Biochemical isolation of a membrane microdomain from resting platelets highly enriched in the plasma membrane glycoprotein CD36

Integrin-Dependent Transient Density Increase in Detergent-Resistant Membrane Rafts in Platelets Activated by Thrombin

Platelet lipid rafts are critical membrane domains for adhesion, aggregation, and clot retraction. Lipid rafts are isolated as a detergent-resistant membrane fraction via sucrose density gradient centrifugation. The platelet detergent-resistant membrane shifted to a higher density on the sucrose density gradient upon thrombin stimulation. The shift peaked at 1 min and returned to the control level at 60 min. During this time, platelets underwent clot retraction and spreading on a fibronectin-coated glass strip. Thrombin induced the transient tyrosine phosphorylation of several proteins in the detergent-resistant membrane raft fraction and the transient translocation of fibrin and myosin to the detergent-resistant membrane raft fraction. The level of phosphatidylserine (36:1) was increased and the level of phosphatidylserine (38:4) was decreased in the detergent-resistant membrane raft fraction via the thrombin stimulation. Furthermore, Glanzmann's thrombasthenia integrin αIIbβ3-deficient platelets underwent no detergent-resistant membrane shift to a higher density upon thrombin stimulation. As the phosphorylation of the myosin regulatory light chain on Ser19 was at a high level in Glanzmann's thrombasthenia resting platelets, thrombin caused no further phosphorylation of the myosin regulatory light chain on Ser19 or clot retraction. These observations suggest that the fibrin-integrin αIIbβ3-myosin axis and compositional change of phosphatidylserine species may be required for the platelet detergent-resistant membrane shift to a higher density upon stimulation with thrombin.

IFITM3 regulates fibrinogen endocytosis and platelet reactivity in nonviral sepsis

Platelets and megakaryocytes are critical players in immune responses. Recent reports suggest infection and inflammation alter the megakaryocyte and platelet transcriptome to induce altered platelet reactivity. We determined whether nonviral sepsis induces differential platelet gene expression and reactivity. Nonviral sepsis upregulated IFN-induced transmembrane protein 3 (IFITM3), an IFN-responsive gene that restricts viral replication. As IFITM3 has been linked to clathrin-mediated endocytosis, we determined whether IFITM3 promoted endocytosis of α-granule proteins. IFN stimulation enhanced fibrinogen endocytosis in megakaryocytes and platelets from Ifitm+/+ mice, but not Ifitm-/- mice. IFITM3 overexpression or deletion in megakaryocytes demonstrated IFITM3 was necessary and sufficient to regulate fibrinogen endocytosis. Mechanistically, IFITM3 interacted with clathrin and αIIb and altered their plasma membrane localization into lipid rafts. In vivo IFN administration increased fibrinogen endocytosis, platelet reactivity, and thrombosis in an IFITM-dependent manner. In contrast, Ifitm-/- mice were completely rescued from IFN-induced platelet hyperreactivity and thrombosis. During murine sepsis, platelets from Ifitm+/+ mice demonstrated increased fibrinogen content and platelet reactivity, which was dependent on IFN-α and IFITMs. Platelets from patients with nonviral sepsis had increases in platelet IFITM3 expression, fibrinogen content, and hyperreactivity. These data identify IFITM3 as a regulator of platelet endocytosis, hyperreactivity, and thrombosis during inflammatory stress.

OxLDL induces membrane structure rearrangement leading to biomechanics alteration and migration deficiency in macrophage

Cholesterol sequestration from plasma membrane has been shown to induce lipid packing disruption, causing actin cytoskeleton reorganization and polymerization, increasing cell stiffness and inducing lysosomal exocytosis in non-professional phagocytes. Similarly, oxidized form of low-density lipoprotein (oxLDL) has also been shown to disrupt lipid organization and packing in endothelial cells, leading to biomechanics alterations that interfere with membrane injury and repair. For macrophages, much is known about oxLDL effects in cell activation, cytokine production and foam cell formation. However, little is known about its impact in the organization of macrophage membrane structured domains and cellular mechanics, the focus of the present study. Treatment of bone marrow-derived macrophages (BMDM) with oxLDL not only altered membrane structure, and potentially the distribution of raft domains, but also induced actin rearrangement, diffuse integrin distribution and cell shrinkage, similarly to observed upon treatment of these cells with MβCD. Those alterations led to decreased migration efficiency. For both treatments, higher co-localization of actin cytoskeleton and GM1 was observed, indicating a similar mechanism of action involving raft-like domain dynamics. Lastly, like MβCD treatment, oxLDL also induced lysosomal spreading in BMDM. We propose that OxLDL induced re-organization of membrane/cytoskeleton complex in macrophages can be attributed to the insertion of oxysterols into the membrane, which lead to changes in lipid organization and disruption of membrane structure, similar to the effect of cholesterol depletion by MβCD treatment. These results indicate that oxLDL can induce physical alterations in the complex membrane/cytoskeleton of macrophages, leading to significant biomechanical changes that compromise cell behavior.

Multidrug Resistance Protein 4 (MRP4/ABCC4): A Suspected Efflux Transporter for Human's Platelet Activation

The main role of platelets is to contribute to hemostasis. However, under pathophysiological conditions, platelet activation may lead to thrombotic events of cardiovascular diseases. Thus, anti-thrombotic treatment is important in patients with cardiovascular disease. This review focuses on a platelet receptor, a transmembrane protein, the Multidrug Resistance Protein 4, MRP4, which contributes to platelet activation by extruding endogenous molecules responsible for their activation and accumulation. The regulation of the intracellular concentration levels of these molecules by MRP4 turned to make the protein suspicious and, at the same time, an interesting regulatory factor of normal platelet function. Especially, the possible role of MRP4 in the excretion of xenobiotic and antiplatelet drugs such as aspirin is discussed, thus imparting platelet aspirin tolerance and correlating the protein with the ineffectiveness of aspirin antiplatelet therapy. Based on the above, this review finally underlines that the development of a highly selective and targeted strategy for platelet MRP4 inhibition will also lead to inhibition of platelet activation and accumulation.

Innate immune receptor clustering and its role in immune regulation

The discovery of receptor clustering in the activation of adaptive immune cells has revolutionized our understanding of the physical basis of immune signal transduction. In contrast to the extensive studies of adaptive immune cells, particularly T cells, there is a lesser, but emerging, recognition that the formation of receptor clusters is also a key regulatory mechanism in host-pathogen interactions. Many kinds of innate immune receptors have been found to assemble into nano- or micro-sized domains on the surfaces of cells. The clusters formed between diverse categories of innate immune receptors function as a multi-component apparatus for pathogen detection and immune response regulation. Here, we highlight these pioneering efforts and the outstanding questions that remain to be answered regarding this largely under-explored research topic. We provide a critical analysis of the current literature on the clustering of innate immune receptors. Our emphasis is on studies that draw connections between the phenomenon of receptor clustering and its functional role in innate immune regulation.

