Linda A. King

The Baculovirus Expression System: A Laboratory Guide

The baculoviruses - introduction the development of baculovirus expression vectors processing of foreign proteins synthesized using baculovirus vectors in insect cells construction of transfer vectors containing the foreign gene insect cell culture media and maintenance of insect cell lines propagation, titration and purification of AcMNPV in cell culture production and selelction of recombinant virus characterization of recombinant viruses scaling up the production of recombinant proteins in insect cells - laboratory bench level propagation of baculoviruses in insect larvae.

Baculovirus Expression Systems for Recombinant Protein Production in Insect Cells

Baculoviruses are lethal pathogens of insects, predominantly of the order Lepidoptera. These viruses have a bi-phasic life cycle, which greatly facilitates their use for biotechnological applications. They were exploited initially as biocontrol agents, and then engineered as protein expression vectors. The baculovirus expression vector system (BEVS) is now widely used for recombinant protein production. More recently they have become a popular choice for development as gene delivery and expression vectors in mammalian cells. This article reviews some of the major developments and patents relating to baculoviruses since their initial use as an expression tool and investigates current technologies alleviating bottlenecks in recombinant gene expression in insect cells.

Evidence for the presence of a low-level, persistent baculovirus infection of Mamestra brassicae insects

A laboratory culture of Mamestra brassicae insects (MbLC) harbours a latent or occult baculovirus that resembles M. brassicae multiple nucleocapsid nucleopolyhedrovirus (MbMNPV). Although conventional extraction techniques have failed to detect the presence of virus in MbLC, control virus-free insects (MbWS) died of an MbMNPV-like infection after being fed MbLC fat-body cells. This suggested that the MbLC cells harboured infectious MbMNPV, albeit at low levels. We have also demonstrated that fat-body cells from MbLC, but not from MbWS, contain mRNA specific for the polyhedrin gene and transcriptional factors that are capable of activating baculovirus late and very late gene promoters linked to a reporter gene encoding chloramphenicol acetyltransferase. Our data provide indirect evidence that the latent MbMNPV in the MbLC insects is maintained as a persistent infection, with the expression of viral genes at a low level.

Efficient transfection of insect cells with baculovirus DNA using electroporation.

Efficient transfection of Spodoptera frugiperda cells with Autographa californica nuclear polyhedrosis virus (AcNPV) DNA has been carried out using the technique of electroporation. The efficiency of transfection was monitored by assaying the extracellular virus in cell culture supernatants 3 days post-electroporation. Maximum titres of 2 x 10(9) p.f.u./ml AcNPV were obtained when using a pulse length of 7.7 ms and a field strength of 500 V/cm. This compared with a titre of 2 x 10(6) p.f.u./ml AcNPV using the standard calcium phosphate transfection method. Cotransfections of wild-type AcNPV DNA and the transfer plasmid pAcRP23-lacZ were also performed using electroporation and gave beta-galactosidase recombinant virus titres of 5 x 10(4) p.f.u./ml; this compared with 5 x 10(2) p.f.u./ml using the calcium phosphate method. The maximum proportion of recombinant virus, 2.9%, was obtained by harvesting the transfection medium after 2 days, and using a pulse length of 2.8 ms and a field strength of 750 V/cm. We therefore conclude that electroporation provides a very efficient method for the transfection of insect cells with baculovirus DNA.

