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

ATP-mediated signalling in the central synapses

ATP released from the synaptic terminals and astrocytes can activate neuronal P2 receptors at a variety of locations across the CNS. Although the postsynaptic ATP-mediated signalling does not bring a major contribution into the excitatory transmission, it is instrumental for slow and diffuse modulation of synaptic dynamics and neuronal firing in many CNS areas. Neuronal P2X and P2Y receptors can be activated by ATP released from the synaptic terminals, astrocytes and microglia and thereby can participate in the regulation of synaptic homeostasis and plasticity. There is growing evidence of importance of purinergic regulation of synaptic transmission in different physiological and pathological contexts. Here, we review the main mechanisms underlying the complexity and diversity of purinergic signalling and purinergic modulation in central neurons.

Abacavir causes leukocyte/platelet crosstalk by activating neutrophil P2X7 receptors thus releasing soluble lectin-like oxidized low-density lipoprotein receptor-1

BACKGROUND AND PURPOSE

Abacavir, an antiretroviral drug used in HIV therapy associated with myocardial infarction, promotes thrombosis through P2X7 receptors. The role of platelets as pro-thrombotic cells is acknowledged whereas that of neutrophils-due to their secretory capacity-is gaining recognition. This study analyses the role of neutrophils-specifically the secretome of abacavir-treated neutrophils (SN_ABC )-in platelet activation that precedes thrombosis.

EXPERIMENTAL APPROACH

Effects of abacavir or SN_ABC on platelet activation and platelet-leukocyte interactions and expression of lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) were analysed by flow cytometry. The secretome was analysed by proteomics. The role of leukocytes in the actions of abacavir was evaluated in a mouse model of thrombosis.

KEY RESULTS

Abacavir induced platelet-leukocyte interactions, not directly via effects of abacavir on platelets, but via activation of neutrophils, which triggered interactions between platelet P-selectin and neutrophil P-selectin glycoprotein ligand-1 (PSGL-1). SN_ABC stimulated platelet activation and platelet-leukocyte interactions through a process that was dependent on LOX-1, neutrophil P2X7 and platelet P2Y1, P2Y12 and P2X1 receptors. Abacavir induced the expression of LOX-1 on neutrophils and of the soluble form of LOX-1 (sLOX-1) in SN_ABC . Neutrophils, LOX-1, P2X7, P2Y1, P2Y12 and P2X1 receptors were required for the pro-thrombotic actions of abacavir in vivo.

CONCLUSION AND IMPLICATIONS

Neutrophils are target cells in abacavir-induced thrombosis. Abacavir released sLOX-1 from neutrophils via activation of their P2X7 receptors, which in turn activated platelets. Hence, sLOX-1 could be the missing link in the cardiovascular risk associated with abacavir.

Molecular mapping of platelet hyperreactivity in diabetes: the stress proteins complex HSPA8/Hsp90/CSK2α and platelet aggregation in diabetic and normal platelets

The molecular understanding of the pathophysiological changes elicited by diabetes in platelets may help in further elucidating the involvement of this pseudo-cell in the increased risk of developing cardiovascular disease and thrombosis in diabetic subjects. We aimed to investigate the differential characteristics of platelets from diabetic patients and nondiabetic controls to unveil the molecular mechanisms behind the increased platelet reactivity in diabetes. We compared platelets from diabetic and control subjects by 2 dimensional-electrophoresis followed by mass spectrometry. Changes in selected differential proteins were validated by immunoprecipitation assays and western blot. Platelet aggregation was measured by light transmittance aggregometry induced by collagen and ADP, and dynamic coagulation analysis of whole blood was measured by thromboelastometry. We observed significant differences in proteins related to platelet aggregation, cell migration, and cell homeostasis. Subjects with diabetes showed higher platelet aggregation and thrombogenicity and higher contents of the stress-related protein complex HSPA8/Hsp90/CSK2α than nondiabetic subjects. Changes in the chaperones HSPA8 and Hsp90, and in CSK2α protein contents correlated with changes in platelet aggregation and blood coagulation activity. In conclusion, the complex HSPA8/Hsp90/CSK2α is involved in diabetes-related platelet hyperreactivity. The role of the HSPA8/Hsp90/CSK2α complex may become a molecular target for the development of future preventive and therapeutic strategies for platelet dysfunction associated with diabetes and its complications.

