Action of highly purified phospholipases on blood platelets. Evidence for an asymmetric distribution of phospholipids in the surface membrane

Exploring the Role of PI3P in Platelets: Insights from a Novel External PI3P Pool

Phosphoinositides (PIs) play a crucial role in regulating intracellular signaling, actin cytoskeleton rearrangements, and membrane trafficking by binding to specific domains of effector proteins. They are primarily found in the membrane leaflets facing the cytosol. Our study demonstrates the presence of a pool of phosphatidylinositol 3-monophosphate (PI3P) in the outer leaflet of the plasma membrane of resting human and mouse platelets. This pool of PI3P is accessible to exogenous recombinant myotubularin 3-phosphatase and ABH phospholipase. Mouse platelets with loss of function of class III PI 3-kinase and class II PI 3-kinase α have a decreased level of external PI3P, suggesting a contribution of these kinases to this pool of PI3P. After injection in mouse, or incubation ex vivo in human blood, PI3P-binding proteins decorated the platelet surface as well as α-granules. Upon activation, these platelets were able to secrete the PI3P-binding proteins. These data sheds light on a previously unknown external pool of PI3P in the platelet plasma membrane that recognizes PI3P-binding proteins, leading to their uptake towards α-granules. This study raises questions about the potential function of this external PI3P in the communication of platelets with the extracellular environment, and its possible role in eliminating proteins from the plasma.

Influence of Cardiometabolic Risk Factors on Platelet Function

Platelets are key players in the thrombotic processes. The alterations of platelet function due to the occurrence of metabolic disorders contribute to an increased trend to thrombus formation and arterial occlusion, thus playing a major role in the increased risk of atherothrombotic events in patients with cardiometabolic risk factors. Several lines of evidence strongly correlate metabolic disorders such as obesity, a classical condition of insulin resistance, dyslipidemia, and impaired glucose homeostasis with cardiovascular diseases. The presence of these clinical features together with hypertension and disturbed microhemorrheology are responsible for the prothrombotic tendency due, at least partially, to platelet hyperaggregability and hyperactivation. A number of clinical platelet markers are elevated in obese and type 2 diabetes (T2DM) patients, including the mean platelet volume, circulating levels of platelet microparticles, oxidation products, platelet-derived soluble P-selectin and CD40L, thus contributing to an intersection between obesity, inflammation, and thrombosis. In subjects with insulin resistance and T2DM some defects depend on a reduced sensitivity to mediators—such as nitric oxide and prostacyclin—playing a physiological role in the control of platelet aggregability. Furthermore, other alterations occur only in relation to hyperglycemia. In this review, the main cardiometabolic risk factors, all components of metabolic syndrome involved in the prothrombotic tendency, will be taken into account considering some of the mechanisms involved in the alterations of platelet function resulting in platelet hyperactivation.

Scrambling of natural and fluorescently tagged phosphatidylinositol by reconstituted G protein-coupled receptor and TMEM16 scramblases

Members of the G protein-coupled receptor and TMEM16 (transmembrane protein 16) protein families are phospholipid scramblases that facilitate rapid, bidirectional movement of phospholipids across a membrane bilayer in an ATP-independent manner. On reconstitution into large unilamellar vesicles, these proteins scramble more than 10,000 lipids/protein/s as measured with co-reconstituted fluorescent nitrobenzoxadiazole (NBD)-labeled phospholipids. Although NBD-labeled phospholipids are ubiquitously used as reporters of scramblase activity, it remains unclear whether the NBD modification influences the quantitative outcomes of the scramblase assay. We now report a refined biochemical approach for measuring the activity of scramblase proteins with radiolabeled natural phosphatidylinositol ([3H]PI) and exploiting the hydrolytic activity of bacterial PI-specific phospholipase C (PI-PLC) to detect the transbilayer movement of PI. PI-PLC rapidly hydrolyzed 50% of [3H]PI in large symmetric, unilamellar liposomes, corresponding to the lipid pool in the outer leaflet. On reconstitution of a crude preparation of yeast endoplasmic reticulum scramblase, purified bovine opsin, or purified Nectria haematococca TMEM16, the extent of [3H]PI hydrolysis increased, indicating that [3H]PI from the inner leaflet had been scrambled to the outer leaflet. Using transphosphatidylation, we synthesized acyl-NBD-PI and used it to compare our PI-PLC-based assay with conventional fluorescence-based methods. Our results revealed quantitative differences between the two assays that we attribute to the specific features of the assays themselves rather than to the nature of the phospholipid. In summary, we have developed an assay that measures scrambling of a chemically unmodified phospholipid by a reconstituted scramblase.

Alterations in plasma membrane promote overexpression and increase of sodium influx through epithelial sodium channel in hypertensive platelets

Platelets are small, anucleated cell fragments that activate in response to a wide variety of stimuli, triggering a complex series of intracellular pathways leading to a hemostatic thrombus formation at vascular injury sites. However, in essential hypertension, platelet activation contributes to causing myocardial infarction and ischemic stroke. Reported abnormalities in platelet functions, such as platelet hyperactivity and hyperaggregability to several agonists, contribute to the pathogenesis and complications of thrombotic events associated with hypertension. Platelet membrane lipid composition and fluidity are determining for protein site accessibility, structural arrangement of platelet surface, and response to appropriate stimuli. The present study aimed to demonstrate whether structural and biochemical abnormalities in lipid membrane composition and fluidity characteristic of platelets from hypertensive patients influence the expression of the Epithelial Sodium Channel (ENaC), fundamental for sodium influx during collagen activation. Wb, cytometry and quantitative Reverse Transcription-Polymerase Chain Reaction (qRT-PCR) assays demonstrated ENaC overexpression in platelets from hypertensive subjects and in relation to control subjects. Additionally, our results strongly suggest a key role of β-dystroglycan as a scaffold for the organization of ENaC and associated proteins. Understanding of the mechanisms of platelet alterations in hypertension should provide valuable information for the pathophysiology of hypertension.

Forty five years with membrane phospholipids, phospholipases and lipid mediators: A historical perspective

Phospholipases play a key role in the metabolism of phospholipids and in cell signaling. They are also a very useful tool to explore phospholipid structure and metabolism as well as membrane organization. They are at the center of this review, covering a period starting in 1971 and focused on a number of subjects in which my colleagues and I have been involved. Those include determination of phospholipid asymmetry in the blood platelet membrane, biosynthesis of lysophosphatidic acid, biochemistry of platelet-activating factor, first attempts to define the role of phosphoinositides in cell signaling, and identification of novel digestive (phospho)lipases such as pancreatic lipase-related protein 2 (PLRP2) or phospholipase B. Besides recalling some of our contributions to those various fields, this review makes an appraisal of the impressive and often unexpected evolution of those various aspects of membrane phospholipids and lipid mediators. It is also the occasion to propose some new working hypotheses.

