Transplantation of skin fibroblasts expressing BMP-2 promotes bone repair more effectively than those expressing Runx2

Genetic engineering for skeletal regenerative medicine

The clinical challenges of skeletal regenerative medicine have motivated significant advances in cellular and tissue engineering in recent years. In particular, advances in molecular biology have provided the tools necessary for the design of gene-based strategies for skeletal tissue repair. Consequently, genetic engineering has emerged as a promising method to address the need for sustained and robust cellular differentiation and extracellular matrix production. As a result, gene therapy has been established as a conventional approach to enhance cellular activities for skeletal tissue repair. Recent literature clearly demonstrates that genetic engineering is a principal factor in constructing effective methods for tissue engineering approaches to bone, cartilage, and connective tissue regeneration. This review highlights this literature, including advances in the development of efficacious gene carriers, novel cell sources, successful delivery strategies, and optimal target genes. The current status of the field and the challenges impeding the clinical realization of these approaches are also discussed.

Dermal fibroblasts genetically modified to express Runx2/Cbfa1 as a mineralizing cell source for bone tissue engineering

Cell-based bone tissue engineering strategies have been effectively applied toward the development of grafting templates for skeletal repair and regeneration, but remain limited by inadequate availability of a robust mineralizing cell source. Dermal fibroblasts have emerged as a particularly promising cell alternative because they are harvested from autologous donors through minimally invasive skin biopsy and display a high capacity for in vitro expansion. In the present study, we investigated retroviral gene delivery of the osteogenic transcription factor Runx2 as a mineralization induction strategy in primary dermal fibroblasts. We demonstrate that constitutive overexpression of Runx2 induced osteogenic gene expression and mineralized nodule deposition in fibroblasts cultured on 3-dimensional fibrous collagen disks in vitro. Fourier transform infrared analysis revealed that Runx2 expressing fibroblasts deposit a carbonate-containing, poorly crystalline hydroxyapatite, whereas control constructs did not contain biologically-equivalent mineral. Importantly, Runx2-transduced fibroblasts formed mineralized templates in vivo after implantation in a subcutaneous, heterotopic site, whereas minimal mineralization was evident in control constructs. Furthermore, immunohistochemical analysis indicated that Runx2-engineered cells co-localized with mineral deposits in vivo, suggesting that nodule formation primarily originated from transplanted donor cells. These results establish Runx2-genetic engineering as a strategy for the conversion of a non-osteogenic cellular phenotype into a mineralizing cell source for bone repair applications. Cellular therapies based on primary dermal fibroblasts would be particularly beneficial for patients with compromised ability to recruit endogenous osteoprogenitors to the site of injury as a result of extreme trauma, age, radiation treatment, or osteolytic disease.

Bone regeneration by regulated in vivo gene transfer using biocompatible polyplex nanomicelles

Gene therapy is a promising strategy for bone regenerative medicine. Although viral vectors have been intensively studied for delivery of osteogenic factors, the immune response inevitably inhibits bone formation. Thus, safe and efficient non-viral gene delivery systems are in high demand. Toward this end, we developed a polyplex nanomicelle system composed of poly(ethyleneglycol) (PEG)-block-catiomer (PEG-b-P[Asp-(DET)]) and plasmid DNA (pDNA). This system showed little cytotoxicity and excellent transfection efficiency to primary cells. By the transfection of constitutively active form of activin receptor-like kinase 6 (caALK6) and runt-related transcription factor 2 (Runx2), the osteogenic differentiation was induced on mouse calvarial cells to a greater extent than when poly(ethylenimine) (PEI) or FuGENE6 were used; this result was due to low cytotoxicity and a sustained gene expression profile. After incorporation into the calcium phosphate cement scaffold, the polyplex nanomicelles were successfully released from the scaffold and transfected surrounding cells. Finally, this system was applied to in vivo gene transfer for a bone defect model in a mouse skull bone. By delivering caALK6 and Runx2 genes from nanomicelles incorporated into the scaffold, substantial bone formation covering the entire lower surface of the implant was induced with no sign of inflammation at 4 weeks. These results demonstrate the first success in in vivo gene transfer with therapeutic potential using polyplex nanomicelles.

