An internal ribosome entry site (IRES)-CreERT2 fusion protein was inserted 8 bp downstream of the stop codon of the paired box gene 7 (Pax7) gene. PAX7 is a marker for satellite cells, adult muscle stem cells required for muscle regeneration after injury. Unlike B6;129-Pax7tm2.1(cre/ERT2)Fan/J mice (Stock No. 012476), in which PAX7 expression is abolished, PAX7 is expressed and functional in these mice. Homozygotes are viable and fertile. Cre-ERT2 fusion gene activity is inducible and only observed following tamoxifen administration. When these mice are bred with mice containing loxP-flanked sequence, tamoxifen-inducible, Cre-mediated recombination will result in deletion of the floxed sequences in the Pax7-expressing cells of the offspring. For example when bred to B6;129-Gt(ROSA)26Sortm1(DTA)Mrc/J mice (Stock No. 010527), tamoxifen induction results in diphtheria toxin A expression in satellite cells, and subsequent ablation of satellite cells. These double mutant mice exhibit complete loss of muscle regeneration, misregulation of fibroblasts, and an increase of muscle connective tissue.
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.
