Topographic targeting and pathfinding errors of retinal axons following overexpression of ephrinA ligands on retinal ganglion cell axons

In the retinotectal projection, the Eph receptor tyrosine kinase ligands ep hrinA2 and ephrinA5 are differentially expressed not only in the tectum, bu t also in a high-nasal-to-low-temporal pattern in the retina. Recently, we have shown that retrovirally driven overexpression of ephrinA2 on retinal a xons leads to topographic targeting errors of temporal axons in that they o vershoot their normal termination zones in the rostral tectum and project o nto the mid- and caudal tectum. The behavior of nasal axons, however, was o nly marginally affected. Here, we show that overexpression of ephrinA5 affe cts the topographic targeting behavior of both temporal and nasal axons. Th ese data reinforce the idea that differential ligand expression on retinal axons contributes to topographic targeting in the retinotectal projection. Additionally, we found that ectopic expression of ephrinA2 and ephrinA5 fre quently leads to pathfinding errors at the chiasm, resulting in an increase d stable ipsilateral projection, (C) 1999, Academic Press.