Ihh and Runx2/Runx3 signaling interact to coordinate early chondrogenesis: a mouse model

Melatonin Delays Arthritis Inflammation and Reduces Cartilage Matrix Degradation through the SIRT1-Mediated NF-κB/Nrf2/TGF-β/BMPs Pathway

Cartilage, a flexible and smooth connective tissue that envelops the surfaces of synovial joints, relies on chondrocytes for extracellular matrix (ECM) production and the maintenance of its structural and functional integrity. Melatonin (MT), renowned for its anti-inflammatory and antioxidant properties, holds the potential to modulate cartilage regeneration and degradation. Therefore, the present study was devoted to elucidating the mechanism of MT on chondrocytes. The in vivo experiment consisted of three groups: Sham (only the skin tissue was incised), Model (using the anterior cruciate ligament transection (ACLT) method), and MT (30 mg/kg), with sample extraction following 12 weeks of administration. Pathological alterations in articular cartilage, synovium, and subchondral bone were evaluated using Safranin O-fast green staining. Immunohistochemistry (ICH) analysis was employed to assess the expression of matrix degradation-related markers. The levels of serum cytokines were quantified via Enzyme-linked immunosorbent assay (ELISA) assays. In in vitro experiments, primary chondrocytes were divided into Control, Model, MT, negative control, and inhibitor groups. Western blotting (WB) and Quantitative RT-PCR (q-PCR) were used to detect Silent information regulator transcript-1 (SIRT1)/Nuclear factor kappa-B (NF-κB)/Nuclear factor erythroid-2-related factor 2 (Nrf2)/Transforming growth factor-beta (TGF-β)/Bone morphogenetic proteins (BMPs)-related indicators. Immunofluorescence (IF) analysis was employed to examine the status of type II collagen (COL2A1), SIRT1, phosphorylated NF-κB p65 (p-p65), and phosphorylated mothers against decapentaplegic homolog 2 (p-Smad2). In vivo results revealed that the MT group exhibited a relatively smooth cartilage surface, modest chondrocyte loss, mild synovial hyperplasia, and increased subchondral bone thickness. ICH results showed that MT downregulated the expression of components related to matrix degradation. ELISA results showed that MT reduced serum inflammatory cytokine levels. In vitro experiments confirmed that MT upregulated the expression of SIRT1/Nrf2/TGF-β/BMPs while inhibiting the NF-κB pathway and matrix degradation-related components. The introduction of the SIRT1 inhibitor Selisistat (EX527) reversed the effects of MT. Together, these findings suggest that MT has the potential to ameliorate inflammation, inhibit the release of matrix-degrading enzymes, and improve the cartilage condition. This study provides a new theoretical basis for understanding the role of MT in decelerating cartilage degradation and promoting chondrocyte repair in in vivo and in vitro cultured chondrocytes.

Xylosyltransferase I mediates the synthesis of proteoglycans with long glycosaminoglycan chains and controls chondrocyte hypertrophy and collagen fibers organization of in the growth plate

Genetic mutations in the Xylt1 gene are associated with Desbuquois dysplasia type II syndrome characterized by sever prenatal and postnatal short stature. However, the specific role of XylT-I in the growth plate is not completely understood. Here, we show that XylT-I is expressed and critical for the synthesis of proteoglycans in resting and proliferative but not in hypertrophic chondrocytes in the growth plate. We found that loss of XylT-I induces hypertrophic phenotype-like of chondrocytes associated with reduced interterritorial matrix. Mechanistically, deletion of XylT-I impairs the synthesis of long glycosaminoglycan chains leading to the formation of proteoglycans with shorter glycosaminoglycan chains. Histological and Second Harmonic Generation microscopy analysis revealed that deletion of XylT-I accelerated chondrocyte maturation and prevents chondrocytes columnar organization and arrangement in parallel of collagen fibers in the growth plate, suggesting that XylT-I controls chondrocyte maturation and matrix organization. Intriguingly, loss of XylT-I induced at embryonic stage E18.5 the migration of progenitor cells from the perichondrium next to the groove of Ranvier into the central part of epiphysis of E18.5 embryos. These cells characterized by higher expression of glycosaminoglycans exhibit circular organization then undergo hypertrophy and death creating a circular structure at the secondary ossification center location. Our study revealed an uncovered role of XylT-I in the synthesis of proteoglycans and provides evidence that the structure of glycosaminoglycan chains of proteoglycans controls chondrocyte maturation and matrix organization.

miR-199a-5p Reduces Chondrocyte Hypertrophy and Attenuates Osteoarthritis Progression via the Indian Hedgehog Signal Pathway

Osteoarthritis (OA), the most common type of arthritis, is an age-associated disease, characterized by the progressive degradation of articular cartilage, synovial inflammation, and degeneration of subchondral bone. Chondrocyte proliferation is regulated by the Indian hedgehog (IHH in humans, Ihh in animals) signaling molecule, which regulates hypertrophy and endochondral ossification in the development of the skeletal system. microRNAs (miRNAs, miRs) are a family of about 22-nucleotide endogenous non-coding RNAs, which negatively regulate gene expression. In this study, the expression level of IHH was upregulated in the damaged articular cartilage tissues among OA patients and OA cell cultures, while that of miR-199a-5p was the opposite. Further investigations demonstrated that miR-199a-5p could directly regulate IHH expression and reduce chondrocyte hypertrophy and matrix degradation via the IHH signal pathway in the primary human chondrocytes. The intra-articular injection of synthetic miR-199a-5p agomir attenuated OA symptoms in rats, including the alleviation of articular cartilage destruction, subchondral bone degradation, and synovial inflammation. The miR-199a-5p agomir could also inhibit the Ihh signaling pathway in vivo. This study might help in understanding the role of miR-199a-5p in the pathophysiology and molecular mechanisms of OA and indicate a potential novel therapeutic strategy for OA patients.

Modeling uniquely human gene regulatory function via targeted humanization of the mouse genome

The evolution of uniquely human traits likely entailed changes in developmental gene regulation. Human Accelerated Regions (HARs), which include transcriptional enhancers harboring a significant excess of human-specific sequence changes, are leading candidates for driving gene regulatory modifications in human development. However, insight into whether HARs alter the level, distribution, and timing of endogenous gene expression remains limited. We examined the role of the HAR HACNS1 (HAR2) in human evolution by interrogating its molecular functions in a genetically humanized mouse model. We find that HACNS1 maintains its human-specific enhancer activity in the mouse embryo and modifies expression of Gbx2, which encodes a transcription factor, during limb development. Using single-cell RNA-sequencing, we demonstrate that Gbx2 is upregulated in the limb chondrogenic mesenchyme of HACNS1 homozygous embryos, supporting that HACNS1 alters gene expression in cell types involved in skeletal patterning. Our findings illustrate that humanized mouse models provide mechanistic insight into how HARs modified gene expression in human evolution.

