Mechanisms involved in regulation of osteoclastic differentiation by mechanical stress-loaded osteoblasts

Significance of mechanical loading in bone fracture healing, bone regeneration, and vascularization

In 1892, J.L. Wolff proposed that bone could respond to mechanical and biophysical stimuli as a dynamic organ. This theory presents a unique opportunity for investigations on bone and its potential to aid in tissue repair. Routine activities such as exercise or machinery application can exert mechanical loads on bone. Previous research has demonstrated that mechanical loading can affect the differentiation and development of mesenchymal tissue. However, the extent to which mechanical stimulation can help repair or generate bone tissue and the related mechanisms remain unclear. Four key cell types in bone tissue, including osteoblasts, osteoclasts, bone lining cells, and osteocytes, play critical roles in responding to mechanical stimuli, while other cell lineages such as myocytes, platelets, fibroblasts, endothelial cells, and chondrocytes also exhibit mechanosensitivity. Mechanical loading can regulate the biological functions of bone tissue through the mechanosensor of bone cells intraosseously, making it a potential target for fracture healing and bone regeneration. This review aims to clarify these issues and explain bone remodeling, structure dynamics, and mechano-transduction processes in response to mechanical loading. Loading of different magnitudes, frequencies, and types, such as dynamic versus static loads, are analyzed to determine the effects of mechanical stimulation on bone tissue structure and cellular function. Finally, the importance of vascularization in nutrient supply for bone healing and regeneration was further discussed.

Wnt signaling modulates macrophage polarization and is regulated by biomaterial surface properties

Macrophages are among the first cells to interact with biomaterials and ultimately determine their integrative fate. Biomaterial surface characteristics like roughness and hydrophilicity can activate macrophages to an anti-inflammatory phenotype. Wnt signaling, a key cell proliferation and differentiation pathway, has been associated with dysregulated macrophage activity in disease. However, the role Wnt signaling plays in macrophage activation and response to biomaterials is unknown. The aim of this study was to characterize the regulation of Wnt signaling in macrophages during classical pro- and anti-inflammatory polarization and in their response to smooth, rough, and rough-hydrophilic titanium (Ti) surfaces. Peri-implant Wnt signaling in macrophage-ablated (MaFIA) mice instrumented with intramedullary Ti rods was significantly attenuated compared to untreated controls. Wnt ligand mRNA were upregulated in a surface modification-dependent manner in macrophages isolated from the surface of Ti implanted in C57Bl/6 mice. In vitro, Wnt mRNAs were regulated in primary murine bone-marrow-derived macrophages cultured on Ti in a surface modification-dependent manner. When macrophageal Wnt secretion was inhibited, macrophage sensitivity to both physical and biological stimuli was abrogated. Loss of macrophage-derived Wnts also impaired recruitment of mesenchymal stem cells and T-cells to Ti implants in vivo. Finally, inhibition of integrin signaling decreased surface-dependent upregulation of Wnt genes. These results suggest that Wnt signaling regulates macrophage response to biomaterials and that macrophages are an important source of Wnt ligands during inflammation and healing.

The effects of P-gp and CYP450 modulated by rifampicin on the steroid-induced osteonecrosis of the femoral head

This study investigated the effects of rifampicin-modulated P-glycoprotein (P-gp) and cytochrome P450 (CYP450) activity on the development of steroid-induced osteonecrosis of the femoral head. Thirty-two rabbits were equally divided into four groups: control group, oral administration group, intramuscular injection group, and local release group, in which rifampicin-loaded artificial bone graft was implanted in the left femur cavity and blank bone graft was implanted in the right femur cavity. Dexamethasone was given 1 week after rifampicin administration. Peripheral P-gp activity and hepatic CYP450 content were investigated 4 weeks later. Hematoxylin and eosin, Massson, and tetracycline-fluorescence staining of the femoral head were compared. In vitro, the effects of intracellular dexamethasone concentration modulated by P-gp on osteoprotegerin (OPG)/receptor activator of nuclear factor κB ligand (RANKL) expression and differentiation of mesenchymal stem cells were further investigated. Peripheral P-gp activity and hepatic CYP450 content in the oral administration group and the intramuscular injection group were significantly higher than those in the local release group. P-gp activity of mesenchymal stem cells in rifampicin-implanted femoral head was significantly higher than that in the blank control. Histological study showed that rifampicin could prevent steroid-induced bone loss and lipid formation, and promote new bone formation and maturation. In vitro study confirmed that intracellular dexamethasone concentration modulated by P-gp could influence the OPG/RANKL ratio and the differentiation of mesenchymal stem cells. Enhanced levels of peripheral P-gp and hepatic CYP450 can reduce the incidence of steroid-induced osteonecrosis of the femoral head. P-gp activity locally enhanced by rifampicin decreases the intracellular steroid concentration, but rifampicin does not have significant effects on peripheral P-gp and hepatic CYP450.

