A comparison of composition and fluidity of multiple sclerosis and normal myelin

Oligodendroglial membrane dynamics in relation to myelin biogenesis

In the central nervous system, oligodendrocytes synthesize a specialized membrane, the myelin membrane, which enwraps the axons in a multilamellar fashion to provide fast action potential conduction and to ensure axonal integrity. When compared to other membranes, the composition of myelin membranes is unique with its relatively high lipid to protein ratio. Their biogenesis is quite complex and requires a tight regulation of sequential events, which are deregulated in demyelinating diseases such as multiple sclerosis. To devise strategies for remedying such defects, it is crucial to understand molecular mechanisms that underlie myelin assembly and dynamics, including the ability of specific lipids to organize proteins and/or mediate protein-protein interactions in healthy versus diseased myelin membranes. The tight regulation of myelin membrane formation has been widely investigated with classical biochemical and cell biological techniques, both in vitro and in vivo. However, our knowledge about myelin membrane dynamics, such as membrane fluidity in conjunction with the movement/diffusion of proteins and lipids in the membrane and the specificity and role of distinct lipid-protein and protein-protein interactions, is limited. Here, we provide an overview of recent findings about the myelin structure in terms of myelin lipids, proteins and membrane microdomains. To give insight into myelin membrane dynamics, we will particularly highlight the application of model membranes and advanced biophysical techniques, i.e., approaches which clearly provide an added value to insight obtained by classical biochemical techniques.

Lipidome and proteome map of myelin membranes

To understand the molecular anatomy of myelin membranes, we performed a large-scale, liquid chromatography-coupled tandem mass spectrometry (LC-MS/MS)-based lipidome and proteome screen on freshly purified human and murine myelin fractions. We identified more than 700 lipid moieties and above 1,000 proteins in the two species, including 284 common lipids and 257 common proteins. This study establishes the first comprehensive map of myelin membrane components in human and mice. Although this study demonstrates many similarities between human and murine myelin, several components have been identified exclusively in each species. Future quantitative validation studies focused on interspecies differences will authenticate the myelin membrane anatomy. The combined lipidome and proteome map presented here can nevertheless be used as a reference library for myelin health and disease.

Membrane saturated fatty acids and disease progression in multiple sclerosis patients

The risk of developing multiple sclerosis is associated with increased dietary intake of saturated fatty acids. We determined the fatty acid composition within the different phospholipid fractions of red blood and peripheral blood mononuclear cell membranes of 31 patients diagnosed with multiple sclerosis and 30 healthy control subjects using gas chromatography. Individual saturated fatty acids were correlated with the severity of neurological outcome as measured by the Kurtzke Expanded Disability Status Scale. Significant increases were found in multiple sclerosis peripheral blood mononuclear cell membrane sphingomyelin C14:0 and phosphatidylinositol C22:0. In the peripheral blood mononuclear cell membranes, C22:0 and C24:0 showed positive correlations, while C14:0, C16:0 and C20:0 showed inverse correlations with the Functional System Scores. In conclusion, this study is in accordance with previous studies that have shown an increase in shorter long-chain SATS in MS patients. In addition, this study also showed that higher C14:0 and C16:0 reflected better disease outcome as demonstrated by the inverse correlation with the EDSS and FSS. We have also characterized the specific SATS, that is, long-chain SATS that may increase the risk of developing MS.

