Phospholipid unsaturation and plasma membrane organization

Lipid Modulation of a Class B GPCR: Elucidating the Modulatory Role of PI(4,5)P2 Lipids

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) lipids have been shown to stabilize an active conformation of class A G-protein coupled receptors (GPCRs) through a conserved binding site, not present in class B GPCRs. For class B GPCRs, previous molecular dynamics (MD) simulation studies have shown PI(4,5)P₂ interacting with the Glucagon receptor (GCGR), which constitutes an important target for diabetes and obesity therapeutics. In this work, we applied MD simulations supported by native mass spectrometry (nMS) to study lipid interactions with GCGR. We demonstrate how tail composition plays a role in modulating the binding of PI(4,5)P₂ lipids to GCGR. Specifically, we find the PI(4,5)P₂ lipids to have a higher affinity toward the inactive conformation of GCGR. Interestingly, we find that in contrast to class A GPCRs, PI(4,5)P₂ appear to stabilize the inactive conformation of GCGR through a binding site conserved across class B GPCRs but absent in class A GPCRs. This suggests differences in the regulatory function of PI(4,5)P₂ between class A and class B GPCRs.

Omega-3 PUFA profoundly affect neural, physiological, and behavioural competences - implications for systemic changes in trophic interactions

In recent decades, much conceptual thinking in trophic ecology has been guided by theories of nutrient limitation and the flow of elements, such as carbon and nitrogen, within and among ecosystems. More recently, ecologists have also turned their attention to examining the value of specific dietary nutrients, in particular polyunsaturated fatty acids (PUFA), among which the omega-3 PUFA, especially eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) play a central role as essential components of neuronal cell membranes in many organisms. This review focuses on a new neuro-ecological approach stemming from the biochemical (mechanistic) and physiological (functional) role of DHA in neuronal cell membranes, in particular in conjunction with G-protein coupled receptors (GPCRs). We link the co-evolution of these neurological functions to metabolic dependency on dietary omega-3 PUFA. We outline ways in which deficiencies in dietary DHA supply may affect, cognition, vision, and behaviour, and ultimately, the biological fitness of consumers. We then review emerging evidence that changes in access to dietary omega-3 PUFA may ultimately have profound impacts on trophic interactions leading to potential changes in community structure and ecosystem functioning that, in turn, may affect the supply of DHA within and across ecosystems, including the supply for human consumption.

Molecular dynamics study of lipid bilayers modeling outer and inner leaflets of plasma membranes of mouse hepatocytes. I. Differences in physicochemical properties between the two leaflets

Outer and inner leaflets of plasma cell membranes have different lipid compositions, and the membrane properties of each leaflet can differ from each other significantly due to these composition differences. However, because of the experimental difficulty in measuring the membrane properties for each leaflet separately, the differences are not well understood at a molecular level. In this study, we constructed two lipid bilayer systems, modeling outer and inner leaflets of plasma membranes of mouse hepatocytes based on experimental composition data. The ion concentration in the interlamellar water phase was also set to match the concentration in extra- and intracellular fluids. The differences in physical properties between the outer and inner leaflets of mouse hepatocyte cell membrane models were investigated by performing 1.2 μs-long all-atomistic molecular dynamics calculations under physiological temperature and pressure conditions (310.15 K and 1 atm). The calculated electron density profiles along the bilayer normal for each model bilayer system captured well the asymmetric feature of the experimental electron density profile across actual cell plasma membranes, indicating that our procedure of modeling the outer and inner leaflets of the cell plasma membranes was satisfactory. We found that compared to the outer leaflet model, the inner leaflet model had a very bulky and soft structure in the lateral direction. To confirm the differences, membrane fluidity was measured from the lateral diffusivity and relaxation times. The fluidity was significantly higher in the inner leaflet model than in the outer leaflet model. We also discuss two topics that are of wide interest in biology, i.e., the interdigitation of acyl tails of lipid molecules between two monolayers and the lateral concentration fluctuation of lipid molecules in the bilayers.

Phospholipids in Human Milk and Infant Formulas: Benefits and Needs for Correct Infant Nutrition

The composition of human milk has served as a basis for the development of infant formulas, which are used when breastfeeding is not possible. Among the human milk nutrients, 50% of the total energetic value corresponds to fat, with a high level of fatty acids and 0.2-2.0% present in the form of phospholipids (PLs). The PL contents and fatty acid distribution in PL species have been investigated as bioactive elements for the production of infant formulas, since they offer potential benefits for the optimum growth and health of the newborn infant. The differences in the amount of PLs and in fatty acid distribution in PL species between human milk and infant formulas can imply biologically significant differences for newborn infants fed with infant formulas versus human milk-mainly due to the greater proportion of sphingomyelin with respect to phosphatidylcholine in infant formulas. The limited information referred to the characterization of fatty acid distribution in PL species in infant formulas or in ingredients used to enrich them merits further research in order to obtain products with benefits similar to those of human milk in terms of infant growth, visual acuity, and neurological development. The present review establishes the scientific basis for helping to adjust formulations to the requirements of infant nutrition.

