A combination of derepression of the lac operator-repressor system with positive induction by glucocorticoid and metal ions provides a high-level-inducible gene expression system based on the human metallothionein-IIA promoter

Mammalian synthetic biology: emerging medical applications

In this review, we discuss new emerging medical applications of the rapidly evolving field of mammalian synthetic biology. We start with simple mammalian synthetic biological components and move towards more complex and therapy-oriented gene circuits. A comprehensive list of ON-OFF switches, categorized into transcriptional, post-transcriptional, translational and post-translational, is presented in the first sections. Subsequently, Boolean logic gates, synthetic mammalian oscillators and toggle switches will be described. Several synthetic gene networks are further reviewed in the medical applications section, including cancer therapy gene circuits, immuno-regulatory networks, among others. The final sections focus on the applicability of synthetic gene networks to drug discovery, drug delivery, receptor-activating gene circuits and mammalian biomanufacturing processes.

Towards evolving a better repressor

Transcriptional regulation is an essential component of all metabolic pathways. At the most basic level, a protein binds to a particular DNA sequence (operator) on the genome and either positively or negatively alters the level of transcription. Together, the protein and its operator form an epigenetic switch that regulates gene expression. In an effort to produce a 'better' switch, we have discovered novel facets of the lac operon that are responsible for optimal functionality. We have uncovered a relationship between operator binding affinity and inducibility and demonstrated that the operator DNA is not a passive component of a genetic switch; it is responsible for establishing binding affinity, specificity as well as translational efficiency. In addition, an operator's directionality can indirectly affect gene expression. Unraveling the basic properties of this classical epigenetic switch demonstrates that multiple factors must be optimized in designing a better switch.

Regulation of transgene expression using an inducible system for improved safety of intervertebral disc gene therapy

STUDY DESIGN

Human nucleus pulposus cells (NPCs) were transduced with an adenoviral vector that expresses Fas Ligand (FasL) and green fluorescent protein (GFP) under the control of a tetracycline-regulated gene expression system to test the transgene control.

OBJECTIVES

To describe the application of a Tet-off gene regulation system for intervertebral disc (IVD) gene therapy.

SUMMARY OF BACKGROUND DATA

Gene therapy has proven its ability to beneficially modulate the biologic processes of the IVD cells in vitro and in vivo. However, we have observed that expression of transgenic growth factors outside the IVD in the event of a misdirected injection has potentially detrimental consequences (e.g., toxicity). To date, a safety system that allows the control transgene expression has not been produced for intradiscal gene therapy.

METHODS

Human NPCs were transduced with Ad/FasL-GFPTET, at 0, 50, 100, and 200 MOI. After 1 day (time 0) cells were cultured in the presence of tetracycline (1, 10, 100 mg/L) for 3 days, and then tetracycline was withdrawn. The transgene expression was evaluated either daily by flow cytometry (from time 0 to day 6) or by imaging the GFP signal (time 0, day 3 and day 9).

RESULTS

NPC expression of GFP 1 day after transduction was proportional to the MOI used. GFP expression was decreased after 3 days of tetracycline administration at all concentrations used. The expression of GFP recovered after removal of tetracycline.

CONCLUSIONS

The transgene expressed by the transduced NPC was efficiently regulated by inclusion of tetracycline in culture media. The presence of tetracycline turns off the protein expression and the subsequent absence allows it to recover again, demonstrating the ability to control gene expression in NPCs. Therefore, we propose a Tet-off inducible system as an efficient tool for modulating the transgene expression to avoid the toxicity that could result from a missed injection.

