Cleavage of aggrecan at the ASN(341)-PHE342 site coincides with the initiation of collagen damage in murine antigen-induced arthritis - A pivotal role for stromelysin 1 in matrix metalloproteinase activity

Objective. The destruction of articular cartilage during arthritis is due t o proteolytic cleavage of the extracellular matrix components, This study i nvestigates the kinetic involvement of metalloproteinases (MMPs) in the deg radation of the 2 major cartilage components, aggrecan and type II collagen , during murine antigen-induced arthritis (AIA). In addition, the role of s tromelysin 1 (SLN-1) induction of MMP-induced neoepitopes was studied. Methods, VDIPEN neoepitopes in aggrecan and collagenase-induced COL2-3/4C n eoepitopes in type II collagen were identified by immunolocalization, Strom elysin 1-deficient knockout (SLN1-KO) mice were used to study SLN-1 involve ment. Results, In AIA, the VDIPEN epitopes in aggrecan appeared after initial pro teoglycan (PG) depletion, The collagenase-induced type II collagen neoepito pes colocalized with VDIPEN epitopes, Remarkably, cartilage from arthritic SLN1-KO mice showed neither the induction of VDIPEN nor collagen cleavage-s ite neoepitopes during AIA, suggesting that stromelysin is a pivotal mediat or in this process, PG depletion, as measured by the loss of Safranin O sta ining, was similar in SLN1-KO mice and wild-type strains. Furthermore, in v itro induction of VDIPEN epitopes in aggrecan and COL2-3/4C epitopes in typ e II collagen, on exposure of cartilage to interleukin-1, could not be acco mplished in SLN1-KO mice,mice, whereas intense staining was achieved for bo th epitopes in cartilage of wild-type strains. Conclusion, This study emphasizes that SLN-1 is essential in the induction of MMP-specific aggrecan and collagen cleavage sites during AIA, It suggest s that SLN-1 is not a dominant enzyme in PG breakdown;vn, but that it activ ates procollagenases and is crucial in the initiation of collagen damage.