Inducible inhibition of eukaryotic gene expression

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.

Genetic toxicities of human teratogens

Birth defects cause a myriad of societal problems and place tremendous anguish on the affected individual and his or her family. Current estimates categorize about 3% of all newborn infants as having some form of birth defect or congenital anomaly. As more precise means of detecting subtle anomalies become available this estimate, no doubt, will increase. Even though birth defects have been observed in newborns throughout history, our knowledge about the causes and mechanisms through which these defects are manifested is limited. For example, it has been estimated that around 20% of all birth defects are due to gene mutations, 5-10% to chromosomal abnormalities, and another 5-10% to exposure to a known teratogenic agent or maternal factor [D.A. Beckman, R.L. Brent, Mechanisms of teratogenesis. Ann. Rev. Pharmacol. Toxicol. 24 (1984) 483-500; K. Nelson, L.B. Holmes Malformations due to presumed spontaneous mutations in newborn infants, N. Engl. J. Med. 320 (1989) 19-23.]. Together, these percentages account for only 30-40%, leaving the etiology of more than half of all human birth defects unexplained. It has been speculated that environmental factors account for no more than one-tenth of all congenital anomalies [D.A. Beckman, R.L. Brent, Mechanisms of teratogenesis, Ann. Rev. Pharmacol. Toxicol. 24 (1984) 483-500]. Furthermore, since there is no evidence in humans that the exposure of an individual to any mutagen measurably increases the risk of congenital anomalies in his or her offspring' [J.F. Crow, C. Denniston, Mutation in human populations, Adv. Human Genet. 14 (1985) 59-121; J.M. Friedman, J.E. Polifka, Teratogenic Effects of Drugs: A Resource for Clinicians (TERIS). The John Hopkins University Press, Baltimore, 1994], the mutagenic activity of environmental agents and drugs as a factor in teratogenesis has been given very little attention. Epigenetic activity has also been given only limited consideration as a mechanism for teratogenesis. As new molecular methods are developed for assessing processes associated with teratogenesis, especially those with a genetic or an epigenetic basis, additional environmental factors may be identified. These are especially important because they are potentially preventable. This paper examines the relationships between chemicals identified as human teratogens (agents that cause birth defects) and their mutagenic activity as evaluated in one or more of the established short-term bioassays currently used to measure such damage. Those agents lacking mutagenic activity but with published evidence that they may otherwise alter the expressions or regulate interactions of the genetic material, i.e. exhibit epigenetic activity, have likewise been identified. The information used in making these comparisons comes from the published literature as well as from unpublished data of the U.S. National Toxicology Program (NTP).

Conditional control of gene expression in the mammary gland

Identifying gene function during mammary gland development and function remains a technical challenge. For example, if a gene deletion is lethal during early embryogenesis, there is no opportunity to study its effects on the development or function of the gland. Similarly, if a dominant gain of gene function alters early mammary gland development, then its specific role during lactation cannot be assessed. Conditional gene expression systems can be used to circumvent these problems. Gene deletions or dominant gain experiments can be performed in an organ or cell type specific manner at specific timepoints using inducible gene expression systems. This review focuses on tetracycline responsive transactivation and Cre-lox systems. Other tetracycline regulatable (tet system) or hormone inducible systems and the Flp recombinase system are discussed as alternative approaches. Each system is described. The advantages and disadvantages of each for studying gene function in the mammary gland are discussed. Finally, the role of mammary gland transplantation in these genetic studies is examined.

Bi-directional gene switching with the tetracycline repressor and a novel tetracycline antagonist

We have screened a panel of tetracycline (tc)-like compounds for their potential use with tc-repressor (tetR) based gene switches. The interaction between tc and tetR appears quite specific, as only tc itself and its close homologues anhydro-tc and doxycycline strongly inhibited DNA binding. However, a single tc-like compound, GR33076X, increased DNA binding of the tetR-VP16 fusion protein, both in eukaryotic cells and in bacteria. We provide evidence that this antagonist of tetracycline is potentially useful for accelerated gene switching, especially in whole animals.

Creation of a reversible on/off system for site-specific in vivo control of exogenous gene activity in the renal glomerulus

Using genetically engineered glomerular mesangial cells, an in vivo gene transfer approach was developed that specifically targets the renal glomerulus. By combining this system with a tetracycline (Tc)-responsive promoter, the present study aimed to create a reversible on/off system for site-specific in vivo control of exogenous gene activity within the glomerulus. In the Tc regulatory system, a Tc-controlled transactivator (tTA) encoded by a regulator plasmid induces target gene transcription by binding to a tTA-responsive promoter located in a response plasmid. Tc inhibits this tTA-dependent transactivation via its affinity for tTA. In double-transfected cells, therefore, the activity of a transgene can be controlled by Tc. Cultured rat mesangial cells were cotransfected with a regulator plasmid and a response plasmid that introduces a beta-galactosidase gene. In vitro, stable double-transfectant MtTAG cells exhibited no beta-galactosidase activity in the presence of Tc. However, following withdrawal of Tc from culture media, expression of beta-galactosidase was induced within 24 h. When Tc was again added, the expression was rapidly resuppressed. Low concentrations of Tc were sufficient to maintain the silent state of tTA-dependent promoter. MtTAG cells were then transferred into the rat glomeruli via renal artery injection. In the isolated chimeric glomeruli, expression of beta-galactosidase was induced ex vivo in the absence of Tc, whereas it was repressed in its presence. When Tc-pretreated MtTAG cells were transferred into the glomeruli of untreated rats, beta-galactosidase expression was induced in vivo within 3 days. Oral administration of Tc dramatically suppressed this induction. These data demonstrate the feasibility of using mesangial cell vectors combined with the Tc regulatory system for site-specific in vivo control of exogenous gene expression in the glomerulus.