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

Loss of p53 compensates osteopenia in murine Mysm1 deficiency

Histone modifications critically contribute to the epigenetic orchestration of bone homeostasis-in part, by modifying the access of transcription factors to specific genes involved in the osteogenic differentiation process of bone marrow mesenchymal stem cells (MSCs) and osteoblasts. Based on our previous finding that histone H2A deubiquitinase 2A-DUB/Mysm1 interacts with the p53 axis in hematopoiesis and tissue development, we analyzed the molecular basis of the skeletal phenotype of Mysm1-deficient mice and dissected the underlying p53-dependent and -independent mechanisms. Visible morphologic, skeletal deformations of young Mysm1-deficient mice-including a kinked and truncated tail and shortened long bones-were associated with osteopenia of long bones. On the cellular level, Mysm1-deficient primary osteoblasts displayed reduced potential to differentiate into mature osteoblasts, as indicated by decreased expression of osteogenic markers. Reduced osteogenic differentiation capacity of Mysm1-deficient osteoblasts was accompanied by an impaired induction of osteogenic transcription factor Runx2. Osteogenic differentiation of Mysm1^-/- MSCs, however, was not compromised in vitro. In line with defective hematopoietic development of Mysm1-deficient mice, Mysm1^-/- osteoclasts had reduced resorption activity and were more prone to apoptosis in TUNEL assays. Skeletal alterations and osteopenia of Mysm1-deficient mice were phenotypically completely rescued by simultaneous ablation of p53 in p53^-/-Mysm1^-/- double-deficient mice-although p53 deficiency did not restore Runx2 expression in Mysm1^-/- osteoblasts on the molecular level but, instead, enhanced proliferation and osteogenic differentiation of MSCs. In summary, our results demonstrate novel roles for Mysm1 in osteoblast differentiation and osteoclast formation, resulting in osteopenia in Mysm1-deficient mice that could be abrogated by the loss of p53 from increased osteogenic differentiation of Mysm1^-/-p53^-/- MSCs.-Haffner-Luntzer, M., Kovtun, A., Fischer, V., Prystaz, K., Hainzl, A., Kroeger, C. M., Krikki, I., Brinker, T. J., Ignatius, A., Gatzka, M. Loss of p53 compensates osteopenia in murine Mysm1 deficiency.

Spatiotemporally Controlled Release of Rho-Inhibiting C3 Toxin from a Protein-DNA Hybrid Hydrogel for Targeted Inhibition of Osteoclast Formation and Activity

In osteoporosis, bone structure can be improved by the introduction of therapeutic molecules inhibiting bone resorption by osteoclasts. Here, biocompatible hydrogels represent an excellent option for the delivery of pharmacologically active molecules to the bone tissue because of their biodegradability, injectability, and manifold functionalization capacity. The present study reports the preparation of a multifunctional hybrid hydrogel from chemically modified human serum albumin and rationally designed DNA building blocks. The hybrid hydrogel combines advantageous characteristics, including rapid gelation through DNA hybridization under physiological conditions and a self-healing and injectable nature with the possibility of specific loading and spatiotemporally controlled release of active proteins, making it an advanced biomaterial for the local treatment of bone diseases, for example, osteoporosis. The hydrogels are loaded with a recombinant Rho-inhibiting C3 toxin, C2IN-C3lim-G205C. This toxin selectively targets osteoclasts and inhibits Rho-signaling and, thereby, actin-dependent processes in these cells. Application of C2IN-C3lim-G205C toxin-loaded hydrogels effectively reduces osteoclast formation and resorption activity in vitro, as demonstrated by tartrate-resistant acid phosphatase staining and the pit resorption assay. Simultaneously, osteoblast activity, viability, and proliferation are unaffected, thus making C2IN-C3lim-G205C toxin-loaded hybrid hydrogels an attractive pharmacological system for spatial and selective modulation of osteoclast functions to reduce bone resorption.

Nanobiomechanics of living cells: a review

Nanobiomechanics of living cells is very important to understand cell-materials interactions. This would potentially help to optimize the surface design of the implanted materials and scaffold materials for tissue engineering. The nanoindentation techniques enable quantifying nanobiomechanics of living cells, with flexibility of using indenters of different geometries. However, the data interpretation for nanoindentation of living cells is often difficult. Despite abundant experimental data reported on nanobiomechanics of living cells, there is a lack of comprehensive discussion on testing with different tip geometries, and the associated mechanical models that enable extracting the mechanical properties of living cells. Therefore, this paper discusses the strategy of selecting the right type of indenter tips and the corresponding mechanical models at given test conditions.

