Genetic evidence that SOST inhibits WNT signaling in the limb

Sall4 regulates downstream patterning genes during limb regeneration

Many salamanders can completely regenerate a fully functional limb. Limb regeneration is a carefully coordinated process involving several defined stages. One key event during the regeneration process is the patterning of the blastema to inform cells of what they must differentiate into. Although it is known that many genes involved in the initial development of the limb are re-used during regeneration, the exact molecular circuitry involved in this process is not fully understood. Several large-scale transcriptional profiling studies of axolotl limb regeneration have identified many transcription factors that are up-regulated after limb amputation. Sall4 is a transcription factor that has been identified to play essential roles in maintaining cells in an undifferentiated state during development and also plays a unique role in limb development. Inactivation of Sall4 during limb bud development results in defects in anterior-posterior patterning of the limb. Sall4 has been found to be up-regulated during limb regeneration in both Xenopus and salamanders, but to date it function has been untested. We confirmed that Sall4 is up-regulated during limb regeneration in the axolotl using qRT-PCR and identified that it is present in the skin cells and also in cells within the blastema. Using CRISPR technology we microinjected gRNAs specific for Sall4 complexed with cas9 protein into the blastema to specifically knockout Sall4 in blastema cells only. This resulted in limb regenerate defects, including missing digits, fusion of digit elements, and defects in the radius and ulna. This suggests that during regeneration Sall4 may play a similar role in regulating the specification of anterior-proximal skeletal elements.

Mechanism and physical activities in bone-skeletal muscle crosstalk

Bone and skeletal muscle work in coordination to maintain the function of the musculoskeletal system, in which skeletal muscle contraction drives the movement of the bone lever system while bone provides insert sites for skeletal muscle through the bone-muscle junction. Existing evidence suggests that factors secreted by skeletal muscle and bone mediate the interaction between the two tissues. Herein, we focused on the relationship between skeletal muscle and bone and the underlying mechanism of the interaction. Exercise can promote bone strength and secrete osteocalcin and insulin-like growth factor I into the blood, thus improving muscle quality. In addition, exercise can also promote myostatin, interleukin-6, Irisin, and apelin in muscles to enter the blood so that they can act on bones to maintain the balance between bone absorption and bone formation. There is a special regulatory axis interleukin-6/osteocalcin between myokines and osteokines, which is mainly influenced by exercise. Therefore, we pay attention to the important factors in the bone-muscle intersection that are affected by exercise, which were found or their functions were expanded, which strengthened the connection between organs of the whole body, highlighting the importance of exercise and contributing to the diagnosis, prevention, and treatment of osteoporosis and sarcopenia in the clinic.

Sostdc1 Suppression in the Absence of Sclerostin Potentiates Anabolic Action of Cortical Bone in Mice

The development of Wnt-based osteoanabolic agents has progressed rapidly in recent years, given the potent effects of Wnt modulation on bone homeostasis. Simultaneous pharmacologic inhibition of the Wnt antagonists sclerostin and Dkk1 can be optimized to create potentiated effects in the cancellous bone compartment. We looked for other candidates that might be co-inhibited along with sclerostin to potentiate the effects in the cortical compartment. Sostdc1 (Wise), like sclerostin and Dkk1, also binds and inhibits Lrp5/6 coreceptors to impair canonical Wnt signaling, but Sostdc1 has greater effects in the cortical bone. To test this concept, we deleted Sostdc1 and Sost from mice and measured the skeletal effects in cortical and cancellous compartments individually. Sost deletion alone produced high bone mass in all compartments, whereas Sostdc1 deletion alone had no measurable effects on either envelope. Mice with codeletion of Sostdc1 and Sost had high bone mass and increased cortical properties (bone mass, formation rates, mechanical properties), but only among males. Combined administration of sclerostin antibody and Sostdc1 antibody in wild-type female mice produced potentiation of cortical bone gain despite no effect of Sostdc1 antibody alone. In conclusion, Sostdc1 inhibition/deletion can work in concert with sclerostin deficiency to improve cortical bone properties. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Sclerostin ablation prevents aortic valve stenosis in mice

The objective of this study was to test the hypothesis that targeting sclerostin would accelerate the progression of aortic valve stenosis. Sclerostin (mouse gene, Sost) is a secreted glycoprotein that acts as a potent regulator of bone remodeling. Antibody therapy targeting sclerostin is approved for osteoporosis but results from a stage III clinical trial showed multiple off-target cardiovascular effects. Wild-type (WT, Sost+/+) and Sost-gene knockout-expression (Null, Sost-/-) mice were generated and maintained to 12 mo of age on a high-cholesterol diet to induce aortic valve stenosis. Mice were examined by echocardiography, histology, and RNAseq. Immortalized valve interstitial cells were developed from each genotype for in vitro studies. Null mice developed a bone overgrowth phenotype, similar to patients with sclerosteosis. Surprisingly, however, WT mice developed hemodynamic signs of aortic valve stenosis, whereas Null mice were unchanged. WT mice had thicker aortic valve leaflets and higher amounts of α-smooth muscle actin, a marker myofibroblast activation and dystrophic calcification, with very little evidence of Runx2 expression, a marker of osteogenic calcification. RNAseq analysis of aortic roots indicated the HOX family of transcription factors was significantly upregulated in Null mice, and valve interstitial cells from Null animals were enriched with Hoxa1, Hoxb2, and Hoxd3 subtypes with downregulated Hoxa7. In addition, Null valve interstitial cells were shown to be less contractile than their WT counterparts. Contrary to our hypothesis, sclerostin targeting prevented hallmarks of aortic valve stenosis and indicates that targeted antibody treatments for osteoporosis may be beneficial for these patients regarding aortic stenosis.NEW & NOTEWORTHY We have found that genetic ablation of the Sost gene (protein: sclerostin) prevents aortic valve stenosis in aged, Western diet mice. This is a new role for sclerostin in the cardiovascular system. To the knowledge of the authors, this is one of the first studies directly manipulating sclerostin in a cardiovascular disease model and the first to specifically study the aortic valve. We also provide a potential new role for Hox genes in cardiovascular disease, noting pan-Hox upregulation in the aortic roots of sclerostin genetic knockouts. The role of Hox genes in postnatal cardiovascular health and disease is another burgeoning field of study to which this article contributes.

