NFAT and Osterix cooperatively regulate bone formation

The effects of PPAR-γ inhibition on gene expression and the progression of induced osteogenic differentiation of human mesenchymal stem cells

Mesenchymal stem cells (MSCs) can differentiate into several cell types, such as osteoblasts and adipocytes, both in vitro and in vivo. Although these two differentiation pathways are distinct from each other, cross-communication between cells of the two lineages exists both systemically and peripherally in the tissue. The transcription factor PPAR-γ, the main switch in adipogenic differentiation of MSCs, has previously been described to have a negative effect on osteogenic differentiation. The aim of this study was to investigate the effect of PPAR-γ inhibition on osteogenic differentiation of human MSCs, in vitro. Extracellular matrix analysis and quantification of osteogenic markers, revealed how these cells respond when the adipogenic differentiation pathway is blocked during induction of osteogenic differentiation. The inhibition leads to a significant increase in mineralization of the extracellular matrix, as well as an increased activity or up-regulated gene expression of alkaline phosphatase, the key enzyme involved in matrix mineralization. Furthermore, it was also demonstrated by microarray analysis, that PPAR-γ inhibition during osteogenic induction leads to a significant up-regulation of a number of genes related to both osteogenesis and adipogenesis such as c10orf10, leptin, GDF5 and KLF15. In conclusion, inhibition of PPAR-γ during induction of osteogenesis leads to increased osteogenic differentiation of human MSCs.

The p38α MAPK function in osteoprecursors is required for bone formation and bone homeostasis in adult mice

BACKGROUND

p38 MAPK activity plays an important role in several steps of the osteoblast lineage progression through activation of osteoblast-specific transcription factors and it is also essential for the acquisition of the osteoblast phenotype in early development. Although reports indicate p38 signalling plays a role in early skeletal development, its specific contributions to adult bone remodelling are still to be clarified.

METHODOLOGY/PRINCIPAL FINDINGS

We evaluated osteoblast-specific deletion of p38α to determine its significance in early skeletogenesis, as well as for bone homeostasis in adult skeleton. Early p38α deletion resulted in defective intramembranous and endochondral ossification in both calvaria and long bones. Mutant mice showed reduction of trabecular bone volume in distal femurs, associated with low trabecular thickness. In addition, knockout mice also displayed decreased femoral cortical bone volume and thickness. Deletion of p38α did not affect osteoclast function. Yet it impaired osteoblastogenesis and osteoblast maturation and activity through decreased expression of osteoblast-specific transcription factors and their targets. Furthermore, the inducible Cre system allowed us to control the onset of p38α disruption after birth by removal of doxycycline. Deletion of p38α at three or eight weeks postnatally led to significantly lower trabecular and cortical bone volume after 6 or 12 months.

CONCLUSIONS

Our data demonstrates that, in addition to early skeletogenesis, p38α is essential for osteoblasts to maintain their function in mineralized adult bone, as bone anabolism should be sustained throughout life. Moreover, our data also emphasizes that clinical development of p38 inhibitors should take into account their potential bone effects.

All-optical regulation of gene expression in targeted cells

Controllable gene expression is always a challenge and of great significance to biomedical research and clinical applications. Recently, various approaches based on extra-engineered light-sensitive proteins have been developed to provide optogenetic actuators for gene expression. Complicated biomedical techniques including exogenous genes engineering, transfection, and material delivery are needed. Here we present an all-optical method to regulate gene expression in targeted cells. Intrinsic or exogenous genes can be activated by a Ca(2+)-sensitive transcription factor nuclear factor of activated T cells (NFAT) driven by a short flash of femtosecond-laser irradiation. When applied to mesenchymal stem cells, expression of a differentiation regulator Osterix can be activated by this method to potentially induce differentiation of them. A laser-induced "Ca(2+)-comb" (LiCCo) by multi-time laser exposure is further developed to enhance gene expression efficiency. This noninvasive method hence provides an encouraging advance of gene expression regulation, with promising potential of applying in cell biology and stem-cell science.

The RUNX2 cistrome in osteoblasts: characterization, down-regulation following differentiation, and relationship to gene expression

RUNX2 is a transcription factor that is first expressed in early osteoblast-lineage cells and represents a primary determinant of osteoblastogenesis. While numerous target genes are regulated by RUNX2, little is known of sites on the genome occupied by RUNX2 or of the gene networks that are controlled by these sites. To explore this, we conducted a genome-wide analysis of the RUNX2 cistrome in both pre-osteoblastic MC3T3-E1 cells (POB) and their mature osteoblast progeny (OB), characterized the two cistromes and assessed their relationship to changes in gene expression. We found that although RUNX2 was widely bound to the genome in POB cells, this binding profile was reduced upon differentiation to OBs. Numerous sites were lost upon differentiation, new sites were also gained; many sites remained common to both cell states. Additional features were identified as well including location relative to potential target genes, abundance with respect to single genes, the frequent presence of a consensus TGTGGT RUNX2 binding motif, co-occupancy by C/EBPβ and the presence of a typical epigenetic histone enhancer signature. This signature was changed quantitatively following differentiation. While RUNX2 binding sites were associated extensively with adjacent genes, the distal nature of the majority of these sites prevented assessment of whether they represented direct targets of RUNX2 action. Changes in gene expression, however, revealed an abundance of genes that contained RUNX2 binding sites and were regulated in concert. These studies establish a basis for further analysis of the role of RUNX2 activity and its function during osteoblast lineage maturation.

Nuclear factor of activated T cells regulates neutrophil recruitment, systemic inflammation, and T-cell dysfunction in abdominal sepsis

The signaling mechanisms regulating neutrophil recruitment, systemic inflammation, and T-cell dysfunction in polymicrobial sepsis are not clear. This study explored the potential involvement of the calcium/calcineurin-dependent transcription factor, nuclear factor of activated T cells (NFAT), in abdominal sepsis. Cecal ligation and puncture (CLP) triggered NFAT-dependent transcriptional activity in the lung, spleen, liver, and aorta in NFAT-luciferase reporter mice. Treatment with the NFAT inhibitor A-285222 prior to CLP completely prevented sepsis-induced NFAT activation in all these organs. Inhibition of NFAT activity reduced sepsis-induced formation of CXCL1, CXCL2, and CXCL5 chemokines and edema as well as neutrophil infiltration in the lung. Notably, NFAT inhibition efficiently reduced the CLP-evoked increases in HMBG1, interleukin 6 (IL-6), and CXCL5 levels in plasma. Moreover, administration of A-285222 restored sepsis-induced T-cell dysfunction, as evidenced by markedly decreased apoptosis and restored proliferative capacity of CD4 T cells. Along these lines, treatment with A-285222 restored gamma interferon (IFN-γ) and IL-4 levels in the spleen, which were markedly reduced in septic mice. CLP-induced formation of regulatory T cells (CD4(+) CD25(+) Foxp3(+)) in the spleen was also abolished in A-285222-treated animals. All together, these novel findings suggest that NFAT is a powerful regulator of pathological inflammation and T-cell immune dysfunction in abdominal sepsis. Thus, our data suggest that NFAT signaling might be a useful target to protect against respiratory failure and immunosuppression in patients with sepsis.

Unilateral coronal craniosynostosis and Down syndrome

There is no known correlation between Down syndrome and craniosynostosis. The authors report 2 infants with trisomy 21 and right unilateral coronal craniosynostosis. Both patients were clinically asymptomatic but displayed characteristic craniofacial features associated with each disorder. One patient underwent a bilateral fronto-orbital advancement and the other underwent an endoscopically assisted strip craniectomy with postoperative helmet therapy. Both patients demonstrated good cosmesis at follow-up.

