High-level expression of exogenous genes by replication-competent retrovirus vectors with an internal ribosomal entry site

Regulated multicistronic expression technology for mammalian metabolic engineering

Contemporary basic research is rapidly revealing increasingly complex molecular regulatory networks which are often interconnected via key signal integrators. These connections among regulatory and catalytic networks often frustrate bioengineers as promising metabolic engineering strategies are bypassed by compensatory metabolic responses or cause unexpected, undesired outcomes such as apoptosis, product protein degradation or inappropriate post- translational modification. Therefore, for metabolic engineering to achieve greater success in mammalian cell culture processes and to become important for future applications such as gene therapy and tissue engineering, this technology must be enhanced to allow simultaneous, in cases conditional, reshaping of metabolic pathways to access difficult-to-attain cell states. Recent advances in this new territory of multigene metabolic engineering are intimately linked to the development of multicistronic expression technology which allows the simultaneous, and in some cases, regulated expression of several genes in mammalian cells. Here we review recent achievements in multicistronic expression technology in view of multigene metabolic engineering.

Nfil3/E4bp4 is required for the development and maturation of NK cells in vivo

Nuclear factor interleukin-3 (Nfil3; also known as E4-binding protein 4) is a basic region leucine zipper transcription factor that has antiapoptotic activity in vitro under conditions of growth factor withdrawal. To study the role of Nfil3 in vivo, we generated gene-targeted Nfil3-deficient (Nfil3^−/−) mice. Nfil3^−/− mice were born at normal Mendelian frequency and were grossly normal and fertile. Although numbers of T cells, B cells, and natural killer (NK) T cells were normal in Nfil3^−/− mice, a specific disruption in NK cell development resulted in severely reduced numbers of mature NK cells in the periphery. This defect was NK cell intrinsic in nature, leading to a failure to reject MHC class I–deficient cells in vivo and reductions in both interferon γ production and cytolytic activity in vitro. Our results confirm the specific and essential requirement of Nfil3 for the development of cells of the NK lineage.

Transgene expression facilitated by the v-src splice acceptor can impair replication kinetics and lead to genomic instability of Rous sarcoma virus-based vectors

Rous sarcoma virus (RSV) can be used for the simple generation of high-titer replication-competent retroviral (RCR) vectors. Retroviruses undergo frequent genomic recombination, however, and vectors with reduced replication kinetics are rapidly overgrown by mutant forms. Vector design is hence critical to vector efficacy. In this study, two different designs of RSV-based RCR vectors were evaluated. Vectors in which transgene expression was facilitated by the v-src splice acceptor were revealed to have greatly reduced replication kinetics and genomic stability in comparison to vectors in which transgene expression was mediated by an internal ribosome entry site in the 3' untranslated region.

Expression and function of Wnt5a in the development of the glandular stomach in the chicken embryo

The epithelium of the chicken embryonic glandular stomach (proventriculus) differentiates into both a glandular and a luminal epithelium, the cells of which express specific marker genes. The subsequent formation and differentiation of the glands then proceed under the influence of the mesenchyme. To search for possible candidates for the mesenchymal factors involved, we have now investigated the expression and function of Wnt5a in this process. Our current results show that Wnt5a is expressed in the mesenchyme during active gland formation and that overexpression of this gene in ovo results in the increased and ectopic expression of some of the marker genes of the luminal and glandular epithelia. In particular, the overexpression of Wnt5a markedly enhances the expression of the embryonic chicken pepsinogen gene, a marker of the glandular epithelium, indicating its role as a mesenchymal factor that regulates the differentiation of the proventricular epithelium.

Characterization of a semi-replicative gene delivery system allowing propagation of complementary defective retroviral vectors

BACKGROUND

Recently, several cancer gene therapy studies have shown that replication-competent retroviral vectors represent a major improvement over replication-defective ones in terms of transgene propagation efficiency. However, this positive effect is somewhat spoiled by the increased risk of dissemination and oncogenesis that replication-competent retroviral vectors entail. To enhance both their integral safety and their transgene capacity, we developed a semi-replication-competent retroviral vector system.

