Overexpression of the transcriptional factor Runx2 in osteoblasts abolishes the anabolic effect of parathyroid hormone in vivo

Suture Cells in a Mechanical Stretching Niche: Critical Contributors to Trans-sutural Distraction Osteogenesis

Trans-sutural distraction osteogenesis has been proposed as an alternative technique of craniofacial remodelling surgery for craniosynostosis correction. Many studies have defined the contribution of a series of biological events to distraction osteogenesis, such as changes in gene expression, changes in suture cell behaviour and changes in suture collagen fibre characteristics. However, few studies have elucidated the systematic molecular and cellular mechanisms of trans-sutural distraction osteogenesis, and no study has highlighted the contribution of cell-cell or cell-matrix interactions with respect to the whole expansion process to date. Therefore, it is difficult to translate largely primary mechanistic insights into clinical applications and optimize the clinical outcome of trans-sutural distraction osteogenesis. In this review, we carefully summarize in detail the literature related to the effects of mechanical stretching on osteoblasts, endothelial cells, fibroblasts, immune cells (macrophages and T cells), mesenchymal stem cells and collagen fibres in sutures during the distraction osteogenesis process. We also briefly review the contribution of cell-cell or cell-matrix interactions to bone regeneration at the osteogenic suture front from a comprehensive viewpoint.

Anabolic actions of PTH in murine models: two decades of insights

Parathyroid hormone (PTH) is produced by the parathyroid glands in response to low serum calcium concentrations where it targets bones, kidneys, and indirectly, intestines. The N-terminus of PTH has been investigated for decades for its ability to stimulate bone formation when administered intermittently (iPTH) and is used clinically as an effective anabolic agent for the treatment of osteoporosis. Despite great interest in iPTH and its clinical use, the mechanisms of PTH action remain complicated and not fully defined. More than 70 gene targets in more than 90 murine models have been utilized to better understand PTH anabolic actions. Because murine studies utilized wild-type mice as positive controls, a variety of variables were analyzed to better understand the optimal conditions under which iPTH functions. The greatest responses to iPTH were in male mice, with treatment starting later than 12 weeks of age, a treatment duration lasting 5-6 weeks, and a PTH dose of 30-60 μg/kg/day. This comprehensive study also evaluated these genetic models relative to the bone formative actions with a primary focus on the trabecular compartment revealing trends in critical genes and gene families relevant for PTH anabolic actions. The summation of these data revealed the gene deletions with the greatest increase in trabecular bone volume in response to iPTH. These included PTH and 1-α-hydroxylase (Pth;1α(OH)ase, 62-fold), amphiregulin (Areg, 15.8-fold), and PTH related protein (Pthrp, 10.2-fold). The deletions with the greatest inhibition of the anabolic response include deletions of: proteoglycan 4 (Prg4, -9.7-fold), low-density lipoprotein receptor-related protein 6 (Lrp6, 1.3-fold), and low-density lipoprotein receptor-related protein 5 (Lrp5, -1.0-fold). Anabolic actions of iPTH were broadly affected via multiple and diverse genes. This data provides critical insight for future research and development, as well as application to human therapeutics. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Regulation of Proliferation, Differentiation and Functions of Osteoblasts by Runx2

Runx2 is essential for osteoblast differentiation and chondrocyte maturation. During osteoblast differentiation, Runx2 is weakly expressed in uncommitted mesenchymal cells, and its expression is upregulated in preosteoblasts, reaches the maximal level in immature osteoblasts, and is down-regulated in mature osteoblasts. Runx2 enhances the proliferation of osteoblast progenitors by directly regulating Fgfr2 and Fgfr3. Runx2 enhances the proliferation of suture mesenchymal cells and induces their commitment into osteoblast lineage cells through the direct regulation of hedgehog (Ihh, Gli1, and Ptch1), Fgf (Fgfr2 and Fgfr3), Wnt (Tcf7, Wnt10b, and Wnt1), and Pthlh (Pthr1) signaling pathway genes, and Dlx5. Runx2 heterozygous mutation causes open fontanelle and sutures because more than half of the Runx2 gene dosage is required for the induction of these genes in suture mesenchymal cells. Runx2 regulates the proliferation of osteoblast progenitors and their differentiation into osteoblasts via reciprocal regulation with hedgehog, Fgf, Wnt, and Pthlh signaling molecules, and transcription factors, including Dlx5 and Sp7. Runx2 induces the expression of major bone matrix protein genes, including Col1a1, Spp1, Ibsp, Bglap2, and Fn1, in vitro. However, the functions of Runx2 in differentiated osteoblasts in the expression of these genes in vivo require further investigation.

