Zheng J Q, Wan J J, Poo M M
Department of Biological Sciences, Columbia University, New York, New York 10027, USA.
J Neurosci. 1996 Feb 1;16(3):1140-9. doi: 10.1523/JNEUROSCI.16-03-01140.1996.
Pathfinding of growing neurites depends on turning of the growth cone in response to extracellular cues. Motile filopodia of the growth cone are known to be critical for mediating contact-dependent guidance of the growth cone. However, whether filopodia also play an essential role in growth cone turning response induced by a diffusible chemotropic substance is unclear. Growth cones of cultured Xenopus spinal neurons exhibited chemotropic turning responses in a gradient of glutamate within a limited range of concentrations. This turning response depends on the activation of the NMDA subtype of glutamate receptors and requires the presence of extracellular Ca2+. Time-lapse differential interference contrast microscopy with quantitative analysis of filopodia dynamics showed a close correlation between an increased number of filopodia on the side of the growth cone facing the glutamate source and the turning. Such filopodia asymmetry was observed within minutes after the onset of the glutamate gradient, before any detectable turning of the growth cone. In Ca(2+)-free medium, no filopodia asymmetry was induced by the glutamate gradient, and no growth cone turning was observed. Furthermore, elimination of filopodia with a low concentration of cytochalasin B completely abolished the turning response without substantially affecting neurite extension. Thus, filopodia may be required for chemotropic guidance of the growth cone, and an asymmetry in filopodia distribution may be an early cellular event responsible for determining the direction the growth cone advances.
正在生长的神经突的路径寻找取决于生长锥响应细胞外信号的转向。已知生长锥的能动丝状伪足对于介导生长锥的接触依赖性导向至关重要。然而，丝状伪足是否在由可扩散的趋化物质诱导的生长锥转向反应中也发挥重要作用尚不清楚。培养的非洲爪蟾脊髓神经元的生长锥在有限浓度范围内的谷氨酸梯度中表现出趋化转向反应。这种转向反应取决于谷氨酸受体NMDA亚型的激活，并且需要细胞外Ca2+的存在。通过对丝状伪足动力学进行定量分析的延时微分干涉对比显微镜显示，生长锥朝向谷氨酸源一侧的丝状伪足数量增加与转向之间存在密切相关性。在谷氨酸梯度开始后的几分钟内，在生长锥出现任何可检测到的转向之前，就观察到了这种丝状伪足不对称性。在无Ca(2+)的培养基中，谷氨酸梯度不会诱导丝状伪足不对称，也未观察到生长锥转向。此外，用低浓度的细胞松弛素B消除丝状伪足完全消除了转向反应，而对神经突延伸没有实质性影响。因此，丝状伪足可能是生长锥趋化导向所必需的，丝状伪足分布的不对称可能是决定生长锥前进方向的早期细胞事件。
