Platelet Prothrombinase Activity and Intracellular Calcium Responses in Patients With Storage Pool Deficiency, Glycoprotein IIb-IIIa Deficiency, or Impaired Platelet Coagulant Activity — A Comparison With Scott Syndrome

A novel GATA1 variant p.G229D causing the defect of procoagulant platelet formation

INTRODUCTION

GATA1 is one of the master transcription factors in hematopoietic lineages development which is crucial for megakaryocytic differentiation and maturation. Previous studies have shown that distinct GATA1 variants are associated with varying severities of macrothrombocytopenia and platelet dysfunction.

OBJECTIVE

To determine the underlying pathological mechanisms of a novel GATA1 variant (c. 686G > A, p. G229D) in a patient with recurrent traumatic muscle hematomas.

METHODS

Comprehensive phenotypic analysis of the patient platelets was performed. Procoagulant platelet formation and function were detected using flow cytometry assay and thrombin generation test (TGT), respectively. The ANO6 expression was measured by qPCR and western blot. The intracellular supramaximal calcium flux was detected by Fluo-5N fluorescent assay.

RESULTS

The patient displayed mild macrothrombocytopenia with defects of platelet granules, aggregation, and integrin αIIbβ3 activation. The percentage of the procoagulant platelet formation of the patient upon the stimulation of thrombin plus collagen was lower than that of the healthy controls (40.9 % vs 49.0 % ± 5.1 %). The patient platelets exhibited a marked reduction of thrombin generation in platelet rich plasma TGT compared to the healthy controls (peak value: ∼70 % of the healthy controls; the endogenous thrombin potential: ∼40 % of the healthy controls). The expression of ANO6 and intracellular calcium flux were impaired, which together with abnormal granules of the patient platelets might contribute to defect of procoagulant platelet function.

CONCLUSIONS

The G229D variant could lead to a novel platelet phenotype characterized by defective procoagulant platelet formation and function, which extended the range of GATA1 variants associated platelet disorders.

Prothrombin consumption as an indicator of hemorrhagic phenotype in mild platelet function disorders

BACKGROUND

The bleeding risk of patients with mild platelet function disorders is difficult to assess and their phenotype remains ill-explored.

AIM

This study was designed to establish a comprehensive biological phenotype of patients with mild platelet function disorders.

METHODS

Twenty patients were included with persistent abnormal light transmission aggregometry (LTA). The ISTH bleeding assessment tool (ISTH-BAT) was assessed to identify laboratory analyses associated with an abnormal hemorrhagic score.

RESULTS

The majority of patients had defects that might affect Gαi protein signaling pathways or minor abnormalities. No LTA nor flow cytometry parameters were associated with an above-normal hemorrhagic score. However, prothrombin consumption, which corresponds to the ratio of serum residual factor II to plasma residual factor II, was significantly higher (p = .006) in the abnormal ISTH-BAT group (mean = 14%, SD = 6) compared with the normal ISTH-BAT group (mean = 8%, SD 4). Prothrombin consumption was significantly associated with ISTH-BAT score (r = .5287, IC 95% 0.0986-0.7924, p = .0165).

CONCLUSION

In this group of patients, there was an association between a pathological bleeding score and increased prothrombin consumption. This test could be used as an additional indicator of platelet function abnormality liable to be related to bleeding risk.

Thrombin generation abnormalities in commonly encountered platelet function disorders

INTRODUCTION

Studies of thrombin generation (TG) with platelet-rich plasma (PRP) and platelet-poor plasma (PPP) have provided insights on bleeding disorders. We studied TG for a cohort with commonly encountered platelet function disorders (PFD).

METHODS

Participants included 40 controls and 31 with PFD due to: nonsyndromic dense granule (DG) deficiency (PFD-DGD, n = 9), RUNX1 haploinsufficiency (n = 6) and aggregation defects from other, uncharacterized causes (n = 16). TG was tested with PRP and PPP samples. As DG store ADP and polyphosphate that enhance platelet-dependent TG, PFD-DGD PRP TG was tested for correction with ADP, polyphosphate and combined additives. Tissue factor pathway inhibitor (TFPI), platelet factor V (FV), and platelet TFPI and ANO6 transcript levels were also evaluated. Findings were tested for associations with TG endpoints and bleeding.

RESULTS

PFD samples had impaired PRP TG, but also impaired PPP TG, with strong associations between their PRP and PPP TG endpoints (P ≤ .005). PFD-DGD PRP TG endpoints showed associations to PPP TG endpoints but not to DG counts, and were improved, but not fully corrected, by adding polyphosphate and agonists. PFD participants had increased plasma TFPI and reduced platelet TFPI (P ≤ .02) but normal levels of platelet FV, and platelet TFPI and ANO6 transcripts levels. PFD plasma TFPI levels showed significant association to several PPP TG endpoints (P ≤ .04). Several PFD PRP TG endpoints showed significant associations to bleeding symptoms, including wound healing problems and prolonged bleeding from minor cuts (P ≤ .04).

CONCLUSION

TG is impaired in commonly encountered PFD, with their PRP TG findings showing interesting associations to symptoms.

Characterization of Procoagulant COAT Platelets in Patients with Glanzmann Thrombasthenia

Patients affected by the rare Glanzmann thrombasthenia (GT) suffer from defective or low levels of the platelet-associated glycoprotein (GP) IIb/IIIa, which acts as a fibrinogen receptor, and have therefore an impaired ability to aggregate platelets. Because the procoagulant activity is a dichotomous facet of platelet activation, diverging from the aggregation endpoint, we were interested in characterizing the ability to generate procoagulant platelets in GT patients. Therefore, we investigated, by flow cytometry analysis, platelet functions in three GT patients as well as their ability to generate procoagulant collagen-and-thrombin (COAT) platelets upon combined activation with convulxin-plus-thrombin. In addition, we further characterized intracellular ion fluxes during the procoagulant response, using specific probes to monitor by flow cytometry kinetics of cytosolic calcium, sodium, and potassium ion fluxes. GT patients generated higher percentages of procoagulant COAT platelets compared to healthy donors. Moreover, they were able to mobilize higher levels of cytosolic calcium following convulxin-plus-thrombin activation, which is congruent with the greater procoagulant activity. Further investigations will dissect the role of GPIIb/IIIa outside-in signalling possibly implicated in the regulation of platelet procoagulant activity.

Should studies on Glanzmann thrombasthenia not be telling us more about cardiovascular disease and other major illnesses?

Glanzmann thrombasthenia (GT) is a rare inherited bleeding disorder caused by loss of αIIbβ3 integrin function in platelets. Most genetic variants of β3 also affect the widely expressed αvβ3 integrin. With brief mention of mouse models, I now look at the consequences of disease-causing ITGA2B and ITGB3 mutations on the non-hemostatic functions of platelets and other cells. Reports of arterial thrombosis in GT patients are rare, but other aspects of cardiovascular disease do occur including deep vein thrombosis and congenital heart defects. Thrombophilic and other risk factors for thrombosis and lessons from heterozygotes and variant forms of GT are discussed. Assessed for GT patients are reports of leukemia and cancer, loss of fertility, bone pathology, inflammation and wound repair, infections, kidney disease, autism and respiratory disease. This survey shows an urgent need for a concerted international effort to better determine how loss of αIIbβ3 and αvβ3 influences health and disease.

