Mechanical strain regulates osteoblast proliferation through integrin-mediated ERK activation

Zinc finger protein 36 like 2-histone deacetylase 1 axis is involved in the bone responses to mechanical stress

The molecular mechanism underlying the promotion of fracture healing by mechanical stimuli remains unclear. The present study aimed to investigate the role of zinc finger protein 36 like 2 (ZFP36L2)-histone deacetylase 1 (HDAC1) axis on the osteogenic responses to moderate mechanical stimulation. Appropriate stimulation of fluid shear stress (FSS) was performed on MC3T3-E1 cells transduced with ZFP36L2 and HDAC1 recombinant adenoviruses, aiming to validate the influence of mechanical stress on the expression of ZFP36L2-HDAC1 and the osteogenic differentiation and mineralization. The results showed that moderate FSS stimulation significantly upregulated the expression of ZFP36L2 in MC3T3-E1 cells (p < 0.01). The overexpression of ZFP36L1 markedly enhanced the levels of osteogenic differentiation markers, including bone morphogenetic protein 2 (BMP2), runt-related transcription factor 2 (RUNX2), alkaline phosphatase (ALP), Osterix, and collagen type I alpha 1 (COL1A1) (p < 0.01). ZFP36L2 accelerated the degradation of HDAC1 by specifically binding to its 3' UTR region, thereby fulfilling its function at the post-transcriptional regulatory gene level and promoting the osteogenic differentiation and mineralization fate of cells. Mechanical unloading notably diminished/elevated the expression of ZFP36L2/HDAC1, decreased bone mineral density and bone volume fraction, hindered the release of osteogenic-related factors and vascular endothelial growth factor in callus tissue (p < 0.01), and was detrimental to fracture healing. Collectively, proper stress stimulation plays a crucial role in facilitating osteogenesis through the promotion of ZFP36L2 and subsequent degradation of HDAC1. Targeting ZFP36L2-HDAC1 axis may provide promising insights to enhance bone defect healing.

Research progress on the regulatory mechanism of integrin-mediated mechanical stress in cells involved in bone metabolism

Mechanical stress is an internal force between various parts of an object that resists external factors and effects that cause an object to deform, and mechanical stress is essential for various tissues that are constantly subjected to mechanical loads to function normally. Integrins are a class of transmembrane heterodimeric glycoprotein receptors that are important target proteins for the action of mechanical stress stimuli on cells and can convert extracellular physical and mechanical signals into intracellular bioelectrical signals, thereby regulating osteogenesis and osteolysis. Integrins play a bidirectional regulatory role in bone metabolism. In this paper, relevant literature published in recent years is reviewed and summarized. The characteristics of integrins and mechanical stress are introduced, as well as the mechanisms underlying responses of integrin to mechanical stress stimulation. The paper focuses on integrin-mediated mechanical stress in different cells involved in bone metabolism and its associated signalling mechanisms. The purpose of this review is to provide a theoretical basis for the application of integrin-mediated mechanical stress to the field of bone tissue repair and regeneration.

Circular RNA-Mediated Regulation of Oral Tissue-Derived Stem Cell Differentiation: Implications for Oral Medicine and Orthodontic Applications

Circular RNAs (circRNAs) are a novel class of endogenous non-coding RNAs (ncRNAs) which unlike linear RNAs, have a covalently closed continuous loop structure. circRNAs are found abundantly in human cells and their biology is complex. They feature unique expression to different types of cells, tissues, and developmental stages. To the present, the functional roles of circular RNAs are not fully understood. They reportedly act as microRNA (miRNA) sponges, therefore having key regulatory functions in diverse physiological and pathological processes. As for dentistry field, lines of evidence indicate that circRNAs play vital roles in the odontogenic and osteogenic differentiation of dental pulp stem cells (DPSCs) and periodontal ligament stem cells (PDLSCs). Abnormal expression of circRNAs have been found in other areas of pathology frequently reflected also in the oral environment, such as inflammation or bone and soft tissue loss. Therefore, circRNAs could be of significant importance in various fields in dentistry, especially in bone and soft tissue engineering and regeneration. Understanding the molecular mechanisms occurring during the regulation of oral biological and tissue remodeling processes could augment the discovery of novel diagnostic biomarkers and therapeutic strategies that will improve orthodontic and other oral therapeutic protocols.

Mechanical induction of osteoanabolic Wnt1 promotes osteoblast differentiation via Plat

Physical activity-induced mechanical stimuli play a crucial role in preserving bone mass and structure by promoting bone formation. While the Wnt pathway is pivotal for mediating the osteoblast response to loading, the exact mechanisms are not fully understood. Here, we found that mechanical stimulation induces osteoblastic Wnt1 expression, resulting in an upregulation of key osteogenic marker genes, including Runx2 and Sp7, while Wnt1 knockdown using siRNA prevented these effects. RNAseq analysis identified Plat as a major target through which Wnt1 exerts its osteogenic influence. This was corroborated by Plat depletion using siRNA, confirming its positive role in osteogenic differentiation. Moreover, we demonstrated that mechanical stimulation enhances Plat expression, which, in turn leads to increased expression of osteogenic markers like Runx2 and Sp7. Notably, Plat depletion by siRNA prevented this effect. We have established that Wnt1 regulates Plat expression by activating β-Catenin. Silencing Wnt1 impairs mechanically induced β-Catenin activation, subsequently reducing Plat expression. Furthermore, our findings showed that Wnt1 is essential for osteoblasts to respond to mechanical stimulation and induce Runx2 and Sp7 expression, in part through the Wnt1/β-Catenin/Plat signaling pathway. Additionally, we observed significantly reduced Wnt1 and Plat expression in bones from ovariectomy (OVX)-induced and age-related osteoporotic mouse models compared with non-OVX and young mice, respectively. Overall, our data suggested that Wnt1 and Plat play significant roles in mechanically induced osteogenesis. Their decreased expression in bones from OVX and aged mice highlights their potential involvement in post-menopausal and age-related osteoporosis, respectively.

ITGB1 and DDR activation as novel mediators in acquired resistance to osimertinib and MEK inhibitors in EGFR-mutant NSCLC

Osimertinib is a third-generation tyrosine kinase inhibitor clinically approved for first-line treatment of EGFR-mutant non-small cell lung cancer (NSCLC) patients. Although an impressive drug response is initially observed, in most of tumors, resistance occurs after different time and an alternative therapeutic strategy to induce regression disease is currently lacking. The hyperactivation of MEK/MAPKs, is one the most common event identified in osimertinib-resistant (OR) NSCLC cells. However, in response to selective drug pressure, the occurrence of multiple mechanisms of resistance may contribute to treatment failure. In particular, the epithelial-to-mesenchymal transition (EMT) and the impaired DNA damage repair (DDR) pathways are recognized as additional cause of resistance in NSCLC thus promoting tumor progression. Here we showed that concurrent upregulation of ITGB1 and DDR family proteins may be associated with an increase of EMT pathways and linked to both osimertinib and MEK inhibitor resistance to cell death. Furthermore, this study demonstrated the existence of an interplay between ITGB1 and DDR and highlighted, for the first time, that combined treatment of MEK inhibitor with DDRi may be relevant to downregulate ITGB1 levels and increase cell death in OR NSCLC cells.

Dkk-1-TNF-α crosstalk regulates MC3T3E1 pre-osteoblast proliferation and differentiation under mechanical stress through the ERK signaling pathway

The study aims to explore the role of the ERK signaling pathway in the crosstalk between Dkk-1 and TNF-α in MC3T3E1 pre-osteoblasts under cyclic tensile/compressive stress. A forced four-point bending system was used to apply cyclic uniaxial tensile/compressive strain (2000 μ, 0.5 Hz) to MC3T3E1 cells. Dkk-1 and TNF-α expression were upregulated in MC3T3E1 cells under compressive strain. Cell proliferation, the cell cycle, osteogenesis-related gene (Wnt5a, Runx2, Osterix) expression, β-catenin expression, and the p-ERK/ERK ratio were significantly enhanced, whereas apoptosis, the RANKL/OPG ratio, and TNF-α expression were significantly attenuated, by Dkk-1 silencing. Dkk-1 expression increased and the effects of Dkk-1 silencing were reversed when exogenous TNF-α was added. Mechanically, TNF-α crosstalked with Dkk-1 through ERK signaling in MC3T3E1 cells. ERK signaling blockade impaired Dkk-1-induced TNF-α expression and TNF-α-mediated Dkk-1 expression. Dkk-1 and TNF-α crosstalked, partially through ERK signaling, in MC3T3E1 cells under compressive/tensile strain, synergistically modulating various biological behaviors of the cells. These findings not only provide mechanical insight into the cellular events and molecular regulation of orthodontic tooth movement (OTM), but also aid the development of novel strategies to accelerate OTM.

