Platelets

Inhibitory Effects of Resveratrol on Platelet Activation Induced by Thromboxane A2Receptor Agonist in Human Platelets

Resveratrol (RSVL), a polyphenolic compound found in red wine is believed to be a contributor in decreasing the incidence of coronary heart disease. Although its primary target is unknown, it blocks platelet aggregation by an ill-defined mechanism. Protein kinase C (PKC), which would redistribute from the cytosol to the platelet membrane upon platelet stimulation, plays a key role in the signal transduction system of platelets in human. In this study, we investigated the effect of RSVL and a PKC inhibitor (DL-erythro-1,3-Dihydroxy-2-aminooctadecane, PKCI) on platelet aggregation induced by a thromboxane A2receptor agonist (U46619, 9,11-Dideoxy-11α, 9α-epoxymethanoprostaglandin F2α) using a platelet aggregometer. We also studied the platelet membranebound fibrinogen (PFig) content and the activity of protein kinase C (PKC) in platelets from healthy volunteers using flow cytometry, and a phosphorimaging system, respectively. Our results showed that RSVL blocked platelet aggregation and PFig content induced by U46619 in a concentration-dependent manner. PKCI and RSVL had an additive effect in inhibiting platelet aggregation and PFig content. Furthermore, RSVL (final concentration 50 μM) remarkably depressed the activity of PKC in the membrane of platelets and the percentage of membrane PKC activity in total PKC activity. Taken together, these results suggested that RSVL suppressed U46619-induced platelet aggregation and PFig content partially through the inhibition of the activity of PKC in platelets.

Platelet proteomics

Platelets are small cell fragments, produced by megakaryocytes, in the bone marrow. They play an important role in hemostasis and diverse thrombotic disorders. They are therefore primary targets of antithrombotic therapies. They are implicated in several pathophysiological pathways, such as inflammation or wound repair. In blood circulation, platelets are activated by several pathways including subendothelial matrix and thrombin, triggering the formation of the platelet plug. Studying their proteome is a powerful approach to understand their biology and function. However, particular attention must be paid to different experimental parameters, such as platelet quality and purity. Several technologies are involved during the platelet proteome processing, yielding information on protein identification, characterization, localization, and quantification. Recent technical improvements in proteomics combined with inter-disciplinary strategies, such as metabolomic, transcriptomics, and bioinformatics, will help to understand platelets biological mechanisms. Therefore, a comprehensive analysis of the platelet proteome under different environmental conditions may contribute to elucidate complex processes relevant to platelet function regarding bleeding disorders or platelet hyperreactivity and identify new targets for antiplatelet therapy.

Comparative study on infection-induced thrombocytopenia among returned travellers

BACKGROUND

Thrombocytopenia is a frequent finding among ill returned travellers and may be caused by a large number of different conditions, including infectious diseases specific or typical for tropical and subtropical regions. In order to assess the diagnostic significance of thrombocytopenia we investigated a large cohort of returned travellers.

METHODS

This was a comparative study in which data collected on 19,473 returned travellers who consulted the outpatient travel clinic of the the University of Munich Hospital between 1999 and 2009 were analysed. Of these, 732 (3.8%) travellers were diagnosed with thrombocytopenia, and their data were compared with those of the remaining 18,741 travellers with normal platelet counts.

RESULTS

Thrombocytopenia was significantly more frequent among patients with malaria (63%), acute human immunodeficiency virus infection (48%), dengue fever/dengue haemorrhagic fever (DF/DHF; 47%), Epstein-Barr virus infectious mononucleosis (23%), paratyphoid/typhoid fever (14%), and rickettsiosis (12%). Malaria and DF/DHF caused 25% of all cases of thrombocytopenia (platelet count <140,000/μl) and 75% of all cases of severe thrombocytopenia (platelet count <30,000/μl). Sex, age, country of origin, duration and type of travel were not significantly correlated with thrombocytopenia. The most frequent travel destinations were Asia (42%), Africa (33%), and Latin America (14%). Travellers to Sub-Saharan Africa (high risk for malaria) and to South/South-east Asia (high risk for DF/DHF) had the highest relative risk for thrombocytopenia.

CONCLUSION

Platelet count among returned travellers is an essential screening parameter, as thrombocytopenia is highly correlated with important infectious diseases, particularly with malaria and DF/DHF.

Platelet Function after Cessation of Chronic Clopidogrel Therapy

Surgeons increasingly encounter patients on clopidogrel therapy who are preparing to undergo surgery. The goal of this study was to examine the change in platelet function after the common clinical scenario of discontinuing chronic clopidogrel therapy in those patients preparing to undergo an elective surgery, and the time course of platelet function recovery after clopidogrel discontinuation. Patients on clopidogrel therapy scheduled for an elective surgical procedure had their platelet function tested using a Verify Now P2Y12 device (Accumetrics, San Diego, CA). Platelet inhibition was evaluated at baseline before clopidogrel discontinuation, and subsequently studied every other day in the week before their scheduled procedure. Mean platelet inhibition was 32.1 per cent on Day 0 (before clopidogrel discontinuation), decreasing to 3.7 per cent on Day 4. Platelet inhibition decreased significantly after discontinuation of clopidogrel in a time-dependent manner ( P = 0.011), although a considerable interindividual variability of P2Y12 reaction units values was observed over the study period. Patients on concomitant proton pump inhibitors and clopidogrel demonstrated a decreased effect of clopidogrel. In conclusion, individual platelet function monitoring may assist the surgeon in perioperative decision-making in patients receiving clopidogrel therapy preparing to undergo elective surgery.

