PAR2 accelerates osteoarthritis-like joint changes in a murine model of post-traumatic osteoarthritis

Background Post-traumatic osteoarthritis (PTOA) is associated with articular cartilage damage and represents a major clinical challenge due to the poor regenerative capability of cartilage. We have recently developed a novel and robust dual injury murine model of PTOA, which combines destabilisation of the medial meniscus (DMM) and cartilage scratch1. This model results in accelerated OA-like symptoms including enhanced osteophyogenesis. Prior studies in the DMM model have demonstrated that absence of proteinase-activated receptor 2 (PAR2) in mice results in significant protection from early OA-like symptoms. 
Objectives To investigate if the absence of PAR2 confers protection in a dual injury PTOA murine model.
Methods PTOA was induced in both male C57BL/6 wild-type (WT) and PAR2-/- mice, via combined destabilisation of the medial meniscus and cartilage scratch (DCS). Twenty-eight days post-surgery, osteophytogenesis and bone changes were monitored using microcomputered tomography. Dynamic weight bearing was assessed as an indirect measurement of pain at day 14.
Results Evaluation of the presence and number of osteophyte revealed no significant differences between WT and PAR2-/- mice at day 28. However, quantification of osteophytes revealed that PAR2-/- mice had significantly smaller osteophytes (p=0.006) with less mineralised bone (p=0.003). Moreover, analysis of metaphyseal trabecular bone on the operated leg showed a significant decrease in % bone volume/tissue volume (BV/TV) (p=0.025). Assessment of pain-related behaviour, using dynamic weight bearing at day 14, demonstrated that PAR2-/- mice exerted less load on their front paws.
Conclusion The findings in this study show that PAR2 plays a role in accelerated OA-like symptoms (i.e., osteophyte formation) in a dual injury model of OA, where both destabilisation of the medial meniscus and cartilage damage drive disease pathology. Furthermore, the loss of PAR2 decreases pain behaviour suggesting that PAR2 is involved in pain sensing. Taken together, these results support the future exploration of PAR2 as a therapeutic target for PTOA.