Global, but not chondrocyte-specific, MT1-MMP deficiency in adult mice causes inflammatory arthritis

Membrane-type I metalloproteinase (MT1-MMP/MMP14) plays a key role in various pathophysiological processes, indicating an unaddressed need for a targeted therapeutic approach. However, mice genetically deficient in Mmp14 show severe defects in development and growth. To investigate the possibility of MT1-MMP inhibition as a safe treatment in adults, we generated global Mmp14 tamoxifen-induced conditional knockout (Mmp14^kd) mice and found that MT1-MMP deficiency in adult mice resulted in severe inflammatory arthritis. Mmp14^kd mice started to show noticeably swollen joints two weeks after tamoxifen administration, which progressed rapidly. Mmp14^kd mice reached a humane endpoint 6 to 8 weeks after tamoxifen administration due to severe arthritis. Plasma TNF-α levels were also significantly increased in Mmp14^kd mice. Detailed analysis revealed chondrocyte hypertrophy, synovial fibrosis, and subchondral bone remodeling in the joints of Mmp14^kd mice. However, global conditional knockout of MT1-MMP in adult mice did not affect body weight, blood glucose, or plasma cholesterol and triglyceride levels. Furthermore, we observed substantial expression of MT1-MMP in the articular cartilage of patients with osteoarthritis. We then developed chondrocyte-specific Mmp14 tamoxifen-induced conditional knockout (Mmp14^chkd) mice. Chondrocyte MT1-MMP deficiency in adult mice also caused apparent chondrocyte hypertrophy. However, Mmp14^chkd mice did not exhibit synovial hyperplasia or noticeable arthritis, suggesting that chondrocyte MT1-MMP is not solely responsible for the onset of severe arthritis observed in Mmp14^kd mice. Our findings also suggest that highly cell-type specific inhibition of MT1-MMP is required for its potential therapeutic use.