Inhibition of mitochondrial calcium transporters alters adp-induced platelet responses.

INTRODUCTION: ADP-stimulated elevation of cytosolic Ca2+ is an important effector mechanism for platelet activation. The rapidly elevating cytosolic Ca2+ is also transported to mitochondrial matrix via Mitochondrial Ca2+ Uniporter (MCU) and extruded via Na+/Ca2+/Li+ Exchanger (NCLX). However, the exact contribution of MCU and NCLX in ADP-mediated platelet responses remains incompletely understood. METHODS AND RESULTS: The present study aimed to elucidate the role of mitochondrial Ca2+ transport in ADP-stimulated platelet responses by inhibition of MCU and NCLX with mitoxantrone (MTX) and CGP37157 (CGP), respectively. As these inhibitory strategies are reported to cause distinct effects on matrix Ca2+ concentration, we hypothesized to observe opposite impact of MTX and CGP on ADP-induced platelet responses. Platelet aggregation profiling was performed by microplate-based spectrophotometery while p-selectin externalization and integrin αIIbß3 activation were analyzed by fluorescent immunolabeling using flow cytometery. Our results confirmed the expression of both MCU and NCLX mRNAs with relatively low abundance of NCLX in human platelets. In line with our hypothesis, MTX caused a dose-dependent inhibition of ADP-induced platelet aggregation without displaying any cytotoxicity. Likewise, ADP-induced p-selectin externalization and integrin αIIbß3 activation was also significantly attenuated in MTX-treated platelets. Concordantly, inhibition of NCLX with CGP yielded an accelerated ADP-stimulated platelet aggregation which was associated with an elevation of p-selectin surface expression and αIIbß3 activation. CONCLUSION: Together, these findings uncover a vital and hitherto poorly characterized role of mitochondrial Ca2+ transporters in ADP-induced platelet activation.

Diagnostic value of antibody-induced procoagulant platelets in heparin-induced thrombocytopenia: communication from the ISTH SSC Subcommittee on Platelet Immunology.

Heparin-induced thrombocytopenia (HIT) is an immune-mediated prothrombotic disorder characterized by a drop in platelet count and an increased risk of thromboembolic events. The accurate diagnosis of HIT involves clinical assessment and laboratory testing with well-characterized functional tests. Recent research has shown the potential of investigating procoagulant platelet formation induced by HIT antibodies. To successfully implement these assays in clinical laboratories, careful consideration of technical and preanalytical factors is crucial. In this communication from the SSC Platelet Immunology, we provide a consensus from experts on the use of flow cytometry in HIT diagnosis, highlighting the importance of standardized protocols.

Pharmacodynamics Monitoring of Emicizumab in Patients with Hemophilia A.

BACKGROUND: Emicizumab is a bispecific antibody mimicking coagulation factor VIII (FVIII) employed to treat patients with hemophilia A (PwHA) regardless of FVIII inhibitor status. The identification of biological markers reflecting the hemostatic competence of patients under emicizumab therapy would have a great clinical value. Unfortunately, emicizumab over-corrects standard coagulation assays, precluding their use for evaluating the hemostatic correction achieved in vivo. Here, we investigated whether global coagulation assays (GCA) would allow monitoring the biological response to non-factor replacement therapy with emicizumab. MATERIALS AND METHODS: Six adults PwHA received a weekly dose of emicizumab of 3 mg/kg during weeks (W) 1 4 and 1.5 mg/kg from W5 onwards. Response to treatment was monitored weekly by emicizumab plasma concentration, thrombin generation (TG), and fibrin clot formation (FCF) and structure. TG and FCF results were compared to patient baseline, FVIII replacement, and healthy donors. RESULTS: TG and FCF significantly increased in PwHA after the loading period, reaching a plateau that lasted until the end of monitoring. Similarly, fibrin clot network became denser with thinner fibrin fibers. However, TG contrary to FCF remained at the lower limits of reference values. Remarkably, despite having similar plateau concentrations of emicizumab some patients showed markedly different degrees of TG and FCF improvement. CONCLUSION: Our study enriches the knowledge on the use of GCA to monitor non-factor replacement therapy, indicating that TG and FCF could act as direct markers of emicizumab biological activity. GCA allow to capture and visualize the individually variable response to emicizumab, leading a step forward to the personalization of patient treatment.

