Expression of bone matrix proteins mRNA during distraction osteogenesis

Bone sialoprotein, but not osteopontin, deficiency impairs the mineralization of regenerating bone during cortical defect healing

Bone healing is a complex multi-step process, which depends on the position and size of the lesion, and on the mechanical stability of the wounded area. To address more specifically the mechanisms involved in cortical bone healing, we created drill-hole defects in the cortex of mouse femur, a lesion that triggers intramembranous repair, and compared the roles of bone sialoprotein (BSP) and osteopontin (OPN), two proteins of the extracellular matrix, in the repair process. Bone regeneration was analyzed by ex vivo microcomputerized X-ray tomography and histomorphometry of bones of BSP-deficient, OPN-deficient and wild-type mice. In all mouse strains, the cortical gap was bridged with woven bone within 2 weeks and no mineralized tissue was observed in the marrow. Within 3 weeks, lamellar cortical bone filled the gap. The amount and degree of mineralization of the woven bone was not affected by OPN deficiency, but cortical bone healing was delayed in BSP-deficient mice due to delayed mineralization. Gene expression studies showed a higher amount of BSP transcripts in the repair bone of OPN-deficient mice, suggesting a possible compensation of OPN function by BSP in OPN-null mice. Our data suggest that BSP, but not OPN, plays a role in primary bone formation and mineralization of newly formed bone during the process of cortical bone healing.

Why high frequency of distraction improved the bone formation in distraction osteogenesis?

Distraction osteogenesis, currently a standard method of bone lengthening, is based upon the "tension-stress principle", as proposed by G.A. Ilizarov. Mechanical stimulation by distraction induces biological responses of skeletal regeneration that is accomplished by a cascade of biologic processes including differentiation of pluripotential tissue, angiogenesis, mineralization, and remodeling. The exact mechanism by which strain stimulates bone formation remains unclear. Distraction rate and rhythm must have great influence on the quality of the newly formed bone generated by mechanical traction. The preliminary results demonstrated that for a given rate higher frequency of distraction improved the bone formation, but the mechanism remains unclear. In this article we present a hypothesis that the reason why higher frequency of distraction improved the bone formation for a given rate is that higher frequency of distraction provides smaller microtrauma to tissues within the gap and longer existence time of the microenvironment stimulating tissues within the gap than low frequency distraction. This hypothesis, if proven to be valid, will not only represent a breakthrough in research of mechanism of distraction osteogenesis, but also will open a new door to the bone regeneration.

The dose of growth factors influences the synergistic effect of vascular endothelial growth factor on bone morphogenetic protein 4-induced ectopic bone formation

Although vascular endothelial growth factor (VEGF) has been shown to act synergistically with bone morphogenetic protein (BMP)2 and BMP4 to promote ectopic endochondral bone formation via cell-based BMP gene therapy, the optimal ratio of VEGF to either of the BMPs required to obtain this beneficial effect remains unclear. In the current study, two cell types (C2C12, NIH/3T3) were retrovirally transduced to express BMP4 only or both BMP4 and VEGF. The resulting groups of cells were tested for their cellular proliferation, in vitro mineralization capacity, survival potential, and ability to undergo ectopic bone formation when implanted into a gluteofemoral muscle pocket created in severe combined immunodeficient mice. Results showed that VEGF inhibited the in vitro calcification of C2C12 and NIH/3T3 cells transduced to express BMP4. In vivo, C2C12 and NIH/3T3 cells expressing BMP4 and VEGF displayed significantly less bone formation than the same cells expressing only BMP4. In vivo, our results indicated that, when the ratio of VEGF to BMP4 is high, a detrimental effect on ectopic bone formation is observed; however, when the ratio is kept low and constant over time, the detrimental effect that VEGF has on ectopic bone formation is lost. Our studies revealed that VEGF's synergistic role in BMP4 induced ectopic bone formation is dose and cell-type dependent, which is an important consideration for cell-based gene therapy and tissue engineering for bone healing.

Biological basis of bone formation, remodeling, and repair-part I: biochemical signaling molecules

The bony biochemical environment is an active and dynamic system that permits and promotes cellular functions that lead to matrix production and ossification. Each component is capable of conveying important regulatory cues to nearby cells, thus effecting gene expression and changes at the cytostructural level. Here, we review the various signaling molecules that contribute to the active and dynamic nature of the biochemical system. These components include hormones, cytokines, and growth factors. We describe their role in regulating bone metabolism. Certain growth factors (i.e., TGF-beta, IGF-1, and VEGF) are described in greater detail because of their potential importance in developing successful tissue-engineering strategies.

Mechanical stimulation alters tissue differentiation and molecular expression during bone healing

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

Mechanical characterization of external fixator stiffness for a rat femoral fracture model

Clinical and experimental studies have shown that several mechanical factors influence the fracture healing process. One such factor, interfragmentary movement, is affected by loading and the stiffness of the fixation device. This study evaluated the stiffness of different external fixation devices for a rat femoral fracture model, using in vitro and analytical methods. The contribution to the stiffness of the fixation construct was dominated by the flexibility of the pins in relation to their offset, diameter, and material properties. The axial stiffness increased with decreasing offset and increasing pin diameter. Titanium pins resulted in significantly lower axial stiffness compared to stainless steel pins of the same design. The fixator body material and fixator length had a less pronounced influence on fixation stiffness. Mechanically characterized external fixation devices will allow in vivo study of the fracture healing process utilizing pre-calculated fracture fixation stiffness. These characterized fixation devices will allow controlled manipulation of the axial and shear interfragmentary movement to achieve a flexible fixation resulting in callus formation compared to a more rigid fixation limiting callus formation in a rat femoral fracture model.

Dynamic processes involved in the pre-vascularization of silk fibroin constructs for bone regeneration using outgrowth endothelial cells

For successful bone regeneration tissue engineered bone constructs combining both aspects, namely a high osteogenic potential and a rapid connection to the vascular network are needed. In this study we assessed the formation of pre-vascular structures by human outgrowth endothelial cells (OEC) from progenitors in the peripheral blood and the osteogenic differentiation of primary human osteoblasts (pOB) on micrometric silk fibroin scaffolds. The rational was to gain more insight into the dynamic processes involved in the differentiation and functionality of both cell types depending on culture time in vitro. Vascular tube formation by OEC was assessed quantitatively at one and 4 weeks of culture. In parallel, we assessed the temporal changes in cell ratios by flow cytometry and in the marker profiles of endothelial and osteogenic markers by quantitative real-time PCR. In terms of OEC, we observed an increase in tube length, tube area, number of nodes and number of vascular meshes within a culture period of 4 weeks, but a decrease in endothelial markers in real-time PCR. At the same time early osteogenic markers were downregulated, while marker expression associated with progressing mineralized matrix was upregulated in later stages of the culture. In addition, deposition of matrix components, such as collagen type I, known as a pro-angiogenic substrate for endothelial cells, appeared to increase with time indicated by immunohistochemistry. In summary, the study suggests a progressing maturation of the tissue construct with culture time which seems to be not effected by culture conditions mainly designed for outgrowth endothelial cells.

Histologic study of the cellular events during rat mandibular distraction osteogenesis

OBJECTIVE

The cellular events, underlying bone regeneration through rat mandibular distraction osteogenesis (DO) was examined using micro computerized tomography (microCT), histology, and histochemistry.

STUDY DESIGN

After 5-day latency, mandibles were distracted at 0.2 mm/12 h for 10 days, and fixed at latency 5 days (L5D), distraction 3, 6, 10 days (D3D, D6D, D10D), and consolidation 1, 3, 6, 10 weeks (C1W, C3W, C6W, C10W).

