ATP-dependent aminophospholipid translocation in erythrocyte vesicles: stoichiometry of transport

The Dual Role of Mesenchymal Stromal Cells and Their Extracellular Vesicles in Carcinogenesis

Simple Summary

Extracellular vesicles (EVs) are membrane structures that play the role of intermediaries between tumor cells and the tumor microenvironment (TME) because they have the ability to transport lipids, transcription factors, mRNA, and proteins. Mesenchymal stem cells (MSCs) are a major component of the TME and may have different effects on tumor progression using EVs. This review includes information about various studies which have reported that EVs from MSCs can have either antitumor or pro-tumor effects, depending on both the tumor type and developmental stage. It provides an overview of the published data on EV MSCs and their effect on tumor cells. In addition, the use of EV MSCs for the development of new methods for treating oncological diseases is described.

Abstract

Mesenchymal stem cells (MSCs) are a major component of the tumor microenvironment (TME) and play an important role in tumor progression. MSCs remodel the extracellular matrix, participate in the epithelial–mesenchymal transition, promote the spread of metastases, and inhibit antitumor immune responses in the TME; however, there are also data pertaining to the antitumor effects of MSCs. MSCs activate the cell death mechanism by modulating the expression of proteins involved in the regulation of the cell cycle, angiogenesis receptors, and proapoptotic proteins. One of the main ways in which MSCs and TME interact is through the production of extracellular vesicles (EVs) by cells. Currently, data on the effects of both MSCs and their EVs on tumor cells are rather contradictory. Various studies have reported that EVs from MSCs can have either antitumor or pro-tumor effects, depending on both the tumor type and developmental stage. In this review, we discuss published data on EV MSCs and their effect on tumor cells. The molecular composition of vesicles obtained from MSCs is also presented in the review. In addition, the use of EV MSCs for the development of new methods for treating oncological diseases is described.

Enzymatic trans-bilayer lipid transport: Mechanisms, efficiencies, slippage, and membrane curvature

The eukaryotic plasma membrane's lipid composition is found to be ubiquitously asymmetric comparing inner and outer leaflets. This membrane lipid asymmetry plays a crucial role in diverse cellular processes critical for cell survival. A specialized set of transmembrane proteins called translocases, or flippases, have evolved to maintain this membrane lipid asymmetry in an energy-dependent manner. One potential consequence of local variations in membrane lipid asymmetry is membrane remodeling, which is essential for cellular processes such as intracellular trafficking. Recently, there has been a surge in the identification and characterization of flippases, which has significantly advanced the understanding of their functional mechanisms. Furthermore, there are intriguing possibilities for a coupling between membrane curvature and flippase activity. In this review we highlight studies that link membrane shape and remodeling to differential stresses generated by the activity of lipid flippases with an emphasis on data obtained through model membrane systems. We review the common mechanistic models of flippase-mediated lipid flipping and discuss common techniques used to test lipid flippase activity. We then compare the existing data on lipid translocation rates by flippases and conclude with potential future directions for this field.

Procoagulant Phosphatidylserine-Exposing Platelets in vitro and in vivo

The physiological heterogeneity of platelets leads to diverse responses and the formation of discrete subpopulations upon platelet stimulation. Procoagulant platelets are an example of such subpopulations, a key characteristic of which is exposure either of the anionic aminophospholipid phosphatidylserine (PS) or of tissue factor on the activated platelet surface. This review focuses on the former, in which PS exposure on a subpopulation of platelets facilitates assembly of the intrinsic tenase and prothrombinase complexes, thereby accelerating thrombin generation on the activated platelet surface, contributing importantly to the hemostatic process. Mechanisms involved in platelet PS exposure, and accompanying events, induced by physiologically relevant agonists are considered then contrasted with PS exposure resulting from intrinsic pathway-mediated apoptosis in platelets. Pathologies of PS exposure, both inherited and acquired, are described. A consideration of platelet PS exposure as an antithrombotic target concludes the review.

Role of Microparticles in the Pathogenesis of Inflammatory Joint Diseases

Rheumatoid arthritis (RA), juvenile idiopathic arthritis (JIA), ankylosing spondylitis (AS), and psoriatic arthritis (PsA) make up a group of chronic immune-mediated inflammatory diseases (IMIDs). The course of these diseases involves chronic inflammation of joints and enthesopathies, which can result in joint damage and disability. Microparticles (MPs) are a group of small spherical membranous vesicles. The structure and cellular origin of MPs, mechanisms that stimulate their secretion and the place of their production, determine their biological properties, which could become manifest in the pathogenesis of immune-mediated inflammatory diseases. Microparticles can stimulate synovitis with proinflammatory cytokines and chemokines. MPs may also contribute to the pathogenesis of rheumatic diseases by the formation of immune complexes and complement activation, pro-coagulation activity, activation of vascular endothelium cells, and stimulation of metalloproteinase production. It seems that in the future, microparticles can become a modern marker of disease activity, a response to treatment, and, possibly, they can be used in the prognosis of the course of arthritis. The knowledge of the complexity of MPs biology remains incomplete and it requires further comprehensive studies to explain how they affect the development of rheumatic diseases. This review focuses on the immunopathogenic and therapeutic role of MPs in chronic immune-mediated inflammatory joint diseases.

The role of microvesicles containing microRNAs in vascular endothelial dysfunction

Many studies have shown that endothelial dysfunction is associated with a variety of cardiovascular diseases. The endothelium is one of the primary targets of circulating microvesicles. Besides, microRNAs emerge as important regulators of endothelial cell function. As a delivery system of microRNAs, microvesicles play an active and important role in regulating vascular endothelial function. In recent years, some studies have shown that microvesicles containing microRNAs regulate the pathophysiological changes in vascular endothelium, such as cell apoptosis, proliferation, migration and inflammation. These studies have provided some clues for the possible roles of microvesicles and microRNAs in vascular endothelial dysfunction‐associated diseases, and opened the door towards discovering potential novel therapeutic targets. In this review, we provide an overview of the main characteristics of microvesicles and microRNAs, summarizing their potential role and mechanism in endothelial dysfunction, and discussing the clinical application and existing problems of microvesicles for better translational applications.

An Update on Novel Therapeutic Warfronts of Extracellular Vesicles (EVs) in Cancer Treatment: Where We Are Standing Right Now and Where to Go in the Future

Extracellular vesicles (EVs) are a heterogeneous group of membrane-bounded vesicles that are believed to be produced and secreted by presumably all cell types under physiological and pathological conditions, including tumors. EVs are very important vehicles in intercellular communications for both shorter and longer distances and are able to deliver a wide range of cargos including proteins, lipids, and various species of nucleic acids effectively. EVs have been emerging as a novel biotherapeutic platform to efficiently deliver therapeutic cargos to treat a broad range of diseases including cancer. This vast potential of drug delivery lies in their abilities to carry a variety of cargos and their ease in crossing the biological membranes. Similarly, their presence in a variety of body fluids makes them a potential biomarker for early diagnosis, prognostication, and surveillance of cancer. Here, we discuss the relatively least and understudied aspects of EV biology and tried to highlight the obstacles and limitations in their clinical applications and also described most of the new warfronts to beat cancer at multiple stages. However, much more challenges still remain to evaluate EV-based therapeutics, and we are very much hopeful that the current work prompts further discovery.