SMAD4 contributes to chondrocyte and osteocyte development

Different cellular and molecular mechanisms contribute to chondrocyte and osteocyte development. Although vital roles of the mothers against decapentaplegic homolog 4 (also called 'SMAD4') have been discussed in different cancers and stem cell-related studies, there are a few reviews summarizing the roles of this protein in the skeletal development and bone homeostasis. In order to fill this gap, we discuss the critical roles of SMAD4 in the skeletal development. To this end, we review the different signalling pathways and also how SMAD4 defines stem cell features. We also elaborate how the epigenetic factors-ie DNA methylation, histone modifications and noncoding RNAs-make a contribution to the chondrocyte and osteocyte development. To better grasp the important roles of SMAD4 in the cartilage and bone development, we also review the genotype-phenotype correlation in animal models. This review helps us to understand the importance of the SMAD4 in the chondrocyte and bone development and the potential applications for therapeutic goals.

Paracrine Interactions Involved in Human Induced Pluripotent Stem Cells Differentiation into Chondrocytes

Osteoarthritis (OA), as a degenerative joint disease, is the most common form of joint disorder that causes pain, stiffness, and other symptoms associated with OA. Various genetic, biomechanical, and environmental factors have a relevant role in the development of OA. To date, extensive efforts are currently being made to overcome the poor self-healing capacity of articular cartilage. Despite the pivotal role of chondrocytes, their proliferation and repair capacity after tissue injury are limited. Therefore, the development of new strategies to overcome these constraints is urgently needed. Recent advances in regenerative medicine suggest that pluripotent stem cells are promising stem cell sources for cartilage repair. Pluripotent stem cells are undifferentiated cells that have the capacity to differentiate into different types of cells and can self-renew indefinitely. In the past few decades, numerous attempts have been made to regenerate articular cartilage by using induced pluripotent stem cells (iPSCs). The potential applications of patient-specific iPSCs hold great promise for regenerative medicine and OA treatment. However, there are different culture conditions for the preparation and characterization of human iPSCs-derived chondrocytes (hiChondrocytes). Recent biochemical analyses reported that several paracrine factors such as TGFb, BMPs, WNT, Ihh, and Runx have been shown to be involved in cartilage cell proliferation and differentiation from human iPSCs. In this review, we summarize and discuss the paracrine interactions involved in human iPSCs differentiation into chondrocytes in different cell culture media.

Growth Factors, Carrier Materials, and Bone Repair

This chapter provides an overview of the growth factors active in bone regeneration and healing. Both normal and impaired bone healing are discussed, with a focus on the spatiotemporal activity of the various growth factors known to be involved in the healing response. The review highlights the activities of most important growth factors impacting bone regeneration, with a particular emphasis on those being pursued for clinical translation or which have already been marketed as components of bone regenerative materials. Current approaches the use of bone grafts in clinical settings of bone repair (including bone grafts) are summarized, and carrier systems (scaffolds) for bone tissue engineering via localized growth factor delivery are reviewed. The chapter concludes with a consideration of how bone repair might be improved in the future.

MiR-218 affects hypertrophic differentiation of human mesenchymal stromal cells during chondrogenesis via targeting RUNX2, MEF2C, and COL10A1

Background

Human mesenchymal stromal cells (MSC) hold hopes for cartilage regenerative therapy due to their chondrogenic differentiation potential. However, undesirable occurrence of calcification after ectopic transplantation, known as hypertrophic degeneration, remains the major obstacle limiting application of MSC in cartilage tissue regeneration approaches. There is growing evidence that microRNAs (miRs) play essential roles in post-transcriptional regulation of hypertrophic differentiation during chondrogenesis. Aim of the study was to identify new miR candidates involved in repression of hypertrophy-related targets.

Methods

The miR expression profile in human articular chondrocytes (AC) was compared to that in hypertrophic chondrocytes derived from human MSC by microarray analysis, and miR expression was validated by qPCR. Putative targets were searched by in silico analysis and validated by miR reporter assay in HEK293T, by functional assays (western blotting and ALP-activity) in transiently transfected SaOS-2 cells, and by a miR pulldown assay in human MSC. The expression profile of miR-218 was assessed by qPCR during in vitro chondrogenesis of MSC and re-differentiation of AC. MSC were transfected with miR-218 mimic, and differentiation outcome was assessed over 28 days. MiR-218 expression was quantified in healthy and osteoarthritic cartilage of patients.

Results

Within the top 15 miRs differentially expressed between chondral AC versus endochondral MSC differentiation, miR-218 was selected as a candidate miR predicted to target hypertrophy-related genes. MiR-218 was downregulated during chondrogenesis of MSC and showed a negative correlation to hypertrophic markers, such as COL10A1 and MEF2C. It was confirmed in SaOS-2 cells that miR-218 directly targets hypertrophy-related COL10A1, MEF2C, and RUNX2, as a gain of ectopic miR-218 mimic caused drop in MEF2C and RUNX2 protein accumulation, with attenuation of COL10A1 expression and significant concomitant reduction of ALP activity. A miR pulldown assay confirmed that miR-218 directly targets RUNX2, MEF2C in human MSC. Additionally, the gain of miR-218 in human MSC attenuated hypertrophic markers (MEF2C, RUNX2, COL10A1, ALPL), although with no boost of chondrogenic markers (GAG deposition, COL2A1) due to activation of WNT/β-catenin signaling. Moreover, no correlation between miR-218 expression and a pathologic phenotype in the cartilage of osteoarthritis (OA) patients was found.

Conclusions

Although miR-218 was shown to target pro-hypertrophic markers MEF2C, COL10A1, and RUNX2 in human MSC during chondrogenic differentiation, overall, it could not significantly reduce the hypertrophic phenotype or boost chondrogenesis. This could be explained by a concomitant activation of WNT/β-catenin signaling counteracting the anti-hypertrophic effects of miR-218. Therefore, to achieve a full inhibition of the endochondral pathway, a whole class of anti-hypertrophic miRs, including miR-218, needs to be taken into consideration.

Exposure to tert-Butylphenyl Diphenyl Phosphate, an Organophosphate Ester Flame Retardant and Plasticizer, Alters Hedgehog Signaling in Murine Limb Bud Cultures

Organophosphate esters have become widely used as flame retardants since the phase out of polybrominated diphenyl ethers. Previously, we demonstrated that some organophosphate esters, such as tert-butylphenyl diphenyl phosphate (BPDP), were more detrimental to endochondral ossification in murine limb bud cultures than one of the major polybrominated diphenyl ethers that they replaced, 2,2',4,4'-tetrabromodiphenyl ether. Here, we used a transcriptomic approach to elucidate the mechanism of action of BPDP in the developing limb. Limb buds collected from gestation day 13 CD1 mouse embryos were cultured for 3 or 24 h in the presence of vehicle, 1 μM, or 10 μM BPDP. RNA sequencing analyses revealed that exposure to 1 µM BPDP for 24 h increased the expression of 5 transcripts, including Ihh, and decreased 14 others, including Gli1, Ptch1, Ptch2, and other targets of Hedgehog (Hh) signaling. Pathway analysis predicted the inhibition of Hh signaling. Attenuation of Hh signaling activity began earlier and reached a greater magnitude after exposure to 10 µM BPDP. Because this pathway is part of the regulatory network governing endochondral ossification, we used a known Hh agonist, purmorphamine, to determine the contribution of Hh signaling inhibition to the negative impact of BPDP on endochondral ossification. Cotreatment of limbs with purmorphamine rescued the detrimental morphological changes in the cartilage template induced by BPDP exposure though it did not restore the expression of key transcription factors, Runx2 and Sp7, to control levels. These data highlight Hh signaling as a developmentally important pathway vulnerable to environmental chemical exposures.

