Regional and cellular localisation of BMPs and their inhibitors' expression in human fractures

Cilostazol Stimulates Angiogenesis and Accelerates Fracture Healing in Aged Male and Female Mice by Increasing the Expression of PI3K and RUNX2

Fracture healing in the aged is associated with a reduced healing capacity, which often results in delayed healing or non-union formation. Many factors may contribute to this deterioration of bone regeneration, including a reduced 'angiogenic trauma response'. The phosphodiesterase-3 (PDE-3) inhibitor cilostazol has been shown to exert pro-angiogenic and pro-osteogenic effects in preclinical studies. Therefore, we herein analyzed in a stable closed femoral fracture model whether this compound also promotes fracture healing in aged mice. Forty-two aged CD-1 mice (age: 16-18 months) were daily treated with 30 mg/kg body weight cilostazol (n = 21) or vehicle (control, n = 21) by oral gavage. At 2 and 5 weeks after fracture, the femora were analyzed by X-ray, biomechanics, micro-computed tomography (µCT), histology, immunohistochemistry, and Western blotting. These analyses revealed a significantly increased bending stiffness at 2 weeks (2.2 ± 0.4 vs. 4.3 ± 0.7 N/mm) and an enhanced bone formation at 5 weeks (4.4 ± 0.7 vs. 9.1 ± 0.7 mm3) in cilostazol-treated mice when compared to controls. This was associated with a higher number of newly formed CD31-positive microvessels (3.3 ± 0.9 vs. 5.5 ± 0.7 microvessels/HPF) as well as an elevated expression of phosphoinositide-3-kinase (PI3K) (3.6 ± 0.8 vs. 17.4 ± 5.5-pixel intensity × 104) and runt-related transcription factor (RUNX)2 (6.4 ± 1.2 vs. 18.2 ± 2.7-pixel intensity × 104) within the callus tissue. These findings indicate that cilostazol accelerates fracture healing in aged mice by stimulating angiogenesis and the expression of PI3K and RUNX2. Hence, cilostazol may represent a promising compound to promote bone regeneration in geriatric patients.

Diclofenac, a NSAID, delays fracture healing in aged mice

Nonsteroidal anti-inflammatory drugs (NSAIDs), such as diclofenac, belong to the most prescribed analgesic medication after traumatic injuries. However, there is accumulating evidence that NSAIDs impair fracture healing. Because bone regeneration in aged patients is subject to significant changes in cell differentiation and proliferation as well as a markedly altered pharmacological action of drugs, we herein analyzed the effects of diclofenac on bone healing in aged mice using a stable closed femoral facture model. Thirty-three mice (male n = 14, female n = 19) received a daily intraperitoneal injection of diclofenac (5 mg/kg body weight). Vehicle-treated mice (n = 29; male n = 13, female n = 16) served as controls. Fractured mice femora were analyzed by means of X-ray, biomechanics, micro computed tomography (μCT), histology and Western blotting. Biomechanical analyses revealed a significantly reduced bending stiffness in diclofenac-treated animals at 5 weeks after fracture when compared to vehicle-treated controls. Moreover, the callus tissue in diclofenac-treated aged animals exhibited a significantly reduced amount of bone tissue and higher amounts of fibrous tissue. Further histological analyses demonstrated less lamellar bone after diclofenac treatment, indicating a delay in callus remodeling. This was associated with a decreased number of osteoclasts and an increased expression of osteoprotegerin (OPG) during the early phase of fracture healing. These findings indicate that diclofenac delays fracture healing in aged mice by affecting osteogenic growth factor expression and bone formation as well as osteoclast activity and callus remodeling.

Pantoprazole impairs fracture healing in aged mice

Proton pump inhibitors (PPIs) belong to the most common medication in geriatric medicine. They are known to reduce osteoclast activity and to delay fracture healing in young adult mice. Because differentiation and proliferation in fracture healing as well as pharmacologic actions of drugs markedly differ in the elderly compared to the young, we herein studied the effect of the PPI pantoprazole on bone healing in aged mice using a murine fracture model. Bone healing was analyzed by biomechanical, histomorphometric, radiological and protein biochemical analyses. The biomechanical analysis revealed a significantly reduced bending stiffness in pantoprazole-treated animals when compared to controls. This was associated with a decreased amount of bone tissue within the callus, a reduced trabecular thickness and a higher amount of fibrous tissue. Furthermore, the number of osteoclasts in pantoprazole-treated animals was significantly increased at 2 weeks and decreased at 5 weeks after fracture, indicating an acceleration of bone turnover. Western blot analysis showed a lower expression of the bone morphogenetic protein-4 (BMP-4), whereas the expression of the pro-angiogenic parameters was higher when compared to controls. Thus, pantoprazole impairs fracture healing in aged mice by affecting angiogenic and osteogenic growth factor expression, osteoclast activity and bone formation.