Intestinal homeostasis and neoplasia studied using conditional transgenesis

Constitutive mouse models of intestinal neoplasia, such as the Apc(min/+) (multiple intestinal neoplasia) mouse have proven valuable tools both for furthering our understanding of tumorigenesis and for the development of therapeutic strategies. However, the in vivo study of a number of genes has been precluded by their absolute requirement during embryonic development. This has led to the development of conditional strategies that allow gene regulation in vivo. This review describes the principal techniques used to achieve conditional transgenesis within the mouse intestine, with a particular focus upon the Cre-Lox and Tet-regulable systems. Further, we discuss how these techniques are being used to dissect the mechanisms governing both normal homeostasis and neoplastic development within the intestine.

Differential effects of c-Ras upon transformation, adipocytic differentiation, and apoptosis mediated by the simian virus 40 large tumor antigen

To investigate the functional relationship between the ability of the simian virus 40 large tumor antigen (TAg) to transform and its ability to block adipocytic differentiation and induce apoptosis, we expressed TAg in C3H10T1/2 (10T1/2)-derived preadipocytes. The results demonstrated that differentiation could be suppressed at lower TAg levels than at the levels required for full neoplastic conversion. Progressively higher TAg levels were accompanied by apoptosis induction in this system. To further examine the role of the cellular Ras protooncogene product (Ras) in TAg function, TAg was expressed in 10T1/2-derived preadipocytes rendered deficient in Ras activity by transfection with inducible or constitutive antisense ras gene constructs. The results indicated that Ras is required for TAg-mediated transformation and for suppression of adipocytic differentiation, while TAg-mediated apoptosis following serum starvation was independent from Ras action. Unexpectedly, our results further demonstrated a dramatic reduction in the levels of the TAg protein itself as differentiation progressed in Ras-knockdown cells, with a concomitant reduction in TAg's ability to induce apoptosis as a result. These findings suggest that Ras, although cytoplasmic, is an integral component of the pathway whereby TAg, an oncoprotein believed to have primarily nuclear targets, suppresses differentiation or induces neoplastic conversion of murine preadipocytes.

Adenovirus E1A requires c-Ras for full neoplastic transformation or suppression of differentiation of murine preadipocytes

We recently demonstrated that the Adenovirus-5 E1A gene products (E1A), known E2F activators, can block the differentiation of murine preadipocytes and that differentiation suppression occurs at lower levels than required for full neoplastic transformation. Progressively higher levels were accompanied by apoptosis induction. To examine the role of the cellular Ras protooncogene product (Ras) in E1A function, E1A was expressed in C3H10T(1/2) (10T(1/2))-derived preadipocytes rendered deficient in Ras activity by transfection with inducible or constitutive antisense ras gene constructs (Ras-knockdowns). The results showed that, although even low amounts of E1A could block the differentiation of 10T(1/2) preadipocytes with normal Ras levels, even the highest E1A levels were unable to block the differentiation or induce transformation of Ras-knockdown preadipocytes. Ras downregulation did not affect E2F activation by E1A. Interestingly, our results further demonstrated a dramatic reduction in the levels of the E1A protein itself as differentiation progressed, with a concomitant reduction in E1A's ability to induce apoptosis as a result. These findings suggest for the first time that Ras, although cytoplasmic, is an integral component of the pathway whereby E1A, an oncoprotein believed to have primarily nuclear targets, suppresses differentiation or induces neoplastic conversion of murine preadipocytes.

