Title: Experimental evaluation of discoidin domain receptor 2 as an ideal target for development of disease-modifying osteoarthritis drugs
Abstract: Purpose: The goal of this study is to evaluate if the complete removal of Ddr2 from the knee joint of mouse adult articular cartilage can delay progression of osteoarthritis prior to or after initiation of articular cartilage degeneration. MMP-13, which has the ability to degrade both aggrecans and type II collagen, would be an ideal target for disease-modifying osteoarthritis drugs. However, the broad biological effects of MMP-13 restrict its application as a target enzyme of inhibitor drugs in the treatment of OA. Recent studies demonstrate that a specific amino acid sequence, from amino acid 594 to 605, on type II collagen is preferentially recognized by DDR2. The expression and subsequent activation of DDR2 were increased in human OA tissues and mouse models of OA, and this was co-localized with elevated expression of MMP-13 in degenerative articular cartilages. Conversely, reduced expression of Ddr2 in the heterozygous Ddr2 knockout condition attenuated progression of articular cartilage degeneration in mouse models of OA. Typically, DDR2 is kept inactivated by the presence of the pericellular matrix, which separates chondrocytes from type II collage in healthy articular cartilage. Once enzymes, such as a serine proteinase, high temperature requirement A1, that are capable of degrading the pericellular molecules expose chondrocytes to type II collagen, DDR2 is activated and then induces expression of MMP-13 leading the degradation of type II collage and proteoglycans resulting in joint destruction and OA. Methods: 1) By use of conditional knock out techniques with aggrecan-CreERt2 mice and floxed Ddr2 mice, Ddr2 was removed from articular cartilage of knee joints in mice at 8 weeks of age via intraperitoneal injection Tamoxifen injection (2mg/10g body weight) for 5 consecutive days (Group A). Mice were subjected to destabilization of the medial meniscus (DMM) or sham surgery at 10 weeks of age. 2) An additional experimental group was subjected to DMM or sham surgery at 10 weeks of age and then Ddr2 was removed by intraperitoneal injection Tamoxifen injection 8 weeks later (Group B). 3) Knee joints from Groups A and B mice and their corresponding controls (n=7) were then collected for morphological analysis. Knee joints from mice in Group A were harvested at 8 weeks (n=7) or 16 weeks (n=9) post-surgery and those from Group B (n=5) at 16 weeks post-surgery. 4) Histology was performed. 5) ORASI Modified Mankin Score was used to evaluate articular cartilage degeneration. 6) Statistically significant differences were determined via T-test. Results: 1) The average modified score for Group A 8 week control was 1.64 (score range 1.5-2) whereas the average modified score with Ddr2 removed was 0.64 (score range 0.5-1) [P<0.05]. 2) The average modified score for Group A 16 week control was 4.67 (score range 4-5) and the average modified score with Ddr2 removed was 1.27 (score range 0.5-3) [P<0.05]. 3) The average modified score for Group B was 1.1 (score range 0.5-2). Conclusions: Conditional removal of Ddr2 in articular cartilage attenuated articular cartilage degeneration in mature knee joints of mouse models of OA. Therefore, inhibiting activity of DDR2, may be considered in treatment of OA in mature joints in humans.