CONICET | Buscador de Institutos y Recursos Humanos

The retinotectal system is a widely studied topographic map. Retinal axons establish ordered connections in the optic tectum (OT). Nasal retinal ganglion cells (RGCs) connect to the caudal OT and temporal RGCs, to the rostral OT. Recognition of guidance cues in the target is the main mechanism of map formation. EphrinAs are guidance cues located in the caudal OT and repel temporal axons activating axonal EphA3 and EphA4. It is unknown whether tectal EphA3 participate in axon guidance and how nasal RGC axons grow towards the caudal OT. The effects of EphA3 on axonal length was analyzed in explants and dissociated neurons of chick embryos retinas. EphA3 promoted axon growth differentially in nasal and temporal cells in a dose dependent manner. Growth cone behavior was studied in time-lapse experiments in which explants were grown over EphA3-expressing cells. Growth cones expanded upon contact with EphA3-expressing cells. Nasal growth cones extended over EphA3-expressing cells but temporal growth cones remained attached. Results suggest that axonal and tectal EphA3 guide axonal growth over the tectal surface, acting as receptors of the repulsive tectal ephrinAs and as stimulatory ligands, respectively. To elucidate if axonal ephrinAs mediate the stimulating effect of EphA3, retinal explants grown on EphA3 were treated with PI-PLC, to shed ephrinAs. EphA3-stimulated nasal cells increased axonal length at low PI-PLC doses, but reduced it at higher doses. Temporal cells showed a weaker response. Non-stimulated cells increased axonal length at every PI-PLC dose, suggesting that GPI-anchor proteins (perhaps ephrinAs) reduce axonal growth. The effect of PI-PLC on EphA3-stimulated axons suggests that a lack of ephrinAs impairs EphA3-elicited axonal growth. EphrinAs could have a double role: repulsive tectal ligands for axonal EphAs and receptors of stimulatory tectal EphA3.