Retroviral vectors for the expression of two genes in human multipotent neural precursors and their differentiated neuronal and glial progeny

High-efficiency transient transduction of human embryonic stem cell-derived neurons with baculoviral vectors

Transient genetic manipulation of human neurons without chromosomal integration of the transgene would be valuable but has been challenging due to the quiescent nature of these postmitotic cells. In this study, we developed a set of baculoviral vectors for transient transduction in nondividing neurons derived from human embryonic stem cells (hESCs). Using a baculoviral vector equipped with the woodchuck hepatitis virus posttranscriptional regulatory element (WPRE), we observed a quick onset of transgene expression as early as day 1 after baculoviral transduction and a high efficiency of up to 80%. Strong transgene expression in the cultured human neurons was observed for more than 1 month and the signal was easily detectable even after 3 months. Using two baculoviral vectors carrying different transgenes, we found that co-transduction at a single neuron level was possible. After transplantation into the brain of nude mice, the baculovirus-transduced human neurons were integrated into the mouse brain and maintained transgene expression for at least 4 weeks, portending the usefulness of this technique in assisting neural transplantation. Therefore, by mediating efficient transient gene expression, baculoviral vectors can provide useful tools for both basic gene function studies in human neurons and therapeutic applications of these cells.

The use of IRES-based bicistronic vectors allows the stable expression of recombinant G-protein coupled receptors such as NPY5 and histamine 4

Stable expression of G protein coupled receptors in cell lines is a crucial tool for the characterization of the molecular pharmacology of receptors and the screening for new antagonists. However, in some instances, many difficulties have been encountered to obtain stable cell lines expressing functional receptors. Here, we addressed the question of vector optimization to establish cell lines expressing the human neuropeptide Y receptor 5 (NPY5-R) or histamine receptor 4 (HH4R). We have compared bicistronic vectors containing viral or cellular internal ribosome entry sites (IRES), co-expressing the receptor and the neomycine resistance gene from a single mRNA, to a bigenic vector containing two distinct promoters upstream each different genes. This study is the first one to validate the use of three cellular IRESs for long-term transgene expression. Our results demonstrate for both NPY5-R and HH4R that the bicistronic vectors with EMCV, VEGF, FGF1A or FGF2 IRES provide clones expressing functional receptors with yields between 25% and 100%. In contrast, the bigenic vector provided no functional clones, related to a low expression of NPY5R mRNA. The cell lines expressing active receptor were stable after more than 50 passages. These data indicate that IRES-based bicistronic vectors are particularly appropriate to establish cell clones expressing active G-coupled protein receptors with a high yield. In the case of NPY5, it was a new way to produce such a stable cell line. Furthermore, the characteristics-presented herein-of this receptor pharmacological property are perfectly in line with those reported in the literature.

Dicistronic MLV-retroviral vectors transduce neural precursors in vivo and co-express two genes in their differentiated neuronal progeny

Dicistronic MLV-based retroviral vectors, in which two IRESes independently initiate the translation of two proteins from a single RNA, have been shown to direct co-expression of proteins in several cell culture systems. Here we report that these dicistronic retroviral vectors can drive co-expression of two gene products in brain cells in vivo. Injection of retroviral vector producer cells leads to the transduction of proliferating precursors in the external granular layer of the cerebellum and throughout the ventricular regions. Differentiated neurons co-expressing both transgenes were observed in the cerebellum and in lower numbers in distant brain regions such as the cortex. Thus, we describe an eukaryotic dicistronic vector system that is capable of transducing mouse neural precursors in vivo and maintaining the expression of genes after cell differentiation.

The leader of human immunodeficiency virus type 1 genomic RNA harbors an internal ribosome entry segment that is active during the G2/M phase of the cell cycle

The 5' leader of the human immunodeficiency virus type 1 (HIV-1) genomic RNA contains highly structured domains involved in key steps of the viral life cycle. These RNA domains inhibit cap-dependent protein synthesis. Here we report that the HIV-1 5' leader harbors an internal ribosome entry site (IRES) capable of driving protein synthesis during the G(2)/M cell cycle phase in which cap-dependent initiation is inhibited. The HIV-1 IRES was delineated with bicistronic mRNAs in in vitro and ex vivo assays. The HIV-1 leader IRES spans nucleotides 104 to 336 and partially overlaps the major determinants of genomic RNA packaging. These data strongly suggest that, as for HIV-1 transcription, IRES-mediated translation initiation could play an important role in virus replication during virus-induced G(2)/M cell cycle arrest.

