Regulation of Wnt/β-catenin signaling within and from osteocytes

Bone has long been known to be responsive to mechanical loading. For at least 25 years it has been known that osteocytes sense mechanical load, and because of their response to mechanical loading, osteocytes are believed to be the mechanosensory cell. The Wnt/β-catenin signaling pathway has been shown to be crucial in bone development. Mutations in LRP5 and SOST, which cause high bone mass, have increased interest in the Wnt pathway as a potential target for osteoporosis therapy and have helped link Wnt/β-catenin signaling to bone's response to mechanical loading. Because of its specificity to osteocytes, the Wnt inhibitor sclerostin is a target for anabolic bone therapies. The response of bone to mechanical loading is critically regulated by osteocytes secreting sclerostin, which binds to Lrp5.This article is part of a Special Issue entitled "The Osteocyte".     

Wnt signaling in bone metabolism

A variety of in vivo models have increased understanding of the role of Wnt signaling in bone since mutations in the LRP5 gene were found in human bone disorders. Canonical Wnt signaling encourages mesenchymal progenitor cells to differentiate into osteoblasts. In osteoblasts, Wnt pathway also promotes proliferation and mineralization, while blocks apoptosis and osteoclastogenesis by increasing the OPG/RANKL ratio. Lrp6-mediated signaling in osteoblasts may regulate osteoclastogenesis. However, the role of canonical Wnt signaling in osteoclasts remains unknown, and our understanding of the role of non-canonical Wnt signaling in bone biology is also not sufficient. As to pharmacological intervention, many levels may be considered to target in Wnt signaling pathway, although tumorigenicity and toxicity to other tissues are important. Mesd might be one of target molecules to increase the quantity of LRP5/6 in the plasma membrane. Since sclerostin is almost exclusively expressed in osteocytes, abrogating sclerostin is the most promising design. T. Kubota is a recipient of JSBMR Encouragement Award of 2006.     

Gain-of-Function Lrp5 Mutation Improves Bone Mass and Strength and Delays Hyperglycemia in a Mouse Model of Insulin-Deficient Diabetes

High fracture rate and high circulating levels of the Wnt inhibitor, sclerostin, have been reported in diabetic patients. We studied the effects of Wnt signaling activation on bone health in a mouse model of insulin-deficient diabetes. We introduced the sclerostin-resistant Lrp5^A214V mutation, associated with high bone mass, in mice carrying the Ins2^Akita mutation (Akita), which results in loss of beta cells, insulin deficiency, and diabetes in males. Akita mice accrue less trabecular bone mass with age relative to wild type (WT). Double heterozygous Lrp5^A214V/Akita mutants have high trabecular bone mass and cortical thickness relative to WT animals, as do Lrp5^A214V single mutants. Likewise, the Lrp5^A214V mutation prevents deterioration of biomechanical properties occurring in Akita mice. Notably, Lrp5^A214V/Akita mice develop fasting hyperglycemia and glucose intolerance with a delay relative to Akita mice (7 to 8 vs. 5 to 6 weeks, respectively), despite lack of insulin production in both groups by 6 weeks of age. Although insulin sensitivity is partially preserved in double heterozygous Lrp5^A214V/Akita relative to Akita mutants up to 30 weeks of age, insulin-dependent phosphorylated protein kinase B (pAKT) activation in vitro is not altered by the Lrp5^A214V mutation. Although white adipose tissue depots are equally reduced in both compound and Akita mice, the Lrp5^A214V mutation prevents brown adipose tissue whitening that occurs in Akita mice. Thus, hyperactivation of Lrp5-dependent signaling fully protects bone mass and strength in prolonged hyperglycemia and improves peripheral glucose metabolism in an insulin independent manner. Wnt signaling activation represents an ideal therapeutic approach for diabetic patients at high risk of fracture. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).     

