Identification of novel regulators associated with early-phase osteoblast differentiation

Notch signaling pathway in osteogenesis, bone development, metabolism, and diseases

The skeletal system provides vital importance to support organ development and functions. The Notch signaling pathway possesses well-established functions in organ development and cellular homeostasis. The Notch signaling pathway comprises five typical ligands (JAG1, JAG2, DLL1, DLL3, and DLL4), four receptors (Notch1-4), and four intracellular domains (NICD1-4). Each component of the Notch signaling pathway has been demonstrated to be fundamental in osteoblast differentiation and bone formation. The dysregulation in the Notch signaling pathway is highly linked with skeletal disorders or diseases at the developmental and postnatal stages. Recent studies have highlighted the importance of the elements of the Notch signaling pathway in the skeletal system, as well as its interaction with signaling, such as Wnt/β-catenin, BMP, TGF-β, FGF, autophagy, and hedgehog (Hh) to construct a potential gene regulatory network to orchestrate osteogenesis and ossification. Our review has provided a comprehensive summary of the Notch signaling pathway in the skeletal system, as well as the insights targeting Notch signaling for innovative potential drug discovery targets or therapeutic interventions to treat bone disorders, such as osteoporosis and osteoarthritis. An in-depth molecular mechanistic strategy to modulate the Notch signaling pathway and its associated signaling pathway will be encouraged for consideration to trigger enhanced therapeutic approaches for bone disorders by defining Notch-regulating drugs for clinical use.

Craniotubular Dysplasia Ikegawa Type: Further Delineation of the Phenotype

Craniotubular Dysplasia Ikegawa type is a sclerosing bone disorder recently identified in five patients from four independent Indian families. It is caused by homozygous or compound heterozygous mutations in TMEM53. Deficient TMEM53 leads to overactive BMP signaling which promotes bone formation. Here, we present another three siblings with intronic mutations in TMEM53, identified by exome sequencing, from a Caucasian family. All three siblings displayed skeletal and radiographic features, similar to the earlier described individuals. All our patients had additional features such as cardiac and urogenital anomalies. Our results confirm the phenotype of CTDI. We discuss whether the additional features in our patients are separate from CTDI or reflect a broader spectrum of the syndrome.

Towards an enhanced understanding of osteoanabolic effects of PTH-induced microRNAs on osteoblasts using a bioinformatic approach

In this study, we used a bioinformatic approach to construct a miRNA-target gene interaction network potentially involved in the anabolic effect of parathyroid hormone analogue teriparatide [PTH (1-34)] on osteoblasts. We extracted a dataset of 26 microRNAs (miRNAs) from previously published studies and predicted miRNA target interactions (MTIs) using four software tools: DIANA, miRWalk, miRDB, and TargetScan. By constructing an interactome of PTH-regulated miRNAs and their predicted target genes, we elucidated signaling pathways regulating pluripotency of stem cells, the Hippo signaling pathway, and the TGF-beta signaling pathway as the most significant pathways in the effects of PTH on osteoblasts. Furthermore, we constructed intersection of MTI networks for these three pathways and added validated interactions. There are 8 genes present in all three selected pathways and a set of 18 miRNAs are predicted to target these genes, according to literature data. The most important genes in all three pathways were BMPR1A, BMPR2 and SMAD2 having the most interactions with miRNAs. Among these miRNAs, only miR-146a-5p and miR-346 have validated interactions in these pathways and were shown to be important regulators of these pathways. In addition, we also propose miR-551b-5p and miR-338-5p for further experimental validation, as they have been predicted to target important genes in these pathways but none of their target interactions have yet been verified. Our wet-lab experiment on miRNAs differentially expressed between PTH (1-34) treated and untreated mesenchymal stem cells supports miR-186-5p from the literature obtained data as another prominent miRNA. The meticulous selection of miRNAs outlined will significantly support and guide future research aimed at discovering and understanding the crucial pathways of osteoanabolic PTH-epigenetic effects on osteoblasts. Additionally, they hold potential for the discovery of new PTH target genes, innovative biomarkers for the effectiveness and safety of osteoporosis-affected treatment, as well as novel therapeutic targets.

Cell signaling and transcriptional regulation of osteoblast lineage commitment, differentiation, bone formation, and homeostasis

The initiation of osteogenesis primarily occurs as mesenchymal stem cells undergo differentiation into osteoblasts. This differentiation process plays a crucial role in bone formation and homeostasis and is regulated by two intricate processes: cell signal transduction and transcriptional gene expression. Various essential cell signaling pathways, including Wnt, BMP, TGF-β, Hedgehog, PTH, FGF, Ephrin, Notch, Hippo, and Piezo1/2, play a critical role in facilitating osteoblast differentiation, bone formation, and bone homeostasis. Key transcriptional factors in this differentiation process include Runx2, Cbfβ, Runx1, Osterix, ATF4, SATB2, and TAZ/YAP. Furthermore, a diverse array of epigenetic factors also plays critical roles in osteoblast differentiation, bone formation, and homeostasis at the transcriptional level. This review provides an overview of the latest developments and current comprehension concerning the pathways of cell signaling, regulation of hormones, and transcriptional regulation of genes involved in the commitment and differentiation of osteoblast lineage, as well as in bone formation and maintenance of homeostasis. The paper also reviews epigenetic regulation of osteoblast differentiation via mechanisms, such as histone and DNA modifications. Additionally, we summarize the latest developments in osteoblast biology spurred by recent advancements in various modern technologies and bioinformatics. By synthesizing these insights into a comprehensive understanding of osteoblast differentiation, this review provides further clarification of the mechanisms underlying osteoblast lineage commitment, differentiation, and bone formation, and highlights potential new therapeutic applications for the treatment of bone diseases.

Effect of Mesenchymal Stem Cells Overexpressing BMP-9 Primed with Hypoxia on BMP Targets, Osteoblast Differentiation and Bone Repair

Bone formation is driven by many signaling molecules including bone morphogenetic protein 9 (BMP-9) and hypoxia-inducible factor 1-alpha (HIF-1α). We demonstrated that cell therapy using mesenchymal stem cells (MSCs) overexpressing BMP-9 (MSCs+BMP-9) enhances bone formation in calvarial defects. Here, the effect of hypoxia on BMP components and targets of MSCs+BMP-9 and of these hypoxia-primed cells on osteoblast differentiation and bone repair was evaluated. Hypoxia was induced with cobalt chloride (CoCl2) in MSCs+BMP-9, and the expression of BMP components and targets was evaluated. The paracrine effects of hypoxia-primed MSCs+BMP-9 on cell viability and migration and osteoblast differentiation were evaluated using conditioned medium. The bone formation induced by hypoxia-primed MSCs+BMP-9 directly injected into rat calvarial defects was also evaluated. The results demonstrated that hypoxia regulated BMP components and targets without affecting BMP-9 amount and that the conditioned medium generated under hypoxia favored cell migration and osteoblast differentiation. Hypoxia-primed MSCs+BMP-9 did not increase bone repair compared with control MSCs+BMP-9. Thus, despite the lack of effect of hypoxia on bone formation, the enhancement of cell migration and osteoblast differentiation opens windows for further investigations on approaches to modulate the BMP-9-HIF-1α circuit in the context of cell-based therapies to induce bone regeneration.

LncMIR181A1HG is a novel chromatin-bound epigenetic suppressor of early stage osteogenic lineage commitment

Bone formation requires osteogenic differentiation of multipotent mesenchymal stromal cells (MSCs) and lineage progression of committed osteoblast precursors. Osteogenic phenotype commitment is epigenetically controlled by genomic (chromatin) and non-genomic (non-coding RNA) mechanisms. Control of osteogenesis by long non-coding RNAs remains a largely unexplored molecular frontier. Here, we performed comprehensive transcriptome analysis at early stages of osteogenic cell fate determination in human MSCs, focusing on expression of lncRNAs. We identified a chromatin-bound lncRNA (MIR181A1HG) that is highly expressed in self-renewing MSCs. MIR181A1HG is down-regulated when MSCs become osteogenic lineage committed and is retained during adipogenic differentiation, suggesting lineage-related molecular functions. Consistent with a key role in human MSC proliferation and survival, we demonstrate that knockdown of MIR181A1HG in the absence of osteogenic stimuli impedes cell cycle progression. Loss of MIR181A1HG enhances differentiation into osteo-chondroprogenitors that produce multiple extracellular matrix proteins. RNA-seq analysis shows that loss of chromatin-bound MIR181A1HG alters expression and BMP2 responsiveness of skeletal gene networks (e.g., SOX5 and DLX5). We propose that MIR181A1HG is a novel epigenetic regulator of early stages of mesenchymal lineage commitment towards osteo-chondroprogenitors. This discovery permits consideration of MIR181A1HG and its associated regulatory pathways as targets for promoting new bone formation in skeletal disorders.