Two novel platelet biotinylation methods and their impact on stored platelet concentrates in a blood bank environment.

BACKGROUND: Storage of platelet concentrates (PCs) has an impact on platelet quality and possibly affects their functions after transfusion. The influence of processing and storage conditions of PCs on their in vivo function upon transfusion is unknown. One option for investigating this question is to implement an ex vivo labeling of human platelets, to analyze them after transfusion into heathy volunteers and/or patients. In this study, we developed two labeling methods employing biotin. METHODS: Two methods of biotinylation were compared to a control (standard PC). The "Bio-Wash" process used washing steps to label all platelets within the PC; for the other method, "Bio-Direct," one fifth of the PC were directly labeled without washing steps. The control and the two biotinylated PCs were analyzed over 7 days of storage. Labeling efficiency, platelet counts, phenotypes, and functions, along with time and costs, were evaluated to select the best process. RESULTS: Both methods achieved a stable labeling through the storage, with similar platelet counts and metabolism in comparison to control PCs. Bio-Wash showed higher activation phenotype and lower aggregation response in comparison to the Bio-Direct method. The Bio-Direct was performed within 1.5 h versus 3 h for the Bio-Wash. However, the Bio-Direct required 12 mg of biotin instead of 8 mg for the other process. CONCLUSION: We set up two methods of biotinylation that can be easily implemented in a blood bank environment. The Bio-Direct process was preferred to the Bio-Wash because of its similarity, from a functional and phenotypic point of view, with standard PCs.

Mechanisms Underlying Dichotomous Procoagulant COAT Platelet Generation-A Conceptual Review Summarizing Current Knowledge.

Procoagulant platelets are a subtype of activated platelets that sustains thrombin generation in order to consolidate the clot and stop bleeding. This aspect of platelet activation is gaining more and more recognition and interest. In fact, next to aggregating platelets, procoagulant platelets are key regulators of thrombus formation. Imbalance of both subpopulations can lead to undesired thrombotic or bleeding events. COAT platelets derive from a common pro-aggregatory phenotype in cells capable of accumulating enough cytosolic calcium to trigger specific pathways that mediate the loss of their aggregating properties and the development of new adhesive and procoagulant characteristics. Complex cascades of signaling events are involved and this may explain why an inter-individual variability exists in procoagulant potential. Nowadays, we know the key agonists and mediators underlying the generation of a procoagulant platelet response. However, we still lack insight into the actual mechanisms controlling this dichotomous pattern (i.e., procoagulant versus aggregating phenotype). In this review, we describe the phenotypic characteristics of procoagulant COAT platelets, we detail the current knowledge on the mechanisms of the procoagulant response, and discuss possible drivers of this dichotomous diversification, in particular addressing the impact of the platelet environment during in vivo thrombus formation.

Thrombocytopathies: Not Just Aggregation Defects-The Clinical Relevance of Procoagulant Platelets.

Platelets are active key players in haemostasis. Qualitative platelet dysfunctions result in thrombocytopathies variously characterized by defects of their adhesive and procoagulant activation endpoints. In this review, we summarize the traditional platelet defects in adhesion, secretion, and aggregation. In addition, we review the current knowledge about procoagulant platelets, focusing on their role in bleeding or thrombotic pathologies and their pharmaceutical modulation. Procoagulant activity is an important feature of platelet activation, which should be specifically evaluated during the investigation of a suspected thrombocytopathy.

High-Dose Epinephrine Enhances Platelet Aggregation at the Expense of Procoagulant Activity.

Platelet activation is characterized by shape change, granule secretion, activation of fibrinogen receptor (glycoprotein IIb/IIIa) sustaining platelet aggregation, and externalization of negatively charged aminophospholipids contributing to platelet procoagulant activity. Epinephrine (EPI) alone is a weak platelet activator. However, it is able to potentiate platelet activation initiated by other agonists. In this work, we investigated the role of EPI in the generation of procoagulant platelets. Human platelets were activated with convulxin (CVX), thrombin (THR) or protease-activated receptor (PAR) agonists, EPI, and combination thereof. Platelet aggregation was assessed by light transmission aggregometry or with PAC-1 binding by flow cytometry. Procoagulant collagen-and-THR (COAT) platelets, induced by combined activation with CVX-and-THR, were visualized by flow cytometry as Annexin-V-positive and PAC-1-negative platelets. Cytosolic calcium fluxes were monitored by flow cytometry using Fluo-3 indicator. EPI increased platelet aggregation induced by all agonist combinations tested. On the other hand, EPI dose-dependently reduced the formation of procoagulant COAT platelets generated by combined CVX-and-THR activation. We observed a decreased Annexin-V-positivity and increased binding of PAC-1 with the triple activation (CVX + THR + EPI) compared with CVX + THR. Calcium mobilization with triple activation was decreased with the higher EPI dose (1,000 µM) compared with CVX + THR calcium kinetics. In conclusion, when platelets are activated with CVX-and-THR, the addition of increasing concentrations of EPI (triple stimulation) modulates platelet response reducing cytosolic calcium mobilization, decreasing procoagulant activity, and enhancing platelet aggregation.

