Mechanically strained cells of the osteoblast lineage organize their extracellular matrix through unique sites of alphavbeta3-integrin expression

Bioinspired oriented calcium phosphate nanocrystal arrays with bactericidal and osteogenic properties

The global diffusion of antibiotic resistance poses a severe threat to public health. Addressing antibiotic-resistant infections requires innovative approaches, such as antibacterial nanostructured surfaces (ANSs). These surfaces, featuring ordered arrays of nanostructures, exhibit the ability to kill bacteria upon contact. However, most currently developed ANSs utilize bioinert materials, lacking bioactivity crucial for promoting tissue regeneration, particularly in the context of bone infections. This study introduces ANSs composed of bioactive calcium phosphate nanocrystals. Two distinct ANSs were created through a biomineralization-inspired growth of amorphous calcium phosphate (ACP) precursors. The ANSs demonstrated efficient antibacterial properties against both Gram-negative (P. aeruginosa) and Gram-positive (S. aureus) antibiotic resistant bacteria, with up to 75% mortality in adhered bacteria after only 4 hours of contact. Notably, the ANS featuring thinner and less oriented nano-needles exhibited superior efficacy attributed to simultaneous membrane rupturing and oxidative stress induction. Moreover, the ANSs facilitate the proliferation of mammalian cells, enhancing adhesion, spreading, and reducing oxidative stress. The ANSs displayed also significant bioactivity towards human mesenchymal stem cells, promoting colonization and inducing osteogenic differentiation. Specifically, the ANS with thicker and more ordered nano-needles demonstrated heightened effects. In conclusion, ANSs introduced in this work have the potential to serve as foundation for developing bone graft materials capable of eradicate site infections while concurrently stimulating bone regeneration. STATEMENT OF SIGNIFICANCE: Nanostructured surfaces with antibacterial properties through a mechano-bactericidal mechanism have shown significant potential in fighting antibiotic resistance. However, these surfaces have not been fabricated with bioactive materials necessary for developing devices that are both antibacterial and able to stimulate tissue regeneration. This study demonstrates the feasibility of creating nanostructured surfaces of ordered calcium phosphate nano-needles through a biomineralization-inspired growth. These surfaces exhibit dual functionality, serving as effective bactericidal agents against Gram-negative and Gram-positive antibiotic-resistant bacteria while also promoting the proliferation of mammalian cells and inducing osteogenic differentiation of human mesenchymal stem cells. Consequently, this approach holds promise in the context of bone infections, introducing innovative nanostructured surfaces that could be utilized in the development of antimicrobial and osteogenic grafts.

The role of the Piezo1 channel in osteoblasts under cyclic stretching: A study on osteogenic and osteoclast factors

OBJECTIVES

Orthodontic tooth movement is a mechanobiological reaction induced by appropriate forces, including bone remodeling. The mechanosensitive Piezo channels have been shown to contribute to bone remodeling. However, information about the pathways through which Piezo channels affects osteoblasts remains limited. Thus, we aimed to investigate the influence of Piezo1 on the osteogenic and osteoclast factors in osteoblasts under mechanical load.

MATERIALS AND METHODS

Cyclic stretch (CS) experiments on MC3T3-E1 were conducted using a BioDynamic mechanical stretching device. The Piezo1 channel blocker GsMTx4 and the Piezo1 channel agonist Yoda1 were used 12 h before the application of CS. MC3T3-E1 cells were then subjected to 15% CS, and the expression of Piezo1, Piezo2, BMP-2, OCN, Runx2, RANKL, p-p65/p65, and ALP was measured using quantitative real-time polymerase chain reaction, western blot, alkaline phosphatase staining, and immunofluorescence staining.

RESULTS

CS of 15% induced the highest expression of Piezo channel and osteoblast factors. Yoda1 significantly increased the CS-upregulated expression of Piezo1 and ALP activity but not Piezo2 and RANKL. GsMTx4 downregulated the CS-upregulated expression of Piezo1, Piezo2, Runx2, OCN, p-65/65, and ALP activity but could not completely reduce CS-upregulated BMP-2.

CONCLUSIONS

The appropriate force is more suitable for promoting osteogenic differentiation in MC3T3-E1. The Piezo1 channel participates in osteogenic differentiation of osteoblasts through its influence on the expression of osteogenic factors like BMP-2, Runx2, and OCN and is involved in regulating osteoclasts by influencing phosphorylated p65. These results provide a foundation for further exploration of osteoblast function in orthodontic tooth movement.

Tissue transglutaminase: a multifunctional and multisite regulator in health and disease

Tissue transglutaminase (TG2) is a widely distributed multifunctional protein involved in a broad range of cellular and metabolic functions carried out in a variety of cellular compartments. In addition to transamidation, TG2 also functions as a Gα signaling protein, a protein disulfide isomerase (PDI), a protein kinase, and a scaffolding protein. In the nucleus, TG2 modifies histones and transcription factors. The PDI function catalyzes the trimerization and activation of heat shock factor-1 in the nucleus and regulates the oxidation state of several mitochondrial complexes. Cytosolic TG2 modifies proteins by the addition of serotonin or other primary amines and in this way affects cell signaling. Modification of protein-bound glutamines reduces ubiquitin-dependent proteasomal degradation. At the cell membrane, TG2 is associated with G protein-coupled receptors (GPCRs), where it functions in transmembrane signaling. TG2 is also found in the extracellular space, where it functions in protein cross-linking and extracellular matrix stabilization. Of particular importance in transglutaminase research are recent findings concerning the role of TG2 in gene expression, protein homeostasis, cell signaling, autoimmunity, inflammation, and hypoxia. Thus, TG2 performs a multitude of functions in multiple cellular compartments, making it one of the most versatile cellular proteins. Additional evidence links TG2 with multiple human diseases including preeclampsia, hypertension, cardiovascular disease, organ fibrosis, cancer, neurodegenerative diseases, and celiac disease. In conclusion, TG2 provides a multifunctional and multisite response to physiological stress.

Transglutaminases are oncogenic biomarkers in human cancers and therapeutic targeting of TGM2 blocks chemoresistance and macrophage infiltration in pancreatic cancer

PURPOSE

Transglutaminases (TGs) are multifunctional enzymes exhibiting transglutaminase crosslinking, as well as atypical GTPase/ATPase and kinase activities. Here, we used an integrated comprehensive analysis to assess the genomic, transcriptomic and immunological landscapes of TGs across cancers.