ClC-3 chloride channel mediates the role of parathyroid hormone [1-34] on osteogenic differentiation of osteoblasts

INTRODUCTION

Different concentrations of parathyroid hormone [1-34] (PTH [1-34]) can have totally opposite effects on osteoblasts. Intermittent stimulation with PTH can significantly increase bone mineral density in vitro, mainly through the protein kinase A (PKA) signaling pathway, which phosphorylates runt-related transcription factor 2 (Runx2). The ClC-3 chloride channel, an important anion channel, can also promote osteogenesis via the Runx2 pathway based on recent studies. The purpose of our study, therefore, is to research whether the ClC-3 chloride channel has an effect on PTH osteodifferentiation in MC3T3-E1 cells.

METHODS AND RESULTS

A cell counting kit (CCK-8) and real-time PCR were used to investigate the impact of different PTH stimulation modes on MC3T3-E1 cell proliferation and osteogenesis-related gene expression, respectively. We found that the minimum inhibitory concentration of PTH was 10-9 M, and the expression of alkaline phosphatase (Alpl) and Runx2 were at the highest levels when treated with 10-9 M PTH. Next, we used real-time PCR and immunofluorescence technique to detect changes in ClC-3 in MC3T3-E1 cells under PTH treatment. The results showed higher expression of the ClC-3 chloride channel at 10-9 M intermittent PTH administration than in the other groups. Finally, we used the ClC-3 siRNA technique to examine the role of the ClC-3 chloride channel in the effect of PTH on the osteogenesis of osteoblasts, and we found an obvious decrease in the expression of bone sialoprotein (Ibsp), osteocalcin (Bglap), osterix (Sp7), Alpl and Runx2, the formation of mineralization nodules as well.

CONCLUSIONS

From the above data, we conclude that the expression of ClC-3 chloride channels in osteoblasts helps them respond to PTH stimulation, which mediates osteogenic differentiation.

Runx2-interacting genes identified by yeast two-hybrid screening of libraries generated from hypertrophic chondrocytes

Runx2, a member of the Runt domain family, is a well-known master transcription factor for osteoblast differentiation. Runx2 has also been shown to play essential roles during chondrocyte hypertrophy, an important late stage of endochondral ossification linking both bone and cartilage development. To identify the co-factors that may interact with Runx2 together to regulate this critical process, we have performed yeast two-hybrid (Y2H) screening using Runx2 as a bait to screen a cDNA library of hypertrophic chondrocytes. The bait expressing cassette was constructed by fusing Runx2 with the pGBKT7 vector containing the Gal4 DNA binding domain (BD). The Mate & Plate libraries were constructed using pGADT7-Rec and cDNAs derived from hypertrophic chondrocytes enriched limb tissues or hypertrophic MCT cells. After co-transformation of pGBKT7-Runx2 and the cDNA libraries, colonies that grew in nutrition deficient medium were selected and subjected to PCR and sequencing analysis. We successfully identified more than 30 candidate genes, including Lectin-1 (Lgals1), Col1a2, Edf1 and Timp-2. We have performed literature review and bioinformatics analysis of these genes using GenePaint. Most of them show ubiquitous expression with Lgals1 show enhanced expression in hypertrophic chondrocytes. We further performed preliminary expression analysis by quantitative PCR and detected differential expression of these candidate genes in proliferative and hypertrophic MCT cells, with Timp-2 significantly (around 3-fold) and Lgals1 moderately (around 1.5 fold) upregulated in hypertrophic MCT cells. Our results suggest that, candidate gene Timp-2 is very likely to interact with Runx2 and together to play essential function during cartilage development, and possibly its homeostasis.

Yap/Taz transcriptional activity in endothelial cells promotes intramembranous ossification via the BMP pathway

Osteogenesis is categorized into two groups based on developmental histology, intramembranous and endochondral ossification. The role of blood vessels during endochondral ossification is well known, while their role in intramembranous ossification, especially the intertissue pathway, is poorly understood. Here, we demonstrate endothelial Yap/Taz is a novel regulator of intramembranous ossification in zebrafish. Appropriate blood flow is required for Yap/Taz transcriptional activation in endothelial cells and intramembranous ossification. Additionally, Yap/Taz transcriptional activity in endothelial cells specifically promotes intramembranous ossification. BMP expression by Yap/Taz transactivation in endothelial cells is also identified as a bridging factor between blood vessels and intramembranous ossification. Furthermore, the expression of Runx2 in pre-osteoblast cells is a downstream target of Yap/Taz transcriptional activity in endothelial cells. Our results provide novel insight into the relationship between blood flow and ossification by demonstrating intertissue regulation.

