressing EGFP and VP2 were produced and expression of VP2 was increased under the optimal condition. In consequence, the CPV-VLP nanoparticles composed of recombinant VP2 subunits were extracted. The purity of VLPs was then evaluated by SDS-PAGE, and the structural integrity and quality of the final product were evaluated by TEM and HA methods. Eventually, the size distribution of the produced biological nanoparticles and their uniformity were determined by the DLS method.

RESULTS

The expression of EGFP protein was confirmed by fluorescent microscopy, and the expression of VP2 protein was evaluated by SDS-PAGE and western blotting. Infected Sf9 insect cells also showed cytopathic effects (CPEs), and the maximum expression of VP2 occurred at MOI of 10 (pfu/cell) at the harvest time of 72 h post-infection (hpi). After performing various stages of purification, buffer exchange, and concentration, the quality and structural integrity of the VLP product were confirmed. The results of the DLS technique showed the presence of uniform particles (PdI below 0.5) with an approximate size of 25 nm.

CONCLUSION

The results indicate BEVS as an appropriate and efficient system for generating CPV-VLPs, and the used method based on two-stage ultracentrifugation was appropriate for purifying these nanoparticles. Produced nanoparticles can be used as the biologic nano-carriers in future studies.

Baculovirus Display of Peptides and Proteins for Medical Applications

Baculoviridae is a large family of arthropod-infective viruses. Recombinant baculoviruses have many applications, the best known is as a system for large scale protein production in combination with insect cell cultures. More recently recombinant baculoviruses have been utilized for the display of proteins of interest with applications in medicine. In the present review we analyze the different strategies for the display of proteins and peptides on the surface of recombinant baculoviruses and provide some examples of the different proteins displayed. We analyze briefly the commercially available systems for recombinant baculovirus production and display and discuss the future of this emerging and powerful technology.

Insect Cells-Baculovirus System for the Production of Difficult to Express Proteins: From Expression Screening for Soluble Constructs to Protein Quality Control

Rapid preparation of proteins for functional and structural analysis is a major challenge both in academia and industry. The number potential targets continuously increases and many are difficult to express proteins which, when produced in bacteria, result in insoluble and/or misfolded recombinant proteins, protein aggregates, or unusable low protein yield. We focus here on the baculovirus expression vector system which is now commonly used for heterologous production of human targets. This chapter describes simple and cost-effective protocols that enable iterative cycles of construct design, expression screening and optimization of protein production. We detail time- and cost-effective methods for generation of baculoviruses by homologous recombination and titer evaluation. Handling of insect cell cultures and preparation of bacmid for cotransfection are also presented.

Optimizing Recombinant Baculovirus Vector Design for Protein Production in Insect Cells

Autographa californica nucleopolyhedrovirus is a very productive expression vector for recombinant proteins in insect cells. Most vectors are based on the polyhedrin gene promoter, which comprises a TAAG transcription initiation motif flanked by 20 base pairs upstream and 47 base pairs downstream before the native ATG. Many transfer vectors also include a short sequence downstream of the ATG, in which case this sequence is mutated to ATT to abolish translation. However, the ATT sequence, or AUU in the mRNA, is known to be leaky. If a target-coding region is placed in the frame with the AUU, then some products will comprise a chimeric molecule with part of the polyhedrin protein. In this study, we showed that if AUU is placed in the frame with a Strep tag and eGFP coding region, we could identify a protein product with both sequences present. Further work examined if alternative codons in lieu of AUG might reduce translation initiation further. We found that AUA was used slightly more efficiently than AUU, whereas AUC was the least efficient at initiating translation. The use of this latter codon suggested that there might also be a slight improvement of protein yield if this is incorporated into expression vectors.

Construction and Characterization of a Novel Bacmid AcBac-Syn Based on a Synthesized Baculovirus Genome

Baculoviruses are large DNA viruses which have been widely used as expression vectors and biological insecticides. Homologous recombination and Bac-to-Bac system have been the main methods for manipulating the baculovirus genome. Recently, we generated a synthetic baculovirus AcMNPV-WIV-Syn1 which fully resembled its parental virus Autographa californica multiple nucleopolyhedrovirus (AcMNPV). Here, we report the modification of AcMNPV-WIV-Syn1 into a novel bacmid, AcBac-Syn, which can be used as a backbone for Bac-to-Bac system. To achieve this, a vector contained a LacZ:attTn7 and egfp cassette was constructed, and recombined with a linearized AcMNPV-WIV-Syn1 genome by transformation-associated recombination in yeast to generate bacmid AcBac-Syn. The bacmid was then transfected to insect cells and the rescued virus showed similar biological characteristics to the wild-type virus in terms of the kinetics of budded virus production, the morphology of occlusion bodies, and the oral infectivity in insect larvae. For demonstration, a red fluorescent protein gene Dsred was transposed into the attTn7 site by conventional Bac-to-Bac method, and the transfection and infection assays showed that AcBac-Syn can be readily used for foreign gene insertion and expression. AcBac-Syn has several advantages over the conventional AcMNPV bacmids, such as it contains an egfp reporter gene which facilitates visualization of virus propagation and titration; its DNA copy numbers could be induced to a higher level in E. coli; and the retaining of the native polyhedrin gene in the genome making it an attractive system for studying the functions of gene related to occlusion body assembly and oral infection.

