The interaction of thrombin with blood platelets

Effect of Systemic Inflammation in the CNS: A Silent History of Neuronal Damage

Central nervous system (CNS) infections including meningitis and encephalitis, resulting from the blood-borne spread of specific microorganisms, provoke nervous tissue damage due to the inflammatory process. Moreover, different pathologies such as sepsis can generate systemic inflammation. Bacterial lipopolysaccharide (LPS) induces the release of inflammatory mediators and damage molecules, which are then released into the bloodstream and can interact with structures such as the CNS, thus modifying the blood-brain barrier's (BBB´s) and blood-cerebrospinal fluid barrier´s (BCSFB´s) function and inducing aseptic neuroinflammation. During neuroinflammation, the participation of glial cells (astrocytes, microglia, and oligodendrocytes) plays an important role. They release cytokines, chemokines, reactive oxygen species, nitrogen species, peptides, and even excitatory amino acids that lead to neuronal damage. The neurons undergo morphological and functional changes that could initiate functional alterations to neurodegenerative processes. The present work aims to explain these processes and the pathophysiological interactions involved in CNS damage in the absence of microbes or inflammatory cells.

Bioactive Nanogels Mimicking the Antithrombogenic Nitric Oxide-Release Function of the Endothelium

Nitric oxide (NO) plays a significant role in controlling the physiology and pathophysiology of the body, including the endothelial antiplatelet function and therefore, antithrombogenic property of the blood vessels. This property of NO can be exploited to prevent thrombus formation on artificial surfaces like extracorporeal membrane oxygenators, which when come into contact with blood lead to protein adsorption and thereby platelet activation causing thrombus formation. However, NO is extremely reactive and has a very short biological half-life in blood, so only endogenous generation of NO from the blood contacting material can result into a stable and kinetically controllable local delivery of NO. In this regards, highly hydrophilic bioactive nanogels are presented which can endogenously generate NO in blood plasma from endogenous NO-donors thereby maintaining a physiological NO flux. It is shown that NO releasing nanogels could initiate cGMP-dependent protein kinase signaling followed by phosphorylation of vasodilator-stimulated phosphoprotein in platelets. This prevents platelet activation and aggregation even in presence of highly potent platelet activators like thrombin, adenosine 5'-diphosphate, and U46619 (thromboxane A2 mimetic).

The Interplay between Non-Esterified Fatty Acids and Plasma Zinc and Its Influence on Thrombotic Risk in Obesity and Type 2 Diabetes

Thrombosis is a major comorbidity of obesity and type-2 diabetes mellitus (T2DM). Despite the development of numerous effective treatments and preventative strategies to address thrombotic disease in such individuals, the incidence of thrombotic complications remains high. This suggests that not all the pathophysiological mechanisms underlying these events have been identified or targeted. Non-esterified fatty acids (NEFAs) are increasingly regarded as a nexus between obesity, insulin resistance, and vascular disease. Notably, plasma NEFA levels are consistently elevated in obesity and T2DM and may impact hemostasis in several ways. A potentially unrecognized route of NEFA-mediated thrombotic activity is their ability to disturb Zn²⁺ speciation in the plasma. Zn²⁺ is a potent regulator of coagulation and its availability in the plasma is monitored carefully through buffering by human serum albumin (HSA). The binding of long-chain NEFAs such as palmitate and stearate, however, trigger a conformational change in HSA that reduces its ability to bind Zn²⁺, thus increasing the ion’s availability to bind and activate coagulation proteins. NEFA-mediated perturbation of HSA-Zn²⁺ binding is thus predicted to contribute to the prothrombotic milieu in obesity and T2DM, representing a novel targetable disease mechanism in these disorders.

COVID-19 and biomarkers of thrombosis: focus on von Willebrand factor and extracellular vesicles

COVID-19, caused by the SARS-CoV-2 virus, is responsible for a pandemic of unparalleled portion over the past century. While the acute phase of infection causes significant morbidity and mortality, post-acute sequelae that can affect essentially any organ system is rapidly taking on an equally large part of the overall impact on human health, quality of life, attempts to return to normalcy and the global economy. Herein, we summarize the potential role of von Willebrand Factor and extracellular vesicles toward understanding the pathophysiology, clinical presentation, duration of illness, diagnostic approach and management of COVID-19 and its sequelae.

Sepsis-Associated Encephalopathy and Blood-Brain Barrier Dysfunction

Sepsis is a life-threatening clinical condition caused by a dysregulated host response to infection. Sepsis-associated encephalopathy (SAE) is a common but poorly understood neurological complication of sepsis, which is associated with increased morbidity and mortality. SAE clinical presentation may range from mild confusion and delirium to severe cognitive impairment and deep coma. Important mechanisms associated with SAE include excessive microglial activation, impaired endothelial barrier function, and blood-brain barrier (BBB) dysfunction. Endotoxemia and pro-inflammatory cytokines produced systemically during sepsis lead to microglial and brain endothelial cell activation, tight junction downregulation, and increased leukocyte recruitment. The resulting neuroinflammation and BBB dysfunction exacerbate SAE pathology and aggravate sepsis-induced brain dysfunction. In this mini-review, recent literature surrounding some of the mediators of BBB dysfunction during sepsis is summarized. Modulation of microglial activation, endothelial cell dysfunction, and the consequent prevention of BBB permeability represent relevant therapeutic targets that may significantly impact SAE outcomes.

