An 18-base-pair sequence in the mouse proalpha1(II) collagen gene is sufficient for expression in cartilage and binds nuclear proteins that are selectively expressed in chondrocytes

Variants in the SOX9 transactivation middle domain induce axial skeleton dysplasia and scoliosis

SOX9 is a crucial transcriptional regulator of cartilage development and homeostasis. Dysregulation of SOX9 is associated with a wide spectrum of skeletal disorders, including campomelic dysplasia, acampomelic campomelic dysplasia, and scoliosis. Yet how SOX9 variants contribute to the spectrum of axial skeletal disorders is not well understood. Here, we report four pathogenic variants of SOX9 identified in a cohort of patients with congenital vertebral malformations. We report a pathogenic missense variant in the transactivation middle (TAM) domain of SOX9 associated with mild skeletal dysplasia and scoliosis. We isolated a Sox9 mutant mouse with an in-frame microdeletion in the TAM domain (Sox9Asp272del), which exhibits skeletal dysplasia including kinked tails, rib cage anomalies, and scoliosis in homozygous mutants. We find that both the human missense and the mouse microdeletion mutations resulted in reduced SOX9 protein stability in cell culture, while Sox9Asp272del mutant mice show decreased SOX9 expression in the growth plate and annulus fibrosus tissues of the spine. This reduction in SOX9 expression was correlated with the reduction of extracellular matrix components, such as tenascin-X and the Adhesion G-protein coupled receptor ADGRG6. In summary, our work identified and modeled a pathologic variant of SOX9 within the TAM domain and demonstrated its importance for SOX9 protein stability. Our work demonstrates that SOX9 stability is important for the regulation of ADGRG6 expression, which is a known regulator of postnatal spine homeostasis, underscoring the essential role of SOX9 dosage in a spectrum of axial skeleton dysplasia in humans.

Fgfr3 enhancer deletion markedly improves all skeletal features in a mouse model of achondroplasia

Achondroplasia, the most prevalent short-stature disorder, is caused by missense variants overactivating the fibroblast growth factor receptor 3 (FGFR3). As current surgical and pharmaceutical treatments only partially improve some disease features, we sought to explore a genetic approach. We show that an enhancer located 29 kb upstream of mouse Fgfr3 (-29E) is sufficient to confer a transgenic mouse reporter with a domain of expression in cartilage matching that of Fgfr3. Its CRISPR/Cas9-mediated deletion in otherwise WT mice reduced Fgfr3 expression in this domain by half without causing adverse phenotypes. Importantly, its deletion in mice harboring the ortholog of the most common human achondroplasia variant largely normalized long bone and vertebral body growth, markedly reduced spinal canal and foramen magnum stenosis, and improved craniofacial defects. Consequently, mouse achondroplasia is no longer lethal, and adults are overall healthy. These findings, together with high conservation of -29E in humans, open a path to develop genetic therapies for people with achondroplasia.

Serinc5 Regulates Sequential Chondrocyte Differentiation by Inhibiting Sox9 Function in Pre-Hypertrophic Chondrocytes

The growth plate is the primary site of longitudinal bone growth with chondrocytes playing a pivotal role in endochondral bone development. Chondrocytes undergo a series of differentiation steps, resulting in the formation of a unique hierarchical columnar structure comprising round, proliferating, pre-hypertrophic, and hypertrophic chondrocytes. Pre-hypertrophic chondrocytes, which exist in the transitional stage between proliferating and hypertrophic stages, are a critical cell population in the growth plate. However, the molecular basis of pre-hypertrophic chondrocytes remains largely undefined. Here, we employed scRNA-seq analysis on fluorescently labeled growth plate chondrocytes for their molecular characterization. Serine incorporator 5 (Serinc5) was identified as a marker gene for pre-hypertrophic chondrocytes. Histological analysis revealed that Serinc5 is specifically expressed in pre-hypertrophic chondrocytes, overlapping with Indian hedgehog (Ihh). Serinc5 represses cell proliferation and Col2a1 and Acan expression by inhibiting the transcriptional activity of Sox9 in primary chondrocytes. Chromatin profiling using ChIP-seq and ATAC-seq revealed an active enhancer of Serinc5 located in intron 1, with its chromatin status progressively activated during chondrocyte differentiation. Collectively, our findings suggest that Serinc5 regulates sequential chondrocyte differentiation from proliferation to hypertrophy by inhibiting Sox9 function in pre-hypertrophic chondrocytes, providing novel insights into the mechanisms underlying chondrocyte differentiation in growth plates.

Chromatin profiling identifies chondrocyte-specific Sox9 enhancers important for skeletal development

The transcription factor SRY-related HMG box 9 (Sox9) is essential for chondrogenesis. Mutations in and around SOX9 cause campomelic dysplasia (CD) characterized by skeletal malformations. Although the function of Sox9 in this context is well studied, the mechanisms that regulate Sox9 expression in chondrocytes remain to be elucidated. Here, we have used genome-wide profiling to identify 2 Sox9 enhancers located in a proximal breakpoint cluster responsible for CD. Enhancer activity of E308 (located 308 kb 5' upstream) and E160 (located 160 kb 5' upstream) correlated with Sox9 expression levels, and both enhancers showed a synergistic effect in vitro. While single deletions in mice had no apparent effect, simultaneous deletion of both E308 and E160 caused a dwarf phenotype, concomitant with a reduction of Sox9 expression in chondrocytes. Moreover, bone morphogenetic protein 2-dependent chondrocyte differentiation of limb bud mesenchymal cells was severely attenuated in E308/E160 deletion mice. Finally, we found that an open chromatin region upstream of the Sox9 gene was reorganized in the E308/E160 deletion mice to partially compensate for the loss of E308 and E160. In conclusion, our findings reveal a mechanism of Sox9 gene regulation in chondrocytes that might aid in our understanding of the pathophysiology of skeletal disorders.

Measuring transcription factor function with cell type-specific somatic transgenesis in chicken embryos

Retroviral-mediated misexpression in chicken embryos has been a powerful research tool for developmental biologists in the last two decades. In the RCASBP retroviral vectors that are widely used for in vivo somatic transgenesis, a coding sequence of interest is under the transcriptional control of a strong viral promoter in the long terminal repeat. While this has proven to be effective for studying secreted signalling proteins, interpretation of the mechanisms of action of nuclear factors is more difficult using this system since it is not clear whether phenotypic effects are cell-autonomous or not, and therefore whether they represent a function of the endogenous protein. Here, we report the consequences of retroviral expression using the RCANBP backbone, in which the transcription factor Dlx5 is expressed under the control of chondrocyte-specific regulatory sequences from the Col2a1 gene. To our knowledge, this is the first demonstration of a tissue-specific phenotype in the chicken embryo.

Sox9 Inhibits Cochlear Hair Cell Fate by Upregulating Hey1 and HeyL Antagonists of Atoh1

It is widely accepted that cell fate determination in the cochlea is tightly controlled by different transcription factors (TFs) that remain to be fully defined. Here, we show that Sox9, initially expressed in the entire sensory epithelium of the cochlea, progressively disappears from differentiating hair cells (HCs) and is finally restricted to supporting cells (SCs). By performing ex vivo electroporation of E13.5-E14.5 cochleae, we demonstrate that maintenance of Sox9 expression in the progenitors committed to HC fate blocks their differentiation, even if co-expressed with Atoh1, a transcription factor necessary and sufficient to form HC. Sox9 inhibits Atoh1 transcriptional activity by upregulating Hey1 and HeyL antagonists, and genetic ablation of these genes induces extra HCs along the cochlea. Although Sox9 suppression from sensory progenitors ex vivo leads to a modest increase in the number of HCs, it is not sufficient in vivo to induce supernumerary HC production in an inducible Sox9 knockout model. Taken together, these data show that Sox9 is downregulated from nascent HCs to allow the unfolding of their differentiation program. This may be critical for future strategies to promote fully mature HC formation in regeneration approaches.

