Gene Expression of Noncollagenous Bone Matrix Proteins in the Limb Joints and Intervertebral Disks of the twy Mouse

The twy (tiptoe walking Yoshimura) mouse is an autosomal recessive mutant manifesting multiple osteochondral lesions characterized by pathologic calcium deposition. To elucidate the pathophysiology of the limb joint lesions and the intervertebral disk lesions of the twy mouse, we assessed the mRNA expression of noncollagenous bone matrix proteins such as osteocalcin, osteonectin, osteopontin, and matrix Gla protein (MGP) by in situ hybridization, but only expression of MGP was observed in association with the pathologic calcium deposits in twy mice. Mild degeneration and abnormal growth of the cartilage in contact with the joint capsule was observed at 5 weeks in the articular cartilage of the ankle joint of the twy mouse, and MGP gene expression was observed at the same time. Simultaneous growth of synovial membrane cells and relatively undifferentiated articular cartilage cells in the knee joint, and of cartilage-like cells near the insertion of the cruciate ligament was observed in the twy mouse, and MGP gene expression was found to be present at the same time. Hypertrophy of abnormally proliferated chondrocyte-like cells, which are different from fibrocartilaginous cells of the annulus fibrosus, was observed in the intervertebral disks of the twy mouse at 3 weeks of age, and MGP gene expression was noted at the same time. These findings suggest that abnormal expression of MGP plays a major role in the pathologic calcification of the twy mouse.