Phosphatidylinositol-4,5-bisphosphate enhances anionic lipid demixing by the C2 domain of PKCα

The C2 domain of PKCα (C2α) induces fluorescence self-quenching of NBD-PS in the presence of Ca²⁺, which is interpreted as the demixing of phosphatidylserine from a mixture of this phospholipid with phosphatidylcholine. Self-quenching of NBD-PS was considerably increased when phosphatidylinositol-4,5-bisphosphate (PIP₂) was present in the membrane. When PIP₂ was the labeled phospholipid, in the form of TopFluor-PIP₂, fluorescence self-quenching induced by the C2 domain was also observed, but this was dependent on the presence of phosphatidylserine. An independent indication of the phospholipid demixing effect given by the C2α domain was obtained by using ²H-NMR, since a shift of the transition temperature of deuterated phosphatidylcholine was observed as a consequence of the addition of the C2α domain, but only in the presence of PIP₂. The demixing induced by the C2α domain may have a physiological significance since it means that the binding of PKCα to membranes is accompanied by the formation of domains enriched in activating lipids, like phosphatidylserine and PIP₂. The formation of these domains may enhance the activation of the enzyme when it binds to membranes containing phosphatidylserine and PIP₂.

Phosphatidylinositol 4,5-bisphosphate decreases the concentration of Ca2+, phosphatidylserine and diacylglycerol required for protein kinase C α to reach maximum activity

The C2 domain of PKCα possesses two different binding sites, one for Ca(2+) and phosphatidylserine and a second one that binds PIP2 with very high affinity. The enzymatic activity of PKCα was studied by activating it with large unilamellar lipid vesicles, varying the concentration of Ca(2+) and the contents of dioleylglycerol (DOG), phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphadidylserine (POPS) in these model membranes. The results showed that PIP2 increased the Vmax of PKCα and, when the PIP2 concentration was 5 mol% of the total lipid in the membrane, the addition of 2 mol% of DOG did not increase the activity. In addition PIP2 decreases K0.5 of Ca(2+) more than 3-fold, that of DOG almost 5-fold and that of POPS by a half. The K0.5 values of PIP2 amounted to only 0.11 µM in the presence of DOG and 0.39 in its absence, which is within the expected physiological range for the inner monolayer of a mammalian plasma membrane. As a consequence, PKCα may be expected to operate near its maximum capacity even in the absence of a cell signal producing diacylglycerol. Nevertheless, we have shown that the presence of DOG may also help, since the K0.5 for PIP2 notably decreases in its presence. Taken together, these results underline the great importance of PIP2 in the activation of PKCα and demonstrate that in its presence, the most important cell signal for triggering the activity of this enzyme is the increase in the concentration of cytoplasmic Ca(2+).

Platelet membrane phospholipid asymmetry: from the characterization of a scramblase activity to the identification of an essential protein mutated in Scott syndrome

Like all eukaryotic cells, platelets maintain plasma membrane phospholipid asymmetry in normal blood circulation via lipid transporters, which control transbilayer movement. Upon platelet activation, the asymmetric orientation of membrane phospholipids is rapidly disrupted, resulting in a calcium-dependent exposure of the anionic phospholipid, phosphatidylserine (PS), at the outer platelet surface. This newly-exposed PS surface is a major component of normal hemostasis because it supports platelet procoagulant function. Binding of blood clotting enzyme complexes to this negatively-charged membrane surface allows a dramatic increase in the rate of conversion of zymogens to active serine proteases, which in turn produce a burst of thrombin leading to the formation of a fibrin clot and further platelet activation. Cells have the capacity to catalyze transbilayer phospholipid exchange via ATP-requiring translocase enzymes (flippases and floppases), which control unidirectional phospholipid transport against a concentration gradient. They also use an energy-independent, calcium-dependent scramblase activity to govern the bidirectional exchange of phospholipids between the two leaflets of the bilayer; this activity is essential for PS exposure during platelet activation. Scramblase activity, biochemically characterized in the 1980s, is deficient in patients with Scott syndrome, a rare inherited bleeding disorder with defective platelet procoagulant activity. Despite considerable efforts, the platelet scramblase protein remained elusive for years but a significant advance has recently been made with the identification of TMEM16F, a membrane protein essential for calcium-dependent PS exposure whose loss of function mutations are found in Scott syndrome. This review recalls historical aspects of platelet membrane asymmetry characterization, summarizes the mechanisms and roles of PS exposure following platelet activation and discusses the recent identification of TMEM16F and its significance in the scrambling process.

Dynamic transbilayer lipid asymmetry

Cells have thousands of different lipids. In the plasma membrane, and in membranes of the late secretory and endocytotic pathways, these lipids are not evenly distributed over the two leaflets of the lipid bilayer. The basis for this transmembrane lipid asymmetry lies in the fact that glycerolipids are primarily synthesized on the cytosolic and sphingolipids on the noncytosolic surface of cellular membranes, that cholesterol has a higher affinity for sphingolipids than for glycerolipids. In addition, P4-ATPases, "flippases," actively translocate the aminophospholipids phosphatidylserine and phosphatidylethanolamine to the cytosolic surface. ABC transporters translocate lipids in the opposite direction but they generally act as exporters rather than "floppases." The steady state asymmetry of the lipids can be disrupted within seconds by the activation of phospholipases and scramblases. The asymmetric lipid distribution has multiple implications for physiological events at the membrane surface. Moreover, the active translocation also contributes to the generation of curvature in the budding of transport vesicles.

Lipid transfer mediated by a recombinant pro-sterol carrier protein 2 for the accurate preparation of asymmetrical membrane vesicles requires a narrow vesicle size distribution: a free-flow electrophoresis study

We applied protein-mediated lipid transfer using recombinant His-tagged pro-sterol carrier protein 2 (pro-SCP2) to prepare asymmetrical membrane vesicles (AMV) featuring an unequal transmembrane distribution of the negative phospholipid egg-phosphatidylglycerol (EPG). Pure egg-phosphatidylcholine (EPC) vesicles were used as the acceptor and EPC:EPG 90:10 mol % vesicles as the donor populations. The changes in surface charge during EPG transfer were used to quantify the degree of asymmetry by free-flow electrophoresis (FFE). The relative deflection in FFE correlated with EPG content in the outer monolayer (x(EPG)). The initial transfer rates and first order rate constants for the transfer process were determined. The addition of pro-SCP2 at a molar protein-to-lipid ratio R(P/L) of (15-20) x 10(-5) accelerated the EPG transfer to half-times of between 2 and 3 h. Thus, the transmembrane redistribution of EPG by flip-flop, which reduces the degree of asymmetry and occurs at half-times of tens of hours, was minimized during the transfer process. We investigated the influence of membrane curvature on the transfer rate using 50 and 100 nm vesicles with very low size distribution widths (RSD of 13-17%). Transfer occurred with a 55.7% higher initial rate between the smaller vesicles. The use of equally sized acceptor and donor populations of such narrow size distributions was shown to be important for the preparation of AMV with a uniform degree of asymmetry. Under these conditions, AMV were obtained after less than 3 h by preparative FFE separation. In the case of the acceptor vesicles, EPG transfer increased x(EPG) to 3 mol %, whereas it was reduced to 6 mol % in the donor vesicles.

