Fluorescence correlation studies of lipid domains in model membranes (Review)

Advances in optical microscopy techniques and single-molecule detection have paved the way to exploring new approaches for investigating membrane dynamics and organization, thereby revealing details on the processing of signals, complex association/dissociation, chemical reactions and transport at and around the membrane. These events rely on a tight regulation of lipid-protein and protein-protein interactions in space and time. Fluorescence Correlation Spectroscopy (FCS) provides exquisite sensitivity in measuring local concentrations, association/dissociation constants, chemical rate constants and, in general, in probing the chemical environment of the species of interest and its interactions with potential partners. Here, we review some applications of FCS to lipid and protein organization in biomimetic membranes with lateral heterogeneities, which share some physico-chemical properties with cellular rafts. What we learn from investigations of lipid-lipid and lipid-protein interactions in simple model membranes can be regarded as an essential basic lecture for studies in more complex cellular membranes.     

Effects of hypothyroidism on mammary and liver lipid metabolism in virgin and late-pregnant rats

Untreated maternal hypothyroidism (hypoT) has serious consequences in offspring development that may result from the effect on lactation of maternal metabolism dysfunction. We studied the effects of prolonged propylthiouracil (PTU)-induced hypoT (0.1% PTU in drinking water starting 8 days before mating until day 21 of pregnancy or for 30 days in virgin rats) on liver and mammary lipid metabolism and serum lipid concentrations. In virgins, hypoT reduced hepatic mRNAs associated with triglyceride (TG) and cholesterol synthesis (including fatty acid synthase and 3-hydroxy-3-methylglutaryl coenzyme A reductase), and induced lobuloalveolar mammary development. Pregnancy increased hepatic mRNAs associated with TG and cholesterol synthesis and uptake (including LDL receptor) and with lipid oxidation, such as acyl CoA oxidase. HypoT decreased mRNAs and the activity of proteins associated with TG synthesis, and mRNAs associated with cholesterol uptake and lipid oxidation. Pregnancy increased mammary mRNAs related to lipid oxidation and decreased cholesterol synthesis, whereas hypoT decreased mRNAs and activities of proteins associated with TG synthesis and decreased epithelial mammary tissue. Virgin and pregnant hypoT rats had increased circulating VLDL + LDL cholesterol. HypoT decreased circulating TGs in pregnant rats. The observed effects of hypoT may result in decreased mammary lipid availability. This, along with the decreased epithelial mammary tissue during lactogenesis, may contribute to the future lactational deficit of hypoT mothers.     

Solid‐state NMR and simulation studies of equinatoxin II N‐terminus interaction with lipid bilayers

The interaction with model membranes of a peptide, EqtII_1–32, corresponding to the N‐terminal region of the pore‐forming toxin equinatoxin II (EqtII) has been studied using solid‐state NMR and molecular dynamics (MD) simulations. The distances between specifically labeled nuclei in [¹⁹F‐para]Phe16‐[1‐¹³C]Leu19 and [¹⁹F‐para]Phe16‐[¹⁵N]Leu23 analogs of EqtII_1–32 measured by REDOR in lyophilized peptide were in agreement with published crystal and solution structures. However, in both DMPC and mixed DMPC:SM membrane environments, significant changes in the distances between the labeled amino acid pairs were observed, suggesting changes in helical content around the experimentally studied region, 16–23, in the presence of bilayers. ¹⁹F‐³¹P REDOR experiments indicated that the aromatic ring of Phe16 is in contact with lipid headgroups in both membrane environments. For the DMPC:SM mixed bilayers, a closer interaction between Phe16 side chains and lipid headgroups was observed, but an increase in distances was observed for both labeled amino acid pairs compared with those measured for EqtII_1–32 in pure DMPC bilayers. The observed differences between DMPC and DMPC:SM bilayers may be due to the greater affinity of EqtII for the latter. MD simulations of EqtII_1–32 in water, on a pure DMPC bilayer, and on a mixed DMPC:SM bilayer indicate significant peptide secondary structural differences in the different environments, with the DMPC‐bound peptide adopting helical formations at residues 16–24, whereas the DMPC:SM‐bound peptide exhibits a longer helical stretch, which may contribute to its enhanced activity against PC:SM compared with pure PC bilayers. Proteins 2010. © 2009 Wiley‐Liss, Inc.     

Studies on the lipid metabolism of the metacercariae of Clinostomum complanatum (Trematoda: Digenea)

Metacercariae of Clinostomum complanatum possess considerable amounts of lipids. Fractionation of the lipids shows triglycerides and phospholipids as the major components whereas cholesterol and free fatty acids are minor components. Furthermore phospholipid fractions by thin layer chromatography reveal lecithin and cephalin as the major polar lipids whereas lysolecithin and lysocephalin are present in small fractions. The specific activity of lipase (E.C. 3.1.1.3) is 150.8 μg free fatty acids liberated/mg protein/h. Epinephrine, testosterone, insulin, sodium fluoride and iodoacetate stimulated, but 2-propanol inhibited, the lipase activity.     