Function of Platelet Glycosphingolipid Microdomains/Lipid Rafts

Lipid rafts are dynamic assemblies of glycosphingolipids, sphingomyelin, cholesterol, and specific proteins which are stabilized into platforms involved in the regulation of vital cellular processes. The rafts at the cell surface play important functions in signal transduction. Recent reports have demonstrated that lipid rafts are spatially and compositionally heterogeneous in the single-cell membrane. In this review, we summarize our recent data on living platelets using two specific probes of raft components: lysenin as a probe of sphingomyelin-rich rafts and BCθ as a probe of cholesterol-rich rafts. Sphingomyelin-rich rafts that are spatially and functionally distinct from the cholesterol-rich rafts were found at spreading platelets. Fibrin is translocated to sphingomyelin-rich rafts and platelet sphingomyelin-rich rafts act as platforms where extracellular fibrin and intracellular actomyosin join to promote clot retraction. On the other hand, the collagen receptor glycoprotein VI is known to be translocated to cholesterol-rich rafts during platelet adhesion to collagen. Furthermore, the functional roles of platelet glycosphingolipids and platelet raft-binding proteins including G protein-coupled receptors, stomatin, prohibitin, flotillin, and HflK/C-domain protein family, tetraspanin family, and calcium channels are discussed.

Platelet membrane lipid rafts protein composition varies following GPVI and CLEC-2 receptors activation

Lipid rafts are membrane microdomains that have been proposed to play an important role in several platelet-signalling cascades, including those mediated by the receptors Glycoprotein VI (GPVI), and C-type lectin domain family 1 member B (CLEC-2), both involved in thrombus formation. We have performed a LC-MS/MS proteomic analysis of lipid rafts isolated from platelets activated through GPVI and CLEC-2 as well as from resting platelets. Our aim was to determine the magnitude of changes in lipid rafts protein composition and to elucidate the relevance of these alterations in platelet function. A number of relevant signalling proteins were found enriched in lipid rafts following platelet activation (such as the tyrosine protein kinases Fyn, Lyn and Yes; the G proteins G(i) and G(z); and cAMP protein kinase). Interestingly, our results indicate that the relative enrichment of lipid rafts in these signalling proteins may not be a consequence of protein translocation to these domains upon platelet stimulation, but the result of a massive loss in cytoskeletal proteins after platelet activation. Thus, this study may help to better understand the effects of platelet activation in the reorganization of lipid rafts and set the basis for further proteomic studies of these membrane microdomains in platelets. SIGNIFICANCE: We performed the first proteomic comparative analysis of lipid rafts- protein composition in platelets activated through GPVI and CLEC-2 receptors and in resting state. We identified a number of signalling proteins essential for platelet activation relatively enriched in platelets activated through both receptors, and we show that lipid rafts reorganization upon platelet activation leads to a loss in cytoskeletal proteins, highly associated to these domains in resting platelets.

Plasma Membrane Lipid Domains as Platforms for Vesicle Biogenesis and Shedding?

Extracellular vesicles (EVs) contribute to several pathophysiological processes and appear as emerging targets for disease diagnosis and therapy. However, successful translation from bench to bedside requires deeper understanding of EVs, in particular their diversity, composition, biogenesis and shedding mechanisms. In this review, we focus on plasma membrane-derived microvesicles (MVs), far less appreciated than exosomes. We integrate documented mechanisms involved in MV biogenesis and shedding, focusing on the red blood cell as a model. We then provide a perspective for the relevance of plasma membrane lipid composition and biophysical properties in microvesiculation on red blood cells but also platelets, immune and nervous cells as well as tumor cells. Although only a few data are available in this respect, most of them appear to converge to the idea that modulation of plasma membrane lipid content, transversal asymmetry and lateral heterogeneity in lipid domains may play a significant role in the vesiculation process. We suggest that lipid domains may represent platforms for inclusion/exclusion of membrane lipids and proteins into MVs and that MVs could originate from distinct domains during physiological processes and disease evolution.

MRP4 (ABCC4) as a potential pharmacologic target for cardiovascular disease

This review focuses on multidrug resistance protein 4 (MRP4 or ABCC4) that has recently been shown to play a role in cAMP homeostasis, a key-pathway in vascular biology and in platelet functions. In vascular system, recent data provide evidence that inhibition of MRP4 prevents human coronary artery smooth muscle cell proliferation in vitro and in vivo, as well as human pulmonary artery smooth muscle cell proliferation in vitro and pulmonary hypertension in mice in vivo. In the heart, MRP4 silencing in adult rat ventricular myocytes results in an increase in intracellular cAMP levels leading to enhanced cardiomyocyte contractility. However, a prolonged inhibition of MRP4 can promote cardiac hypertrophy. In addition, secreted cAMP, through its metabolite adenosine, prevents adrenergically induced cardiac hypertrophy and fibrosis. Finally, MRP4 inhibition in platelets induces a moderate thrombopathy. The localization of MRP4 underlines the emerging concept of cAMP compartmentalization in platelets, which is a major regulatory mechanism in other cells. cAMP storage in platelet dense granules might limit the cAMP cytosolic concentration upon adenylate cyclase activation, a necessary step to induce platelet activation. In this review, we discuss the therapeutic potential of direct pharmacological inhibition of MRP4 in atherothrombotic disease, via its vasodilating and antiplatelet effects.

Role of lipid microdomains in TLR-mediated signalling

Over the last twenty years, evidence has been provided that the plasma membrane is partitioned with microdomains, laterally mobile in the bilayer, providing the necessary microenvironment to specific membrane proteins for signalling pathways to be initiated. We discuss here the importance of such microdomains for Toll-like receptors (TLR) localization and function. First, lipid microdomains favour recruitment and clustering of the TLR machinery partners, i.e. receptors and co-receptors previously identified to be required for ligand recognition and signal transmission. Further, the presence of the so-called Cholesterol Recognition Amino-Acid Consensus (CRAC) sequences in the intracellular juxtamembrane domain of several Toll-like receptors suggests a direct role of cholesterol in the activation process. This article is part of a Special Issue entitled: Lipid-protein interactions.

Co-operation of TLR4 and raft proteins in LPS-induced pro-inflammatory signaling

Toll-like receptor 4 (TLR4) is activated by lipopolysaccharide (LPS), a component of Gram-negative bacteria to induce production of pro-inflammatory mediators aiming at eradication of the bacteria. Dysregulation of the host responses to LPS can lead to a systemic inflammatory condition named sepsis. In a typical scenario, activation of TLR4 is preceded by binding of LPS to CD14 protein anchored in cholesterol- and sphingolipid-rich microdomains of the plasma membrane called rafts. CD14 then transfers the LPS to the TLR4/MD-2 complex which dimerizes and triggers MyD88- and TRIF-dependent production of pro-inflammatory cytokines and type I interferons. The TRIF-dependent signaling is linked with endocytosis of the activated TLR4, which is controlled by CD14. In addition to CD14, other raft proteins like Lyn tyrosine kinase of the Src family, acid sphingomyelinase, CD44, Hsp70, and CD36 participate in the TLR4 signaling triggered by LPS and non-microbial endogenous ligands. In this review, we summarize the current state of the knowledge on the involvement of rafts in TLR4 signaling, with an emphasis on how the raft proteins regulate the TLR4 signaling pathways. CD14-bearing rafts, and possibly CD36-rich rafts, are believed to be preferred sites of the assembly of a multimolecular complex which mediates the endocytosis of activated TLR4.

