Localization of cell-surface components to synaptic regions is an important problem in developmental neurobiology. It has been shown that lateral migration of diffusely-expressed receptors makes a major contribution to receptor clustering at the developing neuromuscular junction. As electric fields offer a convenient means to induce lateral migration of membrane components, their use constitutes a useful experimental system with which to study receptor localization. In particular, cultures of spherical muscle cells from the African frog Xenopus laevis have been shown to cluster receptors in response to such fields. The clustering continues after the field has been terminated, and is specific for acetylcholine receptors. These observations render this a powerful model system with which to probe the mechanisms underlying receptor clustering, because i) the cultured cells form receptor clusters rapidly in response to precisely measurable stimuli, ii) experiments can be conducted in the absence of neurites, cell-substrate contacts, or exogenous factors, and iii) the cultured cells are geometrically simple, permitting high-resolution quantitation of receptor density. The goal of the proposed study is to examine further the mechanisms responsible for the clustering of receptors in the model system afforded by the Xenopus cultures. The experiments will utilize global and local application of external electric fields to cultured Xenopus muscle cells. Following these manipulations, digital video-microscopic analysis of fluorescently labeled cells will be performed in order to characterize the distribution of acetylcholine receptors and other components. Three broad questions will be addressed: 1. What molecular event(s) trigger the field-induced clustering of acetylcholine receptors? 2. How do other post-synaptic molecules behave in response to electric fields? 3. What are the parameters relevant to competition for receptors between neighboring clusters? Examination of these questions will increase our understanding of the in vivo mechanisms responsible for the developmental formation of synaptic contacts. The questions are intellectually crucial within the context of developmental neurobiology, and also have significance with respect to synapse-based disorders. The fundamental importance of receptor clustering is clear from the devastating effects of diseases such as myasthenia gravis, which reduce the number of available receptors at the junction. Moreover, experimental results and theoretical considerations suggest that the mechanisms by which neurons compete for receptors relate to the developmental refinement of synaptic connections, a topic of great interest with respect to developmental disabilities.
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