Released GFR alpha 1 potentiates downstream signaling, neuronal survival, and differentiation via a novel mechanism of recruitment of c-Ret to lipid rafts

Although both c-Ret and GFR alpha1 are required for responsiveness to GDNF, GFR alpha1 is widely expressed in the absence of c-Ret, suggesting alterna tive roles for "ectopic" sites of GFR alpha1 expression. We show that GFR a lpha1 is released by neuronal cells, Schwann cells, and injured sciatic ner ve. c-Ret stimulation in trans by soluble or immobilized GFR alpha1 potenti ates downstream signaling, neurite outgrowth, and neuronal survival, and el icits dramatic localized expansions of axons and growth cones. Soluble GFR alpha1 mediates robust recruitment of c-Ret to lipid rafts via a novel mech anism requiring the c-Ret tyrosine kinase. Activated c-Ret associates with different adaptor proteins inside and outside lipid rafts. These results pr ovide an explanation for the tissue distribution of GFR alpha1, supporting the physiological importance of c-Ret activation in trans as a novel mechan ism to potentiate and diversify the biological responses to GDNF.