Tetracycline-regulated gene expression following direct gene transfer into mouse skeletal muscle

In vivo gene regulation using tetracycline-regulatable systems

Numerous preclinical studies have demonstrated the efficacy of viral gene delivery vectors, and recent clinical trials have shown promising results. However, the tight control of transgene expression is likely to be required for therapeutic applications and in some instances, for safety reasons. For this purpose, several ligand-dependent transcription regulatory systems have been developed. Among these, the tetracycline-regulatable system is by far the most frequently used and the most advanced towards gene therapy trials. This review will focus on this system and will describe the most recent progress in the regulation of transgene expression in various organs, including the muscle, the retina and the brain. Since the development of an immune response to the transactivator was observed following gene transfer in the muscle of nonhuman primate, focus will be therefore, given on the immune response to transgene products of the tetracycline inducible promoter.

Enhanced expression of the high affinity glutamate transporter GLT-1 in C6 glioma cells delays tumour progression in rat

High grade gliomas are known to release excitotoxic concentrations of glutamate, a process thought to contribute to their malignant phenotype through enhanced autocrine stimulation of their proliferation and destruction of the surrounding nervous tissue. A model of C6 glioma cells in which expression of the high affinity glutamate transporter GLT-1 can be manipulated both in vivo and in vitro was used in order to investigate the consequences of increasing glutamate clearance on tumour progression. These cells were grafted in the striatum of Wistar rats and doxycycline was administered after validation of tumour development by magnetic resonance imaging. Both GLT-1 expression examined by immunohistochemistry and glutamate transport activity measured on synaptosomes appeared robustly increased in samples from doxycycline-treated animals. Moreover, these rats showed extended survival times as compared to vehicle-treated animals, an effect that was consistent with volumetric data revealing delayed tumour growth. As constitutive deficiency in glutamate clearance at the vicinity of brain tumours is well established, these data illustrate the potential benefit that could be obtained by enhancing glutamate transport by glioma cells in order to reduce their invasive behaviour.

Inducible nonviral gene expression in the treatment of osteochondral defects

OBJECTIVE

The repair of osteochondral defects with chondrocytes genetically modified to express desired growth factors promises great potential in orthopaedic therapy. Controlled expression of the transgenes is required in many instances. The objective of the present study was to demonstrate the inducibility of tetracycline-responsive transgene expression in osteochondral defects in the knee joint filled with genetically modified chondrocyte implants.

METHODS

An expression plasmid containing the lacZ gene under the control of the minimal CMV promoter fused to the Tet-responsible element (TRE) as well as the reverse transactivator (rtTA2s-M2) was constructed and used to transfect isolated articular chondrocytes from New Zealand white rabbits. rtTA2s-M2 binds to the TRE in the presence of tetracycline and leads to the transcription of the transgene. Different concentrations of DNA and various DNA:lipid ratios were tested to determine best transfection conditions. Transfection efficiency and inducibility were analysed by histochemical analysis and flow-cytometry. To evaluate the system in vivo, collagen-sponges were seeded with transfected autologous chondrocytes and implanted in osteochondral defects in the knees of NZW-rabbits. Gene expression was induced by doxycycline and 3 weeks later, LacZ-expression in isolated knee joints was evaluated in histological sections by X-gal staining.

RESULTS

In vitro 13.5% (+/-1.32) of induced primary chondrocytes were LacZ-positive, while non-induced controls showed a background-staining in 0.6% (+/-0.2). In vivo, upon doxycycline treatment, induction of lacZ-gene-expression could be demonstrated in chondrocytes in 3-week-old, well-integrated implants.

CONCLUSION

For the first time, tetracycline-inducible gene expression is demonstrated to work in the treatment of osteochondral defects. This demonstrates the feasibility for a gene therapy-assisted approach using controlled expression of therapeutic growth factors from transplanted genetically modified chondrocytes.

Tetracycline-regulated secretion of human insulin in transfected primary myoblasts

A mechanism for safely regulating transgene expression will be necessary for gene therapy approaches to endocrine disorders. In this study, a two-plasmid tetracycline-inducible system was used to regulate expression of human proinsulin (hppI1) and a mutated proinsulin construct (hppI4, allowing cleavage by furin) in primary rat soleus myoblasts. In hppI1 and hppI4 transient transfections, the presence of 0.01 and 0.1 microg/ml tetracycline for 48 h inhibited pro/insulin secretion to 19-27% and 7-12%, respectively, compared to tetracycline untreated myoblasts. Following a 48 h tetracycline incubation (1.0 microg/ml), pro/insulin secretion in hppI1 and hppI4 transfected myoblasts was reduced to <4% of that in cells incubated without tetracycline. Pro/insulin secretion equivalent to that of untreated cells was restored following tetracycline withdrawal and incubation for a further 72 h. Conversion of proinsulin to insulin in transfected myoblasts was <1% for hppI1 and >45% for hppI4. In conclusion, regulated insulin secretion has been achieved in a dose-dependent and reversible manner in primary myoblasts.