Circadian BMAL1 regulates mandibular condyle development by hedgehog pathway

OBJECTIVE

Chondrogenesis and endochondral ossification in mandibular condyle play crucial roles in maxillofacial morphogenesis and function. Circadian regulator brain and muscle arnt-like 1 (BMAL1) is proven to be essential for embryonic and postnatal development. The goal of this study was to define the functions of BMAL1 in the embryonic and postnatal growth of mandibular condylar cartilages (MCC).

MATERIALS AND METHODS

Micro-CT, TUNEL staining and EdU assay were performed using BMAL1-deficient mice model, and in vitro experiments were performed using rat chondrocytes isolated from MCC. RNA sequencing in mandibular condyle tissues from Bmal1-/- mice and the age-matched wild-type mice was used for transcriptional profiling at different postnatal stages.

RESULTS

The expression levels of BMAL1 decrease gradually in MCC. BMAL1 is proved to regulate sequential chondrocyte differentiation, and its deficiency can result in the impairment of endochondral ossification of MCC. RNA sequencing reveals hedgehog signalling pathway is the potential target of BMAL1. BMAL1 regulates hedgehog signalling and affects its downstream cascades through directly binding to the promoters of Ptch1 and Ihh, modulating targets of hedgehog signalling which is indispensable for endochondral ossification. Importantly, the short stature phenotypes caused by BMAL1 deficiency can be rescued by hedgehog signalling activator.

CONCLUSIONS

Collectively, these results indicate that BMAL1 plays critical roles on chondrogenesis and endochondral ossification of MCC, giving a new insight on potential therapeutic strategies for facial dysmorphism.

Runx2 plays a central role in Osteoarthritis development

Osteoarthritis (OA) is the most common form of arthritis, is the leading cause of impaired mobility in the elderly, and accounts for more than a third of chronic moderate to severe pain. As a degenerative joint disorder, OA affects the whole joint and results in synovial hyperplasia, degradation of articular cartilage, subchondral sclerosis, osteophyte formation, and chronic pain. Currently, there is no effective drug to decelerate OA progression and molecular targets for drug development have been insufficiently investigated. Anti-OA drug development can benefit from more and precise knowledge of molecular targets for drug development. Runt-related transcription factor 2 (Runx2) is a key transcription factor controlling osteoblast and chondrocyte differentiation and is among the most promising potential therapeutic targets. Notably, Runx2 expression is upregulated in several murine OA models, suggesting a role in disease pathogenesis. In this review article, we summarized recent findings on Runx2 related to OA development and evaluated its potential as a therapeutic target.

The translational potential of this article

A better understanding of the role of Runx2 in osteoarthritis pathogenesis will contribute to the development of novel intervention of osteoarthritis disease.

Overexpressed miR-196a accelerates osteogenic differentiation in osteoporotic mice via GNAS-dependent Hedgehog signaling pathway

Osteoporosis (OP), a common metabolic bone disease, is accompanied by reduced bone mass, bone mineral density (BMD), as well as microstructure destruction of bone. Previously, microRNA-196a-2 (miR-196a-2) and miR-196a-3p were reported for its involvement in BMD. Herein, this study set out to identify the functional relevance of miR-196a in osteogenic differentiation in osteoporotic mice and explore the associated mechanism by establishing an OP mouse model. Guanine nucleotide binding protein, alpha stimulating (GNAS) was verified as a target gene of miR-196a, which was decreased in OP mice. Furthermore, the bone marrow stromal cells (BMSCs) were then extracted from OP mice and treated with miR-196 mimic/inhibitor or small interfering RNA against GNAS to investigate miR-196a interaction with GNAS and the Hedgehog signaling pathway. BMSCs in OP mice transfected with miR-196a mimic or si-GNAS displayed the elevated expression of Smo, ALP, Runx2, and OPN, as well as bone gla protein and tartrate-resistant acid phosphatase, elevated ALP vitality and bone formation ability as well as reduced expression of GNAS and PTCH. Taken conjointly, overexpression of miR-196a repressed GNAS expression by activating the Hedgehog signaling pathway, thus promoting osteogenic differentiation in mice with OP.

RUNX3 suppresses metastasis and stemness by inhibiting Hedgehog signaling in colorectal cancer

Disabled tumor suppressor genes and hyperactive oncogenes greatly contribute to cell fates during cancer development because of their genetic alterations such as somatic mutations. However, little is known about how tumor suppressor genes react to diverse oncogenes during tumor progression. Our previous study showed that RUNX3 inhibits invasiveness by preventing vascular endothelial growth factor secretion and suppressed endothelial cell growth and tube formation in colorectal cancer (CRC). Hedgehog signaling is crucial for the physiological maintenance and self-renewal of stem cells, and its deregulation is responsible for their tumor development. The mechanisms that inhibit this pathway during proliferation remain poorly understood. Here, we found that the tumor suppressor RUNX3 modulates tumorigenesis in response to cancer cells induced by inhibiting oncogene GLI1 ubiquitination. Moreover, we demonstrated that RUNX3 and GLI1 expression were inversely correlated in CRC cells and tissues. We observed a direct interaction between RUNX3 and GLI1, promoting ubiquitination of GLI1 at the intracellular level. Increased ubiquitination of GLI1 was induced by the E3 ligase β-TrCP. This novel RUNX3-dependent regulatory loop may limit the extent and duration of Hedgehog signaling during extension of the tumor initiation capacity. On the basis of our results, identification of agents that induce RUNX3 may be useful for developing new and effective therapies for CRC.

Identification of Gli1-interacting proteins during simvastatin-stimulated osteogenic differentiation of bone marrow mesenchymal stem cells

Simvastatin has been shown to promote osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs). Our study aimed to illuminate the underlying mechanism, with a specific focus on the role of Hedgehog signaling in this process. BMSCs cultured with or without 10^-7  mol/L simvastatin were subjected to evaluation of osteogenic differentiation capacity. Osteogenic markers such as type 1 collagen (COL1) and osteocalcin (OCN), as well as key molecules of Hedgehog signaling molecules, were examined by Western blot and real-time polymerase chain reaction (PCR). Co-immunoprecipitation and mass spectrometry assays were applied to screen for Gli1-interacting proteins. Cyclopamine (Cpn) was used as a Hedgehog signaling inhibitor. Our results indicated that simvastatin increased alkaline phosphatase (ALP) activity; mineralization of extracellular matrix; mRNA expression of ALP, COL1, and OCN; and expression and nuclear translocation of Gli1. Contrasting effects were observed in Cpn-exposed groups, but were partially rescued by the simvastatin treatment. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses indicated that Gli1-interacting proteins were primarily associated with mitogen-activated protein kinase (MAPK) (P = 7.04E^-04 ), hippo, insulin, and glucagon signaling. Further, hub genes identified by protein-protein interaction network analysis included Gli1-interacting proteins such as Ppp2r1a, Rac1, Etf1, and XPO1/CRM1. In summary, the current study showed that the mechanism by which simvastatin stimulates osteogenic differentiation of BMSCs involves activation of Hedgehog signaling, as indicated by interactions with Gli1 and, most notably, the MAPK signaling pathway.

