Differences in the transbilayer and lateral motions of fluorescent analogs of phosphatidylcholine and phosphatidylethanolamine in the apical plasma membrane of bovine aortic endothelial cells

The asymmetric plasma membrane-A composite material combining different functionalities?: Balancing Barrier Function and Fluidity for Effective Signaling

One persistent puzzle in the life sciences is the asymmetric lipid composition of the cellular plasma membrane: while the exoplasmic leaflet is enriched in lipids carrying predominantly saturated fatty acids, the cytoplasmic leaflet hosts preferentially lipids with (poly-)unsaturated fatty acids. Given the high energy requirements necessary for cells to maintain this asymmetry, the question naturally arises regarding its inherent benefits. In this paper, we propose asymmetry to represent a potential solution for harmonizing two conflicting requirements for the plasma membrane: first, the need to build a barrier for the uncontrolled influx or efflux of substances; and second, the need to form a fluid and dynamic two-dimensional substrate for signaling processes. We hence view here the plasma membrane as a composite material, where the exoplasmic leaflet is mainly responsible for the functional integrity of the barrier and the cytoplasmic leaflet for fluidity. We reinforce the validity of the proposed mechanism by presenting quantitative data from the literature, along with multiple examples that bolster our model.

Membrane disorder and phospholipid scrambling in electropermeabilized and viable cells

Membrane electropermeabilization relies on the transient permeabilization of the plasma membrane of cells submitted to electric pulses. This method is widely used in cell biology and medicine due to its efficiency to transfer molecules while limiting loss of cell viability. However, very little is known about the consequences of membrane electropermeabilization at the molecular and cellular levels. Progress in the knowledge of the involved mechanisms is a biophysical challenge. As a transient loss of membrane cohesion is associated with membrane permeabilization, our main objective was to detect and visualize at the single-cell level the incidence of phospholipid scrambling and changes in membrane order. We performed studies using fluorescence microscopy with C6-NBD-PC and FM1-43 to monitor phospholipid scrambling and membrane order of mammalian cells. Millisecond permeabilizing pulses induced membrane disorganization by increasing the translocation of phosphatidylcholines according to an ATP-independent process. The pulses induced the formation of long-lived permeant structures that were present during membrane resealing, but were not associated with phosphatidylcholine internalization. These pulses resulted in a rapid phospholipid flip/flop within less than 1s and were exclusively restricted to the regions of the permeabilized membrane. Under such electrical conditions, phosphatidylserine externalization was not detected. Moreover, this electrically-mediated membrane disorganization was not correlated with loss of cell viability. Our results could support the existence of direct interactions between the movement of membrane zwitterionic phospholipids and the electric field.

Visualizing the growth and dynamics of liquid-ordered domains during lipid bilayer folding in a microfluidic chip

A microfluidic platform enabling optical monitoring of bilayer lipid membrane formation by a new monolayer folding process is described. The thermoplastic chips integrate dried lipid films that are rehydrated by microfluidic perfusion, which enables delivery of lipid-laden air bubbles across a membrane-supporting aperture. As in traditional Montal-Mueller bilayer formation, lipid monolayers are delivered independently to each side of the aperture, thereby allowing asymmetric lipid composition in the resulting bilayer to be achieved. Confocal microscopy is used to image the monolayer folding process, and reveals the growth and dynamics of asymmetric liquid-ordered domains during bilayer stabilization.

Proteins involved in lipid translocation in eukaryotic cells

Since the first discovery of ATP-dependent translocation of lipids in the human erythrocyte membrane in 1984, there has been much evidence of the existence of various ATPases translocating lipids in eukaryotic cell membranes. They include P-type ATPases involved in inwards lipid transport from the exoplasmic leaflet to the cytosolic leaflet and ABC proteins involved in outwards transport. There are also ATP-independent proteins that catalyze the passage of lipids in both directions. Five P-type ATPase involved in lipid transport have been genetically characterized in yeast cells, suggesting a pool of several proteins with partially redundant activities responsible for the regulation of lipid asymmetry. However, expression and purification of individual yeast proteins is still insufficient to allow reconstitution experiments in liposomes. In this review, we want to give an overview over current investigation efforts about the identification and purification of proteins that may be involved in lipid translocation.

