Involvement of BMPs/Smad signaling pathway in mechanical response in osteoblasts

The mechanical regulatory role of ATP13a3 in osteogenic differentiation of pre-osteoblasts

PURPOSE

The process of osteogenic differentiation hinges upon the pivotal role of mechanical signals. Previous studies found that mechanical tensile strain of 2,500 microstrain (με) at a frequency of 0.5 Hz promoted osteogenesis in vitro. However, the mechanism of the mechanical strain influencing osteogenesis at the cellular and molecular levels are not yet fully understood. This study aimed to explore the mechanism of mechanical strain on osteogenic differentiation of MC3T3-E1 cells.

MATERIALS AND METHODS

Proteomics analysis was conducted to explore the mechanical strain that significantly impacted the protein expression. Bioinformatics identified important mechanosensitive proteins and the expression of genes was investigated using real-time PCR. The dual-luciferase assay revealed the relationship between the miRNA and its target gene. Overexpression and downexpression of the gene, to explore its role in mechanically induced osteogenic differentiation and transcriptomics, revealed further mechanisms in this process.

RESULTS

Proteomics and bioinformatics identified an important mechanosensitive lowexpression protein ATP13A3, and the expression of Atp13a3 gene was also reduced. The dual-luciferase assay revealed that microRNA-3070-3p (miR-3070-3p) targeted the Atp13a3 gene. Furthermore, the downexpression of Atp13a3 promoted the expression levels of osteogenic differentiation-related genes and proteins, and this process was probably mediated by the tumor necrosis factor (TNF) signaling pathway.

CONCLUSION

Atp13a3 responded to mechanical tensile strain to regulate osteogenic differentiation, and the TNF signaling pathway regulated by Atp13a3 was probably involved in this process. These novel insights suggested that Atp13a3 was probably a potential osteogenesis and bone formation regulator.

miR-20a: a key regulator of orthodontic tooth movement via BMP2 signaling pathway modulation

AIM

In this study, we aimed to establish a rat tooth movement model to assess miR-20's ability in enhancing the BMP2 signaling pathway and facilitate alveolar bone remodeling.

METHOD

60 male SD rats had nickel titanium spring devices placed between their left upper first molars and incisors, with the right side serving as the control. Forces were applied at varying durations (18h, 24h, 30h, 36h, 42h, 1d, 3d, 5d, 7d, 14d), and their bilateral maxillary molars and surrounding alveolar bones were retrieved for analysis. Fluorescent quantitative PCR was conducted to assess miR-20a, BMP2, Runx2, Bambi and Smad6 gene expression in alveolar bone, and western blot was performed to determine the protein levels of BMP2, Runx2, Bambi, and Smad6 after mechanical loading.

RESULT

We successfully established an orthodontic tooth movement model in SD rats and revealed upregulated miR-20a expression and significantly increased BMP2 and Runx2 gene expression and protein synthesis in alveolar bone during molar tooth movement. Although Bambi and Smad6 gene expression did not significantly increase, their protein synthesis was found to decrease significantly.

CONCLUSION

MiR-20a was found to be involved in rat tooth movement model alveolar bone remodeling, wherein it promoted remodeling by reducing Bambi and Smad6 protein synthesis through the BMP2 signaling pathway.

Mechanically strained osteocyte-derived exosomes contained miR-3110-5p and miR-3058-3p and promoted osteoblastic differentiation

BACKGROUND

Osteocytes are critical mechanosensory cells in bone, and mechanically stimulated osteocytes produce exosomes that can induce osteogenesis. MicroRNAs (miRNAs) are important constituents of exosomes, and some miRNAs in osteocytes regulate osteogenic differentiation; previous studies have indicated that some differentially expressed miRNAs in mechanically strained osteocytes likely influence osteoblastic differentiation. Therefore, screening and selection of miRNAs that regulate osteogenic differentiation in exosomes of mechanically stimulated osteocytes are important.

RESULTS

A mechanical tensile strain of 2500 με at 0.5 Hz 1 h per day for 3 days, elevated prostaglandin E2 (PGE2) and insulin-like growth factor-1 (IGF-1) levels and nitric oxide synthase (NOS) activity of MLO-Y4 osteocytes, and promoted osteogenic differentiation of MC3T3-E1 osteoblasts. Fourteen miRNAs differentially expressed only in MLO-Y4 osteocytes which were stimulated with mechanical tensile strain, were screened, and the miRNAs related to osteogenesis were identified. Four differentially expressed miRNAs (miR-1930-3p, miR-3110-5p, miR-3090-3p, and miR-3058-3p) were found only in mechanically strained osteocytes, and the four miRNAs, eight targeted mRNAs which were differentially expressed only in mechanically strained osteoblasts, were also identified. In addition, the mechanically strained osteocyte-derived exosomes promoted the osteoblastic differentiation of MC3T3-E1 cells in vitro, the exosomes were internalized by osteoblasts, and the up-regulated miR-3110-5p and miR-3058-3p in mechanically strained osteocytes, were both increased in the exosomes, which was verified via reverse transcription quantitative polymerase chain reaction (RT-qPCR).

CONCLUSIONS

In osteocytes, a mechanical tensile strain of 2500 με at 0.5 Hz induced the fourteen differentially expressed miRNAs which probably were in exosomes of osteocytes and involved in osteogenesis. The mechanically strained osteocyte-derived exosomes which contained increased miR-3110-5p and miR-3058-3p (two of the 14 miRNAs), promoted osteoblastic differentiation.

Biomechanical Effects of Mechanical Stress on Cells Involved in Fracture Healing

Fracture healing is a complex staged repair process in which the mechanical environment plays a key role. Bone tissue is very sensitive to mechanical stress stimuli, and the literature suggests that appropriate stress can promote fracture healing by altering cellular function. However, fracture healing is a coupled process involving multiple cell types that balance and limit each other to ensure proper fracture healing. The main cells that function during different stages of fracture healing are different, and the types and molecular mechanisms of stress required are also different. Most previous studies have used a single mechanical stimulus on individual mechanosensitive cells, and there is no relatively uniform standard for the size and frequency of the mechanical stress. Analyzing the mechanisms underlying the effects of mechanical stimulation on the metabolic regulation of signaling pathways in cells such as in bone marrow mesenchymal stem cells (BMSCs), osteoblasts, chondrocytes, and osteoclasts is currently a challenging research hotspot. Grasping how stress affects the function of different cells at the molecular biology level can contribute to the refined management of fracture healing. Therefore, in this review, we summarize the relevant literature and describe the effects of mechanical stress on cells associated with fracture healing, and their possible signaling pathways, for the treatment of fractures and the further development of regenerative medicine.

