Notch1 regulates hepatic thrombopoietin production.

Notch signaling regulates cell-fate decisions in several developmental processes and cell functions. However, a role for Notch in hepatic thrombopoietin (TPO) production remains unclear. We noted thrombocytopenia in mice with hepatic Notch1 deficiency, and so investigated TPO production and other features of platelets in these mice. We found that the liver ultrastructure and hepatocyte function were comparable between control mice and Notch1-deficient mice. However, the Notch1-deficient mice had significantly lower plasma TPO and hepatic TPO mRNA levels, concomitant with lower numbers of platelets and impaired megakaryocyte differentiation and maturation, which were rescued by addition of exogenous TPO. Additionally, JAK2/STAT3 phosphorylation was significantly inhibited in Notch1-deficient hepatocytes, consistent with the RNA-seq analysis. JAK2/STAT3 phosphorylation and TPO production was also impaired in cultured Notch1-deficient hepatocytes after treatment with desialylated platelets. Consistently, hepatocyte-specific Notch1 deletion inhibited JAK2/STAT3 phosphorylation and hepatic TPO production induced by administration of desialylated platelets in vivo. Interestingly, Notch1 deficiency downregulated the expression of HES5 but not HES1. Moreover, desialylated platelets promoted the binding of HES5 to JAK2/STAT3, leading to JAK2/STAT3 phosphorylation and pathway activation in hepatocytes. Hepatocyte Ashwell-Morell receptor (AMR) (asialoglycoprotein receptor 1, ASGR1) physically associates with Notch1 and inhibition of AMR impaired Notch1 signaling activation and hepatic TPO production. Furthermore, blockage of Dll4 on desialylated platelets inhibited hepatocyte Notch1 activation and HES5 expression, JAK2/STAT3 phosphorylation and subsequent TPO production. In conclusion, our study identifies a novel regulatory role of Notch1 in hepatic TPO production, indicating that it might be a target for modulating TPO level.

Tissue inhibitors of metalloproteinases (TIMPs) modulate platelet ADAM10 activity.

Platelet-specific collagen receptor glycoprotein (GP)VI is stable on the surface of circulating platelets but undergoes ectodomain cleavage on activated platelets. Activation-dependent GPVI metalloproteolysis is primarily mediated by A Disintegrin And Metalloproteinase (ADAM) 10. Regulation of platelet ADAMs activity is not well-defined however Tissue Inhibitors of Metalloproteinases (TIMPs) may play a role. As levels of TIMPs on platelets and the control of ADAMs-mediated shedding by TIMPs has not been evaluated, we quantified the levels of TIMPs on the surface of resting and activated platelets from healthy donors by flow cytometry and multiplex ELISA. Variable levels of all TIMPs could be detected on platelets. Plasma contained significant quantities of TIMP1 and TIMP2, but only trace amounts of TIMP3 and TIMP4. Recombinant TIMP3 strongly ablated resting and activated platelet ADAM10 activity, when monitored using a quenched fluorogenic peptide substrate with ADAM10 specificity. Whilst ADAM10-specific inhibitor GI254023X or ethylenediamine tetraacetic acid (EDTA) could modulate ligand-initiated shedding of GPVI, only recombinant TIMP2 achieved a modest (~20%) inhibition. We conclude that some platelet TIMPs are able to modulate platelet ADAM10 activity but none strongly regulate ligand-dependent shedding of GPVI. Our findings provide new insights into the regulation of platelet receptor sheddase activity.

What do we know? Platelet receptor GPVI initiates platelet adhesion and aggregation and is proteolytically cleaved from the activated platelet surfaceThe metalloproteinases responsible belong to the ADAMs family of enzymes which are inhibited by TIMPsWhat did we discover? Plasma contains significant amounts of TIMP1 and TIMP2Circulating platelets bear significant amounts of TIMPs 1, 2, and 3Recombinant TIMP3 strongly inhibits resting and activated platelet ADAM10 activityExogenous addition of TIMP2 mildly blocked ligand-initiated shedding of GPVIWhat is the impact? TIMPs may modulate ADAM10 activity under resting conditions and stabilize GPVI levels in response to platelet activationAnti-GPVI agents are being evaluated as anti-thrombotic agents, however, acute loss of GPVI in trauma or settings of thrombocytopenia is linked with clinical bleedingUnderstanding how GPVI levels are regulated is important as agents that modulate GPVI function are emerging as important therapeutics for clinical applications in Thrombosis and Hemostasis fields.

