EphA/ephrin-A interactions during optic nerve regeneration: restoration of topography and regulation of ephrin-A2 expression.

During visual system development, interactions between Eph tyrosine kinase receptors and their ligands, the ephrins, guide retinal ganglion cell (RGC) axons to their topographic targets in the optic tectum. Here we show that Eph/ephrin interactions are also involved in restoring topography during RGC axon regeneration in goldfish. Following optic nerve crush, EphA/ephrin-A interactions were blocked by intracranial injections of recombinant Eph receptor (EphA3-AP) or phospho-inositol phospholipase-C. Topographic errors with multiple inputs to some tectal loci were detected electrophysiologically and increased projections to caudal tectum demonstrated by RT-97 immunohistochemistry. In EphA3-AP-injected fish, ephrin-A2-expressing cells in the retino-recipient tectal layers were reduced in number compared to controls and their distribution was no longer graded. The findings, supported by in vitro studies, implicate EphA/ephrin-A interactions in restoring precise topography and in regulating ephrin-A2 expression during regeneration.