A narrative review of low-dose rivaroxaban in patients with atherothrombotic cardiovascular disease: vascular protection beyond anticoagulation

Major cardiovascular (CV) events often complicate the natural history of apparently stable atherothrombotic cardiovascular disease (CVD) despite appropriate guideline-based preventive treatment. This finding has been termed residual risk and it has been the focus of recent investigation. New and revisited targets to tackle this so-called residual risk have been proposed, including antithrombotic treatment intensification, further lowering targets of low-density lipoprotein (LDL) cholesterol, novel oral antidiabetic agents with a CV benefit, and drugs to reduce systemic inflammation. In this narrative review, we discuss the evidence, mechanisms and gaps in knowledge concerning the vascular protection derived from low-dose (2.5 mg twice daily) rivaroxaban. On this topic, the main trials (ATLAS ACS 2-TIMI 51, COMPASS and VOYAGER PAD), will be summarized in a comprehensive manner. Indeed, these have shown that a drug developed to prevent thrombus formation (selective Factor Xa inhibition) reduced events that were traditionally platelet-related in concept. Moreover, we propose a simple evidence-based clinically oriented algorithm to thoroughly identify patients at increased risk and who may benefit from this strategy in different clinical scenarios. Low-dose rivaroxaban portrays a novel promising era in atherothrombotic CVD prevention, providing a mechanistic protection beyond traditional strategies in patients overwhelmed by recurrent dismal events.

Surface loops of trypsin-like serine proteases as determinants of function

Trypsin and chymotrypsin-like serine proteases from family S1 (clan PA) constitute the largest protease group in humans and more generally in vertebrates. The prototypes chymotrypsin, trypsin and elastase represent simple digestive proteases in the gut, where they cleave nearly any protein. Multidomain trypsin-like proteases are key players in the tightly controlled blood coagulation and complement systems, as well as related proteases that are secreted from diverse immune cells. Some serine proteases are expressed in nearly all tissues and fluids of the human body, such as the human kallikreins and kallikrein-related peptidases with specialization for often unique substrates and accurate timing of activity. HtrA and membrane-anchored serine proteases fulfill important physiological tasks with emerging roles in cancer. The high diversity of all family members, which share the tandem β-barrel architecture of the chymotrypsin-fold in the catalytic domain, is conferred by the large differences of eight surface loops, surrounding the active site. The length of these loops alters with insertions and deletions, resulting in remarkably different three-dimensional arrangements. In addition, metal binding sites for Na+, Ca2+ and Zn2+ serve as regulatory elements, as do N-glycosylation sites. Depending on the individual tasks of the protease, the surface loops determine substrate specificity, control the turnover and allow regulation of activation, activity and degradation by other proteins, which are often serine proteases themselves. Most intriguingly, in some serine proteases, the surface loops interact as allosteric network, partially tuned by protein co-factors. Knowledge of these subtle and complicated molecular motions may allow nowadays for new and specific pharmaceutical or medical approaches.

Exosite 1 thrombin inhibition with JNJ-64179375 inhibits thrombus formation in a human translational model of thrombosis

AIMS

JNJ-64179375 (hereafter JNJ-9375) is a first-in-class, highly specific, large molecule, exosite 1 thrombin inhibitor. In preclinical studies, JNJ-9375 demonstrated robust antithrombotic protection with a wider therapeutic index when compared to apixaban. The purpose of the present study was to examine for the first time the antiplatelet, anticoagulant and antithrombotic effects of JNJ-9375 in a translational model of ex vivo human thrombosis.

METHODS AND RESULTS

Fifteen healthy volunteers participated in a double-blind randomized crossover study of JNJ-9375 (2.5, 25, and 250 μg/mL), bivalirudin (6 μg/mL; positive control), and matched placebo. Coagulation, platelet activation, and thrombus formation were determined using coagulation assays, flow cytometry, and an ex vivo perfusion chamber, respectively.JNJ-9375 caused concentration-dependent prolongation of all measures of blood coagulation (prothrombin time, activated partial thromboplastin time, and thrombin time; P < 0.001 for all) and agonist selective inhibition of thrombin (0.1 U/mL) stimulated platelet p-selectin expression (P < 0.001) and platelet-monocyte aggregates (P = 0.002). Compared to placebo, JNJ-9375 (250 μg/mL) reduced mean total thrombus area by 41.1% (95% confidence intervals 22.3 to 55.3%; P < 0.001) at low shear and 32.3% (4.9 to 51.8%; P = 0.025) at high shear. Under both shear conditions, there was a dose-dependent decrease in fibrin-rich thrombus (P < 0.001 for both) but not platelet-rich thrombus (P = ns for both).

CONCLUSION

Exosite 1 inhibition with JNJ-9375 caused prolongation of blood coagulation, selective inhibition of thrombin-mediated platelet activation, and reductions in ex vivo thrombosis driven by a decrease in fibrin-rich thrombus formation. JNJ-9375 represents a novel class of anticoagulant with potential therapeutic applications.

Enhancement of hemostatic property of plant derived oxidized nanocellulose-silk fibroin based scaffolds by thrombin loading

To combat post-surgical and traumatic bleeding conditions effective hemostasis is of great importance. The study was designed to investigate the effect of thrombin (Th) loading on hemostatic performance of TEMPO-oxidized cellulose nanofiber (TOCN)-silk fibroin (SF) scaffolds. Addition of SF with TOCN significantly (***P < 0.001) increased blood absorption capacity and improved biocompatibility of TOCN. Thrombin loading potentiated platelet activation and hemostatic property of scaffolds (TOCN-SF-Th) compared to samples without thrombin (TOCN-SF). The hemostatic time of TOCN-SF5-Th in rabbit ear artery bleeding model was reduced (*** P < 0.001) to 114 s from 220 s of TOCN-SF5. Reduction in bleeding time and blood loss of TOCN-SF5-Th in rat tail amputation and liver avulsion model was comparable to commercial hemostat (Floseal). Surface morphology (SEM) of samples applied on bleeding site showed that RBCs and fibrin fiber could strongly interact with TOCN-SF and TOCN-SF-Th scaffolds. The result suggests that TOCN-SF-Th can be a promising candidate for designing hemostatic agents.