Histone deacetylase inhibitors suppress mechanical stress-induced expression of RUNX-2 and ADAMTS-5 through the inhibition of the MAPK signaling pathway in cultured human chondrocytes

OBJECTIVE

To investigate the inhibitory effects and the regulatory mechanisms of histone deacetylase (HDAC) inhibitors on mechanical stress-induced gene expression of runt-related transcription factor (RUNX)-2 and adisintegrin and metalloproteinase with thrombospondin motif (ADAMTS)-5 in human chondrocytes.

METHODS

Human chondrocytes were seeded in stretch chambers at a concentration of 5 × 10(4)cells/chamber. Cells were pre-incubated with or without HDAC inhibitors (MS-275 or trichostatin A; TSA) for 12h, followed by uniaxial cyclic tensile strain (CTS) (0.5Hz, 10% elongation), which was applied for 30 min using the ST-140-10 system (STREX, Osaka, Japan). Total RNA was extracted and the expression of RUNX-2, ADAMTS-5, matrix metalloproteinase (MMP)-3, and MMP-13 at the mRNA and protein levels were examined by real-time polymerase chain reaction (PCR) and immunocytochemistry, respectively. The activation of diverse mitogen-activated protein kinase (MAPK) pathways with or without HDAC inhibitors during CTS was examined by western blotting.

RESULTS

HDAC inhibitors (TSA: 10 nM, MS-275: 100 nM) suppressed CTS-induced expression of RUNX-2, ADAMTS-5, and MMP-3 at both the mRNA and protein levels within 1h. CTS-induced activation of p38 MAPK (p38), extracellular signal-regulated kinase (ERK), and c-Jun N-terminal kinase (JNK) MAPKs was downregulated by both HDAC inhibitors.

CONCLUSION

The CTS-induced expression of RUNX-2 and ADAMTS-5 was suppressed by HDAC inhibitors via the inhibition of the MAPK pathway activation in human chondrocytes. The results of the current study suggested a novel therapeutic role for HDAC inhibitors against degenerative joint disease such as osteoarthritis.

Complement C3a and C5a modulate osteoclast formation and inflammatory response of osteoblasts in synergism with IL-1β

There is a tight interaction of the bone and the immune system. However, little is known about the relevance of the complement system, an important part of innate immunity and a crucial trigger for inflammation. The aim of this study was, therefore, to investigate the presence and function of complement in bone cells including osteoblasts, mesenchymal stem cells (MSC), and osteoclasts. qRT-PCR and immunostaining revealed that the central complement receptors C3aR and C5aR, complement C3 and C5, and membrane-bound regulatory proteins CD46, CD55, and CD59 were expressed in human MSC, osteoblasts, and osteoclasts. Furthermore, osteoblasts and particularly osteoclasts were able to activate complement by cleaving C5 to its active form C5a as measured by ELISA. Both C3a and C5a alone were unable to trigger the release of inflammatory cytokines interleukin (IL)-6 and IL-8 from osteoblasts. However, co-stimulation with the pro-inflammatory cytokine IL-1β significantly induced IL-6 and IL-8 expression as well as the expression of receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG) indicating that complement may modulate the inflammatory response of osteoblastic cells in a pro-inflammatory environment as well as osteoblast-osteoclast interaction. While C3a and C5a did not affect osteogenic differentiation, osteoclastogenesis was significantly induced even in the absence of RANKL and macrophage-colony stimulating factor (M-CSF) suggesting that complement could directly regulate osteoclast formation. It can therefore be proposed that complement may enhance the inflammatory response of osteoblasts and increase osteoclast formation, particularly in a pro-inflammatory environment, for example, during bone healing or in inflammatory bone disorders.

Strain-dependent up-regulation of ephrin-B2 protein in periodontal ligament fibroblasts contributes to osteogenesis during tooth movement

During orthodontic tooth movement, the application of adequate orthodontic forces allows teeth to be moved through the alveolar bone. These forces are transmitted through the periodontal ligaments (PDL) to the supporting alveolar bone and lead to deposition or resorption of bone, depending on whether the tissues are exposed to a tensile or compressive mechanical strain. Fibroblasts within the PDL (PDLF) are considered to be mechanoresponsive. The transduction mechanisms from mechanical loading of the PDLF to the initiation of bone remodeling are not clearly understood. Recently, members of the ephrin/Eph family have been shown to be involved in the regulation of bone homeostasis. For the first time, we demonstrate that PDLF exposed to tensile strain induce the expression of ephrin-B2 via a FAK-, Ras-, ERK1/2-, and SP1-dependent pathway. Osteoblasts of the alveolar bone stimulated with ephrin-B2 increased their osteoblastogenic gene expression and showed functional signs of osteoblastic differentiation. In a physiological setting, ephrin-B2-EphB4 signaling between PDLF and osteoblasts of the alveolar bone might contribute to osteogenesis at tension sites during orthodontic tooth movement.