Red blood cell membrane fluidity in the etiology of multiple sclerosis

Organisms adjust the order, or fluidity, of their cellular membranes in response to changes in their physiochemical environment by adjusting the lipid composition of their membranes. We investigated membrane fluidity using the phospholipid, fatty acid and cholesterol content of red blood cells (RBCs) from multiple sclerosis (MS) patients and correlated this with C-reactive protein (CRP) as well as with the severity of neurological outcome as measured by the Kurtzke Expanded Disability Status Scale (EDSS) and its Functional System Scores. The study group consisted of 31 patients with MS and 30 healthy control subjects. Phospholipids were determined using a colorimetric assay, fatty acids by gas chromatography, cholesterol by an enzymatic assay and CRP by a Beckman nephelometer. Cell membrane fluidity was calculated according to previously established formulae. RBC membrane fluidity as measured by the saturated to polyunsaturated fatty acid ratio was higher in patients than in controls (P = 0.04). The phosphatidylethanolamine saturated to polyunsaturated fatty acid ratio showed highly significant positive correlations with the EDSS and CRP < 5 microg/ml. CRP showed significant inverse correlations with the saturated nature but positive correlations with the ordered-crystalline-phase to liquid-crystalline-phase lipid ratio. In this study we show that membrane fluidity as measured by the relationship between membrane fatty acids, phospholipids and cholesterol is closely interrelated with inflammation and disease outcome in patients with MS. In conclusion, our findings suggest that the membrane lipid composition of patients with MS and, consequently, membrane fluidity are altered, which seems to be influenced by the inflammatory status.

Peptidylarginine deiminase: a candidate factor in demyelinating disease

In earlier studies we demonstrated that an increase in the relative amounts of citrullinated myelin basic protein (MBP) was found in multiple sclerosis (Moscarello et al. 1994). To determine the temporal relationship between the citrullinated MBP and peptidylarginine deiminase (PAD), the enzyme responsible for deiminating arginyl residues in proteins, we studied enzyme activity, enzyme protein, PAD mRNA in a spontaneously demyelinating transgenic mouse model and we correlated the amount of PAD with citrullinated MBP. Both PAD protein as measured in an immunoslot blot method and PAD RNA were elevated. In fractionation studies we showed that the increase in PAD enzyme was due to an increase in the PAD found in membrane fractions and not the soluble PAD (PADII). From our data we concluded that up-regulation of myelin-associated PAD was responsible for the increase in citrullinated MBP in our transgenic mice prior to onset of clinical or pathological signs of demyelination. We postulate that a similar mechanism may be responsible for the increase in citrullinated MBP in multiple sclerosis.

Buoyant density and lipid composition of purified myelin of aging human brain

Purified myelin of human brain from 15 young adult (below 50 years of age) and old (above 70 years of age) autopsy cases each was examined by isopycnic centrifugation in continuous sucrose gradients, and for lipid composition. The mean buoyant density of myelin was the same in both groups. Apparent features of old age were a wide range of density values, less compact myelin bands, and the dissociation of myelin into two bands in six of 15 old cases. Lipid analyses of randomly selected myelin samples of each group revealed an inverse relationship between the total lipid to protein ratio and density of myelin. In old age total lipids decreased by an average 10 mol lipid per mol protein. This decrease was accounted for by cholesterol, phosphatidylserine and cerebrosides. Changes in fatty acid moieties mainly affected sphingolipids. C20:0 and C24:0 of sphingomyelin increased, as did even more markedly the more hydrophilic OH-fatty acids of cerebrosides. Correlations with buoyant density existed for the ratios of cholesterol to protein in young adult cases, and those of galactolipids to protein in old cases. The results suggest that old age is associated with impaired stability and altered lipid composition of myelin.

A calorimetric study of the thermotropic behaviour of mixtures of brain cerebrosides with other brain lipids

