Background: Recent studies have suggested osteocytes as key players in mechanosensation and skeletal metabolism. Results: Simulated microgravity induces an autonomous up-regulation of SOST/sclerostin and RANKL/OPG in a novel osteocytic cell line, Ocy454. Conclusion: Mechanical loading regulates intrinsic osteocyte responses in concert with hormonal and cytokine inputs. Significance: Learning how osteocytes sense mechanical loads would enable novel interventions to prevent disuse-induced bone loss.

Background: Osteocytes, the most abundant cells in adult bone, express PPR. Results: Mice with constitutive PPR deletion in osteocytes demonstrate blunted anabolic and catabolic bone responses and the inability to recruit osteoblasts and osteoclasts upon PTH administration. Conclusion: PPR in osteocytes is needed for a full skeletal response to PTH administration. Significance: PPR signaling in osteocytes is necessary for PTH-driven anabolic effects during osteoporosis therapy.

Cells of the osteoblast lineage are increasingly identified as participants in whole-body metabolism by primarily targeting pancreatic insulin secretion or consuming energy. Osteocytes, the most abundant bone cells, secrete a Wnt-signaling inhibitor called sclerostin. Here we examined three mouse models expressing high sclerostin levels, achieved through constitutive or inducible loss of the stimulatory subunit of G-proteins (Gsα in mature osteoblasts and/or osteocytes). These mice showed progressive loss of white adipose tissue (WAT) with tendency toward increased energy expenditure but no changes in glucose or insulin metabolism. Interestingly beige adipocytes were increased extensively in both gonadal and inguinal WAT and had reduced canonical β-catenin signaling. To determine if sclerostin directly contributes to the increased beige adipogenesis, we engineered an osteocytic cell line lacking Gsα which has high sclerostin secretion. Conditioned media from these cells significantly increased expression of UCP1 in primary adipocytes, and this effect was partially reduced after depletion of sclerostin from the conditioned media. Similarly, treatment of Gsα-deficient animals with sclerostin-neutralizing antibody partially reduced the increased UCP1 expression in WAT. Moreover, direct treatment of sclerostin to wild-type mice significantly increased UCP1 expression in WAT. These results show that osteocytes and/or osteoblasts secrete factors regulating beige adipogenesis, at least in part, through the Wnt-signaling inhibitor sclerostin. Further studies are needed to assess metabolic effects of sclerostin on adipocytes and other metabolic tissues.