Macroscopic domain formation during cooling in the platelet plasma membrane: an issue of low cholesterol content

There has been ample debate on whether cell membranes can present macroscopic lipid domains as predicted by three-component phase diagrams obtained by fluorescence microscopy. Several groups have argued that membrane proteins and interactions with the cytoskeleton inhibit the formation of large domains. In contrast, some polarizable cells do show large regions with qualitative differences in lipid fluidity. It is important to ask more precisely, based on the current phase diagrams, under what conditions would large domains be expected to form in cells. In this work we study the thermotropic phase behavior of the platelet plasma membrane by FTIR, and compare it to a POPC/Sphingomyelin/Cholesterol model representing the outer leaflet composition. We find that this model closely reflects the platelet phase behavior. Previous work has shown that the platelet plasma membrane presents inhomogeneous distribution of DiI18:0 at 24 degrees C, but not at 37 degrees C, which suggests the formation of macroscopic lipid domains at low temperatures. We show by fluorescence microscopy, and by comparison with published phase diagrams, that the outer leaflet model system enters the macroscopic domain region only at the lower temperature. In addition, the low cholesterol content in platelets ( approximately 15 mol%), appears to be crucial for the formation of large domains during cooling.

Polyethylene glycols interact with membrane glycerophospholipids: is this part of their mechanism for hypothermic graft protection?

Polyethylene glycol (PEG), a high-molecular-weight colloid present in new organ preservation solutions, protects against cold ischemia injuries leading to better graft function of transplanted organs. This protective effect cannot be totally explained by immuno-camouflaging property or signaling-pathway modifications. Therefore, we sought for an alternative mechanism dependent on membrane fluidity. Using the Langmuir-Pockles technique, we show here that PEGs interacted with lipid monolayers of defined composition or constituted by a renal cell lipid extract. High-molecular-weight PEGs stabilized the lipid monolayer at low surface pressure. Paradoxically, at high surface pressure, PEGs destabilized the monolayers. Hypothermia reduced the destabilization of saturated monolayer whereas unsaturated monolayer remained unaffected. Modification of ionic strength and pH induced a stronger stabilizing effect of PEG 35,000 Da which could explain its reported higher effectiveness on cold-induced injuries during organ transplantation. This study sheds a new light on PEG protective effects during organ preservation different from all classical hypotheses.

Diacylglycerols, multivalent membrane modulators

Diacylglycerols are second messengers confined to biomembranes and, although relatively simple molecules from the structural point of view, they are able of triggering a surprisingly wide range of biological responses. Diacylglycerols are recognized by a well conserved protein motif, such as the C1 domain. This domain was observed for the first time in protein kinases C but is now known to be present in many other proteins. The effect of diacylglycerols is not limited to binding to C1 domains and they are able to alter the biophysical properties of biomembranes and hence modulate the activity of membrane associated proteins and also facilitate some processes like membrane fusion.

The role of membrane lipids in the induction of macrophage apoptosis by microparticles

Microparticles are membrane-derived vesicles that are released from cells during activation or cell death. These particles can serve as mediators of intercellular cross-talk and induce a variety of cellular responses. Previous studies have shown that macrophages undergo apoptosis after phagocytosing microparticles. Here, we have addressed the hypothesis that microparticles trigger this process via lipid pathways. In these experiments, microparticles induced apoptosis in primary macrophage cells or cell lines (RAW 264.7 or U937) with up to a 5-fold increase. Preincubation of macrophages with phosphatidylinositol-3,5-bisphosphate (PtdIns(3,5)BP) reduced the microparticle-induced apoptosis in a dose-dependent manner. PtdIns(3,5)BP is a specific inhibitor of the acid sphingomyelinase and thus can block the generation of pro-apoptotic ceramides. Similarly, the pre-incubation of macrophages with PtdIns(3,5)BP prevented microparticle-induced upregulation of caspase 8, which is a major target molecule of ceramide action in the apoptosis pathway. PtdIns(3,5)BP, however, had no effect on the spontaneous rate of apoptosis. To evaluate further signaling pathways induced by microparticles, the extracellular signal regulated kinase (ERK-) 1 was investigated. This kinase plays a role in activating phospholipases A2 which cleaves membrane phospholipids into arachidonic acid; microparticles have been suggested to be a preferred substrate for phospholipases A2. As shown in our experiments, microparticles strongly increased the amount of phosphorylated ERK1/2 in RAW 264.7 macrophages in a time-dependent manner, peaking 15 min after co-incubation. Addition of PD98059, a specific inhibitor of ERK1, prevented the increase in apoptosis of RAW 264.7 macrophages. Together, these data suggest that microparticles perturb lipid homeostasis of macrophages and thereby induce apoptosis. These results emphasize the importance of biolipids in the cellular cross-talk of immune cells. Based on the fact that in clinical situations with excessive cell death such as malignancies, autoimmune diseases and following chemotherapies high levels of circulating microparticles might modulate phagocytosing cells, a suppression of the immune response might occur due to loss of macrophages.

Persistence of procoagulant surface expression on activated human platelets: involvement of apoptosis and aminophospholipid translocase activity

BACKGROUND

Activated platelets express a procoagulant surface when the asymmetric distribution of membrane phospholipids is scrambled, leading to phosphatidylserine (PS) exposure. PS expression, associated with apoptosis in nucleated cells, would be expected to be reversed by aminophospholipid translocase (APLT) activity.

OBJECTIVE

To determine whether the procoagulant surface of activated platelets persists after it forms; to examine whether PS expression on platelets is associated with loss of mitochondrial inner membrane potential (DeltaPsi(m)), a hallmark of apoptosis; and to investigate the role of APLT in persistence of PS expression.

METHODS

Platelets were stimulated with thrombin, collagen, a combination of both, or the Ca(2+)-ionophore A23187. Up to 4 h after activation, procoagulant surface expression was measured by annexin A5 binding by flow cytometry and by a prothrombinase assay. Flow cytometry was also used to measure PS expression concurrently with DeltaPsi(m) collapse, using CMXRos. APLT activity in annexin A5-negative and -positive platelets was measured flow cytometrically as the percent of 1-palmitoyl-2-[6-[(7-nitro-2-1,3-benzoxadiazol-4-yl)amino]caproyl]-sn-glycero-3-phosphatidylserine (NBD-PS) translocated from the outer to the inner membrane leaflet.