Role of Sostdc1 in skeletal biology and cancer

Sclerostin domain-containing protein-1 (Sostdc1) is a member of the sclerostin family and encodes a secreted 28–32 kDa protein with a cystine knot-like domain and two N-linked glycosylation sites. Sostdc1 functions as an antagonist to bone morphogenetic protein (BMP), mediating BMP signaling. It also interacts with LRP6, mediating LRP6 and Wnt signaling, thus regulating cellular proliferation, differentiation, and programmed cell death. Sostdc1 plays various roles in the skin, intestines, brain, lungs, kidneys, and vasculature. Deletion of Sostdc1 gene in mice resulted in supernumerary teeth and improved the loss of renal function in Alport syndrome. In the skeletal system, Sostdc1 is essential for bone metabolism, bone density maintenance, and fracture healing. Recently, Sostdc1 has been found to be closely related to the development and progression of multiple cancer types, including breast, renal, gastric, and thyroid cancers. This article summarises the role of Sostdc1 in skeletal biology and related cancers to provide a theoretical basis for the treatment of related diseases.

Sclerostin Depletion Induces Inflammation in the Bone Marrow of Mice

Romosozumab, a humanized monoclonal antibody specific for sclerostin (SOST), has been approved for treatment of postmenopausal women with osteoporosis at a high risk for fracture. Previous work in sclerostin global knockout (Sost^-/-) mice indicated alterations in immune cell development in the bone marrow (BM), which could be a possible side effect in romosozumab-treated patients. Here, we examined the effects of short-term sclerostin depletion in the BM on hematopoiesis in young mice receiving sclerostin antibody (Scl-Ab) treatment for 6 weeks, and the effects of long-term Sost deficiency on wild-type (WT) long-term hematopoietic stem cells transplanted into older cohorts of Sost^-/- mice. Our analyses revealed an increased frequency of granulocytes in the BM of Scl-Ab-treated mice and WT→Sost^-/- chimeras, indicating myeloid-biased differentiation in Sost-deficient BM microenvironments. This myeloid bias extended to extramedullary hematopoiesis in the spleen and was correlated with an increase in inflammatory cytokines TNFα, IL-1α, and MCP-1 in Sost^-/- BM serum. Additionally, we observed alterations in erythrocyte differentiation in the BM and spleen of Sost^-/- mice. Taken together, our current study indicates novel roles for Sost in the regulation of myelopoiesis and control of inflammation in the BM.

Sclerostin-Mediated Impaired Osteogenesis by Fibroblast-Like Synoviocytes in the Particle-Induced Osteolysis Model

Engineered biomaterials are envisioned to replace, augment, or interact with living tissues for improving the functional deformities associated with end-stage joint pathologies. Unfortunately, wear debris from implant interfaces is the major factor leading to periprosthetic osteolysis. Fibroblast-like synoviocytes (FLSs) populate the intimal lining of the synovium and are in direct contact with wear debris. This study aimed to elucidate the effect of Ti particles as wear debris on human FLSs and the mechanism by which they might participate in the bone remodeling process during periprosthetic osteolysis. FLSs were isolated from synovial tissue from patients, and the condition medium (CM) was collected after treating FLSs with sterilized Ti particles. The effect of CM was analyzed for the induction of osteoclastogenesis or any effect on osteogenesis and signaling pathways. The results demonstrated that Ti particles could induce activation of the NFκB signaling pathway and induction of COX-2 and inflammatory cytokines in FLSs. The amount of Rankl in the conditioned medium collected from Ti particle–stimulated FLSs (Ti CM) showed the ability to stimulate osteoclast formation. The Ti CM also suppressed the osteogenic initial and terminal differentiation markers for osteoprogenitors, such as alkaline phosphate activity, matrix mineralization, collagen synthesis, and expression levels of Osterix, Runx2, collagen 1α, and bone sialoprotein. Inhibition of the WNT and BMP signaling pathways was observed in osteoprogenitors after the treatment with the Ti CM. In the presence of the Ti CM, exogenous stimulation by WNT and BMP signaling pathways failed to stimulate osteogenic activity in osteoprogenitors. Induced expression of sclerostin (SOST: an antagonist of WNT and BMP signaling) in Ti particle–treated FLSs and secretion of SOST in the Ti CM were detected. Neutralization of SOST in the Ti CM partially restored the suppressed WNT and BMP signaling activity as well as the osteogenic activity in osteoprogenitors. Our results reveal that wear debris–stimulated FLSs might affect bone loss by not only stimulating osteoclastogenesis but also suppressing the bone-forming ability of osteoprogenitors. In the clinical setting, targeting FLSs for the secretion of antagonists like SOST might be a novel therapeutic approach for preventing bone loss during inflammatory osteolysis.

Role of Physical Activity in Bone-Muscle Crosstalk: Biological Aspects and Clinical Implications

Bone and muscle tissues influence each other through the integration of mechanical and biochemical signals, giving rise to bone-muscle crosstalk. They are also known to secrete osteokines, myokines, and cytokines into the circulation, influencing the biological and pathological activities in local and distant organs and cells. In this regard, even osteoporosis and sarcopenia, which were initially thought to be two independent diseases, have recently been defined under the term "osteosarcopenia", to indicate a synergistic condition of low bone mass with muscle atrophy and hypofunction. Undoubtedly, osteosarcopenia is a major public health concern, being associated with high rates of morbidity and mortality. The best current defence against osteosarcopenia is prevention based on a healthy lifestyle and regular exercise. The most appropriate type, intensity, duration, and frequency of exercise to positively influence osteosarcopenia are not yet known. However, combined programmes of progressive resistance exercises, weight-bearing impact exercises, and challenging balance/mobility activities currently appear to be the most effective in optimising musculoskeletal health and function. Based on this evidence, the aim of our review was to summarize the current knowledge about the role of exercise in bone-muscle crosstalk, highlighting how it may represent an effective alternative strategy to prevent and/or counteract the onset of osteosarcopenia.