Characterization of exosome subpopulations from RBL-2H3 cells using fluorescent lipids

Fluorescent lipid probes were used to track lipid trafficking between parent RBL cells and exosomes. We have checked the intracellular labeling of exosomes ("in vivo labeling") from parent cell incubated with either Bodipy-Cer, Bodipy-PC, or NBD-PC. Bodipy-PC labeled equally cells and exosomes, whereas Bodipy-Cer, a Golgi marker, was enriched in exosomes. Golgi membranes participated effectively in exosome biogenesis since cell incubation with brefeldin A leads to a modified phospholipid/protein ratio in exosomes. At the opposite, NBD-PC, a plasma membrane marker weakly labeled exosome membranes. Sorting of subpopulations indicated that the MHC-II containing exosomes were enriched in Bodipy-PC, whereas tetraspanin(CD 63 or CD81)-containing exosomes are essentially labeled with Bodipy-Cer and Bodipy-PC. These results indicated that RBL released two main subpopulations of exosomes that can be discriminated by their protein and lipid contents. When the bulk of exosomes was labeled after their purification ("in vitro labeling") with either of the above-mentioned lipid probes, the Bodipy-Cer was the only one to incorporate noticeably in all the subpopulations, indicating that the previous results obtained during "in vivo labeling" monitored real intracellular lipid trafficking between organelles and exosomes. Bodipy-Cer was further used as a tool to measure the respective amounts of each subpopulations. CD63, MHC II, and CD81-containing exosomes accounted for 47%, 32%, and 21%, respectively, of total exosomes.

Cholesterol depletion induces solid-like regions in the plasma membrane

Glycosylphosphatidylinositol-linked and transmembrane major histocompatibility complex (MHC) class II I-E(k) proteins, as well as N-(6-tetramethylrhodaminethiocarbamoyl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (Tritc-DHPE), are used as probes to determine the effect of cholesterol concentration on the organization of the plasma membrane at temperatures in the range 22 degrees C-42 degrees C. Cholesterol depletion caused a decrease in the diffusion coefficients for the MHC II proteins and also for a slow fraction of the Tritc-DHPE population. At 37 degrees C, reduction of the total cell cholesterol concentration results in a smaller suppression of the translational diffusion for I-E(k) proteins (twofold) than was observed in earlier work at 22 degrees C (five sevenfold) Vrljic, M., S. Y. Nishimura, W. E. Moerner, and H. M. McConnell. 2005. Biophys. J. 88:334-347. At 37 degrees C, the diffusion of both I-E(k) proteins is Brownian (0.9 < alpha-parameter < 1.1). More than 99% of the protein population diffuses homogeneously when imaged at 65 frames per s. As the temperature is raised from 22 degrees C to 42 degrees C, a change in activation energy is seen at approximately 35 degrees C in the Arrhenius plots. Cytoskeletal effects appear to be minimal. These results are consistent with a previously described model of solid-like domain formation in the plasma membrane.

Ras diffusion is sensitive to plasma membrane viscosity

The cell surface contains a variety of barriers and obstacles that slow the lateral diffusion of glycosylphosphatidylinositol (GPI)-anchored and transmembrane proteins below the theoretical limit imposed by membrane viscosity. How the diffusion of proteins residing exclusively on the inner leaflet of the plasma membrane is regulated has been largely unexplored. We show here that the diffusion of the small GTPase Ras is sensitive to the viscosity of the plasma membrane. Using confocal fluorescence recovery after photobleaching, we examined the diffusion of green fluorescent protein (GFP)-tagged HRas, NRas, and KRas in COS-7 cells loaded with or depleted of cholesterol, a well-known modulator of membrane bilayer viscosity. In cells loaded with excess cholesterol, the diffusional mobilities of GFP-HRas, GFP-NRas, and GFP-KRas were significantly reduced, paralleling the behavior of the viscosity-sensitive lipid probes DiIC(16) and DiIC(18). However, the effects of cholesterol depletion on protein and lipid diffusion in cell membranes were highly dependent on the depletion method used. Cholesterol depletion with methyl-beta-cyclodextrin slowed Ras diffusion by a viscosity-independent mechanism, whereas overnight cholesterol depletion slightly increased both protein and lipid diffusion. The ability of Ras to sense membrane viscosity may represent a general feature of proteins residing on the cytoplasmic face of the plasma membrane.

