DNA hypomethylation during MSC chondrogenesis occurs predominantly at enhancer regions

The methylomic landscape of human articular cartilage development contains epigenetic signatures of osteoarthritis risk

Increasing evidence is emerging to link age-associated complex musculoskeletal diseases, including osteoarthritis (OA), to developmental factors. Multiple studies have shown a functional role for DNA methylation in the genetic mechanisms of OA risk using articular cartilage samples taken from aged individuals, yet knowledge of temporal changes to the methylome during human cartilage development is limited. We quantified DNA methylation at ∼700,000 individual CpGs across the epigenome of developing human chondrocytes in 72 samples ranging from 7 to 21 post-conception weeks. We identified significant changes in 3% of all CpGs and >8,200 developmental differentially methylated regions. We further identified 24 loci at which OA genetic variants colocalize with methylation quantitative trait loci. Through integrating developmental and mature human chondrocyte datasets, we find evidence for functional effects exerted solely in development or throughout the life course. This will have profound impacts on future approaches to translating genetic pathways for therapeutic intervention.

SERPINA3 is a marker of cartilage differentiation and is essential for the expression of extracellular matrix genes during early chondrogenesis

Serine proteinase inhibitors (serpins) are a family of structurally similar proteins which regulate many diverse biological processes from blood coagulation to extracellular matrix (ECM) remodelling. Chondrogenesis involves the condensation and differentiation of mesenchymal stem cells (MSCs) into chondrocytes which occurs during early development. Here, and for the first time, we demonstrate that one serpin, SERPINA3 (gene name SERPINA3, protein also known as alpha-1 antichymotrypsin), plays a critical role in chondrogenic differentiation. We observed that SERPINA3 expression was markedly induced at early time points during in vitro chondrogenesis. We examined the expression of SERPINA3 in human cartilage development, identifying significant enrichment of SERPINA3 in developing cartilage compared to total limb, which correlated with well-described markers of cartilage differentiation. When SERPINA3 was silenced using siRNA, cartilage pellets were smaller and contained lower proteoglycan as determined by dimethyl methylene blue assay (DMMB) and safranin-O staining. Consistent with this, RNA sequencing revealed significant downregulation of genes associated with cartilage ECM formation perturbing chondrogenesis. Conversely, SERPINA3 silencing had a negligible effect on the gene expression profile during osteogenesis suggesting the role of SERPINA3 is specific to chondrocyte differentiation. The global effect on cartilage formation led us to investigate the effect of SERPINA3 silencing on the master transcriptional regulator of chondrogenesis, SOX9. Indeed, we observed that SOX9 protein levels were markedly reduced at early time points suggesting a role for SERPINA3 in regulating SOX9 expression and activity. In summary, our data support a non-redundant role for SERPINA3 in enabling chondrogenesis via regulation of SOX9 levels.

Epigenetic mechanisms of osteoarthritis risk in human skeletal development

The epigenome, including the methylation of cytosine bases at CG dinucleotides, is intrinsically linked to transcriptional regulation. The tight regulation of gene expression during skeletal development is essential, with ~1/500 individuals born with skeletal abnormalities. Furthermore, increasing evidence is emerging to link age-associated complex genetic musculoskeletal diseases, including osteoarthritis (OA), to developmental factors including joint shape. Multiple studies have shown a functional role for DNA methylation in the genetic mechanisms of OA risk using articular cartilage samples taken from aged patients. Despite this, our knowledge of temporal changes to the methylome during human cartilage development has been limited. We quantified DNA methylation at ~700,000 individual CpGs across the epigenome of developing human articular cartilage in 72 samples ranging from 7-21 post-conception weeks, a time period that includes cavitation of the developing knee joint. We identified significant changes in 8% of all CpGs, and >9400 developmental differentially methylated regions (dDMRs). The largest hypermethylated dDMRs mapped to transcriptional regulators of early skeletal patterning including MEIS1 and IRX1. Conversely, the largest hypomethylated dDMRs mapped to genes encoding extracellular matrix proteins including SPON2 and TNXB and were enriched in chondrocyte enhancers. Significant correlations were identified between the expression of these genes and methylation within the hypomethylated dDMRs. We further identified 811 CpGs at which significant dimorphism was present between the male and female samples, with the majority (68%) being hypermethylated in female samples. Following imputation, we captured the genotype of these samples at >5 million variants and performed epigenome-wide methylation quantitative trait locus (mQTL) analysis. Colocalization analysis identified 26 loci at which genetic variants exhibited shared impacts upon methylation and OA genetic risk. This included loci which have been previously reported to harbour OA-mQTLs (including GDF5 and ALDH1A2), yet the majority (73%) were novel (including those mapping to CHST3, FGF1 and TEAD1). To our knowledge, this is the first extensive study of DNA methylation across human articular cartilage development. We identify considerable methylomic plasticity within the development of knee cartilage and report active epigenomic mediators of OA risk operating in prenatal joint tissues.

