Recombinant baculoviruses as mammalian cell gene-delivery vectors

Recombinant baculoviruses used to study estrogen receptor function in human osteosarcoma cells

We report that modified baculoviruses, termed BacMam viruses, can efficiently deliver multiple genes into mammalian cells to generate a heterologous transcription factor/reporter gene system. Using human estrogen receptor (ER) as a model nuclear receptor, we demonstrate how this approach can be successfully applied to assay development in Saos-2 human osteosarcoma cells. BacMam viruses containing full-length cDNAs were constructed for both human ER subtypes, ERalpha and ERbeta, and a third BacMam virus containing an ER-responsive reporter gene cassette. Using these viruses, we found that BacMam-ER expression/reporter constructs could be used to profile the effects of the agonist 17beta-estradiol and the partial agonist raloxifene in human Saos-2 cells. A comparison of assay data obtained with the BacMam-based system with that using standard DNA transfections demonstrates that the two systems are functionally equivalent, giving comparable EC(50) and IC(50) values for estrogen and estrogen plus raloxifene treatments, respectively. Our results indicate that BacMam-mediated gene transfer offers a novel and efficient method for delivery of nuclear receptors and associated genes for mammalian cell-based assay development.

Neuronal gene transfer by baculovirus-derived vectors accommodating a neurone-specific promoter

Recombinant baculoviruses have been employed as gene delivery vectors for mammalian cells, including neurones, during recent years. The aim of the current study was to develop a new recombinant baculovirus vector capable of enhancing gene expression in neurones. A hybrid promoter constructed by fusing the enhancer of human cytomegalovirus (CMV) immediately early promoter to the human platelet-derived growth factor (PDGF) beta-chain promoter was placed into a baculovirus expression cassette. In cultured neurones, baculovirus vectors containing the hybrid promoter augmented transgene expression up to 100-fold greater than that mediated by titre-matched baculovirus vectors with the PDGF promoter alone. Double immunostaining of tissue sections collected from the striatum and the retina injected with the new baculovirus vector demonstrated its specificity in driving gene expression almost exclusively in neurones, confirming the feasibility of using a tissue-specific promoter in the context of baculovirus vectors to provide cell type-specific transgene expression. The attributes of the new baculovirus vector might have practical implications for gene therapy in the nervous system.

A cell-based time-resolved fluorescence assay for selection of antibody reagents for G protein-coupled receptor immunohistochemistry

A cell-based time-resolved fluorescence (celTRF) immunoassay is described for pre-screening antibodies to G protein-coupled receptor (GPCR) peptides that predicts suitability for immunohistochemistry (IHC). Rat GPCRs were expressed in Saos-2 human osteosarcoma cells via recombinant baculoviruses designed for mammalian cell expression, i.e., the transduced cells were used as a "screening lawn". The lawn was fixed and permeabilized similarly to IHC tissue. The celTRF, a dissociation-enhanced lanthanide fluorescence immunoassay (DELFIA), employed Eu-labelled goat anti-rabbit IgG. It exhibited a broad dynamic range upon which enzyme-linked immunosorbant assay (ELISA)-positive affinity-purified anti-peptide antibody reagents were examined for specificity and potency. Over 150 anti-peptide reagents to 27 GPCRs were characterized. All celTRF-positive antibodies were found to be suitable for IHC, whereas ELISA alone did not predict IHC utility. Examples are illustrated with five rabbit anti-neuropeptide FF receptor 1 (NPFF1) antibodies, where a strong correlation between celTRF potency and IHC utility was observed in both applications. In contrast, two high anti-peptide ELISA titer but celTRF-negative antibodies failed to recognize the NPFF1 receptor in IHC. The celTRF assay was performed manually and in an automated fashion, in our case, using a Biomek FX station and Sami scheduling software. The celTRF is the first in vitro automated assay that offers confident pre-selection of antibodies for IHC and the versatility to accommodate the rapid screening of large numbers of GPCRs. The celTRF is readily applicable to other protein target classes.

