Wnt10b increases postnatal bone formation by enhancing osteoblast differentiation

Rough surface topography enhances the activation of Wnt/β-catenin signaling in mesenchymal cells

It is known that the roughness of titanium surfaces affects cell proliferation and differentiation. However, the mechanisms mediating the cellular responses to surface topography are only partially understood. The present study investigated whether Wnt canonical signaling, an important pathway in determining cell fate, is modulated by surface roughness. This study analyzed the behavior of the murine C2C12 mesenchymal cell line on polished or acid-etched, sand-blasted (SLA) commercially pure titanium. When we transfected cells with Wnt3a or wild-type β-catenin and a reporter construct, we found that stimulation of Wnt canonical signaling was enhanced in cells on SLA surfaces. Moreover, more β-catenin translocated to the nucleus in cells on SLA surfaces after stimulation with Wnt3a as evidenced by immunofluorescence. However, when cells were transfected with constitutively active S33Y β-catenin mutant, no difference was observed between the groups. Higher levels of transcripts of Wnt target genes were detected in C2C12 cells cultured on SLA surfaces following transfection with Wnt3a, but the expression of a gene regulating β-catenin degradation, Axin 2, was reduced on SLA surfaces. Inhibition of β-catenin mediated transcription by dnTCF in murine osteoblastic MC3T3 cells, reversed the effects of topography on cell differentiation. Taken together, these results show that surface roughness modulates the responsiveness of mesenchymal cells to Wnt3a, that this requires the control of β-catenin degradation, and that the control of β-catenin signaling by surface topography is accountable for at least part of the effects of surface on cell differentiation.

Lef1DeltaN binds beta-catenin and increases osteoblast activity and trabecular bone mass

Lymphoid enhancer-binding factor (Lef) 1 is a high mobility group protein best known as a Wnt-responsive transcription factor that associates with β-catenin. Lef1ΔN is a short isoform of Lef1 that lacks the first 113 amino acids and a well characterized high affinity β-catenin binding domain present in the full-length protein. Both Lef1 isoforms bind DNA and regulate gene expression. We previously reported that Lef1 is expressed in proliferating osteoblasts and blocks osteocalcin expression. In contrast, Lef1ΔN is only detectable in the later stages of osteoblast differentiation and promotes osteogenesis in vitro. Here, we show that Lef1ΔN retains the ability to interact physically and functionally with β-catenin. Unlike what has been reported in T cells and colon cancer cell lines, Lef1ΔN activated gene transcription in the absence of exogenous β-catenin and cooperated with constitutively active β-catenin to stimulate gene transcription in mesenchymal and osteoblastic cells. Residues at the N terminus of Lef1ΔN were required for β-catenin binding and the expression of osteoblast differentiation genes. To determine the role of Lef1ΔN on bone formation in vivo, a Lef1ΔN transgene was expressed in committed osteoblasts using the 2.3-kb fragment of the type 1 collagen promoter. The Lef1ΔN transgenic mice had higher trabecular bone volume in the proximal tibias and L5 vertebrae. Histological analyses of tibial sections revealed no differences in osteoblast, osteoid, or osteoclast surface areas. However, bone formation and mineral apposition rates as well as osteocalcin levels were increased in Lef1ΔN transgenic mice. Together, our data indicate that Lef1ΔN binds β-catenin, stimulates Lef/Tcf reporter activity, and promotes terminal osteoblast differentiation.

