Genetic engineering in the Post Implantation Uterus using a 4-hydorxytamofen Regulated Cre Recombination

The tamoxifen-inducible PgriCreERT2 mouse model enables temporal Cre recombination in progesterone receptor (Pgr)-expressing reproductive tissues. However, it remains unclear whether the PgriCreERT2 model can be safely activated during pregnancy, as tamoxifen exposure in pregnant mice can adversely affect pregnancy outcomes. Here, we established a 4-hydroxytamoxifen (4-OHT) dosing strategy for inducing Cre recombination after implantation using the PgriCreERT2 model. Pregnant females carrying PgriCreERT2 and a Cre-dependent Rosa26Sun1 reporter were administered a single dose of 4-OHT at gestational day (GD) 8.5. Dose-dependent effects on pregnancy and reporter recombination were evaluated using gross morphology, immunofluorescence, histology, placental and fetal measurements, gestation length, litter size, and resorption rate. A dose of 37.5 mg/kg produced robust SUN1 reporter activation in post-implantation uterine tissues while avoiding the overt adverse effects observed at the higher dose tested. Reporter-positive cells were detected predominantly in the uterus. Placental morphology, steroid hormone receptor (PGR and estrogen receptor alpha (ERα/ESR1)), placental development, and vascularization were preserved following treatment. These findings establish a practical 4-OHT induction regimen for temporally controlled recombination in the PgriCreERT2 model during post-implantation pregnancy and provide a methodological framework for studying uterine cell function during decidualization, placentation, and maternal-fetal development while reducing pregnancy-related confounding effects.