Functional role of circRNAs in osteogenesis: A review

The extracellular matrixes (ECM), as well as the microenvironmental signals, play an essential role in osteogenesis by regulating intercellular pathways. Recently, it has been demonstrated that a newly identified RNA, circular RNA, contributes to the osteogenesis process. Circular RNA (circRNA), the most recently identified RNA, is involved in the regulation of gene expression at transcription to translation levels. The dysregulation of circRNAs has been observed in several tumors and diseases. Also, various studies have shown that circRNAs expression is changed during osteogenic differentiation of progenitor cells. Therefore, understanding the role of circRNAs in osteogenesis might help the diagnosis as well as treatment of bone diseases such as bone defects and osteoporosis. In this review, circRNA functions and the related pathways in osteogenesis have been discussed.

Effect of tension and compression on dynamic alveolar histomorphometry

Here, we tested the hypothesis that tensile and compressive stresses generated in the alveolar bone proper regulate site-specific cellular and functional changes in osteoclasts and osteoblasts. Thirty-two 13-week-old male mice were randomly divided into four groups: two experimental groups with vertical loading obliquely from the palatal side to the buccal side of the maxillary molar (0.9 N) 30 min per day for 8 or 15 days and unloaded controls (n = 8). Calcein and alizarin were administered 8 and 2 days before euthanization, respectively, to detect the time of bone formation. Undecalcified sections parallel to the occlusal plane were prepared on the palatal root and the surrounding alveolar bone in the middle of the root length. The alveolar perimeter was divided into 12 equal regions for site analysis, and the bone histomorphometric parameters were obtained for each region. Data from in vivo microfocus computed tomography were used to construct animal-specific finite element models. 2D stress distribution images were overlain on histology images obtained from the same location. Significant differences in the total perimeter between groups and between loading and unloading in each region were statistically analyzed (α = 0.05). Osteoclast counts and the alizarin label ratio were significantly higher in the loaded group than in the unloaded group in regions where the maximum von Mises and principal tensile stresses were the highest along the perimeter. The label ratio of calcein was significantly lower in the 8-day loaded group than in the unloaded group, indicating that the calcein-labeled surface was resorbed by osteoclasts that appeared during the loading period. The effect of loading was mitigated by an increase in the maximum principal compressive stress. We conclude that bone resorption and bone formation are functions of site-specific tension and compression in the alveolar bone proper, confirming our hypothesis. This finding is critical for the advancement of diagnosis and treatment planning in clinical dentistry.

Interplay between integrins and cadherins to control bone differentiation upon BMP-2 stimulation

Introduction: Upon BMP-2 stimulation, the osteoblastic lineage commitment in C2C12 myoblasts is associated with a microenvironmental change that occurs over several days. How does BMP-2 operate a switch in adhesive machinery to adapt to the new microenvironment and to drive bone cell fate is not well understood. Here, we addressed this question for BMP-2 delivered either in solution or physically bound of a biomimetic film, to mimic its presentation to cells via the extracellular matrix (ECM). Methods: Biommetics films were prepared using a recently developed automated method that enable high content studies of cellular processes. Comparative gene expressions were done using RNA sequencing from the encyclopedia of the regulatory elements (ENCODE). Gene expressions of transcription factors, beta chain (1, 3, 5) integrins and cadherins (M, N, and Cad11) were studied using quantitative PCR. ECM proteins and adhesion receptor expressions were also quantified by Western blots and dot blots. Their spatial organization in and around cells was studied using immuno-stainings. The individual effect of each receptor on osteogenic transcription factors and alkaline phosphatase expression were studied using silencing RNA of each integrin and cadherin receptor. The organization of fibronectin was studied using immuno-staining and quantitative microscopic analysis. Results: Our findings highlight a switch of integrin and cadherin expression during muscle to bone transdifferentiation upon BMP-2 stimulation. This switch occurs no matter the presentation mode, for BMP-2 presented in solution or via the biomimetic film. While C2C12 muscle cells express M-cadherin and Laminin-specific integrins, the BMP-2-induced transdifferentiation into bone cells is associated with an increase in the expression of cadherin-11 and collagen-specific integrins. Biomimetic films presenting matrix-bound BMP-2 enable the revelation of specific roles of the adhesive receptors depending on the transcription factor. Discussion: While β3 integrin and cadherin-11 work in concert to control early pSMAD1,5,9 signaling, β1 integrin and Cadherin-11 control RunX2, ALP activity and fibronectin organization around the cells. In contrast, while β1 integrin is also important for osterix transcriptional activity, Cadherin-11 and β5 integrin act as negative osterix regulators. In addition, β5 integrin negatively regulates RunX2. Our results show that biomimetic films can be used to delinate the specific events associated with BMP-2-mediated muscle to bone transdifferentiation. Our study reveals how integrins and cadherins work together, while exerting distinct functions to drive osteogenic programming. Different sets of integrins and cadherins have complementary mechanical roles during the time window of this transdifferentiation.

Altered mechanotransduction in adolescent idiopathic scoliosis osteoblasts: an exploratory in vitro study

Adolescent idiopathic scoliosis (AIS) is the most prevalent pediatric spinal deformity. We previously demonstrated elongated cilia and an altered molecular mechanosensory response in AIS osteoblasts. The purpose of this exploratory study was to characterize the mechanosensory defect occurring in AIS osteoblasts. We found that cilia length dynamics in response to flow significantly differ in AIS osteoblasts compared to control cells. In addition, strain-induced rearrangement of actin filaments was compromised resulting in a failure of AIS osteoblasts to position or elongate in function of the bidirectional-applied flow. Contrary to control osteoblasts, fluid flow had an inhibitory effect on AIS cell migration. Moreover, flow induced an increase in secreted VEGF-A and PGE2 in control but not AIS cells. Collectively our data demonstrated that in addition to the observed primary cilium defects, there are cytoskeletal abnormalities correlated to impaired mechanotransduction in AIS. Thus, we propose that the AIS etiology could be a result of generalized defects in cellular mechanotransduction given that an adolescent growing spine is under constant stimulation for growth and bone remodeling in response to applied mechanical forces. Recognition of an altered mechanotransduction as part of the AIS pathomechanism must be considered in the conception and development of more effective bracing treatments.

CCL2 is a critical mechano-responsive mediator in crosstalk between osteoblasts and bone mesenchymal stromal cells

It has been known that moderate mechanical loading, like that caused by exercise, promotes bone formation. However, its underlying mechanisms remain elusive. Here we showed that moderate running dramatically improved trabecular bone in mice tibias with an increase in bone volume fraction and trabecular number and a decrease in trabecular pattern factor. Results of immunohistochemical and histochemical staining revealed that moderate running mainly increased the number of osteoblasts but had no effect on osteoclasts. In addition, we observed a dramatic increase in the number of colony forming unit-fibroblast in endosteal bone marrow and the percentage of CD45^- Leptin receptor⁺ (CD45^- LepR⁺ ) endosteal mesenchymal progenitors. Bioinformatics analysis of the transcriptional data from gene expression omnibus (GEO) database identified chemokine c-c-motif ligands (CCL2) as a critical candidate induced by mechanical loading. Interestingly, we found that CCL2 was up-regulated mainly in osteoblastic cells in the tibia of mice after moderate running. Further, we found that mechanical loading up-regulated the expression of CCL2 by activating ERK1/2 pathway, thereby stimulating migration of endosteal progenitors. Finally, neutralizing CCL2 abolished the recruitment of endosteal progenitors and the increased bone formation in mice after 4 weeks running. These results therefore uncover an unknown connection between osteoblasts and endosteal progenitors recruited in the increased bone formation induced by mechanical loading.