Erythrocyte Aging, Protection via Vesiculation: An Analysis Methodology via Oscillatory Flow

We demonstrate that erythrocyte deformations, specifically of a type as occur in splenic flow (Zhu et al., 2017), and of the type that promote vesiculation can be caused by simple, yet tailored, oscillatory shear flow. We show that such oscillatory shear flow provides an ideal environment to explore a wide variety of metabolic and biochemical effects that promote erythrocyte vesiculation. Deformation details, typical of splenic flow, such as in-folding and implications for membrane/skeleton interaction are demonstrated and quantitatively analyzed. We introduce a theoretical, essentially analytical, vesiculation model that directly couples to our more complex numerical, multilevel, model that clearly delineates various fundamental elements, i.e., sub-processes, that are involved and mediate the vesiculation process. This analytical model highlights particulary important vesiculation precursors such as areas of membrane/skeleton disruptions that trigger the vesiculation process. We demonstrate, using flow cytometry, that the deformations we experimentally induce on cells, and numerically simulate, do not induce lethal forms of cell damage but do induce vesiculation as theoretically forecasted. This, we demonstrate, provides a direct link to cell membrane/skeletal damage such as is associated with metabolic and aging damage. An additional noteworthy feature of this approach is the avoidance of artificial devices, e.g., micro-fluidic chambers, in which deformations and their time scales are often unrepresentative of physiological processes such as splenic flow.

Extracellular vesicles as regulators of tumor fate: crosstalk among cancer stem cells, tumor cells and mesenchymal stem cells

The tumor microenvironment comprises a heterogeneous population of tumorigenic and non-tumorigenic cells. Cancer stem cells (CSCs) and mesenchymal stem cells (MSCs) are components of this microenvironment and have been described as key regulators of different aspects of tumor physiology. They act differently on the tumor: CSCs are described as tumor initiators and are associated with tumor growth, drug resistance and metastasis; MSCs can integrate the tumor microenvironment after recruitment and interact with cancer cells to promote tumor modifications. Extracellular vesicles (EVs) have emerged as an important mechanism of cell communication under the physiological and pathological conditions. In cancer, secretion of EVs seems to be one of the main mechanisms by which stem cells interact with other tumor and non-tumor cells. The transfer of bioactive molecules (lipids, proteins and RNAs) compartmentalized into EVs triggers different responses in the target cells, regulating several processes in the tumor as angiogenesis, tumor invasiveness and immune escape. This review focuses on the role of CSCs and MSCs in modulating the tumor microenvironment through secretion of EVs, addressing different aspects of the multidirectional interactions among stem cells, tumor and tumor-associated cells.

Non-signalling energy use in the developing rat brain

Energy use in the brain constrains its information processing power, but only about half the brain's energy consumption is directly related to information processing. Evidence for which non-signalling processes consume the rest of the brain's energy has been scarce. For the first time, we investigated the energy use of the brain's main non-signalling tasks with a single method. After blocking each non-signalling process, we measured oxygen level changes in juvenile rat brain slices with an oxygen-sensing microelectrode and calculated changes in oxygen consumption throughout the slice using a modified diffusion equation. We found that the turnover of the actin and microtubule cytoskeleton, followed by lipid synthesis, are significant energy drains, contributing 25%, 22% and 18%, respectively, to the rate of oxygen consumption. In contrast, protein synthesis is energetically inexpensive. We assess how these estimates of energy expenditure relate to brain energy use in vivo, and how they might differ in the mature brain.

Dopamine induces lipid accumulation, NADPH oxidase-related oxidative stress, and a proinflammatory status of the plasma membrane in H9c2 cells

Excess catecholamine levels are suggested to be cardiotoxic and to underlie stress-induced heart failure. The cardiotoxic effects of norepinephrine and epinephrine are well recognized. However, although cardiac and circulating dopamine levels are also increased in stress cardiomyopathy patients, knowledge regarding putative toxic effects of excess dopamine levels on cardiomyocytes is scarce. We now studied the effects of elevated dopamine levels in H9c2 cardiomyoblasts. H9c2 cells were cultured and treated with dopamine (200 μM) for 6, 24, and 48 h. Subsequently, the effects on lipid accumulation, cell viability, flippase activity, reactive oxygen species (ROS) production, subcellular NADPH oxidase (NOX) protein expression, and ATP/ADP and GTP/GDP levels were analyzed. Dopamine did not result in cytotoxic effects after 6 h. However, after 24 and 48 h dopamine treatment induced a significant increase in lipid accumulation, nitrotyrosine levels, indicative of ROS production, and cell death. In addition, dopamine significantly reduced flippase activity and ATP/GTP levels, coinciding with phosphatidylserine exposure on the outer plasma membrane. Furthermore, dopamine induced a transient increase in cytoplasmic and (peri)nucleus NOX1 and NOX4 expression after 24 h that subsided after 48 h. Moreover, while dopamine induced a similar transient increase in cytoplasmic NOX2 and p47^phox expression, in the (peri)nucleus this increased expression persisted for 48 h where it colocalized with ROS. Exposure of H9c2 cells to elevated dopamine levels induced lipid accumulation, oxidative stress, and a proinflammatory status of the plasma membrane. This can, in part, explain the inflammatory response in patients with stress-induced heart failure.

From trash to treasure: The untapped potential of endothelial microparticles in neurovascular diseases

Discovered in 1947, microparticles (MP) represent a group of sub-micron cell-derived particles isolated by high speed centrifugation. Once regarded as cellular 'trash', in the past decade MP have gained tremendous attention in both basic sciences and medical research both as biomarkers and mediators of infection, injury and response to therapy. Because MP bear cell surface markers derived from parent cells, accumulate in extracellular fluids (plasma, serum, milk, urine, cerebrospinal fluid) MP based tests are being developed commercially as important components in 'liquid biopsy' approaches, providing valuable readouts in cardiovascular disease and cancer, as well as stroke, Alzheimer's disease and Multiple Sclerosis. Importantly, MP have been reported as mobile transport vectors in the intercellular transfer of mRNAs, microRNAs, lipids and proteins. Here we discuss MP structure, properties and functions with particular relevance to neurological and neurovascular diseases.

