EphrinA6 on chick retinal axons is a key component for p75(NTR)-dependent axon repulsion and TrkB-dependent axon branching.

A characteristic of the ephrin/Eph family is their capacity for bi-directional signalling. This means that an ephrin, for example, can function either as a ligand for an Eph 'receptor', or as a receptor for an Eph 'ligand'. A system in which this phenomenon is well studied is the retinotectal projection in which the guidance of retinal ganglion cell (RGC) axons to their target area in the tectum is controlled by both Ephs and ephrins expressed in gradients in both the retina and tectum. Here we have analysed the receptor function of ephrinAs on RGC axons in further detail by focussing on ephrinA6, which is the most strongly expressed ephrinA in the chick retina. EphrinAs are GPI-anchored proteins and therefore require the interaction with transmembrane proteins to exert this receptor function. Previous work has shown that ephrinAs interact on RGC axons in cis with the neurotrophin receptors p75(NTR) and TrkB. P75(NTR) then was shown to be necessary for the repulsion of ephrinA-expressing RGC axons from an EphA substrate and for the downregulation of axon branching. In turn, an interaction of ephrinAs with TrkB as well as an increase in axonal ephrinA expression augments the axon branch-promoting activity of TrkB. We now show that ephrinA6 is the necessary ephrinA component of the repulsive ephrinA/p75(NTR) receptor complex on chick RGC axons as axons lacking ephrinA6 no longer avoid an EphA matrix in stripe assay experiments. We also demonstrate that the branch-promoting activity of TrkB is dependent on ephrinA6 as a knockdown of ephrinA6 renders RGC axons insensitive to BDNF, the high affinity ligand for TrkB. In sum our data further strengthen the hypothesis that a fine-tuned interplay of ephrinAs with p75(NTR) and TrkB is important for the guidance and branching of RGC axons.