Role of Prx1-expressing skeletal cells and Prx1-expression in fracture repair

The healing capacity of bones after fracture implies the existence of adult regenerative cells. However, information on identification and functional role of fracture-induced progenitors is still lacking. Paired-related homeobox 1 (Prx1) is expressed during skeletogenesis. We hypothesize that fracture recapitulates Prx1's expression, and Prx1 expressing cells are critical to induce repair. To address our hypothesis, we used a combination of in vivo and in vitro approaches, short and long-term cell tracking analyses of progenies and actively expressing cells, cell ablation studies, and rodent animal models for normal and defective fracture healing. We found that fracture elicits a periosteal and endosteal response of perivascular Prx1+ cells that participate in fracture healing and showed that Prx1-expressing cells have a functional role in the repair process. While Prx1-derived cells contribute to the callus, Prx1's expression decreases concurrently with differentiation into cartilaginous and bone cells, similarly to when Prx1+ cells are cultured in differentiating conditions. We determined that bone morphogenic protein 2 (BMP2), through C-X-C motif-ligand-12 (CXCL12) signaling, modulates the downregulation of Prx1. We demonstrated that fracture elicits an early increase in BMP2 expression, followed by a decrease in CXCL12 that in turn down-regulates Prx1, allowing cells to commit to osteochondrogenesis. In vivo and in vitro treatment with CXCR4 antagonist AMD3100 restored Prx1 expression by modulating the BMP2-CXCL12 axis. Our studies represent a shift in the current research that has primarily focused on the identification of markers for postnatal skeletal progenitors, and instead we characterized the function of a specific population (Prx1+ cells) and their expression marker (Prx1) as a crossroad in fracture repair. The identification of fracture-induced perivascular Prx1+ cells and regulation of Prx1's expression by BMP2 and in turn by CXCL12 in the orchestration of fracture repair, highlights a pathway in which to investigate defective mechanisms and therapeutic targets for fracture non-union.

Application of Cytokines of the Bone Morphogenetic Protein (BMP) Family in Spinal Fusion - Effects on the Bone, Intervertebral Disc and Mesenchymal Stromal Cells

Low back pain is a prevalent socio-economic burden and is often associated with damaged or degenerated intervertebral discs (IVDs). When conservative therapy fails, removal of the IVD (discectomy), followed by intersomatic spinal fusion, is currently the standard practice in clinics. The remaining space is filled with an intersomatic device (cage) and with bone substitutes to achieve disc height compensation and bone fusion. As a complication, in up to 30% of cases, spinal non-fusions result in a painful pseudoarthrosis. Bone morphogenetic proteins (BMPs) have been clinically applied with varied outcomes. Several members of the BMP family, such as BMP2, BMP4, BMP6, BMP7, and BMP9, are known to induce osteogenesis. Questions remain on why hyper-physiological doses of BMPs do not show beneficial effects in certain patients. In this respect, BMP antagonists secreted by mesenchymal cells, which might interfere with or block the action of BMPs, have drawn research attention as possible targets for the enhancement of spinal fusion or the prevention of non-unions. Examples of these antagonists are noggin, gremlin1 and 2, chordin, follistatin, BMP3, and twisted gastrulation. In this review, we discuss current evidence of the osteogenic effects of several members of the BMP family on osteoblasts, IVD cells, and mesenchymal stromal cells. We consider in vitro and in vivo studies performed in human, mouse, rat, and rabbit related to BMP and BMP antagonists in the last two decades. We give insights into the effects that BMP have on the ossification of the spine. Furthermore, the benefits, pitfalls, and possible safety concerns using these cytokines for the improvement of spinal fusion are discussed.

