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

Objective: Recently it was shown that loading of articular cartilage explants activates TGF beta signaling. Here we investigated if in vivo chondrocytes express permanently high TGF beta signaling, and the consequence of the loss of compressive loading-mediated TGF beta 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 TGF beta 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 TGF beta on proteoglycan level and chondrocyte phenotype (Col10a1 mRNA expression) was analyzed. Results: Unloading of articular cartilage results in rapid loss of TGF beta signaling while subsequent compressive loading swiftly restored this. Loading and exogenous TGF beta enhanced expression of TTGF beta 1 and ALK5. Unloading of explants for 2 weeks resulted in proteoglycan loss and increased Col10a1 expression. Both loading and exogenous TGF beta 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 TGF beta signaling and TGF beta-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. (C) 2016 Osteoarthritis Research Society International. Published by Elsevier Ltd. All rights reserved.