Cell-cell signaling via Eph receptors and ephrins

Abstract

Eph receptors are the largest subfamily of receptor tyrosine kinases regulating cell shape, movements, and attachment. The interactions of the Ephs with their ephrin ligands are restricted to the sites of cell-cell contact since both molecules are membrane attached. This review summarizes recent advances in our understanding of the molecular mechanisms underlining the diverse functions of the molecules during development and in the adult organism. The unique properties of this signaling system that are of highest interest and have been the focus of intense investigations are as follows: (i) the signal is simultaneously transduced in both ligand-expressing cells and receptor-expressing cells, (ii) signaling via the same molecules can generate opposing cellular reactions depending on the context, and (iii) the Ephs and the ephrins are divided into two subclasses with promiscuous intrasubclass interactions, but rarely observed intersubclass interactions.

Figure 1

Activation of bi-directional Eph/ephrin signaling. The current model of signaling initiation involves an initial 1:1 high-affinity interaction between ligands and receptors, followed by tetramerization, oligomerization and clustering of the molecules at the sites of cell-cell contact [4]. This brings about phosphorylation of the cytoplasmic Eph (as well as B-class ephrin) domains and downstream signaling initiation [3]. Kinase activation is controlled by the phosphorylation of residues in the activation loop and the juxtamembrane segment, which affect the inter-lobe(subdomain) dynamics of the kinase domain [–35]. The minimal ligand-binding domain of the receptor is in blue and the ephrin receptor-binding domain is in pink. The kinase and SAM (sterile α-motif) domain is in green and the phosphate groups are in orange.

Figure 2

Schematic representation of the heterotetrameric complex formed between the Eph/ephrin interaction domains. Three interaction interfaces are indicated: dimerization and tetramerization - identified by crystallography [4] and confirmed by mutagenesis [25, 26], as well as a potential oligomerization interface - identified by a random mutagenesis approach [26].

Figure 3

Schematic representation and a comparison of the interaction surfaces in the canonical intra-subclass EphB2/ephrin-B2 [20] and EphB4/ephrin-B2 [24] complexes with the unusual inter-subclass EphB2/ephrin-A5 complex [21*]. While ephrin-B2 has several interacting surface areas with the B-class receptors, the interaction of ephrin-A5 with EphB2 is mostly confined to the hydrophobic receptor surface channel. Peptides “pep” [–41*] and small-molecule compounds “C” can also bind the channel, potentially inhibiting the Eph/ephrin interactions. The distinct architecture of the EphB2/ephrin-A5 complex and the lower affinity of the interaction can be used for the development of interaction assays, including FRET-based ones [22], suitable for high-throughput screening of compound libraries for small-molecule Eph signaling inhibitors “C”.

Figure 4

Cleavage of the Eph-bound ephrins allows for signaling termination and cell separation/repulsion. ADAM10 associates constitutively with the Eph receptors and ephrin binding generates a new ADAM-recognition motif at the Eph/ephrin interface allowing for productive protease positioning and efficient ephrin cleavage in trans. It seems likely that in certain cases cleavage in cis could also occur and that the Eph receptors could also potentially be cleaved off the cell surface. Further cleavage by a γ- secretase internalizes the cytoplasmic tails of the B-class ephrins initiating reverse signaling while the Eph/ephrin complexes are internalized in endosomes. The α- (ADAM10) and γ- secretases are likely retained at the cell surface to participate in another cleavage cycle. The interaction domains of the receptors are in blue and those of the ligands - in red. The protease domain of ADAM10 is in pink, the disintegrin domain - in yellow, and the Cys-rich domain - in green. The γ-secretase is drawn in purple.