Ret-dependent and -independent mechanisms of glial cell line-derived neurotrophic factor signaling in neuronal cells

Glial cell line-derived neurotrophic factor (GDNF) has been shown to signal through a multicomponent receptor complex consisting of the Ret receptor t yrosine kinase and a member of the GFR alpha family of glycosylphosphatidyl inositol-anchored receptors, In the current model of GDNF signaling, Ret de livers the intracellular signal but cannot bind ligand on its own, while GF R alpha s bind ligand but are thought not to signal in the absence of Ret, We have compared signaling pathways activated by GDNF in two neuronal cell lines expressing different complements of GDNF receptors. In a motorneuron- derived cell line expressing Ret and GFR alpha s, GDNF stimulated sustained activation of the Ras/ERK and phosphatidylinositol 3-kinase/Akt pathways, cAMP response element-binding protein phosphorylation, and increased c-fos expression. Unexpectedly, GDNF also promoted biochemical and biological res ponses in a line of conditionally immortalized neuronal precursors that exp ress high levels of GFR alpha s but not Ret, GDNF treatment did not activat e the Ras/ERK pathway in these cells, but stimulated a GFR alpha 1-associat ed Src-like kinase activity in detergent-insoluble membrane compartments, r apid phosphorylation of cAMP response element-binding protein, up-regulatio n of c-fos mRNA, and cell survival. Together, these results offer new insig hts into the dynamics of GDNF signaling in neuronal cells, and indicate the existence of novel signaling mechanisms directly or indirectly mediated by GFR alpha receptors acting in a cell-autonomous manner independently of Re t.