miR‑204 inhibits the osteogenic differentiation of mesenchymal stem cells by targeting bone morphogenetic protein 2

Mesenchymal stem cells (MSCs) are used to investigate regeneration and differentiation. MicroRNA-204 (miR-204) in involved in the Runt-related transcription factor 2/alkaline phosphatase/bone morphogenic protein 2 (Runx2/ALP/BMP2) signaling pathway that regulates bone marrow mesenchymal stem cell (BMSC) differentiation; however, the mechanisms underlying the effects of miR-204 are yet to be determined. The aim of the present study was to investigate the effects of miR-204 on BMSC differentiation. BMSCs were derived from rat bone marrow. The expression levels of Runx2, ALP and BMP2 were measured via reverse transcription-quantitative polymerase chain reaction and western blot analyses following transfection of BMSCs with miR-204 agomir or BMP2 expression vector. The ability of the miR-204 gene to directly bind BMP2 mRNA was assessed using dual-luciferase assays. Ossification was measured via alizarin red stain assays. It was observed that the expression levels of Runx2 and ALP increased over time, whereas those of miR-204 decreased; additionally, miR-204 agomir upregulation inhibited the expression of Runx2, ALP and BMP2 in BMSCs. It was revealed that miR-204 directly interacted with BMP2 mRNA, and that transfection with miR-204 agomir suppressed ossification in BMSCs by targeting the BMP2/Runx2/ALP signaling pathway.

Histological ageing of fractures in infants: a practical algorithm for assessing infants suspected of accidental or non-accidental injury

Aims

This study is the first to systematically document histological features of fractures of known age in infants (≦12 months). It has been used to develop a tabulated database specifically to guide histopathologists to age fractures in children considered to have suffered accidental or non‐accidental injury (NAI). Currently in the United Kingdom there are insufficient pathologists with experience in histological ageing of fractures to meet the medicolegal need for this examination. This study provides a practical tool that will allow those skilled paediatric and forensic pathologists currently involved in assessing infants for evidence of accidental or non‐accidental injury a basis for extending their assessment into this area of unmet need.

Methods and results

One hundred and sixty‐nine fractures of known age at death were obtained from 52 anonymised infants over a period of 32 years (1985–2016 inclusive). Sections stained using haematoxylin and eosin (H&E) and Martius scarlet blue (MSB) were used to identify specific histological features and to relate them to fracture age. In 1999 the data were entered into a tabulated database for fractures accumulated between from 1985 to 1998 inclusive. Thereafter cases were added, and at 2‐yearly intervals the accumulated data were audited against the previous database and adjustments made.

Conclusions

This paper describes the final data set from the 2017 audit. The study was terminated at the end of 2016, as there had been no material changes in the data set for three consecutive audits.

The detection of significant fractures in suspected infant abuse

OBJECTIVE

Skeletal survey is a commonly used means of detecting fractures in infants, and is used as a screen in suspected cases of physical abuse. It is recognised that in live infants, a repeat survey some days after a suspected episode of injury will detect more fractures than one taken shortly after the suspected injury, indicating that the latter lacks sensitivity. In infants who die soon after a suspected episode of physical abuse, the managing clinicians do not have the option of a second survey; however there is the opportunity for the microscopic examination of bones removed at autopsy. Increasingly Osteoarticular Pathology at the Manchester University NHS Foundation Trust (MFT) is being sent samples of bones from infants suspected of inflicted injury for histological examination, both from bones with fractures detected at autopsy or skeletal survey and from posterior ribs and long bone metaphyses (sites of significance in assessing for abusive injury) when there is no evidence of fracture on skeletal survey or autopsy. Here we report the results of an audit of the anonymised data from a series of such cases, to establish the sensitivity of skeletal survey (SS) to detect fractures and to define the medico-legal value of submitting bones for histological examination.

METHODS

This was an audit of skeletal injuries in 38 infants aged <18 months presenting to MFT for specialist histopathological evaluation of suspected non-accidental fractures between January 2011 and June 2017. Histopathological examination was performed on all bones submitted and compared with contact radiography of isolated bones and post-mortem skeletal surveys undertaken by specialist paediatric or musculoskeletal radiologists for the presence of fracture.