We have used a computer-controlled differential scanning calorimeter to determine the phases present in mixtures of the brain galactocerebrosides with other representative brain lipids. There are two types of brain galactocerebroside, those which possess an alpha-hydroxy substituent on the acyl chain (HFA) and those that do not (NFA). In the liquid crystalline state both cerebrosides were miscible with all the lipids studied, but in the gel state they were immiscible with cholesterol and the brain phosphatidylcholines. However, cholesterol mixtures in which the cholesterol mole fraction exceeded one third formed homogeneous metastable gel states on cooling from above the melting point of the cerebroside. Relaxation to the stable two phase state took place slowly over several hours. The solubilities of the galactocerebrosides in the other main brain sphingolipid, sphingomyelin, were much higher. Only in the case of the NFA galactocerebroside and at low mole fractions of sphingomyelin was immiscibility detected. Ternary mixtures of the two cerebrosides with sphingomyelin/cholesterol and phosphatidylcholine/cholesterol (PC/Chol) showed different miscibility characteristics. On cooling from 80 degrees C all mixtures formed homogeneous gel states. However, on standing the cerebrosides separated into discrete gel phases in all mixtures but one, that in which HFA galactocerebrosides were mixed with sphingomyelin and cholesterol. The cerebroside in the mixture with the composition closest to that of myelin, HFA/PC/Chol, melted at 38 degrees C. On scanning guinea pig CNS myelin which had been equilibrated at 5 degrees C a transition was detected with Tmax 33 degrees C. On the basis of comparison with the HFA/PC/Chol mixture we propose that the transition in myelin at this temperature is due to the melting of a galactocerebroside gel phase.

The properties of brain galactocerebroside monolayers

Using a Langmuir film balance we have compared the properties of films of the brain galactocerebrosides at 37 degrees C. There are two types of cerebroside in brain, those with an alpha-hydroxy substituent on the acyl chain (HFA) and those without (NFA). At equivalent pressures the areas of both cerebroside films are significantly less than the areas of films of the brain glycerolipids, the choline and ethanolamine phosphatides. The isotherm of NFA galactocerebrosides has two discontinuities, one at low and one at high film pressure, while the isotherm of HFA galactocerebrosides is a smooth curve at all film pressures. Below the high-pressure transition the area of the NFA film is significantly larger than the area of the HFA film. When compressed beyond the high-pressure transition there is a marked hysteresis between compression and expansion isotherms of the NFA galactocerebrosides. The pressures of both films continue to rise steeply when they are compressed into areas which are too small for them to exist as simple monolayers. We conclude that under compression cerebroside films form bilayer structures; that bilayer formation starts at low pressure and occurs progressively as the HFA cerebroside monolayer is compressed, but occurs more abruptly in the NFA cerebroside monolayer at the high-pressure-transition region of the isotherm. A study of pure cerebrosides with a single defined acyl chain shows that there is a correlation between the relative volumes of the hydrophobic and hydrophilic parts of the molecule and the ease of bilayer formation. The larger the relative volume of the hydrophilic group the more readily the cerebroside forms a bilayer film. Other brain lipids added to cerebroside monolayers have sharply differing effects on their areas. The areas of films containing cholesterol are less than the areas calculated by adding the areas of the pure components multiplied by their mole fractions. On the other hand, the area of phosphatidylcholine-containing films is much larger than calculated.

Dietary supplementation of undernourished rats with soy or safflower oil: effects on myelin polyunsaturated fatty acids

Undernourished suckling rats were administered, by gastric intubation, either soy oil (which is rich in both linoleic and linolenic acids) or safflower oil (which is rich in linoleic acid but deficient in linolenic acid) to determine (1) if dietary supplementation would offset the hypomyelination characteristic of the undernourished, developing brain and (2) to compare myelin fatty acids in normal, undernourished, and oil-supplemented rats. Myelin recovery was not increased by supplementation with either oil. The proportions of C22:4 and C22:6 fatty acids were reduced in myelin of the undernourished rats. Undernourished rats supplemented with either soy or safflower oil had higher than normal proportions of polyunsaturated fatty acids (C20:4 and C22:6). The triene-tetraene ratio in the oil-supplemented rats was lower than in normal controls, indicating that the oil-supplemented rats were not deficient in essential fatty acids. No significant differences were observed between the oil-supplemented groups.

[Perspectives in the use of NMR for the biochemical study of the white matter of the brain]

This paper presents a survey of the function and the biochemical composition of the myelin sheath. Proton nuclear magnetic resonance spectroscopy of extracts of human white matter tissue is a major method for the identification of myelin constituants. We present results of some experiments obtained with normal brain material at 80 MHz and 400 MHz.