Lrp6 Hypomorphic Mutation Affects Bone Mass Through Bone Resorption in Mice and Impairs Interaction With Mesd

Low‐density lipoprotein receptor‐related protein 5 (LRP5) regulates bone acquisition by controlling bone formation. Because roles of LRP6, another co‐receptor for Wnts, in postnatal bone metabolism have not been fully elucidated, we studied bone phenotype in mice harboring an Lrp6 hypomorphic mutation, ringelschwanz (rs), and characterized the mutant protein. First, we performed pQCT, bone histomorphometry, and immunohistochemistry on tibias of Lrp6^rs/rs and Lrp6^+/+ mice and determined biochemical parameters for bone turnover. Lrp6^rs/rs mice exhibited reduced trabecular BMD in pQCT. Bone histomorphometry showed low bone volume and decreased trabecular number, which were associated with increased eroded surface. Urinary deoxypyridinoline excretion was increased in Lrp6^rs/rs mice, whereas levels of serum osteocalcin were comparable between Lrp6^rs/rs mice and wildtype littermates. Increase in cell number and mineralization of calvariae‐derived osteoblasts were not impaired in Lrp6^rs/rs osteoblasts. Rankl expression was increased in Lrp6^rs/rs osteoblasts both in vivo and in vitro, and osteoclastogenesis and bone‐resorbing activity in vitro were accelerated in Lrp6^rs/rs cells. Treatment with canonical Wnt suppressed Rankl expression in both in primary osteoblasts and ST2 cells. Overexpression of Lrp6 also suppressed Rankl expression, whereas the Lrp6 rs mutant protein did not. Functional analyses of the Lrp6 rs mutant showed decreased targeting to plasma membrane because of reduced interaction with Mesoderm development (Mesd), a chaperone for Lrp6, leading to impaired Wnt/β‐catenin signaling. These results indicate that Lrp6‐mediated signaling controls postnatal bone mass, at least partly through the regulation of bone resorption. It is also suggested that the interaction with Mesd is critical for Lrp6 to function.     

In vivo mechanical loading rapidly activates β-catenin signaling in osteocytes through a prostaglandin mediated mechanism

The response of the skeleton to loading appears to be mediated through the activation of the Wnt/β-catenin signaling pathway and osteocytes have long been postulated to be the primary mechanosensory cells in bone. To examine the kinetics of the mechanoresponse of bone and cell types involved in vivo, we performed forearm loading of 17-week-old female TOPGAL mice. β-catenin signaling was observed only in embedded osteocytes, not osteoblasts, at 1 h post-loading, spreading to additional osteocytes and finally to cells on the bone surface by 24 h. This early activation at 1 h appeared to be independent of receptor (Lrp5/6) mediated activation as it occurred in the presence of the inhibitors sclerostin and/or Dkk1. The COX-2 inhibitor, Carprofen, blocked the activation of β-catenin signaling and decline in sclerostin positive osteocytes post-loading implying an important role for prostaglandin. In vitro, PI3K/Akt activation was shown to be required for β-catenin nuclear translocation downstream from prostaglandin in MLO-Y4 osteocyte-like cells supporting this mechanism. Downstream targets of β-catenin signaling, sclerostin and Dkk1, were also examined and found to be significantly downregulated in osteocytes in vivo at 24 h post-loading. The pattern of initially activated osteocytes appeared random and in order to understand this heterogeneous expression, a novel finite element model of the strain field in the ulna was developed, which predicts highly variable local magnitudes of strain experienced by osteocytes. In summary, both in vivo and in vitro models show the rapid activation of β-catenin in response to load through the early release of prostaglandin and that strain fields in the bone are extremely heterogeneous resulting in heterogeneous activation of the β-catenin pathway in osteocytes in vivo.     

