Selected contribution: regulatory pathways involved in mechanical induction of c-fos gene expression in bone cells

Magnetic activation of TREK1 triggers stress signalling and regulates neuronal branching in SH-SY5Y cells

TWIK-related K⁺ 1 (TREK1) is a potassium channel expressed in the nervous system with multiple functions including neurotransmission and is a prime pharmacological target for neurological disorders. TREK1 gating is controlled by a wide range of external stimuli including mechanical forces. Previous work has demonstrated that TREK1 can be mechano-activated using magnetic nanoparticles (MNP) functionalised with antibodies targeted to TREK1 channels. Once the MNP are bound, external dynamic magnetic fields are used to generate forces on the TREK channel. This approach has been shown to drive cell differentiation in cells from multiple tissues. In this work we investigated the effect of MNP-mediated TREK1 mechano-activation on early stress response pathways along with the differentiation and connectivity of neuronal cells using the model neuronal cell line SH-SY5Y. Results showed that TREK1 is well expressed in SH-SY5Y and that TREK1-MNP initiate c-Myc/NF-κB stress response pathways as well as Nitrite production after magnetic stimulation, indicative of the cellular response to mechanical cues. Results also showed that TREK1 mechano-activation had no overall effect on neuronal morphology or expression of the neuronal marker βIII-Tubulin in Retinoic Acid (RA)/Brain-derived Neurotrophic factor (BDNF) differentiated SH-SY5Y but did increase neurite number. These results suggest that TREK1 is involved in cellular stress response signalling in neuronal cells, which leads to increased neurite production, but is not involved in regulating RA/BDNF mediated neuronal differentiation.

c-Fos is a mechanosensor that regulates inflammatory responses and lung barrier dysfunction during ventilator-induced acute lung injury

Background

As one of the basic treatments performed in the intensive care unit, mechanical ventilation can cause ventilator-induced acute lung injury (VILI). The typical features of VILI are an uncontrolled inflammatory response and impaired lung barrier function; however, its pathogenesis is not fully understood, and c-Fos protein is activated under mechanical stress. c-Fos/activating protein-1 (AP-1) plays a role by binding to AP-1 within the promoter region, which promotes inflammation and apoptosis. T-5224 is a specific inhibitor of c-Fos/AP-1, that controls the gene expression of many proinflammatory cytokines. This study investigated whether T-5224 attenuates VILI in rats by inhibiting inflammation and apoptosis.

Methods

The SD rats were divided into six groups: a control group, low tidal volume group, high tidal volume group, DMSO group, T-5224 group (low concentration), and T-5224 group (high concentration). After 3 h, the pathological damage, c-Fos protein expression, inflammatory reaction and apoptosis degree of lung tissue in each group were detected.

Results

c-Fos protein expression was increased within the lung tissue of VILI rats, and the pathological damage degree, inflammatory reaction and apoptosis in the lung tissue of VILI rats were significantly increased; T-5224 inhibited c-Fos protein expression in lung tissues, and T-5224 inhibit the inflammatory reaction and apoptosis of lung tissue by regulating the Fas/Fasl pathway.

Conclusions

c-Fos is a regulatory factor during ventilator-induced acute lung injury, and the inhibition of its expression has a protective effect. Which is associated with the antiinflammatory and antiapoptotic effects of T-5224.

Physical Cues of Matrices Reeducate Nerve Cells

The behavior of nerve cells plays a crucial role in nerve regeneration. The mechanical, topographical, and electrical microenvironment surrounding nerve cells can activate cellular signaling pathways of mechanical transduction to affect the behavior of nerve cells. Recently, biological scaffolds with various physical properties have been developed as extracellular matrix to regulate the behavior conversion of nerve cell, such as neuronal neurite growth and directional differentiation of neural stem cells, providing a robust driving force for nerve regeneration. This review mainly focused on the biological basis of nerve cells in mechanical transduction. In addition, we also highlighted the effect of the physical cues, including stiffness, mechanical tension, two-dimensional terrain, and electrical conductivity, on neurite outgrowth and differentiation of neural stem cells and predicted their potential application in clinical nerve tissue engineering.

Importance of experimental information (metadata) for archived sequence data: case of specific gene bias due to lag time between sample harvest and RNA protection in RNA sequencing

Large volumes of high-throughput sequencing data have been submitted to the Sequencing Read Archive (SRA). The lack of experimental metadata associated with the data makes reuse and understanding data quality very difficult. In the case of RNA sequencing (RNA-Seq), which reveals the presence and quantity of RNA in a biological sample at any moment, it is necessary to consider that gene expression responds over a short time interval (several seconds to a few minutes) in many organisms. Therefore, to isolate RNA that accurately reflects the transcriptome at the point of harvest, raw biological samples should be processed by freezing in liquid nitrogen, immersing in RNA stabilization reagent or lysing and homogenizing in RNA lysis buffer containing guanidine thiocyanate as soon as possible. As the number of samples handled simultaneously increases, the time until the RNA is protected can increase. Here, to evaluate the effect of different lag times in RNA protection on RNA-Seq data, we harvested CHO-S cells after 3, 5, 6, and 7 days of cultivation, added RNA lysis buffer in a time course of 15, 30, 45, and 60 min after harvest, and conducted RNA-Seq. These RNA samples showed high RNA integrity number (RIN) values indicating non-degraded RNA, and sequence data from libraries prepared with these RNA samples was of high quality according to FastQC. We observed that, at the same cultivation day, global trends of gene expression were similar across the time course of addition of RNA lysis buffer; however, the expression of some genes was significantly different between the time-course samples of the same cultivation day; most of these differentially expressed genes were related to apoptosis. We conclude that the time lag between sample harvest and RNA protection influences gene expression of specific genes. It is, therefore, necessary to know not only RIN values of RNA and the quality of the sequence data but also how the experiment was performed when acquiring RNA-Seq data from the database.

Modeling of the Human Bone Environment: Mechanical Stimuli Guide Mesenchymal Stem Cell-Extracellular Matrix Interactions

In bone tissue engineering, the design of in vitro models able to recreate both the chemical composition, the structural architecture, and the overall mechanical environment of the native tissue is still often neglected. In this study, we apply a bioreactor system where human bone-marrow hMSCs are seeded in human femoral head-derived decellularized bone scaffolds and subjected to dynamic culture, i.e., shear stress induced by continuous cell culture medium perfusion at 1.7 mL/min flow rate and compressive stress by 10% uniaxial load at 1 Hz for 1 h per day. In silico modeling revealed that continuous medium flow generates a mean shear stress of 8.5 mPa sensed by hMSCs seeded on 3D bone scaffolds. Experimentally, both dynamic conditions improved cell repopulation within the scaffold and boosted ECM production compared with static controls. Early response of hMSCs to mechanical stimuli comprises evident cell shape changes and stronger integrin-mediated adhesion to the matrix. Stress-induced Col6 and SPP1 gene expression suggests an early hMSC commitment towards osteogenic lineage independent of Runx2 signaling. This study provides a foundation for exploring the early effects of external mechanical stimuli on hMSC behavior in a biologically meaningful in vitro environment, opening new opportunities to study bone development, remodeling, and pathologies.

Regulation of TGFβ Signalling by TRPV4 in Chondrocytes

The growth factor TGFβ and the mechanosensitive calcium-permeable cation channel TRPV4 are both important for the development and maintenance of many tissues. Although TRPV4 and TGFβ both affect core cellular functions, how their signals are integrated is unknown. Here we show that pharmacological activation of TRPV4 significantly increased the canonical response to TGFβ stimulation in chondrocytes. Critically, this increase was only observed when TRPV4 was activated after, but not before TGFβ stimulation. The increase was prevented by pharmacological TRPV4 inhibition or knockdown and is calcium/CamKII dependent. RNA-seq analysis after TRPV4 activation showed enrichment for the TGFβ signalling pathway and identified JUN and SP1 as key transcription factors involved in this response. TRPV4 modulation of TGFβ signalling represents an important pathway linking mechanical signalling to tissue development and homeostasis.

