Differential sensitivity of human platelet P2X1 and P2Y1 receptors to disruption of lipid rafts

Filling of a water-free void explains the allosteric regulation of the β1-adrenergic receptor by cholesterol

Recent high-pressure NMR results indicate that the preactive conformation of the β₁-adrenergic receptor (β₁AR) harbours completely empty cavities of ~100 ų volume, which disappear in the active conformation of the receptor. Here we have localized these cavities using X-ray crystallography of xenon-derivatized β₁AR crystals. One of the cavities is in direct contact with the cholesterol-binding pocket. Solution NMR shows that addition of the cholesterol analogue cholesteryl hemisuccinate impedes the formation of the active conformation of detergent-solubilized β₁AR by blocking conserved G protein-coupled receptor microswitches, concomitant with an affinity reduction of both isoprenaline and G protein-mimicking nanobody Nb80 for β₁AR detected by isothermal titration calorimetry. This wedge-like action explains the function of cholesterol as a negative allosteric modulator of β₁AR. A detailed understanding of G protein-coupled receptor regulation by cholesterol by filling of a dry void and the easy scouting for such voids by xenon may provide new routes for the development of allosteric drugs.

Flow Cytometry Identifies an Early Stage of Platelet Apoptosis Produced by Agonists of the P2X1 and P2X7 Receptors

Platelets express P2X1 receptors and our data also show the expression of P2X7 receptors. We studied the role of both receptors in platelet apoptosis by incubation of PRP with P2X agonists, then centrifuged to remove viable platelets, and analyzed the supernatant by flow cytometry to identify a sparse platelet-derived population that stained with MitoTracker dyes and CD41. BzATP, a potent agonist of P2X receptors, and ABT737, an activator of intrinsic apoptosis, produced altered platelets that stained moderately for annexin V and corresponded to an early stage apoptotic platelet (ESAP). Over a range of BzATP concentrations, we observed a dose-dependent formation of ESAPs between 5 and 500 uM BzATP, together with a variable formation of ESAPs at nanomolar ATP or BzATP (50-200 nM). Production of ESAPs occurred with αβ-meATP, while responses with either BzATP or αβ-meATP showed desensitization at a higher agonist concentration. Formation of ESAPs by either 100 nM or 0.5 mM BzATP was inhibited by preincubation of platelets with latrunculin A, an inhibitor of the actin cytoskeleton that prevents apoptosis. ESAP production was totally inhibited by preincubation of platelets with methyl-beta-cyclodextrin, which removes cholesterol from lipid rafts. Our data show that both P2X1 and P2X7 receptors are localized in platelet lipid rafts where P2X-agonists act to produce early stage apoptotic platelets.

Shear-mediated platelet activation in the free flow II: Evolving mechanobiological mechanisms reveal an identifiable signature of activation and a bi-directional platelet dyscrasia with thrombotic and bleeding features

Shear-mediated platelet activation (SMPA) in the "free flow" is the net result of a range of cell mechanobiological mechanisms. Previously, we outlined three main groups of mechanisms including: 1) mechano-destruction - i.e. additive platelet (membrane) damage; 2) mechano-activation - i.e. activation of shear-sensitive ion channels and pores; and 3) mechano-transduction - i.e. "outside-in" signaling via a range of transducers. Here, we report on recent advances since our original report which describes additional features of SMPA. A clear "signature" of SMPA has been defined, allowing differentiation from biochemically-mediated activation. Notably, SMPA is characterized by mitochondrial dysfunction, platelet membrane eversion, externalization of anionic phospholipids, and increased thrombin generation on the platelet surface. However, SMPA does not lead to integrin αIIbβ3 activation or P-selectin exposure due to platelet degranulation, as is commonly observed in biochemical activation. Rather, downregulation of GPIb, αIIbβ3, and P-selectin surface expression is evident. Furthermore, SMPA is accompanied by a decrease in overall platelet size coupled with a concomitant, progressive increase in microparticle generation. Shear-ejected microparticles are highly enriched in GPIb and αIIbβ3. These observations indicate the enhanced diffusion, migration, or otherwise dispersion of platelet adhesion receptors to membrane zones, which are ultimately shed as receptor-rich PDMPs. The pathophysiological consequence of this progressive shear accumulation phenomenon is an associated dyscrasia of remaining platelets - being both reduced in size and less activatable via biochemical means - a tendency to favor bleeding, while concomitantly shed microparticles are highly prothrombotic and increase the tendency for thrombosis in both local and systemic milieu. These mechanisms and observations offer direct clinical utility in allowing measurement and guidance of the net balance of platelet driven events in patients with implanted cardiovascular therapeutic devices.

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.

