Canonical Wnt Signaling in Osteoblasts Is Required for Osteoclast Differentiation

A Mosaic Variant in CTNNB1/β-catenin as a Novel Cause for Osteopathia Striata With Cranial Sclerosis

CONTEXT

Osteopathia striata with cranial sclerosis (OSCS) is a rare bone disorder with X-linked dominant inheritance, characterized by a generalized hyperostosis in the skull and long bones and typical metaphyseal striations in the long bones. So far, loss-of-function variants in AMER1 (also known as WTX or FAM123B), encoding the APC membrane recruitment protein 1 (AMER1), have been described as the only molecular cause for OSCS. AMER1 promotes the degradation of β-catenin via AXIN stabilization, acting as a negative regulator of the WNT/β-catenin signaling pathway, a central pathway in bone formation.

OBJECTIVE

In this study, we describe a Dutch adult woman with an OSCS-like phenotype, namely, generalized high bone mass and characteristic metaphyseal striations, but no genetic variant affecting AMER1.

RESULTS

Whole exome sequencing led to the identification of a mosaic missense variant (c.876A > C; p.Lys292Asn) in CTNNB1, coding for β-catenin. The variant disrupts an amino acid known to be crucial for interaction with AXIN, a key factor in the β-catenin destruction complex. Western blotting experiments demonstrate that the p.Lys292Asn variant does not significantly affect the β-catenin phosphorylation status, and hence stability in the cytoplasm. Additionally, luciferase reporter assays were performed to investigate the effect of p.Lys292Asn β-catenin on canonical WNT signaling. These studies indicate an average 70-fold increase in canonical WNT signaling activity by p.Lys292Asn β-catenin.

CONCLUSION

In conclusion, this study indicates that somatic variants in the CTNNB1 gene could explain the pathogenesis of unsolved cases of osteopathia striata.

Effects of periodontal and bisphosphonate treatment on the gingival crevicular levels of sclerostin and dickkopf-1 in postmenopausal osteoporosis with and without periodontitis

OBJECTIVE AND BACKGROUND

Both periodontitis and osteoporosis are associated with osteoclast-related bone resorption. Bone metabolism is regulated by wingless-type MMTV integration site family (WNT), and WNT/β-catenin signals are controlled by physiological antagonists including dickkopf-1 (DKK-1) and sclerostin (SOST). This study examined the effects of periodontal and bisphosphonate (BP) treatment on the gingival crevicular fluid (GCF) sclerostin (SOST) and dickkopf-related protein-1 (DKK-1) levels in osteoporotic and systemically healthy postmenopausal women with and without periodontitis.

MATERIALS AND METHODS

A total of 48 postmenopausal women were divided into 4 groups (n = 12) according to periodontal health and osteoporosis status, as follows: Group OP/P: subjects with both osteoporosis and periodontitis; Group P: systemically healthy subjects with periodontitis; Group OP: periodontally healthy subjects with osteoporosis; Group H: systemically and periodontally healthy controls. Clinical data and GCF SOST and DKK-1 levels of the participants were collected at baseline and at 6 and 12 months following the initiation of periodontal and/or BP treatment in the experimental groups. GCF SOST and DKK-1 data were obtained by ELISA.

RESULTS

Clinical improvements were observed in all experimental groups. GCF SOST and DKK1 baseline levels varied significantly between groups due to periodontal disease (p < .001). Following treatment, significant increases in SOST and DKK-1 concentrations and significant decreases in total amounts of SOST were observed in both periodontitis groups (OP/P, P). However, while total amounts of DKK-1 decreased in Group OP/P, in Group P, these amounts had significantly increased at 12 months post-treatment (p < .05). At both 6 and 12 months post-treatment, SOST and DDK1 total amounts in Groups OP/P, OP, and H were similar (p > .05), whereas significant differences were observed between Groups H and P, indicating a deviation from periodontal health in Group P (p < .01).

CONCLUSIONS

Significant changes in GCF SOST and DKK-1 levels were observed among women with osteoporosis who received both periodontal and BP treatment. A more detailed examination of how these treatment protocols can be combined may lead to new therapeutic approaches towards periodontal disease.

An Osteoporosis Susceptibility Allele at 11p15 Regulates SOX6 Expression by Modulating TCF4 Chromatin Binding

Osteoporosis is an age-related complex disease clinically diagnosed with bone mineral density (BMD). Although several genomewide association studies (GWASs) have discovered multiple noncoding genetic variants at 11p15 influencing osteoporosis risk, the functional mechanisms of these variants remain unknown. Through integrating bioinformatics and functional experiments, a potential functional single-nucleotide polymorphism (SNP; rs1440702) located in an enhancer element was identified and the A allele of rs1440702 acted as an allelic specificities enhancer to increase its distal target gene SOX6 (~600 Kb upstream) expression, which plays a key role in bone formation. We also validated this long-range regulation via conducting chromosome conformation capture (3C) assay. Furthermore, we demonstrated that SNP rs1440702 with a risk allele (rs1440702-A) could increase the activity of the enhancer element by altering the binding affinity of the transcription factor TCF4, resulting in the upregulation expression of SOX6 gene. Collectively, our integrated analyses revealed how the noncoding genetic variants (rs1440702) affect osteoporosis predisposition via long-range gene regulatory mechanisms and identified its target gene SOX6 for downstream biomarker and drug development. © 2022 American Society for Bone and Mineral Research (ASBMR).

Integrative Analysis of Exosomal miR-452 and miR-4713 Downregulating NPY1R for the Prevention of Childhood Obesity

Neuropeptides are associated with childhood obesity and exploring their regulatory mechanisms may reveal new insights for novel treatments. Childhood obesity data were downloaded from the GEO database and were used to screen for differentially expressed neuropeptides in patients with obesity. NPY1R expression was significantly upregulated in children with obesity compared to children without obesity (p < 0.05). The GEO database was used to filter differentially expressed miRNAs in patients with obesity. And hsa-mir-4713 and hsa-mir-452 were found significantly downregulated in adipose tissue. The GEO, TRRUST, and TFacts databases were used to screen all transcription factors for differentially expressed genes (DEGs). The potential regulatory networks between the differentially expressed miRNAs, TFs, and neuropeptides were mapped. In the constructed NPY1R regulatory network, the transcription factors TCF4, HEY1, and GATA3 are significantly associated with NPY1R. TCF4 and HEY1 were positively correlated with NPY1R, while GATA3 was negatively correlated with NPY1R. In the clinical peripheral blood samples, NPY1R, TCF4, and HEY1 were significantly more expressed in the obesity and the obesity with fracture group compared to the control group, while there was no statistically significant difference between the obesity group and the obesity with fracture group in terms of expression. The expression of GATA3, miR-452, and miR-4713 was also significantly lower in the obesity and the obesity with fracture groups when compared to the NC group. Therefore, NPY1R, TCF4, HEY1, GATA3, miR-452, and miR-4713 may be risk factors for fracture in obese children. The potential NPY1R regulatory function was exerted by two pathways: positive regulation caused by TCF4 and HEY1 acting on miR-4713 and negative regulation via GATA3 acting on miR-452. Potential NPY1R-related targets for the treatment of childhood obesity were provided in this study.

FAM20C plays a critical role in the development of mouse vertebra

BACKGROUND CONTEXT

Family with sequence similarity 20-member C (FAM20C) is a protein kinase that is responsible for the phosphorylation of many secretory proteins; however, its roles in spine or vertebra development have not be studied.

PURPOSE

The aim of this investigation is to analyze the roles of FAM20C in vertebra development.

STUDY DESIGN/SETTING

A mouse study of the Fam20c gene using conditional knockout to assess the effects of its inactivation on vertebra development.

METHODS

By breeding Sox2-Cre mice with Fam20c^flox/flox mice, Sox2-Cre;Fam20c^flox/flox mice (abbreviated as cKO mice) are created. X-ray radiography, resin-casted scanning electron microscopy, Hematoxylin and Eosin staining, safranin O staining, Goldner's Masson trichrome staining, Von Kossa staining, tartrate-resistant alkaline phosphatase staining, immunohistochemistry staining, Western Immunoblotting and real-time PCR were employed to characterize the vertebrae of cKO mice compared to the normal control mice.

RESULTS

Inactivation of Fam20c in mice results in remarkable spine deformity, severe morphology and mineralization defects, altered levels of osteoblast differentiation markers, reduction of activity of the Wnt/β-catenin signaling pathway and reduced level of osteoclastogenesis in the vertebrae.

