Genetic expression of extracellular matrix proteins correlates with histologic changes during fracture repair

Comparison Between Clinically Available Low-Intensity Pulsed Ultrasound (LIPUS) and a Novel Bimodal Acoustic Signal System for Accelerating Fracture Healing

Low-intensity pulsed ultrasound (LIPUS) accelerates fracture healing by stimulating the production of bone callus and the mineralization process. This study compared a novel bimodal acoustic signal (BMAS) device for bone fracture healing to a clinical LIPUS system (EXOGEN; Bioventus, Durham, NC, USA). Thirty rabbits underwent a bilateral fibular osteotomy. Each rabbits' legs were randomized to receive 20-min treatment daily for 18 days with BMAS or LIPUS. The latter utilizes a longitudinal ultrasonic mode only, while the former employs ultrasound-induced shear stress to promote bone formation. Power Doppler imaging (PDI) was acquired days 0, 2, 4, 7, 11, 14, and 18 post-surgery to monitor treatment response and quantified off-line. X-rays were acquired to evaluate fractures on days 0, 14, 18, and 21. Seventeen rabbits completed the study and were euthanized day 21 post-surgery. The fibulae were analyzed to determine maximum torque, initial torsional stiffness, and angular displacement at failure. ANOVAs and paired t-tests were used to compare pair-wise outcome variables for the two treatment modes on a per rabbit basis. The BMAS system induced better fracture healing with greater stiffness (BMAS 0.21 ± 0.19 versus LIPUS 0.16 ± 0.19 [Formula: see text]cm/°, p = 0.050 ) and maximum torque (BMAS 7.84 ± 5.55 versus LIPUS 6.26 ± 3.46 [Formula: see text]cm, p = 0.022 ) than the LIPUS system. Quantitative PDI assessments showed a higher amount of vascularity with LIPUS than BMAS on days 4 and 18 ( ). In conclusion, the novel BMAS technique achieved better bone fracture healing response than the current Food and Drug Administration (FDA)-approved LIPUS system.

Site-Specific Fracture Healing: Comparison between Diaphysis and Metaphysis in the Mouse Long Bone

The process of fracture healing varies depending upon internal and external factors, such as the fracture site, mode of injury, and mechanical environment. This review focuses on site-specific fracture healing, particularly diaphyseal and metaphyseal healing in mouse long bones. Diaphyseal fractures heal by forming the periosteal and medullary callus, whereas metaphyseal fractures heal by forming the medullary callus. Bone healing in ovariectomized mice is accompanied by a decrease in the medullary callus formation both in the diaphysis and metaphysis. Administration of estrogen after fracture significantly recovers the decrease in diaphyseal healing but fails to recover the metaphyseal healing. Thus, the two bones show different osteogenic potentials after fracture in ovariectomized mice. This difference may be attributed to the heterogeneity of the skeletal stem cells (SSCs)/osteoblast progenitors of the two bones. The Hox genes that specify the patterning of the mammalian skeleton during embryogenesis are upregulated during the diaphyseal healing. Hox genes positively regulate the differentiation of osteoblasts from SSCs in vitro. During bone grafting, the SSCs in the donor’s bone express Hox with adaptability in the heterologous bone. These novel functions of the Hox genes are discussed herein with reference to the site-specificity of fracture healing.

The Effects of Targeted Deliveries of Lovastatin and Tocotrienol on Ossification-Related Gene Expressions in Fracture Healing in an Osteoporosis Rat Model

Osteoporotic drugs are used to prevent fragility fractures, but their role in fracture healing still remains unknown. Thus, alternative agents with suitable mode of delivery are needed to promote fracture healing. This study was performed to investigate the effects of direct deliveries of lovastatin and tocotrienol to fracture sites on ossification-related gene expression in fracture healing in a postmenopausal osteoporosis model. Forty-eight Sprague Dawley female rats were divided into six groups. Group I comprised the sham-operated rats, while Groups II–VI were ovariectomized rats. After 8 weeks, the right tibiae of all rats were fractured and stabilized. Group I and Group II were given two single injections of lovastatin and tocotrienol carriers. Group III was given an estrogen preparation at 64.5 µg/kg daily via oral gavages. Group IV was injected with lovastatin particles (750 µg/kg), while Group V was injected with tocotrienol particles (60 mg/kg). Group VI received two single injections of 750 µg/kg lovastatin particles and 60 mg/kg tocotrienol particles. After 4 weeks, the gene expressions were measured. Group VI showed significantly higher gene expressions of osteocalcin, BMP-2, VEGF-α, and RUNX-2 compared to Group II. In conclusion, combined treatment of lovastatin and tocotrienol upregulated the expression of genes related to fracture healing.

(99m)TC-Methylene diphosphonate uptake at injury site correlates with osteoblast differentiation and mineralization during bone healing in mice

^99mTc-Methylene diphosphonate (^99mTc-MDP) is widely used in clinical settings to detect bone abnormalities. However, the mechanism of ^99mTc-MDP uptake in bone is not well elucidated. In this study, we utilized a mouse tibia injury model, single-photon emission computed tomography (gamma scintigraphy or SPECT), ex vivo micro-computed tomography, and histology to monitor ^99mTc-MDP uptake in injury sites during skeletal healing. In an ex vivo culture system, calvarial cells were differentiated into osteoblasts with osteogenic medium, pulsed with ^99mTc-MDP at different time points, and quantitated for ^99mTc-MDP uptake with a gamma counter. We demonstrated that ^99mTc-MDP uptake in the injury sites corresponded to osteoblast generation in those sites throughout the healing process. The ^99mTc-MDP uptake within the injury sites peaked on day 7 post-injury, while the injury sites were occupied by mature osteoblasts also starting from day 7. ^99mTc-MDP uptake started to decrease 14 days post-surgery, when we observed the highest level of bony tissue in the injury sites. We also found that ^99mTc-MDP uptake was associated with osteoblast maturation and mineralization in vitro. This study provides direct and biological evidence for ^99mTc-MDP uptake in osteoblasts during bone healing in vivo and in vitro.

Local release of magnesium from mesoporous TiO2 coatings stimulates the peri-implant expression of osteogenic markers and improves osteoconductivity in vivo

Local release of Mg ions from titanium implant surfaces has been shown to enhance implant retention and integration. To clarify the biological events that lead to this positive outcome, threaded implants coated with mesoporous TiO2 thin films were loaded with Mg-ions and placed in the tibia of rabbits for 3weeks, after surface characterization. Non-loaded mesoporous coated implants were used as controls. Peri-implant gene expression of a set of osteogenic and inflammatory assays was quantified by means of real-time quantitative polymerase chain reaction. The expression of three osteogenic markers (OC, RUNX-2 and IGF-1) was significantly more pronounced in the test specimens, suggesting that the release of Mg ions directly at the implant sites may stimulate an osteogenic environment. Furthermore, bone healing around implants was evaluated on histological slides and by diffraction-enhanced imaging (DEI), using synchrotron radiation. The histological analysis demonstrated new bone formation around all implants, without negative responses, with a significant increase in the number of threads filled with new bone for test surfaces. DEI analysis attested the high mineral content of the newly formed bone. Improved surface osteoconductivity and increased expression of genes involved in the bone regeneration were found for magnesium-incorporation of mesoporous TiO2 coatings.

The role of bone marrow-derived cells during the bone healing process in the GFP mouse bone marrow transplantation model

Bone healing is a complex and multistep process in which the origin of the cells participating in bone repair is still unknown. The involvement of bone marrow-derived cells in tissue repair has been the subject of recent studies. In the present study, bone marrow-derived cells in bone healing were traced using the GFP bone marrow transplantation model. Bone marrow cells from C57BL/6-Tg (CAG-EGFP) were transplanted into C57BL/6 J wild mice. After transplantation, bone injury was created using a 1.0-mm drill. Bone healing was histologically assessed at 3, 7, 14, and 28 postoperative days. Immunohistochemistry for GFP; double-fluorescent immunohistochemistry for GFP-F4/80, GFP-CD34, and GFP-osteocalcin; and double-staining for GFP and tartrate-resistant acid phosphatase were performed. Bone marrow transplantation successfully replaced the hematopoietic cells into GFP-positive donor cells. Immunohistochemical analyses revealed that osteoblasts or osteocytes in the repair stage were GFP-negative, whereas osteoclasts in the repair and remodeling stages and hematopoietic cells were GFP-positive. The results indicated that bone marrow-derived cells might not differentiate into osteoblasts. The role of bone marrow-derived cells might be limited to adjustment of the microenvironment by differentiating into inflammatory cells, osteoclasts, or endothelial cells in immature blood vessels.