MTH1 protects platelet mitochondria from oxidative damage and regulates platelet function and thrombosis.

Human MutT Homolog 1 (MTH1) is a nucleotide pool sanitization enzyme that hydrolyzes oxidized nucleotides to prevent their mis-incorporation into DNA under oxidative stress. Expression and functional roles of MTH1 in platelets are not known. Here, we show MTH1 expression in platelets and its deficiency impairs hemostasis and arterial/venous thrombosis in vivo. MTH1 deficiency reduced platelet aggregation, phosphatidylserine exposure and calcium mobilization induced by thrombin but not by collagen-related peptide (CRP) along with decreased mitochondrial ATP production. Thrombin but not CRP induced Ca2+-dependent mitochondria reactive oxygen species generation. Mechanistically, MTH1 deficiency caused mitochondrial DNA oxidative damage and reduced the expression of cytochrome c oxidase 1. Furthermore, MTH1 exerts a similar role in human platelet function. Our study suggests that MTH1 exerts a protective function against oxidative stress in platelets and indicates that MTH1 could be a potential therapeutic target for the prevention of thrombotic diseases.

Protein tyrosine phosphatase PTPN22 negatively modulates platelet function and thrombus formation.

Protein tyrosine phosphatase nonreceptor type 22 (PTPN22) is a protein tyrosine phosphatase that negatively regulates T-cell signaling. However, whether it is expressed and functions in platelets remains unknown. Here we investigated the expression and role of PTPN22 in platelet function. We reported PTPN22 expression in both human and mouse platelets. Using PTPN22-/- mice, we showed that PTPN22 deficiency significantly shortened tail-bleeding time and accelerated arterial thrombus formation without affecting venous thrombosis and the coagulation factors VIII and IX. Consistently, PTPN22-deficient platelets exhibited enhanced platelet aggregation, granule secretion, calcium mobilization, lamellipodia formation, spreading, and clot retraction. Quantitative phosphoproteomic analysis revealed the significant difference of phosphodiesterase 5A (PDE5A) phosphorylation in PTPN22-deficient platelets compared with wild-type platelets after collagen-related peptide stimulation, which was confirmed by increased PDE5A phosphorylation (Ser92) in collagen-related peptide-treated PTPN22-deficient platelets, concomitant with reduced level and vasodilator-stimulated phosphoprotein phosphorylation (Ser157/239). In addition, PTPN22 interacted with phosphorylated PDE5A (Ser92) and dephosphorylated it in activated platelets. Moreover, purified PTPN22 but not the mutant form (C227S) possesses intrinsic serine phosphatase activity. Furthermore, inhibition of PTPN22 enhanced human platelet aggregation, spreading, clot retraction, and increased PDE5A phosphorylation (Ser92). In conclusion, our study shows a novel role of PTPN22 in platelet function and arterial thrombosis, identifying new potential targets for future prevention of thrombotic or cardiovascular diseases.

Fc Binding by FcγRIIa Is Essential for Cellular Activation by the Anti-FcγRIIa mAbs 8.26 and 8.2.