In vitro and in vivo osteoblastic differentiation of BMP-2- and Runx2-engineered skeletal myoblasts

Genetic engineering with osteogenic factors is a promising approach for cell-based therapeutics and orthopedic regeneration. However, the relative efficacy of different strategies for inducing osteoblastic differentiation remains unclear and is further complicated by varied delivery vehicles, cell types, and evaluation criteria. In order to elucidate the effects of distinct gene-based strategies, we quantitatively evaluated osteoblastic differentiation and mineralization of primary skeletal myoblasts overexpressing either the BMP-2 growth factor or Runx2 transcription factor. Retroviral delivery of BMP-2 or Runx2 stimulated differentiation into an osteoblastic phenotype, as demonstrated by the induction of osteogenic gene expression, alkaline phosphatase activity, and matrix mineralization in monolayer culture and on collagen scaffolds both in vitro and in an intramuscular site in vivo. In general, BMP-2 stimulated osteoblastic markers faster and to a greater extent than Runx2, although we also identified experimental conditions under which these two factors produced similar effects. Additionally, Runx2-engineered cells did not utilize paracrine signaling via secreted osteogenic factors, in contrast to cells overexpressing BMP-2, as demonstrated by conditioned media studies and activation of Smad signaling. These results emphasize the complexity of gene therapy-based orthopedic therapeutics as an integrated relationship of differentiation state, construct maturation, and paracrine signaling of osteogenic cells. This study is significant in evaluating proposed therapeutic systems and defining a successful strategy for integrating gene medicine and orthopedic regeneration.

Enhanced osseointegration by the chemotactic activity of plasma fibronectin for cellular fibronectin positive cells

Plasma fibronectin (pFN) is known to regulate cell growth, differentiation or survival of osteoblasts in vitro. It is also speculated to be important for the early phase of osseointegration, however, its actual in vivo behavior is unknown. The objective of this study is to clarify the role of pFN during osseointegration. We developed a titanium ion-plated acrylic implant (Ti-acryl) for thin sectioning without removal of the implant. Either Ti-acryl or pFN-coated Ti-acryl (FN-Ti-acryl) was implanted in the mouse femur. Samples were taken on days 1-7 and on day 14 after the operation, and were decalcified and paraffin embedded. The bone healing process and immunofluorescence localization of pFN and cellular fibronectin (cFN), a marker for fibroblastic cells were examined. Simultaneously, the effect of pFN on chemotaxis, proliferation and differentiation of bone marrow stromal cells (BMSCs) was analyzed in vitro. The in vivo results showed that faster direct bone formation was seen for the FN-Ti-acryl group compared to the Ti-acryl group. The in vitro results showed that pFN significantly promoted BMSCs chemotaxis, however, had no effect on proliferation or differentiation. The results indicate that pFN regulated chemotaxis of osteogenic cells and coating the implant with pFN enhanced earlier osseointegration.

Cbfb enhances the osteogenic differentiation of both human and mouse mesenchymal stem cells induced by Cbfa-1 via reducing its ubiquitination-mediated degradation

Core-binding factors are a small family of heterodimeric transcription factors that play critical roles in development. Whereas Cbfa-1, one of the three alpha subunits in the family, is essential for osteogenesis, Cbfb, the only beta subunit, forms heterodimers with different Cbfas to increase their DNA binding affinity by inducing conformational changes. Although defective bone formation was found in both Cbfa-1 and Cbfb knockout animals, the precise role of the latter in osteogenesis remains unclear. To dissect the contribution of Cbfb in osteogenic differentiation of mesenchymal stem cells (MSCs), recombinant adenoviruses carrying Cbfb (AdHACbfb) and Cbfa-1 (AdCbfa-1) were generated and used to infect both the mouse C3H10T1/2 cells and human bone marrow-derived MSCs. Although Cbfb alone failed to trigger osteogenesis of MSCs, it markedly enhanced the gene expression and enzyme activity of alkaline phosphatase as well as osteocalcin activation in those cells overexpressing Cbfa-1. Enhancement of the osteogenic differentiation-inducing effect of Cbfa-1 by Cbfb resulted from an increase in stability of the former due to the suppression of ubiquitination-mediated proteasomal degradation by the latter. Taken together, in addition to defining the role of Cbfb in osteogenic differentiation of MSCs, our results also suggest that the Cbfa-1 and Cbfb coexpressing MSCs might be an appropriate strategy for bone repairing and regeneration therapies. Disclosure of potential conflicts of interest is found at the end of this article.

