Retinal axon growth cones respond to EphB extracellular domains as inhibitory axon guidance cues

Axon pathfinding relies on cellular signaling mediated by growth cone recep tor proteins responding to ligands, or guidance cues, in the environment. E ph proteins are a family of receptor tyrosine kinases that govern axon path way development, including retinal axon projections to CNS targets. Recent examination of EphB mutant mice, however, has shown that axon pathfinding w ithin the retina to the optic disc is dependent on EphB receptors, but inde pendent of their kinase activity. Here we show a function for EphB1, B2 and B3 receptor extracellular domains (ECDs) in inhibiting mouse retinal axons when presented either as substratum-bound proteins or as soluble proteins directly applied to growth cones via micropipettes. In substratum choice as says, retinal axons tended to avoid EphB-ECDs, while time-lapse microscopy showed that exposure to soluble EphB-ECD led to growth cone collapse or oth er inhibitory responses. These results demonstrate that, in addition to the conventional role of Eph proteins signaling as receptors, EphB receptor EC Ds can also function in the opposite role as guidance cues to alter axon be havior. Furthermore, the data support a model in which dorsal retinal gangl ion cell axons heading to the optic disc encounter a gradient of inhibitory EphB proteins which helps maintain tight axon fasciculation and prevents a berrant axon growth into ventral retina. In conclusion, development of neur onal connectivity may involve the combined activity of Eph proteins serving as guidance receptors and as axon guidance cues.