Role of c-Fos in orthodontic tooth movement: an in vivo study using transgenic mice

Objectives

The transcription factor c-Fos controls the differentiation of osteoclasts and is expressed in periodontal ligament cells after mechanical stimulation in vitro. However, it is unclear how c-Fos regulates orthodontic tooth movement (OTM) in vivo. The aim of this study was therefore to analyse OTM in transgenic mice with overexpression of c-Fos.

Materials and methods

We employed c-Fos transgenic mice (c-Fos tg) and wild-type littermates (WT) in a model of OTM induced by Nitinol tension springs that were bonded between the left first maxillary molars and the upper incisors. The unstimulated contralateral side served as an internal control. Mice were analysed by contact radiography, micro-computed tomography, decalcified histology and histochemistry.

Results

Our analysis of the unstimulated side revealed that alveolar bone and root morphology were similar between c-Fos tg and control mice. However, we observed more osteoclasts in the alveolar bone of c-Fos tg mice as tartrate-resistant acid phosphatase (TRAP)-positive cells were increased by 40%. After 12 days of OTM, c-Fos tg mice exhibited 62% increased tooth movement as compared with WT mice. Despite the faster tooth movement, c-Fos tg and WT mice displayed the same amount of root resorption. Importantly, we did not observe orthodontically induced tissue necrosis (i.e. hyalinization) in c-Fos tg mice, while this was a common finding in WT mice.

Conclusion

Overexpression of c-Fos accelerates tooth movement without causing more root resorption.

Clinical relevance

Accelerated tooth movement must not result in more root resorption as higher tissue turnover may decrease the amount of mechanically induced tissue necrosis.

Electronic supplementary material

The online version of this article (10.1007/s00784-020-03503-1) contains supplementary material, which is available to authorized users.

In Vitro Bone Cell Response to Tensile Mechanical Solicitations: Is There an Optimal Protocol?

Bone remodeling is strongly linked to external mechanical signals. Such stimuli are widely used in vitro for bone tissue engineering by applying mechanical solicitations to cell cultures so as to trigger specific cell responses. However, the literature highlights considerable variability in devices and protocols. Here the major biological, mechanical, and technical parameters implemented for in vitro tensile loading applications are reviewed. The objective is to identify which values are used most, and whether there is an optimal protocol to obtain a functional tissue-engineering construct. First, a shift that occurred from fundamental comprehension of bone formation, to its application in rebuilt tissues and clinical fields is shown. Despite the lack of standardized protocols, consensual conditions relevant for in vitro bone development, in particular cell differentiation, could be highlighted. Culture processes are guided by physiological considerations, although out-of-range conditions are sometimes used without implying negative results for the development of rebuilt tissue. Consensus can be found on several parameters, such as strain frequency (1 Hz) or the use of rest periods, but other points have not yet been fully established, especially synergies with other solicitations. It is believed that the present work will be useful to develop new tissue-engineering processes based on stretching.

Hypergravity down-regulates c-fos gene expression via ROCK/Rho-GTP and the PI3K signaling pathway in murine ATDC5 chondroprogenitor cells

Chondrocytes are known to be physiologically loaded with diverse physical factors such as compressive stress, shear stress and hydrostatic pressure. Although the effects of those mechanical stimuli onto various cell models have been widely studied, those of hypergravity have not yet been revealed clearly. Hereby, we hypothesized that the hypergravity affects relative positions of intracellular elements including nucleus and cytoskeletons due to their density differences, triggering mechanotransduction in the cell. The aim of this study was to investigate the effect of hypergravity on c-fos expression in the murine ATDC5 chondroprogenitor cells, as c-fos is a well known key regulator of cell proliferation and differentiation, including in chondrocytes. We first found that hypergravity down-regulated c-fos expression transiently via ROCK/Rho-GTP and PI3K signaling, and the down-regulation was suppressed by inhibition of actin polymerization.

Optimally oriented grooves on dental implants improve bone quality around implants under repetitive mechanical loading

The aim was to investigate the effect of groove designs on bone quality under controlled-repetitive load conditions for optimizing dental implant design. Anodized Ti-6Al-4V alloy implants with -60° and +60° grooves around the neck were placed in the proximal tibial metaphysis of rabbits. The application of a repetitive mechanical load was initiated via the implants (50N, 3Hz, 1800 cycles, 2days/week) at 12weeks after surgery for 8weeks. Bone quality, defined as osteocyte density and degree of biological apatite (BAp) c-axis/collagen fibers, was then evaluated. Groove designs did not affect bone quality without mechanical loading; however, repetitive mechanical loading significantly increased bone-to-implant contact, bone mass, and bone mineral density (BMD). In +60° grooves, the BAp c-axis/collagen fibers preferentially aligned along the groove direction with mechanical loading. Moreover, osteocyte density was significantly higher both inside and in the adjacent region of the +60° grooves, but not -60° grooves. These results suggest that the +60° grooves successfully transmitted the load to the bone tissues surrounding implants through the grooves. An optimally oriented groove structure on the implant surface was shown to be a promising way for achieving bone tissue with appropriate bone quality. This is the first report to propose the optimal design of grooves on the necks of dental implants for improving bone quality parameters as well as BMD. The findings suggest that not only BMD, but also bone quality, could be a useful clinical parameter in implant dentistry.

STATEMENT OF SIGNIFICANCE

Although the paradigm of bone quality has shifted from density-based assessments to structural evaluations of bone, clarifying bone quality based on structural bone evaluations remains challenging in implant dentistry. In this study, we firstly demonstrated that the optimal design of dental implant necks improved bone quality defined as osteocytes and the preferential alignment degree of biological apatite c-axis/collagen fibers using light microscopy, polarized light microscopy, and a microbeam X-ray diffractometer system, after application of controlled mechanical load. Our new findings suggest that bone quality around dental implants could become a new clinical parameter as well as bone mineral density in order to completely account for bone strength in implant dentistry.

Effects of intermittent pressure imitating rolling manipulation on calcium ion homeostasis in human skeletal muscle cells

Background

Homeostasis imbalance of intracellular Ca²⁺ is one of the key pathophysiological factors in skeletal muscle injuries. Such imbalance can cause significant change in the metabolism of Ca²⁺-related biomarkers in skeletal muscle, such as superoxide dismutase (SOD), malondialdehyde (MDA) and creatine kinase (CK). Measurements of these biomarkers can be used to evaluate the degree of damage to human skeletal muscle cells (HSKMCs) injury. Rolling manipulation is the most popular myofascial release technique in Traditional Chinese Medicine. The mechanism of how this technique works in ameliorating muscle injury is unknown. This study aimed to investigate the possible Ca²⁺ mediated effects of intermittent pressure imitating rolling manipulation (IPIRM) of Traditional Chinese Medicine in the injured HSKMCs.

Methods

The normal HSKMCs was used as control normal group (CNG), while the injured HSKMCs were further divided into five different groups: control injured group (CIG), Rolling manipulation group (RMG), Rolling manipulation-Verapamil group (RMVG), static pressure group (SPG) and static pressure-Verapamil group (SPVG). RMG and RMVG cells were cyclically exposed to 9.5-12.5 N/cm² of IPIRM at a frequency of 1.0 Hz for 10 min. SPG and SPVG were loaded to a continuous pressure of 12.5 N/cm² for 10 min. Verapamil, a calcium antagonist, was added into the culture mediums of both RMVG and SPVG groups to block the influx of calcium ion.

