Cell proliferation and apoptosis during fracture healing

FOXO 1 deletion in chondrocytes rescues diabetes-impaired fracture healing by restoring angiogenesis and reducing apoptosis

Introduction

Diabetes mellitus is associated with higher risks of long bone and jaw fractures. It is also associated with a higher incidence of delayed union or non-union. Our previous investigations concluded that a dominant mechanism was the premature loss of cartilage during endochondral bone formation associated with increased osteoclastic activities. We tested the hypothesis that FOXO1 plays a key role in diabetes-impaired angiogenesis and chondrocyte apoptosis.

Methods

Closed fractures of the femur were induced in mice with lineage-specific FOXO1 deletion in chondrocytes. The control group consisted of mice with the FOXO1 gene present. Mice in the diabetic group were rendered diabetic by multiple streptozotocin injections, while mice in the normoglycemic group received vehicle. Specimens were collected 16 days post fracture. The samples were fixed, decalcified, and embedded in paraffin blocks for immunostaining utilizing anti cleaved caspase-3 or CD31 specific antibodies compared with matched control IgG antibody, and apoptosis by the TUNEL assay. Additionally, ATDC5 chondrocytes were examined in vitro by RT-PCR, luciferase reporter and chromatin immunoprecipitation assays.

Results

Diabetic mice had ~ 50% fewer blood vessels compared to normoglycemic mice FOXO1 deletion in diabetic mice partially rescued the low number of blood vessels (p < 0.05). Additionally, diabetes increased caspase-3 positive and apoptotic chondrocytes by 50%. FOXO1 deletion in diabetic animals blocked the increase in both to levels comparable to normoglycemic animals (p < 0.05). High glucose (HG) and high advanced glycation end products (AGE) levels stimulated FOXO1 association with the caspase-3 promoter in vitro, and overexpression of FOXO1 increased caspase-3 promoter activity in luciferase reporter assays. Furthermore, we review previous mechanistic studies demonstrating that tumor necrosis factor (TNF) inhibition reverses impaired angiogenesis and reverses high levels of chondrocyte apoptosis that occur in fracture healing.

Discussion

New results presented here, in combination with recent studies, provide a comprehensive overview of how diabetes, through high glucose levels, AGEs, and increased inflammation, impair the healing process by interfering with angiogenesis and stimulating chondrocyte apoptosis. FOXO1 in diabetic fractures plays a negative role by reducing new blood vessel formation and increasing chondrocyte cell death which is distinct from its role in normal fracture healing.

Reambulation following hindlimb unloading attenuates disuse-induced changes in murine fracture healing

Patients with bone and muscle loss from prolonged disuse have higher risk of falls and subsequent fragility fractures. In addition, fracture patients with continued disuse and/or delayed physical rehabilitation have worse clinical outcomes compared to individuals with immediate weight-bearing activity following diaphyseal fracture. However, the effects of prior disuse followed by physical reambulation on fracture healing cellular processes and adjacent bone and skeletal muscle recovery post-injury remains poorly defined. To bridge this knowledge gap and inform future treatment and rehabilitation strategies for fractures, a preclinical model of fracture healing with a history of prior unloading with and without reambulation was employed. First, skeletally mature male and female C57BL/6J mice (18 weeks) underwent hindlimb unloading by tail suspension (HLU) for 3 weeks to induce significant bone and muscle loss modeling enhanced bone fragility. Next, mice had their right femur fractured by open surgical dissection (stabilized with 24-gauge pin). The, mice were randomly assigned to continued HLU or allowed normal weight-bearing reambulation (HLU + R). Mice given normal cage activity throughout the experiment served as healthy age-matched controls. All mice were sacrificed 4-days (DPF4) or 14-days (DPF14) following fracture to assess healing and uninjured hindlimb musculoskeletal properties (6-10 mice per treatment/biological sex). We found that continued disuse following fracture lead to severely diminished uninjured hindlimb skeletal muscle mass (gastrocnemius and soleus) and femoral bone volume adjacent to the fracture site compared to healthy age-matched controls across mouse sexes. Furthermore, HLU led to significantly decreased periosteal expansion (DPF4) and osteochondral tissue formation by DPF14, and trends in increased osteoclastogenesis (DPF14) and decreased woven bone vascular area (DPF14). In contrast, immediate reambulation for 2 weeks after fracture, even following a period of prolonged disuse, was able to increase hindlimb skeletal tissue mass and increase osteochondral tissue formation, albeit not to healthy control levels, in both mouse sexes. Furthermore, reambulation attenuated osteoclast formation seen in woven bone tissue undergoing disuse. Our results suggest that weight-bearing skeletal loading in both sexes immediately following fracture may improve callus healing and prevent further fall risk by stimulating skeletal muscle anabolism and decreasing callus resorption compared to minimal or delayed rehabilitation regimens.

Hypoxia Preconditioned Serum (HPS) Promotes Osteoblast Proliferation, Migration and Matrix Deposition

Interest in discovering new methods of employing natural growth factor preparations to promote bone fracture healing is becoming increasingly popular in the field of regenerative medicine. In this study, we were able to demonstrate the osteogenic potential of hypoxia preconditioned serum (HPS) on human osteoblasts in vitro. Human osteoblasts were stimulated with two HPS concentrations (10% and 40%) and subsequently analyzed at time points of days 2 and 4. In comparison to controls, a time- and dose-dependent (up to 14.2× higher) proliferation of osteoblasts was observed after 4 days of HPS-40% stimulation with lower lactate dehydrogenase (LDH)-levels detected than controls, indicating the absence of cytotoxic/stress effects of HPS on human osteoblasts. With regards to cell migration, it was found to be significantly faster with HPS-10% application after 72 h in comparison to controls. Further osteogenic response to HPS treatment was evaluated by employing culture supernatant analysis, which exhibited significant upregulation of OPG (Osteoprotegerin) with higher dosage (HPS-10% vs. HPS-40%) and longer duration (2 d vs. 4 d) of HPS stimulation. There was no detection of anti-osteogenic sRANKL (soluble Receptor Activator of NF-κB Ligand) after 4 days of HPS stimulation. In addition, ALP (alkaline phosphatase)-enzyme activity, was found to be upregulated, dose-dependently, after 4 days of HPS-40% application. When assessing ossification through Alizarin-Red staining, HPS dose-dependently achieved greater (up to 2.8× higher) extracellular deposition of calcium-phosphate with HPS-40% in comparison to controls. These findings indicate that HPS holds the potential to accelerate bone regeneration by osteogenic promotion of human osteoblasts.

The specialist in regeneration-the Axolotl-a suitable model to study bone healing?

While the axolotl's ability to completely regenerate amputated limbs is well known and studied, the mechanism of axolotl bone fracture healing remains poorly understood. One reason might be the lack of a standardized fracture fixation in axolotl. We present a surgical technique to stabilize the osteotomized axolotl femur with a fixator plate and compare it to a non-stabilized osteotomy and to limb amputation. The healing outcome was evaluated 3 weeks, 3, 6 and 9 months post-surgery by microcomputer tomography, histology and immunohistochemistry. Plate-fixated femurs regained bone integrity more efficiently in comparison to the non-fixated osteotomized bone, where larger callus formed, possibly to compensate for the bone fragment misalignment. The healing of a non-critical osteotomy in axolotl was incomplete after 9 months, while amputated limbs efficiently restored bone length and structure. In axolotl amputated limbs, plate-fixated and non-fixated fractures, we observed accumulation of PCNA+ proliferating cells at 3 weeks post-injury similar to mouse. Additionally, as in mouse, SOX9-expressing cells appeared in the early phase of fracture healing and amputated limb regeneration in axolotl, preceding cartilage formation. This implicates endochondral ossification to be the probable mechanism of bone healing in axolotls. Altogether, the surgery with a standardized fixation technique demonstrated here allows for controlled axolotl bone healing experiments, facilitating their comparison to mammals (mice).

Localization of VEGF, TGF-β1, BMP-2, and Apoptosis Factors in Hypertrophic Nonunion of Human Tubular Bones

We studied localization of VEGF, TGF-β1, BMP-2, caspase-3, Bcl-2, and TNFα in the callus samples obtained from 5 patients (4 women and 1 man) aged 41-53 years during planned surgery for nonunion and pseudarthrosis of the clavicle (n=1), ulna (n=1), femur (n=1), and tibia (n=2) bones. Two control groups included material of hypertrophied callus (n=3) with consolidated fractures of long bones and samples of intact bones (n=3) obtained by postmortem autopsy of subjects without pathology of the musculoskeletal system. A nonuniform distribution of the studied markers was revealed. Active expression of VEGF was observed in fibroblast-like cells of the fibrous tissue, osteoblasts of the periosteum and osteons. Osteoblasts expressing BMP-2 were localized in the periosteum and the loose connective tissue of the Haversian canals. The number of immunopositive cells expressing TGF-β1 and TNFα in the callus exceeded that in the control and correlated with the expression of caspase-3 in fibroblast-like cells, osteoblasts, chondroblasts, and microvascular endotheliocytes. The results allow considering fracture nonunion as a result of overproduction of cytotoxic and proapoptotic factors in chronic inflammation and dysfunction of the expression of morphogenetic proteins. The morphochemical patterns of the studied markers open up prospects for the development of new methods of pharmacological correction of fracture repair.

