Bone Density Ligand, Sclerostin, Directly Interacts With LRP5 but Not LRP5G171V to Modulate Wnt Activity

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.

Estrogen receptor α mediates proliferation of osteoblastic cells stimulated by estrogen and mechanical strain, but their acute down-regulation of the Wnt antagonist Sost is mediated by estrogen receptor β

Mechanical strain and estrogens both stimulate osteoblast proliferation through estrogen receptor (ER)-mediated effects, and both down-regulate the Wnt antagonist Sost/sclerostin. Here, we investigate the differential effects of ERα and -β in these processes in mouse long bone-derived osteoblastic cells and human Saos-2 cells. Recruitment to the cell cycle following strain or 17β-estradiol occurs within 30 min, as determined by Ki-67 staining, and is prevented by the ERα antagonist 1,3-bis(4-hydroxyphenyl)-4-methyl-5-[4-(2-piperidinylethoxy)phenol]-1H-pyrazole dihydrochloride. ERβ inhibition with 4-[2-phenyl-5,7-bis(trifluoromethyl)pyrazolo[1,5-β]pyrimidin-3-yl] phenol (PTHPP) increases basal proliferation similarly to strain or estradiol. Both strain and estradiol down-regulate Sost expression, as does in vitro inhibition or in vivo deletion of ERα. The ERβ agonists 2,3-bis(4-hydroxyphenyl)-propionitrile and ERB041 also down-regulated Sost expression in vitro, whereas the ERα agonist 4,4′,4″-[4-propyl-(1H)-pyrazol-1,3,5-triyl]tris-phenol or the ERβ antagonist PTHPP has no effect. Tamoxifen, a nongenomic ERβ agonist, down-regulates Sost expression in vitro and in bones in vivo. Inhibition of both ERs with fulvestrant or selective antagonism of ERβ, but not ERα, prevents Sost down-regulation by strain or estradiol. Sost down-regulation by strain or ERβ activation is prevented by MEK/ERK blockade. Exogenous sclerostin has no effect on estradiol-induced proliferation but prevents that following strain. Thus, in osteoblastic cells the acute proliferative effects of both estradiol and strain are ERα-mediated. Basal Sost down-regulation follows decreased activity of ERα and increased activity of ERβ. Sost down-regulation by strain or increased estrogens is mediated by ERβ, not ERα. ER-targeting therapy may facilitate structurally appropriate bone formation by enhancing the distinct ligand-independent, strain-related contributions to proliferation of both ERα and ERβ.

Development of the endochondral skeleton

Much of the mammalian skeleton is composed of bones that originate from cartilage templates through endochondral ossification. Elucidating the mechanisms that control endochondral bone development is critical for understanding human skeletal diseases, injury response, and aging. Mouse genetic studies in the past 15 years have provided unprecedented insights about molecules regulating chondrocyte formation, chondrocyte maturation, and osteoblast differentiation, all key processes of endochondral bone development. These include the roles of the secreted proteins IHH, PTHrP, BMPs, WNTs, and FGFs, their receptors, and transcription factors such as SOX9, RUNX2, and OSX, in regulating chondrocyte and osteoblast biology. This review aims to integrate the known functions of extracellular signals and transcription factors that regulate development of the endochondral skeleton.

Serum β -Catenin Levels Associated with the Ratio of RANKL/OPG in Patients with Postmenopausal Osteoporosis

Objective. To demonstrate the role of Wnt/ β -catenin canonical pathway in postmenopausal osteoporosis by evaluating serum β -catenin levels in patients with postmenopausal osteoporosis and analyzing their possible relationship with serum OPG, RANKL, the ratio of RANKL/OPG, sclerostin, and bone turnover markers. Methods. 480 patients with postmenopausal osteoporosis and 170 healthy postmenopausal women were enrolled in the study. Serum β -catenin, OPG, RANKL, and sclerostin levels were measured by enzyme-linked immunosorbent assay. Bone status was assessed by measuring bone mineral density and bone turnover markers. Estradiol levels were also detected. Results. Serum β -catenin levels were lower in postmenopausal osteoporotic women compared to nonosteoporotic postmenopausal women (26.26 ± 14.81 versus 39.33 ± 5.47 pg/mL, P < 0.001). Serum β -catenin was positively correlated with osteoprotegerin (r = 0.232, P < 0.001) and negatively correlated with the ratio of RANKL/OPG, body mass index, and sclerostin (r = -0.128, P = 0.005; r = -0.117, P = 0.010; r = -0.400, P < 0.001, resp.) in patients with postmenopausal osteoporosis. Conclusion. The results indicate that lower serum β -catenin and concomitantly higher ratio of RANKL/OPG may be involved in the pathogenesis of postmenopausal osteoporosis. Functional communication between RANKL/RANK/OPG system and Wnt pathways plays an important role in postmenopausal osteoporosis.

Sclerostin and DKK1 in postmenopausal osteoporosis treated with denosumab

The bone mass benefits of antiresorbers in postmenopausal osteoporosis are limited by the rapid coupling of decreasing bone resorption with bone formation. Wnt signaling is involved in this coupling process during treatment with bisphosphonates, whereas its role during treatment with the anti-receptor activator of NF-κB ligand (RANKL) antibody denosumab is unknown. The study population includes patients participating in a placebo-controlled trial lasting 36 months: 19 women were on placebo and 24 on subcutaneous 60 mg denosumab every 6 months. All measured parameters (serum C-terminal telopeptide of type I collagen [sCTX], serum bone alkaline phosphatase [bAP], Dickkopf-1 [DKK1], and sclerostin) remained unchanged during the observation period in the placebo group. sCTX and bAP were significantly suppressed by denosumab treatment over the entire follow-up. Denosumab treatment was associated with significant (p < 0.05) increases (28% to 32%) in serum sclerostin over the entire study follow-up. Serum DKK1 significantly decreased within the first 6 months with a trend for further continuous decreases, which reached statistical significance (p < 0.05) versus placebo group from the 18th month onward. The changes in DKK1 were significantly and positively related with the changes in sCTX and bAP and negatively with hip bone mineral density (BMD) changes. The changes in sclerostin were significantly and negatively related only with those of bAP. The changes in bone turnover markers associated with denosumab treatment of postmenopausal osteoporosis is associated with significant increase in sclerostin similar to those seen after long-term treatment with bisphosphonates and significant decrease in DKK1. This latter observation might explain the continuous increase over 5 years in BMD observed during treatment of postmenopausal osteoporosis with denosumab.

