Models of tibial fracture healing in normal and Nf1-deficient mice

Tgif1-deficiency impairs cytoskeletal architecture in osteoblasts by activating PAK3 signaling

Osteoblast adherence to bone surfaces is important for remodeling bone tissue. This study demonstrates that deficiency of TG-interacting factor 1 (Tgif1) in osteoblasts results in altered cell morphology, reduced adherence to collagen type I-coated surfaces, and impaired migration capacity. Tgif1 is essential for osteoblasts to adapt a regular cell morphology and to efficiently adhere and migrate on collagen type I-rich matrices in vitro. Furthermore, Tgif1 acts as a transcriptional repressor of p21-activated kinase 3 (Pak3), an important regulator of focal adhesion formation and osteoblast spreading. Absence of Tgif1 leads to increased Pak3 expression, which impairs osteoblast spreading. Additionally, Tgif1 is implicated in osteoblast recruitment and activation of bone surfaces in the context of bone regeneration and in response to parathyroid hormone 1-34 (PTH 1-34) treatment in vivo in mice. These findings provide important novel insights in the regulation of the cytoskeletal architecture of osteoblasts.

Modeling anabolic and antiresorptive therapies for fracture healing in a mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a genetic bone fragility disorder that features frequent fractures. Bone healing outcomes are contingent on a proper balance between bone formation and resorption, and drugs such as bone morphogenetic proteins (BMPs) and bisphosphonates (BPs) have shown to have utility in modulating fracture repair. While BPs are used for OI to increase BMD and reduce pain and fracture rates, there is little evidence for using BMPs as local agents for fracture healing (alone or with BPs). In this study, we examined wild-type and OI mice (Col1a2^+/G610C ) in a murine tibial open fracture model with (i) surgery only/no treatment, (ii) local BMP-2 (10 µg), or (iii) local BMP-2 and postoperative zoledronic acid (ZA; 0.1 mg/kg total dose). Microcomputed tomography reconstructions of healing fractures indicated BMP-2 was less effective in an OI setting, however, BMP-2 +ZA led to considerable increases in bone volume (+193% WT, p < 0.001; +154% OI, p < 0.001) and polar moment of inertia (+125% WT, p < 0.01; +248% OI, p < 0.05). Tissue histology revealed a thinning of the neocortex of the callus in BMP-2 treated OI bone, but considerable retention of woven bone in the healing callus with BMP + ZA specimens. These data suggest a cautious approach may be warranted with the sole application of BMP-2 in an OI surgical setting as a bone graft substitute. However, this may be overcome by off-label BP administration.

Bone health in RASopathies

The RASopathies are a group of disorders due to pathogenic variants in genes involved in the Ras/MAPK pathway, many of which have overlapping clinical features (e.g., neurofibromatosis type 1, Costello syndrome, cardiofaciocutaneous syndrome and Noonan syndrome) including musculoskeletal manifestations. Osteopenia and osteoporosis are reported in many of the RASopathies suggesting a shared pathogenesis. Even though osteopenia and osteoporosis are often detected and fractures have been reported, the clinical impact of bone mineralization defects on the skeleton of the various syndromes is poorly understood. Further knowledge of the role of the Ras/MAPK pathway on the bone cellular function, and more detailed musculoskeletal phenotyping will be critical in helping to develop therapies to improve bone health in the RASopathies.

Fracture hematoma micro-architecture influences transcriptional profile and plays a crucial role in determining bone healing outcomes

The hematoma that forms between broken fragments of bone serves as a natural fibrin scaffold, and its removal from the defect site delays bone healing. The hypothesis of this study is that the microarchitectural and mechanical properties of the initially formed hematoma has a significant effect on the regulation of the biological process, which ultimately determines the outcome of bone healing. To mimic three healing conditions in the rat femur (normal, delayed, and non-healing bone defects), three different defect sizes of 0.5, 1.5, and 5.0 mm, are respectively used. The analysis of 3-day-old hematomas demonstrates clear differences in fibrin clot micro-architecture in terms of fiber diameter, fiber density, and porosity of the formed fibrin network, which result in different mechanical properties (stiffness) of the hematoma in each model. Those differences directly affect the biological processes involved. Specifically, RNA-sequencing reveals almost 700 differentially expressed genes between normally healing and non-healing defects, including significantly up-regulated essential osteogenic genes in normally healing defects, also differences in immune cell populations, activated osteogenic transcriptional regulators as well as potential novel marker genes. Most importantly, this study demonstrates that the healing outcome has already been determined during the hematoma phase of bone healing, three days post-surgery.

Characterization of the Developing Lacunocanalicular Network During Fracture Repair

Fracture repair is a normal physiological response to bone injury. During the process of bony callus formation, a lacunocanalicular network (LCN) is formed de novo that evolves with callus remodeling. Our aim was the longitudinal assessment of the development and evolution of the LCN during fracture repair. To this end, 45 adult wild‐type C57BL/6 mice underwent closed tibial fracture surgery. Fractured and intact contralateral tibias were harvested after 2, 3, and 6 weeks of bone healing (n = 15/group). High‐resolution micro–computed tomography (μCT) and deconvolution microscopy (DV) approaches were applied to quantify lacunar number density from the calluses and intact bone. On histological sections, Goldner's trichrome staining was used to assess lacunar occupancy, fluorescein isothiocyanate staining to visualize the canalicular network, and terminal deoxynucleotidyl transferase–mediated deoxyuridine triphosphate‐biotin nick end labeling (TUNEL) staining to examine osteocyte apoptosis. Analysis of μCT scans showed progressive decreases in mean lacuna volume over time (−27% 2–3 weeks; −13% 3–6 weeks). Lacunar number density increased considerably between 2 and 3 weeks (+156%). Correlation analysis was performed, showing a positive linear relationship between canalicular number density and trabecular thickness (R² = 0.56, p < 0.001) and an inverse relationship between mean lacuna volume and trabecular thickness (R² = 0.57, p < 0.001). Histology showed increases in canalicular number density over time (+22% 2–3 weeks, +51% 3–6 weeks). Lacunar occupancy in new bone of the callus was high (>90%), but the old cortical bone within the fracture site appeared necrotic as it underwent resorption. In conclusion, our data shows a progressive increase in the complexity of the LCN over time during fracture healing and demonstrates that this network is initiated during the early stages of repair. Further studies are needed to address the functional importance of osteocytes in bone healing, particularly in detecting and translating the effects of micromotion in the fracture. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Characterization of Two Loss-of-Function NF1 Variants in Chinese Patients and Potential Molecular Interpretations of Phenotypes

Neurofibromatosis type 1 (NF1) is a common genetic disorder characterized by cafe'-au-lait spots, skinfold freckles, the formation of neurofibromas, skeletal dysplasia, vascular dysplasia, and an increased risk of malignant tumors. In this study, two Chinese NF1 children troubled with bone lesions or hypertension were reported. A de novo NF1 mutation (c.4925T > A/p.V1642E) and a maternally inherited NF1 mutation (c.4883T > A/p.L1628∗) were identified by molecular sequence. According to the ACMG/AMP guidelines, the c.4925T > A was classified as variants of uncertain significance (VOUS) while the c.4883T > A mutation was identified as likely Pathogenic. Further study found that these two NF1 mutants had lost their function to inhibit the Ras/Erk signaling and the proliferation of cells, which could interpretate some phenotypes of these two NF1 patients. We also observed these two NF1 mutants displayed decreased protein stability with increased ubiquitination levels compared with that of wild-type NF1.

