Localization of Phospholipid Synthesis to Schwann Cells and Axons

Abstract

Quantitative electron microscopic autoradiography was used to detect and characterize endoneurial sites of lipid synthesis in mouse sciatic nerve. Six tritiated phospholipid precursors (choline, serine, methionine, inositol, glycerol, and ethanolamine) and a protein precursor (proline) were individually injected into exposed nerves and after 2 h the mice were perfused with buffered aldehyde. The labeled segments of nerve were prepared for autoradiography with procedures that selectively remove nonincorporated precursors and other aqueous metabolites, while preserving nerve lipids (and proteins). At both the light and electron microscope levels, the major site of phospholipid and protein synthesis was the crescent-shaped perinuclear cytoplasm of myelinating Schwann cells. Other internodal Schwann cell cytoplasm, including that in surface channels, Schmidt-Lanterman incisures, and paranodal regions, was less well labeled than the perinuclear region. Newly formed proteins were selectively located in the Schwann cell nucleus. Lipid and protein formation was also detected in unmyelinated fiber bundles and in endoneurial and perineurial cells. Tritiated inositol was selectively incorporated into phospholipids in both myelinated axons and unmyelinated fibers. Like inositol, glycerol incorporation appeared particularly active in unmyelinated fibers. Quantitative autoradiographic analyses substantiated the following points: myelinating Schwann cells dominate phospholipid and protein synthesis, myelinated axons selectively incorporate tritiated inositol, phospholipid precursors label myelin sheaths and myelinated axons better than proline.

Role of Lipoproteins in the Delivery of Lipids to Axons during Axonal Regeneration

“…Axonal Elongation-In previous studies using the compartment model for culture of neurons (25), we detected no cholesterol biosynthesis, as measured by incorporation of [1][2][3][4][5][6][7][8][9][10][11][12][13][14] C]acetic acid into cholesterol, in axons of rat sympathetic neurons. We have now shown that under normal culture conditions cholesterol is synthesized in the cell body-containing compartment and efficiently transported into the distal axons.…”

“…Incorporation of [1][2][3][4][5][6][7][8][9][10][11][12][13][14] C]Acetate into Cholesterol-Neurons were plated in the center compartment of compartmented dishes and cultured for 14 days. The radioactive cholesterol precursor [1-14 C]acetic acid (10 Ci/ml) was added to the cell body-containing compartment.…”

“…Fig. 2 shows that [1][2][3][4][5][6][7][8][9][10][11][12][13][14] C]acetate had been efficiently incorporated into cholesterol in the cell body-containing compartment by 24 h, and some radiolabeled cholesterol had been transported into the distal axons. After 3.5 days, the amount of radiolabeled cholesterol in the distal axons corresponded to approximately 50% of total neuronal [ 14 C]cholesterol present, and newly synthesized cholesterol had equilibrated throughout all compartments.…”

“…Synthesis of new myelin by Schwann cells in the regenerating peripheral nerve has been extensively studied (1)(2)(3)(4). An interesting model for regeneration of injured peripheral nerve, involving apolipoprotein E (apoE) 1 and the coordinated storage and redistribution of cholesterol, has been proposed (5) and supported by experimental evidence (6). After peripheral nerve injury, axon degeneration and myelin destruction proceed rapidly.…”

Fate of myelin lipids during degeneration and regeneration of peripheral nerve: an autoradiographic study

“…A 5 mm segment of the injected left sciatic nerve was excised from the region of the injection and immersion fixed for 24 hr in 4% glutaraldehyde containing 0.2% digitonin (to prevent the extraction of cholesterol in subsequent steps) (Napolitano and Scallen, 1969;Scallen and Dietert, 1969). Nerve segments were then postfixed in Dalton's chrome osmium for 1 hr, dehydrated rapidly in a series of ice-cold graded acetone solutions to maximize retention of lipids (Gould et al, 1987), and embedded in Spurr's resin. Liquid scintillation spectroscopy of acetone solutions revealed that less than 10% of the incorporated label was extracted by these procedures.…”