Construction of a retrovirus capable of transducing and expressing genes in multipotential embryonic cells

Methodology for Anti-Gene Anti-IGF-I Therapy of Malignant Tumours

The aim of this study was to establish the criteria for methodology of cellular “anti-IGF-I” therapy of malignant tumours and particularly for glioblastoma multiforme. The treatment of primary glioblastoma patients using surgery, radiotherapy, and chemotherapy was followed by subcutaneous injection of autologous cancer cells transfected by IGF-I antisense/triple helix expression vectors. The prepared cell “vaccines” should it be in the case of glioblastomas or other tumours, have shown a change of phenotype, the absence of IGF-I protein, and expression of MHC-I and B7. The peripheral blood lymphocytes, PBL cells, removed after each of two successive vaccinations, have demonstrated for all the types of tumour tested an increasing level of CD8⁺ and CD8⁺28⁺ molecules and a switch from CD8⁺11b⁺ to CD8⁺11. All cancer patients were supervised for up to 19 months, the period corresponding to minimum survival of glioblastoma patients. The obtained results have permitted to specify the common criteria for “anti-IGF-I” strategy: characteristics sine qua non of injected “vaccines” (cloned cells IGF-I(−) and MHC-I(+)) and of PBL cells (CD8⁺ increased level).

Stable gammaretroviral vector expression during embryonic stem cell-derived in vitro hematopoietic development

Unlike conventional gammaretroviral vectors, the murine stem cell virus (MSCV) can efficiently express transgenes in undifferentiated embryonic stem cells (ESCs). However, a dramatic extinction of expression is observed when ESCs are subjected to in vitro hematopoietic differentiation. Here we report the construction of a self-inactivating vector from MSCV, MSinSB, which transmits an intron embedded within the internal transgene cassette to transduced cells. The internal transgene transcriptional unit in MSinSB comprises the composite cytomegalovirus immediate early enhancer-chicken beta-actin promoter and associated 5' splice site positioned upstream of the natural 3' splice site of the gammaretroviral envelope gene, and is configured such that the transgene translational initiation sequence is coincident with the envelope ATG. MSinSB could be produced at titers approaching 10(6) transducing units/ml and directed higher levels of transgene expression in ESCs than a splicing-optimized MSCV-derived vector, MSGV1. Moreover, when transduced ESCs were differentiated into hematopoietic cells in vitro, MSinSB remained transcriptionally active in greater than 90% of the cells, whereas MSGV1 expression was almost completely shut off. Persistent high-level expression of the MSinSB gammaretroviral vector was also demonstrated in murine bone marrow transplant recipients and following in vitro myelomonocytic differentiation of human CD34(+) cord blood stem/progenitor cells.

Generation of replication-defective helper-free vectors based on simian immunodeficiency virus

A systematic study on generating simian immunodeficiency virus (SIV)-based vectors was carried out. The goal was to generate helper-free, replication-defective SIVmac-based vectors at high titers. The general approach was to cotransfect into human 293T cells a plasmid carrying the vector construct along with two helper plasmids that together expressed the SIVmac virion proteins. Initial vectors carried the bacterial beta-galactosidase gene (beta-gal). These vectors had a technical difficulty: "pseudotransduction" of beta-gal protein produced during the 293T cell transfections. As a result, infection of cultures with these vector stocks also resulted in passive transfer into, and X-gal staining of, cells that had not actually been infected by the vector. A second generation of vectors expressing the enhanced jellyfish green fluorescence protein (EGFP) was not subject to this artifact. A systematic study of the SIVmac-based EGFP vectors was carried out. Helper-free vector stocks were obtained when helper plasmids lacking the SIVmac packaging signals were used. By employing envelope helper plasmids derived from different SIVmac isolates, it was possible to generate SIVmac-based vectors pseudotyped with envelope proteins of different cell tropism. Optimization of vector and helper plasmid structures, transfection conditions, and infection procedures ultimately yielded vector titers in excess of 10(6)/ml.

A truncated form of the human CAF-1 p150 subunit impairs the maintenance of transcriptional gene silencing in mammalian cells

Chromatin assembly factor 1 (CAF-1) is a protein complex formed of three subunits, p150, p60, and p48, conserved from the yeast Saccharomyces cerevisiae to humans, which can promote nucleosome assembly onto newly replicated DNA. In S. cerevisiae, deletion of the genes encoding any of the three CAF-1 subunits (cacDelta mutants), although nonlethal, results in a silencing defect of genes packaged into heterochromatin. Here we report on a mammalian cell model that we devised to monitor gene silencing and its reversal in a quantitative manner. This model relies on the use of a cell line stably transfected with a reporter gene in a silenced state. Reversal of reporter gene silencing was achieved upon treatment of the cells with 5-azacytidine, which resulted in the demethylation of the reporter gene copies. We show that expression of a cDNA for the human p150 CAF-1 subunit harboring 5' truncations, but not that of a cDNA encoding the full-length p150 CAF-1 subunit, increases by more than 500-fold the frequency at which transcriptional silencing of the reporter gene copies is reversed in these cells. Reversal of gene silencing is dependent upon expression of a truncated protein, possibly acting as a dominant negative mutant of the wild-type CAF-1, is associated with alterations in chromatin structure as measured by an endonuclease sensitivity assay and is not associated with detectable changes in the methylation status of the silenced genes. These results suggest that the role of CAF-1 in the epigenetic control of gene expression has been conserved between yeast and mammals, despite the lack of DNA methylation in yeast chromatin.