RESULTS

The microCT demonstrated radiopacity at the distraction gap (DG) during C1W, which was filled with new bone at C6W and C10W. At D3D, collagen fibers were aligned along the axis of the distraction vector. At D6D, alkaline phosphatase-positive osteoblasts and intramembranous ossification was observed. Collagen bundles became thicker with new bony trabeculae at D10D. Type II collagen-immunopositive areas first appeared at C1W. At C3W, cartilage tissue and endochondral ossification were found. By C6W, the entire DG had been bridged by new bone. The C10W specimens showed mature lamellar bone.

CONCLUSION

Mandibular DO produces bone through both intramembranous and endochondral ossification.

Molecular mechanisms controlling bone formation during fracture healing and distraction osteogenesis

Fracture healing and distraction osteogenesis have important applications in orthopedic, maxillofacial, and periodontal treatment. In this review, the cellular and molecular mechanisms that regulate fracture repair are contrasted with bone regeneration that occurs during distraction osteogenesis. While both processes have many common features, unique differences are observed in the temporal appearance and expression of specific molecular factors that regulate each. The relative importance of inflammatory cytokines in normal and diabetic healing, the transforming growth factor beta superfamily of bone morphogenetic mediators, and the process of angiogenesis are discussed as they relate to bone repair. A complete summary of biological activities and functions of various bioactive factors may be found at COPE (Cytokines & Cells Online Pathfinder Encyclopedia), http://www.copewithcytokines.de/cope.cgi.

Distraction osteogenesis: advancements in the last 10 years

Distraction is still evolving in the management of patients with orthopedic and craniomaxillofacial deformities. The relationship among latency, gap size, rate, rhythm, and duration of fixation is not totally understood for all of the individual patients and variations in their needs seen in a clinical practice. Numerous factors can influence the quality and quantity of bone seen with distraction, including the age and nutritional status of the patient and the type of distractor that is used.

Molecular Mechanisms Controlling Bone Formation during Fracture Healing and Distraction Osteogenesis

Fracture healing and distraction osteogenesis have important applications in orthopedic, maxillofacial, and periodontal treatment. In this review, the cellular and molecular mechanisms that regulate fracture repair are contrasted with bone regeneration that occurs during distraction osteogenesis. While both processes have many common features, unique differences are observed in the temporal appearance and expression of specific molecular factors that regulate each. The relative importance of inflammatory cytokines in normal and diabetic healing, the transforming growth factor beta superfamily of bone morphogenetic mediators, and the process of angiogenesis are discussed as they relate to bone repair. A complete summary of biological activities and functions of various bioactive factors may be found at COPE (Cytokines & Cells Online Pathfinder Encyclopedia), http://www.copewithcytokines.de/cope.cgi .

An immunohistochemical and ultrastructural study of the pericellular matrix of uneroded hypertrophic chondrocytes in the mandibular condyle of aged c-src-deficient mice

OBJECTIVE

Previous studies indicate that hypertrophic chondrocytes can transdifferentiate or dedifferentiate and redifferentiate into bone cells during the endochondral bone formation. Mandibular condyle in aged c-src-deficient mice has incremental line-like striations consisting of cartilaginous and non-cartilaginous layers, and the former contains intact hypertrophic chondrocytes in uneroded lacunae. The purpose of this study is to determine the phenotype changes of uneroded hypertrophic chondrocytes.

DESIGN

Immunohistochemical and ultrastructural examinations of the pericellular matrix of hypertrophic chondrocytes in the upper, middle, and lower regions of the mandibular condyle were conducted in aged c-src-deficient mice, using several antibodies of cartilage/bone marker proteins.

RESULTS

Co-localisation of aggrecan, type I collagen, and dentin matrix protein-1 (DMP-1) or matrix extracellular phosphoprotein (MEPE) was detected in the pericellular matrix of the middle region. Ultrastructurally, granular substances in the pericellular matrix of the middle region were the remains of upper region chondrocytes, which were mixed with thick collagen fibrils. In the lower region, the width of the pericellular matrix and the amount of collagen fibrils were increased. Versican, type I collagen, DMP-1, and MEPE were detected in the osteocyte lacunae. Additionally, DMP-1 and MEPE were detected in the pericellular matrix of uneroded hypertrophic chondrocytes located in the lower, peripheral region of the mandibular condyle in younger c-src-deficient mice, but not in the aged wild-type mice.

CONCLUSIONS

These results indicate that long-term survived, uneroded hypertrophic chondrocytes, at least in a part, acquire osteocytic characteristics.

Regulation of matrix Gla protein by parathyroid hormone in MC3T3-E1 osteoblast-like cells involves protein kinase A and extracellular signal-regulated kinase pathways

Inhibition of osteoblast-mediated mineralization is one of the major catabolic effects of parathyroid hormone (PTH) on bone. Previously, we showed that PTH induces matrix gamma-carboxyglutamic acid (Gla) protein (MGP) expression and established that this induction is critical for PTH-mediated inhibition of osteoblast mineralization. In the present study, we focus on the mechanism through which PTH regulates MGP expression in osteoblastic MC3T3-E1 cells. Following transient transfection of these cells with a -748 bp murine MGP promoter-luciferase construct (pMGP-luc), PTH (10 (-7) M) induced promoter activity in a time-dependent manner with a maximal four- to six fold induction seen 6 h after PTH treatment. Both H-89 (PKA inhibitor) and U0126 (MEK inhibitor), suppressed PTH induction of MGP promoter activity as well as the MGP mRNA level. In addition, forskolin (PKA activator) stimulated MGP promoter activity and mRNA levels confirming that PKA is one of the signaling molecules required for regulation of MGP by PTH. Co-transfection of MC3T3-E1 cells with pMGP-luc and MEK(SP), a plasmid encoding the constitutively active form of MEK, led to a dose-dependent increase in MGP promoter activity. Both MGP promoter activity and MGP mRNA level were not affected by the protein kinase C (PKC) inhibitor, GF109203X. However, phorbol 12-myristate 13-acetate (PMA), a selective PKC activator induced MGP mRNA expression through activation of extracellular signal-regulated kinase (ERK). Taken together, these results indicate that PTH regulates MGP via both PKA- and ERK-dependent pathways.

Immunolocalization of sibling and RUNX2 proteins during vertical distraction osteogenesis in the human mandible

We tested the hypothesis that mechanical loading of human bone increases expression of the transcription factor RUNX2 and bone matrix proteins osteopontin (OPN), bone sialoprotein (BSP), dentin matrix protein-1 (DMP1), and matrix extracellular phosphoglycoprotein (MEPE). We examined this in tissue sections of atrophic mandibular bone taken from edentulous patients who had undergone distraction osteogenesis. In undistracted bone, weak to moderate staining for OPN and BSP was found in osteoblasts and bone matrix of immature woven bone. RUNX2 was also detectable in osteoblasts and in cells of the periosteum. In woven bone, but not in lamellar bone, a small number of osteocytes stained for all proteins tested. After distraction, staining intensity had increased in the existing old bone and staining was seen in more bone cells than before distraction. We also found a high expression of DMP1 and MEPE in many osteocytes embedded in woven bone and in some osteocytes of lamellar bone not seen before distraction. New bone trabeculae were forming in the fibrous tissue of the distraction gap containing all stages of intramembranous bone formation. Moderate to strong staining was seen for all five proteins tested in osteocytes located in woven bone of these trabeculae and for RUNX2, OPN, and BSP in osteoblasts lining the trabecular surfaces. We conclude that loading of atrophic human jawbone by distraction activates matrix synthesis of bone cells in and around existing bone. Increased staining of DMP1 and MEPE in osteocytes after loading is in line with the concept that these proteins may be involved in signaling the effector cells to adapt the bone structure to its mechanical demands.