Identification of transcriptional factors and key genes in primary osteoporosis by DNA microarray

Background

A number of genes have been identified to be related with primary osteoporosis while less is known about the comprehensive interactions between regulating genes and proteins. We aimed to identify the differentially expressed genes (DEGs) and regulatory effects of transcription factors (TFs) involved in primary osteoporosis.

Material/Methods

The gene expression profile GSE35958 was obtained from Gene Expression Omnibus database, including 5 primary osteoporosis and 4 normal bone tissues. The differentially expressed genes between primary osteoporosis and normal bone tissues were identified by the same package in R language. The TFs of these DEGs were predicted with the Essaghir A method. DAVID (The Database for Annotation, Visualization and Integrated Discovery) was applied to perform the GO (Gene Ontology) and KEGG (Kyoto Encyclopedia of Genes and Genomes) pathway enrichment analysis of DEGs. After analyzing regulatory effects, a regulatory network was built between TFs and the related DEGs.

Results

A total of 579 DEGs was screened, including 310 up-regulated genes and 269 down-regulated genes in primary osteoporosis samples. In GO terms, more up-regulated genes were enriched in transcription regulator activity, and secondly in transcription factor activity. A total 10 significant pathways were enriched in KEGG analysis, including colorectal cancer, Wnt signaling pathway, Focal adhesion, and MAPK signaling pathway. Moreover, total 7 TFs were enriched, of which CTNNB1, SP1, and TP53 regulated most up-regulated DEGs.

Conclusions

The discovery of the enriched TFs might contribute to the understanding of the mechanism of primary osteoporosis. Further research on genes and TFs related to the WNT signaling pathway and MAPK pathway is urgent for clinical diagnosis and directing treatment of primary osteoporosis.

miR-96 promotes osteogenic differentiation by suppressing HBEGF-EGFR signaling in osteoblastic cells

MicroRNAs (miRNAs) are a class of small non-coding RNAs with important roles in various biological and pathological processes, including osteoblast differentiation. Here, we identified miR-96 as a positive regulator of osteogenic differentiation in a mouse osteoblastic cell line (MC3T3-E1) and in mouse bone marrow-derived mesenchymal stem cells. Moreover, we found that miR-96 down-regulates post-transcriptional expression of heparin-binding EGF-like growth factor (HB-EGF) by specifically binding to the 3'untranslated region of HB-EGF mRNA. Furthermore, in MC3T3-E1 cells, miR-96-induced HB-EGF down-regulation suppressed the phosphorylation of epidermal growth factor receptor (EGFR) and of extracellular signal-regulated kinase 1 (ERK1) and AKT, which both lie downstream of EGFR activation. Taken together, miR-96 promotes osteogenic differentiation by inhibiting HB-EGF and by blocking the HB-EGF-EGFR signaling pathway in osteoblastic cells.

Serotonin 2B receptor (5-HT2B R) signals through prostacyclin and PPAR-ß/δ in osteoblasts

Osteoporosis is due to an imbalance between decreased bone formation by osteoblasts and increased resorption by osteoclasts. Deciphering factors controlling bone formation is therefore of utmost importance for the understanding and the treatment of osteoporosis. Our previous in vivo results showed that bone formation is reduced in the absence of the serotonin receptor 5-HT2B, causing impaired osteoblast proliferation, recruitment, and matrix mineralization. In this study, we investigated the signaling pathways responsible for the osteoblast defect in 5-HT2BR(-/-) mice. Notably, we investigated the phospholipase A2 pathway and synthesis of eicosanoids in 5-HT2BR(-/-) compared to wild type (WT) osteoblasts. Compared to control osteoblasts, the lack of 5-HT2B receptors was only associated with a 10-fold over-production of prostacyclin (PGI2). Also, a specific prostacyclin synthase inhibitor (U51605) rescued totally osteoblast aggregation and matrix mineralization in the 5-HT2BR(-/-) osteoblasts without having any effect on WT osteoblasts. Prostacyclin is the endogenous ligand of the nuclear peroxisome proliferator activated receptor ß/δ (PPAR-ß/δ), and its inhibition in 5-HT2BR(-/-) cells rescued totally the alkaline phosphatase and osteopontin mRNA levels, cell-cell adhesion, and matrix mineralization. We conclude that the absence of 5-HT2B receptors leads to the overproduction of prostacyclin, inducing reduced osteoblast differentiation due to PPAR-ß/δ -dependent target regulation and defective cell-cell adhesion and matrix mineralization. This study thus reveals a previously unrecognized cell autonomous osteoblast defect in the absence of 5-HT2BR and highlights a new pathway linking 5-HT2B receptors and nuclear PPAR- ß/δ via prostacyclin.