Transmembrane asymmetry and lateral domains in biological membranes

It is generally assumed that rafts exist in both the external and internal leaflets of the membrane, and that they overlap so that they are coupled functionally and structurally. However, the two monolayers of the plasma membrane of eukaryotic cells have different chemical compositions. This out-of-equilibrium situation is maintained by the activity of lipid translocases, which compensate for the slow spontaneous transverse diffusion of lipids. Thus rafts in the outer leaflet, corresponding to domains enriched in sphingomyelin and cholesterol, cannot be mirrored in the inner cytoplasmic leaflet. The extent to which lipids contribute to raft properties can be conveniently studied in giant unilamellar vesicles. In these, cholesterol can be seen to condense with saturated sphingolipids or phosphatidylcholine to form microm scale domains. However, such rafts fail to model biological rafts because they are symmetric, and because their membranes lack the mechanism that establishes this asymmetry, namely proteins. Biological rafts are in general of nm scale, and almost certainly differ in size and stability in inner and outer monolayers. Any coupling between rafts in the two leaflets, should it occur, is probably transient and dependent not upon the properties of lipids, but on transmembrane proteins within the rafts.

Lipid polarity in brain capillary endothelial cells

Brain capillary endothelial cells (BCEC) represent an epithelial like cell type with continuous tight junctions and polar distributed proteins. In this paper we investigated whether cultured BCEC show a polar distribution of membrane lipids as this was demonstrated for many epithelial cell types. Therefore we applied a high yield membrane fractionation method to isolate pure fractions of the apical and the basolateral plasma membrane (PM) domains. Using a set of methods for lipid analysis we were able to determine the total lipid composition of the whole cells and the PM fractions. Both membrane domains showed a unique lipid composition with clear differences to each other and to the whole cell composition. Three lipid species were polar distributed between the two PM domains. Phosphatidylcholine was enriched in the apical membrane whereas sphingomyelin and glucosylceramide were enriched in the basolateral membrane. The possible function of this lipid polarity for the blood-brain barrier mechanism is the generation of a suitable lipid environment for polar distributed membrane proteins and the generation of two PM domains with different biophysical properties and permeabilities.

Regulation of transbilayer plasma membrane phospholipid asymmetry

Lipids in biological membranes are asymmetrically distributed across the bilayer; the amine-containing phospholipids are enriched on the cytoplasmic surface of the plasma membrane, while the choline-containing and sphingolipids are enriched on the outer surface. The maintenance of transbilayer lipid asymmetry is essential for normal membrane function, and disruption of this asymmetry is associated with cell activation or pathologic conditions. Lipid asymmetry is generated primarily by selective synthesis of lipids on one side of the membrane. Because passive lipid transbilayer diffusion is slow, a number of proteins have evolved to either dissipate or maintain this lipid gradient. These proteins fall into three classes: 1) cytofacially-directed, ATP-dependent transporters ("flippases"); 2) exofacially-directed, ATP-dependent transporters ("floppases"); and 3) bidirectional, ATP-independent transporters ("scramblases"). The flippase is highly selective for phosphatidylserine and functions to keep this lipid sequestered from the cell surface. Floppase activity has been associated with the ABC class of transmembrane transporters. Although they are primarily nonspecific, at least two members of this class display selectivity for their substrate lipid. Scramblases are inherently nonspecific and function to randomize the distribution of newly synthesized lipids in the endoplasmic reticulum or plasma membrane lipids in activated cells. It is the combined action of these proteins and the physical properties of the membrane bilayer that generate and maintain transbilayer lipid asymmetry.