Effect of Mechanical Loading on Bone Regeneration in HA/β-TCP/SF Scaffolds Prepared by Low-Temperature 3D Printing In Vivo

It has been well demonstrated that a dynamic culture environment improves tissue-engineered bone formation in vitro, but little is known about how cyclical mechanical loading induced bone formation in scaffolds in situ. To mimic the organic and inorganic components and multilevel structure of a bony microenvironment, hydroxyapatite/β tricalcium phosphate/silk fibroin(HA/β-TCP/SF) composite scaffolds with macro- and micropores were fabricated in this study. The mechanical properties and structure of the scaffolds were adjusted based on the ratio of organic and inorganic components and three-dimensional (3D) printing parameters. Dynamic sinusoidal loading with different frequencies was applied to the composite scaffold. Mouse bone precursor cells MC3T3-E1 were seeded on the scaffolds, and the cell compatibility of the scaffolds was investigated by MTT, SEM, and HE. The effect of the loading on bone formation in the scaffold in situ was investigated in a rabbit tibia defect model. The scaffold showed viscoelasticity and hysteresis under dynamic sinusoidal loading with different frequencies. With an increase in HA/β-TCP, the stress and modulus of the scaffolds increased. MTT, SEM, and HE results showed that MC3T3-E1 cells could adhere and proliferate on the composite scaffolds. After loading in vivo, the quantity of newly formed bone and the bone volume fraction increased. Micro-CT, undecalcified Van Gieson (VG) staining, and fluorescent double-labeling results suggested that appropriate cyclical mechanical loading at frequencies of 1 and 10 Hz had positive effects on bone formation in situ and it may play a role in clinical bone defect repair.

The effects of strain history on aortic valve interstitial cell activation in a 3D hydrogel environment

Aortic valves (AVs) undergo unique stretch histories that include high rates and magnitudes. While major differences in deformation patterns have been observed between normal and congenitally defective bicuspid aortic valves (BAVs), the relation to underlying mechanisms of rapid disease onset in BAV patients remains unknown. To evaluate how the variations in stretch history affect AV interstitial cell (AVIC) activation, high-throughput methods were developed to impart varied cyclical biaxial stretch histories into 3D poly(ethylene) glycol hydrogels seeded with AVICs for 48 h. Specifically, a physiologically mimicking stretch history was compared to two stretch histories with varied peak stretch and stretch rate. Post-conditioned AVICs were imaged for nuclear shape, alpha smooth muscle actin (αSMA) and vimentin (VMN) polymerization, and small mothers against decapentaplegic homologs 2 and 3 (SMAD 2/3) nuclear activity. The results indicated that bulk gel deformations were accurately transduced to the AVICs. Lower peak stretches lead to increased αSMA polymerization. In contrast, VMN polymerization was a function of stretch rate, with SMAD 2/3 nuclear localization and nuclear shape also trending toward stretch rate dependency. Lower than physiological levels of stretch rate led to higher SMAD 2/3 activity, higher VMN polymerization around the nucleus, and lower nuclear elongation. αSMA polymerization did not correlate with VMN polymerization, SMAD 2/3 activity, nor nuclear shape. These results suggest that a negative feedback loop may form between SMAD 2/3, VMN, and nuclear shape to maintain AVIC homeostatic nuclear deformations, which is dependent on stretch rate. These novel results suggest that AVIC mechanobiological responses are sensitive to stretch history and provide insight into the mechanisms of AV disease.

miR-1306 induces cell apoptosis by targeting BMPR1B gene in the ovine granulosa cells

Bone morphogenetic protein receptor type-1B (BMPR1B) is one of the major gene for sheep prolificacy. However, few studies investigated its regulatory region. In this study, we reported that miR-1306 is a direct inhibitor of BMPR1B gene in the ovine granulosa cells (ovine GCs). We detected a miRNA response element of miR-1306 in the 3’ untranslated region of the ovine BMPR1B gene. Luciferase assay showed that the ovine BMPR1B gene is a direct target of miR-1306. qPCR and western blotting revealed that miR-1306 reduces the expression of BMPR1B mRNA and protein in the ovine granulosa cells. Furthermore, miR-1306 promoted cell apoptosis by suppressing BMPR1B expression in the ovine granulosa cells. Overall, our results suggest that miR-1306 is an epigenetic regulator of BMPR1B, and may serve as a potential target to improve the fecundity of sheep.

MiR-20a: a mechanosensitive microRNA that regulates fluid shear stress-mediated osteogenic differentiation via the BMP2 signaling pathway by targeting BAMBI and SMAD6

Background

MicroRNAs (miRNAs) are crucial regulators of diverse biological and pathological processes. This study aimed to investigate the role of microRNA 20a (miR-20a) in fluid shear stress (FSS)-mediated osteogenic differentiation.

Methods

In the present study, we subjected osteoblast MC3T3-E1 cells or mouse bone marrow stromal cells (BMSCs) to single bout short duration FSS (12 dyn/cm² for 1 hour) using a parallel plate flow system. The expression of miR-20a was quantified by miRNA array profiling and real-time quantitative polymerase chain reaction (qRT-PCR) during FSS-mediated osteogenic differentiation. The expression of osteogenic differentiation markers such as Runt-related transcription factor 2 (RUNX2), alkaline phosphatase (ALP), and SP7 transcription factor (SP7) was detected. Bioinformatics analysis and a luciferase assay were performed to confirm the potential targets of miR-20a.

Results

Osteoblast-expressed miR-20a is sensitive to the mechanical environments of FSS, which are differentially up-regulated during steady FSS-mediated osteogenic differentiation. MiR-20a enhances FSS-induced osteoblast differentiation by activating the bone morphogenetic protein 2 (BMP2) signaling pathway. Both BMP and activin membrane-bound inhibitor (BAMBI) and mothers against decapentaplegic family member 6 (SMAD6) are targets of miR-20a that negatively regulate the BMP2 signaling pathway.