Engineering a root-specific, repressor-operator gene complex

Strong, tissue-specific and genetically regulated expression systems are essential tools in plant biotechnology. An expression system tool called a 'repressor-operator gene complex' (ROC) has diverse applications in plant biotechnology fields including phytoremediation, disease resistance, plant nutrition, food safety, and hybrid seed production. To test this concept, we assembled a root-specific ROC using a strategy that could be used to construct almost any gene expression pattern. When a modified E. coli lac repressor with a nuclear localization signal was expressed from a rubisco small subunit expression vector, S1pt::lacIn, LacIn protein was localized to the nuclei of leaf and stem cells, but not to root cells. A LacIn repressible Arabidopsis actin expression vector A2pot was assembled containing upstream bacterial lacO operator sequences, and it was tested for organ and tissue specificity using beta-glucuronidase (GUS) and mercuric ion reductase (merA) gene reporters. Strong GUS enzyme expression was restricted to root tissues of A2pot::GUS/S1pt::lacIn ROC plants, while GUS activity was high in all vegetative tissues of plants lacking the repressor. Repression of shoot GUS expression exceeded 99.9% with no evidence of root repression, among a large percentage of doubly transformed plants. Similarly, MerA was strongly expressed in the roots, but not the shoots of A2pot::merA/S1pt::lacIn plants, while MerA levels remained high in both shoots and roots of plants lacking repressor. Plants with MerA expression restricted to roots were approximately as tolerant to ionic mercury as plants constitutively expressing MerA in roots and shoots. The superiority of this ROC over the previously described root-specific tobacco RB7 promoter is demonstrated.

Optimizing regulatable gene expression using adenoviral vectors

Inducible gene expression systems have typically encountered limitations, such as pleitropic effects of the inducer, basal leakiness, toxicity of inducing agents and low levels of expression. However, recently non-toxic, tightly regulated control of transgene expression has been reported for several systems, the most frequently cited being the tetracycline gene control system. We have found that the individual components of the Tet system [the Tet transactivators and tetracycline responsive element (TRE)] function optimally to control gene expression when they are incorporated into separate adenoviral vectors. Furthermore, incorporation of the Woodchuck hepatitis virus post-transcriptional enhancer (WPRE) allows a dual vector Tet-regulatable Ad system to be used at very low titres (2 x 10(4)) that elicit a minimal inflammatory response, with no loss of transgene expression or ability to regulate transgene expression. This and similar regulatable systems will benefit studies investigating neuronal gene function and those seeking to develop effective neuronal gene therapy strategies.

Tetracycline-regulated gene expression mediated by a novel chimeric repressor that recruits histone deacetylases in mammalian cells

Regulated gene expression will provide important platforms from which gene functions can be investigated and safer means of gene therapy may be developed. Histone deacetylases have recently been shown to play an important role in regulating gene expression. Here we investigated whether a more tightly controlled expression could be achieved by using a novel chimeric repressor that recruits histone deacetylases to a tetracycline-responsive promoter. This chimeric repressor was engineered by fusing the tetracycline repressor (TetR) with an mSin3-interacting domain of human Mad1 and was shown to bind the tetO(2) element with high affinity, and its binding was efficiently abrogated by doxycycline. The chimeric repressor was shown to directly interact with mSin3 of the histone deacetylase complex. This inducible system was further simplified by using a single vector that contained both a chimeric repressor expression cassette and a tetracycline-responsive promoter. When transiently introduced into mammalian cells, the chimeric repressor system exhibited a significantly lower basal level of luciferase activity (up to 25-fold) than that of the TetR control. When stably transfected into HEK 293 cells, the chimeric repressor system was shown to exert a tight control of green fluorescent protein expression in a doxycycline dose- and time-dependent fashion. Therefore, this novel chimeric repressor provides an effective means for more tightly regulated gene expression, and the simplified inducible system may be used for a broad range of basic and clinical studies.

v-Ras and v-Raf block differentiation of transformable C3H10T1/2-derived preadipocytes at lower levels than required for neoplastic transformation

To investigate the functional relationship between the transforming ability of Ras and its role as an integral component of the differentiative insulin signaling pathway, we introduced a leu61-activated ras gene into a Ras-transformable, C3H10T1/2-derived preadipocytic cell line. The results demonstrate that rasleu61 expression in this line blocks differentiation and that this block appears at lower levels than required for full neoplastic transformation. In addition, to examine whether the inability of Rasleu61 to induce differentiation by replacing the insulin signal could be attributed to its transforming effect in this system, we examined the effect of Rasleu61 at levels below the baseline, by expressing rasleu61 in a series of preadipocytes which were rendered deficient in endogenous c-Ras activity. The results show that even very low Rasleu61 levels, insufficient to restore the growth rate of these cells to normal, blocked rather than enhanced differentiation, indicating that rasleu61 expression alone is not sufficient to promote adipocytic differentiation in this system, even in the absence of neoplastic transformation. Consistent with its established role as a downstream effector of Ras, v-Raf expression mirrored the v-Ras effects upon adipocytic differentiation and transformation.

