Eph and ephrin signaling in the formation of topographic maps

► Topographic maps are used in the visual system to relay spatial information of the visual world. ► Eph and ephrins are cell surface signaling proteins expressed in gradients along each axis of the visual field. ► EphAs and ephrin-As are required for mapping of the nasal–temporal visual field. ► The dorsal–ventral axis of the visual field is mapped using pre-target sorting and EphB/ephrin-B signaling. ► Eph/ephrin signaling leads to changes in actin stabilization in the axon and endocytosis of receptor/ligand complexes. The axonal connections between the retina and its midbrain target, the superior colliculus (SC), is mapped topographically, such that the spatial relationships of cell bodies in the retina are maintained when terminating in the SC. Topographic map development uses a Cartesian mapping system such that each axis of the retina is mapped independently. Along the nasal–temporal mapping axis, EphAs and ephrin-As, are graded molecular cues required for topographic mapping while the dorsal–ventral axis is mapped in part via EphB and ephrin-Bs. Because both Ephs and ephrins are cell surface molecules they can signal in the forward and reverse directions. Eph/ephrin signaling leads to changes in cytoskeletal dynamics that lead to actin depolymerization and endocytosis guiding axons via attraction and repulsion.