Phospholipid metabolism in mouse sciatic nerve in vivo.

To probe the activities of various pathways of lipid metabolism in peripheral nerve, six phospholipid-directed precursors were individually injected into the exposed sciatic nerves of adult mice, and their incorporation into phospholipids and proteins was studied over a 2-week period. Tritiated choline, inositol, ethanolamine, serine, and glycerol were mainly used in phospholipid synthesis; in contrast, methyl-labeled methionine was primarily incorporated into protein. Phosphatidylcholine was the main lipid formed from tritiated choline, glycerol, and methionine precursors. Phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol were the main lipids formed from serine, ethanolamine, and inositol, respectively. With time there was a shift in label among phospholipids, with higher proportions of choline appearing in sphingomyelin, glycerol in phosphatidylserine, ethanolamine in phosphatidylethanolamine (plasmalogen), and inositol in polyphosphoinositides, especially phosphatidylinositol 4,5-bisphosphate. We suggest that the delay in formation of these phospholipids, which are concentrated in peripheral nerve myelin, may, at least in part, be due to their formation at a site(s) distant from the sites where the bulk of Schwann cell lipids are made. We propose that separating the synthesis of these myelin-destined lipids to near the Schwann cell's plasma membrane would facilitate their concentration in peripheral nerve myelin sheaths. At earlier labeling times, ethanolamine and glycerol were more actively incorporated into phosphatidylcholine and phosphatidylinositol, respectively, than later. The transient labeling of these phospholipids may reflect some unique role in peripheral nerve function.