Specificity and sufficiency of EphB1 in driving the ipsilateral retinal projection

At the optic chiasm, retinal ganglion cell (RGC) axons make the decision to either avoid or traverse the midline, a maneuver that establishes the binocular pathways. In mice, the ipsilateral retinal projection arises from RGCs in the peripheral ventrotemporal (VT) crescent of the retina. These RGCs express the guidance receptor EphB1, which interacts with ephrin-B2 on radial glia cells at the optic chiasm to repulse VT axons away from the midline and into the ipsilateral optic tract. However, because VT RGCs express more than one EphB receptor, the sufficiency and specificity of the EphB1 receptor in directing the ipsilateral projection is unclear. In this study, we use in utero retinal electroporation to demonstrate that ectopic EphB1 expression can redirect RGCs with a normally crossed projection to an ipsilateral trajectory. Moreover, EphB1 is specifically required for rerouting RGC projections ipsilaterally, because introduction of the highly similar EphB2 receptor is much less efficient in redirecting RGC fibers, even when expressed at higher surface levels. Introduction of EphB1-EphB2 chimeric receptors into RGCs reveals that both extracellular and juxtamembrane domains of EphB1 are required to efficiently convert RGC projections ipsilaterally. Together, these data describe for the first time functional differences between two highly similar Eph receptors at a decision point in vivo, with EphB1 displaying unique properties that efficiently drives the uncrossed retinal projection.