Showing papers on "Membrane lipids published in 1999"

A proteolytic pathway that controls the cholesterol content of membranes, cells, and blood

Abstract: The integrity of cell membranes is maintained by a balance between the amount of cholesterol and the amounts of unsaturated and saturated fatty acids in phospholipids. This balance is maintained by membrane-bound transcription factors called sterol regulatory element-binding proteins (SREBPs) that activate genes encoding enzymes of cholesterol and fatty acid biosynthesis. To enhance transcription, the active NH2-terminal domains of SREBPs are released from endoplasmic reticulum membranes by two sequential cleavages. The first is catalyzed by Site-1 protease (S1P), a membrane-bound subtilisin-related serine protease that cleaves the hydrophilic loop of SREBP that projects into the endoplasmic reticulum lumen. The second cleavage, at Site-2, requires the action of S2P, a hydrophobic protein that appears to be a zinc metalloprotease. This cleavage is unusual because it occurs within a membrane-spanning domain of SREBP. Sterols block SREBP processing by inhibiting S1P. This response is mediated by SREBP cleavage-activating protein (SCAP), a regulatory protein that activates S1P and also serves as a sterol sensor, losing its activity when sterols overaccumulate in cells. These regulated proteolytic cleavage reactions are ultimately responsible for controlling the level of cholesterol in membranes, cells, and blood.

Aggregation of Lipid Rafts Accompanies Signaling via the T Cell Antigen Receptor

Abstract: The role of lipid rafts in T cell antigen receptor (TCR) signaling was investigated using fluorescence microscopy. Lipid rafts labeled with cholera toxin B subunit (CT-B) and cross-linked into patches displayed characteristics of rafts isolated biochemically, including detergent resistance and colocalization with raft-associated proteins. LCK, LAT, and the TCR all colocalized with lipid patches, although TCR association was sensitive to nonionic detergent. Aggregation of the TCR by anti-CD3 mAb cross-linking also caused coaggregation of raft-associated proteins. However, the protein tyrosine phosphatase CD45 did not colocalize to either CT-B or CD3 patches. Cross-linking of either CD3 or CT-B strongly induced tyrosine phosphorylation and recruitment of a ZAP-70(SH2)2–green fluorescent protein (GFP) fusion protein to the lipid patches. Also, CT-B patching induced signaling events analagous to TCR stimulation, with the same dependence on expression of key TCR signaling molecules. Targeting of LCK to rafts was necessary for these events, as a nonraft- associated transmembrane LCK chimera, which did not colocalize with TCR patches, could not reconstitute CT-B–induced signaling. Thus, our results indicate a mechanism whereby TCR engagement promotes aggregation of lipid rafts, which facilitates colocalization of LCK, LAT, and the TCR whilst excluding CD45, thereby triggering protein tyrosine phosphorylation.

Bricks and mortar of the epidermal barrier

Abstract: A specialized tissue type, the keratinizing epithelium, protects terrestrial mammals from water loss and noxious physical, chemical and mechanical insults. This barrier between the body and the environment is constantly maintained by reproduction of inner living epidermal keratinocytes which undergo a process of terminal differentiation and then migrate to the surface as interlocking layers of dead stratum corneum cells. These cells provide the bulwark of mechanical and chemical protection, and together with their intercellular lipid surroundings, confer water-impermeability. Much of this barrier function is provided by the cornified cell envelope (CE), an extremely tough protein/lipid polymer structure formed just below the cytoplasmic membrane and subsequently resides on the exterior of the dead cornified cells. It consists of two parts: a protein envelope and a lipid envelope. The protein envelope is thought to contribute to the biomechanical properties of the CE as a result of cross-linking of specialized CE structural proteins by both disulfide bonds and N(epsilon)-(gamma-glutamyl)lysine isopeptide bonds formed by transglutaminases. Some of the structural proteins involved include involucrin, loricrin, small proline rich proteins, keratin intermediate filaments, elafin, cystatin A, and desmosomal proteins. The lipid envelope is located on the exterior of and covalently attached by ester bonds to the protein envelope and consists of a monomolecular layer of omega-hydroxyceramides. These not only serve of provide a Teflon-like coating to the cell, but also interdigitate with the intercellular lipid lamellae perhaps in a Velcro-like fashion. In fact the CE is a common feature of all stratified squamous epithelia, although its precise composition, structure and barrier function requirements vary widely between epithelia. Recent work has shown that a number of diseases which display defective epidermal barrier function, generically known as ichthyoses, are the result of genetic defects of the synthesis of either CE proteins, the transglutaminase 1 cross-linking enzyme, or defective metabolism of skin lipids.

