Characterization of lipid composition in stimulated human lymphocytes by 1H-NMR

Recent in vivo NMR studies have raised interest in the structural changes of cellular lipids during proliferative activity. We investigated the changes in plasma membrane lipid and total cell lipid during mitogenically-stimulated proliferation of human peripheral blood lymphocytes by extraction of lipids and assay by 500 MHz 1H-NMR. Resonances were assigned using one- and two-dimensional spectroscopic techniques, and signals unique to certain species of lipid were identified. Choline and ethanolamine-containing lipids, glycerophospholipid backbones, sphingolipids, cholesterol, plasmalogens and triacylglycerols were readily detected. Resolution of a number of lipid species was not possible, despite the use of high-resolution techniques. NMR values for proliferation-induced changes in the most easily determined parameters, namely the total cholesterol to total phospholipid molar ratio, and phosphatidylcholine, phosphatidylethanolamine and sphingolipid composition, were found to agree with traditional methods. Differences in phospholipid and fatty acid profiles were found between plasma membranes and total cell lipid for resting values and for response to mitogen.     

Role of lipid interactions in autoimmune demyelination

A morphological transformation involving loss of adhesion between myelin lamellae and formation of myelin vesicles has been described as a mechanism for demyelination in multiple sclerosis and marmoset experimental allergic encephalomyelitis (EAE). Although protein interactions are involved in maintaining normal myelin structure, we describe here how lipids contribute to myelin stability and how lipid changes in EAE, including increases in lipid polyunsaturation and negatively charged phosphatidylserine (PS), promote demyelination. Three physico-chemical techniques were used to identify these changes: (1) Langmuir monolayer isotherms indicated that EAE white matter lipids were significantly more "expanded" (fluid) than controls. (2) NMR spectroscopy indicated that EAE myelin lipids were more polyunsaturated than controls. (3) High-performance liquid chromatography (HPLC) with an evaporative light scattering detector indicated increased PS in EAE compared to controls, while sphingomyelin (SM), sulfatides and phosphatidylcholine (PC) were decreased. We present a physical model considering electrostatic, van der Waals and undulation forces to quantify the effect of these changes on myelin adhesion at the extracellular interface. Taken together, the isotherm, NMR, HPLC and modeling results support a mechanism for autoimmune demyelination whereby the composition of myelin lipids is altered in a manner that increases myelin fluidity, decreases myelin adhesion, increases membrane curvature, and promotes vesiculation.     

The effect of hypoxia due to iron-deficiency anemia on the physicochemical properties of biological membranes]

The lipid structure and Ca2+ permeability of red blood cell, hepatocyte and cardiomyocyte membranes were determined while investigating the effect of hypoxia caused by iron deficiency anemia upon the structural and functional state of biological membranes. The lipid composition and barrier characteristics of membranes change under conditions of hypoxia caused by experimental iron deficiency anemia. Quantitative changes in the cell membrane lipids may be considered as an important molecular mechanism of Ca2+ transport disorder in membranes, increase of Ca2+ permeability producing its surplus in the cells and subsequent metabolic homeostatic disturbances.     

Membrane Lipid Co-Aggregation with α-Synuclein Fibrils

Amyloid deposits from several human diseases have been found to contain membrane lipids. Co-aggregation of lipids and amyloid proteins in amyloid aggregates, and the related extraction of lipids from cellular membranes, can influence structure and function in both the membrane and the formed amyloid deposit. Co-aggregation can therefore have important implications for the pathological consequences of amyloid formation. Still, very little is known about the mechanism behind co-aggregation and molecular structure in the formed aggregates. To address this, we study in vitro co-aggregation by incubating phospholipid model membranes with the Parkinson’s disease-associated protein, α-synuclein, in monomeric form. After aggregation, we find spontaneous uptake of phospholipids from anionic model membranes into the amyloid fibrils. Phospholipid quantification, polarization transfer solid-state NMR and cryo-TEM together reveal co-aggregation of phospholipids and α-synuclein in a saturable manner with a strong dependence on lipid composition. At low lipid to protein ratios, there is a close association of phospholipids to the fibril structure, which is apparent from reduced phospholipid mobility and morphological changes in fibril bundling. At higher lipid to protein ratios, additional vesicles adsorb along the fibrils. While interactions between lipids and amyloid-protein are generally discussed within the perspective of different protein species adsorbing to and perturbing the lipid membrane, the current work reveals amyloid formation in the presence of lipids as a co-aggregation process. The interaction leads to the formation of lipid-protein co-aggregates with distinct structure, dynamics and morphology compared to assemblies formed by either lipid or protein alone.     

