Reduced expression of BMP-3 due to mechanical loading: a link between mechanical stimuli and tissue differentiation

BMP3 Affects Cortical and Trabecular Long Bone Development in Mice

Bone morphogenetic proteins (BMPs) have a major role in tissue development. BMP3 is synthesized in osteocytes and mature osteoblasts and has an antagonistic effect on other BMPs in bone tissue. The main aim of this study was to fully characterize cortical bone and trabecular bone of long bones in both male and female Bmp3^−/− mice. To investigate the effect of Bmp3 from birth to maturity, we compared Bmp3^−/− mice with wild-type littermates at the following stages of postnatal development: 1 day (P0), 2 weeks (P14), 8 weeks and 16 weeks of age. Bmp3 deletion was confirmed using X-gal staining in P0 animals. Cartilage and bone tissue were examined in P14 animals using Alcian Blue/Alizarin Red staining. Detailed long bone analysis was performed in 8-week-old and 16-week-old animals using micro-CT. The Bmp3 reporter signal was localized in bone tissue, hair follicles, and lungs. Bone mineralization at 2 weeks of age was increased in long bones of Bmp3^−/− mice. Bmp3 deletion was shown to affect the skeleton until adulthood, where increased cortical and trabecular bone parameters were found in young and adult mice of both sexes, while delayed mineralization of the epiphyseal growth plate was found in adult Bmp3^−/− mice.

Non-union bone fractures

The human skeleton has remarkable regenerative properties, being one of the few structures in the body that can heal by recreating its normal cellular composition, orientation and mechanical strength. When the healing process of a fractured bone fails owing to inadequate immobilization, failed surgical intervention, insufficient biological response or infection, the outcome after a prolonged period of no healing is defined as non-union. Non-union represents a chronic medical condition not only affecting function but also potentially impacting the individual's psychosocial and economic well-being. This Primer provides the reader with an in-depth understanding of our contemporary knowledge regarding the important features to be considered when faced with non-union. The normal mechanisms involved in bone healing and the factors that disrupt the normal signalling mechanisms are addressed. Epidemiological considerations and advances in the diagnosis and surgical therapy of non-union are highlighted and the need for greater efforts in basic, translational and clinical research are identified.

BMP signalling in a mechanical context - Implications for bone biology

Bone Morphogenetic Proteins (BMPs) are extracellular multifunctional signalling cytokines and members of the TGFβ super family. These pleiotropic growth factors crucially promote bone formation, remodeling and healing after injury. Additionally, bone homeostasis is systematically regulated by mechanical inputs from the environment, which are incorporated into the bone cells' biochemical response. These inputs range from compression and tension induced by the movement of neighboring muscle, to fluid shear stress induced by interstitial fluid flow in the canaliculi and in the vascular system. Although BMPs are widely applied in a clinic context to promote fracture healing, it is still elusive how mechanical inputs modulate this signalling pathway, hindering an efficient and side-effect free application of these ligands in bone healing. This review aims to summarize the current understanding in how mechanical cues (tension, compression, shear force and hydrostatic pressure) and substrate stiffness modulate BMP signalling. We highlight the time-dependent effects in modulating immediate early up to long-term effects of mechano-BMP crosstalk during bone formation and remodeling, considering the interplay with other already established mechanosensitive pathways, such as MRTF/SRF and Hippo signalling.

A review of the cellular and molecular effects of extracorporeal shockwave therapy

Extracorporeal shockwave therapy (ESWT) is a novel therapeutic modality and its use in promoting connective tissue repair and analgesic effect has been advocated in the literature. It is convenient, cost-effective, and has negligible complications; it therefore bypasses many of the problems associated with surgical interventions. This paper reviews the proposed mechanisms of action in promoting tissue repair and regeneration as well as analysing its efficacy providing an analgesic effect in clinical applications. Further research will be required to not only identify the underlying mechanisms more precisely, but will also be critical for ensuring consistency across the literature so that the most beneficial treatment protocol can be developed. Extracorporeal shockwave therapy stands as a promising alternative modality in promoting tissue repair.

Can skull form predict the shape of the temporomandibular joint? A study using geometric morphometrics on the skulls of wolves and domestic dogs

The temporomandibular joint (TMJ) conducts and restrains masticatory movements between the mammalian cranium and the mandible. Through this functional integration, TMJ morphology in wild mammals is strongly correlated with diet, resulting in a wide range of TMJ variations. However, in artificially selected and closely related domestic dogs, dietary specialisations between breeds can be ruled out as a diversifying factor although they display an enormous variation in TMJ morphology. This raises the question of the origin of this variation. Here we hypothesise that, even in the face of reduced functional demands, TMJ shape in dogs can be predicted by skull form; i.e. that the TMJ is still highly integrated in the dog skull. If true, TMJ variation in the dog would be a plain by-product of the enormous cranial variation in dogs and its genetic causes. We addressed this hypothesis using geometric morphometry on a data set of 214 dog and 60 wolf skulls. We digitized 53 three-dimensional landmarks of the skull and the TMJ on CT-based segmentations and compared (1) the variation between domestic dog and wolf TMJs (via principal component analysis) and (2) the pattern of covariation of skull size, flexion and rostrum length with TMJ shape (via regression of centroid size on shape and partial least squares analyses). We show that the TMJ in domestic dogs is significantly more diverse than in wolves: its shape covaries significantly with skull size, flexion and rostrum proportions in patterns which resemble those observed in primates. Similar patterns in canids, which are carnivorous, and primates, which are mostly frugivorous imply the existence of basic TMJ integration patterns which are independent of dietary adaptations. However, only limited amounts of TMJ variation in dogs can be explained by simple covariation with overall skull geometry. This implies that the final TMJ shape is gained partially independently of the rest of the skull.

