Cartilage tissue enhances proteoglycan retention by nucleus pulposus cells in vitro.

OBJECTIVE To investigate the effect of cartilage on nucleus pulposus (NP) tissue in an in vitro model. METHODS Cells were isolated from bovine NP or articular cartilage and allowed to form tissue in vitro. The NP tissue was grown either alone or in the presence of cartilage tissue (coculture) for up to 4 weeks and examined for histologic appearance, gene expression, and biochemical composition. For selected experiments, NP tissue was grown in coculture with fragments of cartilage end-plate. RESULTS Coculture of in vitro-formed NP tissue with cartilage end-plate tissue resulted in a significant increase in proteoglycan content in the NP tissue by 2 weeks, compared with NP tissue grown alone. Substituting in vitro-formed cartilage tissue for cartilage end-plate also had a positive effect on the NP tissue, suggesting that it was an appropriate substitute for cartilage end-plate. Coculture of NP with in vitro-formed cartilage for 2 weeks increased aggrecan and collagen gene expression compared with that in NP tissue grown alone, and also reduced expression of matrix metalloproteinase 3 (MMP-3), MMP-13, and ADAMTS-5. NP cells from older and younger animals responded similarly to in vitro-formed cartilage. Expression of genes for tumor necrosis factor α (TNFα) and TACE in NP cells was higher when grown in the absence of cartilage. This corresponded with increased TNFα protein levels in the absence of cartilage. CONCLUSION The data suggest that chondrocytes may secrete a factor(s) that positively enhances tissue growth, perhaps by inhibiting TNFα production. This could be a potential mechanism explaining how loss of the cartilage end-plate may contribute to the development of NP degenerative changes.