Low-intensity pulsed ultrasound does not enhance distraction callus in a rabbit model

Low-intensity pulsed ultrasound has been reported to have a positive effect when applied during the consolidation phase of distraction osteogenesis and bone transportation, but the optimal application time has not been determined. We used a rabbit model to determine whether low-intensity pulsed ultrasound applied during the distraction and early consolidation phases of tibial lengthening would have a positive effect on regenerated bone formation. Radiographic analysis showed no differences in regenerated callus area or in percent of callus mineralization between treated and control tibias immediately after distraction or at 1, 2, or 3 weeks after distraction. Similarly, we observed no differences in structural stiffness or maximal torque to failure at 1.5 or 3 weeks after distraction. We detected no differences in bone mineral appositional rates or percent tissue composition measured histologically between groups. Our data do not support the application of low-intensity pulsed ultrasound to regenerated bone during distraction osteogenesis.

A novel rat model for the study of deficits in bone formation in type-2 diabetes

BACKGROUND

There is evidence to suggest that impairment in bone formation and/or turnover is associated with the metabolic abnormalities characteristic of type-2 diabetes mellitus. However, bone regeneration/repair in type-2 diabetes has not been modeled. Using Zucker Diabetic Fatty (ZDF) rats (a model of type-2 diabetes) for tibial distraction osteogenesis (DO), we hypothesized that bone formation within the distraction gap would be impaired.

ANIMALS AND METHODS

Rats were examined for body weight, glycosuria, and glycosemia to confirm the diabetic condition during the study. The rats received placement of the external fixators and osteotomies on the left tibia. Distraction was initiated the following day at 0.2 mm twice a day and continued for 14 days. The lengthened tibiae were harvested and distraction gaps were examined radiographically and histologically.

RESULTS

We found significant reduction in new bone formation in the distraction gaps of the ZDF rats, both radiographically and histologically, compared to lean rats. We found a decrease in a marker of cellular proliferation in the distraction gaps and increased adipose volume in adjacent bone marrow of the ZDF rats.

INTERPRETATION

Our findings suggest that this model might be used to study the contributions of leptin resistance, insulin resistance and/or hyperglycemia to impaired osteoblastogenesis in vivo.

Identification of novel Runx2 targets in osteoblasts: Cell type-specific BMP-dependent regulation of Tram2

Runx2 is an osteoblast master transcription factor and a target for bone morphogenetic protein (BMP) signaling, but our knowledge of events downstream of Runx2 is limited. In this study, we used ChIP Display to discover seven novel genomic regions occupied by Runx2 in living MC3T3-E1 osteoblastic cells. Six of these regions are found within or up to 1-kb away from annotated genes, but only two are found within 5'-gene flanking sequences. One of the newly identified Runx2 target genes is Tram2, whose product facilitates proper folding of type I collagen. We demonstrate that Tram2 mRNA is suppressed in non-osteoblasts when Runx2 is over-expressed, and that this suppression is alleviated upon treatment with BMP-2. Moreover, we show that BMP-induced Runx2 expression in the C3H10T1/2, ST2, C2C12, and MC3T3-E1 cell lines coincides with an increase in Tram2 mRNA levels. Thus, Runx2 may regulate Tram2 expression in a BMP-dependent manner, and Tram2 may participate in the overall osteogenic function of Runx2. Among the other Runx2 target genes discovered in this study are Lnx2, an intracellular scaffolding protein that may play a role in Notch signaling, and Tnfrsf12a, a Tumor Necrosis Factor receptor family member that influences both osteoblast and osteoclast differentiation. Expanding our knowledge of Runx2 target genes, and manipulation of these genes, are warranted to better understand the regulation of osteoblast function and to provide opportunities for the development of new bone anabolics.

Bisphosphonate modulates morphological and mechanical properties in distraction osteogenesis through inhibition of bone resorption

Despite the general clinical acceptance of distraction osteogenesis and much attention to bone formation in this method, little is recognized about activated bone resorption in the regenerated bone. The purpose of this study was to demonstrate the simultaneously activated bone resorption with activated bone formation and to investigate the role and efficacy of bisphosphonate in distraction osteogenesis. Left tibiae of 54 immature rabbits were lengthened for 3 weeks at a rate of 0.7 mm/day after a 1-week lag. Regenerated bone was quantitatively investigated by radiographic bone density, bone histomorphometry, and three-point bending testing. Animals received either vehicle or nitrogen-containing bisphosphonate (N-BP), YM529/ONO5920 at doses of 0.4 mg/kg/w or 0.004 mg/kg/w for 6 weeks. Regenerated bone of the vehicle group showed a radiologically characteristic zone structure containing the osteopenic zones adjacent to the sclerotic zones. The regenerated bone of the 0.4-mg/kg/w group showed no osteopenic zones during the course and eventually became homogeneously radiodense. The bone volume corresponding to the osteopenic zone of this group was 5.6-fold greater compared with that of the vehicle group. The lengthened bone strength of this group was 3.3-fold greater in ultimate force than that of the vehicle group and equivalent to the contralateral tibia. The 0.004-mg/kg/w group had no substantial differences compared with the vehicle group, despite radiological enhancement of the mineralized front as well as somewhat delayed bone resorption. These results demonstrate that not only bone formation but also bone resorption is highly activated in the regenerated bone, implying high bone turnover. Sufficient N-BP caused a notable modulation in morphological properties of the regenerated bone through inhibition of highly activated bone resorption and eventually increased mechanical properties.

Gene identification and analysis of transcripts differentially regulated in fracture healing by EST sequencing in the domestic sheep

BACKGROUND

The sheep is an important model animal for testing novel fracture treatments and other medical applications. Despite these medical uses and the well known economic and cultural importance of the sheep, relatively little research has been performed into sheep genetics, and DNA sequences are available for only a small number of sheep genes.

RESULTS

In this work we have sequenced over 47 thousand expressed sequence tags (ESTs) from libraries developed from healing bone in a sheep model of fracture healing. These ESTs were clustered with the previously available 10 thousand sheep ESTs to a total of 19087 contigs with an average length of 603 nucleotides. We used the newly identified sequences to develop RT-PCR assays for 78 sheep genes and measured differential expression during the course of fracture healing between days 7 and 42 postfracture. All genes showed significant shifts at one or more time points. 23 of the genes were differentially expressed between postfracture days 7 and 10, which could reflect an important role for these genes for the initiation of osteogenesis.

CONCLUSION

The sequences we have identified in this work are a valuable resource for future studies on musculoskeletal healing and regeneration using sheep and represent an important head-start for genomic sequencing projects for Ovis aries, with partial or complete sequences being made available for over 5,800 previously unsequenced sheep genes.

Osteogenic Potentiation of Human Adipose???Derived Stem Cells in a 3-Dimensional Matrix

Adipose-derived stem cells (ADSCs) hold promise for use in tissue engineering. Despite growing enthusiasm for use of ADSCs, there is limited research that has examined their behavior in different in vitro and in vivo systems. The purpose of our study was to evaluate the effect of the extracellular matrix structure and composition on osteogenic differentiation by comparing the osteogenic marker expression of ADSCs grown under 2-dimensional or 3-dimensional cell culture conditions. Group 1 (2-D) included ADSCs raised under conventional cell culture conditions (cells in a 2-D monolayer configuration) (n = 24), and group 2 (3-dimensional) included ADSCs seeded in a collagen gel (cells within a 3-dimensional, biologically active environment) (n = 24). Comparison of ADSC behavior between the 2 groups was analyzed during a 14-day time frame. Osteogenic marker expression (CBFA-1, alkaline phosphatase, osteonectin, osteopontin, Collagen I, and JNK2) was quantified by real-time PCR, and histologic analysis was performed. Histologically, group 1 (2-D) showed cell spreading and deposition of a calcified extracellular matrix. Group 2 (3-dimensional) assumed a disorganized state in the collagen gel, with extension of pseudopodia throughout the matrix. Expression of CBFA-1 was up-regulated immediately in both groups. However, cells in group 2 (3-dimensional) had a more rapid and greater overall expression compared with cells in group 1 (2-D) (250-fold greater at 4 days). At day 14, cells in group 2 (3-dimensional) showed greater expression of all other osteogenic markers than cells in group 1 (2-D) (2.3-fold greater expression of alkaline phosphatase [P < 0.05], 8.4-fold greater expression of osteonectin [P < 0.05], 6.4-fold greater expression of osteopontin [P < 0.05], 2.9-fold greater expression of collagen I [P < 0.05], and 2.5-fold greater expression of JNK2 [P < 0.05]). Our data showed there was a progressive stimulatory effect on ADSCs with regard to osteogenesis when cultured in a 3-dimensional gel compared with a 2-D monolayer.