Transgene expression in zebrafish: A comparison of retroviral-vector and DNA-injection approaches

To assess alternative methods for introducing expressing transgenes into the germ line of zebrafish, transgenic fish that express a nuclear-targeted, enhanced, green fluorescent protein (eGFP) gene were produced using both pseudotyped retroviral vector infection and DNA microinjection of embryos. Germ-line transgenic founders were identified and the embryonic progeny of these founders were evaluated for the extent and pattern of eGFP expression. To compare the two modes of transgenesis, both vectors used the Xenopus translational elongation factor 1-alpha enhancer/promoter regulatory cassette. Several transgenic founder fish which transferred eGFP expression to their progeny were identified. The gene expression patterns are described and compared for the two modes of gene transfer. Transient expression of eGFP was detected 1 day after introducing the transgenes via either DNA microinjection or retroviral vector infection. In both cases of gene transfer, transgenic females produced eGFP-positive progeny even before the zygotic genome was turned on. Therefore, GFP was being provided by the oocyte before fertilization. A transgenic female revealed eGFP expression in her ovarian follicles. The qualitative patterns of gene expression in the transgenic progeny embryos after zygotic induction of gene expression were similar and independent of the mode of transgenesis. The appearance of newly synthesized GFP is detectable within 5-7 h after fertilization. The variability of the extent of eGFP expression from transgenic founder to transgenic founder was wider for the DNA-injection transgenics than for the retroviral vector-produced transgenics. The ability to provide expressing germ-line transgenic progeny via retroviral vector infection provides both an alternative mode of transgenesis for zebrafish work and a possible means of easily assessing the insertional mutagenesis frequency of retroviral vector infection of zebrafish embryos. However, because of the transfer of GFP from oocyte to embryo, the stability of GFP may create problems of analysis in embryos which develop as quickly as those of zebrafish.

Differential expression and function of cadherin-6 during renal epithelium development

The cadherin gene family encodes calcium-dependent adhesion molecules that promote homophilic interactions among cells. During embryogenesis, differential expression of cadherins can drive morphogenesis by stimulating cell aggregation, defining boundaries between groups of cells and promoting cell migration. In this report, the expression patterns of cadherins were examined by immunocytochemistry and in situ hybridization in the embryonic kidney, during the time when mesenchymal cells are phenotypically converted to epithelium and the pattern of the developing nephrons is established. At the time of mesenchymal induction, cadherin-11 is expressed in the mesenchyme but not in the ureteric bud epithelium, which expresses E-cadherin. The newly formed epithelium of the renal vesicle expresses E-cadherin near the ureteric bud tips and cadherin-6 more distally, suggesting that this primitive epithelium is already patterned with respect to progenitor cell types. In the s-shaped body, the cadherin expression patterns reflect the developmental fate of each region. The proximal tubule progenitors express cadherin-6, the distal tubule cells express E-cadherin, whereas the glomeruli express P-cadherin. Ultimately, cadherin-6 is down-regulated whereas E-cadherin expression remains in most, if not all, of the tubular epithelium. Antibodies generated against the extracellular domain of cadherin-6 inhibit aggregation of induced mesenchyme and the formation of mesenchyme-derived epithelium but do not disrupt ureteric bud branching in vitro. These data suggest that cadherin-6 function is required for the early aggregation of induced mesenchymal cells and their subsequent conversion to epithelium.

Transcriptional Silencing of Retroviral Vectors

Although retroviral vector systems have been found to efficiently transduce a variety of cell types in vitro, the use of vectors based on murine leukemia virus in preclinical models of somatic gene therapy has led to the identification of transcriptional silencing in vivo as an important problem. Extinction of long-term vector expression has been observed after implantation of transduced hematopoietic cells as well as fibroblasts, myoblasts and hepatocytes. Here we review the influence of vector structure, integration site and cell type on transcriptional silencing. While down-regulation of proviral transcription is known from a number of cellular and animal models, major insight has been gained from studies in the germ line and embryonal cells of the mouse. Key elements for the transfer and expression of retroviral vectors, such as the viral transcriptional enhancer and the binding site for the tRNA primer for reverse transcription may have a major influence on transcriptional silencing. Alterations of these elements of the vector backbone as well as the use of internal promoter elements from housekeeping genes may contribute to reduce transcriptional silencing. The use of cell culture and animal models in the testing and improvement of vector design is discussed. Copyright 1996 S. Karger AG, Basel

Transfer of single gene-containing long terminal repeats into the genome of mammalian cells by a retroviral vector carrying the cre gene and the loxP site

Retroviral vectors contain viral cis-acting elements to achieve the packaging, reverse transcription, integration, and expression of the retroviral genomic nucleic acid sequence. However, these elements are not useful in the integrated provirus and can be the cause of problems. We have developed a vector which eliminates the majority of these viral elements. This vector, a long terminal repeat (LTR) enhancer-deleted vectors, exploits the Cre-lox recombination system of the P1 bacteriophage. The Cre-lox system is neutral for eukaryotic cells. The 32-nucleotide loxP site is inserted within the U3 of the 3' LTR along with with the gene to be transduced (in place of the viral enhancers). Following the LTR-mediated loxP duplication, the LTRs can be recombined by the Cre enzyme. The structure of the resulting provirus in the host genome corresponds to a single LTR (deleted of the viral enhancers) carrying a single copy of the gene to be transduced. If the Cre expression unit is furnished after the integration of a loxP-containing virus, the efficiency of the recombination is not absolute. If the Cre expression unit is inserted between the two LTRs, only single LTR proviral structures are found following infection by the retroviral vector.

