Fibroblast Growth Factor Receptor-Mediated Rescue of x-Ephrin B1-Induced Cell Dissociation in XenopusEmbryos

Abstract

The Eph family of receptor tyrosine kinases and their membrane-bound ligands, the ephrins, have been implicated in regulating cell adhesion and migration during development by mediating cell-to-cell signaling events. Genetic evidence suggests that ephrins may transduce signals and become tyrosine phosphorylated during embryogenesis. However, the induction and functional significance of ephrin phosphorylation is not yet clear. Here, we report that when we used ectopically expressed proteins, we found that an activated fibroblast growth factor (FGF) receptor associated with and induced the phosphorylation of ephrin B1 on tyrosine. Moreover, this phosphorylation reduced the ability of overexpressed ephrin B1 to reduce cell adhesion. In addition, we identified a region in the cytoplasmic tail of ephrin B1 that is critical for interaction with the FGF receptor; we also report FGF-induced phosphorylation of ephrins in a neural tissue. This is the first demonstration of communication between the FGF receptor family and the Eph ligand family and implicates cross talk between these two cell surface molecules in regulating cell adhesion.Cell adhesion events, coordinated both spatially and temporally, are critical to the development and maintenance of the tissue structures of an organism. Interactions with other cells and extracellular matrix components govern the growth, differentiation, migration, and ultimate location and function of a given cell. An increasing number of proteins expressed at the cell surface have been implicated in regulating cell adhesion and movement within specific developmental contexts. For example, considerable progress has been made in identifying cell surface proteins which regulate neurite outgrowth and pathfinding during nervous system development (9). These proteins include classic cell adhesion molecules such as the neural cell adhesion molecules of the immunoglobulin superfamily (58, 69), members of the cadherin superfamily (20), and integrins (47), as well as a growing number of other protein families including collapsins/semaphorins, neuropilins, netrin receptors (10, 11, 57), various peptide growth factor receptors (49), and receptor-type tyrosine phosphatases (13, 42). These cell surface proteins respond to molecular guidance cues which can arise locally or diffuse from distant sources. In one such case, control of growth cone physiology is achieved at the most intimate level of cell-to-cell contact, when neuronal cell surface proteins interact with attractive or repulsive cues present on the surface of a neighboring cell.The Eph family of receptor tyrosine kinases and their membrane-associated activating ligands, the ephrins, are recent additions to this growing repertoire of cell surface proteins which mediate cell-to-cell interactions. They are highly expressed in the developing and adult nervous system and have been implicated in regulating cell adhesion events in both neural and nonneural tissues including neural crest cell migration (43, 62, 71), hindbrain segmentation (4, 5, 23,...

In Vivo Tyrosine Phosphorylation Sites of Activated Ephrin-B1 and EphB2 from Neural Tissue

“…In addition, growth factor receptor tyrosine kinases may also cause phosphorylation of ephrin-B1 in the retina (10,12,29). Notably, fibroblast growth factor can up-regulate tyrosine phosphorylation of endogenous ephrin-B1 in embryonic retinal tissue (10).…”

“…In addition, growth factor receptor tyrosine kinases may also cause phosphorylation of ephrin-B1 in the retina (10,12,29). Notably, fibroblast growth factor can up-regulate tyrosine phosphorylation of endogenous ephrin-B1 in embryonic retinal tissue (10). Ephrin-B1 Phosphorylation Sites-Various stimuli can induce tyrosine phosphorylation of ephrin-B1, including binding to EphB receptors, activation of Src kinase, platelet-derived growth factor stimulation and, as mentioned above, fibroblast growth factor stimulation (10,12,18).…”

“…Ephrin-B1 Phosphorylation Sites-Various stimuli can induce tyrosine phosphorylation of ephrin-B1, including binding to EphB receptors, activation of Src kinase, platelet-derived growth factor stimulation and, as mentioned above, fibroblast growth factor stimulation (10,12,18). The Tie-2 receptor and fibroblast growth factor receptor 1 can also phosphorylate the cytoplasmic domain of ephrin-B1 in vitro (10,29). Ephrin-B1 is tyrosine-phosphorylated in mouse embryonic tissues (18) and, as we show here, in chick embryonic retina.…”

“…Notably, fibroblast growth factor can up-regulate tyrosine phosphorylation of endogenous ephrin-B1 in embryonic retinal tissue (10). Ephrin-B1 Phosphorylation Sites-Various stimuli can induce tyrosine phosphorylation of ephrin-B1, including binding to EphB receptors, activation of Src kinase, platelet-derived growth factor stimulation and, as mentioned above, fibroblast growth factor stimulation (10,12,18). The Tie-2 receptor and fibroblast growth factor receptor 1 can also phosphorylate the cytoplasmic domain of ephrin-B1 in vitro (10,29).…”

“…This implies that activation of ephrin-B signaling drives cell sorting in the developing hindbrain, similar to activation of Eph receptor signaling (8). Furthermore, ectopic overexpression of ephrin-B1 in Xenopus embryos causes cell dissociation (9,10), an effect that requires the 19 carboxyl-terminal amino acids of the ephrin-B1 cytoplasmic domain (9). The cytoplasmic domain of ephrin-B1 is also required to inhibit oncogenic signaling pathways activated by tyrosine kinases (11,12).…”