CTSK and osteoarthritis: Although various sources of non-osteoclastic CTSK have been reported in pathologic environments, such as the synovium of patients with rheumatoid arthritis (RA) [25], smooth muscle cells in atheroma [26], and articular hyaline cartilage in osteoarthritis [27], the non-osteoclastic cellular sources of CTSK in PD have not been fully explored.