High-bone-mass-producing mutations in the Wnt signaling pathway result in distinct skeletal phenotypes

Mutations among genes that participate in the canonical Wnt signaling pathway can lead to drastically different skeletal phenotypes, ranging from severe osteoporosis to severe osteosclerosis. Many high-bone-mass (HBM) causing mutations that occur in the LRP5 gene appear to impart the HBM phenotype, in part, by increasing resistance to soluble Wnt signaling inhibitors, including sclerostin. Sost loss-of-function mutant mice (Sost knock-out) and Lrp5 gain-of-function mutant mice (Lrp5 HBM knock-in) have high bone mass. These mutants potentially would be predicted to be phenocopies of one another, because in both cases, the sclerostin-Lrp5 interaction is disrupted. We measured bone mass, size, geometry, architecture, and strength in bones from three different genetic mouse models (Sost knock-out, Lrp5 A214V knock-in, and Lrp5 G171V knock-in) of HBM. We found that all three mouse lines had significantly elevated bone mass in the appendicular skeleton and in the cranium. Sost mutants and Lrp5 A214V mutants were statistically indistinguishable from one another in most endpoints, whereas both were largely different from the Lrp5 G171V mutants. Lrp5 G171V mutants preferentially added bone endocortically, whereas Lrp5 A214V and Sost mutants preferentially added bone periosteally. Cranial thickness and cranial nerve openings were similarly altered in all three HBM models. We also assessed serum serotonin levels as a possible mechanism accounting for the observed changes in bone mass, but no differences in serum serotonin were found in any of the three HBM mouse lines. The skeletal dissimilarities of the Lrp5 G171V mutant to the other mutants suggest that other, non-sclerostin-associated mechanisms might account for the changes in bone mass resulting from this mutation.     

The Lrp4R1170Q Homozygous Knock‐In Mouse Recapitulates the Bone Phenotype of Sclerosteosis in Humans

Sclerosteosis is a rare autosomal recessive bone disorder marked by hyperostosis of the skull and tubular bones. Initially, we and others reported that sclerosteosis was caused by loss‐of‐function mutations in SOST, encoding sclerostin. More recently, we identified disease‐causing mutations in LRP4, a binding partner of sclerostin, in three sclerosteosis patients. Upon binding to sclerostin, LRP4 can inhibit the canonical WNT signaling that is known to be an important pathway in the regulation of bone formation. To further investigate the role of LRP4 in the bone formation process, we generated an Lrp4 mutated sclerosteosis mouse model by introducing the p.Arg1170Gln mutation in the mouse genome. Extensive analysis of the bone phenotype of the Lrp4R1170Q/R1170Q knock‐in (KI) mouse showed the presence of increased trabecular and cortical bone mass as a consequence of increased bone formation by the osteoblasts. In addition, three‐point bending analysis also showed that the increased bone mass results in increased bone strength. In contrast to the human sclerosteosis phenotype, we could not observe syndactyly in the forelimbs or hindlimbs of the Lrp4 KI animals. Finally, we could not detect any significant changes in the bone formation and resorption markers in the serum of the mutant mice. However, the serum sclerostin levels were strongly increased and the level of sclerostin in the tibia was decreased in Lrp4R1170Q/R1170Q mice, confirming the role of LRP4 as an anchor for sclerostin in bone. In conclusion, the Lrp4R1170Q/R1170Q mouse is a good model for the human sclerosteosis phenotype caused by mutations in LRP4 and can be used in the future for further investigation of the mechanism whereby LRP4 regulates bone formation. © 2017 American Society for Bone and Mineral Research.     