Atomic view of calcium-induced clustering of phosphatidylserine in mixed lipid bilayers

Membranes play key regulatory roles in biological processes, with bilayer composition exerting marked effects on binding affinities and catalytic activities of a number of membrane-associated proteins. In particular, proteins involved in diverse processes such as vesicle fusion, intracellular signaling cascades, and blood coagulation interact specifically with anionic lipids such as phosphatidylserine (PS) in the presence of Ca(2+) ions. While Ca(2+) is suspected to induce PS clustering in mixed phospholipid bilayers, the detailed structural effects of this ion on anionic lipids are not established. In this study, combining magic angle spinning (MAS) solid-state NMR (SSNMR) measurements of isotopically labeled serine headgroups in mixed lipid bilayers with molecular dynamics (MD) simulations of PS lipid bilayers in the presence of different counterions, we provide site-resolved insights into the effects of Ca(2+) on the structure and dynamics of lipid bilayers. Ca(2+)-induced conformational changes of PS in mixed bilayers are observed in both liposomes and Nanodiscs, a nanoscale membrane mimetic of bilayer patches. Site-resolved multidimensional correlation SSNMR spectra of bilayers containing (13)C,(15)N-labeled PS demonstrate that Ca(2+) ions promote two major PS headgroup conformations, which are well resolved in two-dimensional (13)C-(13)C, (15)N-(13)C, and (31)P-(13)C spectra. The results of MD simulations performed on PS lipid bilayers in the presence or absence of Ca(2+) provide an atomic view of the conformational effects underlying the observed spectra.     

Temperature-induced conformational changes in human tearlipids hydrocarbon chains

As a first step to characterize human meibum and tear lipids, infrared spectroscopy was applied to characterize the molecular structure/conformation and packing of hydrocarbon chains. Temperature-induced phase transitions were fit to a sigmoid equation and were experimentally reproducible and were similar for multiple samples collected from the same person. No hysteresis was observed. Hydration of polar tear lipids increased their phase transition cooperativity, enthalpy and entropy. Hydrophobic interactions in meibum lipid (ML) were stronger than in tear-fluid lipids (TL), as reflected by the higher entropy and enthalpy of the gel to liquid crystalline phase transition of ML. The results of this study provide further evidence of the differences in the composition and structure of ML and TL. The conformational changes observed in the hydrocarbon chains of ML with temperature suggest that the observed therapeutic increased delivery of ML with eye lid heating could be related to the increased disorder in the packing of the hydrocarbon tails. This work also highlights the power of infrared spectroscopy to characterize molecular structure/conformation, and packing of human tear lipids and provides a basis for future studies of tear film lipid composition-structure-function relationships and lipid-protein interactions in relation to age, sex, and dry eye symptoms.     

Mutants of Arabidopsis reveal many roles for membrane lipids

Polyunsaturated acyl lipids constitute approximately 50% of the hydrophobic membrane barriers that delineate the compartments of cells. The composition of these lipids is critically important for many membrane functions and, thus, for proper growth and development of all living organisms. In the model plant Arabidopsis, the isolation of mutants with altered lipid compositions has facilitated biochemical and molecular approaches to understanding lipid metabolism and membrane biogenesis. Just as importantly, the availability of a series of plant lines with specific changes in membrane lipids have provided a new resource to study the structural and adaptive roles of lipids. Now, the sequencing of the Arabidopsis genome, and the development of reverse-genetics approaches provide the tools needed to make additional discoveries about the relationships between lipid structure and membrane function in plant cells.     