CCN3/NOV inhibits BMP-2-induced osteoblast differentiation by interacting with BMP and Notch signaling pathways

We elucidate the role of CCN3/NOV, a member of the CCN family proteins, in osteoblast differentiation using MC3T3-E1 osteoblastic cells. Transduction with CCN3 adenovirus (AdCCN3) alone induced no apparent changes in the expression of osteoblast-related markers, whereas cotransduction with BMP-2 adenovirus (AdBMP-2) and AdCCN3 significantly inhibited the AdBMP-2-induced mRNA expression of Runx2, osterix, ALP, and osteocalcin. Immunoprecipitation-western analysis revealed that CCN3 associated with BMP-2. Compared to transduction with AdBMP-2 alone, cotransduction with AdBMP-2 and AdCCN3 attenuated the expression of phosphorylated Smad1/5/8 and the mRNA for Id1, Id2, and Id3. Transduction with AdCCN3 stimulated the expression of cleaved Notch1, the mRNA expression of Hes1 and Hey1/Hesr1, and the promoter activities of Hes1 and Hey1. The inhibitory effects of CCN3 on the expression of BMP-2-induced osteoblast-related markers were nullified in Hey1-deficient osteoblastic cells. These results indicate that CCN3 exerts inhibitory effects on BMP-2-induced osteoblast differentiation by its involvement of the BMP and Notch signaling pathways.

Identification of a potent combination of osteogenic genes for bone regeneration using embryonic stem (ES) cell‐based sensor

To identify potent bioactive factors for in vivo tissue regeneration by comprehensive screening remains a challenge for regenerative medicine. Here we report the development of an ES cell-based monitoring system for osteogenic differentiation, the identification of a potent combination of osteogenic genes using such a system, and an evaluation of its therapeutic potentials. ES cells were isolated from mice carrying a transgene expressing GFP driven by the 2.3 kb fragment of rat type I collagen alpha1 promoter. Using these cells engineered to fluoresce on osteogenic differentiation, we screened cDNA libraries and combinations of major osteogenesis-related genes. Among them, the combination of constitutively active activin receptor-like kinase 6 (caALK6) and runt-related transcription factor 2 (Runx2) was the minimal unit that induced fluorescence. The combination efficiently induced osteogenic differentiation in various cell types, including terminally differentiated nonosteogenic cells. The cooperative action of the combination occurred through protein stabilization of core binding factor beta (Cbfb), induction of Runx2-Cbfb complex formation, and its DNA binding. Furthermore, transplantation of a monolayer sheet of fibroblasts transduced with the combination achieved bone regeneration within 4 wk in mouse calvarial bone defects. Thus, we successfully identified the potent combination of genes for bone regeneration, which helped broaden cell sources.

Virus-based gene therapy strategies for bone regeneration

Gene therapy has emerged as a promising strategy for the repair and regeneration of damaged musculoskeletal tissues. Application of this paradigm to bone healing has shown enhanced efficacy in preclinical animal studies compared to conventional bone grafting approaches. This review discusses current and emerging virus-based genetic engineering strategies for the delivery of therapeutic molecules which promote skeletal regeneration. Viral gene delivery vectors are discussed in the context of bone repair in order to illustrate the challenges and applications of these methods with tissue-specific examples. Moreover the concepts discussed can be broadly applied to promote healing in a wide range of tissues. We also present important considerations involved in the application of these gene therapy techniques to a variety of osteogenic (e.g. bone marrow-derived cells) and non-osteogenic (e.g. fibroblasts and skeletal myoblasts) cell types. Criteria for the selection of regenerative molecules with soluble versus intracellular modes of action and emerging combinatorial approaches are also discussed. Overall, gene transfer technologies have the potential to overcome limitations associated with existing bone grafting approaches and may enable investigators to design therapies which more closely mimic the complex spatial and temporal cascade of proteins involved in endogenous bone development and repair.