Wnt/β-catenin signaling enhances osteoblastogenic differentiation from human periodontal ligament fibroblasts

Wnt/β-catenin signaling has been known to influence bone formation and homeostasis. In this study, we investigated the canonical Wnt signaling regulation of osteogenic differentiation from periodontal ligament (PDL) fibroblasts. Stimulating PDL fibroblasts with lithium chloride (LiCl), a canonical Wnt activator, significantly increased mineralized nodule and alkaline phosphatase (ALP) activity in a time- and dose-dependent manner. LiCl up-regulated protein expression of osteogenic transcription factors, including the runt-related gene 2, Msx2, and Osterix 2, in the PDL fibroblasts. Treatment of these cells with LiCl also increased the mRNA levels of ALP, FosB, and Fra1 in a dose-dependent manner. Blockage of canonical Wnt signaling by treating the cells with DKK1 inhibited Wnt1-stimulated mRNA expression of these osteogenic factors. Furthermore, pretreatment with DKK1 reduced the ALP activity and matrix mineralization stimulated by Wnt1. Collectively, these results suggest that canonical Wnt signaling leads to the differentiation of PDL fibroblasts into osteogenic lineage with the attendant stimulation of osteogenic transcription factors.

Lymphocyte enhancer-binding factor 1: an essential factor in odontoblastic differentiation of dental pulp cells enzymatically isolated from rat incisors

Lymphocyte enhancer-binding factor 1 (Lef1), a HMG-domain protein, is thought to play important roles in inductive tissue interaction during tooth development. Lef1 knockdown in mice causes arrest at the bud stage in tooth development. As this gene participates in the regulation of a large and diverse set of peptide growth factors in ectomesenchymal cell differentiation of dental papilla, Lef1 appears to be a key factor in odontoblast differentiation. However, the relationship between Lef1 and odontoblast differentiation is still unclear. To analyze the biological roles of Lef1 in regulating odontoblast differentiation, we transiently overexpressed or suppressed Lef1 in cultured dental pulp cells. Lef1-overexpressing cells expressed higher levels of dentin sialoprotein (DSPP), osteocalcin and alkaline phosphatase (ALP) mRNA and formed larger numbers of mineralized nodules compared to control cells. However, Msx-1 expression or cell proliferation was unaffected by overexpression of Lef1. To further examine the role of Lef1 in dental pulp cells, we knocked down Lef1 expression in dental pulp cells using short interfering RNA (siRNA). Transient expression of siRNA against Lef1 markedly reduced Lef1 mRNA levels, and Lef1-suppressed cells expressed lower levels of DSPP, osteocalcin and ALP mRNA compared to control cells. Furthermore, the formation of mineralized nodules was inhibited by siRNA against Lef1; however, neither Msx-1 expression or cell proliferation was inhibited by siRNA against Lef1. These results outline the role of Lef1 in accelerating odontoblast differentiation by regulating DSP and osteocalcin mRNA expression in dental pulp cells, confirming that Lef1 is a key factor for odontoblast differentiation.

Silencing of autocrine motility factor induces mesenchymal-to-epithelial transition and suppression of osteosarcoma pulmonary metastasis

Phosphoglucose isomerase (PGI) is a multifunctional enzyme that functions in glucose metabolism as a glycolytic enzyme catalyzing an interconversion between glucose and fructose inside the cell, while it acts as cytokine outside the cell, with properties that include autocrine motility factor (AMF)-regulating tumor cell motility. Overexpression of AMF/PGI induces epithelial-to-mesenchymal transition with enhanced malignancy. Recent studies have revealed that silencing of AMF/PGI resulted in mesenchymal-to-epithelial transition (MET) of human lung fibrosarcoma cells and breast cancer cells with reduced malignancy. Here, we constructed a hammerhead ribozyme specific against GUC triplet at the position G390 in the human, mouse, and rat AMF/PGI mRNA sequence. Mesenchymal human osteosarcoma MG-63, HS-Os-1, and murine LM8 cells were stably transfected with the ribozyme specific for AMF/PGI. The stable transfectant cells showed effective downregulation of AMF/PGI expression and subsequent abrogation of AMF/PGI secretion, which resulted in morphologic change with reduced growth, motility, and invasion. Silencing of AMF/PGI induced MET, in which upregulation of E-cadherin and cytokeratins, as well as downregulation of vimentin, were noted. The MET guided by AMF/PGI gene silencing induced osteosarcoma MG-63 to terminally differentiate into mature osteoblasts. Furthermore, MET completely suppressed the tumor growth and pulmonary metastasis of LM8 cells in nude mice. Thus, acquisition of malignancy might be completed in part by upregulation of AMF/PGI, and waiver of malignancy might also be controlled by downregulation of AMF/PGI.

