Tamoxifen induces region-specific osteocytic recombination and transiently alters bone structure in Dmp1-Cre-ERT2...

Tamoxifen is widely used to activate Cre-ERT2/LoxP transgenic systems for cell- and time-specific gene recombination. However, its regional and long-term effects on the adult murine skeleton are not well-defined. Here, we sought to identify immediate and long-term regional effects of short-term tamoxifen treatment on gene recombination and on cortical and trabecular bone structure in young adult ^10kbDmp1-Cre-ERT2 mice, which targets gene recombination primarily to osteoblasts and osteocytes. Tamoxifen was administered by four 20 mg/kg bolus injections to 12-week-old mice of both sexes. In cortical bone, tamoxifen induced persistent recombination in osteocytes (detected by Ai9.tdTomato labeling), which was region- and sex-dependent: female metaphyseal cortex showed the highest labeling (in ∼80% of osteocytes) that remained for 12 weeks. However, diaphyseal cortex and males showed less osteocyte labeling (∼60%). In trabecular bone of male and female mice, labeling was observed in ∼90% of osteocytes and in ∼60% of bone surface cells at 14 weeks of age but both substantially declined after a further 12 weeks, particularly in females. In both sexes, tamoxifen transiently accelerated longitudinal bone growth, increased trabecular bone mass, and increased metaphyseal cortical porosity while suppressing radial growth. Bone structure largely normalied by 26 weeks, but the osteocyte lacunocanalicular network remained disordered. This indicates that (i) ^10kbDmp1-Cre-ERT2-induced recombination is not uniform through the skeleton or between sexes, and (ii) short-term tamoxifen administration has both transient and persistent effects on bone structure. Investigators using any tamoxifen-inducible system should test recombination in each site and sex, and control for tamoxifen's effects on skeletal structure.