Molecular dynamics study of lipid bilayers modeling the plasma membranes of mouse hepatocytes and hepatomas

Molecular dynamics (MD) calculations of lipid bilayers modeling the plasma membranes of normal mouse hepatocytes and hepatomas in water have been performed under physiological isothermal-isobaric conditions (310.15 K and 1 atm). The changes in the membrane properties induced by hepatic canceration were investigated and were compared with previous MD calculations included in our previous study of the changes in membrane properties induced by murine thymic canceration. The calculated model membranes for normal hepatocytes and hepatomas comprised 23 and 24 kinds of lipids, respectively. These included phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, sphingomyelin, lysophospholipids, and cholesterol. We referred to previously published experimental values for the mole fraction of the lipids adopted in the present calculations. The calculated structural and dynamic properties of the membranes such as lateral structure, order parameters, lateral self-diffusion constants, and rotational correlation times all showed that hepatic canceration causes plasma membranes to become more ordered laterally and less fluid. Interestingly, this finding contrasts with the less ordered structure and increased fluidity of plasma membranes induced by thymic canceration observed in our previous MD study.

Miscibility of Sphingomyelins and Phosphatidylcholines in Unsaturated Phosphatidylcholine Bilayers

Polyunsaturated phospholipids are common in biological membranes and affect the lateral structure of bilayers. We have examined how saturated sphingomyelin (SM; palmitoyl and stearoyl SM (PSM and SSM, respectively)) and phosphatidylcholine (PC; dipalmitoyl PC and 1-palmitoyl-2-stearoyl PC (DPPC and PSPC, respectively)) segregate laterally to form ordered gel phases in increasingly unsaturated PC bilayers (sn-1: 16:0 and sn-2: 18:1...22:6; or sn-1 and sn-2: 18:1...22:6). The formation of gel phases was determined from the lifetime analysis of trans-parinaric acid. Using calorimetry, we also determined gel phase formation by PSM and DPPC in unsaturated PC mixed bilayers. Comparing PSM with DPPC, we observed that PSM formed a gel phase with less order than DPPC at comparable bilayer concentrations. The same was true when SSM was compared with PSPC. Furthermore, we observed that at equal saturated phospholipid concentration, the gel phases formed were less ordered in unsaturated PCs having 16:0 in sn-1, as compared to PCs having unsaturated acyl chains in both sn-1 and sn-2. The gel phases formed by the saturated phospholipids in unsaturated PC bilayers did not appear to achieve properties similar to pure saturated phospholipid bilayers, suggesting that complete lateral phase separation did not occur. Based on scanning calorimetry analysis, the melting of the gel phases formed by PSM and DPPC in unsaturated PC mixed bilayers (at 45 mol % saturated phospholipid) had low cooperativity and hence most likely were of mixed composition, in good agreement with trans-parinaric acid lifetime data. We conclude that both interfacial properties of the saturated phospholipids and their chain length, as well as the presence of 16:0 in sn-1 of the unsaturated PCs and the total number of cis unsaturations and acyl chain length (18 to 22) of the unsaturated PCs, all affected the formation of gel phases enriched in saturated phospholipids, under the conditions used.

Ceramide-enriched membrane domains--structure and function

Membrane lipids seem to be organized and not randomly distributed in the cell membrane. In particular, sphingolipids seem to interact with cholesterol in the outer leaflet of the cell membrane resulting in the formation of distinct membrane domains, i.e. rafts. The generation of ceramide within rafts alters their biophysical properties and results in the formation of large ceramide-enriched membrane platforms. These platforms serve to cluster receptor molecules and to organize intracellular signalling molecules to facilitate signal transduction via a receptor upon stimulation. Thus, ceramide-enriched membrane domains amplify not only receptor-, but also stress-mediated signalling events. Although many receptors cluster, the molecular mechanisms mediating this important and general event in signal transduction need to be identified.

Docosahexaenoic acid domains: the ultimate non-raft membrane domain

What distinguishes polyunsaturated fatty acids (PUFAs) from less unsaturated fatty acids is the presence of a repeating =CH-CH(2)-CH= unit that produces an extremely flexible structure rapidly isomerizing through conformational states. Docosahexaenoic acid (DHA) with 6 double bonds is the most extreme example. The focus of this review is the profound impact that the high disorder of DHA has on its interaction with cholesterol when the PUFA is incorporated into membrane phospholipids. Results from a battery of biophysical techniques are described. They demonstrate an aversion of DHA for the sterol that drives the lateral segregation of DHA-containing phospholipids into liquid disordered (l(d)) domains that are depleted in cholesterol. These domains are compositionally and organizationally the antithesis of lipid rafts, the much-studied liquid ordered (l(o)) domain that is enriched in predominantly saturated sphingolipids and cholesterol. We hypothesize that the introduction of DHA-rich domains into the plasma membrane where they coexist with lipid rafts is the origin, in part, of the astonishing diversity of health benefits that accrue from dietary consumption of DHA. According to our model, changes in the conformation of signaling proteins when they move between these disparate domains have the potential to modulate cell function.