Excessive mechanical stretch‑mediated osteoblasts promote the catabolism and apoptosis of chondrocytes via the Wnt/β‑catenin signaling pathway

Excessive biomechanical loading is considered an important cause of osteoarthritis. Although the mechanical responses of chondrocytes and osteoblasts have been investigated, their communication during mechanical loading and the underlying molecular mechanisms are not yet fully known. The present study investigated the effects of excessive mechanically stretched osteoblasts on the metabolism and apoptosis of chondrocytes, and also assessed the involvement of the Wnt/β‑catenin signaling pathway. In the present study, rat chondrocytes and osteoblasts were subjected to mechanical tensile strain, and an indirect chondrocyte‑osteoblast co‑culture model was established. Reverse transcription‑quantitative PCR and western blotting were performed to determine the expression levels of genes and proteins of interest. An ELISA was performed to investigate the levels of cytokines, including matrix metalloproteinase (MMP) 13, MMP 3, interleukin‑6 (IL‑6) and prostaglandin E2 (PG E2), released from osteoblasts. Flow cytometry was performed to detect the apoptosis of chondrocytes exposed to stretched osteoblast conditioned culture medium. The levels of MMP 13, IL‑6 and PG E2 increased significantly in the supernatants of stretched osteoblasts compared with the un‑stretched group. By contrast, the mRNA expression levels of Collagen 1a and alkaline phosphatase were significantly decreased in osteoblasts subjected to mechanical stretch compared with the un‑stretched group. The mRNA expression level of Collagen 2a was significantly decreased, whereas the expression levels of MMP 13 and a disintegrin and metalloproteinase with thrombospondin‑like motifs 5 were significantly increased in chondrocytes subjected to mechanical stretch compared with the un‑stretched group. In the co‑culture model, the results indicated that excessive mechanically stretched osteoblasts induced the catabolism and apoptosis of chondrocytes, which was partly inhibited by Wnt inhibitor XAV‑939. The results of the present study demonstrated that excessive mechanical stretch led to chondrocyte degradation and inhibited osteoblast osteogenic differentiation; furthermore, excessive mechanically stretched osteoblasts induced the catabolism and apoptosis of chondrocytes via the Wnt/β‑catenin signaling pathway.

Intestinal changes associated with fluoride exposure in rats: Integrative morphological, proteomic and microbiome analyses

Gastrointestinal signs and symptoms are the first signs of toxicity due to exposure to fluoride (F). This suggests the possibility that lower levels of subchronic F exposure may affect the gut. The aim of this study was to evaluate changes in the morphology, proteome and microbiome of the ileum of rats, after subchronic exposure to F. Male rats ingested water with 0, 10, or 50 mgF/L for thirty days. Treatment with F, regardless of the dose, significantly decreased the density of HuC/D-IR neurons, whereas CGRP-IR and SP-IR varicosities were significantly increased compared to the control group. Increased VIP-IR varicosities were significantly increased only in the group treated with 50 mgF/L. A significant increase in thickness of the tunica muscularis, as well as in the total thickness of the ileum wall was observed at both F doses when compared to controls. In proteomics analysis, myosin isoforms were increased, and Gastrotopin was decreased in F-exposed mice. In the microbiome metagenomics analysis, Class Clostridia was significantly reduced upon exposure to 10 mgF/L. At the higher F dose of 50 mg/L, genus Ureaplasma was significantly reduced in comparison with controls. Morphological and proteomics alterations induced by F were marked by changes associated with inflammation, and alterations in the gut microbiome. Further studies are needed to determine whether F exposure increases inflammation with secondary effects of the gut microbiome, and/or whether primary effects of F on the gut microbiome enhance changes associated with inflammation.

Mechanical loading and the control of stem cell behavior

OBJECTIVE

Mechanical stimulation regulates many cell responses. The present study describes the effects of different in vitro mechanical stimulation approaches on stem cell behavior.

DESIGN

The narrative review approach was performed. The articles published in English language that addressed the effects of mechanical force on stem cells were searched on Pubmed and Scopus database. The effects of extrinsic mechanical force on stem cell response was reviewed and discussed.

RESULTS

Cells sense mechanical stimuli by the function of mechanoreceptors and further transduce force stimulation into intracellular signaling. Cell responses to mechanical stimuli depend on several factors including type, magnitude, and duration. Further, similar mechanical stimuli exhibit distinct cell responses based on numerous factors including cell type and differentiation stage. Various mechanical applications modulate stemness maintenance and cell differentiation toward specific lineages.

CONCLUSIONS

Mechanical force application modulates stemness maintenance and differentiation. Modification of force regimens could be utilized to precisely control appropriate stem cell behavior toward specific applications.

Multiphysics simulation of a compression-perfusion combined bioreactor to predict the mechanical microenvironment during bone metastatic breast cancer loading experiments

Incurable breast cancer bone metastasis causes widespread bone loss, resulting in fragility, pain, increased fracture risk, and ultimately increased patient mortality. Increased mechanical signals in the skeleton are anabolic and protect against bone loss, and they may also do so during osteolytic bone metastasis. Skeletal mechanical signals include interdependent tissue deformations and interstitial fluid flow, but how metastatic tumor cells respond to each of these individual signals remains underinvestigated, a barrier to translation to the clinic. To delineate their respective roles, we report computed estimates of the internal mechanical field of a bone mimetic scaffold undergoing combinations of high and low compression and perfusion using multiphysics simulations. Simulations were conducted in advance of multimodal loading bioreactor experiments with bone metastatic breast cancer cells to ensure that mechanical stimuli occurring internally were physiological and anabolic. Our results show that mechanical stimuli throughout the scaffold were within the anabolic range of bone cells in all loading configurations, were homogenously distributed throughout, and that combined high magnitude compression and perfusion synergized to produce the largest wall shear stresses within the scaffold. These simulations, when combined with experiments, will shed light on how increased mechanical loading in the skeleton may confer anti-tumorigenic effects during metastasis.

Effect of load-induced local mechanical strain on peri-implant bone cell activity related to bone resorption and formation in mice: An analysis of histology and strain distributions

The purpose of this study was to investigate the effect of load-induced local mechanical strain on bone cell activity of peri-implant bone in mice. Titanium implants were placed in the maxillae of 13-week-old male C57BL/6J mice and subjected to intermittent 0.15 N, 0.3 N, or 0.6 N loads for 30 min/day for 6 days. The animals were sacrificed 2 days after the final loading. Unloaded mice were used as controls. An animal-specific three-dimensional finite element model was constructed based on morphological data retrieved from in vivo microfocus computed tomography for each mouse to calculate the mechanical strain distribution. Strain distribution images were overlaid on corresponding histological images of the same site in the same animal. The buccal cervical region of the peri-implant bone was predetermined as the region of interest (ROI). Each ROI was divided by four strain intensity levels: 0-20 με, 20-60 με, 60-100 με, and ≥100 με, and the bone histomorphometric parameters were analyzed by the total area of each strain range for all loaded samples. The distance between the calcified front and calcein labeling as a parameter representing the mineral apposition rate was significantly greater in the areas with strain intensity ≥100 με than in the area with strain intensity <100 με, suggesting that the bone formation activity of osteoblasts was locally enhanced by a higher mechanical strain. However, the shrunken osteocytes and the empty osteocyte lacunae were significantly lower in the highest strain area, suggesting that osteoclastogenesis was more retarded in higher strain areas than in lower strain areas. The histomorphometric parameters were not affected geometrically in the unloaded animals, suggesting that the load-induced mechanical strain caused differences in the histomorphometric parameters. Our findings support the hypothesis that bone cell activity related to bone resorption and formation is local strain-dependent on implant loading.

Purinergic Signaling Mediates PTH and Fluid Flow-Induced Osteoblast Proliferation

Both parathyroid hormone (PTH) and mechanical signals are able to regulate bone growth and regeneration. They also can work synergistically to regulate osteoblast proliferation, but little is known about the mechanisms how PTH and mechanical signals interact with each other during this process. In this study, we investigated responses of MC3T3-E1 osteoblasts to PTH and oscillatory fluid flow. We found that osteoblasts are more sensitive to mechanical signals in the presence of PTH according to ERK1/2 phosphorylation, ATP release, CREB phosphorylation, and cell proliferation. PTH may also reduce the osteoblast refractory period after desensitization due to mechanical signals. We further found that the synergistic responses of osteoblasts to fluid flow or ATP with PTH had similar patterns, suggesting that synergy between fluid flow and PTH may be through the ATP pathway. After we inhibited ATP effects using apyrase in osteoblasts, their synergistic responses to mechanical stimulation and PTH were also inhibited. Additionally, knocking down P2Y2 purinergic receptors can significantly attenuate osteoblast synergistic responses to mechanical stimulation and PTH in terms of ERK1/2 phosphorylation, CREB phosphorylation, and cell proliferation. Thus, our results suggest that PTH enhances mechanosensitivity of osteoblasts via a mechanism involving ATP and P2Y2 purinergic receptors.

Lactoferrin promotes osteogenesis of MC3T3-E1 cells induced by mechanical strain in an extracellular signal-regulated kinase 1/2-dependent manner

INTRODUCTION

This study aimed to investigate the role of lactoferrin (LF) in the mechanical strain-induced osteogenesis of nontransformed osteoblastic cells (MC3T3-E1 cells) and related mechanism.

METHODS

MC3T3-E1 cells were cultured in vitro and treated with 100 μg/mL LF, followed by a 2000 μ mechanical strain load. U0126 was used to determine the role of extracellular signal-regulated kinase 1/2 (Erk1/2). Alizarin red S staining was performed to observe the cell mineralization potential. The osteogenic results were analyzed by reverse transcription-polymerase chain reaction and western blotting.

