Activation of platelet function through G protein-coupled receptors

CLEC-2 is an essential platelet-activating receptor in hemostasis and thrombosis

Damage to the integrity of the vessel wall leads to exposure of the subendothelial extracellular matrix (ECM), triggering platelet activation and aggregation. This process is essential for primary hemostasis but it may also lead to arterial thrombosis. Although the mechanisms underlying platelet activation on the ECM are well explored, it is less clear which receptors mediate cellular activation in a growing thrombus. Here we studied the role of the recently identified C-type lectin-like receptor 2 (CLEC-2) in this process. We show that anti-CLEC-2 antibody treatment of mice leads to complete and highly specific loss of CLEC-2 in circulating platelets for several days. CLEC-2-deficient platelets displayed normal adhesion under flow, but subsequent aggregate formation was severely defective in vitro and in vivo. As a consequence, CLEC-2 deficiency was associated with increased bleeding times and profound protection from occlusive arterial thrombus formation. These results reveal an essential function of CLEC-2 in hemostasis and thrombosis.

Human platelet pathology related to defects in the G-protein signaling cascade

Platelets are highly responsive to signals from their environment. The sensing and processing of some of these stimuli are mediated by G-protein signal transduction cascades. It is well established that proteins involved in signal transduction may be targets for naturally occurring mutations resulting in human diseases. The best-studied molecules in platelets in relation to disease are the G-protein coupled receptors being the most platelet-specific. Many of the other signal transduction genes are often not only present in platelets but also in other tissues. Therefore, the clinical phenotype of signaling defects in platelets, apart from the membrane receptor defects, is seldom isolated to a hemostatic phenotype. Moreover, as platelets are easily accessible cells, and one of the best-studied models regarding signaling, platelets are easily applicable to investigate defects in ubiquitously expressed genes. Apart from a discussion on classical thrombopathies, this review will also deal with the less commonly known relation between platelet signaling defects and disorders with a broader clinical phenotype.

Point-of-care assessment of antiplatelet agents in the perioperative period: a review

The aim of this paper was to review the strengths and limitations of current 'point-of-care' techniques for the detection of antiplatelet drug effects. The review was based on a Medline search for articles with key words related to "platelet function tests", "point-of-care", and "anaesthesia", published in English between January 1996 and September 2008. It was found that global assessments of 'haemostasis', such as the standard thrombelastograph, Sonoclot, Clot Signature Analyser and Hemodyne, are not specific for platelet function and are essentially insensitive to cyclooxygenase inhibitors (aspirin, non-steroidal anti-inflammatory drugs) and P2Y12 antagonists (ticlopidine, clopidogrel). Global assessments of 'platelet function', such as the PFA-100 and PlateletWorks, are more specific for platelet function, but also have limited sensitivity for cyclooxygenase inhibitors and P2Y12 antagonists. The newer devices developed specifically for the assessment of antiplatelet drugs, such as Platelet Mapping, the Impact Cone and Platelet Analyser and the VerifyNow, are more promising, but are not as sensitive as laboratory platelet aggregometry. All three categories of devices detect G(p)II(b)/III(a) antagonists (abciximab, tirofiban, eptifibatide) activity, but not all provide quantitative assessments for monitoring therapy. The limitations appeared to be related to the complexity of platelet function, the multiple pathways of platelet activation, the wide interpatient variability in platelet responses and the interdependence between platelets and other aspects of coagulation. The strengths and limitations of point-of-care devices should be appreciated before they are used to assist clinical decision-making in the perioperative period.

Clinically relevant concentrations of dopamine do not amplify agonist-induced human platelet Ca2+ mobilization or GP IIb IIIa activation and do not accelerate acute coronary thrombosis in dogs

BACKGROUND

Dopamine is an inotrope effective in the short term treatment of acute heart failure - including that caused by coronary artery disease. Catecholamines however can potentiate platelet activation and pre-dispose to coronary thrombosis.

AIMS

Dopamine was studied for effect on agonist induced human platelet Ca mobilization, human platelet GP iib iiia receptor activation and acute coronary thrombosis in dogs. Calcium sensitive indo-1, fluorescent immunostaining and flow cytometry were used for platelet studies while coronary thrombosis was induced in anesthetized dogs via endothelial damage, arterial wall injury and critical stenosis.

RESULTS

Dopamine 10 and 10 M had no effect on the amplitude of the platelet Ca signal evoked by thrombin 0.1 U/mL. Likewise, dopamine 10 M had no effect on GP IIb IIIa activation evoked by ADP 10 M and by thrombin 0.1 U/mL. In dogs, intravenous dopamine 8 ug/Kg/min had no effect on repetitive cycles of acute coronary thrombus formation. In positive control studies, epinephrine increased platelet responsiveness and accelerated canine coronary thrombosis.

