Biomechanical stimulation of osteoblast gene expression requires phosphorylation of the RUNX2 transcription factor

Molecular and Biological Aspects of Orthodontic Tooth Movement: Possibilities for Bioengineering Intervention: A Narrative Review

BACKGROUND

The current review's goal is to examine, with a critical eye, the effect of various biomedical parameters on orthodontic tooth movement in an attempt to provide the reader with related mechanisms of this issue focusing on certain key points.

METHODS

This critical review was conducted using the following keywords in the search strategy: "biomedical molecules", "biomarkers", "orthodontics", "orthodontic tooth movement", "acceleration", "gene therapy", and "stem cells". Cochrane Library, Medline (PubMed), and Scopus were the databases that were used for the electronic search. Studies published until June 2023 were considered.

RESULTS

The use of biomedical approaches in orthodontic tooth movement has been investigated via different procedures and applications. Surgical approaches, biomarkers affecting orthodontic tooth movement, different biological events and mechanisms, RANK, RANK-L, OPG molecular triad, and vibration methods are the basic parameters of biomedical interventions that are examined in the present review.

CONCLUSIONS

The biomedical approach seems to offer a variety of applications to control orthodontic tooth movement. The scarcity of human studies, as well as the high cost and complexity of these methods, currently limit the available accurate data concerning this issue.

A novel CircRNA Circ_0001722 regulates proliferation and invasion of osteosarcoma cells through targeting miR-204-5p/RUNX2 axis

BACKGROUND

Osteosarcoma (OS) is the most prevalent primary fatal bone neoplasm in adolescents and children owing to limited therapeutic methods. Circular RNAs (circRNAs) are identified as vital regulators in a variety of cancers. However, the roles of circRNAs in OS are still unclear.

METHODS

Firstly, we evaluate the differentially expressed circRNAs in 3 paired OS and corresponding adjacent nontumor tissue samples by circRNA microarray assay, finding a novel circRNA, circ_001722, significantly upregulated in OS tissues and cells. The circular structure of candidate circRNA was confirmed through Sanger sequencing, divergent primer PCR, and RNase R treatments. Proliferation of OS cells was evaluated in vitro and in vivo. The microRNA (miRNA) sponge mechanism of circRNAs was verified by dual-luciferase assay and RNA immunoprecipitation assay.

RESULTS

A novel circRNA, circ_001722, is significantly upregulated in OS tissues and cells. Downregulation of circ_0001722 can suppress proliferation and invasion of human OS cells in vitro and in vivo. Computational algorithms predict miR-204-5p can bind with circ_0001722 and RUNX2 mRNA 3'UTR, which is verified by Dual-luciferase assay and RNA immunoprecipitation assay. Further functional experiments show that circ_0001722 competitively binds to miR-204-5p and prevents it to decrease the level of RUNX2, which upregulates proliferation and invasion of human OS cells.

CONCLUSION

Circ_001722 is a novel tumor promotor in OS, and promotes the progression of OS via miR-204-5p/RUNX2 axis.

Incorporation of strontium-containing bioactive particles into PEOT/PBT electrospun scaffolds for bone tissue regeneration

The combination of biomaterials and bioactive particles has shown to be a successful strategy to fabricate electrospun scaffolds for bone tissue engineering. Among the range of bioactive particles, hydroxyapatite and mesoporous bioactive glasses (MBGs) have been widely used for their osteoconductive and osteoinductive properties. Yet, the comparison between the chemical and mechanical characteristics as well as the biological performances of these particle-containing scaffolds have been characterized to a limited extent. In this work, we fabricated PEOT/PBT-based composite scaffolds incorporating either nanohydroxyapatite (nHA), strontium-containing nanohydroxyapatite (nHA_Sr) or MBGs doped with strontium ions up to 15 wt./vol% and 12,5 wt./vol% for nHA and MBG, respectively. The composite scaffolds presented a homogeneous particle distribution. Morphological, chemical and mechanical analysis revealed that the introduction of particles into the electrospun meshes caused a decrease in the fiber diameter and mechanical properties, yet maintaining the hydrophilic nature of the scaffolds. The Sr²⁺ release profile differed according to the considered system, observing a 35-day slowly decreasing release from strontium-containing nHA scaffolds, whereas MBG-based scaffolds showed a strong burst release in the first week. In vitro, culture of human bone marrow-derived mesenchymal stromal cells (hMSCs) on composite scaffolds demonstrated excellent cell adhesion and proliferation. In maintenance and osteogenic media, all composite scaffolds showed high mineralization as well as expression of Col I and OCN compared to PEOT/PBT scaffolds, suggesting their ability to boost bone formation even without osteogenic factors. The presence of strontium led to an increase in collagen secretion and matrix mineralization in osteogenic medium, while gene expression analysis showed that hMSCs cultured on nHA-based scaffolds had a higher expression of OCN, ALP and RUNX2 compared to cells cultured on nHA_Sr scaffolds in osteogenic medium. Yet, cells cultured on MBGs-based scaffolds showed a higher gene expression of COL1, ALP, RUNX2 and BMP2 in osteogenic medium compared to nHA-based scaffolds, which is hypothesized to lead to high osteoinductivity in long term cultures.

How Mechanical and Physicochemical Material Characteristics Influence Adipose-Derived Stem Cell Fate

Adipose-derived stem cells (ASCs) are a subpopulation of mesenchymal stem cells. Compared to bone marrow-derived stem cells, they can be harvested with minimal invasiveness. ASCs can be easily expanded and were shown to be able to differentiate into several clinically relevant cell types. Therefore, this cell type represents a promising component in various tissue engineering and medical approaches (e.g., cell therapy). In vivo cells are surrounded by the extracellular matrix (ECM) that provides a wide range of tissue-specific physical and chemical cues, such as stiffness, topography, and chemical composition. Cells can sense the characteristics of their ECM and respond to them in a specific cellular behavior (e.g., proliferation or differentiation). Thus, in vitro biomaterial properties represent an important tool to control ASCs behavior. In this review, we give an overview of the current research in the mechanosensing of ASCs and current studies investigating the impact of material stiffens, topography, and chemical modification on ASC behavior. Additionally, we outline the use of natural ECM as a biomaterial and its interaction with ASCs regarding cellular behavior.

Mimicking bone matrix through coaxial electrospinning of core-shell nanofibrous scaffold for improving neurogenesis bone regeneration

There is a significant clinical demand for bone repair materials with high efficacy. This study was designed to fabricate nanofibrous scaffolds to promote bone defect regeneration using magnesium doped mesoporous bioactive glass (MBG), a fusion protein Osteocalcin-Osteopontin-Biglycan (OOB), silk fibroin (SF) and nerve growth factor (NGF) for facilitating accelerated bone formation. We found that MBG adsorbed with OOB (OOB@MBG) as core, and SF adsorbed with NGF (SF@NGF) as shell to fabricate the nanofibrous scaffolds (OOB@MBG/NGF@SF) through coaxial electrospinning. OOB@MBG/NGF@SF scaffolds could effectively mimic the component and structure of bone matrix. Interestingly, we observed that OOB@MBG/NGF@SF scaffolds could substantially promote bone mesenchymal stem cells (BMSCs) osteogenesis through stimulating Erk1/2 activated Runx2 and mTOR pathway, and it could also activate the expression level of various osteogenic marker genes. Intriguingly, OOB@MBG/NGF@SF scaffolds could also enhance BMSCs induced neural differentiation cells differentiated into neuron, and activate the expression of the different neuron specific marker genes. Moreover, it was found that OOB@MBG/NGF@SF scaffolds accelerated bone regeneration with neurogenesis, and new neurons were formed in Haversian canal in vivo. Consistent with these observations, we found that Erk1/2 and mTOR signaling pathways also regulated osteogenesis with the neurogenesis process from RNA sequencing result. Overall, our findings provided novel evidence suggesting that OOB@MBG/NGF@SF scaffolds could function as a potential biomaterial in accelerating bone defect regeneration with neurogenesis, as well as in recovering the motor ability and improving the quality of life of patients.