Result

Compared with the CNG (normal cells), SOD activity was remarkably decreased while both MDA content and CK activity were significantly increased in the CIG (injured cells). When the injured cells were treated with the intermittent rolling manipulation pressure (RMG), the SOD activity was significantly increased and MDA content and CK activity were remarkably decreased. These effects were suppressed by adding the calcium antagonist Verapamil into the culture medium in RMVG. On the other hand, exposure to static pressure in SPG and SPVG affected neither the SOD activity nor the MDA content and CK activity in the injured muscle cells regardless of the presence of verapamil or not in the culture medium.

Conclusion

These data suggest that the intermittent rolling pressure with the manipulation could ameliorate HSKMCs injury through a Ca²⁺ dependent pathway. Static pressure did not lead to the same results.

Nano-colloid printing of functionalized PLA-b-PEO copolymers: tailoring the surface pattern of adhesive motif and its effect on cell attachment

In this study, we investigate the preparation of surface pattern of functional groups on poly(lactide) (PLA) surfaces through the controlled deposition of core-shell self-assemblies based on functionalized PLA-b-PEO amphiphilic block copolymers from selective solvents. Through grafting RGDS peptide onto the functionalized copolymer surface, the presented approach enables to prepare PLA surfaces with random and clustered spatial distribution of adhesive motifs. The proposed topography of the adhesion motif was proved by atomic force microscopy techniques using biotin-tagged RGDS peptide grafted on the surface and streptavidin-modified gold nanospheres which bind the tagged RGDS peptides as a contrast agent. The cell culture study under static and dynamic conditions with MG63 osteosarcoma cell line showed that the clustered distribution of RGDS peptides provided more efficient initial cell attachment and spreading, and resistance to cell detachment under dynamic culture compared to randomly distributed RGDS motif when with the same average RGDS peptide concentration.

Megakaryocytes are mechanically responsive and influence osteoblast proliferation and differentiation

Maintenance of bone mass and geometry is influenced by mechanical stimuli. Paradigms suggest that osteocytes embedded within the mineralized matrix and osteoblasts on the bone surfaces are the primary responders to physical forces. However, other cells within the bone marrow cavity, such as megakaryocytes (MKs), are also subject to mechanical forces. Recent studies have highlighted the potent effects of MKs on osteoblast proliferation as well as bone formation in vivo. We hypothesize that MKs are capable of responding to physical forces and that the interactions between these cells and osteoblasts can be influenced by mechanical stimulation. In this study, we demonstrate that two MK cell lines respond to fluid shear stress in culture. Furthermore, using laser capture microdissection, we isolated MKs from histologic sections of murine tibiae that were exposed to compressive loads in vivo. C-fos, a transcription factor shown to be upregulated in response to load in various tissue types, was increased in MKs from loaded relative to non-loaded limbs at a level comparable to that of osteocytes from the same limbs. We also developed a co-culture system to address whether mechanical stimulation of MKs in culture would impact osteoblast proliferation and differentiation. The presence of MKs in co-culture, but not conditioned media, had dramatic effects on proliferation of preosteoblast MC3T3-E1 cells in culture. Our data suggests a minimal decrease in proliferation as well as an increase in mineralization capacity of osteoblasts co-cultured with MKs exposed to shear compared to co-cultures with unstimulated MKs.

Are all osteocytes equal? Multiscale modelling of cortical bone to characterise the mechanical stimulation of osteocytes

Bone continuously adapts its internal structure to accommodate the functional demands of its mechanical environment. This process is orchestrated by a network of mechanosensitive osteocytes that respond to external mechanical signals and recruit osteoblasts and osteoclasts to alter bone mass to meet loading demands. Because of the irregular hierarchical microarchitecture of bone tissue, the precise mechanical stimuli experienced by osteocytes located in different regions of the tissue is not well-understood. The objective of this study is to characterise the local stimulus experienced by osteocytes distributed throughout the tissue structure. Our models predict that an inhomogeneous microstructural strain field contributes to osteocytes receiving vastly different stimuli at the cellular level, depending on their location within the microstructure. In particular, osteocytes located directly adjacent to micropores experienced strain amplifications in their processes of up to nine times the applied global strain. Furthermore, it was found that the principal orientation of lamellar regions was found to contribute significantly to the magnitude of the stimulus being received at the cellular level. These findings indicate that osteocytes are not equal in terms of the mechanical stimulus being received, and we propose that only a subset of osteocytes may be sufficiently stimulated to function as mechanoreceptors.

Application of topical negative pressure (vacuum-assisted closure) to split-thickness skin grafts: a structured evidence-based review

INTRODUCTION

Significant controversy surrounds the effectiveness of negative pressure wound therapy although it has been in use for decades. Although many clinicians favor this modality in relation to its practicality, ease of use especially in complex wounds, it has faced the same challenges as other dressings in relation to evidence base of efficacy in relation to a number of outcome measures. In view of the current financial pressures on health care systems worldwide, this structured review systematically challenges the evidence for perioperative application of topical negative pressure (TNP) to split-thickness skin grafts (STSGs) through evidence-based critical appraisal, and extrapolate the mechanisms of action on the mechanisms through which TNP may aid wound healing. Weighted evidence-based recommendations regarding the impact of TNP on split skin graft quality and quantity of take as outcomes.

METHODS

Phase 1: Structured literature search. Phase 2: Retrieved articles were critically appraised for rigor and methodological validity by 3 independent authors, then stratified according to a validated "levels of evidence" framework. Graded "current best evidence" recommendations could therefore be proposed.

RESULTS

Of the 220 studies retrieved in the initial search, 38 studies satisfied our quality of evidence criteria. Current best evidence supports 2 complementary trends explaining the mechanisms whereby STSG benefits from TNP. Active stimulation of epithelial mitosis: TNP creates mechanical stretch which stimulates multiple signaling pathways up-regulating growth- and mitosis-associated epithelial transcription factors. Topical negative pressure also promotes microcirculatory flow (graft and wound edge), stimulates angiogenesis and basement membrane integrity (grade C). Prevention of complications: significant reduction of graft lift-off by edema, exudates, subgraft hematoma, and reduction of shear when compared to traditional dressings (grade B). Topical negative pressure promotes significant qualitative improvement in the final STSG result studies (level 1B). The role of TNP in prevention of infection is, however, equivocal and further research is required. No evidence of harm from TNP application was reported.

CONCLUSIONS

Topical negative pressure increases quantity and quality of split skin graft take compared to traditional bolster dressings. The advantages are increased in irregularly contoured, technically difficult wounds and suboptimal recipient wound beds where it seems to be the best modality currently available. Large-scale randomized clinical controlled trials remain scanty in all areas of wound dressing research including negative pressure therapy.

