Identification and analysis of type II TGF-β receptors in BMP-9-induced osteogenic differentiation of C3H10T1/2 mesenchymal stem cells

Targeting transforming growth factor beta signaling in metastatic osteosarcoma

Osteosarcoma is a rare type of bone cancer, and half of the cases affect children and adolescents younger than 20 years of age. Despite intensive efforts to improve both chemotherapeutics and surgical management, the clinical outcome for metastatic osteosarcoma remains poor. Transforming growth factor β (TGF-β) is one of the most abundant growth factors in bones. The TGF-β signaling pathway has complex and contradictory roles in the pathogenesis of human cancers. TGF-β is primarily a tumor suppressor that inhibits proliferation and induces apoptosis of premalignant epithelial cells. In the later stages of cancer progression, however, TGF-β functions as a metastasis promoter by promoting tumor growth, inducing epithelial-mesenchymal transition (EMT), blocking antitumor immune responses, increasing tumor-associated fibrosis, and enhancing angiogenesis. In contrast with the dual effects of TGF-β on carcinoma (epithelial origin) progression, TGF-β seems to mainly have a pro-tumoral effect on sarcomas including osteosarcoma (mesenchymal origin). Many drugs that target TGF-β signaling have been developed: neutralizing antibodies that prevent TGF-β binding to receptor complexes; ligand trap employing recombinant Fc-fusion proteins containing the soluble ectodomain of either type II (TβRII) or the type III receptor ((TβRIII), preventing TGF-β from binding to its receptors; antisense nucleotides that reduce TGF-β expression at the transcriptional/translational level; small molecule inhibitors of serine/threonine kinases of the type I receptor (TβRI) preventing downstream signaling; and vaccines that contain cell lines transfected with TβRII antisense genes, or target furin convertase, resulting in reduced TGF-β signaling. TGF-β antagonists have been shown to have effects on osteosarcoma in vitro and in vivo. One of the small molecule TβRI inhibitors, Vactosertib, is currently undergoing a phase 1/2 clinical trial to evaluate its effect on osteosarcoma. Several phase 1/2/3 clinical trials have shown TGF-β antagonists are safe and well tolerated. For instance, Luspatercept, a TGF-β ligand trap, has been approved by the FDA for the treatment of anemia associated with myeloid dysplastic syndrome (MDS) with ring sideroblasts/mutated SF3B1 with acceptable safety. Clinical trials evaluating the long-term safety of Luspatercept are in process.

The effects of Gli1 and Gli2 on BMP9-induced osteogenic differentiation of mesenchymal stem cells

Diseases, such as bone nonunion with bone defects, osteoporosis, etc, seriously endanger people's quality of life, and bone tissue engineering based on mesenchymal stem cells is an effective method to solve such problems. Several studies have shown that BMP9 can effectively promote osteogenic differentiation of MSCs, but the underlying molecular mechanisms are still unclear. Gli1 and Gli2 were important transcription factors and play an important role in the Hedgehog signaling pathway. In this study, we investigated the role of Gli1 and Gli2 in BMP9-induced osteogenic differentiation of MSCs. We found that inhibition of Gli1 and Gli2 weakened BMP9-induced osteogenic differentiation of MSCs, and early osteogenic markers (alkaline phosphatase, ALP), late osteogenic markers (calcium salt deposition), the expression of pivotal osteogenic markers were attenuated, and inhibition of Gli1 and Gli2 weakened the expression of p-Smad1/5/8 and p-p38 induced by BMP9. In conclusion, our study shows that Gli1 and Gli2 play an important role in BMP9-induced osteogenic differentiation.

Early induction of Hes1 by bone morphogenetic protein 9 plays a regulatory role in osteoblastic differentiation of a mesenchymal stem cell line

Bone morphogenic protein 9 (BMP9) is one of the most potent inducers of osteogenic differentiation among the 14 BMP members, but its mechanism of action has not been fully demonstrated. Hes1 is a transcriptional regulator with basic helix-loop-helix (bHLH) domain and is a well-known Notch effector. In this study, we investigated the functional roles of early induction of Hes1 by BMP9 in a mouse mesenchymal stem cell line, ST2. Hes1 mRNA was transiently and periodically induced by BMP9 in ST2, which was inhibited by BMP signal inhibitors but not by Notch inhibitor. Interestingly, Hes1 knockdown in ST2 by siRNA increased the expression of osteogenic differentiation markers such as Sp7 and Ibsp and matrix mineralization in comparison with control siRNA transfected ST2. In contrast, forced expression of Hes1 by using the Tet-On system suppressed the expression of osteogenic markers and matrix mineralization by BMP9. We also found that the early induction of Hes1 by BMP9 suppressed the expression of Alk1, an essential receptor for BMP9. In conclusion, BMP9 rapidly induces the expression of Hes1 via the SMAD pathway in ST2 cells, which plays a negative regulatory role in osteogenic differentiation of mesenchymal stem cells induced by BMP9.

Role of p53 in promoting BMP9‑induced osteogenic differentiation of mesenchymal stem cells through TGF‑β1

Known as a tumour suppressor gene, p53 also plays a key role in controlling the differentiation of mesenchymal stem cells (MSCs). Bone morphogenetic protein 9 (BMP9) has been identified as a potent factor in inducing osteogenic differentiation of MSCs, but its relationship with p53 remains unclear. The present study revealed that TP53 was expressed at higher levels in MSCs from patients with osteoporosis and was associated with the top 10 core central genes found in the current osteoporosis genetic screen. p53 was expressed in C2C12, C3H10T1/2, 3T3-L1, MEFs, and MG-63 cell lines, and could be upregulated by BMP9, as measured by western blotting and reverse-transcription quantitative PCR (RT-qPCR). Furthermore, overexpression of p53 increased the mRNA and protein levels of osteogenic marker Runx2 and osteopontin, as evaluated by western blotting and RT-qPCR in BMP9-induced MSCs, whereas the p53 inhibitor pifithrin (PFT)-α attenuated these effects. The same trend was found in alkaline phosphatase activities and matrix mineralization, as measured by alkaline phosphatase staining and alizarin red S staining. Moreover, p53 overexpression reduced adipo-differentiation markers of PPARγ and lipid droplet formation, as measured by western blotting, RT-qPCR and oil red O staining, respectively, whereas PFT-α facilitated adipo-differentiation in MSCs. In addition, p53 promoted TGF-β1 expression and inhibition of TGF-β1 by LY364947 partially attenuated the effects of p53 on promoting BMP9-induced MSC osteo-differentiation and inhibiting adipo-differentiation. The inhibitory effect of PFT-α on osteogenic markers and the promoting effect on adipogenic markers can be reversed when combined with TGF-β1. TGF-β1 may enhance the promotion of osteo-differentiation of MSCs by p53 through inhibition of adipo-differentiation. Collectively, by promoting BMP9-induced MSCs bone differentiation and inhibiting adipose differentiation, p53 may be a novel therapeutic target for bone-related diseases.