Efficient gene transfer in mouse neural precursors with a bicistronic retroviral vector

Gene transfer into neural precursors is a powerful approach to study the function of specific gene products during nervous system development. Here we describe a retrovirus-based methodology to transduce foreign genes into mouse neural precursors. We used a high-titer bicistronic retroviral vector that encodes a marker gene, placental alkaline phosphatase (plap), and a selection gene, neomycin phosphotransferase II (neoR), under the translational control of two retroviral internal ribosome entry segments. Transduction efficiency even without selection was up to 95% for multipotential neurospheres derived from embryonic striata and grown with basic fibroblast growth factor 2. Expression of plap and neoR was sustained with time in culture and upon differentiation into neurons, astrocytes, and oligodendrocytes, as shown by double immunofluorescence labeling with cell type-specific markers, Western blotting, and neomycin resistance. However, levels of plap were decreased in differentiated oligodendrocytes. Transduction with the same vector of neonatal oligodendrocyte precursors grown in oligospheres consistently resulted in a lower proportion of plap-immunoreactive cells and enhanced cell death in the absence of neomycin. However, plap expression was maintained in some differentiated oligodendrocytes expressing galactocerebroside or myelin basic protein. In that neurospheres can be easily expanded in vitro and factors enabling their differentiation into the three main central nervous system cell types are being elucidated, this methodology could be used in the future to produce large number of transduced, differentiated neural cells.

Evaluation of retroviral vector design in defined chromosomal loci by Flp-mediated cassette replacement

Successful retroviral vector construction is still empirical. Test systems for vector efficiency are based on statistical comparison of numerous infectants with single proviral integrates, since their expression depends on the chromosomal surroundings. More reliable data would be obtained if different vector constructs were studied in an identical chromosomal context. Here, we demonstrate the use of a new method, in which chromosomal sites are provirally tagged in such a way that they can be targeted with other expression cassettes. The original tagging integrate is replaced in one step by the targeting element. This permits a reliable comparison of different retroviral vector configurations, eliminating the influence of neighboring chromosomal elements. We compared different retroviral vector types for coexpression of two genes: a vector containing an internal promoter and a vector with an internal ribosome entry site (IRES) element. In contrast to bicistronic retroviral vectors, dual-promoter proviruses exhibited rapid inactivation of the long terminal repeat (LTR)-driven gene expression. Targeted exchange of the dual-promoter provirus with a bicistronic retroviral cassette resulted in gain of expression stability. The reverse experiment confirmed this promoter interaction phenomenon since initial expression stability from a single-promoter bicistronic provirus was lost by targeted exchange with a dual-promoter cassette. In addition, targeting exchange of the dual-promoter provirus, replacing the LTR with an artificial (Tet) promoter restored expression stability. These observations, valid for various integration sites, prove the strong interaction between the LTR and the internal promoter. Our results have implications for retroviral vector design and suggest that retroviral coexpression of two genes is more predictable in the bicistronic configuration.

N-myc translation is initiated via an internal ribosome entry segment that displays enhanced activity in neuronal cells

Eukaryotic translation can be initiated either by a cap-dependent mechanism or by internal ribosome entry, a process by which ribosomes are directly recruited to structured regions of mRNA upstream of the initiation codon. We analysed the 5' untranslated region (UTR) of the proto-oncogene N-myc, and demonstrated by transfections in a dicistronic vector system that it contains a potent internal ribosome entry segment (IRES). The IRES is similar in length to the c-myc IRES and the activities of these IRESs are comparable in non-neuronal cells. Transfections were also carried out in cell lines derived from neuroblastomas, in which N-myc is expressed, and in a neuronal precursor cell line. In these cells the N-myc IRES is up to seven times more active than that of c-myc, suggesting that neuronal-specific non-canonical trans-acting factors are used by the N-myc but not the c-myc IRES. N-myc expression is increased by gene amplification in many neuroblastomas, but this is the first example of a translational mechanism by which N-myc expression could be further increased. The discovery of an IRES that displays enhanced activity in neuronal cell lines has important potential as a tool for protein expression in neural tissue.

Development of minimal lentivirus vectors derived from simian immunodeficiency virus (SIVmac251) and their use for gene transfer into human dendritic cells

Lentivirus-derived vectors are very promising gene delivery systems since they are able to transduce nonproliferating differentiated cells, while murine leukemia virus-based vectors can only transduce cycling cells. Here we report the construction and characterization of highly efficient minimal vectors derived from simian immunodeficiency virus (SIVmac251). High-fidelity PCR amplification of DNA fragments was used to generate a minimal SIV vector formed from a 5' cytomegalovirus early promoter, the 5' viral sequences up to the 5' end of gag required for reverse transcription and packaging, the Rev-responsive element, a gene-expressing cassette, and the 3' long terminal repeat (LTR). Production of SIV vector particles was achieved by transfecting 293T cells with the vector DNA and helper constructs coding for the viral genes and the vesicular stomatitis virus glycoprotein G envelope. These SIV vectors were found to have transducing titers reaching 10(7) transducing units/ml on HeLa cells and to deliver a gene without transfer of helper functions to target cells. The central polypurine tract can be included in the minimal vector, resulting in a two- to threefold increase in the transduction titers on dividing or growth-arrested cells. Based on this minimal SIV vector, a sin vector was designed by deleting 151 nucleotides in the 3' LTR U3 region, and this SIV sin vector retained high transduction titers. Furthermore, the minimal SIV vector was efficient at transducing terminally differentiated human CD34(+) cell-derived or monocyte-derived dendritic cells (DCs). Results show that up to 40% of human primary DCs can be transduced by the SIV vectors. This opens a new perspective in the field of immunotherapy.