A novel mutation in NF1 gene of patient with Neurofibromatosis type 1: A case report and functional study

BACKGROUND

Neurofibromatosis type 1 is an autosomal dominant inherited disease and caused by NF1 gene mutation. Its clinical manifestations include multiple cafe´-au lait (CAL) spots, skinfold freckling, neurofibroma, bone dysplasia, learning disabilities, and an increased risk of malignancy.

METHODS AND RESULTS

Here, we reported a Chinese patient bearing with a novel NF1 mutation (c.2064delGGATGCAGCGG/p.Gly672AsnfsTer24) and complaining mainly about bone phenotype. Functional studies found that this novel mutation caused the damage of NF1 mRNA and protein levels, and lost the inhibition on Ras/Erk signaling.

CONCLUSION

A novel mutation in NF1 gene was identified and in vitro functional studies were performed, which provided a potential molecular mechanism to explain the bone maldevelopment of patients with neurofibromatosis type 1.

Modeling Complex Orthopedic Trauma in Rodents: Bone, Muscle and Nerve Injury and Healing

Orthopedic injury can occur from a variety of causes including motor vehicle collision, battlefield injuries or even falls from standing. Persistent limb pain is common after orthopedic injury or surgery and presents a unique challenge, as the initiating event may result in polytrauma to bone, muscle, and peripheral nerves. It is imperative that we understand the tissue-specific and multicellular response to this unique type of injury in order to best develop targeted treatments that improve healing and regeneration. In this Mini Review we will first discuss current rodent models of orthopedic trauma/complex orthotrauma. In the second section, we will focus on bone-specific outcomes including imaging modalities, biomechanical testing and immunostaining for markers of bone healing/turnover. In the third section, we will discuss muscle-related pathology including outcome measures of fibrosis, muscle regeneration and tensile strength measurements. In the fourth section, we will discuss nervous system-related pathology including outcome measures of pain-like responses, both reflexive and non-reflexive. In all sections we will consider parallels between preclinical outcome measures and the functional and mechanistic findings of the human condition.

Locally delivered adjuvant biofilm-penetrating antibiotics rescue impaired endochondral fracture healing caused by MRSA infection

Infection is a devastating complication following an open fracture. We investigated whether local rifampin-loaded hydrogel can combat infection and improve healing in a murine model of methicillin-resistant Staphylococcus aureus (MRSA) osteomyelitis. A transverse fracture was made at the tibia midshaft of C57BL/6J mice aged 10-12 weeks and stabilized with an intramedullary pin. A total of 1 × 106 colony-forming units (CFU) of MRSA was inoculated. A collagen-based hydrogel containing low-dose (60 μg) and high-dose (300 μg) rifampin was applied before closure. Postoperative treatment response was assessed through bacterial CFU counts from tissue and hardware, tibial radiographs and microcomputed tomography (μCT), immunohistochemistry, and histological analyses. All untreated MRSA-infected fractures progressed to nonunion by 28 days with profuse MRSA colonization. Infected fractures demonstrated decreased soft callus formation on safranin O stain compared to controls. Areas of dense interleukin-1β stain were associated with poor callus formation. High-dose rifampin hydrogels reduced the average MRSA load in tissue (p < 0.0001) and implants (p = 0.041). Low-dose rifampin hydrogels reduced tissue bacterial load by 50% (p = 0.021). Among sterile models, 88% achieved union compared to 0% of those infected. Mean radiographic union scale in tibia scores improved from 6 to 8.7 with high-dose rifampin hydrogel (p = 0.024) and to 10 with combination local/systemic rifampin therapy (p < 0.0001). μCT demonstrated reactive bone formation in MRSA infection. Histology demonstrated restored fracture healing with bacterial elimination. Rifampin-loaded hydrogels suppressed osteomyelitis, prevented implant colonization, and improved healing. Systemic rifampin was more effective at eliminating infection and improving fracture healing. Further investigation into rifampin-loaded hydrogels is required to correlate these findings with clinical efficacy.

Mast Cell Deficiency Protects Mice from Surgery-Induced Neuroinflammation

Neuroinflammation plays a key role in the occurrence and development of neurodegenerative diseases. Microglia, the resident immune cells in the brain, have been recognized to contribute to neuroinflammation. Previous studies have shown that activated mast cells may be involved in surgery-induced neuroinflammation and neuronal apoptosis by using pharmacological methods. This study is aimed at ascertaining the exactly role of mast cells on neuroinflammation with the mast cell-deficient mice. Adult male C57BL6/J wild-type (WT) and mast cell-deficient (C57BL6/J KitWsh/Wsh (Wsh)) mice underwent tibial fracture surgery. Blood-brain barrier (BBB) breakdown, microglial activation, and neuroinflammatory levels were examined at 1 day after surgery. Surgery-induced BBB breakdown, microglial activation, and neuroinflammatory levels were significantly, pharmacologically reduced using a mast cell stabilizer, cromolyn sodium in WT mice (P < 0.05). These results were reproduced with mast cell deficiency. WT mice administered intraventricularly with cromolyn exhibited reduced BBB breakdown, microglial activation, and neuroinflammatory levels versus vehicle (P < 0.05). But there was no effect of cromolyn versus vehicle in Wsh mice, clarifying the specificity of cromolyn on brain mast cells. These findings demonstrated that activated mast cells promote surgery-induced BBB breakdown and neuroinflammation in mice, and open up a new therapeutic target for neuroinflammation-related diseases.

Fracture Healing in Collagen-Related Preclinical Models of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a genetic bone dysplasia characterized by bone deformities and fractures caused by low bone mass and impaired bone quality. OI is a genetically heterogeneous disorder that most commonly arises from dominant mutations in genes encoding type I collagen (COL1A1 and COL1A2). In addition, OI is recessively inherited with the majority of cases resulting from mutations in prolyl-3-hydroxylation complex members, which includes cartilage-associated protein (CRTAP). OI patients are at an increased risk of fracture throughout their lifetimes. However, non-union or delayed healing has been reported in 24% of fractures and 52% of osteotomies. Additionally, refractures typically go unreported, making the frequency of refractures in OI patients unknown. Thus, there is an unmet need to better understand the mechanisms by which OI affects fracture healing. Using an open tibial fracture model, our study demonstrates delayed healing in both Col1a2 ^G610c/+ and Crtap ^-/- OI mouse models (dominant and recessive OI, respectively) that is associated with reduced callus size and predicted strength. Callus cartilage distribution and chondrocyte maturation were altered in OI, suggesting accelerated cartilage differentiation. Importantly, we determined that healed fractured tibia in female OI mice are biomechanically weaker when compared with the contralateral unfractured bone, suggesting that abnormal OI fracture healing OI may prime future refracture at the same location. We have previously shown upregulated TGF-β signaling in OI and we confirm this in the context of fracture healing. Interestingly, treatment of Crtap ^-/- mice with the anti-TGF-β antibody 1D11 resulted in further reduced callus size and predicted strength, highlighting the importance of investigating dose response in treatment strategies. These data provide valuable insight into the effect of the extracellular matrix (ECM) on fracture healing, a poorly understood mechanism, and support the need for prevention of primary fractures to decrease incidence of refracture and deformity in OI patients. © 2020 American Society for Bone and Mineral Research.

