Macromolecular transfer from glia to the axon.

Axons do not contain polysomes and therefore are unable to synthesize proteins. On the other hand, the nerve cell body has a well-developed protein synthesizing capacity, and proteins are conveyed into the axon from the cell body by orthograde axonal transport. Studies on vertebrate neurones demonstrate that the proteins conveyed from the cell body constitute all of the major structures of the axon including: membranous vesicles, the endoplasmic reticulum, mitochondria, and the cytoskeleton which consists of microtubules, neurofilaments, and microfilaments. Although the nerve cell body is the major source of the macromolecules which provide the structural integrity of the axon, in some axons the glial cells surrounding the axon also supply proteins to the axon. The transfer of newly synthesized proteins from glial cells to the axon is documented most completely in the case of the squid giant axon. The transfer of proteins from glial cells to the neurone can be studied in the giant axon by incubating axons, which have been disconnected from their nerve cell bodies, in a solution which contains labelled amino acids. The glial cells transfer as much as 40 % of their newly synthesized proteins to the giant axon. These transferred proteins can be studied in axoplasm which is separated from the giant axon by extrusion. What are the comparative roles of proteins supplied to the axon by the glial cells and the nerve cell body? To begin to answer this question, we have compared the glial-transfer proteins with the proteins which constitute whole axoplasm. The proteins were analysed by two-dimensional polyacrylamide gel electrophoresis, and axoplasmic proteins were detected by staining the gels with Coomassie blue, whereas the labelled proteins were detected by fluorography of the same gels. The glial-transfer proteins differ substantially from the stained axoplasmic proteins. For example, neurofilament proteins and tubulin are major proteins of the axoplasm but are not represented among the labelled proteins which are transferred from the glial cells. In fact, these proteins are not synthesized to any significant degree by the glial cells. One of the glial-transfer proteins (molecular weight 70000) stands out because it is heavily labelled and is well resolved on the gels. This protein which we have named traversin is present among the stained proteins of whole axoplasm but is a relatively minor component of the axoplasm. In order to determine whether traversin is supplied to the axon by the nerve cell body we analyzed the proteins synthesized in the stellate ganglion which contains the nerve cell bodies of the giant axon. The pattern of proteins synthesized by the stellate ganglion was similar to that of whole axoplasm,