GLIAL-CELL LINE-DERIVED NEUROTROPHIC FACTOR REQUIRES TRANSFORMING-GROWTH-FACTOR-BETA FOR EXERTING ITS FULL NEUROTROPHIC POTENTIAL ON PERIPHERAL AND CNS NEURONS

Numerous studies have suggested that glial cell line-derived neurotrop hic factor (GDNF) is a potent neurotrophic molecule. We show now on a variety of cultured neurons including peripheral autonomic, sensory, a nd CNS dopaminergic neurons that GDNF is not trophically active unless supplemented with TGF-beta. Immunoneutralization of endogenous TGF-be ta provided by serum or TGF-beta-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-beta identifies 60 pg/ml of either factor combined with 2 ng/ml of the othe r factor as the EC50. GDNF/TGF-beta signaling employs activation of ph osphatidylinositol-3 (PI-3) kinase as an intermediate step as shown by the effect of the specific PI-3 kinase inhibitor wortmannin. The syne rgistic action of GDNF and TGF-beta involves protection of glycosylpho sphatidylinositol (GPI)-linked receptors as shown by the restoration o f their trophic effects after phosphatidylinositol-specific phospholip ase C-mediated hydrolysis of GPI-anchored GDNF family receptor alpha. The biological significance of the trophic synergism of GDNF and TGF-b eta is underscored by colocalization of the receptors for TGF-beta and GDNF on all investigated GDNF-responsive neuron populations in vivo. Moreover, the in vivo relevance of the TGF-beta/GDNF synergism is high lighted by the co-storage of TGF-beta and GDNF in secretory vesicles o f a model neuron, the chromaffin cell, and their activity-dependent re lease. Our results broaden the definition of a neurotrophic factor by incorporating the possibility that two factors that lack a neurotrophi c activity when acting separately become neurotrophic when acting in c oncert. Moreover, our data may have a substantial impact on the treatm ent of neurodegenerative diseases.