Conservative Hypomethylation of Mesenchymal Stem Cells and Their Secretome Restored the Follicular Development in Cisplatin-Induced Premature Ovarian Failure Mice

Premature ovarian failure (POF) is one of the main causes of infertility in women under the age of 40 years. Recently, epigenetic reprogramming, particularly DNA hypomethylation, has emerged as a promising strategy to enhance the therapeutic potential of mesenchymal stem cells (MSCs). Thus, it is crucial to elucidate how far global hypomethylation of MSCs genome can maintain their pluripotency and viability and improve their therapeutic effect in chemotherapy-induced POF mice. Herein, the genomic DNA of bone marrow-derived MSCs (BM-MSCs) was hypomethylated by the DNA methyltransferase inhibitor (5-Aza-dC), and the degree of global hypomethylation was assessed by methylation-sensitive HepII/MspI restriction analysis. Next, mildly hypomethylated cells and their secretome were independently transplanted (or infused) in POF mice, established via cisplatin-mediated gonadotoxicity. We found that conservative global hypomethylation of BM-MSCs genome with low doses of 5-Aza-dC (≤0.5 μM) has maintained cell viability and MSCs-specific clusters of differentiation (CD). Engraftment of mildly hypomethylated cells in POF mice, or infusion of their secretome, improved the concentrations of estradiol (E2), follicle-stimulating hormone (FSH), and anti-Mullerian hormone (AMH). Furthermore, mice restored their normal body weight, ovarian size, and ovarian follicle count. This was associated with improved follicular development, where the populations of healthy primordial, primary, secondary, and tertiary follicles were significantly ameliorated, relative to mice transplanted with normally methylated cells. This observational study suggests that transplantation of mildly hypomethylated BM-MSCs cells and their secretome can restore the structural and functional integrity of the damaged ovaries in POF mice. Also, it presents conservative hypomethylation of BM-MSCs and their secretome as a promising alternative to MSCs transplantation.

Single‑cell sequencing, genetics, and epigenetics reveal mesenchymal stem cell senescence in osteoarthritis (Review)

Osteoarthritis (OA) is a chronic joint disease characterized by articular cartilage degeneration, secondary bone hyperplasia, inadequate extracellular matrix synthesis and degeneration of articular cartilage. Mesenchymal stem cells (MSCs) can self‑renew and undergo multidirectional differentiation; they can differentiate into chondrocytes. Aging MSCs have a weakened ability to differentiate, and release various pro‑inflammatory cytokines, which may contribute to OA progression; the other mechanism contributing to OA is epigenetic regulation (for instance, DNA methylation, histone modification and regulation of non‑coding RNA). Owing to the self‑renewal and differentiation ability of MSCs, various MSC‑based exogenous cell therapies have been developed to treat OA. The efficacy of MSC‑based therapy is mainly attributed to cytokines, growth factors and the paracrine effect of exosomes. Recently, extensive studies have been conducted on MSC‑derived exosomes. Exosomes from MSCs can deliver a variety of DNA, RNA, proteins and lipids, thereby facilitating MSC migration and cartilage repair. Therefore, MSC‑derived exosomes are considered a promising therapy for OA. The present review summarized the association between MSC aging and OA in terms of genetics and epigenetics, and characteristics of MSC‑derived exosomes, and the mechanism to alleviate OA cartilage damage.

