Platelet biology and function: plaque erosion vs. rupture.

The leading cause of heart disease in developed countries is coronary atherosclerosis, which is not simply a result of ageing but a chronic inflammatory process that can lead to acute clinical events upon atherosclerotic plaque rupture or erosion and arterial thrombus formation. The composition and location of atherosclerotic plaques determine the phenotype of the lesion and whether it is more likely to rupture or to erode. Although plaque rupture and erosion both initiate platelet activation on the exposed vascular surface, the contribution of platelets to thrombus formation differs between the two phenotypes. In this review, plaque phenotype is discussed in relation to thrombus composition, and an overview of important mediators (haemodynamics, matrix components, and soluble factors) in plaque-induced platelet activation is given. As thrombus formation on disrupted plaques does not necessarily result in complete vessel occlusion, plaque healing can occur. Therefore, the latest findings on plaque healing and the potential role of platelets in this process are summarized. Finally, the clinical need for more effective antithrombotic agents is highlighted.

Platelet polyphosphates are proinflammatory and procoagulant mediators in vivo.

Platelets play a central role in thrombosis, hemostasis, and inflammation. We show that activated platelets release inorganic polyphosphate (polyP), a polymer of 60-100 phosphate residues that directly bound to and activated the plasma protease factor XII. PolyP-driven factor XII activation triggered release of the inflammatory mediator bradykinin by plasma kallikrein-mediated kininogen processing. PolyP increased vascular permeability and induced fluid extravasation in skin microvessels of mice. Mice deficient in factor XII or bradykinin receptors were resistant to polyP-induced leakage. PolyP initiated clotting of plasma via the contact pathway. Ablation of intrinsic coagulation pathway proteases factor XII and factor XI protected mice from polyP-triggered lethal pulmonary embolism. Targeting polyP with phosphatases interfered with procoagulant activity of activated platelets and blocked platelet-induced thrombosis in mice. Addition of polyP restored defective plasma clotting of Hermansky-Pudlak Syndrome patients, who lack platelet polyP. The data identify polyP as a new class of mediator having fundamental roles in platelet-driven proinflammatory and procoagulant disorders.

Plasma Kallikrein as a Forgotten Clotting Factor.

For decades, it was considered that plasma kallikrein's (PKa) sole function within the coagulation cascade is the activation of factor (F)XII. Until recently, the two key known activators of FIX within the coagulation cascade were activated FXI(a) and the tissue factor-FVII(a) complex. Simultaneously, and using independent experimental approaches, three groups identified a new branch of the coagulation cascade, whereby PKa can directly activate FIX. These key studies identified that (1) FIX or FIXa can bind with high affinity to either prekallikrein (PK) or PKa; (2) in human plasma, PKa can dose dependently trigger thrombin generation and clot formation independent of FXI; (3) in FXI knockout murine models treated with intrinsic pathway agonists, PKa activity results in increased formation of FIXa:AT complexes, indicating direct activation of FIX by PKa in vivo. These findings suggest that there is both a canonical (FXIa-dependent) and non-canonical (PKa-dependent) pathway of FIX activation. These three recent studies are described within this review, alongside historical data that hinted at the existence of this novel role of PKa as a coagulation clotting factor. The implications of direct PKa cleavage of FIX remain to be determined physiologically, pathophysiologically, and in the context of next-generation anticoagulants in development.

Clinical utility of the 4S-AF scheme in predicting progression of atrial fibrillation: data from the RACE V study.