Applications of nuclear magnetic resonance in lipid analyses: An emerging powerful tool for lipidomics studies

The role of lipids in cell, tissue, and organ physiology is crucial; as many diseases, including cancer, diabetes, neurodegenerative, and infectious diseases, are closely related to absorption and metabolism of lipids. Mass spectrometry (MS) based methods are the most developed powerful tools to study the synthetic pathways and metabolic networks of cellular lipids in biological systems; leading to the birth of an emerging subject lipidomics, which has been extensively reviewed. Nuclear magnetic resonance (NMR), another powerful analytical tool, which allows the visualization of single atoms and molecules, is receiving increasing attention in lipidomics analyses. However, very little work focusing on lipidomic studies using NMR has been critically reviewed. This paper presents a first comprehensive summary of application of ¹H, ¹³C & ³¹P NMR in lipids and lipidomics analyses. The scientific basis, principles and characteristic diagnostic peaks assigned to specific atoms/molecular structures of lipids are presented. Applications of 2D NMR in mapping and monitoring of the components and their changes in complex lipids systems, as well as alteration of lipid profiling over disease development are also reviewed. The applications of NMR lipidomics in diseases diagnosis and food adulteration are exemplified.     

Membrane interface composition drives the structure and the tilt of the single transmembrane helix protein PMP1: MD studies

PMP1, a regulatory subunit of the yeast plasma membrane H⁺-ATPase, is a single transmembrane helix protein. Its cytoplasmic C-terminus possesses several positively charged residues and interacts with phosphatidylserine lipids as shown through both ¹H- and ²H-NMR experiments. We used all-atom molecular dynamics simulations to obtain atomic-scale data on the effects of membrane interface lipid composition on PMP1 structure and tilt. PMP1 was embedded in two hydrated bilayers, differing in the composition of the interfacial region. The neutral bilayer is composed of POPC (1-palmitoyl-2-oleoyl-3-glycero-phosphatidylcholine) lipids and the negatively charged bilayer is composed of POPC and anionic POPS (1-palmitoyl-2-oleoyl-3-glycero-phosphatidylserine) lipids. Our results were consistent with NMR data obtained previously, such as a lipid sn-2 chain lying on the W28 aromatic ring and in the groove formed on one side of the PMP1 helix. In pure POPC, the transmembrane helix is two residues longer than the initial structure and the helix tilt remains constant at 6 ± 3°. By contrast, in mixed POPC-POPS, the initial helical structure of PMP1 is stable throughout the simulation time even though the C-terminal residues interact strongly with POPS headgroups, leading to a significant increase of the helix tilt within the membrane to 20 ± 5°.     

Exploring membrane selectivity of the antimicrobial peptide KIGAKI using solid-state NMR spectroscopy

The designed antimicrobial peptide KIGAKIKIGAKIKIGAKI possesses enhanced membrane selectivity for bacterial lipids, such as phosphatidylethanolamine and phosphatidylglycerol. The perturbation of the bilayer by the peptide was first monitored using oriented bilayer samples on glass plates. The alignment of POPE/POPG model membranes with respect to the bilayer normal was severely altered at 4 mol% KIGAKI while the alignment of POPC bilayers was retained. The interaction mechanism between the peptide and POPE/POPG bilayers was investigated by carefully comparing three bilayer MLV samples (POPE bilayers, POPG bilayers, and POPE/POPG 4/1 bilayers). KIGAKI induces the formation of an isotropic phase for POPE/POPG bilayers, but only a slight change in the (31)P NMR CSA line shape for both POPE and POPG bilayers, indicating the synergistic roles of POPE and POPG lipids in the disruption of the membrane structure by KIGAKI. (2)H NMR powder spectra show no reduction of the lipid chain order for both POPG and POPE/POPG bilayers upon peptide incorporation, supporting the evidence that the peptide acts as a surface peptide. (31)P longitudinal relaxation studies confirmed that different dynamic changes occurred upon interaction of the peptide with the three different lipid bilayers, indicating that the strong electrostatic interaction between the cationic peptide KIGAKI and anionic POPG lipids is not the only factor in determining the antimicrobial activity. Furthermore, (31)P and (2)H NMR powder spectra demonstrated a change in membrane characteristics upon mixing of POPE and POPG lipids. The interaction between different lipids, such as POPE and POPG, in the mixed bilayers may provide the molecular basis for the KIGAKI carpet mechanism in the permeation of the membrane.     