RESULTS

A total of 318 fractures were detected histologically; of these, 178 (56%) were of the ribs, 119 (37.5%) were of major limb long bones, 10 (3%) were of the skull, and 11 (3.5%) were recorded as 'other'. Excluding refractures, skeletal survey detected 54% of the fractures recorded histologically. No fractures were detected radiologically that were not seen histologically. Generally, for skeletal survey, detection rates improved with the age of the lesion, and rib fractures were more difficult to detect than long bone fractures. Ribs 5-8 were the most frequently fractured ribs, and metaphyses around the knee accounted for most metaphyseal limb long bone fractures undetected by SS.

CONCLUSION

In infants coming to post-mortem, histopathology is more sensitive than SS for the detection of clinically significant fractures. In children suspected of non-accidental injuries but with negative or equivocal SS, sampling of the anterior and posterior end of ribs 5-8 and the bones around the knee for histological examination could reveal clinically unsuspected fractures and significant evidence of physical abuse. 71% of infants showed evidence of old fractures typical of non-accidental injury.

The Good the Bad and the Ugly of Glycosaminoglycans in Tissue Engineering Applications

High sulfation, low cost, and the status of heparin as an already FDA- and EMA- approved product, mean that its inclusion in tissue engineering (TE) strategies is becoming increasingly popular. However, the use of heparin may represent a naïve approach. This is because tissue formation is a highly orchestrated process, involving the temporal expression of numerous growth factors and complex signaling networks. While heparin may enhance the retention and activity of certain growth factors under particular conditions, its binding 'promiscuity' means that it may also inhibit other factors that, for example, play an important role in tissue maintenance and repair. Within this review we focus on articular cartilage, highlighting the complexities and highly regulated processes that are involved in its formation, and the challenges that exist in trying to effectively engineer this tissue. Here we discuss the opportunities that glycosaminoglycans (GAGs) may provide in advancing this important area of regenerative medicine, placing emphasis on the need to move away from the common use of heparin, and instead focus research towards the utility of specific GAG preparations that are able to modulate the activity of growth factors in a more controlled and defined manner, with less off-target effects.

Growth Differentiation Factor 5-Mediated Enhancement of Chondrocyte Phenotype Is Inhibited by Heparin: Implications for the Use of Heparin in the Clinic and in Tissue Engineering Applications

The highly sulfated glycosaminoglycan (GAG) heparin is widely used in the clinic as an anticoagulant, and researchers are now using it to enhance stem cell expansion/differentiation protocols, as well as to improve the delivery of growth factors for tissue engineering (TE) strategies. Growth differentiation factor 5 (GDF5) belongs to the bone morphogenetic protein family of proteins and is vital for skeletal formation; however, its interaction with heparin and heparan sulfate (HS) has not been studied. We identify GDF5 as a novel heparin/HS binding protein and show that HS proteoglycans are vital in localizing GDF5 to the cell surface. Clinically relevant doses of heparin (≥10 nM), but not equivalent concentrations of HS, were found to inhibit GDF5's biological activity in both human mesenchymal stem/stromal cell-derived chondrocyte pellet cultures and the skeletal cell line ATDC5. We also found that heparin inhibited both GDF5 binding to cell surface HS and GDF5-induced induction of Smad 1/5/8 signaling. Furthermore, GDF5 significantly increased aggrecan gene expression in chondrocyte pellet cultures, without affecting collagen type X expression, making it a promising target for the TE of articular cartilage. Importantly, this study may explain the variable (and disappointing) results seen with heparin-loaded biomaterials for skeletal TE and the adverse skeletal effects reported in the clinic following long-term heparin treatment. Our results caution the use of heparin in the clinic and in TE applications, and prompt the transition to using more specific GAGs (e.g., HS derivatives), with better-defined structures and fewer off-target effects.