Establishing and characterizing human stem cells from the apical papilla immortalized by hTERT gene transfer

Stem cells from the apical papilla (SCAPs) are promising candidates for regenerative endodontic treatment and tissue regeneration in general. However, harvesting enough cells from the limited apical papilla tissue is difficult, and the cells lose their primary phenotype over many passages. To get over these challenges, we immortalized human SCAPs with lentiviruses overexpressing human telomerase reverse transcriptase (hTERT). Human immortalized SCAPs (hiSCAPs) exhibited long-term proliferative activity without tumorigenic potential. Cells also expressed mesenchymal and progenitor biomarkers and exhibited multiple differentiation potentials. Interestingly, hiSCAPs gained a stronger potential for osteogenic differentiation than the primary cells. To further investigate whether hiSCAPs could become prospective seed cells in bone tissue engineering, in vitro and in vivo studies were performed, and the results indicated that hiSCAPs exhibited strong osteogenic differentiation ability after infection with recombinant adenoviruses expressing BMP9 (AdBMP9). In addition, we revealed that BMP9 could upregulate ALK1 and BMPRII, leading to an increase in phosphorylated Smad1 to induce the osteogenic differentiation of hiSCAPs. These results support the application of hiSCAPs in tissue engineering/regeneration schemes as a stable stem cell source for osteogenic differentiation and biomineralization, which could be further used in stem cell-based clinical therapies.

The potential regulatory role of BMP9 in inflammatory responses

Inflammation is a protective response of the body to pathogens and injury. Hence, it is particularly important to explore the pathogenesis and key regulatory factors of inflammation. BMP9 is a unique member of the BMP family, which is widely known for its strong osteogenic potential and insensitivity to the inhibition of BMP3. Recently, several studies have reported an underlying pivotal link between BMP9 and inflammation. What is clear, though not well understood, is that BMP9 plays a role in inflammation in a carefully choreographed manner in different contexts. In this review, we have summarized current studies focusing on BMP9 and inflammation in various tissues and the latest advances in BMP9 expression, signal transduction, and crystal structure to better understand the relationship between BMP9 and inflammation. In addition, we also briefly summarized the inflammatory characteristics of some TGF-β superfamily members to provide better insights and ideas for the study of BMP9 and inflammation.

Principles of the activin receptor signaling pathway and its inhibition

This review captures the anabolic and stimulatory effects observed with inhibition of the transforming growth factor β superfamily in muscle, blood, and bone. New medicinal substances that rectify activin, myostatin, and growth differentiation factor 11 signaling give hope to the many whose lives are affected by deterioration of these tissues. The review first covers the origin, structure, and common pathway of activins, myostatin, and growth differentiation factor 11 along with the pharmacodynamics of the new class of molecules designed to oppose the activin receptor signaling pathway. Current terminology surrounding this new class of molecules is inconsistent and does not infer functionality. Adopting inhibitors of the activin receptor signaling pathway (IASPs) as a generic term is proposed because it encapsulates the molecular mechanisms along the pathway trajectory. To conclude, a pragmatic classification of IASPs is presented that integrates functionality and side effects based on the data available from animals and humans. This provides researchers and clinicians with a tool to tailor IASPs therapy according to the need of projects or patients and with respect to side effects.

Osteogenic differentiation cues of the bone morphogenetic protein-9 (BMP-9) and its recent advances in bone tissue regeneration

Bone regeneration using bioactive molecules and biocompatible materials is growing steadily with the advent of the new findings in cellular signaling. Bone Morphogenetic Protein (BMP)-9 is a considerably recent discovery from the BMP family that delivers numerous benefits in osteogenesis. The Smad cellular signaling pathway triggered by BMPs is often inhibited by Noggin. However, BMP-9 is resistant to Noggin, thus, facilitating a more robust cellular differentiation of osteoprogenitor cells into preosteoblasts and osteoblasts. This review encompasses a general understanding of the Smad signaling pathway activated by the BMP-9 ligand molecule with its specific receptors. The robust osteogenic cellular differentiation cue provided by BMP-9 has been reviewed from a bone regeneration perspective with several in vitro as well as in vivo studies reporting promising results for future research. The effect of the biomaterial, chosen in such studies as the scaffold or carrier matrix, on the activity of BMP-9 and subsequent bone regeneration has been highlighted in this review. The non-viral delivery technique for BMP-9 induced bone regeneration is a safer alternative to its viral counterpart. The recent advances in non-viral BMP-9 delivery have also highlighted the efficacy of the protein molecule at a low dosage. This opens a new horizon as a more efficient and safer alternative to BMP-2, which was prevalent among clinical trials; however, BMP-2 applications have reported its downsides during bone defect healing such as cystic bone formation.

Bone morphogenetic protein 9 (BMP9) directly induces Notch effector molecule Hes1 through the SMAD signaling pathway in osteoblasts

Bone morphogenetic protein (BMP) 9 is one of the most osteogenic BMPs, but its mechanism of action has not been fully elucidated. Hes1, a transcriptional regulator with a basic helix-loop-helix domain, is a well-known effector of Notch signaling. Here, we find that BMP9 induces periodic increases of Hes1 mRNA and protein expression in osteoblasts, presumably through an autocrine negative feedback mechanism. BMP9-mediated Hes1 induction is significantly inhibited by an ALK inhibitor and overexpression of Smad7, an inhibitory Smad. Luciferase and ChIP assays revealed that two Smad-binding sites in the 5' upstream region of the mouse Hes1 gene are essential for transcriptional activation by BMP9. Thus, our data indicate that BMP9 induces Hes1 expression in osteoblasts via the Smad signaling pathway.