Physiological and pathophysiological aspects of ceramide

Activation of cells by receptor- and nonreceptor-mediated stimuli not only requires a change in the activity of signaling proteins but also requires a reorganization of the topology of the signalosom in the cell. The cell membrane contains distinct domains, rafts that serve the spatial organization of signaling molecules in the cell. Many receptors or stress stimuli transform rafts by the generation of ceramide. These stimuli activate the acid sphingomyelinase and induce a translocation of this enzyme onto the extracellular leaflet of the cell membrane. Surface acid sphingomyelinase generates ceramide that serves to fuse small rafts and to form large ceramide-enriched membrane platforms. These platforms cluster receptor molecules, recruit intracellular signaling molecules to aggregated receptors, and seem to exclude inhibitory signaling factors. Thus ceramide-enriched membrane platforms do not seem to be part of a specific signaling pathway but may facilitate and amplify the specific signaling elicited by the cognate stimulus. This general function may enable these membrane domains to be critically involved in the induction of apoptosis by death receptors and stress stimuli, bacterial and viral infections of mammalian cells, and the regulation of cardiovascular functions.

Ceramide-enriched membrane domains

Cellular activation involves the re-organization of receptor molecules and the intracellular signalosom in the cell membrane. Recent studies indicate that specialized domains of the cell membrane, termed rafts, are central for the spatial organization of receptors and signaling molecules. Rafts are converted into larger membrane platforms by activity of the acid sphingomyelinase, which hydrolyses raft-sphingomyelin to ceramide. Ceramide molecules spontaneously associate to form ceramide-enriched microdomains, which fuse to large ceramide-enriched membrane platforms. The acid sphingomyelinase is activated by multiple stimuli including CD95, CD40, DR5/TRAIL, CD20, FcgammaRII, CD5, LFA-1, CD28, TNF, the Interleukin-1 receptor, the PAF-receptor, CD14, infection with P. aeruginosa, S. aureus, N. gonorrhoeae, Sindbis-Virus, Rhinovirus, treatment with gamma-irradiation, UV-light, doxorubicin, cisplatin, disruption of integrin-signaling and under some conditions of developmental death. Ceramide-enriched membrane platforms serve the clustering of receptors, the recruitment of intracellular signaling molecules and the exclusion of inhibitory signaling factors and, thus, facilitate signal transduction initiated by the specific stimulus.

Docosahexaenoic acid: membrane properties of a unique fatty acid

Docosahexaenoic acid (DHA) with 22-carbons and 6 double bonds is the extreme example of an omega-3 polyunsaturated fatty acid (PUFA). DHA has strong medical implications since its dietary presence has been positively linked to the prevention of numerous human afflictions including cancer and heart disease. The PUFA, moreover, is essential to neurological function. It is remarkable that one simple molecule has been reported to affect so many seemingly unrelated biological processes. Although details of a molecular mode of action remain elusive, DHA must be acting at a fundamental level common to many tissues that is related to the high degree of conformational flexibility that the multiple double bonds have been identified to confer. One likely target for DHA action is at the cell membrane where the fatty acid is known to readily incorporate into membrane phospholipids. Once esterified into phospholipids DHA has been demonstrated to significantly alter many basic properties of membranes including acyl chain order and "fluidity", phase behavior, elastic compressibility, permeability, fusion, flip-flop and protein activity. It is concluded that DHA's interaction with other membrane lipids, particularly cholesterol, may play a prominent role in modulating the local structure and function of cell membranes.

Dietary n-3 fatty acids influence the lipid composition and physical properties of liver microsomal membranes in diabetic rats

We examined the effect of n-3 fatty acid consumption on the lipid composition and physical properties of liver microsomal membranes in normal and experimental diabetic rats. Lipid analysis showed a significant increase in the cholesterol:phospholipid ratio in membranes of normal animals fed n-3 fatty acids as well as in both groups of diabetic rats. These changes would be in part responsible for the higher fluorescent polarization of DPH (1,6-diphenyl-1,3,5 hexatriene) observed in the diabetic groups compared with the normal ones. These alterations were partially compensated by an increase in the amount of phosphatidylcholine in the diabetic rats fed on n-3 fatty acids. However, proteins also play a role in determining the physical properties of the liver microsomes because in the liposomes derived from them, the fluorescent polarization of DPH decreased in the diabetics fed n-3 fatty acids. Measurements of fluorescence anisotropy of n-AS (2-, 7 and 12 (9 anthroyloxy) stearic acid) probes revealed a restricted rotational mobility in the middle zone of the bilayer. Consistently with this finding there was an elevation in the calculated unsaturation density of the fatty acids at the carbon 8 position. These experiments confirm the lipid abnormalities that take place in experimental diabetes and they show further that n-3 fatty-acid administration causes certain compensatory, and thus beneficial, changes in these abnormalities.