BMPs in bone regeneration: Less is more effective, a paradigm-shift

Worldwide, the clinical application of BMP2 (bone morphogenetic protein 2) has helped an increasing number of patients achieve bone regeneration in a clinical area lacking simple solutions for difficult bone healing situations. In this review, the historical aspects and current critical clinical issues are summarized and positioned against new research findings on efficacy and function of BMP2. Knowledge concerning how the dose of this growth factor as well as its interaction with mechanical loading influences the efficacy of bone regeneration, might open possible future strategies in cases where bony bridging is unachievable so far. In conclusion, it is apparent that there is a substantial need for continued basic research to unravel the details of its function and the underlying signaling pathways involved, to make BMP2 even more relevant and safe in daily clinical use, even though this growth factor has been known for more than 125 years.

BMP growth factor signaling in a biomechanical context

Bone Morphogenetic Proteins (BMPs) are members of the transforming growth factor-β superfamily of secreted polypeptide growth factors and are important regulators in a multitude of cellular processes. To ensure the precise and balanced propagation of their pleiotropic signaling responses, BMPs and their corresponding signaling pathways are subject to tight control. A large variety of regulatory mechanisms throughout different biological levels combines into a complex network and provides the basis for physiological BMP function. This regulatory network not only includes biochemical factors but also mechanical cues. Both BMP signaling and mechanotransduction pathways are tightly interconnected and represent an elaborate signaling network active during development but also during organ homeostasis. Moreover, its dysregulation is associated with a number of human pathologies. A more detailed understanding of this crosstalk in respect to molecular interactions will be indispensable in the future, in particular to understand BMP-related diseases as well as with regard to an efficient clinical application of BMP ligands.

The clinical use of bone morphogenetic proteins revisited: a novel biocompatible carrier device OSTEOGROW for bone healing

PURPOSE

The purpose of this study was to revise the clinical use of commercial BMP2 (Infuse) and BMP7 (Osigraft) based bone devices and explore the mechanism of action and efficacy of low BMP6 doses in a novel whole blood biocompatible device OSTEOGROW.

METHODS

Complications from the clinical use of BMP2 and BMP7 have been systemically reviewed in light of their role in bone remodeling. BMP6 function has been assessed in Bmp6-/- mice by μCT and skeletal histology, and has also been examined in mesenchymal stem cells (MSC), hematopoietic stem cells (HSC) and osteoclasts. Safety and efficacy of OSTEOGROW have been assessed in rats and rabbits.

RESULTS

Clinical use issues of BMP2 and BMP7 have been ascribed to the limited understanding of their role in bone remodeling at the time of device development for clinical trials. BMP2 and BMP7 in bone devices significantly promote bone resorption leading to osteolysis at the endosteal surfaces, while in parallel stimulating exuberant bone formation in surrounding tissues. Unbound BMP2 and BMP7 in bone devices precipitate on the bovine collagen and cause inflammation and swelling. OSTEOGROW required small amounts of BMP6, applied in a biocompatible blood coagulum carrier, for stimulating differentiation of MSCs and accelerated healing of critical size bone defects in animals, without bone resorption and inflammation. BMP6 decreased the number of osteoclasts derived from HSC, while BMP2 and BMP7 increased their number.

CONCLUSIONS

Current issues and challenges with commercial bone devices may be resolved by using novel BMP6 biocompatible device OSTEOGROW, which will be clinically tested in metaphyseal bone fractures, compartments where BMP2 and BMP7 have not been effective.

Mechanical load modulates the stimulatory effect of BMP2 in a rat nonunion model

INTRODUCTION

Local application of bone morphogenetic proteins (BMPs) at the fracture site is known to stimulate bone regeneration. However, recent studies illustrate that the BMP-initiated mineralization may be enhanced by additional mechanical stimulation. Therefore, bone healing was monitored in vivo in order to investigate the effect of mechanical loading on the initiation and maturation of mineralization after cytokine treatment. We hypothesized that the mechanical stimulation would further enhance the efficacy of BMP2 treatment.

METHOD

Female Sprague-Dawley rats underwent a 5-mm defect, stabilized with an external fixator. Type I collagen scaffolds containing 50 μg of BMP2 diluted in a solvent or solvent only were placed into the defects. The BMP2-treated specimens and control specimens were then each divided into two groups: one that underwent mechanical loading and a nonloaded group. In vivo loading began immediately after surgery and continued once per week for the entire 6-week experimental period. For all groups, the newly formed callus tissue was quantitatively evaluated first by in vivo microcomputed tomography at 2, 4, and 6 weeks and further by histologic or histomorphometric analysis at 6 weeks postoperation.