Blood fluidity enhancement by electrical acupuncture stimulation is related to an adrenergic mechanism

We have reported that electrical acupuncture stimulation (ACU) increases blood fluidity by decreasing platelet aggregation. In this study, we investigated the mechanism causing the increase of blood fluidity. The effects of ACU on blood fluidity and platelet adhesion were examined using a Micro Channel Array Flow Analyzer (MC-FAN) and a laser scattering platelet aggregometer (PA-20). Male Wistar rats (7-8 weeks old) were used in the study. ACU (1 or 100 Hz, 3-5 V), which causes slight muscle twitching, was applied to the ZuSanli (ST-36) acupoint for 15 or 60 minutes once/day. Blood samples were collected from the inferior vena cava. ACU applied to ST-36 revealed significant increases in blood fluidity, while platelet adhesion activity decreased, regardless of the difference of stimulus time. The acupuncture had an immediate effect. Even if naloxone was administered during acupuncture stimulus, the blood flow time shortened in a similar way, as in the only acupuncture stimulus group. In addition, the effect of acupuncture on blood fluidity was inhibited by a β-antagonist. The results indicate that ACU affects blood fluidity depending on the acupoints, and that the effect of ACU might involve an endogenous adrenergic mechanism.

Mean platelet volume is increased in patients with deep vein thrombosis

BACKGROUND

Pulmonary thromboembolism is a serious complication in patients with deep vein thrombosis (DVT). Previous studies have demonstrated that platelet activation occurs in patients with DVT. Mean platelet volume (MPV) is a simple and easy method of assessing platelet function. The aim of this study was to assess the MPV, an indicator of platelet activation in patients with DVT.

PATIENTS AND METHODS

The study group consisted of 52 patients with newly diagnosed acute DVT. An age-, gender-, and body mass index-matched control group consisted of 30 healthy volunteers. All patients and control participants underwent ultrasonographic examination. We measured the serum MPV values in patients with DVT and control participants.

RESULTS

Mean platelet volume was significantly higher among patients with DVT when compared with the control group (8.6 ± 0.8 vs 7.7 ± 0.9 fL, respectively; P < .001).

CONCLUSION

We have shown that MPV was significantly elevated in patients with DVT compared to controls.

The effect of vanadium on platelet function

Vanadium pentoxide (V(2)O(5)) inhalation effect on platelet function in mice was explored, as well as the in vitro effect on human platelets. Mouse blood samples were collected and processed for aggregometry and flow cytometry to assess the presence of P-selectin and monocyte-platelet conjugates. Simultaneously, human platelets were processed for aggregometry(.) The mouse results showed platelet aggregation inhibition in platelet-rich-plasma (PRP) at four-week exposure time, and normality returned at eight weeks of exposure, remaining unchanged after the exposure was discontinued after four weeks. This platelet aggregation inhibition effect was reinforced with the in vitro assay. In addition, P-selectin preserved their values during the exposure, until the exposure was discontinued during four weeks, when this activation marker increased. We conclude that vanadium affects platelet function, but further studies are required to evaluate its effect on other components of the hemostatic system.

Blood platelets contain tumor-derived RNA biomarkers

Diagnostic platforms providing biomarkers that are highly predictive for diagnosing, monitoring, and stratifying cancer patients are key instruments in the development of personalized medicine. We demonstrate that tumor cells transfer (mutant) RNA into blood platelets in vitro and in vivo, and show that blood platelets isolated from glioma and prostate cancer patients contain the cancer-associated RNA biomarkers EGFRvIII and PCA3, respectively. In addition, gene-expression profiling revealed a distinct RNA signature in platelets from glioma patients compared with normal control subjects. Because platelets are easily accessible and isolated, they may form an attractive platform for the companion diagnostics of cancer.

A Difficult Diagnosis Case of Prolonged Thrombocytopenia With Sepsis and Disseminated Intravascular Coagulation

A 19-year-old male was admitted because of the trauma due to sepsis-induced disseminated intravascular coagulation (DIC) and multiple organ failure (MOF). We treated with antibiotics, danaparoid, and continuous hemodiafiltration (CHDF). Once he recovered, but after several days, he had septic shock and MOF again. With treatment, the inflammation and MOF improved but the platelet count was less than 1.0 × 10 4/μL. Because of the usage of heparin, we suspected heparin-induced thrombocytopenia (HIT) and measured the HIT antibody and a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13). Heparin-induced thrombocytopenia antibody was positive in the second sepsis but negative in the first sepsis. ADAMTS13 activity was low in both sepses. After stopping CHDF and the usage of heparin, his platelet count improved. Thrombocytopenia is the common and occasional condition for DIC. Heparin-induced thrombocytopenia and thrombotic thrombocytopenic purpura is rare but they must be ruled out in thrombocytopenia with nontypical clinical course, and the assays for HIT antibody and ADAMTS13 activity are useful tools.