Gene delivery in bone tissue engineering: progress and prospects using viral and nonviral strategies

Bone tissue loss as a consequence of the natural aging process or as a result of trauma and degenerative disease has led to the need for procedures to generate cartilage and bone for a variety of orthopedic applications. The ability to transfer genes into multipotential mesenchymal stem cells, while still in its infancy, offers considerable therapeutic hope in a variety of musculoskeletal disorders. However, the choice of gene delivery method is key. This review examines the various techniques and methods currently available to enable gene transfer into a target population from viral methods (transduction) to nonviral (transfection) methods and the limitations associated with each method. The potential applications and current understanding of each method are presented. Given the demographic challenge of an aging population, the ultimate goal remains the development of simple, safe, and reproducible strategies for gene delivery that will address the pressing orthopedic clinical imperatives of many.

Enhanced gene delivery by avidin-displaying baculovirus

Flexible alteration of virus surface properties would be beneficial for enhanced and targeted gene delivery. A useful approach could be based on a high-affinity receptor-ligand pair, such as avidin and biotin. In this study, we have constructed an avidin-displaying baculovirus, Baavi. Avidin display was expected to enhance cell transduction due to the high positive charge of avidin in physiological pH and to provide a binding site for covering the virus with desired biotinylated ligands. Successful incorporation of avidin on the virus envelope was detected by immunoblotting and electron microscopy. Multiple biotin-binding sites per virus were detected with fluorescence-correlation spectroscopy and tight biotin binding was observed using an optical biosensor, IAsys. Baavi showed a 5-fold increase in transduction efficiency in rat malignant glioma cells (BT4C) and a 26-fold increase in rabbit aortic smooth muscle (RAASMC) cells compared to wild-type baculovirus. Enhanced transduction was also observed with biotinylated target cells. Biotinylated epidermal growth factor (EGF) enabled specific targeting of the virus with high efficiency to EGF receptor-expressing (SKOV-3) cells. An additional advantage of the avidin display was demonstrated with biotinylated paramagnetic particles, which enabled magnetic targeting. Altogether, we show that avidin display is a rapid and versatile method to improve viral properties for gene delivery.

Use of the red fluorescent protein as a marker of Kaposi's sarcoma-associated herpesvirus lytic gene expression

A hallmark of all herpesvirus is the ability to exist in either a latent, or lytic, state of replication, enabling the lifelong infection of its host. Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) can efficiently establish a latent infection in a variety of cell types in vitro, making it a valuable model for the study of latency and reactivation. To facilitate the identification of KSHV lytic replication, and allow subsequent experiments with live cells, a recombinant virus, rKSHV.219, was constructed using JSC-1 cells that expresses the red fluorescent protein (RFP) from the KSHV lytic PAN promoter, the green fluorescent protein (GFP) from the EF-1alpha promoter, and with the gene for puromycin resistance as a selectable marker. rKSHV.219 from JSC-1 cells was used to infect Vero cells for purification of the recombinant virus. Vero cells were also used for the production of rKSHV.219 at levels of 10(5)-10(6) infectious units (IU) of virus per milliliter using a combination of KSHV/RTA expressed from a baculovirus vector, BacK50, and butyrate. Virus produced from Vero cells was used to infect human fibroblasts (HF), 293, DU145, T24, HaCaT, and HEp-2 cells, and in all cells except 293 cells, only a latent infection was established with GFP expression, but no RFP expression. In 293 cells, 10-15% of cells showed lytic gene expression. Both primary and immortalized microvascular endothelial cells (MVEC) were also infected with rKSHV.219, and reduced spontaneous lytic replication was found in immortalized cells. In all cells used in this study, rKSHV.219 efficiently established latent infections from which the virus could be reactivated to productive lytic replication. This work also demonstrated strong synergy between KSHV/RTA and butyrate for the activation of KSHV lytic replication and the production of infectious virus.

A rapid and efficient method to express target genes in mammalian cells by baculovirus

AIM: To investigate the modification of baculovirus vector and the feasibility of delivering exogenous genes into mammalian cells with the culture supernatant of Spodoptera frugiperta (Sf9) cells infected by recombinant baculoviruses.