Chondrocyte Homeostasis and Differentiation: Transcriptional Control and Signaling in Healthy and Osteoarthritic Conditions

Chondrocytes are the main cell type in articular cartilage. They are embedded in an avascular, abundant, and specialized extracellular matrix (ECM). Chondrocytes are responsible for the synthesis and turnover of the ECM, in which the major macromolecular components are collagen, proteoglycans, and non-collagen proteins. The crosstalk between chondrocytes and the ECM plays several relevant roles in the regulation of cell phenotype. Chondrocytes live in an avascular environment in healthy cartilage with a low oxygen supply. Although chondrocytes are adapted to anaerobic conditions, many of their metabolic functions are oxygen-dependent, and most cartilage oxygen is supplied by the synovial fluid. This review focuses on the transcription control and signaling responsible for chondrocyte differentiation, homeostasis, senescence, and cell death and the changes that occur in osteoarthritis. The effects of chondroitin sulfate and other molecules as anti-inflammatory agents are also approached and analyzed.

Variants in the SOX9 transactivation middle domain induce axial skeleton dysplasia and scoliosis

SOX9 is an essential transcriptional regulator of cartilage development and homeostasis. In humans, dysregulation of SOX9 is associated with a wide spectrum of skeletal disorders, including campomelic and acampomelic dysplasia, and scoliosis. The mechanism of how SOX9 variants contribute to the spectrum of axial skeletal disorders is not well understood. Here, we report four novel pathogenic variants of SOX9 identified in a large cohort of patients with congenital vertebral malformations. Three of these heterozygous variants are in the HMG and DIM domains, and for the first time, we report a pathogenic variant within the transactivation middle (TAM) domain of SOX9 . Probands with these variants exhibit variable skeletal dysplasia, ranging from isolated vertebral malformation to acampomelic dysplasia. We also generated a Sox9 hypomorphic mutant mouse model bearing a microdeletion within the TAM domain ( Sox9 Asp272del ). We demonstrated that disturbance of the TAM domain with missense mutation or microdeletion results in reduced protein stability but does not affect the transcriptional activity of SOX9. Homozygous Sox9 Asp272del mice exhibited axial skeletal dysplasia including kinked tails, ribcage anomalies, and scoliosis, recapitulating phenotypes observed in human, while heterozygous mutants display a milder phenotype. Analysis of primary chondrocytes and the intervertebral discs in Sox9 Asp272del mutant mice revealed dysregulation of a panel of genes with major contributions of the extracellular matrix, angiogenesis, and ossification-related processes. In summary, our work identified the first pathologic variant of SOX9 within the TAM domain and demonstrated that this variant is associated with reduced SOX9 protein stability. Our finding suggests that reduced SOX9 stability caused by variants in the TAM domain may be responsible for the milder forms of axial skeleton dysplasia in humans.

Emerging RUNX2-Mediated Gene Regulatory Mechanisms Consisting of Multi-Layered Regulatory Networks in Skeletal Development

Skeletal development is tightly coordinated by chondrocytes and osteoblasts, which are derived from skeletal progenitors, and distinct cell-type gene regulatory programs underlie the specification and differentiation of cells. Runt-related transcription factor 2 (Runx2) is essential to chondrocyte hypertrophy and osteoblast differentiation. Genetic studies have revealed the biological functions of Runx2 and its involvement in skeletal genetic diseases. Meanwhile, molecular biology has provided a framework for our understanding of RUNX2-mediated transactivation at a limited number of cis-regulatory elements. Furthermore, studies using next-generation sequencing (NGS) have provided information on RUNX2-mediated gene regulation at the genome level and novel insights into the multiple layers of gene regulatory mechanisms, including the modes of action of RUNX2, chromatin accessibility, the concept of pioneer factors and phase separation, and three-dimensional chromatin organization. In this review, I summarize the emerging RUNX2-mediated regulatory mechanism from a multi-layer perspective and discuss future perspectives for applications in the treatment of skeletal diseases.

Cartilage-Specific Cre Recombinase Transgenes/Alleles in the Mouse

Cartilage is a specialized skeletal tissue with a unique extracellular matrix elaborated by its resident cells, chondrocytes. The tissue presents in several forms, including growth plate and articular cartilage, wherein chondrocytes follow a differential differentiation program and have different fates. The induction of gene modifications in cartilage specifically relies on mouse transgenes and knockin alleles taking advantages of transcriptional elements primarily active in chondrocytes at a specific differentiation stage or in a specific cartilage type. These transgenes/alleles have been widely used to study the roles of specific genes in cartilage development, adult homeostasis, and pathology. As cartilage formation is critical for postnatal life, the inactivation or significant alteration of key cartilaginous genes is often neonatally lethal and therefore hampers postnatal studies. Gold standard approaches to induce postnatal chondrocyte-specific gene modifications include the Cre-loxP and Tet-ON/OFF systems. Selecting the appropriate promoter/enhancer sequences to drive Cre expression is of crucial importance and determines the specificity of conditional gain- or loss-of-function models. In this chapter, we discuss a series of transgenes and knockin alleles that have been developed for gene manipulation in cartilage and we compare their expression patterns and efficiencies.

Biochemical characteristics of the chondrocyte-enriched SNORC protein and its transcriptional regulation by SOX9

Snorc (Small NOvel Rich in Cartilage) has been identified as a chondrocyte-specific gene in the mouse. Yet little is known about the SNORC protein biochemical properties, and mechanistically how the gene is regulated transcriptionally in a tissue-specific manner. The goals of the present study were to shed light on those important aspects. The chondrocyte nature of Snorc expression was confirmed in mouse and rat tissues, in differentiated (day 7) ATDC5, and in RCS cells where it was constitutive. Topological mapping and biochemical analysis brought experimental evidences that SNORC is a type I protein carrying a chondroitin sulfate (CS) attached to serine 44. The anomalous migration of SNORC on SDS-PAGE was due to its primary polypeptide features, suggesting no additional post-translational modifications apart from the CS glycosaminoglycan. A highly conserved SOX9-binding enhancer located in intron 1 was necessary to drive transcription of Snorc in the mouse, rat, and human. The enhancer was active independently of orientation and whether located in a heterologous promoter or intron. Crispr-mediated inactivation of the enhancer in RCS cells caused reduction of Snorc. Transgenic mice carrying the intronic multimerized enhancer drove high expression of a βGeo reporter in chondrocytes, but not in the hypertrophic zone. Altogether these data confirmed the chondrocyte-specific nature of Snorc and revealed dependency on the intronic enhancer binding of SOX9 for transcription.

Histone ChIP-Seq identifies differential enhancer usage during chondrogenesis as critical for defining cell-type specificity

Epigenetic mechanisms are known to regulate gene expression during chondrogenesis. In this study, we have characterized the epigenome during the in vitro differentiation of human mesenchymal stem cells (hMSCs) into chondrocytes. Chromatin immunoprecipitation followed by next‐generation sequencing (ChIP‐seq) was used to assess a range of N‐terminal posttranscriptional modifications (marks) to histone H3 lysines (H3K4me3, H3K4me1, H3K27ac, H3K27me3, and H3K36me3) in both hMSCs and differentiated chondrocytes. Chromatin states were characterized using histone ChIP‐seq and cis‐regulatory elements were identified in chondrocytes. Chondrocyte enhancers were associated with chondrogenesis‐related gene ontology (GO) terms. In silico analysis and integration of DNA methylation data with chondrogenesis chromatin states revealed that enhancers marked by histone marks H3K4me1 and H3K27ac were de‐methylated during in vitro chondrogenesis. Similarity analysis between hMSC and chondrocyte chromatin states defined in this study with epigenomes of cell‐types defined by the Roadmap Epigenomics project revealed that enhancers are more distinct between cell‐types compared to other chromatin states. Motif analysis revealed that the transcription factor SOX9 is enriched in chondrocyte enhancers. Luciferase reporter assays confirmed that chondrocyte enhancers characterized in this study exhibited enhancer activity which may be modulated by DNA methylation and SOX9 overexpression. Altogether, these integrated data illustrate the cross‐talk between different epigenetic mechanisms during chondrocyte differentiation.