METHODS

Gene expression and immune cell infiltration patterns across cancers were obtained from The Cancer Genome Atlas (TCGA) database and Gene Set Enrichment Analysis (GSEA) datasets. Western blotting, immunofluorescence staining, enzyme-linked immunosorbent assays, and orthotopic xenograft models were used to validate our database-derived results.

RESULTS

We found that the overall expression of TGs (designated as the TG score) is significantly upregulated in multiple cancers and related to a worse patient survival. The expression of TG family members can be regulated through multiple mechanisms at the genetic, epigenetic and transcriptional levels. The expression of transcription factors crucial for epithelial-to-mesenchymal transition (EMT) is commonly correlated with the TG score in many cancer types. Importantly, TGM2 expression displays a close connection with chemoresistance to a wide range of chemotherapeutic drugs. We found that TGM2 expression, F13A1 expression and the overall TG score were positively correlated with the infiltration of immune cells in all cancer types tested. Functional and clinical verification revealed that a higher TGM2 expression is linked with a worse patient survival, an increased IC50 value of gemcitabine, and a higher abundance of tumor-infiltrating macrophages in pancreatic cancer. Mechanistically, we found that increased C-C motif chemokine ligand 2 (CCL2) release mediated by TGM2 contributes to macrophage infiltration into the tumor microenvironment.

CONCLUSIONS

Our results reveal the relevance and molecular networks of TG genes in human cancers and highlight the importance of TGM2 in pancreatic cancer, which may provide promising directions for immunotherapy and for addressing chemoresistance.

Association between physical activity and bone mineral density in postmenopausal women: a cross-sectional study from the NHANES 2007-2018

BACKGROUND

Physical activity (PA) is generally encouraged for the treatment of osteoporosis. However, epidemiological statistics on the level of physical activity required for bone health are scarce. The purpose of this research was to analyze the association between PA and total spine bone mineral density (BMD) in postmenopausal women.

METHODS

The research study included postmenopausal women aged ≥ 50 from the National Health and Nutrition Examination Survey. The metabolic equivalent (MET), weekly frequency, and duration of each activity were used to calculate PA. Furthermore, the correlations between BMD and PA were investigated by multivariable weighted logistic regression.

RESULTS

Eventually, 1681 postmenopausal women were included, with a weighted mean age of 62.27 ± 8.18 years. This study found that performing ≥ 38MET-h/wk was linked to a lower risk of osteoporosis after controlling for several covariates. Furthermore, the subgroup analysis revealed that the connection between total spine BMD and moderate-to-vigorous PA was more obvious among postmenopausal women aged < 65 years or individuals with normal BMI (< 25 kg/m²).

CONCLUSION

Physical activity ranging from moderate to vigorous was linked to higher total spine BMD in postmenopausal women.

The role of integrin family in bone metabolism and tumor bone metastasis

Integrins have been the research focus of cell-extracellular matrix adhesion (ECM) and cytokine receptor signal transduction. They are involved in the regulation of bone metabolism of bone precursor cells, mesenchymal stem cells (MSCs), osteoblasts (OBs), osteoclasts (OCs), and osteocytes. Recent studies expanded and updated the role of integrin in bone metabolism, and a large number of novel cytokines were found to activate bone metabolism pathways through interaction with integrin receptors. Integrins act as transducers that mediate the regulation of bone-related cells by mechanical stress, fluid shear stress (FSS), microgravity, hypergravity, extracellular pressure, and a variety of physical factors. Integrins mediate bone metastasis of breast, prostate, and lung cancer by promoting cancer cell adhesion, migration, and survival. Integrin-mediated targeted therapy showed promising prospects in bone metabolic diseases. This review emphasizes the latest research results of integrins in bone metabolism and bone metastasis and provides a vision for treatment strategies.

The cytoskeleton and connected elements in bone cell mechano-transduction

Bone is a mechano-responsive tissue that adapts to changes in its mechanical environment. Increases in strain lead to increased bone mass acquisition, whereas decreases in strain lead to a loss of bone mass. Given that mechanical stress is a regulator of bone mass and quality, it is important to understand how bone cells sense and transduce these mechanical cues into biological changes to identify druggable targets that can be exploited to restore bone cell mechano-sensitivity or to mimic mechanical load. Many studies have identified individual cytoskeletal components - microtubules, actin, and intermediate filaments - as mechano-sensors in bone. However, given the high interconnectedness and interaction between individual cytoskeletal components, and that they can assemble into multiple discreet cellular structures, it is likely that the cytoskeleton as a whole, rather than one specific component, is necessary for proper bone cell mechano-transduction. This review will examine the role of each cytoskeletal element in bone cell mechano-transduction and will present a unified view of how these elements interact and work together to create a mechano-sensor that is necessary to control bone formation following mechanical stress.

Matrix stiffness regulates myocardial differentiation of human umbilical cord mesenchymal stem cells

Myocardial infarction is a cardiovascular disease with high mortality. Human umbilical cord mesenchymal stem cells (hUC-MSCs) with strong self-renewal capacity and multipotency, provide the possibility of replacing injured cardiomyocytes. hUC-MSCs were cultured on polyacrylamide hydrogels with stiffnesses corresponding to Young's modulus of 13-16kPa and 62-68kPa which mimic the stiffnesses of healthy heart tissue and fibrotic myocardium. The expression of early myocardial markers Nkx2.5, GATA4, Mesp1 and the mature myocardial markers cTnT, cTnI, α-actin were detected by RT-PCR and Western Blot, which showed that soft matrix (13-16 kPa) tended to induce the differentiation of hUC-MSCs into myocardium, compared with stiff matrix (62-68 kPa). Piezos are mechanically sensitive non-selective cation channels. The expression of Piezo1 increased with the stiffness gradient of 1-10kPa, 13-16kPa, 35-38kPa and 62-68kPa on the 1^st day, but Piezo2 expression was irregular. The expression of integrin β1 and calcium ions were also higher on stiff substrate than on soft substrate. hUC-MSCs tend to differentiate into myocardium on the matrix stiffness of 13-16 kPa. The relationship among matrix stiffness, Piezo1 and myocardial differentiation needs further validation.

