The mode of mechanical integrin stressing controls intracellular signaling in osteoblasts

Mechanical stretching determines the orientation of osteoblast migration and cell division

Osteoblasts alignment and migration are involved in the directional formation of bone matrix and bone remodeling. Many studies have demonstrated that mechanical stretching controls osteoblast morphology and alignment. However, little is known about its effects on osteoblast migration. Here, we investigated changes in the morphology and migration of preosteoblastic MC3T3-E1 cells after the removal of continuous or cyclic stretching. Actin staining and time-lapse recording were performed after stretching removal. The continuous and cyclic groups showed parallel and perpendicular alignment to the stretch direction, respectively. A more elongated cell morphology was observed in the cyclic group than in the continuous group. In both stretch groups, the cells migrated in a direction roughly consistent with the cell alignment. Compared to the other groups, the cells in the cyclic group showed an increased migration velocity and were almost divided in the same direction as the alignment. To summarize, our study showed that mechanical stretching changed cell alignment and morphology in osteoblasts, which affected the direction of migration and cell division, and velocity of migration. These results suggest that mechanical stimulation may modulate the direction of bone tissue formation by inducing the directional migration and cell division of osteoblasts.

Evidence of the static magnetic field effects on bone-related diseases and bone cells

Static magnetic fields (SMFs), magnetic fields with constant intensity and orientation, have been extensively studied in the field of bone biology both fundamentally and clinically as a non-invasive physical factor. A large number of animal experiments and clinical studies have shown that SMFs have effective therapeutic effects on bone-related diseases such as non-healing fractures, bone non-union of bone implants, osteoporosis and osteoarthritis. The maintenance of bone health in adults depends on the basic functions of bone cells, such as bone formation by osteoblasts and bone resorption by osteoclasts. Numerous studies have revealed that SMFs can regulate the proliferation, differentiation, and function of bone tissue cells, including bone marrow mesenchymal stem cells (BMSCs), osteoblasts, bone marrow monocytes (BMMs), osteoclasts, and osteocytes. In this paper, the effects of SMFs on bone-related diseases and bone tissue cells are reviewed from both in vivo studies and in vitro studies, and the possible mechanisms are analyzed. In addition, some challenges that need to be further addressed in the research of SMF and bone are also discussed.

Emerging modulators for osteogenic differentiation: a combination of chemical and topographical cues for bone microenvironment engineering

Bone presents an intrinsic ability for self-regeneration and repair, however critical defects and large fractures require invasive and time-consuming clinical interventions. As an alternative to current therapy, bone tissue engineering (BTE) has primarily aimed to recreate the bone microenvironment by delivering key biomolecules and/or by modification of scaffolds to guide cell fate towards the osteogenic lineage or other phenotypes that may benefit the bone regeneration mechanism. Considering that bone cells communicate, in their native microenvironment, through biochemical and physical signals, most strategies fail when considering only chemical, geometrical or mechanical cues. This is not representative of the physiological conditions, where the cells are simultaneously in contact and stimulated by several cues. Therefore, this review explores the synergistic effect of biochemical/physical cues in regulating cellular events, namely cell adhesion, proliferation, osteogenic differentiation, and mineralization, highlighting the importance of the combined modifications for the development of innovative bone regenerative therapies.

Long-term dynamic compression enhancement TGF-β3-induced chondrogenesis in bovine stem cells: a gene expression analysis

Background

Bioengineering has demonstrated the potential of utilising mesenchymal stem cells (MSCs), growth factors, and mechanical stimuli to treat cartilage defects. However, the underlying genes and pathways are largely unclear. This is the first study on screening and identifying the hub genes involved in mechanically enhanced chondrogenesis and their potential molecular mechanisms.

Methods

The datasets were downloaded from the Gene Expression Omnibus (GEO) database and contain six transforming growth factor-beta-3 (TGF-β3) induced bovine bone marrow-derived MSCs specimens and six TGF-β3/dynamic-compression-induced specimens at day 42. Screening differentially expressed genes (DEGs) was performed and then analysed via bioinformatics methods. The Database for Annotation, Visualisation, and Integrated Discovery (DAVID) online analysis was utilised to obtain the Gene Ontology (GO) and Kyoto Encyclopaedia of Genes and Genomes (KEGG) pathway enrichment. The protein-protein interaction (PPI) network of the DEGs was constructed based on data from the STRING database and visualised through the Cytoscape software. The functional modules were extracted from the PPI network for further analysis.

Results

The top 10 hub genes ranked by their connection degrees were IL6, UBE2C, TOP2A, MCM4, PLK2, SMC2, BMP2, LMO7, TRIM36, and MAPK8. Multiple signalling pathways (including the PI3K-Akt signalling pathway, the toll-like receptor signalling pathway, the TNF signalling pathway, and the MAPK pathway) may impact the sensation, transduction, and reaction of external mechanical stimuli.

Conclusions

This study provides a theoretical finding showing that gene UBE2C, IL6, and MAPK8, and multiple signalling pathways may play pivotal roles in dynamic compression-enhanced chondrogenesis.

Enhancement of Intracellular Calcium Ion Mobilization by Moderately but Not Highly Positive Material Surface Charges

Electrostatic forces at the cell interface affect the nature of cell adhesion and function; but there is still limited knowledge about the impact of positive or negative surface charges on cell-material interactions in regenerative medicine. Titanium surfaces with a variety of zeta potentials between −90 mV and +50 mV were generated by functionalizing them with amino polymers, extracellular matrix proteins/peptide motifs and polyelectrolyte multilayers. A significant enhancement of intracellular calcium mobilization was achieved on surfaces with a moderately positive (+1 to +10 mV) compared with a negative zeta potential (−90 to −3 mV). Dramatic losses of cell activity (membrane integrity, viability, proliferation, calcium mobilization) were observed on surfaces with a highly positive zeta potential (+50 mV). This systematic study indicates that cells do not prefer positive charges in general, merely moderately positive ones. The cell behavior of MG-63s could be correlated with the materials’ zeta potential; but not with water contact angle or surface free energy. Our findings present new insights and provide an essential knowledge for future applications in dental and orthopedic surgery.

BK ablation attenuates osteoblast bone formation via integrin pathway

Impaired bone formation is one of the major causes of low bone mass and skeletal fragility that occurs in osteoporosis. However, the mechanisms underlying the defects in bone formation are not well understood. Here, we report that big conductance calcium-activated potassium channels (BKs) are required for bone formation and osteoblast function both in vivo and in vitro. By 15 weeks of age, BK knockout (BKO) mice exhibited a decline in bone mineral density and trabecular bone volume of the tibiae and lumbar vertebrae, which were associated with impaired bone formation and osteoblast activity. Mechanistically, BK ablation in bone and bone marrow mesenchymal stem cells (BMSCs) of BKO mice inhibited integrin signaling. Furthermore, the binding of α subunit of BK with integrin β1 protein in osteoblasts was confirmed, and FAK-ERK1/2 signaling was proved to be involved by genetic modification of KCNMA1 (which encodes the α subunit of BK) in ROS17/2.8 osteoblast cells. These findings indicated that BK regulates bone formation by promoting osteoblast differentiation via integrin pathway, which provided novel insight into ion transporter crosstalk with the extracellular matrix in osteoblast regulation and revealed a new potential strategy for intervention in correcting bone formation defects.

