Production of phosphatidylinositol 3,4,5-trisphosphate and phosphatidic acid in platelet rafts: evidence for a critical role of cholesterol-enriched domains in human platelet activation

Antiplatelet activity of astaxanthin in control- and high cholesterol-fed rats mediated by down-regulation of P2Y12, inhibition of NF-κB, and increasing intracellular levels of cAMP

This study evaluated the antiplatelet effect of the plant carotenoid, astaxanthin (ASTX) in rats fed either control or high cholesterol plus cholic acid diet (HCCD) and possible underlying mechanisms. Adult male Wistar rats were divided into four groups (n = 8/each), namely, control (fed normal diet), control + ASTX (10 mg/kg/day), HCCD-fed rats, and HCCD + ASTX-treated rats. Diets and treatments were orally administered daily for 30 days. In both control and HCCD-fed rats, ASTX significantly increased fecal levels of triglycerides and cholesterol, reduced platelet count, prolonged bleeding time, and inhibited platelet aggregation. It also reduced platelet levels of reactive oxygen species (ROS) and Bcl-2; thromboxane B2 (TXB2) release; and the expression of P2Y₁₂, P-selectin, and CD36 receptors. Moreover, the activity NF-κB p65 and Akt was inhibited. Concomitantly, it increased the protein levels of cleaved caspase-3 and vasodilator-stimulated phosphoprotein (p-VASP) as well as intracellular levels of cAMP. However, in HCCD-fed rats, the effects of ASTX were associated with reduced serum levels of ox-LDL-c and fasting plasma glucose levels. In conclusion, antiplatelet effects of ASTX involve ROS scavenging, inhibiting NF-κB activity, down-regulating P2Y₁₂ expression, and increasing intracellular levels of cAMP that are attributed to its antioxidant, hypolipidemic, and anti-inflammatory effects.

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.

'Bending' models of halotropism: incorporating protein phosphatase 2A, ABCB transporters, and auxin metabolism

Salt stress causes worldwide reductions in agricultural yields, a problem that is exacerbated by the depletion of global freshwater reserves and the use of contaminated or recycled water (i.e. effluent water). Additionally, salt stress can occur as cultivated areas are subjected to frequent rounds of irrigation followed by periods of moderate to severe evapotranspiration, which can result in the heterogeneous aggregation of salts in agricultural soils. Our understanding of the later stages of salt stress and the mechanisms by which salt is transported out of cells and roots has greatly improved over the last decade. The precise mechanisms by which plant roots perceive salt stress and translate this perception into adaptive, directional growth away from increased salt concentrations (i.e. halotropism), however, are not well understood. Here, we provide a review of the current knowledge surrounding the early responses to salt stress and the initiation of halotropism, including lipid signaling, protein phosphorylation cascades, and changes in auxin metabolism and/or transport. Current models of halotropism have focused on the role of PIN2- and PIN1-mediated auxin efflux in initiating and controlling halotropism. Recent studies, however, suggest that additional factors such as ABCB transporters, protein phosphatase 2A activity, and auxin metabolism should be included in the model of halotropic growth.

SDF-1α/CXCR4 Signaling in Lipid Rafts Induces Platelet Aggregation via PI3 Kinase-Dependent Akt Phosphorylation

Stromal cell-derived factor-1α (SDF-1α)-induced platelet aggregation is mediated through its G protein-coupled receptor CXCR4 and phosphatidylinositol 3 kinase (PI3K). Here, we demonstrate that SDF-1α induces phosphorylation of Akt at Thr308 and Ser473 in human platelets. SDF-1α-induced platelet aggregation and Akt phosphorylation are inhibited by pretreatment with the CXCR4 antagonist AMD3100 or the PI3K inhibitor LY294002. SDF-1α also induces the phosphorylation of PDK1 at Ser241 (an upstream activator of Akt), GSK3β at Ser9 (a downstream substrate of Akt), and myosin light chain at Ser19 (a downstream element of the Akt signaling pathway). SDF-1α-induced platelet aggregation is inhibited by pretreatment with the Akt inhibitor MK-2206 in a dose-dependent manner. Furthermore, SDF-1α-induced platelet aggregation and Akt phosphorylation are inhibited by pretreatment with the raft-disrupting agent methyl-β-cyclodextrin. Sucrose density gradient analysis shows that 35% of CXCR4, 93% of the heterotrimeric G proteins Gαi-1, 91% of Gαi-2, 50% of Gβ and 4.0% of PI3Kβ, and 4.5% of Akt2 are localized in the detergent-resistant membrane raft fraction. These findings suggest that SDF-1α/CXCR4 signaling in lipid rafts induces platelet aggregation via PI3K-dependent Akt phosphorylation.

Phospholipase C and D regulation of Src, calcium release and membrane fusion during Xenopus laevis development

This review emphasizes how lipids regulate membrane fusion and the proteins involved in three developmental stages: oocyte maturation to the fertilizable egg, fertilization and during first cleavage. Decades of work show that phosphatidic acid (PA) releases intracellular calcium, and recent work shows that the lipid can activate Src tyrosine kinase or phospholipase C during Xenopus fertilization. Numerous reports are summarized to show three levels of increase in lipid second messengers inositol 1,4,5-trisphosphate and sn 1,2-diacylglycerol (DAG) during the three different developmental stages. In addition, possible roles for PA, ceramide, lysophosphatidylcholine, plasmalogens, phosphatidylinositol 4-phosphate, phosphatidylinositol 5-phosphate, phosphatidylinositol 4,5-bisphosphate, membrane microdomains (rafts) and phosphatidylinositol 3,4,5-trisphosphate in regulation of membrane fusion (acrosome reaction, sperm-egg fusion, cortical granule exocytosis), inositol 1,4,5-trisphosphate receptors, and calcium release are discussed. The role of six lipases involved in generating putative lipid second messengers during fertilization is also discussed: phospholipase D, autotaxin, lipin1, sphingomyelinase, phospholipase C, and phospholipase A2. More specifically, proteins involved in developmental events and their regulation through lipid binding to SH3, SH4, PH, PX, or C2 protein domains is emphasized. New models are presented for PA activation of Src (through SH3, SH4 and a unique domain), that this may be why the SH2 domain of PLCγ is not required for Xenopus fertilization, PA activation of phospholipase C, a role for PA during the calcium wave after fertilization, and that calcium/calmodulin may be responsible for the loss of Src from rafts after fertilization. Also discussed is that the large DAG increase during fertilization derives from phospholipase D production of PA and lipin dephosphorylation to DAG.

