Ret-dependent and -independent Mechanisms of Glial Cell Line-derived Neurotrophic Factor Signaling in Neuronal Cells

Abstract

Glial cell line-derived neurotrophic factor (GDNF) has been shown to signal through a multicomponent receptor complex consisting of the Ret receptor tyrosine kinase and a member of the GFR␣ family of glycosylphosphatidylinositol-anchored receptors. In the current model of GDNF signaling, Ret delivers the intracellular signal but cannot bind ligand on its own, while GFR␣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␣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 responses in a line of conditionally immortalized neuronal precursors that express high levels of GFR␣s but not Ret. GDNF treatment did not activate the Ras/ERK pathway in these cells, but stimulated a GFR␣1-associated Src-like kinase activity in detergent-insoluble membrane compartments, rapid phosphorylation of cAMP response element-binding protein, up-regulation of c-fos mRNA, and cell survival. Together, these results offer new insights into the dynamics of GDNF signaling in neuronal cells, and indicate the existence of novel signaling mechanisms directly or indirectly mediated by GFR␣ receptors acting in a cell-autonomous manner independently of Ret.

GDNF/Ret signaling and the development of the kidney

“…Gfra1 is coexpressed with Ret, consistent with its role as a co-receptor, and it can also mediate GDNF signaling independently of Ret in some circumstances. (23)(24)(25) Although Gfra1 is expressed in a slightly broader domain of the UB, and also to some extent in the mesenchyme and forming nephrons, (21,22,26) its expression in cells outside the Ret domain is entirely dispensable for normal kidney development; this was shown by targeting Gfra1 coding sequences into the Ret locus, so it was expressed in the pattern of Ret, and crossing these mice to animals lacking endogenous Gfra1. (27) The key role of GDNF and its receptors in kidney development was initially revealed by the targeted disruption of the Ret, GDNF and Gfra1 genes.…”

Glial Cell Line-Derived Neurotrophic Factor Alters Axon Schwann Cell Units and Promotes Myelination in Unmyelinated Nerve Fibers

“…Demyelination results in loss of caliber and NF-H phosphorylation (de Waegh et al, 1992). Finally, signaling may occur through the GFR␣-1 in the absence of c-ret receptor (Poteryaev et al, 1999;Trupp et al, 1999;Pezeshki et al, 2001). For these reasons, it is premature to ascribe the GDNF effect to either axons or Schwann cells.…”

“…GFR-␣1 does not posses an intracellular domain; and traditionally GDNF action is thought to require the formation of the GFR-␣1 and the Ret tyrosine kinase coreceptor complex (Airaksinen et al, 1999). However, signaling through GFR-␣1 by GDNF can occur even in the absence of Ret coreceptor (Poteryaev et al, 1999;Trupp et al, 1999;Pezeshki et al, 2001).…”

c-Ret–mediated hearing loss in mice with Hirschsprung disease

“…The GDNF exerts its effect on target cells by binding to a GPI-anchored cell surface protein (GFRα1), which, in turn, recruits the receptor tyrosine kinase c-RET to form a multiple-subunit signaling complex. Formation of this complex results in c-RET autophosphorylation and a cascade of intracellular signaling to control cell survival (4)(5)(6)(7).…”