LRP receptors in chondrocytes are modulated by simulated microgravity and cyclic hydrostatic pressure

Mechanical loading is essential for the maintenance of musculoskeletal homeostasis. Cartilage has been demonstrated to be highly mechanoresponsive, but the mechanisms by which chondrocytes respond to mechanical stimuli are not clearly understood. The goal of the study was to determine how LRP4, LRP5, and LRP6 within canonical Wnt-signaling are regulated in simulated microgravity and cyclic hydrostatic pressure, and to investigate the potential role of LRP 4/5/6 in cartilage degeneration. Rat chondrosacroma cell (RCS) pellets were stimulated using either cyclic hydrostatic pressure (1Hz, 7.5 MPa, 4hr/day) or simulated microgravity in a rotating wall vessel (RWV) bioreactor (11RPM, 24hr/day). LRP4/5/6 mRNA expression was assessed by RT-qPCR and LRP5 protein expression was determined by fluorescent immunostaining. To further evaluate our in vitro findings in vivo, mice were subjected to hindlimb suspension for 14 days and the femoral heads stained for LRP5 expression. We found that, in vitro, LRP4/5/6 mRNA expression is modulated in a time-dependent manner by mechanical stimulation. Additionally, LRP5 protein expression is upregulated in response to both simulated microgravity and cyclic hydrostatic pressure. LRP5 is also upregulated in vivo in the articular cartilage of hindlimb suspended mice. This is the first study to examine how LRP4/5/6, critical receptors within musculoskeletal biology, respond to mechanical stimulation. Further elucidation of this mechanism could provide significant clinical benefit for the identification of pharmaceutical targets for the maintenance of cartilage health.

Sostdc1 is expressed in all major compartments of developing and adult mammalian eyes

PURPOSE

This study was conducted in order to study Sostdc1 expression in rat and human developing and adult eyes.

METHODS

Using the yeast signal sequence trap screening method, we identified the Sostdc1 cDNA encoding a protein secreted by the adult rat retinal pigment epithelium. We determined by in situ hybridization, RT-PCR, immunohistochemistry, and western blot analysis Sostdc1 gene and protein expression in developing and postnatal rat ocular tissue sections. We also investigated Sostdc1 immunohistolocalization in developing and adult human ocular tissues.

RESULTS

We demonstrated a prominent Sostdc1 gene expression in the developing rat central nervous system (CNS) and eyes at early developmental stages from E10.5 days postconception (dpc) to E13 dpc. Specific Sostdc1 immunostaining was also detected in most adult cells of rat ocular tissue sections. We also identified the rat ocular embryonic compartments characterized by a specific Sostdc1 immunohistostaining and specific Pax6, Sox2, Otx2, and Vsx2 immunohistostaining from embryonic stages E10.5 to E13 dpc. Furthermore, we determined the localization of SOSTDC1 immunoreactivity in ocular tissue sections of developing and adult human eyes. Indeed, we detected SOSTDC1 immunostaining in developing and adult human retinal pigment epithelium (RPE) and neural retina (NR) as well as in several developing and adult human ocular compartments, including the walls of choroidal and scleral vessels. Of utmost importance, we observed a strong SOSTDC1 expression in a pathological ocular specimen of type 2 Peters' anomaly complicated by retinal neovascularization as well in the walls ofother pathological extra-ocular vessels.  CONCLUSION: As rat Sostdc1 and human SOSTDC1 are dual antagonists of the Wnt/β-catenin and BMP signaling pathways, these results underscore the potential crucial roles of these pathways and their antagonists, such as Sostdc1 and SOSTDC1, in developing and adult mammalian normal eyes as well as in syndromic and nonsyndromic congenital eye diseases.

Sostdc1: A soluble BMP and Wnt antagonist that is induced by the interaction between myeloma cells and osteoblast lineage cells

Multiple myeloma (MM) is characterised by destructive lytic bone disease, caused by induction of bone resorption and impaired bone formation. Our understanding of the molecular mechanisms responsible for osteoblast suppression, are limited. Using the 5T2MM murine model of MM we have previously shown that suppression of the activity of a known inhibitor of bone formation Dikkopf-1 (Dkk1) prevents the development of lytic bone disease. Here we have demonstrated that another potential inhibitor of bone formation, sclerostin domain containing 1 (Sostdc1) is expressed at low levels in MM and osteoblast lineage cells when these cells are grown separately in cell culture but its expression is significantly induced in both cell types when these cells are in contact. The distribution of Sostdc1 staining in bones infiltrated with 5TGM1 myeloma cells in vivo suggested its presence in both myeloma and osteoblast lineage populations when in close proximity. We have also shown that recombinant Sostdc1 inhibits both bone morphogenic proteins (BMP2 and 7) and Wnt signalling in primary osteoblasts and suppresses differentiation of these cells. Together, these findings suggest that Sostdc1 expression in 5TGM1-infiltrated bones as a result of the interaction between myeloma and osteoblast lineage populations, could result in suppression of osteoblast differentiation.

Sostdc1 Regulates NK Cell Maturation and Cytotoxicity

NK cells are innate-like lymphocytes that eliminate virally infected and cancerous cells, but the mechanisms that control NK cell development and cytotoxicity are incompletely understood. We identified roles for sclerostin domain-containing-1 (Sostdc1) in NK cell development and function. Sostdc1-knockout (Sostdc1 ^-/-) mice display a progressive accumulation of transitional NK cells (tNKs) (CD27⁺CD11b⁺) with age, indicating a partial developmental block. The NK cell Ly49 repertoire in Sostdc1 ^-/- mice is also changed. Lower frequencies of Sostdc1 ^-/- splenic tNKs express inhibitory Ly49G2 receptors, but higher frequencies express activating Ly49H and Ly49D receptors. However, the frequencies of Ly49I⁺, G2⁺, H⁺, and D⁺ populations were universally decreased at the most mature (CD27^-CD11b⁺) stage. We hypothesized that the Ly49 repertoire in Sostdc1 ^-/- mice would correlate with NK killing ability and observed that Sostdc1^-/- NK cells are hyporesponsive against MHC class I-deficient cell targets in vitro and in vivo, despite higher CD107a surface levels and similar IFN-γ expression to controls. Consistent with Sostdc1's known role in Wnt signaling regulation, Tcf7 and Lef1 levels were higher in Sostdc1 ^-/- NK cells. Expression of the NK development gene Id2 was decreased in Sostdc1^-/- immature NK and tNK cells, but Eomes and Tbx21 expression was unaffected. Reciprocal bone marrow transplant experiments showed that Sostdc1 regulates NK cell maturation and expression of Ly49 receptors in a cell-extrinsic fashion from both nonhematopoietic and hematopoietic sources. Taken together, these data support a role for Sostdc1 in the regulation of NK cell maturation and cytotoxicity, and identify potential NK cell niches.