Chloride channels are necessary for full platelet phosphatidylserine exposure and procoagulant activity

Platelets enhance thrombin generation at sites of vascular injury by exposing phosphatidylserine during necrosis-like cell death. Anoctamin 6 (Ano6) is required for Ca(2+)-dependent phosphatidylserine exposure and is defective in patients with Scott syndrome, a rare bleeding disorder. Ano6 may also form Cl(-) channels, though the role of Cl(-) fluxes in platelet procoagulant activity has not been explored. We found that Cl(-) channel blockers or removal of extracellular Cl(-) inhibited agonist-induced phosphatidylserine exposure. However, this was not due to direct inhibition of Ca(2+)-dependent scrambling since Ca(2+) ionophore-induced phosphatidylserine exposure was normal. This implies that the role of Ano6 in Ca(2+-)dependent PS exposure is likely to differ from any putative function of Ano6 as a Cl(-) channel. Instead, Cl(-) channel blockade inhibited agonist-induced Ca(2+) entry. Importantly, Cl(-) channel blockers also prevented agonist-induced membrane hyperpolarization, resulting in depolarization. We propose that Cl(-) entry through Cl(-) channels is required for this hyperpolarization, maintaining the driving force for Ca(2+) entry and triggering full phosphatidylserine exposure. This demonstrates a novel role for Cl(-) channels in controlling platelet death and procoagulant activity.

Binding of thrombin-activated platelets to a fibrin scaffold through α(IIb)β₃ evokes phosphatidylserine exposure on their cell surface

Recently, by employing intra-vital confocal microscopy, we demonstrated that platelets expose phosphatidylserine (PS) and fibrin accumulate only in the center of the thrombus but not in its periphery. To address the question how exposure of platelet anionic phospholipids is regulated within the thrombus, an in-vitro experiment using diluted platelet-rich plasma was employed, in which the fibrin network was formed in the presence of platelets, and PS exposure on the platelet surface was analyzed using Confocal Laser Scanning Microscopy. Almost all platelets exposed PS after treatment with tissue factor, thrombin or ionomycin. Argatroban abrogated fibrin network formation in all samples, however, platelet PS exposure was inhibited only in tissue factor- and thrombin-treated samples but not in ionomycin-treated samples. FK633, an α(IIb)β₃ antagonist, and cytochalasin B impaired platelet binding to the fibrin scaffold and significantly reduced PS exposure evoked by thrombin. Gly-Pro-Arg-Pro amide abrogated not only fibrin network formation, but also PS exposure on platelets without suppressing platelet binding to fibrin/fibrinogen. These results suggest that outside-in signals in platelets generated by their binding to the rigid fibrin network are essential for PS exposure after thrombin treatment.

Protease-activated receptor (PAR) 1 and PAR4 differentially regulate factor V expression from human platelets

With the recent interest of protease-activated receptors (PAR) 1 and PAR4 as possible targets for the treatment of thrombotic disorders, we compared the efficacy of protease-activated receptor (PAR)1 and PAR4 in the generation of procoagulant phenotypes on platelet membranes. PAR4-activating peptide (AP)-stimulated platelets promoted thrombin generation in plasma up to 5 minutes earlier than PAR1-AP-stimulated platelets. PAR4-AP-mediated factor V (FV) association with the platelet surface was 1.6-fold greater than for PAR1-AP. Moreover, PAR4 stimulation resulted in a 3-fold greater release of microparticles, compared with PAR1 stimulation. More robust FV secretion and microparticle generation with PAR4-AP was attributable to stronger and more sustained phosphorylation of myosin light chain at serine 19 and threonine 18. Inhibition of Rho-kinase reduced PAR4-AP-mediated FV secretion and microparticle generation to PAR1-AP-mediated levels. Thrombin generation assays measuring prothrombinase complex activity demonstrated 1.5-fold higher peak thrombin levels on PAR4-AP-stimulated platelets, compared with PAR1-AP-stimulated platelets. Rho-kinase inhibition reduced PAR4-AP-mediated peak thrombin generation by 25% but had no significant effect on PAR1-AP-mediated thrombin generation. In conclusion, stimulation of PAR4 on platelets leads to faster and more robust thrombin generation, compared with PAR1 stimulation. The greater procoagulant potential is related to more efficient FV release from intracellular stores and microparticle production driven by stronger and more sustained myosin light chain phosphorylation. These data have implications about the role of PAR4 during hemostasis and are clinically relevant in light of recent efforts to develop PAR antagonists to treat thrombotic disorders.

Protein disulfide isomerase capture during thrombus formation in vivo depends on the presence of β3 integrins

Extracellular protein disulfide isomerase (PDI) is required for platelet thrombus formation and fibrin generation after arteriolar wall injury in live mice. PDI is secreted from platelets and endothelial cells on cellular activation, but the mechanism of capture of secreted PDI within the injured vasculature is unknown. We establish that, like the endothelial β3 integrin α(V)β(3), the platelet integrin α(IIb)β(3) binds PDI. PDI also binds to recombinant β3. Using intravital microscopy, we demonstrate that PDI accumulation at the site of laser-induced arteriolar wall injury is markedly reduced in β3-null (β3(-/-)) mice, and neither a platelet thrombus nor fibrin is generated at the vessel injury site. The absence of fibrin after vascular injury in β3(-/-) mice is because of the absence of extracellular PDI. To evaluate the relative importance of endothelial α(V)β(3) versus platelet α(IIb)β(3) or α(V)β(3), we performed reciprocal bone marrow transplants on wild-type and β3(-/-) mice. PDI accumulation and platelet thrombus formation were markedly decreased after vessel injury in wild-type mice transplanted with β3(-/-) bone marrow or in β3(-/-) mice transplanted with wild-type bone marrow. These results indicate that both endothelial and platelet β3 integrins contribute to extracellular PDI binding at the vascular injury site.

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

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

Evidence of maternal platelet activation, excessive thrombin generation, and high amniotic fluid tissue factor immunoreactivity and functional activity in patients with fetal death

OBJECTIVE

Fetal death can lead to disseminated intravascular coagulation or fetal death syndrome. However, currently it is not clear what are the changes in the coagulation system in patients with a fetal death without the fetal death syndrome. This study was undertaken to determine: (1) whether fetal death in the absence of fetal death syndrome is associated with changes in hemostatic markers in maternal plasma and amniotic fluid; and (2) whether maternal hypertension or placental abruption are associated with further changes in the hemostatic profile of these patients.

METHODS

A cross-sectional study included the following: (1) determination of changes in markers of coagulation and platelet activation in patients with a normal pregnancy (n = 71) and patients with fetal demise (FD) without disseminated intravascular coagulation (n = 65); (2) determination of the amniotic fluid (AF)-tissue factor concentration and activity, as well as the concentrations of thrombin-antithrombin III (TAT) complexes in patients with a normal pregnancy (n = 25) and those with a FD (n = 36) who underwent amniocentesis. Plasma and AF concentrations of TAT complexes and TF (an index of thrombin generation), as well as maternal plasma concentrations of sCD40L (a marker of platelet activation), tissue factor pathway inhibitor (TFPI) and prothrombin fragments (PF) 1 + 2 (also an indicator of in vivo thrombin generation) were measured by ELISA. TF and TFPI activity were measured using chromogenic assays.

RESULTS

(1) patients with FD without hypertension had a higher median maternal plasma sCD40L concentration than normal pregnant women (P < 0.001); (2) patients with FD had a higher median maternal plasma TAT III complexes than women with a normal pregnancy (P < 0.001); (3) the median AF-TF concentration and activity were higher in the FD group than in the normal pregnancy group (P < 0.001 for both); (4) patients with preeclampsia and FD had a higher median maternal plasma immunoreactive TF concentration than both normotensive patients with FD and women with normal pregnancies (P < 0.001 and P = 0.001, respectively); (5) the median plasma TF activity was higher in patients with preeclampsia and FD than that of women with normal pregnancies (P = 0.003); (6) among patients with a FD, those with placental abruption had a higher median AF-TAT complexes concentration than those without abruption (P = 0.0004).