Modulating gene expression using DNA vaccines with different 3'-UTRs influences antibody titer, seroconversion and cytokine profiles

To determine if modulating the amount of foreign antigen produced by a DNA vaccine can influence the overall intensity and cytokine polarization of the ensuing immune response, three different plasmids, each encoding the hepatitis B (HB) surface antigen, were constructed. In each construct, HBs gene expression was driven by the cytomegalovirus immediate early promoter, but differed in the 3'-untranslated regions (3'-UTR) containing the polyadenylation sequence. These 3'-UTR sequences were derived from either the hepatitis B virus (HBVpA), bovine growth hormone (BGHpA), or rabbit beta-globin (betapA). BALB/c mice were immunized intramuscularly with equimolar amounts of each plasmid and blood was collected bi-weekly. Following immunization, total IgG titers correlated with in vitro antigen production levels (from transfected CHO cells), as evidenced by the following response pattern: HBVpA>BGHpA>>betapA. All groups demonstrated a heavy bias toward a Th1 immune response, as evidenced by high serum IgG2a/IgG1 ratios and the predominance of IFN-gamma over IL-4 secretion from cultured splenocytes. In addition, the HBVpA construct resulted in a seroconversion rate of 100%, in comparison to 40-50% in the BGHpA, and 0% in the betapA group. Surprisingly, splenocytes isolated from mice immunized with the betapA construct secreted the highest levels of IFN-gamma. Taken together, these findings suggest that altering the level of gene expression not only affects the overall titer and seroconversion rates of vaccinated animals, but also may play a role in modulating cytokine profiles.

Regulated production of proteins from muscle using gene transfer: potential therapeutic applications

The ability to produce high-level transgene expression following the introduction of genetic material into a host cell has been well documented. Various vectors and methods for in vivo gene delivery have been shown to provide long-term expression from many different tissue types in rodents and large animals. However, many potential therapeutic targets for gene therapy involve the production of proteins that are toxic or lead to undesirable effects if overexpressed. Thus, the ability to achieve regulated gene expression following treatment will be required to ensure the safety of long-acting gene therapy products. Skeletal muscle, in particular, has been widely used as a target for gene therapy protocols, due to the ease of accessibility and ability to produce and secrete some proteins at very high levels. This review focuses on regulated gene therapy systems that are being evaluated for use in muscle, and discusses two classes of system: those dependent on exogenously administered drugs and those dependent on endogenously produced metabolites.

Glucocorticoid-regulated VEGF expression in ischemic skeletal muscle

Vascular endothelial growth factor (VEGF) is a potent neovascular inducer. Gene therapeutic delivery of a plasmid DNA encoding VEGF has been shown to impart collateral vessel development in animal models of hindlimb ischemia. Constitutive, long-lived expression of VEGF through gene transfer, however, may result in hypervascularization and/or leaky blood vessels. To that end, the introduction of regulated VEGF gene transfer technology may provide a safer and more controlled therapy for ischemic tissues. We developed a glucocorticoid-regulated plasmid vector (pNGVL-hAP/GRE(5)-vegf-pA) for modulating VEGF gene expression. This plasmid possessed five tandem repeats of the glucocorticoid-responsive element and adenovirus major-late promoter driving the expression of the VEGF(165) cDNA. Intramuscular delivery of this plasmid to mice, and subsequent treatment with the synthetic glucocorticoid dexamethasone (DEX), led to greatly enhanced VEGF expression. Similar delivery to the gracillis muscle of New Zealand white rabbits that had undergone ligation of their femoral artery to induce ischemia exhibited increased VEGF expression and collateral vessel development only in the presence of DEX. Additionally, reintroduction of DEX at a time point during which initial VEGF transgene levels had subsided resulted in a vigorous reinduction of VEGF transgene expression. This new iteration of VEGF gene delivery provides for fine-tuned angiogenic factor-based therapy for tissues requiring neovascularization.

