Surface immobilization of bone morphogenetic protein 2 via a self-assembled monolayer formation induces cell differentiation

Bone extracellular matrix consists of a network of proteins in which growth factors, like bone morphogenetic protein 2 (BMP-2), are embedded and released upon matrix turnover and degradation. Recombinant human (rh)BMP-2 shows promise in enhancing bone fracture repair, although issues regarding finding a suitable delivery system still limit its extensive clinical use. The aim of this study is to determine which cell activities are triggered by the presentation of immobilized rhBMP-2. For this purpose gold surfaces were first decorated with a self-assembled monolayer consisting of a hetero-bifunctional linker. rhBMP-2 was covalently bound to the surfaces via this linker and used to investigate the cellular responses of C2C12 myoblasts. We show that covalently immobilized rhBMP-2 (iBMP-2) initiates short-term signaling events. Using a BMP-responsive reporter gene assay and western blotting to monitor phosphorylation of Smad1/5/8 we prove that iBMP-2 activates BMP-dependent signal transduction. Furthermore, we demonstrate that iBMP-2 suppresses myotube formation and promotes the osteoblast phenotype in C2C12 cells. The bioactivity of surface-bound rhBMP-2 presented in this study is not due to its release into the medium. As such, our simple approach paves the way for the controlled local presentation of immobilized growth factors, limiting degradation while still maintaining biological activity.     

Immobilized rhBMP-2/succinylated type I atelocollagen gene expression of intracellular signaling molecules on ST2 cells

Recombinant human bone morphogenetic protein-2 (rhBMP-2) chemically-bonded to succinylated type I atelocollagen, a biomaterial carrier with a porous structure, was reported to augment cellular activity of ST2 cells. The Smad protein family has been suggested to play an important role in the intracellular signaling pathway of BMP by its binding to receptors on target cells. However, there has been no study analyzing the downstream genes of the rhBMP-2 induced intracellular signal transduction pathway. The purpose of this study was to examine the effect of immobilized rhBMP-2 on gene expression of intracellular signaling molecules on ST2 cells. Our study showed two expression patterns of downstream genes of rhBMP-2 intracellular signal transduction pathway. In the first pattern, BMPR-IA, Smad 1, and Smad 5 genes showed high basic expression before the addition of rhBMP, and the high level of gene expression continued for long period and decreased in the late stage when rhBMP-2 was immobilized. In the second pattern, Smad 6, Smad 7, and Smad 8 genes showed low basic expression before the addition of rhBMP-2 and a continuous increase from the beginning was followed by a decrease in the late stage when rhBMP-2 was immobilized. Our results also showed that intracellular signaling continued for prolonged period when rhBMP-2 was immobilized to succinylated type I atelocollagen. This study indicated that immobilizing rhBMP-2 is an efficient method to increase bone induction.     

A reporter-cell assay for the detection of BMP-2 immobilized on porous and nonporous materials

Human recombinant bone morphogenetic protein-2 (rhBMP-2) immobilized on the surface of metal implants can facilitate osseointegration. Here, we describe a cell reporter assay useful for quantifying small amounts of immobilized rhBMP-2 on various materials. The peptide was dotted and heat-fixed on titanium, 316L stainless steel, nitrocellulose, or glass, and its distribution was monitored by in situ biotinylation followed by detection with the avidin-biotin method. Bioactivity of rhBMP-2 was demonstrated by means of a confluent layer of osteoblastic MC3T3-E1 cells that evenly covered rhBMP-2-free and rhBMP-2-loaded surface areas, as shown with epifluorescence microscopy of calcein acetoxymethyl (AM)-loaded cells. Expression of osteocalcin, fibronectin, actin, and vimentin increased where cells were located on rhBMP-2 dotted areas, but the signal:noise ratio was too low to bioassay the peptide. However, local pronounced expression of alkaline phosphatase was used to quantify BMP-2 in the range of 5-80 ng/dot by means of a cytochemical color reaction for alkaline phosphatase and image analysis of resulting dots. The lower detection limit was in the order nitrocellulose > glass > titanium > 316L steel. We conclude that the cell reporter assay is useful to assess biological activity of rhBMP-2 even after immobilization on three-dimensional implant materials.     