Fluorescence correlation spectroscopy with single molecule sensitivity on cell and model membranes.

Abstract: We report on the successful application of fluorescence correlation spectroscopy (FCS) to the analysis of single fluorescently labeled lipid analogue molecules diffusing laterally in lipid bilayers, as exemplified by time traces of fluorescence bursts of individual molecules entering and leaving the excitation area. FCS measurements performed on lipid probes in rat basophilic leukemia cell membranes showed deviations from two-dimensional Brownian motion with a single uniform diffusion constant. Giant unilamellar vesicles were employed as model systems to characterize diffusion of fluorescent lipid analogues in both homogeneous and mixed lipid phases with diffusion heterogeneity. Comparing the results of cell membrane diffusion with the findings on the model systems suggests possible explanations for the observations: (a) anomalous subdiffusion in which evanescent attractive interactions with disparate mobile molecules modifies the diffusion statistics; (b) alternatively, probe molecules are localized in microdomains of submicroscopic size, possibly in heterogeneous membrane phases.

A Role for Lipid Rafts in B Cell Antigen Receptor Signaling and Antigen Targeting

Abstract: The B cell antigen receptor (BCR) serves both to initiate signal transduction cascades and to target antigen for processing and presentation by MHC class II molecules. How these two BCR functions are coordinated is not known. Recently, sphingolipid- and cholesterol-rich plasma membrane lipid microdomains, termed lipid rafts, have been identified and proposed to function as platforms for both receptor signaling and membrane trafficking. Here we show that upon cross-linking, the BCR rapidly translocates into ganglioside GM1-enriched lipid rafts that contain the Src family kinase Lyn and exclude the phosphatase CD45R. Both Igα and Lyn in the lipid rafts become phosphorylated, and subsequently the BCR and a portion of GM1 are targeted to the class II peptide loading compartment. Entry into lipid rafts, however, is not sufficient for targeting to the antigen processing compartments, as a mutant surface Ig containing a deletion of the cytoplasmic domain is constitutively present in rafts but when cross-linked does not internalize to the antigen processing compartment. Taken together, these results provide evidence for a role for lipid rafts in the initial steps of BCR signaling and antigen targeting.

Detergent-insoluble glycosphingolipid/cholesterol-rich membrane domains, lipid rafts and caveolae (review).

Abstract: Within the cell membrane glycosphingolipids and cholesterol cluster together in distinct domains or lipid rafts, along with glycosyl-phosphatidylinositol (GPI)-anchored proteins in the outer leaflet and acylated proteins in the inner leaflet of the bilayer. These lipid rafts are characterized by insolubility in detergents such as Triton X-100 at 4 degrees C. Studies on model membrane systems have shown that the clustering of glycosphingolipids and GPI-anchored proteins in lipid rafts is an intrinsic property of the acyl chains of these membrane components, and that detergent extraction does not artefactually induce clustering. Cholesterol is not required for clustering in model membranes but does enhance this process. Single particle tracking, chemical cross-linking, fluorescence resonance energy transfer and immunofluorescence microscopy have been used to directly visualize lipid rafts in membranes. The sizes of the rafts observed in these studies range from 70-370 nm, and depletion of cellular cholesterol levels disrupts the rafts. Caveolae, flask-shaped invaginations of the plasma membrane, that contain the coat protein caveolin, are also enriched in cholesterol and glycosphingolipids. Although caveolae are also insoluble in Triton X-100, more selective isolation procedures indicate that caveolae do not equate with detergent-insoluble lipid rafts. Numerous proteins involved in cell signalling have been identified in caveolae, suggesting that these structures may function as signal transduction centres. Depletion of membrane cholesterol with cholesterol binding drugs or by blocking cellular cholesterol biosynthesis disrupts the formation and function of both lipid rafts and caveolae, indicating that these membrane domains are involved in a range of biological processes.