[P4-ATP-ase Atp8b1/FIC1: structural properties and (patho)physiological functions]

P4-ATP-ases comprise an interesting family among P-type ATP-ases, since they are thought to play a major role in the transfer of phospholipids such as phosphatydylserine from the outer leaflet to the inner leaflet. Isoforms of P4-ATP-ases are partially interchangeable but peculiarities of tissue-specific expression of their genes, intracellular localization of proteins, as well as regulatory pathways lead to the fact that, on the organismal level, serious pathologies may develop in the presence of structural abnormalities in certain isoforms. Among P4-ATP-ases a special place is occupied by ATP8B1, for which several mutations are known that lead to serious hereditary diseases: two forms of congenital cholestasis (PFIC1 or Byler disease and benign recurrent intrahepatic cholestasis) with extraliver symptoms such as sensorineural hearing loss. The physiological function of the Atp8b1/FIC1 protein is known in general outline: it is responsible for transport of certain phospholipids (phosphatydylserine, cardiolipin) for the outer monolayer of the plasma membrane to the inner one. It is well known that perturbation of membrane asymmetry, caused by the lack of Atp8B1 activity, leads to death of hairy cells of the inner ear, dysfunction of bile acid transport in liver-cells that causes cirrhosis. It is also probable that insufficient activity of Atp8b1/FIC1 increases susceptibility to bacterial pneumonia.Regulatory pathways of Atp8b1/FIC1 activity in vivo remain to be insufficiently studied and this opens novel perspectives for research in this field that may allow better understanding of molecular processes behind the development of certain pathologies and to reveal novel therapeutical targets.

Non-signalling energy use in the brain

Energy use limits the information processing power of the brain. However, apart from the ATP used to power electrical signalling, a significant fraction of the brain's energy consumption is not directly related to information processing. The brain spends just under half of its energy on non-signalling processes, but it remains poorly understood which tasks are so energetically costly for the brain. We review existing experimental data on subcellular processes that may contribute to this non-signalling energy use, and provide modelling estimates, to try to assess the magnitude of their ATP consumption and consider how their changes in pathology may compromise neuronal function. As a main result, surprisingly little consensus exists on the energetic cost of actin treadmilling, with estimates ranging from < 1% of the brain's global energy budget up to one-half of neuronal energy use. Microtubule treadmilling and protein synthesis have been estimated to account for very small fractions of the brain's energy budget, whereas there is stronger evidence that lipid synthesis and mitochondrial proton leak are energetically expensive. Substantial further research is necessary to close these gaps in knowledge about the brain's energy-expensive non-signalling tasks.

Membrane lipid interactions in intestinal ischemia/reperfusion-induced Injury

Ischemia, lack of blood flow, and reperfusion, return of blood flow, are a common phenomenon affecting millions of Americans each year. Roughly 30,000 Americans per year experience intestinal ischemia-reperfusion (IR), which is associated with a high mortality rate. Previous studies of the intestine established a role for neutrophils, eicosanoids, the complement system and naturally occurring antibodies in IR-induced pathology. Furthermore, data indicate involvement of a lipid or lipid-like moiety in mediating IR-induced damage. It has been proposed that antibodies recognize exposure of neo-antigens, triggering action of the complement cascade. While it is evident that the pathophysiology of IR-induced injury is complex and multi-factorial, we focus this review on the involvement of eicosanoids, phospholipids and neo-antigens in the early pathogenesis. Lipid changes occurring in response to IR, neo-antigens exposed and the role of a phospholipid transporter, phospholipid scramblase 1 will be discussed.

Cell-derived microparticles in atherosclerosis: biomarkers and targets for pharmacological modulation?

Cardiovascular diseases remain an important cause of morbi-mortality. Atherosclerosis, which predisposes to cardiovascular disorders such as myocardial infarction and stroke, develops silently over several decades. Identification of circulating biomarkers to evaluate cardiovascular event risk and pathology prognosis is of particular importance. Microparticles (MPs) are small vesicles released from cells upon apoptosis or activation. Microparticles are present in blood of healthy individuals. Studies showing a modification of their concentrations in patients with cardiovascular risk factors and after cardiovascular events identify MPs as potential biomarkers of disease. Moreover, the pathophysiological properties of MPs may contribute to atherosclerosis development. In addition, pharmacological compounds, used in the treatment of cardiovascular disease, can reduce plasma MP concentrations. Nevertheless, numerous issues remain to be solved before MP measurement can be applied as routine biological tests to improve cardiovascular risk prediction. In particular, prospective studies to identify the predictive values of MPs in pathologies such as cardiovascular diseases are needed to demonstrate whether MPs are useful biomarkers for the early detection of the disease and its progression.

Mammalian P4-ATPases and ABC transporters and their role in phospholipid transport

Transport of phospholipids across cell membranes plays a key role in a wide variety of biological processes. These include membrane biosynthesis, generation and maintenance of membrane asymmetry, cell and organelle shape determination, phagocytosis, vesicle trafficking, blood coagulation, lipid homeostasis, regulation of membrane protein function, apoptosis, etc. P(4)-ATPases and ATP binding cassette (ABC) transporters are the two principal classes of membrane proteins that actively transport phospholipids across cellular membranes. P(4)-ATPases utilize the energy from ATP hydrolysis to flip aminophospholipids from the exocytoplasmic (extracellular/lumen) to the cytoplasmic leaflet of cell membranes generating membrane lipid asymmetry and lipid imbalance which can induce membrane curvature. Many ABC transporters play crucial roles in lipid homeostasis by actively transporting phospholipids from the cytoplasmic to the exocytoplasmic leaflet of cell membranes or exporting phospholipids to protein acceptors or micelles. Recent studies indicate that some ABC proteins can also transport phospholipids in the opposite direction. The importance of P(4)-ATPases and ABC transporters is evident from the findings that mutations in many of these transporters are responsible for severe human genetic diseases linked to defective phospholipid transport. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

The role of microparticles in the pathogenesis of rheumatic diseases

Microparticles (MPs) are small membrane-bound vesicles that are emerging as important elements in the pathogenesis of rheumatic diseases owing to their pleiotropic effects on thrombosis, vascular reactivity, angiogenesis and inflammation. Released from cells during activation and apoptosis, MPs carry proteins, lipids and nucleic acids, and serve as platforms for enzymatic processes in thrombosis. Furthermore, MPs can transfer cytokines, receptors, RNA and DNA to modulate the properties of target cells. As MPs appear in the blood in increased numbers during rheumatic disease, they represent novel biomarkers that can be used to assess events in otherwise inaccessible tissues. Future research will define further the pathogenetic role of MPs and explore therapeutic strategies to block their release or signaling properties.