Molecular and cell biology of phosphatidylserine and phosphatidylethanolamine metabolism

In this review, the pathways for phosphatidylserine (PS) and phosphatidylethanolamine (PE) biosynthesis, as well as the genes and proteins involved in these pathways, are described in mammalian cells, yeast, and prokaryotes. In mammalian cells, PS is synthesized by a base-exchange reaction in which phosphatidylcholine or PE is substrate for PS synthase-1 or PS synthase-2, respectively. Isolation of Chinese hamster ovary cell mutants led to the cloning of cDNAs and genes encoding these two PS synthases. In yeast and prokaryotes PS is produced by a biosynthetic pathway completely different from that in mammals: from a reaction between CDP-diacylglycerol and serine. The major route for PE synthesis in cultured cells is from the mitochondrial decarboxylation of PS. Alternatively, PE can be synthesized in the endoplasmic reticulum (ER) from the CDP-ethanolamine pathway. Genes and/or cDNAs encoding all the enzymes in these two pathways for PE synthesis have been isolated and characterized. In mammalian cells, PS is synthesized on the ER and/or mitochondria-associated membranes (MAM). PS synthase-1 and -2 are highly enriched in MAM compared to the bulk of ER. Since MAM are a region of the ER that appears to be in close juxtaposition to the mitochondrial outer membrane, it has been proposed that MAM act as a conduit for the transfer of newly synthesized PS into mitochondria. A similar pathway appears to operate in yeast. The use of yeast mutants has led to identification of genes involved in the interorganelle transport of PS and PE in yeast, but so far none of the corresponding genes in mammalian cells has been identified. PS and PE do not act solely as structural components of membranes. Several specific functions have been ascribed to these two aminophospholipids. For example, cell-surface exposure of PS during apoptosis is thought to be the signal by which apoptotic cells are recognized and phagocytosed. Translocation of PS from the inner to outer leaflet of the plasma membrane of platelets initiates the blood-clotting cascade, and PS is an important activator of several enzymes, including protein kinase C. Recently, exposure of PE on the cell surface was identified as a regulator of cytokinesis. In addition, in Escherichia coli, PE appears to be involved in the correct folding of membrane proteins; and in Drosophila, PE regulates lipid homeostasis via the sterol response element-binding protein.

Energy-dependent flip of fluorescence-labeled phospholipids is regulated by nutrient starvation and transcription factors, PDR1 and PDR3

The yeast Saccharomyces cerevisiae readily accumulates short-chain, fluorescent 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-labeled phosphatidylcholine and phosphatidylethanolamine at the nuclear envelope/endoplasmic reticulum and mitochondria. The net intracellular accumulation reflects the sum of their inwardly and outwardly directed transbilayer translocation across the plasma membrane (flip and flop, respectively). The rate of flop is negligible in energy-depleted cells as well as at low temperature (2 degrees C). Although flip is reduced at 2 degrees C, it can still be measured by flow cytometry, allowing the rate of flip, independent of flop, to be characterized at this temperature. Flip requires the energy of the plasma membrane proton electrochemical gradient and is down-regulated as cells pass through the diauxic shift and enter stationary phase. Furthermore, drug-resistant, gain-of-function mutations in the transcription factors, PDR1 and PDR3, result in a dramatic down-regulation of flip in addition to their already established up-regulation of flop. These results imply that down-regulation of the NBD-phospholipid flip pathway is a physiological response to environmental stress.

NBD-labeled phosphatidylcholine and phosphatidylethanolamine are internalized by transbilayer transport across the yeast plasma membrane

The internalization and distribution of fluorescent analogs of phosphatidylcholine (M-C6-NBD-PC) and phosphatidylethanolamine (M-C6-NBD-PE) were studied in Saccharomyces cerevisiae. At normal growth temperatures, M-C6-NBD-PC was internalized predominantly to the vacuole and degraded. M-C6-NBD-PE was internalized to the nuclear envelope/ER and mitochondria, was not transported to the vacuole, and was not degraded. At 2 degrees C, both were internalized to the nuclear envelope/ER and mitochondria by an energy-dependent, N-ethylmaleimide-sensitive process, and transport of M-C6-NBD-PC to and degradation in the vacuole was blocked. Internalization of neither phospholipid was reduced in the endocytosis-defective mutant, end4-1. However, following pre-incubation at 37 degrees C, internalization of both phospholipids was inhibited at 2 degrees C and 37 degrees C in sec mutants defective in vesicular traffic. The sec18/NSF mutation was unique among the sec mutations in further blocking M-C6-NBD-PC translocation to the vacuole suggesting a dependence on membrane fusion. Based on these and previous observations, we propose that M-C6-NBD-PC and M-C6-NBD-PE are transported across the plasma membrane to the cytosolic leaflet by a protein-mediated, energy-dependent mechanism. From the cytosolic leaflet, both phospholipids are spontaneously distributed to the nuclear envelope/ER and mitochondria. Subsequently, M-C6-NBD-PC, but not M-C6-NBD-PE, is sorted by vesicular transport to the vacuole where it is degraded by lumenal hydrolases.