Conclusions

MiR-20a is a novel mechanosensitive miRNA that can enhance osteoblast differentiation in FSS mechanical environments, implying that this miRNA might be a target for bone tissue engineering and orthodontic bone remodeling for regenerative medicine applications.

miR-129-5p restores cardiac function in rats with chronic heart failure by targeting the E3 ubiquitin ligase Smurf1 and promoting PTEN expression

Chronic heart failure (CHF) is a prevalent health concern with complex pathogenesis. This current study set out to estimate the function of the miR-129-5p/Smurf1/PTEN axis on cardiac function injury in CHF. The model of CHF in rats was established. The cardiac function indexes, myocardial tissue damage, and oxidative stress-related factors in CHF rats were evaluated after the interference of Smurf1/miR-129-5p/PTEN. The targeting relationships between miR-129-5p and Smurf1 and between PTEN and Smurf1 were verified. It was found that that after modeling, cardiac functions were impaired, heart/left ventricular/lung weight and the myocardial structure was destroyed, and the degree of fibrosis of myocardial tissue was increased. After Smurf1 knockdown, the cardiac function, myocardial structure, and oxidative stress were improved, and the fibrosis in myocardial tissue was decreased. Smurf1 was a target of miR-129-5p. miR-129-5p could annul the protective effect of Smurf1 silencing on CHF rats. Smurf1 inhibited PTEN expression by promoting PTEN ubiquitination, while miR-129-5p enhanced PTEN expression by inhibiting Smurf1. Meanwhile, overexpression of PTEN annulled the cardiac dysfunction in CHF rats induced by Smurf1. In conclusion, miR-129-5p targeted Smurf1 and repressed the ubiquitination of PTEN, and promoted PTEN expression, thus improving the cardiac function of CHF rats.

Soft surfaces promote astrocytic differentiation of mouse embryonic neural stem cells via dephosphorylation of MRLC in the absence of serum

Astrocytes, which can be obtained from neural stem cells (NSCs) by adding serum and/or recombinant proteins in culture media or by passaging NSCs repeatedly, are expected to be applicable in regenerative medicine for the treatment of neurodegenerative diseases. However, astrocytes obtained using existing methods are costly and have poor quality. The stiffness of culture surfaces has been reported to affect astrocytic differentiation of adult NSCs. However, the influence of surface stiffness on astrocytic differentiation of embryonic NSCs has not yet been reported. In this study, we showed that astrocytic differentiation of embryonic NSCs was increased on soft surfaces (1 kPa and 12 kPa) compared with the NSCs on stiff surfaces (2.8 GPa) in serum-free condition. Furthermore, di-phosphorylated myosin regulatory light chain (PP-MRLC) was decreased in embryonic NSCs cultured on the soft surfaces than the cells on the stiff surfaces. Additionally, astrocytic differentiation of embryonic NSCs was induced by a Ras homolog associated kinase (ROCK) inhibitor, which decreased PP-MRLC in NSCs. These results suggest that decreasing the PP-MRLC of embryonic NSCs on soft surfaces or treating NSCs with a ROCK inhibitor is a good method to prepare astrocytes for application in regenerative medicine.

Axial mechanical loading to ex vivo mouse long bone regulates endochondral ossification and endosteal mineralization through activation of the BMP-Smad pathway during postnatal growth

Mechanical loading contributes to bone development, growth, and metabolism. However, the mechanisms underlying long bone mineralization via changes in loading during the growth period are unclear. The aim of the present study was to investigate the regulatory mechanisms underlying endochondral ossification and endosteal mineralization by developing an ex vivo organ culture model with cyclic axial mechanical loads. The metacarpal bones of 3-week-old C57BL/6 mice were exposed to mechanical loading (0, 7.8, and 78 mN) for 1 h/day for 4 days. Histomorphometry revealed that axial mechanical loading regulated the thickness of the calcified zone in the growth plate and endosteal mineralization in the diaphysis in a load-dependent manner. Mechanical loading also resulted in load-dependent upregulation of endochondral ossification and bone mineralization-related genes, including bone morphogenetic protein 2 (Bmp2). Recombinant human BMP-2 administration caused similar changes in tissue structures. Conversely, inhibition of the BMP-Smad pathway diminished the stimulatory effects of mechanical loading and BMP-2 administration, suggesting that the effects of mechanical loading may be exerted through activation of the BMP-Smad pathway with the results of gene ontology and pathway analyses. Mechanical loading increased alkaline phosphatase activity and decreased carbonic anhydrase IX (Car9) mRNA expression, resulting in a significant pH increase in the culture supernatant. We hypothesize that, through activation of the BMP-Smad pathway, mechanical loading downregulates Car9, which may alkalize the local milieu, thereby inducing bone formation and long bone mineralization. Our results showed that cyclic axial mechanical loading increased endochondral ossification and endosteal mineralization in developing mouse long bones, which may have resulted from changes in the pH, ALP activity, and Pi/PPi of the extracellular environment. These findings advance our understanding of the regulation of mineralization mechanisms by mechanical loading mediated through activation of the BMP-Smad pathway.

Targeting Mechanotransduction in Osteosarcoma: A Comparative Oncology Perspective

Mechanotransduction is the process in which cells can convert extracellular mechanical stimuli into biochemical changes within a cell. While this a normal process for physiological development and function in many organ systems, tumour cells can exploit this process to promote tumour progression. Here we summarise the current state of knowledge of mechanotransduction in osteosarcoma (OSA), the most common primary bone tumour, referencing both human and canine models and other similar mesenchymal malignancies (e.g., Ewing sarcoma). Specifically, we discuss the mechanical properties of OSA cells, the pathways that these cells utilise to respond to external mechanical cues, and mechanotransduction-targeting strategies tested in OSA so far. We point out gaps in the literature and propose avenues to address them. Understanding how the physical microenvironment influences cell signalling and behaviour will lead to the improved design of strategies to target the mechanical vulnerabilities of OSA cells.

The BMP signaling pathway enhances the osteoblastic differentiation of bone marrow mesenchymal stem cells in rats with osteoporosis

Background

This study was conducted with the aim of exploring the effect of the BMP signaling pathway on osteoblastic differentiation in rat bone marrow mesenchymal stem cells (rBMSCs) in rats with osteoporosis (OP).

