Expression of the PEBP2alphaA/AML3/CBFA1 gene is regulated by BMP4/7 heterodimer and its overexpression suppresses type I collagen and osteocalcin gene expression in osteoblastic and nonosteoblastic mesenchymal cells

The Smad Dependent TGF-β and BMP Signaling Pathway in Bone Remodeling and Therapies

Bone remodeling is a continuous process that maintains the homeostasis of the skeletal system, and it depends on the homeostasis between bone-forming osteoblasts and bone-absorbing osteoclasts. A large number of studies have confirmed that the Smad signaling pathway is essential for the regulation of osteoblastic and osteoclastic differentiation during skeletal development, bone formation and bone homeostasis, suggesting a close relationship between Smad signaling and bone remodeling. It is known that Smads proteins are pivotal intracellular effectors for the members of the transforming growth factor-β (TGF-β) and bone morphogenetic proteins (BMP), acting as transcription factors. Smad mediates the signal transduction in TGF-β and BMP signaling pathway that affects both osteoblast and osteoclast functions, and therefore plays a critical role in the regulation of bone remodeling. Increasing studies have demonstrated that a number of Smad signaling regulators have potential functions in bone remodeling. Therefore, targeting Smad dependent TGF-β and BMP signaling pathway might be a novel and promising therapeutic strategy against osteoporosis. This article aims to review recent advances in this field, summarizing the influence of Smad on osteoblast and osteoclast function, together with Smad signaling regulators in bone remodeling. This will facilitate the understanding of Smad signaling pathway in bone biology and shed new light on the modulation and potential treatment for osteoporosis.

Cell-specific expression of runt-related transcription factor 2 contributes to pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a fatal lung disease with limited therapeutic options and unknown etiology. IPF is characterized by epithelial cell injury, impaired cellular crosstalk between epithelial cells and fibroblasts, and the formation of fibroblast foci with increased extracellular matrix deposition (ECM). We investigated the role of runt-related transcription factor 2 (RUNX2), a master regulator of bone development that has been linked to profibrotic signaling. RUNX2 expression was up-regulated in lung homogenates from patients with IPF and in experimental bleomycin-induced lung fibrosis. The RUNX2 level correlated with disease severity as measured by decreased diffusing capacity and increased levels of the IPF biomarker, matrix metalloproteinase 7. Nuclear RUNX2 was observed in prosurfactant protein C-positive hyperplastic epithelial cells and was rarely found in myofibroblasts. We discovered an up-regulation of RUNX2 in fibrotic alveolar epithelial type II (ATII) cells as well as an increase of RUNX2-negative fibroblasts in experimental and human pulmonary fibrosis. Functionally, small interfering RNA-mediated RUNX2 knockdown decreased profibrotic ATII cell function, such as proliferation and migration, whereas fibroblasts displayed activation markers and increased ECM expression after RUNX2 knockdown. This study reveals that RUNX2 is differentially expressed in ATII cells vs. fibroblasts in lung fibrosis, which contributes to profibrotic cell function. Cell-specific targeting of RUNX2 pathways may represent a therapeutic approach for IPF.-Mümmler, C., Burgy, O., Hermann, S., Mutze, K., Günther, A., Königshoff, M. Cell-specific expression of runt-related transcription factor 2 contributes to pulmonary fibrosis.

GATA4 Directly Regulates Runx2 Expression and Osteoblast Differentiation

GATA4 is a zinc‐finger transcription factor that is a pioneer factor in various tissues and regulates tissue‐specific gene regulation. In vivo deletion of Gata4 using Cre‐recombinase under the control of the Col1a1 2.3 kb promoter showed significantly reduced values for trabecular bone properties by microCT analysis of femur and tibia of 14‐week‐old male and female mice, suggesting GATA4 is necessary for maintaining normal adult bone phenotype. Quantitative PCR analysis revealed higher expression of Gata4 in trabecular bone compared with cortical bone, suggesting a role for GATA4 in maintaining normal trabecular bone mass. In vivo and in vitro, reduction of Gata4 correlates with reduced Runx2 gene expression, along with reduced osteoblast mineralization. To determine if Runx2 is a direct target of GATA4, chromatin immunoprecipitation (ChIP) was performed, and it demonstrated that GATA4 is recruited to the two Runx2 promoters and an enhancer region. Furthermore, when Gata4 is knocked down, the chromatin at the Runx2 region is not open, as detected by DNase assays and ChIP with antibodies to the open chromatin marks H3K4me2 (histone 3 lysine 4 dimethylation) and H3K27ac (histone 3 lysine 27 acetylation) and the closed chromatin mark H3K27me2 (histone 3 lysine 27 trimethylation). Together, the data suggest that GATA4 binds near the Runx2 promoter and enhancer and helps maintain open chromatin to regulate Runx2 expression leading to bone mineralization. © 2017 The Authors. JBMR Plus is published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research.

Biological Approaches to Bone Regeneration by Gene Therapy

Safe, effective approaches for bone regeneration are needed to reverse bone loss caused by trauma, disease, and tumor resection. Unfortunately, the science of bone regeneration is still in its infancy, with all current or emerging therapies having serious limitations. Unlike current regenerative therapies that use single regenerative factors, the natural processes of bone formation and repair require the coordinated expression of many molecules, including growth factors, bone morphogenetic proteins, and specific transcription factors. As will be developed in this article, future advances in bone regeneration will likely incorporate therapies that mimic critical aspects of these natural biological processes, using the tools of gene therapy and tissue engineering. This review will summarize current knowledge related to normal bone development and fracture repair, and will describe how gene therapy, in combination with tissue engineering, may mimic critical aspects of these natural processes. Current gene therapy approaches for bone regeneration will then be summarized, including recent work where combinatorial gene therapy was used to express groups of molecules that synergistically interacted to stimulate bone regeneration. Last, proposed future directions for this field will be discussed, where regulated gene expression systems will be combined with cells seeded in precise three-dimensional configurations on synthetic scaffolds to control both temporal and spatial distribution of regenerative factors. It is the premise of this article that such approaches will eventually allow us to achieve the ultimate goal of bone tissue engineering: to reconstruct entire bones with associated joints, ligaments, or sutures. Abbreviations used: BMP, bone morphogenetic protein; FGF, fibroblast growth factor; AER, apical ectodermal ridge; ZPA, zone of polarizing activity; PZ, progress zone; SHH, sonic hedgehog; OSX, osterix transcription factor; FGFR, fibroblast growth factor receptor; PMN, polymorphonuclear neutrophil; PDGF, platelet-derived growth factor; IGF, insulin-like growth factor; TGF-β, tumor-derived growth factor β; CAR, coxsackievirus and adenovirus receptor; MLV, murine leukemia virus; HIV, human immunodeficiency virus; AAV, adeno-associated virus; CAT, computer-aided tomography; CMV, cytomegalovirus; GAM, gene-activated matrix; MSC, marrow stromal cell; MDSC, muscle-derived stem cell; VEGF, vascular endothelial growth factor.

Phenotypic Changes in Dentition of Runx2 Homozygote-null Mutant Mice

Genetic and molecular studies in humans and mice indicate that Runx2 (Cbfa1) is a critical transcriptional regulator of bone and tooth formation. Heterozygous mutations in Runx2 cause cleidocranial dysplasia (CCD), an inherited disorder in humans and mice characterized by skeletal defects, supernumerary teeth, and delayed eruption. Mice lacking the Runx2 gene die at birth and lack bone and tooth development. Our extended phenotypic studies of Runx2 mutants showed that developing teeth fail to advance beyond the bud stage and that mandibular molar organs were more severely affected than maxillary molar organs. Runx2 (−/−) tooth organs, when transplanted beneath the kidney capsules of nude mice, failed to progress in development. Tooth epithelial-mesenchymal recombinations using Runx2 (+/+) and (−/−) tissues indicate that the defect in mesenchyme cannot be rescued by normal dental epithelium. Finally, our molecular analyses showed differential effects of the absence of Runx2 on tooth extracellular matrix (ECM) gene expression. These data support the hypothesis that Runx2 is one of the key mesenchymal factors that influences tooth morphogenesis and the subsequent differentiation of ameloblasts and odontoblasts.

GnRH regulates trophoblast invasion via RUNX2-mediated MMP2/9 expression

STUDY HYPOTHESIS

We hypothesized that Runt-related transcription factor 2 (RUNX2), matrix metalloproteinase (MMP)2 and MMP9 are involved in basal and gonadotrophin-releasing hormone (GnRH)-induced human extravillous trophoblast (EVT) cell invasion.

STUDY FINDING

Our finding indicates that GnRH-induced RUNX2 expression enhances the invasive capacity of EVT cells by modulating the expression of MMP2 and MMP9.