Lipid peroxidation and tissue damage.

Abstract: In recent years it has become apparent that the oxidation of lipids, or lipid peroxidation, is a crucial step in the pathogenesis of several disease states in adult and infant patients. Lipid peroxidation is a process generated naturally in small amounts in the body, mainly by the effect of several reactive oxygen species (hydroxyl radical, hydrogen peroxide etc.). It can also be generated by the action of several phagocytes. These reactive oxygen species readily attack the polyunsaturated fatty acids of the fatty acid membrane, initiating a self-propagating chain reaction. The destruction of membrane lipids and the end-products of such lipid peroxidation reactions are especially dangerous for the viability of cells, even tissues. Enzymatic (catalase, superoxide dismutasse) and nonenzymatic (vitamins A and E) natural antioxidant defence mechanisms exist; however, these mechanisms may be overcome, causing lipid peroxidation to take place. Since lipid peroxidation is a self-propagating chain-reaction, the initial oxidation of only a few lipid molecules can result in significant tissue damage. Despite extensive research in the field of lipid peroxidation it has not yet been precisely determined if it is the cause or an effect of several pathological conditions. Lipid peroxidation has been implicated in disease states such as atherosclerosis, IBD, ROP, BPD, asthma, Parkinson's disease, kidney damage, preeclampsia and others.

Analysis of CD44-containing lipid rafts: Recruitment of annexin II and stabilization by the actin cytoskeleton.

Abstract: CD44, the major cell surface receptor for hyaluronic acid (HA), was shown to localize to detergent-resistant cholesterol-rich microdomains, called lipid rafts, in fibroblasts and blood cells. Here, we have investigated the molecular environment of CD44 within the plane of the basolateral membrane of polarized mammary epithelial cells. We show that CD44 partitions into lipid rafts that contain annexin II at their cytoplasmic face. Both CD44 and annexin II were released from these lipid rafts by sequestration of plasma membrane cholesterol. Partition of annexin II and CD44 to the same type of lipid rafts was demonstrated by cross-linking experiments in living cells. First, when CD44 was clustered at the cell surface by anti-CD44 antibodies, annexin II was recruited into the cytoplasmic leaflet of CD44 clusters. Second, the formation of intracellular, submembranous annexin II-p11 aggregates caused by expression of a trans-dominant mutant of annexin II resulted in coclustering of CD44. Moreover, a frequent redirection of actin bundles to these clusters was observed. These basolateral CD44/annexin II-lipid raft complexes were stabilized by addition of GTPgammaS or phalloidin in a semipermeabilized and cholesterol-depleted cell system. The low lateral mobility of CD44 in the plasma membrane, as assessed with fluorescent recovery after photobleaching (FRAP), was dependent on the presence of plasma membrane cholesterol and an intact actin cytoskeleton. Disruption of the actin cytoskeleton dramatically increased the fraction of CD44 which could be recovered from the light detergent-insoluble membrane fraction. Taken together, our data indicate that in mammary epithelial cells the vast majority of CD44 interacts with annexin II in lipid rafts in a cholesterol-dependent manner. These CD44-containing lipid microdomains interact with the underlying actin cytoskeleton.