Heparin-Induced Thrombocytopenia: A Review of New Concepts in Pathogenesis, Diagnosis, and Management.

Knowledge on heparin-induced thrombocytopenia keeps increasing. Recent progress on diagnosis and management as well as several discoveries concerning its pathogenesis have been made. However, many aspects of heparin-induced thrombocytopenia remain partly unknown, and exact application of these new insights still need to be addressed. This article reviews the main new concepts in pathogenesis, diagnosis, and management of heparin-induced thrombocytopenia.

Sodium-Calcium Exchanger Reverse Mode Sustains Dichotomous Ion Fluxes Required for Procoagulant COAT Platelet Formation.

Procoagulant collagen-and-thrombin (COAT)-activated platelets represent a subpopulation of activated platelets, which retain a coat of prohemostatic proteins and express phosphatidylserine on their surface. Dichotomous intracellular signaling generating procoagulant platelet activity instead of traditional aggregating endpoints is still not fully elucidated. It has been demonstrated that secondary messengers such as calcium and sodium play a critical role in platelet activation. Therefore, we developed a flow cytometric analysis to investigate intracellular ion fluxes simultaneously during generation of aggregating and procoagulant platelets. Human platelets were activated by convulxin-plus-thrombin. Cytosolic calcium, sodium, and potassium ion fluxes were visualized by specific ion probes and analyzed by flow cytometry. We observed high and prolonged intracellular calcium concentration, transient sodium increase, and fast potassium efflux in COAT platelets, whereas aggregating non-COAT platelets rapidly decreased their calcium content, maintaining higher cytosolic sodium, and experiencing lower and slower potassium depletion. Considering these antithetical patterns, we investigated the role of the sodium-calcium exchanger (NCX) during convulxin-plus-thrombin activation. NCX inhibitors, CBDMB and ORM-10103, dose-dependently reduced the global calcium mobilization induced by convulxin-plus-thrombin activation and dose-dependently prevented formation of procoagulant COAT platelets. Our data demonstrate that both NCX modes are used after convulxin-plus-thrombin-induced platelet activation. Non-COAT platelets use forward-mode NCX, thus pumping calcium out and moving sodium in, while COAT platelets rely on reverse NCX function, which pumps additional calcium into the cytosol, by extruding sodium. In conclusion, we described for the first time the critical and dichotomous role of NCX function during convulxin-plus-thrombin-induced platelet activation.

Tissue Factor-Independent Coagulation Correlates with Clinical Phenotype in Factor XI Deficiency and Replacement Therapy.

BACKGROUND: In factor XI (FXI) deficiency, bleeding cannot be predicted by routine analyses. Since FXI is involved in tissue factor (TF)-independent propagation loop of coagulation, we hypothesized that investigating the spatiotemporal separated phases of coagulation (TF-dependent and -independent) could improve diagnostics. OBJECTIVES: This article investigates the correlation of parameters describing TF-dependent and -independent coagulation with the clinical phenotype of FXI deficiency and their ability to assess hemostasis after FXI replacement. METHODS: We analyzed: (1) plasma from healthy controls (n = 53); (2) normal plasma (n = 4) spiked with increasing concentrations of a specific FXI inhibitor (C7P); (3) plasma from FXI-deficient patients (n = 24) with different clinical phenotypes (13 bleeders, 8 non-bleeders, 3 prothrombotics); (4) FXI-deficient plasma spiked with FXI concentrate (n = 6); and (5) plasma from FXI-deficient patients after FXI replacement (n = 7). Thrombin generation was measured with the reference method calibrated automated thrombogram and with Thrombodynamics (TD), a novel global assay differentiating TF-dependent and -independent coagulation. RESULTS: C7P dose-dependently decreased FXI activity, prolonged activated partial thromboplastin time, and hampered TF-independent coagulation. In FXI-deficient bleeders, TD parameters describing TF-independent propagation of coagulation and fibrin clot formation were reduced compared with controls and FXI-deficient nonbleeders and increased in FXI-deficient patients with prothrombotic phenotype. Receiver operating characteristic analysis indicated that TF-independent parameters were useful for discriminating FXI-deficient bleeders from non-bleeders. In FXI-deficient plasma spiked with FXI concentrate and in patients receiving FXI replacement, TD parameters were shifted toward hypercoagulation already at plasma FXI levels around 20%. CONCLUSION: TF-independent coagulation parameters assessed by TD have the potential to identify the clinical phenotype in FXI-deficient patients and to monitor FXI replacement therapy.

