Therapeutic targeting of GPCR Gbetagamma-GRK2 in osteoarthritis

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Abstract Osteoarthritis (OA) is a debilitating disease of the joints involving progressive cartilage degeneration, with no disease modifying therapy available. Chondrocytes, the cellular component of cartilage, regulate cartilage homeostasis by modulating matrix turnover via secretion of proteoglycans and collagen as well as matrix-degrading enzymes. In OA, articular chondrocytes acquire an aberrant phenotype where they undergo differentiation and hypertrophy (CH) with increased production of matrix degrading enzymes leading to cartilage degeneration. G protein-coupled receptors (GPCRs) is a superfamily of receptors coupled to intracellular G-proteins. Agonist binding to a GPCR activates its G- protein a subunit (Ga) physiologic signaling. A parallel G-protein bg subunit (Gbg) activation and recruitment of GPCR kinases (GRK2 particularly) leads to GPCR internalization and signal termination. GRK2 expression and Gbg-GRK2 signaling are pathologically elevated in several diseases, leading to GPCR desensitization and loss of Ga signaling. Importantly, we and others have recently reported the therapeutic efficacy of Gbg-GRK2 inhibition, both by rescuing Ga physiologic activity and by inhibiting direct downstream pathological signaling that promotes cell growth and differentiation; this was achieved using both novel small molecules and FDA-approved drugs. Chondrocytes are rich in type 1 parathyroid hormone receptor (PTH1R, a GRCR), where Gas and PTH1R are key regulators of CH, both are under tight regulation by Gbg-GRK2. However, the role of Gbg-GRK2 signaling in CH and OA remains unknown. Our preliminary work to establish the role of Gbg-GRK2 in CH during OA indicates that GRK2 expression and activity are increased in articular chondrocytes of both OA patients and mice with posttraumatic OA. Interestingly, in vitro and in vivo Gbg-GRK2 inhibition attenuated CH and cartilage degeneration in mice with DMM (surgical destabilization of the medial meniscus to induce OA). These preliminary findings support our overarching hypothesis that: Gbg-GRK2 signaling plays a central role in CH during OA, and its inhibition represents a novel therapeutic strategy for OA. To validate our hypothesis we will use chondrocyte conditional knockout mice and small molecule Gbg and GRK2 inhibitors in an in vivo OA mouse model and utilize different in vitro systems (both murine and human), to (1) determine the role of GRK2 signaling in articular CH in OA, (2) determine the therapeutic efficacy of small molecule Gbg-GRk2 inhibitors in OA, and (3) determine the role of Gbg-GRK2 signaling in PTH1R chondro-protective/regenerative effect in OA. These studies enable us to discover a novel therapeutic strategy and therapeutic agents for OA (both novel and FDA-approved) in a preclinical setting, thus laying the grounds for our future clinical study to establish a novel effective OA therapy.