BMP-2 Induced Signaling Pathways and Phenotypes: Comparisons Between Senescent and Non-senescent Bone Marrow Mesenchymal Stem Cells

The use of BMP-2 in orthopedic surgery is limited by uncertainty surrounding its effects on the differentiation of mesenchymal stem cells (MSCs) and how this is affected by cellular aging. This study compared the effects of recombinant human BMP-2 (rhBMP-2) on osteogenic and adipogenic differentiation between senescent and non-senescent MSCs. Senescent and non-senescent MSCs were cultured in osteogenic and adipogenic differentiation medium containing various concentrations of rhBMP-2. The phenotypes of these cells were compared by performing a calcium assay, adipogenesis assay, staining, real-time PCR, western blotting, and microarray analysis. rhBMP-2 induced osteogenic differentiation to a lesser extent (P < 0.001 and P = 0.005 for alkaline phosphatase activity and Ca²⁺ release) in senescent MSCs regardless of dose-dependent increase in both cells. However, the induction of adipogenic differentiation by rhBMP-2 was comparable between them. There was no difference between these two groups of cells in the adipogenesis assay (P = 0.279) and their expression levels of PPARγ were similar. Several genes such as CHRDL1, NOG, SMAD1, SMAD7, and FST encoding transcription factors were proposed to underlie the different responses of senescent and non-senescent MSCs to rhBMP-2 in microarray analyses. Furthermore, inflammatory, adipogenic, or cell death-related signaling pathways such as NF-kB or p38-MAPK pathways were upregulated by BMP-2 in senescent MSCs, whereas bone forming signaling pathways involving BMP, SMAD, and TGF- ß were upregulated in non-senescent MSCs as expected. This phenomenon explains bone forming dominance by non-senescent MSCs and possible frequent complications such as seroma, osteolysis, or neuritis in senescent MSCs during BMP-2 use in orthopedic surgery.

Carbohydrate responsive element binding protein (ChREBP) negatively regulates osteoblast differentiation via protein phosphatase 2A Cα dependent manner

Carbohydrate responsive element binding protein (ChREBP) is a major transcription factor of lipogenesis regulated by glucose status in the liver. However, the function of ChREBP in osteogenic differentiation is unclear. The present study examined the role of ChREBP in osteoblast differentiation in MC3T3-E1 preosteoblast cell line. The mRNA expression of ChREBP, protein phosphatase 2A catalytic subunit-α (PP2A Cα) and the osteogenic genes such as, DNA-binding protein inhibitor (Id1), runt-related transcription factor-2 (Runx2), and alkaline phosphatase (ALP) was measured by qPCR and RT-PCR. Runx2, ChREBP, and PP2A Cα, protein levels were evaluated by Western blotting. ALP staining experiment was carried out to evaluate ALP enzyme activity, and a luciferase reporter assay was performed to analyze Runx2 transcriptional activity. Expression of ChREBP and PP2A Cα did not change during bone morphogenetic protein-2 (BMP2)-induced osteoblast differentiation. Overexpression of ChREBP reduced the osteogenic genes (Runx2 and ALP) expression and ALP activity, while knockdown of ChREBP had the opposite effects. Overexpression of PP2A Cα increased ChREBP expression, while inhibition of PP2A Cα using okadaic acid not only inhibited the expression of ChREBP, but also restored the mRNA and protein expression of Runx2 and activity of ALP enzyme. These results demonstrate that ChREBP inhibits BMP2-induced osteoblast differentiation in a PP2A Cα- dependent manner.

Inflammation induces endothelial-to-mesenchymal transition and promotes vascular calcification through downregulation of BMPR2

Endothelial‐to‐mesenchymal transition (EndMT) has been unveiled as a common cause for a multitude of human pathologies, including cancer and cardiovascular disease. Vascular calcification is a risk factor for ischemic vascular disorders and slowing calcification may reduce mortality in affected patients. The absence of early biomarkers hampers the identification of patients at risk. EndMT and vascular calcification are induced upon cooperation between distinct stimuli, including inflammatory cytokines and transforming growth factor beta (TGF‐β) family members. However, how these signaling pathways interplay to promote cell differentiation and eventually vascular calcification is not well understood. Using in vitro and ex vivo analysis in animal models and patient‐derived tissues, we have identified that the pro‐inflammatory cytokines tumor necrosis factor alpha (TNF‐α) and interleukin‐1 beta (IL‐1β) induce EndMT in human primary aortic endothelial cells, thereby sensitizing them for BMP‐9‐induced osteogenic differentiation. Downregulation of the BMP type II receptor BMPR2 is a key event in this process. Rather than compromising BMP canonical signal transduction, loss of BMPR2 results in decreased JNK signaling in ECs, thus enhancing BMP‐9‐induced mineralization. Altogether, our results point at the BMPR2–JNK signaling axis as a key pathway regulating inflammation‐induced EndMT and contributing to calcification. © 2018 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

The BMP Pathway and Its Inhibitors in the Skeleton

Bone morphogenetic proteins (BMPs) constitute the largest subdivision of the transforming growth factor-β family of ligands. BMPs exhibit widespread utility and pleiotropic, context-dependent effects, and the strength and duration of BMP pathway signaling is tightly regulated at numerous levels via mechanisms operating both inside and outside the cell. Defects in the BMP pathway or its regulation underlie multiple human diseases of different organ systems. Yet much remains to be discovered about the BMP pathway in its original context, i.e., the skeleton. In this review, we provide a comprehensive overview of the intricacies of the BMP pathway and its inhibitors in bone development, homeostasis, and disease. We frame the content of the review around major unanswered questions for which incomplete evidence is available. First, we consider the gene regulatory network downstream of BMP signaling in osteoblastogenesis. Next, we examine why some BMP ligands are more osteogenic than others and what factors limit BMP signaling during osteoblastogenesis. Then we consider whether specific BMP pathway components are required for normal skeletal development, and if the pathway exerts endogenous effects in the aging skeleton. Finally, we propose two major areas of need of future study by the field: greater resolution of the gene regulatory network downstream of BMP signaling in the skeleton, and an expanded repertoire of reagents to reliably and specifically inhibit individual BMP pathway components.

Ablation of the mammalian lectin galectin-8 induces bone defects in mice

Mice overexpressing galectin-8 [gal-8 transgenic (Tg)], a secreted mammalian lectin, exhibit enhanced bone turnover and reduced bone mass, similar to cases of postmenopausal osteoporosis. Here, we show that gal-8 knockout (KO) mice have increased bone mass accrual at a young age but exhibit accelerated bone loss during adulthood. These phenotypes can be attributed to a gal-8-mediated increase in receptor activator of NF-κB ligand (RANKL) expression that promotes osteoclastogenesis, combined with direct inhibition of osteoblast differentiation, evident by reduced bone morphogenetic protein (BMP) signaling, reduced phosphorylation of receptor regulated mothers against decapentaplegic homolog (R-SMAD) and reduced expression of osteoblast differentiation markers osterix, osteocalcin, runt-related transcription factor 2 (RUNX2), dentin matrix acidic phosphoprotein-1 (DMP1), and alkaline phosphatase. At the same time, gal-8 promotes expression of estrogen receptor α (ESR1). Accordingly, the rate of bone loss is accelerated in ovariectomized, estrogen-deficient gal-8 Tg mice, whereas gal-8 KO mice, having low levels of ESR1, are refractory to ovariectomy. Finally, gal-8 mRNA positively correlates with the mRNA levels of osteoclastogenic markers RANKL, tartrate-resistant acid phosphatase, and cathepsin K in human femurs. Collectively, these findings identify gal-8 as a new physiologic player in the regulation of bone mass.-Vinik, Y., Shatz-Azoulay, H., Hiram-Bab, S., Kandel, L., Gabet, Y., Rivkin, G., Zick, Y. Ablation of the mammalian lectin galectin-8 induces bone defects in mice.

The mixed lineage leukemia 4 (MLL4) methyltransferase complex is involved in transforming growth factor beta (TGF-β)-activated gene transcription

Sma and Mad related (SMAD)-mediated Transforming Growth Factor β (TGF-β) and Bone Morphogenetic Protein (BMP) signaling is required for various cellular processes. The activated heterotrimeric SMAD protein complexes associate with nuclear proteins such as the histone acetyltransferases p300, PCAF and the Mixed Lineage Leukemia 4 (MLL4) subunit Pax Transactivation domain-Interacting Protein (PTIP) to regulate gene transcription.