Genetics of Sost/SOST in sclerosteosis and van Buchem disease animal models

Sclerosteosis and van Buchem disease (VBD) are two rare autosomal recessive disorders that results from osteoblast hyperactivity, in which progressive bone overgrowth leads to very dense bones, distortion of the face, and entrapment of cranial nerves. Sclerosteosis is caused by loss-of-function mutations in the SOST gene which encodes a secreted glycoprotein, sclerostin. VBD is caused by a noncoding deletion that removes a SOST-specific regulatory element in bone. In bone, SOST is expressed predominantly by osteocytes and sclerostin suppresses bone formation by inhibiting the canonical Wnt signaling pathway. Here we describe how human genetics studies in sclerosteosis and VBD patients, in combination with the generation of transgenic and knockout mice, has led to a better understanding of the role of sclerostin in bone metabolism.

Accelerated development of instability-induced osteoarthritis in transgenic mice overexpressing SOST

OBJECTIVE

Sclerostin (SOST), acting as a Wnt antagonist, has been shown to play a key role in regulating bone homestasis, and has also been linked to osteoarthritis (OA) development. Here, we investigated whether overexpressing SOST could affect OA development after destabilization of the medial meniscus (DMM) using SOST transgenic (Tg) mice.

METHODS

Bone and cartilage phenotypes of SOST Tg mice at 10 weeks of age were investigated by dual x-ray absorptiometry (DXA) and histology. Subsequently, 10-week-old SOST Tg mice and their wild-type (WT) littermates were subjected to DMM or sham surgery. Knee joints were isolated to evaluate the cartilage damage and the subchondral bone plate thickness at 2 and 8 weeks post-surgery. The changes of chondrocyte anabolic and catabolic responses after IL-1β or TNFα stimulation, β-catenin signaling and apoptosis were also measured.

RESULTS

Ten-week-old SOST Tg mice were identical to their WT littermate males except that they displayed digit abnormalities and osteopenic, whereas more severe OA was observed in SOST Tg mice at 2 and 8 weeks post-DMM. In addition, DMM resulted in significantly greater subchondral bone changes compared with sham surgery in SOST Tg mice at 8 weeks post-surgery. The accelerated OA in SOST Tg mice may be associated with reduced β-catenin signaling and increased chondrocyte apoptosis.

CONCLUSION

Overexpressing SOST led to accelerated development of instability-induced OA. Our data further highlight that cartilage homeostasis requires finely tuned Wnt signaling.

[SOST knockdown promotes differentiation of osteoblasts MG63 and mesenchymal stem cells C3H10 in an in vitro model of bone metastasis of breast cancer]

OBJECTIVE

To investigate whether SOST is involved in breast cancer MDA-MB-231 cells-induced suppression of differentiation of osteoblast MG63 cells and mesenchymal stem C3H10 cells.

METHODS

SOST-specific small interfering RNA (siRNA) was transfected into breast cancer MDA-MB-231 cells, and the interfering efficiency was verified by RT-PCR. The supernatants were collected from MDA-MB-231 cells in routine culture, cells transfected with SOST siRNA via adenovirus, and cells transfected with empty adenoviral vectors and added in MG63 or C3H10 cell cultures. The changes in the expressions of OPG, OCN, OPN and IBSP in MG63 and C3H10 cells were detected using quantitative real-time PCR, and ALP activity was detected with ALP reading and ALP staining with the cells cultured in routine culture medium and cells in osteogenic induction medium as the negative and positive controls.

RESULTS

The adenovirus Ad-siSOST effectively knocked down the expression of SOST in MDA-MB-231 cells. MG63 cells and C3H10 cells cultured in osteogenic medium showed significantly upregulated expressions of the osteoblast markers OPG, OPN, OCN and IBSP (P<0.01), while co-culture with the supernatant of MDA-MB-231 cells obviously reduced the expressions of the osteoblast markers (P<0.01); the expression of the markers increased again in MG63 and C3H10 cells after treatment with the supernatant of MDA-MB-231 cells transfected with ad-siSOST (P<0.01). ALP activity in MG63 and C3H10 cells exhibited a similar pattern of variations in response to the treatments (P<0.01).

CONCLUSION

In the in vitro model of bone metastasis of breast cancer, the differentiation of MG63 or C3H10 cells is suppressed, which can be partly reversed by knocking down the expression of SOST in the bone metastasis microenvironment.

Sclerostin expression and functions beyond the osteocyte

Sclerostin, the product of the SOST gene, is a secreted inhibitor of Wnt signaling that is produced by osteocytes to regulate bone formation. While it is often considered an osteocyte-specific protein, SOST expression has been reported in numerous other cell types, including hypertrophic chondrocytes and cementocytes. Of interest, SOST/sclerostin expression is altered in certain pathogenic conditions, including osteoarthritis and rheumatic joint disease, and it is unclear whether sclerostin plays a protective role or whether sclerostin may mediate disease pathogenesis. Therefore, as anti-sclerostin antibodies are being developed for the treatment of osteoporosis, it is important to understand the functions of sclerostin beyond the regulation of bone formation.