RESULTS

Mechanical stimulation with BMP2 treatment significantly enhanced mineralized tissue volume and mineral content at 2 weeks. Histological analysis demonstrated a significantly greater area of fibrous connective tissue including bone marrow in the stimulated group, suggesting reconstitution of the endosteal canal and more advanced bone remodeling present in the mechanical loaded group. Both groups receiving BMP2 underwent massive bone formation, achieving bony bridging after only 2 weeks, while both control groups, receiving solvent only, revealed a persisting nonunion, filled with fibrous connective tissue, prolapsed muscle tissue, and a sealed medullary canal at week 6.

CONCLUSION

Mechanical loading further enhanced the efficacy of BMP2 application evidenced by increased mineralized tissue volume and mineralization at the stage of bony callus bridging. These data suggest that already a minimal amount of mechanical stimulation through load bearing or exercise may be a promising adjunct stimulus to enhance the efficacy of cytokine treatment in segmental defects. Further studies are required to elucidate the mechanistic interplay between mechanical and biological stimuli.

BMP2 and mechanical loading cooperatively regulate immediate early signalling events in the BMP pathway

Background

Efficient osteogenic differentiation is highly dependent on coordinated signals arising from growth factor signalling and mechanical forces. Bone morphogenetic proteins (BMPs) are secreted proteins that trigger Smad and non-Smad pathways and thereby influence transcriptional and non-transcriptional differentiation cues. Crosstalk at multiple levels allows for promotion or attenuation of signalling intensity and specificity. Similar to BMPs, mechanical stimulation enhances bone formation. However, the molecular mechanism by which mechanical forces crosstalk to biochemical signals is still unclear.

Results

Here, we use a three-dimensional bioreactor system to describe how mechanical forces are integrated into the BMP pathway. Time-dependent phosphorylation of Smad, mitogen-activated protein kinases and Akt in human fetal osteoblasts was investigated under loading and/or BMP2 stimulation conditions. The phosphorylation of R-Smads is increased both in intensity and duration under BMP2 stimulation with concurrent mechanical loading. Interestingly, the synergistic effect of both stimuli on immediate early Smad phosphorylation is reflected in the transcription of only a subset of BMP target genes, while others are differently affected. Together this results in a cooperative regulation of osteogenesis that is guided by both signalling pathways.

Conclusions

Mechanical signals are integrated into the BMP signalling pathway by enhancing immediate early steps within the Smad pathway, independent of autocrine ligand secretion. This suggests a direct crosstalk of both mechanotransduction and BMP signalling, most likely at the level of the cell surface receptors. Furthermore, the crosstalk of both pathways over longer time periods might occur on several signalling levels.

Hepatoregenerative role of bone morphogenetic protein-9

Bone morphogenetic protein-9 (BMP-9) is a member of the transforming growth factor beta (TGF-β) superfamily of cytokines, which regulate cell growth and differentiation during embryogenesis. Apart of that, the hypoglycemic potential of BMP-9 is of great interest. It has been confirmed that BMP-9, like insulin, improves glycemia in diabetic mice and regulates directional glucose metabolism in hepatocytes; therefore it is proposed to be a candidate hepatic insulin-sensitizing substance (HISS). In liver fibrosis, due to the portocaval shunt, insulin bypasses the organ and the liver undergoes atrophy. Parenteral administration of insulin reverses atrophy by stimulating mitogenic activity of the hepatocytes. Because BMP-9 has a signaling pathway similar to other BMPs and insulin, it is to be expected that BMP-9 has a certain regenerative role in the liver, supporting the above-mentioned is evidence of BMP-9 expression in Dissè's spaces and BMP-7's mitogenic activity in mucosal cells. However, further studies are needed to confirm the possible regenerative role of BMP-9.

Modularity of the oral jaws is linked to repeated changes in the craniofacial shape of african cichlids

The African cichlids of the East-African rift-lakes provide one of the most dramatic examples of adaptive radiation known. It has long been thought that functional decoupling of the oral and pharyngeal jaws in cichlids has facilitated their explosive evolution. Recent research has also shown that craniofacial evolution from radiations in lakes Victoria, Malawi, and Tanganyika has occurred along a shared primary axis of shape divergence, whereby the preorbital region of the skull changes in a manner that is, relatively independent from other head regions. We predicted that the preorbital region would comprise a variational module and used an extensive dataset from each lake that allowed us to test this prediction using a model selection approach. Our findings supported the presence of a preorbital module across all lakes, within each lake, and for Malawi, within sand and rock-dwelling clades. However, while a preorbital module was consistently present, notable differences were also observed among groups. Of particular interest, a negative association between patterns of variational modularity was observed between the sand and rock-dwelling clades, a patter consistent with character displacement. These findings provide the basis for further experimental research involving the determination of the developmental and genetic bases of these patterns of modularity.