The dimerization/packaging sequence is dispensable for both the formation of high-molecular-weight RNA complexes within retroviral particles and the synthesis of proviruses of normal structure

Retroviral particles contain a dimer of two genomic RNA molecules, linked by noncovalent intermolecular bonds. Studies by electron microscopy of viral RNA extracted from virions as well as in vitro studies have implicated a sequence, designated the dimer linkage sequence (DLS), in the dimerization process. The DLS has been localized within a short region encompassing the psi packaging sequence, between nucleotides 212 and 563 for the Moloney murine leukemia retrovirus (MoMLV) RNA. In this report, we show that viral RNAs lacking both the DLS and psi packaging sequences--and even an RNA lacking the first 6,537 nucleotides of MoMLV--can assemble within retroviral particles as high-molecular-weight, slow-migrating, heat-sensitive complexes closely related to those observed for wild-type viral RNAs. Furthermore, we show that proviruses of normal structure are generated upon infection of test cells with retroviral particles which contain the DLS/psi-deleted viral RNAs. These observations demonstrate that the DLS and psi packaging sequences are not essential in cis to form a functional RNA complex for reverse transcription and integration.

Capture of a cellular transcriptional unit by a retrovirus: mode of provirus activation in embryonal carcinoma cells

The expression of murine leukemia provirus in embryonal carcinoma (EC) cells is blocked by a mechanism still incompletely understood. The blockage is not overcome by deleting a large portion of the enhancer region (in U3) in recombinant retroviruses (M-MuLVneo delta Enh). This confirms the presence of negative elements outside the viral 82-bp repeats. However, a few sites in the genomes of EC cells permit M-MuLVneo delta Enh proviral expression. One such site, identified in PCC4, PCC3, and LT, was studied. The complete analysis of the mechanism of activation by Northern (RNA) blotting, cloning, and sequencing of partial cDNA copies of the viral transcript and of the site of integration establishes that viral transcripts are initiated from an upstream host-cell promoter and are spliced from a host donor to a cryptic viral acceptor at position 542 in the Moloney murine leukemia virus (M-MuLV) genome. In consequence, the mature transcripts are host cell-virus fusion transcripts from which M-MuLV sequences, including the cis-active negative elements of the 5' long terminal repeat-containing region, are absent. The provirus integrates apparently randomly into any of the three most proximal introns of the transcriptional unit. The host cell promoter contains a TATA box and 14 potential SpI binding sites included in a 1.0-kb GC-rich island. These elements promote gene expression of recombinant vectors in EC and differentiated cells. The mechanism described points to a mechanism by which retroviruses can be transcribed from upstream nonviral elements and can acquire host genes by 5' annexation of exons.

High-frequency intracellular transposition of a defective mammalian provirus detected by an in situ colorimetric assay

We devised an indicator gene for retrotransposition, nlsLacZRT, which contains the Escherichia coli lacZ gene fused to a nuclear location signal (nlsLacZ), engineered in such a way that the gene is expressed only if the structure in which it has been inserted transposes itself through an RNA intermediate. A cloned murine leukemia retrovirus with an ecotropic host range (Moloney murine leukemia virus), rendered defective by a large deletion encompassing the three viral gag, pol, and env open reading frames, was marked with this indicator gene and introduced by transfection into heterologous feline cells. No beta-galactosidase activity could be detected among the clonal cell population, unless the defective provirus was complemented in trans by the gag-pol gene products. Under these conditions, cell variants which disclosed an easily detectable nuclear blue coloration upon in situ 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside staining were observed. Fluorescence-activated cell sorting of the beta-galactosidase-positive cells, followed by Southern blot analysis, demonstrated an unambiguous correlation between nlsLacZRT activation and retrotransposition of the marked provirus. Transposition occurs at a high frequency (up to 10(-4) events per cell per generation), which is dependent on the level of expression of the gag-pol gene and is concomitant with the release of noninfectious retroviruslike particles which are the hallmarks, but not the intermediates, of the intracellular transposition process.

Promoter interactions in retrovirus vectors introduced into fibroblasts and embryonic stem cells

The activity of the Moloney murine leukemia virus promoter is restricted in mouse embryonic stem cells. Gene expression with retrovirus vectors can be achieved in these cells if internal promoters are used. To address the possible influence of the viral enhancer sequences on expression from the internal promoter, we have constructed high-titer, self-inactivating retrovirus vectors which delete viral regulatory sequences upon integration in the host genome. We show that deleting most of the viral enhancer sequences has no significant effect on viral titer. This enhancer deletion leads to either an increase or a decrease in the amount of RNA transcribed from the internal promoter, but no consistent change can be found with any type of vector. The same changes in expression from the internal promoter observed in embryonic stem cells are also observed in 3T3 fibroblast cells, in which the viral promoter is active. These results indicate that viral regulatory elements influence expression from an internal promoter independently of expression from the virus promoter.

Activator-dependent and activator-independent defective recombinant retroviruses from bovine leukemia virus

The replication-competent bovine leukemia virus (BLV) has been modified for use as a vector for foreign genes. The gag, pol, env, and pX regions of the virus were replaced by an exogenous nuclear location signal LacZ (nlsLacZ) or SVnlsLacZ gene. Transfection of the ovine cell line FLK-BLV, which expresses all BLV proteins from a wild-type provirus, with this viral DNA resulted in a viral titer of 10(4) CFU/ml. The inclusion of a large portion of the gag region did not significantly increase the titer. Both activator-dependent and activator-independent retroviruses were constructed. In activator-dependent vectors, the expression of the insert was dependent on the presence of the Tax protein, which activated the BLV long terminal repeat. In activator-independent vectors, the expression of the insert was constitutive because of the presence of an internal promoter. Infections with the recombinant retrovirus were inhibited by specific neutralizing antibodies. The structure of the transduced genetic material was not rearranged. BLV vectors encoding a reporter nlsLacZ gene, the product of which can be detected in single cells, greatly simplified studies of their biological properties. Determination of the host range of BLV vectors established that BLV-based recombinant retroviruses are effective in the transduction of genes in a variety of species and cell types.