Cyclic hydrostatic pressure stimulates enhanced bone development in the foetal chick femur in vitro

Mechanical loading of bone and cartilage in vivo results in the generation of cyclic hydrostatic forces as bone compression is transduced to fluid pressure in the canalicular network and the joint synovium. It has therefore been suggested that hydrostatic pressure is an important stimulus by which osteochondral cells and their progenitors sense and respond to mechanical loading in vivo. In this study, hydrostatic pressure regimes of 0-279kPa at 0.005-2Hz were applied to organotypically cultured ex vivo chick foetal femurs (e11) for 1hour per day in a custom designed bioreactor for 14days and bone formation assessed by X-ray microtomography and qualified by histology. We found that the mineralised portion of the developing femur cultured under any cyclic hydrostatic pressure regime was significantly larger and/or denser than unstimulated controls but that constant (non-cycling) hydrostatic pressure had no effect on bone growth. Further experiments showed that the increase in bone formation was directly proportional to stimulation frequency (R(2)=0.917), but independent of the magnitude of the pressure applied, whilst even very low frequencies of stimulation (0.005Hz) had significant effects on bone growth. Expression of Type-II collagen in both epiphyses and diaphysis was significantly upregulated (1.48-fold and 1.95-fold respectively), together with osteogenic genes (osteonectin and osteopontin) and the osteocyte maturation marker CD44. This work demonstrates that cyclic hydrostatic pressure promotes bone growth and mineralisation in a developmental model and supports the hypothesis that hydrostatic forces play an important role in regulating bone growth and remodelling in vivo.

Ex vivo real-time observation of Ca(2+) signaling in living bone in response to shear stress applied on the bone surface

Bone cells respond to mechanical stimuli by producing a variety of biological signals, and one of the earliest events is intracellular calcium ([Ca(2+)](i)) mobilization. Our recently developed ex vivo live [Ca(2+)](i) imaging system revealed that bone cells in intact bone explants showed autonomous [Ca(2+)](i) oscillations, and osteocytes specifically modulated these oscillations through gap junctions. However, the behavior and connectivity of the [Ca(2+)](i) signaling networks in mechanotransduction have not been investigated in intact bone. We herein introduce a novel fluid-flow platform for probing cellular signaling networks in live intact bone, which allows the application of capillary-driven flow just on the bone explant surface while performing real-time fluorogenic monitoring of the [Ca(2+)](i) changes. In response to the flow, the percentage of responsive cells was increased in both osteoblasts and osteocytes, together with upregulation of c-fos expression in the explants. However, enhancement of the peak relative fluorescence intensity was not evident. Treatment with 18 α-GA, a reversible inhibitor of gap junction, significantly blocked the [Ca(2+)](i) responsiveness in osteocytes without exerting any major effect in osteoblasts. On the contrary, such treatment significantly decreased the flow-activated oscillatory response frequency in both osteoblasts and osteocytes. The stretch-activated membrane channel, when blocked by Gd(3+), is less affected in the flow-induced [Ca(2+)](i) response. These findings indicated that flow-induced mechanical stimuli accompanied the activation of the autonomous [Ca(2+)](i) oscillations in both osteoblasts and osteocytes via gap junction-mediated cell-cell communication and hemichannel. Although how the bone sense the mechanical stimuli in vivo still needs to be elucidated, the present study suggests that cell-cell signaling via augmented gap junction and hemichannel-mediated [Ca(2+)](i) mobilization could be involved as an early signaling event in mechanotransduction.

Effects of verapamil on the immediate-early gene expression of bone marrow mesenchymal stem cells stimulated by mechanical strain in vitro

Background

To study the effects of verapamil on the immediate-early genes (IEGs) expression of bone marrow mesenchymal stem cells (MSCs) stimulated by cyclic mechanical strain, in order to deduce the role of calcium ion channel in the cell signaling responses of MSCs to mechanical strain.

Material/Methods

MSCs were isolated and cultured, and the passage of 3–6 MSCs were stimulated by mechanical strain and pretreated with or without verapamil. After that, flow cytometry was used to measure the fluorescence intensity of intracellular Ca²⁺ immediately. The expression of early-response genes/proteins (c-fos, c-jun and c-myc) were examined by RT-PCR, immunohistochemistry and Western blot.

Results

Intracellular Ca²⁺ concentration of MSCs significantly changed when stimulated by cyclic strain, and the expression of c-fos, c-jun and c-myc remarkably increased in both mRNA and protein levels, while verapamil pre-treatment partially inhibited these effects (P<0.01).

Conclusions

The changes of the intracellular calcium concentration of MSCs induced by mechanical strain, dependent on the regulation of calcium channel activation, might play a role in the early response of MSCs to cyclic strain.

Uterine distension differentially affects remodelling and distensibility of the uterine vasculature in non-pregnant rats

During pregnancy the mammalian uterine circulation undergoes significant expansive remodelling necessary for normal pregnancy outcome. The underlying mechanisms are poorly defined. The goal of this study was to test the hypothesis that myometrial stretch actively stimulates uterine vascular remodelling by developing a new surgical approach to induce unilateral uterine distension in non-pregnant rats. Three weeks after surgery, which consisted of an infusion of medical-grade silicone into the uterine lumen, main and mesometrial uterine artery and vein length, diameter and distensibility were recorded. Radial artery diameter, distensibility and vascular smooth muscle mitotic rate (Ki67 staining) were also measured. Unilateral uterine distension resulted in significant increases in the length of main uterine artery and vein and mesometrial segments but had no effect on vessel diameter or distensibility. In contrast, there were significant increases in the diameter of the radial arteries associated with the distended uterus. These changes were accompanied by reduced arterial distensibility and increased vascular muscle hyperplasia. In summary, this is the first report to show that myometrial stretch is a sufficient stimulus to induce significant remodelling of uterine vessels in non-pregnant rats. Moreover, the results indicate differential regulation of these growth processes as a function of vessel size and type.

Manipulation of Suspended Single Cells by Microfluidics and Optical Tweezers

Chondrocytes and osteoblasts experience multiple stresses in vivo. The optimum mechanical conditions for cell health are not fully understood. This paper describes the optical and microfluidic mechanical manipulation of single suspended cells enabled by the μPIVOT, an integrated micron resolution particle image velocimeter (μPIV) and dual optical tweezers instrument (OT). In this study, we examine the viability and trap stiffness of cartilage cells, identify the maximum fluid-induced stresses possible in uniform and extensional flows, and compare the deformation characteristics of bone and muscle cells. These results indicate cell photodamage of chondrocytes is negligible for at least 20 min for laser powers below 30 mW, a dead cell presents less resistance to internal organelle rearrangement and deforms globally more than a viable cell, the maximum fluid-induced shear stresses are limited to ~15 mPa for uniform flows but may exceed 1 Pa for extensional flows, and osteoblasts show no deformation for shear stresses up to 250 mPa while myoblasts are more easily deformed and exhibit a modulated response to increasing stress. This suggests that global and/or local stresses can be applied to single cells without physical contact. Coupled with microfluidic sensors, these manipulations may provide unique methods to explore single cell biomechanics.

Connective tissue growth factor in regulation of RhoA mediated cytoskeletal tension associated osteogenesis of mouse adipose-derived stromal cells

Background

Cytoskeletal tension is an intracellular mechanism through which cells convert a mechanical signal into a biochemical response, including production of cytokines and activation of various signaling pathways.

Methods/Principal Findings

Adipose-derived stromal cells (ASCs) were allowed to spread into large cells by seeding them at a low-density (1,250 cells/cm²), which was observed to induce osteogenesis. Conversely, ASCs seeded at a high-density (25,000 cells/cm²) featured small cells that promoted adipogenesis. RhoA and actin filaments were altered by changes in cell size. Blocking actin polymerization by Cytochalasin D influenced cytoskeletal tension and differentiation of ASCs. To understand the potential regulatory mechanisms leading to actin cytoskeletal tension, cDNA microarray was performed on large and small ASCs. Connective tissue growth factor (CTGF) was identified as a major regulator of osteogenesis associated with RhoA mediated cytoskeletal tension. Subsequently, knock-down of CTGF by siRNA in ASCs inhibited this osteogenesis.

Conclusions/Significance

We conclude that CTGF is important in the regulation of cytoskeletal tension mediated ASC osteogenic differentiation.