Perspectives and challenges in regenerative medicine using plasma rich in growth factors

Plasma rich in growth factors (PRGF-Endoret) is an endogenous therapeutic technology that is gaining interest in regenerative medicine due to its potential to stimulate and accelerate tissue healing and bone regeneration. This autologous biotechnology is designed for the in situ delivery of multiple cellular modulators and the formation of a fibrin scaffold, thereby providing different formulations that can be widely used in numerous medical and scientific fields including dentistry, oral implantology, orthopedics, ulcer treatment and tissue engineering among others. Here we discuss the important progress that has been accomplished in this field. Furthermore, a comprehensive outlook of the intriguing therapeutic applications of this technology is presented.

Interindividual variation in platelets and the cardiovascular response to haemorrhage in the pig

The platelet count varies two-fold among healthy individuals. Considering the haemostatic role of platelets, this study evaluated the relation between cardiovascular and metabolic responses to uncontrolled haemorrhage and the pretrauma platelet count in pigs. A laceration liver injury was inflicted in 19 pigs (34 ± 3 kg; mean ± SD). To simulate a prehospital setting, fluid administration was delayed 7 min and was then by lactated Ringer. After 30 min, the fluid administered was by hydroxyethyl starch 130/0.4 to stabilize the blood volume. The platelet count for the pigs was 385 (193-507) × 10⁹/l (median (range)). The injury was similar for all pigs and caused a similar blood loss (1.4 ± 0.4 and 2.4 ± 0.4 l after administration of lactated Ringer and hydroxyethyl starch, respectively) and survival time (79 ± 17 min). At baseline, none of the cardiovascular variables were related to the pretrauma platelet count. After administration of lactated Ringer and hydroxyethyl starch, however, mean arterial pressure (R² = 0.60, P < 0.001 and R² = 0.52, P < 0.01), cardiac output (R² = 0.36, P < 0.05 and R² = 0.84, P < 0.0001), and thus oxygen delivery (R² = 0.38, P < 0.05 and R² = 0.92, P < 0.0001) related to the pretrauma platelet count and at 60 min, that was also the case for standard base excess (R² = 0.37, P < 0.01), bicarbonate (R² = 0.44, P < 0.01), and oxygen uptake (R² = 0.51, P < 0.01). Following a liver trauma in pigs, the immediate cardiovascular and metabolic responses were related to the pretrauma platelet count although it did not affect the blood loss. These results support that platelets exert functions during bleeding beyond their importance for clot formation.

Fibro-α score as a simple and useful non-invasive test for predicting significant liver fibrosis in chronic hepatitis C patients

BACKGROUND AND STUDY AIMS

Non-invasive methods for the assessment of liver fibrosis are clinically important where hepatitis C virus (HCV) is common in Egypt. Our aim was to evaluate the diagnostic performance of a panel of simple blood markers of liver fibrosis in chronic hepatitis C (CHC) patients.

PATIENTS AND METHODS

A total of 199 patients with CHC evaluated for eligibility for antiviral therapy were included. Liver biochemical profile including transaminases, bilirubin, alkaline phosphatase, serum albumin, complete blood count prothrombin time and AFP were estimated. Liver biopsy was done. Statistical analyses were performed by logistic regression and receiver operating characteristic (ROC) curves to assess and compare diagnostic accuracy of blood markers. A stepwise combination algorithm was developed and validated prospectively in 135 additional patients.

RESULTS

α-Foetoprotein (AFP) was the most efficient marker among other markers tested. The areas under the curves (AUCs) of AFP were 0.77 for significant liver fibrosis (F2-F4), 0.75 for advanced liver fibrosis (F3-F4) and 0.76 for liver cirrhosis (F4). The stepwise multivariate discriminant analysis (MDA) selected a novel non-invasive index for discriminating patients with significant liver fibrosis, named Fibro-α score. Fibro-α score=(1.35 (numeric constant) +AFP (IUml(-1))×0.009584+aspartate aminotransferase (AST)/alanine aminotransferase (ALT)×0.243-platelet count (×10(9)l(-1))×0.001624). The Fibro-α score was used for patients with advanced liver fibrosis and liver cirrhosis. The AUCs of Fibro-α score were 0.82 for patients with advanced liver fibrosis and 0.80 for those with cirrhosis. These results were reproduced in a validation study with no significant difference.

CONCLUSION

While liver biopsy is invasive, expensive and, in some settings, impossible to do, Fibro-α score is simple, cheap, non-invasive and may be useful for predicting significant liver fibrosis.