RESULTS AND CONCLUSIONS

Procoagulant surface expression on activated platelets persisted in vitro for at least 4 h; if such persistence occurs in vivo, there are important implications for the propagation of thrombosis. With the physiological stimuli, only 10-20% of the activated platelets expressed PS on their surface, and of these, only a portion exhibited DeltaPsi(m) collapse, indicating that PS expression can be associated with platelet apoptosis, but can also occur independently. APLT activity was very low in the PS-expressing platelet subpopulation for up to 4 h after activation, indicating that the persistence of a procoagulant surface may be attributed, at least in part, to this reduced APLT activity.

Mechanisms of sphingosine and sphingosine 1-phosphate generation in human platelets

The bioactive molecule sphingosine 1-phosphate (S1P) is abundantly stored in platelets and can be released extracellularly. However, although they have high sphingosine (Sph) kinase activity, platelets lack the de novo sphingolipid biosynthesis necessary to provide the substrates. Here, we reveal a generation pathway for Sph, the precursor of S1P, in human platelets. Platelets incorporated extracellular 3H-labeled Sph much faster than human megakaryoblastic cells and rapidly converted it to S1P. Furthermore, Sph formed from plasma sphingomyelin (SM) by bacterial sphingomyelinase (SMase) and neutral ceramidase (CDase) was rapidly incorporated into platelets and converted to S1P, suggesting that platelets use extracellular Sph as a source of S1P. Platelets abundantly express SM, possibly supplied from plasma lipoproteins, at the cell surface. Treating platelets with bacterial SMase resulted in Sph generation at the cell surface, conceivably by the action of membrane-bound neutral CDase. Simultaneously, a time-dependent increase in S1P levels was observed. Finally, we demonstrated that secretory acid SMase also induces S1P increases in platelets. In conclusion, our results suggest that in platelets, Sph is supplied from at least two sources: generation in the plasma followed by incorporation, and generation at the outer leaflet of the plasma membrane, initiated by cell surface SM degradation.

Interaction between phosphatidylserine and the phosphatidylserine receptor inhibits immune responses in vivo

Phosphatidylserine (PS) on apoptotic cells promotes their uptake and induces anti-inflammatory responses in phagocytes, including TGF-beta release. Little is known regarding the effects of PS on adaptive immune responses. We therefore investigated the effects of PS-containing liposomes on immune responses in mice in vivo. PS liposomes specifically inhibited responses to Ags as determined by decreased draining lymph node tissue mass, with reduced numbers of total leukocytes and Ag-specific CD4(+) T cells. There was also a decrease in formation and size of germinal centers in spleen and lymph nodes, accompanied by decreased levels of Ag-specific IgG in blood. Many of these effects were mimicked by an agonistic Ab-specific for the PS receptor. TGF-beta appears to play a critical role in this inhibition, as the inhibitory effects of PS were reversed by in vivo administration of anti-TGF-beta Ab. PS-containing liposomes did not appear to directly inhibit dendritic cell maturation in vitro in response to a variety of stimuli, nor did it prevent their migration to regional lymph nodes in vivo, suggesting that the inhibitory effects may have resulted from complicated interactions between tissue cells and dendritic cells, subsequently inhibiting their ability to productively activate T lymphocytes.

Biology of Lysenin, a Protein in the Coelomic Fluid of the Earthworm Eisenia foetida

Lysenin is a protein of 33?kDa in the coelomic fluid (CF) of the earthworm Eisenia foetida. It differs from other biologically active proteins, such as fetidins, eiseniapore, and coelomic cytolytic factor (CCF-1), that have been found in Eisenia foetida, in terms of both its biochemical and its biological characteristics. The large coelomocytes and free chloragocytes in the typhlosole of Eisenia foetida appear to be the cells that produce lysenin since the mRNA for lysenin and immunoreactive lysenin have been found in these cells. Lysenin binds specifically to sphingomyelin (SM) but not to other phospholipids in cell membranes. After binding to the cell membranes of target cells, lysenin forms oligomers in an SM-dependent manner, with subsequent formation of pores with a hydrodynamic diameter of approximately 3?nm. The biochemical interactions between lysenin and SM in cell membranes are responsible for the pharmacological activities of lysenin and of CF that contains lysenin in vertebrates, such as hemolysis, cytotoxicity, and contraction of smooth muscle in vitro and vasodepressor activity and lethality in vivo. When incubated with SM-liposomes, CF and lysenin lost some or all of their activity, an observation that suggests that SM might be involved in the induction of the various activities of lysenin and CF. However, in general, lysenin is neither cytotoxic nor lethal to invertebrates. An attempt has been made to explain the differences in the responses to lysenin and CF between vertebrates and invertebrates in terms of the presence or absence of SM in the various animals. Among Protostomia, SM is absent in Lophotrochozoa, with the exception of some molluscan species, but it is present in Ecdysozoa, with the exception of Nematomorpha and flies. Among Deuterostomia, Echinodermata and Hemichordata lack SM but SM is found in Chordata. Thus, the difference in terms of the response to lysenin between invertebrates and vertebrates cannot be fully explained by reference to the presence or absence of SM in the organism. Lysenin and its antiserum have made it possible to localize SM in the cell membranes. They should be a useful tool for studies of membrane physiology and the role of SM.

Exposure of platelet membrane phosphatidylserine regulates blood coagulation

This article addresses the role of platelet membrane phosphatidylserine (PS) in regulating the production of thrombin, the central regulatory molecule of blood coagulation. PS is normally located on the cytoplasmic face of the resting platelet membrane but appears on the plasma-oriented surface of discrete membrane vesicles that derive from activated platelets. Thrombin, the central molecule of coagulation, is produced from prothrombin by a complex ("prothrombinase") between factor Xa and its protein cofactor (factor V(a)) that forms on platelet-derived membranes. This complex enhances the rate of activation of prothrombin to thrombin by roughly 150,000 fold relative to factor X(a) in solution. It is widely accepted that the negatively charged surface of PS-containing platelet-derived membranes is at least partly responsible for this rate enhancement, although there is not universal agreement on mechanism by which this occurs. Our efforts have led to an alternative view, namely that PS molecules bind to discrete regulatory sites on both factors X(a) and V(a) and allosterically alter their proteolytic and cofactor activities. In this view, exposure of PS on the surface of activated platelet vesicles is a key regulatory event in blood coagulation, and PS serves as a second messenger in this regulatory process. This article reviews our knowledge of the prothrombinase reaction and summarizes recent evidence leading to this alternative viewpoint. This viewpoint suggests a key role for PS both in normal hemostasis and in thrombotic disease.

Phosphatidylserine (PS) induces PS receptor-mediated macropinocytosis and promotes clearance of apoptotic cells

Efficient phagocytosis of apoptotic cells is important for normal tissue development, homeostasis, and the resolution of inflammation. Although many receptors have been implicated in the clearance of apoptotic cells, the roles of these receptors in the engulfment process have not been well defined. We developed a novel system to distinguish between receptors involved in tethering of apoptotic cells versus those inducing their uptake. Our results suggest that regardless of the receptors engaged on the phagocyte, ingestion does not occur in the absence of phosphatidylserine (PS). Further, recognition of PS was found to be dependent on the presence of the PS receptor (PSR). Both PS and anti-PSR antibodies stimulated membrane ruffling, vesicle formation, and "bystander" uptake of cells bound to the surface of the phagocyte. We propose that the phagocytosis of apoptotic cells requires two events: tethering followed by PS-stimulated, PSR-mediated macropinocytosis.