The effect of omalizumab on hematological and inflammatory parameters in patients with chronic spontaneous urticaria

PURPOSE

The mechanism of action of omalizumab in chronic spontaneous urticaria (CSU) is not exactly known. In previous studies, d-dimer which is a fibrin-degradation product and interleukin-31 which has a role in inflammation were found to be decreased in patients with chronic urticaria treated with omalizumab. However, to our knowledge, there is no study on the effects of omalizumab on the ratio of neutrophil to lymphocyte (NLR) and the ratio of platelet to lymphocyte (PLR) which are inflammatory parameters in patients with CSU in the PubMed database. In this study, we aimed to evaluate the effects of omalizumab on hematological and inflammatory parameters in patients with CSU.

MATERIALS AND METHODS

One hundred six CSU patients treated with omalizumab were evaluated retrospectively. Complete blood count (CBC) and C-reactive protein (CRP) levels before treatment and at the third month of treatment were recorded. NLR and PLR were calculated from the CBC results. CBC parameters, CRP levels, NLR and PLR of the patients before treatment and at the end of third month of treatment were compared.

RESULTS

There was a significant decrease in white blood cell count, platelet count, neutrophil count, CRP level, NLR and PLR and a significant increase in mean platelet volume and eosinophil count with omalizumab treatment in CSU patients (p < 0.05). Basophil count increased; but there was no statistically significant difference (p = 0.293).

CONCLUSION

This study shows that omalizumab has not only anti Ig E effect, but also it may have inhibitory effects on inflammation and coagulation in patients with CSU. Further prospective studies are warranted in order to evaluate the effect of omalizumab on hematological and inflammatory parameters and correlation between omalizumab response rate and these inflammatory parameters in patients with CSU.

Dynamics of pathologic clot formation: a mathematical model

Recent studies have provided evidence of a significant role of the Hageman factor in pathologic clot formation. Since auto-activation of the Hageman factor triggers the intrinsic coagulation pathway, we study the dynamics of pathologic clot formation considering the intrinsic pathway as the predominant mechanism of this process. Our methodological approach to studying the dynamics of clot formation is based on mathematical modelling. Activation of the blood coagulation cascade, particularly its intrinsic pathway, is known to involve platelets. Therefore, equations accounting for the effects of activated platelets on the intrinsic pathway activation are included in our model. This brings about a considerable increase in the values of kinetic constants involved in the model of the principal biochemical processes resulting in clot formation. The purpose of this study is to elucidate the mechanism of pathologic clot formation. Since the time window of thrombolysis is 3-6h, we hypothesize that in many cases the rate of pathologic clot formation is much lower than that of haemostatic clot. This assumption is used to simplify the mathematical model and to estimate kinetic constants of biochemical reactions that initiate pathologic clot formation. The insights we gained from our mathematical model may lead to new approaches to the prophylaxis of pathologic clot formation. We believe that one of the most efficient ways to prevent pathologic clot formation is simultaneous inhibition of activated factors ХII and ХI.

Concurrent and separate inside-out transition of platelet apoptosis and activation markers to the platelet surface

The cell plasma membrane is tightly coupled with the vital processes of apoptosis and activation. In the current study, we investigated exposure of the apoptosis marker phosphatidylserine (PS) and activation marker P-selectin (CD62) on the plasma membrane of anucleate platelets. We found that, depending on triggering stimuli, the plasma membrane of human platelets may exist in four states with predominant exposure of (i) PS but not CD62 (75·9 ± 2·8% of total cells), (ii) CD62 but not PS (86·2 ± 1·3%), (iii) both PS and CD62 (89·6 ± 1·0%) or (iv) neither PS nor CD62 (87·9-97·5%), when platelets were treated at optimal conditions with pro-apoptotic BH3 mimetic ABT-737, thrombin, calcium ionophore A23187 or control diluents, respectively. The dynamics of PS exposure induced by ABT-737 is a slow temperature-dependent process requiring 90 min treatment at 37°C rather than at room temperature for obtaining high levels of PS exposure. In contrast, thrombin-induced CD62 exposure and A23187-induced PS and CD62 exposure showed fast temperature-independent dynamics. This model of selective and concurrent stimulation of PS and/or CD62 transition to the platelet surface provides an experimental horizon for elucidating the roles of plasma membrane markers of platelet apoptosis and activation in platelet clearance.

PAR-1 antagonists: current state of evidence

Vorapaxar (SCH 530348) and atopaxar (E5555) are oral protease-activated receptor-1 (PAR-1) antagonists with high bioavailability. They inhibits thrombin induced platelet aggregation by competitively inhibiting PAR-1. We systematically evaluated the evidence for the efficacy and safety of all PAR-1 antagonists as well as for the individual drugs vorapaxar and atopaxar in different databases-PubMed, EMBASE, Scopus, and Cochrane register of Controlled Clinical Trials (CENTRAL).We selected randomized controlled trials of PAR-1 antagonists that reported on cardiovascular mortality as a clinical outcome. The random-effects Mantel-Haenszel model was used to evaluate the effect of PAR-1 antagonists on cardiovascular mortality. Seven trials were selected (N = 42,355) for analysis. PAR-1 antagonists as a class, as well as individually, were associated with a non-significant numerically lower risk of cardiovascular mortality than that seen with agents used in the control group; RR, 0.93; 95% CI, 0.83-1.04; P = 0.20). No heterogeneity was noted. However, PAR-1 antagonists also appeared to increase the risk of bleeding significantly. PAR-1 antagonists appear to be associated with some reduction in the risk of cardiovascular mortality; however the significantly higher bleeding risk noted with PAR-1 antagonists appear to mandate a very careful selection of patients that may benefit without a substantially increased risk of bleeds.

