Collagen-related Peptide Research Articles

Evolution of Platelet Function and Bleeding in Children and Adults with Chronic Immune Trombocytopenia on Romiplostim Treatment

INTRODUCTION. Platelets in immune thrombocytopenia (ITP) are not only reduced in quantity, but were also reported to have impaired function. There is evidence that this might be the mechanism behind the clinically established fact that severity of bleeding in ITP does not correllate well with the degree of thrombocytopenia. However, there are no established methods or guidelines to predict bleeding risks based on the platelet function tests, and little information about the effects of thrombopoietin mimetics on platelet function.MATERIALS. We studied evolution of platelet function and clinical bleeding symptoms in 32 adult and 44 pediatric patients with chronic ITP 32 over six months of romiplostim treatment using flow cytometry and thrombelastography (TEG). As controls, samples of 20 healthy donors were used. The median age of adult patients was 62 (19-80 years), that of pediatric patients was 10 (1-18 years). All had previous history of treatment (one to three lines of therapy). Platelet function was characterized by flow cytometry before and after activation with SFLLRN plus collagen-related peptide. Levels of CD42b, CD61, CD62P, PAC1, annexin V binding, and mepacrine uptake and release were evaluated. Bleeding score was determined using a standardized consensus-based ITP-specific bleeding assessment tool developed by the International Working Group on ITP. Thrombelastography (TEG) was employed to characterize integral hemostasis function. Investigations were performed in accordance with the Declaration of Helsinki under a protocol approved by the NSPC CHOI Ethical Committee, and written informed consent was obtained from all donors and patients. Comparisons between groups were performed using Mann-Whitney non-parametric test.RESULTS. Bleeding before the start of therapy was observed in 10 adults and 25 children. There was no significant difference in platelet count between the bleeding and non-bleeding adults (median 25 versus 23, respectively). For children, it was significantly (median 19 versus 83, p<0.05) lower for those with bleeding. Both adult's and children's platelets were significantly pre-activated by all parameters (PAC1, CD62p, annexin V) and increased in size (forward scatter, CD42b, CD61, mepacrine uptake) compared with normal ones. The TEG lag time parameters R and K were significantly increased in the ITP patients, while α angle and MA were decreased. Platelet pre-activation, size, and TEG parameter changes were significantly greater (by 50-100%) in the adult patients with bleeding phenotype compared with those who had no hemorrhage. Evaluation of their predictive capabilities with regard to bleeding risks using ROC analysis produced AUC values between 0.78 and 0.91, allowing design of a risk calculator. The results for children were similar when adjusted for the platelet count difference in the bleeding and non-bleeding groups. Romiplostim treatment led to a rapid (<1 month) decrease of procoagulant annexin V-positive platelets in the circulation to the normal level, followed by a more gradual normalization of platelet size and decrease of PAC1-positive platelets over several months. In some adult and pediatric patients, disappearance of clinical bleeding syndromes on romiplostim was associated with normalization of platelet function parameters in the absence of platelet count increase.CONCLUSIONS. Our data indicate that: a) platelets in chronic ITP are pre-activated, large and slowly reacting; b) these changes are associated with bleeding independently of platelet count, and can be used for risk stratification; c) romiplostim treatment decreases platelet size and pre-activation. DisclosuresPanteleev:Amgen: Research Funding.

The GPIbα intracellular tail - role in transducing VWF- and collagen/GPVI-mediated signaling.

The GPIbα-VWF A1 domain interaction is essential for platelet tethering under high shear. Synergy between GPIbα and GPVI signaling machineries has been suggested previously, however its molecular mechanism remains unclear. We generated a novel GPIbα transgenic mouse (GpIbaΔsig/Δsig) by CRISPR-Cas9 technology to delete the last 24 residues of the GPIbα intracellular tail that harbors the 14-3-3 and phosphoinositide-3 kinase binding sites. GPIbαΔsig/Δsig platelets bound von Willebrand factor (VWF) normally under flow. However, they formed fewer filopodia on VWF/botrocetin in the presence of a aIIbb3 blocker, demonstrating that despite normal ligand binding, VWF-dependent signaling is diminished. Activation of GPIbαΔsig/Δsig platelets with ADP and thrombin was normal, but GPIbαΔsig/Δsig platelets stimulated with collagenrelated- peptide (CRP) exhibited markedly decreased P-selectin exposure and aIIbb3 activation, suggesting a role for the GpIba intracellular tail in GPVI-mediated signaling. Consistent with this, while hemostasis was normal in GPIbαΔsig/Δsig mice, diminished tyrosine-phosphorylation, (particularly pSYK) was detected in CRP-stimulated GPIbαΔsig/Δsig platelets as well as reduced platelet spreading on CRP. Platelet responses to rhodocytin were also affected in GPIbαΔsig/Δsig platelets but to a lesser extent than those with CRP. GPIbαΔsig/Δsig platelets formed smaller aggregates than wild-type platelets on collagen-coated microchannels at low, medium and high shear. In response to both VWF and collagen binding, flow assays performed with plasma-free blood or in the presence of aIIbb3- or GPVI-blockers suggested reduced aIIbb3 activation contributes to the phenotype of the GPIbαΔsig/Δsig platelets. Together, these results reveal a new role for the intracellular tail of GPIbα in transducing both VWF-GPIbα and collagen-GPVI signaling events in platelets.

