Outside-inside translocation of aminophospholipids in the human erythrocyte membrane is mediated by a specific enzyme

Phospholipid flippases

Protein mediated phospholipid translocation through membranes has been observed in rat liver endoplasmic reticulum and in the plasma membrane of erythrocytes as well as in a few other cell membranes. Lipid translocation in plasma membranes is ATP dependent and selectively accumulates aminophospholipids on the inner monolayers.

Transbilayer diffusion of phospholipids: dependence on headgroup structure and acyl chain length

The kinetics and thermodynamics of the transmembrane movement (flip-flop) of fluorescent analogs of phosphatidic acid (PA), phosphatidylglycerol (PG), phosphatidylcholine (PC), and phosphatidylethanolamine (PE) were investigated to determine the contributions of headgroup composition and acyl chain length to phospholipid flip-flop. The phospholipid derivatives containing n-octanoic, n-decanoic or n-dodecanoic acid in the sn-1 position and 9-(1-pyrenyl)nonanoic acid in the sn-2 position were incorporated at 3 mol% into sonicated single-bilayer vesicles of 1-palmitoyl-2-oleoyl-sn-glycerol-3-phosphocholine (POPC). The kinetics of diffusion of the pyrene-labeled phospholipids from the outer and inner monolayers of the host vesicles to a large pool of POPC acceptor vesicles were monitored by the time-dependent decrease of pyrene excimer fluorescence. The observed kinetics of transfer were biexponential, with a fast component due to the spontaneous transfer of pyrenyl phospholipids in the outer monolayer of labeled vesicles and a slower component due to diffusion of pyrenyl phospholipid from the inner monolayer of the same vesicles. Intervesicular transfer rates decreased approx. 8-fold for every two carbons added to the first acyl chain. Correspondingly, the free energy of activation for transfer increased approx. 1.3 kcal/mol. With the exception of PE, the intervesicular transfer rates for the different headgroups within a homologous series were nearly the same, with the PC derivative being the fastest. Transfer rates for the PE derivatives were 5-to 7-fold slower than the rates observed for PC. Phospholipid flip-flop, in contrast, was strongly dependent on headgroup composition with a smaller dependence on acyl chain length. At pH 7.4, flip-flop rates increased in the order PC less than PG less than PA less than PE, where the rates for PE were at least 10-times greater than those of the homologous PC derivative. Activation energies for flip-flop were large, and ranged from 38 kcal/mol for the longest acyl chain derivative of PC to 25 kcal/mol for the PE derivatives. Titration of the PA headgroup at pH 4.0 produced an approx. 500-fold increase in the flip-flop rate of PA, while the activation energy decreased 10 kcal/mol. Increasing acyl chain length reduced phospholipid flip-flop rates, with the greatest change observed for the PC analogs, which exhibited an approx. 2-fold decrease in flip-flop rate for every two methylene carbons added to the acyl chain at the sn-1 position.(ABSTRACT TRUNCATED AT 400 WORDS)

Abnormal membrane phospholipid organization in Plasmodium falciparum-infected human erythrocytes

The membrane phospholipid organization in Plasmodium falciparum-infected human erythrocytes was analysed by employing phospholipase A2 and Merocyanine 540 as external membrane probes. Both bee venom and pancreatic phospholipases A2 failed to hydrolyse phosphatidylserine in uninfected human red cells isolated from in vitro P. falciparum cultures. However, these enzymes under identical conditions readily degraded this aminophospholipid in P. falciparum-infected erythrocytes. Phosphatidylethanolamine hydrolysis also increased in parasitized cells. The degree to which these aminophospholipids were cleaved by the enzymes in intact infected cells depended on the developmental stage of the intracellular parasite, and was maximum at the schizont stage. This was consistent with the finding that the 'fluid-sensing' fluorescent dye, Merocyanine 540, readily labelled both the schizont and trophozoite-infected cells but not the fresh, uninfected or ring-infected erythrocytes. These results demonstrate that P. falciparum produces stage-dependent changes in the membrane phospholipid organization of its host erythrocyte.