SIRPα polymorphisms, but not the prion protein, control phagocytosis of apoptotic cells

Prnp(-/-) mice lack the prion protein PrP(C) and are resistant to prion infections, but variable phenotypes have been reported in Prnp(-/-) mice and the physiological function of PrP(C) remains poorly understood. Here we examined a cell-autonomous phenotype, inhibition of macrophage phagocytosis of apoptotic cells, previously reported in Prnp(-/-) mice. Using formal genetic, genomic, and immunological analyses, we found that the regulation of phagocytosis previously ascribed to PrP(C) is instead controlled by a linked locus encoding the signal regulatory protein α (Sirpa). These findings indicate that control of phagocytosis was previously misattributed to the prion protein and illustrate the requirement for stringent approaches to eliminate confounding effects of flanking genes in studies modeling human disease in gene-targeted mice. The plethora of seemingly unrelated functions attributed to PrP(C) suggests that additional phenotypes reported in Prnp(-/-) mice may actually relate to Sirpa or other genetic confounders.

Clot retraction is mediated by factor XIII-dependent fibrin-αIIbβ3-myosin axis in platelet sphingomyelin-rich membrane rafts

Membrane rafts are spatially and functionally heterogenous in the cell membrane. We observed that lysenin-positive sphingomyelin (SM)-rich rafts are identified histochemically in the central region of adhered platelets where fibrin and myosin are colocalized on activation by thrombin. The clot retraction of SM-depleted platelets from SM synthase knockout mouse was delayed significantly, suggesting that platelet SM-rich rafts are involved in clot retraction. We found that fibrin converted by thrombin translocated immediately in platelet detergent-resistant membrane (DRM) rafts but that from Glanzmann's thrombasthenic platelets failed. The fibrinogen γ-chain C-terminal (residues 144-411) fusion protein translocated to platelet DRM rafts on thrombin activation, but its mutant that was replaced by A398A399 at factor XIII crosslinking sites (Q398Q399) was inhibited. Furthermore, fibrin translocation to DRM rafts was impaired in factor XIII A subunit-deficient mouse platelets, which show impaired clot retraction. In the cytoplasm, myosin translocated concomitantly with fibrin translocation into the DRM raft of thrombin-stimulated platelets. Furthermore, the disruption of SM-rich rafts by methyl-β-cyclodextrin impaired myosin activation and clot retraction. Thus, we propose that clot retraction takes place in SM-rich rafts where a fibrin-αIIbβ3-myosin complex is formed as a primary axis to promote platelet contraction.

Increased Gsα within blood cell membrane lipid microdomains in some depressive disorders: an exploratory study

The stimulatory guanine nucleotide binding protein Gs couples many cellular receptors to adenylate cyclase, and the Gsα subunit activates all 9 isoforms of the adenylate cyclase catalytic unit to produce the enzyme product cyclicAMP or cAMP. In prefrontal cortex and cerebellum of unipolar depressive suicides, Rasenick and colleagues have found increased concentrations of Gsα in membrane lipid microdomains (Donati et al., 2008), where the ensconced Gsα is less likely to activate adenylate cyclase by receptor and postreceptor pathways (Allen et al., 2005, 2009). We report that a group of 7 depressed patients (DP-1) had (1) reduced activation of platelet receptor-stimulated adenylate cyclase by both prostaglandins E2 and D2 compared to controls, and (2) reduced postreceptor stimulation of adenylate cyclase by aluminum fluoride ion in both platelets and mononuclear leukocytes when compared to both another group of depressed patients (DP-2, n = 17) and to controls (n = 21). Our observations in the blood cells of the group DP-1 support the findings of Donati et al. (2008), and they reflect the importance of this interaction between the activated Gsα subunit and membrane lipid microdomains in the pathophysiology and treatment of some major depressive disorders.

Timp1 interacts with beta-1 integrin and CD63 along melanoma genesis and confers anoikis resistance by activating PI3-K signaling pathway independently of Akt phosphorylation

BACKGROUND

Anoikis resistance is one of the abilities acquired along tumor progression. This characteristic is associated with metastasis development, since tumorigenic cells must survive independently of cell-matrix interactions in this process. In our laboratory, it was developed a murine melanocyte malignant transformation model associated with a sustained stressful condition. After subjecting melan-a melanocytes to 1, 2, 3 and 4 cycles of anchorage impediment, anoikis resistant cells were established and named 1C, 2C, 3C and 4C, respectively. These cells showed altered morphology and PMA independent cell growth, but were not tumorigenic, corresponding to pre-malignant cells. After limiting dilution of 4C pre-malignant cells, melanoma cell lines with different characteristics were obtained. Previous data from our group showed that increased Timp1 expression correlated with anoikis-resistant phenotype. Timp1 was shown to confer anchorage-independent growth capability to melan-a melanocytes and render melanoma cells more aggressive when injected into mice. However, the mechanisms involved in anoikis regulation by Timp1 in tumorigenic cells are not clear yet.

METHODS

The β1-integrin and Timp1 expression were evaluated by Western blotting and CD63 protein expression by flow cytometry using specific antibodies. To analyze the interaction among Timp1, CD63 and β1-integrin, immunoprecipitation assays were performed, anoikis resistance capability was evaluated in the presence or not of the PI3-K inhibitors, Wortmannin and LY294002. Relative expression of TIMP1 and CD63 in human metastatic melanoma cells was analyzed by real time PCR.

RESULTS

Differential association among Timp1, CD63 and β1-integrins was observed in melan-a melanocytes, 4C pre-malignant melanocytes and 4C11- and 4C11+ melanoma cells. Timp1 present in conditioned medium of melanoma cells rendered melan-a melanocytes anoikis-resistant through PI3-K signaling pathway independently of Akt activation. In human melanoma cell lines, in which TIMP1 and beta-1 integrin were also found to be interacting, TIMP1 and CD63 levels together was shown to correlate significantly with colony formation capacity.

CONCLUSIONS

Our results show that Timp1 is assembled in a supramolecular complex containing CD63 and β1-integrins along melanoma genesis and confers anoikis resistance by activating PI3-K signaling pathway, independently of Akt phosphorylation. In addition, our data point TIMP1, mainly together with CD63, as a potential biomarker of melanoma.

The unfolded von Willebrand factor response in bloodstream: the self-association perspective

von Willebrand factor (vWF) is a multimeric glycoprotein essential for hemostasis after vascular injury, which modulates platelet-surface and platelet–platelet interactions by linking platelet receptors to the extracellular matrix and to each other. The crucial role of vWF in platelet function is particularly apparent when hemodynamic conditions create blood flow with high shear stress. Through multiple functional domains, vWF mediates the attachment of platelets to exposed tissues, where immobilized vWF is able to support a homotypic and/or heterotypic self-association. The self-association of vWF is also supported by a rapidly expanding reservoir of novel evidences that the thiol/disulfide exchange regulates vWF multimer size in the blood circulation. Moreover, in addition to proteolysis and reduction of ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13), the regulation of vWF multimer size and self-association may depend on a disulfide bond reductase activity ascribed to thrombospondin-1 (TSP-1). Along with the classical signaling pathways in activated platelets, evidence is emerging that lipid rafts also play important roles in various phases of hemostasis and thrombosis and facilitate the interaction between the key signaling molecules. Developments in these areas will refine our understanding of the role played by vWF self-association in physiological hemostasis and pathological thrombosis.