Alcohol induced epigenetic perturbations during the inflammatory stage of fracture healing

It is well recognized by orthopedic surgeons that fractures of alcoholics are more difficult to heal successfully and have a higher incidence of non-union, but the mechanism of alcohol's effect on fracture healing is unknown. In order to give direction for the study of the effects of alcohol on fracture healing, we propose to identify gene expression and microRNA changes during the early stages of fracture healing that might be attributable to alcohol consumption. As the inflammatory stage appears to be the most critical for successful fracture healing, this paper focuses on the events at day three following fracture or the stage of inflammation. Sprague-Dawley rats were placed on an ethanol-containing or pair-fed Lieber and DeCarli diet for four weeks prior to surgical fracture. Following insertion of a medullary pin, a closed mid-diaphyseal fracture was induced using a Bonnarens and Einhorn fracture device. At three days' post-fracture, the region of the fracture calluses was harvested from the right hind-limb. RNA was extracted and microarray analysis was conducted against the entire rat genome. There were 35 genes that demonstrated significant increased expression due to alcohol consumption and 20 that decreased due to alcohol. In addition, the expression of 20 microRNAs was increased and six decreased. In summary, while it is recognized that mRNA levels may or may not represent protein levels successfully produced by the cell, these studies reveal changes in gene expression that support the hypothesis that alcohol consumption affects events involved with inflammation. MicroRNAs are known to modulate mRNA and these findings were consistent with much of what was seen with mRNA microarray analysis, especially the involvement of smad4 which was demonstrated by mRNA microarray, microRNA and polymerase chain reaction.

Effects of ultrasound on osteotomy healing in a rabbit fracture model

This study investigated the effects of ultrasound (US) at different frequencies on fracture healing over a three-week period in a rabbit fibular fracture model. Forty-five adult New Zealand White rabbits were divided into five groups: a control group and four groups treated with US frequencies of 0.5, 1.0, 1.5 and 2.0 MHz (0.5 W/cm(2), 200-μs burst, pulsed 1:4). After anesthesia, transverse osteotomy was performed on the fibula bone. This was followed by intravital staining and fluorescence microscopic examination of new bone formation and biomechanical tests of torsional stiffness at the osteotomy site. Results showed that total new bone formation and torsional stiffness of the fibula were greater in all US-treated groups than in the control group. No significant difference was found between any of the four US-treated groups. The US treatment also enhanced bone growth of the sham-treated contralateral fracture site. These results suggest that US treatment at 0.5, 1.0, 1.5 or 2.0 MHz can enhance fracture healing in a rabbit model. Furthermore, the effects of US on fracture healing at present parameters might not be confined locally.

Temporal and spatial expression of osteoactivin during fracture repair

We previously identified osteoactivin (OA) as a novel secreted osteogenic factor with high expression in developing long bones and calvaria, and that stimulates osteoblast differentiation and matrix mineralization in vitro. In this study, we report on OA mRNA and protein expression in intact long bone and growth plate, and in fracture calluses collected at several time points up to 21 days post-fracture (PF). OA mRNA and protein were highly expressed in osteoblasts localized in the metaphysis of intact tibia, and in hypertrophic chondrocytes localized in growth plate, findings assessed by in situ hybridization and immunohistochemistry, respectively. Using a rat fracture model, Northern blot analysis showed that expression of OA mRNA was significantly higher in day-3 and day-10 PF calluses than in intact rat femurs. Using in situ hybridization, we examined OA mRNA expression during fracture healing and found that OA was temporally regulated, with positive signals seen as early as day-3 PF, reaching a maximal intensity at day-10 PF, and finally declining at day-21 PF. At day-5 PF, which correlates with chondrogenesis, OA mRNA levels were significantly higher in the soft callus than in intact femurs. Similarly, we detected high OA protein immunoexpression throughout the reparative phase of the hard callus compared to intact femurs. Interestingly, the secreted OA protein was also detected within the newly made cartilage matrix and osteoid tissue. Taken together, these results suggest the possibility that OA plays an important role in bone formation and serves as a positive regulator of fracture healing.

Noninvasive imaging of bone-specific collagen I expression in a luciferase transgenic mouse model

Luciferase transgenic mice are a very promising tool for noninvasive, quantitative, and longitudinal evaluation of gene expression. The aim of this study was to validate the Col(I)-Luc transgenic mouse model in which the luciferase gene is driven by bone-specific regulatory elements from the mouse collagen α1(I) gene for bioluminescent imaging of bone development and remodeling. We observed strong luciferase activity in skeletal tissues of Col(I)-Luc mice, and observed that the light intensity declined with postnatal bone development. Luciferase activity was enhanced in a tail bone repair model and we were able to monitor the process of ectopic bone formation induced by recombinant human bone morphogenetic protein 2 using bioluminescent imaging. We conclude that Col(I)-Luc transgenic mice can be applied in the field of bone tissue engineering for monitoring bone repair processes and for investigating osteoinductive molecules or scaffolds.

Osteogenic protein-1 overcomes inhibition of fracture healing in the diabetic rat: a pilot study

Type I diabetes mellitus inhibits fracture healing and leads to an increase in complications. As a pilot study, we used a closed fracture model in the diabetic rat to address the question of whether osteogenic protein-1 (OP-1) in a collagen carrier can overcome this inhibition by increasing the area of the newly mineralized callus and femoral torque to failure compared with diabetic animals with fractures treated without OP-1. Diabetes was created in 54 rats by injection of streptozotocin. After 2 weeks, a closed femur fracture was created using a drop-weight impaction device. Each fracture site was immediately opened and treated with or without 25 microg OP-1 in a collagen carrier. Animals were euthanized after 2 or 4 weeks. Fracture healing was assessed by callus area from high-resolution radiographs, callus strength from torsional failure testing, and undecalcified histologic analysis. The area of newly mineralized callus was greater in diabetic animals treated with 25 microg OP-1/carrier compared with diabetic animals with untreated fractures and with fractures treated with carrier alone. This increase in callus area did not translate into an equivalent increase in torque to failure. Osteogenic protein-1 showed some evidence of overcoming the inhibition of fracture healing in the diabetic rat.

rhBMP-2 modulation of gene expression in infected segmental bone defects

The osteoinductive capability of BMPs appears diminished in the setting of acute infection. We applied rhBMP-2 to a segmental defect in a rat femur and measured the expression of key bone formation genes in the presence of acute infection. Types I and II collagen, osteocalcin, and BMP Type II receptor mRNA expression were characterized in 72 Sprague-Dawley rats, which received either bovine collagen carrier with 200 mug rhBMP-2 plus Staphylococcus aureus, carrier with bacteria only, carrier with rhBMP-2 only, or carrier alone. Six animals from each group were euthanized at 1, 2, and 4 weeks. Total RNA was isolated and extracted, and mRNA was determined by real-time comparative quantitative PCR. Infected defects had little expression of collagen I and II and osteocalcin mRNAs, while BMP receptor II expression with infection was greater than carrier-only controls at weeks 2 and 4. Notably, all four genes were upregulated in infected defects in the presence of rhBMP-2. Thus, in a clinical setting with a high risk of infection and nonunion, such as a compound fracture with bone loss, rhBMP-2 may increase the rate and extent of bone formation. Even if infection does occur, rhBMP-2 may allow a quicker overall recovery time.

A role for gamma/delta T cells in a mouse model of fracture healing

OBJECTIVE

Fractures can initiate an immune response that disturbs osteoblastic and osteoclastic cellular homeostasis through cytokine production and release. The aim of our study was to investigate gamma/delta T cells, innate lymphocytes known to be involved in tissue repair, as potential cellular components of the osteoimmune system's response to an in vivo model of bone injury. The absence of such cells or their effector cytokines influences the fate of other responder cells in proliferation, differentiation, matrix production, and ultimate callus formation.

METHODS

Tibia fractures were created in 60 gamma/delta T cell-deficient mice (also called delta T cell receptor [TCR]-knockout mice) and 60 control C57BL/6 mice. Analysis included radiographs, basic histology, mechanical testing, flow cytometry, and immunohistochemical localization of gamma/delta TCR-positive subsets from control animals and of CD44 expression from both groups, as well as enzyme-linked immunosorbent assay for the effector cytokines interleukin-2 (IL-2), interferon-gamma (IFNgamma), and IL-6.

RESULTS

Animals deficient in gamma/delta T cells demonstrated more mature histologic elements and quantitative increases in the expression of major bone (bone sialoprotein) and cartilage (type II collagen) matrix proteins and in the expression of bone morphogenetic protein 2 at a critical reparative phase. Moreover, only gamma/delta T cell-deficient animals had a decrease in the osteoprogenitor antiproliferative cytokines IL-6 and IFNgamma at the reparative phase. The result was improved stability at the repair site and an overall superior biomechanical strength in gamma/delta T cell-deficient mice compared with controls.