Expression of P-selectin (CD62P) on platelets after thrombin and ADP in hypotrophic and healthy, full-term newborns

BACKGROUND

Hypotrophic newborns in comparison with eutrophic newborns demonstrate a reduced blood platelet count and therefore may have haemostasis disorders. Expression of P-selectin (CD62P) on the surface of platelets is a marker of stimulated, activated blood platelets. The ability of platelets to react can be determined after the addition of the following activators: strong (thrombin) and weak (ADP).

MATERIALS AND METHODS

We studied 48 hypotrophic newborns, 25 females and 23 males, weighing less than the 10th centile and 55 healthy, full-term newborns, 25 females and 30 males. Expression of CD62P on the surface of platelets was examined in the native state, after the addition of thrombin or ADP.

RESULTS

Hypotrophic newborns exhibited almost double the percentage of platelets expressing CD62P compared with the control group, 4.21% versus 2.88%. After the addition of thrombin, the percentage was 31.5% versus 12.5%, p < 0.001, whereas after the addition of ADP, the percentage was 9.54% versus 4.5%, p = 0.002.

CONCLUSIONS

Hypotrophic newborns are capable of greater platelet activation in comparison with healthy term newborns. However, gender does not affect the expression of P-selectin.

Selective triggering of platelet apoptosis, platelet activation or both

Anucleate platelets perform two fundamental processes, activation and apoptosis. We elaborated an approach for selective and concurrent stimulation of platelet apoptosis and/or activation, processes important in haemostasis and platelet clearance. Human platelets were treated with BH3 mimetic ABT-737, thrombin, calcium ionophore A23187 and matched diluents. Apoptosis was determined as mitochondrial inner membrane potential (ΔΨm) depolarization and activation as P-selectin exposure. At optimal treatment conditions (90-180 min, 37°C), ABT-737 predominantly induced apoptosis, when 77-81% platelets undergo only ΔΨm depolarization. The ABT-737 impact on ΔΨm depolarization is strongly time- and temperature-dependent, and much higher at 37°C than at room temperature. In contrast, when platelets were treated with thrombin for 15-90 min at either temperature, activation-only was predominantly (79-85%) induced, whereas A23187 triggers both apoptosis and activation (73-81%) when platelets were treated for 15-60 min at 37°C or 15-90 min at room temperature. These data demonstrate that, depending on the triggering stimulus, platelets predominantly undergo ΔΨm depolarization-only, P-selectin exposure-only, or both responses, indicating that platelet apoptosis and activation are different phenomena driven by different mechanisms. The described model provides a basis for studying differential pharmacological manipulation of platelet apoptosis and activation and their role in haemostasis, thrombosis and platelet clearance.

Polyvalent nucleic acid aptamers and modulation of their activity: a focus on the thrombin binding aptamer

Nucleic acid-based aptamers can be selected from combinatorial libraries of synthetic oligonucleotides to bind, with affinity and specificity similar to antibodies, a wide range of biomedically relevant targets. Compared to protein therapeutics, aptamers exhibit significant advantages in terms of size, non-immunogenicity and wide synthetic accessibility. Various chemical modifications have been introduced in the natural oligonucleotide backbone of aptamers in order to increase their half-life, as well as their pharmacological properties. Very effective alternative approaches, devised in order to improve both the aptamer activity and stability, were based on the design of polyvalent aptamers, able to establish multivalent interactions with the target: thus, multiple copies of an aptamer can be assembled on the same molecular- or nanomaterial-based scaffold. In the present review, the thrombin binding aptamers (TBAs) are analyzed as a model system to study multiple-aptamer constructs aimed at improving their anticoagulation activity in terms of binding to the target and stability to enzymatic degradation. Indeed - even if the large number of chemically modified TBAs investigated in the last 20 years has led to encouraging results - a significant progress has been obtained only recently with bivalent or engineered dendritic TBA aptamers, or assemblies of TBAs on nanoparticles and DNA nanostructures. Furthermore, the modulation of the aptamers activity by means of tailored drug-active reversal agents, especially in the field of anticoagulant aptamers, as well as the reversibility of the TBA activity through the use of antidotes, such as porphyrins, complementary oligonucleotides or of external stimuli, are discussed.

Apoptosis in the anucleate platelet

For many years, programmed cell death, known as apoptosis, was attributed exclusively to nucleated cells. Currently, however, apoptosis is also well-documented in anucleate platelets. This review describes extrinsic and intrinsic pathways of apoptosis in nucleated cells and in platelets, platelet apoptosis induced by multiple chemical stimuli and shear stresses, markers of platelet apoptosis, mitochodrial control of platelet apoptosis, and apoptosis mediated by platelet surface receptors PAR-1, GPIIbIIIa and GPIbα. In addition, this review presents data on platelet apoptosis provoked by aging of platelets in vitro during platelet storage, platelet apoptosis in pathological settings in humans and animal models, and inhibition of platelet apoptosis by cyclosporin A, intravenous immunoglobulin and GPIIbIIIa antagonist drugs.