Loss of phospholipid asymmetry and surface exposure of phosphatidylserine is required for phagocytosis of apoptotic cells by macrophages and fibroblasts

Removal of apoptotic cells during tissue remodeling or resolution of inflammation is critical to the restoration of normal tissue structure and function. During apoptosis, early surface changes occur, which trigger recognition and removal by macrophages and other phagocytes. Loss of phospholipid asymmetry results in exposure of phosphatidylserine (PS), one of the surface markers recognized by macrophages. However, a number of receptors have been reported to mediate macrophage recognition of apoptotic cells, not all of which bind to phosphatidylserine. We therefore examined the role of membrane phospholipid symmetrization and PS externalization in uptake of apoptotic cells by mouse macrophages and human HT-1080 fibrosarcoma cells by exposing them to cells that had undergone apoptosis without loss of phospholipid asymmetry. Neither mouse macrophages nor HT-1080 cells recognized or engulfed apoptotic targets that failed to express PS, in comparison to PS-expressing apoptotic cells. If, however, their outer leaflets were repleted with the l-, but not the d-, stereoisomer of sn-1,2-PS by liposome transfer, engulfment by both phagocytes was restored. These observations directly demonstrate that loss of phospholipid asymmetry and PS expression is required for phagocyte engulfment of apoptotic cells and imply a critical, if not obligatory, role for PS recognition in the uptake process.

Lipid phase separation correlates with activation in platelets during chilling

When human platelets are chilled below 22 degrees C, they spontaneously activate, a phenomenon that severely limits their storage life. It has previously been proposed that there is a correlation between cold-induced platelet activation and passage of the membranes through a liquid-crystalline to gel phase transition. Because animal models are essential for developing methods for cold storage of platelets, it is necessary to investigate such a correlation in animal platelets. In this work, horse platelets were used as a model, and it was found that cold-induced morphological activation is related to the lipid phase transition. Using fluorescence microscopy with the lipophilic fluorescent dye 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (Dil-C18), and Fourier transform infrared spectroscopy (FTIR), it was found that lipid phase separation occurs during cooling and low temperature storage. Furthermore, removal of cholesterol from the plasma membrane also induced a phase separation, possibly between specific phospholipid classes. Steady-state fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) and trimethylammonium-DPH (TMA-DPH) were compared in cells and multilamellar vesicles (MLV) composed of platelet lipids. Cholesterol depletion led to a decrease in the fluorescence anisotropy of the two probes, which can be explained by changes in the order of the phospholipid molecules. In addition, the lipid composition and fatty acid profile of the cellular phospholipids were determined. Based of the similarities between horse and human platelets, it is suggested that horse platelets may be used as a model for studying cold-stored platelets. The results are discussed in relation to the possible role of phase separation during cell signalling.

Serine base exchange enzyme in porcine lyophilised platelets: enzyme properties and modulation by AlF4- and different types of heparin

Phosphatidylserine is one of the PKC modulators and thus it may play an important role in signal transduction. Regulation of the synthesis of this phospholipid is not yet clarified. The contrasting reports are possibly related to the existence of different enzymes which, in mammalian tissues, catalyse the exchange between free serine and the nitrogen base of a membrane phospholipid. This study demonstrates that serine base exchange reactions of commercially available lyophilised porcine platelets exhibit similar pH optima, temperature and Ca2+ dependence as observed in fresh tissues. Analysis of fatty acids composition of the three phospholipid classes involved in base exchange reactions also demonstrated a similarity with fresh platelets. Serine and ethanolamine base exchange enzyme activities were assayed in parallel in platelet lysate subjected to preincubation at various temperatures (30-60 degrees C). When dithioerithrol was omitted from the incubation medium, the two base exchange reactions were inhibited with a similar temperature-dependent pattern. Addition of the reducing agent enhanced the sensitivity to preincubation only for the serine base exchange reaction which was inhibited by 80% after preincubation at 45 degrees C. With respect to its regulation, porcine platelet serine base exchange enzyme(s) was inhibited by fluoroalluminate, a widely used G-protein activator, and stimulated by unfractionated heparin. Low mol. wt. heparin did not influence enzyme activity. Unfractionated heparin greatly stimulated SBEE activity assayed at pH 7.4, a pH value far from the optimal pH.

Polyamine regulation of plasma membrane phospholipid flip-flop during apoptosis

During apoptosis, phosphatidylserine (PS) is moved from the plasma membrane inner leaflet to the outer leaflet where it triggers recognition and phagocytosis of the apoptotic cell. Although the mechanisms of PS appearance during apoptosis are not well understood, it is thought that declining activity of the aminophospholipid translocase and calcium-mediated, nonspecific flip-flop of phospholipids play a role. As previous studies in the erythrocyte ghost have shown that polyamines can alter flip-flop of phospholipids, we asked whether alterations in cellular polyamines in intact cells undergoing apoptosis would affect PS appearance, either by altering aminophospholipid translocase activity or phospholipid flip-flop. Cells of the human leukemic cell line, HL-60, were incubated with or without the ornithine decarboxylase inhibitor, difluoromethylornithine (DFMO), and induced to undergo apoptosis by ultraviolet irradiation. Whereas DFMO treatment resulted in profound depletion of putrescine and spermidine (but not spermine), it had no effect on caspase activity, DNA fragmentation, or plasma membrane vesiculation, typical characteristics of apoptosis. Notably, DFMO treatment prior to ultraviolet irradiation did not alter the decline in PS inward movement by the aminophospholipid translocase as measured by the uptake of 6-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)aminocaproyl] (NBD)-labeled PS detected in the flow cytometer. Conversely, the appearance of endogenous PS in the plasma membrane outer leaflet detected with fluorescein isothiocyanate-labeled annexin V and enhanced phospholipid flip-flop detected by the uptake of 1-palmitoyl-1-[6-[(7-nitro-2-1, 3-benzoxadiazol-4-yl)aminocaproyl]-sn-glycero-3-phosphocholine (NBD-PC) seen during apoptosis were significantly inhibited by prior DFMO treatment. Importantly, replenishment of spermidine, by treatment with exogenous putrescine to bypass the metabolic blockade by DFMO, restored both enhanced phospholipid flip-flop and appearance of PS during apoptosis. Such restoration was seen even in the presence of cycloheximide but was not seen when polyamines were added externally just prior to assay. Taken together, these data show that intracellular polyamines can modulate PS appearance resulting from nonspecific flip-flop of phospholipids across the plasma membrane during apoptosis.