CONCLUSIONS

FAM20C plays an essential role in vertebral development; it may regulate vertebral formation through the Wnt/β-catenin signaling pathway.

CLINICAL SIGNIFICANCE

Mutations in the human FAM20C gene are associated with Raine syndrome. The findings of this study provide valuable clues for the clinical management of Raine syndrome regarding spine manifestations in patients.

Bone phenotypes in rheumatology - there is more to bone than just bone

Osteoarthritis, rheumatoid arthritis, psoriatic arthritis, and ankylosing spondylitis, all have one clear common denominator; an altered turnover of bone. However, this may be more complex than a simple change in bone matrix and mineral turnover. While these diseases share a common tissue axis, their manifestations in the area of pathology are highly diverse, ranging from sclerosis to erosion of bone in different regions. The management of these diseases will benefit from a deeper understanding of the local versus systemic effects, the relation to the equilibrium of the bone balance (i.e., bone formation versus bone resorption), and the physiological and pathophysiological phenotypes of the cells involved (e.g., osteoblasts, osteoclasts, osteocytes and chondrocytes). For example, the process of endochondral bone formation in chondrocytes occurs exists during skeletal development and healthy conditions, but also in pathological conditions. This review focuses on the complex molecular and cellular taxonomy of bone in the context of rheumatological diseases that alter bone matrix composition and maintenance, giving rise to different bone turnover phenotypes, and how biomarkers (biochemical markers) can be applied to potentially describe specific bone phenotypic tissue profiles.

Inhibition of Axin1 in osteoblast precursor cells leads to defects in postnatal bone growth through suppressing osteoclast formation

Axin1 is a negative regulator of β-catenin signaling and its role in osteoblast precursor cells remains undefined. In the present studies, we determined changes in postnatal bone growth by deletion of Axin1 in osteoblast precursor cells and analyzed bone growth in newborn and postnatal Axin1Osx mice and found that hypertrophic cartilage area was largely expanded in Axin1Osx KO mice. A larger number of chondrocytes and unabsorbed cartilage matrix were found in the bone marrow cavity of Axin1Osx KO mice. Osteoclast formation in metaphyseal and subchondral bone areas was significantly decreased, demonstrated by decreased TRAP-positive cell numbers, associated with reduction of MMP9- and cathepsin K-positive cell numbers in Axin1Osx KO mice. OPG expression and the ratio of Opg to Rankl were significantly increased in osteoblasts of Axin1Osx KO mice. Osteoclast formation in primary bone marrow derived microphage (BMM) cells was significantly decreased when BMM cells were cultured with conditioned media (CM) collected from osteoblasts derived from Axin1Osx mice compared with BMM cells cultured with CM derived from WT mice. Thus, the loss of Axin1 in osteoblast precursor cells caused increased OPG and the decrease in osteoclast formation, leading to delayed bone growth in postnatal Axin1Osx KO mice.

Loss of Dkk-1 in Osteocytes Mitigates Alveolar Bone Loss in Mice With Periodontitis

Background: Periodontitis is a highly prevalent infection-triggered inflammatory disease that results in bone loss. Inflammation causes bone resorption by osteoclasts, and also by suppression of bone formation via increase of Dickkopf-1 (Dkk-1), an inhibitor of Wnt signaling. Here, we tested the hypothesis that osteocytic Dkk-1 is a key factor in the pathogenesis of periodontitis-induced alveolar bone loss (ABL).

Methods: Twelve-week-old female mice with a constitutive deletion of Dkk-1 specifically in osteocytes (Dkk-1^fl/fl;Dmp1:Cre) were subjected to experimental periodontitis (EP). Cre-negative littermates served as controls. EP was induced by placing a ligature around the upper 2nd left molar, the contralateral side was used as control. Mice were killed after 11 days and maxillae removed for micro-CT and histological analyses. The mRNA expression of Dkk-1, Runx2, Osteocalcin, OPG, RANKL, RANKL/OPG ratio, LEF-1, and TCF-7 were assessed in maxillae, while mRNA expressions of TNF and IL-1 were evaluated on gingiva using real-time PCR. Blood samples were collected for Dkk-1, CTX, and P1NP measurement by ELISA.

Results: The deletion of Dkk-1 in osteocytes prevented ABL in mice with EP, compared to Cre-negative control mice with EP. Micro-CT analysis showed a significant reduction of bone loss (−28.5%) in EP Dkk-1^fl/fl;Dmp1:Cre-positive mice compared to their littermate controls. These mice showed a greater alveolar bone volume, bone mineral density, trabecular number, and trabecular thickness after EP when compared to the Cre-negative controls. The local expression in maxillae as well as the serum levels of Dkk-1 were reduced in Dkk-1^fl/fl;Dmp1:Cre-positive mice with EP. The transgenic mice submitted to EP showed increase of P1NP and reduction of CTX-I serum levels, and increase of TCF-7 expression. Histological analysis displayed less inflammatory infiltrates, a reduction of TNF and IL-1 expressions in the gingiva and fewer osteoclasts in Cre-positive animals with EP. Moreover, in mice with EP, the osteocytic deletion of Dkk-1 enhanced bone formation due to increased expressions of Runx2 and Osteocalcin and decreased expression of RANKL in maxillae.

Conclusion: In summary, Dkk-1 derived from osteocytes plays a crucial role in ABL in periodontitis.

Vitamin D, Inflammation and Osteoporosis in Rheumatoid Arthritis

Patients with Rheumatoid Arthritis (RA) commonly develop osteoporosis and fragility fractures. This fact cannot be explained only with the use of glucocorticoids, known to be detrimental for bone health. RA is characterized by a chronic inflammation caused by the continuous activation of innate and adaptive immunity with proinflammatory cytokines overproduction. This process is detrimental for several organs and physiological processes, including the impairment of bone remodeling. We will briefly review the pathogenesis of inflammation-related bone loss in RA, describing well-known and new molecular pathways and focusing on vitamin D and Parathyroid Hormone role.

Epigenetic Contribution of High-Mobility Group A Proteins to Stem Cell Properties

High-mobility group A (HMGA) proteins have been examined to understand their participation as structural epigenetic chromatin factors that confer stem-like properties to embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), and cancer stem cells (CSCs). The function of HMGA was evaluated in conjunction with that of other epigenetic factors such as histones and microRNAs (miRs), taking into consideration the posttranscriptional modifications (PTMs) of histones (acetylation and methylation) and DNA methylation. HMGA proteins were coordinated or associated with histone and DNA modification and the expression of the factors related to pluripotency. CSCs showed remarkable differences compared with ESCs and iPSCs.

Role of TCF/LEF Transcription Factors in Bone Development and Osteogenesis

Bone formation occurs by two distinct mechanisms, namely, periosteal ossification and endochondral ossification. In both mechanisms, osteoblasts play an important role in the secretion and mineralization of bone-specific extracellular matrix. Differentiation and maturation of osteoblasts is a prerequisite to bone formation and is regulated by many factors. Recent experiments have shown that transcription factors play an important role in regulating osteoblast differentiation, proliferation, and function. Osteogenesis related transcription factors are the central targets and key mediators of the function of growth factors, such as cytokines. Transcription factors play a key role in the transformation of mesenchymal progenitor cells into functional osteoblasts. These transcription factors are closely linked with each other and in conjunction with bone-related signaling pathways form a complex network that regulates osteoblast differentiation and bone formation. In this paper, we discuss the structure of T-cell factor/lymphoid enhancer factor (TCF/LEF) and its role in embryonic skeletal development and the crosstalk with related signaling pathways and factors.

Possible role of Dickkopf-1 protein in the pathogenesis of tympanosclerosis in a rat model

OBJECTIVES

This study aimed to investigate the expression of DKK1 protein in an experimental model of tympanosclerosis and its possible role in the pathogenesis of this disorder.

METHODS

Forty Sprague Dawley rats were included in the study: 20 in the control group (which received no treatment) and 20 in the experimental group (which received an incision to induce tympanosclerosis). Otomicroscopy was performed to observe the development of myringosclerosis. Haematoxylin and eosin staining was performed to observe the morphological changes. Western blot analysis and immunohistochemistry were performed to assess the expression of DKK1 protein.