Differential postprandial incorporation of 20:5n-3 and 22:6n-3 into individual plasma triacylglycerol and phosphatidylcholine molecular species in humans

The mechanisms by which digested fat is absorbed and transported in the circulation are well documented. However, it is uncertain whether the molecular species composition of dietary fats influences the molecular species composition of meal-derived lipids in blood. This may be important because enzymes that remove meal-derived fatty acids from the circulation exhibit differential activities towards individual lipid molecular species. To determine the effect of consuming oils with different molecular compositions on the incorporation of 20:5n-3 and 22:6n-3 into plasma lipid molecular species. Men and women (18–30 years) consumed standardised meals containing 20:5n-5 and 22:6n-3 (total 450 mg) provided by an oil from transgenic Camelina sativa (CSO) or a blended fish oil (BFO) which differed in the composition of 20:5n-3 and 22:6n-3 – containing molecular species. Blood was collected during the subsequent 8 h. Samples were analysed by liquid chromatography-mass spectrometry. The molecular species composition of the test oils was distinct from the composition of plasma triacylglycerol (TG) or phosphatidylcholine (PC) molecular species at baseline and at 1.5 or 6 h after the meal. The rank order by concentration of both plasma PC and TG molecular species at baseline was maintained during the postprandial period. 20:5n-3 and 22:6n-3 were incorporated preferentially into plasma PC compared to plasma TG. Together these findings suggest that the composition of dietary lipids undergoes extensive rearrangement after absorption, such that plasma TG and PC maintain their molecular species composition, which may facilitate lipase activities in blood and/or influence lipoprotein structural stability and function.     

Composition and lipid spatial distribution of HDL particles in subjects with low and high HDL-cholesterol

A low level of high density lipoprotein cholesterol (HDL-C) is a powerful risk factor for cardiovascular disease. However, despite the reported key role of apolipo-proteins, specifically, apoA-I, in HDL metabolism, lipid molecular composition of HDL particles in subjects with high and low HDL-C levels is currently unknown. Here lipidomics was used to study HDL derived from well-characterized high and low HDL-C subjects. Low HDL-C subjects had elevated triacylglycerols and diminished lysophosphatidylcholines and sphingomyelins. Using information about the lipid composition of HDL particles in these two groups, we reconstituted HDL particles in silico by performing large-scale molecular dynamics simulations. In addition to confirming the measured change in particle size, we found that the changes in lipid composition also induced specific spatial distributions of lipids within the HDL particles, including a higher amount of triacylglycerols at the surface of HDL particles in low HDL-C subjects. Our findings have important implications for understanding HDL metabolism and function. For the first time we demonstrate the power of combining molecular profiling of lipoproteins with dynamic modeling of lipoprotein structure.     

Fatty acid composition of lipids in erythrocytes and blood plasma in cadmium intoxication and its correction with unitiol

The effect of cadmium chloride on the fatty acid composition of lipids of the erythrocytes and blood plasma has been studied. It has been established that in the process of cadmium intoxication the decrease of the level of unsaturated fatty acids takes place and the most considerable changes in the content of oleic and arachidonic acids. It makes possible to assume the disturbance of both the erythrocytes membrane structure and metabolism in the erythrocyte. Unitiol injection promotes the normalisation of the indices under investigation.     

Fatty acid composition of erythrocyte and vesicle total lipids during storage of human erythrocytes in protein-free media and in citrate-phosphate-dextrose

We have studied the fatty acid composition of total lipids of erythrocytes and vesicles either during storage of human erythrocytes in their own plasma under blood bank conditions or during incubation at 37 degrees C in protein-free media. Vesicles appear as a heterogeneous population with a diameter of about 130 nm and a varying content of hemoglobin. The fatty acid pattern of vesicle total lipids changes in respect to control erythrocytes, while in erythrocytes it remains fairly constant during the total ageing period. Our results indicate a marked rearrangement of the membrane components during blood storage.     