Wnt10b deficiency results in age-dependent loss of bone mass and progressive reduction of mesenchymal progenitor cells

Wnt10b is a canonical Wnt ligand expressed in developing bone and has been linked to mesenchymal progenitor functions in mice and humans. Because Wnt signaling has been shown to play an important role in progenitor maintenance in a variety of adult tissues, we examined bone deposition and growth rates throughout postnatal development in Wnt10b-null mice. Using bone histomorphometry and micro-computed tomographic (µCT) studies, we demonstrate that trabecular bone deposition is slightly enhanced in Wnt10b-null mice at 1 month of age, followed by progressive loss with age. Importantly, we find that Wnt10b is required for maintenance of adult bone density in multiple backgrounds of inbred mice and that both copies of the Wnt10b gene are required to maintain normal bone density in 6-month-old animals. We go on to show that the loss in trabecular bone in Wnt10b-null mice is associated with a reduction in the number of bone marrow-derived mesenchymal progenitors (MPCs) using in vitro colony-forming unit assays and marker analysis. Analysis of osteogenic gene expression in primary bone marrow stromal cells demonstrated reductions in expression of several osteoblast differentiation markers. Taken together, our results indicate that Wnt10b is uniquely required for maintenance of mesenchymal progenitor activity in adult bone. The results show the significance of studying individual Wnt ligands and their potentially unique contribution in the context of aging and disease.

Wnt signaling in bone

Wnt signaling is involved not only in embryonic development but also in maintenance of homeostasis in postnatal tissues. Multiple lines of evidence have increased understanding of the roles of Wnt signaling in bone since mutations in the LRP5 gene were identified in human bone diseases. Canonical Wnt signaling promotes mesenchymal progenitor cells to differentiate into osteoblasts. The canonical Wnt/β-catenin pathway possibly through Lrp6, a co-receptor for Wnts as well as Lrp5, in osteoblasts regulates bone resorption by increasing the OPG/RANKL ratio. However, endogenous inhibitors of Wnt signaling including sclerostin block bone formation. Regulation of sclerostin appears to be one of the mechanisms of PTH anabolic actions on bone. Since sclerostin is almost exclusively expressed in osteocytes, inhibition of sclerostin is the most promising design. Surprisingly, Lrp5 controls bone formation by inhibiting serotonin synthesis in the duodenum, but not by directly promoting bone formation. Pharmacological intervention may be considered in many components of the canonical Wnt signaling pathway, although adverse effects and tumorigenicity to other tissues are important. More studies will be needed to fully understand how the Wnt signaling pathway actually influences bone metabolism and to assure the safety of new interventions.

Molecular genetic studies of gene identification for osteoporosis: the 2009 update

Osteoporosis is a complex human disease that results in increased susceptibility to fragility fractures. It can be phenotypically characterized using several traits, including bone mineral density, bone size, bone strength, and bone turnover markers. The identification of gene variants that contribute to osteoporosis phenotypes, or responses to therapy, can eventually help individualize the prognosis, treatment, and prevention of fractures and their adverse outcomes. Our previously published reviews have comprehensively summarized the progress of molecular genetic studies of gene identification for osteoporosis and have covered the data available to the end of September 2007. This review represents our continuing efforts to summarize the important and representative findings published between October 2007 and November 2009. The topics covered include genetic association and linkage studies in humans, transgenic and knockout mouse models, as well as gene-expression microarray and proteomics studies. Major results are tabulated for comparison and ease of reference. Comments are made on the notable findings and representative studies for their potential influence and implications on our present understanding of the genetics of osteoporosis.