Dapagliflozin reduces thrombin generation and platelet activation: implications for cardiovascular risk reduction in type 2 diabetes mellitus

Aims/hypothesisPeople with diabetes have an increased cardiovascular risk with an accelerated development of atherosclerosis and an elevated mortality rate after myocardial infarction. Therefore, cardioprotective effects of glucose-lowering therapies are of major importance for the pharmacotherapy of individuals with type 2 diabetes. For sodium–glucose cotransporter 2 inhibitors (SGLT2is), in addition to a reduction in blood glucose, beneficial effects on atherosclerosis, obesity, renal function and blood pressure have been observed. Recent results showed a reduced risk of worsening heart failure and cardiovascular deaths under dapagliflozin treatment irrespective of the diabetic state. However, the underlying mechanisms are yet unknown. Platelets are known drivers of atherosclerosis and atherothrombosis and disturbed platelet activation has also been suggested to occur in type 2 diabetes. Therefore, the present study investigates the impact of the SGLT2i dapagliflozin on the interplay between platelets and inflammation in atherogenesis.MethodsMale, 8-week-old LDL-receptor-deficient (Ldlr−/−) mice received a high-fat, high-sucrose diabetogenic diet supplemented without (control) or with dapagliflozin (5 mg/kg body weight per day) for two time periods: 8 and 25 weeks. In a first translational approach, eight healthy volunteers received 10 mg dapagliflozin/day for 4 weeks.ResultsDapagliflozin treatment ameliorated atherosclerotic lesion development, reduced circulating platelet–leucocyte aggregates (glycoprotein [GP]Ib+CD45+: 29.40 ± 5.94 vs 17.00 ± 5.69 cells, p < 0.01; GPIb+lymphocyte antigen 6 complex, locus G+ (Ly6G): 8.00 ± 2.45 vs 4.33 ± 1.75 cells, p < 0.05) and decreased aortic macrophage infiltration (1.31 ± 0.62 vs 0.70 ± 0.58 ×103 cells/aorta, p < 0.01). Deeper analysis revealed that dapagliflozin decreased activated CD62P-positive platelets in Ldlr−/− mice fed a diabetogenic diet (3.78 ± 1.20% vs 2.83 ± 1.06%, p < 0.01) without affecting bleeding time (85.29 ± 37.27 vs 89.25 ± 16.26 s, p = 0.78). While blood glucose was only moderately affected, dapagliflozin further reduced endogenous thrombin generation (581.4 ± 194.6 nmol/l × min) × 10−9 thrombin vs 254.1 ± 106.4 (nmol/l × min) × 10−9 thrombin), thereby decreasing one of the most important platelet activators. We observed a direct inhibitory effect of dapagliflozin on isolated platelets. In addition, dapagliflozin increased HDL-cholesterol levels. Importantly, higher HDL-cholesterol levels (1.70 ± 0.58 vs 3.15 ± 1.67 mmol/l, p < 0.01) likely contribute to dapagliflozin-mediated inhibition of platelet activation and thrombin generation. Accordingly, in line with the results in mice, treatment with dapagliflozin lowered CD62P-positive platelet counts in humans after stimulation by collagen-related peptide (CRP; 88.13 ± 5.37% of platelets vs 77.59 ± 10.70%, p < 0.05) or thrombin receptor activator peptide-6 (TRAP-6; 44.23 ± 15.54% vs 28.96 ± 11.41%, p < 0.01) without affecting haemostasis.Conclusions/interpretationWe demonstrate that dapagliflozin-mediated atheroprotection in mice is driven by elevated HDL-cholesterol and ameliorated thrombin–platelet-mediated inflammation without interfering with haemostasis. This glucose-independent mechanism likely contributes to dapagliflozin’s beneficial cardiovascular risk profile.Graphical abstract

Janus kinase inhibitors ruxolitinib and baricitinib impair glycoprotein-VI mediated platelet function

