NFAT and Osterix cooperatively regulate bone formation

Regulators of osteoclast differentiation and cell-cell fusion

Osteoclasts are multinuclear giant cells derived from osteoclast/macrophage/dendritic cell common progenitor cells. The most characteristic feature of osteoclasts is multinucleation resulting from cell-cell fusion of mononuclear osteoclasts. Osteoclast cell-cell fusion is considered essential for re-organization of the cytoskeleton, such as the actin-ring and ruffled boarder to seal the resorbing area and to secret protons, respectively, to resorb bone; the fusion process is thus critical for osteoclast function. Various molecules, such as E-cadherin and macrophage fusion receptor (MFR), have been identified as regulators of osteoclast or macrophage cell-cell fusion. Laboratory production of osteoclasts used to be performed in a co-culture of osteoclast progenitors with osteoblastic cells, but recent advances in the identification of nuclear factor of kappa B ligand (RANKL) enabled the isolation of osteoclast-specific molecules involving osteoclast cell-cell fusion and differentiation regulators from purified osteoclast mRNA, since osteoclasts can be formed without osteoblasts. The essential cell-cell fusion regulator, dendritic cell-specific transmembrane protein (DC-STAMP), was isolated by a cDNA subtractive screen between mononuclear macrophages and RANKL-induced multinuclear osteoclasts. The cell-cell fusion of osteoclasts and foreign body giant cells (FBGCs) was completely abrogated in DC-STAMP-deficient mice in vivo and in vitro. Bone resorbing activity was significantly reduced but was still detected in DC-STAMP-deficient osteoclasts. DC-STAMP expression is positively regulated by two transcriptional factors: nuclear factor of activated T cells 1 (NFATc1) and c-Fos, both of which are essential for osteoclast differentiation. Furthermore, a novel osteoclastogenesis-regulating pathway involving two transcriptional repressors [B cell lymphoma 6 (Bcl6) and B lymphocyte-induced maturation protein 1 (Blimp1)] under RANKL stimulation has been discovered. The expression of osteoclastic genes such as DC-STAMP, NFATc1, and Cathepsin K, as well as osteoclast differentiation, was inhibited by Bcl6. Bcl6-deficient mice showed enhanced osteoclastogenesis and reduced bone mass, whereas osteoclast-specific Blimp1-conditional knockout mice showed elevated Bcl6 expression, osteoclastic gene expression, and osteoclast differentiation and increased bone mass. In this review, recent advances in our understanding of the regulators of osteoclast differentiation and cell-cell fusion are discussed.

Tacrolimus treatment increases bone formation in patients with rheumatoid arthritis

Tacrolimus is a calcineurin inhibitor, and it is used for the treatment of rheumatoid arthritis (RA). It works by inhibiting nuclear factor of activated T cells and inducting immunosuppression. This study aims to evaluate the influence of tacrolimus on the bone metabolism of patients with RA. Twenty-eight RA patients in three centers received tacrolimus 3 mg once daily for 24 weeks. Blood samples for evaluating bone metabolism and cytokines were collected at Weeks 0 and 24. We measured the serum C-telopeptide of type I collagen (sCTx-I), osteocalcin and inflammatory cytokines. We analyzed the data using the Kruskal-Wallis test and Spearman's correlation. IL-2 and IL-6 were significantly decreased after the administration of tacrolimus (p = 0.027 and p = 0.024). There was no significant difference in the serum level of sCTx-I before and after treatment. The level of serum osteocalcin at Week 24 was significantly increased compared to the level at Week 0 (p = 0.002). The increase of osteocalcin was correlated with the reductions of IL-2 and IFN-γ (r = 0.405, p = 0.033 and r = 0.380, p = 0.046, respectively). Tacrolimus treatment increased bone formation markers in RA patients. This suggests that tacrolimus may play a role to inhibit bone erosion by increasing bone formation as well as improving the clinical symptoms of RA.

Physiological notch signaling maintains bone homeostasis via RBPjk and Hey upstream of NFATc1

Notch signaling between neighboring cells controls many cell fate decisions in metazoans both during embryogenesis and in postnatal life. Previously, we uncovered a critical role for physiological Notch signaling in suppressing osteoblast differentiation in vivo. However, the contribution of individual Notch receptors and the downstream signaling mechanism have not been elucidated. Here we report that removal of Notch2, but not Notch1, from the embryonic limb mesenchyme markedly increased trabecular bone mass in adolescent mice. Deletion of the transcription factor RBPjk, a mediator of all canonical Notch signaling, in the mesenchymal progenitors but not the more mature osteoblast-lineage cells, caused a dramatic high-bone-mass phenotype characterized by increased osteoblast numbers, diminished bone marrow mesenchymal progenitor pool, and rapid age-dependent bone loss. Moreover, mice deficient in Hey1 and HeyL, two target genes of Notch-RBPjk signaling, exhibited high bone mass. Interestingly, Hey1 bound to and suppressed the NFATc1 promoter, and RBPjk deletion increased NFATc1 expression in bone. Finally, pharmacological inhibition of NFAT alleviated the high-bone-mass phenotype caused by RBPjk deletion. Thus, Notch-RBPjk signaling functions in part through Hey1-mediated inhibition of NFATc1 to suppress osteoblastogenesis, contributing to bone homeostasis in vivo.

Osteoporosis is a disease caused by disruption of the balance between bone formation and resorption resulting in a net loss of bone mass. Although anti-resorptive agents are the current mainstay of osteoporosis therapy, novel strategies to promote bone formation are critically needed for more effective prevention and treatment of the disease. Notch signaling, an evolutionally conserved mechanism among multi-cellular organisms, was recently shown to control bone formation and therefore represents a potential target pathway for novel bone-promoting therapeutics. In this study we elucidate the intracellular signaling mechanism through which Notch controls bone formation, providing a molecular framework that may guide future drug development.

P2X₇-mediated calcium influx triggers a sustained, PI3K-dependent increase in metabolic acid production by osteoblast-like cells

The P2X₇ receptor is an ATP-gated cation channel expressed by a number of cell types, including osteoblasts. Genetically modified mice with loss of P2X₇ function exhibit altered bone formation. Moreover, activation of P2X₇ in vitro stimulates osteoblast differentiation and matrix mineralization, although the underlying mechanisms remain unclear. Because osteogenesis is associated with enhanced cellular metabolism, our goal was to characterize the effects of nucleotides on metabolic acid production (proton efflux) by osteoblasts. The P2X₇ agonist 2',3'-O-(4-benzoylbenzoyl)ATP (BzATP; 300 μM) induced dynamic membrane blebbing in MC3T3-E1 osteoblast-like cells (consistent with activation of P2X₇ receptors) but did not induce cell death. Using a Cytosensor microphysiometer, we found that 9-min exposure to BzATP (300 μM) caused a dramatic increase in proton efflux from MC3T3-E1 cells (∼2-fold), which was sustained for at least 1 h. In contrast, ATP or UTP (100 μM), which activate P2 receptors other than P2X₇, failed to elicit a sustained increase in proton efflux. Specific P2X₇ receptor antagonists A 438079 and A 740003 inhibited the sustained phase of the BzATP-induced response. Extracellular Ca²⁺ was required during P2X₇ receptor stimulation for initiation of sustained proton efflux, and removal of extracellular glucose within the sustained phase abolished the elevation elicited by BzATP. In addition, inhibition of phosphatidylinositol 3-kinase blocked the maintenance but not initiation of the sustained phase. Taken together, we conclude that brief activation of P2X₇ receptors on osteoblast-like cells triggers a dramatic, Ca²⁺-dependent stimulation of metabolic acid production. This increase in proton efflux is sustained and dependent on glucose and phosphatidylinositol 3-kinase activity.

