Effects of Cyclic Tensile Forces on the Expression of Vascular Endothelial Growth Factor (VEGF) and Macrophage-colony-stimulating Factor (M-CSF) in Murine Osteoblastic MC3T3-E1 Cells

Cyclic tensile force modifies calvarial osteoblast function via the interplay between ERK1/2 and STAT3

BACKGROUND

Mechanical therapies, such as distraction osteogenesis, are widely used in dental clinics. During this process, the mechanisms by which tensile force triggers bone formation remain of interest. Herein, we investigated the influence of cyclic tensile stress on osteoblasts and identified the involvement of ERK1/2 and STAT3.

MATERIALS AND METHODS

Rat clavarial osteoblasts were subjected to tensile loading (10% elongation, 0.5 Hz) for different time periods. RNA and protein levels of osteogenic markers were determined using qPCR and western blot after inhibition of ERK1/2 and STAT3. ALP activity and ARS staining revealed osteoblast mineralization capacity. The interaction between ERK1/2 and STAT3 was investigated by immunofluorescence, western blot, and Co-IP.

RESULTS

The results showed that tensile loading significantly promoted osteogenesis-related genes, proteins and mineralized nodules. In loading-induced osteoblasts, inhibition of ERK1/2 or STAT3 decreased osteogenesis-related biomarkers significantly. Moreover, ERK1/2 inhibition suppressed STAT3 phosphorylation, and STAT3 inhibition disrupted the nuclear translocation of pERK1/2 induced by tensile loading. In the non-loading environment, inhibition of ERK1/2 hindered osteoblast differentiation and mineralization, while STAT3 phosphorylation was elevated after ERK1/2 inhibition. STAT3 inhibition also increased ERK1/2 phosphorylation, but did not significantly affect osteogenesis-related factors.

CONCLUSION

Taken together, these data suggested that ERK1/2 and STAT3 interacted in osteoblasts. ERK1/2-STAT3 were sequentially activated by tensile force loading, and both affected osteogenesis during the process.

Cyclic tensile forces enhance the angiogenic properties of HUVECs by promoting the activities of human periodontal ligament cells

BACKGROUND

This study aimed to investigate whether human periodontal ligament (PDL) cells secrete pro-angiogenic factors that induce the vascularization of surrounding bone tissue under tensile stress.

METHODS

Quantitative real-time PCR and Western blotting were used to analyze the mRNA and protein expression levels of vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), Angiopoietin-I (Ang-I), connective tissue growth factor （CTGF）, and macrophage colony-stimulating factor (M-CSF) in PDL cells after tensile force treatments of different durations. Enzyme-linked immunosorbent assay was used to measure the VEGF concentration in the supernatants of cell cultures. Cell viability assay, wound healing assay, and tube formation assay were performed to evaluate the angiogenic behaviors of human umbilical vein endothelial cells (HUVECs).

RESULTS

The mRNA expression and protein expression of VEGF, bFGF, Ang-I, and M-CSF was increased in the cells that received 6 to 48 hours of tensile force treatment. And, the VEGF level in the supernatant significantly increased in the human PDL cell cultures stressed for 6 to 48 hours. The abilities of HUVECs to proliferate, migrate, and form tubes were enhanced in media conditioned with tensile-stressed human PDL cells. Hence, tensile force induced human PDL cells to express and release pro-angiogenic factors enhancing the proliferation, migration, and angiogenic capacity of HUVECs.

CONCLUSION

Tensile stress induced human PDL cells to express and release pro-angiogenic factors, including VEGF, bFGF, Ang-I, and M-CSF, thereby enhancing the proliferation, migration, and angiogenic capacity of HUVECs.

Vibration acceleration promotes endochondral formation during fracture healing through cellular chondrogenic differentiation

Vibration acceleration through whole body vibration has been reported to promote fracture healing. However, the mechanism responsible for this effect remains unclear. Purpose of this study was to determine whether vibration acceleration directly affects cells around the fracture site and promotes endochondral ossification. Four-week-old female Wistar Hannover rats were divided into two groups (vibration [V group] and control [C group]). The eighth ribs on both sides were cut vertically using scissors. From postoperative day 3 to 11, vibration acceleration using Power Plate^® (30 Hz, low amplitude [30-Low], 10 min/day) was applied in the V group. Mature calluses appeared earlier in the V group than in the C group by histological analysis. The GAG content in the fracture callus on day 6 was significantly higher in the V group than in the C group. The mRNA expressions of SOX-9, aggrecan, and Col-II in the fracture callus on day 6 and Col-X on day 9 were significantly higher in the V group than in the C group. For in vitro analysis, four different conditions of vibration acceleration (30 or 50 Hz with low or high amplitude [30-Low, 30-High, 50-Low, and 50-High], 10 min/day) were applied to a prechondrogenic cell (ATDC5) and an undifferentiated cell (C3H10T1/2). There was no significant difference in cell proliferation between the control and any of the four vibration conditions for both cell lines. For both cell lines, alcian blue staining was greater under 30-Low and 50-Low conditions than under control as well as 30-High and 50-High conditions on days 7 and 14. Vibration acceleration under 30-L condition upregulated chondrogenic gene expressions of SOX-9, aggrecan, Col-II, and Col-X. Low-amplitude vibration acceleration can promote endochondral ossification in the fracture healing in vivo and chondrogenic differentiation in vitro.