Targeting Runx2 expression in hypertrophic chondrocytes impairs endochondral ossification during early skeletal development

Runx2 is a known master transcription factor for osteoblast differentiation, as well as an essential regulator for chondrocyte maturation. Recently, more and more data has shown that Runx2 regulates hypertrophic chondrocyte-specific type X collagen gene (Col10a1) expression in different species. However, how Runx2 regulation of Col10a1 expression impacts chondrocyte maturation, an essential step of endochondral bone formation, remains unknown. We have recently generated transgenic mice in which Flag-tagged Runx2 was driven by a cell-specific Col10a1 control element. Significantly increased level of Runx2 and Col10a1 mRNA transcripts were detected in transgenic mouse limbs at both E17.5 (embryonic day 17.5) and P1 (post-natal day1) stages, suggesting an in vivo correlation of Runx2 and Col10a1 expression. Surprisingly, skeletal staining suggested delayed ossification in both the axial and the appendicular skeleton of transgenic mice from E14.5 until P6. Histological analysis showed elongated hypertrophic zones in transgenic mice, with less von Kossa and TUNEL staining in long bone sections at both E17.5 and P1 stages, suggesting defective mineralization due to delayed chondrocyte maturation or apoptosis. Indeed, we detected increased level of anti-apoptotic genes B-cell leukemia/lymphoma 2, Osteopontin, and Sox9 in transgenic mice by real-time RT-PCR. Moreover, immunohistochemistry and Western blotting analysis also suggested increased Sox9 expression in hypertrophic chondrocytes of transgenic mice. Together, our data suggest that targeting Runx2 in hypertrophic chondrocytes upregulates expression of Col10a1 and other marker genes (such as Sox9). This will change the local matrix environment, delay chondrocyte maturation, reduce apoptosis and matrix mineralization, and eventually, lead to impaired endochondral ossification.

Insulin like growth factor-I: a critical mediator of the skeletal response to parathyroid hormone

This review focuses on the mechanisms by which PTH stimulates both osteoblast and osteoclast function, emphasizing the critical role that IGF-I plays in these processes. After reviewing the current literature on the skeletal actions of PTH and the modulation of IGF action on bone by the different IGF-binding proteins, the review then examines studies from mouse models in which IGF-I or its receptor have been selectively deleted in different cells of the skeletal system, in particular osteoprogenitors, mature osteoblasts, and osteoclasts. Mice in which IGF-I production has been deleted from all cells are deficient in both bone formation and bone resorption with few osteoblasts or osteoclasts in bone in vivo, reduced osteoblast colony forming units, and an inability of either the osteoblasts or osteoclast precursors to support osteoclastogenesis in vitro. Mice in which the IGF-I receptor is specifically deleted in mature osteoblasts have a mineralization defect in vivo, and bone marrow stromal cells from these mice fail to mineralize in vitro. Mice in which the IGF-I receptor is deleted in osteoprogenitor cells have a marked reduction in osteoblast proliferation and differentiation leading to osteopenia. Finally mice lacking the IGF-I receptor in their osteoclasts have increased bone and decreased osteoclast formation. PTH fails to stimulate bone formation in the mice lacking IGF-I or its receptor in osteoblasts or enhance the signaling between osteoblasts and osteoclasts through RANKL/RANK and Ephrin B2/Eph B4, emphasizing the role IGF-I signaling plays in cell-communication per se and as stimulated by PTH.

Changes in serum runt-related transcription factor 2 levels after a 6-month treatment with recombinant human parathyroid hormone in patients with osteoporosis

BACKGROUND

The mechanisms regulating the anabolic response of the skeleton for recombinant human PTH (1- 34) [rhPTH (1-34)] administration has not been fully elucidated.

AIM

The aim of this study was to evaluate the effect of rhPTH (1-34) on serum levels of runt-related transcription factor 2 (Runx2) in women with osteoporosis.