METHODS

The semi-replication-competent retroviral vector system is based on two transcomplementing replication-defective retroviral vectors termed gag-pol vector (GPv) and env vector (Ev). Vector propagation was monitored in vitro and in solid tumors in vivo, using different reporter transgenes for GPv and Ev. Systemic vector dissemination and leukemogenesis was assessed by direct intravenous vector injection and subsequent bone marrow transplantation, in MLV-sensitive mice.

RESULTS

In vitro and in vivo the semi-replication-competent retroviral vectors propagate transgenes almost as efficiently as replication-competent ones. The semi-replication-competent retroviral vector system does not lead to detectable dissemination or leukemogenesis as does the replication-competent vector or the parental virus. Additionally, the vector duo allows co-propagation of different transgenes as well as mobilization of a third replication-defective vector.

CONCLUSIONS

This study is an initial proof of principle for the use of complementary retroviral vectors to deliver and propagate transgenes in vitro and in solid tumors in vivo, but with reduced pathogenicity compared to its parental virus. In-between replication-defective and replication-competent retroviral vectors, this semi-replicative system offers good grounds for its application in in vitro studies and allows envisioning its further development for cancer gene therapy.

Use and comparison of different internal ribosomal entry sites (IRES) in tricistronic retroviral vectors

BACKGROUND

Polycistronic retroviral vectors that contain several therapeutic genes linked via internal ribosome entry sites (IRES), provide new and effective tools for the co-expression of exogenous cDNAs in clinical gene therapy protocols. For example, tricistronic retroviral vectors could be used to genetically modify antigen presenting cells, enabling them to express different co-stimulatory molecules known to enhance tumor cell immunogenicity.

RESULTS

We have constructed and compared different retroviral vectors containing two co-stimulatory molecules (CD70, CD80) and selectable marker genes linked to different IRES sequences (IRES from EMCV, c-myc, FGF-2 and HTLV-1). The tricistronic recombinant amphotropic viruses containing the IRES from EMCV, FGF-2 or HTLV-1 were equally efficient in inducing the expression of an exogenous gene in the transduced murine or human cells, without displaying any cell type specificity. The simultaneous presence of several IRESes on the same mRNA, however, can induce the differential expression of the various cistrons. Here we show that the IRESes of HTLV-1 and EMCV interfere with the translation induced by other IRESes in mouse melanoma cells. The IRES from FGF-2 did however induce the expression of exogenous cDNA in human melanoma cells without any positive or negative regulation from the other IRESs present within the vectors. Tumor cells that were genetically modified with the tricistronic retroviral vectors, were able to induce an in vivo anti-tumor immune response in murine models.

CONCLUSION

Translation of the exogenous gene is directed by the IRES and its high level of expression not only depends on the type of cell that is transduced but also on the presence of other genetic elements within the vector.

GENE DELIVERY INTO THE CHICKEN EMBRYO BY USING REPLICATION-COMPETENT RETROVIRAL VECTORS

Rous sarcoma virus (RSV)-derived retroviral vectors have allowed for efficient gene transfer into the chicken embryo which is a classical model for studying vertebrate development. Current evidence reveals that this method can be used for regionally restricted expression, inducible expression, and for interfering with endogenous gene function, suggesting that gain-of-function and loss-of-function strategies for specific genes can be achieved spatially and temporally in the avian embryo. Thus, retroviral-mediated gene transfer into the chicken embryo coupled with a wide variety of strategies is now an important tool to address specific biological questions in the vertebrate.

Hox proteins functionally cooperate with the GC box-binding protein system through distinct domains