Inducible regulation of Runx2-stimulated osteogenesis

Ex vivo gene therapy is a promising approach to orthopedic regenerative medicine. These strategies typically focus on the constitutive overexpression of osteogenic factors to induce osteoblastic differentiation and matrix mineralization. However, the unregulated production of osteoinductive molecules has also resulted in abnormal bone formation and tumorigenesis. To address these limitations, this work describes a retroviral system to deliver the Runx2 osteoblastic transcription factor under control of the tetracycline-inducible (tet-off) promoter in primary skeletal myoblasts. Runx2 expression was tightly regulated by anhydrotetracyline (aTc) concentration in cell culture media. Osteoblastic gene expression, alkaline phosphatase activity, and matrix mineralization were also controlled by aTc in a dose-dependent manner. Additionally, osteoblastic differentiation was temporally regulated by adding and removing aTc from the culture media. Engineered cells were seeded onto collagen scaffolds and implanted intramuscularly in the hind limbs of syngeneic mice. In vivo mineralization by these constructs was regulated by supplementing the drinking water with aTc, as demonstrated by micro-computed tomography and histological analyses. Collectively, these results present a novel system for regulating osteoblastic differentiation of a clinically relevant autologous cell source. This system is significant to developing controlled and effective orthopedic gene therapy strategies and studying the regulation of osteoblastic differentiation.

Mineralization capacity of Runx2/Cbfa1-genetically engineered fibroblasts is scaffold dependent

Development of tissue-engineered constructs for skeletal regeneration of large critical-sized defects requires the identification of a sustained mineralizing cell source and careful optimization of scaffold architecture and surface properties. We have recently reported that Runx2-genetically engineered primary dermal fibroblasts express a mineralizing phenotype in monolayer culture, highlighting their potential as an autologous osteoblastic cell source which can be easily obtained in large quantities. The objective of the present study was to evaluate the osteogenic potential of Runx2-expressing fibroblasts when cultured in vitro on three commercially available scaffolds with divergent properties: fused deposition-modeled polycaprolactone (PCL), gas-foamed polylactide-co-glycolide (PLGA), and fibrous collagen disks. We demonstrate that the mineralization capacity of Runx2-engineered fibroblasts is scaffold dependent, with collagen foams exhibiting ten-fold higher mineral volume compared to PCL and PLGA matrices. Constructs were differentially colonized by genetically modified fibroblasts, but scaffold-directed changes in DNA content did not correlate with trends in mineral deposition. Sustained expression of Runx2 upregulated osteoblastic gene expression relative to unmodified control cells, and the magnitude of this expression was modulated by scaffold properties. Histological analyses revealed that matrix mineralization co-localized with cellular distribution, which was confined to the periphery of fibrous collagen and PLGA sponges and around the circumference of PCL microfilaments. Finally, FTIR spectroscopy verified that mineral deposits within all Runx2-engineered scaffolds displayed the chemical signature characteristic of carbonate-containing, poorly crystalline hydroxyapatite. These results highlight the important effect of scaffold properties on the capacity of Runx2-expressing primary dermal fibroblasts to differentiate into a mineralizing osteoblastic phenotype for bone tissue engineering applications.

Glucocorticoid-induced osteogenesis is negatively regulated by Runx2/Cbfa1 serine phosphorylation

Glucocorticoid hormones have complex stimulatory and inhibitory effects on skeletal metabolism. Endogenous glucocorticoid signaling is required for normal bone formation in vivo, and synthetic glucocorticoids, such as dexamethasone, promote osteoblastic differentiation in several in vitro model systems. The mechanism by which these hormones induce osteogenesis remains poorly understood. We demonstrate here that the coordinate action of dexamethasone and the osteogenic transcription factor Runx2/Cbfa1 synergistically induces osteocalcin and bone sialoprotein gene expression, alkaline phosphatase activity, and biological mineral deposition in primary dermal fibroblasts. Dexamethasone decreased Runx2 phosphoserine levels, particularly on Ser125, in parallel with the upregulation of mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1) through a glucocorticoid-receptor-mediated mechanism. Inhibition of MKP-1 abrogated the dexamethasone-induced decrease in Runx2 serine phosphorylation, suggesting that glucocorticoids modulate Runx2 phosphorylation via MKP-1. Mutation of Ser125 to glutamic acid, mimicking constitutive phosphorylation, inhibited Runx2-mediated osteoblastic differentiation, which was not rescued by dexamethasone treatment. Conversely, mutation of Ser125 to glycine, mimicking constitutive dephosphorylation, markedly increased osteoblastic differentiation, which was enhanced by, but did not require, additional dexamethasone supplementation. Collectively, these results demonstrate that dexamethasone induces osteogenesis, at least in part, by modulating the phosphorylation state of a negative-regulatory serine residue (Ser125) on Runx2. This work identifies a novel mechanism for glucocorticoid-induced osteogenic differentiation and provides insights into the role of Runx2 phosphorylation during skeletal development.