Investigation of interaction of human platelet membrane components with anticoagulant drugs Abciximab and Eptifibatide

Abciximab (Abci) and eptifibatide (Epti) are antiaggregate drugs which may reduce thrombotic complications in acute coronary syndromes. The aim of this work was the investigation of the interaction between the phospholipid-GPIIb/IIIa glycoprotein complex and Abci or Epti, and the influence of these drugs on the phospholipid ratio in the platelet membrane. The interaction between the phospholipid-GPIIb/IIIa glycoprotein complex and antiaggregate drugs were investigated using the Surface Plasmon Resonance Imaging technique (SPRI). Phospholipids phosphatidylinositol (PI), phosphatidylserine (PS), phosphatidylethanolamine (PE), phosphatidylcholine (PC) and sphingomyelin (SM) were first immobilized onto the gold chip surface. The phospholipid ratio in the platelet membrane was determined by the HPLC. Only PI, PS, PE and PC were determined. Human platelets treated 'in vitro' with Abci or Epti exhibit changes in the phospholipid ratio in the platelet membrane. The ratio of PS decreases and PC rises. The SPRI distinctly shows interactions between phospholipids and glycoprotein GPIIb/IIIa, and between the phospholipid-glycoprotein GPIIb/IIIa complex and Abci or Epti. The interaction between phospholipids and glycoprotein GPIIb/IIIa is growing in the sequence: PI<<SM<PE<PC<PS. The interaction between phospholipid-glycoprotein GPIIb/IIIa complex and Abci/Epti is growing in the sequence: PS<PI<PC<PE<SM. SPRI was proved to be excellent tool for observation of such interactions.

[Blood platelets and biological response to 'danger' signals and subsequent inflammation: towards a new paradigm?]

Blood platelets are cellular elements of primary haemostasis. During the last decade research on platelets has been subsequently based on this paradigm, with separate observations on issues such as the ability for platelets to bind infectious agents or even engulf them, to drop in counts in case of evolving infectious processes, etc. More recently, novel work has set up bases for novel functions for platelets, as members of functional immune cells, principally in innate immunity but capable of influencing adaptive immunity. Platelets are thus essential to haemostasis and to inflammation, questioning their essential functionality and the set up of a novel paradigm: could platelets be tissue-repairing cells? Such an assumption would open an entire new field of investigations. The present "State of the Art" essay attempts to discuss the main arguments on this.

Antiplatelet effects of Spatholobus suberectus via inhibition of the glycoprotein IIb/IIIa receptor

ETHNOPHARMACOLOGICAL RELEVANCE

The vine stem of Spatholobus suberectus is a widely used blood-activating and stasis-dispelling medicine for the treatment of diseases related to blood stasis syndrome in traditional medicine in Korea, Japan, and China.

AIM OF THE STUDY

To demonstrate the clinical effects of Spatholobus suberectus against blood stasis syndromes using in vitro and in vivo platelet aggregation studies and to investigate its exact mechanisms.

MATERIALS AND METHODS

We extracted vine stems of Spatholobus suberectus, using 95% EtOH (SSE) and investigated its antiplatelet activity on platelet aggregation induced by collagen and ADP in human platelet-rich plasma (PRP). For the mechanism study, a glycoprotein IIb/IIIa (GP IIb/IIIa) assay using flow cytometric analysis and a thromboxane A(2) (TXA(2)) assay were performed. In addition, we investigated the effects of SSE in a thromboembolic mouse model.

RESULTS

SSE significantly inhibited ADP- and collagen-induced platelet aggregation in human PRP concentration-dependently without affecting plasma clotting time. It also significantly inhibited fibrinogen binding to the GP IIb/IIIa receptor and partly inhibited the formation of TXA(2). In the in vivo study, oral administration of SSE dose-dependently suppressed the death of thromboembolism model mice induced by intravenous injection of collagen plus epinephrine.

CONCLUSIONS

SSE showed antiplatelet activity without anticoagulant effects mainly through the inhibition of fibrinogen binding to the GP IIb/IIIa receptor. Our current results support the clinical usage of SSE in the East Asian region treating atherothrombotic diseases and may represent a new natural source to develop antiplatelet agents.

Fatigue in adult patients with primary immune thrombocytopenia

BACKGROUND

 Patients with primary immune thrombocytopenia (ITP) commonly describe symptoms of fatigue. However, hematologists rarely consider fatigue a manifestation of ITP.

OBJECTIVES

  To document the prevalence of fatigue among patients with ITP and to determine the patient characteristics that are associated with fatigue.   Using a cross-sectional design, we surveyed 1871 members of the UK ITP Support Association [585 (31%) responded], and 93 patients enrolled in the Oklahoma (US) ITP Registry [68 (73%) responded] with questions about their ITP and with validated symptom assessment scales for fatigue, daytime sleepiness, and orthostatic symptoms.

RESULTS

  The prevalence of fatigue among both UK (39%) and US (22%) patients was significantly greater than expected compared with normal subjects (P<0.0001 and P<0.0001 respectively). In univariate analysis of the combined cohorts, fatigue was associated with a platelet count <100000/μL, treatment with steroids, bleeding symptoms, presence of other medical conditions, daytime sleepiness, and orthostatic symptoms. Fatigue was not associated with age, gender, duration of ITP, or splenectomy status. Multivariate analysis of the combined cohorts was stratified for the presence or absence of bleeding symptoms. Among 107 patients with bleeding symptoms, fatigue was independently associated with a platelet count <100000/μL and female gender. Among 491 patients without bleeding symptoms, fatigue was independently associated with a platelet count <30000/μL, presence of other medical conditions, daytime sleepiness, and orthostatic symptoms.