Emerging therapeutic targets for osteoporosis treatment

INTRODUCTION

To date, osteoporosis still remains a major public health burden especially for the aging populations. Over the last few decades treatments for osteoporosis have largely focused on anti-resorptive agents represented by bisphosphonates and estrogen therapy that dominated the market. Unsatisfactory efficacy, non-specificity and long-term safety of current therapies necessitate the need for new targets effectively preventing and treating of osteoporosis.

AREAS COVERED

This review expatiates on the mechanism of osteoporosis occurrence and bone remodeling cycle in detail. New targets of antiresorptive and anabolic agents based on the functions of osteoblasts and osteoclasts as well as associated signaling pathways are outlined.

EXPERT OPINION

Advanced understanding in the fields of bone remodeling, functions of osteoblasts, osteoclasts and osteocytes associated with osteoporosis occurrence offers the emerging bone-resorptive or bone-formative targets. Currently, molecules involving RANK-RANKL-OPG system and Wnt/β-catenin signaling pathway act as the most promising targets.

An update of human mesenchymal stem cell biology and their clinical uses

In the past decade, an increasing urge to develop new and novel methods for the treatment of degenerative diseases where there is currently no effective therapy has lead to the emerging of the cell therapy or cellular therapeutics approach for the management of those conditions where organ functions are restored through transplantation of healthy and functional cells. Stem cells, because of their nature, are currently considered among the most suitable cell types for cell therapy. There are an increasing number of studies that have tested the stromal stem cell functionality both in vitro and in vivo. Consequently, stromal (mesenchymal) stem cells (MSCs) are being introduced into many clinical trials due to their ease of isolation and efficacy in treating a number of disease conditions in animal preclinical disease models. The aim of this review is to revise MSC biology, their potential translation in therapy, and the challenges facing their adaptation in clinical practice.

Key role for the transcriptional factor, osterix, in spine development

BACKGROUND CONTEXT

Osterix (Osx) is an important transcriptional factor for bone formation; however, its role in spine development has not been determined.

PURPOSE

The goal of the present study is to observe the role of Osx in spine development.

STUDY DESIGN/SETTING

Deletion and overexpression of Osx were achieved in Osx knockout and transgenic mice, respectively, to determine the effect of Osx on spine development.

METHODS

With 2.3-kb type I collagen as a promoter, Osx were deleted in mice, and overexpression of Osx was obtained in Osx transgenic mice. Video, X-ray radiology, histology, tartrate-resistant acid phosphatase staining, Safranin O staining, and immunohistochemistry were used to assess the effect of Osx deletion and overexpression on spine development. This study was supported by National Science Foundation of China (81271935) and Foundation of State Key Laboratory of Trauma, Burns and combined injury (SKLZZ SKLZZ201124). No potential conflict of interest to disclose.

RESULTS

Overexpression of Osx did not have an obvious effect on spine development, whereas deletion of Osx led to severe spine deformities that included wedged vertebrae, spinal stenosis, and congenital scoliosis. Also, Osx deactivation resulted in shortened vertebrae and excessive bone volume in the vertebrae. TRAP staining showed that activity of osteoclasts decreased in Osx-null mice, and examination with TdT-mediated dUTP nick end labeling revealed that the apoptosis rate at the growth plate decreased significantly in Osx-null mice. Excessive formation of bone was positive for Safranin O staining.

CONCLUSIONS

Osx plays an important role in spine development, and its deactivation leads to severe spine deformities.

Deregulation of bone forming cells in bone diseases and anabolic effects of strontium-containing agents and biomaterials

Age-related bone loss and osteoporosis are associated with bone remodeling changes that are featured with decreased trabecular and periosteal bone formation relative to bone resorption. Current anticatabolic therapies focusing on the inhibition of bone resorption may not be sufficient in the prevention or reversal of age-related bone deterioration and there is a big need in promoting osteoblastogenesis and bone formation. Enhanced understanding of the network formed by key signaling pathways and molecules regulating bone forming cells in health and diseases has therefore become highly significant. The successful development of agonist/antagonist of the PTH and Wnt signaling pathways are profits of the understanding of these key pathways. As the core component of an approved antiosteoporosis agent, strontium takes its effect on osteoblasts at multilevel through multiple pathways, representing a good example in revealing and exploring anabolic mechanisms. The recognition of strontium effects on bone has led to its expected application in a variety of biomaterial scaffolds used in tissue engineering strategies aiming at bone repairing and regeneration. While summarizing the recent progress in these respects, this review also proposes the new approaches such as systems biology in order to reveal new insights in the pathology of osteoporosis as well as possible discovery of new therapies.

Aortic valve sclerosis in mice deficient in endothelial nitric oxide synthase

Risk factors for fibrocalcific aortic valve disease (FCAVD) are associated with systemic decreases in bioavailability of endothelium-derived nitric oxide (EDNO). In patients with bicuspid aortic valve (BAV), vascular expression of endothelial nitric oxide synthase (eNOS) is decreased, and eNOS(-/-) mice have increased prevalence of BAV. The goal of this study was to test the hypotheses that EDNO attenuates profibrotic actions of valve interstitial cells (VICs) in vitro and that EDNO deficiency accelerates development of FCAVD in vivo. As a result of the study, coculture of VICs with aortic valve endothelial cells (vlvECs) significantly decreased VIC activation, a critical early phase of FCAVD. Inhibition of VIC activation by vlvECs was attenuated by N(G)-nitro-l-arginine methyl ester or indomethacin. Coculture with vlvECs attenuated VIC expression of matrix metalloproteinase-9, which depended on stiffness of the culture matrix. Coculture with vlvECs preferentially inhibited collagen-3, compared with collagen-1, gene expression. BAV occurred in 30% of eNOS(-/-) mice. At age 6 mo, collagen was increased in both bicuspid and trileaflet eNOS(-/-) aortic valves, compared with wild-type valves. At 18 mo, total collagen was similar in eNOS(-/-) and wild-type mice, but collagen-3 was preferentially increased in eNOS(-/-) mice. Calcification and apoptosis were significantly increased in BAV of eNOS(-/-) mice at ages 6 and 18 mo. Remarkably, these histological changes were not accompanied by physiologically significant valve stenosis or regurgitation. In conclusion, coculture with vlvECs inhibits specific profibrotic VIC processes. In vivo, eNOS deficiency produces fibrosis in both trileaflet and BAVs but produces calcification only in BAVs.

Osterix/Sp7 regulates biomineralization of otoliths and bone in medaka (Oryzias latipes)

Osterix/Sp7 is a zinc finger transcription factor and critical regulator of osteoblast differentiation, maturation and activity. Osterix expression has also been described in non-skeletal tissues but functional analyses are lacking. In the present study, we show that in the teleost model medaka, osterix is present as two alternatively spliced transcripts, osx_tv1 and osx_tv2. Knock-down of osx_tv1 and/or osx_tv2 results in mineralization loss in early intramembranous bones while cartilage formation is mostly unaffected. Formation of the parasphenoid, the earliest mineralized bone in the medaka skeleton, is impaired and fails to recover at later stages. Ossification of later bones, such as the operculum and cleithrum, is delayed but recovers during further development. In the axial skeleton, formation of the neural arches and centra is strongly delayed. In vivo analyses using osterix:nlGFP and osteocalcin:GFP transgenic medaka and whole mount in situ hybridization suggest that bone defects observed after knock-down of osterix are caused by a delay of osteoblast maturation and activity. Furthermore, we analyzed expression profile and function of osterix during ear and otolith formation. We show that osterix is expressed in otic placodes at the otic vesicle stage and that its knock-down results in a loss of otoliths. Taken together, we show that osterix is required for bone formation in a teleost fish and that its important regulatory functions are conserved between teleosts and mammals. Furthermore, we provide the first functional evidence for a role of Osterix in a non-skeletal tissue, i.e. the otoliths.