Tonic Calcium-Activated Chloride Current Sustained by ATP Release and Highly Desensitizing Human P2X1 Receptors

Extracellular adenosine triphosphate (ATP) participates in maintaining the vascular tone in the CNS, particularly in the retina, via the tonic activity of ligand gated activated P2X1 receptors. P2X1 receptors are characterized by their high affinity for ATP and their strong desensitization to concentrations of ATP that are 200-fold lower than their EC₅₀. The mechanism behind P2X1 tonic activity remains unclear. In this study, we expressed human P2X1 (hP2X1) homomeric receptors in Xenopus oocytes to explore the relationship between ATP release from oocytes at rest, hP2X1, and Ca²⁺-activated Cl^- channels. Our results indicate that Xenopus oocytes release ATP at rest via vesicular exocytosis, and this process is a constitutive phenomenon independent of extracellular Ca²⁺. Our results also indicate that hP2X1 receptors are able to sustain a tonic activity of Ca²⁺-activated Cl^- channels. In the presence of extracellular Ca²⁺ the activity of hP2X1 receptors is greatly amplified by its coupling with Ca²⁺-activated Cl^- channels. Future studies addressing the relationship between hP2X1 receptors and Ca²⁺-activated Cl^- channels in vascular smooth muscle cells should provide information about additional mechanisms that regulate the vascular tone and their potential as pharmaceutical targets. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Emerging Diversity in Lipid-Protein Interactions

Membrane lipids interact with proteins in a variety of ways, ranging from providing a stable membrane environment for proteins to being embedded in to detailed roles in complicated and well-regulated protein functions. Experimental and computational advances are converging in a rapidly expanding research area of lipid-protein interactions. Experimentally, the database of high-resolution membrane protein structures is growing, as are capabilities to identify the complex lipid composition of different membranes, to probe the challenging time and length scales of lipid-protein interactions, and to link lipid-protein interactions to protein function in a variety of proteins. Computationally, more accurate membrane models and more powerful computers now enable a detailed look at lipid-protein interactions and increasing overlap with experimental observations for validation and joint interpretation of simulation and experiment. Here we review papers that use computational approaches to study detailed lipid-protein interactions, together with brief experimental and physiological contexts, aiming at comprehensive coverage of simulation papers in the last five years. Overall, a complex picture of lipid-protein interactions emerges, through a range of mechanisms including modulation of the physical properties of the lipid environment, detailed chemical interactions between lipids and proteins, and key functional roles of very specific lipids binding to well-defined binding sites on proteins. Computationally, despite important limitations, molecular dynamics simulations with current computer power and theoretical models are now in an excellent position to answer detailed questions about lipid-protein interactions.

P2Y11 Receptors: Properties, Distribution and Functions

The P2Y₁₁ receptor is a G protein-coupled receptor that is stimulated by endogenous purine nucleotides, particularly ATP. Amongst P2Y receptors it has several unique properties; (1) it is the only human P2Y receptor gene that contains an intron in the coding sequence; (2) the gene does not appear to be present in the rodent genome; (3) it couples to stimulation of both phospholipase C and adenylyl cyclase. Its absence in mice and rats, along with a limited range of selective pharmacological tools, has hampered the development of our knowledge and understanding of its properties and functions. Nonetheless, through a combination of careful use of the available tools, suppression of receptor expression using siRNA and genetic screening for SNPs, possible functions of native P2Y₁₁ receptors have been identified in a variety of human cells and tissues. Many are in blood cells involved in inflammatory responses, consistent with extracellular ATP being a damage-associated signalling molecule in the immune system. Thus proposed potential therapeutic applications relate, in the main, to modulation of acute and chronic inflammatory responses.

P2X1 Receptors Amplify FcγRIIa-Induced Ca2+ Increases and Functional Responses in Human Platelets

Platelets express key receptors of the innate immune system such as FcγRIIa and Toll-like receptors (TLR). P2X1 cation channels amplify the platelet responses to several major platelet stimuli, particularly glycoprotein (GP)VI and TLR2/1, whereas their contribution to Src tyrosine kinase-dependent FcγRIIa receptors remains unknown. We investigated the role of P2X1 receptors during activation of FcγRIIa in human platelets, following stimulation by cross-linking of an anti-FcγRIIa monoclonal antibody (mAb) IV.3, or bacterial stimulation with Streptococcus sanguinis . Activation was assessed in washed platelet suspensions via measurement of intracellular Ca ²⁺ ([Ca ²⁺ ] _i ) increases, ATP release and aggregation. P2X1 activity was abolished by pre-addition of α,β-meATP, exclusion of apyrase or the antagonist NF449. FcγRIIa activation evoked a robust increase in [Ca ²⁺ ] _i (441 ± 33 nM at 30 μg/mL mAb), which was reduced to a similar extent (to 66–70% of control) by NF449, pre-exposure to α,β-meATP or apyrase omission, demonstrating a significant P2X1 receptor contribution. FcγRIIa activation-dependent P2X1 responses were partially resistant to nitric oxide (NO), but abrogated by 500 nM prostacyclin (PGI ₂ ). Aggregation responses to bacteria and FcγRIIa activation were also inhibited by P2X1 receptor desensitization (to 66 and 42% of control, respectively). However, FcγRIIa-mediated tyrosine phosphorylation and ATP release were not significantly altered by the loss of P2X1 activity. In conclusion, we show that P2X1 receptors enhance platelet FcγRIIa receptor-evoked aggregation through an increase in [Ca ²⁺ ] _i downstream of the initial tyrosine phosphorylation events and early dense granule release. This represents a further route whereby ATP-gated cation channels can contribute to platelet-dependent immune responses in vivo.

