Objective: To study the roles of calpains in the synovial joint in rheumatoid arthritis (RA) and osteoarthritis (OA) and to verify the hypothesis that calpains present in the synovial fluid come from the synovium.
Methods: We performed immunohistochemical, biochemical, and immunoblotting analyses for calpains in synovial tissues, synovial cell cultures, and synovial fluids.
Results: Immunohistochemical staining of RA synovium demonstrated specific cytoplasmic staining of cells in the synovial lining layer, storomal fibroblasts, and endothelial cells. OA synovium showed almost the same intensity and distribution of calpain staining. DEAE-cellulose chromatography of RA and OA synovial extracts and synovial fluids showed a peak of caseinolytic activity attributable to calpain, as well as an inhibitory peak of calpastatin, a specific inhibitor protein of calpains. Immunoblotting using the anticalpain antibody from the calpain peak of RA and OA synovium and synovial fluid showed identity with the heavy subunit of calpain (80 kd). Similarly, calpain existed in the same form (80 kd) in conditioned media (supernatant) obtained from synovial cell cultures, as well as in the synoviocytes. The total specific activity of the 2 calpains in the synovial fluid of RA patients was higher than that of calpastatin.
Conclusion: The findings suggest that the extracellular appearance of calpains could be due to the secretion of these proteins from the synovial cells and that calpains may play a role in cartilage damage of RA and OA that occurs in synovial joints.