Phenotypic stability of the human MG-63 osteoblastic cell line at different passages

One of the most popular cell lines in osteogenesis studies is the human osteoblastic line MG-63. For cell biological investigation, it is important that the cells remain stable in their phenotype over several passages in cell culture. MG-63 cells can be used to provide fundamental insights into cell--material interaction. The aim of this study is to present a systematic characterization of the physiological behavior of MG-63 cells in the range of passages 5-30. Significant cell physiology processes during the first 24 h, including cell morphology, availability of adhesion receptors, cell cycle phases, as well as the expression of the signaling proteins Akt, GSK3a/b, IkB-α, ERK1/2, p38-MAPK, and intracellular calcium ion mobilization, remained stable over the entire range of passages P5-P30. Due to these stable characteristics in a wide range of cell culture passages, MG-63 cells can be considered as a suitable in vitro model to analyze the biocompatibility and biofunctionality of implant materials.

Enhanced calcium ion mobilization in osteoblasts on amino group containing plasma polymer nanolayer

Background

Biomaterial modifications—chemical and topographical—are of particular importance for the integration of materials in biosystems. Cells are known to sense these biomaterial characteristics, but it has remained unclear which physiological processes bio modifications trigger. Hence, the question arises of whether the dynamic of intracellular calcium ions is important for the characterization of the cell–material interaction. In our prior research we could demonstrate that a defined geometrical surface topography affects the cell physiology; this was finally detectable in a reduced intracellular calcium mobilization after the addition of adenosine triphosphate (ATP).

Results

This new contribution examines the cell physiology of human osteoblasts concerning the relative cell viability and the calcium ion dynamic on different chemical modifications of silicon–titanium (Ti) substrates. Chemical modifications comprising the coating of Ti surfaces with a plasma polymerized allylamine (PPAAm)-layer or with a thin layer of collagen type-I were compared with a bare Ti substrate as well as tissue culture plastic. For this purpose, the human osteoblasts (MG-63 and primary osteoblasts) were seeded onto the surfaces for 24 h. The relative cell viability was determined by colorimetric measurements of the cell metabolism and relativized to the density of cells quantified using crystal violet staining. The calcium ion dynamic of osteoblasts was evaluated by the calcium imaging analysis of fluo-3 stained vital cells using a confocal laser scanning microscope. The positively charged nano PPAAm-layer resulted in enhanced intracellular calcium ion mobilization after ATP-stimulus and cell viability. This study underlines the importance of the calcium signaling for the manifestation of the cell physiology.

Conclusions

Our current work provides new insights into the intracellular calcium dynamic caused by diverse chemical surface compositions. The calcium ion dynamic appears to be a sensitive parameter for the cell physiology and, thus, may represent a useful approach for evaluating a new biomaterial. In this regard, reliable in vitro-tests of cell behavior at the interface to a material are crucial steps in securing the success of a new biomaterial in medicine.

Potential role for a specialized β3 integrin-based structure on osteocyte processes in bone mechanosensation

Osteocyte processes are an order of magnitude more sensitive to mechanical loading than their cell bodies. The mechanisms underlying this remarkable mechanosensitivity are not clear, but may be related to the infrequent αV β3 integrin sites where the osteocyte cell processes attach to canalicular walls. These sites develop dramatically elevated strains during load-induced fluid flow in the lacunar-canalicular system and were recently shown to be primary sites for osteocyte-like MLO-Y4 cell mechanotransduction. These αV β3 integrin sites lack typical integrin transduction mechanisms. Rather, stimulation at these sites alters Ca2+ signaling, ATP release and membrane potential. In the current studies, we tested the hypothesis that in authentic osteocytes in situ, key membrane proteins implicated in osteocyte mechanotransduction are preferentially localized at or near to β3 integrin-foci. We analyzed these spatial relationships in mouse bone osteocytes using immunohistochemistry combined with Structured Illumination Super Resolution Microscopy, a method that permits structural resolution at near electron microscopy levels in tissue sections. We discovered that the purinergic channel pannexin1, the ATP-gated purinergic receptor P2 × 7R and the low voltage transiently opened T-type calcium channel CaV3.2-1 all reside in close proximity to β3 integrin attachment foci on osteocyte processes, suggesting a specialized mechanotransduction complex at these sites. We further confirmed this observation on isolated osteocytes in culture using STochasitc Optical Resonance Microscopy. These findings identify a possible structural basis for the unique mechanosensation and transduction capabilities of the osteocyte process. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:642-652, 2018.

Responsiveness of voltage-gated calcium channels in SH-SY5Y human neuroblastoma cells on micropillar substrates

In this study, poly-L-lactic acid micropillar substrates were fabricated to evaluate the influence of topographic substrates on cell morphological and functional characteristics, such as spreading area, voltage-gated calcium channels (VGCCs) and membrane potential. The proliferation, spreading area, perimeter and circularity of SH-SY5Y cells interfaced with different substrates were first investigated. In addition, the cytoskeleton and focal adhesion of a cell as important manifestations of cell morphology were analyzed by immunofluorescence. VGCC responsiveness was evaluated by measuring the dynamic changes in intracellular Ca²⁺ evoked by 50 mM extracellular K⁺. To determine study whether the differences in VGCC responsiveness were caused by the differences in VGCC gene expression, the expression of N/L- type VGCCs was determined by qPCR and fluorescence staining. Notably, improved measurement of the membrane potential with potentiometric fluorescent dye TMRM was applied to determine the membrane potential of SH-SY5Y cells. Results indicated that the SH-SY5Y cells were deformed significantly to adapt to the substrates; however, no distinct effect on the proliferative ability of SH-SY5Y cells was observed. The micropillar substrates markedly influenced VGCC responsiveness, which correlated strongly with cell spreading but not with VGCC expression. The resting membrane potential of SH-SY5Y cells cultured on different substrates also changed, but no effect on responsiveness of VGCC was observed. These results suggest that the effect of the micropillar substrates on cell VGCC responsiveness may be attributed to changes in the functionality of the ion channel itself. Thus, topographic substrates can be used to engineer cell functionality in cell-based drug screening.

Osteopontin inhibits osteoblast responsiveness through the down-regulation of focal adhesion kinase mediated by the induction of low-molecular weight protein tyrosine phosphatase

Osteopontin (OPN), a major marker of osteogenic differentiation, suppresses osteoblast responses to mechanical stress and cytokines, including HGF and PDGF. These OPN-induced effects are mediated through focal adhesion kinase inactivation by the induction of low–molecular weight protein tyrosine phosphatase.