FcγR activity underpins the role of antibodies in both protective immunity and auto-immunity and importantly, the therapeutic activity of many monoclonal antibody therapies. Some monoclonal anti-FcγR antibodies activate their receptors, but the properties required for cell activation are not well defined. Here we examined activation of the most widely expressed human FcγR; FcγRIIa, by two non-blocking, mAbs, 8.26 and 8.2. Crosslinking of FcγRIIa by the mAb F(ab')2 regions alone was insufficient for activation, indicating activation also required receptor engagement by the Fc region. Similarly, when mutant receptors were inactivated in the Fc binding site, so that intact mAb was only able to engage receptors via its two Fab regions, again activation did not occur. Mutation of FcγRIIa in the epitope recognized by the agonist mAbs, completely abrogated the activity of mAb 8.26, but mAb 8.2 activity was only partially inhibited indicating differences in receptor recognition by these mAbs. FcγRIIa inactivated in the Fc binding site was next co-expressed with the FcγRIIa mutated in the epitope recognized by the Fab so that each mAb 8.26 molecule can contribute only three interactions, each with separate receptors, one via the Fc and two via the Fab regions. When the Fab and Fc binding were thus segregated onto different receptor molecules receptor activation by intact mAb did not occur. Thus, receptor activation requires mAb 8.26 Fab and Fc interaction simultaneously with the same receptor molecules. Establishing the molecular nature of FcγR engagement required for cell activation may inform the optimal design of therapeutic mAbs.

Loss of the exocyst complex component EXOC3 promotes hemostasis and accelerates arterial thrombosis.

The exocyst is an octameric complex comprising 8 distinct protein subunits, exocyst complex components (EXOC) 1 to 8. It has an established role in tethering secretory vesicles to the plasma membrane, but its relevance to platelet granule secretion and function remains to be determined. Here, EXOC3 conditional knockout (KO) mice in the megakaryocyte/platelet lineage were generated to assess exocyst function in platelets. Significant defects in platelet aggregation, integrin activation, α-granule (P-selectin and platelet factor 4), dense granule, and lysosomal granule secretion were detected in EXOC3 KO platelets after treatment with a glycoprotein VI (GPVI)-selective agonist, collagen-related peptide (CRP). Except for P-selectin exposure, these defects were completely recovered by maximal CRP concentrations. GPVI surface levels were also significantly decreased by 14.5% in KO platelets, whereas defects in proximal GPVI signaling responses, Syk and LAT phosphorylation, and calcium mobilization were also detected, implying an indirect mechanism for these recoverable defects due to decreased surface GPVI. Paradoxically, dense granule secretion, integrin activation, and changes in surface expression of integrin αIIb (CD41) were significantly increased in KO platelets after protease-activated receptor 4 activation, but calcium responses were unaltered. Elevated integrin activation responses were completely suppressed with a P2Y12 receptor antagonist, suggesting enhanced dense granule secretion of adenosine 5'-diphosphate as a critical mediator of these responses. Finally, arterial thrombosis was significantly accelerated in KO mice, which also displayed improved hemostasis determined by reduced tail bleeding times. These findings reveal a regulatory role for the exocyst in controlling critical aspects of platelet function pertinent to thrombosis and hemostasis.

Immobilized collagen prevents shedding and induces sustained GPVI clustering and signaling in platelets.

Collagen, the most thrombogenic constituent of blood vessel walls, activates platelets through glycoprotein VI (GPVI). In suspension, following platelet activation by collagen, GPVI is cleaved by A Disintegrin And Metalloproteinase (ADAM)10 and ADAM17. In this study, we use single-molecule localization microscopy and a 2-level DBSCAN-based clustering tool to show that GPVI remains clustered along immobilized collagen fibers for at least 3 hours in the absence of significant shedding. Tyrosine phosphorylation of spleen tyrosine kinase (Syk) and Linker of Activated T cells (LAT), and elevation of intracellular Ca2+, are sustained over this period. Syk, but not Src kinase-dependent signaling is required to maintain clustering of the collagen integrin α2ß1, whilst neither is required for GPVI. We propose that clustering of GPVI on immobilized collagen protects GPVI from shedding in order to maintain sustained Src and Syk-kinases dependent signaling, activation of integrin α2ß1, and continued adhesion.

Platelet phenotype and function in the absence of splenic sequestration (Review).