ATP8A1 activity and phosphatidylserine transbilayer movement

The asymmetric distribution of the amino-containing phospholipids, phosphatidyl-serine (PS) and phosphatidyl-ethanolamine (PE), across the two leaflets of red blood cell (RBC) membrane is essential to the function and survival of the cell. PS and PE are sequestered in the inner leaflet by an ATP-dependent transport activity of a membrane protein known as the RBC flippase that specifically moves amino-phospholipids from the outer to the inner leaflet. The enucleated RBC lacks the means to replace damaged enzymes and inactivation of the flippase can lead to the unwarranted exposure of PS on the cell surface. Loss in the ability to maintain phospholipid asymmetry is exacerbated in RBC disorders and PS-exposing RBCs present in the circulation play a significant role in the pathology of hemoglobinopathies. We identified the Atp8a1 protein, a member of the family of the P(4)-type ATPases, as a RBC flippase candidate. Atp8a1 is expressed in RBC precursors and is present in the membrane of mature red cells. The flippase activity of the protein was established in purified secretory vesicles of Saccharomyces cerevisiae. ATPase activity was stimulated by PS and PE. In addition, Atp8a1 can move PS molecules across the leaflets of the vesicle membrane in presence of ATP.

Circulating microparticles: pathophysiology and clinical implications

Microparticles (MP) derived from vascular endothelium or circulating blood cells circulate in the peripheral blood. They originate from blebbing and shedding from cell membrane surfaces in physiological and pathological conditions and are present in low concentrations in normal plasma. Increased levels are generated by a number of mechanisms including platelet activation, direct vascular endothelial damage, thrombin activity on the cell surface, C5b-9 activation, and PF4-heparin-antibody interaction. Several techniques are currently used to study the generation and nature of circulating microparticles. In particular, the genesis and role of microparticles, derived from platelets, endothelial cells and monocytes, in sepsis (especially meningococcal-induced), heparin-induced thrombocytopenia (HIT), thrombotic thrombocytopenic purpura (TTP), aplastic anaemia, paroxysmal nocturnal haemoglobinuria (PNH) and sickle cell disease (SCD) have been well studied, and provide important insights into the underlying diseases. A defect in the ability to form microparticles leads to the severe bleeding disorder of Scott syndrome, which in turn provides a revealing insight into the physiology of coagulation. In addition the complex role of microparticles in vascular and cardiovascular diseases is an area of immense interest, that promises to yield important advances into diagnosis and therapy.

Translocation of Ethanolamine Phosphoglyceride is Required for Initiation of Apoptotic Death in OLN-93 Oligodendroglial Cells

The possible interplay between extracellular signal-regulated protein kinase (ERK) activation and ethanolamine phosphoglycerides (PG) membrane bilayer translocation following oxidative stress (OS) (0.5 mM H2O2/0.05 mM Fe2+), was examined in oligodendroglia, OLN93, cells with altered plasma membrane PG composition. Cells supplemented with 50 microM docosahexaenoic acid (DHA, 22:6n3) to increase the number of potential double bond targets for OS in ethanolamine-PG (EPG) were compared to cells with diminished content of EPG, attained by the addition of 0.5 mM N,N-dimethylethanolamine (dEa). After 30 min OS, EPG translocation accompanied by sustained ERK activation and nuclear translocation culminating in apoptosis was found in DHA-supplemented cells in contrast to no EPG translocation, a brief ERK activation, but no nuclear translocation, and no cell death in DHA/dEa-supplemented cells. DHA/dEa-supplemented cells pretreated with the protein-tyrosine phosphatases inhibitor Na3VO4 followed by OS, although expressing a sustained ERK activation and nuclear translocation, failed to show apoptosis and lacked EPG translocation. In DHA-supplemented cells U0126, a MEK inhibitor, prevented ERK activation and EPG translocation and protected from cell death. These findings most likely indicate that ERK activation is an indispensable component for the signaling cascades leading to EPG translocation but only activation of the latter is leading to OS-induced apoptotic cell death.

Flippases and vesicle-mediated protein transport

The best-understood mechanisms for generating transport vesicles in the secretory and endocytic pathways involve the localized assembly of cytosolic coat proteins such as clathrin, coat protein complex (COP)I and COPII onto membranes. These coat proteins can deform membranes by themselves, but accessory proteins might help to generate the tight curvature needed to form a vesicle. Enzymes that pump phospholipid from one leaflet of the bilayer to the other (flippases) can deform membranes by creating an imbalance in the phospholipid number between the two leaflets. Recent studies describe a requirement for the yeast Drs2p family of P-type ATPases in both phospholipid translocation and protein transport in the secretory and endocytic pathways. This indicates that flippases work with coat proteins to form vesicles.

Involvement of sodium in early phosphatidylserine exposure and phospholipid scrambling induced by P2X7 purinoceptor activation in thymocytes

Extracellular ATP (ATP(ec)), a possible effector in thymocyte selection, induces thymocyte death via purinoceptor activation. We show that ATP(ec) induced cell death by apoptosis, rather than lysis, and early phosphatidylserine (PS) exposure and phospholipid scrambling in a limited thymocyte population (35-40%). PS externalization resulted from the activation of the cationic channel P2X7 (formerly P2Z) receptor and was triggered in all thymocyte subsets although to different proportions in each one. Phospholipid movement was dependent on ATP(ec)-induced Ca(2+) and/or Na(+) influx. At physiological external Na(+) concentration, without external Ca(2+), PS was exposed in all ATP(ec)-responsive cells. In contrast, without external Na(+), physiological external Ca(2+) concentration promoted a submaximal response. Altogether these data show that Na(+) influx plays a major role in the rapid PS exposure induced by P2X7 receptor activation in thymocytes.

Scott syndrome, a bleeding disorder caused by defective scrambling of membrane phospholipids

Normal quescent cells maintain membrane lipid asymmetry by ATP-dependent membrane lipid transporters, which shuttle different phospholipids from one leaflet to the other against their respective concentration gradients. When cells are challenged, membrane lipid asymmetry can be perturbed resulting in exposure of phosphatidylserine [PS] at the outer cell surface. Translocation of PS from the inner to outer membrane leaflet of activated blood platelets and platelet-derived microvesicles provides a catalytic surface for interacting coagulation factors. This process is dramatically impaired in Scott syndrome, a rare congenital bleeding disorder, underscoring the indispensible role of PS in hemostasis. This also testifies to a defect of a protein-catalyzed scrambling of membrane phospholipids. The Scott phenotype is not restricted to platelets, but can be demonstrated in other blood cells as well. The functional aberrations observed in Scott syndrome have increased our understanding of transmembrane lipid movements, and may help to identify the molecular elements that promote the collapse of phospholipid asymmetry during cell activation and apoptosis.