Specific, Sensitive, and Stable Reporting of Human Mesenchymal Stromal Cell Chondrogenesis

IMPACT STATEMENT

The promoter characterized in this study has been made accessible as a resource for the skeletal tissue engineering and regenerative medicine community. When combined with suitable reporter vectors, the resulting tools can be used for noninvasive and/or high-throughput screening of test conditions for stimulating chondrogenesis by candidate stem/progenitor cells. As demonstrated in this study, they can also be used with small animal imaging platforms to monitor the chondrogenic activity of implanted progenitors within orthotopic models of bone and cartilage repair.

Insights into Gene Regulatory Networks in Chondrocytes

Chondrogenesis is a key developmental process that molds the framework of our body and generates the skeletal tissues by coupling with osteogenesis. The developmental processes are well-coordinated by spatiotemporal gene expressions, which are hardwired with gene regulatory elements. Those elements exist as thousands of modules of DNA sequences on the genome. Transcription factors function as key regulatory proteins by binding to regulatory elements and recruiting cofactors. Over the past 30 years, extensive attempts have been made to identify gene regulatory mechanisms in chondrogenesis, mainly through biochemical approaches and genetics. More recently, newly developed next-generation sequencers (NGS) have identified thousands of gene regulatory elements on a genome scale, and provided novel insights into the multiple layers of gene regulatory mechanisms, including the modes of actions of transcription factors, post-translational histone modifications, chromatin accessibility, the concept of pioneer factors, and three-dimensional chromatin architecture. In this review, we summarize the studies that have improved our understanding of the gene regulatory mechanisms in chondrogenesis, from the historical studies to the more recent works using NGS. Finally, we consider the future perspectives, including efforts to improve our understanding of the gene regulatory landscape in chondrogenesis and potential applications to the treatment of chondrocyte-related diseases.

SOX9 is dispensable for the initiation of epigenetic remodeling and the activation of marker genes at the onset of chondrogenesis

SOX9 controls cell lineage fate and differentiation in major biological processes. It is known as a potent transcriptional activator of differentiation-specific genes, but its earliest targets and its contribution to priming chromatin for gene activation remain unknown. Here, we address this knowledge gap using chondrogenesis as a model system. By profiling the whole transcriptome and the whole epigenome of wild-type and Sox9-deficient mouse embryo limb buds, we uncover multiple structural and regulatory genes, including Fam101a, Myh14, Sema3c and Sema3d, as specific markers of precartilaginous condensation, and we provide evidence of their direct transactivation by SOX9. Intriguingly, we find that SOX9 helps remove epigenetic signatures of transcriptional repression and establish active-promoter and active-enhancer marks at precartilage- and cartilage-specific loci, but is not absolutely required to initiate these changes and activate transcription. Altogether, these findings widen our current knowledge of SOX9 targets in early chondrogenesis and call for new studies to identify the pioneer and transactivating factors that act upstream of or along with SOX9 to prompt chromatin remodeling and specific gene activation at the onset of chondrogenesis and other processes.

Skeletal Characterization of the Fgfr3 Mouse Model of Achondroplasia Using Micro-CT and MRI Volumetric Imaging

Achondroplasia, the most common form of dwarfism, affects more than a quarter million people worldwide and remains an unmet medical need. Achondroplasia is caused by mutations in the fibroblast growth factor receptor 3 (FGFR3) gene which results in over-activation of the receptor, interfering with normal skeletal development leading to disproportional short stature. Multiple mouse models have been generated to study achondroplasia. The characterization of these preclinical models has been primarily done with 2D measurements. In this study, we explored the transgenic model expressing mouse Fgfr3 containing the achondroplasia mutation G380R under the Col2 promoter (Ach). Survival and growth rate of the Ach mice were reduced compared to wild-type (WT) littermates. Axial skeletal defects and abnormalities of the sternebrae and vertebrae were observed in the Ach mice. Further evaluation of the Ach mouse model was performed by developing 3D parameters from micro-computed tomography (micro-CT) and magnetic resonance imaging (MRI). The 3-week-old mice showed greater differences between the Ach and WT groups compared to the 6-week-old mice for all parameters. Deeper understanding of skeletal abnormalities of this model will help guide future studies for evaluating novel and effective therapeutic approaches for the treatment of achondroplasia.

Transcriptional Network Controlling Endochondral Ossification

Endochondral ossification is the fundamental process of skeletal development in vertebrates. Chondrocytes undergo sequential steps of differentiation, including mesenchymal condensation, proliferation, hypertrophy, and mineralization. These steps, which are required for the morphological and functional changes in differentiating chondrocytes, are strictly regulated by a complex transcriptional network. Biochemical and mice genetic studies identified chondrogenic transcription factors critical for endochondral ossification. The transcription factor sex-determining region Y (SRY)-box 9 (Sox9) is essential for early chondrogenesis, and impaired Sox9 function causes severe chondrodysplasia in humans and mice. In addition, recent genome-wide chromatin immunoprecipitation-sequencing studies revealed the precise regulatory mechanism of Sox9 during early chondrogenesis. Runt-related transcription factor 2 promotes chondrocyte hypertrophy and terminal differentiation. Interestingly, endoplasmic reticulum (ER) stress-related transcription factors have recently emerged as novel regulators of chondrocyte differentiation. Here we review the transcriptional mechanisms that regulate endochondral ossification, with a focus on Sox9.

A Novel Regulatory Mechanism of Type II Collagen Expression via a SOX9-dependent Enhancer in Intron 6

Type II collagen α1 is specific for cartilaginous tissues, and mutations in its gene are associated with skeletal diseases. Its expression has been shown to be dependent on SOX9, a master transcription factor required for chondrogenesis that binds to an enhancer region in intron 1. However, ChIP sequencing revealed that SOX9 does not strongly bind to intron 1, but rather it binds to intron 6 and a site 30 kb upstream of the transcription start site. Here, we aimed to determine the role of the novel SOX9-binding site in intron 6. We prepared reporter constructs that contain a Col2a1 promoter, intron 1 with or without intron 6, and the luciferase gene. Although the reporter constructs were not activated by SOX9 alone, the construct that contained both introns 1 and 6 was activated 5-10-fold by the SOX9/SOX5 or the SOX9/SOX6 combination in transient-transfection assays in 293T cells. This enhancement was also observed in rat chondrosarcoma cells that stably expressed the construct. CRISPR/Cas9-induced deletion of intron 6 in RCS cells revealed that a 10-bp region of intron 6 is necessary both for Col2a1 expression and SOX9 binding. Furthermore, SOX9, but not SOX5, binds to this region as demonstrated in an electrophoretic mobility shift assay, although both SOX9 and SOX5 bind to a larger 325-bp fragment of intron 6 containing this small sequence. These findings suggest a novel mechanism of action of SOX5/6; namely, the SOX9/5/6 combination enhances Col2a1 transcription through a novel enhancer in intron 6 together with the enhancer in intron 1.