BMP signaling is required for adult skeletal homeostasis and mediates bone anabolic action of parathyroid hormone

Bmp2 and Bmp4 genes were ablated in adult mice (KO) using a conditional gene knockout technology. Bones were evaluated by microcomputed tomography (μCT), bone strength tester, histomorphometry and serum biochemical markers of bone turnover. Drill-hole was made at femur metaphysis and bone regeneration in the hole site was measured by calcein binding and μCT. Mice were either sham operated (ovary intact) or ovariectomized (OVX), and treated with human parathyroid hormone (PTH), 17β-estradiol (E2) or vehicle. KO mice displayed trabecular bone loss, diminished osteoid formation and reduced biomechanical strength compared with control (expressing Bmp2 and Bmp4). Both osteoblast and osteoclast functions were impaired in KO mice. Bone histomorphomtery and serum parameters established a low turnover bone loss in KO mice. Bone regeneration at the drill-hole site in KO mice was lower than control. However, deletion of Bmp2 gene alone had no effect on skeleton, an outcome similar to that reported previously for deletion of Bmp4 gene. Both PTH and E2 resulted in skeletal preservation in control-OVX, whereas in KO-OVX, E2 but not PTH was effective which suggested that the skeletal action of PTH required Bmp ligands but E2 did not. To determine cellular effects of Bmp2 and Bmp4, we used bone marrow stromal cells in which PTH but not E2 stimulated both Bmp2 and Bmp4 synthesis leading to increased Smad1/5 phosphorylation. Taken together, we conclude that Bmp2 and Bmp4 are essential for maintaining adult skeletal homeostasis and mediating the anabolic action of PTH.

Mammalian tolloid proteinases: role in growth factor signalling

Tolloid proteinases are essential for tissue patterning and extracellular matrix assembly. The members of the family differ in their substrate specificity and activity, despite sharing similar domain organization. The mechanisms underlying substrate specificity and activity are complex, with variation between family members, and depend on both multimerization and substrate interaction. In addition, enhancers, such as Twisted gastrulation (Tsg), promote cleavage of tolloid substrate, chordin, to regulate growth factor signalling. Although Tsg and mammalian tolloid (mTLD) are involved in chordin cleavage, no interaction has been detected between them, suggesting Tsg induces a change in chordin to increase susceptibility to cleavage. All members of the tolloid family bind the N terminus of latent TGFβ‐binding protein‐1, providing support for their role in TGFβ signalling.

BMP2 Regulation of CXCL12 Cellular, Temporal, and Spatial Expression is Essential During Fracture Repair

The cellular and humoral responses that orchestrate fracture healing are still elusive. Here we report that bone morphogenic protein 2 (BMP2)-dependent fracture healing occurs through a tight control of chemokine C-X-C motif-ligand-12 (CXCL12) cellular, spatial, and temporal expression. We found that the fracture repair process elicited an early site-specific response of CXCL12(+)-BMP2(+) endosteal cells and osteocytes that was not present in unfractured bones and gradually decreased as healing progressed. Absence of a full complement of BMP2 in mesenchyme osteoprogenitors (BMP2(cKO/+)) prevented healing and led to a dysregulated temporal and cellular upregulation of CXCL12 expression associated with a deranged angiogenic response. Healing was rescued when BMP2(cKO/+) mice were systemically treated with AMD3100, an antagonist of CXCR4 and agonist for CXCR7 both receptors for CXCL12. We further found that mesenchymal stromal cells (MSCs), capable of delivering BMP2 at the endosteal site, restored fracture healing when transplanted into BMP2(cKO/+) mice by rectifying the CXCL12 expression pattern. Our in vitro studies showed that in isolated endosteal cells, BMP2, while inducing osteoblastic differentiation, stimulated expression of pericyte markers that was coupled with a decrease in CXCL12. Furthermore, in isolated BMP2(cKO/cKO) endosteal cells, high expression levels of CXCL12 inhibited osteoblastic differentiation that was restored by AMD3100 treatment or coculture with BMP2-expressing MSCs that led to an upregulation of pericyte markers while decreasing platelet endothelial cell adhesion molecule (PECAM). Taken together, our studies show that following fracture, a CXCL12(+)-BMP2(+) perivascular cell population is recruited along the endosteum, then a timely increase of BMP2 leads to downregulation of CXCL12 that is essential to determine the fate of the CXCL12(+)-BMP2(+) to osteogenesis while departing their supportive role to angiogenesis. Our findings have far-reaching implications for understanding mechanisms regulating the selective recruitment of distinct cells into the repairing niches and the development of novel pharmacological (by targeting BMP2/CXCL12) and cellular (MSCs, endosteal cells) interventions to promote fracture healing.