AIMS: The recent 4S-AF (scheme proposed by the 2020 ESC AF guidelines to address stroke risk, symptom severity, severity of AF burden and substrate of AF to provide a structured phenotyping of AF patients in clinical practice to guide therapy and assess prognosis) scheme has been proposed as a structured scheme to characterize patients with atrial fibrillation (AF). We aimed to assess whether the 4S-AF scheme predicts AF progression in patients with self-terminating AF. METHODS AND RESULTS: We analysed 341 patients with self-terminating AF included in the well-phenotyped Reappraisal of Atrial Fibrillation: Interaction between HyperCoagulability, Electrical remodelling, and Vascular Destabilization in the Progression of AF (RACE V) study. Patients had continuous monitoring with implantable loop recorders or pacemakers. AF progression was defined as progression to persistent or permanent AF or progression of self-terminating AF with >3% burden increase. Progression of AF was observed in 42 patients (12.3%, 5.9% per year). Patients were given a score based on the components of the 4S-AF scheme. Mean age was 65 [interquartile range (IQR) 58-71] years, 149 (44%) were women, 103 (49%) had heart failure, 276 (81%) had hypertension, and 38 (11%) had coronary artery disease. Median CHA2DS2-VASc (the CHA2DS2-VASc score assesses thromboembolic risk. C, congestive heart failure/left ventricular dysfunction; H, hypertension; A2, age ≥ 75 years; D, diabetes mellitus; S2, stroke/transient ischaemic attack/systemic embolism; V, vascular disease; A, age 65-74 years; Sc, sex category (female sex)) score was 2 (IQR 2-3), and median follow-up was 2.1 (1.5-2.6) years. The average score of the 4S-AF scheme was 4.6 ± 1.4. The score points from the 4S-AF scheme did not predict the risk of AF progression [odds ratio (OR) 1.1 95% CI 0.88-1.41, C-statistic 0.53]. However, excluding the symptoms domain, resulting in the 3S-AF (4S-AF scheme without the domain symptom severity, only including stroke risk, severity of AF burden and substrate of AF) scheme, predicted the risk of progression (OR 1.59 95% CI 1.15-2.27, C-statistic 0.62) even after adjusting for sex and age. CONCLUSIONS: In self-terminating AF patients, the 4S-AF scheme does not predict AF progression. The 3S-AF scheme, excluding the symptom domain, may be a more appropriate score to predict AF progression. TRIAL REGISTRATION NUMBERS: Clinicaltrials.gov NCT02726698 for RACE V.

Relationships between coagulation factors and thrombin generation in a general population with arterial and venous disease background.

BACKGROUND: The current study aims to identify the relationships between coagulation factors and plasma thrombin generation in a large population-based study by comparing individuals with a history of arterial or venous thrombosis to cardiovascular healthy individuals. METHODS: This study comprised 502 individuals with a history of arterial disease, 195 with history of venous thrombosis and 1402 cardiovascular healthy individuals (reference group) from the population-based Gutenberg Health Study (GHS). Calibrated Automated Thrombography was assessed and coagulation factors were measured by means of BCS XP Systems. To assess the biochemical determinants of TG variables, a multiple linear regression analysis, adjusted for age, sex and antithrombotic therapy, was conducted. RESULTS: The lag time, the time to form the first thrombin, was mainly positively associated with the natural coagulant and anti-coagulant factors in the reference group, i.e. higher factors result in a longer lag time. The same determinants were negative for individuals with a history of arterial or venous thrombosis, with a 10 times higher effect size. Endogenous thrombin potential, or area under the curve, was predominantly positively determined by factor II, VIII, X and IX in all groups. However, the effect sizes of the reported associations were 4 times higher for the arterial and venous disease groups in comparison to the reference group. CONCLUSION: This large-scale analysis demonstrated a stronger effect of the coagulant and natural anti-coagulant factors on the thrombin potential in individuals with a history of arterial or venous thrombosis as compared to healthy individuals, which implicates sustained alterations in the plasma coagulome in subjects with a history of thrombotic vascular disease, despite intake of antithrombotic therapy.

Mouse venous thrombosis upon silencing of anticoagulants depends on tissue factor and platelets, not FXII or neutrophils.