Multivalent adhesion molecule 7 clusters act as signaling platform for host cellular GTPase activation and facilitate epithelial barrier dysfunction

Vibrio parahaemolyticus is an emerging bacterial pathogen which colonizes the gastrointestinal tract and can cause severe enteritis and bacteraemia. During infection, V. parahaemolyticus primarily attaches to the small intestine, where it causes extensive tissue damage and compromises epithelial barrier integrity. We have previously described that Multivalent Adhesion Molecule (MAM) 7 contributes to initial attachment of V. parahaemolyticus to epithelial cells. Here we show that the bacterial adhesin, through multivalent interactions between surface-induced adhesin clusters and phosphatidic acid lipids in the host cell membrane, induces activation of the small GTPase RhoA and actin rearrangements in host cells. In infection studies with V. parahaemolyticus we further demonstrate that adhesin-triggered activation of the ROCK/LIMK signaling axis is sufficient to redistribute tight junction proteins, leading to a loss of epithelial barrier function. Taken together, these findings show an unprecedented mechanism by which an adhesin acts as assembly platform for a host cellular signaling pathway, which ultimately facilitates breaching of the epithelial barrier by a bacterial pathogen.

Vibrio parahaemolyticus is a bacterial pathogen which occurs in marine and estuarine environments. It is a main cause of gastrointestinal illness following the consumption of raw or undercooked seafood. In immunocompromised people, the bacteria can sometimes enter the bloodstream and cause septicemia, a serious and often fatal condition. V. parahaemolyticus attaches to host tissues using adhesive proteins. Multivalent Adhesion Molecule (MAM) 7 is an adhesin which helps the bacteria to hold onto the host cells early on during infection. It does so by binding two different molecules on the host, a protein (fibronectin) and phospholipids called phosphatidic acids. We show that MAM7 does not only play a role in sticking to host cells. By forming adhesin clusters on the host surface and binding to host lipids, it triggers signaling processes in the host. These include activation of RhoA, an important mediator of cytoskeletal dynamics. By doing so, MAM7 perturbs proteins at cellular junctions, which normally maintain the cells in the gut as a tightly sealed layer protective of environmental influences. When bacteria use MAM7 to attach to the intestine, the seals between cells break, permitting bacteria to cross the barrier and cause infection of underlying tissues.

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.

Optogenetic control of PIP3: PIP3 is sufficient to induce the actin-based active part of growth cones and is regulated via endocytosis

Phosphatidylinositol-3,4,5-trisphosphate (PIP3) is highly regulated in a spatiotemporal manner and plays multiple roles in individual cells. However, the local dynamics and primary functions of PIP3 in developing neurons remain unclear because of a lack of techniques for manipulating PIP3 spatiotemporally. We addressed this issue by combining optogenetic control and observation of endogenous PIP3 signaling. Endogenous PIP3 was abundant in actin-rich structures such as growth cones and "waves", and PIP3-rich plasma membranes moved actively within growth cones. To study the role of PIP3 in developing neurons, we developed a PI3K photoswitch that can induce production of PIP3 at specific locations upon blue light exposure. We succeeded in producing PIP3 locally in mouse hippocampal neurons. Local PIP3 elevation at neurite tips did not induce neurite elongation, but it was sufficient to induce the formation of filopodia and lamellipodia. Interestingly, ectopic PIP3 elevation alone activated membranes to form actin-based structures whose behavior was similar to that of growth-cone-like "waves". We also found that endocytosis regulates effective PIP3 concentration at plasma membranes. These results revealed the local dynamics and primary functions of PIP3, providing fundamental information about PIP3 signaling in neurons.

Rho/Ras-GTPase-dependent and -independent activity of clostridial glucosylating toxins

Clostridial glucosylating toxins are the main virulence factors of clostridia responsible for gangrene and/or colitis. These toxins have been well characterized to inactivate Rho/Ras-GTPases through glucosylation. However, the signalling pathways downstream of Rho/Ras-GTPases leading to the intracellular effects of these toxins are only partially known. Rac-dependent modification of focal adhesion complexes and phosphoinositide metabolism seem to be key processes involved in actin filament depolymerization and disorganization of intercellular junctions. In addition, clostridial glucosylating toxins induce Rho/Ras-independent intracellular effects such as activation of mitogen-activated protein kinase pathways, which are used by some of these toxins to trigger an inflammatory response.

One lipid, multiple functions: how various pools of PI(4,5)P(2) are created in the plasma membrane

Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)] is a minor lipid of the inner leaflet of the plasma membrane that controls the activity of numerous proteins and serves as a source of second messengers. This multifunctionality of PI(4,5)P(2) relies on mechanisms ensuring transient appearance of PI(4,5)P(2) clusters in the plasma membrane. One such mechanism involves phosphorylation of PI(4)P to PI(4,5)P(2) by the type I phosphatidylinositol-4-phosphate 5-kinases (PIP5KI) at discrete membrane locations coupled with PI(4)P delivery/synthesis at the plasma membrane. Simultaneously, both PI(4)P and PI(4,5)P(2) participate in anchoring PIP5KI at the plasma membrane via electrostatic bonds. PIP5KI isoforms are also selectively recruited and activated at the plasma membrane by Rac1, talin, or AP-2 to generate PI(4,5)P(2) in ruffles and lamellipodia, focal contacts, and clathrin-coated pits. In addition, PI(4,5)P(2) can accumulate at sphingolipid/cholesterol-based rafts following activation of distinct membrane receptors or be sequestered in a reversible manner due to electrostatic constrains posed by proteins like MARCKS.

A central role of GPIb-IX in the procoagulant function of platelets that is independent of the 45-kDa GPIbα N-terminal extracellular domain

Activated platelets become procoagulant and efficiently promote the conversion of prothrombin to thrombin. A role of the GPIb-V-IX complex has long been postulated in view of the decreased prothrombin consumption in Bernard-Soulier patients. We evaluated the impact of GPIb-V-IX deficiency and the requirement for the GPIbα extracellular domain. In GPIbβ−/− mice, thrombin generation was profoundly decreased in tissue factor– or collagen-related peptide (CRP)–activated platelet-rich plasma and in washed platelets supplemented with normal plasma or with FVa, FXa, and prothrombin. Phosphatidylserine (PS) exposure was similarly decreased in response to thrombin, CRP, or CRP + PAR4 peptide despite a normal platelet phospholipid composition. The hypothesis that these defects originate from lack of the GPIbα N-terminal domain was evaluated after its removal from normal mouse and human platelets with Nk protease or O-sialoglycoprotein endopeptidase. Unexpectedly, the treated platelets exhibited normal thrombin generation and PS exposure, indicating that GPIb-V-IX regulates procoagulant activity independently of its GPIbα-binding region. These results suggested a more general structuring role through intracellular cytoskeleton-anchoring portions regulating responses leading to PS exposure. This hypothesis was supported by the decreased calcium mobilization observed in GPIbβ−/− platelets in response to several agonists, some acting independently of GPIb, in contrast to the normal calcium responses in Nk protease–treated platelets.