Recent developments in bone anabolic therapy for osteoporosis

Osteoporosis is a disorder in which there is a net bone loss and microarchitectural deterioration with an increased risk of bone fracture because of uncoupling of bone formation and bone resorption. The treatment of osteoporosis aims to inhibit bone resorption by osteoclasts and/or promote bone formation by osteoblasts. However, most of the current approaches for treating osteoporosis focus on inhibiting bone resorption. As the only US FDA-approved anabolic agent, the recombinant human parathyroid hormone is recommended for consecutive 2-year period treatment in a clinical setting. Therefore, it is highly desirable to identify novel bone anabolic agents or approaches for osteoporosis treatment. In this review, the authors introduce a new bone anabolic therapy by means of RNAi strategy. Specifically, the authors also discuss the current status and perspectives for RNAi as a novel anabolic approach in the treatment of osteoporosis.

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.

Transforming growth factor β1 inhibits bone morphogenic protein (BMP)-2 and BMP-7 signaling via upregulation of Ski-related novel protein N (SnoN): possible mechanism for the failure of BMP therapy?

BACKGROUND

Bone morphogenic proteins (BMPs) play a key role in bone formation. Consequently, it was expected that topical application of recombinant human (rh)BMP-2 and rhBMP-7 would improve the healing of complex fractures. However, up to 36% of fracture patients do not respond to this therapy. There are hints that a systemic increase in transforming growth factor β1 (TGFβ1) interferes with beneficial BMP effects. Therefore, in the present work we investigated the influence of rhTGFβ1 on rhBMP signaling in primary human osteoblasts, with the aim of more specifically delineating the underlying regulatory mechanisms.

METHODS

BMP signaling was detected by adenoviral Smad-binding-element-reporter assays. Gene expression was determined by reverse transcription polymerase chain reaction (RT-PCR) and confirmed at the protein level by western blot. Histone deacetylase (HDAC) activity was determined using a test kit. Data sets were compared by one-way analysis of variance.

RESULTS

Our findings showed that Smad1/5/8-mediated rhBMP-2 and rhBMP-7 signaling is completely blocked by rhTGFβ1. We then investigated expression levels of genes involved in BMP signaling and regulation (for example, Smad1/5/8, TGFβ receptors type I and II, noggin, sclerostin, BMP and activin receptor membrane bound inhibitor (BAMBI), v-ski sarcoma viral oncogene homolog (Ski), Ski-related novel protein N (SnoN) and Smad ubiquitination regulatory factors (Smurfs)) and confirmed the expression of regulated genes at the protein level. Smad7 and SnoN were significantly induced by rhTGFβ1 treatment while expression of Smad1, Smad6, TGFβRII and activin receptor-like kinase 1 (Alk1) was reduced. Elevated SnoN expression was accompanied by increased HDAC activity. Addition of an HDAC inhibitor, namely valproic acid, fully abolished the inhibitory effect of rhTGFβ1 on rhBMP-2 and rhBMP-7 signaling.

CONCLUSIONS

rhTGFβ1 effectively blocks rhBMP signaling in osteoblasts. As possible mechanism, we postulate an induction of SnoN that increases HDAC activity and thereby reduces the expression of factors required for efficient BMP signaling. Thus, inhibition of HDAC activity may support bone healing during rhBMP therapy in patients with elevated TGFβ serum levels.

Mechanotransduction in bone tissue: The A214V and G171V mutations in Lrp5 enhance load-induced osteogenesis in a surface-selective manner

Mechanotransduction in bone requires components of the Wnt signaling pathway to produce structurally adapted bone elements. In particular, the Wnt co-receptor LDL-receptor-related protein 5 (LRP5) appears to be a crucial protein in the mechanotransduction cascades that translate physical tissue deformation into new bone formation. Recently discovered missense mutations in LRP5 are associated with high bone mass (HBM), and the altered function of these proteins provide insight into LRP5 function in many skeletal processes, including mechanotransduction. We further investigated the role of LRP5 in bone cell mechanotransduction by applying mechanical stimulation in vivo to two different mutant mouse lines, which harbor HBM-causing missense mutations in Lrp5. Axial tibia loading was applied to mature male Lrp5 G171V and Lrp5 A214V knock-in mice, and to their wild type controls. Fluorochrome labeling revealed that 3 days of loading resulted in a significantly enhanced periosteal response in the A214V knock in mice, whereas the G171V mice exhibited a lowered osteogenic threshold on the endocortical surface. In summary, our data further highlight the importance of Lrp5 in bone cell mechanotransduction, and indicate that the HBM-causing mutations in Lrp5 can alter the anabolic response to mechanical stimulation in favor of increased bone gain.

Increasing participation of sclerostin in postnatal bone development, revealed by three-dimensional immunofluorescence morphometry

Confocal immunofluorescence tiling imaging revealed the spatio-temporal distributions of osterix and sclerostin in femurs from 3-day-old, 2-week-old and 4-week-old rats to be reciprocally exclusive at the tissue level. Further quantitative three-dimensional immuno fluorescence morphometry demonstrated the increasing distribution of sclerostin in the osteocytic lacuno-canalicular system specifically in diaphysis, which paralleled the cooperative participation and depletion of osterix and β-catenin in adjacent periosteum cells. Treating MC3T3-E1 cells with BIO (a GSK3 inhibitor) induced the stabilization of β-catenin and nuclear translocation of osterix, and negatively regulated osteocalcin/BGLAP and Dmp1. These results collectively demonstrate that the increasing distribution of sclerostin in diaphyseal cortical bone appears to be involved in the attenuation of osterix and β-catenin in adjacent periosteum cells, thus possibly contributing to osteoblast maturation and reducing the osteoblast formation at this bone site. Our confocal microscopy-based imaging analyses provide a comprehensive and detailed view of the spatio-temporal distribution of sclerostin, β-catenin and osterix at the tissue to subcellular level in a coherent manner, and uncovered their spatio-temporal cooperation in postnatal bone development, thus providing evidence that they link skeletogenic growth and functional bone development.

Wnt/β-catenin signaling and disease

The WNT signal transduction cascade controls myriad biological phenomena throughout development and adult life of all animals. In parallel, aberrant Wnt signaling underlies a wide range of pathologies in humans. In this Review, we provide an update of the core Wnt/β-catenin signaling pathway, discuss how its various components contribute to disease, and pose outstanding questions to be addressed in the future.

Characterization of the interaction of sclerostin with the low density lipoprotein receptor-related protein (LRP) family of Wnt co-receptors

LRP5 and LRP6 are proteins predicted to contain four six-bladed β-propeller domains and both bind the bone-specific Wnt signaling antagonist sclerostin. Here, we report the crystal structure of the amino-terminal region of LRP6 and using NMR show that the ability of sclerostin to bind to this molecule is mediated by the central core of sclerostin and does not involve the amino- and carboxyl-terminal flexible arm regions. We show that this structured core region interacts with LRP5 and LRP6 via an NXI motif (found in the sequence PNAIG) within a flexible loop region (loop 2) within the central core region. This sequence is related closely to a previously identified motif in laminin that mediates its interaction with the β-propeller domain of nidogen. However, the NXI motif is not involved in the interaction of sclerostin with LRP4 (another β-propeller containing protein in the LRP family). A peptide derived from the loop 2 region of sclerostin blocked the interaction of sclerostin with LRP5/6 and also inhibited Wnt1 but not Wnt3A or Wnt9B signaling. This suggests that these Wnts interact with LRP6 in different ways.

