Here, we present a protocol for the modulation of transgene expression using pEUI(+) singular gene switch by tebufenozide treatment.
Precise control of transgene expression is desirable in biological and clinical studies. However, because the binary feature of currently employed gene switches requires the transfer of two therapeutic expression units concurrently into a single cell, the practical application of the system for gene therapy is limited. To simplify the transgene expression system, we generated a gene switch designated as pEUI(+) encompassing a complete set of transgene expression modules in a single vector. Comprising of the GAL4 DNA-binding domain and modified EcR (GvEcR), a minimal VP16 activation domain fused with a GAL4 DNA-binding domain, as well as a modified Drosophila ecdysone receptor (EcR), the newly developed singular gene switch is highly responsive to the administration of a chemical inducer in a time- and dosage-dependent manner. The pEUI(+) vector is a potentially powerful tool for improving the control of transgene expression in both biological research and pre-clinical studies. Here, we present a detailed protocol for modulation of a transient and stable transgene expression using pEUI(+) vector by the treatment of tebufenozide (Teb). Additionally, we share important guidelines for the use of Teb as a chemical inducer.
Several different gene switches have been explored for their ability to precisely regulate transgene expression in cell culture and animal models, with varying degrees of success1,2,3. Potential gene switch systems should meet several stringent criteria, including: precise directional expression of the transgene, dosage- and time-dependent responsiveness to inducers, negligible leakiness of the promoter, reversibility of transgene activation through inducer removal, and inducer toxicity levels tolerable to cell lines and laboratory animals1,2,3. Several small molecule inducers have been exploited, including tetracycline4,5, rapamycin6,7,8, mifepristone9,10, and ecdysone11,12,13,14. In general, these systems require at least two plasmids containing two or three discrete expression units, one or two of which compose a regulatory element (inducer plasmid) that binds a small molecule inducer to gain transcriptional activity; and another plasmid (effector plasmid) containing the transgene under the control of a regulatory DNA region that allows the access of the inducer-bound regulatory element. The main drawback of the binary system is that it requires the concomitant introduction of two vectors (inducer and effector) into the target cell. In transient transgene expression experiments, the simultaneous transfection of two plasmids inevitably produces a population of cells that is singly transfected with either the inducer or the effector, or co-transfected unequally. In addition, the binary system requires at least two rounds of antibiotic selection to establish stable cell lines, which is time-consuming and laborious. Thus, combining all of the components required for transgene expression and regulation into a single vector would be ideal to guarantee the concomitant expression of all the required elements in a single cell and allow for the regulation of transgene expression through treatment with small molecule inducers.
To overcome the shortcomings of binary gene switches, we recently developed a singular gene switch, using a Drosophila melanogaster EcR-based gene inducible system15. Ecdysone mediates gene expression when it binds to the EcR/ultraspiracle (USP) heterodimer, which in turn induces binding of the EcR to DNA regulatory elements3,11. The vertebrate retinoid X receptor (RXR), an ortholog of insect USP, recruits EcR to form an active transcriptional activator16. Thus, for the targeted expression of a transgene in vertebrate cells or tissue, the EcR and RXR must be co-expressed simultaneously before being stimulated by ecdysone or its agonists. Since the heterodimeric feature of the EcR-based gene switch can be influenced by the endogenous RXR level, Padidam et al. replaced the EcR DNA-binding and activation domains with heterologous GAL4 DNA-binding, herpes simplex virus protein vmw65 (VP16) activation domains fused to the EcR ligand-binding domain, which then became unresponsive to endogenous RXR and formed a homodimer to gain transcriptional activity17. The EcR-based gene switch was further improved by constructing a chimeric protein comprised of minimal VP16 activation domain combined with GvEcR, which formed a homodimer and localized in the cytosol in the absence of the ecdysone agonist, Teb16,18. By binding to Teb, the GvEcR changed its subcellular localization from the cytosol to the nucleus to recognize a hybrid promoter comprised of zebrafish E1b minimal promoter combined with a ten tandem repeated upstream activation sequences (UASs) to initiate the transcription of the target gene16.