Tissue factor, coagulation factor XII, platelets, and neutrophils are implicated as important players in the pathophysiology of (experimental) venous thrombosis (VT). Their role became evident in mouse models in which surgical handlings were required to provoke VT. Combined inhibition of the natural anticoagulants antithrombin (Serpinc1) and protein C (Proc) using small interfering RNA without additional triggers also results in a venous thrombotic phenotype in mice, most notably with vessel occlusion in large veins of the head. VT is fatal but is fully rescued by thrombin inhibition. In the present study, we used this VT mouse model to investigate the involvement of tissue factor, coagulation factor XII, platelets, and neutrophils. Antibody-mediated inhibition of tissue factor reduced the clinical features of VT, the coagulopathy in the head, and fibrin deposition in the liver. In contrast, genetic deficiency in, and small interfering RNA-mediated depletion of, coagulation factor XII did not alter VT onset, severity, or thrombus morphology. Antibody-mediated depletion of platelets fully abrogated coagulopathy in the head and liver fibrin deposition. Although neutrophils were abundant in thrombotic lesions, depletion of circulating Ly6G-positive neutrophils did not affect onset, severity, thrombus morphology, or liver fibrin deposition. In conclusion, VT after inhibition of antithrombin and protein C is dependent on the presence of tissue factor and platelets but not on coagulation factor XII and circulating neutrophils. This study shows that distinct procoagulant pathways operate in mouse VT, dependent on the triggering stimulus.

Plasma Kallikrein Contributes to Coagulation in the Absence of Factor XI by Activating Factor IX.

OBJECTIVES: FXIa (factor XIa) induces clot formation, and human congenital FXI deficiency protects against venous thromboembolism and stroke. In contrast, the role of FXI in hemostasis is rather small, especially compared with FIX deficiency. Little is known about the cause of the difference in phenotypes associated with FIX deficiency and FXI deficiency. We speculated that activation of FIX via the intrinsic coagulation is not solely dependent on FXI(a; activated FXI) and aimed at identifying an FXI-independent FIX activation pathway. Approach and Results: We observed that ellagic acid and long-chain polyphosphates activated the coagulation system in FXI-deficient plasma, as could be demonstrated by measurement of thrombin generation, FIXa-AT (antithrombin), and FXa-AT complex levels, suggesting an FXI bypass route of FIX activation. Addition of a specific PKa (plasma kallikrein) inhibitor to FXI-deficient plasma decreased thrombin generation, prolonged activated partial thromboplastin time, and diminished FIXa-AT and FXa-AT complex formation, indicating that PKa plays a role in the FXI bypass route of FIX activation. In addition, FIXa-AT complex formation was significantly increased in F11-/- mice treated with ellagic acid or long-chain polyphosphates compared with controls and this increase was significantly reduced by inhibition of PKa. CONCLUSIONS: We demonstrated that activation of FXII leads to thrombin generation via FIX activation by PKa in the absence of FXI. These findings may, in part, explain the different phenotypes associated with FIX and FXI deficiencies.

Evaluation of the analytical performance of the PC100 platelet counter.