CONCLUSIONS

Our findings indicate that: (1) mothers with a FD have evidence of increased in vivo thrombin generation and platelet activation than women with normal pregnancies; (2) patients with a FD and hypertension had a higher degree of TF activation than those with fetal death but without hypertension; (3) the AF of women with a FD had a higher median TF concentration and activity than that of normal pregnant women. AF can be a potential source for tissue factor and it participates in the development of fetal death syndrome in patients with a retained dead fetus.

The value of proteomics for the diagnosis of a platelet-related bleeding disorder

Familial bleeding problems are frequently difficult to diagnose because currently used clinical tests cannot identify intracellular molecular defects of platelets. Using platelet proteomics, a comprehensive analytical tool, we diagnosed a family with severe bleeding problems of unknown origin with Quebec Platelet Disorder. Prior to proteomic analysis, we determined platelet counts, presence of glycoprotein (GP) Ib and GPIIb/IIIa, platelet aggregation, dense granule content and release, plasma levels of fibrinogen, Factor XIII and fibrin degradation products in four family members. Abnormalities were detected in platelet aggregation studies, which revealed variably reduced responses to ADP, collagen and epinephrine with concomitantly decreased ATP/serotonin secretion. In addition, D-dimer levels were significantly elevated 72 hours after in vitro thrombin stimulation of platelet-rich plasma. Together with the autosomal dominant inheritance and the delayed onset of bleeding in two of the four patients these results did not support any known platelet disorder. Therefore, the proteome of platelet lysates separated by one-dimensional SDS-PAGE was analysed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Platelet proteomics showed reduced amounts of alpha-granule proteins multimerin, fibrinogen and thrombospondin-1 in patient compared to control samples suggestive of Quebec Platelet Disorder. The diagnosis of Quebec Platelet Disorder was confirmed by urokinase-specific Western blots. Urokinase causes the degradation of alpha-granule proteins in this disorder. Diagnosis of rare bleeding disorders has important implications for prophylactic and acute treatment of bleeding patients. This is the first report using proteomics to identify a familial platelet defect.

Glanzmann's thrombasthenia: an overview

Glanzmann's thrombasthenia (GT) is an autosomal recessive inherited bleeding disorder due to a defect in platelet function. The hallmark of this disease is severely reduced/absent platelet aggregation in response to multiple physiological agonists. Bleeding signs in GT include epistaxis, bruising, gingival hemorrhage, gastrointestinal hemorrhage, hematuria, menorrhagia, and hemarthrosis. Homozygous or compound heterozygous mutations in the genes of GPIIb and GPIIIa lead to GT. A patient with GT, with no possible causative mutations in GPIIb and GPIIIa genes, may harbor defects in a regulatory element affecting the transcription of these 2 genes. GT occurs in high frequency in certain ethnic populations with an increased incidence of consanguinity such as in Indians, Iranians, Iraqi Jews, Palestinian and Jordanian Arabs, and French Gypsies. Carrier detection in GT is important to control the disorder in family members. Carrier detection can be done both by protein analysis and direct gene analysis.

Diagnostic approach to platelet function disorders

Platelet disorders are common bleeding disorders, with a variety of congenital and acquired causes. The diagnostic evaluation of platelet disorders challenges both clinicians and clinical laboratories, as testing for these conditions is complex, not well standardized and time consuming. An understanding of normal platelet function has provided insights on the pathogenesis of many platelet function disorders. Knowledge of the key features of platelet disorders aids their diagnostic assessment. Tests for aggregation, secretion and dense granule defects continue to be the most helpful for the evaluation of suspected platelet function disorders.

Differential regulation of platelet aggregation and aminophospholipid exposure by calpain

Aggregation, exposure of procoagulant phospholipids and shedding of microparticles are platelet responses that depend on activating conditions. To determine how these different responses are interconnected, we simultaneously measured fibrinogen (Fg) binding and aminophospholipid exposure on activated platelets by means of flow cytometry. Low calcium ionophore (A23187) concentrations induced Fg binding but not annexin V binding. In contrast, high A23187 concentrations induced annexin V binding but not Fg binding. Collagen, both alone and in the presence of thrombin, induced both Fg and annexin V binding. Dual labelling was found on 38 +/- 9% of platelets stimulated by thrombin plus collagen. The regulatory role of calpain in these platelet functions was investigated. When calpain was partially inhibited by 2 microg/ml calpeptin, Fg binding still occurred but aminophospholipid exposure was limited. By contrast, complete inhibition of calpain by 100 microg/ml calpeptin or E64d decreased Fg binding but enhanced aminophospholipid exposure. In these latter conditions, cytosolic calcium-extruding systems were inhibited. The results suggest that (i) conditions that favour aminophospholipid exposure tend to decrease the aggregation process and (ii) calpain determines the switch to either aggregation or aminophospholipid exposure by controlling intracellular calcium.

Identification of genes involved in Ca2+ ionophore A23187-mediated apoptosis and demonstration of a high susceptibility for transcriptional repression of cell cycle genes in B lymphoblasts from a patient with Scott syndrome

Background

In contrast to other agents able to induce apoptosis of cultured cells, Ca²⁺ ionophore A23187 was shown to elicit direct activation of intracellular signal(s). The phenotype of the cells derived from patients having the hemorrhagic disease Scott syndrome, is associated with an abnormally high proportion of apoptotic cells, both in basal culture medium and upon addition of low ionophore concentrations in long-term cultures. These features are presumably related to the mutation also responsible for the defective procoagulant plasma membrane remodeling. We analyzed the specific transcriptional re-programming induced by A23187 to get insights into the effect of this agent on gene expression and a defective gene regulation in Scott cells.

Results

The changes in gene expression upon 48 hours treatment with 200 nM A23187 were measured in Scott B lymphoblasts compared to B lymphoblasts derived from the patient's daughter or unrelated individuals using Affymetrix microarrays. In a similar manner in all of the B cell lines, results showed up-regulation of 55 genes, out of 12,000 represented sequences, involved in various pathways of the cell metabolism. In contrast, a group of 54 down-regulated genes, coding for histones and proteins involved in the cell cycle progression, was more significantly repressed in Scott B lymphoblasts than in the other cell lines. These data correlated with the alterations of the cell cycle phases in treated cells and suggested that the potent effect of A23187 in Scott B lymphoblasts may be the consequence of the underlying molecular defect.

Conclusion

The data illustrate that the ionophore A23187 exerts its pro-apoptotic effect by promoting a complex pattern of genetic changes. These results also suggest that a subset of genes participating in various steps of the cell cycle progress can be transcriptionally regulated in a coordinated fashion. Furthermore, this research brings a new insight into the defect in cultured Scott B lymphoblasts, leading to hypothesize that a mutated gene plays a role not only in membrane remodeling but also in signal transduction pathway(s) leading to altered transcriptional regulation of cell cycle genes.