BMP9 is a potential therapeutic agent for use in oral and maxillofacial bone tissue engineering

Oral and maxillofacial surgery is often challenging due to defective bone healing owing to the microbial environment of the oral cavity, the additional involvement of teeth and esthetic concerns. Insufficient bone volume as a consequence of aging and some oral and maxillofacial surgical procedures, such as tumor resection of the jaw, may further impact facial esthetics and cause the failure of certain procedures, such as oral and maxillofacial implantation. Bone morphogenetic protein (BMP) 9 (BMP9) is one of the most effective BMPs to induce the osteogenic differentiation of different stem cells. A large cross-talk network that includes the BMP9, Wnt/β, Hedgehog, EGF, TGF-β and Notch signaling pathways finely regulates osteogenesis induced by BMP9. Epigenetic control during BMP9-induced osteogenesis is mainly dependent on histone deacetylases (HDACs), microRNAs (miRNAs) and long noncoding RNAs (lncRNAs), which adds another layer of complexity. As a result, all these factors work together to orchestrate the molecular and cellular events underlying BMP9-related tissue engineering. In this review, we summarize our current understanding of the SMAD-dependent and SMAD-independent BMP9 pathways, with a particular focus on cross-talk and cross-regulation between BMP9 and other major signaling pathways in BMP9-induced osteogenesis. Furthermore, recently discovered epigenetic regulation of BMP9 pathways and the molecular and cellular basis of the application of BMP9 in tissue engineering in current oral and maxillofacial surgery and other orthopedic-related clinical settings are also discussed.

TGFβ1 induces bone formation from BMP9-activated Bone Mesenchymal Stem Cells, with possible involvement of non-canonical pathways

Reconstruction of bone defects is one of the most substantial and difficult clinical challenges in orthopedics. Transforming growth factor beta 1 (TGFβ1) might play an important role in stimulating osteogenic differentiation of bone morphogenetic protein 9 (BMP9)-induced C3H10T1/2 mesenchymal stem cells. In our current study, we examined the potential synergy between TGFβ1 and BMP9 in promoting the osteogenesis of C3H10T1/2 cells, and whether such effects could contribute to bone formation in vivo. Our experiment data indicated that TGFβ1 could increase the expression of osteogenic markers and the formation of mineralized calcium nodules in, while suppressing the proliferation of, BMP9-induced C3H10T1/2 cells. Furthermore, mice intramuscularly injected with BMP9/TGFβ1-transduced C3H10T1/2 cells into the gastrocnemius muscle on their tibiae developed ectopic bone masses with more mature osteoid structures, compared to those grafted with cells expressing BMP9/RFP. Subsequent mechanistic studies found that TGFβ1-induced enhancement of osteogenesis in BMP9-overexpressing C3H10T1/2 cells was accompanied by augmented expression of heat shock protein 47 (HSP47), a collagen-specific molecular chaperone essential for collagen biosynthesis, and can be attenuated by pirfenidone, a known anti-fibrotic inhibitor. Interestingly, protein microarray analysis suggested that TGFβ1/BMP9-dependent osteogenesis of C3H10T1/2 cells seemed to involve several non-canonical signaling pathways such as Janus kinase-signal transducer and activator of transcription, phosphoinositide-3-kinase-protein kinase B, and mitogen-activated protein kinase. These results provided further evidence that TGFβ1 could promote bone formation from BMP9-induced C3H10T1/2 cells and shed important light on the underlying molecular mechanisms.

A PINCH-1-Smurf1 signaling axis mediates mechano-regulation of BMPR2 and stem cell differentiation

Mechanical cues from extracellular matrix exert strong effects on stem cell differentiation. This study finds that a signaling axis consisting of PINCH-1, Smurf1, and BMPR2 senses mechanical signals from extracellular matrix and regulates BMP signaling and mesenchymal stem cell differentiation.

Mechano-environment plays multiple critical roles in the control of mesenchymal stem cell (MSC) fate decision, but the underlying signaling mechanisms remain undefined. We report here a signaling axis consisting of PINCH-1, SMAD specific E3 ubiquitin protein ligase 1 (Smurf1), and bone morphogenetic protein type 2 receptor (BMPR2) that links mechano-environment to MSC fate decision. PINCH-1 interacts with Smurf1, which inhibits the latter from interacting with BMPR2 and consequently suppresses BMPR2 degradation, resulting in augmented BMP signaling and MSC osteogenic differentiation (OD). Extracellular matrix (ECM) stiffening increases PINCH-1 level and consequently activates this signaling axis. Depletion of PINCH-1 blocks stiff ECM-induced BMP signaling and OD, whereas overexpression of PINCH-1 overrides signals from soft ECM and promotes OD. Finally, perturbation of either Smurf1 or BMPR2 expression is sufficient to block the effects of PINCH-1 on BMP signaling and MSC fate decision. Our findings delineate a key signaling mechanism through which mechano-environment controls BMPR2 level and MSC fate decision.

The wonders of BMP9: From mesenchymal stem cell differentiation, angiogenesis, neurogenesis, tumorigenesis, and metabolism to regenerative medicine

Although bone morphogenetic proteins (BMPs) initially showed effective induction of ectopic bone growth in muscle, it has since been determined that these proteins, as members of the TGF-β superfamily, play a diverse and critical array of biological roles. These roles include regulating skeletal and bone formation, angiogenesis, and development and homeostasis of multiple organ systems. Disruptions of the members of the TGF-β/BMP superfamily result in severe skeletal and extra-skeletal irregularities, suggesting high therapeutic potential from understanding this family of BMP proteins. Although it was once one of the least characterized BMPs, BMP9 has revealed itself to have the highest osteogenic potential across numerous experiments both in vitro and in vivo, with recent studies suggesting that the exceptional potency of BMP9 may result from unique signaling pathways that differentiate it from other BMPs. The effectiveness of BMP9 in inducing bone formation was recently revealed in promising experiments that demonstrated efficacy in the repair of critical sized cranial defects as well as compatibility with bone-inducing bio-implants, revealing the great translational promise of BMP9. Furthermore, emerging evidence indicates that, besides its osteogenic activity, BMP9 exerts a broad range of biological functions, including stem cell differentiation, angiogenesis, neurogenesis, tumorigenesis, and metabolism. This review aims to summarize our current understanding of BMP9 across biology and the body.