Pharmacologic dissection of the overlapping impact of heat shock protein family members on platelet function

BACKGROUND

Platelets play a pivotal role in hemostasis, wound healing, and inflammation, and are thus implicated in a variety of diseases, including cancer. Platelet function is associated with release of granule content, cellular shape change, and upregulation of receptors that promote establishment of a thrombus and maintenance of hemostasis.

OBJECTIVES

The role of heat shock proteins (Hsps) in modulating platelet function has been studied for a number of years, but comparative roles of individual Hsps have not been thoroughly examined.

METHODS

We utilized a panel of specific inhibitors of Hsp40, Hsp70, Hsp90, and Grp94 (the endoplasmic reticulum homolog of Hsp90) to assess their impact on several aspects of platelet function.

RESULTS

Inhibition of each of the aforementioned Hsps reduced alpha granule release. In contrast, there was some selectivity in impacts on dense granule release. Thromboxane synthesis was impaired after exposure to inhibitors of Hsp40, Hsp90, and Grp94, but not after inhibition of Hsp70. Both expression of active glycoprotein IIb/IIIa (GPIIb/IIIa) and fibrinogen-induced platelet shape change were diminished by our inhibitors. In contrast, aggregation was selectively abrogated after inhibition of Hsp40 or Hsp90. Lastly, activated platelet-cancer cell interactions were reduced by inhibition of both Hsp70 and Grp94.

CONCLUSIONS

These data suggest the importance of Hsp networks in regulating platelet activity.

Effects of purine-scaffold inhibitors on HUVECs: Involvement of the purinergic pathway and interference with ATP. Implications for preventing the adverse effects of extracellular Grp94

Background

Extracellular Glucose-regulated protein94 (Grp94) is linked to pathological conditions disrupting the obligatory intracellular location of this Heat Shock Protein (HSP). In plasma, Grp94 is linked to IgG in complexes that drive adverse effects on vascular cells and are biomarker of gastro-intestinal cancer. By blocking ATP site in different HSPs, purine-scaffold inhibitors are used as promising anti-cancer compounds, but their effects on vasculature are not known.

Methods

We tested the capacity of two purine-scaffold inhibitors, PU-H71 and PU-WS13, to prevent the binding of Grp94 to IgG and to antagonize the effects of Grp94 and native Grp94-IgG complexes on HUVECs in different experimental conditions.

Results

PU-H71 and PU-WS13 blocked Grp94 and the formation of Grp94-IgG complexes in absence of cells. Instead, in presence of HUVECs rather than Grp94 PU-inhibitors targeted cells causing stimulation of Akt and VEGF pathways and displaying angiogenic-like effects similar to, although less intense than that provoked by Grp94 and Grp94-IgG complexes. Unlike Grp94 and Grp94-IgG complexes, PU-inhibitors also activated the purinergic pathway and increased the expression of the ATP receptor P2X7. Effects of PU-inhibitors on HUVECs were reversed by ATP and in presence of ATP PU-inhibitors were again able to block Grp94.

Conclusions

PU-inhibitors can display direct effects on endothelial cells by targeting the ATP receptor P2X7. In absence of ATP, PU-inhibitors preferentially bind to cells rather than Grp94. ATP antagonizes the PU-inhibitor binding to cells thus restoring the capacity to block Grp94 and Grp94-IgG complex formation. Results have implications for enhancing the therapeutic efficacy of PU-inhibitors against circulating pathogenic Grp94.

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Extracellular Grp94 forms pathogenic complexes with IgG.

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PU-inhibitors block the Grp94-IgG complex formation in absence of cells.

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PU-inhibitors target cells and activate the purinergic pathway.

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Effects of PU-inhibitors on cells are reversed by ATP.

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ATP restores the capacity of PU-inhibitors to block the Grp94-IgG complex formation.

Organization of ATP-gated P2X1 receptor intracellular termini in apo and desensitized states