Thrombin as a multi-functional enzyme. Focus on in vitro and in vivo effects

Thrombin is the central protease in the coagulation cascade and one of the most extensively studied of all enzymes. In addition to its recognised role in the coagulation cascade and haemostasis, thrombin is known to have multiple pleiotropic effects, which mostly have been shown only in in vitro studies: it plays a role in inflammation and cellular proliferation and displays a mitogen activity on smooth muscle cells and endothelial cells, predominantly by activation of angiogenesis. In vivo , thrombin effects were examined in animal models of intravenous or intraarterial thrombin infusion. An extensive literature search regarding in vivo data showed that i) thrombin administered as a bolus causes microembolism, ii) thrombin infused slowly at steady-state conditions (up to 1.6 U/kg/min) leads to bleeds but not to intravascular clotting, iii) large quantity of thrombin infused at low rates (0.05 U/kg/min) does not have any measurable effect, and iv) thrombin increases vascular permeability leading to tissue damage. Although several decades of research on thrombin functions have provided a framework for understanding the biology of thrombin, animal and human studies with use of newer laboratory techniques are still needed to confirm the pleiotropic thrombin functions shown in in vitro studies.

Thrombin and vascular inflammation

Vascular endothelium is a key regulator of homeostasis. In physiological conditions it mediates vascular dilatation, prevents platelet adhesion, and inhibits thrombin generation. However, endothelial dysfunction caused by physical injury of the vascular wall, for example during balloon angioplasty, acute or chronic inflammation, such as in atherothrombosis, creates a proinflammatory environment which supports leukocyte transmigration toward inflammatory sites. At the same time, the dysfunction promotes thrombin generation, fibrin deposition, and coagulation. The serine protease thrombin plays a pivotal role in the coagulation cascade. However, thrombin is not only the key effector of coagulation cascade; it also plays a significant role in inflammatory diseases. It shows an array of effects on endothelial cells, vascular smooth muscle cells, monocytes, and platelets, all of which participate in the vascular pathophysiology such as atherothrombosis. Therefore, thrombin can be considered as an important modulatory molecule of vascular homeostasis. This review summarizes the existing evidence on the role of thrombin in vascular inflammation.

Thrombin mediates severe neurovascular injury during ischemia

BACKGROUND AND PURPOSE

Cerebral ischemia initiates cascades of pathological events such as edema, blood-brain barrier breakdown, and tissue degeneration. Thrombin activation is a key step in coagulation, and thrombin has recently been shown to mediate endothelial permeability and cellular toxicity in vitro. We examined the effect of thrombin on vasculature during ischemia in vivo.

METHODS

Focal ischemia was induced in adult Sprague-Dawley rats by occlusion of the middle cerebral artery for 4 hours followed by a short period of reperfusion. High-molecular-weight fluorescein isothiocyanate-dextran was injected before surgery to label the severe vascular disruption. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining was used to identify dying cells, which were quantified with manual counts. Intra-arterial thrombin or intravenous thrombin inhibitors were infused during ischemia and reperfusion.

RESULTS

Infusion of thrombin (3 U/kg) intra-arterially during ischemia greatly enlarged the volume of severe vascular disruption, as visualized by fluorescein isothiocyanate-dextran extravasation (P<0.05). Thrombin also promoted blood-brain barrier leakage of IgG during ischemia. Vascular disruption was blocked by intravenous infusion of the direct thrombin inhibitor argatroban (1.69 mg/kg, P<0.05). Greater numbers of dying cells were found in regions of severe vascular disruption, and interventions that reduced vascular leakage also reduced the numbers of dying cells.

CONCLUSIONS

Thrombin mediates severe vascular disruption during ischemia and thrombin inhibitors may partially ameliorate vascular disruption. Further work is needed to establish whether thrombin, entering parenchyma due to increased vascular permeability, augments neurotoxicity during ischemia.

Thrombin induces platelet activation in the absence of functional protease activated receptors 1 and 4 and glycoprotein Ib-IX-V

Three different surface receptors mediate thrombin-induced activation and aggregation of human blood platelets: the protease activated receptors 1 and 4 (PAR1 and PAR4), and the glycoprotein (GP) Ibalpha of the GPIb-IX-V complex. However, their relative contribution in the stimulation of specific intracellular signaling pathways by thrombin remains largely controversial. In this work, we have shown that activation of PAR1 and PAR4 by thrombin or by selective activating peptides stimulated phospholipase C, tyrosine kinases, as well as the small GTPase Rap1b, promoted actin polymerization and cytoskeleton reorganization. When platelets were desensitized for both PAR1 and PAR4, high doses of thrombin, were unable to activate Rap1b, but produced a still evident stimulation of phospholipase C, as documented by the measurement of intracellular Ca(2+) mobilization and protein kinase C activation. These events were abrogated upon proteolysis of GPIbalpha by the metalloproteinase mocarhagin. In PAR1- and PAR4-desensitized platelets, thrombin also induced tyrosine phosphorylation of some substrates, but, surprisingly, this event was largely independent of GPIbalpha binding, as it persisted upon platelet treatment with mocarhagin. Similarly, thrombin-induced actin polymerization and cytoskeleton reorganization were only minimally altered upon PAR1 and PAR4 inactivation and GPIbalpha proteolysis. Interestingly, none of these events were elicited by enzymatically inactive thrombin. Finally we found that GPIbalpha cleavage reduced, but did not abrogate, platelet aggregation in PAR1- and PAR4-desensitized platelets. These results identify a novel pathway for platelet activation operated by thrombin independently of PAR1, PAR4 and GPIbalpha.

