Cathepsin K Non-Osteoclast Cells in the Skeletal System: Function, Models, Identity, and Therapeutic Implications.

Cathepsin K (Ctsk) is a cysteine protease of the papain superfamily initially identified in differentiated osteoclasts; it plays a critical role in degrading the bone matrix. However, subsequent and studies based on animal models elucidate novel subpopulations of Ctsk-expressing cells, which display markers and properties of mesenchymal stem/progenitor cells. This review introduces the function, identity, and role of Ctsk cells and their therapeutic implications in related preclinical osseous disorder models. It also summarizes the available models for studying Ctsk cells and their progeny. Further investigations of detailed properties and mechanisms of Ctsk cells in transgenic models are required to guide potential therapeutic targets in multiple diseases in the future.