The human P2X1 receptor (hP2X1R) is a trimeric ligand-gated ion channel opened by extracellular ATP. The intracellular amino and carboxyl termini play significant roles in determining the time-course and regulation of channel gating-for example, the C terminus regulates recovery from the desensitized state following agonist washout. This suggests that the intracellular regions of the channel have distinct structural features. Studies on the hP2X3R have shown that the intracellular regions associate to form a cytoplasmic cap in the open state of the channel. However, intracellular features could not be resolved in the agonist-free apo and ATP-bound desensitized structures. Here we investigate the organization of the intracellular regions of hP2X1R in the apo and ATP-bound desensitized states following expression in HEK293 cells. We couple cysteine scanning mutagenesis of residues R25-G30 and H355-R360 with the use of bi-functional cysteine reactive cross-linking compounds of different lengths (MTS-2-MTS, BMB, and BM(PEG)2), which we use as molecular calipers. If two cysteine residues come into close proximity, we predict they will be cross-linked and result in ∼66% of the receptor subunits running on a Western blot as dimers. In the control construct (C349A) that removed the free cysteine C349, and some cysteine-containing mutants, cross-linker treatment does not result in dimerization. However, we detect efficient dimerization for R25C, G30C, P358C, K359C, and R360C. This selective pattern indicates that there is structural organization to these regions in the apo and desensitized states in a native membrane environment. The existence of such precap (apo) and postcap (desensitized) organization of the intracellular domains would facilitate efficient gating of the channel.

17-allylamino-17-demethoxygeldanamycin impeded chemotherapy through antioxidant activation via reducing reactive oxygen species-induced cell death

Hyperthermia enhances the anticancer effects of thymidylate synthase (TYMS) inhibitors (raltitrexed, RTX) and improves the precise biochemical mechanisms partially through enhancement of intracellular drug absorption. Recent research focuses on the potential anticancer drug target Heat Shock Protein 90 (HSP90), which could increase the sensitivity of cancer cells to TYMS inhibitors; however, with different HSP90 inhibitors, several research studies finally showed a poor efficacy in preclinical or clinical research. Here, we showed that 17-allylamino-17-demethoxygeldanamycin (17-AAG, HSP90 inhibitor) affects the efficacy of chemotherapy through antioxidant activation-induced resistance. In this study, we found that RTX, alone or in combination with hyperthermia, triggers reactive oxygen species (ROS) exposure and thus induces cell death. Also, the addition of hyperthermia showed more ROS exposure and function. The pharmacologic inhibition of HSP90 reversed the effects of chemotherapeutical treatments, while the overexpression of HSP90 showed no relation with these effects, which demonstrated that dysregulation of HSP90 might have a significant impact on chemotherapeutic treatments. The addition of 17-AAG increased the activation of antioxidant with increased antioxidant enzymes, thus affecting the RTX efficacy.

ATP-Gated P2X Receptor Channels: Molecular Insights into Functional Roles

In the nervous system, ATP is co-stored in vesicles with classical transmitters and released in a regulated manner. ATP from the intracellular compartment can also exit the cell through hemichannels and following shear stress or membrane damage. In the past 30 years, the action of ATP as an extracellular transmitter at cell-surface receptors has evolved from somewhat of a novelty that was treated with skepticism to purinergic transmission being accepted as having widespread important functional roles mediated by ATP-gated ionotropic P2X receptors (P2XRs). This review focuses on work published in the last five years and provides an overview of ( a) structural studies, ( b) the molecular basis of channel properties and regulation of P2XRs, and ( c) the physiological and pathophysiological roles of ATP acting at defined P2XR subtypes.

P2X4 receptor re-sensitization depends on a protonation/deprotonation cycle mediated by receptor internalization and recycling

KEY POINTS

Re-sensitization of P2X₄ receptors depends on a protonation/de-protonation cycle Protonation and de-protonation of the receptors is achieved by internalization and recycling of P2X₄ receptors via acidic compartments Protonation and de-protonation occurs at critical histidine residues within the extracellular loop of P2X₄ receptors Re-sensitization is blocked in the presence of the receptor agonist ATP ABSTRACT: P2X₄ receptors are members of the P2X receptor family of cation-permeable, ligand-gated ion channels that open in response to the binding of extracellular ATP. P2X₄ receptors are implicated in a variety of biological processes, including cardiac function, cell death, pain sensation and immune responses. These physiological functions depend on receptor activation on the cell surface. Receptor activation is followed by receptor desensitization and deactivation upon removal of ATP. Subsequent re-sensitization is required to return the receptor into its resting state. Desensitization and re-sensitization are therefore crucial determinants of P2X receptor signal transduction and responsiveness to ATP. However, the molecular mechanisms controlling desensitization and re-sensitization are not fully understood. In the present study, we provide evidence that internalization and recycling via acidic compartments is essential for P2X₄ receptor re-sensitization. Re-sensitization depends on a protonation/de-protonation cycle of critical histidine residues within the extracellular loop of P2X₄ receptors that is mediated by receptor internalization and recycling. Interestingly, re-sensitization under acidic conditions is completely revoked by receptor agonist ATP. Our data support the physiological importance of the unique subcellular distribution of P2X₄ receptors that is predominantly found within acidic compartments. Based on these findings, we suggest that recycling of P2X₄ receptors regulates the cellular responsiveness in the sustained presence of ATP.

