Creb5 coordinates synovial joint formation with the genesis of articular cartilage

While prior work has established that articular cartilage arises from Prg4-expressing perichondrial cells, it is not clear how this process is specifically restricted to the perichondrium of synovial joints. We document that the transcription factor Creb5 is necessary to initiate the expression of signaling molecules that both direct the formation of synovial joints and guide perichondrial tissue to form articular cartilage instead of bone. Creb5 promotes the generation of articular chondrocytes from perichondrial precursors in part by inducing expression of Wif1, which blocks a Wnt5a autoregulatory loop in the perichondrium. Postnatal deletion of Creb5 in the articular cartilage leads to loss of both flat superficial zone articular chondrocytes coupled with a loss of both Prg4 and Wif1 expression; and a non-cell autonomous up-regulation of Ctgf. Our findings indicate that Creb5 promotes both joint formation and the subsequent development of articular chondrocytes by disrupting a Wnt5a positive-feedback loop in the perichondrium. We have identified genes expressed in bovine superficial zone articular chondrocytes whose expression was altered following shRNA-mediated knockdown of the transcription factor, Creb5. We performed RNA-Seq analysis of bovine superficial zone articular chondrocytes that had been infected with lentivirus programmed to express either a control shScramble or shCreb5. We cultured the infected superficial zone articular chondrocytes for forty-eight hours in the presence of either the EGFR ligand, TGFalpha (condition A), or in the presence of both TGFalpha and TGFbeta-2 (condition AB).