Inflammatory bone loss: pathogenesis and therapeutic intervention

Bone is a tissue undergoing continuous building and degradation. This remodelling is a tightly regulated process that can be disturbed by many factors, particularly hormonal changes. Chronic inflammation can also perturb bone metabolism and promote increased bone loss. Inflammatory diseases can arise all over the body, including in the musculoskeletal system (for example, rheumatoid arthritis), the intestine (for example, inflammatory bowel disease), the oral cavity (for example, periodontitis) and the lung (for example, cystic fibrosis). Wherever inflammatory diseases occur, systemic effects on bone will ensue, as well as increased fracture risk. Here, we discuss the cellular and signalling pathways underlying, and strategies for therapeutically interfering with, the inflammatory loss of bone.

Interactions between extracellular signal-regulated kinase 1/2 and p38 MAP kinase pathways in the control of RUNX2 phosphorylation and transcriptional activity

RUNX2, a key transcription factor for osteoblast differentiation, is regulated by ERK1/2 and p38 MAP kinase-mediated phosphorylation. However, the specific contribution of each kinase to RUNX2-dependent transcription is not known. Here we investigate ERK and p38 regulation of RUNX2 using a unique P-RUNX2-specific antibody. Both MAP kinases stimulated RUNX2 Ser319 phosphorylation and transcriptional activity. However, a clear preference for ERK1 versus p38α/β was found when the ability of these MAPKs to phosphorylate and activate RUNX2 was compared. Similarly, ERK1 preferentially bound to a consensus MAPK binding site on RUNX2 that was essential for the activity of either kinase. To assess the relative contribution of ERK1/2 and p38 to osteoblast gene expression, MC3T3-E1 preosteoblast cells were grown in control or ascorbic acid (AA)-containing medium ± BMP2/7. AA-induced gene expression, which requires collagen matrix synthesis, was associated with parallel increases in P-ERK and RUNX2-S319-P in the absence of any changes in P-p38. This response was blocked by ERK, but not p38, inhibition. Significantly, in the presence of AA, BMP2/7 synergistically stimulated RUNX2 S319 phosphorylation and transcriptional activity without affecting total RUNX2 and this response was totally dependent on ERK/MAPK activity. In contrast, although p38 inhibition partially blocked BMP-dependent transcription, it did not affect RUNX2 S319 phosphorylation, suggesting the involvement of other phosphorylation sites and/or transcription factors in this response. Based on this work, we conclude that extracellular matrix and BMP regulation of RUNX2 phosphorylation and transcriptional activity in osteoblasts is predominantly mediated by ERK rather than p38 MAPKs.

Effect of cyclosporine-A on orthodontic tooth movement in rats

OBJECTIVE

The objective of this study is to examine the effect of cyclosporine-A (CsA) on the rate of orthodontic tooth movement in rats.

SETTING AND SAMPLE POPULATION

This is a randomized controlled trial with a split-mouth design in Sprague-Dawley rats.

MATERIAL AND METHODS

Eighteen rats, divided at random in two groups, were fed with 8 mg/kg CsA (experiment) or mineral oil (control) daily after initial healing of bilateral maxillary second molar removal. All rats received orthodontic coil springs (10 cN) secured to the maxillary incisors and first molars at the rights side, while no springs were placed at the left. Distances between first and third molars were measured on days 0, 3, 6, and 12. After sacrificing on day 12, the alveolar ridges of the maxillae were sectioned and blood samples were collected for serum tartrate-resistant acid phosphatase (TRAP)-5b level detection and for histology, respectively.

RESULTS

Significantly larger changes in intermolar distances were found after orthodontic force application in the CsA group at days 3 and 12 when compared with the control group. The inter-radicular dental alveolus of CSA-fed rats was osteopenic. Significantly increased TRAP-5b serum level was noted in the CsA group when compared with the control group.

CONCLUSIONS

We suggest that CsA enhanced the rate of orthodontic tooth movement. The osteopenia and the increased osteoclastic activity could be the underlying factors.

The Involvement of TRP Channels in Bone Homeostasis

Calcium and bone homeostasis are intimately related. On the one hand, bone relies on a sufficient supply of calcium to maintain its structural and mechanical properties and thus largely depends on calcium absorption in the intestine and calcium reabsorption in the kidney. On the other hand, bone serves as a calcium reserve from which calcium is mobilized to maintain normal calcium levels in blood. A negative external calcium balance will therefore at all times impair skeletal integrity. In addition to the external calcium balance, skeletal homeostasis also depends on the proper differentiation and functioning of bone cells, which relies for a large part on intracellular Ca(2+) signaling. Members of the transient receptor potential (TRP) family of ion channels affect skeletal homeostasis by mediating processes involved in the extracellular as well as intracellular Ca(2+) balance, including intestinal calcium absorption (TRPV6), renal calcium reabsorption (TRPV5), and differentiation of osteoclasts (TRPV1, TRPV2, TRPV4, TRPV5), chondrocytes (TRPV4), and possibly osteoblasts (TRPV1). In this review, we will give a brief overview of the systemic calcium homeostasis and the intracellular Ca(2+) signaling in bone cells with special focus on the TRP channels involved in these processes.

Swine atrophic rhinitis caused by pasteurella multocida toxin and bordetella dermonecrotic toxin

Atrophic rhinitis is a widespread and economically important swine disease caused by Pasteurella multocida and Bordetella bronchiseptica. The disease is characterized by atrophy of the nasal turbinate bones, which results in a shortened and deformed snout in severe cases. P. multocida toxin and B. bronchiseptica dermonecrotic toxin have been considered to independently or cooperatively disturb the osteogenesis of the turbinate bone by inhibiting osteoblastic differentiation and/or stimulating bone resorption by osteoclasts. Recently, the intracellular targets and molecular actions of both toxins have been clarified, enabling speculation on the intracellular signals leading to the inhibition of osteogenesis.

Orai1-mediated calcium entry plays a critical role in osteoclast differentiation and function by regulating activation of the transcription factor NFATc1

Bone diseases such as postmenopausal osteoporosis are primarily caused by excessive formation and activity of osteoclasts (OCLs). Receptor activator of nuclear factor-κB ligand (RANKL) is a key initiating cytokine for OCL differentiation and function. RANKL induces calcium (Ca(2+)) oscillations, resulting in selective and robust induction of nuclear factor of activated T cells c1 (NFATc1), a Ca(2+)-responsive transcription factor that drives osteoclastogenesis. Store-operated Ca(2+) entry (SOCE) is a major Ca(2+) influx pathway in most nonexcitable cell types and is activated by any stimulus that depletes Ca(2+) stores in the endoplasmic reticulum. Although the role of Orai1, a SOCE channel in the plasma membrane, in maintaining Ca(2+) oscillations and transactivation of NFAT in other cell types is well known, its contribution to osteoclastogenesis remains unclear. We show here that silencing of the Orai1 gene with viral delivery of shRNA reduces SOCE and inhibits RANKL-induced osteoclastogenesis of RAW264.7 cells, a murine monocyte/macrophage cell line, by suppressing the induction of NFATc1. This was accompanied by defective induction of OCL-specific genes, such as tartrate-resistant acid phosphatase and immunoreceptor OCL-associated receptor, which are known to be direct transcriptional targets of NFATc1 during osteoclastogenesis. In addition, maturation of OCLs was abrogated by defective cell fusion of pre-OCLs depleted of Orai1, consistent with defective RANKL-mediated induction of d2 isoform of vacuolar ATPase V(o) domain that is involved in cell fusion of pre-OCLs. We found that the functional bone resorbing capacity was severely impaired in OCLs depleted of Orai1, potentially related to the observed decrease in the induction of cathepsin K, a major bone matrix degrading protease. Our results indicate that Orai1 plays a critical role in the differentiation and function of OCLs, suggesting that Orai1 might be a potential therapeutic target for the treatment or prevention of bone loss caused by OCLs.