Angiotensin receptor blockade mimics the effect of exercise on recovery after orthopaedic trauma by decreasing pain and improving muscle regeneration

KEY POINTS

Our tibial fracture orthopaedic injury model in mice recapitulates the major manifestations of complex trauma, including nociceptive sensitization, bone fracture, muscle fibrosis and muscle fibre loss. Delayed exercise after complex orthopaedic trauma results in decreased muscle fibrosis and improved pain Losartan, an angiotensin-receptor blocker with anti-fibrotic abilities, recapitulates the effect of exercise on post-injury recovery and may provide an enhanced recovery option for those who are unable to exercise after injury ABSTRACT: Chronic pain and disability after limb injury are major public health problems. Early mobilization after injury improves functional outcomes for patients, although when and how to implement rehabilitation strategies remains a clinical challenge. Additionally, whether the beneficial effects of exercise can be reproduced using pharmacological tools remains unknown and may benefit patients who are unable to exercise as a result of immobilization. We developed a murine model of orthopaedic trauma combining tibia fracture and pin fixation with muscle damage. Behavioural measures included mechanical nociceptive thresholds and distances run on exercise wheels. Bone healing was quantified using microcomputed tomagraphic scanning, and muscle fibre size distribution and fibrosis were followed using immunohistochemistry. We found that the model provided robust mechanical allodynia, fibrosis and a shift to smaller average muscle fibre size lasting up to 5 weeks from injury. We also observed that allowing 'late' (weeks 1-2) rather than 'early' (weeks 0-1) exercise after injury resulted in greater overall running activity and greater reversal of allodynia. In parallel, the late running paradigm was associated with reduced muscle fibrosis, earlier increase in muscle fibre diameter and a short-term benefit in reducing callus volume. Providing the anti-fibrotic angiotensin receptor blocker losartan to mice in drinking water reduced both allodynia and muscle fibrosis. Combining losartan and late exercise provided no additional benefit. We conclude that early healing after orthopaedic trauma must be allowed prior to the initiation of exercise to achieve optimal pain, functional and physiological outcomes and that losartan is a viable candidate for translational studies.

Lack of Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) Disturbs Callus Formation

Pituitary adenylate cyclase–activating polypeptide (PACAP) is a naturally secreted signaling peptide and has important regulatory roles in the differentiation of the central nervous system and its absence results in disorders in femur development. PACAP has an important function in prevention of oxidative stress or mechanical stress in chondrogenesis but little is known about its function in bone regeneration. A new callus formation model was set to investigate its role in bone remodeling. Fracturing was 5 mm distal from the proximal articular surface of the tibia and the depth was 0.5 mm. Reproducibility of callus formation was investigated with CT 3, 7, and 21 days after the operation. Absence of PACAP did not alter the alkaline phosphatase (ALP) activation in PACAP KO healing process. In developing callus, the expression of collagen type I increased in wild-type (WT) and PACAP KO mice decreased to the end of healing process. Expression of the elements of BMP signaling was disturbed in the callus formation of PACAP KO mice, as bone morphogenic protein 4 (BMP4) and 6 showed an early reduction in bone regeneration. However, elevated Smad1 expression was demonstrated in PACAP KO mice. Our results indicate that PACAP KO mice show various signs of disturbed bone healing and suggest PACAP compensatory and fine tuning effects in proper bone regeneration.

Tissue Morphology and Antigenicity in Mouse and Rat Tibia: Comparing 12 Different Decalcification Conditions

Conventional bone decalcification is a time-consuming process and is therefore unsuitable for clinical applications and time-limited research projects. Consequently, we compared the effect of four different decalcification solutions applied at three different temperatures, and assessed the rate of decalcification and the implications on tissue morphology and antigenicity of mouse and rat tibiae. Bones were decalcified with 10% ethylenediaminetetraacetic acid (EDTA), 10% formic acid, 5% hydrochloric acid, and 5% nitric acid at 4C, 25C, and 37C. Decalcification in both species was fastest in nitric acid at 37C and slowest in EDTA at 4C. Histological and immunohistochemical staining confirmed that the conventional protocols of EDTA at 4C and 25C remain the best option regarding the quality of tissue preservation. Whereas formic acid at 4C is a good alternative saving about 90% of the decalcification time, hydrochloric and nitric acids should be avoided particularly in case of rat tibia. By contrast, due to their smaller size, mouse tibiae had shorter decalcification times and tolerated higher temperatures and exposure to acids much better. In conclusion, this study demonstrated that depending on the specific research question and sample size, alternative decalcification methods could be used to decrease the time of decalcification while maintaining histological accuracy.

Preclinical models for orthopedic research and bone tissue engineering

In this review, we broadly define and discuss the preclinical rodent models that are used for orthopedics and bone tissue engineering. These range from implantation models typically used for biocompatibility testing and high-throughput drug screening, through to fracture and critical defect models used to model bone healing and severe orthopedic injuries. As well as highlighting the key methods papers describing these techniques, we provide additional commentary based on our substantive practical experience with animal surgery and in vivo experimental design. This review also briefly touches upon the descriptive and functional outcome measures and power calculations that are necessary for an informative study. Obtaining informative and relevant research outcomes can be very dependent on the model used, and we hope this evaluation of common models will serve as a primer for new researchers looking to undertake preclinical bone studies. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:832-840, 2018.

Osthole Promotes Bone Fracture Healing through Activation of BMP Signaling in Chondrocytes

Osthole is a bioactive coumarin derivative and has been reported to be able to enhance bone formation and improve fracture healing. However, the molecular mechanism of Osthole in bone fracture healing has not been fully defined. In this study we determined if Osthole enhances bone fracture healing through activation of BMP2 signaling in mice. We performed unilateral open transverse tibial fracture procedure in 10-week-old C57BL/6 mice which were treated with or without Osthole. Our previous studies demonstrated that chondrocyte BMP signaling is required for bone fracture healing, in this study we also performed tibial fracture procedure in Cre-negative and Col2-Cre;Bmp2^flox/flox conditional knockout (KO) mice (Bmp2^Col2Cre) to determine if Osthole enhances fracture healing in a BMP2-dependent manner. Fracture callus tissues were collected and analyzed by X-ray, micro-CT (μCT), histology, histomorphometry, immunohistochemistry (IHC), biomechanical testing and quantitative gene expression analysis. In addition, mouse chondrogenic ATDC5 cells were cultured with or without Osthole and the expression levels of chondrogenic marker genes were examined. The results demonstrated that Osthole promotes bone fracture healing in wild-type (WT) or Cre^- control mice. In contrast, Osthole failed to promote bone fracture healing in Bmp2^Col2Creconditional KO mice. In the mice receiving Osthole treatment, expression of cartilage marker genes was significantly increased. We conclude that Osthole could promote bone strength and enhance fracture healing by activation of BMP2 signaling. Osthole may be used as an alternative approach in the orthopaedic clinic for the treatment of fracture healing.

Closed head experimental traumatic brain injury increases size and bone volume of callus in mice with concomitant tibial fracture

Concomitant traumatic brain injury (TBI) and long bone fracture are commonly observed in multitrauma and polytrauma. Despite clinical observations of enhanced bone healing in patients with TBI, the relationship between TBI and fracture healing remains poorly understood, with clinical data limited by the presence of several confounding variables. Here we developed a novel trauma model featuring closed-skull weight-drop TBI and concomitant tibial fracture in order to investigate the effect of TBI on fracture healing. Male mice were assigned into Fracture + Sham TBI (FX) or Fracture + TBI (MULTI) groups and sacrificed at 21 and 35 days post-injury for analysis of healing fractures by micro computed tomography (μCT) and histomorphometry. μCT analysis revealed calluses from MULTI mice had a greater bone and total tissue volume, and displayed higher mean polar moment of inertia when compared to calluses from FX mice at 21 days post-injury. Histomorphometric results demonstrated an increased amount of trabecular bone in MULTI calluses at 21 days post-injury. These findings indicate that closed head TBI results in calluses that are larger in size and have an increased bone volume, which is consistent with the notion that TBI induces the formation of a more robust callus.