Lipids in freshwater ecosystems

Abstract: 1. Determination of Total Lipid, Lipid Classes, and Fatty Acids in Aquatic Samples.- 1.1. Introduction.- 1.2. Results and Discussion.- 1.2.1. Sampling and Storage.- 1.2.2. Lipid Extraction.- 1.2.3. Determination of Total Lipid.- 1.2.4. Determination of Lipid Classes.- 1.2.5. Determination of Fatty Acids and Carbon Number Profiles.- 1.3. Conclusion.- References.- 2. Fatty Acids as Trophic and Chemical Markers in Freshwater Ecosystems.- 2.1. Introduction.- 2.2. Nomenclature.- 2.3. Characteristics of Fatty Acid Markers for Trophic Studies.- 2.4. Primary Sources and Trophic Transfer of Fatty Acids.- 2.4.1. Fatty Acid Composition of Algae and Cyanobacteria.- 2.4.2. Fatty Acids as Trophic Markers of Algae and Cyanobacteria.- 2.4.3. Fatty Acid Composition of Bacteria.- 2.4.4. Fatty Acids as Trophic Markers of Bacteria.- 2.4.5. Fatty Acid Markers from Allochthonous Sources.- 2.4.6. Fatty Acids as Trophic Markers in Vertebrates.- 2.5. Research Needs.- 2.6. Conclusions.- References.- 3. Irradiance and Lipid Production in Natural Algal Populations.- 3.1. Introduction.- 3.2. Metabolism and Reallocation.- 3.2.1. Lipids in Relation to Other Macromolecular Classes.- 3.2.2. Diel Versus Light-Phase Allocation and Synthesis.- 3.2.3. Budgets for Overnight Activity.- 3.2.4. Reallocation Among Lipid Classes.- 3.3. Irradiance and Lipid Synthesis.- 3.3.1. Photosynthetic Parameters.- 3.3.2. Light Saturation Parameter, Ik.- 3.3.3. Production Efficiency Parameter, ?.- 3.3.4. Areal Lipid Production.- 3.3.5. Implications of the Irradiance Response.- 3.4. Conclusions.- 3.5. Research Directions.- References.- 4. Lipids in Freshwater Zooplankton: Selected Ecological and Physiological Aspects.- 4.1. Introduction.- 4.2. Usefulness of Areal Energy Reserve Estimates.- 4.3. Time Course of Lipid Deposition/Loss.- 4.4. Lipids as Indices of Stress.- 4.4.1. Ratio of Storage to Membrane Lipids.- 4.4.2. Maternal Lipid Investment.- 4.4.3. Visible Lipid Energy Stores.- 4.4.4. Fatty Acid Composition and Abundance.- 4.5. Ultraviolet Radiation and Zooplankton Lipids.- 4.6. Research Needs and Suggested Future Directions.- 4.6.1. Geographical Disparities.- 4.6.2. Physicochemical Disparities.- 4.6.3. Essential Fatty Acids.- 4.6.4. Effects of Temperature Changes.- 4.6.5. Diapause.- 4.6.6. Lipids as Allelopathic Compounds and Chemical Feeding Deterrents.- 4.7. Conclusions.- References.- 5. Lipid Dietary Dependencies in Zooplankton.- 5.1. Introduction.- 5.2. Methods.- 5.2.1. Microparticle Preparation.- 5.2.2. Dietary Supplement Experiments.- 5.2.3. Characterization of the Natural Algal Diet.- 5.2.4. Algal Counting and Autoradiography.- 5.3. Results.- 5.3.1. Supplement Experiments with Natural Populations.- 5.3.2. Lake Waynewood Experiment: October 1989.- 5.3.2.1. Algal Diet.- 5.3.2.2. Response byDaphnia.- 5.4. Discussion.- References.- 6. Seasonal Dynamics of Lipids in Freshwater Benthic Invertebrates.- 6.1. Introduction.- 6.2. Results and Discussion.- 6.2.1. Slope and Profundal Zones.- 6.2.1.1. Crustacea: Amphipoda.- 6.2.1.2. Mysidacea.- 6.2.2. Shelf and Nearshore Zones.- 6.2.2.1. Crustacea: Amphipoda.- 6.2.2.2. Annelida: Oligochaeta.- 6.2.2.3. Insecta.- 6.2.2.4. Mollusca: Bivalvia.- 6.3. Research Needs.- 6.4. Conclusions.- References.- 7. Ecological Role of Lipids in the Health and Success of Fish Populations.- 7.1. Introduction.- 7.2. Results and Discussion.- 7.2.1. Overwinter Starvation and Survival.- 7.2.2. Energy Allocation Strategies.- 7.2.3. Reproductive Development and Early Life History.- 7.2.4. Lipids and Environmental Stress.