Renal osteodystrophy: alpha-Heremans Schmid glycoprotein/fetuin-A, matrix GLA protein serum levels, and bone histomorphometry

BACKGROUND

Fetuin-A of hepatic origin circulates in large amounts in serum, but also is expressed in bone, where it is an inhibitor of transforming growth factor beta (TGF-beta)/bone morphogenetic protein (BMP) proteins. Together with matrix GLA protein (MGP), fetuin-A is able to make up a complex with calcium and phosphate that is more soluble than calcium and phosphate alone, preventing its deposition in extraskeletal tissues. Experimental results suggested that this complex is made at bone tissue level. The aim of this study is to evaluate whether serum fetuin-A and MGP are influenced by type of renal osteodystrophy, they correlate with bone histomorphometric and histodynamic parameters, and/or serum levels may influence bone turnover.

METHODS

Thirty-eight hemodialysis patients who volunteered to undergo a bone biopsy were studied. Patients (27 men, 11 women) had a mean age of 55.2 +/- 11.8 years and dialysis vintage of 75.7 +/- 57.4 months. They were not administered vitamin D or drugs connected with mineral metabolism. They underwent transiliac bone biopsy after tetracycline labeling. Biopsies were performed for histological, histomorphometric, and histodynamic evaluation and aluminum histochemistry. Serum fetuin-A and MGP were measured by using enzyme-linked immunosorbent assay kits.

RESULTS

Serum fetuin-A levels were significantly less than normal, whereas MGP levels were less than the normal average. Fetuin-A levels in patients with hyperparathyroidism, mixed osteodystrophy, and low-turnover osteodystrophy were 0.219 +/- 0.1, 0.27 +/- 0.1, and 0.197 +/- 0.1 ng/mL, respectively (P = not significant). Fetuin-A level significantly correlated inversely with values for several histomorphometric parameters, such as osteoid volume (OV/BV), osteoblastic surface (Ob.S/BS), osteoid surface (OS/BS), and osteoclastic surface (Oc.S/BS). Logistic regression showed odds ratios of 5.3 and 4.9 for the association of high fetuin-A levels with low values for OS/BS and Ob.S/BS, respectively. Results of multiple regression analysis with intact parathyroid hormone and fetuin-A levels as independent variables and OV/BV and Ob.S/BS as dependent variables showed that independent variables correlated significantly with dependent variables, positively for intact parathyroid hormone levels and inversely for fetuin-A levels. MGP levels in patients with hyperparathyroidism, mixed osteodystrophy, and low-turnover osteodystrophy were not significantly different (3.94 +/- 0.86, 3.40 +/- 0.99, and 5.64 +/- 2.4 nmol/L, respectively). By dividing MGP serum values into tertiles, mean values for OV/BV were different (analysis of variance, P < 0.04), with a greater value in the higher MGP tertile. By exclusion of 3 extravariant cases (>3 SDs greater than the mean), 1 case for each type of osteodystrophy, a significant correlation between bone formation rate and MGP serum level was found (P < 0.05). In addition, a significant correlation was found between MGP level and trabecular thickness.

CONCLUSION

Fetuin-A and MGP levels correlated with bone formation parameters. This association could be caused by an effect of these proteins on bone formation, presumably mediated by the TGF-beta/BMP system. Fetuin-A, as opposed to MGP, is known to inhibit the TGF-beta/BMP complex, a protein-cytokine system that appears to be an important regulator of bone formation and probably a factor with an important role in renal osteodystrophy.

Matricellular proteins: Extracellular modulators of bone development, remodeling, and regeneration

Matricellular proteins are components of the extracellular matrix which are highly expressed in the developing and mature skeleton. Members of this protein class serve as biological mediators of cell function by interacting directly with cells or by modulating the activity of growth factors, proteases, and other extracellular matrix proteins. Although skeletons of matricellular protein-null mice are grossly normal, they each display unique deficiencies that are often magnified under pathological conditions. In addition, bone cells from wild-type and matricellular protein-null mice behave differently in various in vitro models of bone matrix synthesis and turnover. In this review, osteopontin, bone sialoprotein, tenascin C, SPARC, and thrombospondins 1 and 2 will each be discussed in the context of bone cell biology. Because the biological effects of matricellular proteins are largely context dependent, in vivo and in vitro results must be considered together in order to fully appreciate the specific contributions that matricellular proteins make to bone physiology and pathophysiology. In particular, it is clear that although matricellular proteins are not required for bone development and function, the proteins act to modulate post-natal bone structure in response to aging, ovariectomy, mechanical loading, and bone regeneration.

Reconstruction of #7 facial cleft with distraction-assisted in situ osteogenesis (DISO): role of recombinant human bone morphogenetic protein-2 with Helistat-activated collagen implant

A case involving concomitant presentation of a #7 lateral facial cleft with a complete cleft of the ipsilateral lip, alveolus, and palate is presented. The mandibular defect was Pruzansky III with a foreshortened body, absent ramus and absent masseter. Taking advantage of developmental field theory, reconstruction of the osseous defect was undertaken using the autogenous periosteum as a source of mesenchymal stem cells. Expansion of the periosteum was followed by implantation of Helistat (Integra Life Sciences, Plainsboro, NJ) collagen sponge saturated with recombinant human bone morphogenetic protein-2. Stimulation of this distraction-induced envelope by rhBMP-2 resulted in abundant production of bicortical membranous bone in situ within 12 weeks. The neoramus was subsequently suspended from the cranial base, and a temporalis muscle transfer was used to provide motor control of the jaw. Synthesis of bone in this manner is termed DISO (distraction-assisted in situ osteogenesis). The biologic rationale and clinical implications of DISO are discussed.

Dynamic expression of sparc precedes formation of skeletal elements in the Medaka (Oryzias latipes)

Sparc is a secreted calcium-binding glycoprotein that regulates mineralization of bone tissues in mammals. In other vertebrates, its function remains largely unclear. Here, we describe the isolation, genomic organization and expression of the sparc gene in the teleost Medaka (Oryzias latipes), an established vertebrate model for developmental studies. During earliest stages of Medaka embryogenesis, sparc is expressed in the sclerotome compartment of the somites that gives rise to precursor cells of the axial skeleton. Importantly, in this area its expression precedes that of twist-1, which is a crucial regulator of osteoblast formation. Dynamic expression is also found in the floor plate of the neural tube and the notochord. Both structures are passed by migrating skeletal precursors shortly before they differentiate and form the vertebrae. In general, sparc is expressed before the formation and mineralization of bone elements and expression of bone markers like collagen type 1a in the fins and axial skeleton of Medaka embryos. It is also expressed in several non-skeletal tissues of embryos and adult fish, suggesting possible other functions not related to bone mineralization. Taken together, the Medaka sparc gene represents an excellent marker for early sclerotome development. Its restricted and highly dynamic expression suggests a novel function during migration of sclerotome cells and their differentiation into early vertebrae. This marker thus allows the analysis of early skeletal development and formation of extracellular bone matrix in this vertebrate model.