RESULTS

The expression of Col1, Alp, Ocn, Bsp, and Opn mRNA and p-Erk1/2 proteins was significantly upregulated under mechanical strain load. In addition, mineralized nodule formation was increased. After adding LF, the expression of the biomarkers and the formation of mineralized nodules were further promoted. On treatment with the Erk1/2 inhibitor U0126, the expression of Col1, Alp, and p-Erk1/2 mRNA and protein was significantly downregulated.

CONCLUSIONS

These findings demonstrate that LF promotes osteogenic activity by activating osteogenesis-related biomarkers, corroborating that the effects of mechanical strain depend on Erk1/2 signaling pathway.

Long-Term Effect of Honeycomb β-Tricalcium Phosphate on Zygomatic Bone Regeneration in Rats

In recent years, artificial bones with high biocompatibility have been developed for hard tissue reconstruction. However, current bone replacement methods are inadequate for large defects, causing infection, exposure, and damage. We have developed a new honeycomb β-tricalcium phosphate (TCP) material, which achieved good bone regeneration after implantation in a rat complete zygomatic bone defect. In this study, we further investigated the ability of honeycomb β- TCP for remodeling after bone regeneration as a long-term result. Bone morphogenic protein (BMP)-2-free honeycomb β-TCP (TCP group) and honeycomb β-TCP with BMP-2 (BMP group) were implanted in the zygomatic bone of rats. Micro-computed tomography was performed to track the zygomatic bone morphology, and specimens were histologically examined for osteogenesis and remodeling. In the TCP group, no bone formation was observed at 1 month, but it was observed at 6 months. Bone formation was observed in the BMP group at 1 month, and β-TCP absorption reproducing the zygomatic bone morphology was observed at 6 months. This honeycomb β-TCP with BMP-2 may provide appropriate remodeling that reproduces good bone formation in the early stage and good morphology in the long term, offering an alternative bone reconstruction material to vascularized bone grafts.

Uniaxial Static Strain Promotes Osteoblast Proliferation and Bone Matrix Formation in Distraction Osteogenesis In Vitro

Objective

We aimed at investigating the effects of uniaxial static strain on osteoblasts in distraction osteogenesis (DO).

Methods

To simulate the mechanical stimulation of osteoblasts during DO, 10% uniaxial static strain was applied to osteoblasts using a homemade multiunit cell stretching and compressing device. Before and after applying strain stimulation, the morphological changes of osteoblasts were observed by inverted phase-contrast microscopy, Coomassie blue staining, and immunofluorescence. Alkaline phosphatase (ALP) activity, mRNA levels (proliferating cell nuclear antigen [PCNA], ALP, Runx2, osteocalcin [OCN], collagen type I, hypoxia-inducible factor- [HIF-] 1α, and vascular endothelial growth factor [VEGF]), and protein levels (Runx2, OCN, collagen type I, HIF-1α, and VEGF) were evaluated by using ALP kit, real-time quantitative reverse transcription-polymerase chain reaction, western blot, and enzyme-linked immunosorbent assay.

Results

After the mechanical stimulation, the cytoskeleton microfilaments were rearranged, and the cell growth direction of the osteoblasts became ordered, with their direction being at an angle of about 45° from the direction of strain. The proliferation of osteoblasts and the expression levels of mRNA and protein of ALP, Runx2, OCN, collagen type I, HIF-1α, and VEGF were significantly higher than in the nonstretch control groups.

Conclusion

Our homemade device can exert uniaxial static strain and promote the proliferation of osteoblasts and bone matrix formation. It can be used to simulate the mechanical stimulation of osteoblasts during DO.

Mindin serves as a tumour suppressor gene during colon cancer progression through MAPK/ERK signalling pathway in mice

Mindin is important in broad spectrum of immune responses. On the other hand, we previously reported that mindin attenuated human colon cancer development by blocking angiogenesis through Egr-1-mediated regulation. However, the mice original mindin directly suppressed the syngenic colorectal cancer (CRC) growth in our recent study and we aimed to further define the role of mindin during CRC development in mice. We established the mouse syngeneic CRC CMT93 and CT26 WT cell lines with stable mindin knock-down or overexpression. These cells were also subcutaneously injected into C57BL/6 and BALB/c mice as well as established a colitis-associated colorectal cancer (CAC) mouse model treated with lentiviral-based overexpression and knocked-down of mindin. Furthermore, we generated mindin knockout mice using a CRISPR-Cas9 system with CAC model. Our data showed that overexpression of mindin suppressed cell proliferation in both of CMT93 and CT26 WT colon cancer cell lines, while the silencing of mindin promoted in vitro cell proliferation via the ERK and c-Fos pathways and cell cycle control. Moreover, the overexpression of mindin significantly suppressed in vivo tumour growth in both the subcutaneous transplantation and the AOM/DSS-induced CAC models. Consistently, the silencing of mindin reversed these in vivo observations. Expectedly, the tumour growth was promoted in the CAC model on mindin-deficient mice. Thus, mindin plays a direct tumour suppressive function during colon cancer progression and suggesting that mindin might be exploited as a therapeutic target for CRC.

Mechanical stimulation improves osteogenesis and the mechanical properties of osteoblast-laden RGD-functionalized polycaprolactone/hydroxyapatite scaffolds

This article presents the results of the combined effects of RGD (arginine-glycine-aspartate) functionalization and mechanical stimulation on osteogenesis that could lead to the development of implantable robust tissue-engineered mineralized constructs. Porous polycaprolactone/hydroxyapatite (PCL/HA) scaffolds are functionalized with RGD-C (arginine-glycine-aspartate-cysteine) peptide. The effects of RGD functionalization are then explored on human fetal osteoblast cell adhesion, proliferation, osteogenic differentiation (alkaline phosphatase activity), extracellular matrix (ECM) production, and mineralization over 28 days. The effects of RGD functionalization followed by mechanical stimulation with a cyclic fluid shear stress of 3.93 mPa in a perfusion bioreactor are also elucidated. The tensile properties (Young's moduli and ultimate tensile strengths) of the cell-laden scaffolds are measured at different stages of cell culture to understand how the mechanical properties of the tissue-engineered structures evolve. RGD functionalization is shown to promote initial cell adhesion, proliferation, alkaline phosphatase (ALP) activity, and ECM production. However, it does not significantly affect mineralization and tensile properties. Mechanical stimulation after RGD functionalization is shown to further improve the ALP activity, ECM production, mineralization, and tensile properties, but not cell proliferation. The results suggest that combined RGD functionalization and mechanical stimulation of cell-laden PCL/HA scaffolds can be used to accelerate the regeneration of robust bioengineered bone structures.

Proper mechanical stress promotes femoral head recovery from steroid-induced osteonecrosis in rats through the OPG/RANK/RANKL system

Background

Long-term use of steroid may lead to osteonecrosis of the femoral head (ONFH). Mechanical stress may help bone formation and remodeling. This study aimed to probe the role of mechanical stress in the femoral head recovery in rats.

Methods

Rat models with ONFH were induced by steroid. Rats were subjected to different levels of mechanical stress (weight-bearing training), and then the morphology and bone density of femoral head of rats were measured. The mRNA and protein levels of the OPG/RANK/RANKL axis in rat femoral head were assessed. Gain- and loss-of function experiments of OPG were performed to identify its role in femoral head recovery following stress implement. The ex vivo cells were extracted and the effects of stress and OPG on osteogenesis in vitro were explored.

Results

Steroid-induced ONFH rats showed decreased bone density and increased bone spaces, as well as necrotic cell colonies and many cavities in the cortical bones and trabeculars. Proper mechanical stress or upregulation of OPG led to decreased RANK/RANKL expression and promoted femoral head recovery from steroid-induced osteonecrosis. However, excessive mechanical stress might impose too much load on the femurs thus leading even retard femoral head recovery process. In addition, the in vitro experimental results supported that proper stress and overexpression of OPG increased the osteogenesis of ex vivo cells of femoral head.

Conclusion

This study provided evidence that proper mechanical stress promoted femoral head recovery from steroid-induced osteonecrosis through the OPG/RANK/RANKL system, while overload might inhibit the recovery process. This study may offer novel insights for ONFH treatment.

Revealing hidden information in osteoblast's mechanotransduction through analysis of time patterns of critical events

BACKGROUND

Mechanotransduction in bone cells plays a pivotal role in osteoblast differentiation and bone remodelling. Mechanotransduction provides the link between modulation of the extracellular matrix by mechanical load and intracellular activity. By controlling the balance between the intracellular and extracellular domains, mechanotransduction determines the optimum functionality of skeletal dynamics. Failure of this relationship was suggested to contribute to bone-related diseases such as osteoporosis.