Regulation of P2X Purinergic Receptor Signaling by Cholesterol

P2X receptors are cation-selective channels that are activated by the binding of extracellular ATP. They have a high permeability to Ca²⁺, Na⁺, and K⁺ and are expressed widely throughout the nervous, immune, cardiovascular, skeletal, gastrointestinal, respiratory, and endocrine systems. Seven mammalian subtypes of P2X receptor subunits have been identified, P2X1-7, and those that function as homotrimeric receptors (P2X1, 2, 3, 4, and 7) are targeted to lipid rafts, although they show limited resistance to solubilization by Triton X-100. Recent crystal structures of P2X3 and P2X4 receptors have provided considerable high-resolution information about the architecture of this family of receptors and yet the molecular details of how they are regulated by cholesterol are unknown. Currents mediated by the P2X1-4 receptors are either inhibited or relatively insensitive to cholesterol depletion, but there is no clear evidence to support the direct binding of cholesterol to these receptors. In contrast, the activity of the low-affinity, proinflammatory P2X7 receptor is potentiated by cholesterol depletion and regions within the proximal C-terminus play an important role in coupling changes in cholesterol to the gating of the pore. Based upon our understanding of the lipid signaling events that are triggered downstream of P2X7 receptor activation, a change in the levels of cholesterol may contribute to the sensitization of receptor currents and the dilation of the pore that occurs following prolonged, high-level stimulation. This chapter focuses on the regulation of P2X7 receptor signaling by cholesterol and our current understanding of the mechanisms that underlie this.

Calcium Signalling through Ligand-Gated Ion Channels such as P2X1 Receptors in the Platelet and other Non-Excitable Cells

Ligand-gated ion channels on the cell surface are directly activated by the binding of an agonist to their extracellular domain and often referred to as ionotropic receptors. P2X receptors are ligand-gated non-selective cation channels with significant permeability to Ca(2+) whose principal physiological agonist is ATP. This chapter focuses on the mechanisms by which P2X1 receptors, a ubiquitously expressed member of the family of ATP-gated channels, can contribute to cellular responses in non-excitable cells. Much of the detailed information on the contribution of P2X1 to Ca(2+) signalling and downstream functional events has been derived from the platelet. The underlying primary P2X1-generated signalling event in non-excitable cells is principally due to Ca(2+) influx, although Na(+) entry will also occur along with membrane depolarization. P2X1 receptor stimulation can lead to additional Ca(2+) mobilization via a range of routes such as amplification of G-protein-coupled receptor-dependent Ca(2+) responses. This chapter also considers the mechanism by which cells generate extracellular ATP for autocrine or paracrine activation of P2X1 receptors. For example cytosolic ATP efflux can result from opening of pannexin anion-permeable channels or following damage to the cell membrane. Alternatively, ATP stored in specialised secretory vesicles can undergo quantal release via the process of exocytosis. Examples of physiological or pathophysiological roles of P2X1-dependent signalling in non-excitable cells are also discussed, such as thrombosis and immune responses.

Blood cells: an historical account of the roles of purinergic signalling

The involvement of purinergic signalling in the physiology of erythrocytes, platelets and leukocytes was recognised early. The release of ATP and the expression of purinoceptors and ectonucleotidases on erythrocytes in health and disease are reviewed. The release of ATP and ADP from platelets and the expression and roles of P1, P2Y(1), P2Y(12) and P2X1 receptors on platelets are described. P2Y(1) and P2X(1) receptors mediate changes in platelet shape, while P2Y(12) receptors mediate platelet aggregation. The changes in the role of purinergic signalling in a variety of disease conditions are considered. The successful use of P2Y(12) receptor antagonists, such as clopidogrel and ticagrelor, for the treatment of thrombosis, myocardial infarction and stroke is discussed.

The Transmembrane Domains of β and IX Subunits Mediate the Localization of the Platelet Glycoprotein Ib-IX Complex to the Glycosphingolipid-enriched Membrane Domain

We have previously reported that the structural elements of the GP Ib-IX complex required for its localization to glycosphingolipid-enriched membranes (GEMs) reside in the Ibβ and IX subunits. To identify them, we generated a series of cell lines expressing mutant GP Ibβ and GP IX where 1) the cytoplasmic tails (CTs) of either or both GP Ibβ and IX are truncated, and 2) the transmembrane domains (TMDs) of GP Ibβ and GP IX were swapped with the TMD of a non-GEMs associating molecule, human transferrin receptor. Sucrose density fractionation analysis showed that the removal of either or both of the CTs from GP Ibβ and GP IX does not alter GP Ibα-GEMs association when compared with the wild type. In contrast, swapping of the TMDs of either GP Ibβ or GP IX with that of transferrin receptor results in a significant loss (∼ 50%) of GP Ibα from the low density GEMs fractions, with the largest effect seen in the dual TMD-replaced cells (> 80% loss) when compared with the wild type cells (100% of GP Ibα present in the GEMs fractions). Under high shear flow, the TMD-swapped cells adhere poorly to a von Willebrand factor-immobilized surface to a much lesser extent than the previously reported disulfide linkage dysfunctional GP Ibα-expressing cells. Thus, our data demonstrate that the bundle of GP Ibβ and GP IX TMDs instead of their individual CTs is the structural element that mediates the β/IX complex localization to the membrane GEMs, which through the α/β disulfide linkage brings GP Ibα into the GEMs.