We investigated the functional role of PTIP and PTIP Interacting protein 1 (PA1) in relation to TGF-β-activated SMAD signaling. We immunoprecipitated PTIP and PA1 with all SMAD family members to identify the TGF-β and not BMP-specific SMADs as interacting proteins. Gene silencing experiments of MLL4 and the subunits PA1 and PTIP confirm TGF-β-specific genes to be regulated by the MLL4 complex, which links TGF-β signaling to transcription regulation by the MLL4 methyltransferase complex.

Transcriptome analysis for the identification of cellular markers related to trabecular meshwork differentiation

Background

Development of primary open-angle glaucoma (POAG) is associated with the malfunctioning trabecular meshwork (TM). Cell therapy offers great potential for the treatment of POAG, but requires the generation of functional TM cells in vitro to replace the lost/dysfunctional cells. TM differentiation in vitro from various stem cell types must be monitored by the expression of specific markers. However, no single definitive marker of the TM has been identified.

Results

To identify robust markers of TM differentiation, we performed global transcriptome profiling using high-density oligonucleotide microarray on ex vivo TM tissue and cultured TM progenitors. Corneal and scleral tissues were also used in the analysis. After removal of genes expressed in the cornea and sclera, 18 genes were identified that were differentially expressed in the TM relative to the other samples. CDH23, F5, KCNAB1, FGF9, SPP1, and HEY1 were selected among the genes highly expressed in the TM, together with BDNF which was repressed, compared to progenitors for further investigation. Expression analysis by qPCR verified the differential expression and immunofluorescence of the anterior segment confirmed strong expression in the TM.

Conclusions

Three independent cohort of expression studies have identified novel markers, fitting in identifying TM cells and in evaluating directed TM differentiation in vitro.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3758-7) contains supplementary material, which is available to authorized users.

Bone Morphogenetic Proteins

Bone morphogenetic proteins (BMPs), originally identified as osteoinductive components in extracts derived from bone, are now known to play important roles in a wide array of processes during formation and maintenance of various organs including bone, cartilage, muscle, kidney, and blood vessels. BMPs and the related "growth and differentiation factors" (GDFs) are members of the transforming growth factor β (TGF-β) family, and transduce their signals through type I and type II serine-threonine kinase receptors and their intracellular downstream effectors, including Smad proteins. Furthermore, BMP signals are finely tuned by various agonists and antagonists. Because deregulation of the BMP activity at multiple steps in signal transduction is linked to a wide variety of human diseases, therapeutic use of activators and inhibitors of BMP signaling will provide potential avenues for the treatment of the human disorders that are caused by hypo- and hyperactivation of BMP signals, respectively.

Effective Spatial Separation of PC12 and NIH3T3 Cells by the Microgrooved Surface of Biocompatible Polymer Substrates

Most organs and tissues are composed of more than one type of cell that is spatially separated and located in different regions. This study used a microgrooved poly(lactic-co-glycolic acid) (PLGA) substrate to guide two types of cocultured cells to two spatially separated regions. Specifically, PC12 pheochromocytoma cells are guided to the inside of microgrooves, whereas NIH3T3 fibroblasts are guided to the ridge area in between neighboring parallel microgrooves. In addition, the microgrooved structures can significantly promote the proliferation and neural differentiation of PC12 cells as well as the osteogenic differentiation of NIH3T3 cells. Therefore, the microgrooved PLGA surface with separated PC12 and NIH3T3 cells can serve as a potential model system for studying nerve reconstruction in bone-repairing scaffolds.

Bone morphogenetic protein 7 (BMP-7) influences tendon-bone integration in vitro

Introduction

Successful graft ingrowth following reconstruction of the anterior cruciate ligament is governed by complex biological processes at the tendon-bone interface. The aim of this study was to investigate in an in vitro study the effects of bone morphogenetic protein 7 (BMP-7) on tendon-bone integration.

Materials and Methods

To study the biological effects of BMP-7 on the process of tendon-bone-integration, two independent in vitro models were used. The first model involved the mono- and coculture of bovine tendon specimens and primary bovine osteoblasts with and without BMP-7 exposure. The second model comprised the mono- and coculture of primary bovine osteoblasts and fibroblasts. Alkaline phosphatase (ALP), lactate dehydrogenase (LDH), lactate and osteocalcin (OCN) were analyzed by ELISA. Histological analysis and electron microscopy of the tendon specimens were performed.

Results

In both models, positive effects of BMP-7 on ALP enzyme activity were observed (p<0.001). Additionally, similar results were noted for LDH activity and lactate concentration. BMP-7 stimulation led to a significant increase in OCN expression. Whereas the effects of BMP-7 on tendon monoculture peaked during an early phase of the experiment (p<0.001), the cocultures showed a maximal increase during the later stages (p<0.001). The histological analysis showed a stimulating effect of BMP-7 on extracellular matrix formation. Organized ossification zones and calcium carbonate-like structures were only observed in the BMP-stimulated cell cultures.

Discussion

This study showed the positive effects of BMP-7 on the biological process of tendon-bone integration in vitro. Histological signs of improved mineralization were paralleled by increased rates of osteoblast-specific protein levels in primary bovine osteoblasts and fibroblasts.

Conclusion

Our findings indicated a role for BMP-7 as an adjuvant therapeutic agent in the treatment of ligamentous injuries, and they emphasized the importance of the transdifferentiation process of tendinous fibroblasts at the tendon-bone interface.

Identification of differentiation-stage specific markers that define the ex vivo osteoblastic phenotype

The phenotype of osteoblastic (OB) cells in culture is currently defined using a limited number of markers of low sensitivity and specificity. For the clinical use of human skeletal (stromal, mesenchymal) stem cells (hMSC) in therapy, there is also a need to identify a set of gene markers that predict in vivo bone forming capacity. Thus, we used RNA sequencing to examine changes in expression for a set of skeletally-related genes across 8 time points between 0 and 12days of ex vivo OB differentiation of hMSC. We identified 123 genes showing significant temporal expression change. Hierarchical clustering and Pearson's correlation generated 4 groups of genes: early stage differentiation genes (peak expression: 0-24h, n=28) which were enriched for extracellular matrix organisation, e.g. COL1A1, LOX, and SERPINH1; middle stage differentiating genes (peak expression days: 3 and 6, n=20) which were enriched for extracellular matrix/skeletal system development e.g. BMP4, CYP24A1, and TGFBR2; and late stage differentiation genes (peak expression days: 9 and 12, n=27) which were enriched for bone development/osteoblast differentiation, e.g. BMP2 and IGF2. In addition, we identified 13 genes with bimodal temporal expression (2 peaks of expression: days 0 and 12) including VEGFA, PDGFA and FGF2. We examined the specificity of the 123 genes' expression in skeletal tissues and thus propose a set of ex vivo differentiation-stage-specific markers (n=21). In an independent analysis, we identified a subset of genes (n=20, e.g. ELN, COL11A1, BMP4) to predict the bone forming capacity of hMSC and another set (n=20, e.g. IGF2, TGFB2, SMAD3) associated with the ex vivo phenotype of hMSC obtained from osteoporotic patients.

Microarray meta-analysis identifies evolutionarily conserved BMP signaling targets in developing long bones

In vertebrates, BMP signaling has been demonstrated to be sufficient for bone formation in several tissue contexts. This suggests that genes necessary for bone formation are expressed in a BMP signaling dependent manner. However, till date no gene has been reported to be expressed in a BMP signaling dependent manner in bone. Our aim was to identify such genes. On searching the literature we found that several microarray experiments have been conducted where the transcriptome of osteogenic cells in absence and presence of BMP signaling activation have been compared. However, till date, there is no evidence to suggest that any of the genes found to be upregulated in presence of BMP signaling in these microarray analyses is indeed a target of BMP signaling in bone. We wanted to utilize this publicly available information to identify candidate BMP signaling target genes in vivo. We performed a meta-analysis of six such comparable microarray datasets. This analysis and subsequent experiments led to the identification of five targets of BMP signaling in bone that are conserved both in mouse and chick. Of these Lox, Klf10 and Gpr97 are likely to be direct transcriptional targets of BMP signaling pathway. Dpysl3, is a novel BMP signaling target identified in our study. Our data demonstrate that Dpysl3 is important for osteogenic differentiation of mesenchymal cells and is involved in cell secretion. We have demonstrated that the expression of Dpysl3 is co-operatively regulated by BMP signaling and Runx2. Based on our experimental data, in silico analysis of the putative promoter-enhancer regions of Bmp target genes and existing literature, we hypothesize that BMP signaling collaborates with multiple signaling pathways to regulate the expression of a unique set of genes involved in endochondral ossification.