Advances in the molecular regulation of endothelial BMP9 signalling complexes and implications for cardiovascular disease

Bone morphogenetic protein 9 (BMP9), a member of the transforming growth factor β (TGFβ) superfamily, is a circulating vascular quiescence and endothelial protective factor, accounting for the majority of BMP activities in plasma. BMP9 and BMP10 bind preferentially to the high-affinity type I receptor activin receptor-like kinase 1 on vascular endothelial cells. Recently, many reports have highlighted the important roles of BMP9 in cardiovascular disease, particularly pulmonary arterial hypertension. In vivo, BMP9 activity and specificity are determined by tightly regulated protein-protein recognition with cognate receptors and a co-receptor, and may also be influenced by other proteins present on the endothelial cell surface (such as low-affinity receptors) and in circulation (such as TGFβ family ligands competing for the same receptors). In this review, we summarise recent findings on the role and therapeutic potential of BMP9 in cardiovascular disease and review the current understanding of how the extracellular protein-protein interaction milieu could play a role in regulating endothelial BMP9 signalling specificity and activity.

Dentinogenesis and Tooth-Alveolar Bone Complex Defects in BMP9/GDF2 Knockout Mice

Tooth development is regulated by sequential and reciprocal epithelium-mesenchymal interactions and their related molecular signaling pathways, such as bone morphogenetic proteins (BMPs). Among the 14 types of BMPs, BMP9 (also known as growth differentiation factor 2) is one of the most potent BMPs to induce osteogenic differentiation of mesenchymal stem cells. The purpose of this study was to examine potential roles of BMP9 signaling in tooth development. First, we detected the expression pattern of BMP9 in tooth germ during postnatal tooth development, and we found that BMP9 was widely expressed in odontoblasts, ameloblasts, dental pulp cells, and osteoblasts in alveolar bones. Then, we established a BMP9-KO mouse model. Gross morphological examination revealed that the tooth cusps of BMP9-KO mice were significantly abraded with shorter roots. Micro-computed tomography and three-dimensional reconstruction analysis indicated that the first molars of the BMP9-KO mice exhibited a reduced thickness dentin, enlarged pulp canals, and shortened roots, resembling the phenotypes of the common hereditary dental disease dentinogenesis imperfecta. Further, the alveolar bone of the BMP9-KO mutants was found to be shorter and had a decreased mineral density and trabecular thickness and bone volume fraction compared with that of the wild-type control. Mechanistically, we demonstrated that both dentin sialophosphoprotein and dentin matrix protein 1 were induced in dental stem cells by BMP9, whereas their expression was reduced when BMP9 was silenced. Further studies are required to determine whether loss of or decreased BMP9 expression is clinically associated with dentinogenesis imperfecta. Collectively, our results strongly suggest that BMP9 may play an important role in regulating dentinogenesis and tooth development. Further research is recommended into the therapeutic uses of BMP9 to regenerate traumatized and diseased tissues and for the bioengineering of replacement teeth.

N-(3-methoxybenzyl)-(9Z,12Z,15Z)-octadecatrienamide promotes bone formation via the canonical Wnt/β-catenin signaling pathway

Osteoporosis is characterized by low bone mineral density and microarchitectural deterioration of bone tissue. N-(3-methoxybenzyl)-(9Z,12Z,15Z)-octadecatrienamide (MBOC) is one of the macamides isolated from Maca (Lepidium meyenii Walp.), a cruciferous plant from the Andes of Peru. In this study, C3H/10T1/2 mesenchymal stem cells were treated with MBOC in osteogenic induction medium. An ovariectomized (OVX) mouse model was used to investigate the effect of 1-month MBOC treatment on the prevention of postmenopausal osteoporosis. Remarkably, trabecular thickness, trabecular number, and bone volume/tissue volume of the distal femoral metaphysis were significantly increased in OVX + MBOC mice compared with OVX mice, as revealed by microcomputed tomography analysis. Trabecular separation was decreased in OVX + MBOC mice compared with OVX mice. Consistently, MBOC increased the levels of osteocalcin and runt-related transcription factor 2 in OVX mice, as well as the expression of runt-related transcription factor 2, osterix, and alkaline phosphatase in C3H/10T1/2 cells. Mechanistically, MBOC activates the canonical Wnt/β-catenin signaling pathway via inhibiting phosphorylation of GSK-3β at Tyr216 and maintaining β-catenin expression. Collectively, the current study demonstrates the robustness of MBOC in the induction of mesenchymal stem cells osteogenic differentiation and consequent bone formation, suggesting that MBOC may be a potentially effective drug to treat postmenopausal osteoporosis.

Clinical significance linked to functional defects in bone morphogenetic protein type 2 receptor, BMPR2

Bone morphogenetic protein type 2 receptor (BMPR2) is one of the transforming growth factor-β (TGF-β) superfamily receptors, performing diverse roles during embryonic development, vasculogenesis, and osteogenesis. Human BMPR2 consists of 1,038 amino acids, and contains functionally conserved extracellular, transmembrane, kinase, and C-terminal cytoplasmic domains. Bone morphogenetic proteins (BMPs) engage the tetrameric complex, composed of BMPR2 and its corresponding type 1 receptors, which initiates SMAD proteins-mediated signal transduction leading to the expression of target genes implicated in the development or differentiation of the embryo, organs and bones. In particular, genetic alterations of BMPR2 gene are associated with several clinical disorders, including representative pulmonary arterial hypertension, cancers, and metabolic diseases, thus demonstrating the physiological importance of BMPR2. In this mini review, we summarize recent findings regarding the molecular basis of BMPR2 functions in BMP signaling, and the versatile roles of BMPR2. In addition, various aspects of experimentally validated pathogenic mutations of BMPR2 and the linked human diseases will also be discussed, which are important in clinical settings for diagnostics and treatment.

Hmox1 promotes osteogenic differentiation at the expense of reduced adipogenic differentiation induced by BMP9 in C3H10T1/2 cells

Mesenchymal stem cells (MSCs) are multipotent progenitors that can differentiate into a variety of cell types under proper stimuli. Bone morphogenetic protein 9 (BMP9) is able to simultaneously induce both adipogenic and osteogenic differentiation of MSCs although the regulatory molecules involved remain to be fully identified and characterized. Heme oxygenase 1 (Hmox1) plays an essential role not only in fat metabolism, but also in bone development. In the present study, we investigated the functional role of Hmox1 in BMP9-induced osteogenic/adipogenic differentiation in MSCs line C3H10T1/2 and probed the possible mechanism involved. We found that BMP9 promoted the endogenous expression of Hmox1 in C3H10T1/2 cells. Overexpression of Hmox1 or cobalt protoporphyrin (CoPP), an inducer of Hmox1, increased BMP9-induced osteogenic differentiation in vitro. Subcutaneous stem cell implantation in nude mice further confirmed that Hmox1 potentiated BMP9-induced ectopic bone formation in vivo. In contrast, Hmox1 reduced BMP9-induced adipogenic differentiation in C3H10T1/2 cells. Although had no obvious effect on BMP9-induced Smad1/5/8 phosphorylation, Hmox1 enhanced phosphorylation of p38, and AKT, while decreased phosphorylation of ERK1/2. Furthermore, Hmox1 increased total β-catenin protein level, and promoted the nuclear translocation of β-catenin in C3H10T1/2 cells. Taken together, our study strongly suggests that Hmox1 is likely to potentiate osteogenic differentiation and yet decrease adipogenic differentiation induced by BMP9 possibly through regulation of multiple signaling pathways.