Protection From Glucocorticoid‐Induced Osteoporosis by Anti‐Catabolic Signaling in the Absence of Sost/Sclerostin

Excess of glucocorticoids, either due to disease or iatrogenic, increases bone resorption and decreases bone formation and is a leading cause of osteoporosis and bone fractures worldwide. Improved therapeutic strategies are sorely needed. We investigated whether activating Wnt/β‐catenin signaling protects against the skeletal actions of glucocorticoids, using female mice lacking the Wnt/β‐catenin antagonist and bone formation inhibitor Sost. Glucocorticoids decreased the mass, deteriorated the microarchitecture, and reduced the structural and material strength of bone in wild‐type (WT), but not in Sost^–/– mice. The high bone mass exhibited by Sost^–/– mice is due to increased bone formation with unchanged resorption. However, unexpectedly, preservation of bone mass and strength in Sost^–/– mice was due to prevention of glucocorticoid‐induced bone resorption and not to restoration of bone formation. In WT mice, glucocorticoids increased the expression of Sost and the number of sclerostin‐positive osteocytes, and altered the molecular signature of the Wnt/β‐catenin pathway by decreasing the expression of genes associated with both anti‐catabolism, including osteoprotegerin (OPG), and anabolism/survival, such as cyclin D1. In contrast in Sost^–/– mice, glucocorticoids did not decrease OPG but still reduced cyclin D1. Thus, in the context of glucocorticoid excess, activation of Wnt/β‐catenin signaling by Sost/sclerostin deficiency sustains bone integrity by opposing bone catabolism despite markedly reduced bone formation and increased apoptosis. This crosstalk between glucocorticoids and Wnt/β‐catenin signaling could be exploited therapeutically to halt resorption and bone loss induced by glucocorticoids and to inhibit the exaggerated bone formation in diseases of unwanted hyperactivation of Wnt/β‐catenin signaling. © 2016 American Society for Bone and Mineral Research.     

Reversing LRP5‐Dependent Osteoporosis and SOST Deficiency–Induced Sclerosing Bone Disorders by Altering WNT Signaling Activity

The bone formation inhibitor sclerostin encoded by SOST binds in vitro to low‐density lipoprotein receptor‐related protein (LRP) 5/6 Wnt co‐receptors, thereby inhibiting Wnt/β‐catenin signaling, a central pathway of skeletal homeostasis. Lrp5/LRP5 deficiency results in osteoporosis‐pseudoglioma (OPPG), whereas Sost/SOST deficiency induces lifelong bone gain in mice and humans. Here, we analyzed the bone phenotype of mice lacking Sost (Sost^?/?), Lrp5 (Lrp5^?/?), or both (Sost^?/?;Lrp5^?/?) to elucidate the mechanism of action of Sost in vivo. Sost deficiency–induced bone gain was significantly blunted in Sost^?/?;Lrp5^?/? mice. Yet the Lrp5 OPPG phenotype was fully rescued in Sost^?/?;Lrp5^?/? mice and most bone parameters were elevated relative to wild‐type. To test whether the remaining bone increases in Sost^?/?;Lrp5^?/? animals depend on Lrp6, we treated wild‐type, Sost^?/?, and Sost^?/?;Lrp5^?/? mice with distinct Lrp6 function blocking antibodies. Selective blockage of Wnt1 class–mediated Lrp6 signaling reduced cancellous bone mass and density in wild‐type mice. Surprisingly, it reversed the abnormal bone gain in Sost^?/? and Sost^?/?;Lrp5^?/? mice to wild‐type levels irrespective of enhancement or blockage of Wnt3a class‐mediated Lrp6 activity. Thus, whereas Sost deficiency–induced bone anabolism partially requires Lrp5, it fully depends on Wnt1 class–induced Lrp6 activity. These findings indicate: first, that OPPG syndrome patients suffering from LRP5 loss‐of‐function should benefit from principles antagonizing SOST/sclerostin action; and second, that therapeutic WNT signaling inhibitors may stop the debilitating bone overgrowth in sclerosing disorders related to SOST deficiency, such as sclerosteosis, van Buchem disease, and autosomal dominant craniodiaphyseal dysplasia, which are rare disorders without viable treatment options. © 2014 American Society for Bone and Mineral Research.     