Mechanically Loading Cell/Hydrogel Constructs with Low-Intensity Pulsed Ultrasound for Bone Repair

Low-intensity pulsed ultrasound (LIPUS) has been shown to be effective for orthopedic fracture repair and nonunion defects, but the specific mechanism behind its efficacy is still unknown. Previously, we have shown a measurable acoustic radiation force at LIPUS intensities traditionally used for clinical treatment and have applied this force to osteoblastic cells encapsulated in type I collagen hydrogels. Our goal in this study is to provide insight and inform the appropriate design of a cell therapy approach to bone repair in which osteoblasts are embedded in collagen hydrogels, implanted into a bony defect, and then transdermally stimulated using LIPUS-derived acoustic radiation force to enhance bone formation at the earliest time points after bone defect repair. To this end, in this study, we demonstrate the ability to measure local hydrogel deformations in response to LIPUS-induced acoustic radiation force and reveal that hydrogel deformation varies with both LIPUS intensity and hydrogel stiffness. Specifically, hydrogel deformation is positively correlated with LIPUS intensity and this deformation is further increased by reducing the stiffness of the hydrogel. We have also shown that encapsulated osteoblastic cells respond to increases in LIPUS intensity by upregulating both cyclooxygenase 2 and prostaglandin E₂ (PGE₂), both implicated in new bone formation and well-established responses to the application of fluid forces on osteoblast cells. Finally, we demonstrate that combining an increase in LIPUS with a three-dimensional culture environment upregulates both markers beyond their expression noted from either experimental condition alone, suggesting that both LIPUS and hydrogel encapsulation, when combined and modulated appropriately, can enhance osteoblastic response considerably. These studies provide important information toward a clinically relevant cell therapy treatment for bone defects that allows the transdermal application of mechanical loading to bone defects without physically destabilizing the defect site.

Impact of mechanical stretch on the cell behaviors of bone and surrounding tissues

Mechanical loading is recognized to play an important role in regulating the behaviors of cells in bone and surrounding tissues in vivo. Many in vitro studies have been conducted to determine the effects of mechanical loading on individual cell types of the tissues. In this review, we focus specifically on the use of the Flexercell system as a tool for studying cellular responses to mechanical stretch. We assess the literature describing the impact of mechanical stretch on different cell types from bone, muscle, tendon, ligament, and cartilage, describing individual cell phenotype responses. In addition, we review evidence regarding the mechanotransduction pathways that are activated to potentiate these phenotype responses in different cell populations.

Current status of temporomandibular joint disorders and the therapeutic system derived from a series of biomechanical, histological, and biochemical studies

This article was designed to report the current status of temporomandibular joint disorders (TMDs) and the therapeutic system on the basis of a series of clinical, biomechanical, histological and biochemical studies in our research groups. In particular, we have focused on the association of degenerative changes of articular cartilage in the mandibular condyle and the resultant progressive condylar resorption with mechanical stimuli acting on the condyle during the stomatognathic function. In a clinical aspect, the nature and prevalence of TMDs, association of malocclusion with TMDs, association of condylar position with TMDs, association of craniofacial morphology with TMDs, and influences of TMDs, TMJ-osteoarthritis (TMJ-OA) in particular, were examined. In a biomechanical aspect, the nature of stress distribution in the TMJ from maximum clenching was analyzed with finite element method. In addition, the pattern of stress distribution was examined in association with varying vertical discrepancies of the craniofacial skeleton and friction between the articular disk and condyle. The results demonstrated an induction of large compressive stresses in the anterior and lateral areas on the condyle by the maximum clenching and the subsequent prominent increases in the same areas of the mandibular condyle as the vertical skeletal discrepancy became more prominent. Increase of friction at the articular surface was also indicated as a cause of larger stresses and the relevant disk displacement, which further induced an increase in stresses in the tissues posterior to the disks, indicating an important role of TMJ disks as a stress absorber. In a histological or biological aspect, increase in TMJ loading simulated by vertical skeletal discrepancy, which has already been revealed by the preceding finite element analysis or represented by excessive mouth opening, produced a decrease in the thickness of cartilage layers, an increase in the numbers of chondroblasts and osteoclasts and the subsequent degenerative changes in the condylar cartilage associated with the expression of bone resorption-related factors. In a biochemical or molecular and cellular aspect, excessive mechanical stimuli, irrespective of compressive or tensile stress, induced HA fragmentation, expression of proinflammatory cytokines, an imbalance between matrix metalloproteinases and the tissue inhibitors, all of which are assumed to induce lower resistance to external stimuli and degenerative changes leading to bone and cartilage resorption. Excessive mechanical stimuli also reduced the synthesis of superficial zone protein in chondrocytes, which exerts an important role in the protection of cartilage and bone layers from the degenerative changes. It is also revealed that various cytoskeletal changes induced by mechanical stimuli are transmitted through a stretch-activated or Ca2+channel. Finally, on the basis of the results from a series of studies, it is demonstrated that optimal intra-articular environment can be achieved by splint therapy, if indicated, followed by occlusal reconstruction with orthodontic approach in patients with myalgia of the masticatory muscles, and TMJ internal derangement or anterior disk displacement with or without reduction. It is thus shown that orthodontic treatment is available for the treatment of TMDs and the long-term stability after treatment.