Hey bHLH transcription factors

Hey bHLH transcription factors are direct targets of canonical Notch signaling. The three mammalian Hey proteins are closely related to Hes proteins and they primarily repress target genes by either directly binding to core promoters or by inhibiting other transcriptional activators. Individual candidate gene approaches and systematic screens identified a number of Hey target genes, which often encode other transcription factors involved in various developmental processes. Here, we review data on interaction partners and target genes and conclude with a model for Hey target gene regulation. Furthermore, we discuss how expression of Hey proteins affects processes like cell fate decisions and differentiation, e.g., in cardiovascular, skeletal, and neural development or oncogenesis and how this relates to the observed developmental defects and phenotypes observed in various knockout mice.

BMP signaling in development and diseases: a pharmacological perspective

Bone morphogenetic protein (BMP) signaling has been implicated in several processes during embryonic development and in adult tissue homeostasis. Maintenance of many organs such as skin, intestinal villi, bones and bone marrow requires continuous regeneration and subsequent differentiation of stem cells in order to maintain organ shape and size necessary for proper functioning. Although BMPs were initially identified as osteogenic factors present in demineralized bone capable of inducing ectopic bone formation, it is now evident that BMPs perform several other functions during embryonic development as well as during the adult life of an organism. Many disorders have been linked to either the BMPs or the molecules functioning downstream of BMP signaling pathway. This review summarizes the existing literature describing the role of BMP signaling during embryonic development and in adult tissue homeostasis to provide a perspective on pharmacological interventions of BMP signaling pathway to mitigate several disease conditions.

Mineralization content alters osteogenic responses of bone marrow stromal cells on hydroxyapatite/polycaprolactone composite nanofiber scaffolds

Synthetic tissue scaffolds have a high potential impact for patients experiencing osteogenesis imperfecta. Using electrospinning, tissue scaffolds composed of hydroxyapatite/polycaprolactone (HAp/PCL) composite nanofibers were fabricated with two different HAp concentrations—1% and 10% of the solid scaffold weight. After physico-chemical scaffold characterization, rat bone marrow stromal cells were cultured on the composite scaffolds in maintenance medium and then in osteogenic medium. Quantitative PCR, colorimetric assays, immunofluorescent labeling, and electron microscopy measured osteogenic cell responses to the HAp/PCL scaffolds. In maintenance conditions, both Hap/PCL scaffolds and control scaffolds supported cell colonization through seven days with minor differences. In osteogenic conditions, the 10% HAp scaffolds exhibited significantly increased ALP assay levels at week 3, consistent with previous reports. However, qPCR analysis demonstrated an overall decrease in bone matrix-associated genes on Hap/PCL scaffolds. Osteopontin and osteocalcin immunofluorescent microscopy revealed a trend that both mineralized scaffolds had greater amounts of both proteins, though qPCR results indicated the opposite trend for osteopontin. Additionally, type I collagen expression decreased on HAp scaffolds. These results indicate that cells are sensitive to minor changes in mineral content within nanofibers, even at just 1% w/w, and elucidating the sensing mechanism may lead to optimized osteogenic scaffold designs.

Microarray expression analysis of genes and pathways involved in growth plate cartilage injury responses and bony repair

The injured growth plate cartilage is often repaired by a bone bridge which causes bone growth deformities. Whilst previous studies have identified sequential inflammatory, fibrogenic, osteogenic and bone remodelling responses involved in the repair process, the molecular pathways which regulated these cellular events remain unknown. In a rat growth plate injury model, tissue from the injury site was collected across the time-course of bone bridge formation using laser capture microdissection and was subjected to Affymetrix microarray gene expression analysis. Real Time PCR and immunohistochemical analyses were used to confirm changes in levels of expression of some genes identified in microarray. Four major functional groupings of differentially expressed genes with known roles in skeletal development were identified across the time-course of bone bridge formation, including Wnt signalling (SFRP1, SFRP4, β-catenin, Csnk2a1, Tcf7, Lef1, Fzd1, Fzd2, Wisp1 and Cpz), BMP signalling (BMP-2, BMP-6, BMP-7, Chrd, Chrdl2 and Id1), osteoblast differentiation (BMP-2, BMP-6, Chrd, Hgn, Spp1, Axin2, β-catenin, Bglap2) and skeletal development (Chrd, Mmp9, BMP-1, BMP-6, Spp1, Fgfr1 and Traf6). These studies provide insight into the molecular pathways which act cooperatively to regulate bone formation following growth plate cartilage injury and highlight potential therapeutic targets to limit bone bridge formation.

Wnt/β-catenin pathway regulates bone morphogenetic protein (BMP2)-mediated differentiation of dental follicle cells

BACKGROUND AND OBJECTIVE

Bone morphogenetic protein 2 (BMP2)-induced osteogenic differentiation has been shown to occur through the canonical Wnt/βcatenin pathway, whereas factors promoting canonical Wnt signaling in cementoblasts inhibit cell differentiation and promote cell proliferation in vitro. The aim of this study was to investigate whether putative precursor cells of cementoblasts, dental follicle cells (murine SVF4 cells), when stimulated with BMP2, would exhibit changes in genes/proteins associated with the Wnt/β-catenin pathway.

MATERIAL AND METHODS

SVF4 cells were stimulated with BMP2, and the following assays were carried out: (i) Wnt/β-catenin pathway activation assessed by western blotting, β-catenin/transcription factor (TCF) reporter assays and expression of the lymphoid enhancer-binding factor-1 (Lef1), transcription factor 7 (Tcf7), Wnt inhibitor factor 1 (Wif1) and Axin2 (Axin2) genes; and (ii) cementoblast/osteoblast differentiation assessed by mineralization in vitro, and by the mRNA levels of runt-related transcription factor 2 (Runx2), osterix (Osx), alkaline phosphatase (Alp), osteocalcin (Ocn) and bone sialoprotein (Bsp), determined by quantitative PCR after treatment with wingless-type MMTV integration site family, member 3A (WNT3A) and knockdown of β-catenin.

RESULTS

WNT3A induced β-catenin nuclear translocation and up-regulated the transcriptional activity of a canonical Wnt-responsive reporter, suggesting that the Wnt/β-catenin pathway functions in SVF4 cells. Activation of Wnt signaling with WNT3A suppressed BMP2-mediated induction of cementoblast/osteoblast maturation of SVF4 cells. However, β-catenin knockdown showed that the BMP2-induced expression of cementoblast/osteoblast differentiation markers requires endogenous β-catenin. WNT3A down-regulated transcripts for Runx2, Alp and Ocn in SVF4 cells compared with untreated cells. In contrast, BMP2 induction of Bsp transcripts occurred independently of Wnt/β-catenin signaling.

CONCLUSION

These data suggest that stabilization of β-catenin by WNT3A inhibits BMP2-mediated induction of cementoblast/osteoblast differentiation in SVF4 cells, although BMP2 requires endogenous Wnt/β-catenin signaling to promote cell maturation.

HLA-B-associated transcript 3 (Bat3/Scythe) negatively regulates Smad phosphorylation in BMP signaling

Members of the transforming growth factor-β (TGF-β) superfamily participate in numerous biological phenomena in multiple tissues, including in cell proliferation, differentiation, and migration. TGF-β superfamily proteins therefore have prominent roles in wound healing, fibrosis, bone formation, and carcinogenesis. However, the molecular mechanisms regulating these signaling pathways are not fully understood. Here, we describe the regulation of bone morphogenic protein (BMP) signaling by Bat3 (also known as Scythe or BAG6). Bat3 overexpression in murine cell lines suppresses the activity of the Id1 promoter normally induced by BMP signaling. Conversely, Bat3 inactivation enhances the induction of direct BMP target genes, such as Id1, Smad6, and Smad7. Consequently, Bat3 deficiency accelerates the differentiation of primary osteoblasts into bone, with a concomitant increase in the bone differentiation markers Runx2, Osterix, and alkaline phosphatase. Using biochemical and cell biological analyses, we show that Bat3 inactivation sustains the C-terminal phosphorylation and nuclear localization of Smad1, 5, and 8 (Smad1/5/8), thereby enhancing biological responses to BMP treatment. At the mechanistic level, we show that Bat3 interacts with the nuclear phosphatase small C-terminal domain phosphatase (SCP) 2, which terminates BMP signaling by dephosphorylating Smad1/5/8. Notably, Bat3 enhances SCP2–Smad1 interaction only when the BMP signaling pathway is activated. Our results demonstrate that Bat3 is an important regulator of BMP signaling that functions by modulating SCP2–Smad interaction.