Several Janus kinase (JAK) inhibitors (jakinibs) have recently been approved to treat inflammatory, autoimmune and hematological conditions. Despite emerging roles for JAKs and downstream signal transducer and activator of transcription (STAT) proteins in platelets, it remains unknown whether jakinibs affect platelet function. Here, we profile platelet biochemical and physiological responses in vitro in the presence of five different clinically relevant jakinibs, including ruxolitinib, upadacitinib, oclacitinib, baricitinib and tofacitinib. Flow cytometry, microscopy and other assays found that potent JAK1/2 inhibitors baricitinib and ruxolitinib reduced platelet adhesion to collagen, as well as platelet aggregation, secretion and integrin αIIbβ3 activation in response to the glycoprotein VI (GPVI) agonist collagen-related peptide (CRP-XL). Western blot analysis demonstrated that jakinibs reduced Akt phosphorylation and activation following GPVI activation, where ruxolitinib and baricitinib prevented DAPP1 phosphorylation. In contrast, jakinibs had no effects on platelet responses to thrombin. Inhibitors of GPVI and JAK signaling also abrogated platelet STAT5 phosphorylation following CRP-XL stimulation. Additional pharmacologic experiments supported roles for STAT5 in platelet secretion, integrin activation and cytoskeletal responses. Together, our results demonstrate that ruxolitinib and baricitinib have inhibitory effects on platelet function in vitro and support roles for JAK/STAT5 pathways in GPVI/ITAM mediated platelet function.

MTOR regulates GPVI-mediated platelet activation

BackgroundDue to mTOR (mammalian/mechanistic target of rapamycin) gene-loss mice die during embryonic development, the role of mTOR in platelets has not been evaluated using gene knockout technology.MethodsA mouse model with megakaryocyte/platelet-specific deletion of mTOR was established, and be used to evaluate the role of mTOR in platelet activation and thrombus formation.ResultsmTOR−/− platelets were deficient in thrombus formation when grown on low-concentration collagen-coated surfaces; however, no deficiency in thrombus formation was observed when mTOR−/− platelets were perfused on higher concentration collagen-coated surfaces. In FeCl3-induced mouse mesenteric arteriole thrombosis models, wild-type (WT) and mTOR−/− mice displayed significantly different responses to low-extent injury with respect to the ratio of occluded mice, especially within the first 40 min. Additionally, mTOR−/− platelets displayed reduced aggregation and dense granule secretion (ATP release) in response to low doses of the glycoprotein VI (GPVI) agonist collagen related peptide (CRP) and the protease-activated receptor-4 (PAR4) agonist GYPGKF-NH2; these deficiencies were overcame by stimulation with higher concentration agonists, suggesting dose dependence of the response. At low doses of GPVI or PAR agonist, the activation of αIIbβ3 in mTOR−/− platelets was reduced. Moreover, stimulation of mTOR−/− platelets with low-dose CRP attenuated the phosphorylation of S6K1, S6 and Akt Ser473, and increased the phosphorylation of PKCδ Thr505 and PKCε Ser729. Using isoform-specific inhibitors of PKCs (δ, ɛ, and α/β), we established that PKCδ/ɛ, and especially PKCδ but not PKCα/β or PKCθ, may be involved in low-dose GPVI-mediated/mTOR-dependent signaling.ConclusionThese observations indicate that mTOR plays an important role in GPVI-dependent platelet activation and thrombus formation.

Evidence that GPVI is Expressed as a Mixture of Monomers and Dimers, and that the D2 Domain is not Essential for GPVI Activation.

Collagen has been proposed to bind to a unique epitope in dimeric glycoprotein VI (GPVI) and the number of GPVI dimers has been reported to increase upon platelet activation. However, in contrast, the crystal structure of GPVI in complex with collagen-related peptide (CRP) showed binding distinct from the site of dimerization. Further fibrinogen has been reported to bind to monomeric but not dimeric GPVI. In the present study, we have used the advanced fluorescence microscopy techniques of single-molecule microscopy, fluorescence correlation spectroscopy (FCS) and bioluminescence resonance energy transfer (BRET), and mutagenesis studies in a transfected cell line model to show that GPVI is expressed as a mixture of monomers and dimers and that dimerization through the D2 domain is not critical for activation. As many of these techniques cannot be applied to platelets to resolve this issue, due to the high density of GPVI and its anucleate nature, we used Förster resonance energy transfer (FRET) to show that endogenous GPVI is at least partially expressed as a dimer on resting and activated platelet membranes. We propose that GPVI may be expressed as a monomer on the cell surface and it forms dimers in the membrane through diffusion, giving rise to a mixture of monomers and dimers. We speculate that the formation of dimers facilitates ligand binding through avidity.

Acquired platelet GPVI receptor dysfunction in critically ill patients with sepsis

Glycoprotein VI (GPVI), the platelet immunoreceptor tyrosine activating motif (ITAM) receptor for collagen, plays a striking role on vascular integrity in animal models of inflammation and sepsis. Understanding ITAM-receptor signaling defects in humans suffering from sepsis may improve our understanding of the pathophysiology, especially during disease onset. In a pilot study, platelets from 15 patients with sepsis were assessed consecutively at day of admission, day 5 to 7, and the day of intensive care unit (ICU) discharge and subjected to comprehensive analyses by flow cytometry, aggregometry, and immunoblotting. Platelet function was markedly reduced in all patients. The defect was most prominent after GPVI stimulation with collagen-related peptide. In 14 of 15 patients, GPVI dysfunction was already present at time of ICU admission, considerably before the critical drop in platelet counts. Sepsis platelets failed to transduce the GPVI-mediated signal to trigger tyrosine phosphorylation of Syk kinase or LAT. GPVI deficiency was partially inducible in platelets of healthy donors through coincubation in whole blood, but not in plasma from patients with sepsis. Platelet aggregation upon GPVI stimulation increased only in those patients whose condition ameliorated. As blunted GPVI signaling occurred early at sepsis onset, this defect could be exploited as an indicator for early sepsis diagnosis, which needs to be confirmed in prospective studies.