Modulation of rabbit platelet aggregation and calcium mobilization by platelet cholesterol content

Hypercholesterolemia is one of the major contributing factors in atherosclerosis and the development of cardiovascular disease. Platelets from hypercholesterolemic rabbit have an increased cholesterol content and a hypersensitivity to endogenous aggregating agonists. Although rabbit has been widely used in studies of hypercholesterolemia, the precise role of platelet cholesterol in rabbit platelet activation has not been studied. In the present study, to determine the direct role of cholesterol on rabbit platelet activation, we examined the effect of in vitro modulation of cholesterol content on platelet activation. Cholesterol-depleted rabbit platelets by the treatment with methyl-beta-cyclodextrin showed decreased platelet aggregation by physiological agonists such as thrombin, adenosine diphosphate, and collagen. The inhibition of thrombin-induced aggregation in cholesterol-depleted platelets was restored by cholesterol repletion in platelets. The cholesterol depletion also inhibited Ca(2+) mobilization, which plays a pivotal role in the platelet activation induced by physiological agonists. We showed that the Ca(2+) influx pathway is strongly suppressed by cholesterol depletion more than Ca(2+) release from intracellular Ca(2+) stores in platelets stimulated with thrombin. Furthermore, platelet aggregation induced by PMA, a potent protein kinase C activator, was also depressed by cholesterol depletion. On the other hand, cholesterol enrichment in platelets augmented thrombin-induced aggregation and Ca(2+) mobilization. These findings suggest that cholesterol plays a critical role in regulating rabbit platelet activation, and provides fundamental information regarding hypercholesterolemia-mediated effects on cells in the rabbit model.

Rac1 inactivation by lethal toxin from Clostridium sordellii modifies focal adhesions upstream of actin depolymerization

Inactivation of different small GTPases upon their glucosylation by lethal toxin from Clostridium sordellii strain IP82 (LT-82) is already known to lead to cell rounding, adherens junction (AJ) disorganization and actin depolymerization. In the present work, we observed that LT-82 induces a rapid dephosphorylation of paxillin, a protein regulating focal adhesion (FA), independently of inactivation of paxillin kinases such as Src, Fak and Pyk2. Among the small GTPases inactivated by this toxin, including Rac, Ras, Rap and Ral, we identified Rac1, as responsible for paxillin dephosphorylation using cells overexpressing Rac1(V12). Rac1 inactivation by LT-82 modifies interactions between proteins from AJ and FA complexes as shown by pull-down assays. We showed that in Triton X-100-insoluble membrane proteins from these complexes, namely E-cadherin, beta-catenin, p120-catenin and talin, are decreased upon LT-82 intoxication, a treatment that also induces a rapid decrease in cell phosphoinositide content. Therefore, we proposed that Rac inactivation by LT-82 alters phosphoinositide metabolism leading to FA and AJ complex disorganization and actin depolymerization.

N-cadherin/p120 catenin association at cell-cell contacts occurs in cholesterol-rich membrane domains and is required for RhoA activation and myogenesis

p120 catenin is a major regulator of cadherin stability at cell-cell contacts and a modulator of Rho GTPase activities. In C2C12 myoblasts, N-cadherin is stabilized at cell contacts through its association with cholesterol-rich membrane domains or lipid rafts (LR) and acts as an adhesion-activated receptor that activates RhoA, an event required for myogenesis induction. Here, we report that association of p120 catenin with N-cadherin at cell contacts occurs specifically in LR. We demonstrate that interaction of p120 catenin with N-cadherin is required for N-cadherin association with LR and for its stabilization at cell contacts. LR disruption inhibits myogenesis induction and N-cadherin-dependent RhoA activation as does the perturbation of the N-cadherin-p120 catenin complex after p120 catenin knockdown. Finally, we observe an N-cadherin-dependent accumulation of RhoA at phosphatidylinositol 4,5-bisphosphate-enriched cell contacts which is lost after LR disruption. Thus, a functional N-cadherin-catenin complex occurs in cholesterol-rich membrane microdomains which allows the recruitment of RhoA and the regulation of its activity during myogenesis induction.

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.

Membrane interactions of G proteins and other related proteins

Guanine nucleotide-binding proteins, G proteins, propagate incoming messages from receptors to effector proteins. They switch from an inactive to active state by exchanging a GDP molecule for GTP, and they return to the inactive form by hydrolyzing GTP to GDP. Small monomeric G proteins, such as Ras, are involved in controlling cell proliferation, differentiation and apoptosis, and they interact with membranes through isoprenyl moieties, fatty acyl moieties, and electrostatic interactions. This protein-lipid binding facilitates productive encounters of Ras and Raf proteins in defined membrane regions, so that signals can subsequently proceed through MEK and ERK kinases, which constitute the canonical MAP kinase signaling cassette. On the other hand, heterotrimeric G proteins undergo co/post-translational modifications in the alpha (myristic and/or palmitic acid) and the gamma (farnesol or geranylgeraniol) subunits. These modifications not only assist the G protein to localize to the membrane but they also help distribute the heterotrimer (Galphabetagamma) and the subunits generated upon activation (Galpha and Gbetagamma) to appropriate membrane microdomains. These proteins transduce messages from ubiquitous serpentine receptors, which control important functions such as taste, vision, blood pressure, body weight, cell proliferation, mood, etc. Moreover, the exchange of GDP by GTP is triggered by nucleotide exchange factors. Membrane receptors that activate G proteins can be considered as such, but other cytosolic, membranal or amphitropic proteins can accelerate the rate of G protein exchange or even activate this process in the absence of receptor-mediated activation. These and other protein-protein interactions of G proteins with other signaling proteins are regulated by their lipid preferences. Thus, G protein-lipid interactions control the features of messages and cell physiology.