Membrane Protein Production and Purification from Escherichia coli and Sf9 Insect Cells

A major obstacle to studying membrane proteins by biophysical techniques is the difficulty in producing sufficient amounts of materials for functional and structural studies. To overexpress the target membrane protein heterologously, especially an eukaryotic protein, a key step is to find the optimal host expression system and perform subsequent expression optimization. In this chapter, we describe protocols for screening membrane protein production using bacterial and insect cells, solubilization screening, large-scale production, and commonly used affinity chromatography purification methods. We discuss general optimization conditions, such as promoters and tags, and describe current techniques that can be used in any laboratory without specialized expensive equipment. Especially for insect cells, GFP fusions are particularly useful for localization and in-gel fluorescence detection of the proteins on SDS-PAGE. We give detailed protocols that can be used to screen the best expression and purification conditions for membrane protein study.

Expressing Cloned Genes for Protein Production, Purification, and Analysis

Obtaining high quantities of a specific protein directly from native sources is often challenging, particularly when dealing with human proteins. To overcome this obstacle, many researchers take advantage of heterologous expression systems by cloning genes into artificial vectors designed to operate within easily cultured cells, such as Escherichia coli, Pichia pastoris (yeast), and several varieties of insect and mammalian cells. Heterologous expression systems also allow for easy modification of the protein to optimize expression, mutational analysis of specific sites within the protein and facilitate their purification with engineered affinity tags. Some degree of purification of the target protein is usually required for functional analysis. Purification to near homogeneity is essential for characterization of protein structure by X-ray crystallography or nuclear magnetic resonance (NMR) and characterization of the biochemical and biophysical properties of a protein, because contaminating proteins almost always adversely affect the results. Methods for producing and purifying proteins in several different expression platforms and using a variety of vectors are introduced here.

A Comprehensive Guide to the Commercial Baculovirus Expression Vector Systems for Recombinant Protein Production

The Baculovirus Expression Vector System (BEVS) is a workhorse for recombinant protein expression for over thirty-five years. Ever since it was first used to overexpress the human IFN-β protein, the system has been engineered and modified several times for quick and easy expression and scale-up of the recombinant proteins. Multiple gene assemblies performed on the baculovirus genome using synthetic biology methods lead to optimized overexpression of the multiprotein complexes. Nowadays, several commercially available BEVS platforms offer a variety of customizable features, and often it is confusing which one to choose for a novice user. This short review is intended to be a one-stop guide to the commercially available baculovirus technology for heterologous protein expression in the insect cells, which users can refer to choose from popular and desirable BEVS products or services.

Construction of the Antheraea pernyi (Lepidoptera: Saturniidae) Multicapsid Nucleopolyhedrovirus Bacmid System

In this study, we established the Antheraea pernyi multicapsid nucleopolyhedrovirus (AnpeNPV) bacmid system for the construction of a Bac-to-Bac expression system and the generation of virus mutants. The CopyRight pSMART BAC cloning vector harboring the chloramphenicol resistance gene was introduced into the AnpeNPV genome to produce the AnpeNPV bacmid that could be propagated in Escherichia coli with stable replication. The enhanced green fluorescent protein (EGFP) was successfully expressed in both Tn-Hi5 cells and A. pernyi pupae using the AnpeNPV Bac-to-Bac expression system. To generate the AnpeNPV mutants, we developed the AnpeNPV bacmid/λ Red recombination system that facilitated the deletion of viral genes from the AnpeNPV genome. The genes cathepsin and chitinase were deleted and a derivative AnpeNPV Bac-to-Bac expression system was constructed. Furthermore, we demonstrated that the novel expression system could be used to express human epidermal growth factor in A. pernyi pupae. Taken together, the AnpeNPV bacmid system provides a powerful tool to create the AnpeNPV Bac-to-Bac expression system for protein expression in A. pernyi pupae. Further, it helps to knock-out genes from the AnpeNPV genome with λ Red recombination system for identification of the role of viral genes involved in regulating gene expression, DNA replication, virion structure, and infectivity during the AnpeNPV infection process.

Wide spectrum and high frequency of genomic structural variation, including transposable elements, in large double-stranded DNA viruses

Our knowledge of the diversity and frequency of genomic structural variation segregating in populations of large double-stranded (ds) DNA viruses is limited. Here, we sequenced the genome of a baculovirus (Autographa californica multiple nucleopolyhedrovirus [AcMNPV]) purified from beet armyworm (Spodoptera exigua) larvae at depths >195,000× using both short- (Illumina) and long-read (PacBio) technologies. Using a pipeline relying on hierarchical clustering of structural variants (SVs) detected in individual short- and long-reads by six variant callers, we identified a total of 1,141 SVs in AcMNPV, including 464 deletions, 443 inversions, 160 duplications, and 74 insertions. These variants are considered robust and unlikely to result from technical artifacts because they were independently detected in at least three long reads as well as at least three short reads. SVs are distributed along the entire AcMNPV genome and may involve large genomic regions (30,496 bp on average). We show that no less than 39.9 per cent of genomes carry at least one SV in AcMNPV populations, that the vast majority of SVs (75%) segregate at very low frequency (<0.01%) and that very few SVs persist after ten replication cycles, consistent with a negative impact of most SVs on AcMNPV fitness. Using short-read sequencing datasets, we then show that populations of two iridoviruses and one herpesvirus are also full of SVs, as they contain between 426 and 1,102 SVs carried by 52.4–80.1 per cent of genomes. Finally, AcMNPV long reads allowed us to identify 1,757 transposable elements (TEs) insertions, 895 of which are truncated and occur at one extremity of the reads. This further supports the role of baculoviruses as possible vectors of horizontal transfer of TEs. Altogether, we found that SVs, which evolve mostly under rapid dynamics of gain and loss in viral populations, represent an important feature in the biology of large dsDNA viruses.