Embryonic stem cell virus, a recombinant murine retrovirus with expression in embryonic stem cells

The expression of Moloney murine leukemia virus and vectors derived from it is restricted in undifferentiated mouse embryonal carcinoma and embryonal stem (ES) cells. We have developed a retroviral vector, the murine embryonic stem cell virus (MESV), that is active in embryonal carcinoma and ES cells. MESV was derived from a retroviral mutant [PCC4-cell-passaged myeloproliferative sarcoma virus (PCMV)] expressed in embryonal carcinoma cells but not in ES cells. The enhancer region of PCMV was shown to be functional in both cell types, but sequences within the 5' untranslated region of PCMV were found to restrict viral expression in ES cells. Replacement of this region by related sequences obtained from the dl-587rev retrovirus results in MESV, a modified PCMV virus that confers G418 resistance to fibroblasts and ES cells with similar efficiencies. Expression of MESV in ES cells is mediated by transcriptional regulatory elements within the 5' long terminal repeat of the viral genome.

Development of a retroviral vector for inducible expression of transforming growth factor beta 1

A retroviral vector system for the expression of exogenous genes under the control of an inducible promoter was developed. By utilizing this system, the cDNA for human transforming growth factor beta 1 (TGF-beta 1) was inserted into a retroviral vector under the control of an internal mouse metallothionein promoter and introduced via infection into normal rat kidney fibroblasts (NRK-49F) and epithelial cells (NRK-52E), Chinese hamster ovary cells (CHO), and the human monocytic cell line U937. Control of TGF-beta 1 expression, achieved by Cd2+ induction of vector-encoded TGF-beta 1 mRNA, was cell line specific and resulted in a concomitant increase in neutralizable TGF-beta 1 production by the cells. Autocrine stimulation of vector-containing cells by vector-encoded TGF-beta 1 was detected by an increase in soft-agar colony formation of NRK-49F infectants compared with that of the control cells. In addition, the use of a second internal promoter in a retroviral vector of similar design allowed isolation of stable infectants from a cell line (CHO) in which the viral long terminal repeat does not function efficiently.

Immortalization of bipotential and plastic glio-neuronal precursor cells

Permanent clonal cell lines from newborn mouse striatum have been established after transfer of the simian virus 40 large tumor oncogene by means of a retroviral vector. Some of the lines obtained displayed properties of bipotential and plastic glio-neuronal precursors. Depending on the culture conditions, these cells express either the glial fibrillary acidic protein or neurofilaments. In addition, the cells can display adrenergic, D1 and D2 dopaminergic, muscarinic, and 5-hydroxytryptamine type 2 serotoninergic receptors, which are coupled either to the adenylate cyclase or to the phosphatidylinositol signaling pathways. The panel of receptors for neurotransmitters exhibited by these lines closely resembles that of primary striatal neurons. Results suggest that plastic common precursors of astrocytes and neurons persist in the striatum at a late developmental stage. As these permanent cell lines constitute an unlimited source of homogenous cell material, we suggest that they should be useful for molecular and pharmacological studies on the mechanisms and regulation of signal transduction as well as the commitment, plasticity, and differentiation of neural cells.

Expression of human adenosine deaminase in murine hematopoietic cells

Multiple replication-defective retrovirus vectors were tested for their ability to transfer and express human adenosine deaminase in vitro and in vivo in a mouse bone marrow transplantation model. High-titer virus production was obtained from vectors by using both a retrovirus long terminal repeat promoter and internal transcriptional units with human c-fos and herpes virus thymidine kinase promoters. After infection of primary murine bone marrow with one of these vectors, human adenosine deaminase was detected in 60 to 85% of spleen colony-forming units and in the blood of 14 of 14 syngeneic marrow transplant recipients. This system offers the opportunity to assess methods for increasing efficiency of gene transfer, for regulation of expression of foreign genes in hematopoietic progenitors, and for long-term measurement of the stability of expression in these cells.

Expression of human adenosine deaminase in murine hematopoietic cells

Multiple replication-defective retrovirus vectors were tested for their ability to transfer and express human adenosine deaminase in vitro and in vivo in a mouse bone marrow transplantation model. High-titer virus production was obtained from vectors by using both a retrovirus long terminal repeat promoter and internal transcriptional units with human c-fos and herpes virus thymidine kinase promoters. After infection of primary murine bone marrow with one of these vectors, human adenosine deaminase was detected in 60 to 85% of spleen colony-forming units and in the blood of 14 of 14 syngeneic marrow transplant recipients. This system offers the opportunity to assess methods for increasing efficiency of gene transfer, for regulation of expression of foreign genes in hematopoietic progenitors, and for long-term measurement of the stability of expression in these cells.

