The P2X1ion channel in platelet function

Molecular Aspects of Pathophysiology of Platelet Receptors

Receptor is a dynamic instrumental surface protein that helps to interact with specific molecules to respond accordingly. Platelet is the smallest in size among the blood components, but it plays many pivotal roles to maintain hemostasis involving its surface receptors. It (platelet) has cell adhesion receptors (e.g., integrins and glycoproteins), leucine-rich repeats receptors (e.g., TLRs, glycoprotein complex, and MMPs), selectins (e.g., CLEC, P-selectin, and CD), tetraspanins (e.g., CD and LAMP), transmembrane receptors (e.g., purinergic—P2Y and P2X1), prostaglandin receptors (e.g., TxA2, PGH2, and PGI2), immunoglobulin superfamily receptors (e.g., FcRγ and FcεR), etc. on its surface. The platelet receptors (e.g., glycoproteins, protease-activated receptors, and GPCRs) during platelet activation are over expressed and their granule contents are secreted (including neurotransmitters, cytokines, and chemokines) into circulation, which are found to be correlated with different physiological conditions. Interestingly, platelets promote metastasis through circulation protecting from cytolysis and endogenous immune surveillance involving several platelets receptors. The updated knowledge about different types of platelet receptors in all probable aspects, including their inter- and intra-signaling mechanisms, are discussed with respect to not only its (platelets) receptor type but also under different pathophysiological conditions.

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.

C-type lectin like receptor 2 (CLEC-2) signals independently of lipid raft microdomains in platelets

C-type lectin like receptor 2 (CLEC-2) has been reported to activate platelets through a lipid raft-dependent manner. Secreted ADP potentiates CLEC-2-mediated platelet aggregation. We have investigated whether the decrease in CLEC-2-mediated platelet aggregation, previously reported in platelets with disrupted rafts, is a result of the loss of agonist potentiation by ADP. We disrupted platelet lipid rafts with methyl-β-cyclodextrin (MβCD) and measured signaling events downstream of CLEC-2 activation. Lipid raft disruption decreases platelet aggregation induced by CLEC-2 agonists. The inhibition of platelet aggregation by the disruption of lipid rafts was rescued by the exogenous addition of epinephrine but not 2-methylthioadenosine diphosphate (2MeSADP), which suggests that lipid raft disruption effects P2Y12-mediated Gi activation but not Gz. Phosphorylation of Syk (Y525/526) and PLCγ2 (Y759), were not affected by raft disruption in CLEC-2 agonist-stimulated platelets. Furthermore, tyrosine phosphorylation of the CLEC-2 hemi-ITAM was not effected when MβCD disrupts lipid rafts. Lipid rafts do not directly contribute to CLEC-2 receptor activation in platelets. The effects of disruption of lipid rafts in in vitro assays can be attributed to inhibition of ADP feedback that potentiates CLEC-2 signaling.

[Activators, receptors and signal transduction pathways of blood platelets]

Platelet participation in hemostatic plug formation requires transition into an activated state (or, rather, variety of states) upon action of agonists like ADP, thromboxane A , collagen, thrombin, and others. The mechanisms of action for different agonists, their receptors and signaling pathways associated with them, as well as the mechanisms of platelet response inhibition are the subject of the present review. Collagen exposed upon vessel wall damage induced initial platelet attachment and start of thrombus formation, which involves numerous processes such as aggregation, activation of integrins, granule secretion and increase of intracellular Ca2+. Thrombin, ADP, thromboxane A , and ATP activated platelets that were not initially in contact with the wall and induce additional secretion of activating substances. Vascular endothelium and secretory organs also affect platelet activation, producing both positive (adrenaline) an d negative (prostacyclin, nitric oxide) regulators, thereby determining the relation of activation and inhibition signals, which plays a significant role in the formation of platelet aggregate under normal and pathological conditions. The pathways of platelet signaling are still incompletely understood, and their exploration presents an important objective both for basic cell biology and for the development of new drugs, the methods of diagnostics and of treatment of hemostasis disorders.

Platelet biology: the role of shear

Platelets are anucleated fragments produced by megakaryocytes that circulate in the blood. Platelets are involved in the initial cellular response to damaged endothelium and migrate to this area to prevent excessive bleeding. What is becoming more acknowledged over the last few decades is that blood flow (hemodynamics) plays a critical role in platelet function. The purpose of this review is to summarize the current understanding of platelet biology with particular focus on the role of hemodynamics. The emerging concept of shear microgradients, which are challenging the traditional model of platelet function, will also be introduced in the review.