Decreased serum sclerostin levels in patients with primary hyperparathyroidism: a cross-sectional and a longitudinal study

Decreased serum sclerostin was evident in patients with primary hyperparathyroidism and was inversely related to parathyroid hormone (PTH). Sclerostin normalized earlier than biochemical bone turnover markers (BTMs) following parathyroidectomy.

INTRODUCTION

There is limited information on the changes of serum sclerostin in conditions with chronic PTH excess in humans. The main objectives of the present study were to: (1) examine cross-sectionally the changes of serum sclerostin levels in patients with primary hyperparathyroidism (PHPT), (2) study the time course changes in serum sclerostin in PHPT patients following parathyroidectomy (PTX) followed up longitudinally for 12 months, and (3) compare the changes in serum sclerostin to that of BTMs.

METHODS

We studied 60 PHPT patients and compared them with 74 PTX patients together with 268 age- and sex-matched healthy controls. Also, we followed 27 PTX patients longitudinally at 2, 4, 6, 10, 30, 60, 180, and 360 days postoperatively. Serum sclerostin, BTMs, and minerals were measured. Also, bone mineral density was determined by dual energy X-ray absorptiometry.

RESULTS

Patients with PHPT exhibited significantly lower mean serum sclerostin [mean, in picomoles per liter; 95% confidence interval (CI)] (28.98; 27.94-30.03) than that obtained for PTX patients (37.01; 35.75-38.27) and healthy controls (46.22; 45.13-47.31) (P < 0.0001, for each case), respectively. Serum PTH inversely correlated with serum sclerostin (r = -0.651, P < 0.0001). Serum sclerostin was normalized in PTX patients by the tenth day postoperatively and remained within the expected reference range thereafter.

CONCLUSIONS

Significantly decreased serum sclerostin was evidenced in PHPT patients as compared with PTX and euparathyroid controls. The inverse PTH and sclerostin relationship suggests that sclerostin is downregulated by PTH in humans. Serum sclerostin normalized earlier than BTMs following parathyroidectomy.

Expression of sclerostin in the developing zebrafish (Danio rerio) brain and skeleton

Sclerostin is a highly conserved, secreted, cystine-knot protein which regulates osteoblast function. Humans with mutations in the sclerostin gene (SOST), manifest increased axial and appendicular skeletal bone density with attendant complications. In adult bone, sclerostin is expressed in osteocytes and osteoblasts. Danio rerio sclerostin-like protein is closely related to sea bass sclerostin, and is related to chicken and mammalian sclerostins. Little is known about the expression of sclerostin in early developing skeletal or extra-skeletal tissues. We assessed sclerostin (sost) gene expression in developing zebrafish (D. rerio) embryos with whole mount is situ hybridization methods. The earliest expression of sost mRNA was noted during 12h post-fertilization (hpf). At 15 hpf, sost mRNA was detected in the developing nervous system and in Kupffer's vesicle. At 18, 20 and 22 hpf, expression in rhombic lip precursors was seen. By 24 hpf, expression in the upper and lower rhombic lip and developing spinal cord was noted. Expression in the rhombic lip and spinal cord persisted through 28 hpf and then diminished in intensity through 44 hpf. At 28 hpf, sost expression was noted in developing pharyngeal cartilage; expression in pharyngeal cartilage increased with time. By 48 hpf, sost mRNA was clearly detected in the developing pharyngeal arch cartilage. Sost mRNA was abundantly expressed in the pharyngeal arch cartilage, and in developing pectoral fins, 72, 96 and 120 hpf. Our study is the first detailed analysis of sost gene expression in early metazoan development.

Bisphosphonate treatment of postmenopausal osteoporosis is associated with a dose dependent increase in serum sclerostin

The benefits coming from long-term treatment of postmenopausal osteoporosis with bisphophonates are limited by a coupled decrease in bone formation. The objective of this study is to determine whether this decrease in bone formation is associated with changes in serum levels of the WNT signaling antagonist sclerostin or Dickkopf-1 (DKK1). This is an ancillary observation from patients participating in a 12 months, phase 2, randomized clinical trial. We analyzed 107 patients given either monthly intramuscular neridronate (12.5, 25 or 50 mg) or placebo. Serum C-terminal telopeptide of type I collagen (sCTX, a bone-resorption marker) decreased by 61%, 75% and 73% in the 12.5, 25 and 50 mg dose groups, respectively. Mean changes in bone alkaline phosphatase (bAP) at 12 months were -47%, -60.0% and -52.6% in the groups receiving 12.5, 25 or 50 mg neridronate, respectively. Serum DKK1 remained unchanged at all time points in the 3 groups. Serum sclerostin increased versus placebo group gradually and significantly only in patients treated with 25 or 50 mg neridronate monthly, reaching 138-148% of baseline values (P<0.001). Changes in serum sclerostin at 12 months were negatively correlated with changes in bAP (P<0.001) even when data were adjusted for sCTX changes and only treated patients were included. In conclusions, decreased bone formation after several months of bisphosphonate therapy is associated with increased serum levels of sclerostin. This might suggest that Wnt signaling may play a role in the coupling between resorption and formation.

Update on bone anabolics in osteoporosis treatment: rationale, current status, and perspectives

Osteoporosis is defined as low bone mineral density associated with skeletal fractures secondary to minimal or no trauma, most often involving the spine, the hip, and the forearm. The decrease in bone mineral density is the consequence of an unbalanced bone remodeling process, with higher bone resorption than bone formation. Osteoporosis affects predominantly postmenopausal women, but also older men. This chronic disease represents a considerable medical and socioeconomic burden for modern societies. The therapeutic options for the treatment of osteoporosis have so far comprised mostly antiresorptive drugs, in particular bisphosphonates and more recently denosumab, but also calcitonin and, for women, estrogens or selective estrogen receptor modulators. These drugs have limitations, however, in particular the fact that they lead to a low turnover state where bone formation decreases with the decrease in bone-remodeling activity. In this review, we discuss the alternative class of osteoporosis drugs, i.e. bone anabolics, their biology, and the perspectives they offer for our therapeutic armamentarium. We focus on the two main osteoanabolic pathways identified as of today: PTH, the only anabolic drug currently on the market; and activation of canonical Wnt signaling through inhibition of the endogenous inhibitors sclerostin and dickkopf1. Each approach is based on a different molecular mechanism, but most recent evidence suggests that these two pathways may actually converge, at least in part. Whereas recombinant human PTH treatment is being revisited with different formulations and attempts to regulate endogenous PTH secretion via the calcium-sensing receptor, antibodies to sclerostin and dickkopf1 are currently in clinical trials and may prove to be even more efficient at increasing bone mass, possibly independent of bone turnover. Each of these anabolic approaches has its own limitations and safety issues, but the prospects of effective anabolic therapy for osteoporosis are indeed bright.

