Metabolically-Incorporated Deuterium in Myelin Localized by Neutron Diffraction and Identified by Mass Spectrometry

Abstract

Myelin is a natural and dynamic multilamellar membrane structure that continues to be of significant biological and neurological interest, especially its biosynthesis and assembly in the context of its normal formation and renewal, and pathological breakdown. To explore further the usefulness of neutron diffraction in the structural analysis of myelin, we investigated the use of in vivo labeling by metabolically incorporating via drinking water nontoxic levels of deuterium (2H; D) into pregnant dams and their developing embryos. All of the mice were sacrificed when the pups were about 60 days old. Myelinated nerves were dissected, fixed in glutaraldehyde and examined by neutron diffraction. Parallel samples that were unfixed were frozen for mass spectrometry (MS). Analysis of the neutron diffraction patterns of the sciatic nerves from deuterium-fed mice (D mice) versus the controls (H mice) showed no appreciable differences in myelin periodicity, but major differences in the intensities of the Bragg peaks. The neutron scattering density profiles showed an appreciable increase in density at the center of the membrane bilayer in the D mice, particularly in the pups. MS analysis of the lipids isolated from the trigeminal nerves demonstrated that the level of D was greater in the pups compared to their mother: 97.6% +/- 2.0% (n=54; range 89.6% to 99.6%) versus 60.6% +/- 26.4% (n=27; range 11.4% to 97.3%). Deuteration in the mother also varied by lipid species, and among lipid subspecies. Three molecular species of phosphatidylcholine (PC), phosphatidylinositol (PI), and diglycerol (DG) were the most deuterated lipids, and sulfatide (SHexCer), one species of sphingomyelin (SM), and triacylglycerol (TG) were the least deuterated. The distribution pattern of deuterium in the D pups was always bell-shaped, and the average number of D atoms ranged from a low of ~4 in fatty acid (FA), to a high of ~9 in cerebroside (HexCer). By contrast, in D dam only about one third of the lipids had symmetric bell-shaped distributions; most had more complex, overlapping distributions that were weighted toward a lower average number of D atom labels. The average number of D atoms ranged from a low of ~3 to 4 in FA and in one species of SHexCer, to a high of 6 to 7 in HexCer and SM. In D pups, the consistently high level of deuteration can be attributed to their de novo lipogenesis during gestation and continuation of rapid myelination postnatally. In D dam, the widely varying levels of deuteration of lipids likely depends on the relative metabolic stability of the particular lipid species during myelin maintenance in the mature animal. Our current findings demonstrate that stably-incorporated D label can be detected and localized using neutron diffraction in a complex tissue such as myelin; and moreover, that MS can be used to screen a broad range of deuterated lipid species to monitor differential rates of lipid turnover. In addition to helping to develop a comprehensive understanding of the de novo synthesis and turnover of specific lipids in normal and abnormal myelin, our results also suggest application to myelin proteins, as well as more broadly to the molecular constituents of other biological tissues.