The inducible lactose operator-repressor system is functional in the whole animal

Mouse liver cell lines that bear a stably integrated lactose operon repressor (lacI) gene and a Ha-ras gene linked to a lactose operator-containing SV40 early promoter were generated. When grown in medium containing more than 0.1 mM isopropyl beta-D-thiogalactoside (IPTG), the Ha-ras gene was induced up to 20-fold. Maximum induction of Ha-ras gene expression occurred after 12 h of exposure. The tumorigenicity of these cell lines in syngeneic mice was enhanced when the mice were maintained on drinking water containing 12.5 mM IPTG. Ha-ras gene expression in tumors was strongly induced in the presence of IPTG in vivo. Induction of Ha-ras gene expression in mice was consistently observed after 48 hr of exposure to drinking water containing IPTG. This system provides an approach for studying the function of oncogene in vivo as well as other genes of interest.

Regulation of the CYP1A1 promoter in transgenic mice: an exquisitely sensitive on-off system for cell specific gene regulation

Mammalian cytochrome P-450s in the CYP1A gene family catalyse the oxidation of a wide range of drugs and foreign compounds resulting in their excretion. These enzymes are highly inducible by a range of compounds, including polycyclic aromatic hydrocarbons such as 3-methylcholanthrene (3-MC) and dioxins. Analysis of the CYP1A1 promoter has identified dioxin responsive enhancer elements which mediate the induction response. In order to evaluate this promoter as an in vivo regulatable expression system and to gain further insights into the tissue specific regulation of this gene, an 8.5 kb genomic fragment of the rat CYP1A1 promoter was cloned upstream of the lacZ reporter gene. This construct was used to generate transgenic mice and three independent lines were expanded for further study. The regulation of beta-galactosidase expression was determined in mock and 3-MC-treated mice in an extensive range of tissues. In untreated animals no transgene expression was detectable over non-transgenic controls. Treatment with 3-MC caused a profound increase in transgene expression (> 1,000-fold) in many tissues including liver, adrenal, kidney and intestine. Inducible transgene expression was also detectable in many of the other tissues including the spleen, lung, pancreas and the reproductive organs. Although the absolute levels of induction varied, no significant differences in the pattern of transgene expression were observed between the three different transgenic mouse lines. In addition, the pattern of transgene expression correlated closely with the reported regulation of CYP1A1 protein. These results indicate that the CYP1A1 promoter can drive expression of heterologous genes in a truly on/off manner in a variety of tissues and cell types which will allow the expression of other proteins to be controlled in vivo. This reporter system also provides a model for establishing the environmental and hormonal factors regulating the expression of the CYP1A1 gene.

Ras is involved in gap junction closure in proliferating fibroblasts or preadipocytes but not in differentiated adipocytes

A decrease in gap junctional, intercellular communication (GJIC) has been associated with cells neoplastically transformed by a variety of factors. To investigate the role of the Ras oncogene product in gap junction function, a panel of murine C3H10T1/2 (10T1/2) fibroblasts was constructed in which the levels of ras gene expression could be effectively up- or down-regulated. Intercellular communication was measured using a novel technique of in situ electroporation of adherent cells on a partly conductive slide. The introduction of increasing amounts of activated Ras(leu61) in mouse 10T1/2 fibroblasts proportionally reduced GJIC, while the downregulation of endogenous c-ras gene expression increased junctional permeability. These results indicate that Ras plays an important role in the junction closure pathway leading to the proliferation of normal cells. However, differentiation of c-Ras-deficient preadipocytes entirely abolished their initially extensive GJIC, indicating that junction closure in response to adipocytic differentiation is independent of Ras.