Osteoimmunology in orthodontic tooth movement

The skeletal and immune systems share a multitude of regulatory molecules, including cytokines, receptors, signaling molecules, and signaling transducers, thereby mutually influencing each other. In recent years, several novel insights have been attained that have enhanced our current understanding of the detailed mechanisms of osteoimmunology. In orthodontic tooth movement, immune responses mediated by periodontal tissue under mechanical force induce the generation of inflammatory responses with consequent alveolar bone resorption, and many regulators are involved in this process. In this review, we take a closer look at the cellular/molecular mechanisms and signaling involved in osteoimmunology and at relevant research progress in the context of the field of orthodontic tooth movement.

Influence of heating and cyclic tension on the induction of heat shock proteins and bone-related proteins by MC3T3-E1 cells

Stress conditioning (e.g., thermal, shear, and tensile stress) of bone cells has been shown to enhance healing. However, prior studies have not investigated whether combined stress could synergistically promote bone regeneration. This study explored the impact of combined thermal and tensile stress on the induction of heat shock proteins (HSPs) and bone-related proteins by a murine preosteoblast cell line (MC3T3-E1). Cells were exposed to thermal stress using a water bath (44°C for 4 or 8 minutes) with postheating incubation (37°C for 4 hours) followed by exposure to cyclic strain (equibiaxial 3%, 0.2 Hz, cycle of 10-second tensile stress followed by 10-second rest). Combined thermal stress and tensile stress induced mRNA expression of HSP27 (1.41 relative fold induction (RFI) compared to sham-treated control), HSP70 (5.55 RFI), and osteopontin (1.44 RFI) but suppressed matrix metalloproteinase-9 (0.6 RFI) compared to the control. Combined thermal and tensile stress increased vascular endothelial growth factor (VEGF) secretion into the culture supernatant (1.54-fold increase compared to the control). Therefore, combined thermal and mechanical stress preconditioning can enhance HSP induction and influence protein expression important for bone tissue healing.

Influence of cyclical mechanical loading on osteogenic markers in an osteoblast-fibroblast co-culture in vitro: tendon-to-bone interface in anterior cruciate ligament reconstruction

PURPOSE

We aimed to evaluate the influence of cyclical mechanical loading on osteoblasts and fibroblasts, and co-cultures of both in vitro, simulating the conditions of the tendon-to-bone interface in anterior cruciate ligament reconstruction.

METHODS

Osteoblast-like cells (OBL) and tendon-derived rodent fibroblasts (TDF) were cultured alone or in co-culture to simulate the tendon-to-bone interface. Cyclical loading was applied for one hour twice a day for three days, with a frequency of 1 Hz and 3 % strain. Alkaline phosphatase (AP), osteocalcin (OC), collagen type 1 (COL1A1), and bone morphogenetic protein 2 (BMP-2) gene expression and protein deposition were detected by real-time polymerase chain reaction (qPCR) and immunocytochemical analysis.

RESULTS

Mechanical loading significantly decreased AP, OC, and COL1A1 gene expression in both OBL and TDF, compared to non-loaded culture. However, mechanical load increased gene expression of the same marker genes including BMP-2 during co-culture. Immunocytochemistry demonstrated increased deposition of corresponding proteins in the same range, independent of culture conditions. Higher depositions of BMP-2 were shown under loading conditions for osteoblast and TDF monocultures. Prolongation of mechanical loading resulted in cell detachment and spheroid formation.

CONCLUSION

Cyclical mechanical loading caused downregulation of genes involved in osteointegration and osteoinduction, such as OC, ALP, and COL1A1 in monocultures of osteoblasts and fibroblasts; co-cultures lacked this phenomenon. Immunocytochemistry and qPCR analysis showed slight upregulations of marker genes and corresponding proteins. This might be due to the potential stabilising effects of osteoblast-fibroblast cross talk in the co-culture environment, simulating fibrocartilage formation at the tendon-to-bone interface.