Inhibitory effects of P2Y12 receptor antagonists on TRAP-induced platelet aggregation, procoagulant activity, microparticle formation and intracellular calcium responses in patients with acute coronary syndromes

Thrombin induces platelet aggregation and membrane rearrangements leading to enhanced procoagulant activity and microparticle production, all of which are thought to contribute to thrombus formation in patients with acute coronary syndromes (ACS). Clopidogrel, an adenosine diphosphate (ADP) receptor antagonist acting at the P2Y(12) receptor, has been shown to provide clinical benefit in ACS. We aimed to investigate the effects of clopidogrel ex vivo and another ADP-antagonist, AR-C69931MX in vitro on thrombin receptor activating peptide (TRAP)-induced platelet aggregation, procoagulant activity, microparticle formation and [Ca(2+)]i responses in patients with ACS. Measurements were performed in platelet-rich plasma using aggregometry and flow cytometry (n = 12). Clopidogrel (300 mg loading dose plus 75 mg daily) significantly inhibited TRAP-induced aggregation, procoagulant activity (annexin V binding) and microparticle production (all P < 0.05) but not as extensively as AR-C69931MX (400 nmol/l). [Ca(2+)]i responses induced by a combination of TRAP and ADP designed to mimic the physiological effects of released ADP showed that clopidogrel partially and AR-C69931MX completely removed the ADP component of the [Ca(2+)]i responses (n = 6). The results provide new information on the mechanisms involved in the beneficial effects of P2Y(12) antagonists in patients with ACS.

Synergistic Effect of Thrombin on Collagen-Induced Platelet Procoagulant Activity Is Mediated Through Protease-Activated Receptor-1

OBJECTIVE

In the blood coagulation process, the rate of thrombin formation is critically dependent on phosphatidylserine (PtdSer) at the surface of activated platelets. Thrombin synergistically enhances the collagen-induced platelet procoagulant response. The objective of this study is to elucidate the mechanism of this synergistic action with a focus on the intracellular Ca2+ concentration ([Ca2+]i) and the various platelet receptors for thrombin.

METHODS AND RESULTS

We demonstrate that procoagulant activity is related to a sustained increased [Ca2+]i, which in turn depends on extracellular Ca2+ influx. Increased PtdSer exposure coincides with increased [Ca2+]i and was observed in a subpopulation (approximately 14%) of the platelets after stimulation with thrombin plus collagen. 2D2-Fab fragments against the thrombin binding site on GPIbalpha made clear that this receptor did not signal for platelet procoagulant activity. Inhibition of protease-activated receptor 1 (PAR-1) and PAR-4 by selective intracellular inhibitors and selective desensitization of these receptors revealed that PAR-1, but not PAR-4, activation is a prerequisite for both sustained elevations in [Ca2+]i and procoagulant activity induced by collagen plus thrombin.

CONCLUSIONS

The interaction of thrombin with PAR-1 mediates a synergistic effect on collagen-induced procoagulant activity by inducing a sustained elevation in [Ca2+]i in a subpopulation of platelets.

Role of protease-activated and ADP receptor subtypes in thrombin generation on human platelets

The activated platelet surface serves as an integral part of the prothrombinase complex upon activation by potent platelet agonists such as thrombin and collagen. We determined the receptor specificity through which thrombin was enhancing collagen-induced thrombin generation. Whereas SFLLRN or AYPGKF alone produced minimal thrombin generation or phosphatidylserine exposure through protease activated receptor (PAR) stimulation, they caused a leftward shift in the collagen-induced thrombin generation dose-response curve. Although SFLLRN or AYPGKF potentiated collagen-induced thrombin generation, neither of them potentiated to the same extent as thrombin. However, SFLLRN and AYPGKF together potentiated collagen-induced thrombin generation to the same extent as thrombin. We conclude that thrombin mediates its procoagulant activity through activation of both PAR1 and PAR4 receptors. Similarly, neither PAR1 nor PAR4 stimulation alone mimicked the annexin V-binding response caused by thrombin stimulation. The combination of PAR activating peptides caused minimal increases in annexin V binding, but caused significant thrombin generation, suggesting that events other than phosphatidylserine exposure may play a role in platelet prothrombinase complex formation. We also investigated the ability of ADP to potentiate agonist-induced thrombin generation. Whereas P2Y(1) antagonism did not affect collagen or thrombin-induced thrombin generation, P2Y(12) antagonism did decrease both collagen- and thrombin-induced thrombin generation, suggesting that ADP potentiates thrombin generation primarily through the P2Y(12) receptor. Collectively, these results suggest that stimulation of both the PAR1 and PAR4 receptors are necessary for thrombin-induced procoagulant activity, and that the P2Y(12) receptor, but not the P2Y(1) receptor, is responsible for the potentiation of agonist-induced platelet procoagulant activity.

Differential involvement of the P2Y1 and P2Y12 receptors in platelet procoagulant activity

OBJECTIVE

In vivo, activated platelets contribute to the initiation of thrombin generation through the exposure of phosphatidylserine to form a procoagulant catalytic surface and through platelet-leukocyte interactions, which lead to the exposure of leukocyte tissue factor (TF). On the basis of observations that the platelet P2Y1 and P2Y12 receptors both contribute to thrombosis and thrombin formation in an in vivo model of TF-induced thromboembolism, we further characterized the role of these receptors in thrombin generation.

METHODS AND RESULTS

By using the selective P2 antagonists MRS2179 and AR-C69931MX, the P2Y12 receptor was found to be involved in thrombin-induced exposure of PS on isolated platelets and consequently in TF-induced thrombin formation in platelet-rich plasma. By contrast, the P2Y1 receptor was not involved in phosphatidylserine exposure nor in thrombin generation in platelet-rich plasma. In addition, both receptors were found to contribute to the interactions between platelets and leukocytes mediated by platelet P-selectin exposure, which result in TF exposure at the surface of leukocytes.

CONCLUSIONS

Overall, these results point to a differential involvement of the 2 platelet ADP receptors in the generation of thrombin and provide further evidence for the relevance of molecules targeting these receptors as antithrombotic agents.

Cellular microparticles: what are they bad or good for?

Microparticles are fragments released from the plasma membrane of most stimulated or apoptotic cells. After having long been considered inert cell debris, of possible value for the diagnosis of cell activation or death, there is increasing documented evidence that they can interact with neighboring or remote cells, in which case they acquire a pathophysiologic potential. On the one hand, deleterious microparticles stemming from activated cells can elicit an adverse response from other cells, themselves undergoing membrane vesiculation, leading to pathogenic amplification. On the other hand, since they are thought to reflect a balance between cell stimulation, proliferation, and death, it is conceivable that they are discerned as sensors for the maintenance of homeostasis in multicellular organisms. Because vesiculation is an integral part of the plasma-membrane remodeling process, with the transverse migration of procoagulant phosphatidylserine from the cytoplasmic to the exoplasmic leaflet as the central event, the majority of released microparticles are thought to fulfill a hemostatic function under physiologic conditions. This is particularly true when they originate from platelets, with possible deviation towards thrombosis when produced in excess. Owing to these procoagulant properties, the hemostasis laboratory offers the most appropriate tools for the assessment of the in vivo significance of microparticles.

Abciximab inhibits procoagulant activity but not the release reaction upon collagen- or clot-adherent platelets

In addition to inhibiting platelet (plt) aggregation abciximab, a glycoprotein (GP) IIb/IIIa antagonist, reduces coagulation in blood or platelet-rich plasma. We assessed the effects of abciximab (10 micro g mL(-1)) on adhesion-dependent procoagulant activity (PCA) of plt upon: (i) collagen to model initial adhesion; or (ii) plasma clot or fibrin to model a preformed thrombus with retained thrombin activity. In a two-stage assay gel-filtered plt (GFP) first adhered on collagen, plasma clot, or fibrin, and plt activation was traced with platelet factor 4 (PF 4) release. Second, PCA was measured on adherent plt (i) by soluble prothrombin fragments (F1 + 2); and (ii) chromogenically by adding defibrinated plasma and thromboplastin. Abciximab inhibited aggregation upon collagen-adherent plt both in the absence and presence of plasma. In contrast, without plasma abciximab enhanced plt deposition to fibrin surfaces depending on thrombin generation and fibrin polymerization. However, abciximab reduced PCA and generation of F1 + 2 on adherent plt surface-independently by 35%, whereas PF 4 release persisted. Also, a GP Ib inhibitor, mAb SZ2, attenuated PCA by 40% alone, and by 65% together with abciximab, leaving 35% of PCA unaltered. Abciximab decreased generation of new thrombin on both collagen- and clot-adherent plt. However, abciximab did not inhibit alpha-granule release, suggesting distinct pathways for PCA and release reaction. Deposition of isolated plt on clots in the presence of abciximab was dependent on thrombin and polymerizing plt-derived fibrin(ogen). Due to local consumption of natural anticoagulants adjacent to a preformed thrombus the antithrombotic effect of abciximab benefits from additional inhibition of thrombin.