Membrane sidedness of biosynthetic pathways involved in the production of lysophosphatidic acid

Lysophosphatidic acid (LPA) is a novel phospholipid mediator with diverse biological activities such as smooth muscle contraction, and proliferative effects or modifications of cytoskeleton. Activated blood platelets are the best identified source, explaining accumulation of LPA in serum upon blood coagulation. However, the metabolic pathways responsible for LPA synthesis are still poorly known. Using a model of human erythrocytes treated with the calcium ionophore A23187, we have shown that type II secretory phospholipase A2 (sPLA2) is able to produce LPA by hydrolyzing phosphatidic acid exposed on the cell surface after phospholipid scrambling. A similar mechanism does not appear to occur in platelets, where inhibitors of sPLA2 or genetic lack of the enzyme do not modify LPA production. However, this does not definitely eliminate the possibility that LPA is also produced in platelets in the external leaflet of the membrane by other phospholipases, which have to be better characterized.

Lipid composition of seven APTT reagents in relation to heparin sensitivity

The phospholipid content of different activated partial thromboplastin time (APTT) reagents was determined and compared to heparin sensitivity. The seven reagents included were those most widely used amongst participants of the U.K. National External Quality Assessment Scheme (NEQAS) at the time of study. Heparin sensitivity was assessed using the APTT ratios obtained by more than 300 NEQAS participants on five plasmas prepared from patients receiving unfractionated heparin. The concentrations of three neutral lipids and six phospholipids present in the seven APTT reagents were determined by high-performance thin-layer chromatography (HPTLC) and densitometry. Both the concentrations and the relative percentages of individual phospholipid components varied markedly between reagents. The total phospholipid concentration included a 12-fold range from 16 to 205 microgram/ml. Phosphatidylserine (PS) was completely lacking from one reagent prepared from vegetable material and ranged from 3 to 22 microgram/ml in the other six reagents containing extracts from animal tissue. The concentration of phosphatidylcholine ranged from 3 to 109 microgram/ml. There was no demonstrable relationship between the concentration of any individual lipid components and heparin sensitivity. However, the relative percentage phospholipid composition was important since a lower % of PS or phosphatidylinositol (PI) correlated with increasing heparin sensitivity.

Annexin V inhibits phosphatidylserine-induced intrauterine growth restriction in mice

To investigate the role of phosphatidylserine (PS) derived from the activated platelets in placental circulation, we established an artificial PS-induced intrauterine growth restriction (IUGR) model in mice and examined whether annexin V, a PS-binding anticoagulant protein, could prevent the development of IUGR in the animal experiments. The ICR mice were injected in the tail vein on days 8, 11 and 14 of pregnancy with filtered PS/phosphatidylcholine (PC) microvesicles (<0.3 microm in maximal diameter). The microvesicles have a procoagulant activity which was inhibited by annexin V in a dose-dependent fashion. The mice were killed on day 18 of pregnancy and the placental and fetal weights were measured. The placental tissue specimens were examined microscopically. PS/PC vesicles induced significant reductions in fetal weights compared with PC vesicles alone. The placental tissue revealed severe congestion with fibrin depositions although the lung and kidney tissue specimens showed minimal histological changes. PS/PC microvesicles with recombinant annexin V showed no significant reductions in fetal weights in mice with PS/PC vesicles alone. These results suggest that PS from the activated platelets induces IUGR by enhancing the coagulation cascade in the placental circulation and that annexin V from the villous trophoblast prevents the development of IUGR.

Mechanisms by which elevated intracellular calcium induces S49 cell membranes to become susceptible to the action of secretory phospholipase A2

Exposure of S49 lymphoma cells to exogenous group IIA or V secretory phospholipase A2 (sPLA2) caused an initial release of fatty acid followed by resistance to further hydrolysis by the enzyme. This refractoriness was overcome by exposing cells to palmitoyl lysolecithin. This effect was specific in terms of lysophospholipid structure. Induction of membrane susceptibility by lysolecithin involved an increase in cytosolic calcium and was duplicated by incubating the cells with calcium ionophores such as ionomycin. Lysolecithin also activated cytosolic phospholipase A2 (cPLA2). Inhibition of this enzyme attenuated the ability of lysolecithin (but not ionomycin) to induce susceptibility to sPLA2. Lysolecithin or ionomycin caused concurrent hydrolysis of both phosphatidylethanolamine and phosphatidylcholine implying that transbilayer movement of phosphatidylethanolamine occurred upon exposure to these agents but that susceptibility is not simply due to exposure of a preferred substrate (i.e. phosphatidylethanolamine) to the enzyme. Microvesicles were apparently released from the cells upon addition of lysolecithin or ionomycin. Both these vesicles and the remnant cell membranes were susceptible to sPLA2. Together these data suggest that lysolecithin induces susceptibility through both cPLA2-dependent and -independent pathways. Whereas elevated cytosolic calcium was required for both pathways, it was sufficient only for the cPLA2-independent pathway. This cPLA2-independent pathway involved changes in cell membrane structure associated with transbilayer phospholipid migration and microvesicle release.

Sphingolipid metabolism during human platelet activation

Thrombin activation of human platelets causes release of sphingosine-1-phosphate from platelets and an increase in sphingosine levels. Sphingosine-1-phosphate is also known to potentiate platelet aggregation. Thus, these sphingolipids may serve as second messengers during platelet activation making it possible that another sphingolipid, ceramide, might play a role in platelet function. Platelets are known to contain sphingomyelinase activity and hydrolysis of sphingomyelin by this enzyme yields phosphocholine and ceramide. Since ceramide is thought to exert its effects through regulation of protein kinases and phosphatases, both of which are involved in platelet function, it is possible that ceramide produced during platelet activation could be involved in regulating signal transduction events. To investigate this possibility, potential changes in levels of ceramide and sphingomyelin in resting and thrombin-activated platelets have been evaluated. Thin-layer chromatographic analysis of the total mass or of radiolabeled (14C-palmitate or 14C-serine) pools of ceramide and sphingomyelin did not reveal any significant changes in the concentrations of either of these molecules during platelet activation. In addition, activation of platelets labeled with [14C-choline]-sphingomyelin did not lead to production of 14C-phosphocholine, suggesting that platelet activation did not cause sphingomyelinase to hydrolyze the [14C-choline]-sphingomyelin. Taken together, our results suggest that ceramide does not serve as a second messenger during platelet aggregation.

Cholesterol relieves the inhibitory effect of sphingomyelin on type II secretory phospholipase A2

Secretory type II phospholipase A2 (sPLA2) is inhibited by sphingomyelin (SPH); cholesterol either mixed with the model glycerophospholipid substrate or added to the assay medium as separated liposomes counteracts this inhibition efficiently. The inhibition of fatty acid release assayed by quantitative gas chromatography-MS is observed when SPH is added to erythrocyte membranes as the substrate instead of a readily hydrolysable phosphatidylethanolamine/phosphatidylserine model mixture. Hydrolysis of SPH by Staphylococcus aureus sphingomyelinase suppresses its inhibitory potency. The addition of cholesterol to SPH liposomes with a 1:1 stoichiometry relieves completely the inhibition of sPLA2 exerted by SPH. The mechanism of inhibition suggested by the binding assay is that sPLA2 binds with affinity to the SPH interface, after either phase segregation at the assay temperature or on the pure SPH liposomes added to the incubation medium. Cholesterol is shown to suppress the binding affinity of the enzyme for the SPH interface. A model for inhibition is suggested in which binding of the sphingosine moiety is competitive for sPLA2 (inhibition) or for cholesterol (release of the enzyme).