From mesenchymal niches to engineered in vitro model systems: Exploring and exploiting biomechanical regulation of vertebrate hedgehog signalling

Tissue patterning is the result of complex interactions between transcriptional programs and various mechanical cues that modulate cell behaviour and drive morphogenesis. Vertebrate Hedgehog signalling plays key roles in embryogenesis and adult tissue homeostasis, and is central to skeletal development and the osteogenic differentiation of mesenchymal stem cells. The expression of several components of the Hedgehog signalling pathway have been reported to be mechanically regulated in mesodermal tissue patterning and osteogenic differentiation in response to external stimulation. Since a number of bone developmental defects and skeletal diseases, such as osteoporosis, are directly linked to aberrant Hedgehog signalling, a better knowledge of the regulation of Hedgehog signalling in the mechanosensitive bone marrow-residing mesenchymal stromal cells will present novel avenues for modelling these diseases and uncover novel opportunities for extracellular matrix-targeted therapies. In this review, we present a brief overview of the key molecular players involved in Hedgehog signalling and the basic concepts of mechanobiology, with a focus on bone development and regeneration. We also highlight the correlation between the activation of the Hedgehog signalling pathway in response to mechanical cues and osteogenesis in bone marrow-derived mesenchymal stromal cells. Finally, we propose different tissue engineering strategies to apply the expanding knowledge of 3D material-cell interactions in the modulation of Hedgehog signalling in vitro for fundamental and translational research applications.

Effect of high content nanohydroxyapatite composite scaffolds prepared via melt extrusion additive manufacturing on the osteogenic differentiation of human mesenchymal stromal cells

The field of bone tissue engineering seeks to mimic the bone extracellular matrix composition, balancing the organic and inorganic components. In this regard, additive manufacturing (AM) of high content calcium phosphate (CaP)-polymer composites holds great promise towards the design of bioactive scaffolds. Yet, the biological performance of such scaffolds is still poorly characterized. In this study, melt extrusion AM (ME-AM) was used to fabricate poly(ethylene oxide terephthalate)/poly(butylene terephthalate) (PEOT/PBT)-nanohydroxyapatite (nHA) scaffolds with up to 45 wt% nHA, which presented significantly enhanced compressive mechanical properties, to evaluate their in vitro osteogenic potential as a function of nHA content. While osteogenic gene upregulation and matrix mineralization were observed on all scaffold types when cultured in osteogenic media, human mesenchymal stromal cells did not present an explicitly clear osteogenic phenotype, within the evaluated timeframe, in basic media cultures (i.e. without osteogenic factors). Yet, due to the adsorption of calcium and inorganic phosphate ions from cell culture media and simulated body fluid, the formation of a CaP layer was observed on PEOT/PBT-nHA 45 wt% scaffolds, which is hypothesized to account for their bone forming ability in the long term in vitro, and osteoconductivity in vivo.

Regulation of transforming growth factor-β1-stimulation of Runx2 acetylation for matrix metalloproteinase 13 expression in osteoblastic cells

Transforming growth factor beta 1 (TGF-β1) functions as a coupling factor between bone development and resorption. Matrix metalloproteinase 13 (MMP13) is important in bone remodeling, and skeletal dysplasia is caused by a deficiency in MMP13 expre-ssion. Runx2, a transcription factor is essential for bone development, and MMP13 is one of its target genes. TGF-β1 promoted Runx2 phosphorylation, which was necessary for MMP13 production in osteoblastic cells, as we previously shown. Since the phosphorylation of some proteins causes them to be degraded by the ubiquitin/proteasome pathway, we hypothesized that TGF-β1 might stabilize the phosphorylated Runx2 protein for its activity by other post-translational modification (PTM). This study demonstrated that TGF-β1-stimulated Runx2 acetylation in rat osteoblastic cells. p300, a histone acetyltransferase interacted with Runx2, and it promoted Runx2 acetylation upon TGF-β1-treatment in these cells. Knockdown of p300 decreased the TGF-β1-stimulated Runx2 acetylation and MMP13 expression in rat osteoblastic cells. TGF-β1-treatment stimulated the acetylated Runx2 bound at the MMP13 promoter, and knockdown of p300 reduced this effect in these cells. Overall, our studies identified the transcriptional regulation of MMP13 by TGF-β1 via Runx2 acetylation in rat osteoblastic cells, and these findings contribute to the knowledge of events presiding bone metabolism.

Epigenetic modifications of histones during osteoblast differentiation

In bone biology, epigenetics plays a key role in mesenchymal stem cells' (MSCs) commitment towards osteoblasts. It involves gene regulatory mechanisms governed by chromatin modulators. Predominant epigenetic mechanisms for efficient osteogenic differentiation include DNA methylation, histone modifications, and non-coding RNAs. Among these mechanisms, histone modifications critically contribute to altering chromatin configuration. Histone based epigenetic mechanisms are an essential mediator of gene expression during osteoblast differentiation as it directs the bivalency of the genome. Investigating the importance of histone modifications in osteogenesis may lead to the development of epigenetic-based remedies for genetic disorders of bone. Hence, in this review, we have highlighted the importance of epigenetic modifications such as post-translational modifications of histones, including methylation, acetylation, phosphorylation, ubiquitination, and their role in the activation or suppression of gene expression during osteoblast differentiation. Further, we have emphasized the future advancements in the field of epigenetics towards orthopaedical therapeutics.

Design of novel functionalized collagen-chitosan-MBG scaffolds for enhancing osteoblast differentiation in BMSCs

Collagen and chitosan are two different kinds of natural biodegradable polymers commonly used in the regeneration of bone defects. Mesoporous bioactive glass (MBG) is a type of favorable bone filler which can effectively constitute an enlarged microenvironment to facilitate an exchange of important factors between the cells and scaffolds. Here we prepared a collagen-chitosan-MBG (C-C-MBG) scaffold which displayed significantly increased proliferation, differentiation and mineralization in bone mesenchymal stem cells (BMSCs). Additionally, we found that the scaffold can stimulate extra-cellular signal regulated kinase 1/2 (Erk1/2) activated Runx2 pathway, which is the predominant signaling pathway involved in osteoblast differentiation. Consistently, we observed that the scaffold can markedly enhance the expression ofType I collagen, Osteopontin(Opn), andRunx2, which are important osteoblastic marker genes implicated in the process of osteoblast differentiation. Therefore, we conclude that the composite scaffold can significantly promote the differentiation of BMSCs into osteoblasts by activating Erk1/2-Runx2 pathway. Our finding thereby implies that the C-C-MBG scaffold can possibly act as a potential biomaterial in the bone regeneration.

14-3-3β isoform is specifically acetylated at Lys51 during differentiation to the osteogenic lineage

The 14-3-3 protein family binds and regulates hundreds of serine/threonine phosphorylated proteins as an essential component of many signaling networks. Specific biological functions are currently been discovered for each of its seven isoforms in mammals. These proteins have been traditionally considered unregulated; however, its acetylation in an essential lysine residue, causing its inactivation, was recently published. Here, we studied the acetylation state of this lysine 49/51 during the osteogenic differentiation of human adipose-derived stem cells. We found that during this process, the levels of 14-3-3β (but not its isoform 14-3-3γ) acK49/51 increase, representing the first report linking this PTM to a specific isoform and a cellular process. Our results suggested that this posttranslational modification could be catalyzed by the HBO1 acetyltransferase, as overexpression of HBO1 increased specifically 14-3-3 acK49/51 acetylation. Acetylated 14-3-3 proteins are located primarily in the nucleus, where their active state has been described to bind H3 histones and many transcription factors. The inhibition of the expression of different isoforms showed that the specific silencing of the 14-3-3β gene, but not γ, increased significantly the osteogenic potential of the cells. This result correlated to the increase in acetylation of 14-3- 3β Lys 49/51 during osteogenesis. The possible role of this PTM in osteogenesis is discussed.

High glucose promotes mineralization via bone morphogenetic protein 4-Smad signals in early stage of osteoblast differentiation

Diabetes mellitus is associated with bone fragility. Although osteoblast maturation is disturbed in patients with diabetes mellitus, the involvement of high glucose (HG) in different stages of osteoblast maturation is unclear. We used MC3T3-E1 cells, a murine osteoblastic cell line. The cells were incubated in high glucose medium (16.5 and 27.5 mM) with three different time courses: throughout 21 days, only first 7 days (early stage) and only last 7 days (late stage). Mineralization assay showed that HG throughout 21 days increased mineralization compared with control (5.5 mM). In the time course experiment, HG increased mRNA expression of Alp, osteocalcin (Ocn), runt-related transcription factor 2 and osterix on days 3 and 5. By contrast, long-term treatment with HG (14 and 21 days) decreased expression of these osteoblastic markers. HG only during early stage enhanced mineralization, while HG only during late stage had no effects. HG increased the expression of bone morphogenetic protein (BMP) 4 and enhanced phosphorylation of Smad1/5/8. Treatment with a BMP receptor antagonist LDN193189 prevented the HG-induced mineralization during early stage of osteoblast differentiation, indicating that HG in the early stage promotes mineralization by BMP4. In conclusion, the study demonstrates that continuous HG treatment might enhance early osteoblast differentiation but disturbs osteoblast maturation, and that BMP-4-Smad signal might be involved in the HG-induced differentiation and mineralization of osteoblasts.