Osteogenic alveolar distraction: A review of the literature

OBJECTIVE

Alveolar distraction is a relatively novel procedure by which alveolar bone and underlying mucosa are regenerated. The low predictability of other vertical or horizontal bone regeneration methods has increased interest in this promising technique. This article was designed to review published clinical and experimental results on alveolar distraction, including basic research in other disciplines (maxillofacial and orthopedic distraction) related to or with influence on alveolar distraction.

STUDY DESIGN

A review of the international literature was performed to summarize results of clinical and experimental studies on alveolar distraction and on distraction at other anatomical sites that contribute important findings on tissue biology, molecular mechanisms, and other factors that influence and participate in the alveolar distraction process.

RESULTS

Research into alveolar distraction has addressed the latency phase, distraction phase, and consolidation phase, yielding highly variable results. Little experimental research has been carried out on this procedure, and most publications are clinical studies with a short follow-up period. Published studies have reported a high rate of complications, attributable to our current lack of understanding of the process.

CONCLUSIONS

Definitive conclusions on alveolar distraction are hampered by the lack of clinical and experimental studies to date. Greater knowledge of the factors that influence the distraction process will lead to a more predictable and efficacious distraction technique and a better distractor design.

Relationships between tissue dilatation and differentiation in distraction osteogenesis

Mechanical factors modulate the morphogenesis and regeneration of mesenchymally derived tissues via processes mediated by the extracellular matrix (ECM). In distraction osteogenesis, large volumes of new bone are created through discrete applications of tensile displacement across an osteotomy gap. Although many studies have characterized the matrix, cellular and molecular biology of distraction osteogenesis, little is known about relationships between these biological phenomena and the local physical cues generated by distraction. Accordingly, the goal of this study was to characterize the local physical environment created within the osteotomy gap during long bone distraction osteogenesis. Using a computational approach, we quantified spatial and temporal profiles of three previously identified mechanical stimuli for tissue differentiation-pressure, tensile strain and fluid flow-as well as another candidate stimulus-tissue dilatation (volumetric strain). Whereas pressure and fluid velocity throughout the regenerate decayed to less than 31% of initial values within 20 min following distraction, tissue dilatation increased with time, reaching steady state values as high as 43% strain. This dilatation created large reductions and large gradients in cell and ECM densities. When combined with previous findings regarding the effects of strain and of cell and ECM densities on cell migration, proliferation and differentiation, these results indicate two mechanisms by which tissue dilatation may be a key stimulus for bone regeneration: (1) stretching of cells and (2) altering cell and ECM densities. These results are used to suggest experiments that can provide a more mechanistic understanding of the role of tissue dilatation in bone regeneration.

Effects of distraction forces and frequency of distraction on bony regeneration

The rate and frequency of distraction have a decisive influence on the regenerative process. We tested a newly developed hydraulic osteodistractor in 12 pigs, which were assigned to have continuous and intermittent osteodistraction of the mandible after osteotomy. The forces necessary to distract the mandible were recorded during intermittent distraction. These data were then used for continuous distraction of the bone. Continuous osteodistraction resulted in intramembranous regeneration of bone, whereas intermittent osteodistraction caused chondroid ossification in the regeneration of the bone. Continuous osteodistraction caused speedier regeneration, and distraction forces were lower than with intermittent distraction.

Cyclical articular joint loading leads to cartilage thinning and osteopontin production in a novel in vivo rabbit model of repetitive finger flexion

OBJECTIVE

An in vivo rabbit model of repetitive joint flexion and loading was used to characterize the morphological effects of cyclical loading on articular cartilage.

DESIGN

The forepaw digits of eight anesthetized New Zealand White adult female rabbits were repetitively flexed at 1 Hz with a mean peak digit load of 0.42 N for 2 h per day for 60 cumulative hours. Metacarpophalangeal joints were collected from loaded and contra-lateral control limbs, fixed, decalcified, embedded, and thin-sectioned. Serial sections were stained for histology or for immunohistochemistry. Morphometric data including the mean thicknesses of the uncalcified cartilage and of the calcified cartilage were collected from digital photomicrographs of safranin O-stained sections. The number of cells stained with anti-osteopontin antibody was counted.

RESULTS

We observed a decrease in uncalcified cartilage mean thickness with no significant change in calcified cartilage thickness. We also observed a significant increase in the number of cells positive for osteopontin (OPN) in the uncalcified cartilage. These changes occurred without overt cartilage surface degeneration.

CONCLUSIONS

Cyclical loading leads to changes at the tissue and cellular levels in articular cartilage. These changes are suggestive of tidemark advancement and may indicate a reactivation of cartilage mineralization steps analogous to endochondral ossification. This novel in vivo rabbit model of repetitive flexion and loading can be used to investigate the effects of cyclical loading on articular joints.

Wavy-Walled Bioreactor Supports Increased Cell Proliferation and Matrix Deposition in Engineered Cartilage Constructs

Hydrodynamic forces in bioreactors can decisively influence extracellular matrix deposition in engineered cartilage constructs. In the present study, the reduced fluid shear, high-axial mixing environment provided by a wavy-walled bioreactor was exploited in the cultivation of cartilage constructs using polyglycolic acid scaffolds seeded with bovine articular chondrocytes. Increased growth as defined by weight, cell proliferation and extracellular matrix deposition was observed in cartilage constructs from wavy-walled bioreactors in comparison with those from spinner flasks cultured under the same conditions. The wet weight composition of 4-week constructs from the wavy-walled bioreactor was similar to that of spinner flask constructs, but the former were 60% heavier due to equally higher incorporation of extracellular matrix and 30% higher cell population. It is most likely that increased construct matrix incorporation was a result of increased mitotic activity of chondrocytes cultured in the environment of the wavy-walled bioreactor. A layer of elongated cells embedded in type I collagen formed at the periphery of wavy-walled bioreactor and spinner flask constructs, possibly as a response to local shear forces. On the basis of the robustness and reproducibility of the extracellular matrix composition of cartilage constructs, the wavy-walled bioreactor demonstrated promise as an experimental cartilage tissue-engineering vessel. Increased construct growth in the wavy-walled bioreactor may lead to enhanced mechanical properties and expedited in vitro cultivation.

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.

Bone formation in beta-tricalcium phosphate-filled bone defects of the rat femur: morphometric analysis and expression of bone related protein mRNA

The purpose of the current study was to evaluate the bone formation when beta-tricalcium phosphate (TCP) was implanted in bone defects of rat femurs. beta-TCP granules were applied to defects created in the femurs of 65 male rats who were sacrificed 3, 7, 10, 14 or 30 days later. Bone tissues were embedded in paraffin, serial sections were cut and then stained with hematoxylin-eosin. Histomorphometric analyses were also conducted. Furthermore, total mRNAs were extracted, homogenized, and reverse transcribed, after which quantitative PCR assays were conducted with a LightCycler using the double-stranded DNA dye Syber Green I with primers for either rat osteopontin or osteocalcin. Tissues in defects without beta-TCP were used as controls. The amount of newly formed bone tissue in the beta-TCP implanted group was significantly greater in both the side areas and the central area of defects than in the control group. Expressions of osteopontin and osteocalcin mRNAs of cells in the defects of the experimental group were up-regulated compared with the control group at all time periods. Taken together, these results prove that beta-TCP is an appropriate material for osteoconduction and promotes bone formation in bone defects.