Caveolae, caveolin, and cavins: potential targets for the treatment of cardiac disease

Caveolae are omega-shaped membrane invaginations present in essentially all cell types of the cardiovascular system, including endothelial cells, smooth muscle cells, macrophages, cardiac myocytes, and fibroblasts. Numerous functions have been ascribed to this omega-shaped structure. Caveolae are enriched with different signaling molecules and ion channel regulatory proteins and function both in protein trafficking and signal transduction in these cell types. Caveolins are the structural proteins that are necessary for the formation of caveola membrane domains. Mechanistically, caveolins interact with a variety of downstream signaling molecules, as, for example, Src-family tyrosine kinase, p42/44 mitogen-activated protein (MAP) kinase, and endothelial nitric oxide synthase (eNOS) and hold the signal transducers in the inactive condition until activated with proper stimulus. Caveolae are gradually acquiring increasing attention as cellular organelles contributing to the pathogenesis of several structural and functional processes including cardiac hypertrophy, atherosclerosis, and heart failure. At present, very little is known about the role of caveolae in cardiac function and dysfunction, although recent studies with caveolin knock-out mouse have shown that caveolae and caveolins play a pivotal role in various human pathobiological conditions. This review will discuss the possible role and mechanism of action of caveolae and caveolins in different cardiac diseases.

Platelet signaling-a primer

OBJECTIVE

To review the receptors and signal transduction pathways involved in platelet plug formation and to highlight links between platelets, leukocytes, endothelium, and the coagulation system.

DATA SOURCES

Original studies, review articles, and book chapters in the human and veterinary medical fields.

DATA SYNTHESIS

Platelets express numerous surface receptors. Critical among these are glycoprotein VI, the glycoprotein Ib-IX-V complex, integrin α(IIb) β(3) , and the G-protein-coupled receptors for thrombin, ADP, and thromboxane. Activation of these receptors leads to various important functional events, in particular activation of the principal adhesion receptor α(IIb) β(3) . Integrin activation allows binding of ligands such as fibrinogen, mediating platelet-platelet interaction in the process of aggregation. Signals activated by these receptors also couple to 3 other important functional events, secretion of granule contents, change in cell shape through cytoskeletal rearrangement, and procoagulant membrane expression. These processes generate a stable thrombus to limit blood loss and promote restoration of endothelial integrity.

CONCLUSIONS

Improvements in our understanding of how platelets operate through their signaling networks are critical for diagnosis of unusual primary hemostatic disorders and for rational antithrombotic drug design.

Cytoskeletal control of CD36 diffusion promotes its receptor and signaling function

The mechanisms that govern receptor coalescence into functional clusters--often a critical step in their stimulation by ligand--are poorly understood. We used single-molecule tracking to investigate the dynamics of CD36, a clustering-responsive receptor that mediates oxidized LDL uptake by macrophages. We found that CD36 motion in the membrane was spatially structured by the cortical cytoskeleton. A subpopulation of receptors diffused within linear confinement regions whose unique geometry simultaneously facilitated freedom of movement along one axis while increasing the effective receptor density. Co-confinement within troughs enhanced the probability of collisions between unligated receptors and promoted their clustering. Cytoskeleton perturbations that inhibited diffusion in linear confinement regions reduced receptor clustering in the absence of ligand and, following ligand addition, suppressed CD36-mediated signaling and internalization. These observations demonstrate a role for the cytoskeleton in controlling signal transduction by structuring receptor diffusion within membrane regions that increase their collision frequency.

Regulation of platelet function by class B scavenger receptors in hyperlipidemia

Platelets constitutively express class B scavenger receptors CD36 and SR-BI, 2 closely related pattern recognition receptors best known for their roles in lipoprotein and lipid metabolism. The biological role of scavenger receptors in platelets is poorly understood. However, in vitro and in vivo data suggest that class B scavenger receptors modulate platelet function and contribute significantly to thrombosis by sensing pathological or physiological ligands, inducing prothrombotic signaling, and increasing platelet reactivity. Platelet CD36 recognizes a novel family of endogenous oxidized choline phospholipids that accumulate in plasma of hyperlipidemic mice and in plasma of subjects with low high-density lipoprotein levels. This interaction leads to the activation of specific signaling pathways and promotes platelet activation and thrombosis. Platelet SR-BI, on the other hand, plays a critical role in the induction of platelet hyperreactivity and accelerated thrombosis under conditions associated with increased platelet cholesterol content. Intriguingly, oxidized high-density lipoprotein, an SR-BI ligand, can suppress platelet function. These recent findings demonstrate that platelet class B scavenger receptors play roles in thrombosis in dyslipidemia and may contribute to acute cardiovascular events in vivo in hypercholesterolemia.

Palmitoylation of CD36/FAT regulates the rate of its post-transcriptional processing in the endoplasmic reticulum

CD36/FAT is a transmembrane glycoprotein that functions in the cellular uptake of long-chain fatty acids and also as a scavenger receptor. As such it plays an important role in lipid homeostasis and, pathophysiologically, in the progression of type 2 diabetes and atherosclerosis. CD36 expression is tightly regulated at the levels of both transcription and translation. Here we show that its expression and location are also regulated post-translationally, by palmitoylation. Although palmitoylation of CD36 was not required for receptor maturation and cell surface expression, inhibition of palmitoylation either pharmacologically with cerulenin or by mutation of the relevant cysteines delayed processing at the ER and trafficking through the secretory pathway. The absence of palmitoylation also reduced the half life of the CD36 protein. Additionally, the CD36 palmitoylation mutant did not incorporate efficiently into lipid rafts, a site known to be required for its function of fatty acid uptake, and this reduced the efficiency of uptake of oxidized low density lipoprotein. These findings provide an added level of sophistication where translocation of CD36 to the plasma membrane may be physiologically regulated by palmitoylation.

The platelet glycoprotein Ib-IX-V complex anchors lipid rafts to the membrane skeleton: implications for activation-dependent cytoskeletal translocation of signaling molecules

BACKGROUND

The glycoprotein (GP) Ib-IX-V complex attaches platelets to areas of endothelial damage by binding von Willebrand factor (VWF), an interaction that transmits intracellular activation signals. These signals require that the complex associates with both lipid rafts and the membrane cytoskeleton, but it is not clear whether the same GPIb-IX-V subpopulation associates with both structures.

OBJECTIVES

To determine which subpopulation of GPIb-IX-V associates with lipid rafts, and the consequences of that interaction.

METHODS

We analyzed the content of proteins (particularly the GPIb-IX-V complex) and lipids in rafts from detergent lysates of platelets before and after removal of the actin cytoskeleton alone or both the actin cytoskeleton and membrane skeleton (by successive centrifugations of 15,800 x g and 100,000 x g).

RESULTS

In unstimulated platelets, little raft-associated GPIb-IX-V sedimented with the actin skeleton; most was removed by sedimentation of the membrane skeleton. The Src family kinase Lyn followed the same pattern. In VWF-activated platelets, almost all of the GPIb-IX-V complex and Lyn in rafts sedimented with the actin cytoskeleton, consistent with a previously described crosslinking of the membrane and actin skeletal structures following platelet activation. Disruption of the GPIbalpha-filamin linkage with N-ethylmaleimide prevented depletion of raft-associated GPIb-IX-V by skeletal sedimentation. Not all raft-associated proteins and lipids followed this pattern.

CONCLUSION

These results suggest that the raft association and cytoskeletal linkage of the GPIb-IX-V complex are interrelated, and both are required for optimal receptor function, perhaps because raft association attracts signaling proteins and membrane skeletal association allows these proteins to move en masse to new locations.