- 7.2.4.1. Contaminant Effects.- 7.2.4.2. Thermal Effects.- 7.2.4.3. Other Stressors.- References.- 8. Lipids and Essential Fatty Acids in Aquatic Food Webs: What Can Freshwater Ecologists Learn from Mariculture?.- 8.1. Introduction.- 8.2. Results and Discussion.- 8.2.1. Some Important Lipids and Fatty Acids.- 8.2.1.1. Essential Fatty Acids.- 8.2.1.2. Lipid Classes.- 8.2.2. Methodological Considerations.- 8.2.3. Physiological Requirements of Marine Animals.- 8.2.3.1. General Evaluation of EFA Requirements.- 8.2.3.2. Anabolic Processes and Growth.- 8.2.3.3. Membrane Transport and Metabolism.- 8.2.3.4. Regulation of Metabolism.- 8.2.3.5. General Considerations and Concluding Remarks.- 8.2.4. Methods for Evaluation of EFA Requirements.- 8.2.5. Symptoms of EFA Deficiency.- 8.2.6. Fatty Acid Transport and Metabolism in Food Webs.- 8.2.6.1. Algae.- 8.2.6.1.1. Lipids of Algae.- 8.2.6.1.2. Essential Fatty Acids of Algae.- 8.2.6.1.3. Conclusion.- 8.2.6.2. Zooplankton.- 8.2.6.2.1. Lipid Content and Lipid Composition.- 8.2.6.2.2. Fatty Acids of TAG-Zooplankton.- 8.2.6.3. Fish.- 8.2.6.3.1. Lipid.- 8.2.6.3.2. Fatty Acid Composition.- 8.2.6.4. General Conclusions.- 8.2.7. Relevance of Mariculture Research.- 8.2.8. Evaluation of Ecological Effects of Essential Fatty Acids.- 8.3. Concluding Remarks.- References.- 9. Influence of Lipids on the Bioaccumulation and Trophic Transfer of Organic Contaminants in Aquatic Organisms.- 9.1. Introduction.- 9.1.1. Sources of Contaminant Gain and Loss in Aquatic Systems.- 9.1.2. Organism Adiposity and Internal Distribution of Contaminants.- 9.1.3. Nonlipid Factors Affecting Internal Distributions.- 9.2. Prediction of Bioconcentration and Bioaccumulation.- 9.2.1. Bioconcentration.- 9.2.2. Bioaccumulation.- 9.3. Factors Affecting Prediction.- 9.3.1. Methods for Measuring Lipid Content.- 9.3.2. Lipid Composition and Bioaccumulation.- 9.4. Mimicking Bioconcentration with Semipermeable Membrane Devices.- 9.5. Toxicity and the Role of Lipid.- 9.5.1. Release of Sequestered Contaminant During Metabolism.- 9.5.2. Lipids and Membrane Narcosis.- 9.5.3. Effect of Toxins on Lipid Metabolism and Function.- 9.6. Relevance of Food Chain Transfer to Bioaccumulation.- 9.6.1. Relevance of Trophic Transfer to Bioaccumulation...- 9.6.2. Role of Lipids in Food Chain Accumulation.- 9.6.2.1. Fugacity Model.- 9.6.2.2. Mechanism for Trophic Transfer.- 9.6.3. Factors Affecting Trophic Transfer.- 9.6.3.1. Assimilation Efficiency.- 9.6.3.2. Miscellaneous Factors Affecting Assimilation.- 9.7. Biomagnification and Organism Lipids.- 9.7.1. Is Biomagnification Real?.- 9.7.2. A Lipid-Based Model for Biomagnification.- 9.7.3. Current Issues in Biomagnification and Relationship to Lipids.- 9.8. Lipids and Transgenerational Transfer of Contaminants.- 9.9. Conclusions.- References.- 10. Lipids in Water-Surface Microlayers and Foams.- 10.1. Introduction.- 10.2. Basic Physicochemistry of Surface Microlayers.- 10.3. Basic Structure of Foams.- 10.4. Sampling Techniques.- 10.5. Physicochemical Processes at the Surface Microlayers.- 10.6. Lipids in the Water-Surface Microlayers and Foams.- 10.6.1. Total Lipids and Major Lipid Classes.- 10.6.2. Fatty Acids.- 10.7. Research Needs.- 10.8. Final Remarks.- References.- 11. Comparison of Lipids in Marine and Freshwater Organisms.- 11.1. Introduction.- 11.2. Discussion.- 11.2.1. Lipid Classes.- 11.2.2. Sterols and Cholesterol.- 11.2.3. Wax Esters and Triacylglycerols.- 11.2.4. Fatty Acids.- 11.2.5. Furan and Some Other Unusual Fatty Acids.- 11.2.6. Ether Lipids.- 11.2.7. Prostanoids.- 11.3. Conclusions.- References.