Magnitude-dependent response of osteoblasts regulated by compressive stress

The present study aimed to investigate the role of magnitude in adaptive response of osteoblasts exposed to compressive stress. Murine primary osteoblasts and MC3T3-E1 cells were exposed to compressive stress (0, 1, 2, 3, 4, and 5 g/cm2) in 3D culture. Cell viability was evaluated, and expression levels of Runx2, Alp, Ocn, Rankl, and Opg were examined. ALP activity in osteoblasts and TRAP activity in RAW264.7 cells co-cultured with MC3T3-E1 cells were assayed. Results showed that compressive stress within 5.0 g/cm2 did not influence cell viability. Both osteoblastic and osteoblast-regulated osteoclastic differentiation were enhanced at 2 g/cm2. An increase in stress above 2 g/cm2 did not enhance osteoblastic differentiation further but significantly inhibited osteoblast-regualted osteoclastic differentiation. This study suggested that compressive stress regulates osteoblastic and osteoclastic differentiation through osteoblasts in a magnitude-dependent manner.

Static compression regulates OPG expression in periodontal ligament cells via the CAMK II pathway

Objective

This study aimed to investigate the potential role of CAMK II pathway in the compression-regulated OPG expression in periodontal ligament cells (PDLCs).

Material and Methods

The PDL tissue model was developed by 3-D culturing human PDLCs in a thin sheet of poly lactic-co-glycolic acid (PLGA) scaffolds, which was subjected to static compression of 25 g/cm² for 3, 6 and 12 h, with or without treatment of KN-93. After that, the expression of OPG, RANKL and NFATC2 was investigated through real-time PCR and western blot analysis.

Results

After static compression, the NFATC2 and RANKL expression was significantly up-regulated, while partially suppressed by KN-93 for 6 and 12 h respectively. The OPG expression was significantly down-regulated by compression in 3 h, started to elevate in 6 h, and significantly up-regulated in 12 h. The up-regulation after 12 h was significantly suppressed by KN-93.

Conclusions

Long-term static compression increases OPG expression in PDLCs, at least partially, via the CAMK II pathway.

Cyclic mechanical stretch promotes energy metabolism in osteoblast-like cells through an mTOR signaling-associated mechanism

Energy metabolism is essential for maintaining function and substance metabolism in osteoblasts. However, the role of cyclic stretch in regulating osteoblastic energy metabolism and the underlying mechanisms remain poorly understood. In this study, we found that cyclic stretch (10% elongation at 0.1 Hz) significantly enhanced glucose consumption, lactate levels (determined using a glucose/lactate assay kit), intracellular adenosine triphosphate (ATP) levels (quantified using rLuciferase/Luciferin reagent) and the mRNA expression of energy metabolism-related enzymes [mitochondrial ATP synthase, L-lactate dehydrogenase A (LDHA) and enolase 1; measured by RT-qPCR], and increased the phosphorylation levels of Akt, mammalian target of rapamycin (mTOR) and p70s6k (measured by western blot analysis) in human osteoblast‑like MG‑63 cells. Furthermore, the inhibition of Akt or mTOR with an antagonist (wortmannin or rapamycin) suppressed the stretch-induced increase in glucose consumption, lactate levels, intracellular ATP levels and the expression of mitochondrial ATP synthase and LDHA, indicating the significance of the Akt/mTOR/p70s6k pathway in regulating osteoblastic energy metabolism in response to mechanical stretch. Thus, we concluded that cyclic stretch regulates energy metabolism in MG‑63 cells partially through the Akt/mTOR/p70s6k signaling pathway. The present findings provide novel insight into osteoblastic mechanobiology from the perspective of energy metabolism.