The role of agonist-induced activation and inhibition for the regulation of purinergic receptor expression in human platelets

INTRODUCTION

Adenosine diphosphate (ADP) as physiological activator of human platelets mediates its effects via three purinergic receptors: P2Y1, P2Y12 and P2X1. The inhibition of P2Y12 is used pharmacologically to suppress aggregation underlining the physiological significance of this receptor. Since the regulation of purinergic receptor expression has not thoroughly been investigated yet, this study analyzed the content of purinergic receptors on the platelet surface membrane upon activation and inhibition.

MATERIALS AND METHODS

The surface expression of purinergic receptors was measured by flow cytometry using two different polyclonal antibodies as basal values and after incubation with thrombin receptor activating peptide (TRAP-6) or with inhibitors DEA/NO, MAHMA/NO or Prostaglandin E1 (PGE1). Western blot analysis was used to confirm inhibitory effects.

RESULTS

Both investigated antibodies revealed a significant increase of purinergic receptor expression upon TRAP-6 stimulation. The NO donors, DEA/NO and MAHMA/NO, did not influence basal or TRAP-6 stimulated values. PGE1 did not affect basal receptor expression, but diminished TRAP-6 stimulated purinergic receptor expression in a dose-dependent manner.

CONCLUSIONS

In summary, TRAP-6 induced platelet activation leads to an elevation of purinergic receptor expression. In contrast to other surface ligands, this effect is not suppressed by cGMP-mediated inhibition, but almost completely abrogated by enhanced cAMP-mediated signaling as induced by PGE1.

Heat-shock protein 72 promotes platelet aggregation induced by various platelet activators in rats

Increase of thrombus in the coronary arteries is positively correlated with the level of heat-shock protein 72 (HSP72) in the blood of patients with acute myocardial infarction (AMI). Platelet aggregation participates in thrombus formation on ruptured plaque in AMI. In this study, we aimed to clarify the role of HSP72 in thrombus formation by evaluating the effects of HSP72 on platelet aggregation. Platelet aggregation activities were measured in platelet-rich plasma obtained from male Sprague-Dawley rats with or without the platelet activators, such as adenosine diphosphate (ADP), collagen, thrombin receptor-activating peptide-6 (TRAP-6), ristocetin, and arachidonic acid. Changes in aggregation were estimated by the co-addition of recombinant HSP72 and anti-HSP72 antibodies. Our results showed that addition of HSP72 increased platelet aggregation in the presence of low concentrations of ADP, collagen, TRAP-6, ristocetin, and arachidonic acid. Increased platelet aggregation stimulated by ADP and HSP72 was reduced by the co-addition of anti-HSP72 antibodies. Thus, these findings suggested that HSP72 was released extracellularly in response to stress, promoting thrombus formation and AMI. Additionally, treatment with anti-HSP72 antibodies may control platelet aggregation induced by extracellular HSP72.

Endotoxin-induced autocrine ATP signaling inhibits neutrophil chemotaxis through enhancing myosin light chain phosphorylation

Although the neutrophil recruitment cascade during inflammation has been well described, the molecular players that halt neutrophil chemotaxis remain unclear. In this study, we found that lipopolysaccharide (LPS) was a potent stop signal for chemotactic neutrophil migration. Treatment with an antagonist of the ATP receptor (P2X1) in primary human neutrophils or knockout of the P2X1 receptor in neutrophil-like differentiated HL-60 (dHL-60) cells recovered neutrophil chemotaxis. Further observations showed that LPS-induced ATP release through connexin 43 (Cx43) hemichannels was responsible for the activation of the P2X1 receptor and the subsequent calcium influx. Increased intracellular calcium stopped neutrophil chemotaxis by activating myosin light chain (MLC) through the myosin light chain kinase (MLCK)-dependent pathway. Taken together, these data identify a previously unknown function of LPS-induced autocrine ATP signaling in inhibiting neutrophil chemotaxis by enhancing MLC phosphorylation, which provides important evidence that stoppage of neutrophil chemotaxis at infectious foci plays a key role in the defense against invading pathogens.

