p75NTR Mediates Ephrin-A Reverse Signaling Required for Axon Repulsion and Mapping

SummaryReverse signaling by ephrin-As upon binding EphAs controls axon guidance and mapping. Ephrin-As are GPI-anchored to the membrane, requiring that they complex with transmembrane proteins that transduce their signals. We show that the p75 neurotrophin receptor (NTR) serves this role in retinal axons. p75NTR and ephrin-A colocalize within caveolae along retinal axons and form a complex required for Fyn phosphorylation upon binding EphAs, activating a signaling pathway leading to cytoskeletal changes. In vitro, retinal axon repulsion to EphAs by ephrin-A reverse signaling requires p75NTR, but repulsion to ephrin-As by EphA forward signaling does not. Constitutive and retina-specific p75NTR knockout mice have aberrant anterior shifts in retinal axon terminations in superior colliculus, consistent with diminished repellent activity mediated by graded ephrin-A reverse signaling induced by graded collicular EphAs. We conclude that p75NTR is a signaling partner for ephrin-As and the ephrin-A- p75NTR complex reverse signals to mediate axon repulsion required for guidance and mapping