Unloading results in rapid loss of TGFbeta signaling in articular cartilage: role of loading-induced TGFbeta signaling in maintenance of articular chondrocyte phenotype?

Abstract

OBJECTIVE: Recently it was shown that loading of articular cartilage explants activates TGFbeta signaling. Here we investigated if in vivo chondrocytes express permanently high TGFbeta signaling, and the consequence of the loss of compressive loading-mediated TGFbeta signaling on chondrocyte function and phenotype. METHOD: Bovine articular cartilage explants were collected within 10 min post mortem and stained immediately and after 30, 60 and 360 min for phosphorylated-Smad2, indicating active TGFbeta signaling. Explants were unloaded for 48 h and subsequently repeatedly loaded with a compressive load of 3 MPa. In addition, explants were cultured unloaded for 2 weeks and the effect of loading or exogenous TGFbeta on proteoglycan level and chondrocyte phenotype (Col10a1 mRNA expression) was analyzed. RESULTS: Unloading of articular cartilage results in rapid loss of TGFbeta signaling while subsequent compressive loading swiftly restored this. Loading and exogenous TGFbeta enhanced expression of TGFbeta1 and ALK5. Unloading of explants for 2 weeks resulted in proteoglycan loss and increased Col10a1 expression. Both loading and exogenous TGFbeta inhibited elevated Col10a1 expression but not proteoglycan loss. CONCLUSION: Our data might imply that in vivo regular physiological loading of articular cartilage leads to enduring TGFbeta signaling and TGFbeta-induced gene expression. We propose a hypothetical model in which loading activates a self-perpetuating system that prevents hypertrophic differentiation of chondrocytes and is crucial for cartilage homeostasis.