Methods

The bilateral ovaries of female SD rats were resected for the establishment of a rat OP model. The osteoblastic differentiation of isolated rBMSCs was identified through osteogenic induction. Adipogenetic induction and flow cytometry (FCM) were used to detect adipogenic differentiation and the expression of rBMSC surface markers. The rBMSCs were grouped into the blank group, NC group, si-BMP2 group, and oe-BMP2 group. The expression levels of key factors and osteogenesis-related factors were determined by Western blot and quantitative real-time polymerase chain reaction (qRT-PCR). The formation of calcified nodules was observed by alizarin red staining. ALP activity was measured by alkaline phosphatase staining.

Results

The rats with OP had greater weight but decreased bone mineral density (BMD) than normal rats (all P < 0.01). The rBMSCs from rats with OP were capable of osteoblastic differentiation and adipogenic differentiation and showed high expression of CD44 (91.3 ± 2.9%) and CD105 (94.8 ± 2.1%). Compared with the blank group, the oe-BMP2 group had elevated BMP-2 and Smad1 levels and an increase in calcified nodules and ALP-positive staining areas (all P < 0.05). Moreover, the expression levels of Runx2, OC, and OPN in the oe-BMP2 group were relatively higher than those in the blank group (all P < 0.05). The findings in the si-BMP2 group were opposite to those in the oe-BMP2 group.

Conclusion

BMP signaling pathways activated by BMP-2 can promote the osteoblastic differentiation of rBMSCs from rats with OP.

Bench-to-bedside strategies for osteoporotic fracture: From osteoimmunology to mechanosensation

Osteoporosis is characterized by a decrease in bone mass and strength, rendering people prone to osteoporotic fractures caused by low-energy forces. The primary treatment strategy for osteoporotic fractures is surgery; however, the compromised and comminuted bones in osteoporotic fracture sites are not conducive to optimum reduction and rigid fixation. In addition, these patients always exhibit accompanying aging-related disorders, including high inflammatory status, decreased mechanical loading and abnormal skeletal metabolism, which are disadvantages for fracture healing around sites that have undergone orthopedic procedures. Since the incidence of osteoporosis is expected to increase worldwide, orthopedic surgeons should pay more attention to comprehensive strategies for improving the poor prognosis of osteoporotic fractures. Herein, we highlight the molecular basis of osteoimmunology and bone mechanosensation in different healing phases of elderly osteoporotic fractures, guiding perioperative management to alleviate the unfavorable effects of insufficient mechanical loading, high inflammatory levels and pathogen infection. The well-informed pharmacologic and surgical intervention, including treatment with anti-inflammatory drugs and sufficient application of antibiotics, as well as bench-to-bedside strategies for bone augmentation and hardware selection, should be made according to a comprehensive understanding of bone biomechanical properties in addition to the remodeling status of osteoporotic bones, which is necessary for creating proper biological and mechanical environments for bone union and remodeling. Multidisciplinary collaboration will facilitate the improvement of overall osteoporotic care and reduction of secondary fracture incidence.

Load-induced osteogenic differentiation of mesenchymal stromal cells is caused by mechano-regulated autocrine signaling

Mechanical boundary conditions critically influence the bone healing process. In this context, previous in vitro studies have demonstrated that cyclic mechanical compression alters migration and triggers osteogenesis of mesenchymal stromal cells (MSC), both processes being relevant to healing. However, it remains unclear whether this mechanosensitivity is a direct consequence of cyclic compression, an indirect effect of altered supply or a specific modulation of autocrine bone morphogenetic protein (BMP) signaling. Here, we investigate the influence of cyclic mechanical compression (ε = 5% and 10%, f = 1 Hz) on human bone marrow MSC (hBMSC) migration and osteogenic differentiation in a 3D biomaterial scaffold, an in vitro system mimicking the mechanical environment of the early bone healing phase. The open-porous architecture of the scaffold ensured sufficient supply even without cyclic compression, minimizing load-associated supply alterations. Furthermore, a large culture medium volume in relation to the cell number diminished autocrine signaling. Migration of hBMSCs was significantly downregulated under cyclic compression. Surprisingly, a decrease in migration was not associated with increased osteogenic differentiation of hBMSCs, as the expression of RUNX2 and osteocalcin decreased. In contrast, BMP2 expression was significantly upregulated. Enabling autocrine stimulation by increasing the cell-to-medium ratio in the bioreactor finally resulted in a significant upregulation of RUNX2 in response to cyclic compression, which could be reversed by rhNoggin treatment. The results indicate that osteogenesis is promoted by cyclic compression when cells condition their environment with BMP. Our findings highlight the importance of mutual interactions between mechanical forces and BMP signaling in controlling osteogenic differentiation.

MiR-26b-3p regulates osteoblast differentiation via targeting estrogen receptor α

BACKGROUND

Understanding of the molecular mechanisms of miRNAs involved in osteoblast differentiation is important for the treatment of bone-related diseases.

METHODS

MC3T3-E1 cells were induced to osteogenic differentiation by culturing with bone morphogenetic protein 2 (BMP2). After transfected with miR-26b-3p mimics or inhibitors, the osteogenic differentiation of MC3T3-E1 cells was detected by ALP and ARS staining. Cell viability was analyzed by MTT. The expressions of miR-26b-3p and osteogenic related markers and signaling were examined by qPCR and western blot. Direct binding of miR-26b-3p and ER-α were determined by dual luciferase assay.

RESULTS

miR-26b-3p was significantly down-regulated during osteoblast differentiation. Overexpression of miR-26b-3p inhibited osteoblast differentiation, while inhibition of miR-26b-3p enhanced osteoblast differentiation. Further studies demonstrated miR-26b-3p inhibited the expression of estrogen receptor α (ER-α) by directly targeting to the CDS region of ER-α mRNA. Overexpression of ER-α rescued the suppression effects of miR-26b-3p on osteoblast differentiation, while knockdown of ER-α reversed the upregulation of osteoblast differentiation induced by knockdown of miR-26b-3p.

CONCLUSION

Our study demonstrates that miR-26b-3p suppresses osteoblast differentiation of MC3T3-E1 cells via directly targeting ER-α.

