Bidirectional ephrin/Eph signaling in synaptic functions

Abstract

Eph receptors, the largest family of receptor tyrosine kinases, and their membrane bound ligands, the ephrins, are involved in multiple developmental and adult processes within and outside of the nervous system. Bi-directional signaling from both the receptor and the ligand is initiated by ephrin-Eph binding upon cell–cell contact, and involves interactions with distinct subsets of downstream signaling molecules related to specific functions. In the CNS, Ephs and ephrins act as attractive/repulsive, migratory and cell adhesive cues during development and participate in synaptic functions in adult animals. In this review, we will focus on recent findings highlighting the functions of ephrin/Eph signaling in dendritic spine morphogenesis, synapse formation and synaptic plasticity.

Introduction

Erythropoietin-producing hepatocellular carcinoma (Eph) receptors form the largest known family of receptor tyrosine kinases (Eph Nomenclature Committee, 1997). Currently, 16 genes (EphA1-10, EphB1-6) have been identified in the vertebrate genome (Pasquale, 2005) and 14 of them are present in mammals (Murai and Pasquale, 2003). All Eph receptors are transmembrane proteins with highly conserved extra- and intracellular domains. The extracellular part of the Eph receptors includes a N-terminal ligand binding domain, a cysteine-rich region and two fibronectin type III repeats (Yamaguchi and Pasquale, 2004). Following the juxtamembrane region is a tyrosine kinase domain, followed by a sterile-α-motif (SAM), and a type-II PSD-95/Disc large/ZO-1 (PDZ) binding motif at the carboxyl terminus (Kullander and Klein, 2002) (Fig. 1). Eph receptors can undergo homo- as well as heterodimerization (Freywald et al., 2002), which is mediated directly by the extracellular cysteine-rich region, the fibronectin type III repeats (Lackmann et al., 1998) and the SAM motif (Stapleton et al., 1999, Thanos et al., 1999) or indirectly through PDZ protein interactions (Fanning and Anderson, 1999). The Eph family receptors are divided into two groups based on the similarity of their extracellular domain sequences (Eph Nomenclature Committee, 1997), which coincidently corresponds to their binding affinities for their respective ligands, the ephrins (Table 1).

In the vertebrate genome, nine Eph Receptor Interacting Proteins (ephrin) ligands have been identified and classified based on their attachment to the cell membrane (ephrinA1–6 and ephrinB1–3). The A-ephrin ligands are bound to the membrane via a glycosylphosphoinositol (GPI) anchor while B-ephrins are type-I transmembrane proteins. Both classes of ligands contain a 20-kDa receptor-binding domain, which consists of approximately 180 amino acids (Nikolov et al., 2005). The cytoplasmic tail of the B-ligands is short and consists of approximately 80 amino acids. The last 33 amino acids of the ephrinB ligands display 95% sequence homology (100% conserved between ephrinB1 and B2) with a conserved type-II PDZ-binding motif at the C-terminus (Kullander and Klein, 2002, Martinez and Soriano, 2005). Phosphorylation of several tyrosine residues in the ephrinB intracellular domain upon receptor binding enables their direct interactions with SH2/SH3 adaptor proteins, which play important roles in ephrinB reverse signaling (Nakada et al., 2006, Ran and Song, 2005, Tanaka et al., 2005) (Fig. 1).

The receptor–ligand interactions between Ephs and ephrins follow a general rule that A-ligands interact preferentially with A-receptors and B-ligands with B-receptors. The only exceptions found so far are that EphA4 and EphB2 interact with ephrinB2/3 and ephrinA5, respectively (Grunwald et al., 2004, Himanen et al., 2004) (Table 1). Within each subfamily, the receptor–ligand interaction is believed to be rather promiscuous. High affinity heterodimers are formed between Ephs and ephrins upon cell–cell contact. The receptor-ligand heterodimers, in a 2:2 ratio, form tetramers (Himanen and Nikolov, 2002, Himanen et al., 1998, Himanen et al., 2001, Nikolov et al., 2005). The extracellular domains on both receptors and ligands mediate tetramerization and may even enhance subfamily specificity (Himanen et al., 2001, Himanen et al., 2004, Nikolov et al., 2005). Tetramers may then form higher order aggregates at higher concentrations (Himanen et al., 2001), and may cluster into lipid raft microdomains on the cell membrane when they interact with cytoplasmic PDZ proteins such as GRIP (Bruckner et al., 1999). High-density clusters of Eph-ephrin complexes are believed to serve as signaling centers for the localization, concentration and activation of intracellular signaling molecules (Bruckner and Klein, 1998, Bruckner et al., 1999, Murai and Pasquale, 2003). An interesting characteristic of Eph receptors and ephrin ligands is that they are capable of bidirectional signaling. Signaling pathways directly associated with Eph receptor activation are termed “forward” and those with ephrin activations, “reverse” signaling (Holland et al., 1996, Murai and Pasquale, 2003) (Fig. 1).