Identification and purification of aminophospholipid flippases

Transbilayer phospholipid asymmetry is a common structural feature of most biological membranes. This organization of lipids is generated and maintained by a number of phospholipid transporters that vary in lipid specificity, energy requirements and direction of transport. These transporters can be divided into three classes: (1) bidirectional, non-energy dependent 'scramblases', and energy-dependent transporters that move lipids (2) toward ('flippases') or (3) away from ('floppases') the cytofacial surface of the membrane. One of the more elusive members of this family is the plasma membrane aminophospholipid flippase, which selectively transports phosphatidylserine from the external to the cytofacial monolayer of the plasma membrane. This review summarizes the characteristics of aminophospholipid flippase activity in intact cells and describes current strategies to identify and isolate this protein. The biochemical characteristics of candidate flippases are critically compared and their potential role in flippase activity is evaluated.

Alteration of the lateral organization of the plasma membrane of Chinese hamster ovary cells by synthetic lipopeptide, Pam3Cys-Ser-Lys4

The cationic lipohexapeptide (S)-[2, 3-bis(palmitoyloxy)-(2RS)-propyl]-N-palmitoyl-(R)-Cys-(S)-Ser-(S)- Lys 4-OH, trihydrochloride (Pam3Cys-Ser-Lys4) is a synthetic analog of the triacylated N-terminal part of bacterial lipoproteins. In this study we addressed the question of whether Pam3Cys-Ser-Lys4 could modify the organization of the plasma membrane of Chinese hamster ovary cells. 1-Acyl-2-[6-(7-nitro-2-1, 3-benzoxadiazol-4-yl)amino]caproyl]-sn-glycero-3-phosphocholine (C6-NBD-PC) diffusion was followed by fluorescence recovery after photobleaching experiments carried out on the plasma membrane of Chinese hamster ovary cells. Incubation of cells in the presence of Pam3Cys-Ser-Lys4 induced an increase in the lateral diffusion coefficient and in the immobile fraction of C6-NBD-PC probes. Various control experiments have shown that the increase in the immobile fraction was not due to probe internalization induced by Pam3Cys-Ser-Lys4. Back-exchange experiments showed that a good correlation exists between the fractions of immobilized probes and nonextractable probes in the plasma membrane of Chinese hamster ovary cells. A useful way to analyze the origin of probe immobilization (micrometer-sized domains or aggregated patches of proteins) is to carry out fluorescence recovery after photobleaching experiments at variable observation radii. This type of experiment, carried out on the plasma membrane of Chinese hamster ovary cells incubated with Pam3Cys-Ser-Lys4, confirmed that the lipopeptide induced the aggregation of proteins of Chinese hamster ovary plasma membrane. Lipids which were trapped inside these aggregates were thus prevented from diffusing at long range in the plasma membrane plane and behave as an immobile fraction.

Protein-mediated inward translocation of phospholipids occurs in both the apical and basolateral plasma membrane domains of epithelial cells