Continuous cyclic stretch induces osteoblast alignment and formation of anisotropic collagen fiber matrix

Bone tissue geometry shows a highly anisotropic architecture, which is derived from its genetic regulation and mechanical environment. Osteoblasts are responsible not only for bone formation, through the secretion of collagen type I, but also for sensing the mechanical stimuli due to bone surface strain. Mechanotransduction by osteoblasts is therefore considered one of the regulators of anisotropic bone tissue morphogenesis. The orientation of osteoblasts and the secreted collagen matrix was successfully regulated by applying a continuous mechanical stress on osteoblasts for a long period. Under a continuous cyclic stretch of 4% magnitude at a rate of 2 cycles min(-1), osteoblasts reoriented their actin stress fibers in the direction that minimizes the strain applied to them. Extended culture of up to 2weeks resulted in the formation of collagen fibers in the extracellular spaces, and the preferred orientation of these fibers was parallel to the direction of cell elongation. To the best of our knowledge, this is the first report to establish anisotropic bone matrix architecture following the alignment of osteoblasts under mechanical stimuli for long-term cultivation.

Strain waveform dependence of stress fiber reorientation in cyclically stretched osteoblastic cells: effects of viscoelastic compression of stress fibers

Actin stress fibers (SFs) of cells cultured on cyclically stretched substrate tend to reorient in the direction in which a normal strain of substrate becomes zero. However, little is known about the mechanism of this reorientation. Here we investigated the effects of cyclic stretch waveform on SF reorientation in osteoblastic cells. Cells adhering to silicone membranes were subjected to cyclic uniaxial stretch, having one of the following waveforms with an amplitude of 8% for 24 h: triangular, trapezoid, bottom hold, or peak hold. SF reorientation of these cells was then analyzed. No preferential orientation was observed for the triangular and the peak-hold waveforms, whereas SFs aligned mostly in the direction with zero normal strain (∼55°) with other waveforms, especially the trapezoid waveform, which had a hold time both at loaded and unloaded states. Viscoelastic properties of SFs were estimated in a quasi-in situ stress relaxation test using intact and SF-disrupted cells that maintained their shape on the substrate. The dynamics of tension FSFsacting on SFs during cyclic stretching were simulated using these properties. The simulation demonstrated that FSFsdecreased gradually during cyclic stretching and exhibited a compressive value (FSFs< 0). The magnitude and duration time of the compressive forces were relatively larger in the group with a trapezoid waveform. The frequency of SF orientation had a significant negative correlation with the applied compressive forces integrated with time in a strain cycle, and the integrated value was largest with the trapezoid waveform. These results may indicate that the applied compressive forces on SFs have a significant effect on the stretch-induced reorientation of SFs, and that SFs realigned to avoid their compression. Stress relaxation of SFs might be facilitated during the holding period in the trapezoid waveform, and depolymerization and reorientation of SFs were significantly accelerated by their viscoelastic compression.

Reciprocal osteoblastic and osteoclastic modulation in co-cultured MG63 osteosarcoma cells and human osteoclast precursors

Osteosarcoma is usually associated with a disturbed bone metabolism. The aim of this work was to characterize the reciprocal interactions between MG63 osteosarcoma cells and osteoclasts, in a co-culture system. Co-cultures were characterized throughout 21 days for the osteoclastogenic response and the expression of osteoblastic markers. Monocultures of MG63 cells and peripheral blood mononuclear cell (PBMC) and co-cultures of PBMC + human bone marrow cells (hBMC) were also performed. Compared to PBMC cultures, co-cultures yielded significantly increased gene expression of osteoclast-related markers, tartarate-acid resistant phosphatase (TRAP) activity, TRAP-positive multinucleated cells, cells with actin rings and vitronectin receptors (VNR) and calcitonin receptors (CTR) and calcium phosphate resorbing ability. Results showed that the development of functional osteoclasts required a very low number of MG63 cells, suggesting a high osteoclastogenic-triggering capacity of this cell line. Subjacent mechanisms involved the pathways MEK and NF-kB, although with a lower relevance than that observed on PBMC monocultures or co-cultures of hBMC + PBMC; PGE2 production also had a contribution. Compared to MG63 cell monocultures, the co-culture expressed lower levels of COL1 and ALP, and higher levels of BMP-2, suggesting that PBMC also modulated the osteoblastic behavior. While M-CSF appeared to be involved in the osteoclastogenic response on the MG63 + PBMC co-cultures, RANKL does not seem to be a key player in the process. On the other hand, sphingosine-1-phosphate production might contribute to the modulation of the osteoblastic behavior. Results suggest that the reciprocal modulation between osteosarcoma and osteoclastic cells might contribute to the disturbed bone metabolism associated with bone tumors.