Mesenchymal stem cells under epigenetic control - the role of epigenetic machinery in fate decision and functional properties

Mesenchymal stem cells (mesenchymal stromal cells, MSC) are multipotent stem cells that can differentiate into cells of at least three mesodermal lineages, namely adipocytes, osteoblasts, and chondrocytes, and have potent immunomodulatory properties. Epigenetic modifications are critical regulators of gene expression and cellular differentiation of mesenchymal stem cells (MSCs). Epigenetic machinery controls MSC differentiation through direct modifications to DNA and histones. Understanding the role of epigenetic machinery in MSC is crucial for the development of effective cell-based therapies for degenerative and inflammatory diseases. In this review, we summarize the current understanding of the role of epigenetic control of MSC differentiation and immunomodulatory properties.

Perturbation of 3D nuclear architecture, epigenomic dysregulation and aging, and cannabinoid synaptopathy reconfigures conceptualization of cannabinoid pathophysiology: part 1-aging and epigenomics

Much recent attention has been directed toward the spatial organization of the cell nucleus and the manner in which three-dimensional topologically associated domains and transcription factories are epigenetically coordinated to precisely bring enhancers into close proximity with promoters to control gene expression. Twenty lines of evidence robustly implicate cannabinoid exposure with accelerated organismal and cellular aging. Aging has recently been shown to be caused by increased DNA breaks. These breaks rearrange and maldistribute the epigenomic machinery to weaken and reverse cellular differentiation, cause genome-wide DNA demethylation, reduce gene transcription, and lead to the inhibition of developmental pathways, which contribute to the progressive loss of function and chronic immune stimulation that characterize cellular aging. Both cell lineage-defining superenhancers and the superanchors that control them are weakened. Cannabis exposure phenocopies the elements of this process and reproduces DNA and chromatin breakages, reduces the DNA, RNA protein and histone synthesis, interferes with the epigenomic machinery controlling both DNA and histone modifications, induces general DNA hypomethylation, and epigenomically disrupts both the critical boundary elements and the cohesin motors that create chromatin loops. This pattern of widespread interference with developmental programs and relative cellular dedifferentiation (which is pro-oncogenic) is reinforced by cannabinoid impairment of intermediate metabolism (which locks in the stem cell-like hyper-replicative state) and cannabinoid immune stimulation (which perpetuates and increases aging and senescence programs, DNA damage, DNA hypomethylation, genomic instability, and oncogenesis), which together account for the diverse pattern of teratologic and carcinogenic outcomes reported in recent large epidemiologic studies in Europe, the USA, and elsewhere. It also accounts for the prominent aging phenotype observed clinically in long-term cannabis use disorder and the 20 characteristics of aging that it manifests. Increasing daily cannabis use, increasing use in pregnancy, and exponential dose-response effects heighten the epidemiologic and clinical urgency of these findings. Together, these findings indicate that cannabinoid genotoxicity and epigenotoxicity are prominent features of cannabis dependence and strongly indicate coordinated multiomics investigations of cannabinoid genome-epigenome-transcriptome-metabolome, chromatin conformation, and 3D nuclear architecture. Considering the well-established exponential dose-response relationships, the diversity of cannabinoids, and the multigenerational nature of the implications, great caution is warranted in community cannabinoid penetration.

Comparison of genome-wide DNA methylation patterns between antler precartilage and cartilage

Deer antlers are the only mammalian organs that can fully regenerate after being lost and provide a valuable model for cartilage development. As one of the best-studied epigenetic mechanisms, DNA methylation is known to engage in organ and tissue development. This study aimed to investigate the role of DNA methylation in antler chondrogenesis by comparing whole-genome DNA methylation between precartilage and cartilage. Quantitative reverse transcription PCR (RT-qPCR) showed significant differences in the expression levels of DNA methyltransferase genes (DNMT1, DNMT3A, and DNMT3B) between precartilage and cartilage. Subsequently, we obtained DNA methylation profiles of antler precartilage and cartilage tissues by whole-genome bisulfite sequencing. Although sequencing data indicated that overall methylation levels at CpG and non-CpG sites were similar between precartilage and cartilage, 140,784 differentially methylated regions (DMRs, P < 0.05) and 3,941 DMR-related genes were identified. Gene ontology (GO) analysis of DMR-related genes demonstrated some significantly enriched GO terms (P < 0.05) related to chondrogenesis, including insulin receptor binding, collage trimer, integrin binding, and extracellular matrix structural constituent. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of DMR-related genes uncovered that the PI3K/AKT, cortisol synthesis and secretion, glycosaminoglycan biosynthesis-keratan sulfate, Hippo, and NF-κB signaling pathways might play a pivotal role in the transition of precartilage to cartilage. Moreover, we found that 25 DMR-related genes, including CD44, IGF1, ITGAV, ITGB1, RUNX1, COL2A1, COMP, and TAGLN, were most likely involved in antler chondrogenesis. In conclusion, this study revealed the genome-wide DNA methylation patterns of antler precartilage and cartilage, which may contribute to understanding the epigenetic regulation of antler chondrogenesis.