Molecular Programming of Perivascular Stem Cell Precursors

Pericytes have been shown to act as precursors of resident adult stem cells in stromal tissues in vivo. When expanded in vitro these cells are capable of giving rise to multiple mesenchymal cell types, irrespective of their tissue of origin. This phenomenon of multi-lineage differentiation is only observed in culture, whereas in vivo, stromal stem cell differentiation is restricted to tissue-specific cell types. An important unanswered question is how a single, widely distributed cell type (a pericyte) gives rise to stem cells with tissue-specific functions and attributes. Using a combination of transcriptomics and epigenomics we have compared the molecular status of two populations of stromal stem cell precursors. Using a LacZ transgene insertion that is expressed in pericytes but not in stem cells, we were able to compare pericyte populations from two different tissues, mouse incisors and bone marrow. Pericytes, freshly isolated from mouse incisors and bone marrow, exhibited transcriptomes and epigenetic landscapes that were extensively different, reflecting their tissue of origin and future in vivo differentiation potential. Dspp, an odontoblast differentiation gene, as well as additional odontogenic genes, are shown to be expressed in dental pulp-derived pericytes. These genetic loci are also decorated with histone modifications indicative of a transcriptionally active chromatin state. In bone marrow pericytes, a major osteogenic differentiation gene, Runx2, is not expressed but is marked by both active and repressive histones and therefore primed to be expressed. Polycomb repressor complex 1 analysis showed that key genes involved in the induction of adipogenesis, chondrogenesis, and myogenesis are targeted by Ring1b and therefore stably repressed. This indicates that pericyte populations are molecularly obstructed from differentiating down certain lineages in vivo. Stem Cells 2018;36:1890-15.

Knockdown Indian Hedgehog (Ihh) does not delay Fibular Fracture Healing in genetic deleted Ihh mice and pharmaceutical inhibited Ihh Mice

The objective of this study was to determine if Ihh is required for fracture healing. Fibular fracture was created in adult Col2a1-CreER^T2; Ihh^fl/fl mice. Ihh^fl/fl mice received Tamoxifen (TM) to delete Ihh. WT mice received Cyclopamine to inhibit Hh pathway. Callus tissue properties and Ihh pathway were analyzed at 1, 2, and 3 weeks post-fracture by X-ray, micro-CT, mechanical test, RT-PCR and immunohistochemistry. Deleted Ihh was evidenced by the occurrence of growth plate closure in the Ihh^fl/fl mice by X-ray 3 weeks after TM treatment. All mice showed fracture healing at 3 weeks post-operation. Histology analysis indicated that, compared to the control, cartilage area was less in fracture sites from Ihh deficient animals by either genetic deletion or drug inhibition at 1 and 2 weeks post-fracture. Ihh immunostaining and its mRNA level were diminished in the fracture callus in Ihh reduced mice. There was no significant difference in BV/TV, BMD and mechanical test. Interruption to Ihh pathway by either genetic or pharmaceutical approach didn’t affect fibular fracture healing in these mice. This surprised finding implicates that the deleted Ihh does not affect fracture healing in this model.

The art of building bone: emerging role of chondrocyte-to-osteoblast transdifferentiation in endochondral ossification

There is a worldwide epidemic of skeletal diseases causing not only a public health issue but also accounting for a sizable portion of healthcare expenditures. The vertebrate skeleton is known to be formed by mesenchymal cells condensing into tissue elements (patterning phase) followed by their differentiation into cartilage (chondrocytes) or bone (osteoblasts) cells within the condensations. During the growth and remodeling phase, bone is formed directly via intramembranous ossification or through a cartilage to bone conversion via endochondral ossification routes. The canonical pathway of the endochondral bone formation process involves apoptosis of hypertrophic chondrocytes followed by vascular invasion that brings in osteoclast precursors to remove cartilage and osteoblast precursors to form bone. However, there is now an emerging role for chondrocyte-to-osteoblast transdifferentiation in the endochondral ossification process. Although the concept of "transdifferentiation" per se is not recent, new data using a variety of techniques to follow the fate of chondrocytes in different bones during embryonic and post-natal growth as well as during fracture repair in adults have identified three different models for chondrocyte-to-osteoblast transdifferentiation (direct transdifferentiation, dedifferentiation to redifferentiation, and chondrocyte to osteogenic precursor). This review focuses on the emerging models of chondrocyte-to-osteoblast transdifferentiation and their implications for the treatment of skeletal diseases as well as the possible signaling pathways that contribute to chondrocyte-to-osteoblast transdifferentiation processes.

The art of building bone: emerging role of chondrocyte-to-osteoblast transdifferentiation in endochondral ossification

There is a worldwide epidemic of skeletal diseases causing not only a public health issue but also accounting for a sizable portion of healthcare expenditures. The vertebrate skeleton is known to be formed by mesenchymal cells condensing into tissue elements (patterning phase) followed by their differentiation into cartilage (chondrocytes) or bone (osteoblasts) cells within the condensations. During the growth and remodeling phase, bone is formed directly via intramembranous ossification or through a cartilage to bone conversion via endochondral ossification routes. The canonical pathway of the endochondral bone formation process involves apoptosis of hypertrophic chondrocytes followed by vascular invasion that brings in osteoclast precursors to remove cartilage and osteoblast precursors to form bone. However, there is now an emerging role for chondrocyte-to-osteoblast transdifferentiation in the endochondral ossification process. Although the concept of "transdifferentiation" per se is not recent, new data using a variety of techniques to follow the fate of chondrocytes in different bones during embryonic and post-natal growth as well as during fracture repair in adults have identified three different models for chondrocyte-to-osteoblast transdifferentiation (direct transdifferentiation, dedifferentiation to redifferentiation, and chondrocyte to osteogenic precursor). This review focuses on the emerging models of chondrocyte-to-osteoblast transdifferentiation and their implications for the treatment of skeletal diseases as well as the possible signaling pathways that contribute to chondrocyte-to-osteoblast transdifferentiation processes.

Exendin-4 promotes the osteogenic differentiation of osteoblasts via the Hedgehog/Gli1 signaling pathway

This study aimed to investigate the effect and mechanisms of Exendin-4 mediated-Hedgehog/Gli1 signaling pathway on the differentiation of osteoblasts in mouse. The alkaline phosphate activity, alizarin red staining and expression of Gli1, GLP-1R, Hedgehog, Runx2 and osteocalcin were analyzed using PCR and Western blot analysis after treating the osteoblastic cell line MC3T3-E1 with Exendin-4. Osteoblasts were treated with Gli1-siRNA and Hedgehog receptor antagonist Cyclopamine (Cy) and analyzed for their impact on the Hedgehog/Gli1 signaling pathway. Our results showed that optimal treatment of Exendin-4 was 7 days at 10^-7 mol/L. Exendin-4 significantly promoted osteoblast formation in the cell line in a dose-dependent manner and up-regulated the expression of GLP-1R, Hedgehog and Gli1. Gli1-siRNA significantly down regulated the expression of Gli1 and Runx2, and offset Exendin-4-induced osteoblast differentiation. Similarly, Cy offset Exendin-4-induced Gli1 up-regulation. It is clear that Exendin-4 can promote the osteogenic differentiation of osteoblasts through Hedgehog/Gli1 signaling pathway.

