Novel roles of glycosaminoglycans in the degradation of type I collagen by cathepsin K

Glycosaminoglycans (GAGs) and collagen are the major organic components of bone matrix. However, their roles and functional relationships remain elusive. To investigate the role of GAGs in bone matrix degradation, the effects of GAGs on collagen were examined under acidic conditions that recapitulate the microenvironment of osteoclast resorption pits. We found that sulfated GAGs protect collagen fibrils against acid denaturation. Scanning electron microscopy demonstrated that collagen fibrils retain the fibril structure at pH 4.0 in the presence of chondroitin 6-sulfate. By surface plasmon resonance analysis, we found that sulfated GAGs, but not non-sulfated GAGs, bind to triple-helix type I collagen below pH 4.5. The binding of collagen in an acidic solution was dependent upon the GAG sugar chain length. Functionally, the acid-resistant collagen fibrils generated in the presence of sulfated GAGs were resistant to cathepsin K degradation in vitro below pH 4.0. As the pH increased from 4.0 to 5.0, the acid-resistant collagen fibrils were degraded by cathepsin K. Our results highlight the possibility that the interaction between GAGs and collagen under acidic conditions has a regulatory impact on cathepsin K-mediated bone degradation.