Stably transmitted triple-promoter retroviral vectors and their use in transformation of primary mammalian cells

Retroviral vectors were constructed which coexpressed three inserted genes from independent transcriptional promoters in singly infected cells. Several such triple-promoter vectors were constructed with various combinations of oncogenes and selectable drug resistance genes. All expressed three mRNAs of the expected size in infected cells. One vector expressing the v-Ha-ras, v-myc, and neo genes was characterized in detail. This retrovirus did not undergo rearrangement during the process of infection, as judged by Southern analysis, and infection of primary rat embryo fibroblasts demonstrated that ras-myc-cotransformed cells could be selected in G418. This demonstration that retroviral vectors can be used to express three cistrons independently increases their value as gene transfer vehicles, particularly for studies involving oncogene cooperation in primary cells.

Stably transmitted triple-promoter retroviral vectors and their use in transformation of primary mammalian cells

Retroviral vectors were constructed which coexpressed three inserted genes from independent transcriptional promoters in singly infected cells. Several such triple-promoter vectors were constructed with various combinations of oncogenes and selectable drug resistance genes. All expressed three mRNAs of the expected size in infected cells. One vector expressing the v-Ha-ras, v-myc, and neo genes was characterized in detail. This retrovirus did not undergo rearrangement during the process of infection, as judged by Southern analysis, and infection of primary rat embryo fibroblasts demonstrated that ras-myc-cotransformed cells could be selected in G418. This demonstration that retroviral vectors can be used to express three cistrons independently increases their value as gene transfer vehicles, particularly for studies involving oncogene cooperation in primary cells.

Construction and characterization of a retroviral vector demonstrating efficient expression of cloned cDNA sequences

We describe the construction and properties of a retroviral expression vector, designated pMV-7, designed to transfer unselected cDNAs and produce their encoded proteins in recipient cells. The vector is flanked by the long terminal repeats (LTRs) of the Moloney murine sarcoma virus (MSV) and contains the selectable drug resistance gene neo under the regulation of the herpes simplex virus (HSV) thymidine kinase (tk) promoter. Unique Eco RI and Hind III sites facilitate the introduction of sequences whose transcription is regulated by the 5' LTR. We have inserted cDNAs encoding: (i) the human lymphocyte antigen T4, (ii) the human lymphocyte antigen T8, and (iii) the murine hypoxanthine-guanine phosphoribosyl transferase (HPRT), into the pMV-7 vector. These constructions were used to transduce recipient cells to the neo+ phenotype. In each case, functional assays demonstrated that 65-92% of the neo+ clones produced the appropriate protein encoded by its corresponding cDNA. These clones were characterized further by analyzing the expression of vector-regulated transcripts. The neo+T4+ clones expressed a single full-length LTR-to-LTR transcript as detected by a T4 probe. The neo+T8+ clones, however, expressed both a full-length LTR-to-LTR transcript and an additional smaller transcript as detected by a T8 probe. This smaller transcript probably resulted from the utilization of cryptic signals which control 3' RNA processing. Furthermore, all of the neo+ clones expressed a transcript that initiated from the tk promoter, contained the neo gene, and used polyadenylation signals provided by the 3' LTR. Thus, the pMV-7 vector is capable of high-efficiency transfer and high-frequency expression of the cDNA-encoded protein.

An indicator gene to demonstrate intracellular transposition of defective retroviruses

An indicator gene for detection and quantitation of RNA-mediated transposition was constructed (neoRT). It was inserted into a Moloney murine leukemia provirus (Mo-MLV) deleted for the envelope gene to test for intracellular transposition of defective retroviruses [Mo-MLV(neo)RT]. NeoRT contains the selectable neo gene (which confers resistance to the drug G418), inactivated by a polyadenylylation sequence inserted between the neo promotor and coding sequence. The polyadenylylation sequence is flanked (on the antisense strand of the DNA) by a donor and an acceptor splice site so as to be removed upon passage of the provirus through an RNA intermediate. 3T3 cells transfected with the defective Mo-MLV(neo)RT provirus are sensitive to G418. After trans-complementation with Mo-MLV, viral transcripts confer resistance to G418 upon infection of test cells. In the resistant cells, the polyadenylylation sequence has been removed, as a result in most cases of precise splicing of the intronic domain. Retrotransposition of the defective Mo-MLV(neo)RT provirus was demonstrated by submitting transfected G418-sensitive clones to selection. Between 1 and 10 G418-resistant clones were obtained per 10(7) cells. Several possess additional copies, with evidence for precise removal of the intronic domain. By using target test cells in coculture experiments, extracellular intermediates of retrotransposition could not be detected.

Proviral sequences that restrict retroviral expression in mouse embryonal carcinoma cells

Embryonal carcinoma (EC) cells are nonpermissive for retrovirus replication. Restriction of retroviral expression in EC cells was studied by using DNA transfection techniques. To investigate the activity of the Moloney murine leukemia virus (M-MuLV)enhancer and promoter sequences, the M-MuLV long terminal repeat and the defined long terminal repeat deletions were linked to neo structural gene sequences that encode resistance to the neomycin analog G418. Transient expression data and drug resistance frequencies support the findings that the M-MuLV enhancer is not active in EC cells but that promoter sequences are functional. In addition, a proviral DNA fragment that encodes the leader RNA sequence of a M-MuLV recombinant retrovirus was found to restrict expression specifically in EC cells. Deletion analysis of the leader fragment localized the inhibitory sequences to a region that spans the M-MuLV tRNA primer binding site. It is not known whether restriction occurs at a transcriptional or posttranscriptional level, but steady-state RNA levels in transient expression assays were significantly reduced.