Intermolecular interaction of phosphatidylinositol with the lipid raft molecules sphingomyelin and cholesterol

Diacylphosphatidylinositol (PI) is the starting reactant in the process of phosphatidylinositide-related signal transduction mediated through the lipid raft domain. We investigated intermolecular interactions of PI with major raft components, sphingomyelin (SM) and cholesterol (Chol), using surface pressure-molecular area (π-A) isotherm measurements. The classical mean molecular area versus composition plot showed that the measured mean molecular areas are smaller in PI/Chol mixed monolayers and larger in PI/SM mixed monolayers than those calculated on the basis of the ideal additivity. These results indicate that PI interacts attractively with Chol and repulsively with SM. In addition, we energetically evaluated the interaction of PI with SM/Chol mixtures and found that the mixing energy of PI/SM/Chol ternary monolayers decreased as the molar ratio of Chol to SM increased. In order to quantitatively analyze the distribution of PI we calculated the chemical potentials of mixing of PI into the SM/Chol mixed monolayer and into the dioleoylphosphatidylcholine (DOPC) monolayer, which was used as a model for the fluid matrix, on the basis of partial molecular area analysis. Analysis using the chemical potential of mixing of PI suggested that partition of PI molecules between these two monolayers can be changed by a factor of about 1.7 in response to change in Chol molar fraction in the SM/Chol mixed monolayer from 0.3 to 0.6 when the concentration of PI in the DOPC monolayer is kept constant at 7 mol%.

Assembly and trafficking of heterotrimeric G proteins

To be activated by cell surface G protein-coupled receptors, heterotrimeric G proteins must localize at the cytoplasmic surface of plasma membranes. Moreover, some G protein subunits are able to traffic reversibly from the plasma membrane to intracellular locations upon activation. This current topic will highlight new insights into how nascent G protein subunits are assembled and how they arrive at plasma membranes. In addition, recent reports have increased our knowledge of activation-induced trafficking of G proteins. Understanding G protein assembly and trafficking will lead to a greater understanding of novel ways that cells regulate G protein signaling.

A FRET analysis to unravel the role of cholesterol in Rac1 and PI 3-kinase activation in the InlB/Met signalling pathway

The signalling pathway for the hepatocyte growth factor receptor, Met/HGF-R, is hijacked by the bacterial surface protein InlB to induce Listeria monocytogenes entry into non-phagocytic cells. We previously showed that Listeria invades host cells by interacting with specialized microdomains of the host plasma membrane called lipid rafts. In this study, we analysed in living cells signalling events that are crucial for Listeria entry using a fluorescence resonance energy transfer-based microscopic method. Phosphoinositide (PI) 3-kinase activity and Rac1 signalling induced by Listeria interacting with epithelial cells were monitored as well as signalling induced by soluble InlB and the Met natural ligand HGF. We found that InlB and HGF induced similar kinetics of PI 3-kinase and Rac1 activation. PI 3-kinase activation was upstream and independent of Rac1 activation. Cholesterol-depletion experiments were performed to address the role of lipid rafts in Met signalling. The amount of 3'-phosphoinositides produced by PI 3-kinase was not affected by cholesterol depletion, while their membrane dynamic was cholesterol-dependent. Rac1 activation, downstream from PI 3-kinase, was cholesterol-dependent suggesting that the spatial distribution of 3'-phosphoinositides within membrane microdomains is critical for Rac1 activation and consequently for F-actin assembly at bacterial entry site.

Phosphatidic acid regulates the affinity of the murine phosphatidylinositol 4-phosphate 5-kinase-Ibeta for phosphatidylinositol-4-phosphate

Type I phosphatidylinositol 4-phosphate 5-kinase (PI4P5K) catalyzes the phosphorylation of phosphatidylinositol 4 phosphate [PI(4)P] at carbon 5, producing phosphatidylinositol 4,5 bisphosphate [PI(4,5)P2]. Phosphatidic acid (PA) activates PI4P5K in vitro and plays a central role in the activation of PIP5K pathways in vivo. This report demonstrates that actin fiber formation in murine fibroblasts involves PA activation of PIP5Ks and defines biochemical interactions between PA and the PIP5Ks. Inhibition of phospholipase D production of PA results in the loss of actin fibers. Overexpression of the beta isoform of the type I murine phosphatidylinositol 4-phosphate 5-kinase (mPIP5K-Ibeta) maintains actin fiber structure in the face of phospholipase D inhibition. PA activates mPIP5K-Ibeta by direct binding to mPIP5K-Ibeta through both electrostatic and hydrophobic interactions, with the fatty acid acyl chain length and degree of saturation acting as critical determinants of binding and activation. Furthermore, kinetic analysis suggests that phosphorylation of the PI(4)P substrate does not follow classical Michaelis-Menten kinetics. Instead, the kinetic data are consistent with a model in which mPIP5K-Ibeta initially binds to the lipid micelle and subsequently binds the PI(4)P substrate. In addition, the kinetics indicate substrate inhibition, suggesting that mPIP5K-Ibeta contains an inhibitory PI(4)P-binding site. These results suggest a model in which mPIP5K-Ibeta is surrounded by PI(4)P, but is unable to catalyze its conversion to PI(4,5)P2 unless PA is bound.

Glycoprotein VI agonists have distinct dependences on the lipid raft environment

BACKGROUND

It has been reported that the association of glycoprotein VI (GPVI) with lipid rafts regulates GPVI signaling in platelets.

OBJECTIVE

Secreted adenosine 5'-diphosphate (ADP) potentiates GPVI-induced platelet aggregation at particular agonist concentrations. We have investigated whether the decrease in GPVI signaling, previously reported in platelets with disrupted rafts, is a result of the loss of agonist potentiation by ADP.

METHODS

We disrupted platelet lipid rafts with methyl-beta-cyclodextrin and measured signaling events downstream of GPVI activation.

RESULTS

Lipid raft disruption decreases aggregation induced by low concentrations of convulxin, but this decrease is almost eliminated in the presence of ADP antagonists. Signaling indicators, such as protein phosphorylation and calcium mobilization, were not affected by raft disruption in collagen or convulxin stimulated platelets. Interestingly, however, raft disruption directly reduced GPVI signaling induced by collagen-related peptide.

CONCLUSIONS

Lipid rafts do not directly contribute to signaling by the physiologic agonist collagen. The effects of disruption of lipid rafts in in vitro assays can be attributed to inhibition of ADP feedback that potentiates GPVI signaling.