The spleen, in addition to its role in immunity, plays key roles in erythrocyte maintenance and platelet sequestration. Loss of the spleen via splenectomy occurs in approximately 6.4 to 7.1 per 100 000 people per year globally, commonly as a life-saving emergency procedure in trauma and a therapeutic procedure in hematological and hematological malignant conditions. It is associated with increased risk of life-threatening infection and thromboembolism, presumably via loss of splenic function, but the underlying mechanisms behind post-splenectomy thromboembolism are unclear. The splenectomized individual has a two-fold risk of thromboembolism as compared to non-splenectomized individuals and the risk of thromboembolism is elevated both post-operatively and in the longer term. Although those splenectomized for hematological conditions or hematological malignant conditions are at highest risk for thromboembolism, an increase in thromboembolic outcomes is also observed amongst individuals splenectomized for trauma, suggesting underlying disease state is only a partial factor. Although the physiological role of the splenic platelet pool on platelets is unclear, platelet changes after splenectomy suggest that the spleen may play a role in maintaining platelet quality and function. In hypersplenic conditions, sequestration can increase to sequester up to 72% of the total platelet mass. Following splenectomy, a thrombocytosis is commonly seen secondary to the loss of the ability to sequester platelets. Abnormal platelet quality and function have been observed as a consequence of splenectomy. These platelet defects seen after splenectomy may likely contribute to the increase in post-splenectomy thromboembolism. Here we draw upon the literature to characterize the post-splenectomy platelet and its potential role in post-splenectomy thromboembolism.

Targeting shear gradient activated von Willebrand factor by the novel single-chain antibody A1 reduces occlusive thrombus formation in vitro.

Intraluminal thrombus formation precipitates conditions such as acute myocardial infarction and disturbs local blood flow resulting in areas of rapidly changing blood flow velocities and steep gradients of blood shear rate. Shear rate gradients are known to be pro-thrombotic with an important role for the shear-sensitive plasma protein von Willebrand factor (VWF). Here, we developed a single-chain antibody (scFv) that targets a shear gradient specific conformation of VWF to specifically inhibit platelet adhesion at sites of SRGs but not in areas of constant shear. Microfluidic flow channels with stenotic segments were used to create shear rate gradients during blood perfusion. VWF-GPIbα interactions were increased at sites of shear rate gradients compared to constant shear rate of matched magnitude. The scFv-A1 specifically reduced VWF-GPIbα binding and thrombus formation at sites of SRGs but did not block platelet deposition and aggregation under constant shear rate in upstream sections of the channels. Significantly, the scFv A1 attenuated platelet aggregation only in the later stages of thrombus formation. In the absence of shear, direct binding of scFv-A1 to VWF could not be detected and scFV-A1 did not inhibit ristocetin induced platelet agglutination. We have exploited the pro-aggregatory effects of SRGs on VWF dependent platelet aggregation and developed the shear-gradient sensitive scFv-A1 antibody that inhibits platelet aggregation exclusively at sites of shear rate gradients. The lack of VWF inhibition in non-stenosed vessel segments places scFV-A1 in an entirely new class of anti-platelet therapy for selective blockade of pathological thrombus formation while maintaining normal haemostasis.

Phosphoproteomic Analysis of Platelets in Severe Obesity Uncovers Platelet Reactivity and Signaling Pathways Alterations.

OBJECTIVE: Obesity is associated with a proinflammatory and prothrombotic state that supports atherosclerosis progression. The goal of this study was to gain insights into the phosphorylation events related to platelet reactivity in obesity and identify platelet biomarkers and altered activation pathways in this clinical condition. Approach and Results: We performed a comparative phosphoproteomic analysis of resting platelets from obese patients and their age- and gender-matched lean controls. The phosphoproteomic data were validated by mechanistic, functional, and biochemical assays. We identified 220 differentially regulated phosphopeptides, from at least 175 proteins; interestingly, all were up-regulated in obesity. Most of the altered phosphoproteins are involved in SFKs (Src-family kinases)-related signaling pathways, cytoskeleton reorganization, and vesicle transport, some of them validated by targeted mass spectrometry. To confirm platelet dysfunction, flow cytometry assays were performed in whole blood indicating higher surface levels of GP (glycoprotein) VI and CLEC (C-type lectin-like receptor) 2 in platelets from obese patients correlating positively with body mass index. Receiver operator characteristics curves analysis suggested a much higher sensitivity for GPVI to discriminate between obese and lean individuals. Indeed, we also found that obese platelets displayed more adhesion to collagen-coated plates. In line with the above data, soluble GPVI levels-indicative of higher GPVI signaling activation-were almost double in plasma from obese patients. CONCLUSIONS: Our results provide novel information on platelet phosphorylation changes related to obesity, revealing the impact of this chronic pathology on platelet reactivity and pointing towards the main signaling pathways dysregulated.