WHAT IS KNOWN ALREADY

GnRH is expressed in first-trimester placenta and exerts pro-invasive effects on EVT cells in vitro. RUNX2 regulates MMP2 and MMP9 expression and is often associated with invasive phenotypes.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

First-trimester human placenta (n = 9) was obtained from women undergoing elective termination of pregnancy. The localization of RUNX2, MMP2 and MMP9 in first-trimester human placenta was examined by immunohistochemistry. Primary or immortalized (HTR-8/SVneo) EVT cells were treated alone or in combination with GnRH, GnRH antagonist Antide, MAPK kinase inhibitor PD98095, phosphatidylinositol 3-kinase inhibitor LY294002, MMP2/9 inhibitor or small interfering RNAs (siRNAs) targeting RUNX2, MMP2 and/or MMP9. Protein and mRNA levels were measured by western blot and RT-PCR, respectively. Cell invasiveness was evaluated by transwell Matrigel or collagen I invasion assays.

MAIN RESULTS AND THE ROLE OF CHANCE

RUNX2, MMP2 and MMP9 were detected in the cell column regions of human first-trimester placental villi. GnRH treatment increased RUNX2 mRNA and protein levels in HTR-8/SVneo cells and primary EVTs, and these effects were attenuated by co-treatment with Antide, PD98095 or LY294002. Down-regulation of RUNX2 by siRNA reduced basal and GnRH-induced MMP2/9 expression and cell invasion. Moreover, pharmacological inhibition or siRNA-mediated knockdown of MMP2/9 reduced basal and GnRH-induced cell invasion.

LIMITATIONS, REASONS FOR CAUTION

The lack of an in vivo model is the major limitation of our in vitro study.

WIDER IMPLICATIONS OF THE FINDINGS

Our findings provide important insight into the functions of the GnRH - GnRH receptor system in early implantation and placentation.

LARGE SCALE DATA

Not applicable.

STUDY FUNDING AND COMPETING INTERESTS

This research was supported by Canadian Institutes of Health Research (Grant #143317) to P.C.K.L. The authors have nothing to disclose.

Defect in Runx2 gene accelerates ureteral obstruction-induced kidney fibrosis via increased TGF-β signaling pathway

Runt-related transcription factor 2 (Runx2) plays an important role in bone formation and de novo synthesis of proteins, including type 1 collagen. Runx2 has a potent effect on signaling of transforming growth factor (TGF)-β and vice versa, implicating its significant role in fibrosis. Chronic renal failure comprises fibrosis, characterized as an increase in TGF-β signaling, and expression of α-smooth muscle actin (α-SMA), and extracellular matrix proteins. Here, we evaluated the role of Runx2 in ureteral obstruction (UO)-induced kidney fibrosis using mice whose Runx2 gene expression is genetically down-regulated. UO caused tubular atrophy and dilation, expansion of interstitium, and increased expression of collagens and α-SMA with a concomitant decrease in expression of Runx2. Deficiency of Runx2 gene (Runx2(+/-) mice) showed higher expression of collagens and α-SMA in the kidney following UO compared to wild type (Runx2(+/+)) mice. UO-induced activation of TGF-β signaling was higher in the Runx2(+/-) kidney than Runx2(+/+) kidney, suggesting an inhibitory effect of Runx2 on TGF-β signaling in kidney fibrosis. Besides, overexpression of the Runx2 gene using an adenoviral vector in kidney tubule cells resulted in attenuated TGF-β-induced Smad3 phosphorylation and expressions of α-SMA and collagen I. Furthermore, Runx2 gene deficient mouse embryonic fibroblasts induced greater activation of Smad3 and expression of α-SMA in response to TGF-β. Collectively, Runx2 plays a protective role in UO-induced kidney fibrosis by inhibition of TGF-β signaling, suggesting Runx2 as a novel target for protection against fibrosis-related diseases such as chronic renal failure.

Osteoblast physiology in normal and pathological conditions

Osteoblasts are mononucleated cells that are derived from mesenchymal stem cells and that are responsible for the synthesis and mineralization of bone during initial bone formation and later bone remodelling. Osteoblasts also have a role in the regulation of osteoclast activity through the receptor activator of nuclear factor κ-B ligand and osteoprotegerin. Abnormalities in osteoblast differentiation and activity occur in some common human diseases such as osteoporosis and osteoarthritis. Recent studies also suggest that osteoblast functions are compromised at sites of focal bone erosion in rheumatoid arthritis.

Post-translational Regulation of Runx2 in Bone and Cartilage

The Runx2 gene product is essential for mammalian bone development. In humans, Runx2 haploinsufficiency results in cleidocranial dysplasia, a skeletal disorder characterized by bone and dental abnormalities. At the molecular level, Runx2 acts as a transcription factor for genes expressed in hypertrophic chondrocytes and osteoblasts. Runx2 gene expression and protein function are regulated on multiple levels, including transcription, translation, and post-translational modification. Furthermore, Runx2 is involved in numerous protein-protein interactions, most of which either activate or repress transcription of target genes. In this review, we discuss expression of Runx2 during development as well as the post-translational regulation of Runx2 through modification by phosphorylation, ubiquitination, and acetylation.

Bmp2 and Bmp4 genetically interact to support multiple aspects of mouse development including functional heart development

Bone morphogenetic proteins (BMPs) have multiple roles during embryogenesis. Current data indicate that the dosage of BMPs is tightly regulated for normal development in mice. Since Bmp2 or Bmp4 homozygous mutant mice show early embryonic lethality, we generated compound heterozygous mice for Bmp2 and Bmp4 to explore the impact of lowered dosage of these BMP ligands. Genotyping pups bred between Bmp2 and Bmp4 heterozygous mice revealed that the ratio of adult compound heterozygous mice for Bmp2 and Bmp4 is much lower than expected. During embryogenesis, the compound heterozygous embryos showed several abnormalities, including defects in eye formation, body wall closure defects, and ventricular septal defects (VSD) in the heart. However, the ratio of the compound heterozygous embryos was the same as expected. Caesarean sections at E18.5 revealed that half of the compound heterozygotes died soon after birth, and the majority of the dead individuals exhibited VSD. Survivors were able to grow to adults, but their body weight was significantly lower than control littermates. They demonstrated progressive abnormalities in the heart, eventually showing a branched leaflet in atrioventricular valves. These results suggest that the dosage of both BMP2 and 4 is critical for functional heart formation during embryogenesis and after birth.

Effect of BMP-2 and BMP-7 homodimers and a mixture of BMP-2/BMP-7 homodimers on osteoblast adhesion and growth following culture on a collagen scaffold

In the present study, we studied the involvement of BMP-2 and BMP-7 as homodimers and as a mixture of homodimers in bone regeneration using an engineered bone model. The engineered bone model consisted of a collagen scaffold populated with osteoblasts that acted as a carrier for the BMPs. BMP-2, BMP-7 and a mixture of BMP-2/BMP-7 were used at final concentrations of 10 and 100 ng ml(-1). Osteoblasts seeded onto a collagen scaffold were cultured for 24 h before being stimulated with the BMPs. Four days later, osteoblast adhesion to and growth on the scaffold were assessed. Osteocalcin, IL-6, metalloproteinase (MMP-2 and MMP-9) and protease inhibitor (TIMP-1 and TIMP-2) mRNA and protein levels were measured. Our results showed that the BMP-2, BMP-7 and a mixture of BMP-2/BMP-7 all promoted osteoblast growth on the collagen scaffold, with the mixture of BMP-2/BMP-7 enhancing the most growth. BMP-2 and the mixture of BMP-2/BMP-7 enhanced osteocalcin (an osteoblast differentiation marker) mRNA expression and protein secretion, likely via the IL-6 pathway given that IL-6 secretion was upregulated by BMP-7 and a mixture of BMP-2/BMP-7. BMPs promote extracellular matrix production by inhibiting MMP-2 mRNA and increasing TIMP-1 and TIMP-2 mRNA expressions and protein secretions. BMP-2, BMP-7 and the mixture of BMP-2/BMP-7 could promote bone regeneration via different mechanisms involving IL-6 and MMP inhibitors.

[Immunohistochemical study on collagen I content in the gingiva in cleidocranial dysplasia]

BACKGROUND

Patients with cleidocranial dysplasia (CCD) present a thickend and fibrotic gingiva.

PURPOSE

To the best of our knowledge it was analysed for the first time, whether this is correlated with an increased rate of collagen I in oral mucosa.

PATIENTS AND METHODS

27 soft tissue biopsies of six CCD-patients and 17 tissue samples of 12 healthy persons were labled with a monoclonal antibody against collagen I and the bound antibodies were detected with alkaline phosphatase-anti-alkaline phophatase-kit. The histological slices were analysed by a digital image recognition software under a fully automated microscope and the rate of collagen I was converted into amounts of grey tones.