CONCLUSION

Clinically relevant concentrations of dopamine did not amplify agonist induced human platelet Ca activation, GPiib iiia expression or experimental canine coronary thrombosis--providing a degree of reassurance concerning this versatile inotrope.

Platelet receptors and signaling in the dynamics of thrombus formation

Hemostasis and pathological thrombus formation are dynamic processes that require a co-ordinated series of events involving platelet membrane receptors, bidirectional intracellular signals, and release of platelet proteins and inflammatory substances. This review aims to summarize current knowledge in the key steps in the dynamics of thrombus formation, with special emphasis on the crucial participation of platelet receptors and signaling in this process. Initial tethering and firm adhesion of platelets to the exposed subendothelium is mediated by glycoprotein (GP) Ib/IX/V complex and collagen receptors, GP VI and alpha(2)beta(1) integrin, in the platelet surface, and by VWF and fibrillar collagen in the vascular site. Interactions between these elements are largely influenced by flow and trigger signaling events that reinforce adhesion and promote platelet activation. Thereafter, soluble agonists, ADP, thrombin, TxA(2), produced/released at the site of vascular injury act in autocrine and paracrine mode to amplify platelet activation and to recruit circulating platelets to the developing thrombus. Specific interactions of these agonists with their G-protein coupled receptors generate inside-out signaling leading to conformational activation of integrins, in particular alpha(IIb)beta(3), increasing their ligand affinity. Binding of alpha(IIb)beta(3) to its ligands, mainly fibrinogen, supports processes such as clot retraction and platelet aggregation. Stabilization of thrombi is supported by the late wave of signaling events promoted by close contact between aggregated platelets. The best known contact-dependent signaling is outside-in signaling through alphaIb beta(3), but new ones are being clarified such as those mediated by interaction of Eph receptors with ephrins, or by Sema 4D and Gas-6 binding to their receptors. Finally, newly identified mechanisms appear to control thrombus growth, including back-shifting of activated integrins and actuation of compensatory molecules such as ESAM or PECAM-1. The expanding knowledge of thrombotic disease is expected to translate into the development of new drugs to help management and prevention of thrombosis.

Inhibition of platelet GPIIb-IIIa and P-selectin expression by aspirin is impaired by stress hyperglycemia

Increased aspirin resistance may contribute to the increase in thrombotic events observed in patients with type 2 diabetes. In this study, we examined if acute exposure to increased plasma glucose impaired the inhibitory effects of aspirin on platelet activation. Whole-blood samples were incubated with 100 (euglycemia), 200, 300, and 600 mg/dl glucose followed by incubation with aspirin [acetylsalicylic acid (ASA)]. Using flow cytometry, GPIIb-IIIa and P-selectin were analyzed in unstimulated and arachidonic acid (AA)-stimulated platelets. In euglycemic blood, AA caused a significant increase in platelet GPIIb-IIIa expression [unstimulated: 59.5+/-8.2 total fluorescence intensity (TFI), AA stimulated: 319.6+/-42.7 TFI, P=.002] and P-selectin (4.4+/-0.7 and 179.5+/-38.5 TFI, P<.001). In vitro, ASA significantly inhibited both GPIIb-IIIa expression (36.5%) and P-selectin expression (81%; P<.005). However, increased blood glucose (200 mg/dl) significantly impaired the inhibitory effect of ASA (84% for GPIIb-IIIa, P<.005; 48% for P-selectin, P=NS). Increasing glucose to 600 mg/dl completely overwhelmed the inhibitory effect of ASA. A statistically significant interaction between glucose concentration and ASA dose was found (P<.001 for GPIIb-IIIa and P=.004 for P-selectin). In vitro, concentration-dependent stress hyperglycemia significantly impaired the inhibitory effects of aspirin on human platelet GPIIb-IIIa and P-selectin expression. Under acute hyperglycemic conditions, the effectiveness of ASA to inhibit platelets via the AA-activation pathway may be significantly reduced.

GNAS defects identified by stimulatory G protein alpha-subunit signalling studies in platelets

CONTEXT

GNAS is an imprinted region that gives rise to several transcripts, antisense transcripts, and noncoding RNAs, including transcription of RNA encoding the alpha-subunit of the stimulatory G protein (Gsalpha). The complexity of the GNAS cluster results in ubiquitous genomic imprints, tissue-specific Gsalpha expression, and multiple genotype-phenotype relationships. Phenotypes resulting from genetic and epigenetic abnormalities of the GNAS region include Albright's hereditary osteodystrophy, pseudohypoparathyroidism types Ia (PHPIa) and Ib (PHPIb), and pseudopseudohypoparathyroidism (PPHP).

OBJECTIVE

The aim was to study the complex GNAS pathology by a functional test as an alternative to the generally used but labor-intensive erythrocyte complementation assay.