Superior Osteo-Inductive and Osteo-Conductive Properties of Trabecular Titanium vs. PEEK Scaffolds on Human Mesenchymal Stem Cells: A Proof of Concept for the Use of Fusion Cages

Fusion cages composed of titanium and its alloys are emerging as valuable alternative to standard polyetheretherketone (PEEK) ones routinely used in cervical and lumbar spine surgery. Aim of this study was to evaluate osteo-inductive and osteo-conductive ability of an innovative trabecular titanium (T-Ti) scaffold on human mesenchymal stem cells (hMSCs), in both absence and presence of biochemical osteogenic stimuli. Same abilities were assessed on PEEK and standard 2D plastic surface, the latter meant as gold-standard for in vitro differentiation studies. hMSCs adhered and colonized both T-Ti and PEEK scaffolds. In absence of osteogenic factors, T-Ti triggered osteogenic induction of MSCs, as demonstrated by alkaline phosphatase activity and calcium deposition increments, while PEEK and standard 2D did not. Addition of osteogenic stimuli reinforced osteogenic differentiation of hMSCs cultured on T-Ti in a significantly higher manner with respect to standard 2D plastic culture surfaces, whereas PEEK almost completely abolished the process. T-Ti driven differentiation towards osteoblasts was confirmed by gene and marker expression analyses, even in absence of osteogenic stimuli. These results clearly indicate superior in vitro osteo-inductive and osteo-conductive capacity of T-Ti compared to PEEK, and make ground for further studies supporting the use of T-Ti cages to improve bone fusion.

Making and shaping endochondral and intramembranous bones

Skeletal elements have a diverse range of shapes and sizes specialized to their various roles including protecting internal organs, locomotion, feeding, hearing, and vocalization. The precise positioning, size, and shape of skeletal elements is therefore critical for their function. During embryonic development, bone forms by endochondral or intramembranous ossification and can arise from the paraxial and lateral plate mesoderm or neural crest. This review describes inductive mechanisms to position and pattern bones within the developing embryo, compares and contrasts the intrinsic vs extrinsic mechanisms of endochondral and intramembranous skeletal development, and details known cellular processes that precisely determine skeletal shape and size. Key cellular mechanisms are employed at distinct stages of ossification, many of which occur in response to mechanical cues (eg, joint formation) or preempting future load‐bearing requirements. Rapid shape changes occur during cellular condensation and template establishment. Specialized cellular behaviors, such as chondrocyte hypertrophy in endochondral bone and secondary cartilage on intramembranous bones, also dramatically change template shape. Once ossification is complete, bone shape undergoes functional adaptation through (re)modeling. We also highlight how alterations in these cellular processes contribute to evolutionary change and how differences in the embryonic origin of bones can influence postnatal bone repair.

Compares and contrasts Endochondral and intramembranous bone development

Reviews embryonic origins of different bones

Describes the cellular and molecular mechanisms of positioning skeletal elements.

Describes mechanisms of skeletal growth with a focus on the generation of skeletal shape

Vitexin enhances osteoblast differentiation through phosphorylation of Smad and expression of Runx2 at in vitro and ex vivo

Vitexin (apigenin-8-C-d-glucopyranoside) is a flavonoid isolated from natural sources. It has been employed as an anti-oxidant, anti-inflammatory, and anti-cancer agent, and is used as a traditional Chinese medicine to treat a variety of illnesses. The present study investigated the effect of vitexin on osteoblast differentiation of C3H10T1/2 mesenchymal stem cells, MC3T3-E1 preosteoblast, mouse calvarial primary cells, and primary bone marrow stem cells (BMSCs). RT-PCR and quantitative PCR demonstrated that vitexin increased mRNA expression of the osteogenic genes distal-less homeobox 5 (Dlx5) and Runxt-related transcription factor 2 (Runx2). Vitexin also increased the Dlx5 and Runx2 protein levels, Smad1/5/9 phosphorylation, and alkaline phosphatase (ALP) activity. In addition, vitexin increased Runx2-luciferase activity. Moreover, knockdown of Runx2 attenuated the increase in ALP activity induced by vitexin. These results demonstrate that vitexin enhances osteoblast differentiation via Runx2.

Epigenetic Regulators of Mesenchymal Stem/Stromal Cell Lineage Determination

PURPOSE OF REVIEW

Although many signalling pathways have been discovered to be essential in mesenchymal stem/stromal (MSC) differentiation, it has become increasingly clear in recent years that epigenetic regulation of gene transcription is a vital component of lineage determination, encompassing diet, lifestyle and parental influences on bone, fat and cartilage development.

RECENT FINDINGS

This review discusses how specific enzymes that modify histone methylation and acetylation or DNA methylation orchestrate the differentiation programs in lineage determination of MSC and the epigenetic changes that facilitate development of bone related diseases such as osteoporosis. The review also describes how environmental factors such as mechanical loading influence the epigenetic signatures of MSC, and how the use of chemical agents or small peptides can regulate epigenetic drift in MSC populations during ageing and disease. Epigenetic regulation of MSC lineage commitment is controlled through changes in enzyme activity, which modifies DNA and histone residues leading to alterations in chromatin structure. The co-ordinated epigenetic regulation of transcriptional activation and repression act to mediate skeletal tissue homeostasis, where deregulation of this process can lead to bone loss during ageing or osteoporosis.

The Mechanical Microenvironment in Breast Cancer

Mechanotransduction is the interpretation of physical cues by cells through mechanosensation mechanisms that elegantly translate mechanical stimuli into biochemical signaling pathways. While mechanical stress and their resulting cellular responses occur in normal physiologic contexts, there are a variety of cancer-associated physical cues present in the tumor microenvironment that are pathological in breast cancer. Mechanistic in vitro data and in vivo evidence currently support three mechanical stressors as mechanical modifiers in breast cancer that will be the focus of this review: stiffness, interstitial fluid pressure, and solid stress. Increases in stiffness, interstitial fluid pressure, and solid stress are thought to promote malignant phenotypes in normal breast epithelial cells, as well as exacerbate malignant phenotypes in breast cancer cells.

Delayed tooth movement in Runx2+/- mice associated with mTORC2 in stretch-induced bone formation

Runt-related transcription factor 2 (Runx2) is an essential transcription factor for osteoblast differentiation, and is activated by mechanical stress to promote osteoblast function. Cleidocranial dysplasia (CCD) is caused by mutations of RUNX2, and CCD patients exhibit malocclusion and often need orthodontic treatment. However, treatment is difficult because of impaired tooth movement, the reason of which has not been clarified. We examined the amount of experimental tooth movement in Runx2^+/− mice, the animal model of CCD, and investigated bone formation on the tension side of experimental tooth movement in vivo. Continuous stretch was conducted to bone marrow stromal cells (BMSCs) as an in vitro model of the tension side of tooth movement. Compared to wild-type littermates the Runx2^+/− mice exhibited delayed experimental tooth movement, and osteoid formation and osteocalcin (OSC) mRNA expression were impaired in osteoblasts on the tension side of tooth movement. Runx2 heterozygous deficiency delayed stretch-induced increase of DNA content in BMSCs, and also delayed and reduced stretch-induced alkaline phosphatase (ALP) activity, OSC mRNA expression, and calcium content of BMSCs in osteogenic medium. Furthermore Runx2^+/− mice exhibited delayed and suppressed expression of mammalian target of rapamycin (mTOR) and rapamycin-insensitive companion of mTOR (Rictor), essential factors of mTORC2, which is regulated by Runx2 to phosphorylate Akt to regulate cell proliferation and differentiation, in osteoblasts on the tension side of tooth movement in vivo and in vitro. Loss of half Runx2 gene dosage inhibited stretch-induced PI3K dependent mTORC2/Akt activity to promote BMSCs proliferation. Furthermore, Runx2^+/− BMSCs in osteogenic medium exhibited delayed and suppressed stretch-induced expression of mTOR and Rictor. mTORC2 regulated stretch-elevated Runx2 and ALP mRNA expression in BMSCs in osteogenic medium. We conclude that Runx2^+/− mice present a useful model of CCD patients for elucidation of the molecular mechanisms in bone remodeling during tooth movement, and that Runx2 plays a role in stretch-induced proliferation and osteogenesis in BMSCs via mTORC2 activation.