RESULTS

The amount of grey tones reached from 11.909 to 15.319 in the CCD-group, and from 2752 to 12.556 in the control group. The U-Test of Mann, Whitney and Wilcoxon for two independent samples generated a rank sum of 91,50 for CCD-patients, and of 79,50 for the control group. The Z-value was 3,246, the p-value 0,005. "Fisher's exact test" identified a p-value of 0,0003.

CONCLUSIONS

The rate of collagen I in the oral mucosa seems to be increased significantly in CCD. This could explain the typical thick and fibrotic consistency of the gingiva and could be one reason for the delayed or missing dentition.

Alendronate inhibits PTH (1-34)-induced bone morphogenetic protein expression in MC3T3-E1 preosteoblastic cells

The bisphosphonate class of antiresorptive drugs and active forms of parathyroid hormone (PTH (1-34)) have been used clinically to enhance bone mass and density in patients with osteoporosis. Abundant evidence suggests that the mechanism by which PTH (1-34) increases bone density is stimulation of osteoblast differentiation. Although bisphosphonates have been classically thought to increase bone density by inhibiting osteoclasts, there is increasing evidence to suggest that bisphosphonates have direct stimulatory effects on osteoblast differentiation. Interestingly, in patients with osteoporosis, combination therapy with bisphosphonates and PTH (1-34) is not synergistic in increasing bone density; bisphosphonates appear to blunt the effect of PTH (1-34). To begin to understand the mechanism governing the effects of these agents on osteoblasts and a possible explanation for their apparent antagonism, we examined the expression of several bone morphogenetic proteins (BMPs) in MC3T3-E1 preosteoblastic cells either untreated, or treated with alendronate, parathyroid hormone, or a combination of the two agents. We find by reverse transcriptase-polymerase chain reaction (RT-PCR) that while alendronate fails to induce the expression of any of the BMPs tested, several BMPs are induced by PTH (1-34). The induction of the PTH (1-34)-inducible BMPs is blocked with simultaneous alendronate treatment. These data suggest that alendronate interferes with PTH (1-34)-induced BMP gene transcription and provides a possible basis for the antagonism observed between the two agents in increasing bone density.

Effect of thrombin peptide 508 (TP508) on bone healing during distraction osteogenesis in rabbit tibia

Thrombin-related peptide 508 (TP508) accelerates bone regeneration during distraction osteogenesis (DO). We have examined the effect of TP508 on bone regeneration during DO by immunolocalization of Runx2 protein, a marker of osteoblast differentiation, and of osteopontin (OPN) and bone sialoprotein (BSP), two late markers of the osteoblast lineage. Distraction was performed in tibiae of rabbits over a period of 6 days. TP508 (30 or 300 μg) or vehicle was injected into the distraction gap at the beginning and end of the distraction period. Two weeks after active distraction, tissue samples were harvested and processed for immunohistochemical analysis. We also tested the in vitro effect of TP508 on Runx2 mRNA expression in osteoblast-like (MC3T3-E1) cells by polymerase chain reaction analysis. Runx2 and OPN protein were observed in preosteoblasts, osteoblasts, osteocytes of newly formed bone, blood vessel cells and many fibroblast-like cells of the soft connective tissue. Immunostaining for BSP was more restricted to osteoblasts and osteocytes. Significantly more Runx2- and OPN-expressing cells were seen in the group treated with 300 μg TP508 than in the control group injected with saline or with 30 μg TP508. However, TP508 failed to increase Runx2 mRNA levels significantly in MC3T3-E1 cells after 2–3 days of exposure. Our data suggest that TP508 enhances bone regeneration during DO by increasing the proportion of cells of the osteoblastic lineage. Clinically, TP508 may shorten the healing time during DO; this might be of benefit when bone regeneration is slow.

Callus formation enhanced by BMP-7 ex vivo gene therapy during distraction osteogenesis in rats

This study was designed to observe the effects of bone morphogenetic protein-7 (BMP-7) ex vivo gene therapy on callus formation during rat mandibular distraction osteogenesis (DO). Fifty-four Sprague-Dawley rats underwent osteodistraction of the right mandible and were then randomly divided into three groups. Immediately after distraction, autologous bone marrow mesenchymal stem cells (MSCs) transfected with BMP-7, MSCs untransfected with BMP-7, and physiological saline were injected into the distraction gaps of the mandibles in groups A, B, and C, respectively. Nine animals from each group were euthanized at 2 and 6 weeks after completion of distraction. The distracted mandibles were removed and processed for radiographic, histological, immunohistochemical, and scanning electron microscopic examinations as well as Ca/P ratio analysis. Group A animals showed greater bone formation and earlier mineralization in the distracted callus when compared with group B. Similarly increased callus formation was found in group B than group C. Positive immunostaining of BMP-7 was observed in the distracted callus in all groups. However, BMP-7 expression was much stronger in group A compared with groups B and C. The results of this study suggest that BMP-7-mediated ex vivo gene transfer based on MSCs may accelerate callus formation in distraction osteogenesis and facilitate consolidation. Local gene therapy may ultimately be an alternative or supplemental approach to DO enhancement, especially for patients whose osteogenic potentials are compromised by diseases such as osteoporosis, severe trauma, and postoncologic irradiation.

Expression of RUNX2 isoforms: involvement of cap-dependent and cap-independent mechanisms of translation

RUNX2, a major regulator of skeletogenesis, is expressed as type-I and type-II isoforms. Whereas most eukaryotic mRNAs are translated by the cap-dependent scanning mechanism, translation of many mRNAs including type-I and type-II RUNX2 mRNAs has been reported to be initiated by a cap independent internal ribosomal entry site (IRES). Since the dicistronic plasmid assay used to demonstrate IRES has been questioned, we investigated the presence of IRES in RUNX2 mRNAs using dicistronic plasmid and mRNA assays. Our results show that the dicistronic plasmid assay cannot be used to demonstrate IRES in RUNX2 mRNAs because the intercistronic region of dicistronic plasmids containing the 5'-UTRs of both RUNX2 mRNAs operates as a cryptic promoter. In dicistronic mRNA transfection studies the 5'-UTRs of both RUNX2 mRNAs exhibited no IRES activity. When transfected into osteoblastic cells, monocistronic reporter mRNA preceded by the 5'-UTR of type-II RUNX2 (Type-II-FLuc-A100) was translated to a high degree only in the presence of a functional cap (m(7)GpppG); in contrast, luciferase mRNA preceded by the 5'-UTR of type-I RUNX2 mRNA (Type-I-FLuc-A100) was translated poorly in the presence of either m(7)GpppG or a nonfunctional cap (ApppG). Notably, in transfected cells inhibitors of cap-dependent translation suppressed the translation of m(7)GpppG-capped Type-II-FLuc-A100, but not ApppG-capped reporter mRNA preceded by the IRES-containing hepatitis C virus (HCV) 5'-UTR. Our study demonstrates that type-II RUNX2 mRNA is translated by the cap-dependent mechanism. Although efficient translation of type-I RUNX2 mRNA appears to require a process other than cap-dependent, the mechanism of type-I RUNX2 mRNA translation remains to be resolved.

Parathyroid hormone controls receptor activator of NF-kappaB ligand gene expression via a distant transcriptional enhancer

RANKL, a protein essential for osteoclast development and survival, is stimulated by parathyroid hormone (PTH) via a PTH receptor 1/cyclic AMP (cAMP)/protein kinase A (PKA)/CREB cascade, exclusively in osteoblastic cells. We report that a bacterial artificial chromosome-based transcriptional reporter construct containing 120 kb of RANKL 5'-flanking region was stimulated by dibutyryl-cAMP in stromal/osteoblastic cells, but not other cell types. Full cAMP responsiveness was dependent upon a conserved 715-bp region located 76 kb upstream from the transcription start site, which we identified by sequential deletion analysis and by comparison of human and mouse genomic sequences in silico. This region contained conserved consensus sequences which bound CREB and the osteoblast-specific transcription factor Runx2, and when mutated blunted cAMP responsiveness. Overexpression of Runx2 potentiated cAMP responsiveness of the endogenous RANKL gene in a cell-type-specific manner. Lastly, PTH responsiveness of the endogenous RANKL gene was abrogated in mice from which we deleted this conserved upstream region. Thus, PTH responsiveness of the RANKL gene is determined by a distant regulatory region that responds to cAMP in a cell-type-specific manner and Runx2 may contribute to such cell-type specificity.