Hox genes encode a transcriptional factor that plays a key role in regulating position-specific cartilage pattern formation. We found that Hoxa-13 and Hoxd-13, which are members of the Abd-B subfamily of Hox genes and are crucial for the autopod development of the limb, stimulate transcription from the Bmp-4 promoter. This stimulation was dependent on the GC box within the promoter and independent of the putative Hox protein binding site. The stimulation by HoxA-13 was remarkably enhanced by cotransfection with members of a family of zinc finger GC box binding transcriptional factors including Sp1. The stimulation was suppressed by another Abd-B Hox protein, HoxA-11, indicating that each Abd-B Hox protein has a different effect on the target genes through the Sp1 system. We have identified multiple functional domains involved in transcriptional regulation, including three independent transcriptional activation domains (ADs) in HoxA-13. AD1 and AD3 in helices 1 and 2 of the homeodomain individually cooperate with Sp1-dependent stimulation. The homeodomain is also required for cooperation of the AD with Sp1. By contrast, AD2 strongly activates transcription in an Sp1-independent manner only when the homeodomain has been removed. These observations indicate that HoxA-13 regulates transcription through multiple pathways. In addition, we found that a helix 3 mutation of the HoxA-13 homeodomain behaves as a dominant negative form.

Down-regulation of endodermal Shh is required for gland formation in chicken stomach

During the development of the proventriculus (glandular stomach) of the chicken embryo, the endodermal epithelium invades into the surrounding mesenchyme and forms glands. The glandular epithelial cells produce pepsinogen, while the non-glandular (luminal) epithelial cells secrete mucus. Sonic hedgehog is expressed uniformly in the proventricular epithelium before gland formation, but its expression ceases in gland cells. Here we present evidence that down-regulation of Sonic hedgehog is necessary for gland formation in the epithelium using a specific inhibitor of Sonic hedgehog signaling and virus mediated overexpression of Sonic hedgehog. We also show that gland formation is not induced by down-regulation of Sonic hedgehog alone; a mesenchymal influence is also required.

Kinetics of v-src-induced epithelial-mesenchymal transition in developing glandular stomach

The oncogene function in primary epithelial cells is largely unclear. Recombination organ cultures in combination with the stable and transient gene transfer techniques by retrovirus and electroporation, respectively, enable us to transfer oncogenes specifically into primary epithelial cells of the developing avian glandular stomach (proventriculus). In this system, the epithelium and mesenchyme are mutually dependent on each other for their growth and differentiation. We report here that either stable or transient expression of v-src in the epithelium causes budding and migration of epithelial cells into mesenchyme. In response to the transient expression of v-Src or a constitutive active mutant of MEK, we observed immediate downregulation of the Sonic hedgehog gene and subsequent elimination of E-cadherine expression in migrating cells, suggesting the involvement of MAP kinase signaling pathway in these processes. v-src-expressing cells that were retained in the epithelium underwent apoptosis (anoikis) and detached from the culture. Continuous expression of v-src by, for example, Rous sarcoma virus (RSV) was required for the epithelial cells to acquire the ability to express type I collagen and fibronectin genes (mesenchymal markers), and finally to establish the epithelial-mesenchymal transition. These observations would partly explain why RSV does not apparently cause carcinoma formation, but induces sarcomas exclusively.

A replication-competent feline leukemia virus, subgroup A (FeLV-A), tagged with green fluorescent protein reporter exhibits in vitro biological properties similar to those of the parental FeLV-A

We previously established that lymphoid tumors could be induced in cats by intradermal injection of ecotropic feline leukemia virus (FeLV), subgroup A, plasmid DNA. In preparation for in vivo experiments to study the cell-to-cell pathway for the spread of the virus from the site of inoculation, the green fluorescent protein (GFP) transgene fused to an internal ribosome entry site (IRES) was inserted after the last nucleotide of the env gene in the ecotropic FeLV-A Rickard (FRA) provirus. The engineered plasmid was transfected into feline fibroblast cells for production of viruses and determination of GFP expression. The virions produced were highly infectious, and the infected cells could continue to mediate strong expression of GFP after long-term propagation in culture. Similar to parental virus, the transgene-containing ecotropic virus demonstrated recombinogenic activity with endogenous FeLV sequences in feline cells to produce polytropic recombinant FeLV subgroup B-like viruses which also contained the IRES-GFP transgene in the majority of recombinants. To date, the engineered virus has been propagated in cell culture for up to 8 months without diminished GFP expression. This is the first report of a replication-competent FeLV vector with high-level and stable expression of a transgene.