METHODS

Sixty post-menopausal women with osteoporosis (EO group) and 45 control subjects (NC group) were enrolled in this study. The EO group received daily injection of 20 μg rhPTH (1-34) plus oral 500 mg elemental calcium and 400 IU vitamin D3 for 6 months. Runx2 and Matrix metalloproteinase 13 (MMP-13) were measured with commercially available enzyme-linked immunosorbent assay kits. Bone mineral density (BMD) was also measured before and 6 months after rhPTH (1-34) treatment.

RESULTS

Serum total Ca2+, phosphate, and bone-specific alkaline phosphatase were significantly increased (p<0.05 or p<0.01), and the lumbar spine BMD (LS-BMD) was also increased by 4% in patients with osteoporosis after treatment with rhPTH (1-34) (p<0.05). On the contrary, serum Runx2 and MMP-13 were significantly decreased at post treatment (13.1% and 36.6%, respectively, p<0.05 and p<0.01). At baseline, serum Runx2 positively correlated with MMP-13 (r=0.74, p<0.01), the correction remained after adjusting for age and body mass index.

CONCLUSION

The daily injection of rhPTH (1-34) was able to stimulate bone formation. The therapy of 20 μg rhPTH (1- 34) for 6 months resulted in decrease of serum Runx2 and MMP-13. These changes might reflect the increase of active osteoblasts and the better bone homeostasis.

Effects of risedronate in Runx2 overexpressing mice, an animal model for evaluation of treatment effects on bone quality and fractures

Young mice overexpressing Runx2 specifically in cells of the osteoblastic lineage failed to gain bone mass and exhibited a dramatic increase in bone resorption, leading to severe osteopenia and spontaneous vertebral fractures. The objective of the current study was to determine whether treatment with a bisphosphonate (risedronate, Ris), which reduces fractures in postmenopausal as well as in juvenile osteoporosis, was able to improve bone quality and reduce vertebral fractures in mice overexpressing Runx2. Four-week-old female Runx2 mice received Ris at 2 and 10 μg/kg subcutaneously twice a week for 12 weeks. Runx2 and wild-type mice received vehicle (Veh) as control. We measured the number of new fractures by X-ray and bone mineral density (BMD) by DEXA. We evaluated bone quality by histomorphometry, micro-CT, and Fourier transform infrared imaging (FTIRI). Ris at 20 μg/kg weekly significantly reduced the average number of new vertebral fractures compared to controls. This was accompanied by significantly increased BMD, increased trabecular bone volume, and reduced bone remodeling (seen in indices of bone resorption and formation) in the vertebrae and femoral metaphysis compared to Runx2 Veh. At the femur, Ris also increased cortical thickness. Changes in collagen cross-linking seen on FTIRI confirmed that Runx2 mice have accelerated bone turnover and showed that Ris affects the collagen cross-link ratio at both forming and resorbing sites. In conclusion, young mice overexpressing Runx2 have high bone turnover-induced osteopenia and spontaneous fractures. Ris at 20 μg/kg weekly induced an increase in bone mass, changes in bone microarchitecture, and decreased vertebral fractures.

Inducible expression of Runx2 results in multiorgan abnormalities in mice

Runx2 is a transcription factor controlling skeletal development, and is also expressed in extraskeletal tissues where its function is not well understood. Existing Runx2 mutant and transgenic mouse models do not allow the necessary control of Runx2 expression to understand its functions in different tissues. We generated conditional, doxycyline-inducible, triple transgenic mice (CMV-Cre;ROSA26-neo(flox/+)-rtTA;Tet-O-Runx2) to investigate the effects of wide spread overexpression of Runx2. Osteoblasts isolated from CMV-Cre;ROSA26-neo(flox/+)-rtTA; Tet-O-Runx2 mice demonstrated a dose-dependent effect of doxycycline to stimulate Runx2 transgene expression. Doxycycline administration to CMV-Cre;ROSA26-neo(flox/+)-rtTA;Tet-O-Runx2 mice induced Runx2 transgene expression in all tissues tested, with the highest levels observed in kidney, ovary, and bone. Runx2 overexpression resulted in deceased body size and reduced viability. With regard to bone, Runx2 overexpressing mice paradoxically displayed profound osteopenia and diminished osteogenesis. Induced expression of Runx2 in extraskeletal tissues resulted in ectopic calcification and induction of the osteogenic program in a limited number of tissues, including lung and muscle. In addition, the triple transgenic mice showed evidence of a myeloproliferative disorder and an apparent inhibition of lymphocyte development. Thus, overexpression of Runx2 both within and outside of the skeleton can have diverse biological effects. Use of tissue specific Cre mice will allow this model to be used to conditionally and inducibly overexpress Runx2 in different tissues and provide a means to study the post-natal tissue- and cell context-dependent functions of Runx2.