Epigenetic control of mesenchymal stem cells orchestrates bone regeneration

Recent studies have revealed the vital role of MSCs in bone regeneration. In both self-healing bone regeneration processes and biomaterial-induced healing of bone defects beyond the critical size, MSCs show several functions, including osteogenic differentiation and thus providing seed cells. However, adverse factors such as drug intake and body senescence can significantly affect the functions of MSCs in bone regeneration. Currently, several modalities have been developed to regulate MSCs' phenotype and promote the bone regeneration process. Epigenetic regulation has received much attention because of its heritable nature. Indeed, epigenetic regulation of MSCs is involved in the pathogenesis of a variety of disorders of bone metabolism. Moreover, studies using epigenetic regulation to treat diseases are also being reported. At the same time, the effects of epigenetic regulation on MSCs are yet to be fully understood. This review focuses on recent advances in the effects of epigenetic regulation on osteogenic differentiation, proliferation, and cellular senescence in MSCs. We intend to illustrate how epigenetic regulation of MSCs orchestrates the process of bone regeneration.

Differential expression and methylation patterns of NFATC1, NADSYN1 and JAK3 gene in equine chondrocytes expanded in monolayer culture

Ex vivo expansion of chondrocytes in monolayer (ML) culture for therapeutic purposes is burdened with difficulties related to the loss of cartilaginous phenotype. Epigenetic mechanisms responsible for regulation of gene expression are believed to underlie chondrocyte dedifferentiation. We have inspected the relevance of DNA methylation alterations for passage-related differential expression of NFATC1 gene involved in hard connective tissue turnover and development, NADSYN1 influencing redox metabolism, and JAK3 - an important driver of inflammation. We have assessed relative amount of transcript abundance and performed DNA bisulfite sequencing of upstream located elements. It seems that anabolic-like effects of chondrogenic differentiation were observed in form of NFATC1 and NADSYN1 upregulation in chondrocytes at the earlier stages of passaging whereas JAK3 upregulation at the 11th passage was the sign of chondrocytes dedifferentiation. Summarizing the inversely correlated DNA methylation and expression patterns in NFATC1 and JAK3 locus might be relevant for cellular dedifferentiation during chondrocyte expansion in monolayer. Obtained results are supportive for further studies on the role of encoded proteins in regenerative biology of articular cartilage using in vitro expanded chondrocytes.

Epigenetic therapy targeting bone marrow mesenchymal stem cells for age-related bone diseases

As global aging accelerates, the prevention and treatment of age-related bone diseases are becoming a critical issue. In the process of senescence, bone marrow mesenchymal stem cells (BMSCs) gradually lose the capability of self-renewal and functional differentiation, resulting in impairment of bone tissue regeneration and disorder of bone tissue homeostasis. Alteration in epigenetic modification is an essential factor of BMSC dysfunction during aging. Its transferability and reversibility provide the possibility to combat BMSC aging by reversing age-related modifications. Emerging evidence demonstrates that epigenetic therapy based on aberrant epigenetic modifications could alleviate the senescence and dysfunction of stem cells. This review summarizes potential therapeutic targets for BMSC aging, introduces some potential approaches to alleviating BMSC aging, and analyzes its prospect in the clinical application of age-related bone diseases.