Effects of Runx2 genetic engineering and in vitro maturation of tissue-engineered constructs on the repair of critical size bone defects

Genetic and tissue engineering strategies are being pursued to address the clinical limitations of current bone grafting materials. Based on our previous work demonstrating that overexpression of the Runx2 osteoblastic transcription factor and in vitro construct maturation synergistically enhanced in vivo mineralization in an ectopic site (Byers et al., Tissue Eng 2004;10:1757-1766), we examined the effects of these two parameters on the repair of critical size bone defects. Primary rat bone marrow stromal cells transduced with Runx2 or control (no Runx2 insert) retroviral vector were seeded onto 3D fused deposition-modeled polycaprolactone scaffolds. Runx2-modified cells produced biologically-equivalent mineralized matrices at nearly 2-fold higher rates than control cells. Constructs cultured in vitro for 1 day (immature) or 21 days (mineralized) were subsequently implanted into critical size calvaria defects in syngeneic rats, and bone healing was analyzed by micro-CT and histomorphometry at 28 days. Runx2-modified and control constructs precultured for 1 day healed to a greater extent than defects receiving no implant. Cell-free scaffolds yielded equivalent levels of bone formation as constructs precultured for 1 day. Interestingly, defects treated with control cell-seeded constructs precultured for 21 days exhibited low bone formation compared to other construct treatments, and repair was comparable to empty defects. In contrast, Runx2-modified constructs precultured for 21 days contained twice as much bone as control constructs precultured for 21 days and equivalent levels of new bone as cell-free and 1 day precultured constructs. These results demonstrate interplay between Runx2 genetically-modified cells and in vitro construct maturation in bone healing responses.

Gene-enhanced tissue engineering for dental hard tissue regeneration: (1) overview and practical considerations

Gene-based therapies for tissue regeneration involve delivering a specific gene to a target tissue with the goal of changing the phenotype or protein expression profile of the recipient cell; the ultimate goal being to form specific tissues required for regeneration. One of the principal advantages of this approach is that it provides for a sustained delivery of physiologic levels of the growth factor of interest.

This manuscript will review the principals of gene-enhanced tissue engineering and the techniques of introducing DNA into cells. Part 2 will review recent advances in gene-based therapies for dental hard tissue regeneration, specifically as it pertains to dentin regeneration/pulp capping and periodontal regeneration.

Induction of osteoblast differentiation markers in human dermal fibroblasts: potential application to bone tissue engineering

Tissue engineered constructs have the potential to be used as replacements for current bone graft technologies. One component necessary for bone tissue engineering is a readily available, osteogenic cell source. Human dermal fibroblasts may have the potential to differentiate along an osteoblastic lineage, making them a candidate for use in bone tissue engineering applications. The objective of this study was to validate the ability of dermal fibroblasts to express gene and protein markers of osteoblastic differentiation and to explore their potential, in combination with biomaterial scaffolds and signaling factors, for use in bone tissue engineering.

Scaffold-based bone engineering by using genetically modified cells

The first generation of clinically applied tissue engineering concepts in the area of skin, cartilage and bone marrow regeneration was based on the isolation, expansion and implantation of cells from the patient's own tissue. Although successful in selective treatments, tissue engineering needs to overcome major challenges to allow widespread clinical application with predictable outcomes. One challenge is to present the cells in a matrix to the implantation site to allow the cells to survive the wound healing contraction forces, tissue remodeling in certain tissues such as bone and biomechanical loading. Hence, several tissue engineering strategies focus on the development of load-bearing scaffold/cell constructs. From a cell source point of view, bone engineers face challenges to isolate and expand cells with the highest potential to form osseous tissue along with harvesting tissue without extensive donor site morbidity. A major hurdle to tissue engineering is de-differentiation and limited ability to control cell phenotype following in vitro expansion. Due to early successes with genetic engineering, bone tissue engineers have used different strategies to genetically alter various types of mesenchymal cells to enhance the mineralization capacity of tissue-engineered scaffold/cell constructs. Although the development of multi-component scaffold/osteogenic cell constructs requires a combination of interdisciplinary research strategies, the following review is limited to describe the general aspects of bone engineering and to present overall directions of technology platforms, which include a genetic engineering component. This paper reviews the most recent work in the field and discusses the concepts developed and executed by a collaborative effort of the multi-disciplinary teams of the two authors.

