Graded and Lamina-Specific Distributions of Ligands of EphB Receptor Tyrosine Kinases in the Developing Retinotectal System

Abstract

Molecular gradients have been postulated to control the topographic mapping of retinal axons in their central targets. Based initially on their expression patterns, and more recently on functional studies, members of the EphA subfamily of receptor tyrosine kinases and their ephrin-A ligands have been implicated in the guidance of retinal axons along the anterior–posterior axis of the chick optic tectum. The report that a receptor of the EphB subfamily, EphB2/Cek5/Nuk/Sek3, is expressed in a high ventral to low dorsal gradient in the developing chick retina and is present on ganglion cell axons suggests that it may be involved in the mapping of retinal axons along the corresponding dorsal–ventral axis of the tectum. To address this issue, we have determined the expression and distribution of ephrin-B1/LERK-2/Cek5-L and ephrin-B2/LERK-5/Htk-L/ELF-2, ligands for EphB2, in the developing chick retinotectal system using riboprobes, immunocytochemistry, and receptor affinity probes. Both ephrin-B1 and ephrin-B2 transcripts are expressed in a high dorsal to low ventral gradient in the developing retina, complementary to the distribution of EphB2. Ephrin-B1 and ephrin-B2 proteins are predominantly found in the developing plexiform layers, suggesting a role in the development of intraretinal connections. Neither protein is detected on ganglion cell axons. In tectum, ephrin-B1 transcripts are expressed in a high dorsal to low ventral gradient in the neuroepithelium and the protein is present along the processes of radial glia and is concentrated at their endfeet in the stratum opticum, at the time retinal axons are growing through it. This distribution of ephrin-B1 suggests that it influences retinal axon mapping along the dorsal–ventral tectal axis and may also be involved in intratectal development. In contrast, ephrin-B2 transcripts and protein are localized to the deeper retinorecipient laminae in the tectum at the time retinal axons begin to arborize in them, suggesting that this ligand may influence the laminar patterning of retinal axon terminations.