Prevention of stroke and dementia by statin therapy: Experimental and clinical evidence of their pleiotropic effects

Stroke and dementia are major causes of disability in most countries. Epidemiological studies have demonstrated that statins (3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors) are likely to reduce the risk for developing these formidable disorders. The favorable outcomes in statin users may be attributable to not only cholesterol-dependent actions, but also various cholesterol-independent actions called "pleiotropic effects." Several clinical trials have suggested that statins decrease the incidence of stroke, especially ischemic stroke. Statins improve endothelial function, inhibit platelet activation, reduce blood coagulability, and suppress inflammatory reactions, all of which may contribute to the beneficial effects of the therapy. Statins also reduce the risk of vasospasm caused by subarachnoid hemorrhage (SAH). In addition, statins might inhibit the development and progression of Alzheimer's disease (AD), the dominant type of dementia in most industrialized countries, upstream of the amyloid cascade. In vitro studies have shown that statins modulate the metabolism of the beta-amyloid precursor protein (APP) and reduce the extracellular level of its proteolytic product, amyloid-beta (Abeta). The aggregated Abeta is cytotoxic, leading to formation of neurofibrillary tangles and neuronal loss in the brain. Inflammatory processes are active in AD and may contribute significantly to AD pathology. We review the experimental background regarding the pleiotropic effects of statins and summarize clinical trials that examined the preventative effects of statin therapy on stroke and dementia. We include current trials in which statin therapy is initiated within 24 hr of onset of acute ischemic stroke.

Norepinephrine and endothelin activate diacylglycerol kinases in caveolae/rafts of rat mesenteric arteries: agonist-specific role of PI3-kinase

The phosphatidylinositol (PI) signaling pathway mediates norepinephrine (NE)- and endothelin-1 (ET-1)-stimulated vascular smooth muscle contraction through an inositol-trisphosphate-induced rise in intracellular calcium and diacylglycerol (DG) activation of protein kinase C (PKC). Subsequent activation of DG kinases (DGKs) metabolizes DG to phosphatidic acid (PA), potentially regulating PKC activity. Because precise regulation and spatial restriction of the PI pathway is necessary for specificity, we have investigated whether this occurs within caveolae/rafts, specialized plasma membrane microdomains implicated in vascular smooth muscle contraction. We show that components of the PI signaling cascade-phosphatidylinositol 4,5-bisphosphate (PIP(2)), PA, and DGK-theta are present in caveolae/rafts prepared from rat mesenteric small arteries. Stimulation with NE or ET-1 induced [(33)P]PIP(2) hydrolysis solely within caveolae/rafts. NE stimulated an increase in DGK activity in caveolae/rafts alone, whereas ET-1 activated DGK in caveolae/rafts and noncaveolae/rafts; however, [(33)P]PA increased in all fractions with both agonists. Previously, we reported that NE activated DGK-theta in a phosphatidylinositol 3-kinase (PI3-kinase)-dependent manner; here, we describe PI3-kinase-dependent DGK activation and [(33)P]PA production in caveolae/rafts in response to NE but not ET-1. Additionally, PKB, a potential activator of DGK-theta, translocated to caveolae/rafts in response to NE but not ET-1, and PI3-kinase inhibition prevented this. Furthermore, PI3-kinase inhibition reduced the sensitivity of contraction to NE but not ET-1. Our study shows that caveolae/rafts are major sites of vasoconstrictor hormone activation of the PI pathway in intact small arteries and suggest a link between lipid signaling events within caveolae/rafts and contraction.

Characterization and comparison of raft-like membranes isolated by two different methods from rat submandibular gland cells

Lipid rafts are defined as cholesterol and sphingolipid enriched domains in biological membranes. Their role in signalling and other cellular processes is widely accepted but the methodology used for their biochemical isolation and characterization remains controversial. Raft-like membranes from rat submandibular glands were isolated by two different protocols commonly described in the literature; one protocol was based on selective solubilization by Triton X-100 at low temperature and the other protocol consisted in extensive sonication. In both cases a low density vesicular fraction was obtained after ultracentrifugation in a sucrose density gradient. These fractions contained about 20% of total cholesterol but less than 8% of total proteins, and were more rigid than bulk membranes. Fatty acid analyses revealed a similar composition of raft-like membranes isolated by the two different methods, which was characterized by an enrichment in saturated fatty acids in detriment of polyunsaturated acids when compared with the whole cell membranes. Protein profile of detergent resistant membranes or raft-like membranes prepared by sonication was assessed by silver staining after SDS-PAGE and by MALDI-TOF. Both analyses provided evidence of a different protein composition of the Triton X-100 and sonication preparations. Immunoblot experiments revealed that raft-like membranes prepared by detergent extraction or sonication were free of Golgi apparatus or endoplasmic reticulum protein markers (beta-COP and calnexin, respectively) and that they were not substantially contaminated by transferrin receptor (a non-raft protein). While caveolin-1 was highly enriched in raft-like membranes prepared by the two methods, the P2X(7) receptor was enriched in raft-like membrane fractions prepared by sonication, but almost undetectable in the detergent resistant membranes. It can be concluded that both methods can be used to obtain raft-like membranes, but that detergent may affect protein interactions responsible for their association with different membrane domains.

Chondrocytes utilize a cholesterol-dependent lipid translocator to externalize phosphatidylserine

During endochondral ossification, growth plate chondrocytes release plasma membrane (PM) derived matrix vesicles (MV), which are the site of initial hydroxyapatite crystal formation. MV constituents which facilitate the mineralization process include the integral membrane ectoenzymes alkaline phosphatase (ALPase) and nucleotide pyrophosphatase phosphodiesterase (NPP1/PC-1), along with a phosphatidylserine- (PS-) rich membrane surface that binds annexins and calcium, resulting in enhanced calcium entry into MV. In this study, we determined that chick growth plate MV were highly enriched in membrane raft microdomains containing high levels of cholesterol, glycophosphatidylinositol- (GPI-) anchored ALPase, and phosphatidylserine (PS) localized to the external leaflet of the bilayer. To determine how such membrane microdomains arise during chondrocyte maturation, we explored the role of PM cholesterol-dependent lipid assemblies in regulating the activities of lipid translocators involved in the externalization of PS. We first isolated and determined the composition of detergent-resistant membranes (DRMs) from chondrocyte PM. DRMs isolated from chondrocyte PM were enhanced in ganglioside 1 (GM1) and cholesterol as well as GPI-anchored ALPase. Furthermore, these membrane domains were enriched in PS (localized to the external leaflet of the bilayer) and had significantly higher ALPase activity than non-cholesterol-enriched domains. To understand the role of cholesterol-dependent lipid assemblies in the externalization of PS, we measured the activities of two lipid transporters involved in PS externalization, aminophospholipid translocase (APLT) and phospholipid scramblase (PLSCR1), during maturation of a murine chondrocytic cell line, N1511. In this report, we provide the first evidence that maturing chondrocytes express PLSCR1 and have scramblase activity. We propose that redistribution of PS is dependent on an increase in phospholipid scramblase activity and a decrease in APLT activity. Lastly, we show that translocator activity is most likely to be modulated by membrane cholesterol levels through a membrane raft microdomain.

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.