Immunolocalization of P2Y1 and TPalpha receptors in platelets showed a major pool associated with the membranes of alpha -granules and the open canalicular system

P2Y(1) and thromboxane-prostanoid-alpha (TPalpha) receptors on platelets belong to the G-protein-coupled 7-transmembrane domain family. They transmit signals for shape change, mobilization of calcium, and platelet aggregation. Immunogold labeling with a monoclonal antibody (MoAb) to the amino-terminal domain of P2Y(1) and a polyclonal antibody to the C-terminal domain of TPalpha revealed that while present at the platelet surface, both receptors were abundantly represented inside the platelet. Specifically, receptors were found in membranes of alpha-granules and elements of the open-canalicular system. A similar organization was found in mature megakaryocytes. Activation of platelets by adenosine diphosphate (ADP) and the thromboxane A(2) (TXA(2)) analog, I-BOP [1S-(1 alpha,2 beta(5Z),3 alpha-(1E,3S)4 alpha)-7-(3-(3- hydroxy-4-(p-iodophenoxy)-1-butenyl)-7-oxabicyclo(2.2.1)hept-2-yl)-5-heptenoic acid], increased the labeling of both P2Y(1) and TPalpha at the surface and in intracellular pools, suggesting that activation resulted in greater antibody accessibility to the receptor. A return to a platelet discoid shape and to basal values of labeling accompanied receptor desensitization. Platelets lacking the P2Y(12) ADP receptor normally expressed P2Y(1) and TPalpha, both before and after activation. Studies with the anti-ligand-induced binding site (anti-LIBS) MoAb, AP-6, confirmed that stored fibrinogen associated with internal pools of alpha(IIb)beta(3) at the start of secretion in a microenvironment containing agonist receptors. Pharmacologic antagonism of ADP or TXA(2) receptors in antithrombotic therapy may need to take into account blockade of internal receptor pools.

A hereditary bleeding disorder of dogs caused by a lack of platelet procoagulant activity

We have discovered a novel canine hereditary bleeding disorder with the characteristic features of Scott syndrome, a rare defect of platelet procoagulant activity. Affected dogs were from a single, inbred colony and experienced clinical signs of epistaxis, hyphema, intramuscular hematoma, and prolonged bleeding with cutaneous bruising after surgery. The hemostatic abnormalities identified were restricted to tests of platelet procoagulant activity, whereas platelet count, platelet morphology under light microscopy, bleeding time, clot retraction, and platelet aggregation and secretion in response to thrombin, collagen, and adenosine diphosphate stimulation were all within normal limits. Washed platelets from the affected dogs demonstrated approximately twice normal clotting times in a platelet factor 3 availability assay and, in a prothrombinase assay, generated only background levels of thrombin in response to calcium ionophore, thrombin, or combined thrombin plus collagen stimulation. While platelet phospholipid content was normal, flow cytometric analyses revealed diminished phosphatidylserine exposure and a failure of microvesiculation in response to calcium ionophore, thrombin, and collagen stimulation. Pedigree studies indicate a likely homozygous recessive inheritance pattern of the defect. These findings confirm the importance of platelet procoagulant activity for in vivo hemostasis and provide a large animal model for studying agonist-induced signal transduction, calcium mobilization, and effector pathways involved in the late platelet response of transmembrane phospholipid movement and membrane vesiculation.

A novel immunocytochemical assay for the detection of serotonin in platelets

A method for the rapid, inexpensive and easy detection of platelet serotonin (5-hydroxytryptamine, 5-HT) is not currently available. Consequently, many patients suffering from unresolved platelet-related bleeding disorders are not examined for a possible platelet 5-HT deficiency. The direct measurement of 5-HT concentration with high-performance liquid chromatography (HPLC) or serotonin enzyme-linked immunosorbent assay (ELISA) is costly and highly demanding. Indirect methods, which determine the content of ATP or calcium with lumi-aggregometry or electron microscopy, rely upon the assumption that the ATP or calcium concentration is equivalent to that of 5-HT. We have developed a fluorescence-based assay for 5-HT that can be performed within 2 h on fresh or frozen samples using a fluorescence microscope or a flow cytometer. The assay requires only 0.2 ml of platelet-rich plasma and might therefore be of particular interest for paediatric patients. Samples from control and patient donors were analysed for 5-HT with the new immunocytochemical assay in comparison with HPLC and/or 5-HT ELISA. Patients with Hermansky-Pudlak syndrome were readily identified. The new assay was also reliable in cases where the 5-HT content of dense granules was not correlated with the calcium or ATP content, such as in calcium deficiency or in the presence of selective serotonin reuptake inhibitors.

Platelet aggregation through prothrombinase activation induced by non-aggregant doses of platelet agonists

Activation of the prothrombinase complex, which catalyzes the formation of thrombin from prothrombin, is crucial for the (patho)physiological processes of hemostasis and thrombosis. We here report that washed platelets supplemented with prothrombin can be irreversibly aggregated with otherwise non-aggregant doses of adenosine diphosphate (10 micromol/l), thrombin (0.06 U/ml), or collagen (1 microg/ml). Prothrombinase-catalyzed prothrombin to thrombin conversion most probably supports this aggregation response, since inhibitors of thrombin (hirudin or heparin) and an inhibitor of activated factor X (DX-9065a) impair the response. A certain degree of agonist-induced platelet activation seems to be required for this prothrombin-supported aggregation response, since prothrombin alone does not induce aggregation, and blockade of glycoprotein Ia/IIa with a specific antibody inhibits the platelet aggregation response to collagen and prothrombin. These results may suggest that activation of the prothrombinase complex could be a common step of the platelet response to distinct agonists, which may be achieved at low levels of platelet stimulation.