CONCLUSIONS

  Fatigue is a common symptom among patients with ITP. These data provide the basis for future studies to define the clinical importance of fatigue in ITP.

Platelets participate in synovitis via Cox-1-dependent synthesis of prostacyclin independently of microparticle generation

In addition to the well-described role of platelets in thrombosis, a growing body of evidence implicates platelets in diverse inflammatory responses. We recently showed platelets can contribute to the pathophysiology of inflammatory arthritis via IL-1- containing microparticles. In this study, we demonstrate that platelets, and not platelet microparticles, actively contribute to synovitis via production of proinflammatory prostacyclin in an autoimmune arthritis model. Using both genetic and pharmacologic approaches, we establish that paracrine production of prostacyclin proceeds in the absence of cyclooxygenase-2. Furthermore, we also demonstrate that prostacyclin generation can arise via transcellular collaboration between platelets and fibroblast-like synoviocytes. In addition to shedding light on an unappreciated pathway of lipid synthesis in arthritis, we further delineate a novel effector activity by which platelets can contribute to inflammatory disease.

Preparation of platelet concentrates

Platelets are specialized blood cells that play central roles in physiologic and pathologic processes of hemostasis, wound healing, host defense, thrombosis, inflammation, and tumor metastasis. Activation of platelets is crucial for platelet function that includes a complex interplay of adhesion, signaling molecules, and release of bioactive factors. Transfusion of platelet concentrates is an important treatment component for thrombocytopenia and bleeding. Recent progress in high-throughput mRNA and protein profiling techniques has advanced the understanding of platelet biological functions toward identifying novel platelet-expressed and secreted proteins, analyzing functional changes between normal and pathologic states, and determining the effects of processing and storage on platelet concentrates for transfusion. It is important to understand the different standard methods of platelet preparation and how they differ from the perspective for use as research samples in clinical chemistry. Two simple methods are described here for the preparation of research-scale platelet samples from whole blood, and detailed notes are provided about the methods used for the preparation of platelet concentrates for transfusion.

Safety and antithrombotic efficacy of moderate platelet count reduction by thrombopoietin inhibition in primates

Most heart attacks and strokes are caused by blood clots (thrombi) that block the vasculature. Because disease-causing arterial thrombosis depends on blood platelets, platelet inhibitors such as aspirin and clopidogrel effectively decrease the risk of thrombosis; however, they also impair platelet-dependent hemostasis that staunches bleeding from wounds and can therefore produce excessive bleeding. Experimental studies show that a reduction in the number of platelets also inhibits thrombosis, but these treatments also interfere with platelet function. Because normal hemostasis requires that the platelet concentration remain within a physiological range in the circulation, we evaluated whether lowering the number of circulating platelets--but only to a value still within the normal range--by inhibiting platelet formation in the bone marrow inhibits acute thrombogenesis in baboons. We reduced the platelet count with an inhibitor against the megakaryocyte-promoting hormone thrombopoietin and then showed that experimental occlusive thrombogenesis on collagen-coated vascular grafts was reduced, without impairment of primary hemostasis. These results suggest that suppressing platelet production without interfering with the hemostatic function of platelets may offer a safe alternative to current therapies for prevention of stroke and heart attack triggered by blood clotting.

Pathogen sensing, subsequent signalling, and signalosome in human platelets

Beyond haemostasis, platelets exert a potent role in innate immunity and particularly in its inflammatory arm. The extent of this action remains however debatable, despite clear - and old - evidence of a link between platelets and infection. Platelets can sense infectious pathogens by pathogen recognition receptors and they can even discriminate between various types of infectious signatures. In reply, they can shape their capacity to respond by activating a signalosome and by producing different profiles of pro-inflammatory cytokines and related products. The links between pathogen sensing, signalosome activation and protein production, and their finely tuned regulation are still under investigation since platelets lack a nucleus and thus, canonical molecular biology and genomics apparati.

The effect of low molecular weight heparin (enoxaparin) on enhanced coagulation induced by crystalloid haemodilution

The purpose of this study was to establish whether a low molecular weight heparin (enoxaparin) attenuated or abolished the enhanced coagulation induced by crystalloid fluid therapy. Twenty young, healthy male volunteers were injected subcutaneously with either enoxaparin 40 mg or saline on two separate occasions one week apart, in a randomised, blinded study. Twelve hours later, a blood sample was taken for thrombelastography analysis and haematocrit. Saline 14 ml.kg⁻¹ was then infused over thirty minutes and thrombelastography and haematocrit measurements repeated. There was a significant post-dilutional difference in the alpha angle (p = 0.002) and k-time (p = 0.001) between the two groups. There was a trend towards reduced shortening of r-time in the enoxaparin group compared to the saline control (p = 0.18). The findings suggest that enoxaparin diminished acceleration of clot formation due to haemodilution.