Role of bone morphogenetic protein 2 in the crosstalk between endothelial progenitor cells and mesenchymal stem cells

In recent studies, we and others have demonstrated that bone morphogenetic protein-2 (BMP-2) promotes vascularization, inhibits hypoxic cell death of cancer cells and may be involved in tumor angiogenesis. The activation of circulating endothelial progenitor cells (EPCs) and mesenchymal stem cells (MSCs) represents a crucial factor in the process of postnatal neovascularization. BMP-2 protein expression has been detected in several tumor tissues and BMP receptors are expressed in EPCs and MSCs. We therefore analysed the influence of recombinant human (rh) BMP-2 on the function of human EPCs and human bone marrow derived MSCs. Treatment of EPCs isolated from peripheral blood with rhBMP-2 did not induce any significant changes in EPC viability but induced a dose-dependent activation of chemotaxis. Incubation of human MSCs isolated from bone marrow aspirates with rhBMP-2 revealed no significant effect on MSC proliferation. Incubation of EPCs with supernatants of MSCs significantly increased the cell viability compared to controls cultivated with endothelial cell medium. Protein and mRNA expression of the vascular endothelial growth factor (VEGF) family member, placental growth factor (PlGF), which is known to be involved in the expansion and recruitment of EPCs, was induced in MSCs after treatment with rhBMP-2. We conclude that tumor- associated BMP-2 secretion might promote tumor angiogenesis by chemotactic effects on EPCs circulating in the peripheral blood and by increased secretion of paracrine angiogenic growth factors including PlGF in MSCs of the tumor stroma.     

Retention and activity of BMP-2 in hyaluronic acid-based scaffolds in vitro.

Bone morphogenetic protein-2 (BMP-2) delivered in a suitable implantable matrix has the potential to repair local skeletal defects by inducing new bone formation from undifferentiated pluripotent stem cells resident in host tissue. In this study, we examined in vitro the potential of a derivatized hyaluronic acid (Hyaff-11) scaffold as a delivery vehicle for recombinant human BMP-2 (rhBMP-2) in bone and cartilage repair therapies. Hyaff-11 scaffolds were fabricated using a phase inversion/particulate leaching method and soak-loaded with rhBMP-2. In vitro release kinetics of rhBMP-2, demonstrated using enzyme-linked immunosorbant assay and alkaline phosphatase (ALP) assay revealed a slow, sustained rhBMP-2 release during 28 days, with a cumulative release of 31.82% of the initial rhBMP-2 loaded. rhBMP-2 was released in bioactive form as demonstrated by ALP induction of pluripotent cell line, C3H10T1/2 (T1/2), down the osteoblast lineage when incubated with the release supernatants. rhBMP-2 retention in Hyaff-11 scaffolds was greater than that from collagen gels, which released most of the initially loaded rhBMP-2 by 14 days. rhBMP-2-loaded Hyaff-11 scaffolds were also seeded with T1/2 cells and evaluated at 3, 7, 14, and 28 days for viability and expression of osteoblast phenotype. Cells remained viable throughout the study and expressed a time- and dose-dependent ALP and osteocalcin expression in the rhBMP-2 groups. Based on these observations, Hyaff-11 scaffolds may be suitable delivery systems for rhBMP-2 in bone/cartilage repair because of their ability to retain rhBMP-2, release low levels of bioactive rhBMP-2 to the local environment in a sustained manner, and stimulate differentiation of pluripotent stem cells.     

The role of recombinant human bone morphogenetic protein-2 in PLGA capsules at an extraskeletal site of the rat.

Bone morphogenetic protein-2 (BMP-2) is a member of the transforming growth factor-beta (TGF-beta) superfamily and has strong bone-inductive activity in vivo. To examine the role of BMP-2 in an extraskeletal site of rat using a controlled release system of peptides, we encapsulated the recombinant human BMP-2 (rhBMP-2) with poly(DL-lactide-co-glycolide) (PLGA) and implanted the rhBMP-2/PLGA capsules in the subcutaneous area of rats. Upon histochemical examination, it was found that bone-inducing cells having alkaline phosphatase (ALP) activity appeared around the capsules by the suitably released rhBMP-2. In addition, the temporal histological examination showed that direct bone formation without cartilage occurred in the process of this ectopic bone induction. These data indicate that the role of rhBMP-2 in the extraskeletal site of rats is to induce the differentiation of mesenchymal cells into the osteoblasts.Copyright 1999 John Wiley & Sons, Inc.     