Subtype-specific control of P2X receptor channel signaling by ATP and Mg2+

The identity and forms of activating ligands for ion channels are fundamental to their physiological roles in rapid electrical signaling. P2X receptor channels are ATP-activated cation channels that serve important roles in sensory signaling and inflammation, yet the active forms of the nucleotide are unknown. In physiological solutions, ATP is ionized and primarily found in complex with Mg(2+). Here we investigated the active forms of ATP and found that the action of MgATP(2-) and ATP(4-) differs between subtypes of P2X receptors. The slowly desensitizing P2X2 receptor can be activated by free ATP, but MgATP(2-) promotes opening with very low efficacy. In contrast, both free ATP and MgATP(2-) robustly open the rapidly desensitizing P2X3 subtype. A further distinction between these two subtypes is the ability of Mg(2+) to regulate P2X3 through a distinct allosteric mechanism. Importantly, heteromeric P2X2/3 channels present in sensory neurons exhibit a hybrid phenotype, characterized by robust activation by MgATP(2-) and weak regulation by Mg(2+). These results reveal the existence of two classes of homomeric P2X receptors with differential sensitivity to MgATP(2-) and regulation by Mg(2+), and demonstrate that both restraining mechanisms can be disengaged in heteromeric channels to form fast and sensitive ATP signaling pathways in sensory neurons.

Emerging roles of store-operated Ca²⁺ entry through STIM and ORAI proteins in immunity, hemostasis and cancer

Store-operated Ca(2+) entry (SOCE) is an important Ca(2+) influx pathway, which is defined by the fact that depletion of intracellular Ca(2+) stores, mainly the endoplasmic reticulum (ER), triggers the opening of Ca(2+) channels in the plasma membrane. The best characterized SOC channel is the Ca(2+) release-activated Ca(2+) (CRAC) channel, which was first described in cells of the immune system but has since been reported in many different cell types. CRAC channels are multimers of ORAI family proteins, of which ORAI1 is the best characterized. They are activated by stromal interaction molecules (STIM) 1 and 2, which respond to the depletion of intracellular Ca(2+) stores with oligomerization and binding to ORAI proteins. The resulting SOCE is critical for the physiological function of many cell types including immune cells and platelets. Recent studies using cell lines, animal models and primary cells from human patients with defects in SOCE have highlighted the importance of this Ca(2+) entry mechanism in a variety of pathophysiological processes. This review focuses on the role of SOCE in immunity to infection, allergy, hemostasis and cancer.

Molecular and functional properties of P2X receptors--recent progress and persisting challenges

ATP-gated P2X receptors are trimeric ion channels that assemble as homo- or heteromers from seven cloned subunits. Transcripts and/or proteins of P2X subunits have been found in most, if not all, mammalian tissues and are being discovered in an increasing number of non-vertebrates. Both the first crystal structure of a P2X receptor and the generation of knockout (KO) mice for five of the seven cloned subtypes greatly advanced our understanding of their molecular and physiological function and their validation as drug targets. This review summarizes the current understanding of the structure and function of P2X receptors and gives an update on recent developments in the search for P2X subtype-selective ligands. It also provides an overview about the current knowledge of the regulation and modulation of P2X receptors on the cellular level and finally on their physiological roles as inferred from studies on KO mice.

Platelet receptors

Well-understood functions for "traditional" platelet receptors are described, but "newer" receptors are equally discussed. Receptors are described biochemically (structure, ligand(s), protein partners, and function) and whenever possible, their clinical importance (mutations, polymorphisms, syndrome) are highlighted.