Effects of constrained dynamic loading, CKIP‑1 gene knockout and combination stimulations on bone loss caused by mechanical unloading

Mechanical stimulation plays an important role in maintaining the growth and normal function of the skeletal system. Mechanical unloading occurs, for example, in astronauts spending long periods of time in space or in patients on prolonged bed rest, and causes a rapid loss of bone mass. Casein kinase 2‑interacting protein‑1 (CKIP‑1) is a novel negative bone regulation factor that has been demonstrated to reduce bone loss and enhance bone formation. The aim of this study was to investigate the effect of constrained dynamic loading (Loading) in combination with CKIP‑1 gene knockout (KO) on unloading‑induced bone loss in tail‑suspension mice. The blood serum metabolism index [alkaline phosphatase (ALP) activity and osteocalcin (OCN) levels], tibia mechanical behavior (including bone trabecular microstructure parameters and tibia biomechanical properties), osteoblast‑related gene expression [ALP, OCN, collagen I and bone morphogenetic protein‑2 and osteoprotegerin (OPG)] and osteoclast‑related gene expression [receptor activators of NF‑kB ligand (RANKL)] were measured. The results demonstrated that mice experienced a loss of bone mass after four weeks of tail suspension compared with a wild type group. The mechanical properties, microarchitecture and mRNA expression were significantly increased in mice after Loading + KO treatment (P<0.05). Furthermore, compared with loading or KO alone, the ratio of OPG/RANKL was increased in the combined treatment group. The combined effect of Loading + KO was greater than that observed with loading or KO alone (P<0.05). The present study demonstrates that Loading + KO can counter unloading‑induced bone loss, and combining the two treatments has an additive effect. These results indicate that combined therapy could be a novel strategy for the clinical treatment of disuse osteoporosis associated with space travel or bed rest.

Precise spatial restriction of BMP signaling in developing joints is perturbed upon loss of embryo movement

Dynamic mechanical loading of synovial joints is necessary for normal joint development, as evidenced in certain clinical conditions, congenital disorders and animal models where dynamic muscle contractions are reduced or absent. Although the importance of mechanical forces on joint development is unequivocal, little is known about the molecular mechanisms involved. Here, using chick and mouse embryos, we observed that molecular changes in expression of multiple genes analyzed in the absence of mechanical stimulation are consistent across species. Our results suggest that abnormal joint development in immobilized embryos involves inappropriate regulation of Wnt and BMP signaling during definition of the emerging joint territories, i.e. reduced β-catenin activation and concomitant upregulation of pSMAD1/5/8 signaling. Moreover, dynamic mechanical loading of the developing knee joint activates Smurf1 expression; our data suggest that Smurf1 insulates the joint region from pSMAD1/5/8 signaling and is essential for maintenance of joint progenitor cell fate.

Role of Protein Phosphatase 1 in Angiogenesis and Odontoblastic Differentiation of Human Dental Pulp Cells

INTRODUCTION

The aims of this study were to examine the immunolocalization of protein phosphatase 1 (PP1) in developing mouse pulp tissue and to explore the role of PP1 in odontoblastic differentiation and in vitro angiogenesis in human dental pulp cells (HDPCs).

METHODS

Immunolocalization of PP1 was assessed in developing mouse pulp tissue. Odontogenic differentiation was examined by alkaline phosphatase activity, alizarin red staining, and reverse transcriptase polymerase chain reaction. Angiogenesis was evaluated by endothelial cell migration and capillary tube formation. Signaling pathways were analyzed by Western blotting and confocal immunofluorescence.

RESULTS

PP1 expression was detected in preodontoblasts, odontoblasts, dental pulp cells, and endothelial cells within pulp tissue during the crown formed, root formation, and root completion stages. PP1 messenger RNA (mRNA) and protein levels were up-regulated at the late mineralization stage during odontogenic differentiation of HDPCs. The PP1 activator C2 ceramide increased alkaline phosphatase activity, mineralized nodule formation, and mRNA expression of dentin matrix protein 1 and dentin sialophosphoprotein. In contrast, knockdown by PP1 small interfering RNA inhibited odontoblastic differentiation. Moreover, PP1 activator up-regulated mRNA expression of angiogenic genes in HDPCs and increased the migration and capillary tube formation of endothelial cells, whereas PP1 small interfering RNA showed opposite effects. C2 ceramide increased levels of bone morphogenetic protein 2, phosphorylation of Smad 1/5/8, and mRNA expression of runt-related transcription factor 2 and osterix.

CONCLUSIONS

This study provides the first evidence that PP1 might be a potent regulator of developing pulp tissue in vivo and odontoblastic differentiation and angiogenesis in HDPCs in vitro and may have clinical implications for pulp/dentin regeneration or reparative dentinogenesis.

BMP4 promotes mouse iPS cell differentiation to male germ cells via Smad1/5, Gata4, Id1 and Id2

Generation of male germ cells from pluripotent cells could provide male gametes for treating male infertility and offer an ideal model for unveiling molecular mechanisms of spermatogenesis. However, the influence and exact molecular mechanisms, especially downstream effectors of BMP4 signaling pathways, in male germ cell differentiation of the induce pluripotent stem (iPS) cells, remain unknown. This study was designed to explore the role and mechanism of BMP4 signaling in the differentiation of mouse iPS cells to male germ cells. Embryoid body (EB) formation and recombinant BMP4 or Noggin were utilized to evaluate the effect of BMP4 on male germ cell generation from mouse iPS cells. Germ cell-specific genes and proteins as well as the downstream effectors of BMP4 signaling pathway were assessed using real-time PCR and Western blots. We found that BMP4 ligand and its multiple receptors, including BMPR1a, BMPR1b and BMPR2, were expressed in mouse iPS cells. Real-time PCR and Western blots revealed that BMP4 could upregulate the levels of genes and proteins for germ cell markers in iPS cells-derived EBs, whereas Noggin decreased their expression in these cells. Moreover, Smad1/5 phosphorylation, Gata4 transcription and the transcripts of Id1 and Id2 were enhanced by BMP4 but decreased when exposed to Noggin. Collectively, these results suggest that BMP4 promotes the generation of male germ cells from iPS cells via Smad1/5 pathway and the activation of Gata4, Id1 and Id2 This study thus offers novel insights into molecular mechanisms underlying male germ cell development.

