Sclerostin, the protein product of the Sost gene, is a potent inhibitor of bone formation. Among bone cells, sclerostin is found nearly exclusively in the osteocytes, the cell type that historically has been implicated in sensing and initiating mechanical signaling. The recent discovery of the antagonistic effects of sclerostin on Lrp5 receptor signaling, a crucial mediator of skeletal mechanotransduction, provides a potential mechanism for the osteocytes to control mechanotransduction, by adjusting their sclerostin (Wnt inhibitory) signal output to modulate Wnt signaling in the effector cell population. We investigated the mechanoregulation of Sost and sclerostin under enhanced (ulnar loading) and reduced (hindlimb unloading) loading conditions. Sost transcripts and sclerostin protein levels were dramatically reduced by ulnar loading. Portions of the ulnar cortex receiving a greater strain stimulus were associated with a greater reduction in Sost staining intensity and sclerostin-positive osteocytes (revealed via in situ hybridization and immunohistochemistry, respectively) than were lower strain portions of the tissue. Hindlimb unloading yielded a significant increase in Sost expression in the tibia. Modulation of sclerostin levels appears to be a finely tuned mechanism by which osteocytes coordinate regional and local osteogenesis in response to increased mechanical stimulation, perhaps via releasing the local inhibition of Wnt/Lrp5 signaling.Low bone mass and poor bone structure are two major risk factors for osteoporotic fracture (1, 2). A simple yet effective means to enhance bone mass and architecture is through mechanical stimulation of the resident bone cell population (3, 4). Mechanical loading (e.g. exercise) improves bone mass and strength by stimulating the addition of new bone onto surfaces experiencing high strains, whereas surfaces that experience small strains largely remain quiescent. This phenomenon occurs both across the skeleton (limb bones adapt to locomotive loading, whereas nonbearing bones (e.g. skull) do not) and within a loaded bone (tension/compression surfaces undergo bone formation, whereas surfaces straddling the neutral bending axis do not). The cellular mechanisms involved in directing new bone formation to the high strain regions of a loaded bone are unclear, but elucidation of these mechanisms would provide an attractive target for pharmaceutical intervention aimed at mimicking the adaptive response to loading (5).Despite these gaps in our understanding, significant progress has been made in delineating some of the basic mechanisms of mechanotransduction in bone, in large part because of the creation of genetically engineered mice. A key finding in this arena is the requirement for Wnt signaling through Lrp5 (the low density lipoprotein receptor-related protein 5) in mechanically induced bone formation. We reported recently that mice engineered with a loss-of-function mutation in Lrp5 recapitulate the low bone mass phenotype observed in humans with inactivating mutations of LRP...

Both aging and loss of sex steroids have adverse effects on skeletal homeostasis, but whether and how they may influence each others negative impact on bone remains unknown. We report herein that both female and male C57BL/6 mice progressively lost strength (as determined by load-to-failure measurements) and bone mineral density in the spine and femur between the ages of 4 and 31 months. These changes were temporally associated with decreased rate of remodeling as evidenced by decreased osteoblast and osteoclast numbers and decreased bone formation rate; as well as increased osteoblast and osteocyte apoptosis, increased reactive oxygen species levels, and decreased glutathione reductase activity and a corresponding increase in the phosphorylation of p53 and p66 shc , two key components of a signaling cascade that are activated by reactive oxygen species and influences apoptosis and lifespan. Exactly the same changes in oxidative stress were acutely reproduced by gonadectomy in 5-month-old females or males and reversed by estrogens or androgens in vivo as well as in vitro. We conclude that the oxidative stress that underlies physiologic organismal aging in mice may be a pivotal pathogenetic mechanism of the age-related bone loss and strength. Loss of estrogens or androgens accelerates the effects of aging on bone by decreasing defense against oxidative stress.Age-related loss of bone mass and strength is an invariable feature of human biology, affecting women and men alike. Moreover, population-based studies demonstrate that substantial bone loss begins as early as the 20s in young adult women and men, long before any hormonal changes (1).3 The extent to which estrogen deficiency contributes to age-related bone loss and the slower rate of decline of bone mass and strength during the late postmenopausal years, and the molecular and cellular mechanisms of such putative interactions, are unknown.The universality of age-associated bone loss irrespective of sex steroid status notwithstanding, age is by far a more critical determinant of fracture risk than bone mass in humans indicating that age-related increase in fracture risk reflects a loss of bone strength that is only partly accounted for by loss of bone mass (2). Whereas an increased propensity to fall due to agerelated decline in neuromuscular function is a factor, there are also age-related changes in the bone itself. Such changes include disrupted architecture, altered composition of the bone mineral and matrix, delayed repair of fatigue microdamage, excessive turnover, and inadequate bone size (3-7). The most recently appreciated qualitative factor is loss of osteocytes (8, 9), former osteoblasts entombed into the mineralized matrix. Osteocyte death may influence the signals necessary for mechanical adaptation and repair and also lead to long term changes in bone hydration. The anti-apoptotic effect of sex steroids on osteocytes, which has been well documented in mice, rats, and humans (10 -12), may contribute to their anti-fracture efficacy independently of...

