The Mechanism Of GPR4 Regulation In Osteoarthritis

Osteoarthritis（OA）,is characterized by pain,cartilage degradation,synovial inflammation,subchondral bone sclerosis and osteophyte formation.Increasingly studies have demonstrated that inflammatory diseases can decrease the p H of the extracellular environment,the same phenomena was found in patients with osteoarthritis,indicating that the change of p H may play a crucial role in the development of osteoarthritis.Unfortunately,recent studies only focused on the inflammatory response,but not p H change.Therefore,this study took joint acid-base balance as a breakthrough to study the regulation of proton-sensitive G protein-coupled receptor（GPCR）,a molecular sensor that senses extracellular acidification,in the development of osteoarthritis.It has been reported that GPR4 can sense extracellular acidification and it plays a regulatory role in colitis,nephritis,intestinal fibrosis and other diseases,however,whether GPR4 can regulate OA process remains unclear.We found that the expression of proton-sensitive receptor GPR4 was significantly up-regulated both in human and mouse OA cartilage.Our results showed that lentivirusmediated overexpression of GPR4 in the joint of mice accelerated the progress of osteoarthritis,including promoting articular cartilage damage,synovial hyperplasia and osteophyte formation,while the deletion of Gpr4 effectively attenuated the development of OA in DMM-operated mice and aging mice.We also found that GPR4 accelerated extracellular matrix degradation by up-regulating the expression of matrixdegrading enzymes（MMP13,MMP3,ADAMTS4）.To clarify the downstream signaling pathway（s）of GPR4,we selected 22 OA-related signaling pathway by using related inhibitors.The results showed that GPR4 upregulated the expression of the chemokine CXCL12,leading to the activation of NF-κB/MEK signaling and resulted into the degradation of articular cartilage.Critically,inhibition of GPR4 with the antagonist NE52-QQ57 reduced the extracellular matrix degradation of human articular cartilage explants.Moreover,injection of NE52-QQ57 into the articular cavity of mice effectively delayed the injury of articular cartilage and the generation of osteophytes.In conclusion,we first elucidated that proton-sensitive GPR4 is a key regulator in the progress of osteoarthritis pathogenesis and it can be a potent target for osteoarthritis treatment.