Studies on sickled erythrocytes provide evidence that the asymmetric distribution of phosphatidylserine in the red cell membrane is maintained by both ATP-dependent translocation and interaction with membrane skeletal proteins

In order to study factors which are involved in maintenance of phosphatidylserine (PS) asymmetry within the human red cell membrane, we measured the effect of ATP-depletion and of membrane skeleton/lipid bilayer uncoupling induced by sickling on the distribution of PS within the membrane bilayer of sickle cells. Trace amounts of radiolabeled PS were introduced into the outer membrane leaflet of both fresh and ATP-depleted reversibly sickled cells (RSCs), using a non-specific lipid transfer protein purified from bovine liver. The equilibration of the newly introduced PS over the two halves of the bilayer was monitored by treatment of the cells with phospholipase A2 which selectively hydrolyzes only those molecules present in the outer membrane leaflet. Within 1 h after insertion into fresh RSCs, only 10% of the labeled PS was accessible to the action of phospholipase A2. This fraction was markedly increased when the cells were subsequently deoxygenated. Prolonged deoxygenation of RSCs, deprived of their ATP after incorporation of radiolabeled PS, caused enhanced phospholipase A2-induced hydrolysis of radiolabeled PS. Similarly, phospholipase A2-induced hydrolysis of endogenous PS in intact RSCs was markedly enhanced when ATP-depleted, but not when fresh cells, were incubated under nitrogen for 3.5 h. Deoxygenated ATP-depleted RSCs markedly enhanced the rate of thrombin formation in the presence of purified coagulation factors Xa, Va, prothrombin and Ca2+. This enhancement appeared to be dependent on the duration of incubation under nitrogen. This phenomenon, indicating the presence of increasing amounts of endogenous PS in the outer membrane leaflet, was not observed when either fresh RSCs or ATP-depleted normal erythrocytes were incubated under nitrogen. Our present observations provide evidence that, in addition to the interaction of PS with the skeletal proteins, an ATP-dependent translocation of PS is required to maintain its absolute asymmetric distribution in the human erythrocyte membrane.

Ion regulation of phosphatidylserine and phosphatidylethanolamine outside-inside translocation in human erythrocytes

In previous publications, we have shown, by using spin-labeled derivatives, that the translocation of phosphatidylserine and phosphatidylethanolamine from the outer to the inner monolayer of human erythrocyte membrane is a protein-mediated phenomenon, which requires hydrolisable Mg2+-ATP. The inhibition by intracellular Ca2+ (0.2 microM) or by extracellularly added vanadate (50 microM) was reported (Seigneuret, M. and Devaux, P.F. (1984) Proc. Natl. Acad. Sci. USA 81, 3751-3755; Zachowski, A., Favre, E., Cribier, S., Hervé, P. and Devaux, P.F. (1986) Biochemistry 25, 2585-2590). The present article gives further insight into the effects of intracellular and extracellular ions on the aminophospholipid translocation in human erythrocytes. By measuring the cell ATP concentration, we now show that the inhibitory effect of intracellular calcium on spin-labeled aminophospholipid translocation is partly due to the ATP depletion, which follows the increased consumption by the calcium pump. However, a direct inhibitory effect of cytosolic Ca2+ on the aminophospholipid translocase can be demonstrated by measuring the initial rate of aminophospholipid translocation in the presence of variable amounts of intracellular calcium, at fixed ATP concentrations. Moreover, the transmembrane equilibrium distribution of phosphatidylserine and phosphatidylethanolamine are affected differently by Ca2+: when cytosolic Ca2+ concentration is increased, alteration of phosphatidylethanolamine distribution begins as soon as the inward translocation is affected by Ca2+ (approx. 50 nM), whereas phosphatidylserine distribution remains unchanged within a large inhibitory range of cytosolic Ca2+ concentrations and decreases above 0.2 microM of free Ca2+ within the cytosol. Decrease of the intracellular Mg2+ concentration below its physiological value (approx. 2 mM) results in the inhibition of aminophospholipid inward transport, whereas increase of Mg2+ concentration does not modify this transport. If Mn2+ is substituted for Mg2+, part of the aminophospholipid translocation is maintained, whereas if Co2+ is substituted for Mg2+, the rapid translocation is completely abolished. Concentrations as high as a millimolar of extracellular Ca2+, Mg2+ or Mn2+ have no effect on the aminophospholipid translocation. The less usual cations Cr3+, Fe2+, Cu2+, Sn2+ and Eu3+ are also uneffective. With extracellular Ni2+ or Co2+, some inhibition can be observed, half inhibition by Ni2+ corresponding to 500 microM. Vanadyl (VO2+), on the other hand, is a potent inhibitor of the aminophospholipid translocation when applied on the extracellular surface, half-inhibition being reached around 30 microM.(ABSTRACT TRUNCATED AT 400 WORDS)

Loss of phospholipid asymmetry in dilauroylphosphatidylcholine induced plasma membrane vesicles from human platelets