Exploring the Ca2+-dependent synaptic dynamics in vibro-dissociated cells

Dynamic alteration of the synaptic strength is one of the most important processes occurring in the nervous system. Combination of electrophysiology, confocal imaging and molecular biology led to significant advances in this research field. Yet, a progress in this area, in particular in studies of changes in the quantal behavior of central synapses and impact of glial cells on individual synapses, is hampered by technical difficulties of resolving small quantal synaptic currents. In this paper we will show how the technique of non-enzymatic vibro-dissociation, which enables to isolate living neurons avoiding artifacts of cell culture and preserving functional synapse, can be used to obtain a valuable information on fine details and mechanisms of synaptic plasticity. In particular, we will describe our recent results on Ca²⁺-dependent modulation of the postsynaptic AMPA and NMDA receptors in the individual synaptic boutons.

Several adaptor proteins promote intracellular localisation of the transporter MRP4/ABCC4 in platelets and haematopoietic cells

The multidrug resistance protein 4 (MRP4/ABCC4) has been identified as an important transporter for signalling molecules including cyclic nucleotides and several lipid mediators in platelets and may thus represent a novel target to interfere with platelet function. Besides its localisation in the plasma membrane, MRP4 has been also detected in the membrane of dense granules in resting platelets. In polarised cells it is localised at the basolateral or apical plasma membrane. To date, the mechanism of MRP4 trafficking has not been elucidated; protein interactions may regulate both the localisation and function of this transporter. We approached this issue by searching for interacting proteins by in vitro binding assays, followed by immunoblotting and mass spectrometry, and by visualising their co-localisation in platelets and haematopoietic cells. We identified the PDZ domain containing scaffold proteins ezrin-binding protein 50 (EBP50/NHERF1), postsynaptic density protein 95 (PSD95), and sorting nexin 27 (SNX27), but also the adaptor protein complex 3 subunit β3A (AP3B1) and the heat shock protein HSP90 as putative interaction partners of MRP4. The knock-down of SNX27, PSD95, and AP3B1 by siRNA in megakaryoblastic leukaemia cells led to a redistribution of MRP4 from intracellular structures to the plasma membrane. Inhibition of HSP90 led to a diminished expression and retention of MRP4 in the endoplasmic reticulum. These results indicate that MRP4 localisation and function are regulated by multiple protein interactions. Changes in the adaptor proteins can hence lead to altered localisation and function of the transporter.

ATP from synaptic terminals and astrocytes regulates NMDA receptors and synaptic plasticity through PSD-95 multi-protein complex

Recent studies highlighted the importance of astrocyte-secreted molecules, such as ATP, for the slow modulation of synaptic transmission in central neurones. Biophysical mechanisms underlying the impact of gliotransmitters on the strength of individual synapse remain, however, unclear. Here we show that purinergic P2X receptors can bring significant contribution to the signalling in the individual synaptic boutons. ATP released from astrocytes facilitates a recruitment of P2X receptors into excitatory synapses by Ca²⁺-dependent mechanism. P2X receptors, co-localized with NMDA receptors in the excitatory synapses, can be activated by ATP co-released with glutamate from pre-synaptic terminals and by glia-derived ATP. An activation of P2X receptors in turn leads to down-regulation of postsynaptic NMDA receptors via Ca²⁺-dependent de-phosphorylation and interaction with PSD-95 multi-protein complex. Genetic deletion of the PSD-95 or P2X4 receptors obliterated ATP-mediated down-regulation of NMDA receptors. Impairment of purinergic modulation of NMDA receptors in the PSD-95 mutants dramatically decreased the threshold of LTP induction and increased the net magnitude of LTP. Our findings show that synergistic action of glia- and neurone-derived ATP can pre-modulate efficacy of excitatory synapses and thereby can have an important role in the glia-neuron communications and brain meta-plasticity.

HSP90 Regulation of P2X7 Receptor Function Requires an Intact Cytoplasmic C-Terminus

P2X7 receptors (P2X7Rs) are ATP-gated ion channels that display the unusual property of current facilitation during long applications of agonists. Here we show that facilitation disappears in chimeric P2X7Rs containing the C-terminus of the P2X2 receptor (P2X2R), and in a truncated P2X7R missing the cysteine-rich domain of the C-terminus. The chimeric and truncated receptors also show an apparent decreased permeability to N-methyl-d-glucamine(+) (NMDG(+)). The effects of genetic modification of the C-terminus on NMDG(+) permeability were mimicked by preapplication of the HSP90 antagonist geldanamycin to the wild-type receptor. Further, the geldanamycin decreased the shift in the reversal potential of the ATP-gated current measured under bi-ionic NMDG(+)/Na(+) condition without affecting the ability of the long application of agonist to facilitate current amplitude. Taken together, the results suggest that HSP90 may be essential for stabilization and function of P2X7Rs through an action on the cysteine-rich domain of the cytoplasmic the C-terminus.

