Introduction
For studies of gene function in higher eukaryotes or to generate transgenic disease models inducible gene expression systems are very powerful tools. The first attempts to develop such systems were based on components derived from endogenous eukaryotic regulatory systems, such as heat-shock, metallothionein and hormone dependent promoters. However, these systems have only been of limited use and were mostly restricted to the cell culture level due to pleiotropic effects elicited by the inducing agent interacting with components of the eukaryotic transcriptional machinery or overall inefficient regulation (Yarranton, 1992; Christopherson et al., 1992). These difficulties have led to investigate systems derived from evolutionary distant organisms. Among these Cre/lox, Flp/FRT (Sauer and Henderson, 1989; O'Gorman et al. 1991) and the tetracycline inducible gene switch (Tet Switch, Gossen and Bujard, 1992) are the most successful conditional systems. Cre/Lox and Flp/FRT are recombination-based systems, introducing irreversible genetic changes and consequently are "one-time switches" which do not allow multiple On/Off-cycles in an isogenic background. By contrast, the tetracycline-based systems which act by controlling expression at the transcriptional level are truly conditional circuits allowing reversible phenotypic alterations. Modulation of gene expression via the Tet Switch depends on the effector substance tetracycline, in particular doxycycline (Dox), an antibiotic with well characterized pharmacological properties and more than 40 years of clinical experience.
The Tet Switch was first established in tissue culture and meanwhile has been used to generate many Dox-dependent cell lines of different origins (species, tissues). Those studies unequivocally proved that the Tet Switch provides tight and reversible control of gene expression. Subsequently the Tet Switch was adapted to all major model organisms: including yeast (Gari et al., 1997), Dictyostelium (Blaauw et al., 2000), C. elegans (Han and Guan, 1999), insects (Thomas et al., 2000), Zebrafish (Huang et al., 2005), Xenopus (Camacho-Venegas et al., 1998), mice (Furth et al., 1994), rats (Tesson et al., 1999) and non-human primates (Lamartina et al., 2003). Especially in mice, the Tet System is used as an efficient tool to study gene function and has provided fundamental insights into complex biological processes. More than 170 mouse lines with specific characteristics are available through international repositories and many more have been published (>500 are listed on TET Systems' website. These already existing mice offer the opportunity to be adapted to new applications thus minimizing the efforts required to generate new mouse models. An impressive example for this strategy is a transactivator mouse line under control of the calcium-calmodulin kinase II (CamKIIa) promoter which was generated in 1996 by the group of Eric Kandel at Columbia University (Mayford et al., Science 1996). Use of this mouse line which allowed regulated gene expression in specific brain regions , generated ground-breaking insights into mechanisms of synaptic plasticity, learning and memory. The same line is still successfully used today by various research groups in crossings with different Tet-responder lines and may be the most ‘recycled’ single Tet-transactivator mouse line. The CamKIIa mouse line not only provides an impressive example on how specific single transgenic Tet mice can be used to facilitate the generation of new double transgenic lines but also allows the direct phenotypic comparison with newly generated double transgenic mice.
Since its original publication the Tet Technology has been modified and further developed not only by its inventors but by many scientists worldwide. Besides the numerous transgenic mouse lines which can be obtained from repositories like The Jackson Labs, EMMA and RIKEN, a wealth of reagents like transactivator- or indicator cell lines, expression plasmids and viral vectors are available to the research community which allows easy adaption of the system to specific problems. With 8 000 scientific publications in peer reviewed scientific journals the Tet Technology has been extensively validated and is not only the most widely used system for regulated gene expression in higher eukaryotes but one of the most widely used molecular biology technologies.