A Hidden Bone Receptor Could Open The Door To Reversing Osteoporosis

A mouse study points to a new path against osteoporosis.

16 Mar 2026, 18:43 by Tudor Tarita · ZME Science

Schematic of osteoporosis development that leads to bone loss. Credit: Wikimedia Commons

Osteoporosis is a common disorder in which bones lose density and become more fragile. Over time, that raises the risk of fractures, especially in older adults and in women after menopause. Several drugs can slow bone loss or lower fracture risk, but researchers are still looking for treatments that rebuild bone more effectively and with fewer side effects.

A new study, published in Signal Transduction and Targeted Therapy, focuses on a protein-coupled receptor called GPR133, also known as ADGRD1. Researchers in Germany and China found that mice missing this receptor developed weaker bones, while mice treated with an experimental compound that activates it built stronger bones. The same effect appeared in a mouse model of postmenopausal osteoporosis, where the compound improved several measures of bone loss.

Healthy bone depends on a balance between bone formation and bone breakdown. Osteoporosis develops when the body removes old bone faster than it replaces it. In this study, GPR133 appeared to support bone-building cells directly and, at least indirectly, limit bone breakdown.

A Hidden Receptor That May Be Key to Reversing Osteoporosis

The GPR133 receptor is expressed by osteoblasts, the cells that make bone. The study suggests that this receptor helps those cells sense two kinds of input: mechanical strain and signals relayed by PTK, a protein on the osteoblast surface. It’s a reminder that bone is a living tissue that constantly remodels itself in response to use and load.

When GPR133 was missing, bone formation dropped, while bone-resorbing osteoclasts became more active, shifting the balance toward bone loss.

Mice lacking GPR133 had lower bone mineral density, thinner cortical bone, and weaker femurs in bending tests. Removing GPR133 specifically from osteoblast precursors caused similar changes, indicating that the receptor acts directly in bone-building cells. Female mice showed a similar pattern, suggesting the effect is not limited to one sex in mice.

To test whether they could target this pathway with a drug, the researchers used a small molecule agonist called AP503. AP503 promoted osteoblast differentiation and mineralization in cell experiments. In mice, four weeks of AP503 treatment increased bone volume, raised bone formation rates, and improved mechanical strength in animals with functional GPR133. In the ovariectomy model, commonly used to mimic postmenopausal osteoporosis, AP503 brought several bone measures closer to normal and partly normalized changes in osteoblasts, osteocytes, and osteoclasts.

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“Using the substance AP503, which was only recently identified via a computer-assisted screen as a stimulator of GPR133, we were able to significantly increase bone strength in both healthy and osteoporotic mice,” said Professor Ines Liebscher of Leipzig University, one of the study’s lead investigators.

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The researchers also found that AP503 and exercise seemed to reinforce each other. In mice, both improved bone measures on their own, but together they produced broader gains.

Early Promise, Not a Cure

Electron scan microscope image of osteocyte. Credit: Wikimedia Commons

The biggest limitation of the study is that it was done in mice. While the findings show that GPR133 plays an important role in bone remodeling in animal models and that activating it with AP503 can improve bone density and strength, there’s no guarantee that the same approach will work safely or effectively in people. Mouse bone differs from human bone, and many treatments that look promising in preclinical studies fail in later testing.

Even so, the results give researchers a clearer direction. GPR133 now looks like a promising preclinical drug target because it appears to influence both sides of bone remodeling: the formation of new bone and the breakdown of old bone. That is especially important in osteoporosis, where those two processes fall out of balance. The authors also suggest that some human variants of the GPR133 gene may increase the risk of early-onset osteoporosis or other forms of bone loss, though that idea still needs direct study.