Invasion by Toxoplasma gondii Establishes a Moving Junction That Selectively Excludes Host Cell Plasma Membrane Proteins on the Basis of Their Membrane Anchoring

Abstract: The protozoan parasite Toxoplasma gondii actively penetrates its host cell by squeezing through a moving junction that forms between the host cell plasma membrane and the parasite. During invasion, this junction selectively controls internalization of host cell plasma membrane components into the parasite-containing vacuole. Membrane lipids flowed past the junction, as shown by the presence of the glycosphingolipid GM1 and the cationic lipid label 1.1′-dihexadecyl-3-3′-3-3′-tetramethylindocarbocyanine (DiIC16). Glycosylphosphatidylinositol (GPI)-anchored surface proteins, such as Sca-1 and CD55, were also readily incorporated into the parasitophorous vacuole (PV). In contrast, host cell transmembrane proteins, including CD44, Na+/K+ ATPase, and β1-integrin, were excluded from the vacuole. To eliminate potential differences in sorting due to the extracellular domains, parasite invasion was examined in host cells transfected with recombinant forms of intercellular adhesion molecule 1 (ICAM-1, CD54) that differed in their mechanism of membrane anchoring. Wild-type ICAM-1, which contains a transmembrane domain, was excluded from the PV, whereas both GPI-anchored ICAM-1 and a mutant of ICAM-1 missing the cytoplasmic tail (ICAM-1–Cyt−) were readily incorporated into the PV membrane. Our results demonstrate that during host cell invasion, Toxoplasma selectively excludes host cell transmembrane proteins at the moving junction by a mechanism that depends on their anchoring in the membrane, thereby creating a nonfusigenic compartment.

Amphitropic proteins: regulation by reversible membrane interactions (Review)

Abstract: What do Src kinase, Ras-guanine nucleotide exchange factor, cytidylyltransferase, protein kinase C, phospholipase C, vinculin, and DnaA protein have in common? These proteins are amphitropic, that is, they bind weakly (reversibly) to membrane lipids, and this process regulates their function. Proteins functioning in transduction of signals generated in cell membranes are commonly regulated by amphitropism. In this review, the strategies utilized by amphitropic proteins to bind to membranes and to regulate their membrane affinity are described. The recently solved structures of binding pockets for specific lipids are described, as well as the amphipathic alpha-helix motif. Regulatory switches that control membrane affinity include modulation of the membrane lipid composition, and modification of the protein itself by ligand binding, phosphorylation, or acylation. How does membrane binding modulate the protein's function? Two mechanisms are discussed: (1) localization with the substrate, activator, or downstream target, and (2) activation of the protein by a conformational switch. This paper also addresses the issue of specificity in the cell membrane targetted for binding.

Is ascorbic acid an antioxidant for the plasma membrane

Abstract: Ascorbic acid, or vitamin C, is a primary antioxidant in plasma and within cells, but it can also interact with the plasma membrane by donating electrons to the α-tocopheroxyl radical and a trans-plasma membrane oxidoreductase activity. Ascorbate-derived reducing capacity is thus transmitted both into and across the plasma membrane. Recycling of α-tocopherol by ascorbate helps to protect membrane lipids from peroxidation. However, neither the mechanism nor function of the ascorbate-dependent oxidoreductase activity is known. This activity has typically been studied using extracellular ferricyanide as an electron acceptor. Whereas an NADH:ferricyanide reductase activity is evident in open membranes, ascorbate is the preferred electron donor within cells. The oxidoreductase may be a single membrane-spanning protein or may only partially span the membrane as part of a trans-membrane electron transport chain composed of a cytochrome or even hydrophobic antioxidants such as α-tocopherol or ubiquinol-10. Furthe...

