Eph/ephrin Signaling and Biology of Mesenchymal Stromal/Stem Cells

Abstract

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.

Keywords: Eph; Ephrin; MSC.

Figure 1

Eph and ephrin molecules. The two families of ephrins present an extracellular receptor-binding domain (RBD) that interacts with Eph, but while ephrin-A ligands have a GPI-binding domain, ephrin-B are transmembrane proteins with an intracellular kinase domain. Eph are transmembrane proteins with an extracellular region composed of a ligand-binding domain (LBD) to interact with ephrins; a Cys-rich region formed by sushi and epidermal growth factor (EGF)–like domains; and two fibronectin-type III (FN III) repeats. This is followed by a transmembrane region, the Tyr kinase domain (TK), the sterile alpha motif (SAM), and the PDZ domain, forming the intracellular side of Eph receptors (modified from [37]).

Figure 2

Effects of Eph/ephrin-Fc fusion protein treatment on the morphology of cultured BM-MSCs. (A) Control, Fc protein-treated MSCs attach properly to culture plates showing actin (red) and vimentin (green) filaments arranged throughout cytoplasm. (B) On the other hand, Eph/ephrin-Fc-treated cultures contained cellular masses with numerous nuclei (blue) (arrow) and a less expanded network of actin and vimentin filaments. Scale bars: 50 mm.

Figure 3

Growth curves and cell proliferation of both WT (wild type) and Eph-B-deficient BM-MSCs. (A) Cell content (×10⁶) of WT and Eph-B-deficient MSCs at different days of culture. Note the significant reduced cellularity of Eph-B2-LacZ MSCs. (B) Percentage of cycling cells in the different cultures. Lower cell proportions occurred in the Eph-B2-LacZ cultures as compared to the other ones (* p value < 0.05) (n = 5).

Figure 4

Immunomodulatory effects of MSCs on activated splenocytes. Immunomodulation mediated by either WT or Eph-B-deficient MSCs cocultured with activated splenocytes with either PHA (A) or MLR (B). In both experimental conditions, cocultures established with WT or Eph-B2-LacZ MSCs, but not those containing Eph-B2^−/− and Eph-B3^−/− MSCs, showed significant reduced splenocyte numbers as compared with activated splenocytes cultured alone (gray columns) (* p value < 0.05) (n = 5).