Generation and characterization of a novel inducible Sost_P2A_CreERT2 mouse model with high specificity for osteocytes.

We have developed a novel Sost_P2A_CreERT2 model, in which the CreERT2 fusion sequence was knocked into the 5'UTR of the endogenous Sost gene (both exons left undisturbed) and is preceded by an upstream P2A linker sequence to promote separate translation of the CreERT2 message. These mice were crossed with the Ai9 (TdTomato, TdT) mouse, and the Sost_P2A_CreERT2/Ai9 male and female offspring were aged for either 2 or 5 mo. Cre activity was then induced by 5 daily injections of 75 mg/kg tamoxifen and TdT expression was examined in skeletal and soft tissues 7 days after the final tamoxifen injection. Quantitation of reporter positive osteocytes showed lowest percent expression in femoral trabecular bone ranging from 24% to 49% with 0.3%-4.7% positive cells or "leakiness" in vehicle injected mice. Femoral cortical bone, L2 vertebra, and calvaria showed higher induction ranging from 60% to 90% depending on the age and sex of the mice, with 1%-18% leakiness. TdT expression was not observed in any cells on the periosteal or endosteal bone surfaces or in the bone marrow. No TdT expression was observed at either age or sex in muscle, brain, lung, kidney, or other soft tissues including heart. However, TdT positive cells were observed in the ascending aorta, appearing to be vascular smooth muscle cells. Lowering the dose of tamoxifen to a single injection of 10 mg/kg induced minimal expression in the aorta while inducing significant expression in only in osteocytes. In summary, this novel Sost_P2A_CreERT2 model shows high specificity for osteocytes and can be fine-tuned with varying doses of tamoxifen.