Using antibodies against P2Y and P2X receptors in purinergic signaling research

The broad expression pattern of the G protein-coupled P2Y receptors has demonstrated that these receptors are fundamental determinants in many physiological responses, including neuromodulation, vasodilation, inflammation, and cell migration. P2Y receptors couple either G(q) or G(i) upon activation, thereby activating different signaling pathways. Ionotropic ATP (P2X) receptors bind extracellular nucleotides, a signal which is transduced within the P2X protein complex into a cation channel opening, which usually leads to intracellular calcium concentration elevation. As such, this family of proteins initiates or shapes several cellular processes including synaptic transmission, gene expression, proliferation, migration, and apoptosis. The ever-growing range of applications for antibodies in the last 30 years attests to their major role in medicine and biological research. Antibodies have been used as therapeutic tools in cancer and inflammatory diseases, as diagnostic reagents (flow cytometry, ELISA, and immunohistochemistry, to name a few applications), and in widespread use in biological research, including Western blot, immunoprecipitation, and ELISPOT. In this article, we will showcase several of the advances that scientists around the world have achieved using the line of antibodies developed at Alomone Labs for P2Y and P2X receptors.

Anti-platelet therapy: ADP receptor antagonists

The P2Y(12) receptor on platelets with which ADP interacts has an important role in promoting platelet function and thereby platelet involvement in both haemostasis and thrombosis. Agents that act as antagonists at this receptor are thus likely to provide effective antithrombotic therapy, provided that there are no adverse effects on haemostasis. Here we describe the ADP receptor antagonists that are available and in development. We also consider their mode of action and ask whether there are additional mechanisms through which they exert their inhibitory effects on platelet function.

PKC inhibition markedly enhances Ca2+ signaling and phosphatidylserine exposure downstream of protease-activated receptor-1 but not protease-activated receptor-4 in human platelets

BACKGROUND

Cytosolic calcium concentration is a critical regulator of platelet activation, and so platelet Ca(2+) signaling must be tightly controlled. Thrombin-induced Ca(2+) signaling is enhanced by inhibitors of protein kinase C (PKC), suggesting that PKC negatively regulates the Ca(2+) signal, although the mechanisms by which this occurs and its physiological relevance are still unclear.

OBJECTIVES

To investigate the mechanisms by which PKC inhibitors enhance thrombin-induced Ca(2+) signaling, and to determine the importance of this pathway in platelet activation.

METHODS

Cytosolic Ca(2+) signaling was monitored in fura-2-loaded human platelets. Phosphatidylserine (PS) exposure, a marker of platelet procoagulant activity, was measured by annexin V binding and flow cytometry.

RESULTS

PKC inhibition by bisindolylmaleimide-I (BIM-I) enhanced α-thrombin-induced Ca(2+) signaling in a concentration-dependent manner. PAR1 signaling, activated by SFLLRN, was enhanced much more strongly than PAR4, activated by AYPGKF or γ-thrombin, which is a potent PAR4 agonist but a poor activator of PAR1. BIM-I had little effect on α-thrombin-induced signaling following treatment with the PAR1 antagonist, SCH-79797. BIM-I enhanced Ca(2+) release from intracellular stores and Ca(2+) entry, as assessed by Mn(2+) quench. However, the plasma membrane Ca(2+) ATPase inhibitor, 5(6)-carboxyeosin, did not prevent the effect of BIM-I. PKC inhibition strongly enhanced α-thrombin-induced PS exposure, which was reversed by blockade of PAR1.

CONCLUSIONS

Together, these data show that when PAR1 is stimulated, PKC negatively regulates Ca(2+) release and Ca(2+) entry, which leads to reduced platelet PS exposure.

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.

Platelets at work in primary hemostasis

When platelet numbers are low or when their function is disabled, the risk of bleeding is high, which on the one hand indicates that in normal life vascular damage is a rather common event and that hence the role of platelets in maintaining a normal hemostasis is a continuously ongoing physiological process. Upon vascular injury, platelets instantly adhere to the exposed extracellular matrix resulting in platelet activation and aggregation to form a hemostatic plug. This self-amplifying mechanism nevertheless requires a tight control to prevent uncontrolled platelet aggregate formation that eventually would occlude the vessel. Therefore endothelial cells produce inhibitory compounds such as prostacyclin and nitric oxide that limit the growth of the platelet thrombus to the damaged area. With this review, we intend to give an integrated survey of the platelet response to vascular injury in normal hemostasis.

Platelet physiology and antiplatelet agents

Apart from the central beneficial role platelets play in hemostasis, they are also involved in atherothrombotic diseases. Here, we review the current knowledge of platelet intracellular signal transduction pathways involved in platelet adhesion, activation, amplification of the activation signal and aggregation, as well as pathways limiting platelet aggregation. A thorough understanding of these pathways allows explanation of the mechanism of action of existing antiplatelet agents, but also helps to identify targets for novel drug development.