Gene therapy for Parkinson's disease

Significant progress has been made in the field of gene therapy for Parkinson's disease (PD). Successful vehicles for gene transfer into the central nervous system have been developed and clinical efficacy and safety have both been shown in various animal models of PD. Further optimisation of dosing, timing and location of gene therapy delivery as well as the ability to regulate and prolong gene expression will be important for the commencement of human trials. Current gene therapy models for PD have focused on two treatment strategies. One is the replacement of biosynthetic enzymes for dopamine synthesis and the second strategy is the addition of neurotrophic factors for protection and restoration of dopaminergic neurones. Concepts of neuroprotection and restoration of the nigrostriatal pathway will become important themes for future genetic treatment strategies for PD and may include, in addition to neurotrophic factors, genes to prevent apoptosis or detoxify free radical species. This review will highlight the recent literature on gene therapy for PD and summarise general approaches to gene therapy.

Stringent control of gene expression in vivo by using novel doxycycline-dependent trans-activators

The tetracycline (Tet)-dependent regulatory system has been widely used for controlling gene expression. The Tet-on version of the system, in which the reverse Tet-responsive transcriptional activator (rtTA) is positively regulated by Tet or its analogs, such as doxycycline (Dox), is of potential utility for gene therapy applications in humans. However, rtTA may display a high basal activity, especially when delivered in vivo by using episomal vectors such as plasmids. Two novel Dox-inducible activators, called rtTA2(S)-S2 and rtTA2(S)-M2, which have a significantly lower basal activity than rtTA in stably transfected cell lines, have been described. In this study we tested the capability of these trans-activators to control expression of mouse erythropoietin (mEpo) and to modulate hematocrit (Hct) increase in vivo on delivery of plasmids into quadriceps muscles of adult mice by DNA electroinjection. Both rtTA2(S)-M2 and rtTA2(S)-S2 displayed a considerably lower background activity and higher window of induction than rtTA in vivo. Moreover, a stringent control of mEpo gene expression and Hct levels in the absence of any background activity was maintained over a 10-month period by injecting as little as 1 microg of a single plasmid containing the rtTA2(S)-S2 expression cassette and the Tet-responsive mEpo cDNA. This constitutes the first report of a stringent ligand-dependent control of gene expression in vivo obtained by delivering a single plasmid encoding both the trans-activator and the regulated gene. Notably, the rtTA2(S)-S2-based system was induced by oral doses of doxycycline comparable to those normally used in clinical practice in humans.

TGF-alpha-driven tumor growth is inhibited by an EGF receptor tyrosine kinase inhibitor

The simultaneous presence of the EGFR and its ligand TGF-alpha in human tumor tissues suggests that autocrine TGF-alpha stimulation drives tumor growth. Here we show that autocrine TGF-alpha stimulation does cause increased tumor growth in vivo, an effect that was proven to be mediated via EGFR activation, and that this TGF-alpha/EGFR autocrine loop was accessible to an EGFR specific tyrosine kinase inhibitor. Clones of the EGFR expressing glioma cell line U-1242 MG were transfected with TGF-alpha cDNA using a tetracycline-inhibitory system for gene expression. TGF-alpha expression was inhibited by the presence of tetracycline, and subcutaneous tumors forming from cell lines injected into nude mice could be inhibited by feeding mice tetracycline. We confirmed that TGF-alpha mRNA and protein were present in these tumors and that, subsequently, the endogenous EGFR was activated. Tumor growth could be inhibited by an EGFR specific tyrosine kinase inhibitor of the type 4-(3-chloroanilino)-6,7-dimethoxy-quinazoline, administered daily by intraperitoneal injection, thereby interrupting the autocrine loop.

MUSEAP, a novel reporter gene for the study of long-term gene expression in immunocompetent mice

The improvement of gene therapy vectors would benefit from the availability of a reporter gene that can be used for long-term studies in immunocompetent laboratory animals. We describe the construction and characterization of a novel reporter gene, murine secreted embryonic alkaline phosphatase (MUSEAP). We demonstrate by gene transfer in skeletal muscle of immunocompetent mice that MUSEAP is efficiently secreted and detected in the bloodstream and that injection of an increasing dose of DNA leads to a dose-dependent increase of plasma MUSEAP activity. We also show that the expression of MUSEAP under the control of a constitutive promoter is stable for 1 year and that the activity of MUSEAP in the bloodstream reflects the changes in the transcription rate of its gene. These properties make MUSEAP the only reporter gene that can be used for somatic gene transfer into immunocompetent mice in order to study the impact of gene transfer vectors of metabolic, developmental or environmental factors on long-term gene expression.

