Ret Is a Multifunctional Coreceptor that Integrates Diffusible- and Contact-Axon Guidance Signals

Growing axons encounter multiple guidance cues, but it is unclear how separate signals are resolved and integrated into coherent instructions for growth cone navigation. We report that glycosylphosphati-dylinositol (GPI)-anchored ephrin-As function as "reverse'' signaling receptors for motor axons when contacted by transmembrane EphAs present in the dorsal limb. Ephrin-A receptors are thought to depend on transmembrane coreceptors for transmitting signals intracellularly. We show that the receptor tyrosine kinase Ret is required for motor axon attraction mediated by ephrin-A reverse signaling. Ret also mediates GPI-anchored GFR alpha 1 signaling in response to GDNF, a diffusible chemoattractant in the limb, indicating that Ret is a multifunctional coreceptor for guidance molecules. Axons respond synergistically to coactivation by GDNF and EphA ligands, and these cooperative interactions are gated by GFR alpha 1 levels. Our studies uncover a hierarchical GPI-receptor signaling network that is constructed from combinatorial components and integrated through Ret using ligand coincidence detection.