Control of Dkk-1 ameliorates chondrocyte apoptosis, cartilage destruction, and subchondral bone deterioration in osteoarthritic knees

OBJECTIVE

Perturbation of Wnt signaling components reportedly regulates chondrocyte fate and joint disorders. The Wnt inhibitor Dkk-1 mediates remodeling of various tissue types. We undertook this study to examine whether control of Dkk-1 expression prevents joint deterioration in osteoarthritic (OA) knees.

METHODS

Anterior cruciate ligament transection-and collagenase-induced OA in rat knees was treated with end-capped phosphorothioate Dkk-1 antisense oligonucleotide (Dkk-1-AS). Articular cartilage destruction, cartilage degradation markers, bone mineral density (BMD), and subchondral trabecular bone volume of injured knee joints were measured using Mankin scoring, enzyme-linked immunosorbent assay, dual x-ray absorptiometry, and histomorphometry. Dkk-1-responsive molecule expression and apoptotic cells in knee tissue were detected by quantitative reverse transcriptase-polymerase chain reaction, immunoblotting, and TUNEL staining.

RESULTS

Up-regulated Dkk-1 expression was associated with increased Mankin score and with increased serum levels of cartilage oligomeric matrix protein and C-telopeptide of type II collagen (CTX-II) during OA development. Dkk-1-AS treatment alleviated OA-associated increases in Dkk-1 expression, Mankin score, cartilage fibrillation, and serum cartilage degradation markers. Dkk-1-AS also alleviated epiphyseal BMD loss and subchondral bone exposure associated with altered serum levels of osteocalcin and CTX-I. The treatment abrogated chondrocyte/osteoblast apoptosis and subchondral trabecular bone remodeling in OA. Dkk-1 knockdown increased levels of nuclear beta-catenin and phosphorylated Ser(473)-Akt but attenuated expression of inflammatory factors (Toll-like receptor 4 [TLR-4], TLR-9, interleukin-1beta, and tumor necrosis factor alpha), the apoptosis regulator Bax, matrix metalloproteinase 3, and RANKL in OA knee joints.

CONCLUSION

Interference with the cartilage- and bone-deleterious actions of Dkk-1 provides therapeutic potential for alleviating cartilage destruction and subchondral bone damage in OA knee joints.

Alteration of Notch signaling in skeletal development and disease

Notch signaling is an evolutionarily conserved mechanism for specifying and regulating organogenesis and tissue renewal. Human and mouse genetic studies have demonstrated mutations in many components of the Notch signaling pathway that cause skeletal patterning defects. More recently, the in vivo effects of Notch signaling on osteoblast specification, proliferation, and differentiation have been demonstrated in addition to its regulation of osteoclast activity. However, while our understanding of canonical Notch signaling in skeletal biology is rapidly evolving, the role of noncanonical Notch signaling is still poorly understood. In a pathologic context, aberration of Notch signaling is also associated with osteosarcoma. These studies raise the question of how Notch may interact with other signaling pathways, such as Wnt. Finally, manipulation of Notch signaling for bone-related diseases remains complex because of the temporal and context-dependent nature of Notch signaling during mesenchymal stem cell and osteoblast differentiation.

Osteocyte Wnt/beta-catenin signaling is required for normal bone homeostasis

Beta-Catenin-dependent canonical Wnt signaling plays an important role in bone metabolism by controlling differentiation of bone-forming osteoblasts and bone-resorbing osteoclasts. To investigate its function in osteocytes, the cell type constituting the majority of bone cells, we generated osteocyte-specific beta-catenin-deficient mice (Ctnnb1(loxP/loxP); Dmp1-Cre). Homozygous mutants were born at normal Mendelian frequency with no obvious morphological abnormalities or detectable differences in size or body weight, but bone mass accrual was strongly impaired due to early-onset, progressive bone loss in the appendicular and axial skeleton with mild growth retardation and premature lethality. Cancellous bone mass was almost completely absent, and cortical bone thickness was dramatically reduced. The low-bone-mass phenotype was associated with increased osteoclast number and activity, whereas osteoblast function and osteocyte density were normal. Cortical bone Wnt/beta-catenin target gene expression was reduced, and of the known regulators of osteoclast differentiation, osteoprotegerin (OPG) expression was significantly downregulated in osteocyte bone fractions of mutant mice. Moreover, the OPG levels expressed by osteocytes were higher than or comparable to the levels expressed by osteoblasts during skeletal growth and at maturity, suggesting that the reduction in osteocytic OPG and the concomitant increase in osteocytic RANKL/OPG ratio contribute to the increased number of osteoclasts and resorption in osteocyte-specific beta-catenin mutants. Together, these results reveal a crucial novel function for osteocyte beta-catenin signaling in controlling bone homeostasis.

