Translational control maintains cartilage homeostasis and regulates osteoarthritis progression

Abstract

Purpose: The unique properties of articular cartilage are defined by quiescent, differentiated articular chondrocytes whose homeostasis is maintained through a strict balance between anabolic and catabolic processes. This balance is shifted toward catabolic activities in osteoarthritis (OA), a disease that involves all joint tissues and disrupts the structural integrity of articular cartilage. In OA, articular chondrocytes become more hypertrophic-ike, and abnormally increase not only extracellular matrix (ECM) production but also the expression of ECM-degrading enzymes. While chondrocyte hypertrophy is indispensable for endochondral bone formation, in OA it mediates detrimental changes that lead to cartilage destruction. Our previous work established that translational control of gene expression is critical for dedifferentiation of articular chondrocytes in OA. We showed that fine-tuned control of protein synthesis by translational repressor 4E-BP (eIF4E-Binding Protein) is required for proper chondrocyte homeostasis. By binding to the m7-GTP cap-binding protein eIF4E, 4E-BP inhibits cap-dependent mRNA translation by restricting the incorporation of eIF4E into a multi-subunit initiation factor called eIF4F, which is needed to recruit 40S ribosomes to mRNA. We hypothesized that proper translational control is crucial for maintaining healthy cartilage phenotype. We therefore investigated expression of which genes is controlled (completely or partially) at the translational level. Methods: To identify the pool of mRNAs that are translationally control in OA cartilage we first compared translation efficiencies of mRNAs by analyzing polysome profiles of rat articular chondrocytes (RAC) untreated and treated with Interleukin 1β (IL-1β). Expression of several translationally regulated mRNAs was then compared to their expression in human and rodent OA cartilage. Additionally, the eIF4F inhibitor was used in vivo to determine if restriction of eIF4F activity affects OA progression in ACLT rat model. Results: We identified 617 mRNAs that are translationally regulated by IL-1β. One of them is the orphan receptor Nr4a1, whose protein but not mRNA level is significantly increased in OA cartilage in vivo, highlighting the exceptional potential of our approach for discovering new possible targets in OA cartilage. Modulating Nr4a1 expression in IL-1β-treated articular chondrocytes effects expression of MMP13, the metalloproteinase that has a predominant role in OA. Intra-articular injection of 4E1RCat, an inhibitor of cap-dependent translation not only precludes Fn and Nr4a upregulation in a rodent OA model, but also significantly delays cartilage degeneration in the ACLT rat model. Conclusions: We identified 617 mRNAs that are translationally regulated by IL-1β. One of them is the orphan receptor Nr4a1, whose protein but not mRNA level is significantly increased in OA cartilage in vivo, highlighting the exceptional potential of our approach for discovering new possible targets in OA cartilage. Modulating Nr4a1 expression in IL-1β-treated articular chondrocytes effects expression of MMP13, the metalloproteinase that has a predominant role in OA. Intra-articular injection of 4E1RCat, an inhibitor of cap-dependent translation not only precludes Fn and Nr4a upregulation in a rodent OA model, but also significantly delays cartilage degeneration in the ACLT rat model.