Effect of stretching force on the cells of epithelial rests of malassez in vitro

Background and Objective. The aim of this study was to investigate the behavior of cells from epithelial rest of Malassez (ERM) against stretching force. Material and Methods. ERM-cultured cells were stretched for 1 hour, at the cycle of 18% elongation for 1 second followed by 1-second relaxation. The cells without addition of stretching force were used as controls. The cells were observed by immunohistochmical staining using actin 0, 12, 24, 36, 48, and 72 hours. Furthermore, expressions of HSP70-, VEGF-, and OPN-mRNAs of cells were also evaluated using quantitative RT-PCR. Results. Actin filaments were randomly orientated in the cytoplasm in the control group, whereas in the stretching group, actin filaments were orientated comparatively parallel to the stretching direction. Expression of HSP70-mRNA in the stretching group was significantly higher than that of control group at 12, 24, 36 hours (P < .05). Expression of VEGF-mRNA in the stretching group was significantly higher than that of control group at 24, 36, 48, and 72 hours (P < .05). Expression of OPN-mRNA in the stretching group was significantly higher than that of control group at 12 and 24 hours (P < .05). Conclusion. ERM cells response against the stretching force by expressing HSP70, VEGF, and OPN.

In vitro bone growth responds to local mechanical strain in three-dimensional polymer scaffolds

Mechanical stimulation plays a key role in healing and remodelling of bone tissue in vivo, and is used in bone tissue regeneration strategies in vitro. Although macroscopic compression of three-dimensional (3-D) seeded constructs can increase bone formation, it is not yet reported how this response is related to differences in local mechanical strains inside the scaffolds. In this study, we experimentally test the hypothesis that differences in local average of heterogeneous strains in a polymer scaffold will correlate with induced differences in the local biological response. Twenty-four poly(L-lactic acid) porous scaffolds seeded with rat bone cells were cultured first for 2 and 3 weeks under static conditions, respectively. Then for 1 week, half of the scaffolds were cyclically compressed (1.5%, 1 Hz), 1 h daily, with continuous perfusion (0.1 ml/min). The remaining half was kept under static conditions. The pore-surface strains in the scaffolds at the start of culture were calculated with micro-finite element modelling based on micro-Computed Tomography (microCT) images. The locations of mineralized nodules were determined from microCT images and coupled to the calculated strains. Detectable mineralized nodules (>10(3) microm3) were only present in the loaded samples. Averages of absolute principal strains at the start of culture were significantly higher at nodule sites than at sites without a nodule. The results support the hypothesis that regenerating bone tissue in a 3-D porous scaffold responds to local mechanical strain. The methodology presented in this study can contribute design optimisation of tissue regeneration strategies relying on mechanical stimulation.

Activation of RhoA and FAK induces ERK-mediated osteopontin expression in mechanical force-subjected periodontal ligament fibroblasts

The precise mechanism by which Rho kinase translates the mechanical signals into OPN up-regulation in force-exposed fibroblasts has not been elucidated. Human periodontal ligament fibroblasts (hPLFs) were exposed to mechanical force by centrifuging the culture plates at a magnitude of 50 g/cm(2) for 60 min. At various times of the force application, they were processed for analyzing cell viability, trypan blue exclusion, and OPN expression at protein and RNA levels. Cellular mechanism(s) of the force-induced OPN up-regulation was also examined using various kinase inhibitors or antisense oligonucleotides specific to mechanosensitive factors. Centrifugal force up-regulated OPN expression and induced a rapid and transient increase in the phosphorylation of focal adhesion kinase (FAK), extracellular signal-regulated kinase (ERK), and Elk1. Pharmacological blockade of RhoA/Rho-associated coiled coil-containing kinase (ROCK) signaling markedly reduced force-induced FAK and ERK1/2 phosphorylation. Transfecting hPLFs with FAK antisense oligonucleotide diminished ERK1/2 activation and force-induced OPN expression. Further, ERK inhibitor inhibited significantly OPN expression, Elk1 phosphorylation, and activator protein-1 (AP-1)-DNA binding activation, but not FAK phosphorylation, in the force-applied cells. These results demonstrate that FAK signaling plays critical roles in force-induced OPN expression in hPLFs through interaction with Rho/ROCK as upstream effectors and ERK-Elk1/ERK-c-Fos as downstream effectors.

Mechanical force induces type I collagen expression in human periodontal ligament fibroblasts through activation of ERK/JNK and AP-1

Type I collagen (COL I) is the predominant collagen in the extracellular matrix of periodontal ligament (PDL), and its expression in PDL fibroblasts (PLF) is sensitive to mechanical force. However, the mechanism by which PLF induces COL I to respond to mechanical force is unclear. This study examined the nature of human PLF in mediating COL I expression in response to centrifugal force. Signal transduction pathways in the early stages of mechanotransduction involved in the force-driven regulation of COL I expression were also investigated. Centrifugal force up-regulated COL I without cytotoxicity and activated extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 kinase. ERK and JNK inhibitor blocked the expression of COL I but p38 kinase inhibitor had no effect. Centrifugal force activated activator protein-1 (AP-1) through dimerization between c-Fos and c-Jun transcription factors. ERK and JNK inhibitors also inhibited AP-1-DNA binding, c-Fos nuclear translocation, and c-Jun phosphorylation that were increased in the force-exposed PLF. Further, transfecting the cells with c-Jun antisense oligonucleotides almost completely abolished the force-induced increase of c-Jun phosphorylation and COL I induction. Our findings suggest that mechanical signals are transmitted into the nucleus by ERK/JNK signaling pathways and then stimulate COL I expression through AP-1 activation in force-exposed human PLF.

Type II cGMP-dependent protein kinase mediates osteoblast mechanotransduction

Continuous bone remodeling in response to mechanical loading is critical for skeletal integrity, and interstitial fluid flow is an important stimulus for osteoblast/osteocyte growth and differentiation. However, the biochemical signals mediating osteoblast anabolic responses to mechanical stimulation are incompletely understood. In primary human osteoblasts and murine MC3T3-E1 cells, we found that fluid shear stress induced rapid expression of c-fos, fra-1, fra-2, and fosB/DeltafosB mRNAs; these genes encode transcriptional regulators that maintain skeletal integrity. Fluid shear stress increased osteoblast nitric oxide (NO) synthesis, leading to activation of cGMP-dependent protein kinase (PKG). Pharmacological inhibition of the NO/cGMP/PKG signaling pathway blocked shear-induced expression of all four fos family genes. Induction of these genes required signaling through MEK/Erk, and Erk activation was NO/cGMP/PKG-dependent. Treating cells with a membrane-permeable cGMP analog partly mimicked the effects of fluid shear stress on Erk activity and fos family gene expression. In cells transfected with small interfering RNAs (siRNA) specific for membrane-bound PKG II, shear- and cGMP-induced Erk activation and fos family gene expression was nearly abolished and could be restored by transducing cells with a virus encoding an siRNA-resistant form of PKG II; in contrast, siRNA-mediated repression of the more abundant cytosolic PKG I isoform was without effect. Thus, we report a novel function for PKG II in osteoblast mechanotransduction, and we propose a model whereby NO/cGMP/PKG II-mediated Erk activation and induction of c-fos, fra-1, fra-2, and fosB/DeltafosB play a key role in the osteoblast anabolic response to mechanical stimulation.