An embryonic DNA-binding protein specific for the promoter of the retrovirus long terminal repeat

Retrovirus expression is restricted in embryonal carcinoma (EC) cells but not in many differentiated cell lines. We used a very sensitive gel retardation assay to detect sequence-specific DNA-binding proteins in crude nuclear extracts obtained from EC and differentiated cells. Four binding sites were mapped in the noncoding sequences of the amphotropic murine leukemia virus. Strong binding to the CCAAT consensus sequence located in the promoter was specifically observed with EC nuclear extract. The binding protein is called EPBF (embryonal promoter-binding factor), and it is a candidate for the repressor of retrovirus transcription.

An embryonic DNA-binding protein specific for the promoter of the retrovirus long terminal repeat

Retrovirus expression is restricted in embryonal carcinoma (EC) cells but not in many differentiated cell lines. We used a very sensitive gel retardation assay to detect sequence-specific DNA-binding proteins in crude nuclear extracts obtained from EC and differentiated cells. Four binding sites were mapped in the noncoding sequences of the amphotropic murine leukemia virus. Strong binding to the CCAAT consensus sequence located in the promoter was specifically observed with EC nuclear extract. The binding protein is called EPBF (embryonal promoter-binding factor), and it is a candidate for the repressor of retrovirus transcription.

Proviral sequences that restrict retroviral expression in mouse embryonal carcinoma cells

Embryonal carcinoma (EC) cells are nonpermissive for retrovirus replication. Restriction of retroviral expression in EC cells was studied by using DNA transfection techniques. To investigate the activity of the Moloney murine leukemia virus (M-MuLV)enhancer and promoter sequences, the M-MuLV long terminal repeat and the defined long terminal repeat deletions were linked to neo structural gene sequences that encode resistance to the neomycin analog G418. Transient expression data and drug resistance frequencies support the findings that the M-MuLV enhancer is not active in EC cells but that promoter sequences are functional. In addition, a proviral DNA fragment that encodes the leader RNA sequence of a M-MuLV recombinant retrovirus was found to restrict expression specifically in EC cells. Deletion analysis of the leader fragment localized the inhibitory sequences to a region that spans the M-MuLV tRNA primer binding site. It is not known whether restriction occurs at a transcriptional or posttranscriptional level, but steady-state RNA levels in transient expression assays were significantly reduced.

Retroviral vector gene expression in F9 embryonal carcinoma cells

When F9 embryonal carcinoma (EC) cells are infected with retroviral vectors, the efficiency of expression of selectable genes is considerably lower than that in mouse fibroblasts infected with the same retroviral vectors. In this study, several retroviral vectors with regulatory sequences placed immediately 5' to a selectable gene were constructed, packaged, and used to infect mouse fibroblasts and F9 EC cells. With selection as an assay, there was a hierarchy of relative expression in F9 cells compared with that in mouse fibroblasts. These internally placed regulatory sequences are the source of the mRNAs detected in F9 EC cells, while both retroviral long-terminal-repeat promoters and internal promoters are the source of steady-state mRNAs in mouse fibroblasts. This effect was observable with both the internally placed herpes simplex virus thymidine kinase promoter and the Moloney murine leukemia virus promoter.

Two distinct sequence elements mediate retroviral gene expression in embryonal carcinoma cells

Moloney murine leukemia virus (M-MuLV) and M-MuLV-derived retroviral vectors are not expressed in early mouse embryos or in embryonal carcinoma cells. M-MuLV-derived mutants or M-MuLV-related variants which transduce the neomycin phosphotransferase gene can, however, induce drug resistance in embryonal carcinoma cells with high efficiency. In this study we investigated the sequences critical for retroviral gene expression in two different embryonal carcinoma cell lines, F9 and PCC4. We show that two synergistically acting sequence elements mediate expression in embryonal carcinoma cells. One of these is located within the U3 region of the viral long terminal repeat, and the second one is in the 5' untranslated region of the retrovirus. The latter element, characterized by a single point mutation, affects the level of stable RNA in infected cells, suggesting a regulatory mechanism similar to that of human immunodeficiency virus in human T cells.

Rous sarcoma virus is integrated but not expressed in chicken early embryonic cells

We have developed a protocol that allows us to infect chicken early embryonic (CEE) cells with high efficiency. This was achieved by exposing the CEE cells to a semicontinuous dose of Rous sarcoma virus (RSV) for a period of 20 hr. Southern blot analysis indicated that an average of one proviral copy is integrated per embryonic cell. However, there was no production of infectious viral particles by the cells containing the proviral genome, although low levels of full-length genomic RNA could be detected by RNA transfer blot analysis. These low RNA levels contrast with the 100- to 1000-fold higher levels found in RSV-infected chicken embryo fibroblasts. We conclude that in cells derived from pregastrulating chicken embryos, RSV DNA is integrated into the cell genome but fails to be expressed in an efficient manner. These primary cells can therefore be used to identify factors involved in regulation of retroviral gene expression in normal cells. Such factors may also be instrumental in elucidating basic mechanisms involved in gene regulation during early development in higher vertebrates.

Expression of human class II major histocompatibility complex antigens using retrovirus vectors

Retrovirus vectors [direct orientation (DO) vectors] that permit the simultaneous expression of an inserted protein-coding sequence and a dominant-acting selectable marker have been constructed. In these vectors, an internal simian virus 40 or human metallothionein promoter sequence serves to drive the expression of the bacterial neomycin phosphotransferase or guanine-xanthine phosphoribosyltransferase genes, whereas the viral long terminal repeat sequences are utilized to promote expression of inserted sequences. In some of the vectors, the viral 5' splice site, normally used in the biogenesis of the subgenomic env-encoding mRNA, has been eliminated. These vectors yield high transient and stable titers of virus after transfection of viral packaging cell lines, show little or no depression of virus titer with a variety of inserts, and faithfully transmit recombinant proviral sequences to recipient cells. To characterize the expression potential of these vectors, a variety of inserts encoding the alpha and beta subunits of the human major histocompatibility complex class II antigen HLA-DR have been introduced into these vectors. NIH 3T3 cells infected by viruses containing HLA-DR alpha or beta cDNAs express these proteins as shown by immunoprecipitation of metabolically labeled extracts. In addition, through the sequential infection of cells with retrovirus constructions expressing two different selectable markers, both subunits of the class II antigen have been introduced into NIH 3T3 cells. Such infected cells express HLA-DR molecules at the cell surface.