Antagonists of LRP6 regulate PTH-induced cAMP generation

LRP6 is a common coreceoptor for different G protein-coupled seven-transmembrane receptors in production of cAMP. Extracelluar proteins sclerostin and DKK1, initially identified as antagonists for Wnt signaling by binding to LRP6, are negative regulators for bone formation. Here, we show that both sclerostin and DKK1 inhibit PTH-stimulated cAMP production. In addition, PTH suppresses expression of sclerostin in osteocytes in mice. We also found that sclerostin and DKK1 binds to LRP6 as antagonists to increase the availability of LRP6 to facilitate PTH signaling in a positive-feedback fashion. These studies reveal a previously unrecognized function of sclerostin and DKK1, which provides an alternative explanation for the application of sclerostin and DKK1 neutralization on enhancing bone formation as a potential therapy for skeletal diseases.

The sclerostin-bone protein interactome

The secreted glycoprotein, sclerostin alters bone formation. To gain insights into the mechanism of action of sclerostin, we examined the interactions of sclerostin with bone proteins using a sclerostin affinity capture technique. Proteins from decalcified rat bone were captured on a sclerostin-maltose binding protein (MBP) amylose column, or on a MBP amylose column. The columns were extensively washed with low ionic strength buffer, and bound proteins were eluted with buffer containing 1M sodium chloride. Eluted proteins were separated by denaturing sodium-dodecyl sulfate gel electrophoresis and were identified by mass spectrometry. Several previously unidentified full-length sclerostin-interacting proteins such as alkaline phosphatase, carbonic anhydrase, gremlin-1, fetuin A, midkine, annexin A1 and A2, and collagen α1, which have established roles in bone formation or resorption processes, were bound to the sclerostin-MBP amylose resin but not to the MBP amylose resin. Other full-length sclerostin-interacting proteins such as casein kinase II and secreted frizzled related protein 4 that modulate Wnt signaling were identified. Several peptides derived from proteins such as Phex, asporin and follistatin that regulate bone metabolism also bound sclerostin. Sclerostin interacts with multiple proteins that alter bone formation and resorption and is likely to function by altering several biologically relevant pathways in bone.

Sclerostin expression is induced by BMPs in human Saos-2 osteosarcoma cells but not via direct effects on the sclerostin gene promoter or ECR5 element

Sclerostin is a secreted inhibitor of Wnt signaling and plays an essential role in the regulation of bone mass. The expression of sclerostin is largely restricted to osteocytes although its mode of transcriptional regulation is not well understood. We observed regulated expression of sclerostin mRNA and protein that was directly correlated with the mineralization response in cultured human Saos-2 osteosarcoma cells and rat primary calvarial cells. Sclerostin mRNA and protein levels were increased following treatment of cells with BMP2, BMP4 and BMP7. Analysis of deletion mutants from the -7.4 kb upstream region of the human sclerostin promoter did not reveal any specific regions that were responsive to BMPs, Wnt3a, PTH, TGFβ1 or Activin A in Saos-2 cells. The downstream ECR5 element did not show enhancer activity in Saos-2 cells and also was not affected when Saos-2 cells were treated with BMPs or PTH. Genome-wide microarray analysis of Saos-2 cells treated with BMP2 showed significant changes in expression of several transcription factors with putative consensus DNA binding sites in the region of the sclerostin promoter. However, whereas most factors tested showed either a range of inhibitory activity (DLX family, MSX2, HEY1, SMAD6/7) or lack of activity on the sclerostin promoter including SMAD9, only MEF2B showed a positive effect on both the promoter and ECR5 element. These results suggest that the dramatic induction of sclerostin gene expression by BMPs in Saos-2 cells occurs indirectly and is associated with late stage differentiation of osteoblasts and the mineralization process.

The optimal dose of recombinant human osteogenic protein-1 enhances differentiation of mouse osteoblast-like cells: an in vitro study

OBJECTIVE

There is no certain conclusion on the effect of recombinant human Osteogenic Protein-1 (OP-1, BMP-7) on the proliferation of the osteoblast-like cell line, MC3T3-E1. Furthermore, the optimal dose of rhOP-1 on cell differentiation still needs to be elucidated. This investigation aims to delineate the biofunctional characteristics of rhOP-1 in inducing osteoblastogenesis of MC3T3-E1 through in vitro time-course and dose-response studies.

DESIGN

MC3T3-E1 cells were cultured for 1, 4, 7 days with the addition of different rhOP-1 concentrations (0, 10, 20, 50, 100, 200, 400 ng/ml), and cell proliferation and cell differentiation were examined.

RESULTS

MC3T3-E1 cell proliferation was stimulated by rhOP-1 in a dose-dependent manner (0-400 ng/ml) on day 1, whereas on day 4 and 7, it was still stimulated at low concentrations (10, 20, 50 ng/ml) but inhibited at high ones (200, 400 ng/ml). The alkaline phosphatase (ALP) activity, osteocalcin (OC) production, collagen deposition and extracellular matrix mineralization were dramatically elevated by rhOP-1 treatment, as a function of culture time and rhOP-1 concentration, and all of them reached a plateau at the concentration of 200 ng/ml. Real-time quantitative RT-PCR results showed Runx2, AKP-2, OC and Nog mRNA expressions increased in a dose- and time-dependent manner, and their expressions were significantly higher at high rhOP-1 concentrations than that of low ones. No significant differences were found between the effects of 200 ng/ml rhOP-1 and 400 ng/ml rhOP-1 on the differentiation of MC3T3-E1 cells, except the expression of Nog mRNA, whose expression level was much higher at 400 ng/ml than that at 200 ng/ml.

CONCLUSIONS

These results suggest that cell proliferation of MC3T3-E1 is depended on culture time and rhOP-1 concentration, rhOP-1 could stimulate the differentiation of MC3T3-E1 cells and the optimal concentration could be 200 ng/ml.

Update on Wnt signaling in bone cell biology and bone disease

For more than a decade, Wnt signaling pathways have been the focus of intense research activity in bone biology laboratories because of their importance in skeletal development, bone mass maintenance, and therapeutic potential for regenerative medicine. It is evident that even subtle alterations in the intensity, amplitude, location, and duration of Wnt signaling pathways affects skeletal development, as well as bone remodeling, regeneration, and repair during a lifespan. Here we review recent advances and discrepancies in how Wnt/Lrp5 signaling regulates osteoblasts and osteocytes, introduce new players in Wnt signaling pathways that have important roles in bone development, discuss emerging areas such as the role of Wnt signaling in osteoclastogenesis, and summarize progress made in translating basic studies to clinical therapeutics and diagnostics centered around inhibiting Wnt pathway antagonists, such as sclerostin, Dkk1 and Sfrp1. Emphasis is placed on the plethora of genetic studies in mouse models and genome wide association studies that reveal the requirement for and crucial roles of Wnt pathway components during skeletal development and disease.