Effect of aberrant DNA methylation on cancer stem cell properties

DNA methylation, as an epigenetic mechanism, occurs by adding a methyl group of cytosines in position 5 by DNA methyltransferases and has essential roles in cellular function, especially in the transcriptional regulation of embryonic and adult stem cells. Hypomethylation and hypermethylation cause either the expression or inhibition of genes, and there is a tight balance between regulating the activation or repression of genes in normal cellular activity. Abnormal methylation is well-known hallmark of cancer development and progression and can switch normal stem cells into cancer stem cells. Cancer Stem Cells (CSCs) are minor populations of tumor cells that exhibit unique properties such as self-regeneration, resistance to chemotherapy, and high ability of metastasis. The purpose of this paper is to show how aberrant DNA methylation accumulation affects self-renewal, differentiation, multidrug-resistant, and metastasis processes in cancer stem cells.

Effects of Berberine on the Chondrogenic Differentiation of Embryonic Limb Skeletal Progenitors

Introduction

Berberine (BBR) is an isoquinoline plant alkaloid with demonstrated anti-inflammatory, anti-tumor and immunosuppressive pharmacological properties that functions via multiple signaling pathways and epigenetic modulators. Numerous studies have proposed BBR as a promising therapeutic agent for joint cartilage degeneration, and other connective tissue diseases.

Purpose and Methods

This work aimed to evaluate the effects of BBR on the growth and differentiation of embryonic skeletal progenitors using the limb mesoderm micromass culture assay.

Results

Our findings show that at difference of its apoptotic influence on a variety of tumor tissues, cell death was not induced in skeletal progenitors by the addition of 12 or 25 µM BBR concentration to the culture medium. Morphological and transcriptional analysis revealed dual and opposite effects of BBR treatments on chondrogenesis depending on the stage of differentiation of the cultured progenitors. At early stage of culture, BBR was a potent chondrogenic inhibitor, while chondrogenesis was intensified in treatments at advanced stages of culture. The chondrogenic promoting effect was accompanied by a moderate upregulation of gene markers of prehypertrophic cartilage, including ColXa1, alkaline phosphatase Alpl, Runx2, and Indian Hedgehog Ihh. We further observed a positive transcriptional influence of BBR in the expression of DNA methyltransferase genes, Dnmt1, Dnmt3a and Dnmt3b, suggesting a potential involvement of epigenetic factors in its effects.

Conclusion

Our study uncovers a new pharmacological influence of BBR in cartilage differentiation that must be taken into account in designing clinical protocols for its employment in the treatment of cartilage degenerative diseases.

DNMT3B Is an Oxygen-Sensitive De Novo Methylase in Human Mesenchymal Stem Cells

The application of physiological oxygen (physoxia) concentrations is becoming increasingly commonplace within a mammalian stem cell culture. Human mesenchymal stem cells (hMSCs) attract widespread interest for clinical application due to their unique immunomodulatory, multi-lineage potential, and regenerative capacities. Descriptions of the impact of physoxia on global DNA methylation patterns in hMSCs and the activity of enzymatic machinery responsible for its regulation remain limited. Human bone marrow-derived mesenchymal stem cells (BM-hMSCs, passage 1) isolated in reduced oxygen conditions displayed an upregulation of SOX2 in reduced oxygen conditions vs. air oxygen (21% O2, AO), while no change was noted for either OCT-4 or NANOG. DNA methylation marks 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC) showed decreases in 2% O2 environment (workstation) (2% WKS). DNMT3B (DNA methyltransferase 3B) and TET1 (Ten-eleven translocation enzyme 1) displayed reduced transcription in physoxia. Consistent with transcriptional downregulation, we noted increased promoter methylation levels of DNMT3B in 2% WKS accompanied by reduced DNMT3B and TET1 protein expression. Finally, a decrease in HIF1A (Hypoxia-inducible factor 1A) gene expression in 2% WKS environment correlated with protein levels, while HIF2A was significantly higher in physoxia correlated with protein expression levels vs. AO. Together, these data have demonstrated, for the first time, that global 5mC, 5hmC, and DNMT3B are oxygen-sensitive in hMSCs. Further insights into the appropriate epigenetic regulation within hMSCs may enable increased safety and efficacy development within the therapeutic ambitions.