Appearance of voltage-gated calcium channels following overexpression of ATPase II cDNA in neuronal HN2 cells

ATPase II (a Mg2+-ATPase) is also believed to harbor aminophospholipid translocase (APTL) activity, which is responsible for the translocation of phosphatidylserine (PS) from the outer leaflet of the plasma membrane to the inner. To test this hypothesis we overexpressed the mouse ATPase II cDNA in the neuronal HN2 cells. In addition to a dramatic increase in APTL activity, we also made the unexpected observation that expression of the mouse ATPase II cDNA from the vector pCMV6 resulted in the appearance of calcium current. Although the hybrid cell line HN2 or a line (HN2V32) obtained by expressing a heterologous gene from the same expression vector showed no calcium current, both ATPase II-overexpressing clones (HN2A12 and HN2A22) showed significant barium conductance. This current was due to calcium channels because it was blocked almost completely by 100 microM CdCl2 and it had a significant N-type component since it was blocked by 38.5% in the presence of 5 microM omega-conotoxin (omega-CTX). Western blot analysis using an antibody against the N-type calcium-channel alpha1B subunit revealed a dramatic increase in expression of this protein in the HN2A12 and HN2A22 cell lines. Our results suggest that ATPase II also harbors APTL activity. In view of the prior knowledge that APTL activity is inhibited by an increase in calcium, our results also suggest that APTL expression exerts a negative feedback regulation on itself by inducing expression of channels that cause an influx of calcium ions. The mechanism of this regulation could reveal important information on a possible cross-regulation between these two families of proteins in neuronal cells.

Autoimmune disease as a cause of reproductive failure

High-risk pregnancy is the most common clinical association with antiphospholipid antibodies; the principal manifestations are pregnancy loss and early preeclampsia. Membership in this family of antibodies is continually growing and includes antibodies against a variety of phospholipids, phospholipid-protein complexes, and phospholipid-binding proteins. The current information in the literature is inadequate to clearly implicate a subgroup of antiphospholipid antibodies or a particular pathophysiologic mechanism as being responsible for poor pregnancy outcomes. It is clear, however, that prevalent diagnostic tests for LA and aCL are extremely useful to identify many of these patients, but are inadequate for diagnosis of all patients with autoimmune pregnancy loss or to elucidate the pathophysiology. Many patients who present clinically with autoimmune-like pregnancy complications currently are negative in tests for LA or aCL, but have antibodies against annexin V, phosphatidylserine, or other relevant antigens. The greatest risk for a complicated pregnancy is conveyed by a subgroup of antibodies that affect the normal function of placental trophoblast. As clinical laboratory tests designed to detect more members of the antiphospholipid antibody family become available, understanding of this complicated disease (APS) will increase.

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.

Chemical control of phospholipid distribution across bilayer membranes

Most biological membranes possess an asymmetric transbilayer distribution of phospholipids. Endogenous enzymes expend energy to maintain the arrangement by promoting the rate of phospholipid translocation, or flip-flop. Researchers have discovered ways to modify this distribution through the use of chemicals. This review presents a critical analysis of the phospholipid asymmetry data in the literature followed by a brief overview of the maintenance and physiological consequences of phospholipid asymmetry, and finishes with a list of chemical ways to alter phospholipid distribution by enhancement of flip-flop.

Identification of a functional role for lipid asymmetry in biological membranes: Phosphatidylserine-skeletal protein interactions modulate membrane stability

Asymmetric distribution of phospholipids is ubiquitous in the plasma membranes of many eukaryotic cells. The majority of the aminophospholipids are located in the inner leaflet whereas the cholinephospholipids are localized predominantly in the outer leaflet. Several functional roles for asymmetric phospholipid distribution in plasma membranes have been suggested. Disruption of lipid asymmetry creates a procoagulant surface on platelets and serves as a trigger for macrophage recognition of apoptotic cells. Furthermore, the dynamic process of phospholipid translocation regulates important cellular events such as membrane budding and endocytosis. In the present study, we used the red cell membrane as the model system to explore the contribution of phospholipid asymmetry to the maintenance of membrane mechanical properties. We prepared two different types of membranes in terms of their phospholipid distribution, one in which phospholipids were scrambled and the other in which the asymmetric distribution of phospholipids was maintained and quantitated their mechanical properties. We documented that maintenance of asymmetric distribution of phospholipids resulted in improved membrane mechanical stability. The greater difficulty in extracting the spectrin-actin complex at low-ionic strength from the membranes with asymmetric phospholipid distribution further suggested the involvement of interactions between aminophospholipids in the inner leaflet and skeletal proteins in modulating mechanical stability of the red cell membrane. These findings have enabled us to document a functional role of lipid asymmetry in regulating membrane material properties.

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.

Lipid translocation across the plasma membrane of mammalian cells

The plasma membrane, which forms the physical barrier between the intra- and extracellular milieu, plays a pivotal role in the communication of cells with their environment. Exchanging metabolites, transferring signals and providing a platform for the assembly of multi-protein complexes are a few of the major functions of the plasma membrane, each of which requires participation of specific membrane proteins and/or lipids. It is therefore not surprising that the two leaflets of the membrane bilayer each have their specific lipid composition. Although membrane lipid asymmetry has been known for many years, the mechanisms for maintaining or regulating the transbilayer lipid distribution are still not completely understood. Three major players have been presented over the past years: (1) an inward-directed pump specific for phosphatidylserine and phosphatidylethanolamine, known as aminophospholipid translocase; (2) an outward-directed pump referred to as 'floppase' with little selectivity for the polar headgroup of the phospholipid, but whose actual participation in transport of endogenous lipids has not been well established; and (3) a lipid scramblase, which facilitates bi-directional migration across the bilayer of all phospholipid classes, independent of the polar headgroup. Whereas a concerted action of aminophospholipid translocase and floppase could, in principle, account for the maintenance of lipid asymmetry in quiescent cells, activation of the scramblase and concomitant inhibition of the aminophospholipid translocase causes a collapse of lipid asymmetry, manifested by exposure of phosphatidylserine on the cell surface. In this article, each of these transporters will be discussed, and their physiological importance will be illustrated by the Scott syndrome, a bleeding disorder caused by impaired lipid scrambling. Finally, phosphatidylserine exposure during apoptosis will be briefly discussed in relation to inhibition of translocase and simultaneous activation of scramblase.

Internalization of phospholipids from the plasma membrane of human osteoblasts depends on the lipid head group

The redistribution of spin- or fluorescence-labeled phospholipid analogs across the plasma membrane of human osteoblast cells, either in suspension or grown as monolayers, was investigated. After incorporation into the outer membrane leaflet, analogs of the aminophospholipids phosphatidylserine and phosphatidylethanolamine moved rapidly to the inner monolayer, whereas the choline-containing analogs of phosphatidylcholine and sphingomyelin disappeared more slowly from the outer leaflet. The fast inward movement of the aminophospholipids became reduced after lowering the intracellular ATP, suggesting the presence of an aminophospholipid translocase activity in the plasma membrane of these cells. From these data, a transverse phospholipid asymmetry in osteoblasts can be inferred with the aminophospholipids mainly concentrated in the inner monolayer and the choline-containing phospholipids in the outer leaflet. A similar pattern of phospholipid internalization was inferred for osteoblasts from human osteoporotic bones and for a human osteosarcoma cell line. The relevance of the enrichment of phosphatidylserine in the cytoplasmic membrane leaflet for calcification in skeletal tissues is emphasized.