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.

Structural basis of Wnt signaling inhibition by Dickkopf binding to LRP5/6

LDL receptor-related proteins 5 and 6 (LRP5/6) are coreceptors for Wnt growth factors, and also bind Dkk proteins, secreted inhibitors of Wnt signaling. The LRP5/6 ectodomain contains four β-propeller/EGF-like domain repeats. The first two repeats, LRP6(1-2), bind to several Wnt variants, whereas LRP6(3-4) binds other Wnts. We present the crystal structure of the Dkk1 C-terminal domain bound to LRP6(3-4), and show that the Dkk1 N-terminal domain binds to LRP6(1-2), demonstrating that a single Dkk1 molecule can bind to both portions of the LRP6 ectodomain and thereby inhibit different Wnts. Small-angle X-ray scattering analysis of LRP6(1-4) bound to a noninhibitory antibody fragment or to full-length Dkk1 shows that in both cases the ectodomain adopts a curved conformation that places the first three repeats at a similar height relative to the membrane. Thus, Wnts bound to either portion of the LRP6 ectodomain likely bear a similar spatial relationship to Frizzled coreceptors.

Crystal structures of the extracellular domain of LRP6 and its complex with DKK1

Low-density-lipoprotein (LDL) receptor-related proteins 5 and 6 (LRP5/6) are Wnt co-receptors essential for Wnt/β-catenin signaling. Dickkopf 1 (DKK1) inhibits Wnt signaling by interacting with the extracellular domains of LRP5/6 and is a drug target for multiple diseases. Here we present the crystal structures of a human LRP6-E3E4-DKK1 complex and the first and second halves of human LRP6's four propeller-epidermal growth factor (EGF) pairs (LRP6-E1E2 and LRP6-E3E4). Combined with EM analysis, these data demonstrate that LRP6-E1E2 and LRP6-E3E4 form two rigid structural blocks, with a short intervening hinge that restrains their relative orientation. The C-terminal domain of DKK1 (DKK1c) interacts with the top surface of the LRP6-E3 YWTD propeller and given their structural similarity, probably also that of the LRP6-E1 propeller, through conserved hydrophobic patches buttressed by a network of salt bridges and hydrogen bonds. Our work provides key insights for understanding LRP5/6 structure and the interaction of LRP5/6 with DKK, as well as for drug discovery.

α-Actinin-3 deficiency is associated with reduced bone mass in human and mouse

Bone mineral density (BMD) is a complex trait that is the single best predictor of the risk of osteoporotic fractures. Candidate gene and genome-wide association studies have identified genetic variations in approximately 30 genetic loci associated with BMD variation in humans. α-Actinin-3 (ACTN3) is highly expressed in fast skeletal muscle fibres. There is a common null-polymorphism R577X in human ACTN3 that results in complete deficiency of the α-actinin-3 protein in approximately 20% of Eurasians. Absence of α-actinin-3 does not cause any disease phenotypes in muscle because of compensation by α-actinin-2. However, α-actinin-3 deficiency has been shown to be detrimental to athletic sprint/power performance. In this report we reveal additional functions for α-actinin-3 in bone. α-Actinin-3 but not α-actinin-2 is expressed in osteoblasts. The Actn3(-/-) mouse displays significantly reduced bone mass, with reduced cortical bone volume (-14%) and trabecular number (-61%) seen by microCT. Dynamic histomorphometry indicated this was due to a reduction in bone formation. In a cohort of postmenopausal Australian women, ACTN3 577XX genotype was associated with lower BMD in an additive genetic model, with the R577X genotype contributing 1.1% of the variance in BMD. Microarray analysis of cultured osteoprogenitors from Actn3(-/-) mice showed alterations in expression of several genes regulating bone mass and osteoblast/osteoclast activity, including Enpp1, Opg and Wnt7b. Our studies suggest that ACTN3 likely contributes to the regulation of bone mass through alterations in bone turnover. Given the high frequency of R577X in the general population, the potential role of ACTN3 R577X as a factor influencing variations in BMD in elderly humans warrants further study.

Wnt antagonists bind through a short peptide to the first β-propeller domain of LRP5/6

The Wnt pathway inhibitors DKK1 and sclerostin (SOST) are important therapeutic targets in diseases involving bone loss or damage. It has been appreciated that Wnt coreceptors LRP5/6 are also important, as human missense mutations that result in bone overgrowth (bone mineral density, or BMD, mutations) cluster to the E1 propeller domain of LRP5. Here, we report a crystal structure of LRP6 E1 bound to an antibody, revealing that the E1 domain is a peptide recognition module. Remarkably, the consensus E1 binding sequence is a close match to a conserved tripeptide motif present in all Wnt inhibitors that bind LRP5/6. We show that this motif is important for DKK1 and SOST binding to LRP6 and for inhibitory function, providing a detailed structural explanation for the effect of the BMD mutations.

Involvement of Wnt activation in the micromechanical vibration-enhanced osteogenic response of osteoblasts

BACKGROUND

Low-magnitude vibration has been widely used as a tool for rehabilitation, enhancing physical performance, and stimulating bone development. Although mechanical stimulation generated by vibrations is regarded as important factor in bone remodeling, the underlying cellular and molecular regulatory mechanisms of this response, which may be important in the development of new mechanobiological strategies, currently remain unclear.

METHODS

In this study, to investigate the mechanobiological mechanisms of vibration-enhanced osteogenic responses in osteoblasts, MC3T3-E1 cells were subjected to vibrations of different amplitude (0.06, 0.14, 0.32, 0.49, 0.66, and 0.8 × g) at 40 Hz for 30 min/day over 3 days. The osteogenesis-related transcription factors Wnt10B, Sclerostin, OPG, and RANKL were analyzed for mRNA and protein expression.

RESULTS

The results revealed that protein expression of Wnt10B and OPG was increased in a magnitude-dependent manner by mechanical vibrations at amplitudes of 0.06, 0.14, 0.32, and 0.49 × g; the maximum increases were 2.4-fold (p < 0.001) and 7.9-fold (p < 0.001), respectively, at 0.49 × g. Sclerostin and RANKL levels were reduced at all amplitudes. On the basis of mRNA levels, the reduced expression of RANKL was further downregulated (p < 0.05) whereas OPG expression was further increased (p < 0.01) when the MC3T3-E1 cells were treated with LiCl compared with the effects of vibration alone.

CONCLUSIONS

The findings may indicate that Wnt signaling is involved in mechanotransduction at low-magnitude vibration; this may provide a cellular basis, and impetus for further development of, biomechanically based intervention for enhancing bone strength and accelerating implant osseointegration.