Role of local vitamin D signaling and cellular calcium transport system in bone homeostasis

Mouse genetic studies have demonstrated that the 1,25-dihydroxyvitamin D [1,25(OH)2D] endocrine system is required for calcium (Ca(2+)) and bone homeostasis. These studies reported severe hypocalcemia and impaired bone mineralization associated with rickets in mutant mice. Specific phenotypes of these mice with an engineered deletion of 1,25(OH)2D cell signaling resemble the features observed in humans with the same congenital disease or severe 1,25(OH)2D deficiency. Decreased active intestinal Ca(2+) absorption because of reduced expression of epithelial Ca(2+) channels is a crucial mechanism that contributes to the major phenotypes observed in the mutant mice. The importance of intestinal Ca(2+) absorption supported by 1,25(OH)2D-mediated transport was further emphasized by the observation that Ca(2+) supplementation rescues hypocalcemia and restores bone mineralization in both patients and mice lacking 1,25(OH)2D signaling. This observation questions the direct role of 1,25(OH)2D signaling in bone tissue. Studies regarding tissue-specific manipulation of 1,25(OH)2D function have provided a consensus on this issue by demonstrating a direct action of 1,25(OH)2D on cells in bone tissue through bone metabolism and mineral homeostasis. In addition, movement of Ca(2+) from the bone as a result of osteoclastic bone resorption also provides a large Ca(2+) supply in Ca(2+) homeostasis; however, the system controlling Ca(2+) homeostasis in osteoclasts has not been fully identified. Transient receptor potential vanilloid (TRPV) 4 mediates Ca(2+) influx during the late stage of osteoclast differentiation, thereby regulating the Ca(2+) signaling essential for cellular events during osteoclast differentiation; however, the system-modifying effect of TRPV4 activity should be determined. Furthermore, it remains unknown how local Ca(2+) metabolism participates in systemic Ca(2+) homeostasis through bone remodeling. New insights are therefore required to understand this issue.

Understanding the role of Notch in osteosarcoma

The Notch pathway has been described as an oncogene in osteosarcoma, but the myriad functions of all the members of this complex signaling pathway, both in malignant cells and nonmalignant components of tumors, make it more difficult to define Notch as simply an oncogene or a tumor suppressor. The cell-autonomous behaviors caused by Notch pathway manipulation may vary between cell lines but can include changes in proliferation, migration, invasiveness, oxidative stress resistance, and expression of markers associated with stemness or tumor-initiating cells. Beyond these roles, Notch signaling also plays a vital role in regulating tumor angiogenesis and vasculogenesis, which are vital aspects of osteosarcoma growth and behavior in vivo. Further, osteosarcoma cells themselves express relatively low levels of Notch ligand, making it likely that nonmalignant cells, especially endothelial cells and pericytes, are the major source of Notch activation in osteosarcoma tumors in vivo and in patients. As a result, Notch pathway expression is not expected to be uniform across a tumor but likely to be highest in those areas immediately adjacent to blood vessels. Therapeutic targeting of the Notch pathway is likewise expected to be complicated. Most pharmacologic approaches thus far have focused on inhibition of gamma secretase, a protease of the presenilin complex. This enzyme, however, has numerous other target proteins that would be expected to affect osteosarcoma behavior, including CD44, the WNT/β-catenin pathway, and Her-4. In addition, Notch plays a vital role in tissue and organ homeostasis in numerous systems, and toxicities, especially GI intolerance, have limited the effectiveness of gamma secretase inhibitors. New approaches are in development, and the downstream targets of Notch pathway signaling also may turn out to be good targets for therapy. In summary, a full understanding of the complex functions of Notch in osteosarcoma is only now unfolding, and this deeper knowledge will help position the field to better utilize novel therapies as they are developed.

NFATc1 and NFATc2 repress spontaneous osteoarthritis

Osteoarthritis (OA) was once viewed originally as a mechanical disease of "wear and tear," but advances made during the past two decades suggest that abnormal biomechanics contribute to active dysregulation of chondrocyte biology, leading to catabolism of the cartilage matrix. A number of signaling and transcriptional mechanisms have been studied in relation to the regulation of this catabolic program, but how they specifically regulate the initiation or progression of the disease is poorly understood. Here, we demonstrate that cartilage-specific ablation of Nuclear factor of activated T cells c1 (Nfatc1) in Nfatc2(-/-) mice leads to early onset, aggressive OA affecting multiple joints. This model recapitulates features of human OA, including loss of proteoglycans, collagen and aggrecan degradation, osteophyte formation, changes to subchondral bone architecture, and eventual progression to cartilage effacement and joint instability. Consistent with the notion that NFATC1 is an OA-suppressor gene, NFATC1 expression was significantly down-regulated in paired lesional vs. macroscopically normal cartilage samples from OA patients. The highly penetrant, early onset, and severe nature of this model make it an attractive platform for the preclinical development of treatments to alter the course of OA. Furthermore, these findings indicate that NFATs are key suppressors of OA, and regulating NFATs or their transcriptional targets in chondrocytes may lead to novel disease-modifying OA therapies.

Effects of SiO2, SrO, MgO, and ZnO dopants in tricalcium phosphates on osteoblastic Runx2 expression

Calcium phosphate materials share a compositional similarity to natural bone, which makes them excellent for use in orthopedic applications. Although these materials are osteoconductive, they lack strong osteoinductive capabilities and recent research has focused on the addition of biologics and pharmacologics with varying successes. In this study, trace elements that have been proven to play important roles in bone health and bone formation were incorporated into β-tricalcium phosphate compacts in their oxide forms (SiO2, ZnO, SrO, and MgO). Cell material interactions were characterized using human fetal preosteoblastic cells. An MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) assay was used to evaluate cellular proliferation. Cellular differentiation was evaluated using an enzymatic colorimetric alkaline phosphatase assay as well as immunohistochemistry for Runt-related transcription factor 2 (Runx2) expression. Results prove ZnO and MgO to be effective mitogenic factors and SiO2, ZnO, and SrO to be capable of inducing rapid cellular differentiation. MgO was found to have little effect on the modulation of osteoblastic differentiation, likely due to more aggressive inherent cellular regulation of Mg(2+). In addition to the results from the study, a signaling mechanism is proposed as to the action of the dopants for further consideration.

Zinc inhibits osteoclast differentiation by suppression of Ca2+-Calcineurin-NFATc1 signaling pathway

BACKGROUND

Zinc, an essential trace element, inhibits osteoclast differentiation in vitro and in vivo. The molecular mechanism for the inhibitory effect of zinc, however, is poorly understood. The purpose of this study was to investigate the effect of zinc and determine its molecular mechanism on receptor activator of NF-κB ligand (RANKL)-induced osteoclastogenesis in mouse bone marrow-derived monocyte cells (BMMs) and RAW264.7 cells.