INTRODUCTION: Platelet count can be altered in various diseases and treatments and measuring it may provide better insight into the expected outcome. So far, quantification of platelet count is done within laboratory conditions by using established hematology analyzers, whereas a point-of-care device could be used for this purpose outside of the clinical laboratories. AIM: Our aim was to assess the closeness of agreement between a newly developed point-of-care PC100 platelet counter and two reference methods (Sysmex® XP-300, Sysmex® XN-9000) in measuring platelet counts in whole blood and platelet-rich-plasma (PRP). METHOD: Whole blood was obtained from 119 individuals, of which 74 were used to prepare PRP samples. Whole blood platelet count was measured by the two reference methods and the PC100 platelet counter. PRP was prepared from the whole blood and platelet count was adjusted to the range of 250-3600 × 103/µl and measured with the PC100 platelet counter and Sysmex® XP-300. RESULTS: A median difference of - 1.35% and - 2.98% occurred in whole blood platelet count between the PC100 platelet counter and the Sysmex® XP-300 and Sysmex® XN-9000, respectively. A strong linear correlation (r ≥ 0.98) was seen in both cases and regression equations indicated neither a constant nor a proportional bias between the methods. Direct comparison of the two reference methods revealed a median difference of - 1.15% and a strongly linear relationship (r = 0.99). Platelet count in PRP resulted in a median difference of 1.42% between the PC100 platelet counter and the reference method, Sysmex® XP-300. While the difference between two methods increased with concentration of platelets in PRP, a strong linear relationship remained throughout the whole measuring interval indicated by the high correlation coefficient (r = 0.99). Assessment of the predicted bias at predefined platelet counts showed that the bias in platelet counts falls within the acceptance criterion for both whole blood and PRP measurements. CONCLUSIONS: Our results show that the PC100 platelet counter can be used interchangeably with the reference methods for determining platelet counts.

Serial markers of coagulation and inflammation and the occurrence of clinical pulmonary thromboembolism in mechanically ventilated patients with SARS-CoV-2 infection; the prospective Maastricht intensive care COVID cohort.

BACKGROUND: The incidence of pulmonary thromboembolism is high in SARS-CoV-2 patients admitted to the Intensive Care. Elevated biomarkers of coagulation (fibrinogen and D-dimer) and inflammation (c-reactive protein (CRP) and ferritin) are associated with poor outcome in SARS-CoV-2. Whether the time-course of fibrinogen, D-dimer, CRP and ferritin is associated with the occurrence of pulmonary thromboembolism in SARS-CoV-2 patients is unknown. We hypothesise that patients on mechanical ventilation with SARS-CoV-2 infection and clinical pulmonary thromboembolism have lower concentrations of fibrinogen and higher D-dimer, CRP, and ferritin concentrations over time compared to patients without a clinical pulmonary thromboembolism. METHODS: In a prospective study, fibrinogen, D-dimer, CRP and ferritin were measured daily. Clinical suspected pulmonary thromboembolism was either confirmed or excluded based on computed tomography pulmonary angiography (CTPA) or by transthoracic ultrasound (TTU) (i.e., right-sided cardiac thrombus). In addition, patients who received therapy with recombinant tissue plasminogen activator were included when clinical instability in suspected pulmonary thromboembolism did not allow CTPA. Serial data were analysed using a mixed-effects linear regression model, and models were adjusted for known risk factors (age, sex, APACHE-II score, body mass index), biomarkers of coagulation and inflammation, and anticoagulants. RESULTS: Thirty-one patients were considered to suffer from pulmonary thromboembolism ((positive CTPA (n = 27), TTU positive (n = 1), therapy with recombinant tissue plasminogen activator (n = 3)), and eight patients with negative CTPA were included. After adjustment for known risk factors and anticoagulants, patients with, compared to those without, clinical pulmonary thromboembolism had lower average fibrinogen concentration of - 0.9 g/L (95% CI: - 1.6 - - 0.1) and lower average ferritin concentration of - 1045 µg/L (95% CI: - 1983 - - 106) over time. D-dimer and CRP average concentration did not significantly differ, 561 µg/L (- 6212-7334) and 27 mg/L (- 32-86) respectively. Ferritin lost statistical significance, both in sensitivity analysis and after adjustment for fibrinogen and D-dimer. CONCLUSION: Lower average concentrations of fibrinogen over time were associated with the presence of clinical pulmonary thromboembolism in patients at the Intensive Care, whereas D-dimer, CRP and ferritin were not. Lower concentrations over time may indicate the consumption of fibrinogen related to thrombus formation in the pulmonary vessels.

Thrombin-Fibrin(ogen) Interactions, Host Defense and Risk of Thrombosis.