DRAGON, a GPI-anchored membrane protein, inhibits BMP signaling in C2C12 myoblasts

Bone morphogenetic proteins (BMPs) induce osteoblastic differentiation of myoblasts via binding to cell surface receptors. Repulsive guidance molecules (RGMs) have been identified as BMP co-receptors. We report here that DRAGON/RGMb, a member of the RGM family, suppressed BMP signaling in C2C12 myoblasts via a novel mechanism. All RGMs were expressed in C2C12 cells that were differentiated into myocytes and osteoblastic cells, but RGMc was not detected in immature cells. In C2C12 cells, only DRAGON suppressed ALP and Id1 promoter activities induced by BMP-4 or by constitutively activated BMP type I receptors. This inhibition by DRAGON was dependent on the secretory form of the von Willbrand factor type D domain. DRAGON even suppressed BMP signaling induced by constitutively activated Smad1. Over-expression of neogenin did not alter the inhibitory capacity of DRAGON. Taken together, these findings indicate that DRAGON may be an inhibitor of BMP signaling in C2C12 myoblasts. We also suggest that a novel molecule(s) expressed on the cell membrane may mediate the signal transduction of DRAGON in order to suppress BMP signaling in C2C12 myoblasts.     

The effects of recombinant human BMP-7, prepared from a COS-7 expression system,on the proliferation and differentiation of rat newborn calvarial osteoblasts

A family of proteins, the bone morphogenetic proteins (BMPs), which promote osteoblast differentiation and bone mineralization, have recently been identified. One, BMP-7, has shown the ability to induce cartilage and bone formation processes. In this report, the possibility that other cell lines, to CHO cells, may also be available as host cells for the expression of hBMP-7 was validated. Recombinant human BMP (rhBMP)-7 was produced in COS-7 cells, as a processed mature disulfide-linked homodimer, with an apparent molecular weight of 36,000. Examination of the expressions of the markers characteristic of osteoblast phenotypes showed that the rhBMP-7 specifically stimulated the inductions of alkaline phosphatase (ALP) (5-fold increase at 100 ng of rhBMP-7/ml), parathyroid hormone (PTH)-mediated intracellular cAMP production (4-fold increase at 100 ng of rhBMP-7/ml) and osteocalcin synthesis (5-fold increase at 100 ng of rhBMP- 7/ml). In summary, the in vitro mineralization assay results provide evidence that the rhBMP-7 peptide, produced by COS-7 expression system, possesses intact biological activity. A similar pattern of biological activity was observed for the BMP-7 in COS-7 cells compared to the corresponding CHO cell expression system. Thus, these findings can be experimentally utilized for the production of rhBMPs for in vitro or in vivo studies.     

Dura mater stimulates human adipose-derived stromal cells to undergo bone formation in mouse calvarial defects

Human adipose-derived stromal cells (hASCs) have a proven capacity to aid in osseous repair of calvarial defects. However, the bone defect microenvironment necessary for osseous healing is not fully understood. In this study, we postulated that the cell-cell interaction between engrafted ASCs and host dura mater (DM) cells is critical for the healing of calvarial defects. hASCs were engrafted into critical sized calvarial mouse defects. The DM-hASC interaction was manipulated surgically by DM removal or by insertion of a semipermeable or nonpermeable membrane between DM and hASCs. Radiographic, histologic, and gene expression analyses were performed. Next, the hASC-DM interaction is assessed by conditioned media (CM) and coculture assays. Finally, bone morphogenetic protein (BMP) signaling from DM was investigated in vivo using novel BMP-2 and anti-BMP-2/4 slow releasing scaffolds. With intact DM, osseous healing occurs both from host DM and engrafted hASCs. Interference with the DM-hASC interaction dramatically reduced calvarial healing with abrogated BMP-2-Smad-1/5 signaling. Using CM and coculture assays, mouse DM cells stimulated hASC osteogenesis via BMP signaling. Through in vivo manipulation of the BMP-2 pathway, we found that BMP-2 plays an important role in DM stimulation of hASC osteogenesis in the context of calvarial bone healing. BMP-2 supplementation to a defect with disrupted DM allowed for bone formation in a nonhealing defect. DM is an osteogenic cell type that both participates in and stimulates osseous healing in a hASC-engrafted calvarial defect. Furthermore, DM-derived BMP-2 paracrine stimulation appears to play a key role for hASC mediated repair.     