The healing of alveolar bone defects with novel bio-implants composed of Ad-BMP9-transfected rDFCs and CHA scaffolds

Cells, scaffolds, and growth factors play important roles in bone regeneration. Bone morphogenetic protein 9 (BMP9), a member of BMP family, could facilitate osteogenesis by regulating growth factors and promoting angiogenesis. Similar to other stem cells, rat dental follicle stem cells (rDFCs), the precursor cells of cementoblasts, osteoblasts and periodontal ligament cells, can self-renew and exhibit multipotential capacity. Coralline hydroxyapatite (CHA) has good biocompatibility and conductivity required for bone tissue engineering. Here, we reported that BMP9 could enhance the osteogenic differentiation of rDFCs in cell culture. Moreover, our results suggested that BMP9 acted through the Smad1/5/8 signaling pathway. We also produced a novel scaffold that encompasses bio-degradable CHA seeded with recombinant adenoviruses expressing BMP9-transfected rDFCs (Ad-BMP9-transfected rDFCs). With this implant, we achieved more alveolar bone regeneration in the alveolar bone defect compared to blank group, CHA group and rDFCs group. Our results provided a novel bio-implants composed of Ad-BMP9-transfected rDFCs and CHA scaffolds and its mechanism is regarding the activation of Smad1/5/8 signaling pathway in BMP9-induced rDFCs osteogenesis.

RUNX1 Plays an Important Role in Mediating BMP9-Induced Osteogenic Differentiation of Mesenchymal Stem Cells Line C3H10T1/2, Murine Multi-Lineage Cells Lines C2C12 and MEFs

As one of the least studied bone morphogenetic proteins (BMPs), BMP9 is highly capable of promoting osteogenic differentiation. However, the underlying mechanism involved remains largely unknown. Recent studies have demonstrated that RUNX1 (runt-related transcription factor 1) is essential in osteoblast/chondrocyte maturation. In this study, we investigated the function of RUNX1 in BMP9-induced osteogenic of murine mesenchymal stem cell line (C3H10T1/2) and murine multi-lineage cell lines (C2C12 and MEFs). Our data showed that BMP9 promoted the endogenous expression of RUNX1 in C3H10T1/2, C2C12 and MEFs. Moreover, RUNX1 was probably a direct target of BMP9/Smad signaling. BMP9-induced osteogenic differentiation was enhanced by overexpression of RUNX1, whereas inhibited by knockdown RUNX1 in C3H10T1/2, C2C12 and MEFs. Further mechanism studies demonstrated that RUNX1 might affect BMP9-induced phosphorylation of Smad1/5/8, but not the phosphorylation of p38 and ERK1/2.Our results suggest that RUNX1 may be an essential modulator in BMP9- induced osteogenic differentiation of MSCs (Mesenchymal stem cells).

Benzo[a]pyrene impedes self-renewal and differentiation of mesenchymal stem cells and influences fracture healing

Mesenchymal stem cells (MSCs) are implicated in the bone-forming process during fracture repair. Benzo[a]pyrene (BaP)-a cigarette smoke component and powerful motivator of the aryl hydrocarbon receptor (Ahr)-unfavorably influences bone condition and osteoblast differentiation. The first thing we noticed decreases self-renewal and differentiation of human bone marrow mesenchymal stem (hBM-MSCs) from smokers and activates Ahr signaling in MSCs by up-regulating the Ahr target gene cytochrome P450 (CYP) 1B1 expression. In vitro studies, we employed C3H10T1/2 and bone marrow mesenchymal stem cells (BM-MSCs) with BaP and discovered that BaP impaired innate properties of MSCs. Further investigation into MSCs showed that exposure to BaP activated Ahr signaling and inhibited TGF-β1/SMAD4 and TGF-β1/ERK/AKT signaling pathways. Corresponding with the outcomes, tibial fracture calluses produced by BaP-administered rats appeared to delay healing. This effect of BaP was abrogated by resveratrol, a natural Ahr antagonist, in vitro and in vivo. These data demonstrated that Ahr may play a key role in BaP-impaired innate properties by inhibiting SMAD-dependent signaling pathways TGF-β1/SMAD4 and SMAD-independent TGF-β1/ERK/AKT signaling pathways. Furthermore, resveratrol inhibited MSCs from adverse effects caused by BaP.

Bone Morphogenetic Protein-9 Enhances Osteogenic Differentiation of Human Periodontal Ligament Stem Cells via the JNK Pathway

Bone morphogenetic protein-9 (BMP9) shows great osteoinductive potential in bone regeneration. Periodontal ligament stem cells (PDLSCs) with multi-differentiation capability and low immunogenicity are increasingly used as seed cells for periodontal regenerative therapies. In the present study, we investigated the potent osteogenic activity of BMP9 on human PDLSCs (hPDLSCs), in which the c-Jun N-terminal kinase (JNK) pathway is possibly involved. Our results showed that JNK inhibition by the specific inhibitor SP600125 or adenovirus expressing small interfering RNA (siRNA) targeting JNK (AdR-si-JNK) significantly decreased BMP9-induced gene and protein expression of early and late osteogenic markers, such as runt-related transcription factor 2 (Runx2), alkaline phosphatase (ALP), osteopontin (OPN), and osteocalcin (OCN), in hPDLSCs. We also confirmed the in-vivo positive effect of JNKs on ectopic bone formation induced by hPDLSCs injected into the musculature of athymic nude mice and BMP9 ex vivo gene delivery. For the cellular mechanism, we found that BMP9 activated the phosphorylation of JNKs and Smad2/3, and that JNKs may engage in cross-talk with the Smad2/3 pathway in BMP9-mediated osteogenesis.

