Retrovirol gene transfer in Xenopus cell lines and embryos

Progress with schistosome transgenesis

Genome sequences for Schistosoma japonicum and Schistosoma mansoni are now available. The schistosome genome encodes ~13,000 protein encoding genes for which the function of only a minority is understood. There is a valuable role for transgenesis in functional genomic investigations of these new schistosome gene sequences. In gain-of-function approaches, transgenesis can lead to integration of transgenes into the schistosome genome which can facilitate insertional mutagenesis screens. By contrast, transgene driven, vector-based RNA interference (RNAi) offers powerful loss-of-function manipulations. Our laboratory has focused on development of tools to facilitate schistosome transgenesis. We have investigated the utility of retroviruses and transposons to transduce schistosomes. Vesicular stomatitis virus glycoprotein (VSVG) pseudotyped murine leukemia virus (MLV) can transduce developmental stages of S. mansoni including eggs. We have also observed that the piggyBac transposon is transpositionally active in schistosomes. Approaches with both VSVG-MLV and piggyBac have resulted in somatic transgenesis and have lead to integration of active reporter transgenes into schistosome chromosomes. These findings provided the first reports of integration of reporter transgenes into schistosome chromosomes. Experience with these systems is reviewed herewith, along with findings with transgene mediated RNAi and germ line transgenesis, in addition to pioneering and earlier reports of gene manipulation for schistosomes.

Pantropic retroviruses as a transduction tool for sea urchin embryos

Sea urchins are an important model for experiments at the intersection of development and systems biology, and technical innovations that enhance the utility of this model are of great value. This study explores pantropic retroviruses as a transduction tool for sea urchin embryos, and demonstrates that pantropic retroviruses infect sea urchin embryos with high efficiency and genomically integrate at a copy number of one per cell. We successfully used a self-inactivation strategy to both insert a sea urchin-specific enhancer and disrupt the endogenous viral enhancer. The resulting self-inactivating viruses drive global and persistent gene expression, consistent with genomic integration during the first cell cycle. Together, these data provide substantial proof of principle for transduction technology in sea urchin embryos.

Establishment, characterization and immortalization of a fibroblast cell line from the Chinese red belly toad Bombina maxima skin

The skin of the amphibian Bombina maxima is rich in biologically active proteins and peptides, most of which have mammalian analogues. The physiological functions of most of the mammalian analogues are still unknown. Thus, Bombina maxima skin may be a promising model to reveal the physiological role of these proteins and peptides because of their large capacity for secretion. To investigate the physiological role of these proteins and peptides in vitro, a fibroblast cell line was successfully established from Bombina maxima tadpole skin. The cell line grew to form a monolayer with cells of a uniform shape and abundant rough endoplasmic reticulum, which are typical characteristics of fibroblasts. Further identification at a molecular level revealed that they strongly expressed the fibroblast marker protein vimentin. The chromosome number of these cells is 2n = 28, and most of them were diploid. Growth property analysis showed that they grew well for 14 passages. However, cells showed decreased proliferative ability after passage 15. Thus, we tried to immortalize the cells through the overexpression of SV40 T antigen. After selecting by G418, cells stably expressed SV40 large T antigen and showed enhanced proliferative ability and increased telomerase activity. Signal transduction analysis revealed functional p42 mitogen-activated protein (MAP) kinase in immortalized Bombina maxima dermal fibroblasts. Primary fibroblast cells and the immortalized fibroblast cells from Bombina maxima cultured in the present study can be used to investigate the physiological role of Bombina maxima skin-secreted proteins and peptides. In addition, the methods for primary cell culturing and cell immortalization will be useful for culturing and immortalizing cells from other types of amphibians.

Transduction of Schistosoma japonicum schistosomules with vesicular stomatitis virus glycoprotein pseudotyped murine leukemia retrovirus and expression of reporter human telomerase reverse transcriptase in the transgenic schistosomes

Although draft genome sequences of two of the major human schistosomes, Schistosoma japonicum and Schistosoma mansoni are available, the structures and characteristics of most genes and the influence of exogenous genes on the metabolism of schistosomes remain uncharacterized. Furthermore, which functional genomics approaches will be tractable for schistosomes are not yet apparent. Here, the vesicular stomatitis virus glycoprotein (VSVG)-pseudotyped pantropic retroviral vector pBABE-puro was modified to incorporate the human telomerase reverse transcriptase gene (hTERT) as a reporter, under the control of the retroviral long terminal repeat (LTR). Pseudotyped virions were employed to transduce S. japonicum to investigate the utility of retrovirus-mediated transgenesis of S. japonicum and the activity of human telomerase reverse transcriptase as a reporter transgene in schistosomes. Schistosomules perfused from experimentally infected rabbits were cultured for 6 days after exposure to the virions after which genomic DNAs from virus exposed and control worms were extracted. Analysis of RNA from transduced parasites and immunohistochemistry of thin parasite sections revealed expression of hTERT in the transduced worms. Expression of hTERT was also confirmed by immunoblot analysis. These findings indicated that S. japonicum could be effectively transduced by VSVG-pseudotyped retrovirus carrying the hTERT gene. Given the potential of hTERT to aid in derivation of immortalized cells, these findings suggest that this pantropic retroviral approach can be employed to transduce cells from specific tissues and organs of schistosomes to investigate the influence of transgene hTERT on growth and proliferation of schistosome cells.

