Axonal transport of phospholipids in rat visual system.

Abstract

Abstract— Seventeen day old rats were injected intraocularly with a phospholipid precursor, [32P]phosphate, and a glycoprotein precursor, [3H]fucose. Animals were killed between 1 h and 21 days later, and structures of the visual pathway (retina, optic nerve, optic tract, lateral geniculate body, and superior colliculus) were dissected. Radioactivity in phospholipids ([32P] in solvent‐extracted material) and in glycoproteins ([3H] in solvent‐extracted residue) was determined. Incorporation of [3H]fucose into retinal glycoproteins peaked at 6–8 h. Labelled glycoproteins were present in superior colliculus by 2h after injection, indicating a rapid rate of transport; maximal labelling was at 8–10 h after injection. Incorporation of [32P]phosphate into retinal phospholipids peaked at 1 day after injection. Phospholipids were also rapidly transported since label was present in the superior colliculus by 3 h after injection: however, maximal labelling did not occur until 5–6 days. These results indicate that newly synthesized phospholipids enter a preexisting pool, part of which is later committed to transport at a rapid rate.Transported phospholipids were catabolized at the nerve endings with a maximum half‐life of several days; there was minimal recycling of precursor label.Lipids were fractionated by thin‐layer chromatography, and radioactivity in individual phospholipid classes determined. Choline and ethanolamine phosphoglycerides were the major transported phospholipids, together accounting for approx 85% of the total transported lipid radioactivity. At early time points, the ratio of radioactivity in choline phosphoglycerides to that in ethanolamine phosphoglycerides increased in structures progressively removed from the site of synthesis (retina) but by 2 days approached a constant value. In each structure, choline phosphoglyceride‐ethanolamine phosphoglyceride radioactivity ratios decreased with time, rapidly at first, but plateaued by 2 days. These results indicate that choline phosphoglycerides are committed to transport sooner than ethanolamine phosphoglycerides.Some experiments were also conducted using [2‐3H]glycerol as a phospholipid precursor. Results concerning incorporation of this precursor into individual phospholipid classes and their subsequent axonal transport were comparable to those obtained using [32P]phosphate, with the following exceptions: (a) incorporation of [2‐3H]glycerol into retinal phospholipids was relatively rapid (near‐maximal levels at 1 h after injection) although transport to the superior colliculus showed an extended time course very similar to [32P]‐labelled lipids; (b) [2‐3H]glycerol was somewhat less efficient than [32P]phosphate in labelling lipids committed to transport relative to labelling those which remained in the retina; and (c) [2‐3H]glycerol did not label plasmalogens.