METHODS: Two recombinant baculoviruses (BacV-CMV-EGFPA, BacV-CMV-EGFPB) containing CMV-EGFP expression cassette were constructed. HepG2 cells were directly incubated with the culture supernatant of Sf9 cells infected by recombinant baculoviruses, and reporter gene transfer and expression efficiencies were analyzed by flow cytometry (FCM). The optimal transduction conditions were investigated by FCM assay in HepG2 cells. Gene-transfer and expression efficiencies in HepG2 or CV1 cells by baculovirus vectors were compared with lipofectAMINE, recombinant retrovirus and vaccinia virus expression systems. Twenty different mammalian cell lines were used to investigate the feasibility of delivering exogenous genes into different mammalian cells with the culture supernatant of infected Sf9 cells.

RESULTS: CMV promoter could directly express reporter genes in Sf9 cells with a relatively low efficiency. Target cells incubated with the 1:1 diluted culture supernatant (moi = 50) for 12 h at 37 °C could achieve the highest transduction and expression efficiencies with least impairment to cell viability. Under similar conditions the baculovirus vector could achieve the highest gene-transfer and expression efficiency than lipofectAMINE, recombinant retrovirus and vaccinia virus expression systems. Most mammalian cell lines could be transduced with recombinant baculovirus. In primate adherent culture cells the recombinant baculovirus could arrive the highest infection and expression efficiencies, but it was not very satisfactory in the cell lines from mice and suspended culture cells.

CONCLUSION: Mammalian cells incubated with the culture supernatant of infected Sf9 cells could serve as a very convenient way for rapid and efficient expression of foreign genes in mammalian cells, but it might be more suitable for primate adherent culture cells.

Improved display of synthetic IgG-binding domains on the baculovirus surface

Improved display of foreign protein moieties in combination with beneficial alteration of the viral surface properties should be of value for targeted and enhanced gene delivery. Here, we describe a vector based on Autographa californica multiple nucleopolyhedrovirus (AcMNPV) displaying synthetic IgG-binding domains (ZZ) of protein A fused to the transmembrane anchor of vesicular stomatitis virus (VSV) G protein. This display vector was equipped with a GFP/EGFP expression cassette enabling fluorescent detection in both insect and mammalian cells. The virus construct displayed the biologically active fusion protein efficiently and showed increased binding capacity to IgG. As the display is carried out using a membrane anchor of foreign origin, gp64 is left intact for virus entry, which may increase gene expression in the transduced mammalian cells. In addition, the viral vector can be targeted to any desired cell type via binding of ZZ domains when an appropriate IgG antibody is available.

Australian funnel-web spiders: master insecticide chemists

Arthropods are the most diverse animal group on the planet. Their ability to inhabit a vast array of ecological niches has inevitably brought them into conflict with humans. Although only a small minority are classified as pest species, they nevertheless destroy about a quarter of the world's annual crop production and transmit an impressive array of pathogens of human and veterinary public health importance. Arthropod pests have been controlled almost exclusively with chemical insecticides since the introduction of DDT in the 1940s. However, the evolution of resistance to many insecticides, coupled with increased awareness of the potential environmental and human and animal health impacts of these chemicals, has stimulated the search for new insecticidal compounds, novel molecular targets, and alternative control methods. Spider venoms are complex chemical cocktails that have evolved to kill or paralyze arthropod prey, and they represent a largely untapped reservoir of insecticidal compounds. This review focuses on several families of invertebrate-specific peptide neurotoxins that were isolated from the venom of Australian funnel-web spiders. These peptides are promising insecticide leads because of their selectivity for invertebrates and activity on previously unvalidated targets. These toxins should facilitate the development of novel target-based screens for new insecticide leads, while their mapped pharmacophores will provide templates for rational design of mimetics that act at these target sites. Furthermore, genes encoding these toxins can be used to improve the efficacy of insect-specific viruses.