Sostdc1 deficiency accelerates fracture healing by promoting the expansion of periosteal mesenchymal stem cells

Loss of Sostdc1, a growth factor paralogous to Sost, causes the formation of ectopic incisors, fused molars, abnormal hair follicles, and resistance to kidney disease. Sostdc1 is expressed in the periosteum, a source of osteoblasts, fibroblasts and mesenchymal progenitor cells, which are critically important for fracture repair. Here, we investigated the role of Sostdc1 in bone metabolism and fracture repair. Mice lacking Sostdc1 (Sostdc1(-/-)) had a low bone mass phenotype associated with loss of trabecular bone in both lumbar vertebrae and in the appendicular skeleton. In contrast, Sostdc1(-/-) cortical bone measurements revealed larger bones with higher BMD, suggesting that Sostdc1 exerts differential effects on cortical and trabecular bone. Mid-diaphyseal femoral fractures induced in Sostdc1(-/-) mice showed that the periosteal population normally positive for Sostdc1 rapidly expands during periosteal thickening and these cells migrate into the fracture callus at 3days post fracture. Quantitative analysis of mesenchymal stem cell (MSC) and osteoblast populations determined that MSCs express Sostdc1, and that Sostdc1(-/-) 5day calluses harbor >2-fold more MSCs than fractured wildtype controls. Histologically a fraction of Sostdc1-positive cells also expressed nestin and α-smooth muscle actin, suggesting that Sostdc1 marks a population of osteochondral progenitor cells that actively participate in callus formation and bone repair. Elevated numbers of MSCs in D5 calluses resulted in a larger, more vascularized cartilage callus at day 7, and a more rapid turnover of cartilage with significantly more remodeled bone and a thicker cortical shell at 21days post fracture. These data support accelerated or enhanced bone formation/remodeling of the callus in Sostdc1(-/-) mice, suggesting that Sostdc1 may promote and maintain mesenchymal stem cell quiescence in the periosteum.

[Sclerostin expression in periodontal ligaments during movement of orthodontic teeth in rats]

OBJECTIVE

This study aims to observe the expression of Sclerostin during movement of orthodontic teeth and determine the effect of this protein on remodeling of periodontal tissues.

METHODS

Twenty-four Wistar rats were chosen. Orthodontic forces were applied between the bilateral incisor and first molar to achieve mesial movement. Rats in each group were executed at different time points (0, 1, 3, 5, 7, 14 d). Morphology of periodontal tissue was observed by hematoxylin-eosin (HE) staining. The number of osteoclasts were observed by tartrate-resistant acid phosphatase (TRAP) staining. Sclerostin expression were observed by immunohistochemical staining.

RESULTS

HE staining revealed that the resorption of alveolar bone intensified with prolonged movement. Results of immunohistochemical and TRAP staining revealed that Sclerostin expression and number of osteoclasts were related to duration of movement of orthodontic tooth. After staining for 5 days, the number of osteoclasts and Sclerostin expression reached their peak and then began to decline. The numbers of osteoclasts and the expression level of Sclerostin were higher at the compressive side than those at the tensive side.

CONCLUSION

Sclerostin affected orthodontic tooth movement by inhibiting the Wnt signaling pathway and by indirectly or directly controlling bone morphogenetic protein.

Sclerostin antibody treatment improves fracture outcomes in a Type I diabetic mouse model

Type 1 diabetes mellitus (T1DM) patients have osteopenia and impaired fracture healing due to decreased osteoblast activity. Further, no adequate treatments are currently available that can restore impaired healing in T1DM; hence a significant need exists to investigate new therapeutics for treatment of orthopedic complications. Sclerostin (SOST), a WNT antagonist, negatively regulates bone formation, and SostAb is a potent bone anabolic agent. To determine whether SOST antibody (SostAb) treatment improves fracture healing in streptozotocin (STZ) induced T1DM mice, we administered SostAb twice weekly for up to 21days post-fracture, and examined bone quality and callus outcomes at 21days and 42days post-fracture (11 and 14weeks of age, respectively). Here we show that SostAb treatment improves bone parameters; these improvements persist after cessation of antibody treatment. Markers of osteoblast differentiation such as Runx2, collagen I, osteocalcin, and DMP1 were reduced, while an abundant number of SP7/osterix-positive early osteoblasts were observed on the bone surface of STZ calluses. These results suggest that STZ calluses have poor osteogenesis resulting from failure of osteoblasts to fully differentiate and produce mineralized matrix, which produces a less mineralized callus. SostAb treatment enhanced fracture healing in both normal and STZ groups, and in STZ+SostAb mice, also reversed the lower mineralization seen in STZ calluses. Micro-CT analysis of calluses revealed improved bone parameters with SostAb treatment, and the mineralized bone was comparable to Controls. Additionally, we found sclerostin levels to be elevated in STZ mice and β-catenin activity to be reduced. Consistent with its function as a WNT antagonist, SostAb treatment enhanced β-catenin activity, but also increased the levels of SOST in the callus and in circulation. Our results indicate that SostAb treatment rescues the impaired osteogenesis seen in the STZ induced T1DM fracture model by facilitating osteoblast differentiation and mineralization of bone.

The WNT system: background and its role in bone

WNTs are extracellular proteins that activate different cell surface receptors linked to canonical and noncanonical WNT signalling pathways. The Wnt genes were originally discovered as important for embryonic development of fruit flies and malignant transformation of mouse mammary cancers. More recently, WNTs have been implicated in a wide spectrum of biological phenomena and diseases. During the last decade, several lines of clinical and preclinical evidence have indicated that WNT signalling is critical for trabecular and cortical bone mass, and this pathway is currently an attractive target for drug development. Based on detailed knowledge of the different WNT signalling pathways, it appears that it might be possible to develop drugs that specifically target cortical and trabecular bone. Neutralization of a bone-specific WNT inhibitor is now being evaluated as a promising anabolic treatment for patients with osteoporosis. Here, we provide the historical background to the discoveries of WNTs, describe the different WNT signalling pathways and summarize the current understanding of how these proteins regulate bone mass by affecting bone formation and resorption.