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Experimental tooth movement is delayed in Runx2^+/− mice compared with wild-type mice.

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Runx2 plays a role in stretch-induced proliferation and differentiation of BMSCs via mTORC2 activation.

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Runx2^+/− mice are useful model to clarify the mechanical stress-induced bone remodeling in CCD patients.

Synergistic stimulating effect of 2-hydroxymelatonin and BMP-4 on osteogenic differentiation in vitro

2-Hydroxymelatonin is a metabolite produced when melatonin 2-hydroxylase catalyzes melatonin. Recent studies have reported the important roles of melatonin in bone metabolism. However, the roles of 2-hydroxymelatonin in bone metabolism remains poorly understood. The purpose of this study is to present evidence of the effect of 2-hydroxymelatonin on osteogenic differentiation in C2C12 cells. In this study, we demonstrated the synergistic stimulating effect of 2-hydroxymelatonin and bone morphogenetic protein (BMP)-4 on osteogenic differentiation in vitro, using alkaline phosphatase (ALP) staining, Alizarin red S (ARS) staining, qPCR, and luciferase reporter assay. The combination of 2-hydroxymelatonin and BMP-4 revealed a synergistic effect on osteogenic differentiation in vitro. This finding provides evidence that optimal concentrations of both 2-hydroxymelatonin and BMP-4 are beneficial for anabolic effects on bone in vitro.

Symphytum officinale augments osteogenesis in human bone marrow-derived mesenchymal stem cells in vitro as they differentiate into osteoblasts

ETHNOPHARMACOLOGICAL RELEVANCE

Mesenchymal stem cells (MSCs) are multipotent stem cells possessing regenerative potential. Symphytum officinale (SO) is a medicinal plant and in homoeopathic literature, believed to accelerate bone healing.

AIM OF THE STUDY

This study aimed to determine if homoeopathic doses of SO could augment osteogenesis in MSCs as they differentiate into osteoblasts in vitro.

MATERIALS AND METHODS

Bone marrow samples were obtained from patients who underwent bone grafting procedures (n = 15). MSCs were isolated, expanded and characterized by flow cytometry (CD90, CD105). Cytotoxicity of SO was evaluated by MTT assay. Osteogenic differentiation was induced in MSCs with β-glycerophosphate, ascorbic acid and dexamethasone over 2 weeks. Different homoeopathic doses of SO (MT, 3C, 6C, 12C and 30C) were added to the basic differentiation medium (BDM) and efficiency of MSCs differentiating into osteoblasts were measured by evaluating expression of Osteocalcin using flow cytometry, and alkaline phosphatase activity using ELISA. Gene expression analyses for osteoblast markers (Runx-2, Osteopontin and Osteocalcin) were evaluated in differentiated osteoblasts using qPCR.

RESULTS

Flow cytometry (CD90, CD105) detected MSCs isolated from bone marrow (93-98%). MTT assay showed that the selected doses of SO did not induce any cytotoxicity in MSCs (24 hours). The efficiency of osteogenic differentiation (2 weeks) for different doses of Symphytum officinale was determined by flow cytometry (n = 10) for osteoblast marker, Osteocalcin, and most doses of Symphytum officinale enhanced osteogenesis. Interestingly, gene expression analysis for Runx-2 (n = 10), Osteopontin (n = 10), Osteocalcin (n = 10) and alkaline phosphatase activity (n = 8) also showed increased osteogenesis with the addition of Symphytum officinale to BDM, specially mother tincture.

CONCLUSIONS

Our findings suggest that homoeopathic dose (specially mother tincture) of Symphytum officinale has the potential to enhance osteogenesis.

Improving cell distribution on 3D additive manufactured scaffolds through engineered seeding media density and viscosity

In order to ensure the long-term in vitro and in vivo functionality of cell-seeded 3D scaffolds, an effective and reliable method to control cell seeding efficiency and distribution is crucial. Static seeding on 3D additive manufactured scaffolds made of synthetic polymers still remains challenging, as it often results in poor cell attachment, high cell sedimentation and non-uniform cell distribution, due to gravity and to the intrinsic macroporosity and surface chemical properties of the scaffolds. In this study, the biocompatible macromolecules dextran and Ficoll (Ficoll-Paque) were used for the first time as temporary supplements to alter the viscosity and density of the seeding media, respectively, and improve the static seeding output. The addition of these macromolecules drastically reduced the cell sedimentation velocities, allowing for homogeneous cell attachment to the scaffold filaments. Both dextran and Ficoll-Paque -based seeding methods supported human mesenchymal stromal cells viability and osteogenic differentiation post-seeding. Interestingly, the improved cell distribution led to increased matrix production and mineralization compared to scaffolds seeded by conventional static method. These results suggest a simple and universal method for an efficient seeding of 3D additive manufactured scaffolds, independent of their material and geometrical properties, and applicable for bone and various other tissue regeneration. STATEMENT OF SIGNIFICANCE: Additive manufacturing has emerged as one of the desired technologies to fabricate complex and patient-specific 3D scaffolds for bone regeneration. Along with the technology, new synthetic polymeric materials have been developed to meet processability requirements, as well as the mechanical properties and biocompatibility necessary for the application. Yet, there is still lack of methodology for a universal cell seeding method applicable to all additive manufactured 3D scaffolds regardless of their characteristics. We believe that our simple and reliable method, which is based on adjusting the cell settling velocity to aid cell attachment, could potentially help to maximize the efficiency, and therefore, functionality of cell-seeded constructs. This is of great importance when aiming for both in vitro and future clinical applications.

LAPTM5 is transactivated by RUNX2 and involved in RANKL trafficking in osteoblastic cells

The present study aimed to investigate the role of lysosomal-associated transmembrane protein 5 (LAPTM5) in osteoclast differentiation induced by osteoblasts. The results demonstrated that the expression levels of LAPTM5 were downregulated following runt-related transcription factor 2 (RUNX2) silencing and upregulated following RUNX2 overexpression in ST2 cells. Chromatin immunoprecipitation analysis identified the binding of RUNX2 to the LAPTM5 promoter at the −1176 to −1171 position. Dual-luciferase reporter assays confirmed that RUNX2 directly activated the LAPTM5 gene. The concentration of receptor activator of nuclear factor-κB ligand (RANKL) protein in the cytoplasm and in the media was significantly increased following LAPTM5 knockdown. LAPTM5 silencing in ST2 cells enhanced osteoclastic differentiation of co-cultured RAW264.7 cells. The present study indicated that expression of LAPTM5 was regulated by the interaction of RUNX2 with its promoter region and that LAPTM5 was involved in the trafficking of RANKL. These findings suggested a possible coupling mechanism between osteogenesis and osteoclastogenesis in which RUNX2 may be involved in osteoclast differentiation through the regulation of the lysosome-associated genes that modulate RANKL expression.

Chondrocyte-Specific RUNX2 Overexpression Accelerates Post-traumatic Osteoarthritis Progression in Adult Mice

RUNX2 is a transcription factor critical for chondrocyte maturation and normal endochondral bone formation. It promotes the expression of factors catabolic to the cartilage extracellular matrix and is upregulated in human osteoarthritic cartilage and in murine articular cartilage following joint injury. To date, in vivo studies of RUNX2 overexpression in cartilage have been limited to forced expression in osteochondroprogenitor cells preventing investigation into the effects of chondrocyte-specific RUNX2 overexpression in postnatal articular cartilage. Here, we used the Rosa26^Runx2 allele in combination with the inducible Col2a1^CreERT2 transgene or the inducible Acan^CreERT2 knock-in allele to achieve chondrocyte-specific RUNX2 overexpression (OE) during embryonic development or in the articular cartilage of adult mice, respectively. RUNX2 OE was induced at embryonic day 13.5 (E13.5) for all developmental studies. Histology and in situ hybridization analyses suggest an early onset of chondrocyte hypertrophy and accelerated terminal maturation in the limbs of the RUNX2 OE embryos compared to control embryos. For all postnatal studies, RUNX2 OE was induced at 2 months of age. Surprisingly, no histopathological signs of cartilage degeneration were observed even 6 months following induction of RUNX2 OE. Using the meniscal/ligamentous injury (MLI), a surgical model of knee joint destabilization and meniscal injury, however, we found that RUNX2 OE accelerates the progression of cartilage degeneration following joint trauma. One month following MLI, the numbers of MMP13-positive and TUNEL-positive chondrocytes were significantly greater in the articular cartilage of the RUNX2 OE joints compared to control joints and 2 months following MLI, histomorphometry and Osteoarthritis Research Society International (OARSI) scoring revealed decreased cartilage area in the RUNX2 OE joints. Collectively, these results suggest that although RUNX2 overexpression alone may not be sufficient to initiate the OA degenerative process, it may predetermine the rate of OA onset and/or progression following traumatic joint injury. © 2019 American Society for Bone and Mineral Research.