Ablation of Proliferating Osteoblast Lineage Cells After Fracture Leads to Atrophic Nonunion in a Mouse Model

Nonunion is defined as the permanent failure of a fractured bone to heal, often necessitating surgical intervention. Atrophic nonunions are a subtype that are particularly difficult to treat. Animal models of atrophic nonunion are available; however, these require surgical or radiation-induced trauma to disrupt periosteal healing. These methods are invasive and not representative of many clinical nonunions where osseous regeneration has been arrested by a "failure of biology". We hypothesized that arresting osteoblast cell proliferation after fracture would lead to atrophic nonunion in mice. Using mice that express a thymidine kinase (tk) "suicide gene" driven by the 3.6Col1a1 promoter (Col1-tk), proliferating osteoblast lineage cells can be ablated upon exposure to the nucleoside analog ganciclovir (GCV). Wild-type (WT; control) and Col1-tk littermates were subjected to a full femur fracture and intramedullary fixation at 12 weeks age. We confirmed abundant tk+ cells in fracture callus of Col-tk mice dosed with water or GCV, specifically many osteoblasts, osteocytes, and chondrocytes at the cartilage-bone interface. Histologically, we observed altered callus composition in Col1-tk mice at 2 and 3 weeks postfracture, with significantly less bone and more fibrous tissue. Col1-tk mice, monitored for 12 weeks with in vivo radiographs and micro-computed tomography (μCT) scans, had delayed bone bridging and reduced callus size. After euthanasia, ex vivo μCT and histology showed failed union with residual bone fragments and fibrous tissue in Col1-tk mice. Biomechanical testing showed a failure to recover torsional strength in Col1-tk mice, in contrast to WT. Our data indicates that suppression of proliferating osteoblast-lineage cells for at least 2 weeks after fracture blunts the formation and remodeling of a mineralized callus leading to a functional nonunion. We propose this as a new murine model of atrophic nonunion. © 2021 American Society for Bone and Mineral Research (ASBMR).

Dynamics of Renewal of Cell Populations of the Bone Tissue on the Surface of Titanium Implants with Bioactive Coating during Fracture Modeling in Rats

Localization of PCNA, CD44, osteocalcin, Mdm2, p53, and caspase-3 on the surface of implant with calcium phosphate and hydroxyapatite coating was studied by immunocytochemical method in a model of femur fracture in rats. PCNA⁺, Ost⁺, CD44⁺, and Mdm²⁺ cells were found in the periosteum, in the layer of the outer surrounding plates, and in the connective tissue of the Haversian canals. Cell density increased on day 7 after fracture and then decreased by day 30. The number of p53⁺ and CASP3⁺ cells reached a maximum on day 14 (they were predominantly located in the periosteum and bone plates adjacent to it) and decreased by day 30. Calcium phosphate coating stimulated proliferative activity of cells at the early stages of the regeneration phase and apoptotic death at the later stages. Components of coating can be viewed as a positioning clue for differentiation of mesenchymal stromal cells. The effectiveness of reparative osteogenesis is determined by the balance of proliferative and destructive factors at the site of the fracture healing. This process can be optimized with various nanostructured materials with osteoinductive properties, in particular bioresorbable calcium phosphate coatings on titanium implants. However, the influence of these components on the state of cambial cells, their differentiation, and positioning in the repair zone is unknown.

The effect of calcium phosphate biodegradable coatings of titanium implants on cell differentiation and apoptosis in rat bone tissue after experimental fracture

BACKGROUND

The effectiveness of bone repair is determined by the balance of proliferative and destructive factors in the fracture union site. It can be enhanced by using various nanostructured materials possessing osteoinductive properties, in particular titanium implants with biodegradable calcium phosphate coatings. The effects of these coatings on the state of stem cells, their differentiation and distribution in the repair zone is unknown.

OBJECTIVE

To study the dynamics of proliferation, differentiation, and apoptosis of stem cells after experimental fracture followed by implantation of titanium implants with calcium phosphate coatings.

METHODS

The localization of proliferation (PCNA) and differentiation (CD44 and osteocalcin) factors and apoptotic molecules (MDM2, p53, caspase-3) was studied in a rat femoral fracture model with implant placement. Titanium implant screws with bioactive calcium phosphate and hydroxyapatite coatings formed by plasma electrolytic oxidation were used in the study. Experimental rats were arranged into three groups (15 animals per group): control group; rats implanted with uncoated implants; and rats implanted with coated implants. Control rats were subject to a similar fracture as experimental ones and were allowed to heal conservatively. Rats from all groups were sampled on days 7, 14, and 30 after injury.

RESULTS

Low-differentiated PCNA-, osteocalcin-, and CD44-immunopositive cells were localized around the implant in the inner layer of the periosteum, layer of outer circumferential lamellae, and connective tissue lining of haversian canals. The spatial density of cells expressing the above proliferation and differentiation factors, as well as that of MDM2-immunoreactive cells, increased on day 7 and decreased by day 30 after injury. The spatial density of apoptotic cells reached the maximum on day 14 after injury. They were mainly found in the inner layer of the periosteum and outer circumferential lamellae. p53- and caspase-3-positive cells occurred on the surface of the concentric lamellae surrounding haversian canals and under the periosteum. Their spatial density decreased by day 30 after injury.

CONCLUSIONS

Calcium phosphate coatings stimulate cell proliferation at early stages of fracture restoration and apoptotic cell death at later stages. Coating components may provide positional information guiding the differentiation of mesenchymal stromal cells. A change in the activity of apoptotic factors, osteocalcin, and CD44 is caused by gene induction in response to the diffusion of calcium phosphate compounds from coating to surrounding tissue.

Review on material parameters to enhance bone cell function in vitro and in vivo

Bone plays critical roles in support, protection, movement, and metabolism. Although bone has an innate capacity for regeneration, this capacity is limited, and many bone injuries and diseases require intervention. Biomaterials are a critical component of many treatments to restore bone function and include non-resorbable implants to augment bone and resorbable materials to guide regeneration. Biomaterials can vary considerably in their biocompatibility and bioactivity, which are functions of specific material parameters. The success of biomaterials in bone augmentation and regeneration is based on their effects on the function of bone cells. Such functions include adhesion, migration, inflammation, proliferation, communication, differentiation, resorption, and vascularization. This review will focus on how different material parameters can enhance bone cell function both in vitro and in vivo.

Effects of ultrasound, radial extracorporeal shock waves, and electrical stimulation on rat bone defect healing

Fractures associated with osteoporosis are a major public health concern. Current treatments for fractures are limited to surgery or fixation, leading to long-term bedrest, which is linked to increased mortality. Alternatively, utilization of physical agents has been suggested as a promising therapeutic approach for fractures. Here, we examined the effects of ultrasound, radial extracorporeal shock waves, and electrical stimulation on normal or osteoporotic fracture healing. Femoral bone defects were created in normal or ovariectomized rats. Rats were divided into four groups: untreated, and treated with ultrasound, shock waves, or electrical stimulation after surgery. Samples were collected at 2 or 4 weeks after surgery, and the healing process was evaluated with micro-CT, histological, and immunohistochemical analyses. Ultrasound at intensities of 0.5 and 1.0 W/cm² , but not 0.05 W/cm² , accelerated new bone formation. Shock wave exposure also increased newly formed bone, but formed abnormal periosteal callus around the defect site. Conversely, electrical stimulation did not affect the healing process. Ultrasound exposure increased osteoblast activity and cell proliferation and decreased sclerostin-positive osteocytes. We demonstrated that higher-intensity ultrasound and radial extracorporeal shock waves accelerate fracture healing, but shock wave treatment may increase the risk of periosteal callus formation.

The beneficial effect of cold atmospheric plasma on parameters of molecules and cell function involved in wound healing in human osteoblast-like cells in vitro

The aim of this study was to analyse the effect of cold atmospheric plasma (CAP) on human osteoblast-like cells in vitro. Additionally, underlying intracellular mechanisms were to be studied. Human osteoblast-like (MG63) cells were exposed to CAP for 60 s. The effects of CAP on key molecules essential for the wound healing response were studied using real-time PCR, ELISA and immunocytochemistry. For studying intracellular signalling pathways, MAP kinase MEK 1/2 was blocked. Cell viability was analysed by an XTT assay and with an EVE automated cell counter. Cell migration was examined by an in vitro wound healing assay.CAP exposition on osteoblast-like cells caused a significant upregulation of interleukin (IL)-1β, IL-6, IL-8, tumor necrosis factor (TNF)α, cyclooxygenase (COX)2, collagen (COL) 1α, matrix metalloproteinase (MMP)1, Ki67, proliferating-cell-nuclear-antigen (PCNA) and chemokine ligand (CCL)2 mRNA expression at 1 day. Interestingly, after blocking of MAP kinase, CAP-induced upregulation of Ki67 was inhibited by 57%. Moreover, CAP treatment improved significantly osteoblast-like cell viability as compared to untreated cells at 1 day. Beneficial effect of CAP treatment was shown by an in vitro wound healing assay, displaying a significant faster wound closure. Our findings provide evidence that CAP exposure effects gene and protein regulation in human osteoblast-like cells. Furthermore, CAP treatment has a positive impact on wound closure in an in vitro setting and might improve existing concepts of hard tissue regeneration in the future.