Osteocytes and WNT: the Mechanical Control of Bone Formation

Mechanical loading is of pivotal importance in the maintenance of skeletal homeostasis, but the players involved in the transduction of mechanical stimuli to promote bone maintenance have long remained elusive. Osteocytes, the most abundant cells in bone, possess mechanosensing appendices stretching through a system of bone canaliculi. Mechanical stimulation plays an important role in osteocyte survival and hence in the preservation of bone mechanical properties, through the maintenance of bone hydratation. Osteocytes can also control the osteoblastic differentiation of mesenchymal precursors in response to mechanical loading by modulating WNT signaling pathways, essential regulators of cell fate and commitment, through the protein sclerostin. Mutations of Sost, the sclerostin-encoding gene, have dramatic effects on the skeleton, indicating that osteocytes may act as master regulators of bone formation and localized bone remodeling. Moreover, the development of sclerostin inhibitors is opening new possibilities for bone regeneration in orthopedics and the dental field.

Polymorphisms of the WNT10B gene, bone mineral density, and fractures in postmenopausal women

Wnt ligands are important regulators of skeletal homeostasis. Wnt10B tends to stimulate the differentiation of common mesenchymal precursors toward the osteoblastic lineage, while inhibiting adipocytic differentiation. Hence, we decided to explore the association of WNT10B allelic variants with bone mineral density and osteoporotic fractures. A set of tag SNPs capturing most common variations of the WNT10B gene was genotyped in 1438 Caucasian postmenopausal women, including 146 with vertebral fractures and 432 with hip fractures. We found no association between single SNPs and spine or hip bone mineral density (BMD). In the multilocus analysis, some haplotypes showed a slight association with spine BMD (P = 0.03), but it was not significant after multiple-test correction. There was no association between genotype and vertebral or hip fractures. Transcripts of WNT10B and other Wnt ligands were detected in human bone samples by real-time PCR. However, there was no relationship between genotype and RNA abundance. Thus, WNT10B is expressed in the bone microenvironment and may be an important regulator of osteoblastogenesis, but we have not found evidence for a robust association of common WNT10B gene allelic variants with either BMD or fractures in postmenopausal women.

Wnt signaling as a therapeutic target for bone diseases

BACKGROUND

There is a need to develop new bone anabolic agents because current bone regeneration regimens have limitations. The Wingless-type MMTV integration site (Wnt) pathway has emerged as a regulator of bone formation and regeneration.

OBJECTIVE

To review the molecular basis for Wnt pathway modulation and discuss strategies that target it and improve bone mass.

METHODS

Data in peer-reviewed reports and meeting abstracts are discussed.

RESULTS/CONCLUSIONS

Neutralizing inhibitors of Wnt signaling have emerged as promising strategies. Small-molecule inhibitors of glycogen synthase kinase 3beta increase bone mass, lower adiposity and reduce fracture risk. Neutralizing antibodies to Dickkopf 1, secreted Frizzled-related protein 1 and sclerostin produce similar outcomes in animal models. These drugs are exciting breakthroughs but are not without risks. The challenges include tissue-specific targeting and consequently, long-term safety.

