Experimental Evaluation of Discoid Domain Receptor 2 as an Ideal Target for Development of Disease-Modifying Osteoarthritis Drugs

Abstract

Abstract: Osteoarthritis (OA) affects 250 million people worldwide. Currently, no targets for disease-modifying osteoarthritis drugs exist. Matrix metalloproteinase-13 (MMP-13) would make it an ideal target; however, its broad biological effects restrict its application as a target enzyme of inhibitory drugs in the treatment of OA. The expression and activation of discoidin domain receptor 2 (DDR2) is increased in human OA tissues and mouse models of OA and was co-localized with elevated expression of MMP-13 in degenerative articular cartilages. In healthy articular cartilage, DDR2 is kept inactivated by the pericellular matrix, which separates the receptor from its ligand, type II collagen. Once enzymes capable of degrading the pericellular molecules expose chondrocytes to type II collagen, DDR2 is activated and induces expression of MMP-13 leading to degradation of type II collagen and proteoglycans resulting in joint destruction and OA. We tested the hypothesis that 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. To accomplish this goal, conditional knock out techniques were used 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 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. An additional experimental group was subjected to DMM or sham surgery at 10 weeks of age and then DDR2 was removed by intraperitoneal Tamoxifen injection 8 weeks later (Group B). Knee joints from mice in Group A and their corresponding controls were harvested at 8 weeks or 16 weeks post-surgery and mice from Group B and their controls were harvested at 16 weeks post surgery. Histology was performed and the OARSI Modified Mankin Score was used to evaluate articular cartilage degeneration. Statistically significant differences were determined via T-test. We found the average modified score for Group A 8 week control was 1.64 (n=7) whereas with Ddr2 removed was 0.64 (n=7) [P<0.05]. 2) The average modified score for Group A 16 week control was 4.67 (n=7) and with Ddr2 removed was 1.27 (n=9) [P<0.05]. 3) The average modified score for Group B was 1.1 (n=5). In conclusion, conditional removal of Ddr2 in articular cartilage attenuated articular cartilage degeneration in mature knee joints of mouse models of OA.