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Estrogen receptor-alpha (ERα) regulates bone mass and is implicated in bone tissue's response to mechanical loading. The effects of ERα deletion in mice depend on sex, anatomical location, and the cellular stage at which ERα is removed. Few studies have investigated the effect of age on the role of ERα in skeletal maintenance and functional adaptation. We previously demonstrated that bone mass and adaptation to loading were altered in growing 10-week-old female and male mice lacking ERα in mature osteoblasts and osteocytes (pOC-ERαKO). Here our goal was to determine the effects of ERα and mechanical loading in skeletally-mature adult mice. We subjected 26-week-old skeletally-mature adult pOC-ERαKO and littermate control (LC) mice of both sexes to two weeks of in vivo cyclic tibial loading. ERα deletion in male mice did not alter bone mass or the response to loading. Adult female pOC-ERαKO mice had reduced cancellous and cortical bone mass and increased adaptation to high-magnitude mechanical loading compared to LC mice. Thus, ERα deletion from mature osteoblasts reduced the bone mass and increased the mechanoadaptation of adult female but not male mice. Additionally, compared to our previous work in young mice, adult female mice had greatly reduced mechanoadaptation and adult male mice retained most of their mechanoadaptation with age.

Objective Reduced subchondral bone mass and increased remodeling are associated with early stage OA. However, the direct effect of low subchondral bone mass on the risk and severity of OA development is unclear. We sought to determine the role of low bone mass resulting from a bone-specific loss of estrogen signaling in load-induced OA development using female osteoblast-specific estrogen receptor-alpha knockout (pOC-ERαKO) mice. Methods Osteoarthritis was induced by cyclic mechanical loading applied to the left tibia of 26-week-old female pOC-ERαKO and littermate control mice at peak loads of 6.5N, 7N, or 9N for 2 weeks. Cartilage damage and thickness, osteophyte development, and joint capsule fibrosis were assessed from histological sections. Subchondral bone morphology was analyzed by microCT. The correlation between OA severity and intrinsic bone parameters was determined. Results The loss of ERα in bone resulted in an osteopenic subchondral bone phenotype, but did not directly affect cartilage health. Following two weeks of cyclic tibial loading to induce OA pathology, pOC-ERαKO mice developed more severe cartilage damage, larger osteophytes, and greater joint capsule fibrosis compared to littermate controls. Intrinsic bone parameters negatively correlated with measures of OA severity in loaded limbs. Conclusions Subchondral bone osteopenia resulting from bone-specific loss of estrogen signaling was associated with increased severity of load-induced OA pathology, suggesting that reduced subchondral bone mass directly exacerbates load-induced OA development. Bone-specific changes associated with estrogen loss may contribute to the increased incidence of OA in post-menopausal women.

Mechanical loading is an anabolic stimulus that increases bone mass, and thus a promising method to counteract osteoporosis-related bone loss. The mechanism of this anabolism remains unclear, and needs to be established for both cortical and cancellous envelopes individually. We hypothesized that cortical and cancellous bone display different gene expression profiles at baseline and in response to mechanical loading. To test this hypothesis, the left tibiae of 10-week-old female C57Bl/6 mice were subjected to one session of axial tibial compression (9N, 1200 cycles, 4Hz triangle waveform) and euthanized 3 and 24 hours following loading. The right limb served as the contralateral control. We performed RNA-seq on marrow-free metaphyseal samples from the cortical shell and the cancellous core to determine differential gene expression at baseline (control limb) and in response to load. Differential expression was verified with qPCR. Cortical and cancellous bone exhibited distinctly different transcriptional profiles basally and in response to mechanical loading. More genes were differentially expressed with loading at 24 hours with more genes downregulated at 24 hours than at 3 hours in both tissues. Enhanced Wnt signaling dominated the response in cortical bone at 3 and 24 hours, but in cancellous bone only at 3 hours. In cancellous bone at 24 hours many muscle-related genes were downregulated. These findings reveal key differences between cortical and cancellous genetic regulation in response to mechanical loading. Future studies at different time points and multiple loading sessions will add to our knowledge of cortical and cancellous mechanotransduction with the potential to identify new targets for mouse genetic knockout studies and drugs to treat osteoporosis.