Phloretin Suppresses Bone Morphogenetic Protein-2-Induced Osteoblastogenesis and Mineralization via Inhibition of Phosphatidylinositol 3-kinases/Akt Pathway

Phloretin has pleiotropic effects, including glucose transporter (GLUT) inhibition. We previously showed that phloretin promoted adipogenesis of bone marrow stromal cell (BMSC) line ST2 independently of GLUT1 inhibition. This study investigated the effect of phloretin on osteoblastogenesis of ST2 cells and osteoblastic MC3T3-E1 cells. Treatment with 10 to 100 µM phloretin suppressed mineralization and expression of osteoblast differentiation markers, such as alkaline phosphatase (ALP), osteocalcin (OCN), type 1 collagen, runt-related transcription factor 2 (Runx2), and osterix (Osx), while increased adipogenic markers, peroxisome proliferator-activated receptor γ (PPARγ), CCAAT/enhancer-binding protein α (C/EBPα), fatty acid-binding protein 4, and adiponectin. Phloretin also inhibited mineralization and decreased osteoblast differentiation markers of MC3T3-E1 cells. Phloretin suppressed phosphorylation of Akt in ST2 cells. In addition, treatment with a phosphatidylinositol 3-kinase (PI3K)/Akt inhibitor, LY294002, suppressed the mineralization and the expression of osteoblast differentiation markers other than ALP. GLUT1 silencing by siRNA did not affect mineralization, although it decreased the expression of OCN and increased the expression of ALP, Runx2, and Osx. The effects of GLUT1 silencing on osteoblast differentiation markers and mineralization were inconsistent with those of phloretin. Taken together, these findings suggest that phloretin suppressed osteoblastogenesis of ST2 and MC3T3-E1 cells by inhibiting the PI3K/Akt pathway, suggesting that the effects of phloretin may not be associated with glucose uptake inhibition.

FGF adsorbed mesoporous bioactive glass with larger pores in enhancing bone tissue engineering

Mesoporous bioactive glass (MBG) is performed as a bone tissue engineering material because of its good bioactivity, biocompatibility and osteoinducion characteristics. Here, we propose MBG with larger pores (MBG-L) adsorbed fibroblast growth factor (FGF) to facilitate osteoblast differentiation and matrix mineralization. Specifically, we observed that MBG-L promotes calcium deposit precipitation in vitro. In addition, adhesion, proliferation, differentiation and matrix mineralization were promoted after osteoblast cultured on MBG-L/FGF. Interestingly, we found that the transcriptional activity of the critical transcription factor Runx2 was increased through MAPK pathway after osteoblast cultured on MBG-L/FGF. Support for this result, we found that the expression of osteoblastic marker genes, Osteocalcin (Ocn), Osteopontin (Opn), and Runx2 were increased. Thus, our findings provided that MBG-L/FGF could be a promising new material in bone tissue engineering.

Periosteal progenitors contribute to load-induced bone formation in adult mice and require primary cilia to sense mechanical stimulation

BACKGROUND

The fully developed adult skeleton adapts to mechanical forces by generating more bone, usually at the periosteal surface. Progenitor cells in the periosteum are believed to differentiate into bone-forming osteoblasts that contribute to load-induced adult bone formation, but in vivo evidence does not yet exist. Furthermore, the mechanism by which periosteal progenitors might sense physical loading and trigger differentiation is unknown. We propose that periosteal osteochondroprogenitors (OCPs) directly sense mechanical load and differentiate into bone-forming osteoblasts via their primary cilia, mechanosensory organelles known to be involved in osteogenic differentiation.

METHODS

We generated a diphtheria toxin ablation mouse model and performed ulnar loading and dynamic histomorphometry to quantify the contribution of periosteal OCPs in adult bone formation in vivo. We also generated a primary cilium knockout model and isolated periosteal cells to study the role of the cilium in periosteal OCP mechanosensing in vitro. Experimental groups were compared using one-way analysis of variance or student's t test, and sample size was determined to achieve a minimum power of 80%.

RESULTS

Mice without periosteal OCPs had severely attenuated mechanically induced bone formation and lacked the mineralization necessary for daily skeletal maintenance. Our in vitro results demonstrate that OCPs in the periosteum uniquely sense fluid shear and exhibit changes in osteogenic markers consistent with osteoblast differentiation; however, this response is essentially lost when the primary cilium is absent.

CONCLUSIONS

Combined, our data show that periosteal progenitors are a mechanosensitive cell source that significantly contribute to adult skeletal maintenance. More importantly, an OCP population persists in the adult skeleton and these cells, as well as their cilia, are promising targets for bone regeneration strategies.

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.

Evaluation of osteoblast differentiation and function when cultured on mesoporous bioactive glass adsorbed with testosterone

Mesoporous bioactive glass (MBG), a kind of porous materials with great osteoconductive and osteoinductive ability, shows promising application in bone tissue engineering due to its high specific surface area, orderly channel structure, and large pore volume. Here we reported that the proliferation, differentiation, and mineralization were promoted in MC3T3-E1 cells cultured on MBG which adsorbed with testosterone (MBG/T). We found that transcriptional activity of Runx2 which is a critical transcription factor is increased in MC3T3-E1 cells cultured on MBG/T. Intriguingly, we observed that ERK phosphorylation was enhanced in MC3T3-E1 cells cultured on MBG/T. We showed that activated Runx2 in MC3T3-E1 cells cultured on MBG/T is through Erk1/2 phosphorylation. Consistent with this result, we also found that the expression of osteoblastic marker genes were increased. Therefore, we concluded that osteoblast differentiation and mineralization was enhanced after cells cultured on MBG/T through Erk1/2-activated Runx2 pathway. Our findings provided that MBG/T is a potential material in the process of bone repair.

Osteoblast versus Adipocyte: Bone Marrow Microenvironment-Guided Epigenetic Control

The commitment and differentiation of bone marrow mesenchymal stem cells (MSCs) is tightly controlled by the local environment ensuring lineage differentiation balance and bone homeostasis. However, pathological conditions linked with osteoporosis have changed the bone marrow microenvironment, shifting MSCs’ fate to favor adipocytes over osteoblasts, and consequently leading to decreased bone mass with marrow fat accumulation. Multiple questions related to the underlying mechanisms remain to be answered. As recent findings have confirmed the fundamental role of the epigenetic mechanism in connecting environmental signals with gene expression and stem cell differentiation, a regulatory network in the bone marrow microenvironment, epigenetic modulation, gene expression, and MSC differentiation begins to emerge. This review discusses how pathological environmental factors affect MSCs’ fate by epigenetic modulating lineage-specific genes. We conclude that manipulating local environments and/or the epigenetic regulatory machinery that target the adipocyte differentiation pathway might be a therapeutic implication of bone loss diseases such as osteoporosis.

Inhibition of adenosine monophosphate-activated protein kinase suppresses bone morphogenetic protein-2-induced mineralization of osteoblasts via Smad-independent mechanisms

Previous studies showed that adenosine monophosphate-activated protein kinase (AMPK), which plays as an intracellular energy sensor, promotes the differentiation and mineralization of osteoblasts via enhancing expression of bone morphogenetic protein (BMP)-2, which is a potent inducer of osteoblastogenesis. Thus, the aim of this study was to examine the roles of AMPK in BMP-2-induced osteoblastogenesis. We used a murine osteoblastic cell line MC3T3-E1 and a murine marrow stromal cell line ST2. BMP-2 (50 and 100 ng/mL) stimulated alkaline phosphatase (ALP) activity and enhanced mineralization of MC3T3-E1 cells, while the effects of BMP-2 were partly abolished by an inhibitor of AMPK, ara-A (0.1 mM). Real-time PCR showed that BMP-2 significantly increased the mRNA expressions of Alp, osteocalcin (Ocn), Runx2, Osterix and Dlx-5 in MC3T3-E1 cells, while co-incubation of ara-A significantly decreased the BMP-2-stimulated expression of Alp, Ocn, and Runx2. Moreover, co-incubation of ara-A suppressed the BMP-2-induced upregulation of Alp and Ocn in ST2 cells. Western blot analysis showed that BMP-2 phosphorylated Smad1/5 although it did not affect AMPK phosphorylation in MC3T3-E1 cells. Furthermore, a BMP receptor inhibitor LDN-193189 inhibited the phosphorylation of Smad1/5, but did not affect AMPK. In addition, co-incubation of ara-A did not affect BMP-2-induced phosphorylation of Smad1/5. These findings suggest that the inhibition of AMPK activation reduces the osteo-inductive effects of BMP-2 by decreasing the expression of Alp, Ocn, and Runx2 through Smad-independent mechanisms in osteoblastic cells.