Activation of focal adhesion kinase induces extracellular signal-regulated kinase-mediated osteogenesis in tensile force-subjected periodontal ligament fibroblasts but not in osteoblasts

The exact mechanism by which focal adhesion kinase (FAK) translates mechanical signals into osteogenesis differentiation in force-subjected cells has not been elucidated. The responses to different forces differ according to the origin of cells and the type of stress applied. Therefore, the recruitment of osteoclast and osteoblast progenitor cells, and the balanced activation of these cells around and within the periodontal ligament (PDL) are essential for alveolar bone remodeling. Cells within the PDL and MG63 cells were subjected to tensile forces of -100 kPa for different periods of time. At various times during the tensile force application, they were processed for the purpose of analyzing cell viability, cell cycle, and osteogenic protein. The effect of small interfering RNA transfection targeting FAK was also evaluated. Tensile force enhanced a rapid increase in the phosphorylation of FAK and up-regulated osteogenic protein expression in PDL cells, but not in MG63 cells. Transfecting PDL cells with FAK antisense oligonucleotide diminished alkaline phosphatase and osteocalcin secretion. These findings suggest that tensile force activates FAK pathways in PDL cells, which down-regulate immune cytokine and up-regulate osteogenic protein.

New insights into adhesion signaling in bone formation

Mineralized tissues that are protective scaffolds in the most primitive species have evolved and acquired more specific functions in modern animals. These are as diverse as support in locomotion, ion homeostasis, and precise hormonal regulation. Bone formation is tightly controlled by a balance between anabolism, in which osteoblasts are the main players, and catabolism mediated by the osteoclasts. The bone matrix is deposited in a cyclic fashion during homeostasis and integrates several environmental cues. These include diffusible elements that would include estrogen or growth factors and physicochemical parameters such as bone matrix composition, stiffness, and mechanical stress. Therefore, the microenvironment is of paramount importance for controlling this delicate equilibrium. Here, we provide an overview of the most recent data highlighting the role of cell-adhesion molecules during bone formation. Due to the very large scope of the topic, we focus mainly on the role of the integrin receptor family during osteogenesis. Bone phenotypes of some deficient mice as well as diseases of human bones involving cell adhesion during this process are discussed in the context of bone physiology.

Short-term mechanical stress inhibits osteoclastogenesis via suppression of DC-STAMP in RAW264.7 cells

Mechanical stress is an important factor in bone homeostasis, which is maintained by a balance between bone resorption by osteoclasts and bone formation by osteoblasts. However, little is known about the effects of mechanical stress on osteoclast differentiation. In this study, we examined the effects of short-term mechanical stress on osteoclastogenesis by applying tensile force to RAW264.7 cells stimulated with receptor activator of nuclear factor-κB ligand (RANKL) using a Flexercell tension system. We counted the number of osteoclasts that were tartrate-resistant acid phosphatase (TRAP)-positive and multinucleated (two or more nuclei) with or without application of mechanical stress for 24 h. Osteoclast number was lower after mechanical stress compared with no mechanical stress. Furthermore, mechanical stress for up to 24 h caused downregulation of osteoclast-specific gene expression and fusion-related molecule [dendritic cell specific transmembrane protein (DC-STAMP), osteoclast stimulatory transmembrane protein (OC-STAMP), E-cadherin, Integrin αV and Integrin β3] mRNA levels. Protein expression of DC-STAMP decreased with mechanical stress for 24 h compared to the control without mechanical stress, whereas the expression of E-cadherin, Integrin αV and Integrin β3 was slightly decreased. Nuclear factor of activated T cells c1 (NFATc1) mRNA levels were decreased at 6 h and increased at 12 and 24 h compared with the control. The levels of NFATc2, NFATc3 mRNA did not change compared with the control group. By contrast, mechanical stress for 24 h significantly enhanced NFAT transcriptional activity compared with the control, despite a decrease in DC-STAMP mRNA and protein levels. These results suggest that short-term mechanical stress strongly inhibits osteoclastogenesis through the downregulation of DC-STAMP and other fusion-related molecules and that short-term mechanical stress induces a negative regulatory mechanism that cancels the enhancement of NFAT transcriptional activity.

Vascularized bone tissue engineering: approaches for potential improvement

Significant advances have been made in bone tissue engineering (TE) in the past decade. However, classical bone TE strategies have been hampered mainly due to the lack of vascularization within the engineered bone constructs, resulting in poor implant survival and integration. In an effort toward clinical success of engineered constructs, new TE concepts have arisen to develop bone substitutes that potentially mimic native bone tissue structure and function. Large tissue replacements have failed in the past due to the slow penetration of the host vasculature, leading to necrosis at the central region of the engineered tissues. For this reason, multiple microscale strategies have been developed to induce and incorporate vascular networks within engineered bone constructs before implantation in order to achieve successful integration with the host tissue. Previous attempts to engineer vascularized bone tissue only focused on the effect of a single component among the three main components of TE (scaffold, cells, or signaling cues) and have only achieved limited success. However, with efforts to improve the engineered bone tissue substitutes, bone TE approaches have become more complex by combining multiple strategies simultaneously. The driving force behind combining various TE strategies is to produce bone replacements that more closely recapitulate human physiology. Here, we review and discuss the limitations of current bone TE approaches and possible strategies to improve vascularization in bone tissue substitutes.