RESULTS

A hybrid mechanical and agent-based model (Mech-ABM), simulating mechanotransduction in a single osteoblast under external mechanical perturbations, was utilised to simulate and examine modulation of the activation dynamics of molecules within mechanotransduction on the cellular response to mechanical stimulation. The number of molecules and their fluctuations have been analysed in terms of recurrences of critical events. A numerical approach has been developed to invert subordination processes and to extract the direction processes from the molecular signals in order to derive the distribution of recurring events. These predict that there are large fluctuations enclosing information hidden in the noise which is beyond the dynamic variations of molecular baselines. Moreover, studying the system under different mechanical load regimes and altered dynamics of feedback loops, illustrate that the waiting time distributions of each molecule are a signature of the system's state.

CONCLUSIONS

The behaviours of the molecular waiting times change with the changing of mechanical load regimes and altered dynamics of feedback loops, presenting the same variation of patterns for similar interacting molecules and identifying specific alterations for key molecules in mechanotransduction. This methodology could be used to provide a new tool to identify potent molecular candidates to modulate mechanotransduction, hence accelerate drug discovery towards therapeutic targets for bone mass upregulation.

Diffuse microdamage in bone activates anabolic response by osteoblasts via involvement of voltage-gated calcium channels

INTRODUCTION

Matrix damage sustained by bone tissue is repaired by the concerted action of bone cells. Previous studies have reported extracellular calcium ([Ca²⁺]_E) efflux to originate from regions of bone undergoing diffuse microdamage termed as "diffuse microdamage-induced calcium efflux" (DMICE). DMICE has also been shown to activate and increase intracellular calcium ([Ca²⁺]_I) signaling in osteoblasts via the involvement of voltage-gated calcium channels (VGCC). Past studies have assessed early stage (< 1 h) responses of osteoblasts to DMICE. The current study tested the hypothesis that DMICE has longer-term sustained effect such that it induces anabolic response of osteoblasts.

MATERIALS AND METHODS

Osteoblasts derived from mouse calvariae were seeded on devitalized bovine bone wafers. Localized diffuse damage was induced in the vicinity of cells by bending. The response of osteoblasts to DMICE was evaluated by testing gene expression, protein synthesis and mineralized nodule formation.

RESULTS

Cells on damaged bone wafers showed a significant increase in RUNX2 and Osterix expression compared to non-loaded control. Also, RUNX2 and Osterix expression were suppressed significantly when the cells were treated with bepridil, a non-selective VGCC inhibitor, prior to loading. Significantly higher amounts of osteocalcin and mineralized nodules were synthesized by osteoblasts on diffuse damaged bone wafers, while bepridil treatment resulted in a significant decrease in osteocalcin production and mineralized nodule formation.

CONCLUSION

In conclusion, this study demonstrated that DMICE activates anabolic responses of osteoblasts through activation of VGCC. Future studies of osteoblast response to DMICE in vivo will help to clarify how bone cells repair diffuse microdamage.

Effect of focal adhesion kinase inhibition on osteoblastic cells grown on titanium with different topographies

Objective

The present study aimed to investigate the participation of focal adhesion kinases (FAK) in interactions between osteoblastic cells and titanium (Ti) surfaces with three different topographies, namely, untreated (US), microstructured (MS), and nanostructured (NS).

Methodology

Osteoblasts harvested from the calvarial bones of 3-day-old rats were cultured on US, MS and NS discs in the presence of PF-573228 (FAK inhibitor) to evaluate osteoblastic differentiation. After 24 h, we evaluated osteoblast morphology and vinculin expression, and on day 10, the following parameters: gene expression of osteoblastic markers and integrin signaling components, FAK protein expression and alkaline phosphatase (ALP) activity. A smooth surface, porosities at the microscale level, and nanocavities were observed in US, MS, and NS, respectively.

Results

FAK inhibition decreased the number of filopodia in cells grown on US and MS compared with that in NS. FAK inhibition decreased the gene expression of Alp, bone sialoprotein, osteocalcin, and ALP activity in cells grown on all evaluated surfaces. FAK inhibition did not affect the gene expression of Fak, integrin alpha 1 ( Itga1 ) and integrin beta 1 ( Itgb1 ) in cells grown on MS, increased the gene expression of Fak in cells grown on NS, and increased the gene expression of Itga1 and Itgb1 in cells grown on US and NS. Moreover, FAK protein expression decreased in cells cultured on US but increased in cells cultured on MS and NS after FAK inhibition; no difference in the expression of vinculin was observed among cells grown on all surfaces.

Conclusions

Our data demonstrate the relevance of FAK in the interactions between osteoblastic cells and Ti surfaces regardless of surface topography. Nanotopography positively regulated FAK expression and integrin signaling pathway components during osteoblast differentiation. In this context, the development of Ti surfaces with the ability to upregulate FAK activity could positively impact the process of implant osseointegration.

Periodic-Mechanical-Stimulus Enhanced Osteogenic Differentiation of Mesenchymal Stem Cells on Fe3O4/Mineralized Collagen Coatings

Mechanical stimulus has been demonstrated to be critical to stem cell fate commitment and tissue repair. However, it still remains a challenge to remote control of the mechanical stimulus acting on cells. Here, we designed a magnetic Fe₃O₄/mineralized collagen coating on titanium substrate to regulate the osteogenic differentiation of mesenchymal stem cells (MSCs). The mode and intensity of the mechanical stimulus acting on cells could be controlled by adjusting the remote applied magnetic field. We demonstrated that the adhesion, proliferation, and differentiation of MSCs were strongly dependent on the mode and intensity of the mechanical stimuli. Strikingly, the periodic mechanical stimulus (12 h every other day, PMS) showed the significantly up-regulated expression of osteogenesis-related markers, ALP, compared to that of the static mechanical stimulus mode. The reason is proposed as (1) initially, PMS mode enables the coatings to have appropriate surface mechanical properties for promoting focal adhesion, integrin expression, and cytoskeleton development of MSCs, letting MSCs have good capability of accepting as well as transferring mechanical stimuli; (2) during MSCs growth, PMS mode may effectively manipulate MSCs cytoskeleton development and movement, and mechanotranduction mechanism could be well activated; thus, MSCs osteogenic differentiation is enhanced. This work therefore provides a novel strategy to engineer bioactive coatings with remote control over the intensity and mode of the mechanical stimulus acting on cells, and would have an impact on the design of smart biomaterial surfaces for orthopedic applications.

Splicing factor-modulated generation of mechano growth factor regulates physiological processes in osteoblasts under mechanical stimuli

Mechanical stimuli influence various physiological processes in osteoblasts. We previously showed that mechano-growth factor (MGF), a splicing variant of insulin-like growth factor 1, is highly expressed in osteoblasts in response to mechanical stimuli. This study aims to explore the systemic functions of MGF in osteoblasts, and the mechanisms by which mechanical stress regulates the alternative splicing of Igf1 to generate MGF. We found that MGF promoted the proliferation and migration of osteoblasts, while it inhibited their differentiation via Erk1/2 pathway. Furthermore, cyclic stretching upregulated the expression of ASF/SF2, which in turn regulated the expression of MGF. Our findings indicate that mechanical stimuli influence the physiological responses of osteoblasts by increasing the expression of MGF, which is regulated by splicing factors.

Conditional Activation of NF-κB Inducing Kinase (NIK) in the Osteolineage Enhances Both Basal and Loading-Induced Bone Formation

Studies from global loss-of-function mutants suggest that alternative NF-κB downstream of NF-κB inducing kinase (NIK) is a cell-intrinsic negative regulator of osteogenesis. However, the interpretation of the osteoblast and/or osteocyte contribution to the bone phenotype is complicated by simultaneous osteoclast defects in these models. Therefore, we turned to a transgenic mouse model to investigate the direct role of NIK in the osteolineage. Osx-Cre;NT3 animals (NT3-Cre +), which bear a constitutively active NIK allele (NT3) driven by Osx-Cre, were compared with their Cre-negative, Control (Ctrl) littermates. NT3-Cre + mice had elevated serum P1NP and CTX levels. Despite this high turnover state, µCT showed that constitutive activation of NIK resulted in a net increase in basal bone mass in both cortical and cancellous compartments. Furthermore, NT3-Cre + mice exhibited a greater anabolic response following mechanical loading compared with controls. We next performed RNA-Seq on nonloaded and loaded tibias to elucidate possible mechanisms underlying the increased bone anabolism seen in NT3-Cre + mice. Hierarchical clustering revealed two main transcriptional programs: one loading-responsive and the other NT3 transgene-driven. Gene ontology (GO) analysis indicated a distinct upregulation of receptor, kinase, and growth factor activities including Wnts, as well as a calcium-response signature in NT3-Cre + limbs. The promoters of these GO-term associated genes, including many known to be bone-anabolic, were highly enriched for multiple κB recognition elements (κB-RE) relative to the background frequency in the genome. The loading response in NT3-Cre + mice substantially overlapped (>90%) with Ctrl. Surprisingly, control animals had 10-fold more DEGs in response to loading. However, most top DEGs shared between genotypes had a high incidence of multiple κB-RE in their promoters. Therefore, both transcriptional programs (loading-responsive and NT3 transgene-driven) are modulated by NF-κB. Our studies uncover a previously unrecognized role for NF-κB in the promotion of both basal and mechanically stimulated bone formation. © 2019 American Society for Bone and Mineral Research.