Synergy between genetic and tissue engineering: Runx2 overexpression and in vitro construct development enhance in vivo mineralization

Tissue engineering has emerged as a promising strategy to generate bone-grafting substrates. These approaches, however, are limited by an insufficient supply of committed osteoprogenitor cells and dedifferentiation of osteogenic cells during in vitro culture. To address these limitations, we engineered bone marrow stromal cells to constitutively express the osteoblastic transcription factor Runx2/Cbfa1, using retroviral gene delivery. These Runx2-modified cells were integrated into three-dimensional polymeric scaffolds to create tissue-engineered constructs. Compared with control stromal cells, Runx2 overexpression significantly upregulated osteoblastic differentiation and mineralization in vitro and in vivo in an ectopic, nonosseous subcutaneous site. More importantly, in vitro construct development to create a mineralized template before implantation dramatically enhanced subsequent in vivo mineralized tissue formation, providing a novel templating tissue-engineering strategy to improve in vivo mineralization. Finally, Runx2 overexpression and in vitro construct development synergistically enhanced in vivo mineralization compared with in vitro construct development or genetic engineering alone. This work provides a novel integrated genetic and tissue-engineering strategy to create mineralized templates for generating robust bone-grafting material.

Runx2/Cbfa1-genetically engineered skeletal myoblasts mineralize collagen scaffolds in vitro

Genetic engineering of progenitor and stem cells is an attractive approach to address cell sourcing limitations associated with tissue engineering applications. Bone tissue engineering represents a promising strategy to repair large bone defects, but has been limited in part by the availability of a sustained, mineralizing cell source. This study examined the in vitro mineralization potential of primary skeletal myoblasts genetically engineered to overexpress Runx2/Cbfa1, an osteoblastic transcriptional regulator essential to bone formation. These cells were viable at the periphery of 3D fibrous collagen scaffolds for 6 weeks of static culture. Exogenous Runx2 expression induced osteogenic differentiation and repressed myogenesis in these constructs relative to controls. Runx2-modified cells deposited significant amounts of mineralized matrix and hydroxyapatite, as determined by microcomputed tomography, histological analysis, and Fourier transform infrared spectroscopy, whereas scaffolds seeded with control cells exhibited no mineralized regions. Although mineralization by Runx2-engineered cells was confined to the periphery of the construct, colocalizing with cell viability, it was sufficient to increase the compressive modulus of constructs 30-fold relative to controls. This work demonstrates that Runx2 overexpression in skeletal myoblasts may address current obstacles of bone tissue engineering by providing a potent cell source for in vitro mineralization and construct maturation. Additionally, the use of genetic engineering methods to express downstream control factors and transcriptional regulators, in contrast to soluble signaling molecules, represents a robust strategy to enhance cellular activities for tissue engineering applications.

Bone morphogenetic protein-2 induces differentiation of multipotent C3H10T1/2 cells into osteoblasts, chondrocytes, and adipocytes in vivo and in vitro

Mesenchymal stem cells capable of differentiating into multiple cell types are potentially useful therapeutically for regeneration of bone and cartilaginous tissues. Multipotent fibroblastic C3H10T1/2 cells are known to differentiate into osteoblasts, chondrocytes, and adipocytes in response to certain growth factors. In this study we compared the effects of bone morphogenetic protein (BMP)-2 on the differentiation of C3H10T1/2 and MC3T3-E1 preosteoblastic cells. Incubation of these cells with BMP-2 resulted in a dose- and time-dependent increase in alkaline phosphatase activity, but the increase in MC3T3-E1 cells was much higher than that in C3H10T1/2 cells. BMP-2 also induced differentiation of C3H10T1/2 cells but not MC3T3-E1 cells into chondrocytes and adipocytes. Reverse transcription-polymerase chain reaction analysis showed that these commitments were accompanied by the specific expression of osteocalcin, aggrecan, and PPARgamma. To investigate the in vivo differential property, these cells were inoculated with BMP-2 in a diffusion chamber and transplanted into the mouse peritoneal cavity for 4 weeks. The transplanted C3H10T1/2 cells formed mineralized bone containing chondrocytes and adipocytes, whereas MC3T3-E1 created only bony tissue. Our results indicate that BMP-2 can induce the differentiation of C3H10T1/2 into osteoblasts, chondrocytes, and adipocytes in both in vivo and in vitro conditions, and that C3H10T1/2 could be used to examine the BMP-2-induced regulatory mechanisms of mesenchymal differentiation.