The effects of galactolipid depletion on the structure of a photosynthetic membrane

The galactolipids monogalactosyldiglyceride and digalactosyldiglyceride together comprise more than 77% of the photosynthetic membrane lipids of higher plant chloroplasts. We have isolated a lipase from the chloroplasts of runner beans (Phaseolus vulgaris) which is highly specific for these galactolipids. This galactolipase promotes the hydrolysis of monogalactosyldiglyceride and digalactosyldiglyceride, in the process liberating two free fatty acids into the membrane bilayer, leaving the residual galactosyl glyceride group to diffuse into the aqueous bulk phase. Isolated spinach photosynthetic membranes were treated with this enzyme preparation and changes in membrane composition were studied with thin layer chromatography (for lipids), gel electrophoresis (proteins), and freeze-etching (membrane structure). After 30 min of lipolysis, nearly 100% of the galactolipids had been converted into membrane-associated fatty acids and water-soluble galactosyl glycerides. SDS PAGE showed that two proteins, one of which is possibly associated with the reaction center of photosystem II, were removed by the treatment. Despite the minor nature of changes in membrane protein composition, freeze-fracture and freeze-etch studies showed that striking changes in membrane structure had taken place. The large freeze-fracture particle on the E fracture face had disappeared in stacked regions of the membrane system. In addition, a tetrameric particle visible at the inner surface of the membrane had apparently dissociated into individual monomeric particles. The fact that these two structures are so dramatically affected by the loss of galactolipids strongly suggests that these lipids play a crucial role in maintaining their structure. Both structures are believed to be different views of the same transmembrane unit: a membrane-spanning complex associated with photosystem II. Our results are consistent with two possible interpretations: the intramembrane particles may be lipidic in nature, and hence lipolysis causes their disappearance; or galactolipids are necessary for the organization of a complex photosystem II-associated structure which is composed of a number of different molecular species.     

Solid-State NMR-Restrained Ensemble Dynamics of a Membrane Protein in Explicit Membranes

Solid-state NMR has been used to determine the structures of membrane proteins in native-like lipid bilayer environments. Most structure calculations based on solid-state NMR observables are performed using simulated annealing with restrained molecular dynamics and an energy function, where all nonbonded interactions are represented by a single, purely repulsive term with no contributions from van der Waals attractive, electrostatic, or solvation energy. To our knowledge, this is the first application of an ensemble dynamics technique performed in explicit membranes that uses experimental solid-state NMR observables to obtain the refined structure of a membrane protein together with information about its dynamics and its interactions with lipids. Using the membrane-bound form of the fd coat protein as a model membrane protein and its experimental solid-state NMR data, we performed restrained ensemble dynamics simulations with different ensemble sizes in explicit membranes. For comparison, a molecular dynamics simulation of fd coat protein was also performed without any restraints. The average orientation of each protein helix is similar to a structure determined by traditional single-conformer approaches. However, their variations are limited in the resulting ensemble of structures with one or two replicas, as they are under the strong influence of solid-state NMR restraints. Although highly consistent with all solid-state NMR observables, the ensembles of more than two replicas show larger orientational variations similar to those observed in the molecular dynamics simulation without restraints. In particular, in these explicit membrane simulations, Lys⁴⁰, residing at the C-terminal side of the transmembrane helix, is observed to cause local membrane curvature. Therefore, compared to traditional single-conformer approaches in implicit environments, solid-state NMR restrained ensemble simulations in explicit membranes readily characterize not only protein dynamics but also protein-lipid interactions in detail.     