Osteogenic Differentiation of Mesenchymal Stem Cells with Silica-Coated Gold Nanoparticles for Bone Tissue Engineering

Multifunctional nanofibrous scaffolds for effective bone tissue engineering (BTE) application must incorporate factors to promote neovascularization and tissue regeneration. In this study, silica-coated gold nanoparticles Au(SiO2) were tested for their ability to promote differentiation of human mesenchymal stem cells (hMSCs) into osteoblasts. Biocompatible poly-ε-caprolactone (PCL), PCL/silk fibroin (SF) and PCL/SF/Au(SiO2) loaded nanofibrous scaffolds were first fabricated by an electrospinning method. Electrospun nanofibrous scaffolds were characterized for fiber architecture, porosity, pore size distribution, fiber wettability and the relevant mechanical properties using field emission scanning electron microscopy (FESEM), porosimetry, determination of water contact angle, measurements by a surface analyzer and tabletop tensile-tester measurements. FESEM images of the scaffolds revealed beadless, porous, uniform fibers with diameters in the range of 164 ± 18.65 nm to 215 ± 32.12 nm and porosity of around 88-92% and pore size distribution around 1.45-2.35 µm. Following hMSCs were cultured on the composite scaffolds. Cell-scaffold interaction, morphology and proliferation of were analyzed by FESEM analysis, MTS (3-(4,5-dimethyl thiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium inner salt) and CMFDA (5-choromethyl fluorescein acetate) dye assays. Osteogenic differentiation of MSCs into osteogenic cells were determined by alkaline phosphatase (ALP) activity, mineralization by alizarin red S (ARS) staining and osteocalcin expression by immunofluorescence staining. The results revealed that the addition of SF and Au(SiO2) to PCL scaffolds enhanced the mechanical strength, interconnecting porous structure and surface roughness of the scaffolds. This, in turn, led to successful osteogenic differentiation of hMSCs with improved cell adhesion, proliferation, differentiation, mineralization and expression of pro-osteogenic cellular proteins. This provides huge support for Au(SiO2) as a suitable material in BTE.

Trauma in the Diabetic Limb

Poorly controlled diabetes with comorbid manifestations negatively affects outcomes in lower extremity trauma, increasing the risk of short-term and long-term complications. Management strategies of patients with diabetes that experience lower extremity trauma should also include perioperative management of hyperglycemia to reduce adverse and serious adverse events.

Enrichment of miR-126 enhances the effects of endothelial progenitor cell-derived microvesicles on modulating MC3T3-E1 cell function via Erk1/2-Bcl-2 signalling pathway

OBJECTIVE

To evaluate whether EPC-MVs could promote bone regeneration by directly regulating osteoblast through miR-126. The underlying mechanisms were also explored.

METHODS

EPCs were isolated from bone marrow mononuclear cells. EPC-MVs were collected from EPCs cultured medium. The lentivirus was used to induce miR-126 over-expression in EPCs and EPC-MVs. miR-126 expression was detected by qRT-PCR. The proliferation, migration, apoptosis and differentiation abilities of osteoblast cells MC3T3-E1 were analysed in the presence or absence of EPC-MVs or miR-126 overexpressed EPC-MVs (EPC-MVs-miR126). The proteins of Erk1/2 and Bcl-2 were analysed by western blot. Erk1/2 inhibitor was used for pathway exploration.

RESULTS

EPC-MVs reduced apoptosis and promoted proliferation and migration of MC3T3-E1 cells, which could be enhanced by miR-126 enrichment (p< 0.05). Neither EPC-MVs nor EPC-MVs-miR126 had an effect on MC3T3-E1 cell osteogenic differentiation (p> 0.05). EPC-MVs-miR126 had better effects than EPC-MVs on upregulating the expressions of p-Erk1/2 and Bcl-2, which were abolished by Erk1/2 inhibitor. ERK1/2-Bcl-2 activity plays a crucial role in the regulation of EPC-MVs/EPC-MVs-miR126 on the effect of MC3T3-E1 cells.

CONCLUSION

EPC-MVs promote proliferation and migration of MC3T3-E1 cell while reduced apoptosis via the miR-126/Erk1/2-Bcl-2 pathway. A combination of EPC-MVs and miR-126 might provide novel therapeutic targets for bone regeneration and fracture healing through regulating osteoblast.

Identification of key gene networks associated with fracture healing using αSMA‑labeled progenitor cells

The aim of the present study was to investigate the key gene network in fracture healing. The dataset GSE45156 was downloaded from the Gene Expression Omnibus. Differentially expressed genes (DEGs) were identified using the linear models for microarray data package of Bioconductor. Subsequently, Gene Ontology (GO) functional and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses were conducted for DEGs in day 2 and 6 fractured samples via the Database for Annotation, Visualization and Integrated Discovery. Furthermore, protein-protein interactions (PPIs) of DEGs were analyzed and a PPI network was constructed. A total of 774 and 1,172 DEGs were identified in day 2 and 6 fractured samples, respectively, compared with unfractured controls. Of the DEGs in day 2 and 6 fractured samples, various upregulated DEGs, including protein kinase C α (Prkca) and B-cell lymphoma antagonist/killer 1 were significantly enriched in GO terms associated with cell death, and certain downregulated DEGs, including fms-related tyrosine kinase 1 (Flt1), nitric oxide synthase 3 (Nos3), bone morphogenetic protein 4 (Bmp4) and Notch1 were enriched in GO terms associated with angiogenesis. Furthermore, a series of downregulated DEGs were enriched in the Notch signaling pathway, including hes family bHLH transcription factor 1 and Notch1. Certain DEGs had a high degree and interacted with each other, including Flt1, Nos3, Bmp4 and Notch1, and Prkca and ras-related C3 botulinum toxin substrate 3. The up and downregulated DEGs may exert critical functions by interactively regulating angiogenesis or apoptosis.

Parathyroid Hormone-Related Protein Protects Osteoblastic Cells From Oxidative Stress by Activation of MKP1 Phosphatase

Oxidative damage is an important contributor to the morphological and functional changes in osteoporotic bone. Aging increases the levels of reactive oxygen species (ROS) that cause oxidative stress and induce osteoblast apoptosis. ROS modify several signaling responses, including mitogen-activated protein kinase (MAPK) activation, related to cell survival. Both parathyroid hormone (PTH) and its bone counterpart, PTH-related protein (PTHrP), can regulate MAPK activation by modulating MAPK phosphatase-1 (MKP1). Thus, we hypothesized that PTHrP might protect osteoblasts from ROS-induced apoptosis by targeting MKP1. In osteoblastic MC3T3-E1 and MG-63 cells, H₂ O₂ triggered p38, JNK, ERK and p66^Shc phosphorylation, and cell apoptosis. Meanwhile, PTHrP (1-37) rapidly but transiently increased ERK and Akt phosphorylation without affecting p38, JNK, or p66^Shc activation. H₂ O₂ -induced p38 and ERK phosphorylation and apoptosis were both decreased by pre-treatment with specific kinase inhibitors or PTHrP (1-37) in both osteoblastic cell types. These dephosphorylating and prosurvival actions of PTHrP (1-37) were prevented by a phosphatase inhibitor cocktail, the phosphatase MKP1 inhibitor sanguinarine or a MKP1 siRNA. PTHrP (1-37) promptly enhanced MKP1 protein and gene expression and MKP1-dependent catalase activity in osteoblastic cells. Furthermore, exposure to PTHrP (1-37) adsorbed in an implanted hydroxyapatite-based ceramic into a tibial defect in aging rats increased MKP1 and catalase gene expression in the healing bone area. Our findings demonstrate that PTHrP counteracts the pro-apoptotic actions of ROS by a mechanism dependent on MKP1-induced dephosphorylation of MAPKs in osteoblasts. J. Cell. Physiol. 232: 785-796, 2017. © 2016 Wiley Periodicals, Inc.

Growth and differentiation of a long bone in limb development, repair and regeneration

Repair from traumatic bone fracture is a complex process that includes mechanisms of bone development and bone homeostasis. Thus, elucidation of the cellular/molecular basis of bone formation in skeletal development would provide valuable information on fracture repair and would lead to successful skeletal regeneration after limb amputation, which never occurs in mammals. Elucidation of the basis of epimorphic limb regeneration in amphibians would also provide insights into skeletal regeneration in mammals, since the epimorphic regeneration enables an amputated limb to re-develop the three-dimensional structure of bones. In the processes of bone development, repair and regeneration, growth of the bone is achieved through several events including not only cell proliferation but also aggregation of mesenchymal cells, enlargement of cells, deposition and accumulation of extracellular matrix, and bone remodeling.