Critical regulation of bone morphogenetic protein-induced osteoblastic differentiation by Delta1/Jagged1-activated Notch1 signaling

Functional involvement of the Notch pathway in osteoblastic differentiation has been previously investigated using the truncated intracellular domain, which mimics Notch signaling by interacting with the DNA-binding protein CBF-1. However, it is unclear whether Notch ligands Delta1 and Jagged1 also induce an identical cellular response in osteoblastic differentiation. We have shown that both Delta1 and Jagged1 were expressed concomitantly with Notch1 in maturating osteoblastic cells during bone regeneration and that overexpressed and immobilized recombinant Delta1 and Jagged1 alone did not alter the differentiated state of MC3T3-E1 and C2C12 cells. However, they augmented bone morphogenetic protein-2 (BMP2)-induced alkaline phosphatase activity and the expression of several differentiation markers, except for osteocalcin, and ultimately enhanced calcified nodule and in vivo ectopic bone formation of MC3T3-E1. In addition, both ligands transmitted signal through the CBF-1-dependent pathway and stimulated the expression of HES-1, a direct target of Notch pathway. To test the necessity of Notch signaling in BMP2-induced differentiation, Notch signaling was inhibited by the dominant negative extracellular domain of Notch1, specific inhibitor, or small interference RNA. These treatments decreased alkaline phosphatase activity as well as the expression of other differentiation markers and inhibited the promoter activity of Id-1, a target gene of the BMP pathway. These results indicate the functional redundancy between Delta1 and Jagged1 in osteoblastic differentiation whereby Delta1/Jagged1-activated Notch1 enhances BMP2-induced differentiation through the identical signaling pathway. Furthermore, our data also suggest that functional Notch signaling is essential not only for BMP2-induced osteoblast differentiation but also for BMP signaling itself.

Exogenous Runx2 Expression Enhances in Vitro Osteoblastic Differentiation and Mineralization in Primary Bone Marrow Stromal Cells

Bone marrow stromal cells represent a promising cell source for cell-based therapeutic and bone tissue-engineering applications, but are restricted by a low frequency in healthy marrow, an age-related decrease in osteogenic capacity, and a propensity for dedifferentiation during in vitro expansion. To address these limitations, retroviral gene delivery was used to examine the effects of sustained and elevated expression of the Runx2 osteoblastic transcription factor on osteoblastic gene and protein expression and mineralization in primary rat bone marrow stromal cells. Runx2 overexpression upregulated several osteoblast-specific genes, including collagen type I and osteocalcin, and enhanced alkaline phosphatase activity and biological mineral deposition. Forced Runx2 expression in combination with dexamethasone increased matrix mineralization compared with exogenous Runx2 expression or dexamethasone treatment alone, whereas dexamethasone-free control cultures displayed minimal mineralization. These additive effects suggest complementary interactions between Runx2 and dexamethasone-responsive regulatory factors. Finally, Runx2 overexpression in stromal cell cultures undergoing considerable in vitro expansion resulted in higher matrix mineralization capacity compared with controls, which completely lost the ability to produce mineralized matrix even in the presence of dexamethasone. These findings provide a novel strategy for cell-based therapeutic applications requiring significant numbers of osteogenic cells to synthesize mineralized constructs for the treatment of large bone defects.

