During the development process cell migration and adhesion are the main forces involved in morphing the cells into critical anatomical structures. The ability of a cell to migrate to its correct destination depends heavily on signaling at the cell membrane. Erythropoietin producing hepatocellular carcinoma (EPH) receptors and their ligands, the ephrins (EPH receptors interacting proteins, EFNs), orchestrates the precise control necessary to guide a cell to its destination. They are expressed in all tissues of a developing embryo and are involved in multiple developmental processes such as axon guidance, cardiovascular and skeletal development and tissue patterning. In addition, EPH receptors and EFNs are expressed in developing and mature synapses in the nervous system, where they may have a role in regulating synaptic plasticity and long-term potentiation. Activation of EPHB receptors in neurons induces the rapid formation and enlargement of dendritic spines, as well as rapid synapse maturation (Dalva et al. 2007). On the other hand, EPHA4 activation leads to dendritic spine elimination (Murai et al. 2003, Fu et al. 2007).
EPH receptors are the largest known family of receptor tyrosine kinases (RTKs), with fourteen total receptors divided into either A- or B-subclasses: EPHA (1-8 and 10) and EPHB (1-4 and 6). EPH receptors can have overlapping functions, and loss of one receptor can be partially compensated for by another EPH receptor that has similar expression pattern and ligand-binding specificities. EPH receptors have an N-terminal extracellular domain through which they bind to ephrin ligands, a short transmembrane domain, and an intracellular cytoplasmic signaling structure containing a canonical tyrosine kinase catalytic domain as well as other protein interaction sites. Ephrins are also sub-divided into an A-subclass (A1-A5), which are tethered to the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor, and a B-subclass (B1-B3), members of which have a transmembrane domain and a short, highly conserved cytoplasmic tail lacking endogenous catalytic activity. The interaction between EPH receptors and its ligands requires cell-cell interaction since both molecules are membrane-bound. Close contact between EPH receptors and EFNs is required for signaling to occur. EPH/EFN-initiated signaling occurs bi-directionally into either EPH- or EFN-expressing cells or axons. Signaling into the EPH receptor-expressing cell is referred as the forward signal and signaling into the EFN-expressing cell, the reverse signal. (Dalva et al. 2000, Grunwald et al. 2004, Davy & Robbins 2000, Cowan et al. 2004)