Biosynthesis and intra-axonal transport of proteins during neuronal regeneration

Intraocular injections of radiolabeled amino acids permitted the study of protein biosynthesis in goldfish retinal ganglion cells and the distribution of this material to the synapse via intra-axonal transport, during regeneration of the fish visual system. There is a 3-fold increase in amino acid incorporated within the ganglion cell layer compared to intact contralateral controls by 10 days after axotomy. Tubulin and actin are the major proteins synthesized and axonally transported. In addition, glycosylation of proteins is also increased, with the appearance of three fucosylated polypeptides in regenerating optic fibers not detected in controls. The peak of amino acid incorporation is a 5-fold increase which occurs about 30 days after cutting and which is paralleled by increased amounts of axonally transported proteins to the optic fibers. Growing axons reach the optic tectum 30 days after transection, carrying newly synthesized protein towards the site of synapse formation. A minor polypeptide of high molecular weight is synthesized by regenerating ganglion cells and is slowly transported axonally to the optic tectum. It can be detected only within regenerating neurons during later stages of growth. Amino acid incorporation and axonal transport return to near control levels after the 50th day, approximately when recovery of sight occurs. Overall, retinal ganglion cells show significant increases in the biosynthesis, in the intra-axonal transport, and in the glycosylation of proteins during regeneration. Major synthetic events are enhanced production of structural proteins, which persist throughout the regrowth process. Other synthetic events include the production of minor high molecular weight polypeptides which are not detected within the intact visual system.