Genomic stability of murine leukemia viruses containing insertions at the Env-3' untranslated region boundary

Retroviruses containing inserts of exogenous sequences frequently eliminate the inserted sequences upon spread in susceptible cells. We have constructed replication-competent murine leukemia virus (MLV) vectors containing internal ribosome entry site (IRES)-transgene cassettes at the env-3' untranslated region boundary in order to examine the effects of insert sequence and size on the loss of inserts during viral replication. A virus containing an insertion of 1.6 kb replicated with greatly attenuated kinetics relative to wild-type virus and lost the inserted sequences in a single infection cycle. In contrast, MLVs containing inserts of 1.15 to 1.30 kb replicated with kinetics only slightly attenuated compared to wild-type MLV and exhibited much greater stability, maintaining their genomic integrity over multiple serial infection cycles. Eventually, multiple species of deletion mutants were detected simultaneously in later infection cycles; once detected, these variants rapidly dominated the population and thereafter appeared to be maintained at a relative equilibrium. Sequence analysis of these variants identified preferred sites of recombination in the parental viruses, including both short direct repeats and inverted repeats. One instance of insert deletion through recombination with an endogenous retrovirus was also observed. When specific sequences involved in these recombination events were eliminated, deletion variants still arose with the same kinetics upon virus passage and by apparently similar mechanisms, although at different locations in the vectors. Our results suggest that while lengthened, insert-containing genomes can be maintained over multiple replication cycles, preferential deletions resulting in loss of the inserted sequences confer a strong selective advantage.

Endogenous AP-1 levels necessary for oncogenic activity are higher than those sufficient to support normal growth

We investigated the role of endogenous AP-1 in human tumor cell lines by introducing SupJunD-1, a dominant-negative mutant of AP-1, using vesicular stomatitis virus G protein (VSV-G)-pseudotyped retrovirus vectors. Single inoculation of six human tumor cell lines, originating from osteosarcomas, non-small cell lung carcinomas or cervical carcinomas, with recombinant SupJunD-1 virus at a high multiplicity of infection readily inhibited colony formation in soft agar. We detected no significant changes in expression levels of AP-1 components c-Jun or Fra-1, adhesion molecules CD44 or E-cadherin, or cell cycle regulator p53, which are encoded by genes previously reported to be under the control of AP-1 in some mouse or human cell lines. By varying the dosage of VSV-G-pseudotyped retrovirus, we were able to change the proviral copy number of supjunD-1 from 1 to approximately 10 and monitor suppression of endogenous AP-1 function as assessed by growth characteristics of the tumor cell lines, we found a SupJunD-1 dosage which significantly suppressed anchorage-independent growth without affecting the cellular growth in monolayer cultures at all. We conclude that endogenous AP-1 levels necessary for oncogenic activity are much higher than those sufficient to support normal growth.

Gene transfer into chicken embryos by retrovirus vectors

While chickens have many properties that are advantageous for embryological studies, their genetic analysis has been restricted. However, by using retrovirus vector systems in combination with classical techniques of experimental developmental biology, it has recently become possible to analyze the function of genes involved in the development of this organism. Avian retrovirus vectors are unique in that they can be divided into two categories: replication-competent and replication-defective (replication-incompetent). By choosing the vectors correctly, there are many experimental applications of these vectors such as induction of constitutive (or regulated) gene expression in a restricted region of tissues, organs and embryos; cell lineage analysis; and formation of concentration gradients of morphogens in micromass cultures. In this paper, several retrovirus vectors available for the chicken will be introduced and their applications in developmental biology will be reviewed.

The concentric structure of the developing gut is regulated by Sonic hedgehog derived from endodermal epithelium

The embryonic gut of vertebrates consists of endodermal epithelium, surrounding mesenchyme derived from splanchnic mesoderm and enteric neuronal components derived from neural crest cells. During gut organogenesis, the mesenchyme differentiates into distinct concentric layers around the endodermal epithelium forming the lamina propria, muscularis mucosae, submucosa and lamina muscularis (the smooth muscle layer). The smooth muscle layer and enteric plexus are formed at the outermost part of the gut, always some distance away from the epithelium. How this topographical organization of gut mesenchyme is established is largely unknown. Here we show the following: (1) Endodermal epithelium inhibits differentiation of smooth muscle and enteric neurons in adjacent mesenchyme. (2) Endodermal epithelium activates expression of patched and BMP4 in adjacent non-smooth muscle mesenchyme, which later differentiates into the lamina propria and submucosa. (3) Sonic hedgehog (Shh) is expressed in endodermal epithelium and disruption of Shh-signaling by cyclopamine induces differentiation of smooth muscle and a large number of neurons even in the area adjacent to epithelium. (4) Shh can mimic the effect of endodermal epithelium on the concentric stratification of the gut. Taken together, these data suggest that endoderm-derived Shh is responsible for the patterning across the radial axis of the gut through induction of inner components and inhibition of outer components, such as smooth muscle and enteric neurons.