Baculovirus-Derived Vectors for Immunization and Therapeutic Applications

Baculoviruses are arthropod-specific, enveloped viruses with circular, supercoiled double-stranded deoxyribonucleic acid genomes. While many viruses are studied to seek solutions for their adverse impact on human, veterinary, and plant health, the study of baculoviruses was stimulated initially by their potential utility to control insect pests. Later, the utility of baculovirus as gene expression vectors was evidenced leading to numerous applications. Several strategies are employed to obtain recombinant viruses that express large quantities of heterologous proteins. A major step forward was the development of bacmid technology (the construction of bacterial artificial chromosomes containing the genome of the baculovirus) which allows the manipulation of the baculovirus genome in bacteria. With this technology, foreign genes can be introduced into the bacmid by homologous and site-directed recombination or by transposition. Baculoviruses have been used to explore fundamental questions in molecular biology such as the nature of programmed cell-death. Moreover, the ability of baculoviruses to transduce mammalian cells led to the consideration of their use as gene-therapy and vaccine vectors. Strategies for genetic engineering of baculoviruses have been developed to meet the requirements of new application areas. Display of foreign proteins on the surface of virions or in nucleocapsid structures, the assembly of expressed proteins to form virus-like particles or protein complexes have been explored and validated as vaccines. The aim of this chapter is to update the areas of application of the baculoviruses in protein expression, alternative vaccine designs and gene therapy of infectious diseases and genetic disorders. Finally, we review the baculovirus-derived products on the market and in the pipeline for biomedical and veterinary use.

Construction of a Baculovirus Derivative to Produce Linearized Antheraea pernyi (Lepidoptera: Saturniidae) Multicapsid Nucleopolyhedrovirus Genomic DNA

In the Antheraea pernyi multicapsid nucleopolyhedrovirus (AnpeNPV)-based expression vector system, the frequency of homologous recombination events between wild-type AnpeNPV DNA and the transfer vector is low, resulting in a small amount of recombinant virus. Previous reports have indicated that linearized baculovirus DNA can increase the proportion of recombinant virus relative to the total progeny. To improve the recombination efficiency, we constructed a linearized derivative of AnpeNPV, referred to as AnpeNPVPhEGFP-AvrII, in which egfp flanked by AvrII restriction sites was located at the polyhedrin locus and driven by the polyhedrin promoter. Linear AnpeNPV DNA was obtained by the treatment of AnpeNPVPhEGFP-AvrII genomic DNA with AvrII endonuclease. The infectivity and recombinogenic activity between the linearized and circular viral DNA were evaluated by quantitative real-time polymerase chain reactions. We demonstrated that the linearized AnpeNPV DNA produced only small numbers of infectious budded viruses, accounting for approximately 4.5% of the budded virus production of wild-type AnpeNPV DNA in A. pernyi pupae. However, the linearized AnpeNPV DNA substantially increased recombinant virus production after cotransfection with an appropriate transfer vector; relative abundance of the recombinant virus was approximately 5.5-fold higher than that of the wild-type AnpeNPV DNA in A. pernyi pupae. The linearization of AnpeNPV DNA will facilitate the purification of recombinant viruses using the AnpeNPV-based expression vector system and the construction of an AnpeNPV-based bacmid system.

Baculovirus entire ORF1629 is not essential for viral replication

It is generally accepted that ORF1629 is essential for baculovirus replication, which has enabled isolation of recombinant viruses in a baculovirus expression system using linearized viral DNA. ORF1629-defective viruses cannot replicate in insect cells; only recombinant virus with complete ORF1629 restoration by recombination can propagate, allowing for pure isolation and the development of bacmids for easy selection of recombinant viruses. We inadvertently found proliferation in insect cells of a bacmid lacking a complete ORF1629. PCR indicated no other viruses but a lack of complete ORF1629 in the proliferated bacmid, suggesting that the baculovirus propagated without a complete ORF1629. Lack of ORF1629 decreased the virus growth rate and yield; it also increased the occlusion body (OB) size but decreased its yield. These results were confirmed for Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) and Bombyx mori NPV (BmNPV). Thus, entire ORF1629 is not essential for viral replication, though it does affect the virus growth rate, yield, and size and OB production.

Overview of the Baculovirus Expression System

This unit provides information on the replication cycle of insect baculovirus to provide an understanding of how this virus has been adapted for use as an expression vector for recombinant proteins in insect cells. We provide an overview of the virus structure and its unique bi-phasic replication cycle, which has been exploited in developing the virus as an expression vector. We also review the development of the baculovirus expression vector system (BEVS), from the mid-1980s to the present day in which the BEVS is now an established tool for the production of a range of recombinant proteins and multi-protein complexes including virus-like particles. We describe advances made to the BEVS to allow the rapid and easy production of recombinant viruses and developments to improve protein yield. We finish by describing the application of recombinant BacMam as vectors for the delivery of genes into mammalian and human cells. © 2018 by John Wiley & Sons, Inc.

CRISPR-Cas9 vectors for genome editing and host engineering in the baculovirus-insect cell system

The baculovirus-insect cell system (BICS) has been widely used to produce many different recombinant proteins for basic research and is being used to produce several biologics approved for use in human or veterinary medicine. Early BICS were technically complex and constrained by the relatively primordial nature of insect cell protein glycosylation pathways. Since then, recombination has been used to modify baculovirus vectors-which has simplified the system-and transform insect cells, which has enhanced its protein glycosylation capabilities. Now, CRISPR-Cas9 tools for site-specific genome editing are needed to facilitate further improvements in the BICS. Thus, in this study, we used various insect U6 promoters to construct CRISPR-Cas9 vectors and assessed their utility for site-specific genome editing in two insect cell lines commonly used as hosts in the BICS. We demonstrate the use of CRISPR-Cas9 to edit an endogenous insect cell gene and alter protein glycosylation in the BICS.