Peroxiredoxin II is an antioxidant enzyme that negatively regulates collagen-stimulated platelet function

Collagen-induced platelet signaling is mediated by binding to the primary receptor glycoprotein VI (GPVI). Reactive oxygen species produced in response to collagen have been found to be responsible for the propagation of GPVI signaling pathways in platelets. Therefore, it has been suggested that antioxidant enzymes could down-regulate GPVI-stimulated platelet activation. Although the antioxidant enzyme peroxiredoxin II (PrxII) has emerged as having a role in negatively regulating signaling through various receptors by eliminating H2O2 generated upon receptor stimulation, the function of PrxII in collagen-stimulated platelets is not known. We tested the hypothesis that PrxII negatively regulates collagen-stimulated platelet activation. We analyzed PrxII-deficient murine platelets. PrxII deficiency enhanced GPVI-mediated platelet activation through the defective elimination of H2O2 and the impaired protection of SH2 domain-containing tyrosine phosphatase 2 (SHP-2) against oxidative inactivation, which resulted in increased tyrosine phosphorylation of key components for the GPVI signaling cascade, including Syk, Btk, and phospholipase Cγ2. Interestingly, PrxII-mediated antioxidative protection of SHP-2 appeared to occur in the lipid rafts. PrxII-deficient platelets exhibited increased adhesion and aggregation upon collagen stimulation. Furthermore, in vivo experiments demonstrated that PrxII deficiency facilitated platelet-dependent thrombus formation in injured carotid arteries. This study reveals that PrxII functions as a protective antioxidant enzyme against collagen-stimulated platelet activation and platelet-dependent thrombosis.

Ca2+ influx through P2X1 receptors amplifies P2Y1 receptor-evoked Ca2+ signaling and ADP-evoked platelet aggregation

Many cells express both P2X cation channels and P2Y G-protein-coupled receptors that are costimulated by nucleotides released during physiologic or pathophysiologic responses. For example, during hemostasis and thrombosis, ATP-gated P2X1 channels and ADP-stimulated P2Y1 and P2Y12 G-protein coupled receptors play important roles in platelet activation. It has previously been reported that P2X1 receptors amplify P2Y1-evoked Ca(2+) responses in platelets, but the underlying mechanism and influence on function is unknown. In human platelets, we show that maximally activated P2X1 receptors failed to stimulate significant aggregation but could amplify the aggregation response to a submaximal concentration of ADP. Costimulation of P2X1 and P2Y1 receptors generated a superadditive Ca(2+) increase in both human platelets and human embryonic kidney 293 (HEK293) cells via a mechanism dependent on Ca(2+) influx rather than Na(+) influx or membrane depolarization. The potentiation, due to an enhanced P2Y1 response, was observed if ADP was added up to 60 seconds after P2X1 activation. P2X1 receptors also enhanced Ca(2+) responses when costimulated with type 1 protease activated and M1 muscarinic acetylcholine receptors. The P2X1-dependent amplification of Gq-coupled [Ca(2+)]i increase was mimicked by ionomycin and was not affected by inhibition of protein kinase C, Rho-kinase, or extracellular signal-regulated protein kinase 1/2, which suggests that it results from potentiation of inositol 1,4,5-trisphosphate receptors and/or phospholipase C. We conclude that Ca(2+) influx through P2X1 receptors amplifies Ca(2+) signaling through P2Y1 and other Gq-coupled receptors. This represents a general form of co-incidence detection of ATP and coreleased agonists, such as ADP at sites of vascular injury or synaptic transmitters acting at metabotropic Gq-coupled receptors.

The trafficking and targeting of P2X receptors

The functional expression of P2X receptors at the plasma membrane is dependent on their trafficking along secretory and endocytic pathways. There are seven P2X receptor subunits, and these differ in their subcellular distributions because they have very different trafficking properties. Some are retained within the endoplasmic reticulum (ER), while others are predominantly at the cell surface or within endosomes and lysosomes. Changes in recruitment of receptors to and from the plasma membrane provides a way of rapidly up- or down-regulating the cellular response to adenosine triphosphate (ATP). An additional layer of regulation is the targeting of these receptors within the membranes of each compartment, which affects their stability, function and the nature of the effector proteins with which they form signaling complexes. The trafficking and targeting of P2X receptors is regulated by their interactions with other proteins and with lipids and we can expect this to vary in a cell-type specific manner and in response to changes in the environment giving rise to differences in receptor activity and function.

P2X and P2Y nucleotide receptors as targets in cardiovascular disease

Endogenous nucleotides have widespread actions in the cardiovascular system, but it is only recently that the P2X and P2Y receptor subtypes, at which they act, have been identified and subtype-selective agonists and antagonists developed. These advances have greatly increased our understanding of the physiological and pathophysiological functions of P2X and P2Y receptors, but investigation of the clinical usefulness of selective ligands is at an early stage. Nonetheless, the evidence considered in this review demonstrates clearly that various cardiovascular disorders, including vasospasm, hypertension, congestive heart failure and cardiac damage during ischemic episodes, may be viable targets. With further development of novel, selective agonists and antagonists, our understanding will continue to improve and further therapeutic applications are likely to be discovered.

