High amplification of a densovirus-derived vector in larval and adult tissues of Drosophila

Complete Genome Sequence of a Putative Densovirus Infecting the Carrot Psyllid Bactericera trigonica

Here, we report the genome of a putative densovirus infecting the carrot psyllid Bactericera trigonica, obtained by inverse PCR and named Bactericera trigonica densovirus (BtDNV). The ambisense genome of BtDNV is identical in structure to those of the ambidensoviruses, and its encoded proteins share the highest sequence identity with the Asian citrus psyllid Diaphorina citri densovirus.

Here, we report the genome of a putative densovirus infecting the carrot psyllid Bactericera trigonica, obtained by inverse PCR and named Bactericera trigonica densovirus (BtDNV). The ambisense genome of BtDNV is identical in structure to those of the ambidensoviruses, and its encoded proteins share the highest sequence identity with the Asian citrus psyllid Diaphorina citri densovirus.

Linear Lepidopteran ambidensovirus 1 sequences drive random integration of a reporter gene in transfected Spodoptera frugiperda cells

BACKGROUND

The Lepidopteran ambidensovirus 1 isolated from Junonia coenia (hereafter JcDV) is an invertebrate parvovirus considered as a viral transduction vector as well as a potential tool for the biological control of insect pests. Previous works showed that JcDV-based circular plasmids experimentally integrate into insect cells genomic DNA.

METHODS

In order to approach the natural conditions of infection and possible integration, we generated linear JcDV-gfp based molecules which were transfected into non permissive Spodoptera frugiperda (Sf9) cultured cells. Cells were monitored for the expression of green fluorescent protein (GFP) and DNA was analyzed for integration of transduced viral sequences. Non-structural protein modulation of the VP-gene cassette promoter activity was additionally assayed.

RESULTS

We show that linear JcDV-derived molecules are capable of long term genomic integration and sustained transgene expression in Sf9 cells. As expected, only the deletion of both inverted terminal repeats (ITR) or the polyadenylation signals of NS and VP genes dramatically impairs the global transduction/expression efficiency. However, all the integrated viral sequences we characterized appear "scrambled" whatever the viral content of the transfected vector. Despite a strong GFP expression, we were unable to recover any full sequence of the original constructs and found rearranged viral and non-viral sequences as well. Cellular flanking sequences were identified as non-coding ones. On the other hand, the kinetics of GFP expression over time led us to investigate the apparent down-regulation by non-structural proteins of the VP-gene cassette promoter.

CONCLUSION

Altogether, our results show that JcDV-derived sequences included in linear DNA molecules are able to drive efficiently the integration and expression of a foreign gene into the genome of insect cells, whatever their composition, provided that at least one ITR is present. However, the transfected sequences were extensively rearranged with cellular DNA during or after random integration in the host cell genome. Lastly, the non-structural proteins seem to participate in the regulation of p9 promoter activity rather than to the integration of viral sequences.

Regulation of Junonia coenia densovirus P9 promoter expression

Transcriptional activity of the Junonia coenia densovirus (JcDNV) P9 promoter depends on a 557-bp sequence located within the overlapping 3' sequences for viral capsid and nonstructural genes. Utilizing a somatic transformation assay to assess JcDNV promoter activity in Drosophila melanogaster and Plodia interpunctella, viral sequences were subjected to deletional analysis. Removal of a 685-bp fragment reduced P9-driven expression to background levels. Inclusion of a second expression cassette demonstrated vector persistence and confirmed somatic transformation. P9 promoter-driven expression was restored by insertion of a 557-bp JcDNV fragment or by inclusion of a heterologous baculovirus hr5 enhancer. Consensus polycomb transcriptional factor binding sites were identified within the 557-bp fragment, which suggests a potential role in regulating densoviral transcription.

Somatic transformation efficiencies and expression patterns using the JcDNV and piggyBac transposon gene vectors in insects

A somatic transformation gene vector that exploits the genomic integration properties of Junonia coenia lepidopteran densovirus (JcDNV) sequences in vivo has been developed. JcDNV somatic transformation vectors are derivatives of plasmids containing an interrupted genome of JcDNV that provide efficient, robust vectors that can be used to examine regulation of chromosomally integrated transgenes in insects. Microinjection of JcDNV plasmids into syncytial embryos of Drosophila melanogaster or the lepidopterans Plodia interpunctella, Ephestia kuehniella or Trichoplusia ni resulted in persistent transgene expression throughout development. Inclusion of transgenes with tissue-specific promoters resulted in expression patterns canonical with phenotypes of piggyBac germline transformants. Somatic transformation required the presence of the viral inverted terminal repeat in cis only and did not depend upon non-structural viral proteins.