Soluble TLT-1 modulates platelet-endothelial cell interactions and actin polymerization

Triggering receptor expressed on myeloid cells (TREM) like transcript-1 (TLT-1) is a membrane protein receptor found in alpha-granules of platelets and megakaryocytes. Upon platelet activation TLT-1 is rapidly brought to the surface of platelets. Recently, we demonstrated that activated platelets release a soluble form of TLT-1 (sTLT-1) that is found in serum but not in the plasma of healthy individuals and can enhance platelet aggregation in vitro. Furthermore, evaluation of patients diagnosed with inflammatory diseases, such as sepsis, show that these patients have significantly elevated levels of sTLT-1 in their blood. Accordingly, mice deficient in TLT-1 are predisposed to bleeding in response to an inflammatory challenge; however, the mechanism of TLT-1 function remains unknown. In this investigation, we demonstrate an increase in the amount of platelets that adhere to endothelial cell monolayers in the presence of recombinant sTLT-1 (rsTLT-1). Additionally, we present evidence that rsTLT-1 increases platelet adherence to glass slides by stimulating actin polymerization in platelets, as determined by increased staining of rodamine phalloidin. These results suggest that during inflammation, sTLT-1 may mediate hemostasis by enhancing actin polymerization, resulting in increased platelet aggregation and adherence to the endothelium.

Adipocytokines in atherothrombosis: focus on platelets and vascular smooth muscle cells

Visceral obesity is a relevant pathological condition closely associated with high risk of atherosclerotic vascular disease including myocardial infarction and stroke. The increased vascular risk is related also to peculiar dysfunction in the endocrine activity of adipose tissue responsible of vascular impairment (including endothelial dysfunction), prothrombotic tendency, and low-grade chronic inflammation. In particular, increased synthesis and release of different cytokines, including interleukins and tumor necrosis factor-α (TNF-α), and adipokines—such as leptin—have been reported as associated with future cardiovascular events. Since vascular cell dysfunction plays a major role in the atherothrombotic complications in central obesity, this paper aims at focusing, in particular, on the relationship between platelets and vascular smooth muscle cells, and the impaired secretory pattern of adipose tissue.

Platelet retraction force measurements using flexible post force sensors

Platelets play an important role in hemostasis by forming a thrombotic plug that seals the vessel wall and promotes vascular healing. After platelets adhere and aggregate at the wound site, their next step is to generate contractile forces through the coordination of physicochemical interactions between actin, myosin, and alpha(IIb)beta(3) integrin receptors that retract the thrombus' size and strengthen its adhesion to the exposed matrix. Although platelet contractile forces (PCF) are a definitive feature of hemostasis and thrombosis, there are few approaches that can directly measure them. In this study, we describe the development of an approach to measure PCF in microthrombi using a microscopic flexible post force sensor array. Quasi-static measurements and live microscopic imaging of thrombin-activated platelets on the posts were conducted to assay the development of PCF to various hemostatic conditions. Microthrombi were observed to produce forces that monotonically increased with thrombin concentration and activation time, but forces subsided when thrombin was removed. PCF results were statistically similar on arrays of posts printed with fibronectin or fibrinogen. PCF measurements were combined with clot volume measurements to determine that the average force per platelet was 2.1 +/- 0.1 nN after 60 min, which is significantly higher than what has been measured with previous approaches. Overall, the flexible post arrays for PCF measurements are a promising approach for evaluating platelet functionality, platelet physiology and pathology, the impacts of different matrices or agonists on hemostatic responses, and in providing critical information regarding platelet activity that can guide new hemostatic or thrombotic strategies.

A double-blind, randomized study on prevention and existence of a rebound phenomenon of platelets after cessation of clopidogrel treatment

OBJECTIVES

The goal of this study was to assess whether a platelet rebound exists and whether it can be attenuated by clopidogrel tapering.

BACKGROUND

Clinical studies have reported a clustering of thrombotic events after stopping clopidogrel treatment. The hypothesis of a rebound phenomenon of platelets has been declared causative, but its existence has never been confirmed. Tapering of clopidogrel over a certain period of time before stopping the drug completely might provide a way to attenuate this supposed phenomenon.

METHODS

Patients (n = 69) receiving clopidogrel treatment due to prior drug-eluting stent placement and planning to stop clopidogrel were recruited in a double-blind, randomized trial. Patients were randomized to either receive a pre-specified tapering regimen (tapering group; n = 35) for 4 weeks with complete discontinuation of clopidogrel thereafter or continue a daily clopidogrel intake for 4 more weeks with abrupt discontinuation afterwards (off group; n = 34). Platelet aggregation (PA) was assessed with light transmission aggregometry (LTA) and multiple electrode aggregometry (MEA) simultaneously at study inclusion and at weeks 2 to 8 after randomization. The primary end point was the highest value of adenosine diphosphate-induced PA measured with LTA in the weeks after complete cessation of clopidogrel in both groups.

RESULTS

The highest values of adenosine diphosphate-induced PA after complete cessation of clopidogrel were similar between both groups (p = 0.21 with LTA, and p = 0.55 with MEA).

CONCLUSIONS

Tapering of clopidogrel does not result in lower PA values after clopidogrel withdrawal. The course of PA values after clopidogrel cessation provides no evidence for the existence of a rebound phenomenon of platelets.