Identification of contractile P2Y1, P2Y6, and P2Y12 receptors in rat intrapulmonary artery using selective ligands

ATP and UDP constrict rat intrapulmonary arteries, but which receptors mediate these actions is unclear. Here, we used selective agonists and antagonists, along with measurements of P2Y receptor expression, to characterize the receptor subtypes involved. Isometric tension was recorded from endothelium-denuded rat intrapulmonary artery rings (i.d. 200-500 μm) mounted on a wire myograph. Expression of P2Y receptor subtype expression was determined by using reverse transcription-polymerase chain reaction with receptor-specific oligonucleotide primers. The selective P2Y(1) agonist (N)-methanocarba-2-methylthioadenosine-5'-O-diphosphate (MRS2365) induced small, concentration-dependent contractions that were inhibited by the P2Y(1) antagonist N(6)-methyl-2'-deoxyadenosine-3',5'-bisphosphate (MRS2179). Contractions evoked by ATP were unaffected by MRS2179, but inhibited by approximately one-third by the P2Y(12) antagonist N(6)-(2-methylthiomethyl)-2-(3,3,3-trifluoropropylthio)dichloro-methylene ATP (AR-C69931MX). Combined blockade of P2X1 and P2Y(12) receptors virtually abolished the response to ATP. ADP also evoked contractions that were abolished by AR-C69931MX. The selective P2Y(6) receptor agonist 3-(2-oxo-2-phenylethyl)-UDP (PSB 0474) evoked concentration-dependent contractions and was approximately three times more potent than UDP, but the P2Y(14) agonist UDP-glucose had no effect. Contractions evoked by UDP were inhibited by the P2Y(6) receptor antagonist N,N'-1,4-butanediylbis-N'-(3-isothiocyanatophenyl)thiourea (MRS2578), but not the cysteinyl leukotriene 1 (CysL(1)) antagonist 3-(3-(2-(7-chloro-2-quinolinyl)ethenyl)phenyl)((3-dimethylamino-3-oxopropyl)thio)methyl)thiopropanoic acid (MK571). Higher concentrations of MRS2578 inhibited contractions to KCl, so they were not studied further. mRNA for P2Y(1), P2Y(6), and P2Y(12) receptors was identified. Our working model is that P2Y(12) and P2X1 receptors are present in rat intrapulmonary arteries and together mediate ATP-induced vasoconstriction. Contractile P2Y(6), but not P2Y(14) or CysLT(1), receptors are also present and are a major site through which UDP evokes constriction.

Heat shock protein 90 inhibitors reduce trafficking of ATP-gated P2X1 receptors and human platelet responsiveness

We have used selective inhibitors to determine whether the molecular chaperone heat shock protein 90 (HSP90) has an effect on both recombinant and native human P2X1 receptors. P2X1 receptor currents in HEK293 cells were reduced by ∼70–85% by the selective HSP90 inhibitor geldanamycin (2 μm, 20 min). This was associated with a speeding in the time course of desensitization as well as a reduction in cell surface expression. Imaging in real time of photoactivatable GFP-tagged P2X receptors showed that they are highly mobile. Geldanamycin almost abolished this movement for P2X1 receptors but had no effect on P2X2 receptor trafficking. P2X1/2 receptor chimeras showed that the intracellular N and C termini were involved in geldanamycin sensitivity. Geldanamycin also inhibited native P2X1 receptor-mediated responses. Platelet P2X1 receptors play an important role in hemostasis, contribute to amplification of signaling to a range of stimuli including collagen, and are novel targets for antithrombotic therapies. Platelet P2X1 receptor-, but not P2Y1 receptor-, mediated increases in intracellular calcium were reduced by 40–45% following HSP90 inhibition with geldanamycin or radicicol. Collagen stimulation leads to ATP release from platelets, and calcium increases to low doses of collagen were also reduced by ∼40% by the HSP90 inhibitors consistent with an effect on P2X1 receptors. These studies suggest that HSP90 inhibitors may be as effective as selective antagonists in regulating platelet P2X1 receptors, and their potential effects on hemostasis should be considered in clinical studies.

Activation and regulation of purinergic P2X receptor channels

Mammalian ATP-gated nonselective cation channels (P2XRs) can be composed of seven possible subunits, denoted P2X1 to P2X7. Each subunit contains a large ectodomain, two transmembrane domains, and intracellular N and C termini. Functional P2XRs are organized as homomeric and heteromeric trimers. This review focuses on the binding sites involved in the activation (orthosteric) and regulation (allosteric) of P2XRs. The ectodomains contain three ATP binding sites, presumably located between neighboring subunits and formed by highly conserved residues. The detection and coordination of three ATP phosphate residues by positively charged amino acids are likely to play a dominant role in determining agonist potency, whereas an AsnPheArg motif may contribute to binding by coordinating the adenine ring. Nonconserved ectodomain histidines provide the binding sites for trace metals, divalent cations, and protons. The transmembrane domains account not only for the formation of the channel pore but also for the binding of ivermectin (a specific P2X4R allosteric regulator) and alcohols. The N- and C- domains provide the structures that determine the kinetics of receptor desensitization and/or pore dilation and are critical for the regulation of receptor functions by intracellular messengers, kinases, reactive oxygen species and mercury. The recent publication of the crystal structure of the zebrafish P2X4.1R in a closed state provides a major advance in the understanding of this family of receptor channels. We will discuss data obtained from numerous site-directed mutagenesis experiments accumulated during the last 15 years with reference to the crystal structure, allowing a structural interpretation of the molecular basis of orthosteric and allosteric ligand actions.