CCN1/Cyr61 Is Required in Osteoblasts for Responsiveness to the Anabolic Activity of PTH

CCN1/Cyr61 is a dynamically expressed matricellular protein that serves regulatory functions in multiple tissues. Previous studies from our laboratory demonstrated that CCN1 regulates bone maintenance. Using an osteoblast and osteocyte conditional knockout mouse model (Ccn1^OCN ), we found a significant decrease in trabecular and cortical bone mass in vivo, in part through suppression of Wnt signaling since the expression of the Wnt antagonist sclerostin (SOST) is increased in osteoblasts lacking CCN1. It has been established that parathyroid hormone (PTH) signaling also suppresses SOST expression in bone. We therefore investigated the interaction between CCN1 and PTH-mediated responses in this study. We find that loss of Ccn1 in osteoblasts leads to impaired responsiveness to anabolic intermittent PTH treatment in Ccn1^OCN mice in vivo and in osteoblasts from these mice in vitro. Analysis of Ccn1^OCN mice demonstrated a significant decrease in parathyroid hormone receptor-1 (PTH1R) expression in osteoblasts in vivo and in vitro. We investigated the regulatory role of a non-canonical integrin-binding domain of CCN1 because several studies indicate that specific integrins are critical to mechanotransduction, a PTH-dependent response, in bone. These data suggest that CCN1 regulates the expression of PTH1R through interaction with the αvβ3 and/or αvβ5 integrin complexes. Osteoblasts that express a mutant form of CCN1 that cannot interact with αvβ3/β5 integrin demonstrate a significant decrease in mRNA and protein expression of both PTH1R and αv integrin. Overall, these data suggest that the αvβ3/β5-binding domain of CCN1 is required to endow PTH signaling with anabolic activity in bone cells. © 2020 American Society for Bone and Mineral Research (ASBMR).

Molecular Aspects of Thyroid Calcification

In thyroid cancer, calcification is mainly present in classical papillary thyroid carcinoma (PTC) and in medullary thyroid carcinoma (MTC), despite being described in benign lesions and in other subtypes of thyroid carcinomas. Thyroid calcifications are classified according to their diameter and location. At ultrasonography, microcalcifications appear as hyperechoic spots ≤ 1 mm in diameter and can be named as stromal calcification, bone formation, or psammoma bodies (PBs), whereas calcifications > 1 mm are macrocalcifications. The mechanism of their formation is still poorly understood. Microcalcifications are generally accepted as a reliable indicator of malignancy as they mostly represent PBs. In order to progress in terms of the understanding of the mechanisms behind calcification occurring in thyroid tumors in general, and in PTC in particular, we decided to use histopathology as the basis of the possible cellular and molecular mechanisms of calcification formation in thyroid cancer. We explored the involvement of molecules such as runt-related transcription factor-2 (Runx-2), osteonectin/secreted protein acidic and rich in cysteine (SPARC), alkaline phosphatase (ALP), bone sialoprotein (BSP), and osteopontin (OPN) in the formation of calcification. The present review offers a novel insight into the mechanisms underlying the development of calcification in thyroid cancer.

Enzyme-catalysed polymer cross-linking: Biocatalytic tools for chemical biology, materials science and beyond

Intermolecular cross-linking is one of the most important techniques that can be used to fundamentally alter the material properties of a polymer. The introduction of covalent bonds between individual polymer chains creates 3D macromolecular assemblies with enhanced mechanical properties and greater chemical or thermal tolerances. In contrast to many chemical cross-linking reactions, which are the basis of thermoset plastics, enzyme catalysed processes offer a complimentary paradigm for the assembly of cross-linked polymer networks through their predictability and high levels of control. Additionally, enzyme catalysed reactions offer an inherently 'greener' and more biocompatible approach to covalent bond formation, which could include the use of aqueous solvents, ambient temperatures, and heavy metal-free reagents. Here, we review recent progress in the development of biocatalytic methods for polymer cross-linking, with a specific focus on the most promising candidate enzyme classes and their underlying catalytic mechanisms. We also provide exemplars of the use of enzyme catalysed cross-linking reactions in industrially relevant applications, noting the limitations of these approaches and outlining strategies to mitigate reported deficiencies.

Methylsulfonylmethane increases osteogenesis and regulates the mineralization of the matrix by transglutaminase 2 in SHED cells

Methylsulfonylmethane (MSM) is a naturally occurring, sulfate-containing, organic compound. It has been shown to stimulate the differentiation of mesenchymal stem cells into osteoblast-like cells and bone formation. In this study, we investigated whether MSM influences the differentiation of stem cells from human exfoliated deciduous teeth (SHED) into osteoblast-like cells and their osteogenic potential. Here, we report that MSM induced osteogenic differentiation through the expression of osteogenic markers such as osterix, osteopontin, and RUNX2, at both mRNA and protein levels in SHED cells. An increase in the activity of alkaline phosphatase and mineralization confirmed the osteogenic potential of MSM. These MSM-induced effects were observed in cells grown in basal medium but not osteogenic medium. MSM induced transglutaminase-2 (TG2), which may be responsible for the cross-linking of extracellular matrix proteins (collagen or osteopontin), and the mineralization process. Inhibition of TG2 ensued a significant decrease in the differentiation of SHED cells and cross-linking of matrix proteins. A comparison of mineralization with the use of mineralized and demineralized bone particles in the presence of MSM revealed that mineralization is higher with mineralized bone particles than with demineralized bone particles. In conclusion, these results indicated that MSM could promote differentiation and osteogenic potential of SHED cells. This osteogenic property is more in the presence of mineralized bone particles. TG2 is a likely cue in the regulation of differentiation and mineral deposition of SHED cells in response to MSM.

Development of 3D-printed PLGA/TiO2 nanocomposite scaffolds for bone tissue engineering applications

Porous scaffolds were 3D-printed using poly lactic-co-glycolic acid (PLGA)/TiO2 composite (10:1 weight ratio) for bone tissue engineering applications. Addition of TiO2 nanoparticles improved the compressive modulus of scaffolds. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) revealed an increase in both glass transition temperature and thermal decomposition onset of the composite compared to pure PLGA. Furthermore, addition of TiO2 was found to enhance the wettability of the surface evidenced by reducing the contact angle from 90.5 ± 3.2 to 79.8 ± 2.4 which is in favor of cellular attachment and activity. The obtained results revealed that PLGA/TiO2 scaffolds significantly improved osteoblast proliferation compared to pure PLGA (p < 0.05). Furthermore, osteoblasts cultured on PLGA/TiO2 nanocomposite showed significantly higher ALP activity and improved calcium secretion compared to pure PLGA scaffolds (p < 0.05).