Signaling Cascades Governing Cdc42-Mediated Chondrogenic Differentiation and Mensenchymal Condensation

Endochondral ossification consists of successive steps of chondrocyte differentiation, including mesenchymal condensation, differentiation of chondrocytes, and hypertrophy followed by mineralization and ossification. Loss-of-function studies have revealed that abnormal growth plate cartilage of the Cdc42 mutant contributes to the defects in endochondral bone formation. Here, we have investigated the roles of Cdc42 in osteogenesis and signaling cascades governing Cdc42-mediated chondrogenic differentiation. Though deletion of Cdc42 in limb mesenchymal progenitors led to severe defects in endochondral ossification, either ablation of Cdc42 in limb preosteoblasts or knockdown of Cdc42 in vitro had no obvious effects on bone formation and osteoblast differentiation. However, in Cdc42 mutant limb buds, loss of Cdc42 in mesenchymal progenitors led to marked inactivation of p38 and Smad1/5, and in micromass cultures, Cdc42 lay on the upstream of p38 to activate Smad1/5 in bone morphogenetic protein-2-induced mesenchymal condensation. Finally, Cdc42 also lay on the upstream of protein kinase B to transactivate Sox9 and subsequently induced the expression of chondrocyte differential marker in transforming growth factor-β1-induced chondrogenesis. Taken together, by using biochemical and genetic approaches, we have demonstrated that Cdc42 is involved not in osteogenesis but in chondrogenesis in which the BMP2/Cdc42/Pak/p38/Smad signaling module promotes mesenchymal condensation and the TGF-β/Cdc42/Pak/Akt/Sox9 signaling module facilitates chondrogenic differentiation.

The transcription factors SOX9 and SOX5/SOX6 cooperate genome-wide through super-enhancers to drive chondrogenesis

SOX9 is a transcriptional activator required for chondrogenesis, and SOX5 and SOX6 are closely related DNA-binding proteins that critically enhance its function. We use here genome-wide approaches to gain novel insights into the full spectrum of the target genes and modes of action of this chondrogenic trio. Using the RCS cell line as a faithful model for proliferating/early prehypertrophic growth plate chondrocytes, we uncover that SOX6 and SOX9 bind thousands of genomic sites, frequently and most efficiently near each other. SOX9 recognizes pairs of inverted SOX motifs, whereas SOX6 favors pairs of tandem SOX motifs. The SOX proteins primarily target enhancers. While binding to a small fraction of typical enhancers, they bind multiple sites on almost all super-enhancers (SEs) present in RCS cells. These SEs are predominantly linked to cartilage-specific genes. The SOX proteins effectively work together to activate these SEs and are required for in vivo expression of their associated genes. These genes encode key regulatory factors, including the SOX trio proteins, and all essential cartilage extracellular matrix components. Chst11, Fgfr3, Runx2 and Runx3 are among many other newly identified SOX trio targets. SOX9 and SOX5/SOX6 thus cooperate genome-wide, primarily through SEs, to implement the growth plate chondrocyte differentiation program.

Distinct Transcriptional Programs Underlie Sox9 Regulation of the Mammalian Chondrocyte

Sox9 encodes an essential transcriptional regulator of chondrocyte specification and differentiation. When Sox9 nuclear activity was compared with markers of chromatin organization and transcriptional activity in primary chondrocytes, we identified two distinct categories of target association. Class I sites cluster around the transcriptional start sites of highly expressed genes with no chondrocyte-specific signature. Here, Sox9 association reflects protein-protein association with basal transcriptional components. Class II sites highlight evolutionarily conserved active enhancers that direct chondrocyte-related gene activity through the direct binding of Sox9 dimer complexes to DNA. Sox9 binds through sites with sub-optimal binding affinity; the number and grouping of enhancers into super-enhancer clusters likely determines the levels of target gene expression. Interestingly, comparison of Sox9 action in distinct chondrocyte lineages points to similar regulatory strategies. In addition to providing insights into Sox family action, our comprehensive identification of the chondrocyte regulatory genome will facilitate the study of skeletal development and human disease.

How to build an inducible cartilage-specific transgenic mouse

Transgenic mice are used to study the roles of specific proteins in an intact living system. Use of transgenic mice to study processes in cartilage, however, poses some challenges. First of all, many factors involved in cartilage homeostasis and disease are also crucial factors in embryogenesis. Therefore, meddling with these factors often leads to death before birth, and mice who do survive cannot be considered normal. The build-up of cartilage in these mice is altered, making it nearly impossible to truly interpret the role of a protein in adult cartilage function.An elegant way to overcome these limitations is to make transgenic mice time- and tissue-specific, there by omitting side-effects in tissues other than cartilage and during embryology. This review discusses the potential building blocks for making an inducible cartilage-specific transgenic mouse. We review which promoters can be used to gain chondrocyte-specificity - all chondrocytes or a specific subset thereof - as well as different systems that can be used to enable inducibility of a transgene.

Fibroblast growth factor and canonical WNT/β-catenin signaling cooperate in suppression of chondrocyte differentiation in experimental models of FGFR signaling in cartilage

Aberrant fibroblast growth factor (FGF) signaling disturbs chondrocyte differentiation in skeletal dysplasia, but the mechanisms underlying this process remain unclear. Recently, FGF was found to activate canonical WNT/β-catenin pathway in chondrocytes via Erk MAP kinase-mediated phosphorylation of WNT co-receptor Lrp6. Here, we explore the cellular consequences of such a signaling interaction. WNT enhanced the FGF-mediated suppression of chondrocyte differentiation in mouse limb bud micromass and limb organ cultures, leading to inhibition of cartilage nodule formation in micromass cultures, and suppression of growth in cultured limbs. Simultaneous activation of the FGF and WNT/β-catenin pathways resulted in loss of chondrocyte extracellular matrix, expression of genes typical for mineralized tissues and alteration of cellular shape. WNT enhanced the FGF-mediated downregulation of chondrocyte proteoglycan and collagen extracellular matrix via inhibition of matrix synthesis and induction of proteinases involved in matrix degradation. Expression of genes regulating RhoA GTPase pathway was induced by FGF in cooperation with WNT, and inhibition of the RhoA signaling rescued the FGF/WNT-mediated changes in chondrocyte cellular shape. Our results suggest that aberrant FGF signaling cooperates with WNT/β-catenin in suppression of chondrocyte differentiation.

A novel SOX9 H169Q mutation in a family with overlapping phenotype of mild campomelic dysplasia and small patella syndrome

The phenotypic similarities have been demonstrated between non-lethal campomelic dysplasia (CD) and small patella syndrome (SPS), in which different genetic defects have been identified. We report on a familial case of skeletal dysplasia with overlapping phenotype of mild CD and SPS, including defective ischio-pubic ossification, elongated femoral neck, hypoplastic patellae, and increased space between the first and the second toes (sandal gap). Direct sequencing analysis demonstrated a novel missense mutation (p.H169Q) within the coding region of the SOX9 gene and negative for TBX4 mutations. Functional analysis of the p.H169Q mutant revealed reduced but not fully abolished transactivation capacity of the mutated protein. Retained residual SOX9 function might contribute to an extremely mild CD phenotype in the present cases. © 2013 Wiley Periodicals, Inc.

A far-upstream (-70 kb) enhancer mediates Sox9 auto-regulation in somatic tissues during development and adult regeneration

SOX9 encodes a transcription factor that presides over the specification and differentiation of numerous progenitor and differentiated cell types, and although SOX9 haploinsufficiency and overexpression cause severe diseases in humans, including campomelic dysplasia, sex reversal and cancer, the mechanisms underlying SOX9 transcription remain largely unsolved. We identify here an evolutionarily conserved enhancer located 70-kb upstream of mouse Sox9 and call it SOM because it specifically activates a Sox9 promoter reporter in most Sox9-expressing somatic tissues in transgenic mice. Moreover, SOM-null fetuses and pups reduce Sox9 expression by 18–37% in the pancreas, lung, kidney, salivary gland, gut and liver. Weanlings exhibit half-size pancreatic islets and underproduce insulin and glucagon, and adults slowly recover from acute pancreatitis due to a 2-fold impairment in Sox9 upregulation. Molecular and genetic experiments reveal that Sox9 protein dimers bind to multiple recognition sites in the SOM sequence and are thereby both necessary and sufficient for enhancer activity. These findings thus uncover that Sox9 directly enhances its functions in somatic tissue development and adult regeneration through SOM-mediated positive auto-regulation. They provide thereby novel insights on molecular mechanisms controlling developmental and disease processes and suggest new strategies to improve disease treatments.