Mechanics and mechano-biology of fracture healing in normal and osteoporotic bone

Fracture repair, which aims at regaining the functional competence of a bone, is a complex and multifactorial process. For the success of fracture repair biology and mechanics are of immense importance. The biological and mechanical environments must be compatible with the processes of cell and tissue proliferation and differentiation. The biological environment is characterized by the vascular supply and by many biochemical components, the biochemical milieu. A good vascular supply is a prerequisite for the initiation of the fracture repair process. The biochemical milieu involves complex interactions among local and systemic regulatory factors such as growth factors or cytokines. The mechanical environment is determined by the local stress and strain within the fracture. However, the local stress and strain is not accessible, and the mechanical environment, therefore, is described by global mechanical factors, e.g., gap size or interfragmentary movement. The relationship between local stress and strain and the global mechanical factors can be obtained by numerical models (Finite Element Model). Moreover, there is considerable interaction between biological factors and mechanical factors, creating a biomechanical environment for the fracture healing process. The biomechanical environment is characterized by osteoblasts and osteocytes that sense the mechanical signal and express biological markers, which effect the repair process. This review will focus on the effects of biomechanical factors on fracture repair as well as the effects of age and osteoporosis.

Collagen Type I Increases Bone Remodelling around Hydroxyapatite Implants in the Rat Tibia

The early interface reaction of cancellous bone to a nanocrystalline hydroxyapatite (HA) cement containing 3 wt% collagen type I (HA/Coll) with a setting under physiological temperature and pH was observed using immunohistochemical techniques. Pure HA served as a control. Cylinders with a diameter of 2 mm were implanted into the proximal tibia of 72 adult Wistar rats. Histological sections of 6 animals were prepared after 1, 2, 4, 6, 14 and 28 days. First, osteoblast-like cells as well as a marked reaction for osteonectin, osteopontin and its ligand CD44 were observed as early as 2 days after implantation at the interface around HA/Coll implants. Further, reactivity for ED1 and cathepsin D, both markers for phagocytotic cells, appeared earlier and stronger around HA/Coll. In cell counts, a significantly higher average number of ED1- and cathepsin D-positive phagocytotic cells was observed around the HA/Coll implants on days 6 (p < 0.01), 14 and 28 (p < 0.05). The number of osteopontin-positive cells was significantly higher around HA/Coll implants at days 6 and 14 (p < 0.05). Two weeks after the implantation, first islands of newly formed woven bone were observed around the HA/Coll implant, but not around the control implant. The amount of direct bone contact after 28 days averaged 28% around pure HA and 51% around HA/Coll implants (p < 0.05). While both implants displayed a good osteoconductivity, a higher bone remodelling activity was observed around collagen-containing HA implants compared to pure HA implants. It appears that the addition of collagen to HA implants can enhance both phagocytotic and osteogenic processes. This may result in an earlier acceptance and better osseointegration of the HA/Coll implants into the surrounding tissue.

An In Vitro Model for Dissecting Distraction Osteogenesis

Distraction osteogenesis (DO) is a mechanotransduction process capable of generating viable osseous tissue by the gradual separation of osteotomized bone edges. Several variables are implicated in DO: magnitude of mechanical strain, distraction rate, and type of distracted bone. The combination of these factors acts on different types of cells inducing apoptosis, cell proliferation, and differentiation. The elucidation of the molecular mechanisms has important clinical implications because it may facilitate the use of recombinant proteins or gene therapy to accelerate bone regeneration. Previous reports have analyzed several molecules such as extracellular matrix proteins, cytokines, bone morphogenetic proteins, hormones, and angiogenic factors. Moreover, a single protein can have multifunctional roles. With such a huge number of mechanical, histologic, cellular, and molecular variables, there is the need to have a cell culture model that enables the selection of the effect of a specific strength to a single cell type at different time points and with or without cytokines. The analysis of the genetic profiling of a cell line cultured on an equibiaxial stretch device has such characteristic. Because there is a recruitment and commitment of preosteoblastic cells during bone lengthening and no previous report has focus on them, the authors used a preosteoblast MC3T3-E1 cell line to detect the early molecular effects of distraction on mesenchymal cells. By using DNA microarrays containing 15,000 clones, the authors identified several genes the expression of which was significantly up- or down-regulated. The differentially expressed genes cover a broad range of biological processes: cell growth, metabolism, morphogenesis, cell communication, response to stress, and cell death. The data reported are the first genetic portrait of stretched preosteoblasts. They can be relevant in the better understanding of the molecular mechanism of DO and as a model for comparing the effect of distraction on different cell lines and primary cultures, rate and strength of distraction, and with or without cytokines.

The role of bone morphogenetic proteins BMP-2 and BMP-4 and their related postreceptor signaling system (Smads) in distraction osteogenesis of the mandible

Distraction osteogenesis has become a widely used clinical approach in the treatment of craniofacial and orthopedic disorders. The exact biological mechanism of bone formation remains illusive, however. The aim of this study was to evaluate the role of bone morphogenetic protein-2, bone morphogenetic protein-4, and transforming growth factor-beta superfamily-related postreceptor signaling glycoproteins Smads 1 through 5 in distraction osteogenesis. Twelve sheep randomly divided into two groups were distracted to 24 mm at 1 or 4 mm/d using a submandibular osteotomy and an external distractor. After a 5-week fixation period, the mandibles were harvested. Employing immunohistochemical techniques, the sections were investigated for the previous antigens. Osteoblasts and periosteal lining cells were strongly positive. The matrix did not stain for the antigens investigated. Osteocytes demonstrated weak staining for the antigens. No significant difference between the groups was noted. In fracture healing, bone morphogenetic proteins 2 and 4 have been localized to the cambial layer of the periosteum, where healing occurs by intramembranous ossification. Their diffuse staining of the osteoblasts in the distracted region supports a similar role in distraction osteogenesis, where bone formation is predominantly through intramembranous ossification. Furthermore, bone morphogenetic proteins 2 and 4 have been demonstrated to promote mesenchymal cell conversion to osteoblasts. This is similar to the process observed in distraction osteogenesis. The presence of related Smads confirms postreceptor activity of these bone morphogenetic proteins in the process of distraction osteogenesis. This study supports induction of bone morphogenetic proteins 2 and 4, their related postreceptor signaling system (Smads), and intramembranous bone formation associated with mechanical strain in distraction osteogenesis.

Matrix-mediated retention of osteogenic differentiation potential by human adult bone marrow stromal cells during ex vivo expansion

During prolonged cultivation ex vivo, adult bone marrow stromal stem cells (BMSCs) undergo two probably interdependent processes, replicative aging and a decline in differentiation potential. Recently, our results with primary human fibroblasts indicated that growth on denatured collagen (DC) matrix results in the reduction of the rate of cellular aging. The present study has been undertaken to test whether the growth of human BMSCs under the same conditions would translate into preservation of cellular aging-attenuated functions, such as the ability to express HSP70 in response to stress as well as of osteogenic differentiation potential. We report here that growth of BMSCs on a DC matrix versus tissue culture polystyrene significantly reduced one of the main manifestations of cellular aging, the attenuation of the ability to express a major protective stress response component, HSP70, increased the proliferation capacity of ex vivo expanded BMSCs, reduced the rate of morphological changes, and resulted in a dramatic increase in the retention of the potential to express osteogenic-specific functions and markers upon treatment with osteogenic stimulants. BMSCs are a promising and increasingly important cell source for tissue engineering as well as cell and gene therapeutic strategies. For use of BMSCs in these applications, ex vivo expansion is necessary to obtain a sufficient, therapeutically useful, number of cells; however, this results in the loss of differentiation potential. This problem is especially acute in older patients where more extensive in vitro expansion of smaller number of stem/progenitor cells is needed. The finding that growth on certain biomaterials preserves aging-attenuated functions, enhances proliferation capacity, and maintains differentiation potential of BMSCs indicates a promising approach to address this problem.