Characterization of Procoagulant COAT Platelets in Patients with Glanzmann Thrombasthenia.

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

How to Capture the Bleeding Phenotype in FXI-Deficient Patients.

Factor XI (FXI) is a serine protease involved in the propagation phase of coagulation and in providing clot stability. Several mutations in the F11 gene lead to FXI deficiency, a rare mild bleeding disorder. Current laboratory methods are unable to assess bleeding risk in FXI-deficient patients, because the degree of bleeding tendency does not correlate with plasma FXI activity as measured by routine coagulometric aPTT-based assays. Bleeding manifestations are highly variable among FXI-deficient patients and FXI replacement therapy can be associated with an increased thrombotic risk. A correct evaluation of the patient hemostatic potential is crucial to prevent under- or overtreatment. In recent years, different research groups have investigated the use of global coagulation assays as alternative for studying the role of FXI in hemostasis and identifying the clinical phenotype of FXI deficiency. This brief review article summarizes the main features of coagulation factor XI and its deficiency and resumes the principle axes of research and methods used to investigate FXI functions.

Biomarkers of liver dysfunction correlate with a prothrombotic and not with a prohaemorrhagic profile in patients with cirrhosis.

BACKGROUND & AIMS: Different liver dysfunction biomarkers are used to assess the bleeding risk of patients with cirrhosis, either as such or included in bleeding risk assessment scores. Since the current model of coagulation in patients with cirrhosis describes a procoagulant tendency with increasing severity according to Child-Pugh stage, we decided to investigate the relation between liver dysfunction biomarkers and thrombin generation. Our aim was to verify their adequacy for bleeding risk assessment. METHODS: We performed a prospective single-centre study including 260 patients with liver cirrhosis. Thrombin generation was measured using ST Genesia® Thrombin Generation System without and with thrombomodulin in order to assess the role of proteins C and S. Relations between thrombin generation and Child-Pugh/model for end-stage liver disease (MELD) scores, prothrombin time (PT)/international normalised ratio (INR), activated partial thromboplastin time (aPTT), factor V activity, albumin, and total bilirubin were assessed. RESULTS: Thrombomodulin-mediated inhibition of thrombin generation was significantly decreased in patients with liver cirrhosis compared with healthy donors (p <0.0001) and in Child-Pugh B and C compared with A (p <0.0001 [A-B], 0.4515 [B-C], <0.0001 [A-C]). Thrombomodulin-mediated inhibition significantly decreased with increasing PT/INR, aPTT, and total bilirubin levels and with decreasing factor V activity and albumin levels. CONCLUSIONS: Worsening liver dysfunction biomarkers reflect an increasing prothrombotic profile in patients with liver cirrhosis. In particular, prolonged PT/INR and aPTT as well as decreasing factor V activity are related to an increasing thrombotic risk and not to an increasing bleeding risk. These parameters should not be used to assess bleeding risk due to haemostatic anomalies in patients with liver cirrhosis. Alternative biomarkers for bleeding risk assessment in patients with liver cirrhosis need to be developed. LAY SUMMARY: We demonstrate that the laboratory parameters used to assess bleeding risk of patients with liver disease, e.g. prothrombin time/international normalised ratio (PT/INR) and activated partial thromboplastin time (aPTT), are inadequate for this purpose because they are correlated with a prothrombotic coagulation profile. In this article, we highlight the need for alternative parameters to assess bleeding risk in patients with liver disease.

