Estrogen receptor α in osteocytes regulates trabecular bone formation in female mice

• The mice lacking ERα in osteocytes (ERαΔOcy/ΔOcy) were generated using Dmp1-Cre and ERα flox mice.
• Female ERαΔOcy/ΔOcy mice exhibited trabecular bone loss due to reduced bone formation.
• Tail-suspension-induced bone loss was confirmed in trabecular bone, not in cortical bone, of female ERαΔOcy/ΔOcy mice.
• Osteocytes obtained from ERαΔOcy/ΔOcy mice highly expressed Wnt antagonists, such as Mdk and Sostdc1.

Estrogens are well known steroid hormones necessary to maintain bone health. In addition, mechanical loading, in which estrogen signaling may intersect with the Wnt/β-catenin pathway, is essential for bone maintenance. As osteocytes are known as the major mechanosensory cells embedded in mineralized bone matrix, osteocyte ERα deletion mice (ERαΔOcy/ΔOcy) were generated by mating ERα floxed mice with Dmp1-Cre mice to determine the role of ERα in osteocytes. Trabecular bone mineral density of female, but not male ERαΔOcy/ΔOcy mice was significantly decreased. Bone formation parameters in ERαΔOcy/ΔOcy were significantly decreased while osteoclast parameters were unchanged. This suggests that ERα in osteocytes exerts osteoprotective function by positively controlling bone formation. To identify potential targets of ERα, gene array analysis of Dmp1-GFP osteocytes sorted by FACS from ERαΔOcy/ΔOcy and control mice was performed. Gene expression microarray followed by gene ontology analyses revealed that osteocytes from ERαΔOcy/ΔOcy highly expressed genes categorized in ‘Secreted’ when compared to control osteocytes. Among them, expression of Mdk and Sostdc1, both of which are Wnt inhibitors, was significantly increased without alteration of expression of the mature osteocyte markers such as Sost and β-catenin. Moreover, hindlimb suspension experiments showed that trabecular bone loss due to unloading was greater in ERαΔOcy/ΔOcy mice without cortical bone loss. These data suggest that ERα in osteocytes has osteoprotective functions in trabecular bone formation through regulating expression of Wnt antagonists, but conversely plays a negative role in cortical bone loss due to unloading.