Impaired angiogenesis during fracture healing in GPCR kinase 2 interacting protein-1 (GIT1) knock out mice

G protein coupled receptor kinase 2 (GRK2) interacting protein-1 (GIT1), is a scaffold protein that plays an important role in angiogenesis and osteoclast activity. We have previously demonstrated that GIT1 knockout (GIT1 KO) mice have impaired angiogenesis and dysregulated osteoclast podosome formation leading to a reduction in the bone resorbing ability of these cells. Since both angiogenesis and osteoclast-mediated bone remodeling are involved in the fracture healing process, we hypothesized that GIT1 participates in the normal progression of repair following bone injury. In the present study, comparison of fracture healing in wild type (WT) and GIT1 KO mice revealed altered healing in mice with loss of GIT1 function. Alcian blue staining of fracture callus indicated a persistence of cartilagenous matrix in day 21 callus samples from GIT1 KO mice which was temporally correlated with increased type 2 collagen immunostaining. GIT1 KO mice also showed a decrease in chondrocyte proliferation and apoptosis at days 7 and 14, as determined by PCNA and TUNEL staining. Vascular microcomputed tomography analysis of callus samples at days 7, 14 and 21 revealed decreased blood vessel volume, number, and connection density in GIT1 KO mice compared to WT controls. Correlating with this, VEGF-A, phospho-VEGFR2 and PECAM1 (CD31) were decreased in GIT1 KO mice, indicating reduced angiogenesis with loss of GIT1. Finally, calluses from GIT1 KO mice displayed a reduced number of tartrate resistant acid phosphatase-positive osteoclasts at days 14 and 21. Collectively, these results indicate that GIT1 is an important signaling participant in fracture healing, with gene ablation leading to reduced callus vascularity and reduced osteoclast number in the healing callus.

Increased levels of apoptosis and p53 in partial-thickness supraspinatus tendon tears

PURPOSE

The role of apoptosis in the progression of rotator cuff tendinopathy remains poorly understood. In particular, the extent of apoptosis in the partially torn supraspinatus tendon has not been well examined.

METHODS

Biopsies were obtained from nine partially torn supraspinatus tendons, from the matched intact subscapularis tendons, and from 10 reference subscapularis tendons. Immunohistochemistry was used to assess the density of apoptotic cells (activated caspase-3; Asp175), proliferation (Ki67), and p53 (M7001), a key protein involved in regulating cell death. The Bonar scale was used to evaluate tendon degeneration.

RESULTS

The density of apoptotic tendon cells and the density of cells expressing p53 were significantly increased in both the partially torn supraspinatus tendons and in the matched subscapularis tendons, compared with uninjured reference tendons. The Bonar score revealed significant tendon degeneration in the partially torn supraspinatus tendons compared with both matched and reference subscapularis tendons. Tendon cell proliferation was significantly increased in the partially torn supraspinatus tendons compared with reference subscapularis tendons.

CONCLUSIONS

Partial-thickness tears of the supraspinatus tendon demonstrated an increased density of apoptotic, p53+ tendon cells. The fact that apoptosis was accompanied by increased tendon cell proliferation suggests that apoptosis may be related to an ongoing injury-repair process. Increased tenocyte apoptosis may be a relatively early feature in rotator cuff tendinopathy and could represent a possible target for therapeutic intervention.

B2A, a receptor modulator, increases the growth of pluripotent and preosteoblast cells through bone morphogenetic protein receptors

B2A (B2A2-K-NS) is a synthetic, multidomain peptide which is being developed to augment spinal fusion and bone repair locally. Using pluripotent mesenchymal cells of murine and human origin, we show that B2A-induced cell proliferation in a modest but dose-dependent manner. However, essentially all human tumor lines tested were not responsive or were weakly responsive to B2A. B2A treatment activated extracellular signal-regulated kinases 1 and 2 (ERK1/2), and the proliferation was partially blocked by an mitogen-activated protein kinase (MEK) inhibitor. The bone morphogenetic protein (BMP) type I receptor kinase inhibitors depressed B2A-induced proliferation. Upregulation of bone morphogenetic protein 2 was not involved, as noggin, DAN, or chordin did not block B2A-induced proliferation. These data suggest that B2A-induced proliferation results from cell-type-specific activation of bone morphogenetic protein receptor, which, in turn, regulates ERK1/2 activity. B2A-induced proliferation, acting through ERK1/2, is a phenomenon that, while not strictly related to the ability of B2A to augment BMP-induced differentiation via the small mothers against decapentaplegic pathway, may ultimately contribute to bone repair in vivo.

Salubrinal promotes healing of surgical wounds in rat femurs

Phosphorylation of eukaryotic initiation factor 2α (eIF2α), transiently activated by various cellular stresses, is known to alleviate stress-induced cellular damage. Here, we addressed a question: does elevation of eIF2α phosphorylation by salubrinal (a pharmacological inhibitor of eIF2α dephosphorylation) enhance healing of bone wounds? We hypothesized that salubrinal would accelerate a closure of surgically generated bone holes by modifying expression of stress-sensitive genes. To examine this hypothesis, we employed a rat wound model. Surgical wounds were generated on anterior and posterior femoral cortexes, and salubrinal was locally administered on the anterior side. The results showed that, compared to a contralateral control, the size of surgical wounds was reduced by 10.8 % (day 10) and 18.0 % (day 20) on the anterior side (both p < 0.001), and 4.1 % (day 10; p < 0.05) and 11.1 % (day 20; p < 0.001) on the posterior side. In addition, salubrinal locally elevated cortical thickness and increased BMD and BMC. Pharmacokinetic analysis revealed that subcutaneous injection of salubrinal transiently increased its concentration in plasma followed by a rapid decrease within 24 h, and its half-life in plasma was 1.2 h. Salubrinal altered the phosphorylation level of eIF2α as well as the mRNA levels of ATF3, ATF4, and CHOP, and suppressed cell death induced by stress to the endoplasmic reticulum. In summary, the results herein demonstrate that subcutaneous administration of salubrinal accelerates healing of surgically generated bone holes through the modulation of eIF2α phosphorylation.

BMP4 localization and PCNA expression during distraction osteogenesis of the porcine mandible

This study characterized sequential molecular and cellular events in the porcine mandibular distraction osteogenesis (DO) wound. Nineteen Yucatan minipigs were divided into three treatment groups: Group A, unilateral mandibular distraction with 0 day latency, 1mm/day rate for 12 days, 24 days fixation (n=16); Group B, acute lengthening 12 mm (n=2); Group C, sham control (n=1). Group A was further divided by death date: mid-DO (n=5), end-DO (n=4), mid-fixation (n=5) and end-fixation (n=2). Groups B and C were killed on postoperative day 36, corresponding to end-fixation. Specimens were stained for proliferating cell nuclear antigen (PCNA) and bone morphogenetic protein-4 (BMP4). Cellular proliferation (PCNA) was assessed quantitatively and BMP4 staining was assessed on a semi-quantitative scale. Progenitor cell proliferation was greatest during mid-DO and decreased from end-DO through end-fixation. Proliferation in the acute lengthening group was elevated relative to sham control and comparable to end-DO. BMP4 staining intensity (localized to the periosteal cambium layer) was greatest during mid- and end-DO, decreased at mid-fixation and was undetectable at end-fixation. Progenitor cell proliferation and BMP4 expression are greatest during mid-DO and decrease progressively thereafter. At the time of death of the acute lengthening group, only increased cell proliferation was demonstrated.

Endogenous tissue engineering: PTH therapy for skeletal repair

Based on its proven anabolic effects on bone in osteoporosis patients, recombinant parathyroid hormone (PTH(1-34)) has been evaluated as a potential therapy for skeletal repair. In animals, the effect of PTH(1-34) has been investigated in various skeletal repair models such as fractures, allografting, spinal arthrodesis and distraction osteogenesis. These studies have demonstrated that intermittent PTH(1-34) treatment enhances and accelerates the skeletal repair process via a number of mechanisms, which include effects on mesenchymal stem cells, angiogenesis, chondrogenesis, bone formation and resorption. Furthermore, PTH(1-34) has been shown to enhance bone repair in challenged animal models of aging, inflammatory arthritis and glucocorticoid-induced bone loss. This pre-clinical success has led to off-label clinical use and a number of case reports documenting PTH(1-34) treatment of delayed-unions and non-unions have been published. Although a recently completed phase 2 clinical trial of PTH(1-34) treatment of patients with radius fracture has failed to achieve its primary outcome, largely because of effective healing in the placebo group, several secondary outcomes are statistically significant, highlighting important issues concerning the appropriate patient population for PTH(1-34) therapy in skeletal repair. Here, we review our current knowledge of the effects of PTH(1-34) therapy for bone healing, enumerate several critical unresolved issues (e.g., appropriate dosing regimen and indications) and discuss the long-term potential of this drug as an adjuvant for endogenous tissue engineering.

Tenocyte apoptosis in the torn rotator cuff: a primary or secondary pathological event?

