ABSTRACT
The development of disease-modifying pharmacologic therapy for osteoarthritis (OA) currently faces major obstacles largely because the regulatory mechanisms for the function of adult articular chondrocytes remain unclear. We previously demonstrated that lack of Nfat1, one of the nuclear factor of activated T cells (NFAT) transcription factors, causes OA-like changes in adult mice. This study aimed to identify whether Nfat1 specifically regulates adult articular chondrocyte function and its age-dependent regulatory mechanism using both Nfat1-deficient and wild-type mice. Deletion of Nfat1 did not induce OA-like articular chondrocyte dysfunction (e.g., overexpression of proinflammatory cytokines and matrix-degrading proteinases) until the adult stage. RNAi-mediated Nfat1 knockdown caused dysfunction of wild-type adult articular chondrocytes. Nfat1 expression in wild-type articular chondrocytes was low in the embryonic but high in the adult stage. Chromatin immunoprecipitation assays demonstrated that an increase in Nfat1 expression in articular chondrocytes was associated with increased H3K4me2 (a histone modification linked to transcriptional activation), whereas a decrease in Nfat1 expression in articular chondrocytes was correlated with increased H3K9me2 (a histone modification linked to transcriptional repression). Knockdown of lysine-specific demethylase-1 (Lsd1) in embryonic articular chondrocytes upregulated Nfat1 expression concomitant with increased H3K4me2 at the Nfat1 promoter. Knockdown of Jmjc-containing histone demethylase-2a (Jhdm2a) in 6-month articular chondrocytes downregulated Nfat1 expression concomitant with increased H3K9me2 at the Nfat1 promoter. These results suggest that Nfat1 is an essential transcriptional regulator of chondrocyte homeostasis in adult articular cartilage. Age-dependent Nfat1 expression in articular chondrocytes is regulated by dynamic histone methylation, one of the epigenetic mechanisms that regulate gene transcription.
Subject(s)
Aging/genetics , Cartilage, Articular/pathology , Chondrocytes/metabolism , Epigenesis, Genetic , Histones/metabolism , NFATC Transcription Factors/metabolism , Animals , Base Sequence , Chondrocytes/pathology , Chromatin Immunoprecipitation , Gene Knockdown Techniques , Histone Demethylases , Jumonji Domain-Containing Histone Demethylases/metabolism , Lysine/metabolism , Methylation , Mice , Molecular Sequence Data , NFATC Transcription Factors/deficiency , Osteoarthritis/metabolism , Osteoarthritis/pathology , Oxidoreductases, N-Demethylating/metabolism , Promoter Regions, Genetic/genetics , RNA Interference , RNA, Small Interfering/metabolism , Up-Regulation/geneticsABSTRACT
Osteoarthritis (OA) is the most common form of joint disease in middle-aged and older individuals. Previous studies have shown that over-expression of matrix-degrading proteinases and proinflammatory cytokines is associated with osteoarthritic cartilage degradation. However, it remains unclear which transcription factors regulate the expression of these cartilage-degrading molecules in articular chondrocytes. This study demonstrated that mice lacking Nfat1, a member of the nuclear factor of activated T cells (NFAT) transcription factors, exhibited normal skeletal development but displayed loss of type II collagen (collagen-2) and aggrecan with over-expression of specific matrix-degrading proteinases and proinflammatory cytokines in young adult articular cartilage of load-bearing joints. These initial changes are followed by articular chondrocyte proliferation/clustering, progressive articular surface destruction, periarticular chondro-osteophyte formation and exposure of thickened subchondral bone, all of which resemble human OA. Forced expression of Nfat1 delivered with lentiviral vectors in cultured 3 month-old primary Nfat1 knockout (Nfat1(-/-)) articular chondrocytes partially or completely rescued the abnormal catabolic and anabolic activities of Nfat1(-/-) articular chondrocytes. These new findings revealed a previously unrecognized critical role of Nfat1 in maintaining the physiological function of differentiated adult articular chondrocytes through regulating the expression of specific matrix-degrading proteinases and proinflammatory cytokines. Nfat1 deficiency causes OA due to an imbalance between the catabolic and anabolic activities of adult articular chondrocytes, leading to articular cartilage degradation and failed repair activities in and around articular cartilage. These results may provide new insights into the aetiology, pathogenesis and potential therapeutic strategies for osteoarthritis.