SOX9 and myocardin counteract each other in regulating vascular smooth muscle cell differentiation

Transdifferentiation of vascular smooth muscle cells (VSMC) into chondrogenic cells contributes significantly to vascular calcification during the pathogenesis of atherosclerosis. However, the transcriptional mechanisms that control such phenotypic switch remain unclear. This process is characterized by the induction of Sox9 and Col2a1 genes accompanied by the repression of myocardin (Myocd) and SMC differentiation markers such as SM22, SM α-actin and SM-MHC. Here we explore the regulatory role of SOX9, the master regulator for chondrogenesis, in modulating SMC marker gene expression. qRT-PCR and luciferase assays show that over-expression of SOX9 inhibits SMC gene transcription and promoter activities induced by myocardin, the master regulator of smooth muscle differentiation. Such suppression is independent of the CArG box in the SMC promoters but dependent on myocardin. EMSA assay further shows that SOX9 neither participates in SRF (serum response factor) binding to the CArG box nor interacts with SRF, while co-immunoprecipitation demonstrates an association of SOX9 with myocardin. Conversely, myocardin suppresses SOX9-mediated chondrogenic gene Col2a1 expression. These findings provide the first mechanistic insights into the important regulatory role of SOX9 and myocardin in controlling the transcription program during SMC transdifferentiation into chondrocytes.

Basic helix-loop-helix transcription factor Twist1 inhibits transactivator function of master chondrogenic regulator Sox9

Canonical Wnt signaling strongly inhibits chondrogenesis. Previously, we identified Twist1 as a critical downstream mediator of Wnt in repression of chondrocyte differentiation. However, the mechanistic basis for the antichondrogenic activity of Twist1 has not heretofore been established. Here, we show that Twist1 suppresses cartilage development by directly inhibiting the transcriptional activity of Sox9, the master regulator of chondrogenesis. Twist1, through its carboxyl-terminal Twist-box, binds to the Sox9 high mobility group DNA-binding domain, inhibiting Sox9 transactivation potential. In chondrocyte precursor cells, Twist1, in a Twist-box-dependent manner, inhibits Sox9-dependent activation of chondrocyte marker gene expression by blocking Sox9-enhancer DNA association. These findings identify Twist1 as an inhibitor of Sox9 and further suggest that the balance between Twist1 and Sox9 may determine the earliest steps of chondrogenesis.

Nkx3.2 promotes primary chondrogenic differentiation by upregulating Col2a1 transcription

BACKGROUND

The Nkx3.2 transcription factor promotes chondrogenesis by forming a positive regulatory loop with a crucial chondrogenic transcription factor, Sox9. Previous studies have indicated that factors other than Sox9 may promote chondrogenesis directly, but these factors have not been identified. Here, we test the hypothesis that Nkx3.2 promotes chondrogenesis directly by Sox9-independent mechanisms and indirectly by previously characterized Sox9-dependent mechanisms.

METHODOLOGY/PRINCIPAL FINDINGS

C3H10T1/2 pluripotent mesenchymal cells were cultured with bone morphogenetic protein 2 (BMP2) to induce endochondral ossification. Overexpression of wild-type Nkx3.2 (WT-Nkx3.2) upregulated glycosaminoglycan (GAG) production and expression of type II collagen α1 (Col2a1) mRNA, and these effects were evident before WT-Nkx3.2-mediated upregulation of Sox9. RNAi-mediated inhibition of Nkx3.2 abolished GAG production and expression of Col2a1 mRNA. Dual luciferase reporter assays revealed that WT-Nkx3.2 upregulated Col2a1 enhancer activity in a dose-dependent manner in C3H10T1/2 cells and also in N1511 chondrocytes. In addition, WT-Nkx3.2 partially restored downregulation of GAG production, Col2 protein expression, and Col2a1 mRNA expression induced by Sox9 RNAi. ChIP assays revealed that Nkx3.2 bound to the Col2a1 enhancer element.

CONCLUSIONS/SIGNIFICANCE

Nkx3.2 promoted primary chondrogenesis by two mechanisms: Direct and Sox9-independent upregulation of Col2a1 transcription and upregulation of Sox9 mRNA expression under positive feedback system.

Exportin 4 interacts with Sox9 through the HMG Box and inhibits the DNA binding of Sox9

Sox9 is a transcription factor that is required for tissue development in mammals. In general, such transcription factors require co-regulators for precise temporal and spatial control of the activation and inactivation of the numerous genes necessary for precise development during embryogenesis. Here we identify a new Sox9 co-regulator: Using affinity chromatography with immobilized Sox9 protein, we identified exportin 4 (Exp4) as an interacting protein of Sox9 in human cultured cells. Interaction between endogenous Exp4 and Sox9 proteins was confirmed in the human osteosarcoma U2OS cells by immunoprecipitation experiments using anti-Sox9 antibody. siRNA depletion of Exp4 enhanced transcription of Sox9 target genes in U2OS cells, but did not affect nuclear localization of Sox9. These results suggest that Exp4 regulates Sox9 activity in the nucleus. Furthermore we found that the HMG box of Sox9 was responsible for binding to Exp4, and the HMG box was required for suppression of Sox9-mediated transcription. This contrasts with the known Sox9 co-regulators which bind to its transcriptional activation domain. Chromatin immunoprecipitation analyses revealed that Exp4 prevents Sox9 binding to the enhancers of its target genes. These results demonstrate that Exp4 acts as a Sox9 co-regulator that directly regulates binding of Sox9 to its target genes.

The effect of quercetin on expression of SOX9 and subsequent release of type II collagen in spheno-occipital synchondroses of organ-cultured mice

OBJECTIVE

To identify the expressions of SOX9 and type II collagen in spheno-occipital synchondrosis in response to quercetin, using a mouse in vitro model.

MATERIALS AND METHODS

A total of 50 one-day-old male BALB/c mice were randomly assigned to the control and experimental groups. Each group was subdivided into five different time points, which were 6, 24, 48, 72, and 168 hours, and each subgroup contained 5 mice (n  =  5). In the experimental group, the spheno-occipital synchondrosis was immersed in the BGJb medium + quercetin dihydrate 1 µM. In the control group, the spheno-occipital synchondrosis was immersed in the BGJb medium. Tissue sections were subjected to immunohistochemical staining for SOX9 and type II collagen expressions.

RESULTS

Quantitative analysis revealed there was a statistically significant increase of 32.31% (P < .001) in the expression of SOX9 between experimental groups and control groups at 24 hours. Furthermore, there was a statistically significant increase of 22.99% (P < .001) in the expression of type II collagen between experimental groups and control groups at 72 hours.

CONCLUSION

The expressions of SOX9 and type II collagen in the spheno-occipital synchondrosis can be increased by quercetin.

Arid5a cooperates with Sox9 to stimulate chondrocyte-specific transcription

SRY-box-containing gene 9 (Sox9) is an essential transcription factor in chondrocyte lineage determination and differentiation. Recent studies demonstrated that Sox9 controls the transcription of chondrocyte-specific genes in association with several other transcriptional regulators. To further understand the molecular mechanisms by which Sox9 influences transcriptional events during chondrocyte differentiation, we attempted to identify transcriptional partners of Sox9 and to examine their roles in chondrocyte differentiation. We isolated AT-rich interactive domain-containing protein 5a (Arid5a; also known as Mrf1) as an activator of the Col2a1 gene promoter from an ATDC5 cDNA library. Arid5a was highly expressed in cartilage and induced during chondrocyte differentiation. Furthermore, Arid5a physically interacted with Sox9 in nuclei and up-regulated the chondrocyte-specific action of Sox9. Overexpression of Arid5a stimulated chondrocyte differentiation in vitro and in an organ culture system. In contrast, Arid5a knockdown inhibited Col2a1 expression in chondrocytes. In addition, Arid5a binds directly to the promoter region of the Col2a1 gene and stimulates acetylation of histone 3 in the region. Our results suggest that Arid5a may directly interact with Sox9 and thereby enhance its chondrocyte-specific action.