Runx2/Cbfa1 stimulates transdifferentiation of primary skeletal myoblasts into a mineralizing osteoblastic phenotype

Runx2, a transcriptional activator downstream of bone morphogenetic protein (BMP) signaling, is essential to osteoblastic differentiation and bone formation and maintenance. BMPs activate complex signaling networks, utilizing numerous signaling molecules and transcription factors to induce expression of osteoblastic markers in mesenchymal cell types. However, the role of Runx2 in this process, particularly in an environment independent of the other regulatory elements modulated by BMPs, remains poorly understood. In the present study, we used retroviral gene delivery to examine the effects of sustained Runx2 expression in primary myoblasts. Runx2 inhibited myogenesis, as demonstrated by suppression of MyoD and myogenin mRNA levels and reduced myotube formation. Additionally, Runx2-stimulated osteogenesis including osteoblastic gene expression, alkaline phosphatase activity, and biological mineral deposition. Notably, these osteogenic markers were induced to significantly greater levels than those observed in BMP-2-treated controls. These results demonstrate that direct exogenous expression of the Runx2 transcription factor, only one of numerous downstream targets of BMP signaling, is sufficient to induce transdifferentiation of myogenic cells into a mineralizing osteogenic lineage. This work underscores the potency of Runx2 as a regulator of osteogenesis and cell differentiation and provides new insights into the plasticity of committed mesenchymal cells.

Isolation and characterization of a mesenchymal cell line that differentiates into osteoblasts in response to BMP-2 from calvariae of GFP transgenic mice

We established the clonal mesenchymal cell line, GFP-C3 (C3), which differentiates into osteoblasts in response to BMP-2 from calvariae of newborn green fluorescence protein (GFP) transgenic mice. This cell line cultured with control medium expressed low levels of alkaline phosphatase (ALP) activity and osterix mRNA and undetectable ALP and osteocalcin mRNA. Incubation of these cells with rhBMP-2 increased ALP activity dose-dependently and induced substantial levels of ALP, osteocalcin and osterix mRNA expression. C3 cells infected with adenovirus vector encoding BMP-2 (AdBMP-2) or Runx2 (AdRunx2) showed greatly increased ALP mRNA expression in a time-dependent fashion. Transduction with AdRunx2-induced expression of ALP and osteocalcin mRNA, but not osterix mRNA by day 3. Transduction with AdBMP-2 induced apparent expression of ALP and osterix mRNA by day 1 after transduction, but induced only weak expression of osteocalcin mRNA day 3 after transduction. Transplantation of C3 cells transduced with AdBMP-2 into back subfascia in wild-type mice with a complex of poly-d,l-lactic-co-glycolic acid/gelatin sponge (PGS) generated ectopic bone formation involving GFP-positive osteoblasts and osteocytes 2 weeks after transplantation. C3 cells transduced with AdRunx2 or AdLacZ failed to induce ectopic bone formation. Transplantation of C3 cells transduced with AdBMP-2 into craniotomy defects in wild-type mice using PGS as a carrier induced bone formation 2 weeks after transplantation, and replaced defects 4 weeks after transplantation. C3 cells transduced with AdRunx2 failed to induce bone repair after transplantation into craniotomy defects. These results indicate that C3 cells retain differentiation potential into osteoblasts in response to BMP-2. They are useful tools for analyzing the process of osteoblast differentiation in vivo after transplantation.

Reciprocal roles of MSX2 in regulation of osteoblast and adipocyte differentiation

Mice deficient in the Msx2 gene manifest defects in skull ossification and a marked reduction in bone formation associated with decreases in osteoblast numbers, thus suggesting that Msx2 is involved in bone formation. However, the precise role of Msx2 during osteoblast differentiation is not fully understood. In the present study, we investigated the role of Msx2 in the regulation of osteoblast differentiation in the multipotent mesenchymal cell lines C3H10T1/2 and C2C12 and in murine primary osteoblasts. Introduction of Msx2 induced alkaline phosphatase activity in C3H10T1/2 and C2C12 cells and promoted the calcification of murine primary osteoblasts. This effect of Msx2 was also observed in mesenchymal cells isolated from Runx2-deficient mice. Interestingly the expression of Msx2 was induced by bone morphogenetic protein 2 treatment in Runx2-deficient mesenchymal cells. In contrast, Msx2 diminished peroxisome proliferator-activated receptor gamma (PPARgamma) expression and adipogenesis of the preadipocytic cell line 3T3-F442A. Moreover Msx2 inhibited the transcriptional activity of PPARgamma, CCAAT/enhancer-binding protein beta (C/EBPbeta), and C/EBPdelta and blocked adipocyte differentiation of mesenchymal cells induced by overexpression of PPARgamma, C/EBPalpha, C/EBPbeta, or C/EBPdelta. These data indicate that Msx2 promotes osteoblast differentiation independently of Runx2 and negatively regulates adipocyte differentiation through inhibition of PPARgamma and the C/EBP family.