The translocation of spin-labeled analogues of phosphatidylcholine (4-doxylpentanoyl-PC, SL-PC), phosphatidylethanolamine (SL-PE), phosphatidylserine (SL-PS), and sphingomyelin (SL-SM) from the outer to the inner leaflet of the plasma membrane bilayer was investigated in dog kidney MDCK II and human colon Caco-2 cells. Disappearance from the outer leaflet was assayed using back-exchange to serum albumin. Experiments with cells in suspension as well as with polarized cells on filters were performed at reduced temperatures (10 and 20 degreesC) to suppress endocytosis and hydrolysis of spin-labeled lipids. For both epithelial cell lines, a fast ATP-dependent inward movement of the aminophospholipids SL-PS and SL-PE was found, while SL-SM was only slowly internalized without any effect of ATP depletion. The kinetics of redistribution of SL-PC were clearly different between the two cell lines. In MDCK II cells, SL-PC was rapidly internalized in an ATP-dependent and N-ethylmaleimide-sensitive manner and at a rate similar to that of the aminophospholipids. In contrast, in Caco-2 cells the inward movement of SL-PC was much slower than that of the aminophospholipids, did not depend on ATP, and was not N-ethylmaleimide-sensitive. Inhibitor studies indicated that the outward-translocating multidrug resistance P-glycoprotein present in these cells did not affect the kinetics of inward translocation. Internalization was always similar on the apical and basolateral cell surface, suggesting the presence of the same phospholipid translocator(s) on both surface domains of epithelial cells. We propose that Caco-2 cells contain the well-known aminophospholipid translocase, while MDCK II cells contain either two translocases, namely, the aminophospholipid translocase and a phosphatidylcholine-specific translocase, or one translocase of a new type, translocating aminophospholipids as well as phosphatidylcholine.

Undifferentiated HL-60 cells internalize an antitumor alkyl ether phospholipid more rapidly than resistant K562 cells

In this study, we confirmed a previous finding that 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (methyl-PAF) expresses higher antineoplastic activity against the promyelocytic leukemia cell line HL-60, than against the erythroleukemic cell line K562, and intended to clarify the reason for this. Using an albumin back-exchange method, we measured the rates of binding and internalization of [3H]methyl-PAF by HL-60 and K562 cells. We found that methyl-PAF associated very rapidly and to similar extents with the two types of cells at low concentrations of extracellular bovine serum albumin, but that when bound to the cell surface, it was internalized into HL-60 cells faster than into K562 cells. The internalization of methyl-PAF by HL-60 cells was concentration-independent, intracellular ATP-independent and susceptible to thiol group-modifying reagents and cytochalasin B. Thus the inward transbilayer movement of methyl-PAF seems to occur by cytochalasin B-sensitive protein-mediated mechanism based on passive diffusion not requiring energy, in which SH-groups of protein play a critical role. We also found that the internalization of 1-hexadecanoyl-2-(4,4-difluoro-5,7- dimethyl-4-bora-3a, 4a-diaza-s-indacene-3-pentanoyl)-sn-glycero-3-phosphocholine (Bodipy-C5-PC), whose structure resembles that of methyl-PAF, into HL-60 cells was faster than that into K562 cells. Using a combination of an albumin back-exchange method and observation by confocal laser scanning microscopy, we next examined the intracellular distribution of this fluorescent phospholipid probe after its internalization. Intracellular membranes, especially those peripheral to nuclei, were fluorescence-labeled in both HL-60 and K562 cells, but fluorescence of the nuclear membranes was weak, suggesting that this probe seems mainly to accumulate in intracellular granules, and may interact directly with several key enzymes for phospholipid metabolism, leading to cell injury. Because the difference between the internalization rates of methyl-PAF in HL-60 and K562 cells was correlated with their different susceptibilities to the cytotoxic effect of methyl-PAF, we suggest that the capacities for uptake of methyl-PAF and its accumulation in intracellular membranes are critical factor for its induction of apoptosis. (c) 1998 Elsevier Science B.V.

12-O-tetradecanoylphorbol-13-acetate inhibits aminophospholipid translocase activity and modifies the lateral motions of fluorescent phospholipid analogs in the plasma membrane of bovine aortic endothelial cells