Differential involvement of Wnt signaling in Bmp regulation of cancellous versus periosteal bone growth

Bone morphogenetic proteins(Bmp) are well-known to induce bone formation following chondrogenesis,but the direct role of Bmp signaling in the osteoblast lineage is not completely understood. We have recently shown that deletion of the receptor Bmpr1 a in the osteoblast lineage with Dmp1-Cre reduces osteoblast activity in general but stimulates proliferation of preosteoblasts specifically in the cancellous bone region,resulting in diminished periosteal bone growth juxtaposed with excessive cancellous bone formation.Because expression of sclerostin(SOST), a secreted Wnt antagonist, is notably reduced in the Bmpr1 adeficient osteocytes, we have genetically tested the hypothesis that increased Wnt signaling might mediate the increase in cancellous bone formation in response to Bmpr1 a deletion. Forced expression of human SOST from a Dmp1 promoter fragment partially rescues preosteoblast hyperproliferation and cancellous bone overgrowth in the Bmpr1 a mutant mice, demonstrating functional interaction between Bmp and Wnt signaling in the cancellous bone compartment. To test whether increased Wnt signaling can compensate for the defect in periosteal growth caused by Bmpr1 a deletion, we have generated compound mutants harboring a hyperactive mutation(A214 V) in the Wnt receptor Lrp5. However, the mutant Lrp5 does not restore periosteal bone growth in the Bmpr1 a-deficient mice. Thus, Bmp signaling restricts cancellous bone accrual partly through induction of SOST that limits preosteoblast proliferation, but promotes periosteal bone growth apparently independently of Wnt activation.     

Modulation of Wnt signaling influences fracture repair

While the importance of Wnt signaling in skeletal development and homeostasis is well documented, little is known regarding its function in fracture repair. We hypothesized that activation and inactivation of Wnt signaling would enhance and impair fracture repair, respectively. Femoral fractures were generated in Lrp5 knockout mice (Lrp5?/?) and wild‐type littermates (Lrp5+/+), as well as C57BL/6 mice. Lrp5?/? and Lrp5+/+ mice were untreated, while C57BL/6 mice were treated 2×/week with vehicle or anti‐Dkk1 antibodies (Dkk1 Ab) initiated immediately postoperatively (Day 0) or 4 days postoperatively (Day 4). Fractures were radiographed weekly until sacrifice at day 28, followed by DXA, pQCT, and biomechanical analyses. Lrp5?/? mice showed impaired repair compared to Lrp5+/+ mice, as evidenced by reduced callus area, BMC, BMD, and biomechanical properties. The effects of Dkk1 Ab treatment depended on the timing of initiation. Day 0 initiation enhanced repair, with significant gains seen for callus area, BMC, BMD, and biomechanical properties, whereas Day 4 initiation had no effect. These results validated our hypothesis that Wnt signaling influences fracture repair, with prompt activation enhancing repair and inactivation impairing it. Furthermore, these data suggest that activation of Wnt signaling during fracture repair may have clinical utility in facilitating fracture repair. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 28:928–936, 2010     

Osteocyte‐Secreted Wnt Signaling Inhibitor Sclerostin Contributes to Beige Adipogenesis in Peripheral Fat Depots

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. © 2016 American Society for Bone and Mineral Research.     

Potential Role for Therapies Targeting DKK1, LRP5, and Serotonin in the Treatment of Osteoporosis

Osteoporosis is a common disorder in which diminished bone mass leads to progressive microarchitectural skeletal deterioration and increased fracture risk. Our understanding of both normal and pathologic bone biology continues to evolve, and with it our grasp of the highly coordinated relationships between primary bone cells (osteoblasts, osteoclasts, and osteocytes) and the complex molecular signals bone cells use to integrate signals derived from other organ systems, including the immune, hematopoietic, gastrointestinal, and central nervous systems. It is now clear that the Wnt signaling pathway is central to regulation of both skeletal modeling and remodeling. Herein, we discuss components of the Wnt signaling pathway (DKK1, an endogenous soluble inhibitor of Wnt signaling) and LRP5 (a plasma membrane-localized Wnt co-receptor) as potential future targets for osteoporosis therapy. Finally, we discuss the current controversial role for serotonin in skeletal metabolism, and the potential role of future therapies targeting serotonin for osteoporosis treatment.     