Integrins in disguise - mechanosensors in Saccharomyces cerevisiae as functional integrin analogues

The ability to sense external mechanical stimuli is vital for all organisms. Integrins are transmembrane receptors that mediate bidirectional signalling between the extracellular matrix (ECM) and the cytoskeleton in animals. Thus, integrins can sense changes in ECM mechanics and can translate these into internal biochemical responses through different signalling pathways. In the model yeast species Saccharomyces cerevisiae there are no proteins with sequence similarity to mammalian integrins. However, we here emphasise that the WSC-type (Wsc1, Wsc2, and Wsc3) and the MID-type (Mid2 and Mtl1) mechanosensors in yeast act as partial functional integrin analogues. Various environmental cues recognised by these mechanosensors are transmitted by a conserved signal transduction cascade commonly referred to as the PKC1-SLT1 cell wall integrity (CWI) pathway. We exemplify the WSC- and MID-type mechanosensors functional analogy to integrins with a number of studies where they resemble the integrins in terms of both mechanistic and molecular features as well as in the overall phenotypic consequences of their activity. In addition, many important components in integrin-dependent signalling in humans are conserved in yeast; for example, Sla1 and Sla2 are homologous to different parts of human talin, and we propose that they together might be functionally similar to talin. We also propose that the yeast cell wall is a prominent cellular feature involved in sensing a number of external factors and subsequently activating different signalling pathways. In a hypothetical model, we propose that nutrient limitations modulate cell wall elasticity, which is sensed by the mechanosensors and results in filamentous growth. We believe that mechanosensing is a somewhat neglected aspect of yeast biology, and we argue that the physiological and molecular consequences of signal transduction initiated at the cell wall deserve more attention.

Effect of Duration of Exposure to Fluoride and Type of Diet on Lipid Parameters and De Novo Lipogenesis

The effect of duration of chronic treatment with fluoride (F, 50 mg/L as NaF) on the lipid profile, lipid droplets and triglycerides (TG) in liver was evaluated in mice with nonalcoholic fatty liver disease (NAFLD) previously induced by hyperlipidic diet and in animals fed normocaloric diet. In addition, the effect of F administered for a short period (20 days) was evaluated on de novo lipogenesis, by nuclear magnetic resonance. GRP78, Apo-E, and sterol regulatory element-binding protein (SREBP) were quantified by Western blotting. Our data indicate that F interferes in lipid metabolism and lipid droplets, having a different action depending on the exposure time and type of diet administered. F improved lipid parameters and reduced steatosis only when administered for a short period of time (up to 20 days) to animals fed normocaloric diet. However, when NAFLD was already installed, lipid parameters were only slightly improved at 20 days of treatment, but no effect was observed on the degree of steatosis. In addition, lipid profile was in general impaired when the animals were treated with F for 30 days, regardless of the diet. Moreover, F did not alter de novo lipogenesis in animals with installed NAFLD. Furthermore, hyperlipidic diet increased F accumulation in the body. GRP78 increased, while Apo-E and SREBP decreased in the F-treated groups. Our results provide new insights on how F affects lipid metabolism depending on the available energy source.

PTH1R signalling regulates the mechanotransduction process of cementoblasts under cyclic tensile stress

Objective

To investigate the regulatory role of type I parathyroid hormone receptor (PTH1R) signalling in the mechanotransduction process of cementoblasts under cyclic tensile stress (CTS).

Materials and methods

Immortalized cementoblast cell line OCCM-30 were employed and subjected to cyclic tensile strain applied by a four-point bending system. The expression of PTHrP and PTH1R, as well as cementoblastic transcription factor Runx-2, Osterix, and extracellular matrix protein COL-1 and OPN were assessed by quantitative real-time polymerase chain reaction and western blot analysis. PTH1R expression was knocked down by siPTH1R transfection, and the alteration of cementoblastic biomarkers expression was examined to evaluate the function of PTH1R. Furthermore, to investigate possible downstream molecules, expression of signal molecule ERK1/2 with or without siPTH1R transfection, and the effect of ERK inhibitor PD98059 on the expression of cementoblastic biomarkers was also examined.

Results

Cyclic tensile strain elevated the expression of PTHrP and PTH1R, as well as cementoblastic biomarkers Runx-2, Osterix, COL-1, and OPN in a time-dependent manner, which was inhibited by siPTH1R transfection. The expression of phosphorylated ERK1/2 was upregulated time-dependently under cyclic stretch, which was also inhibited by siPTH1R transfection, and pretreatment of p-ERK1/2 inhibitor PD98059 undermined the increase of Runx-2, Osterix, COL-1, and OPN prominently.

Conclusion

The findings of the present study indicate that PTH1R signalling plays a regulatory role in the CTS induced cementoblastic differentiation in mature cementoblasts, and ERK1/2 is essentially involved as a downstream intracellular signal molecule in this mechanotransduction process.

Low-level fluoride exposure reduces glycemia in NOD mice

Water fluoridation is the most widespread measure to prevent dental caries but its relationship with the development of type-1 diabetes (T1D), which has been increasing by 2-5% worldwide, is not quite well understood.

AIM

This study evaluated if fluoride (F) administered in the drinking water can prevent or reduce the development of T1D in non-obese diabetic (NOD) mice, as well as to explore the underlying mechanisms.

MATERIALS AND METHODS

Twenty-four weaning NOD mice received water containing 0, 10 or 50 ppm F for 21 days. Plasma glucose and insulin were analyzed. Quantitative proteomic analysis was conducted in the liver and gastrocnemius muscle.

RESULTS

Animals treated with 10 ppm F had significantly lower glucose levels than the control group, but there was no significant difference among the groups in relation to insulin. The % of β-cell function was significantly higher in the 10 ppm F group. Changes in the proteomic profile of muscle and liver were seen among the groups. In the muscle, the 10 ppm F group presented, when compared with control, increased expression of proteins involved in energy metabolism. The 50 ppm F group, compared with control, presented increased expression of proteins related to muscle contraction, differentiation of brown adipose tissue and apoptosis. For the liver, the 10 ppm F group had increase in proteins involved in energy metabolism and protein synthesis, in respect to control. There was also an increase in isoforms of Glutathione S transferase, which was confirmed by Western blotting. In the group treated with 50 ppm F, proteins related to ROS metabolism and energetic metabolism were altered.

CONCLUSION

Increased expression of antioxidant proteins by treatment with low F concentration may possibly help to explain protection against the development of T1D, which should be better explored in future mechanistic studies.

Molecular Basis for Periodontal Ligament Adaptation to In Vivo Loading

A soft food diet leads to changes in the periodontal ligament (PDL). These changes, which have been recognized for more than a century, are ascribed to alterations in mechanical loading. While these adaptive responses have been well characterized, the molecular, cellular, and mechanical mechanisms underlying the changes have not. Here, we implicate Wnt signaling in the pathoetiology of PDL responses to underloading. We show that Wnt-responsive cells and their progeny in the PDL space exhibit a burst in proliferation in response to mastication. If an animal is fed a soft diet from the time of weaning, then this burst in Wnt-responsive cell proliferation is quelled; as a consequence, both the PDL and the surrounding alveolar bone undergo atrophy. Returning these animals to a hard food diet restores the Wnt signaling in PDL. These data provide, for the first time, a molecular mechanism underlying the adaptive response of the PDL to loading.

Quantitative in situ time-series evaluation of osteoblastic collagen synthesis under cyclic strain using second-harmonic-generation microscopy

The aim of this study is to evaluate the osteoblastic collagen synthesis under mechanical stimulation using second-harmonic-generation (SHG) microscopy. We apply SHG microscopy to monitor the collagen fibers synthesized by osteoblast-like cells (MC3T3-E1) without the need for fixation and staining. To quantitatively evaluate the influence of mechanical stimulation on osteoblastic collagen synthesis, we compare SHG images of osteoblast-synthesized collagen fibers with and without a cyclic stretch stimulus applied using a lab-made stretching device. We acquire SHG images every 7 days for 3 weeks at different stimulus conditions (5 min/day and 3 h/day with a strain magnitude of 5% and a frequency of 0.5 Hz). Image analysis of the average SHG intensity indicates that the amount of osteoblastic collagen synthesis is significantly enhanced by the cyclic stretch compared with the nonstretched condition, while there is no significant difference between the two mechanical stimulation conditions. Furthermore, the maturity of the collagen fibers in the early stage of bone formation is not affected by the mechanical stimulation. The results can be used in bone regenerative medicine to apply feedback control of collagen synthesis by artificial stimulation.