Compression-dependent Up-regulation of Ephrin-A2 in PDL Fibroblasts Attenuates Osteogenesis

Members of the ephrin/Eph family have recently been shown to be involved in the regulation of bone homeostasis in a murine model. The activation of the EphB4 receptor on osteoblasts by its ligand ephrin-B2 led to stimulation of osteoblastogenesis and therefore to bone formation. The activation of ephrin-A2-EphA2 signaling on osteoblasts inhibited the activation of osteoblast-specific gene expression, leading to bone resorption. Fibroblasts within the periodontal ligament periodontal ligament may be one of the first responders to orthodontic forces. Periodontal ligament fibroblasts (PDLF) are mechanoresponsive. Members of the ephrin/Eph family might link mechanical forces received by PDLF with the regulation of osteoblastogenesis on osteoblasts of the alveolar bone. To study whether ephrin-A2 is modulated upon compression, we subjected human primary PDLF to static compressive forces (30.3 g/cm2). Static compressive forces significantly induced the expression of ephrin-A2, while the expression of ephrin-B2 was significantly down-regulated. Moreover, osteoblasts of the alveolar bone stimulated with ephrin-A2 in vitro significantly suppressed their osteoblastogenic gene expression (RUNX2, ALPL) and decreased signs of osteoblastic differentiation, as demonstrated by a significantly reduced ALP activity. Together, these findings establish a role for this ligand/receptor system linking mechanical forces with the regulation of osteogenesis during orthodontic tooth movement.

Non-resorbing osteoclasts induce migration and osteogenic differentiation of mesenchymal stem cells

Osteoclast activity has traditionally been regarded as restricted to bone resorption but there is some evidence that also non-resorbing osteoclasts might influence osteoblast activity. The aim of the present study was to further investigate the hypothesis of an anabolic function of non-resorbing osteoclasts by investigating their capability to recruit mesenchymal stem cells (MSC) and to provoke their differentiation toward the osteogenic lineage. Bone-marrow-derived human MSC were exposed to conditioned media (CM) derived from non-resorbing osteoclast cultures, which were generated from human peripheral blood monocytes. Osteogenic marker genes (transcription factor Runx2, bone sialoprotein, alkaline phosphatase (AP), and osteopontin) were significantly increased. Osteogenic differentiation (OD) was also proved by von Kossa and AP staining occurred in the same range as in MSC cultures stimulated with osteogenic supplements. Chemotactic responses of MSC were measured with a modified Boyden chamber assay. CM from osteoclast cultures induced a strong migratory response in MSC, which was greatly reduced in the presence of an anti-human platelet-derived growth factor (PDGF) receptor beta antibody. Correspondingly, significantly increased PDGF-BB concentrations were measured in the CM using a PDGF-BB immunoassay. CM derived from mononuclear cell cultures did not provoke MSC differentiation and had a significantly lower migratory effect on MSC suggesting that the effects were specifically mediated by osteoclasts. In conclusion, it can be suggested that human non-resorbing osteoclasts induce migration and OD of MSC. While effects on MSC migration might be mainly due to PDGF-BB, the factors inducing OD remain to be elucidated.

Mechanosensitive promoter region in the human HB-GAM gene

Mechanical loading is essential for maintaining bone mass in the adult skeleton. However, the underlying process of the transfer of the physical stimulus into a biochemical response, which is termed mechanotransduction is poorly understood. Mechanotransduction results in the modulation of gene expression through specific transcription factor binding sites in the promoter region of mechanosensitive genes. In the present study, we demonstrate that the expression of HB-GAM, which is known to have stimulating effects on osteogenic differentiation, is rapidly induced by mechanical loading in hMSC-TERT4 cells. Analysis of the human HB-GAM gene upstream regulatory region with luciferase reporter gene assays revealed that the upregulation of HB-GAM expression occurred at the transcriptional level and was mainly dependent on the HB-GAM promoter region most upstream containing three potential AP-1 binding motifs.

Periprosthetic strain magnitude-dependent upregulation of type I collagen synthesis in human osteoblasts through an ERK1/2 pathway

Human osteoblasts sense mechanical stimulation and synthesise type I collagen in periprosthetic osseointegration following total hip arthroplasty. However, the regulation of type I collagen synthesis by periprosthetic strain is unclear because the cellular-level strain magnitude remains unknown to date. Fortunately, the tissue-level strain in implanted femurs is measurable. According to the mechanism of strain amplification, the tissue-level strain was amplified 20 times to stretch human osteoblasts in this study. Elongation of 0.8-3.2% enhanced the mRNA level of type I collagen, whereas the release of procollagen type I C propeptide only increased at 2.4% and 3.2% elongation. Type I collagen expression increased with the activation of ERK1/2 phosphorylation in a strain-magnitude-dependent manner, whereas JNK and P38 were unaffected. The responses were completely inhibited by blocking the ERK1/2 pathway with U0126. The results indicate that type I collagen synthesis in human osteoblasts depends on the level of periprosthetic strain and ERK1/2 activation.

