Scaffoldless tissue‐engineered cartilage for studying transforming growth factor beta‐mediated cartilage formation

Reduced transforming growth factor beta (TGF-beta) signaling is associated with osteoarthritis (OA). TGF-beta is thought to act as a chondroprotective agent and provide anabolic cues to cartilage, thus acting as an OA suppressor in young, healthy cartilage. A potential approach for treating OA is to identify the factors that act downstream of TGF-beta's anabolic pathway and target those factors to promote cartilage regeneration or repair. The aims of the present study were to (a) develop a scaffoldless tissue-engineered cartilage model with reduced TGF-beta signaling and disrupted cartilage formation and (b) validate the system for identifying the downstream effectors of TGF-beta that promote cartilage formation. Sox9 was used to validate the model because Sox9 is known to promote cartilage formation and TGF-beta regulates Sox9 activity. Primary bovine articular chondrocytes were grown in Transwell supports to form cartilage tissues. An Alk5/TGF-beta type I receptor inhibitor, SB431542, was used to attenuate TGF-beta signaling, and an adenovirus encoding FLAG-Sox9 was used to drive the expression of Sox9 in the in vitro-generated cartilage. SB431542-treated tissues exhibited reduced cartilage formation including reduced thicknesses and reduced proteoglycan staining compared with control tissue. Expression of FLAG-Sox9 in SB431542-treated cartilage allowed the formation of cartilage despite antagonism of the TGF-beta receptor. In summary, we developed a three-dimensional in vitro cartilage model with attenuated TGF-beta signaling. Sox9 was used to validate the model for identification of anabolic agents that counteract loss of TGF-beta signaling. This model has the potential to identify additional anabolic factors that could be used to repair or regenerate damaged cartilage.