The transcription factor Znf219 regulates chondrocyte differentiation by assembling a transcription factory with Sox9

Sox9 is an essential transcription factor for chondrogenesis by regulating the expression of chondrogenic genes. However, its regulatory mechanism is not fully understood. To address this, we attempted to identify the transcriptional partners of Sox9 by screening the cDNA library of the chondrogenic cell line ATDC5 using the collagen 2α1 (Col2α1) gene promoter fused to a luciferase reporter gene. One of the positive clones encoded the Znf219 gene. Whole mount in situ hybridization experiments indicated that Znf219 mRNA was specifically expressed in the developing limb buds where Col2α1 and Sox9 were strongly expressed. Znf219 markedly enhanced the transcriptional activity of Sox9 on the Col2a1 gene promoter. In addition, Znf219 is physically associated with Sox9 and is colocalized with Sox9 in the nucleus. We also found that overexpression of Znf219 profoundly increased Sox9-induced mRNA expression of Col2a1, aggrecan and Col11a2. Consistently, knockdown of Znf219 decreased the Sox9-induced mRNA expression of these genes. Furthermore, a dominant-negative mutant Znf219 inhibited Bmp2-induced chondrocyte differentiation. Our results suggest that Znf219 plays an important role in the regulation of chondrocyte differentiation as a transcriptional partner of Sox9.

Identification of SOX9 interaction sites in the genome of chondrocytes

BACKGROUND

Our previous work has provided strong evidence that the transcription factor SOX9 is completely needed for chondrogenic differentiation and cartilage formation acting as a "master switch" in this differentiation. Heterozygous mutations in SOX9 cause campomelic dysplasia, a severe skeletal dysmorphology syndrome in humans characterized by a generalized hypoplasia of endochondral bones. To obtain insights into the logic used by SOX9 to control a network of target genes in chondrocytes, we performed a ChIP-on-chip experiment using SOX9 antibodies.

METHODOLOGY/PRINCIPAL FINDINGS

The ChIP DNA was hybridized to a microarray, which covered 80 genes, many of which are involved in chondrocyte differentiation. Hybridization peaks were detected in a series of cartilage extracellular matrix (ECM) genes including Col2a1, Col11a2, Aggrecan and Cdrap as well as in genes for specific transcription factors and signaling molecules. Our results also showed SOX9 interaction sites in genes that code for proteins that enhance the transcriptional activity of SOX9. Interestingly, a strong SOX9 signal was also observed in genes such as Col1a1 and Osx, whose expression is strongly down regulated in chondrocytes but is high in osteoblasts. In the Col2a1 gene, in addition to an interaction site on a previously identified enhancer in intron 1, another strong interaction site was seen in intron 6. This site is free of nucleosomes specifically in chondrocytes suggesting an important role of this site on Col2a1 transcription regulation by SOX9.

CONCLUSIONS/SIGNIFICANCE

Our results provide a broad understanding of the strategies used by a "master" transcription factor of differentiation in control of the genetic program of chondrocytes.

The main product of specialized tissues regulates cell life and may cause neoplastic transformation

Many tissues and cells in vertebrates are highly specialized and devoted to a single function through the action of a single molecule, that we call the "main product" (MP) of the cell. The hypothesis here proposed is that these MPs control all aspects of the cell life, namely activity, division, differentiation and apoptosis. Evidences supporting this hypothesis are reported for the immune system, pancreatic beta-cells, melanocytes, connective tissues, thyroid cells, skin and erythroid cells. In all cases cell division and differentiation is promoted by a normal activity of the MP, while hyperactivity leads to cell apoptosis. Evidences are also provided that alterations of the activity of the MP may elicit pathological disorders; in particular mutations altering the structure of the MP may elicit tumoural transformation.

Sox9 family members negatively regulate maturation and calcification of chondrocytes through up-regulation of parathyroid hormone-related protein

Sox9 is a transcription factor that plays an essential role in chondrogenesis and has been proposed to inhibit the late stages of endochondral ossification. However, the molecular mechanisms underlying the regulation of chondrocyte maturation and calcification by Sox9 remain unknown. In this study, we attempted to clarify roles of Sox9 in the late stages of chondrocyte differentiation. We found that overexpression of Sox9 alone or Sox9 together with Sox5 and Sox6 (Sox5/6/9) inhibited the maturation and calcification of murine primary chondrocytes and up-regulated parathyroid hormone-related protein (PTHrP) expression in primary chondrocytes and the mesenchymal cell line C3H10T1/2. Sox5/6/9 stimulated the early stages of chondrocyte proliferation and development. In contrast, Sox5/6/9 inhibited maturation and calcification of chondrocytes in organ culture. The inhibitory effects of Sox5/6/9 were rescued by treating with anti-PTHrP antibody. Moreover, Sox5/6/9 bound to the promoter region of the PTHrP gene and up-regulated PTHrP gene promoter activity. Interestingly, we also found that the Sox9 family members functionally collaborated with Ihh/Gli2 signaling to regulate PTHrP expression and chondrocyte differentiation. Our results provide novel evidence that Sox9 family members mediate endochondral ossification by up-regulating PTHrP expression in association with Ihh/Gli2 signaling.

Interference by adrenaline with chondrogenic differentiation through suppression of gene transactivation mediated by Sox9 family members

In contrast to osteoblasts, little attention has been paid to the functional expression of adrenergic signaling machineries in chondrocytes. Expression of mRNA was for the first time demonstrated for different adrenergic receptor (AdR) subtypes in chondrogenic ATDC5 cells and mouse metatarsals isolated before vascularization in culture, but not for other molecules related to adrenergic signaling. In neonatal mouse tibial sections, beta(2)AdR and alpha(2a)AdR mRNA expression was found in chondrocytes at different developmental stages by in situ hybridization. Exposure to adrenaline significantly suppressed expression of several maturation markers through the cAMP/protein kinase A pathway activated by beta(2)AdR without affecting cellular proliferation in both cultured ATDC5 cells and metatarsals. Adrenaline also significantly inhibited gene transactivation by sry-type HMG box 9 (Sox9) family members essential for chondrogenic differentiation in a manner prevented by the general betaAdR antagonist propranolol, with a concomitant significant decrease in the levels of Sox6 mRNA and corresponding protein, in ATDC5 cells and primary cultured mouse costal chondrocytes. Systemic administration of propranolol significantly promoted the increased expression of mRNA for collagen I and collagen X, but not for collagen II, in callus of fractured femur in mice. These results suggest that adrenaline may interfere with chondrogenic differentiation through downregulation of Sox6 expression for subsequent suppression of gene transactivation mediated by Sox9 family members after activation of beta(2)AdR expressed by chondrocytes.

The transcriptional activity of Sox9 in chondrocytes is regulated by RhoA signaling and actin polymerization

In this study, we demonstrate that dedifferentiation of round primary chondrocytes into a fibroblast morphology correlates with a profound induction of RhoA protein and stress fibers. Culture of dedifferentiated chondrocytes in alginate gel induces a precipitous loss of RhoA protein and a loss of stress fibers concomitant with the reexpression of the chondrocyte differentiation program. We have found that chondrogenesis in limb bud micromass cultures similarly entails a loss of RhoA protein and that expression of dominant negative RhoA in such cultures can markedly enhance chondrogenesis. Consistent with these results, expression of the Rho antagonist C3 transferase can restore chondrocyte gene expression in dedifferentiated chondrocytes grown on plastic. Transfection of cells with agents that block actin polymerization enhance the ability of either exogenous Sox9 or a Gal4 DBD-Sox9 fusion protein to activate gene expression. Interestingly, the enhancement of Sox9 function by actin depolymerization requires both protein kinase A (PKA) activity and a PKA phosphorylation site in Sox9 (S181) that is known to enhance Sox9 transcriptional activity. Lastly, we demonstrate that RhoA-mediated modulation of actin polymerization regulates the ability of Sox9 to both activate chondrocyte-specific markers and maintain its own expression in chondrocytes via a positive feedback loop.

