Axonal Transport of Choline Lipids in Normal and Regenerating Rat Sciatic Nerve

Abstract

Biochemical methods were used to study the time course of transport of choline phospholipids (labeled by the injection of [3H]choline into the ventral horn of the lumbar spinal cord) in rat sciatic nerve. Autoradiographic methods were used to localize the transported lipid within motor axons. Transported phospholipid, primarily phosphatidylcholine, present in the nerve at 6 h, continued to accumulate over the following 12 days. No discrete waves of transported lipid were observed (a small wave of radioactive phospholipid moving at the high rate would have been missed); the amounts of radioactive lipid increased uniformly along the entire sciatic nerve. In light‐microscope autoradiographs, a class of large‐caliber axons, presumably motor axons, retained the labeled lipid. Some lipid, even at 6 h, was seen within the myelin sheaths. Later, the labeling of the myelin relative to axon increased. The continued accumulation of choline phospholipids in the axons probably signifies their prolonged release from cell bodies and their retention in various axonal membranes, including the axolemma. The build‐up of these phospholipids in myelin probably represents their transfer from the axons to the myelin sheaths surrounding them. When nerves are crushed and allowed to regenerate for 6 or 12 days, choline phospholipids transported during these times enter the regenerating nerve. In light and electron microscope autoradiographs, transported lipid was seen to be localized primarily in the regenerating axons. However, grains overlay the adjacent Schwann cell cytoplasm, indicating transported lipids were transferred from the regenerating axons to the associated Schwann cells. In addition, some cells not associated with growing axons were labeled, suggesting that phosphatidylcholine and possibly acetylcholine, carried to the regenerating axons by axonal transport, were actively metabolized in the terminal, with released choline label being used by other cells. These results demonstrate that axonal transport supplies mature and growing axons and their glial cells with choline phospholipids.

Removal of retrogradely transported material from rat lumbosacral alpha-motor axons by paranodal axon-schwann cell networks

“…Spencer and Thomas, 1974). Earlier studies have presented evidence that small molecules (, 1 kDa) of neuronal origin, such as phospholipids Gould et al, 1982;Ledeen et al, 1992;Toews et al, 1988), and nucleosides and polyamines Lindquist et al, 1985), can be transferred from the axon to the myelinsupporting glial cells under physiological conditions. Some types of invertebrate periaxonal glia also seem to have a rapid access to a variety of larger tracer molecules after their introduction into the neuron (Goldstein et al, 1982;Grossfeld, 1991;Grossfeld et al, 1988;Lieberman et al, 1994;Viancour et al, 1981).…”

Phosphatidate phosphohydrolase in purified rat brain myelin

“…Although the cytidine ("Kennedy") pathway is localized primarily in the endoplasmic reticulum (Wilgram and Kennedy, 1963), recent studies have demonstrated the presence in myelin of virtually all the enzymes needed to convert diacylglycerol to phosphatidylcholine and phosphatidylethanolamine (see Introduction). The existence of such enzymes in myelin had been suggested by earlier studies which indicated that certain substrates are transferred from the axon to myelin where they are incorporated in situ into specific lipids (Droz et al, 1978(Droz et al, , 1981Haley and Ledeen, 1979;Toews and Morell, 1981;Alberghina et al, 1982;Gould et al, 1982;Ledeen and Haley, 1983). The process was selective since other myelin lipids (e.g., gangliosides, sulfatides) appeared not to be labeled by this route (Haley and Ledeen, 1979), and the corresponding synthetic enzymes could not be detected (Wu and Ledeen, 1980).…”

Ethanolamine Kinase Activity in Purified Myelin of Rat Brain

“…A growing number of lipid-metabolizing enzymes, including several involved in phospholipid biosynthesis, have been shown to occur in purified CNS myelin (for review, see Suzuki, 1980;Norton, 1981; Norton and Cammer, 1984;Ledeen, 1984). The existence of such enzymes in myelin had been suggested by several studies (Droz et al, 1978, 198 1;Haley and Ledeen, 1979;Toews and Morell, 1981;Alberghina et al, 1982;Gould et al, 1982;Ledeen and Haley, 1983) indicating transfer of various substrates from the axon to myelin, these substances then becoming enzymatically incorporated into myelin lipids. We have previously shown myelin to possess CDP-ethanolamine: 1,2-diacyl-sn-glycerol ethanolaminephosphotransferase (EC 2.7.8.1) (Wu and Ledeen, 1980) and CTP:phosphoethanolamine cytidylyltransferase (EC 2.7.7.14) (Kunishita and Ledeen, 1984), the two enzymes that complete the synthesis of phosphatidylethanolamine.…”