Variability observed in mechano-regulated in vivo tissue differentiation can be explained by variation in cell mechano-sensitivity

Computational simulations of tissue differentiation have been able to capture the main aspects of tissue formation/regeneration observed in animal experiments-except for the considerable degree of variability reported. Understanding and modelling the source of this variability is crucial if computational tools are to be developed for clinical applications. The objective of this study was to test the hypothesis that differences in cell mechano-sensitivity between individuals can explain the variability of tissue differentiation patterns observed experimentally. Simulations of an experiment of tissue differentiation in a mechanically loaded bone chamber were performed. Finite element analysis was used to determine the biophysical environment, and a lattice-modelling approach was used to simulate cell activity. Differences in cell mechano-sensitivity among individuals were modelled as differences in cell activity rates, with the activation of cell activities regulated by the mechanical environment. Predictions of the tissue distribution in the chambers produced the two different classes of results found experimentally: (i) chambers with a layer of bone across the chamber covered by a layer of cartilage on top and (ii) chambers with almost no bone, mainly fibrous tissue and small islands of cartilage. This indicates that the differing cellular response to the mechanical environment (i.e., subject-specific mechano-sensitivity) could be a reason for the different outcomes found when implants (or tissue engineered constructs) are used in a population.

Action of recombinant human BMP-2 on fracture healing in rabbits is dependent on the mechanical environment

The utility of recombinant human bone morphogenetic protein-2 (rhBMP-2) in inducing bone formation in fractures of bone is well known. However, the influence of the mechanical environment on the actions of rhBMP-2 on fracture healing is not clear. An experimental model of fractures of the tibia in rabbits was developed and utilized to investigate the role of mechanical environment on rhBMP-2 action. A 1 mm osteotomy gap was stabilized by either a low- or high-stiffness fixator (LSF or HSF, respectively), and local treatment with rhBMP-2 in an absorbable collagen sponge (ACS) was evaluated. The results of the investigation were analysed by both histomorphometry and biomechanics. The LSF caused an increase in mineralized periosteal callus compared to HSF, the rhBMP-2 in ACS accelerated fracture healing only in the LSF group but not in the HSF group. The area of mineralized tissue in interfragmentary callus was determined by fixation stiffness and not by BMP treatment. rhBMP-2 caused higher bone resorption in the endosteal callus during the late stages of fracture healing, but these histological differences did not affect the mechanical properties. Biomechanical evaluation showed only differences at 3 weeks between LSF-rhBMP-2 and LSF-ACS. The bending and torsional properties were higher in the rhBMP-2/ACS group compared to ACS alone at 3 weeks.

Microtopographically patterned surfaces promote the alignment of tenocytes and extracellular collagen

This paper investigates the role of microtopographical features on the cytomorphology, alignment, proliferation and gene expression of tenocytes. We made use of simple microfabrication approaches to create surfaces patterned with topographical features suitable for in vitro studies of tenocytes. These surfaces were composed of glass substrates patterned with polymeric ridges spaced from 50 to 250 microm apart. Our studies demonstrate that the microgrooves differentially impact tenocyte shape, alignment and matrix organization along the direction of grooves. Groove widths significantly influenced cellular alignment, with 50 microm grooved patterns affecting alignment most substantially. Polarized light microscopy demonstrated that mature collagen fibers were denser and more oriented within 50 microm patterns. None of the patterns had a significant effect on the expression of genes linked to proliferation or extracellular matrix synthesis, although time in culture profoundly influenced both gene groups. COMP mRNA expression was moderately increased in tenocytes seeded onto 250 microm grooves, but there was no overall beneficial phenotypic effect of aligned growth. The results of this study indicate that microtopography affects cell density and alignment of tenocytes and leads to the deposition of an aligned collagen matrix, but does not significantly impact matrix gene expression or cell phenotype. These outcomes provide insights into the biology of tendon regeneration, thus providing guidance in the design of clinical procedures for tendon repair.

Recent advances in BMP receptor signaling

Bone Morphogenetic Proteins (BMPs) play an important role during organ development and during regeneration after tissue damage. BMPs signal via transmembrane serine/threonine kinase receptors. From our current understanding heteromeric complexes of type I and type II receptors are required for signal propagation. Presently, three type I and three type II receptors are known to bind BMPs with different affinities. Ligands and receptors eventually oligomerize via defined modes into signaling complexes. Co-receptors recruit into these complexes to either inhibit or to promote signaling. The Smad pathway, initiated by phosphorylation through the activated type I receptors, results in transcriptional regulation of early target genes. However, on its way to the nucleus, Smads represent signaling platforms for other pathways, which eventually finetune BMP signal transduction. We also describe BMP-induced signaling cascades leading to cytoskeletal rearrangements, non-transcriptional and non-Smad pathways. BMPs induce a plethora of different cellular effects ranging from stem cell maintenance, migration, differentiation, proliferation to apoptosis. The molecular mechanism, by which the same ligand induces these manifold effects, depends on the cellular context. Here we try to give a current picture of the most important players in regulating and directing BMP signaling towards the desired cellular outcome. Examples of BMP action during development, but also physiological and pathophysiological conditions in the adult organism are presented.