Mechanisms of BMP-Receptor Interaction and Activation

Bone morphogenetic proteins (BMPs), together with the eponymous transforming growth factor (TGF) β and the Activins form the TGFβ superfamily of ligands. This protein family comprises more than 30 structurally highly related proteins, which determine formation, maintenance, and regeneration of tissues and organs. Their importance for the development of multicellular organisms is evident from their existence in all vertebrates as well as nonvertebrate animals. From their highly specific functions in vivo either a strict relation between a particular ligand and its cognate cellular receptor and/or a stringent regulation to define a distinct temperospatial expression pattern for the various ligands and receptor is expected. However, only a limited number of receptors are found to serve a large number of ligands thus implicating highly promiscuous ligand-receptor interactions instead. Since in tissues a multitude of ligands are often found, which signal via a highly overlapping set of receptors, this raises the question how such promiscuous interactions between different ligands and their receptors can generate concerted and highly specific cellular signals required during embryonic development and tissue homeostasis.

BMP-9 expression in human traumatic heterotopic ossification: a case report

Background

Heterotopic ossification (HO) is defined as the abnormal formation of mature bone in soft tissue, notably skeletal muscle. The morbidity of HO in polytraumatized patients impacts the functional outcome, impairs rehabilitation, and increases costs due to subsequent surgical interventions.

Case presentation

We present the case of a 34-year-old African male who developed severe HO around his right hip 11 days after a major trauma. Immunohistochemical analyses of resected tissue revealed that several BMPs were expressed in the HO, including highly osteogenic BMP-9.

Conclusions

To the best of our knowledge, this is the first report of local BMP expression, notably BMP-9, in traumatic HO, and suggests that BMP-9, possibly through mrSCs, can contribute to HO formation in soft tissues when a suitable microenvironment is present.

CCN3 protein participates in bone regeneration as an inhibitory factor

CCN3, a member of the CCN protein family, inhibits osteoblast differentiation in vitro. However, the role of CCN3 in bone regeneration has not been well elucidated. In this study, we investigated the role of CCN3 in bone regeneration. We identified the Ccn3 gene by microarray analysis as a highly expressed gene at the early phase of bone regeneration in a mouse bone regeneration model. We confirmed the up-regulation of Ccn3 at the early phase of bone regeneration by RT-PCR, Western blot, and immunofluorescence analyses. Ccn3 transgenic mice, in which Ccn3 expression was driven by 2.3-kb Col1a1 promoter, showed osteopenia compared with wild-type mice, but Ccn3 knock-out mice showed no skeletal changes compared with wild-type mice. We analyzed the bone regeneration process in Ccn3 transgenic mice and Ccn3 knock-out mice by microcomputed tomography and histological analyses. Bone regeneration in Ccn3 knock-out mice was accelerated compared with that in wild-type mice. The mRNA expression levels of osteoblast-related genes (Runx2, Sp7, Col1a1, Alpl, and Bglap) in Ccn3 knock-out mice were up-regulated earlier than those in wild-type mice, as demonstrated by RT-PCR. Bone regeneration in Ccn3 transgenic mice showed no significant changes compared with that in wild-type mice. Phosphorylation of Smad1/5 was highly up-regulated at bone regeneration sites in Ccn3 KO mice compared with wild-type mice. These results indicate that CCN3 is up-regulated in the early phase of bone regeneration and acts as a negative regulator for bone regeneration. This study may contribute to the development of new strategies for bone regeneration therapy.

Advanced BMP gene therapies for temporal and spatial control of bone regeneration

Spatial and temporal patterns of bone morphogenetic protein (BMP) signaling are crucial to the assembly of appropriately positioned and shaped bones of the face and head. This review advances the hypothesis that reconstitution of such patterns with cutting-edge gene therapies will transform the clinical management of craniofacial bone defects attributed to trauma, disease, or surgical resection. Gradients in BMP signaling within developing limbs and orofacial primordia regulate proliferation and differentiation of mesenchymal progenitors. Similarly, vascular and mesenchymal cells express BMPs in various places and at various times during normal fracture healing. In non-healing fractures of long bones, BMP signaling is severely attenuated. Devices that release recombinant BMPs promote healing of bone in spinal fusions and, in some cases, of open fractures, but cannot control the timing and localization of BMP release. Gene therapies with regulated expression systems may provide substantial improvements in efficacy and safety compared with protein-based therapies. Synthetic gene switches, activated by pharmacologics or light or hyperthermic stimuli, provide several avenues for the non-invasive regulation of the expression of BMP transgenes in both time and space. Through new gene therapy platforms such as these, active control over BMP signaling can be achieved to accelerate bone regeneration.