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.

Patients with long bone fracture have altered Caveolin-1 expression in their peripheral blood mononuclear cells

INTRODUCTION

Fracture triggers a cascade of systemic and local responses including inflammatory mediator signaling, chemotaxis, osteogenic cell recruitment, differentiation and proliferation at the fracture site. Early signaling between immune cells and repair cells in fracture repair is not well understood. Caveolin-1, a 21-24 kDa membrane protein plays key roles in transmembrane signaling. This study was to investigate the expression of caveolin-1 in human peripheral blood mononuclear cells (PBMNCs) following long bone fracture.

METHODS

PBMNCs were obtained from healthy volunteers or fracture patients at three time points following fracture by density-gradient-centrifugation procedure. Caveolin-1 gene expression and protein characterization was examined by semi-quantitative RT-PCR, immunocytochemistry and Western blot analysis.

RESULTS

Caveolin-1 mRNA and protein was expressed at low levels in the PBMNCs of non-fracture samples. In contrast, caveolin-1 expression was greatly increased in the PBMNCs of fracture patients 9-12 days and reduced at 16-21 days following long bone fracture.

CONCLUSION

The identification of caveolin-1 in PBMNCs and osteoblasts makes this cellular domain a new focus for further investigation. We speculate that caveolin-1 expression in PBMNCs and osteoblasts play an important role in signal transduction during the early stages of fracture healing and may be an indicator for normal or abnormal fracture repair.

Phospholipase D in human platelets: presence of isoenzymes and participation of autocrine stimulation during thrombin activation

Phospholipase D (PLD), which hydrolyzes phosphatidylcholine to phosphatidic acid (PA) and choline, is present in human platelets. Thrombin and other agonists have been shown to activate PLD but the precise mechanisms of activation and PLDs role in platelet activation remains unclear. We measured thrombin-stimulated PLD activity in platelets as formation of phosphatidylethanol. Since no specific PLD inhibitors exist, we investigated possible roles for PLD in platelets by correlating PLD activity with platelet responses such as thrombin-mediated secretion and F-actin formation (part of platelet shape change). Extracellular Ca2+ potentiated thrombin-stimulated PLD, but did not stimulate PLD in the absence of thrombin. Thrombin-induced PLD activity was enhanced by secreted ADP and binding of fibrinogen to its receptors. In contrast to others, we also found a basal PLD activity. Comparison of time courses and dose responses of platelets with PLD showed many points of correlation between PLD activation and lysosomal secretion and F-actin formation. The finding of different PLD activities suggested that different PLD isoenzymes exist in platelets as reported for other cells. Here we present evidence for the presence of both PLD1 and PLD2 in platelets by use of specific antibodies with immunoblotting and immunohistochemistry. Both isoforms were randomly localized in resting platelets, but became rapidly translocated to the proximity of the plasma membrane upon thrombin stimulation, thus indicating a role for PLD in platelet activation.

Lipoprotein binding preference of CD36 is altered by filipin treatment

The class B scavenger receptor CD36 binds multiple ligands, including oxidized and native lipoprotein species. CD36 and the related receptor SR-B1 have been localized to caveolae, domains that participate in cell signaling, transcytosis, and regulation of cellular cholesterol homeostasis. Previous work has indicated that the ligand preference of CD36 may depend on the cell type in which it is expressed. To determine if the presence or absence of caveolae is the determining factor for lipoprotein preference, we treated CHO-CD36 and C32 cells with filipin. Filipin treatment rapidly increased the binding capacity of CD36 for the native lipoproteins HDL and LDL, but did not affect the binding capacity of CD36 for oxidized LDL. Filipin treatment affected the distribution of caveolin and CD36 suggesting that the presence caveolae may modulate the ligand preference of CD36. However, its molecular mechanism how CD36 and caveolin interaction in regulating lipoprotein transport remains to be further studied.

CD36: implications in cardiovascular disease

CD36 is a broadly expressed membrane glycoprotein that acts as a facilitator of fatty acid uptake, a signaling molecule, and a receptor for a wide range of ligands, including apoptotic cells, modified forms of low density lipoprotein, thrombospondins, fibrillar beta-amyloid, components of Gram positive bacterial walls and malaria infected erythrocytes. CD36 expression on macrophages, dendritic and endothelial cells, and in tissues including muscle, heart, and fat, suggest diverse roles, and indeed, this is truly a multi-functional receptor involved in both homeostatic and pathological conditions. Despite an impressive increase in our knowledge of CD36 functions, in depth understanding of the mechanistic aspects of this protein remains elusive. This review focuses on CD36 in cardiovascular disease-what we know, and what we have yet to learn.

Membrane lipid rafts coordinate estrogen-dependent signaling in human platelets

The impact of estrogens on the viability of cardiovascular system and their ability to regulate platelet function is still an open and debated question. We have previously shown that estrogen is able to significantly potentiate the aggregation induced by low doses of thrombin and to initiate a rapid and reversible signaling pathway mediated by ERbeta-directed activation of the tyrosine kinases Src and Pyk2 at the level of the plasma membrane. Lipid rafts are critical, cholesterol-enriched membrane domains, which play a major role in blood platelet activation processes. In this work, we investigated the role of lipid rafts in 17beta-estradiol signaling in human platelets. We observed that membrane rafts were essential for both 17beta-estradiol-dependent potentiation of platelet aggregation induced by subthreshold concentrations of thrombin and 17beta-estradiol-induced phosphorylation of Src. 17beta-estradiol caused the reversible translocation of ERbeta to the raft fractions and promoted the rapid and transient recruitment to, and activation within the membrane raft domains of the tyrosine kinases Src and Pyk2. The raft integrity was essential with this respect, as these effects of 17beta-estradiol were completely inhibited by cholesterol depletion. This paper provides evidence for the first time that membrane lipid rafts coordinate estrogen signaling in human platelets.

Lipid rafts facilitate the interaction of PECAM-1 with the glycoprotein VI-FcR gamma-chain complex in human platelets

Glycoprotein (GP) VI, the main signaling receptor for collagen on platelets, is expressed in complex with the FcR gamma-chain. The latter contains an immunoreceptor tyrosine-based activation motif, which becomes phosphorylated, initiating a signaling cascade leading to the rapid activation and aggregation of platelets. Previous studies have shown that signaling by immunoreceptor tyrosine-based activation motif-containing receptors is counteracted by signals from receptors with immunoreceptor tyrosine-based inhibitory motifs. Here we show, by immunoprecipitation, that the GPVI-FcR gamma-chain complex associates with the immunoreceptor tyrosine-based inhibitory motif-containing receptor, PECAM-1. In platelets stimulated with collagen-related peptide (CRP-XL), tyrosine phosphorylation of PECAM-1 precedes that of the FcR gamma-chain, implying direct regulation of the former. The GPVI-FcR gamma-chain complex and PECAM-1 were present in both lipid raft and soluble fractions in human platelets; this distribution was unaltered by activation with CRP-XL. Their association occurred in lipid rafts and was lost after lipid raft depletion using methyl-beta-cyclodextrin. We propose that lipid raft clustering facilitates the interaction of PECAM-1 with the GPVI-FcR gamma-chain complex, leading to the down-regulation of the latter.