Osteopontin (OPN) is an osteogenic marker protein. Osteoblast functions are affected by inflammatory cytokines and pathological conditions. OPN is highly expressed in bone lesions such as those in rheumatoid arthritis. However, local regulatory effects of OPN on osteoblasts remain ambiguous. Here we examined how OPN influences osteoblast responses to mechanical stress and growth factors. Expression of NO synthase 1 (Nos1) and Nos2 was increased by low-intensity pulsed ultrasound (LIPUS) in MC3T3-E1 cells and primary osteoblasts. The increase of Nos1/2 expression was abrogated by both exogenous OPN overexpression and recombinant OPN treatment, whereas it was promoted by OPN-specific siRNA and OPN antibody. Moreover, LIPUS-induced phosphorylation of focal adhesion kinase (FAK), a crucial regulator of mechanoresponses, was down-regulated by OPN treatments. OPN also attenuated hepatocyte growth factor–induced vitamin D receptor (Vdr) expression and platelet-derived growth factor–induced cell mobility through the repression of FAK activity. Of note, the expression of low–molecular weight protein tyrosine phosphatase (LMW-PTP), a FAK phosphatase, was increased in both OPN-treated and differentiated osteoblasts. CD44 was a specific OPN receptor for LWW-PTP induction. Consistently, the suppressive influence of OPN on osteoblast responsiveness was abrogated by LMW-PTP knockdown. Taken together, these results reveal novel functions of OPN in osteoblast physiology.

Magnetic forces and magnetized biomaterials provide dynamic flux information during bone regeneration

The fascinating prospect to direct tissue regeneration by magnetic activation has been recently explored. In this study we investigate the possibility to boost bone regeneration in an experimental defect in rabbit femoral condyle by combining static magnetic fields and magnetic biomaterials. NdFeB permanent magnets are implanted close to biomimetic collagen/hydroxyapatite resorbable scaffolds magnetized according to two different protocols . Permanent magnet only or non-magnetic scaffolds are used as controls. Bone tissue regeneration is evaluated at 12 weeks from surgery from a histological, histomorphometric and biomechanical point of view. The reorganization of the magnetized collagen fibers under the effect of the static magnetic field generated by the permanent magnet produces a highly-peculiar bone pattern, with highly-interconnected trabeculae orthogonally oriented with respect to the magnetic field lines. In contrast, only partial defect healing is achieved within the control groups. We ascribe the peculiar bone regeneration to the transfer of micro-environmental information, mediated by collagen fibrils magnetized by magnetic nanoparticles, under the effect of the static magnetic field. These results open new perspectives on the possibility to improve implant fixation and control the morphology and maturity of regenerated bone providing "in site" forces by synergically combining static magnetic fields and biomaterials.

HOW DO BONE CELLS SENSE MECHANICAL LOADING?

Influenced by gravidity, bone tissue experiences stronger or lighter deformation according to the strength of the activities of daily life. Activities resulting in impact are particularly known to stimulate osteogenesis, thus reducing bone mass loss. Knowing how bone cells recognize the mechanical deformation imposed to the bone and trigger a series of biochemical chain reactions is of crucial importance for the development of therapeutic and preventive practices in orthopaedic activity. There is still a long way to run until we can understand the whole process, but current knowledge has shown a strong progression, with researches being conducted focused on therapies. For a mechanical sign to be transformed into a biological one (mechanotransduction), it must be amplified at cell level by the histological structure of bone tissue, producing tensions in cell membrane proteins (integrins) and changing their spatial structure. Such change activates bindings between these and the cytoskeleton, producing focal adhesions, where cytoplasmatic proteins are recruited to enable easier biochemical reactions. Focal adhesion kinase (FAK) is the most important one being self-activated when its structure is changed by integrins. Activated FAK triggers a cascade of reactions, resulting in the activation of ERK-1/2 and Akt, which are proteins that, together with FAK, regulate the production of bone mass. Osteocytes are believed to be the mechanosensor cells of the bone and to transmit the mechanical deformation to osteoblasts and osteoclasts. Ionic channels and gap junctions are considered as intercellular communication means for biochemical transmission of a mechanical stimulus. These events occur continuously on bone tissue and regulate bone remodeling.

Integrin-β1, not integrin-β5, mediates osteoblastic differentiation and ECM formation promoted by mechanical tensile strain

BACKGROUND

Mechanical strain plays a great role in growth and differentiation of osteoblast. A previous study indicated that integrin-β (β1, β5) mediated osteoblast proliferation promoted by mechanical tensile strain. However, the involvement of integrin-β in osteoblastic differentiation and extracellular matrix (ECM) formation induced by mechanical tensile strain, remains unclear.

RESULTS

After transfection with integrin-β1 siRNA or integrin-β5 siRNA, mouse MC3T3-E1 preosteoblasts were cultured in cell culture dishes and stimulated with mechanical tensile strain of 2500 microstrain (με) at 0.5 Hz applied once a day for 1 h over 3 or 5 consecutive days. The cyclic tensile strain promoted osteoblastic differentiation of MC3T3-E1 cells. Transfection with integrin-β1 siRNA attenuated the osteoblastic diffenentiation induced by the tensile strain. By contrast, transfection with integrin-β5 siRNA had little effect on the osteoblastic differentiation induced by the strain. At the same time, the result of ECM formation promoted by the strain, was similar to the osteoblastic differentiation.

CONCLUSION

Integrin-β1 mediates osteoblast differentiation and osteoblastic ECM formation promoted by cyclic tensile strain, and integrin-β5 is not involved in the osteoblasts response to the tensile strain.

A comprehensive review on the role of various materials in the osteogenic differentiation of mesenchymal stem cells with a special focus on the association of heat shock proteins and nanoparticles

Mesenchymal stem cells (MSCs) have important roles in the area of regenerative medicine and clinical applications due to their pluripotent nature. Osteogenic differentiation of MSCs has been studied extensively using various stimulants to develop models of bone repair. There are several factors that enhance the differentiation of MSCs into bone tissues. This review focuses on the effects of various inducers on the osteoblast differentiation of MSCs at different stages of cellular development. We discuss the various growth factors, hormones, vitamins, cytokines, chemical stimulants, and mechanical forces applied in bioreactors that play an essential role in the proliferation, differentiation, and matrix mineralization of stem cells during osteogenesis. Various nanoparticles have also been used recently for the same purpose and the results are promising. Moreover, we review the role of various stresses, including thermal stress, and the subsequent involvement of heat shock proteins as inducers of the proliferation and differentiation of osteoblasts. We also report how various proteasome inhibitors have been shown to induce proliferation and osteogenic differentiation of MSCs in a number of cases. In this communication, the role of peptide-based scaffolds in osteoblast proliferation and differentiation is also reviewed. Based on the reviewed information, this article proposes novel possibilities for the enhancement of proliferation, differentiation, and migration of osteoblasts from MSCs. © 2014 S. Karger AG, Basel.

Mechanotransduction in bone: Intervening in health and disease

Mechanotransduction has been proven to be one of the most significant variables in bone remodeling and its alterations have been shown to result in a variety of bone diseases. Osteoporosis, Paget’s disease, orthopedic disorders, osteopetrosis as well as hyperparathyroidism and hyperthyroidism all comprise conditions which have been linked with deregulated bone remodeling. Although the significance of mechanotransduction for bone health and disease is unquestionable, the mechanisms behind this important process have not been fully understood. This review will discuss the molecules that have been found to be implicated in mechanotransduction, as well as the mechanisms underlying bone health and disease, emphasizing on what is already known as well as new molecules potentially taking part in conveying mechanical signals from the cell surface towards the nucleus under physiological or pathologic conditions. It will also focus on the model systems currently used in mechanotransduction studies, like osteoblast-like cells as well as three-dimensional constructs and their applications among others. It will also examine the role of mechanostimulatory techniques in preventing and treating bone degenerative diseases and consider their applications in osteoporosis, craniofacial development, skeletal deregulations, fracture treatment, neurologic injuries following stroke or spinal cord injury, dentistry, hearing problems and bone implant integration in the near future.