Cyclic stretch-induced nuclear localization of transcription factors results in increased nuclear targeting of plasmids in alveolar epithelial cells

BACKGROUND

We have shown previously that cyclic stretch corresponding to that experienced by the pulmonary epithelium during normal breathing enhances nonviral gene transfer and expression in alveolar epithelial cells by increasing plasmid intracellular trafficking. Although reorganization of the microtubule and actin cytoskeletons by cyclic stretch is necessary for increased plasmid trafficking, the role of nuclear entry in this enhanced trafficking has not been elucidated.

METHODS

Alveolar epithelial cells were subjected to biaxial cyclic stretch (10% change in surface area at 0.5 Hz) and assayed for RNA expression, nuclear localization and activation of key transcription factors. Stretched epithelial cells were transfected with plasmids via electroporation and exposed to inhibitors of transcription factor activation.

RESULTS

When assayed by in situ hybridization, more plasmids were localized to the nuclei of cells that were stretched following electroporation compared to unstretched cells. Cyclic stretch also increases the nuclear localization of multiple transcription factors thought to be involved in plasmid nuclear entry, including AP1, AP2, NF-kappaB and NF1. Specific inhibition of the nuclear import of AP1 and/or NF-kappaB abolishes the enhanced plasmid nuclear localization seen with stretch.

CONCLUSIONS

Nuclear entry of plasmids is thought to be mediated by the binding of proteins that chaperone the DNA through the nuclear pore. Stretch-enhanced nuclear localization of transcription factors increases nuclear targeting of plasmids, whereas inhibition of the nuclear import of specific transcription factors abrogated stretch-enhanced plasmid nuclear localization. Taken together, these results suggest that cyclic stretch increases gene trafficking in the cytoplasm and at the nuclear envelope.

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.

Mechanical regulation of osteoclastic genes in human osteoblasts

Bone adaptation to mechanical load is accompanied by changes in gene expression of bone-forming cells. Less is known about mechanical effects on factors controlling bone resorption by osteoclasts. Therefore, we studied the influence of mechanical loading on several key genes modulating osteoclastogenesis. Human osteoblasts were subjected to various cell stretching protocols. Quantitative RT-PCR was used to evaluate gene expression. Cell stretching resulted in a significant up-regulation of receptor activator of nuclear factor-kappaB ligand (RANKL) immediate after intermittent loading (3x3h, 3x6h, magnitude 1%). Continuous loading, however, had no effect on RANKL expression. The expression of osteoprotegerin (OPG), macrophage-colony stimulating factor (M-CSF), and osteoclast inhibitory lectin (OCIL) was not significantly altered. The data suggested that mechanical loading could influence osteoclasts recruitment by modulating RANKL expression in human osteoblasts and that the effects might be strictly dependent on the quality of loading.

Undifferentiated human mesenchymal stem cells (hMSCs) are highly sensitive to mechanical strain: transcriptionally controlled early osteo-chondrogenic response in vitro

OBJECTIVE

Physical cues play a crucial role in skeletogenesis and osteochondral regeneration. Although human mesenchymal stem cells (hMSCs) offer considerable therapeutic potential, little is known about the molecular mechanisms that control their differentiation. We hypothesized that mechanical strain might be an inherent stimulus for chondrogenic and/or osteogenic differentiation in undifferentiated hMSCs, where c-Fos (FOS) might play a major role in mechanotransduction.

METHOD

hMSCs from 10 donors were intermittently stimulated by cyclic tensile strain (CTS) at 3000 mustrain for a period of 3 days. Differential gene expression of strained and unstrained hMSCs was analysed by real-time RT-PCR for several marker genes, including the transcription factors FOS, RUNX2, SOX9, and others. Additionally, alkaline phosphatase activity (ALP) was determined kinetically.

RESULTS

The application of CTS significantly stimulated the expression levels of the early chondrogenic and osteogenic marker genes (SOX9, LUM, DCN; RUNX2, SPARC, SPP1, ALPL); this was accompanied by stimulation of ALP activity (+38%+/-12 standard error of mean, P<0.05). Matrix analysis revealed that the osteo-chondrogenic response followed a coordinated expression pattern, in which FOS was attributed to early osteogenic but not chondrogenic differentiation.

CONCLUSION

Undifferentiated hMSCs are highly sensitive to mechanical strain with a transcriptionally controlled osteo-chondrogenic differentiation response in vitro.

Relationships between genetic, environmental and nutritional factors influencing osteoporosis in laying hens

1. The effects upon bone quality of feeding limestone in flour or particulate form and housing type (cage or aviary) in lines of hens divergently selected for high (H) or low (L) bone strength over 7 generations were investigated. 2. As in previous generations, highly significant phenotypic differences between lines were observed in all measured bone traits at peak egg production (25 weeks) and towards the end of production (56 weeks) in both cage and aviary systems. 3. At 25 weeks there were no significant effects on bone variables of feeding particulate limestone although a significant reduction in osteoclast number was observed at this age. By 56 weeks osteoclast numbers were further reduced in hens fed particulate limestone and beneficial effects on some bone variables were observed in this treatment group. 4. The genotypic and dietary improvements upon bone quality were independent and additive at both ages. There were very few interactive effects. 5. Hens with the freedom to move in an aviary environment during the laying period had improved bone status compared to caged siblings. Environmental and genotypic effects were additive. 6. There were no effects of line on egg production although H line hens had slightly higher egg production by 56 weeks. Egg numbers were unaffected by diet. Eggshell thickness and strength were unaffected by line but hens fed particulate limestone had thicker- and stronger-shelled eggs over the production period as a whole. 7. We conclude that; (a) genetic selection is extremely effective in improving bone strength and resistance to osteoporosis; (b) allowing hens freedom to exercise can also improve bone strength but may increase the risk of keel damage if they do not have genetically-improved bone status; (c) feeding hens a particulate form of limestone from 15 weeks onwards can also increase bone strength and eggshell quality; (d) genetics, environment and nutrition all have independent and additive effects on bone status in laying hens but the relative effectiveness of these factors is genetics > environment > nutrition.

VOCCs and TREK-1 ion channel expression in human tenocytes

Mechanosensitive and voltage-gated ion channels are known to perform important roles in mechanotransduction in a number of connective tissues, including bone and muscle. It is hypothesized that voltage-gated and mechanosensitive ion channels also may play a key role in some or all initial responses of human tenocytes to mechanical stimulation. However, to date there has been no direct investigation of ion channel expression by human tenocytes. Human tenocytes were cultured from patellar tendon samples harvested from five patients undergoing routine total knee replacement surgery (mean age: 66 yr; range: 63-73 yr). RT-PCR, Western blotting, and whole cell electrophysiological studies were performed to investigate the expression of different classes of ion channels within tenocytes. Human tenocytes expressed mRNA and protein encoding voltage-operated calcium channel (VOCC) subunits (Ca alpha(1A), Ca alpha(1C), Ca alpha(1D), Ca alpha(2)delta(1)) and the mechanosensitive tandem pore domain potassium channel (2PK(+)) TREK-1. They exhibit whole cell currents consistent with the functional expression of these channels. In addition, other ionic currents were detected within tenocytes consistent with the expression of a diverse array of other ion channels. VOCCs and TREK channels have been implicated in mechanotransduction signaling pathways in numerous connective tissue cell types. These mechanisms may be present in human tenocytes. In addition, human tenocytes may express other channel currents. Ion channels may represent potential targets for the pharmacological management of chronic tendinopathies.