Cellular Ras partly mediates gap junction closure by the polyoma virus middle tumor antigen

Endogenous, cellular Ras proteins (c-Ras) mediate the transforming action of the polyoma virus middle Tumor antigen (mT), which is accompanied by elimination of gap junctional, intercellular communication (GJIC). In this report we show that reducing the c-Ras content of murine C3H10T1/2 fibroblasts (10T1/2) through the expression of an anti-sense ras gene, increased GJIC by 60-80% mT totally eliminated GJIC in normal 10T1/2 cells but it reduced GJIC no more than 50% in the c-Ras deficient lines. These results indicate that endogenous c-Ras is at least partly responsible for the mT-induced gap junction closure.

Parameters affecting the use of the lac repressor system in eukaryotic cells and transgenic animals

Elements of the lactose operon were used to study parameters affecting gene expression in cultured cells and transgenic animals. A Lac repressor protein containing a nuclear transport signal was shown to inhibit expression of a reporter gene by interacting with lac operator sequences. In cultured cells, operator sequence, operator placement and induction parameters were all shown to be important for obtaining tight repression of a reporter gene followed by high level expression upon induction. Induction levels were also dependent on the reporter gene, with the luciferase gene yielding higher induction levels than the chloramphenicol acetyltransferase gene. In transgenic animals, the lacI mRNA was not detected in the C57BL/6 mouse strain until the animal was exposed to a demethylating agent. After 5-azacytidine treatment, expression of lacI mRNA was detected in the brain, heart, kidney, lung and ovary. In the FVB transgenic mouse strain, expression of lacI mRNA was detected without 5-azacytidine treatment in the kidney, liver, lung, and testes. Preliminary experiments with double transgenic animals containing both lacI and operator/luciferase transgenes showed a decrease in luciferase expression compared to the luciferase-only animals in both tissue extracts and transgenic fetal primary cultures, although IPTG induction was not achieved in these animals or primary cultures. The applicability and challenges of the system for regulation of gene expression are discussed.

An engineered PGK promoter and lac operator-repressor system for the regulation of gene expression in mammalian cells

Previous reports have demonstrated that the Escherichia coli lac repressor can operate effectively in mammalian cells to repress expression of genes driven by modified viral or metallothionein (MT) promoters. We have developed a more general expression system using the promoter from the PGK1 gene (encoding murine 3-phosphoglycerate kinase) which is widely expressed in almost all cell types, including early embryonic and ES (embryonic stem) cells. Firstly, we engineered the lac repressor to include a nuclear localisation signal and placed it under control of the PGK1 promoter. Efficient nuclear localisation of the repressor was demonstrated by mobility-shift assays and immunofluorescence detection. For the target vectors, we modified the wild-type (wt) PGK1 promoter to include lac operator (lacO) sites for binding of the lac repressor and compared a number of different lacO positions and arrangements based on proximity to the native start points for transcription (tsp) and translation. In the absence of repressor, we observed reduced expression of the neo reporter gene for some placements of the lacO, but wt expression for placements near the tsp. When both target and repressor were present in the cells, we observed that the expression of neo could be strongly suppressed and reversibly regulated by induction with IPTG. In particular, for a promoter which contained two spaced lacO replacing native sequence around the major tsp, we observed 90-95% repression by the lac repressor for the neo reporter gene and up to 98% repression for the cat reporter gene. Efficient derepression by IPTG was observed in both cases.(ABSTRACT TRUNCATED AT 250 WORDS)

Inducible vectors for expression in mammalian cells

The most recent developments in mammalian cell inducible expression systems have involved the use of bacterial gene control elements and viral transactivator proteins. The combination of hybrid viral transactivator and bacterial repressor proteins, and simple chemical inducers can provide induction ratios of over 1000-fold. These developments will have applications in both cell-based research and the generation of transgenic animals.