Increased vascular delivery and efficacy of chemotherapy after inhibition of platelet-derived growth factor-B

A relationship exists between defects in bone morphogenetic protein (BMP) signaling and formation of hamartoma and adenoma in the gastric epithelium; however, the role of BMP signaling in the progression of diffuse-type gastric carcinoma remains unknown. We investigated whether BMP functions as a tumor suppressor in human diffuse-type gastric carcinoma using three different human diffuse-type gastric carcinoma cell lines (OCUM-12, HSC-39, and OCUM-2MLN). Overexpression of the dominant-negative form of BMP-2/4-specific type I receptor (ALK-3) in OCUM-12 and HSC-39 cells accelerated their growth in vivo. BMP-4 induced cell cycle arrest in these cells via p21 induction through the SMAD pathway. Moreover, overexpression of the constitutively active form of ALK-3 in HSC-39 and OCUM-2MLN cells suppressed the proliferation of these cells in vitro and in vivo. Our findings suggest that BMP-2 and BMP-4 function as potent tumor suppressors in diffuse-type gastric carcinoma.     

Bone morphogenetic protein-2 and -7 mediate the anabolic function of nucleus pulposus cells with discrete mechanisms

Bone morphogenetic proteins (BMPs) play roles in promoting cell anabolism, especially in extracellular matrix production. The difference between BMP members in their capacity to modulate intervertebral disc cell activity is yet to be defined. BMP-7/OP-1 has been shown to retard disc degeneration. We compared the activity of BMP-7 with that of BMP-2 on nucleus pulposus (NP) cell phenotype and function, and investigated how they differentially affect the gene expression profiles of signaling cascade components in human NP cells under degenerative states. We found that while both BMP-2 and BMP-7 enhanced matrix production of bovine NP cells, BMP-7 is more potent than BMP-2 at various dosages (50–800 ng/ml). BMP-7 exerted a relatively stronger stimulation on sulfated glycosaminoglycan production and proliferation in human NP cells. Degenerated NP cells showed an overall weaker response to the BMPs than non-degenerated cells, and were more sensitive to BMP-7 than BMP-2 stimulation. Compared to BMP-2, BMP-7 not only induced the gene expression of canonical BMP components, but also evoked changes in MAPKs as well as CREB1 and EP300 gene expression in degenerated NP cells, suggesting potential activation of the cAMP dependent protein kinase related pathways. In contrast to BMP-2, BMP-7 concomitantly inhibited the expression of profibrotic genes. We propose that BMP-2 and BMP-7, and likely other BMPs, may operate multifaceted but discrete molecular machineries that give rise to their different capacity in regulating NP cell phenotype. Further investigations into such differential capacity may possibly derive alternative cues important for IVD repair or engineering.     

Analysis of hydrolyzable polyethylene glycol hydrogels and deproteinized bone mineral as delivery systems for glycosylated and non-glycosylated bone morphogenetic protein-2