Kinetic and thermodynamic aspects of lipid translocation in biological membranes

A theoretical analysis of the lipid translocation in cellular bilayer membranes is presented. We focus on an integrative model of active and passive transport processes determining the asymmetrical distribution of the major lipid components between the monolayers. The active translocation of the aminophospholipids phosphatidylserine and phosphatidylethanolamine is mathematically described by kinetic equations resulting from a realistic ATP-dependent transport mechanism. Concerning the passive transport of the aminophospholipids as well as of phosphatidylcholine, sphingomyelin, and cholesterol, two different approaches are used. The first treatment makes use of thermodynamic flux-force relationships. Relevant forces are transversal concentration differences of the lipids as well as differences in the mechanical states of the monolayers due to lateral compressions. Both forces, originating primarily from the operation of an aminophospholipid translocase, are expressed as functions of the lipid compositions of the two monolayers. In the case of mechanical forces, lipid-specific parameters such as different molecular surface areas and compression force constants are taken into account. Using invariance principles, it is shown how the phenomenological coefficients depend on the total lipid amounts. In a second approach, passive transport is analyzed in terms of kinetic mechanisms of carrier-mediated translocation, where mechanical effects are incorporated into the translocation rate constants. The thermodynamic as well as the kinetic approach are applied to simulate the time-dependent redistribution of the lipid components in human red blood cells. In the thermodynamic model the steady-state asymmetrical lipid distribution of erythrocyte membranes is simulated well under certain parameter restrictions: 1) the time scales of uncoupled passive transbilayer movement must be different among the lipid species; 2) positive cross-couplings of the passive lipid fluxes are needed, which, however, may be chosen lipid-unspecifically. A comparison of the thermodynamic and the kinetic approaches reveals that antiport mechanisms for passive lipid movements may be excluded. Simulations with kinetic symport mechanisms are in qualitative agreement with experimental data but show discrepancies in the asymmetrical distribution for sphingomyelin.

Apoptosis-independent alterations in membrane dynamics induced by curcumin

Curcumin is a well-known natural compound with antiinflammatory properties. Its antiproliferative effect and ability to modulate apoptotic response are considered essential in cancer therapy. The physicochemical properties of curcumin suggest membranous localization, which prompted an investigation of the mechanisms of membrane disturbances evoked by curcumin. We chose the erythrocyte as a convenient model for studying membrane effects of curcumin and showed its nonspecific, apoptosis-independent way of action. Curcumin was found to expand the cell membrane, inducing echinocytosis. Changes in cell shape were accompanied by transient exposure of phosphatidylserine. Membrane asymmetry was recovered by the action of aminophospholipid translocase, which remained active in the presence of curcumin. Lipids rearrangements and drug partitioning caused changes of lipid fluidity. Such nonspecific effects of curcumin on cellular membranes would produce artifacts of apoptosis measurement, since several methods are based on membrane changes.

Regulatory mechanisms of transmembrane phospholipid distributions and pathophysiological implications of transbilayer lipid scrambling

The various phospholipid classes that comprise mammalian cell membranes are distributed over both leaflets of the bilayer in a non-random fashion. While a specific and ATP-dependent transporter is responsible for rapid inward movement of aminophospholipids, its inhibition does not lead to spontaneous redistribution of lipids. Conditions of cellular activation which are accompanied with increased levels of intracellular Ca2+ may cause a collapse of lipid asymmetry by switching on an ATP-independently operating scramblase, which accelerates bidirectional movement of all phospholipid classes. The most prominent change in transmembrane lipid distribution is surface exposure of phosphatidylserine (PS), the more so since conditions which activate scramblase in most if not all cases lead to inhibition of aminophospholipid translocase activity, which will prevent PS from being pumped back to the inner leaflet of the membrane. Surface-exposed PS serves at least two important physiological functions: it promotes blood coagulation and offers a recognition signal for clearance by macrophages and other cells of the reticuloendothelial system. As such, PS exposure may form an important early event in the process of apoptosis to ensure rapid removal of these cells in order to avoid release of their inflammatory contents. Defective regulation of transbilayer lipid distribution may result in clinical manifestations such as in the Scott syndrome, a bleeding disorder caused by an impaired scramblase activity. Conversely, excessive PS exposure may lead to thrombosis or may explain formation of so-called antiphospholipid antibodies as occurring in patients with antiphospholipid syndrome.

Energy requirements for two aspects of phospholipid metabolism in mammalian brain

Previous estimates have placed the energy requirements of total phospholipid metabolism in mammalian brain at 2% or less of total ATP consumption. This low estimate was consistent with the very long half-lives (up to days) reported for fatty acids esterified within phospholipids. However, using an approach featuring analysis of brain acyl-CoA, which takes into account dilution of the precursor acyl-CoA pool by recycling of fatty acids, we reported that half-lives of fatty acids in phospholipids are some 100 times shorter (min-h) than previously thought. Based on these new estimates of short half-lives, palmitic acid and arachidonic acid were used as prototype fatty acids to calculate energy consumption by fatty acid recycling at the sn-1 and sn-2 positions of brain phospholipids. We calculated that the energy requirements for reacylation of fatty acids into lysophospholipids are 5% of net brain ATP consumption. We also calculated ATP requirements for maintaining asymmetry of the aminophospholipids, phosphatidylserine and phosphatidylethanolamine across brain membrane bilayers. This asymmetry is maintained by a translocase at a stoichiometry of 1 mol of ATP per mol of phospholipid transferred in either direction across the membrane. The energy cost of maintaining membrane bilayer asymmetry of aminophospholipids at steady-state was calculated to be 8% of total ATP consumed. Taken together, deacylation-reacylation and maintenance of membrane asymmetry of phosphatidylserine and phosphatidylethanolamine require about 13% of ATP consumed by brain as a whole. This is a lower limit for energy consumption by processes involving phospholipids, as other processes, including phosphorylation of polyphosphoinositides and de novo phospholipid biosynthesis, were not considered.