Spatial segregation of BMP/Smad signaling affects osteoblast differentiation in C2C12 cells

BACKGROUND

Bone morphogenetic proteins (BMPs) are involved in a plethora of cellular processes in embryonic development and adult tissue homeostasis. Signaling specificity is achieved by dynamic processes involving BMP receptor oligomerization and endocytosis. This allows for spatiotemporal control of Smad dependent and non-Smad pathways. In this study, we investigate the spatiotemporal regulation within the BMP-induced Smad transcriptional pathway.

METHODOLOGY/PRINCIPAL FINDINGS

Here we discriminate between Smad signaling events that are dynamin-dependent (i.e., require an intact endocytic pathway) and dynamin-independent. Inhibition of dynamin-dependent endocytosis in fluorescence microscopy and fractionation studies revealed a delay in Smad1/5/8 phosphorylation and nuclear translocation after BMP-2 stimulation of C2C12 cells. Using whole genome microarray and qPCR analysis, we identified two classes of BMP-2 induced genes that are differentially affected by inhibition of endocytosis. Thus, BMP-2 induced gene expression of Id1, Id3, Dlx2 and Hey1 is endocytosis-dependent, whereas BMP-2 induced expression of Id2, Dlx3, Zbtb2 and Krt16 is endocytosis-independent. Furthermore, we demonstrate that short term inhibition of endocytosis interferes with osteoblast differentiation as measured by alkaline phosphatase (ALP) production and qPCR analysis of osteoblast marker gene expression.

CONCLUSIONS/SIGNIFICANCE

Our study demonstrates that dynamin-dependent endocytosis is crucial for the concise spatial activation of the BMP-2 induced signaling cascade. Inhibition of endocytic processes during BMP-2 stimulation leads to altered Smad1/5/8 signaling kinetics and results in differential target gene expression. We show that interfering with the BMP-2 induced transcriptional network by endocytosis inhibition results in an attenuation of osteoblast differentiation. This implies that selective sensitivity of gene expression to endocytosis provides an additional mechanism for the cell to respond to BMP in a context specific manner. Moreover, we suggest a novel Smad dependent signal cascade induced by BMP-2, which does not require endocytosis.

Time series gene expression profiling and temporal regulatory pathway analysis of BMP6 induced osteoblast differentiation and mineralization

Background

BMP6 mediated osteoblast differentiation plays a key role in skeletal development and bone disease. Unfortunately, the signaling pathways regulated by BMP6 are largely uncharacterized due to both a lack of data and the complexity of the response.

Results

To better characterize the signaling pathways responsive to BMP6, we conducted a time series microarray study to track BMP6 induced osteoblast differentiation and mineralization. These temporal data were analyzed using a customized gene set analysis approach to identify temporally coherent sets of genes that act downstream of BMP6. Our analysis identified BMP6 regulation of previously reported pathways, such as the TGF-beta pathway. We also identified previously unknown connections between BMP6 and pathways such as Notch signaling and the MYB and BAF57 regulatory modules. In addition, we identify a super-network of pathways that are sequentially activated following BMP6 induction.

Conclusion

In this work, we carried out a microarray-based temporal regulatory pathway analysis of BMP6 induced osteoblast differentiation and mineralization using GAGE method. This novel temporal analysis is more informative and powerful than the classical static pathway analysis in that: (1) it captures the interconnections between signaling pathways or functional modules and demonstrates the even higher level organization of molecular biological systems; (2) it describes the temporal perturbation patterns of each pathway or module and their dynamic roles in osteoblast differentiation. The same set of experimental and computational strategies employed in our work could be useful for studying other complex biological processes.

Dose-dependent differential effects of risedronate on gene expression in osteoblasts

Bisphosphonates have multiple effects on bone. Their actions on osteoclasts lead to inhibition of bone resorption, at least partially through apoptosis. Effects on osteoblasts vary, with modifications in the molecule and concentration both resulting in qualitatively different responses. To understand the mechanism of the differential effects of high and low bisphosphonate concentrations on osteoblast activity, we compared the effects of 10⁻⁸ M and 10⁻⁴ M risedronate on gene expression in UMR-106 rat osteoblastic cells. Two targeted arrays, an 84-gene signaling array and an 84-gene osteogeneic array were used. Gene expression was measured at 1 and 24 h. Although some genes were regulated similarly by low and high concentrations of the drug, there was also differential regulation. At 1 h, 11 genes (1 signaling and 10 osteogenesis) were solely regulated by the low concentration, and 7 genes (3 signaling, 4 osteogenesis) were solely regulated by the high concentration. At 24 h, 8 genes (3 signaling, 5 osteogenesis) were solely regulated by the low concentration and 30 genes (16 signaling and 14 osteogenesis) were solely regulated by the high concentration. Interestingly, the low, but not the high concentration of risedronate transiently and selectively upregulated several genes associated with cell differentiation. A number of genes related to apoptosis were regulated, and could be involved in effects of bisphosphonates to promote osteoblast apoptosis. Also, observed gene changes associated with decreased angiogenesis and decreased metastasis could, if they occur in other cell types, provide a basis for the effectiveness of bisphosphonates in the prevention of cancer metastases.

Mesenchymal stem cells as therapeutics and vehicles for gene and drug delivery

Mesenchymal stem cells (MSCs) possess a set of several fairly unique properties which make them ideally suited both for cellular therapies/regenerative medicine, and as vehicles for gene and drug delivery. These include: 1) relative ease of isolation; 2) the ability to differentiate into a wide variety of seemingly functional cell types of both mesenchymal and non-mesenchymal origin; 3) the ability to be extensively expanded in culture without a loss of differentiative capacity; 4) they are not only hypoimmunogenic, but they produce immunosuppression upon transplantation; 5) their pronounced anti-inflammatory properties; and 6) their ability to home to damaged tissues, tumors, and metastases following in vivo administration. In this review, we summarize the latest research in the use of mesenchymal stem cells in regenerative medicine, as immunomodulatory/anti-inflammatory agents, and as vehicles for transferring both therapeutic genes in genetic disease and genes designed to destroy malignant cells.

Stem cell-related gene expression in clonal populations of mesenchymal stromal cells from bone marrow

Decline in the frequency of potent mesenchymal stem cells (MSCs) has been implicated in ageing and degenerative diseases. Increasing the circulating stem cell population can lead to renewed recruitment of these potent cells at sites of damage. Therefore, identifying the ideal cells for ex vivo expansion will form a major pursuit of clinical applications. This study is a follow-up of previous work that demonstrated the occurrence of fast-growing multipotential cells from the bone marrow samples. To investigate the molecular processes involved in the existence of such varying populations, gene expression studies were performed between fast- and slow-growing clonal populations to identify potential genetic markers associated with stemness using the quantitative real-time polymerase chain reaction comprising a series of 84 genes related to stem cell pathways. A group of 10 genes were commonly overrepresented in the fast-growing stem cell clones. These included genes that encode proteins involved in the maintenance of embryonic and neural stem cell renewal (sex-determining region Y-box 2, notch homolog 1, and delta-like 3), proteins associated with chondrogenesis (aggrecan and collagen 2 A1), growth factors (bone morphogenetic protein 2 and insulin-like growth factor 1), an endodermal organogenesis protein (forkhead box a2), and proteins associated with cell-fate specification (fibroblast growth factor 2 and cell division cycle 2). Expression of diverse differentiation genes in MSC clones suggests that these commonly expressed genes may confer the maintenance of multipotentiality and self-renewal of MSCs.

Epidermal growth factor and bone morphogenetic proteins upregulate osteoblast proliferation and osteoblastic markers and inhibit bone nodule formation

OBJECTIVE

The aim of this study was to investigate the in vitro osteogenic activity of EGF in association with bone morphogenetic proteins BMP2 and BMP7.

METHODS

SaOS-2 (osteoblast-like cell line from human osteosarcoma) were cultured in the presence of EGF and BMPs for various culture periods to assess (a) cell proliferation by MTT assay, (b) Runx2, alkaline phosphatase (ALP) and osteocalcin (OC) mRNA expression using quantitative RT-PCR and ELISA, and (c) bone tissue mineralization using Alizarin Red staining.

