Endocytosis of EphA receptors is essential for the proper development of the retinocollicular topographic map.

Endocytosis of Eph-ephrin complexes may be an important mechanism for converting cell-cell adhesion to a repulsive interaction. Here, we show that an endocytosis-defective EphA8 mutant forms a complex with EphAs and blocks their endocytosis in cultured cells. Further, we used bacterial artificial chromosome transgenic (Tg) mice to recapitulate the anterior>posterior gradient of EphA in the superior colliculus (SC). In mice expressing the endocytosis-defective EphA8 mutant, the nasal axons were aberrantly shifted to the anterior SC. In contrast, in Tg mice expressing wild-type EphA8, the nasal axons were shifted to the posterior SC, as predicted for the enhanced repellent effect of ephrinA reverse signalling. Importantly, Rac signalling was shown to be essential for EphA-ephrinA internalization and the subsequent nasal axonal repulsion in the SC. These results indicate that endocytosis of the Eph-ephrin complex is a key mechanism by which axonal repulsion is generated for proper guidance and topographic mapping.

The EphA8 receptor induces sustained MAP kinase activation to promote neurite outgrowth in neuronal cells.

Recent studies in our laboratory demonstrate that ligand-mediated activation of the EphA8 receptor critically regulates cell adhesion and migration. In this report, we show that the EphA8 receptor induces neurite outgrowth in NG108-15 cells in the absence of ligand stimulation. Using various deletion mutants lacking specific intracytoplasmic regions, we confirm that the tyrosine kinase domain of EphA8 is important for inducing neurite outgrowth. However, the tyrosine kinase activity of EphA8 is not crucial for neurite outgrowth induction. Treatment with various inhibitors further reveals that the mitogen-activated protein kinase (MAPK) signaling pathway is critical for neurite outgrowth induced by EphA8. Consistent with these results, EphA8 expression induced a sustained increase in the activity of MAPK, whereas ligand-mediated EphA8 activation had no further modulatory effects on MAP kinase activity. Additionally, activated MAPK relocalized from the cytoplasm to the nucleus in response to EphA8 transfection. These results collectively suggest that the EphA8 receptor is capable of inducing a sustained increase in MAPK activity, thereby promoting neurite outgrowth in neuronal cells.

The p110 gamma PI-3 kinase is required for EphA8-stimulated cell migration.

This study provides evidence that treatment with preclustered ephrin A5-Fc results in a substantial increase in the stability of the p110 gamma PI-3 kinase associated with EphA8, thereby enhancing PI-3 kinase activity and cell migration on a fibronectin substrate. In contrast, co-expression of a lipid kinase-inactive p110 gamma mutant together with EphA8 inhibits ligand-stimulated PI-3 kinase activity and cell migration on a fibronectin substrate, suggesting that the mutant has a dominant negative effect against the endogenous p110 gamma PI-3 kinase. Significantly, the tyrosine kinase activity of EphA8 is not important for either of these processes. Taken together, our results demonstrate that the stimulation of cell migration on a fibronectin substrate by the EphA8 receptor depends on the p110 gamma PI-3 kinase but is independent of a tyrosine kinase activity.