Changes in DNA methylation accompany changes in gene expression during chondrocyte hypertrophic differentiation in vitro

During osteoarthritis (OA), articular chondrocytes undergo phenotypic changes that resemble developmental patterns characteristic of growth plate chondrocytes. These phenotypic alterations lead to a hypertrophy-like phenotype characterized by altered production of extracellular matrix constituents and increased collagenase activity, which, in turn, results in cartilage destruction in OA disease. Recent studies have shown that the phenotypic instability and dysregulated gene expression in OA are associated with changes in DNA methylation patterns. Subsequent efforts have aimed to identify changes in DNA methylation with functional impact in OA disease, to potentially uncover therapeutic targets. Here, we paired an in vitro 3D/pellet culture system that mimics chondrocyte hypertrophy with RNA sequencing (RNA-Seq) and enhanced reduced representation of bisulfite sequencing (ERRBS) to identify transcriptomic and epigenomic changes in murine primary articular chondrocytes undergoing hypertrophy-like differentiation. We identified hypertrophy-associated changes in DNA methylation patterns in vitro. Integration of RNA-Seq and ERRBS datasets identified associations between changes in methylation and gene expression. Our integrative analyses showed that hypertrophic differentiation of articular chondrocytes is accompanied by transcriptomic and epigenomic changes in vitro. We believe that our integrative approaches have the potential to uncover new targets for therapeutic intervention.

Dynamic chromatin accessibility landscape changes following interleukin-1 stimulation

Dynamic modifications of chromatin allow rapid access of the gene regulatory machinery to condensed genomic regions facilitating subsequent gene expression. Inflammatory cytokine stimulation of cells can cause rapid gene expression changes through direct signalling pathway-mediated transcription factor activation and regulatory element binding.

Here we used the Assay for Transposase Accessible Chromatin with high-throughput sequencing (ATAC-seq) to assess regions of the genome that are differentially accessible following treatment of cells with interleukin-1 (IL-1). We identified 126,483 open chromatin regions, with 241 regions significantly differentially accessible following stimulation, with 64 and 177 more or less accessible, respectively. These differentially accessible regions predominantly correspond to regions of the genome marked as enhancers. Motif searching identified an overrepresentation of a number of transcription factors, most notably RelA, in the regions becoming more accessible, with analysis of ChIP-seq data confirmed RelA binding to these regions. A significant correlation in differential chromatin accessibility and gene expression was also observed.

Functionality in regulating gene expression was confirmed using CRISPR/Cas9 genome-editing to delete regions that became more accessible following stimulation in the genes MMP13, IKBKE and C1QTNF1. These same regions were also accessible for activation using a dCas9-transcriptional activator and showed enhancer activity in a cellular model.

Together, these data describe and functionally validate a number of dynamically accessible chromatin regions involved in inflammatory signalling.

Osteoclast-derived exosomal let-7a-5p targets Smad2 to promote the hypertrophic differentiation of chondrocytes

The invasion of osteoclasts into the cartilage via blood vessels advances the process of endochondral ossification, and dysregulation of dynamic intercellular interactions results in skeletal dysplasias. Although the regulation of osteoclasts by growth plate chondrocytes has been reported in detail, the effect of osteoclasts on chondrocytes remains to be determined. In this study, ATDC5 cells and bone marrow mesenchymal stem cells were differentiated into chondrocytes and treated with conditioned medium obtained from bone marrow macrophages differentiated to osteoclast precursors and osteoclasts. Exosomes were inhibited in conditioned medium or isolated directly from osteoclasts to further determine whether osteoclast-derived exosomes play an important role in chondrocyte hypertrophy. Additionally, exosomal miRNAs were detected, and let-7a-5p was selected as an miRNA with significantly increased expression in osteoclast-derived exosomes. Experiments were performed to verify the potential target Smad2 and investigate how let-7a-5p affected chondrocytes. The results suggest that both osteoclast precursors and osteoclasts promote chondrocyte hypertrophy and that the promotive effect of osteoclasts is more significant than that of osteoclast precursors. Osteoclast-derived exosomes promote the hypertrophic differentiation of chondrocytes. Moreover, osteoclast-derived exosomal let-7a-5p inhibits Smad2 to decrease the transforming growth factor-β-induced inhibition of chondrocyte hypertrophy. Our research reveals the role of osteoclasts in the regulation of chondrocytes and provides insights into the highly coordinated intercellular process of endochondral ossification.

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.