Coating of titanium implants with type-I collagen

PURPOSE

Type-I collagen, the major structural protein in bone, has beneficial properties regarding bone regeneration. Little is known about the potential effects of collagen coating on orthopedic implants.

METHODS

3 to 6 microg/cm2 of lyophilized type-I collagen was absorbed on titanium rods. Six coated and uncoated pins of 0.9 mm diameter were inserted into the tibia of adult male Wistar rats for 1, 2, 4, 7, 14, and 28 days. Specimens were embedded in methacrylate-based Technovit 9100N resin. From one portion cutting and grinding sections were obtained. The implant was removed from the other half that was depolymerized, sectioned, and mounted for immunohistochemistry.

RESULTS

At day 4, the interface around the collagen-coated implants displayed a granulation tissue with higher numbers of cathepsin D-positive mononucleated cells compared to the uncoated implants (p<0.05). Active osteoblasts, reactive for osteopontin, were increased around the collagen-coated pins at day 4 and 7 (p<0.01). After 28 days of implantation, direct bone contact averaged 74.9% around the collagen-coated implants and 62.1% around uncoated implants (NS). The amount of newly formed bone averaged 76.3% around the collagen-coated pins and 67.8% around the uncoated pins (NS). The histomorphometric findings were confirmed by SRmicroCT in two specimens.

CONCLUSIONS

The earlier observation of mononuclear phagocytozing cells and the earlier and higher expression of bone-specific matrix proteins suggest an increased early bone remodeling around titanium pins through collagen coating. A tendency towards increased bone formation was observed around the coated implants.

Intermittent parathyroid hormone (1-34) enhances mechanical strength and density of new bone after distraction osteogenesis in rats

Distraction osteogenesis is used both for leg lengthening and for bone transportation in the treatment of fractures and nonunions. The main problem with this method is that the time until full recovery may be up to a year, partly because of the time needed for the new formed bone to consolidate and become strong enough for weight bearing. We have studied whether intermittent parathyroid hormone (PTH(1-34)) could accelerate the consolidation of new formed bone after distraction osteogenesis in rats. Forty-seven, 3-months-old male Sprague-Dawley rats underwent lengthening of the right femur using an external fixator. After a middiaphyseal osteotomy and a 7-day latency period, the callus was distracted during 10 days, with a distraction rate of 0.25 mm twice a day. The consolidation time was either 20 days or 40 days after distraction was completed. A dose of 60 microg of human PTH(1-34)/kg body weight/injection or vehicle was given every second day beginning 30 days before the rats were killed. Both femura of each rat were subjected to mechanical testing and dual-energy X-ray absorptiometry. Blinded histological examination was done for the distracted femura. In the 20 days consolidation experiment, PTH(1-34) increased ultimate load (56%), stiffness (117%), total regenerate callus volume (58%), callus BMC (24%) and histologic bone density (35%) compared to untreated distraction osteogenesis specimens. In the 40 days consolidation experiment, PTH(1-34) increased ultimate load (54%), stiffness (55%), callus BMC (33%) and histologic bone density (23%) compared to untreated distraction osteogenesis specimens. Total regenerate callus volume was unchanged. The contralateral femur also became stronger, stiffer and denser under PTH(1-34) treatment, but to a lesser degree. PTH(1-34) might become useful to shorten the consolidation time after distraction osteogenesis in humans.

The role of angiogenesis in a murine tibial model of distraction osteogenesis

Distraction osteogenesis (DO) is one of the most dramatic in vivo applications of mechanical stimulation as a means of inducing bone regeneration. A simple and reproducible murine model of tibia distraction osteogenesis was developed using a monolateral fixator. Bone formation was assessed histologically over a 35-day time course. The steady state expression of a broad family of angiogenesis-associated genes was assessed by microarray hybridization analyses over the same time course, while the immediate gene response that was induced during each cycle of distraction was assessed at 30 min and 8 h after the first and last rounds of activation of the fixator. Distraction osteogenesis promoted new bone formation primarily through an intramembranous process with maximal osteogenesis during the active distraction period. Histological analysis also showed that dense cortical bone continued to be formed, during the consolidation phase, for 2 weeks after distraction ended. The analysis of steady state mRNA expression levels over the time course of DO showed that VEGF-A and neuropilin, an alternate receptor for VEGF-A, both angiopoietin (Ang) 1 and 2 factors, and the Ang receptor Tie2 were the critical angiogenic factors during DO. A key transcriptional regulator of many of the angiogenic factors, hypoxia-induced factor1alpha (Hif-1a), the FGF binding protein pleiotropin/OSF1, and multiple MMP(s), were also induced during the active distraction period. Examination of the expression of angiogenic factors that were induced after each cycle of activation, demonstrated that Hif-1a, Nrp1, and VEGF-A were all cyclically induced after each increment of distraction. These results suggest that these factors are early mediators that are produced by distraction and contribute toward the processes that promote bone formation. These experiments represent the first step in defining the molecular mechanisms that regulate skeletal regeneration and the functional relationship between angiogenesis and osteogenesis during distraction osteogenesis.

Biologie moléculaire de la distraction osseuse

Distraction osteogenesis has become a mainstay in bone engineering and the recent application of this technique to the membranous craniofacial skeleton has significantly improved our armamentarium for reconstructive craniomaxillofacial procedures. However, if the biomechanical, histological and ultrastructural changes associated with distraction osteogenesis have been widely described, the molecular mechanisms governing the formation of new bone in the interfragmental gap of gradually distracted bone segments remain largely unclear. Recently, our laboratory has described a rat mandibular distraction model that provides an excellent environment for deciphering the molecular mechanisms that mediate distraction osteogenesis. In this Article, we present the hypotheses and current research that have furthered our knowledge of the molecular mechanisms that govern distraction osteogenesis. Recent studies have implicated a growing number of cytokines that are intimately involved in the regulation of bone synthesis and turnover. The gene regulation of numerous cytokines (Transforming Growth Factor-B, Bone Morphogenetic Proteins, Insulin-like Growth Factor-1, Fibroblast Growth Factor-2) during distraction osteogenesis have been best characterized and will be discussed in this text. We believe that novel systems like the rat model will facilitate our understanding of the biomolecular mechanisms that mediate membranous distraction osteogenesis and will ultimately guide the development of targeted-strategies designed to accelerate bone healing.

Identification of promoter regions involved in cell- and developmental stage-specific osteopontin expression in bone, kidney, placenta, and mammary gland: an analysis of transgenic mice

Cell-specific expression of GFP under the control of different lengths of the osteopontin promoter in transgenic mice identified the positive and negative regulatory regions for respective cell types. The results provide new insights for physiological and pathological expression of the osteopontin gene.

INTRODUCTION

Osteopontin (OPN) is a major non-collagenous bone matrix protein that is involved in normal and pathological calcification and is expressed in a tissue-specific manner. To investigate how such tissue-specific OPN gene expression is regulated in vivo, transgenic mice expressing the green fluorescent protein (GFP) reporter gene controlled by different lengths of the OPN promoter were generated.

MATERIALS AND METHODS

Cell- and developmental stage-specific osteopontin expression in transgenic mice was examined by Northern blotting, immunoblotting, fluorescence examination, and in situ hybridization and compared with those of OPN.

