GPR133: A Receptor That Rebuilds Bone and Strengthens Muscle
Leipzig University researchers identified GPR133 (ADGRD1), an adhesion GPCR receptor, as a dual bone-and-muscle regulator. A computationally discovered compound called AP503 activates it, simultaneously boosting osteoblast activity (bone building) and suppressing osteoclast activity (bone breaking). In mouse studies, AP503 significantly increased bone density in both healthy and osteoporotic subjects. GPR133 is also mechanosensitive — activated by physical forces, which may explain why weight-bearing exercise strengthens bone.
Scientists at Leipzig University identified GPR133 (also called ADGRD1), a receptor in the adhesion GPCR family, as a dual regulator of bone formation and muscle strength. A compound called AP503, discovered through computer-assisted screening, activates GPR133 and produced significant bone density increases in mouse studies. ## Mechanism GPR133 activation simultaneously increases osteoblast activity (bone-building cells) and decreases osteoclast activity (bone-breaking cells). Mice lacking functional GPR133 developed low bone density early in life — essentially osteoporosis. AP503 treatment reversed this in both healthy and osteoporotic mice. Earlier research by the same team demonstrated that AP503 also improves skeletal muscle strength through the same receptor, making it a dual bone-muscle target. ## Clinical Significance Osteoporosis affects approximately 6 million people in Germany alone, predominantly post-menopausal women. Current treatments mostly slow bone loss; AP503 could potentially *rebuild* bone. The dual bone-plus-muscle benefit is particularly valuable for aging populations, addressing both fracture risk and the fall risk that causes fractures. ## Mechanosensitivity GPR133 is activated by physical forces — movement and pressure on bone tissue. This may explain the well-established link between weight-bearing exercise and bone density. Running, jumping, and resistance training apply the mechanical forces that activate this receptor pathway naturally. ## Supporting Evidence for Natural Activation While nothing is known to directly activate GPR133 naturally beyond mechanical force, several interventions support the same osteoblast/osteoclast balance: prunes (5-6 per day preserved hip bone density in post-menopausal women), vitamin K2 from fermented foods (natto, sauerkraut, aged cheese), strontium-rich foods (cabbage, parsley, nuts), and rutin (buckwheat, asparagus, citrus peel). ## Current Status Mouse studies only — no human clinical trials. Part of over 10 years of adhesion GPCR research at Leipzig, within Collaborative Research Centre 1423. Published in *Signal Transduction and Targeted Therapy* (2025).