Blackmore Murray, Letourneau Paul C
Department of Neuroscience, University of Minnesota, Minneapolis, Minnesota 55455, USA.
Dev Neurobiol. 2007 Jun;67(7):976-86. doi: 10.1002/dneu.20395.
It is now well established that new proteins are synthesized in the distal segments of elongating axons, where they may play an essential role in some guidance decisions. It remains unclear, however, whether distal protein synthesis also plays an essential role in axon growth per se. Previous in vitro experiments have shown that blocking protein synthesis in distal axons has no effect on the rate of axonal advance. However, because these experiments were performed in vitro and over a relatively short time period, the role of distal protein synthesis over longer periods and in a native tissue environment remained untested. Here, we tested whether protein synthesis in distal axons plays an essential role in the elongation of descending axons in the embryonic spinal cord. We developed an in situ model of the brainstem-spinal projection of the embryonic chick, and developed a split-chamber method in which inhibitors of proteins synthesis could be applied independently to cell bodies in the brainstem or to distal axons in the spinal cord. When protein synthesis was blocked in distal axons, axon growth remained robust for 2 days, which is the length of the experiment. However, when protein synthesis was blocked only in the brainstem, axonal elongation in the spinal cord ceased within 6 h. These data showed that protein synthesis in the distal axon is not essential to continue the advance of axons. Rather, essential proteins are synthesized more proximally and then transported rapidly to the distal axon.
现在已经充分证实，新蛋白质是在正在延长的轴突的远端段合成的，在那里它们可能在某些导向决策中发挥重要作用。然而，远端蛋白质合成是否在轴突生长本身中也发挥重要作用仍不清楚。先前的体外实验表明，阻断远端轴突中的蛋白质合成对轴突前进的速度没有影响。然而，由于这些实验是在体外进行的，且时间相对较短，远端蛋白质合成在更长时期和天然组织环境中的作用仍未得到检验。在这里，我们测试了远端轴突中的蛋白质合成在胚胎脊髓中下行轴突的延长过程中是否发挥重要作用。我们建立了胚胎鸡脑干 - 脊髓投射的原位模型，并开发了一种分隔室方法，通过该方法可以将蛋白质合成抑制剂分别应用于脑干中的细胞体或脊髓中的远端轴突。当远端轴突中的蛋白质合成被阻断时，轴突生长在2天内（即实验时长）仍保持强劲。然而，当仅阻断脑干中的蛋白质合成时，脊髓中的轴突伸长在6小时内就停止了。这些数据表明，远端轴突中的蛋白质合成对于轴突的继续前进并非必不可少。相反，必需蛋白质是在更靠近近端的位置合成，然后迅速运输到远端轴突。
