Proteoglycan production is required in initial stages of new cartilage matrix formation but inhibits integrative cartilage repair

The optimal stimulus to repair or regenerate cartilage is not known. We therefore modulated collagen deposition, collagen crosslinking and GAG deposition simultaneously during cartilage matrix production and integrative repair, creating more insight into their role in cartilage repair processes. Insulin-like growth factor 1 (IGF-1; increases proteoglycan and collagen synthesis), beta-aminopropionitrile (BAPN; a reversible inhibitor of collagen crosslinking) and para-nitrophenyl-beta-D-xyloside (PNPX; interferes with proteoglycan production) were used. Bovine articular chondrocytes were cultured in alginate beads for 3 weeks with or without IGF-1, BAPN or PNPX alone and in all possible combinations, followed by 3 weeks in control medium. DNA content, GAG and collagen deposition and collagen crosslinks were determined. Cartilage constructs were cultured under the same conditions and histologically analysed for integration of two opposing cartilage matrices. In alginate cultures, inhibition of collagen crosslinking with BAPN, in combination with promotion of matrix synthesis using IGF1, was most beneficial for matrix deposition. Addition of PNPX was always detrimental for matrix deposition. For integration of opposing cartilage constructs, the combination of BAPN, IGF1 and temporary prevention of proteoglycan formation with PNPX was most beneficial. When a new matrix is produced, proteoglycans are important to retain collagen in the matrix. When two already formed cartilage matrices have to integrate, a temporary absence of proteoglycans and temporary inhibition of collagen crosslinking might be more beneficial in combination with stimulation of collagen production, e.g. by IGF1. Therefore, the choice of soluble factors to promote cartilage regeneration depends on the type of therapy that will be used.