Phospholipid composition of the mammalian red cell membrane can be rationalized by a superlattice model

Although the phospholipid composition of the erythrocyte membrane has been studied extensively, it remains an enigma as to how the observed composition arises and is maintained. We show here that the phospholipid composition of the human erythrocyte membrane as a whole, as well as the composition of its individual leaflets, is closely predicted by a model proposing that phospholipid head groups tend to adopt regular, superlattice-like lateral distributions. The phospholipid composition of the erythrocyte membrane from most other mammalian species, as well as of the platelet plasma membrane, also agrees closely with the predictions of the superlattice model. Statistical analyses indicate that the agreement between the observed and predicted compositions is highly significant, thus suggesting that head group superlattices may indeed play a central role in the maintenance of the phospholipid composition of the erythrocyte membrane.

Impaired redistribution of aminophospholipids with distinctive cell shape change during Ca2+-induced activation of platelets from a patient with Scott syndrome

We have investigated phospholipid redistribution, membrane vesicle shedding, shape change, and granule release following A23187 activation of platelets from a patient with Scott syndrome, characterized by impaired transmembrane migration of phosphatidylserine (PS) accompanied by haemorrhagic complications, and two of her children. Electron spin resonance spectroscopy measurement of phospholipids redistribution showed that the internalization of PS was unaffected by the disorder but, after activation, PS exposure was significantly reduced in platelets from the homozygous-type patient. Vesicle shedding was also reduced in these platelets. However, the slow redistribution of phosphatidylcholine was similar to that observed in normal platelets. When treated with calpeptin, platelets from the homozygous-type patient, unlike normal or heterozygous Scott syndrome platelets, showed a smoothly rounded shape without filopods after activation. Following A23187 activation of normal platelets, filopod formation was consecutive to the re-exposition of aminophospholipids on the outer leaflet of the plasma membrane, and the existence of a floppase (outward aminoPLs translocase) has been suggested. In homozygous Scott syndrome platelets the deficiency in PS re-exposition, the absence of filopod formation, and low vesicle shedding are correlated with each other, and argue in favour of a disruption of the proposed floppase activity.

Effect of a glycerol-containing hypotonic medium on erythrocyte phospholipid asymmetry and aminophospholipid transport during storage

Previous studies from our laboratory have shown that under blood bank storage conditions red blood cell (RBC) ATP and lipid content were better maintained in a glycerol-containing hypotonic experimental additive solution (EAS 25) than in the conventional storage medium Adsol. The objective of this study was to determine the mechanism of the protective effect of EAS 25, by measuring transmembrane phospholipid asymmetry and the membrane integrity of stored RBCs. Split units of packed RBCs were stored in either EAS 25 or Adsol. RBCs were analyzed after 0, 42, and 84 days and vesicles shed from stored RBCs were analyzed after 84 days of storage. Phospholipid asymmetry was measured by phospholipase A2 digestion (RBCs) and activation of the prothrombinase complex (RBCs, vesicles). RBC membrane exhibited a significantly greater (P < 0.01) amount of phosphatidylethanolamine externalized after storage in Adsol than in EAS 25 (44.3% +/- 11.7 vs. 25.3% +/- 5.7, respectively). Prothrombin converting activities in RBCs were significantly lower than in shed vesicles (P < 0.001) suggesting the presence of phosphatidylserine in the outer monolayer of vesicle, but not in RBC membranes. The rates of inwardly-directed aminophospholipid transport in RBCs decreased by 50% and glutathione levels decreased by approximately 50% in both media. RBC cholesterol and phospholipid content of stored RBCs remained significantly greater (P < 0.01) in EAS 25 than in Adsol. The results indicate that despite comparable reduction in the rate of aminophospholipid transport and reduced GSH concentrations, RBC phospholipid asymmetry was better maintained during storage in EAS 25 than in Adsol. The data suggest that glycerol in the hypotonic EAS helps preserve RBC lipid organization and membrane integrity during storage.

Snake venom cardiotoxins-structure, dynamics, function and folding

Snake cardiotoxins are highly basic (pI > 10) small molecular weight (approximately 6.5 kDa), all beta-sheet proteins. They exhibit a broad spectrum of interesting biological activities. The secondary structural elements in these toxins include antiparallel double and triple stranded beta-sheets. The three dimensional structures of these toxins reveal an unique asymmetric distribution of the hydrophobic and hydrophilic amino acids. The 3D structures of closely related snake venom toxins such as neurotoxins and cardiotoxin-like basic proteins (CLBP) fail to show similar pattern(s) in the distribution of polar and nonpolar residues. Recently, many novel biological activities have been reported for cardiotoxins. However, to-date, there is no clear structure-function correlation(s) available for snake venom cardiotoxins. The aim of this comprehensive review is to summarize and critically evaluate the progress in research on the structure, dynamics, function and folding aspects of snake venom cardiotoxins.

Modulation of human type II secretory phospholipase A2 by sphingomyelin and annexin VI

Conjectural results have been reported on the capacity of inflammatory secreted phospholipase A2 (sPLA2) to hydrolyse mammalian membrane phospholipids. Development of an assay based on the release of non-esterified fatty acids by the enzyme acting on the organized phospholipid mixture constituting the membrane matrix has led to the identification of two prominent effectors, sphingomyelin (SPH) and annexin. Recombinant human type II sPLA2 hydrolyses red-cell membrane phospholipids with a marked preference for the inner leaflet. This preference is apparently related to the high content of SPH in the outer leaflet, which inhibits sPLA2. This inhibition by SPH is specific for sPLA2. Cholesterol counteracts the inhibition of sPLA2 by SPH, suggesting that the SPH-to-cholesterol ratio accounts in vivo for the variable susceptibility of cell membranes to sPLA2. Different effects were observed of the presence of the non-hydrolysable D-alpha-dipalmitoyl phosphatidylcholine (D-DPPC), which renders the membranes rigid but does not inhibit sPLA2. Annexin VI was shown, along with other annexins, to inhibit sPLA2 activity by sequestering the phospholipid substrate. The present study has provided the first evidence that annexin VI, in concentrations that inhibit hydrolysis of purified phospholipid substrates, stimulated the hydrolysis of membrane phospholipids by sPLA2. The activation requires the presence of membrane proteins. The effect is specific for type II sPLA2 and is not reproducible with type I PLA2. The activation by annexin VI of sPLA2 acting on red cell membranes results in the preferential release of polyunsaturated fatty acids. It suggests that type II sPLA2, in conjunction with annexin VI, might be involved in the final step of endocytosis and/or exocytosis providing the free polyunsaturated fatty acids acting synergistically to cause membrane fusion.

