Evidence for bidirectional transverse diffusion of spin-labeled phospholipids in the plasma membrane of guinea pig blood cells

Flow cytometric analysis of transmembrane phospholipid movement in bull sperm

Fluorescent phospholipids are useful to investigate phospholipid dynamics in biological membranes. We used flow cytometry to investigate transbilayer phospholipid movement in live sperm cells. Acyl-labeled N-4-nitrobenzo-2-oxa-1,3-diazole (NBD) -phosphatidylcholine (-PC), -phosphatidylethanolamine (-PE), or -phosphatidylserine (-PS) were incorporated into sperm cells, and the transbilayer location was determined by extraction of probe from cell with excess bovine serum albumin (BSA) or by chemical destruction of probe by sodium dithionite. Using these methods, we have measured the head group specific outer leaflet to inner leaflet movement (flip) of the aminophospholipids NBD-PS and NBD-PE. The fluorescent phospholipids moved inward across the plasma membrane with half-times of 1.8, 2.5, and 11.2 min, for NBD-PS, NBD-PE, and NBD-PC and reached apparent equilibrium levels of 88%, 94%, and 32% inside, respectively. The inward movement of NBD-PE was inhibited by sulfhydryl reagents, elevated intracellular Ca2+, and depletion of cellular ATP. Analysis of the kinetics of NBD-PE and -PS extraction by BSA allows determination of the rates for outward movement (flop) across the plasma membrane. Half-times for flop were 4.7 and 4.5 min for NBD-PS and -PE, respectively. Based on these measurements, a simple model of NBD-phospholipid equilibria was developed and fit to the kinetic data. Computer-generated fits reflected major features of the experimental data and provide a potential tool for predicting the dynamics of endogenous lipids.

Loss of phospholipid asymmetry in human platelet plasma membrane after 1-12 days of storage. An ESR study

We used paramagnetic analogs of endogenous phospholipids to study modification of phospholipid distribution in platelet plasma membranes during aging. Asymmetrical distributions and translocation kinetics were very different for spin-labeled phosphatidylserine and spin-labeled phosphatidylcholine in fresh platelet plasma membranes. In freshly prepared platelets and up to day 7, spin-labeled phosphatidylserine very rapidly penetrated to the inner leaflet of the platelet plasma membrane. However, spin-labeled phosphatidylcholine was mainly retained on the external leaflet. From day 7 to day 9, the two translocation kinetics became identical with symmetrical distribution of both spin-labeled phospholipids at equilibrium. Inhibition of translocase activity and modification of membrane stability accounted for these transformations. The rapid re-exposition of spin-labeled phosphatidylserine after stimulation by the calcium ionophore A23187, measured in fresh platelet concentrates, persisted up to day 9 but disappeared between day 10 and day 12. From day 7 to day 9, a strong cytoskeleton proteolysis and marked decrease in intracellular ATP were observed. Moreover, complete suppression of beta-N-acetyl glucosaminidase secretion and vesicle formation after A23187 stimulation of aged platelets indicated that platelets could no longer be activated beyond day 9. Taken together, these results showed that during in vitro aging there are metabolic and membrane modifications in platelet similar to those described for platelet activation. In addition, all of the observed events occurred simultaneously between day 7 and day 9. These results highlight the importance of maintaining plasma membrane asymmetry to increase the hemostatic effectiveness of transfused platelet concentrates.

Phospholipids in animal eukaryotic membranes: transverse asymmetry and movement

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Quantitative comparison between aminophospholipid translocase activity in human erythrocytes and in K562 cells

Spin-labeled phospholipids were used to determine the transbilayer movement of phospholipids in human erythrocytes, in K562 cells and in human neonatal red cells. The erythroleukemia cell line, K562, as well as human neonatal red cells, which are rich in reticulocytes, were considered as representative of human erythrocyte precursor cells. In the nucleated cells, the difference between outside-inside movement of aminophospholipids and that of phosphatidylcholine or sphingomyelin analogues allowed us to discriminate between lipid internalization due to aminophospholipid translocase activity and to endocytosis. From the initial rates of aminophospholipid inward movement, we inferred that the activity of the aminophospholipid translocase is higher in the precursor cells than in mature erythrocytes.

Phospholipid asymmetry and transmembrane diffusion in photoreceptor disc membranes

The phospholipid asymmetry and flip-flop rate in rod outer segment (ROS) disc membranes has been investigated by both a spin-label reduction method and a novel electron-electron double resonance (ELDOR) method. At 39 degrees C in the absence of ATP, spin-labeled derivatives of phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS) undergo rapid transmembrane diffusion, with a half-time of less than 10 min. At equilibrium, approximately 53% of the spin-labeled PC, 37% of the spin-labeled PE, and 18% of the spin-labeled PS reside in the inner (intradiscal) monolayer. The rapid transmembrane diffusion of phospholipids is partially inhibited by N-ethylmaleimide, suggesting that the process is mediated by proteins in the disc membrane.