Incubation of human platelets with unilamellar vesicles composed of dilauroylphosphatidylcholine (DLPC) induces shedding of small vesicular structures from the platelet plasma membrane. No significant cell lysis is observed during the process of shedding. Isolated spicules contain the major membrane glycoproteins, Ib, IIb, and IIIa, which are used to define the sidedness of the spicule membrane. These glycoproteins are completely susceptible to chymotrypsin treatment, whereas cytoskeletal proteins are inaccessible towards this enzyme. This demonstrates that the spicule membranes have a right-side-out orientation in as far as membrane proteins are concerned. Isolated spicules were 30-fold more active than platelets in stimulating prothrombin conversion to thrombin by the prothrombinase complex (factors Xa, Va and Ca2+). The increased prothrombinase activity reflects an increased amount of phosphatidylserine in the outer leaflet of the spicule membrane. Protein analysis of platelet spicules and native platelets reveals a number of differences, the most conspicuous of which is the virtual absence of myosin in the spicule preparations. It is proposed that a lack of myosin produces a different cytoskeletal organization in the spicules. This enables phosphatidylserine to become exposed at the outer surface of the spicule membrane.

Correlation between calpain-mediated cytoskeletal degradation and expression of platelet procoagulant activity. A role for the platelet membrane-skeleton in the regulation of membrane lipid asymmetry?

The relationship between platelet calpain-activity and platelet procoagulant-activity was investigated by comparison of the time course of their generation after platelet stimulation by calcium ionophore A23187, or by the combined action of collagen and thrombin, or during exposure of platelets to the local anesthetics dibucaine or tetracaine. In addition, the Ca2+ dose-response curves of both activities in intact platelets, obtained by stimulation with A23187 in the presence of Ca2+/HEDTA-buffers, were compared. Platelet procoagulant activity was determined by assaying for prothrombinase activity in the presence of saturating concentrations of factors Xa, Va, and prothrombin. Platelet calpain activity was monitored by the degradation of its major substrates (filamin, talin, myosin) and the formation of their fragments as judged from protein patterns after gel electrophoresis. Platelet stimulation by A23187 resulted in a fast increase in prothrombinase activity, reaching its maximum level after about 20 seconds. Filamin and talin were completely hydrolysed within 15 s, and myosin was partly degraded between 15 and 30 s after platelet activation. When platelets were activated by collagen plus thrombin, prothrombinase activity was generated with a sigmoid time course, the steepest increase being observed between 1 and 2 min after platelet activation. Proteolysis of filamin and talin occurred between 0.5 and 1.5 min after platelet activation, while degradation of myosin became visible after 2 to 2.5 min. Dibucaine and tetracaine were both found to be potent stimulators of prothrombinase activity, with half-maximal activities obtained at 0.7 and 2.8 mM, respectively. Using suboptimal concentrations of both local anesthetics, it was found that the generation of prothrombinase activity closely paralleled that of calpain activity over a time course of 1 hour. Ca2+ titration of intact platelets using A23187 and Ca2+/HEDTA buffers, revealed half-maximal response at about 15 microM free Ca2+ for both calpain and prothrombinase activity. These findings strongly suggest a causal relationship between generation of a procoagulant platelet surface and calpain-mediated degradation of filamin, talin, and myosin. Since an increased procoagulant activity reflects an increased exposure of phosphatidylserine at the platelet outer surface, the present findings suggest that platelet cytoskeletal proteins are involved in the regulation of membrane lipid asymmetry.

Polarity of the Forssman glycolipid in MDCK epithelial cells

To determine whether epithelial plasma membrane glycolipids are polarized in a manner analogous to membrane proteins, MDCK cells grown on permeable filters were analyzed for the expression of Forssman ceramide pentasaccharide, the major neutral glycolipid in these cells. In contrast to a recent report which described exclusive apical localization of the Forssman glycolipid (Hansson, G.C., Simons, K. and Van Meer, G. (1986) EMBO J. 5, 483-489), immunofluorescence and immunoelectron microscopic staining revealed the Forssman glycolipid on both the apical and basolateral surfaces of polarized cells. Immunoblots indicated that the Forssman antigen was detectable only on glycolipids and not on proteins. Analysis of metabolically labeled glycolipids released into the apical and basal culture medium, either as shed membrane vesicles or in budding viruses, also demonstrated the presence of the Forssman glycolipid on both apical and basolateral membranes of polarized cells. Quantitation of the released glycolipid indicated that the Forssman glycolipid was concentrated in the apical membrane. These results are consistent with previous reports which described quantitative enrichment of glycolipids in the apical domain of several epithelia.