RESULTS

At day 15, sclerotic lesions were observed in 70 per cent of the tympanic membranes. Inflammatory infiltration and hyaline degeneration markedly appeared in the tympanic membranes and middle-ear mucosa. DKK1 protein was mainly distributed in the cytoplasm of epithelial cells, which were widely distributed in the tympanic membranes and middle-ear mucosa. The expression of DKK1 protein was significantly decreased in the calcified experimental ears.

CONCLUSION

DKK1 protein is involved in the pathogenesis of tympanosclerosis by regulating the Wnt/β-catenin signalling pathway.

Alcohol-related deficient fracture healing is associated with activation of FoxO transcription factors in mice

The process of fracture healing is complex, and poor or incomplete healing remains a significant health problem. Proper fracture healing relies upon resident mesenchymal stem cell (MSC) differentiation into chondrocytes and osteoblasts, which are necessary for callus formation and ossification. Alcohol abuse is a leading contributor to poor fracture healing. Although the mechanism behind this action is unknown, excessive alcohol consumption is known to promote systemic oxidative stress. The family of FoxO transcription factors is activated by oxidative stress, and FoxO activation antagonizes Wnt signaling, which regulates mesenchymal stem cell differentiation. We hypothesize that alcohol exposure increases oxidative stress leading to deficient fracture repair by activating FoxO transcription factors within the fracture callus which disrupts chondrogenesis of mesenchymal stem cells. Our laboratory has developed an experimental model of delayed fracture union in mice using ethanol administration. We have found that ethanol administration significantly decreases external, cartilaginous callus formation, and hallmarks of endochondral ossification, and these changes are concomitant with increases in FoxO expression and markers of activation in fracture callus tissue of these mice. We were able to prevent these alcohol-induced effects with the administration of the antioxidant n-acetyl cysteine (NAC), suggesting that alcohol-induced oxidative stress produces the perturbed endochondral ossification and FoxO expression. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:2106-2115, 2016.

Teriparatide and denosumab combination therapy and skeletal metabolism

Several therapies are available for osteoporis. Understanding the bone turnover changes and their mutual realtionship gives an overall view and might lead to a target therapy INTRODUCTION: The aim of this study is to compare the changes in bone turnover markers in patients treated with either denosumab alone, teriparatide (TPTD) alone, or in a third therapeutic scheme, when TPTD was added to patients previously treated with denosumab.

METHODS

Fifty-nine women over 65 years old with severe postmenopausal osteoporosis (evidence of at least two moderate-severe vertebral fractures) were enrolled in the study. Serum samples were collected every 3 months. They were assayed for intact N-propeptide of type I collagen (P1NP), C-terminal telopeptide of type I collagen (CTX), intact parathyroid hormone (PTH), 25 hydroxy-vitamin D (25 OHD), Sclerostin (SOST), and Dickkopf-related protein 1 (DKK1). Bone mass density was assessed by dual-energy X-ray absorptiometry at the lumbar spine and at the total hip.

RESULTS

In the groups treated only with TPTD or with denosumab, bone turnover markers increased and decreased, respectively. In TPTD group, a later significant increase in DKK1 was observed, while in denosumab group, a progressive increase in SOST was associated with a progressive significant decrease in DKK1. In the group treated first with denosumab and in which TPTD was added 3 months later, both CTX and P1NP increased 3 months after the beginning of TPTD. The strong effect of denosumab on bone turnover seems to be reversed by TPTD treatment.

CONCLUSIONS

In this study, we showed that TPTD is able to express its biological activity even when bone turnover is fully suppressed by denosumab treatment. The combination therapy is associated with significant increases in both DKK1 and SOST.

Secondary osteoporosis in collagen-induced arthritis rats

Numerous studies have demonstrated that rheumatoid arthritis (RA) is often associated with bone loss; however, few experiments have focused on cancellous and cortical bone changes in rats during the process of arthritis. We have investigated bone changes in rats with collagen-induced arthritis (CIA) and have explored the characteristics of how RA induces osteoporosis by means of bone histomorphometry, bone biomechanics studies, bone mineral density studies, micro computer tomography, enzyme-linked immunosorbant assay, immunohistochemistry, and Western blot analysis. Bone mineral density of the femur and lumbar vertebrae and biomechanical properties of the femur were decreased in CIA rats. Trabecular bone volume of the tibia and lumbar vertebrae was decreased whereas bone resorption was increased in CIA rats. Bone formation of the tibial shaft in periosteal surfaces was decreased in CIA rats. Furthermore, the trabecular bone loss in CIA rats was severer at 16 weeks than at 8 weeks, as was cortical bone loss. The serum level of tumor necrosis factor α in CIA rats was increased, and the expression of dickkopf 1 and that of receptor activator of nuclear factor κB (RANKL) ligand (RANKL) in the ankle joints were also increased, but the expression of osteoprotegerin (OPG) was decreased. We conclude that CIA rats developed systemic osteoporosis, and that osteoporosis became more serious with CIA development. The mechanism may be related to the increase of bone resorption in cancellous bone cause by upregulation of the expression of DKK-1 and regulation of the RANKL/RANK/OPG signaling pathway, and the decrease of bone formation in cortical bone caused by an increase in the expression of DKK-1.

Signaling pathways effecting crosstalk between cartilage and adjacent tissues: Seminars in cell and developmental biology: The biology and pathology of cartilage

Endochondral ossification, the mechanism responsible for the development of the long bones, is dependent on an extremely stringent coordination between the processes of chondrocyte maturation in the growth plate, vascular expansion in the surrounding tissues, and osteoblast differentiation and osteogenesis in the perichondrium and the developing bone center. The synchronization of these processes occurring in adjacent tissues is regulated through vigorous crosstalk between chondrocytes, endothelial cells and osteoblast lineage cells. Our knowledge about the molecular constituents of these bidirectional communications is undoubtedly incomplete, but certainly some signaling pathways effective in cartilage have been recognized to play key roles in steering vascularization and osteogenesis in the perichondrial tissues. These include hypoxia-driven signaling pathways, governed by the hypoxia-inducible factors (HIFs) and vascular endothelial growth factor (VEGF), which are absolutely essential for the survival and functioning of chondrocytes in the avascular growth plate, at least in part by regulating the oxygenation of developing cartilage through the stimulation of angiogenesis in the surrounding tissues. A second coordinating signal emanating from cartilage and regulating developmental processes in the adjacent perichondrium is Indian Hedgehog (IHH). IHH, produced by pre-hypertrophic and early hypertrophic chondrocytes in the growth plate, induces the differentiation of adjacent perichondrial progenitor cells into osteoblasts, thereby harmonizing the site and time of bone formation with the developmental progression of chondrogenesis. Both signaling pathways represent vital mediators of the tightly organized conversion of avascular cartilage into vascularized and mineralized bone during endochondral ossification.

Normal hematopoiesis and lack of β-catenin activation in osteoblasts of patients and mice harboring Lrp5 gain-of-function mutations

Osteoblasts are emerging regulators of myeloid malignancies since genetic alterations in them, such as constitutive activation of β-catenin, instigate their appearance. The LDL receptor-related protein 5 (LRP5), initially proposed to be a co-receptor for Wnt proteins, in fact favors bone formation by suppressing gut-serotonin synthesis. This function of Lrp5 occurring in the gut is independent of β-catenin activation in osteoblasts. However, it is unknown whether Lrp5 can act directly in osteoblast to influence other functions that require β-catenin signaling, particularly, the deregulation of hematopoiesis and leukemogenic properties of β-catenin activation in osteoblasts, that lead to development of acute myeloid leukemia (AML). Using mice with gain-of-function (GOF) Lrp5 alleles (Lrp5(A214V)) that recapitulate the human high bone mass (HBM) phenotype, as well as patients with the T253I HBM Lrp5 mutation, we show here that Lrp5 GOF mutations in both humans and mice do not activate β-catenin signaling in osteoblasts. Consistent with a lack of β-catenin activation in their osteoblasts, Lrp5(A214V) mice have normal trilinear hematopoiesis. In contrast to leukemic mice with constitutive activation of β-catenin in osteoblasts (Ctnnb1(CAosb)), accumulation of early myeloid progenitors, a characteristic of AML, myeloid-blasts in blood, and segmented neutrophils or dysplastic megakaryocytes in the bone marrow, are not observed in Lrp5(A214V) mice. Likewise, peripheral blood count analysis in HBM patients showed normal hematopoiesis, normal percentage of myeloid cells, and lack of anemia. We conclude that Lrp5 GOF mutations do not activate β-catenin signaling in osteoblasts. As a result, myeloid lineage differentiation is normal in HBM patients and mice. This article is part of a Special Issue entitled: Tumor Microenvironment Regulation of Cancer Cell Survival, Metastasis, Inflammation, and Immune Surveillance edited by Peter Ruvolo and Gregg L. Semenza.