RESULTS

In BMMs, zinc treatment during osteoclast differentiation decreased RANKL-induced osteoclast formation in a dose-dependent manner. We show that zinc suppressed the mRNA levels of nuclear factor of activated T-cells, cytoplasmic 1 (Nfatc1). Zinc also accumulated phospho-Nfatc1 (p-Nfatc1) in the cytosol in a dose-dependent manner and inhibited the translocation of Nfatc1 to the nucleus in RAW264.7 cells. Zinc suppressed the activities of Nfatc1 in the nucleus without changing the activities of NF-κB in RAW264.7 cells. In contrast, calcineurin activity decreased in response to zinc but its protein level was unchanged. RANKL-induced Ca2+ oscillations were inhibited by zinc treatment, but phospho-phospholipase Cγ1 (p-PLCγ1), the upstream signaling molecule of Ca2+ oscillations, was unaffected. Moreover, a constitutively active form of Nfatc1 obviously rescued suppression of osteoclastogenesis by zinc.

CONCLUSIONS

Taken together, these results demonstrate for the first time that the inhibitory effect of zinc during osteoclastogesis is caused by suppressing the Ca2+-Calcineurin-NFATc1 signaling pathway. Thus, zinc may be a useful therapeutic candidate for the prevention of bone loss caused by NFATc1 activation in osteoclasts.

Inhibitory Effect of Chrysanthemum zawadskii Herbich var. latilobum Kitamura Extract on RANKL-Induced Osteoclast Differentiation

Chrysanthemum zawadskii Herbich var. latilobum Kitamura, known as “Gujulcho” in Korea, has been used in traditional medicine to treat various inflammatory diseases, including rheumatoid arthritis. However, these effects have not been tested on osteoclasts, the bone resorbing cells that regulate bone metabolism. Here, we investigated the effects of C. zawadskii Herbich var. latilobum Kitamura ethanol extract (CZE) on osteoclast differentiation induced by treatment with the receptor activator of NF-κB ligand (RANKL). CZE inhibited osteoclast differentiation and formation in a dose-dependent manner. The inhibitory effect of CZE on osteoclastogenesis was due to the suppression of ERK activation and the ablation of RANKL-stimulated Ca²⁺-oscillation via the inactivation of PLCγ2, followed by the inhibition of CREB activation. These inhibitory effects of CZE resulted in a significant repression of c-Fos expression and a subsequent reduction of NFATc1, a key transcription factor for osteoclast differentiation, fusion, and activation in vitro and in vivo. These results indicate that CZE negatively regulates osteoclast differentiation and may be a therapeutic candidate for the treatment of various bone diseases, such as postmenopausal osteoporosis, rheumatoid arthritis, and periodontitis.

Comparing the gene expression profile of stromal cells from human cord blood and bone marrow: lack of the typical "bone" signature in cord blood cells

With regard to the bone-regenerative capacity, bone marrow stromal cells (BMSC) can still be termed the "gold standard." Nevertheless, neonatal stromal cells from cord blood (CB) feature advantages concerning availability, immaturity, and proliferation potential. The detailed gene expression analysis and overexpression of genes expressed differentially provide insight into the inherent capacity of stromal cells. Microarray and qRT-PCR analyses revealed closely related gene expression patterns of two stromal cell populations derived from CB. In contrast to the CB-derived cell types, BMSC displayed high expression levels of BSP, OSX, BMP4, OC, and PITX2. Lentiviral overexpression of BSP but not of OSX in CB-cells increased the capacity to form a mineralized matrix. BMP4 induced the secretion of proteoglycans during chondrogenic pellet culture and extended the osteogenic but reduced the adipogenic differentiation potential. BMSC revealed the typical osteogenic gene expression signature. In contrast, the CB-derived cell types exhibited a more immature gene expression profile and no predisposition towards skeletal development. The absence of BSP and BMP4-which were defined as potential key players affecting the differentiation potential-in neonatal stromal cells should be taken into consideration when choosing a cell source for tissue regeneration approaches.

MicroRNA expression profiling of human bone marrow mesenchymal stem cells during osteogenic differentiation reveals Osterix regulation by miR-31

Osteogenesis is the result of a complex sequence of events that involve the differentiation of mesenchymal stem cells (MSC) into osteoblasts. MSCs are multipotent adult stem cells that can give rise to different cell types of the mesenchymal germ layer. The differentiation fate of MSCs depends on the microenvironmental signals received by these cells and is tightly regulated by multiple pathways that lead to the activation of specific transcription factors. Among the transcription factors involved in osteogenic differentiation Osterix (Sp7) plays a key role and has been shown to be fundamental for bone homeostasis. However, the molecular events governing the expression of this transcription factor are not fully understood. In this study we set out to investigate the changes in the microRNA (miRNA) expression that occur during the osteogenic differentiation of bone marrow-derived MSCs. To this purpose, we analyzed the miRNA expression profile of MSCs deriving from 3 donors during the differentiation and mineralization processes by microarray. 29 miRNAs were significantly and consistently modulated during the osteogenic differentiation and 5 during the mineralization process. Interestingly, most of the differentially expressed miRNAs have been reported to be implicated in stemness maintenance, differentiation and/or oncogenesis. Subsequently, we focused our attention on the regulation of Osterix by miRNAs and demonstrated that one of the miRNAs differentially modulated during osteogenic differentiation, miR-31, controls Osterix expression through association to the 3' untranslated region of this transcription factor. By analyzing miR-31 and Osterix expression levels we found an inverse miRNA-target expression trend during osteogenic differentiation and in osteosarcoma cell lines. Moreover, the inhibition of the microRNA activity led to an increase in the endogenous expression of Osterix. Our results define a miRNA signature characterizing the osteogenic differentiation of MSCs and provide evidence for the involvement of miR-31 in the regulation of the bone-specific transcription factor Osterix.

NFAT signaling in osteoblasts regulates the hematopoietic niche in the bone microenvironment

Osteoblasts support hematopoietic cell development, including B lymphopoiesis. We have previously shown that the nuclear factor of activated T cells (NFAT) negatively regulates osteoblast differentiation and bone formation. Interestingly, in smooth muscle, NFAT has been shown to regulate the expression of vascular cellular adhesion molecule-1 (VCAM-1), a mediator of cell adhesion and signaling during leukocyte development. To examine whether NFAT signaling in osteoblasts regulates hematopoietic development in vivo, we generated a mouse model expressing dominant-negative NFAT driven by the 2.3 kb fragment of the collagen-αI promoter to disrupt NFAT activity in osteoblasts (dnNFAT^OB). Bone histomorphometry showed that dnNFAT^OB mice have significant increases in bone volume (44%) and mineral apposition rate (131%) and decreased trabecular thickness (18%). In the bone microenvironment, dnNFAT^OB mice displayed a significant increase (87%) in Lineage⁻cKit⁺Sca-1⁺ (LSK) cells and significant decreases in B220⁺CD19⁻IgM⁻ pre-pro-B cells (41%) and B220⁺CD19⁺IgM⁺ immature B cells (40%). Concurrent with these findings, LSK cell differentiation into B220⁺ cells was inhibited when cocultured on differentiated primary osteoblasts harvested from dnNFAT^OB mice. Gene expression and protein levels of VCAM-1 in osteoblasts decreased in dnNFAT^OB mice compared to controls. These data suggest that osteoblast-specific NFAT activity mediates early B lymphopoiesis, possibly by regulating VCAM-1 expression on osteoblasts.