A model system for studying postnatal myogenesis with tetracycline-responsive, genetically engineered clonal myoblasts in vitro and in vivo

The aim of this work was to introduce a tetracycline-responsive (Tet-off) gene expression system into myoblasts in order to regulate a reporter gene not only in vitro but also particularly in muscles implanted with these engineered myoblasts. Mouse myoblasts from a long-term culture (i28 cells) were transfected initially to generate and characterize two stable master clones expressing tetracycline-responsive transactivator protein tTA. Like parental i28 myoblasts, these clones differentiated well in vitro. The second step introduced the firefly (Photinus pyralis) luciferase gene into one of the stable tTA clones producing double transfectants expressing luciferase in the absence of tetracycline. Addition of tetracycline (1 microg ml(-1)) resulted in at least 100-fold decreases in luciferase activity within 8 h in both growing and differentiating myoblast cultures. Enzyme activity was rapidly restored after tetracycline was removed (8 h). After successful implantation of these myoblasts into damaged mouse muscles, luciferase expression in the matured progeny cells could be regulated by oral application of doxycycline for at least 1 month. The tetracycline-responsive master clones are potentially powerful tools for studying the function of various genes in postnatal myogenesis.

Targeted gene regulation and gene ablation

Investigations of the mechanisms involved in appropriate, developmentally regulated tissue-specific gene transcription have laid the foundations for transgenic and gene-therapy technologies directing specific induction or ablation of genes of interest in a tissue-restricted manner. This technology has further evolved to allow for temporal control of gene expression and ablation. Genes can now be switched on and off or be ablated by administering exogenous compounds. These technologies are based on the development of ligand-inducible transcription factors or recombinases that regulate gene expression or ablation by the administration of specific ligands and should lead to animal models that are better suited for investigating the molecular basis of human disease. This review describes the evolution, components and applications of systems that are currently being employed in transgenic and mutant-mouse technology for the conditional regulation of gene expression and ablation.

Oxygen tension and a pharmacological switch in the regulation of transgene expression for gene therapy

BACKGROUND

The combination of physiologically and pharmacologically controlled elements may provide a means to ensure both the regulation and the safety of transgene expression--two major goals in gene therapy.

METHODS

A two-gene modulation system was developed that uses the following three levels of control: (i) the hypoxia-responsive element directing the transcription of the tetracycline-controlled transactivator (tTA); (ii) part of the oxygen-degradation domain limiting the production of tTA in normoxia; and (iii) the tetracycline switch of the transactivator activity (the tet-off system).

RESULTS

This triple-control system allowed high expression of the gene of interest (luciferase or erythropoietin) by transfected cells upon hypoxia and low expression under normoxia or in the presence of tetracycline. This control of transgene expression was also obtained in mouse tumors.

CONCLUSIONS

This multiple-control system is of interest for spatially restricting transgene expression into hypoxic tumors, and for finely adjusting the expression level of a therapeutic protein to the oxygen supply in medical applications such as neoangiogenesis or the erythropoietin-mediated treatment of anemia.

Muc4/sialomucin complex, an intramembrane modulator of ErbB2/HER2/Neu, potentiates primary tumor growth and suppresses apoptosis in a xenotransplanted tumor

Overexpression of the membrane mucin MUC4/Sialomucin complex (SMC) has been observed during malignant progression of mammary tumors in both humans and rats, suggesting that deregulation of MUC4/SMC expression might facilitate development of these malignancies. As previously reported, overexpression of SMC results in suppression of both cell adhesion and immune killing of tumor cells. SMC also acts as a ligand for ErbB2/Neu, modulating phosphorylation of the receptor tyrosine kinase in the presence and absence of heregulin. The present studies investigated the effect of Muc4/SMC up-regulation on primary tumor growth using a tetracycline-inducible SMC expression system in a xenotransplanted tumor model. SMC up-regulation provoked rapid growth of transfected A375 melanoma in nude mice. Up-regulation of SMC, however, did not significantly increase proliferation of A375 cells in vitro. Instead, a strong suppression of apoptosis was observed in situ in SMC-overexpressing tumors. These data suggest that Muc4/SMC expression promotes tumor growth in vivo at least in part via suppression of tumor cell apoptosis. Importantly, reduction of apoptosis was also observed in vitro, indicating that anti-apoptotic effect of SMC is independent of tumor-host interactions. These findings strongly suggest that SMC up-regulation alters intracellular signaling to favor cell survival, providing for the first time evidence for the regulation of programmed cell death by a gene of the MUC family.