Tissue transglutaminase is involved in mechanical load-induced osteogenic differentiation of human ligamentum flavum cells

Mechanical load-induced osteogenic differentiation might be the key cellular event in the calcification and ossification of ligamentum flavum. The aim of this study was to investigate the influence of tissue transglutaminase (TGM2) on mechanical load-induced osteogenesis of ligamentum flavum cells. Human ligamentum flavum cells were obtained from 12 patients undergoing lumbar spine surgery. Osteogenic phenotypes of ligamentum flavum cells, such as alkaline phosphatase (ALP), Alizarin red-S stain, and gene expression of osteogenic makers were evaluated following the administration of mechanical load and BMP-2 treatment. The expression of TGM2 was evaluated by real-time PCR, Western blotting, and enzyme-linked immunosorbent assay (ELISA) analysis. Our results showed that mechanical load in combination with BMP-2 enhanced calcium deposition and ALP activity. Mechanical load significantly increased ALP and OC gene expression on day 3, whereas BMP-2 significantly increased ALP, OPN, and Runx2 on day 7. Mechanical load significantly induced TGM2 gene expression and enzyme activity in human ligamentum flavum cells. Exogenous TGM2 increased ALP and OC gene expression; while, inhibited TG activity significantly attenuated mechanical load-induced and TGM2-induced ALP activity. In summary, mechanical load-induced TGM2 expression and enzyme activity is involved in the progression of the calcification of ligamentum flavum.

Comparison of tissue transglutaminase 2 and bone biological markers osteocalcin, osteopontin and sclerostin expression in human osteoporosis and osteoarthritis

Osteoporosis (OP) and osteoarthritis (OA) are the most common joint diseases, with a high incidence in the elderly population. OP is characterized by trabecular bone remodeling and reabsorption, whereas articular cartilage and subchondral bone remodeling are major features of OA. Although classically considered as independent or even conflicting processes, clinical coexistence of OP and OA was recently described. Transglutaminase 2 (TG2) expression is considered a biomarker of OA, but its role in osteoporotic bone remodeling is still uncertain. We investigated TG2 and bone biological markers (Osteocalcin, Osteopontin, and Sclerostin) in osteoporotic and osteoarthritic osteocartilagineous tissue (n = 54) and human chondrocyte cultures in vitro by immunohistochemistry, immunofluorescence and RT-PCR. Histomorphometric evaluation of bone trabecular remodeling was also performed. In cartilage, TG2 expression was faint in control and OP and significantly less than in OA and OP + OA chondrocytes; the opposite was found for Osteocalcin, whereas Osteopontin and Sclerostin expression was similar. In the subchondral trabecular bone, osteocytes/osteoblasts TG2 expression was slight and similar comparing control, OP, OA, and OP + OA group, whereas Osteocalcin and Osteopontin expression was lower in OP compared to control, OA and OP + OA. Increased TG2 and reduced Osteocalcin expression were maintained in human osteoarthritic chondrocytes in vitro. Histomorphometric analysis confirmed reduced trabecular bone mass in OP and OP + OA compared with OA patients. TG2 represented a suitable biomarker of osteoarthritic chondrocyte activation, whereas osteocalcin and osteopontin characterized osteoporotic osteocyte/osteoblast changes; differences were lost in OP + OA patients, suggesting careful consideration when coexistence of the two diseases occurs.

Preclinical models for in vitro mechanical loading of bone-derived cells

It is well established that bone responds to mechanical stimuli whereby physical forces are translated into chemical signals between cells, via mechanotransduction. It is difficult however to study the precise cellular and molecular responses using in vivo systems. In vitro loading models, which aim to replicate forces found within the bone microenvironment, make the underlying processes of mechanotransduction accessible to the researcher. Direct measurements in vivo and predictive modeling have been used to define these forces in normal physiological and pathological states. The types of mechanical stimuli present in the bone include vibration, fluid shear, substrate deformation and compressive loading, which can all be applied in vitro to monolayer and three-dimensional (3D) cultures. In monolayer, vibration can be readily applied to cultures via a low-magnitude, high-frequency loading rig. Fluid shear can be applied to cultures in multiwell plates via a simple rocking platform to engender gravitational fluid movement or via a pump to cells attached to a slide within a parallel-plate flow chamber, which may be micropatterned for use with osteocytes. Substrate strain can be applied via the vacuum-driven FlexCell system or via a four-point loading jig. 3D cultures better replicate the bone microenvironment and can also be subjected to the same forms of mechanical stimuli as monolayer, including vibration, fluid shear via perfusion flow, strain or compression. 3D cocultures that more closely replicate the bone microenvironment can be used to study the collective response of several cell types to loading. This technical review summarizes the methods for applying mechanical stimuli to bone cells in vitro.

A comparative study of mechanical strain, icariin and combination stimulations on improving osteoinductive potential via NF-kappaB activation in osteoblast-like cells

Background

The combination of drugs and exercise was the effective treatment in bone injure and rebuilding in clinic. As mechanical strain has potential in inducing the differentiation of osteoblasts in our previous study, the further research to investigate the combination of mechanical strain and icariin stimulation on inducing osteoblast proliferation, differentiation and the possible mechanism in MC3T3-E1 cell line.

Methods

A whole cell enzyme-linked immunosorbent assay that detects the bromodeoxyuridine incorporation during DNA synthesis was applied to evaluate the proliferation. The mRNA expression of alkaline phosphatase (ALP), osteocalcin (OCN), type I collagen (Col I), bone morphogenetic protein-2 (BMP-2) and BMP-4 was detected by real-time reverse-transcription polymerase chain reaction. The activity of ALP was analyzed by ELISA and the protein expression of OCN, Col I and BMP-2 was assessed by western blot. Moreover, the activity of nuclear transcription factor kappa-B (NF-κB) signaling pathway was investigated with the expression of inhibitor of κB (IκB) α, phosphorylation of IκB-α (P-IκB-α), p65, P-p65 by western blot.

Results

We observed that compared to single mechanical strain or icariin stimulation, the mRNA and protein expressions of ALP (P < 0.05 or P < 0.01), OCN (P < 0.01) and Col I (P < 0.05 or P < 0.01) were increased significantly by the combination of mechanical strain and icariin stimulation. Moreover, the combination of mechanical strain and icariin stimulation could up-regulate the expression of BMP-2 (P < 0.01) and BMP-4 compared to single mechanical strain or icariin stimulation. The combination of mechanical strain and icariin stimulation could activate NF-κB signaling pathway by increasing the expression of IκB α, P-IκB-α, p65, P-p65 (P < 0.01).