Calcineurin/nuclear factor of activated T cells (NFAT) signaling in cobalt-chromium-molybdenum (CoCrMo) particles-induced tumor necrosis factor-α (TNFα) secretion in MLO-Y4 osteocytes

Aseptic loosening is the devastating long term complication of total hip arthroplasty and orthopedic implant debris has been shown to trigger an intense inflammatory reaction leading to resorption of the bone matrix. Inflammatory cytokines, such as tumor necrosis factor-α (TNFα), have been implicated in this process and osteocytes may play a role in its production. We previously demonstrated that cobalt-chromium-molybdenum (CoCrMo) particles upregulate TNFα production by MLO-Y4 osteocytes in vitro, but the underlying mechanism has not been elucidated. Based on previous studies by others, we hypothesized that the calcineurin-nuclear factor of activated T cells (NFAT) pathway mediates CoCrMo particle-induced TNFα production in MLO-Y4 osteocytes. MLO-Y4 osteocytes exposed to CoCrMo particle treatment resulted in a rapid and significant increase in calcineurin activity. We also demonstrate that CoCrMo particle-induced upregulation of TNFα is reduced to control levels with calcineurin-NFAT inhibitors and this was also confirmed at mRNA level. Moreover, we demonstrate the localization of NFATs in MLO-Y4 osteocytes and that NFAT1 and 2 translocate to the nucleus upon CoCrMo particle treatment. Our results suggest that calcineurin-NFAT signaling is involved in TNFα production by MLO-Y4 osteocytes after CoCrMo particle treatment.

Midterm clinico-radiologic findings of an open label observation study of add-on tacrolimus with biologics or non-biologic DMARDs

Tacrolimus (TAC) suppresses immune-inflammation by an intermediary inhibition of calcineurin activation in the treatment of rheumatoid arthritis (RA). Various combination therapies for RA have been reported to be superior to monotherapies. The aim was therefore to study add-on TAC in a combination with biologics (BIO) and/or non-BIO disease-modifying anti-rheumatic drugs (DMARDs) in treatment-resistant patients. In eight RA patients, TAC was added on to BIO (TAC/BIO group) and in forty-one to non-BIO DMARDs (TAC/non-BIO group). The mean C-reactive protein (CRP) decreased from 33 mg/l at the baseline to 16 mg/l at first year in the TAC/BIO group (P < 0.05), from 41 to 14 mg/l in the TAC/non-BIO group (P < 0.05); the mean DAS28-CRP (28 joint count) disease activity score decreased from 5.3 to 4.4 in the TAC/BIO group (P < 0.05) and from 5.0 to 3.9 in the TAC/non-BIO group (P < 0.05). The median of Δ modified total Sharp score decreased from 43 during the year preceding the baseline to 3 during the first year of the follow-up in the TAC/BIO group (P < 0.05) and from 22 to 0 during the second year in the TAC/non-BIO group (P < 0.05). Twenty-six adverse events occurred in this study in 26 patients (53% in all); however, the only severe adverse event was one case of an atypical mycobacterial disease (2%). The combination therapy of TAC with BIO or non-BIO DMARDs represents an effective and relatively safe mode of therapy in treatment-resistant RA.

The tumor necrosis factor type 2 receptor plays a protective role in tumor necrosis factor-α-induced bone resorption lacunae on mouse calvariae

Tumor necrosis factor (TNF)-α exerts its biological function via TNF type 1 and type 2 receptors (TNFR1 and TNFR2). We have previously reported that bone resorption induced by lipopolysaccharide (LPS) in TNFR2-deficient mice is accelerated compared to that in wild-type (WT) mice. Although these results suggested that TNFR2 might have a protective role in bone resorption, we could not exclude the possibility that TNFR2 has no role in bone resorption. To clarify the role of TNFR2, we developed a TNF-α-induced bone resorption model using cholesterol-bearing pullulan nanogel as a TNF-α carrier to minimize the influence of inflammatory cytokines other than TNF-α. Injections of human TNF-α (hTNF), an agonist of mouse TNFR1, stimulated bone resorption lacunae on the calvariae in WT mice, but mouse TNF-α (mTNF), an agonist of both mouse TNFR1 and TNFR2, could not. To eliminate the possibility that the TNFR1 agonistic effects of hTNF were stronger than those of mTNF, we used the same model in TNFR2-deficient mice. Injection of mTNF resulted in clear bone resorption lacunae to the same extent observed after using hTNF in the TNFR2-deficient mice. Histomorphometric analysis of osteoclast number supported the observed changes in bone resorption lacunae. These data suggest that TNFR2 has a protective role in TNF-α-induced bone resorption.

Retinoic acid inhibits NFATc1 expression and osteoclast differentiation

Ingestion of excess vitamin A appears to correlate with an increased fracture risk, an outcome that is likely mediated by retinoic acids (RAs); these are vitamin A metabolites that have dramatic effects on skeletal development. We studied the impacts of RA and isoform-specific RA receptor (RAR) agonists (α, β, and γ) on osteoclast formation (osteoclastogenesis) in two model systems: RAW264.7 cells and murine bone marrow-derived monocytes. The pan-RAR agonists, all-trans and 9-cis RA, inhibited receptor activator of nuclear factor kappa B ligand (RANKL)-mediated osteoclast differentiation in a concentration-dependent manner. Isoform-specific RAR agonists (α, β, and γ) also inhibited osteoclastogenesis, with the RARα agonist producing the most consistent reductions in both osteoclast number and size and total area covered. Inhibition of osteoclastogenesis correlated with reductions in expression, DNA binding, and nuclear abundance of nuclear factor of activated T cells c1 (NFATc1), a transcription factor critical for osteoclastogenesis. The upregulation of three NFATc1-responsive genes, cathepsin K, dendritic cell-specific transmembrane protein and osteoclast-associated receptor were similarly reduced following RA or RAR agonist exposure. These results suggest that RA blocks in vitro RANKL-mediated osteoclastogenesis by decreasing NFATc1 function.

The skeleton: a multi-functional complex organ: new insights into osteoblasts and their role in bone formation: the central role of PI3Kinase

Studies on bone development, formation and turnover have grown exponentially over the last decade in part because of the utility of genetic models. One area that has received considerable attention has been the phosphatidylinositol 3-kinase (PI3K) signaling pathway, which has emerged as a major survival network for osteoblasts. Genetic engineering has enabled investigators to study downstream effectors of PI3K by directly overexpressing activated forms of AKT in cells of the skeletal lineage or deleting Pten that leads to a constitutively active AKT. The results from these studies have provided novel insights into bone development and remodeling, critical processes in the lifelong maintenance of skeletal health. This paper reviews those data in relation to recent advances in osteoblast biology and their potential relevance to chronic disorders of the skeleton and their treatment.