Establishment of a human cell line for the detection of demethylating agents

To detect xenobiotics affecting the DNA methylation pattern we have established a human clonal cell line (A4/4) derived from the 293 epithelial cells. This cell line is stable after more than 1 year in culture and we report here its characteristics. The A4/4 cells carry the Escherichia coli lacI gene and the symmetric lacO sequence upstream of the beta-galactosidase coding gene (lacZ). Both sequences have been independently integrated into the genomic DNA. The lacZ gene is under the control of the metal-inducible mouse metallothionein-I (mMT-I) promoter. Vector integration followed by cell ageing in culture gave rise to the methylation of the 5'CpG3' sites in the mMT-I promoter and the 5' part of the lacZ gene. The reactivation of the lacZ gene by 5-azacytidine (5-Aza-CR) treatment allowed a tremendous lacZ expression which is correlated with demethylation in the mMT-I promoter and its neighboring regions. This enhanced transcription is easily quantified by measuring the beta-galactosidase activity in cell extracts. 5-Aza-CR, the specific isopropyl beta-D-thiogalactoside inducer, and heavy metal ions added together trigger an increase of the beta-galactosidase activity up to 600-fold over the basal level. These cells can be used for a rapid assessment of the demethylating potential of environmental chemicals.

Regulation of the Escherichia coli lac operon expressed in human cells

We have investigated the use of various Epstein-Barr virus (EBV)-based vectors bearing the two components of the Escherichia coli lac operator-repressor (lacO, lacI) complex. Our aim was to develop a model system of gene expression by looking at the transcription of the bacterial beta-galactosidase coding gene (lacZ) in 293 human embryonic kidney cells. Several vectors have been built carrying different promoters upstream of the lacI and lacZ genes and in which natural or synthetic operator sequences were inserted in the 5' part of the lacZ gene. In transient expression assays we achieved efficient lacZ gene repression which could be released by the specific inducer isopropyl beta-D-thiogalactoside (IPTG). A stable transformed cell line carrying two EBV-derived plasmids with the building blocks of the lac operator/repressor system was established. This cell line allowed us to achieve a wide range of lacZ gene regulation. In this cell line IPTG alone could remove the repression to trigger a 5-fold increase of lacZ expression. Heavy metal ions, which induced the mouse metallothionein I promoter located upstream of the lacZ gene, added together with IPTG gave rise to a 40-fold induction of lacZ expression.

Targeting the Escherichia coli lac repressor to the mammalian cell nucleus

We have previously shown that about 90% of total Escherichia coli lac repressor synthesized in mammalian cells is located in the cytoplasm [Hu and Davidson, Cell 48 (1987) 555-566]. To target a functional lac repressor to the nucleus, we mutated 10 nucleotides at the 3' end of the coding sequence, thus adding the nuclear localization signal of the simian virus 40 large-T antigen to the C terminus of the repressor. The mutant lacI gene and the wild-type (wt) gene, both in standard animal cell expression vectors, driven by the promoter of the Rous sarcoma virus long terminal repeat, were stably transfected into three rodent cell lines. In confirmation of our previous results, only about 10% of the wt repressor, but all of the mutant protein, was localized in the nucleus. DNase I footprint analyses showed that the mutant repressor retained the same operator DNA-binding specificity as wt repressor. Furthermore, both repressor-operator complexes could be dissociated by addition of isopropyl-beta-D-thiogalactopyranoside in vitro. However, the ratio of number of repressor molecules per nucleus that, by in vitro assay, could bind to the operator sequence to the number of monomer repressor polypeptides per nucleus, as determined by Western blotting, was about 1:4 for the wt repressor and about 1:30 for the mutant repressor. This suggests that: (a) the mutant repressor assembles into tetramers inefficiently; and/or (b) it has reduced binding affinity to the operator sequence; and/or (c) it has higher binding affinity to nonspecific DNA.