Vector-based RNA interference of cathepsin B1 in Schistosoma mansoni

In helminth parasites, proteolytic enzymes have been implicated in facilitating host invasion, moulting, feeding, and evasion of the host immune response. These key functions render them potential targets for anti-parasite chemotherapy and immunotherapy. Schistosomes feed on host blood and the digested haemoglobin is their major source of amino acids. Haemoglobin digestion is essential for parasite development, growth, and reproduction. We recently reported the use of pseudotyped Moloney murine leukaemia virus to accomplish transformation of Schistosoma mansoni. Here, we report the design of a viral vector expressing a dsRNA hairpin to silence expression of the schistosome cathepsin B1 (SmCB1) gene. We observed 80% reduction in transcript level 72 h after virus exposure and complete silencing of enzyme activity in transduced worms. This is the first report using this technology in any helminth parasite. It will facilitate the evaluation of potential drug targets and biochemical pathways for novel interventions in schistosomes.

Production of vesicular stomatitis virus G glycoprotein (VSV-G) pseudotyped retroviral vectors

Retrovirus pseudotype is defined as the genome of one retrovirus encapsidated by the envelope protein of a second virus. The host range of the pseudotype is that of the virus donating the envelope protein. Two procedures that use 293GP cells, which are derived from human kidney 293 cells, are described here. The first is based on the high transient transfection efficiency of 293 cells. The retroviral construct and an expression plasmid for VSV-G are co-transfected into 293GP cells that stably express MLV gag and pol proteins. Transiently generated virus is then harvested during consecutive days following DNA transfection. The second procedure involves stable 293GP cell lines containing the VSV-G gene under the control of a promoter whose activity is regulated by tetracycline. Cell lines containing the retroviral vector of interest are established under noninduced conditions. Infectious virus can be harvested following the induction of VSV-G expression in these cell lines.

Transgenesis of schistosomes: approaches employing mobile genetic elements

Draft genome sequences for Schistosoma mansoni and Schistosoma japonicum are now available. However, the identity and importance of most schistosome genes have yet to be determined. Recently, progress has been made towards the genetic manipulation and transgenesis of schistosomes. Both loss-of-function and gain-of-function approaches appear to be feasible in schistosomes based on findings described in the past 5 years. This review focuses on reports of schistosome transgenesis, specifically those dealing with the transformation of schistosomes with exogenous mobile genetic elements and/or their endogenous relatives for the genetic manipulation of schistosomes. Transgenesis mediated by mobile genetic elements offers a potentially tractable route to introduce foreign genes to schistosomes, a means to determine the importance of schistosome genes, including those that could be targeted in novel interventions and the potential to undertake large-scale forward genetics by insertional mutagenesis.

Transduction of Schistosoma mansoni by vesicular stomatitis virus glycoprotein-pseudotyped Moloney murine leukemia retrovirus

Retroviral transduction of cultured schistosomes offers a potential means to establish transgenic lines of schistosomes and thereby to facilitate the elucidation of schistosome gene function and expression. The Moloney murine leukemia retroviral (MMLV) vector pLNHX was modified to incorporate EGFP or luciferase reporter genes under control of schistosome endogenous gene promoters from the spliced leader RNA and HSP70 genes. These constructs and a plasmid encoding vesicular stomatitis virus glycoprotein (VSVG) were utilized along with GP2-293 cells to produce replication incompetent retrovirus particles pseudotyped with the VSVG envelope. Exposure of several developmental stages, including sporocysts, of Schistosoma mansoni to these virions was facilitated by incubation with polybrene and/or by centrifugation. The early stages of binding and uptake of virus to the parasite tegument were demonstrated by the immunofluorescence colocalization of VSVG envelope and retroviral capsid proteins. Southern hybridization analysis indicated the integration of proviral forms of the MMLV constructs in genomic DNA isolated from the virus exposed schistosomes. Furthermore, analysis of RNA isolated from virus treated parasites demonstrated the presence of transcripts encoding reporter transgenes. Together these results indicated productive transduction by VSVG pseudotyped MMLV of cultured schistosomes, and suggest a tractable route forward towards heritable schistosome transgenesis.