Genetic and molecular control of osterix in skeletal formation

Osteoblast differentiation is a multi-step process where mesenchymal cells differentiate into osteoblast lineage cells including osteocytes. Osterix (Osx) is an osteoblast-specific transcription factor which activates a repertoire of genes during differentiation of preosteoblasts into mature osteoblasts and osteocytes. The essential role of Osx in the genetic program of bone formation and in bone homeostasis is well established. Osx mutant embryos do not form bone and fail to express osteoblast-specific marker genes. Inactivation of Osx in mice after birth causes multiple skeletal phenotypes including lack of new bone formation, absence of resorption of mineralized cartilage, and defects in osteocyte maturation and function. Since Osx is a major effector in skeletal formation, studies on Osx gained momentum over the last 5-7 years and implicated its important function in tooth formation as well as in healing of bone fractures. This review outlines mouse genetic studies that establish the essential role of Osx in bone and tooth formation as well as in healing of bone fractures. We also discuss the recent advances in regulation of Osx expression, which is under control of a transcriptional network, signaling pathways, and epigenetic regulation. Finally, we summarize important findings on the positive and negative regulation of Osx's transcriptional activity through protein-protein interactions in expression of its target genes during osteoblast differentiation. In particular, the identification of the histone demethylase NO66 as an Osx-interacting protein, which negatively regulates Osx activity opens further avenues in studying epigenetic control of Osx target genes during differentiation and maturation of osteoblasts.

MicroRNA-214 suppresses osteogenic differentiation of C2C12 myoblast cells by targeting Osterix

Osterix (Osx) is an osteoblast-specific transcription factor that is essential for osteoblast differentiation and bone formation. Osx-null mice, which exhibit a complete absence of bone formation and arrested osteoblast differentiation, die immediately after birth. However, our understanding of the regulatory mechanism of Osx expression remains poor. MicroRNAs (miRNAs) are a class of small non-coding RNAs that play pivotal roles in diverse biological processes, including the development, differentiation, proliferation, survival, and oncogenesis of cells and organisms. In this study, we aimed to investigate the impact of miRNAs on Osx expression. Bioinformatic analyses predicted that miR-214 would be a potential regulator of Osx. The direct binding of miR-214 to the Osx 3' untranslated region (3' UTR) was demonstrated by a luciferase reporter assay using a construct containing the Osx 3' UTR. Deletion mutant construction revealed that the Osx 3' UTR contained two miR-214 binding sites. MiR-214 expression was inversely correlated with Osx expression in Saos-2 and U2OS cells. The forced expression of miR-214 in Saos-2 cells led to a reduction in the level of Osx protein. Moreover, the role of miR-214 in the osteogenic differentiation of C2C12 cells was investigated. We found that the osteogenic differentiation of C2C12 cells was enhanced by the downregulation of miR-214 expression, as measured by increased alkaline phosphatase activity and matrix mineralization. Taken together, these results indicate that miR-214 is a novel regulator of Osx, and that it plays an important role in the osteogenic differentiation of C2C12 cells as a suppressor.

Advances in the regulation of osteoclasts and osteoclast functions

Osteoclasts are derived from mononuclear hematopoietic myeloid lineage cells, which are formed in the bone marrow and are attracted to the bloodstream by factors, including sphingsine-1 phosphate. These circulating precursors are attracted to bone surfaces undergoing resorption by chemokines and other factors expressed at these sites, where they fuse to form multinucleated bone-resorbing cells. All aspects of osteoclast formation and functions are regulated by macrophage-colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL), cytokines essential for osteoclast formation and expressed by a variety of cell types, including osteoblast lineage cells. Since the discovery of RANKL in the mid-1990s, mouse genetic and molecular studies have revealed numerous signaling pathways activated by RANKL and M-CSF. More recent studies indicate that osteoclasts and their precursors regulate immune responses and osteoblast formation and functions by means of direct cell-cell contact through ligands and receptors, such as ephrins and Ephs, and semaphorins and plexins, and through expression of clastokines. There is also growing recognition that osteoclasts are immune cells with roles in immune responses beyond mediating the bone destruction that can accompany them. This article reviews recent advances in the understanding of the molecular mechanisms regulating osteoclast formation and functions and their interactions with other cells in normal and pathologic states.

Inhibition of protein kinase-D promotes cartilage repair at injured growth plate in rats

INTRODUCTION

Injured growth plate cartilage is often repaired by bony tissue, causing bone growth defects in children. Currently, mechanisms for the undesirable repair remain unclear and there are no biological treatments available to prevent the associated bone growth defects. Osterix is known as a vital transcription factor for osteoblast differentiation which is critical for normal bone formation and bone repair, and osterix is known to be regulated by protein kinase-D; however it is unknown whether protein kinase-D-osterix signalling plays any roles in the bony repair of injured growth plate.

METHODS

Using a rat model, this study investigated potential roles of protein kinase-D (PKD) in regulating expression of osteogenic transcription factor osterix and the growth plate bony repair. 4 days post injury at the proximal tibial growth plate, rats received four once-daily injections of vehicle or 2.35 mg/kg gö6976 (a PKD inhibitor), and growth plate tissues collected at day 10 were examined histologically and molecularly. In addition, effects of PKD inhibition on osteogenic and chondrogenic differentiation were examined in vitro using rat bone marrow mesenchymal stromal cells.

RESULTS

Compared to vehicle control, PKD inhibition caused a decrease in bone volume (p<0.05), an increase in % of mesenchymal tissue (p<0.01), and an increase in cartilaginous tissue within the injury site. Consistently, gö6976 treatment tended to decrease expression of bone-related genes (osterix, osteocalcin) and increase levels of cartilage-related genes (Sox9, collagen-2a, collagen-10a1). In support, in vitro experiments showed that gö6976 presence in the primary rat marrow stromal cell culture resulted in a decrease of alkaline phosphatase(+) CFU-f colonies formed (p<0.05) and an increase in collagen-2a expression in chondrogenic pellet culture (p<0.05).

CONCLUSION

These studies suggest that PKD is important for growth plate bony repair and its inhibition after growth plate injury may result in less bone formation and potentially more cartilage repair.

P2 receptor networks regulate signaling duration over a wide dynamic range of ATP concentrations

The primordial intercellular signaling molecule ATP acts through two families of cell-surface P2 receptors - the P2Y family of G-protein-coupled receptors and the P2X family of ligand-gated cation channels. Multiple P2 receptors are expressed in a variety of cell types. However, the significance of these networks of receptors in any biological system remains unknown. Using osteoblasts as a model system, we found that a low concentration of ATP (10 µM, ATPlow) induced transient elevation of cytosolic Ca(2+), whereas a high concentration of ATP (1 mM, ATPhigh) elicited more sustained elevation. Moreover, graded increases in the Ca(2+) signal were achieved over a remarkable million-fold range of ATP concentrations (1 nM to 1 mM). Next, we demonstrated that ATPlow caused transient nuclear localization of the Ca(2+)-regulated transcription factor NFATc1; whereas, ATPhigh elicited more sustained localization. When stimulated with ATPhigh, osteoblasts from P2X7 loss-of-function mice showed only transient Ca(2+)-NFATc1 signaling; in contrast, sustained signaling was observed in wild-type cells. Additional experiments revealed a role for P2Y receptors in mediating transient signaling induced by low ATP concentrations. Thus, distinct P2 receptors with varying affinities for ATP account for this wide range of sensitivity to extracellular nucleotides. Finally, ATPhigh, but not ATPlow, was shown to elicit robust expression of the NFAT target gene Ptgs2 (encoding COX-2), consistent with a crucial role for the duration of Ca(2+)-NFAT signaling in regulating target gene expression. Taken together, ensembles of P2 receptors provide a mechanism by which cells sense ATP over a wide concentration range and transduce this input into distinct cellular signals.