Overexpression of osteoprotegerin promotes preosteoblast differentiation to mature osteoblasts

OBJECTIVE

The hypothesis of the present study is that overexpression of osteoprotegerin (OPG) promotes preosteoblast maturation.

MATERIALS AND METHODS

The preosteoblast cell line MC3T3-E1 was transfected with OPG overexpression. OPG expression was confirmed by enzyme-linked immunosorbent assay (ELISA) and Western blot. Changes in the transcription factors in OPG-expressing cells were assessed by real-time polymerase quantitative polymerase chain reaction (RT-qPCR). Alkaline phosphate (ALP) expression was measured by ELISA.

RESULTS

The success of stable transfection of MC3T3-E1 cells with OPG overexpression was confirmed by MoFlow sorting followed by G418 selection. RT-qPCR showed that expression of RunX2, the most important osteoblast differentiation controlling factor, was suppressed. Smad1 and Akt1, as well as ALP, were upregulated in the OPG overexpressing cells.

CONCLUSION

Results from the present study provide evidence that overexpression of OPG in preosteoblasts promotes its differentiation into mature osteoblasts.

Strontium ranelate decreases the incidence of new caudal vertebral fractures in a growing mouse model with spontaneous fractures by improving bone microarchitecture

Young mice over-expressing Runx2 fail to gain bone relative to wild type mice with growth and present spontaneous fractures. It allows, for the first time in rodents, direct assessment of anti-fracture efficacy of strontium ranelate which was able to decrease caudal vertebrae fracture incidence through an improvement of trabecular and cortical architecture.

INTRODUCTION

The aim was to investigate whether strontium ranelate was able to decrease fracture incidence in mice over-expressing Runx2, model of severe developmental osteopenia associated with spontaneous vertebral fractures.

METHODS

Transgenic mice and their wild type littermates were treated by oral route with strontium ranelate or vehicle for 9 weeks. Caudal fracture incidence was assessed by repeated X-rays, resistance to compressive loading by biochemical tests, and bone microarchitecture by histomorphometry.

RESULTS

Transgenic mice receiving strontium ranelate had significantly fewer new fractures occurring during the 9 weeks of the study (-60%, p < 0.05). In lumbar vertebrae, strontium ranelate improves resistance to compressive loading (higher ultimate force to failure, +120%, p < 0.05) and trabecular microarchitecture (higher bone volume and trabecular number, lower trabecular separation, +60%, +50%, -39%, p < 0.05) as well as cortical thickness (+17%, p < 0.05). In tibiae, marrow cavity cross-section area and equivalent diameter were lower (-39%, -21%, p < 0.05). The strontium level in plasma and bone was in the same range as the values measured in treated postmenopausal women.

CONCLUSIONS

This model allows, for the first time, direct assessment of anti-fracture efficacy of strontium ranelate treatment in rodents. In these transgenic mice, strontium ranelate was able to decrease caudal vertebral fracture incidence through an improvement of trabecular and cortical architecture.

Bone loss induced by Runx2 over-expression in mice is blunted by osteoblastic over-expression of TIMP-1

The Runx2 gene is essential for osteoblast differentiation and function. In vivo over-expression of Runx2 in osteoblasts increases bone resorption, and blocks terminal osteoblast differentiation. Several lines of evidence suggest that osteoblastic matrix metalloproteinases (MMPs) could contribute to the increased bone resorption observed in mice over-expressing Runx2 (Runx2 mice). The goal of our study was to use a transgenic approach to find out whether the inhibition of osteoblastic MMPs can reduce the bone loss induced by the over-expression of Runx2. We analyzed the effect of the in vivo over-expression of the TIMP-1 in osteoblasts on the severe osteopenic phenotype in Runx2 mice. Females with the different genotypes (WT, Runx2, TIMP-1 and TIMP-1/Runx2) were analyzed for bone density, architecture, osteoblastic and osteoclastic activity and gene expression using qPCR. TIMP-1 over-expression reduces the bone loss in adult Runx2 mice. The prevention of the bone loss in TIMP-1/Runx2 mice was due to a combination of reduced bone resorption and sustained bone formation. We present evidence that the ability of osteoblastic cells to induce osteoclastic differentiation is lower in TIMP-1/Runx2 mice than in Runx2 mice, probably due to a reduction in the expression of RANK-L and of the Runx2 transgene. Osteoblast primary cells from TIMP-1/Runx2 mice, but not from Runx2 mice, were able to differentiate into fully mature osteoblasts producing high osteocalcin levels. In conclusion, our findings suggest that osteoblastic MMPs can affect osteoblast differentiation. Our work also indicates that osteoblastic MMPs are partly responsible for the bone loss observed in Runx2 transgenic mice.