Heat shock protein 70 regulates platelet integrin activation, granule secretion and aggregation

Molecular chaperones that support protein quality control, including heat shock protein 70 (Hsp70), participate in diverse aspects of cellular and physiological function. Recent studies have reported roles for specific chaperone activities in blood platelets in maintaining hemostasis; however, the functions of Hsp70 in platelet physiology remain uninvestigated. Here we characterize roles for Hsp70 activity in platelet activation and function. In vitro biochemical, microscopy, flow cytometry, and aggregometry assays of platelet function, as well as ex vivo analyses of platelet aggregate formation in whole blood under shear, were carried out under Hsp70-inhibited conditions. Inhibition of platelet Hsp70 blocked platelet aggregation and granule secretion in response to collagen-related peptide (CRP), which engages the immunoreceptor tyrosine-based activation motif-bearing collagen receptor glycoprotein VI (GPVI)-Fc receptor-γ chain complex. Hsp70 inhibition also reduced platelet integrin-αIIbβ3 activation downstream of GPVI, as Hsp70-inhibited platelets showed reduced PAC-1 and fibrinogen binding. Ex vivo, pharmacological inhibition of Hsp70 in human whole blood prevented the formation of platelet aggregates on collagen under shear. Biochemical studies supported a role for Hsp70 in maintaining the assembly of the linker for activation of T cells signalosome, which couples GPVI-initiated signaling to integrin activation, secretion, and platelet function. Together, our results suggest that Hsp70 regulates platelet activation and function by supporting linker for activation of T cells-associated signaling events downstream of platelet GPVI engagement, suggesting a role for Hsp70 in the intracellular organization of signaling systems that mediate platelet secretion, "inside-out" activation of platelet integrin-αIIbβ3, platelet-platelet aggregation, and, ultimately, hemostatic plug and thrombus formation.

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.

Purinergic control of inflammation and thrombosis: Role of P2X1 receptors

Inflammation shifts the hemostatic mechanisms in favor of thrombosis. Upon tissue damage or infection, a sudden increase of extracellular ATP occurs, that might contribute to the crosstalk between inflammation and thrombosis. On platelets, P2X1 receptors act to amplify platelet activation and aggregation induced by other platelet agonists. These receptors critically contribute to thrombus stability in small arteries. Besides platelets, studies by our group indicate that these receptors are expressed by neutrophils. They promote neutrophil chemotaxis, both in vitro and in vivo. In a laser-induced injury mouse model of thrombosis, it appears that neutrophils are required to initiate thrombus formation and coagulation activation on inflamed arteriolar endothelia. In this model, by using P2X1−/ − mice, we recently showed that P2X1 receptors, expressed on platelets and neutrophils, play a key role in thrombus growth and fibrin generation. Intriguingly, in a model of endotoxemia, P2X1−/ − mice exhibited aggravated oxidative tissue damage, along with exacerbated thrombocytopenia and increased activation of coagulation, which translated into higher susceptibility to septic shock. Thus, besides its ability to recruit neutrophils and platelets on inflamed endothelia, the P2X1 receptor also contributes to limit the activation of circulating neutrophils under systemic inflammatory conditions. Taken together, these data suggest that P2X1 receptors are involved in the interplay between platelets, neutrophils and thrombosis. We propose that activation of these receptors by ATP on neutrophils and platelets represents a new mechanism that regulates thrombo-inflammation.