Osteoprotegerin inhibits aortic valve calcification and preserves valve function in hypercholesterolemic mice

BACKGROUND

There are no rigorously confirmed effective medical therapies for calcific aortic stenosis. Hypercholesterolemic Ldlr (-/-) Apob (100/100) mice develop calcific aortic stenosis and valvular cardiomyopathy in old age. Osteoprotegerin (OPG) modulates calcification in bone and blood vessels, but its effect on valve calcification and valve function is not known.

OBJECTIVES

To determine the impact of pharmacologic treatment with OPG upon aortic valve calcification and valve function in aortic stenosis-prone hypercholesterolemic Ldlr (-/-) Apob (100/100) mice.

METHODS

Young Ldlr (-/-) Apob (100/100) mice (age 2 months) were fed a Western diet and received exogenous OPG or vehicle (N = 12 each) 3 times per week, until age 8 months. After echocardiographic evaluation of valve function, the aortic valve was evaluated histologically. Older Ldlr (-/-) Apob (100/100) mice were fed a Western diet beginning at age 2 months. OPG or vehicle (N = 12 each) was administered from 6 to 12 months of age, followed by echocardiographic evaluation of valve function, followed by histologic evaluation.

RESULTS

In Young Ldlr (-/-) Apob (100/100) mice, OPG significantly attenuated osteogenic transformation in the aortic valve, but did not affect lipid accumulation. In Older Ldlr (-/-) Apob (100/100) mice, OPG attenuated accumulation of the osteoblast-specific matrix protein osteocalcin by ∼80%, and attenuated aortic valve calcification by ∼ 70%. OPG also attenuated impairment of aortic valve function.

CONCLUSIONS

OPG attenuates pro-calcific processes in the aortic valve, and protects against impairment of aortic valve function in hypercholesterolemic aortic stenosis-prone Ldlr (-/-) Apob (100/100) mice.

Coordinated transcriptional regulation of bone homeostasis by Ebf1 and Zfp521 in both mesenchymal and hematopoietic lineages

Bone homeostasis is maintained by the coupled actions of hematopoietic bone-resorbing osteoclasts (OCs) and mesenchymal bone-forming osteoblasts (OBs). Here we identify early B cell factor 1 (Ebf1) and the transcriptional coregulator Zfp521 as components of the machinery that regulates bone homeostasis through coordinated effects in both lineages. Deletion of Zfp521 in OBs led to impaired bone formation and increased OB-dependent osteoclastogenesis (OC-genesis), and deletion in hematopoietic cells revealed a strong cell-autonomous role for Zfp521 in OC progenitors. In adult mice, the effects of Zfp521 were largely caused by repression of Ebf1, and the bone phenotype of Zfp521(+/-) mice was rescued in Zfp521(+/-):Ebf1(+/-) mice. Zfp521 interacted with Ebf1 and repressed its transcriptional activity. Accordingly, deletion of Zfp521 led to increased Ebf1 activity in OBs and OCs. In vivo, Ebf1 overexpression in OBs resulted in suppressed bone formation, similar to the phenotype seen after OB-targeted deletion of Zfp521. Conversely, Ebf1 deletion led to cell-autonomous defects in both OB-dependent and cell-intrinsic OC-genesis, a phenotype opposite to that of the Zfp521 knockout. Thus, we have identified the interplay between Zfp521 and Ebf1 as a novel rheostat for bone homeostasis.

Nuclear receptors in bone physiology and diseases

During the last decade, our view on the skeleton as a mere solid physical support structure has been transformed, as bone emerged as a dynamic, constantly remodeling tissue with systemic regulatory functions including those of an endocrine organ. Reflecting this remarkable functional complexity, distinct classes of humoral and intracellular regulatory factors have been shown to control vital processes in the bone. Among these regulators, nuclear receptors (NRs) play fundamental roles in bone development, growth, and maintenance. NRs are DNA-binding transcription factors that act as intracellular transducers of the respective ligand signaling pathways through modulation of expression of specific sets of cognate target genes. Aberrant NR signaling caused by receptor or ligand deficiency may profoundly affect bone health and compromise skeletal functions. Ligand dependency of NR action underlies a major strategy of therapeutic intervention to correct aberrant NR signaling, and significant efforts have been made to design novel synthetic NR ligands with enhanced beneficial properties and reduced potential negative side effects. As an example, estrogen deficiency causes bone loss and leads to development of osteoporosis, the most prevalent skeletal disorder in postmenopausal women. Since administration of natural estrogens for the treatment of osteoporosis often associates with undesirable side effects, several synthetic estrogen receptor ligands have been developed with higher therapeutic efficacy and specificity. This review presents current progress in our understanding of the roles of various nuclear receptor-mediated signaling pathways in bone physiology and disease, and in development of advanced NR ligands for treatment of common skeletal disorders.

Advances in osteoclast biology reveal potential new drug targets and new roles for osteoclasts

Osteoclasts are multinucleated myeloid lineage cells formed in response to macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL) by fusion of bone marrow-derived precursors that circulate in the blood and are attracted to sites of bone resorption in response to factors, such as sphingosine-1 phosphate signaling. Major advances in understanding of the molecular mechanisms regulating osteoclast functions have been made in the past 20 years, mainly from mouse and human genetic studies. These have revealed that osteoclasts express and respond to proinflammatory and anti-inflammatory cytokines. Some of these cytokines activate NF-κB and nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) signaling to induce osteoclast formation and activity and also regulate communication with neighboring cells through signaling proteins, including ephrins and semaphorins. Osteoclasts also positively and negatively regulate immune responses and osteoblastic bone formation. These advances have led to development of new inhibitors of bone resorption that are in clinical use or in clinical trials; and more should follow, based on these advances. This article reviews current understanding of how bone resorption is regulated both positively and negatively in normal and pathologic states.

Regulation of the bone-restricted IFITM-like (Bril) gene transcription by Sp and Gli family members and CpG methylation

BACKGROUND

BRIL is a bone-specific membrane protein that is involved in osteogenesis imperfecta type V.

RESULTS

Bril transcription is activated by Sp1, Sp3, OSX, and GLI2 and by CpG demethylation.

CONCLUSION

Regulation of Bril involves trans-acting factors integrating at conserved promoter elements and epigenetic modifications.

SIGNIFICANCE

Identification of the mechanisms governing Bril transcription is important to understand its role in skeletal biology. Bril encodes a small membrane protein present in osteoblasts. In humans, a single recurrent mutation in the 5'-UTR of BRIL causes osteogenesis imperfecta type V. The exact function of BRIL and the mechanism by which it contributes to disease are still unknown. The goal of the current study was to characterize the mechanisms governing Bril transcription in humans, rats, and mice. In the three species, as detected by luciferase reporter assays in UMR106 cells, we found that most of the base-line regulatory activity was localized within ∼250 bp upstream of the coding ATG. Co-transfection experiments indicated that Sp1 and Sp3 were potent inducers of the promoter activity, through the binding of several GC-rich boxes. Osterix was a weak activator but acted cooperatively with Sp1 and GLI2 to synergistically induce the BRIL promoter. GLI2, a mediator of hedgehog signaling pathway, was also a potent activator of BRIL through a single GLI binding site. Correspondingly, agonists of the hedgehog pathway (purmorphamine and Indian hedgehog) in MC3T3 osteoblasts led to increased BRIL levels. The BRIL promoter activity was also found to be negatively modulated through two different mechanisms. First, the ZFP354C zinc finger protein repressed basal and Sp1-induced activity. Second, CpG methylation of the promoter region correlated with an inactive state and prevented Sp1 activation. The data provide the very first analyses of the cis- and trans-acting factors regulating Bril transcription. They revealed key roles for the Sp members and GLI2 that possibly cooperate to activate Bril when the promoter becomes demethylated.

