Calpain of Synovia and synovial fluid in arthritis

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Calpain, also called Calcium-activated neutral proteinase (CAMP) , is a calcium ion-dependent neutral cysteine proteinase. Two forms of calpain are present, and they differ in their Ca^<2+> requirements for activation : mu-calpain, or calpain I requires low concentrations (micromolar) and m-calpain, or calpain II requires high concentrations (millimolar) . Specific inhibitor protein, calpastatin, is also known to exist. Calpains have been known to be intracellular proteinases. Recent studies elucidated a new aspect of calpain as a matrix proteinase with its potent proteinase activities against cartilage proteoglycan. Osteoarthritic synovial fluid contains mu-calpain, m-calpain and calpastatin and exerting proteoglycanse activities extracellularly (Arthritis Rheum 1990) . The present study was performed with the object of elucidating responsible cells for secretion of calpain into synovial fluid and to clarify function of calpain in arthritis. Synovial tissues from rheumatoid arthritis (RA) and Osteoarthritis (OA) was stained immunohistochemically using anti-calpain antibody. Calpain immunoreactivity was demonstrated in synovial lining cell, endothelial cells, and interstitial fibroblast. Immunoblot of synovial tissues, synovial fluid, cultured synovial fibroblast, culture medium showed specific band of calpain. Calpain activity was assayd in synovial fluids of RA and OA.The results showed both degrading activity of calpain and inhibitory activity of calpastatin. Total degrading activity was higher than inhibitory activity. These data strongly suggest that calpain is secreted from synovial cells into synovial fluid and that calpain contributes to degradation of cartilage matrix as well as other known matrix proteinases in osteoarthritis.