Baculovirus expression: old dog, new tricks

Since its inception more than 30 years ago, the baculovirus expression vector system (BEVS) has been used prolifically to produce heterologous proteins for research and development. In the cell, a cornerstone of biological activity are multiprotein complexes, catalyzing essential functions. BEVS has been uniquely successful to unlock such complex assemblies for high-resolution structural and functional analysis. Synthetic biology approaches have been implemented to optimize multigene assembly methods, accelerating upstream processes. Specialized baculoviral genomes are being created with functions tailored to enhance production of particular target protein classes. Here we comment on current and emerging developments in the field and their potential to accelerate protein complex research.

Generation of virus like particles for epizootic hemorrhagic disease virus

Epizootic hemorrhagic disease virus (EHDV) is a distinct species within the genus Orbivirus, within the family Reoviridae. The epizootic hemorrhagic disease virus genome comprises ten segments of linear, double stranded (ds) RNA, which are packaged within each virus particle. The EHDV virion has a three layered capsid-structure, generated by four major viral proteins: VP2 and VP5 (outer capsid layer); VP7 (intermediate, core-surface layer) and VP3 (innermost, sub-core layer). Although EHDV infects cattle sporadically, several outbreaks have recently occurred in this species in five Mediterranean countries, indicating a potential threat to the European cattle industry. EHDV is transmitted by biting midges of the genus Culicoides, which can travel long distances through wind-born movements (particularly over water), increasing the potential for viral spread in new areas/countries. Expression systems to generate self-assembled virus like particles (VLPs) by simultaneous expression of the major capsid-proteins, have been established for several viruses (including bluetongue virus). This study has developed expression systems for production of EHDV VLPs, for use as non-infectious antigens in both vaccinology and serology studies, avoiding the risk of genetic reassortment between vaccine and field strains and facilitating large scale antigen production. Genes encoding the four major-capsid proteins of a field strain of EHDV-6, were isolated and cloned into transfer vectors, to generate two recombinant baculoviruses. The expression of these viral genes was assessed in insect cells by monitoring the presence of specific viral mRNAs and by western blotting. Electron microscopy studies confirmed the formation and purification of assembled VLPs.

Recombinant Baculovirus Isolation

Although there are several different methods available of making recombinant baculovirus expression vectors (reviewed in Chapter 3 ), all require a stage in which insect cells are transfected with either the virus genome alone (Bac-to-Bac(®) or BaculoDirect™, Invitrogen) or virus genome and transfer vector. In the latter case, this allows the natural process of homologous recombination to transfer the foreign gene, under control of the polyhedrin or other baculovirus gene promoter, from the transfer vector to the virus genome to create the recombinant virus. Previously, many methods required a plaque-assay to separate parental and recombinant virus prior to amplification and use of the recombinant virus. Fortunately, this step is no longer required for most systems currently available. This chapter provides an overview of the historical development of increasingly more efficient systems for the isolation of recombinant baculoviruses (Chapter 3 provides a full account of the different systems and transfer vectors available). The practical details cover: transfection of insect cells with either virus DNA or virus DNA and plasmid transfer vector; a reliable plaque-assay method that can be used to separate recombinant virus from parental (nonrecombinant) virus where this is necessary; methods for the small-scale amplification of recombinant virus; and subsequent titration by plaque-assay or real-time polymerase chain reaction (PCR). Methods unique to the Bac-to-Bac(®) system are also covered and include the transformation of bacterial cells and isolation of bacmid DNA ready for transfection of insect cells.

Baculovirus Transfer Vectors

The production of a recombinant baculovirus expression vector normally involves mixing infectious virus DNA with a plasmid-based transfer vector and then co-transfecting insect cells to initiate virus infection. The aim of this chapter is to provide an update on the range of baculovirus transfer vectors currently available. Some of the original transfer vectors developed are now difficult to obtain but generally have been replaced by superior reagents. We focus on those that are available commercially and should be easy to locate. These vectors permit the insertion of single or multiple genes for expression, or the production of proteins with specific peptide tags that aid subsequent protein purification. Others have signal peptide coding regions permitting protein secretion or plasma membrane localization. A table listing the transfer vectors also includes information on the parental virus that should be used with each one. Methods are described for the direct insertion of a recombinant gene into the virus genome without the requirement for a transfer vector. The information provided should enable new users of the system to choose those reagents most suitable for their purposes.

Development of a recombination system for the generation of occlusion positive genetically modified Anticarsia gemmatalis multiple nucleopolyhedrovirus

Anticarsia gemmatalis is an important pest in legume crops in South America and it has been successfully controlled using Anticarsia gemmatalis Multiple Nucleopolyhedrovirus (AgMNPV) in subtropical climate zones. Nevertheless, in temperate climates its speed of kill is too slow. Taking this into account, genetic modification of AgMNPV could lead to improvements of its biopesticidal properties. Here we report the generation of a two-component system that allows the production of recombinant AgMNPV. This system is based on a parental AgMNPV in which the polyhedrin gene (polh) was replaced by a bacterial β-galactosidase (lacZ) gene flanked by two target sites for the homing endonuclease I-PpoI. Co-transfection of insect cells with linearized (I-PpoI-digested) parental genome and a transfer vector allowed the restitution of polh and the expression of a heterologous gene upon homologous recombination, with a low background of non-recombinant AgMNPV. The system was validated by constructing a recombinant occlusion-positive (polh⁺) AgMNPV expressing the green fluorescent protein gene (gfp). This recombinant virus infected larvae normally per os and led to the expression of GFP in cell culture as well as in A. gemmatalis larvae. These results demonstrate that the system is an efficient method for the generation of recombinant AgMNPV expressing heterologous genes, which can be used for manifold purposes, including biotechnological and pharmaceutical applications and the production of orally infectious recombinants with improved biopesticidal properties.

