A defect of sphingolipid metabolism modifies the properties of normal appearing white matter in multiple sclerosis

Maintaining the appropriate complement and content of lipids in cellular membranes is critical for normal neural function. Accumulating evidence suggests that even subtle perturbations in the lipid content of neurons and myelin can disrupt their function and may contribute to myelin and axonal degradation. In this study, we determined the composition and quantified the content of lipids and sterols in normal appearing white matter (NAWM) and normal appearing grey matter (NAGM) from control and multiple sclerosis brain tissues by electrospray ionization tandem mass spectrometry. Our results suggest that in active-multiple sclerosis, there is a shift in the lipid composition of NAWM and NAGM to a higher phospholipid and lower sphingolipid content. We found that this disturbance in lipid composition was reduced in NAGM but not in NAWM of inactive-multiple sclerosis. The pattern of disturbance in lipid composition suggests a metabolic defect that causes sphingolipids to be shuttled to phospholipid production. Modelling the biophysical consequence of this change in lipid composition of NAWM indicated an increase in the repulsive force between opposing bilayers that could explain decompaction and disruption of myelin structure

Glycans of myelin proteins.

P0 is a myelin glycoprotein of the peripheral nervous system (PNS). It can bind as many as 6 different glycans, all at its one and unique glycosylation site, Asn93.This is in contrast to other myelin glycoproteins which have also one glycosylation site: PMP22 at Asn36, MOG at Asn31, but can bind only one glycan type containing usually HNK-1 epitope. The MAG has 10 glycosylation sites, and glycoprotein OMpg has 11 glycosylation sites, some of them contain HNK-1 epitope. The HNK-1 epitope is involved in cell interactions that controls cell type-specific neurite outgrowth, regeneration, myelin stability and is target for autoimmune IgM antibodies in human demyelinating neuropathies of the PNS. Beside myelin P0 glycoprotein, no data are available on other myelin glycoproteins. In this paper we review and analyze all published data on the chemical structure of glycans containing HNK-1 epitopes or sialic acid of bovine P0, PMP22, MOG and MAG. For bovine P0, the MW of glycan is in the range of 1294.56(GP3) Da - 2279.94(GP5) Da. The pI of glycosylated P0 protein varies from pH 9.32-9.46. The most charged is a glycan MS2 containing 3 sulfate groups and one glucuronic acid; the less charged is glycan BA2. All glycans contain 1 fucose and 1 galactose. The most mannose rich are MS2 and GP4, each has 4 mannoses; OPPE1 contains 5 N-acetylglucosamines and 1 sulfated glucuronic acid, GP4 contains 1 sialic acid

Tetrameric assembly of full-sequence protein zero myelin glycoprotein by synchrotron x-ray scattering.

Highly purified myelin P0 glycoprotein was solubilized to 1-8 mg/ml in 0.1% sodium dodecyl sulfate (SDS), and the solution structure of the P0 assembly was studied using synchrotron x-ray scattering. The full-length P0, which was isolated from bovine intradural roots, included both the extracellular and cytoplasmic domains of the molecule. At the higher concentrations (4, 6, and 8 mg/ml, respectively), an x-ray intensity maximum was observed at 316 A, 245 A, and 240 A Bragg spacing. Because the position of this intensity depended on P0 concentration, it is most likely due to interparticle interference. By contrast, the position of a second intensity maximum, which was at approximately 40 A Bragg spacing, was invariant with P0 concentration. This latter intensity was accounted for by monodispersed, 80 A-diameter particles that are composed of eight, approximately 30 A-diameter spheres. Chemical parameters suggest that the 80 A particles correspond to the size of a tetramer of P0 molecules. Therefore, the approximately 30 A spheres would correspond to the sizes of the extracellular and cytoplasmic domains for each of the P0 monomers. The invariance of the second intensity maximum with P0 concentration indicates that the structure of the 80 A-diameter, tetrameric particles is unaltered. According to the liquid model for interparticle interference from charged spheres, the 80 A-diameter particle has 10 negative surface charges which likely arise from negatively charged SDS molecules bound to the transmembrane domain of P0. This binding, however, apparently does not alter the tetrameric assembly of P0, suggesting that intermolecular interactions involving extracellular domains and cytoplasmic domains likely stabilize this assembly. Some of our results have been published in abstract form (Inouye, H., H. Tsuruta, D. A. Kirschner, J. Sedzik, and K. Uyemura. Abstracts of the 4th International School and Symposium on Synchrotron Radiation in Natural Science, June 15-20, 1998. Ustron-Jaszowiec, Poland. p. 31)