The association between CD36 and Lyn protein tyrosine kinase is mediated by lipid

CD36 is a transmembrane glycoprotein receptor that engages in signal transduction implicated in important physiological and pathophysiological events. CD36 in platelets has been shown physically and functionally to associate with members of the Src family of protein tyrosine kinases, Fyn, Lyn, and Yes, but the nature of this important association has never been rigorously examined. Here, we show that CD36 does not associate with Lyn through a protein-mediated interaction. In COS cells transfected with both CD36 and Lyn these molecules did not co-precipitate, suggesting a requirement for an intermediary molecule absent from the COS cells. Yeast two-hybrid analysis confirmed that the carboxylterminal cytoplasmic tail of CD36 did not bind Lyn directly, and no Lyn binding protein bound to CD36 in a cDNA library screen. Conversely, when the CD36-Lyn association seen in platelets was analysed by biophysical parameters, dissociation occurred at 37 degrees C and also by solubilisation in octylglucoside, indicative of a lipid-mediated association. Since both CD36 and Lyn are enriched in Triton X-100-insoluble rafts at the plasma membrane, these findings point to the importance of raft-associated lipids in CD36-mediated signal transduction.

The shedding activity of ADAM17 is sequestered in lipid rafts

The tumor necrosis factor-alpha (TNF) converting enzyme (ADAM17) is a metalloprotease-disintegrin responsible for the cleavage of several biologically active transmembrane proteins. However, the substrate specificity of ADAM17 and the regulation of its shedding activity are still poorly understood. Here, we report that during its transport through the Golgi apparatus, ADAM17 is included in cholesterol-rich membrane microdomains (lipid rafts) where its prodomain is cleaved by furin. Consequently, ADAM17 shedding activity is sequestered in lipid rafts, which is confirmed by the fact that metalloproteinase inhibition increases the proportion of ADAM17 substrates (TNF and its receptors TNFR1 and TNFR2) in lipid rafts. Membrane cholesterol depletion increases the ADAM17-dependent shedding of these substrates demonstrating the importance of lipid rafts in the control of this process. Furthermore, ADAM17 substrates are present in different proportions in lipid rafts, suggesting that the entry of each of these substrates in these particular membrane microdomains is specifically regulated. Our data support the idea that one of the mechanisms regulating ADAM17 substrate cleavage involves protein partitioning in lipid rafts.

The active metabolite of Clopidogrel disrupts P2Y12 receptor oligomers and partitions them out of lipid rafts

P2Y12, a G protein-coupled receptor that plays a central role in platelet activation has been recently identified as the receptor targeted by the antithrombotic drug, clopidogrel. In this study, we further deciphered the mechanism of action of clopidogrel and of its active metabolite (Act-Met) on P2Y12 receptors. Using biochemical approaches, we demonstrated the existence of homooligomeric complexes of P2Y12 receptors at the surface of mammalian cells and in freshly isolated platelets. In vitro treatment with Act-Met or in vivo oral administration to rats with clopidogrel induced the breakdown of these oligomers into dimeric and monomeric entities in P2Y12 expressing HEK293 and platelets respectively. In addition, we showed the predominant association of P2Y12 oligomers to cell membrane lipid rafts and the partitioning of P2Y12 out of rafts in response to clopidogrel and Act-Met. The raft-associated P2Y12 oligomers represented the functional form of the receptor, as demonstrated by binding and signal transduction studies. Finally, using a series of receptors individually mutated at each cysteine residue and a chimeric P2Y12/P2Y13 receptor, we pointed out the involvement of cysteine 97 within the first extracellular loop of P2Y12 in the mechanism of action of Act-Met.

CO as a cellular signaling molecule

Many biological functions of heme oxygenase (HO), such as cytoprotection against oxidative stress, vasodilation, neurotransmission in the central or peripheral nervous systems, and anti-inflammatory, anti-apoptotic, or anti-proliferative potential, have been attributed to its enzymatic byproduct carbon monoxide (CO), although roles for biliverdin/bilirubin and iron have also been proposed. In addition to these well-characterized effects, recent findings reveal that HO-derived CO may act as an oxygen sensor and circadian modulator of heme biosynthesis. In lymphocytes, CO may participate in regulatory T cell function. A number of the known signaling effects of CO depend on stimulation of soluble guanylate cyclase and/or activation of mitogen-activated protein kinases (MAPK). Furthermore, modulation of caveolin-1 status may serve as an essential component of certain aspects of CO action, such as growth control. In this review, we summarize recent findings of the beneficial or detrimental effects of endogenous CO with an emphasis on the signaling pathways and downstream targets that trigger the action of this gas.

Opposite effect of caveolin-1 in the metabolism of high-density and low-density lipoproteins

Receptors of the scavenger class B family were reported to be localized in caveolae, the cell surface microdomains rich in free cholesterol and glycosphyngolipids, which are characterized by the presence of caveolin-1. Parenchymal hepatic and hepatoma HepG2 cells express very low levels of caveolin-1. In the present study, stable transformants of HepG2 cells expressing caveolin-1 were generated to address the effect of caveolin-1 on receptor activity. Compared to normal cells, these cells show higher (125)I-bovine serum albumin (BSA) uptake and cholesterol efflux, two indicators of functional caveolae. By immunoprecipitation, cell fractionation and confocal analyses, we found that caveolin-1 is well colocalized with the cluster of differentiation-36 (CD36) and the low-density lipoprotein (LDL) receptor (LDLr) but to a lesser extent with the scavenger receptor class B type I (SR-BI) in HepG2 cells expressing caveolin-1. However, caveolin-1 expression favors the dimerization of SR-BI. Two clones of cells expressing caveolin-1 were investigated for their lipoprotein metabolism activity. Compared to normal cells, these cells show a 71-144% increase in (125)I-LDL degradation. The analysis of the cholesteryl esters (CE)-selective uptake (CE association minus protein association) revealed that the expression of caveolin-1 in HepG2 cells decreases by 59%-73% LDL-CE selective uptake and increases high-density lipoprotein (HDL)-CE selective uptake by 44%-66%. We conclude that the expression of caveolin-1 in HepG2 cells moves the balance of LDL degradation/CE selective uptake towards degradation and favors HDL-CE selective uptake. Thus, in the normal hepatic parenchymal situation where caveolin-1 is poorly expressed, LDL-CE selective uptake is the preferred pathway.

Adhesive surface determines raft composition in platelets adhered under flow

Adhesion to von Willebrand factor (VWF) induces platelet spreading, whereas adhesion to collagen induces aggregation. Here we report that cholesterol-rich domains (CRDs) or rafts play a critical role in clustering of receptors that control these responses. Platelets adhered to VWF and collagen show CRDs concentrated in filopodia which contain both the VWF receptor glycoprotein (GP) Ibalpha and the collagen receptor GPVI. Biochemical analysis of CRDs shows a threefold enrichment of GPIbalpha (but not GPVI) in VWF-adhered platelets and a fourfold enrichment of GPVI (but not GPIbalpha) in collagen-adhered platelets. Depletion of cholesterol (i) leaves the initial adhesion unchanged, (ii) inhibits spreading on VWF and aggregate formation on collagen, (iii) leaves filopodia formation intact, and (iv) reduces the localization in filopodia of GPIbalpha but not of GPVI. These data show that the adhesive substrate determines the composition of CRDs, and that cholesterol is crucial for redistribution of GPIbalpha but not of GPVI.

