916530.fig.002
Figure 2: Schematic diagram showing that cartilage matrix homeostasis is adjusted dynamically by TGF-β signaling pathway. Activated TGF-β binds to the TGF-β type I receptor ALK1, resulting in phosphorylation of Smad1, Smad5, or Smad8, which form a complex with the co-Smad Smad4 and translocate to the nucleus to promote cartilage matrix anabolism by modifying gene expression. Meanwhile, the activated TGF-β type II receptor ALK5 results in phosphorylation of Smad2 or Smad3 which form a complex with the co-Smad Smad4 and translocate to the nucleus to promote cartilage matrix catabolism by modifying gene expression.