Suppression of bone formation by osteoclastic expression of semaphorin 4D

Most of the currently available drugs for osteoporosis inhibit osteoclastic bone resorption; only a few drugs promote osteoblastic bone formation. It is thus becoming increasingly necessary to identify the factors that regulate bone formation. We found that osteoclasts express semaphorin 4D (Sema4D), previously shown to be an axon guidance molecule, which potently inhibits bone formation. The binding of Sema4D to its receptor Plexin-B1 on osteoblasts resulted in the activation of the small GTPase RhoA, which inhibits bone formation by suppressing insulin-like growth factor-1 (IGF-1) signaling and by modulating osteoblast motility. Sema4d-/- mice, Plxnb1-/- mice and mice expressing a dominant-negative RhoA specifically in osteoblasts showed an osteosclerotic phenotype due to augmented bone formation. Notably, Sema4D-specific antibody treatment markedly prevented bone loss in a model of postmenopausal osteoporosis. Thus, Sema4D has emerged as a new therapeutic target for the discovery and development of bone-increasing drugs.

The regulation of valvular and vascular sclerosis by osteogenic morphogens

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

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

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

The transcriptional activity of osterix requires the recruitment of Sp1 to the osteocalcin proximal promoter

The transcription factor osterix (Osx/Sp7) is required for osteogenic differentiation and bone formation in vivo. While Osx can act at canonical Sp1 DNA-binding sites and/or interact with NFATc1 to cooperatively regulate transcription in some osteoblast promoters, little is known about the molecular details by which Osx regulates osteocalcin (OCN) transcription. We previously identified in the OCN proximal promoter a minimal C/T-rich motif, termed OCN-CxRE (connexin-response element) that binds Sp1 and Sp3 in a gap junction-dependent manner. In the present study, we hypothesized that Osx could act via this non-canonical Sp1/Sp3-binding element to regulate OCN transcription. OCN promoter luciferase reporter assays show that Osx alone is an insufficient activator that requires Sp1, but not Sp3, to synergistically stimulate OCN promoter activity. Moreover, promoter deletion analyses demonstrate that both the Sp1/Sp3-binding OCN-CxRE (-70 to -57) and the -92 to -87 region of the OCN proximal promoter are critical for Osx/Sp1 synergistic activities. Our data show that Sp1 influences Osx activity by enhancing Osx occupancy on the OCN promoter, perhaps via physical interactions between the two transcription factors. Finally, alteration of the expression of the gap junction protein connexin43 modulates the recruitment of both Sp1 and Osx to the OCN promoter. In total, our data are strongly in support of Sp1 as an essential transcription factor required for Osx recruitment and transactivation of the OCN promoter. Further, these data lend insight into a mechanism by which alteration of connexin43 impacts osteogenesis in vitro and in vivo.

Type XII collagen regulates osteoblast polarity and communication during bone formation

Type XII collagen–null mice have fragile bones with disorganized collagen fiber arrangement, decreased bone matrix formation, and delayed osteoblast differentiation.

Differentiated osteoblasts are polarized in regions of bone deposition, demonstrate extensive cell interaction and communication, and are responsible for bone formation and quality. Type XII collagen is a fibril-associated collagen with interrupted triple helices and has been implicated in the osteoblast response to mechanical forces. Type XII collagen is expressed by osteoblasts and localizes to areas of bone formation. A transgenic mouse null for type XII collagen exhibits skeletal abnormalities including shorter, more slender long bones with decreased mechanical strength as well as altered vertebrae structure compared with wild-type mice. Col12a^−/− osteoblasts have decreased bone matrix deposition with delayed maturation indicated by decreased bone matrix protein expression. Compared with controls, Col12a^−/− osteoblasts are disorganized and less polarized with disrupted cell–cell interactions, decreased connexin43 expression, and impaired gap junction function. The data demonstrate important regulatory roles for type XII collagen in osteoblast differentiation and bone matrix formation.

NELL-1, an osteoinductive factor, is a direct transcriptional target of Osterix

NELL-1 is a novel secreted protein associated with premature fusion of cranial sutures in craniosynostosis that has been found to promote osteoblast cell differentiation and mineralization. Our previous study showed that Runx2, the key transcription factor in osteoblast differentiation, transactivates the NELL-1 promoter. In this study, we evaluated the regulatory involvement and mechanisms of Osterix, an essential transcription factor of osteoblasts, in NELL-1 gene expression and function. Promoter analysis showed a cluster of potential Sp1 sites (Sp1/Osterix binding sites) within approximately 70 bp (from -71 to -142) of the 5' flanking region of the human NELL-1 transcriptional start site. Luciferase activity in our NELL-1 promoter reporter systems was significantly decreased in Saos-2 cells when Osterix was overexpressed. Mutagenesis study demonstrated that this suppression is mediated by the Sp1 sites. The binding specificity of Osterix to these Sp1 sites was confirmed in Saos-2 cells and primary human osteoblasts by EMSA in vitro and ChIP assay in vivo. ChIP assay also showed that Osterix downregulated NELL-1 by affecting binding of RNA polymerase II to the NELL-1 promoter, but not by competing with Runx2 binding to the OSE2 sites. Moreover, NELL-1 mRNA levels were significantly decreased when Osterix was overexpressed in Saos-2, U2OS, Hela and Glioma cells. Correspondingly, knockdown of Osterix increased NELL-1 transcription and osteoblastic differentiation in both Saos-2 cells and primary human osteoblasts. These results suggest that Osterix is a direct transcriptional regulator with repressive effect on NELL-1 gene expression, contributing to a delicate balance of regulatory effects on NELL-1 transcription with Runx2, and may play a crucial role in osteoblast differentiation and mineralization. These findings also extend our understanding of the molecular mechanism of Runx2, Osterix, and NELL-1 and demonstrate their crosstalk during osteogenesis.

Antibody-mediated osseous regeneration: a novel strategy for bioengineering bone by immobilized anti-bone morphogenetic protein-2 antibodies

Bone regeneration often requires harvesting of autologous bone with significant potential morbidity and cost. Recombinant human bone morphogenetic protein (rhBMP)-2 has been approved by the U.S. Food and Drug Administration for specific regenerative indications. However, administration of exogenous growth factors has many drawbacks. The objective of the present proof-of-concept study was to determine whether immobilized anti-BMP-2 antibodies (Abs) could capture endogenous BMP-2 in local sites to mediate osteogenesis, a strategy we refer to as antibody-mediated osseous regeneration (AMOR). We have generated a murine anti-BMP-2 monoclonal antibody library, which was tested along with commercially available Abs in vitro and in vivo for their ability to mediate AMOR. In vitro studies demonstrated that only some anti-BMP-2 Abs tested formed immune complexes with BMP-2, which can bind to BMP cellular receptor, whereas other BMP-2/anti-BMP-2 complexes failed to bind. To investigate whether anti-BMP-2 Abs were able to mediate AMOR in vivo, anti-BMP-2 Abs were immobilized on absorbable collagen sponge (ACS) and surgically placed in rat calvarial defects. Microcomputed tomography analysis of live animals at 2, 4, and 6 weeks demonstrated that some anti-BMP-2 Abs immobilized on ACS mediated significant bone regeneration, whereas other clones did not mediate any bone regeneration. In situ BMP-2 and osteocalcin expression was investigated by immunohistochemistry. Results demonstrated higher BMP-2 and osteocalcin expression in sites with increased bone regeneration. Results provide first evidence for the ability of anti-BMP2 Abs to form an immune complex with endogenous BMP-2 and mediate bone regeneration in vivo, suggesting a promising therapeutic method for tissue engineering.