RESULTS

EGF alone was able to stimulate osteoblast growth in a time-dependent manner. When mixed with BMP2, BMP7, and their combination, EGF greatly promoted osteoblast growth, compared to the BMP- and EGF-stimulated cells, suggesting a possible synergistic effect between EGF and BMPs on osteoblast growth. Stimulation with EGF, EGF/BMP2, and EGF/BMP2/BMP7 for 7 days upregulated Runx2 mRNA expression by the osteoblasts. EGF downregulated ALP mRNA expression, which was recovered when the BMP2/BMP7 combination was added to the osteoblast culture. Tested on OC mRNA expression, EGF had no effect and inhibited the enhancing effect of BMP2 and BMP7 on osteocalcin expression. The bone mineralization assay showed that EGF reduced both the number and size of the bone nodules. This reducing effect was observable even in the presence of BMP2 and BMP7.

CONCLUSION

This study demonstrated that EGF may act in the early phase to promote osteoblast growth and specific marker expression rather than the late phase involving cell differentiation/mineralization.

Inhibition of methylation decreases osteoblast differentiation via a non-DNA-dependent methylation mechanism

S-adenosylmethionine (SAM)-dependent methylation of biological molecules including DNA and proteins is rapidly being uncovered as a critical mechanism for regulation of cellular processes. We investigated the effects of reduced SAM-dependent methylation on osteoblast differentiation by using periodate oxidized adenosine (ADOX), an inhibitor of SAM-dependent methyltransferases. The capacity of this agent to modulate osteoblast differentiation was analyzed under non-osteogenic control conditions and during growth factor-induced differentiation and compared with the effect of inhibition of DNA methylation by 5-Aza-2'-deoxycytidine (5-Aza-CdR). Without applying specific osteogenic triggers, both ADOX and 5-Aza-CdR induced mRNA expression of the osteoblast markers Alp, Osx, and Ocn in murine C2C12 cells. Under osteogenic conditions, ADOX inhibited differentiation of both human mesenchymal stem cells and C2C12 cells. Gene expression analysis of early (Msx2, Dlx5, Runx2) and late (Alp, Osx, Ocn) osteoblast markers during bone morphogenetic protein 2-induced C2C12 osteoblast differentiation revealed that ADOX only reduced expression of the late phase Runx2 target genes. By using a Runx2-responsive luciferase reporter (6xOSE), we showed that ADOX reduced the activity of Runx2, while 5-Aza-CdR had no effect. Taken together, our data suggest that decreased SAM-dependent methyltransferase activity leads to impaired osteoblast differentiation via non-DNA-dependent methylation mechanisms and that methylation is a regulator of Runx2-controlled gene expression.

Bone morphogenetic protein receptors and signal transduction

Bone morphogenetic proteins (BMPs) exhibit broad spectra of biological activities in various tissues, including bone, cartilage, blood vessels, heart, kidney, neurons, liver and lung. BMPs are members of the transforming growth factor-beta (TGF-beta) family that bind to type II and type I serine-threonine kinase receptors, and transduce signals through Smad and non-Smad signalling pathways. Recent findings have revealed that BMP signalling is finely tuned by various mechanisms in both positive and negative fashions. Perturbations of BMP signalling pathways are linked to a wide variety of clinical disorders, including vascular diseases, skeletal diseases and cancer. Administration of recombinant BMP ligands and increasing endogenous expression of BMPs provide therapeutic effects on some diseases. The recent development of BMP receptor inhibitors may also prove useful for some clinical diseases induced by hyperactivation of the BMP signalling pathways.

Identification of differentially expressed genes between osteoblasts and osteocytes

Osteocytes represent the most abundant cellular component of mammalian bones with important functions in bone mass maintenance and remodeling. To elucidate the differential gene expression between osteoblasts and osteocytes we completed a comprehensive analysis of their gene profiles. Selective identification of these two mature populations was achieved by utilization of visual markers of bone lineage cells. We have utilized dual GFP reporter mice in which osteocytes are expressing GFP (topaz) directed by the DMP1 promoter, while osteoblasts are identified by expression of GFP (cyan) driven by 2.3 kb of the Col1a1 promoter. Histological analysis of 7-day-old neonatal calvaria confirmed the expression pattern of DMP1GFP in osteocytes and Col2.3 in osteoblasts and osteocytes. To isolate distinct populations of cells we utilized fluorescent activated cell sorting (FACS). Cell suspensions were subjected to RNA extraction, in vitro transcription and labeling of cDNA and gene expression was analyzed using the Illumina WG-6v1 BeadChip. Following normalization of raw data from four biological replicates, 3444 genes were called present in all three sorted cell populations: GFP negative, Col2.3cyan(+) (osteoblasts), and DMP1topaz(+) (preosteocytes and osteocytes). We present the genes that showed in excess of a 2-fold change for gene expression between DMP1topaz(+) and Col2.3cyan(+) cells. The selected genes were classified and grouped according to their associated gene ontology terms. Genes clustered to osteogenesis and skeletal development such as Bmp4, Bmp8a, Dmp1, Enpp1, Phex and Ank were highly expressed in DMP1topaz(+)cells. Most of the genes encoding extracellular matrix components and secreted proteins had lower expression in DMP1topaz(+) cells, while most of the genes encoding plasma membrane proteins were increased. Interestingly a large number of genes associated with muscle development and function and with neuronal phenotype were increased in DMP1topaz(+) cells, indicating some new aspects of osteocyte biology. Although a large number of genes differentially expressed in DMP1topaz(+) and Col2.3cyan(+) cells in our study have already been assigned to bone development and physiology, for most of them we still lack any substantial data. Therefore, isolation of osteocyte and osteoblast cell populations and their subsequent microarray analysis allowed us to identify a number or genes and pathways with potential roles in regulation of bone mass.

Gene expression profile on chitosan/rhBMP-2 films: A novel osteoinductive coating for implantable materials

This study focusses on the gene expression profile related to a new rhBMP-2 carrier material, chitosan film. This film could be suitable for use as an osteoinductive coating of commercially available titanium implants. The developed material was characterized, biocompatibility was tested and the cellular response was extensively characterized by transcriptional expression studies. Finally, in vivo studies were carried out to confirm the osteoinductivity of the developed coating. Results show good material properties for cell adhesion and proliferation. Presented data show cellular differentiation to the osteoblastic phenotype due to rhBMP-2, with a 90% common transcriptional response between the control rhBMP-2 treatment and the developed chitosan/rhBMP-2 film. The growing surface also had an influence on the observed cellular response and was quantified as 7% of the total. These results indicate that both the growth factor and the material induce a cell response, but this is mainly driven by the osteoinductor factor. In vivo, new bone formation and early vascularization was observed around chitosan/rhBMP-2 coated titanium pieces implanted in mouse muscle. In contrast, control implants did not induce this reaction. This work, therefore, shows both in vitro and in vivo that chitosan/rhBMP-2 film is a promising osteoinductive coating for titanium implantable materials.

Notch1 represses osteogenic pathways in aortic valve cells

Calcific aortic stenosis is the third leading cause of adult heart disease and the most common form of acquired valvular disease in developed countries. However, the molecular pathways leading to calcification are poorly understood. We reported two families in which heterozygous mutations in NOTCH1 caused bicuspid aortic valve and severe aortic valve calcification. NOTCH1 is part of a highly conserved signaling pathway involved in cell fate decisions, cell differentiation, and cardiac valve formation. In this study, we examined the mechanism by which NOTCH1 represses aortic valve calcification. Heterozygous Notch1-null (Notch1(+/)(-)) mice had greater than fivefold more aortic valve calcification than age- and sex-matched wildtype littermates. Inhibition of Notch signaling in cultured sheep aortic valve interstitial cells (AVICs) also increased calcification more than fivefold and resulted in gene expression typical of osteoblasts. We found that Notch1 normally represses the gene encoding bone morphogenic protein 2 (Bmp2) in murine aortic valves in vivo and in aortic valve cells in vitro. siRNA-mediated knockdown of Bmp2 blocked the calcification induced by Notch inhibition in AVICs. These findings suggest that Notch1 signaling in aortic valve cells represses osteoblast-like calcification pathways mediated by Bmp2.