[Bone mechanobiology, an emerging field: a review]

INTRODUCTION

Mechanobiology, at the interface between biology and biophysics, studies the impact of mechanical forces on tissues, cells and biomolecules. The application of orthodontic forces, followed by induced tooth displacement, is a striking example of its clinical application.

OBJECTIVE

The purpose of this article was to compile a review of the literature on the subject of mechanobiology; from its detection at bone level to the presentation of stimulated intracellular pathways.

MATERIALS AND METHODS

The literature search was conducted on the Pubmed database in April 2018, with associations of the terms "mechanobiology", "orthodontics", "cell culture", "physiopathology".

RESULTS

Three major areas of research were selected: highlighting of the phenomenon and its application in the field of bone biology; the cellular effectors of mechanobiology and its clinical applications. The use of mechanobiology in dentofacial orthopedics opens up a new field of reflection for clinicians regarding future advances in orthodontics.

Identification and characterization of circular RNAs involved in mechanical force-induced periodontal ligament stem cells

Circular RNAs (circRNAs) play critical roles in signal transduction during cell proliferation, differentiation, and apoptosis in a posttranscriptional manner. Recently, circRNAs have been proved to be a large class of animal RNAs with regulatory potency. However, whether circRNAs can respond to mechanical force (MF) and impact on human periodontal ligament stem cells (PDLSCs) and the orthodontic tooth movement (OTM) process remain unknown. Here, we investigated the circRNAs expression patterns in PDLSCs induced by MF and found that circRNAs were responsive to the MF in PDLSCs. Through the valid reads' distribution analysis, we found that the majority of reads in both the control PDLSCs and the MF-induced PDLSCs were distributed in exons. Then we analyzed Gene Ontology terms of genes that overlap with or are neighbors of the stress-responsive circRNAs and found unique enrichment patterns in biological processes, molecular function, and cellular component of PDLSCs. Next, we predicted the possible functions of circRNAs through circRNAs-miRNAs networks. We found that one circRNA may regulate one or several miRNA/miRNAs and one miRNA may interact with one or multiple circRNA/circRNAs. Importantly, a number of circRNAs were predicted to directly or indirectly regulate miRNAs-mediated osteogenic differentiation in mesenchymal stem cells. For instance, circRNA3140 was highly and widely associated with microRNA-21, which plays a critical role in MF-induced osteogenic differentiation of PDLSCs. Taken together, these findings reveal a previously unrecognized mechanism that MF can induce the expression changes of circRNAs in PDLSCs, which may modulate the OTM process and the alveolar bone remodeling.

Mechanical Strain Regulates Osteoblast Proliferation Through Ca2+-CaMK-CREB Signal Pathway

Objective To investigate the effects of mechanical strain on Ca²⁺-calmodulin dependent kinase (CaMK)-cAMP response element binding protein (CREB) signal pathway and proliferation of osteoblasts.Methods Using a four-point bending device, MC3T3-E1 cells were exposed to mechanical tensile strains of 2500 µs and 5000 µs at 0.5 Hz respectively. The intracellular free Ca²⁺ ([Ca²⁺]i) concentration and calmodulin activity were assayed by fluorospectrophotometry, CaMK II β, CREB, and phosphorylated (activated) CREB (p-CREB) were assessed by Western blot, and cells proliferation was assayed with MTT. Pretreatment with verapamil was carried out to block Ca²⁺ channel, and inhibitor U73122 was used to inhibit phospholipase C (PLC).Results Mechanical strains of 2500 µs and 5000 µs for 1 to 10 minutes both increased [Ca²⁺]i level of the cells. The 2500 µs strain, a periodicity of 1 h/d for 3 days, activated calmodulin, elevated protein levels of CaMK II β and p-CREB, and promoted cells proliferation, which were attenuated by pretreatment of verapamil or U73122. The effects of 5000 µs strain on calmodulin, CaMK II β, p-CREB and proliferation were contrary to 2500 µs strain.Conclusion The mechanical strain regulates osteoblasts proliferation through Ca²⁺-CaMK-CREB signal pathway via Ca²⁺ channel and PLC/IP₃ transduction cascades.

The major β-catenin/E-cadherin junctional binding site is a primary molecular mechano-transductor of differentiation in vivo

In vivo, the primary molecular mechanotransductive events mechanically initiating cell differentiation remain unknown. Here we find the molecular stretching of the highly conserved Y654-β-catenin-D665-E-cadherin binding site as mechanically induced by tissue strain. It triggers the increase of accessibility of the Y654 site, target of the Src42A kinase phosphorylation leading to irreversible unbinding. Molecular dynamics simulations of the β-catenin/E-cadherin complex under a force mimicking a 6 pN physiological mechanical strain predict a local 45% stretching between the two α-helices linked by the site and a 15% increase in accessibility of the phosphorylation site. Both are quantitatively observed using FRET lifetime imaging and non-phospho Y654 specific antibody labelling, in response to the mechanical strains developed by endogenous and magnetically mimicked early mesoderm invagination of gastrulating Drosophila embryos. This is followed by the predicted release of 16% of β-catenin from junctions, observed in FRAP, which initiates the mechanical activation of the β-catenin pathway process.

Fabrication and optimization of Nanodiamonds-composited poly(ε-caprolactone) fibrous matrices for potential regeneration of hard tissues

Background

Electrospun fibrous matrices are of great importance for tissue engineering and drug delivery device. However, relatively low mechanical strength of the fibrous matrix is one of the major disadvantages. NDs with a positive charge were selected to enhance the mechanical property of a composited fibrous matrix by inducing the intermolecular interaction between NDs and polymer chain. We prepared ND-composited poly (ε-caprolactone) (PCL) fibrous matrices by electrospinning and evaluated their performance in terms of mechanical strength and cell behaviors.

Methods

A predetermined amounts of NDs (0.5, 1, 2 and 3 wt%) were added into PCL solution in a mixture of chloroform and 2,2,2-trifluoroethanol (8:2). ND-composited PCL (ND/PCL) fibrous matrices were prepared by electrospinning method. The tensile properties of the ND/PCL fibrous matrices were analyzed by using a universal testing machine. Mouse calvaria-derived preosteoblast (MC3T3-E1) was used for cell proliferation, alkaline phosphatase (ALP) assay, and Alizarin Red S staining.

Results

The diameters of the fibrous matrices were adjusted to approximately 1.8 μm by changing process variables. The intermolecular interaction between NDs and PCL polymers resulted in the increased tensile strength and the favorable interfacial adhesion in the ND/PCL fibrous matrices. The ND/PCL fibrous matrix with 1 wt% of ND had the highest tensile strength among the samples and also improved proliferation and differentiation of MC3T3-E1 cells.

Conclusions

Compared to the other samples, the ND/PCL fibrous matrix with 1 wt% of ND concentration exhibited superior performances for MC3T3 cells. The ND/PCL fibrous matrix can be potentially used for bone and dental tissue engineering.

N-AC-l-Leu-PEI-mediated miR-34a delivery improves osteogenic differentiation under orthodontic force

Rare therapeutic genes or agents are reported to control orthodontic bone remodeling. MicroRNAs have recently been associated with bone metabolism. Here, we report the in vitro and in vivo effects of miR-34a on osteogenic differentiation under orthodontic force using an N-acetyl-L-leucine-modified polyethylenimine (N-Ac-l-Leu-PEI) carrier. N-Ac-l-Leu-PEI exhibited low cytotoxicity and high miR-34a transfection efficiency in rat bone mineral stem cells and local alveolar bone tissue. After transfection, miR-34a enhanced the osteogenic differentiation of Runx2 and ColI, Runx2 and ColI protein levels, and early osteogenesis function under orthodontic strain in vitro. MiR-34a also enhanced alveolar bone remodeling under orthodontic force in vivo, as evidenced by elevated gene and protein expression, upregulated indices of alveolar bone anabolism, and diminished tooth movement. We determined that the mechanism miR-34a in osteogenesis under orthodontic force may be associated with GSK-3β. These results suggested that miR-34a delivered by N-Ac-l-Leu-PEI could be a potential therapeutic target for orthodontic treatment.