Soluble glycoprotein VI is a predictor of major bleeding in patients with suspected heparin-induced thrombocytopenia.

We have shown that patients with suspected heparin-induced thrombocytopenia (HIT) have a high incidence of major bleeding. Recent studies have implicated elevated soluble glycoprotein VI (sGPVI) levels as a potential risk factor for bleeding. We sought to determine if elevated sGPVI plasma levels are associated with major bleeding events in patients with suspected HIT. We used a cohort of 310 hospitalized adult patients with suspected HIT who had a blood sample collected at the time HIT was suspected. Plasma sGPVI levels were measured by using enzyme-linked immunosorbent assay. Patients were excluded who had received a platelet transfusion within 1 day of sample collection because of the high levels of sGPVI in platelet concentrates. We assessed the association of sGPVI (high vs low) with International Society on Thrombosis and Haemostasis major bleeding events by multivariable logistic regression, adjusting for other known risk factors for bleeding. Fifty-four patients were excluded due to recent platelet transfusion, leaving 256 patients for analysis. Eighty-nine (34.8%) patients had a major bleeding event. Median sGPVI levels were significantly elevated in patients with major bleeding events compared with those without major bleeding events (49.09 vs 31.93 ng/mL; P < .001). An sGPVI level >43 ng/mL was independently associated with major bleeding after adjustment for critical illness, sepsis, cardiopulmonary bypass surgery, and degree of thrombocytopenia (adjusted odds ratio, 2.81; 95% confidence interval, 1.51-5.23). Our findings suggest that sGPVI is associated with major bleeding in hospitalized patients with suspected HIT. sGPVI may be a novel biomarker to predict bleeding risk in patients with suspected HIT.

Pharmacological Blockade of Glycoprotein VI Promotes Thrombus Disaggregation in the Absence of Thrombin.

OBJECTIVE: Atherothrombosis occurs upon rupture of an atherosclerotic plaque and leads to the formation of a mural thrombus. Computational fluid dynamics and numerical models indicated that the mechanical stress applied to a thrombus increases dramatically as a thrombus grows, and that strong inter-platelet interactions are essential to maintain its stability. We investigated whether GPVI (glycoprotein VI)-mediated platelet activation helps to maintain thrombus stability by using real-time video-microscopy. Approach and Results: We showed that GPVI blockade with 2 distinct Fab fragments promoted efficient disaggregation of human thrombi preformed on collagen or on human atherosclerotic plaque material in the absence of thrombin. ACT017-induced disaggregation was achieved under arterial blood flow conditions, and its effect increased with wall shear rate. GPVI regulated platelet activation within a growing thrombus as evidenced by the loss in thrombus contraction when GPVI was blocked, and the absence of the disaggregating effect of an anti-GPVI agent when the thrombi were fully activated with soluble agonists. The GPVI-dependent thrombus stabilizing effect was further supported by the fact that inhibition of any of the 4 key immunoreceptor tyrosine-based motif signalling molecules, src-kinases, Syk, PI3Kß, or phospholipase C, resulted in kinetics of thrombus disaggregation similar to ACT017. The absence of ACT017-induced disaggregation of thrombi from 2 afibrinogenemic patients suggests that the role of GPVI requires interaction with fibrinogen. Finally, platelet disaggregation of fibrin-rich thrombi was also promoted by ACT017 in combination with r-tPA (recombinant tissue plasminogen activator). CONCLUSIONS: This work identifies an unrecognized role for GPVI in maintaining thrombus stability and suggests that targeting GPVI could dissolve platelet aggregates with a poor fibrin content.

Loss of GPVI and GPIbα contributes to trauma-induced platelet dysfunction in severely injured patients.