Osteogenic differentiation of human mesenchymal stem cells is regulated by bone morphogenetic protein-6

Bone marrow-derived mesenchymal stem cells (MSC) are multipotent, self-renewing, mesodermal-origin stem cells that are sequestered in the endosteal compartment. MSC are maintained in a relative state of quiescence in vivo but in response to a variety of physiological and pathological stimuli, proliferate and differentiate into osteoblasts, chondrocytes, adipocytes, or hematopoiesis-supporting stromal cells. Little is understood regarding the cellular or molecular events underlying MSC fate decisions. We report that human MSC (hMSC) cultured in defined, serum-free conditions respond to a narrow spectrum of growth factors with osteogenic commitment, differentiation, and hydroxyapatite deposition. Of the osteogenic factors we examined, only treatment with bone morphogenetic protein (BMP) results in osteoinduction under defined serum-free conditions. Among BMP-2, 4, 6, and 7, BMP-6 is the most consistent and potent regulator of osteoblast differentiation and, of these BMPs, only BMP-6 gene expression is detected prior to hMSC osteoblast differentiation. Addition of exogenous BMP-6 to hMSC induces the expression or upregulation of a repertoire of osteoblast-related genes including type I collagen, osteocalcin, bone sialoprotein, and their regulatory transcription factors Cbfa1/Runx2, and Osterix. This translates into increased production of osteogenic extracellular matrix (ECM) with subsequent hydroxyapatite deposition.

Where tendons and ligaments meet bone: attachment sites ('entheses') in relation to exercise and/or mechanical load

Entheses (insertion sites, osteotendinous junctions, osteoligamentous junctions) are sites of stress concentration at the region where tendons and ligaments attach to bone. Consequently, they are commonly subject to overuse injuries (enthesopathies) that are well documented in a number of sports. In this review, we focus on the structure-function correlations of entheses on both the hard and the soft tissue sides of the junction. Particular attention is paid to mechanical factors that influence form and function and thus to exploring the relationship between entheses and exercise. The molecular parameters indicative of adaptation to mechanical stress are evaluated, and the basis on which entheses are classified is explained. The application of the 'enthesis organ' concept (a collection of tissues adjacent to the enthesis itself, which jointly serve the common function of stress dissipation) to understanding enthesopathies is considered and novel roles of adipose tissue at entheses are reviewed. A distinction is made between different locations of fat at entheses, and possible functions include space-filling and proprioception. The basic anchorage role of entheses is considered in detail and comparisons are explored between entheses and other biological 'anchorage' sites. The ability of entheses for self-repair is emphasized and a range of enthesopathies common in sport are reviewed (e.g. tennis elbow, golfer's elbow, jumper's knee, plantar fasciitis and Achilles insertional tendinopathies). Attention is drawn to the degenerative, rather than inflammatory, nature of most enthesopathies in sport. The biomechanical factors contributing to the development of enthesopathies are reviewed and the importance of considering the muscle-tendon-bone unit as a whole is recognized. Bony spur formation is assessed in relation to other changes at entheses which parallel those in osteoarthritic synovial joints.

Runx2-mediated regulation of the zinc finger Osterix/Sp7 gene

The zinc finger transcription factor Osterix (Osx) regulates bone formation and osteoblast differentiation in vitro and in vivo. We investigated the transcriptional mechanisms underlying the mouse Osx expression by isolating and characterizing its 5' upstream region. We performed 5' RACE on mRNA isolated from murine chondroprogenitor cells and determined a cap site of Osx approximately -99 nucleotides upstream of the initiation codon. Sequence analysis of this TATA-less promoter shows several putative response elements for Sox9, VDRE, Runx and Sp1. Transfection of the Osx promoter driving the luciferase reporter gene into C3H10T1/2 and ATDC5 cells shows a strong basal promoter activity between 565 bp and 2 kb. Deletion mutant analyses show that the most proximal 852 kb of the Osx promoter contains the highest activating domains, while strong repressive domains were identified between 1.8 and 2 kb. Over-expression experiments indicate that Runx2 significantly transactivates the Osx promoter by at least 2 fold indicating that Osx is downstream of Runx2 in mesenchymal cells. This up-regulation was abrogated when the Runx2 responsive element on the Osx promoter was mutated. Finally, we show that Runx2 specifically binds to this DNA element in the Osx promoter. Thus our results show for the first time Osx transcriptional regulation through the bone and cartilage related transcription factor Runx2.

Multiple mechanisms are involved in inhibition of osteoblast differentiation by PTHrP and PTH in KS483 Cells

We examined the mechanism by which PTHrP and PTH inhibit KS483 osteoblastic differentiation. We show that PTHrP and PTH inhibit differentiation downstream of early BMP signaling and downregulated components of the hedgehog (Hh) signaling cascade. In addition, PTHrP and PTH repressed RunX2 and osx expression. Overexpression of either gene, however, could not relieve PTHrP and PTH's inhibitory actions. Our data suggest that multiple parallel mechanisms are involved in the inhibition of osteoblast differentiation and matrix mineralization by PTHrP and PTH.

INTRODUCTION

PTH-related peptide (PTHrP) and PTH are potent inhibitors of osteoblast differentiation in vitro by as yet unexplained mechanisms.

MATERIALS AND METHODS

We treated murine bone marrow stromal cells and the mesenchymal progenitor cell line KS483 with PTHrP and PTH in combination with either BMPs or hedgehog (Hh) and measured early and late markers of osteoblast differentiation and studied the expression of RunX2 and Osterix (osx). In addition, we examined the PTHrP and PTH response in stable KS483 cells overexpressing either RunX2 or osx.

RESULTS

PTHrP and PTH inhibited BMP- and Hh-induced osteogenesis downstream of early BMP signaling and by downregulation of components of the Hh signaling cascade. PTHrP and PTH prevented the upregulation of RunX2 expression associated with osteoblast differentiation in an indirect response. However, PTHrP and PTH could still inhibit differentiation, and particularly matrix mineralization, of cells expressing RunX2. In addition, PTHrP and PTH potently downregulated osx expression only in mature osteoblasts in an intermediate early response, but osx overexpression could not relieve the inhibitory effects of PTHrP and PTH on matrix mineralization.

CONCLUSIONS

Our data suggest that, besides transcriptional repression of RunX2 and osx, other mechanisms in parallel with or downstream of RunX2 and osx are involved in the inhibition of osteoblast differentiation and matrix mineralization by PTHrP and PTH in vitro.

Bone morphogenetic protein-6 promotes osteoblastic prostate cancer bone metastases through a dual mechanism

Prostate cancer frequently metastasizes to bone where it forms osteoblastic lesions through unknown mechanisms. Bone morphogenetic proteins (BMP) are mediators of skeletal formation. Prostate cancer produces a variety of BMPs, including BMP-6. We tested the hypothesis that BMP-6 contributes to prostate cancer-induced osteosclerosis at bone metastatic sites. Prostate cancer cells and clinical tissues produced BMP-6 that increased with aggressiveness of the tumor. Prostate cancer-conditioned medium induced SMAD phosphorylation in the preosteoblast MC3T3 cells, and phosphorylation was diminished by anti-BMP-6 antibody. Prostate cancer-conditioned medium induced mineralization of MC3T3 cells, which was blocked by both the BMP inhibitor noggin and anti-BMP-6. Human fetal bones were implanted in severe combined immunodeficient mice and after 4 weeks, LuCaP 23.1 prostate cancer cells were injected both s.c. and into the bone implants. Anti-BMP-6 or isotype antibody administration was then initiated. Anti-BMP-6 reduced LuCaP 23.1-induced osteoblastic activity, but had no effect on its osteolytic activity. This was associated with increased osteoblast numbers and osteoblast activity based on bone histomorphometric evaluation. As endothelin-1 has been implicated in bone metastases, we measured serum endothelin-1 levels but found they were not different among the treatment groups. In addition to decreased bone production, anti-BMP-6 reduced intraosseous, but not s.c., tumor size. We found that BMP-2, BMP-4, BMP-6, and BMP-7 had no direct effect on prostate cancer cell growth, but BMP-2 and BMP-6 increased the in vitro invasive ability of prostate cancer cell. These data show that prostate cancer promotes osteoblastic activity through BMP-6 and that, in addition to its bone effects, suggest that BMPs promote the ability of the prostate cancer cells to invade the bone microenvironment.

Relationship of bone morphogenetic protein expression during osteoblast differentiation to wild type p53

We have previously shown p53 to have a specific role in osteoblast differentiation by its ability to regulate expression of certain bone specific proteins. In this study, we show mineralized matrix formation in vivo to be directly related to the presence of wild type p53 in osteoblastic osteosarcoma cells. In order to further understand the importance of p53 in differentiation, we investigated the relationship between p53 and Bone Morphogenetic Proteins (BMPs) (BMP 1, 2, 3A, 3B (GDF-10), 4, 5, 6, 7, 8A and 8B) during osteoblast differentiation. The expression of several BMPs were tested using RNase Protection Assay in differentiating ROS17/2.8 osteoblastic osteosarcoma cells. The expression of BMPs 1, 2, 3a, 3b and 7 showed time dependent modulation during in vitro differentiation. In order to determine if p53 has a role in this process, we used a murine osteosarcoma cell line stably expressing a temperature sensitive p53. Cells were exposed to ascorbic acid and glycerophosphates to hasten in vitro osteoblast differentiation and maintained either at 32 or 37 degrees C for expression of the wild type or mutant p53 phenotype. The expression of BMP-2, BMP-4 and BMP-7 were modulated in a p53 dependent fashion. We were able to confirm the p53 dependency of BMP-2 independently by RT-PCR. While BMP-2 expression was evident in the presence of both wild type and mutant p53, regulated expression was seen only in cells expressing wild type p53. Transient over expression of wild type p53 did not result in the same BMP-2 response as stable expression showing that the presence of p53 may be important for an orderly development of osteoblast differentiation rather than a direct effect on gene expression. The functional relationship between p53 and these bone specific markers is discussed.