Stably Transformed Insect Cell Lines: Tools for Expression of Secreted and Membrane‐anchored Proteins and High‐throughput Screening Platforms for Drug and Insecticide Discovery

Insect cell-based expression systems are prominent amongst current expression platforms for their ability to express virtually all types of heterologous recombinant proteins. Stably transformed insect cell lines represent an attractive alternative to the baculovirus expression system, particularly for the production of secreted and membrane-anchored proteins. For this reason, transformed insect cell systems are receiving increased attention from the research community and the biotechnology industry. In this article, we review recent developments in the field of insect cell-based expression from two main perspectives, the production of secreted and membrane-anchored proteins and the establishment of novel methodological tools for the identification of bioactive compounds that can be used as research reagents and leads for new pharmaceuticals and insecticides.

Junonia coenia densovirus-based vectors for stable transgene expression in Sf9 cells: influence of the densovirus sequences on genomic integration

The invertebrate parvovirus Junonia coenia densovirus (JcDNV) shares similarities with terminal hairpins and nonstructural (NS) protein activities of adeno-associated virus (AAV) despite their evolutionary divergence (B. Dumas, M. Jourdan, A. M. Pascaud, and M. Bergoin, Virology, 191:202-222, 1992, and C. Ding, M. Urabe, M. Bergoin, and R. M. Kotin, J. Virol. 76:338-345, 2002). We demonstrate here that persistent transgene expression in insect cells results from stable integration of transfected JcDNV-derived vectors into the host genome. To assess the integrative properties of JcDNV vectors, the green fluorescent protein (GFP) gfp marker gene was fused in frame into the major open reading frame (ORF1) of the viral sequence under the control of the P9 capsid protein promoter. In addition, the influence of the nonstructural proteins on the posttransfection maintenance of the vectors was examined by interruption of one or all three NS ORFs. Following transfection of Sf9 cells with each of the JcDNV constructs, clones showing persistent GFP expression were isolated. Structural analyses revealed that the majority of the JcDNV plasmid sequence was integrated into the genome of the fluorescent clones. Integration was observed whether or not NS proteins were expressed. However, the presence of NS genes in the constructs greatly influenced the number of integrated copies and their distribution in the host genome. Disruption of NS genes expression resulted in integration of head-to-tail concatemers at multiple sites within the genome. Further analyses demonstrated that the cis JcDNV 5' inverted terminal repeat region was the primary site of recombination. Sequence analyses of integration junctions showed rearrangements of both flanking and internal sequences for most integrations. These findings demonstrate that JcDNV vectors integrate into insect cells in a manner similar to AAV plasmids in mammalian cells.

Linkage and mapping analysis of a non-susceptibility gene to densovirus (nsd-2) in the silkworm, Bombyx mori

Nonsusceptibility to Bombyx mori densovirus type 2 (BmDNV-2) is controlled by a recessive non-susceptibility gene, nsd-2 (non-susceptibility to DNV-2) in B. mori. Taking advantage of a lack of crossing over in females, reciprocal backcrossed F1 (BF1) progeny were used for linkage analysis and mapping of nsd-2 using silkworm strains C124 and 902, which are classified as being highly susceptible and non-susceptible to DNV-2, respectively. BF1 larvae were inoculated twice with DNV-2 virus at the first and second instar stages. DNA was extracted from each of the surviving fifth instar larvae and analysed by RFLP inheritance patterns using probes specific to each of the 28 linkage groups of B. mori. Our results indicated that the non-susceptibility gene was linked to linkage group 17, since all surviving larvae showed the homozygous profile of strain 902 in their genotype. The other linkage groups showed mixtures of heterozygous and homozygous genotypes, indicating an independent assortment. A linkage map of 30.6 cM was constructed for linkage group 17 with nsd-2 mapped at 24.5 cM and three closely linked cDNA markers were identified.