Trauma-induced coagulopathy (TIC) is a complex, multifactorial failure of hemostasis that occurs in 25% of severely injured patients and results in a fourfold higher mortality. However, the role of platelets in this state remains poorly understood. We set out to identify molecular changes that may underpin platelet dysfunction after major injury and to determine how they relate to coagulopathy and outcome. We performed a range of hemostatic and platelet-specific studies in blood samples obtained from critically injured patients within 2 hours of injury and collected prospective data on patient characteristics and clinical outcomes. We observed that, although platelet counts were preserved above critical levels, circulating platelets sampled from trauma patients exhibited a profoundly reduced response to both collagen and the selective glycoprotein VI (GPVI) agonist collagen-related peptide, compared with those from healthy volunteers. These responses correlated closely with overall clot strength and mortality. Surface expression of the collagen receptors GPIbα and GPVI was reduced on circulating platelets in trauma patients, with increased levels of the shed ectodomain fragment of GPVI detectable in plasma. Levels of shed GPVI were highest in patients with more severe injuries and TIC. Collectively, these observations demonstrate that platelets experience a loss of GPVI and GPIbα after severe injury and translate into a reduction in the responsiveness of platelets during active hemorrhage. In turn, they are associated with reduced hemostatic competence and increased mortality. Targeting proteolytic shedding of platelet receptors is a potential therapeutic strategy for maintaining hemostatic competence in bleeding and improving the efficacy of platelet transfusions.

Shedding of soluble glycoprotein VI is neither affected by animal-derived antibeta-2-glycoprotein 1 antibodies nor IgG fractions from patients with systemic lupus erythematosus.

: Antibeta-2-glycoprotein 1 (antiß2GP1) antibodies are associated with increased risk of thrombosis in patients with systemic lupus erythematosus (SLE). The specific effect(s) of antiß2GP1 antibodies on platelets are unclear. Platelet activation in response to antiplatelet antibodies has been shown to induce shedding of the ectodomain of the platelet collagen receptor, glycoprotein VI (GPVI), releasing soluble GPVI (sGPVI). The aim of this study was to therefore determine whether antiß2GP1 antibodies, and/or purified IgG fractions, from patients with SLE shed sGPVI from platelets. We determined sGPVI levels in platelet poor plasma from SLE patients with/without antiß2GP1 antibodies (n = 37), as well as in platelet-rich plasma from healthy donors treated with either SLE-derived IgG fractions containing antiß2GP1, animal-derived antiß2GP1, or isotype control antibodies. Levels of sGPVI were higher in three SLE-derived platelet poor plasma with antiß2GP1 antibodies (103.52 ±â€Š12.32 ng/ml) compared with those without (28.11 ±â€Š12.73 ng/ml). Neither SLE-derived IgG fractions containing antiß2GP1 antibodies, nor animal-derived antiß2GP1 antibodies induced significant shedding of sGPVI from healthy donor platelets compared with isotype controls. These results suggest that antiß2GP1 antibodies do not affect shedding of sGPVI, and therefore collagen-mediated platelet signalling pathways. The shedding activity in SLE patients may be due to factors other than antiß2GP1 antibodies, for example, metalloproteinases.

Fibrin exposure triggers αIIbß3-independent platelet aggregate formation, ADAM10 activity and glycoprotein VI shedding in a charge-dependent manner.