The Anti‐Osteoanabolic Function of Sclerostin Is Blunted in Mice Carrying a High Bone Mass Mutation of Lrp5

Activating mutations of the putative Wnt co‐receptor Lrp5 or inactivating mutations of the secreted molecule Sclerostin cause excessive bone formation in mice and humans. Previous studies have suggested that Sclerostin functions as an Lrp5 antagonist, yet clear in vivo evidence was still missing, and alternative mechanisms have been discussed. Moreover, because osteoblast‐specific inactivation of β‐catenin, the major intracellular mediator of canonical Wnt signaling, primarily affected bone resorption, it remained questionable, whether Sclerostin truly acts as a Wnt signaling antagonist by interacting with Lrp5. In an attempt to address this relevant question, we generated a mouse model (Col1a1‐Sost) with transgenic overexpression of Sclerostin under the control of a 2.3‐kb Col1a1 promoter fragment. These mice displayed the expected low bone mass phenotype as a consequence of reduced bone formation. The Col1a1‐Sost mice were then crossed with two mouse lines carrying different high bone mass mutations of Lrp5 (Lrp5^A170V and Lrp5^G213V), both of them potentially interfering with Sclerostin binding. Using µCT‐scanning and histomorphometry we found that the anti‐osteoanabolic influence of Sclerostin overexpression was not observed in Lrp5^A213V/A213V mice and strongly reduced in Lrp5^A170V/A170V mice. As a control we applied the same strategy with mice overexpressing the transmembrane Wnt signaling antagonist Krm2 and found that the anti‐osteoanabolic influence of the Col1a1‐Krm2 transgene was not affected by either of the Lrp5 mutations. Taken together, our data support the concept that Sclerostin inhibits bone formation through Lrp5 interaction, yet their physiological relevance remains to be established. © 2015 American Society for Bone and Mineral Research.     

Mechanism of Sclerostin/Lrp4 in the regulation of vascular smooth muscle cell calcification

Objective To investigate the possible mechanism of sclerostin/Lrp4 in calcification of VSMC induced by high phosphorus and the protective effect of Ginkgo biloba extract. Methods Aortic vascular smooth muscle cells (VSMCs) of SD rats were extracted and identified. VSMCs were divided into normal control group, high phosphorus induced calcification group (10 mmol/L β-glycerophosphate+50 μg/ml ascorbic acid), and high phosphorus induced calcification+Ginkgo biloba extract intervention group (10 mmol/L β-glycerophosphate+50 μg/ml ascorbic acid+0.5 mg/ml GBE), cultured in different mediums for 14 days. Vonkossa staining and alizarin red staining were used to detect the calcification of VSMCs. The mRNA level of BGP was detected by real time PCR, and the protein expressions of sclerostin and Lrp4 were detected by Western blot. Results Compared with normal control group, vonkossa staining and alizarin red staining showed significant calcium deposition in calcification group. Compared with calcification group, calcium salt deposition was significantly reduced in GBE treatment group. Real time PCR results showed β-catenin and BGP mRNA expressions in VSMC calcification group were higher than those in normal control group (P＜0.05). mRNA expressions of β-catenin and BGP in GBE treatment group were lower than those in calcification group (all P＜0.05). Compared with normal control group, the protein expression of sclerostin was increased, but the protein expression of Lrp4 was decreased in calcified group (all P＜0.05). Compared with calcification group, the protein expression of sclerostin decreased and the protein expression of Lrp4 increased in GBE treatment group (all P＜0.05). Conclusions High phosphorus can induce VSMC calcification by activating Wn/β-catenin signaling pathway. Sclerostin/Lrp4 is involved in hyperphosphine-induced VSMC calcification. GBE can reduce the high phosphorus induced VSMC calcification by regulating the Wnt/β-catenin signaling pathway.     