Modulating the actin cytoskeleton affects mechanically induced signal transduction and differentiation in mesenchymal stem cells

Mechanical interactions of mesenchymal stem cells (MSC) with the environment play a significant role in controlling the diverse biological functions of these cells. Mechanical forces are transduced by integrins to the actin cytoskeleton that functions as a scaffold to switch mechanical signals into biochemical pathways. To explore the significance of cytoskeletal mechanisms in human MSC we modulated the actin cytoskeleton using the depolymerising drugs cytochalasin D (CytD) and latrunculin A (LatA), as well as the stabilizing drug jasplakinolide (Jasp) and examined the activation of the signalling molecules ERK and AKT during mechanical loading. All three drugs provoked significant changes in cell morphology and organisation of the cytoskeleton. Application of mechanical forces to β1-integrin receptors using magnetic beads without deformation of the cell shape induced a phosphorylation of ERK and AKT. Of the two drugs that inhibited the cytoskeletal polymerization, LatA completely blocked the activation of ERK and AKT due to mechanical forces, whereas CytD inhibited the activation of AKT but not of ERK. Activation of both signalling molecules by integrin loading was not affected due to cell treatment with the cytoskeleton stabilizing drug Jasp. To correlate the effects of the drugs on mechanically induced activation of AKT and ERK with parameters of MSC differentiation, we studied ALP activity as a marker for osteogenic differentiation and examined the uptake of fat droplets as marker for adipogenic differentiation in the presence of the drugs. All three drugs inhibited ALP activity of MSC in osteogenic differentiation medium. Adipogenic differentiation was enhanced by CytD and Jasp, but not by LatA. The results indicate that modulation of the cytoskeleton using perturbing drugs can differentially modify both mechanically induced signal transduction and MSC differentiation. In addition to activation of the signalling molecules ERK and AKT, other cytoskeletal mechanisms are involved in MSC differentiation.

Targeting integrins to promote bone formation and repair

The ageing skeleton experiences a progressive decline in the rate of bone formation, which can eventually result in osteoporosis--a common disease characterized by reduced bone mass and altered bone microarchitecture which can result in fractures. One emerging therapy involves the identification of molecules that target bone-marrow mesenchymal stromal cells (MSCs) and promote their differentiation into osteoblasts, thereby counteracting bone loss. This Review highlights the discovery that some integrins, a family of heterodimeric transmembrane proteins that can interact with matrix proteins and generate intracellular signals, can be targeted to promote homing of MSCs to bone, osteogenic differentiation and bone formation. Specifically, priming of the α(5)β(1) integrin, which is required for osteoblastic differentiation of MSCs, leads to increased bone formation and improved bone repair in mice. Additionally, treatment with a peptidomimetic ligand of the α(4)β(1) integrin coupled to an agent with a high affinity for bone improves the homing of MSCs to bone and promotes osteoblast differentiation and bone formation, leading to increased bone mass in osteopenic mice. Strategies that target key integrins expressed by MSCs might, therefore, translate into improved therapies for age-related bone loss and possibly other disorders.

Chip-based comparison of the osteogenesis of human bone marrow- and adipose tissue-derived mesenchymal stem cells under mechanical stimulation

Adipose tissue-derived stem cells (ASCs) are considered as an attractive stem cell source for tissue engineering and regenerative medicine. We compared human bone marrow-derived mesenchymal stem cells (hMSCs) and hASCs under dynamic hydraulic compression to evaluate and compare osteogenic abilities. A novel micro cell chip integrated with microvalves and microscale cell culture chambers separated from an air-pressure chamber was developed using microfabrication technology. The microscale chip enables the culture of two types of stem cells concurrently, where each is loaded into cell culture chambers and dynamic compressive stimulation is applied to the cells uniformly. Dynamic hydraulic compression (1 Hz, 1 psi) increased the production of osteogenic matrix components (bone sialoprotein, oateopontin, type I collagen) and integrin (CD11b and CD31) expression from both stem cell sources. Alkaline phosphatase and Alrizarin red staining were evident in the stimulated hMSCs, while the stimulated hASCs did not show significant increases in staining under the same stimulation conditions. Upon application of mechanical stimulus to the two types of stem cells, integrin (β1) and osteogenic gene markers were upregulated from both cell types. In conclusion, stimulated hMSCs and hASCs showed increased osteogenic gene expression compared to non-stimulated groups. The hMSCs were more sensitive to mechanical stimulation and more effective towards osteogenic differentiation than the hASCs under these modes of mechanical stimulation.

Mechanical strain regulates osteoblast proliferation through integrin-mediated ERK activation

Mechanical strain plays a critical role in the proliferation, differentiation and maturation of bone cells. As mechanical receptor cells, osteoblasts perceive and respond to stress force, such as those associated with compression, strain and shear stress. However, the underlying molecular mechanisms of this process remain unclear. Using a four-point bending device, mouse MC3T3-E1 cells was exposed to mechanical tensile strain. Cell proliferation was determined to be most efficient when stimulated once a day by mechanical strain at a frequency of 0.5 Hz and intensities of 2500 µε with once a day, and a periodicity of 1 h/day for 3 days. The applied mechanical strain resulted in the altered expression of 1992 genes, 41 of which are involved in the mitogen-activated protein kinase (MAPK) signaling pathway. Activation of ERK by mechanical strain promoted cell proliferation and inactivation of ERK by PD98059 suppressed proliferation, confirming that ERK plays an important role in the response to mechanical strain. Furthermore, the membrane-associated receptors integrin β1 and integrin β5 were determined to regulate ERK activity and the proliferation of mechanical strain-treated MC3T3-E1 cells in opposite ways. The knockdown of integrin β1 led to the inhibition of ERK activity and cell proliferation, whereas the knockdown of integrin β5 led to the enhancement of both processes. This study proposes a novel mechanism by which mechanical strain regulates bone growth and remodeling.

Modulation of embryonic mesenchymal progenitor cell differentiation via control over pure mechanical modulus in electrospun nanofibers

As the potential range of stem cell applications in tissue engineering continues to grow, the appropriate scaffolding choice is necessary to create tightly defined artificial microenvironments for each target organ. These microenvironments determine stem cell fate via control over differentiation. In this study we examined the specific effects of scaffold stiffness on embryonic mesenchymal progenitor cell behavior. Mechanically distinct scaffolds having identical microstructures and surface chemistries were produced utilizing core-shell electrospinning. The modulus of core-shell poly(ether sulfone)-poly(ε-caprolactone) (PES-PCL) fibers (30.6 MPa) was more than four times that of pure PCL (7.1 MPa). The results for chondrogenic and osteogenic differentiation of progenitor cells on each scaffold indicate that the lower modulus PCL fibers provided more appropriate microenvironments for chondrogenesis, evident by a marked up-regulation of chondrocytic Sox9, collagen type 2, and aggrecan gene expression and chondrocyte-specific extracellular matrix glycosaminoglycan production. In contrast, the stiffer core-shell PES-PCL fibers supported enhanced osteogenesis by promoting osteogenic Runx2, alkaline phosphatase, and osteocalcin gene expression, as well as alkaline phosphatase activity. The findings demonstrate that the microstructural stiffness/modules of a scaffold and the pliability of individual fibers may play a critical role in controlling stem cell differentiation. Regulation of cytoskeletal organization may occur via a "dynamic scaffold" leading to the subsequent intracellular signaling events that control differentiation-specific gene expression.

