Introduction: Osteoarthritis (OA) is a degenerative joint disease affecting a large population of people. The mechanism of this highly prevalent disease is not fully understood. Currently there is no effective disease-modifying treatment for OA. The purpose of this study was two-fold: 1) to investigate the role of MMP13 in the development of OA; and 2) to evaluate the efficacy of the MMP13 inhibitor CL82198 as a pharmacologic treatment for preventing OA progression. Methods: To investigate the role of the endogenous Mmp13 gene in OA development, tamoxifen was administered to two-week-old Col2CreER;Mmp13 ^fx/fx (Mmp13 ^Col2ER ) and Cre-negative control mice for five days. OA was induced by meniscal-ligamentous injury (MLI) when the mice were 10 weeks old and MLI or sham-operated joints were harvested 4, 8, 12, or 16 weeks after surgery. To evaluate the efficacy of CL82198, MLI surgery was performed on 10-week-old wild type mice. CL82198 or saline was administered to the mice daily beginning immediately after the surgery for up to 16 weeks. The joint tissues collected from both experiments were evaluated by cartilage grading, histology/histomorphometry, immunohistochemistry (IHC), and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining. The ability of CL82198 to inhibit MMP13 activity in vitro was confirmed by ELISA. Results: The OA progression was decelerated in Mmp13 ^Col2ER mice 8, 12, and 16 weeks post-surgery. Cartilage grading by blinded observers confirmed decreased articular cartilage degeneration in Mmp13 ^Col2ER mice at 8, 12 and 16 weeks compared to Cre-negative mice. Histomorphometric analysis demonstrated that Mmp13 ^Col2ER mice had a higher articular cartilage area and thickness at 12 and 16 weeks post-surgery compared to the control mice. Results of IHC revealed greater type II collagen and proteoglycan expression in Mmp13 ^Col2ER mice. Chondrocyte apoptosis, as determined by TUNEL staining, was higher in control mice compared to Mmp13 ^Col2ER mice. CL82198 inhibited MMP13 activity in conditioned media from vehicle (> 85%) or bone morphogenetic protein 2 (BMP2)-treated (> 90%) primary murine sternal chondrocytes. Intraperitoneal injection of CL82198 decelerated MLI-induced OA progression, increased type II collagen and proteoglycan levels, and inhibited chondrocyte apoptosis compared to saline treatment as determined by OA grading, histology, histomorphometry, IHC, and TUNEL staining, respectively. Conclusions: Mmp13 is critical for OA progression and pharmacologic inhibition of MMP13 is an effective strategy to decelerate articular cartilage loss in a murine model of injury-induced knee OA.

Objective: While transforming growth factor β (TGFβ) signaling plays a critical role in chondrocyte metabolism, the TGFβ signaling pathways and target genes involved in cartilage homeostasis and the development of osteoarthritis (OA) remain unclear. Using an in vitro cell culture method and an in vivo mouse genetic approach, we undertook this study to investigate TGFβ signaling in chondrocytes and to determine whether Mmp13 and Adamts5 are critical downstream target genes of TGFβ signaling.Methods: TGFβ receptor type II (TGFβRII)–conditional knockout (KO) (TGFβRII^Col2ER) mice were generated by breeding TGFβRII^flox/flox mice with Col2‐CreER–transgenic mice. Histologic, histomorphometric, and gene expression analyses were performed. In vitro TGFβ signaling studies were performed using chondrogenic rat chondrosarcoma cells. To determine whether Mmp13 and Adamts5 are critical downstream target genes of TGFβ signaling, TGFβRII/matrix metalloproteinase 13 (MMP‐13)– and TGFβRII/ADAMTS‐5–double‐KO mice were generated and analyzed.Results: Inhibition of TGFβ signaling (deletion of the Tgfbr2 gene in chondrocytes) resulted in up‐regulation of Runx2, Mmp13, and Adamts5 expression in articular cartilage tissue and progressive OA development in TGFβRII^Col2ER mice. Deletion of the Mmp13 or Adamts5 gene significantly ameliorated the OA‐like phenotype induced by the loss of TGFβ signaling. Treatment of TGFβRII^Col2ER mice with an MMP‐13 inhibitor also slowed OA progression.Conclusion: Mmp13 and Adamts5 are critical downstream target genes involved in the TGFβ signaling pathway during the development of OA.