Membrane fluidity and fatty acids in multiple sclerosis patients

Abstract

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS), which leads to neuronal demyelination, and eventually to oligodendrocyte and axon loss, with subsequent lesion formation. The wide distribution of lesions in the CNS results in a variety of clinical features, such as cognitive impairment, vertigo, spasticity, ataxia tremors, progressive quadriparesis, pain and depression. Currently no cure exists for CNS disorders, resulting in a decline in quality of life, and an economic burden on society. Metabolic disturbances, especially lipid metabolic abnormalities, have been implicated in the development of MS. Although the disease cannot be cured, disease-modifiers, such as interferon beta, glatiramer acetate and mitoxantrone, as well as fatty acid supplementatlon have been used to delay the progression of the disease. Membrane fatty acids are precursors for mediators of inflammation, the eicosanoids, which are produced soon after stimulation and which regulate a number of inflammatory effects, such as the induction of fever, vasodilation and production of macrophage- and Iymphocyte-derived cytokines. Eicosanoids, in contrast to their fatty acid precursors, have a short half-life and are therefore difficult to measure. The objective in the present study was to determine the role of fatty acids from South African MS patients, by measuring the fatty acid composition of phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI) and sphingomyelin (SM) phospholipids in the plasma, red blood cell (RBC) and peripheral blood mononuclear cell (PBMC) membranes and correlate abnormalities with the neurological outcome as measured by the Kurtzke Expanded Disability Status Scale (EDSS) and inflammation assessed by C-reactive protein (CRP). A second objective was to establish whether possible changes in membrane lipids (phospholipids, fatty acids and cholesterol) would have an effect on membrane fluidity, and whether this would correlate with the EDSS and CRP. The plasma, RBC and PBMC membrane lipid composition from 31 white female patients with MS and 30 age- and gender-matched control subjects were assessed. Fatty acids were quanflfied by gas chromatography (GC), phospholipids by colorimetric and cholesterol by enzymatic assays. Membrane fluidity, as measured by the membrane lipid composition, was calculated, using previously established formulae, and includes the following: the saturated nature of the membrane was measured by the phospholipid PC+PS/PE+PS ratio, fluidity and permeability were measured by the cholesterol concentratlon and the cholesterol to total phospholipid ratio and membrane deformability was measured by the phospholipid PE to PS ratio. Membrane fluidity was also measured by the ordered-erystalline-phase to liquidcrystalline- phase lipid composition, which correlates with the phospholipid PE to PC ratio. The membrane saturated (SATS) to polyunsaturated fatty acid (PUFA) ratio was further used as an indication- of the fluidity status of the membranes. CRP was measured in all participants using a Beckman nephelometer. In MS, the n-6 fatty acids, particularly C18:2n-6, C20:4n-6 and C22:4n-6, were significantly decreased in plasma, RBC and/or PBMC membranes. In addition, the relationship between C20:3n-6 and C20:4n-6 showed a metabolic disturbance in both RBC and PBMC membranes from patients with MS, as compared to the control group. C20:4n-6 showed significant inverse correlations with the EDSS and CRP in MS patients, indicating that loss of these fatty acids from membranes correlated with higher disability as well as with increased inflammation. There were significant increases in free fatty acids C18:2n-6 and C20:4n-6 in plasma from MS patients. Saturated fatty acids, SM C14:0 and PI C22:0 were significantly increased in PBMC membranes from MS patients, and SM C14:0, C16:0 and C20:0 showed inverse correlations with the Functional System Scores. In contrast, the longer-ehain SATS, C22:0 and C24:0 showed positive correlations with the Functional System Scores. Red blood cell membrane fluidity as measured by the SATS to PUFA ratio was significantly higher in patients than in controls. In patients with CRP ~ 5.00 Ilglml the ratio showed significant inverse correlation with disease outcome. The saturated nature correlated positively, whilst the .ordered-erystalline-phase to liquid-crystalline-phase lipid ratio correlated inversely with the Functional System Scores. In this study it was consistently shown that C20:4n-6, or its precursor and elongation products, C18:2n-6 and C22:4n-6 respectively, was lower in plasma, RBC and/or PBMC membranes from MS patients. Red blood cells lack the desaturase enzymes and depend on fatty acids sourced from the plasma. Therefore, lower C20:4n-6 in the RBC membranes from MS patients may be due to depleted plasma stores, or an indication of an increased demand of this fatty acid elsewhere. Furthermore, this study has demonstrated that lower RBC C20:4n-6, with an increase in plasma FFA C20:4n6, resulted in worse disease outcome, perhaps due to the pro-inflammatory effect of eicosanoid production. This. study also characterized the specific SATS, that is, longer-ehain SATS that may increase the risk of developing MS, as higher shorter-ehain SATS, C14:0 and C16:0 reflected better disease outcome, demonstrated by the inverse correlation with the EDSS and FSS. Lastly, this study has shown that in the presence of uncontrolled inflammation such as in MS, the altered lipid composition indirectly compromised cell membrane, structure and fluidity, and thereby contributed to the disease progression in MS patients.