The transcription factor Lc-Maf participates in Col27a1 regulation during chondrocyte maturation

The transcription factor Lc-Maf, which is a splice variant of c-Maf, is expressed in cartilage undergoing endochondral ossification and participates in the regulation of type II collagen through a cartilage-specific Col2a1 enhancer element. Type XXVII and type XI collagens are also expressed in cartilage during endochondral ossification, and so enhancer/reporter assays were used to determine whether Lc-Maf could regulate cartilage-specific enhancers from the Col27a1 and Col11a2 genes. The Col27a1 enhancer was upregulated over 4-fold by Lc-Maf, while the Col11a2 enhancer was downregulated slightly. To confirm the results of these reporter assays, rat chondrosarcoma (RCS) cells were transiently transfected with an Lc-Maf expression plasmid, and quantitative RT-PCR was performed to measure the expression of endogenous Col27a1 and Col11a2 genes. Endogenous Col27a1 was upregulated 6-fold by Lc-Maf overexpression, while endogenous Col11a2 was unchanged. Finally, in situ hybridization and immunohistochemistry were performed in the radius and ulna of embryonic day 17 mouse forelimbs undergoing endochondral ossification. Results demonstrated that Lc-Maf and Col27a1 mRNAs are coexpressed in proliferating and prehypertrophic regions, as would be predicted if Lc-Maf regulates Col27a1 expression. Type XXVII collagen protein was also most abundant in prehypertrophic and proliferating chondrocytes. Others have shown that mice that are null for Lc-Maf and c-Maf have expanded hypertrophic regions with reduced ossification and delayed vascularization. Separate studies have indicated that Col27a1 may serve as a scaffold for ossification and vascularization. The work presented here suggests that Lc-Maf may affect the process of endochondral ossification by participating in the regulation of Col27a1 expression.

Egr-1 inhibits the expression of extracellular matrix genes in chondrocytes by TNFalpha-induced MEK/ERK signalling

Introduction

TNFα is increased in the synovial fluid of patients with rheumatoid arthritis and osteoarthritis. TNFα activates mitogen-activated kinase kinase (MEK)/extracellular regulated kinase (ERK) in chondrocytes; however, the overall functional relevance of MEK/ERK to TNFα-regulated gene expression in chondrocytes is unknown.

Methods

Chondrocytes were treated with TNFα with or without the MEK1/2 inhibitor U0126 for 24 hours. Microarray analysis and real-time PCR analyses were used to identify genes regulated by TNFα in a MEK1/2-dependent fashion. Promoter/reporter, immunoblot, and electrophoretic mobility shift assays were used to identify transcription factors whose activity in response to TNFα was MEK1/2 dependent. Decoy oligodeoxynucleotides bearing consensus transcription factor binding sites were introduced into chondrocytes to determine the functionality of our results.

Results

Approximately 20% of the genes regulated by TNFα in chondrocytes were sensitive to U0126. Transcript regulation of the cartilage-selective matrix genes Col2a1, Agc1 and Hapln1, and of the matrix metalloproteinase genes Mmp-12 and Mmp-9, were U0126 sensitive – whereas regulation of the inflammatory gene macrophage Csf-1 was U0126 insensitive. TNFα-induced regulation of Sox9 and NFκB activity was also U0126 insensitive. Conversely, TNFα-increased early growth response 1 (Egr-1) DNA binding was U0126 sensitive. Transfection of chondrocytes with cognate Egr-1 oligodeoxynucleotides attenuated the ability of TNFα to suppress Col2a1, Agc1 or Hapln1 mRNA expression.

Conclusions

Our results suggest that MEK/ERK and Egr1 are required for TNFα-regulated catabolic and anabolic genes of the cartilage extracellular matrix, and hence may represent potential targets for drug intervention in osteoarthritis or rheumatoid arthritis.

Sox9, a key transcription factor of bone morphogenetic protein-2-induced chondrogenesis, is activated through BMP pathway and a CCAAT box in the proximal promoter

Mouse embryonic fibroblasts (MEFs) can be differentiated into fully functional chondrocytes in response to bone morphogenetic protein-2 (BMP-2). The expression of Sox9, a critical transcription factor for the multiple steps of chondrogenesis, has been reported to be upregulated during this process. But the molecular mechanisms by which BMP-2 promotes chondrogenesis still remain largely unknown. The aim of the present study was therefore to investigate the underlying mechanism. In the MEFs, BMP-2 efficiently induced Sox9 expression along with chondrogenic differentiation in a time- and dose-dependent manner. SB203580, a specific inhibitor for p38 pathway, blocked BMP-2-induced chondrogenic differentiation as well as Sox9 expression and its transactivation of downstream genes. Forced expression of Smad6, a natural antagonist for BMP/Smad pathway, only inhibited Sox9 protein function without rendering any effects on its mRNA expression. A CCAAT box was identified in Sox9 promoter as the cis-elements responsible for BMP-2 stimulation. This study provides insight into the mechanisms underlying BMP-2-regulated Sox9 expression and activity in MEFs, and suggests differential roles of BMP-2/p38 and BMP-2/Smad pathways in modulating the function of Sox9 during chondrogenesis.

Bone morphogenetic protein-2 stimulates chondrogenic expression in human nasal chondrocytes expanded in vitro

Articular cartilage contains an extracellular matrix with characteristic macromolecules such as type II collagen. Because this tissue is avascular and mature chondrocytes do not proliferate, cartilage lesions have a limited capacity for healing after trauma. Autologous chondrocyte implantation (ACI) is widely used for the treatment of patients with focal damage to articular cartilage. However, this method faces a major issue: dedifferentiation of chondrocytes occurs during the long-term culture necessary for mass cell production. The aim of this study was to determine if the step of cell amplification required for ACI could benefit from the use of bone morphogenetic protein (BMP)-2, a potent regulator of chondrogenic expression. Chondrocytes were isolated from human nasal cartilage, a hyaline cartilage like articular cartilage and were serially cultured in monolayers. After one, two or three passages, BMP-2 was used to evaluate the chondrogenic potential of the dedifferentiated chondrocytes, at the gene and protein level. We found that BMP-2 can reactivate the program of chondrogenic expression in dedifferentiated chondrocytes. To gain insight into the molecular mechanisms involved in the responsiveness of chondrocytes to BMP-2, we examined the phosphorylation of Smad proteins and the interaction of the Sry-type high-mobility-group box (Sox) transcription factors with the cartilage-specific enhancer of the type II procollagen gene. Our results show that BMP-2 acts by stimulating Smad phosphorylation and by enhancing DNA-binding of the Sox transcription factors to the specific enhancer of the type II procollagen gene. Thus, this study reveals the potential use of BMP-2 as a stimulatory agent in conventional ACI strategies.

L-Sox5 and Sox6 drive expression of the aggrecan gene in cartilage by securing binding of Sox9 to a far-upstream enhancer

The Sry-related high-mobility-group box transcription factor Sox9 recruits the redundant L-Sox5 and Sox6 proteins to effect chondrogenesis, but the mode of action of the trio remains unclear. We identify here a highly conserved 359-bp sequence 10 kb upstream of the Agc1 gene for aggrecan, a most essential cartilage proteoglycan and key marker of chondrocyte differentiation. This sequence directs expression of a minimal promoter in both embryonic and adult cartilage in transgenic mice, in a manner that matches Agc1 expression. The chondrogenic trio is required and sufficient to mediate the activity of this enhancer. It acts directly, Sox9 binding to a critical cis-acting element and L-Sox5/Sox6 binding to three additional elements, which are cooperatively needed. Upon binding to their specific sites, L-Sox5/Sox6 increases the efficiency of Sox9 binding to its own recognition site and thereby robustly potentiates the ability of Sox9 to activate the enhancer. L-Sox5/Sox6 similarly secures Sox9 binding to Col2a1 (encoding collagen-2) and other cartilage-specific enhancers. This study thus uncovers critical cis-acting elements and transcription factors driving Agc1 expression in cartilage and increases understanding of the mode of action of the chondrogenic Sox trio.

