Mice hemizygous for the GCE transgene are viable and fertile, with a human glial fibrillary acidic protein (hGFAP) promoter sequence directing expression of a Cre-estrogen receptor (Cre-ERT2) fusion protein. This Cre-ERT2 fusion protein is only active when it binds to the estrogen analog 4-hydroxytamoxifen (OHT). Specifically, Cre recombinase expression is observed in subventricular zone (SVZ), dentate gyrus (DG), celebellum, mesencephalon, diencephalon, and cerebral cortex. Cre expression was also enriched in the sub-granular zone of the DG and the cerebellar Purkinje layer. When these transgenic mice are bred with mice containing a loxP-flanked sequence, tamoxifen-inducible Cre-mediated recombination is expected to result in deletion of the floxed sequences in the Cre recombinase-expressing tissues of the offspring.
The Cre-ERT2 fusion protein consists of Cre recombinase fused to a triple mutant form of the human estrogen receptor which does not bind its natural ligand (17β-estradiol) at physiological concentrations but will bind the synthetic estrogen receptor ligands 4-hydroxytamoxifen (OHT or tamoxifen) and, with lesser sensitivity, ICI 182780. Restricted to the cytoplasm, Cre-ERT2 can only gain access to the nuclear compartment after exposure to tamoxifen. To counteract the mixed estrogen agonist effects of tamoxifen injections, which can result in late fetal abortions in pregnant mice, progesterone may be coadministered.
