DDX5 inhibits hyaline cartilage fibrosis and degradation in osteoarthritis via alternative splicing and G-quadruplex unwinding

Hyaline cartilage fibrosis is one of the important reasons for the poor prognosis of advanced osteoarthritis (OA), and is the main factor leading to joint stiffness and deformity. However, the mechanism of hyaline cartilage fibrosis remains largely unclear. Here we found that DDX5, one of the most important members of the DEAD-box RNA helicase family, was dramatically downregulated in the degenerated articular cartilage of aged mice, destabilization of the medial meniscus murine model as well as patients with OA. Chondrocyte-specific deletion of Ddx5 led to much severe and early OA lesions in murine model. In vitro and in vivo experimental findings showed that DDX5 deficiency increased the hyaline cartilage fibrosis phenotype by upregulating collagen type I (COL1) expression and downregulating collagen type II (COL2) expression. In addition, loss of DDX5 aggravated cartilage degradation through inducing the production of cartilage-degrading enzymes. Mechanistically, on one hand, DDX5 was enriched at the Col2 promoter, was extremely proficient at unfolding Col2 promoter G4-DNA and activated Col2 transcription, which explains why DDX5 deficiency directly leads to a decrease in Col2 transcription. On the other hand, DDX5-induced exon 25 skipping of Fn1-AS and exon 14 skipping of Plod2-AS produced two alternative splicing transcripts, but the weaken DDX5 resulted in abundance of the Fn1-AS-WT and Plod2-AS-WT variants, which promotes transcription of fibrosis-related genes (Col1, Acta2, etc) and extracellular matrix degradation genes (Mmp13, Nos2, etc), respectively. Our data suggest that strategies aimed at upregulation of DDX5 will be of great value for the treatment of cartilage fibrosis and damage in OA.