Mechanical stimulation alters tissue differentiation and molecular expression during bone healing

Further understanding of how mechanical cues modulate skeletal tissue differentiation can identify potential means of enhancing repair following injury or disease. Prior studies examined the effects of mechanical loading on osteogenesis, chondrogenesis, and fibrogenesis in an effort to enhance bony union. However, exploring how mechanical stimuli can divert the bone healing process towards formation of other mesenchymal tissues, as an endpoint, may elucidate new avenues for repair and regeneration of tissues such as cartilage and fibrous tissue. This study investigated the use of mechanical stimulation to promote cartilage rather than bone formation within an osteotomy. Our overall goal was to define skeletal tissue distribution and molecular expression patterns induced by the stimulation. Retired breeder Sprague-Dawley rats (n = 85) underwent production of a mid-diaphyseal, transverse femoral osteotomy followed by external fixation. Beginning on postoperative day 10 and continuing for 1, 2, or 4 weeks, a cyclic bending motion (+35 degrees/-25 degrees at 1 Hz) was applied in the sagittal plane for 15 min/day for 5 consecutive days/week. Control animals experienced continuous rigid fixation. Histological and molecular analyses indicated that stimulation substantially altered normal bone healing. Stimulated specimens exhibited an increase in cartilage volume over time, while control specimens demonstrated bony bridging. Stimulation induced upregulation of cartilage-related genes (COL2A1 and COL10A1) and downregulation of bone morphogenetic proteins (BMPs) -4, -6 and -7. However, BMP-3 was upregulated with stimulation. These findings illustrate that mechanical cues can selectively modulate osteogenesis and chondrogenesis in vivo, and suggest a potential basis for treatment regimens for injured or diseased cartilaginous tissues.

Alendronate prevents collapse in mechanically loaded osteochondral grafts A bone chamber study in rats

BACKGROUND

Subchondral bone necrosis is important in osteonecrosis, Mb Kienboeck, intraarticular fractures or osteochondral grafting. As revascularization follows, bone resorption may lead to collapse in load bearing areas during the remodeling. Bisphosphonates are potent osteoclast inhibitors. Our hypothesis was that local bisphosphonate treatment of an osteochondral graft, in a high load environment, would protect the subchondral bone from collapse and maintain the joint architecture during remodeling. To investigate this, we used a rat bone chamber model to subject a necrotic osteochondral graft to a large mechanical load during remodeling.

METHOD

Cylindrical osteochondral grafts were taken from the patellar groove of rats, one end of the cylinder being the joint surface. The grafts were frozen, thawed and treated with alendronate. The length of the cylinder was measured and the grafts were placed in the chambers, which were inserted into the proximal tibia of rats. The chambers were left to heal in for two weeks to allow establishment of a vascular supply, and then the transplanted osteochondral plugs were mechanically loaded for 4 weeks, once a day with 10 cycles of 2 MPa pressure at 0.16 Hz.

RESULTS

At harvest, the graft length had decreased during remodeling in 5 of the 6 untreated controls, but only in 2 out of 8 alendronate-treated rats (p = 0.05). Histologically, the bone graft in the non-treated controls was resorbed in the remodeled part of the graft, whereas in the alendronate-treated rats a dense trabecular bone was found consisting of both new bone and graft.

INTERPRETATION

Local treatment of the graft with bisphosphonate diminishes the risk of collapse during revascularization and bone remodeling in a mechanically loaded osteochondral graft. This could be useful in a variety of situations when bone remodeling occurs after a necrosis close to a joint, either spontaneously after osteonecrosis or a fracture, or after surgical procedures such as mosaic-plasty or other osteochondral grafting.

Application of rhBMP-7 and platelet-rich plasma in the treatment of long bone non-unions: a prospective randomised clinical study on 120 patients

The purpose of this prospective randomised clinical study was to compare the efficacy of recombinant bone morphogenetic protein 7 (rhBMP-7) and platelet-rich plasma (PRP) as bone-stimulating agents in the treatment of persistent fracture non-unions. One hundred and twenty patients were randomised into two treatment groups (group rhBMP-7 vs. group PRP). Sixty patients with sixty fracture non-unions were assigned to each group (median age: 44 years, range 19-65, for the rhBMP-7 group and 41 years, range 21-62, for the PRP group, respectively). In the rhBMP-7 group, there were 15 tibial non-unions, 10 femoral, 15 humeral, 12 ulnar, and 8 radial non-unions. In the PRP group, there were 19 tibial non-unions, 8 femoral, 16 humeral, 8 ulnar, and 9 radial non-unions. The median number of operations performed prior to our intervention was 2 (range 1-5) and 2 (range 1-5) with autologous bone graft being used in 23 and 21 cases for the rhBMP-7 and PRP groups, respectively. Both clinical and radiological union occurred in 52 (86.7%) cases of the rhBMP-7 group compared to 41 (68.3%) cases of the PRP group, with a lower median clinical and radiographic healing time observed in the rhBMP-7 group (3.5 months vs. 4 months and 8 months vs. 9 months, respectively). This study supports the view that in the treatment of persistent long bone non-unions, the application of rhBMP-7 as a bone-stimulating agent is superior compared to that of PRP with regard to their clinical and radiological efficacy.