Identification of the first deletion in the LRP5 gene in a patient with autosomal dominant osteopetrosis type I

In the last decade, the low-density lipoprotein receptor-related protein 5 (LRP5) gene, coding for a coreceptor in the canonical Wnt signalling pathway, has been shown to play an important role in regulating bone mass and to be involved in the pathogenesis of several bone disorders. Here we describe a patient who presented with a clinical picture of Autosomal Dominant Osteopetrosis type I (ADO I), in whom we could identify the first deletion in the LRP5 gene causing increased bone mass. This mutation caused the in-frame deletion of two amino acids in the fourth blade of the first propeller of the protein, namely the highly conserved glycine at position 171 and the following glutamate residue. In vitro studies suggested that the pathogenic effect of this novel mutation could be due to a decreased inhibition of Wnt signalling by the antagonistic proteins sclerostin and Dickkopf-1, encoded respectively by the SOST and DKK1 genes, in the presence of mutated LRP5. Our results highlight an increasing molecular heterogeneity in LRP5-related bone diseases.

Dissecting molecular differences between Wnt coreceptors LRP5 and LRP6

Low-density lipoprotein receptor-related proteins 5 and 6 (LRP5 and LRP6) serve as Wnt co-receptors for the canonical β-catenin pathway. While LRP6 is essential for embryogenesis, both LRP5 and LRP6 play critical roles for skeletal remodeling, osteoporosis pathogenesis and cancer formation, making LRP5 and LRP6 key therapeutic targets for cancer and disease treatment. LRP5 and LRP6 each contain in the cytoplasmic domain five conserved PPPSPxS motifs that are pivotal for signaling and serve collectively as phosphorylation-dependent docking sites for the scaffolding protein Axin. However existing data suggest that LRP6 is more effective than LRP5 in transducing the Wnt signal. To understand the molecular basis that accounts for the different signaling activity of LRP5 and LRP6, we generated a series of chimeric receptors via swapping LRP5 and LRP6 cytoplasmic domains, LRP5C and LRP6C, and studied their Wnt signaling activity using biochemical and functional assays. We demonstrate that LRP6C exhibits strong signaling activity while LRP5C is much less active in cells. Recombinant LRP5C and LRP6C upon in vitro phosphorylation exhibit similar Axin-binding capability, suggesting that LRP5 and LRP6 differ in vivo at a step prior to Axin-binding, likely at receiving phosphorylation. We identified between the two most carboxyl PPPSPxS motifs an intervening "gap4" region that appears to account for much of the difference between LRP5C and LRP6C, and showed that alterations in this region are sufficient to enhance LRP5 PPPSPxS phosphorylation and signaling to levels comparable to LRP6 in cells. In addition we provide evidence that binding of phosphorylated LRP5 or LRP6 to Axin is likely direct and does not require the GSK3 kinase as a bridging intermediate as has been proposed. Our studies therefore uncover a new and important molecular tuning mechanism for differential regulation of LRP5 and LRP6 phosphorylation and signaling activity.

A rate-limiting role for Dickkopf-1 in bone formation and the remediation of bone loss in mouse and primate models of postmenopausal osteoporosis by an experimental therapeutic antibody

Genetic studies have linked both osteoporotic and high bone mass phenotypes to low-density lipoprotein receptor-related proteins (LRP4, LRP5, and LRP6). LRPs are receptors for inhibitory Dickkopf-1 (DKK1) protein, and treatment modalities that modulate LRP/DKK1 binding therefore may act as stimulators of bone mass accrual. Here, we report that RH2-18, a fully human monoclonal anti-DKK1 antibody elicits systemic pharmacologic bone efficacy and new bone formation at endosteal bone surfaces in vivo in a mouse model of estrogen-deficiency-induced osteopenia. This was paralleled by partial-to-complete resolution of osteopenia (bone mineral density) at all of the skeletal sites investigated in femur and lumbar-vertebral bodies and the restoration of trabecular bone microarchitecture. More importantly, testing of RH2-18 in adult, osteopenic rhesus macaques demonstrated a rate-limiting role of DKK1 at multiple skeletal sites and responsiveness to treatment. In conclusion, this study provides pharmacologic evidence for the modulation of DKK1 bioactivity in the adult osteopenic skeleton as a viable approach to resolve osteopenia in animal models. Thus, data described here suggest that targeting DKK1 through means such as a fully human anti-DKK1-antibody provides a potential bone-anabolic treatment for postmenopausal osteoporosis.

Anabolic and catabolic regimens of human parathyroid hormone 1-34 elicit bone- and envelope-specific attenuation of skeletal effects in Sost-deficient mice

PTH is a potent calcium-regulating factor that has skeletal anabolic effects when administered intermittently or catabolic effects when maintained at consistently high levels. Bone cells express PTH receptors, but the cellular responses to PTH in bone are incompletely understood. Wnt signaling has recently been implicated in the osteo-anabolic response to the hormone. Specifically, the Sost gene, a major antagonist of Wnt signaling, is down-regulated by PTH exposure. We investigated this mechanism by treating Sost-deficient mice and their wild-type littermates with anabolic and catabolic regimens of PTH and measuring the skeletal responses. Male Sost(+/+) and Sost(-/-) mice were injected daily with human PTH 1-34 (0, 30, or 90 μg/kg) for 6 wk. Female Sost(+/+) and Sost(-/-) mice were continuously infused with vehicle or high-dose PTH (40 μg/kg · d) for 3 wk. Dual energy x-ray absorptiometry-derived measures of intermittent PTH (iPTH)-induced bone gain were impaired in Sost(-/-) mice. Further probing revealed normal or enhanced iPTH-induced cortical bone formation rates but concomitant increases in cortical porosity among Sost(-/-) mice. Distal femur trabecular bone was highly responsive to iPTH in Sost(-/-) mice. Continuous PTH (cPTH) infusion resulted in equal bone loss in Sost(+/+) and Sost(-/-) mice as measured by dual energy x-ray absorptiometry. However, distal femur trabecular bone, but not lumbar spine trabecular bone, was spared the bone-wasting effects of cPTH in Sost(-/-) mice. These results suggest that changes in Sost expression are not required for iPTH-induced anabolism. iPTH-induced resorption of cortical bone might be overstimulated in Sost-deficient environments. Furthermore, Sost deletion protects some trabecular compartments, but not cortical compartments, from bone loss induced by high-dose PTH infusion.

Lrp5 functions in bone to regulate bone mass

The human skeleton is affected by mutations in low-density lipoprotein receptor-related protein 5 (LRP5). To understand how LRP5 influences bone properties, we generated mice with osteocyte-specific expression of inducible Lrp5 mutations that cause high and low bone mass phenotypes in humans. We found that bone properties in these mice were comparable to bone properties in mice with inherited mutations. We also induced an Lrp5 mutation in cells that form the appendicular skeleton but not in cells that form the axial skeleton; we observed that bone properties were altered in the limb but not in the spine. These data indicate that Lrp5 signaling functions locally, and they suggest that increasing LRP5 signaling in mature bone cells may be a strategy for treating human disorders associated with low bone mass, such as osteoporosis.