Bone morphogenetic proteins (BMP), in particular BMP-2, are the growth factors primarily responsible for osteoinduction. A knowledge of interactions between bone substitute materials and growth factor variants is crucial to designing bone substitutes with an ideal release profile. Here we compare glycosylated and non-glycosylated recombinant human bone morphogenetic protein-2 (rhBMP-2) either incorporated into a hydrolyzable polyethylene glycol (PEG) hydrogel developed as a slow release system or adsorbed to a deproteinized bovine bone matrix (DBBM), a clinically well-established bone substitute material. rhBMP-2 loaded materials were immersed in cell culture medium and rhBMP-2 concentration profiles in the supernatant were determined by an enzyme-linked immunosorbent assay. The corresponding biological activities were assessed in vitro by alkaline phosphatase activity assay. We show a strong affinity of rhBMP-2 for DBBM and reduced biological activity after its release from PEG hydrogels. Glycosylated rhBMP-2 was significantly less affected by the hydrogel and interacted significantly more strongly with DBBM than non-glycosylated rhBMP-2. We therefore question the combination of PEG hydrogels with DBBM as a rhBMP-2 delivery system over DBBM alone, since rhBMP-2 released from the hydrogel will be trapped by DBBM. Moreover, our results suggest that glycosylated rhBMP-2 is favorable in combination with PEG hydrogels, since its activity is better preserved, whereas in combination with DBBM non-glycosylated rhBMP-2 is favorable, benefiting from an initially higher concentration of free rhBMP-2.     

New scaffold for recombinant human bone morphogenetic protein-2

A bioactive and resorbable scaffold is necessary to exhibit the osteoinductive potency of recombinant human bone morphogenetic protein-2 (rhBMP-2). In a previous study, we found that synthetic octacalcium phosphate (OCP) enhances bone regeneration and is replaced by newly formed bone after it is resorbed. We hypothesized that OCP may be useful as an effective scaffold for rhBMP-2 to enhance bone regeneration. To test this hypothesis, the present study was designed to investigate whether an OCP/BMP composite implant could more effectively enhance bone regeneration. A critical-sized defect was made in a rat calvarium and 1. 15 mg of OCP combined with 10 microg of rhBMP-2 (OCP/BMP 10 microg), 2. 15 mg of OCP combined with 1 microg of rhBMP-2 (OCP/BMP 1 microg), or 3. OCP (OCP alone) was implanted into the defect and fixed at 4 or 8 weeks after implantation. The percentage of newly formed bone (n-Bone%) in the defect was determined by a histomorphometrical analysis. A statistical analysis showed that n-Bone% with OCP/BMP was significantly higher than that with OCP at both time points, whereas the difference in n-Bone% between OCP/BMP 10 microg and OCP/BMP 1 microg was not significant. The present results suggest that OCP can be used as an effective scaffold for rhBMP-2 and this OCP delivery system may be able to reduce the standard effective dose of rhBMP-2, which would be beneficial because low doses (<100 microg/g OCP) of rhBMP-2 enhance bone regeneration.     

Mixing conditions for cell scaffolds affect the bone formation induced by bone engineering with human bone marrow stromal cells, beta-tricalcium phosphate granules, and rhBMP-2

Bone regenerative medicine via tissue engineering is expected to be an alternative treatment for conventional autogenous bone graft, as it is less invasive. One of the best triads for bone engineering is bone marrow stromal cells, calcium phosphate ceramics, and bone morphogenetic protein (BMP). However, the optimal mixing conditions for BMP-induced osteoblasts and ceramic granules remain unclear. Therefore, we investigated the effect of the mixing conditions for cell scaffolds on the bone-forming potential. The cells were mixed with beta-tricalcium phosphate (beta-TCP) granules followed by osteoblast induction with recombinant human BMP-2 (rhBMP-2) (first mixture), or were first induced with rhBMP-2 on plastic dishes and then mixed with the beta-TCP granules (last mixture) just prior to the operation. Both the first and last mixtures were transplanted into nude mice subcutaneously, with the amount of bone formation analyzed histomorphometrically. In addition, cell numbers and alkaline phosphatase (ALP) activity before transplantation was determined in both the mixtures. In vitro analyses revealed that cell numbers were greater in the last mixture, whereas ALP activity was greater in the first mixture. In vivo analyses revealed that the first mixture was much more osteogenic than the last mixture with respect to new bone formation and osteocalcin synthesis. These data suggest that cell-scaffold mixing conditions have a significant influence on the bone-forming capacity via bone engineering and that first mixture might be the optimal condition for rhBMP-2-induction of human osteoblasts.