Expression of Wnt3a, Wnt10b, β-catenin and DKK1 in periodontium during orthodontic tooth movement in rats

OBJECTIVE

To investigate the expression of Wnt3a, Wnt10b, β-catenin and DKK1 in the periodontal ligament (PDL) during orthodontic tooth movement (OTM) in rats.

MATERIALS AND METHODS

Nickel-titanium closed-coil springs were used to deliver an initial 50 g mesial force to the left maxillary first molars in 30 rats. The force was kept constant for 1, 3, 5, 7, 10 and 14 days until the animals were sacrificed. The right maxillary molars without force application served as control. Paraffin-embedded sections of the upper jaws were prepared for histological and immunohistochemical analyses to detect Wnt3a, Wnt10b, β-catenin and DKK1 expression in PDL.

RESULTS

Wnt3a, Wnt10b, β-catenin and DKK1 were expressed on both the ipsilateral and contralateral sides of PDL in each group. After the application of orthodontic force, the expression of β-catenin and DKK1 was initially increased and then decreased on both sides, with maximal levels of expression at day 7 and day 10, respectively. On the compression side, Wnt3a and Wnt10b levels started to increase at day 5, while on the tension side, these two molecules began to increase at day 1. Furthermore, the expression levels of Wnt3a, Wnt10b, and β-catenin were much stronger on the tension side than on the compression side at any of the observation points, while DKK1 level was much higher on the compression side.

CONCLUSION

Wnt3a, Wnt10b, β-catenin and DKK1 expression may be related to the periodontal tissue remodeling following the application of an orthodontic force in rats. These observations suggest that the Wnt/β-catenin signaling pathway may play a crucial role in periodontal tissue remodeling during OTM.

Association of serum Dkk-1 levels with β-catenin in patients with postmenopausal osteoporosis

Wnt signaling plays an important role in the bone development and remodeling. The Wnt antagonist Dkk-1 is a potent inhibitor of bone formation. The aims of this study were firstly to compare the serum Dkk-1 levels in postmenopausal osteoporosis patients with age-matched healthy controls, and secondly, to assess the possible relationship between Dkk-1 and β-catenin, sclerostin, or bone turnover markers [CTX, PINP, N-MID-OT and 25(OH)D] in the setting of postmenopausal osteoporosis. A total of 350 patients with postmenopausal osteoporosis and 150 age-matched healthy controls were enrolled, and the serum levels of Dkk-1, β-catenin, sclerostin, OPG, and RANKL were detected by ELISA, and bone turnover markers [CTX, PINP, N-MID-OT and 25(OH)D] were measured by Roche electrochemiluminescence system in two groups. Serum Dkk-1 levels were significantly higher in postmenopausal osteoporosis group than in control group (P<0.001). Univariate analyses revealed that serum Dkk-1 levels were weakly negatively correlated to β-catenin (r=-0.161, P=0.003) and OPG (r=-0.106, P=0.047), while multiple regression analysis showed a negative correlation between serum Dkk-1 levels with β-catenin (β=-0.165, P=0.009) and BMD (β=-0.139, P=0.027), and a positive correlation between serum Dkk-1 levels and CTX (β=0.122, P=0.040) in postmenopausal osteoporosis group. No similar correlations ware observed in control group. The results provided evidence for the role of Dkk-1 in bone metabolism and demonstrated the link of Dkk-1 and Wnt/β-catenin in some ways.

Association between polymorphisms in sclerostin, dickkopfs and secreted frizzled-related protein genes and bone mineral density in postmenopausal Korean women

BACKGROUND/AIMS

The purpose of this study was to investigate the association between single nucleotide polymorphisms (SNPs) in sclerostin (SOST), dickkopf (DKK), secreted frizzled-related protein (sFRP) genes and bone mineral density (BMD) in postmenopausal Korean women.

METHODS

The SOST, Wnt inhibitory factor 1 (WIF1), sFRP1,sFRP2,sFRP3, sFRP4, sFRP5, DKK1, DKK2 and DKK3 polymorphisms were analyzed in 399 postmenopausal Korean women. Serum levels of bone turnover markers were measured, and BMDs at the lumbar spine and femoral neck were also examined.

RESULTS

No significant differences in adjusted BMD at the lumbar spine and femoral neck were noted according to any single and combined polymorphisms measured in SOST, DKKs and sFRPs. However, osteoporosis at the femoral neck was 2.35 times more frequently observed in the AA genotype of the sFRP4 c.958C>A polymorphism compared to the non-AA genotype (95% CI 1.09-5.08, p = 0.03). Also, the CC genotype of the sFRP3 c.970C>G polymorphism had a higher rate of osteoporosis at the femoral neck compared to the GC genotype (OR 8.47, 95% CI 1.37-52.63, p = 0.049).

CONCLUSIONS

Our results suggest that the sFRP3 c.970C>G and sFRP4 c.958C>A polymorphisms may be genetic factors associated with the prevalence of osteoporosis at the femoral neck in postmenopausal Korean women.

The role of midkine in skeletal remodelling

Bone tissue is subjected to continuous remodelling, replacing old or damaged bone throughout life. In bone remodelling, the coordinated activities of bone-forming osteoblasts and bone-resorbing osteoclasts ensure the maintenance of bone mass and strength. In early life, the balance of these cellular activities is tightly regulated by various factors, including systemic hormones, the mechanical environment and locally released growth factors. Age-related changes in the activity of these factors in bone remodelling can result in diseases with low bone mass, such as osteoporosis. Osteoporosis is a systemic and age-related skeletal disease characterized by low bone mass and structural degeneration of bone tissue, predisposing the patient to an increased fracture risk. The growth factor midkine (Mdk) plays a key role in bone remodelling and it is expressed during bone formation and fracture repair. Using a mouse deficient in Mdk, our group have identified this protein as a negative regulator of bone formation and mechanically induced bone remodelling. Thus, specific Mdk antagonists might represent a therapeutic option for diseases characterized by low bone mass, such as osteoporosis.

LINKED ARTICLES

This article is part of a themed section on Midkine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-4.

The effects of hedgehog on RNA binding protein Msi1 during the osteogenic differentiation of human cord blood-derived mesenchymal stem cells

Human umbilical cord blood (UCB)-derived mesenchymal stem cells (MSCs) are useful tools for regenerative medicine due to their capacity for self-renewal and multi-lineage differentiation. The appropriate clinical application of MSCs for regenerative medicine requires an integrated understanding of multiple signaling pathways that regulate cell proliferation, stemness and differentiation. However, the potential molecular mechanisms mediating these functions are not completely understood. The effects of hedgehog (Hh) signaling on the osteogenic differentiation of MSCs are still controversial, and the underlying mechanisms are unclear. In the present study, we evaluated the direct effects of Hh signaling on the osteogenic differentiation of hUCB-MSCs and investigated potential downstream regulatory mechanisms responsible for Hh signaling. We observed that Hh signaling acts as a negative regulator of osteogenic differentiation through the suppression of RNA-binding Msi1, which in turn suppresses the expression of Wnt1 and the miR-148 family, especially miR-148b. Moreover, Hh and Msi1 are considered to be potential stemness markers of hUCB-MSCs due to their differentiation-dependent expression profiles. This study provides new insights into mechanisms regulating MSC differentiation and may have implications for a variety of therapeutic applications in the clinic.