Thermosensing via transmembrane protein–lipid interactions

Cell membranes are composed of a lipid bilayer containing proteins that cross and/or interact with lipids on either side of the two leaflets. The basic structure of cell membranes is this bilayer, composed of two opposing lipid monolayers with fascinating properties designed to perform all the functions the cell requires. To coordinate these functions, lipid composition of cellular membranes is tailored to suit their specialized tasks. In this review, we describe the general mechanisms of membrane–protein interactions and relate them to some of the molecular strategies organisms use to adjust the membrane lipid composition in response to a decrease in environmental temperature. While the activities of all biomolecules are altered as a function of temperature, the thermosensors we focus on here are molecules whose temperature sensitivity appears to be linked to changes in the biophysical properties of membrane lipids. This article is part of a Special Issue entitled: Lipid–protein interactions.     

Effect of lignocaine on blood lipid in relation to partition coefficient and biological activity.

To correlate lipophilicity of lignocaine with changes in lipid composition of blood as a result of in vitro incubation with the drug, phosphorus content and fatty acid compositions of blood lipids before and after lignocaine treatment have been compared with those of a standard phospholipid, lecithin, under similar conditions of drug treatment. The change in fatty acid constituents has been correlated with the biological activity (both therapeutic and toxic) of lignocaine.     

Metabolite changes in BT4C rat gliomas undergoing ganciclovir-thymidine kinase gene therapy-induced programmed cell death as studied by 1H NMR spectroscopy in vivo,ex vivo,and in vitro

Programmed cell death was induced by HSV-tk gene therapy in rat BT4C glioma cells, and metabolite changes associated with cell damage were monitored in vivo by 1H NMR spectroscopy and ex vivo by high resolution magic angle spinning (HRMAS) 1H NMR, and in vitro in perchloric acid extracts of tumors. Metabolite concentrations, as quantified in vivo using water as an internal reference and in vitro in extracts, were correlated with cell density. The results showed that both in vivo and in vitro glycine and creatine concentrations followed volume-averaged cell density, whereas that of total choline-containing compounds was unaffected by a cell loss approaching 60%. Meanwhile, both saturated and unsaturated 1H NMR visible lipids increased. HRMAS 1H NMR spectroscopy of the tumor samples at 14.1 tesla demonstrated the presence of nucleotide peaks from adenosine and uridine nucleotides in glioma samples ex vivo. The assignment of a doublet at 7.95 ppm to UDP was confirmed by spiking experiments of tumor extracts in conjunction with 1H and 31P NMR spectroscopy. HRMAS also resolved the choline-containing peak at 3.2 ppm in vivo into resonances from choline (3.20 ppm), phosphocholine (3.22 ppm), glycerophosphocholine (3.24 ppm), and taurine (3.26 ppm). These resonances were uncorrelated with temporal progression through programmed cell death. Our results show that 1H NMR-detected lipids and some of the small molecular weight metabolites respond to gene therapy. However, the choline-containing compounds are unaffected by severe decline in cell density. The latter observation supports the idea that triacylglycerols, rather than membrane phospholipids, are the key components of 1H NMR visible lipids, and it also casts doubt on the validity of resonance of choline-containing compounds as a diagnostic marker of programmed cell death in vivo.     

Brain tissue characterisation by infrared imaging in a rat glioma model

Pathological changes associated with the development of brain tumor were investigated by Fourier transform infrared microspectroscopy (FT-IRM) with high spatial resolution. Using multivariate statistical analysis and imaging, all normal brain structures were discriminated from tumor and surrounding tumor tissues. These structural changes were mainly related to qualitative and quantitative changes in lipids (tumors contain little fat) and were correlated to the degree of myelination, an important factor in several neurodegenerative disorders. Lipid concentration and composition may thus be used as spectroscopic markers to discriminate between healthy and tumor tissues. Additionally, we have identified one peculiar structure all around the tumor. This structure could be attributed to infiltrative events, such as peritumoral oedema observed during tumor development. Our results highlight the ability of FT-IRM to identify the molecular origin that gave rise to the specific changes between healthy and diseased states. Comparison between pseudo-FT-IRM maps and histological examinations (Luxol fast blue, Luxol fast blue-cresyl violet staining) showed the complementarities of both techniques for early detection of tissue abnormalities.