Phosphatidylserine expression on the platelet membrane of patients with myeloproliferative disorders and its effect on platelet-dependent thrombin formation

Recently, the asymmetric distribution of phospholipids in eukaryotic cell membranes has been appreciated and been found to be dependent on the activity of a number of enzymes. The expression of phosphatidylserine (PS), a negatively charged phospholipid, on the platelets of patients with polycythemia vera (P vera) and essential thrombocythemia (ET) was compared to that in normal individuals. The effect of platelet aggregation on PS expression was determined. Exposure of PS on platelets obtained from patients with P vera and ET and from age- and sex-matched healthy volunteers was measured by fluorescein-labeled Annexin V binding to platelets and by the platelets' thrombin-generating capacity determined by the prothrombinase assay. PLatelet prothrombinase activity (mean +/- standard deviation [SD]), as measured by thrombin generation, was 2.32+/-2.2 micro/mL in the P vera group and 1.55+/-1.0 micro/mL in the control group (p=0.3). PS expression as measured by Annexin V binding (mean +/- SD) was 2.6+/-2.4 % in the P vera group versus 1.55+/-1.2% among controls (p=0.03). In the ET group, prothrombinase activity (mean +/- SD) was 1.0+/-0.6 micro/mL and 2.1+/-0.9 micro/mL in the control group (p=0.006). Annexin V binding (mean +/- SD) was 4.8+/-4.2% in the ET group and 2.77+/-2.1% among control subjects (p=0.09). When the prothrombinase assay was performed after addition of adenosine diphosphate (ADP) to the platelets, there was a significant increase in thrombin generation in the myeloproliferative disorder (MPD) group (3.1+/-2.0 micro/mL) compared to the thrombin generated by unstimulated myeloproliferative disorder platelets (2.07+/-1.69 micro/mL) (p=0.0006). An increase in thrombin generation was seen in the ADP-stimulated platelet samples in all ten paired samples studied. Likewise, the addition of ADP to control platelets increased thrombin generation from 2.0+/-1.0 micro/mL in unstimulated platelets to 4.3+/-1.6 micro/mL in ADP-treated platelets (p=0.0006). Thrombin generation increased in all of the ADP-stimulated platelet samples compared to the untreated platelets. There was however, no difference in the increased thrombin generation when ADP-stimulated platelets from MPD patient and control subjects were compared (p=0.3). Results indicate that some patients with MPDs may show increased PS expression on platelet surface. When analyzed overall, there was a tendency toward greater PS expression in the P vera and ET patient groups; however, the increase did not reach statistical significance. This increase was noted in both the prothrombinase assay the Annexin V binding assay. We have also shown that stimulation of platelets by addition of the agonist ADP results in enhanced PS expression, which appears increase the thrombogenic potential of the platelets as demonstrated by the enhanced thrombin generation demonstrated by these platelets in the prothrombinase assay. There was no difference in the degree of PS expression in response to ADP stimulation between MPD and control platelets. Results show that PS expression and platelet-dependent thrombin generation is variable in patients with MPDs. This expression is increased after platelet aggregation occurs. The role of PS expression in the thromboembolic complications of MPD patients should be studied further.

The storage defects in grey platelet syndrome and alphadelta-storage pool deficiency affect alpha-granule factor V and multimerin storage without altering their proteolytic processing

Among proteins stored in alpha-granules, multimerin and factor V share unusual features: they bind to each other, are proteolysed to unique forms and are stored eccentrically in alpha-granules. These unique features of their processing led us to study these proteins in alpha delta storage pool deficiency (alphadelta-SPD) and grey platelet syndrome (GPS, alpha-SPD), two conditions known to impair alpha-granule protein storage. Platelet factor V and multimerin were severely reduced in GPS, whereas they ranged from reduced to normal in alphadelta-SPD. The platelet levels of factor V and multimerin in these disorders indicated multimerin deficiency was not predictive of platelet factor V deficiency, although it reduced the amount of multimerin associated with platelet factor V. In GPS only, the defect in storing proteins was associated with increased multimerin and multimerin-factor V complexes in plasma. Like normal platelets, GPS and alphadelta-SPD platelets contained factor V mainly in granules. Platelet factor V and multimerin were proteolysed to normal platelet forms in GPS and alphadelta-SPD platelets, indicating that these conditions preserve some aspects of normal alpha-granule protein processing. Although we found factor V can be stored in platelets deficient in multimerin, our data indicate that multimerin storage influences the point at which multimerin binds factor V.

Greater inhibition of platelet procoagulant activity by antibody-derived glycoprotein IIb--IIIa inhibitors than by peptide and peptidomimetic inhibitors

Antibody-derived GPIIb-IIIa antagonists, such as the c7E3 Fab fragment abciximab, have been shown to inhibit platelet procoagulant activity as well as platelet aggregation. Whether low-molecular-weight peptide-derived and peptidomimetic antagonists also inhibit platelet procoagulant activity in a similar manner has not been fully investigated. We compared the effects of the antibody-derived antagonists c7E3 Fab and m10E5 IgG, the peptide-derived antagonists eptifibatide, MK-852 and RGDS, and the peptidomimetic RO44--9883 on platelet procoagulant activity and on the stimulated cytosolic calcium increases that promote procoagulant activity. Procoagulant activity was measured as prothrombinase activity in gel-filtered platelets, activated by collagen plus thrombin or collagen alone, with and without stirring. The stimulated increases in cytosolic calcium were measured in parallel samples of platelets loaded with fura-2AM. Both c7E3 and m10E5 inhibited prothrombinase activity by 40--50% under all conditions of activation tested and inhibited cytosolic calcium increases to a similar extent in stirred, but not unstirred, platelets. The low-molecular-weight antagonists caused significantly less inhibition of prothrombinase activity in collagen plus thrombin-stimulated platelets, and produced no inhibition but rather a slight enhancement of activity in platelets stimulated by collagen alone. These antagonists also had little or no effect on the cytosolic calcium increases in stirred platelets. These differential effects of antibody-derived versus non-antibody GPIIb-IIIa antagonists on procoagulant activity may be a factor contributing to the differing anti-thrombotic effects of these antagonists seen in clinical trials.

Platelet factor V New York: a defect in factor V distinct from that in factor V Quebec resulting in impaired prothrombinase generation

Studies were performed on a patient with a longstanding bleeding disorder whose major defects were impaired platelet prothrombinase activity in the absence of added factor Va, and a platelet factor V value that was either decreased or at the lower limit of normal when assayed on multiple occasions. In contrast, plasma factor V values were consistently normal. Unlike Scott Syndrome, in which platelet prothrombinase activity is decreased in both the presence and absence of added factor V, her platelets appeared to utilize added factor Va normally in supporting the generation of prothrombinase activity. These findings suggest an intrinsic defect in platelet factor V as the basis of her platelet prothrombinase defect. This defect appears to be different than that described in the Quebec platelet disorder (factor V Quebec). Immunoblot analyses of washed platelet lysates demonstrated a pattern of variably sized factor V molecules that was entirely similar to that observed in normal platelets, and both the heavy and light chains of her factor V after thrombin cleavage were of the same size as that observed in normal platelets. In addition, her platelet multimerin was normal and immunoblot analysis excluded the type of generalized granular protein defect and pathological proteolysis that has been suggested to explain the factor V defect in the Quebec platelet disorder. The findings in this patient thus suggest a new type of platelet factor V defect as the basis for the impaired capacity of her activated platelets to support prothrombinase generation. The findings further support an important role for platelet factor V in hemostasis.

The effect of storage on the expression of platelet membrane phosphatidylserine and the subsequent impacton the coagulant function of stored platelets

BACKGROUND

Platelet concentrates (PCs) derived from whole blood and stored under standard blood bank conditions undergo changes that are referred to as the platelet storage lesion. This study assesses the effect of PC preparation and storage on the distribution of phosphatidylserine (PS) in the platelet membrane and the effect that this distribution may have on the thrombogenic potential of stored PCs.

STUDY DESIGN AND METHODS

Fresh platelets and PCs donated by healthy donors were obtained. PCs derived from platelet-rich plasma were studied on Day 1, Day 3, and Day 6 of storage under blood bank conditions.

RESULTS

Platelet aggregation after exposure to the platelet agonists ADP and epinephrine singly declined progressively, but, when ADP and epinephrine in combination and collagen and thrombin in combination were used as agonists, the decline in platelet aggregation was less marked. PS expression as measured by Annexin V binding (mean and SD) was 2.02 +/- 0.93 percent in fresh platelet samples and increased to 5.39 +/- 4.2 percent on Day 1, 22. 1 +/- 7.1 percent on Day 3, and 39.5 +/- 12.1 percent on Day 6. Platelet prothrombinase activity (mean +/- SD) as measured by thrombin generation increased from 1.49 +/- 0.7 micro per mL in fresh platelet samples to 3.68 +/- 1.1 micro per mL in Day 1 platelets (p<0.001), 5.15 +/- 2.5 micro per mL in Day 3 platelets (p<0.001), and 4.65 +/- 2.48 micro per mL in Day 6 platelets (p<0. 001).