Cartilage-derived morphogenetic proteins induce osteogenic gene expression in the C2C12 mesenchymal cell line

Cartilage-derived morphogenetic protein-1, -2, and -3 (CDMP-1, -2, and -3) are members of the bone morphogenetic protein (BMP) family and have been shown to exhibit a variety of biological activities. In the present study, effects of these CDMPs on the temporal and spatial expression of genes in the pluripotent mesenchymal cell line C2C12 were examined. Cells cultured in the presence of CDMPs lost the characteristic elongated shape of myoblasts. At the molecular level, CDMP treatment did not change the mRNA expression of MyoD, aggrecan, Six1, and tendin. Scleraxis mRNA level was reduced by CDMP treatment. CDMP-1 and -3, but not CDMP-2, stimulated expression of osteogenic markers, such as alkaline phosphatase (AP), osteocalcin (OC), BSP, and type I collagen, in a dose- and time-dependent manner. With few exceptions, the three CDMPs changed, with different potencies, the expression profile of different members of the BMP family in a similar temporal pattern. Except at the late phase of treatment, CDMP treatment did not change the expression of ActR-IA, BMPR-IA, BMPR-IB, BMPR-II, and ALK-7 mRNAs. Based on the current data, the CDMPs appear to be able to stimulate the C2C12 cells to differentiate into the osteoblast pathway.

Differential regulation of dentin sialophosphoprotein expression by Runx2 during odontoblast cytodifferentiation

Dentin sialophosphoprotein (DSPP) consists of dentin sialoprotein (DSP) and dentin phosphoprotein (DPP). The spatial-temporal expression of DSPP is largely restricted during differentiational stages of dental cells. DSPP plays a vital role in tooth development. It is known that an osteoblast-specific transcription factor, Runx2, is essential for osteoblast differentiation. However, effects of Runx2 on DSPP transcription remain unknown. Here, we studied different roles of Runx2 in controlling DSPP expression in mouse preodontoblast (MD10-F2) and odontoblast (MO6-G3) cells. Two Runx2 isoforms were expressed in preodontoblast and odontoblast cells, and in situ hybridization assay showed that DSPP expression increased, whereas Runx2 was down-regulated during odontoblast differentiation and maturation. Three potential Runx2 sites are present in promoters of mouse and rat DSPP genes. Runx2 binds to these sites as demonstrated by electrophoretic mobility shift assay and supershift experiments. Mutations of Runx2 sites in mouse DSPP promoter resulted in a decline of promoter activity in MD10-F2 cells compared with an increase of its activity in MO6-G3 cells. Multiple Runx2 sites were more active than a single site in regulating the DSPP promoter. Furthermore, forced overexpression of Runx2 isoforms induced increases of endogenous DSPP protein levels in MD10-F2 cells but reduced its expression in MO6-G3 cells consistent with the DSPP promoter analysis. Thus, our results suggest that differential positive and negative regulation of DSPP by Runx2 is dependent on use of cytodifferentiation of dental ectomesenchymal-derived cells that may contribute to the spatial-temporal expression of DSPP during tooth development.

Cell type-specific regulation of the TGF-beta-responsive alpha2(I) collagen gene by CpG methylation

The alpha2(I) collagen gene shows cell type-specific expression, however, the mechanism behind this specificity remains to be determined. We demonstrate here that transforming growth factor-beta (TGF-beta)-mediated induction of alpha2(I) collagen gene is regulated by DNA methylation in a cell type-specific manner. Human alpha2(I) collagen mRNA and type I collagen protein were expressed in normal human fibroblasts (NHF), and also strongly enhanced by TGF-beta; they were not detected in HaCaT, HeLa, or HepG2 cells (termed "collagen-induction resistant (CIR) cells") even following stimulation with TGF-beta. On the other hand, the transcriptional activity of exogenously transfected alpha2(I) collagen promoter was clearly up-regulated by TGF-beta in the CIR cells as well as in NHF. In the CIR cells, CpG clusters around the transcription start site of the alpha2(I) collagen gene were heavily methylated, whereas no methylation was detected in NHF. Moreover, alpha2(I) collagen gene was reactivated in the CIR cells by 5-Aza-2-deoxycytidine (5-AdC) treatment to some extent. However, demethylation by 5-AdC was limited and it was unable to recover the TGF-beta responsiveness. In NHF, the alpha2(I) collagen gene has a Smad3-accessible chromatin structure and acetylated histones in the promoter regions. By contrast, in the CIR cells, Smad3 failed to bind to the chromatin and histones were not acetylated in this area. Furthermore, in vitro methylation of the reporter gene containing the alpha2(I) collagen promoter significantly reduced both basal and TGF-beta-induced enhancement of the transcriptional activity in NHF. Thus, we propose that alpha2(I) collagen gene provides the first example of the TGF-beta responsive gene whose cell type-specificity is regulated by CpG methylation.

The bone-specific expression of Runx2 oscillates during the cell cycle to support a G1-related antiproliferative function in osteoblasts

The Runx2 (CBFA1/AML3/PEBP2alphaA) transcription factor promotes skeletal cell differentiation, but it also has a novel cell growth regulatory activity in osteoblasts. We addressed here whether Runx2 activity is functionally linked to cell cycle-related mechanisms that control normal osteoblast proliferation and differentiation. We found that the levels of Runx2 gene transcription, mRNA and protein, are each up-regulated with cessation of cell growth (i.e. G(0)/G(1) transition) in preconfluent MC3T3 osteoblastic cells that do not yet express mature bone phenotypic gene expression. Cell growth regulation of Runx2 is also observed in primary calvarial osteoblasts and other osteoblastic cells with relatively normal cell growth characteristics, but not in osteosarcoma cells (e.g. SAOS-2 and ROS17/2.8). Runx2 levels are cell cycle-regulated in MC3T3 cells with respect to the G(1)/S and M/G(1) transitions: oscillates from maximal expression levels during early G(1) to minimal levels during early S phase and mitosis. However, in normal or immortalized (e.g. ATDC5) chondrocytic cells, Runx2 expression is suppressed during quiescence, and Runx2 levels are not regulated during G(1) and S phase in ATDC5 cells. Antisense or small interfering RNA-mediated reduction of the low physiological levels of Runx2 in proliferating MC3T3 cells does not accelerate cell cycle progression. However, forced expression of Runx2 suppresses proliferation of MC3T3 preosteoblasts or C2C12 mesenchymal cells which have osteogenic potential. Forced elevation of Runx2 in synchronized MC3T3 cells causes a delay in G(1). We propose that Runx2 levels and function are biologically linked to a cell growth-related G(1) transition in osteoblastic cells.

Altered gene expression in human cleidocranial dysplasia dental pulp cells

Cleidocranial dysplasia (CCD) is an autosomal dominant disorder characterised by defects of bone and tooth development. The dental manifestations in CCD patients include supernumerary teeth, delayed tooth eruption, tooth hypoplasia and absence of cellular cementum formation. This disorder is associated with mutations in the osteoblast-specific transcription factor Runx2. To identify morphological and molecular alterations associated with CCD dental tissues, human primary dental pulp cell cultures were established from age- and sex-matched CCD and normal patients. Dental pulp cells were compared for general morphology, proliferation rates, and gene expression profiles using cDNA microarray technology. CCD pulp cells were about four-fold larger than normal cells, however the normal pulp proliferation rates were two- and three-fold greater at time points tested than the CCD cells. Of the 226 genes analysed by blot microarray, 18.6% displayed significant differences at least two-fold in expression levels. This includes 25 genes (11.1%) that were up-regulated, while 17 (7.5%) that were down-regulated in the CCD cells as compared to the normal cells. Expression of selected genes was further verified by quantitative real-time polymerase chain reaction (qRT-PCR). Comparison between the CDD and normal cells revealed that gene expression of cytokines and growth factors, such as leukemia inhibitory factor (LIF), interleukin-6 (IL-6) and transforming growth factor beta receptor II (TGF-betaRII) and vascular endothelial growth factor B (VEGFB) were higher while bone morphogenetic protein 2 (BMP2) was lower in the CCD cells. Furthermore, potential Runx2 binding sites were found in all putative target gene promoters. This study suggests that in addition to bone and tooth cell differentiation, Runx2 may be involved in controlling cell growth during tooth development.