An intracellular simian malarial parasite (Plasmodium knowlesi) induces stage-dependent alterations in membrane phospholipid organization of its host erythrocyte

The membrane phospholipid organization in monkey erythrocytes harbouring different developmental stages of the simian malarial parasite Plasmodium knowlesi was studied using phospholipase A2 from two different sources and Merocyanine 540 as the external-membrane probes. Experiments were done to confirm that the phospholipases did not penetrate into the infected cells or hydrolyse phospholipids during membrane isolation. The parasite-free erythrocyte membrane was isolated by differential centrifugation or by using the cationic beads Affi-Gel 731. The purity of the membranes was established by optical and electron microscopy, and by assaying the parasite-specific enzyme glutamate dehydrogenase. About 10% of the phosphatidylethanolamine and none of phosphatidylserine were hydrolysed by the phospholipases in intact normal monkey erythrocytes. However, accessibility of these aminophospholipids to the enzymes was significantly enhanced in the infected cells under identical conditions. The degree of this enhancement depended on the developmental stage of the intracellular parasite, but not on the parasitaemia levels in the infected monkeys, and increased with the parasite growth inside the cells. Analogously, Merocyanine 540 was found to label the trophozoite- or schizont-infected erythrocytes, but not the ring-infected or normal cells. These results demonstrate that the intracellular malarial parasite produces stage-dependent alterations in the membrane phospholipid organization of its host erythrocyte.

Maintenance of membrane phospholipid asymmetry. Lipid-cytoskeletal interactions or lipid pump?

Two models for the mechanism of maintenance of lipid asymmetry in erythrocytes are considered: binding of internal lipids to cytoskeletal proteins, and pumping of internal lipids from the outside to the inside of the cell. Analysis of the kinetics of lipid internalization suggests that the first model is more likely, and that the apparent pumping of lipids represents the activity of an ATP-dependent lipid flip/flop catalyst.

Phospholipid outside-inside translocation in lymphocyte plasma membranes is a protein-mediated phenomenon

We have measured the transbilayer diffusion of spin-labeled analogs of sphingomyelin, phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine in pig lymphocyte plasma membrane. At 4 degrees C and 37 degrees C the aminophospholipids are rapidly transported from the outer to the inner leaflet of the membrane, whereas the choline-containing phospholipids experience a slower diffusion. This selectivity is abolished after cell treatment by SH-group reagents indicating that the aminophospholipid translocation is protein-dependent and must be driven by a system analogous to the one existing in the human red cell membrane. The fact that the selectivity exists at low temperature, that it does not depend on cytoskeleton integrity and that there is a competition between the two aminophospholipids show that this translocation is not purely an endocytic process.

Phosphatidylserine decarboxylase: generation of asymmetric vesicles and determination of the transbilayer distribution of fluorescent phosphatidylserine in model membrane systems

Large unilamellar vesicles (LUV) that contained a fluorescent analog of phosphatidylserine (NBD-PS) were used in model systems to determine the feasibility of employing phosphatidylserine decarboxylase (PS-decarboxylase) to generate asymmetric vesicles and to determine the transbilayer distribution of PS. PS-decarboxylase prepared by sonication of Escherichia coli JA 200 pLC 8-47 was found to be stable in detergent-free buffers and catalyzed the conversion of NBD-PS to NBD-phosphatidylethanolamine (NBD-PE). PS-decarboxylase was capable of decarboxylating virtually all of the NBD-PS present in the outer leaflet of LUV containing a symmetric or asymmetric (outside only) distribution of NBD-PS, but not NBD-PS present in the inner leaflet of the vesicles. The ability of PS-decarboxylase to decarboxylate only NBD-PS located in the outer leaflet of the vesicles was independently verified by resonance energy transfer (between NBD-PS and (lissamine) rhodamine B-labeled phosphatidylethanolamine) and by derivatization with trinitrobenzenesulfonic acid (TNBS). These techniques revealed that the exchangeable pool (the fraction of NBD-PS on the outer leaflet) and the respective fraction of Tnp-(NBD-PS) formed were equivalent to the extent of PS-decarboxylase-mediated decarboxylation of NBD-PS to NBD-PE. These results show that PS-decarboxylase can be used to generate asymmetric vesicles (i.e., PS inside, PE outside) and determine the intrabilayer distribution of PS in model membranes.