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

Abstract

SUMMARY 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 glycosylphosphatidylinositol (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 co-receptors 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 GFRα1 signaling in response to GDNF, a diffusible chemoattractant in the limb - indicating that Ret is a multifunctional co-receptor for guidance molecules. Axons respond synergistically to co-activation by GDNF and EphA ligands and these cooperative interactions are gated by GFRα1 levels. Our studies uncover a hierarchical GPI-receptor signaling network that is constructed from combinatorial components and integrated through Ret using ligand coincidence detection.

Contact-dependent Signaling

“…53 In contrast, interaction between ephrin-A and the p75 neurotrophin receptor causes reverse signaling from Eph to ephrin-A that results in axon repulsion. 53 Notch ligands. The Notch signaling pathway is a highly conserved juxtacrine cell communication system, involved in cell-fate specification, formation of growthorganizing boundaries, stem cell maintenance, proliferation, apoptosis, and migration.…”

“…This occurs by interaction of ephrin-A with the receptor rearranged during transfection (Ret) in the ephrin-bearing cell. 53 In the presence of the Ret ligand, glial cell-derived neurotrophic factor (GDNF), and its co-receptor, GDNF family receptor α1 (GFRα1), axon attraction is potentiated. 53 In contrast, interaction between ephrin-A and the p75 neurotrophin receptor causes reverse signaling from Eph to ephrin-A that results in axon repulsion.…”

Lipid Raft-Excluded GFRα1/Ret Fails to Transmit GDNF SignalingIn Vivo

“…Spinal motor axon innervation of the dorsal limb mesenchyme is controlled in part by GDNF:GFR␣1/Ret signaling (Kramer et al, 2006;Dudanova et al, 2010). In addition to expressing GDNF, the dorsal limb tissue is rich in EphA, and motor axons innervating it express its binding partner, GPI-anchored ephrinA, which is also thought to reside in lipid rafts (Bonanomi et al, 2012). EphA:ephrinA signaling also depends on Ret as a coreceptor, and the coincidence of GDNF and EphA produce synergistic attractive growth cone responses (Bonanomi et al, 2012).…”

“…In addition to expressing GDNF, the dorsal limb tissue is rich in EphA, and motor axons innervating it express its binding partner, GPI-anchored ephrinA, which is also thought to reside in lipid rafts (Bonanomi et al, 2012). EphA:ephrinA signaling also depends on Ret as a coreceptor, and the coincidence of GDNF and EphA produce synergistic attractive growth cone responses (Bonanomi et al, 2012). In GFR␣1-TM mouse limbs, the dorsal nerve fails to grow as axons are all redirected ventrally, a phentoype that has previously been reported in mice with defective GDNF or EphA pathways (Helmbacher et al, 2000;Kramer et al, 2006;Gould et al, 2008;Bonanomi et al, 2012).…”

“…GDNF is an important neuronal survival factor (Moore et al, 1996), and an axon guidance cue for limb-innervating spinal motor neurons (Kramer et al, 2006;Dudanova et al, 2010;Bonanomi et al, 2012). A diffusible ligand, GDNF binds to a glycosylphosphatidylinositol (GPI)-anchored receptor, GDNF family receptor ␣ 1 (GFR␣1), that is found within membrane fractions that are thought to contain lipid rafts.…”