Clinical premise: Osteoarthritis (OA), the most common form of joint disorder, is a debilitating joint disease involving progressive cartilage degeneration, with no disease modifying therapy available. Thus, efforts to elucidate new molecular target and mechanism that can be utilized to prevent and/or treat the OA are of utmost importance. Recently, an orphan GPCR protein-the GPR68 has been identified and characterized in musculoskeletal tissues including bone and intervertebral disc, however its role in cartilage and OA is not known. Our preliminary analyses demonstrate that GPR68 was robustly expressed in cartilage derived from human OA patient and experimental model of OA in mice, indicating its role in OA pathogenesis. Using IDG/Pharos database, we identified `Inflammatory Process' a major GO term (Gene Ontology) associated with GPR68. In this project, we propose to establish the functional role of GPR68 in inflammatory signaling in osteoarthritic chondrocytes and ascertain its significance in OA pathogenesis. Scientific premise: We provide compelling preliminary evidence of the following: 1. GPR68 was robustly expressed in human and mice cartilage and its expression was significantly higher in OA cartilage compared to healthy cartilage; 2. GPR68 mRNA and protein levels was higher in high-grade OA cartilage as compared to low-grade OA cartilage; 3. Mechanical stimulation of primary human chondrocytes resulted in increased expression of GPR68; 4. Gpr68 expression was significantly higher in mice cartilage of surgically induced OA (DMM surgery) as compared to sham control; 5. IL1β stimulation of OA chondrocytes resulted in significantly increased expression of GPR68; 6. Knockdown of GPR68 in OA chondrocytes alleviated IL-1β induced expression of inflammatory genes. Hypothesis: GPR68 mediates inflammatory and catabolic effects leading to OA progression and thus suppression of GPR68 attenuates the inflammation and matrix degeneration in OA joints. Specific objectives: To verify this hypothesis, we will establish the functional role of GPR68 in inflammatory and catabolic pathways in human OA chondrocytes (Aim1). We will then define the role of Gpr68 in cartilage degradation in surgically induced osteoarthritis in vivo (Aim 2). Impact: Identifying the pathophysiological role of GPR68 in the regulation of chondrocyte inflammatory and catabolic activity will lead to the potential development of novel therapeutic strategies to treat OA, thus laying the foundation for future clinical study to establish a novel effective OA modifying drug.

Project Narrative The proposed work will capitalize on the genome sequencing revolution to determine the role of specific proteins in cartilage degeneration. We will focus our studies on an orphan GPCR protein, the GRP68, which is present in cartilage, and will determine its function in the pathology of osteoarthritis. Once our experiments are complete, we will have a druggable target to start testing treatments modalities to prevent joint disease progression in the future.