DESIGN AND PATIENTS

We report the first platelet-based diagnostic test for Gsalpha hypofunction, supported by clinical, biochemical, and molecular data for six patients with PHPIa or PPHP and nine patients with PHPIb. The platelet test is based on the inhibition of platelet aggregation by cAMP, produced after Gsalpha stimulation.

RESULTS

Platelets are easily accessible, and platelet aggregation responses were found to reflect Gsalpha signaling defects in patients, in concordance with the patient's phenotype and genotype. Gsalpha hypofunction in PHPIa and PPHP patients with GNAS mutations was clearly detected by this method. Mildly decreased or normal Gsalpha function was detected in patients with PHPIb with either an overall or exon 1A-only epigenetic defect, respectively. Platelet Gsalpha expression was reduced in both PHPIb patient groups, whereas XLalphas was up-regulated only in PHPIb patients with the broad epigenetic defect.

CONCLUSION

The platelet-based test is a novel tool for establishing the diagnosis of Gsalpha defects, which may otherwise be quite challenging.

Elevated platelet activation in patients with atopic dermatitis and psoriasis: increased plasma levels of beta-thromboglobulin and platelet factor 4

BACKGROUND

Beyond their role in hemostasis and thrombosis, platelets are important for modulating inflammatory reactions. Activated platelets play a role in the pathomechanism of inflammatory diseases such as asthma, but little is known about platelet activation in chronic skin inflammation, including atopic dermatitis (AD) and psoriasis. Furthermore, the relationship between platelet activation and disease severity is not understood. This work was performed to investigate plasma levels of beta-thromboglobulin (beta-TG) and platelet factor 4 (PF4) as platelet activation markers in patients with AD or psoriasis, and to determine the relationships between these markers and disease severity.

METHODS

Plasma levels of beta-TG and PF4 were measured by enzyme-linked immunoassay in 22 healthy controls, 44 patients with AD, and 16 patients with psoriasis. The relationships between these markers and the scoring AD (SCORAD) index, blood eosinophilia, serum IgE and serum lactate dehydrogenase were investigated in AD patients, and relationships with the psoriasis area and severity index (PASI) score were examined in psoriatic patients.

RESULTS

Plasma beta-TG and PF4 levels were significantly higher in patients with AD or psoriasis compared with healthy controls. beta-TG and PF4 levels correlated with the SCORAD index, and PF4 levels correlated with PASI scores. Elevated beta-TG and PF4 levels were significantly reduced after treatments.

CONCLUSIONS

Our results show that blood platelets are activated in patients with AD or psoriasis, suggesting that activated platelets play a role in the pathomechanism of chronic skin inflammation. Furthermore, plasma beta-TG and PF4 may be markers for the severity of AD and psoriasis.

Effect of pH on the interaction between zwitterions and titanium oxide

The isoelectric points (IEPs) of two zwitterions, glycine and both-terminals-terminated poly(ethylene glycol) (NH(2)-PEG-COOH), were determined from the titration curves, and the thicknesses of zwitterion layers immobilized on titanium (Ti) with immersion and electrodeposition at various pH based on IEPs were evaluated with ellipsometry to investigate the effect of pH and the immobilization technique on the interactions between the zwitterions and the Ti surface. From the titration curves, pK(1), pK(2), and the IEP of glycine were determined as 2.8, 8.9, and 5.9, respectively, and pK(1), pK(2), and the IEP of NH(2)-PEG-COOH were determined as 2.1, 11.7, and 6.9, respectively. At a certain specific pH, (+)H(3)N-CH(2)-COO(-) or (+)H(3)N-PEG-COO(-) was formed by hydrolysis of glycine or NH(2)-PEG-COOH. In addition, the Ti surface was negatively charged at this pH. As a result, for immersion, the electrostatic reactivity between terminal groups of zwitterions and hydroxyl groups on the Ti surface was the highest and the thickness of the immobilized layer was significantly the largest at pH 12. For electrodeposition, glycine, with its lower molecular weight, was more easily attracted to the Ti surface than NH(2)-PEG-COOH, which has a higher molecular weight, while the thickness of the immobilized layer was the greatest at pH 12 in both zwitterions.