To simplify the previously reported EcR-based gene switches16,18, we combined the binary features of the system into a single vector equipped with all the required elements for stimulating transgene expression, and then designated the newly generated vector, pEUI(+) (GenBank accession number: KP123436, Figure 1A)15. The effector responding to the GvEcR driver (hereafter driver) consists of ten tandem UAS repeats next to an E1b minimal promoter followed by a multiple cloning site (MCS) with EcoRI, PmlI, NheI, BmtI, AfeI, and AflII recognition sequences (Figure 1B). Additionally, to facilitate the selection of transfected cells, an SV40 promoter-driven puromycin N-acetyl-transferase (PAC) gene was placed between the driver and effector regions to maintain stable cell lines (Figure 1).
We describe a detailed protocol for the transient and stable modulation of transgene expression using pEUI(+). In addition, we provide detailed instructions for the successful use of Teb as an ecdysone agonist.
1. Transgene Subcloning
2. Transient Transfection and Teb Treatment
3. Generation of Stable HEK293 Cell Lines with Genomic Integration of the pEUI(+) Gene Switch
4. Depletion of Teb
The GvEcR-based singular gene switch is depicted in Figure 1A. The pEUI(+) vector was optimized for regulated transgene expression by the treatment with Teb. The effector region of pEUI(+) comprises a 10xUAS and an E1b minimal promoter followed by an MCS containing EcoRI, PmlI, NheI, BmtI, AfeI, and AflII restriction enzyme recognition sites. An SV40 poly(A) signal was added behind the MCS (Figure 1B). A PAC gene was inserted between the driver and effector regions of pEUI(+) to confer resistance to puromycin.
To evaluate the activity and leakiness of pEUI(+), we subcloned EGFP into the EcoRI site of the MCS (pEUI(+)-EGFP), transiently transfected the plasmid construct into HEK293 cells, and administered Teb at different concentrations for 24 h (Figure 2). While mock vector transfectants did not show fluorescent signals regardless of Teb treatment (Figure 2A, B), pEUI(+)-EGFP transfectants became progressively sensitized to the increased amount of Teb (Figure 2C–F). More importantly, pEUI(+)-EGFP transfectants without treatment of Teb did not elicit any detectable EGFP signals under a fluorescent microscope (Figure 2C).
To further validate the responsiveness of pEUI(+) to Teb, we subcloned a FLAG epitope-tagged ANKRD13A gene into the EcoRI site of pEUI(+), transferred the construct into HEK293 cells, and then subjected them to puromycin selection for 3 weeks to establish a desired cell line with the ANKRD13A transgene. We analyzed the expression of the ANKRD13A transgene after a 24 h treatment with Teb and the established cell line responded well (Figure 3). We further demonstrated the reversibility of the pEUI(+) gene switch by depleting Teb from the cultural media. As shown in Figure 3, ANKRD13A expression was no longer detectable when we cultured the cells in Teb-free media.
Figure 1. Schematic diagram of pEUI(+). (A) The vector map of pEUI(+) was constructed. A CMV promoter drives constitutive expression of GvEcR. The Teb-bound GvEcR will translocate into the nucleus and bind the 10xUAS cis-acting regulatory sequences to stimulate expression of the transgene inserted into the MCS. (B) The sequence information between the square brackets in (A) contains the entire effector sequences of pEUI(+). Arrows represent the individual restriction enzyme recognition sites included in the MCS. Please click here to view a larger version of this figure.
Figure 2. GvEcR-based singular gene switch responds to the treatment of Teb. HEK293 cells were transfected with the specified DNA constructs. After 24 h of transfection, the cells were stimulated for 24 h with Teb at the indicated concentration. EGFP transgene activity was observed under a fluorescent microscope. Mock transfectants with pEUI(+) did not show any detectable fluorescence with (A) or without (B) Teb treatment. (C–E) The fluorescent intensity in pEUI(+)-EGFP-transfected cells increases in a Teb dosage-dependent manner. Scale bar, 10 µm. Please click here to view a larger version of this figure.
Figure 3. HEK293 cells stably harboring pEUI(+)/FLAG-ANKRD13A respond reversibly to the administration of Teb. FLAG epitope-tagged ANKRD13A expression was triggered by Teb (1.5 µM) treatment. After 24 h of treatment with Teb, the cells were switched to Teb drop-out media for the indicated amount of time. The relative amount of ANKRD13A was measured by Western blotting with anti-FLAG antibody. The endogenous expression level of α-tubulin was measured with anti-α-tubulin antibody as a control. The white asterisk indicates an unidentified cross-reactive species. Please click here to view a larger version of this figure.