Controlled differentiation of human bone marrow stromal cells using magnetic nanoparticle technology

Targeting and differentiating stem cells at sites of injury and repair is an exciting and promising area for disease treatment and reparative medicine. We have investigated remote magnetic field activation of magnetic nanoparticle-tagged mechanosensitive receptors on the cell membrane of human bone marrow stromal cells (HBMSCs) for use in osteoprogenitor cell delivery systems and activation of differentiation in vitro and in vivo toward an osteochondral lineage. HBMSC-labeled with magnetic beads coated with antibodies or peptides to the transmembrane ion channel stretch activated potassium channel (TREK-1) or arginine–glycine–aspartic acid were cultured in monolayer or encapsulated into polysaccharide alginate/chitosan microcapsules. Upregulation in gene expression was measured in magnetic particle-labeled HBMSCs in response to TREK-1 activation over a short period (7 days) with an increase in mRNA levels of Sox9, core binding factor alpha1 (Cbfa1), and osteopontin. Magnetic particle-labeled HBMSCs encapsulated into alginate chitosan capsules were exposed to magnetic forces both in vitro and in vivo intermittently for 21 days. After 21 days the encapsulated, magnetic particle-labeled HBMSCs in vivo were viable as evidenced by extensive cell tracker green fluorescence. The mechanical stimulation of HBMSCs labeled with TREK-1 magnetic nanoparticle receptors enhanced expression of type-1 collagen in vitro with increases in proteoglycan matrix, core binding factor alpha1 (Cbfa1) and collagen synthesis, and extracellular matrix production and elevated the expression of type-1 and type-2 collagen in vivo. Additionally, the magnetically remote stimulation of HBMSCs labeled with magnetic nanoparticle arginine–glycine–aspartic acid considerably enhanced proteoglycan and collagen synthesis and extracellular matrix production and elevated the expression of type-1 and type-2 collagen in vivo and in vitro. Osteogenic mechanosensitive receptor manipulation by magnetic nanotechnology can induce the differentiation of osteoprogenitor cell populations toward an osteogenic lineage. These cell manipulation strategies offer tremendous therapeutic opportunities in soft and hard tissue repair.

Calcium phosphate surfaces promote osteogenic differentiation of mesenchymal stem cells

Although studies in vivo revealed promising results in bone regeneration after implantation of scaffolds together with osteogenic progenitor cells, basic questions remain how material surfaces control the biology of mesenchymal stem cells (MSC). We used human MSC derived from bone marrow and studied the osteogenic differentiation on calcium phosphate surfaces. In osteogenic differentiation medium MSC differentiated to osteoblasts on hydroxyapatite and BONITmatrix^®, a degradable xerogel composite, within 14 days. Cells revealed a higher alkaline phosphatase (ALP) activity and increased RNA expression of collagen I and osteocalcin using real-time RTPCR compared with cells on tissue culture plastic. To test whether material surface characteristics alone are able to stimulate osteogenic differentiation, MSC were cultured on the materials in expansion medium without soluble additives for osteogenic differentiation. Indeed, cells on calcium phosphate without osteogenic differentiation additives developed to osteoblasts as shown by increased ALP activity and expression of osteogenic genes, which was not the case on tissue culture plastic. Because we reasoned that the stimulating effect on osteogenesis by calcium phosphate surfaces depends on an altered cell–extracellular matrix interaction we studied the dynamic behaviour of focal adhesions using cells transfected with GFP labelled vinculin. On BONITmatrix^®, an increased mobility of focal adhesions was observed compared with cells on tissue culture plastic. In conclusion, calcium phosphate surfaces are able to drive MSC to osteoblasts in the absence of osteogenic differentiation supplements in the medium. An altered dynamic behaviour of focal adhesions on calcium phosphate surfaces might be involved in the molecular mechanisms which promote osteogenic differentiation.

Extracellular matrix-specific focal adhesions in vascular smooth muscle produce mechanically active adhesion sites

Integrin-mediated mechanotransduction in vascular smooth muscle cells (VSMCs) plays an important role in the physiological control of tissue blood flow and vascular resistance. To test whether force applied to specific extracellular matrix (ECM)-integrin interactions could induce myogenic-like mechanical activity at focal adhesion sites, we used atomic force microscopy (AFM) to apply controlled forces to specific ECM adhesion sites on arteriolar VSMCs. The tip of AFM probes were fused with a borosilicate bead (2 ~ 5 microm) coated with fibronectin (FN), collagen type I (CNI), laminin (LN), or vitronectin (VN). ECM-coated beads induced clustering of alpha(5)- and beta(3)-integrins and actin filaments at sites of bead-cell contact indicative of focal adhesion formation. Step increases of an upward (z-axis) pulling force (800 ~ 1,600 pN) applied to the bead-cell contact site for FN-specific focal adhesions induced a myogenic-like, force-generating response from the VSMC, resulting in a counteracting downward pull by the cell. This micromechanical event was blocked by cytochalasin D but was enhanced by jasplakinolide. Function-blocking antibodies to alpha(5)beta(1)- and alpha(v)beta(3)-integrins also blocked the micromechanical cell event in a concentration-dependent manner. Similar pulling experiments with CNI, VN, or LN failed to induce myogenic-like micromechanical events. Collectively, these results demonstrate that mechanical force applied to integrin-FN adhesion sites induces an actin-dependent, myogenic-like, micromechanical event. Focal adhesions formed by different ECM proteins exhibit different mechanical characteristics, and FN appears of particular relevance in its ability to strongly attach to VSMCs and to induce myogenic-like, force-generating reactions from sites of focal adhesion in response to externally applied forces.

Mechanotransduction in human bone: in vitro cellular physiology that underpins bone changes with exercise

Bone has a remarkable ability to adjust its mass and architecture in response to a wide range of loads, from low-level gravitational forces to high-level impacts. A variety of types and magnitudes of mechanical stimuli have been shown to influence human bone cell metabolism in vitro, including fluid shear, tensile and compressive strain, altered gravity and vibration. Therefore, the current article aims to synthesize in vitro data regarding the cellular mechanisms underlying the response of human bone cells to mechanical loading. Current data demonstrate commonalities in response to different types of mechanical stimuli on the one hand, along with differential activation of intracellular signalling on the other. A major unanswered question is, how do bone cells sense and distinguish between different types of load? The studies included in the present article suggest that the type and magnitude of loading may be discriminated by overlapping mechanosensory mechanisms including (i) ion channels; (ii) integrins; (iii) G-proteins; and (iv) the cytoskeleton. The downstream signalling pathways identified to date appear to overlap with known growth factor and hormone signals, providing a mechanism of interaction between systemic influences and the local mechanical environment. Finally, the data suggest that exercise should emphasize the amount of load rather than the number of repetitions.