Effect of benzyl alcohol on phospholipid transverse mobility in human erythrocyte membrane

The effect of benzyl alcohol on the transverse mobility and repartition of phospholipids in the human erythrocyte membrane was investigated using electron spin resonance and morphological modification of red blood cells. Transmembrane internalization rates and equilibrium distribution in red blood cells of short-chain spin-labeled phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine were strongly modified by treatment with 10-70 mM benzyl alcohol. A dual effect was observed: (a) at 4 degrees C and 37 degrees C there was an N-ethylmaleimide-sensitive, long lasting and fully reversible increase in the spin-labeled phosphatidylserine and phosphatidylethanolamine internalization rate; (b) at 37 degrees C, an enhancement of N-ethylmaleimide-insensitive fluxes of all the labeled phospholipids through the membrane occurred. Both effects were dose-dependent. Erythrocytes submitted to benzyl alcohol incubation also showed dose-dependent shape changes: an immediate one from discocytes to echinocytes, followed by a slower N-ethylmaleimide- and ATP-dependent change to stomatocytes. Moreover, benzyl alcohol treatment was shown to lead to enhanced hydrolysis of intracellular ATP. All the effects of benzyl alcohol can be described as an accumulation of labeled phosphatidylethanolamine (and labeled phosphatidylcholine at 37 degrees C) in the inner leaflet. This can be interpreted as a perturbation of the erythrocyte membrane, leading to an energy-consuming specific increase in aminophospholipid translocase activity, in addition to a slow and passive bidirectional flux of all phospholipids at 37 degrees C.

A model for the extracellular release of PAF: the influence of plasma membrane phospholipid asymmetry

Recent studies suggesting that cellular activation leads to enhanced transbilayer movement of phospholipids and loss of plasma membrane phospholipid asymmetry lead us to hypothesize that such events may govern the release of PAF, a potent, but variably release, lipid mediator synthesized by numerous inflammatory cells. To model these membrane events, we studied the transbilayer movement of PAF across the human erythrocyte and erythrocyte ghost plasma membrane, membranes with documented phospholipid asymmetry which can be deliberately manipulated. Utilizing albumin to extract outer leaflet PAF, transbilayer movement of PAF was shown to be significantly enhanced in erythrocytes and ghosts altered to lose membrane asymmetry when compared to movement in those with native membrane asymmetry. Verification of membrane changes was demonstrated using merocyanine 540 (MC540), a dye which preferentially stains loosely packed or hydrophobic membranes, and acceleration of the modified Russell's viper venom clotting assay by externalized anionic phospholipids. Utilizing the erythrocyte ghost loaded with PAF in either the outer or the inner leaflet, enhanced transbilayer movement to the opposite leaflet was seen to accompany loss of membrane asymmetry. Studies utilizing ghosts loaded with albumin intracellularly demonstrated that 'acceptor' molecules binding PAF further influence the disposition of PAF across the plasma membrane. Taken together, these findings suggest that the net release of PAF from activated inflammatory cells will depend on localization of PAF to the plasma membrane, transbilayer movement, which is facilitated by alteration of membrane phospholipid asymmetry, and removal from the membrane by extracellular and intracellular 'acceptor' molecules.

Transmembrane movements of lipids

Membranes allow the rapid passage of unchanged lipids. Phospholipids on the other hand diffuse very slowly from one monolayer to another with a half-time of several hours. This slow spontaneous movement in a pure lipid bilayer can be selectively modulated in biological membranes by intrinsic proteins. In microsomes, and probably in bacterial membranes, non-specific phospholipid flippases allow the rapid redistribution of newly synthesized phospholipids. In eukaryotic plasma membranes, aminophospholipid translocase selectively pumps phosphatidylserine (PS) and phosphatidylethanolamine (PE) from the outer to the inner leaflet and establishes a permanent lipid asymmetry. The discovery of an aminophospholipid translocase in chromaffin granules proves that eukaryotic organelles may also contain lipid translocators.

