Persistence of graded EphA/Ephrin-A expression in the adult frog visual system

Many studies have demonstrated the involvement of the EphA family of receptor tyrosine kinases and their ligands, ephrin-A2 and -A5, in the development of the temporonasal axis of the retinotectal/collicular map, but the role of these molecules in optic nerve regeneration has not been well studied. Noting that the characteristic gradients of the EphA/ephrin-A family that are expressed topographically in the retina and tectum of embryonic chicks and mice tend to disappear after birth, we took as our starting point an analysis of EphA and ephrin-A expression in leopard frogs (Rana pipiens and utricularia), species capable of regenerating the retinotectal map as adults. For the EphA family to be involved in the regeneration, one would expect these topographic gradients to persist in the adult or, if downregulated after metamorphosis, to be reexpressed after optic nerve injury. Using EphA3 receptor and ephrin-A5 ligand alkaline phosphatase in situ affinity probes (RAP and LAP, respectively) in whole-mount applications, we report that reciprocally complementary gradients of RAP and LAP binding persist in the optic tract and optic tectum of postmetamorphic frogs, including mature adults. EphA expression in temporal retinal axons in the optic tract was significantly reduced after nerve section but returned during regeneration. However, ephrin-A expression in the tectal parenchyma was not significantly elevated by either eye removal, with degeneration of optic axons, or during regeneration of the retinotectal projection. Thus, the present study has demonstrated a persisting expression of EphA/ephrin-A family members in the retinal axons and tectal parenchyma that may help guide regenerating fibers, but we can offer no evidence for an upregulation of ephrin-A expression in conjunction with optic nerve injury.