The platelet receptor for type III collagen (TIIICBP) is present in platelet membrane lipid microdomains (rafts)

Platelet interactions with collagen are orchestrated by the presence or the migration of platelet receptor(s) for collagen into lipid rafts, which are specialized lipid microdomains from the platelet plasma membrane enriched in signalling proteins. Electron microscopy shows that in resting platelets, TIIICBP, a receptor specific for type III collagen, is present on the platelet membrane and associated with the open canalicular system, and redistributes to the platelet membrane upon platelet activation. After platelet lysis by 1% Triton X-100 and the separation of lipid rafts on a discontinuous sucrose gradient, TIIICBP is recovered in lipid raft-containing fractions and Triton X-100 insoluble fractions enriched in cytoskeleton proteins. Platelet aggregation, induced by type III collagen, was inhibited after disruption of the lipid rafts by cholesterol depletion, whereas platelet adhesion under static conditions did not require lipid raft integrity. These results indicate that TIIICBP, a platelet receptor involved in platelet interaction with type III collagen, is localized within platelet lipid rafts where it could interact with other platelet receptors for collagen (GP VI and alpha2beta1 integrin) for efficient platelet activation.

Receptors, rafts, and microvesicles in thrombosis and inflammation

Hemostasis at sites of blood vessel injury and its pathologic counterpart, thrombosis, involve a complex interplay between several blood elements: soluble proteins of the blood coagulation system, blood cells (most prominently platelets) and cell fragments, and elements of the vessel wall (endothelial cells and, at sites of injury, the exposed matrix and deeper cellular components). In this review, we focus on ways in which specialized membrane microdomains known as lipid rafts are involved in various phases of hemostasis and thrombosis.

Triacylglycerol is synthesized in a specific subclass of caveolae in primary adipocytes

A principal metabolic function of adipocytes is to synthesize triacylglycerol (TG) from exogenous fatty acids. The level of fatty acids has to be tightly controlled in the adipocyte, as they can act as detergents that rapidly dissolve the plasma membrane, causing cell lysis if allowed to accumulate. Fatty acids therefore have to be efficiently converted to TG and stored in the central lipid droplet. We report that in intact primary adipocytes exogenous oleic acid was taken up and directly converted to TG in the plasma membrane, in a novel subclass of caveolae that specifically contains the protein perilipin. Isolated caveolae catalyzed de novo TG synthesis from oleic acid and glycerol 3-phosphate. Electron microscopy revealed the presence of caveolin and perilipin in caveolae and in lipid-laden bulbs in the plasma membrane, and fluorescence microscopy demonstrated colocalization of fatty acids/TG with caveolin and perilipin at the plasma membrane. A second caveolae fraction was isolated, which lacked perilipin and the triacylglycerol synthesizing enzymes. Both caveolae fractions contained caveolin-1 and the insulin receptor. The findings demonstrate that specific subclasses of caveolae carry out specific functions in cell metabolism. In particular, triacylglycerol is synthesized at the site of fatty acid entry in one of these caveolae classes.

Vectorial proteomics reveal targeting, phosphorylation and specific fragmentation of polymerase I and transcript release factor (PTRF) at the surface of caveolae in human adipocytes

Caveolae, the specialized invaginations of plasma membranes, formed sealed vesicles with outwards-orientated cytosolic surface after isolation from primary human adipocytes. This morphology allowed differential, vectorial identification of proteins at the opposite membrane surfaces by proteolysis and MS. Extracellular-exposed caveolae-specific proteins CD36 and copper-containing amine oxidase were concealed inside the vesicles and resisted trypsin treatment. The cytosol-orientated caveolins were efficiently digested by trypsin, producing peptides amenable to direct MS sequencing. Isolation of peripheral proteins associated with the cytosolic surface of caveolae revealed a set of proteins that contained nuclear localization signals, leucine-zipper domains and PEST (amino acid sequence enriched in proline, glutamic acid, serine and threonine) domains implicated in regulation by proteolysis. In particular, PTRF (polymerase I and transcript release factor) was found as a major caveolae-associated protein and its co-localization with caveolin was confirmed by immunofluorescence confocal microscopy. PTRF was present at the surface of caveolae in the intact form and in five different truncated forms. Peptides (44 and 45 amino acids long) comprising both the PEST domains were sequenced by nanospray-quadrupole-time-of-flight MS from the full-length PTRF, but were not found in the truncated forms of the protein. Two endogenous cleavage sites corresponding to calpain specificity were identified in PTRF; one of them was in a PEST domain. Both cleavage sites were flanked by mono- or diphosphorylated sequences. The phosphorylation sites were localized to Ser-36, Ser-40, Ser-365 and Ser-366 in PTRF. Caveolae of human adipocytes are proposed to function in targeting, relocation and proteolytic control of PTRF and other PEST-domain-containing signalling proteins.

Rho and Rho-kinase Mediate Thrombin-induced Phosphatidylinositol 4-Phosphate 5-Kinase Trafficking in Platelets

Phosphatidylinositol 4-phosphate 5-kinase (PIP5K) catalyzes the rate-limiting step in the production of phosphatidylinositol 4,5-bisphosphate (PIP(2)), a signaling phospholipid that contributes to actin dynamics. We have shown in transfected tissue culture cells that PIP5K translocates from the cytosol to the plasma membrane following agonist-induced stimulation of Rho family GTPases. Nonetheless, it is unclear whether Rho GTPases induce PIP5K relocalization in platelets. We used PIP5K isoform-specific immunoblotting and lipid kinase assays to examine the intracellular localization of PIP5K in resting and activated platelets. Using differential centrifugation to separate the membrane skeleton, actin filaments and associated proteins, and cytoplasmic fractions, we found that PIP5K isoforms were translocated from cytosol to actin-rich fractions following stimulation of the thrombin receptor. PIP5K translocation was detectable within 30 s of stimulation and was complete by 2-5 min. This agonist-induced relocalization and activation of PIP5K was inhibited by 8-(4-parachlorophenylthio)-cAMP, a cAMP analogue that inhibits Rho and Rac. In contrast, 8-(4-parachlorophenylthio)-cGMP, a cGMP analogue that inhibits Rac but not Rho, did not affect PIP5K translocation and activation. This suggests that Rho GTPase may be an essential regulator of PIP5K in platelets. Consistent with this hypothesis, we found that C3 exotoxin (a Rho-specific inhibitor) and HA1077 (an inhibitor of the Rho effector, Rho-kinase) also eliminated PIP5K activation and trafficking into the membrane cytoskeleton. Thus, these data indicate that Rho GTPase and its effector Rho-kinase have an intimate relationship with the trafficking and activation of platelet PIP5K. Moreover, these data suggest that relocalization of platelet PIP5K following agonist stimulation may play an important role in regulating the assembly of the platelet cytoskeleton.