Spin label studies of erythrocytes with abnormal lipid composition: comparison of red cells in a hereditary hemolytic syndrome and lecithin: Cholesterol acyltransferase deficiency

Erythrocytes from patients with familial lecithin:cholesterol acyltransferase (LCAT) deficiency have been shown to exhibit an increase in membrane fluidity which is surprisingly small in view of the extensive alterations both in membrane lipicl composition (namely, an elevation in cholesterol and phosphatidylcholine contents as well as a decrease in phosphatidylethanolamine) and in the functional properties of these cells. In the hope of deriving some information concerning the interrelationship between the structural and functional abnormalities, we have used the spin probe 5-doxyl stearic acid to investigate the temperature-dependent fluidity properties of red cells from two patients with a hereditary hemolytic syndrome (HHS) whose red cells are also characterized by qualitatively similar alterations in phosphatidylcholine and phosphatidylethanolamine but, unlike those in LCAT deficiency, have relatively normal levels of membrane cholesterol. A small increase in membrane fluidity of HHS erythrocytes equivalent to that previously observed in LCAT deficiency was found, indicating that membrane cholesterol level does not exert an important modulatory influence on membrane fluidity in these cells. It is concluded that while the distinct patterns of structural and functional erythrocyte alterations in these two disorders cannot be explained on the basis of differences in bulk membrane fluidity, the marginally increased fluidity may underlie the abnormalities in osmotic fragility and membrane p-nitrophenylphosphatase activity which are shared in common by both types of modified red cells.     

Comparison of raloxifene and atorvastatin effects on serum lipids composition of healthy post-menopausal women

The aim of this study was to evaluate the effects of the selective oestrogen receptor modulator, raloxifene, and those of statin, atorvastatin, in reducing the cardiovascular risks associated with the post-menopausal status. A detailed study of serum lipid concentrations was performed in four groups of post-menopausal women receiving either placebo, raloxifene or atorvastatin alone or their combination for the period of three months.Group A (raloxifene) showed significant decrease in total cholesterol levels (P < 0.05) and an increase in phospholipids concentration (P < 0.05), followed by a marked reduction in low-density lipoprotein cholesterol (LDL-C) levels (P < 0.01) and ApoB amounts (P < 0.001). Additionally, ApoA-I concentration was significantly increased (P < 0.01).Group B (atorvastatin) presented decreased cholesterol (P < 0.05) and triglycerides levels (P < 0.01), followed by elevated high-density lipoprotein cholesterol (HDL-C) concentration (P < 0.05) and low LDL-C amounts (P < 0.001). ApoA-I was significantly increased (P < 0.001) whereas ApoB was reduced (P < 0.001).The combined treatment in Group C (raloxifene and atorvastatin) showed significant changes in the majority of serum lipids. In particular, total cholesterol was reduced (P < 0.001), as well as triglycerides (P < 0.001) levels. Phospholipids were raised (P < 0.01) whereas LDL-C was reduced (P < 0.001) as was ApoB (P < 0.001). Furthermore, ApoA-I was elevated (P < 0.001). A further attempt to evaluate each treatment group was performed and the significance of these results is discussed. (Mol Cell Biochem 261: 71–75, 2004)     

Lipid composition of adult Foleyella agamae

Full-size image

& Obiamiwe B. A. 1986. Lipid composition of adult Foleyella agamae. International Journal for Parasitology 16: 655–657. The lipid and fatty acid composition of the filarial parasite Foleyella agamae were investigated. Total lipids accounted for 7.05% of the parasite fresh weight. Neutral lipids comprised 56.34% of the total and polar lipids 43.66%. The major lipid classes detected include sterol esters, cholesterol, phosphatidyl choline and phosphatidyl ethanolamine. Fatty acids varying in chain length from 10 carbon atoms through 20 carbon atoms were identified in the total lipid extract. The 18 carbon fatty acids formed the predominant components. The 20 carbon fatty acids were confined to the polar lipds.     

Effects of temperature on ether lipid composition of Caldariella acidophila

The composition of the ether lipids of a strain of Caldariella acidophila, with respect to the different numbers of cyclizations of the biphytanyl components, is shown to differ between the various complex lipid classes, but the degree of cyclization increases systematically with the growth temperature in the range 75–89°. The problem of distinguishing adaptive from phyletic features in archaebacterial lipids is considered.     