Role of shear-stress-induced VEGF expression in endothelial cell survival

Vascular endothelial growth factor (VEGF) plays a crucial role in developmental and pathological angiogenesis. Expression of VEGF in quiescent adult tissue suggests a potential role in the maintenance of mature blood vessels. We demonstrate, using a Vegf-lacZ reporter mouse model, that VEGF is expressed by arterial but not by venous or capillary endothelial cells (ECs) in vivo. Using an in vitro model, we show that arterial shear stress of human umbilical vein ECs (HUVECs) decreases apoptosis and increases VEGF expression, which is mediated by the induction of Krüppel-like factor 2 (KLF2). Additionally, shear stress stimulates the expression of VEGF receptor 2 (VEGFR2) and is associated with its activation. Knockdown of VEGF in shear stressed HUVECs blocks the protective effect of shear stress, resulting in EC apoptosis equivalent to that in control ECs cultured under static conditions. Similarly, treatment of ECs subjected to arterial shear stress with the VEGF receptor tyrosine kinase inhibitor SU1498, or VEGFR2 neutralizing antiserum, led to increased apoptosis, demonstrating that the mechanoprotection from increased shear is mediated by VEGFR2. Taken together, these studies suggest that arterial flow induces VEGF-VEGFR2 autocrine-juxtacrine signaling, which is a previously unidentified mechanism for vascular EC survival in adult arterial blood vessels.

Response of a preosteoblastic cell line to cyclic tensile stress conditioning and growth factors for bone tissue engineering

Bone regeneration can be accelerated by utilizing mechanical stress and growth factors (GFs). However, a limited understanding exists regarding the response of preosteoblasts to tensile stress alone or with GFs. We measured cell proliferation and expression of heat-shock proteins (HSPs) and other bone-related proteins by preosteoblasts following cyclic tensile stress (1%-10% magnitude) alone or in combination with bone morphogenetic protein-2 (BMP-2) and transforming growth factor-β1 (TGF-β1). Tensile stress (3%) with GFs induced greater gene upregulation of osteoprotegerin (3.3 relative fold induction [RFI] compared to sham-treated samples), prostaglandin E synthase 2 (2.1 RFI), and vascular endothelial growth factor (VEGF) (11.5 RFI), compared with samples treated with stimuli alone or sham-treated samples. The most significant increases in messenger RNA expression occurred with GF addition to either static-cultured or tensile-loaded (1% elongation) cells for the following genes: HSP47 (RFI=2.53), cyclooxygenase-2 (RFI=72.52), bone sialoprotein (RFI=11.56), and TGF-β1 (RFI=8.05). Following 5% strain with GFs, VEGF secretion increased 64% (days 3-6) compared with GF alone and cell proliferation increased 23% compared with the sham-treated group. GF addition increased osteocalcin secretion but decreased matrix metalloproteinase-9 significantly (days 3-6). Tensile stress and GFs in combination may enhance bone regeneration by initiating angiogenic and anti-osteoclastic effects and promote cell growth.

Regulation of subchondral bone osteoblast metabolism by cyclic compression

OBJECTIVE

Recent data have shown that abnormal subchondral bone remodeling plays an important role in osteoarthritis (OA) onset and progression, and it was suggested that abnormal mechanical pressure applied to the articulation was responsible for these metabolic changes. This study was undertaken to evaluate the effects of cyclic compression on osteoblasts from OA subchondral bone.

METHODS

Osteoblasts were isolated from sclerotic and nonsclerotic areas of human OA subchondral bone. After 28 days, the osteoblasts were surrounded by an abundant extracellular matrix and formed a resistant membrane, which was submitted to cyclic compression (1 MPa at 1 Hz) for 4 hours. Gene expression was evaluated by reverse transcription-polymerase chain reaction. Protein production in culture supernatants was quantified by enzyme-linked immunosorbent assay or visualized by immunohistochemistry.

RESULTS

Compression increased the expression of genes coding for interleukin-6 (IL-6), cyclooxygenase 2, RANKL, fibroblast growth factor 2, IL-8, matrix metalloproteinase 3 (MMP-3), MMP-9, and MMP-13 but reduced the expression of osteoprotegerin in osteoblasts in both sclerotic and nonsclerotic areas. Colα1(I) and MMP-2 were not significantly affected by mechanical stimuli. Nonsclerotic osteoblasts were significantly more sensitive to compression than sclerotic ones, but after compression, differences in messenger RNA levels between nonsclerotic and sclerotic osteoblasts were largely reduced or even abolished. Under basal conditions, sclerotic osteoblasts expressed similar levels of α5, αv, β1, and β3 integrins and CD44 as nonsclerotic osteoblasts but 30% less connexin 43, an important mechanoreceptor.

CONCLUSION

Genes involved in subchondral bone sclerosis are mechanosensitive. After compression, nonsclerotic and sclerotic osteoblasts expressed a similar phenotype, suggesting that compression could be responsible for the phenotype changes in OA subchondral osteoblasts.

