Neurons are dependent upon their highly unusual shape for function. The axon is an exceptionally long "electric wire" extension of the neuron and conducts the electrical signals generated in the nervous system. In developing, embryonic vertebrates, a substantial portion of the elongation of axons occurs as a result of tension applied to the axon by the expansion of the growing embryo. Previous studies on cultured neurons suggest that tension, exerted by the motile growth cone, is responsible for axonal elongation. The filamentous proteins of the cytoplasm (actin), microtubules, and intermediate filaments compose a dynamic skeletal structure for cells, i.e. a cytoskeleton, that underlies the ability of axons to elongate. Mechanical tension is expected to regulate the cytoskeleton during axonal elongation. This idea is supported by thermodynamic considerations that suggest the assembly of filaments is affected by mechanical forces. This research project proposes to investigate the affect of mechanical tension on the elongation and cytoskeleton of cultured, embryonic chick neurons. The experiments will provide direct measurements of the tension within axons through the use of force-calibrated glass needles. The role of the cytoskeleton in producing or stabilizing the mechanical force will be determined through the used of anti-cytoskeletal drugs known to halt axonal elongation. The mechanical forces involved in experimentally induced elongation of axons will be achieved by towing the axons with an appropriately paced motor. This growth is a model for the elongation of axons within the expanding displacements of force- calibrated glass needles. Anti-cytoskeletal drugs will be used to assess the contribution of various cytoskeletal components. This work is most clearly relevant to obtaining a better understanding of the cellular and molecular basis of the development of the nervous system. The study has a broad scientific relevance because axonal elongation is a special case of the general function of the cytoskeleton to elaborate and maintain cell shape in eukaryotes. A better understanding of the relationship of cell shape, mechanical force and the cytoskeleton is particularly interesting because of recent evidence that cell shape plays a fundamental role in regulation of cell division, in the responsiveness of cells to signal molecules such as hormones and growth factors, and the loss of normal regulation in disease states.