CONCLUSION

The evidence for a role of gamma/delta T cells in the context of skeletal injury demonstrates the importance of the immune system's effect on bone biology, which is relevant to the field of osteoimmunology, and offers a potential molecular platform from which to develop essential therapeutic strategies.

Genetic network and pathway analysis of differentially expressed proteins during critical cellular events in fracture repair

Bone repair consists of inflammation, intramembranous ossification, chondrogenesis, endochondral ossification, and remodeling. To better understand the translational regulation of these distinct but interrelated cellular events, we used the second generation of BD Clontechtrade mark Antibody Microarray to dissect and functionally characterize proteins differentially expressed between intact and fractured rat femur at each of these cellular events. Genetic network analysis showed that proteins differentially expressed within a given cellular event tend to be physically or functionally correlated. Seventeen such interacting networks were established over five cellular events that were most frequently associated with cell cycle, cell death, cell-to-cell signaling and interaction, and cell growth and proliferation. Eighteen molecular pathways were significantly enriched during the bone repair process, of which ERK/MAPK, NF-kB, PDGF, and T-cell receptor signaling pathways were significant during three or more cellular events. The analyses revealed dynamic temporal expression patterns and cellular-event-specific functions. The inflammation event on Day 1 was characteristic of the cell cycle-related molecular changes. The relative quiet stage of intramembranous ossification on Day 4 and the molecularly most active stage of chondrogenesis on Day 7 were featured by coordinated cell death and cell-proliferation signals. Endochondral ossification on Day 14 experienced a clear transition from the molecular/cellular function to the physiological system development/function. The osteoclast-mediated remodeling on Day 28 was highlighted by the integrin signaling pathway. The distinct changes in protein expression during these cellular events provide a molecular basis for developing cellular event-targeted therapeutic strategy to accelerate bone healing.

Patterns and localization of gene expression during intramembranous bone regeneration in the rat femoral marrow ablation model

Tissue formation and repair are dependent upon cascades of biological events, but the signals involved and the possible gene coexpression patterns during intramembranous bone repair are only poorly understood. We sought to place this mode of regeneration in context by profiling quantitative gene expression for a panel of 39 genes between days 1 and 14 following rat femoral marrow ablation. In situ hybridization was employed to localize a subset of genes. Additionally, principal components analysis was conducted to identify underlying factors suggestive of coexpression patterns. During inflammation (days 1-5), several genes, including cyclooxygenase-1 and -2, showed downregulation. Other proinflammatory cytokines, tumor necrosis factor-alpha and interleukin-1beta, exhibited increasing levels around day 5. During repair (days 3-10), growth factors, receptors, and inhibitor genes for transforming growth factor- beta; basic fibroblast growth factor; bone morphogenetic proteins 2, 4, and 7; vascular endothelial growth factor; and insulin-like growth factor-I were upregulated. In addition, the gene for core binding factor-alpha1 and markers of osteoblast function such as alkaline phosphatase, collagen type I, osteonectin, osteopontin, and osteocalcin had peak expression at day 5 or 7. The remodeling phase (days 10-14) was characterized by peaks for cytokines associated with osteoclastic activity including receptor activator of nuclear factor-kappaB, receptor activator of nuclear factor-kappaB ligand (RANKL), cathepsin K, tumor necrosis factor-alpha, interleukin-6, and cyclooxygenase-2. In situ hybridization showed that the most common sites of increased signal were within osteoblastic cells on trabecular and endosteal surfaces. Principal components analysis identified eight underlying factors that together explained over 80% of the variance in the data.

Human parathyroid hormone (1-34) accelerates the fracture healing process of woven to lamellar bone replacement and new cortical shell formation in rat femora

This study aimed to test whether intermittent treatment of human parathyroid hormone [hPTH(1-34)] disturbs or accelerates the fracture healing process using rat surgical osteotomy model. One hundred five, 5-week-old SD rats were allocated to vehicle control (CNT) and four PTH groups; 10 and 30 microg/kg of hPTH(1-34) treatment before surgery (P10, P30), and treatment before and after surgery (C10, C30). All animals were given subcutaneous injections three times a week for 3 weeks. Then, fractures were produced by transversely cutting the midshaft of bilateral femora and fixing with intramedullary wire. Human PTH(1-34) treatment was continued in C10 and C30 groups until sacrifice at 3, 6, and 12 weeks after surgery. The femora were assessed by peripheral quantitative computed tomography, three-point bending mechanical test, and histomorphometry. Total cross-sectional area was not significantly different among all groups at any time point. At 3 weeks after surgery, the lamellar bone/callus area was significantly increased in C10 and C30 groups compared to the other groups. At 6 weeks, remodeling of woven bone to lamellar bone in the callus was almost complete in all groups. At 12 weeks, percent new cortical shell area was significantly higher in C10 and C30 groups compared to the other groups, and the ultimate load in mechanical testing was significantly higher in C30 group than in CNT, P10, and P30 groups. Intermittent PTH treatment at 30 microg/kg before and after osteotomy accelerated the healing process as evidenced by earlier replacement of woven bone to lamellar bone, increased new cortical shell formation, and increased the ultimate load up to 12 weeks after osteotomy.

Effect of low- and high-intensity pulsed ultrasound on collagen post-translational modifications in MC3T3-E1 osteoblasts

Different intensities of pulsed ultrasound have distinct biological effects on bone mineralization in the process of bone fracture repair, even across a narrow range (e.g., 30-120 mW/cm(2)). The aim of our study was to elucidate the effect of low-intensity (30 mW/cm(2)) and high-intensity (120 mW/cm(2)) pulsed ultrasound on collagen metabolism by using MC3T3-E1 osteoblasts. Of special interest was the relationship between posttranslational collagen quality and prostaglandin E(2) activity. Cells with or without a cyclooxygenase-2 inhibitor, NS398, were exposed every day for four consecutive days to high-level or low-level intensities of pulsed ultrasound. We examined the, expression patterns of cyclooxygenase-2, lysyl oxidase, telopeptidyl lysyl hydroxylase (TLH), and helical lysyl hydroxylase by real-time polymerase chain reaction analysis. Quantitative analyses of reducible immature and nonreducible mature cross-links were also performed. Ultrasound at 30 mW/cm(2) upregulated TLH messenger RNA (mRNA) expression and enzyme activity compared to the control and resulted in increased relative concentrations of telopeptidyl hydroxylysine-derived cross-links. In addition to upregulated lysyl oxidase mRNA expression, increased total reducible and nonreducible cross-links were observed by 30 mW/cm(2) exposure compared to the control. In contrast, ultrasound at 120 mW/cm(2) had no obvious effect on collagen metabolism owing to high levels of endogenous prostaglandin E(2) induced by ultrasound. Our results showed that (1) low-intensity, but not high-intensity, ultrasound may accelerate the formation of the unique molecular packing of collagen fibers conducive to bone mineralization and that (2) the high dose of endogenous prostaglandin E(2) induced by pulsed ultrasound may be detrimental to calcifiable cross-link formation.

Intensity-related differences in collagen post-translational modification in MC3T3-E1 osteoblasts after exposure to low- and high-intensity pulsed ultrasound

Low-intensity pulsed ultrasound (LIPUS) has distinct effects on biologic mineralization at intensities of <100 mW/cm2. Intensity-dependent differences in the pattern of accelerated mineralization may be due to different alterations in regulation of collagenous matrix formation. However, little is known about the influence of LIPUS on collagen metabolism in the context of mineralization processes. Therefore, we attempted to evaluate differential effects of two intensities of pulsed ultrasound (30 vs. 120 mW/cm2) on collagen post-translational modification and mineralization in osteoblastic MC3T3-E1 cells. Murine osteoblastic MC3T3-E1 cells were exposed to pulsed ultrasound (1.5-MHz, 200-ms burst sine wave at 1.0-kHz frequency, either 30 or 120 mW/cm2 SATA, for 20 min/day from Day 14 to Day 35 postconfluence). Expression patterns of lysyl oxidase (LO), procollagen-lysine, 2-oxyglutarate, 5-dioxigenase 1 (PLOD1, LH1), and 2 (PLOD2, LH2) was examined using quantitative PCR. Quantitative analysis of reducible immature cross-links (dihydroxylysinonorleucine, hydroxylysinonorleucine, and lysinonorleucine) and nonreducible mature cross-links (pyridinoline and deoxypyridinoline) as well as analysis of the maturation of immature to mature cross-links were performed. Exposure to 30 mW/cm2 LIPUS upregulated LH2 mRNA expression and enzyme activity compared to controls. It was associated with increased relative amounts of telopeptidyl hydroxylysine (Hyl)-derived cross-links beginning on Day 14, upregulated LO mRNA expression, increased total reducible and nonreducible cross-links, and increased ratios of newly formed nonreducible to reducible cross-links. Similarities in the pattern of cross-link formation and calcium deposition in matrices between 30 mW/cm2 LIPUS-treated MC3T3-E1 cultures and bone suggest that 30 mW/cm2 LIPUS may promote the maturation of collagenous matrix as a scaffold for calcification. In contrast, exposure to 120 mW/cm2 ultrasound increased calcium accumulation compared to control at Day 35, but increases were delayed until Day 25. No differences in the extent and pattern of cross-links were observed compared to controls. These results suggest that the promotion of mineralization induced by 120 mW/cm2 may be attributed to other factors involved in mineralization process rather than cross-link pattern. Our results demonstrated the existence of differential effects of lower versus higher intensities of ultrasound on mineralization processes in vitro.