Platelets amplify inflammation in arthritis via collagen-dependent microparticle production

In addition to their pivotal role in thrombosis and wound repair, platelets participate in inflammatory responses. We investigated the role of platelets in the autoimmune disease rheumatoid arthritis. We identified platelet microparticles--submicrometer vesicles elaborated by activated platelets--in joint fluid from patients with rheumatoid arthritis and other forms of inflammatory arthritis, but not in joint fluid from patients with osteoarthritis. Platelet microparticles were proinflammatory, eliciting cytokine responses from synovial fibroblasts via interleukin-1. Consistent with these findings, depletion of platelets attenuated murine inflammatory arthritis. Using both pharmacologic and genetic approaches, we identified the collagen receptor glycoprotein VI as a key trigger for platelet microparticle generation in arthritis pathophysiology. Thus, these findings demonstrate a previously unappreciated role for platelets and their activation-induced microparticles in inflammatory joint diseases.

Comparison of the relative activities of inducing platelet apoptosis stimulated by various platelet-activating agents

Apoptosis-like events are known to occur in anuclear platelets. Although the mechanisms responsible for these events are still not completely understood, studies suggested that some platelet agonists can activate platelet apoptosis. However, the relative activities of various platelet agonists in inducing apoptosis have not yet been investigated. In the present study we explored this issue, and attempted to identify the correlation between platelet activation and apoptosis. In a platelet aggregation study, there were no significant differences respectively stimulated by arachidonic acid (AA; 100 microM), ADP (20 microM), collagen (10 microg/mL), thrombin (0.1 U/mL), U46619 (10 microM), and A23187 (5 microM). In a subsequent study, we fixed these concentrations of agonists to further compare their relative activities in inducing platelet apoptosis. Our results found that thrombin, U46619, and A23187 possess stronger activities than the other agonists in inducing platelet apoptosis (i.e., phosphatidylserine exposure, mitochondrial membrane potential depolarization, eukaryotic initiation factor (eIF)2alpha, and caspase activation). On the other hand, AA induced no apoptotic events in platelets. Based on this approach, we demonstrated for the first time that thrombin, U46619, and A23187, but not AA, possess stronger activity in inducing platelet apoptosis. In addition, we also found that platelet activation might not necessarily be associated with the occurrence of platelet apoptosis. The in vivo physiological function of the apoptotic machinery in platelets is not yet clearly understood, and needs to be further investigated in the future.

Thrombocytopenia and its Related Factors: A Hospital-based, Cross-sectional Study

Introduction: The objective of this study was to explore the association between thrombocytopenia and its related factors. Materials and Methods: This was a hospital-based, cross-sectional study. We retrospectively analysed the medical records of all patients who received periodic health examinations at a medical centre located at Taichung in Taiwan between 2000 and 2004. In all, 5585 subjects were included for further analysis. A complete physical examination, laboratory survey and abdominal ultrasonography were performed on each subject. The t-test, chi-square test and multivariate logistic regression analysis were used. Results: The subjects consisted of 3123 men (55.9%) and 2462 women (44.1%). The mean age was 49.4 ± 12.3 years (range, 20 to 87). The overall prevalence of thrombocytopenia was found to be 0.5%, higher in men than in women (0.6% vs 0.4%, P = 0.504). After controlling for the other covariates, multivariate logistic regression analysis exhibited that the factors significantly related to thrombocytopenia were increasing age (OR, 1.04; 95% CI, 1.004-1.08), anti-HCV positive (OR, 5.24; 95% CI, 2.08-13.20), liver cirrhosis (OR, 7.93; 95% CI, 2.28-27.62), and splenomegaly (OR, 18.86; 95% CI, 6.86-51.87). Conclusion: It is advisable to further check the hepatic status, if thrombocytopenia is noted. Key words: Hepatitis C, Liver cirrhosis, Splenomegaly, Thrombocytopenia

The platelet receptor CLEC-2 is active as a dimer

The platelet receptor CLEC-2 binds to the snake venom toxin rhodocytin and the tumor cell surface protein podoplanin. Binding of either of these ligands promotes phosphorylation of a single tyrosine residue in the YXXL motif in the intracellular domain of CLEC-2. Phosphorylation of this tyrosine initiates binding of spleen tyrosine kinase (Syk) and triggers further downstream signaling events and ultimately potent platelet activation and aggregation. However, it is unclear how a single YXXL motif can interact efficiently with Syk, which usually recognizes two tandem YXXL repeats presented as an immunoreceptor tyrosine-based activation motif (ITAM). Using bioluminescence resonance energy transfer, coimmunopreciptitation, recombinant protein expression and analytical gel filtration chromatography, surface plasmon resonance, Western blotting, multiangle light scattering (MALS), and analytical ultracentrifugation, we show that CLEC-2 exists as a non-disulfide-linked homodimer which could allow each Syk molecule to interact with two YXXL motifs, one from each CLEC-2 monomer.