Terminal osteoblast differentiation, mediated by runx2 and p27KIP1, is disrupted in osteosarcoma

The molecular basis for the inverse relationship between differentiation and tumorigenesis is unknown. The function of runx2, a master regulator of osteoblast differentiation belonging to the runt family of tumor suppressor genes, is consistently disrupted in osteosarcoma cell lines. Ectopic expression of runx2 induces p27^KIP1, thereby inhibiting the activity of S-phase cyclin complexes and leading to the dephosphorylation of the retinoblastoma tumor suppressor protein (pRb) and a G1 cell cycle arrest. Runx2 physically interacts with the hypophosphorylated form of pRb, a known coactivator of runx2, thereby completing a feed-forward loop in which progressive cell cycle exit promotes increased expression of the osteoblast phenotype. Loss of p27^KIP1 perturbs transient and terminal cell cycle exit in osteoblasts. Consistent with the incompatibility of malignant transformation and permanent cell cycle exit, loss of p27^KIP1 expression correlates with dedifferentiation in high-grade human osteosarcomas. Physiologic coupling of osteoblast differentiation to cell cycle withdrawal is mediated through runx2 and p27^KIP1, and these processes are disrupted in osteosarcoma.

Control of RUNX2 isoform expression: The role of promoters and enhancers

The three mammalian RUNX genes constitute the family of runt domain transcription factors that are involved in the regulation of a number of developmental processes such as haematopoiesis, osteogenesis and neuronal differentiation. All three genes show a complex temporo-spatial pattern of expression. Since the three proteins are probably mutually interchangeable with regard to function, most of the specificity of each family member seems to be based on a tightly controlled regulation of expression. While RUNX gene expression is driven by two promoters for each gene, the promoter sequence alone does not seem to suffice for a proper expressional control. This review focuses on the available evidence for the existence of such control mechanisms and studies aiming at discovering cis-acting regulatory sequences of the RUNX2 gene.

Runx2: A master organizer of gene transcription in developing and maturing osteoblasts

Runx2 is essential for osteoblast development and proper bone formation. A member of the Runt domain family of transcription factors, Runx2 binds specific DNA sequences to regulate transcription of numerous genes and thereby control osteoblast development from mesenchymal stem cells and maturation into osteocytes. Although necessary for gene transcription and osteoblast development, Runx2 is not sufficient for optimal gene expression or bone formation. Runx2 cooperates with numerous proteins, including transcription factors and cofactors, is posttranslationally modified, and associates with the nuclear matrix to integrate a variety of signals and organize crucial events during osteoblast development and maturation. Consistent with its role as a master organizer, alterations in Runx2 expression levels are associated with skeletal diseases. Runx2 haploinsufficiency causes cleidocranial dysplasia, while Runx2 overexpression is common in many bone-metastatic cancers. In this review, we summarize the molecular mechanisms by which Runx2 integrates signals through coregulatory interactions, and discuss how its role as a master organizer may shift depending on promoter structure, developmental cues, and cellular context.

The promyelotic leukemia zinc finger promotes osteoblastic differentiation of human mesenchymal stem cells as an upstream regulator of CBFA1

Ossification of the posterior longitudinal ligament of the spine (OPLL) is the leading cause of myelopathy in Japan and is diagnosed by ectopic bone formation in the paravertebral ligament. OPLL is a systemic high bone mass disease with a strong genetic background. To detect genes relevant to the pathogenesis of OPLL, we performed a cDNA microarray analysis of systematic gene expression profiles during the osteoblastic differentiation of ligament cells from OPLL patients (OPLL cells), patients with a disorder called ossification of yellow ligament (OYL), and non-OPLL controls together with human mesenchymal stem cells (hMSCs) after stimulating them with osteogenic differentiation medium (OS). Twenty-four genes were up-regulated during osteoblastic differentiation in OPLL cells. Zinc finger protein 145 (promyelotic leukemia zinc finger or PLZF) was one of the highly expressed genes during osteoblastic differentiation in all the cells examined. We investigated the roles of PLZF in the regulation of osteoblastic differentiation of hMSCs and C2C12 cells. Small interfering RNA-mediated gene silencing of PLZF resulted in a reduction in the expression of osteoblast-specific genes such as the alkaline phosphatase, collagen 1A1 (Col1a1), Runx2/core-binding factor 1 (Cbfa1), and osteocalcin genes, even in the presence of OS in hMSCs. The expression of PLZF was unaffected by the addition of bone morphogenetic protein 2 (BMP-2), and the expression of BMP-2 was not affected by PLZF in hMSCs. In C2C12 cells, overexpression of PLZF increased the expression of Cbfa1 and Col1a1; on the other hand, the overexpression of CBFA1 did not affect the expression of Plzf. These findings indicate that PLZF plays important roles in early osteoblastic differentiation as an upstream regulator of CBFA1 and thereby might participate in promoting the ossification of spinal ligament cells in OPLL patients.

Aged mice require full transcription factor, Runx2/Cbfa1, gene dosage for cancellous bone regeneration after bone marrow ablation

Runx2 is prerequisite for the osteoblastic differentiation in vivo. To elucidate Runx2 gene functions in adult bone metabolism, we conducted bone marrow ablation in Runx2 heterozygous knockout mice and found that aged (but not young) adult Runx2 heterozygous knockout mice have reduced new bone formation capacity after bone marrow ablation. We also found that bone marrow cells from aged Runx2 heterozygous knockout mice have reduced ALP(+) colony-forming potential in vitro. This indicates that full Runx2 dosage is needed for the maintenance of osteoblastic activity in adult mice.

INTRODUCTION

Null mutation of the Runx2 gene results in total loss of osteoblast differentiation, and heterozygous Runx2 deficiency causes cleidocranial dysplasia in humans and mice. However, Runx2 gene functions in adult bone metabolism are not known. We therefore examined the effects of Runx2 gene function in adult mice with heterozygous loss of the Runx2 gene.

MATERIALS AND METHODS

Bone marrow ablation was conducted in young adult (2.5 +/- 0.5 months old) or aged adult (7.5 +/- 0.5 months old) Runx2 heterozygous knockout mice and wildtype (WT) littermates. Cancellous bone regeneration was evaluated by 2D microCT.

RESULTS

Although new bone formation was observed after bone marrow ablation in the operated bone marrow cavity of WT mice, such bone formation was significantly reduced in Runx2 heterozygous knockout mice. Interestingly, this effect was observed specifically in aged but not young adult mice. Runx2 heterozygous deficiency in aged mice significantly reduced the number of alkaline phosphatase (ALP)(+) cell colonies in the bone marrow cell cultures, indicating a reduction in the numbers of osteoprogenitor cells. Such effects of heterozygous Runx2 deficiency on osteoblasts in vitro was specific to the cells from aged adult mice, and it was not observed in the cultures of marrow cells from young adult mice.

CONCLUSION

These results indicate that full gene dosage of Runx2 is required for cancellous bone formation after bone marrow ablation in adult mice.

Effects of osteogenic protein-1 (OP-1, BMP-7) on gene expression in cultured medial collateral ligament cells

Osteogenic protein-1 (OP-1, also called BMP-7), a member of the BMP family and the TGF-beta superfamily, induces formation of new bone and cartilage, but also regulates a wide array of processes. In the present study, the expression of several characteristic biochemical markers of ligaments, such as Six1, Scleraxis, aggrecan, and type I collagen in primary cultures of adult rat medial collateral ligament (MCL) cells was determined. The effects of OP-1 on cell proliferation and on gene expression were subsequently examined. OP-1 stimulated cell proliferation, alkaline phosphatase (AP) activity, and the steady-state mRNA levels of the transcription factor Runx2/Cbfa1 in a dose- and time-dependent manner. The mRNA levels of type I collagen only increased slightly, but the activity of the cloned collagen promoter increased by 2-fold in transiently transfected MCL cells. OP-1 also stimulated aggrecan mRNA expression. The mRNA levels of Six1 and Scleraxis were not detectably altered by OP-1. In control cultures, the steady-state mRNA levels of ActR-I, BMPR-IA, BMPR-IB, and BMPR-II increased as a function of time in culture. The mRNA levels of BMP-1 and -4 increased significantly after 12 days, but those of BMP-2 and -6 did not change. The GDF-1, -3, -5, -6, and -8 mRNA levels in the control cultures also increased as a function of time. OP-1 treatment stimulated mRNA expression of BMPR-IA and BMPR-II, but had little effect on ActR-I and BMPR-IB mRNA expression. OP-1 lowered the BMP-1, -2, and -6 mRNA levels without changing the BMP-4 mRNA level. OP-1 treatment also reduced the mRNA levels of GDFs detected. In summary, the present study demonstrated that OP-1 stimulated cell proliferation and mRNA expression of several biochemical markers in this ligament cell culture model and established the spatial and temporal appearance of several members of the TGF-beta superfamily.

