Generation and characterization of transgenic mice expressing tamoxifen-inducible cre-fusion protein specifically in mouse liver

The estrogen receptor fusion system in mouse models: a reversible switch

Reversible regulatory mouse models have significantly contributed to our understanding of normal tissue and cancer biology, providing the opportunity to temporally control initiation, progression, and evolution of physiological and pathological events. The tamoxifen inducible system, one of the best-characterized "reversible switch" models, has a number of beneficial features. In this system, the hormone-binding domain of the mammalian estrogen receptor is used as a heterologous regulatory domain. Upon ligand binding, the receptor is released from its inhibitory complex and the fusion protein becomes functional. We summarize the advantages and drawbacks of the system, describe several mouse models that rely on it, and discuss potential improvements that could render it even more useful and versatile.

Split-CreERT2: temporal control of DNA recombination mediated by split-Cre protein fragment complementation

Background

DNA recombination technologies such as the Cre/LoxP system advance modern biological research by allowing conditional gene regulation in vivo. However, the precise targeting of a particular cell type at a given time point has remained challenging since spatial specificity has so far depended exclusively on the promoter driving Cre recombinase expression. We have recently established split-Cre that allows DNA recombination to be controlled by coincidental activity of two promoters, thereby increasing spatial specificity of Cre-mediated DNA recombination. To allow temporal control of split-Cre-mediated DNA recombination we have now extended split-Cre by fusing split-Cre proteins with the tamoxifen inducible ERT2 domain derived from CreERT2.

Methodology/Principal Findings

In the split-CreERT2 system, Cre-mediated DNA recombination is controlled by two expression cassettes as well as the time of tamoxifen application. By using two independent Cre-dependent reporters in cultured cells, the combination of NCre-ERT2+ERT2-CCre was identified as having the most favorable properties of all constructs tested, showing an induction ratio of about 10 and EC₅₀-values for 4-hydroxy-tamoxifen of 10 nM to 70 nM.

Conclusions/Significance

These characteristics of split-CreERT2 in vitro indicate that split-CreERT2 will be well suited for inducing DNA recombination in living mice harboring LoxP-flanked alleles. In this way, split-CreERT2 will provide a new tool of modern genetics allowing spatial and temporal precise genetic access to cell populations defined by the simultaneous activity of two promoters.

Regulated gene insertion by steroid-induced PhiC31 integrase

Nonviral integration systems are widely used genetic tools in transgenesis and play increasingly important roles in strategies for therapeutic gene transfer. Methods to efficiently regulate the activity of transposases and site-specific recombinases have important implications for their spatiotemporal regulation in live transgenic animals as well as for studies of their applicability as safe vectors for genetic therapy. In this report, strategies for posttranslational induction of a variety of gene-inserting proteins are investigated. An engineered hormone-binding domain, derived from the human progesterone receptor, hPR891, and specifically recognized by the synthetic steroid mifepristone, is fused to the Sleeping Beauty, Frog Prince, piggyBac and Tol2 transposases as well as to the Flp and ΦC31 recombinases. By analyzing mifepristone-directed inducibility of gene insertion in cultured human cells, efficient posttranslational regulation of the Flp recombinase and the ΦC31 integrase is documented. In addition, fusion of the ΦC31 integrase with the ER^T2 modified estrogen receptor hormone-binding domain results in a protein, which is inducible by a factor of 22-fold and retains 75% of the activity of the wild-type protein. These inducible ΦC31 integrase systems are important new tools in transgenesis and in safety studies of the ΦC31 integrase for gene therapy applications.