Mesenchymal stromal/stem cells (MSCs) have emerged as important therapeutic agents, owing to their easy isolation and culture, and their remarkable immunomodulatory and anti-inflammatory properties. However, MSCs constitute a heterogeneous cell population which does not express specific cell markers and has important problems for in vivo homing, and factors regulating their survival, proliferation, and differentiation are largely unknown. Accordingly, in the present article, we review the current evidence on the relationships between Eph kinase receptors, their ephrin ligands, and MSCs. These molecules are involved in the adult homeostasis of numerous tissues, and we and other authors have demonstrated their expression in human and murine MSCs derived from both bone marrow and adipose tissue, as well as their involvement in the MSC biology. We extend these studies providing new results on the effects of Eph/ephrins in the differentiation and immunomodulatory properties of MSCs.

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.