Little information exists on the contribution of apoptosis to pathological tendon changes in rotator cuff tendinopathy. The purpose of this study was to quantitate the rate of tenocyte apoptosis in torn supraspinatus tendons and in the matched intact subscapularis and to examine the potential relation between apoptotic index (AI) and tendon pathology. In addition, the authors examined tenocyte density, proliferation rate and p53 gene expression patterns to gain further insight into relevant pathological mechanisms in the torn suprapinatus. 15 torn supraspinatus tendons with matched intact subscapularis tendon samples and 10 reference subscapularis samples were collected. Immunohistochemistry was used to define the AI (F7-26), proliferation rate (Ki67) and presence of p53 (M7001). Tendon degeneration was evaluated according to the Bonar scale. Expression of p53 and relevant genes (n=84) was examined on a subset of samples using microfluidic arrays. The AI was significantly increased in torn supraspinatus tendon and matched subscapularis tendon (R² =0.5742; p=0.0005). Cell density and proliferation rate were also elevated in torn supraspinatus compared with reference subscapularis tendons (p<0.05). A significant increase in p53 occurred specifically in torn supraspinatus tendon (p<0.05), and several genes encoding p53-inhibiting proteins were downregulated in association, including HDAC1 (p<0.05), MDM4 (p<0.001) and PPM1D (p<0.05). Our results suggest that tenocyte apoptosis results from more than one mechanism in the injured rotator cuff, including both intrinsic factors related specifically to the torn supraspinatus tendon, as well as a more generalised effect on the adjacent subscapularis tendon.

Accelerated cartilage resorption by chondroclasts during bone fracture healing in osteoprotegerin-deficient mice

Receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG), a decoy receptor of RANKL, maintain bone mass by regulating the differentiation of osteoclasts, which are bone-resorbing cells. Endochondral bone ossification and bone fracture healing involve cartilage resorption, a less well-understood process that is needed for replacement of cartilage by bone. Here we describe the role of OPG produced by chondrocytes in chondroclastogenesis. Fracture healing in OPG(-/-) mice showed faster union of the fractured bone, faster resorption of the cartilaginous callus, and an increased number of chondroclasts at the chondroosseous junctions compared with that in wild-type littermates. When a cultured pellet of OPG(-/-) chondrocytes was transplanted beneath the kidney capsule, the pellet recruited many chondroclasts. The pellet showed the ability to induce tartrate-resistant acid phosphatase-positive multinucleated cells from RAW 264.7 cells in vitro. Finally, OPG(-/-) chondrocytes (but not wild-type chondrocytes) cultured with spleen cells induced many tartrate-resistant acid phosphatase-positive multinucleated cells. The expression of RANKL and OPG in chondrocytes was regulated by several osteotropic factors including 1,25-dihydroxyvitamin D(3), PTHrP, IL-1alpha, and TNF-alpha. Thus, local OPG produced by chondrocytes probably controls cartilage resorption as a negative regulator for chondrocyte-dependent chondroclastogenesis.

Thrombospondin-2 influences the proportion of cartilage and bone during fracture healing

Thrombospondin-2 (TSP2) is a matricellular protein with increased expression during growth and regeneration. TSP2-null mice show accelerated dermal wound healing and enhanced bone formation. We hypothesized that bone regeneration would be enhanced in the absence of TSP2. Closed, semistabilized transverse fractures were created in the tibias of wildtype (WT) and TSP2-null mice. The fractures were examined 5, 10, and 20 days after fracture using microCT, histology, immunohistochemistry, quantitative RT-PCR, and torsional mechanical testing. Ten days after fracture, TSP2-null mice showed 30% more bone by microCT and 40% less cartilage by histology. Twenty days after fracture, TSP2-null mice showed reduced bone volume fraction and BMD. Mice were examined 5 days after fracture during the stage of neovascularization and mesenchymal cell influx to determine a cellular explanation for the phenotype. TSP2-null mice showed increased cell proliferation with no difference in apoptosis in the highly cellular fracture callus. Although mature bone and cartilage is minimal 5 days after fracture, TSP2-null mice had reduced expression of collagen IIa and Sox9 (chondrocyte differentiation markers) but increased expression of osteocalcin and osterix (osteoblast differentiation markers). Importantly, TSP2-null mice had a 2-fold increase in vessel density that corresponded with a reduction in vascular endothelial growth factor (VEGF) and Glut-1 (markers of hypoxia inducible factor [HIF]-regulated transcription). Finally, by expressing TSP2 using adenovirus starting 3 days after fracture, chondrogenesis was restored in TSP2-null mice. We hypothesize that TSP2 expressed by cells in the fracture mesenchyme regulates callus vascularization. The increase in vascularity increases tissue oxemia and decreases HIF; thus, undifferentiated cells in the callus develop into osteoblasts rather than chondrocytes. This leads to an alternative strategy for achieving fracture healing with reduced endochondral ossification and enhanced appositional bone formation. Controlling the ratio of cartilage to bone during fracture healing has important implications for expediting healing or promoting regeneration in nonunions.

Quantifying osteoblast and osteocyte apoptosis: challenges and rewards

Since the initial demonstration of the phenomenon in murine and human bone sections approximately 10 yr ago, appreciation of the biologic significance of osteoblast apoptosis has contributed greatly not only to understanding the regulation of osteoblast number during physiologic bone remodeling, but also the pathogenesis of metabolic bone diseases and the pharmacology of some of the drugs used for their treatment. It is now appreciated that all major regulators of bone metabolism including bone morphogenetic proteins (BMPs), Wnts, other growth factors and cytokines, integrins, estrogens, androgens, glucocorticoids, PTH and PTH-related protein (PTHrP), immobilization, and the oxidative stress associated with aging contribute to the regulation of osteoblast and osteocyte life span by modulating apoptosis. Moreover, osteocyte apoptosis has emerged as an important regulator of remodeling on the bone surface and a critical determinant of bone strength, independently of bone mass. The detection of apoptotic osteoblasts in bone sections remains challenging because apoptosis represents only a tiny fraction of the life span of osteoblasts, not unlike a 6-mo-long terminal illness in the life of a 75-yr-old human. Importantly, the phenomenon is 50 times less common in human bone biopsies because human osteoblasts live longer and are fewer in number. Be that as it may, well-controlled assays of apoptosis can yield accurate and reproducible estimates of the prevalence of the event, particularly in rodents where there is an abundance of osteoblasts for inspection. In this perspective, we focus on the biological significance of the phenomenon for understanding basic bone biology and the pathogenesis and treatment of metabolic bone diseases and discuss limitations of existing techniques for quantifying osteoblast apoptosis in human biopsies and their methodologic pitfalls.

The response of costal cartilage to mechanical injury in mice

BACKGROUND

The healing potential of articular cartilage in response to injury is poor, because articular cartilage lacks blood vessels or perichondrium. Costal cartilage is covered with a vascularized perichondrium, which is known to have chondrogenic potential. The purpose of this study was to determine whether costal cartilage can heal in response to mechanical injury.

METHODS

Sixty-five ICR mice were used. Under anesthesia, the left tenth costal cartilage was dissected using microscissors. At 1 day and at 1, 2, 3, and 12 weeks after injury, the mice were killed and paraffin sections were prepared. Safranin O staining, in situ hybridization of type II collagen, and immunostaining for CD44 were performed. Localization of cell proliferation was performed using immunohistochemistry with bromodeoxyuridine monoclonal antibody. In situ detection of apoptosis (deoxynucleotidyl transferase-mediated dUTP nick end labeling) was performed using an Apop Tag Kit.

RESULTS

From 1 to 2 weeks after costal cartilage injury, bromodeoxyuridine-positive cells were observed in the perichondrium. Two weeks after injury, the dissected cartilage fragments had combined with newly formed safranin O-positive tissue. Type II collagen mRNA was strongly expressed in the cells of the newly formed tissue. Apoptosis was detected in newly formed cartilaginous tissue at 1 and 2 weeks after injury. The cartilage fragments failed to unite at 12 weeks after injury. CD44 immunoreactivity was detected on the surface of the cavity between the cartilage fragments.

CONCLUSION

Although the dissected fragments of costal cartilage can combine with newly formed cartilaginous tissue temporarily, they fail to unite ultimately.

Enhanced bone regeneration associated with decreased apoptosis in mice with partial HIF-1alpha deficiency

HIF-1alpha activates genes under hypoxia and was hypothesized to regulate bone regeneration. Surprisingly, HIF-1alpha+/- fracture calluses are larger, stronger, and stiffer than HIF-1alpha+/+ calluses because of decreased apoptosis. These data identify apoptosis inhibition as a means to enhance bone regeneration.

INTRODUCTION

Bone regeneration subsequent to fracture involves the synergistic activation of multiple signaling pathways. Localized hypoxia after fracture activates hypoxia-inducible factor 1alpha (HIF-1alpha), leading to increased expression of HIF-1 target genes. We therefore hypothesized that HIF-1alpha is a key regulator of bone regeneration.

MATERIALS AND METHODS

Fixed femoral fractures were generated in mice with partial HIF-1alpha deficiency (HIF-1alpha+/-) and wildtype littermates (HIF-1alpha+/+). Fracture calluses and intact contralateral femurs from postfracture days (PFDs) 21 and 28 (N=5-10) were subjected to microCT evaluation and four-point bending to assess morphometric and mechanical properties. Molecular analyses were carried out on PFD 7, 10, and 14 samples (N=3) to determine differential gene expression at both mRNA and protein levels. Finally, TUNEL staining was performed on PFD 14 samples (N=2) to elucidate differential apoptosis.

RESULTS

Surprisingly, fracture calluses from HIF-1alpha+/- mice exhibited greater mineralization and were larger, stronger, and stiffer. Microarray analyses focused on hypoxia-induced genes revealed differential expression (between genotypes) of several genes associated with the apoptotic pathway. Real-time PCR confirmed these results, showing higher expression of proapoptotic protein phosphatase 2a (PP2A) and lower expression of anti-apoptotic B-cell leukemia/lymphoma 2 (BCL2) in HIF-1alpha+/+ calluses. Subsequent TUNEL staining showed that HIF-1alpha+/+ calluses contained larger numbers of TUNEL+ chondrocytes and osteoblasts than HIF-1alpha+/- calluses.