Quantitation of cholesterol incorporation into extruded lipid bilayers

Cholesterol incorporation into lipid bilayers, in the form of multilamellar vesicles or extruded large unilamellar vesicles, has been quantitated. To this aim, the cholesterol contents of bilayers prepared from phospholipid:cholesterol mixtures 33-75 mol% cholesterol have been measured and compared with the original mixture before lipid hydration. There is a great diversity of cases, but under most conditions the actual cholesterol proportion present in the extruded bilayers is much lower than predicted. A quantitative analysis of the vesicles is thus required before any experimental study is undertaken.     

The sensitivity of lipid domains to small perturbations demonstrated by the effect of Triton

The hypothesis of lipid rafts describes functional domains in biological membranes. It is often assumed that rafts form by spontaneous de-mixing of certain lipids and that they can be isolated as detergent-resistant membrane particles (DRMs) using the detergent Triton X-100 (TX). Here, we present a model that describes the process of domain formation in membranes in the presence and in the absence of TX. We measure the interactions between TX and an equimolar mixture of sphingomyelin (SM), cholesterol (Cho), and 1-palmitoyl-2-oleoyl-3-sn-glycero-phosphatidylcholine (POPC) (1:1:1, mol) by means of isothermal titration calorimetry. Comparison with pure POPC membranes reveals a very unfavorable interaction between TX and SM/Cho, which causes a substantial tendency to segregate these molecules into separate, DRM-like (SM-rich) and fluid (TX-rich), domains. If rafts are indeed formed by spontaneous de-mixing of PC and SM/Cho, they must be very sensitive, and perturbations caused by techniques used to study rafts could lead to misleading results. If, however, rafts are much more stable than PC-SM-Cho domains, there must be an unknown raft stabilizer. Subtle changes of such a promoter could serve to modulate raft function.     

Comparative ultrastructural topography of the gut epithelia of some trematodes

The gut epithelia of six species of digenetic trematodes, Clonorchis sinensis, Eurytrema pancreaticum, Haematoloechus lobatus, Echinostoma hortense, Schistosoma japonicum and Fasciola hepatica, were studied with scanning and also transmission electron microscopy. Morphological differences in cytoplasmic projections of the gut of adult flukes were demonstrated stereoscopically among these species. The cytoplasmic projections vary considerably in shape, but are roughly separated into three groups by their essential forms: ribbon-shaped narrow type in C. sinensis and E. pancreaticum, broad, triangular with filamentous extensions distally and/or marginally as in F. hepatica and E. hortense, and broad, sheet-like or triangular with the distal ends blunt or rounded as in H. lobatus and S. japonicum. This character appears rather constant, without regional differences in the gut. No marked correlation was found between the gut projections of the parasites and their host or food. There are also specific discriminations in the ultrastructure of the cellular organization among the species examined.     

Dynamics of lipid content during early development of freshwater salmon <Emphasis Type="Italic">Salmo salar</Emphasis> L.

Dynamics of lipid and phospholipid content was studied during early development of freshwater salmon Salmo salar L. from blastodisc formation (3 h) to hatching (108 days) as well as in eggs before fertilization. High and stable content of total lipids including structural phospholipids as well as relatively high content of triglycerides and its slight increase at the time of hatching have been demonstrated, which can indicate their utilization as the main energy source after hatching under conditions of deficient food and low fry activity for some time. Accumulation of a certain level of lipids in eggs before spawning is required for embryonic development and high survival after hatching. The significance of increasing and decreasing levels of structural lipids modulating membrane enzyme activities in metabolic changes before hatching is discussed.     

Histochemical studies on the distribution of alkaline phosphatase, acid phosphatase, 5-nucleotidase and ATPase in various reproductive tissues of certain digenetic trematodes.

Out of other functions performed by vitellaria in digenetic trematodes, their role in the formation of shell globules and shell membrane of the capsule, as well as in the excretion of iron with the help of vitamin C is very important. The present histochemical work shows the localization of certain enzymes in different parts of the reproductive system of ten species of trematodes viz.: Neopronocephalus triangularis Mehra, 1932; Glossimetra orientalis Mehra, 1937; Orientodiscus lobatus Srivastava, 1938; Eumegacetes artemii Mehra, 1935; Ganeo tigrinus mehra et Negi, 1928; Encyclometra caudata Dollfus, 1928; Thapariella udaipurensis Gupta and Sharma, 1970; Paradistomoides indicum Narain et Das, 1929; Patagifer wesleyi Verma, 1936; Proalarioides tropidonotus Vidyarthi, 1937 and indicates their functional significance. The hydrolytic enzymes (alkaline phosphatase, acid phosphatase, 5-nucleotidase and ATPase) are suggestive of their involvement in the uptake of certain nutrients, glycogen and lipoprotein being very significant among others. The four enzymes could also be detected in testes, ovary, uterus, cirrus sac and egg shell. The possible functional significance of each enzyme has been discussed.     