Co-expression of BMPs and BMP-inhibitors in human fractures and non-unions

Bone morphogenetic proteins (BMPs) are increasingly being used clinically to enhance fracture repair and healing of non-unions. However, the potential efficacy of supraphysiological dosing for clinical results warrants further clarification of the BMP signaling pathway in human fracture healing. As BMP signaling can be fine-tuned at numerous levels, the role of BMP-inhibitors has become a major focus. The aim of the present study was to document co-expression of BMPs, pSmad 1/5/8, and BMP-inhibitors in human fracture callus and human non-unions. Using human tissue of fracture callus (n=14) and non-unions (n=4) we documented expression of BMPs (BMP2, BMP3 and BMP7), pSmad 1/5/8 and the BMP-inhibitors noggin, gremlin, chordin, Smad-6, Smad-7 and BAMBI. Co-expression of pSmad 1/5/8, BMPs and BMP-inhibitors was noted in the osteoblasts of fracture callus as well as of non-unions. Expression of BMP-inhibitors was generally stronger in non-unions than in fracture callus. The most pertinent differences were noted in the cartilaginous tissue components. Expression of BMP2 in chondrocytes was markedly decreased in non-unions compared to fracture callus and that of BMP7 was almost completely absent. Expression of BMP-inhibitors was almost the same in osteoblasts, chondrocytes and fibroblasts of fracture callus and well as in non-unions. Interestingly, although BMP ligands were present in the chondrocytes and fibroblasts of non-unions, they did not co-express pSmad 1/5/8 suggesting that BMP signaling may have been inhibited at some point before Smad 1/5/8 phosphorylation. These results suggest co-expression of BMP, pSmad 1/5/8 and BMP-inhibitors occurs in human fracture callus as well as non-unions but the relative expression of BMPs vs. BMP-inhibitors was different between these two tissue types. In contrast to our expectations, the expression of BMP inhibitors was comparable between fracture callus and non-unions, whereas the expression of BMPs was notably lower in the cartilaginous component of the non-unions in comparison to fracture callus. Based on these results, we believe that aberrations in the BMP-signaling pathway in the cartilaginous component of fracture healing could influence clinical fracture healing. An imbalance between the local presence of BMP and BMP-inhibitors may switch the direction towards healing or non-healing of a fracture.

Temporal and spatial vascularization patterns of unions and nonunions: role of vascular endothelial growth factor and bone morphogenetic proteins

BACKGROUND

Failure of fracture-healing with nonunion is a major clinical problem. Angiogenesis is closely linked to bone regeneration, but the role of angiogenesis in nonunion formation remains unclear. Because established nonunions are well vascularized, we hypothesized that lack of vascular endothelial growth factor (VEGF) expression and vascularization during the early time course of fracture-healing determine nonunion formation.

METHODS

In seventy-two CD-1 mice, a femoral osteotomy with a gap size of 1.80 mm (nonunion group) or a gap size of 0.25 mm (union group) was created and stabilized by a pin-clip technique. Healing was analyzed after three, seven, fourteen, twenty-one, twenty-eight, and seventy days by micro-computed tomography and histomorphometry. Vascularization was determined in different healing zones by immunohistochemical staining of PECAM-1 (platelet-endothelial cell adhesion molecule). Additional animals were analyzed after seven, fourteen, and twenty-one days with Western blot analysis of VEGF, bone morphogenetic protein (BMP)-2, and BMP-4 expression.

RESULTS

Micro-computed tomography and histomorphometry showed complete bone-bridging in the union group, whereas animals in the nonunion group showed atrophic nonunion formation. Vascularization increased from day 3 to day 7 in both groups, with a subsequent decrease after fourteen days. However, overall vascularization did not differ between unions and nonunions over time. It is of interest that vascularization within the endosteal healing zone was even higher in nonunions than in unions after fourteen days. Expression of VEGF was significantly higher in nonunions, while expression of BMP-2 and 4 and proliferating cell nuclear antigen were found significantly reduced compared with unions.