Development of a novel high-throughput surrogate assay to measure HIV envelope/CCR5/CD4-mediated viral/cell fusion using BacMam baculovirus technology

The initial event by which M-tropic HIV strains gain access to cells is via interaction of the viral envelope protein gp120 with the host cell CCR5 coreceptor and CD4. Inhibition of this event reduces viral fusion and entry into cells in vitro. The authors have employed BacMam baculovirus-mediated gene transduction to develop a cell/cell fusion assay that mimics the HIV viral/cell fusion process and allows high-throughput quantification of this fusion event. The assay design uses human osteosarcoma (HOS) cells stably transfected with cDNAs expressing CCR5, CD4, and long terminal repeat (LTR)-luciferase as the recipient host cell. An HEK-293 cell line transduced with BacMam viral constructs to express the viral proteins gp120, gp41, tat, and rev represents the virus. Interaction of gp120 with CCR5/CD4 results in the fusion of the 2 cells and transfer of tat to the HOS cell cytosol; tat, in turn, binds to the LTR region on the luciferase reporter and activates transcription, resulting in an increase in cellular luciferase activity. In conclusion, the cell/cell fusion assay developed has been demonstrated to be a robust and reproducible high-throughput surrogate assay that can be used to assess the effects of compounds on gp120/CCR5/CD4-mediated viral fusion into host cells.

Lentiviral vectors pseudotyped with baculovirus gp64 efficiently transduce mouse cells in vivo and show tropism restriction against hematopoietic cell types in vitro

The envelope glycoprotein from vesicular stomatitis virus (VSV-G) has been used extensively to pseudotype lentiviral vectors, but has several drawbacks including cytotoxicity, potential for priming of immune responses against transgene products through efficient transduction of antigen-presenting cells (APCs) and sensitivity to inactivation by human complement. As an alternative to VSV-G, we extensively characterized lentiviral vectors pseudotyped with the gp64 envelope glycoprotein from baculovirus both in vitro and in vivo. We demonstrated for the first time that gp64-pseudotyped vectors could be delivered efficiently in vivo in mice via portal vein injection. Following delivery, the efficiency of mouse cell transduction and the transgene expression is comparable to VSV-G-pseudotyped vectors. In addition, we found that gp64-pseudotyped lentiviral vectors could efficiently transduce a variety of cell lines in vitro, although gp64 showed a more restricted tropism than VSV-G, with especially poor ability to transduce hematopoietic cell types including dendritic cells (DCs). Although we found that gp64-pseudotyped vectors are also sensitive to inactivation by human complement, gp64 nevertheless has advantages over VSV-G, because of its lack of cytotoxicity and narrower tropism. Consequently, gp64 is an attractive alternative to VSV-G because it can efficiently transduce cells in vivo and may reduce immune responses against the transgene product or viral vector by avoiding transduction of APCs such as DCs.

Baculovirus display of fusion protein of Peste des petits ruminants virus and hemagglutination protein of Rinderpest virus and immunogenicity of the displayed proteins in mouse model

Recombinant Bombyx mori nucleopolyhedroviruses (BmNPV) displaying the immunodominant ectodomains of fusion glycoprotein (F) of Peste des petitis ruminants virus (PPRV) and the hemagglutinin protein (H) of Rinderpest virus (RPV), on the budded virions as well as the surface of the infected host cells have been constructed. The F and H protein sequences were inserted in-frame within the amino-terminal region of BmNPV envelope glycoprotein GP64 expressing under the strong viral polyhedrin (polh) promoter. We improved the recombinant virus selection in BmNPV by incorporating the green fluorescent protein gene (gfp) as selection marker under a separate promoter within the transfer cassette harboring the desired genes. Following infection of the insect larvae or the host-derived BmN cells with these recombinant BmNPVs, the expressed GP64 fusion proteins were displayed on the host cell surface and the budded virions. The antigenic epitopes of the recombinant proteins were properly displayed and the recombinant virus particles induced immune response in mice against PPRV or RPV.

The homologous region sequence (hr1) of Autographa californica multinucleocapsid polyhedrosis virus can enhance transcription from non-baculoviral promoters in mammalian cells