High extracellular calcium-induced NFATc3 regulates the expression of receptor activator of NF-κB ligand in osteoblasts

Nuclear factor of activated T cell (NFAT) is a key transcription factor for receptor activator of NF-κB ligand (RANKL)-induced osteoclast differentiation. However, it is unclear whether NFAT plays a role in the expression of RANKL in osteoblasts. High extracellular calcium ([Ca(2+)](o)) increases intracellular calcium, enhances RANKL expression in osteoblasts/stromal cells, and induces osteoclastogenesis in a coculture of osteoblasts and hematopoietic bone marrow cells. Because intracellular calcium signaling activates the calcineurin/NFAT pathway, we examined the role of NFAT activation on high [Ca(2+)](o)-induced RANKL expression in MC3T3-E1 subclone 4 (MC4) cells. Among the family of NFAT transcription factors, expression of NFATc1 and NFATc3, but not NFATc2, NFATc4 or NFAT5, was observed in MC4 cells. High [Ca(2+)](o) increased the expression levels of NFATc1, NFATc3 and RANKL. Cyclosporin A and FK506, inhibitors of calcineurin phosphatase, blocked high [Ca(2+)](o)-induced expression of NFAT and RANKL. Knockdown of NFATc1 and NFATc3 by siRNA prevented high [Ca(2+)](o)-induced RANKL expression, whereas overexpression of NFATc1 and NFATc3 induced RANKL expression. Furthermore, overexpressed NFATc1 upregulated NFATc3 expression, but NFATc1 knockdown decreased NFATc3 expression. Chromatin immunoprecipitation and reporter assay results showed that NFATc3, but not NFATc1, directly binds to the RANKL promoter and stimulates RANKL expression. In summary, these results demonstrate that high [Ca(2+)](o) increases expression of RANKL via activation of the calcineurin/NFAT pathway in osteoblasts. In addition, high [Ca(2+)](o) induces the activation and expression of NFATc1; NFATc3 expression and activity are subsequently increased; and NFATc3 directly binds to the RANKL promoter to increase its expression.

Osteomyelosclerosis, anemia and extramedullary hematopoiesis in mice lacking the transcription factor NFATc2

BACKGROUND

Nuclear factors of activated T cells (NFAT) are transcription factors that are central to cytokine production in activated T cells and regulate the development and differentiation of various tissues. NFATc2 is expressed in hematopoietic stem cells and regulated during myeloid commitment in a lineage-specific manner. The biological role of NFATc2 in hematopoiesis is, however, unclear.

DESIGN AND METHODS

In the present study, we analyzed steady-state hematopoiesis in young (<3 months) and old (>12 months) mice lacking NFATc2. Complete blood counts were performed in the peripheral blood, bone marrow and spleen. Using cytological and histological analyses, the blood cell differential was determined. Colony-formation assays were used to determine the differentiation potential of hematopoietic cells. Bone cell cultures were derived from the bone marrow, and bone remodeling markers were determined in the serum.

RESULTS

NFATc2(-/-) mice older than 12 months were anemic and thrombocytopenic. The bone marrows of these mice showed a markedly reduced number of hematopoietic cells, of which megakaryocytic and erythroid lineages were most affected. While the number of hematopoietic progenitor cells in NFATc2-deficent bone marrow was reduced, the myeloid differentiation potential of these cells remained intact. Aged NFATc2(-/-) mice showed ossification of their bone marrow space and developed extramedullary hematopoiesis in the spleen. Ex vivo differentiation assays revealed an intrinsic defect of NFATc2-deficient stromal cells, in which NFATc2(-/-) osteoblasts differentiated more efficiently than wild-type cells, whereas osteoclast differentiation was impaired.

CONCLUSIONS

Our data suggest that NFATc2 may play a role in the maintenance of steady-state hematopoiesis and bone remodeling in adult organisms.

Osteoimmunology - Unleashing the concepts

Osteoimmunology is an emerging field of research dedicated to the relationship between the immune processes and the bone metabolism of various inflammatory bone diseases. The regulatory mechanisms governing the osteoclast and osteoblast are critical for understanding the health and disease of the skeletal system. These interactions are either by cell to cell contact or by the secretion of immune regulatory mediators like cytokines and chemokines by immune cells that are governed by the RANKL (TRANCE)-RANK- OPG axis. TRANCE-RANK signaling has served as a cornerstone of osteoimmunology research. There is increased recognition of the importance of the inflammatory and immune responses in the pathogenesis of periodontal disease. Thus, this field has provided a framework for studying the mechanisms underlying periodontal destruction. As bone homeostasis is mainly regulated by both the immune and endocrine systems, there emerged osteoimmunoendocrinology where adipokines take the lead. This review focuses on the underlying concepts of osteoimmunology, its relation to Periodontics.

Pannexin 3 functions as an ER Ca(2+) channel, hemichannel, and gap junction to promote osteoblast differentiation

Pannexin 3 functions as an essential protein for Ca²⁺ and ATP transport and cell–cell communication during osteoblast differentiation

The pannexin proteins represent a new gap junction family. However, the cellular functions of pannexins remain largely unknown. Here, we demonstrate that pannexin 3 (Panx3) promotes differentiation of osteoblasts and ex vivo growth of metatarsals. Panx3 expression was induced during osteogenic differentiation of C2C12 cells and primary calvarial cells, and suppression of this endogenous expression inhibited differentiation. Panx3 functioned as a unique Ca²⁺ channel in the endoplasmic reticulum (ER), which was activated by purinergic receptor/phosphoinositide 3-kinase (PI3K)/Akt signaling, followed by activation of calmodulin signaling for differentiation. Panx3 also formed hemichannels that allowed release of ATP into the extracellular space and activation of purinergic receptors with the subsequent activation of PI3K–Akt signaling. Panx3 also formed gap junctions and propagated Ca²⁺ waves between cells. Blocking the Panx3 Ca²⁺ channel and gap junction activities inhibited osteoblast differentiation. Thus, Panx3 appears to be a new regulator that promotes osteoblast differentiation by functioning as an ER Ca²⁺ channel and a hemichannel, and by forming gap junctions.

Segmental bone defects: from cellular and molecular pathways to the development of novel biological treatments

Several conditions in clinical orthopaedic practice can lead to the development of a diaphyseal segmental bone defect, which cannot heal without intervention. Segmental bone defects have been traditionally treated with bone grafting and/or distraction osteogenesis, methods that have many advantages, but also major drawbacks, such as limited availability, risk of disease transmission and prolonged treatment. In order to overcome such limitations, biological treatments have been developed based on specific pathways of bone physiology and healing. Bone tissue engineering is a dynamic field of research, combining osteogenic cells, osteoinductive factors, such as bone morphogenetic proteins, and scaffolds with osteoconductive and osteoinductive attributes, to produce constructs that could be used as bone graft substitutes for the treatment of segmental bone defects. Scaffolds are usually made of ceramic or polymeric biomaterials, or combinations of both in composite materials. The purpose of the present review is to discuss in detail the molecular and cellular basis for the development of bone tissue engineering constructs.