Genetic inhibition of PPARγ S112 phosphorylation reduces bone formation and stimulates marrow adipogenesis

A common feature of many skeletal diseases is the accumulation of marrow fat. A reciprocal relationship exists between osteogenesis and adipogenesis in bone marrow that is mediated by the relative activity of PPARγ and RUNX2 transcription factors. The ERK/MAPK pathway is an important inducer of MSC differentiation to osteoblasts and an inhibitor of adipogenesis that functions by phosphorylating RUNX2 and PPARγ. To begin to assess the importance of this regulation in vivo, we examined the consequences of blocking one arm of this pathway, PPARγ S112 phosphorylation, by evaluating the bone phenotype of PPARγ S112A mutant mice. This mutation prevents MAPK phosphorylation and inhibition of PPARγ transcriptional activity. Both male and female PPARγ S112A mice had decreased tibial and vertebral BV/TV and decreased trabecular bone relative to wild type littermates. These results were explained by a decrease in bone formation and osteoblast activity in the absence of changes in resorption. In contrast, marrow adipose tissue, adipocyte markers and serum adiponectin were all dramatically increased. Bone marrow stromal cells isolated from PPARγ S112A mice had elevated PPARγ and preferentially differentiated to adipocytes while total and phosphorylated RUNX2 and osteoblastogenesis were inhibited, indicating that the PPARγ S112A mutation affects bone in a cell autonomous manner. Changes in osteoblast/adipocyte lineage allocation in MSC cultures were also seen where CFU-OBs were reduced with a parallel increase in CFU-AD. This study emphasizes the importance of PPARγ phosphorylation in controlling bone mass and marrow adiposity and demonstrates how a regulatory mutation in PPARγ previously associated with peripheral fat metabolism can have broader effects on bone homeostasis that may in turn affect whole body energy metabolism.

miRNA-133a-5p Inhibits the Expression of Osteoblast Differentiation-Associated Markers by Targeting the 3' UTR of RUNX2

Recent studies have recognized the involvement of microRNAs (miRNAs) in the development of osteoporosis, which regulate the balance between osteogenesis and osteoclasis. In this study, we investigated the regulation by miRNA-133a-5p on the osteoblast differentiation-associated markers in the mouse osteoblast-like MC3T3-E1 cells by RUNX2. First, we manipulated the miRNA-133a level in the MC3T3-E1 cells with 20 or 40 nM miR-133a-5p mimics, miR-133a-5p inhibitor, or scramble miRNA. Then, we quantified with real-time polymerase chain reaction (qRT-PCR) the expression of Collagen I, osteocalcin (OCN), and osteopontin (OPN) in the miR-133a-5p-manipulated MC3T3-E1 cells. And the confocal microscopy was also utilized to confirm the regulation by miR-133a-5p on the expression of the three molecules. We also investigated the extracellular matrix (ECM) mineralization and the alkaline phosphatase (ALP) activity in the miR-133a-5p-manipulated MC3T3-E1 cells. In addition, we explored the possible targeting by miR-133a-5p on RUNX2, which was a well-recognized promoter to osteoblast differentiation, with luciferase reporter, qRT-PCR, and Western blotting assay. Results demonstrated that the miRNA-133a-5p mimics markedly reduced, whereas the miRNA-133a-5p inhibitor significantly promoted the expression of Collagen I, OCN, and OPN, the ECM mineralization, and the ALP activity in MC3T3-E1 cells. The alignment analysis demonstrated a high homology between miRNA-133a-5p and the 3' UTR of RUNX2. Moreover, the luciferase reporter assay demonstrated that miRNA-133a-5p targeted the 3' UTR of RUNX2, and inhibited the expression of RUNX2 in both mRNA and protein levels. In conclusion, we identified the inhibition by miRNA-133a-5p to the expression of osteoblast differentiation markers, to the ECM mineralization, and to the ALP activity in MC3T3-E1 cells, by targeting the 3' UTR of RUNX2. Our study suggests that miRNA-133a-5p might be an important target to inhibit osteoblast differentiation in osteoporosis.

Regulation of osteogenesis of human amniotic mesenchymal stem cells by sodium butyrate

Human amniotic membrane-derived mesenchymal stem cells (hAMSCs) draw great interests for regenerative medicine due to convenient availability and low immunogenicity. However, suboptimal culture conditions limit their application. In recent years, small molecules have proven powerful in regulating stem cell fates and can be applied to stimulate their function. In the present study, the impacts of sodium butyrate (NaBu), a histone deacetylase inhibitor (HDACi), on hAMSCs were investigated. It was shown that NaBu at a low concentration inhibited cell proliferation by arresting cell cycle at G0/G1 rather than inducing apoptosis. When NaBu was supplemented at a concentration of <1.0 mM for 3 days during osteogenic induction, significantly more mineralized nodules were generated and the expression of osteogenesis-related genes (ALP, Runx2, Opn, and Ocn) and proteins (Col1a1, OPN, OCN, Runx2, and TAZ) were both significantly enhanced. However, a higher concentration (1.0 mM) and longer exposure time (14 days) of NaBu showed no such effects, which may be partially attributed to both the increased expression of histone deacetylase 8 (HDAC8) and reduced level of H3K9-Ace, thus leading to the transcriptional inhibition during osteogenesis. Further, it was indicated that ERK might be involved in the stimulatory effects of NaBu. These findings may be helpful to develop an efficient culture process for hAMSCs towards bone regeneration.

Dorsal root ganglion neurons regulate the transcriptional and translational programs of osteoblast differentiation in a microfluidic platform

Innervation by the sensory nervous system plays a key role in skeletal development and in orchestration of bone remodeling and regeneration. However, it is unclear how and in which bone cells can sensory nerves act to control these processes. Here, we show a microfluidic coculture system comprising dorsal root ganglion (DRG) neurons and mesenchymal stem cells (MSCs) that more faithfully represents the in vivo scenario of bone sensory innervation. We report that DRG neurons promote the osteogenic differentiation capacity of MSCs, by mediating the increase of alkaline phosphatase activity and the upregulation of osteoblast-specific genes. Furthermore, we show that DRG neurons have a positive impact on Cx43 levels in MSCs during osteoblastogenesis, especially at an early stage of this process. Conversely, we described a negative impact of DRG neurons on MSCs N-cadherin expression at a later stage. Finally, we demonstrate a cytoplasmic accumulation of β-catenin translocation into the nucleus, and subsequently Lymphoid Enhancer Binding Factor 1—responsive transcriptional activation of downstream genes in cocultured MSCs. Together, our study provides a robust body of evidence that the direct interaction of DRG neurons with MSCs in a bone-like microenvironment leads to an enhancement of osteoblast differentiation potential of MSCs. The osteogenic effect of DRG neurons on MSCs is mediated through the regulation of Cx43 and N-cadherin expression and activation of the canonical/β-catenin Wnt signaling pathway.

Biomechanical regulation of drug sensitivity in an engineered model of human tumor

Predictive testing of anticancer drugs remains a challenge. Bioengineered systems, designed to mimic key aspects of the human tumor microenvironment, are now improving our understanding of cancer biology and facilitating clinical translation. We show that mechanical signals have major effects on cancer drug sensitivity, using a bioengineered model of human bone sarcoma. Ewing sarcoma (ES) cells were studied within a three-dimensional (3D) matrix in a bioreactor providing mechanical loadings. Mimicking bone-like mechanical signals within the 3D model, we rescued the ERK1/2-RUNX2 signaling pathways leading to drug resistance. By culturing patient-derived tumor cells in the model, we confirmed the effects of mechanical signals on cancer cell survival and drug sensitivity. Analyzing human microarray datasets, we showed that RUNX2 expression is linked to poor survival in ES patients. Mechanical loadings that activated signal transduction pathways promoted drug resistance, stressing the importance of introducing mechanobiological cues into preclinical tumor models for drug screening.