Osteoblast Differentiation at a Glance

Ossification is a tightly regulated process, performed by specialized cells called osteoblasts. Dysregulation of this process may cause inadequate or excessive mineralization of bones or ectopic calcification, all of which have grave consequences for human health. Understanding osteoblast biology may help to treat diseases such as osteogenesis imperfecta, calcific heart valve disease, osteoporosis, and many others. Osteoblasts are bone-building cells of mesenchymal origin; they differentiate from mesenchymal progenitors, either directly or via an osteochondroprogenitor. The direct pathway is typical for intramembranous ossification of the skull and clavicles, while the latter is a hallmark of endochondral ossification of the axial skeleton and limbs. The pathways merge at the level of preosteoblasts, which progress through 3 stages: proliferation, matrix maturation, and mineralization. Osteoblasts can also differentiate into osteocytes, which are stellate cells populating narrow interconnecting passages within the bone matrix.

The key molecular switch in the commitment of mesenchymal progenitors to osteoblast lineage is the transcription factor cbfa/runx2, which has multiple upstream regulators and a wide variety of targets. Upstream is the Wnt/Notch system, Sox9, Msx2, and hedgehog signaling. Cofactors of Runx2 include Osx, Atf4, and others. A few paracrine and endocrine factors serve as coactivators, in particular, bone morphogenetic proteins and parathyroid hormone. The process is further fine-tuned by vitamin D and histone deacetylases. Osteoblast differentiation is subject to regulation by physical stimuli to ensure the formation of bone adequate for structural and dynamic support of the body. Here, we provide a brief description of the various stimuli that influence osteogenesis: shear stress, compression, stretch, micro- and macrogravity, and ultrasound. A complex understanding of factors necessary for osteoblast differentiation paves a way to introduction of artificial bone matrices.

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.

Mechanical strain affects some microRNA profiles in pre-oeteoblasts

MicroRNAs (miRNAs) are important regulators of cell proliferation, differentiation and function. Mechanical strain is an essential factor for osteoblast proliferation and differentiation. A previous study revealed that a physiological mechanical tensile strain of 2500 microstrain (με) at 0.5 Hz applied once a day for 1 h over 3 consecutive days promoted osteoblast differentiation. However, the mechanoresponsive miRNAs of these osteoblasts were not identified. In this study, we applied the same mechanical tensile strain to in vitro cultivated mouse MC3T3-E1 pre-osteoblasts and identified the mechanoresponsive miRNAs. Using miRNA microarray and qRT-PCR assays, the expression patterns of miRNAs were evaluated and 5 of them were found to be significantly different between the mechanical loading group and the control group: miR-3077-5p, 3090-5p and 3103-5p were significantly upregulated and miR-466i-3p and 466h-3p were downregulated. Bioinformatics analysis revealed possible target genes for these differentially expressed miRNAs. Some target genes correlated with osteoblast differentiation. These findings indicated that the mechanical strain changed the expression levels of these miRNAs. This might be a potential regulator of osteoblast differentiation and responses to mechanical strain.

Activation of PPAR-γ inhibits differentiation of rat osteoblasts by reducing expression of connective tissue growth factor

Long-term treatment with an agonist of peroxisome proliferator-activated receptor (PPAR)-γ is associated with bone fractures in the clinical practice. However, the mechanisms underlying the fractures are not fully understood. This study was aimed to examine the effect of rosiglitazone (an agonist of PPAR-γ) of different doses on the proliferation, differentiation, and transforming growth factor beta 1 (TGF-β1)-induced expression of connective tissue growth factor (CTGF) in primary rat osteoblasts in vitro. Osteoblasts were isolated from newly born SD rats and treated with different doses of rosiglitazone (0-20 μmol/L). The proliferation and differentiation of osteoblasts were measured by MTT assay and NPP assay, respectively. The expression of CTGF was determined by RT-PCR and Western blotting. The results showed that most isolated osteoblasts displayed strong alkaline phosphatase (ALP) activity and treatment with different doses of rosiglitazone did not affect their proliferation, but significantly inhibited the differentiation of osteoblasts in a dose-dependent manner. Moreover, treatment with different doses of rosiglitazone significantly reduced the TGF-β1-induced CTGF mRNA transcription and protein expression in a dose-dependent manner in rat osteoblasts. It was concluded that the activation of PPAR-γ may inhibit the differentiation of osteoblasts by reducing the TGF-β1-induced CTGF expression in vitro.

BMP growth factor signaling in a biomechanical context

Bone Morphogenetic Proteins (BMPs) are members of the transforming growth factor-β superfamily of secreted polypeptide growth factors and are important regulators in a multitude of cellular processes. To ensure the precise and balanced propagation of their pleiotropic signaling responses, BMPs and their corresponding signaling pathways are subject to tight control. A large variety of regulatory mechanisms throughout different biological levels combines into a complex network and provides the basis for physiological BMP function. This regulatory network not only includes biochemical factors but also mechanical cues. Both BMP signaling and mechanotransduction pathways are tightly interconnected and represent an elaborate signaling network active during development but also during organ homeostasis. Moreover, its dysregulation is associated with a number of human pathologies. A more detailed understanding of this crosstalk in respect to molecular interactions will be indispensable in the future, in particular to understand BMP-related diseases as well as with regard to an efficient clinical application of BMP ligands.

Mechanosignaling in bone health, trauma and inflammation

SIGNIFICANCE

Mechanosignaling is vital for maintaining the structural integrity of bone under physiologic conditions. These signals activate and suppress multiple signaling cascades regulating bone formation and resorption. Understanding these pathways is of prime importance to exploit their therapeutic potential in disorders associated with bone loss due to disuse, trauma, or disruption of homeostatic mechanisms.

RECENT ADVANCES

In the case of cells of the bone, an impressive amount of data has been generated that provides evidence of a complex mechanism by which mechanical signals can maintain or disrupt cellular homeostasis by driving transcriptional regulation of growth factors, matrix proteins and inflammatory mediators in health and inflammation. Mechanical signals act on cells in a magnitude dependent manner to induce bone deposition or resorption. During health, physiological levels of these signals are essential for maintaining bone strength and architecture, whereas during inflammation, similar signals can curb inflammation by suppressing the nuclear factor kappa B (NF-κB) signaling cascade, while upregulating matrix synthesis via mothers against decapentaplegic homolog and/or Wnt signaling cascades. Contrarily, excessive mechanical forces can induce inflammation via activation of the NF-κB signaling cascade.