Bone tissue engineering and spinal fusion: the potential of hybrid constructs by combining osteoprogenitor cells and scaffolds

In this paper, we discuss the current knowledge and achievements on bone tissue engineering with regard to spinal fusion and highlight the technique that employs hybrid constructs of porous scaffolds with bone marrow stromal cells. These hybrid constructs potentially function in a way comparable to the present golden standard, the autologous bone graft, which comprises besides many other factors, a construct of an optimal biological scaffold with osteoprogenitor cells. However, little is known about the role of the cells in autologous grafts, and especially survival of these cells is questionable. Therefore, more research will be needed to establish a level of functioning of hybrid constructs to equal the autologous bone graft. Spinal fusion models are relevant because of the increasing demand for graft material related to this procedure. Furthermore, they offer a very challenging environment to further investigate the technique. Anterior and posterolateral animal models of spinal fusion are discussed together with recommendations on design and assessment of outcome parameters.

Molecular cloning and characterization of Mustang, a novel nuclear protein expressed during skeletal development and regeneration

Bone regeneration occurs as a series of events that requires temporal and spatial orchestration of numerous cell types guided by the transcriptional activity of thousands of genes, as recently demonstrated by our laboratory. Using the rat femoral fracture model, bioinformatics, cloning, expression assays, fusion proteins, and transfection, we report on the identification and characterization of one such differentially expressed gene, termed Mustang (musculoskeletal temporally activated novel gene). Mustang encodes for an 82 amino acid nuclear protein with no homology to any known protein family. However, other species homologues (mouse, human, cow) were identified within EST (expressed sequence tag) databases. Nuclear localization was confirmed using a GFP-Mustang fusion protein. Using in situ hybridization, Mustang expression was localized to differentiating periosteal osteogenic cells, proliferating chondrocytes, and osteoblasts of the fracture callus. Unlike adult tissues, developing embryos abundantly express Mustang, especially in mesenchymal condensations of limbs, vertebral perichondrium, and mesenchymal cells of the intervertebral discs. Although the precise function of Mustang is unknown, its unique pattern of expression during bone development and regeneration, absence in adult tissues (except skeletal muscle and tendon), and nuclear localization suggest that Mustang is involved in the development and regeneration of the mammalian musculoskeletal system.

Determination of the complete cDNA sequence of rat type II collagen and evaluation of distinct expression patterns of types IIA and IIB procollagen mRNAs during fracture repair in rats

Elucidating the molecular mechanisms that underlie fracture healing is crucial to understanding and devising strategies for the management of fractures, especially those associated with a pathological condition such as diabetes or old age. Cartilage formation, and therefore the expression of type II collagen by chondrocytes, is a critical step in frac-ture healing. Two forms of type II collagen, IIA and IIB, are known to be produced by alternative splicing of the Alpha(1) (II) procollagen gene. We have followed the patterns of expression of these two forms of type II collagen to determine the nature of chondrocyte recruitment during fracture healing. First, we sequenced the rat collagen type II cDNA to design the primers. Second, using a competitive quantitative reverse transcription-mediated polymerase chain reaction, we provide evidence that (1) there is a basal level of type IIA collagen expression during the early stages of fracture healing; (2) transient but sharp up-regulation of IIA expression occurs concomitant with chondrogenesis and endochondral ossification; and (3) type IIB collagen is the predominant mRNA variant expressed at virtually all times during fracture repair.

Spatial and temporal localization of transforming growth factor-beta1, bone morphogenetic protein-2, and platelet-derived growth factor-A in healing tooth extraction sockets in a rabbit model

PURPOSE

The purpose of this study was to test the hypothesis that spatial and temporal localization of growth factors transforming growth factor (TGF)-beta1, bone morphogenetic protein (BMP)-2, and platelet derived growth factor (PDGF)-A in a rabbit tooth extraction model correlate with the histologic events contributing toward healing.

MATERIALS AND METHODS

Twenty-four male New Zealand White rabbits were used in the study. Incisor teeth were extracted from both jaws, and the healing extraction socket with the surrounding jaw bone was harvested at 48 hours, 4 days, and 1, 2, 4, 8, 12, and 16 weeks. Tissues were fixed, decalcified, and processed for hematoxylin and eosin and immunohistochemical staining. The sections were stained to detect TGF-beta1, BMP-2, and PDGF-A. Stained sections were then imaged, and an automated computer program was used to detect the brown 3,3'-diaminobenzidine regions representing the location of growth factors of interest. The data were collected in terms of percentage area and intensity of stain, and an analysis of variance was conducted (Tukey-Kramer and Scheffe's test) for statistical comparison between different time points, jaws, and growth factors. These results were also compared with hematoxylin and eosin-stained histologic specimens obtained at similar time points.

RESULTS

Spatial and temporal differences in localization of TGF-beta1, BMP-2, and PDGF-A were observed across all time frames in both jaws. Statistically significant differences in percentage area and intensity of brown diaminobenzidine stain were detected temporally between TGF-beta1, BMP-2, and PDGF-A (P < or =.0001).

CONCLUSION

The results of this study showed positive correlation between histologic events and the spatial and temporal localization of TGF-beta1, BMP-2, and PDGF-A in a rabbit tooth extraction model.

The effect of roughness on biophysical stimuli at the bone-cartilage interface

Numerical simulations are often used to investigate the effect of mechanical environment on fracture healing. Although these models exhibit biologically relevant mechanical parameters at the bone-callus interface, this interface is modelled as perfectly smooth when in fact it is rough. In this study, a macro-micro-two-scale finite element model was used to determine if roughness significantly alters calculated local mechanical parameters. An idealized fracture healing poroelastic model with a small micro-modelled sub-domain of cartilaginous callus adjacent to rough bone was subjected to cyclic loading. The shear stress, tangential fluid velocity, and pore pressure were investigated. With roughness similar to that at the growth plate, solid matrix shear stress differed substantially with interface roughness, whereas interstitial fluid velocity and pore pressure were only slightly affected. Hence, when modelling local micro-mechanical environments near hard-soft tissue interfaces, interface roughness should be considered.

Enhanced expression of Runx2/PEBP2alphaA/CBFA1/AML3 during fracture healing

The cause of the dramatic increase in expression of the osteopontin gene during fracture healing was studied in a mouse experimental model. Semiquantitative reverse transcription-polymerase chain reaction, Northern blotting, and in situ hybridization analysis showed that the enhanced expression took place prior to callus formation. The change in the expression pattern of collagenous and noncollagenous bone matrix proteins in addition to Ets-1 and Runx2, major transcription factors of osteopontin, were examined and compared to that of osteopontin. Although Ets-1 expression showed no significant change during fracture healing, enhanced expression of Runx2 corresponding to that of osteopontin was observed. Furthermore, in situ hybridization demonstrated that osteopontin-expressing cells also express the Runx2 gene. The results indicated the possibility that Runx2 is a major regulator of osteopontin during fracture healing.

Determination of variations in gene expression during fracture healing

The genetic make-up and physiological state of a cell or tissue in an organism interact to determine the level at which specific genes are expressed. Identifying genes differentially expressed between 2 genetic or physiological states often gives insight into the molecular mechanisms controlled by the process in question. Various methods have been devised to identify differentially expressed genes and to quantify the expression of differentially regulated genes at the RNA or protein level. These methods are most accurate when the experimental samples are derived from highly controlled and reproducible sources, such as cultured cells. However, no simple in vitro models have been developed to study all biological processes and some are still best studied in the context of the whole organism. Using bone fracture healing as a model, we quantified the expression of 2 housekeeping and 2 regulatory genes during this complex biological process to determine the statistical parameters required to study differential gene expression in tissue samples derived from entire organisms. Our analysis shows that 5 samples in each group are needed to identify genes differentially expressed by a factor of 3 between 2 physiological or genetic states.

Controlled induction of a pseudarthrosis: a study using a rodent model

OBJECTIVES

This study aimed to test the hypothesis that under standardized mechanical and biologic conditions, the process of indirect bone repair in a rodent species could be manipulated to form a reproducible, atrophic, fibrous pseudarthrosis.

DESIGN

The model used comprised a mid-diaphyseal, transverse osteotomy in the rat femur, stabilized via a precision miniature external fixator, a constant axial fixation stiffness being defined by a specific frame geometry.