Genetic engineering of the unsaturation of fatty acids in membrane lipids alters the tolerance of Synechocystis to salt stress

Abstract: The role of unsaturated fatty acids in membrane lipids in the tolerance of the photosynthetic machinery to salt stress was studied by comparing the desA−/desD− mutant of Synechocystis sp. PCC 6803, which contained monounsaturated fatty acids, with the wild-type strain, which contained a full complement of polyunsaturated fatty acids. In darkness, the loss of oxygen-evolving photosystem II activity in the presence of 0.5 M NaCl or 0.5 M LiCl was much more rapid in desA−/desD− cells than in wild-type cells. Oxygen-evolving activity that had been lost during incubation with 0.5 M NaCl in darkness returned when cells were transferred to conditions that allowed photosynthesis or respiration. Recovery was much greater in wild-type than in desA−/desD− cells, and it was prevented by lincomycin. Thus, the unsaturation of fatty acids is important in the tolerance of the photosynthetic machinery to salt stress. It appears also that the activity and synthesis of the Na+/H+ antiporter system might be suppressed under high-salt conditions and that this effect can be reversed, in part, by the unsaturation of fatty acids in membrane lipids.

Adsorbed and free lipid bilayers at the solid-liquid interface

Abstract: Single and double phosphocholine (DPPC and DSPC) bilayers adsorbed at the silicon-water interface have been prepared and characterised. The second bilayer, called “free bilayer”, is a novel highly hydrated system floating at \(\)above the first one. Robust and reproducible preparation has been possible thanks to a combination of Langmuir-Blodgett and Langmuir-Schaeffer techniques. Carefully optimised neutron reflectivity measurements have allowed a precise non-destructive characterisation of the structure, hydration and roughness of the layers. This work opens new possibilities for the investigation of the interaction between membrane lipids and soluble proteins, in particular peptides too small to be visible with other techniques.

Archaeobacterial ether lipid liposomes (archaeosomes) as novel vaccine and drug delivery systems.

Abstract: Liposomes are artificial, spherical, closed vesicles consisting of one or more lipid bilayer(s). Liposomes made from ester phospholipids have been studied extensively over the last 3 decades as artificial membrane models. Considerable interest has been generated for applications of liposomes in medicine, including their use as diagnostic reagents, as carrier vehicles in vaccine formulations, or as delivery systems for drugs, genes, or cancer imaging agents. The objective of this article is to review the properties and potential applications of novel liposomes made from the membrane lipids of Archaeobacteria (Archaea). These lipids are unique and distinct from those encountered in Eukarya and Bacteria. Polar glycerolipids make up the bulk of the membrane lipids, with the remaining neutral lipids being primarily squalenes and other hydrocarbons. The polar lipids consist of regularly branched, and usually fully saturated, phytanyl chains of 20, 25, or 40 carbon length, with the 20 and 40 being most common. The phytanyl chains are attached via ether bonds to the sn-2,3 carbons of the glycerol backbone(s). It has been shown only recently that total polar lipids of archaeobacteria, and purified lipid fractions therefrom, can form liposomes. We refer to liposomes made with any lipid composition that includes ether lipids characteristic of Archaeobacteria as archaeosomes to distinguish them from vesicles made from the conventional lipids obtained from eukaryotic or eubacterial sources or their synthetic analogs. In general, archaeosomes demonstrate relatively higher stabilities to oxidative stress, high temperature, alkaline pH, action of phospholipases, bile salts, and serum proteins. Some archaeosome formulations can be sterilized by autoclaving, without problems such as fusion or aggregation of the vesicles. The uptake of archaeosomes by phagocytic cells can be up to 50-fold greater than that of conventional liposome formulations. Studies in mice have indicated that systemic administration of several test antigens entrapped within certain archaeosome compositions give humoral immune responses that are comparable to those obtained with the potent but toxic Freund's adjuvant. Archaeosome compositions can be selected to give a prolonged, sustained immune response, and the generation of a memory response. Tissue distribution studies of archaeosomes administered via various systemic and peroral routes indicate potential for targeting to specific organs. All in vitro and in vivo studies performed to date indicate that archaeosomes are safe and do not invoke any noticeable toxicity in mice. The stability, tissue distribution profiles, and adjuvant activity of archaeosome formulations indicate that they may offer a superior alternative to the use of conventional liposomes, at least for some biotechnology applications.