RASopathies: unraveling mechanisms with animal models

RASopathies are developmental disorders caused by germline mutations in the Ras-MAPK pathway, and are characterized by a broad spectrum of functional and morphological abnormalities. The high incidence of these disorders (∼1/1000 births) motivates the development of systematic approaches for their efficient diagnosis and potential treatment. Recent advances in genome sequencing have greatly facilitated the genotyping and discovery of mutations in affected individuals, but establishing the causal relationships between molecules and disease phenotypes is non-trivial and presents both technical and conceptual challenges. Here, we discuss how these challenges could be addressed using genetically modified model organisms that have been instrumental in delineating the Ras-MAPK pathway and its roles during development. Focusing on studies in mice, zebrafish and Drosophila, we provide an up-to-date review of animal models of RASopathies at the molecular and functional level. We also discuss how increasingly sophisticated techniques of genetic engineering can be used to rigorously connect changes in specific components of the Ras-MAPK pathway with observed functional and morphological phenotypes. Establishing these connections is essential for advancing our understanding of RASopathies and for devising rational strategies for their management and treatment.

Summary: Developmental disorders caused by germline mutations in the Ras-MAPK pathway are called RASopathies. Studies with animal models, including mice, zebrafish and Drosophila, continue to enhance our understanding of these diseases.

Xylan hemicellulose improves chitosan hydrogel for bone tissue regeneration

The hemicellulose xylan, which has immunomodulatory effects, has been combined with chitosan to form a composite hydrogel to improve the healing of bone fractures. This thermally responsive and injectable hydrogel, which is liquid at room temperature and gels at physiological temperature, improves the response of animal host tissue compared with similar pure chitosan hydrogels in tissue engineering models. The composite hydrogel was placed in a subcutaneous model where the composite hydrogel is replaced by host tissue within 1 week, much earlier than chitosan hydrogels. A tibia fracture model in mice showed that the composite encourages major remodeling of the fracture callus in less than 4 weeks. A non-union fracture model in rat femurs was used to demonstrate that the composite hydrogel allows bone regeneration and healing of defects that with no treatment are unhealed after 6 weeks. These results suggest that the xylan/chitosan composite hydrogel is a suitable bone graft substitute able to aid in the repair of large bone defects.

Tibial fracture exacerbates traumatic brain injury outcomes and neuroinflammation in a novel mouse model of multitrauma

Multitrauma is a common medical problem worldwide, and often involves concurrent traumatic brain injury (TBI) and bone fracture. Despite the high incidence of combined TBI and fracture, preclinical TBI research commonly employs independent injury models that fail to incorporate the pathophysiologic interactions occurring in multitrauma. Here, we developed a novel mouse model of multitrauma, and investigated whether bone fracture worsened TBI outcomes. Male mice were assigned into four groups: sham-TBI+sham-fracture (SHAM); sham-TBI+fracture (FX); TBI+sham-fracture (TBI); and TBI+fracture (MULTI). The injury methods included a closed-skull weight-drop TBI model and a closed tibial fracture. After a 35-day recovery, mice underwent behavioral testing and magnetic resonance imaging (MRI). MULTI mice displayed abnormal behaviors in the open-field compared with all other groups. On MRI, MULTI mice had enlarged ventricles and diffusion abnormalities compared with all other groups. These changes occurred in the presence of heightened neuroinflammation in MULTI mice at 24 hours and 35 days after injury, and elevated edema and blood-brain barrier disruption at 24 hours after injury. Together, these findings indicate that tibial fracture worsens TBI outcomes, and that exacerbated neuroinflammation may be an important factor that contributes to these effects, which warrants further investigation.

Animal models of scoliosis

Multiple techniques designed to induce scoliotic deformity have been applied across many animal species. We have undertaken a review of the literature regarding experimental models of scoliosis in animals to discuss their utility in comprehending disease aetiology and treatment. Models of scoliosis in animals can be broadly divided into quadrupedal and bipedal experiments. Quadrupedal models, in the absence of axial gravitation force, depend upon development of a mechanical asymmetry along the spine to initiate a scoliotic deformity. Bipedal models more accurately mimic human posture and consequently are subject to similar forces due to gravity, which have been long appreciated to be a contributing factor to the development of scoliosis. Many effective models of scoliosis in smaller animals have not been successfully translated to primates and humans. Though these models may not clarify the aetiology of human scoliosis, by providing a reliable and reproducible deformity in the spine they are a useful means with which to test interventions designed to correct and prevent deformity.

Anti-DKK1 antibody promotes bone fracture healing through activation of β-catenin signaling

In this study we investigated if Wnt/β-catenin signaling in mesenchymal progenitor cells plays a role in bone fracture repair and if DKK1-Ab promotes fracture healing through activation of β-catenin signaling. Unilateral open transverse tibial fractures were created in CD1 mice and in β-catenin(Prx1ER) conditional knockout (KO) and Cre-negative control mice (C57BL/6 background). Bone fracture callus tissues were collected and analyzed by radiography, micro-CT (μCT), histology, biomechanical testing and gene expression analysis. The results demonstrated that treatment with DKK1-Ab promoted bone callus formation and increased mechanical strength during the fracture healing process in CD1 mice. DKK1-Ab enhanced fracture repair by activation of endochondral ossification. The normal rate of bone repair was delayed when the β-catenin gene was conditionally deleted in mesenchymal progenitor cells during the early stages of fracture healing. DKK1-Ab appeared to act through β-catenin signaling to enhance bone repair since the beneficial effect of DKK1-Ab was abrogated in β-catenin(Prx1ER) conditional KO mice. Further understanding of the signaling mechanism of DKK1-Ab in bone formation and bone regeneration may facilitate the clinical translation of this anabolic agent into therapeutic intervention.

Activated protein C (APC) can increase bone anabolism via a protease-activated receptor (PAR)1/2 dependent mechanism

Activated Protein C (APC) is an anticoagulant with strong cytoprotective properties that has been shown to promote wound healing. In this study APC was investigated for its potential orthopedic application using a Bone Morphogenetic Protein 2 (rhBMP-2) induced ectopic bone formation model. Local co-administration of 10 µg rhBMP-2 with 10 µg or 25 µg APC increased bone volume at 3 weeks by 32% (N.S.) and 74% (p<0.01) compared to rhBMP-2 alone. This was associated with a significant increase in CD31+ and TRAP+ cells in tissue sections of ectopic bone, consistent with enhanced vascularity and bone turnover. The actions of APC are largely mediated by its receptors endothelial protein C receptor (EPCR) and protease-activated receptors (PARs). Cultured pre-osteoblasts and bone nodule tissue sections were shown to express PAR1/2 and EPCR. When pre-osteoblasts were treated with APC, cell viability and phosphorylation of ERK1/2, Akt, and p38 were increased. Inhibition with PAR1 and sometimes PAR2 antagonists, but not with EPCR blocking antibodies, ameliorated the effects of APC on cell viability and kinase phosphorylation. These data indicate that APC can affect osteoblast viability and signaling, and may have in vivo applications with rhBMP-2 for bone repair.