CONCLUSIONS

We conclude that partial HIF-1alpha deficiency results in decreased chondrocytic and osteoblastic apoptosis, thereby allowing the development of larger, stiffer calluses and enhancing bone regeneration. Furthermore, apoptosis inhibition may be a promising target for developing new treatments to accelerate bone regeneration.

Smoking delays chondrogenesis in a mouse model of closed tibial fracture healing

Smoking delays the healing process and increases morbidity associated with many common musculoskeletal disorders, including long bone fracture. In the current study, a murine model of tibial fracture healing was used to test the hypothesis that smoking delays chondrogenesis after fracture. Mice were divided into two groups, a nonsmoking control group and a group exposed to cigarette smoke for 1 month prior to surgical tibial fracture. Mice were euthanized at 7, 14, and 28 days after surgery. The outcomes measured were immunohistochemical staining for type II collagen protein expression as a marker of cartilage matrix and proliferating cell nuclear antigen (PCNA) staining to measure proliferation at the site of injury. Toluidine blue staining and histomorphometry were used to quantify areas of cartilaginous and noncartilaginous fracture callus. Radiographs were analyzed for evidence of remodeling after injury. At day 7 after injury, mice exposed to cigarette smoke had a smaller fracture callus with less cartilage matrix compared to controls. Proliferation was present at high levels in both groups at this time point, but proliferating cells had a more immature morphology in the smoking group. At day 14, chondrogenesis was more active in smokers compared to controls, while a higher percentage of bone was present in the control animals. At day 28, X-ray analysis revealed a larger fracture callus remaining in the smoking animals. Together, these findings show that the chondrogenic phase of tibial fracture healing is delayed by smoking. This study represents, to our knowledge, the first analysis of molecular and cellular mechanisms of healing in a smoking mouse fracture model.

Effects of cigarette smoking on early medial collateral ligament healing in a mouse model

Cigarette smoking delays the healing process and increases morbidity associated with many common musculoskeletal disorders such as medial collateral ligament (MCL) injury. In the current study, a murine model of MCL healing was used to test the hypothesis that smoking impairs extracellular matrix synthesis after injury. Mice were divided into two groups, a nonsmoking control group and a group exposed to smoke for 2 months prior to surgical MCL injury. Mice were euthanized at 3 and 7 days after surgery. Subsequently, propidium iodine staining was used to quantify cellular density of injured and sham ligaments. Immunohistochemical staining and in situ hybridization to mRNA were used to detect proliferation, apoptosis, and type I collagen gene expression at the site of injury. Cell density increased significantly from baseline to 7 days after injury in control mice. In mice exposed to cigarette smoke, there was a significantly lower cellular density compared to controls at this time point (p=0.01). There was no difference in proliferation between groups at the site of injury, and the low level of proliferation observed was not sufficient to account for the large increase in cell density by day 7. No evidence of apoptosis was observed in any of the groups at the site of injury. Type I collagen gene expression was higher in controls compared to smokers at day 7. Almost all of the cells in the substance of the injured MCL at day 7 were spindle-shaped and expressed type I collagen, suggesting that increased cell density from day 3 to day 7 represented an increase in ligament cells rather than an increased inflammatory response. We conclude that the decreased cellular density and type I collagen expression in the injured ligament of mice exposed to smoke begin to provide a cellular and molecular basis for delayed or deficient early healing in these animals.

Osteochondritis dissecans of the elbow: Histopathological assessment of the articular cartilage and subchondral bone with emphasis on their damage and repair

Osteochondritis dissecans (OCD) of the elbow is a localized injury of the articular cartilage and subchondral bone that is commonly seen in the young athlete. In the present study, the extent of damage and repair on the articular cartilage and subchondral bone was examined histologically using specimens of 25 osteochondral cylinders and seven loose bodies obtained from 25 young athletes who had undergone osteochondral autograft surgery. Terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling (TUNEL) assays for detecting apoptotic cells and immunohistochemistry of matrix metalloproteinases (MMP) were performed on the osteochondral cylinder specimens. The histological findings of the OCD of the elbow showed that the articular cartilage exhibited degenerative change, mimicking osteoarthritis, and was markedly damaged as the lesion progressed. TUNEL-positive cells and MMP-3- and -13-expressing cells were distributed in the degenerative articular cartilage and reparative fibrocartilage tissue. Separation occurred at either the deep articular cartilage or the subchondral bone, with the former being dominant in the early OCD lesions. The present results suggest that the primary pathological changes in OCD of the elbow were due to damage of articular cartilage induced by repeated stress following degenerative and reparative process of articular cartilage and subchondral fracturing, and separation subsequently occurred on the cartilage and developed onto the subchondral bone in its advanced stages.

Recombinant human growth/differentiation factor-5 (rhGDF-5) induced bone formation in murine calvariae

OBJECTIVES

Growth/differentiation factor-5 (GDF-5), a member of the transforming growth factor-beta superfamily, shows a close structural relationship to bone morphogenetic proteins and plays crucial roles in skeletal morphogenesis. Recombinant human (rh) GDF-5 was reported as a suitable factor for enhancing healing in bone defect and inducing ectopic bone formation. The purpose of the present study was to investigate the mechanism of bone formation induced by rhGDF-5 in murine calvariae by radiological, histological and immunohistochemical methods. Cell proliferation was also examined in vitro.

MATERIAL AND METHODS

Cells including primary osteoblasts, periosteum cells and connective tissue fibroblasts were isolated enzymatically from neonatal murine calvariae or head skin. In the presence or absence of rhGDF-5, cell proliferation was estimated by tetrazolium reduction assay. To examine the mechanism of osteoinduction, rhGDF-5/atelocollagen (AC) composite or 0.01 N HCl/AC composite were injected into murine calvariae subcutaneously. Tissue was examined radiologically, histologically and immunohistochemically.

RESULTS

In the presence of rhGDF-5, proliferation of primary osteoblasts, periosteum cells, and connective tissue fibroblasts was increased significantly in culture. Immunohistochemical observations showed cells at the site injected with rhGDF-5/AC displayed immunoreactivity for proliferating cell nuclear antigen (PCNA). Newly formed bone- and cartilage-like tissue contained chondrocyte osteocyte and osteoclastic cells, and were immunoreactive for both type I and II collagen.

CONCLUSION

Exposure to GDF-5 promotes proliferation and differentiation of calvarial cells, which give rise to ectopic bone formation.

Programmed cell death in the regenerating deer antler

Antlers are the only mammalian appendages capable of epimorphic regeneration and thus provide a unique model for investigating the mechanisms that underlie mammalian regeneration. Antlers elongate by a modified endochondral ossification process while intramembranous ossification takes place concurrently around the antler shaft. In this study, sites of apoptosis in the growing antler tip were identified by TUNEL staining and related to cell proliferation, as determined by PCNA staining. Bcl-2 and bax were identified by RT-PCR and bax was also immunolocalized in tissue sections. The apoptotic index was high in perichondrium, undifferentiated mesenchymal cells and cellular periosteum but was low in skin. The proliferation index was high in mesenchyme, skin (specifically in hair follicles) and cellular periosteum; it was low in fibrous perichondrium and periosteum, and barely detectable in cartilage. Both bcl-2 and bax were found to be more highly expressed in the perichondrium/mesenchyme and non-mineralized cartilage than in skin and mineralized cartilage. Bax was immunolocalized in mesenchyme cells, chondroprogenitors, chondrocytes, osteoblasts, osteocytes and osteoclasts. In conclusion, this study shows that programmed cell death plays a necessary role in regenerating antlers, as it does during skeletal development, bone growth and bone remodelling. The high level of apoptosis and proliferation in mesenchymal progenitor cells confirms that this represents the antler 'growth zone'. In fact, the percentage of TUNEL-positive cells in the mesenchymal growth zone (up to 64%) is higher than that recorded in any other adult tissue. This extensive cell death probably reflects the phenomenal rate of morphogenesis and tissue remodelling that takes place in a growing antler. The local and/or systemic factors that control the balance between cell growth and apoptosis in antler tissues now need to be determined.

Osteocyte apoptosis and osteoclast presence in chicken radii 0-4 days following osteotomy

Osteocyte apoptosis caused by load-induced microdamage is followed by osteoclastic bone remodeling, and a causal link between apoptosis and repair has been suggested. The objectives of the present study were to use a chick model to examine the incidence of osteocyte apoptosis and the presence of osteoclasts during the first 96 hours following an osteotomy, prior to extensive callus mineralization. Osteotomies were performed on the right radii of 24 chicks at 23-24 days of age. The left radii served as controls. Radii were collected and processed at six time points following surgery (0, 12, 24, 48, 72, and 96 hours). Decalcified bone tissue sections were stained either for apoptosis using a modified TUNEL procedure or for tartrate-resistant acid phosphatase to identify osteoclasts in the intracortical and periosteal envelopes. The percentage of apoptotic osteocytes, as well as osteoclast counts (n/mm or n/mm2) were quantified in four regions (0-1, 1-2, 2-4, and 4-8 mm from the site of the osteotomy; regions 1-4, respectively) in the osteotomized radii and in the same measured areas in the control radii. Data for osteocyte apoptosis and osteoclasts in the control limb were subtracted from the osteotomized limb data to identify differences due to surgical influence. The incidence of osteocyte apoptosis was significantly higher at 12, 24, 48, and 72 hours versus 0 hours following osteotomy, and the response was highest in region 1; however, there was no interaction between time and region. Intracortical osteoclast counts (n/mm2) were elevated after 48 hours, and the response was similar in all regions. The data demonstrate that osteocyte apoptosis occurs within 24 hours in response to an osteotomy and temporally precedes an increase in osteoclast presence. Hence, osteocyte apoptosis may play a role in signaling during the bone healing process.