Canonical Wnts function as potent regulators of osteogenesis by human mesenchymal stem cells

Genetic evidence indicates that Wnt signaling is critically involved in bone homeostasis. In this study, we investigated the functions of canonical Wnts on differentiation of adult multipotent human mesenchymal stem cells (hMSCs) in vitro and in vivo. We observe differential sensitivities of hMSCs to Wnt inhibition of osteogenesis versus adipogenesis, which favors osteoblastic commitment under binary in vitro differentiation conditions. Wnt inhibition of osteogenesis is associated with decreased expression of osteoblastic transcription factors and inhibition of c-Jun N-terminal kinase and p38 mitogen-activated protein kinase activation, which are involved in osteogenic differentiation. An hMSC subpopulation exhibits high endogenous Wnt signaling, the inhibition of which enhances osteogenic and adipogenic differentiation in vitro. In an in vivo bone formation model, high levels of Wnt signaling inhibit de novo bone formation by hMSCs. However, hMSCs with exogenous expression of Wnt1 but not stabilized β-catenin markedly stimulate bone formation by naive hMSCs, arguing for an important role of a canonical Wnt gradient in hMSC osteogenesis in vivo.

Association analysis of WNT10B with bone mass and structure among individuals of African ancestry

Wnts comprise a family of secreted growth factors that regulate the development and maintenance of many organs. Recently, Wnt10b was shown to stimulate osteoblastogenesis and bone formation in mice. To evaluate further the role of Wnt10b in bone health in humans, we performed bidirectional sequencing of approximately 8 kb of the WNT10B gene region in 192 individuals (96 African, 96 white) to identify single nucleotide polymorphisms (SNPs). We identified 19 SNPs with minor allele frequency (MAF) > or =0.01. Ten of these SNPs were not present in the NCBI dbSNP database (build 127), whereas 10 of the 20 SNPs (50%) reported in dbSNP were not verified. We initially genotyped seven tagging SNPs that captured common (MAF > or = 0.05) variation in the region with r (2) > 0.80 and a potentially functional SNP in exon 5 in 1035 Afro-Caribbean men > or =40 yr of age. Association analysis showed three SNPs in a 3' region of linkage disequilibrium that were associated with DXA measures of hip BMD. Associations between two of these three SNPs (rs1051886, rs3741627) with hip BMD were replicated in an additional 980 Afro-Caribbean men (p < 0.05), in the combined sample of 2015 men (p < or = 0.006), and in 416 individuals > or =18 yr of age (mean, 44 yr) belonging to eight extended, multigenerational Afro-Caribbean families with mean family size >50 (3535 relative pairs; p < 0.05). Further analysis showed that rs1051886 and rs3741627 were associated with cortical cross-sectional area, periosteal circumference, and BMC in the radius, such that individuals with the minor alleles had lower biomechanical indices of long-bone bending strength. This analysis implicates the WNT10B locus as a genetic element in the regulation of bone mass and structural geometry.

Quantitative trait loci, genes, and polymorphisms that regulate bone mineral density in mouse

This is an in silico analysis of data available from genome-wide scans. Through analysis of QTL, genes and polymorphisms that regulate BMD, we identified 82 BMD QTL, 191 BMD-associated (BMDA) genes, and 83 genes containing known BMD-associated polymorphisms (BMDAP). The catalogue of all BMDA/BMDAP genes and relevant literatures are provided. In total, there are substantially more BMDA/BMDAP genes in regions of the genome where QTL have been identified than in non-QTL regions. Among 191 BMDA genes and 83 BMDAP genes, 133 and 58 are localized in QTL regions, respectively. The difference was still noticeable for the chromosome distribution of these genes between QTL and non-QTL regions. These results have allowed us to generate an integrative profile of QTL, genes, polymorphisms that determine BMD. These data could facilitate more rapid and comprehensive identification of causal genes underlying the determination of BMD in mouse and provide new insights into how BMD is regulated in humans.