Impaired osteoblast differentiation in annexin A2- and -A5-deficient cells

Annexins are a class of calcium-binding proteins with diverse functions in the regulation of lipid rafts, inflammation, fibrinolysis, transcriptional programming and ion transport. Within bone, they are well-characterized as components of mineralizing matrix vesicles, although little else is known as to their function during osteogenesis. We employed shRNA to generate annexin A2 (AnxA2)- or annexin A5 (AnxA5)-knockdown pre-osteoblasts, and determined whether proliferation or osteogenic differentiation was altered in knockdown cells, compared to pSiren (Si) controls. We report that DNA content, a marker of proliferation, was significantly reduced in both AnxA2 and AnxA5 knockdown cells. Alkaline phosphatase expression and activity were also suppressed in AnxA2- or AnxA5-knockdown after 14 days of culture. The pattern of osteogenic gene expression was altered in knockdown cells, with Col1a1 expressed more rapidly in knock-down cells, compared to pSiren. In contrast, Runx2, Ibsp, and Bglap all revealed decreased expression after 14 days of culture. In both AnxA2- and AnxA5-knockdown, interleukin-induced STAT6 signaling was markedly attenuated compared to pSiren controls. These data suggest that AnxA2 and AnxA5 can influence bone formation via regulation of osteoprogenitor proliferation, differentiation, and responsiveness to cytokines in addition to their well-studied function in matrix vesicles.

Reduced renal calcium excretion in the absence of sclerostin expression: evidence for a novel calcium-regulating bone kidney axis

The kidneys contribute to calcium homeostasis by adjusting the reabsorption and excretion of filtered calcium through processes that are regulated by parathyroid hormone (PTH) and 1α,25-dihydroxyvitamin D3 (1α,25[OH]2D3). Most of the filtered calcium is reabsorbed in the proximal tubule, primarily by paracellular mechanisms that are not sensitive to calcium-regulating hormones in physiologically relevant ways. In the distal tubule, however, calcium is reabsorbed by channels and transporters, the activity or expression of which is highly regulated and increased by PTH and 1α,25(OH)2D3. Recent research suggests that other, heretofore unrecognized factors, such as the osteocyte-specific protein sclerostin, also regulate renal calcium excretion. Clues in this regard have come from the study of humans and mice with inactivating mutations of the sclerostin gene that both have increased skeletal density, which would necessitate an increase in intestinal absorption and/or renal reabsorption of calcium. Deletion of the sclerostin gene in mice significantly diminishes urinary calcium excretion and increases fractional renal calcium reabsorption. This is associated with increased circulating 1α,25(OH)2D3 levels, whereas sclerostin directly suppresses 1α-hydroxylase in immortalized proximal tubular cells. Thus, evidence is accumulating that sclerostin directly or indirectly reduces renal calcium reabsorption, suggesting the presence of a novel calcium-excreting bone-kidney axis.

To BMP or not to BMP during vertebrate limb bud development

The analysis of vertebrate limb bud development provides insight of general relevance into the signaling networks that underlie the controlled proliferative expansion of large populations of mesenchymal progenitors, cell fate determination and initiation of differentiation. In particular, extensive genetic analysis of mouse and experimental manipulation of chicken limb bud development has revealed the self-regulatory feedback signaling systems that interlink the main morphoregulatory signaling pathways including BMPs and their antagonists. It this review, we showcase the key role of BMPs and their antagonists during limb bud development. This review provides an understanding of the key morphoregulatory interactions that underlie the highly dynamic changes in BMP activity and signal transduction as limb bud development progresses from initiation and setting-up the signaling centers to determination and formation of the chondrogenic primordia for the limb skeletal elements.

Developments in sclerostin biology: regulation of gene expression, mechanisms of action, and physiological functions

The SOST gene, which encodes the protein sclerostin, was identified through genetic linkage analysis of sclerosteosis and van Buchem's disease patients. Sclerostin is a secreted glycoprotein that binds to the low-density lipoprotein receptor-related proteins 4, 5, and 6 to inhibit Wnt signaling. Since the initial discovery of sclerostin, much understanding has been gained into the role of this protein in the regulation of skeletal biology. In this article, we discuss the latest findings in the regulation of SOST expression, sclerostin mechanisms of action, and the potential utility of targeting sclerostin in conditions of low bone mass.

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.

Sost and its paralog Sostdc1 coordinate digit number in a Gli3-dependent manner

WNT signaling is critical in most aspects of skeletal development and homeostasis, and antagonists of WNT signaling are emerging as key regulatory proteins with great promise as therapeutic agents for bone disorders. Here we show that Sost and its paralog Sostdc1 emerged through ancestral genome duplication and their expression patterns have diverged to delineate non-overlapping domains in most organ systems including musculoskeletal, cardiovascular, nervous, digestive, reproductive and respiratory. In the developing limb, Sost and Sostdc1 display dynamic expression patterns with Sost being restricted to the distal ectoderm and Sostdc1 to the proximal ectoderm and the mesenchyme. While Sostdc1^–/– mice lack any obvious limb or skeletal defects, Sost^–/– mice recapitulate the hand defects described for Sclerosteosis patients. However, elevated WNT signaling in Sost^–/–; Sostdc1^–/– mice causes misregulation of SHH signaling, ectopic activation of Sox9 in the digit 1 field and preaxial polydactyly in a Gli1- and Gli3-dependent manner. In addition, we show that the syndactyly documented in Sclerosteosis is present in both Sost^–/– and Sost^–/–; Sostdc1^–/– mice, and is driven by misregulation of Fgf8 in the AER, a region lacking Sost and Sostdc1 expression. This study highlights the complexity of WNT signaling in skeletal biology and disease and emphasizes how redundant mechanism and non-cell autonomous effects can synergize to unveil new intricate phenotypes caused by elevated WNT signaling.

Expression and functional study of extracellular BMP antagonists during the morphogenesis of the digits and their associated connective tissues

The purpose of this study is to gain insight into the role of BMP signaling in the diversification of the embryonic limb mesodermal progenitors destined to form cartilage, joints, and tendons. Given the importance of extracellular BMP modulators in in vivo systems, we performed a systematic search of those expressed in the developing autopod during the formation of the digits. Here, we monitored the expression of extracellular BMP modulators including: Noggin, Chordin, Chordin-like 1, Chordin-like 2, Twisted gastrulation, Dan, BMPER, Sost, Sostdc1, Follistatin, Follistatin-like 1, Follistatin-like 5 and Tolloid. These factors show differential expression domains in cartilage, joints and tendons. Furthermore, they are induced in specific temporal patterns during the formation of an ectopic extra digit, preceding the appearance of changes that are identifiable by conventional histology. The analysis of gene regulation, cell proliferation and cell death that are induced by these factors in high density cultures of digit progenitors provides evidence of functional specialization in the control of mesodermal differentiation but not in cell proliferation or apoptosis. We further show that the expression of these factors is differentially controlled by the distinct signaling pathways acting in the developing limb at the stages covered by this study. In addition, our results provide evidence suggesting that TWISTED GASTRULATION cooperates with CHORDINS, BMPER, and NOGGIN in the establishment of tendons or cartilage in a fashion that is dependent on the presence or absence of TOLLOID.