Interplay of mechanical loading and growth factors in the mandibular condyle

The mandibular condyle is an important growth site in the developing mandible. The growth of the condyle is known to be highly adaptable to functional factors. This property is exploited in orthodontics for the treatment of class II malocclusions and mandibular asymmetries. However, there is an ongoing debate on the efficacy of functional appliances. The comparison of experimental studies is complicated by the lack of detailed analyses of the load distribution within the condyle. In spite of this, there is a large body of evidence showing that mechanical manipulation of the condyle induces metabolic changes, and changes in the expression of growth factors and other signalling molecules. This review aims to give an overview of the role of growth factors in the condyle with special emphasis on their responsiveness to mechanical perturbation.

Critical issues in translational and clinical research for the study of new technologies to enhance bone repair

Osteoporosis increases fracture risk, especially in metaphyseal bone. Fractures seriously impair function and quality of life and incur large direct and indirect costs. Although the prevention of fractures is certainly the option, a fast and uneventful healing process is optimal when fractures do occur. Many new therapeutic strategies have been developed to accelerate fracture-healing or to diminish the complication rate during the course of fracture-healing. However, widely accepted guidelines are needed to demonstrate the positive or negative interactions of bioactive substances, drugs, and other agents that are being used to promote fracture-healing. For each study design, the primary study goal should be indicated. Outcome variables should include both objective and subjective parameters. The guidelines should be harmonized between European and American regulatory authorities to ensure comparability of results of studies and to foster global harmonization of regulatory requirements.

Histological development and dynamic expression of Bmp2-6 mRNAs in the embryonic and postnatal mouse cranial base

The cranial base is formed by endochondral ossification and is characterized by the presence of the synchondrosis growth centers. The aim of this study was to describe the histological development of the mouse midsagittal cranial base area from embryonic day 10 (E10) to the postnatal age of 2 months. The Bmp family of signaling molecules serves important functions in embryo and bone development and may therefore play a significant role in the early formation of the cranial base. To investigate this, we analyzed the mRNA pattern of expression of Bmp2-6 in the mouse cranial base from E10 to 5 days postnatally using radioactive in situ hybridization. We found that the formation of the mouse cranial base corresponds to that of rat and proceeds in a caudorostral sequence. Moreover, all Bmps studied showed distinct and overlapping developmentally regulated expression domains. Bmp2, Bmp5, and Bmp6 were expressed in the early mesenchymal condensations. Later, Bmp2, Bmp3, Bmp4, and Bmp5 were detected in the perichondrium and in the adjacent mesenchyme. Subsequently, Bmp2 and Bmp6 expressions were confined to hypertrophic chondrocytes, while Bmp3, Bmp4, and Bmp5 were expressed in the osteoblasts of the trabecular bone and bone collar. Interestingly, Bmp3 was uniquely expressed postnatally in the resting zone of the synchondrosis growth center, suggesting a role in the regulation of cranial base growth. These results suggest that Bmp signaling may serve specific and synergistic functions at different key stages of cranial base development and growth.

Systemically administered bone morphogenetic protein-6 restores bone in aged ovariectomized rats by increasing bone formation and suppressing bone resorption

Although recombinant human bone morphogenetic proteins (BMPs) are used locally for treating bone defects in humans, their systemic effect on bone augmentation has not been explored. We have previously demonstrated that demineralized bone (DB) from ovariectomized (OVX) rats cannot induce bone formation when implanted ectopically at the subcutaneous site. Here we showed in vitro that 17beta-estradiol (E2) specifically induced expression of Bmp6 mRNA in MC3T3-E1 preosteoblastic cells and that bone extracts from OVX rats lack BMPs. Next we demonstrated that 125I-BMP-6 administered systemically accumulated in the skeleton and also restored the osteoinductive capacity of ectopically implanted DB from OVX rats. BMP-6 applied systemically to aged OVX rats significantly increased bone volume and mechanical characteristics of both the trabecular and cortical bone, the osteoblast surface, serum osteocalcin and osteoprotegerin levels, and decreased the osteoclast surface, serum C-telopeptide, and interleukin-6. E2 was significantly less effective, and was not synergistic with BMP-6. Animals that discontinued BMP-6 therapy maintained bone mineral density gains for another 12 weeks. BMP-6 increased in vivo the bone expression of Acvr-1, Bmpr1b, Smad5, alkaline phosphatase, and collagen type I and decreased expression of Bmp3 and BMP antagonists, chordin and cerberus. These results show, for the first time, that systemically administered BMP-6 restores the bone inductive capacity, microarchitecture, and quality of the skeleton in osteoporotic rats.