MAIN OUTCOME MEASUREMENTS

The repair process for both 0.5-mm and 3.0-mm gap osteotomies was characterized using radiography, dual-energy x-ray absorptiometry, histologic assessment of standardized longitudinal sections, and postmortem mechanical testing.

RESULTS

Healing of the defect was highly reproducible, bone union being attained at around 5 weeks postoperatively with a 0.5-mm gap. Increasing the gap width to 3.0 mm resulted consistently in a pseudarthrosis.

CONCLUSION

These two reproducible patterns of repair can now be used to elucidate the underlying molecular mechanisms controlling the extent and progression of connective tissue differentiation in indirect bone repair without the additional variable of a nonstandardized mechanical environment.

Transcriptional profiling of bone regeneration. Insight into the molecular complexity of wound repair

The healing of skeletal fractures is essentially a replay of bone development, involving the closely regulated, interdependent processes of chondrogenesis and osteogenesis. Using a rat femur model of bone healing to determine the degree of transcriptional complexity of these processes, suppressive subtractive hybridization (SSH) was performed between RNA isolated from intact bone to that of callus from post-fracture (PF) days 3, 5, 7, and 10 as a means of identifying up-regulated genes in the regenerative process. Analysis of 3,635 cDNA clones revealed 588 known genes (65.8%, 2392 clones) and 821 expressed sequence tags (ESTs) (31%, 1,127). The remaining 116 cDNAs (3.2%) yielded no homology and presumably represent novel genes. Microarrays were then constructed to confirm induction of expression and determine the temporal profile of all isolated cDNAs during fracture healing. These experiments confirmed that approximately 90 and approximately 80% of the subtracted known genes and ESTs are up-regulated (> or = 2.5-fold) during the repair process, respectively. Clustering analysis revealed subsets of genes, both known and unknown, that exhibited distinct expression patterns over 21 days (PF), indicating distinct roles in the healing process. Additionally, this transcriptional profiling of bone repair revealed a host of activated signaling molecules and even pathways (i.e. Wnt). In summary, the data demonstrate, for the fist time, that the healing process is exceedingly complex, involves thousands of activated genes, and indicates that groups of genes rather than individual molecules should be considered if the regeneration of bone is to be accelerated exogenously.

Localization of nitric oxide synthases during fracture healing

Previously, we have reported that nitric oxide synthases (NOSs), which generate NO, modulate fracture healing. However, the cellular sources of the NOS isoforms during the course of fracture healing have not been studied systematically. The purpose of this study was to localize the cellular distribution of NOS isoforms (inducible NOS [iNOS], endothelial NOS [eNOS], and neuronal NOS [bNOS]) by in situ hybridization and immunohistology after femoral fractures in rats. The iNOS signal was detected during the initial stages (on day 4 and day 7) of fracture healing in 52 +/- 2% (mean +/- SE, n = 7) of cells within the intramembranous region, along the edge of the periosteal callus. The iNOS signal in callus cells declined to an undetectable level on day 14. eNOS was detected during the middle stages (on day 7 and day 14) of fracture healing in cells lining the blood vessels and also in 49 +/- 3% of cells in the chondral region. The bNOS signal was found to be increased at the later stages (day 14 and day 21) of fracture healing in 51 +/- 3% of cells at the junction between fibrous tissue and cartilage within the fibrochondral region. In summary, the,expression of NOS isoforms during fracture healing was time dependent and cellular distinctive.

Prolonged culture of HOS 58 human osteosarcoma cells with 1,25-(OH)2-D3, TGF-beta, and dexamethasone reveals physiological regulation of alkaline phosphatase, dissociated osteocalcin gene expression, and protein synthesis and lack of mineralization

Cultured rodent osteoblastic cells reiterate the phenotypic differentiation and maturation of osteoblasts seen in vivo. As previously shown, the human osteosarcoma cell line HOS 58 represents a differentiated stage of osteoblast development. The potential of HOS 58 for still further in vitro differentiation suggests the line can serve as a model of osteoblast maturation. Using this cell line, we have investigated the influence of 1,25-(OH)2-D3 (D3), TGF-beta and Dexamethasone (Dex) on proliferation and on the protein and mRNA levels of alkaline phosphatase (AP), procollagen 1 (Col 1), and osteocalcin (Oc), as well as mineralization during 28 days in culture. AP mRNA and protein were highly expressed throughout the culture period with further increase of protein AP activity at constant gene expression levels. A differentiation inhibiting effect of either TGF-beta or Dex was seen. Col 1 was investigated without the use of ascorbic acid and showed only minor changes during culture time or stimulation. The gene expression for Oc increased continually whereas protein synthesis peaked at confluence and decreased thereafter. TGF-beta and Dex treatments decreased Oc mRNA and protein levels. Stimulation by D3 was maximal at day 7 with a decrease thereafter. HOS 58 cells showed no mineralization capacity when stimulated with different agents, as measured by energy-dispersive X-ray microanalysis. This was not due to absence of Cbfa1 expression. In conclusion, the HOS 58 osteosarcoma cell line represents a differentiated cell line with highly expressed and physiologically regulated AP expression during further differentiation in culture. We observed a dissociation between osteocalcin gene expression and protein secretion which may contribute to the lack of mineralization in this cell line.

Inhibitory effect of bFGF on endochondral heterotopic ossification

Basic fibroblast growth factor (bFGF) is reported to stimulate repair of fracture and bony defects in in vivo animal studies. However, most studies performed in vitro demonstrate inhibitory effect of bFGF on cartilage and bone differentiation. To understand the discrepancy observed in in vivo and in vitro studies, we evaluated the effect of bFGF on chondro-osteogenesis initiated by bone matrix powder (MP). MP was implanted in the murine hamstring muscles with or without administration of bFGF. Injection of 1 microg of bFGF markedly reduced the size of heterotopic bone induced by MP, as detected by X-ray. Injection of 10 microg of bFGF completely inhibited ossification and only fibrous tissues were observed at the site of MP implantation. The expressions of alkaline phosphatase and osteocalcin mRNAs, markers for bone differentiation, were completely suppressed by 10 microg of bFGF. These results demonstrate the inhibitory effect of bFGF on endochondral ossification in vivo, implicating a precaution for its use in musculo-skeletal disorders.

Diminished callus size and cartilage synthesis in alpha 1 beta 1 integrin-deficient mice during bone fracture healing

Integrins mediate cell adhesion to extracellular matrix components. Integrin alpha 1 beta 1 is a collagen receptor expressed on many mesenchymal cells, but mice deficient in alpha 1 integrin (alpha1-KO) have no gross structural defects. Here, the regeneration of a fractured long bone was studied in alpha1-KO mice. These mice developed significantly less callus tissue than the wild-type (WT) mice, and safranin staining revealed a defect in cartilage formation. The mRNA levels of nine extracellular matrix genes in calluses were evaluated by Northern blotting. During the first 9 days the mRNA levels of cartilage-related genes, including type II collagen, type IX collagen, and type X collagen, were lower in alpha1-KO mice than in WT mice, consistent with the reduced synthesis of cartilaginous matrix appreciated in tissue sections. Histological observations also suggested a diminished number of chondrocytes in the alpha 1-KO callus. Proliferating cell nuclear antigen staining revealed a reduction of mesenchymal progenitors at the callus site. Although, the number of mesenchymal stem cells (MSCs) obtained from WT and alpha 1-KO whole marrow was equal, in cell culture the proliferation rate of the MSCs of alpha 1-KO mice was slower, recapitulating the in vivo observation of reduced callus cell proliferation. The results demonstrate the importance of proper collagen-integrin interaction in fracture healing and suggest that alpha1 integrin plays an essential role in the regulation of MSC proliferation and cartilage production.

In vitro effects of low-intensity ultrasound stimulation on the bone cells

Mechanical perturbations serve as extracellular signals to a variety of cells, including bone cells. Low-intensity pulsed ultrasound produces significant multifunctional effects that are directly relevant to bone formation and resorption. Ultrasound stimulation has been shown to accelerate bone-defect healing and trabecular bone regeneration. In this study, we use an in vitro bone cell culture model to investigate the effect of low-intensity pulsed ultrasound. The rat alveolar mononuclear cell-calvaria osteoblast coculture system was used in this study. Before treatment, the bone cells were cultured for 3 days to facilitate their attachment and differentiation. Then, ultrasound exposure (frequency = 1 MHz, intensity = 0.068 W/cm(2)) or sham exposure for 20 min per day was applied until the end of the experiment. Half of the culture media were obtained on the 4th, 5th, 6th, 7th, 8th, 9th, and 10th days for the analysis of cytokines and biochemical parameters. At the end of the experiment, cells were fixed and stained for identification and quantification of the osteoblast and osteoclast cells. After low-intensity pulse ultrasound stimulation, the osteoblast cell counts were significantly increased, whereas the osteoclast cell counts were significantly decreased. The total alkaline phosphatase amount in the culture medium was increased after 7 days of ultrasound stimulation, and tumor necrosis factor-alpha in ultrasound-stimulated bone cells was significantly increased after the 7th day of culture and reached 474.77% of the control medium on the 10th day of culture. The results of this study suggest that low-intensity ultrasound treatment may have a stimulatory effect on bone-healing processes.