Nfatc2 is a primary response gene of Nell-1 regulating chondrogenesis in ATDC5 cells

Nell-1 is a growth factor required for normal skeletal development and expression of extracellular matrix proteins required for bone and cartilage cell differentiation. We identified the transcription factor nuclear factor of activated T cells (Nfatc2) as a primary response gene of Nell-1 through a microarray screen, with validation using real-time polymerase chain reaction (PCR). We investigated the effects of recombinant Nell-1 protein on the chondrogenic cell line ATDC5 and primary mouse chondrocytes. The osteochondral transcription factor Runx2 was investigated as a possible intermediary between Nell-1 and Nfatc2 using adenoviral overexpression of wild-type and dominant-negative Runx2. Nell-1 transiently induced both transcription and translation of Nfatc2, an effect inhibited by transduction of dominant-negative Runx2, suggesting that Runx2 was necessary for Nfatc2 induction. Differentiation assays revealed inhibitory effects of Nell-1 on ATDC5 cells. Although proliferation was unaffected, expression of chondrocyte-specific genes was decreased, and cartilage nodule formation and proteoglycan accumulation were suppressed. siRNA knockdown of Nfatc2 significantly reversed these inhibitory effects. To elucidate the relationship between Nell-1, Runx2, and Nfatc2 in vivo, their presence and distribution were visualized in femurs of wild-type and Nell1-deficient mice at both neonatal and various developmental stages using immunohistochemistry. All three proteins colocalized in the perichondrium of wild-type femurs but stained weakly or were completely absent in Nell1-deficient femurs at neonatal stages. Thus Nfatc2 likely plays an important role in Nell-1-mediated osteochondral differentiation in vitro and in vivo. To our knowledge, this is the first demonstration that Nfatc2 is a primary response gene of Nell-1.

Characterization of the osteoblast-specific transmembrane protein IFITM5 and analysis of IFITM5-deficient mice

Interferon-inducible transmembrane protein 5 (IFITM5) is an osteoblast-specific membrane protein whose expression peaks around the early mineralization stage during the osteoblast maturation process. To investigate IFITM5 function, we first sought to identify which proteins interact with IFITM5. Liquid chromatography mass spectrometry revealed that FK506-binding protein 11 (FKBP11) co-immunoprecipitated with IFITM5. FKBP11 is the only protein it was found to interact with in osteoblasts, while IFITM5 interacts with several proteins in fibroblasts. FKBPs are involved in protein folding and immunosuppressant binding, but we could not be sure that IFITM5 participated in these activities when bound to FKBP11. Thus, we generated Ifitm5-deficient mice and analyzed their skeletal phenotypes. The skeletons, especially the long bones, of homozygous mutants (Ifitm5(-/-)) were smaller than those of heterozygous mutants (Ifitm5(+/-)), although we did not observe any significant differences in bone morphometric parameters. The effect of Ifitm5 deficiency on bone formation was more significant in newborns than in young and adult mice, suggesting that Ifitm5 deficiency might have a greater effect on prenatal bone development. Overall, the effect of Ifitm5 deficiency on bone formation was less than we expected. We hypothesize that this may have resulted from a compensatory mechanism in Ifitm5-deficient mice.

Bone regeneration and stem cells

This invited review covers research areas of central importance for orthopaedic and maxillofacial bone tissue repair, including normal fracture healing and healing problems, biomaterial scaffolds for tissue engineering, mesenchymal and foetal stem cells, effects of sex steroids on mesenchymal stem cells, use of platelet-rich plasma for tissue repair, osteogenesis and its molecular markers. A variety of cells in addition to stem cells, as well as advances in materials science to meet specific requirements for bone and soft tissue regeneration by addition of bioactive molecules, are discussed.

Noncanonical Wnt signaling in vertebrate development, stem cells, and diseases

Wnt signaling regulates many aspects of vertebrate development and adult stem cells. Deregulation of Wnt signaling causes development defect and cancer. The signaling is categorized in two pathways: canonical and noncanonical. Both pathways are initiated by Wnt ligands and Frizzled receptors. Canonical pathway leads to β-catenin:T-cell factor/lymphoid enhancer factor-mediated gene expression, which regulates proliferation and differentiation of cells. Noncanonical Wnt signaling is mediated by intracellular calcium ion and JNK. This signaling leads to NFAT, a key transcriptional factor regulating gene expression. In addition, β-catenin:T-cell factor/lymphoid enhancer factor-mediated gene expression is downregulated by CaMKII-TAK1-NLK. Cellular polarity and motility are the main outcomes of the signaling. During development, noncanonical Wnt signaling is required for tissue formation. Recent studies have shown that atypical cadherin Flamingo contributes to noncanonical Wnt signaling by directing the migration of cells. Also, noncanonical Wnt signaling is required for maintenance of adult stem cells. In the field of cancer research, noncanonical Wnt signaling has been considered a tumor suppressor; however, recent evidence has shown that the signaling also enhances cancer progression in the later stages of disease. In this review, we describe and discuss components of noncanonical Wnt signaling, diseases caused by deregulation of the signaling, regulation of adult stem cells by the signaling, and implications in cancer biology.

Sodium/myo-inositol cotransporter 1 and myo-inositol are essential for osteogenesis and bone formation

myo-Inositol (MI) plays an essential role in several important processes of cell physiology, is involved in the neural system, and provides an effective treatment for some psychiatric disorders. Its role in osteogenesis and bone formation nonetheless is unclear. Sodium/MI cotransporter 1 (SMIT1, the major cotransporter of MI) knockout (SMIT1(-/-)) mice with markedly reduced tissue MI levels were used to characterize the essential roles of MI and SMIT1 in osteogenesis. SMIT1(-/-) embryos had a dramatic delay in prenatal mineralization and died soon after birth owing to respiratory failure, but this could be rescued by maternal MI supplementation. The rescued SMIT1(-/-) mice had shorter limbs, decreased bone density, and abnormal bone architecture in adulthood. Deletion of SMIT1 resulted in retarded postnatal osteoblastic differentiation and bone formation in vivo and in vitro. Continuous MI supplementation partially restored the abnormal bone phenotypes in adult SMIT1(-/-) mice and strengthened bone structure in SMIT1(+/+) mice. Although MI content was much lower in SMIT1(-/-) mesenchymal cells (MSCs), the I(1,4,5)P(3) signaling pathway was excluded as the means by which SMIT1 and MI affected osteogenesis. PCR expression array revealed Fgf4, leptin, Sele, Selp, and Nos2 as novel target genes of SMIT1 and MI. SMIT1 was constitutively expressed in multipotential C3H10T1/2 and preosteoblastic MC3T3-E1 cells and could be upregulated during bone morphogenetic protein 2 (BMP-2)-induced osteogenesis. Collectively, this study demonstrated that deficiency in SMIT1 and MI has a detrimental impact on prenatal skeletal development and postnatal bone remodeling and confirmed their essential roles in osteogenesis, bone formation, and bone mineral density (BMD) determination.