Conclusion

The combination of mechanical strain and icariin stimulation could activate the NF-κB pathway to improve the proliferation, differentiation of osteoblast-like cells.

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.

MicroRNA-218, microRNA-191*, microRNA-3070a and microRNA-33 are responsive to mechanical strain exerted on osteoblastic cells

MicroRNA (miRNA) is an important regulator of cell differentiation and function. Mechanical strain is important in the growth and differentiation of osteoblasts. Therefore, mechanresponsive miRNA may be important in the response of osteoblasts to mechanical strain. The purpose of the present study was to select and identify the mechanoresponsive miRNAs of osteoblasts. Mouse osteoblastic MC3T3-E1 cells were cultured in cell culture dishes and stimulated with a mechanical tensile strain of 2,50 με at 0.5 Hz, and the activity of alkaline phosphatase (ALP), mRNA levels of ALP, osteocalcin (OCN), and collagen type I (Col I), and protein levels of bone morphogenetic proteins (BMPs) in the cell culture medium were assayed. Following miRNA microarray and reverse transcription-quantitative polymerase chain reaction analyses, differentially expressed miRNAs in the mechanically strained cells and unstrained cells were selected and identified. Using bioinformatics analysis, the target genes of the miRNAs were then predicted. The results revealed that the mechanical strain of 2,500 με increased the activity of ALP, the mRNA levels of ALP, OCN and Col I, and the protein levels of bone morphogenetic protein(BMP)-2 and BMP-4 Continuous mechanical stimulation for 8 h had the most marked stimulant effects. miR-218, miR-191*, miR-3070a and miR-33 were identified as differentially expressed miRNAs in the mechanically strained MC3T3-E1 cells. Certain target genes of these four miRNAs were involved in osteoblastic differentiation. These findings indicated that a mechanical strain of 2,500 με, particularly for a period of 8 h, promoted osteoblastic differentiation, and the four mechanoresponsive miRNAs identified may be a potential regulator of osteoblastic differentiation and their response to mechanical strain.

Mechanical strain using 2D and 3D bioreactors induces osteogenesis: implications for bone tissue engineering

Fracture healing is a complicated process involving many growth factors, cells, and physical forces. In cases, where natural healing is not able, efforts have to be undertaken to improve healing. For this purpose, tissue engineering may be an option. In order to stimulate cells to form a bone tissue several factors are needed: cells, scaffold, and growth factors. Stem cells derived from bone marrow or adipose tissues are the most useful in this regard. The differentiation of the cells can be accelerated using mechanical stimulation. The first part of this chapter describes the influence of longitudinal strain application. The second part uses a sophisticated approach with stem cells on a newly developed biomaterial (Sponceram) in a rotating bed bioreactor with the administration of bone morphogenetic protein-2. It is shown that such an approach is able to produce bone tissue constructs. This may lead to production of larger constructs that can be used in clinical applications.

Comparison of osteogenic medium and uniaxial strain on differentiation of endometrial stem cells

Background:

Mechanotransduction plays a pivotal role in remodeling and repair of skeletal tissues. This mechanism has been widely used in bone tissue engineering especially under in vitro conditions. To date, various stem cells have been used for this purpose. The present study was the first to evaluate the effect of mechanical loading on differentiation of human endometrial stem cells (hESCs) to osteoblasts.

Materials and Methods:

Adhesion of endometrial stem cells after isolation and culture on a silicone membrane covered with collagen was evaluated under scanning electron microscope (SEM). Twenty-four hours after cell culture on the membrane and ensuring appropriate cell adhesion, a group of cells in a conventional culture medium received 3% static uniaxial strain. In the positive control group, cells cultured on the membrane were placed in an osteogenic medium without receiving any mechanical strain. The negative control group was placed in a regular medium and received no strain either. Two weeks later, cultured cells were evaluated for expression of osteogenic markers using immunofluorescence staining and real-time polymerase chain reaction (PCR). Data of real-time PCR was analyzed by ANOVA. P < 0.05 was considered statistically significant.

Results:

SEM analysis revealed adequate cell adhesion to the membrane after 24 h. Two weeks after loading, expression of markers in the positive control group was significantly higher compared to test group.

Conclusion:

We can conclude that static uniaxial strain exerted on hESCs results in their differentiation to osteoblasts. However, this magnitude of static strain in the tested time period cannot yield excellent differentiation when compared to the osteogenic medium.

p38-MAPK signaling pathway is not involved in osteogenic differentiation during early response of mesenchymal stem cells to continuous mechanical strain

Mechanical stimuli play a significant role in the regulation of bone remodeling during orthodontic tooth movement. However, the correlation between mechanical strain and bone remodeling is still poorly understood. In this study, we used a model of continuous mechanical strain (CMS) on bone mesenchymal stem cells (BMSCs) to investigate the proliferation and osteogenic differentiation of BMSCs and the mechanism of mechano-transduction. A CMS of 10 % at 1 Hz suppressed the proliferation of BMSCs and induced early osteogenic differentiation within 48 h by activating Runx2 and increasing alkaline phosphatase (ALP) activity and mRNA expression of osteogenesis-related genes (ALP, collagen type I, and osteopontin). Regarding mitogen-activated protein kinase (MAPK) activation, CMS induced phased phosphorylation of p38 consisting of a rapid induction of p38 MAPK at 10 min and a rapid decay after 1 h. Furthermore, the potent p38 inhibitor SB203580 blocked the induction of p38 MAPK signaling, but had little effect on subsequent osteogenic events. These results demonstrate that mechanical strain may act as a stimulator to induce the differentiation of BMSCs into osteoblasts, which is a vital function for bone formation in orthodontic tooth movement. However, activation of the p38 signaling pathway may not be involved in this process.

Mechanical strain promotes osteoblast ECM formation and improves its osteoinductive potential

BACKGROUND

The extracellular matrix (ECM) provides a supportive microenvironment for cells, which is suitable as a tissue engineering scaffold. Mechanical stimulus plays a significant role in the fate of osteoblast, suggesting that it regulates ECM formation. Therefore, we investigated the influence of mechanical stimulus on ECM formation and bioactivity.

