Ultrastructural localization of slow retrograde axonal transport: an autoradiographic study

We used [3H]-N-succinimidylpropionate ([3H]-N-SP) to covalently label endogenous intra-axonal proteins within the nerve in order to study their bidirectional transport. At the time of injection virtually all of the labeled proteins are found at the injection site. At later times specific patterns of labeled proteins are found within the nerve both proximal to and distal from the injection site, as a result of retrograde and anterograde axonal transport, respectively. We undertook the current study to determine the ultrastructural distribution of the [3H]-N-SP-labeled transported proteins in the nerve. One microliter of [3H]-N-SP was injected subepineurially in sciatic nerve, and 5 days later the nerves were processed either for light and electron microscopic autoradiography or for gel electrophoresis and fluorography. At the injection site the labeled proteins are predominantly myelin proteins. Distally, a pattern similar to that described for slow anterograde transport is seen. Proximal to the injection site a constellation dominated by the 68-kilodalton protein is seen. Light microscopic autoradiography shows diffuse labeling both in axons and in myelin at the injection site, with predominant axonal labeling distant from the injection site. Electron microscopic autoradiography of segments distal to the injection site show silver grains which are distributed within the axoplasm without apparent relationship to organelles. In contrast, segments proximal to the injection site show silver grains which seem closely related to membrane-bound organelles, predominantly mitochondria. These results suggest that slow retrograde transport has a unique subcellular distribution that is distinct from that of slow anterograde transport.