Osteocyte regulation of bone mineral: a little give and take

Osteocytes actively participate in almost every phase of mineral handling by bone. They regulate the mineralisation of osteoid during bone formation, and they are also a major RANKL-producing cell. Osteocytes are thus able to liberate bone mineral by regulating osteoclast differentiation and activity in response to a range of stimuli, including bone matrix damage, bone disuse and mechanical unloading, oestrogen deficiency, high-dose glucocorticoid and chemotherapeutic agents. At least some of these activities may be regulated by the osteocyte-secreted product, sclerostin. There is also mounting evidence that in addition to regulating phosphate homeostasis systemically, osteocytes contribute directly to calcium homeostasis in the mature skeleton. Osteocyte cell death and the local loss of control of bone mineralisation may be the cause of focal hypermineralisation of bone and osteopetrosis, as seen in aging and pathology. The sheer number of osteocytes in bone means that "a little give and take" in terms of regulation of bone mineral content translates into a powerful whole organism effect.

Decreased and inactivated nuclear factor kappa B 1 (p50) in human degenerative calcified aortic valve

BACKGROUND

Degenerative aortic valve calcification is an important factor in aortic stenosis and incompetence, but the pathogenesis is unclear. The purpose of the present study was to observe the expression of p50 in degenerative calcified aortic valves, which may provide a potential therapeutic target.

METHODS

Fifteen cases of degenerative calcified aortic valve constituted the experimental group, and 10 aortic valves from patients who had undergone the Bentall operation constituted the control group.

RESULTS

Immunostaining demonstrated that α-smooth muscle actin was highly expressed in valvular interstitial cells in the experimental group and that the percentage of CD68-positive cells was significantly higher in degenerative calcified aortic valves. Using bone gamma-carboxyglutamate protein as a marker of calcification showed that osteoblasts were significantly increased in the experimental valves. Western blot showed that p50 was more highly expressed and activated in the control group compared with the experimental group. Immunohistochemistry confirmed this finding and showed that p50 was principally localized to the endothelial cells of uncalcified valves, suggesting that it might play an important role in maintaining valve function.

CONCLUSIONS

Inhibition of p50 activity in endothelial cells might lead to calcification in degenerative calcified aortic valves.

Sclerostin stimulates osteocyte support of osteoclast activity by a RANKL-dependent pathway

Sclerostin is a product of mature osteocytes embedded in mineralised bone and is a negative regulator of bone mass and osteoblast differentiation. While evidence suggests that sclerostin has an anti-anabolic role, the possibility also exists that sclerostin has catabolic activity. To test this we treated human primary pre-osteocyte cultures, cells we have found are exquisitely sensitive to sclerostin, or mouse osteocyte-like MLO-Y4 cells, with recombinant human sclerostin (rhSCL) and measured effects on pro-catabolic gene expression. Sclerostin dose-dependently up-regulated the expression of receptor activator of nuclear factor kappa B (RANKL) mRNA and down-regulated that of osteoprotegerin (OPG) mRNA, causing an increase in the RANKL∶OPG mRNA ratio. To examine the effects of rhSCL on resulting osteoclastic activity, MLO-Y4 cells plated onto a bone-like substrate were primed with rhSCL for 3 days and then either mouse splenocytes or human peripheral blood mononuclear cells (PBMC) were added. This resulted in cultures with elevated osteoclastic resorption (approximately 7-fold) compared to untreated co-cultures. The increased resorption was abolished by co-addition of recombinant OPG. In co-cultures of MLO-Y4 cells with PBMC, SCL also increased the number and size of the TRAP-positive multinucleated cells formed. Importantly, rhSCL had no effect on TRAP-positive cell formation from monocultures of either splenocytes or PBMC. Further, rhSCL did not induce apoptosis of MLO-Y4 cells, as determined by caspase activity assays, demonstrating that the osteoclastic response was not driven by dying osteocytes. Together, these results suggest that sclerostin may have a catabolic action through promotion of osteoclast formation and activity by osteocytes, in a RANKL-dependent manner.