METHODS

Mouse osteoblastic MC3T3-E1 cells were cultured in cell culture dishes and stimulated with mechanical tensile strain. After removing the cells, the ECMs coated on dishes were prepared. The ECM protein and calcium were assayed and MC3T3-E1 cells were re-seeded on the ECM-coated dishes to assess osteoinductive potential of the ECM.

RESULTS

The cyclic tensile strain increased collagen, bone morphogenetic protein 2 (BMP-2), BMP-4, and calcium levels in the ECM. Compared with the ECM produced by unstrained osteoblasts, those of mechanically stimulated osteoblasts promoted alkaline phosphatase activity, elevated BMP-2 and osteopontin levels and mRNA levels of runt-related transcriptional factor 2 (Runx2) and osteocalcin (OCN), and increased secreted calcium of the re-seeded MC3T3-E1 cells.

CONCLUSION

Mechanical strain promoted ECM production of osteoblasts in vitro, increased BMP-2/4 levels, and improved osteoinductive potential of the ECM. This study provided a novel method to enhance bioactivity of bone ECM in vitro via mechanical strain to osteoblasts.

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.

Tissue transglutaminase (TG2) activity regulates osteoblast differentiation and mineralization in the SAOS-2 cell line

Tissue transglutaminase (type II, TG2) has long been postulated to directly promote skeletal matrix calcification and play an important role in ossification. However, limited information is available on the expression, function and modulating mechanism of TG2 during osteoblast differentiation and mineralization. To address these issues, we cultured the well-established human osteosarcoma cell line SAOS-2 with osteo-inductive conditioned medium and set up three time points (culture days 4, 7, and 14) to represent different stages of SAOS-2 differentiation. Osteoblast markers, mineralization, as well as TG2 expression and activity, were then assayed in each stage. Furthermore, we inhibited TG activity with cystamine and then checked SAOS-2 differentiation and mineralization in each stage. The results showed that during the progression of osteoblast differentiation SAOS-2 cells presented significantly high levels of osteocalcin (OC) mRNA, bone morphogenetic protein-2 (BMP-2) and collagen I, significantly high alkaline phosphatase (ALP) activity, and the increased formation of calcified matrix. With the same tendency, TG2 expression and activity were up-regulated. Furthermore, inhibition of TG activity resulted in a significant decrease of OC, collagen I, and BMP-2 mRNA and of ALP activity and mineralization. This study demonstrated that TG2 is involved in osteoblast differentiation and may play a role in the initiation and regulation of the mineralization processes. Moreover, the modulating effects of TG2 on osteoblasts may be related to BMP-2.

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.

Megakaryocytes regulate expression of Pyk2 isoforms and caspase-mediated cleavage of actin in osteoblasts

The proliferation and differentiation of osteoblast (OB) precursors are essential for elaborating the bone-forming activity of mature OBs. However, the mechanisms regulating OB proliferation and function are largely unknown. We reported that OB proliferation is enhanced by megakaryocytes (MKs) via a process that is regulated in part by integrin signaling. The tyrosine kinase Pyk2 has been shown to regulate cell proliferation and survival in a variety of cells. Pyk2 is also activated by integrin signaling and regulates actin remodeling in bone-resorbing osteoclasts. In this study, we examined the role of Pyk2 and actin in the MK-mediated increase in OB proliferation. Calvarial OBs were cultured in the presence of MKs for various times, and Pyk2 signaling cascades in OBs were examined by Western blotting, subcellular fractionation, and microscopy. We found that MKs regulate the temporal expression of Pyk2 and its subcellular localization. We also found that MKs regulate the expression of two alternatively spliced isoforms of Pyk2 in OBs, which may regulate OB differentiation and proliferation. MKs also induced cytoskeletal reorganization in OBs, which was associated with the caspase-mediated cleavage of actin, an increase in focal adhesions, and the formation of apical membrane ruffles. Moreover, BrdU incorporation in MK-stimulated OBs was blocked by the actin-polymerizing agent, jasplakinolide. Collectively, our studies reveal that Pyk2 and actin play an important role in MK-regulated signaling cascades that control OB proliferation and may be important for therapeutic interventions aimed at increasing bone formation in metabolic diseases of the skeleton.

Potential for osteogenic and chondrogenic differentiation of MSC

The introduction of mesenchymal stem cells (MSC) into the field of tissue engineering for bone and cartilage repair is a promising development, since these cells can be expanded ex vivo to clinically relevant numbers and, after expansion, retain their ability to differentiate into different cell lineages. Mesenchymal stem cells isolated from various tissues have been intensively studied and characterized by many research groups. To obtain functionally active differentiated tissue, tissue engineered constructs are cultivated in vitro statically or dynamically in bioreactors under controlled conditions. These conditions include special cell culture media, addition of signalling molecules, various physical and chemical factors and the application of different mechanical stimuli. Oxygen concentration in the culture environment is also a significant factor which influences MSC proliferation, stemness and differentiation capacity. Knowledge of the different aspects which affect MSC differentiation in vivo and in vitro will help researchers to achieve directed cell fate without the addition of differentiation agents in concentrations above the physiological range.

Mechanical signaling through the cytoskeleton regulates cell proliferation by coordinated focal adhesion and Rho GTPase signaling

The notion that cell shape and spreading can regulate cell proliferation has evolved over several years, but only recently has this been linked to forces from within and upon the cell. This emerging area of mechanical signaling is proving to be wide-spread and important for all cell types. The microenvironment that surrounds cells provides a complex spectrum of different, simultaneously active, biochemical, structural and mechanical stimuli. In this milieu, cells probe the stiffness of their microenvironment by pulling on the extracellular matrix (ECM) and/or adjacent cells. This process is dependent on transcellular cell-ECM or cell-cell adhesions, as well as cell contractility mediated by Rho GTPases, to provide a functional linkage through which forces are transmitted through the cytoskeleton by intracellular force-generating proteins. This Commentary covers recent advances in the underlying mechanisms that control cell proliferation by mechanical signaling, with an emphasis on the role of 3D microenvironments and in vivo extracellular matrices. Moreover, as there is much recent interest in the tumor-stromal interaction, we will pay particular attention to exciting new data describing the role of mechanical signaling in the progression of breast cancer.