Cyclooxygenase-2 selective inhibition suppresses restoration of tibial trabecular bone formation in association with restriction of osteoblast maturation in skeletal reloading after hindlimb elevation of mice

To clarify the role of cyclooxygenase-2 (COX-2) in acute recovery of trabecular bone in reloaded hindlimbs of tail-suspended mice, we administered a COX-2 selective inhibitor in the mice during the reloading period after unloading. Experiments were conducted on 140 male C57BL/6J mice (8 weeks old). They were divided into ground control (GC) and unloading by tail suspension (UL) groups. On day 7, Group GC was divided into Groups GC+Vehicle (Veh) and GC+Celecoxib (Cel), while Group UL mice were fed on the ground [reloading (RL)] after 7-day unloading and were then divided into Groups RL+Veh and RL+Cel. Bone histomorphometry, osteogenic cell development, and mRNA expression of osteogenic molecules were assessed. At 5 days after reloading, the increase of bone formation rate and the ratio of osteocalcin mRNA expression per CFU-f colony in Group RL+Cel were significantly decreased compared with those in Group RL+Veh, while alkaline phosphatase-positive (ALP+) CFU-f formation and the ratios of cbfa-1, osterix, and type 1 collagen mRNA expression per CFU-f colony increased to the same levels in both RL groups. At 14 days after reloading, decreased bone volume by unloading in RL+Veh recovered to the same level as that of GC+Veh, but that in RL+Cel did not recover completely. The increase of c-fos mRNA expression in bone marrow cells at 1, 24, and 48 h after reloading, osteocalcin mRNA at 6 h, and osterix mRNA at 24 h were suppressed by COX-2 inhibitor. These data indicate that the COX-2 selective inhibitor celecoxib suppresses the restoration of tibial trabecular bone formation and the acute recovery of trabecular bone. These actions are closely related to restriction of c-fos and osteocalcin mRNA expressions and osteoblast differentiation in bone marrow cells.

Biodegradable scaffolds – delivery systems for cell therapies

The use of cells as therapies for disease, repair and regeneration of tissues is one of the new challenges in modern therapeutics. To facilitate the ability to localise, condition and protect cells, biodegradable scaffolds are being developed that will improve the efficiency of these treatments. Thus, cell delivery systems, either scaffolds or capsules, play a pivotal role in the success of these techniques. This review discusses these novel approaches. The selection of scaffold materials is addressed alongside issues of biocompatibility. The current research developments in smart scaffolds, which focus on the formation of biomimetic scaffolds, new fabrication techniques capable of controlling architecture and microstructure of scaffolds, and the production of injectable and in situ crosslinked scaffolds, are outlined. Finally, the continuing challenges that will drive future research in the cell therapies are highlighted.

Influence of mechanical and biological signals on gene expression in human MG-63 cells: evidence for a complex interplay between hydrostatic compression and vitamin D3 or TGF-beta1 on MMP-1 and MMP-3 mRNA levels

Biological mediators can influence the activity and differentiation of bone cells. 1,25-dihydroxy-vitamin D3 (1,25-(OH)2D3) is known to induce differentiation of precursors into mature osteoblasts, and transforming growth factor-beta1 (TGF-beta1) can modulate the activity of bone cells leading to alterations in proliferation and gene expression patterns. Bone-derived cells were loaded via intermittent cyclic hydrostatic pressure (icHP) on cells under basal conditions and in the presence of 1,25-(OH)2D3 or TGF-beta1. Evaluating the effects of loading on the cells allowed for a comparison to be made between responsiveness to biomechanical and biochemical stimuli and their potential interplay. The effects of icHP on mRNA levels for the specific genes involved in bone remodelling and differentiation were measured in MG-63 cells using reverse transcription-polymerase chain reaction (RT-PCR). The mRNA levels for matrix metalloproteinase-1 and -3 (MMP-1 and MMP-3) were significantly, and uniquely, increased (p < 0.001) in cells exposed to icHP under serum-free conditions for 4-12 h. However, mRNA levels for MMP-3, but not MMP-1, were significantly enhanced in cells subjected to static hydrostatic pressure (HP). Treatment of cells with 1,25-(OH)2D3 resulted in increased (p < 0.001) mRNA levels for osteocalcin and decreased (p < 0.001) mRNA levels for both MMP-1 and MMP-3. In cells exposed to icHP and 1,25-(OH)2D3, the mRNA levels for both MMP-1 and MMP-3 were elevated (p < 0.001) compared with hormone alone, but not to the same degree (p < 0.01) as cells subjected to icHP alone. Addition of TGF-beta1 to cells led to increases in cell proliferation and expression of collagen I, as well as decreases in expression of osteocalcin and MMP-1 and MMP-3. Exposure of cells to icHP and TGF-beta1 again led to unique and significant increases in expression of MMP-1 and MMP-3. No changes in mRNA levels for glyceraldehyde-3-phosphate dehydrogenase (GAPDH) or any of the other 9 genes assessed, including those for MMP-2 and MMP-13, were detected under any of the conditions described. Therefore, icHP can induce alterations in mRNA levels for a specific subset of genes in both premature and mature osteoblasts. Such stimuli can modulate the impact of potent biological mediators in defining patterns of gene expression by bone cells and potentially modify function in vivo.

Signal transduction pathways involved in mechanical regulation of HB-GAM expression in osteoblastic cells

Protein kinase C (PKC), protein kinase A (PKA), prostaglandin synthesis, and various mitogen-activated protein kinases (MAPKs) have been reported to be activated in bone cells by mechanical loading. We studied the involvement of these signal transduction pathways in the downregulation of HB-GAM expression in osteoblastic cells after cyclic stretching. Specific antagonists and agonists of these signal transduction pathways were added to cells before loading and to non-loaded control cells. Quantitative RT-PCR was used to evaluate gene expression. The data demonstrated that the extracellular signal-regulated kinase (ERK) 1/2 pathway, PKC, PKA, p38, and c-Jun N-terminal kinase MAPK participated in the mechanical downregulation of HB-GAM expression, whereas prostaglandin synthesis did not seem to be involved.

Roles of MAP kinases in the regulation of bone matrix gene expressions in human osteoblasts by oscillatory fluid flow

We investigated the effects of oscillatory flow in regulating the gene expressions of type I collagen (COL1, the main component of human bone tissues) and osteopontin (OPN, the key gene for calcium deposition) in human osteoblast-like (MG-63) cells, and the roles of mitogen-activated protein kinases (MAPKs) in this regulation. The cells were subjected to oscillatory flow (0.5 +/- 4 dyn/cm(2)) or kept under static condition for various time periods (15 min, 30 min, 1 h, 2 h, 4 h, 8 h, and 16 h). Oscillatory flow caused significant up-regulations of both COL1 and OPN gene expressions over the 16 h of study, and a transient activation of MAPKs was starting at 15 min and declining to basal level in 2 h. The flow-induction of COL1 was blocked by an ERK inhibitor (PD98059) and reduced by a JNK inhibitor (SP600125), whereas that of OPN was abolished by PD98059. Analysis of the cis-elements in the COL1 and OPN promoters suggests the involvement of transacting factors Elk-1 and AP-1 in the transcription regulation. The ERK inhibitor (PD98059) blocked Elk-1 phosphorylation, as well as COL1 and OPN gene expression. The JNK inhibitor (SP600125) abolished c-jun phosphorylation and COL1 expression. These results suggest that the flow-induction of OPN was mediated through the ERK-Elk1-OPN pathway, and that COL1 was regulated by both the ERK-Elk1-COL1 and JNK-c-JUN-COL1 pathway.

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.

