Correspondence

Abstract

Glial cell line-derived neurotrophic factor was initially identified as a survival factor for developing midbrain dopamine neurons (for reviews, see Refs 17 and 19). Subsequent studies have demonstrated a more wide-spread role for glial cell line-derived neurotrophic factor in the developing and adult CNS. In the adult rat brain, for instance, prior administration of glial cell line-derived neurotrophic factor protects nigrostriatal dopamine neurons from 6-hydroxydopamine-induced damage. When given several weeks after 6-hydroxydopamine injection, glial cell line-derived neurotrophic factor also restores the function of these neurons. Glial cell line-derived neurotrophic factor attenuates excitotoxin-induced cell death in the striatum and hippocampal formation and protective effects of glial cell line-derived neurotrophic factor following axotomy have been reported for spinal motor neurons and basal forebrain cholinergic neurons. These findings suggest that glial cell line-derived neurotrophic factor may be a protective/restorative agent for a diverse population of neurons and imply that it may be a useful therapeutic tool for a variety of neurodegenerative diseases including Parkinson's, Huntington's and Alzheimer's diseases. The potential receptor mediating the pleiotropic effects of glial cell line-derived neurotrophic factor has been characterized only recently as a novel glycosyl-phosphatidylinositol-linked protein, GDNFR-alpha. Because GDNFR-alpha is a cell surface receptor, an additional protein(s) was thought to be involved in the glial cell line-derived neurotrophic factor signalling cascade. The identity of the likely candidate, ret, was inferred initially from indirect evidence. Not only were there remarkable similarities in the distribution of glial cell line-derived neurotrophic factor and the proto-oncogene ret in the developing rat and mouse brain, but also in the phenotype of glial cell line-derived neurotrophic factor knockout mice and mice with ret mutations. Mice with either mutation exhibited pronounced renal and enteric abnormalities, implicating the receptor tyrosine kinase protein product of the ret proto-oncogene as the glial cell line-derived neurotrophic factor signalling protein. More conclusive evidence showing that activation of GDNFR-alpha by glial cell line-derived neurotrophic factor induces phosphorylation of ret has confirmed ret as a signalling protein for glial cell line-derived neurotrophic factor. Preliminary results showing that 6-hydroxydopamine lesions of the substantia nigra markedly reduced ret messenger RNA expression, established its localization to presumably glial cell line-derived neurotrophic factor-responsive dopamine neurons in the nigrostriatal pathway. In contrast, it is not clear whether other glial cell line-derived neurotrophic factor-responsive neurons in the CNS, such as the basal forebrain cholinergic neurons and striatal neurons, also express ret, nor is it evident whether levels of the protein are regulated by disruption of the respective pathways. The present study shows that dense networks of ret immunoreactivity are distributed throughout the nigrostriatal pathway, with lower densities of staining in other brain regions, including the septohippocampal pathway. Following extensive unilateral 6-hydroxydopamine lesions of the medial forebrain bundle, ret immunoreactivity in the substantia nigra and striatum was reduced significantly, to a similar extent as tyrosine hydroxylase immunoreactivity. In contrast, excitotoxic lesions of the striatum, achieved by intrastriatal quinolinic acid injections, resulted in increased ret staining in this brain region. In addition, marked decrements in septal ret immunoreactivity were consequent to complete transections of the fimbria-fornix.