The in vitro and in vivo treatment effects of overexpressed lentiviral vector-mediated human BMP2 gene in the femoral bone marrow stromal cells of osteoporotic rats

This study aimed to compare the treatment effects of lentiviral vector-mediated hBMP2 which was overexpressed in the femoral bone marrow stromal cells of osteoporotic rats through genetic infection in vitro and in vivo. Comparison of the two transgenic effects may be crucial to determining the lentivirus infection method to be used. Following a comparison of the rat bone marrow stromal cells (rBMSCs) in osteoporotic (MSCs OVX) and normal (MSCs CON) groups, the lentiviral vector-mediated human bone morphogenetic protein 2 (hBMP2), which overexpressed the BMSCs of osteoporotic rats in vitro (rBMSCs in OE group), was constructed. The osteogenic ability in the overexpressed (OE) group was then compared to that of the MSCs CON. The rBMSCs in the OE group (transplants of genetic infection in vitro) and the lentivirus-containing solution (injected material of genetic infection in vivo) were injected into the femurs. The treatment effect of each group was compared via bone mineral density (BMD) and bone histomorphometry. The hBMP2-modified osteoporosis rBMSCs formed by genetic infection in vitro (n=7) had an ameliorated treatment effect on the femur as compared to that of the in vivo (n=7) (BMD: 0.315 vs. 0.19 g/cm2, P<0.01; bone histomorphometry: For bone trabeculars (Tb.Ar/T.Ar): 0.301 vs. 0.114, P<0.01; for trabecular thickness (Tb.Th): 43.54 vs. 21.39 µm, P<0.01; for trabecular separation (Tb.Sp): 115.7 vs. 304.87 µm, P<0.01). The results showed that the treatment effects of osteoporotic rBMSCs on local osteoporosis performed by genetic infection were improved in vitro as compared to those in vivo.

Osteoporosis in chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is being regarded as a heterogeneous disease with clinically significant pulmonary and extrapulmonary manifestations, such as emphysema, cardiovascular disease and osteoporosis. Osteoporosis is characterized by low bone mass and microarchitectural deterioration of bone tissue, leading to enhanced bone fragility and, consequently, an increased risk of fracture. Fractures resulting from osteoporosis might contribute to increased morbidity and mortality, particularly in COPD patients. The high prevalence of osteoporosis in COPD patients is assumed to be due to common risk factors, such as older age and tobacco smoking, and COPD-specific risk factors, such as systemic inflammation, vitamin D deficiency and the use of oral or inhaled corticosteroids. This review provides a state-of-the-art summary of the prevalence, pathophysiology, diagnosis, risk factors and treatment of osteoporosis in COPD patients. It also discusses potential mechanisms linking COPD with osteoporosis.

Autosomal dominant osteopetrosis revisited: lessons from recent studies

Systematic studies of autosomal dominant osteopetrosis (ADO) were followed by the identification of underlying mutations giving unique possibilities to perform translational studies. What was previously designated ADO1 turned out to be a high bone mass phenotype caused by a missense mutation in the first propeller of LRP5, a region of importance for binding inhibitory proteins. Thereby, ADO1 cannot be regarded as a classical form of osteopetrosis but must now be considered a disease of LRP5 activation. ADO (Albers-Schönberg disease, or previously ADO2) is characterized by increased number of osteoclasts and a defect in the chloride transport system (ClC-7) of importance for acidification of the resorption lacuna (a form of Chloride Channel 7 Deficiency Osteopetrosis). Ex vivo studies of osteoclasts from ADO have shown that cells do form normally but have reduced resorption capacity and an expanded life span. Bone formation seems normal despite decreased osteoclast function. Uncoupling of formation from resorption makes ADO of interest for new strategies for treatment of osteoporosis. Recent studies have integrated bone metabolism in whole-body energy homeostasis. Patients with ADO may have decreased insulin levels indicating importance beyond bone metabolism. There seems to be a paradigm shift in the treatment of osteoporosis. Targeting ClC-7 might introduce a new principle of dual action. Drugs affecting ClC-7 could be antiresorptive, still allowing ongoing bone formation. Inversely, drugs affecting the inhibitory site of LRP5 might stimulate bone formation and inhibit resorption. Thereby, these studies have highlighted several intriguing treatment possibilities, employing novel modes of action, which could provide benefits to the treatment of osteoporosis.

Deletion of beta catenin in hypertrophic growth plate chondrocytes impairs trabecular bone formation

In order to elucidate the role of β-catenin in hypertrophic cartilage zone of the growth plate, we deleted the β-catenin gene ctnnb1specifically from hypertrophic chondrocytes by mating ctnnb1(fl/fl) mice with BAC-Col10a1-Cre-deleter mice. Surprisingly, this resulted in a significant reduction of subchondral trabecular bone formation in BACCol10Cre; ctnnb1(Δ/Δ) (referred to as Cat-ko) mice, although Cre expression was restricted to hypertrophic chondrocytes. The size of the Col10a1 positive hypertrophic zone was normal, but qRT-PCR revealed reduced expression of Mmp13, and Vegfa in Cat-ko hypertrophic chondrocytes, indicating impaired terminal differentiation. Immunohistological and in situ hybridization analysis revealed the substantial deficiency of collagen I positive mature osteoblasts, but equal levels of osterix-positive cells in the subchondral bone marrow space of Cat-ko mice, indicating that the supply of osteoblast precursor cells was not reduced. The fact that in Cat-ko mice subchondral trabeculae were lacking including their calcified cartilage core indicated a strongly enhanced osteoclast activity. In fact, TRAP staining as well as in situ hybridization analysis of Mmp9 expression revealed denser occupation of the cartilage erosion zone with enlarged osteoclasts as compared to the control growth plate, suggesting increased RANKL or reduced osteoprotegerin (Opg) activity in this zone. This notion was confirmed by qRT-PCR analysis of mRNA extracted from cultured hypertrophic chondrocytes or from whole epiphyses, showing increased Rankl mRNA levels in Cat-ko as compared to control chondrocytes, whereas changes in OPG levels were not significant. These results indicate that β-catenin levels in hypertrophic chondrocytes play a key role in regulating osteoclast activity and trabecular bone formation at the cartilage-bone interface by controlling RANKL expression in hypertrophic chondrocytes.

Acute effects of glucocorticoids on serum markers of osteoclasts, osteoblasts, and osteocytes

The aim of this study was to investigate the acute effects of oral glucocorticoids in doses used in clinical practice on biochemical indices of the function of osteoclasts, osteoblasts, and osteocytes. In 17 adult patients suffering from various medical pathologies requiring systemic steroid therapy that were never before treated with glucocorticoids, glucocorticoid treatment was initiated (mean prednisolone equivalent dose of 23.1 ± 12.7 mg/day, range 10-50). Fasting morning serum concentrations of osteocalcin (OC), amino-terminal propeptide of type I procollagen (PINP), type 1 collagen cross-linked C-telopeptide (βCTX), soluble receptor activator of nuclear factor kappaB ligand (sRANKL), osteoprotegerin (OPG), sclerostin, Dickkopf-1 (Dkk-1), and high-sensitivity C-reactive protein (hsCRP) were measured at baseline and on three consecutive days. Significant reductions in serum OC, PINP, OPG, sclerostin, and hsCRP were observed during 96 h of glucocorticoid administration, while serum βCTX showed a significant percentual increase. A significant positive correlation was found between serum concentrations of Dkk-1 and βCTX after 96 h of treatment with glucocorticoids. A significant drop in serum sclerostin, OPG, and OC observed in this study may reflect the rapid glucocorticoid-induced apoptosis of osteocytes.

In vivo and in vitro effects of a novel anti-Dkk1 neutralizing antibody in multiple myeloma

Over-expression of the protein Dickkopf-1 (Dkk1) has been associated with multiple myeloma bone disease. Previous reports with the use of anti-Dkk1 neutralizing Ab directed strategies have demonstrated a pro-anabolic effect with associated anti-myeloma activity in 2 in vivo mouse models. However new insights on the role of the wnt pathway in osteoclasts (OC) are emerging and the potential effect of a neutralizing Ab to Dkk1 in osteoclastogenesis remains to be elucidated. In order to better define the effect of an anti-Dkk1 neutralizing Ab on osteoclastogenesis and myeloma, we studied a novel anti-Dkk1 monoclonal Ab in our preclinical models. In vivo data confirmed the pro-anabolic and anti-MM effect. In vitro data in part confirmed the in vivo observation, suggesting an indirect anti-MM effect secondary to inhibition of osteoclastogenesis and thus the interaction between MM and bone microenvironment. However, when studies on osteoclastogenesis were extended to samples derived from MM patients, we observed a variable response to anti-Dkk1 treatment without correlation to expression of surface receptors for Dkk1 in OCs suggesting potential heterogeneity in the efficacy of such a strategy. In conclusion, Dkk1 is a promising target for the treatment of both MM and bone disease, and ongoing clinical studies will help elucidate its efficacy.