BACKGROUND: Collagen and fibrin engagement and activation of glycoprotein (GP) VI induces proteolytic cleavage of the GPVI ectodomain generating shed soluble GPVI (sGPVI). Collagen-mediated GPVI shedding requires intracellular signalling to release the sGPVI, mediated by A Disintegrin And Metalloproteinase 10 (ADAM10); however, the precise mechanism by which fibrin induces GPVI shedding remains elusive. Plasma sGPVI levels are elevated in patients with coagulopathies, sepsis, or inflammation and can predict onset of sepsis and sepsis-related mortality; therefore, it is clinically important to understand the mechanisms of GPVI shedding under conditions of minimal collagen exposure. OBJECTIVES: Our aim was to characterize mechanisms by which fibrin-GPVI interactions trigger GPVI shedding. METHODS: Platelet aggregometry, sGPVI ELISA, and an ADAM10 fluorescence resonance energy transfer assay were used to measure fibrin-mediated platelet responses. RESULTS: Fibrin induced αIIbß3-independent washed platelet aggregate formation, GPVI shedding, and increased ADAM10 activity, all of which were insensitive to pre-treatment with inhibitors of Src family kinases but were divalent cation- and metalloproteinase-dependent. In contrast, treatment of washed platelets with other GPVI ligands, collagen, and collagen-related peptide caused αIIbß3-dependent platelet aggregation and GPVI release but did not increase constitutive ADAM10 activity. CONCLUSIONS: Fibrin engages GPVI in a manner that differs from other GPVI ligands. Inclusion of polyanionic molecules disrupted fibrin-induced platelet aggregate formation and sGPVI release, suggesting that electrostatic charge may play a role in fibrin/GPVI engagement. It may be feasible to exploit this property and specifically disrupt GPVI/fibrin interactions whilst sparing GPVI/collagen engagement.Fibrin engages GPVI in a manner that differs from other GPVI ligands. Inclusion of polyanionic molecules disrupted fibrin-induced platelet aggregate formation and sGPVI release, suggesting that electrostatic charge may play a role in fibrin/GPVI engagement. It may be feasible to exploit this property and specifically disrupt GPVI/fibrin interactions whilst sparing GPVI/collagen engagement.

Successful renal denervation decreases the platelet activation status in hypertensive patients.

AIMS: To determine whether renal denervation (RDN) in hypertensive patients affects the platelet activation status. METHODS AND RESULTS: We investigated the effect of RDN on the platelet activation status in 41 hypertensive patients undergoing RDN. Ambulatory blood pressure (BP), plasma sympathetic neurotransmitter Neuropeptide Y, and platelet activation markers were measured at baseline, at 3 months, and 6 months after RDN. RDN significantly decreased BP at 3 months (150.6 ± 11.3/80.9 ± 11.4 mmHg to 144.7 ± 12.0/77.1 ± 11.1 mmHg; P < 0.01) and at 6 months (144.3 ± 13.8/78.3 ± 11.1 mmHg; P < 0.01). Plasma levels of the sympathetic neurotransmitter Neuropeptide Y, an indicator of sympathetic nerve activity, were significantly decreased at 3 months (0.29 ± 0.11 ng/mL to 0.23 ± 0.11 ng/mL; P < 0.0001) and at 6 months (0.22 ± 0.12 ng/mL; P < 0.001) after RDN. This was associated with a reduction in platelet membrane P-selectin expression (3 months, P < 0.05; 6 months, P < 0.05), soluble P-selectin (6 months, P < 0.05), circulating numbers of platelet-derived extracellular vesicles (EVs) (3 months, P < 0.001; 6 months, P < 0.01), and phosphatidylserine expressing EVs (3 months, P < 0.001; 6 months, P < 0.0001), indicative of a reduction in platelet activation status and procoagulant activity. Only patients who responded to RDN with a BP reduction showed inhibition of P-selectin expression at 3 months (P < 0.05) and 6 months (P < 0.05) as well as reduction of glycoprotein IIb/IIIa activation at 3 months (P < 0.05). Notably, 13 patients who took aspirin did not show significant reduction in platelet P-selectin expression following RDN. CONCLUSION: Our results imply a connection between the sympathetic nervous system and the platelet activation status and provide a potential mechanistic explanation by which RDN can have favourable effects towards reducing cardiovascular complications.

Plasma levels of the soluble form of the FcγRIIa receptor vary with receptor polymorphisms and are elevated in rheumatoid arthritis.

Soluble forms of the low-affinity immunoglobulin receptor FcγRIIa (sFcγRIIa) lacking the cytoplasmic tail have been reported in plasma however the mechanism and functional consequences are unknown. This study aimed to evaluate mechanisms of FcγRIIa release compared to GPVI release from platelets, and examine whether genetic polymorphisms at positions 27 and 131 within FcγRIIa correlate with platelet FcγRIIa stability and function. Enzyme-linked immunosorbent assays (ELISAs) were used to measure plasma sFcγRIIa and sGPVI levels. FcγRIIa genotype at positions 27 and 131 was evaluated. sFcγRIIa levels were not significantly different between non-131HH and 131HH but were significantly lower in 27W than non-27W. Treatment of platelets with aggregated immunoglobulin (Ig) G induced release of FcγRIIa and GPVI, but only sGPVI release was statistically significant, required functional FcγRIIa, and was blocked by inhibitors of signaling pathways and metalloproteinases. This indicated that sFcγRIIa was not released from platelets by metalloproteolysis. sFcγRIIa levels were not correlated with sGPVI levels in healthy individuals however levels of sFcγRIIa and sGPVI in plasma from patients with rheumatoid arthritis (RA) were significantly elevated above levels found in healthy individuals. Elevated level of sFcγRIIa in RA patients may reflect active immune-based arthritis and be predictive of active inflammation.