The Autographa californica multinucleocapsid polyhedrosis virus homologous region sequence hr1 enhances transcription from the viral polyhedrin promoter in Spodoptera frugiperda insect cells and independently functions as an origin of replication (ori) sequence. The binding of the host nuclear protein, hr1-binding protein (hr1-BP), is crucial for the enhancer activity (Habib, S., Pandey, S., Chatterji, U., Burma, S., Ahmad, R., Jain, A., and Hasnain, S. E. (1996) DNA Cell Biol. 15, 737-747 and Habib, S., and Hasnain, S. E. (1996) J. Biol. Chem. 271, 28250-28258). We demonstrate that hr1 can also enhance transcription from non-baculoviral promoters like cytomegalovirus and hsp70 in mammalian cells but does not support ori activity in these cells. Unlike insect cells, hr1 can also function in mammalian cells as an enhancer when present in trans. hr1 DNA sequence binds with high affinity and specificity to nuclear factors in the mammalian cells. The insect hr1-BP- and the hr1-BP-like proteins from mammalian cells (mhr1-BP) have different properties with respect to ion requirements, DNA groove binding, and molecular size. When mammalian cells are infected with a recombinant baculovirus containing two promoters, the baculovirus polyhedrin and Drosophila hsp70 gene promoter, the hsp70 gene promoter alone is active in these cells, and this activity is further enhanced by the presence of an additional hr1 in the recombinant virus. hr1 may thus also have a role in baculovirus-mediated gene delivery in mammalian cells.

Improved generation of recombinant baculovirus genomes in Escherichia coli

An improved method for the generation of recombinant baculoviruses by Tn7-mediated transposition is described. The method is based on the modified donor vector (pBVboost) and an improved selection scheme of the baculovirus bacmids in Escherichia coli with a mutated SacB gene. Recombinant bacmids can be generated at a frequency of approximately 10(7)/microg of donor vector with a negligible background. This easy-to-use and efficient pBVboost system provides the basis for a high-throughput generation of recombinant baculoviruses as well as a more convenient way to produce single viruses. The introduced selection scheme is also useful for the construction of other vectors by transposition in E.coli.

In vitro and in vivo gene delivery by recombinant baculoviruses

Although recombinant baculovirus vectors can be an efficient tool for gene transfer into mammalian cells in vitro, gene transduction in vivo has been hampered by the inactivation of baculoviruses by serum complement. Recombinant baculoviruses possessing excess envelope protein gp64 or other viral envelope proteins on the virion surface deliver foreign genes into a variety of mammalian cell lines more efficiently than the unmodified baculovirus. In this study, we examined the efficiency of gene transfer both in vitro and in vivo by recombinant baculoviruses possessing envelope proteins derived from either vesicular stomatitis virus (VSVG) or rabies virus. These recombinant viruses efficiently transferred reporter genes into neural cell lines, primary rat neural cells, and primary mouse osteal cells in vitro. The VSVG-modified baculovirus exhibited greater resistance to inactivation by animal sera than the unmodified baculovirus. A synthetic inhibitor of the complement activation pathway circumvented the serum inactivation of the unmodified baculovirus. Furthermore, the VSVG-modified baculovirus could transduce a reporter gene into the cerebral cortex and testis of mice by direct inoculation in vivo. These results suggest the possible use of the recombinant baculovirus vectors in combination with the administration of complement inhibitors for in vivo gene therapy.

Palmitoylation of the Autographa californica multicapsid nucleopolyhedrovirus envelope glycoprotein GP64: mapping, functional studies, and lipid rafts

Budded virions (BV) of the baculovirus Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) contain a major envelope glycoprotein known as GP64, which was previously shown to be palmitoylated. In the present study, we used truncation and amino acid substitution mutations to map the palmitoylation site to cysteine residue 503. Palmitoylation of GP64 was not detected when Cys503 was replaced with alanine or serine. Palmitoylation-minus forms of GP64 were used to replace wild-type GP64 in AcMNPV, and these viruses were used to examine potential functions of GP64 palmitoylation in the context of the infection cycle. Analysis by immunoprecipitation and cell surface studies revealed that palmitoylation of GP64 did not affect GP64 synthesis or its transport to the cell surface in Sf9 cells. GP64 proteins lacking palmitoylation also mediated low-pH-triggered membrane fusion in a manner indistinguishable from that of wild-type GP64. Cells infected with viruses expressing palmitoylation-minus forms of GP64 produced infectious virions at levels similar to those from cells infected with wild-type AcMNPV. In combination, these data suggest that virus entry and exit in Sf9 cells were not significantly affected by GP64 palmitoylation. To determine whether GP64 palmitoylation affected the association of GP64 with membrane microdomains, the potential association of GP64 with lipid raft microdomains was examined. These experiments showed that: (i) AcMNPV-infected Sf9 cell membranes contain lipid raft microdomains, (ii) GP64 association with lipid rafts was not detected in infected Sf9 cells, and (iii) GP64 palmitoylation did not affect the apparent exclusion of GP64 from lipid raft microdomains.