In vitro modulation of rat adipocyte ghost membrane fluidity by cholesterol oxysterols

The effects of cholesterol and cholesterol-derived oxysterols (cholestanone, cholestenone, coprostanone and epicoprostanol) on adipocyte ghost membrane fluidity were studied using a fluorescence depolarization method. The fluorescence anisotropy of the treated membranes was determined using 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH). Cholestanone and cholesterol decreased membrane fluidity at both the concentrations tested (10 & 50 microM) while the rest of the sterols did not exert any significant effect on membrane fluidity. In the presence of epinephrine, cholestanone partitioned more towards the lipid core but cholesterol partitioning was not affected. The fusion activation energies (delta E) obtained for membranes preincubated with cholestanone (8.6 kcal/mol) and cholesterol (8.2 kcal/mol) were not significantly different from that of untreated membranes (8.3 kcal/mol). Membranes preincubated with cholestanone and cholesterol did not exhibit any change in lipid phase throughout the temperature range (10-45 degrees C) tested. The sterols were found to inhibit fisetin-induced phospholipid methylation in isolated rat adipocytes in the rank order of cholesterol > epicoprostanol > cholestanone = cholestenone = coprostanone, while basal methylation was unaffected. When adipocytes were preincubated with the sterols before the addition of fisetin, cholestanone and cholestenone showed 74% and 66% inhibition of maximal methylation respectively. These results indicated that cholesterol oxysterols interact differently with rat adipocyte membranes, with cholestanone interacting more with phospholipids located at the inner lipid bilayer (e.g. phosphatidylethanolamine) while cholesterol interacts more with phosphatidylcholine located at the outer lipid bilayer. This differential interaction may cause selective changes in membrane fluidity at different depths of the bilayer and thus may modulate the activities of membrane-bound proteins such as enzymes and receptors.

Membrane properties of metastatic and non-metastatic cells cultured from C3H mice injected with LM fibroblasts

Little is known regarding the membrane properties of metastatic cells as compared to non-metastatic tumor cells. In order to remove variables such as site of growth and nutrition, C3H mice and LM fibroblasts were used as a model system to derive cell lines from local tumors and lung metastases. LM cells were injected subcutaneously into C3H mice and local skin tumors and secondary lung tumors were isolated, cultured in vitro and analyzed. The activities of lipid-sensitive membrane enzymes, membrane lipid composition, and membrane structure were correlated with metastatic ability. Plasma membranes and microsomes of the cultured metastatic cells had 3.8 +/- 0.5- and 5.4 +/- 0.6-fold elevated 5'-nucleotidase activity, respectively, as compared to plasma membranes and microsomes of cultured non-metastatic cells. The mitochondria of cultured metastatic cells had 3.5 +/- 0.5-fold decreased succinate-dependent cytochrome-c reductase activity as compared to mitochondria of the cultured non-metastatic cells. The lipids of plasma membranes from the metastatic cells had 30 +/- 2% and 46 +/- 7% lower phosphatidylinositol and sterol/phospholipid ratio, respectively, and 30 +/- 3% increased unsaturated/saturated fatty acid as compared to cultured non-metastatic cells. The lower sterol/phospholipid ratio correlated with a 30 +/- 1% lower level of cytosolic sterol carrier protein in the cultured metastatic cells as compared to cultured non-metastatic cells. Multifrequency phase and modulation fluorometry in conjunction with the fluorescence probe, 1,6-diphenyl-1,3,5-hexatriene, was used to determine the static and dynamic aspects of membrane fluidity. The plasma membranes and microsomes of cultured metastatic cells were more fluid than those of cultured non-metastatic cells as indicated by 24 +/- 3% and 7 +/- 1%, respectively, lower limiting anisotropy of 1,6-diphenyl-1,3,5-hexatriene in the membranes of the metastatic as compared to non-metastatic cells.

Role of polyunsaturated fatty acids and lipid peroxidation in LM fibroblast plasma membrane transbilayer structure

The effects of polyunsaturated fatty acids and lipid peroxidation on LM fibroblast plasma membrane individual leaflet sterol distribution and structural order were examined. The cytofacial (inner) leaflet was more rigid and contained more sterol than the exofacial (outer) leaflet. The static (limiting anisotropy) and dynamic (rotational relaxation time) structural components of diphenylhexatriene (DPH) motion in each leaflet were determined by phase and modulation fluorometry measurements combined with leaflet-specific quenching by trinitrophenyl groups. Polyunsaturated fatty acids, incorporated into the membrane phospholipids by culture medium supplementation, decreased the limiting anisotrophy of DPH in the cytofacial but not the exofacial leaflet thereby abolishing the transbilayer difference in fluidity. Peroxidation by Fe(II) + H2O2 resulted in a rigidification (increase in limiting anisotropy and rotational relaxation time) of the plasma membrane exofacial leaflet, regardless of whether the membranes contained saturated and monounsaturated fatty acids or were enriched in either linoleate or linolenate. The structure of the cytofacial leaflet reported by DPH was unaffected. Plasma membrane transbilayer sterol distribution, measured by leaflet-specific quenching of dehydroergosterol fluorescence, indicated that 20-28% of the sterol was localized in the exofacial leaflet. Polyunsaturated fatty acid supplementation of LM fibroblasts resulted in a complete reversal of plasma membrane transbilayer sterol distribution (72-76% exofacial leaflet). Sterol transbilayer distribution between the membrane leaflets was completely resistant to alteration by exposure to crosslinking agents and peroxidation in control plasma membranes and by peroxidation in linoleate- or linolenate-supplemented membranes.