In vivo interaction between RGS4 and calmodulin visualized with FRET techniques: possible involvement of lipid raft

Regulators of G-protein signaling (RGS) are a family of proteins which accelerate intrinsic GTP-hydrolysis on heterotrimeric G-protein-alpha-subunits. Although it has been suggested that the function of RGS4 is reciprocally regulated by competitive binding of the membrane phospholipid, phosphatidylinositol-3,4,5,-trisphosphate(PtdIns(3,4,5)P(3)), and Ca(2+)/calmodulin (CaM), it remains to be shown that these interactions occur in vivo. Here, using fluorescence resonance energy transfer (FRET) techniques, we show that an elevation of intracellular Ca(2+) concentration by ionomycin increased the FRET efficiency from ECFP (a variant of cyan fluorescent protein)-labeled calmodulin to Venus (a variant of yellow fluorescent protein)-labeled RGS4. The increase in FRET efficiency was greatly attenuated by pre-treating the cells with methyl-beta-cyclodextrin, which depletes membrane cholesterol and thus disrupts lipid rafts. These results provide the first demonstration of a Ca(2+)-dependent interaction between RGS4 and CaM in vivo and show that association in lipid rafts of the plasma membrane might be involved in this physiological regulation of RGS proteins.

Plasma membrane organization is essential for balancing competing pseudopod- and uropod-promoting signals during neutrophil polarization and migration

Exposure of neutrophils to chemoattractant induces cell polarization and migration. These behaviors require the asymmetric activation of distinct signaling pathways and cytoskeletal elements in the protruding pseudopod at the front of cells and the retracting uropod at the rear. An important outstanding question is, how does the organization of the plasma membrane participate in establishing asymmetry during polarization and migration? To answer this question, we investigated the function of cholesterol, a lipid known to influence membrane organization. Using controlled cholesterol depletion, we found that a cholesterol-dependent membrane organization enabled cell polarization and migration by promoting uropod function and suppressing ectopic pseudopod formation. At a mechanistic level, we showed that cholesterol was directly required for suppressing inappropriate activation of the pseudopod-promoting Gi/PI3-kinase signaling pathway. Furthermore, cholesterol was required for dampening Gi-dependent negative feedback on the RhoA signaling pathway, thus enabling RhoA activation and uropod function. Our findings suggest a model in which a cholesterol-dependent membrane organization plays an essential role in the establishment of cellular asymmetry by balancing the activation and segregating the localization of competing pseudopod- and uropod-inducing signaling pathways during neutrophil polarization and migration.

Platelet GPIb-IX-V-dependent signaling

Although the signaling pathways related to GPIb-IX-V have not been fully elucidated, an accumulating body of evidence suggests that phospholipase C (PLC)gamma2 activation, subsequent Ca++ release and oscillations constitute an essential signal transduction pathway related to GPIb-IX-V. Src family kinases are required for PLCgamma2 activation, while FcR gamma-chain/Fc gammaRIIA may be dispensable for PLCgamma2 activation. Although PI-3K serves to potentiate various signaling events culminating in alpha(IIb)beta3 activation, PI-3K activity may be dispensable for Src-PLCgamma2 activation in GPIb-IX-V-mediated signaling. Glycosphingolipid-enriched microdomains (GEMs) appear to provide platforms for the signal transduction pathway related to GIb-IX-V, as the interaction between GPIb-IX-V and Src or PLCgamma2 tyrosine phosphorylation occurs exclusively in GEMs.

Lipid rafts are required in Galpha(i) signaling downstream of the P2Y12 receptor during ADP-mediated platelet activation

ADP is important in propagating hemostasis upon its secretion from activated platelets in response to other agonists. Lipid rafts are microdomains within the plasma membrane that are rich in cholesterol and sphingolipids, and have been implicated in the stimulatory mechanisms of platelet agonists. We sought to determine the importance of lipid rafts in ADP-mediated platelet activation via the G protein-coupled P2Y1 and P2Y12 receptors using lipid raft disruption by cholesterol depletion with methyl-beta-cyclodextrin. Stimulation of cholesterol-depleted platelets with ADP resulted in a reduction in the extent of aggregation but no difference in the extent of shape change or intracellular calcium release. Furthermore, repletion of cholesterol to previously depleted membranes restored ADP-mediated platelet aggregation. In addition, P2Y12-mediated inhibition of cAMP formation was significantly decreased upon cholesterol depletion from platelets. Stimulation of cholesterol-depleted platelets with agonists that depend upon Galpha(i) activation for full activation displayed significant loss of aggregation and secretion, but showed restoration when simultaneously stimulated with the Galpha(z)-coupled agonist epinephrine. Finally, Galpha(i) preferentially localizes to lipid rafts as determined by sucrose density centrifugation. We conclude that Galpha(i) signaling downstream of P2Y12 activation, but not Galpha(q) or Galpha(z) signaling downstream of P2Y1 or alpha2A activation, respectively, has a requirement for lipid rafts that is necessary for its function in ADP-mediated platelet activation.

UV-C light induces raft-associated acid sphingomyelinase and JNK activation and translocation independently on a nuclear signal

The initiation of UV light-induced signaling in mammalian cells is largely considered to be subsequent to DNA damage. Several studies have also described ceramide (CER), a lipid second messenger, as a major contributor in mediating UV light-induced c-Jun N-terminal kinase (JNK) activation and cell death. It is demonstrated here that UV-C light irradiation of U937 cells results in the activation and translocation of a Zn2+-independent acid sphingomyelinase, leading to CER accumulation in raft microdomains. These CER-enriched rafts aggregate and play a functional role in JNK activation. The observation that UV-C light also induced CER generation and the externalization of acid sphingomyelinase and JNK in human platelets conclusively rules out the involvement of a nuclear signal generated by DNA damage in the initiation of a UV light response, which is generated at the plasma membrane.

pH-dependent domain formation in phosphatidylinositol polyphosphate/phosphatidylcholine mixed vesicles

Phosphatidylinositol polyphosphates (PI-PPs) have been shown to mediate a large variety of physiological processes by attracting proteins to specific cellular sites. Such site-specific signaling requires local accumulation of PI-PPs, and in light of the rich headgroup functionality, it is conceivable that hydrogen bond formation between adjacent headgroups is a contributing factor to the formation of PI-PP-enriched domains. To explore the significance of hydrogen bond formation for the mutual interaction of PI-PPs, this study aims to characterize the pH-dependent phase behavior of phosphatidylcholine/phosphatidylinositol bisphosphate and trisphosphate mixed vesicles by differential scanning calorimetry, infrared transmission spectroscopy, and fluorescence resonance energy transfer measurements. For pH values >7-7.5, the experiments yielded results consistent with dipalmitoylphosphatidylcholine/dipalmitoylphosphatidylinositol polyphosphate gel phase demixing, whereas for moderately acidic conditions, an enhanced mixing was observed. Similarly, this pH-dependent formation of PI-PP-enriched domains was also found for the physiologically important fluid phase. The stability of PI-PP-enriched domains and to some extent the pH dependence of the domain formation was governed by the number as well as the position of the phosphomonoester groups at the inositol ring.