Identification of human P2X1 receptor-interacting proteins reveals a role of the cytoskeleton in receptor regulation

P2X1 receptors are ATP-gated ion channels expressed by smooth muscle and blood cells. Carboxyl-terminally His-FLAG-tagged human P2X1 receptors were stably expressed in HEK293 cells and co-purified with cytoskeletal proteins including actin. Disruption of the actin cytoskeleton with cytochalasin D inhibited P2X1 receptor currents with no effect on the time course of the response or surface expression of the receptor. Stabilization of the cytoskeleton with jasplakinolide had no effect on P2X1 receptor currents but decreased receptor mobility. P2X2 receptor currents were unaffected by cytochalasin, and P2X1/2 receptor chimeras were used to identify the molecular basis of actin sensitivity. These studies showed that the intracellular amino terminus accounts for the inhibitory effects of cytoskeletal disruption similar to that shown for lipid raft/cholesterol sensitivity. Stabilization of the cytoskeleton with jasplakinolide abolished the inhibitory effects of cholesterol depletion on P2X1 receptor currents, suggesting that lipid rafts may regulate the receptor through stabilization of the cytoskeleton. These studies show that the cytoskeleton plays an important role in P2X1 receptor regulation.

P2X1 receptor-mediated vasoconstriction of afferent arterioles in angiotensin II-infused hypertensive rats fed a high-salt diet

Experiments tested the hypothesis that P2 receptor reactivity is impaired in angiotensin (Ang) II hypertensive rats fed an 8%NaCl diet (Ang II+HS). Juxtamedullary afferent arteriolar autoregulatory behavior was determined over a pressure range of 65 to 200 mm Hg. Arteriolar responsiveness to P2X1 (β,γ-methylene ATP) or P2Y2 receptor (uridine triphosphate) activation was determined in vitro. Systolic blood pressure averaged 126±3 and 225±4 mm Hg in control and Ang II+HS rats, respectively (P<0.05). In control kidneys, β,γ-methylene ATP (10(-8) to 10(-4) mol/L) reduced arteriolar diameter by 8±3%, 13±5%, 19±5%, 22±6%, and 24±9%, respectively, whereas uridine triphosphate reduced diameter by 2±1%, 2±2%, 9±3%, 37±7%, and 58±7%. Autoregulation was markedly blunted in Ang II+HS kidneys, with arteriolar diameter remaining essentially unchanged when perfusion pressure increased to 200 mm Hg compared with a 40±2% decline in diameter observed in normal kidneys over the same pressure range (P<0.05). P2X1 receptor-mediated vasoconstriction was significantly attenuated in Ang II+HS kidneys. β,γ-Methylene ATP reduced arteriolar diameter by 1±1%, 3±2%, 6±1%, 9±3%, and 7±1%, respectively (P<0.05), versus control rats. Similar patterns were noted when hypertensive perfusion pressures were used. Uridine triphosphate-mediated responses were unchanged in Ang II+HS rats compared with control, indicating preservation of P2Y2 receptor function. Ang II+HS blunted P2X1-mediated increases in intracellular Ca2+ concentration in preglomerular smooth muscle cells. Therefore, Ang II+HS rats exhibit attenuated afferent arteriolar responses to P2X1 receptor stimulation. These data support the hypothesis that P2X1 receptors are important for pressure-mediated autoregulatory responses. Impairment of P2X1 receptor function may explain the hypertension-induced decline in renal autoregulatory capability.

The P2X1 receptor and platelet function

Extracellular nucleotides are ubiquitous signalling molecules, acting via the P2 class of surface receptors. Platelets express three P2 receptor subtypes, ADP-dependent P2Y1 and P2Y12 G-protein-coupled receptors and the ATP-gated P2X1 non-selective cation channel. Platelet P2X1 receptors can generate significant increases in intracellular Ca(2+), leading to shape change, movement of secretory granules and low levels of α(IIb)β(3) integrin activation. P2X1 can also synergise with several other receptors to amplify signalling and functional events in the platelet. In particular, activation of P2X1 receptors by ATP released from dense granules amplifies the aggregation responses to low levels of the major agonists, collagen and thrombin. In vivo studies using transgenic murine models show that P2X1 receptors amplify localised thrombosis following damage of small arteries and arterioles and also contribute to thromboembolism induced by intravenous co-injection of collagen and adrenaline. In vitro, under flow conditions, P2X1 receptors contribute more to aggregate formation on collagen-coated surfaces as the shear rate is increased, which may explain their greater contribution to localised thrombosis in arterioles compared to venules within in vivo models. Since shear increases substantially near sites of stenosis, anti-P2X1 therapy represents a potential means of reducing thrombotic events at atherosclerotic plaques.

Lipid raft association and cholesterol sensitivity of P2X1-4 receptors for ATP: chimeras and point mutants identify intracellular amino-terminal residues involved in lipid regulation of P2X1 receptors

Cholesterol-rich lipid rafts act as signaling microdomains and can regulate receptor function. We have shown in HEK293 cells recombinant P2X1-4 receptors (ATP-gated ion channels) are expressed in lipid rafts. Localization to flotillin-rich lipid rafts was reduced by the detergent Triton X-100. This sensitivity to Triton X-100 was concentration- and subunit-dependent, demonstrating differential association of P2X1-4 receptors with lipid rafts. The importance of raft association to ATP-evoked P2X receptor responses was determined in patch clamp studies. The cholesterol-depleting agents methyl-β-cyclodextrin or filipin disrupt lipid rafts and reduced P2X1 receptor currents by >90%. In contrast, ATP-evoked P2X2-4 receptor currents were unaffected by lipid raft disruption. To determine the molecular basis of cholesterol sensitivity, we generated chimeric receptors replacing portions of the cholesterol-sensitive P2X1 receptor with the corresponding region from the insensitive P2X2 receptor. These chimeras identified the importance of the intracellular amino-terminal region between the conserved protein kinase C site and the first transmembrane segment for the sensitivity to cholesterol depletion. Mutation of any of the variant residues between P2X1 and P2X2 receptors in this region in the P2X1 receptor (residues 20-23 and 27-29) to cysteine removed cholesterol sensitivity. Cholesterol depletion did not change the ATP sensitivity or cell surface expression of P2X1 receptors. This suggests that cholesterol is normally needed to facilitate the opening/gating of ATP-bound P2X1 receptor channels, and mutations in the pre-first transmembrane segment region remove this requirement.