The cell biology of parathyroid hormone in osteoblasts

Continuous exposure to parathyroid hormone (PTH) is associated with catabolic effects, whereas intermittent exposure to low doses of PTH is associated with anabolic effects. By controlling osteoblast function, PTH increases bone formation on cancellous, endocortical, and periosteal bone surfaces. In general, PTH does not affect the replication of uncommitted osteoblast progenitors but suppresses proliferation of committed osteoprogenitors. Intermittent PTH promotes osteoblast differentiation, in part, by its ability to promote exit from the cell cycle, to activate Wnt signaling in osteoblasts, and to inhibit the Wnt antagonist sclerostin in osteocytes. Insulin-like growth factor-1 is also required for the actions of PTH to increase osteoblast numbers. Intermittent PTH prolongs osteoblast survival in rodents by mechanisms that involve activation and proteolytic degradation of Runx2. PTH’s ability to orchestrate a dynamic range of signaling cascades that determine osteoblast fate may explain both its catabolic and beneficial actions on the skeleton.     

Sclerostin Mediates Bone Response to Mechanical Unloading Through Antagonizing Wnt/β‐Catenin Signaling

Reduced mechanical stress leads to bone loss, as evidenced by disuse osteoporosis in bedridden patients and astronauts. Osteocytes have been identified as major cells responsible for mechanotransduction; however, the mechanism underlying the response of bone to mechanical unloading remains poorly understood. In this study, we found that mechanical unloading of wildtype mice caused decrease of Wnt/β‐catenin signaling activity accompanied by upregulation of Sost. To further analyze the causal relationship among these events, Sost gene targeting mice were generated. We showed that sclerostin selectively inhibited Wnt/β‐catenin in vivo, and sclerostin suppressed the activity of osteoblast and viability of osteoblasts and osteocytes. Interestingly, Sost^?/? mice were resistant to mechanical unloading‐induced bone loss. Reduction in bone formation in response to unloading was also abrogated in the mutant mice. Moreover, in contrast to wildtype mice, Wnt/β‐catenin signaling was not altered by unloading in Sost^?/? mice. Those data implied that sclerostin played an essential role in mediating bone response to mechanical unloading, likely through Wnt/β‐catenin signaling. Our findings also indicated sclerostin is a promising target for preventing disuse osteoporosis.     

Effects of PTH on osteocyte function

Osteocytes are ideally positioned to detect and respond to mechanical and hormonal stimuli and to coordinate the function of osteoblasts and osteoclasts. However, evidence supporting the involvement of osteocytes in specific aspects of skeletal biology has been limited mainly due to the lack of suitable experimental approaches. Few crucial advances in the field in the past several years have markedly increased our understanding of the function of osteocytes. The development of osteocytic cell lines initiated a plethora of in vitro studies that have provided insights into the unique biology of osteocytes and continue to generate novel hypotheses. Genetic approaches using promoter fragments that direct gene expression to osteocytes allowed the generation of mice with gain or loss of function of particular genes revealing their role in osteocyte function. Furthermore, evidence that Sost/sclerostin is expressed primarily in osteocytes and inhibits bone formation by osteoblasts, fueled research attempting to identify regulators of this gene as well as other osteocyte products that impact the function of osteoblasts and osteoclasts. The discovery that parathyroid hormone (PTH), a central regulator of bone homeostasis, inhibits sclerostin expression generated a cascade of studies that revealed that osteocytes are crucial target cells of the actions of PTH. This review highlights these investigations and discusses their significance for advancing our understanding of the mechanisms by which osteocytes regulate bone homeostasis and for developing therapies for bone diseases targeting osteocytes. This article is part of a Special Issue entitled "The Osteocyte".