Characteristics of a thyroid hormone responsive reporter gene transduced into a Xenopus laevis cell line using lentivirus vector

We introduced a self-inactivation (SIN) lentivirus vector (LV) into Xenopus laevis cell lines and established a permanent cell line expressing a reporter gene in a 3,5,3'-l-triiodothyronine (T(3)) dependent manner. The SIN LV contained the luciferase gene downstream from the X. laevis T(3)-response elements (TREs) and the SV40 promoter, and the enhanced green fluorescent protein (EGFP) gene downstream from the cytomegalovirus (CMV) promoter. It was integrated into the genome of X. laevis XL58, XTC2, and KR cells. The SIN LV transduced the X. laevis cells as efficiently as mammalian cells; however, the expression of EGFP in the transgene decreased with increasing culture time. A cell clone exhibiting the highest TH-dependent luciferase gene expression (XL58-TRE-Luc clone) was isolated from the EGFP-positive XL58 cell pool and characterized. The minimum effective concentration of T(3) that significantly induced the luciferase gene expression was 10(-11)M in the XL58-TRE-Luc clone. The application of the luciferase gene assay using the permanent XL58-TRE-Luc clone for the screening of thyroid-disrupting chemicals revealed that tetrachlorobisphenol A, at 10(-6)M, had a weak T(3)-agonist activity, whereas trichlorobisphenol A, at 10(-8) - 10(-6)M had a weak T(3)-antagonist activity. Our results indicated that the permanent X. laevis cell line containing a T(3)-response transgene could be used as a bioassay, with small intra-assay variation, for the rapid screening, identification, and characterization of the thyroid-disrupting chemicals.

Lipofection strategy for the study of Xenopus retinal development

The analysis of gene function during retinal development can be addressed by perturbing gene expression either by inhibition or by overexpression in desired regions and at defined stages of development. An in vivo lipofection strategy has been applied for stage-specific and region-specific expression of genes in Xenopus retina. Due to colipofection efficiency, this strategy enables us to study functional interaction of genes by lipofecting multiple expression constructs. This lipofection technique also allows us to transfect morpholino oligonucleotides into retinoblasts to block gene function. We present here various aspects of this technique, including recent improvements and modifications.

Many nonmammalian cells exhibit postentry blocks to transduction by gammaretroviruses pseudotyped with various viral envelopes, including vesicular stomatitis virus G glycoprotein

Previous studies have suggested that Moloney murine leukemia virus (MoMLV)-based vectors pseudotyped with the vesicular stomatitis virus G glycoprotein (VSV-G) have extensive ability to transduce nonmammalian cells. However, we have identified multiple cell lines from fish (FHM), mosquitoes (Mos-55), moths (Sf9 and High-5), flies (S2), and frogs (XPK2) that are not efficiently transduced by MoMLV-based vectors pseudotyped with many different viral envelope proteins, including VSV-G, while the same vectors are functional in these cells following transfection. A comparison of MoMLV-based vector transduction in mammalian and nonmammalian cells shows that the nonmammalian cells exhibit blocks at either entry, reverse transcription, or integration. Additionally, VSV-G-pseudotyped MoMLV-based vector transduction is attenuated in the zebrafish cell line ZF4 at entry and/or reverse transcription, whereas other transduction processes are unaffected. We show that the variation of transduction by MoMLV-based vectors in mammalian and nonmammalian cells is not due to differences in culture conditions or cell division rate but is likely the result of divergence in cellular factors required for retroviral transduction.

Pantropic retroviral vectors mediate gene transfer and expression in Entamoeba histolytica

Transformation of Entamoeba histolytica has been previously reported, but the foreign genes have all been replicated episomally. Pantropic retroviral vectors based on the Moloney murine leukemia virus with the envelope glycoprotein of vesicular stomatitis virus (VSV-G) have an extremely broad host range and can be concentrated to high titer. To investigate whether these pseudotyped, pantropic vectors can mediate gene transfer and expression in E. histolytica, we constructed a retroviral vector, in which a hygromycin phosphotransferase is expressed from the E. histolytica actin promoter. Data confirm the infection, integration, and expression of a foreign gene mediated by the provirus. To our knowledge, this is the most evolutionarily distant example of successful integration and expression of a mammalian retrovirus. Pantropic retroviral vectors may thus facilitate genetic analysis in species lacking transformation systems.

Pantropic retroviral vectors integrate and express in cells of the malaria mosquito, Anopheles gambiae

The lack of efficient mechanisms for stable genetic transformation of medically important insects, such as anopheline mosquitoes, is the single most important impediment to progress in identifying novel control strategies. Currently available techniques for foreign gene expression in insect cells in culture lack the benefit of stable inheritance conferred by integration. To overcome this problem, a new class of pantropic retroviral vectors has been developed in which the amphotropic envelope is completely replaced by the G glycoprotein of vesicular stomatitis virus. The broadened host cell range of these particles allowed successful entry, integration, and expression of heterologous genes in cultured cells of Anopheles gambiae, the principle mosquito vector responsible for the transmission of over 100 million cases of malaria each year. Mosquito cells in culture infected with a pantropic vector expressing hygromycin phosphotransferase from the Drosophila hsp70 promoter were resistant to the antibiotic hygromycin B. Integrated provirus was detected in infected mosquito cell clones grown in selective media. Thus, pantropic retroviral vectors hold promise as a transformation system for mosquitoes in vivo.