The role of scavenger receptor class B type I (SR-BI) in lipid trafficking. defining the rules for lipid traders

The scavenger receptor class B type I (SR-BI) is a 509-amino acid, 82 kDa glycoprotein, with two cytoplasmic C- and N-terminal domains separated by a large extracellular domain. The aim of this review is to define the role of SR-BI as a lipoprotein receptor responsible for selective uptake of cholesteryl esters (CE) from high density lipoprotein (HDL) and low density lipoprotein (LDL) and free cholesterol (FC) efflux to lipoprotein acceptors. These activities depend on lipoprotein binding to its extracellular domain and subsequent lipid exchange at the plasma membrane. CE selective uptake supplies cholesterol to liver and steroidogenic tissues, for biliary cholesterol secretion and steroid hormone synthesis. Genetically modified mice have confirmed SR-BI's major role in tissue cholesterol uptake and in reverse cholesterol transport, i.e. cholesterol turnover. Accordingly, cellular cholesterol level, estrogens and trophic hormones regulate SR-BI expression by both transcriptional and post-transcriptional mechanisms. Importantly, mouse SR-BI overexpression has both corrective and preventive effects on atherosclerosis. Human SR-BI has very similar tissue distribution, binding properties and lipid transfer activities compared to rodent SR-BI. However, human plasma has most of its cholesterol in LDL. Thus, there is considerable interest to develop anti-atherogenic strategies involving human SR-BI-mediated increases in reverse cholesterol transport through HDL and/or LDL.

Important role of raft aggregation in the signaling events of cold-induced platelet activation

When human platelets are chilled below 20 degrees C, they undergo cold-induced activation. We have previously shown that cold activation correlates with the main phospholipid phase transition (10-20 degrees C) and induces the formation of large raft aggregates. In addition, we found that the glycoprotein CD36 is selectively enriched within detergent-resistant membranes (DRMs) of cold-activated platelets and is extremely sensitive to treatment with methyl-beta-cyclodextrin (MbetaCD). Here, we further studied the partitioning of downstream signaling molecules within the DRMs. We found that the phospholipase Cgamma2 (PLCgamma2) and the protein tyrosine kinase Syk do not partition exclusively within the DRMs, but their distribution is perturbed by cholesterol extraction. In addition, PLCgamma2 activity increases in cold-activated cells compared to resting platelets and is entirely inhibited after treatment with MbetaCD. The Src-family protein tyrosine kinases Src and Lyn preferentially partition within the DRMs and are profoundly affected by removal of cholesterol. These kinases are non-redundant in cold-activation. CD36, active Lyn, along with inactive Src and PLCgamma2 co-localize in small raft complexes in resting platelets. Cold-activation induces raft aggregation, resulting in changes in the activity of these proteins. These data suggest a crucial role of raft aggregation in the early events of cold-induced platelet activation.

Lipid rafts and HIV pathogenesis: virion-associated cholesterol is required for fusion and infection of susceptible cells

We have shown that HIV budding occurs at cholesterol-rich membrane microdomains called lipid rafts (Nguyen and Hildreth, J Virol 2000;74:3264-3272). This observation prompted us to examine the role in HIV entry of cholesterol in the membrane of cells. We recently reported that host cell cholesterol is required for HIV infection (Liao et al., AIDS Res Hum Retroviruses 2001;17:1009-1019). In the present study we examined the role of virion-associated cholesterol in HIV infection by modulating the cholesterol content of virions and infected cells with 2-hydoxypropyl-beta-cyclodextrin (beta-cyclodextrin). Our results show that removal of cholesterol from the membrane of HIV-infected cells dramatically lowered virus release and that virions released from cholesterol-depleted cells are minimally infectious. Exposure of infectious HIV particles to beta-cyclodextrin resulted in a dose-dependent inactivation of the virus. In both cases, the effect was attributable to loss of cholesterol and could be reversed by replenishing cholesterol. beta-Cyclodextrin-treated, noninfectious HIV retained its ability to bind cells. Western blot, p24 core ELISA, and reverse transcription assays indicated that virions remained intact after treatment with beta-cyclodextrin at concentrations that abolished infectivity. Electron microscopy revealed that beta-cyclodextrin-treated HIV had a morphology very similar to that of untreated virus. R18 fluorescence dequenching studies showed that beta-cyclodextrin-treated HIV did not fuse to the membrane of susceptible cells. Dequenching was restored by replenishing virion-associated cholesterol. The results indicate that cholesterol in HIV particles is strictly required for fusion and infectivity. These observations in combination with those of past studies indicate beta-cyclodextrin to be an excellent candidate for use as a chemical barrier for AIDS prophylaxis.

Resistance of cellular membrane antigens to solubilization with Triton X-100 as a marker of their association with lipid rafts--analysis by flow cytometry

Lipid rafts are specialized micro-domains of the plasma membrane enriched in glycosphingolipid and cholesterol that play important role in signal transduction, membrane trafficking, and cell adhesion. A distinct feature of lipid rafts is their resistance to solubilization with non-ionic detergent Triton X-100 (TX-100). In this study, we used flow cytometry to evaluate TX-100 resistance of 74 cell membrane molecules expressed on normal human peripheral blood lymphocytes (PBL), thymocytes, and 12 lymphoid cell lines. Resistance of membrane molecules to solubilization with TX-100 was determined by comparing the intensities of fluorescence of cells treated with TX-100 or left untreated. The majority of antigens analyzed were easily solubilized with TX-100 that resulted in decreased fluorescence intensity. However, a group of antigens showed TX-100 resistance in the range of 20-100%. These included all glycosylphosphatidylinositol (GPI)-anchored antigens under study, as well as some glycolipid and trans-membrane antigens. With the few exceptions, antigen resistance to solubilization with TX-100 was stable parameter, which did not depend on cell type in which it was analyzed. There was a good correspondence between the antigens showing resistance to solubilization with TX-100 as evaluated by our flow cytometry method, and the antigens that were previously demonstrated in detergent-resistant membranes using a more standard method of physical fractionation. Taken collectively, our data suggest that flow cytometry is a useful method for rapid evaluation of the possible association of a membrane antigen with lipid rafts.

RETRACTED: The glycoprotein Ib-IX-V complex mediates localization of factor XI to lipid rafts on the platelet membrane

Factor XI binds to activated platelets where it is efficiently activated by thrombin. The factor XI receptor is the platelet membrane glycoprotein (GP) Ib-IX-V complex (Baglia, F. A., Badellino, K. O., Li, C. Q., Lopez, J. A., and Walsh, P. N. (2002) J. Biol. Chem. 277, 1662-1668), a significant fraction of which exists within lipid rafts on stimulated platelets (Shrimpton, C. N., Borthakur, G., Larrucea, S., Cruz, M. A., Dong, J. F., and Lopez, J. A. (2002) J. Exp. Med. 196, 1057-1066). Lipid rafts are membrane microdomains enriched in cholesterol and sphingolipids implicated in localizing membrane ligands and in cellular signaling. We now show that factor XI was localized to lipid rafts in activated platelets ( approximately 8% of total bound) but not in resting platelets. Optimal binding of factor XI to membrane rafts required prothrombin (and Ca2+) or high molecular weight kininogen (and Zn2+), which are required for factor XI binding to platelets. An antibody to GPIb (SZ-2) that disrupts factor XI binding to the GPIb-IX-V complex also disrupted factor XI-raft association. The isolated recombinant Apple 3 domain of factor XI, which mediates factor XI binding to platelets, also completely displaces factor XI from membrane rafts. To investigate the physiological relevance of the factor XI-raft association, the structural integrity of lipid rafts was disrupted by cholesterol depletion utilizing methyl-beta-cyclodextrin. Cholesterol depletion completely prevented FXI binding to lipid rafts, and initial rates of factor XI activation by thrombin on activated platelets were inhibited >85%. We conclude that factor XI is localized to GPIb in membrane rafts and that this association is important for promoting the activation of factor XI by thrombin on the platelet surface.