Inducible cell labeling and lineage tracking during fracture repair

Mouse models incorporating inducible Cre-ERT2/LoxP recombination coupled with sensitive fluorescent reporter lines are being increasingly used to track cell lineages in vivo. In this study we use two inducible reporter strains, Ai9iCol2a1 (Ai9×Col2a1-creERT2) to track contribution of chondrogenic progenitors during bone regeneration in a closed fracture model and Ai9i UBC (Ai9×UBC-creERT2) to examine methods for inducing localized recombination. By comparing with Ai9 littermate controls as well as inducible reporter mice not dosed with tamoxifen, we revealed significant leakiness of the CreERT2 system, particularly in the bone marrow of both lines. These studies highlight the challenges associated with highly sensitive reporters that may be activated without induction in tissues where the CreERT2 fusion is expressed. Examination of the growth plate in the Ai9iCol2a1 strain showed cells of the osteochondral lineage (cell co-staining with chondrocyte and osteoblast markers) labeled with the tdTom reporter. However, no such labeling was noted in healing fractures of Ai9iCol2a1 mice. Attempts to label a single limb using intramuscular injection of 4-hydroxytamoxifen in the Ai9i UBC strain resulted in complete labeling of the entire animal, comparable to intraperitoneal injection. While a challenge to interpret, these data are nonetheless informative regarding the limitations of these inducible reporter models, and justify caution and expansive controls in future studies using such models.

Maximizing bone formation in posterior spine fusion using rhBMP-2 and zoledronic acid in wild type and NF1 deficient mice

Spinal pseudarthrosis is a well described complication of spine fusion surgery in NF1 patients. Reduced bone formation and excessive resorption have been described in NF1 and anti-resorptive agents may be advantageous in these individuals. In this study, 16 wild type and 16 Nf1(+/-) mice were subjected to posterolateral fusion using collagen sponges containing 5 µg rhBMP-2 introduced bilaterally. Mice were dosed twice weekly with 0.02 mg/kg zoledronic acid (ZA) or sterile saline. The fusion mass was assessed for bone volume (BV) and bone mineral density (BMD) by microCT. Co-treatment using rhBMP-2 and ZA produced a significant increase (p < 0.01) in BV of the fusion mass compared to rhBMP-2 alone in both wild type mice (+229%) and Nf1(+/-) mice (+174%). Co-treatment also produced a significantly higher total BMD of the fusion mass compared to rhBMP-2 alone in both groups (p < 0.01). Despite these gains with anti-resorptive treatment, Nf1(+/-) deficient mice still generated less bone than wild type controls. TRAP staining on histological sections indicated an increased osteoclast surface/bone surface (Oc.S/BS) in Nf1(+/-) mice relative to wild type mice, and this was reduced with ZA treatment.

Modulation of endochondral ossification by MEK inhibitors PD0325901 and AZD6244 (Selumetinib)

MEK inhibitors (MEKi) PD0325901 and AZD6244 (Selumetinib) are drugs currently under clinical investigation for cancer treatment, however the Ras-MAPK pathway is also an important mediator of normal bone cell differentiation and function. In this study we examined the effects of these compounds on endochondral processes using both in vitro and in vivo models. Treatment with PD0325901 or AZD6244 significantly increased Runx2 and Alkaline phosphate gene expression in calvarial osteoblasts and decreased TRAP+ cells in induced osteoclast cultures. To test the effects of these drugs on bone healing, C57/Bl6 mice underwent a closed tibial fracture and were treated with PD0325901 or AZD6244 at 10mg/kg/day. Animals were culled at day 10 and at day 21 post-fracture for analysis of the fracture callus and the femoral growth plate in the contralateral leg. MEKi treatment markedly increased cartilage volume in the soft callus at day 10 post-fracture (+60% PD0325901, +20% AZD6244) and continued treatment led to a delay in cartilage remodeling. At the growth plate, we observed an increase in the height of the hypertrophic zone relative to the proliferative zone of +78% in PD0325901 treated mice. Osteoclast surface was significantly decreased both at the terminal end of the growth plate and within the fracture calluses of MEKi treated animals. The mechanistic effects of MEKi on genes encoding cartilage matrix proteins and catabolic enzymes were examined in articular chondrocyte cultures. PD0325901 or AZD6244 led to increased matrix protein expression (Col2a1 and Acan) and decreased expression of catabolic factors (Mmp13 and Adamts-5). Taken together, these data support the hypothesis that MEKi treatment can impact chondrocyte hypertrophy, matrix resorption, and fracture healing. These compounds can also affect bone architecture by expanding the hypertrophic zone of the growth plate and reducing osteoclast surface systemically.

Hyperactive transforming growth factor-β1 signaling potentiates skeletal defects in a neurofibromatosis type 1 mouse model

Dysregulated transforming growth factor beta (TGF-β) signaling is associated with a spectrum of osseous defects as seen in Loeys-Dietz syndrome, Marfan syndrome, and Camurati-Engelmann disease. Intriguingly, neurofibromatosis type 1 (NF1) patients exhibit many of these characteristic skeletal features, including kyphoscoliosis, osteoporosis, tibial dysplasia, and pseudarthrosis; however, the molecular mechanisms mediating these phenotypes remain unclear. Here, we provide genetic and pharmacologic evidence that hyperactive TGF-β1 signaling pivotally underpins osseous defects in Nf1(flox/-) ;Col2.3Cre mice, a model which closely recapitulates the skeletal abnormalities found in the human disease. Compared to controls, we show that serum TGF-β1 levels are fivefold to sixfold increased both in Nf1(flox/-) ;Col2.3Cre mice and in a cohort of NF1 patients. Nf1-deficient osteoblasts, the principal source of TGF-β1 in bone, overexpress TGF-β1 in a gene dosage-dependent fashion. Moreover, Nf1-deficient osteoblasts and osteoclasts are hyperresponsive to TGF-β1 stimulation, potentiating osteoclast bone resorptive activity while inhibiting osteoblast differentiation. These cellular phenotypes are further accompanied by p21-Ras-dependent hyperactivation of the canonical TGF-β1-Smad pathway. Reexpression of the human, full-length neurofibromin guanosine triphosphatase (GTPase)-activating protein (GAP)-related domain (NF1 GRD) in primary Nf1-deficient osteoblast progenitors, attenuated TGF-β1 expression levels and reduced Smad phosphorylation in response to TGF-β1 stimulation. As an in vivo proof of principle, we demonstrate that administration of the TGF-β receptor 1 (TβRI) kinase inhibitor, SD-208, can rescue bone mass deficits and prevent tibial fracture nonunion in Nf1(flox/-) ;Col2.3Cre mice. In sum, these data demonstrate a pivotal role for hyperactive TGF-β1 signaling in the pathogenesis of NF1-associated osteoporosis and pseudarthrosis, thus implicating the TGF-β signaling pathway as a potential therapeutic target in the treatment of NF1 osseous defects that are refractory to current therapies.

NF1 is a critical regulator of muscle development and metabolism

There is emerging evidence for reduced muscle function in children with neurofibromatosis type 1 (NF1). We have examined three murine models featuring NF1 deficiency in muscle to study the effect on muscle function as well as any underlying pathophysiology. The Nf1(+/-) mouse exhibited no differences in overall weight, lean tissue mass, fiber size, muscle weakness as measured by grip strength or muscle atrophy-recovery with limb disuse, although this model lacks many other characteristic features of the human disease. Next, muscle-specific knockout mice (Nf1muscle(-/-)) were generated and they exhibited a failure to thrive leading to neonatal lethality. Intramyocellular lipid accumulations were observed by electron microscopy and Oil Red O staining. More mature muscle specimens lacking Nf1 expression taken from the limb-specific Nf1Prx1(-/-) conditional knockout line showed a 10-fold increase in muscle triglyceride content. Enzyme assays revealed a significant increase in the activities of oxidative metabolism enzymes in the Nf1Prx1(-/-) mice. Western analyses showed increases in the expression of fatty acid synthase and the hormone leptin, as well as decreased expression of a number of fatty acid transporters in this mouse line. These data support the hypothesis that NF1 is essential for normal muscle function and survival and are the first to suggest a direct link between NF1 and mitochondrial fatty acid metabolism.