Pannexin-1 hemichannel-mediated ATP release together with P2X1 and P2X4 receptors regulate T-cell activation at the immune synapse

Engagement of T cells with antigen-presenting cells requires T-cell receptor (TCR) stimulation at the immune synapse. We previously reported that TCR stimulation induces the release of cellular adenosine-5'-triphosphate (ATP) that regulates T-cell activation. Here we tested the roles of pannexin-1 hemichannels, which have been implicated in ATP release, and of various P2X receptors, which serve as ATP-gated Ca(2+) channels, in events that control T-cell activation. TCR stimulation results in the translocation of P2X1 and P2X4 receptors and pannexin-1 hemichannels to the immune synapse, while P2X7 receptors remain uniformly distributed on the cell surface. Removal of extracellular ATP or inhibition, mutation, or silencing of P2X1 and P2X4 receptors inhibits Ca(2+) entry, nuclear factors of activated T cells (NFAT) activation, and induction of interleukin-2 synthesis. Inhibition of pannexin-1 hemichannels suppresses TCR-induced ATP release, Ca(2+) entry, and T-cell activation. We conclude that pannexin-1 hemichannels and P2X1 and P2X4 receptors facilitate ATP release and autocrine feedback mechanisms that control Ca(2+) entry and T-cell activation at the immune synapse.

Two-pore potassium ion channels are inhibited by both G(q/11)- and G(i)-coupled P2Y receptors

Two-pore potassium (K(2P)) ion channels and P2Y receptors modulate the activity of neurones and are targets for the treatment of neuronal disorders. Here we have characterised their interaction. In cells coexpressing the Galpha(i)-coupled hP2Y(12) receptor, ADP and ATP significantly inhibited hK(2P)2.1 currents. This was abolished by pertussis toxin (PTX), the hP2Y(12) antagonist AR-C69931MX, the hP2Y(1) antagonist MRS2179 and by mutating potential PKA/PKC phosphorylation sites in the channel C terminal. In cells coexpressing the Galpha(q/11)-coupled hP2Y(1) receptor, ADP and ATP also inhibited hK(2P)2.1 currents, which were abolished by MRS2179, but unaffected by AR-C69931MX and PTX. When both receptors were coexpressed with K(2P)2.1 channels, ADP-induced inhibition was antagonised by AR-C69913MX and MRS2179, but not PTX. Thus, both Galpha(q/11)- and Galpha(i)-coupled P2Y receptors inhibit K(2P) channels and the action of hP2Y(12) receptors appears to involve co-activation of endogenous hP2Y(1) receptors. This represents a novel mechanism by which P2Y receptors may modulate neuronal activity.

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.

The dual nature of extracellular ATP as a concentration-dependent platelet P2X1 agonist and antagonist

Patient groups subject to higher occurrence of stroke (e.g., people with diabetes, cystic fibrosis, pulmonary hypertension) have reduced release of ATP from their erythrocytes (ERYs) when subjected to flow-induced deformation or pharmacological stimuli. These same groups also have platelets that are more adhesive in comparison to controls. Here we show platelet aggregation, and inhibition of that aggregation, is affected by free Ca(2+) entering the platelet through the ATP-gated P2X1 receptor. The addition of ATP (10 microM) increased the platelet NO by 26.7 +/- 7.7%. This value was decreased significantly to below basal levels in the presence of NF 449 (p < 0.001), an inhibitor of the P2X1 receptor on the platelet. Aggregation profiles measured in the presence of ATP revealed that when the P2X1 receptor was blocked, or when the measurements were performed in Ca(2+) free buffer, platelet aggregation was nearly eliminated. Our findings employing standard aggregation measurements suggest that ATP behaves as a platelet inhibitor below 1.6 x 10(-19) moles ATP per platelet; however, above this value, ATP behaves as a platelet activator. These findings suggesting a dual nature of ATP with regard to platelet behavior were confirmed by passing platelets over endothelial cells that were coated in the channels of a microfluidic device. Importantly, it was determined that ERY-derived ATP release was a major determinant of platelet adhesion to the endothelium. These findings may have implications in anti-platelet drug design as most current therapies focus on the inhibition of P2Y-type receptors. Moreover, through the use of microfluidic technologies, we have provided in vitro evidence for a possible relationship between ERY properties and platelet behavior in vivo.