Platelet-derived extracellular vesicles express NADPH oxidase-1 (Nox-1), generate superoxide and modulate platelet function

BackgroundPlatelets release platelet-derived extracellular vesicles (PDEVs) upon activation – in a process that is regulated by generation of reactive oxygen species (ROS). Platelet NADPH oxidase-1 (Nox-1) contributes to ROS generation and thrombus formation downstream of the collagen receptor GPVI. ObjectivesWe aimed to investigate whether PDEVs contain Nox-1 and whether this is relevant for PDEV-induced platelet activation. MethodsPDEVs were isolated through serial centrifugation after platelet activation with thrombin receptor agonist TRAP-6 (activated PDEVs) or in the absence of agonist (resting PDEVs). The physical properties of PDEVs were analyzed through nanoparticle tracking analysis. Nox-1 levels, fibrinogen binding and P-selectin exposure were measured using flow cytometry, and protein levels quantified by immunoblot analysis. ROS were quantified using DCF fluorescence and electron paramagnetic resonance. ResultsNox-1 was found to be increased on the platelet outer membrane upon activation and was present in PDEVs. PDEVs induced platelet activation, while co-addition of GPVI agonist collagen-related peptide (CRP) did not potentiate this response. PDEVs were shown to be able to generate superoxide in a process at least partially mediated by Nox-1, while Nox-1 inhibition with ML171 (also known as 2-APT) did not influence PDEV production. Finally, inhibition of Nox-1 abrogated PDEV-mediated platelet activation. ConclusionsPDEVs are able to generate superoxide, bind to and activate platelets in a process mediated by Nox-1. These data provide novel mechanisms by which Nox-1 potentiates platelet responses, thus proposing Nox-1 inhibition as a feasible strategy to treat and prevent thrombotic diseases.

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

AbstractThe 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.

Structural characterization of a novel GPVI-nanobody complex reveals a biologically active domain-swapped GPVI dimer

AbstractGlycoprotein VI (GPVI) is the major signaling receptor for collagen on platelets. We have raised 54 nanobodies (Nb), grouped into 33 structural classes based on their complementary determining region 3 loops, against recombinant GPVI-Fc (dimeric GPVI) and have characterized their ability to bind recombinant GPVI, resting and activated platelets, and to inhibit platelet activation by collagen. Nbs from 6 different binding classes showed the strongest binding to recombinant GPVI-Fc, suggesting that there was not a single dominant class. The most potent 3, Nb2, 21, and 35, inhibited collagen-induced platelet aggregation with nanomolar half maximal inhibitory concentration (IC50) values and inhibited platelet aggregation under flow. The binding KD of the most potent Nb, Nb2, against recombinant monomeric and dimeric GPVI was 0.6 and 0.7 nM, respectively. The crystal structure of monomeric GPVI in complex with Nb2 revealed a binding epitope adjacent to the collagen-related peptide (CRP) binding groove within the D1 domain. In addition, a novel conformation of GPVI involving a domain swap between the D2 domains was observed. The domain swap is facilitated by the outward extension of the C-C′ loop, which forms the domain swap hinge. The functional significance of this conformation was tested by truncating the hinge region so that the domain swap cannot occur. Nb2 was still able to displace collagen and CRP binding to the mutant, but signaling was abolished in a cell-based NFAT reporter assay. This demonstrates that the C-C′ loop region is important for GPVI signaling but not ligand binding and suggests the domain-swapped structure may represent an active GPVI conformation.

