Neurotrophic signalling by GDNF and its receptors

• Many investigators have sought a polypeptide which would alleviate the neuronal loss associated with Parkinson's disease--specifically dopaminergic neurons of the ventral mesencephalon. The survival of this population of neurons has been known for some time to be promoted by soluble factors present in the conditioned media of glial cell lines. It was from one of these cell lines, B49 rat glioma, that the glial cell line derived neurotrophic factor (GDNF) was initially isolated based upon its ability to promote dopamine uptake in primary cultures prepared from ventral midbrain of embryonic day 14 (E14) rats. Although GDNF was originally reported to be highly specific for dopaminergic neurons, subsequent studies have demonstrated other potent neuroprotective actions such as promoting the survival of cultured and lesioned motor neurons. We began this study by PCR cloning of the GDNF cDNA, production and purification of recombinant protein and analysis of GDNF activities on various peripheral ganglia of the developing chick and rat. These results, combined with our GDNF mRNA expression studies, suggested that GDNF functions as a target-derived trophic factor for several populations of peripheral ganglia during development. Subsequently, we utilized fibroblasts engineered to secrete high levels of the neurotrophic factor to identify noradrenergic neurons of the locus coeruleus as a population which responds to GDNF survival promoting activities following pharmacological lesion. Subsequent efforts have centered around the identification and characterization of GDNF receptors. In addition to our finding that GDNF activities are mediated by the c-ret proto-oncogene, we have isolated two GDNF receptor-alpha homologues (GFRa2,3) which mediate GDNF binding to the RET receptor. We have also carried out in situ hybridization studies of the mRNA expression of GDNF and its receptors in the normal rat brain and in several regions following mechanical and pharmacological lesion. Our most recent efforts involve the elucidation of the intracellular signalling events following GDNF binding to Ret as well as other non-Ret signalling receptors.

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