CRITICAL ISSUES

Given the osteogenic potential of mechanical signals, it is imperative to exploit their therapeutic efficacy for the treatment of bone disorders. Here we review select signaling pathways and mediators stimulated by mechanical signals to modulate the strength and integrity of the bone.

FUTURE DIRECTIONS

Understanding the mechanisms of mechanotransduction and its effects on bone lay the groundwork for development of nonpharmacologic mechanostimulatory approaches for osteodegenerative diseases and optimal bone health.

Osteoblasts subjected to mechanical strain inhibit osteoclastic differentiation and bone resorption in a co-culture system

Bone remodeling is strictly mediated by the coupled activities of osteoblasts and osteoclasts, which are responsible for bone formation and resorption, respectively. Although many papers have been published on the mechanical responses of osteoblasts and osteoclasts, little is known about their communication during mechanical loading. In this study, a novel co-culture system was first established using Transwell culture inserts; MC3T3-E1 cells were embedded in the lower compartment of the inserts, and RAW264.7 cells were co-cultured in the upper compartment. The MC3T3-E1 cells were subjected to a mechanical strain of 2500 με at 0.5 Hz to investigate the effect of strain-loaded osteoblasts on co-cultured osteoclasts. The results showed that osteoblast-like cells were activated with an increase of alkaline phosphatase (ALP) activities. The strain-conditioned medium caused decreased activity of tartrate-resistant acid phosphatase and reduced the number of mature multinucleated osteoclasts, which subsequently resulted in the suppressed formation of resorption pits. The expression levels of cathepsin-K and matrix metalloproteinase-9 were also depressed by the strain-conditioned medium. In addition, we found that the expression ratio between osteoprotegerin (OPG) and receptor activator of NF-kB ligand in osteoblasts was significantly up-regulated due to the enhanced levels of OPG. In summary, we conclude that the strain-stimulated osteoblasts inhibited the differentiation and bone resorption of osteoclasts and that the mechanism was associated with the increased secretion of OPG in osteoblasts.

Bone formation in rabbit cancellous bone explant culture model is enhanced by mechanical load

Background

When studying and designing an artificial bone in vitro with similar features and functionality of natural bone by tissue engineering technology, the culturing environment, especially the mechanical environment is supposed to be an important factor, because a suitable mechanical environment in vitro may improve the adaptability of the planted-in tissue engineering bone in the body. Unfortunately, up to now, the relationship between mechanical stimuli and natural bone growth has not yet been precisely determined, and it is so imperative for a prior study on effect of mechanical loading on growth of the natural bone cultured in vitro.

Methods

Under sterile conditions, explant models of rabbit cancellous bone with 3 mm in thickness and 8 mm in diameter were prepared and cultured in a dynamic loading and circulating perfusion bioreactor system. By Micro-CT scanning, a 3D model for finite element (FEM) analysis was achieved. According to the results of FEM analysis and physiological load bearing capacity of the natural bone, these models were firstly subjected to mechanical load with 1Hz frequency causing average apparent strain of 1000 μϵ, 2000 μϵ, 3000 μϵ and 4000 μϵ respectively for 30 min every day, activities of alkaline phosphatase (AKP) were detected on the 5^th and the 14^th loading day and on the 14^th and the 21^st day, mechanical properties, tissue mineral density (TMD) of the bone explant models were investigated and Von-kossa staining and fluorescence double labeling assays were conducted to evaluate whether there were fresh osteoid in the bone explant models. In addition, Western blot, Elisa and Real-time PCR were employed to analyze expression of Collagen-I (COL-1), bone morphogenetic protein-2 (BMP-2) and osteoprotegerin (OPG) protein and RNA.

Results

The explant models of rabbit cancellous bone prepared under sterile conditions grew well in the bioreactor system. With the increasing culturing time and load levels, bone explant models in groups with 1000 μϵ and 2000 μϵ average apparent strain experienced improving mechanical properties and TMD (P<0.05), and results of Von-kossa staining and fluorescence double labeling also showed apparent fresh osteoid formation. Under the same loading conditions, a up-regulations in protein and RNA of COL-1, BMP-2 and OPG were detected, especially, relative genes notably expressed after 21 days.

Conclusion

Our study demonstrated that mechanical load could improve function and activity of osteoblasts in explant models of cancellous bone. Through regulations of COL-1, OPG and BMP-2 secreted by osteoblasts, the mechanical load could improve the tissue structural density and stiffness due to formation of fresh osteoid.

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.

Mechanical load modulates the stimulatory effect of BMP2 in a rat nonunion model

INTRODUCTION

Local application of bone morphogenetic proteins (BMPs) at the fracture site is known to stimulate bone regeneration. However, recent studies illustrate that the BMP-initiated mineralization may be enhanced by additional mechanical stimulation. Therefore, bone healing was monitored in vivo in order to investigate the effect of mechanical loading on the initiation and maturation of mineralization after cytokine treatment. We hypothesized that the mechanical stimulation would further enhance the efficacy of BMP2 treatment.

METHOD

Female Sprague-Dawley rats underwent a 5-mm defect, stabilized with an external fixator. Type I collagen scaffolds containing 50 μg of BMP2 diluted in a solvent or solvent only were placed into the defects. The BMP2-treated specimens and control specimens were then each divided into two groups: one that underwent mechanical loading and a nonloaded group. In vivo loading began immediately after surgery and continued once per week for the entire 6-week experimental period. For all groups, the newly formed callus tissue was quantitatively evaluated first by in vivo microcomputed tomography at 2, 4, and 6 weeks and further by histologic or histomorphometric analysis at 6 weeks postoperation.

RESULTS

Mechanical stimulation with BMP2 treatment significantly enhanced mineralized tissue volume and mineral content at 2 weeks. Histological analysis demonstrated a significantly greater area of fibrous connective tissue including bone marrow in the stimulated group, suggesting reconstitution of the endosteal canal and more advanced bone remodeling present in the mechanical loaded group. Both groups receiving BMP2 underwent massive bone formation, achieving bony bridging after only 2 weeks, while both control groups, receiving solvent only, revealed a persisting nonunion, filled with fibrous connective tissue, prolapsed muscle tissue, and a sealed medullary canal at week 6.