Maintenance of lipid asymmetry in red blood cells and ghosts: effect of divalent cations and serum albumin on the transbilayer distribution of phosphatidylserine

The maintenance of lipid asymmetry in the plasma membrane of human red blood cells (RBC) was investigated by assessing the equilibrium distribution of exogenously inserted NBD-labeled phosphatidylserine (PS) and endogenous PS in RBC and hypotonically lysed ghosts. PS distribution was determined by the ability to 'back-exchange' NBD-lipids into acceptor membranes and bovine serum albumin, and by prothrombinase complex assay for endogenous PS. To maintain the normal asymmetric distribution of PS in RBC, ghosts required Mg2+ in the lysis buffer. The inclusion of Ca2+, even in the presence of Mg2+ resulted in complete randomization of endogenous and exogenously inserted PS. These results indicate that NBD-labeled PS analogs faithfully monitor the distribution of endogenous PS during ghost preparation. In contrast, treatment of RBC with bovine serum albumin had no effect on the distribution of endogenous PS, although it resulted in a time-dependent movement of NBD-labeled PS from the inner to the outer leaflet (flop). This phenomenon was dependent on continuous incubation in the presence of albumin and could not be duplicated when pure acceptor membranes were used.

Aminophospholipid translocation in erythrocytes: evidence for the involvement of a specific transporter and an endofacial protein

The transport of exogenously supplied fluorescent analogues of aminophospholipids from the outer to inner leaflet in red blood cells (RBC) is dependent upon the oxidative status of membrane sulfhydryls. Oxidation of a sulfhydryl on a 32-kDa membrane protein by pyridyldithioethylamine (PDA) has been previously shown [Connor & Schroit (1988) Biochemistry 27, 848-851] to inhibit the transport of NBD-labeled phosphatidylserine (NBD-PS). In the present study, other sulfhydryl oxidants were examined to determine whether additional sites are involved in the transport process. Our results show that diamide inhibits the transport of NBD-PS via a mechanism that is independent of the 32-kDa site. This is shown by the inability of diamide to block labeling of the 32-kDa sulfhydryl with 125I-labeled PDA and to protect against PDA-mediated inhibition of NBD-PS transport. diamide-mediated inhibition, but not PDA-mediated inhibition, could be reversed by reduction with cysteamine or endogenous glutathione. Similarly, treatment of RBC with 5,5'-dithiobis(2-nitrobenzoic acid), which depletes endogenous glutathione and induces oxidation of endofacial proteins [Reglinski et al. (1988) J. Biol. Chem. 263, 12360-12366], inhibited NBD-PS transport in a manner analogous to diamide. Once established, the asymmetric distribution of NBD-PS could not be altered by oxidation of either site. These data indicate that a second site critical to the transport of aminophospholipids resides on the endofacial surface and suggest that the transport of aminophospholipids across the bilayer membrane of RBC depends on a coordinated and complementary process between a cytoskeletal component and the 32-kDa membrane polypeptide; both must be operative for transport to proceed.

Asymmetric distribution of phospholipids in the membrane of vesicles released during in vitro maturation of guinea pig reticulocytes: evidence precluding a role for "aminophospholipid translocase"

Guinea pig reticulocytes lose their transferrin (Tf) binding activity during maturation, in the form of vesicles (exosomes) released into the extracellular medium. Vesicles were prepared from cultures of reticulocytes to study the possible externalization of a particular membrane-associated activity, i.e., that of "aminophospholipid translocase." Analysis of the peptide composition of these vesicles revealed that the major proteins are the Tf receptor and another peptide (70kDa), which is probably the "clathrin-uncoating ATPase" described by Johnstone et al. (1987). The exosome had a lipid composition similar to erythrocyte membrane, although with a lightly but significantly lower phosphatidylethanolamine content. The aminophospholipid distribution in the vesicle membrane was determined by fluorescamine labeling. The exosomes showed an asymmetric aminophospholipid distribution similar to that of erythrocytes. "Aminophospholipid translocase" activity was absent, as no transverse diffusion of spin-labeled phospholipids occurred over more than 2 hours at 37 degrees C.

Control of transmembrane lipid asymmetry in chromaffin granules by an ATP-dependent protein

The Ca2+-dependent binding of annexin proteins to secretory granule membranes seems to be involved in the early stage of exocytosis. Binding studies have shown that these proteins have a specificity for phosphatidylserine (PtdS) interfaces. Furthermore, aminolipids are necessary for contact and fusion between lipid vesicles or between liposomes and chromaffin granules. Thus, PtdS must be present on the granule outer (cytoplasmic) monolayer. We report here that chromaffin granules possess a mechanism to maintain PtdS orientation, comparable to the ATP-dependent aminophospholipid translocase from human erythrocytes. The translocase, in granules, selectively transports PtdS from the luminal to the cytoplasmic monolayer, provided the incubation medium contains ATP. As this protein shares several properties with the granule vanadate-sensitive ATPase II, we infer that this ATPase, of relative molecular mass 115,000, is the protein responsible for aminophospholipid translocation. This is the first evidence for an ATP-dependent specific phospholipid 'flippase' in intracellular organelles.