Wnt/β-catenin Signaling in Normal and Cancer Stem Cells

The ability of Wnt ligands to initiate a signaling cascade that results in cytoplasmic stabilization of, and nuclear localization of, β-catenin underlies their ability to regulate progenitor cell differentiation. In this review, we will summarize the current knowledge of the mechanisms underlying Wnt/β-catenin signaling and how the pathway regulates normal differentiation of stem cells in the intestine, mammary gland, and prostate. We will also discuss how dysregulation of the pathway is associated with putative cancer stem cells and the potential therapeutic implications of regulating Wnt signaling.

Differential expression of sclerostin in adult and juvenile mouse calvariae

BACKGROUND

An understanding of the molecular mechanisms controlling bone formation is central to skeletal tissue engineering efforts. The observation that immature animals are able to heal calvarial defects while adult animals are not has proven to be a useful tool for examining these mechanisms. Thus, the authors compared expression of sclerostin, a bone inhibitor, between the calvariae of juvenile and adult mice.

METHODS

Parietal bone was harvested from juvenile (6-day-old; n = 20) and adult (60-day-old; n = 20) mice. Sclerostin transcript and protein levels were compared between the parietal bone of juvenile and adult mice using polymerase chain reaction, Western blotting, and immunohistochemistry. Finally, osteoblasts from the parietal bone of juvenile and adult mice were harvested and cultured under osteogenic differentiation conditions with and without recombinant sclerostin (200 ng/ml). Terminal osteogenic differentiation was assessed at 21 days with alizarin red staining.

RESULTS

Polymerase chain reaction, Western blot analysis, and immunohistochemistry all confirmed greater expression of sclerostin in the parietal bone of adult mice when compared with that of juvenile mice. Osteoblasts, whether from juvenile or adult parietal bones, demonstrated reduced capacity for osteogenic differentiation when exposed to recombinant sclerostin.

CONCLUSIONS

Given the role of sclerostin in inhibiting bone formation, the authors' findings suggest that differences in expression levels of sclerostin may play a role in the differential regenerative capacity of calvariae from juvenile and adult animals. These findings suggest it as a potential target to abrogate in future tissue engineering studies.

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.

LRP6 mediates cAMP generation by G protein-coupled receptors through regulating the membrane targeting of Gα(s)

Ligand binding to certain heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) stimulates the rapid synthesis of cyclic adenosine monophosphate (cAMP) through the G protein α(s) subunit, which activates adenylyl cyclase (AC). We found that the transmembrane receptor low-density lipoprotein receptor-related protein 6 (LRP6), a co-receptor for Wnt proteins, bound to the Gα(s)βγ heterotrimer and that knockdown of LRP6 attenuated cAMP production by various GPCRs, including parathyroid hormone receptor 1 (PTH1R). Knockdown of LRP6 disrupted the localization of Gα(s) to the plasma membrane, which led to a decrease in the extent of coupling of Gα(s) to PTH1R and inhibited the production of cAMP and the activation of cAMP-dependent protein kinase (PKA) in response to PTH. PKA phosphorylated LRP6, which enhanced the binding of Gα(s) to LRP6, its localization to the plasma membrane, and the production of cAMP in response to PTH. Decreased PTH-dependent cAMP production was observed in single cells in which LRP6 was knocked down or mutated at the PKA site by monitoring the cAMP kinetics. Thus, we suggest that the binding of Gα(s) to LRP6 is required to establish a functional GPCR-Gα(s)-AC signaling pathway for the production of cAMP, providing an additional regulatory component to the current GPCR-cAMP paradigm.

Lrp4: A novel modulator of extracellular signaling in craniofacial organogenesis

The low-density lipoprotein (LDL) receptor family is a large evolutionarily conserved group of transmembrane proteins. It has been shown that LDL receptor family members can also function as direct signal transducers or modulators for a broad range of cellular signaling pathways. We have identified a novel mode of signaling pathway integration/coordination that occurs outside cells during development that involves an LDL receptor family member. Physical interaction between an extracellular protein (Wise) that binds BMP ligands and an Lrp receptor (Lrp4) that modulates Wnt signaling, acts to link these two pathways. Mutations in either Wise or Lrp4 in mice produce multiple, but identical abnormalities in tooth development that are linked to alterations in BMP and Wnt signaling. Teeth, in common with many other organs, develop by a series of epithelial-mesenchymal interactions, orchestrated by multiple cell signaling pathways. In tooth development, Lrp4 is expressed exclusively in epithelial cells and Wise mainly in mesenchymal cells. Our hypothesis, based on the mutant phenotypes, cell signaling activity changes and biochemical interactions between Wise and Lrp4 proteins, is that Wise and Lrp4 together act as an extracellular mechanism of coordinating BMP and Wnt signaling activities in epithelial-mesenchymal cell communication during development.

Emerging therapeutic opportunities for skeletal restoration

Osteoporosis, a syndrome characterized by thin bones and fractures, has become more prevalent in both women and men. Established therapies for treating this disorder consist primarily of drugs that prevent bone loss, such as the bisphosphonates and selective oestrogen receptor modulators. Although these drugs have been shown to reduce fractures in randomized trials, there is an urgent need for treatments that could lower fracture risk further without additional adverse effects. The introduction of parathyroid hormone (teriparatide), which significantly increases bone mineral density, albeit for a relatively short duration, raised expectations that drugs that stimulate bone formation might cure osteoporosis. After outlining current approaches for treating osteoporosis, this Review focuses on emerging therapeutic opportunities for osteoporosis that are based on recent insights into skeletal physiology. Such novel strategies offer promise not only for reducing age-related bone loss and the associated risk of fractures but also for restoring bone mineral density to healthy levels.

Spatiotemporal expression of sclerostin in odontoblasts during embryonic mouse tooth morphogenesis

INTRODUCTION

Sclerostin is the product of the SOST gene. Loss-of-function mutations in the SOST gene result in a high bone mass phenotype, thus confirming that sclerostin is a negative regulator of bone mass. SOST knockdown in humans also causes oral and dental malformations. However, the relationship between sclerostin and tooth development is unclear.

METHODS

Using immunohistochemical techniques, we investigated sclerostin expression during fetal mouse tooth development and adult mouse tooth morphogenesis.

RESULTS

Sclerostin was expressed in the secretory odontoblasts located along the ameloblasts of fetal mouse tooth germ and adult incisor. Sclerostin expression was also observed in the fetal and adult osteocytes in the jaw bone.

CONCLUSION

These results suggest that sclerostin, one of the important regulatory factors of differentiated odontoblast function, may usable in vital pulp therapy.

