Glial Cell Line-Derived Neurotrophic Factor Requires Transforming Growth Factor-β for Exerting Its Full Neurotrophic Potential on Peripheral and CNS Neurons

摘要

Numerous studies have suggested that glial cell line-derived neurotrophic factor (GDNF) is a potent neurotrophic molecule. We show now on a variety of cultured neurons including peripheral autonomic, sensory, and CNS dopaminergic neurons that GDNF is not trophically active unless supplemented with TGF-β. Immunoneutralization of endogenous TGF-β provided by serum or TGF-β-secreting cells, as e.g., neurons, in culture abolishes the neurotrophic effect of GDNF. The dose–response relationship required for the synergistic effect of GDNF and TGF-β identifies 60 pg/ml of either factor combined with 2 ng/ml of the other factor as the EC50. GDNF/TGF-β signaling employs activation of phosphatidylinositol-3 (PI-3) kinase as an intermediate step as shown by the effect of the specific PI-3 kinase inhibitor wortmannin. The synergistic action of GDNF and TGF-β involves protection of glycosylphosphatidylinositol (GPI)-linked receptors as shown by the restoration of their trophic effects after phosphatidylinositol-specific phospholipase C-mediated hydrolysis of GPI-anchored GDNF family receptor α. The biological significance of the trophic synergism of GDNF and TGF-β is underscored by colocalization of the receptors for TGF-β and GDNF on all investigated GDNF-responsive neuron populationsin vivo. Moreover, the in vivo relevance of the TGF-β/GDNF synergism is highlighted by the co-storage of TGF-β and GDNF in secretory vesicles of a model neuron, the chromaffin cell, and their activity-dependent release. Our results broaden the definition of a neurotrophic factor by incorporating the possibility that two factors that lack a neurotrophic activity when acting separately become neurotrophic when acting in concert. Moreover, our data may have a substantial impact on the treatment of neurodegenerative diseases.