The Gq and G12 families of heterotrimeric G proteins report functional selectivity

Receptors coupled to the G(q) and G(12) families of heterotrimeric G proteins have surfaced rarely in the context of functional selectivity and always indirectly. We explore here the differential engagement of G(q) and G(13) (of the G(12) family) by the thromboxane A(2) receptor alpha (TPalpha), via agonist-effected [(35)S]-guanosine 5'-O-(3-thio)triphosphate binding when the G proteins themselves are used as reporters. We find for TPalpha introduced into human embryonic kidney 293 cells and for the receptor expressed normally in human platelets an agonist-selective engagement of G(q) versus G(13). Pinane thromboxane A(2) (PTA(2)) activates G(q) in preference to G(13), whereas 8-iso-prostaglandin F(2alpha) activates G(13) in preference to G(q). 9,11-Dideoxy-9alpha,11alpha-methanoepoxy-prosta-5Z,13E-dien-1-oic acid (U46619), in contrast, exhibits no preference. Reserve of receptor in relation to G protein and of G protein in relation to downstream events is apparent in some instances but does not have a bearing on selectivity. Activation of G proteins by PTA(2) is right-shifted from binding of the ligand to receptor, a manifestation of which is a bimodal action: PTA(2) is an antagonist at low concentrations and an agonist at higher concentrations. We posit two populations of TPalpha, or two intrinsic sites of ligand binding, with selectivity evident not only in terms of the G proteins activated but properties of antagonism versus agonism.

G(12)/G(13)-mediated signalling in mammalian physiology and disease

The human genome encodes hundreds of G-protein-coupled receptors. Their intracellular effects, however, are mediated by only four families of heterotrimeric G proteins: G(s), G(i)/G(o), G(q)/G(11) and G(12)/G(13). Progress in the knowledge about the G(12)/G(13) family has somewhat lagged behind because their downstream effectors remained unknown for several years, and tools to specifically interfere with G(12)/G(13)-mediated signalling were, therefore, missing. However, with the identification of G(12)/G(13)-regulated signalling pathways and the recent application of new techniques, such as conditional gene inactivation, RNA interference or expression of inhibitory proteins, new insights into the in vivo functions of this G-protein family have been gained. It has become clear that this pathway regulates cellular proliferation, movement and morphology in many different organs and that it is centrally involved in various diseases including cancer and cardiovascular disorders. Here, we focus on recent progress made in the analyses of the in vivo functions of mammalian G(12)/G(13)-mediated signalling.

Acute arterial thrombosis in the absence of inflammation: the stress-related anti-inflammatory hormone ACTH participates in platelet-mediated thrombosis

OBJECTIVE

Despite the reciprocal relationship that exists between inflammation and thrombosis, we asked whether thrombosis can develop without inflammation, and whether stress-related hormones (ACTH and cortisol) influence platelet-mediated thrombosis.

METHODS

We investigated the role of ACTH and cortisol in platelet aggregation, as well as on the circulating levels of IL-6 in pigs subjected to different treatments. In control animals, deep vessel wall injury (DVWI) was induced in the right common carotid artery, while in the animals under study DVWI was induced 60 min after ACTH administration (subgroup 1) or not at all (subgroup 2). In an ex vivo study we evaluated whether ACTH or cortisol modulates platelet aggregation. Indeed, we assessed whether blocking the P2Y platelet receptors inhibits the effect of ACTH on platelet aggregation. Finally, we assessed whether ACTH mobilizes intracellular calcium and modulates intracellular cAMP in platelets ex vivo.

RESULTS

We found that the suppression of inflammation following ACTH administration was accompanied by acute arterial thrombosis in the zone of injury in vivo. Furthermore, ACTH but not cortisol amplifies the platelet aggregation induced ex vivo by agonists. Platelets do not express ACTH receptors which may explain why ACTH does not reduce intracellular levels of cAMP in platelets. Nevertheless, supraphysiological concentrations of ACTH increase calcium mobilization in platelets.

CONCLUSION

These results indicate for the first time that ACTH may fulfil an important role in acute arterial thrombosis by increasing the platelet aggregation induced by agonists, probably via a G(q)-coupled pathway.

Rac1 is essential for phospholipase C-gamma2 activation in platelets

Platelet activation at sites of vascular injury is triggered through different signaling pathways leading to activation of phospholipase (PL) Cbeta or PLCgamma2. Active PLCs trigger Ca(2+) mobilization and entry, which is a prerequisite for adhesion, secretion, and thrombus formation. PLCbeta isoenzymes are activated downstream of G protein-coupled receptors (GPCRs), whereas PLCgamma2 is activated downstream of immunoreceptor tyrosine-based activation motif (ITAM)-coupled receptors, such as the major platelet collagen receptor glycoprotein (GP) VI or CLEC-2. The mechanisms underlying PLC regulation are not fully understood. An involvement of small GTPases of the Rho family (Rho, Rac, Cdc42) in PLC activation has been proposed but this has not been investigated in platelets. We here show that murine platelets lacking Rac1 display severely impaired GPVI- or CLEC-2-dependent activation and aggregation. This defect was associated with impaired production of inositol 1,4,5-trisphosphate (IP(3)) and intracellular calcium mobilization suggesting inappropriate activation of PLCgamma2 despite normal tyrosine phosphorylation of the enzyme. Rac1 ( -/- ) platelets displayed defective thrombus formation on collagen under flow conditions which could be fully restored by co-infusion of ADP and the TxA(2) analog U46619, indicating that impaired GPVI-, but not G-protein signaling, was responsible for the observed defect. In line with this, Rac1 ( -/- ) mice were protected in two collagen-dependent arterial thrombosis models. Together, these results demonstrate that Rac1 is essential for ITAM-dependent PLCgamma2 activation in platelets and that this is critical for thrombus formation in vivo.

