RESEARCH PROGRAM: This program project brings together four investigators with a long standing research interest in neurodevelopment and specifically in the structure and development of the neuromuscular junction. This program project application has its origins in part in this long-standing collaboration and research interest and in part in the new avenues of investigation that have been made available by the revolution in eucaryotic molecular biology. While many molecules that regulate axon target interactions have been identified, the roles they play in vivo are still poorly understood. This lag between identification of specific molecules and the understanding of the roles they play in development is in part due to the fact that molecular studies of vertebrates have been limited to tissue culture where it is difficult to achieve the full repertoire of developmental processes. Now, new methods in gene transfer provide ways to circumvent this problem. It is now possible to over-express genes in specific tissues of transgenic mice, to produce transgenic cells in wild-type animals with viral vectors and to mutate genes by homologous recombination. In this program project, the investigators propose to use all of these techniques to initiate a molecular genetic analysis of neuromuscular development in the mouse. The investigators bring to this project a complementary set of expertise. Dr. Lichtman's ability to observe synapse in living mice over long periods of time permits detailed studies of synapse elimination and rearrangement. Dr. Merlie's molecular biology expertise will facilitate the necessary gene transfer experiments. Dr. Sanes' history and knowledge of the extracellular matrix and immunohistochemistry will help analyze the neuromuscular junction in genetically modified mice over long periods of time. Finally, Dr. Snider's work on the neurotrophins and their receptors as well as his clinical perspective will allow this program project to include neuronal synapse and growth factors.