A role for the EPH ligand ephrin-A3 in entorhino-hippocampal axon targeting

Neurons of layers II and III of the entorhinal cortex constitute the major afferent connection of the hippocampus. The molecular mechanisms that targe t the entorhinal axons to specific layers in the hippocampus are not known. EphA5, a member of the Eph receptor family, which has been shown to play c ritical roles in axon guidance, is expressed in the entorhinal cortex, the origin of the perforant pathway. In addition, ligands that interact with Ep hA5 are expressed in distinct hippocampal regions during development of the entorhino-hippocampal projection. Of these ligands, ephrin-A3 mRNA is loca lized both in the granular cell layer of the dentate gyrus and in the pyram idal cell layer of the cornu ammonis, whereas ephrin-A5 mRNA is only expres sed in the pyramidal cell layer of the cornu ammonis. In the dentate gyrus, the ligand protein is not present in the termination zone of the entorhina l efferents (the outer molecular layer of the dentate gyrus) but is concent rated in the inner molecular layer into which entorhinal efferents do not g row. We used outgrowth and stripe assays to test the effects of ephrin-A3 a nd ephrin-A5 on the outgrowth behavior of entorhinal axons. This functional analysis revealed that entorhinal neurites were repelled by ephrin-A3 but not by ephrin-A5. These observations suggest that ephrin-A3 plays an import ant role in the layer-specific termination of the perforant pathway and tha t this ligand may interact with the EphA5 receptor to restrict entorhinal a xon terminals in the outer molecular layer of the dentate gyrus.