Cathepsin K activity-dependent regulation of osteoclast actin ring formation and bone resorption

Cathepsin K is responsible for the degradation of type I collagen in osteoclast-mediated bone resorption. Collagen fragments are known to be biologically active in a number of cell types. Here, we investigate their potential to regulate osteoclast activity. Mature murine osteoclasts were seeded on type I collagen for actin ring assays or dentine discs for resorption assays. Cells were treated with cathepsins K-, L-, or MMP-1-predigested type I collagen or soluble bone fragments for 24 h. The presence of actin rings was determined fluorescently by staining for actin. We found that the percentage of osteoclasts displaying actin rings and the area of resorbed dentine decreased significantly on addition of Cathepsin K-digested type I collagen or bone fragments, but not with Cathepsin L or MMP-1 digests. Counterintuitively, actin ring formation was found to decrease in the presence of the cysteine proteinase inhibitor LHVS and in Cathepsin K-deficient osteoclasts. However, Cathepsin L deficiency or the general MMP Inhibitor GM6001 had no effect on the presence of actin rings. Predigestion of the collagen matrix with Cathepsin K, but not by Cathepsin L or MMP-1 resulted in an increased actin ring presence in Cathepsin K-deficient osteoclasts. These studies suggest that Cathepsin K interaction with type I collagen is required for 1) the release of cryptic Arg-Gly-Asp motifs during the initial attachment of osteoclasts and 2) termination of resorption via the creation of autocrine signals originating from type I collagen degradation.