Mechanical Force in the Cytoskeleton Underlying Neuronal Shape and Growth

神经元的功能依赖于它们极不寻常的形状。轴突是神经元的一根特别长的“电线”延伸，并传导神经系统中产生的电信号。在发育中的胚胎脊椎动物中，轴突伸长的很大一部分是由于生长中的胚胎膨胀对轴突施加的张力而发生的。先前对培养神经元的研究表明，运动生长锥施加的张力是轴突伸长的原因。细胞质的丝状蛋白（肌动蛋白）、微管和中间丝构成细胞的动态骨架结构，即细胞骨架，是轴突伸长能力的基础。在轴突伸长期间，机械张力有望调节细胞骨架。这一观点得到了热力学考虑的支持，即细丝的组装受到机械力的影响。本研究旨在探讨机械张力对培养的胚胎鸡神经元伸长和细胞骨架的影响。实验将通过使用力校准玻璃针直接测量轴突内的张力。细胞骨架在产生或稳定机械力中的作用将通过使用已知的阻止轴突伸长的抗细胞骨架药物来确定。实验诱导轴突伸长的机械力将通过适当的速度马达牵引轴突来实现。这种增长是在力校准玻璃针的扩展位移中轴突伸长的模型。抗细胞骨架药物将用于评估各种细胞骨架成分的贡献。这项工作显然与更好地理解神经系统发育的细胞和分子基础有关。该研究具有广泛的科学意义，因为轴突伸长是真核生物细胞骨架精心设计和维持细胞形状的一般功能的特殊情况。更好地理解细胞形状、机械力和细胞骨架之间的关系尤其有趣，因为最近有证据表明，细胞形状在细胞分裂的调节、细胞对激素和生长因子等信号分子的反应以及疾病状态下正常调节的丧失中起着重要作用。

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.