Osterix induces Col1a1 gene expression through binding to Sp1 sites in the bone enhancer and proximal promoter regions

Bone-specific transcription factors promote differentiation of mesenchymal precursors toward the osteoblastic cell phenotype. Among them, Runx2 and Osterix have been widely accepted as master osteogenic factors, since neither Runx2 nor Osterix null mice form mature osteoblasts. Recruitment of Osterix to a number of promoters of bone-specific genes has been shown. However, little is known about the functional interactions between Osterix and the Col1a1 promoter. In this study we determined in several mesenchymal and osteoblastic cell types that either BMP-2 or Osterix overexpression increased Col1a1 transcription. We identified consensus Sp1 sequences, located in the proximal promoter and in the bone-enhancer, as Osterix binding regions in the Col1a1 promoter in vitro and in vivo. Furthermore, we show that p38 or Erk MAPK signaling is required for maximal transcriptional effects on Col1a1 expression. Runx2 has been shown to activate Col1a1 expression through binding to sites which are located close to the Sp1 sites where Osterix binds. Our data show that overexpression of Runx2 and Osterix leads to a cooperative effect on the expression of the Col1a1 endogenous gene and its promoter reporter construct. These effects mainly affect the long isoform of Osterix which suggest that the two Osterix isoforms might display some differential effects on the transactivation of bone-specific genes.

Development of the endochondral skeleton

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

Nanog promotes osteogenic differentiation of the mouse mesenchymal cell line C3H10T1/2 by modulating bone morphogenetic protein (BMP) signaling

How the pluripotency of stem cells is maintained and the role of transcription factors in this maintenance remain major questions. In the present study, in order to clarify the mechanism underlying the pluripotency of stem cells for the advancement of regenerative medicine, we examined the effect of forced Nanog expression in mesenchymal cells, with a particular focus on osteogenic differentiation. The human mesenchymal stromal cells (hMSCs) or mouse mesenchymal cell line C3H10T1/2 cells were transduced with the Nanog gene or control green fluorescent protein (GFP) gene by using retrovirus vectors. Short-term, forced Nanog gene expression had few effects on the terminal osteogenic differentiation of either hMSCs or C3H10T1/2 cells. To determine its long-term effects, we established C3H10T1/2 cells expressing Nanog constitutively. Constitutive Nanog expression strongly induced osteogenic differentiation of C3H10T1/2 cells. In regard to cell proliferation, constitutive Nanog expression only repressed the proliferation of the cells treated with rhBMP-2. Moreover, Nanog also had the potential to promote the proliferation of C3H10T1/2 cells in the absence of rhBMP-2. Constitutive Nanog expression enhanced phosphorylation of Smad1/5/8 and suppressed Cdk4 and cyclinD1. The promoter activities of both the osteocalcin and Id-1 genes were activated in cells expressing Nanog constitutively. To identify downstream molecules of Nanog involved in the promotion of osteogenic differentiation, we performed a DNA microarray analysis and discovered that NFATc1 was one of the downstream effectors of Nanog. These results indicate that Nanog functions as a modulator of BMP signaling in C3H10T1/2 cells probably through a genome reprogramming process.

Transcriptional regulatory cascades in Runx2-dependent bone development

The development of the musculoskeletal system is a complex process that involves very precise control of bone formation and growth as well as remodeling during postnatal life. Although the understanding of the transcriptional mechanisms of osteogenesis has increased considerably, the molecular regulatory basis, especially the gene regulatory network of osteogenic differentiation, is still poorly understood. This review provides the reader with an overview of the key transcription factors that govern bone formation, highlighting their function and regulation linked to Runt-related transcription factor 2 (Runx2). Runx2 as the master transcription factor of osteoblast differentiation, Twist, Msh homeobox 2 (Msx2), and promyelocytic leukemia zinc-finger protein (PLZF) acting upstream of Runx2, Osterix (Osx) acting downstream of Runx2, and activating transcription factor 4 (ATF4) and zinc-finger protein 521 (ZFP521) acting as cofactors of Runx2 are discussed, and their relevance for tissue engineering is presented. References are provided for more in-depth personal study.

Nuclear factor of activated T-cells (NFAT)C2 inhibits Notch receptor signaling in osteoblasts

Notch receptors regulate osteoblastogenesis, and Notch activation induces cleavage and nuclear translocation of the Notch intracellular domain (NICD), which associates with Epstein-Barr virus latency C-promoter binding factor-1/suppressor of hairless/lag-1 (CSL) and induces transcription of Notch target genes, such as hairy enhancer of split-related with YRPW motif (Hey)1 and Hey2. Nuclear factors of activated T-cells (NFAT) are transcription factors that regulate osteoclastogenesis, but their function in osteoblasts is not clear. Notch inhibits NFATc1 transcription, but interactions between Notch and NFAT are understood poorly. To determine the regulation of NFAT expression by Notch, osteoblasts from Rosa(Notch) mice, where NICD is transcribed following excision of a loxP flanked STOP cassette, were used. Alternatively, wild-type C57BL/6 osteoblasts were exposed to the Notch ligand Delta-like (Dll)1 to induce Notch signaling or to bovine serum albumin as control. In Rosa(Notch) osteoblasts, Notch suppressed NFATc1 expression, increased Nfatc2 mRNA by post-transcriptional mechanisms, and had no effect on NFATc3 and NFATc4 transcripts. Induction of Nfatc2 transcripts by Notch was confirmed in C57BL/6 osteoblasts exposed to Dll1. To investigate NFATc2 function in osteoblasts, constitutively active NFATc2 was overexpressed in Rosa(Notch) osteoblasts. NFATc2 suppressed Notch transactivation and expression of Hey genes. Electrophoretic mobility shift assays revealed that NFATc2 and CSL bind to similar DNA sequences, and chromatin immunoprecipitation indicated that NFATc2 displaced CSL from the Hey2 promoter. The effects of NICD and NFATc2 in Rosa(Notch) osteoblasts were assessed, and both proteins inhibited osteoblast function. In conclusion, Notch stabilizes Nfatc2 transcripts, NFATc2 suppresses Notch signaling, and both proteins inhibit osteoblast function.

Deletion of Orai1 alters expression of multiple genes during osteoclast and osteoblast maturation

Store-operated Ca(2+) entry (SOCE) is a major Ca(2+) influx pathway in most non-excitable cell types and Orai1 was recently identified as an essential pore-subunit of SOCE channels. Here we investigate the physiological role of Orai1 in bone homeostasis using Orai1-deficient mice (Orai1(-/-)). Orai1(-/-) mice developed osteopenia with decreased bone mineral density and trabecular bone volume. To identify the nature and origin of the bone defect, bone-resorbing osteoclasts and bone-forming osteoblasts from Orai1(-/-) mice were examined. Orai1-mediated SOCE was completely abolished in Orai1(-/-) osteoclast precursor cells and osteoclastogenesis in vitro from Orai1(-/-) mice was impaired due to a defect in cell fusion of pre-osteoclasts. Also, resorption activity in vitro was comparable but the size of pits formed by Orai1(-/-) osteoclasts was smaller. We next assessed the role of Orai1 in osteoblast differentiation and function by using a pre-osteoblast cell line, as well as primary osteoblasts from wild-type and Orai1(-/-) mice. SOCE in MC3T3-E1 pre-osteoblastic cells was inactivated by lentiviral overexpression of a pore-dead Orai1 mutant. Lack of SOCE in MC3T3-E1 had no effect on alkaline phosphatase staining and expression but substantially inhibited mineralized nodule formation. Consistent with this finding, Orai1-mediated SOCE was markedly reduced in Orai1(-/-) osteoblast precursor cells and osteoblastogenesis in vitro from Orai1(-/-) stromal cells showed impaired mineral deposition but no change in differentiation. This indicates that Orai1 is involved in the function but not in the differentiation of osteoblasts. Together, these results suggest that Orai1 plays a critical role in bone homeostasis by regulating both osteoblasts and osteoclasts.