Cbfa1 couples chondrocytes maturation and endochondral ossification in rat mandibular condylar cartilage

Core binding factor a1 (Cbfa1) is a crucial transcription factor for osteoblasts differentiation and chondrocytes maturation in embryonic skeletal genesis, but little is known about its function in mandibular condylar growth. The aim of this study was to determine the temporal and spatial pattern of Cbfa1 expression in condylar cartilage during natural growth. Mandibular condyles were harvested from 50 female Sprague-Dawley rats at age of 38, 42, 49, 56 and 65 days. Alcian blue and PAS staining was used for histological analysis. Type A antibody raised against Cbfa1 isoform II was observed in the pre-hypertrophic and hypertrophic chondrocytes in condylar cartilage, and in the mature osteocytes in trabecular bone. Type B antibody raised against 17 aa sequence present after the Runt domain was detected in tartrate resistant acid phosphatase (TRAP) positive osteoclasts in the erosive front of cartilage, and also in the osteoblasts on the sub-chondral bone surface. In situ hybridisation was carried out with a probe containing a fragment in exon 8 of the cDNA. Cbfa1 transcripts were localised in the osteoblasts and chondrocytes, but not in osteoclasts. Quantitative analysis demonstrated that both types of Cbfa1 proteins reached their maximum level on day 56, which coincided with the terminal maturation of hypertrophic chondrocytes and the aggregation of mineralisation deposits in extracellular matrix. These results suggest that Cbfa1 is a master gene controlling the functions of all the skeletal cell lineages by synthesising different functional isoforms. Furthermore, Cbfa1 couples the process of chondrocytes maturation, extracellular matrix mineralisation and degradation, as well as osteoblasts invasion during endochondral bone formation. Beyond its function on embryonic development, Cbfa1 regulates the postnatal growth of mandibular condyle.

RUNX2 alleles associated with BMD in Scottish women; interaction of RUNX2 alleles with menopausal status and body mass index

Bone mineral density (BMD) is influenced by both environmental and genetic factors. We previously reported the association of the RUNX2 A allele with increased bone mineral density (BMD) and protection against a common form of osteoporotic fracture within a Geelong population. We genotyped 991 women from a Scottish cohort to decipher the role of RUNX2 alleles in regulating BMD. The alleles of RUNX2 within the glutamine-alanine repeat were determined by MspA1I restriction digest. Allele frequencies estimated from Scottish cohort were G allele, 0.87 +/- 0.01; A allele, 0.08 +/- 0.01; and 11Ala alanine deletion allele, 0.05 +/- 0.01. Analysis of covariance (ANCOVA) was used to adjust for the covariates weight and age for BMD at the femoral neck (FN). The A allele was associated with higher FN BMD (P = 0.035) within a postmenopausal subgroup of the population (n = 312). The effect of RUNX2 A alleles increased with increasing weight; A alleles were associated with FN BMD in those above the median BMI (BMI > 25), while no association was observed in thin/normal (BMI </= 25) postmenopausal women. Glutamine variants and an alanine insertion were identified within the group. These data suggest that the RUNX2 alleles are associated with BMD in a menopause- and weight-dependent manner.

Selective Deficiency of the “Bone-related” Runx2-II Unexpectedly Preserves Osteoblast-mediated Skeletogenesis

Runx2 (runt-related transcription factor 2) is a master regulator of skeletogenesis. Distinct promoters in the Runx2 gene transcribe the "bone-related" Runx2-II and non-osseous Runx2-I isoforms that differ only in their respective N termini. Existing mutant mouse models with both isoforms deleted exhibit an arrest of osteoblast and chondrocyte maturation and the complete absence of mineralized bone, but they do not distinguish the separate functions of the two N-terminal isoforms. To elucidate the function of the bone-related isoform, we generated selective Runx2-II-deficient mice by the targeted deletion of the distal promoter and exon 1. Homozygous Runx2-II-deficient (Runx2-II(-/-)) mice unexpectedly formed axial, appendicular, and craniofacial bones derived from either intramembranous ossification or mesenchymal cells of the bone collar, but they failed to form the posterior cranium and other bones derived from endochondral ossification. Heterozygous Runx2-II-deficient mice had grossly normal skeletons, but were osteopenic. The commitment of mesenchymal cells ex vivo to the osteoblast lineage occurred in Runx2-II(-/-) mice, but osteoblastic gene expression was impaired. Chondrocyte maturation appeared normal, but the zone of hypertrophic chondrocytes was not transformed into metaphyseal bone, leading to widened growth plates in Runx2-II(-/-) mice. Compensatory increments in Runx2-I expression occurred in Runx2-II(-/-) mice but were not sufficient to normalize osteoblastic maturation or transcriptional activity. Our findings support distinct functions of Runx2-II and -I in the control of skeletogenesis. Runx2-I is sufficient for early osteoblastogenesis and intramembranous bone formation, whereas Runx2-II is necessary for complete osteoblastic maturation and endochondral bone formation.

Osteogenic protein-1 (OP-1, BMP-7) induces osteoblastic cell differentiation of the pluripotent mesenchymal cell line C2C12

The effects of Osteogenic Protein-1 (OP-1, BMP-7) on the differentiation of the pluripotent mesenchymal cell line, C2C12, were examined. OP-1 at 50 ng/ml partially inhibited myotube formation in C2C12 cells, while OP-1 at 200 ng/ml completely inhibited myotube formation and induced the formation of cells displaying osteoblastic morphology. High concentrations of OP-1 elevated the alkaline phosphatase (AP) activity dramatically, both as a function of time and OP-1 concentration. Osteocalcin (OC) mRNA expression was detected as early as 8 days in OP-1-treated cultures and subsequently increased considerably. Expression of bone sialoprotein (BSP) mRNA was low in control cultures and stimulated by OP-1. Collagen type I mRNA expression was enhanced by OP-1 during the early days in culture, but gradually decreased thereafter. MyoD mRNA expression, high in control cultures, was suppressed by OP-1 in a dose- and time-dependent manner. OP-1 enhanced ActR-I mRNA expression and significantly elevated the mRNA expressions of BMP-1, BMP-4, BMP-5, GDF-6, and GDF-8. The present results indicate that OP-1 is a potent inducer of C2C12 differentiation into osteoblastic cells.

Functional Notch signaling is required for BMP4-induced inhibition of myogenic differentiation

The bone morphogenetic protein (BMP) and Notch signaling pathways are crucial for cellular differentiation. In many cases, the two pathways act similarly; for example, to inhibit myogenic differentiation. It is not known whether this inhibition is caused by distinct mechanisms or by an interplay between Notch and BMP signaling. Here we demonstrate that functional Notch signaling is required for BMP4-mediated block of differentiation of muscle stem cells, i.e. satellite cells and the myogenic cell line C2C12. Addition of BMP4 during induction of differentiation dramatically reduced the number of differentiated satellite and C2C12 cells. Differentiation was substantially restored in BMP4-treated cultures by blocking Notch signaling using either the gamma-secretase inhibitor L-685,458 or by introduction of a dominant-negative version of the Notch signal mediator CSL. BMP4 addition to C2C12 cells increased transcription of two immediate Notch responsive genes, Hes1 and Hey1, an effect that was abrogated by L-685,458. A 3 kb Hey1-promoter reporter construct was synergistically activated by the Notch 1 intracellular domain (Notch 1 ICD) and BMP4. The BMP4 mediator SMAD1 mimicked BMP activation of the Hey1 promoter. A synthetic Notch-responsive promoter containing no SMAD1 binding sites responded to SMAD1, indicating that DNA-binding activity of SMAD1 is not required for activation. Accordingly, Notch 1 ICD and SMAD1 interacted in binding experiments in vitro. Thus, the data presented here provide evidence for a direct interaction between the Notch and BMP signaling pathways, and indicate that Notch has a crucial role in the execution of certain aspects of BMP-mediated differentiation control.

Prostate cancer bone metastases promote both osteolytic and osteoblastic activity

Advanced prostate cancer is frequently accompanied by the development of metastasis to bone. In the past, prostate cancer bone metastases were characterized as being osteoblastic (i.e., increasing bone density) based on radiographs. However, emerging evidence suggests that development of prostate cancer bone metastases requires osteoclastic activity in addition to osteoblastic activity. The complexities of how prostate tumor cells influence bone remodeling are just beginning to be elucidated. Prostate cancer cells produce a variety of pro-osteoblastic factors that promote bone mineralization. For example, both bone morphogenetic proteins and endothelin-1 have well recognized pro-osteoblastic activities and are produced by prostate cancer cells. In addition to factors that enhance bone mineralization prostate cancer cells produced factors that promote osteoclast activity. Perhaps the most critical pro-osteoclastogenic factor produced by prostate cancer cells is receptor activator of NFkappaB ligand (RANKL), which has been shown to be required for the development of osteoclasts. Blocking RANKL results in inhibiting prostate cancer-induced osteoclastogenesis and inhibits development and progression of prostate tumor growth in bone. These findings suggest that targeting osteoclast activity may be of therapeutic benefit. However, it remains to be defined how prostate cancer cells synchronize the combination of osteoclastic and osteoblastic activity. We propose that as the bone microenvironment is changed by the developing cancer, this in turn influences the prostate cancer cells' balance between pro-osteoclastic and pro-osteoblastic activity. Accordingly, the determination of how the prostate cancer cells and bone microenvironment crosstalk are important to elucidate how prostate cancer cells modulate bone remodeling.