Influence of specimen preparation on the identification of phospholipids by the phospholipase A2-gold method in mineralizing cartilage and bone

The role of phospholipids in biological mineralization has been hypothesized but not fully elucidated. In order to identify phospholipids at the ultrastructural level in the mineralizing extracellular matrix, rat epiphyseal cartilage and metaphyseal bone have been labeled with the phospholipase A(2) (PLA(2))-gold method. The specificity and the efficiency of phospholipid detection have been evaluated by postembedding labeling of sections from epoxy- or hydrophilic resin-embedded samples, and by preembedding labeling of cryosectioned samples. The efficiency of the labeling was higher in cryosections than in hydrophilic resin-embedded specimens, while lower efficiency was found in epoxy resin-embedded samples. A 2- to 6-fold increase of the labeling density in calcified with respect to uncalcified areas of cartilage and bone has been found, depending on the specimen preparation used. The labeling intensity was significantly higher, at the periphery of the calcifying nodules in the epiphyseal cartilage matrix and in the calcifying osteoid, while the fully calcified bone matrix presented a weak labeling. Matrix vesicles, which are considered a possible source of extracellular phospholipids, appeared labeled in cryosections and in epoxy resin-embedded samples after a preincubation with PLA(2), which also increased the labeling of the intracellular membranes. The localization of phospholipids in the areas of initial mineralization suggests some hypotheses on the possible involvement of these molecules in the mineral phase deposition process.

Inhibition of platelet type II phospholipase A2 by an acylamino phospholipid does not alter arachidonate liberation

An acylamino phospholipid analogue (2-(R)-N-palmitoylnorleucinol-1-phosphoglycol or (R)-PNPG) was examined for its inhibitory effects against type II phospholipase A2 (PLA2) acting on membranes from Escherichia coli. Using two enzyme sources (rat platelet membranes or recombinant human type II PLA2), (R)-PNPG inhibited phospholipid hydrolysis to a maximal value of 80-85%, half-maximal effect being attained at a substrate/inhibitor molar ratio of 80-250. In contrast, (S)-PNPG was 12-fold less potent and thus provided a control for possible non-specific effects of these polar lipids. However, both analogues exerted only marginal effects on the liberation of [3H]arachidonic acid from rat platelets challenged with calcium ionophore A23187. Since, among various animal species, rat platelets contain by far the highest amounts of this enzyme, our data rule out any possible involvement of secretory PLA2 in arachidonic acid liberation from platelet phospholipids, cytosolic PLA2 appearing in this case as the best candidate able to regulate eicosanoid biosynthesis.

Secretory phospholipase A2 generates the novel lipid mediator lysophosphatidic acid in membrane microvesicles shed from activated cells

Nonpancreatic secretory phospholipase A2 (sPLA2) displays proinflammatory properties; however, its physiological substrate is not identified. Although inactive toward intact cells, sPLA2 hydrolyzed phospholipids in membrane microvesicles shed from Ca(2+)-loaded erythrocytes as well as from platelets and from whole blood cells challenged with inflammatory stimuli. sPLA2 was stimulated upon degradation of sphingomyelin (SPH) and produced lysophosphatidic acid (LPA), which induced platelet aggregation. Finally, lysophospholipid-containing vesicles and sPLA2 were detected in inflammatory fluids in relative proportions identical to those used in vitro. We conclude that upon loss of phospholipid asymmetry, cell-derived microvesicles provide a preferential substrate for sPLA2. SPH hydrolysis, which is provoked by various cytokines, regulates sPLA2 activity, and the novel lipid mediator LPA can be generated by this pathway.

Platelet secretory phospholipase A2 fails to induce rabbit platelet activation and to release arachidonic acid in contrast with venom phospholipases A2

The ability of platelet secretory phospholipase A2 (sPLA2) to induce platelet activation was investigated. sPLA2 (group II) contained in an activated platelet supernatant, as well as high concentrations of purified recombinant platelet sPLA2, failed to induce platelet activation. Furthermore, sPLA2 did not modify platelet activation induced by various agonists. The possible relationship between the failure of this enzyme to induce platelet activation and its origin (mammalian) or its structural group (group II) was then investigated, using pancreatic PLA2s (group I) and venom PLA2s from groups I, II and III. All venom PLA2s induced platelet activation that was accompanied by the liberation of arachidonic acid and was abolished by aspirin. In contrast, as observed for platelet sPLA2, enzymes from hog or bovine pancreas were unable to induce platelet activation even when used at high concentrations. Interestingly, PLA2 able to induce platelet activation efficiently hydrolyse phosphatidylcholine, while those inactive on platelets did not. Taken together, these results suggest that the catalytic activity of added PLA2 is necessary but not sufficient to induce platelet activation. Moreover, the ability of PLA2 to induce platelet activation is not related to its structural group (I, II, III) but rather to its origin (venom vs. mammalian) and capacity to hydrolyse phosphatidylcholine, the major phospholipid of the outer leaflet of the plasma membrane.

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.

Phospholipid asymmetry in plasma membrane vesicles derived from BHK cells

The transbilayer distribution of phospholipids in plasma membrane vesicles derived from BHK cells by treatment with iodoacetamide or fluoride and merocyanine 540 has been examined by exposing the vesicles to bee venom phospholipase A2 (PLA2) or to Bacillus cereus sphingomyelinase. The results show that almost all of the phosphatidylserine (PS) is on the inner lipid leaflet and most of the sphingomyelin is on the outer lipid leaflet. In contrast, about 50% of the phosphatidylcholine (PC) and 30-40% of the phosphatidylethanolamine (PE) is rapidly degraded by PLA2 and thus appears to be present on the surface of the vesicles. The pools of PC and PE which are accessible only slowly to PLA2 are degraded with halftimes of about 5 h and 2 h, respectively, and it is suggested that this rate reflects the rate of transbilayer migration of these lipids. We conclude that the profound energy depletion caused by treatment with iodoacetamide or fluoride does not alter the asymmetric distribution of PS across the plasma membrane but does have a marked effect on the transbilayer distribution of PE. Residual cells after treatment with fluoride and MC540 were also exposed to PLA2. The results were broadly in agreement with those obtained with vesicles, suggesting that the vesicles were representative of the BHK cell plasma membrane in terms of phospholipid asymmetry. Fluoride or MC540 added separately caused little vesicle release but did lead to significant loss of phospholipid asymmetry. When centrifuged on a sucrose density gradient, vesicles were separated into two major fractions accounting for about two thirds and about 20%, respectively, of total phospholipid but no significant differences were seen in the transbilayer phospholipid asymmetry of the two fractions.