Neural EGFL-Like 1 Regulates Cartilage Maturation through Runt-Related Transcription Factor 3-Mediated Indian Hedgehog Signaling

The pro-chondrogenic function of runt-related transcription factor 2 (Runx2) was previously considered to be dependent on direct binding with the promoter of Indian hedgehog (Ihh)-the major regulator of chondrocyte differentiation, proliferation, and maturation. The authors' previous studies identified neural EGFL like 1 (Nell-1) as a Runx2-responsive growth factor for chondrogenic differentiation and maturation. In this study, it was further revealed that the pro-chondrogenic activities of Nell-1 also rely on Ihh signaling, by showing: i) Nell-1 significantly elevated Ihh signal transduction; ii) Nell-1 deficiency markedly reduced Ihh activation in chondrocytes; and iii) Nell-1-stimulated chondrogenesis was significantly reduced by the specific hedgehog inhibitor cyclopamine. Importantly, the authors demonstrated that Nell-1-responsive Ihh signaling and chondrogenic differentiation extended to Runx2^-/- models in vitro and in vivo. In Runx2^-/- chondrocytes, Nell-1 stimulated the expression and signal transduction of Runx3, another transcription factor required for complete chondrogenic differentiation and maturation. Furthermore, knocking down Runx3 in Runx2^-/- chondrocytes abolished Nell-1's stimulation of Ihh-associated molecule expression, which validates Runx3 as a major mediator of Nell-1-stimulated Ihh activation. For the first time, the Runx2→Nell-1→Runx3→Ihh signaling cascade during chondrogenic differentiation and maturation has been identified as an alternative, but critical, pathway for Runx2 to function as a pro-chondrogenic molecule via Nell-1.

RUNX3 plays an important role in mediating the BMP9-induced osteogenic differentiation of mesenchymal stem cells

Although bone morphogenetic protein 9 (BMP9) is highly capable of promoting the osteogenic differentiation of mesenchymal stem cells (MSCs) both in vitro and in vivo, the molecular mechanisms involved remain to be fully elucidated. Runt-related transcription factor (RUNX)3 is an essential regulator of osteoblast/chondrocyte maturation. However, the exact role of RUNX3 in BMP9 osteoinductive activity is unknown. In this study, we sought to investigate the functional role of RUNX3 in the BMP9-induced osteogenic differentiation of MSCs. We found that BMP9 upregulated the endogenous expression of RUNX3 in MSCs. The overexpression or/and knockdown of RUNX3 both increased the levels of alkaline phosphatase (ALP) a marker of BMP9-induced early osteogenic differentiation. Nevertheless, matrix mineralization, a marker of BMP9-induced late osteogenic differentiation was enhanced by the overexpression of RUNX3, whereas it was inhibited by the knockdown of RUNX3. The BMP9-induced expression of osteogenic pivotal transcription factors [inhibitor of differentiation (Id)3, distal-less homeobox 5 (DLX5) and RUNX2)] was further increased by the overexpression of RUNX3; however, it was reduced by the knockdown of RUNX3. However, the expression levels of Id1 and Id2 were both enhanced by the overexpression or/and knockdown of RUNX3. The BMP9-induced phosphorylation of Smad1/5/8 was increased with the overexpression of RUNX3, and yet was decreased with the knockdown of RUNX3. Collectively, our findings suggest that RUNX3 is an essential modulator of the BMP9-induced osteoblast lineage differentiation of MSCs.

Curcumin Inhibits Chondrocyte Hypertrophy of Mesenchymal Stem Cells through IHH and Notch Signaling Pathways

Using tissue engineering technique to repair cartilage damage caused by osteoarthritis is a promising strategy. However, the regenerated tissue usually is fibrous cartilage, which has poor mechanical characteristics compared to hyaline cartilage. Chondrocyte hypertrophy plays an important role in this process. Thus, it is very important to find out a suitable way to maintain the phenotype of chondrocytes and inhibit chondrocyte hypertrophy. Curcumin deriving from turmeric was reported with anti-inflammatory and anti-tumor pharmacological effects. However, the role of curcumin in metabolism of chondrocytes, especially in the chondrocyte hypertrophy remains unclear. Mesenchymal stem cells (MSCs) are widely used in cartilage tissue engineering as seed cells. So we investigated the effect of curcumin on chondrogenesis and chondrocyte hypertrophy in MSCs through examination of cell viability, glycosaminoglycan synthesis and specific gene expression. We found curcumin had no effect on expression of chondrogenic markers including Sox9 and Col2a1 while hypertrophic markers including Runx2 and Col10a1 were down-regulated. Further exploration showed that curcumin inhibited chondrocyte hypertrophy through Indian hedgehog homolog (IHH) and Notch signalings. Our results indicated curcumin was a potential agent in modulating cartilage homeostasis and maintaining chondrocyte phenotype.

Gene expression profile in human induced pluripotent stem cells: Chondrogenic differentiation in vitro, part B

The development of human induced pluripotent stem cells (hiPSCs) is considered a turning point in tissue engineering. However, more data are required to improve understanding of key aspects of the cell differentiation process, including how specific chondrogenic processes affect the gene expression profile of chondrocyte‑like cells and the relative value of cell differentiation markers. The main aims of the present study were as follows: To determine the gene expression profile of chondrogenic‑like cells derived from hiPSCs cultured in mediums conditioned with HC‑402‑05a cells or supplemented with transforming growth factor β3 (TGF‑β3), and to assess the relative utility of the most commonly‑used chondrogenic markers as indicators of cell differentiation. These issues are relevant with regard to the use of human fibroblasts in the reprogramming process to obtain hiPSCs. Human fibroblasts are derived from mesoderm and thus share a wide range of properties with chondrocytes, which originate from the mesenchyme. The hiPSCs were obtained from human primary dermal fibroblasts during a reprogramming process. Two methods, both involving embryoid bodies (EB), were used to obtain chondrocytes from the hiPSCs: EBs formed in the presence of a chondrogenic medium with TGF‑β3 (10 ng/ml) and EBs formed in a medium conditioned with growth factors from HC‑402‑05a cells. Based on reverse transcription-quantitative polymerase chain reaction analysis, the results demonstrated that hiPSCs are capable of effective chondrogenic differentiation, with the cells obtained in the HC‑402‑05a medium presenting with morphological features and markers characteristic of mature human chondrocytes. In contrast, cells differentiated in the presence of TGF‑β3 presented with certain undesirable hypertrophic characteristics. Several genes, most notably runt‑related transcription factor 2, transforming growth factor β2 and transforming growth factor β3, were good markers of advanced and late hiPSC chondrogenic differentiation, whereas transforming growth factor β3I, II, III receptors and bone morphogenetic protein-2, bone morphogenetic protein-4 and growth differentiation factor 5 were less valuable. These findings provide valuable data on the use of stem cells in cartilage tissue regeneration.