BMP9 a possible alternative drug for the recently withdrawn BMP7? New perspectives for (re-)implementation by personalized medicine

Promotion of rhBMP2 and rhBMP7 for the routine use to support fracture healing has been hampered by high costs, safety concerns and reasonable failure rates, imposing restrictions in its clinical use. Since there is little debate regarding its treatment potential, there is rising need for a better understanding of the mode of action of these BMPs to overcome its drawbacks and promote more efficacious treatment strategies for bone regeneration. Recently, BMP9, owing to its improved osteogenic potential, is gaining attention as a promising therapeutic alternative. Our study aimed at identifying specific gene expression patterns which may predict and explain individual responses to rhBMP7 and rhBMP9 treatments. Therefore, we investigated the effect of rhBMP7 and rhBMP9 on primary human osteoblasts from 110 donors and corresponding THP-1-derived osteoclasts. This was further compared with each other and our reported data on rhBMP2 response. Based on the individual donor response, we found three donor groups profiting from rhBMP treatment either directly via stimulation of osteoblast function or viability and/or indirectly via inhibition of osteoclasts. The response on rhBMP7 treatment correlated with expression levels of the genes BAMBI, SOST, Noggin, Smad4 and RANKL, while the response of rhBMP9 correlated to the expression levels of Alk6, Endoglin, Smurf1, Smurf2, SOST and RANKL in these donors. Noteworthy, rhBMP9 treatment showed significantly increased osteogenic activity (AP activity and Smad nuclear translocation) when compared to the two clinically used rhBMPs. Based on patient's respective expression profiles, clinical application of rhBMP9 either solely or in combination with rhBMP2 and/or rhBMP7 can become a promising new approach to fit the patient's needs to promote fracture healing.

Osteogenic potential of rhBMP9 combined with a bovine-derived natural bone mineral scaffold compared to rhBMP2

OBJECTIVES

Combination therapies of growth factors and scaffolds for bone tissue engineering are becoming routine for clinical use. BMP9 has previously been characterized as one of the most osteogenic inducers among the BMP superfamily; however, up until recently, BMP9 has only been available through adenovirus transfection experiments (gene therapy). While recombinant human (rh)BMP2 is regarded as the gold standard for bone regeneration with recombinant growth factors, recently the successful development of rhBMP9 brings intriguing new possibilities for future clinical use. The purpose of this pioneering study was to investigate the effects of rhBMP9 in comparison with rhBMP2 on an in vitro cell behavior of bone-forming osteoblasts when combined with a bone grafting material.

MATERIAL AND METHODS

Undifferentiated mouse ST2 stromal bone marrow cells were seeded onto bovine-derived natural bone mineral (NBM) particles treated with (i) control, (ii) rhBMP2 (10 ng/ml), (iii) rhBMP2 (100 ng/ml), (iv) rhBMP9 (10 ng/ml) and (v) rhBMP9 (100 ng/ml). The effects of rhBMPs were compared for cell adhesion at 8 h, cell proliferation at 1, 3 and 5 days and osteoblast differentiation as assessed by real-time PCR at 3 and 14 days for genes encoding Runx2, collagen1alpha2 (COL1a2), alkaline phosphatase (ALP) and osteocalcin (OCN). Furthermore, ALP staining and alizarin red staining were used to investigate localization of osteoblast differentiation marker and mineralization on NBM.

RESULTS

Although neither rhBMP2 nor rhBMP9 influenced cell attachment to NBM particles, both were able to stimulate cell proliferation at 3 days. Furthermore, all concentrations of rhBMPs were able to significantly induce mRNA levels of Runx2, COL1a2 and OCN at 3 days. Interestingly, only rhBMP9 was able to significantly upregulate mRNA levels of ALP up to eightfold, and ALP staining up to 25-fold, when compared to rhBMP2. In addition, only rhBMP9 (100 ng/ml) significantly increased alizarin red staining when compared to control and rhBMP2 (10 ng/ml) samples.

CONCLUSION

These results demonstrate that both rhBMP2 and rhBMP9 have osteopromotive properties on osteoblast differentiation. It was found that rhBMP9 additionally stimulated the osteopromotive potential of osteoblasts when compared to rhBMP2 by demonstrating higher levels of ALP expression and alizarin red staining. Further animal studies comparing both recombinant proteins are necessary to further characterize the osteoinductive potential of BMP9.

Hedgehog signaling is involved in the BMP9-induced osteogenic differentiation of mesenchymal stem cells

Nonunion is a serious complication of a bone fracture that may occur in any bone of the skeletal system. It occurs when a broken bone fails to heal. Mesenchymal stem cell (MSC)-based tissue engineering technology has been considered an efficient method to improve the healing rate of nonunions. Although previous studies have demonstrated that bone morphogenetic protein 9 (BMP9) is highly capable of promoting the osteogenic differentiation of MSCs, the mechanisms involved remain largely unclear. In the present study, we investigated the possible involvement and the detailed role of Hedgehog (Hh) signaling in the BMP9-induced osteogenic differentiation of MSCs. It was found that BMP9 exerts an effect on Hh signaling in MSCs. The expression levels of early markers of BMP9-induced osteogenic differentiation, such as alkaline phosphatase (ALP) activity, and late markers of osteogenic differentiation, such as matrix mineralization, as well as the expression levels of osteopontin (OPN) and osteocalcin (OCN) were decreased by the Hh signaling inhibitor, cyclopamine, whereas these levels were increased by the Hh signaling agonist, purmorphamine. Furthermore, the BMP9-induced transcriptional activity of Smad1/5/8 and the expression of pivotal osteogenic transcription factors were reduced by cyclopamine, and were increased by purmorphamine. Taken together, our results demonstrate that BMP9 exerts an effect on Hh signaling in MSCs. What is most noteworthy, however, is that the inhibition or enhancement of Hh signaling resulted in the reduction and augmentation of the BMP9-induced osteogenic differentiation of MSCs, respectively, suggesting that Hh signaling is involved and plays a regulatory role in the osteogenic differentiation of MSCs induced by BMP9.

The high affinity ALK1-ligand BMP9 induces a hypertrophy-like state in chondrocytes that is antagonized by TGFβ1

OBJECTIVE

In osteoarthritic cartilage, expression of the receptor ALK1 correlates with markers of deleterious chondrocyte hypertrophy. Recently, bone morphogenetic protein 9 (BMP9) was identified as a high affinity ligand for ALK1. Therefore, we studied if BMP9 signaling results in expression of hypertrophy markers in chondrocytes. Furthermore, because transforming growth factorß1 (TGFβ1) is a well known anti-hypertrophic factor, the interaction between BMP9 and TGFβ1 signaling was also studied.

