| Glial cell line-derived growth factor (GDNF), neurturin, persephin, and artemin form a new family of neurotrophic factors called GDNFs. The biological functions of GDNFs are mediated by two-component receptors consisting of the signaling component c-RET, and the GFR binding component, of which four members have been identified. GDNF and neurturin have been shown to enhance the survival of midbrain dopaminergic neurons and spinal cord motoneurons. However, the signaling mechanisms and the biological functions of GDNFs are largely unknown. In particular, it is unclear whether GDNFs plays any role in synaptic transmission and synapse development. My dissertation is aimed at a number of questions. First, to identify the molecular components of the GDNF signaling pathway, I cloned the second GDNF receptor, GFR 2, and showed that it interacts with GDNF, leading to the activation of MAP kinase (MAPK) and neurite-outgrowth in PC12 cells. Second, I demonstrated that in cultured ventral mesencephalic neurons, although GDNF and BDNF both activate the MAPK and CREB pathways, the kinetics and pharmacology of their activation are quite different. The activation of MAPK and CREB induced by BDNF is sustained and insensitive to the MAPK kinase inhibitor, PD, whereas the activation of MAPK and CREB by GDNF is transient and sensitive to PD. BDNF, but not GDNF, up-regulates the expression of the neuronal proteins calbindin and SNAP25. GDNF, but not BDNF, promotes axon fasciculation. Thus, differential signaling of the same pathways by BDNF and GDNF may lead to different cellular functions. Third, I used the neuromuscular junction (NMJ) as a model system to study the role of GDNFs in synaptic transmission. I demonstrated that GDNF enhances synaptic transmission by up-regulating frequenin, a calcium binding protein, and this modulation is achieved through potentiation of Ca++ channels. I also showed that GDNF and neurturin have long-term but not acute effects on synaptic transmission at the NMJ, and both act in a target-derived manner. Finally, both GDNF and neurturin promote neurite growth, and increase the number of synaptic varicosities on motoneurons. These results suggest that GDNF and neurturin may play an important role in the development of the neuromuscular synapses. |