Neural EGFL-Like 1 Is a Downstream Regulator of Runt-Related Transcription Factor 2 in Chondrogenic Differentiation and Maturation

Recent studies indicate that neural EGFL-like 1 (Nell-1), a secretive extracellular matrix molecule, is involved in chondrogenic differentiation. Herein, we demonstrated that Nell-1 serves as a key downstream target of runt-related transcription factor 2 (Runx2), a central regulator of chondrogenesis. Unlike in osteoblast lineage cells where Nell-1 and Runx2 demonstrate mutual regulation, further studies in chondrocytes revealed that Runx2 tightly regulates the expression of Nell-1; however, Nell-1 does not alter the expression of Runx2. More important, Nell-1 administration partially restored Runx2 deficiency-induced impairment of chondrocyte differentiation and maturation in vitro, ex vivo, and in vivo. Mechanistically, although the expression of Nell-1 is highly reliant on Runx2, the prochondrogenic function of Nell-1 persisted in Runx2^-/- scenarios. The biopotency of Nell-1 is independent of the nuclear import and DNA binding functions of Runx2 during chondrogenesis. Nell-1 is a key functional mediator of chondrogenesis, thus opening up new possibilities for the application of Nell-1 in cartilage regeneration.

MicroRNA-467g inhibits new bone regeneration by targeting Ihh/Runx-2 signaling

MicroRNAs are important post transcriptional regulators of gene expression and play critical role in osteoblast differentiation. In this study we report miR-467g, an uncharacterized novel miRNA, in regulation of osteoblast functions. Over-expression of miR-467g inhibited osteoblast differentiation. Target prediction analysis tools and experimental validation by luciferase 3' UTR reporter assay identified Runx-2 as a direct target of miR-467g. Over expression of miR-467g in osteoblasts down regulated Runx-2 and Ihh signaling components. Furthermore, silencing of miR-467g was done to see its role in Ihh and Runx-2 mediated bone healing and regeneration in a drill hole injury model in BALB/c mice. Silencing of miR-467g led to significant increase in new bone regeneration and Ihh and Runx-2 localization at injury site in a day dependent manner. In conclusion, miR-467g negatively regulates osteogenesis by targeting Ihh/Runx-2 signaling. We, thus, propose that therapeutic approaches targeting miR-467g could be useful in enhancing the new bone formation.

Osteogenesis requires FAK-dependent collagen synthesis by fibroblasts and osteoblasts

Focal adhesion kinase (FAK) is critical in adhesion-dependent signaling, but its role in osteogenesis in vivo is ill defined. We deleted Fak in fibroblasts and osteoblasts in Floxed-Fak mice bred with those expressing Cre-recombinase driven by 3.6-kb α1(I)-collagen promoter. Compared with wild-type (WT), conditional FAK-knockout (CFKO) mice were shorter (2-fold; P < 0.0001) and had crooked, shorter tails (50%; P < 0.0001). Microcomputed tomography analysis showed reduced bone volume (4-fold in tails; P < 0.0001; 2-fold in mandibles; P < 0.0001), whereas bone surface area/bone volume increased (3-fold in tails; P < 0.0001; 2.5-fold in mandibles; P < 0.001). Collagen density and fiber alignment in periodontal ligament were reduced by 4-fold (P < 0.0001) and 30% (P < 0.05), respectively, in CFKO mice. In cultured CFKO osteoblasts, mineralization at d 7 and mineralizing colony-forming units at d 21 were 30% (P < 0.0001) and >3-fold less than WT, respectively. Disruptions of FAK function in osteoblasts by conditional knockout, siRNA-knockdown, or FAK inhibitor reduced mRNA and protein expression of Runx2 (>30%), Osterix (>25%), and collagen-1 (2-fold). Collagen synthesis was abrogated in WT osteoblasts with Runx2 knockdown and in Fak-null fibroblasts transfected with an FAK kinase domain mutant or a kinase-impaired mutant (Y397F). These data indicate that FAK regulates osteogenesis through transcription factors that regulate collagen synthesis.-Rajshankar, D., Wang, Y., McCulloch, C. A. Osteogenesis requires FAK-dependent collagen synthesis by fibroblasts and osteoblasts.

BMP2 induces chondrogenic differentiation, osteogenic differentiation and endochondral ossification in stem cells

Bone morphogenetic protein 2 (BMP2), a member of the transforming growth factor-β (TGF-β) super-family, is one of the main chondrogenic growth factors involved in cartilage regeneration. BMP2 is known to induce chondrogenic differentiation in various types of stem cells in vitro. However, BMP2 also induces osteogenic differentiation and endochondral ossification in mesenchymal stem cells (MSCs). Although information regarding BMP2-induced chondrogenic and osteogenic differentiation within the same system might be essential for cartilage tissue engineering, few studies concerning these issues have been conducted. In this study, BMP2 was identified as a regulator of chondrogenic differentiation, osteogenic differentiation and endochondral bone formation within the same system. BMP2 was used to regulate chondrogenic and osteogenic differentiation in stem cells within the same culture system in vitro and in vivo. Any changes in the differentiation markers were assessed. BMP2 was found to induce chondrogenesis and osteogenesis in vitro via the expression of Sox9, Runx2 and its downstream markers. According to the results of the subcutaneous stem cell implantation studies, BMP2 not only induced cartilage formation but also promoted endochondral ossification during ectopic bone/cartilage formation. In fetal limb cultures, BMP2 promoted chondrocyte hypertrophy and endochondral ossification. Our data reveal that BMP2 can spontaneously induce chondrogenic differentiation, osteogenic differentiation and endochondral bone formation within the same system. Thus, BMP2 can be used in cartilage tissue engineering to regulate cartilage formation but has to be properly regulated for cartilage tissue engineering in order to retain the cartilage phenotype.

Noggin inactivation affects the number and differentiation potential of muscle progenitor cells in vivo

Inactivation of Noggin, a secreted antagonist of Bone Morphogenetic Proteins (BMPs), in mice leads, among others, to severe malformations of the appendicular skeleton and defective skeletal muscle fibers. To determine the molecular basis of the phenotype, we carried out a histomorphological and molecular analysis of developing muscles Noggin(-/-) mice. We show that in 18.5 dpc embryos there is a marked reduction in muscle fiber size and a failure of nuclei migration towards the cell membrane. Molecularly, the absence of Noggin results in an increased BMP signaling in muscle tissue as shown by the increase in SMAD1/5/8 phosphorylation, concomitant with the induction of BMP target genes such as Id1, 2, 3 as well as Msx1. Finally, upon removal of Noggin, the number of mesenchymal Pax7(+) muscle precursor cells is reduced and they are more prone to differentiate into adipocytes in vitro. Thus, our results highlight the importance of Noggin/BMP balance for myogenic commitment of early fetal progenitor cells.