Secreted and transmembrane wnt inhibitors and activators

Signaling by the Wnt family of secreted glycoproteins plays important roles in embryonic development and adult homeostasis. Wnt signaling is modulated by a number of evolutionarily conserved inhibitors and activators. Wnt inhibitors belong to small protein families, including sFRP, Dkk, WIF, Wise/SOST, Cerberus, IGFBP, Shisa, Waif1, APCDD1, and Tiki1. Their common feature is to antagonize Wnt signaling by preventing ligand-receptor interactions or Wnt receptor maturation. Conversely, the Wnt activators, R-spondin and Norrin, promote Wnt signaling by binding to Wnt receptors or releasing a Wnt-inhibitory step. With few exceptions, these antagonists and agonists are not pure Wnt modulators, but also affect additional signaling pathways, such as TGF-β and FGF signaling. Here we discuss their interactions with Wnt ligands and Wnt receptors, their role in developmental processes, as well as their implication in disease.

WNT signaling in bone homeostasis and disease: from human mutations to treatments

Low bone mass and strength lead to fragility fractures, for example, in elderly individuals affected by osteoporosis or children with osteogenesis imperfecta. A decade ago, rare human mutations affecting bone negatively (osteoporosis-pseudoglioma syndrome) or positively (high-bone mass phenotype, sclerosteosis and Van Buchem disease) have been identified and found to all reside in components of the canonical WNT signaling machinery. Mouse genetics confirmed the importance of canonical Wnt signaling in the regulation of bone homeostasis, with activation of the pathway leading to increased, and inhibition leading to decreased, bone mass and strength. The importance of WNT signaling for bone has also been highlighted since then in the general population in numerous genome-wide association studies. The pathway is now the target for therapeutic intervention to restore bone strength in millions of patients at risk for fracture. This paper reviews our current understanding of the mechanisms by which WNT signalng regulates bone homeostasis.

Sclerostin and Dickkopf-1 as therapeutic targets in bone diseases

The processes of bone growth, modeling, and remodeling determine the structure, mass, and biomechanical properties of the skeleton. Dysregulated bone resorption or bone formation may lead to metabolic bone diseases. The Wnt pathway plays an important role in bone formation and regeneration, and expression of two Wnt pathway inhibitors, sclerostin and Dickkopf-1 (DKK1), appears to be associated with changes in bone mass. Inactivation of sclerostin leads to substantially increased bone mass in humans and in genetically manipulated animals. Studies in various animal models of bone disease have shown that inhibition of sclerostin using a monoclonal antibody (Scl-Ab) increases bone formation, density, and strength. Additional studies show that Scl-Ab improves bone healing in models of bone repair. Inhibition of DKK1 by monoclonal antibody (DKK1-Ab) stimulates bone formation in younger animals and to a lesser extent in adult animals and enhances fracture healing. Thus, sclerostin and DKK1 are emerging as the leading new targets for anabolic therapies to treat bone diseases such as osteoporosis and for bone repair. Clinical trials are ongoing to evaluate the effects of Scl-Ab and DKK1-Ab in humans for the treatment of bone loss and for bone repair.

Targeted deletion of Sost distal enhancer increases bone formation and bone mass

The Wnt antagonist Sost has emerged as a key regulator of bone homeostasis through the modulation of Lrp4/5/6 Wnt coreceptors. In humans, lack of Sclerostin causes sclerosteosis and van Buchem (VB) disease, two generalized skeletal hyperostosis disorders that result from hyperactive Wnt signaling. Unlike sclerosteosis, VB patients lack SOST coding mutations but carry a homozygous 52 kb noncoding deletion that is essential for the transcriptional activation of SOST in bone. We recently identified a putative bone enhancer, ECR5, in the VB deletion region, and showed that the transcriptional activity of ECR5 is controlled by Mef2C transcription factor in vitro. Here we report that mice lacking ECR5 or Mef2C through Col1-Cre osteoblast/osteocyte-specific ablation result in high bone mass (HBM) due to elevated bone formation rates. We conclude that the absence of the Sost-specific long-range regulatory element ECR5 causes VB disease in rodents, and that Mef2C is the main transcription factor responsible for ECR5-dependent Sost transcriptional activation in the adult skeleton.

The transformation suppressor gene Reck is required for postaxial patterning in mouse forelimbs

The membrane-anchored metalloproteinase-regulator RECK has been characterized as a tumor suppressor. Here we report that mice with reduced Reck-expression show limb abnormalities including right-dominant, forelimb-specific defects in postaxial skeletal elements. The forelimb buds of low-Reck mutants have an altered dorsal ectoderm with reduced Wnt7a and Igf2 expression, and hypotrophy in two signaling centers (i.e., ZPA and AER) that are essential for limb outgrowth and patterning. Reck is abundantly expressed in the anterior mesenchyme in normal limb buds; mesenchyme-specific Reck inactivation recapitulates the low-Reck phenotype; and some teratogens downregulate Reck in mesenchymal cells. Our findings illustrate a role for Reck in the mesenchymal-epithelial interactions essential for mammalian development.