DESIGN

Primary chondrocytes were isolated from bovine cartilage and stimulated with BMP9 and/or TGFβ1 to measure intracellular signaling via pSmads with the use of Western blot. Expression of Smad-responsive genes or hypertrophy-marker genes was measured using qPCR. To confirm observations on TGFβ/Smad3 responsive genes, a Smad3-dependent CAGA12-luc transcriptional reporter assay was performed in the chondrocyte G6 cell line.

RESULTS

In primary chondrocytes, BMP9 potently induced phosphorylation of Smad1/5 and Smad2 to a lesser extent. BMP9-induced Smad1/5 phosphorylation was rapidly (2 h) reflected in gene expression, whereas Smad2 phosphorylation was not. Remarkably, BMP9 and TGFβ1 dose-dependently synergized on Smad2 phosphorylation, and showed an additive effect on expression of Smad3-dependent genes like bSerpine1 after 24 h. The activation of the TGFβ/Smad3 signaling cascade was confirmed using the CAGA12-luc transcriptional reporter. BMP9 selectively induced bAlpl and bColX expression, which are considered early markers of cellular hypertrophy, but this was potently antagonized by addition of a low dose of TGFβ1.

CONCLUSIONS

This study shows that in vitro in chondrocytes, BMP9 potently induces pSmad1/5 and a chondrocyte hypertrophy-like state, which is potently blocked by TGFβ1. This observation underlines the importance of TGFβ1 in maintenance of chondrocyte phenotype.

Activation of JNKs is essential for BMP9-induced osteogenic differentiation of mesenchymal stem cells

Although BMP9 is highly capable of promoting osteogenic differentiation of mesenchymal stem cell (MSCs), the molecular mechanism involved remains to be fully elucidated. Here, we explore the possible involvement and detail role of JNKs (c-Jun N-terminal kinases) in BMP9-induced osteogenic differentiation of MSCs. It was found that BMP9 stimulated the activation of JNKs in MSCs. BMP9-induced osteogenic differentiation of MSCs was dramatically inhibited by JNKs inhibitor SP600125. Moreover, BMP9-activated Smads signaling was decreased by SP600125 treatment in MSCs. The effects of inhibitor are reproduced with adenoviruses expressing siRNA targeted JNKs. Taken together, our results revealed that JNKs was activated in BMP9-induced osteogenic differentiation of MSCs. What is most noteworthy, however, is that inhibition of JNKs activity resulted in reduction of BMP9-induced osteogenic differentiation of MSCs, implying that activation of JNKs is essential for BMP9 osteoinductive activity. [BMB Reports 2013; 46(8):422-427]

Analysis and characterization of the functional TGFβ receptors required for BMP6-induced osteogenic differentiation of mesenchymal progenitor cells

Although BMP6 is highly capable of inducing osteogenic differentiation of mesenchymal progenitor cells (MPCs), the molecular mechanism involved remains to be fully elucidated. Using dominant negative (dn) mutant form of type I and type II TGFβ receptors, we demonstrated that three dn-type I receptors (dnALK2, dnALK3, dnALK6), and three dn-type II receptors (dnBMPRII, dnActRII, dnActRIIB), effectively diminished BMP6- induced osteogenic differentiation of MPCs. These findings suggested that ALK2, ALK3, ALK6, BMPRII, ActRII and ActRIIB are essential for BMP6-induced osteogenic differentiation of MPCs. However, MPCs in this study do not express ActRIIB. Moreover, RNA interference of ALK2, ALK3, ALK6, BMPRII and ActRII inhibited BMP6-induced osteogenic differentiation in MPCs. Our results strongly suggested that BMP6-induced osteogenic differentiation of MPCs is mediated by its functional TGFβ receptors including ALK2, ALK3, ALK6, BMPRII, and ActRII.

BMP signaling in mesenchymal stem cell differentiation and bone formation

Bone morphogenetic proteins (BMPs) are members of the TGF-β superfamily and have diverse functions during development and organogenesis. BMPs play a major role in skeletal development and bone formation, and disruptions in BMP signaling cause a variety of skeletal and extraskeletal anomalies. Several knockout models have provided insight into the mechanisms responsible for these phenotypes. Proper bone formation requires the differentiation of osteoblasts from mesenchymal stem cell (MSC) precursors, a process mediated in part by BMP signaling. Multiple BMPs, including BMP2, BMP6, BMP7 and BMP9, promote osteoblastic differentiation of MSCs both in vitro and in vivo. BMP9 is one of the most osteogenic BMPs yet is a poorly characterized member of the BMP family. Several studies demonstrate that the mechanisms controlling BMP9-mediated osteogenesis differ from other osteogenic BMPs, but little is known about these specific mechanisms. Several pathways critical to BMP9-mediated osteogenesis are also important in the differentiation of other cell lineages, including adipocytes and chondrocytes. BMP9 has also demonstrated translational promise in spinal fusion and bone fracture repair. This review will summarize our current knowledge of BMP-mediated osteogenesis, with a focus on BMP9, by presenting recently completed work which may help us to further elucidate these pathways.

Bone morphogenetic protein-9 activates Smad and ERK pathways and supports human osteoclast function and survival in vitro

BMP-9 is a potent osteogenic factor; however, its effects on osteoclasts, the bone-resorbing cells, remain unknown. To determine the effects of BMP-9 on osteoclast formation, activity and survival, we used human cord blood monocytes as osteoclast precursors that form multinucleated osteoclasts in the presence of RANKL and M-CSF in long-term cultures. BMP-9 did not affect osteoclast formation, but adding BMP-9 at the end of the culture period significantly increased bone resorption compared to untreated cultures, and reduced both the rate of apoptosis and caspase-9 activity. BMP-9 also significantly downregulated the expression of pro-apoptotic Bid, but only after RANKL and M-CSF, which are both osteoclast survival factors, had been eliminated from the culture medium. To investigate the mechanisms involved in the effects of BMP-9, we first showed that osteoclasts expressed some BMP receptors, including BMPR-IA, BMPR-IB, ALK1, and BMPR-II. We also found that BMP-9 was able to induce the phosphorylation of Smad-1/5/8 and ERK 1/2 proteins, but did not induce p38 phosphorylation. Finally, knocking down the BMPR-II receptor abrogated the BMP-9-induced ERK-signaling, as well as the increase in bone resorption. In conclusion, these results show for the first time that BMP-9 directly affects human osteoclasts, enhancing bone resorption and protecting osteoclasts against apoptosis. BMP-9 signaling in human osteoclasts involves the canonical Smad-1/5/8 pathway, and the ERK pathway.