Fibrinogen is a well-known risk factor for arterial and venous thrombosis. Its function is not restricted to clot formation, however, as it partakes in a complex interplay between thrombin, soluble plasma fibrinogen, and deposited fibrin matrices. Fibrinogen, like thrombin, participates predominantly in hemostasis to maintain vascular integrity, but executes some important pleiotropic effects: firstly, as observed in thrombin generation experiments, fibrin removes thrombin from free solution by adsorption. The adsorbed thrombin is protected from antithrombins, notably α2-macroglobulin, and remains physiologically active as it can activate factors V, VIII, and platelets. Secondly, immobilized fibrinogen or fibrin matrices activate monocytes/macrophages and neutrophils via Mac-1 interactions. Immobilized fibrin(ogen) thereby elicits a pro-inflammatory response with a reciprocal stimulating effect of the immune system on coagulation. In contrast, soluble fibrinogen prohibits recruitment of these immune cells. Thus, while fibrin matrices elicit a procoagulant response, both directly by protecting thrombin and indirectly through the immune system, high soluble fibrinogen levels might protect patients due to its immune diminutive function. The in vivo influence of the 'protective' plasma fibrinogen versus the 'pro-thrombotic' fibrin matrices on thrombosis should be explored in future research.

Thrombin generation in cardiovascular disease and mortality - results from the Gutenberg Health Study.

Thrombin generation may be a potential tool to improve risk stratification for cardiovascular diseases. This study aims to explore the relation between thrombin generation and cardiovascular risk factors, cardiovascular diseases, and total mortality. For this study, N=5000 subjects from the population-based Gutenberg Health Study were analysed in a highly standardized setting. Thrombin generation was assessed by the Calibrated Automated Thrombogram method at 1 and 5 pM tissue factors trigger in platelet poor plasma. Lag time, endogenous thrombin potential, and peak height were derived from the thrombin generation curve. Sex-specific multivariable linear regression analysis adjusted for age, cardiovascular risk factors, cardiovascular diseases and therapy, was used to assess clinical determinants of thrombin generation. Cox regression models adjusted for age, sex, cardiovascular risk factors and vitamin K antagonists investigated the association between thrombin generation parameters and total mortality. Lag time was positively associated with obesity and dyslipidaemia for both sexes (p<0.0001). Obesity was also positively associated with endogenous thrombin potential in both sexes (p<0.0001) and peak height in males (1 pM tissue factor, p=0.0048) and females (p<0.0001). Cox regression models showed an increased mortality in individuals with lag time (1 pM tissue factor, hazard ratio=1.46, [95% CI: 1.07; 2.00], p=0.018) and endogenous thrombin potential (5 pM tissue factor, hazard ratio = 1.50, [1.06; 2.13], p=0.023) above the 95th percentile of the reference group, independent of the cardiovascular risk profile. This large-scale study demonstrates traditional cardiovascular risk factors, particularly obesity, as relevant determinants of thrombin generation. Lag time and endogenous thrombin potential were found as potentially relevant predictors of increased total mortality, which deserves further investigation.

Roles of Coagulation Proteases and PARs (Protease-Activated Receptors) in Mouse Models of Inflammatory Diseases.

Activation of the blood coagulation cascade leads to fibrin deposition and platelet activation that are required for hemostasis. However, aberrant activation of coagulation can lead to thrombosis. Thrombi can cause tissue ischemia, and fibrin degradation products and activated platelets can enhance inflammation. In addition, coagulation proteases activate cells by cleavage of PARs (protease-activated receptors), including PAR1 and PAR2. Direct oral anticoagulants have recently been developed to specifically inhibit the coagulation proteases FXa (factor Xa) and thrombin. Administration of these inhibitors to wild-type mice can be used to determine the roles of FXa and thrombin in different inflammatory diseases. These results can be compared with the phenotypes of mice with deficiencies of either Par1 (F2r) or Par2 (F2rl1). However, inhibition of coagulation proteases will have effects beyond reducing PAR signaling, and a deficiency of PARs will abolish signaling from all proteases that activate these receptors. We will summarize studies that examine the roles of coagulation proteases, particularly FXa and thrombin, and PARs in different mouse models of inflammatory disease. Targeting FXa and thrombin or PARs may reduce inflammatory diseases in humans.

