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

Platelet extracellular vesicles and the secretory interactome join forces in health and disease

Extracellular vesicles (EVs) are small membrane-bound vesicles released by cells under various conditions. They are found in the extracellular milieu in all biological fluids. As the concentrations, contents, and origin of EVs can change during inflammation, the assessment of EVs can be used as a proxy of cellular activation. Here, we review the literature regarding EVs, more particularly those released by platelets and their mother cells, the megakaryocytes. Their cargo includes cytokines, growth factors, organelles (mitochondria and proteasomes), nucleic acids (messenger and non-coding RNA), transcription factors, and autoantigens. EVs may thus contribute to intercellular communication by facilitating exchange of material between cells. EVs also interact with other molecules secreted by cells. In autoimmune diseases, EVs are associated with antibodies secreted by B cells. By definition, EVs necessarily comprise a phospholipid moiety, which is thus the target of secreted phospholipases also abundantly expressed in the extracellular milieu. We discuss how platelet-derived EVs, which represent the majority of the circulating EVs, may contribute to immunity through the activity of their cargo or in combination with the secretory interactome.

Extraction and identification of platelet‑derived microparticles

Microparticles are carriers of signals for intracellular signal transduction. These carriers include proteins, mRNAs, microRNAs and other bioactive substances. Platelets are a major source of circulating microparticles, and microparticles are closely associated with the development of certain cardiovascular diseases. In the present study, a method for separating, extracting and identifying platelet‑derived microparticles was developed and differences in the expression of surface proteins on microparticles harvested from platelets stimulated by vortexing or treatment with thrombin was investigated. The counts, composition, sizes and inner structures of microparticles were determined using flow cytometry and transmission electron microscopy. Additionally, it was demonstrated that platelets could be readily activated, and a large quantity of microparticles with varying complex compositions, structures and sizes were derived from activated platelets. High purity platelet‑derived microparticles were obtained by gradient centrifugation. However, the microparticles derived from platelets stimulated by thrombin treatment or vortexing differed significantly in the levels of CD63. The present study aimed to provide improved options for the extraction and identification of microparticles.

Cell membrane extensions, generated by mechanical constraint, are associated with a sustained lipid raft patching and an increased cell signaling

Platelet activation triggers an imbalance in plasma membrane phospholipids by a specific aminophospholipid outflux, resulting in filopodia formation. Similarly, the addition of a phospholipid excess in the outer leaflet of the plasma membrane induces cellular extensions and actin polymerization. The implication of membrane microdomains in sustaining these mechanical constraints remains, however, unknown and was investigated in human platelets and mouse fibroblasts. The disruption of lipid rafts by cholesterol depletion prevents actin polymerization and formation of cellular extensions. Phospholipid excess triggers raft patching underneath the cell extensions, recruitment of protein raft markers and increase of tyrosine phosphorylation of raft proteins. Using a mass spectrometric analysis of isolated platelet rafts, we identified tyrosine kinases and proteins implicated in the formation of cell membrane extensions, cell adhesion and motility. They are recruited to rafts in response to a mechanical constraint. Taken together, our results demonstrate that exogenous phospholipid addition causes a modulation of the lateral plasma membrane organization and an activation of the cell signaling triggering actin remodeling and the formation of cellular protrusions. Raft disruption abolishes these processes, demonstrating that their integrity is crucial for cell shape changes in response to a mechanical constraint on plasma membrane.

Membrane binding and anticoagulant properties of protein S natural variants

INTRODUCTION

Protein S (PS) is a vitamin K-dependent plasma glycoprotein with a key role in the control of coagulation pathway on phospholipid membranes. We compared anticoagulant and membrane binding properties of PS altered by natural mutations (N217S, DelI203D204) affecting the epidermal growth factor like-domain 4 (EGF4) and causing PS deficiency.

MATERIALS AND METHODS

Binding of recombinant, immunopurified PS (rPS) to several conformation-specific antibodies, to C4BP and to phospholipid liposomes was investigated by ELISA. PS binding to cells was analysed by flow cytometry. PS inhibitory activities were studied in plasma and purified systems.

RESULTS AND CONCLUSIONS

Conformational changes produced by mutations were revealed by mapping with calcium-dependent antibodies. The immunopurified recombinant mutants (rPS) showed at 200-800 nM concentration reduced inhibition of coagulation (rPS217S, 10.2-17.3%; rPSDelI203D204, 5.8-8.9% of rPSwt) in FXa 1-stage clotting assay with APC. In thrombin generation assays the inhibition of ETP was reduced to 51.6% (rPS217S) and 24.1% (rPSDelI203D204) of rPSwt. A slightly shortened lag time (minutes) was also observed (rPS217S, 2.58; rPSDelI203D204, 2.33; rPSwt, 3.17; PS deficient plasma, 2.17). In flow cytometry analysis both mutants efficiently bound apoptotic cells in adhesion or in suspension. The affinity for phosphatidylserine-rich vesicles (apparent Kd: rPSwt 27.7+/-1.6 nM, rPS217S 146.0+/-16.1 nM and rPSDelI203D204 234.1+/-28.1 nM) was substantially increased by membrane oxidation (10.9+/-0.6, 38.2+/-3.5 and 81.4+/-6.0 nM), which resulted in a virtually normal binding capacity of mutants at physiological PS concentration. These properties help to define the molecular bases of PS deficiency, and provide further elements for PS-mediated bridging of coagulation and inflammation.