Lipid characteristics of RSV-transformed Balb/c 3T3 cell lines with different spontaneous metastatic potentials

To determine whether a metastatic phenotype may be correlated with a characteristic lipid pattern, we compared the lipid composition of low metastasizing Balb/c 3T3 cells transformed by the B77 strain of Rous sarcoma virus (B77-3T3 cells) with that of a subclone isolated by growth in 0.6% agar, the B77-AA6 cells, which exhibit a high capacity for spontaneous metastasis. B77-3T3 cells revealed characteristics in their lipid composition common to other systems of transformed cells, i.e., an accumulation of ether-linked lipids, a reduction of the more complex gangliosides, an increase of oleic acid (18:1) and a decrease of arachidonic (20:4) and C22 polyunsaturated fatty acids in phospholipids. High metastatic B77-AA6 cells showed: a) an even more marked decrease of complex gangliosides; b) a more pronounced increase of 18:1 and decrease of 20:4 and 22 polyunsaturated fatty acids in certain phospholipid classes; and c) a higher percentage of alkyl-acyl subfractions in both phosphatidylcholine and phosphatidylethanolamine than B77-3T3 cells. Comparing the data for other systems of metastatic cells with those of lipid studies of spontaneously metastasizing B77-AA6 cell system leads us to conclude that the metastatic phenotype is characterized by a change in ether-linked lipids, rather than in fatty acids.

Manipulation of plasma membrane physical state affects desaturase activity in rat lymphocytes

Thymocytes, obtained from young rats, were incubated in the presence of either diheptadecanoylphosphatidylcholine or dioleylphosphatidylcholine vesicles and desaturation of either [1-14C]stearic acid or [1-14C]linoleic acid was followed in the endoplasmic reticulum. Incubation with diheptadecanoylphosphatidylcholine resulted in an accumulation of heptadecanoic acid in the plasma membrane, but not in the endoplasmic reticulum and mitochondria, and an increase in membrane ordering as assessed by diphenylhexatriene fluorescence polarization. A shift to higher temperature of the phase separation in the plasma membrane was also observed. Both delta 9 and delta 6 desaturase activities were enhanced in these cells, with delta 6 responding more intensly. Accumulation of oleic acid in the plasma membrane could not be observed when the cells were incubated in the presence of dioleylphosphatidylcholine vesicles, but all the membranes separated, including the microsomes, became more fluid. This can be attributed to removal of cholesterol by the vesicles. Fluidization of plasma membrane and endoplasmic reticulum depressed the conversion of stearate to oleate and linoleate to gamma-linolenate. It is concluded that there is an exchange of information between the plasma membrane and the endoplasmic reticulum in order to maintain the proper fluidity relationships and that this occurs without transfer of lipids from the former to the latter.

Increase of erythrocyte resistance to hemolysis and modification of membrane lipids induced by hemodialysis

Mechanical fragility, deformability and lipid composition of erythrocyte membranes were studied in 22 uremic dialysed patients before and after dialysis in comparison to controls. While deformability was not affected by a dialysis session, osmotic and saponin resistances to hemolysis were significantly increased after dialysis (p less than 0.001). The lipid composition of erythrocyte membranes was also altered during dialysis, with a notable increase in cholesterol and a different phospholipid distribution, i.e. phosphatidylcholine and sphingomyelin significantly decreased (p less than 0.001). Likewise, the fatty acid distribution showed large variations with an increase in polyunsaturated fatty acid. The acute effect of dialysis on the membrane lipid composition seems to be linked to erythrocyte alterations. Its significance is discussed in relation to the asymmetric transverse distribution of phospholipids previously described in human erythrocyte.

Spin label study of the effect of ticlopidine on platelets

The effect of ticlopidine on platelet membrane fluidity was investigated using a spin label technique. Ticlopidine, when orally administered to rats, increased both the order parameter and the motion parameter, indicating a decrease in the membrane fluidity of platelets. On the other hand, the order parameter and the motion parameter decreased markedly when the platelets were aggregated by thrombin. Ticlopidine inhibited the thrombin-induced aggregation of platelets and caused a slight increase in order parameter and motion parameter in thrombin-aggregated platelets. Judging from sodium dodecyl sulfate polyacrylamide gel electrophoresis, ticlopidine did not modify the electrophoretic pattern of platelet proteins appreciably. Ticlopidine decreased cholesterol/phospholipids molar ratio and increased slightly total amounts of proteins of the platelets. These results indicate that the inhibitory action by ticlopidine was accompanied by changes in membrane fluidity, and these changes were due to a perturbation of the membrane phospholipid core of the platelets by ticlopidine and/or its metabolites.

