The role of muscle cells in regulating cartilage matrix production.

Muscle is one of the tissues located in close proximity to cartilage tissue. Although it has been suggested that muscle could influence skeletal development through generating mechanical forces by means of contraction, very little is known regarding whether muscle cells release biochemical signals to regulate cartilage gene expression. We tested the hypothesis that muscle cells directly regulate cartilage matrix production by analyzing chondrocytes cocultured with muscle cells in 2D or 3D conditions. We found that chondrocytes cultured with C2C12 muscle cells exhibited enhanced alcian blue staining and elevated expression of collagen II and collagen IX proteins. Although nonmuscle cells did not promote cartilage matrix production, converting them into muscle cells enhanced their pro-chondrogenic activity. Furthermore, muscle cell-conditioned medium led to increased cartilage matrix production, suggesting that muscle cells secrete pro-chondrogenic factors. Taken together, our study suggests that muscle cells may play an important role in regulating cartilage gene expression. This result may ultimately lead to the discovery of novel factors that regulate cartilage formation and homeostasis, and provide insights into improving the strategies for regenerating cartilage.