Wnt signaling in bone metabolism

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

Regulation of bone formation by osteoclasts involves Wnt/BMP signaling and the chemokine sphingosine-1-phosphate

Under most conditions, resorbed bone is nearly precisely replaced in location and amount by new bone. Thus, it has long been recognized that bone loss through osteoclast-mediated bone resorption and bone replacement through osteoblast-mediated bone formation are tightly coupled processes. Abundant data conclusively demonstrate that osteoblasts direct osteoclast differentiation. Key questions remain, however, as to how osteoblasts are recruited to the resorption site and how the amount of bone produced is so precisely controlled. We hypothesized that osteoclasts play a crucial role in the promotion of bone formation. We found that osteoclast conditioned medium stimulates human mesenchymal stem (hMS) cell migration and differentiation toward the osteoblast lineage as measured by mineralized nodule formation in vitro. We identified candidate osteoclast-derived coupling factors using the Affymetrix microarray. We observed significant induction of sphingosine kinase 1 (SPHK1), which catalyzes the phosphorylation of sphingosine to form sphingosine 1-phosphate (S1P), in mature multinucleated osteoclasts as compared with preosteoclasts. S1P induces osteoblast precursor recruitment and promotes mature cell survival. Wnt10b and BMP6 also were significantly increased in mature osteoclasts, whereas sclerostin levels decreased during differentiation. Stimulation of hMS cell nodule formation by osteoclast conditioned media was attenuated by the Wnt antagonist Dkk1, a BMP6-neutralizing antibody, and by a S1P antagonist. BMP6 antibodies and the S1P antagonist, but not Dkk1, reduced osteoclast conditioned media-induced hMS chemokinesis. In summary, our findings indicate that osteoclasts may recruit osteoprogenitors to the site of bone remodeling through SIP and BMP6 and stimulate bone formation through increased activation of Wnt/BMP pathways.

Targeting the Wnt signaling pathway to augment bone formation

Recent discoveries in humans and mice have revealed that the Wnt (Wingless and Int-1) signaling pathway is responsible for a complex array of functions in maintaining bone homeostasis. The Wnt proteins are key modulators of mesenchymal lineage specification and regulate most aspects of osteoblast physiology and postnatal bone acquisition by controlling the differentiation and activity of osteoblasts and osteoclasts. Initial reports have indicated that activators of Wnt signaling are potent promoters of osteogenesis; however, systemic hyperactivation of the canonical Wnt pathway could potentially accelerate neoplastic transformation and subsequent tumor growth. Alternatively, recent investigations of natural soluble antagonists of Wnt signaling in bone suggest the possibilities of bone-specific therapies targeting the negative regulators of Wnt pathway, especially sclerostin. With this new knowledge, novel pharmacologic interventions that alter Wnt signaling are being evaluated for the management of osteoporosis. In this article, we briefly describe the Wnt signaling elements, their characterized role in bone, and summarize the current knowledge on the potential to enhance bone formation through the manipulation of Wnt signaling antagonists.

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.

Osteoclast-osteoblast communication

Cells in osteoclast and osteoblast lineages communicate with each other through cell-cell contact, diffusible paracrine factors and cell-bone matrix interaction. Osteoclast-osteoblast communication occurs in a basic multicellular unit (BMU) at the initiation, transition and termination phases of bone remodeling. At the initiation phase, hematopoietic precursors are recruited to the BMU. These precursors express cell surface receptors including c-Fms, RANK and costimulatory molecules, such as osteoclast-associated receptor (OSCAR), and differentiate into osteoclasts following cell-cell contact with osteoblasts, which express ligands. Subsequently, the transition from bone resorption to formation is mediated by osteoclast-derived 'coupling factors', which direct the differentiation and activation of osteoblasts in resorbed lacunae to refill it with new bone. Bidirectional signaling generated by interaction between ephrinB2 on osteoclasts and EphB4 on osteoblast precursors facilitates the transition. Such interaction is likely to occur between osteoclasts and lining cells in the bone remodeling compartment (BRC). At the termination phase, bone remodeling is completed by osteoblastic bone formation and mineralization of bone matrix. Here, we describe molecular communication between osteoclasts and osteoblasts at distinct phases of bone remodeling.