Predicting therapeutic value in the lead optimization phase of drug discovery

Recombinant and natural cellular assays for human G-protein-coupled receptors are used to optimize initial lead molecules obtained from screening. Although the activity of these molecules can be assessed on human genotype receptors, there is increasing evidence that cells impose a phenotypic selectivity to molecules in various cellular backgrounds. This opens the possibility of dissimulations between activity seen in lead optimization assays and the intended therapeutic value in humans. This review discusses the mechanisms by which cells can impose phenotypic selectivity on molecules and approaches to reduce this practical problem for drug discovery.

Transduction of cultured fish cells with recombinant baculoviruses

Five fish cell lines were tested for their ability to be transduced by Ac-CAlacZ, a recombinant baculovirus that is capable of expressing a beta-galactosidase reporter gene from the CAG promoter (consisting of a cytomegalovirus enhancer element, a chicken actin promoter and rabbit beta-globin termination sequences). TO (Tilapia ovary), EPC (carp), CHH-1 (Chum salmon heart fibroblast) and CHSE-214 (chinook salmon embryo) cells were transducible, as demonstrated by an in situ beta-galactosidase assay, whereas RTG-2 (rainbow trout gonad) cells were not. The EPC cell line was used for more detailed studies on baculovirus transduction. The transduction frequency was found to be higher at 28 degrees C than at 21 degrees C. Addition of the histone deacetylase inhibitor sodium butyrate increased the number of blue cells detected 5- to 7-fold. The m.o.i. was positively correlated with transduction frequency, although the relationship did not appear to be strictly linear, as has been observed with mammalian cells. The temperature at which baculoviruses were adsorbed to EPC cells did not affect levels of beta-galactosidase expression. We also examined expression levels of beta-galactosidase in EPC cells after infection with a baculovirus construct that overexpresses the vesicular stomatitis virus G protein and displays it on the virion surface. Expression levels with this virus were approximately 15-fold higher than were observed with Ac-CAlacZ.

The processing of human mitochondrial leucyl-tRNA synthetase in the insect cells

A His-tagged full-length cDNA of human mitochondrial leucyl-tRNA synthetase was expressed in a baculovirus system. The N-terminal sequence of the enzyme isolated from the mitochondria of insect cells was found to be IYSATGKWTKEYTL, indicating that the mitochondrial targeting signal peptide was cleaved between Ser39 and Ile40 after the enzyme precursor was translocated into mitochondria. The enzyme purified from mitochondria catalyzed the leucylation of Escherichia coli tRNA(1)(Leu)(CAG) and Aquifex aeolicus tRNA(Leu)(GAG) with higher catalytic activity in the leucylation of E. coli tRNA(Leu) than that previously expressed in E. coli without the N-terminal 21 residues.

Viruses as gene delivery vectors: application to gene function, target validation, and assay development

A Biochemical Pharmacology Discussion Group Conference, was held at the headquarters of the New York Academy of Sciences on December 4, 2001 as part of an ongoing series designed to highlight and review areas important to modern drug development (Figure 1). Briefly introduced by Tom Kost (GlaxoSmithKline) and Michael Lotze (University of Pittsburgh), the focus was on the intersection of genomics, proteomics, and now "viromics." The latter term refers to the use of viruses and viral gene transfer to explore the complexity arising from the vast array of new targets available from the human and murine genomes. Indeed, access to large numbers of genes using viral vectors is a key tool for drug discovery and drug delivery. With 38,000 genes identified within the human genome, only 5000 are considered readily druggable. Generating tools such as these to validate targets represents a major part of the armamentarium of the postgenomic scientist. During the last 12 years alone, there have been over 26,000 publications on virus vectors. Many of them have been found useful in target validation, assay development, and evaluation in in vivo models and gene therapy. Thus, there is now an extensive knowledge base for several viral vectors, with unique attributes within each of them providing versatility, efficiency, and ease of use. The individual scientists presenting at the meeting illustrated many of the unique and useful characteristics of such vector systems including retrovirus, adenovirus, herpes virus, simbis virus, and baculovirus.