Attenuated Wnt/β-catenin signalling mediates methotrexate chemotherapy-induced bone loss and marrow adiposity in rats

Cancer chemotherapy often causes significant bone loss, marrow adiposity and haematopoietic defects, yet the underlying mechanisms and recovery potential remain unclear. Wnt/β-catenin signalling is integral to the regulation of osteogenesis, adipogenesis and haematopoiesis; using a rat model, the current study investigated roles of this signalling pathway in changes to bone marrow stromal and haematopoietic cell differentiation after chemotherapy with methotrexate (MTX), a commonly used antimetabolite. MTX treatment in rats (5 daily administrations at 0.75 mg/kg) has previously been found to decrease bone volume and increase marrow fat, which was associated with increased osteoclastogenesis in haematopoietic cells and with an osteogenesis to adipogenesis switch in bone marrow stromal cells of treated rats. In the current study, on day 6 after the first MTX dose we found that accompanying these changes as well as a suppressed haematopoietic cellularity but increased granulocyte/macrophage differentiation potential, there was an increase in mRNA expression of Wnt antagonists sFRP-1 and Dkk-1 in bone, a reduction in nuclear β-catenin protein in bone marrow stromal cells, and decreased mRNA levels of β-catenin target genes lef-1, cyclin D1 and survivin, suggesting reduced activation of Wnt/β-catenin signalling in the bone during MTX-induced damage. Concurrent administration of BIO, a GSK-3β inhibitor that stabilises β-catenin, partially abrogated the MTX-induced transient changes in osteogenic/adipogenic commitment, granulocyte/macrophage lineage differentiation and osteoclast number. These findings demonstrate a potentially important role of Wnt/β-catenin signalling in MTX chemotherapy-induced cellular changes to the bone marrow microenvironment.

Noncanonical Wnt signaling promotes osteoclast differentiation and is facilitated by the human immunodeficiency virus protease inhibitor ritonavir

Wnt proteins that signal via the canonical Wnt/β-catenin pathway directly regulate osteoblast differentiation. In contrast, most studies of Wnt-related effects on osteoclasts involve indirect changes. While investigating bone mineral density loss in the setting of human immunodeficiency virus (HIV) infection and its treatment with the protease inhibitor ritonavir (RTV), we observed that RTV decreased nuclear localization of β-catenin, critical to canonical Wnt signaling, in primary human and murine osteoclast precursors. This occurred in parallel with upregulation of Wnt5a and Wnt5b transcripts. These Wnts typically stimulate noncanonical Wnt signaling, and this can antagonize the canonical Wnt pathway in many cell types, dependent upon Wnt receptor usage. We now document RTV-mediated upregulation of Wnt5a/b protein in osteoclast precursors. Recombinant Wnt5b and retrovirus-mediated expression of Wnt5a enhanced osteoclast differentiation from human and murine monocytic precursors, processes facilitated by RTV. In contrast, canonical Wnt signaling mediated by Wnt3a suppressed osteoclastogenesis. Both RTV and Wnt5b inhibited canonical, β-catenin/T cell factor-based Wnt reporter activation in osteoclast precursors. RTV- and Wnt5-induced osteoclast differentiation were dependent upon the receptor-like tyrosine kinase Ryk, suggesting that Ryk may act as a Wnt5a/b receptor in this context. This is the first demonstration of a direct role for Wnt signaling pathways and Ryk in regulation of osteoclast differentiation, and its modulation by a clinically important drug, ritonavir. These studies also reveal a potential role for noncanonical Wnt5a/b signaling in acceleration of bone mineral density loss in HIV-infected individuals, and illuminate a potential means of influencing such processes in disease states that involve enhanced osteoclast activity.

Effects of HIV infection and antiretroviral therapy with ritonavir on induction of osteoclast-like cells in postmenopausal women

SUMMARY

Ritonavir (RTV) is a commonly used antiretroviral associated with bone loss. We show that peripheral blood mononuclear cells (PBMCs) from human immunodeficiency virus (HIV)-positive women on RTV are more likely to differentiate into osteoclast-like cells when cultured with their own sera than PBMCs and sera from HIV- women or HIV+ on other antiretrovirals.

INTRODUCTION

RTV increases differentiation of human adherent PBMCs to functional osteoclasts in vitro, and antiretroviral regimens containing RTV have been associated with low bone mineral density (BMD) and bone loss.

METHODS

BMD, proresorptive cytokines, bone turnover markers (BTMs), and induction of osteoclast-like cells from adherent PBMCs incubated either with macrophage colony-stimulating factor (MCSF) and receptor activator of nuclear factor κB ligand (RANKL) or with autologous serum were compared in 51 HIV- and 68 HIV+ postmenopausal women.

RESULTS

BMD was lower, and serum proresorptive cytokines and BTMs were higher in HIV+ versus HIV- women. Differentiation of osteoclast-like cells from adherent PBMCs exposed to either MCSF/RANKL or autologous serum was greater in HIV+ women. Induction of osteoclast-like cells was greater from PBMCs exposed to autologous sera from HIV+ women on RTV-containing versus other regimens (172 ± 14% versus 110 ± 10%, p < 0.001). Serum-based induction of osteoclast-like cells from adherent PBMCs correlated with certain BTMs but not BMD.

CONCLUSIONS

HIV infection and antiretroviral therapy are associated with higher BTMs and increased differentiation of osteoclast-like cells from adherent PBMCs, especially in women on regimens containing RTV. HIV+ postmenopausal women receiving RTV may be at greater risk for bone loss.

Osteoclast activity and subtypes as a function of physiology and pathology--implications for future treatments of osteoporosis

Osteoclasts have traditionally been associated exclusively with catabolic functions that are a prerequisite for bone resorption. However, emerging data suggest that osteoclasts also carry out functions that are important for optimal bone formation and bone quality. Moreover, recent findings indicate that osteoclasts have different subtypes depending on their location, genotype, and possibly in response to drug intervention. The aim of the current review is to describe the subtypes of osteoclasts in four different settings: 1) physiological, in relation to turnover of different bone types; 2) pathological, as exemplified by monogenomic disorders; 3) pathological, as identified by different disorders; and 4) in drug-induced situations. The profiles of these subtypes strongly suggest that these osteoclasts belong to a heterogeneous cell population, namely, a diverse macrophage-associated cell type with bone catabolic and anabolic functions that are dependent on both local and systemic parameters. Further insight into these osteoclast subtypes may be important for understanding cell-cell communication in the bone microenvironment, treatment effects, and ultimately bone quality.

Evolution of the parathyroid hormone family and skeletal formation pathways

Bone is considered to be a feature of higher vertebrates and one of the features that was required for the movement from water onto land. But there are a number of evolutionarily important species that have cartilaginous skeletons, including sharks. Both bony and cartilaginous fish are believed to have a common ancestor who had a bony skeleton. A number of factors and pathways have been shown to be involved in the development and maintenance of bony skeleton including the Wnt pathway and the parathyroid hormone gene family. The study of these pathways and factors in cartilaginous animals may shed light on the evolution of the vertebrate skeleton.

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.

Association between polymorphisms in Wnt signaling pathway genes and bone mineral density in postmenopausal Korean women

OBJECTIVE

The purpose of this study was to investigate the association between single nucleotide polymorphisms in Wnt signal pathway genes and circulating osteoprotegerin (OPG), soluble receptor activator of the nuclear factor-κB ligand (sRANKL) levels, bone turnover markers, and bone mineral density (BMD) in postmenopausal Korean women.

METHODS

Wnt9a c256G>A; low-density lipoprotein receptor-related protein (LRP) 5 c266A>G, c2245C>G, c3893C>T, and c4099G>A; secreted frizzled-related protein (sFRP) 4 c1019G>A; axin II c148C>T and c1615G>A; glycogen synthase kinase binding protein (GBP) c455C>A; β-catenin c94G>T and c101G>T; T-cell factor 1 c663G>T, c734C>T, and c766G>A; and adenomatous polyposis coli c5465T>A polymorphisms were analyzed in 392 postmenopausal Korean women. Serum levels of OPG, sRANKL, and bone turnover markers were measured, and BMDs at the lumbar spine and femoral neck were examined.

RESULTS

Wnt9a c256G>A, LRP5 c2245C>G and c4099G>A, axin II c1615G>A, GBP c455C>A, β-catenin c94G>T and c101G>T, and T-cell factor 1 c663G>T and c734C>T single nucleotide polymorphisms were not observed. Among the genes showing polymorphisms, only the sFRP4 c1019G>A polymorphism was associated with BMD. The AA genotype in the sFRP4 c1019G>A polymorphism showed significantly lower lumbar spine BMD and a higher serum bone alkaline phosphatase level than did the GG genotype and showed a 6.39 times higher risk for osteoporosis at the lumbar spine compared with the GG genotype. No significant differences in bone turnover markers, OPG, and sRANKL were detected among the other single genotypes or the LRP haplotype genotype.