Resurrection of DNA function in vivo from an extinct genome

There is a burgeoning repository of information available from ancient DNA that can be used to understand how genomes have evolved and to determine the genetic features that defined a particular species. To assess the functional consequences of changes to a genome, a variety of methods are needed to examine extinct DNA function. We isolated a transcriptional enhancer element from the genome of an extinct marsupial, the Tasmanian tiger (Thylacinus cynocephalus or thylacine), obtained from 100 year-old ethanol-fixed tissues from museum collections. We then examined the function of the enhancer in vivo. Using a transgenic approach, it was possible to resurrect DNA function in transgenic mice. The results demonstrate that the thylacine Col2A1 enhancer directed chondrocyte-specific expression in this extinct mammalian species in the same way as its orthologue does in mice. While other studies have examined extinct coding DNA function in vitro, this is the first example of the restoration of extinct non-coding DNA and examination of its function in vivo. Our method using transgenesis can be used to explore the function of regulatory and protein-coding sequences obtained from any extinct species in an in vivo model system, providing important insights into gene evolution and diversity.

The three SoxC proteins--Sox4, Sox11 and Sox12--exhibit overlapping expression patterns and molecular properties

The group C of Sry-related high-mobility group (HMG) box (Sox) transcription factors has three members in most vertebrates: Sox4, Sox11 and Sox12. Sox4 and Sox11 have key roles in cardiac, neuronal and other major developmental processes, but their molecular roles in many lineages and the roles of Sox12 remain largely unknown. We show here that the three genes are co-expressed at high levels in neuronal and mesenchymal tissues in the developing mouse, and at variable relative levels in many other tissues. The three proteins have conserved remarkable identity through evolution in the HMG box DNA-binding domain and in the C-terminal 33 residues, and we demonstrate that the latter residues constitute their transactivation domain (TAD). Sox11 activates transcription several times more efficiently than Sox4 and up to one order of magnitude more efficiently than Sox12, owing to a more stable α-helical structure of its TAD. This domain and acidic domains interfere with DNA binding, Sox11 being most affected and Sox4 least affected. The proteins are nevertheless capable of competing with one another in reporter gene transactivation. We conclude that the three SoxC proteins have conserved overlapping expression patterns and molecular properties, and might therefore act in concert to fulfill essential roles in vivo.

Regulation of Sox9 activity by crosstalk with nuclear factor-kappaB and retinoic acid receptors

Introduction

Sox9 and p300 cooperate to induce expression of cartilage-specific matrix proteins, including type II collagen, aggrecan and link protein. Tumour necrosis factor (TNF)-α, found in arthritic joints, activates nuclear factor-κB (NF-κB), whereas retinoic acid receptors (RARs) are activated by retinoid agonists, including all-trans retinoic acid (atRA). Like Sox9, the activity of NF-κB and RARs depends upon their association with p300. Separately, both TNF-α and atRA suppress cartilage matrix gene expression. We investigated how TNF-α and atRA alter the expression of cartilage matrix genes.

Methods

Primary cultures of rat chondrocytes were treated with TNF-α and/or atRA for 24 hours. Levels of transcripts encoding cartilage matrix proteins were determined by Northern blot analyses and quantitative real-time PCR. Nuclear protein levels, DNA binding and functional activity of transcription factors were assessed by immunoblotting, electrophoretic mobility shift assays and reporter assays, respectively.

Results

Together, TNF-α and atRA diminished transcript levels of cartilage matrix proteins and Sox9 activity more than each factor alone. However, neither agent altered nuclear levels of Sox9, and TNF-α did not affect protein binding to the Col2a1 48-base-pair minimal enhancer sequence. The effect of TNF-α, but not that of atRA, on Sox9 activity was dependent on NF-κB activation. Furthermore, atRA reduced NF-κB activity and DNA binding. To address the role of p300, we over-expressed constitutively active mitogen-activated protein kinase kinase kinase (caMEKK)1 to increase p300 acetylase activity. caMEKK1 enhanced basal NF-κB activity and atRA-induced RAR activity. Over-expression of caMEKK1 also enhanced basal Sox9 activity and suppressed the inhibitory effects of TNF-α and atRA on Sox9 function. In addition, over-expression of p300 restored Sox9 activity suppressed by TNF-α and atRA to normal levels.

Conclusion

NF-κB and RARs converge to reduce Sox9 activity and cartilage matrix gene expression, probably by limiting the availability of p300. This process may be critical for the loss of cartilage matrix synthesis in inflammatory joint diseases. Therefore, agents that increase p300 levels or activity in chondrocytes may be useful therapeutically.

Functional gene screening system identified TRPV4 as a regulator of chondrogenic differentiation

Sox9 is a transcription factor that is essential for chondrocyte differentiation and chondrocyte-specific gene expression. However, the precise mechanism of Sox9 activation during chondrogenesis is not fully understood. To investigate this mechanism, we performed functional gene screening to identify genes that activate SOX9-dependent transcription, using full-length cDNA libraries generated from a murine chondrogenic cell line, ATDC5. Screening revealed that TRPV4 (transient receptor potential vanilloid 4), a cation channel molecule, significantly elevates SOX9-dependent reporter activity. Microarray and quantitative real time PCR analyses demonstrated that during chondrogenesis in ATDC5 and C3H10T1/2 (a murine mesenchymal stem cell line), the expression pattern of TRPV4 was similar to the expression patterns of chondrogenic marker genes, such as type II collagen and aggrecan. Activation of TRPV4 by a pharmacological activator induced SOX9-dependent reporter activity, and this effect was abolished by the addition of the TRPV antagonist ruthenium red or by using a small interfering RNA for TRPV4. The SOX9-dependent reporter activity due to TRPV4 activation was abrogated by both EGTA and a calmodulin inhibitor, suggesting that the Ca2+/calmodulin signal is essential in this process. Furthermore, activation of TRPV4 in concert with insulin activity in ATDC5 cells or in concert with bone morphogenetic protein-2 in C3H10T1/2 cells promoted synthesis of sulfated glycosaminoglycan, but activation of TRPV4 had no effect alone. We showed that activation of TRPV4 increased the steady-state levels of SOX9 mRNA and protein and SOX6 mRNA. Taken together, our results suggest that TRPV4 regulates the SOX9 pathway and contributes to the process of chondrogenesis.

Control of cell fate and differentiation by Sry-related high-mobility-group box (Sox) transcription factors

Maintain stemness, commit to a specific lineage, differentiate, proliferate, or die. These are essential decisions that every cell is constantly challenged to make in multi-cellular organisms to ensure proper development, adult maintenance, and adaptability. SRY-related high-mobility-group box (Sox) transcription factors have emerged in the animal kingdom to help cells effect such decisions. They are encoded by 20 genes in humans and mice. They share a highly conserved high-mobility-group box domain that was originally identified in SRY, the sex-determining gene on the Y chromosome, and that has derived from a canonical high-mobility-group domain characteristic of chromatin-associated proteins. The high-mobility-group box domain binds DNA in the minor groove and increases its DNA binding affinity and specificity by interacting with many types of transcription factors. It also bends DNA and may thereby confer on Sox proteins a unique and critical role in the assembly of transcriptional enhanceosomes. Sox proteins fall into eight groups. Most feature a transactivation or transrepression domain and thereby also act as typical transcription factors. Each gene has distinct expression pattern and molecular properties, often redundant with those in the same group and overlapping with those in other groups. As a whole the Sox family controls cell fate and differentiation in a multitude of processes, such as male differentiation, stemness, neurogenesis, and skeletogenesis. We review their specific molecular properties and in vivo roles, stress recent advances in the field, and suggest directions for future investigations.