Apical localization of PMCA2w/b is lipid raft-dependent

Alternative splicing of the first intracellular loop differentially targets plasma membrane calcium ATPase (PMCA) isoform 2 to the apical or basolateral membrane in MDCK cells. To determine if the targeting is affected by lipid interactions, we stably expressed PMCA2w/b and PMCA2z/b in MDCK cells, and analyzed the PMCA distribution by confocal fluorescence microscopy and membrane fractionation. PMCA2w/b showed clear apical and lateral distribution, whereas PMCA2z/b was mainly localized to the basolateral membrane. A significant fraction of PMCA2w/b partitioned into low-density membranes associated with lipid rafts. Depletion of membrane cholesterol by methyl-beta-cyclodextrin resulted in reduced lipid raft association and a striking loss of PMCA2w/b from the apical membrane, whereas the lateral localization of PMCA2z/b remained unchanged. Our data indicate that alternative splicing differentially affects the lipid interactions of PMCA2w/b and PMCA2z/b and that the apical localization of PMCA2w/b is lipid raft-dependent and sensitive to cholesterol depletion.

Lipid rafts/caveolae as microdomains of calcium signaling

Ca(2+) is a major signaling molecule in both excitable and non-excitable cells, where it serves critical functions ranging from cell growth to differentiation to cell death. The physiological functions of these cells are tightly regulated in response to changes in cytosolic Ca(2+) that is achieved by the activation of several plasma membrane (PM) Ca(2+) channels as well as release of Ca(2+) from the internal stores. One such channel is referred to as store-operated Ca(2+) channel that is activated by the release of endoplasmic reticulum (ER) Ca(2+) which initiates store-operated Ca(2+) entry (SOCE). Recent advances in the field suggest that some members of TRPCs and Orai channels function as SOCE channels. However, the molecular mechanisms that regulate channel activity and the exact nature of where these channels are assembled and regulated remain elusive. Research from several laboratories has demonstrated that key proteins involved in Ca(2+) signaling are localized in discrete PM lipid rafts/caveolar microdomains. Lipid rafts are cholesterol and sphingolipid-enriched microdomains that function as unique signal transduction platforms. In addition lipid rafts are dynamic in nature which tends to scaffold certain signaling molecules while excluding others. By such spatial segregation, lipid rafts not only provide a favorable environment for intra-molecular cross-talk but also aid to expedite the signal relay. Importantly, Ca(2+) signaling is shown to initiate from these lipid raft microdomains. Clustering of Ca(2+) channels and their regulators in such microdomains can provide an exquisite spatiotemporal regulation of Ca(2+)-mediated cellular function. Thus in this review we discuss PM lipid rafts and caveolae as Ca(2+)-signaling microdomains and highlight their importance in organizing and regulating SOCE channels.

Platelet receptor recognition and cross-talk in collagen-induced activation of platelets

Comprehensive mapping of protein-binding sites within human collagen III has allowed the recognition motifs for integrin alpha(2)beta(1) and VWF A3 domain to be identified. Glycoprotein VI-binding sites are understood, although less well defined. This information, together with recent developments in understanding collagen fiber architecture, and crystal structures of the receptor collagen-binding domains, allows a coherent model for the interaction of collagen with the platelet surface to be developed. This complements our understanding of the orchestration of receptor presentation by membrane microdomains, such that the polyvalent collagen surface may stabilize signaling complexes within the heterogeneous receptor composition of the lipid raft. The ensuing interactions lead to the convergence of signals from each of the adhesive receptors, mediated by FcR gamma-chain and/or FcgammaRIIa, leading to concerted and co-operative platelet activation. Each receptor has a shear-dependent role, VWF/GpIb essential at high shear, and alpha(2)beta(1) at low and intermediate shear, whilst GpVI provides core signals that contribute to enhanced integrin affinity, tighter binding to collagen and consequent platelet activation.

Primary and secondary agonists can use P2X(1) receptors as a major pathway to increase intracellular Ca(2+) in the human platelet

In the platelet, it is well established that many G-protein- and tyrosine kinase-coupled receptors stimulate phospholipase-C-dependent Ca(2+) mobilization; however, the extent to which secondary activation of adenosine 5'-triphosphate (ATP)-gated P2X(1) receptors contributes to intracellular Ca(2+) responses remains unclear. We now show that selective inhibition of P2X(1) receptors substantially reduces the [Ca(2+)](i) increase evoked by several important agonists in human platelets; for collagen, thromboxane A(2), thrombin, and adenosine 5'-diphoshate (ADP) the maximal effect was a reduction to 18%, 34%, 52%, and 69% of control, respectively. The direct contribution of P2X(1) to the secondary Ca(2+) response was far greater than that of either P2Y receptors activated by co-released ADP, or via synergistic P2X(1):P2Y interactions. The relative contribution of P2X(1) to the peak Ca(2+) increase varied with the strength of the initial stimulus, being greater at low compared to high levels of stimulation for both glycoprotein VI and PAR-1, whereas P2X(1) contributed equally at both low and high levels of stimulation of thromboxane A(2) receptors. In contrast, only strong stimulation of P2Y receptors resulted in significant P2X(1) receptor activation. ATP release was detected by soluble luciferin:luciferase in response to all agonists that stimulated secondary P2X(1) receptor activation. However, P2X(1) receptors were stimulated earlier and to a greater extent than predicted from the average ATP release, which can be accounted for by a predominantly autocrine mechanism of activation. Given the central role of [Ca(2+)](i) increases in platelet activation, these studies indicate that ATP should be considered alongside ADP and thromboxane A(2) as a significant secondary platelet agonist.