The effect of TNFα secreted from macrophages activated by titanium particles on osteogenic activity regulated by WNT/BMP signaling in osteoprogenitor cells

Wear particles are the major cause of osteolysis associated with failure of implant following total joint replacement. During this pathologic process, activated macrophages mediate inflammatory responses to increase osteoclastogenesis, leading to enhanced bone resorption. In osteolysis caused by wear particles, osteoprogenitors present along with macrophages at the implant interface may play significant roles in bone regeneration and implant osteointegration. Although the direct effects of wear particles on osteoblasts have been addressed recently, the role of activated macrophages in regulation of osteogenic activity of osteoblasts has scarcely been studied. In the present study, we examined the molecular communication between macrophages and osteoprogenitor cells that may explain the effect of wear particles on impaired bone forming activity in inflammatory bone diseases. It has been demonstrated that conditioned medium of macrophages challenged with titanium particles (Ti CM) suppresses early and late differentiation markers of osteoprogenitors, including alkaline phosphatase (ALP) activity, collagen synthesis, matrix mineralization and expression of osteocalcin and Runx2. Moreover, bone forming signals such as WNT and BMP signaling pathways were inhibited by Ti CM. Interestingly, TNFα was identified as a predominant factor in Ti CM to suppress osteogenic activity as well as WNT and BMP signaling activity. Furthermore, Ti CM or TNFα induces the expression of sclerostin (SOST) which is able to inhibit WNT and BMP signaling pathways. It was determined that over-expression of SOST suppressed ALP activity, whereas the inhibition of SOST by siRNA partially restored the effect of Ti CM on ALP activity. This study highlights the role of activated macrophages in regulation of impaired osteogenic activity seen in inflammatory conditions and provides a potential mechanism for autocrine regulation of WNT and BMP signaling mediated by TNFα via induction of SOST in osteprogenitor cells.

The regulation of valvular and vascular sclerosis by osteogenic morphogens

Vascular calcification increasingly afflicts our aging, dysmetabolic population. Once considered only a passive process of dead and dying cells, vascular calcification has now emerged as a highly regulated form of biomineralization organized by collagenous and elastin extracellular matrices. During skeletal bone formation, paracrine epithelial-mesenchymal and endothelial-mesenchymal interactions control osteochondrocytic differentiation of multipotent mesenchymal progenitor cells. These paracrine osteogenic signals, mediated by potent morphogens of the bone morphogenetic protein and wingless-type MMTV integration site family member (Wnt) superfamilies, are also active in the programming of arterial osteoprogenitor cells during vascular and valve calcification. Inflammatory cytokines, reactive oxygen species, and oxylipids-increased in the clinical settings of atherosclerosis, diabetes, and uremia that promote arteriosclerotic calcification-elicit the ectopic vascular activation of osteogenic morphogens. Specific extracellular and intracellular inhibitors of bone morphogenetic protein-Wnt signaling have been identified as contributing to the regulation of osteogenic mineralization during development and disease. These inhibitory pathways and their regulators afford the development of novel therapeutic strategies to prevent and treat valve and vascular sclerosis.

Lrp5 and Lrp6 redundantly control skeletal development in the mouse embryo

The role of Wnt signaling in osteoblastogenesis in the embryo remains to be fully established. Although β-catenin, a multifunctional protein also mediating canonical Wnt signaling, is indispensable for embryonic osteoblast differentiation, the roles of the key Wnt co-receptors Lrp5 and Lrp6 are unclear. Indeed, global deletion of either Lrp5 or Lrp6 did not overtly affect osteoblast differentiation in the mouse embryo. Here, we generated mice lacking both receptors specifically in the embryonic mesenchyme and observed an absence of osteoblasts in the embryo. In addition, the double-deficient embryos developed supernumerary cartilage elements in the zeugopod, revealing an important role for mesenchymal Lrp5/6 signaling in limb patterning. Importantly, the phenotypes of the Lrp5/6 mutant closely resembled those of the β-catenin-deficient embryos. These phenotypes are likely independent of any effect on the adherens junction, as deletion of α-catenin, another component of the complex, did not cause similar defects. Thus, Lrp5 and 6 redundantly control embryonic skeletal development, likely through β-catenin signaling.

Bone overgrowth-associated mutations in the LRP4 gene impair sclerostin facilitator function

Humans lacking sclerostin display progressive bone overgrowth due to increased bone formation. Although it is well established that sclerostin is an osteocyte-secreted bone formation inhibitor, the underlying molecular mechanisms are not fully elucidated. We identified in tandem affinity purification proteomics screens LRP4 (low density lipoprotein-related protein 4) as a sclerostin interaction partner. Biochemical assays with recombinant proteins confirmed that sclerostin LRP4 interaction is direct. Interestingly, in vitro overexpression and RNAi-mediated knockdown experiments revealed that LRP4 specifically facilitates the previously described inhibitory action of sclerostin on Wnt1/β-catenin signaling. We found the extracellular β-propeller structured domain of LRP4 to be required for this sclerostin facilitator activity. Immunohistochemistry demonstrated that LRP4 protein is present in human and rodent osteoblasts and osteocytes, both presumed target cells of sclerostin action. Silencing of LRP4 by lentivirus-mediated shRNA delivery blocked sclerostin inhibitory action on in vitro bone mineralization. Notably, we identified two mutations in LRP4 (R1170W and W1186S) in patients suffering from bone overgrowth. We found that these mutations impair LRP4 interaction with sclerostin and its concomitant sclerostin facilitator effect. Together these data indicate that the interaction of sclerostin with LRP4 is required to mediate the inhibitory function of sclerostin on bone formation, thus identifying a novel role for LRP4 in bone.

Prostaglandin E2 signals through PTGER2 to regulate sclerostin expression

The Wnt signaling pathway is a robust regulator of skeletal homeostasis. Gain-of-function mutations promote high bone mass, whereas loss of Lrp5 or Lrp6 co-receptors decrease bone mass. Similarly, mutations in antagonists of Wnt signaling influence skeletal integrity, in an inverse relation to Lrp receptor mutations. Loss of the Wnt antagonist Sclerostin (Sost) produces the generalized skeletal hyperostotic condition of sclerosteosis, which is characterized by increased bone mass and density due to hyperactive osteoblast function. Here we demonstrate that prostaglandin E₂ (PGE₂), a paracrine factor with pleiotropic effects on osteoblasts and osteoclasts, decreases Sclerostin expression in osteoblastic UMR106.01 cells. Decreased Sost expression correlates with increased expression of Wnt/TCF target genes Axin2 and Tcf3. We also show that the suppressive effect of PGE₂ is mediated through a cyclic AMP/PKA pathway. Furthermore, selective agonists for the PGE₂ receptor EP2 mimic the effect of PGE₂ upon Sost, and siRNA reduction in Ptger2 prevents PGE₂-induced Sost repression. These results indicate a functional relationship between prostaglandins and the Wnt/β-catenin signaling pathway in bone.