Prevention of fracture healing in rats by an inhibitor of angiogenesis

Angiogenesis is considered essential to fracture healing, but its role in the healing process remains poorly understood. Angiogenesis inhibitors, which block new blood vessel formation by specifically targeting vascular cells, are currently under development for use in cancer chemotherapy, and are potentially powerful tools for defining the consequences of angiogenic impairment on fracture healing. In this study, we directly tested the effects of the angiogenesis inhibitor TNP-470 on the healing of closed femoral fractures in an established rat model system. Beginning 1 day after fracture, animals received either angiogenesis inhibitor at a therapeutically effective antitumor dose, or a weight-adjusted amount of carrier vehicle. The progress of fracture healing was assessed at weekly intervals for 21 days by radiography and histology; functional assessment was carried out at day 24 by biomechanical testing. By all three criteria, treatment with the angiogenesis inhibitor completely prevented fracture healing. Formation of both callus and periosteal woven bone were suppressed, indicating that both the intramembranous and endochondral pathways of osteogenesis were affected. The resulting tissue resembled "atrophic nonunions" often seen clinically in cases of failed fracture healing, but rarely achieved in animal models. These results show that angiogenesis is essential to very early stages of fracture healing, and suggest this model system may be useful for understanding the mechanisms underlying fracture nonunions due to vascular impairment. Finally, the data raise the possibility that impairment of fracture healing may be an adverse effect of clinical treatments with antiangiogenic drugs.

Spatial and temporal gene expression in chondrogenesis during fracture healing and the effects of basic fibroblast growth factor

Chondrogenesis is an essential component of endochondral fracture healing, though the molecular and cellular events by which it is regulated have not been fully elucidated. In this study, we used a rat model of closed fracture healing to determine the spatial and temporal expression of genes for cartilage-specific collagens. Furthermore, to determine the effects of basic fibroblast growth factor (bFGF) on chondrogenesis in fracture healing, we injected 100 microg recombinant human bFGF into the fracture site immediately after fracture. In normal calluses, pro-alpha1(II) collagen mRNA (COL2A1) was detected in proliferative chondrocytes beginning on day 4 after the fracture, and pro-alpha1(X) collagen mRNA (COL10A1) in hypertrophic chondrocytes beginning on day 7. In FGF-injected calluses, the cartilage enlarged in size significantly. On day 14, both COL2A1- and COL10A1-expressing cells were more widely distributed, and the amounts of COL2A1 and COL10A1 mRNAs were both approximately 2-fold increased when compared with uninjected fractures. Temporal patterns of expression for these genes were, however, identical to those found in normal calluses. The number of proliferating cell nuclear antigen-positive cells was increased in the non-cartilaginous area in the bFGF-injected calluses by day 4. The present molecular analyses demonstrate that a single injection of bFGF enhances the proliferation of chondroprogenitor cells in fracture callus, and thus contributes to the formation of a larger cartilage. However, maturation of chondrocytes and replacement of the cartilage by osseous tissue are not enhanced by exogenous bFGF, and this results in the prolonged cartilaginous callus phase. We conclude that, in the healing of closed fractures of long bones, exogenous bFGF has a capacity to enlarge the cartilaginous calluses, but not to induce more rapid healing.

The E11 osteoblastic lineage marker is differentially expressed during fracture healing

Given the complexity of the fracture healing process and the involvement of a multitude of cells, we hypothesize that a very large number of genes would be transcriptionally regulated during the repair process. To identify genes that play a role during bone regeneration in cellular events, such as proliferation, migration, adhesion, and differentiation, we employed differential display and compared mRNA populations isolated from postfracture (PF) day 3 calluses to those of intact (contralateral) rat femurs. One such gene whose expression was upregulated at PF day 3 is identified as the osteoblastic lineage marker, E11 antigen. E11 is a cell membrane protein localized predominantly on osteoblasts and osteocytes. In this study we show that E11 mRNA expression is consistently upregulated during fracture repair, with elevated levels (tenfold) seen as early as PF day 3. These high levels of expression were maintained throughout all of the early stages of repair examined (PF day 3-21 calluses) and paralleled those of another osteoblastic marker, osteopontin. Similarly, high protein levels were detected throughout the reparative phase of the callus, particularly in osteoblasts, preosteocytes, and osteocytes, localized within the newly made osteoid. No labeling was detected in fibroblasts, proliferating chondrocytes, or hypertrophic chondrocytes, consistent with previous results. Taken together, these results suggest that the E11 antigen is indeed a suitable marker for both osteoblasts and osteocytes and that it plays a significant role in osteogenesis.

Alteration of fracture stability influences chondrogenesis, osteogenesis and immigration of macrophages

Mechanical conditions at the fracture line determine the mode of fracture healing (osteonal versus non-osteonal bone union). The aim of this study was to investigate the influence of differing degrees of fracture stability on the time course of chondrogenesis, enchondral ossification and immigration of macrophages into the fracture callus. Using a fracture model of the rat's tibia, histological (Azan staining), immunohistological (antibodies directed against the macrophage-specific surface antigen ED2), and molecular biological techniques (expression of the mRNA of the cartilage-specific collagen IX, osteocalcin - a marker for mature osteoblasts - and the macrophage-specific macrosialin) were employed. In terms of histology and molecular biology (collagen IX mRNA expression) chondrogenesis in the fracture gap continued for longer in less stable fractures. In more stable fractures bone formation - identified by osteocalcin mRNA expression - increased from day 12 onwards. The expression of the macrophage-specific surface antigen ED2 and the mRNA of macrosialin was more pronounced but of shorter duration in the more stable fractures. This study shows that differing degrees of fracture stability not only influence the interplay between osteogenesis and chondrogenesis but also alter the kinetics of macrophage immigration into the fracture callus. These findings could aid in better understanding the cytobiologic mechanisms of callus formation and may suggest that macrophages are an important factor not only in soft tissue healing but also in bone healing.

Hypoxia regulates osteoblast gene expression

Vascular disruption secondary to fracture creates a hypoxic gradient of injury wherein the oxygen tension at the center of the wound is very low. In vivo this hypoxic microenvironment stimulates the expression of a variety of cytokines from inflammatory cells, fibroblasts, endothelial cells, and osteoblasts. In order to begin to dissect this complex system, we have examined the effects of hypoxia on isolated osteoblast gene expression in vitro. Understanding gene expression in this system may facilitate the development of targeted therapeutic modalities designed to accelerate fracture repair and reduce complications. Using an established model of in vitro hypoxia, we have analyzed the expression of genes involved in bone matrix production and turnover. Subconfluent neonatal rat calvarial osteoblasts were exposed to hypoxia (pO(2) = 35-40 mm Hg) and total cellular RNA was collected at 0, 3, 6, 24, and 48 h. Northern analysis was used to analyze the expression patterns of (1) transforming growth factors (TGFs)-beta1, -beta2, and -beta3 and their type I receptor; (2) collagens I and III; and (3) tissue inhibitor of metalloproteinase-1. We have demonstrated a marked elevation of TGF-beta1 gene expression within 3 h of hypoxia. Although neither TGF-beta2 nor TGF-beta3 expression was affected by hypoxia, the TGF-beta type I receptor was substantially upregulated within 6 h. In addition, extracellular matrix scaffolding molecules (collagens I and III) were markedly, but differentially, upregulated. Finally, we have demonstrated that the expression of an inhibitor of extracellular matrix turnover, the tissue inhibitor of metalloproteinase-1, was strikingly decreased in response to hypoxia. These results imply that hypoxia can affect osseous healing by altering the expression of cytokines, bone-specific extracellular matrix molecules, and their regulators.