Quantitative analysis of gene suppression in integrated retrovirus vectors

Previously, we described a retrovirus vector that contained two genes: a 5' gene under transcriptional control of the viral long terminal repeat and a 3' gene under transcriptional control of the herpes simplex virus thymidine kinase promoter. By using a biological assay, we found that expression of the 5' gene is suppressed when there was selection for the 3' gene and expression of the 3' gene is suppressed when there is selection for the 5' gene (M. Emerman and H. M. Temin, Cell 39:459-467, 1984). In the present study, we replaced the thymidine kinase promoter with stronger promoters, and we measured expression of the genes in the retrovirus vectors by enzyme activity and RNA analysis. We found that all of the vectors displayed gene suppression when analyzed biochemically, although not when analyzed biologically. The suppressed genes produced about 10 to 50% as much product as when they were selected. The amount of suppression depended on whether the suppressed gene was 5' or 3' to the selected gene and from which promoter the suppressed gene was transcribed. The amount of suppression correlated with a decrease in the amount of steady-state RNA transcribed from the suppressed gene's promoter.

Comparison of promoter suppression in avian and murine retrovirus vectors

Previously, we described "promoter suppression" in infectious retrovirus vectors with two genes and an internal promoter. Here, we examined several parameters of promoter suppression and found that the amount of suppression in an integrated retrovirus vector was dependent both on whether the vector was derived from spleen necrosis virus or murine leukemia virus and on which internal promoter was present in the vector. Murine leukemia virus vectors showed less suppression than analogous spleen necrosis virus vectors. Furthermore, the amount of suppression was not dependent on either the relative strengths of the promoters nor the distance between the promoters. Moreover, we found that in vectors in which one promoter was suppressed, there was an inverse correlation between the DNaseI sensitivity of the chromatin surrounding a promoter and the suppression of its expression.

Transgenesis by means of blastocyst-derived embryonic stem cell lines

This study demonstrates that blastocyst-derived embryonic stem cells (ES cells) can be used as a vehicle for transgenesis. The method is nearly as efficient as other methods, and the introduced neomycin phosphotransferase (neo) gene is stably transmitted through several generations with no apparent loss in G418 resistance. An important factor contributing to the efficiency of this process is the rigorous selection, before blastocyst injection, of genetically transformed cells for in vitro developmental pluripotency. One of the advantages of the ES cell route to transgenesis is that it provides investigators with the opportunity to screen for the desired genetic alterations before reintroducing the ES cells into the animal.

Increase of retroviral infection in vitro by the binding of antiretroviral antibodies

Monoclonal antiretrovirus antibodies were assayed for their ability to influence retrovirus infection in vitro. Some antibodies increased murine cell infection by an ecotropic virus and both murine and human cell infection by an amphotropic virus. The stability of these viruses was not modified, suggesting that antibody-virus complexes may be more infectious than free virus particles.

Use of a recombinant retrovirus to study post-implantation cell lineage in mouse embryos

We show that a gene introduced into cells of mouse embryos by a retrovirus can serve as a heritable marker for the study of cell lineage in vivo. We constructed a defective recombinant retrovirus in which the Escherichia coli beta-galactosidase (lacZ) gene is inserted in the genome of a Muloney murine leukemia virus (M-MuLV). Expression of lacZ was detected with a histochemical stain that can be applied to cultured cells and embryonic tissue. Infection of cultured cells showed that lacZ has no detectable deleterious effects on cell viability or growth, that the enzyme is stably expressed in the progeny of infected cells for many generations in the absence of selective pressure, and that the virus can induce lacZ in a variety of cell types. Following injection of the virus into mid-gestation mouse embryos, clones of lacZ-positive cells were detected in skin, skull, meninges, brain, visceral yolk sac, and amnion. We identified the cell types comprising a series of lacZ-positive clones in the visceral yolk sac and skin to learn the lineage relationships of the labelled cells. In each tissue, we obtained evidence that several cell types have a pluripotential ancestor and that cell fate is progressively restricted as development proceeds.

Retroviral mutants efficiently expressed in embryonal carcinoma cells

The myeloproliferative sarcoma virus (MPSV) is a unique member of the Moloney murine sarcoma virus family. Due to mutations in the U3 region of its long terminal repeat, MPSV has an expanded host range that includes cells of the hematopoietic compartment. Using a MPSV recombinant containing the gene for neomycin-resistance (NeoR-MPSV), we demonstrate that the host range of MPSV also includes undifferentiated F9 embryonal carcinoma cells. Transfer of G418-resistance with NeoR-MPSV to F9 cells is almost as efficient as G418-resistance transfer to fibroblasts, in contrast to G418-resistance transfer to PCC4 embryonal carcinoma cells, which is at least 3 orders of magnitude lower. To isolate NeoR-MPSV mutants that are efficiently expressed in PCC4 cells, G418-resistant PCC4 cell lines were induced to differentiate, and the provirus was rescued by superinfection with murine leukemia virus. Viral isolates (PCMV-5 and -6; PCMV = PCC4 cell-passaged NeoR-MPSV) were obtained and assayed for expression in embryonal carcinoma cells. The efficiency of NeoR transfer was equally as high in both F9 and PCC4 as in fibroblasts. mos oncogene expression was unaltered as judged by transformation capability. No gross alteration in the coding region and in the long terminal repeat was detectable by restriction enzyme analysis. NeoR-MPSV and its mutants PCMV-5 and -6 can thus be utilized as vectors for the efficient transduction of genes into embryonic cells.

