Motor neuron diseases (MND) are a group of degenerative disorders characterized by progressive weakness and atrophy of skeletal muscle leading to eventual death of affected individuals. The selective degeneration of upper and/or lower motor neurons (MN) is the hallmark of these diseases, the most common of which in adults is Amyotrophic Lateral Sclerosis (ALS). Little clinical success has been reached for the treatment of ALS and other MNDs. Neurotrophic factors (NF) have been considered as potential agents for treatment of these diseases; however, several NFs failed to demonstrate any beneficial effects in ALS trials. It is well recognized that a major issue is the peripheral delivery of these factors. Our primary interest has focused on the GDNF family ligands, since GDNT is the most potent MN survival factor known to date. Understanding the mechanisms, by which GDNF acts to promote MN survival during development and following neuronal injury, as well as its effect on the innervation of target tissue, may give insight into MN biology and help define a therapeutic value for GDNF in the treatment of MN diseases. A transgenic approach will be used to study the importance of GDNF on MNs in normal and pathological conditions, thus allowing us to compare the effects of GDNF over expression in MN target tissue with that in the Central Nervous System (CNS). It has been shown that the beneficial effects of some NFs indeed depend on the route of delivery. In this application we will test the hypothesis that the route of delivery of GDNF may significantly impact its survival effects on MNs, and that GDNF over expression in transgenic mice will rescue MNs from programmed cell death (PCD), and produce long-term survival of MNs after axotomy. In addition, we will test GDNF's potential protective effects in mouse models of MN disease, and determine if its effectiveness is dependent on the site of GDNF expression (CNS vs. peripheral target tissue). In specific aim 1, we will study in vivo the role of GDNF (delivered via different routes) on MN PCD. In aim 2, we will determine whether GDNF promotes both long-term survival of MN following axotomy, and permanent prevention of synapse elimination at the neuromuscular junction. In aim 3, we will test the possible protective effects of GDNF in a murine model of Familial ALS, along with the importance of its route of administration. And in aim 4, we will test the protective effect of GDNF in a naturally occurring mouse model of MN degeneration, progressive motor neuronopathy (pmn) mice. These studies should provide new insight into the in vivo role of GDNF on MNs in both normal and pathological conditions, and define the differences between the various modes of GDNF delivery to MNs. They will also further define the potential of GDNIF in treating MNDs, and help design suitable strategies for its delivery.