Histomorphological and proliferative characterization of developing periosteal neochondrocytes in vitro

Periosteal chondrogenesis is relevant to cartilage repair and fracture healing. Cell proliferation is a limiting factor of cartilage production. We used an in vitro organ culture model to test the hypothesis that proliferative activity correlates with cell morphology. One hundred and four periosteal explants from 26 two-month old New Zealand rabbits were cultured for up to 42 days. They were analyzed histomorphologically, and immunohistochemically with proliferative cell nuclear antigen (PCNA). The periosteal neocartilage displayed a consistent zonal pattern of chondrocyte cell shapes. The flat cell zone from day 7 to 21, consisted of uniform-sized small spindle-shaped cells. The round cell zone, which appeared on day 14, consisted of variable-sized round cells averaging 510 +/- 250 microm2 in area. They were subdivided into small round (<510 microm2) and large round cells (>510 microm2). The proliferative index was highest in the small round cell group (32 +/- 6%), intermediate in the flat cell group (27 +/- 6%), and lowest in the large round cell group (20 +/- 7%) (P < 0.001). Furthermore, the proliferative indices in the round cell group were inversely proportional to cell size. Therefore, (1) there is a sequential progression of cell morphology during periosteal chondrogenesis, (2) cell differentiation is arrested prior to terminal differentiation for some cells and not for others, and (3) proliferative activity is strongly related to cell morphology. This organ culture model provides us with opportunities to study the regulation of terminal chondrocyte differentiation and the control of cell proliferation. This will contribute to our understanding of cartilage repair, fracture healing and growth plate physiology.

Low-intensity pulsed ultrasound accelerates rat femoral fracture healing by acting on the various cellular reactions in the fracture callus

Low-intensity pulsed ultrasound (LIPUS) has been shown to accelerate fracture healing in both animal models and clinical trials, but the mechanism of action remains unclear. In fracture healing, various consecutive cellular reactions occurred until repair. We investigated whether the advanced effects of LIPUS depended on the duration and timing of LIPUS treatment in a rat closed femoral fracture model to determine the target of LIPUS in the healing process. Sixty-nine Long-Evans male rats that have bilateral closed femoral fractures were used. The right femur was exposed to LIPUS (30 mW/cm2 spatial and temporal average [SATA], for 20 minutes/day), and the left femur was used as a control. Rats were divided into four groups according to timing and duration of treatment (Ph-1, days 1-8; Ph-2, days 9-16; Ph-3, days 17-24; throughout [T], days 1-24 after the fracture). Animals were killed on day 25. After radiographs and microfocus X-ray computed tomography (muCT) tomograms were taken, the hard callus area (HCA), bone mineral content (BMC) at the fracture site, and mechanical torsion properties were measured, and histological analysis was conducted. Interestingly, the maximum torque of the LIPUS-treated femur was significantly greater than that of the controls in all groups without any changes in HCA and BMC. The multiviewing of three-dimensional (3D) muCT reconstructions and histology supported our findings that the partial LIPUS treatment time was able to accelerate healing, but longer treatment was more effective. These results suggest that LIPUS acts on some cellular reactions involved in each phase of the healing process such as inflammatory reaction, angiogenesis, chondrogenesis, intramembranous ossification, endochondral ossification, and bone remodeling.

Long-term effect of incadronate disodium (YM-175) on fracture healing of femoral shaft in growing rats

The aim of this study was to investigate the long-term effect of incadronate on fracture healing of the femoral shaft in rats. Female Sprague-Dawley 8-week-old rats were injected subcutaneously (sc) with either vehicle (V group) or two doses of incadronate (10 microg/kg and 100 microg/kg) three times a week for 2 weeks. Right femoral diaphysis was then fractured and fixed with intramedullary stainless wire. Just after fracture, incadronate treatment was stopped in pretreatment groups (P groups: P-10 and P-100) or continued in continuous treatment groups (C groups: C-10 and C-100). All rats were killed at 25 weeks or 49 weeks after surgery. Fractured femur was evaluated radiologically and mechanically and then stained in Villanueva bone stain and embedded in methyl methacrylate. Undecalcified cross-sections from the fracture area were evaluated microradiologically and histomorphometrically. Radiographic observation showed that the fracture line disappeared in all groups. Cross-sectional area in the C-100 group was the biggest among all groups and in the C-10 group was larger than that in the V group at 25 weeks. Histological and histomorphometric observations showed that the process of fracture healing was delayed under continuous treatment with incadronate as evidenced by the delay of both lamellar cortical shell formation and resolution of original cortex in C groups. Percent linear labeling perimeter, mineral apposition rate (MAR), and bone formation rate (BFR) in C groups significantly decreased compared with the other groups, indicating that the callus remodeling was suppressed under continuous treatment, especially with a high dose. Mechanical study showed that the stiffness and ultimate load of the fractured femur in the C 100 group were the highest among all groups at both 25 weeks and 49 weeks. In conclusion, this study showed that long-term continuous treatment with incadronate delayed the process of fracture healing of femur in rats, especially under high dose but it did not impair the recovery of mechanical integrity of the fracture.

Expression of bone morphogenetic proteins during membranous bone healing

For the reconstructive plastic surgeon, knowledge of the molecular biology underlying membranous fracture healing is becoming increasingly vital. Understanding the complex patterns of gene expression manifested during the course of membranous fracture repair will be crucial to designing therapies that augment poor fracture healing or that expedite normal osseous repair by strategic manipulation of the normal course of gene expression. In the current study, we present a rat model of membranous bone repair. This model has great utility because of its technical simplicity, reproducibility, and relatively low cost. Furthermore, it is a powerful tool for analysis of the molecular regulation of membranous bone repair by immunolocalization and/or in situ hybridization techniques. In this study, an osteotomy was made within the caudal half of the hemimandible, thus producing a stable bone defect without the need for external or internal fixation. The healing process was then catalogued histologically in 28 Sprague-Dawley rats that were serially killed at 1, 2, 3, 4, 5, 6, and 8 weeks after operation. Furthermore, using this novel model, we analyzed, within the context of membranous bone healing, the temporal and spatial expression patterns of several members of the bone morphogenetic protein (BMP) family, known to be critical regulators of cells of osteoblast lineage. Our data suggest that BMP-2/-4 and BMP-7, also known as osteogenic protein-1 (OP-1), are expressed by osteoblasts, osteoclasts, and other more primitive mesenchymal cells within the fracture callus during the early stages of membranous fracture healing. These proteins continue to be expressed during the process of bone remodeling, albeit less prominently. The return of BMP-2/-4 and OP-1 immunostaining to baseline intensity coincides with the histological appearance of mature lamellar bone. Taken together, these data underscore the potentially important regulatory role played by the bone morphogenetic proteins in the process of membranous bone repair.

Temporal expression of the chondrogenic and angiogenic growth factor CYR61 during fracture repair

The repair of a fractured bone is a complex biological event that essentially recapitulates embryonic development and requires the activity of a number of different cell types undergoing proliferation, migration, adhesion, and differentiation, while at the same time expressing a host of different genes. To identify such genes, we employed differential display and compared messenger RNA (mRNA) populations isolated from postfracture (PF) day 5 calluses to those of intact rat femurs. One such gene in which expression was up-regulated at PF day 5 is identified as CYR61, a member of the CCN family of secreted regulatory proteins. CYR61 is a growth factor that stimulates chondrogenesis and angiogenesis. We show that its mRNA expression during fracture repair is regulated temporally, with elevated levels seen as early as PF day 3 and day 5, rising dramatically at PF day 7 and day 10, and finally declining at PF day 14 and day 21. At the highest peak of expression (PF day 7 and day 10, which correlates with chondrogenesis), CYR61 mRNA levels are approximately 10-fold higher than those detected in intact femurs. Similarly, high protein levels are detected throughout the reparative phase of the callus, particularly in fibrous tissue and periosteum, and in proliferating chondrocytes, osteoblasts, and immature osteocytes. The secreted form of CYR61 also was detected within the newly made osteoid. No labeling was detected in hypertrophic chondrocytes or in mature cortical osteocytes. These results suggest that CYR61 plays a significant role in cartilage and bone formation and may serve as an important regulator of fracture healing.