Ethanol's effects on cortical adenylate cyclase activity

The effects of ethanol on beta-adrenergic receptor-coupled adenylate cyclase (AC) of mouse cerebral cortex were examined. The addition of ethanol (20-500 mM) to incubation mixtures containing cortical membranes demonstrated that ethanol could increase AC activity and potentiate the stimulatory effects of guanylyl-imidodiphosphate [Gpp(NH)p] on AC activity. Ethanol increased the rate of activation of AC by guanine nucleotides and concomitantly decreased the EC50 for magnesium required to achieve maximal stimulation of cortical AC. The EC50 values for Gpp(NH)p and isoproterenol stimulation of AC activity were also altered by ethanol. Ethanol was capable of stimulating AC extracted by use of digitonin. The AC activity in the digitonin extract was no longer sensitive to the addition of Gpp(NH)p or NaF, but was still stimulated by ethanol. We propose multiple sites of action for ethanol in stimulating cortical AC activity. These sites include actions at the beta-adrenergic receptor, at the G/F coupling proteins, and at the catalytic unit of cortical AC. Comparison of ethanol's actions on cortical beta receptor coupled AC activity with prior reported actions of ethanol on striatal dopamine (DA)-sensitive AC indicated differential sensitivities of these two AC systems to ethanol. These differences may be determined by specific coupling characteristics of the striatal and cortical AC systems or by differences in the plasma membranes in which striatal and cortical AC systems are located.

Role of membrane lipid asymmetry in aging

Recent advances in our understanding of the asymmetric distribution of lipids across nervous system membranes coupled with the application of biophysical techniques to examine transbilayer structure and function have led to the formulation of a new hypothesis. The author hopes that the insights presented herein will stimulate investigation into this developing new field. The theory provides an approach to correlation the accumulation of nervous tissue membrane peroxidative and cross-linking damage, the loss of transbilayer lipid asymmetry, and loss of transbilayer neuroendocrine, transport, secretory and immunoregulatory functions. Central to this scheme is the role of membrane lipid asymmetry in regulation to and/or coupling of transbilayer functions.

The fluidity of plasma membranes from ethanol-treated rat liver

Male Wistar rats were maintained for 35-40 days on a liquid diet containing 36% of calories as ethanol. Ethanol was replaced by carbohydrates in the isocaloric diet fed to control animals. The effect of ethanol consumption has been studied on the fluorescence polarization of rat liver plasma membranes and artificial lipid vesicles and on the lipid composition of the membranes. Fluorescence polarization in both membranes and vesicles was determined using DPH and TMA-DPH as fluorescence markers; from these data, the polarization term (ro/r-l)-1 and flow activation energy (delta E) were calculated. The ethanol consumption induces a more fluid environment within the membrane core of liver plasma membranes; the ethanol-fed rat membranes are more resistant to the in vitro effect of ethanol disordering the membrane structure. Vesicles obtained with lipids from either control membranes or ethanol-fed rat membranes were treated with ethanol and the changes in polarization paralleled to those exhibited by the membranes. The absence of phase transitions and of delta E changes was also shown in temperature-dependence studies. The lower cholesterol content found in ethanol-fed rat plasma membranes might be responsible for observed variations in the microviscosity.

Measles-virus-persistent infection in BGM cells. Modification of the incorporation of [3H]arachidonic acid and [14C]stearic acid into lipids

In BGM cells chronically infected with measles virus, although the composition of the phospholipids is unaltered, the fatty acid composition is modified. Uninfected, lytic and persistently infected cells were labelled with [3H]arachidonic acid and [14C]stearic acid and their metabolic fate analysed. No difference in the total incorporation was observed in the different systems. However, the [14C]stearic acid and [3H]arachidonic acid were incorporated up to 2-fold and 13-fold respectively greater into the neutral lipid of persistently infected compared with that of uninfected cells. Both radioactive fatty acids were specifically accumulated in the triacylglycerol and non-esterified fatty acids fractions. Lytically infected cells were similar to uninfected cells. Although there was no significant difference in the incorporation of radioactivity into the total phospholipid in either system, there was a large decrease in [3H]arachidonic acid incorporated into phosphatidylethanolamine and to a lesser extent phosphatidylcholine and phosphatidylinositol in persistently infected cells. [14C]Stearic acid incorporation was also reduced in phosphatidylcholine and phosphatidylethanolamine fractions of persistently infected cells.

Elevated membrane cholesterol concentrations inhibit glucagon-stimulated adenylate cyclase

A method was devised which increases the cholesterol concentration of rat liver plasma membranes by exchange from cholesterol-rich liposomes at low temperature (4 degrees C). When the cholesterol concentration of liver plasma membranes is increased, there is an increase in lipid order as detected by a decrease in mobility of an incorporated fatty acid spin probe. This is accompanied by an inhibition of adenylate cyclase activity. The various ligand-stimulated adenylate cyclase activities exhibit different sensitivities to inhibition by cholesterol, with inhibition of glucagon-stimulated greater than fluoride-stimulated greater than basal activity. The bilayer-fluidizing agent benzyl alcohol is able to reverse the inhibitory effect of cholesterol on adenylate cyclase activity in full. The thermostability of fluoride-stimulated cyclase is increased in the cholesterol-rich membranes. Elevated cholesterol concentrations abolish the lipid-phase separation occurring at 28 degrees C in native membranes as detected by an incorporated fatty acid spin probe. This causes Arrhenius plots of glucagon-stimulated adenylate cyclase activity to become linear, rather than exhibiting a break at 28 degrees C. It is suggested that the cholesterol contents of both halves of the bilayer are increased by the method used and that inhibition of adenylate cyclase ensues, owing to the increase in lipid order and promotion of protein-protein and specific cholesterol-phospholipid interactions.