A review of inherited platelet disorders with guidelines for their management on behalf of the UKHCDO

The inherited platelet disorders are an uncommon cause of symptomatic bleeding. They may be difficult to diagnose (and are likely to be under-diagnosed) and pose problems in management. This review discusses the inherited platelet disorders summarising the current state of the art with respect to investigation and diagnosis and suggests how to manage bleeding manifestations with particular attention to surgical interventions and the management of pregnancy.

Antigen recognition induces phosphatidylserine exposure on the cell surface of human CD8+ T cells

In eukaryotic cells the phospholipid phosphatidylserine (PS) is restricted to the inner plasma-membrane leaflet. This lipid asymmetry, which is maintained by the concerted action of phospholipid transport proteins, is mainly lost during apoptosis. Here, we demonstrate that primary human CD8+ cytotoxic T lymphocytes (CTLs) expose PS on T-cell receptor (TCR)-mediated antigen (Ag) recognition. In contrast to PS externalization on apoptotic cells, activation-induced PS exposure is less pronounced and reversible. Fluorescence microscopic analysis revealed that PS is distributed nonhomogenously over the plasma membrane and concentrated in membrane lipid raft domains at the immunologic synapse. By studying the activity of PS transport proteins using a fluorescence-labeled PS analogue, we found that activation of CTLs inhibited the flippase-mediated inward-directed PS transport without affecting the outward transport. Shielding of exposed PS by annexin V protein during Ag recognition diminished cytokine secretion, activation, and cell-to-cell clustering of Ag-specific CTLs. In summary, our data demonstrate for the first time that externalized PS on Ag-stimulated CTLs is linked to T-cell activation and probably involved in cell-to-cell contact formation at the immunologic synapse.

From normal to pathological hemostasis

PURPOSE

To review the evolution of knowledge on physiological hemostasis and the main abnormalities that may interfere with hemostasis in the perioperative period.

METHODS

Narrative review of the literature, including relevant papers published in English.

PRINCIPAL FINDINGS

Physiological hemostasis controls blood fluidity and rapidly induces hemostatic plug formation in order to stop or limit bleeding. The three distinct phases of the hemostatic process, primary hemostasis, coagulation and fibrinolysis are closely linked to each other and precisely regulated in order to efficiently close vessel wounds, promote vascular healing and maintain vessel patency. Primary hemostasis is the result of complex interactions between the vascular wall, platelets and adhesive proteins. Initiation of the coagulation pathway in vivo is secondary to the exposure of tissue factor (TF) and the formation of TF/VIIa complex which can activate both FIX and FX. This initiation phase is followed by a propagation phase with amplification of thrombin generation. Several control mechanisms exist for localizing fibrin formation to the site of injury including tissue factor pathway inhibitor, protein C system, antithrombin, and glycosaminoglycans on the vessel wall. Fibrinolysis is also a highly regulated system that controls fibrin dissolution. Both constitutive and acquired hemostasic defects exist. The consequences of these abnormalities are highly variable according to the type of defect, and to the genetic and environmental background.

CONCLUSION

Hemostasis is one of the most complex physiological self-defence systems, not only involved in control of blood fluidity but also interfering in major physiopathological processes. The evolution of our knowledge of the physiology of hemostasis has numerous implications for therapy.

Mechanical constraint imposed on plasma membrane through transverse phospholipid imbalance induces reversible actin polymerization via phosphoinositide 3-kinase activation

Platelets were used to explore the effect of membrane curvature induced by phospholipid excess on cell shape and on organization of the actin cytoskeleton. We showed that the addition of short chain analogues of phospholipids to the outer leaflet of plasma membrane of resting platelets immediately induced a shape change with long filopodia formation containing newly polymerized actin. Cells recovered rapidly their discoid shape and their initial F-actin content only with the phosphatidylserine analogue, which was transported to the inner leaflet by aminophospholipid translocase. Filopodia formation and actin polymerization were inhibited in platelets pre-incubated with cytochalasin D. Both wortmannin and LY294002, two unrelated inhibitors of phosphoinositide 3-kinase, considerably reduced actin polymerization and filopodia formation. Phospholipid imbalance was accompanied by a reversible translocation of phosphoinositide 3-kinase from cytoplasm to plasma membrane. In agreement with a role for PI 3-kinase, when phospholipids were added to platelets, PtdIns(3,4)P2 increased two-fold and Akt protein was partly phosphorylated. A similar shape change was also observed in nocodazole-treated L929 fibroblasts which were incubated with the similar phospholipid analogues. In those nucleated cells, where the microtubule cytoskeleton was disrupted, a major actin-dependent membrane extension was induced by addition of short chain phospholipids that required the functional integrity of PI 3-kinase. We conclude that any physical constraint acting on plasma membrane and resulting on local changes in membrane curvature is sufficient to initiate transient actin polymerization via phosphoinositide 3-kinase activation.