Gene expression of TGF-beta, TGF-beta receptor, and extracellular matrix proteins during membranous bone healing in rats

Poorly healing mandibular fractures and osteotomies can be troublesome complications of craniomaxillofacial trauma and reconstructive surgery. Gene therapy may offer ways of enhancing bone formation by altering the expression of desired growth factors and extracellular matrix molecules. The elucidation of suitable candidate genes for therapeutic intervention necessitates investigation of the endogenously expressed patterns of growth factors during normal (i.e., successful) fracture repair. Transforming growth factor beta1 (TGF-beta1), its receptor (Tbeta-RII), and the extracellular matrix proteins osteocalcin and type I collagen are thought to be important in long-bone (endochondral) formation, fracture healing, and osteoblast proliferation. However, the spatial and temporal expression patterns of these molecules during membranous bone repair remain unknown. In this study, 24 adult rats underwent mandibular osteotomy with rigid external fixation. In addition, four identically treated rats that underwent sham operation (i.e., no osteotomy) were used as controls. Four experimental animals were then killed at each time point (3, 5, 7, 9, 23, and 37 days after the procedure) to examine gene expression of TGF-beta1 and Tbeta-RII, osteocalcin, and type I collagen. Northern blot analysis was used to compare gene expression of these molecules in experimental animals with that in control animals (i.e., nonosteotomized; n = 4). In addition, TGF-beta1 and T-RII proteins were immunolocalized in an additional group of nine animals killed on postoperative days 3, 7, and 37. The results of Northern blot analysis demonstrated a moderate increase (1.7 times) in TGF-beta1 expression 7 days postoperatively; TGF-beta1 expression returned thereafter to near baseline levels. Tbeta-RII mRNA expression was downregulated shortly after osteotomy but then increased, reaching a peak of 1.8 times the baseline level on postoperative day 9. Osteocalcin mRNA expression was dramatically downregulated shortly after osteotomy and remained low during the early phases of fracture repair. Osteocalcin expression trended slowly upward as healing continued, reaching peak expression by day 37 (1.7 times the control level). In contrast, collagen type IalphaI mRNA expression was acutely downregulated shortly after osteotomy, peaked on postoperative days 5, and then decreased at later time points. Histologic samples from animals killed 3 days after osteotomy demonstrated TGF-beta1 protein localized to inflammatory cells and extracellular matrix within the fracture gap, periosteum, and peripheral soft tissues. On postoperative day 7, TGF-beta1 staining was predominantly localized to the osteotomized bone edges, periosteum, surrounding soft tissues, and residual inflammatory cells. By postoperative day 37, complete bony healing was observed, and TGF-beta1 staining was localized to the newly formed bone matrix and areas of remodeling. On postoperative day 3, Tbeta-RII immunostaining localized to inflammatory cells within the fracture gap, periosteal cells, and surrounding soft tissues. By day 7, Tbeta-RII staining localized to osteoblasts of the fracture gap but was most intense within osteoblasts and mesenchymal cells of the osteotomized bone edges. On postoperative day 37, Tbeta-RII protein was seen in osteocytes, osteoblasts, and the newly formed periosteum in the remodeling bone. These observations agree with those of previous in vivo studies of endochondral bone formation, growth, and healing. In addition, these results implicate TGF-beta1 biological activity in the regulation of osteoblast migration, differentiation, and proliferation during mandibular fracture repair. Furthermore, comparison of these data with gene expression during mandibular distraction osteogenesis may provide useful insights into the treatment of poorly healing fractures because distraction osteogenesis has been shown to be effective in the management of these difficult clinical cases.

Transient chondrogenic phase in the intramembranous pathway during normal skeletal development

Calvarial and facial bones form by intramembranous ossification, in which bone cells arise directly from mesenchyme without an intermediate cartilage anlage. However, a number of studies have reported the emergence of chondrocytes from in vitro calvarial cell or organ cultures and the expression of type II collagen, a cartilage-characteristic marker, in developing calvarial bones. Based on these findings we hypothesized that a covert chondrogenic phase may be an integral part of the normal intramembranous pathway. To test this hypothesis, we analyzed the temporal and spatial expression patterns of cartilage characteristic genes in normal membranous bones from chick embryos at various developmental stages (days 12, 15 and 19). Northern and RNAse protection analyses revealed that embryonic frontal bones expressed not only the type I collagen gene but also a subset of cartilage characteristic genes, types IIA and XI collagen and aggrecan, thus resembling a phenotype of prechondrogenic-condensing mesenchyme. The expression of cartilage-characteristic genes decreased with the progression of bone maturation. Immunohistochemical analyses of developing embryonic chick heads indicated that type II collagen and aggrecan were produced by alkaline phosphatase activity positive cells engaged in early stages of osteogenic differentiation, such as cells in preosteogenic-condensing mesenchyme, the cambium layer of periosteum, the advancing osteogenic front, and osteoid bone. Type IIB and X collagen messenger RNAs (mRNA), markers for mature chondrocytes, were also detected at low levels in calvarial bone but not until late embryonic stages (day 19), indicating that some calvarial cells may undergo overt chondrogenesis. On the basis of our findings, we propose that the normal intramembranous pathway in chicks includes a previously unrecognized transient chondrogenic phase similar to prechondrogenic mesenchyme, and that the cells in this phase retain chondrogenic potential that can be expressed in specific in vitro and in vivo microenvironments.

Anabolic response of mouse bone-marrow-derived stromal cell clone ST2 cells to low-intensity pulsed ultrasound

The effects of 20-min exposure to low-intensity, pulsed ultrasound were investigated in ST2 cells of bone marrow stromal origin. They responded to ultrasound with elevated levels of IGF mRNAs, osteocalcin, and bone sialoprotein mRNAs. The upregulated expression of these messages appeared in a biphasic manner, with the first peak resistant to the protein synthesis inhibitor cycloheximide, and a second peak that was eliminated by NS398, an inhibitor of the inducive prostaglandin G/H synthase (cyclooxygenase-2). A cumulative effect of mechanical loading called the memory effect, which has been observed in vivo, can be explained from such a biphasic anabolic reaction mediated by prostaglandins. The upregulation of IGF or osteocalcin mRNAs can be observed even at 24 h after the initiation of the 20-min exposure to ultrasound. Our results suggest that this low-intensity, pulsed ultrasound, which has been clinically used to accelerate the healing processes of fractured bone, induces a direct anabolic reaction of osteogenic cells, leading to bone matrix formation.

Normal osteoconduction and repair in and around submerged highly bisphosphonate-complexed hydroxyapatite implants in rat tibiae

BACKGROUND

Ceramic hydroxyapatite implants have been used in dentistry for their unique compatibility with alveolar bone. Recently it was reported that bisphosphonates may be beneficial in preventing alveolar bone destruction associated with natural and experimental periodontal disease. Furthermore, bisphosphonate does prevent resorption of alveolar bone following mucoperiosteal flap surgery. We undertook a preliminary study evaluating the effects of highly bisphosphonate-complexed hydroxyapatite implants on osteoconduction and repair in rat tibiae.

METHODS

Porous hydroxyapatite implants were pre-incubated in 10(-2)M bisphosphonate solutions at pH 3.49 and pH 7.32. The implants had a diameter of 2.1 mm and a height of 2 mm and adsorbed 115 microg bisphosphonate in vitro. Bisphosphonate/hydroxyapatite implants and plain hydroxyapatite implants were inserted in opposite tibial metaphyses of 35 rats. The measurement errors for the mineral density (MD) of the implants and the proximal trabecular mineral bone density (TD) were estimated by peripheral computed tomography and the bone mineral density (BMD) measurement error by dual x-ray absorptiometry.

RESULTS

The measurement errors for the MD of the implants and the TD by peripheral computed tomography were 0.81% and 1.96%, respectively ex vivo. The BMD measurement error estimated by dual x-ray absorptiometry was 0.51% ex vivo. TD and BMD for bisphosphonate/hydroxyapatite implants were insignificantly higher compared to plain hydroxyapatite implants. Bisphosphonate/hydroxyapatite pre-incubated at pH 7.32 were found to be nondegradable implants, while bisphosphonate/hydroxyapatite (pH 3.49) implants were slowly degradable and lost a significant 5% of their density. Histologically, all bisphosphonate/hydroxyapatite implants appeared to be fully integrated and effective as bone replacement material in rat tibial bone. They exhibited vascularization and osteoconduction of tibial bone growth along and inside their porous structure.

CONCLUSIONS

Our study suggests that normal osteoconduction and repair occurred in and around the highly bisphosphonate-complexed hydroxyapatite implants in rat tibiae.

Cartilage-derived retinoic acid-sensitive protein and type II collagen expression during fracture healing are potential targets for Sox9 regulation

Cartilage-derived retinoic acid-sensitive protein (CD-RAP) and mRNA were examined in the mouse fracture model by immunohistochemistry and Northern blot analysis and compared with the expression of type II collagen. We also studied the expression of the transcription factor Sox9, reported to enhance type II collagen and CD-RAP gene expression in vitro. CD-RAP was first detected in immature chondrocytes on day 5. Intense signals for CD-RAP were found in fracture cartilage on days 7 and 9. CD-RAP decreased at the phase of endochondral ossification. Throughout fracture healing, CD-RAP was detected in cartilage and not in bone or fibrous tissue, thus CD-RAP may be a molecular marker of cartilage formation during fracture healing. Northern blot analysis revealed similar changes in CD-RAP and type II collagen mRNA levels. However, with respect to protein levels, CD-RAP decreased faster than type II collagen implying the stability is lower than type II collagen. Increased levels of Sox9 mRNA and protein were detected on day 5 and coincided with the initial increase of CD-RAP and type II collagen mRNAs. Sox9 mRNA levels declined with the progress of chondrocyte hypertrophy, followed by a concomitant decrease in CD-RAP and type II collagen mRNA levels. These changes in Sox9 expression compared with the cartilage-specific genes (CD-RAP and type II collagen) suggest that cell differentiation during fracture healing may be controlled by specific transcriptional factors which regulate phenotypic changes of the cells.