Atrial fibrillation progression risk factors and associated cardiovascular outcome in well-phenotyped patients: data from the AF-RISK study.

AIMS: Atrial fibrillation (AF) is a progressive disease, but identifying patients at risk for AF progression is challenging. We aimed to identify factors associated with AF progression. METHODS AND RESULTS: Atrial fibrillation progression was assessed in 392 patients with recent-onset paroxysmal or persistent AF included in the prospective, observational, multicentre identification of a risk profile to guide atrial fibrillation (AF-RISK) study. Progression of AF was assessed by Holter monitoring and 2-week event recorder at baseline and 1-year follow-up. AF progression was defined as: (i) doubling in AF burden at 1 year compared to baseline with a minimum AF burden of 10% in paroxysmal AF; or (ii) transition from paroxysmal to persistent or permanent AF; or (iii) persistent to permanent AF. Age was 60 ± 11 years, 62% were men, and 83% had paroxysmal AF. At 1 year, 52 (13%) had AF progression (11% in paroxysmal; 26% in persistent AF). Multivariable logistic regression showed that left atrial volume [odds ratio (OR) per 10 mL 1.251, 95% confidence interval (CI) 1.078-1.450; P < 0.001], N-terminal pro-B-type natriuretic peptide (NT-proBNP; OR per standard deviation increase 1.583, 95% CI 1.099-2.281; P = 0.014), and plasminogen activator inhibitor-1 (PAI-1; OR per standard deviation increase 0.660, 95% CI 0.472-0.921; P = 0.015) were associated with AF progression. In an additional follow-up of 1.9 (0.9-3.3) years patients with AF progression developed more cardiovascular events and all-cause mortality (12.4%/year vs. 2.3%/year, P < 0.001). CONCLUSION: Atrial fibrillation progression occurred in 13% of patients with recent-onset AF during 1-year follow-up. Left atrial volume, NT-proBNP, and PAI-1 were associated with AF progression. Patients with AF progression had a higher event rate. TRIAL REGISTRATION NUMBER: Clinicaltrials.gov NCT01510210.

Additive effect of erythropoietin use on exercise-induced endothelial activation and hypercoagulability in athletes.

PURPOSE: Recombinant human erythropoietin (rHuEPO) is known to increase thrombotic risk in patients and might have similar effects in athletes abusing the drug. rHuEPO is prohibited by anti-doping legislation, but this risk has not been investigated thoroughly. This analysis was designed to evaluate whether rHuEPO impacts hemostatic profile and endothelial and platelet activation markers in trained subjects, and whether the combination with exercise affects exercise induced alterations. METHODS: This double-blind, randomized, placebo-controlled trial enrolled healthy, trained male cyclists aged 18-50 years. Participants were randomly allocated (1:1) to receive subcutaneous injections of rHuEPO (epoetin-ß; mean dose 6000 IU per week) or placebo (0.9% NaCl) for 8 weeks. Subjects performed five maximal exercise tests and a road race, coagulation and endothelial/platelet markers were measured at rest and directly after each exercise effort. RESULTS: rHuEPO increased P-selectin (+ 7.8% (1.5-14.5), p = 0.02) and E-selectin (+ 8.6% (2.0-15.7), p = 0.01) levels at rest. Maximal exercise tests significantly influenced all measured coagulation and endothelial/platelet markers, and in the rHuEPO group maximal exercise tests led to 15.3% ((7.0-24.3%), p = 0.0004) higher E-selectin and 32.1% ((4.6-66.8%), p = 0.0207) higher Platelet factor 4 (PF4) levels compared to the placebo group. CONCLUSION: In conclusion, rHuEPO treatment resulted in elevated E- and P-selectin levels in trained cyclists, indicating enhanced endothelial activation and/or platelet reactivity. Exercise itself induces hypercoagulability, and the combination of rHuEPO and exercise increased E-selectin and PF4 levels more than either intervention alone. Based on this, exercise potentially increases thrombotic risk, a risk that might be enhanced in combination with rHuEPO use.