Transforming growth factor-beta prevents osteoblast apoptosis induced by skeletal unloading via PI3K/Akt, Bcl-2, and phospho-Bad signaling

Loss of mechanical loading induces rapid bone loss resulting from reduced osteoblastogenesis and decreased bone formation. The signaling mechanisms involved in this deleterious effect on skeletal metabolism remain poorly understood. We have previously shown that hindlimb suspension in rats increases osteoblast apoptosis associated with decreased phosphatidylinositol 3-kinase (PI3K) signaling. In this study, we investigated whether transforming growth factor (TGF)-beta2 may prevent the altered signaling and osteoblast apoptosis induced by skeletal unloading in vivo. Hindlimb suspension-induced decreased bone volume was associated with reduced alpha(5)beta(1)-integrin protein levels and PI3K/Akt signaling in unloaded bone. Continuous administration of TGF-beta2 using osmotic minipumps prevented the decreased alpha(5)beta(1)-integrin expression and the reduced PI3K/Akt signaling in unloaded bone, resulting in the prevention of osteoblast apoptosis. We also show that TGF-beta2 prevented the decreased Bcl-2 levels induced by unloading, which suggests that TGF-beta2 targets Bcl-2 via PI3K/Akt to prevent osteoblast apoptosis in unloaded bone. Furthermore, we show that TGF-beta2 prevented the decrease in phosphorylated Bad, the inactive form of the proapoptotic protein Bad, induced by unloading. These results identify a protective role for TGF-beta2 in osteoblast apoptosis induced by mechanical unloading via the alpha(5)beta(1)/PI3K/Akt signaling cascade and downstream Bcl-2 and phospho-Bad survival proteins. We thus propose a novel role for TGF-beta2 in protection from unloading-induced apoptosis in vivo.

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.

The role of extracellular matrix, integrins, and cytoskeleton in mechanotransduction of centrifugal loading

The study was aimed to investigate the role of the "extracellular matrix (ECM)-integrins-cytoskeleton" signal pathway in mechanotransduction of centrifugal loading. MG-63 osteoblasts were exposed to centrifugal loading at 209xg for 10 min. Uncentrifuged cells and centrifuged cells that have been trypsinized and suspended in liquors were designed as control. The changes in F-actin and alpha-actin cytoskeleton, gene transcription of ECM components, and integrins expression were analyzed by LSCM, Real-Time RT-PCR and FCM, respectively. A temporary and fast reversible change was observed in F-actin and alpha-actin cytoskeleton. And the change was paralleled with the fast autoregulation in gene transcription of ECM components of fibronection, osteopontin and Collagen I, and integrins expression of both alpha2 and beta1 subunits. The result suggested that cytoskeleton was a possible mechanical sensor to centrifugal stimuli, and the cytoskeleton regulation to centrifugal loading was in an ECM-dependent and integrin-mediated manner.

Magnetic targeting of mechanosensors in bone cells for tissue engineering applications

Mechanical signalling plays a pivotal role in maintaining bone cell function and remodelling of the skeleton. Our previous work has highlighted the potential role of mechano-induction in tissue engineering applications. In particular, we have highlighted the potential for using magnetic particle techniques for tissue engineering applications. Previous studies have shown that manipulation of integrin attached magnetic particles leads to changes in intracellular calcium signalling within osteoblasts. However, due to the phenomenon of particle internalisation, previous studies have typically focused on short-term stimulation experiments performed within 1-2 h of particle attachment. For tissue engineering applications, bone tissue growth occurs over a period of 3-5 weeks. To date, no study has investigated the cellular responses elicited from osteoblasts over time following stimulation with internalised magnetic particles. Here, we demonstrate the long-term biocompatibility of 4.5 microm RGD-coated particles with osteoblasts up to 21 days in culture, and detail a time course of responses elicited from osteoblasts following mechanical stimulation with integrin attached magnetic particles (<2h post attachment) and internalised particles (>48h post attachment). Mechanical manipulation of both integrin attached and internalised particles were found to induce intracellular calcium signalling. It is concluded that magnetic particles offer a tool for applying controlled mechanical forces to osteoblasts, and can be used to stimulate intracellular calcium signalling over prolonged periods of time. Magnetic particle technology presents a potentially valuable tool for tissue engineering which permits the delivery of highly localised mechano-inductive forces directly to cells.

Skeletal unloading induces osteoblast apoptosis and targets α5β1-PI3K-Bcl-2 signaling in rat bone

The mechanisms underlying the altered osteoblastogenesis and bone loss in response to disuse are incompletely understood. Using the rat tail suspension model, we studied the effect of skeletal unloading on osteoblast and osteocyte apoptosis. Tail suspension for 2 to 7 days decreased tibial bone mass and induced early apoptotic loss of osteoblasts and delayed apoptotic loss of osteocytes. Surrenal gland weight and plasma corticosterone levels did not differ in loaded and unloaded rats at any time point, indicating that osteoblast/osteocyte apoptosis occurred independently of endogenous glucocorticoids. The mechanistic basis for the disuse-induced osteoblast/osteocyte apoptosis was examined. We found that alpha5beta1 integrin and phosphorylated phosphatidyl-inositol-3 kinase (p-PI3K) protein levels were transiently decreased in unloaded metaphyseal long bone compared to loaded bones. In contrast, p-FAK and p-ERK p42/44 levels were not significantly altered. Interestingly, the reduced p-PI3K levels in unloaded long bone was associated with decreased levels of the survival protein Bcl-2 with unaltered Bax levels, causing increased Bax/Bcl-2 levels. The results indicate that skeletal unloading in rats induces a glucocorticoid-independent, immediate increase in osteoblast apoptosis associated with decreased alpha5beta1-PI3K-Bcl-2 survival pathway in rat bone, which may contribute to the altered osteoblastogenesis and osteopenia induced by unloading.

ERK 1/2 activation in enhanced osteogenesis of human mesenchymal stem cells in poly(lactic‐glycolic acid) by cyclic hydrostatic pressure

The aim of this study was to identify the signal transduction pathways and mechano-transducers that play critical roles in the processes induced by changes in cyclic hydrostatic pressure and fluid shear in 3-dimensional (3D) culture systems. Mesenchymal stem cells were loaded into a polymeric scaffold and divided into three groups according to the stress treatment: static, fluid shear, and hydrostatic pressure with fluid shear. Cells were exposed daily to a hydrostatic pressure of 0.2 MPa for 1 min followed by 14 min rest with fluid flow at 30 rpm. Protein extracts were analyzed by Western blot for extracellular signal-regulated kinase 1/2 (ERK1/2). The complexes were cultured under the mechanical stimuli for 21 days with or without phospho-ERK1/2 inhibitor (U0126) and evaluated by RT-PCR, calcium contents, and immunohistochemistry. Under conditions of mechanical stimulation, the activation of ERK1/2 was sustained or increased with time. U0126 suppressed mechanical stimuli-induced expression of osteocalcin. In addition, calcium contents and the degrees of osteocalcin and osteopontin staining were decreased by this inhibitor. These results demonstrate that mechanical stimuli, particularly hydrostatic pressure with fluid shear, enhance osteogenesis in 3D culture systems via ERK1/2 activation.