Baculoviruses and nucleosome management

Negatively-supercoiled-ds DNA molecules, including the genomes of baculoviruses, spontaneously wrap around cores of histones to form nucleosomes when present within eukaryotic nuclei. Hence, nucleosome management should be essential for baculovirus genome replication and temporal regulation of transcription, but this has not been documented. Nucleosome mobilization is the dominion of ATP-dependent chromatin-remodeling complexes. SWI/SNF and INO80, two of the best-studied complexes, as well as chromatin modifier TIP60, all contain actin as a subunit. Retrospective analysis of results of AcMNPV time course experiments wherein actin polymerization was blocked by cytochalasin D drug treatment implicate actin-containing chromatin modifying complexes in decatenating baculovirus genomes, shutting down host transcription, and regulating late and very late phases of viral transcription. Moreover, virus-mediated nuclear localization of actin early during infection may contribute to nucleosome management.

Expression of Equine Interleukin-18 by Baculovirus Expression System and Its Biologic Activity

The equine interleukin-18 (IL-18) cDNA that contains the coding sequence was cloned and a recombinant baculovirus, named AcEIL-18, was constructed. The recombinant protein of the equine IL-18 was expressed by AcEIL-18 and its expression was confirmed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting. Insect cells infected with AcEIL-18 secreted a precursor IL-18 with 24 kilo dalton (kDa) into the culture supernatant. Western blot analysis showed that mature equine IL-18 about 18 kDa was also confirmed without co-expression of caspase-1. Culture supernatant from AcEIL-18 infected cells showed a synergistic effect with recombinant human interleukin-12 for induction of interferon-gamma gene expression in equine peripheral mononuclear cells, indicating that the recombinant equine IL-18 expressed in this study also has biological activity without any treatment.

Use of bacterial artificial chromosomes in baculovirus research and recombinant protein expression: current trends and future perspectives

The baculovirus expression system is one of the most successful and widely used eukaryotic protein expression methods. This short review will summarise the role of bacterial artificial chromosomes (BACS) as an enabling technology for the modification of the virus genome. For many years baculovirus genomes have been maintained in E. coli as bacterial artificial chromosomes, and foreign genes have been inserted using a transposition-based system. However, with recent advances in molecular biology techniques, particularly targeting reverse engineering of the baculovirus genome by recombineering, new frontiers in protein expression are being addressed. In particular, BACs have facilitated the propagation of disabled virus genomes that allow high throughput protein expression. Furthermore, improvement in the selection of recombinant viral genomes inserted into BACS has enabled the expression of multiprotein complexes by iterative recombineering of the baculovirus genome.

A seroepidemiologic study of Reston ebolavirus in swine in the Philippines

BACKGROUND

Ebola viruses cause viral hemorrhagic fever in humans and non-human primates and are endemic in Africa. Reston ebolavirus (REBOV) has caused several epizootics in cynomolgus monkeys (Macaca fascicularis) but is not associated with any human disease. In late 2008, REBOV infections were identified in swine for the first time in the Philippines.

METHODS

A total of 215 swine sera collected at two REBOV-affected farms in 2008, in Pangasinan and Bulacan, were tested for the presence of REBOV-specific antibodies using multiple serodiagnosis systems. A total of 98 swine sera collected in a non-epizootic region, Tarlac, were also tested to clarify the prevalence of REBOV infection in the general swine population in the Philippines.

RESULTS

Some 70 % of swine sera at the affected farms were positive for REBOV antibodies in the multiple serodiagnosis systems. On the other hand, none of the swine sera collected in Tarlac showed positive reactions in any of the diagnosis systems.

CONCLUSIONS

The high prevalence of REBOV infection in swine in the affected farms in 2008 suggests that swine is susceptible for REBOV infection. The multiple serological assays used in the study are thought to be useful for future surveillance of REOBV infection in swine in the Philippines.

A bi-cistronic baculovirus expression vector for improved recombinant protein production

Baculoviruses are one of the most studied insect viruses both in basic virology research and in biotechnology applications. Incorporating an internal ribosome entry site (IRES) into the baculovirus genome generates bi-cistronic baculoviruses expression vectors that produce two genes of interest. The bi-cistronic baculoviruses also facilitate recombinant virus isolation and titer determination when the green fluorescent protein was co-expressed. Furthermore, when the secretion proteins were co-expressed with the cytosolic green fluorescent protein, the cell lysis and cytosolic protein released into the culture medium could be monitored by the green fluorescence, thus facilitating purification of the secreted proteins.