Multiple ways to switch platelet integrins on and off

In the classical concept of platelet integrin activation, it is considered that unidirectional conformational changes of alpha(IIb)beta(3) and alpha(2)beta(1) regulate the adhesiveness of platelets for fibrin(ogen) and collagen, respectively. Here, we summarize recent evidence that these conformational changes: (i) can also occur in the reverse direction; and (ii) are not independent events. Platelet stimulation through the P2Y(12) receptors provokes only transient alpha(IIb)beta(3) activation via signaling routes involving phosphoinositide 3-kinases and Rap1b. Furthermore, alpha(IIb)beta(3) can be secondarily inactivated in platelets with prolonged high Ca(2+) rises, which expose phosphatidylserine and bind coagulation factors. Thus, platelet stimulation with strong agonists (collagen and thrombin) also results in transient integrin activation. Integrin alpha(2)beta(1) is found to be activated by a mechanism that is directly linked to alpha(IIb)beta(3) activation. Integrin alpha(2)beta(1) can adopt different activation states, depending on the trigger. Conclusively, reversibility and synchrony of platelet integrin activation are newly identified mechanisms to restrict thrombus growth and to allow optimal coagulation factor binding. Back-shifting of activated integrins towards their resting state may be a novel goal of antithrombotic medication.

Strategies for antiplatelet targets and agents

Platelets play a key role in thrombosis and haemostasis, which can be either beneficial or deleterious depending on the circumstance. Antiplatelet therapy is the 'cornerstone' in the prevention and treatment of thrombotic diseases. Platelet activation is a complex process known as transmembrane signaling which then serves to activate the platelet via a cascade of biochemical interactions. Currently available strategies of antiplatelet therapy are generally based on the signaling pathway of platelet activation, which possess the characters that mono-agent, mono-target, and mainly irreversible inhibition. Therefore, both established and novel antiplatelet agents have their own pros and cons and such problems as resistance, drug-drug interaction, discontinuation and monitoring, etc. have been appeared. Due to the problems existing in current antiplatelet agents, future new strategies for antiplatelet targets, agent-developing and treatment might probably include three aspects: targeting the factors associated with platelet hyperactivity, developing novel antiplatelet agents with multiple targets, mild and reversible properties from natural products, and keeping healthy lifestyle and emphasizing prevention.

In vitro norepinephrine significantly activates isolated platelets from healthy volunteers and critically ill patients following severe traumatic brain injury

Introduction

Norepinephrine, regularly used to increase systemic arterial blood pressure and thus improve cerebral perfusion following severe traumatic brain injury (TBI), may activate platelets. This, in turn, could promote microthrombosis formation and induce additional brain damage.

Methods

The objective of this study was to investigate the influence of norepinephrine on platelets isolated from healthy volunteers and TBI patients during the first two post-traumatic weeks. A total of 18 female and 18 male healthy volunteers of different age groups were recruited, while 11 critically ill TBI patients admitted consecutively to our intensive care unit were studied. Arterial and jugular venous platelets were isolated from norepinephrine-receiving TBI patients; peripheral venous platelets were studied in healthy volunteers. Concentration-dependent functional alterations of isolated platelets were analyzed by flow cytometry, assessing changes in surface P-selectin expression and platelet-derived microparticles before and after in vitro stimulation with norepinephrine ranging from 10 nM to 100 μM. The thrombin receptor-activating peptide (TRAP) served as a positive control.

Results

During the first week following TBI, norepinephrine-mediated stimulation of isolated platelets was significantly reduced compared with volunteers (control). In the second week, the number of P-selectin- and microparticle-positive platelets was significantly decreased by 60% compared with the first week and compared with volunteers. This, however, was associated with a significantly increased susceptibility to norepinephrine-mediated stimulation, exceeding changes observed in volunteers and TBI patients during the first week. This pronounced norepinephrine-induced responsiveness coincided with increased arterio-jugular venous difference in platelets, reflecting intracerebral adherence and signs of cerebral deterioration reflected by elevated intracranial pressure and reduced jugular venous oxygen saturation.

Conclusion

Clinically infused norepinephrine might influence platelets, possibly promoting microthrombosis formation. In vitro stimulation revealed a concentration- and time-dependent differential level of norepinephrine-mediated platelet activation, possibly reflecting changes in receptor expression and function. Whether norepinephrine should be avoided in the second post-traumatic week and whether norepinephrine-stimulated platelets might induce additional brain damage warrant further investigations.