Time for treating bone fracture using rhBMP-2: a randomised placebo controlled mouse fracture trial

Although the mechanisms of osteoinduction by bone morphogenic proteins (BMPs) are increasingly understood, the most appropriate time to administer BMPs exogenously is yet to be clarified. The purpose of this study was to investigate when BMP may be administered to a fracture arena to maximise the enhancement of healing. Forty mice with externally fixed left femoral fractures were randomised into four groups: Group I, the control group was given a placebo of 30 microl saline at day 0; Groups II, III and IV were given 30 microl saline plus 2.5 microg rhBMP-2, at post-operative days 0, 4 or 8, respectively. Sequential radiographs were taken at days 0, 8, 16. On day 22 the mice were sacrificed and both femora were harvested for biomechanical assessment in 3-point bending and histological evaluation. Radiographic analysis indicated that healing of fractures in Groups II and III was significantly greater (p < 0.05) than those in Groups I and IV, at both 16 and 22 days post-fracture. The highest median bone mineral content at the fracture site was evidenced in Group III and II. Furthermore, Group III also had the highest relative ultimate load values, followed by Groups II, IV and I. Greater percentage peak loads were observed between Group I and both Groups II and III (p < 0.05). Histological examination confirmed that at 22 days post-fracture, only fractures in Groups II and III had united with woven bone, and Groups I and IV still had considerable amounts of fibrous tissue and cartilage at the fracture gap. Data presented herein indicates that there is a time after fracture when rhBMP administration is most effective, and this may be at the time of surgery as well as in the early fracture healing phases.

The role of protease-activated receptor-1 in bone healing

Protease-activated receptor (PAR)-1, a G-protein-coupled receptor activated by thrombin, mediates thrombin-induced proliferation of osteoblasts. The current study was undertaken to define the role of PAR-1 in bone repair. Holes were drilled transversely through the diaphysis of both tibiae of PAR-1-null and wild-type mice. Three days later, fewer cells had invaded the drill site from adjacent bone marrow in PAR-1-null mice than in wild-type mice, and a lower percentage of cells were labeled with [(3)H]thymidine in PAR-1-null drill sites. More osteoclasts were also observed in the drill site of PAR-1-null mice than in wild-type mice 7 days after drilling. New mineralized bone area was less in the drill site and on the adjacent periosteal surface in PAR-1-null mice than in wild-type mice at day 9. From day 14, no obvious differences could be seen between PAR-1-null and wild-type tibiae. In vitro thrombin caused a dose-dependent increase in proliferation of bone marrow stromal cells isolated from wild-type mice but not PAR-1-null mice. Thrombin stimulated survival of bone marrow stromal cells from both wild-type and PAR-1-null mice, but it did not affect bone marrow stromal cell migration in either wild-type or PAR-1-null cells. The results indicate that PAR-1 plays an early role in bone repair.

Effects of pleiotrophin (PTN) over-expression on mouse long bone development, fracture healing and bone repair

Pleiotrophin (PTN) was found to have potent effects on regulation of osteoblast recruitment, proliferation and differentiation. The present study examined the long-term effects of targeted PTN over-expression on bone development and repair in a transgenic mouse model. Femurs and tibiae from the PTN transgenic mice and the wild type mice at age 1, 2, 4, 6, 12 and 24 months were collected, and examined by radiography, peripheral quantitative computed tomography (pQCT), histology and mechanical testing. Age-matched PTN and the control mice received a standardized femoral fracture, followed by regular x-rays and sacrificed at day 16 post-fracture for histology examination. A cortical hole was drilled on the tibiae of age-matched PTN and wild type mice, collagen sponge with either saline, 100 ng of rhBMP-2 or rhPTN was implanted in the holes, and animals were sacrificed 10 days later, subject to pQCT and histology examinations. During early stages of bone development, the PTN mice had advanced bone growth in length and maturation, but the difference diminished in later life. The fracture healing was impaired in the PTN mice, and there was delayed callus formation and remodelling. The cortical holes treated with BMP-2 in the PTN mice had significantly less trabecular bone formation. The current study confirmed that the targeted PTN over-expression in mouse bone has moderate enhancing effects on early bone development; but the bones become brittle in later life. Fracture healing was impaired in the adult PTN mice and this may be due to inhibitory effects of PTN over-expression on BMP-2 mediated bone induction.

Characterization of a closed femur fracture model in mice

OBJECTIVES

The goal of this study was to develop and characterize a closed femur fracture model for mice that can be used for the molecular and genetic analysis of fracture healing.

STUDY DESIGN

Longitudinal time study of species-specific fracture healing.

METHODS

A protocol was developed for creating reproducible, closed femur fractures in mice. Impending fractures were stabilized by retrograde insertion of a 0.01-inch-diameter, stainless steel wire into the intramedullary canal. The intramedullary wire was held in place with a wedge made from the first 2 mm of a 30-gauge needle. Fractures were produced by 3-point bending. Fracture healing was assessed by radiography, histology, and torsional mechanical testing.

RESULTS

The mouse femur fracture technique produced good results with minimal loss of animals. Of the 246 mice used in the study, 22 mice were excluded due to poor fracture quality (8), loss of fracture stabilization (6), or to anesthesia death (8). Radiography showed a consistent pattern of fracture healing between mice with peak fracture callus volume evident at 10 (15 mice) to 14 days (18 mice) after fracture. Fracture bridging was apparent in all 3-week postfracture radiographs (35 mice). Histologic examination of 117 specimens at 9 time points showed chondrocyte differentiation within the fracture callus by 7 days after fracture, endochondral ossification occurring by 10 days after fracture, and bone remodeling evident as early as 3 weeks after fracture. Despite radiologic and histologic evidence of fracture bridging after 3 weeks, torsional mechanical testing of 68 mice at 3, 4, 6, and 12 weeks after fracture (group size of 15 to 18 mice at each time point) indicated that significant increases in structural or material strength did not occur until 6 to 12 weeks after fracture.

CONCLUSIONS

Femur fracture healing in mice follows a typical endochondral ossification pathway with fracture bridging occurring approximately 1 week faster in mice than rats. This fracture model is amenable to the molecular and genetic analysis of fracture healing using different inbred, transgenic, and knockout strains of mice.

Time- and Concentration-Dependent Effects of Dissolution Products of 58S Sol–Gel Bioactive Glass on Proliferation and Differentiation of Murine and Human Osteoblasts

Bone loss is a significant clinical problem, and treatments utilizing donated graft material are limited. To meet future demands in the healthcare industry, there has been a shift of outlook toward the use of bioactive materials for tissue regeneration. A number of in vivo and in vitro studies have highlighted the potential of the bioactive glass ceramic 45S5 Bioglass as a synthetic regenerative scaffold. The application of sol-gel processing techniques has led to the synthesis of mesoporous bioactive glasses with greater textural and compositional variety. In this study, we evaluated the effects of supplemented tissue culture medium containing up to 203 ppm silica prepared by static soaking of particles of 58S sol-gel bioactive glass (58% SiO(2), 33% CaO, 9% P(2)O(5)) on the in vitro proliferation and differentiation of murine and human primary osteoblasts. These extracts had a higher silica content than those used previously in studies of 45S5 Bioglass, because of the faster rates of ion exchange permitted by the higher surface area-to-volume ratio of mesoporous glass. We found that osteoblasts from both species increased their proliferation in response to the glass-conditioned medium. In addition, the extent to which supplemented medium could alter cell differentiation varied with time in culture. Proliferation induced by supplemented medium paralleled effects induced by treatment with basic fibroblast growth factor, a known mitogenic growth factor for osteoblasts. Bone nodule formation was also increased by exposure to the glass-conditioned medium and this effect was positively correlated with the dose of glass used to prepare the medium. Apoptosis was stimulated by glass-conditioned medium in murine osteoblasts, but inhibited in human osteoblasts. These data demonstrate the bioactive effects of dissolution products derived from sol-gel materials on primary osteoblasts and complements in vivo studies that indicate the suitability of this material as a bone graft substitute.