CONCLUSIONS

Our results suggest that the sFRP4 c1019G>A polymorphism may be one of the genetic factors affecting lumbar spine BMD in postmenopausal Korean women.

SPARC functions as an inhibitor of adipogenesis

Adipogenesis, a key step in the pathogenesis of obesity, involves extensive ECM remodeling, changes in cell-ECM interactions, and cytoskeletal rearrangement. Matricellular proteins regulate cell-cell and cell-ECM interactions. Evidence in vivo and in vitro indicates that the prototypic matricellular protein, SPARC, inhibits adipogenesis and promotes osteoblastogenesis. Herein we discuss mechanisms underlying the inhibitory effect of SPARC on adipogenesis. SPARC enhances the Wnt/β-catenin signaling pathway and regulates the expression and posttranslational modification of collagen. SPARC might drive preadipocytes away from the status of growth arrest and therefore prevent terminal differentiation. SPARC could also decrease WAT deposition through its negative effects on angiogenesis. Therefore, several stages of white adipose tissue accumulation are sensitive to the inhibitory effects of SPARC.

Osteoblastogenic effects of dexamethasone through upregulation of TAZ expression in rat mesenchymal stem cells

Transcriptional coactivator with PDZ-binding motif (TAZ), a beta-catenin-like molecule, drives mesenchymal stem cell (MSC) to differentiate into osteoblast lineage through co-activation of Runx2-dependent gene transcription and repression of peroxisome proliferator-activated receptorgamma (PPARgamma)-dependent gene transcription. Dexamethasone (DEX), a synthetic and widely used glucocorticoid, affects osteogenesis. However, the signaling pathway by which DEX affects osteoblastic differentiation remains obscure. In this study, we found that DEX at the concentration of 10(-8)M enhanced calcium deposition, TAZ, bone morphogenetic protein 2 (BMP-2) and alkaline phosphatase (ALP) expression during osteoblastic differentiation. RU486, an antagonist of glucocorticoid receptor, blocked the improvement of TAZ expression while MSCs were treated with 10(-8)M DEX. Moreover, higher concentration (10(-7)M) of DEX robustly suppressed TAZ and ALP expression in MSCs. These findings suggest that TAZ is not only involved in the signal pathway of BMP-2-induced osteoblastic differentiation, but also involved in the signaling pathway of DEX-induced osteoblastic differentiation, supporting the notion that TAZ is a convergence point of two signaling pathways, BMP-2 signaling pathway and Wnt-beta-catenin signaling pathway.

Assessment of circulating Dickkopf-1 with a new two-site immunoassay in healthy subjects and women with breast cancer and bone metastases

The aim of our study was to investigate the sex- and age-related changes of serum Dickkopf-1 (Dkk-1), a soluble inhibitor of the Wnt signaling pathway, in healthy individuals and in patients with breast cancer (BC) and bone metastases (BM) using a new ELISA. Association of serum Dkk-1 with markers of bone turnover was also investigated. Serum Dkk-1 measurements were performed using a commercial sandwich ELISA in 150 healthy men, 175 healthy pre- and postmenopausal women (20-65 years), 22 women with BC and BM (mean age 63 years), and 16 women with BC and metastases at sites other than bone (mean age 53 years). Intra- and interassay coefficients of variation were below 7% and 12%, respectively. The detection limit was determined to be 0.018 microg/L. In healthy women and men, Dkk-1 did not change with age. Serum Dkk-1 modestly correlated with serum bone alkaline phosphatase (r = 0.19, P = 0.013) and serum C-terminal cross-linking telopeptide of type I collagen (r = 0.19, P = 0.014) in women but not in men. Dkk-1 levels were higher in women with BC and BM (5.57 +/- 5.50 microg/L) than in healthy age-matched controls (3.47 +/- 1.47 microg/L, P < 0.0001) and women with metastases at sites other than bone (3.57 +/- 1.66 microg/L, P = 0.0003). In conclusion, serum Dkk-1 is stable with age in healthy women and men and increases in patients with BC and BM. Measurements of circulating Dkk-1 with this new ELISA may be useful for the clinical investigation of patients with malignant bone diseases.

Signaling networks that control the lineage commitment and differentiation of bone cells

Osteoblasts and osteoclasts are the two major bone cells involved in the bone remodeling process. Osteoblasts are responsible for bone formation while osteoclasts are the bone-resorbing cells. The major event that triggers osteogenesis and bone remodeling is the transition of mesenchymal stem cells into differentiating osteoblast cells and monocyte/macrophage precursors into differentiating osteoclasts. Imbalance in differentiation and function of these two cell types will result in skeletal diseases such as osteoporosis, Paget's disease, rheumatoid arthritis, osteopetrosis, periodontal disease, and bone cancer metastases. Osteoblast and osteoclast commitment and differentiation are controlled by complex activities involving signal transduction and transcriptional regulation of gene expression. Recent advances in molecular and genetic studies using gene targeting in mice enable a better understanding of the multiple factors and signaling networks that control the differentiation process at a molecular level. This review summarizes recent advances in studies of signaling transduction pathways and transcriptional regulation of osteoblast and osteoclast cell lineage commitment and differentiation. Understanding the signaling networks that control the commitment and differentiation of bone cells will not only expand our basic understanding of the molecular mechanisms of skeletal development but will also aid our ability to develop therapeutic means of intervention in skeletal diseases.

WNT/beta-catenin signaling is involved in regulation of osteoclast differentiation by human immunodeficiency virus protease inhibitor ritonavir: relationship to human immunodeficiency virus-linked bone mineral loss

Untreated human immunodeficiency virus (HIV) infection is accompanied by reduced bone mineral density, which appears to be exacerbated by certain HIV protease inhibitors (PIs). The mechanisms leading to this apparent paradox, however, remain unclear. We have previously shown that, the HIV envelope glycoprotein gp120 used at levels similar those in plasmas of untreated HIV(+) patients, induced expression of the osteoclast (OC) differentiation factor RANKL in CD4+ T cells. In addition, the HIV PI ritonavir abrogated the interferon-gamma-mediated degradation of the RANKL nuclear adapter protein TRAF6, a physiological block to RANKL activity. Here, using oligonucleotide microarrays and quantitative polymerase chain reaction, we explored potential upstream mechanisms for these effects. Ritonavir, but not the HIV PIs indinavir or nelfinavir, up-regulated the production of transcripts for OC growth factors and the non-canonical Wnt Proteins 5B and 7B as well as activated promoters of nuclear factor-kappaB signaling, but suppressed genes involved in canonical Wnt signaling. Similarly, ritonavir blocked the cytoplasmic to nuclear translocation of beta-catenin, the molecular node of the Wnt signaling pathway, in association with enhanced beta-catenin ubiquitination. Exposure of OC precursors to LiCl, an inhibitor of the canonical Wnt antagonist GSK-3beta, suppressed OC differentiation, as did adenovirus-mediated overexpression of beta-catenin. These data identify, for the first time, a biologically relevant role for Wnt signaling via beta-catenin in isolated OC precursors and the modulation of Wnt signaling by ritonavir. The reversal of these ritonavir-mediated changes by interferon-gamma provides a model for possible intervention in this metabolic complication of HIV therapy.

SPARC inhibits adipogenesis by its enhancement of beta-catenin signaling

SPARC (secreted protein acidic and rich in cysteine) modulates interactions between cells and extracellular matrix and is enriched in white adipose tissue. We have reported that SPARC-null mice accumulate significantly more fat than wild-type mice and maintain relatively high levels of serum leptin. We now show that SPARC inhibits adipogenesis in vitro. Specifically, recombinant SPARC inhibited (a) adipocyte differentiation of stromal-vascular cells isolated from murine white adipose tissue and (b) the expression of adipogenic transcription factors and adipocyte-specific genes. SPARC induced the accumulation and nuclear translocation of beta-catenin and subsequently enhanced the interaction of beta-catenin and T cell/lymphoid enhancer factor 1. The activity of integrin-linked kinase was required for the effect of SPARC on beta-catenin accumulation as well as extracellular matrix remodeling. During adipogenesis, fusiform preadipocytes change into sphere-shaped adipocytes and convert the extracellular matrix from a fibronectin-rich stroma to a laminin-rich basal lamina. SPARC retarded the morphological changes exhibited by preadipocytes during differentiation. In the presence of SPARC, the deposition of fibronectin was enhanced, and that of laminin was inhibited; in parallel, the expression of alpha5 integrin was enhanced, and that of alpha6 integrin was inhibited. Lithium chloride, which enhances the accumulation of beta-catenin, also inhibited the expression of alpha6 integrin. These findings demonstrate a role for SPARC in adipocyte morphogenesis and in signaling processes leading to terminal differentiation.