CONCLUSION

These results show that PS expression increases after preparation of PCs from platelet-rich plasma and rises progressively during platelet storage under blood bank conditions. Furthermore, the greater PS expression is associated with increased platelet- dependent thrombin-generating capacity.

Procoagulant expression in platelets and defects leading to clinical disorders

Hemostasis is a result of interactions between fibrillar structures in the damaged vessel wall, soluble components in plasma, and cellular elements in blood represented mainly by platelets and platelet-derived material. During formation of a platelet plug at the damaged vessel wall, factors IXa and VIIIa form the "tenase" complex, leading to activation of factor X on the surface of activated platelets. Subsequently, factors Xa and Va form the "prothrombinase" complex, which catalyzes the formation of thrombin from prothrombin, leading to fibrin formation. An enhanced expression of negatively charged phosphatidylserine in the outer membrane leaflet resulting from a breakdown of the phospholipid asymmetry is essential for the formation of the procoagulant surface. An ATP-driven and inward-acting aminophospholipid "translocase" and a "floppase" counterbalancing this have been postulated to maintain the dynamic state of phospholipid asymmetry. A phospholipid-nonspecific "scramblase," believed to be responsible for the fast breakdown of the asymmetry during cell activation, has recently been isolated from erythrocytes, cloned, and characterized. An intracellular calcium-binding segment and one or more thioesterified fatty acids are probably of importance for calcium-induced activation of this transporter protein. Cytosolic calcium ions also activate the calcium-dependent protease calpain associated with shedding of microvesicles from the transformed platelet membrane. These are shed with a procoagulant surface and with surface-exposed P-selectin from the alpha-granules. Theoretically, therefore, microvesicles can be involved in both coagulation and inflammation. Scott syndrome is probably caused by a defect in the activation of an otherwise normal scramblase, resulting in a relatively severe bleeding tendency. In Stormorken syndrome, the patients demonstrate a spontaneous surface expression of aminophospholipids. Activated platelets and the presence of procoagulant microvesicles have been demonstrated in several clinical conditions, such as thrombotic and idiopathic thrombocytopenia, disseminated intravascular coagulation, and HIV-1 infection, and have been found to be associated with fibrin in thrombosis. Procoagulant microvesicles may also be formed from other cells as a result of apoptosis.

Pathological events in platelets of Wiskott-Aldrich syndrome patients

The Wiskott-Aldrich syndrome (WAS) is a severe X-linked platelet/immunological disorder arising from mutations of the gene WASP. At the clinical level, the major platelet abnormalities are small size and low number, both partially correctable by splenectomy. To identify underlying pathological events, we examined WAS platelets at various stages of their lifetime. In spleen sections from WAS patients, fluorescence microscopy showed dramatic co-localization of markers of platelets (CD41) and macrophages (CD68) compared to non-thrombocytopenic controls, suggesting that WAS splenic macrophages are involved in platelet removal. Study of isolated WAS blood platelets by flow cytometry showed substantial enhancement of surface exposure of phosphatidylserine (PS), a signal for engulfment by macrophages. Isolated resting WAS platelets were also aberrantly susceptible to microparticle release, and plasma samples of WAS patients contained > 5 times normal numbers of platelet-derived microparticles which may explain the small size of circulating platelets. Measurements with the Ca2+ sensitive dye fluo-3 revealed significantly increased Ca2+ levels, 310 +/- 13 nmol/l for WAS platelets versus 106 +/- 12 nmol/l for normal platelets, and also prolongation of agonist-induced Ca2+ flux. Cumulatively, these studies identify abnormal events occurring in WAS platelets: increased Ca2+ levels and enhancement of two Ca2+ dependent processes, PS exposure and microparticle release; these abnormal events may contribute to the in vivo decrease of platelet number and reduction of platelet size in this disease.

Comparative study of antiplatelet drugs in vitro: distinct effects of cAMP-elevating drugs and GPIIb/IIIa antagonists on thrombin-induced platelet responses

Among various categories of antiplatelet drugs, cAMP-elevating agents and GP IIb/IIIa antagonists have been reported to inhibit platelet aggregation stimulated by a wide variety of platelet agonists. To clarify the qualitative difference between these two agents, their effects on various platelet responses in washed platelets evoked by thrombin (0.05 U/mL) were compared in vitro. Two types of cAMP-elevating drugs, cilostazol (a phosphodiesterase III inhibitor) and prostaglandin E1 (an adenylate cyclase activator), both inhibited platelet aggregation, thromboxane A2 formation, and platelet factor 4 release in a concentration-dependent manner. In addition, both agents suppressed intracellular Ca++ elevation induced by thrombin. However, two classes of GP IIb/IIIa antagonists, abciximab (Fab fragment of antibody) and tirofiban (a synthetic compound), showed no inhibitory effects against thromboxane A2 formation and platelet factor 4 release, although these drugs inhibited platelet aggregation. Essentially the same results were obtained in platelet-rich plasma stimulated with high concentration (100 microM) of thrombin receptor activating peptide. In contrast to these different profiles on thromboxane A2 formation and release reaction, both cAMP-elevating agents and GP IIb/IIIa antagonists potently suppressed procoagulant activity in thrombin-stimulated platelets. These results suggest that the development of platelet procoagulant activity induced by thrombin is exclusively dependent on platelet aggregation or aggregation-dependent processes. These observations also indicate that cAMP-elevating agents possess wider inhibitory effects on platelet responses evoked by strong agonists than GP IIb/IIIa antagonists.

Platelet dense granules: structure, function and implications for haemostasis

The advances that have been made over the last decade in microscopic, biochemical, molecular, and genetic techniques have led to substantial improvement in our understanding of platelet dense granule structure and function, and the implications of dense granule deficiencies for haemostasis. However, much has still to be learned. For example, what is the specific mechanism of docking and fusion that occurs during dense granule exocytosis? What are the roles of dense granule membrane proteins during exocytosis or after expression on the surface of activated platelets? Finally, how do the genetic defects identified in HPS and CHS result in the clinical phenotype of these diseases, and what does this tell us about the origin and function of the affected subcellular organelles?