The calcium sensor STIM1 is an essential mediator of arterial thrombosis and ischemic brain infarction

Platelet activation and aggregation are essential to limit posttraumatic blood loss at sites of vascular injury but also contributes to arterial thrombosis, leading to myocardial infarction and stroke. Agonist-induced elevation of [Ca²⁺]_i is a central step in platelet activation, but the underlying mechanisms are not fully understood. A major pathway for Ca²⁺ entry in nonexcitable cells involves receptor-mediated release of intracellular Ca²⁺ stores, followed by activation of store-operated calcium (SOC) channels in the plasma membrane. Stromal interaction molecule 1 (STIM1) has been identified as the Ca²⁺ sensor in the endoplasmic reticulum (ER) that activates Ca²⁺ release–activated channels in T cells, but its role in mammalian physiology is unknown. Platelets express high levels of STIM1, but its exact function has been elusive, because these cells lack a normal ER and Ca²⁺ is stored in a tubular system referred to as the sarcoplasmatic reticulum. We report that mice lacking STIM1 display early postnatal lethality and growth retardation. STIM1-deficient platelets have a marked defect in agonist-induced Ca²⁺ responses, and impaired activation and thrombus formation under flow in vitro. Importantly, mice with STIM1-deficient platelets are significantly protected from arterial thrombosis and ischemic brain infarction but have only a mild bleeding time prolongation. These results establish STIM1 as an important mediator in the pathogenesis of ischemic cardio- and cerebrovascular events.

Differential regulation of RhoA-mediated signaling by the TPalpha and TPbeta isoforms of the human thromboxane A2 receptor: independent modulation of TPalpha signaling by prostacyclin and nitric oxide

In humans, thromboxane (TX) A₂ signals through the TPα and TPβ isoforms of the TXA₂ receptor that exhibit common and distinct roles. For example, Gq/phospholipase (PL)Cβ signaling by TPα is directly inhibited by the vasodilators prostacyclin and nitric oxide (NO) whereas that signaling by TPβ is unaffected. Herein, we investigated whether TPα and/or TPβ regulate G₁₂/Rho activation and whether that signaling might be differentially regulated by prostacyclin and/or NO. Both TPα and TPβ independently regulated RhoA activation and signaling in clonal cells over-expressing TPα or TPβ and in primary human aortic smooth muscle cells (1° AoSMCs). While RhoA-signaling by TPα was directly impaired by prostacyclin and NO through protein kinase (PK)A- and PKG-dependent phosphorylation, respectively, signaling by TPβ was not directly affected by either agent. Collectively, while TPα and TPβ contribute to RhoA activation, our findings support the hypothesis that TPα is involved in the dynamic regulation of haemostasis and vascular tone, such as in response to prostacyclin and NO. Conversely, the role of TPβ in such processes remains unsolved. Data herein provide essential new insights into the physiologic roles of TPα and TPβ and, through studies in AoSMCs, reveal an additional mode of regulation of VSM contractile responses by TXA₂.

Modifying murine von Willebrand factor A1 domain for in vivo assessment of human platelet therapies

The A1 domain of von Willebrand factor (VWF-A1) plays a crucial role in hemostasis and thrombosis by initiating platelet adhesion at sites of arterial injury through interactions with the platelet receptor glycoprotein Ib alpha (GPIbalpha). Here we report that murine VWF-A1 supports limited binding of human platelets. However, atomic models of GPIbalpha-VWF-A1 complexes identified an electrostatic 'hot-spot' that, when mutated in murine VWF-A1, switches its binding specificity from mouse to human GPIbalpha. Furthermore, mice expressing this mutant VWF-A1 display a bleeding phenotype that can be corrected by infusion of human platelets. Mechanistically, human platelets correct the phenotype by forming occlusive thrombi, an event that can be abrogated by blockade of GPIbalpha or by the preadministration of inhibitors of platelet activation or adhesion (clopidogrel (Plavix) and abciximab (ReoPro), respectively). Thus, by modifying a protein interface, we have generated a potential biological platform for preclinical screening of antithrombotics that specifically target human platelets.

Regulation of platelet dense granule secretion by the Ral GTPase-exocyst pathway

Non-hydrolyzable GTP analogues, such as guanosine 5'-(beta, gamma-imido)triphosphate (GppNHp), induce granule secretion from permeabilized platelets in the absence of increased intracellular Ca(2+). Here, we show that the GppNHp-induced dense granule secretion from permeabilized platelets occurred concomitantly with the activation of small GTPase Ral. This secretion was inhibited by the addition of GTP-Ral-binding domain (RBD) of Sec5, which is a component of the exocyst complex known to function as a tethering factor at the plasma membrane for vesicles. We generated an antibody against Sec5-RBD, which abolished the interaction between GTP-Ral and the exocyst complex in vitro. The addition of this antibody inhibited the GppNHp-induced secretion. These data indicate that Ral mediates the GppNHp-induced dense granule secretion from permeabilized platelets through interaction with its effector, the exocyst complex. Furthermore, GppNHp enhanced the Ca(2+) sensitivity of dense granule secretion from permeabilized platelets, and this enhancement was inhibited by Sec5-RBD. In intact platelets, the association between Ral and the exocyst complex was induced by thrombin stimulation with a time course similar to that of dense granule secretion and Ral activation. Taken together, our results suggest that the Ral-exocyst pathway participates in the regulation of platelet dense granule secretion by enhancing the Ca(2+) sensitivity of the secretion.