A study of platelets in the armadillochaetophractus villosus(xenarthra, dasypodidae)

Nothing is known about haemostasis in Xenarthra, a widely distributed Order of American mammalians. Chaetophractus villosus, a member of the Dasypodidae family of this group, which is easily adapted to captivity, is of growing interest for biomedical research. In this work, we studied platelet number, MPV, ultrastructure of the platelets by SEM and TEM, and aggregation responses to ADP and ristocetin in this species. Blood samples were obtained by cardiac puncture in 20 anaesthetised animals. Platelet count and MPV were evaluated at the beginning and at the end of a 3-year experimental period, to detect possible variations related to time of captivity. SEM and TEM were done by routine methods adapted to the material, and aggregation response to ADP and ristocetin were evaluated by the Born method. The parameters studied did not show any sex-related differences, nor did the platelet count change during captivity. Nevertheless, MPV decreased during this period. Platelets were ultrastructurally similar to those of other mammals and human beings and responded to proven agonists. Data provided in this study will contribute to the understanding of the haemostatic process in this species.

Effect of the psoralen-based photochemical pathogen inactivation on mitochondrial DNA in platelets

Photochemical treatment (PCT) of platelet concentrates, using amotosalen HCl and UVA-light, inactivates pathogens by forming adducts between amotosalen and nucleic acids. The impact of the photochemical treatment on pathogens and leukocytes has been studied extensively. Yet little is known about the effect of PCT on nucleic acids in platelets. Platelets contain viable mitochondria and mitochondrial DNA (mtDNA) and this study aimed at evaluating the amotosalen modifications on platelet mtDNA. We applied two independent but complementary molecular assays to investigate qualitative as well as quantitative aspects of the psoralen-mediated DNA modifications in platelet mtDNA. The amotosalen-DNA modification density was measured using (14)C-labeled amotosalen. Amotosalen (150 microM) yielded 4.0 +/- 1.2 psoralen adducts per 1,000 bp in mtDNA after irradiation with 3 J/cm(2) UVA. Furthermore, we tested if the PCT-induced DNA modifications could be detected by a PCR assay. On the basis of PCR inhibition due to amotosalen-DNA adducts, mtDNA-specific PCR assays were developed and tested for their specificity and sensitivity. Our data revealed that mtDNA in platelets is substantially modified by PCT and that these modifications can be documented by a PCR inhibition system.

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.

[Platelet immunology and the immune response]

Platelets exert not only hemostatic activities, but also pro-inflammatory effects. Platelet-linked inflammation seems essentially related to their capacity of secreting cytokines, chemokines and related molecules. This activity is important in terms of concentration of secreted products. This secretory function confers to platelets a regulatory role in immunity. Besides, platelets do exhibit non-self infectious danger detection molecules on their surfaces, belonging in particular to the "Toll-like receptor family"; through this property, platelets can bind infectious agents but also deliver differential signals for the secretion of cytokines and chemokines. Platelets, which are non-nucleated cells deprived of nuclear DNA, possess however some cellular machinery which permits intracellular signalling and even the production of RNA transcripts for certain cytokines. Last, platelets express variant surface determinants of hemostatic molecules (referred to as HPA antigens) along with HLA class I variant molecules, the function of which on platelets is still unknown. An intriguing question is to reconcile those diverse properties and to understand whether the pro-inflammatory secretory process can affect the immunogenicity of transfused, allogeneic, platelet components.

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.

Enrichment of human platelet membranes for proteomic analysis

Platelets (thrombocytes) are the smallest human blood cells and are pivotal in processes of hemostasis and thrombosis. Central to their function, the activation of platelets includes a complex interplay of adhesion and signalling molecules mediated via the plasma and inner membrane. Because platelets are enucleated, the analysis of the proteome is the best way to approach their biology. Here, we employ mass spectrometry (MS)-based proteomics to characterise membrane proteins derived from non-stimulated human platelets. This protocol details the extraction and purification of platelet membrane proteins from whole blood using SDS-PAGE in conjunction with LC-MS/MS. This method allowed the identification, and characterization of 207 platelet membrane proteins (PMP) from approximately 9.95 x 10(9) platelets (16).

Donor platelets stored for at least 3 days can elicit activation marker expression by the recipient's blood mononuclear cells: an in vitro study

BACKGROUND

Recent studies have demonstrated that infused platelets (PLTs) can promote inflammation. The objective of this study was to evaluate the impact of storage of transfusion-grade PLTs on the peripheral blood mononuclear cells (PBMNCs) of the recipient.

STUDY DESIGN AND METHODS

An in vitro cell model system was established to measure the degree of activation of donor PLTs during 5 days of their storage and then to measure immune cell activation by detecting marker expression in coculture experiments.

RESULTS

The level of soluble CD62p increased significantly by Day 3, and membrane expression of CD62p increased significantly from Day 2, indicating some degree of PLT activation over time during storage (p < 0.05). Donor PLTs and PBMNC subsets (monocytes, B cells, and T cells) from recipients were cocultured for 48 hours. The number of PLT-PBMNC subset doublets detected by flow cytometry was correlated with the PLT storage time after Day 3 (p < 0.05), indicating consistent binding of PLTs to PBMNCs. The results of these experiments showed that there was a consistent and significant increase in expression of conventional activation markers of T cells, B cells, and monocytes compared with appropriate controls (p < 0.05 to <0.01).

CONCLUSION

The results of this study indicate that, from Day 3 onward, activation markers are consistently expressed on PLTs. From these results, we conclude that activated PLTs may affect PBMNC interactions in recipients.