NMDA enhances stretching-induced differentiation of osteoblasts through the ERK1/2 signaling pathway

Activation of the excitatory neurotransmitter N-methyl-d-aspartate (NMDA) and stretching both increase Ca(2+) influx in osteoblastic cells. We postulated that NMDA would enhance the osteoblastic cell's response to stretching. The goal of this study was to investigate, in the presence of the neurotransmitter NMDA, the effect of mechanical loading on osteoblast's stage of differentiation and the mitogen-activated protein kinase (MAPK) signaling pathway associated with it. Rat primary osteoblastic cells were subjected to cyclic, equibiaxial stretch for 48 h in the presence or absence of NMDA. Pretreatment with 0.5 mM NMDA significantly enhanced the stretching magnitude-dependent increase in osteogenesis markers. MK801, an antagonist of NMDA receptors, abolished those responses. To further study the mechanism of this response, osteoblastic cells were stretched for 5, 15, or 60 min in the absence of NMDA. Cyclic stretch induced a rapid increase in extracellular signal-regulated kinase ERK1/2 phosphorylation with the peak at 15 min, but no changes were noted in p38 and JNK pathway signaling. NMDA could enhance ERK1/2 phosphorylation stimulated by stretching. U0126, an inhibitor of ERK1/2, blocked the increase in osteogenesis markers. In conclusion, the current study demonstrates that there is a synergistic effect between mechanical stimulation and NMDA in osteoblasts. ERK1/2 signaling may be the common pathway in the increased response to stretching in the presence of NMDA in osteoblastic cells.

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.

[Metabolic bone diseases]

Osteomalacia is caused by impaired vitamin D receptor (VDR) signaling, calcium deficiency, and altered bone mineralization. This can be due to insufficient sunlight exposure, malabsorption, reduced D hormone activation in chronic kidney disease, and rare alterations of VDR signaling and phosphate metabolism. Leading symptoms are bone pain, muscular cramps, and increased incidence of falls in the elderly. The adequate respective countermeasures are to optimize the daily intake of calcium and vitamin D3 and to replace active D hormone and phosphate if deficient. Osteoporosis is characterized by bone fragility fractures upon minor physical impact. Indications for diagnosis and treatment can be established by estimating the absolute fracture risk, taking into account bone mineral density, age, gender, and individual risk factors. Exercise, intervention programs to avoid falls, and specific drugs are capable of substantially reducing fracture risk even in the elderly. Secondary osteoporosis primarily requires both bone-altering medications and effective treatment of underlying diseases.

Mechanical stress-mediated Runx2 activation is dependent on Ras/ERK1/2 MAPK signaling in osteoblasts

The sequence of biochemical events involved in mechanical stress-induced signaling in osteoblastic cells remains unclear. Runx2, a transcription factor involved in the control of osteoblast differentiation, has been identified as a target of mechanical stress-induced signaling in osteoblastic cells. In this study, uniaxial sinusoidal stretching (15% strain, 115% peak-to-peak, at 1/12 Hz) stimulated the differentiation of osteoblast-like MC3T3-E1 cells and rat primary osteoblastic cells by activating Runx2. We examined the involvement of diverse mitogen-activated protein kinase (MAPK) pathways in the activation of Runx2 during mechanical stress. Mechanical stress increased alkaline phosphatase activity, a marker of osteoblast differentiation, increased the expression of the osteoblast-specific extracellular matrix (ECM) protein osteocalcin, and induced Runx2 activation, along with increased osterix expression. Furthermore, activation of ERK1/2 and p38 MAPKs increased significantly. U0126, a selective inhibitor of ERK1/2, completely blocked Runx2 activation during periods of mechanical stress, but the p38 MAPK-selective inhibitor SB203580 did not alter nuclear phosphorylation of Runx2. Small interfering RNA (siRNA) targeting Rous sarcoma kinase (RAS), an upstream regulator of both ERK1/2 and p38 MAPKs, inhibited stretch-induced ERK1/2 activation, but not mechanically induced p38 MAPK activity. Furthermore, mechanically induced Runx2 activation was inhibited by Ras depletion, using siRNA. These findings indicate that mechanical stress regulates Runx2 activation and favors osteoblast differentiation through the activation of MAPK signal transduction pathways and Ras/Raf-dependent ERK1/2 activation, independent of p38 MAPK signaling.