The Role of Osteocyte Estrogen Receptor β in Bone Mass Maintenance and Tibial Stiffness Following Sex Hormone Withdrawal in Male and Female Mice

Sex hormones are essential regulators of skeletal maintenance, but the cell-specific mechanisms by which osteocytes mediate the skeletal consequences of sex hormone deficiency remain incompletely understood. Osteocyte estrogen receptor β (Ot-ERβ) has been implicated in sex-specific regulation of bone mass, particularly in male mice, yet its role in coordinating bone morphology and mechanical competence following sex hormone withdrawal is unclear. In this study, male and female mice with osteocyte-targeted ERβ deletion (ERβ-dOT) and littermate controls were subjected to orchiectomy (ORX), ovariectomy (OVX), or sham surgery at 20 weeks of age. Four weeks later, vertebral and tibial bone morphology were assessed by micro-computed tomography, and tibial mechanical behavior was evaluated using strain gauge-calibrated, microCT-based finite element modeling. ORX induced substantial cancellous bone loss in the lumbar vertebra and proximal tibia of male mice regardless of genotype. In cortical bone, however, ORX reduced tibial cortical area and minimum moment of inertia in male littermate controls, whereas these cortical deficits were attenuated in male ERβ-dOT mice. Consistent with these morphological changes, ORX increased finite element-predicted peak tensile and compressive strains in tibial cortical and cancellous compartments and reduced whole-bone stiffness in male controls, but these mechanical deteriorations were largely prevented by Ot-ERβ deletion. In contrast, OVX produced modest changes in female tibial cortical geometry and increased cancellous bone strains, but these responses were not strongly dependent on Ot-ERβ. Together, these findings reveal that Ot-ERβ mediates the skeletal response to sex hormone withdrawal in a sex- and compartment-dependent manner. Specifically, Ot-ERβ contributes to ORX-induced deterioration of tibial cortical morphology and mechanical competence in male mice, whereas it is largely dispensable for OVX-induced skeletal changes in female mice. This work highlights osteocyte ERβ as a sex-specific regulator linking hormonal status, bone morphology, and load-induced strain environments.