Neuropeptide Y modulates fracture healing through Y1 receptor signaling

Neuropeptide Y acting via it's Y1 receptor represents a powerful pathway in the control of bone mass. The global or osteoblast-specific Y1 receptor deletion induces pronounced bone anabolic effects in mice. However, the contribution of Y1 receptor deletion in bone repair/healing remained to be clarified. Therefore, in this study we characterized the role of Y1 receptor deletion in fracture healing. Closed tibial fractures were generated in germline (Y1 (-/-) ) and osteoblastic-specific Y1 receptor knockout mice. The progression of tibial repair monitored from 1- until 6-weeks post-fracture demonstrated that in Y1 (-/-) mice there is a delay in fracture repair, as seen by a decrease in bone callus volume and callus strength. Moreover, the histological features included elevated avascular and cartilage area and consequently delayed cartilage removal, and hence impaired union. Interestingly, this delay in bone repair was not related directly to Y1 receptors expressed by mature osteoblasts. These findings suggest that the global absence of the Y1 receptor delays fracture healing, through impairing the early phases of fracture repair to achieve bony union. The data acquired on the role of Y1 receptor signaling disruption in bone regeneration is critical for the design of future therapeutic strategies.

Modelling for conflict: the legacy of ballistic research and current extremity in vivo modelling

Extremity ballistic injury is unique and the literature intended to guide its management is commonly misinterpreted. In order to care for those injured in conflict and conduct appropriate research, clinicians must be able to identify key in vivo studies, understand their weaknesses and desist the propagation of miscited and misunderstood ballistic dogma. This review provides the only inclusive critical overview of key studies of relevance to military extremity injury. In addition, the non-ballistic studies of limb injury, stabilisation and contamination that will form the basis from which future small animal extremity studies are constructed are presented. With an awareness of the legacy of military wound models and an insight into available generic models of extremity injury and contamination, research teams are well placed to optimise future military extremity injury management.

Hyperactive Ras/MAPK signaling is critical for tibial nonunion fracture in neurofibromin-deficient mice

Neurofibromatosis type 1 (NF1) is a common genetic disorder affecting 1 in 3500 individuals. Patients with NF1 are predisposed to debilitating skeletal manifestations, including osteopenia/osteoporosis and long bone pseudarthrosis (nonunion fracture). Hyperactivation of the Ras/mitogen-activated protein kinase (MAPK) pathway in NF1 is known to underlie aberrant proliferation and differentiation in cell lineages, including osteoclast progenitors and mesenchymal stem cells (MSCs) also known as osteoblast progenitors (pro-OBLs). Our current study demonstrates the hyper Ras/MAPK as a critical pathway underlying the pathogenesis of NF1-associated fracture repair deficits. Nf1-deficient pro-OBLs exhibit Ras/MAPK hyperactivation. Introduction of the NF1 GTPase activating-related domain (NF1 GAP-related domain) in vitro is sufficient to rescue hyper Ras activity and enhance osteoblast (OBL) differentiation in Nf1(-/-) pro-OBLs and NF1 human (h) MSCs cultured from NF1 patients with skeletal abnormalities, including pseudarthrosis or scoliosis. Pharmacologic inhibition of mitogen-activated protein kinase kinase (MEK) signaling with PD98059 partially rescues aberrant Erk activation while enhancing OBL differentiation and expression of OBL markers, osterix and osteocalcin, in Nf1-deficient murine pro-OBLs. Similarly, MEK inhibition enhances OBL differentiation of hMSCs. In addition, PD98059 rescues aberrant osteoclast maturation in Nf1 haploinsufficient bone marrow mononuclear cells (BMMNCs). Importantly, MEK inhibitor significantly improves fracture healing in an NF1 murine model, Col2.3Cre;Nf1(flox/-). Collectively, these data indicate the Ras/MAPK cascade as a critical pathway in the pathogenesis of bone loss and pseudarthrosis related to NF1 mutations. These studies provide evidence for targeting the MAPK pathway to improve bone mass and treat pseudarthrosis in NF1.

Deterioration of fracture healing in the mouse model of NF1 long bone dysplasia

Neurofibromatosis type 1 (NF1) is an autosomal dominant genetic disease resulting from inactivating mutations in the gene encoding the protein neurofibromin. NF1 manifests as a heritable susceptibility to tumours of neural tissue mainly located in the skin (neurofibromas) and pigmented skin lesions. Besides these more common clinical manifestations, many NF1 patients (50%) have abnormalities of the skeleton. Long bones are often affected (usually the tibia) and the clinical signs range from bowing to spontaneous fractures and non-unions. Here we present the analysis of bone fracture healing in the Nf1(Prx1)-knock-out mouse, a model of NF1 long bone dysplasia. In line with previously reported cortical bone injury results, fracture healing was impaired in Nf1(Prx1) mice. We showed that the defective fracture healing in Nf1(Prx1) mice is characterized by diminished cartilaginous callus formation and a thickening of the periosteal bone. These changes are paralleled by fibrous tissue accumulation within the fracture site. We identify a population of fibrous tissue cells within the Nf1 deficient fracture as alpha-smooth muscle actin positive myofibroblasts. Additionally, histological and in-situ hybridization analysis reveal a direct contact of the fracture site with muscle fascia, suggesting a possible involvement of muscle derived cells in the fracture deterioration.

Interplay of Nkx3.2, Sox9 and Pax3 regulates chondrogenic differentiation of muscle progenitor cells

Muscle satellite cells make up a stem cell population that is capable of differentiating into myocytes and contributing to muscle regeneration upon injury. In this work we investigate the mechanism by which these muscle progenitor cells adopt an alternative cell fate, the cartilage fate. We show that chick muscle satellite cells that normally would undergo myogenesis can be converted to express cartilage matrix proteins in vitro when cultured in chondrogenic medium containing TGFß3 or BMP2. In the meantime, the myogenic program is repressed, suggesting that muscle satellite cells have undergone chondrogenic differentiation. Furthermore, ectopic expression of the myogenic factor Pax3 prevents chondrogenesis in these cells, while chondrogenic factors Nkx3.2 and Sox9 act downstream of TGFß or BMP2 to promote this cell fate transition. We found that Nkx3.2 and Sox9 repress the activity of the Pax3 promoter and that Nkx3.2 acts as a transcriptional repressor in this process. Importantly, a reverse function mutant of Nkx3.2 blocks the ability of Sox9 to both inhibit myogenesis and induce chondrogenesis, suggesting that Nkx3.2 is required for Sox9 to promote chondrogenic differentiation in satellite cells. Finally, we found that in an in vivo mouse model of fracture healing where muscle progenitor cells were lineage-traced, Nkx3.2 and Sox9 are significantly upregulated while Pax3 is significantly downregulated in the muscle progenitor cells that give rise to chondrocytes during fracture repair. Thus our in vitro and in vivo analyses suggest that the balance of Pax3, Nkx3.2 and Sox9 may act as a molecular switch during the chondrogenic differentiation of muscle progenitor cells, which may be important for fracture healing.