Bone morphogenetic protein-5, a key molecule that mediates differentiation in MC3T3E1 osteoblast cell line

Bone morphogenetic protein-5 (BMP-5) is a member of the TGF receptor-β family with osteoinductive property. However, its physiological role in osteoblast differentiation is not defined. This study highlights the importance of BMP-5 in MC3T3E1 osteoblast differentiation. Pre-osteoblasts exposed to osteogenic media (ascorbic acid, 50 µg/ml and β-glycerophosphate, 10 mM) showed high protein expression of BMP-5 in cell lysates and cell culture supernatants, which peaked during early time-points of differentiation and declined with onset of mineralization. Attenuation of endogenous BMP-5 protein expression by RNA interference downregulated the expression of type I collagen (COLIA1), an early osteoblast differentiation marker but not osteocalcin, a late osteoblast differentiation marker. Further experiments to analyze the cell signaling components revealed that BMP-5 modulates COLIA1 expression via p38-Runx2 axis involving Runx2 (Ser19) phosphorylation. These effects were also observed when recombinant BMP-5 was added to pre-osteoblast cultures reinforcing the fact that BMP-5 is a modulator of COLIA1 expression. We conclude that BMP-5 has stage-specific role to play during MC3T3E1 osteoblast differentiation in part by autocrine p38/Runx2/COLIA1 signaling. © 2017 BioFactors, 43(4):558-566, 2017.

Function and mechanism of mesoporous bioactive glass adsorbed epidermal growth factor for accelerating bone tissue regeneration

Mesoporous bioactive glass (MBG) has been demonstrated to play a vital role in bone tissue engineering due to its bioactivity, biocompatibility, and osteoinduction properties. Here, we report that MBG grafted with an amino group (MBG-NH₂) and MBG-NH₂ adsorbed epidermal growth factor (EGF) (MBG-NH₂/EGF) sustained-release EGF, and MBG-NH2/EGF could accelerate osteoblast differentiation and mineralization in MC3T3-E1 cells. We found that MBG-NH₂ could promote bone-like deposit formation and Ca deposition in vitro. Intriguingly, we observed that MBG-NH₂/EGF enhanced MC3T3-E1 cell adhesion. We also showed that extracellular signal-regulated kinase 1/2 (ERK1/2) was phosphorylated when MC3T3-E1 cells were cultured on MBG-NH₂/EGF. Interestingly, the transcription factor Runx2, important for osteoblast differentiation, was also activated when MC3T3-E1 cells were cultured on MBG-NH₂/EGF. We showed that MC3T3-E1 cells cultured on MBG-NH₂/EGF activating Runx2 was through ERK1/2 phosphorylation. Consistent with this survey, we observed that MC3T3-E1 cells cultured on MBG-NH₂/EGF accelerated osteoblastic marker gene expressions, including osteopontin (Opn) and osteocalcin (Ocn). Taken together, we conclude that the osteoblast differentiation and mineralization were accelerated in MC3T3-E1 cells cultured on MBG-NH₂/EGF through ERK-activated Runx2 pathway. These findings support the idea that MBG-NH₂/EGF is a potential biomaterial for bone tissue repair in bone defect-related diseases.

Control of the Osteoblast Lineage by Mitogen-Activated Protein Kinase Signaling

PURPOSE OF THE REVIEW

This review will provide a timely assessment of MAP kinase actions in bone development and homeostasis with particular emphasis on transcriptional control of the osteoblast lineage.

RECENT FINDINGS

ERK and p38 MAP kinases function as transducers of signals initiated by the extracellular matrix, mechanical loading, TGF-β, BMPs and FGF2. MAPK signals may also affect and/or interact with other important pathways such as WNT and HIPPO. ERK and p38 MAP kinase pathways phosphorylate specific osteogenic transcription factors including RUNX2, Osterix, ATF4 and DLX5. For RUNX2, phosphorylation at specific serine residues initiates epigenetic changes in chromatin necessary for decondensation and increased transcription. MAPK also suppresses marrow adipogenesis by phosphorylating and inhibiting PPARγ, which may explain the well-known relationship between reduced skeletal loading and marrow fat accumulation.

SUMMARY

MAPKs transduce signals from the extracellular environment to the nucleus allowing bone cells to respond to changes in hormonal/growth factor signaling and mechanical loading thereby optimizing bone structure to meet physiological and mechanical needs of the body.

Intermittent parathyroid hormone (PTH) promotes cementogenesis and alleviates the catabolic effects of mechanical strain in cementoblasts

Background

External root resorption, commonly starting from cementum, is a severe side effect of orthodontic treatment. In this pathological process and repairing course followed, cementoblasts play a significant role. Previous studies implicated that parathyroid hormone (PTH) could act on committed osteoblast precursors to promote differentiation, and inhibit apoptosis. But little was known about the role of PTH in cementoblasts. The purpose of this study was to investigate the effects of intermittent PTH on cementoblasts and its influence after mechanical strain treatment.

Results

Higher levels of cementogenesis- and differentiation-related biomarkers (bone sialoprotein (BSP), osteocalcin (OCN), Collagen type I (COL1) and Osterix (Osx)) were shown in 1–3 cycles of intermittent PTH treated groups than the control group. Additionally, intermittent PTH increased alkaline phosphatase (ALP) activity and mineralized nodules formation, as measured by ALP staining, quantitative ALP assay, Alizarin red S staining and quantitative calcium assay. The morphology of OCCM-30 cells changed after mechanical strain exertion. Expression of BSP, ALP, OCN, osteopontin (OPN) and Osx was restrained after 18 h mechanical strain. Furthermore, intermittent PTH significantly increased the expression of cementogenesis- and differentiation-related biomarkers in mechanical strain treated OCCM-30 cells.

Conclusions

Taken together, these data suggested that intermittent PTH promoted cementum formation through activating cementogenesis- and differentiation-related biomarkers, and attenuated the catabolic effects of mechanical strain in immortalized cementoblasts OCCM-30.

Electronic supplementary material

The online version of this article (doi:10.1186/s12860-017-0133-0) contains supplementary material, which is available to authorized users.

MAP Kinase-Dependent RUNX2 Phosphorylation Is Necessary for Epigenetic Modification of Chromatin During Osteoblast Differentiation

RUNX2, an essential transcription factor for osteoblast differentiation and bone formation is activated by ERK/MAP kinase-dependent phosphorylation. However, relationship between these early events and specific epigenetic modifications of chromatin during osteoblast differentiation have not been previously examined. Here, we explore these relationships using chromatin immunoprecipitation (ChIP) to detect chromatin modifications in RUNX2-binding regions of Bglap2 and Ibsp. Growth of MC3T3-E1c4 preosteoblast cells in differentiation conditions rapidly induced Bglap2 and lbsp mRNAs. For both genes, osteogenic stimulation increased chromatin-bound P-ERK, P-RUNX2, p300, and RNA polymerase II as well as histone H3K9 and H4K5 acetylation. The level of H3K4 di-methylation, another gene activation-associated histone mark, also increased. In contrast, levels of the gene repressive marks, H3K9 mono-, di-, and tri-methylation in the same regions were reduced. Inhibition of MAP kinase signaling blocked differentiation-dependent chromatin modifications and Bglap2 and Ibsp expression. To evaluate the role of RUNX2 phosphorylation in these responses, RUNX2-deficient C3H10T1/2 cells were transduced with adenovirus encoding wild type or phosphorylation site mutant RUNX2 (RUNX2 S301A/S319A). Wild type RUNX2, but not the non-phosphorylated mutant, increased H3K9 and H4K5 acetylation as well as chromatin-associated P-ERK, p300, and polymerase II. Thus, RUNX2 phosphorylation is necessary for subsequent epigenetic changes required for osteoblast gene expression. Taken together, this study reveals a molecular mechanism through which osteogenic genes are controlled by a MAPK and P-RUNX2-dependent process involving epigenetic modifications of specific promoter regions. J. Cell. Physiol. 232: 2427-2435, 2017. © 2016 Wiley Periodicals, Inc.