Mechanosensitivity of human osteosarcoma cells and phospholipase C β2 expression

Bone adapts to mechanical load by osteosynthesis, suggesting that osteoblasts might respond to mechanical stimuli. We therefore investigated cell proliferation and phospholipase C (PLC) expression in osteoblasts. One Hertz uniaxial stretching at 4000 microstrains significantly increased the proliferation rates of human osteoblast-like osteosarcoma cell line MG-63 and primary human osteoblasts. However, U-2/OS, SaOS-2, OST, and MNNG/HOS cells showed no significant changes in proliferation rate. We investigated the expression pattern of different isoforms of PLC in these cell lines. We were able to detect PLC beta1, beta3, gamma1, gamma2, and delta1 in all cells, but PLC beta2 was only detectable in the mechanosensitive cells. We therefore investigated the possible role of PLC beta2 in mechanotransduction. Inducible antisense expression for 24h inhibited the translation of PLC beta1 in U-2/OS cells by 35% and PLC beta2 in MG-63 by 29%. Fluid shear flow experiments with MG-63 lacking PLC beta2 revealed a significantly higher level of cells losing attachment to coverslips and a significantly lower number of cells increasing intracellular free calcium.

Stretch-induced phosphorylation of ERK1/2 depends on differentiation stage of osteoblasts

The goal of this study was to investigate the effect of mechanical loading on osteoblasts and extracellular signal-regulated kinase (ERK1/2) signaling in relation to osteoblast differentiation and mineralization. A human osteoblast cell line (SV-HFO) was triggered to differentiate to the advanced state of mineralization by addition of the osteogenic factors dexamethasone and beta-glycerophosphate. Osteoblasts were subjected to cyclic, equibiaxial stretch for 5, 15, or 60 min at different stages of differentiation (days 7, 14, and 21). Baseline (static) phosphorylated ERK1/2 and total ERK1/2 levels gradually increased during osteoblast differentiation. Cyclic stretch induced a rapid increase in ERK1/2 phosphorylation with a maximum between 5 and 15 min. Prolonged stretching for 60 min resulted in a decrease of phosphorylated ERK1/2 towards baseline level, suggesting a desensitization mechanism. The effect of stretch on ERK1/2 phosphorylation was strongest at later stages of differentiation (days 14 and 21). At day 21, the increase of ERK1/2 phosphorylation in response to stretch was significantly lower in non-differentiating than in differentiating osteoblasts. This could not be explained by differences in cell density, but did correlate with the formation of extracellular matrix, collagen fibrils. Mineralization of the extracellular matrix did not lead to a further increase of ERK1/2 phosphorylation. In conclusion, the current study demonstrates that the extent of activation of the ERK1/2 pathway is dependent on the differentiation or functional stage of the osteoblast. The presence of an extracellular matrix, but not per se mineralization, seems to be the predominant determinant of osteoblastic response to strain.

Expression of the mechanosensitive 2PK+ channel TREK-1 in human osteoblasts

TREK-1 is a mechanosensitive member of the two-pore domain potassium channel family (2PK+) that is also sensitive to lipids, free fatty acids (including arachidonic acid), temperature, intracellular pH, and a range of clinically relevant compounds including volatile anaesthetics. TREK-1 is known to be expressed at high levels in excitable tissues, such as the nervous system, the heart and smooth muscle, where it is believed to play a prominent role in controlling resting cell membrane potential and electrical excitability. In this report, we use RT-PCR, Western blotting and immunohistochemistry to confirm that human derived osteoblasts and MG63 cells express TREK-1 mRNA and protein. In addition, we show gene expression of TREK2c and TRAAK channels. Furthermore, whole cell patch clamp electrophysiology demonstrates that these cells express a spontaneously active, outwardly rectifying potassium "background leak" current that shares many similarities to TREK-1. The outward current is largely insensitive to TEA and Ba2+, and is sensitive to application of lysophosphatidylcholine (LPC). In addition, blocking TREK-1 channel activity is shown to upregulate bone cell proliferation. It is concluded that human osteoblasts functionally express TREK-1 and that these channels contribute, at least in part, to the resting membrane potential of human osteoblast cells. We hypothesise a possible role for TREK-1 in mechanotransduction, leading to bone remodelling.

A comparison of strain and fluid shear stress in stimulating bone cell responses--a computational and experimental study

Bone undergoes continuous remodeling in response to mechanical loading. However, the underlying mechanisms by which bone cells respond to their changing mechanical environment, that is, strain in the load-bearing matrix or fluid flow through the canalicular network, are not well understood. It has been established in vitro that bone cells respond differently to substrate strain and fluid shear stress treatments. Uncovering the mechanical basis of these differences represents a significant challenge to our understanding of cellular mechanotransduction and bone remodeling. To investigate this problem, we developed a biomechanical model of an adherent cell, to test the hypothesis that bone cells respond differently to 0.6 Pa fluid shear stress and 1,000 mu(epsilon) substrate strain stimulation because of qualitative and quantitative differences in the cellular deformation caused. Fluid shear stress loading conditions resulted in maximum displacements at the apical surface of the cell approximately 8 times higher than those due to strain at the cell-substrate interface and also caused higher stressing of all parts of the cell. Significantly, this shows that the deforming effects of fluid shear stress and strain on a cellular level are qualitatively different, which may provide a basis for explaining differences in bone cell responses to both stimuli as reported in several studies. Although our approach to modeling the morphology and complex physical environment of an adherent cell is certainly simplified, our results do show independent roles for fluid flow and strain as mechanical stimuli and highlight the importance of deformation on a cellular level in bone physiology.

Signal transduction and mechanical stress

Bone undergoes a constant process of remodeling in which mass is retained or lost in response to the relative activity of osteoblasts and osteoclasts. Weight-bearing exercise-which is critical for retaining skeletal integrity-promotes osteoblast function, whereas a lack of mechanical stimulation, as seen during spaceflight or prolonged bed rest, can lead to osteoporosis. Thus, understanding mechanotransduction at the cellular level is key to understanding basic bone biology and devising new treatments for osteoporosis. Various mechanical stimuli have been studied as in vitro model systems and have been shown to act through numerous signaling pathways to promote osteoblast activity. Here, we examine the various types of stress and the sequential response of transduction pathways that result in changes in gene expression and the ensuing proliferation of osteoblasts.

The stress reaction and its molecular events: splicing variants

The growth of cells and tissues is regulated by stress. When body is injured, it manifests a large spectrum of metabolic, endocrine, and immune alterations, which is named stress reaction. Among them, the production of growth factors may play a critical role. For osteoblasts and myoblasts, IGF-I has been shown to be involved in the process of cells in response to overloads. There are two splicing forms, one is IGF-Ea, the other is the IGF-IEb in the rodents and corresponds to IGF-IEc in humans. The latter is markedly up-regulated in response to overloads. Therefore, it has been named mechanogrowth factor. The link between the mechanical stimulus and the gene expression represents a new and important area in cell science. Understanding the process of splicing in IGF-I helps one to investigate the mechanotransduction of cells in response to mechanical stimulation at molecular level.

Mechanical regulation of HB-GAM expression in bone cells

Bone adaption upon mechanical stimulation is accompanied by changes in gene expression. In this context we investigated the influence of mechanical loading on heparin binding growth associated molecule (HB-GAM) expression, an extracellular matrix molecule which in cell culture has been shown to stimulate the differentiation of osteoblasts. We obtained information on the participating signal transduction pathways using a mitogenic loading regimen. Specific inhibitors of various signal transduction pathways were added to loaded cells and to unloaded controls. By semi-quantitative PCR studies we demonstrated a rapid decrease of HB-GAM expression in primary osteoblasts and SaOs-2 cells by 20-30% upon mechanical loading within 30min. We showed that the RGD-integrin interaction is involved in the regulation of HB-GAM expression. Furthermore, integrity of the cytoskeleton, stretch-activated, and voltage-sensitive Ca(2+) channels as well as gap junctional communication are necessary for the downregulation of HB-GAM expression by mechanical loading.

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.