HSA-MCM mice express MerCreMer double fusion protein under the control of the human ACTA1 (actin, alpha 1, skeletal muscle) promoter. Heterozygous mice are viable and fertile. Homozygotes are also viable but exhibit significanly reduced fertility. Of note, the MerCreMer double fusion protein has substantially greater Cre recombinase activity with less promiscuity compared with the CreMer single fusion protein. When HSA-MCM mice are bred with mice containing loxP-flanked sequences, tamoxifen-inducible Cre-mediated recombination is expected to result in deletion of the floxed sequences in skeletal muscles of the limbs, face/tongue, and diaphragm of the offspring.
When cre mice are treated with tamoxifen and then fed a high fat diet, glucose tolerance and circulating insulin levels are significantly improved. [Batran et al. Diabetologia June 2018].
The MerCreMer double fusion protein consists of Cre recombinase flanked on each end with a mutated murine estrogen receptor (mer) ligand binding domain (amino acids 281-599, G525R); 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, MerCreMer 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.