RESULTS AND CONCLUSIONS

The line bearing the -5505 to +14 region of the OPN promoter was shown by Northern blotting and immunoblotting to express GFP in the same cells that express endogenous OPN (osteoblasts, hypertrophic chondrocytes, renal and mammary gland epithelial cells, and granulated metrial gland [GMG] placental cells) at the same stage in development. Thus, the 5.5-kb -5505 to +14 promoter region is sufficient for proper tissue-specific OPN expression. The lines carrying shorter segments of the OPN promoter showed different expression patterns. These patterns revealed a putative cis-acting element in the -5269 to -5263 region that restricts OPN expression to hypertrophic chondrocytes and a mammary gland-specific expressing element and a GMG cell-specific enhancing element in the -5505 to -3156 region. Furthermore, the -3155 to -1576 region seems to contain positive renal epithelial cell- and GMG cell-specific expression motif(s) as well as a negative regulatory element that prevents OPN expression in fibroblasts. Moreover, the -1576 to -910 region seems to contain a positive osteoblast-specific-expressing element. Thus, the 5.5-kb OPN promoter contains multiple cis-acting elements encoding positive and negative cell-specific regulatory systems.

Characterization of the development of ectopic chondroid/bone matrix and chondrogenic/osteogenic cells during osteoinduction by rhBMP-2: a histochemical and ultrastructural study

OBJECTIVE

To investigate the characteristics of ectopic chondroid/bone matrix and chondrogenic/osteogenic cells induced by recombinant human bone morphogenetic protein-2 (rhBMP-2).

MATERIALS AND METHODS

rhBMP-2 (5 microg) combined with atelocollagen was implanted into calf muscles of rats and removed on days 7, 10, 14, 21, or 28. Tissue sections were examined using: (i) hematoxylin/Alcian blue/Sirius red stain, (ii) enzyme histochemistry for alkaline phosphatase (ALP) and tartrate-resistant acid phosphatase activity, (iii) immunohistochemistry for types I, II, and X collagen, and (iv) electron microscopy.

RESULTS

On day 7, numerous fibroblast-like cells with ALP activity were present on the pellet rim. On day 10, chondroid matrix (CM) had formed, contained both type I collagen and proteoglycans, and often continued into the BMP pellet. On day 14, bone-like matrix formed around hypertrophic chondrocytes simultaneously with endochondral ossification. Coexpression of types I and II collagen within chondrocytes and osteocytes was observed throughout the time course of the experiment.

CONCLUSION

These results suggest that fibroblast-like cells invading the pellet differentiate into chondrocytes and form CM under the scaffold of the carrier component. It appears that some chondrocytes change their phenotype to produce the bone-like matrix and remain within the endochondral bone. This process enables rapid osteogenesis to occur.

Mechanical Strain Affects Dura Mater Biological Processes: Implications for Immature Calvarial Healing

The human brain grows rapidly during the first 2 years of life. This growth generates tensile strain in the overlying dura mater and neurocranium. Interestingly, it is largely during this 2-year growth period that infants are able to reossify calvarial defects. This clinical observation is important because it suggests that calvarial healing is most robust during the period of active intracranial volume expansion. With a rat model, it was previously demonstrated that immature dura mater proliferates more rapidly and produces more osteogenic cytokines and markers of osteoblast differentiation than does mature dura mater. It was therefore hypothesized that mechanical strain generated by the growing brain induces immature dura mater proliferation and increases osteogenic cytokine expression necessary for growth and healing of the overlying calvaria. Human and rat (n = 40) intracranial volume expansion was calculated as a function of age. These calculations demonstrated that 83 percent of human intracranial volume expansion is complete by 2 years of age and 90 percent of Sprague-Dawley rat intracranial volume expansion is achieved by 2 months of age. Next, the maximal daily circumferential tensile strains that could be generated in immature rat dura mater were calculated, and the corresponding daily biaxial tensile strains in the dura mater during this 2-month period were determined. With the use of a three-parameter monomolecular growth curve, it was calculated that rat dura mater experiences daily equibiaxial strains of at most 9.7 percent and 0.1 percent at birth (day 0) and 60 days of age, respectively. Because it was noted that immature dural cells may experience tensile strains as high as approximately 10 percent, neonatal rat dural cells were subjected to 10 percent equibiaxial strain in vitro, and dural cell proliferation and gene expression profiles were analyzed. When exposed to mechanical strain, immature dural cells rapidly proliferated (5.8-fold increase in proliferating cell nuclear antigen expression at 24 hours). Moreover, mechanical strain induced marked up-regulation of dural cell osteogenic cytokine production; transforming growth factor-beta1 messenger RNA levels increased 3.4-fold at 3 hours and fibroblast growth factor-2 protein levels increased 4.5-fold at 24 hours and 5.6-fold at 48 hours. Finally, mechanical strain increased dural cell expression of markers of osteoblast differentiation (2.8-fold increase in osteopontin levels at 3 hours). These findings suggest that mechanical strain can induce changes in dura mater biological processes and gene expression that may play important roles in coordinating the growth and healing of the neonatal calvaria.

Difference of osteopontin gene regulation between bone and kidney

Osteopontin is a sialoprotein that is expressed in various cells. It plays a variety of important roles in cell adhesion, migration, signaling, calcification, and immunity. Its diverse functions indicate that the regulation of osteopontin may also vary extensively among tissues. Although osteopontin promoter has been studied in vitro, in vivo analyses may be more appropriate for elucidating osteopontin's functions. In an attempt to investigate osteopontin gene expression, we generated transgenic mice in which the bacterial beta-galactosidase reporter gene was conjugated downstream of osteopontin promoter. The osteopontin promoter was a mouse -910 bp upstream fragment, which we had previously found functional in 3T3 cells. Among 34 transgenic founders, 13 mice were transgenic, as determined with the polymerase chain reaction. Osteopontin and beta-galactosidase signals were evaluated with in situ hybridization. Among the 13 transgenic mice, 3 were beta-galactosidase-positive. In these transgenic mice, osteopontin signals were observed in bones and kidneys, whereas beta-galactosidase message was detected only in bones. This suggests that the -910 bp osteopontin promoter is active in bones but not in kidneys. These data imply that the promoter region required for osteopontin expression in kidneys may differ from that in bones.

Osteopontin expression in osteoblasts and osteocytes during bone formation under mechanical stress in the calvarial suture in vivo

To clarify the role of OPN in bone formation under mechanical stress, we examined the expression and the function of OPN in bone using an expansion force-induced osteogenesis model. Our results indicated that OPN expression was enhanced during the bone formation and that OPN would be one of the positive factors for the bone formation under mechanical stress.

INTRODUCTION

Bone formation is known to be stimulated by mechanical stress; however, molecules involved in stress-dependent regulation of bone formation have not yet been fully characterized. Extracellular matrix proteins such as osteopontin (OPN) could play a role in mediation of the mechanical stress signal to osteoblasts. However, the function of OPN in bone formation under mechanical force is not known. Therefore, we examined the expression and the role of OPN in bone formation in vivo under tensile mechanical stress.

MATERIALS AND METHODS

Sagittal sutures of mice were subjected to expansion mechanical stress by setting orthodontic spring wires, and OPN expression during bone formation within the suture gap was examined.

RESULTS

Expansion of the sutures resulted in bone formation at the edges of the parietal bones within the sagittal suture. Immunohistochemical analysis revealed abundant accumulation of OPN protein in the matrix of newly formed bone on the inner edge of the parietal bone within the mechanically expanded sutures. Osteoblasts forming bone within the suture subjected to tensile stress also exhibited high levels of OPN protein expression. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis indicated that OPN mRNA expression was enhanced in wild-type calvariae subjected to expansion force compared with the control calvariae where dead spring wires were set without expansion stress. In addition, type I collagen mRNA was also expressed in the calvariae under the mechanical stimuli. To understand the function of OPN, sagittal sutures in OPN-deficient mice were subjected the expansion stress, and bone formation within the suture to fill the expanded gap was compared with that observed in wild-type mice. OPN deficiency reduced bone formation at the edge of the parietal bone in contact with the expanded suture gap.

CONCLUSIONS

These observations revealed that OPN plays a pivotal role in bone formation under tensile mechanical stress.