Focal adhesion kinase regulation of mechanotransduction and its impact on endothelial cell functions

Vascular endothelial cells lining the blood vessels form the interface between the bloodstream and the vessel wall and as such they are continuously subjected to shear and cyclic stress from the flowing blood in the lumen. Additional mechanical stimuli are also imposed on these cells in the form of substrate stiffness transmitted from the extracellular matrix components in the basement membrane, and additional mechanical loads imposed on the lung endothelium as the result of respiration or mechanical ventilation in clinical settings. Focal adhesions (FAs) are complex structures assembled at the abluminal endothelial plasma membrane which connect the extracellular filamentous meshwork to the intracellular cytoskeleton and hence constitute the ideal checkpoint capable of controlling or mediating transduction of bidirectional mechanical signals. In this review we focus on focal adhesion kinase (FAK), a component of FAs, which has been studied for a number of years with regards to its involvement in mechanotransduction. We analyzed the recent advances in the understanding of the role of FAK in the signaling cascade(s) initiated by various mechanical stimuli with particular emphasis on potential implications on endothelial cell functions.

Comparison of the biological response of osteoblasts after tension and compression

The aim of this study was to investigate the difference in the biological response of osteoblasts when stretched and compressed. A cellular cyclic tension and compression apparatus (CCTCA) was designed to stretch and compress cells under the same conditions. After stretching or compressing MC3T3-E1 with continuously increased strain for 5 hours, cellular cytoskeletal modulation was detected by immunohistochemical assay with actin antibody. Real-time polymerase chain reaction was performed at 1, 3, and 5 hours to detect local factors related to bone remodelling. Statistical analysis was undertaken with analysis of variance and the Kruskal-Wallis. Following stretching or compression for 5 hours, MC3T3-E1 attached to the culture dishes grew well. Compared with the control, the microfilaments orientated parallel with each other and were clearly observed by laser scanning confocal microscope after 5 hours of stretching. The morphology of MC3T3-E1 cells was thinner and longer than the control. However, microfilaments presented a disordered arrangement after 5 hours of compression, and the MC3T3-E1 cells decreased in size. Gene expression of Wnt10b and Lrp5 increased during tension but more in the compression groups at 1, 3, and 5 hours. The ratio of osteoprotegerin to receptor activator for nuclear factor kappa B ligand increased in the tension group compared with the control but decreased in the compression group at 5 hours.

Transglutaminase-mediated oligomerization promotes osteoblast adhesive properties of osteopontin and bone sialoprotein

Tissue transglutaminase (TG2) is a widely distributed, protein-crosslinking enzyme having a prominent role in cell adhesion as a β1 integrin co-receptor for fibronectin. In bone and teeth, its substrates include the matricellular proteins osteopontin (OPN) and bone sialoprotein (BSP). The aim of this study was to examine effects of TG2-mediated crosslinking and oligomerization of OPN and BSP on osteoblast cell adhesion. We show that surfaces coated with oligomerized OPN and BSP promote MC3T3-E1/C4 osteoblastic cell adhesion significantly better than surfaces coated with the monomeric form of the proteins. Both OPN and BSP oligomer-adherent cells showed more cytoplasmic extensions than those cells grown on the monomer-coated surfaces indicative of increased cell connectivity. Our study suggests a role for TG2 in promoting the cell adhesion function of two matricellular substrate proteins prominent in bone, tooth cementum and certain tumors.

Mammary gland ECM remodeling, stiffness, and mechanosignaling in normal development and tumor progression

Cells of the mammary gland are in intimate contact with other cells and with the extracellular matrix (ECM), both of which provide not only a biochemical context, but a mechanical context as well. Cell-mediated contraction allows cells to sense the stiffness of their microenvironment, and respond with appropriate mechanosignaling events that regulate gene expression and differentiation. ECM composition and organization are tightly regulated throughout development of the mammary gland, resulting in corresponding regulation of the mechanical environment and proper tissue architecture. Mechanical regulation is also at play during breast carcinoma progression, as changes in ECM deposition, composition, and organization accompany breast carcinoma. These changes result in stiffer matrices that activate mechanosignaling pathways and thereby induce cell proliferation, facilitate local tumor cell invasion, and promote progression. Thus, understanding the role of forces in the mammary gland is crucial to understanding both normal developmental and pathological processes.

Human adipose-derived side population stem cells cultured on demineralized bone matrix for bone tissue engineering

BACKGROUND

Tissue engineering of new bone relies on the combination and application of osteoconductive, osteoinductive, and osteogenic elements. Natural scaffolds, such as demineralized bone matrix (DBM), contain collagenous networks with growth factors such as bone morphogenetic protein-2. Stem cells from readily available sources, including discarded adipose tissue, have the propensity to differentiate into bone. The present study examines a multi-component technique consisting of a novel side population of adipose stem cells cultured on DBM for tissue engineering applications.

METHODS

Adipose-derived side population stem cells were cultured on DBM for up to 14 days. Cell proliferation, alkaline phosphatase activity, extracellular matrix protein production, and calcium-containing mineral deposit formation were assayed. Ectopic bone formation in a rat model was also evaluated.

RESULTS

Side population stem cells attached to and proliferated on DBM while generating markers of new bone formation. When these cell/substrate composites were implanted into an ectopic model, newly formed bone was 30% greater than that of DBM alone.

CONCLUSIONS

Novel populations of adipose-derived stem cells cultured on DBM compose a system that develops new bone matrix in vitro and in vivo. This strategy provides a novel approach using naturally occurring materials for bone repair in tissue engineering applications.

Application of different strain regimes in two-dimensional and three-dimensional adipose tissue-derived stem cell cultures induces osteogenesis: implications for bone tissue engineering

Mechanical strain has become an important tool in tissue engineering for progenitor cell differentiation. Furthermore, it is used to enhance the mechanical properties of engineered tissue constructs. Although strain amplitude and frequency are well investigated and optimal values are known; application of various strain schemes regarding duration and repetition are not described in literature. In this study, we therefore applied singular and repetitive cyclic strain (1 Hz, 5%) of 15 min short-time strain and longer strain durations up to 8 h. Additionally, a gradually increasing strain scheme starting with short-time strain and consecutive elongated strain periods was applied. The cultivation surface was planar silicone on one hand and a three-dimensionally structured collagen I mesh on the other hand. Adipose tissue-derived mesenchymal stem cells and an osteogenic model cell line (MG-63) were exposed to these strain regimes and post-strain cell viability, osteogenic marker gene expression, and matrix mineralization were investigated. Upregulation of alkaline phosphatase, osteocalcin, osteopontin, and BMP-2/4 revealed that even short-time strain can enhance osteogenic differentiation. Elongation and repetition of strain, however, resulted in a decline of the observed short-time strain effects, which we interpret as positively induced cellular adaptation to the mechanically active surroundings. With regard to cellular adaptation, the gradually increasing strain scheme was especially advantageous.