Vas deferens neuro-effector junction: from kymographic tracings to structural biology principles

The vas deferens is a simple bioassay widely used to study the physiology of sympathetic neurotransmission and the pharmacodynamics of adrenergic drugs. The role of ATP as a sympathetic co-transmitter has gained increasing attention and furthered our understanding of its role in sympathetic reflexes. In addition, new information has emerged on the mechanisms underlying the storage and release of ATP. Both noradrenaline and ATP concur to elicit the tissue smooth muscle contractions following sympathetic reflexes or electrical field stimulation of the sympathetic nerve terminals. ATP and adenosine (its metabolic byproduct) are powerful presynaptic regulators of co-transmitter actions. In addition, neuropeptide Y, the third member of the sympathetic triad, is an endogenous modulator. The peptide plus ATP and/or adenosine play a significant role as sympathetic modulators of transmitter's release. This review focuses on the physiological principles that govern sympathetic co-transmitter activity, with special interest in defining the motor role of ATP. In addition, we intended to review the recent structural biology findings related to the topology of the P2X1R based on the crystallized P2X4 receptor from Danio rerio, or the crystallized adenosine A2A receptor as a member of the G protein coupled family of receptors as prototype neuro modulators. This review also covers structural elements of ectonucleotidases, since some members are found in the vas deferens neuro-effector junction. The allosteric principles that apply to purinoceptors are also reviewed highlighting concepts derived from receptor theory at the light of the current available structural elements. Finally, we discuss clinical applications of these concepts.

Amplification of human platelet activation by surface pannexin-1 channels

BACKGROUND

Pannexin-1 (Panx1) forms an anion-selective channel with a permeability up to ~1 kDa and represents a non-lytic, non-vesicular ATP release pathway in erythrocytes, leukocytes and neurons. Related connexin gap junction proteins have been reported in platelets; however, the expression and function of the pannexins remain unknown.

OBJECTIVE

To determine the expression and function of pannexins in human plate-lets, using molecular, cellular and functional techniques.

METHODS

Panx1 expression in human platelets was det-ermined using qPCR and antibody-based techniques. Contributions of Panx1 to agonist-evoked efflux of cytoplasmic calcein, Ca(2+) influx, ATP release and aggregation were assessed in washed platelets under conditions where the P2X1 receptor response was preserved (0.32 U mL(-1) apyrase). Thrombus formation in whole blood was assessed in vitro using a shear chamber assay. Two structurally unrelated and widely used Panx1 inhibitors, probenecid and carbenoxolone, were used throughout this study, at concentrations that do not affect connexin channels.

RESULTS

PANX1, but not PANX2 or PANX3, mRNA was detected in human platelets. Furthermore, Panx1 protein is glycosylated and present on the plasma membrane of platelets, and displays weak physical association with P2X1 receptors. Panx1 inhibition blocked thrombin-evoked efflux of calcein, and reduced Ca(2+) influx, ATP release, platelet aggregation and thrombus formation under arterial shear rates in vitro. The Panx1-dependent contribution was not additive to that of P2X1 receptors.

CONCLUSIONS

Panx1 is expressed on human platelets and amplifies Ca(2+) influx, ATP release and aggregation through the secondary activation of P2X1 receptors. We propose that Panx1 represents a novel target for the management of arterial thrombosis.

P2X1 receptors localized in lipid rafts mediate ATP motor responses in the human vas deferens longitudinal muscles

To assess the role of the P2X1 receptors (P2X1R) in the longitudinal and circular layers of the human vas deferens, ex vivo-isolated strips or rings were prepared from tissue biopsies to record isometric contractions. To ascertain its membrane distribution, tissue extracts were analyzed by immunoblotting following sucrose gradient ultracentrifugation. ATP, alpha,beta-methylene ATP, or electrical field stimulation elicited robust contractions of the longitudinal layer but not of the circular layer which demonstrated inconsistent responses. Alpha,beta-methylene ATP generated stronger and more robust contractions than ATP. In parallel, prostatic segments of the rat vas deferens were examined. The motor responses in both species were not sustained but decayed within the first minute, showing desensitization to additional applications. Cross-desensitization was established between alpha,beta-methylene ATP or ATP-evoked contractions and electrical field stimulation-induced contractions. Full recovery of the desensitized motor responses required more than 30 min and showed a similar pattern in human and rat tissues. Immunoblot analysis of the human vas deferens extracts revealed a P2X1R oligomer of approximately 200 kDa under nonreducing conditions, whereas dithiothreitol-treated extracts showed a single band of approximately 70 kDa. The P2X1R was identified in ultracentrifugation fractions containing 15%-29% sucrose; the receptor localized in the same fractions as flotillin-1, indicating that it regionalized into smooth muscle lipid rafts. In conclusion, ATP plays a key role in human vas deferens contractile responses of the longitudinal smooth muscle layer, an effect mediated through P2X1Rs.

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.