ABSTRACT
The transcription factor SOX9 is believed to be the master regulator of chondrogenesis. SOX8 is another SOX group E transcription factor with a high degree of homology to SOX9. Here, we demonstrate that SOX8 mRNA levels decrease during in vitro dedifferentiation of human articular chondrocytes and increase during chondrogenic differentiation of mesenchymal stromal cells. Knockdown of SOX9 reduced the expression of SOX8, COL2A1, and a range of other chondrogenic molecules. SOX8 knockdown reduced the expression of a large number of overlapping chondrogenic molecules, but not SOX9. Neither siSOX9 nor siSOX8 altered expression of the hypertrophic marker gene COL10A1. siSOX9, but not siSOX8 led to upregulation of hypertrophy associated genes MMP13 and ALPL. Transfection of synthetic SOX5, 6, and 9 mRNA trio upregulated SOX8, COL2A1, and ACAN, but not COL10A1 mRNA. Replacement of synthetic SOX9 by SOX8 in the SOX trio showed similar but lower chondrogenic effect. We conclude that SOX8 expression is regulated by SOX9, and that both together with SOX5 and SOX6 are required as a SOX quartet for transcription of COL2A1 and a large number of other chondrogenic molecules. Neither SOX8 nor SOX9 affect COL10A1 expression, but SOX9 inhibits chondrocyte hypertrophy through inhibition of MMP13 and ALPL expression.
Subject(s)
Cell Differentiation , Chondrogenesis , Mesenchymal Stem Cells/cytology , Mesenchymal Stem Cells/metabolism , SOXE Transcription Factors/metabolism , Cell Dedifferentiation/genetics , Cell Differentiation/genetics , Chondrocytes/metabolism , Chondrogenesis/genetics , Gene Expression Regulation , Gene Knockdown Techniques , Humans , RNA, Messenger/genetics , RNA, Messenger/metabolism , RNA, Small Interfering/metabolism , Reproducibility of Results , SOX9 Transcription Factor/metabolism , SOXE Transcription Factors/genetics , TransfectionABSTRACT
Lesions of hyaline cartilage do not heal spontaneously, and represent a therapeutic challenge. In vitro engineering of articular cartilage using cells and biomaterials may prove to be the best solution. Patients with osteoarthritis (OA) may require tissue engineered cartilage therapy. Chondrocytes obtained from OA joints are thought to be involved in the disease process, and thus to be of insufficient quality to be used for repair strategies. Bone marrow (BM) derived mesenchymal stem cells (MSCs) from healthy donors may represent an alternative cell source. We have isolated chondrocytes from OA joints, performed cell culture expansion and tissue engineering of cartilage using a disc-shaped alginate scaffold and chondrogenic differentiation medium. We performed real-time reverse transcriptase quantitative PCR and fluorescence immunohistochemistry to evaluate mRNA and protein expression for a range of molecules involved in chondrogenesis and OA pathogenesis. Results were compared with those obtained by using BM-MSCs in an identical tissue engineering strategy. Finally the two populations were compared using genome-wide mRNA arrays. At three weeks of chondrogenic differentiation we found high and similar levels of hyaline cartilage-specific type II collagen and fibrocartilage-specific type I collagen mRNA and protein in discs containing OA and BM-MSC derived chondrocytes. Aggrecan, the dominant proteoglycan in hyaline cartilage, was more abundantly distributed in the OA chondrocyte extracellular matrix. OA chondrocytes expressed higher mRNA levels also of other hyaline extracellular matrix components. Surprisingly BM-MSC derived chondrocytes expressed higher mRNA levels of OA markers such as COL10A1, SSP1 (osteopontin), ALPL, BMP2, VEGFA, PTGES, IHH, and WNT genes, but lower levels of MMP3 and S100A4. Based on the results presented here, OA chondrocytes may be suitable for tissue engineering of articular cartilage.
Subject(s)
Cartilage, Articular/cytology , Chondrocytes/cytology , Mesenchymal Stem Cells/cytology , Osteoarthritis/pathology , Tissue Engineering/methods , Aged , Aggrecans/genetics , Aggrecans/metabolism , Alginates/chemistry , Bone Morphogenetic Protein 2/genetics , Bone Morphogenetic Protein 2/metabolism , Cartilage, Articular/metabolism , Cell Differentiation/genetics , Cells, Cultured , Chondrocytes/metabolism , Chondrogenesis/genetics , Collagen Type I/genetics , Collagen Type I/metabolism , Collagen Type II/genetics , Collagen Type II/metabolism , Gene Expression , Glucuronic Acid/chemistry , Hedgehog Proteins/genetics , Hedgehog Proteins/metabolism , Hexuronic Acids/chemistry , Humans , Immunohistochemistry , Mesenchymal Stem Cells/metabolism , Middle Aged , Osteoarthritis/genetics , Osteoarthritis/metabolism , Osteopontin/genetics , Osteopontin/metabolism , Reverse Transcriptase Polymerase Chain Reaction , Time Factors , Tissue Scaffolds/chemistry , Vascular Endothelial Growth Factor A/genetics , Vascular Endothelial Growth Factor A/metabolism , Wnt Proteins/genetics , Wnt Proteins/metabolismABSTRACT
BACKGROUND: For safe clinical application of engineered cartilage made from mesenchymal stem cells (MSCs), molecular mechanisms for chondrogenic differentiation must be known in detail. Changes in gene expression and extracellular matrix synthesis have been extensively studied, but the epigenomic modifications underlying these changes have not been described. To this end we performed whole-genome chromatin immunoprecipitation and deep sequencing to quantify six histone modifications, reduced representation bisulphite sequencing to quantify DNA methylation and mRNA microarrays to quantify gene expression before and after 7 days of chondrogenic differentiation of MSCs in an alginate scaffold. To add to the clinical relevance of our observations, the study is based on primary bone marrow-derived MSCs from four donors, allowing us to investigate inter-individual variations. RESULTS: We see two levels of relationship between epigenetic marking and gene expression. First, a large number of genes ontogenetically linked to MSC properties and the musculoskeletal system are epigenetically prepatterned by moderate changes in H3K4me3 and H3K9ac near transcription start sites. Most of these genes remain transcriptionally unaltered. Second, transcriptionally upregulated genes, more closely associated with chondrogenesis, are marked by H3K36me3 in gene bodies, highly increased H3K4me3 and H3K9ac on promoters and 5' end of genes, and increased H3K27ac and H3K4me1 marking in at least one enhancer region per upregulated gene. Within the 7-day time frame, changes in promoter DNA methylation do not correlate significantly with changes in gene expression. Inter-donor variability analysis shows high level of similarity between the donors for this data set. CONCLUSIONS: Histone modifications, rather than DNA methylation, provide the primary epigenetic control of early differentiation of MSCs towards the chondrogenic lineage.