Prospects for gene therapy for inherited cardiomyopathies

Over the last few years the genes responsible for a number of genetic diseases of the cardiovascular system have been identified. These have included X-linked and autosomal dominant dilated cardiomyopathy, and hypertrophic cardiomyopathy. Genetic heterogeneity has been described in both of these diseases but a commonality of function has been apparent: defects in cytoskeletal proteins cause dilated cardiomyopathy and mutations in sarcomeric proteins cause hypertrophic cardiomyopathy. This led us to develop a 'final common pathway' hypothesis as a framework for selecting candidate genes for mutation screening in families with these diseases. The characterization of gene mutations has led to the development of therapies specifically targeting the defective protein or the pathway in which it is involved. These have included the use of pharmaceutical agents to replace or to antagonize the mutated protein, and replacement of the defective gene with a functional one (gene therapy). While early studies using gene therapy vectors were promising, translating studies in animals to viable therapeutic options for humans has remained problematic. There have been many publications describing the use of vectors to transduce target cells for the correction of gene defects, including recombinant retroviruses, adenoviruses, and adeno-associated viruses, as well as non-viral vectors. In this review we will discuss the identification of gene defects associated with cardiomyopathies, and the potential of gene therapy for the treatment of these diseases, as well as addressing some concerns related to the use of adenovirus-based vectors, a virus known to be an etiologic agent of acquired dilated cardiomyopathy.

Components of vectors for gene transfer and expression in mammalian cells

Progress in diverse scientific fields has been realized partly by the continued refinement of mammalian gene expression vectors. A growing understanding of biological processes now allows the design of vector components to meet specific objectives. Thus, gene expression in a tissue-selective or ubiquitous manner may be accomplished by selecting appropriate promoter/enhancer elements; stabilization of labile mRNAs may be effected through removal of 3' untranslated regions or fusion to heterologous stabilizing sequences; protein targeting to selected tissues or different organelles is carried out using specific signal sequences; fusion moieties effect the detection, enhanced yield, surface expression, prolongation of half-life, and facile purification of recombinant proteins; and careful tailoring of the codon content of heterologous genes enhances protein production from poorly translated transcripts. The use of viral as well as nonviral genetic elements in vectors allows the stable replication of episomal elements without the need for chromosomal integration. The development of baculovirus vectors for both transient and stable gene expression in mammalian cells has expanded the utility of such vectors for a broad range of cell types. Internal ribosome entry sites are now widely used in many applications that require coexpression of different genes. Progress in gene targeting techniques is likely to transform gene expression and amplification in mammalian cells into a considerably less labor-intensive operation. Future progress in the elucidation of eukaryotic protein degradation pathways holds promise for developing methods to minimize proteolysis of specific recombinant proteins in mammalian cells and tissues.

Gene therapy for Parkinson's disease: review and update

Gene transfer technology is under exploration to find therapies for the treatment of Parkinson's disease (PD) and other neurodegenerative disorders. The technology of genetic transfer can also be used as a neurobiological tool to understand the role of various genes in animal models of neurodegeneration. We describe the general approaches to gene therapy for neurodegeneration, with specific attention to commonly used methodologies. Current gene therapy models for PD are then described in two parts: genetic transfer of the biosynthetic enzymes for dopamine synthesis, and genetic transfer of the genes encoding neurotrophic factors protective for dopaminergic neurones. Future strategies for the genetic treatment of PD, such as the introduction of genes to prevent apoptosis or to detoxify free radical species are also discussed. Limitations of current approaches, such as the length and regulation of transgene expression, as well as strategies to overcome those limitations, are emphasised where possible. Gene therapy remains a promising but as yet theoretical approach to the treatment of PD in humans. However, current results in animal models predict eventual therapeutic applications.

Cytochrome c transcriptional activation and mRNA stability during contractile activity in skeletal muscle

We evaluated contractile activity-induced alterations in cytochrome c transcriptional activation and mRNA stability with unilateral chronic stimulation (10 Hz, 3 h/day) of the rat tibialis anterior (TA) muscle for 1, 2, 3, 4, 5, and 7 days (n = 3-11/group). Transcriptional activation was assessed by direct plasmid DNA injection into the TA with a chloramphenicol acetyltransferase (CAT) reporter gene linked to 326 bp of the cytochrome c promoter. Cytochrome c mRNA in stimulated muscles increased by 1.3- to 1. 7-fold above control between 1 and 7 days. Cytochrome c protein was increased after 5 days of stimulation to reach levels that were 1. 9-fold higher than control by 7 days. Cytochrome c mRNA stability, determined with an in vitro decay assay, was greater in stimulated TA than in control between 2 and 4 days, likely mediated by the induction of a cytosolic factor. In contrast, cytochrome c transcriptional activation was elevated only after 5 days of stimulation when mRNA stability had returned to control levels. Thus the contractile activity-induced increase in cytochrome c mRNA was due to an early increase in mRNA stability, followed by an elevation in transcriptional activation, leading to an eventual increase in cytochrome c protein levels.