The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) is a potent mitogenic factor which can replace the growth promoting activity of basic fibroblast growth factor (bFGF) on bovine aortic endothelial cells. However, TPA-treated cells lose their strict contact inhibition at confluence, which is a characteristic of cells grown in the presence of bFGF. We have examined whether these changes could be related to modifications of the transbilayer and lateral motions of fluorescent lipids, namely 1-acyl-2-[6-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]caproyl]-p hosphatidylcholine (C6-NBD-PC), -phosphatidylserine (C6-NBD-PS), and -phosphatidylethanolamine (C6-NBD-PE) inserted in the outer leaflet of the cell plasma membrane. In TPA-treated cells, the three fluorescent phospholipids remained located in the outer leaflet for at least 1 h at 20 degrees C after their insertion, indicating a blockade of the aminophospholipid translocase activity which is normally present in the plasma membrane of bFGF-treated cells. TPA also induced a large increase in the percentage of C6-NBD-PC and C6-NBD-PE probes which were free to diffuse laterally. The mobile fractions M reached values of approximately 100% for the two lipids, while for bFGF-treated cells they were found around 85 and 75%, respectively. For the C6-NBD-PS probe, M remained unchanged in bFGF and TPA-treated cells, at around 85%. TPA treatment also induced a twofold increase in the lateral diffusion coefficients of C6-NBD-PC and C6-NBD-PE, while that of C6-NBD-PS remained nearly unchanged. These effects of TPA may be related to the observed loss of differentiated properties of vascular endothelial cells and not to its mitogenic properties.

Aminophospholipid translocase activity in JEG-3; a choriocarcinoma model of cytotrophoblast differentiation

The plasma membrane is characterized by a non-symmetrical distribution of phospholipids; the outer monolayer of the plasma membrane consists primarily of phosphatidylcholine (PC), and the aminophospholipids, phosphatidylserine (PS) and phosphatidylethanolamine (PE), preferentially reside in the inner monolayer. Asymmetry is maintained by a membrane associated ATP-dependent aminophospholipid translocase that preferentially relocates PS and PE from the outer to the inner monolayer. Although in most cells the translocase minimizes expression of PS on the outer surface, differentiating trophoblasts express increasing levels of surface PS. One possible explanation of prolonged PS externalization is that trophoblasts lack an effective aminophospholipid translocase. To test this hypothesis, fluorescent PC and PS analogues, NBD-PC and NBD-PS, were introduced into the plasma membrane of a choriocarcinoma model of trophoblast, JEG-3 cells. After incubation, the fluorescent lipid remaining on the outer monolayer was removed by incubation with fetal bovine serum. JEG-3 cells selectively translocated 80 per cent of the NBD-PS without significant translocation of NBD-PC. The process was significantly inhibited by N-ethylmaleimide (NEM) and vanadate. It is concluded that this model of trophoblast contains an active aminophospholipid translocase.

7-nitrobenz-2-oxa-1,3-diazole-4-yl-labeled phospholipids in lipid membranes: differences in fluorescence behavior

Steady-state and time-resolved fluorescence properties of the 7-nitrobenz-2-oxa-1, 3-diazole-4-yl (NBD) fluorophore attached either to the sn-2 acyl chain of various phospholipids (phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidic acid) or to the polar headgroup of phosphatidylethanolamine were studied after insertion of these NBD-labeled lipid probes into unilamellar vesicles of phosphatidylcholine, phosphatidylglycerol, phosphatidic acid, and phosphatidylserine. The fluorescence response of the NBD group was observed to strongly depend on the chemical structure and physical state of the host phospholipids and on the chemical structure of the lipid probe itself. Among the various fluorescence parameters studied, i.e., Stokes' shifts, lifetimes, and quantum yields, the quantum yields were by far the most affected by these structural and environmental factors, whereas the Stokes' shifts were practically unaffected. Thus, depending on the phospholipid probe and the host phospholipid, the fluorescence emission of the NBD group was found to vary by a factor of up to 5. Careful analysis of the data shows that for the various couples of probe and host lipid molecules studied, deexcitation of the fluorophore was dominated by nonradiative deactivation processes. This great sensitivity of the NBD group to environmental factors originates from its well-known solvatochromic properties, and comparison of these knr values with those obtained for n-propylamino-NBD in a set of organic solvents covering a large scale of polarity indicates that in phospholipids, the NBD fluorophore experiences a dielectric constant of around 27-41, corresponding to a medium of relatively high polarity. From these epsilon values and on the basis of models of the dielectric transition that characterizes any water-phospholipid interface, it can be inferred that for all of the phospholipid probes and host phospholipids tested, the NBD group is located in the region of the polar headgroups, near the phosphoglycerol moiety of the lipids.