Secretable Human Platelet-Derived Factor V Originates From the Plasma Pool

Factor Va (FVa), derived from plasma or released from stimulated platelets, is the essential protein cofactor of the prothrombinase complex. Plasma-derived factor V (FV) is synthesized by the liver, whereas the source of the platelet-derived cofactor has not been unambiguously identified. Megakaryocytes, platelet precursors, are known to synthesize platelet proteins and to endocytose proteins from plasma (ie, fibrinogen) and then package these proteins into -granules. To determine which mechanism accounts for FV presence in platelets, two patients heterozygous for FVLeiden who underwent allogeneic transplantation from homozygous FV wild-type donors (bone marrow [BM] or liver) were studied. Patient JMW, whose skin biopsy specimen showed heterozygous FVLeiden, received a BM transplant from a wild-type homozygous FV donor as analyzed from posttransplant peripheral blood cells. Patient FW, whose native liver is heterozygous for FVLeiden, received a homozygous wild-type FV liver. Because each individual has two distinct genetic pools of factor V in liver and megakaryocytes, it was possible to determine whether secretable platelet-derived FV was normal or contained the FVLeiden mutation. Platelet-derived FVa released from thrombin-activated platelets from a normal individual, an individual heterozygous for the FVLeiden mutation, and the two patients was incubated with phospholipid vesicles and activated protein C (APC). Western blotting analyses using a monoclonal antibody that allows distinction between platelet-derived FVa and FVaLeiden subsequent to APC-catalyzed cleavage were then performed. Based on the accumulation of proteolytic fragments derived from APC-induced cleavage, analyses of platelet-derived FVa from JMW demonstrated both normal FVa and FVaLeiden consistent with a plasma-derived origin of the secretable platelet-derived FVa. Western blotting analyses of the APC-cleaved platelet-derived FVa from FW showed a wild-type phenotype, despite the presence of a FVLeiden allele in her megakaryocyte genome, also consistent with a plasma origin of her secretable platelet-derived FVa. Platelets do not appear to endocytose the plasma cofactor, because a 35-hour incubation of platelet-rich plasma with 125I-factor V showed no specific association/uptake of the radiolabeled ligand with the platelet pellet. Collectively, these results show for the first time that the majority of secretable platelet-derived factor V is endocytosed by megakaryocytes from plasma and is not exclusively synthesized by these cells, as previously believed.

© 1998 by The American Society of Hematology.

Secretable Human Platelet-Derived Factor V Originates From the Plasma Pool

Factor Va (FVa), derived from plasma or released from stimulated platelets, is the essential protein cofactor of the prothrombinase complex. Plasma-derived factor V (FV) is synthesized by the liver, whereas the source of the platelet-derived cofactor has not been unambiguously identified. Megakaryocytes, platelet precursors, are known to synthesize platelet proteins and to endocytose proteins from plasma (ie, fibrinogen) and then package these proteins into -granules. To determine which mechanism accounts for FV presence in platelets, two patients heterozygous for FVLeiden who underwent allogeneic transplantation from homozygous FV wild-type donors (bone marrow [BM] or liver) were studied. Patient JMW, whose skin biopsy specimen showed heterozygous FVLeiden, received a BM transplant from a wild-type homozygous FV donor as analyzed from posttransplant peripheral blood cells. Patient FW, whose native liver is heterozygous for FVLeiden, received a homozygous wild-type FV liver. Because each individual has two distinct genetic pools of factor V in liver and megakaryocytes, it was possible to determine whether secretable platelet-derived FV was normal or contained the FVLeiden mutation. Platelet-derived FVa released from thrombin-activated platelets from a normal individual, an individual heterozygous for the FVLeiden mutation, and the two patients was incubated with phospholipid vesicles and activated protein C (APC). Western blotting analyses using a monoclonal antibody that allows distinction between platelet-derived FVa and FVaLeiden subsequent to APC-catalyzed cleavage were then performed. Based on the accumulation of proteolytic fragments derived from APC-induced cleavage, analyses of platelet-derived FVa from JMW demonstrated both normal FVa and FVaLeiden consistent with a plasma-derived origin of the secretable platelet-derived FVa. Western blotting analyses of the APC-cleaved platelet-derived FVa from FW showed a wild-type phenotype, despite the presence of a FVLeiden allele in her megakaryocyte genome, also consistent with a plasma origin of her secretable platelet-derived FVa. Platelets do not appear to endocytose the plasma cofactor, because a 35-hour incubation of platelet-rich plasma with 125I-factor V showed no specific association/uptake of the radiolabeled ligand with the platelet pellet. Collectively, these results show for the first time that the majority of secretable platelet-derived factor V is endocytosed by megakaryocytes from plasma and is not exclusively synthesized by these cells, as previously believed.

© 1998 by The American Society of Hematology.

Blood flow and antithrombotic drug effects

This paper reviews the importance of blood flow phenomena in models of experimental thrombosis used for measuring antithrombotic drug efficacy. The characteristics of these systems and their application for studies with human blood and in animal models are considered. Central to these investigations has been the development of various types of perfusion chambers in which a thrombogenic test surface is exposed to flowing blood under well-defined conditions of blood flow and device geometry. Such perfusion chambers, which have been used in vitro, ex vivo, and in vivo by insertion into arteriovenous shunts in various animal species, have allowed reproducible testing of both conventional and experimental agents. Shear-dependent antithrombotic effects have been observed with anticoagulants such as heparin and with selective inhibitors of thrombin, factor Xa, and factor VIIa. However, the degree of shear dependency depends on the chemical composition of the thrombogenic surface; for example, anticoagulant effects may be more pronounced on a tissue factor-rich surface than on a collagen-rich surface, particularly at venous or low arterial shear rates. Platelet inhibitors such as aspirin, thromboxane antagonists, or inhibitors of von Willebrand factor platelet interactions are also shear dependent, being more efficient at high shear rates. In contrast, inhibitors of adenosine diphosphate-dependent platelet reactions or antagonists of the platelet membrane glycoprotein IIb/IIIa complex are shear rate independent. At very high shear rates characteristic of severely stenotic arteries, aspirin loses its direct antithrombotic effect, whereas adenosine diphosphate pathway inhibitors and GP IIb/IIIa antagonists are still beneficial. In general, results obtained with many of these models have predicted antithrombotic efficacy in human beings when comparisons were possible. Therefore shear-dependent models of experimental thrombosis are routinely used in the evaluation of antithrombotic pharmacologic agents, both preclinically and clinically.

Mouse models of Hermansky Pudlak syndrome: a review

Hermansky Pudlak Syndrome (HPS) is a recessively inherited disease affecting the contents and/or the secretion of several related subcellular organelles including melanosomes, lysosomes, and platelet dense granules. It presents with disorders of pigmentation, prolonged bleeding, and ceroid deposition, often accompanied by severe fibrotic lung disease and colitis. In the mouse, the disorder is clearly multigenic, caused by at least 14 distinct mutations. Studies on the mouse mutants have defined the granule abnormalities of HPS and have shown that the disease is associated with a surprising variety of phenotypes affecting many tissues. This is an exciting time in HPS research because of the recent molecular identification of the gene causing a major form of human HPS and the expected identifications of several mouse HPS genes. Identifications of mouse HPS genes are expected to increase our understanding of intracellular vesicle trafficking, lead to discovery of new human HPS genes, and suggest diagnostic and therapeutic approaches toward the more severe clinical consequences of the disease.

Networking in the hemostatic system. Integrin alphaiibbeta3 binds prothrombin and influences its activation

Prothrombin activation is a pivotal event in thrombosis and hemostasis because thrombin can mediate fibrin formation and can activate and aggregate platelets. Platelet aggregation depends upon the binding of adhesive proteins to integrin alphaIIbbeta3 on the platelet surface. In the present study, a novel interface between the blood coagulation system and platelets is demonstrated by showing that 1) prothrombin binds to alphaIIbbeta3 and 2) this interaction accelerates prothrombin activation. Prothrombin bound to purified alphaIIbbeta3 in a specific, saturable, and divalent cation-dependent manner. This interaction was inhibited by certain monoclonal antibodies to alphaIIbbeta3, by the alphaIIbbeta3 ligands fibrinogen and RGD peptides, but not by thrombin or unrelated proteins. Prothrombin also interacted with alphaIIbbeta3 on resting and stimulated platelets as demonstrated by soluble ligand binding and platelet adhesion assays. Activation of prothrombin by Factor Xa alone or Factor Xa-Va was accelerated by alphaIIbbeta3, and this enhancement was blocked by a monoclonal antibody that inhibited prothrombin binding to the receptor. Taken together, these data identify a previously unrecognized linkage between platelets and the blood coagulation system that may have a significant regulatory consequence.