Biomechanical forces exert anabolic effects on osteoblasts by activation of SMAD 1/5/8 through type 1 BMP receptor

Osteoblasts are mechanosensitive cells, which respond to biomechanical stimuli to regulate the bone structure through anabolic and catabolic gene regulation. To examine the effects of mechanical forces on the osteogenic responses through the SMAD signaling in osteoblasts, the cells were cultured in well-characterized mechanoresponsive 3-D scaffolds and exposed to 10% dynamic compressive strain (Cmp) at 1 Hz. Subsequently, SMAD phosphorylation and osteogenic gene induction was examined. Osteoblasts cultured in 3-D scaffolds exhibited increased constitutive SMAD 1/5/8 phosphorylation, as compared to monolayers cultures. This SMAD 1/5/8 phosphorylation was further upregulated after 10, 30 and 60 min in response to Cmp, exhibiting a peak activation at 30 min. No significant changes in SMAD2 phosphorylation were observed, suggesting signals generated by Cmp may not activate the Transforming Growth Factor-β signaling cascade. Subsequently, biomechanical stimulation-induced SMAD 1/5/8 phosphorylation upregulated the expression of osteogenic genes such as Osteoprotegrin, Msx2 and Runx2. Dorsomorphin, a selective inhibitor of the bone morphogenetic protein (BMP) receptor type 1 (BMPR1), blocked Cmp-induced SMAD 1/5/8 phosphorylation, as well as Osteoprotegrin, Msx2 and Runx2 gene expression. Collectively, the present findings demonstrate that biomechanical stimulation of osteoblasts activates SMAD 1/5/8 in the BMP signaling pathway through BMPR1 and may enhance osteogenesis by upregulating SMAD-dependent osteogenic genes.

Mechanical Stimulation Mediates Gene Expression in MC3T3 Osteoblastic Cells Differently in 2D and 3D Environments

Successful bone tissue engineering requires the understanding of cellular activity in three-dimensional (3D) architectures and how it compares to two-dimensional (2D) architecture. We developed a perfusion culture system that utilizes fluid flow to mechanically load a cell-seeded 3D scaffold. This study compared the gene expression of osteoblastic cells in 2D and 3D cultures, and the effects of mechanical loading on gene expression in 2D and 3D cultures. MC3T3-E1 osteoblastlike cells were seeded onto 2D glass slides and 3D calcium phosphate scaffolds and cultured statically or mechanically loaded with fluid flow. Gene expression of OPN and FGF-2 was upregulated at 24 h and 48 h in 3D compared with 2D static cultures, while collagen 1 gene expression was downregulated. In addition, while flow increased OPN in 2D culture at 48 h, it decreased both OPN and FGF-2 in 3D culture. In conclusion, gene expression is different between 2D and 3D osteoblast cultures under static conditions. Additionally, osteoblasts respond to shear stress differently in 2D and 3D cultures. Our results highlight the importance of 3D mechanotransduction studies for bone tissue engineering applications.

Characterisation of the osteoclastogenic potential of human osteoblastic and fibroblastic conditioned media

Although M-CSF and RANKL are sufficient to promote in vitro osteoclastogenesis, in vivo this is a complex process which requires the action of many signalling molecules and cellular crosstalks. In this work, isolated or combined conditioned media, obtained from human adult skin fibroblast and bone marrow cells, were tested for their osteoclastogenic potential, through an indirect co-culture system, in the absence of recombinant M-CSF and RANKL. Osteoclastogenesis was assessed on human peripheral blood mononuclear cells (PBMC) and CD14+ cell cultures by quantification of total protein content, tartrate-resistant acid phosphatase (TRAP) activity, presence of multinucleated cells positive for TRAP, RT-PCR of TRAP, CATK, CA2, c-myc and c-src and presence of multinucleated cells displaying actin rings, vitronectin and calcitonin receptors. Cultures supplemented with M-CSF and RANKL were used as positive controls. It was observed that the conditioned medium from dexamethasone osteogenic-induced bone marrow cell cultures displayed the highest osteoclastogenic potential, with similar behaviour to that observed in the presence of both M-CSF and RANKL. Comparatively, fibroblastic conditioned medium elicited a slightly lower osteoclastogenic response. Combination of both conditioned media resulted in a significant increase of TRAP activity. On the other hand, conditioned medium from non-osteogenic-induced bone marrow cell cultures presented the lowest osteoclastogenic potential. These results were observed for both PBMC and CD14+ cell cultures, suggesting that fibroblast and osteoblast cells are able to modulate osteoclastogenesis in the absence of physical cell-cell interactions. In addition, osteoclastogenic potential of bone marrow cells increases with their osteoblastic differentiation.