Transbilayer movement of fluorescent and spin-labeled phospholipids in the plasma membrane of human fibroblasts: a quantitative approach

All phospholipids in the plasma membrane of eukaryotic cells are subject to a slow passive transbilayer movement. In addition, aminophospholipids are recognized by the so-called aminophospholipid translocase, and are rapidly moved from the exoplasmic to the cytoplasmic leaflet of the plasma membrane at the expense of ATP hydrolysis. Though these principal pathways of transbilayer movement of phospholipids probably apply to all eukaryotic plasma membranes, studies of the actual kinetics of phospholipid redistribution have been largely confined to non-nucleated cells (erythrocytes). Experiments on nucleated cells are complicated by endocytosis and metabolism of the lipid probes inserted into the plasma membrane. Taking these complicating factors into account, we performed a detailed kinetic study of the transbilayer movement of short-chain fluorescent (N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl); NBD) and, for the first time, spin-labeled analogues of phosphatidylcholine (PC), -ethanolamine (PE), -serine (PS), and sphingomyelin (SM) in the plasma membrane of cultured human gingival fibroblasts. At 20 degrees C, the passive transbilayer diffusion of NBD analogues was very slow, and the choline-containing NBD analogues were internalized predominantly by endocytosis. Spin-labeled analogues of PC and SM showed higher passive transbilayer diffusion rates, and probably entered the cell by both passive transbilayer movement and endocytosis. In contrast, the rapid uptake of NBD- and spin-labeled aminophospholipid analogues could be mainly ascribed to the action of the aminophospholipid translocase, since it was inhibited by ATP depletion and N-ethylmaleimide pretreatment. The initial velocity of NBD-aminophospholipid translocation was eight to ten times slower than that of the corresponding spin-labeled lipid, and the half-times of redistribution of NBD-PS and spin-labeled PS were 7.2 and 3.6 minutes, respectively. Our data indicate that in human fibroblasts the initial velocity of aminophospholipid translocation is at least one order of magnitude higher than that in human erythrocytes, which should be sufficient to maintain the phospholipid asymmetry in the plasma membrane.

Lipid domains and lipid/protein interactions in biological membranes

In the fluid mosaic model of membranes, lipids are organized in the form of a bilayer supporting peripheral and integral proteins. This model considers the lipid bilayer as a two-dimensional fluid in which lipids and proteins are free to diffuse. As a direct consequence, both types of molecules would be expected to be randomly distributed within the membrane. In fact, evidences are accumulating to indicate the occurrence of both a transverse and lateral regionalization of membranes which can be described in terms of micro- and macrodomains, including the two leaflets of the lipid bilayer. The nature of the interactions responsible for the formation of domains, the way they develop and the time- and space-scale over which they exist represent today as many challenging problems in membranology. In this report, we will first consider some of the basic observations which point to the role of proteins in the transverse and lateral regionalization of membranes. Then, we will discuss some of the possible mechanisms which, in particular in terms of lipid/protein interactions, can explain lateral heterogenities in membranes and which have the merit of providing a thermodynamic support to the existence of lipid domains in membranes.

Protein-dependent translocation of aminophospholipids and asymmetric transbilayer distribution of phospholipids in the plasma membrane of ram sperm cells

We have investigated the transbilayer movement of phospholipids in the plasma membrane of ram sperm cells using spin- and fluorescence-labeled lipid analogues. After incorporation into the outer leaflet, phosphatidylcholine (PC) and sphingomyelin (SM) moved slowly to the inner cytoplasmic leaflet, whereas phosphatidylserine (PS) and phosphatidylethanolamine (PE) rapidly disappeared from the exoplasmic monolayer. Variation of the initial velocity of the relocation kinetics vs the amount of analogue incorporated into the membrane suggests a saturability of the transbilayer movement of aminophospholipids. ATP depletion or pretreatment with N-ethylmaleimide of ram sperm cells reduced the fast inward motion of PS and PE, indicating a protein-mediated aminophospholipid translocation. The results suggest for the plasma membrane of ram sperm cells the presence of an aminophospholipid translocase and an asymmetric transversal lipid distribution with aminophospholipids preferentially located in the inner leaflet and choline-containing phospholipids in the outer leaflet. The relevance of the transversal segregation of phospholipids for membrane fusion processes occurring during fertilization is discussed.