Baculovirus expression systems for production of recombinant proteins in insect and mammalian cells

Baculovirus vector systems are extensively used for the expression of foreign gene products in insect and mammalian cells. New advances increase the possibilities and applications of the baculovirus expression system, which makes it possible to express multiple genes simultaneously within a single infected insect cell and to obtain multimeric proteins functionally similar to their natural analogs. Recombinant viruses with expression cassettes active in mammalian cells are used to deliver and express genes in mammalian cells in vitro and in vivo. Further improvement of the baculovirus expression system and its adaptation to specific target cells can open up a wide variety of applications. The review considers recent achievements in the use of modified baculoviruses to express recombinant proteins in eukaryotic cells, advantages and drawbacks of the baculovirus expression system, and ways to optimize the expression of recombinant proteins in both insect and mammalian cell lines.

Expression and characterization of bluetongue virus serotype 21 VP7 antigen: C-terminal truncated protein has significantly reduced antigenicity

In the present study, group-specific antigen VP7 of bluetongue virus (BTV) serotype 21 isolated from cattle in Tochigi prefecture in Japan in 1994 was characterized by sequencing and expression. Gene was amplified from cDNA synthesized on viral dsRNA using reverse-transcriptase-PCR. Nucleotide sequence of this isolate showed high similarity with other published BTV VP7 sequences. Full-length and C-terminal truncated forms of VP7 were expressed in insect cells by a baculovirus gene expression system under control of the viral polyhedrin promoter. Expression of full-length recombinant VP7 was confirmed by immunoprecipitation with VP7 specific monoclonal antibody (8A3B.6, ATCC). Recombinant proteins expressed with or without 6x His-tag showed good expression levels in TN5 cells and reacted well with the monoclonal antibody in the indirect ELISA. However C-terminal truncated VP7 with His-tag failed to react with this monoclonal antibody, while poor antigenicity was evident when it was reacted with infected bovine serum. Reduced antigenicity of the latter suggested that C-terminal truncation affects 8A3B.6 epitope construction probably via inhibition of VP7 trimer structure formation.

Baculovirus-based display and gene delivery systems

The baculovirus expression vector system has been used extensively to produce numerous proteins originating from both prokaryotic and eukaryotic sources. In addition to easy cloning techniques and abundant viral propagation, the system's insect cell environment provides eukaryotic post-translational modification machinery. The recently established eukaryotic molecular biology tool, the baculovirus display vector system (BDVS), allows the combination of genotype with phenotype, enabling presentation of foreign peptides or even complex proteins on the baculoviral envelope or capsid. This strategy is important because it can be used to enhance viral binding and entry to mammalian cells as well as to produce antibodies against the displayed antigen. In addition, the technology should enable modifications of intracellular behavior, that is, trafficking of recombinant "nanoparticles," a highly relevant feature for studies of targeted gene or protein delivery. This article discusses the design and potential uses of insect-derived baculoviral display vectors.

New baculovirus expression tools for recombinant protein complex production

Most eukaryotic proteins exist as large multicomponent assemblies with many subunits, which act in concert to catalyze specific cellular activities. Many of these molecular machines are only present in low amounts in their native hosts, which impede purification from source material. Unraveling their structure and function at high resolution will often depend on heterologous overproduction. Recombinant expression of multiprotein complexes for structural studies can entail considerable, sometimes inhibitory, investment in both labor and materials, in particular if altering and diversifying of the individual subunits are necessary for successful structure determination. Our laboratory has addressed this challenge by developing technologies that streamline the complex production and diversification process. Here, we review several of these developments for recombinant multiprotein complex production using the MultiBac baculovirus/insect cell expression system which we created. We also addressed parallelization and automation of gene assembly for multiprotein complex expression by developing robotic routines for multigene vector generation. In this contribution, we focus on several improvements of baculovirus expression system performance which we introduced: the modifications of the transfer plasmids, the methods for generation of composite multigene baculoviral DNA, and the simplified and standardized expression procedures which we delineated using our MultiBac system.

Genetic modification of a baculovirus vector for increased expression in insect cells

Generating large amounts of recombinant protein in transgenic animals is often challenging and has a number of drawbacks compared to cell culture systems. The baculovirus expression vector system (BEVS) uses virus-infected insect cells to produce recombinant proteins to high levels, and these are usually processed in a similar way to the native protein. Interestingly, since the development of the BEVS, the virus most often used (Autographa californica multi-nucleopolyhedovirus; AcMNPV) has been little altered genetically from its wild-type parental virus. In this study, we modified the AcMNPV genome in an attempt to improve recombinant protein yield, by deleting genes that are non-essential in cell culture. We deleted the p26, p10 and p74 genes from the virus genome, replacing them with an antibiotic selection cassette, allowing us to isolate recombinants. We screened and identified recombinant viruses by restriction enzyme analysis, PCR and Western blot. Cell viability analysis showed that the deletions did not improve the viability of infected cells, compared to non-deletion viruses. However, expression studies showed that recombinant protein levels for the deletion viruses were significantly higher than the expression levels of non-deletion viruses. These results confirm that there is still great potential for improving the BEVS, further increasing recombinant protein expression yields and stability in insect cells.

Mutational analysis of the herpes simplex virus type 1 DNA packaging protein UL33

The UL33 protein of herpes simplex virus type 1 (HSV-1) is thought to be a component of the terminase complex that mediates the cleavage and packaging of viral DNA. In this study we describe the generation and characterization of a series of 15 UL33 mutants containing insertions of five amino acids located randomly throughout the 130-residue protein. Of these mutants, seven were unable to complement the growth of the UL33-null virus dlUL33 in transient assays and also failed to support the cleavage and packaging of replicated amplicon DNA into capsids. The insertions in these mutants were clustered between residues 51 and 74 and between 104 and 116, within the most highly conserved regions of the protein. The ability of the mutants to interact with the UL28 component of the terminase was assessed in immunoprecipitation and immunofluorescence assays. All four mutants with insertions between amino acids 51 and 74 were impaired in this interaction, whereas two of the three mutants in the second region (with insertions at positions 111 and 116) were not affected. These data indicate that the ability of UL33 to interact with UL28 is probably necessary, but not sufficient, to support viral growth and DNA packaging.