Osteoclast migration, differentiation and function: novel therapeutic targets for rheumatic diseases

RA is a chronic autoimmune disease characterized by joint synovial inflammation and progressive cartilage/bone destruction. Although various kinds of RA drug have been developed worldwide, there are currently no established methods for preventing RA-associated bone destruction, the most severe outcome of this disease. One of the major pathogenic factors in arthritic bone destruction is the enhanced activity of osteoclasts at inflammatory sites. Osteoclasts are bone-resorbing giant polykaryons that differentiate from mononuclear macrophage/monocyte-lineage haematopoietic precursors. Upon stimulation by cytokines, such as M-CSF and RANK ligand, osteoclast precursor monocytes migrate and attach onto the bone surface (migration). They then fuse with each other to form giant cells (differentiation) and mediate bone resorption (function). In this review, we summarize the current understanding regarding the mechanisms underlying these three dynamic steps of osteoclastic activity and discuss novel lines of osteoclast-targeted therapies that will impact future treatment of RA.

Myeloma cells inhibit non-canonical wnt co-receptor ror2 expression in human bone marrow osteoprogenitor cells: effect of wnt5a/ror2 pathway activation on the osteogenic differentiation impairment induced by myeloma cells

Multiple myeloma (MM) is characterized by the impaired osteogenic differentiation of human mesenchymal stromal cells (hMSCs). Canonical Wnt signaling is critical for the regulation of bone formation, however, recent evidence suggests that the non-canonical Wnt agonist Wnt5a stimulates human osteoblastogenesis through its co-receptor Ror2. The effects of MM cells on non-canonical Wnt signaling and the effect of the activation of this pathway on MM-induced osteoblast exhaustion are not known and were investigated in this study. We found that the osteogenic differentiation of bone marrow hMSCs toward osteoprogenitor cells (PreOB) significantly increased Ror2 expression, and that MM cells inhibit Ror2 expression by PreOB in co-culture by inhibiting the non-canonical Wnt5a signaling. The activation of the non-canonical Wnt pathway in hMSCs by means of Wnt5a treatment and the overexpression of Wnt5 or Ror2 by lentiviral vectors increased the osteogenic differentiation of hMSCs and blunted the inhibitory effect of MM in co-culture. Consistently, Wnt5a inhibition by specific small interfering RNA reduced the hMSC expression of osteogenic markers. Our findings demonstrate that the Wnt5a/Ror2 pathway is involved in the pathophysiology of MM-induced bone disease and that the activation of the non-canonical Wnt5a/Ror2 pathway in hMSCs increases osteogenic differentiation and may counterbalance the inhibitory effect of MM cells.

[Gap junctions and bone remodeling]

Gap junctions are specialized plasma membrane structures consisting of transmembrane channels that directly link the cytoplasms of adjoining cells and mediate the reciprocal exchange of ions and low molecular weight molecules (<1200 Da). Structural studies have demonstrated that each gap junctional channel is formed by the extracellular interaction of two hemi-channels (connexons). Each connexon is a hexameric assembly of protein subunits (connexins), which delineate an aqueous pore. Connexins are homologous proteins encoded by a multigene family and are named according to their predicted molecular weight. Connexin 43, widely distributed in different cell types, is the main gap junction protein expressed in human bone cells, although Cx45 and Cx46 have been reported to be expressed as well. Bone remodeling requires coordinated activity among osteoblasts and osteoclasts. Osteoblasts (bone forming cells) are derived from mesenchymal stem cells, and osteoclasts (bone resorbing cells), are multinucleated cells of monocyte/macrophage origin. Here, we review what is known regarding the structure of gap junctions and the mechanisms regulating bone remodeling, and discuss the evidence suggesting that gap junctional intercellular communication contributes to the bone remodeling.

Mice lacking the calcineurin inhibitor Rcan2 have an isolated defect of osteoblast function

Calcineurin-nuclear factor of activated T cells signaling controls the differentiation and function of osteoclasts and osteoblasts, and regulator of calcineurin-2 (Rcan2) is a physiological inhibitor of this pathway. Rcan2 expression is regulated by T(3), which also has a central role in skeletal development and bone turnover. To investigate the role of Rcan2 in bone development and maintenance, we characterized Rcan2(-/-) mice and determined its skeletal expression in T(3) receptor (TR) knockout and thyroid-manipulated mice. Rcan2(-/-) mice had normal linear growth but displayed delayed intramembranous ossification, impaired cortical bone formation, and reduced bone mineral accrual during development as well as increased mineralization of adult bone. These abnormalities resulted from an isolated defect in osteoblast function and are similar to skeletal phenotypes of mice lacking the type 2 deiodinase thyroid hormone activating enzyme or with dominant-negative mutations of TRα, the predominant TR isoform in bone. Rcan2 mRNA was expressed in primary osteoclasts and osteoblasts, and its expression in bone was differentially regulated in TRα and TRβ knockout and thyroid-manipulated mice. However, in primary osteoblast cultures, T(3) treatment did not affect Rcan2 mRNA expression or nuclear factor of activated T cells c1 expression and phosphorylation. Overall, these studies establish that Rcan2 regulates osteoblast function and its expression in bone is regulated by thyroid status in vivo.

Strontium ranelate rebalances bone marrow adipogenesis and osteoblastogenesis in senescent osteopenic mice through NFATc/Maf and Wnt signaling

With aging, bone marrow mesenchymal stromal cell (MSC) osteoblast differentiation decreases whereas MSC differentiation into adipocytes increases, resulting in increased adipogenesis and bone loss. Here, we investigated whether activation of cell signaling by strontium ranelate (SrRan) can reverse the excessive adipogenic differentiation associated with aging. In murine MSC cultures, SrRan increased Runx2 expression and matrix mineralization and decreased PPARγ2 expression and adipogenesis. This effect was associated with increased expression of the Wnt noncanonical representative Wnt5a and adipogenic modulator Maf and was abrogated by Wnt- and nuclear factor of activated T-cells (NFAT)c antagonists, implying a role for Wnt and NFATc/Maf signaling in the switch in osteoblastogenesis to adipogenesis induced by SrRan. To confirm this finding, we investigated the effect of SrRan in SAMP6 senescent mice, which exhibit decreased osteoblastogenesis, increased adipogenesis, and osteopenia. SrRan administration at a clinically relevant dose level increased bone mineral density, bone volume, trabecular thickness and number, as shown by densitometric, microscanning, and histomorphometric analyses in long bones and vertebrae. This attenuation of bone loss was related to increased osteoblast surface and bone formation rate and decreased bone marrow adipocyte volume and size. The restoration of osteoblast and adipocyte balance induced by SrRan was linked to increased Wnt5a and Maf expression in the bone marrow. The results indicate that SrRan acts on lineage allocation of MSCs by antagonizing the age-related switch in osteoblast to adipocyte differentiation via mechanisms involving NFATc/Maf and Wnt signaling, resulting in increased bone formation and attenuation of bone loss in senescent osteopenic mice.