Bone morphogenetic protein 3 expression pattern in rat condylar cartilage, femoral cartilage and mandibular fracture callus

Mandibular condylar cartilage differs from primary cartilage in morphological organization of the chondrocytes and in responses to biomechanical stress and humoral factors. For the first time, we describe the expression of Bmp3 mRNA in relation to types I, II and X collagen mRNA (as determined by in situ hybridization) in chondrocytes of the rat mandibular condylar cartilage, femoral articular cartilage, femoral growth plate cartilage, and temporal cartilage, which transiently appeared in the reparative response stage of mandibular ramus fracture healing. In all cartilages evaluated, Bmp3 was expressed in proliferating chondrocytes that expressed type I collagen in condylar cartilage, articular cartilage, and temporal cartilage appearing during fracture healing. Bmp3 was also found in hypertrophic chondrocytes that expressed type X collagen mRNA in all cartilages evaluated. Furthermore, in remodeling bone, Bmp3 mRNA was strongly expressed in active osteoblast cells in periosteal reaction layers formed after fracture. These findings suggest that Bmp3 expression in a special layer of typical articular cartilage may be regulated by mechanical stress stimulation. We also found that Bmp3 was expressed in the periosteal layers of the bone segments near the fracture site during fracture healing.

Temporomandibular joint morphology following post-traumatic ankylosis in 26 patients

This study evaluated the joint morphology on coronal computed tomography of ankylosed temporomandibular joints in 26 patients. All patients developed ankylosis following blunt trauma. Post-ankylosis joint morphology was assessed to determine if the precursor condylar fracture could be identified and this was compared to the condylar fracture prevalence to determine if any condylar fractures have an increased risk factor for ankylosis. Mean age at presentation was 20.9+/-14.41 years (range 6-58) and mean age at injury was 13.84+/-13.81 years (range 3-53). Thirty-seven joints were ankylosed in 26 patients (11 bilateral, 15 unilateral) with 27 joints in 19 patients showing vestiges of a medially dislocated condylar fracture (72.9% of joints). The prevalence of MDCF at our unit over a period of 6 months was 16.8% (16 of 95 condylar fractures). This suggests that a medially dislocated condylar fracture is more likely to ankylose than other condylar fractures. A hypothesis is proposed to explain this increased risk.

Expression of bone morphogenetic proteins, receptors, and tissue inhibitors in human fetal, adult, and osteoarthritic articular cartilage

Coordinate expression of BMPs and their receptors and inhibitors is likely necessary for physiologic BMP regulation and activity. To characterize the expression of such factors in fetal, normal adult, and end-stage osteoarthritic articular cartilage, samples from these sources were analyzed. PCR-amplified sequences (BMPs 1-11), receptors (IA, IB, II), TGF-beta1, TGF-beta2, inhibitors noggin and follistatin, CDMP-1, COMP, and GAPDH from cDNAs generated from extracted total RNA were resolved by gel electrophoresis. Protein levels of BMPs 3, 7, and 8 were also analyzed by SDS-PAGE and Western blotting. RT-PCR revealed that BMPs 1, 2, 4-6, and 11, BMPR-IA and II, noggin, follistatin, CDMP-1, COMP, and GAPDH mRNAs were expressed in similar fashion in both fetal and adult (normal or osteoarthritic) cartilage. BMPs 9 and 10 mRNAs were not expressed in either group. BMPs 7, 8, and BMPR-IB mRNAs were consistently expressed in fetal but not in adult cartilage. BMP-3 mRNA was expressed in fetal and normal adult, but not in osteoarthritic samples. TGF-beta1 was expressed in both adult normal and osteoarthritic, but not fetal, samples. Similarly, Western blotting demonstrated BMPs 7 and 8 to be present in fetal but not in adult samples. BMP-3 protein was present in fetal and adult normal samples, to a lesser extent, but absent in osteoarthritic cartilage.

Temporal and spatial expression of bone morphogenetic proteins in extracorporeal shock wave-promoted healing of segmental defect

Extracorporeal shock wave (ESW) is a noninvasive acoustic wave, which has recently been demonstrated to promote bone repair. The actual healing mechanism triggered by ESW has not yet been identified. Bone morphogenetic proteins (BMP) have been implicated as playing an important role in bone development and fracture healing. In this study, we aimed to examine the involvement of BMP-2, BMP-3, BMP-4, and BMP-7 expression in ESW promotion of fracture healing. Rats with a 5-mm segmental femoral defect were given ESW treatment using 500 impulses at 0.16 mJ/mm(2). Femurs and calluses were subjected to immunohistochemistry and RT-PCR assay 1, 2, 4, and 8 weeks after treatment. Histological observation demonstrated that fractured femurs received ESW treatment underwent intensive mesenchymal cell aggregation, hypertrophic chondrogenesis, and endochondral/intramembrane ossification, resulting in the healing of segmental defect. Aggregated mesenchymal cells at the defect, chondrocytes at the hypertrophic cartilage, and osteoblasts adjunct to newly formed woven bone showed intensive proliferating cell nuclear antigen expression. ESW treatment significantly promoted BMP-2, BMP-3, BMP-4, and BMP-7 mRNA expression of callus as determined by RT-PCR, and BMP immunoreactivity appeared throughout the bone regeneration period. Mesenchymal cells and immature chondrocytes showed intensive BMP-2, BMP-3, and BMP-4 immunoreactivity. BMP-7 expression was evident on osteoblasts located at endochondral ossification junction. Our findings suggest that BMP play an important role in signaling ESW-activated cell proliferation and bone regeneration of segmental defect.