BMPs are necessary for stomach gland formation in the chicken embryo: a study using virally induced BMP-2 and Noggin expression

Epithelial-mesenchymal interactions are necessary for the normal development of various digestive organs. In chicken proventriculus (glandular stomach), morphogenesis and differentiation of the epithelium depend upon the inductive signals coming from underlying mesenchyme. However, the nature of such signals is still unclear despite extensive analyses carried out using experimental tissue recombinations. In this study we have examined the possible involvement of bone morphogenetic proteins (BMPs) in the formation of stomach glands in the chicken embryo. Analysis of the expression patterns of BMP-2, −4 and −7 showed that these BMPs were present in the proventricular mesenchyme prior to the initiation of the proventricular gland formation. BMP-2 expression, in particular, was restricted to the proventriculus among anterior digestive organs. Virus-mediated BMP-2 overexpression resulted in an increase in the number of glands formed. Moreover, ectopic expression of Noggin, which antagonizes the effect of BMPs, in the proventricular mesenchyme or epithelium, led to the complete inhibition of gland formation, indicating that BMP signals are necessary for the proventricular gland formation. These findings suggest that BMPs are of prime importance as mesenchymal signals for inducing proventricular glands.

Expression of heterologous genes from an IRES translational cassette in replication competent murine leukemia virus vectors

We describe replication competent retroviruses capable of expressing heterologous genes during multiple rounds of infection. An internal ribosome entry site (IRES) from encephalomyocarditis virus was inserted in the U3 region of Akv- and SL3-3-murine leukemia viruses (MLV) to direct translation of neo or the enhanced green fluorescence protein gene (EGFP). Akv-MLV's with IRES-neo and IRES-EGFP cassettes replicated with titers of about 10(6) infectious units/ml while SL3-3-MLV with IRES-neo gave about 10(3)-fold lower titers. Interestingly, RNA analysis showed a drastic reduction in the amount of spliced env mRNA for the SL3-3 derived vector relative to the Akv derived vectors, seemingly contributing to its low replication capacity. The EGFP expressing Akv-MLV was genetically stable for multiple rounds of infection; marker-cassette deletion revertants appeared after several replication rounds and these revertants only slowly became dominant in the virus population.

Production and design of more effective avian replication-incompetent retroviral vectors

Retroviral vectors have been invaluable tools for studies of development in vertebrates. Their use has been somewhat constrained, however, by the low viral titers typically obtained with replication-incompetent vectors, particularly of the avian type. We have addressed this problem in several ways. We optimized the transient production of avian replication-incompetent viruses in a series of cell lines. One of the optimal cell lines was the mammalian line 293T, which was surprising in light of previous reports that avian viral replication was not supported by mammalian cells. We also greatly increased the efficiency of viral infection. Pseudotyping with the vesicular stomatitus virus G (VSV-G) protein led to an over 350-fold increase in the efficiency of infection in ovo relative to infection with virus particles bearing an avian retroviral envelope protein. To further increase the utility of the system, we developed new Rous sarcoma virus (RSV)-based replication-incompetent vectors, designed to express a histochemical marker gene, human placental alkaline phosphatase, as well as an additional gene. These modified retroviral vectors and the VSV-G pseudotyping technique constitute significant improvements that allow for expanded use of avian replication-incompetent viral vectors in ovo.