Compressive force induces VEGF production in periodontal tissues

During orthodontic tooth movement, the activation of the vascular system in the compressed periodontal ligament (PDL) is an indispensable process in tissue remodeling. We hypothesized that compressive force would induce angiogenesis of PDL through the production of vascular endothelial growth factor (VEGF). We examined the localization of VEGF in rat periodontal tissues during experimental tooth movement in vivo, and the effects of continuous compressive force on VEGF production and angiogenic activity in human PDL cells in vitro. PDL cells adjacent to hyalinized tissue and alveolar bone on the compressive side showed marked VEGF immunoreactivity. VEGF mRNA expression and production in PDL cells increased, and conditioned medium stimulated tube formation. These results indicate that continuous compressive force enhances VEGF production and angiogenic activity in PDL cells, which may contribute to periodontal remodeling, including angiogenesis, during orthodontic tooth movement.

The polycystic kidney disease 1 (Pkd1) gene is required for the responses of osteochondroprogenitor cells to midpalatal suture expansion in mice

Mechanical stress is known to modulate postnatal skeletal growth and development. However, the mechanisms underlying the mechanotransduction are not fully understood. Polycystin-1 (PC1) is a promising candidate among proteins that may play a role in the process as it has been shown to function as a flow sensor in renal epithelium and it is known to be important for skeletal development. To investigate whether PC1 is involved in mechanotransduction in skeletal tissues, mice with a conditional deficiency for PC1 in neural crest cells, osteoblasts or chondrocytes were subjected to midpalatal suture expansion. Dynamic bone labeling revealed that new bone formation in response to expansion was significantly reduced in Wnt1Cre;Pkd1 mice, as the suture area containing new bone was 14.0+/-3.4% in mutant mice versus 65.0+/-3.8% in control mice at 2 weeks (p<0.001). In contrast, stress-induced new bone formation was not affected in OsxCre;Pkd1 mice. The increase in cell proliferation and differentiation into osteoblasts, seen in wild-type mice 1 day after force delivery, was not observed until 14 days in Wnt1Cre;Pkd1 mice. TUNEL labeling showed a significant increase in apoptotic suture cells at days 1 and 3 (from 7.0+/-0.5% to 13.5+/-1.4% at day 1 and from 4.6+/-1.1% to 10.5+/-1.7% at day 3, p<0.05). Abnormal ossification of nasal cartilage of Wnt1Cre;Pkd1 mice was accelerated upon suture expansion. Such ossification was also observed, but to a lesser extent in Col2a1-ERCre;Pkd1 mice. Transcript levels of Runx2 and MMP13 were significantly increased in the nasal cartilage of Wnt1Cre;Pkd1 mice compared to controls (p<0.05 and p<0.001, respectively), and in mutant mice with expansion versus without expansion (p<0.05 and p<0.001, respectively). Lack of PC1 in chondroprogenitor cells also resulted in increased cell apoptosis and an altered arrangement of chondrocytes in nasal cartilage. These results indicate that PC1 plays a critical role in the response of osteochondroprogenitor cells to the mechanical tissue stress induced by midpalatal suture expansion. They also suggest that the combination of an in vivo mechanical model, such as midpalatal suture expansion, with conditional deficiency for proteins that play a role in mechanotransduction, represents a powerful experimental strategy to explore underlying mechanisms.

VEGF and M-CSF levels in periodontal tissue during tooth movement

It has been reported that both vascular endothelial growth factor (VEGF) and macrophage colonystimulating factor (M-CSF) can induce osteoclast recruitment. Thus, VEGF and M-CSF are considered to be closely involved in the bone remodeling process. The purpose of this study was to evaluate changes in VEGF and M-CSF expression during orthodontic treatment. The expression of VEGF and M-CSF mRNA in osteoblasts and fibroblasts was detected by in situ hybridization during experimental tooth movement in mice. Furthermore, the canine retraction side and the control side of orthodontic patients were compared, revealing a statistically significant increase in both VEGF and M-CSF concentrations in gingival crevicular fluid. These results suggest that orthodontic tooth movement causes an increase in VEGF and M-CSF levels. These factors may induce bone remodeling via osteoclastic bone resorption.