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Glucocorticoid-induced osteoporosis may be due, in part, to increased apoptosis of osteocytes and osteoblasts, and bisphosphonates (BPs) are effective in the management of this condition. We have tested the hypothesis that BPs suppress apoptosis in these cell types. Etidronate, alendronate, pamidronate, olpadronate, or amino-olpadronate (IG9402, a bisphosphonate that lacks antiresorptive activity) at 10 -9 to 10 -6 M prevented apoptosis of murine osteocytic MLO-Y4 cells, whether it was induced by etoposide, TNF-α, or the synthetic glucocorticoid dexamethasone. BPs also inhibited apoptosis of primary murine osteoblastic cells isolated from calvaria. Similar antiapoptotic effects on MLO-Y4 and osteoblastic cells were seen with nanomolar concentrations of the peptide hormone calcitonin. The antiapoptotic effect of BPs and calcitonin was associated with a rapid increase in the phosphorylated fraction of extracellular signal regulated kinases (ERKs) and was blocked by specific inhibitors of ERK activation. Consistent with these in vitro results, alendronate abolished the increased prevalence of apoptosis in vertebral cancellous bone osteocytes and osteoblasts that follows prednisolone administration to mice. These results suggest that the therapeutic efficacy of BPs or calcitonin in diseases such as glucocorticoidinduced osteoporosis may be due, in part, to their ability to prevent osteocyte and osteoblast apoptosis.

Whether the negative impact of excess glucocorticoids on the skeleton is due to direct effects on bone cells, indirect effects on extraskeletal tissues, or both is unknown. To determine the contribution of direct effects of glucocorticoids on osteoblastic/osteocytic cells in vivo, we blocked glucocorticoid action on these cells via transgenic expression of 11beta-hydroxysteroid dehydrogenase type 2, an enzyme that inactivates glucocorticoids. Osteoblast/osteocyte-specific expression was achieved by insertion of the 11beta-hydroxysteroid dehydrogenase type 2 cDNA downstream from the osteoblast-specific osteocalcin promoter. The transgene did not affect normal bone development or turnover as demonstrated by identical bone density, strength, and histomorphometry in adult transgenic and wild-type animals. Administration of excess glucocorticoids induced equivalent bone loss in wild-type and transgenic mice. As expected, cancellous osteoclasts were unaffected by the transgene. However, the increase in osteoblast apoptosis that occurred in wild-type mice was prevented in transgenic mice. Consistent with this, osteoblasts, osteoid area, and bone formation rate were significantly higher in glucocorticoid-treated transgenic mice compared with glucocorticoid-treated wild-type mice. Glucocorticoid-induced osteocyte apoptosis was also prevented in transgenic mice. Strikingly, the loss of vertebral compression strength observed in glucocorticoid-treated wild-type mice was prevented in the transgenic mice, despite equivalent bone loss. These results demonstrate for the first time that excess glucocorticoids directly affect bone forming cells in vivo. Furthermore, our results suggest that glucocorticoid-induced loss of bone strength results in part from increased death of osteocytes, independent of bone loss.