Inhibition of sclerostin by monoclonal antibody increases bone formation, bone mass, and bone strength in aged male rats

The purpose of this study was to evaluate the effects of sclerostin inhibition by treatment with a sclerostin antibody (Scl-AbII) on bone formation, bone mass, and bone strength in an aged, gonad-intact male rat model. Sixteen-month-old male Sprague-Dawley rats were injected subcutaneously with vehicle or Scl-AbII at 5 or 25 mg/kg twice per week for 5 weeks (9-10/group). In vivo dual-energy X-ray absorptiometry (DXA) analysis showed that there was a marked increase in areal bone mineral density of the lumbar vertebrae (L(1) to L(5) ) and long bones (femur and tibia) in both the 5 and 25 mg/kg Scl-AbII-treated groups compared with baseline or vehicle controls at 3 and 5 weeks after treatment. Ex vivo micro-computed tomographic (µCT) analysis demonstrated improved trabecular and cortical architecture at the fifth lumbar vertebral body (L(5) ), femoral diaphysis (FD), and femoral neck (FN) in both Scl-AbII dose groups compared with vehicle controls. The increased cortical and trabecular bone mass was associated with a significantly higher maximal load of L(5) , FD, and FN in the high-dose group. Bone-formation parameters (ie, mineralizing surface, mineral apposition rate, and bone-formation rate) at the proximal tibial metaphysis and tibial shaft were markedly greater on trabecular, periosteal, and endocortical surfaces in both Scl-AbII dose groups compared with controls. These results indicate that sclerostin inhibition by treatment with a sclerostin antibody increased bone formation, bone mass, and bone strength in aged male rats and, furthermore, suggest that pharmacologic inhibition of sclerostin may represent a promising anabolic therapy for low bone mass in aged men.

Patients with primary hyperparathyroidism have lower circulating sclerostin levels than euparathyroid controls

OBJECTIVE

In vitro and in vivo studies in animal models have shown that parathyroid hormone (PTH) inhibits the expression of the SOST gene, which encodes sclerostin, an osteocyte-derived negative regulator of bone formation. We tested the hypothesis that chronic PTH excess decreases circulating sclerostin in humans.

DESIGN

We studied 25 patients with elevated serum PTH concentrations due to primary hyperparathyroidism (PHPT) and 49 patients cured from PHPT after successful parathyroidectomy (PTx; euparathyroid controls (EuPTH)).

METHODS

We measured plasma PTH and serum sclerostin levels and the serum markers of bone turnover alkaline phosphatase, P1NP, and β-CTX.

RESULTS

As expected by the design of the study, mean plasma PTH was significantly higher (P<0.001) in PHPT patients (15.3 pmol/l; 95% confidence interval (CI): 11.1-19.5) compared with that of EuPTH controls (4.1 pmol/l; 95% CI: 3.6-4.5). PHPT patients had significantly lower serum sclerostin values compared with those in EuPTH subjects (30.5 pg/ml; 95% CI: 26.0-35.1 vs 45.4 pg/ml; 95% CI: 40.5-50.2; P<0.001) and healthy controls (40.0 pg/ml; 95% CI: 37.1-42.9; P=0.01). Plasma PTH concentrations were negatively correlated with serum sclerostin values (r=-0.44; P<0.001). Bone turnover markers were significantly correlated with PTH, but not with sclerostin.

CONCLUSION

Patients with PHPT have significantly lower serum sclerostin values compared with PTx controls with normal PTH concentrations. The negative correlation between PTH and sclerostin suggests that SOST is downregulated by PTH in humans.

Genetics of osteoporosis: perspectives for personalized medicine

Osteoporosis is the most common metabolic bone disorder worldwide. At least 15 genes (e.g., ESR1, LRP5, SOST, OPG, RANK and RANKL) have been confirmed as osteoporosis susceptibility genes, and another 30 have been highlighted as promising susceptibility genes. Notably, these genes are clustered in three biological pathways: the estrogen endocrine pathway, the Wnt/β-catenin signaling pathway and the RANK/RANKL/osteoprotegerin (OPG) pathway. In this article, using data pertaining to these three biological pathways as examples, we illustrate possible principles of personalized therapy for osteoporosis. In particular, we propose to use inhibitors (e.g., denosumab) of the RANK/RANKL/OPG signaling pathway to circumvent resistance to estrogen-replacement therapy: a novel idea resulting from the consideration of a mechanistic link between the estrogen endocrine pathway and the RANK/RANKL/OPG signaling pathway. In addition, we call for more attention to be focused on rare variants of major effects in future studies.

Distinct modes of inhibition by sclerostin on bone morphogenetic protein and Wnt signaling pathways

Sclerostin is expressed by osteocytes and has catabolic effects on bone. It has been shown to antagonize bone morphogenetic protein (BMP) and/or Wnt activity, although at present the underlying mechanisms are unclear. Consistent with previous findings, Sclerostin opposed direct Wnt3a-induced but not direct BMP7-induced responses when both ligand and antagonist were provided exogenously to cells. However, we found that when both proteins are expressed in the same cell, sclerostin can antagonize BMP signaling directly by inhibiting BMP7 secretion. Sclerostin interacts with both the BMP7 mature domain and pro-domain, leading to intracellular retention and proteasomal degradation of BMP7. Analysis of sclerostin knock-out mice revealed an inhibitory action of sclerostin on Wnt signaling in both osteoblasts and osteocytes in cortical and cancellous bones. BMP7 signaling was predominantly inhibited by sclerostin in osteocytes of the calcaneus and the cortical bone of the tibia. Our results suggest that sclerostin exerts its potent bone catabolic effects by antagonizing Wnt signaling in a paracrine and autocrine manner and antagonizing BMP signaling selectively in the osteocytes that synthesize simultaneously both sclerostin and BMP7 proteins.

Sclerostin-erbB-3 interactions: modulation of erbB-3 activity by sclerostin

To gain insights into the mechanism of action of sclerostin, a protein that regulates bone mass, we performed yeast two-hybrid analyses using human SOST (sclerostin) cDNA cloned into pGBKT7 DNA-binding domain vector as a bait, and a normalized, high-complexity, universal cDNA library in a GAL4 activating domain vector. We identified an interaction between sclerostin and the carboxyl-terminal portion of the receptor tyrosine-protein kinase erbB-3. To determine the biological relevance of this interaction, we treated MC3T3-E1 mouse osteoblast cells transfected with either a SOST expression plasmid or a control vector, with recombinant heregulin/neuregulin. Phospho-p44/42 (Thr202/Tyr204) MAPK was assessed in heregulin/neuregulin treated cells. We observed an increase in phospho-p44/42 (Thr202/Tyr204) MAPK concentrations in SOST transfected cells but not in cells transfected with a control vector, thus demonstrating a modulatory effect of sclerostin on heregulin/neuregulin signaling in osteoblasts. The data demonstrate that sclerostin functions in part, by modulating the activity of erbB-3.