Flow Cytometric Monitoring of Dynamic Cytosolic Calcium, Sodium, and Potassium Fluxes Following Platelet Activation.

The interaction of platelet agonists with their respective membrane receptors triggers intracellular signaling, among which cytosolic ion fluxes play an important role in activation processes. While the key contribution of intercellular free calcium is accepted, sodium and potassium roles in platelet activation have been less investigated in recent studies. Here, we implemented a novel flow-cytometric method to monitor over time cytosolic free calcium, sodium, and potassium ion fluxes upon platelet activation and we demonstrate the feasibility of real-time visualization of ion kinetics, in particular with a focus on sodium and potassium. Platelets were loaded with selective ion indicators, Fluo-3 (Ca2+ ), ION NaTRIUM Green-2 (Na+ ), and ION Potassium Green-2 (K+ ). Fluorescence was monitored by flow cytometry. After measurement of a stable baseline, platelets were activated and ion indicator fluorescence was acquired over time, up to 10 min. Platelets were activated with either thromboxane analogue U46619, ADP, thrombin, TRAP6 (PAR-1 agonist), AYPGKF (PAR-4 agonist), convulxin (collagen receptor GPVI agonist), or combinations thereof. We evaluated preanalytical parameters (in particular dye loading time and concentration) to implement an accurate method. Subsequently, we characterized cytosolic calcium, sodium, and potassium kinetics in response to platelet agonists. We observed different patterns of agonist synergism. In conclusion, the present work highlights the use of cytosolic ion monitoring by flow cytometry to investigate characteristic calcium, sodium, and potassium mobilization patterns following platelet activation. This easy technique opens a new way to analyze signaling in different platelet subpopulations and it should prove useful for investigating platelet pathophysiology. © 2020 International Society for Advancement of Cytometry.

Thrombin generation in a woman with heterozygous factor V Leiden and combined oral contraceptives: A case report.

Combined oral contraceptives and factor V Leiden mutation are multiplicative risk factors for venous thromboembolism. However, it remains unknown whether this multiplicative effect is reflected in thrombin generation assays. We report here the evolution of the thrombin generation profile while taking combined oral contraceptives and after their discontinuation in a woman with heterozygous factor V Leiden mutation. The proband exhibited a distinctly prothrombotic thrombin generation profile including markedly decreased thrombomodulin (TM) sensitivity, compared to the control population. This profile possibly reflected a high thrombotic risk. After discontinuation of combined oral contraceptives, thrombin generation and TM sensitivity improved greatly, leaving only a slightly prothrombotic profile. Therefore, the multiplied thrombotic risk occurring with simultaneous combined oral contraceptives and factor V Leiden mutation is reflected by a thrombin generation assay performed without and with TM. This could be a promising tool to identify women taking combined oral contraceptives at high risk for venous thromboembolism. Further studies are needed to verify this hypothesis.

Hemostatic Alterations in Patients With Cirrhosis: From Primary Hemostasis to Fibrinolysis.

In the setting of liver cirrhosis (LC), profound hemostatic changes occur, which affect primary hemostasis, coagulation, and fibrinolysis. They involve prohemorrhagic and prothrombotic alterations at each of these steps. Patients with cirrhosis exhibit multifactorial thrombocytopenia and in vitro thrombocytopathy, counterbalanced by increased von Willebrand factor. The resultant shift is difficult to assess, but overall these changes probably result in a rebalanced primary hemostasis. Concerning coagulation, the reduced activity of coagulation factors is counterbalanced by an increase in factor VIII (produced by liver sinusoidal endothelial cells), a decrease of the natural anticoagulants, and complex changes, including changes in circulating microparticles, cell-free DNA, and neutrophil extracellular traps. Overall, these alterations result in a procoagulant state. As for fibrinolysis, increased tissue-type and urokinase-type plasminogen activators, a relatively decreased plasminogen activator inhibitor 1, and decreased levels of thrombin-activatable fibrinolysis inhibitor and α2-antiplasmin are counterbalanced by decreased plasminogen and a decreased fibrin clot permeability. Whether and how these changes shift fibrinolysis remains to be determined. Overall, the current consensus is that in patients with cirrhosis, the hemostasis is shifted toward a procoagulant state. We review the published evidence for the concept of LC as a prothrombotic state, discuss discordant data, and highlight the impact of the underlying cause of LC on the resultant imbalance.