Identification of Proteinase Activated Receptor (PAR) cleaving enzymes in human osteoarthritis knee joint synovial fluids.

Objective: Osteoarthritis (OA) is the most prevalent joint disorder with incidence increasing worldwide. Mechanistic insights into OA pathophysiology are still evolving and there are currently no disease-modifying OA drugs available. It is well established that an increase in proteolytic enzyme activity is linked to progressive degradation of the cartilage in OA. Proteolytic enzymes can also trigger inflammation through activation of a family of G-protein coupled receptors (GPCRs) called the Proteinase Activated Receptors (PARs). Here we sought to characterize the PAR activating enzyme repertoire in human OA knee joint fluids. Methods: Human knee joint synovial fluids derived from twenty-five OA patients and four healthy donors were screened for PAR cleavage activity using novel genetically encoded human PAR biosensor expressing cells. The class or type of enzymes cleaving the PARs was further characterized using enzyme-selective inhibitors and enzyme-specific fluorogenic substrates. Results: Activity of PAR1, PAR2 and PAR4 activating enzymes were identified at substantially different levels in OA patients relative to healthy knee joint synovial fluids. Using enzyme class or type selective inhibitors and fluorogenic substrates we found that serine proteinases, including thrombin-like enzymes, trypsin-like enzymes, and matrix metalloproteinases are the major PAR activating enzymes present in the OA knee synovial fluids. Conclusions: Multiple enzymes activating PAR1, PAR2 and PAR4 are present in OA joint fluids. PAR signalling can trigger pro-inflammatory responses and targeting PARs has been proposed as a therapeutic approach in OA. Knowledge of the PAR activators present in the human knee joint will guide study of relevant signaling events and enable future development of novel PAR targeted therapies for OA and other inflammatory joint diseases.