Abstract
The Eph receptor tyrosine kinases and their membrane-bound ligands, the ephrins, are involved in a variety of developmental processes such as axonal guidance, cell migration, cell adhesion, proliferation, and differentiation. In addition to repulsive effects, ephrins can also induce attractive responses. Up to now, little was known about the underlying signaling mechanisms that regulate attractive versus repulsive effects. In this study, we show that ephrin-A5 enhances the motility of cortical neurons that is dependent on the activity of Src-family kinases (SFKs). Ephrin-A5 further changes the adhesive properties of neurons by inducing the formation of cell aggregates. Using the stripe assay, we found that the motogenic effect of ephrin-A5 is the result of repulsive ephrin-A interactions. Blocking SFK function leads to a conversion of repulsion into adhesion, suggesting that SFKs can act as a biological switch for the response of EphA receptors. Finally, we discovered a ligand-induced release of membrane particles containing EphA receptors, suggesting membrane ripping as a novel mechanism to overcome the "ephrin paradox" of repulsion after high-affinity receptor-ligand binding.