Antagonism of cysteinyl leukot-riene receptor 1 (cysLT1R) by montelukast regulates differentiation of MC3T3-E1 cells under overloaded mechanical environment

Long-term exposure to overloaded mechanical environment induces bone fatigue damage symptoms and osteoblast damages. Montelukast is a selective cysteinyl leukot-riene receptor 1 (cysLT1R) antagonist, which has been used for the treatment of bronchial asthma in clinics. In the current study, we have identified a novel pharmacological role of montelukast by finding that it has protective properties against overload damage in osteoblastic MC3T3-E1 cells. Firstly, our results show that CysLT1R is expressed in MC3T3-E1 cells. Mechanical tensile strain of 5000-7000 με resulted in a significant upregulation of CysLT1R in osteoblastic MC3T3-E1 cells in an intensity dependent manner. Secondly, MTT assay indicates that loading with 5000 με mechanical strain inhibited cell proliferation, which was suppressed by montelukast treatment. Furthermore, montelukast promotes cell differentiation by increasing the expression of ALP and RUNX2. Alizarin Red S staining assay showed that montelukast abolished the inhibitory effects of overload mechanics on osteoblast mineralization. Mechanistically, the effect of montelukast on osteoblastic differentiation acted by activating the extracellular regulated protein kinases (ERK) pathway. The obtained results suggested that montelukast promotes proliferation and differentiation in osteoblasts exposed to overload mechanics.

The Effect of Uniaxial Mechanical Stretch on Wnt/β-Catenin Pathway in Bone Mesenchymal Stem Cells

Wnt/β-catenin signal is required in bone formation and remodling, but little is known about whether Wnt/β-catenin signal could promote osteogenic differentiation of rat bone marrow mesenchymal stem cells (BMSCs) after uniaxial mechanical stretch. In this study, rat BMSCs were identified by flow cytometry and used for mechanical stretch. A custom-made uniaxial dynamic stretch apparatus was applied for rat BMSCs stretch. There were 2 groups in the study: the mechanical stretch group and the nonstretch control group. Cell morphology, alkaline phosphatase (ALP) activity, mRNA levels (Wnt3a, Lrp5, β-catenin, as well as Runx2 were evaluated using quantitative real-time reverse transcription-polymerase chain reaction) and protein levels (β-catenin and Runx2 were examined using western blot) were observed in both groups. The authors finally found that not only the cell proliferation, ALP activity, but also mRNA expression of Wnt3a, Lrp5, β-catenin, and Runx2 in BMSCs were markedly elevated by mechanical stretch than the controls. Protein levels of β-catenin and Runx2 were significantly higher than that of control as well. Activation of mechanical stretch was partially reversed by DKK-1, a classical inhibitor of Wnt/β-catenin signal. These results demonstrate that uniaxial mechanical stretch could stimulate osteogenic differentiation and proliferation of BMSCs by activating the Wnt/β-catenin signaling.

Mechanical Stress Regulates Osteogenesis and Adipogenesis of Rat Mesenchymal Stem Cells through PI3K/Akt/GSK-3β/β-Catenin Signaling Pathway

Osteogenesis and adipogenesis of bone marrow mesenchymal stem cells (BMSCs) are regarded as being of great importance in the regulation of bone remodeling. In this study, rat BMSCs were exposed to different levels of cyclic mechanical stress generated by liquid drops and cultured in general medium or adipogenic medium. Markers of osteogenic (Runx2 and Collagen I) and adipogenic (C/EBPα, PPARγ, and lipid droplets) differentiation were detected using Western blot and histological staining. The protein levels of members of the phosphatidylinositol 3-kinase (PI3K)/Akt/glycogen synthase kinase 3β (GSK-3β)/β-catenin signaling pathway were also examined. Results showed that small-magnitude stress significantly upregulated Runx2 and Collagen I and downregulated PPARγ and C/EBPα expression in BMSCs cultured in adipogenic medium, while large-magnitude stress reversed the effect when compared with unloading groups. The PI3K/Akt signaling pathway could be strongly activated by mechanical stimulation; however, large-magnitude stress led to decreased activation of the signaling pathway when compared with small-magnitude stress. Activation of β-catenin with LiCl led to increased expression of Runx2 and Collagen I and reduction of C/EBPα and PPARγ expression in BMSCs. Inhibition of PI3K/Akt signaling partially blocked the expression of β-catenin. Taken together, our results indicate that mechanical stress-regulated osteogenesis and adipogenesis of rat BMSCs are mediated, at least in part, by the PI3K/Akt/GSK-3β/β-catenin signaling pathway.

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.

YAP and ERK mediated mechanical strain-induced cell cycle progression through RhoA and cytoskeletal dynamics in rat growth plate chondrocytes

Yes-associated protein (YAP) and extracellular signal-regulated kinase (ERK) have been considered as key regulators in tissue homeostasis, organ development, and tumor formation. However, the roles of YAP and ERK in the mediating strain mechanosensing in the growth plate cartilage have not been determined. In this study, chondrocytes obtained from the growth plate cartilage of 2-week-old Sprague-Dawley rats were subjected to the mechanical strain with different magnitudes and durations at a frequency of 0.5 Hz. We found that YAP and ERK activation in response to mechanical strain was time and magnitude dependent. Pretreatment with a RhoA inhibitor (C3 toxin) or a microfilament cytoskeleton disrupting reagent (cytochalasin D) could suppress their activation. In addition, activated YAP and ERK were able to induce cell cycle progression by up-regulating the expression of cell cycle-related genes. These results shed new light on the function of YAP and ERK in mechanical strain-promoted growth plate development. Our results also provided evidence that RhoA and cytoskeletal dynamics are required for this mechanotransduction. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1121-1129, 2016.

Tensile force on human macrophage cells promotes osteoclastogenesis through receptor activator of nuclear factor κB ligand induction

Little is known about the effects of tensile forces on osteoclastogenesis by human monocytes in the absence of mechanosensitive cells, including osteoblasts and fibroblasts. In this study we consider the effects of tensile force on osteoclastogenesis in human monocytes. The cells were treated with receptor activator of nuclear factor κB ligand (RANKL) to promote osteoclastogenesis. Then,expression and secretion of cathepsin K were examined. RANKL and the formation of osteoclasts during the osteoclast differentiation process under continual tensile stress were evaluated by Western blot. It was also found that -100 kPa or lower induces RANKL-enhanced tartrate-resistant acid phosphatase activity in a dose-dependent manner. Furthermore, an increased tensile force raises the expression and secretion of cathepsin K elevated by RANKL, and is concurrent with the increase of TNF-receptor-associated factor 6 induction and nuclear factor κB activation. Overall, the current report demonstrates that tensile force reinforces RANKL-induced osteoclastogenesis by retarding osteoclast differentiation. The tensile force is able to modify every cell through dose-dependent in vitro RANKL-mediated osteoclastogenesis, affecting the fusion of preosteoclasts and function of osteoclasts. However, tensile force increased TNF-receptor-associated factor 6 expression. These results are in vitro findings and were obtained under a condition of tensile force. The current results help us to better understand the cellular roles of human macrophage populations in osteoclastogenesis as well as in alveolar bone remodeling when there is tensile stress.

Fluoride Intensifies Hypercaloric Diet-Induced ER Oxidative Stress and Alters Lipid Metabolism

BACKGROUND

Here, we evaluated the relationship of diet and F-induced oxidative stress to lipid metabolism in the liver of rats eating normocaloric or hypercaloric diets for two time periods (20 or 60 days).

METHODS

Seventy-two 21-day-old Wistar rats were divided into 2 groups (n = 36) based on the type of diet they were eating; each of these groups was then further divided into another two groups (n = 18) based on the time periods of either 20 or 60 days, for a total of four groups. Each of these was divided into 3 subgroups (n = 6 animals/subgroup), dependent on the dose of F administered in the drinking water (0 mg/L(control), 15 mg/L or 50 mg/L). After the experimental period, blood samples and the liver were collected. Plasma samples were analyzed for HDL, cholesterol and triglycerides. Western blots were performed to probe for GRP78, Erp29, SOD2, Apo-E and SREBP in hepatic tissues.

RESULTS

As expected,the expression of target proteins involved in oxidative stress increased in the F-treated groups, especially in liver tissue obtained from animals eating a hypercaloric diet. Most changes in the lipid levels and pathological conditions were seen earlier in the time period, at day 20. The morphometric analyses showed a reduction in steatosis in groups on ahypercaloric diet and treated with 50 mg F/L compared to the control, while no changes were obtained in normocaloric-fed rats. Accordingly, plasma TG was reduced in the F-treated group. The reduced expression of Apo-E in a time- and diet-dependent pattern may account for the particular decrease in steatosis in hypercaloric-fed F-treated rats.

CONCLUSIONS

These results suggest that F changes liver lipid homeostasis, possibly because of the induction of oxidative stress, which seems to be higher in animals fed hypercaloric diets.