Ethanol effects on striatal dopamine receptor-coupled adenylate cyclase and on striatal opiate receptors

The perturbation of neuronal cell membranes by ethanol may result in specific functional changes through modification of the activity of various membrane-bound proteins. In mouse striatum, adenylate cyclase, a membrane-bound enzyme, is coupled to dopamine, as well as to opiate, receptors. Ethanol stimulates striatal adenylate cyclase activity by modifying the regulatory protein ("G-protein")-adenylate cyclase interaction to produce an increased amount of activated enzyme. This action is additive with the effects of dopamine on adenylate cyclase. Ethanol also modifies striatal opiate receptor-effector coupling processes. In the presence of ethanol, opiate receptor affinity is altered, and this alteration is modified by GTP, suggesting that ethanol influences the interaction of the opiate receptor complex with the G-protein. Our results suggest that ethanol can affect receptor-effector coupling, including the binding of opiate agonists to their receptors, through its membrane-disordering capacity, and that particular systems may react in a relatively specific manner with ethanol.

Effects of ethanol on Arrhenius parameters and activity of mouse striatal adenylate cyclase

Arrhenius plots of basal and dopamine (DA)-stimulated adenylate cyclase activities exhibited discontinuities at 20 degrees, while the plot of fluoride-stimulated adenylate cyclase activity was linear over the studied temperature range. None of the Arrhenius parameters were altered by in vitro addition of ethanol (75 or 750 mM) to enzyme assay mixtures, and Arrhenius parameters were found to be unchanged when enzyme obtained from animals rendered tolerant to, and physically dependent on, ethanol was assayed. The differences between the response to ethanol of adenylate cyclase and the response of other membrane-bound enzymes [e.g. (Na+-K+)ATPase], as measured by Arrhenius plots, may indicate different sites of action of ethanol. When the specific activity of adenylate cyclase was examined, ethanol was found to stimulate activity at all temperatures tested. The dose-response curve for ethanol activation of basal adenylate cyclase activity was shifted to the right for enzyme obtained from mice chronically treated with ethanol. Analysis of the data indicated that activation of adenylate cyclase by ethanol (as well as by DA) was an entropy-driven process. Since ethanol treatment did not affect the Arrhenius parameters, which appear to be associated with membrane lipids, it is suggested that enzyme activation by ethanol results from direct effects on the enzyme or regulatory protein. Resistance to this effect occurs through changes in protein conformation following chronic ethanol treatment.

Transbilayer distribution of phospholipid fatty acyl chains in photoreceptor membrane

The transverse distribution of the fatty acyl chains of the major phospholipids over the two faces of the photoreceptor membranes has been determined in bovine rod outer segment (stacked disk) preparations. For this purpose, the fatty acid composition of the phospholipids has been analyzed before and after treatment with trinitrobenzenesulfonic acid and phospholipase D. The latter agents are used under conditions in which they have been demonstrated to attack only the outer (cytoplasmic) face of the membrane. After treatment with trinitrobenzenesulfonic acid or phospholipase D, the fatty acid composition of the unreacted phospholipids is the same as that before treatment, regardless of the extent of modification or hydrolysis attained. The fatty acid composition of phosphatidic acid, resulting from phospholipase D action, also remains unchanged during progressive hydrolysis. These results indicate that the fatty acyl chains of the major phospholipids have the same composition on either side of the disk membrane. Together with our previously published evidence for the distribution of the major phospholipids in rod outer segment disk membranes, this means that both the phospholipids and their fatty acyl chains have a remarkably symmetrical distribution over the two membrane faces. On the basis of literature data it is concluded that this approximate symmetry reflects the high mobility of the entire phospholipid pool of disk membranes, thus including appreciable transbilayer movements of the phospholipids.

Plasma membrane aminophospholipid distribution in transformed murine fibroblasts

The possibility that the asymmetric distribution of aminophospholipids may be an intrinsic property of mammalian plasma membranes was examined in LM cells, a transformed murine fibroblast cell line. The cells were grown in suspension culture in a chemically defined medium without lipid, protein, or serum and then treated with 2,4,6-trinitrobenzenesulfonic acid (TNBS). A maximum of 4% of LM cell plasma membrane phosphatidylethanolamine and 5% of the phosphatidylserine was labelled with TNBS. Furthermore, long chain and unsaturated fatty acids were preferentially esterified to the non-derivatized phosphatidylethanolamine (inner monolayer) as compared to phosphatidylethanolamine derivatized with TNBS (outer monolayer). Isethionyl acetimidate, an alternative non-penetrating reagent, confirmed the results obtained with TNBS and provided supportive evidence for the highly asymmetric distribution of phosphatidylethanolamine; 6% of the phosphatidylethanolamine was labelled with isethionyl acetimidate. When the penetrating reagent methylacetimidate was used, more than 80% of the phosphatidylethanolamine was derivatized. Although the growing of the LM cells in 10% calf serum significantly increased plasma membrane phosphatidylcholine while decreasing phosphatidylethanolamine, calf serum had no significant effect on phosphatidylethanolamine or phosphatidylserine asymmetry.

