Analysis of the apparent biphasic axonal transport kinetics of fucosylated glycoproteins

Abstract

Following intraocular injection of [3H]fucose, which labels many glycoproteins of retinal ganglion cells, the accumulation of transported radioactivity arriving at the superior colliculus (nerve terminals) peaks within a few hours and decays with a time course of hours. Then, over a period of several days, radioactivity again accumulates at the superior colliculus and then decays with a half-life of days. The second peak also represents fast transported material since it occurs almost simultaneously along the optic nerve and tract as well as at the nerve endings. Such data have been interpreted as evidence for both a group of rapidly released, rapidly transported glycoproteins (first peak) and a group of slowly released but rapidly transported glycoproteins (second peak). We investigated this supposition by studying in more detail the metabolism of some individual fucosylated proteins in both the retina and superior colliculus. We noted that much of the radioactivity incorporated in fucosylated glycoproteins at the retina was rapidly metabolized (with a turnover on the order of hours), while the remainder of the fucosylated moieties had a metabolic half-life on the order of days. This was also true of the metabolic behavior of several individual glycoproteins, selected for study because they are major components of the group committed to transport and accumulating in two waves at the superior colliculus. In other experiments we injected [35S]methionine intraocularly and examined the metabolism in the retina and the kinetics of transport to the superior colliculus of the peptide backbone of these same individual proteins. In contrast to the two waves of accumulation of radioactivity from [3H]fucose, accumulation of radioactivity of the peptide backbone of the same glycoproteins was monophasic. Our explanation of these data involves the presence of two types of fucose moieties on the peptides. One group of fucose moieties is labile and is lost from the peptide backbone over a period of hours. Other fucose moieties are approximately as metabolically stable as the peptide backbones to which they are attached. The actual peptide backbones of the glycoproteins are committed to rapid transport over a period of several days. Thus, the first (and most prominent) peak of transported radioactivity in [3H]fucosylated glycoproteins does not represent a discrete phase of transport but, rather, is the summation of kinetics of gradual arrival of proteins and the rapid drop in their specific radioactivity as the more labile moieties of [3H]fucose are lost.(ABSTRACT TRUNCATED AT 400 WORDS)