Cleavage of fibromodulin in cartilage explants involves removal of the N-terminal tyrosine sulfate-rich region by proteolysis at a site that is sensitive to matrix metalloproteinase-13.

Integrity of cartilage fails in joint disease. The current work aimed to identify candidate active proteinases in joint diseases using an in vitro model for cartilage degradation induced by interleukin-1. A critical event in the process of cartilage destruction in joint disease is the failure of the collagen fiber network to maintain integrity. Proteins binding to the surface of the fibers are likely early points of failure. Fibromodulin, a member of the leucine-rich repeat protein family, is one predominant protein in cartilage and is known for its roles in the formation of collagen fibrils and sustained interaction with these formed fibers. Cleavage removes the tyrosine sulfate-rich region in the N terminus of fibromodulin. Whereas fibromodulin bound to collagen in tissue was digested, purified fibromodulin was not cleaved. In contrast an N-terminal 10-kDa fragment, Gln19-Lys98, of the protein generated by Lys-C digestion contains the cleavage site and was a substrate cleaved by the enzyme in medium from stimulated cultures. In solution, digestion of this substrate with matrix metalloproteinase (MMP)-2, -9, -8, and -13 demonstrated that only MMP-13 was capable to efficiently cleave it. The cleavage product obtained after MMP-13 digestion was identical to that observed in cleaved fibromodulin from cartilage explant cultures stimulated with interleukin-1. MMP-13 treatment of fresh articular cartilage also produced the fragment under study. The elucidation of the enzyme responsible for such cleavage may lead to treatment modalities involving its selective inhibition for patients suffering from arthritis. The known structure of the fragments permits the generation of neo-epitope antibodies to the cleavage site, which can be used to detect ongoing cartilage degradation in patients with arthritic disease, an important adjunct in monitoring disease progression, active disease, and efficacy of treatment.

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