PTH and PTHrP signaling in osteoblasts

The striking clinical benefit of PTH in osteoporosis began a new era of skeletal anabolic agents. Several studies have been performed, new studies are emerging out and yet controversies remain on PTH anabolic action in bone. This review focuses on the molecular aspects of PTH and PTHrP signaling in light of old players and recent advances in understanding the control of osteoblast proliferation, differentiation and function.

Nmp4/CIZ suppresses parathyroid hormone-induced increases in trabecular bone

The nucleocytoplasmic shuttling transcription factor Nmp4/CIZ (nuclear matrix protein 4/cas interacting zinc finger protein) is a ubiquitously expressed protein that regulates both cytoplasmic and nuclear activities. In the nucleus, Nmp4/CIZ represses transcription of genes crucial to osteoblast differentiation and genes activated by various anabolic stimuli, including parathyroid hormone (PTH). We investigated the role of Nmp4/CIZ in the PTH-induced increase in bone by engineering mice with loss-of-function mutations in the Nmp4/CIZ gene, and treating 10-week-old female mice with anabolic doses of human PTH (1-34) at 30 microg/kg/day, 7 day/week, for 7 weeks or vehicle control. The untreated, baseline phenotype of the Nmp4-null mice between 8 and 16 weeks of age included a modest but significant increase in bone mineral density (BMD) and bone mineral content (BMC) compared to wild-type (WT) mice. Type I collagen mRNA expression was moderately elevated in the femurs of the Nmp4-null mice. The Nmp4 mutant alleles decreased body weight by 4% when expressed on a mixed background but the same alleles on a pure B6 background yielded a significant, 15% increase in body weight among the KO mice, compared to their WT controls. Hormone treatment equally enhanced BMD and BMC over vehicle-treated mice in both the WT and Nmp4-null groups but Nmp4-KO mice exhibited a significantly greater PTH-induced acquisition of femoral trabecular bone as compared to WT mice. These data support our hypothesis that Nmp4/CIZ is a transcriptional attenuator that suppresses osteoid synthesis and PTH-mediated acquisition of cancellous bone. J. Cell. Physiol. 219: 734-743, 2009. (c) 2009 Wiley-Liss, Inc.

Zfp521 antagonizes Runx2, delays osteoblast differentiation in vitro, and promotes bone formation in vivo

Zfp521, a 30 C2H2 Kruppel-like zinc finger protein, is expressed at high levels at the periphery of early mesenchymal condensations prefiguring skeletal elements and in all developing bones in the perichondrium and periosteum, in osteoblast precursors and osteocytes, and in chondroblast precursors and growth plate prehypertrophic chondrocytes. Zfp521 expression in cultured mesenchymal cells is decreased by BMP-2 and increased by PTHrP, which promote and antagonize osteoblast differentiation, respectively. In vitro, Zfp521 overexpression reduces the expression of several downstream osteoblast marker genes and antagonizes osteoblast differentiation. Zfp521 binds Runx2 and represses its transcriptional activity, and Runx2 dose-dependently rescues Zfp521's inhibition of osteoblast differentiation. In contrast, osteocalcin promoter-targeted overexpression of Zfp521 in osteoblasts in vivo results in increased bone formation and bone mass. We propose that Zfp521 regulates the rate of osteoblast differentiation and bone formation during development and in the mature skeleton, in part by antagonizing Runx2.

Histamine promotes osteoclastogenesis through the differential expression of histamine receptors on osteoclasts and osteoblasts

