Glial Cell Line-Derived Neurotrophic Factor Rescues Target-Deprived Sympathetic Spinal Cord Neurons But Requires Transforming Growth Factor-β as Cofactor In Vivo

Glial cell line-derived neurotrophic factor (GDNF) is a potent neurotrophic factor for several populations of CNS and peripheral neurons. Synthesis and storage of GDNF by the neuron-like adrenal medullary cells suggest roles in adrenal functions and/or in the maintenance of spinal cord neurons that innervate the adrenal medulla. We show that unilateral adrenomedullectomy causes degeneration of all sympathetic preganglionic neurons within the intermediolateral column (IML) of spinal cord segments T7–T10 that project to the adrenal medulla. In situ hybridization revealed that IML neurons express the glycosylphosphatidylinositol-linked α receptor 1 and c-Ret receptors, which are essential for GDNF signaling. IML neurons also display immunoreactivity for transforming growth factor-β (TGF-β) receptor II. Administration of GDNF (recombinant human, 1 μg) in Gelfoam implanted into the medullectomized adrenal gland rescued all Fluoro-Gold-labeled preganglionic neurons projecting to the adrenal medulla after four weeks. Cytochrome c applied as a control protein was not effective. The protective effect of GDNF was prevented by co-administration to the Gelfoam of neutralizing antibodies recognizing all three TGF-β isoforms but not GDNF. This suggests that the presence of endogenous TGF-β was essential for permitting a neurotrophic effect of GDNF. Our data indicate that GDNF has a capacity to protect a population of autonomic spinal cord neurons from target-deprived cell death. Furthermore, our results demonstrate for the first time that the previously reported requirement of TGF-β for permitting trophic actions of GDNF in vitro (Krieglstein et al., 1998) also applies to the in vivo situation.