The main drawback of using a binary gene expression switch is the need to simultaneously deliver two separate plasmids (driver and effector) into the target cell. This can lead to an unequal distribution of plasmids and result in an inconsistent response of the switch to the inducers1,2,3. Another shortcoming of the two-vector system is that a minimum of two rounds of antibiotic selection are required to identify promising cell lines. Therefore, a gene-inducible cassette comprising one unit has long been sought for use in biological research. The pEUI(+) singular transgene expression switch is contained in a single vector equipped with all regulatory units required for transgene stimulation. Thus, the induction level of the transgene, subcloned in pEUI(+), is dependent on the amount of DNA transfected in the transient transfection experiment and dosage of the chemical inducer. In addition, we further tested the usage of pEUI(+) vector by generating a stable cell line containing a single copy of the ANKRD13A transgene15. The established cell line showed high sensitivity to Teb treatment (Figure 3). Collectively its high sensitivity, reversibility, and low leakiness15, along with limited variation in expression level make pEUI(+) a valuable molecular and cellular tool for manipulating target gene expression.
Among various inducible gene systems, we selected the GvEcR-dependent gene induction cassette. The GvEcR has several advantages over other systems: first, the lipophilic nature of several ecdysone analogs used as chemical inducers (including Teb) is advantageous for efficient penetration of cellular membranes21; second, there are no EcR orthologs in vertebrates, so ecdysteroid agonists do not influence the function of endogenously expressed nuclear receptors21,22; third, ecdysteroids are innocuous in vertebrates and rapidly cleared from the circulation system in vivo21,23; and fourth, since numerous ecdysone agonists have been developed thus far, by choosing optimally-sized small molecules, researchers can avoid undesirable complications13,22,24. Although the pEUI(+) shows high sensitivity to Teb, testing the responsiveness of pEUI(+) to other ecdysteroid agonists may prove valuable. The existence of non-steroidal EcR agonists, such as methoxyfenozide, that are more soluble in water than Teb24 may widen the scope of applications of pEUI(+). Though the expression of transgene in pEUI(+) is relatively weaker than that in a Tet-On system15, the level of transgene induction could be augmented by the addition of a multi-copy of the VP16 transactivation domain (data not shown). Recent gene therapy applications have focused on the safe delivery of a target gene into cells or tissues that lack the respective functional gene product owing to genetic disease25,26,27. However, non-regulated transgene under the control of tissue-specific or viral constitutive promoters can elicit hyper-expression of the target gene, which elevates the possibility of local tissue damage28,29. Thus, the application of a reliable gene switch for gene therapy could avoid unwanted complications. The pEUI(+) vector provides a convenient and powerful tool to control transgene expression in biological and clinical studies. Currently, we are developing a singular lentiviral vector based on GvEcR to increase the application range of our system.
The authors have nothing to disclose.
The authors thank S-Y Choi (Chonnam National University) for valuable comments and thorough reading of our manuscript. This work was supported by a research fund of Chungnam National University.
pEUI(+) | TransLab | The patent license was transferred to TransLab Inc. | |
Gene-Fect Transfection Reagent | TransLab | TLC-001 | |
HEK293 | ATCC | CRL-1573 | |
Tebufenozide | Fluka | 31652 | |
Cloning cylinder | Sigma | CLS31668 | |
Puromycin | Corning | 58-58-2 | |
Antibiotics | Gibco | 15240-062 | |
Trypsin-EDTA | Welgene | LS015-01 | |
DMEM | Welgene | LM001-05 | |
Fetal Bovine Serum | Welgene | S001-01 | |
Cell culture dish | SPL life science | 11090 | |
mouse FLAG M2 | Sigma | F3165 | |
anti-alpha tubulin antibody | Calbiochem | CP06 | |
Cloning cylinder | Sigma | CLS31668 | |
NucleoBond Xtra Midi | Macherey-Nagel | 740410.1 | |
M-PER Mammalian Protein Extraction Reagent |
Thermo Fisher | 78505 | |
100X Protease inhibitor Cock. III | T&I | BPI-9200 |