Bone morphogenetic proteins and C-FOS: early signals in endochondral bone formation

Posttraumatic intra-articular heterotopic ossification of the shoulder joint in a 15-month-old boy

Heterotopic ossification (HO) can result from a single severe injury, repeated microtrauma, central nervous system injury, extensive burns, or muscular bleeding due to hemophilia. Although relatively rare in childhood and extremely rare within a joint, HO should be included in the differential diagnosis of an intra-articular mass when indicated by clinical, radiographic, and histologic findings. Here, we report a posttraumatic intra-articular HO of the shoulder joint in a 15-month-old boy without underlying hematologic disease. Intra-articular HO in a healthy infant has not been reported previously in the published literature. Because of the unusual presentation and location in a patient of such young age, tumorigenic conditions were considered in the differential diagnosis.

A review on macrophage responses to biomaterials

Macrophages are the dominant infiltrating cells that respond rapidly to biomaterial implantation in soft and hard tissues. These cells and their fused morphologic variants, multinucleated giant cells or foreign body giant cells, usually remain at biomaterial-tissue interfaces for the lifetime of the device in vivo. As a component of the immune system, macrophage activities are closely related to immune responses, inflammation and foreign body responses. However, macrophages also mediate biodegradation of bioresorbable materials via phagocytosis and extracellular degradation. In addition, macrophages are essential for effective tissue regeneration as they regulate the recruitment, proliferation and differentiation of target cells, such as fibroblasts, osteoblasts, endothelial cells and keratinocytes during healing processes.

Current Topics in Pharmacological Research on Bone Metabolism: Molecular Basis of Ectopic Bone Formation Induced by Mechanical Stress

Ectopic bone formation (EBF) is frequently found in various tissues and affects the prognosis of diseases accompanied by EBF. Although the mechanism of EBF remains unclear, several local factors that influence the progression of EBF have been proposed. We have been focusing on the role of mechanical stress as a local factor in EBF in spinal ligament tissues, that is, ossification of the posterior longitudinal ligament (OPLL), which causes serious neurological deficiencies. Transcriptome analyses revealed that the expressions of several marker genes related to bone remodeling were enhanced after exposure of ligament cells derived from OPLL patients (OPLL cells) to cyclic stretching as a type of mechanical stress. However, no significant alterations in gene expressions were detected after cyclic stretching of ligament cells derived from non-OPLL patients. OPLL cells exposed to cyclic stretching released several autocrine/paracrine factors that are known to mediate bone remodeling. These results suggest that OPLL cells have been transformed into cells that are highly sensitive to mechanical stress, which may induce the progression of OPLL. These observations provide information regarding the role of mechanical stress in the process of EBF.

Bone growth factors in maxillofacial skeletal reconstruction

A literature review was performed to survey the available information on the potential of bone growth factors in skeletal reconstruction in the maxillofacial area. The aim of this review was to characterize the biological and developmental nature of the growth factors considered, their molecular level of activity and their osteogenic potential in craniofacial bone repair and reconstruction. A total of 231 references were selected for evaluation by the content of the abstracts. All growth factors considered have a fundamental role in growth and development. In postnatal skeletal regeneration, PDGF plays an important role in inducing proliferation of undifferentiated mesenchymal cells. It is an important mediator for bone healing and remodelling during trauma and infection. It can enhance bone regeneration in conjunction with other growth factors but is unlikely to provide entirely osteogenic properties itself. IGFs have an important role in general growth and maintenance of the body skeleton. The effect of local application of IGFs alone in craniofacial skeletal defects has not yet shown a clear potential for enhancement of bone regeneration in the reported dosages. The combination of IGF-I with PDGF has been effective in promoting bone regeneration in dentoalveolar defects around implants or after periodontal bone loss. TGFbeta alone in skeletal reconstruction appears to be associated with uncertain results. The presence of committed cells is required for enhancement of bone formation by TGFbeta. It has a biphasic effect, which suppresses proliferation and osteoblastic differentiation at high concentrations. BMPs, BMP2, BMP4 and BMP7 in particular, appear to be the most effective growth factors in terms of osteogenesis and osseous defect repair. Efficacy of BMPs for defect repair is strongly dependent on the type of carrier and has been subject to unknown factors in clinical feasibility trials resulting in ambiguous results. The current lack of clinical data may prolong the period until this factor is introduced into routine clinical application. PRP is supposed to increase proliferation of undifferentiated mesenchymal cells and to enhance angiogenesis. There is little scientific evidence about the benefit of PRP in skeletal reconstructive and preprosthetic surgery yet and it is unlikely that peri-implant bone healing or regeneration of local bone into alloplastic material by the application of PRP alone will be significantly enhanced.