Targeted Inactivation of Murine Band 3 (AE1) Gene Produces a Hypercoagulable State Causing Widespread Thrombosis In Vivo

Only 5% to 10% of band 3 null mice survive the neonatal period. To determine the cause of death, 3 adult and 11 newborn band 3 null mice were submitted for histopathologic examination. All but 1 pup showed evidence of thrombosis including: (1) large thrombotic lesions in the heart, which were partially organized, calcified in some fields, and endothelialized, indicating a process that developed premortem (3 of 3 adults and 6 of 11 pups). (2) Subcapsular necrotic areas in the liver suggestive of premortem ischemic events caused by arteriolar occlusions (8 of 11 pups). (3) Large vein thrombi (4 of 11 pups). To investigate the etiology of this hypercoagulable state, we have used the Russell’s viper venom test (RVV) to show that red blood cells (RBCs) from band 3 null mice significantly shorten the RVV clotting time of normal plasma in a dose-dependent fashion, whereas RBCs from normal mice have no effect, suggesting that the membrane of band 3 null RBCs provides a suitable surface for activation of the prothrombinase complex. Using flow cytometry, we have examined the phosphatidylserine (PS)-specific binding of fluorescein isothiocyanate (FITC)-annexin V to normal and band 3 null RBCs. A subpopulation of cells (3% to 5% of RBCs) with increased FITC-annexin V binding was detected in band 3 null RBCs as compared with normal RBCs. Furthermore, the entire cell population of band 3 null RBCs shows a measurable increase in the mean fluorescence intensity, suggesting that band 3 null RBCs may have increased PS exposure on the outer membrane leaflet. These findings are further supported by direct fluorescence microscopy of normal and band 3 null RBCs labeled with FITC-annexin V. Based on these observations, we postulate that the high mortality of band 3 null mice may be related to a hypercoagulable state, which appears to originate from changes in the phospholipid composition of the membrane leading to PS exposure on the outer leaflet.

© 1998 by The American Society of Hematology.

Targeted Inactivation of Murine Band 3 (AE1) Gene Produces a Hypercoagulable State Causing Widespread Thrombosis In Vivo

Only 5% to 10% of band 3 null mice survive the neonatal period. To determine the cause of death, 3 adult and 11 newborn band 3 null mice were submitted for histopathologic examination. All but 1 pup showed evidence of thrombosis including: (1) large thrombotic lesions in the heart, which were partially organized, calcified in some fields, and endothelialized, indicating a process that developed premortem (3 of 3 adults and 6 of 11 pups). (2) Subcapsular necrotic areas in the liver suggestive of premortem ischemic events caused by arteriolar occlusions (8 of 11 pups). (3) Large vein thrombi (4 of 11 pups). To investigate the etiology of this hypercoagulable state, we have used the Russell’s viper venom test (RVV) to show that red blood cells (RBCs) from band 3 null mice significantly shorten the RVV clotting time of normal plasma in a dose-dependent fashion, whereas RBCs from normal mice have no effect, suggesting that the membrane of band 3 null RBCs provides a suitable surface for activation of the prothrombinase complex. Using flow cytometry, we have examined the phosphatidylserine (PS)-specific binding of fluorescein isothiocyanate (FITC)-annexin V to normal and band 3 null RBCs. A subpopulation of cells (3% to 5% of RBCs) with increased FITC-annexin V binding was detected in band 3 null RBCs as compared with normal RBCs. Furthermore, the entire cell population of band 3 null RBCs shows a measurable increase in the mean fluorescence intensity, suggesting that band 3 null RBCs may have increased PS exposure on the outer membrane leaflet. These findings are further supported by direct fluorescence microscopy of normal and band 3 null RBCs labeled with FITC-annexin V. Based on these observations, we postulate that the high mortality of band 3 null mice may be related to a hypercoagulable state, which appears to originate from changes in the phospholipid composition of the membrane leading to PS exposure on the outer leaflet.

© 1998 by The American Society of Hematology.

Phosphoinositide metabolism and shape control in sheep red blood cells

Metabolic depletion of sheep red blood cells leads to decreased intracellular concentrations of ATP and reduced glutathione as well as degradation of phosphoinositides. In sheep red blood cells, depletion of ATP induced two types of shape transformation: one early phase involving formation of protrusions on the cell surface similar to those observed upon depletion of human red blood cells; and one late phase, in which the sheep red blood cells develop long, rod-shaped projections. During the initial stages of shape changes, degradation of the phosphoinositides parallels the discocyte-echinocyte transformation, thus giving further support to a shape-controlling mechanism based on the bilayer-couple hypothesis. However, formation of the long projections does not coincide with turnover of the phosphoinositides but rather with the level of reduced glutathione. This indicates that development of these rod-like extensions on the cell surface is induced by oxidative processes that may well involve cross-linking of membrane skeleton proteins.

Aminophospholipid translocase and proteins involved in transmembrane phospholipid traffic

The transmembrane distribution of phospholipids in the membranes of eukaryotic cells depends on specific proteins (called flippases). The aminophospholipid translocase is responsible for the sequestration of phosphatidylserine and phosphatidylethanolamine in the cytosolic leaflet of plasma membranes. Several laboratories are presently working on the identification, purification and cloning of this Mg-ATPase, first recognized in the human red cell membrane. In accordance with the 1992 hypothesis of Higgins and Gottesman, proteins of the MDR1 family appear to be able to translocate certain phospholipids from the inner to the outer monolayer of the plasma membrane. It has been reported in particular that expression of the human MDR3 and mouse mdr2 genes promote translocation of long chain phosphatidylcholine, while expression of the MDR1 gene stimulates the outward motion of phospholipids possessing at least one short chain. ATP-independent flippases activities were recognized not only in microsomes but also in Golgi membranes.

Characterization of the correlation between ATP-dependent aminophospholipid translocation and Mg2+-ATPase activity in red blood cell membranes

Pseudosubstrates and inhibitors of ATPases were studied with respect to their capability to modulate the kinetic behavior of Mg2+-ATPase and aminophospholipid translocation in red blood cell ghosts. ATP was substituted by the pseudosubstrates of P-type ATPases acetyl phosphate and p-nitrophenyl phosphate. With both pseudosubstrates, aminophospholipid translocation from the outer to the inner leaflets of resealed erythrocyte ghosts could be observed, although with a significantly decreased velocity compared to that in presence of ATP, both with respect to phosphate hydrolysis and translocation. Similarly, the apparent affinities for the pseudosubstrates were much lower than for ATP. Among the inhibitors studied, suramin acted as a competitive inhibitor of ATP towards both Mg2+-ATPase activity and aminophospholipid translocation. However, the inhibition of translocation occurred at a higher inhibitor concentration than the inhibition of Mg2+-ATPase activity. With elaiophylin, only a partial inhibition of Mg2+-ATPase activity could be detected, but translocation of labeled phosphatidylserine was almost completely abolished. With eosin Y, an almost complete inhibition of both Mg2+-ATPase activity and translocation could be achieved. The observed responses of aminophospholipid translocation to ATPase inhibitors strongly suggest that a P-type ATPase, part of which displays a Mg2+-ATPase activity, is involved in aminophospholipid translocation.