Conditional Deletion of Fgfr3 in Chondrocytes leads to Osteoarthritis-like Defects in Temporomandibular Joint of Adult Mice

Osteoarthritis (OA) in the temporomandibular joint (TMJ) is a common degenerative disease in adult, which is characterized by progressive destruction of the articular cartilage. To investigate the role of FGFR3 in the homeostasis of TMJ cartilage during adult stage, we generated Fgfr3^f/f; Col2a1-CreER^T2 (Fgfr3 cKO) mice, in which Fgfr3 was deleted in chondrocytes at 2 months of age. OA-like defects were observed in Fgfr3 cKO TMJ cartilage. Immunohistochemical staining and quantitative real-time PCR analyses revealed a significant increase in expressions of COL10, MMP13 and AMAMTS5. In addition, there was a sharp increase in chondrocyte apoptosis at the Fgfr3 cKO articular surface, which was accompanied by a down-regulation of lubricin expression. Importantly, the expressions of RUNX2 and Indian hedgehog (IHH) were up-regulated in Fgfr3 cKO TMJ. Primary Fgfr3 cKO chondrocytes were treated with IHH signaling inhibitor, which significantly reduced expressions of Runx2, Col10, Mmp13 and Adamts5. Furthermore, the IHH signaling inhibitor partially alleviated OA-like defects in the TMJ of Fgfr3 cKO mice, including restoration of lubricin expression and improvement of the integrity of the articular surface. In conclusion, our study proposes that FGFR3/IHH signaling pathway plays a critical role in maintaining the homeostasis of TMJ articular cartilage during adult stage.

The Transcription Factor Hand1 Is Involved In Runx2-Ihh-Regulated Endochondral Ossification

The developing long bone is a model of endochondral ossification that displays the morphological layers of chondrocytes toward the ossification center of the diaphysis. Indian hedgehog (Ihh), a member of the hedgehog family of secreted molecules, regulates chondrocyte proliferation and differentiation, as well as osteoblast differentiation, through the process of endochondral ossification. Here, we report that the basic helix-loop-helix transcription factor Hand1, which is expressed in the cartilage primordia, is involved in proper osteogenesis of the bone collar via its control of Ihh production. Genetic overexpression of Hand1 in the osteochondral progenitors resulted in prenatal hypoplastic or aplastic ossification in the diaphyses, mimicking an Ihh loss-of-function phenotype. Ihh expression was downregulated in femur epiphyses of Hand1-overexpressing mice. We also confirmed that Hand1 downregulated Ihh gene expression in vitro by inhibiting Runx2 transactivation of the Ihh proximal promoter. These results demonstrate that Hand1 in chondrocytes regulates endochondral ossification, at least in part through the Runx2-Ihh axis.

Multifaceted signaling regulators of chondrogenesis: Implications in cartilage regeneration and tissue engineering

Defects of articular cartilage present a unique clinical challenge due to its poor self-healing capacity and avascular nature. Current surgical treatment options do not ensure consistent regeneration of hyaline cartilage in favor of fibrous tissue. Here, we review the current understanding of the most important biological regulators of chondrogenesis and their interactions, to provide insight into potential applications for cartilage tissue engineering. These include various signaling pathways, including: fibroblast growth factors (FGFs), transforming growth factor β (TGF-β)/bone morphogenic proteins (BMPs), Wnt/β-catenin, Hedgehog, Notch, hypoxia, and angiogenic signaling pathways. Transcriptional and epigenetic regulation of chondrogenesis will also be discussed. Advances in our understanding of these signaling pathways have led to promising advances in cartilage regeneration and tissue engineering.

Differential effects of tyrosine-rich amelogenin peptide on chondrogenic and osteogenic differentiation of adult chondrocytes

Current approaches to treat osteoarthritis (OA) are insufficient. Autologous chondrocyte implantation (ACI) has been used for the past decade to treat patients with OA or focal cartilage defects. However, a number of complications have been reported post-ACI, including athrofibrosis and symptomatic hypertrophy. Thus, a long-term ACI strategy should ideally incorporate methods to ‘prime’ autologous chondrocytes to form a cartilage-specific matrix and suppress hypertrophic mineralization. The objective of this study is to examine the effects of tyrosine-rich amelogenin peptide (TRAP; an isoform of the developmental protein amelogenin) on human articular cartilage cell (HAC) chondrogenic differentiation and hypertrophic mineralization in vitro. Effects of chemically synthesized TRAP on HAC chondrogenic differentiation were determined by assessing: (1) sGAG production; (2) Alcian blue staining for proteoglycans; (3) collagen type II immunostaining; and (4) expression of the chondrogenic genes SOX9, ACAN and COL2A1. Hypertrophic mineralization was assayed by: (1) ALP expression; (2) Alizarin red staining for Ca⁺²-rich bone nodules; (3) OC immunostaining; and (4) expression of the osteogenic/hypertrophic genes Ihh and BSP. Chemically synthesized TRAP was found to suppress terminal osteogenic differentiation of HACs cultured in hypertrophic mineralization-like conditions, an effect mediated via down-regulation of the Ihh gene. Moreover, TRAP was found to augment chondrogenic differentiation of HACs via induction of SOX9 gene expression when cells were cultured in pro-chondrogenic media. The results obtained from this proof-of-concept study motivate further studies on the use of TRAP as part of a preconditioning regimen in autologous chondrocyte implantation procedures for OA patients and patients suffering from focal cartilage defects.

The Regulatory Role of Signaling Crosstalk in Hypertrophy of MSCs and Human Articular Chondrocytes

Hypertrophic differentiation of chondrocytes is a main barrier in application of mesenchymal stem cells (MSCs) for cartilage repair. In addition, hypertrophy occurs occasionally in osteoarthritis (OA). Here we provide a comprehensive review on recent literature describing signal pathways in the hypertrophy of MSCs-derived in vitro differentiated chondrocytes and chondrocytes, with an emphasis on the crosstalk between these pathways. Insight into the exact regulation of hypertrophy by the signaling network is necessary for the efficient application of MSCs for articular cartilage repair and for developing novel strategies for curing OA. We focus on articles describing the role of the main signaling pathways in regulating chondrocyte hypertrophy-like changes. Most studies report hypertrophic differentiation in chondrogenesis of MSCs, in both human OA and experimental OA. Chondrocyte hypertrophy is not under the strict control of a single pathway but appears to be regulated by an intricately regulated network of multiple signaling pathways, such as WNT, Bone morphogenetic protein (BMP)/Transforming growth factor-β (TGFβ), Parathyroid hormone-related peptide (PTHrP), Indian hedgehog (IHH), Fibroblast growth factor (FGF), Insulin like growth factor (IGF) and Hypoxia-inducible factor (HIF). This comprehensive review describes how this intricate signaling network influences tissue-engineering applications of MSCs in articular cartilage (AC) repair, and improves understanding of the disease stages and cellular responses within an OA articular joint.

The suture provides a niche for mesenchymal stem cells of craniofacial bones

Bone tissue undergoes constant turnover supported by stem cells. Recent studies showed that perivascular mesenchymal stem cells (MSCs) contribute to the turnover of long bones. Craniofacial bones are flat bones derived from a different embryonic origin than the long bones. The identity and regulating niche for craniofacial-bone MSCs remain unknown. Here, we identify Gli1+ cells within the suture mesenchyme as the main MSC population for craniofacial bones. They are not associated with vasculature, give rise to all craniofacial bones in the adult and are activated during injury repair. Gli1+ cells are typical MSCs in vitro. Ablation of Gli1+ cells leads to craniosynostosis and arrest of skull growth, indicating that these cells are an indispensable stem cell population. Twist1(+/-) mice with craniosynostosis show reduced Gli1+ MSCs in sutures, suggesting that craniosynostosis may result from diminished suture stem cells. Our study indicates that craniofacial sutures provide a unique niche for MSCs for craniofacial bone homeostasis and repair.

