Diversity in the axonal transport of structural proteins: major differences between optic and spinal axons in the rat

Open Access
Publisher Website
Google Scholar
Abstract
Investigations of slow axonal transport reveal variation in both protein composition and the rate of movement. However, these studies involve a variety of nerve preparations in different species, and most lack the resolution needed to determine the kinetics of identified proteins. We have compared the axonal transport of slow-transported proteins in retinal ganglion cells and spinal motor neurons of young rats. Nine proteins that contribute to axonal structures were examined: the neurofilament triplet (NFT), alpha and beta tubulin, actin, fodrin, calmodulin, and clathrin. Axonally transported proteins were pulse- labeled by intraocular or intracord injections of 35S-methionine. After allowing sufficient time for labeled slow-component proteins to enter the spinal or optic nerves, consecutive 2–3 mm nerve segments were subjected to SDS-PAGE. Fluorographs were used as templates for locating the gel regions containing the above polypeptides, and the radioactivity in these regions was measured by liquid-scintillation spectrometry. In retinal ganglion cells, the peak of tubulin labeling advanced at 0.36 mm/d in association with the NFT and fodrin. The cotransport of tubulin and the NFT identified this complex as the slower subcomponent of slow transport, termed slow component a (SCa) and representing the movement of the microtubule-neurofilament network. The peaks of actin and calmodulin labeling were cotransported at 2.3 mm/d in near-register with peaks of fodrin and clathrin labeling. These 4 proteins, moving ahead of the NFT, identified this complex as SCb-- the faster subcomponent of slow transport, which represents the movement of the cytoplasmic matrix and microtrabecular lattice. Both subcomponents had the same composition and rate as that reported for the optic axons of guinea pigs and rabbits, establishing a basic mammalian pattern. In spinal motor axons, the SCa tubulin peak advanced at 1.3 mm/d, and the SCb actin and calmodulin peaks were cotransported at 3.1 mm/d. Unlike optic axons, SCa in motor axons was more heavily labeled than SCb, and included labeled peaks of actin, clathrin, and calmodulin moving in register with the SCa tubulin peak. Actin was the most heavily labeled of these SCb proteins moving with SCa, and it left a higher plateau of radioactivity behind the advancing SCa peak. The SDS-PAGE labeling pattern for SCb did not differ from that seen in optic axons, except that some tubulin was found to form a peak that advanced in register with the actin and calmodulin peaks.(ABSTRACT TRUNCATED AT 400 WORDS)