Temperature-sensitive liposomal formulation

Abstract: A liposome contains an active agent and has a gel-phase lipid bilayer membrane comprising phospholipid and a surface active agent The phospholipids are the primary lipid source for the lipid bilayer membrane and the surface active agent is contained in the bilayer membrane in an amount sufficient to increase the percentage of active agent released at the phase transition temperature of the lipid bilayer, compared to that which would occur in the absence of the surface active agent The surface active agent is present in the lipid bilayer membrane so as to not destabilize the membrane in the gel phase

Microdomain-dependent Regulation of Lck and Fyn Protein-Tyrosine Kinases in T Lymphocyte Plasma Membranes

Abstract: Src family protein-tyrosine kinases are implicated in signaling via glycosylphosphatidylinositol (GPI)-anchored receptors. Both kinds of molecules reside in opposite leaflets of the same sphingolipid-enriched microdomains in the lymphocyte plasma membrane without making direct contact. Under detergent-free conditions, we isolated a GPI-enriched plasma membrane fraction, also containing transmembrane proteins, selectively associated with sphingolipid microdomains. Nonionic detergents released the transmembrane proteins, yielding core sphingolipid microdomains, limited amounts of which could also be obtained by detergent-free subcellular fractionation. Protein-tyrosine kinase activity in membranes containing both GPI-anchored and transmembrane proteins was much lower than in core sphingolipid microdomains but was strongly reactivated by nonionic detergents. The inhibitory mechanism acting on Lck and Fyn kinases in these membranes was independent of the protein-tyrosine phosphatase CD45 and was characterized as a mixed, noncompetitive one. We propose that in lymphocyte plasma membranes, Lck and Fyn kinases exhibit optimal activity when juxtaposed to the GPI- and sphingolipid-enriched core microdomains but encounter inhibitory conditions in surrounding membrane areas that are rich in glycerophospholipids and contain additional transmembrane proteins.

The Role of MIP in Lens Fiber Cell Membrane Transport

Abstract: MIP has been hypothesized to be a gap junction protein, a membrane ion channel, a membrane water channel and a facilitator of glycerol transport and metabolism. These possible roles have been indirectly suggested by the localization of MIP in lens gap junctional plaques and the properties of MIP when reconstituted into artificial membranes or exogenously expressed in oocytes. We have examined lens fiber cells to see if these functions are present and whether they are affected by a mutation of MIP found in CatFr mouse lens. Of these five hypothesized functions, only one, the role of water channel, appears to be true of fiber cells in situ. Based on the rate of volume change of vesicles placed in a hypertonic solution, fiber cell membrane lipids have a low water permeability (pH2O) on the order of 1 micron/sec whereas normal fiber cell membrane pH2O was 17 micron/sec frog, 32 micron/sec rabbit and 43 micron/sec mouse. CatFr mouse lens fiber cell pH2O was reduced by 13 micron/sec for heterozygous and 30 micron/sec for homozygous mutants when compared to wild type. Lastly, when expressed in oocytes, the pH2O conferred by MIP is not sensitive to Hg2+ whereas that of CHIP28 (AQP1) is blocked by Hg2+. The fiber cell membrane pH2O was also not sensitive to Hg2+ whereas lens epithelial cell pH2O (136 micron/sec in rabbit) was blocked by Hg2+. With regard to the other hypothesized roles, fiber cell membrane or lipid vesicles had a glycerol permeability on the order of 1 nm/sec, an order of magnitude less than that conferred by MIP when expressed in oocytes. Impedance studies were employed to determine gap junctional coupling and fiber cell membrane conductance in wild-type and heterozygous CatFr mouse lenses. There was no detectable difference in either coupling or conductance between the wild-type and the mutant lenses.