Genetic and transcriptional control of bone formation

An exquisite interplay of developmental cues, transcription factors, and coregulatory and signaling proteins support formation of skeletal elements of the jaw during embryogenesis and dynamic remodeling of alveolar bone in postnatal life. These molecules promote initial condensation of the mesenchyme, commitment of the mesenchymal progenitor to osteogenic lineage cells, and differentiation of committed osteoblasts to mature osteocytes within mineralized bone. Parallel regulatory networks promote formation of the functional osteoclast from mononuclear cells to support continuous bone remodeling within the alveolar bone. With an ever expanding list of new regulatory factors, the complexities of the molecular mechanisms that control gene expression in skeletal cells are being further appreciated. This article examines the multifunctional roles of prominent nuclear proteins, cytokines, hormones, and paracrine factors that control osteogenesis.

Reciprocal regulation of Notch and nuclear factor of activated T-cells (NFAT) c1 transactivation in osteoblasts

Notch are transmembrane receptors involved in the determination of cell fate. Nuclear factor of activated T-cells (NFAT)c are transcription factors that control cell differentiation and function. We tested whether Notch and NFAT signaling pathways interacted in osteoblastic cells. Notch signaling was induced in ST-2 cells using vectors expressing Notch1 intracellular domain (NICD), and in Rosa(Notch) osteoblastic cells by Cre recombinase-mediated excision of a loxP-flanked STOP cassette cloned between the Rosa26 promoter and NICD. NFATc1 was induced in Rosa(Notch) osteoblastic cells by transducing an adenoviral vector expressing constitutively active NFATc1. Notch inhibited NFAT transactivation and NFATc1 transcription. In ST-2 cells, suppression of NFAT transactivation by Notch was reversed by constitutively active cGMP-dependent protein kinase type II. NFATc1 inhibited the transactivation of Notch target genes, and competed for binding to DNA with the Notch interacting protein Epstein-Barr virus latency C promoter binding factor-1, suppressor of hairless, Lag-1 (CSL). Co-immunoprecipitation and confocal microscopy demonstrated that NFATc1 and CSL interacted. Studies on the effects of NICD and NFATc1 on the differentiation and function of osteoblastic cells demonstrated that NICD and NFATc1 inhibited expression of osteoblast gene markers in Rosa(Notch) osteoblasts, but only NICD suppressed the commitment of bone marrow stromal cells to the osteoblastic lineage. In conclusion, NICD and NFATc1 reciprocally inhibit their signaling pathways, and form a regulatory network to control their activity in osteoblasts.

The skeletal subsystem as an integrative physiology paradigm

Homeostatic bone remodeling depends on precise regulation of osteoblast-osteoclast coupling through intricate endocrine, immune, neuronal, and mechanical factors. The osteoblast-osteoclast model of bone physiology with layers of regulatory complexity can be investigated as a component of a local skeletal subsystem or as a part of a complete whole-body system. In this review, we flip the traditional investigative paradigm of scientific experimentation ("bottom-top research") to a "top-bottom" approach using systems biology. We first establish the intricacies of the two-cell model at the molecular signaling level. We then provide, on a systems level, an integrative physiologic approach involving many recognized organ-level subsystems having direct and/or indirect effects on bone remodeling. Lastly, a hypothetical model of bone remodeling based on frequency and amplitude regulatory mechanisms is presented. It is hoped that by providing a thorough model of skeletal homeostasis, future progress can be made in researching and treating skeletal morbidities.

Expression of master regulatory genes controlling skeletal development in benign cartilage and bone forming tumors

Recent progress in skeletal molecular biology has led to the clarification of the transcriptional mechanisms of chondroblastic and osteoblastic lineage differentiation. Three master transcription factors-Sox9, Runx2, and Osterix-were shown to play an essential role in determining the skeletal progenitor cells' fate. The present study evaluates the expression of these factors in 4 types of benign bone tumors-chondromyxoid fibroma, chondroblastoma, osteoid osteoma, and osteoblastoma-using immunohistochemistry and tissue microarrays. Osteoid osteoma and osteoblastoma showed strong nuclear expression of Osterix and Runx2. In contrast, only a few chondroblastomas showed positive nuclear expression of Osterix. Strong nuclear expression of Sox9 was detected in all chondroblastomas, whereas nearly half of the osteoblastomas showed focal weak cytoplasmic expression of Sox9.

Maf promotes osteoblast differentiation in mice by mediating the age-related switch in mesenchymal cell differentiation

Aging leads to the disruption of the homeostatic balance of multiple biological systems. In bone marrow multipotent mesenchymal cells undergo differentiation into various anchorage-dependent cell types, including osteoblasts and adipocytes. With age as well as with treatment of antidiabetic drugs such as thiazolidinediones, mesenchymal cells favor differentiation into adipocytes, resulting in an increased number of adipocytes and a decreased number of osteoblasts, causing osteoporosis. The mechanism behind this differentiation switch is unknown. Here we show an age-related decrease in the expression of Maf in mouse mesenchymal cells, which regulated mesenchymal cell bifurcation into osteoblasts and adipocytes by cooperating with the osteogenic transcription factor Runx2 and inhibiting the expression of the adipogenic transcription factor Pparg. The crucial role of Maf in both osteogenesis and adipogenesis was underscored by in vivo observations of delayed bone formation in perinatal Maf(-/-) mice and an accelerated formation of fatty marrow associated with bone loss in aged Maf(+/-) mice. This study identifies a transcriptional mechanism for an age-related switch in cell fate determination and may provide a molecular basis for novel therapeutic strategies against age-related bone diseases.

Coordination of chondrogenesis and osteogenesis by hypertrophic chondrocytes in endochondral bone development

Mammalian bones have three distinct origins (paraxial mesoderm, lateral plate mesoderm, and neural crest) and undergo two different modes of formation (intramembranous and endochondral). Bones derived from the paraxial mesoderm and lateral plate mesoderm mainly form through the endochondral process. During this process, hypertrophic chondrocytes play a vital role in inducing osteogenesis. So far, a number of published papers have provided evidence that chondrocyte hypertrophy and osteoblast differentiation are controlled by a variety of signaling pathways and factors; however, little is known about their hierarchy (which are upstream? which are most potent?). In this review, we discuss the signaling pathways and transcriptional factors regulating chondrocyte hypertrophy and osteoblast differentiation based on the evidence that has been reported and confirmed by multiple independent groups. We then discuss which factor would provide the most coherent evidence for its role in endochondral ossification.