Ligand-dependent regulation of plasmid-based transgene expression in vivo

As gene therapy advances, the ability to regulate transgene expression will become paramount for safety and efficacy. In this study, we investigate the ability of the mifepristone-dependent GeneSwitch system to regulate the expression of trangenes delivered to mice by nonviral methods. Two plasmids, one encoding the chimeric GeneSwitch protein, the other an inducible transgene for secreted human placental alkaline phosphatase (SEAP), were delivered to the hind-limb muscles of adult mice. Modulation of the level of secretion of the transgene product into serum was achieved by intraperitoneal administration of low doses of the drug mifepristone (MFP). The EC50 for induction of transgene expression by MFP was 0.03 +/- 0.005 mg/kg. The maximal level of transgene expression after induction was equal to or higher than that displayed by a plasmid driven by the CMV enhancer/promoter. The average magnitude of induction was 14- to 19-fold. Multiple rounds of drug-dependent regulation of transgene expression in vivo were demonstrated. In BALB/c mice, the ability to regulate transgene expression persisted for approximately 3 weeks, until the appearance of neutralizing antibodies to the secreted transgene product. In immune-deficient mice, the ability to repetitively regulate transgene expression persisted for at least 5 weeks. Although the dynamic range of regulation needs improvement, the plasmid-based GeneSwitch system has features that are attractive for gene therapy applications.

Main features of DNA-based immunization vectors

DNA-based immunization has initiated a new era of vaccine research. One of the main goals of gene vaccine development is the control of the levels of expression in vivo for efficient immunization. Modifying the vector to modulate expression or immunogenicity is of critical importance for the improvement of DNA vaccines. The most frequently used vectors for genetic immunization are plasmids. In this article, we review some of the main elements relevant to their design such as strong promoter/enhancer region, introns, genes encoding antigens of interest from the pathogen (how to choose and modify them), polyadenylation termination sequence, origin of replication for plasmid production in Escherichia coli, antibiotic resistance gene as selectable marker, convenient cloning sites, and the presence of immunostimulatory sequences (ISS) that can be added to the plasmid to enhance adjuvanticity and to activate the immune system. In this review, the specific modifications that can increase overall expression as well as the potential of DNA-based vaccination are also discussed.

Tetracycline-regulatable expression vectors tightly regulate in vitro gene expression of secreted proteins

The regulation of gene expression by the tetracycline system has attracted a high level of interest in the recent past. However, expression of secreted proteins has not been evaluated precisely. In this study, we constructed two versions of a one-plasmid system containing the elements necessary for the regulation of gene expression. The regulatable elements and the selectable marker (Neor) were set up in two different configurations, pTRIN31 and pTRIN76. With these two regulatable versions, the levels of protein expression after transfection into the NIH/3T3 cell line were measured by insertion of three different genes encoding the secreted proteins (hGH, ApoE3, hGM-CSF). The maximum levels of gene expression obtained with the pTRIN76-derived plasmids were 100ng/24h/106 cells for hGH, 427ng/24h/106 cells for ApoE3 and 108ng/24h/106 cells for hGM-CSF. For the pTRIN31-derived plasmids the maximum levels were 2.7ng/24h/106 cells for hGH and 47ng/24h/106 for ApoE3. Both plasmids give rise to an expression of the transfected gene that can be tightly regulated by three different molecules: tetracycline, minocycline and doxycycline. The levels of the secreted proteins are below the detectable level when the reporter genes are repressed. This repression is reversible within 48h after the regulator has been removed from the medium.

Regulation of gene expression in vivo following transduction by two separate rAAV vectors

Control of gene expression is important to gene therapy for purposes of both dosing and safety. In vivo regulation of gene expression was demonstrated following co-injection of two separate recombinant adeno-associated virus vectors, one encoding an inducible murine erythropoietin transgene and the other a transcriptional activator, directly into the skeletal muscle of adult immunocompetent mice. Transcription was controlled by systemic administration or withdrawal of tetracycline over an 18 week period, demonstrating that the two vectors were capable of transducing the same cell. Cellular or humoral immune responses against the transactivator protein were not detected.

Dose-dependent doxycycline-mediated adrenocorticotropic hormone secretion from encapsulated Tet-on proopiomelanocortin Neuro2A cells in the subarachnoid space

We previously reported that polymer-encapsulated mouse neuroblastoma cells that are capable of secreting beta-endorphin may reduce pain sensitivity in rats after capsule implantation into the cerebrospinal fluid (CSF)-filled subarachnoid space of the spinal cord. The neuroblastoma cells carry the proopiomelanocortin (POMC) gene that encodes the precursor of adrenocorticotropic hormone (ACTH) and beta-endorphin. To control the expression of these hormones in the present study, a promoter that is inducible by administration of tetracycline derivatives such as doxycycline (Dox) was linked to the POMC gene. Encapsulated cells in the CSF space of rats stimulated by four intraperitoneal doses of Dox responded with ACTH expression as determined in a subsequence 36-hr in vitro incubation. The amount of ACTH released was dependent on the in vivo Dox dose. These findings indicate that gene expression in xenogeneic cells in the CSF space can be manipulated by injection of a relatively innocuous drug, and suggest that this system may be applicable to cell transplantation therapy in patients with central nervous system diseases that require temporary control of ligand delivery.