Integrin-dependent interaction of lipid rafts with the actin cytoskeleton in activated human platelets

Dynamic connections between actin filaments and the plasma membrane are crucial for the regulation of blood platelet functions. Protein complexes associated with αIIbβ3 integrin-based cytoskeleton structures are known to play a role in these processes. However, mechanisms involving lateral organizations of the plasma membrane remain to be investigated. Here, we demonstrate that a large fraction of platelet lipid rafts specifically associates with the actin cytoskeleton upon activation. This association was inhibited by antagonists of fibrinogen-αIIbβ3 binding and did not occur in type I Glanzman's thrombasthenic platelets. The raft-cytoskeleton interaction is a reversible process correlating with the intensity and stability of platelet aggregation. Although only a minor fraction of αIIbβ3 was recovered in rafts upon activation, this integrin specifically upregulated the level of PtdIns(4,5)P2 in membrane microdomains and induced the recruitment of several actin-modulating proteins known to directly or indirectly interact with this lipid. Controlled disruption of rafts did not affect αIIbβ3-mediated platelet aggregation in response to high concentrations of thrombin but significantly inhibited fibrin clot retraction. We propose that rafts participate in the organization of membrane-cytoskeleton interactions where αIIbβ3-mediated tension forces apply during the late phase of platelet activation.

Domain formation in phosphatidylinositol monophosphate/phosphatidylcholine mixed vesicles

Phosphoinositides have been shown to control membrane trafficking events by targeting proteins to specific cellular sites, which requires a tight regulation of phosphoinositide generation and turnover as well as a high degree of compartmentalization. To shed light on the processes that lead to the formation of phosphoinositide-enriched microdomains, phosphatidylcholine/phosphatidylinositol monophosphate (phosphatidylinositol-3-phosphate (PI-3P), -4-phosphate (PI-4P), or -5-phosphate (PI-5P)) mixed vesicles were investigated by calorimetric (DSC) Fourier transform infrared spectroscopic (FTIR), and fluorescence resonance energy transfer (FRET) measurements. The experiments furnished results consistent with a pH-dependent formation of phosphatidylinositol monophosphate-enriched microdomains. The domain formation was most pronounced between pH approximately 7 and approximately 9.5, whereas slightly acidic pH values (pH 4) resulted in the disintegration of the domains. This pH-dependent phosphatidylcholine/phosphatidylinositol monophosphate demixing was observed for the gel phase (FTIR experiments) as well as for the fluid lipid phase (FRET measurements). The observed microdomains are presumably stabilized by hydroxyl/hydroxyl as well as hydroxyl/phosphomonoester and phosphodiester interactions. While the pH dependence of the mutual phosphatidylinositol monophosphate interaction was largely the same for all investigated phosphatidylinositol monophosphates, it turned out that the relative stability of phosphatidylinositol monophosphate-enriched microdomains (pH 7-9.5) was governed by the position of the phosphomonoester group at the inositol ring (PI-4P > PI-5P > PI-3P). Demixing was also observed for phosphatidylcholine/phosphatidylinositol mixed vesicles; however, in this case the microdomain formation was only slightly affected by pH changes.

Cholesterol depletion by methyl-beta-cyclodextrin enhances myoblast fusion and induces the formation of myotubes with disorganized nuclei

The formation of a skeletal muscle fiber begins with the withdrawal of committed mononucleated precursors from the cell cycle. These myoblasts elongate while aligning with each other, guided by recognition between their membranes. This step is followed by cell fusion and the formation of long striated multinucleated myotubes. We used methyl-beta-cyclodextrin (MCD) in primary cultured chick skeletal muscle cells to deplete membrane cholesterol and investigate its role during myogenesis. MCD promoted a significant increase in the expression of troponin T, enhanced myoblast fusion, and induced the formation of large multinucleated myotubes with nuclei being clustered centrally and not aligned at the cell periphery. MCD myotubes were striated, as indicated by sarcomeric alpha-actinin staining, and microtubule and desmin filament distribution was not altered. Pre-fusion MCD-treated myoblasts formed large aggregates, with cadherin and beta-catenin being accumulated in cell adhesion contacts. We also found that the membrane microdomain marker GM1 was not present as clusters in the membrane of MCD-treated myoblasts. Our data demonstrate that cholesterol is involved in the early steps of skeletal muscle differentiation.

Mast cell- and dendritic cell-derived exosomes display a specific lipid composition and an unusual membrane organization

Exosomes are small vesicles secreted from multivesicular bodies, which are able to stimulate the immune system leading to tumour cell eradication. We have analysed lipids of exosomes secreted either upon stimulation from rat mast cells (RBL-2H3 cells), or constitutively from human dendritic cells. As compared with parent cells, exosomes displayed an enrichment in sphingomyelin, but not in cholesterol. Phosphatidylcholine content was decreased, but an enrichment was noted in disaturated molecular species as in phosphatidylethanolamines. Lyso(bis)phosphatidic acid was not enriched in exosomes as compared with cells. Fluorescence anisotropy demonstrated an increase in exosome-membrane rigidity from pH 5 to 7, suggesting their membrane reorganization between the acidic multivesicular body compartment and the neutral outer cell medium. NMR analysis established a bilayer organization of exosome membrane, and ESR studies using 16-doxyl stearic acid demonstrated a higher flip-flop of lipids between the two leaflets as compared with plasma membrane. In addition, the exosome membrane exhibited no asymmetrical distribution of phosphatidylethanolamines. Therefore exosome membrane displays a similar content of the major phospholipids and cholesterol, and is organized as a lipid bilayer with a random distribution of phosphatidylethanolamines. In addition, we observed tight lipid packing at neutral pH and a rapid flip-flop between the two leaflets of exosome membranes. These parameters could be used as a hallmark of exosomes.