Effect of membrane composition on lipid oxidation in liposomes

To study the effect of membrane composition on the oxidation of liposomes, different systems were prepared by adding one component at time to phosphatidylcholine (Epikuron 200). In particular, the effect of cholesterol and its ester, cholesterol stearate, on membrane structure and oxidation was studied. A first screening of the structure and net charge of the different preparation was made by means of z-potential and size measurements. Then the liposomes were oxidized by using a hydrophilic radical initiator, the (2,2-azobis(2-amidinopropane) hydrochloride, AAPH, which thermally decomposes to give a constant radical flux in water. The oxidation of liposomes, monitored by following the absorbance of the primary products of oxidation at 234 nm, was shown to be dependent on the composition of the liposomal bilayer and so on its biophysical properties. In addition, size and z-potential measurements gathered in the time course of the peroxidation reaction, revealed that the oxidation induced a modification of the superficial characteristics of the membrane bilayer so as to change its charge at the shear plane (z-potential). This behaviour was shared by all liposomal preparations independent of the composition. The change in sizes of the different liposomal preparation, instead, followed different trends, being more stable both in control samples and in oxidized ones when cholesterol was present. From the analysis of the results, it can be concluded that cholesterol affects the oxidation induced by hydrophilic radical initiator of model membranes by changing the biophysical properties of the phospholipid bilayer. The rigidity induced by cholesterol at temperatures above the T_m makes the membrane more resistant to radical attack from an external aqueous phase and this in turn delays the start of the reaction. The decrease of z-potential of the liposomal particles induced by the oxidation process can be an important clue to understand the mechanisms involved in the etiology of important diseases.     

Correlation between the extent of coronary atherosclerosis and lipid profile

Increased concentration of low density lipoprotein (LDL) cholesterol or decreased level of high density lipoprotein (HDL) cholesterol are important risk factors for coronary atherosclerosis. However, an independent association of triglycerides (TG) with atherosclerosis is uncertain.The aim of this prospective study was to evaluate the relationship between serum lipid levels and the extent of coronary atherosclerosis in patients with suspected coronary artery disease (CAD) and no previous myocardial infarction who were not treated with lipids lowering therapy or low-lipid diet.The study was conducted in 141 patients (53.6 ± 7.8 years old; 32 female) who underwent a routine coronary angiography for CAD diagnosis. A modified angiographic Gensini Score (GS) was used to reflect the extent of coronary atherosclerosis. Fasting serum lipid concentrations were determined using cholesterol esterase/peroxidase (CHOD/PAP) enzymatic method for total cholesterol and its fractions and lipase glycerol kinase (GPO/PAP) enzymatic method TG evaluation. The association of Gensini Score with variables characterising lipid profile was analysed with the use of Pearson correlation (r co-efficient; p value).GS was positively correlated with total cholesterol (r = 0.404; p < 0.001), LDL cholesterol (r = 0.484; p < 0.001) and TG (r = 0.235; p = 0.005). There was a negative correlation between Gensini Score and HDL cholesterol (r = –0.396; p < 0.001).In angina pectoris patients with no previous myocardial infarction, the extent of coronary atherosclerosis is positively correlated with pro-atherogenic lipids, i.e. total cholesterol, LDL cholesterol and TG and negatively correlated with antiatherogenic HDL cholesterol.     

Comparative studies on the glycoprotein composition of mammalian platelets.

1. The membrane glycoprotein composition of the blood platelets of 13 mammalian species has been compared by SDS-polyacrylamide gel electrophoresis. 2. A basic pattern of 2-3 predominant high molecular weight glycoprotein bands was observed, however species differences in their relative rates of migration and abundance were apparent. 3. Wide species differences in the number and rate of migration of the acidic glycopeptides released by trypsin digestion of washed platelet suspensions were observed following polyacrylamide gel electrophoresis in the absence of SDS.     

Comparative studies on human skin fibroblasts: Life span and lipid metabolism in medium containing fetal bovine or human serum

Summary Three strains of human skin fibroblasts were cultivated in nutrient medium supplemented either with human serum or fetal bovine serum, and growth and lipid synthesis were compared. Rates of cellular growth were similar in both kinds of medium, but the replicative life spans of all three strains were curtailed significantly in human-serum medium. Incorporation of label into the major classes of neutral lipids from [¹⁴C]acetate and³H₂O was increased also in human-serum medium. Since human serum contained higher concentrations of cholesterol known to reduce endogenous cholesterol synthesis, these results were unexpected. Nonlipid factors in human serum may account for the shortened cellular life spans and increased lipogenesis and perhaps for the potential to develop atherosclerosis. Supported by grants from the Ontario Heart Foundation and Medical Research Council of Canada during the tenure of a Senior Research Fellowship from the Ontario Heart Foundation (J.T.C.) and a Scholarship from the Medical Research Council of Canada (S.G.).