Lipid composition and microviscosity of subcellular fractions from rabbit thymocytes. Differences in the microviscosity of plasma membranes from subclasses of thymocytes

There are indications from freeze-fracture experiments that subclasses of rabbit thymocytes show different mobilities of plasma membrane components. Consequently, one would expect differences in the fluidity of the plasma membrane. For this reason, rabbit thymocytes were separated on a Ficoll/Metrizoate gradient yielding three subclasses representing various levels of cell differentiation. These thymocyte subclasses did not show any significant differences in the degree of fluorescence polarization using the probe 1,6-diphenyl-1,3,5-hexatriene. The fluorescence polarization of the plasma membrane may be overshadowed by the contribution of all cellular lipids due to penetration of the fluorescent probe into the cell. Therefore, plasma membranes were isolated from rabbit thymocytes using a cell-disrupting pump, differential centrifugation, and sucrose density gradient centrifugation. As shown by biochemical and electron microscopical analyses, plasma membranes with a high degree of purity were obtained. As expected the plasma membrane fractions showed a higher microviscosity than the other subcellular fractions. This was attributed to a higher cholesterol to phospholipid molar ratio and a higher degree of saturation of phospholipid fatty acid chains. Subsequently, the microviscosity was measured of plasma membrane preparations obtained from two main subclasses of thymocytes representing mature and immature lymphocytes. The immature thymocytes yielded two plasma membrane fractions with higher microviscosity than the mature cells. These finding is in line with earlier observed differences in the glycerol-induced clustering of intramembranous particles. Furthermore, the results of this study support the view that the fluorescence polarization technique applied to whole cells does not exclusively monitor the plasma membrane.

Gross structural changes in isolated liver cell plasma membranes upon binding of insulin

The addition of 10(-9) M insulin to a suspension of rat liver plasma membranes increases the overall lipid microviscosity, eta, by about 10--20%. The effect is confined to physiological concentrations of the hormone and is highly specific. The specificity was demonstrated in experiments where insulin analogues were added to liver plasma membranes and where insulin was added to human erythrocyte membranes. In both of these experiments practically no change in eta was detected. Upon in vitro enrichment of the membrane cholesterol, eta exceeded the level mediated by insulin binding, and the addition of 10(-9) M insulin to the cholesterol-enriched membranes did not further increase eta. Concomitant to the increase in eta upon insulin binding, the overall degree of exposure of the membrane protein, presumably to both sides of the membrane, is substantially increased. This effect is in line with the notion of vertical displacement of membrane proteins induced by changes in eta. The observed structural modulation can account for the effect of insulin on unrelated membrane responses, as well as for the negative cooperativity of insulin binding.

The effects of serum albumin and phospholipid on sterol excretion in tissue culture cells

Serum albumin was as effective as whole serum or alpha-globulins in facilitating sterol release from strain L mouse fibroblasts. Commercial bovine serum albumin preparations, however, had markedly different absolute effects in this regard. These differences were attributable to the variation in phospholipid content of these products. All but one of these albumins enhanced sterol release when supplemented with phospholipid. The exception was fatty acid-poor albumin which contained an adequate amount of phospholipid. Among the phospholipids examined, lecithin proved to be most effective, while phosphatidylethanolamine had little potentiating influence. As the unsaturation of the test lecithins increased, enhancement of sterol release decreased. The potentiating effect of the phospholipid was in turn dependent on the protein used, since the phenomenon was not observed with non-serum proteins like ovalbumin or with non-transport serum proteins such as gamma-globulins. The results of these studies raise the possibility that serum albumin together with phospholipid can play an important role in sterol release in tissue culture cells.

Effect of exogenous fatty acids on the retention of phospholipid acyl groups by mouse L fibroblasts

Exogenous oleic or linoleic acid, given at a high but nontoxic level (1 mg fatty acid/day for 20 . 10(6) cells in 50 ml medium), caused substantial redistribution of the otherwise permanently retained phospholipid acyls in mouse L fibroblasts. 18--40% of the preformed phospholipid acyls were shifted to triglycerides but most returned to phospholipids when the supply of exogenous fatty acid was removed. The phospholipid acyls could be reshuttled back to triglycerides again whenever an adequate amount of exogenous fatty acid was provided. Daily changes of medium containing oleic acid bound to bovine serum albumin caused a still greater total loss of phospholipid acyls into the medium. The removal of the prelabeled phospholipid acyls also occurred with phospholipid acyls which had been synthesized from [1-(14C)]acetate 3 days earlier. The results demonstrate the fact that the apparent permanently retained phospholipid acyl groups found in L-cells could in fact be displaced through experimental manipulations.