CONCLUSION

Mechanical loading further enhanced the efficacy of BMP2 application evidenced by increased mineralized tissue volume and mineralization at the stage of bony callus bridging. These data suggest that already a minimal amount of mechanical stimulation through load bearing or exercise may be a promising adjunct stimulus to enhance the efficacy of cytokine treatment in segmental defects. Further studies are required to elucidate the mechanistic interplay between mechanical and biological stimuli.

BMP2 and mechanical loading cooperatively regulate immediate early signalling events in the BMP pathway

Background

Efficient osteogenic differentiation is highly dependent on coordinated signals arising from growth factor signalling and mechanical forces. Bone morphogenetic proteins (BMPs) are secreted proteins that trigger Smad and non-Smad pathways and thereby influence transcriptional and non-transcriptional differentiation cues. Crosstalk at multiple levels allows for promotion or attenuation of signalling intensity and specificity. Similar to BMPs, mechanical stimulation enhances bone formation. However, the molecular mechanism by which mechanical forces crosstalk to biochemical signals is still unclear.

Results

Here, we use a three-dimensional bioreactor system to describe how mechanical forces are integrated into the BMP pathway. Time-dependent phosphorylation of Smad, mitogen-activated protein kinases and Akt in human fetal osteoblasts was investigated under loading and/or BMP2 stimulation conditions. The phosphorylation of R-Smads is increased both in intensity and duration under BMP2 stimulation with concurrent mechanical loading. Interestingly, the synergistic effect of both stimuli on immediate early Smad phosphorylation is reflected in the transcription of only a subset of BMP target genes, while others are differently affected. Together this results in a cooperative regulation of osteogenesis that is guided by both signalling pathways.

Conclusions

Mechanical signals are integrated into the BMP signalling pathway by enhancing immediate early steps within the Smad pathway, independent of autocrine ligand secretion. This suggests a direct crosstalk of both mechanotransduction and BMP signalling, most likely at the level of the cell surface receptors. Furthermore, the crosstalk of both pathways over longer time periods might occur on several signalling levels.

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.

Involvement of p38MAPK/NF-κB signaling pathways in osteoblasts differentiation in response to mechanical stretch

Bone morphogenetic proteins (BMPs) are known to be important in osteoblasts' response to mechanical stimuli. BMPs/Smad signaling pathway has been demonstrated to play a regulatory role in the mechanical signal transduction in osteoblasts. However, little is currently known about the Smad independent pathway in osteoblasts differentiation in mechanical loading. In this study, MC3T3-E1 cells were subjected to mechanical stretch of 2000 micro-stain (με) at 0.5 Hz, in order to investigate the involvement of p38MAPK and NF-κB signaling pathways in mechanical response in osteoblasts. We found BMP-2/BMP-4 were up-regulated by mechanical stretch via the earlier activation of p38MAPK and NF-κB signaling pathways, which enhanced osteogenic gene expressions including alkaline phosphatase (ALP), collagen type I (Col I) and osteocalcin (OCN), and the expressions of these osteogenic genes were remarkably decreased with Noggin (an inhibitor for BMPs signals) pretreatment. Furthermore, BMP-2/BMP-4 expressions were suppressed by PDTC, an inhibitor of NF-κB pathway and SB203580, an inhibitor of p38MAPK pathway, respectively, leading to the declined levels of ALP, Col I and OCN. Interestingly, blocking in p38MAPK pathway can also cause the inactivation of NF-κB pathway in mechanical stretch. Collectively, the results indicate during mechanical stretch p38MAPK and NF-κB signaling pathways are activated first, and then up-regulate BMP-2/BMP-4 to enhance osteogenic gene expressions. Moreover, p38MAPK and NF-κB signals have cross-talk in regulation of BMP-2/BMP-4 in mechanical response.

Noggin is novel inducer of mesenchymal stem cell adipogenesis: implications for bone health and obesity

Noggin is a glycosylated-secreted protein known so far for its inhibitory effects on bone morphogenetic protein (BMP) signaling by sequestering the BMP ligand. We report here for the first time a novel mechanism by which noggin directly induces adipogenesis of mesenchymal stem cells independently of major human adipogenic signals through C/EBPδ, C/EBPα and peroxisome proliferator-activated receptor-γ. Evaluation of a possible mechanism for noggin-induced adipogenesis of mesenchymal stem cells identified the role of Pax-1 in mediating such differentiation. The relevance of elevated noggin levels in obesity was confirmed in a preclinical, immunocompetent mouse model of spontaneous obesity and in human patients with higher body mass index. These data clearly provide a novel role for noggin in inducing adipogenesis and possibly obesity and further indicates the potential of noggin as a therapeutic target to control obesity.

Wnt/β-catenin expression does not correlate with serum alkaline phosphatase concentration in canine osteosarcoma patients

Osteosarcoma is an aggressive malignancy of the bone and an increase in serum alkaline phosphatase concentration has clinical prognostic value in both humans and canines. Increased serum alkaline phosphatase concentration at the time of diagnosis has been associated with poorer outcomes for osteosarcoma patients. The biology underlying this negative prognostic factor is poorly understood. Given that activation of the Wnt signaling pathway has been associated with alkaline phosphatase expression in osteoblasts, we hypothesized that the Wnt/β-catenin signaling pathway would be differentially activated in osteosarcoma tissue based on serum ALP status. Archived canine osteosarcoma samples and primary canine osteosarcoma cell lines were used to evaluate the status of Wnt/β-catenin signaling pathway activity through immunohistochemical staining, western immunoblot analyses, quantitative reverse-transcription polymerase chain reaction, and a Wnt-responsive promoter activity assay. We found no significant difference in β-catenin expression or activation between OSA populations differing in serum ALP concentration. Pathway activity was mildly increased in the primary OSA cell line generated from a patient with increased serum ALP compared to the normal serum ALP OSA cell line. Further investigation into the mechanisms underlying differences in serum ALP concentration is necessary to improve our understanding of the biological implications of this negative prognostic indicator.