Axon-myelin transfer of phospholipids and phospholipid precursors. Labeling of myelin phosphoinositides through axonal transport.
Previous studies have provided evidence for axon-to-myelin transfer of intact lipids and lipid precursors for reutilization by myelin enzymes. Several of the lipid constituents of myelin showed significant contralateral/ipsilateral ratios of incorporated radioactivity, indicative of axonal origin, whereas proteins and certain other lipids did not participate in this transfer-reutilization process. The present study will examine the labeling of myelin phosphoinositides by this pathway. Both 32PO4 and [3H]inositol were injected monocularly into 7-9-wk-old rabbits and myelin was isolated 7 or 21 days later from pooled optic tracts and superior colliculi. In total lipids 32P counts of the isolated myelin samples showed significant contralateral/ipsilateral ratios as well as increasing magnitude of contralateral-ipsilateral differences during the time interval. Thin-layer chromatographic isolation of the myelin phosphoinositides revealed significant 32P-labeling of these species, with PIP and PIP2 showing time-related increases. This resembled the labeling pattern of the major phospholipids from rabbit optic system myelin in a previous study and suggested incorporation of axon-derived phosphate by myelin-associated enzymes. The 32P label in PI, on the other hand, remained constant between 7 and 21 days, suggesting transfer of intact lipid. This was supported by the labeling pattern with [3H]inositol, which also showed no increase over time for PI. These results suggest axon-myelin transfer of intact PI followed by myelin-localized incorporation of axon-derived phosphate groups into PIP and PIP2. The general topic of axon-myelin transfer of phospholipids and phospholipid precursors is reviewed.
D000818:Animals; D001370:Axonal Transport; D001369:Axons / Q000502:physiology*; D007294:Inositol / Q000378:metabolism; D008959:Models, Neurological; D009186:Myelin Sheath / Q000502:physiology*; D010710:Phosphates / Q000378:metabolism; D010716:Phosphatidylinositols / Q000378:metabolism*; D010743:Phospholipids / Q000378:metabolism*; D010761:Phosphorus Radioisotopes; D011817:Rabbits; D013477:Superior Colliculi / Q000502:physiology*; D014316:Tritium; D014795:Visual Pathways / Q000502:physiology*
D010710:Phosphates; D010716:Phosphatidylinositols; D010743:Phospholipids; D010761:Phosphorus Radioisotopes; D014316:Tritium; D007294:Inositol