Thymosin beta 4 is associated with RUNX2 expression through the Smad and Akt signaling pathways in mouse dental epithelial cells

In previous studies by our group, we reported that thymosin beta 4 (Tb4) is closely associated with the initiation and development of the tooth germ, and can induce the expression of runt-related transcription factor 2 (RUNX2) during the development of the tooth germ. RUNX2 regulates the expression of odontogenesis-related genes, such as amelogenin, X-linked (Amelx), ameloblastin (Ambn) and enamelin (Enam), as well as the differentiation of osteoblasts during bone formation. However, the mechanisms through which Tb4 induces the expression of RUNX2 remain unknown. In the present study, we employed a mouse dental epithelial cell line, mDE6, with the aim to elucidate these mechanisms. The mDE6 cells expressed odontogenesis-related genes, such as Runx2, Amelx, Ambn and Enam, and formed calcified matrices upon the induction of calcification, thus showing characteristics of odontogenic epithelial cells. The expression of odontogenesis-related genes, and the calcification of the mDE6 cells were reduced by the inhibition of phosphorylated Smad1/5 (p-Smad1/5) and phosphorylated Akt (p-Akt) proteins. Furthermore, we used siRNA against Tb4 to determine whether RUNX2 expression and calcification are associated with Tb4 expression in the mDE6 cells. The protein expression of p-Smad1/5 and p-Akt in the mDE6 cells was reduced by treatment with Tb4-siRNA. These results suggest that Tb4 is associated with RUNX2 expression through the Smad and PI3K-Akt signaling pathways, and with calcification through RUNX2 expression in the mDE6 cells. This study provides putative information concerning the signaling pathway through which Tb4 induces RUNX2 expression, which may help to understand the regulation of tooth development and tooth regeneration.

O-GlcNAc modification of the runt-related transcription factor 2 (Runx2) links osteogenesis and nutrient metabolism in bone marrow mesenchymal stem cells

Runx2 is the master switch controlling osteoblast differentiation and formation of the mineralized skeleton. The post-translational modification of Runx2 by phosphorylation, ubiquitinylation, and acetylation modulates its activity, stability, and interactions with transcriptional co-regulators and chromatin remodeling proteins downstream of osteogenic signals. Characterization of Runx2 by electron transfer dissociation tandem mass spectrometry revealed sites of O-linked N-acetylglucosamine (O-GlcNAc) modification, a nutrient-responsive post-translational modification that modulates the action of numerous transcriptional effectors. O-GlcNAc modification occurs in close proximity to phosphorylated residues and novel sites of arginine methylation within regions known to regulate Runx2 transactivation. An interaction between Runx2 and the O-GlcNAcylated, O-GlcNAc transferase enzyme was also detected. Pharmacological inhibition of O-GlcNAcase (OGA), the enzyme responsible for the removal of O-GlcNAc from Ser/Thr residues, enhanced basal (39.9%) and BMP2/7-induced (43.3%) Runx2 transcriptional activity in MC3T3-E1 pre-osteoblasts. In bone marrow-derived mesenchymal stem cells differentiated for 6 days in osteogenic media, inhibition of OGA resulted in elevated expression (24.3%) and activity (65.8%) of alkaline phosphatase (ALP) an early marker of bone formation and a transcriptional target of Runx2. Osteogenic differentiation of bone marrow-derived mesenchymal stem cells in the presence of BMP2/7 for 8 days culminated in decreased OGA activity (39.0%) and an increase in the abundance of O-GlcNAcylated Runx2, as compared with unstimulated cells. Furthermore, BMP2/7-induced ALP activity was enhanced by 35.6% in bone marrow-derived mesenchymal stem cells differentiated in the presence of the OGA inhibitor, demonstrating that direct or BMP2/7-induced inhibition of OGA is associated with increased ALP activity. Altogether, these findings link O-GlcNAc cycling to the Runx2-dependent regulation of the early ALP marker under osteoblast differentiation conditions.

Indian Hedgehog, a critical modulator in osteoarthritis, could be a potential therapeutic target for attenuating cartilage degeneration disease

The Hedgehog (Hh) family of proteins consists of Indian hedgehog (Ihh), sonic hedgehog (Shh), and desert hedgehog (Dhh). These proteins serve as essential regulators in a variety of developmental events. Ihh is mainly produced and secreted by prehypertrophic chondrocytes and regulates chondrocyte hypertrophy and endochondral bone formation during growth plate development. Tissue-specific deletion of the Ihh gene (targeted by Col2a1-Cre) causes early lethality in mice. Transgenic mice with induced Ihh expression exhibit increased chondrocyte hypertrophy and cartilage damage resembling human osteoarthritis (OA). During OA development, chondrocytes recapitulate the differentiation process that happens during the fetal status and which does not occur to an appreciable degree in adult articular cartilage. Ihh expression is up-regulated in human OA cartilage, and this upregulation correlates with OA progression and changes in chondrocyte morphology. A genetic study in mice further showed that conditional deletion of Ihh in chondrocytes attenuates OA progression, suggesting the possibility that blocking Ihh signaling can be used as a therapeutic approach to prevent or delay cartilage degeneration. However, Ihh gene deletion is currently not a therapeutic option as it is lethal in animals. RNA interference (RNAi) provides a means to knockdown Ihh without the severe side effects caused by chemical inhibitors. The currently available delivery methods for RNAi are nanoparticles and liposomes. Both have problems that need to be addressed. In the future, it will be necessary to develop a safe and effective RNAi delivery system to target Ihh signaling for preventing and treating OA.

Sox9 potentiates BMP2-induced chondrogenic differentiation and inhibits BMP2-induced osteogenic differentiation

Bone morphogenetic protein 2 (BMP2) is one of the key chondrogenic growth factors involved in the cartilage regeneration. However, it also exhibits osteogenic abilities and triggers endochondral ossification. Effective chondrogenesis and inhibition of BMP2-induced osteogenesis and endochondral ossification can be achieved by directing the mesenchymal stem cells (MSCs) towards chondrocyte lineage with chodrogenic factors, such as Sox9. Here we investigated the effects of Sox9 on BMP2-induced chondrogenic and osteogenic differentiation of MSCs. We found exogenous overexpression of Sox9 enhanced the BMP2-induced chondrogenic differentiation of MSCs in vitro. Also, it inhibited early and late osteogenic differentiation of MSCs in vitro. Subcutaneous stem cell implantation demonstrated Sox9 potentiated BMP2-induced cartilage formation and inhibited endochondral ossification. Mouse limb cultures indicated that BMP2 and Sox9 acted synergistically to stimulate chondrocytes proliferation, and Sox9 inhibited BMP2-induced chondrocytes hypertrophy and ossification. This study strongly suggests that Sox9 potentiates BMP2-induced MSCs chondrogenic differentiation and cartilage formation, and inhibits BMP2-induced MSCs osteogenic differentiation and endochondral ossification. Thus, exogenous overexpression of Sox9 in BMP2-induced mesenchymal stem cells differentiation may be a new strategy for cartilage tissue engineering.