Altered membrane phospholipid organization and erythrophagocytosis in E beta-thalassemia

The underlying cause behind the accelerated destruction of erythrocytes in the bone marrow and in the peripheral circulation, accompanying the beta-thalassemic syndromes is still not clearly understood. The present investigation demonstrates that increased phagocytosis of erythrocytes in E beta-thalassemia is inhibited by the presence of phosphatidylserine (PS) vesicles, suggesting a PS-'PS-receptor' type of interaction in premature recognition of these erythrocytes by macrophages. Increased exposure of both aminophospholipids phosphatidylethanolamine (PE) and PS was demonstrated by fluorescamine labeling and annexin binding, respectively. The slower rate of translocation of PS across the bilayer suggested that this contributed towards the increased exposure of PS in E beta-thalassemic erythrocytes.

Regulatory mechanisms in maintenance and modulation of transmembrane lipid asymmetry: pathophysiological implications

The two leaflets of the plasma membrane of eukaryotic cells differ in lipid composition: the outer leaflet comprises mainly neutral choline containing phospholipids, whereas the aminophospholipids reside almost exclusively in the cytoplasmic leaflet. The importance of transmembrane lipid asymmetry may be judged from the fact that the cell invests energy to maintain this situation for which at least two regulatory mechanisms are held responsible. A translocase, selective for aminophospholipids, acts as an ATP-dependent pump for rapid inward movement of phosphatidylserine (PS) and phosphatidylethanolamine; in addition, a non-selective, but also ATP-dependent pump causes outward movement of phospholipids, be it at a much lower rate compared to the inward transport by the aminophospholipid translocase. These two systems, acting in concert, are thought to be the main players in the maintenance of a dynamic equilibrium of the phospholipids over both membrane leaflets. Dissipation of membrane lipid asymmetry can be elicited in different cell types under a variety of conditions; in particular, platelets upon activation rapidly lose their normal plasma membrane lipid distribution, but also in other blood cells, lipid asymmetry can be lost, be it at a much lower rate and extent than in platelets. A putative protein, referred to as "scramblase' has been described, which requires the continuous presence of elevated intracellular Ca(2+)-levels, to allow a rapid, non-selective and bidirectional transbilayer movement of phospholipids. Although scrambling of lipids does not require ATP as such, preliminary studies suggest the possible involvement of one or more phosphorylated proteins. The most prominent consequence of the loss of phospholipid asymmetry is exposure of PS in the outer leaflet of the plasma membrane. Surface-exposed PS serves several important physiological functions: it promotes assembly of enzyme complexes of the coagulation cascade, it forms a signal for cell-cell recognition, which is important for cell scavenging processes. Surface-exposure of PS is an early phenomenon of apoptosis and appears to be involved in efficient removal of these cells. In addition, PS in the outer leaflet of cells is thought to play a role in cell fusion processes. It may be clear from the foregoing, that the amount of PS present at the cell surface needs to be tightly controlled, and that an impairment of this process leads to either excessive- or diminished exposition of PS which may have several pathophysiological consequences.

Phospholipid asymmetry in red blood cells and spectrin-free vesicles during prolonged storage

Erythrocytes and spectrin-free DMPC-induced vesicles released from the cells were incubated for 3 weeks at 6 degrees C under conditions of metabolic ATP-depletion. Phosphatidylserine (PS) asymmetry was monitored during this period by use of the prothrombinase assay. Prothrombinase activities measured at the beginning of the incubation period indicated that approximately 0.06% of PS was located at the outer layer of the red cell membrane, whereas in DMPC-induced vesicles approximately 1.5% the PS was exposed on the outside. After completion of the incubation period PS exposure on the outside of red cells and vesicles was increased by no more than 5-fold. On the other hand, with vesicles prepared with a significantly increased (4-fold) ATP-content to sustain translocase activity, the incubation process resulted in a surprisingly high (20-fold) increase of PS exposure. With vanadate, an inhibitor of the aminophospholipid translocase, included in the incubation medium, the redistribution of PS was even more pronounced. These observations indicate that PS asymmetry in spectrin-free vesicles can not be directly correlated to either ATP content or translocase activity and suggest that besides the aminophospholipid translocase and the membrane skeleton, other mechanisms must be involved in maintaining phospholipid asymmetry.

Colocalization of Rh polypeptides and the aminophospholipid transporter in dilauroylphosphatidylcholine-induced erythrocyte vesicles

Cytoskeleton-free vesicles released from human red blood cells (RBC) transport exogenously supplied aminophospholipid analogues from the vesicle's outer to inner leaflet at rates comparable to those of normal RBC (Beleznay et al. (1993) Biochemistry 32, 3146-3152). Because polypeptides associated with the Rh blood group system have been implicated in the transbilayer movement of phosphatidylserine (PS), we investigated the relationship and co-localization of the aminophospholipid translocase and Rh in dilauroylphosphatidylcholine-induced RBC vesicles. The transbilayer movement of fluorescent (NBD-PS) and photoactivatable (125I-N3-PS) PS in RBC vesicles was ATP-and temperature-dependent. Inhibition of PS transport by sulfhydryl reagents could be accomplished by direct vesicle treatment or by treating RBC before vesiculation. In the case of diamide- and pyridyldithioethylamine-mediated inhibition, NBD-PS transport could be restored by reduction with dithiothreitol, indicating that the movement of the PS transporter into the emerging vesicle was independent of the oxidative status of membrane sulfhydryls. The presence of Rh polypeptides in the vesicles was verified by direct immunoprecipitation of isotopically-labeled Rh and semi-quantified by antibody adsorption assays. Similar to the movement of the PS transporter, localization of Rh polypeptides in the vesicle membrane was independent of the red cell's oxidative status. These results show that the PS translocase and Rh-related proteins colocalize in RBC vesicles suggesting that these proteins may be members of a multicomponent complex that plays a role in lipid movement and the generation of membrane lipid asymmetry.

Expression of phosphatidylserine-dependent antigens on the surface of differentiating BeWo human choriocarcinoma cells

PROBLEM

Antiphospholipid antibodies (aPLs) are associated with pregnancy loss, pregnancy-induced hypertension, and intrauterine growth retardation. We have previously reported that phosphatidylserine (PS)-dependent antigens are expressed in formalin-fixed cells concurrent with differentiation in a choriocarcinoma model (BeWo) of cytotrophoblast. That study, however, could not differentiate between cytoplasmic or surface antigen expression.

METHOD

Three monoclonal aPLs that differentiate between PS- and cardiolipin (CL)-dependent antigens were reacted with BeWo, with or without forskolin activation, before fixation, and antibody binding was evaluated by immunoperoxidase techniques.

RESULTS

Activation with forskolin induced a PS-dependent antigenic determinant on the surface on BeWo cells. CL-reactive monoclonal antibodies did not react with the cell surface, whether forskolin treated or not.

CONCLUSION

These observations demonstrate that a PS-dependent antigen is expressed on the surface of a model of differentiating cytotrophoblastic cells and should be accessible in vivo to circulating aPLs.