In vivo models of bone repair

This review is aimed at clinicians appraising preclinical trauma studies and researchers investigating compromised bone healing or novel treatments for fractures. It categorises the clinical scenarios of poor healing of fractures and attempts to match them with the appropriate animal models in the literature.

We performed an extensive literature search of animal models of long bone fracture repair/nonunion and grouped the resulting studies according to the clinical scenario they were attempting to reflect; we then scrutinised them for their reliability and accuracy in reproducing that clinical scenario.

Models for normal fracture repair (primary and secondary), delayed union, nonunion (atrophic and hypertrophic), segmental defects and fractures at risk of impaired healing were identified. Their accuracy in reflecting the clinical scenario ranged greatly and the reliability of reproducing the scenario ranged from 100% to 40%.

It is vital to know the limitations and success of each model when considering its application.

A murine model of neurofibromatosis type 1 tibial pseudarthrosis featuring proliferative fibrous tissue and osteoclast-like cells

Neurofibromatosis type 1 (NF1) is a common genetic condition caused by mutations in the NF1 gene. Patients often suffer from tissue-specific lesions associated with local double-inactivation of NF1. In this study, we generated a novel fracture model to investigate the mechanism underlying congenital pseudarthrosis of the tibia (CPT) associated with NF1. We used a Cre-expressing adenovirus (AdCre) to inactivate Nf1 in vitro in cultured osteoprogenitors and osteoblasts, and in vivo in the fracture callus of Nf1(flox/flox) and Nf1(flox/-) mice. The effects of the presence of Nf1(null) cells were extensively examined. Cultured Nf1(null)-committed osteoprogenitors from neonatal calvaria failed to differentiate and express mature osteoblastic markers, even with recombinant bone morphogenetic protein-2 (rhBMP-2) treatment. Similarly, Nf1(null)-inducible osteoprogenitors obtained from Nf1 MyoDnull mouse muscle were also unresponsive to rhBMP-2. In both closed and open fracture models in Nf1(flox/flox) and Nf1(flox/-) mice, local AdCre injection significantly impaired bone healing, with fracture union being <50% that of wild type controls. No significant difference was seen between Nf1(flox/flox) and Nf1(flox/-) mice. Histological analyses showed invasion of the Nf1(null) fractures by fibrous and highly proliferative tissue. Mean amounts of fibrous tissue were increased upward of 10-fold in Nf1(null) fractures and bromodeoxyuridine (BrdU) staining in closed fractures showed increased numbers of proliferating cells. In Nf1(null) fractures, tartrate-resistant acid phosphatase-positive (TRAP+) cells were frequently observed within the fibrous tissue, not lining a bone surface. In summary, we report that local Nf1 deletion in a fracture callus is sufficient to impair bony union and recapitulate histological features of clinical CPT. Cell culture findings support the concept that Nf1 double inactivation impairs early osteoblastic differentiation. This model provides valuable insight into the pathobiology of the disease, and will be helpful for trialing therapeutic compounds.

Myogenic progenitors contribute to open but not closed fracture repair

BACKGROUND

Bone repair is dependent on the presence of osteocompetent progenitors that are able to differentiate and generate new bone. Muscle is found in close association with orthopaedic injury, however its capacity to make a cellular contribution to bone repair remains ambiguous. We hypothesized that myogenic cells of the MyoD-lineage are able to contribute to bone repair.

METHODS

We employed a MyoD-Cre+:Z/AP+ conditional reporter mouse in which all cells of the MyoD-lineage are permanently labeled with a human alkaline phosphatase (hAP) reporter. We tracked the contribution of MyoD-lineage cells in mouse models of tibial bone healing.

RESULTS

In the absence of musculoskeletal trauma, MyoD-expressing cells are limited to skeletal muscle and the presence of reporter-positive cells in non-muscle tissues is negligible. In a closed tibial fracture model, there was no significant contribution of hAP+ cells to the healing callus. In contrast, open tibial fractures featuring periosteal stripping and muscle fenestration had up to 50% of hAP+ cells detected in the open fracture callus. At early stages of repair, many hAP+ cells exhibited a chondrocyte morphology, with lesser numbers of osteoblast-like hAP+ cells present at the later stages. Serial sections stained for hAP and type II and type I collagen showed that MyoD-lineage cells were surrounded by cartilaginous or bony matrix, suggestive of a functional role in the repair process. To exclude the prospect that osteoprogenitors spontaneously express MyoD during bone repair, we created a metaphyseal drill hole defect in the tibia. No hAP+ staining was observed in this model suggesting that the expression of MyoD is not a normal event for endogenous osteoprogenitors.

CONCLUSIONS

These data document for the first time that muscle cells can play a significant secondary role in bone repair and this knowledge may lead to important translational applications in orthopaedic surgery. Please see related article: http://www.biomedcentral.com/1741-7015/9/136.

JNK inhibitors increase osteogenesis in Nf1-deficient cells

Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder that is associated with a variety of manifestations, including orthopedic complications such as scoliosis and tibial pseudarthrosis. Orthopedic management of these skeletal complications is rendered more challenging due to a lack of standardized adjunctive pharmacotherapies. NF1 leads to disruption of the canonical Ras/Raf-1/MEK/ERK axis, and this has been associated with defects in bone anabolism. The roles of other non-canonical Ras effector pathways, such as the c-Jun N-terminal Kinase (JNK) pathway, are less well understood. In this study we examine the effects of an anthrapyrazolone inhibitor of JNK (SP600125) on inducible osteoprogenitors as well as Nf1-deficient and Nf1-null primary osteoblasts. C2C12 cells, which are highly responsive to rhBMP-2, were examined with exogenous rhBMP-2 and a range of SP600125 doses. Based on the expression of early and late bone markers and matrix mineralization, 10 μM SP600125 was found to be pro-osteogenic whether delivered concurrent with or following 2 days of rhBMP-2 treatment. Aberrant JNK activity was identified in Nf1-deficient osteoprogenitors (increased rhBMP-2 induced phospho-c-Jun) and in Nf1-null mature osteoblasts (increased total c-Jun). Next, SP600125 was used to treat these cells and was found to facilitate osteogenesis in Nf1-deficient osteoprogenitors, and in Nf1-null osteoblasts when given in conjunction with rhBMP-2. Outcome measures included alkaline phosphatase activity, matrix mineralization, and osteogenic gene expression. In summary, JNK inhibitors represent a class of potentially useful adjunctive agents for orthopedic medicine, particularly in the context of NF1.

Distal tibial fracture repair in a neurofibromatosis type 1-deficient mouse treated with recombinant bone morphogenetic protein and a bisphosphonate

Congenital pseudarthrosis of the tibia is an uncommon manifestation of neurofibromatosis type 1 (NF1), but one that remains difficult to treat due to anabolic deficiency and catabolic excess. Bone grafting and more recently recombinant human bone morphogenetic proteins (rhBMPs) have been identified as pro-anabolic stimuli with the potential to improve the outcome after surgery. As an additional pharmaceutical intervention, we describe the combined use of rhBMP-2 and the bisphosphonate zoledronic acid in a mouse model of NF1-deficient fracture repair. Fractures were generated in the distal tibiae of neurofibromatosis type 1-deficient (Nf1(+/-)) mice and control mice. Fractures were open and featured periosteal stripping. All mice received 10 μg rhBMP-2 delivered in a carboxymethylcellulose carrier around the fracture as an anabolic stimulus. Bisphosphonate-treated mice also received five doses of 0.02 mg/kg zoledronic acid given by intraperitoneal injection. When only rhBMP but no zoledronic acid was used to promote repair, 75% of fractures in Nf1(+/-) mice remained ununited at three weeks compared with 7% of controls (p < 0.001). Systemic post-operative administration of zoledronic acid halved the rate of ununited fractures to 37.5% (p < 0.07). These data support the concept that preventing bone loss in combination with anabolic stimulation may improve the outcome following surgical treatment for children with congenital pseudarthosis of the tibia and NF1.