Silencing of a targeted protein in in vivo platelets using a lentiviral vector delivering short hairpin RNA sequence

OBJECTIVE

Because platelets are anucleate cells having a limited life span, direct gene manipulation cannot in principle be used to investigate the involvement of a specific signal transduction pathway in platelet activation. In this study, we examined whether the expression of a short hairpin RNA (shRNA) sequence in hematopoietic stem cells is maintained during megakaryocyte differentiation, thus resulting in inhibition of targeted protein in platelets.

METHODS AND RESULTS

To identify platelets derived from transduced stem cells, we generated a lentiviral vector that simultaneously expresses the shRNA sequence driven by the U6 promoter and GFP under the control of the glycoprotein (GP) Ib alpha promoter. Transplantation of mouse bone marrow cells transduced with the vector facilitated specifically mark platelets derived from the transduced cells. Transplantation of cells transduced with shRNA sequence targeting integrin alphaIIb caused a significant reduction of integrin alphaIIb beta3 (alphaIIb beta3) expression in GFP-positive platelets. It also inhibited alphaIIb beta3 activation assessed by the binding of JON/A, an antibody that recognizes activated alphaIIb beta3. Talin-1 silencing by the same method resulted in normal alphaIIb beta3 expression but deficient inside-out alphaIIb beta3 signaling.

CONCLUSIONS

shRNA expression driven by the U6 promoter is preserved during megakaryopoiesis. This method facilitates functional analysis of targeted protein in platelet activation.

Characterization of a new peptide agonist of the protease-activated receptor-1

A new peptide (TFRRRLSRATR), derived from the c-terminal of human platelet P2Y(1) receptor, was synthesized and its biological function was evaluated. This peptide activated platelets in a concentration-dependent manner, causing shape change, aggregation, secretion and calcium mobilization. Of the several receptor antagonists tested, only BMS200261, a protease activated receptor 1 (PAR-1) specific antagonist, totally abolished the peptide-induced platelet aggregation, secretion and calcium mobilization. The TFRRR-peptide-pretreated washed platelets failed to aggregate in response to SFLLRN (10 microM) but not to AYPGKF (500 microM). In addition, in mouse platelets, peptide concentrations up to 600 microM failed to cause platelet activation, indicating that the TFRRR-peptide activated platelets through the PAR-1 receptor, rather than through the PAR-4 receptor. The shape change induced by 10 microM peptide was totally abolished by Y-27632, an inhibitor of p160(ROCK) which is a downstream mediator of G12/13 pathways. The TFRRR-peptide, YFLLRNP, and the physiological agonist thrombin selectively activated G12/13 pathways at low concentrations and began to activate both Gq and G12/13 pathways with increasing concentrations. Similar to SFLLRN, the TFRRR-peptide caused phosphorylation of Akt and Erk in a P2Y(12) receptor-dependent manner, and p-38 MAP kinase activation in a P2Y(12)-independent manner. The effects of this peptide are elicited by the first six amino acids (TFRRRL) whereas the remaining peptide (LSRATR), TFERRN, or TFEERN had no effects on platelets. We conclude that TFRRRL activates human platelets through PAR-1 receptors.

Computational and functional evaluation of a microfluidic blood flow device

The development of microfluidic devices supporting physiological blood flow has the potential to yield biomedical technologies emulating human organ function. However, advances in this area have been constrained by the fact that artificial microchannels constructed for such devices need to achieve maximum chemical diffusion as well as hemocompatibility. To address this issue, we designed an elastomeric microfluidic flow device composed of poly (dimethylsiloxane) to emulate the geometry and flow properties of the pulmonary microcirculation. Our chip design is characterized by high aspect ratio (width > height) channels in an orthogonally interconnected configuration. Finite element simulations of blood flow through the network design chip demonstrated that the apparent pressure drop varied in a linear manner with flow rate. For simulated flow rates <250 mul min, the simulated pressure drop was <2000 Pa, the flow was laminar, and hemolysis was minimal. Hemolysis rate, assayed in terms of [total plasma hemoglobin (TPH) (sample - control)/(TPH control)] during 6 and 12 hour perfusions at 250 mul/min, was <5.0% through the entire period of device perfusion. There was no evidence of microscopic thrombus at any channel segment or junction under these perfusion conditions. We conclude that a microfluidic blood flow device possessing asymmetric and interconnected microchannels exhibits uniform flow properties and preliminary hemocompatibility. Such technology should foster the development of miniature oxygenators and similar biomedical devices requiring both a microscale reaction volume and physiological blood flow.