Smads, p38 and ERK1/2 are involved in BMP9-induced osteogenic differentiation of C3H10T1/2 mesenchymal stem cells

Although previous studies have demonstrated that BMP9 is highly capable of inducing osteogenic differentiation of mesenchymal stem cells, the molecular mechanism involved remains to be fully elucidated. In this study, we showed that BMP9 simultaneously promotes the activation of Smad1/5/8, p38 and ERK1/2 in C3H10T1/2 cells. Knockdown of Smad4 with RNA interference reduced nuclear translocation of Smad1/5/8, and disrupted BMP9-induced osteogenic differentiation. BMP9-induced osteogenic differentiation was blocked by p38 inhibitor SB203580, whereas enhanced by ERK1/2 inhibitor PD98059. SB203580 decreased BMP9-activated Smads singling, and yet PD98059 stimulated Smads singling in C3H10T1/2 cells. The effects of inhibitor were reproduced with adenovirus expressing siRNA targeted p38 and ERK1/2, respectively. Taken together, our findings revealed that Smads, p38 and ERK1/2 are involved in BMP9-induced osteogenic differentiation. Also, it is noteworthy that p38 and ERK1/2 may play opposing regulatory roles in mediating BMP9-induced osteogenic differentiation of C3H10T1/2 cells.

P38 and ERK1/2 MAPKs act in opposition to regulate BMP9-induced osteogenic differentiation of mesenchymal progenitor cells

Although previous studies have demonstrated that BMP9 is highly capable of inducing osteogenic differentiation and bone formation, the precise molecular mechanism involved remains to be fully elucidated. In this current study, we explore the possible involvement and detail effects of p38 and ERK1/2 MAPKs on BMP9-indcued osteogenic differentiation of mesenchymal progenitor cell (MPCs). We find that BMP9 simultaneously stimulates the activation of p38 and ERK1/2 in MPCs. BMP9-induced early osteogenic marker, such as alkaline phosphatase (ALP), and late osteogenic markers, such as matrix mineralization and osteocalcin (OC) are inhibited by p38 inhibitor SB203580, whereas enhanced by ERK1/2 inhibitor PD98059. BMP9-induced activation of Runx2 and Smads signaling are reduced by SB203580, and yet increased by PD98059 in MPCs. The in vitro effects of inhibitors are reproduced with adenoviruses expressing siRNA targeted p38 and ERK1/2, respectively. Using mouse calvarial organ culture and subcutaneous MPCs implantation, we find that inhibition of p38 activity leads to significant decrease in BMP9-induced osteogenic differentiation and bone formation, however, blockage of ERK1/2 results in effective increase in BMP9-indcued osteogenic differentiation in vivo. Together, our results reveal that p38 and ERK1/2 MAPKs are activated in BMP9-induced osteogenic differentiation of MPCs. What is most noteworthy, however, is that p38 and ERK1/2 act in opposition to regulate BMP9-induced osteogenic differentiation of MPCs.

TGF-β and BMP signaling in osteoblast differentiation and bone formation

Transforming growth factor-beta (TGF-β)/bone morphogenic protein (BMP) signaling is involved in a vast majority of cellular processes and is fundamentally important throughout life. TGF-β/BMPs have widely recognized roles in bone formation during mammalian development and exhibit versatile regulatory functions in the body. Signaling transduction by TGF-β/BMPs is specifically through both canonical Smad-dependent pathways (TGF-β/BMP ligands, receptors and Smads) and non-canonical Smad-independent signaling pathway (e.g. p38 mitogen-activated protein kinase pathway, MAPK). Following TGF-β/BMP induction, both the Smad and p38 MAPK pathways converge at the Runx2 gene to control mesenchymal precursor cell differentiation. The coordinated activity of Runx2 and TGF-β/BMP-activated Smads is critical for formation of the skeleton. Recent advances in molecular and genetic studies using gene targeting in mice enable a better understanding of TGF-β/BMP signaling in bone and in the signaling networks underlying osteoblast differentiation and bone formation. This review summarizes the recent advances in our understanding of TGF-β/BMP signaling in bone from studies of genetic mouse models and human diseases caused by the disruption of TGF-β/BMP signaling. This review also highlights the different modes of cross-talk between TGF-β/BMP signaling and the signaling pathways of MAPK, Wnt, Hedgehog, Notch, and FGF in osteoblast differentiation and bone formation.

BMP2 and VEGF promote angiogenesis but retard terminal differentiation of osteoblasts in bone regeneration by up-regulating Id1

Inadequate vascularization limits the repair of bone defects. In order to improve angiogenesis and accelerate osteogenesis, the synergism of co-cultured cells with genetic modification in bone regeneration was investigated in this study. Endothelial progenitor cells (EPCs) and bone marrow stem cells (BMSCs) were transfected with the genes of vascular endothelial growth factor (VEGF) and bone morphogenetic protein 2 (BMP2) by adenovirus, respectively. The co-cultured cells, designated as four groups including BMSC + EPC, Ad-BMP2-BMSC + EPC, BMSC + Ad-VEGF-EPC, and Ad-BMP2-BMSC + Ad-VEGF-EPC groups, were seeded on an alginate gel and then implanted into rat intramuscularly to evaluate the effects on angiogenesis and osteogenesis. Both VEGF and BMP2 could induce the overexpression of inhibitor of DNA-binding 1(Id1) gene which significantly promoted tube formation in vitro and increase the amount of blood vessels in the Ad-BMP2-BMSC + Ad-VEGF-EPC group after implantation. Nevertheless, overexpression of Id1 retarded the terminal differentiation of osteoblasts and the bone formation. Later, osteogenic gene expression at transcriptional level, calcium nodules, and alkaline phosphatase (ALP) activity showed a gradual decrease and the amount of newly formed osteogenesis area exhibited a small increase in the Ad-BMP2-BMSC + Ad-VEGF-EPC group. This finding suggests that a balanced regulation of Id1 expression in VEGF-EPCs and BMP2-BMSCs may be critical to cell-based and gene-based approaches for bone regeneration.