Platelet Activation through GPVI Receptor: Variability of the Response

Platelets are nuclear-free cell fragments responsible for preventing blood loss in case of the vascular wall injury. After a contact with collagen exposed when the vessel is damaged, platelets become activated through the receptor glycoprotein GPVI. This leads to platelet shape change, degranulation, aggregation, and procoagulant responses. The aim of this study was to observe changes in the concentration of Ca2+ in the cytosol and functional responses of platelets upon stimulation through the GPVI receptor. The study involved healthy adult volunteers and house mice Mus musculus of the C57Bl6 line (wild type). Calcium signaling was monitored by Fura-Red fluorophore fluorescence using BD FACS Canto II flow cytometer. Functional responses were observed on a flow cytometer by binding of human fibrinogen conjugated to a fluorescent label or by Biola optical aggregometry. When tested on a cytometer, platelets were activated by collagen-related peptide CRP and when tested by aggregometry, platelets were activated using collagen. As a result of the study, the following phenomenon was revealed: despite a significant variability in the human platelet cytosolic Ca2+ levels in response to stimulation, the variability in activation of platelet integrins, shape change, and the aggregation response was significantly lower. No variability in the platelet cytosolic Ca2+ levels in response to stimulation was observed in mice. The observed variability of the calcium response of platelets could be caused by differences in the GPVI expression or polymorphisms of the GPVI receptor gene in the studied donors. Similarities in the functional responses of platelets with different signaling suggest a variable contribution of the phosphoinositide branch of intracellular signaling in platelets in response to activation of GPVI.

Functional phenotype of phosphoinositide 3-kinase p85α-null platelets characterized by an impaired response to GP VI stimulation

AbstractPhosphoinositide 3-kinases (PI3Ks), a family of lipid kinases comprising 3 classes with multiple isoforms, have been shown to participate in different phases of platelet signaling. To investigate the roles that enzymes play in platelet function in vivo and determine which isoforms are important for particular signaling events, we analyzed platelet function of gene knockout mice deficient in the p85α regulatory subunit of heterodimeric class IA PI3K. The kinase activity of p85α–/– platelets was only 5% of the activity of platelets from wild-type littermates. Platelet aggregation induced by adenosine diphosphate (ADP), thrombin, U46619, phorbol 12-myristate 13-acetate (PMA), or botrocetin was not defective in p85α–/– mice, compared with wild-type animals. In contrast, aggregation induced by collagen and collagen-related peptide (CRP) was partially but readily impaired in p85α–/– mice. Both P-selectin expression and fibrinogen binding in response to CRP were also decreased to a similar extent in p85α–/– platelets. Platelets from p85α–/– mice appeared to spread poorly over a CRP-coated surface with intact filopodial protrusions. Significant attenuation of CRP-induced tyrosine phosphorylation in known PI3K effectors such as Btk, Tec, PKB/Akt, and phospholipase Cγ2 were observed with p85α–/– platelets, whereas no alteration was noted in upstream molecules of Syk, LAT, and SLP-76. Considered as a whole, these results provide the first genetic evidence that PI3K p85α plays a significant role in platelet function, almost exclusively in the glycoprotein (GP) VI/Fc receptor γ chain complex-mediated signaling pathway.

Bleeding Diathesis in Mice Lacking JAK2 in Platelets

The tyrosine kinase JAK2 is a critical component of intracellular JAK/STAT cytokine signaling cascades that is prevalent in hematopoietic cells such as hematopoietic stem and progenitor cells (HSPCs), megakaryocytes (MKs), and platelets. Individuals expressing somatic activating JAK2 mutations such as JAK2V617F commonly develop myeloproliferative neoplasms (MPNs) associated with serious complications, including venous and arterial thrombosis, a leading cause of mortality. Here, we investigated the role of JAK2 in hemostasis and thrombosis using Jak2fl/flPf4-Cre (Jak2Plt-/-) mice specifically lacking JAK2 within the platelet lineage. Jak2Plt-/- mice developed severe thrombocytosis with a 5-fold increase in circulating platelet number, MK hyperplasia, and splenomegaly. Jak2Plt-/-platelets were of normal size and the expression of major membrane surface glycoproteins was indistinguishable from controls, except for the integrin β3, which was reduced by 20%. Despite the thrombocytosis, Jak2Plt-/- mice had a severe bleeding diathesis, as evidenced by: 1) prolonged tail bleeding time; 2) failure to occlude in a ferric chloride-induced carotid artery injury model; and 3) failure to form stable thrombi in a laser-induced cremaster muscle injury model. Jak2Plt-/- platelets spread poorly on immobilized collagen or on immobilized fibrinogen following GPVI stimulation with the collagen-related peptide (CRP). Jak2Plt-/- platelets had defective α-granule secretion and integrin αIIbβ3 activation following stimulation with CRP, but not thrombin, and showed aggregation defects with low-doses of CRP. Together, the data support a GPVI-specific impairment in platelets lacking JAK2, a notion that was supported by impaired intracellular signaling following GPVI stimulation, as assessed by protein tyrosine phosphorylation. Jak2Plt-/- platelets adhered poorly to type I collagen under arterial shear rates in whole blood. However, JAK2 deletion in platelets did not alter plasma von Willebrand factor (VWF) levels or botrocetin-mediated binding of plasma VWF to GPIbα. Together, the results underline a critical role for JAK2 in platelet GPVI signaling and hemostatic function, which likely contributes to venous and arterial thrombosis observed in patients with MPNs with the activating JAK2V617F mutation. Disclosures No relevant conflicts of interest to declare.