New insights into BMP-7 mediated osteoblastic differentiation of primary human mesenchymal stem cells

Bone Morphogenetic Proteins (BMPs) are members of the TGF-beta superfamily of growth factors. Several BMPs exhibit osteoinductive bioactivities, and are critical for bone formation in both developing and mature skeletal systems. BMP-7 (OP-1) is currently used clinically in revision of posterolateral spine fusions and long bone non-unions. The current study characterizes BMP-7 induced gene expression during early osteoblastic differentiation of human mesenchymal stem cells (hMSC). Primary hMSC were treated with BMP-7 for 24 or 120 h and gene expression across the entire human genome was evaluated using Affymetrix HG-U133 Plus 2.0 Arrays. 955 probe sets representing 655 genes and 95 ESTs were identified as differentially expressed and were organized into three major expression profiles (Profiles A, B and C) by hierarchical clustering. Genes from each profile were classified according to biochemical pathway analyses. Profile A, representing genes upregulated by BMP-7, revealed strong enrichment for established osteogenic marker genes, as well as several genes with undefined roles in osteoblast function, including MFI2, HAS3, ADAMTS9, HEY1, DIO2 and FGFR3. A functional screen using siRNA suggested roles for MFI2, HEY1 and DIO2 in osteoblastic differentiation of hMSC. Profile B contained genes transiently downregulated by BMP-7, including numerous genes associated with cell cycle regulation. Follow-up studies confirmed that BMP-7 attenuates cell cycle progression and cell proliferation during early osteoblastic differentiation. Profile C, comprised of genes continuously downregulated by BMP-7, exhibited strong enrichment for genes associated with chemokine/cytokine activity. Inhibitory effects of BMP-7 on cytokine secretion were verified by analysis of enriched culture media. Potent downregulation of CHI3L1, a potential biomarker for numerous joint diseases, was also observed in Profile C. A focused evaluation of BMP, GDF and BMP inhibitor expression elucidated feedback loops modulating BMP-7 bioactivity. BMP-7 was found to induce BMP-2 and downregulate GDF5 expression. Transient knockdown of BMP-2 using siRNA demonstrated that osteoinductive properties associated with BMP-7 are independent of endogenous BMP-2 expression. Noggin was identified as the predominant inhibitor induced by BMP-7 treatment. Overall, this study provides new insight into key bioactivities characterizing early BMP-7 mediated osteoblastic differentiation.

Signaling networks that control the lineage commitment and differentiation of bone cells

Osteoblasts and osteoclasts are the two major bone cells involved in the bone remodeling process. Osteoblasts are responsible for bone formation while osteoclasts are the bone-resorbing cells. The major event that triggers osteogenesis and bone remodeling is the transition of mesenchymal stem cells into differentiating osteoblast cells and monocyte/macrophage precursors into differentiating osteoclasts. Imbalance in differentiation and function of these two cell types will result in skeletal diseases such as osteoporosis, Paget's disease, rheumatoid arthritis, osteopetrosis, periodontal disease, and bone cancer metastases. Osteoblast and osteoclast commitment and differentiation are controlled by complex activities involving signal transduction and transcriptional regulation of gene expression. Recent advances in molecular and genetic studies using gene targeting in mice enable a better understanding of the multiple factors and signaling networks that control the differentiation process at a molecular level. This review summarizes recent advances in studies of signaling transduction pathways and transcriptional regulation of osteoblast and osteoclast cell lineage commitment and differentiation. Understanding the signaling networks that control the commitment and differentiation of bone cells will not only expand our basic understanding of the molecular mechanisms of skeletal development but will also aid our ability to develop therapeutic means of intervention in skeletal diseases.

Specific expression of BMP2/4 ortholog in biomineralizing tissues of corals and action on mouse BMP receptor

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor beta superfamily, and have been identified by their ability to induce bone formation in vertebrates. The biomineral-forming process, called biomineralization, is a widespread process, present in all kingdoms of living organisms and among which stony corals are one of the major groups of calcifying animals. Here, we report the presence of a BMP2/4 ortholog in eight species of adult corals. The synthesis of such a protein by the calcifying epithelium of corals suggests that coral BMP2/4 plays a role in skeletogenesis, making BMP the first common protein involved in biomineralization among Eumetazoans. In addition we show that recombinant coral BMP2/4 is able to inhibit human BMP2-induced osteoblastic differentiation in mesenchymal C2C12 cells. We suggest that this inhibition results from a competition between coral BMP2/4 and human BMP2, indicating conservation of binding affinity of BMP and its receptor during evolution from corals to vertebrates. Further studies are needed to understand interactions between coral BMP2/4 and its receptors, and, thus, the action of BMP2/4 in adult corals.

Hey1 basic helix-loop-helix protein plays an important role in mediating BMP9-induced osteogenic differentiation of mesenchymal progenitor cells

Pluripotent mesenchymal stem cells (MSCs) are bone marrow stromal progenitor cells that can differentiate into osteogenic, chondrogenic, adipogenic, and myogenic lineages. We previously demonstrated that bone morphogenetic protein (BMP) 9 is one of the most potent and yet least characterized BMPs that are able to induce osteogenic differentiation of MSCs both in vitro and in vivo. Here, we conducted gene expression-profiling analysis and identified that Hey1 of the hairy/Enhancer of split-related repressor protein basic helix-loop-helix family was among the most significantly up-regulated early targets in BMP9-stimulated MSCs. We demonstrated that Hey1 expression was up-regulated at the immediate early stage of BMP9-induced osteogenic differentiation. Chromatin immunoprecipitation analysis indicated that Hey1 may be a direct target of the BMP9-induced Smad signaling pathway. Silencing Hey1 expression diminished BMP9-induced osteogenic differentiation both in vitro and in vivo and led to chondrogenic differentiation. Likewise, constitutive Hey1 expression augmented BMP9-mediated bone matrix mineralization. Hey1 and Runx2 were shown to act synergistically in BMP9-induced osteogenic differentiation, and Runx2 expression significantly decreased in the absence of Hey1, suggesting that Runx2 may function downstream of Hey1. Accordingly, the defective osteogenic differentiation caused by Hey1 knockdown was rescued by exogenous Runx2 expression. Thus, our findings suggest that Hey1, through its interplay with Runx2, may play an important role in regulating BMP9-induced osteoblast lineage differentiation of MSCs.

Gene expression profile of the bone microenvironment in human fragility fracture bone

Osteoporosis (OP) is a common age-related systemic skeletal disease, with a strong genetic component, characterised by loss of bone mass and strength, which leads to increased bone fragility and susceptibility to fracture. Although some progress has been made in identifying genes that may contribute to OP disease, much of the genetic component of OP has yet to be accounted for. Therefore, to investigate the molecular basis for the changes in bone causally involved in OP and fragility fracture, we have used a microarray approach. We have analysed altered gene expression in human OP fracture bone by comparing mRNA in bone from individuals with fracture of the neck of the proximal femur (OP) with that from age-matched individuals with osteoarthritis (OA), and control (CTL) individuals with no known bone pathology. The OA sample set was included because an inverse association, with respect to bone density, has been reported between OA and the OP individuals. Compugen H19K oligo human microarray slides were used to compare the gene expression profiles of three sets of female samples comprising, 10 OP-CTL, 10 OP-OA, and 10 OA-CTL sample pairs. Using linear models for microarray analysis (Limma), 150 differentially expressed genes in OP bone with t scores >5 were identified. Differential expression of 32 genes in OP bone was confirmed by real time PCR analysis (p<0.01). Many of the genes identified have known or suspected roles in bone metabolism and in some cases have been implicated previously in OP pathogenesis. Three major sets of differentially expressed genes in OP bone were identified with known or suspected roles in either osteoblast maturation (PRRX1, ANXA2, ST14, CTSB, SPARC, FST, LGALS1, SPP1, ADM, and COL4A1), myelomonocytic differentiation and osteoclastogenesis (TREM2, ANXA2, IL10, CD14, CCR1, ADAM9, CCL2, CTGF, and KLF10), or adipogenesis, lipid and/or glucose metabolism (IL10, MARCO, CD14, AEBP1, FST, CCL2, CTGF, SLC14A1, ANGPTL4, ADM, TAZ, PEA15, and DOK4). Altered expression of these genes and others in these groups is consistent with previously suggested underlying molecular mechanisms for OP that include altered osteoblast and osteoclast differentiation and function, and an imbalance between osteoblastogenesis and adipogenesis.