Transgenesis approaches for functional analysis of peptidergic cells in the silkworm Bombyx mori

The domestic silkworm, Bombyx mori represents an insect model of great scientific and economic importance. Besides the establishment of a stable germline transformation using the PiggyBac vector, technically feasible methods for in vivo gene delivery and transient gene expression were developed using viral based vectors, especially Sindbis viruses and baculoviruses. The recombinant baculovirus, Autographa californica multiple nucleopolyhedrovirus (AcMNPV), commonly used for large-scale protein production in permissive cell lines or insects, has been used for foreign gene transfer into specific peptidergic cells of B. mori in vivo. Since targeted gene expression is essential for functional analysis of neuropeptide genes and their receptors, the baculovirus-mediated gene transfer can serve as a reliable approach in reverse genetic studies in the silkworm. We review various strategies employing the baculovirus vector system for transient expression of molecular markers and transcription factors in specific peptidergic cells to investigate their roles in B. mori. We also use this system for functional analysis of neuropeptide signaling in the ecdysis behavioral sequence. Our data indicate that the AcMNPV vector is suitable for efficient delivery of foreign genes and their expression directed into specific peptidergic neurons and endocrine cells of B. mori larvae and pupae. However, some modifications of the vector and steps for optimization are necessary to minimize negative effects of viral infection on the host development. The transient gene expression using the AcMNPV and other virus vectors are promising tools for analysis of molecular mechanisms underlying various neuroendocrine processes during development of B. mori.

Recombinant protein production by a Kaiko-baculovirus system

Nucleopolyhedrovirus, a baculovirus, generates many intranuclear polyhedra in lepidopterous insects. The replacement of the polyhedra gene with a target gene, under a potent polyhedrin promoter, is widely used to express recombinant proteins. In this chapter, we describe the application of a highly efficient and reproducible baculovirus expression system with high throughput using Kaiko (silkworm).

Baculovirus-insect cell expression systems

In the early 1980s, the first-published reports of baculovirus-mediated foreign gene expression stimulated great interest in the use of baculovirus-insect cell systems for recombinant protein production. Initially, this system appeared to be the first that would be able to provide the high production levels associated with bacterial systems and the eukaryotic protein processing capabilities associated with mammalian systems. Experience and an increased understanding of basic insect cell biology have shown that these early expectations were not completely realistic. Nevertheless, baculovirus-insect cell expression systems have the capacity to produce many recombinant proteins at high levels and they also provide significant eukaryotic protein processing capabilities. Furthermore, important technological advances over the past 20 years have improved upon the original methods developed for the isolation of baculovirus expression vectors, which were inefficient, required at least some specialized expertise and, therefore, induced some frustration among those who used the original baculovirus-insect cell expression system. Today, virtually any investigator with basic molecular biology training can relatively quickly and efficiently isolate a recombinant baculovirus vector and use it to produce their favorite protein in an insect cell culture. This chapter will begin with background information on the basic baculovirus-insect cell expression system and will then focus on recent developments that have greatly facilitated the ability of an average investigator to take advantage of its attributes.

Characterization of a new Autographa californica multiple nucleopolyhedrovirus (AcMNPV) polyhedra mutant

In the very late phase of baculovirus infection, virions are occluded in a crystalline matrix called polyhedra, which is mainly composed of polyhedrin. This protein is highly conserved among baculoviruses and changes in its amino acid sequence may lead to mutant polyhedra. During the purification of an AcMNPV recombinant virus, a mutant virus was isolated. Structural and ultrastrutural analysis by light and transmission electron microscopy (TEM) of insect cells infected with this mutant virus did not show polyhedra formation and differed from the wild-type infection by the presence of a proteinaceous mass dispersed in the cytoplasm and nucleus of the infected cells, which was confirmed by immunogold labelling to be polyhedrin. The polyhedrin gene was amplified by PCR and sequenced. The only change observed was the substitution of a G to a T at the nucleotide +352, which resulted in a Val to Phe change. A recombinant virus was constructed by transferring the mutant gene into a polyhedrin negative virus. The phenotype of this recombinant virus was the same as the mutant one, confirming that this single mutation alone was responsible for the mutant phenotype.

Generation of baculovirus vectors for the high-throughput production of proteins in insect cells

The baculovirus expression system is one of the most popular methods used for the production of recombinant proteins but has several complex steps which have proved inherently difficult to adapt to a multi-parallel process. We have developed a bacmid vector that does not require any form of selection pressure to separate recombinant virus from non-recombinant parental virus. The method relies on homologous recombination in insect cells between a transfer vector containing a gene to be expressed and a replication-deficient bacmid. The target gene replaces a bacterial replicon at the polyhedrin loci, simultaneously restoring a virus gene essential for replication. Therefore, only recombinant virus can replicate facilitating the rapid production of multiple recombinant viruses on automated platforms in a one-step procedure. Using this vector allowed us to automate the generation of multiple recombinant viruses with a robotic liquid handler and then rapidly screen infected insect cell supernatant for the presence of secreted proteins.