Prospective Study Reveals Increased Platelet Function Associated with Multiple Myeloma and Its Treatment

Background: Multiple Myeloma (MM) is a rare incurable bone marrow cancer characterised by a malignant proliferation of plasma cells. MM is usually preceded by a premalignant and benign Monoclonal Gammopathy of Undetermined Significance (MGUS). The incidence of arterial and venous thrombosis in MM is substantially higher than in the normal population, however the cause of this increased thrombosis risk and the impact of MM on platelet function is unclear. Treatments for both newly diagnosed and relapsed/refractory patients with MM include Immunomodulatory drugs (IMiDs) such as thalidomide/lenalidomide-based combinations. These treatments improve considerably patient outcomes, however iMiD treatment also increases the risk of thrombotic complications in these patients. Aims: In this prospective study we explored the impact of MM and its treatment on platelet function. Methods: High throughput functional analysis was performed using platelets from normal healthy controls (n=31) and patients with MGUS (n=18), smouldering multiple myeloma (SMM, n= 20), and MM (26). The MM group was further divided into 3 treatment cohorts; (1) no treatment, (2) treatment with proteasome inhibitor (PI) and dexamethasone (Dex), and (3) treatment with PI, Dex, immunomodulatory drug (iMiD) and direct oral anticoagulant. Platelet aggregation and activation (fibrinogen binding and P-selectin exposure) were measured in response to a concentration range of agonists including ADP, the thrombin receptor agonist TRAP-6, collagen, collagen-related peptide (CRP), a thromboxane receptor agonist U46619 and epinephrine. Cereblon protein was detected in platelet protein extracts by immunoblot analysis. Results: Consistent with previous reports, modestly increased VWF and factor VIII levels were detected in MM patients, but no additional differences in coagulation parameters were detected in patient groups compared to normal healthy controls (other than expected due to anticoagulant usage). Platelet aggregation in response to each agonist was increased significantly in the MM patient group compared to the normal healthy controls, suggesting that platelet reactivity is elevated in MM patients through a common mechanism that is shared by different activation pathways or the involvement of multiple mechanisms. P-selectin exposure on platelets from MM patients was not significantly different from normal healthy donors, indicating that enhanced platelet reactivity in MM is specifically through modulation of integrin αIIbβ3 activation, fibrinogen binding and therefore enhanced aggregation. The effects of treatment on platelet function in patients on iMiD vs. non iMiD treatment were assessed. In the iMiD treatment group, patient platelets aggregated in response to lower concentrations of ADP, collagen, epinephrine and CRP in samples taken post-treatment compared to those taken before and during treatment. This demonstrates an increased sensitivity to platelet activation in these patients induced by treatment. Immunoblot analysis revealed that platelets contain cereblon, a therapeutic target of lenalidomide. The potential direct effects of iMiDs on platelets in vitro was therefore explored. Lenalidomide treatment (10mM) increased the ability of platelets to aggregate in response to low concentrations of each agonist tested when compared to normal controls. Conclusions: Platelet reactivity is increased in multiple myeloma and increased further upon iMiD treatment. The presence of the key therapeutic target for iMiDs in platelets and the ability of lenalidomide to modulate platelet function directly, reveals new avenues for investigation to determine the underlying mechanism of action. Disclosures Laffan: CSL: Consultancy; Pfizer: Consultancy; Sobi: Consultancy; Roche: Consultancy; LFB: Consultancy; Shire: Consultancy; Octapharma: Consultancy; Bayer: Speakers Bureau; Roche-Chugai: Speakers Bureau; Takeda: Speakers Bureau; Leo-Pharma: Speakers Bureau; Pfizer: Speakers Bureau. Shapiro:Bayer: Consultancy, Speakers Bureau; Pfizer: Consultancy, Speakers Bureau; NovoNordisk: Consultancy, Speakers Bureau; Sobi: Consultancy, Speakers Bureau; Chugai/Roche: Consultancy, Speakers Bureau; Shire/Takeda: Consultancy, Speakers Bureau. Thakurta:Oxford University: Other: visiting professor; Bristol Myers Squibb: Current Employment, Current equity holder in publicly-traded company. Ramasamy:Takeda: Research Funding; Janssen: Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees; Bristol Myers Squibb: Research Funding; Amgen: Research Funding; Amgen: Honoraria; Takeda: Honoraria; Sanofi: Honoraria; Oncopeptides: Honoraria; Takeda: Speakers Bureau; Abbvie: Membership on an entity's Board of Directors or advisory committees; GSK: Membership on an entity's Board of Directors or advisory committees; Oncopeptides: Membership on an entity's Board of Directors or advisory committees; Sanofi: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria; Bristol Myers Squibb: Honoraria; Janssen: Membership on an entity's Board of Directors or advisory committees; Bristol Myers squibb: Membership on an entity's Board of Directors or advisory committees. Gibbins:Bristol Myers Squibb: Research Funding; Arena Pharmaceuticals: Research Funding.