Protease-activated receptors are potential regulators in the development of arterial endofibrosis in high-performance athletes.

OBJECTIVE: High-performance athletes can develop symptomatic arterial flow restriction during exercise caused by endofibrosis. The pathogenesis is poorly understood; however, coagulation enzymes, such as tissue factor (TF) and coagulation factor Xa, might contribute to the fibrotic process, which is mainly regulated through activation of protease-activated receptors (PARs). Therefore, the aim of this explorative study was to evaluate the presence of coagulation factors and PARs in endofibrotic tissue, which might be indicative of their potential role in the natural development of endofibrosis. METHODS: External iliac arterial specimens with endofibrosis (n = 19) were collected during surgical interventions. As control, arterial segments of the external iliac artery (n = 20) were collected post mortem from individuals with no medical history of cardiovascular disease who donated their body to medical science. Arteries were paraffinized and cut in tissue sections for immunohistochemical analysis. Positive staining within lesions was determined with ImageJ software (National Institutes of Health, Bethesda, Md). RESULTS: Endofibrotic segments contained a neointima, causing intraluminal stenosis, which was highly positive for collagen (+150%; P < .01) and elastin (+148%; P < .01) in comparison with controls. Intriguingly, endofibrosis was not limited to the intima because collagen (+213%) and elastin (+215%) were also significantly elevated in the media layer of endofibrotic segments. These findings were accompanied by significantly increased α-smooth muscle actin-positive cells, morphologically compatible with the presence of myofibroblasts. In addition, PAR1 and PAR4 and the membrane receptor TF were increased as well as coagulation factor X. CONCLUSIONS: We showed that myofibroblasts and the accompanying collagen and elastin synthesis might be key factors in the development of endofibrosis. The special association with increased presence of PARs, factor X, and TF suggests that protease-mediated cell signaling could be a contributing component in the mechanisms leading to endofibrosis.

Role of platelets, neutrophils, and factor XII in spontaneous venous thrombosis in mice.

Recently, platelets, neutrophils, and factor XII (FXII) have been implicated as important players in the pathophysiology of venous thrombosis. Their role became evident in mouse models in which surgical handling was used to provoke thrombosis. Inhibiting anticoagulation in mice by using small interfering RNA (siRNA) targeting Serpinc1 and Proc also results in a thrombotic phenotype, which is spontaneous (no additional triggers) and reproducibly results in clots in the large veins of the head and fibrin deposition in the liver. This thrombotic phenotype is fatal but can be fully rescued by thrombin inhibition. The mouse model was used in this study to investigate the role of platelets, neutrophils, and FXII. After administration of siRNAs targeting Serpinc1 and Proc, antibody-mediated depletion of platelets fully abrogated the clinical features as well as microscopic aspects in the head. This was corroborated by strongly reduced fibrin deposition in the liver. Whereas neutrophils were abundant in siRNA-triggered thrombotic lesions, antibody-mediated depletion of circulating Ly6G-positive neutrophils did not affect onset, severity, or thrombus morphology. In addition, absence of circulating neutrophils did not affect quantitative liver fibrin deposition. Remarkably, siRNA-mediated depletion of plasma FXII accelerated the onset of the clinical phenotype; mice were affected with more severe thrombotic lesions. To summarize, in this study, onset and severity of the thrombotic phenotype are dependent on the presence of platelets but not circulating neutrophils. Unexpectedly, FXII has a protective effect. This study challenges the proposed roles of neutrophils and FXII in venous thrombosis pathophysiology.