Internal ribosome entry sites (IRESs): reality and use

IRESs are known to recruit ribosomes directly, without a previous scanning of untranslated region of mRNA by the ribosomes. IRESs have been found in a number of viral and cellular mRNAs. Experimentally, IRESs are commonly used to direct the expression of the second cistrons of bicistronic mRNAs. The mechanism of action of IRESs is not fully understood and a certain number of laboratories were not successful in using them in a reliable manner. Three observations done in our laboratory suggested that IRESs might not work as functionally as it was generally believed. Stem loops added before IRESs inhibited mRNA translation. When added into bicistronic mRNAs, IRESs initiated translation of the second cistrons efficiently only when the intercistronic region contained about 80 nucleotides, and they did not work any more effectively with intercistronic regions containing at least 300-400 nucleotides. Conversely, IRESs inserted at any position into the coding region of a cistron interrupted its translation and initiated translation of the following cistron. The first two data are hardly compatible with the idea that IRESs are able to recruit ribosomes without using the classical scanning mechanism. IRESs are highly structured and cannot be scanned by the 40S ribosomal subunit. We suggest that IRESs are short-circuited and are essentially potent stimulators favoring translation in particular physiological situations.

Developmental patterning in chondrocytic cultures by morphogenic gradients: BMP induces expression of indian hedgehog and noggin

BACKGROUND

The maturation of chondrocytes is essential for endochondral bone formation. The Indian Hedgehog (Ihh) gene is expressed in prehypertropic chondrocytes and has been proposed to regulate chondrocyte maturation. While such secretary factors as PTHrP and BMP are thought to be involved in Ihh expression, the mechanism of the restricted expression of Ihh is not clear.

RESULTS

Using primary chondrocytes, we have developed here a modified micromass culture (MM-C) system that allows the formation of concentration gradients of secreted factors, expressed either endogenously or retrovirally, from each of plural micromass cultures on a single plate. Using this system, we determined that chondrocytes create the inhibitory micro-environment, partly dependent on PTHrP secretion, for the Ihh expression. We also showed that retrovirally induced BMP-2 induces the expression of both Ihh and Noggin (encoding the BMP-inactivating protein), and we further present evidence that a negative-feedback loop involving Noggin might account for the precise localization of BMP signalling for Ihh induction.

CONCLUSION

These results suggest that the expression of the Ihh gene in cartilage is regulated by several mechanisms that include the secretion of inhibitory factors (including PTHrP) and the negative-feed back loop formed by BMPs and Noggin.

Inducible expression of herpes simplex virus thymidine kinase from a bicistronic HIV1 vector

The possibility of protecting human CD4+ lymphocytes from human immunodeficiency virus type 1 (HIV1) infection, through a suicide mechanism elicited by the HIV1 transcription apparatus itself, offers a potentially useful approach for gene therapy of the acquired immunodeficiency syndrome. A replication-defective lentiviral HIV1 vector (HYIRES-TK) was designed to carry both the hygromycin (Hy) phosphotransferase gene for positive selection and the thymidine kinase (TK) gene of herpes simplex virus driven by the viral long terminal repeat (LTR). The internal ribosome entry site (IRES) from encephalomyocarditis virus was placed between the two genes for their efficient simultaneous translation. Transient expression of active TK into transfected COS-1 cells was shown to be induced by Tat and Rev over a detectable basal level. By providing the missing viral proteins in trans, recombinant viruses were generated and used to transduce Jurkat cells. The Hy-resistant population of cells was sensitive to ganciclovir (GCV) and acyclovir (ACV), a result consistent with a basal level of TK expression. Cocultivation of transduced cells with cells chronically infected with HIV in the presence of 10 microM ACV, a concentration non-toxic for the uninfected cells, resulted in increased killing of cells transduced with the HY-IRES-TK vector. These data indicate that two genes can be expressed from the viral LTR in the context of an HIV1 vector, with the aid of an IRES sequence. The expression is inducible by the HIV proteins Tat and Rev and it is possible to specifically kill infected cells with subtoxic concentrations of drug. To decrease the sensitivity of the transduced cells towards GCV, a variant vector expressing a truncated TK was constructed. The truncated version was expressed at levels similar to those of wild-type TK but induced sensitivity towards GCV in transduced cells that was intermediate between that of untransduced cells and of cells expressing wild-type TK.