GSK669, a NOD2 receptor antagonist, inhibits thrombosis and oxidative stress via targeting platelet GPVI

Background and PurposePreviously, we discovered that the activation of nucleotide-binding oligomerization domain 2 (NOD2) enhances platelet activation. We here investigated the antiplatelet and antithrombotic potential of GSK669, a NOD2 antagonist. Experimental ApproachEffects of GSK669 on platelet functions, reactive oxygen species (ROS) and proinflammatory cytokine generation were detected. NOD2-/- platelets were used to confirm GSK669 target. The interaction between GSK669 and glycoprotein VI (GPVI) was detected using surface plasmon resonance (SPR) spectroscopy. GPVI downstream signaling was examined by Western blot. The antithrombotic and antioxidative effects were investigated using mouse mesenteric arteriole thrombosis model and pulmonary embolism model. Key ResultsGSK669 significantly inhibits platelet proinflammatory cytokine release induced by muramyl dipeptide, platelet aggregation, ATP release, and ROS generation induced by collagen and collagen related peptide (CRP). Platelet spreading and clot retraction are also inhibited. GSK669 also decreases collagen-induced phosphorylation of Src, Syk, PLCγ2, and Akt. The antiplatelet effect of GSK669 is NOD2-independent and mediated by GPVI antagonism. Consistent with its antiplatelet activity as a GPVI antagonist, GSK669 inhibits platelet adhesion on collagen in flow condition. Notably, GSK669 inhibits mouse mesenteric arteriole thrombosis similarly to aspirin without bleeding. The antithrombotic effect of GSK669 is further confirmed in the pulmonary embolism model; decreased malonaldehyde (MDA) and increased superoxide dismutase (SOD) levels in mouse plasma reveal a significant antioxidant effect of GSK669. Conclusion and ImplicationsBeyond its anti-inflammatory effect as a NOD2 antagonist, GSK669 is also an efficient and safe antiplatelet agent combined with antioxidant effect by targeting GPVI. An antiplatelet agent bearing antioxidative and anti-inflammatory activities without bleeding risk may have therapeutic advantage over current antiplatelet drugs for atherothrombosis.

Annexin A7 is a critical regulator of Ca2+ mobilization and lipid metabolism during platelet activation and arterial thrombosis

Abstract Background Platelet activation after contact to subendothelial collagen following atherosclerotic plaque rupture can lead to arterial thrombosis with acute thrombotic vascular occlusion. Annexin A7 (AnxA7) is an intracellular Ca2+- and phospholipid-binding protein that participates in the regulation of prostaglandin production in inflammatory diseases, but also in cell survival and tumor growth. Objective In the present study, we aimed to determine the role of AnxA7 for platelet Ca2+ signaling and lipid metabolism in platelet activation and arterial thrombosis in gene-targeted mice lacking annexin A7 (Anxa7−/−). Results AnxA7 is strongly expressed in platelets of platelet-rich human coronary thrombi aspirated from patients with acute ST elevation myocardial infarction. Functionally, platelet aggregation and dense granule secretion were significantly abrogated in Anxa7−/− platelets as compared to wildtype platelets (Anxa7+/+) after activation with collagen or collagen-related peptide (CRP), a specific agonist of the major platelet collagen receptor glycoprotein VI (GPVI). Further, in vitro thrombus formation on a collagen-coated surface under high arterial shear rates was significantly diminished in Anxa7-deficient platelets, and thrombotic vascular occlusion after FeCl3-induced injury in vivo was blunted in Anxa7−/−bone marrow chimeric mice, but no prolongation of bleeding time was observed. Moreover, Anxa7−/− platelets showed a significant reduction of IP3 production due to an abolished phospholipase C (PLC) gamma2 phosphorylation resulting in an abolished increase of [Ca2+]i after platelet activation with CRP. Moreover, we could show by quantitative lipidomics analysis that annexin A7 critically affects platelet oxylipid metabolism following activation of GPVI-dependent platelet signalling since Anxa7−/− platelets showed a significant reduction of the bioactive metabolites thromboxane A2 and 12(S)-hydroxy-eicosatetraenoic acid (12(S)-HETE) levels as well as significantly reduced levels of several other prostaglandins following stimulation with collagen or CRP. Finally, defective PLCgamma2 phosphorylation, IP1 production and blunted increase of [Ca2+]i in Anxa7−/− platelets could be rescued by exogenous addition of 12(S)-HETE indicating that AnxA7 is a critical regulator of the platelet oxygenase 12-lipoxygenase (12-LOX) in GPVI-dependent platelet Ca2+ signalling during arterial thrombosis following activation by collagen. Conclusions The present study reveals annexin A7 as a critical regulator of oxylipid metabolism and Ca2+ signaling in GPVI-dependent platelet activation. Anxa7-deficiency further results in decreased in vitro and in vivo thrombus formation, but does not affect bleeding time. In conclusion, annexin A7 plays an important role in platelet signaling during arterial thrombosis and thus, may reflect a promising target for novel antiplatelet strategies. Funding Acknowledgement Type of funding source: Public grant(s) – National budget only. Main funding source(s): German Research Foundation (Deutsche Forschungsgemeinschaft, DFG)

Flow Cytometric Assessment of AKT Signaling in Platelet Activation: An Alternative Diagnostic Tool for Small Volumes of Blood.

