EphB3 receptor negatively regulates osteogenesis in mice

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 the capacity of adipose tissue-derived mesenchymal stromal cells (Ad-MSC) derived from either WT, EphB2-and EphB3-KO mice to differentiate into bone tissue. Differentiation capacities were evaluated in cultured MSC by RT-qPCR and histological staining, revealing that whereas EphB2-/- MSC cultured in a specific medium expressed mainly pro-adipogenic transcription factors, EphB3-/- MSC showed abundant osteogenic transcripts, such as Runx2, Msx2 and Osterix. In addition, the lack of EphB3 signaling alters the genetic profile of differentiating Ad-MSC, 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 two animal models of induced osteoporosis (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 did they show increased urine Ca2+ or blood serum CTX-1. The proportions of osteoprogenitor cells and pre-osteoblasts were also found to be significantly higher in EphB3-KO mice, and the osteoclasts significantly reduced, as compared to WT and EphB2-KO mice. We conclude that EphB3 acts as a negative regulator of the osteogenic differentiation, and its absence prevents the development of experimentally-induced osteoporosis.