In addition to the numerous roles of histamine in both the immune and nervous systems, previous studies have suggested that this bioamine might also be involved in bone metabolism. Following our observations of impaired bone resorption in ovariectomized rats after histamine receptor antagonist treatment, we focused in this study on osteoclasts and osteoclast precursors. We looked for a direct action of histamine on these cells using both in vivo and in vitro approaches. In vivo, we triggered a remodeling sequence in rat mandibular bone and treated the animals with either histamine or histamine receptor antagonists. Histamine was shown to increase the number of osteoclasts and osteoclast precursors whereas antagonists of histamine receptor-1 and -2 decreased both osteoclast recruitment and resorption. In vitro, spleen cells from histamine-deficient mice were treated with receptor activator for nuclear factor kappa B ligand and macrophage colony stimulating factor, giving rise to both reduced numbers of osteoclasts and decreased resorption on dentin slices. Histamine enhanced resorption in these cultures in a dose-dependent manner. In addition, we identified osteoclast precursors as a source of histamine. In contrast, histamine increased the receptor activator for nuclear factor kappa B ligand/osteoprotegerin ratio in primary osteoblasts that did not secrete histamine. We observed a differential expression of histamine receptor-1 and -2 mRNAs in both primary osteoclasts and osteoblasts, confirming their functional roles with selective antagonists. Thus, histamine acts directly on osteoclasts, osteoclast precursors, and osteoblasts, promoting osteoclastogenesis through autocrine/paracrine mechanisms.

Dlx5, a positive regulator of osteoblastogenesis, is essential for osteoblast-osteoclast coupling

The homeodomain protein Dlx5 is an activator of Runx2 (a key regulator of osteogenesis) and is thought to be an important regulator of bone formation. At present, however, the perinatal lethality of Dlx5-null mice has hampered the elucidation of its function in osteogenesis. Here we provide the first analysis of the effects of Dlx5 inactivation on bone development. Femurs of Dlx5-null mouse embryos at the end of gestation exhibit a reduction in both total and trabecular bone volume associated with increased trabecular separation and reduced trabecular number. These parameters are often associated with pathological conditions characterized by reduced osteoblast activity and increased bone resorption. Dlx5(-/-) osteoblasts in culture display reduced proliferation and differentiation rate and reduction of Runx2, Osx, Osteocalcin and Bone Sialoprotein expression. In addition to impaired osteoblast function, Dlx5(-/-) femurs exhibit significant increases in osteoclast number. As Dlx5 is not expressed by osteoclasts, we suggest that its osteoblastic expression might control osteoblast/osteoclast coupling. Cultured Dlx5(-/-) osteoblasts displayed a higher RANKL/OPG ratio. Furthermore, Dlx5(-/-) osteoblasts induced a higher number of TRAP-positive multinucleated cells in normal spleen cultures with a globally increased resorption activity. These findings suggest that Dlx5 is a central regulator of bone turnover as it activates bone formation directly and bone resorption indirectly.

Molecular genetic studies of gene identification for osteoporosis

This review comprehensively summarizes the most important and representative molecular genetics studies of gene identification for osteoporosis published up to the end of September 2007. It is intended to constitute a sequential update of our previously published reviews covering the available data up to the end of 2004. Evidence from candidate gene-association studies, genome-wide linkage and association studies, as well as functional genomic studies (including gene-expression microarray and proteomics) on osteogenesis and osteoporosis, are reviewed separately. Studies of transgenic and knockout mice models relevant to osteoporosis are summarized. The major results of all studies are tabulated for comparison and ease of reference. Comments are made on the most notable findings and representative studies for their potential influence and implications on our present understanding of genetics of osteoporosis. The format adopted by this review should be ideal for accommodating future new advances and studies.

The serotonin 5-HT2B receptor controls bone mass via osteoblast recruitment and proliferation

The monoamine serotonin (5-HT), a well-known neurotransmitter, is also important in peripheral tissues. Several studies have suggested that 5-HT is involved in bone metabolism. Starting from our original observation of increased 5-HT(2B) receptor (5-HT(2B)R) expression during in vitro osteoblast differentiation, we investigated a putative bone phenotype in vivo in 5-HT(2B)R knockout mice. Of interest, 5-HT(2B)R mutant female mice displayed reduced bone density that was significant from age 4 months and had intensified by 12 and 18 months. This histomorphometrically confirmed osteopenia seems to be due to reduced bone formation because 1) the alkaline phosphatase-positive colony-forming unit capacity of bone marrow precursors was markedly reduced in the 5-HT(2B)R mutant mice from 4 to 12 months of age, 2) ex vivo primary osteoblasts from mutant mice exhibited reduced proliferation and delayed differentiation, and 3) calcium incorporation was markedly reduced in osteoblasts after 5-HT(2B)R depletion (produced genetically or by pharmacological inactivation). These findings support the hypothesis that the 5-HT(2B)R receptor facilitates osteoblast recruitment and proliferation and that its absence leads to osteopenia that worsens with age. We show here, for the first time, that the 5-HT(2B)R receptor is a physiological mediator of 5-HT in bone formation and, potentially, in the onset of osteoporosis in aging women.