Osteoclasts control osteoblast chemotaxis via PDGF-BB/PDGF receptor beta signaling

Background

Bone remodeling relies on the tightly regulated interplay between bone forming osteoblasts and bone digesting osteoclasts. Several studies have now described the molecular mechanisms by which osteoblasts control osteoclastogenesis and bone degradation. It is currently unclear whether osteoclasts can influence bone rebuilding.

Methodology/Principal Findings

Using in vitro cell systems, we show here that mature osteoclasts, but not their precursors, secrete chemotactic factors recognized by both mature osteoblasts and their precursors. Several growth factors whose expression is upregulated during osteoclastogenesis were identified by DNA microarrays as candidates mediating osteoblast chemotaxis. Our subsequent functional analyses demonstrate that mature osteoclasts, whose platelet-derived growth factor bb (PDGF-bb) expression is reduced by siRNAs, exhibit a reduced capability of attracting osteoblasts. Conversely, osteoblasts whose platelet-derived growth factor receptor β (PDGFR-β) expression is reduced by siRNAs exhibit a lower capability of responding to chemotactic factors secreted by osteoclasts.

Conclusions/Significance

We conclude that, in vitro mature osteoclasts control osteoblast chemotaxis via PDGF-bb/PDGFR-β signaling. This may provide one key mechanism by which osteoclasts control bone formation in vivo.

The developmental basis of skeletal cell differentiation and the molecular basis of major skeletal defects

Vertebrate skeletal differentiation retains elements from simpler phyla, and reflects the differentiation of supporting tissues programmed by primary embryonic development. This developmental scheme is driven by homeotic genes expressed in sequence, with subdivision of skeletal primordia driven by a combination of seven transmembrane-pass receptors responding to Wnt-family signals, and by bone morphogenetic family signals that define borders of individual bones. In sea-dwelling vertebrates, an essentially complete form of the skeleton adapted by the land-living vertebrates develops in cartilage, based on type II collagen and hydrophilic proteoglycans. In bony fishes, this skeleton is mineralized to form a solid bony skeleton. In the land-living vertebrates, most of the skeleton is replaced by an advanced vascular mineralized skeleton based on type I collagen, which reduces skeletal mass while facilitating use of skeletal mineral for metabolic homeostasis. Regulation of the mammalian skeleton, in this context, reflects practical adaptations to the needs for life on land that are related to ancestral developmental signals. This regulation includes central nervous system regulation that integrates bone turnover with overall metabolism. Recent work on skeletal development, in addition, demonstrates molecular mechanisms that cause developmental bone diseases.

Differential requirement for beta-catenin in epithelial and fiber cells during lens development

Recent studies implicate Wnt/beta-catenin signaling in lens differentiation (Stump, R. J., et al., 2003. A role for Wnt/beta-catenin signaling in lens epithelial differentiation. Dev Biol;259:48-61). Beta-catenin is a component of adherens junctions and functions as a transcriptional activator in canonical Wnt signaling. We investigated the effects of Cre/LoxP-mediated deletion of beta-catenin during lens development using two Cre lines that specifically deleted beta-catenin in whole lens or only in differentiated fibers, from E13.5. We found that beta-catenin was required in lens epithelium and during early fiber differentiation but appeared to be redundant in differentiated fiber cells. Complete loss of beta-catenin resulted in an abnormal and deficient epithelial layer with loss of E-cadherin and Pax6 expression as well as abnormal expression of c-Maf and p57(kip2) but not Prox1. There was also disrupted fiber cell differentiation, characterized by poor cell elongation, decreased beta-crystallin expression, epithelial cell cycle arrest at G(1)-S transition and premature cell cycle exit. Despite cell cycle arrest there was no induction of apoptosis. Mutant fiber cells displayed altered apical-basal polarity as evidenced by altered distribution of the tight junction protein, ZO1, disruption of apical actin filaments and abnormal deposition of extracellular matrix, resulting in a deficient lens capsule. Loss of beta-catenin also affected the formation of adhesion junctions as evidenced by dissociation of N-cadherin and F-actin localization in differentiating fiber cells. However, loss of beta-catenin from terminally differentiating fibers had no apparent effects on adhesion junctions between adjacent embryonic fibers. These data indicate that beta-catenin plays distinct functions during lens fiber differentiation and is involved in both Wnt signaling and adhesion-related mechanisms that regulate lens epithelium and early fiber differentiation.

Msx2 exerts bone anabolism via canonical Wnt signaling

Msx2 is a homeodomain transcription factor first identified in craniofacial bone and human femoral osteoblasts. We hypothesized that Msx2 might activate skeletal Wnt signaling. Therefore, we analyzed the effects of CMV-Msx2 transgene (Msx2Tg) expression on skeletal physiology and composition. Skeletal Msx2 expression was increased 2-3-fold by Msx2Tg, with expanded protein accumulation in marrow, secondary ossification centers, and periosteum. Microcomputed tomography established increased bone volume in Msx2Tg mice, with increased numbers of plate-like trabeculae. Histomorphometry revealed increased bone formation in Msx2Tg mice versus non-Tg siblings, arising from increased osteoblast numbers. While decreasing adipogenesis, Msx2Tg increased osteogenic differentiation via mechanisms inhibited by Dkk1, an antagonist of Wnt receptors LRP5 and LRP6. Bone from Msx2Tg mice elaborated higher levels of Wnt7 canonical agonists, with diminished Dkk1, changes that augment canonical signaling. Analysis of non-Tg and Msx2Tg siblings possessing the TOPGAL reporter confirmed this; Msx2Tg up-regulated skeletal beta-galactosidase expression (p </= 0.01), along with Wnt7a and Wnt7b, and reduced circulating Dkk1. To better understand molecular mechanisms, we studied C3H10T1/2 osteoprogenitor cells. As in bone, Msx2 increased Wnt7 genes and down-regulated Dkk1, while inducing the osteoblast gene alkaline phosphatase. Msx2-directed RNA interference increased Dkk1 expression and promoter activity, while reducing Wnt7a, Wnt7b, and alkaline phosphatase. Moreover, Msx2 inhibited Dkk1 promoter activity and reduced RNA polymerase association with Dkk1 chromatin. RNA interference-mediated knockdown of Wnt7a, Wnt7b, and LRP6 significantly reduced Msx2-induced alkaline phosphatase. Msx2 exerts bone anabolism in part by reducing Dkk1 expression and enhancing Wnt signaling, thus promoting osteogenic differentiation of skeletal progenitors.

Increasing Wnt signaling in the bone marrow microenvironment inhibits the development of myeloma bone disease and reduces tumor burden in bone in vivo

There is increasing evidence to suggest that the Wnt signaling pathway plays a critical role in the pathogenesis of myeloma bone disease. In the present study, we determined whether increasing Wnt signaling within the bone marrow microenvironment in myeloma counteracts development of osteolytic bone disease. C57BL/KaLwRij mice were inoculated intravenously with murine 5TGM1 myeloma cells, resulting in tumor growth in bone and development of myeloma bone disease. Lithium chloride (LiCl) treatment activated Wnt signaling in osteoblasts, inhibited myeloma bone disease, and decreased tumor burden in bone, but increased tumor growth when 5TGM1 cells were inoculated subcutaneously. Abrogation of beta-catenin activity and disruption of Wnt signaling in 5TGM1 cells by stable overexpression of a dominant-negative TCF4 prevented the LiCl-induced increase in subcutaneous growth but had no effect on LiCl-induced reduction in tumor burden within bone or on osteolysis in myeloma-bearing mice. Together, these data highlight the importance of the local microenvironment in the effect of Wnt signaling on the development of myeloma bone disease and demonstrate that, despite a direct effect to increase tumor growth at extraosseous sites, increasing Wnt signaling in the bone marrow microenvironment can prevent the development of myeloma bone disease and inhibit myeloma growth within bone in vivo.