Trypsin cleavage of the baculovirus occlusion-derived virus attachment protein P74 is prerequisite in per os infection

Baculovirus occlusion-derived virions (ODVs) contain a number of infectivity factors essential for the initiation of infection in larval midgut cells. Deletion of any of these factors neutralizes infectivity by the per os route. We have observed that P74 of the group I alphabaculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is N-terminally cleaved when a soluble form of the protein was incubated with insect midgut tissues under alkaline conditions and that cleavage was prevented by soybean trypsin inhibitor (SBTI). Presently, biological assays were carried out that suggest SBTI inhibits and trypsin enhances baculovirus per os infectivity. We developed a method to rescue per os infectivity of a P74 null virus involving co-transfection of viral DNA with a plasmid that transiently expresses p74. We used this plasmid rescue method to functionally characterize P74. A series of site-directed mutants were generated at the N terminus to evaluate if trypsin cleavage sites were necessary for function. Mutagenesis of R195, R196 and R199 compromised per os infectivity and rendered P74 resistant to midgut trypsin.

Overview of the baculovirus expression system

Baculoviruses have emerged as a popular system for overproducing recombinant proteins in eukaryotic cells. This unit gives an overview of the baculovirus expression system, including discussion of the baculovirus life cycle, and post-translational modifications that occur in insect cells. In addition, the steps for overproducing proteins in the baculovirus systems are described along with recommendations for choosing an appropriate baculovirus vector and DNA, and reagents and equipment necessary for implementing the whole overexpression system.

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 recombinant nucleoprotein-based diagnostic systems for Lassa fever

Diagnostic systems for Lassa fever (LF), a viral hemorrhagic fever caused by Lassa virus (LASV), such as enzyme immunoassays for the detection of LASV antibodies and LASV antigens, were developed using the recombinant nucleoprotein (rNP) of LASV (LASV-rNP). The LASV-rNP was expressed in a recombinant baculovirus system. LASV-rNP was used as an antigen in the detection of LASV-antibodies and as an immunogen for the production of monoclonal antibodies. The LASV-rNP was also expressed in HeLa cells by transfection with the expression vector encoding cDNA of the LASV-NP gene. An immunoglobulin G enzyme-linked immunosorbent assay (ELISA) using LASV-rNP and an indirect immunofluorescence assay using LASV-rNP-expressing HeLa cells were confirmed to have high sensitivity and specificity in the detection of LASV-antibodies. A novel monoclonal antibody to LASV-rNP, monoclonal antibody 4A5, was established. A sandwich antigen capture (Ag-capture) ELISA using the monoclonal antibody and an anti-LASV-rNP rabbit serum as capture and detection antibodies, respectively, was then developed. Authentic LASV nucleoprotein in serum samples collected from hamsters experimentally infected with LASV was detected by the Ag-capture ELISA. The Ag-capture ELISA specifically detected LASV-rNP but not the rNPs of lymphocytic choriomeningitis virus or Junin virus. The sensitivity of the Ag-capture ELISA in detecting LASV antigens was comparable to that of reverse transcription-PCR in detecting LASV RNA. These LASV rNP-based diagnostics were confirmed to be useful in the diagnosis of LF even in institutes without a high containment laboratory, since the antigens can be prepared without manipulation of the infectious viruses.

Impact of a basement membrane-degrading protease on dissemination and secondary infection of Autographa californica multiple nucleopolyhedrovirus in Heliothis virescens (Fabricus)

ScathL is a cathepsin L-like cysteine protease from the flesh fly, Sarcophaga peregrina, that digests components of the basement membrane (BM) during insect metamorphosis. A recombinant baculovirus that expresses ScathL (AcMLF9.ScathL) kills larvae of the tobacco budworm, Heliothis virescens, significantly faster than the wild-type virus and triggers melanization and tissue fragmentation in infected larvae shortly before death. As BMs are a potential barrier to the spread of baculovirus secondary infection to other tissues in the host, this study tested the hypothesis that the rapid death of insects infected with AcMLF9.ScathL was caused by accelerated secondary infection resulting from the degradation of host BMs by ScathL. Viruses expressing catalytically active or inactive ScathL were used to examine the effects of ScathL activity on budded virus release into the haemocoel during infection, the production of polyhedra in infected larvae and the rate of infection of the gut, trachea, haemocytes, fat body and Malpighian tubules. It was concluded that the enhanced insecticidal efficacy of the recombinant baculovirus that expresses ScathL does not result from altered tissue tropism or accelerated systemic infection. Implications for the role of the BM as a barrier to baculovirus dissemination within the host insect are discussed.

Recombinant baculovirus isolation

Although there are several different methods available of making recombinant baculovirus expression vectors (reviewed in Chapter 3), all require a stage in which insect cells are transfected with either the virus genome alone (Bac-to-Bac or BaculoDirect, Invitrogen) or virus genome and transfer vector. In the latter case, this allows the natural process of homologous recombination to transfer the foreign gene, under control of the polyhedrin or other baculovirus gene promoter, from the transfer vector to the virus genome to create the recombinant virus. Additionally, many systems require a plaque-assay to separate parental and recombinant virus prior to amplification and use of the recombinant virus. This chapter provides an overview of the historical development of increasingly more efficient systems for the isolation of recombinant baculoviruses (Chapter 3 provides a full account of the different systems and transfer vectors available). The practical details cover: transfection of insect cells with either virus DNA or virus DNA and plasmid transfer vector; a reliable plaque-assay method that can be used to separate recombinant virus from parental (nonrecombinant) virus where this is necessary; methods for the small-scale amplification of recombinant virus; and subsequent titration by plaque-assay. Methods unique to the Bac-to-Bac system are also covered and include the transformation of bacterial cells and isolation of bacmid DNA ready for transfection of insect cells.