Wnt/beta-catenin signaling is a normal physiological response to mechanical loading in bone

A preliminary expression profiling analysis of osteoblasts derived from tibia explants of the high bone mass LRP5 G171V transgenic mice demonstrated increased expression of canonical Wnt pathway and Wnt/beta-catenin target genes compared with non-transgenic explant derived osteoblasts. Therefore, expression of Wnt/beta-catenin target genes were monitored after in vivo loading of the tibia of LRP5 G171V transgenic mice compared with non-transgenic mice. Loading resulted in the increased expression of Wnt pathway and Wnt/beta-catenin target genes including Wnt10B, SFRP1, cyclin D1, FzD2, WISP2, and connexin 43 in both genotypes; however, there was a further increased in transcriptional response with the LRP5 G171V transgenic mice. Similar increases in the expression of these genes (except cyclin D1) were observed when non-transgenic mice were pharmacologically treated with a canonical Wnt pathway activator, glycogen synthase kinase 3beta inhibitor and then subjected to load. These in vivo results were further corroborated by in vitro mechanical loading experiments in which MC3T3-E1 osteoblastic cells were subjected to 3400 microstrain alone for 5 h, which increased the expression of Wnt10B, SFRP1, cyclin D1, FzD2, WISP2, and connexin 43. Furthermore, when MC3T3-E1 cells were treated with either glycogen synthase kinase 3beta inhibitor or Wnt3A to activate Wnt signaling and then subjected to load, a synergistic up-regulation of these genes was observed compared with vehicle-treated cells. Collectively, the in vivo and in vitro mechanical loading results support that Wnt/beta-catenin signaling is a normal physiological response to load and that activation of the Wnt/beta-catenin pathway enhances the sensitivity of osteoblasts/osteocytes to mechanical loading.

Integrins β1, α6, and α3contribute to mechanical strain-induced differentiation of fetal lung type II epithelial cells via distinct mechanisms

Mechanical forces regulate lung maturation in the fetus by promoting type II epithelial differentiation. However, the cell surface receptors that transduce these mechanical cues into cellular responses remain largely unknown. When distal lung type II epithelial cells isolated from embryonic day 19 rat fetuses were cultured on flexible plates coated with laminin, fibronectin, vitronectin, collagen, or elastin and exposed to a level of mechanical strain (5%) similar to that observed in utero, transmembrane signaling responses were induced under all conditions, as measured by ERK activation. However, mechanical stress maximally increased expression of the type II cell differentiation marker surfactant protein C when cells were cultured on laminin substrates. Strain-induced alveolar epithelial differentiation was inhibited by interfering with cell binding to laminin using soluble laminin peptides (IKVIV or YIGSR) or blocking antibodies against integrin β1, α3, or α6. Additional studies were carried out with substrates coated directly with different nonactivating anti-integrin antibodies. Blocking integrin β1and α6binding sites inhibited both cell adhesion and differentiation, whereas inhibition of α3prevented differentiation without altering cell attachment. These data demonstrate that various integrins contribute to mechanical control of type II lung epithelial cell differentiation on laminin substrates. However, they may act via distinct mechanisms, including some that are independent of their cell anchoring role.

Magnetic micro- and nanoparticle mediated activation of mechanosensitive ion channels

Most cells are known to respond to mechanical cues, which initiate biochemical signalling pathways and play a role in cell membrane electrodynamics. These cues can be transduced either via direct activation of mechanosensitive (MS) ion channels or through deformation of the cell membrane and cytoskeleton. Investigation of the function and role of these ion channels is a fertile area of research and studies aimed at characterizing and understanding the mechanoactive regions of these channels and how they interact with the cytoskeleton are fundamental to discovering the specific role that mechanical cues play in cells. In this review, we will focus on novel techniques, which use magnetic micro- and nanoparticles coupled to external applied magnetic fields for activating and investigating MS ion channels and cytoskeletal mechanics.

Control of focal adhesion dynamics by material surface characteristics

The mechanisms of cell adhesion to the extracellular matrix (ECM) which are of fundamental importance for function, survival, and growth of cells involve the formation of focal adhesions to facilitate integrin signaling. Recently, it became evident that focal adhesions are not stable but move to enable cell migration and ECM formation. We examined the number, size, and dynamic behavior of focal adhesions in living MG-63 osteoblastic cells, which were cultured on titanium surfaces with different roughnesses and on stainless steel (SS). As a marker for focal adhesions we used GFP-tagged vinculin, a cytoskeletal protein. Focal adhesions were smaller on titanium and on SS than on collagen-coated glass coverslips. The corundum-blasted rough surface of titanium induced the smallest adhesions. On all the surfaces that we have tested, we observed a mobility of focal adhesions. On collagen-coated coverslips focal adhesions moved with a speed of 60 nm/min. The speed was reduced on titanium and still more restricted on SS. The topography did not affect the mobility of focal adhesions. We conclude that on the material surfaces that we have studied a reduced mobility of focal adhesions may strengthen the linkages between cell and ECM but impair the ability to dynamically organize and remodel the ECM. The results may have a great impact in the functional evaluation of tailored biomaterial surfaces for the application in tissue engineering.

Skeletal phenotype of growing transgenic mice that express a function-perturbing form of beta1 integrin in osteoblasts

Skeletal modeling entails the deposition of large amounts of extracellular matrix (ECM) to form structures tailored to withstand increasing mechanical loads during rapid growth. Specific ECM molecules bind to integrin receptors on the cell surface, thereby triggering a cascade of signaling events that affect critical cell functions. To evaluate the role of integrins during skeletal growth, transgenic mice were engineered to express a function-perturbing fragment of beta1 integrin consisting of the transmembrane domain and cytoplasmic tail under the control of the osteocalcin promoter (TG mice). Thus, transgene expression was targeted to mature cells of the osteoblast lineage, and herein we show that cultured cells resembling osteocytes from 90-day-old TG mice display impaired adhesion to collagen I, a ligand for beta1 integrin. To determine the influence of beta1 integrin on bones that are responsible for providing structural support during periods of rapid growth, we examined the phenotype of the appendicular skeleton in TG mice compared to wild type (WT) mice. According to radiographs, bones from mice of both genotypes between 14 and 90 days of age appeared similar in gross structure and density, although proximal tibiae from 35-90 days old TG mice were less curved than those of WT mice (72-92% TG/WT). Although there were only mild and transient differences in absolute bone mass and strength, once normalized to body mass, the tibial dry mass (79.1% TG/WT females), ash mass (78.5% TG/WT females), and femoral strength in torsion (71.6% TG/WT females) were reduced in TG mice compared to WT mice at 90 days of age. Similar effects of genotype on bone mass and curvature were observed in 1-year-old retired breeders, indicating that these phenotypic differences between TG and WT mice were stable well into adulthood. Effects of genotype on histomorphometric indices of cancellous bone turnover were minimal and evident only transiently during growth, but when present they demonstrated differences in osteoblast rather than osteoclast parameters. Together, these results suggest that integrin signals generated during growth enhance the acquisition of a skeletal mass, structure, and strength to withstand the mechanical loads generated by weight-bearing.