All-Trans Retinoic Acid Impairs Platelet Function and Thrombus Formation and Inhibits Protein Kinase CßI/δ Phosphorylation.

All-trans retinoic acid (ATRA) is widely used for induction of complete remission in patients with acute promyelocytic leukemia (APL). ATRA also regulates protein kinase C (PKC) activity. Therapeutic use of ATRA reportedly interferes with hemostatic function in APL patients, including effects on coagulation or other vascular cells, although effects of ATRA on platelets remain unclear. This study aims to investigate the effect of therapeutic-relevant doses of ATRA on platelet function. Human platelets were preincubated with ATRA (0-20 µM) for 1 hour at 37°C, followed by analysis of aggregation, granule secretion, receptor expression by flow cytometry, platelet spreading, or clot retraction. Additionally, ATRA (10 mg/kg) was injected intraperitoneally into mice and tail bleeding time and arterial thrombus formation were evaluated. ATRA inhibited platelet aggregation and adenosine triphosphate release induced by collagen (5 µg/mL) or thrombin (0.05 U/mL) in a dose-dependent manner without affecting P-selectin expression or surface levels of glycoprotein (GP) Ibα, GPVI, or αIIbß3. ATRA-treated platelets demonstrated reduced spreading on immobilized fibrinogen or collagen and reduced thrombin-induced clot retraction together with reduced phosphorylation of Syk and PLCγ2. In addition, ATRA-treated mice displayed significantly impaired hemostasis and arterial thrombus formation in vivo. Further, in platelets stimulated with either collagen-related peptide or thrombin, ATRA selectively inhibited phosphorylation of PKCßI (Ser661) and PKCδ (Thr505), but not PKCα or PKCßII phosphorylation (Thr638/641). In conclusion, ATRA inhibits platelet function and thrombus formation, possibly involving direct or indirect inhibition of PKCßI/δ, indicating that ATRA might be beneficial for the treatment of thrombotic or cardiovascular diseases.

Illustrated State-of-the-Art Capsules of the ISTH 2019 Congress in Melbourne, Australia.

The 27th Congress of the International Society of Thrombosis and Haemostasis (ISTH) is an international conference held July 6-10, 2019, in Melbourne, the capital of the state of Victoria, Australia. The ISTH congress has previously been held every other year, with the Scientific and Standardization Committee (SSC) meeting held annually, until 2019 when it became one combined annual meeting of the ISTH and SSC. The conference covers clinical and basic aspects of hemostasis and thrombosis, and this year includes 5 Plenary lectures and >50 State of Art (SOA) lectures, presented by internationally recognized speakers, as well as numerous oral session and poster presentations selected from submitted abstracts, including many early career and reach the world support recipients. This SOA review article in RPTH contains concise Illustrated Review Articles or 'Capsules' consisting of short text, three references and a figure, with topics including stroke, cancer-associated thrombosis, hemophilia, coagulation, the interface between infection and inflammation, and in the experimental and discovery areas, megakaryocyte biology and platelet production, structure-function of key receptors and coagulation factors, and emerging new roles for thrombotic/hemostatic factors. Together, these articles highlight novel findings which will advance knowledge and with the potential to change clinical practice and improve outcomes. It is hoped that conference attendees and followers will enjoy utilizing the images for ongoing education and during the conference for live tweeting during sessions, to assist in the broadcasting and promotion of the science to those unable to attend, or who have chosen to attend a concurrent session. Use #IllustratedReview and #ISTH2019 on social media.