Mechanisms balancing skeletal matrix synthesis and degradation

Bone is regulated by evolutionarily conserved signals that balance continuous differentiation of bone matrix-producing cells against apoptosis and matrix removal. This is continued from embryogenesis, where the skeleton differentiates as a solid mass and is shaped into separate bones by cell death and proteolysis. The two major tissues of the skeleton are avascular cartilage, with an extracellular matrix based on type II collagen and hydrophilic proteoglycans, and bone, a stronger and lighter material based on oriented type I collagen and hydroxyapatite. Both differentiate from the same mesenchymal stem cells. This differentiation is regulated by a family of related signals centred on bone morphogenic proteins. Fibroblast growth factors, Indian hedgehog and parathyroid hormone-related protein are important in determining the type of matrix and the relation of skeletal and non-skeletal structures. Removal of mineralized matrix involves apoptosis of matrix cells and differentiation of acid-secreting cells (osteoclasts) from macrophage precursors. Key regulators of matrix removal are signals in the tumour-necrosis-factor family. Osteoclasts dissolve bone by isolating a region of the matrix and secreting HCl and proteinases at that site. Successive cycles of removal and replacement allow growth, repair and remodelling. The signals for bone turnover are predominantly cell-membrane-associated, allowing very specific spatial regulation. In addition to its support function, bone is a reservoir of Ca2+, PO3-(4) and OH-. Secondary modulation of mineral secretion and bone degradation are mediated by humoral signals, including parathyroid hormone and vitamin D, as well as the cytokines that also regulate the underlying cell differentiation.

Bone morphogenetic proteins: relevance in spine surgery

Bone morphogenetic proteins (BMPs) are low molecular weight glycoproteins that play a vital role in the development and maturation of skeletal tissue. Bone morphogenetic protein-induced mesenchymal cell recruitment and differentiation leads to the formation of chondroblasts and osteoblasts leading to the formation of de novo bone. Overwhelming pre-clinical and clinical evidence has suggested a promising role for BMPs for anterior and posterolateral spinal fusion. Strength of this approach lies in the potential ability of these growth factors to reverse inhibitory conditions common in the clinical setting and enabling predictable fusion. However, several issues related to carriers, costs, and dosages still need to be consecutively addressed. Gene therapy techniques producing in vivo osteoinductive factors and utilizing minimally invasive approaches are attractive options being developed for the future.

Matrix GLA protein, a regulatory protein for bone morphogenetic protein-2

Matrix GLA protein (MGP) has been identified as a calcification inhibitor in cartilage and vasculature. Part of this effect may be attributed to its influence on osteoinductive activity of bone morphogenetic protein-2 (BMP-2). To detect binding between MGP and BMP-2, we performed immunoprecipitation using MGP and BMP-2 tagged with FLAG and c-Myc. The results showed co-precipitation of BMP-2 with MGP. To quantify the effect of MGP on BMP-2 activity, we assayed for alkaline phosphatase activity and showed a dose-dependent effect. Low levels of MGP relative to BMP-2 (<1-fold excess) resulted in mild enhancement of osteoinduction, whereas intermediate levels (1-15-fold excess) resulted in strong inhibition. High levels of MGP (>15-fold excess), however, resulted in pronounced enhancement of the osteoinductive effect of BMP-2. Cross-linking studies showed that inhibitory levels of MGP abolished BMP-2 receptor binding. Immunoblotting showed a corresponding decrease in activation of Smad1, part of the BMP signaling system. Enhancing levels of MGP resulted in increased Smad1 activation. To determine the cellular localization of BMP-2 in the presence of MGP, binding assays were performed on whole cells and cell-synthesized matrix. Inhibitory levels of MGP yielded increased matrix binding of BMP-2, suggesting that MGP inhibits BMP-2 in part via matrix association. These results suggest that MGP is a BMP-2 regulatory protein.