CONCLUSIONS

Because vascularization during the early time course of fracture-healing was not impaired despite the failure of bone-healing in nonunions, we rejected our hypothesis and accepted the null hypothesis that nonunion formation is not due to failure of VEGF-mediated angiogenesis. Failure of fracture-healing was associated with a decreased expression of BMP-2 and 4 and a disturbed ratio of angiogenic to osteogenic growth factors, which may be responsible for nonunion.

CLINICAL RELEVANCE

Because the intrinsic angiogenic response during nonunion formation was sufficient for adequate vascularization, treatment strategies for nonunions should focus on the stimulation of osteogenesis rather than on the stimulation of angiogenesis.

Local BMP-2 application can rescue the delayed osteotomy healing in a rat model

Delayed healing is still a severe complication in the clinic. The aim of the present study was to investigate the effect of locally delivered BMP-2 incorporated in a poly(d,l-lactide) (PDLLA) implant coating in a rat model with delayed tibial healing. The healing delay in this model is not caused by mechanical instability or additional tissue manipulation and presents therefore a common and challenging clinical situation of impaired healing. Radiological, histological and biomechanical evaluations were performed at days 5, 10, 28, 42, and 84 after tibial osteotomy. The control group showed a delayed healing without complete bridging and without reaching the biomechanical stability of the contralateral tibiae after 84 days. The mechanical stability of the BMP-treated tibiae showed a significant increase at days 28 and 42 compared to the control group and exceeded the stability of the intact contralateral tibiae. Less cartilage was detected at day 28 and the mineralisation was significantly enhanced at day 42 due to the local BMP application. Looking at the early healing phase (day 10) a reduced vascularisation was seen in the BMP group. This reflects the situation seen during normal healing, whereas the delayed healing in the present model had an increased vascularisation. The present study clearly demonstrates that local BMP-2 application can stimulate delayed healing in a clinically relevant animal model.

Bone Morphogenic Protein--mRNA upregulation after exposure to low frequency electric field

PURPOSE

For many years, our laboratory has been investigating different biological substrates for the effects of electromagnetic stimulation proposed in orthopaedic treatments. The results show an acceleration of differentiation at the expense of proliferation. This study using microarray analysis is focused on the cellular mechanisms involved.

METHODS

A microarray analysis (Affymetrix) allowing the screening of the expression of 38,500 genes was used on epidermal cells sampled from three different human donors and distributed within each donor in seven groups of 12 explants, stimulated at different times, to compare control. Modifications of the expression of BMP-2, 4 and 7 were studied at days four, seven and 12.

RESULTS

The expression of BMP-2 was significantly increased at day 12 on the stimulated samples. J(4) and J(7) did not show any significant difference nor did the expression of BMP-4 and 7 at the different times.

CONCLUSION

The results obtained in previous experiments on cellular substrates, bone embryonic tissue and clinical series were all consistent with the increase of BMP-2. Other publications have confirmed an increase of BMP-2 under electric or electromagnetic stimulation. The increase of BMP-2 appears as an effect of the electromagnetic field stimulations applied in orthopaedics. This observation contributes towards possible indications and a better understanding of the cellular mechanism.

Altered relative expression of BMPs and BMP inhibitors in cartilaginous areas of human fractures progressing towards nonunion

The present study was conducted to evaluate the hypothesis that an imbalance in the local production of bone morphogenetic proteins (BMPs) and BMP inhibitors exists within the cartilaginous intermediate of nonhealing fractures. Biopsies were recovered intraoperatively from human fractures that, upon follow-up, were found to heal normally or become nonunions. The samples were examined by immunohistochemistry to determine the expression of BMP-2, BMP-14, and the BMP inhibitors noggin and chordin. Expression was determined semiquantitatively based on the area of positive staining per area of cartilage and by determining the number of positively staining cells and the intensity of staining. There was a significant reduction in BMP-2 and BMP-14 expression in cartilaginous areas of nonhealing fractures compared to healing fractures. However, there was no difference in the expression of the BMP inhibitors between the two groups of fractures. This imbalance in the expression of BMPs and BMP inhibitors within cartilaginous areas of developing nonunions may account for their reduced bone forming ability. These data suggest strategies for preventing the development of nonunions by altering levels of BMPs and their inhibitors within fracture sites.