Comparison between Ca2+-induced scrambling of various fluorescently labelled lipid analogues in red blood cells

Treatment of red blood cells with calcium and ionomycin causes activation of the lipid scramblase, a putative membrane protein catalysing flip-flop of (phospho)lipids. Various fluorescent 1-oleoyl-2-[6(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino] caproyl (C(6)-NBD) analogues were tested for transbilayer movement across the plasma membrane of red blood cells. Among these phospholipid analogues were phosphatidylgalactose, phosphatidylmaltose and phosphatidylmaltotriose, which were obtained from C(6)-NBD-phosphatidylcholine by phospholipase D-catalysed transphosphatidylation. The inward movement after the onset of scrambling was monitored by extraction of the non-internalized probe with BSA. We demonstrate that both the amino group and the size of the headgroup determine the kinetics of lipid scrambling, and that lipids with a ceramide backbone migrate much more slowly than glycerophospholipids with the same headgroup.

Phosphatidylserine, a death knell

Virtually every cell in the body restricts phosphatidylserine (PS) to the inner leaflet of the plasma membrane by energy-dependent transport from the outer to the inner leaflet of the bilayer. Apoptotic cells of all types rapidly randomize the asymmetric distribution, bringing PS to the surface where it serves as a signal for phagocytosis. A myriad of phagocyte receptors have been implicated in the recognition of apoptotic cells, among them a PS receptor, yet few ligands other than PS have been identified on the apoptotic cell surface. Since apoptosis and the associated exposure of PS on the cell surface is probably over 600 million years old, it is not surprising that evolution has appropriated aspects of this process for specialized purposes such as blood coagulation, membrane fusion and erythrocyte differentiation. Failure to efficiently remove apoptotic cells may contribute to inflammatory responses and autoimmune diseases resulting from chronic, inappropriate exposure of PS.

Heterogeneity in microparticle formation and exposure of anionic phospholipids at the plasma membrane of single adherent platelets

Adherent platelets were examined for their ability to form microvesicles and procoagulant sites for thrombin formation. Epifluorescence and phase-contrast microscopy were employed to visualize shape changes, changes in intracellular Ca(2+) levels ([Ca(2+)](i)), vesiculation of the plasma membrane and appearance of anionic phospholipids in the outer leaflet of the plasma membrane, as probed by annexin V binding. In the absence of extracellular Ca(2+) two stable populations of adherent platelets were observed. The majority of the adherent platelets were fully spread and about 10% remained in a non-spread dendritic state. In the presence of extracellular Ca(2+) vesiculation at the surface of spread platelets occurred at a rather slow rate (10% of the platelets after 20 min) concomitantly with an increase in [Ca(2+)](i) and binding of annexin V. However, a small fraction of the adherent platelets ( approximately 1%) responded much faster. Ionomycin-enhanced influx of Ca(2+) in dendritic platelets resulted in a rapid transformation of these platelets into inflated, balloon-shaped, platelets having a diameter of 2.0+/-0.7 microm without notable microvesicle formation. In contrast, fully spread platelets retained their shape but obtained frayed edges as a result of microvesicle formation. Confocal scanning fluorescence microscopy indicated that annexin V bound to very distinct sites at the outer plasma membrane of spread as well as balloon-shaped platelets. Inhibition of platelet calpain activity suppressed ionomycin-enhanced microvesicle formation and ballooning of platelets, but not annexin V binding. These findings indicate that vesiculation and ballooning, but not the exposure of phosphatidylserine at the outer leaflet of the adherent platelet membrane, are associated with cytoskeleton destruction. Altogether, the data suggest a similar relationship between [Ca(2+)](i) and the formation of platelet procoagulant sites as reported for platelets in suspension. However, the present investigations on single adherent platelets reveal for the first time that adhesion and spreading of platelets is not necessarily associated with the appearance of procoagulant sites. Secondly, an unexpected diversity was observed among adherent platelets with respect to sensitivity to Ca(2+)-induced generation of procoagulant sites and Ca(2+)-induced vesiculation of plasma membrane. It is tempting to speculate that this diversity is of importance for the procoagulant response of platelets to a hemostatic challenge elicited by an injured vessel wall.