Transcriptional regulation of the human Runx2/Cbfa1 gene promoter by bone morphogenetic protein-7

It is well established that core binding factor Runx2/Cbfa1 is required for osteoblast recruitment and differentiation from mesenchymal stem cells. Transcriptional regulation of the Runx2/Cbfa1 gene by osteogenic factors such as bone morphogenetic proteins (BMPs) plays an important role in the stimulation of bone formation by these cytokines. BMP7 (also termed OP-1) is a member of the transforming growth factor beta (TGF-beta) superfamily and induces osteoblast differentiation from mesenchymal precursor stem cells in vitro as well as bone formation in vivo. This study examines the effects of BMP7 on markers of osteoblast differentiation and specifically on human Runx2/Cbfa1 gene transcription in a mouse C2C12 myoblast cell line where it induces expression of both alkaline phosphatase (ALP) and endogenous Runx2/Cbfa1. To further understand the mechanisms of human Runx2/Cbfa1 transcriptional regulation by BMP7, we cloned 3.0 kb of the human Runx2/Cbfa1 gene 5'-upstream flanking region and created a series of promoter deletions cloned into luciferase-based reporter vectors (Runx2/Cbfa1/Luc). Sequence data revealed six copies of the osteoblastic cis-acting element (OSE2) in the proximal promoter region. In C2C12 cells transiently transfected with Runx2/Cbfa1/Luc deletion constructs, transcriptional activity of Runx2/Cbfa1 was upregulated up to 2-fold after 24 h of BMP7 treatment. Mutational analysis demonstrated that the minimal responsive promoter region for BMP7-regulated transcription maps to a proximal -74 OSE2 site. Electromobility shift assays with C2C12 cellular extracts indicate that BMP7 increases binding of OSE2 promoter sequences, and supershift assays with anti-Runx2/Cbfa1 antibodies demonstrate that Runx2/Cbfa1 is part of the nucleoprotein complex binding OSE2. Together, these data indicate BMP7 can upregulate Runx2/Cbfa1 gene expression in C2C12 myoblast cells, and suggest that Runx2/Cbfa1 may bind to OSE2 elements within its own promoter to autoregulate gene transcription in differentiating osteoblasts.

Bone morphogenetic protein-2 restores mineralization in glucocorticoid-inhibited MC3T3-E1 osteoblast cultures

The anti-glucocorticoid potential of BMP-2 in osteoblasts was tested in MC3T3-E1 cells using dexamethasone (1 microM) and rhBMP-2 (10 or 100 ng/ml). rhBMP-2 restored mineralization but not condensation or collagen accumulation. These results demonstrate the potential and limitations of BMPs in counteracting glucocorticoids.

INTRODUCTION

Pharmacologic glucocorticoids (GCs) inhibit osteoblast function and induce osteoporosis. Bone morphogenetic proteins (BMPs) stimulate osteoblast differentiation and bone formation. Here we tested the anti-glucocorticoid potential of BMP-2 in cultured osteoblasts.

MATERIALS AND METHODS

MC3T3-E1 cells were treated with dexamethasone (DEX; 1 microM) and/or recombinant human BMP-2 (rhBMP-2; 10 or 100 ng/ml). Culture progression was characterized by cell cycle profiling, biochemical assays for DNA, alkaline phosphatase (ALP), collagen, and calcium, and by reverse transcriptase-polymerase chain reaction (RT-PCR) of osteoblast phenotypic mRNAs. Mineralization was characterized by Alizarin red and von Kossa staining and by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD).

RESULTS

DEX inhibited differentiation-related cell cycle, nodule formation, collagen accumulation, osteocalcin, and BMP-2 gene expression as well as mineralization. Replenishment of GC-inhibited cultures with 10 or 100 ng/ml rhBMP-2 dramatically rescued mineral deposition. The rhBMP-2-rescued mineral was bone-like apatite nearly identical to the mineral of control cultures. The rhBMP-2 rescue was associated with increased mRNA levels for alpha1(I) collagen, osteocalcin, and Cbfa1 types I and II, as well as ALP activity. In contrast, rhBMP-2 did not rescue the GC-inhibited differentiation-related cell cycle, nodule formation, or collagen accumulation. When administered alone, rhBMP-2 also increased the mRNA levels for alpha1(I) collagen, osteocalcin, and Cbfa1 types I and II, as well as ALP activity. However, treatment with rhBMP-2 alone inhibited cell cycle progression, nodule formation, and collagen accumulation. Surprisingly, in contrast to its rescue of mineralization in DEX-treated cultures, rhBMP-2 inhibited mineralization in the absence of DEX. In parallel to its bimodal effect on mineralization, rhBMP-2 stimulated endogenous BMP-2 mRNA in the presence of DEX, but inhibited endogenous BMP-2 mRNA in the absence of DEX.

CONCLUSIONS

Suppression of BMP-2 gene expression plays a pivotal role in GC inhibition of osteoblast differentiation. However, the inability of rhBMP-2 to rescue the entire osteoblast phenotype suggests BMP-2-independent inhibitory effects of CCs. BMP-2 exerts both positive and negative effects on osteoblasts, possibly depending on the differentiation stage and/or the existing BMP signaling.

Morphogenesis and dysmorphogenesis of the appendicular skeleton

Cartilage patterning and differentiation are prerequisites for skeletal development through endochondral ossification (EO). Multipotential mesenchymal cells undergo a complex process of cell fate determination to become chondroprogenitors and eventually differentiate into chondrocytes. These developmental processes require the orchestration of cell-cell and cell-matrix interactions. In this review, we present limb bud development as a model for cartilage patterning and differentiation. We summarize the molecular and cellular events and signaling pathways for axis patterning, cell condensation, cell fate determination, digit formation, interdigital apoptosis, EO, and joint formation. The interconnected nature of these pathways underscores the effects of genetic and teratogenic perturbations that result in skeletal birth defects. The topics reviewed also include limb dysmorphogenesis as a result of genetic disorders and environmental factors, including FGFR, GLI3, GDF5/CDMP1, Sox9, and Cbfa1 mutations, as well as thalidomide- and alcohol-induced malformations. Understanding the complex interactions involved in cartilage development and EO provides insight into mechanisms underlying the biology of normal cartilage, congenital disorders, and pathologic adult cartilage.

Spatial expression of Cbfa1/Runx2 isoforms in teeth and characterization of binding sites in the DSPP gene

Cbfa1/Runx2 is an essential transcription factor for osteoblast and odontoblast differentiation. Heterogeneous mutations of Cbfa1 gene result in cleidocranial dysplasia, an autosomal dominant syndrome, characterized by abnormal skeletal genesis and dental disorders. Recently three Cbfa1/Runx isoforms (Pebp2 alpha A/type I, til-1/type II, and Osf2/type III) have been identified that differ in their amino-terminal sequences. The precise roles of Cbfa1/Runx2 isoforms in odontoblast development are not known. The purpose of this study was to determine and compare expression patterns of the three Cbfa1/Runx2 isoforms in newborn tooth organs. Toward this aim, we developed three probes: type I and type II, which specifically hybridize with Pebp2 alpha A and til-1, respectively, and type II/III, which hybridizes with osf2 and partially with til-1. In addition, Cbfa1/Runx2 binding sites were identified in the regulatory elements of mouse dentin sialophosphoprotein (mDSPP) gene, which encodes a matrix protein expressed during odontogenesis. In situ hybridization performed with the specific Cbfa1/Runx2 isoform probes demonstrated that all isoforms are expressed in teeth and bone. The type I isoform was expressed at higher levels than isoforms type II and type II/III in developing newborn mouse incisors. Genomic mDSPP clones were isolated and characterized containing approximately 2.6 kb of the promoter region. Computer analysis of the promoter segment and intron 1 revealed a number of potential transcriptional factor binding sites including five Cbfa1/Runx2 binding sites, three in the promoter region and two within intron 1. DNA-protein assay and antibody supershift experiments showed that these binding sites interact with nuclear extracts isolated from the mouse odontoblast cell line MO6-G3. Further characterization of the functional role of Cbfa1/Runx2 in the regulation of the mDSPP gene expression is being investigated.