Clinical aspects of platelet inhibitors and thrombus formation

The platelet, once thought to be solely involved in clot formation, is now known to be a key mediator in various others processes such as inflammation, thrombosis, and atherosclerosis. Supported by the wealth of evidence from clinical trials demonstrating their benefits in patient outcomes, antiplatelet agents have become paramount in the prevention and management of various diseases involving the cardiovascular, cerebrovascular, and peripheral arterial systems. Despite being among the most widely used and studied classes of medical therapies, new discoveries regarding important clinical aspects and properties of these agents continue to be made. As our understanding of platelet biology expands, more effective and safer novel therapies continue to be developed. The use of more refined agents in conjunction with a better understanding of their effects will further the ability to provide more optimized care on an individual basis.

Protease-activated receptor signaling: new roles and regulatory mechanisms

PURPOSE OF REVIEW

Protease-activated receptors are G-protein-coupled receptors that transmit cellular responses to coagulant proteases in a variety of cell types in the vasculature and other tissues. Several other proteases can activate protease-activated receptors in vitro and may affect their function in vivo. While a role for these receptors in hemostasis and thrombosis has been established, their functions in inflammatory and other responses have yet to be fully elucidated. In addition, the mechanisms responsible for protease and cell type-specific signaling mediated by these receptors are largely undefined. Here, we highlight recent advances in understanding the roles and regulation of protease-activated receptor signaling.

RECENT FINDINGS

Recent studies have increased our knowledge of the function of protease-activated receptor signaling in platelets and its contribution to thrombosis. In other cell types, recent work has revealed new connections between these receptors and signaling effectors important for vascular development and inflammatory responses. Other studies have advanced our understanding of protease and cell type-specific responses as well as novel regulatory mechanisms for control of protease-activated receptor signaling.

SUMMARY

Thus, elucidating the signaling and regulatory mechanisms of protease-activated receptors in various tissues and cell types is important for understanding their biological function as well as for designing therapeutic strategies to control their function.

In vivo thrombus formation in murine models

Platelets play a central role in hemostasis, but also in atherothrombosis, as they rapidly adhere to tissue and to one another as a response to any vascular injury. This process involves a large number of surface receptors, signaling pathways, and enzymatic cascades as well as their complex interplay. Although in vitro experiments proved successful in both identifying new receptors and pathways and developing potent and selective antithrombotic drugs, in vitro research cannot mimic the myriad hemodynamic and spatiotemporal cellular and molecular interactions that occur during the generation and propagation of thrombi in vivo. Animal models, and, with the availability of genetically modified mouse strains and of modern intravital imaging techniques, mouse models in particular, have opened new ways to identify both individual roles and the interplay of platelet proteins in complex in vivo settings. In vivo models revealed the important role of, eg, Gas6 or blood coagulation factor XII in thrombus formation, and results obtained in in vivo models raised the interesting possibility that (physiologic) hemostasis and (pathologic) thrombosis might represent 2 mechanistically different processes. This review summarizes in vivo findings that contributed significantly to our understanding of hemostatic and thrombotic processes and which may help to guide future research.

Modèles murins de pathologies plaquettaires

Platelet-related diseases correspond to functional defects or abnormal production (thrombopoiesis) of hereditary and immunological origins. Recent progress in the manipulation of the mouse genome (transgenesis, gene inactivation or insertion) has resulted in the generation of numerous strains exhibiting defective platelet function or production. Some strains reproduce known hereditary diseases affecting haemostasis (Glanzmann thrombasthenia, Bernard-Soulier syndrome (BSS) or thrombopoiesis (Wiscott-Aldrich or May-Hegglin syndrome). More often the mutated strains have no human equivalent and represent useful models to study: (i) the role of adhesive or signalling receptors or of signalling proteins in platelet-dependent haemostasis and thrombosis or; (ii) to study the poorly characterized mechanisms of thrombopoiesis, which implicate transcription factors (GATA, Fli1), growth factors and receptors (TPO, cMPL), and cytoskeletal or contractile proteins (tubulin, myosin). Additional mouse strains result from the selection of spontaneous mutants many of which affect intracellular platelet granules, representing models of storage pool diseases (SPD) such as the Gray platelet syndrome (alphaSPD) or Hermansky-Pudlack syndrome (deltaSPD). More recently, a systematic chemical mutagenesis approach has also identified genes involved in thrombopoiesis and platelet survival. Finally, mouse models of auto- or allo-immune thrombocytopenia have been developed to study the mechanisms of platelet destruction or removal.