Increased mean platelet volume and C-reactive protein levels in patients with chronic urticaria with a positive autologous serum skin test

INTRODUCTION

Activation of the coagulation cascade resulting in thrombin production is a prominent feature of exacerbations in chronic spontaneous urticaria (CU). Autologous serum skin test (ASST) causes wheal-and-flare reactions in 30% to 50% of CU cases.

OBJECTIVE

The aim of this study was to evaluate the clinical and laboratory data in patients with CU with positive and negative ASST. To understand the role of platelets in CU, we investigated the relation between CU clinical severity, platelet count and their mean platelet volume (MPV).

METHODS

Clinical and laboratory data were prospectively collected from 373 patients with CU who attended our Allergy and Clinical immunology Clinic during the period 2003 to 2007. The laboratory data were compared with 46 healthy subjects.

RESULTS

There were no significant differences in platelet counts between the groups, nevertheless the platelets in ASST-positive CU patients were characterized by a higher MPV (9.12 +/- 1.25 fl), than that in ASST-negative patients (7.95 +/- 1.08 fl; P = 0.039) and control group (7.72 +/- 1.04 fl; P = 0.007). There was a significant positive correlation between CU severity score and MPV in ASST-positive patients (r = 0.44; P < 0.001) but not in ASST-negative patients. Higher levels of C-reactive protein (5.31 +/- 2.74 mg/L) were measured in the ASST-positive CU group compared with the ASST-negative CU group (2.53 +/- 1.27; P = 0.029) and the control group (2.34 +/- 1.38; P = 0.003).

CONCLUSION

CU with positive ASST is characterized with higher clinical severity, increased MPV and C-reactive protein.

L-carnitine modulates blood platelet oxidative stress

The oxidative stress induced by acute exertion may interfere with blood platelet activation. The beneficial effect of L-carnitine (gamma-trimethylamino-beta-hydroxybutyric acid) on oxidative stress in blood platelets has not been fully investigated; however, different studies indicate that this compound modulates platelet functions. The aim of our study was to assess the effects of L-carnitine on platelet activation and oxidative/nitrative protein damage (determined by the levels of protein carbonyl groups, thiol groups, and 3-nitrotyrosine residues) in resting blood platelets or platelets treated with peroxynitrite (ONOO(-), a strong physiological oxidant) in vitro. We also investigated the effects of L-carnitine on the level of platelet glutathione and on the formation of superoxide anion radicals O2(-*), lipid peroxidation measured by thiobarbituric acid reactive substances (TBARS) in blood platelets stimulated by thrombin (a strong physiological agonist), and platelet aggregation induced by adenosine diphosphate (a strong physiological stimulator). We have observed that carnitine decreases platelet activation (measured by platelet aggregation, the generation of O2(-*), and TBARS production). Moreover, our results in vitro demonstrate that carnitine may protect against oxidation of thiol groups induced by ONOO(-). Thus, carnitine may have some protectory effects against oxidative changes induced in blood platelets.

A new type of thrombin inhibitor, noncytotoxic phospholipase A2, from the Naja haje cobra venom

Thrombin is a key enzyme in the blood coagulation cascade and is also involved in carcinogenesis; therefore, its inhibitors are of fundamental and clinical importance. Snake venoms are widely used as sources of proteins that affect blood coagulation. We have isolated a new protein, called TI-Nh, from the Naja haje cobra venom. TI-Nh is a mixed-type inhibitor of thrombin (K(i) of 72.8 nM for a synthetic peptide substrate) and effectively inhibits thrombin-induced platelet aggregation with an IC(50) value of 0.2 nM. At concentrations up to approximately 50 nM, at which the thrombin-clotting time is substantially prolonged, TI-Nh exerts no detectable effects on both the intrinsic and extrinsic pathways of the coagulation cascade. It does not hydrolyze either fibrinogen or thrombin. Although TI-Nh bears structural features typical of group IB phospholipases A(2) (PLA(2)s), it possesses relatively weak enzymatic activity and is nontoxic to PC12 cells at concentrations up to 15 microM. Nevertheless, TI-Nh evokes neurite outgrowth in these cells at a concentration of approximately 1 microM, similar to cytotoxic snake PLA(2)s with strong enzymatic activity. TI-Nh is the first thrombin inhibitor found in the venom of the Elapidae snake family, and it is the first phospholipase shown to inhibit thrombin.

Epinephrine induces intracellular Ca2+mobilization in thrombin-desensitized platelets: a role for GPIb-IX-V

