Matrix regulation of osteoclast bone resorption mediated by cathepsin K

Objectives: Cathepsin K (catK) is responsible for the degradation of type I collagen in osteoclast mediated bone resorption. As collagen fragments are known to be biologically active we investigated their potential to regulate osteoclast activity. Methods: Soluble collagen and bone powder were subject to degradation reactions by catK, L and MMP-1. Soluble degradation products or GRGDS peptides were added to isolated murine osteoclasts seeded on type I collagen substrate. Alternatively, the collagen substrate was predigested with catK or L. Wild type, catK and catL deficient osteoclasts were also treated with cysteine proteinase inhibitor LHVS and MMP inhibitor GM6001. After 24h, osteoclasts were stained with FITC-phalloidin and actin rings were counted as a percentage of total osteoclast number. Results: We found that the percentage of osteoclasts displaying actin rings decreased significantly on addition of cat K digested type I collagen or bone fragments, but not with catL or MMP1 digests. Inhibition by collagen fragments could be partially abrogated by the presence of a vitronectin receptor blocking antibody, suggesting they interact directly with the integrin receptor. Counterintuitively, actin ring formation was found to decrease in the presence of cysteine proteinase inhibitor LHVS and in catK-deficient osteoclasts. However, catL-deficiency or the general MMP inhibitor GM6001 had no effect on the presence of actin rings. Predigestion of the collagen matrix with catK, but not by catL or MMP1, resulted in an increased actin ring presence in cathepsin K-deficient osteoclasts. Conclusions: These studies suggest that catK interaction with type I collagen is required for a) the release of cryptic RGD motifs during the initial attachment of osteoclasts and b) termination of resorption via the creation of autocrine signals. This is a novel regulatory role for catK and collagen fragments in bone resorption. NIH grants AR 46182 and DK 072070.