Effect of proteolipid protein on central nervous system myelin membrane fluidity

The effect which intrinsic (proteolipid) protein has on fluidity of central nervous system myelin membrane was measured through differences in temperature-dependent anisotropy of the lipid-soluble fluorescence probe, 1,6-diphenyl-1,3,5-hexatriene (DPH), in multilamellar vesicles (MLV) prepared from total myelin lipids in the presence and absence of proteolipid protein. Very little difference was observed in the anisotropies of DPH incorporated into intact myelin membrane vesicles compared with MLV reconstituted from total myelin lipid plus proteolipid protein but excluding myelin basic protein. In contrast, a significant decrease (P less than 0.01) in anisotropy was observed when MLV prepared from total myelin lipids depleted of proteolipid protein were compared with vesicles containing proteolipid protein. Given the different distributions of myelin basic protein and proteolipid protein suggested by freeze-fracture, neutron and X-ray diffraction studies, and the fact that the hydrophobic DPH probe is known to distribute in the non-polar regions of lipid bilayers, we interpret the marked decrease in anisotropy when proteolipid protein is excluded from MLV to suggest that at least part of the proteolipid is distributed in the hydrocarbon region of the MLV. These findings are consistent with the earlier physical studies and recent postulations that extensive hydrophobic segments exist in proteolipid protein and that these hydrophobic segments are buried in the myelin lipid bilayer and alternate with hydrophilic extra-membrane segments.

Size and surface charge properties of myelin vesicles from normal and diseased (multiple sclerosis) brain

Differences have been observed between myelin vesicles prepared from normal human central nervous system and from white matter of patients who died with multiple sclerosis (MS). The mean cross-sectional area of the vesicles was 5.69 +/- 0.17 micron 2 from normal myelin and 3.71 +/- 0.28 micron 2 for diseased myelin. Vesicle size was reduced to 4.08 +/- 0.21 micron 2 when normal myelin vesicles were prepared in the presence of 0.1 mM EDTA. The presence of Ca2+ during the preparation of the vesicles had no effect on the mean cross-sectional area. In the case of MS myelin vesicles, 0.1 mM EDTA had no effect on vesicle size, whereas the presence of Ca2+ increased the vesicle size from 3.71 +/- 0.28 to 5.40 +/- 0.31 micron 2. Electrokinetic analysis revealed that the electrophoretic mobility of normal myelin vesicles was -5.169 +/- 0.193 X 10(-8) compared with -6.093 +/- 0.202 X 10(-8) m2 s-1 V-1 for the MS myelin vesicles. The presence of 0.1 mM EDTA increased the electrophoretic mobility of the normal vesicles to -6.483 +/- 0.151 X 10(-8) m2 s-1 V-1 but did not significantly affect that of the MS vesicles. Addition of 0.1 mM Ca2+ decreased the electrophoretic mobility of both normal and MS vesicles to similar mobilities. From these data, the surface charge densities were calculated for both normal and MS myelin vesicles and found to be -2.93 and -5.39 mV m-1, respectively. The phase transition temperature determined by wide-angle x-ray diffraction studies was 63 degrees C for normal myelin vesicles and 43 degrees C for MS myelin vesicles.(ABSTRACT TRUNCATED AT 250 WORDS)

Alteration of lipid-phase behavior in multiple sclerosis myelin revealed by wide-angle x-ray diffraction

Wide-angle x-ray diffraction studies revealed that the lipid-phase transition temperature of multiple sclerosis (MS) myelin was about 20 degrees C lower than that of normal myelin, indicating differences in the physical organization of the bilayer. The transition temperature of liposomes prepared from total lipid extracts of normal myelin was 12 degrees C lower than that for corresponding intact myelin, demonstrating that the protein of normal myelin had a substantial ordering effect on the lipid bilayer. The transition temperature for liposomes of MS myelin lipid was essentially similar to that for isolated MS myelin. Because the protein/phospholipid ratio was higher in MS myelin, and no difference in degree of fatty acid saturation was observed, the inability of MS myelin protein to organize the lipid reflects a qualitative difference in the proteins.