Pharmacologic control of a humanized gene therapy system implanted into nude mice

Systemic delivery of specific therapeutic proteins by a parenteral route of administration is a recognized practice in the management of several gene defects and acquired diseases. As an alternative to repetitive parenteral administration, gene therapy may provide a novel means for systemic delivery of therapeutic proteins while improving patient compliance and therapeutic efficacy. However, for gene therapy to be an efficacious and safe approach to the clinical management of such diseases, gene expression must be tightly regulated. These investigations demonstrate precise in vivo control of protein expression from cells that are engineered to secrete human growth hormone (hGH) in response to stimulation by rapamycin. The cells were implanted intramuscularly into nu/nu mice and stimulated by intravenous or oral administration of rapamycin. In vivo experiments demonstrate that the activity and pharmacokinetics of rapamycin determine the level of serum hGH that result from the engineered cells. In addition, responsiveness of the cells to rapamycin, number of cells implanted, hGH expression kinetics, and the pharmacokinetics of hGH itself, also influence the circulating levels of hGH after rapamycin stimulation. Controlled manipulation of several of these parameters, either independently or in combination, allows for precise regulation of circulating hGH concentration in vivo.

Long-term erythropoietin expression in rodents and non-human primates following intramuscular injection of a replication-defective adenoviral vector

Erythropoietin (Epo)-responsive anemia is a debilitating complication of chronic renal failure and human immunodeficiency virus (HIV) infection that effects more than 150,000 Americans. Patients with Epo-responsive anemias are currently treated with repeated injections of recombinant human Epo. In the studies described in this report, we have examined the safety and efficacy of using a single intramuscular (i.m.) injection of replication-defective adenoviral vectors (RDAd) encoding Epo for the treatment of Epo-responsive anemias in both mice and non-human primates. Our results demonstrate that there is a threshold dose of virus (2.5-8 x 10(7) pfu/gram of body weight) which is required to obtain long-term Epo expression and polycythemia in both species. A single i.m. injection of mice with 10(9) pfu of an RDAd encoding murine Epo (AdmEpo) resulted in elevations in hematocrits from control values of 49 +/- 0.9% to treated values of 81 +/- 3%, which were stable for more than 1 year. Similarly, a single i.m. injection of a monkey with 4 x 10(11) pfu of an RDAd-encoding simian Epo (AdsEpo) resulted in elevations of hematocrits from control levels of 40% to treated levels of > or =70%, which were stable for 84 days. Intramuscular injection of monkeys with AdsEpo appeared to be safe in that we did not detect abnormalities in chest X-rays, serum chemistries, hematologic, or clotting profiles (apart from elevated hematocrits) or organ histologies during the 84-day time course of the experiment. Taken together, these results suggest the feasibility of using i.m. injection of RDAd for the treatment of Epo-responsive anemias in humans.

Novel retroviral vector transferring a suicide gene and a selectable marker gene with enhanced gene expression by using a tetracycline-responsive expression system

A retroviral vector for the enhanced expression of the herpes simplex virus thymidine kinase (HSV tk) gene was developed by using a tetracycline-responsive expression system (TRES). The two components of the TRES, the chimeric transactivator (tTA) and the corresponding tTA-binding cis element (tetO), were both incorporated into a retroviral vector and resulted in high levels of tk gene expression from tetO in target cells. Amphotropic virus supernatants from stable producer cells, generated by the retroviral vector containing the TRES, gave titers of 10(4) to 10(5) G418-resistant CFU/ml on murine NIH 3T3 cells. The retroviral vector (G1tTA-[tetOTkINa]R), in which tetO was used in the opposite orientation relative to viral transcription, was capable of transducing tk and neo genes into murine NIH 3T3 cells to yield a high level of tk gene expression. TK enzyme activity in NIH 3T3 cells transduced by this vector was 417-fold higher than in control cells. This increased TK activity was returned to basal levels in the presence of tetracycline. The level of tk gene expression driven by tetO from G1tTA-[tetOTkINa]R vector in NIH 3T3 cells was fourfold higher at both the mRNA level and the TK enzyme level than that produced by the long terminal repeat of G1Tk1SvNa, the vector being used in the ongoing brain tumor gene therapy trial. Retroviral vectors containing the TRES may be useful therefore in achieving higher levels of tk gene expression, which should facilitate gene therapy approaches in the treatment of cancer.