Shape changes and vesicle fission of giant unilamellar vesicles of liquid-ordered phase membrane induced by lysophosphatidylcholine

Liquid-ordered phase (lo phase) of lipid membranes has properties that are intermediate between those of liquid-crystalline phase and those of gel phase and has attracted much attention in both biological and biophysical aspects. Rafts in the lo phase in biomembranes play important roles in cell function of mammalian cells such as signal transduction. In this report, we have prepared giant unilamellar vesicles (GUVs) of lipid membranes in the lo phase and investigated their physical properties using phase-contrast microscopy and fluorescence microscopy. GUVs of dipalmitoyl-phosphatidylcholine (DPPC)/cholesterol membranes and also GUVs of sphingomyelin (SM)/cholesterol membranes in the lo phase in water were formed at 20-37 degrees C successfully, when these membranes contained >/=30 mol % cholesterol. The diameters of GUVs of DPPC/cholesterol and SM/cholesterol membranes did not change from 50 to 28 degrees C, supporting that the membranes of these GUVs were in the lo phase. To elucidate the interaction of a substance with a long hydrocarbon chain with the lo phase membrane, we investigated the interaction of low concentrations (less than critical micelle concentration) of lysophosphatidylcholine (lyso-PC) with DPPC/cholesterol GUVs and SM/cholesterol GUVs in the lo phase. We found that lyso-PC induced several shape changes and vesicle fission of these GUVs above their threshold concentrations in water. The analysis of these shape changes indicates that lyso-PC can be partitioned into the external monolayer in the lo phase of the GUV from the aqueous solution. Threshold concentrations of lyso-PC in water to induce the shape changes and vesicle fission increased greatly with a decrease in chain length of lyso-PC. Thermodynamic analysis of this result indicates that shape changes and vesicle fission occur at threshold concentrations of lyso-PC in the membrane. These new findings on GUVs of the lo phase membranes indicate that substances with a long hydrocarbon chain such as lyso-PC can enter into the lo phase membrane and also the raft in the cell membrane. We have also proposed a mechanism for the lyso-PC-induced vesicle fission of GUVs.

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 plasma membrane Ca2+pump from proximal kidney tubules is exclusively localized and active in caveolae

Plasma membrane Ca2+-ATPase is involved in the fine-tuned regulation of intracellular Ca2+. In this study, the presence of Ca2+-ATPase in caveolae from kidney basolateral membranes was investigated. With the use of a discontinuous sucrose gradient, we show that Ca2+-ATPase is exclusively located and fully active in caveolin-containing microdomains. Treatment with methyl-beta-cyclodextrin--a cholesterol chelator--leads to a spreading of both caveolin and completely inactive Ca2+-ATPase toward high-density fractions. These data support the view that Ca2+ fluxes mediated by Ca2+-ATPase in kidney epithelial cells occur only in caveolae, being strictly dependent on the integrity of these microdomains.

Cholesterol-dependent partitioning of PtdIns(4,5)P2 into membrane domains by the N-terminal fragment of NAP-22 (neuronal axonal myristoylated membrane protein of 22 kDa)

A myristoylated peptide corresponding to the N-terminus of NAP-22 (neuronal axonal myristoylated membrane protein of 22 kDa) causes the quenching of the fluorescence of BODIPY-TMR-labelled PtdIns(4,5) P2 in bilayers of 1-palmitoyl-2-oleoyl phosphatidylcholine containing 40 mol% cholesterol and 0.1 mol% BODIPY-PtdIns(4,5)2. Both fluorescence spectroscopy and total internal reflectance fluorescence microscopy revealed the cholesterol-dependent nature of PtdIns(4,5) P2-enriched membrane-domain formation.

Impact of cholesterol depletion on shape changes, actin reorganization, and signal transduction in neutrophil-like HL-60 cells

Stimulation of neutrophils with chemotactic peptide induces actin reorganization, formation of actin-rich protrusions, and development of polarity. Shape changes and actin polymerization can also be induced by phorbol ester-mediated direct activation of protein kinase C (PKC). We have investigated the role of cholesterol in stimulus-dependent motile events and in activation of signaling pathways in neutrophil-like differentiated HL-60 cells. Depletion of plasma membrane cholesterol using methyl-beta-cyclodextrin (MbetaCD) prevented chemotactic peptide and phorbol ester-induced shape changes and increases in cytoskeletal actin. Cholesterol depletion almost completely suppressed chemotactic peptide-mediated activation of p42/44 mitogen-activated protein kinase (MAPK). Phosphorylation of protein kinase B on Thr-308, which is indicative of activation of phosphatidylinositol 3-kinase, was in contrast only partially inhibited. Stimulus-mediated membrane recruitment of different PKC isoforms was differentially affected by treatment of cells with MbetaCD. Membrane recruitment of PKCalpha induced by chemotactic peptide or phorbol ester was suppressed, whereas that of PKCbetaII was only partially affected. Membrane association of PKCdelta was almost insensitive to cholesterol depletion. In summary, our results implicate an important role of cholesterol-containing lipid microdomains (rafts) especially in chemotactic peptide-induced activation of MAPK pathways and in chemotactic peptide- and phorbol ester-mediated activation of PKCalpha.

Lipid rafts: heterogeneity on the high seas

Lipid rafts are membrane microdomains that are enriched in cholesterol and glycosphingolipids. They have been implicated in processes as diverse as signal transduction, endocytosis and cholesterol trafficking. Recent evidence suggests that this diversity of function is accompanied by a diversity in the composition of lipid rafts. The rafts in cells appear to be heterogeneous both in terms of their protein and their lipid content, and can be localized to different regions of the cell. This review summarizes the data supporting the concept of heterogeneity among lipid rafts and outlines the evidence for cross-talk between raft components. Based on differences in the ways in which proteins interact with rafts, the Induced-Fit Model of Raft Heterogeneity is proposed to explain the establishment and maintenance of heterogeneity within raft populations.

Cholesterol modulates P-glycoprotein activity in human peripheral blood mononuclear cells

P-glycoprotein (P-gp) is expressed in a wide range of cell types including peripheral blood mononuclear cells (PBMCs) where it may restrict intracellular accumulation of substrates like antineoplastic agents, HIV protease inhibitors, or rhodamine123. P-gp is known to be located in membrane microdomains, whose structure and function are susceptible to cholesterol alterations. This study evaluated the effect of cholesterol alteration in human PBMCs on P-gp activity. Whereas cholesterol depletion had no effect, cholesterol repletion of depleted cells significantly decreased intracellular rhodamine123 concentrations in lymphocytes to 32.2%+/-2.7 (p<0.001) and to 41.9%+/-3.5 (p<0.001) in monocytes. After cholesterol saturation of native cells intracellular rhodamine123 fluorescence decreased to 12.4%+/-1.6 (p<0.001) in lymphocytes and 12.9%+/-3.5 (p<0.001) in monocytes. These data demonstrate that elevated cellular cholesterol levels can markedly increase P-gp activity in human PBMCs.