Nuclear receptor profile in calvarial bone cells undergoing osteogenic versus adipogenic differentiation

Nuclear receptors (NRs) are key regulators of cell function and differentiation. We examined NR expression during osteogenic versus adipogenic differentiation of primary mouse calvarial osteoblasts (MOBs). MOBs were cultured for 21 days in osteogenic or adipogenic differentiation media. von Kossa and Oil Red O staining, and qRT-PCR of marker genes and 49 NRs were performed. PCR amplicons were subcloned to establish correct sequences and absolute standard curves. Forty-three NRs were detected at days 0-21. Uncentered average linkage hierarchical clustering identified four expression clusters: NRs (1) upregulated during osteogenic, but not adipogenic, differentiation, (2) upregulated in both conditions, with greater upregulation during adipogenic differentiation, (3) upregulated equally in both conditions, (4) downregulated during adipogenic, but not osteogenic, differentiation. One-way ANOVA with contrast revealed 20 NRs upregulated during osteogenic differentiation and 12 NRs upregulated during adipogenic differentiation. Two-way ANOVA demonstrated that 18 NRs were higher in osteogenic media, while 9 NRs were higher in adipogenic media. The time effect revealed 16 upregulated NRs. The interaction of condition with time revealed 6 NRs with higher expression rate during adipogenic differentiation and 3 NRs with higher expression rate during osteogenic differentiation. Relative NR abundance at days 0 and 21 were ranked. Basal ranking changed at least 5 positions for 13 NRs in osteogenic media and 9 NRs in adipogenic media. Osteogenic and adipogenic differentiation significantly altered NR expression in MOBs. These differences offer a fingerprint of cellular commitment and may provide clues to the underlying mechanisms of osteogenic versus adipogenic differentiation.

Zfp64 participates in Notch signaling and regulates differentiation in mesenchymal cells

Notch signaling is required for multiple aspects of tissue and cell differentiation. In this study, we identified zinc finger protein 64 (Zfp64) as a novel coactivator of Notch1. Zfp64 is associated with the intracellular domain of Notch1, recruited to the promoters of the Notch target genes Hes1 and Hey1, and transactivates them. Zfp64 expression is under the control of Runx2, and is upregulated by direct transactivation of its promoter. Zfp64 suppresses the myogenic differentiation of C2C12 cells and promotes their osteoblastic differentiation. Our data demonstrate two functions of Zfp64: (1) it is a downstream target of Runx2 and, (2) its cognate protein acts as a coactivator of Notch1, which suggests that Zfp64 mediates mesenchymal cell differentiation by modulating Notch signaling.

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.

Osteopenia in Sparc (osteonectin)-deficient mice: characterization of phenotypic determinants of femoral strength and changes in gene expression

Sparc null mutants have been generated independently via targeted mutations in exons 4 and 6. Previous studies have identified low-turnover osteopenia in the 129Sv/C57BL/6 exon 4 knockout. Since both Sparc null mutations result in complete absence of Sparc protein, similar phenotypic outcomes are likely. However, genetic background (strain) and/or linkage disequilibrium effects can influence phenotype. Different inactivating mutations should be tested in various mouse strains; similar phenotypic outcomes can then confidently be assigned to the mutated gene. We have evaluated the bone phenotype in the 129Sv/EvSparc(tm1cam) exon 6 knockout at 4 and 9 mo, using physical measurement, mechanical strength tests, and DXA scanning. We have also quantified bone marrow adiposity and circulating leptin levels to assess adipose tissue metabolism. 129Sv/EvSparc(tm1cam) null mice show decreased bone mineral density and bone mineral content and increased mechanical fragility of bone, in line with previous studies. Differences were also noted. Increased body weight and levels of bone marrow adiposity but decreased circulating leptin concentrations were identified at 4, but not 9 mo, and 129Sv/EvSparc(tm1cam) null mice also had shorter femurs. Molecular phenotyping was carried out using mouse HGMP NIA microarrays with cortical femur samples at various ages, using semiquantitative RT-PCR validation. We identified 429 genes highly expressed in normal bone. Six genes (Sparc, Zfp162, Bysl, E2F4, two ESTs) are differentially regulated in 129Sv/EvSparc(tm1cam) cortical femur vs. 129Sv/Ev controls. We confirm low-turnover osteopenia as a feature of the Sparc null phenotype, identifying the usefulness of this mouse as a model for human osteoporosis.

BMP-2 promotes differentiation of osteoblasts and chondroblasts in Runx2-deficient cell lines

To investigate the molecular mechanism underlying the differentiation of osteoblasts and chondroblasts, we established a clonal cell lines, RD-C6, from Runx2-deficient mouse embryos. RD-C6 cells expressed almost undetectable levels of phenotypes related to osteoblast and chondroblast differentiation at basal culture condition, whereas treatment with recombinant human bone morphogenetic protein-2 (rhBMP-2) or transduction of BMP-2 by adenovirus effectively induced this cell line to express mRNA related to the differentiation of osteoblasts and chondroblasts including alkaline phosphatase, osteocalcin, and osterix. Transduction of Runx2 also induced the expression of these mRNA in RD-C6 cells. BMP-2 transduction increased expression levels of mRNA for Msx2 and Dlx5, but Runx2 transduction induced no significant increases in expression levels of these mRNA. Microarray analysis using RD-C6 cells with or without rhBMP-2 treatment demonstrated that BMP-2 upregulated 66 genes including 13 transcription-related molecules such as Id1, Id2, Id4, Hey1, Smad6, Smad7, and Msx2. To confirm bone and cartilage formation ability of RD-C6 cells, we transplanted RD-C6 cells into the peritoneal cavity of athymic mice using diffusion chambers with rhBMP-2. RD-C6 cells generated unmineralized cartilage but not bone. These results indicate that BMP-2 induces Runx2-deficient cells to express markers related to osteoblast and chondroblast differentiation using a Runx2-independent pathway, but it failed to induce these cells to differentiate into bone-forming osteoblasts and mature chondrocytes.

Signaling and transcriptional regulation in osteoblast commitment and differentiation

The major event that triggers osteogenesis is the transition of mesenchymal stem cells into bone forming, differentiating osteoblast cells. Osteoblast differentiation is the primary component of bone formation, exemplified by the synthesis, deposition and mineralization of extracellular matrix. Although not well understood, osteoblast differentiation from mesenchymal stem cells is a well-orchestrated process. Recent advances in molecular and genetic studies using gene targeting in mouse enable a better understanding of the multiple factors and signaling networks that control the differentiation process at a molecular level. Osteoblast commitment and differentiation are controlled by complex activities involving signal transduction and transcriptional regulation of gene expression. We review Wnt signaling pathway and Runx2 regulation network, which are critical for osteoblast differentiation. Many other factors and signaling pathways have been implicated in regulation of osteoblast differentiation in a network manner, such as the factors Osterix, ATF4, and SATB2 and the TGF-beta, Hedgehog, FGF, ephrin, and sympathetic signaling pathways. This review summarizes the recent advances in the studies of signaling transduction pathways and transcriptional regulation of osteoblast cell lineage commitment and differentiation. The knowledge of osteoblast commitment and differentiation should be applied towards the development of new diagnostic and therapeutic alternatives for human bone diseases.

Dickkopf homolog 1 mediates endothelin-1-stimulated new bone formation

Tumor-produced endothelin-1 (ET-1) stimulates osteoblasts to form new bone and is an important mediator of osteoblastic bone metastasis. The anabolic actions of ET-1 in osteoblasts were investigated by gene microarray analyses of murine neonatal calvarial organ cultures. Targets of ET-1 action were validated by real-time RT-PCR in murine primary osteoblast cultures. IL-6, IL-11, the CCN (CYR61, CTGF, NOV) family members cysteine-rich protein 61 and connective tissue growth factor, inhibin beta-A, serum/glucocorticoid regulated kinase, receptor activator of nuclear factor kappaB ligand, snail homolog 1, tissue inhibitor of metalloproteinase 3, and TG-interacting factor transcripts were increased by ET-1. ET-1 decreased the transcript for the Wnt signaling pathway inhibitor, dickkopf homolog 1 (Dkk1). Calvarial organ cultures treated with ET-1 had lower concentrations of DKK1 protein in conditioned media than control cultures. High DKK1 concentrations in bone marrow suppress bone formation in multiple myeloma. We hypothesized that the converse occurs in osteoblastic bone metastasis, where ET-1 stimulates osteoblast activity by reducing autocrine production of DKK1. Recombinant DKK1 blocked ET-1-mediated osteoblast proliferation and new bone formation in calvarial organ cultures, whereas a DKK1-neutralizing antibody increased osteoblast numbers and new bone formation. ET-1 directed nuclear translocation of beta-catenin in osteoblasts, indicating activation of the Wnt signaling pathway. The data suggest that ET-1 increases osteoblast proliferation and new bone formation by activating the Wnt signaling pathway through suppression of the Wnt pathway inhibitor DKK1.