Quantitative analysis of gene suppression in integrated retrovirus vectors

Previously, we described a retrovirus vector that contained two genes: a 5' gene under transcriptional control of the viral long terminal repeat and a 3' gene under transcriptional control of the herpes simplex virus thymidine kinase promoter. By using a biological assay, we found that expression of the 5' gene is suppressed when there was selection for the 3' gene and expression of the 3' gene is suppressed when there is selection for the 5' gene (M. Emerman and H. M. Temin, Cell 39:459-467, 1984). In the present study, we replaced the thymidine kinase promoter with stronger promoters, and we measured expression of the genes in the retrovirus vectors by enzyme activity and RNA analysis. We found that all of the vectors displayed gene suppression when analyzed biochemically, although not when analyzed biologically. The suppressed genes produced about 10 to 50% as much product as when they were selected. The amount of suppression depended on whether the suppressed gene was 5' or 3' to the selected gene and from which promoter the suppressed gene was transcribed. The amount of suppression correlated with a decrease in the amount of steady-state RNA transcribed from the suppressed gene's promoter.

Introduction of genes into preimplantation mouse embryos by use of a defective recombinant retrovirus

Two-cell and four-cell preimplantation mouse embryos were cocultured in vitro with fibroblasts producing the recombinant retrovirus M-MuLV-neo. No wildtype helper virus was detected in these cultures. Of the embryos that survived the in vitro cultivation and the reimplantation into foster mothers, 2 of 15 that were tested contained the proviral genome. The provirus integrated as a single copy at a unique site. We estimate that approximately equal to 20% of the cells in each of the two transgenic fetuses contained the provirus.

Expression of foreign genes from retroviral vectors in mouse teratocarcinoma chimaeras

The formation of chimaeric mice from embryonic stem cells (EC and ES) carrying and exposing foreign genes is described. Two retroviral vectors were used to stably introduce genes, by virus infection and selection into EC and ES cells. In the first series EC cell clones were isolated that contained a single intact copy of the vector, which expressed the neomycin resistance gene (neo) from the 5' long terminal repeat (LTR). Following the formation of chimaeras with one of the clones, expression of the foreign gene was found in all chimaeric tissues examined. In the second series, a vector was used which contained the OK10 v-myc oncogene under the control of the 5' LTR as well as the neo gene expressed from an internal thymidine kinase (TK) promoter. ES cell clones were again isolated and used to form chimaeras. Although expression of the TK neo transcripts was maintained in the chimaeric tissues analysed, no expression of the v-myc transcripts from the 5' LTR was detected. These results show that genes can be introduced into mice, using clones of embryonic stem cells selected in vitro and that the expression of the genes can be maintained probably throughout development and in all tissues. The advantages this approach offers compared with DNA injection into eggs and the use of retroviruses as vectors for introducing genes into mice are discussed.

Insertion of a bacterial gene into the mouse germ line using an infectious retrovirus vector

Using a Moloney leukemia virus vector containing the bacterial neo gene, we demonstrate that retrovirus vectors can be used to introduce genes into the mouse germ line. Infection of preimplantation embryos with the vector MLV-NEO.1 resulted in integration of neo sequences in approximately equal to 10% of the progeny mice. One of these animals, mouse F.2, contained approximately six MLV-NEO.1 proviruses at independent integration sites, each present at less than a single copy per cell. This mosaic mouse transmitted one of these proviruses to her offspring, producing a line of transgenic mice carrying a full-length, unrearranged MLV.NEO.1 provirus at a single chromosomal integration site. Mice homozygous at this MLV-NEO.1 locus have also been produced. No expression of the neo gene has been detected in the transgenic mice, either by screening of primary bone marrow or lung cells for resistance to G418 or by RNA transfer blot analysis of RNA from several tissues. In addition, the neo gene was found to be extensively methylated in the transgenic mice; however, treatment of primary cells with 5-azacytidine did not induce G418 resistance. The inactivity of the MLV-NEO.1 provirus in transgenic mice and potential means of eliciting neo expression under these conditions are discussed.

Insertion of the bacterial gpt gene into the germ line of mice by retroviral infection

Mouse substrains genetically transmitting the exogenous Moloney murine leukemia virus (Mo-MuLV) at a single locus have been derived previously by infection of preimplantation embryos. Here we explore the potential of retroviral vectors for transferring nonviral genes into the germ line of mice. Preimplantation mouse embryos were cocultivated with a cell line that produces a recombinant retrovirus whose genome carries the Escherichia coli gene gpt. We show that the vector sequence was inserted into the genome of the embryo and into the germ line at a frequency similar to that for the Mo-MuLV-helper sequence. A new mouse strain, Mgpt-1, was developed that is homozygous for a single MSVgpt proviral genome. The proviral sequences were highly methylated and not expressed in tissues of Mgpt-1 mice. When cells derived from transgeneic animals were treated with 5-azacytidine, the proviral sequences were not methylated and were transcriptionally activated. These results indicate that nonviral genes that are under the control of the viral long terminal repeat are inactivated when transferred into the germ line of animals.