IN VITRO HOST RANGE OF AUTOGRAPHA CALIFORNICA NUCLEOPOLYHEDROVIRUS RECOMBINANTS LACKING FUNCTIONAL P35, IAP1 OR IAP2

We have examined the host range in different insect cell lines of Autographa californica nucleopolyhedrovirus (AcMNPV) recombinants lacking p35, iap1 or iap2. These genes encode, or are predicted to encode, anti-apoptotic proteins. Abrogation of p35 reduced the ability of AcMNPV to replicate in permissive cell lines derived from Spodoptera frugiperda insects by inducing apoptosis. In semi-permissive cell lines, such as Lymantria dispar and Spodoptera littoralis cells, we observed cytopathic effects after infection with AcMNPV but little virus production. Infection of these cells by AcMNPV lacking p35 resulted in apoptosis. However, p35-deficient viruses were still able to replicate normally in Trichoplusia ni, Mamestra brassicae and Panolis flammea cell lines. Disruption of AcMNPV iap1 and iap2 was found not to affect virus replication in any of the cell lines. It was also possible to disrupt both iap1 and iap2 in the same virus without loss of infectivity. A virus without iap1 and p35 demonstrated identical growth characteristics and host range to a virus lacking p35. We conclude that in cells which respond to AcMNPV infection by initiating programmed cell death, the p35 gene product alone is sufficient to inhibit apoptosis. Removal of iap1 or iap2 has no effect on virus replication, even in cell lines which do not undergo apoptosis in response to AcMNPV infection. Our results with two semi-permissive cell lines further indicate that whilst p35 is important in blocking block apoptosis, other factors are involved in restricting AcMNPV replication within these cells.

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.

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. Biotechnol. Bioeng.

Synthesis of functional GABAA receptors in stable insect cell lines

We have synthesised the β1‐subunit of the bovine GABAA receptor in stable, continuous insect (Spodoptera frugiperda) cell lines. A cDNA was integrated randomly into the insect cell genome under control of a baculovirus immediate early (IE‐1) gene promoter. Transformed cells were obtained by co‐transfection of the insect cells with pIEK 1 .GRβ1, encoding the β1 subunit cDNA, and pIEK1 .neo, encoding the neomycin resistance gene. G‐418‐resistant clones were selected and expanded into continuous cell lines synthesising functional, GABA‐gated, homo‐oligomeric chloride channels. These cell lines had significant advantages over the transient baculovirus expression system for the characterisation of receptors using electrophysiological recording techniques.

Improved expression of secreted and membrane-targeted proteins in insect cells.

Secretory and membrane‐bound proteins are generally produced in lower amounts in insect cells compared with cytoplasmic and nuclear proteins. There may be many reasons for this, including degradation of recombinant proteins by proteases, competition for cellular resources between native and recombinant proteins, and physical blockage of the secretory pathways. In the present study, we describe the construction of a baculovirus in which chiA (chitinase) and cath (cathepsin) genes have been deleted and show improved recombinant protein expression using this vector. We confirmed the complete removal of both genes by PCR, restriction enzyme analysis and enzyme assays, and the modified virus DNA was shown to be stable in bacterial cells over multiple passages. A selection of recombinant genes were inserted into the double‐deletion virus and their expression levels compared with recombinant viruses that had single or no gene deletions. In all instances, the double‐deletion viruses showed greatly enhanced levels of protein production for both secreted and nuclear/cytoplasmic proteins. In summary, we have conclusively demonstrated the importance of this deletion vector for the high‐level production of recombinant proteins.

Sequence recombination and conservation of Varroa destructor virus-1 and deformed wing virus in field collected honey bees (Apis mellifera).

We sequenced small (s) RNAs from field collected honeybees (Apis mellifera) and bumblebees ( Bombus pascuorum ) using the Illumina technology. The sRNA reads were assembled and resulting contigs were used to search for virus homologues in GenBank. Matches with Varroa destructor virus-1 (VDV1) and Deformed wing virus (DWV) genomic sequences were obtained for A. mellifera but not B . pascuorum . Further analyses suggested that the prevalent virus population was composed of VDV-1 and a chimera of 5’-DWV-VDV1-DWV-3’. The recombination junctions in the chimera genomes were confirmed by using RT-PCR, cDNA cloning and Sanger sequencing. We then focused on conserved short fragments (CSF, size > 25 nt) in the virus genomes by using GenBank sequences and the deep sequencing data obtained in this study. The majority of CSF sites confirmed conservation at both between-species (GenBank sequences) and within-population (dataset of this study) levels. However, conserved nucleotide positions in the GenBank sequences might be variable at the within-population level. High mutation rates (Pi>10%) were observed at a number of sites using the deep sequencing data, suggesting that sequence conservation might not always be maintained at the population level. Virus-host interactions and strategies for developing RNAi treatments against VDV1/DWV infections are discussed.