Sex-specific differences in genetic and nongenetic determinants of mean platelet volume: results from the Gutenberg Health Study.

Mean platelet volume (MPV), a measure of platelet size, is a potential biological marker of platelet function. To date, a comprehensive analysis including known genetic and nongenetic factors that determine MPV is still lacking. MPV has been evaluated in 15 010 individuals from the population-based Gutenberg Health Study. Genetic information was available for 4175 individuals. Our results showed that age (ß, 0.0346; 95% confidence interval [CI], 0.0255 to 0.0436), cardiovascular risk factors (CVRFs) such as smoking (ß, 0.178; 95% CI, 0.128 to 0.229), hypertension (ß, 0.05; 95% CI, 0.00289 to .0981), and high glucose level (ß, 0.00179; 95% CI, 0.0006 to 0.00299) were linked with higher MPV in males only. Intake of oral contraceptives (ß, 0.150; 95% CI, 0.0649 to 0.236) and menstruation (ß, 0.123; 95% CI, 0.0231 to 0.224) were strongly associated with higher MPV in females. Seven single nucleotide polymorphisms (SNPs) for females and 4 SNPs for males were associated with higher MPV. The full model, including age, CVRFs, laboratory parameters, medications, and genetic variation, explained 20.4% of the MPV variance in females and 18.6% in males. The curves of cumulative mortality, stratified for sex, showed worse survival for males only with MPV > 9.96 fL vs MPV ≤ 9.96 fL (P < .0001). This study provides evidence for heterogeneity in the profile of determinants for MPV between sexes. The observed interactions between genetic variability, CVRFs, and MPV and its association with the development of cardiovascular disease or thrombotic risk need to be further investigated.

Hypercoagulability causes atrial fibrosis and promotes atrial fibrillation.

AIMS: Atrial fibrillation (AF) produces a hypercoagulable state. Stimulation of protease-activated receptors by coagulation factors provokes pro-fibrotic, pro-hypertrophic, and pro-inflammatory responses in a variety of tissues. We studied the effects of thrombin on atrial fibroblasts and tested the hypothesis that hypercoagulability contributes to the development of a substrate for AF. METHODS AND RESULTS: In isolated rat atrial fibroblasts, thrombin enhanced the phosphorylation of the pro-fibrotic signalling molecules Akt and Erk and increased the expression of transforming growth factor ß1 (2.7-fold) and the pro-inflammatory factor monocyte chemoattractant protein-1 (6.1-fold). Thrombin also increased the incorporation of 3H-proline, suggesting enhanced collagen synthesis by fibroblasts (2.5-fold). All effects could be attenuated by the thrombin inhibitor dabigatran. In transgenic mice with a pro-coagulant phenotype (TMpro/pro), the inducibility of AF episodes lasting >1 s was higher (7 out of 12 vs. 1 out of 10 in wild type) and duration of AF episodes was longer compared with wild type mice (maximum episode duration 42.8 ± 68.4 vs. 0.23 ± 0.39 s). In six goats with persistent AF treated with nadroparin, targeting Factor Xa-mediated thrombin generation, the complexity of the AF substrate was less pronounced than in control animals (LA maximal activation time differences 23.3 ± 3.1 ms in control vs. 15.7 ± 2.1 ms in nadroparin, P < 0.05). In the treated animals, AF-induced α-smooth muscle actin expression was lower and endomysial fibrosis was less pronounced. CONCLUSION: The hypercoagulable state during AF causes pro-fibrotic and pro-inflammatory responses in adult atrial fibroblasts. Hypercoagulability promotes the development of a substrate for AF in transgenic mice and in goats with persistent AF. In AF goats, nadroparin attenuates atrial fibrosis and the complexity of the AF substrate. Inhibition of coagulation may not only prevent strokes but also inhibit the development of a substrate for AF.