SPP1 and Alzheimer disease: Therefore, we investigated the following: (1) the correlation between AD and OPN expression in the serum and synovial tissue of patients with RA, (2) the regulatory effect of AD on OPN expression and osteoclast precursor migration in synovial fibroblasts and (3) the effect of AD and OPN on disease progression, osteoclastogenesis and bone erosion in CIA mice.