Fibroblast growth factor 2 internal ribosome entry site (IRES) activity ex vivo and in transgenic mice reveals a stringent tissue-specific regulation

Fibroblast growth factor 2 (FGF-2) is a powerful mitogen involved in proliferation, differentiation, and survival of various cells including neurons. FGF-2 expression is translationally regulated; in particular, the FGF-2 mRNA contains an internal ribosome entry site (IRES) allowing cap-independent translation. Here, we have analyzed FGF-2 IRES tissue specificity ex vivo and in vivo by using a dual luciferase bicistronic vector. This IRES was active in most transiently transfected human and nonhuman cell types, with a higher activity in p53 -/- osteosarcoma and neuroblastoma cell lines. Transgenic mice were generated using bicistronic transgenes with FGF-2 IRES or encephalomyocarditis virus (EMCV) IRES. Measurements of luciferase activity revealed high FGF-2 IRES activity in 11-d-old embryos (E11) but not in the placenta; activity was high in the heart and brain of E16. FGF-2 IRES activity was low in most organs of the adult, but exceptionally high in the brain. Such spatiotemporal variations were not observed with the EMCV IRES. These data, demonstrating the strong tissue specificity of a mammalian IRES in vivo, suggest a pivotal role of translational IRES- dependent activation of FGF-2 expression during embryogenesis and in adult brain. FGF-2 IRES could constitute, thus, a powerful tool for gene transfer in the central nervous system.

Analysis of 4070A envelope levels in retroviral preparations and effect on target cell transduction efficiency

A number of stable producer cell lines for high-titer Mo-MuLV vectors have been constructed. Development has previously centered on increasing end-point titers by producing maximal levels of Mo-MuLV Gag/Pol, envelope glycoproteins, and retroviral RNA genomes. We describe the production yields and transduction efficiency characteristics of two Mo-MuLV packaging cell lines, FLYA13 and TEFLYA. Although they both produce 4070A-pseudotyped retroviral vectors reproducibly at >1 x 10(6) LFU ml(-1), the transduction efficiency of unconcentrated and concentrated virus from FLYA13 lines is poor compared with vector preparations from TEFLYA lines. A powerful inhibitor of retroviral transduction is secreted by FLYA13 packaging cells. We show that the inhibitory factor does not affect transduction of target cells by RD114-pseudotyped vectors. This suggests that the inhibitory factor functions at the level of envelope-receptor interactions. Phosphate starvation of target cells shows a two-fold increase in Pit2 receptor mRNA and causes some improvement in FLYA13 virus transduction efficiency. Western blots show that FLYA13 viral samples contain an eight-fold higher ratio of 4070A envelope to p30gag than that of virus produced by TEFLYA producer cell lines. This study correlates overexpression of 4070A envelope glycoprotein in retroviral preparations with a reduction of transduction efficiency at high multiplicities of infection. We suggest that TEFLYA packaging cells express preferable levels of 4070A compared with FLYA13, which not only enables high-titer stocks to be generated, but also facilitates a high efficiency of transduction of target cells.

Comparison of bicistronic retroviral vectors containing internal ribosome entry sites (IRES) using expression of human interleukin-12 (IL-12) as a readout

BACKGROUND

Many gene therapy applications require the co-ordinated delivery of more than one reading frame. We wished to systematically compare IRES in the context of a retroviral vector to determine which was the most effective for protein production and viral titre. To do this we monitored expression of IL-12, as co-ordinated expression of both p35 and p40 subunits is required for production of the active heterodimer.

METHODS

Retroviral vectors were constructed to express human IL-12 in which an IRES initiates translation of the p40 subunit, with the IRES optimally aligned to the initiation codon of p40. Vectors containing an IRES from either polio virus (PV), encephalomyocarditis virus (EMCV), foot and mouth disease virus (FMDV) or murine leukaemia virus (MLV) were compared with a vector expressing IL-12 as a single protein (Flexi-12; in which the two IL-12 subunits are linked by a peptide).

RESULTS

All vectors produced high titre virus and directed synthesis of IL-12 in target cells. The bicistronic vectors containing the IRES from EMCV and PV were the most effective in infected 3T3 cells, producing up to 40 ng IL-12/10(6) cells/48 h, similar to the 50 ng IL-12/10(6) cells/48 h obtained with Flexi-12. The IRES from PV was the most efficient in human melanoma cells.

CONCLUSIONS

Bicistronic retroviral vectors have been constructed that effectively transduce target cells and produce high levels of protein. Target cell specificity of IRES function was observed. The combination of Flexi-12 and the IRES from PV will be useful in the generation of vectors expressing IL-12 with a second protein such as IL-2 for transduction of melanoma cells.