Doxycycline-mediated quantitative and tissue-specific control of gene expression in transgenic mice

The tet regulatory system in which doxycycline (dox) acts as an inducer of specifically engineered RNA polymerase II promoters was transferred into transgenic mice. Tight control and a broad range of regulation spanning up to five orders of magnitude were monitored dependent on the dox concentration in the water supply of the animals. Administration of dox rapidly induces the synthesis of the indicator enzyme luciferase whose activity rises over several orders of magnitude within the first 4 h in some organs. Induction is complete after 24 h in most organs analyzed. A comparable regulatory potential was revealed with the tet regulatory system where dox prevents transcription activation. Directing the synthesis of the tetracycline-controlled transactivator (tTA) to the liver led to highly specific regulation in hepatocytes where, in presence of dox, less than one molecule of luciferase was detected per cell. By contrast, a more than 10(5)-fold activation of the luciferase gene was observed in the absence of the antibiotic. This regulation was homogeneous throughout but stringently restricted to hepatocytes. These results demonstrate that both tetracycline-controlled transcriptional activation systems provide genetic switches that permit the quantitative control of gene activities in transgenic mice in a tissue-specific manner and, thus, suggest possibilities for the generation of a novel type of conditional mutants.

Myoblast-mediated expression of colony stimulating factor-1 (CSF-1) in the cytokine-deficient op/op mouse

The osteopetrotic (op/op) mouse lacks colony stimulating factor-1 (CSF-1) due to an inactivating mutation in the CSF-1 gene. Intramuscular transplantation of engineered myoblasts was used to introduce CSF-1 into the circulation of op/op mice. The CSF-1 cDNA was introduced into C2C12 mouse myoblasts in culture using retroviral mediated gene transfer. Upon transplantation into the skeletal muscle of mutant mice, physiological levels of the cytokine were achieved systemically and elicited a biological response: circulating monocytes were induced. Howvever, both circulating CSF-1 levels and the induction of monocytes were transient. Analysis of the site of cell transplantation revealed local changes that may account for the transience of serum cytokine levels. Macrophage markers were induced in muscle tissue implanted with CSF-1 expressing myoblasts: c-fms, the CSF-1 receptor as well as the lineage-restricted antigen F4/80. We propose that in addition to CSF-1 clearance by Kupffer cells of the liver, macrophages that accumulated at the site of cell transplantation bound the CSF-1 produced by the muscle cell transplants, precluding the sustained release of this cytokine into the systemic circulation. Our studies also revealed that damage to muscle caused during cell transplantation or by freeze injury resulted in the accumulation of macrophages in op/op mouse muscle tissue. Indeed, op/op mice were fully capable of regenerating injured muscle suggesting the presence of as yet unidentified CSF-1-independent factors capable of generating macrophages that presumably participate in tissue remodeling in this cytokine-deficient mouse.

Rapid retroviral delivery of tetracycline-inducible genes in a single autoregulatory cassette

We describe a single autoregulatory cassette that allows reversible induction of transgene expression in response to tetracycline (tet). This cassette contains all of the necessary components previously described by others on two separate plasmids that are introduced sequentially over a period of months [Gossen, M. & Bujard, H. (1992) Proc. Natl. Acad. Sci. USA 89, 5547-5551]. The cassette is introduced using a retrovirus, allowing transfer into cell types that are difficult to transfect. Thus, populations of thousands of cells, rather than a few clones, can be isolated and characterized within weeks. To avoid potential interference of the strong retroviral long terminal repeat enhancer and promoter elements with the function of the tet-regulated cytomegalovirus minimal promoter, the vector is self-inactivating, eliminating transcription from the long terminal repeat after infection of target cells. Tandem tet operator sequences and the cytomegalovirus minimal promoter drive expression of a bicistronic mRNA, leading to transcription of the gene of interest (lacZ) and the internal ribosome entry site controlled transactivator (Tet repressor-VP16 fusion protein). In the absence of tet, there is a progressive increase in transactivator by means of an autoregulatory loop, whereas in the presence of tet, gene expression is prevented. Northern blot, biochemical, and single cell analyses have all shown that the construct yields low basal levels of gene expression and induction of one to two orders of magnitude. Thus, the current cassette of the retroviral construct (SIN-RetroTet vector) allows rapid delivery of inducible genes and should have broad applications to cultured cells, transgenic animals, and gene therapy.