Cathepsin K knockout mice develop osteopetrosis due to a deficit in matrix degradation but not demineralization.

Cathepsin K is a cysteine protease expressed predominantly in osteoclasts. Activated cathepsin K cleaves key bone matrix proteins and is believed to play an important role in degrading the organic phase of bone during bone resorption. Mutations in the human cathepsin K gene have been demonstrated to be associated with a rare skeletal dysplasia, pycnodysostosis. The degree of functional activity of the mutated forms of cathepsin K in these individuals has not been elucidated, but is predicted to be low or absent. To study the role of cathepsin K in bone resorption, we have generated mice deficient in the cathepsin K gene. Histologic and radiographic analysis of the mice revealed osteopetrosis of the long bones and vertebrae, and abnormal joint morphology. X-ray microcomputerized tomography images allowed quantitation of the increase in bone volume, trabecular thickness, and trabecular number in both the primary spongiosa and the metaphysis of the proximal tibiae. Not all bones were similarly affected. Chondrocyte differentiation was normal. The mice also had abnormalities in hematopoietic compartments, particularly decreased bone marrow cellularity and splenomegaly. The heterozygous animals appeared normal. Close histologic examination of bone histology revealed fully differentiated osteoclasts apposed to small regions of demineralized bone. This strongly suggests that cathepsin K-deficient osteoclasts are capable of demineralizing the extracellular matrix but are unable to adequately remove the demineralized bone. This is entirely consistent with the proposed function of cathepsin K as a matrix-degrading proteinase in bone resorption.

Osteopetrosis and osteoporosis: two sides of the same coin.

Together, osteoporosis and osteopetrosis comprise a substantial proportion of the bone diseases that severely affect humans. In order to understand and effectively treat these disorders, an understanding of the mechanisms controlling bone remodelling is essential. While numerous animal models of bone disease have been generated, the lack of correlation between these animal models and human disease has limited their utility in terms of defining therapeutic strategies. The generation and analysis of cathepsin K knockout mice has resulted in a model for pycnodysostosis, a rare human osteopetrotic disease, and is now providing considerable insights into both osteoclast function and potential therapeutic strategies for the treatment of bone disease. This review highlights the importance of genes such as cathepsin K in understanding bone remodelling and illustrates a new trend towards understanding bone disease as a complete entity rather than as a series of unrelated disorders.