The lack of EphB3 receptor prevents bone loss in mouse models of osteoporosis

Bone homeostasis is a complex process in which some Eph kinase receptors and their ephrin ligands appear to be involved. In the present study, we address this issue by examining, both in vitro and in vivo, the role of EphB2 and EphB3 in mesenchymal stromal/stem cell (MSC) differentiation into bone tissue. This was first evaluated by quantitative reverse transcription PCR (RT-qPCR) and histological staining in MSCs cultured in specific mediums revealing that although EphB2-/- MSCs mainly expressed pro-adipogenic transcription factors, EphB3-/- MSCs showed abundant osteogenic transcripts, such as Runx2, Msx2, and Sp7. To clarify the underlying molecular mechanisms, we found that the lack of EphB3 signaling alters the genetic profile of differentiating MSCs, reducing the expression of many inhibitory molecules and antagonists of the BMP signaling pathway, and increasing Bmp7 expression, a robust bone inductor. Then, to confirm the osteogenic role of EphB3 in vivo, we studied the condition of 2 mouse models of induced bone loss (ovariectomy or long-term glucocorticoid treatment). Interestingly, in both models, both WT and EphB2-/- mice equally developed the disease but EphB3-/- mice did not exhibit the typical bone loss, nor an increase in urine Ca2+ or blood serum CTX-1. This phenotype in EphB3-KO mice could be due to their significantly higher proportions of osteoprogenitor cells and preosteoblasts, and their lower number of osteoclasts, as compared with WT and EphB2-KO mice. Thus, we conclude that EphB3 acts as a negative regulator of the osteogenic differentiation, and its absence prevents bone loss in mice subjected to ovariectomy or dexamethasone treatment. Osteoporosis affects more than 200 million people, mostly women. Our work shows that the EphB3 receptor restricts bone formation, and its absence prevents bone loss in osteoporotic mice. The bone protection observed in EphB3-deficient mice is due to the presence of more bone-forming cells and fewer bone-degrading cells. Molecularly, we found that when there's no EphB3 in mesenchymal stem cells, some bone-promoting genes are increased while many inhibitors are reduced. Therefore, this receptor could become a key target for new therapies that would help to improve the quality of life for those suffering from bone diseases. We're really excited to share our findings with a broad audience, including patients, healthcare professionals, researchers, and the life sciences industry. Graphical Abstract