Magnitude-dependent response of osteoblasts regulated by compressive stress

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

Mechanical stimulation promote the osteogenic differentiation of bone marrow stromal cells through epigenetic regulation of Sonic Hedgehog

Mechanical unloading leads to bone loss and disuse osteoporosis partly due to impaired osteoblastogenesis of bone marrow stromal cells (BMSCs). However, the underlying molecular mechanisms of this phenomenon are not fully understood. In this study, we demonstrated that cyclic mechanical stretch (CMS) promotes osteoblastogenesis of BMSCs both in vivo and in vitro. Besides, we found that Hedgehog (Hh) signaling pathway was activated in this process. Inhibition of which by either knockdown of Sonic hedgehog (Shh) or treating BMSCs with Hh inhibitors attenuated the osteogenic effect of CMS on BMSCs, suggesting that Hh signaling pathway acts as an endogenous mediator of mechanical stimuli on BMSCs. Furthermore, we demonstrated that Shh expression level was regulated by DNA methylation mechanism. Chromatin Immunoprecipitation (ChIP) assay showed that DNA methyltransferase 3b (Dnmt3b) binds to Shh gene promoter, leading to DNA hypermethylation in mechanical unloading BMSCs. However, mechanical stimulation down-regulates the protein level of Dnmt3b, results in DNA demethylation and Shh expression. More importantly, we found that inhibition of Dnmt3b partly rescued bone loss in HU mice by mechanical unloading. Our results demonstrate, for the first time, that mechanical stimulation regulates osteoblastic genes expression via direct regulation of Dnmt3b, and the therapeutic inhibition of Dnmt3b may be an efficient strategy for enhancing bone formation under mechanical unloading.

Discoidin Receptor 2 Controls Bone Formation and Marrow Adipogenesis

Cell-extracellular matrix (ECM) interactions play major roles in controlling progenitor cell fate and differentiation. The receptor tyrosine kinase, discoidin domain receptor 2 (DDR2), is an important mediator of interactions between cells and fibrillar collagens. DDR2 signals through both ERK1/2 and p38 MAP kinase, which stimulate osteoblast differentiation and bone formation. Here we show that DDR2 is critical for skeletal development and differentiation of marrow progenitor cells to osteoblasts while suppressing marrow adipogenesis. Smallie mice (Ddr2slie/slie ), which contain a nonfunctional Ddr2 allele, have multiple skeletal defects. A progressive decrease in tibial trabecular bone volume/total volume (BV/TV) was observed when wild-type (WT), Ddr2wt/slie , and Ddr2slie/slie mice were compared. These changes were associated with reduced trabecular number (Tb.N) and trabecular thickness (Tb.Th) and increased trabecular spacing (Tb.Sp) in both males and females, but reduced cortical thickness only in Ddr2slie/slie females. Bone changes were attributed to decreased bone formation rather than increased osteoclast activity. Significantly, marrow fat and adipocyte-specific mRNA expression were significantly elevated in Ddr2slie/slie animals. Additional skeletal defects include widened calvarial sutures and reduced vertebral trabecular bone. To examine the role of DDR2 signaling in cell differentiation, bone marrow stromal cells (BMSCs) were grown under osteogenic and adipogenic conditions. Ddr2slie/slie cells exhibited defective osteoblast differentiation and accelerated adipogenesis. Changes in differentiation were related to activity of runt-related transcription factor 2 (RUNX2) and PPARγ, transcription factors that are both controlled by MAPK-dependent phosphorylation. Specifically, the defective osteoblast differentiation in calvarial cells from Ddr2slie/slie mice was associated with reduced ERK/MAP kinase and RUNX2-S319 phosphorylation and could be rescued with a constitutively active phosphomimetic RUNX2 mutant. Also, DDR2 was shown to increase RUNX2-S319 phosphorylation and transcriptional activity while also increasing PPARγ-S112 phosphorylation, but reducing its activity. DDR2 is, therefore, important for maintenance of osteoblast activity and suppression of marrow adipogenesis in vivo and these actions are related to changes in MAPK-dependent RUNX2 and PPARγ phosphorylation. © 2016 American Society for Bone and Mineral Research.

Protein Phosphatase PP5 Controls Bone Mass and the Negative Effects of Rosiglitazone on Bone through Reciprocal Regulation of PPARγ (Peroxisome Proliferator-activated Receptor γ) and RUNX2 (Runt-related Transcription Factor 2)

Peroxisome proliferator-activated receptor γ (PPARγ) and runt-related transcription factor 2 (RUNX2) are key regulators of mesenchymal stem cell (MSC) differentiation toward adipocytes and osteoblasts, respectively. Post-translational modifications of these factors determine their activities. Dephosphorylation of PPARγ at Ser-112 is required for its adipocytic activity, whereas phosphorylation of RUNX2 at serine 319 (Ser-319) promotes its osteoblastic activity. Here we show that protein phosphatase 5 (PP5) reciprocally regulates each receptor by targeting each serine. Mice deficient in PP5 phosphatase have increased osteoblast numbers and high bone formation, which results in high bone mass in the appendicular and axial skeleton. This is associated with a substantial decrease in lipid-containing marrow adipocytes. Indeed, in the absence of PP5 the MSC lineage allocation is skewed toward osteoblasts and away from lipid accumulating adipocytes, although an increase in beige adipocyte gene expression is observed. In the presence of rosiglitazone, PP5 translocates to the nucleus, binds to PPARγ and RUNX2, and dephosphorylates both factors, resulting in activation of PPARγ adipocytic and suppression of RUNX2 osteoblastic activities. Moreover, shRNA knockdown of PP5 results in cells refractory to rosiglitazone treatment. Lastly, mice deficient in PP5 are resistant to the negative effects of rosiglitazone on bone, which in wild type animals causes a 50% decrease in trabecular bone mass. In conclusion, PP5 is a unique phosphatase reciprocally regulating PPARγ and RUNX2 activities in marrow MSC.

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.

Mechanical stimulation orchestrates the osteogenic differentiation of human bone marrow stromal cells by regulating HDAC1

Mechanical stimulation and histone deacetylases (HDACs) have essential roles in regulating the osteogenic differentiation of bone marrow stromal cells (BMSCs) and bone formation. However, little is known regarding what regulates HDAC expression and therefore the osteogenic differentiation of BMSCs during osteogenesis. In this study, we investigated whether mechanical loading regulates HDAC expression directly and examined the role of HDACs in mechanical loading-triggered osteogenic differentiation and bone formation. We first studied the microarrays of samples from patients with osteoporosis and found that the NOTCH pathway and skeletal development gene sets were downregulated in the BMSCs of patients with osteoporosis. Then we demonstrated that mechanical stimuli can regulate osteogenesis and bone formation both in vivo and in vitro. NOTCH signaling was upregulated during cyclic mechanical stretch (CMS)-induced osteogenic differentiation, whereas HDAC1 protein expression was downregulated. The perturbation of HDAC1 expression also had a significant effect on matrix mineralization and JAG1-mediated Notch signaling, suggesting that HDAC1 acts as an endogenous attenuator of Notch signaling in the mechanotransduction of BMSCs. Chromatin immunoprecipitation (ChIP) assay results suggest that HDAC1 modulates the CMS-induced histone H3 acetylation level at the JAG1 promoter. More importantly, we found an inhibitory role of Hdac1 in regulating bone formation in response to hindlimb unloading in mice, and pretreatment with an HDAC1 inhibitor partly rescued the osteoporosis caused by mechanical unloading. Our results demonstrate, for the first time, that mechanical stimulation orchestrates genes expression involved in the osteogenic differentiation of BMSCs via the direct regulation of HDAC1, and the therapeutic inhibition of HDAC1 may be an efficient strategy for enhancing bone formation under mechanical stimulation.

Mitotic Inheritance of mRNA Facilitates Translational Activation of the Osteogenic-Lineage Commitment Factor Runx2 in Progeny of Osteoblastic Cells

Epigenetic mechanisms mediate the acquisition of specialized cellular phenotypes during tissue development, maintenance and repair. When phenotype-committed cells transit through mitosis, chromosomal condensation counteracts epigenetic activation of gene expression. Subsequent post-mitotic re-activation of transcription depends on epigenetic DNA and histone modifications, as well as other architecturally bound proteins that "bookmark" the genome. Osteogenic lineage commitment, differentiation and progenitor proliferation require the bone-related runt-related transcription factor Runx2. Here, we characterized a non-genomic mRNA mediated mechanism by which osteoblast precursors retain their phenotype during self-renewal. We show that osteoblasts produce maximal levels of Runx2 mRNA, but not protein, prior to mitotic cell division. Runx2 mRNA partitions symmetrically between daughter cells in a non-chromosomal tubulin-containing compartment. Subsequently, transcription-independent de novo synthesis of Runx2 protein in early G1 phase results in increased functional interactions of Runx2 with a representative osteoblast-specific target gene (osteocalcin/BGLAP2) in chromatin. Somatic transmission of Runx2 mRNAs in osteoblasts and osteosarcoma cells represents a versatile mechanism for translational rather than transcriptional induction of this principal gene regulator to maintain osteoblast phenotype identity after mitosis.