Fracture repair with ultrasound: clinical and cell-based evaluation

Fracture repair continues to be widely investigated, both within the clinical realm and at the fundamental research level, in part due to the fact that 5% to 10% of fractures result in either delayed union or nonunion, depending on the duration of incomplete healing. Beyond the temporal delay in repair, nonunions share the same unifying characteristic: all periosteal and endosteal repair processes have stopped and the fracture will not heal without surgical intervention. A less-invasive alternative method--low-intensity pulsed ultrasound--has shown promise as a treatment for delayed unions and nonunions and as a method to facilitate distraction osteogenesis. In this paper, we summarize the clinical effectiveness of low-intensity pulsed ultrasound with regard to fracture repair, treatment of nonunion, and distraction osteogenesis and we discuss the results of a multitude of published studies that have sought to elucidate the mechanisms behind that effectiveness through research on low-intensity pulsed ultrasound exposure on osteoblasts and osteoblast precursors. When evaluated clinically, low-intensity pulsed ultrasound was shown to enhance bone repair (most commonly noted as a decrease in healing time), although variations in patient population hindered a definitive claim to clinical effectiveness. In vitro cellular evaluation and in vivo studies on animal models have revealed an increase in cell proliferation, protein synthesis, collagen synthesis, membrane permeability, integrin expression, and increased cytosolic Ca(2+) levels as well as other increased indicators of bone repair in response to low-intensity pulsed ultrasound exposure. Many of the cellular responses to low-intensity pulsed ultrasound mirror the cellular responses to fluid-induced shear flow, suggesting a link between the two as one potential mechanism of action. The considerable amount of information that has been revealed about the behavior of osteoblasts under low-intensity pulsed ultrasound exposure suggests that the exact mechanism of action is complex. It is clear, however, that considerable progress is being made toward uncovering these mechanisms, which has served to encourage the use of low-intensity pulsed ultrasound in new applications. It is posited that successful noninvasive treatment strategies such as low-intensity pulsed ultrasound may be combined with other conventional and novel tissue-regeneration strategies to develop new treatments for large-scale bone defects.

Mechanical force-induced midpalatal suture remodeling in mice

Mechanical stress is an important epigenetic factor for regulating skeletal remodeling, and application of force can lead to remodeling of both bone and cartilage. Chondrocytes, osteoblasts and osteoclasts all participate and interact with each other in this remodeling process. To study cellular responses to mechanical stimuli in a system that can be genetically manipulated, we used mouse midpalatal suture expansion in vivo. Six-week-old male C57BL/6 mice were subjected to palatal suture expansion by opening loops with an initial force of 0.56 N for the periods of 1, 3, 5, 7, 14 or 28 days. Periosteal cells in expanding sutures showed increased proliferation, with Ki67-positive cells representing 1.8+/-0.1% to 4.5+/-0.4% of total suture cells in control groups and 12.0+/-2.6% to 19.9+/-1.2% in experimental/expansion groups (p<0.05). Starting at day 1, cells expressing alkaline phosphatase and type I collagen were seen. New cartilage and bone formation was observed at the oral edges of the palatal bones at day 7; at the nasal edges only bone formation without cartilage appeared to occur. An increase in osteoclast numbers suggested increased bone remodeling, ranging from 60 to 160% throughout the experimental period. Decreased Saffranin O staining after day 3 suggested decreased proteoglycan content in the secondary cartilage. Micro-CT showed a significant increase in maxillary width at days 14 and 28 (from 2334+/-4 microm to 2485+/-3 microm at day 14 and from 2383+/-5 microm to 2574+/-7 microm at day 28, p<0.001). The suture width was increased at days 14 and 28, except in the oral third region at day 28 (from 48+/-5 microm to 36+/-4 microm, p<0.05). Bone volume/total volume was significantly reduced at days 14 and 28 (50.2+/-0.7% vs. 68.0+/-3.7% and 56.5+/-1.0% vs. 60.9+/-1.3%, respectively, p<0.05), indicative of increased bone marrow space. These findings demonstrate that expansion forces across the midpalatal suture promote bone resorption through activation of osteoclasts and bone and cartilage formation via increased proliferation and differentiation of periosteal cells. Mouse midpalatal suture expansion would be useful in further studies of the ability of mineralized tissues to respond to mechanical stimulation.

Effects of different magnitudes of mechanical strain on Osteoblasts in vitro

In addition to systemic and local factors, mechanical strain plays a crucial role in bone remodeling during growth, development, and fracture healing, and especially in orthodontic tooth movement. Although many papers have been published on the effects of mechanical stress on osteoblasts or osteoblastic cells, little is known about the effects of different magnitudes of mechanical strain on such cells. In the present study, we investigated how different magnitudes of cyclic tensile strain affected osteoblasts. MC3T3-E1 osteoblastic cells were subjected to 0%, 6%, 12% or 18% elongation for 24h using a Flexercell Strain Unit, and then the mRNA and protein expressions of osteoprotegerin (OPG) and receptor activator of nuclear factor-kappaB ligand (RANKL) were examined. The results showed that cyclic tensile strain induced a magnitude-dependent increase (0%, 6%, 12%, and 18%) in OPG synthesis and a concomitant decrease in RANKL mRNA expression and sRANKL release from the osteoblasts. Furthermore, the induction of OPG mRNA expression by stretching was inhibited by indomethacin or genistein, and the stretch-induced reduction of RANKL mRNA was inhibited by PD098059. These results indicate that different magnitudes of cyclic tensile strain influence the biological behavior of osteoblasts, which profoundly affects bone remodeling.