Long bone fracture repair in mice harboring GFP reporters for cells within the osteoblastic lineage

GFP reporter mice previously developed to assess levels of osteoblast differentiation were employed in a tibial long bone fracture model using a histological method that preserves fluorescent signals in non-decalcified sections of bone. Two reporters, based on Col1A1 (Col3.6GFPcyan) and osteocalcin (OcGFPtpz) promoter fragments, were bred into the same mice to reflect an early and late stage of osteoblast differentiation. Three observations were apparent from this examination. First, the osteoprogenitor cells that arise from the flanking periosteum proliferate and progress to fill the fracture zone. These cells differentiate to osteoblasts, chondrocytes, to from the outer cortical shell. Second, the hypertrophic chondrocytes are dispersed and the cartilage matrix mineralized by the advancing Col3.6+ osteoblasts. The endochondral matrix is removed by the following osteoclasts. Third, a new cortical shell develops over the cartilage core and undergoes a remodeling process of bone formation on the inner surface and resorption on the outer surface. The original fractured cortex undergoes resorption as the outer cortical shell remodels inward to become the new diaphyseal bone. The fluorescent microscopy and GFP reporter mice used in this study provide a powerful tool for appreciating the molecular and cellular processes that control these fundamental steps in fracture repair, and may provide a basis for understanding fracture nonunion.

Local low-dose lovastatin delivery improves the bone-healing defect caused by Nf1 loss of function in osteoblasts

Postfracture tibial nonunion (pseudoarthrosis) leads to lifelong disability in patients with neurofibromatosis type I (NF1), a disorder caused by mutations in the NF1 gene. To determine the contribution of NF1 in bone healing, we assessed bone healing in the Nf1(ob) (-/-) conditional mouse model lacking Nf1 specifically in osteoblasts. A closed distal tibia fracture protocol and a longitudinal study design were used. During the 21- to 28-day postfracture period, callus volume, as expected, decreased in wild-type but not in Nf1(ob) (-/-) mice, suggesting delayed healing. At these two time points, bone volume (BV/TV) and volumetric bone mineral density (vBMD) measured by 3D micro-computed tomography were decreased in Nf1(ob) (-/-) callus-bridging cortices and trabecular compartments compared with wild-type controls. Histomorphometric analyses revealed the presence of cartilaginous remnants, a high amount of osteoid, and increased osteoclast surfaces in Nf1(ob) (-/-) calluses 21 days after fracture, which was accompanied by increased expression of osteopontin, Rankl, and Tgfbeta. Callus strength measured by three-point bending 28 days after fracture was reduced in Nf1(ob) (-/-) versus wild-type calluses. Importantly, from a clinical point of view, this defect of callus maturation and strength could be ameliorated by local delivery of low-dose lovastatin microparticles, which successfully decreased osteoid volume and cartilaginous remnant number and increased callus BV/TV and strength in mutant mice. These results thus indicate that the dysfunctions caused by loss of Nf1 in osteoblasts impair callus maturation and weaken callus mechanical properties and suggest that local delivery of low-dose lovastatin may improve bone healing in NF1 patients.

Skeletal abnormalities in neurofibromatosis type 1: approaches to therapeutic options

The skeleton is frequently affected in individuals with neurofibromatosis type 1, and some of these bone manifestations can result in significant morbidity. The natural history and pathogenesis of the skeletal abnormalities of this disorder are poorly understood and consequently therapeutic options for these manifestations are currently limited. The Children's Tumor Foundation convened an International Neurofibromatosis Type 1 Bone Abnormalities Consortium to address future directions for clinical trials in skeletal abnormalities associated with this disorder. This report reviews the clinical skeletal manifestations and available preclinical mouse models and summarizes key issues that present barriers to optimal clinical management of skeletal abnormalities in neurofibromatosis type 1. These concepts should help advance optimal clinical management of the skeletal abnormalities in this disease and address major difficulties encountered for the design of clinical trials.

Fixation compliance in a mouse osteotomy model induces two different processes of bone healing but does not lead to delayed union

Delayed unions are a problematic complication of fracture healing whose pathophysiology is not well understood. Advanced molecular biology methods available with mice would be advantageous for investigation. In humans, decreased fixation rigidity and poor reduction are generally associated with delayed unions. In this study, these two factors were combined to observe their effect on bone healing in mice. Two plates with locking screws, one with 14 the bending stiffness of the other, were used to stabilize a 0.45mm gap osteotomy. muCT, radiographs, 4pt-bending tests and histological analysis demonstrated that the different plate types led to two different healing pathways. The less flexible bridging plate induced only intramembranous ossification whereas the more flexible bridging plate induced a mixture of endochondral and intramembranous ossification. However, the different plates led to a delay in healing of only 3-5 days in the period between 14 and 21 post-operative days. In mice, considerable fixation flexibility is necessary to induce secondary bone healing similar to that which occurs in humans, but this was not sufficient to induce a substantial delay in bone healing as would be expected in humans.

The contribution of different cell lineages to bone repair: exploring a role for muscle stem cells

An anabolic response driven by osteoblasts is critical for the process of bone healing. Current evidence suggests that these osteoblasts may arise from multiple tissue types and cell lineages. Stem cells present in the bone marrow, periosteum, local soft tissues, vasculature, and/or circulation have been shown to have osteogenic potential. Transplanted cells from these sources have also been shown to incorporate into induced ectopic bone or repaired bone. While these experiments demonstrate the latent capacity of different lineages to assume an osteoblastic phenotype under pro-osteogenic conditions, the actual contribution of the different lineages to various repair situations in vivo remains unclear. This review explores the data arising from different bone formation and repair models. We propose a model suggesting that cells arising from the local tissues, particularly muscle cells, may play an important role in fracture repair under situations where the periosteal and/or bone marrow progenitor populations are depleted.

Neurofibromin regulates somatic growth through the hypothalamic-pituitary axis

To study the role of the neurofibromatosis-1 (NF1) gene in mammalian brain development, we recently generated mice in which Nf1 gene inactivation occurs in neuroglial progenitor cells using the brain lipid binding protein (BLBP) promoter. We found that Nf1(BLBP)CKO mice exhibit significantly reduced body weights and anterior pituitary gland sizes. We further demonstrate that the small anterior pituitary size reflects loss of neurofibromin expression in the hypothalamus, leading to reduced growth hormone releasing hormone, pituitary growth hormone (GH) and liver insulin-like growth factor-1 (IGF1) production. Since neurofibromin both negatively regulates Ras activity and positively modulates cAMP levels, we examined the signaling pathway responsible for these abnormalities. While BLBP-mediated expression of an activated Ras molecule did not recapitulate the body weight and hypothalamic/pituitary defects, treatment of Nf1(BLBP)CKO mice with rolipram to increase cAMP levels resulted in a partial restoration of the body weight phenotype. Furthermore, conditional expression of the Ras regulatory GAP domain of neurofibromin also did not rescue the body weight or Igf1 mRNA defects in Nf1(BLBP)CKO mice. Collectively, these data demonstrate a critical role for neurofibromin in hypothalamic-pituitary axis function and provide further insights into the short stature and GH deficits seen in children with NF1.