Mechanical stimulation and mitogen-activated protein kinase signaling independently regulate osteogenic differentiation and mineralization by calcifying vascular cells

Ectopic calcification of vascular tissue is associated with several cardiovascular pathologies and likely involves active regulation by vascular smooth muscle cells and osteoblast-like vascular cells. This process often occurs in sites with altered mechanical environments, suggesting a role for mechanical stimuli in calcification. In this study, we investigated the effect of mechanical stimulation on the proliferation, osteogenic differentiation, calcification, and mitogen-activated protein kinase (MAPK) signaling in calcifying vascular cells (CVCs), a subpopulation of aortic smooth muscle cells putatively involved in vascular calcification. Application of equibiaxial cyclic strain (7%, 0.25 Hz) to CVCs had no effect on cell proliferation, but accelerated alkaline phosphatase expression and significantly increased mineralization by 3.1-fold over unstrained cells. Fluid motion in the absence of strain also enhanced mineralization, but to a lesser degree. Because MAPK pathways mediate mechanically regulated osteoblast differentiation, we tested whether similar signaling was involved in mineralization by CVCs. In static cultures, pharmacological inhibition of the extracellular signal-regulated kinase (ERK1/2), p38 MAPK, and c-Jun N-terminal kinase pathways significantly attenuated mineral production by as much as -94%, compared with uninhibited CVCs. Strikingly, although mechanical stimulation activated each of the MAPK pathways, inhibition of these pathways had no effect on the mechanically induced enhancement of alkaline phosphatase activity or mineralization. These novel data indicate that mechanical signals regulate calcification by CVCs, and although MAPK signaling is critical to CVC osteogenic differentiation and mineralization, it is not involved directly in transduction of mechanical signals to regulate these processes under the conditions utilized in this study.

Mechanical strain on osteoblasts activates autophosphorylation of focal adhesion kinase and proline-rich tyrosine kinase 2 tyrosine sites involved in ERK activation

The mechanisms involved in the mechanical loading-induced increase in bone formation remain unclear. In this study, we showed that cyclic strain (CS) (10 min, 1% stretch at 0.25 Hz) stimulated the proliferation of overnight serum-starved ROS 17/2.8 osteoblast-like cells plated on type I collagen-coated silicone membranes. This increase was blocked by MEK inhibitor PD-98059. Signaling events were then assessed 0 min, 30 min, and 4 h after one CS period with Western blotting and coimmunoprecipitation. CS rapidly and time-dependently promoted phosphorylation of both ERK2 at Tyr-187 and focal adhesion kinase (FAK) at Tyr-397 and Tyr-925, leading to the activation of the Ras/Raf/MEK pathway. Cell transfection with FAK mutated at Tyr-397 completely blocked ERK2 Tyr-187 phosphorylation. Quantitative immunofluorescence analysis of phosphotyrosine residues showed an increase in focal adhesion plaque number and size in strained cells. CS also induced both Src-Tyr-418 phosphorylation and Src to FAK association. Treatment with the selective Src family kinase inhibitor pyrazolopyrimidine 2 did not prevent CS-induced FAK-Tyr-397 phosphorylation suggesting a Src-independent activation of FAK. CS also activated proline-rich tyrosine kinase 2 (PYK2), a tyrosine kinase highly homologous to FAK, at the 402 phosphorylation site and promoted its association to FAK in a time-dependent manner. Mutation of PYK2 at the Tyr-402 site prevented the ERK2 phosphorylation only at 4 h. Intra and extracellular calcium chelators prevented PYK2 activation only at 4 h. In summary, our data showed that osteoblast response to mitogenic CS was mediated by MEK pathway activation. The latter was induced by ERK2 phosphorylation under the control of FAK and PYK2 phosphorylation orchestrated in a time-dependent manner.

Physical stress by magnetic force accelerates differentiation of human osteoblasts

We examined the effect of magnetic force on differentiation of cultured human osteoblasts. Magnetic microparticles (MPs) were introduced into the cytoplasm of a human osteoblast cell line and the cells were cultured in a magnetic field (MF) in group MP-MF. Three groups of controls were used: cells without MPs were cultured out of MF (group C), cells without MPs were cultured in MF (group MF), and cells with MPs were cultured out of MF (group MP). The cells in group MP-MF became larger and were elongated along the axis of the magnetic poles. Appearance of alkaline phosphatase (AlPase) activity, formation of bone nodules, and calcium deposition were accelerated depending on the intensity of the magnetic field. It takes longer culture in the other three groups to exhibit these changes. Core-binding factor A1 (Cbfa1: transcription factor for osteoblast differentiation) and osteocalcin (a bone-matrix protein involved in controlling osteogenesis) were expressed earlier or stronger in group MP-MF than the other groups. Then we compared phosphorylation of mitogen-activated protein kinase (MAPK) between group MP-MF and group C. Phosphorylation of p38(MAPK) (p38) was increased in group MP-MF, while total p38 as well as total and phosphorylated forms of MAPK/ERK 1/2 and SAPK/JNK were not changed between the two groups. When a p38 inhibitor, SB 203580, was added to the culture medium in group C, AlPase activity, formation of bone nodules, and calcium deposits were completely inhibited. On the other hand, they were inhibited only partially by a MAPK/ERK 1/2 inhibitor, U-0126. Based on these results, it is concluded that (1) osteoblast differentiation is accelerated by a magnetic force, (2) this acceleration is mainly attributed to the activation of p38 phosphorylation, and (3) the stimulus induced by a magnetic field offers a new approach to osteoblast differentiation.

Focal adhesion kinase pp125FAK interacts with the large conductance calcium-activated hSlo potassium channel in human osteoblasts: potential role in mechanotransduction

Molecular events of mechanotransduction in osteoblasts are poorly defined. We show that the mechanosensitive BK channels open and recruit the focal adhesion kinase FAK in osteoblasts on hypotonic shock. This could convert mechanical signals in biochemical events, leading to osteoblast activation.

INTRODUCTION

Mechanical strains applied to the skeleton influence bone remodeling and architecture mainly through the osteoblast lineage. The molecular mechanisms involved in osteoblastic mechanotransduction include opening of mechanosensitive cation channels and the activation of protein tyrosine kinases, notably FAK, but their interplay remains poorly characterized. The large conductance K+ channel (BK) seems likely as a bone mechanoreceptor candidate because of its high expression in osteoblasts and its ability to open in response to membrane stretch or hypotonic shock. Propagation of the signals issued from the mechanosensitivity of BK channels inside the cell likely implies complex interactions with molecular partners involved in mechanotransduction, notably FAK.

METHODS

Interaction of FAK with the C terminus of the hSlo alpha-subunit of BK was investigated using the yeast two-hybrid system as well as immunofluorescence microscopy and coimmunoprecipitation experiments with a rabbit anti-hslo antibody on MG63 and CAL72 human osteosarcoma cell lines and on normal human osteoblasts. Mapping of the FAK region interacting with hSlo was approached by testing the ability of hSlo to recruit mutated ot truncated FAK proteins.

RESULTS

To the best of our knowledge, we provide the first evidence of the physical association of FAK with the intracellular part of hslo. We show that FAK/hSlo interaction likely takes place through the Pro-1-rich domain situated in the C-terminal region of the kinase. FAK/hSlo association occurs constitutively at a low, but appreciable, level in human osteosarcoma cells and normal human osteoblasts that express endogenous FAK and hSlo. In addition, we found that application of an hypo-osmotic shock to these cells induced a sustained activation of BK channels associated to a marked increase in the recruitment of FAK on hSlo.

CONCLUSIONS

Based on these data, we propose that BK channels might play a triggering role in the signaling cascade induced by mechanical strains in osteoblasts.