Protein synthesis in distal axons is not required for axon growth in the embryonic spinal cord

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.