Connective tissue growth factor (CTGF) transactivates nuclear factor of activated T-cells (NFAT) in cells of the osteoblastic lineage

Connective tissue growth factor (CTGF), a member of the Cyr 61, CTGF, Nov (CCN) family of proteins, regulates multiple cellular functions. Overexpression of CTGF in vivo causes osteopenia, but in vitro CTGF can induce osteoblastogenesis. To investigate mechanisms involved in the effects of CTGF on osteoblastic cell differentiation, we examined whether CTGF modifies the activity of nuclear factor of activated T-cells (NFATc) 1, a transcription factor that cooperates with osterix in the formation of new bone. CTGF increased the transactivation of a transiently transfected reporter construct, where 9 NFAT binding sites direct the expression of luciferase (9xNFAT-Luc) and the activity of the Regulators of calcineurin 1 exon 4 (Rcan1.4) promoter, an NFAT target gene. We postulated that CTGF could modify the phosphorylation of NFAT by regulating glycogen synthase kinase 3beta (GSK3beta). CTGF increased the mRNA levels of Protein kinase cyclic guanosine monophosphate (cGMP) dependent type II (Prkg2), the gene encoding for cGMP dependent protein kinase II (cGKII) which phosphorylates GSK3beta. Accordingly, CTGF induced GSK3beta phosphorylation and decreased the active pool of GSK3beta, a kinase that phosphorylates NFAT and leads to its nuclear export. As a consequence, CTGF favored the nuclear localization of NFATc1. Downregulation of PRKG2 by RNA interference reversed the effect of CTGF on the transactivation of the 9xNFAT reporter construct and the Rcan 1.4 promoter, confirming the role of cGKII in the activation of NFAT by CTGF. In conclusion, CTGF enhances NFAT signaling through the induction of cGKII and the phosphorylation of GSK3beta.

p38 regulates expression of osteoblast-specific genes by phosphorylation of osterix

Osterix, a zinc finger transcription factor, is specifically expressed in osteoblasts and osteocytes of all developing bones. Because no bone formation occurs in Osx-null mice, Osterix is thought to be an essential regulator of osteoblast differentiation. We report that, in several mesenchymal and osteoblastic cell types, BMP-2 induces an increase in expression of the two isoforms of Osterix arising from two alternative promoters. We identified a consensus Sp1 sequence (GGGCGG) as Osterix binding regions in the fibromodulin and the bone sialoprotein promoters in vitro and in vivo. Furthermore, we show that Osterix is a novel substrate for p38 MAPK in vitro and in vivo and that Ser-73 and Ser-77 are the regulatory sites phosphorylated by p38. Our data also demonstrate that Osterix is able to increase recruitment of p300 and Brg1 to the promoters of its target genes fibromodulin and bone sialoprotein in vivo and that it directly associates with these cofactors through protein-protein interactions. Phosphorylation of Osterix at Ser-73/77 increased its ability to recruit p300 and SWI/SNF to either fibromodulin or bone sialoprotein promoters. We therefore propose that Osterix binds to Sp1 sequences on target gene promoters and that its phosphorylation by p38 enhances recruitment of coactivators to form transcriptionally active complexes.

Essential role of nuclear factor of activated T cells (NFAT)-mediated Wnt signaling in osteoblast differentiation induced by strontium ranelate

The antiosteoporotic treatment strontium ranelate (SrRan) was shown to increase bone mass and strength by dissociating bone resorption and bone formation. To identify the molecular mechanisms of action of SrRan on osteoblasts, we investigated its effects on calcineurin-NFAT (nuclear factor of activated T cells) signaling, an important calcium sensitive pathway controlling bone formation. Using murine MC3T3-E1 and primary murine osteoblasts, we demonstrate that SrRan induces NFATc1 nuclear translocation, as shown by immunocytochemical and Western blot analyses. Molecular analysis showed that SrRan increased NFATc1 transactivation in osteoblasts, an effect that was fully abrogated by the calcineurin inhibitors cyclosporin A or FK506, confirming that SrRan activates NFATc1 signaling in osteoblasts. This has functional implications because calcineurin inhibitors blunted the enhanced osteoblast replication and expression of the osteoblast phenotypic markers Runx2, alkaline phosphatase, and type I collagen induced by SrRan. We further found that SrRan increased the expression of Wnt3a and Wnt5a as well as beta-catenin transcriptional activity in osteoblasts, and these effects were abolished by calcineurin inhibitors. The Wnt inhibitors sFRP1 and DKK1 abolished SrRan-induced osteoblast gene expression. Furthermore, blunting the Wnt5a receptor Ryk or RhoA that acts downstream of Ryk abrogated cell proliferation and osteoblast gene expression induced by SrRan. These results indicate that activation of NFATc1 and downstream canonical and non-canonical Wnt signaling pathways mediate SrRan-induced osteoblastic cell replication and differentiation, which provides novel insights into the mechanisms of action of this antiosteoporotic agent in osteoblastogenesis.

The p38 MAPK pathway is essential for skeletogenesis and bone homeostasis in mice

Nearly every extracellular ligand that has been found to play a role in regulating bone biology acts, at least in part, through MAPK pathways. Nevertheless, much remains to be learned about the contribution of MAPKs to osteoblast biology in vivo. Here we report that the p38 MAPK pathway is required for normal skeletogenesis in mice, as mice with deletion of any of the MAPK pathway member-encoding genes MAPK kinase 3 (Mkk3), Mkk6, p38a, or p38b displayed profoundly reduced bone mass secondary to defective osteoblast differentiation. Among the MAPK kinase kinase (MAP3K) family, we identified TGF-beta-activated kinase 1 (TAK1; also known as MAP3K7) as the critical activator upstream of p38 in osteoblasts. Osteoblast-specific deletion of Tak1 resulted in clavicular hypoplasia and delayed fontanelle fusion, a phenotype similar to the cleidocranial dysplasia observed in humans haploinsufficient for the transcription factor runt-related transcription factor 2 (Runx2). Mechanistic analysis revealed that the TAK1-MKK3/6-p38 MAPK axis phosphorylated Runx2, promoting its association with the coactivator CREB-binding protein (CBP), which was required to regulate osteoblast genetic programs. These findings reveal an in vivo function for p38beta and establish that MAPK signaling is essential for bone formation in vivo. These results also suggest that selective p38beta agonists may represent attractive therapeutic agents to prevent bone loss associated with osteoporosis and aging.

Common variants in FLNB/CRTAP, not ARHGEF3 at 3p, are associated with osteoporosis in southern Chinese women

SUMMARY

We performed an association study of five candidate genes within chromosome 3p14-25 in 1,080 Chinese female subjects. Polymorphisms in FLNB/CRTAP are associated with bone mineral density (BMD) in Chinese.

INTRODUCTION

Chromosomal region 3p14-25 has shown strong evidence of linkage to BMD in genome-wide linkage scans. The variants responsible for this linkage signal, nonetheless, remain obscure.

METHODS

Thirty SNPs in five positional and functional candidate genes within 3p14-25 (PPARG, CRTAP, TDGF1, PTHR1, and FLNB) and rs7646054 in the ARHGEF3 gene were genotyped in a case-control cohort of 1,080 Chinese females. Allelic and haplotypic association were tested using logistic regression analysis implemented in PLINK software. Potential transcription factor binding sites were predicted with MatInspector.

RESULTS

Multiple SNPs and haplotypes in FLNB were significantly associated with BMDs, with the strongest association between lumbar spine BMD and rs9828717 (p = 0.005). SNP rs7623768 and the haplotype G-C of rs4076086-rs7623768 in CRTAP were associated with femoral neck BMD (p = 0.009 and p = 0.003, respectively). PTHR1 showed haplotypic associations with lumbar spine and femoral neck BMD (p = 0.02 and p = 0.044, respectively). Nevertheless, the association between rs7646054 in ARHGEF3 and BMD observed in Caucasians was not replicated in our samples. Comparative genomics analysis indicated that rs9828717 is located within a highly conserved region. The minor T allele at rs9828717 may lead to loss of binding site for nuclear factor of activated T cells which binds and triggers the transcriptional program of osteoblasts.

CONCLUSIONS

Our data suggest that variants in FLNB and CRTAP at 3p are involved in BMD regulation in southern Chinese.