The diagnosis of platelet function disorder in children is challenging. Light transmission aggregometry is the gold standard for platelet function disorders. However, large blood volumes are required. Currently, there are no existing tools for the diagnosis of platelet function disorders that use small blood volumes. AKT signaling plays a central role in platelet activation during hemostasis and might be visualized by flow cytometry. Platelet-rich plasma obtained by centrifugation of citrated blood from healthy volunteers was activated with arachidonic acid, thrombin receptor activating peptide-6 (TRAP-6), collagen, adenosine diphosphate ADP, collagen-related peptide (CRP), and epinephrine. After platelet activation, the phosphorylation of AKT was assessed by flow cytometer using a Navios cytometer. Healthy volunteers showed a reproducible phosphorylation of AKT upon activation. In comparison to nonactivated platelets, we documented an increase in pAKT expression with all agonists. Especially TRAP-6 and CRP caused considerable increase in percentage of pAKT expression throughout all the tested healthy volunteers. An activation of the AKT-signal pathway by different agonists can clearly be detected on the flow cytometer, indicating that the visualization of signaling in platelets by flow cytometry might be an efficient alternative for light transmission aggregometry to test platelet function in children.

Cysteine sulfenylation by CD36 signaling promotes arterial thrombosis in dyslipidemia

AbstractArterial thrombosis in the setting of dyslipidemia promotes clinically significant events, including myocardial infarction and stroke. Oxidized lipids in low-density lipoproteins (oxLDL) are a risk factor for athero-thrombosis and are recognized by platelet scavenger receptor CD36. oxLDL binding to CD36 promotes platelet activation and thrombosis by promoting generation of reactive oxygen species. The downstream signaling events initiated by reactive oxygen species in this setting are poorly understood. In this study, we report that CD36 signaling promotes hydrogen peroxide flux in platelets. Using carbon nucleophiles that selectively and covalently modify cysteine sulfenic acids, we found that hydrogen peroxide generated through CD36 signaling promotes cysteine sulfenylation of platelet proteins. Specifically, cysteines were sulfenylated on Src family kinases, which are signaling transducers that are recruited to CD36 upon recognition of its ligands. Cysteine sulfenylation promoted activation of Src family kinases and was prevented by using a blocking antibody to CD36 or by enzymatic degradation of hydrogen peroxide. CD36-mediated platelet aggregation and procoagulant phosphatidylserine externalization were inhibited in a concentration-dependent manner by a panel of sulfenic acid–selective carbon nucleophiles. At the same concentrations, these probes did not inhibit platelet aggregation induced by the purinergic receptor agonist adenosine diphosphate or the collagen receptor glycoprotein VI agonist collagen-related peptide. Selective modification of cysteine sulfenylation in vivo with a benzothiazine-based nucleophile rescued the enhanced arterial thrombosis seen in dyslipidemic mice back to control levels. These findings suggest that CD36 signaling generates hydrogen peroxide to oxidize cysteines within platelet proteins, including Src family kinases, and lowers the threshold for platelet activation in dyslipidemia.

ASC regulates platelet activation and contributes to thrombus formation independent of NLRP3 inflammasome

BackgroundPlatelets are critical mediators of vascular homeostasis and thrombosis, and also contribute to the development of inflammation. NLRP3 inflammasome is a cytosolic multi-protein complex that consists of NLRP3, ASC and caspase-1, and regulates IL-1β-mediated inflammation. Method and ResultsUsing two mouse models of thrombosis (i.e., occlusion of the middle cerebral artery and inferior vena cava), we found that thrombus formation was significantly enhanced in ASC-deficient (ASC−/−) mice, compared to that in wild-type (WT) and IL-1β−/− mice. ASC deficiency had no effects on blood coagulation parameters (i.e., prothrombin time [PT] and activated partial thromboplastin time [APTT]). Platelets from WT mice express ASC, but neither NLRP3 nor caspase-1. ASC deficiency significantly enhanced the expression of P-selectin and GPIIb/IIIa in response to a GPVI agonist (collagen-related peptide [CRP]), but not to thrombin, in platelets. CRP induced ASC speck formation in WT platelets. ASC deficiency also enhanced cytosolic Ca2+ elevation and phosphorylation of ERK1/2 and Akt in platelets. ConclusionOur results demonstrate that ASC negatively regulates GPVI signaling in platelets and enhances thrombus formation, independent of NLRP3 inflammasome and IL-1β, and provide novel insights into the link between inflammation and thrombosis.

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

AbstractTrauma-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.