Recruitment of mesenchymal stem cells and expression of TGF-beta 1 and VEGF in the early stage of shock wave-promoted bone regeneration of segmental defect in rats

Extracorporeal shock wave (ESW) treatment has recently been established as a method to enhance bone repair. Here, we reported that ESW-promoted healing of segmental defect via stimulation of mesenchymal stem cell recruitment and differentiation into bone forming cells. Rats with a segmental femoral defect were exposed to a single ESW treatment (0.16 mJ/mm(2), 1 Hz, 500 impulses). Cell morphology and histological changes in the defect region were assessed 3, 7, 14, and 28 days post-treatment. Presence of mesenchymal stem cell was assayed by immuno-staining for RP59, a recently discovered marker, and also production of TGF-beta 1 and VEGF was monitored. ESW treatment increased total cell density and the proportion of RP59 positive cells in the defect region. High numbers of round- and cuboidal-shaped cells strongly expressing RP59 were initially found. Later, the predominant cell type was spindle-shaped fibroblastic cells, subsequently, aggregates of osteogenic and chondrogenic cells were observed. Histological observation suggested that bone marrow stem cells were progressively differentiated into osteoblasts and chondrocytes. RP59 staining was initially intense and decreased with the appearance of expression depended on the differentiation states of osteogenic and chondrogenic cells during the regeneration phase. Mature chondrocytes and osteoblasts exhibited only slight RP59 immuno-reactivity. Expression of TGF-beta 1 and VEGF-A mRNA in the defect tissues was also significantly increased (P<0.05) after ESW treatment as determined by RT-PCR. Intensive TGF-beta 1 immuno-reactivity was induced immediately, whereas a lag period was observed for VEGF-A. Chondrocytes and osteoblasts at the junction of ossified cartilage clearly exhibited VEGF-A expression. Our findings suggest that recruitment of meseoblasts at the junction of ossified cartilage clearly exhibited mesenchymal stem cells is a critical step in bone reparation that is enhanced by ESW treatment. TGF-beta 1 and VEGF-A are proposed to play a chemotactic and mitogenic role in recruitment and differentiation of mesenchymal stem cells.

Shock wave application enhances pertussis toxin protein-sensitive bone formation of segmental femoral defect in rats

Extracorporeal shock waves (ESWs) elicit a dose-dependent effect on the healing of segmental femoral defects in rats. After ESW treatment, the segmental defect underwent progressive mesenchymal aggregation, endochondral ossification, and hard callus formation. Along with the intensive bone formation, there was a persistent increase in TGF-beta1 and BMP-2 expression. Pretreatment with pertussis toxin reduced ESW-promoted callus formation and gap healing, which presumably suggests that Gi proteins mediate osteogenic signaling.

INTRODUCTION

Extracorporeal shock waves (ESWs) have previously been used to promote bone repair. In our previous report, we found that ESWs promoted osteogenic differentiation of mesenchymal cells through membrane perturbation and activation of Ras protein. In this report, we show that ESWs elicit a dose-dependent effect on the healing of segmental defects and that Gi proteins play an important role in mediating ESW stimulation.

MATERIALS AND METHODS

Rats with segmental femoral defects were subjected to ESW treatment at different energy flux densities (EFD) and impulses. Bone mass (mineral density and calcium content), osteogenic activities (bone alkaline phosphatase activity and osteocalcin content), and immunohistochemistry were assessed.

RESULTS

An optimal ESW energy (500 impulses at 0.16 mJ/mm2 EFD) stimulated complete bone healing without complications. ESW-augmented healing was characterized by significant increases (p < 0.01) in callus size, bone mineral density, and bone tissue formation. With exposure to ESW, alkaline phosphatase activity and osteocalcin production in calluses were found to be significantly enhanced (p < 0.05). After ESW treatment, the histological changes we noted included progressive mesenchymal aggregation, endochondral ossification, and hard callus formation. Intensive bone formation was associated with a persistent increase in transforming growth factor-beta 1 (TGF-beta1) and bone morphogenetic protein-2 (BMP-2) expression, suggesting both growth factors were active in ESW-promoted bone formation. We also found that pertussis toxin, an inhibitor of membrane-bound Gi proteins, significantly reduced (p < 0.01) ESW promotion of callus formation and fracture healing.

CONCLUSION

ESW treatments enhanced bone formation and the healing of segmental femoral defects in rats. It also seems likely that TGF-beta1 and BMP-2 are important osteogenic factors for ESW promotion of fracture healing, presumably through Gi protein-mediated osteogenic signaling.

Expression of fibroblast growth factor receptor-3 (FGFR3), signal transducer and activator of transcription-1, and cyclin-dependent kinase inhibitor p21 during endochondral ossification: differential role of FGFR3 in skeletal development and fracture repair

Increasing evidence suggests that fibroblast growth factor receptor-3 (FGFR3) is a negative regulator of endochondral bone growth; however, its role during skeletal repair is unknown. Using a rat model of closed femoral fracture healing, we analyzed the spatial and temporal expression of FGFR3. To assess a possible role for FGFR3 during healing, we also analyzed the spatial and temporal expression of signal transducer and activator of transcription-1 (STAT1) and cyclin-dependent kinase inhibitor p21, important mediators of FGFR3 signaling. Before these experiments, we studied the spatial expression of FGFR3 during skeletal development in mouse embryos. At 16.5 and 19.5 d post coitum, FGFR3 mRNA was strongly expressed in resting and proliferating chondrocytes but weakly in hypertrophic chondrocytes and not in osteoblasts. In contrast, during fracture repair, it was strongly expressed in prehypertrophic chondrocytes, and the expression level reached a maximum on d 14. Immunoreactivity for STAT1 was detected in the cytoplasm of chondrocytes on d 4 and 7 and both in the cytoplasm and nucleus of hypertrophic chondrocytes on d 14. Furthermore, FGFR3, STAT1, and p21 exhibited a similar temporal expression profile, suggesting that FGFR3-mediated STAT1-p21 signaling plays a role in fracture repair. These results indicate a differential role of FGFR3 in skeletal development and fracture repair.

A reliable externally fixated murine femoral fracture model that accounts for variation in movement between animals

Fifty-two CFLP mice had an open femoral diaphyseal osteotomy held in compression by a four-pin external fixator. The movement of 34 of the mice in their cages was quantified before and after operation, until sacrifice at 4, 8, 16 or 24 days. Thirty-three specimens underwent histomorphometric analysis and 19 specimens underwent torsional stiffness measurement. The expected combination of intramembranous and endochondral bone formation was observed, and the model was shown to be reliable in that variation in the histological parameters of healing was small between animals at the same time point, compared to the variation between time-points. There was surprisingly large individual variation in the amount of animal movement about the cage, which correlated with both histomorphometric and mechanical measures of healing. Animals that moved more had larger external calluses containing more cartilage and demonstrated lower torsional stiffness at the same time point. Assuming that movement of the whole animal predicts, at least to some extent, movement at the fracture site, this correlation is what would be expected in a model that involves similar processes to those in human fracture healing. Models such as this, employed to determine the effect of experimental interventions, will yield more information if the natural variation in animal motion is measured and included in the analysis.

Rapid new bone tissue remodeling during distraction osteogenesis is associated with apoptosis

During the process of distraction osteogenesis new bone forms and undergoes rapid remodeling. Apoptosis may be one of the regulatory mechanisms governing the removal of the redundant callus during distraction osteogenesis. A rabbit tibial lengthening model was used and lengthened at 0.7 mm/day for 3 weeks. The regenerating tissues from the distraction gap were examined for apoptotic changes by transmission electron microscopy (TEM) and the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling (TUNEL) method. Osteoclastic bone resorption activities were demonstrated by tartrate resistant acid phosphatase (TRAP) staining. The apoptotic cells were mainly present in the transitional regions between the fibrous tissue and the new bone in the mineralization front, and close to or on the new bone surfaces near the center of the regenerate. The TUNEL labeling was greatly reduced in the mature bone near the osteotomied bone ends. TEM examination confirmed the presence of cells with apoptotic changes at various regions of the regenerate. TRAP staining revealed that osteoclastic bone resorption activities in the regenerate were in a similar pattern of distribution to those of the TUNEL labeling. The localization of apoptotic cells at the different regions of the regenerate, accompanied by the osteoclast activities, suggest that apoptosis is closely related to bone formation and remodeling during distraction osteogenesis.

Synergistic enhancement of bone formation and healing by stem cell-expressed VEGF and bone morphogenetic protein-4

We investigated the interaction between angiogenic and osteogenic factors in bone formation and bone healing with ex vivo gene therapy using muscle-derived stem cells genetically engineered to express human bone morphogenetic protein-4 (BMP4), VEGF, or VEGF-specific antagonist (soluble Flt1). Our results show that although VEGF alone did not improve bone regeneration, it acted synergistically with BMP4 to increase recruitment of mesenchymal stem cells, to enhance cell survival, and to augment cartilage formation in the early stages of endochondral bone formation. These early effects, coupled with accelerated cartilage resorption, eventually led to a significant enhancement of bone formation and bone healing. The beneficial effect of VEGF on bone healing elicited by BMP4 depends critically on the ratio of VEGF to BMP4, with an improper ratio leading to detrimental effects on bone healing. Finally, we show that soluble Flt1 inhibits bone formation elicited by BMP4. Thus, VEGF plays an important role in bone formation elicited by BMP4, and it can significantly enhance BMP4-elicited bone formation and regeneration through multiple mechanisms. This study has important implications for the formulation of new strategies to improve bone healing through increasing mesenchymal stem cell recruitment and survival, in combination with muscle-derived stem cell-based gene therapy.