Ectopic bone formation after temporal muscle transposition for facial paralysis

This report describes the case of a patient with congenital bilateral facial paralysis in whom ectopic bone formation developed following temporal muscle transposition. Ectopic bone formation was first noticed 4 years after surgery. Whether the ossification is a result of the transfer of periosteum or the osteogenic capacity of muscular tissue is still unknown.

Expression of bone morphogenetic proteins and receptors in sarcomas

Bone morphogenetic proteins, which are capable of inducing mesenchymal tissue to form bone in mammals, have been implicated as important in normal skeletal development. The expression of bone morphogenetic proteins and their receptors were studied in 36 osteosarcoma specimens, six Ewing's sarcomas, 20 synovial sarcomas, and 20 chondrosarcomas by reverse transcriptase-polymerase chain reaction, and the findings were correlated with clinical data. Bone morphogenetic protein-2, and -4 messages were detected in most sarcoma samples. Bone morphogenetic protein-6 expression was detected in 22 of 32 osteosarcomas and seven of eight chondrosarcomas. Bone morphogenetic protein-7 and receptor IB were not detected in sarcoma samples but were detected in three osteosarcoma cell lines and one malignant fibrous histiocytoma cell line. Expression of bone morphogenetic protein receptor II was found in 25 of 36 osteosarcomas, eight of 20 chondrosarcomas, four of six Ewing's sarcomas, and 15 of 20 synovial sarcoma samples. Expression of bone morphogenetic protein type II receptor was found to correlate with metastasis in osteosarcomas, which suggests that the bone morphogenetic protein pathway may participate in tumor aggressiveness or progression. The expression of bone morphogenetic protein receptor II in metastatic synovial sarcoma and dedifferentiated chondrosarcoma lesions also supports this hypothesis. The current study showed that the ligands for bone morphogenetic protein receptors, bone morphogenetic proteins-2, -4, and -6 also are expressed in osteosarcoma and other sarcoma tissues, indicating a potential for autocrine or paracrine growth stimulation in these tumors.

Id genes are direct targets of bone morphogenetic protein induction in embryonic stem cells

Bone morphogenetic proteins (BMPs) are morphogenetic signaling molecules essential for embryonic patterning. To obtain molecular insight into the influence of BMPs on morphogenesis, we searched for new genes directly activated by BMP signaling. In vitro cultured mouse embryonic stem (ES) cells were used, cultivated in chemically defined growth medium (CDM). CDM-cultured ES cells responded very selectively to stimulation by various mesoderm inducers (BMP2/4, activin A, and basic fibroblast growth factor). BMP2/4 rapidly induced transcript levels of the homeobox genes Msx-1 and Msx-2 and the proto-oncogene JunB, whereas c-jun transcripts displayed delayed albeit prolonged increase. Using differential display cDNA cloning, six direct BMP target genes were identified. These include Id3, which showed strong mRNA induction, and the moderately induced Cyr61, DEK, and eIF4AII genes, as well as a gene encoding a GC-binding protein. Besides Id3, also the Id1 and Id2 genes were activated by BMP4 in both ES cells and a range of different cell lines. Id genes encode negative regulators of basic helix-loop-helix transcription factors. In vivo we observed local ectopic expression of Id3 and Msx-2 mRNAs in Ft/+ embryos at overlapping regions of ectopic Bmp4 misexpression. We therefore propose that the Msx and Id genes are direct target genes of embryonic BMP4 signaling in vivo.

Macrophysiologic roles of a delivery system for vulnerary factors needed for bone regeneration

Traditional histology identifies three components of bone: cells, an extracellular mineralized organic matrix, and a lymphatic-vascular component. Specialized bone cells known as osteoblasts promote bone regeneration. Clinically, this property has been exploited by surgeons with autografts and bank bone preparations to restore deficient form and function to almost every aspect of the skeleton. Unfortunately, these therapies can be inadequate for patients with panskeletal trauma. Therefore, a suitable alternative may be a laboratory-derived product consisting of a vulnerary factor and delivery system. The integration of a laboratory-engineered product in an osseous wound environment is a formidable challenge demanding a keen appreciation of the product's macrophysiologic roles in wound healing biology. Consequently, the purposes for this paper are 1) to define briefly macrophysiology relevant to a delivery system for vulnerary molecules and bone regeneration; 2) to review a key family of bone regenerating molecules, the bone morphogenetic proteins (BMPs); and 3) to relate delivery system engineering with bone regeneration.