In this work we have investigated the ability of epinephrine to trigger the release of intracellular Ca2+ in thrombin-desensitized platelets. Addition of thrombin to platelets in the presence of extracellular EGTA caused a rapid and transient release of Ca2+ from intracellular stores and rendered platelets unresponsive to a second addition of the same agonist. Although epinephrine alone had no effect on intracellular Ca2+ mobilization, its addition to thrombin-desensitized platelets was associated to a rapid and evident secondary release of intracellular Ca2+. This effect of epinephrine was not observed when platelets were desensitized with other agonists able to induce phospholipase C activation, including convulxin, U46619, and ADP. Although the platelet receptor for epinephrine is coupled to the Gi family member Gz, no secondary Ca2+ release was seen in thrombin-desensitized platelets upon stimulation of other Gi-coupled receptors, including the P2Y12 receptor and the CXCR4. Addition of hirudin to thrombin-desensitized platelets prevented epinephrine-promoted secondary release of Ca2+, indicating that thrombin, rather than epinephrine itself, is actually responsible for this event as a consequence of thrombin receptors resensitization. Studies with platelets stimulated with specific PAR1- and PAR4- activating peptides proved that neither one of these thrombin receptors were involved in the secondary epinephrine-assisted Ca2+ release. Moreover, we found that thrombin was still able to induce a reduced, but evident release of Ca2+ from internal stores in PAR1- and PAR4-desensitized platelets, which could be followed by a secondary Ca2+ release upon subsequent addition of epinephrine. Importantly, both the primary and the secondary Ca2+ release induced by thrombin and epinephrine in PAR1- and PAR4-desensitized platelets were abrogated upon cleavage of GPIbalpha by the metalloproteinase mocarhagin. These results demonstrate a direct role of thrombin binding to GPIb-IX-V in the mobilization of Ca2+ from intracellular stores, and reveal that epinephrine can restore this process in desensitized platelets, thus prolonging the effect of thrombin stimulation.

Crystallography and protein–protein interactions: biological interfaces and crystal contacts

Crystallography is commonly used for studying the structures of protein–protein complexes. However, a crystal structure does not define a unique protein–protein interface, and distinguishing a ‘biological interface’ from ‘crystal contacts’ is often not straightforward. A number of computational approaches exist for distinguishing them, but their error rate is high, emphasizing the need to obtain further data on the biological interface using complementary structural and functional approaches. In addition to reviewing the computational and experimental approaches for addressing this problem, we highlight two relevant examples. The first example from our laboratory involves the structure of acyl-CoA thioesterase 7, where each domain of this two-domain protein was crystallized separately, but both yielded a non-functional assembly. The structure of the full-length protein was uncovered using a combination of complementary approaches including chemical cross-linking, analytical ultracentrifugation and mutagenesis. The second example involves the platelet glycoprotein Ibα–thrombin complex. Two groups reported the crystal structures of this complex, but all the interacting interfaces differed between the two structures. Our computational analysis did not fully resolve the reasons for the discrepancies, but provided interesting insights into the system. This review highlights the need to complement crystallographic studies with complementary experimental and computational approaches.

Application of 2-dimensional difference gel electrophoresis (2D-DIGE) to the study of thrombin-activated human platelet secretome

Thrombin is an agonist inducing platelet activation. We combined two-dimensional difference gel electrophoresis (2D-DIGE) and mass spectrometry (MALDI-TOF MS) to analyse differentially expressed proteins secreted from thrombin-stimulated platelets. Human washed platelets, from healthy volunteers, were stimulated with thrombin 0.5 U/ml at 37 degrees C without stirring and the secreted proteins were resolved by 2D-DIGE. By image analysis, 1094 spots were detected in the 2D gel. The spots whose mean intensity showed at least a five-fold change intensity increase or decrease in the thrombin-activated platelet gel in comparison with unstimulated control were digested by trypsin and subjected to MALDI-TOF MS analysis. Peptides from mass spectra of in-gel digest samples were matched against available databases, using the Mascot search engine (Matrix Science) for peptide mass fingerprint. In the activated platelet secretome, transferrin, glutathione-transferase, WD repeat protein, ER-60, thrombospondin-1 precursor and thrombospondin were the most abundant. Also lamin A, a nuclear protein, not previously identified in platelets, appeared to be released. The novel strategy to combine 2D-DIGE with MALDI-TOF MS is a useful approach for a quantitative analysis of the effect of thrombin on the secretome profile of human platelets.

Mutation-prone points in thrombin receptor

BACKGROUND

Thrombin receptor (TR) is a G-protein-coupled receptor that transmits cellular responses to coagulant proteases in a variety of cell types in the vasculature and other tissues. Mutation within TR can be seen. Presently, the prediction of protein nanostructure and function is a great challenge in the proteomics and structural genomics era.

METHODS

To identify the points vulnerable to mutation is a new trend directed at expanding the knowledge on disorders in genomic and proteomic levels of diseases. In this paper, the author performed a bioinformatics analysis to find the mutation-prone positions in the amino acid sequence of TR. To identify those points in TR, a new bioinformatics tool, namely, GlobPlot was used.

RESULTS

According to this work, no position was identified to be resistant to mutation.

CONCLUSION

This means that TR is a very highly genetically unstable molecule. Thousands of types of mutation can be expected. Of interest, only a few sense mutations are mentioned in clinical settings. Therefore, many occulted sense mutations might still be detected.

In vitro evaluation of platelet concentrates during storage: Platelet counts and markers of platelet destruction

BACKGROUND

Differences in platelet counts are observed by use of automated haematology analyzers making interlaboratory comparison difficult.

MATERIALS AND METHODS

Twenty-eight single-donor platelet concentrates (PCs) were collected. Platelet concentration and markers of platelet destruction were investigated during storage for 11/12 days.

RESULTS

Increasing impedance-immunoplatelet ratio was observed during storage, correlating to platelet fragments, large platelets, platelet density and cell-lysis. High variability was observed for optical-immunoplatelet ratio.

CONCLUSION

Immunoplatelet count or correction factor calculated by impedance-immunoplatelet ratio should be used to confirm that platelet unit meets platelet count requirements or to compare results from clinical trials. Optical platelet count is not recommended.

In vitro evaluation of metabolic changes and residual platelet responsiveness in photochemical treated and gamma-irradiated single-donor platelet concentrates during long-term storage

BACKGROUND

Photochemical treatment (PCT) prevents replication of pathogens in platelet concentrates (PCs) by cross-linking nucleic acids and thus affects all cells containing DNA or RNA.

