TrkA Talks to Ret

Abstract

Nerve growth factor (NGF), which is a ligand for the Trk receptor tyrosine kinases, has two roles in sympathetic nerve cells: survival and differentiation. Sympathetic neurons also rely on other neurotrophins, such as members of the glial-cell line-derived nuerotrophic factor family ligands (GFLs). GFLs activate the receptor tyrosine kinase Ret through interaction with a coreceptor, GFRα, and then the GFL-GFRα complex activates Ret. Tsui-Pierchala et al. provide evidence that NGF may activate Ret independently from GFLs to regulate mature sympathetic neuron growth, metabolism, and gene expression. Superior cervical ganglia (SCG) sympathetic neurons, in vivo and in isolated primary cultures, demonstrated increased Ret phosphorylation as a function of age. The isolated neurons expressed both a short isoform of Ret (Ret9) and a longer isoform of Ret (Ret51), and the increase in phosphorylation was specific to Ret51, despite both isoforms having the same extracellular domain. The age-dependent increase in Ret phosphorylation was not the result of activation of the receptor by GFLs: (i) only one isoform exhibited the increased phosphorylation, (ii) conditioned medium from older cells (21 days in culture) did not stimulate Ret phosphorylation in younger cells (5 days in culture), (iii) removal of the GFRα cofactor did not inhibit the phosphorylation, and (iv) washing of the cultures to remove autocrine factors did not inhibit phosphorylation. The cultured neurons were maintained in medium containing NGF, and the presence of NGF was required for the Ret phosphorylation, Ret phosphorylation was inhibited by antagonists of the TrkA receptor, not the p75 neurotrophin receptor. Injection of antiserum against NGF into rats inhibited Ret phosphorylation in SCG. Analysis of the possible signaling pathways from TrkA to Ret suggested that the mitogen-activated protein kinase pathway and the phosphatidylinositol 3-kinase pathway were not mediating NGF-stimulated Ret phosphorylation, although pharmacological inhibition of either of these two pathways did inhibit NGF-stimulated increase in Ret expression. The stimulation of Ret phosphorylation by NGF was a long-term response, requiring 48 hours to peak. Cultured SCG neurons from ret –/– mice were compared for NGF-mediated regulation of both cell size (size of the cell body) and metabolism and were found to be smaller with decreased stimulation of NGF-dependent gene expression (tyrosine hydroxylase, microtubule-associated protein-2, and neurofilament-light). Thus, NGF activation of Ret appears to contribute to mature sympathetic neuronal trophic responses to NGF, through an as-yet-uncharacterized GFL-independent mechanism. These results are discussed by Dechant. B. A. Tsui-Pierchala, J. Milbrandt, E. M. Johnson Jr., NGF utilizes c-Ret via a novel GFL-independent, inter-RTK signaling mechanism to maintain the trophic status of mature sympathetic neurons. Neuron 33 , 261-273 (2002). [Online Journal] G. Dechant, Chat in the trophic web: NGF activates Ret by inter-RTK signaling. Neuron 33 , 156-158 (2002). [Online Journal]