Tissue transglutaminase expression and activity in human ligamentum flavum cells derived from thoracic ossification of ligamentum flavum

STUDY DESIGN

The study was undertaken to compare the expression and activity of tissue transglutaminase (TG2) in human ligamentum flavum cells derived from ossification of the ligamentum flavum (OLF) and non-OLF patients.

OBJECTIVE

To determine whether TG2 is involved in the pathologic process of OLF.

SUMMARY OF BACKGROUND DATA

OLF is a disease characterized by heterotopic formation of new bone in the flavum ligament. Recently, TG2 is proved to directly promote skeletal matrix mineralization and play an important role in the ossification. TG2 activity is vital to the differentiation of osteoblasts and the formation of mineralization. But whether TG2 is involved in the pathologic process of OLF is unknown. We investigated the relations between TG2 expression and OLF.

METHODS

OLF and non-OLF cells were cultured and osteocalcin, bone morphogenetic protein-2(BMP-2) and TG2 mRNA expressions were assayed by reverse transcription polymerase chain reaction. Meanwhile, alkaline phosphatase activity and calcified nodules were compared between OLF and non-OLF cells. To detect TG2 expression, Western blot and immunohistochemical analysis were carried out, and TG2 activity was compared between OLF and non-OLF cells.

RESULTS

Our experiments demonstrated that OLF cells showed osteoblast-like activity and increased mRNA expression of BMP-2. More interesting, compared with non-OLF cells, OLF cells showed elevated expression levels of TG2 mRNA and protein, as well as enzyme activity.

CONCLUSION

TG2 expression and enzyme activity are upregulated in the OLF cells and TG2 may be involved in the pathologic process of OLF.

Nmp4/CIZ inhibits mechanically induced beta-catenin signaling activity in osteoblasts

Cellular mechanotransduction, the process of converting mechanical signals into biochemical responses within cells, is a critical aspect of bone health. While the effects of mechanical loading on bone are well recognized, elucidating the specific molecular pathways involved in the processing of mechanical signals by bone cells represents a challenge and an opportunity to identify therapeutic strategies to combat bone loss. In this study we have for the first time examined the relationship between the nucleocytoplasmic shuttling transcription factor nuclear matrix protein-4/cas interacting zinc finger protein (Nmp4/CIZ) and beta-catenin signaling in response to a physiologic mechanical stimulation (oscillatory fluid shear stress, OFSS) in osteoblasts. Using calvaria-derived osteoblasts from Nmp4-deficient and wild-type mice, we found that the normal translocation of beta-catenin to the nucleus in osteoblasts that is induced by OFSS is enhanced when Nmp4/CIZ is absent. Furthermore, we found that other aspects of OFSS-induced mechanotransduction generally associated with the beta-catenin signaling pathway, including ERK, Akt, and GSK3beta activity, as well as expression of the beta-catenin-responsive protein cyclin D1 are also enhanced in cells lacking Nmp4/CIZ. Finally, we found that in the absence of Nmp4/CIZ, OFSS-induced cytoskeletal reorganization and the formation of focal adhesions between osteoblasts and the extracellular substrate is qualitatively enhanced, suggesting that Nmp4/CIZ may reduce the sensitivity of bone cells to mechanical stimuli. Together these results provide experimental support for the concept that Nmp4/CIZ plays an inhibitory role in the response of bone cells to mechanical stimulation induced by OFSS.

Focal adhesion kinase is important for fluid shear stress-induced mechanotransduction in osteoblasts

Mechanical loading of bone is important for maintenance of bone mass and structural stability of the skeleton. When bone is mechanically loaded, movement of fluid within the spaces surrounding bone cells generates fluid shear stress (FSS) that stimulates osteoblasts, resulting in enhanced anabolic activity. The mechanisms by which osteoblasts convert the external stimulation of FSS into biochemical changes, a process known as mechanotransduction, remain poorly understood. Focal adhesions are prime candidates for transducing external stimuli. Focal adhesion kinase (FAK), a nonreceptor tyrosine kinase found in focal adhesions, may play a key role in mechanotransduction, although its function has not been directly examined in osteoblasts. We examined the role of FAK in osteoblast mechanotransduction using short interfering RNA (siRNA), overexpression of a dominant negative FAK, and FAK(-/-) osteoblasts to disrupt FAK function in calvarial osteoblasts. Osteoblasts were subjected to varying periods oscillatory fluid flow (OFF) from 5 min to 4 h, and several physiologically important readouts of mechanotransduction were analyzed including: extracellular signal-related kinase 1/2 phosphorylation, upregulation of c-fos, cyclooxygenase-2, and osteopontin, and release of prostaglandin E(2). Osteoblasts with disrupted FAK signaling exhibited severely impaired mechanical responses in all endpoints examined. These data indicate the importance of FAK for both short and long periods of FSS-induced mechanotransduction in osteoblasts.

PEG attachment to osteoblasts enhances mechanosensitivity

Fluid flow induces proliferation and differentiation of osteoblasts, and fibrous structure like a primary cilium on a cell surface contributes to flow sensing and flow-driven gene regulation. We address a question: Does attachment of synthetic polymers on a cell surface enhance mechanosensitivity of osteoblasts? Using MC3T3 osteoblast cells (C4 clone) and a PEG polymer, one of whose termini was covalently linked to a succinimidyl succinate group (functionalized PEG-PEGSS), we examined attachment of PEGSS to osteoblasts and evaluated its effects on the mRNA expression of stress-responsive genes. AFM images exhibited globular PEGSS conformation of approximately 100 nm in size, and SEM images confirmed the attachment of a cluster of pancake-like PEGSS molecules on the osteoblast surface. Compared to control cells incubated with unfunctionalized PEG, real-time PCR revealed that RNA upregulation of c-fos, egr1, ATF3 and Cox2 genes was magnified in the cells incubated with PEGSS. These results support a PEG-induced increase in mechanosensitivity of osteoblasts and indicate that the described approach would be useful to accelerate growth and development of osteoblasts for bone repair and tissue engineering.

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.