STUDY DESIGN AND METHODS

Fourteen double-dose single-donor PCs were divided into two study arms. The double-dose PCs were split in two identical units, PCT and conventional control PCs. Study Arm A consisted of seven PCT PCs with corresponding untreated controls, whereas Study Arm B consisted of seven PCT PCs with corresponding gamma-irradiated control. Metabolic changes and agonist-induced platelet (PLT) response were evaluated during storage for up to 12 days.

RESULTS

Higher rate of PLT destruction, illustrated by reduced PLT content, increased lactate dehydrogenase levels, and higher CD61+ microparticle formation rate, were observed after PCT. Generally PCT accelerated metabolic changes in PCs and reduced agonist-induced (collagen or thrombin receptor activator peptide [TRAP]) aggregation responses. Flow cytometric analysis of CD62P and CD42b (GPIbalpha) expression showed higher spontaneous PLT activation in PCT PCs from 5 days of storage. Correspondingly, a reduced capacity for up regulation of CD62P expression and down regulation of CD42b was observed in PCT PLTs after stimulation by the agonists ADP or TRAP.

CONCLUSION

Generally reduced in vitro PLT quality was observed after PCT during storage for up to 12 days, with marked reduction from 5 days of storage. Compared to conventional PCs, reduced agonist-induced aggregation and glycoprotein expression were observed after PCT during storage, corresponding to significantly higher level of spontaneous PLT activation in PCT PCs. Clinical studies of efficacy and safety of PCT PCs stored for more than 5 days are recommended.

Pregnancy-associated changes in the hemostatic system

Pregnancy, from implantation to parturition, presents unique and profound challenges to a women's hemostatic system. During pregnancy, potentially catastrophic bleeding can occur during implantation and endovascular trophoblast invasion of the maternal spiral arteries. The risk of hemorrhage reaches a peak during the third stage of labor when the placenta is shorn from the decidua basalis exposing 120 spiral arteries largely denuded of their smooth muscle, and thus, their ability to constrict in response to injury. These challenges are met by dramatic changes in the local uterine, and systemic hemostatic systems. The net effect of these changes is to increase the efficiency of clotting and to impair fibrinolysis. Unfortunately, they also lead to an increase in the prevalence of venous thromboembolism, which is otherwise uncommon in reproductive age women.

Thrombin-triggered platelet apoptosis

BACKGROUND

Thrombin is primarily known as a coagulation factor and as an inducer of platelet activation and aggregation. It has been reported that thrombin modulates apoptosis of nucleated cells.

OBJECTIVES

The current study investigated whether thrombin can affect apoptosis in anucleated human platelets.

METHODS

Using flow cytometry, we studied platelet apoptosis at the single-cell level, analyzing markers of mitochondrial and cytoplasmic apoptosis. Western blotting was also employed, in addition to flow cytometry, for determining the expression of Bcl-2 family proteins.

RESULTS

We found that human alpha-thrombin induced four key manifestations of apoptosis in human platelets: (i) mitochondrial inner transmembrane potential (DeltaPsi m) depolarization; (ii) strong expression of pro-apoptotic Bax and Bak proteins but only weak expression of anti-apoptotic Bcl-2 protein; (iii) caspase-3 activation; and (iv) phosphatidylserine (PS) exposure.

CONCLUSIONS

This study demonstrates that, aside from its 'classical' function as an inducer of platelet activation, thrombin can trigger platelet apoptosis, where it acts as a death ligand. These data indicate that thrombin triggers platelet apoptosis by impacting on several intracellular apoptotic targets, including shifting the balance between Bcl-2 regulatory proteins in a pro-apoptotic direction, depolarizing the inner mitochondrial membrane, activating the executioner caspase-3, and stimulating aberrant exposure of PS on the platelet surface.

Prevention of platelet glycoprotein IIb/IIIa activation by 3,4-methylenedioxy-beta-nitrostyrene, a novel tyrosine kinase inhibitor

Binding fibrinogen to activated glycoprotein (GP)IIb/IIIa is the final common pathway of platelet aggregation and has become a successful target for antiplatelet therapy. In the present study, we found that a small chemical compound, 3,4-methyl-enedioxy-beta-nitrostyrene (MNS), exhibited potent and broad-spectrum inhibitory effects on human platelet aggregation caused by various stimulators. Moreover, addition of MNS to human platelets that had been aggregated by ADP caused a rapid disaggregation. We demonstrated that the antiaggregatory activity of MNS is due to inhibition of GPIIb/IIIa activation by measuring the binding amount of PAC-1 in platelets. In contrast, MNS is not a direct antagonist of GPIIb/IIIa, because MNS did not affect fibrinogen binding to fixed ADP-stimulated platelets. By investigating how MNS inhibits GPIIb/IIIa activation, we found that MNS potently inhibited the activity of tyrosine kinases (Src and Syk) and prevented protein tyrosine phosphorylation and cytoskeletal association of GPIIb/IIIa and talin, but it had no direct effects on protein kinase C, Ca2+ mobilization, Ca2+-dependent enzymes (myosin light chain kinase and calpain), and arachidonic acid metabolism, and it did not affect the cellular levels of cyclic nucleotides. Therefore, MNS represents a new class of tyrosine kinase inhibitor that potently prevents GPIIb/IIIa activation and platelet aggregation without directly affecting other signaling pathways required for platelet activation. Because MNS inhibits GPIIb/IIIa functions in a manner different from GPIIb/IIIa antagonists, this feature may provide a new strategy for treatment of platelet-dependent thrombosis.