Expression of bone morphogenetic proteins and cartilage-derived morphogenetic proteins during osteophyte formation in humans

Bone- and cartilage-derived morphogenetic proteins (BMPs and CDMPs), which are TGFbeta superfamily members, are growth and differentiation factors that have been recently isolated, cloned and biologically characterized. They are important regulators of key events in the processes of bone formation during embryogenesis, postnatal growth, remodelling and regeneration of the skeleton. In the present study, we used immunohistochemical methods to investigate the distribution of BMP-2, -3, -5, -6, -7 and CDMP-1, -2, -3 in human osteophytes (abnormal bony outgrowths) isolated from osteoarthritic hip and knee joints from patients undergoing total joint replacement surgery. All osteophytes consisted of three different areas of active bone formation: (1) endochondral bone formation within cartilage residues; (2) intramembranous bone formation within the fibrous tissue cover and (3) bone formation within bone marrow spaces. The immunohistochemistry of certain BMPs and CDMPs in each of these three different bone formation sites was determined. The results indicate that each BMP has a distinct pattern of distribution. Immunoreactivity for BMP-2 was observed in fibrous tissue matrix as well as in osteoblasts; BMP-3 was mainly present in osteoblasts; BMP-6 was restricted to young osteocytes and bone matrix; BMP-7 was observed in hypertrophic chondrocytes, osteoblasts and young osteocytes of both endochondral and intramembranous bone formation sites. CDMP-1, -2 and -3 were strongly expressed in all cartilage cells. Surprisingly, BMP-3 and -6 were found in osteoclasts at the sites of bone resorption. Since a similar distribution pattern of bone morphogenetic proteins was observed during embryonal bone development, it is suggested that osteophyte formation is regulated by the same molecular mechanism as normal bone during embryogenesis.

CDMP-2 induces bone or tendon-like tissue depending on mechanical stimulation

Cartilage derived morphogenetic proteins (CDMPs, also known as growth and differentiation factors, GDFs) are a subgroup of the bone morphogenetic protein (BMP) gene family. As most BMPs, they are known to induce cartilage or bone formation when implanted subcutaneously or intramuscularly on an appropriate carrier. However, similar implantation experiments with CDMPs have also reported the formation of a tendon-like tissue, without any cartilage or bone. A solution to this apparent contradiction might be offered by the mechanical tissue differentiation theory, suggesting that tissue differentiation depends on the mechanical environment. This study analyzes the response to CDMP-2 implants at different sites and under different loading conditions in the rat. Collagen sponges carrying CDMP-2 were implanted subcutaneously, intramuscularly or inside a freshly created defect in the achilles tendon. Large amounts of bone were induced subcutaneously, smaller amounts intramuscularly, and in the tendons, only small amounts of bone or cartilage were seen in few animals. Thus, the amount of bone appeared inversely related to the degree of mechanical stimulus. To confirm this, CDMP was also injected into tendon defects that were either loaded or partially unloaded. All the unloaded tendons showed bone induction after one CDMP-2 injection, whereas only 4 of 10 loaded ones showed any cartilage or bone (p = 0.0005). Single injections of a similar dose of CDMP-2 have previously been shown to augment tendon repair by increasing the size of the tendon callus. This study suggests that the response to CDMP-2 is dependent on the mechanical situation at the site where it is applied.

Expression and activation of the BMP-signaling components in human fracture nonunions

BACKGROUND

The potential use of bone morphogenetic proteins (BMPs) to promote bone-healing is of great interest to orthopaedic surgeons. Although the complex mechanism leading from the local presence of BMP (whether endogenous or exogenous) to bone formation is increasingly understood, limited information is available as to whether endogenous BMPs, their receptors, or other molecules involved in their signal transduction, such as Smad1, are present or disappear during the development of fracture nonunions. The purpose of the present study was to determine, by immunohistochemical analysis, whether BMPs, BMP receptors, or Smad1 disappear from tissues during the development of a fracture nonunion.

METHODS

Twenty-one patients (average age, sixty-one years; range, thirty to eighty-five years) with a delayed union (four patients) or a nonunion (seventeen patients) were included. The average duration of the delayed union or nonunion was twenty-two months (range, 3.5 to 120 months). With use of immunohistochemical analysis, we studied the localization of BMP-2, BMP-4, and BMP-7 and their receptors BMPR-IA, BMPR-IB, and BMPR-II as well as pSmad1. With use of a pSmad1 antibody, we also studied whether the BMP receptors that were expressed were activated.

RESULTS

The immunohistochemical localization of all seven BMP-signaling components was demonstrated in seventeen (81%) of the twenty-one patients. The remaining four patients lacked one or more of the components. Areas of newly formed bone had the highest percentage of positively staining cells, with the staining generally decreasing in areas remote from bone formation. However, even in areas of dense fibrous tissue and in specimens that lacked newly formed bone, immunostaining was still present. The staining patterns showed co-localization of the BMP-2, BMP-4, and BMP-7 proteins with the BMP receptors. The presence of pSmad1 signified the activated state of the BMP receptors, which implies that the BMP signal is transduced inside the cell.